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Behavioral assessment of mice lacking D1A dopamine receptors
Abstract
Dopamine D1A receptor-deficient mice were assessed in a wide variety of tasks chosen to reflect the diverse roles of this receptor subtype in behavioural regulation. The protocol included examination of exploration and locomotor activity in an open field, a test of sensorimotor orienting, both place and cue learning in the Morris water maze, and assessment of simple associative learning in an olfactory discrimination task. Homozygous mice showed broad-based impairments that were characterized by deficiencies in initiating movement and/or reactivity to external stimuli. Data obtained from flash evoked potentials indicated that these deficits did not reflect gross visual impairments. The partial reduction in D1A receptors in the heterozygous mice did not affect performance in most tasks, although circumscribed deficits in some tasks were observed (e.g., failure to develop a reliable spatial bias in the water maze). These findings extend previous behavioural studies of null mutant mice lacking D1A receptors and provide additional support for the idea that the D1A receptor participates in a wide variety of behavioural functions. The selective impairments of heterozygous mice in a spatial learning task suggest that the hippocampal/cortical dopaminergic system may be uniquely vulnerable to the partial loss of the D1A receptor.
... The role of DA in arousal was initially proposed by Jones et al. Smith et al. 1998) who found that lesioning of DA cells in VTA and SNc of cats induced behavioural states characterized by akinesia, hypertonus and decreased arousal. However, due to the general lack in lesion specificity, it was hard to solely ascribe a role for DA in these behaviours. ...
... More recent genetic targeting of DA receptors has begun to unravel the specific roles of DA in regulating various aspects of behaviour, in particular its role in modulating motivation, mood and sleep. For example, knocking out D1 subtype receptors (D1 KO mice) induced decreased movement initiation, reaction time to external stimuli and grooming bouts (Smith et al. 1998;Cromwell et al. 1998). Knocking out D1 receptors decreased spontaneous activity and impaired motor coordination (Baik et al. 1995;Kelly et al. 1998;Vallone et al. 2002). ...
Dopamine, a neurotransmitter produced in various brain regions, is implicated in regulation of motor control, reward, mood and addiction. Depression is a serious disorder affecting the day-to-day activities of patients that also imposes a substantial financial burden on society. The following chapter offers an expansive demonstration of the heterogeneous dopamine system in the brain, by describing some of the advances made in unraveling the various dopaminergic neural pathways and molecular mechanisms involved in depression. It also sheds some light on how these same neural pathways might be responsible for the disturbances in sleep and circadian rhythms experienced by patients suffering from depression.
... exhibits the most conserved sequence of all the DARs, featuring an extended C-terminus and a shortened third intracellular loop (ICL3) when compared to the D2like receptor family. Of all the individual DAR knockout mice, the D1DAR knockout exhibits the most severe phenotypes, including spatial learning deficits, 4 hyperactivity, 5 and abnormal memory retention, 6 emphasizing the functional importance of this receptor subtype. The D5DAR (also called D1B) is the remaining receptor subtype that comprises the D1-like family and will be discussed in a later section of this chapter. ...
... The human D4DAR gene possesses four exons and is located on chromosome 11p15. 5 203,204 The most common is the 4-repeat (D4.4) allele followed by the D4.7 or D4.2, depending on the population 205 ( Fig. 3.1.3). The 48bp repeat, with intra-and interspecies variations in both the sequences and the number of copies of the repeats, is also found in the D4 receptor of nonhuman primates but not in the rat D4 receptor. ...
Dopamine receptors are rhodopsin-like seven-transmembrane receptors (also called G protein-coupled receptors) that mediate the central and peripheral actions of dopamine. Dopamine receptors are most abundant in pituitary and brain, particularly in the basal forebrain, but are also found in the retina and in peripheral organs such as the kidney. Stimulation of dopamine receptors modulates natriuresis in the kidney, as well as cell division and hormone synthesis and secretion in the pituitary. Brain dopamine receptors regulate movement and locomotion, motivation, and working memory. Five subtypes of mammalian dopamine receptors have been identified that are divided into D1-like (D1, D5) or D2-like (D2, D3, D4) subgroups. The D1-like receptors couple primarily to the Gs family of G proteins (Gs and Golf), whereas the D2-like receptors couple primarily to the Gi/o family. This chapter discusses the molecular pharmacology of the five dopamine receptor subtypes.
... Dopamine 1-and 5-receptor (D1R and D5R) activation is linked to spatial learning and memory processing (Bethus et al., 2010;O'Carroll et al., 2006;Sawaguchi and Goldman-Rakic, 1991;Silva et al., 2012). Constitutive deletion of the D1R significantly impairs regular and reversal learning on the watermaze task (El-Ghundi et al., 1999;Granado et al., 2008;Holmes et al., 2001;Karasinska et al., 2000;Smith et al., 1998). However, constitutive D5R deletion has not shown deficits in water-maze spatial learning and memory (Holmes et al., 2001). ...
... Our finding that the primary site of D1R expression is the DG, and not CA1, is consistent with some previous findings (Fremeau et al., 1991;Mansour et al., 1992;Mu et al., 2011). However, other previous studies have focused on hippocampal CA1 D1R activation as the driver of synaptic plasticity, learning, and memory (Huang and Kandel, 1995;Lemon and Manahan-Vaughan, 2006;Li et al., 2003;Ortiz et al., 2010;Smith et al., 1998). Some of these previous studies attributed the observed deficits to CA1 D1Rs, which may be due to sparse expression of CA1 D1Rs (Gangarossa et al., 2012). ...
Activation of prefrontal cortical (PFC), striatal, and hippocampal dopamine 1-class receptors (D1R and D5R) is necessary for normal spatial information processing. Yet the precise role of the D1R versus the D5R in the aforementioned structures, and their specific contribution to the water-maze spatial learning task remains unknown. D1R- and D5R-specific in situ hybridization probes showed that forebrain restricted D1R and D5R KO mice (F-D1R/D5R KO) displayed D1R mRNA deletion in the medial (m)PFC, dorsal and ventral striatum, and the dentate gyrus (DG) of the hippocampus. D5R mRNA deletion was limited to the mPFC, the CA1 and DG hippocampal subregions. F-D1R/D5R KO mice were given water-maze training and displayed subtle spatial latency differences between genotypes and spatial memory deficits during both regular and reversal training. To differentiate forebrain D1R from D5R activation, forebrain restricted D1R KO (F-D1R KO) and D5R KO (F-D5R KO) mice were trained on the water-maze task. F-D1R KO animals exhibited escape latency deficits throughout regular and reversal training as well as spatial memory deficits during reversal training. F-D1R KO mice also showed perseverative behavior during the reversal spatial memory probe test. In contrast, F-D5R KO animals did not present observable deficits on the water-maze task. Because F-D1R KO mice showed water-maze deficits we tested the necessity of hippocampal D1R activation for spatial learning and memory. We trained DG restricted D1R KO (DG-D1R KO) mice on the water-maze task. DG-D1R KO mice did not present detectable spatial memory deficit, but did show subtle deficits during specific days of training. Our data provides evidence that forebrain D5R activation plays a unique role in spatial learning and memory in conjunction with D1R activation. Moreover, these data suggest that mPFC and striatal, but not DG D1R activation are essential for spatial learning and memory.
... Normal spatial learning ability has also been observed in mice engineered to lack D3 receptors (Karasinska et al., 2000) and D5 receptors (Holmes et al., 2001). Conversely, Drd1-/-mice did exhibit impairments in spatial learning and memory (El-Ghundi et al., 1999;Smith et al., 1998). The lack of impairment in spatial learning in Drd4-/-mice does not have to be interpreted as if D4 receptors expressed in the hippocampus do not participate in circuits related to this complex cognitive process. ...
... In the hippocampus, genetic, and pharmacological evidence suggests that D1Rs play an important role in regulating longterm potentiation and specific forms of learning and memory (Huang and Kandel, 1995;Smith et al., 1998;El-Ghundi et al., 1999;Li et al., 2003;Hansen and Manahan-Vaughan, 2014). Although both D1R and D5R are present in the hippocampus, targeted deletion of the D1R but not D5R results in deficits in hippocampal LTP (Granado et al., 2008;Ortiz et al., 2010), contextual fear conditioning (Ortiz et al., 2010;Sariñana et al., 2014) and spatial learning (Granado et al., 2008;Ortiz et al., 2010;Sariñana and Tonegawa, 2016), therefore suggesting a predominant role for the D1R in these processes. ...
The dopamine D1 receptor (D1R) is a Gα s/olf -coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα s or Gα olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.
... A selective monoamine oxidase B inhibitor L-deprenyl alleviated acquisition and probe trial performance deficits caused by scopolamine (Yavich et al., 1993;Gelowitz et al., 1994). Similarly, DA antagonists impair MWM learning (McNamara and Skelton, 1993) and transgenic mice lacking DA D1 receptors showed severely impaired MWM acquisition (Smith et al., 1998); and administration of D1 receptor agonists enhanced acquisition performance (Hersi et al., 1995). Likely, administration of smilagenin (He et al., 2019) or overexpression pre-enkephalin in the striatum (Bissonnette et al., 2014) to increase TH positive neurons in SN significantly improved the locomotor ability. ...
Dysfunction of the central noradrenergic and dopaminergic systems is the primary neurobiological characteristic of Parkinson's disease (PD). Importantly, neuronal loss in the locus coeruleus (LC) that occurs in early stages of PD may accelerate progressive loss of dopaminergic neurons. Therefore, restoring the activity and function of the deficient noradrenergic system may be an important therapeutic strategy for early PD. In the present study, the lentiviral constructions of transcription factors Phox2a/2b, Hand2 and Gata3, either alone or in combination, were microinjected into the LC region of the PD model VMAT2 Lo mice at 12 and 18 month age. Biochemical analysis showed that microinjection of lentiviral expression cassettes into the LC significantly increased mRNA levels of Phox2a, and Phox2b, which were accompanied by parallel increases of mRNA and proteins of dopamine β-hydroxylase (DBH) and tyrosine hydroxylase (TH) in the LC. Furthermore, there was considerable enhancement of DBH protein levels in the frontal cortex and hippocampus, as well as enhanced TH protein levels in the striatum and substantia nigra. Moreover, these manipulations profoundly increased norepinephrine and dopamine concentrations in the striatum, which was followed by a remarkable improvement of the spatial memory and locomotor behavior. These results reveal that over-expression of these transcription factors in the LC improves noradrenergic and dopaminergic activities and functions in this rodent model of PD. It provides the necessary groundwork for the development of gene therapies of PD, and expands our understanding of the link between the LC-norepinephrine and dopamine systems during the progression of PD.
... Several animal models were developed in order to understand the role of dopamine in neurodevelopment. For example, knockout (KO) mice for the dopaminergic enzyme tyrosine hydroxylase (TH) and dopaminergic receptors show several behavioral alterations with respect to motor behavior, feeding behavior, reward behavior, and cognition (Baik et al., 1995;El-Ghundi et al., 2003;Maldonado et al., 1997;Smith et al., 1998;Szczypka et al., 1999;Xu et al., 1994). ...
Dopaminergic signaling and neurodevelopment alterations are associated with several neuropsychiatric disorders. Knockout mice for dopamine transporters (DAT) as well as site-specific knockout mice lacking dopaminergic D2 autoreceptors in dopaminergic neurons (DA-D2RKO) display behavioral alterations such as hyperlocomotion and abnormal prepulse inhibition. However, it is possible that dopaminergic imbalances may have different effects during varied neurodevelopmental windows. In our previous study, we observed that elevated levels of dopamine during the perinatal developmental window increased exploratory behavior of juvenile (4-week-old) Swiss female mice and impaired hedonic behavior in males. In this study, we investigated whether these behavioral alterations persist through young adulthood. In order to do so, we administered daily doses of L-Dopa to mice pups beginning from postnatal day 1 (PD1) to PD5. At the age of 8 weeks, we submitted the young adult males and females to the open field test, elevated plus maze, forced swimming test, and sucrose preference test. We observed that augmentation of dopamine levels during the perinatal developmental window increased locomotor behavior in females, but not males. We also observed an increase in anxiety-behavior in females and anxiolytic-like behavior in males. In addition, we observed stress-coping behavior in males and an increase of hedonic behavior in females. Our results show that dopamine signaling is important for behavioral development and that transient imbalances of dopamine levels can cause permanent behavioral alterations – alterations which are different in males than in females. These data may help in better understanding the spectrum of symptoms associated with different neuropsychiatric disorders.
... Several studies suggest that activity of D1 neurons mainly influences both eating and mood. 1) Activation of D1-MSNs in the NAc increases food intake, and inhibition suppresses food consumption, while the level of D2 receptors has no effect on eating behavior (Zhu et al., 2016). 2) Pharmacological inhibition or genetic deletion of the D1 receptors disrupts normal feeding and induces anxiety (Lutz et al., 2001;Smith et al., 1998;Wall et al., 2011). 3) Chronic restraint stress in mice that induces anorexia leading to weight loss and anhedonia, is linked to altered excitatory transmission selectively onto D1-MSNs (Lim et al., 2012). ...
Dieting induces depression and anxiety among other emotional symptoms. Animal models indicate that repeated access to palatable foods such as sugar induces depression and anxiety-like behavior when the food is no longer available. However, the neurobiological mechanisms of how dietary restriction influences mood have not been fully understood. We used the two-bottle sucrose choice paradigm as an overeating and withdrawal model. Withdrawal after lengthy sucrose overeating elicited depression and anxiety-like behavior, which was reversed by sucrose reinstatement. In the nucleus accumbens (NAc) of sucrose withdrawal animals, dopamine levels and cAMP response element binding protein (CREB) activity were significantly reduced, while the inwardly rectifying K+ channel, Kir2.1, was significantly elevated. In addition, overexpression of Kir2.1 selectively in neurons expressing dopamine D1 receptors was sufficient to induce negative mood-linked behavior in the absence of sucrose overeating experience. As elevated K+ channels reduce neuronal excitability, a sucrose withdrawal-induced increase in Kir2.1 expression is able to decrease NAc activity, which provides a cellular basis for depression and anxiety-like behavior in animals.
... Although hippocampal D 1 /D 5 receptors may play a disproportionate role in the persistence of hippocampal memory, it has also been implicated in facilitating the induction of E-LTP (reviewed in [21]) and, thereby, the entry of information into earlier memory [39]. Since available pharmacological agonists and antagonists of dopamine D 1 -like receptors do not discriminate D 1 and D 5 receptors [40], numerous gene knockout studies were conducted in order to elucidate the precise function of D 1 and D 5 receptors in roles of hippocampal synaptic plasticity and memory [41][42][43][44][45][46][47] (reviewed in [21]). Yet, differentiating the function of hippocampal D 1 and D 5 receptors may seem like a daunting task, because there is a caveat in global knockout studies in that they lack regional selectivity. ...
Most everyday memories including many episodic-like memories that we may form automatically in the hippocampus (HPC) are forgotten, while some of them are retained for a long time by a memory stabilization process, called initial memory consolidation. Specifically, the retention of everyday memory is enhanced, in humans and animals, when something novel happens shortly before or after the time of encoding. Converging evidence has indicated that dopamine (DA) signaling via D 1 /D 5 receptors in HPC is required for persistence of synaptic plasticity and memory, thereby playing an important role in the novelty-associated memory enhancement. In this review paper, we aim to provide an overview of the key findings related to D 1 /D 5 receptor-dependent persistence of synaptic plasticity and memory in HPC, especially focusing on the emerging evidence for a role of the locus coeruleus (LC) in DA-dependent memory consolidation. We then refer to candidate brain areas and circuits that might be responsible for detection and transmission of the environmental novelty signal and molecular and anatomical evidence for the LC-DA system. We also discuss molecular mechanisms that might mediate the environmental novelty-associated memory enhancement, including plasticity-related proteins that are involved in initial memory consolidation processes in HPC.
... The generation of D 1 and D 5 genetically modified mice has helped elucidate critical functions of the two receptors in multiple physiological processes (Smith et al., 1998;Miyamoto et al., 2001;Montague et al., 2001;Hollon et al., 2002;Karlsson et al., 2008). In particular, D 1 receptor null mutants have deficits in higher order cognitive functions such as working memory (Drago et al., 1994;Xu et al., 1994;Holmes et al., 2001;Xing et al., 2012). ...
Pharmacological studies indicate that dopamine D1-like receptors (D1 and D5) are critically involved in cognitive function. However, the lack of pharmacological ligands selective for either the D1 or D5 receptors has made it difficult to determine the unique contributions of the D1-like family members. To circumvent these pharmacological limitations, we used D5 receptor homozygous (-/-) and heterozygous (+/-) knockout mice, to identify the specific role of this receptor in higher order cognitive functions. We identified a novel role for D5 receptors in the regulation of spatial working memory and temporal order memory function. The D5 mutant mice acquired a discrete paired-trial variable-delay T-maze task at normal rates. However, both D5+/- and D5-/- mice exhibited impaired performance compared to D5+/+ littermates when a higher burden on working memory faculties was imposed. In a temporal order object recognition task, D5+/- exhibited significant memory deficits. No D5-dependent differences in locomotor functions and interest in exploring objects were evident. Molecular biomarkers of dopaminergic functions within the prefrontal cortex (PFC) revealed a selective gene-dose effect on Akt phosphorylation at Ser473 with increased levels in D5-/- knockout mice. A trend toward reduced levels in CaMKKbeta brain-specific band (64 kDa) in D5-/- compared to D5+/+ was also evident. These findings highlight a previously unidentified role for D5 receptors in working memory function and associated molecular signatures within the PFC.
... In the current study, animals receiving IMI presented an increase in the freezing and rearing behaviors, suggesting that IMI could increase the emotionality, thereby decreasing the locomotor activity. This could be related to the exploration acting as a good index of stress response in rodents (Smith et al., 1988). Thus, the open-field results obtained in this experiment demonstrated the presence of a stress-like state in IMI (0.5 and 1.0 mg/kg bw)-treated rats. ...
... Sponges were all identical, measuring 8.5cm x 7cm x 4cm, with a 2cm hole cut in the centre, and were only used for one testing day with an individual animal. Kellogg's Froot Loops used in the study were shown to be a good reinforcement in operant tasks [25]. All the sessions were recorded by a video camera positioned directly above the operant box to allow observation during trials and subsequent off-line analysis. ...
L-Lactate (LL) is an essential cellular metabolite which can be used to generate energy. In addition, accumulating evidence suggests that LL is used for inter-cellular signalling. Some LL-sensitive receptors have been identified but we recently proposed that there may be yet another unknown G-protein coupled receptor (GPCR) sensitive to LL in the brain. Olfactory receptors (ORs) represent the largest family of GPCRs and some of them are expressed outside the olfactory system, including brain, making them interesting candidates for non-olfactory LL signalling. One of the “ectopically” expressed ORs, Olfr78 in mice (Olr59 in rats and OR51E2 in humans), reportedly can be activated by LL. This implies that both rodents and humans should be able to detect the LL odour. Surprisingly, this has never been demonstrated. Here we show that mice can detect the odour of LL in odour detection and habituation-dishabituation tasks, and discriminate it from peppermint and vanilla odours. Behaviour of the Olfr78 null mice and wildtype mice in odour detection task was not different, indicating that rodents are equipped with more than one LL-sensitive OR. Rats were also able to use the smell of LL as a cue in an odour-reward associative learning task. When presented to humans, more than 90% of participants detected a smell of LL in solution. Interestingly, LL was perceived differently than acetate or propionate—LL was preferentially reported as a pleasant sweet scent while acetate and propionate were perceived as repulsive sour/acid smells. Subjective perception of LL smell was different in men and women. Taken together, our data demonstrate that both rodents and humans are able to detect the odour of LL. Moreover, in mice, LL perception is not purely mediated by Olfr78. Discovery of further LL-sensitive OR might shed the light on their contribution to LL signalling in the body.
... D 1 receptor antagonists strongly disrupt locomotor activity in animals, but the results obtain with the D 1 receptor-deficient mice have been inconsistent. A decreased (Drago et al. 1996;El-Ghundi et al. 1998;Smith et al. 1998;Tomiyama et al. 2002) or increased spontaneous activity of these mice has been reported Centonze et al. 2003;McNamara et al. 2003). Compared to the D 1 and D 2 receptors, the D 3 , D 4 and D 5 receptors do not appear to regulate spontaneous activity. ...
... Experiments combining targeted gene knock-out (KO) of the D 1 receptor and the behavioral effects of cocaine also suggest that the D 1 receptor is involved in the locomotor-stimulating effects of cocaine. For example, although mice that have been genetically altered to lack the D 1 DA gene (homozygous KO) have relatively normal levels of spontaneous locomotor activity (Zhang et al. 1994;Smith et al. 1998; but see Xu et al. 1994), they have a reduced locomotor response to cocaine relative to heterozygous and wild-type mice (Xu et al. 2000). Further evidence supporting a role for D 1 -like DA receptors in the locomotor stimulating effects of cocaine comes from studies of D 1 -like antagonists. ...
... The Drd1 I116S mutant rats might share phenotypic similarities with Drd1-knockout mice, but comparison is complicated by variable findings in these mice. For instance, it has been reported that horizontal locomotor activity is unaltered (Drago et al., 1996), increased (Waddington et al., 2005a) or decreased (Smith et al., 1998) in Drd1-knockout mice. Furthermore, it has been reported that Drd1-knockout mice display both reduced (Cromwell et al., 1998) and increased selfgrooming behaviour (Clifford and Waddington, 1998), whereas we found no changes in self-grooming in Drd1 I116S mutant rats. ...
Social cognition is an endophenotype that is impaired in schizophrenia and several other (comorbid) psychiatric disorders. One of the modulators of social cognition is dopamine, but its role is not clear. The effects of dopamine are mediated through dopamine receptors, including the dopamine D1 receptor (Drd1). Because today's Drd1 receptor agonists are not Drd1 selective, pharmacological tools are not sufficient to delineate the role of the Drd1. We describe a novel rat model with a genetic mutation in the Drd1, in which we measured basic behavioural phenotypes and social cognition. The I116S mutation was predicted to render the receptor less stable. In line with this computational prediction, the Drd1 mutation led to a decreased transmembrane insertion of Drd1, while Drd1 expression, as measured by Drd1 mRNA levels, remained unaffected. Due to decreased transmembrane Drd1 insertion, the mutant rats displayed normal basic motoric and neurological parameters, as well as locomotor activity and anxiety-like behaviour. However, measures of social cognition like social interaction, scent marking, pup ultrasonic vocalizations and sociability, were strongly reduced in the mutant rats. This profile of the Drd1 mutant rat offers the field of neuroscience a novel genetic rat model to study a series of psychiatric disorders including schizophrenia, autism, depression, bipolar disorder and drug addiction.
... D1R KO mice have abnormal locomotor behaviour that ranges from hypoactive to hyperactive depending on the experimental conditions. Although in several studies it was reported that these mice were hyperactive in a novel environment and during the dark phase of the light-dark cycle (Granado et al., 2008;Xu et al., 1994), others reported an increased latency to move in an open field consistent with a hypoactive phenotype (Smith et al., 1998). ...
The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion.
... D 1 receptor antagonists strongly disrupt locomotor activity in animals, but the results obtain with the D 1 receptor-deficient mice have been inconsistent. A decreased (Drago et al. 1996;El-Ghundi et al. 1998;Smith et al. 1998;Tomiyama et al. 2002) or increased spontaneous activity of these mice has been reported Centonze et al. 2003;McNamara et al. 2003). Compared to the D 1 and D 2 receptors, the D 3 , D 4 and D 5 receptors do not appear to regulate spontaneous activity. ...
Dopamine is an important neurotransmitter in the brain and controls various functions, including motor activity, cognition, motivation, emotion, food intake and endocrine secretions. The dopaminergic systems responsible for these functions have received much attention, namely because their dysfunctions are involved in the etiology or treatment of several pathological conditions, including schizophrenia, Parkinson's disease, Tourette's syndrome, attentiondeficit and hyperactivity disorder and hyperprolactinemia. It has long been thought that only two receptor subtypes, the D1 and D 2 receptors, which were initially defined on the basis of their distinct transduction mechanisms and pharmacological profiles (Spano et al. 1978; Kebabian and Calne 1979), mediated the pleiotropic actions of dopamine. At the time, it was recognized that the target of antiparkinsonian drugs and of antipsychotic drugs was the D2 receptor. In spite of various proposals for additional subtypes, the dual classification of dopamine receptors has gained general acceptance. The cloning of a D2 receptor cDNA (Bunzow et al. 1988) and the subsequent demonstration that two splicing variants of this receptor exist, the D2S and D2L, the latter with an additional 29-aminoacid sequence (Giros et al. 1989), was followed by the cloning of a D1 receptor cDNA (Dearry et al. 1990); (Sunahara et al. 1990); (Zhou et al. 1990; Monsma et al. 1991). Thus, molecular cloning outcomes met the expectations regarding the diversity of dopamine receptors, as perceived at the time. However, only one week after publication of the reports on cloning of the D1 receptor, a novel dopamine receptor subtype was identified, the D3 receptor (Sokoloff et al. 1990). It was followed soon later by the cloning of the D4 (Van Tol et al. 1991) and D 5 (Sunahara et al. 1991) receptors, which were also far from being expected. Structure and functions of dopamine receptors, and their implications to pathological conditions have been reviewed earlier (Civelli et al. 1993; Sokoloff et al. 1995; Missale et al. 1998), at a time when no highly specific tools were available. Notably, a wealth of novel information has been gained from the identification of highly selective ligands for some dopamine receptor subtypes, the generation of dopamine receptor subtype-targeted mutant mice and subtypespecific antibodies, and the advances of molecular genetics. This review intends to discuss this new information and to examine its implication for our knowledge of dopamine-related brain disorders and their treatment.
... D1 knockout mice demonstrate spatial learning deficits (El-Ghundi et al., 1999), and D1a (a subtype of the D1 receptor) knockout mice show broad impairments in the initiation of learning cue related tasks (Smith et al., 1998). However, the idea of receptors being impaired versus not impaired may be a simplistic viewpoint of DA interaction at the D1 receptor. ...
Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients is the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. Experimental therapies based upon cortical and hippocampal neuroprotection have not translated clinically. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Striatal damage causes deficits in executive function, learning, and memory. Dopamine and cAMP regulated phosphoprotein 32 (DARPP-32), expressed within striatal medium spiny neurons, is known to regulate several substrates of cognition. We found that controlled cortical impact injury in rats produces a chronic decrease in DARPP-32 threonine-34 phosphorylation and increase in protein phosphatase-1 activity. There is no effect of injury on threonine-75 phosphorylation or DARPP-32 protein. Amantadine has known benefits on post-TBI cognitive deficits and when given daily for two weeks reversed the DARPP-32 and protein phosphatase-1 changes. Amantadine also decreased the phosphorylation of threonine-75 consistent with activity as a partial N-methyl-D-aspartic acid receptor antagonist and partial dopamine agonist. FK-506, also known as tacrolimus, is a calcineurin inhibitor that has been shown to decrease cell death in the hippocampus following a fluid percussion experimental TBI. Calcineurin is also an important regulator of DARPP-32 phosphorylation in the striatum. We evaluated the effect of FK-506 on the hippocampus and DARPP-32 in the striatum to better detail its effects after a TBI. An acute administration of FK-506 following controlled cortical impact reversed the effects of TBI on DARPP-32 phosphorylation seen chronically. We then evaluated the effect of a combined drug therapy on cognitive deficits post TBI. An acute treatment with FK-506 post TBI followed by chronic Amantadine therapy demonstrated an improvement in both motor behavior and Morris water maze deficits seen following TBI. Neither drug produced benefit when given alone. These data demonstrate that DARPP-32 represents a promising new therapeutic target for TBI induced cognitive deficits.
Most everyday memories, including numerous episodic memories formed automatically in the hippocampus, are forgotten. However, some memories are retained for extended periods through a memory stabilization process known as cellular or initial memory consolidation. Notably, in both animals and humans, the retention of everyday memories is enhanced during novel experiences occurring shortly before or after memory encoding, a process known as synaptic tagging and capture (STC). A growing body of evidence suggests that dopamine signaling via D1/D5 receptors in the hippocampus is crucial for the persistence of synaptic plasticity and memory, highlighting its significant role in novelty-associated memory enhancement. This chapter presents an overview of key findings related to the persistence of synaptic plasticity and memory in the hippocampus through hippocampal D1/D5 receptor dependency, with special emphasis on the emerging role of the locus coeruleus (LC) in novelty-associated dopamine-dependent memory consolidation. Furthermore, two distinct dopaminergic systems are explored (the ventral tegmental area (VTA)-hippocampal and LC-hippocampal systems), and the specialization mechanisms of each system in different memory consolidation processes are discussed. Additionally, the anatomical and molecular foundations of D1/D5 receptor-mediated signaling in the LC-hippocampal system are examined. Finally, the molecular mechanisms possibly underlying distinct novelty-associated memory enhancement are discussed, including the involvement of plasticity-related proteins (PRPs) in the stabilization of structural and functional changes at potentiated synapses, culminating in initial memory consolidation in the hippocampus.
Purpose of Review
CNS stimulants have been the treatment of choice among children with attention-deficit hyperactivity disorder (ADHD) ages 6 and older, but their effectiveness and tolerability are major concerns. There is an unmet need for dopamine receptor-specific pharmacotherapy to improve the effectiveness and tolerability. Here, we conducted a scoping review of the literature to evaluate the current understanding of specific receptors and how they may relate to various phenotypes and behaviors in ADHD.
Recent Findings
ADHD is the most common pediatric neurobehavioral disorder and is associated with significant impairment and long-term negative outcomes. The pathophysiology of ADHD is related to dopamine (DA) and dopamine receptor (DAR) dysregulation in the brain. There is growing evidence that specific dopamine receptor subtypes are associated with specific symptoms and behaviors associated with ADHD, such as motor and attention dysfunction.
Summary
This study provides a scoping review of the up-to-date knowledge on specific DAR subtypes and how they may be implicated in the pathophysiology and or symptoms of ADHD. Knowledge of DAR and how they relate to the underlying disease process of ADHD may aid in developing targeted treatment options for ADHD with improved efficacy and tolerability.
How to optimize deep-brain stimulation
Deep-brain stimulation as presently used in clinical settings, for example, to treat Parkinson’s disease, does not differentiate between different neural circuitries. Considerable improvements could thus be achieved with selective stimulation that targets particular neuronal populations. Spix et al . used optogenetics to develop a clever electrical stimulation protocol that enhances cell-type specificity (see the Perspective by Haas). The authors managed to drive population-specific neuromodulation in a brain region called the external globus pallidus with brief bursts of electrical stimulation, which then yielded a long-lasting effect in a mouse model of Parkinson’s disease. —PRS
At each time in our life, we choose one or few behaviors, while suppressing many other behaviors. This is the basic mechanism in the basal ganglia, which is done by tonic inhibition and selective disinhibition. Dysfunctions of the basal ganglia then cause 2 types of disorders (difficulty in initiating necessary actions and difficulty in suppressing unnecessary actions) that occur in Parkinson’s disease. The basal ganglia generate such opposite outcomes through parallel circuits: The direct pathway for initiation and indirect pathway for suppression. Importantly, the direct pathway processes good information and the indirect pathway processes bad information, which enables the choice of good behavior and the rejection of bad behavior. This is mainly enabled by dopaminergic inputs to these circuits. However, the value judgment is complex because the world is complex. Sometimes, the value must be based on recent events, thus is based on short-term memories. Or, the value must be based on historical events, thus is based on long-term memories. Such memory-based value judgment is generated by another parallel circuit originating from the caudate head and caudate tail. These circuit-information mechanisms allow other brain areas (e.g., prefrontal cortex) to contribute to decisions by sending information to these basal ganglia circuits. Moreover, the basal ganglia mechanisms (i.e., what to choose) are associated with cerebellum mechanisms (i.e., when to choose). Overall, multiple levels of parallel circuits in and around the basal ganglia are essential for coordinated behaviors. Understanding these circuits is useful for creating clinical treatments of disorders resulting from the failure of these circuits.
The modulation of interval timing by dopamine (DA) has been well-established over decades of research. The nature of this modulation, however, has remained controversial: While the pharmacological evidence has largely suggested that time intervals are overestimated with higher DA levels, more recent optogenetic work has shown the opposite effect. In addition, a large body of work has asserted DA's role as a ``reward prediction error" (RPE), or a teaching signal that allows the basal ganglia to learn to predict future rewards in reinforcement learning tasks. Whether these two seemingly disparate accounts of DA may be related has remained an open question. By taking a reinforcement learning-based approach to interval timing, we show here that the RPE interpretation of DA naturally extends to its role as a modulator of timekeeping, and furthermore, that this view reconciles the seemingly conflicting observations. We derive a biologically plausible, DA-dependent plasticity rule that can modulate the rate of timekeeping in either direction, and whose effect depends on the timing of the DA signal itself. This bidirectional update rule can account for the results from pharmacology and optogenetics, as well as the behavioral effects of reward rate on interval timing, and the temporal selectivity of striatal neurons. Hence, by adopting a single RPE interpretation of DA, our results take a step towards unifying computational theories of reinforcement learning and interval timing.
In this chapter we highlight four applications of joint modeling to various contexts where the integration of neural and behavioral data proved useful in unveiling the underlying cognitive dynamics that were not apparent with behavioral data alone. In the first application, we discuss how structural information in the form of diffusion weighted imaging (DWI) can be used to better constrain a model of evidence accumulation fit to data from a perceptual decision making task with a speed accuracy manipulation. Second, we discuss a recent extension of this approach to functional imaging data obtained using magnetic resonance imaging (fMRI) on a trial-by-trial level. In particular, we discuss how pre-stimulus information can be used to enhance predictions about the speed and accuracy of decision making processes once the stimulus is presented. Third, we discuss how the joint modeling approach can be used to exploit the temporal resolution provided by electroencephalography (EEG) and the spatial resolution provided by fMRI to better constrain a cognitive model. We show that while these two measures provide similar information about the decision making process, the information is not identical within a subject, and as a result, having more neural covariates can enhance model predictions for withheld data. Finally, we discuss how joint modeling can be applied to single-unit recording data from monkeys, highlighting that our approach is not modality specific.
Spatial memory deficits are a common hallmark of psychiatric conditions, possibly due to a genetic predisposition. Thus, unravelling the relationship between genes and memory might suggest novel therapeutic targets and pathogenetic pathways. Genetic deletions are known to lead to memory deficits (post-deletion "forgetfulness" genes, PDF), or, in few instances to improve spatial memory (post-deletion "hypermnesic" genes, PDH). To assess this topic, we performed a meta-analytic approach on memory behavior in knock-out mice. We screened 300 studies from PubMed and retrieved 87 genes tested for possible effects on spatial memory. This database was crossed with the Allen Brain Atlas (brain distribution) and the Enrichr (gene function) databases. The results show that PDF genes have higher expression level in several ventral brain structures, particularly the encephalic trunk and in the hypothalamus. Moreover, part of these genes are implicated in synaptic functions. Conversely, the PDH genes are associated to G-protein coupled receptors downstream signalling. Some candidate drugs were also found to interfere with some of the PDH genes, further suggesting that this approach might help in identifying drugs to improve memory performance in psychiatric conditions.
Much of the motor impairment associated with Parkinson's disease is thought to arise from pathological activity in the networks formed by the basal ganglia (BG) and motor cortex. To evaluate several hypotheses proposed to explain the emergence of pathological oscillations in Parkinsonism, we investigated changes to the directed connectivity in BG networks following dopamine depletion. We recorded local field potentials (LFPs) in the cortex and basal ganglia of rats rendered Parkinsonian by injection of 6-hydroxydopamine (6-OHDA) and in dopamine-intact controls. We performed systematic analyses of the networks using a novel tool for estimation of directed interactions (Non-Parametric Directionality, NPD). We also used a 'conditioned' version of the NPD analysis which reveals the dependence of correlation between two signals upon a third reference signal. We find evidence of dopamine dependency of both low beta (14-20 Hz) and high beta/low gamma (20-40 Hz) directed network interactions. Notably, 6-OHDA lesions were associated with enhancement of the cortical "hyper-direct" connection to the subthalamic nucleus (STN) and its feedback to the cortex and striatum. We find that pathological beta synchronization resulting from 6-OHDA lesioning is widely distributed across the network and cannot be located to any individual structure. Further, we provide evidence that high beta/gamma oscillations propagate through the striatum in a pathway that is independent of STN. Rhythms at high beta/gamma show susceptibility to conditioning that indicates a hierarchical organization when compared to low beta. These results further inform our understanding of the substrates for pathological rhythms in salient brain networks in Parkinsonism.
In addition to their well-known role in skeletal movements, the basal ganglia control saccadic eye movements (saccades) by means of their connection to the superior colliculus (SC). The SC receives convergent inputs from cerebral cortical areas and the basal ganglia. To make a saccade to an object purposefully, appropriate signals must be selected out of the cortical inputs, in which the basal ganglia play a crucial role. This is done by the sustained inhibitory input from the substantia nigra pars reticulata (SNr) to the SC. This inhibition can be removed by another inhibition from the caudate nucleus (CD) to the SNr, which results in a disinhibition of the SC. The basal ganglia have another mechanism, involving the external segment of the globus pallidus and the subthalamic nucleus, with which the SNr-SC inhibition can further be enhanced. The sensorimotor signals carried by the basal ganglia neurons are strongly modulated depending on the behavioral context, which reflects working memory, expectation, and attention. Expectation of reward is a critical determinant in that the saccade that has been rewarded is facilitated subsequently. The interaction between cortical and dopaminergic inputs to CD neurons may underlie the behavioral adaptation toward purposeful saccades.
Dopamine is widely involved in behaviors related to motor activity, cognition, motivation, and reward. Dopamine signal is transduced through the dopamine receptor gene family. The dopamine D1 receptor (D1R) is highly expressed in the striatum, and is responsible for regulating the motor function. Recently, we have reported that the knockdown (KD) mice in which D1R was conditionally eliminated at adult stage, displayed a hypoactivity in the home cage than wild type mice; however, conventional D1R knockout (KO) mice show hyperactive phenotypes. In order to assess whether the difference in the time of eliminating D1R expression affects the behavioral phenotypes, we generated D1R KD mice at the postnatal and adult stages, and compared their motor function with D1R KO mice. Consequently, D1R KD at postnatal and adult stages resulted in severe locomotive defects compared with D1R KO mice. These results suggested that D1R has versatile functions, and the knockdown timing greatly influences the normal motor activity in the adolescent to adult stages.
The motor symptoms of both Parkinson’s disease and focal dystonia arise from dysfunction of the basal ganglia, and are improved by pallidotomy or deep brain stimulation of the Globus Pallidus interna (GPi). However, Parkinson’s disease is associated with a greater degree of basal ganglia-dependent learning impairment than dystonia. We attempt to understand this observation in terms of a comparison of the electrophysiology of the output of the basal ganglia between the two conditions. We use the natural experiment offered by Deep Brain Stimulation to compare GPi local field potential responses in subjects with Parkinson’s disease compared to subjects with dystonia performing a forced-choice decision-making task with sensory feedback. In dystonic subjects, we found that auditory feedback was associated with the presence of high gamma oscillations nestled on a negative deflection, morphologically similar to sharp wave ripple complexes described in human rhinal cortex. These were not present in Parkinson’s disease subjects. The temporal properties of the high gamma burst were modified by incorrect trial performance compared to correct trial performance. Both groups exhibited a robust low frequency response to ‘incorrect’ trial performance in dominant GPi but not non-dominant GPi at theta frequency. Our results suggest that cellular processes associated with striatum-dependent memory function may be selectively impaired in Parkinson’s disease even if dopaminergic drugs are administered, but that error detection mechanisms are preserved.
Significance
Cocaine-induced synaptic plasticity in the nucleus accumbens is implicated in neural adaptations that underlie addiction. Here we demonstrate that Wiskott-Aldrich syndrome protein (WASP) family verprolin homologous protein 1 (WAVE1) plays a selective role in medium spiny projection neurons that express D1 dopamine receptors (D1-MSNs) in the cellular and behavioral actions of cocaine. Our results suggest that chronic exposure to cocaine, followed by withdrawal, potentiates the signaling capacity of D1 dopamine and NMDA glutamate receptors, allowing WAVE1 activation in response to an acute cocaine challenge. WAVE1 activation is associated with morphological and functional decreases in glutamatergic synapses on D1-MSNs. Thus, we propose that WAVE1 is involved in a negative feedback mechanism of regulation of glutamatergic synapses as a part of the process of cocaine-induced synaptic plasticity.
Over the past two decades, there has been an explosion of tools that allow researchers to manipulate and observe the function of specific neural populations and their projections. These tools, including optogenetics, chemogenetics, cell-type-specific ablation, and fluorescent calcium indicators, have been especially useful in studying the function of cell types in the basal ganglia, where the majority of neurons are essentially indistinguishable by morphology or physiology. Although recent work has mostly supported the classical model of basal ganglia structure and function, cell-type-specific methods have revealed nuances in the role of striatal circuits in reward and action selection. These tools have also provided evidence for an expanded role of the basal ganglia in reinforcement and emotional states. Moving forward, new technologies will be needed to study the basal ganglia on a circuit level to better understand how cell types interact to shape behavior.
In the present study, we generated a novel parvalbumin (PV)-Cre rat model and conducted detailed morphological and electrophysiological investigations of axons from PV neurons in globus pallidus (GP). The GP is considered as a relay nucleus in the indirect pathway of the basal ganglia (BG). Previous studies have used molecular profiling and projection patterns to demonstrate cellular heterogeneity in the GP; for example, PV-expressing neurons are known to comprise approximately 50% of GP neurons and represent majority of prototypic neurons that project to the subthalamic nucleus and/or output nuclei of BG, entopeduncular nucleus and substantia nigra (SN). The present study aimed to identify the characteristic projection patterns of PV neurons in the GP (PV-GP neurons) and determine whether these neurons target dopaminergic or GABAergic neurons in SN pars compacta (SNc) or reticulata (SNr), respectively. We initially found that (1) 57% of PV neurons co-expressed Lim-homeobox 6, (2) the PV-GP terminals were preferentially distributed in the ventral part of dorsal tier of SNc, (3) PV-GP neurons formed basket-like appositions with the somata of tyrosine hydroxylase, PV, calretinin and cholecystokinin immunoreactive neurons in the SN, and (4) in vitro whole-cell recording during optogenetic photo-stimulation of PV-GP terminals in SNc demonstrated that PV-GP neurons strongly inhibited dopamine neurons via GABAA receptors. These results suggest that dopamine neurons receive direct focal inputs from PV-GP prototypic neurons. The identification of high-contrast inhibitory systems on dopamine neurons might represent a key step toward understanding the BG function.
We develop a methodology for testing computational hypotheses about neural functionality articulated in models at the systems level of description. In this approach, the first step is to attempt the construction of a model of the underlying brain system which is consistent with the known anatomy and physiology, but which is also able to exhibit functional properties consistent with a putative computational hypothesis. If this is successful, the second step consists of including additional known pathways into the model and testing the new models to see whether they show an improvement in functional performance (using appropriate performance metrics). A positive outcome is taken as evidence in support of the hypothesis. A final step is to construct ‘control’ models by including pathways that are not consistent with biological data. In this case a performance detriment is taken as support for the hypothesis. The methodology is applied to the basal ganglia, and builds on a previously published model of this system (Gurney et al 2001 Biol. Cybern. 84 401–23) which was based on the hypothesis that the basal ganglia perform action selection. The realistically constrained models show a selection benefit, while control models show a decrement in selection ability. These results, taken together, provide further validation of our selection hypothesis of basal ganglia function.
Unidirectional connections from the cortex to the matrix of the corpus striatum initiate the cortico-basal ganglia (BG)-thalamocortical loop, thought to be important in momentary action selection and in longer-term fine tuning of behavioural repertoire; a discrete set of striatal compartments, striosomes, has the complementary role of registering or anticipating reward that shapes corticostriatal plasticity. Re-entrant signals traversing the cortico-BG loop impact predominantly frontal cortices, conveyed through topographically ordered output channels; by contrast, striatal input signals originate from a far broader span of cortex, and are far more divergent in their termination. The term 'disclosed loop' is introduced to describe this organisation: a closed circuit that is open to outside influence at the initial stage of cortical input. The closed circuit component of corticostriatal afferents is newly dubbed 'operative', as it is proposed to establish the bid for action selection on the part of an incipient cortical action plan; the broader set of converging corticostriatal afferents is described as contextual. A corollary of this proposal is that every unit of the striatal volume, including the long, C-shaped tail of the caudate nucleus, should receive a mandatory component of operative input, and hence include at least one area of BG-recipient cortex amongst the sources of its corticostriatal afferents. Individual operative afferents contact twin classes of GABAergic striatal projection neuron (SPN), distinguished by their neurochemical character, and onward circuitry. This is the basis of the classic direct and indirect pathway model of the cortico-BG loop. Each pathway utilises a serial chain of inhibition, with two such links, or three, providing positive and negative feedback, respectively. Operative co-activation of direct and indirect SPNs is, therefore, pictured to simultaneously promote action, and to restrain it. The balance of this rival activity is determined by the contextual inputs, which summarise the external and internal sensory environment, and the state of ongoing behavioural priorities. Notably, the distributed sources of contextual convergence upon a striatal locus mirror the transcortical network harnessed by the origin of the operative input to that locus, thereby capturing a similar set of contingencies relevant to determining action. The disclosed loop formulation of corticostriatal and subsequent BG loop circuitry, as advanced here, refines the operating rationale of the classic model and allows the integration of more recent anatomical and physiological data, some of which can appear at variance with the classic model. Equally, it provides a lucid functional context for continuing cellular studies of SPN biophysics and mechanisms of synaptic plasticity.
Changes in neuronal firing in the subthalamic nucleus have emerged as one of the cardinal features of Parkinson’s disease. A central role for this region in Parkinson’s disease can be inferred from the success of subthalamic lesions in non-human primates (Bergman et al., 1990) and of subthalamic deep brain stimulation (Kumar et al., 1998) in alleviating the symptoms of Parkinson’s disease. Recordings in humans with Parkinson’s disease during stereotaxic surgery have revealed that subthalamic neurons have a markedly increased rate of firing with increased bursting (Hutchinson et al., 1998). The principal mechanism leading to this effect has been traditionally considered to be a decreased inhibitory input from the external pallidum (Albin et al., 1989). However, several experimental data suggest that this explanation is not sufficient for understanding how dopamine depletion leads to subthalamic dysfunction (Chesselet and Delfs, 1996; Hassani et al., 1996; Levy et al., 1997). It is likely that changes in the activity of other subthalamic inputs, coupled with reactive changes in subthalamic receptors and/or transduction pathways, need to be taken into account to understand this key feature of Parkinson’s disease. Increased activity of subthalamic neurons can be modeled in rats with a unilateral lesion of the nigrostriatal pathway (Kreiss et al., 1997; Hassani et al., 1996). Therefore, this model can be used to determine the cellular and molecular mechanisms induced in the subthalamic nucleus by the loss of dopaminergic neurons. Here we review recent studies conducted in our laboratory to examine the changes induced by chronic dopamine depletion in the subthalamic responses to local drug administration.
In an attempt to understand the potential contribution of various neurotransmitter systems to the elaboration of symptoms associated with central nervous systems disorders in humans, we have studied genetically modified animal models. A mouse in which the dopamine transporter gene (DAT-KO) has been inactivated provides a model of hyperdopaminergia. This mouse displays phenotypes of hyperactivity, impaired cognitive tasks, sensorimotor gating, habituation and attention. These behavioral impairments can be corrected by antipsychotic drugs and as such recapitulate some manifestations of psychotic behaviors. However, the potent calming effect of psychostimulants on the hyperlocomotion phenotype of these mice suggests some relevance to symptoms associated with attention deficit hyperactivity disorder (ADHD). In the DAT-KO mice, our results reveal an interplay between the dopamine, glutamate and serotonin systems in controlling these behaviors. A mouse that carries a hypomorphic allele of the NR1 subunit of the NMDA receptor provides a model for a hypofunctioning glutamate system. NR1 mutant mice display behavioral abnormalities that are consistent with other pharmacological models of psychotic disorders (PCP and MK-801 intoxication), and these behaviors can be ameliorated more effectively by atypical rather than by typical antipsychotics. Thus, animals in which specific genes have been genetically modified can provide valuable models that can mimic certain disease symptoms.
Since the pioneering work of Whittier and Mettler (1949) showing that small electrolytic lesion of the subthalamic nucleus (STN) induces violent involuntary movements of the contralateral limbs in non-human primates, the exact mechanisms by which the STN plays such a powerful role in the control of motor behaviors have been the subject of intensive research. In the 1980’s, the introduction of sensitive immunocytochemical and tract-tracing techniques, combined with single-unit recording in normal and parkinsonian monkeys, led to major breakthroughs in our knowledge of the critical role of this nucleus in the functional circuitry of the basal ganglia (Albin et al., 1989; Bergman et al., 1990; Wichmann and DeLong, 1996). The STN, which is the only excitatory glutamatergic structure of the basal ganglia (Smith and Parent, 1988), is now considered to be a major source of excitatory drive to basal ganglia output nuclei (Kitai and Kita, 1987). Observations in l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP)-treated monkeys showing that the STN activity is increased after dopamine depletion led to the development of novel surgical therapies aimed at silencing STN outflow to basal ganglia output nuclei (Bergman et al., 1990; Aziz et al., 1991). Such therapies are currently used worldwide as efficient treatment for Parkinson’s disease in humans (Starr et al., 1998). The recent demonstration that the interconnections between the globus pallidus (GP) and the STN act as a pacemaker of neuronal oscillations observed in various basal ganglia structures after dopamine depletion is further evidence that the STN plays a critical role in mediating basal ganglia functions in both normal and pathological conditions (Plenz and Kitai, 1999).
Targeted gene mutation provides a powerful tool for dissecting the biological substrates of neuropsychiatric diseases. Transgenic mice contain a new gene, such as the human gene for a disease, or an extra copy of a normal mouse gene. Knockout mice contain a DNA construct that effectively deletes a gene from the mouse genome. The targeted gene mutation approach is particularly useful for testing discrete hypotheses about genes linked to major psychiatric syndromes. Neurochemical, anatomical, neurophysiological, and behavioral sequelae of the mutation of a homologous gene in mice are compared to the symptoms characterizing the human disease state (Burright et al., 1997; Campbell and Gold, 1996; Crawley, 1999, 2000; Crawley and Paylor, 1997; Crawley et al., 1997; Jucker and Ingram, 1997; Kieffer, 1999; Nelson and Young, 1997; Picciotto, 1999).
Excellent pharmacological tools are available for manipulation of various neuropharmacological components of the dopamine system. However, a major drawback of the pharmacological approach is that almost all drug antagonists or agonists have multiple sites of action, certainly at higher doses. A molecular biological approach provides a means of selectively manipulating the genes that encode the proteins responsible for a given neuropharmacological site with little concern of crosstalk or pharmacological interaction. The cloning of the genes responsible for encoding dopamine receptors and transporter proteins, as well as the proteins responsible for the synthesis of dopamine, has provided the molecular information necessary to decrease or eliminate these proteins and assess function. Such a knockout approach has a number of advantages over traditional pharmacological approaches but also a number of disadvantages.
Tourette's syndrome (TS) is characterized by the presence of chronic motor and vocal tics and is commonly associated with a variety of behavioral and emotional problems. In his manuscript published in 1885, Gilles de la Tourette (1) noted no anatomical or pathological cause in the syndrome that bears his name and referred scientists interested in pursuing pathophysiological mechanisms to the field of psychology. Although much information has been acquired pertaining to the underlying anatomy and physiology of tic disorders, many perplexing questions remain. The fact that tics resolve or diminish in many individuals suggests the possibility of a developmental alteration rather than a fixed or progressive disorder. There is convincing evidence that cortico-striatal-thalamo-cortical (aka frontal-subcortical) pathways are involved in the expression of TS and its accompanying neuropsychiatric problems, but the precise location(s) remains speculative. A twopathway model of circuits (direct and indirect) through the basal ganglia is often cited in discussions of hyperkinetic and hypokinetic movement disorders, but this concept represents an oversimplification of complex interactions and newer models have been proposed. Intrinsic neurotransmitters utilized within cortico-striatal-thalamo-cortical pathways are well established, but each has its own complex system of message transduction as well as MD: KURLAN, JOB: 03329, PAGE: interaction with other transmitter agents. Although TS is generally accepted as a genetic disorder, environmental factors, such as streptococci infection, have been proposed as contributing factors.
Sylvain Doré, Kenji Sampei, Shozo Goto, Nabil J Alkayed, Daniel Guastella, Seth Blackshaw, Michela Gallagher, Richard J Traystman, Patricia D Hurn, Raymond C Koehler, and Solomon H Snyder. (1999) Heme Oxygenase-2 Is Neuroprotective in Cerebral Ischemia. Mol. Med. 1999 Oct;5(10):656–663.
In contrast to γ-aminobutyric acid (GABA)ergic neurons which are ubiquitous in the brain, dopaminergic systems are restricted to a few well-characterized pathways. Dopaminergic cell bodies are for the most part concentrated in the mesencephalon and give rise to three main pathways: the nigrostriatal (or mesostriatal) system, innervating the caudate putamen (or striatum) and other regions of the basal ganglia, the mesolimbic system, innervating the nucleus accumbens and other parts of the limbic system, and the mesocortical pathway, innervating the prefrontal cortex (see Chesselet 1999). All these dopaminergic systems interact with GABAergic neurons both at the level of their cell bodies and in their terminal regions. However, the mesostriatal system has been more extensively studied because the loss of these dopaminergic neurons leads to Parkinson’s disease, and GABAergic neurons normally controlled by nigrostriatal dopamine are thought to play a critical role in the symptoms of the disease (see Chesselet and Delfs 1996).
The striatum is the major division of the basal ganglia, a group of subcortical nuclei involved in a variety of processes including motor, associative, cognitive and mnemonic functions. The dorsal division of the basal ganglia consists of the striatum, the globus pallidus (GP, external segment of the globus pallidus, or GPe, in primates), entopeduncular nucleus (EP, internal segment of globus pallidus, or GPi, in primates), the subthalamic nucleus (STN), and the substantia nigra (SN). The SN is divided into two main parts, the dorsal pars compacta (SNC), in which the dopaminergic nigrostriatal neurons reside, and the more ventral pars reticulata (SNR). The ventral division of the basal ganglia, which is primarily associated with limbic functions, consists of the ventral striatum or nucleus accumbens, ventral pallidum, and ventral tegemental area.
Initial classification of dopamine (DA) receptors into D1 and D2 subtypes on the basis of stimulatory and no/inhibitory linkage to adenylyl cyclase (AC), respectively (Spano et al 1978; Kebabian and Calne 1979), endured in substance for approximately a decade until the molecular cloning of D1 and D2 receptors provided additional criteria for distinguishing these two receptors in terms of genomic structure/localization, primary structure and mRNA tissue distribution profile. Thereafter, primarily during the early 1990s, further molecular cloning studies revealed the mammalian DA receptor family to be yet more heterogeneous (see Missale et al. 1998; Neve and Neve 1997; Niznik 1994): in particular, cloning both of primate D1 (rodent homologue D1A ) and of primate D5 (rodent homologue D1B ) receptors indicated the original designation of D1 to encompass a family of D1-like receptors whose properties are t he focus of this chapter, in juxtaposition with a family of D2-like receptors (D2L/S, D3, D4) whose properties are the focus of subsequent chapters.
This chapter discusses the role of dopamine in addiction. Research over the past 50 years has revealed that the mesocorticolimbic dopamine system has an essential role in the acute reinforcing effects of psychostimulant drugs and a contributory role in the acute reinforcing effects of nonstimulant drugs of abuse. Mesocorticolimbic dopamine systems contribute to motivational withdrawal and relapse with all drugs of abuse, and dopamine, by interacting with key elements of brain hormonal stress systems, also has a prominent role in individual differences for the vulnerability to initiate aspects of stimulant addiction that may extend to other drugs of abuse.
Previous studies (G. E. Ploeger, B. M. Spruijt, & A. R. Cools, 1992) showed that low doses of systemically injected haloperidol affected spatial learning in the Morris water maze. This study investigated effects of intra-accumbens injections of haloperidol on spatial learning. To control for motivation and sensorimotor coordination, the researchers trained the rats to escape onto a visible platform. Low doses (50–100 ng) of haloperidol impaired spatial learning, whereas escaping on a visible platform was undisturbed. The 500-ng dose of haloperidol completely blocked acquisition because of combined learning and motor impairments. Retrieval of an acquired escape response was unaffected by 500 ng haloperidol. The data show that mesolimbic dopaminergic activity is involved in the acquisition of spatial localization. The results are related to studies demonstrating the involvement of the nucleus accumbens in cue-directed behaviors.
The conditioned place preference technique was used to assess the affective properties of the direct dopamine D1 agonist, SKF38393, and the direct D2 agonist, LY171555 (quinpirole). A three compartment apparatus was used: the animals' pre-experimental preference for the two choice compartments was equal and, within each experimental group, half the rats received drug pairings in each choice compartment. Intraperitoneal injections of SKF38393 produced conditioned place aversions at all doses tested (1.0-4.0 mg/kg); LY171555 produced weak conditioned place preferences at 1.0 and 2.0 mg/kg, but no reliable effect at 4.0 mg/kg. Bilateral intra-accumbens microinjections of SKF38393 produced strong preferences at all doses tested (0.5-2.0 micrograms/side); LY171555 produced strong preferences at two doses (0.5 and 1.0 micrograms/side) and no effect at a third dose (2.0 micrograms/side). These results suggest that activation of either D1 or D2 receptors in the nucleus accumbens can produce reward, and that D1 receptors (and possibly also D2 receptors) located elsewhere in the brain or in the periphery may mediate aversive effects.
The effect of posttraining intracerebral injections of the indirect dopamine (DA) agonist d-amphetamine, the direct D2 agonist LY 171555, and the direct D1 agonist SKF-38393 on the acquisition of two 8-arm radial maze tasks were examined. On a win-stay task, a light cue signaled the location of food in 4 randomly selected maze arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Posttraining intracaudate injection of d-amphetamine (10.0 and 15.0 micrograms), LY 171555 (2.0 micrograms), and SKF-38393 (5.0 micrograms) all improved win-stay acquisition in relation to saline-injected controls. In contrast, posttraining intrahippocampal injection of DA agonists had no effect on win-stay acquisition. On a win-shift task, rats were allowed to obtain food from 4 randomly selected maze arms, followed by a delay period in which they were removed from the maze. They were returned to the maze for a retention test in which only those arms that had not been visited before the delay contained food. Posttraining intrahippocampal (but not intracaudate) injection of d-amphetamine (5.0 micrograms), LY 171555 (2.0 micrograms), and SKF-38393 (5.0 micrograms) all improved win-shift retention in relation to saline-injected controls. The results demonstrate a double dissociation of hippocampus and caudate nucleus memory functions and show that posttraining injection of both D1 and D2 agonists modulate the memory processes subserved by both hippocampus and caudate nucleus.
Of the five known dopamine receptors, D1A and D2 represent the major subtypes expressed in the striatum of the adult brain. Within the striatum, these two subtypes are differentially distributed in the two main neuronal populations that provide direct and indirect pathways between the striatum and the output nuclei of the basal ganglia. Movement disorders, including Parkinson disease and various dystonias, are thought to result from imbalanced activity in these pathways. Dopamine regulates movement through its differential effects on D1A receptors expressed by direct output neurons and D2 receptors expressed by indirect output neurons. To further examine the interaction of D1A and D2 neuronal pathways in the striatum, we used homologous recombination to generate mutant mice lacking functional D1A receptors (D1A-/-). D1A-/- mutants are growth retarded and die shortly after weaning age unless their diet is supplemented with hydrated food. With such treatment the mice gain weight and survive to adulthood. Neurologically, D1A-/- mice exhibit normal coordination and locomotion, although they display a significant decrease in rearing behavior. Examination of the striatum revealed changes associated with the altered phenotype of these mutants. D1A receptor binding was absent in striatal sections from D1A-/- mice. Striatal neurons normally expressing functional D1A receptors are formed and persist in adult homozygous mutants. Moreover, substance P mRNA, which is colocalized specifically in striatal neurons with D1A receptors, is expressed at a reduced level. In contrast, levels of enkephalin mRNA, which is expressed in striatal neurons with D2 receptors, are unaffected. These findings show that D1A-/- mice exhibit selective functional alterations in the striatal neurons giving rise to the direct striatal output pathway.
The critical analysis of the physiological, functional, and anatomical responses of the eye to the introduction of exogenous compounds is an integral part of many toxicity studies. Clinical studies conducted with biomicroscopy, indirect ophthalmoscopy, and histopathology allow for the detection of anatomical changes. Electrophysiology and psychophysics provide functional measures of the visual system that complement anatomical analysis.
The application of modern molecular biological methods has had an increasing and dramatic impact upon the discipline of molecular neuropharmacology. This is particularly true for the study of neurotransmitter receptors, where the use of recombinant DNA techniques has resulted in the cloning of multiple and sometimes unexpected receptor subtypes for a given neurotransmitter and, in some cases, the cloning of receptors for which no neurotransmitter is known. Within the past couple of years, it has become readily apparent that dopamine receptors will be no exception to this trend. Five different dopamine receptors have now been cloned and identified using molecular biological techniques, while only a few years ago only two receptor subtypes were thought to exist. David Sibley and Frederick Monsma review the molecular characteristics of the recently cloned dopamine receptors and discuss prospects for the cloning and identification of additional subtypes in this receptor family.
This review highlights the use of transgenic mice and gene targeting in the study of reproduction, pituitary gene expression, and cell lineage. Since 1980 numerous applications of transgenic animal technology have been reported. Altered phenotypes resulting from transgene expression demonstrated that introduced genes can exert profound effects on animal physiology. Transgenic mice have been important for the study of hormonal and developmental control of gene expression because gene expression in whole animals often requires more DNA sequence information than is necessary for expression in cell cultures. This point is illustrated by studies of pituitary glycoprotein hormone alpha- and beta-subunit gene expression (Kendall et al., Mol Endocrinol 1994; in press [1]. Transgenic mice have also been invaluable for producing animal models of cancer and other diseases and testing the efficacy of gene therapy. In addition, cell-cell interactions and cell lineage relationships have been explored by cell-specific expression of toxin genes in transgenic mice. Recent studies suggest that attenuated and inducible toxins hold promise for future transgene ablation experiments. Since 1987, embryonic stem (ES) cell technology has been used to create numerous mouse strains with targeted gene alterations, contributing enormously to our understanding of the functional importance of individual genes. For example, the unexpected development of gonadal tumors in mice with a targeted disruption of the inhibin gene revealed a potential role for inhibin as a tumor suppressor (Matzuk et al., Nature 1992:360: 313-319 [2]. The transgenic and ES cell technologies will undoubtedly continue to expand our understanding and challenge our paradigms in reproductive biology.
: Clinical disorders that affect the basal ganglia, such as Parkinson's disease, are believed to result from an imbalance in the activity of the two main output pathways of the striatum, the direct and indirect striatal output pathways. Dopamine (DA) modulates the activity of these pathways through the select distribution of the D1 and D2 DA-receptor subtypes on neurons, which give rise to the direct and indirect striatal output pathways, respectively. Studies employing quantitative in situ hybridization histochemistry have been used to analyze changes in levels of gene-regulated peptides and immediate early genes in direct and indirect striatal output neurons in response to selective D1- and D2-agonist treatments. D2-receptor activation reverses and D1-receptor activation increases gene regulation in indirect and direct striatal output neurons, respectively. These findings suggest that functional activity in the two output pathways of the striatum are oppositely regulated by DA by segregation of the D1- and D2-receptor subtypes on different striatal neuron populations. A proposed pharmacologic strategy for the treatment of Parkinson's disease based on these findings is suggested.
Dopamine (5 to 50 Μg) applied bilaterally to the nucleus accumbens of reserpine-nialamide pretreated rats produced a marked dose-dependent rise in coordinated locomotor activity, devoid of stereotypies such as gnawing, rearing and licking seen after dopamine application (50 Μg) to the neostriatum. The locomotor activity was completely blocked by pimozide, but not by phenoxybenzamine. The effects of apomorphine or d-noradrenaline were similar to those of dopamine. In contrast, l-noradrenaline produced a “convulsive” syndrome devoid of coordinated locomotor activity, and this convulsive syndrome could be completely blocked by phenoxybenzamine but not by pimozide. Release of endogenous dopamine by d- or l-amphetamine (10 and 50 Μg) in the nucleus accumbens produced a rise in coordinated activity, the d-isomer was about 4 times as potent as the l-isomer, and the effect of the d-isomer was blocked completely by α-methyltyrosine. Bilateral application of trifluoperazine (2.5 Μg) to the nucleus accumbens completely blocked the effect of systemically administered d-amphetamine (1.5 and 3.0 mg/kg), but similar application to the area of the central nucleus of the amygdala or the neostriatum was much less effective. Partial protection of the endogenous dopamine stores against the depleting action of reserpine by local application of metatyramine to the nucleus accumbens resulted in a higher level of basal activity than in control animals. Application of dopamine or noradrenaline to the area of the central nucleus of the amygdala or to the olfactory tubercles did not lead to any consistent changes in locomotor activity.
The nucleus accumbens and olfactory tubercles contained most of the dopamine in the limbic forebrain, with noradrenaline more evenly distributed.
These data suggest that the nucleus accumbens plays an important role in the locomotor activity in rats.
The antisense DNA method has been used successfully not onlyin vitro but also within vivo systems to block effectively the expression of specific genes. An increasing number of studies have shown that antisense
DNA administered directly into the brain can modify various kinds of behaviors. These findings strongly suggest that the antisense
DNA method can be widely used as a powerful tool for the study of the molecular bases of behavior. In addition to traditional
methods of behavioral genetics, the antisense DNA method may provide a new approach for the study of the effects of gene in
behavioral function. In this article, we review recent studies reportingin vivo effects of antisense DNA on brain function and behavior.
Using the Morris swimming pool test of spatial navigation, medial caudate-putamen lesions in rats produce impairments in the acquisition and retention of both place and cue tasks, and impair the selection of normal navigation strategies. Also described are some novel features of spatial navigation behaviour displayed by control animals in cue and place tasks that provide insights into the performance of the caudate-putamen rats. Analyses of the swim patterns on postacquisition probe trials, in which the target platform was removed or relocated, showed that the strategy used by the caudate-putamen lesioned rats was dependent upon the task that they were required to solve. Control rats used a place strategy and distal visual cues to identify the location of the start points, the routes from the start points to the platform, and the location of the platform on both the cue and place tasks. The caudate-putamen lesioned rats used distal visual cues and a place strategy only to acquire the place task. They solved the cue task using a taxon strategy consisting of a combination of proximal and position response cues. The results suggest that when necessary, medial caudate-putamen lesioned rats, like normal rats, can use place strategies for spatial navigation, but if an alternate, perhaps simpler, taxon solution is available they seemingly ignore place information and navigate using the simpler strategy. The deficit, which has features of a neglect rather than a loss of ability per se, suggests that medial caudate-putamen neural systems are involved in the selection of alternative strategies in spatial navigation tasks.
Intra-accumbens d-amphetamine caused a dose-dependent hyperactivity which was shown to involve the release of newly synthesized dopamine. The amphetamine response was antagonised by apomorphine in a narrow dose range. This antagonistic effect of apomorphine was specifically inhibited by the neuroleptic agents pimozide and haloperidol in doses too low to inhibit the amphetamine response per se. Also, apomorphine exerted its antagonistic effect only when administered directly into the nucleus accumbens, not when injected into the caudateputamen or tuberculum olfactorium. Further, doses of intra-accumbens apomorphine which antagonised the amphetamine response reduced the homovanillic acid content of the nucleus accumbens but not that of the caudateputamen or tuberculum olfactorium. The apomorphine induced change in accumbens homovanillic acid was antagonised by haloperidol at doses effective in the behavioural experiments. Finally, the ability of apomorphine to antagonise amphetamine hyperactivity was abolished following the intra-accumbens injection of 6-OHDA (the amphetamine response being maintained after lesion) which specifically destroyed dopamine nerve terminals in the nucleus accumbens. This data is forwarded to support a general conclusion that intra-accumbens apomorphine antagonises the hyperactivity induced by intra-accumbens amphetamine by an action at neuroleptic sensitive sites within the nucleus accumbens and which may be located on nerve terminals, i.e. presynaptic receptors. The ability of neuroleptic agents to antagonise at postsynaptic dopamine receptors (antagonise the amphetamine response per se) was compared with an ability to reverse the apomorphine antagonism of the amphetamine response at a hypothesised ‘presynaptic receptor’. A differential dose could not be determined for clozapine of thioridazine. For pimozide and haloperidol the ratio of pre- to postsynaptic activity was small, whilst (−)-sulpiride was 5 fold more effective to inhibit pre- than postsynaptic receptors. This data provides support for the hypothesis that there may exist more than one neuroleptic mechanism in the nucleus accumbens which contribute to an overall control of motor function via the mesolimbic system.
The family of genes encoding G-protein-coupled dopamine receptors continues to grow with the recent cloning of a fifth member. The availability of these clones has revolutionized the dopamine receptor field. Expression of individual dopamine receptors is permitting the detailed analysis of their pharmacology and coupling to second messenger systems, while probes based on the receptors' nucleotide sequences are being used to gain new insights into their tissue distribution and genetics.
The distribution of the messenger RNA encoding the dopamine D5 receptor was determined in the rat brain by in situ hybridization. Using [35S]-labelled riboprobes to either the rat or human D5 receptor, this mRNA was localized to the hippocampus and the parafascicular nucleus of the thalamus. This mRNA could not be visualized in the more traditional brain regions associated with dopaminergic cell bodies or projection fields. This unusual distribution suggests a novel function in the brain for this subtype of the dopamine receptor.
Central dopaminergic transmission has been implicated in memory processes. The present experiments examined the effects of several direct acting dopaminergic agents on performance of a delayed-non-match-to-sample radial arm maze task. Preadministration of apomorphine (D1-D2 agonist; 0.25, 0.5, and 1.0 mg/kg), quinpirole (D2 agonist; 0.1 mg/kg), or SKF38393 (D1 agonist; 3 mg/kg) increased the latency of choices but did not affect any index of accuracy with a 1 h retention interval. Post-training administration of quinpirole (0.1, 0.2, 1.0, and 2.0 mg/kg), SKF38393 (0.3, 3.0, and 6.0 mg/kg), sulpiride (D2 antagonist; 3, 10, and 30 mg/kg), or SCH23390 (D1 antagonist; 0.01, 0.1, and 1.0 mg/kg) also did not affect accuracy, although quinpirole produced a dose-dependent increase in the latency of choices, assessed 10 h post-treatment. For comparison, pretraining and post-training administration of the benzodiazepine chlordiazepoxide (1, 3, 5 mg/kg) was also tested and produced dose-dependent impairments in mnemonic performance at either a 1 or 4 h retention interval. The effects of chlordiazepoxide are consistent with evidence indicating that GABAergic agents can influence memory processes. In contrast, the present findings indicate that (peripheral administration of dopaminergic agents IS) not sufficient to alter the mnemonic processes required for accurate performance of this DNMTS-RAM task.
We investigated the effect of the selective D1 dopamine antagonist, SCH23390, on the establishment of a pipradrol-conditioned place preference (CPP). Among various doses of pipradrol (6.25-75.0 mg/kg, SC), a CPP was established at 25.0 mg/kg. SCH23390 (0.16 mg/kg, IP) blocked the establishment of a CPP by this dose of pipradrol. The results suggest that pipradrol produces a rewarding effect and that this effect may involve activation of D1 dopamine receptors.
Regardless of the field of application, the raison d'etre of transgenic animals is to study gene regulation and function. With increasing frequency, mammalian genes are being isolated with no concomitant knowledge of their function. The human genome mapping initiative will undoubtedly produce a cornucopia of such genes. While the merit of taking a transgenic route to study genes of unknown function is axiomatic, the choices of strategies for gene regulation in vivo may not be fully appreciated. This review will address two main points: first, the targeted and regulated expression of genes, and second, the structural and functional ablation of genes.
On the basis of experiments made on striatal membranes, Leff and Creese [Molec. Pharmac. (1985)27, 184–192] have proposed that tritiated dopamine binds to a high-affinity agonist state of D1 dopamine receptors (D1h) which adopt this conformation when they are associated with the GTP-binding protein involved in the transduction process. Quantitative autoradiography was thus used to look for the distribution of these D1h sites in the rat brain and to compare it with that of D1 receptors labelled with [3H]7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine ([3H]SCH23390), a D1 antagonist. The effects of unilateral 6-hydroxydopamine lesion of the ascending dopamine pathways on the density of [3H]dopamine D1h and [3H]SCH23390 binding sites in the striatum and the nucleus accumbens were also analysed.
Over the past 10 years significant progress has been made in techniques for manipulating the genome of the animal. Production of transgenic mice has led to important insights into the regulation of gene expression, the molecular basis of cancer, immunology, and developmental biology. The tools necessary to generate transgenic mice are becoming widely available, making it possible to study a variety of problems. In this review a description of the strategies being used to address problems of interest in cell physiology using transgenic mice is given. Elucidation of the rules governing the regulation of gene expression now permits the targeted expression of a protein to a particular organ or cell type within an organ. Overexpression of proteins, expression of foreign or mutant proteins, mislocalization of proteins, and directed elimination of proteins are all procedures that can now be used to generate interesting animal models for physiological studies. The applications of these techniques to a variety of problems in normal and abnormal physiology are discussed in this review.
The application of modern molecular biological methods has had an increasing and dramatic impact upon the discipline of molecular neuropharmacology. This is particularly true for the study of neurotransmitter receptors, where the use of recombinant DNA techniques has resulted in the cloning of multiple and sometimes unexpected receptor subtypes for a given neurotransmitter and, in some cases, the cloning of receptors for which no neurotransmitter is known. Within the past couple of years, it has become readily apparent that dopamine receptors will be no exception to this trend. Five different dopamine receptors have now been cloned and identified using molecular biological techniques, while only a few years ago only two receptor subtypes were thought to exist. David Sibley and Frederick Monsma review the molecular characteristics of the recently cloned dopamine receptors and discuss prospects for the cloning and identification of additional subtypes in this receptor family.
The effects of the dopamine D2 receptor agonist quinpirole (LY 171555) on locomotor activity and margin time (thigmotaxis or wall-hugging) were measured for 2 h in rats injected either s.c. (vehicle, 0.02, 2.0 mg/kg) or directly into either the dorsal striatum or nucleus accumbens (vehicle, 0.1, 1.0, 10, 20 or 40 micrograms bilaterally in each site). In all groups, margin time decreased as drug dose increased. As in previous research, quinpirole given s.c. decreased locomotor activity at a low dose and had a biphasic effect on locomotor activity at the high dose. Both of these effects were also elicited by quinpirole injected directly into the dorsal striatum; 10 and 20 micrograms decreased locomotion immediately, while 40 micrograms led to both the immediate decrease and a later increase. In contrast, the lowest doses of quinpirole (0.1 and 1.0 microgram) injected into the nucleus accumbens led to an increase in locomotion from 20 to 60 min, while the higher doses led only to the early decrease. Thus, both the locomotor activating and inhibiting effects of quinpirole are found in both the nucleus accumbens and the dorsal striatum, but the differing dose-response relationships indicate that the mechanisms are not the same in these two brain regions.
D1, a subtype of the dopamine receptors, is widely distributed in the nervous system and has been shown to be positively coupled to adenylate cyclase. Using a combination of in vitro receptor autoradiographic and in situ hybridization techniques, the present study examines the co-distribution of D1 receptor binding sites and D1 receptor messenger RNA in adjacent rat brain sections. D1 receptor binding sites were labeled using the selective antagonist [3H]SCH23390 (4.6 nM) in the presence of 1 microM ketanserin, while the D1 receptor messenger RNA was visualized with a 35S-labeled riboprobe corresponding to a region between transmembrane domains III and VI of the rat D1 receptor (bp 383-843). Analysis of serial sections suggested a good agreement between D1 receptor binding and messenger RNA in several brain regions, including the paleocortex, caudate-putamen, nucleus accumbens, amygdala and suprachiasmatic nucleus. Marked discrepancies between D1 receptor binding and messenger RNA were observed in other brain regions including the entopeduncular and subthalamic nuclei, substantia nigra (pars reticulata), hippocampus and cerebellum. While technical considerations may contribute to these results, much of the discordance between the distributions is likely due to the differential localization D1 receptor messenger RNA in cell bodies and receptor binding sites on fibers and may provide insights into receptor synthesis, transport and membrane insertion. In the basal ganglia, for instance, D1 receptors are synthesized in the striatum and are either transported to efferent projections in areas such as the substantia nigra, or remain localized in striatal cells bodies. Ibotenic acid lesions in the striatum are consistent with these conclusions and demonstrate a coordinate loss of D1 receptor binding and messenger RNA in the caudate-putamen that is accompanied by a degeneration of fibers projecting to substantia nigra and a loss of D1 binding in the pars reticulata. Neurons in the dentate gyrus and in the granular layer of the cerebellum, on the other hand, synthesize D1 receptors and transport them entirely to either their dendritic or axonal fields, respectively, in the molecular layer. This analysis provides a better understanding of dopaminergic receptor systems in the CNS and their anatomical organization.
1. In the retinal inner nuclear layer of the majority of species, a dopaminergic neuronal network has been visualized in either amacrine cells or the so-called interplexiform cells. 2. Binding studies of retinal dopamine receptors have revealed the existence of both D1- as well D2-subtypes. The D1-subtype was characterized by labeled SCH 23390 (Kd ranging from 0.175 to 1.6 nM and Bmax from 16 to 482 fmol/mg protein) and the D2-subtype by labelled spiroperidol (Kd ranging from 0.087 to 1.35 nM and Bmax from 12 to 1500 fmol/mg protein) and more selectively by iodosulpiride (Kd 0.6 nM and Bmax 82 fmol/mg protein) or methylspiperone (Kd 0.14 nM and Bmax 223 fmol/mg protein). 3. Retinal dopamine receptors have been also shown to be positively coupled with adenylate cyclase activity in most species, arguing for the existence of D1-subtype, whereas in some others (lower vertebrates and rats), a negative coupling (D2-subtype) has been also detected in peculiar pharmacological conditions implying various combinations of dopamine or a D2-agonist with a D1-antagonist or a D2-antagonist in the absence or presence of forskolin. 4. A subpopulation of autoreceptors of D2-subtype (probably not coupled to adenylate cyclase) also seems to be involved in the modulation of retinal dopamine synthesis and/or release. 5. Light/darkness conditions can affect the sensitivity of retinal dopamine D1 and/or D2-receptors, as studied in binding or pharmacological experiments (cAMP levels, dopamine synthesis, metabolism and release). 6. Visual function(s) of retinal dopamine receptors were connected with the regulation of electrical activity and communication (through gap junctions) between horizontal cells mediated by D1 and D2 receptor stimulation. Movements of photoreceptor cells and migration of melanin granules in retinal pigment epithelial cells as well as synthesis of melatonin in photoreceptors were on the other hand mediated by the stimulation of D2-receptors. 7. Other physiological functions of dopamine D1-receptors respectively in rabbit and in embryonic avian retina would imply the modulation of acetylcholine release and the inhibition of neuronal growth cones.
The D1/D2 dopamine receptor classification is widely accepted. However, intense investigative efforts over the last several years using pharmacological, biochemical and behavioral approaches have produced results that are increasingly difficult to reconcile with the existence of only two dopamine receptor subtypes. Recent developments, including cloning of the cDNAs and/or genes for several members of the large family of G-protein-coupled receptors, have revealed that heterogeneity in the pharmacological or biochemical characteristics of individual receptors often indicates the presence of previously unsuspected molecular subtypes. In this article, Marc Caron and colleagues have assembled the main lines of evidence that suggest the presence of several novel subtypes for both D1 and D2 dopamine receptors and predict that molecular cloning will, in the near future, confirm their existence.
The present study examined the role of D1 and D2 receptors in mediating locomotor activity induced by dopamine (DA) agonists after injection into the nucleus accumbens (Acb). The D1 receptor agonist SKF38393 (as the racemic mixture) induced a dose-related increase in activity when injected bilaterally (1-10 micrograms/side). At a dose of 1 microgram/side, only the R-enantiomer was active. The SKF38393 (10 micrograms/side)-induced activity was antagonized by the D1 receptor antagonist SCH23390 (0.5 mg/kg i.p.), by the D2 receptor antagonist spiperone (0.1 mg/kg, i.p.), but not by the 5-HT2 antagonist ketanserin (1 mg/kg, i.p.). Another D1 agonist, CY208 243, also induced a moderate increase in activity when injected into the Acb (2 and 8 micrograms/side), but this was of much less intensity and of shorter duration than that produced by SKF38393. The D2 receptor agonist quinpirole slightly increased activity when administered into the Acb (0.3-3 micrograms/side), with the magnitude and duration of the response, however, being much less than that produced by SKF38393. The locomotor stimulant effects of SKF38393 (5 micrograms/side), CY208 243 (2 micrograms/side) and quinpirole (1 microgram/side) were blocked by the depletion of catecholamines with reserpine (5 mg/kg s.c., 24 h pretreatment) and alpha-methyl-p-tyrosine (200 mg/kg, i.p.). However, when SKF38393 and quinpirole were injected concurrently into the Acb at doses of 5 and 1 microgram/side respectively, a marked locomotor stimulation occurred in catecholamine-depleted rats. Furthermore, SKF38393 (1 microgram/side) or CY208 243 (2 micrograms/side), injected concurrently with quinpirole (0.3 microgram/side), into the Acb of rats with intact DA stores produced an at least additive effect on locomotor activity. These results suggest that both D1 and D2 receptor stimulation in the Acb is required for the expression of locomotor effects. Furthermore, D1 and D2 receptors in this nucleus appear to interact positively with each other, and may mediate the additive locomotor stimulatory effects induced by concurrent systemic administration of selective D1 and D2 agonists.
Repetitive jaw movements (RJM in the rat can be produced in a dose-dependent manner with the selective D1 agonist, SKF 38393. Administration of the protein coupling agent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to rats pretreated with a D2 receptor blocker resulted in a 70-30% reduction of D1 dopamine receptors, but only a 10% reduction of D2 receptors in the rat caudate. Twenty-four hours following EEDQ, the RJM response to SKF 38393 was assessed. The massive selective reduction of the D1 receptor density was found not to modify the rate of RJM induced by SKF 38393 in that dose response curves in control and EEDQ-treated rats were essentially identical. These data provide evidence to indicate that there is a functional D1 receptor reserve for D1-mediated RJM behavior.
High-affinity binding sites for the D1 dopamine receptor antagonist [3H]SCH 23390 were identified in membranes from human putamen, frontal cortex and calf retina. In frontal cortex binding not only occurred to D1 but also to 5-HT2 receptors. In retina and putamen no binding to 5-HT2 receptors was detected. All further binding experiments in frontal cortex were carried out in the presence of 20 nM mianserin to prevent binding to 5-HT2 receptors. In the 3 tissues, antagonist competition curves were monophasic, whereas competition curves with the agonist dopamine revealed the presence of two binding sites, one having high affinity (RH) (32% in retina, 28% in putamen, and 15% in frontal cortex) and the other having low affinity (RL). In retina, the addition of 100 microM GTP caused a full conversion of RH into RL. In contrast, in frontal cortex, RH sites were not altered by 400 microM GTP or 100 microM 5'-guanylyl-imidodi-phosphate (Gpp(NH)p). In putamen, both guanine nucleotides provoked only a partial conversion of RH into RL. Dopamine (100 microM) produced a 220% and 56% increase in cAMP production in respectively retina and putamen homogenates, while no increase was observed in frontal cortex homogenate. These data may suggest that not all D1-receptors are coupled to the adenylate cyclase system.
Ex vivo D-1 or D-2 receptor binding in the striatum was reduced by 65-78% after treatment with EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) in combination with either the D-2 antagonist, raclopride, or the D-1 antagonist, SCH 23390, respectively. EEDQ induced a 65% reduction in D-1 receptor binding and a 51% decrease in cAMP production in striatal homogenates. Selective D-2 receptor inactivation inhibited the stereotyped behaviour induced by the mixed D-1/D-2 agonist, apomorphine, or by the D-2 agonist, quinpirole, when given alone and in combination with the D-1 agonist, SK&F 38393. Selective inactivation of D-1 receptors did not inhibit the behavioural effects of quinpirole when given alone and in combination with the D-1 agonists, SK&F 81297, SK&F 38393 or SK&F 75670. Likewise, the effect of apomorphine was unchanged. These results indicate that a normal density of D-2 receptors is critical for the expression of the stereotyped behaviour induced by DA agonists. In contrast, there is a large surplus of D-1 receptors to enable the response to a D-2 agonist. This is particularly illustrated by the persistent behavioural effects of the partial D-1 agonist, SK&F 75670, in rats with up to a 78% decrease in D-1 receptor binding.
Populations of DA neurons innervating the component nuclei of the amygdaloid complex differ in their inferred density of innervation, estimated rate of impulse activity, and adaptive response to the prolonged administration of antipsychotic drugs. Mesoamygdaloid DA neurons have in common the absence of tonically inhibitory, nerve terminal autoreceptors regulating DA synthesis, the nonassociation with a DA-stimulated adenylate cyclase, and the regulation of DA synthesis by receptor-mediated neuronal feedback mechanisms and end-product inhibition. The output of the amygdaloid complex appears to be organized into distinct functions subserved by component nuclei. The present findings suggest a differing role for DA afferents in modulating the functional output of discrete nuclei. The significance of this focal influence will be speculative pending a more complete understanding of the physiology of DA neurotransmission in the amygdaloid complex. Populations of DA neurons innervating discrete amygdaloid nuclei exhibit a composite of mechanisms of regulation and signal transduction and pharmacology that differ from that of other mesotelencephalic DA systems. These comparisons highlight the fact that the nucleus accumbens and olfactory tubercle do not represent or reflect DA neurotransmission in the limbic system. The study of the physiology, pharmacology, and pathology of mesolimbic DA neurons can and should extend beyond the nucleus accumbens and olfactory tubercle to the amygdala and other brain structures central to the organization of the limbic system. It is our opinion that the term "mesolimbic" DA system has purely anatomical connotations and that a more specific terminology (e.g., meso-central amygdaloid nuclear) would express the functional organization of this system more accurately.
Both in vivo and in vitro treatments with the irreversible protein-modifying reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), were used to investigate rat striatal D1 dopamine receptor/effector interactions. Peripherally administered EEDQ markedly reduced D1 dopamine receptor binding and D1 dopamine receptor-stimulated adenylate cyclase in a dose-dependent manner. However, EEDQ administered in vivo did not result in functional modification of either the guanine nucleotide-regulatory protein (Ns) or the catalytic subunit of striatal adenylate cyclase as assessed via guanine nucleotide- or forskolin-stimulated cAMP production. Interestingly, the loss in D1 dopamine receptor binding did not correlate directly with observed reductions in dopamine-stimulated adenylate cyclase activity; 40% of D1 dopamine receptor binding was lost with no significant reduction in the Vmax of dopamine-stimulated adenylate cyclase activity. Conversely, the reduction by EEDQ of the adenylate cyclase activity stimulated by the partial agonist SKF38393 was reduced in parallel with EEDQ-induced reductions in the D1 dopamine receptor Bmax. However, when SKF38393-stimulated adenylate cyclase activity was potentiated by forskolin, approximately 30% of receptors could be lost with no significant reduction in cAMP production, resembling the pattern observed utilizing the full agonist dopamine. In vivo pretreatment with the specific D1 antagonist, SCH23390, prevented reductions in dopamine-stimulated adenylate cyclase activity and D1 dopamine receptor binding, suggesting that EEDQ acts at the ligand recognition site of the receptor. Unlike in vivo treatment, in vitro EEDQ treatment resulted in dose-dependent decreases in catalytic subunit activity as assessed by forskolin-stimulated cAMP production, indicating that, in vitro, the adenylate cyclase catalytic subunit is vulnerable to EEDQ-induced modification. These data indicate that EEDQ is an effective tool for elucidating the mechanisms and biochemistry of D1 dopamine receptor/effector coupling.
The reduction of motor activity elicited in rats by a subcutaneous injection of a small dose of apomorphine was reversed by pretreatment of the nucleus accumbens with haloperidol (10 pg), sulpiride (10 pg) or desenkephalin-gamma-endorphin (DE gamma E) (100 pg or 10 ng). These doses of the compounds did not change motor activity in placebo-treated rats. Pretreatment of the nucleus caudatus with the same neuroleptics or DE gamma E did not diminish the effect of subcutaneously administered low doses of apomorphine. A small dose of apomorphine decreased motor activity when it was injected directly into the nucleus accumbens. This effect was dose dependently antagonized by subcutaneous pretreatment with DE gamma E. It is suggested that the hypoactivity elicited by small doses of apomorphine is exclusively mediated by dopaminergic systems in the nucleus accumbens.
Two highly inbred strains of mice were found to differ in habituation of activity repeatedly assessed in a toggle-box exploration task. The recombinant inbred (RI) strains derived from their cross resembled either one or the other parent strain, suggesting that a single gene exerts a marked influence on this behavior. The influence of an acute ethanol injection on activity in an open field was found to differ among 19 inbred strains. In 6 strains significant decreases in activity from the previous day's scores were seen; in two strains activity increased; and in 11 strains, no significant change was seen. Genetic specificity must, therefore, be considered in the interpretation of pharmacologic substrates for activity in mice. Lines of mice selectively bred for high and low open-field activity are suggested to offer an ideal subject population for neuropharmacologic studies.
Stimulation of dopamine receptors in the nucleus accumbens results in enhanced locomotor activity. The behavioural pattern showed the same characteristics as seen after injection of ergometrine, i.e. continuous exploring activity, except that the time for the appearance of locomotor stimulation was shorter. This finding favours the assumption that ergometrine has dopamine stimulating properties in the rat brain. The experiment provided evidence that the mesolimbic dopamine system with cell bodies in the A 10 group and nerve terminals in the nucleus accumbens and tuberculum olfactorium is functionally different from the nitrostriatal dopamine system.
The distribution of the D3 and D2 dopamine receptor subtypes in forebrain regions of the basal ganglia and mesocorticolimbic system was determined. This was assessed through combined fluorescent visualization of subtype selective anti-peptide antibodies for these cloned receptors and detection of their ligand recognition sites using the D2 subfamily antagonist,N-(p-aminophenethyl) spiperone (NAPS fluoroprobe). The double-labeling technique enabled direct comparison of the cloned receptor proteins and NAPS fluoroprobe binding in vitro. The application of these two methods together produced results comparable to single-labeling paradigms. Functional D3 receptors, defined as the coincident fluorescence of the D3 receptor antisera and fluoroprobe binding, were detected in the core region of the nucleus accumbens and exhibited a laminated expression pattern in the frontal cortex. D3 receptor protein was expressed robustly in neurons of the dorsolateral striatum, but showed an intense neuropil reaction in the globus pallidus. Functional D2 receptors, defined as the coincident fluorescence of the D2 receptor antisera and fluoroprobe binding, were detected in the frontal cortex and the medial shell of the nucleus accumbens. Thus, heterogeneities occurred in the cellular expression of functional D3 and D2 receptors in forebrain dopaminoceptive areas. D3 appears more related to basal ganglia and structures involved with motoric behavior, while D2 was associated with regions associated with cognitive/affective functions.
The notion that oligonucleotides can modulate gene-specific expression was established more than a decade ago. Recent advances in molecular genetics have broadened the armamentarium used to manipulate gene expression in biological systems including triplex DNA, antisense RNA/DNA, and ribozymes (catalytic RNA). These oligonucleotides demonstrated important early application to the elucidation of cellular signaling pathways. More recently, studies with these agents have probed their utility as potential therapeutic agents, especially in the realm of cancer. With the implementation of gene therapy in early clinical trials, oligonucleotide-mediated suppression of gene expression has emerged as an important strategy for gene therapy. This review will discuss the current knowledge in this field, focusing on the biology of triplex DNA, antisense oligonucleotides, and ribozymes.
Dopaminergic neuronal pathways arise from mesencephalic nuclei and project axons to the striatum, cortex, limbic system and hypothalamus. Through these pathways dopamine affects many physiological functions, such as the control of coordinated movement and hormone secretion. Here we have studied the physiological involvement of the dopamine D2 receptors in dopaminergic transmission, using homologous recombination to generate D2-receptor-deficient mice. Absence of D2 receptors leads to animals that are akinetic and bradykinetic in behavioural tests, and which show significantly reduced spontaneous movements. This phenotype presents analogies with symptoms characteristic of Parkinson's disease. Our study shows that D2 receptors have a key role in the dopaminergic control of nervous function. These mice have therapeutic potential as a model for investigating and correcting dysfunctions of the dopaminergic system.
Previous studies (G. E. Ploeger, B. M. Spruijt, & A. R. Cools, 1992) showed that low doses of systemically injected haloperidol affected spatial learning in the Morris water maze. This study investigated effects of intra-accumbens injections of haloperidol on spatial learning. To control for motivation and sensorimotor coordination, the researchers trained the rats to escape onto a visible platform. Low doses (50-100 ng) of haloperidol impaired spatial learning, whereas escaping on a visible platform was undisturbed. The 500-ng dose of haloperidol completely blocked acquisition because of combined learning and motor impairments. Retrieval of an acquired escape response was unaffected by 500 ng haloperidol. The data show that mesolimbic dopaminergic activity is involved in the acquisition of spatial localization. The results are related to studies demonstrating the involvement of the nucleus accumbens in cue-directed behaviors.
The effects of treatment with dopamine (DA) D1-agonist SKF38393 and D2-agonist quinpirole on morphine-induced hyperlocomotion were investigated in mice. Morphine-induced hyperlocomotion was increased by approximately 2.0-fold in SKF38393 (10 nmol, i.c.v.)-treated mice. Pretreatment with SCH23390 antagonized the enhancing effect of SKF38393. In contrast, pretreatment with quinpirole (10 nmol, i.c.v.) reduced morphine-induced hyperlocomotion. Morphine significantly increased DA metabolite levels, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the limbic forebrain (nucleus accumbens and olfactory tubercle). This elevation of DA metabolites by treatment with morphine was not modified by the co-administration of SKF38393. These results suggest that the activation of D1-receptors in the limbic forebrain may enhance the expression of morphine-induced hyperlocomotion.
The brain dopaminergic system is a critical modulator of basal ganglia function and plasticity. To investigate the contribution of the dopamine D1 receptor to this modulation, we have used gene targeting technology to generate D1 receptor mutant mice. Histological analyses suggested that there are no major changes in general anatomy of the mutant mouse brains, but indicated that the expression of dynorphin is greatly reduced in the striatum and related regions of the basal ganglia. The mutant mice do not respond to the stimulant and suppressive effects of D1 receptor agonists and antagonists, respectively, and they exhibit locomotor hyperactivity. These results suggest that the D1 receptor regulates the neurochemical architecture of the striatum and is critical for the normal expression of motor activity.
The effects of systemic administration of the full dopamine D1 receptor agonist A-77636 on acetylcholine release in rat frontal cortex and hippocampus were studied using in vivo microdialysis. Administration of A-77636 (4 mumol/kg s.c.) greatly (> 230%) increased both cortical and hippocampal acetylcholine release for more than 3 h; at a lower dose (1 mumol/kg s.c.) A-77636 significantly stimulated cortical but not hippocampal acetylcholine release. The effect of the higher dose of A-77636 on cortical acetylcholine release was blocked by the dopamine D1 receptor antagonist SCH 23390 (300 micrograms/kg s.c.). These results confirm that stimulation of dopamine D1 receptors facilitates cortical and hippocampal acetylcholine release in vivo, and indicate that these two structures are differentially sensitive to this effect. They also raise the possibility that dopamine D1 receptor agonists may be useful in the treatment of cortical and hippocampal acetylcholine deficit-related syndromes.
The extent to which acetylcholine (ACh) release in the hippocampus is regulated by dopaminergic mechanisms was assessed using in vivo microdialysis in freely moving rats. Systemic administration of the dopamine (DA) receptor agonist apomorphine (1.0 mg/kg) or the specific D1 agonist CY 208-243 (1.0 mg/kg) increased microdialysate concentrations of ACh in the hippocampus. The D2 receptor agonist quinpirole (0.5 mg/kg) produced a small but statistically significant decrease in hippocampal ACh release. d-Amphetamine (2.0 mg/kg) increased ACh release, an effect that was blocked by the D1 receptor antagonist SCH 23390 (0.3 mg/kg) but not by the D2 antagonist raclopride (1.0 mg/kg). These findings suggest that endogenous DA stimulates septohippocampal cholinergic neurons primarily via actions at D1 receptors. In addition, these results are similar to previous findings regarding the dopaminergic regulation of cortical ACh release, and suggest that the anatomical continuum formed by basal forebrain cholinergic neurons that project to the cortex and hippocampus acts as a functional unit, at least with respect to its regulation by DA.
The brain mesoaccumbens dopamine system is intricately involved in the psychomotor stimulant activities of cocaine. However, the extent to which different dopamine receptors mediate these effects has not yet been firmly established. The present study used dopamine D1 receptor mutant mice produced by gene targeting to investigate the role of this receptor in the effects induced by cocaine. In contrast with wild-type mice, which showed a dose-dependent increase in locomotion, D1 mutant mice exhibited a dose-dependent decrease. Electrophysiological studies of dopamine-sensitive nucleus accumbens neurons demonstrated a marked reduction in the inhibitory effects of cocaine on the generation of action potentials. In addition, the inhibitory effects of dopamine as well as D1 and D2 agonists were almost completely abolished, whereas those of serotonin were unaffected. D2-like dopamine receptor binding was also normal. These results demonstrate the essential role of the D1 receptor in the locomotor stimulant effects of cocaine and in dopamine-mediated neurophysiological effects within the nucleus accumbens.
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During the last decade, an understanding of the causes of many human diseases has progressed rapidly, in large measure because of the development of technologies that allow us to identify the genes that are involved. Identification of a gene that is suspected to play an important role in a particular disease opens up a whole new dimension of research to understand the molecular events that underlie the cause of that disorder. A crucial step in this process is often the development of an animal model of the disease. Again, the last decade has seen rapid advances in our ability to create such models, particularly in mice. Technologies that allow for the addition, alteration, or elimination of individual genes from the genome to create a transgenic mouse are now routine. The advantages of having a transgenic mouse model of a human disease are many. These animals often provide the first unequivocal proof that a particular gene is responsible for causing the pathological changes that occur with disease. They also can provide a system to carefully dissect the successive events that lead to the disease state, and can provide a custom-designed whole animal system to test potential therapies to treat and eventually cure the disease. Most important, new concepts relating to gene expression and gene function in disease often emerge from such transgenic studies. This review will illustrate several examples in which transgenic animals have contributed significantly to the evolution of concepts of the underlying mechanisms of human disease.
Whereas biochemical and pharmacological studies indicated that there were two subclasses of dopamine receptor (D1, D2) the application of molecular biology techniques has defined at least six dopamine receptor isoforms. These may be divided into D1-like (D1, D5) and D2-like (D2(short), D2(long), D3, D4) subfamilies on the basis of their structural and pharmacological properties. In this commentary the common properties of these dopamine receptor species are described, including the predicted structures of seven transmembrane α-helices, amino acid homologies and conserved amino acids that may play important structural and functional roles. The D1-like and D2-like receptor isoforms have individual properties and these are described in terms of their structures, pharmacological and biochemical properties and localizations in different brain regions. The existence of multiple dopamine receptor isoforms is important for understanding how certain drugs achieve their therapeutic effects and how unwanted side effects arise. This is considered for the anti-parkinsonian and anti-schizophrenic drugs.
The present study examined the effects of the dopamine D1 receptor subtypes agonist SK&F 38393 on locomotor activities after bilateral microinjection (0.00, 0.01, 0.1, 1.0, 10.0 micrograms) into the nucleus accumbens (Acb). The dose of 0.1 microgram elicited the highest response rate across measures of locomotion, rearing and stereotypy behavior. On the other hand, the largest dose of 10.0 micrograms was associated with significant increase in center time behaviors. The data were supportive of the hypothesis that dose-related locomotor activities elicited by microinjections of SK&F 38393 into the Acb are independently mediated by D1 receptors.
1. The objective of Exp. 1 was to determine whether intracerebroventricular (ICV) injection of cathinone CATH (8.0-32 micrograms) would produce a dose-dependent conditioned place preference (CPP) and/or activation in rats. Results indicate that rats conditioned with 16 or 32 micrograms doses of CATH significantly increased the time spent in their less preferred side, whereas rats conditioned with the 8.0 micrograms dose failed to show any shift from baseline preference. The 16 and 32 micrograms doses of CATH also significantly (p < .004) increased activity by more than 65% of baseline. 2. Exp. 2 was designed to determine whether ICV pretreatment with a dopamine release inhibitor CGS 10746B (CGS; 15 micrograms/rat) would block place conditioning produced by CATH. The results demonstrate that CGS pretreatment effectively blocked CATH-induced place conditioning and the CATH-induced elevation of activity.
Rats were given unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway and permanent indwelling cannula were surgically implanted into the non-lesioned side of the brain; cannula were used for direct injections of dopamine antagonists into the pars reticulata region of the non-lesioned substantia nigra. The selective D1 receptor antagonist, SCH 23390, was injected intranigrally at various concentrations (3.0, 1.5, 1.0, 0.6, or 0.3 mM) just prior to an intraperitoneal injection of amphetamine. SCH 23390 dose-dependently inhibited amphetamine-induced rotational behavior with the highest doses completely blocking rotational behavior in some animals. An intranigral injection of the selective D2 receptor antagonist, (-)-sulpiride (1.0 mM), did not produce a significant reduction in amphetamine-induced rotational behavior whereas an equivalent molar concentration of SCH 23390 (1.0 mM) produced a significant 62% reduction in amphetamine-induced rotational behavior. A concentration of SCH 23390 that produced a 50% reduction in rotational behavior when injected directly into the substantia nigra was unable to produce a significant reduction in rotational behavior when injected directly into the striatum. The effects of intranigral injections of SCH 23390 on apomorphine-induced rotational behavior were directly opposite to that observed for amphetamine-induced rotational behavior; contralateral rotational behavior increased relative to baseline measures. These data support the hypothesis that dopamine release in the midbrain may act as a neuromodulator of motor behavior, and that D1 receptors play a functional role in this process.