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The Metabotropic Glutamate Receptors: Structure and Functions
Abstract
Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to act only on ligand-gated receptor channels--termed NMDA, AMPA and kainate receptors--involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory synapses.
... On the basis of sequence homology, agonist potency and signal transduction mechanisms the mGluRs have been subdivided into three different groups (Pin and Duvoisin 1995) ( Fig. 1.15). Group I (mGluRla-d and mGluR5a,b) receptors are characterised by the stimulation of phospholipase C (PLC) which is revealed by an increase in phosphoinositide turnover, and calcium release from internal stores (Pin and Duvoisin 1995;Nakanishi 1992;Simoncini 1993). The G proteins involved in the activation of PLC by group I mGluRs have not been clearly identified. ...
... The inhibition observed, however, is often smaller than that obtained with group II mGluRs (50%), which suggests that either group III mGluRs do not have a very high density of expression in the membrane of these cells or that there is an inappropriate coupling of these receptors to this transduction pathway. However, the tranduction is totally inhibited by PTX suggesting that the G-protein in this coupling is of the Gj family (Pin and Duvoisin 1995). The mGluRG subtype which is found exclusively in retinol cells, however, is thought to have a different transduction pathway since it is thought to couple to a cGMP phosphodiesterase in its native environment (Shiells and Falk 1992 ...
... A major role for the mGluR has been suggested in plastic changes within the CNS. Both long-term potentiation (LTP) (Bashir et al. 1993) and long-term depression (LTD) (Aiba et al. 1994;Pin and Duvoisin 1995) may depend upon the activation of mGluRs. ...
p>In this study, in vitro spinal cord preparations have been used to investigate the possible role of excitatory amino acids in nociceptive mechanisms. The in vitro spinal cord dorsal horns showed good viability which was demonstrated by the development of spontaneous dorsal root activity a nd an evoked dorsal root reflex (DRR) 2-3 hours following dissection. Both these activities were blocked by the addition of manganese to the bathing medium, demonstrating them to be of synaptic origin. The DRR evoked by stimulation of an adjacent dorsal root showed evidence of both excitatory and inhibitory phases, similar to that described for hamster spinal cords (Bagust et al. 1982; Bagust et al. 1985a; Bagust et al. 1989). Stimulation of a lumbar dorsal root gave rise to dorsal horn field potentials. Investigations were made into the effects of the N-methyl-D-aspartate (NMDA) antagonists, 801 and the non-NMDA antagonist CNQX on fast and slow dorsal horn field potentials elictied by low and high intensity dorsal root stimulation respectively. None of the NMDA antagonists had any significant effect on fast wave dorsal root field potentials at concentrations up to 100 [jM, FC/NA a nd M K - 8 01 significantly inhibited the slow wave field potential. The non-NMDA antagonist CNQX significantly inhibited the synaptic components of both the fast and slow wave dorsal horn field potentials. It was concluded that low-threshold, non-noxious sensory transmission is mediated by non-NMDA glutamate receptors and that NMDA glutamate receptors have a role in the transmission of high-threshold, noxious sensory information in the dorsal horn of the spinal cord. arcaine sulphate, MK- but D-AP57 - CID - A P5 a nd 7 - CIKYNA, agonist (mGluR) group I The metabotropic glutamate receptor the synaptic components of the fast wave field potential. However, neither the group I antagonist L-AP3 nor the group II antagonist EGLU had any significant effect. The group III mGluR agonist L-AP4 significantly reduced the fast wave synaptic components. These results suggested a possible involvement of group I, II and III mGluRs in the processing of non-noxious information in the dorsal horn. The slow wave field potential elicited by high threshold dorsal root stimulation was inhibited by trans-ACPD, L-AP3 and EGLU. The group I mGluR agonist DHPG facilitated the slow wave at low concentrations. L-AP4 had no significant effect. It was therefore concluded that group I and II mGluRs but not group III mGluRs have a role in the processing of noxious sensory information in the spinal cord. trans-ACPD inhibited Expression of c-fos has previously been demonstrated in the dorsal horn of in vitro spinal cord preparations taken from 19-23 day old rats following high intensity dorsal root stimulation (Zhang et al. 1998). The expression of c-fos occurred only when dorsal root stimulation was sufficient to excite C fibres. Addition of 0 . %M capsaicin to the bathing medium induced intense c-fos staining in the absence of dorsal root stimulation. This provided a positive control suggesting that C fibre activation induces c-fos expression. Cords obtained from animals treated at 1 day old with capsaicin (50mg/kg) to destroy afferent C fibres showed a significant decrease in the number of Fos-positive cells induced by high intensity dorsal root stimulation when compared to vehicle treated animals. The effect of D-AP5, 7-CI KYNA and CNQX on pain-related expression of the immediate early gene c-fos was also investigated. Both D-AP5 and 7-CI KYNA significantly reduced the number of Fos-positive cells suggesting that NMDA receptors have a role in mediating expression. CNQX had no significant effect. This study has confirmed that the in vitro spinal cord is a valuable tool for the investigation of nociceptive processing and modulation in the dorsal horn of the spinal cord. The results support the suggestion that NMDA and group I and II metabotropic glutamate receptors acting at glutamatergic synapses in the dorsal horn of the spinal cord have a role in processing or modulating nociceptive input from C-fibre afferents.</p
... Metabotropic glutamatergic receptors (mGluR)[homodimers], which are mainly distributed in corticolimbic areas, can be broadly divided into three categories according to their structures, ligand recognition profiles and coupling to cellular transduction systems: group I (mGluR1 and 5) receptors, group II (mGluR2 and 3), and group III (mGluR4, 6,7 and 8) (32). ...
... Group I mGluR antagonists mimic certain properties of NMDA antagonists to some extent such as their influence on mood, as well as their neuroprotective effects (32,33). This similarity could be explained by the indirect facilitation of NMDA receptors by presynaptic group I receptors, and thus promoting glutamate release in cortico-limbic areas such as the amygdala (34). ...
... While group I receptors are positively coupled with Phospholipase C via Gq, group II and III receptors, on the other hand, are positively coupled to Gi, leading to an inhibitory effect on adenylyl cyclase (32,36) which subsequently suppresses the release of glutamate (37,38) and in turn modulates depressive states. As shown in Figure 1, mGluR 2/3 antagonists are thought to enhance synaptic glutamate levels, thereby boosting AMPAR transmission and firing rates and extracellular monoamine levels. ...
Depressive disorders are among the most common psychiatric conditions and contribute to significant morbidity. Even though the use of antidepressants revolutionized the management of depression and had a tremendous positive impact on the patient's outcome, a significant proportion of patients with major depressive disorder (MDD) show no or partial or response even with adequate treatment. Given the limitations of the prevailing monoamine hypothesis-based pharmacotherapy, glutamate and glutamatergic related pathways may offer an alternative and a complementary option for designing novel intervention strategies. Over the past few decades, there has been a growing interest in understanding the neurobiological underpinnings of glutamatergic dysfunctions in the pathogenesis of depressive disorders and the development of new pharmacological and non-pharmacological treatment options. There is a growing body of evidence for the efficacy of neuromodulation techniques, including transcranial magnetic stimulation, transcutaneous direct current stimulation, transcranial alternating current stimulation, and photo-biomodulation on improving connectivity and neuroplasticity associated with depression. This review attempts to revisit the role of glutamatergic neurotransmission in the etiopathogenesis of depressive disorders and review the current neuroimaging, neurophysiological and clinical evidence of these neuromodulation techniques in the pathophysiology and treatment of depression.
... mGluRs were indeed the first members of class C G-Protein Coupled Receptor (GPCR) family (Pin et al., 2003). Once the eight subtypes were identified, they were classified in three groups according to their sequence identity, transduction mechanism and pharmacological profile ( Fig. 1) (Conn and Pin, 1997;Pin and Acher, 2002;Pin et al., 1999;Pin and Duvoisin, 1995). ...
... Among the GPCRs, class C receptors hold 7 transmembrane helices (7TM) as members of the other classes. Yet their hallmark is a large extracellular amino-terminal domain (ATD) composed of a binding domain linked to the 7TM by a cysteine-rich domain (CRD) for most of them but the GABA B receptor where the CRD is replaced by a stalk domain (Conn and Pin, 1997;Niswender and Conn, 2010;Pin and Duvoisin, 1995;Shaye et al., 2020). ...
... Initially this ATD was represented as a linear thread ( Fig. 2A) (Pin and Duvoisin, 1995) until P. O'Hara and colleagues reported a similarity between the ATD of mGlu1 and Leucine-Isoleucine-Valine Periplasmic Binding Protein (LIVBP), a bacterial protein for which an X-ray structure was available (O'Hara et al., 1993;Sack et al., 1989). Their discovery was based on sensitive sequence analysis techniques revealing 22.3% identity over the first 496 N-terminal residues. ...
Metabotropic glutamate receptors (mGluRs) have been discovered almost four decades ago. Since then, their pharmacology has been largely developed as well as their structural organization. Indeed mGluRs are attractive therapeutic targets for numerous psychiatric and neurological disorders because of their modulating role of synaptic transmission. The more recent drug discovery programs have mostly concentrated on allosteric modulators. However, orthosteric agonists and antagonists have remained unavoidable pharmacological tools as, although not expected, many of them can reach the brain, or can be modified to reach the brain. This review focuses on the most common orthosteric ligands as well as on the few allosteric modulators interacting with the glutamate binding domain. The 3D-structures of these ligands at their binding sites are reported. For most of them, X-Ray structures or docked homology models are available. Because of the high conservation of the binding site, subtype selective agonists were not easy to find. Yet, some were discovered when extending their chemical structures in order to reach selective sites of the receptors.
... 14 On the basis of homology in gene sequences (30-60%), pharmacology, and signaling pathways, the metabotropic glutamate receptors have been categorized into three groups: Group I (mGluR1 and mGluR5), Group II (mGluR2 and mGluR3) and Group III (mGluRs 4, 6, 7 and 8). 7,[15][16][17][18][19] The genes that code for the multiple receptor subtypes and their chromosomal localization in humans are enlisted in Table 1. Alternative splicing events give rise to receptor isoforms differing in their nature, localization, and the C-terminus length. ...
... 61 mGluR5a and mGluR5b are the two receptor splice variants, differing only in the addition of a short segment of 32 amino acids to the cytoplasmic domain of mGluR5b. 62 Group II and III mGluRs mediate signaling via pertussis toxin (PTX) sensitive G i/o by inhibiting adenylyl cyclase activity, i.e., inhibitory cAMP cascade, regulation of voltage-sensitive channels: inhibit Ca 2+ channels, and activate K + channels, 11,15 as well as the activation of PI3-K and MAPK/ ERK pathways and, are primarily located in the pre-and extra-synaptic elements. 9,63,64 Phosphorylation of these receptors by G-protein coupled receptor kinases (GRKs) and protein kinases affects signaling, trafficking, and desensitization. ...
The non-essential amino acid glutamate acts as a major excitatory neurotransmitter and plays a significant role in the central nervous system (CNS). It binds with two different types of receptors, ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs), responsible for the postsynaptic excitation of neurons. They are important for memory, neural development and communication, and learning. Endocytosis and subcellular trafficking of the receptor are essential for the regulation of receptor expression on the cell membrane and excitation of the cells. The endocytosis and trafficking of the receptor are dependent on its type, ligand, agonist, and antagonist present. This chapter discusses the types of glutamate receptors, their subtypes, and the regulation of their internalization and trafficking. The roles of glutamate receptors in neurological diseases are also briefly discussed.
Keyword
Glutamate, Metabotropic glutamate receptors, Ionotropic glutamate receptors, Clathrin-mediated endocytosis, Signaling, Neurological diseases
... According to previous studies, amino acid neurotransmitters (AANTs), which are an important component of central neurotransmitters (NTs), can either activate or inhibit cGMP formation [17][18][19]. AANTs include excitatory AANTs, such as glutamate (Glu) and aspartate (Asp), as well as inhibitory AANTs, such as γ-aminobutyric acid (GABA), all of which are strongly associated with the mechanism of action of anesthetics [20,21]. Furthermore, AANTs have been linked to brain injury, neurodegeneration, analgesia, learning, and memory. ...
... These results suggested that the anesthetic and analgesic effects of Xyl may be produced by activating α2-adrenergic receptors and GABA receptors, while inhibiting NMDA receptors, leading to the inhibition of the NO-cGMP signaling pathway. In addition, Hammond [17] and Pin and Duvoisin [18] reported that excitatory AANTs activated metabotropic Glu receptors and modulated neurotransmission through the regulation of second messenger systems, including inhibiting adenylyl cyclase and potentiating cAMP formation and cGMP formation. However, many factors can influence the anesthetic effect of Xyl. ...
Background:
Xylazole (Xyl) is a veterinary anesthetic that is structurally and functionally similar to xylazine. However, the effects of Xyl in vitro remain unknown.
Objectives:
This study aimed to investigate the anesthetic mechanism of Xyl using fetal rat nerve cells treated with Xyl.
Methods:
Fetal rat nerve cells cultured for seven days were treated with 10, 20, 30, and 40 μg/ mL Xyl for 0, 5, 10, 15, 20, 25, 30, 45, 60, 90, and 120 min. Variations of amino acid neurotransmitters (AANTs), Nitric oxide-Cyclic GMP (NO-cGMP) signaling pathway, and ATPase were evaluated.
Results:
Xyl decreased the levels of cGMP and NO in nerve cells. Furthermore, Xyl affected the AANT content and Na+-K+-ATPase and Ca2+-Mg2+-ATPase activity in nerve cells. These findings suggested that Xyl inhibited the NO-cGMP signaling pathway in nerve cells in vitro.
Conclusions:
This study provided new evidence that the anesthetic and analgesic effects of Xyl are related to the inhibition of the NO-cGMP signaling pathway.
... All mGlu receptors can be activated by the endogenous orthosteric agonist glutamate, though with different potencies (J. P. Pin and Duvoisin 1995). Moreover, group-selective compounds (i.e. ...
... EC50 of glutamate for mGlu4 is in µM range) (J. P. Pin and Duvoisin 1995). Activation of the mGlu4 receptor leads to activation of its main signalling pathway through inhibition of adenylyl cyclase by Gαi, which will be discussed in more detail in the next subchapter. ...
Transmembrane receptors are proteins that translate extracellular information into intracellular signals. These proteins are categorised based on the way they transduce signals through the plasma membrane. In many cases such transduction involves the formation of dimers, either ligand-induced or constitutive dimers. For example, while the enzyme-linked receptors, such as the Epidermal Growth Factor (EGF) receptor, may be ligand-induced dimers, the G protein-coupled receptors (GPCR) activated by the neurotransmitter glutamate are covalently linked constitutive dimers. Understanding how a signal can be transduced from the extracellular domain into the cell is important to develop novel drugs targeting such receptors.The EGF receptor is a member of the receptor tyrosine kinase (RTK) class and is involved in cell differentiation and proliferation. Inhibition of the EGF receptor by medication is used as a successful treatment for a variety of cancers, but side effects and resistance to medication remain common major obstacles. The full-length EGF receptor crystal structure is unknown, which leaves ambiguity to the precise activation process and the effect of inhibitors. Its activation mechanism has been described as either ligand-induced dimerization or conformational changes within pre-assembled dimers. This is followed by phosphorylation of the intracellular domain of the receptor and the generation of intracellular signalling cascades. By generating a fluorescent conformational biosensor, we showed that the activation of the EGF receptor by its endogenous agonists results from ligand-induced dimerization. We also found that some non-competitive tyrosine kinase (TK) inhibitors also induce dimer formation, without activating the receptor, through a direct contact of the TK domains. Strikingly, internalization induced by its endogenous ligand is not blocked by the non-competitive inhibitors, demonstrating it does not require TK domain activity and is likely mainly dependent on receptor dimer conformation. This is supported by our observation that TK inhibitors promoting dimer formation slowed down the internalization process. This finding shows that the activation/dimerization process of the EGF receptor can be changed by inhibitors and that internalization of the EGF receptor is regulated by monomeric/dimeric conformations rather than by phosphorylation of the receptor.In contrast to the EGF receptor, the metabotropic glutamate (mGlu) 4 receptor is a constitutive dimer that is important for the regulation of many synapses in the brain. Recently, an mGlu2-mGlu4 heterodimer was proven to exist in the brain, but its function remains unknown due to the lack of tools selectively controlling it. To further investigate mGlu4 homo- and heterodimers, an innovative nanobody that discriminates between these receptors was developed. The nanobody is selective for the human mGlu4 receptor, and stabilizes the active homodimer conformation, making it a full agonist. By combining molecular dynamics simulations and site-directed mutagenesis, the epitope and mechanism of action of the nanobody were identified, revealing a new way of activating mGlu receptors. Conversely, this nanobody is incapable of activating heterodimeric mGlu2-mGlu4, where it acts as positive allosteric modulator through asymmetric activation of the heterodimer. This nanobody is the first pharmacological tool that discriminates between homo- and heterodimers.Overall, our studies revealed how important is the conformation of dimeric receptors to control their signalling, as shown with the TK inhibitors controlling to conformation of the TK domain-induced dimers and the internalization process of the EGF receptor, or by showing that nanobodies can control specifically a receptor dimer made of specific subunits. These findings show the importance of allosteric and asymmetric regulation of dimeric transmembrane receptors.
... We next examined why mGluR2 and mGluR3 act differently towards Aβ1-42 peptides' production while sharing an amino acid sequence homology of approximately 70% 25 and being coupled to the same Gα subunit. It has been described that APP can bind to the N-terminal sushi domain (SD1) of GB1a subunit of the GABAB receptor and this complex decreases the availability of APP to be processed by the β-secretase, thus limiting the production of the Aβ peptides in the endosomal compartment 26 . ...
Immunotherapy of Alzheimer′s disease (AD) is a promising approach to reduce the accumulation of amyloid-beta (Aβ), a critical event in the onset of the disease. Targeting the group II metabotropic glutamate receptors, mGlu2 and mGlu3, could be important in controlling Aβ production, although their respective contribution remains unclear due to the lack of selective tools. Here, we show that enhancing mGlu2 receptor activity increases Aβ1-42 peptide production whereas activation of mGlu3 has no effect. We show that such a difference likely results from the direct interaction of APP with mGlu3, but not with mGlu2 receptors, that prevents APP amyloidogenic cleavage and Aβ1-42 peptides production. We then show that chronic treatments of the AD model 5xFAD mice with a brain-penetrating mGlu2-potentiating nanobody accelerated amyloid aggregation and exacerbated memory deficits, but had no effect in control mice. Our results confirm that a selective mGluR2 activation exacerbates AD disease development, suggesting that therapeutic benefices could be obtained with blockers of this receptor. Our study also provides the proof-of-concept that chronic administration of nanobodies targeting neuroreceptors can be envisioned to treat brain diseases.
... Metabotropic glutamate receptors (mGluR 1-8) are divided into 3 groups and belong to a heterogeneous family of G protein-coupled receptors [12]. It is assumed that they function as presynaptic regulatory mechanisms that control the release of neurotransmitters and, accordingly, modulate brain excitability through presynaptic, postsynaptic, and glial mechanisms [13]. ...
Hyperprolactinemia is one of the common adverse events of antipsychotic therapy. The role of genetic factors in the development of drug-induced side effects is being actively investigated. The present study examined the association of two polymorphisms rs2237748 and rs2299472 in the GRM8 gene encoding the glutamate metabotropic receptor type 8 with antipsychotic-induced hyperprolactinemia in 536 patients with schizophrenia from several regions of Siberia (Russia). The investigated polymorphisms are not associated with drug-induced hyperprolactinemia in patients with schizophrenia. There were no associations of the GRM8 gene polymorphisms with serum prolactin levels in patients taking antipsychotic therapy. Our results did not confirm the involvement of the GRM8 rs2237748 and rs2299472 in the development of antipsychotic-induced hyperprolactinemia.
... The vesicle's internal glutamate content is estimated to be 100 mmol/L, and when it is released, it causes an excitatory postsynaptic potential (EPSP) that is predominantly linked to AMPA receptor activation and has faster kinetics than NMDA receptors while having a lower glutamate affinity [27]. Glutamate metabotropic receptors show a minor similarity to the GABA-B receptor and share significant sequence homology with other metabotropic receptors [28][29][30]. ...
The bidirectional communication among the different peptide neurotransmitters and their receptors influences brain, immunity, and behavior. Among the peptide neurotransmitters, Glutamate is the primary excitatory while; gamma-aminobutyrate (γ-GABA), is the inhibitory neurotransmitter. Glutamatergic/GABAergic imbalances are seen in many neurological and autoimmune disorders. With an aim to understand more deeply the intricacies of glutamate/GABA homeostasis, we provide a critical review of glutamate, glycine and GABA peptide neurotransmitters and their role in the brain, behavior, and immunity. Another aspect of maintaining this homeostasis has its origin in the gut-brain-axis which influences mood and behavior via the bidirectional biochemical exchange network between central (CNS) and enteric nervous system (ENS). This present review also provides evidence of the cross-talk between glutamate, glycine, and GABA along the microbiotagut- brain axis, thus any variations in this axis bear the consequences of the pathological condition. Drugs like alcohol, Benzodiazepines (Barbiturates) and neurosteroids inhibit the excitatory action of glutamate leading to an overall increase of glutamate/GABA ratio that causes relaxation of nerves. However, these drugs are misused and abused among drug addicts and now their commercial production is either banned or downsized and heavily monitored. Because only a limited number of drug molecules are considered in pharmaceutics and clinics as antidepressants, it is essential to focus on alternate peptide modulator analogues which are safe, eco-friendly and can be used as drugs to relieve stress and anxiety. In this review, we present a synopsis of the studies on synthetic GABAergic agonists or GABA modulators that can be targeted for future therapeutics and clinics.
... iGluRs are located on the head of postsynaptic dendritic spines, mediate fast excitatory neurotransmission, and are divided into three different subtypes: N-methyl-D-aspartate (NMDA), α-amino-3-hydroxyl-5methyl-4-isoxazole-propionate (AMPA), and kainic acid (KA) [12]. mGluRs are located on both presynaptic terminals and postsynaptic dendritic spines [13][14][15]. mGluRs mediate slower, modulatory neurotransmission, and based on their intracellular signaling and pharmacological properties, are categorized into three families. Group I mGluRs consists of mGluR1 and mGluR5 receptors, Group II mGluRs consists of mGluR2 and mGluR3 receptors, and Group III consists of mGluR4, mGluR6, mGluR7 and mGluR8 receptors [12]. ...
Cocaine is a psychostimulant that is one of the most widely used illicit drugs, particularly in America. Cocaine addiction is a chronic relapsing disease that is characterized by drug craving and loss of inhibitory control. Animal models of psychiatric diseases are essential to identify underlying neural circuitry and to test the effectiveness of novel pharmacotherapies to prevent relapse. Current research using animal models indicates that type 5 metabotropic glutamate receptors may be of particular importance to the onset and maintenance of cocaine addiction. This literature review provides a general overview of the glutamate system, and the animal models frequently used in the study of addiction and summarizes peer-reviewed research focused on cocaine-induced adaptations to the type 5 metabotropic glutamate receptors in mice. Cocaine administration in mouse models induces a range of neural changes in the brain, reflecting the neuroadaptations associated with cocaine addiction. Cocaine-induced adaptations to type 5 metabotropic glutamate receptor vary by brain region and by methodological constraints. Key neural changes that occur in the mouse brain following cocaine administration include adaptations in the dopaminergic and glutamatergic systems. The interplay between mGluR5 and the dopamine system plays a significant role in the neurobiological adaptations that drive cocaine addiction. Lastly, we cover the potential efficacy of targeting this receptor as a novel therapeutic option to prevent cocaine relapse. Selective antagonists of this receptor have been studied for their potential therapeutic effects in mouse models of cocaine addiction. These compounds reduce the conditioned responses to drug-associated cues and reduce the motivation to seek cocaine, thereby inhibiting relapse-like behavior, and have been found to modulate synaptic plasticity in brain regions involved in addiction, such as the nucleus accumbens and prefrontal cortex. Overall, these compounds demonstrate promising effects in mouse models of cocaine addiction.
... The metabotropic glutamate receptors are categorized into three distinct subtypes, based on their structure and functions: group I, comprising mGluR1 and mGluR5; group II, comprising mGluR2 and mGluR3; and group III, comprising mGluR4 and mGluR6-8. The mGluR belongs to the G-protein-coupled receptors Pharmaceuticals 2023, 16, 1127 2 of 16 (GPCRs), which represent the largest class of drug targets, accounting for over 40% of marketed drugs [4,5]. Among these receptors, mGluR5 has been shown to play critical roles in synaptic plasticity and neuronal development [6,7]. ...
The metabotropic glutamate receptor subtype 5 (mGluR5) is a class C G-protein-coupled receptor (GPCR) that has been implicated in various neuronal processes and, consequently, in several neuropsychiatric or neurodevelopmental disorders. Over the past few decades, mGluR5 has become a major focus for pharmaceutical companies, as an attractive target for drug development, particularly through the therapeutic potential of its modulators. In particular, allosteric binding sites have been targeted for better specificity and efficacy. In this context, Positron Emission Tomography (PET) appears as a useful tool for making decisions along a drug candidate’s development process, saving time and money. Thus, PET provides quantitative information about a potential drug candidate and its target at the molecular level. However, in this area, particular attention has to be given to the interpretation of the PET signal and its conclusions. Indeed, the complex pharmacology of both mGluR5 and radioligands, allosterism, the influence of endogenous glutamate and the choice of pharmacokinetic model are all factors that may influence the PET signal. This review focuses on mGluR5 PET radioligands used at several stages of central nervous system drug development, highlighting advances and setbacks related to the complex pharmacology of these radiotracers.
... 12 Historically, these genes have been categorized into three groups based on sequence homology, pharmacology, and signal transduction mechanisms. 13 The Group 1 mGluRs consist of mGluR1 and mGluR5, and almost locate exclusively on postsynaptic membrane. They critically modulate synaptic transmission, plasticity, and adaptive behaviors. ...
Background:
Anti-metabotropic glutamate receptor 5 (mGluR5) encephalitis is a rare and under-recognized autoimmune encephalitis. This study is conducted to characterize its clinical and neuroimaging features.
Methods:
Twenty-nine patients with anti-mGluR5 encephalitis (15 new cases identified in this study and 14 previously reported cases) were included in this study and their clinical features were characterized. Brain MRI volumetric analysis using FreeSurfer software was performed in 9 new patients and compared with 25 healthy controls at both early (≤6 months of onset) and chronic (>1 year of onset) disease stages.
Results:
The common clinical manifestations of anti-mGluR5 encephalitis included cognitive deficits (n = 21, 72.4%), behavioral and mood disturbances (n = 20, 69%), seizures (n = 16, 55.2%), and sleep disorder (n = 13, 44.8%). Tumors were observed in 7 patients. Brain MRI T2/FLAIR signal hyperintensities were observed predominantly in mesiotemporal and subcortical regions in 75.9% patients. MRI volumetric analysis demonstrated significant amygdala enlargement in both early and chronic disease stages compared to healthy controls (P < 0.001). Twenty-six patients had complete or partial recovery, one remained stable, one died and one was lost to follow-up.
Conclusion:
Our findings demonstrated that cognitive impairment, behavioral disturbance, seizures, and sleep disorder are the prominent clinical manifestations of anti-mGluR5 encephalitis. Most patients showed a good prognosis with full recovery, even in the paraneoplastic disease variants. The amygdala enlargement in the early and chronic disease stages is a distinct MRI feature, which exploratively offer a valuable perspective for the study of the disease processes.
... Excitotoxicity was one of the first mechanisms of ischemic cell death to be identified and one of the most intensively studied, with the term "excitotoxicity" describing the process by which excess glutamate overactivates NMDA receptors (NMDARs) and induces neuronal toxicity (Choi et al., 1988;Garthwaite et al., 1992). There are two types of glutamate receptors: ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors (Pin and Duvoisin, 1995;Ferraguti and Shigemoto, 2006). The ionotropic receptors include kainate (KA) receptors, alpha-amino-3-hydroxy-5methyl-4-isoxazole propionic acid (AMPA) receptors, and N-methyl-D-aspartate (NMDA) receptors (Takahashi, 2019;Burada et al., 2020). ...
Glutamate plays an important role in excitotoxicity and ferroptosis. Excitotoxicity occurs through over-stimulation of glutamate receptors, specifically NMDAR, while in the non-receptor-mediated pathway, high glutamate concentrations reduce cystine uptake by inhibiting the System Xc-, leading to intracellular glutathione depletion and resulting in ROS accumulation, which contributes to increased lipid peroxidation, mitochondrial damage, and ultimately ferroptosis. Oxidative stress appears to crosstalk between excitotoxicity and ferroptosis, and it is essential to maintain glutamate homeostasis and inhibit oxidative stress responses in vivo. As researchers work to develop natural compounds to further investigate the complex mechanisms and regulatory functions of ferroptosis and excitotoxicity, new avenues will be available for the effective treatment of ischaemic stroke. Therefore, this paper provides a review of the molecular mechanisms and treatment of glutamate-mediated excitotoxicity and ferroptosis.
... The eight mGluRs are divided into three families based on sequence identity and receptor function. Group I (mGluR1 and mGluR5) stimulates phospholipase C via coupling to Gαq/11, which leads to elevated intracellular Ca 2+ levels, whereas group II (mGluR2 and mGluR3) and group III (mGluR4, mGluR6, mGluR7, and mGluR8) activate Gαi/o, which can inhibit adenylate cyclase or regulate ion channels (25,26). Despite this functional divergence, mGluR subtypes share a common dimer interface (15) and highly conserved residues at the endogenous ligand-binding pocket (27). ...
Upon ligand binding to a G protein-coupled receptor (GPCR), extracellular signals are transmitted into a cell through sets of residue interactions that translate ligand binding into structural rearrangements. These interactions needed for functions impose evolutionary constraints so that, on occasion, mutations in one position may be compensated by other mutations at functionally coupled positions. To quantify the impact of amino acid substitutions in the context of major evolutionary divergence in the GPCR sub-family of metabotropic glutamate receptors (mGluRs), we combined two phylogenetic-based algorithms, Evolutionary Trace (ET) and Covariation ET (CovET), to infer potential structure-function couplings and roles in mGluRs. We found a subset of evolutionarily important residues at known functional sites and evidence of coupling among distinct structural clusters in mGluR. Additionally, experimental mutagenesis and functional assays confirmed that some highly covariant residues are coupled, revealing their synergy. Collectively, these findings inform a critical step towards understanding the molecular and structural basis of amino acid variation patterns within mGluRs and provide insight for drug development, protein engineering, and analysis of naturally occurring variants.
... Glutamate acts through two main classes of ionotropic glutamate receptors, N-methyl-D-aspartate (NMDA) receptors and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, as well as via metabotropic glutamate receptors (mGlu) receptors. There are eight types of mGlu receptors that can be grouped according to their expression and function [6,7]. Group-I mGlu receptors comprise mGlu1 and mGlu5, which are expressed post-synaptically in the CNS [8,9] and are coupled to Gq proteins, whose activation leads to the formation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol [10,11]. ...
Glutamate is the major excitatory neurotransmitter in the central nervous system, and there is evidence that Group-I metabotropic glutamate receptors (mGlu1 and mGlu5) have established roles in excitatory neurotransmission and synaptic plasticity. While glutamate is abundantly present in the gut, it plays a smaller role in neurotransmission in the enteric nervous system. In this study, we examined the roles of Group-I mGlu receptors in gastrointestinal function. We investigated the expression of Grm1 (mGlu1) and Grm5 (mGlu5) in the mouse myenteric plexus using RNAscope in situ hybridization. Live calcium imaging and motility analysis were performed on ex vivo preparations of the mouse colon. mGlu5 was found to play a role in excitatory enteric neurotransmission, as electrically-evoked calcium transients were sensitive to the mGlu5 antagonist MPEP. However, inhibition of mGlu5 activity did not affect colonic motor complexes (CMCs). Instead, inhibition of mGlu1 using BAY 36-7620 reduced CMC frequency but did not affect enteric neurotransmission. These data highlight complex roles for Group-I mGlu receptors in myenteric neuron activity and colonic function.
... The postsynaptic responses of neurons to glutamate in the hippocampus are mediated by a range of ionotropic and metabotropic glutamate receptors [1,2]. Ionotropic glutamate layers of the neocortex, the molecular layer of the cerebellum, the pre-subiculum, and the septum ( Figure 1A,C,D,F). ...
Tau pathology is a hallmark of Alzheimer’s disease (AD) and other tauopathies, but how pathological tau accumulation alters the glutamate receptor dynamics driving synaptic dysfunction is unclear. Here, we determined the impact of tau pathology on AMPAR expression, density, and subcellular distribution in the hippocampus of P301S mice using immunoblot, histoblot, and quantitative SDS-digested freeze-fracture replica labeling (SDS-FRL). Histoblot and immunoblot showed differential regulation of GluA1 and GluA2 in the hippocampus of P301S mice. The GluA2 subunit was downregulated in the hippocampus at 3 months while both GluA1 and GluA2 subunits were downregulated at 10 months. However, the total amount of GluA1-4 was similar in P301S mice and in age-matched wild-type mice. Using quantitative SDS-FRL, we unraveled the molecular organization of GluA1-4 in various synaptic connections at a high spatial resolution on pyramidal cell spines and interneuron dendrites in the CA1 field of the hippocampus in 10-month-old P301S mice. The labeling density for GluA1-4 in the excitatory synapses established on spines was significantly reduced in P301S mice, compared to age-matched wild-type mice, in the strata radiatum and lacunosum-moleculare but unaltered in the stratum oriens. The density of synaptic GluA1-4 established on interneuron dendrites was significantly reduced in P301S mice in the three strata. The labeling density for GluA1-4 at extrasynaptic sites was significantly reduced in several postsynaptic compartments of CA1 pyramidal cells and interneurons in the three dendritic layers in P301S mice. Our data demonstrate that the progressive accumulation of phospho-tau is associated with alteration of AMPARs on the surface of different neuron types, including synaptic and extrasynaptic membranes, leading to a decline in the trafficking and synaptic transmission, thereby likely contributing to the pathological events taking place in AD.
... The GRM2 and GRM3 genes encode human mGlu2 and mGlu3 receptor proteins, respectively [mGlu2 receptor accession numbers: NP_000830 (human), NP_001099181 (rat), and NP_001153825 (mouse); mGlu3 receptor accession numbers: NP_000831 (human), NP_001099182 (rat), and NP_862898 (mouse)] (Tanabe et al., 1992;Flor et al., 1995;Nicoletti et al., 2011). The amino acid sequence of the mGlu2 receptor shares approximately 70% homology with the mGlu3 receptor (Pin and Duvoisin, 1995). While no evidence for alternative splicing of GRM2 was observed, alternative splicing of GRM3 resulted in three transcript (mRNA) splice variants different from the canonical full-length GRM3 transcript: GRM3Δ2 (lacking exon 2), GRM3Δ4 (lacking exon 4), and GRM3Δ2Δ3 (lacking exons 2 and 3) (Sartorius et al., 2006). ...
Group II metabotropic glutamate (mGlu) receptors (mGlu2/3) are Gαi/o-coupled receptors and are primarily located on presynaptic axonal terminals in the central nervous system. Like ionotropic glutamate receptors, group II mGlu receptors are subject to regulation by posttranslational phosphorylation. Pharmacological evidence suggests that several serine/threonine protein kinases possess the ability to regulate mGlu2/3 receptors. Detailed mapping of phosphorylation residues has revealed that protein kinase A (PKA) phosphorylates mGlu2/3 receptors at a specific serine site on their intracellular C-terminal tails in heterologous cells or neurons, which underlies physiological modulation of mGlu2/3 signaling. Casein kinases promote mGlu2 phosphorylation at a specific site. Tyrosine protein kinases also target group II receptors to induce robust phosphorylation. A protein phosphatase was found to specifically bind to mGlu3 receptors and dephosphorylate the receptor at a PKA-sensitive site. This review summarizes recent progress in research on group II receptor phosphorylation and the phosphorylation-dependent regulation of group II receptor functions. We further explore the potential linkage of mGlu2/3 phosphorylation to various neurological and neuropsychiatric disorders, and discuss future research aimed at analyzing novel biochemical and physiological properties of mGlu2/3 phosphorylation.
... All eight subtypes of mGluR which are classified into 3 groups, are expressed in the hippocampus [10]. The group II mGluRs (mGluR2, mGluR3) and group III mGluRs (mGluR4, mGluR6, mGluR7, mGluR8) are negatively linked to adenylnyl cyclase and suppress excitatory synaptic transmission at presynaptic sites [11]. The group 1 mGluRs (mGluR1, mGluR5) are coupled to inositol phospholipid hydrolysis and induce subsequent [Ca 2+ ] i increases by releasing Ca 2+ from IP 3 -sensitive intracellular stores [12,13], which can induce sustained depolarization through activation of TRPC channels [14]. ...
Group 1 metabotropic glutamate receptors (mGluRs) can positively affect postsynaptic neuronal excitability and epileptogenesis. The objective of the present study was to determine whether group 1 mGluRs might be involved in synaptically-induced intracellular free Ca2+ concentration ([Ca2+]i) spikes and neuronal cell death induced by 0.1 mM Mg2+ and 10 µM glycine in cultured rat hippocampal neurons from embryonic day 17 fetal Sprague-Dawley rats using imaging methods for Ca2+ and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays for cell survival. Reduction of extracellular Mg2+ concentration ([Mg2+]o) to 0.1 mM induced repetitive [Ca2+]i spikes within 30 sec at day 11.5. The mGluR5 antagonist 6-Methyl-2-(phenylethynyl) pyridine (MPEP) almost completely inhibited the [Ca2+]i spikes, but the mGluR1 antagonist LY367385 did not. The group 1 mGluRs agonist, 3,5-dihydroxyphenylglycine (DHPG), significantly increased the [Ca2+]i spikes. The phospholipase C inhibitor U73122 significantly inhibited the [Ca2+]i spikes in the absence or presence of DHPG. The IP3 receptor antagonist 2-aminoethoxydiphenyl borate or the ryanodine receptor antagonist 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate also significantly inhibited the [Ca2+]i spikes in the absence or presence of DHPG. The TRPC channel inhibitors SKF96365 and flufenamic acid significantly inhibited the [Ca2+]i spikes in the absence or presence of DHPG. The mGluR5 antagonist MPEP significantly increased the neuronal cell survival, but mGluR1 antagonist LY367385 did not. These results suggest a possibility that mGluR5 is involved in synaptically-induced [Ca2+]i spikes and neuronal cell death in cultured rat hippocampal neurons by releasing Ca2+ from IP3 and ryanodine-sensitive intracellular stores and activating TRPC channels.
... For this reason, these processes are typically investigated in freely behaving rats and mice [93]. MGlu5 is positively coupled to phospholipase C and activation of the receptor leads to the generation of inositol trisphospate (IP3) [24,94,95]. The binding of IP3 to receptors on the endoplasmic reticulum leads to release of calcium into the cytosol and subsequent activation of enzymes, such as protein kinase C (PKC), that are stimulated by intracellular Ca 2+ elevations [96][97][98][99]. ...
The metabotropic glutamate (mGlu) receptor family consists of group I receptors (mGlu1 and mGlu5) that are positively coupled to phospholipase-C and group II (mGlu2 and mGlu3) and III receptors (mGlu4-8) that are negatively coupled to adenylyl cyclase. Of these, mGlu5 has emerged as a key factor in the induction and maintenance of persistent (> 24 h) forms of hippocampal synaptic plasticity. Studies in freely behaving rodents have revealed that mGlu5 plays a pivotal role in the stabilisation of hippocampal long-term potentiation (LTP) and long-term depression (LTD) that are tightly associated with the acquisition and retention of knowledge about spatial experience. In this review article we shall address the state of the art in terms of the role of mGlu5 in forms of hippocampal synaptic plasticity related to experience-dependent information storage and present evidence that normal mGlu5 function is central to these processes.
... Metabotropic glutamate receptors are coupled to a variety of second messenger systems vm G proteins. To date, eight different metabotropic glutamate receptors have been cloned and subsequently expressed in cell lines (Schoepp et al., 1999); (Pin and Duvoisin, 1995). ...
p> Caenorhabditis elegans is a well described nematode employed as a genetic model organism for studies into function, differentiation, development, and morphology of simple nervous and muscular systems. However, essential information regarding its physiology and pharmacology are lacking. To address this, electrophysiological recording techniques from pharyngeal muscle cells were developed to determine the ionic basis of the resting membrane potential and action potential. Resting membrane potential of pharyngeal muscle cells were relatively unaffected by changes in the extracellular concentrations of Cl<sup>-</sup>, Na<sup>+</sup> or Ca<sup>++</sup>. However, variations in extracellular concentrations of K<sup>+</sup>, or exposure to ouabain, both elicited a depolarisation, although the depolarisations observed during elevations of extracellular K<sup>+</sup> were less than would be predicted if the membrane potential were completely dependent on K<sup>+</sup>. It can be concluded therefore, that the resting membrane potential is largely determined by a K<sup>+</sup> permeability, and a ouabain-sensitive, electrogenic pump. Action potential height was reduced or increased in concentration-dependent manner following exposure to low or high extracellular Ca<sup>++</sup> concentrations respectively. Furthermore, the L-type Ca<sup>++</sup> channel blockers, verapamil and nifedipine, both reduced action potential amplitude and duration. This suggests a role for an L-type Ca<sup>++</sup> channel in the action potential. However, action potentials persisted in Ca<sup>++</sup> free saline. Action potential duration increased or decreased in a concentration-dependent manner following exposure to low or high Ca<sup>++</sup> concentrations respectively. This suggests that the repolarisation phase is partly determined by a Ca<sup>++</sup> activated K<sup>+</sup> channel. Possibly the most surprising finding was the absolute dependence of pharyngeal action potential generation on extracellular Na<sup>+</sup> concentration, especially as extensive searches of the C. elegans genome have failed to find any obvious candidate for a voltage-gated Na<sup>+</sup> channel.</p
... To date, eight distinct metabotropic glutamate receptor subtypes (termed mGluRl-GluR8) have been identified by molecular cloning (Conn and Pin, 1997;Pin and Duvoisin, 1995 loop and the carboxy tail region appear to be critical for coupling of mGluR to G proteins, rather than the third intracellular loop (Dingledine and Conn, 2000). ...
p>Glutamate is the principal neurotransmitter in mammals and its signaling is conserved between Caenorhabditis elegans and mammals. An important role of glutamate transmission in C.elegans has been established by genetic investigations of ionotropic glutamate receptors (glr-J, nmr-1), glutamate-gated chloride channels (avr-15) and vesicular glutamate transporters (eat-4). Metabotropic glutamate receptors are hypothesized to modulate the transmission properties of synapses in response to the magnitude and frequency of glutamate signaling. Three mGluR-like genes (mgl-1, mgl-2 and mlg-3) have been identified in C. elegans accordingly. The aim of this MPhil/PhD project is to investigate the roles of metabotropic glutamate receptors (mGluRs) in regulating behaviour, using the nematode C.elegans as a model organism. We obtained, backcrossed and characterized mgl mutants. Accordingly we assume that mgl-1 (1811). mgl-2(tm355) and mgl-3(tm!766) are functional null mutants. We also made double mutants and triple mutants by outcrossing. Behavioural assays were carried out on the mutants, including longevity, growth rates, body bends, pharyngeal pumping, thrashing, forward and backward movement, locomotion towards bacteria, Osmotic Avoidance. The analysis on the mutant strains suggest? that mgl-1 (tml811) mutants have a selectively disrupted switch between forward and backward locomotion. Backward movement is regulated during foraging. When food source is detected by worms, backward movement dramatically decreases. Locomotion towards bacteria is disrupted in mgl-2(tm355) mutants, they could not get to the food source as fast as the wild type. mgt-3 (tml 766) behaves normally in all the tested assays. Existing and self generated reporter strains have allowed us :o study mgl expression patterns. An analysis of MGL-3::GFP suggests mgl-3 is expressed in NSM and some other neurons, which would support further behavioral analysis. This behavioural analysis of mgl mutants and expression pattern study of MGL provides an insight of neuromodulatory roles for these G-protein coupled receptors and suggest they play a broad role in defined circuits that integrate complex behaviour.</p
... Les récepteurs métabotropiques au glutamate sont des récepteurs à sept domaines transmembranaires couplés à des protéines G qui sont capables de moduler l'excitabilité neuronale via l'activation d'effecteurs et de seconds messagers (Pin and Duvoisin, 1995). Figure 11), ainsi que de leurs homologies de séquence et de leur sélectivité pharmacologique (Nakanishi, 1992). ...
Le trouble de l'usage d'alcool (TUA) est une pathologie chronique et hautement récidivante caractérisée par un usage compulsif d'alcool. Ces dernières années, un nombre grandissant d'études suggère un rôle clé du glutamate dans les aspects renforçant et motivationnel de l'alcool. Durant cette thèse, nous avons donc cherché à déterminer si l'utilisation de modulateurs pharmacologiques des transmissions glutamatergiques pouvait permettre de réduire les propriétés addictives de l'alcool 1) dans un modèle préclinique de binge drinking, caractérisé par une consommation excessive d'alcool sur une courte période de temps et 2) dans un modèle d'alcoolodépendance marqué par un fort état émotionnel négatif au cours du sevrage. Dans un premier temps, nous avons démontré que la N-acétylcystéine, un précurseur de la cystéine, nécessaire au fonctionnement des échangeurs cystine/glutamate (xc-), permet de réduire la consommation d'alcool et la rechute dans ces 2 modèles de TUA. De façon intéressante, nous avons pu constater que la N-acétylcystéine est plus efficace chez les animaux alcoolodépendants (possédant moins de transporteurs xc-). Pour finir, nous avons révélé que le LSP2-9166, un nouvel agoniste orthostérique des récepteurs métabotropiques au glutamate du groupe III, permet également de réduire la consommation d'alcool, la motivation et la rechute dans le modèle de binge drinking. Ces résultats s'ajoutent à un nombre croissant d'études montrant que les récepteurs métabotropiques au glutamate jouent un rôle clé dans la dépendance à l'alcool, faisant d'eux un nouvel espoir thérapeutique pour le traitement du TUA
... Les récepteurs métabotropiques au glutamate (mGluR) sont des protéines à 7 domaines transmembranaires couplés à des protéines G. Huit sous-types de mGluR ont été clonés (mGluR1 à mGluR8) et classés en trois groupes en fonction de leurs homologies de séquence polypeptidique et du second messager impliqué (Pin and Duvoisin, 1995 (Minakami et al., 1993;Pin et al., 1992). Les extrémités intracellulaires de ces variants sont différentes ce qui leur confère des interactions spécifiques avec les protéines intracellulaires responsables de la régulation de leur adressage au niveau membranaire. ...
L'alcool (EtOH) est une des substances d'abus les plus consommées en France chez les adolescents comme chez les adultes. La consommation d'EtOH induit des déficits mnésiques en perturbant les phénomènes de plasticité synaptique de type potentialisation à long terme (PLT) et dépression à long terme (DLT) dépendants du récepteur NMDA (PLTNMDA et DLTNMDA), et qui constituent la base cellulaire des apprentissages et de la mémoire, notamment au niveau de l'hippocampe. La composition en sous-unités GluN2A et GluN2B du récepteur NMDA peut influencer l'induction de ces deux formes de plasticité synaptique selon le modèle théorique de Bienenstock et al. (1982). De plus, la plasticité synaptique est sous l'influence de mécanismes épigénétiques et/ou de facteurs de transcription. A l'heure actuelle, les mécanismes cellulaires qui sous-tendent la perturbation de la plasticité synaptique suite à la consommation d'EtOH demeurent mal compris. Durant ma thèse, j'ai testé l'hypothèse que les perturbations de la plasticité synaptique dépendante du NMDA impliqueraient une modulation des sous-unités GluN2A et GluN2B dans un modèle de binge drinking-like chez le rat jeune adulte et dans un modèle d'alcoolisation chronique chez des souris adultes. Afin de comprendre les mécanismes sous-tendant ces perturbations, j’ai étudié l'implication des facteurs épigénétiques et le rôle d'un facteur de transcription de la famille des heat shock factor, HSF2. Pour cela, j'ai utilisé la technique d'enregistrement de potentiel de champs somatique et dendritique (potentiel postsynaptique excitateur NMDA ; PPSE-NMDA) dans l'aire CA1 de tranches d'hippocampe. De manière intéressante, nos résultats montrent que les deux types d'alcoolisation, aigue et chronique, augmentent la sensibilité du PPSE-NMDA à un antagoniste de la sous-unité GluN2B alors que la sensibilité à l'antagoniste de la sous-unité GluN2A diminue. Chez le rat jeune adulte, ces modifications sont accompagnées d'une forte réduction de la DLTNMDA et d'un déficit d'apprentissage (test de reconnaissance de nouvel objet). Dans ce modèle, l'inhibition de l'activité des enzymes HDACs, responsables de la désacétylation des histones, prévient l'ensemble des effets de l’EtOH (sensibilité pharmacologique du PPSE-NMDA, DLTNMDA et apprentissage). Concernant HSF2 et avant toute alcoolisation, des souris adultes hsf-/- présentent une absence de DLTNMDA accompagnée d'une plus grande sensibilité du PPSE-NMDA à un antagoniste GluN2B comparé à des souris sauvages. L'exposition chronique à l'EtOH induit chez les souris sauvages, une abolition de la DLTNMDA accompagnée d'une augmentation de la sensibilité du PPSE-NMDA à un antagoniste GluN2B et à une diminution de la sensibilité de ce signal à un antagoniste GluN2A. En revanche, les souris hsf-/- ne présentent aucune de ces modifications. Ainsi, l'ensemble de mes travaux de thèse montre que quel que soit le type d'alcoolisation, aigue ou chronique, mais aussi l'espèce animale utilisée, rat ou souris, l'EtOH induit des adaptations du réseau hippocampique qui consiste en une augmentation de la sensibilité du PPSE-NMDA à un antagoniste GluN2B et en une diminution de sa sensibilité à un antagoniste GluN2A ; modifications qui accompagnent une abolition de la DLT. Cette réponse globale à l'EtOH mettrait en jeu des facteurs épigénétiques modulant l'état d'acétylation de l'ADN et des facteurs transcriptionnels de type heat shock
... mGluR3 belongs to a group of G protein-coupled receptors that are coupled to inhibitory Gi / Go proteins. Activated mGluR3 inhibits adenylate cyclase activity, reduces cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) formation, and is able to inhibit voltage-sensitive Ca 2+ channels [39][40][41][42]. Herein, we found that NAAG was negatively correlated with Glu at 0-12 h after HI, which indicating NAAG transformed with Glu. ...
Abstract Background Synapses can adapt to changes in the intracerebral microenvironment by regulation of presynaptic neurotransmitter release and postsynaptic neurotransmitter receptor expression following hypoxic ischemia (HI) injury. The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) exerts a protective effect on neurons after HI and may be involved in maintaining the function of synaptic networks. In this study, we investigated the changes in the expression of NAAG, glutamic acid (Glu) and metabotropic glutamate receptors (mGluRs), as well as the dynamic regulation of neurotransmitters in the brain after HI, and assessed their effects on synaptic plasticity of the cerebral cortex. Methods Thirty-six Yorkshire newborn pigs (3-day-old, males, 1.0–1.5 kg) were selected and randomly divided into normal saline (NS) group (n = 18) and glutamate carboxypeptidase II inhibition group (n = 18), both groups were divided into control group, 0–6 h, 6–12 h, 12–24 h, 24–48 h and 48–72 h groups (all n = 3) according to different post-HI time. The content of Glu and NAAG after HI injury were detected by 1H-MRS scanning, immunofluorescence staining of mGluRs, synaptophysin (syph) along with postsynaptic density protein-95 (PSD95) and transmission electron microscopy were performed. ANOVA, Tukey and LSD test were used to compare the differences in metabolite and protein expression levels among subgroups. Correlation analysis was performed using Pearson analysis with a significance level of α = 0.05. Results We observed that the NAAG and mGluR3 expression levels in the brain increased and then decreased after HI and was significantly higher in the 12–24 h (P
... The mGluRs belong to class c G-protein-coupled receptors (GPCRs), and so far, eight subtypes have been identified. These subtypes are further divided into three sub-categories according to phenotypes and intracellular signalling [6][7][8]. Group I consists of mGluR1 and mGluR5 that couple to Gα q/11 G-proteins, promoting intracellular Ca 2+ efflux [9,10]. Group II contains mGluR2 and mGluR3; and mGluR4, mGluR6, mGluR7, and mGluR8 belong to group III mGluRs [8]. ...
Metabotropic glutamate receptors (mGluRs; members of class C G-protein-coupled receptors) have been shown to modulate excitatory neurotransmission, regulate presynaptic extracellular glutamate levels, and modulate postsynaptic ion channels on dendritic spines. mGluRs were found to activate myriad signalling pathways to regulate synapse formation, long-term potentiation, autophagy, apoptosis, necroptosis, and pro-inflammatory cytokines release. A notorious expression pattern of mGluRs has been evident in several neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and schizophrenia. Among the several mGluRs, mGluR5 is one of the most investigated types of considered prospective therapeutic targets and potential diagnostic tools in neurodegenerative diseases and neuropsychiatric disorders. Recent research showed mGluR5 radioligands could be a potential tool to assess neurodegenerative disease progression and trace respective drugs’ kinetic properties. This article provides insight into the group I mGluRs, specifically mGluR5, in the progression and possible therapy for PD.
... There are eight major mGluRs and several splice variants, subclassified into three groups (I, II, and III) based on the structure, G-protein coupling or function, and ligand selectivity: Group I (mGluR1 and mGluR5) is linked to activation of phospholipase C (PLC); and Groups II (mGluR2 and mGluR3) and III (mGluR4, mGluR6, mGluR7, and mGluR8) are linked to inhibition of adenylate cyclase to change levels of cyclic adenosine monophosphate [32,33]. mGluR1 is one of the most abundantly expressed mGluRs in the mammalian brain [16]. ...
Delayed cerebral ischemia (DCI) remains a challenging but very important condition, because DCI is preventable and treatable for improving functional outcomes after aneurysmal subarachnoid hemorrhage (SAH). The pathologies underlying DCI are multifactorial. Classical approaches to DCI focus exclusively on preventing and treating the reduction of blood flow supply. However, recently, glutamate-mediated neuroelectric disruptions, such as excitotoxicity, cortical spreading depolarization and seizures, and epileptiform discharges, have been reported to occur in high frequencies in association with DCI development after SAH. Each of the neuroelectric disruptions can trigger the other, which augments metabolic demand. If increased metabolic demand exceeds the impaired blood supply, the mismatch leads to relative ischemia, resulting in DCI. The neuroelectric disruption also induces inverted vasoconstrictive neurovascular coupling in compromised brain tissues after SAH, causing DCI. Although glutamates and the receptors may play central roles in the development of excitotoxicity, cortical spreading ischemia and epileptic activity-related events, more studies are needed to clarify the pathophysiology and to develop novel therapeutic strategies for preventing or treating neuroelectric disruption-related DCI after SAH. This article reviews the recent advancement in research on neuroelectric disruption after SAH.
... Metabotropic glutamate receptors (mGluRs) are GPCRs mediating the slow neuromodulatory actions of glutamate, and they represent attractive drug targets for several neurological and psychiatric disorders (Nicoletti et al., 2011;Niswender and Conn, 2010). On the basis of their sequence similarity, transduction mechanisms, and ligand-binding profiles, mGluRs are classified into group I (mGluR1 and mGluR5), group II (mGluR2 and mGluR3) and group III receptors (mGluR4, mGluR6, mGluR7 and mGluR8) (Pin and Duvoisin, 1995). Group I mGluRs couple primarily to Gq and stimulate phosphoinositide hydrolysis and intracellular calcium release. ...
Dopamine replacement therapy with l-DOPA is the most efficacious symptomatic treatment for Parkinson's disease, but its utility is limited by the development of motor fluctuations and abnormal involuntary movements (dyskinesia) in a majority of the patients. These complications are attributed to the combined effects of dopaminergic degeneration and non-physiological reinstatement of dopamine transmission by the standard oral medications. There is substantial evidence that this altered state of dopamine transmission causes pathophysiological changes to a variety of non-dopaminergic neurotransmitter systems in the brain. This evidence has prompted an interest in developing drugs that target non-dopaminergic receptors for the purpose of improving l-DOPA-induced dyskinesia and/or motor fluctuations. We here review all the most important categories of non-dopaminergic targets that have been investigated so far, but with a particular focus on modulators of glutamatergic and serotonergic transmission, which continue to inspire significant efforts towards clinical translation. In particular, we discuss both the experimental rationale and the clinical experience thus far gained from studying 5-HT1A and 5-HT1B receptor agonists, NMDA and AMPA receptor antagonists, mGluR5 negative allosteric modulators, mGluR4 positive allosteric modulators, and adenosine A2a receptor antagonists. We also review compounds with complex pharmacological properties that are already used clinically or about to enter an advanced phase of clinical development (amantadine, safinamide, zonisamide, pridopidine, mesdopetam). We conclude with an outlook on possible directions to address unmet needs and improve the chance of successful translation in this therapeutic area.
The world has witnessed a revolution in therapeutics with the development of biological medicines such as antibodies and antibody fragments, notably nanobodies. These nanobodies possess unique characteristics including high specificity and modulatory activity, making them promising candidates for therapeutic applications. Identifying their binding mode is essential for their development. Experimental structural techniques are effective to get such information, but they are expensive and time-consuming. Here, we propose a computational approach, aiming to identify the epitope of a nanobody that acts as an agonist and a positive allosteric modulator at the rat metabotropic glutamate receptor 5. We employed multiple structure modeling tools, including various artificial intelligence algorithms for epitope mapping. The computationally identified epitope was experimentally validated, confirming the success of our approach. Additional dynamics studies provided further insights on the modulatory activity of the nanobody. The employed methodologies and approaches initiate a discussion on the efficacy of diverse techniques for epitope mapping and later nanobody engineering.
Group I metabotropic glutamate receptors (mGluRs) have diverse functions in some fundamental neuronal processes, including modulation of synaptic plasticity and dysregulation of these receptors could lead to various neuropsychiatric disorders. Trafficking of group I mGluRs plays critical roles in controlling the precise spatio-temporal localization and activity of these receptors, both of which contribute to proper downstream signaling. Using "molecular replacement" approach in hippocampal neurons derived from mice of both sexes, we demonstrate a critical role for the post-synaptic density protein Norbin in regulating the ligand-induced internalization of group I mGluRs. We show that Norbin associates with protein kinase A (PKA) through its N-terminus and anchors mGluR5 through its C-terminus, both of which are necessary for the ligand-mediated endocytosis of mGluR5, a member of the group I mGluR family. A point mutation (A687G) at the C-terminus of Norbin inhibits the binding of Norbin to mGluR5 and blocks mGluR5 endocytosis. Finally, we demonstrate an important mechanism by which Norbin regulates mGluR-mediated AMPAR endocytosis in hippocampal neurons, a cellular correlate for mGluR-dependent synaptic plasticity. Norbin, through its PKA-binding regions recruits PKA to AMPARs upon activation of mGluRs and deletion of the PKA-binding regions of Norbin inhibits mGluR-triggered AMPAR endocytosis. We further report that Norbin is important specifically for the mGluR-mediated AMPAR endocytosis, but not for NMDAR-dependent AMPAR endocytosis. Thus, this study unravels a novel role for Norbin in the internalization of mGluRs and mGluR-mediated AMPAR endocytosis that can have clinical relevance to the function of group I mGluRs in pathological processes.Significance StatementThe post-synaptic protein Norbin interacts with mGluR5 and both of them have been implicated in disorders like schizophrenia. However, the mechanistic basis underlying the regulation of mGluRs by Norbin remains elusive. We have identified Norbin as an essential mediator of ligand-mediated endocytosis of group I mGluRs. Mechanistically, Norbin N-terminus associates with PKA and C-terminus binds to mGluR5 to coordinate receptor internalization. A point mutation NorA687G inhibits endocytosis by disrupting this interaction. Additionally, Norbin is critical for the recruitment of PKA to AMPARs upon activation of group I mGluRs that assists in mGluR-mediated AMPAR endocytosis. Thus, Norbin has a dual function in the hippocampus; regulation of mGluR internalization and PKA-dependent modulation of mGluR-mediated AMPAR endocytosis, a prerequisite for mGluR-mediated synaptic plasticity.
Schizophrenia is a severe and debilitating psychiatric disorder characterized by early cognitive deficits, emotional and behavioral abnormalities resulted by a dysfunctional gene x environment interaction. Genetic and epigenetic abnormalities in cortical parvalbumin-positive GABAergic interneurons lead to alterations in glutamate-mediated excitatory neurotransmission, synaptic plasticity, and neuronal development. Epigenetic alterations during pregnancy or early phases of postnatal life are associated with schizophrenia vulnerability as well as inflammatory processes which are at the basis of brain pathology. An epigenetic animal model of schizophrenia showed specific changes in promoter DNA methylation activity of genes related to schizophrenia such as reelin, BDNF and GAD67, and altered expression and function of mGlu2/3 receptors in the frontal cortex. Although antipsychotic medications represent the main treatment for schizophrenia and generally show an optimal efficacy profile for positive symptoms and relatively poor efficacy for negative or cognitive symptoms, a considerable percentage of individuals show poor response, do not achieve a complete remission, and approximately 30 % of patients show treatment-resistance. Here, we explore the potential role of epigenetic abnormalities linked to metabotropic glutamate 2/3 receptors changes in expression and function as key molecular factors underlying the difference in response to antipsychotics.
Alzheimer disease is one of the most challenging demons in our society due to its very high prevalence and its clinical manifestations which cause deterioration of cognition, intelligence, and emotions - the very capacities that distinguish Homo sapiens from other animal species. Besides the personal, social, and economical costs, late stages of AD are vivid experiences for the family, relatives, friends, and general observers of the progressive ruin of an individual who turns into a being with lower mental and physical capacities than less evolved species. A human brain with healthy cognition, conscience, and emotions can succeed in dealing with most difficulties that life may pose. Without these capacities, the same person probably cannot. Due, in part, to this emotional impact, the absorbing study of AD has generated, over the years, a fascinating and complex story of theories, hypotheses, controversies, fashion swings, and passionate clashes, together with tremendous efforts and achievements geared to improve understanding of the pathogenesis and treatment of the disorder. Familal AD is rare and linked to altered genetic information associated with three genes. Sporadic AD (sAD) is much more common and multifactorial. A major point of clinical discussion has been, and still is, establishing the differences between brain aging and sAD. This is not a trivial question, as the neuropathological and molecular characteristics of normal brain aging and the first appearance of early stages of sAD-related pathology are not easily distinguishable in most individuals. Another important point is confidence in assigning responsibility for the beginning of sAD to a few triggering molecules, without considering the wide number of alterations that converge in the pathogenesis of aging and sAD. Genetic risk factors covering multiple molecular signals are increasing in number. In the same line, molecular pathways are altered at early stages of sAD pathology, currently grouped under the aegis of normal brain aging, only to increase massively at advanced stages of the process. Sporadic AD is here considered an inherent, natural part of human brain aging, which is prevalent in all humans, and variably present or not in a few individuals in other species. The progression of the process has devastating effects in a relatively low percentage of human beings eventually evolving to dementia. The continuum of brain aging and sAD implies the search for a different approach in the study of human brain aging at the first stages of the biological process, and advances in the use of new technologies aimed at slowing down the molecular defects underlying human brain aging and sAD at the outset, and transfering information and tasks to AI and coordinated devices.
Group I metabotropic glutamate receptors (mGluRs) play important roles in many neuronal processes and are believed to be involved in synaptic plasticity underlying the encoding of experience, including classic paradigms of learning and memory. These receptors have also been implicated in various neurodevelopmental disorders, such as Fragile X syndrome and autism. Internalization and recycling of these receptors in the neuron are important mechanisms to regulate the activity of the receptor and control the precise spatio-temporal localization of these receptors. Applying a "molecular replacement" approach in hippocampal neurons derived from mice, we demonstrate a critical role for protein interacting with C kinase 1 (PICK1) in regulating the agonist-induced internalization of mGluR1. We show that PICK1 specifically regulates the internalization of mGluR1 but it does not play any role in the internalization of the other member of group I mGluR family, mGluR5. Various regions of PICK1 viz., the N-terminal acidic motif, PDZ domain and BAR domain play important roles in the agonist-mediated internalization of mGluR1. Finally, we demonstrate that PICK1-mediated internalization of mGluR1 is critical for the resensitization of the receptor. Upon knockdown of endogenous PICK1, mGluR1s stayed on the cell membrane as inactive receptors, incapable of triggering the MAP-kinase signaling. They also could not induce AMPAR endocytosis, a cellular correlate for mGluR-dependent synaptic plasticity. Thus, this study unravels a novel role for PICK1 in the agonist-mediated internalization of mGluR1 and mGluR1-mediated AMPAR endocytosis that might contribute to the function of mGluR1 in neuropsychiatric disorders.
Metabotropic glutamate receptor 7 (mGlu7) is a G protein coupled receptor that has demonstrated promise as a therapeutic target across a number of neurological and psychiatric diseases. Compounds that modulate the activity of mGlu7, such as positive and negative allosteric modulators (PAMs and NAMs), may represent new therapeutic strategies to modulate receptor activity. The endogenous neurotransmitter associated with the mGlu receptor family, glutamate, exhibits low efficacy and potency in activating mGlu7 and surrogate agonists, such as the compound L-AP4, are often used for receptor activation and compound profiling. To understand the implications of the use of such agonists in the development of PAMs, we performed a systematic evaluation of receptor activation using a system in which mutations can be made in either protomer of the mGlu7 dimer; we employed mutations that prevent interaction with the orthosteric site as well as the G-protein coupling site of the receptor. We then measured increases in calcium levels downstream of a promiscuous G protein to assess the effects of mutations in one of the two protomers in the presence of two different agonists and three positive allosteric modulators. Our results reveal that distinct PAMs, for example VU0422288 and VU6006459, do exhibit different maximal levels of potentiation with L-AP4 versus glutamate, but there appear to be common stable receptor conformations that are shared among all of the compounds examined here. Significance Statement This manuscript describes the systematic evaluation of the mGlu7 agonists glutamate and L-AP4 in the presence and absence of three distinct potentiators examining possible mechanistic differences. These findings demonstrate that mGlu7 potentiators display subtle variances in response to glutamate versus L-AP4.
Allosteric modulation of mGlu1 represents a viable therapeutic target for treating numerous CNS disorders. While multiple chemically distinct mGlu1 positive (PAMs) and negative (NAMs) allosteric modulators have been identified, drug discovery paradigms have not included rigorous pharmacological analysis. In the present study, we hypothesised existing mGlu1 allosteric modulators possess unappreciated probe dependent or biased pharmacology. Using HEK293A cells stably expressing human mGlu-1 , we screened mGlu1 PAMs and NAMs from divergent chemical scaffolds for modulation of different mGlu1 orthosteric agonists in intracellular calcium (iCa2+) mobilisation and inositol monophosphate (IP1) accumulation assays. Operational models of agonism and allosterism were used to derive estimates for important pharmacological parameters such as affinity, efficacy and cooperativity. Modulation of glutamate and quisqualate-mediated iCa2+ mobilisation revealed probe dependence at the level of affinity and cooperativity for both mGlu1 PAMs and NAMs. We also identified the previously described mGlu5 selective NAM PF-06462894 as an mGlu1 NAM with a different pharmacological profile to other NAMs. Differential profiles were also observed when comparing ligand pharmacology between iCa2+ mobilisation and IP1 accumulation. The PAMs Ro67-4853 and CPPHA displayed apparent negative cooperativity for modulation of quisqualate affinity, and the NAMs CPCCOEt and PF-06462894 had a marked reduction in cooperativity with quisqualate in IP1 accumulation and upon extended incubation in iCa2+ mobilisation assays. These data highlight the importance of rigorous assessment of mGlu1 modulator pharmacology to inform future drug discovery programs for mGlu1 allosteric modulators. Significance Statement mGlu1 positive and negative allosteric modulators have therapeutic potential in multiple CNS disorders. We show that chemically distinct modulators display differential pharmacology with different orthosteric ligands and across divergent signalling pathways at human mGlu1 Such complexities in allosteric ligand pharmacology should be considered in future mGlu1 allosteric drug discovery programmes.
Teneurin C-terminal associated peptide (TCAP) is an ancient bioactive peptide that is highly conserved in metazoans. TCAP administration reduces cellular and behavioural stress in vertebrate and urochordate models, yet despite numerous studies in higher animals, there is limited knowledge of its role in invertebrates. In particular, there are no studies on TCAP’s effects on the heart of any metazoan, which is a critical organ in the stress response. We used the Sydney rock oyster (SRO) as an invertebrate model to investigate a potential role for sroTCAP in regulating cardiac activity, including during stress. sroTCAP is localized to the neural innervation network of the SRO heart, and suggested binding with various heart proteins related to metabolism and stress, including SOD, GAPDH and metabotropic glutamate receptor. Intramuscular injection of sroTCAP (10 pmol) significantly altered the expression of heart genes that are known to regulate remodelling processes under different conditions, and modulated several gene families responsible for stress mitigation. sroTCAP (1 and 10 pmol) was shown to cause transient bradycardia (heart rate was reduced by up to 63% and for up to 40 min post-administration), indicative of an unstressed state. In summary, this study has established a role for a TCAP in the regulation of cardiac activity through modulation of physiological and molecular components associated with energy conservation, stress and adaptation. This represents a novel function for TCAP and may have implications for higher-order metazoans.
P2X3 receptors and group II metabotropic glutamate receptors (mGluRs) have been found to be expressed in primary sensory neurons. P2X3 receptors participate in a variety of pain processes, while the activation of mGluRs has an analgesic effect. However, it's still unclear whether there is a link between them in pain. Herein, we reported that the group II mGluR activation inhibited the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. Group II mGluR agonist LY354740 concentration-dependently decreased P2X3 receptor-mediated and α,β-methylene-ATP (α,β-meATP)-evoked inward currents in DRG neurons. LY354740 significantly suppressed the maximum response of P2X3 receptor to α,β-meATP, but did not change their affinity. Inhibition of ATP currents by LY354740 was blocked by the group II mGluR antagonist LY341495, also prevented by the intracellular dialysis of either the Gi/o protein inhibitor pertussis toxin, the cAMP analog 8-Br-cAMP, or the protein kinase A (PKA) inhibitor H-89. Moreover, LY354740 decreased α,β-meATP-induced membrane potential depolarization and action potential bursts in DRG neurons. Finally, intraplantar injection of LY354740 also relieved α,β-meATP-induced spontaneous nociceptive behaviors and mechanical allodynia in rats by activating peripheral group Ⅱ mGluRs. These results indicated that peripheral group II mGluR activation inhibited the functional activity of P2X3 receptors via a Gi/o protein and cAMP/PKA signaling pathway in rat DRG neurons, which revealed a novel mechanism underlying analgesic effects of peripheral group II mGluRs.
Dysregulation of striatal dopamine is considered to be an important driver of pathophysiological processes in schizophrenia. Despite being one of the main origins of dopaminergic input to the striatum, the (dys)functioning of the substantia nigra (SN) has been relatively understudied in schizophrenia. Hence, this paper aims to review different molecular aspects of nigral functioning in patients with schizophrenia compared to healthy controls by integrating post-mortem and molecular imaging studies. We found evidence for hyperdopaminergic functioning in the SN of patients with schizophrenia (i.e. increased AADC activity in antipsychotic-free/-naïve patients and elevated neuromelanin accumulation). Reduced GABAergic inhibition (i.e. decreased density of GABAergic synapses, lower VGAT mRNA levels and lower mRNA levels for GABAA receptor subunits), excessive glutamatergic excitation (i.e. increased NR1 and Glur5 mRNA levels and a reduced number of astrocytes), and several other disturbances implicating the SN (i.e. immune functioning and copper concentrations) could potentially underlie this nigral hyperactivity and associated striatal hyperdopaminergic functioning in schizophrenia. These results highlight the importance of the SN in schizophrenia pathology and suggest that some aspects of molecular functioning in the SN could potentially be used as treatment targets or biomarkers.
Monosodium glutamat (MSG) 1800’lü yıllardan beri lezzet artırıcı katkı maddesi olarak, işlenmiş ve paketlenmiş tuzlu veya tatlı gıdalarda kullanılmaktadır. Yapılan birçok çalışma MSG kullanımının çok sayıda yapısal ve fonksiyonel bozukluklara yol açabileceğini ortaya çıkarmıştır. Son yıllarda MSG kullanımının çok artması gıda güvenliği konusunda endişelerinde artmasına neden olmuştur.
The point where two neurons communicate is called the synapse (Jessell and Kandel 1993). The word “synapse” comes from the Greek verb συνάπτω (bring together, join), and it was introduced in 1897 by Michael Foster (1836–1907) at the suggestion of Arthur Woollgar Verrall (1851–1912) which was an English classical scholar (Tansey 1997). The attribution of the first usage of the term to the Nobel Laureate Charles Scott Sherrington (1857–1952) is probably wrong.
Glutamate and GABA are the main synaptic neurotransmitters in the hippocampus. However, their actions are not limited only to the local postsynaptic zone. These amino acids can be released into the extrasynaptic space by glutamate and GABA reuptake, glial exocytosis, osmotic shock, and spillover (flowing out of the synaptic cleft). Glutamate and GABA receptors are also located on various parts of neurons and glial cells. Depending on the subcellular distribution of these receptors, their subunit composition, and the matabotropic/ionotropic functions, the effects of extracellular glutamate and GABA differ. The present review discusses the general principles of the organization of diffusion-based glutamatergic and GABAergic systems of extrasynaptic neurotransmission, the interaction of these systems with synaptic transmission, and the interaction of diffusion signals with each other.
The carotid body (CB) contributes significantly to oxygen sensing. It is unclear, however, whether glutamatergic signaling is involved in the CB response to hypoxia. Previously, we reported that ionotropic glutamate receptors (iGluRs) and multiple glutamate transporters are present in the rat CB. Except for iGluRs, glutamate receptors also include metabotropic glutamate receptors (mGluRs), which are divided into the following groups: Group I (mGluR1/5); group II (mGluR2/3); group III (mGluR4/6/7/8). We have studied the expression of group I mGluRs in the rat CB and its physiological function response to acute hypoxia. To further elucidate the states of mGluRs in the CB, this study’s aim was to investigate the expression of group II and III mGluRs and the response of rat CB to acute hypoxia. We used reverse transcription-polymerase chain reaction (RT-PCR) to observed mRNA expression of GRM2/3/4/6/7/8 subunits by using immunostaining to show the distribution of mGluR2 and mGluR8. The results revealed that the GRM2/3/4/6/7/8 mRNAs were expressed in both rat and human CB. Immunostaining showed that mGluR2 was localized in the type I cells and mGluR8 was localized in type I and type II cells in the rat CB. Moreover, the response of CB to acute hypoxia in rats was recorded by in vitro carotid sinus nerve (CSN) discharge. Perfusion of group II mGluRs agonist or group III mGluRs agonist (LY379268 or L-SOP) was applied to examine the effect of group II and III mGluRs on rat CB response to acute hypoxia. We found that LY379268 and L-SOP inhibited hypoxia-induced enhancement of CSN activity. Based on the above findings, group II and III mGluRs appear to play an inhibitory role in the carotid chemoreceptor response to acute hypoxia.
The thalamus is a key structure in the mammalian brain, providing a hub for communication within and across distributed forebrain networks. Research in this area has undergone a revolution in the last decade, with findings that suggest an expanded role for the thalamus in sensory processing, motor control, arousal regulation, and cognition. Moving beyond previous studies of anatomy and cell neurochemistry, scientists have expanded into investigations of cognitive function, and harness new methods and theories of neural computation. This book provides a survey of topics at the cutting edge of this field, covering basic anatomy, evolution, development, physiology and computation. It is also the first book to combine these disciplines in one place, highlighting the interdisciplinary nature of thalamus research, and will be an essential resource for students and experts in biology, medicine and computer science.
There is still no effective treatment for central nervous system (CNS) pathologies, including cerebral ischemia, neurotrauma, and neurodegenerative diseases in which the Glu/GABA balance is disturbed with associated excitotoxicity. It is thus important to search for new efficacious therapeutic strategies. Preclinical studies on the role of metabotropic glutamate receptors (mGluRs) in neuroprotection conducted over the years show that these receptors may have therapeutic potential in these CNS disorders. However, clinical trials, especially for treating Parkinson's disease, have been unsatisfactory. This review focuses on the specific role of group III mGluRs in neuroprotection in experimental in vitro and in vivo models of excitotoxicity/neurotoxicity using neurotoxins as well as ischemia, traumatic brain injury, and neurodegenerative diseases such as Parkinson's disease, Alzheimer's diseases, and multiple sclerosis. The review highlights recent preclinical studies in which group III mGluR ligands (especially those acting at mGluR4 or mGluR7) were administered after damage, thus emphasizing the importance of the therapeutic time window in the treatment of ischemic stroke and traumatic brain injury. From a clinical standpoint, the review also highlights studies using group III mGluR agonists with favorable neuroprotective efficacy (histological and functional) in experimental ischemic stroke, including healthy normotensive and-hypertensive rats. This review also summarizes possible mechanisms underlying the neuroprotective activity of the group III mGluR ligands, which may be helpful in developing more effective and safe therapeutic strategies.
Therefore, to fully assess the role of these receptors in neuroprotection, it is necessary to uncover new selective ligands, primarily those stimulating mGlu4 and mGlu7 receptors.
Astrocytes are the major glial cells in the brain, which play a supporting role in the energy and nutritional supply of neurons. They were initially regarded as passive space-filling cells, but the latest progress in the study of the development and function of astrocytes highlights their active roles in regulating synaptic transmission, formation, and plasticity. In the concept of “tripartite synapse,” the bidirectional influence between astrocytes and neurons, in addition to their steady-state and supporting function, suggests that any negative changes in the structure or function of astrocytes will affect the activity of neurons, leading to neurodevelopmental disorders. The role of astrocytes in the pathophysiology of various neurological and psychiatric disorders caused by synaptic defects is increasingly appreciated. Understanding the roles of astrocytes in regulating synaptic development and the plasticity of neural circuits could help provide new treatments for these diseases.
Imipramine belongs to a group of tricyclic antidepressants (TCAs). It has been also documented that its antidepressant activity connects with the modulation of cytosolic phospholipase A2 (cPLA2) and arachidonic acid (AA) turnover. Through this mechanism, imipramine can indirectly modify glutamate (Glu) transmission. Additionally, it has been shown that chronic treatment with imipramine results in the upregulation of the metabotropic glutamate receptor subtype 5 (mGlu5 receptor) in the hippocampus of rats. Our previous study revealed that manipulation of the AA pathway via inhibition of cyclooxygenase-2 (COX-2) by selective COX-2 inhibitor (NS398) could effectively modulate the behavior of mice treated with imipramine. Here, we hypothesized that COX-2 inhibition could similarly to imipramine influence mGlu5 receptor, and thus NS398 can modulate the effect of imipramine on Glu. Moreover, such regulation changes should correspond with alterations in neurotransmission. Increased cPLA activity after imipramine administration may change the activity of the AA pathway and the endocannabinoid metabolism, e.g., 2-Arachidonyl-glycerol (2-AG). To verify the idea, mGlu5 receptor level was investigated in the hippocampus (HC) and prefrontal cortex (PFC) of mice treated for 7 or 14 days with imipramine and/or COX-2 inhibitor: NS398. Western blot and PCR analyses were conducted. Moreover, the excitatory (Glu) and inhibitory (gamma-aminobutyric acid; GABA) neurotransmitters were measured using HPLC and 2-AG using ELISA.
A time-dependent change in mGlu5 receptor and COX-2 protein level, COX-2 expression, and 2-AG level in the PFC after imipramine administration was found. Up-regulation of mGlu5 receptor after NS398 was found in HC and PFC. A structure-dependent shift between excitatory vs. inhibitory transmission was detected when NS398 and imipramine were co-administered.
Metabotropic glutamate (mGlu) receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Metabotropic Glutamate Receptors [347]) are a family of G protein-coupled receptors activated by the neurotransmitter glutamate [138]. The mGlu family is composed of eight members (named mGlu1 to mGlu8) which are divided in three groups based on similarities of agonist pharmacology, primary sequence and G protein coupling to effector: Group-I (mGlu1 and mGlu5), Group-II (mGlu2 and mGlu3) and Group-III (mGlu4, mGlu6, mGlu7 and mGlu8) (see Further reading).Structurally, mGlu are composed of three juxtaposed domains: a core G protein-activating seven-transmembrane domain (TM), common to all GPCRs, is linked via a rigid cysteine-rich domain (CRD) to the Venus Flytrap domain (VFTD), a large bi-lobed extracellular domain where glutamate binds. mGlu form constitutive dimers, cross-linked by a disulfide bridge. The structures of the VFTD of mGlu1, mGlu2, mGlu3, mGlu5 and mGlu7 have been solved [198, 271, 264, 399]. The structure of the 7 transmembrane (TM) domains of both mGlu1 and mGlu5 have been solved, and confirm a general helical organization similar to that of other GPCRs, although the helices appear more compacted [87, 429, 61]. Recent advances in cryo-electron microscopy have provided structures of full-length mGlu receptor dimers [189]. Studies have revealed the possible formation of heterodimers between either group-I receptors, or within and between group-II and -III receptors [88]. First well characterized in transfected cells, co-localization and specific pharmacological properties also suggest the existence of such heterodimers in the brain [266].[436, 143, 279]. Beyond heteromerization with other mGlu receptor subtypes, increasing evidence suggests mGlu receptors form heteromers and larger order complexes with class A GPCRs (reviewed in [138]). The endogenous ligands of mGlu are L-glutamic acid, L-serine-O-phosphate, N-acetylaspartylglutamate (NAAG) and L-cysteine sulphinic acid. Group-I mGlu receptors may be activated by 3,5-DHPG and (S)-3HPG [30] and antagonized by (S)-hexylhomoibotenic acid [232]. Group-II mGlu receptors may be activated by LY389795 [265], LY379268 [265], eglumegad [350, 430], DCG-IV and (2R,3R)-APDC [351], and antagonised by eGlu [168] and LY307452 [421, 103]. Group-III mGlu receptors may be activated by L-AP4 and (R,S)-4-PPG [128]. An example of an antagonist selective for mGlu receptors is LY341495, which blocks mGlu2 and mGlu3 at low nanomolar concentrations, mGlu8 at high nanomolar concentrations, and mGlu4, mGlu5, and mGlu7 in the micromolar range [183]. In addition to orthosteric ligands that directly interact with the glutamate recognition site, allosteric modulators that bind within the TM domain have been described. Negative allosteric modulators are listed separately. The positive allosteric modulators most often act as ‘potentiators’ of an orthosteric agonist response, without significantly activating the receptor in the absence of agonist.
More than a century of dedicated research has resulted in what we now know, and what we think we know, about synapses and neural circuits. This piece asks to what extent some of the major advances – both theoretical and practical – have resulted from carefully considered theory, or experimental design: endeavors that aim to address a question, or to refute an existing hypothesis. It also, however, addresses the important part that serendipity and chance have played. There are cases where hypothesis driven research has resulted in important progress. There are also examples where a hypothesis, a model, or even an experimental approach – particularly one that seems to provide welcome simplification – has become so popular that it becomes dogma and stifles advance in other directions. The nervous system rejoices in complexity, which should neither be ignored, nor run from. The emergence of testable “rules” that can simplify our understanding of neuronal circuits has required the collection of large amounts of data that were difficult to obtain. And although those collecting these data have been criticized for not advancing hypotheses while they were “collecting butterflies,” the beauty of the butterflies always enticed us toward further exploration.
A novel metabotropic glutamate receptor, mGluR8, was identified by screening a mouse retina cDNA library. This receptor is most related to mGluR4, mGluR7, and mGluR6 (74%, 74%, and 70% identical amino acid residues, respectively). Similar to these receptors, stimulation by L-glutamate or L-2-amino-4-phosphonobutyrate (L-APB) of Chinese hamster ovary (CHO) cells stably transfected with mGluR8 result in the inhibition of forskolin-stimulated adenylyl cyclase. In situ hybridization studies revealed a strong expression of the mGluR8 gene in the olfactory bulb, accessory olfactory bulb, and mammillary body. A weaker expression was found in the retina, and in scattered cells in the cortex and hindbrain. During development, the distribution of mGluR8 expression was more widespread. These results extend the diversity of metabotropic glutamate receptors in the CNS. Because at least two APB receptors are expressed in the retina, the use of this drug to block selectively the ON pathway needs to be reconsidered. The pharmacology and expression of mGluR8 in mitral/tufted cells suggest it could be a presynaptic receptor modulating glutamate release by these cells at their axon terminals in the entorhinal cortex.
The mammalian thalamus forms an obligatory relay for nearly all sensory information that reaches the cerebral cortex. The transmission of sensory information by the thalamus varies in a state-dependent manner, such that during slow wave sleep or drowsiness thalamic responsiveness is markedly reduced, whereas during the waking, attentive state transmission is enhanced. Although activation of brainstem inputs to thalamic neurons has long been assumed to underlie this gating of sensory transfer through the thalamus, numerically the largest input to thalamic relay neurons derives from layer VI cells of the cerebral cortex. Here we report that activation of corticothalamic fibers causes a prolonged excitatory postsynaptic potential in guinea pig dorsal lateral geniculate relay neurons resulting from the reduction of a potassium conductance, consistent with the activation of glutamatergic "metabotropic" receptors. This slow depolarization can switch firing of thalamic neurons from the burst firing mode, which is prevalent during slow wave sleep, to the single spike mode, which is prevalent during waking, thereby facilitating transmission of sensory information through the thalamus. This prolonged enhancement of thalamic transfer may allow the cerebral cortex to gate or control selective fields of sensory inputs in a manner that facilitates arousal, attention, and cognition.
The AP4 (2-amino-4-phosphonobutyrate) receptor is a presynaptic glutamate receptor that inhibits transmitter release via an unknown mechanism. We examined the action of L-AP4 on voltage-dependent calcium currents and excitatory synaptic transmission on cultured olfactory bulb neurons using whole-cell voltage-clamp methods. In neurons dialyzed with GTP, L-AP4 inhibited high-threshold calcium currents evoked in barium solutions. The inhibition was irreversible in the presence of GTP-gamma-S and blocked by removing intracellular Mg2+ or by preincubation with pertussis toxin (PTX), consistent with the involvement of a PTX-sensitive G-protein. Dialysis with staurosporine or buffering of intracellular calcium to pCa less than 8 did not block the action of L-AP4, suggesting that protein phosphorylation or release of intracellular calcium stores was not involved in calcium current inhibition under these experimental conditions. PTX also blocked the L-AP4-induced inhibition of monosynaptic EPSPs evoked by intracellular stimulation of cultured mitral cells. These results suggest that the presynaptic AP4 receptor is a G-protein-coupled glutamate receptor, and that inhibition of calcium influx by a membrane-delimited action of a G-protein may account for L-AP4-induced presynaptic inhibition.
A cDNA clone for a new metabotropic glutamate receptor, mGluR5, was isolated through polymerase chain reaction-mediated DNA amplification by using primer sequences conserved among the metabotropic glutamate receptor (mGluR) family and by the subsequent screening of a rat brain cDNA library. The cloned receptor consists of 1171 amino acid residues and exhibits a structural architecture common to the mGluR family, possessing a large extracellular domain preceding the seven putative membrane-spanning segments. mGluR5 shows the highest sequence similarity to mGluR1 among the mGluR members and is coupled to the stimulation of phosphatidylinositol hydrolysis/Ca2+ signal transduction in Chinese hamster ovary cells transfected with the cloned cDNA. This receptor also resembles mGluR1 in its agonist selectivity and antagonist responses; the potency rank order of agonists for mGluR5 was determined to be quisqualate greater than L-glutamate greater than or equal to ibotenate greater than trans-1-aminocyclopentane-1,3-dicarboxylate. Blot and in situ hybridization analyses indicated that mGluR5 mRNA is widely distributed in neuronal cells of the central nervous system and is expressed differently from mGluR1 mRNA in many brain regions. This investigation thus demonstrates that there is an additional mGluR subtype which closely resembles mGluR1 in its signal transduction and pharmacological properties and is expressed in specialized neuronal cells in the central nervous system.
2‐(Carboxycyclopropyl)glycines (CCGs) are conformationally restricted glutamate analogues and consist of eight isomers including l ‐ and d ‐forms. The agonist potencies and selectivities of these compounds for metabotropic glutamate receptors (mGluRs) were studied by examining their effects on the signal transduction of representative mGluR1, mGluR2 and mGluR4 subtypes in Chinese hamster ovary cells expressing the individual cloned receptors.
Two extended isomers of l ‐CCG, l ‐CCG‐I and l ‐CCG‐II, effectively stimulated phosphatidylinositol hydrolysis in mGluR1‐expressing cells. The rank order of potencies of these compounds was l ‐glutamate > l ‐CCG‐I > l ‐CCG‐II.
l ‐CCG‐I and l ‐CCG‐II were effective in inhibiting the forskolin‐stimulated adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) accumulation in mGluR2‐expressing cells. Particularly, l ‐CCG‐I was a potent agonist for mGluR2 with an EC 50 value of 3 × 10 ⁻⁷ m , which was more than an order of potency greater than that of l ‐glutamate.
l ‐CCG‐I evoked an inhibition of the forskolin‐stimulated cyclic AMP production characteristic of mGluR4 with a potency comparable to l ‐glutamate.
In contrast to the above compounds, the other CCG isomers showed no appreciable effects on the signal transduction involved in the three mGluR subtypes.
6 This investigation demonstrates not only the importance of a particular isomeric structure of CCGs in the interaction with the mGluRs but also a clear receptor subtype specificity for the CCG‐receptor interaction, and indicates that the CCG isomers would serve as useful agonists for investigation of functions of the mGluR family.
Metabotropic glutamate receptor (mGluR) is highly expressed in cerebellar Purkinje cells. The purpose of this study was pharmacological and immunocytochemical characterization of the mGluR in single cerebellar neurons, especially Purkinje cells. Ca2+ imaging with fura-2 in cultured cerebellar neurons, identified immunocytochemically, was used to record the direct effects of drugs in stable conditions. In addition, the expression of mGluR was examined, and expression of the intracellular receptor for inositol trisphosphate (IP3) produced by mGluR activation was studied immunocytochemically with specific antibodies. Purkinje neurons and some other neurons showed Ca(2+)-mobilizing responses to mGluR agonists. These responses were mediated by mGluR because they were not blocked by ionotropic GluR antagonists, were independent of the caffeine-sensitive Ca2+ pool, and were blocked by inhibitors of IP3-induced Ca2+ release. This is the first pharmacological characterization of mGluR at single Purkinje cells. The results differed as follows from those in earlier studies in which phosphoinositide turnover of the entire population of cerebellar cells was monitored: (1) the mGluR responses were not blocked by pertussis toxin or D,L-2-amino-3-phosphonopropionic acid; (2) glutamate was a potent agonist, whereas L-aspartate was ineffective; and (3) the dose-response relationship showed an all-or-none tendency. The metaboltropic response of Purkinje cells changed markedly during development, with a sharp peak after day 4 of culture, whereas mGluR and IP3 receptor proteins increased steadily during maturation. This apparent desensitization of mGluR was not blocked by inhibitors of protein kinase C (PKC) or ADP-ribosyltransferase. The metabotropic responses were mainly localized to the center of the somata of Purkinje cells even on day 4, whereas both receptor proteins were expressed throughout the cell. These results suggest that the function of mGluR is spatially and developmentally controlled by a posttranslational mechanism involving a mechanism other than phosphorylation by PKC or ADP-ribosylation.
Following microinjection into the nucleus tractus solitarius (NTS), the effects of glutamate on the baroreceptor reflex are poorly antagonized by kynurenic acid and DL-2-amino-5-phosphonovaleric acid, suggesting the possible involvement of metabotropic glutamate receptors in this response. The metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depolarized neurons located medial to the tractus solitarius (TS) at the level of the area postrema in coronal sections of the rat NTS. This effect was mimicked by glutamate and was not blocked by antagonists at alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate or NMDA receptors. 1S,3R-ACPD also produced an inward current under voltage clamp that was not accompanied by a rise in [Ca2+]i, monitored with the Ca(2+)-sensitive dye fura-2. Conversely, the muscarinic agonist carbachol produced an outward current and a rise in [Ca2+]i. 1S,3R-ACPD reduced both the excitatory and the inhibitory postsynaptic current resulting from single electrical stimuli in the region of the TS. High-frequency stimulation of the TS produced an inward current in the presence of AMPA/kainate and NMDA receptor blockers. This current had similar properties to that produced by 1S,3R-ACPD. Thus, metabotropic glutamate receptors may mediate a component of excitatory transmission in the NTS.
We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.
Lipid derivatives play a major role as second messengers. A large body of evidence has accumulated on the mechanisms leading to their formation and on their role in cell regulation (reviewed in Dennis et al. 1991). They are formed by the action of different phospholipases (A2, C, D) acting on membrane phospholipids. Of particular physiological relevance has been the elucidation of the hormonal regulation of these enzymes. A large number of receptors for hormones and neurotransmitters are coupled to cellular phospholipases and regulate the cytosolic levels of second messengers such as Ca2+, diacylglycerol, inositol trisphosphate, and arachidonic acid. The activation of phospholipases can be indirectly induced by a receptormediated increase in cytosolic Ca2+, or it can be due to the activation of a heterotrimeric GTP-binding (G) protein directly coupled to the enzyme.
SIGNAL transduction for the characteristic long-term desensitization of glutamate receptors in Purkinje cells was investigated with wedge recordings from rat cerebellar slices. Long-term desensitization was induced specifically in the AMPA-selective subtype of glutamate receptors following brief exposure to 100 [mu]M quisqualate. It was abolished either by treatment of the rat with pertussis toxin or by perfusion of a slice with BAPTA-AM, L-NMMA, hemoglobin, or inhibitor of PKG. Brief application of AMPA alone did not cause desensitization, but in combination with t-ACPD, sodium nitroprusside, or 8-bromo-cGMP, AMPA produced desensitization similar to that induced by quisqualate. These results indicate that the desensitization arises from activation of AMPA receptors in association with activation of metabotropic glutamate receptors, the latter leading to Ca2+ elevation to nitric oxide (NO) production to cGMP synthesis, and eventually to activation of PKG.
: D,L-2-Amino-3-phosphonopropionate (AP-3), a proposed metabotropic receptor antagonist, produced a concentration-dependent increase in the formation of inositol 1,4,5-trisphosphate in rat hippocampal slices. The response was maximal at 1 mM and completely due to the l-isomer. d,l-AP-3 was half as efficacious as (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist of this receptor. The response produced by maximally effective concentrations of l-AP-3 and 1S,3R-ACPD together for 5 min was not significantly different from that produced by 1S,3R-ACPD alone. However, pretreatment for 40 min with either 1 mM l-AP-3 or D,L-AP-3 completely inhibited the response to 1S,3R-ACPD. This inhibition was long-lasting (wash-resistant) and was reversed by reduction of the extracellular Ca2+ concentration. Also, pretreatment for 40 min with 1S,3R-ACPD reduced, but did not completely block, the response to readdition of 1S,3R-ACPD. l-AP-3 (1 mM) also produced a stereoselective 2.3-fold increase in the efflux of glutamate from the hippocampal slices. These data suggest that incubation of hippocampal slices with AP-3 induces a time-dependent desensitization of the metabotropic response by a mechanism that is dependent on extracellular Ca2+. The possible roles of receptor occupancy and inhibition of glutamate uptake by AP-3 are also discussed.
Activation of phosphoinositide metabolism is an early event in signal transduction for a number of neurotransmitters and hormones. In primary cultures of rat neurocortical cells, various excitatory amino acids stimulate inositol phosphate production with a rank order of potency of quisqualate > ibotenate > glutamate > kainate, N-methyl-d-aspartate > -amino-3-hydroxyl-5-methyl-4-isoxazole propionate. This response to excitatory amino acids was insensitive to a variety of excitatory amino acid antagonists including 6-cyano-7-nitroquinoxaline-2,3-dione, 3–3(2-carboxypiperazine-4-yl)propyl-l-phosphonate, and 2-amino-4-phosphonobutyrate. The individual responses of quisqualate-, ibotenate-, and kainate-stimulated inositol phosphate production were not additive. These results suggest that phosphoinositide metabolism activated by excitatory amino acids is mediated by a unique quisqualate-preferring receptor that is not antagonized by known N-methyl-d-aspartate and non-N-methyl-d-aspartate antagonists, and is relatively insensitive to -amino-3-hydroxyl-5-methyl-4-isoxazole propionate.
The possible roles of G-protein coupled metabotropic glutamate receptors in central nervous function are currently the focus of intensive investigation. The complexity of effects produced by agonists at these receptors probably reflects the activity of a range of sub-types. The metabotropic glutamate receptors first described are linked to phospholipase C, mediating phosphoinositide hydrolysis and release of Ca2+ from intracellular stores. A substance generally considered to be a selective agonist for these receptors is (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). This substance not only stimulates phosphoinositide hydrolysis, but also inhibits cyclic AMP formation. A family of metabotropic glutamate receptors, incorporating both phospholipase C- and adenylcyclase-linked sub-types has been cloned. Various effects of metabotropic glutamate receptor agonists on membrane ion fluxes and synaptic events have been reported, including neuronal depolarization and/or excitation, hyperpolarization, inhibition of Ca(2+)-dependent and voltage-gated K+ currents, potentiation of N-methyl-D-aspartate-induced responses, depression of synaptic excitation and either induction or augmentation of long-term potentiation. To clarify the role of metabotropic glutamate receptors in central nervous activity and to aid the characterization of the various receptor types that may be involved, a range of highly selective agonists and antagonists is required. To date, currently available antagonists such as L-2-amino-3-phosphonopropionate and L-aspartic acid-beta-hydroxamate appear to be unselective and insufficiently potent. We report here the actions of three phenylglycine derivatives, the particular agonist and/or antagonist properties of which may help to elucidate the roles of metabotropic glutamate receptors in central nervous activity.(ABSTRACT TRUNCATED AT 250 WORDS)
Pharmacological studies of glutamate receptor stimulation of the phosphoinositide (PI) system have demonstrated that this response is blocked by several agents: 2-amino-3-phosphonopropionate (AP3), phorbol esters and in some preparations pertussis toxin. In electrophysiological studies of CA1 pyramidal neurons, we have found that pertussis toxin and AP3 (1–2 mM) do not block either the membrane depolarization or inhibition of the slow afterhyperpolarization elicited by trans-1-aminocyclopentyl-1,3-dicarboxylate (ACPD; 30 μM), a selective agonist of the PI-linked glutamate receptor. However, phorbol 12,13-diacetate (1–1.5 μM) which itself blocks the slow afterhyperpolarization, completely blocks the membrane depolarizing response elicited by ACPD. These results add to growing evidence for heterogeneity among PI-linked glutamate receptor responses.
Excitatory amino acids (EAAs) are known to stimulate neurohormone release through the activation of ion-channel-linked receptors (ionotropic receptors). Here we report that a receptor for EAAs linked to polyphosphoinositide hydrolysis (metabotropic receptor) is also present at the hypothalamus where its expression is developmentally regulated. Stimulation of [3H]inositolmonophosphate ([3H]InsP) formation by quisqualate (EC50 = 1.5 μM), ibotenate (EC50 = 100 μM) and trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD; EC50 = 30 μM) is extremely high (up to 50-fold) in the first 10 days of postnatal life, progressively declines during maturation and is virtually absent in the adult. Stimulation of phosphoinositide hydrolysis by quisqualate, ibotenate and t-ACPD is more pronounced than that induced by classical neurotransmitters that stimulate inositol phosphate formation such as norepinephrine and carbamylcholine. Agonists of the ionotropic glutamate receptor such as kainate, NMDA and α-amino-3-hydroxy-5-methyl-5-isoxazolpropionate (AMPA), do not modify inositol phosphate accumulation in hypothalamic slices. The selective antagonist of quisqualate metabotropic receptor, d,l-2-amino-3-phosphonopropionate (AP3), produces a slight stimulation of phosphoinositide hydrolysis, but potently antagonizes the stimulation produced by quisqualate and t-ACPD.
Many receptors that sense the environment effect intracellular regulation through stimulation of heterotrimeric G proteins and the consequences thereof. While prominence was originally given to the α-subunits of G proteins as the pathway for downstream regulation, very active roles for the βγ-subunits have emerged in the past year. Recent experiments highlight the versatility of βγ-subunits in these regulatory pathways, but also emphasize some fundamental questions that remain.
Inositol phosphate synthesis elicited by excitatory amino acids was measured in rat forebrain synaptoneurosomes in presence of Li(+). Quisqualate (QA) was the most potent excitatory amino acid inducing inositol phosphate formation. This QA action was not blocked by any of the usual antagonists [glutamate-amino-methyl-sulphonate (GAMS); glutamate-diethyl-ester (GDEE); ?-d-glutamyl-glycine (?-DGG)] known to inhibit the QA-induced depolarization. The same was found for the most potent and selective QA antagonist reported so far [6-nitro-7-cyanoquinoxaline-2,3-dion (FG 9065)]. In addition, dl-?-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) a potent depolarizing agonist at the quisqualate receptor subtype was about 300 times less potent than quisqualate in increasing inositol phosphate accumulation. Our results provide the first pharmacological evidence indicating that a new quisqualate receptor subtype, tentatively termed sAA(2) is responsible for inositol phosphate formation.
The activity of a synthetic compound, cis-1-aminocyclopentane-1,3-dicarboxylic acid, which possesses a conformationally very restricted molecule, has been compared with that of L-glutamic acid and a number of other compounds as a neuronal excitant. The new compound is about 10-fold more active than glutamate and its rigidity indicates that it reacts only with glutamate receptors and not with those responsive to aspartate.
Three cDNA clones, mGluR2, mGluR3, and mGluR4, were isolated from a rat brain cDNA library by cross-hybridization with the cDNA for a metabotropic glutamate receptor (mGluR1). The cloned receptors show considerable sequence similarity with mGluR1 and possess a large extracellular domain preceding the seven putative membrane-spanning segments. mGluR2 is expressed in some particular neuronal cells different from those expressing mGluR1 and mediates an efficient inhibition of forskolin-stimulated cAMP formation in cDNA-transfected cells. The mGluRs thus form a novel family of G protein-coupled receptors that differ in their signal transduction and expression patterns.
It is well documented that the effects of excitatory amino acid (EAA) agonists on phosphoinositide hydrolysis involve a GTP-binding protein-linked or "metabotropic" receptor mechanism. The mechanisms by which EAAs alter cyclic AMP levels in brain slices, however, are not yet clear. In this study, the selective metabotropic EAA agonist trans-(+-)-1-aminocyclopentane-1,3-dicarboxylic acid and its isomers were examined for effects on basal and forskolin-stimulated cyclic AMP formation in slices of the rat hippocampus. Trans-(+-)-1-Aminocyclopentane-1,3-dicarboxylic acid had little effect on basal cyclic AMP but inhibited forskolin-stimulated cyclic AMP formation in a biphasic manner. The 1S,3R isomer of 1-aminocyclopentane-1,3-dicarboxylic acid produced potent but only partial (approximately 50%) inhibition of forskolin-stimulated cyclic AMP formation. 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid fully inhibited forskolin-stimulated cyclic AMP but with lower potency than the 1S,3R isomer. These results show that in addition to the formation of phosphoinositide-derived second messengers, the cellular consequences of selectively activating hippocampal metabotropic EAA receptors include an alteration of cellular cyclic AMP levels.
The signal transduction and pharmacological properties of a metabotropic glutamate receptor, mGluR1, were studied in CHO cells permanently expressing the cloned receptor. mGluR1 stimulated phosphatidylinositol (PI) hydrolysis in the potency rank order of quisqualate greater than L-glutamate greater than or equal to ibotenate greater than L-homocysteine sulfinate greater than or equal to trans-ACPD. This receptor also evoked the stimulation of cAMP formation and arachidonic acid release with comparable agonist potencies. DL-AP3 and L-AP4, the effective antagonists reported for glutamate-stimulated PI hydrolysis in brain slices, showed no appreciable effects on mGluR1, suggesting the existence of an additional subtype of this receptor family. Pertussis toxin and phorbol ester produced distinct effects on the three transduction cascades, implying that mGluR1 independently links to the multiple transduction pathways probably through different G proteins.
To examine whether multiple subtypes of the excitatory amino acid (EAA) receptor coupled to phosphoinositide (PPI) hydrolysis exist, we have pharmacologically characterized the PPI response in neonatal and adult rat brain. Activation of PPI hydrolysis was determined by the accumulation of [3H]inositol monophosphate in brain slices prelabeled with [3H]inositol. In neonatal hippocampus, D,L-2-amino-3-phosphonopropionic acid (AP3; 1 mM) inhibited the cis-1-aminocyclopentane-1,3-dicarboxylic acid (IUPAC nomenclature; ACPD; 100 microM)- and quisqualate (Quis; 100 microM)-stimulated PPI hydrolysis by 73 and 66%, respectively, but had no effect in neonatal cerebellum. In adult hippocampus, AP3 stimulated PPI hydrolysis with potency and efficacy comparable to those of Quis and ACPD and completely masked the Quis concentration-response curve. In adult cerebellum, only Quis behaved as a full agonist on the PPI response. The Quis concentration-response curve was shifted rightward with a fourfold decrease in potency in the presence of ACPD (5 mM), whereas it was nearly additive with the PPI response induced by AP3 (5 mM). Thus, our data reveal significant developmental and brain regional differences in metabotropic EAA receptor responses and support the notion that this receptor is heterogeneous, in both a regionally specific and a developmentally dependent manner.
Little is known about the in vivo function of the GTP-binding protein-coupled "metabotropic" excitatory amino acid (EAA) receptor. In vitro studies on agonist-induced brain phosphoinositide hydrolysis have shown that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid is a highly selective and efficacious metabotropic EAA agonist. We have recently reported that in vivo unilateral intrastriatal injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid induces transient extrapyramidal motor activation that manifests itself as contralateral turning. In this study, we fully characterized the onset of turning behavior following intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid injection and the possible involvement of striatal dopamine neurons in the mediation of this effect. Rats were anesthetized with the short-acting agent halothane to allow for rapid surgical recovery and thus early behavioral measurements. Intrastriatal (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 mumol/2 microliters) produced an incremental increase in contralateral turning starting at 1 h and plateauing 3-6 h after injection (peak effect, 39.1 +/- 6.7 rotations per 5 min). Dopamine depletion with alpha-methyl-DL-p-tyrosine (250 mg/kg i.p., 80% depletion) resulted in greater than 85% inhibition of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced contralateral turning. The dopamine antagonist haloperidol (0.3 mg/kg i.p.) produced 48% inhibition of the (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid response. In time course studies, turning behavior correlated with increases in levels of the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid. These results suggest a functional interaction between the metabotropic EAA receptor and the dopaminergic system in the striatum.
D,L-2-Amino-3-phosphonopropionate (AP-3), a proposed metabotropic receptor antagonist, produced a concentration-dependent increase in the formation of inositol 1,4,5-trisphosphate in rat hippocampal slices. The response was maximal at 1 mM and completely due to the L-isomer. D,L-AP-3 was half as efficacious as (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist of this receptor. The response produced by maximally effective concentrations of L-AP-3 and 1S,3R-ACPD together for 5 min was not significantly different from that produced by 1S,3R-ACPD alone. However, pretreatment for 40 min with either 1 mM L-AP-3 or D,L-AP-3 completely inhibited the response to 1S,3R-ACPD. This inhibition was long-lasting (wash-resistant) and was reversed by reduction of the extracellular Ca2+ concentration. Also, pretreatment for 40 min with 1S,3R-ACPD reduced, but did not completely block, the response to readdition of 1S,3R-ACPD. L-AP-3 (1 mM) also produced a stereoselective 2.3-fold increase in the efflux of glutamate from the hippocampal slices. These data suggest that incubation of hippocampal slices with AP-3 induces a time-dependent desensitization of the metabotropic response by a mechanism that is dependent on extracellular Ca2+. The possible roles of receptor occupancy and inhibition of glutamate uptake by AP-3 are also discussed.
Ca2+ mobilisation induced by L-glutamate (Glu) and acetylcholine (ACh) has been studied in cultured cerebellar granule cells using digital fluorescence microscopy. The ability of Glu-receptor activation to mobilise Ca2+ was decreased when [Ca2+]o was lowered to 10 microM (from 1.8 mM). It was enhanced when [Ca2+]i was raised using 25 mM external K+ or by N-methyl-D-aspartate (NMDA), which selectively activates a distinct Glu-receptor subtype. The enhancement was dependent on entry of external Ca2+. In contrast, the ability of ACh receptor activation to mobilise Ca2+ was not affected by these conditions. Furthermore, pretreatment with pertussis toxin inhibited Ca2+ mobilisation in response to Glu-receptor activation without affecting mobilisation in response to ACh. However, activation of both receptors mobilised Ca2+ from a common, thapsigargin-sensitive pool. We conclude that there are differences in the Ca2+ mobilization pathways for the two receptor systems in cerebellar granule cells. The Ca(2+)-sensitivity of this Ca2+ mobilizing Glu receptor may have implications for its function in neuronal synaptogenesis and plasticity.
Metabotropic glutamate receptors (mGluRs) have been recently described as a family of guanine nucleotide-binding regulatory protein-coupled receptors with multiple signal transduction pathways. At least one of these receptors appears to be negatively coupled to adenylyl cyclase when stably expressed in transfected cells. We have studied how activation of native mGluRs modulates cyclic AMP (cAMP) formation in brain slices prepared from rats at different ages. 1S,3R-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,1R-ACPD), a selective agonist of mGluRs, slightly increased basal cAMP formation but reduced forskolin-stimulated cAMP formation in adult hippocampal slices, in agreement with previous results. The action of 1S,3R-ACPD on basal cAMP formation was not reproduced by the ionotropic receptor agonists N-methyl-D-aspartate, kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and was antagonised by L-2-amino-3-phosphonopropionate (L-AP-3). L-AP-3, however, did not prevent but rather mimicked the inhibitory action of 1S,3R-ACPD on forskolin-stimulated cAMP formation. In hippocampal slices from 1-, 8-, or 15-day-old rats, 1S,3R-ACPD increased basal cAMP formation but failed to reduce the action of forskolin. A similar development pattern of modulation was observed in hypothalamic slices with the difference that 1S,3R-ACPD did not stimulate basal cAMP formation in the hypothalamus of adult animals. These results suggest that inhibition of forskolin-stimulated cAMP formation by 1S,3R-ACPD is mediated by a specific mGluR subtype that is preferentially expressed in the adult.
The specific glutamate metabotropic receptor agonist 1S,3R-aminocyclopentane dicarboxylate (ACPD), but not its inactive enantiomer 1R,3S-ACPD, induced a slowly developing potentiation of synaptic transmission in rat hippocampal slices. This effect was independent of its ability to potentiate responses mediated by the activation of N-methyl-D-aspartate receptors. Perfusion with 1S,3R-ACPD provides, therefore, a means of chemically inducing a form of long-term potentiation.
The cloning and sequencing of porcine lutropin/choriogonadotropin (LH/hCG) receptor messenger RNAs have shown the presence of a full-length receptor (pLHR-A) and of shorter variants lacking either the transmembrane and the intracellular domains (pLHR-B and pLHR-C) or only the transmembrane domain (pLHR-D). Moreover, immunoblotting of testicular membrane extracts has detected 85-, 68-, and 45-48-kDa proteins reacting with antireceptor antibodies. Transfection experiments were performed to assign the protein species to the various messenger RNAs and to study the function of the various receptor species. COS-7 and L-cells transfected with an expression vector encoding full-length receptor pLHR-A yielded a protein of apparent molecular mass of 105 kDa. This corresponded to the complete receptor which had undergone a different glycosylation pattern to that found in testis, since after digestion with peptide N-glycosidase F both the 105-kDa COS-7 protein and the 85-kDa testicular glycoprotein yielded a holoprotein of approximately 63 kDa. Transfection with pLHR-A also yielded a high proportion of the 68-kDa glycoprotein which was shown by digestion with endoglycosidase H to be a high-mannose precursor of the full-length receptor. The existence of a large pool of precursor species in both transfected cells and Leydig cells evokes possible physiological regulations at the level of receptor maturation.(ABSTRACT TRUNCATED AT 250 WORDS)
Using an in vitro slice preparation, we studied the effects, on parallel fiber (PF)-mediated EPSPs, of coactivation of metabotropic-glutamate receptors and of voltage-gated calcium (Ca) channels of Purkinje cells (PCs) by bath application of 50 microM trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) and by direct depolarization of the cells, respectively. These effects were compared with changes in synaptic efficacy obtained when alpha-amino-3hydroxy-5-methylisoxalone-4-propionate (AMPA) receptors of PCs were also activated through stimulation of PFs during the pairing protocol, as well as when similar experiments were performed without trans-ACPD in the bath. In a control medium, pairing for 1 min of PF-mediated EPSPs evoked at 1 Hz with Ca spikes evoked by steady depolarization of PCs (n = 13) led to LTD of synaptic transmission in 9 cases whereas for the others EPSPs were not affected. No LTD occurred in 9 out of 10 other cells tested when PF stimulation was omitted during the 1 min period of Ca spike firing of PCs. Bath application of 50 microM trans-ACPD, in conjunction with the same pairing protocol as before (n = 8), led to a significantly larger LTD of PF-mediated EPSPs after washing out of this drug. Moreover, a clear-cut LTD of PF-mediated EPSPs was also observed in 5 of the 8 other cells, when PF stimulation was omitted during Ca spike firing in the presence of trans-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)
The glutamate receptors mediate excitatory neurotransmission in the brain and are important in memory acquisition, learning,
and some neurodegenerative disorders. This receptor family is classified in three groups: the N-methyl-D-aspartate (NMDA),
alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate, and metabotropic receptors. Recent molecular studies
have shown that many receptor subtypes exist in all three groups of the receptors and exhibit heterogeneity in function and
expression patterns. This article reviews the molecular and functional diversity of the glutamate receptors and discusses
their implications for integrative brain function.
Hydropathicity analysis of 39 G-protein-coupled receptors (GPCR) reveals seven hydrophobic stretches corresponding to membrane spanning alpha-helices. The alignment of the primary sequences shows a high degree of homology in the GPCR transmembrane regions. 3D models of 39 GPCRs were generated using the refined model of bacteriorhodopsin as a template. Five cationic neurotransmitter receptors (serotonergic 5-HT2, dopaminergic D2, muscarinic m2, adrenergic alpha 2 and beta 2 receptors) were taken as prototypes and studied in detail. The 3D models of the cationic neurotransmitter receptors, together with their primary structure comparison, indicate that the agonist binding site is located near the extracellular face of the receptor and involves residues of the membrane-spanning helices 3, 4, 5, 6, and 7. The binding site consists of a negatively-charged Asp located at the middle of transmembrane helix 3 and a hydrophobic pocket containing conserved aromatic residues on helices 4, 5, 6, and 7. To define the precise receptor-ligand interactions, the natural neurotransmitters were docked into the binding sites. Residues responsible for the affinity, selectivity, and eventually stereospecificity of dopamine, adrenaline, noradrenaline, serotonin, and acetylcholine for their receptors were identified. The ligands are involved in electrostatic interactions as well as hydrogen bonds and specific hydrophobic aromatic interactions. All the GPCRs possess invariant hinge residues, which might be responsible for a conformational change during agonist binding and therefore influence dissociation and association of G-proteins to the receptors. The role of hydrophobic interactions and hydrogen bonds in the conformational change of the receptors, modulating the coupling to the G-protein, is discussed with regard to these residues. The models are in agreement with published data obtained from mutagenesis and labeling studies and represent important working hypotheses to direct future mutagenesis studies. They also enable structure-activity relationship studies and more rational drug design. The 3D models of other G-protein-coupled receptors have been generated in a similar way.
To investigate the relation between the function of Ca(2+)-activated K+ channels and phosphoinositide turnover, we have examined the physiological and pharmacological characteristics of ionotropic and metabotropic quisqualate (QA) receptors in rat cerebellar Purkinje cells during development using the slice-patch method combined with Ca2+ imaging. The typical response to QA obtained from a rat on postnatal day (PND) 21 consisted of three components: (1) a fast inactivating inward current, (2) a slow inward current, and (3) a slow outward current. The slow inward current was abolished in Ca(2+)-free medium, while the fast inactivating inward current and the slow outward current remained unaffected. The slow outward current which appeared to be activated via a metabotropic receptor was not observed in the Purkinje cell of PND 7 rat, in which dendrites were poorly developed but its amplitude increased linearly with PND. QA caused significant increases in [Ca2+]i in the fully developed dendritic region of the Purkinje cells even in Ca(2+)-free medium, suggesting a dendritic localization of the metabotropic receptors.
Whole-cell patch-clamp recordings were obtained from on-bipolar cells in, or isolated from, retinal slices prepared from dogfish retina. The properties of the cGMP-activated conductance of on-bipolar cells were compared with that of rod photoreceptors. The on-bipolar cell cGMP-activated channel was blocked by L-cis-diltiazem, a block which was strongly voltage dependent. However, this channel is not identical with that of photoreceptors. The location of the L-cis-diltiazem blocking site and its accessibility in the channel are not the same as in rods. The voltage dependence of block suggests that the blocking site, although near the intracellular side of the channel, is accessible to the positively charged form of L-cis-diltiazem only from the outward facing side of the channel. Furthermore, in contrast to rod channels, the conductance of the on-bipolar cell channels is unaltered by the removal of external divalent cations.
Whole-cell patch-clamp recordings were obtained from light-responsive on-bipolar cells in retinal slices of the dogfish. Inclusion of the A-subunit of pertussis toxin in the patch-pipette solution resulted in an increase in inward current and membrane conductance, and a block of light-evoked currents of on-bipolar cells. The opposite effect was obtained with the A-subunit of cholera toxin, which blocked light responses, and induced an outward current and a decrease in membrane conductance. These actions were NAD+ dependent. The results show that the G-protein(s) linking glutamate receptors to a cGMP cascade in on-bipolar cells possess sites which are ADP-ribosylated by pertussis and cholera toxins, with no homology to the adenylate cyclase system but possibly with a homology to transducin. Furthermore, inclusion of H-7, a kinase inhibitor in the patch-pipette solution, or of a non-hydrolysable ATP analogue (AMP-PNP) had no effect on light responses, membrane conductance or dark current of on-bipolar cells, suggesting that the components of this cGMP cascade are unlikely to be regulated by protein kinases.
We examined glutamate-mediated neurotoxicity in cortical cell cultures pretreated with 1-5 micrograms/ml tetanus toxin to attenuate the Ca(2+)-dependent release of neurotransmitters. Efficacy of the tetanus toxin pretreatment was suggested by blockade of electrical burst activity induced by Mg2+ removal and by reduction of glutamate efflux induced by high K+. Tetanus toxin reduced neuronal injury produced by brief exposure to elevated extracellular K+ or to glutamate, situations in which release of endogenous excitatory neurotransmitter is likely to play a role. Furthermore, although glutamate efflux evoked by anoxic conditions may occur largely via Ca(2+)-independent transport, tetanus toxin attenuated both glutamate efflux and neuronal injury following combined oxygen and glucose deprivation. With prolonged exposure periods, the neuroprotective efficacy of tetanus toxin was comparable to that of NMDA receptor antagonists. Presynaptic inhibition of Ca(2+)-dependent glutamate release may be a valuable approach to attenuating hypoxic-ischemic brain injury.
The selective metabotropic glutamate receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) stimulates phosphoinositide hydrolysis and elicits several physiological responses in rat hippocampal slices. However, recent studies suggest that the physiological effects of trans-ACPD in the hippocampus are mediated by activation of a receptor that is distinct from the phosphoinositide hydrolysis-linked receptor. Previous experiments indicate that cyclic AMP mimics many of the physiological effects of trans-ACPD in hippocampal slices. Furthermore, recent cloning and biochemistry experiments indicate that multiple metabotropic glutamate receptor subtypes exist, some of which are coupled to yet unidentified effector systems. Thus, we performed a series of experiments to test the hypothesis that ACPD increases cyclic AMP levels in hippocampal slices. We report that 1S,3R- and 1S,3S-ACPD (but not 1R,3S-ACPD) induce a concentration-dependent increase in cyclic AMP accumulation in hippocampal slices. This effect was blocked by the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonoproprionic acid but not by selective antagonists of ionotropic glutamate receptors. Furthermore, our results suggest that 1S,3R-ACPD-stimulated increases in cyclic AMP accumulation are not secondary to increases in cell firing or to activation of phosphoinositide hydrolysis.
Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.
We have previously reported that, in the rat dorsolateral septal nucleus (DLSN), metabotropic glutamate receptor (met-GluR) agonists evoked a slow depolarization accompanied by an increase in membrane conductance and burst firing. We have speculated that the burst firing elicited by met-GluR agonists may be due to activation or enhancement of a non-specific cation current, which exists in some DLSN neurons. Now we report that a slow afterdepolarization (sADP) mediated by a non-specific cation current was potentiated by both 1S,3R-ACPD and quisqualate. In addition, met-GluR agonists unmask a sADP in DLSN neurons which did not show a sADP under control conditions. Our data suggest that a non-specific cation current can be potentiated by activation of the met-GluR.
Intracellular recordings were obtained from neurones of the basolateral nucleus of the amygdala (BLA) and glutamate-mediated EPSPs evoked by stimulation of the stria terminalis (ST). The conformationally restricted analogue of glutamate trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) caused a dose-dependent reduction in EPSP amplitude, EC50 approximately 50 microM. This effect was mimicked by the glutamate autoreceptor agonist, L-aminophosphonobutyric acid (L-APB, 50 microM). Furthermore, the effects of submaximal concentrations (50 microM) of trans-ACPD and L-APB were additive. The reduction in EPSP amplitude is observed with concentrations of both drugs that have no effect on either the resting membrane potential or the input resistance of BLA neurones. In addition, these compounds can reduce EPSP amplitude but not the response to exogenous application of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA) suggesting activation of presynaptic receptors. These findings suggest that both trans-ACPD and L-APB act at presynaptic glutamate receptors on glutamatergic afferents to reduce excitatory transmission in the BLA.
The role of quisqualate (QUIS) metabotropic receptors in the synaptic transmission in the striatum was investigated using the cortico-striatal slice preparation. Low concentrations (1-30 microM) of trans-1-amino-cyclopentyl-1,3- dicarboxylic acid (t-ACPD), a selective agonist of QUIS metabotropic receptors, decreased glutamate-mediated synaptic potentials (EPSPs) evoked in the striatum by the stimulation of cortico-striatal fibers. This agonist decreased also GABA-mediated depolarizing synaptic potentials evoked by intrastriatal stimulation in the presence of 6-cyano-7-nitro-quinoxaline-2,3- dione (CNQX); this effect was less potent than the action of t-ACPD on glutamate-mediated potentials. Low concentrations of t-ACPD did not affect the intrinsic membrane properties of striatal neurons and their postsynaptic responses to exogenous glutamate and GABA. Higher concentrations (50-100 microM) to t-ACPD caused membrane depolarizations and inward currents in several neurons. Our data suggest that low concentrations of t-ACPD selectively reduce synaptic transmission while higher concentrations of this agonist may cause a direct excitatory action on striatal neurons.