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Hyperpolarization of serotonergic neurons by serotonin and LSD: Studies in brain slices showing increased K+ conductance
Abstract
Serotonin and LSD hyperpolarized serotonergic dorsal raphe neurons in rat midbrain slices; the hyperpolarizations were accompanied by a decrease in input resistance, suggesting an increase in potassium conductance as one possible mechanism. Reversal potentials for serotonin and LSD-induced hyperpolarizations showed a shift of approximately 18 mV for a two-fold change in extracellular potassium concentration; this shift was close to that predicted by the Nernst equation for a potassium-dependent conductance.
... Intriguingly, recent findings in optogenetic studies on learned helplessness in rodents highlight a neural circuit featuring the ventromedial prefrontal cortex (vmPFC) and dorsal raphe nucleus (DRN) 46 that was also explored in earlier decades of psychedelic research but has since been largely neglected. 47 Below we present the learned helplessness paradigm and the key studies that led to its development, and then show how current preclinical and neuroimaging findings on psychedelics demonstrate some overlap with this framework on both biological and behavioral levels. We conclude by proposing that learned helplessness could be a productive model in psychedelic research. ...
... Further studies found that LSD hyperpolarized DRN neurons in rat brain slices in a similar manner to 5-HT, suggesting shared action on the serotonergic autoreceptors. 47 However, this theory does not hold true for all classes of psychedelics, as psychedelic phenethylamines have very little affinity for the 5-HT 1A receptor as compared with their affinity for the 5-HT 2A receptor (e.g., 2,5dimethoxy-4-iodoamphetamine [DOI]), yet can still modulate the activity of the DRN. 91,93,101 In particular, phenethylamines have been shown to suppress DRN firing, especially in the ventral portion. ...
... These include: the aforementioned temperature changes that are straightforwardly related to conduction velocity, the repeated observation of an alpha wave frequency increase by LSD (Fink, 1969;Carhart-Harris et al., 2016), an increase in the frequency of model nervous systems by LSD (Wright, Mocrhead, & Welsh, 1962), the enhanced release of the glutamate by LSD and subsequent increase in the frequency of EPSPs and EPSCs (Muschamp et al., 2004), the promotion of social behavior by LSD that requires excitatory neurotransmission but not inhibitory neurons (De Gregorio et al., 2021), and an increase in specific and nonspecific currents by LSD in neuronal cultures (Aghajanian & Lakoski, 1984;Parker, Panicker, & Miledi, 1990;Garratt, Alreja, & Aghajanian, 1993). The alpha wave frequency increase is particularly compelling, as the 10 hertz rhythm is deemed to be a "fulcrum" (Garcia-Rill et al., 2016) or carrier wave for the brain and is scaled with temperature (cf. ...
... For this reason, action potentials may be conceived as "all or none" adsorption processes (Ling, 1984), or as electrochemical pulses for which selective ion permeabilities are not required (Heimburg & Jackson, 2005;Heimburg, 2017). Minimally, this would allow for the reinterpretation of LSD-and serotonin-induced potassium conductance when implied by the Nernst equation (Aghajanian & Lakoski, 1984). More generally, this allows for the tying of many and ostensibly unrelated observations into a coherent biochemical framework. ...
A general assumption in lysergic acid diethylamide (LSD) research is that LSD’s experiential effects are basically hallucinatory, varying in the degree of distortion. This likely derives from the widely and perhaps implicitly held position of indirect realism which, though accepting an objective world, assumes the brain is generating within the mind not only imaginative images and memory images but also perceptual images—with no substantive difference in their qualia. However, the origin of perceptual images permeated with qualia is an aspect of the “hard problem” of consciousness. And without a solution to the origin of images, any theory of LSD’s seemingly distorted images of the perceptual world is ungrounded. Currently, a model describing LSD’s perceptual effects in terms of direct perception is created by joining Henri Bergson’s panpsychism and J. J. Gibson’s ecological optics. In this model, different doses of LSD produce different timescale changes in perception, and these changes are veridical specifications of the external world, not hallucinatory. In this context, the brain is a very concrete dynamical device, one not captured by an abstract computational framework. A testable model is proposed by which LSD increases rate processes on the relevant timescale. A variety of biochemical data are argued as being productively understood in the context of Gilbert Ling’s association-induction hypothesis, and a primary sequence analysis of the “inductive index”—an amino acid score that is central to this hypothesis— is consistent with the importance of 5-hydroxytryptamine receptor 2A. The proposed model is compared to recent progress in structural biology.
... By what cellular mechanisms do aminergic neurons, of which DRN cells are but one example, cease discharging before and during REM sleep? Pacemaker potentials in the DRN have recently been identified (Aghajanian & Lakoski 1984), and their possible role in the REM-off discharge pattern remains to be explored. Several lines of evidence suggest a pontine reticular influence on DRN neurons. ...
... The same applies to serotonin. Several different actions of serotonin have been found in vertebrates (e.g., Wood & Mayer 1979;Vandermaelen & Aghajanian 1980;Aghajanian & Lakoski 1984;Hounsgaard & Kiehn 1985;Parker, Gundersen & Miledi 1985;Van Dongen, Grillner & Hokfelt 1986). The actions of noradrenaline and serotonin (and most other neurotransmitters) are different from excitation and inhibition, and this has far-reaching implications for their interactions. ...
... Both inhibitory and excitatory responses to 5-HT have been reported in many mammalian CNS preparations (see review by VanderMaelen, 1985). Inhibitory responses have been observed in the dorsal raphe nucleus (Aghajanian and Lakoski, 1984) rat dorsal lateral geniculate (Rogawski and Aghajanian, 1980) rat hippocampus (Segal, 1980;, guinea pig hippocampus (Jahnsen, 1980), and striatum from rabbit (Herz and Zieglgansberger, 1968) and rat (Bevan et al., 1975;Davies and Tongroach, 1978). A proposed mechanism for the inhibitory effect is an increase in potassium conductance (Jahnsen, 1980;Segal, 1980;Aghajanian and Lakoski, 1984;. ...
... Inhibitory responses have been observed in the dorsal raphe nucleus (Aghajanian and Lakoski, 1984) rat dorsal lateral geniculate (Rogawski and Aghajanian, 1980) rat hippocampus (Segal, 1980;, guinea pig hippocampus (Jahnsen, 1980), and striatum from rabbit (Herz and Zieglgansberger, 1968) and rat (Bevan et al., 1975;Davies and Tongroach, 1978). A proposed mechanism for the inhibitory effect is an increase in potassium conductance (Jahnsen, 1980;Segal, 1980;Aghajanian and Lakoski, 1984;. Excitatory responses to 5-HT have been observed in a variety of CNS regions, including the guinea pig hippocampus (Jahnsen, 1980), rat neostriatum (Bevan et al., 1975;Park et al., 1982) rat facial motoneurons (VanderMaelen and Aghajanian, 1982), and cultured mouse spinal cord (Cottrell and Green, 1982). ...
Serotonin (5-HT) produced 3 different types of responses in neurons of mouse hippocampal and striatal cell cultures. These 3 responses have been characterized in terms of their pharmacological specificity, physiological mechanism, and dependence on cytoplasmic components. The most frequently observed response was inhibitory and was the result of a receptor-mediated activation of an inwardly rectifying potassium conductance. Typically, the response peaked within 1–3 sec of agonist application and did not exhibit desensitization. 5-Methoxy-N,N- dimethyltryptamine also produced this response in both striatal and hippocampal cultures and had no effect on the other 5-HT currents observed in this study. The selective 5-HT agonists--8-hydroxy-2-(di-n- propylamino)-tetralin, 1-(m-chlorophenyl) piperazine, and 1-(2- methoxyphenyl) piperazine--did not activate this outward current response. Methysergide did not block the 5-HT-activated outward current and often acted as an agonist. The response was lost in low-series- resistance recordings which facilitate solution exchange between the patch electrode and the cell. The loss of this response was prevented by using high-resistance patch electrodes, which retard this exchange. The 2 other responses described in this study were excitatory. They were seen less often than the inhibitory response. One of the excitatory responses was fast, with a time to peak of approximately 200 msec and a duration of 2–4 sec. The other was slow, with a time to peak of 7–10 sec and a duration of approximately 30–40 sec. Both of these responses were accompanied by a conductance increase. The fast excitatory response reversed at depolarized potentials and desensitized with a rate that varied with voltage. Metoclopramide and d-tubocurarine completely and reversibly blocked this fast excitatory response, while methysergide had no effect. The fast excitatory response was not lost during intracellular dialysis of cells in cultures from either striatum or hippocampus. In cultures from both brain regions, the slow excitatory response was blocked by methysergide. The slow excitatory response was lost even in patch-clamp recordings with high-resistance electrodes. This response was similar to responses to dopamine, norepinephrine, and forskolin, all of which are known to activate adenylate cyclase in the CNS.
... In addition to release in projection regions, vesicular serotonin (5-hydroxytryptamine; 5-HT) release occurs locally in the DRN at somatic (Kaushalya et al. 2008;Colgan et al. 2009), dendritic (de Kock et al. 2006;Colgan et al. 2012) and axonal sites (Bruns et al. 2000) where it modulates the activity of DRN neurons (Pineyro & Blier, 1999;Adell et al. 2002;Michelsen et al. 2008;Andrade et al. 2015). Release of serotonin can activate inhibitory Gα i/o-coupled 5-HT 1A autoreceptors that inhibit serotonergic neuron impulse activity through the opening of inwardly rectifying potassium (GIRK) channels (Aghajanian & Lakoski, 1984;De Vivo & Maayani, 1986;Bayliss et al. 1997;Katayama et al. 1997;Gantz et al. 2015) and inhibition of voltage-dependent Ca 2+ channels (Penington & Kelly, 1990;Penington et al. 1991). By suppressing pacemaker firing, 5-HT 1A autoreceptors regulate serotonin levels both locally in the dorsal raphe and in terminal projection regions (Aghajanian & Lakoski, 1984;Hjorth & Sharp, 1991;Portas et al. 1996;Adell et al. 2002;Michelsen et al. 2008), thereby influencing behaviours such as anxiety and stress (Richardson-Jones et al. 2010). ...
... Release of serotonin can activate inhibitory Gα i/o-coupled 5-HT 1A autoreceptors that inhibit serotonergic neuron impulse activity through the opening of inwardly rectifying potassium (GIRK) channels (Aghajanian & Lakoski, 1984;De Vivo & Maayani, 1986;Bayliss et al. 1997;Katayama et al. 1997;Gantz et al. 2015) and inhibition of voltage-dependent Ca 2+ channels (Penington & Kelly, 1990;Penington et al. 1991). By suppressing pacemaker firing, 5-HT 1A autoreceptors regulate serotonin levels both locally in the dorsal raphe and in terminal projection regions (Aghajanian & Lakoski, 1984;Hjorth & Sharp, 1991;Portas et al. 1996;Adell et al. 2002;Michelsen et al. 2008), thereby influencing behaviours such as anxiety and stress (Richardson-Jones et al. 2010). ...
Key points:
In the dorsal raphe nucleus, it is known that serotonin release activates metabotropic 5-HT1A autoreceptors located on serotonin neurons that leads to an inhibition of firing through the activation of G-protein-coupled inwardly rectifying potassium channels. We found that in mouse brain slices evoked serotonin release produced a 5-HT1A receptor-mediated inhibitory postsynaptic current (IPSC) that resulted in only a transient pause in firing. While spillover activation of receptors contributed to evoked IPSCs, serotonin reuptake transporters prevented pooling of serotonin in the extrasynaptic space from activating 5-HT1A -IPSCs. As a result, the decay of 5-HT1A -IPSCs was independent of the intensity of stimulation or the probability of transmitter release. These results indicate that evoked serotonin transmission in the dorsal raphe nucleus mediated by metabotropic 5-HT1A autoreceptors may occur via point-to-point synapses rather than by paracrine mechanisms.
Abstract:
In the dorsal raphe nucleus (DRN), feedback activation by Gαi/o -coupled 5-HT1A autoreceptors reduces the excitability of serotoninergic neurons, which decreases serotonin release both locally within the DRN and in projection regions. Serotonin transmission within the DRN is thought to occur via transmitter spillover and paracrine activation of extrasynaptic receptors. Here, we tested the volume transmission hypothesis in mouse DRN brain slices by recording 5-HT1A receptor-mediated inhibitory postsynaptic currents (5-HT1A -IPSCs) generated by the activation of G-protein-coupled inwardly rectifying potassium channels (GIRKs). We found that in the DRN of ePET1-EYFP mice, which selectively express enhanced yellow fluorescent protein in serontonergic neurons, the local release of serotonin generated 5-HT1A -IPSCs in serotonin neurons that rose and fell within a second. The transient activation of 5-HT1A autoreceptors resulted in brief pauses in neuron firing that did not alter the overall firing rate. The duration of 5-HT1A -IPSCs was primarily shaped by receptor deactivation due to clearance via serotonin reuptake transporters. Slowing diffusion with dextran prolonged the rise and reduced the amplitude the IPSCs and the effects were potentiated when uptake was inhibited. By examining the decay kinetics of IPSCs, we found that while spillover may allow for the activation of extrasynaptic receptors, efficient uptake by serotonin reuptake transporters (SERTs) prevented the pooling of serotonin from prolonging the duration of transmission when multiple inputs were active. Together the results suggest that the activation of 5-HT1A receptors in the DRN results from the local release of serotonin rather than the extended diffusion throughout the extracellular space.
... 5-HT-induced neuronal depolarization have also been detected in slice preparations of the nucleus accumbens [60], neocortex [61], dentate gyrus of the hippocampus [62], and all have the pharmacological characteristics which bear the hallmark of the 5-HT2A receptor. The excitatory responses mediate by 5-HT2A receptor activation are associated with a reduction of potassium conductance [63]. These responses are blocked by both selective (MDL 100 907) and non-selective 5-HT2A receptor antagonists [64,65]. ...
... Activation of 5-HT2C receptors also appears to depolarize pyramidal neurones in rat pyriform cortex. This response is blocked by spiperone, ritanserin and LY 53857 at higher concentrations than those needed to block 5-HT2A receptor-mediated responses in the same preparation [63] [64]. ...
... The electrophysiological features to identify 5-HT cells were the following: a regular discharging pattern, a slow firing rate and a long-lasting biphasic positive-negative waveform (2 ms). As a pharmacological criteria, the response to short perfusion of 5-HT (50-100 μM, 1 min) was used (Aghajanian and Lakoski, 1984;Hajós et al., 1996;Mendiguren and Pineda, 2009;Mendiguren et al., 2022). Only the neurons that showed the mentioned electrophysiological features and were inhibited by 5-HT were selected for the study. ...
The pharmacological profile of cannabigerol (CBG), which acid form constitutes the main precursor of the most abundant cannabinoids, has been scarcely studied. It has been reported to target α2-adrenoceptor and 5-HT1A receptor. The locus coeruleus (LC) and the dorsal raphe nucleus (DRN) are the main serotonergic (5-HT) and noradrenergic (NA) areas in the rat brain, respectively. We aimed to study the effect of CBG on the firing rate of LC NA cells and DRN 5-HT cells and on α2-adrenergic and 5-HT1A autoreceptors by electrophysiological techniques in male Sprague-Dawley rat brain slices. The effect of CBG on the novelty-suppressed feeding test (NSFT) and the elevated plus maze test (EPMT) and the involvement of the 5-HT1A receptor was also studied. CBG (30 μM, 10 min) slightly changed the firing rate of NA cells but failed to alter the inhibitory effect of NA (1-100 µM). However, in the presence of CBG the inhibitory effect of the selective α2-adrenoceptor agonist UK14304 (10 nM) was decreased. Perfusion with CBG (30 μM, 10 min) did not change the firing rate of DRN 5-HT cells or the inhibitory effect of 5-HT (100 μM, 1 min) but it reduced the inhibitory effect of ipsapirone (100 nM). CBG failed to reverse ipsapirone-induced inhibition whereas perfusion with the 5-HT1A receptor antagonist WAY100635 (30 nM) completely restored the firing rate of DRN 5-HT cells. In the EPMT, CBG (10 mg/kg, i.p.) significantly increased the percentage of time the rats spent on the open arms and the number of head-dipping but it reduced the anxiety index. In the NSFT, CBG decreased the time latency to eat in the novel environment but it did not alter home-cage consumption. The effect of CBG on the reduction of latency to feed was prevented by pretreatment with WAY100635 (1 mg/kg, i.p.). In conclusion, CBG hinders the inhibitory effect produced by selective α2-adrenoceptor and 5-HT1A receptor agonists on the firing rate of NA-LC and 5-HT-DRN neurons by a yet unknown indirect mechanism in rat brain slices and produces anxiolytic-like effects through 5-HT1A receptor.
... DRN 5-HT cells were identified by their electrophysiological features: a regular discharging pattern, a slow firing rate, and a long-lasting biphasic positive-negative waveform (2 ms). Moreover, pharmacological criteria such as the inhibitory response to bath application of 5-HT (50-100 μM, 1 min) at the beginning of the experiment were used to confirm the identity of 5-HT cells (Aghajanian and Lakoski, 1984Hajós and Sharp, 1996Mendiguren and Pineda, 2009). We only considered as 5-HT neurons those cells that fulfilled both electrophysiological and pharmacological criteria. ...
Cannabidiol (CBD), the main non-psychoactive cannabinoid found in the cannabis plant, elicits several pharmacological effects via the 5-HT 1A receptor. The dorsal raphe nucleus (DRN) is the main serotonergic cluster in the brain that expresses the 5-HT 1A receptor. To date, the effect of CBD on the neuronal activity of DRN 5-HT cells and its interaction with somatodendritic 5-HT 1A autoreceptors have not been characterized. Our aim was to study the effect of CBD on the firing activity of DRN 5-HT cells and the 5-HT 1A autoreceptor activation by electrophysiological and calcium imaging techniques in male Sprague–Dawley rat brain slices. Perfusion with CBD (30 μM, 10 min) did not significantly change the firing rate of DRN 5-HT cells or the inhibitory effect of 5-HT (50–100 μM, 1 min). However, in the presence of CBD (30 μM, 10 min), the inhibitory effects of 8-OH-DPAT (10 nM) and ipsapirone (100 nM) were reduced by 66% and 53%, respectively. CBD failed to reverse ipsapirone-induced inhibition, whereas perfusion with the 5-HT 1A receptor antagonist WAY100635 (30 nM) completely restored by 97.05 ± 14.63% the firing activity of 5-HT cells. Administration of AM251 (1 µM), MDL100907 (30 nM), or picrotoxin (20 μM) did not change the blockade produced by CBD (30 μM) on ipsapirone-induced inhibition. Our study also shows that CBD failed to modify the KCl (15 mM, 4 min)-evoked increase in [Ca ²⁺ ] i or the inhibitory effect of ipsapirone (1 μM, 4 min) on KCl-evoked [Ca ²⁺ ] i . In conclusion, CBD does not activate 5-HT 1A autoreceptors, but it hindered the inhibitory effect produced by selective 5-HT 1A receptor agonists on the firing activity of DRN 5-HT cells through a mechanism that does not involve CB 1 , 5-HT 2A, or GABA A receptors. Our data support a negative allosteric modulation of DRN somatodendritic 5-HT 1A receptor by CBD.
... Of note, some 5-HT neurons have been shown to discharge at higher frequency but they are also inhibited by 5-HT 1A receptor stimulation (Hajós et al., 1995). The decrease is due to the stimulation of K + conductance at the level of the cell body (Aghajanian and Lakoski, 1984;Montalbano et al., 2015). The control exerted by the 5-HT 1A autoreceptor would appear to be phasic, e.g. ...
The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2B, 5-HT7 receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.
... This study demonstrates that local activation of 5-HT1A receptors by 8-OH-DPAT emulates the effects that serotonin and fluoxetine evoked in the electrical activity of RTn neurons; additionally, this effect is attenuated by WAY-100635, a specific antagonist of such receptors. Serotonin 5-HT1A receptors are G protein-coupled receptors (GPCRs), since the activation of Gi/Go proteins (Innis et al. 1988;Zhang et al. 1994) is closely linked to the opening frequency of the inwardly rectifying potassium (GIRK) channels (Aghajanian and Lakoski 1984;Haj-Dahmane et al. 1991;Penington et al. 1993;Gantz et al. 2013;Courtney and Ford 2016); therefore, the increase in the potassium conductance through 5-HT1A receptor activation may explain the decrease in the spiking frequency of RTn neurons that we show in this work. ...
The reticular thalamic nucleus (RTn) is a thin shell of GABAergic neurons that covers the dorsal thalamus that regulate the global activity of all thalamic nuclei. RTn controls the flow of information between thalamus and cerebral cortex since it receives glutamatergic information from collaterals of thalamo-cortical (TCs) and cortico-thalamic neurons. It also receives aminergic information from several brain stem nuclei, including serotonergic fibers originated in the dorsal raphe nucleus. RTn neurons express serotonergic receptors including the 5-HT1A subtype, however, the role of this receptor in the RTn electrical activity has been scarcely analyzed. In this work, we recorded in vivo the unitary spontaneous electrical activity of RTn neurons in anesthetized rats; our study aimed to obtain information about the effects of 5-HT1A receptors in RTn neurons. Local application of fluoxetine (a serotonin reuptake inhibitor) increases burst firing index accompanied by a decrease in the basal spiking rate. Local application of different doses of serotonin and 8-OH-DPAT (a specific 5-HT1A receptor agonist) causes a similar response to fluoxetine effects. Local 5-HT1A receptors blockade produces opposite effects and suppresses the effect by 8-OH-DPAT. Our findings indicate the presence of a serotonergic tonic discharge in the RTn that increases the burst firing index and simultaneously decreases the basal spiking frequency through 5-HT1A receptors activation.
... Les autorécepteurs 5-HT1A sont quant à eux impliqués dans la balance inhibitrice de l'activité des neurones 5-HT. En effet, la sérotonine en se fixant sur ce récepteur, entraîne l'hyperpolarisation du neurone à travers l'ouverture d'un canal potassique à rectification entrante (Kir) (Aghajanian and Lakoski, 1984;Montalbano et al., 2015). Cette hyperpolarisation a pour conséquence une diminution de la fréquence de décharge du neurone qui est associée à une diminution de la libération de sérotonine dans les aires de projections (Blier and de Montigny, 1987; Haj-autorécepteurs 5-HT1A dépendait uniquement d'une libération synaptique, l'activité de ces neurones s'en retrouverait erratique. ...
Les études épidémiologiques mettent en évidence que les patients atteint de diabète de type 2 (DT2) ont deux fois plus de risque de souffrir de dépression majeure (DM), un trouble mental caractérisé par une tristesse intense et/ou une anhédonie. Plus précisément, les données de la littérature indiquent que la résistance à l’insuline, qui est la caractéristique majeure du DT2, est positivement corrélée à la sévérité des symptômes dépressifs. Etant donné le rôle essentiel que joue la neurotransmission sérotoninergique (5-HT) dans la physiopathologie de la DM, nous avons émis l’hypothèse que la résistance à l’insuline sélective de ce système neuronal est responsable des troubles de l’humeur associés au DT2. En ce sens, des altérations du système 5-HT ont été observées dans un modèle de troubles émotionnels associés à un DT2. En effet, le régime obésogène utilisé dans ce modèle induit une altération significative des propriétés électrophysiologiques des neurones 5-HT du Raphé dorsal (DR) ainsi qu’une diminution des taux de sérotonine. Cette étude vise également à déterminer l’action de l’insuline cérébrale sur le comportement émotionnel et le système sérotoninergique. En utilisant une approche par électrophysiologie ex-vivo, nous avons pu observer que l’insuline modulait positivement l’activité des neurones 5-HT du RD. Nous avons également pu mettre en évidence un effet de type anxiolytique de l’action de l’insuline sur le cerveau par voie intranasale. Cet effet est accompagné de diminution des taux de sérotonine (5-HT) tissulaires dans certaines structures cérébrales impliquées dans la régulation de l’anxiété. Enfin, nous avons utilisé un modèle transgénique grâce auquel nous avons invalidé sélectivement le récepteur à l’insuline dans les neurones 5-HT. L’étude comportementale a permis de mettre en évidence une diminution du comportement de type anxieux chez ces animaux, associé à une diminution de l’activité des neurones sérotoninergiques du RD. Ensemble ces données suggèrent que l’insuline peut moduler le comportement émotionnel notamment via le système sérotoninergique. Ces éléments pourront contribuer à la possible découverte de nouveaux traitements et à la prise en charge des troubles de l’humeur chez les patients atteint de DT2.
... Of note, some 5-HT neurons have been shown to discharge at higher frequency but they are also inhibited by 5-HT 1A receptor stimulation (Hajós et al., 1995). The decrease is due to the stimulation of K + conductance at the level of the cell body (Aghajanian and Lakoski, 1984;Montalbano et al., 2015). The control exerted by the 5-HT 1A autoreceptor would appear to be phasic, e.g. ...
The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2B, 5-HT7 receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.
... The DRN contains serotonergic neurons and non-serotonergic cells, such as GABAergic (Nanopoulos et al. 1982) or glutamatergic (Clements et al. 1991) neurons. Serotonergic neurons are modulated by Gi/Go protein-coupled 5-HT 1A receptors (Aghajanian and Lakoski 1984), which activation leads to hyperpolarization of serotonergic cells (Courtney and Ford 2016). Non-serotonergic cells synapse with serotonergic neurons to directly regulate their firing activity within the nucleus (local circuitry). ...
The main noradrenergic and serotonergic nuclei in the central nervous system (CNS) are the locus coeruleus (LC) and the dorsal raphe nucleus (DRN). These brain areas, located in the brainstem, play a pivotal role in the control of various functions and behaviors that are altered by cannabinoids (i.e., pain, arousal, mood, anxiety, or sleep-wake cycle). Anatomical, neurochemical, and functional data suggest that cannabinoids regulate both central noradrenergic and serotonergic neurotransmission. Thus, strong evidence has shown that the firing activity of LC and DRN monoamine neurons or the synthesis/release of noradrenaline (NA) and serotonin (5-HT) in the projection areas are all affected by cannabinoid administration. Herein, we propose that interaction between the endocannabinoid system and the noradrenergic-serotonergic systems could account for some of the anxiolytic, antidepressant, and antinociceptive effects of cannabinoids or the disruption of attention/sleep induced by these drugs.
... Numerous anatomical data show that 5-HT 1A receptors are autoreceptors being expressed by 5-HT neurons themselves (Gozlan et al., 1983;Miquel et al., 1994;Riad et al., 2000). The stimulation of somatodendritic 5-HT 1A receptor enhances K þ conductance through G proteinegated inwardly rectifying K þ (GIRK) thereby leading to inhibitory influences (Aghajanian & Lakoski, 1984;Montalbano et al., 2015). This control appears to be phasic in nature as the administration of antagonists usually does not modulate the firing activity of 5-HT neurons (Martin et al., 1999). ...
It has been known for several years that serotonin (5-hydroxytryptamine, 5-HT) acts on neuronal cell excitability in various complex organisms of the animal kingdom including humans. 5-HT stimulates a variety of 5-HT receptors which can transiently and locally alter the ion conductance in neurons, ultimately leading to change the activity of the whole neurobiological network. We have summarized here some evidence showing that 5-HT through its multiple 5-HT receptor subtypes controls the excitability of various neuronal cell populations in numerous ways. These controls evolve under various circumstances. The 5-HT modulation of excitability is more complex when we consider network level, and we have explored the meaning of this unclear notion in term of mechanisms in different organisms. Finally, we describe the 5-HT control of epilepsy which represents the extreme neuronal excitability disturbance.
... 5-HT 1A R stimulation inhibits adenylyl cyclase (AC) activity and opens K+ channels, playing an inhibitory role in cells expressing the receptor. The stimulation of somatodendritic 5-HT 1A R enhances K + conductance through G-protein-gated inwardly rectifying K + (GIRK) channels, thereby leading to inhibitory influences (Aghajanian and Lakoski, 1984;Montalbano et al., 2015). Numerous other pathways have been described depending on the tissue (Fig. 2). ...
The constitutive activity of different serotonin receptors (5-HTRs) toward intracellular signaling pathways has been proposed to have physiological and pathological importance. Inverse agonists block the constitutive activity and can be used to probe and silence such a spontaneous activity. The constitutive activity of 5-HTRs can be observed in various heterologous systems of expression in vitro (very high for 5-HT2CR; very low for 5-HT2AR). The demonstration of the existence of this activity in native tissues and ultimately in integrative neurobiology and behavior is a real pharmacological challenge. Irrespective of the existence of mutants or polymorphisms that could alter the constitutive activity of 5-HTRs, evidence suggests that spontaneous activity of 5-HT2CR could impact the activity of neurobiological networks and that of 5-HT6R and 5-HT7R the developmental morphogenesis. Some findings exist for 5-HT2BR and 5-HT2AR in diverse though rare conditions. The existence of a constitutive activity for 5-HT1AR, 5-HT1B/1DR, and 5-HT4R is still poorly supported. When identified, the constitutive activity may differ according to brain location, state of activity (phasic in nature), and intracellular signaling pathways. A very few studies have reported aberrant constitutive activity of 5-HTRs in animal models of human diseases and patients. The purpose of this review is a critical examination of the available neuropharmacological data on the constitutive activity of 5-HTRs to determine whether this activity is an essential component of the serotonergic system transmission and it may be a possible target for CNS drug development.
... The presynaptic 5-HT 1A receptors are located in the raphe nuclei, where they act as cell body auto-receptors to inhibit the firing rate of 5-HT neurons. On the other hand, the postsynaptic 5-HT 1A receptors are located in the limbic and cortical regions, where they also attenuate firing activity, which indicates that 5-HT 1A receptors bring about a negative feedback influence on firing activity in the brain (Aghajanian and Lakoski, 1984;Blier and De Montigny, 1987;Blier and Ward, 2003). ...
Depression, an emotion regulation disorder, is a prevalent mental illness in the world. Meanwhile, traditional Chinese medicine (TCM) has been increasingly regarded as a promising and effective alternative therapy approach for patients with depression. Despite many years of research on depression, the current understanding of the pathological mechanism of depression based on TCM theories is still in its infancy. Due to the lack of scientific evidence in the past, TCM is not fully recognized by researchers around the world. This review firstly summarizes the pathogenesis and etiology of depression in terms of both Eastern and Western medical systems. Secondly, it adopts an integrated Eastern and Western approach to propose some plausible neurophysiological pathways linking the liver, spleen, and heart functions explicated in TCM theory. The aim of this theoretical review is to bridge the knowledge gap between Eastern and Western medicine, which may better explain the pathology of depression.
... These neurons express the serotonergic markers tryptophan hydroxylase (TPH2), and 5-HT transporter (SERT), and also the negative autoreceptors, 5-HT 1A and 5-HT 1B receptors, whose expression is restricted to somatodendritic compartments of 5-HT neurons, and to axonal terminals, respectively (Riad et al, 2000). The 5-HT 1A autoreceptor activation elicits an outward current carried through G protein-coupled inwardly-rectifying potassium channels (GIRK) of the Kir3 family leading to membrane hyperpolarization and inhibition of 5-HT neuron firing (Aghajanian and Lakoski, 1984). The presence of synaptic vesicles in dendrites of 5-HT neurons led to the suggestion that autoinhibition is mediated via dendritic release of 5-HT, for review see (Andrade et al, 2015). ...
Serotonin is a neurotransmitter involved in many psychiatric diseases. In humans, a lack of 5-HT2B receptors is associated with serotonin-dependent phenotypes, including impulsivity and suicidality. A lack of 5-HT2B receptors in mice eliminates the effects of molecules that directly target serotonergic neurons including amphetamine derivative serotonin releasers, and selective serotonin reuptake inhibitor antidepressants. In this work, we tested the hypothesis that 5-HT2B receptors directly and positively regulate raphe serotonin neuron activity. By ex vivo electrophysiological recordings, we report that stimulation by the 5-HT2B receptor agonist, BW723C86, increased the firing frequency of serotonin Pet1-positive neurons. Viral overexpression of 5-HT2B receptors in these neurons increased their excitability. Furthermore, in vivo 5-HT2B-receptor stimulation by BW723C86 counteracted 5-HT1A autoreceptor-dependent reduction in firing rate and hypothermic response in wild-type mice. By a conditional genetic ablation that eliminates 5-HT2B receptor expression specifically and exclusively from Pet1-positive serotonin neurons (Htr2b5-HTKO mice), we demonstrated that behavioral and sensitizing effects of MDMA (3,4-methylenedioxy-methamphetamine), as well as acute behavioral and chronic neurogenic effects of the antidepressant fluoxetine, require 5-HT2B receptor expression in serotonergic neurons. In Htr2b5-HTKO mice, dorsal raphe serotonin neurons displayed a lower firing frequency compared to control Htr2blox/lox mice as assessed by in vivo extracellular recordings and a stronger hypothermic effect of 5-HT1A-autoreceptor stimulation was observed. The increase in head-twitch response to DOI (2,5-dimethoxy-4-iodoamphetamine) further confirmed the lower serotonergic tone resulting from the absence of 5-HT2B receptors in serotonin neurons. Together, these observations indicate that the 5-HT2B receptor acts as a direct positive modulator of serotonin Pet1-positive neurons in an opposite way as the known 5-HT1A-negative autoreceptor.
... Immunocytochemical and electron microscopic studies have shown that the CB 1 receptor is mainly located at GABAergic and glutamatergic axon terminals in the CNS, whereas low labelling is present at somatodendritic sites [52,127]. Thus, one of the main functions attributed to the endocannabinoid system in the brain is to modulate the excitatory and inhibitory synapses [56], which has been related to CB 1 receptor. Yet, the role of the CB 2 receptor in the regulation of CNS neurotransmission has not been widely studied. ...
The cannabinoid system is composed of Gi/o protein-coupled cannabinoid type 1 receptor (CB1) and cannabinoid type 2 (CB2) receptor and endogenous compounds. The CB1 receptor is widely distributed in the central nervous system (CNS) and it is involved in the regulation of common physiological functions. At the neuronal level, the CB1 receptor is mainly placed at GABAergic and glutamatergic axon terminals, where it modulates excitatory and inhibitory synapses. To date, the involvement of CB2 receptor in the regulation of neurotransmission in the CNS has not been clearly shown. The majority of noradrenergic (NA) cells in mammalian tissues are located in the locus coeruleus (LC) while serotonergic (5-HT) cells are mainly distributed in the raphe nuclei including the dorsal raphe nucleus (DRN). In the CNS, NA and 5-HT systems play a crucial role in the control of pain, mood, arousal, sleep-wake cycle, learning/memory, anxiety, and rewarding behaviour. This review summarizes the electrophysiological, neurochemical and behavioural evidences for modulation of the NA/5-HT systems by cannabinoids and the CB1 receptor. Cannabinoids regulate the neuronal activity of NA and 5-HT cells and the release of NA and 5-HT by direct and indirect mechanisms. The interaction between cannabinoid and NA/5-HT systems may underlie several behavioural changes induced by cannabis such as anxiolytic and antidepressant effects or side effects (e.g. disruption of attention). Further research is needed to better understand different aspects of NA and 5-HT systems regulation by cannabinoids, which would be relevant for their use in therapeutics.
... The combined treatment may be necessary in order to produce through FGFR1 activation a significant uncoupling of the 5-HT1A autoreceptor to the GIRK channel. Indeed, electrophysiological studies in vivo and in midbrain slices indicate that stimulation of 5-HT receptors hyperpolarizes dorsal raphe 5-HT neurons, by activating an inwardly rectifying K + conductance mediated by GIRK channels (Aghajanian and Vandermaelen, 1982;Aghajanian and Lakoski, 1984;Williams et al., 1988;Penington et al., 1993a,b;Bayliss et al., 1997). In keeping with these findings, it has been shown that GIRK1, GIRK2 and GIRK3 are expressed in dorsal raphe 5-HT neurons (Karschin et al., 1996;Fairchild et al., 2003;Saenz del Burgo et al., 2008). ...
The FGFR1-5-HT1A heteroreceptor complexes are involved in neuroplasticity in the rat hippocampus and in the mesencephalic raphe 5-HT nerve cells. There exists a 5-HT1A protomer enhancement of FGFR1 protomer signaling. Acute and 10 day treatment with intracerebroventricular (i.c.v.) FGF-2 and the 5-HT1A agonist 8-OH-DPAT produced enhanced antidepressant effects in the forced swim test (FST). We studied in the current work the disturbances in the FGFR1-5-HT1A heterocomplexes in a genetic rat model of depression, the Flinders sensitive line (FSL) rats of Sprague-Dawley (SD) origin, by means of neurochemical, neurophysiological and behavioral techniques. In control SD rats, the FGFR1 agonist SUN11602 and FGF2 produced a significant reduction of G protein-coupled inwardly rectifying K+ channel (GIRK) currents induced by 8-OH-DPAT in the CA1 area of the hippocampus. In FSL rats, only i.c.v. 8-OH-DPAT alone treatment produced a significant reduction in the immobility time. The combined i.c.v. treatment (FGF2 + 8-OH-DPAT) in FSL rats did not cause a significant decrease in immobility time in the FST. However, in the SD rats this combined treatment produced a significant reduction. Furthermore, in the FSL rat a significant increase in the density of FGFR1-5-HT1A proximity ligation assay (PLA) positive clusters was only found after i.c.v. 8-OH-DPAT treatment alone in the CA2 and CA3 areas. In the SD rat a significant increase in the density of specific PLA clusters was only observed in the CA2 area of the i.c.v. combined treatment (FGF2 + 8-OH-DPAT) group. No treatment led to significant changes in the PLA clusters of the dorsal raphe in the FSL rat. However, significant changes in the density of specific PLA clusters were only found in the dorsal raphe of SD rats after combined treatment and treatment with 8-OH-DPAT alone. The results indicate that in FSL rats compared with SD rats alterations may develop in the ability of 8-OH-DPAT and combined FGFR1 and 5-HT1A agonist treatment to increase the density of FGFR1-5-HT1A heteroreceptor complexes of the dorsal raphe. It is proposed that such deficits in FSL rats may possibly reflect a failure of the combined agonist treatment to uncouple the 5-HT1A autoreceptors from the GIRK channels. This may contribute to the failure of producing antidepressant-like effects in the FSL rat by combined agonist treatment as seen in the SD rat. The antidepressant-like effects seen with the 5-HT1A agonist alone treatment in FSL but not in SD rats may instead involve significant increases in the FGFR1-5-HT1A complexes of the CA2 and CA3 areas of the hippocampus.
... The dorsal raphe (DR) is a serotonergic (5-HT) midbrain nucleus that is highly implicated in the etiology of several mood disorders, including depression (Jans et al. 2007;Olivier 2015). The activity of this nucleus is critically regulated by 5-HT 1A receptors, which, upon stimulation, activate G protein-coupled inwardly rectifying K + (GIRK) channels, leading to a reduction in membrane excitability and an inhibition of action potentials (Aghajanian and Lakoski 1984;Bayliss et al. 1997). In addition to 5-HT 1A receptor-mediated regulation, the GABAergic system also provides inhibitory control of local DR activity and descending excitatory inputs (Gervasoni et al. 2000;Celada et al. 2001;Varga et al. 2001Varga et al. , 2003Amat et al. 2005;Challis et al. 2014). ...
The serotonergic tone of the dorsal raphe (DR) is regulated by 5-HT1A receptors, which negatively control serotonergic activity via the activation of G protein-coupled inwardly rectifying K+ (GIRK) channels. In addition, DR activity is modulated by local GABAergic transmission, which is believed to play a key role in the development of mood-related disorders. Here, we sought to characterize the role of GIRK2 subunit-containing channels on the basal electrophysiological properties of DR neurons and to investigate whether the presynaptic and postsynaptic activities of 5-HT1A, GABAB, and GABAA receptors are affected by Girk2 gene deletion. Whole-cell patch-clamp recordings in brain slices from GIRK2 knockout mice revealed that the GIRK2 subunit contributes to maintenance of the resting membrane potential and to the membrane input resistance of DR neurons. 5-HT1A and GABAB receptor-mediated postsynaptic currents were almost absent in the mutant mice. Spontaneous and evoked GABAA receptor-mediated transmissions were markedly reduced in GIRK2 KO mice, as the frequency and amplitude of spontaneous IPSCs were reduced, the paired-pulse ratio was increased and GABA-induced whole-cell currents were decreased. Similarly, the pharmacological blockade of GIRK channels with tertiapin-Q prevented the 5-HT1A and GABAB receptor-mediated postsynaptic currents and increased the paired-pulse ratio. Finally, deletion of the Girk2 gene also limited the presynaptic inhibition of GABA release exerted by 5-HT1A and GABAB receptors. These results indicate that the properties and inhibitory activity of DR neurons are highly regulated by GIRK2 subunit-containing channels, introducing GIRK channels as potential candidates for studying the pathophysiology and treatment of affective disorders.
... In a recent study, we showed similar incidence of DR neuronal inhibition (83%) during long-train mPFC-HFS across a broad range of neuronal firing rates in anaesthetized rats, suggesting a non-specific inhibitory effect on DR neurons (Srejic et al., 2015). Electrophysiological and pharmacological criteria including low frequency firing rate, wide spikes, and a hyperpolarizing response to 5-HT 1A receptor agonists, initially used to distinguish between 5-HT and unidentified cells (Aghajanian and Lakoski, 1984;Aghajanian and Vandermaelen, 1982;Vandermaelen and Aghajanian, 1983) have become inadequate for proper categorization of DR neurons (Calizo et al., 2011;Kocsis et al., 2006). A number of ultrastructural studies reported that identification of 5-HT neurons based on these criteria alone produced a number of false-negative and false-positive results (Allers and Sharp, 2003;Kirby et al., 2003;Li et al., 2001). ...
... The 1AR functions as both a presynaptic autoreceptor and a postsynaptic heteroreceptor (Blier et al, 1987). Presynaptic autoreceptors are located on the cell body and dendrites of serotonergic neurons in the raphe nuclei where they serve to negatively inhibit their own activity and suppress the release of serotonin (Aghajanian and Lakoski, 1984). Postsynaptic heteroreceptors are found mainly on glutamatergic and GABAergic pyramidal cells in limbic and cortical regions innervated by serotonergic raphe cells, where they inhibit the firing of target neurons and regulate the function of other neurotransmitter systems (Azmitia et al, 1996; Hall et al, 1997; Palchaudhuri and Flugge, 2005). ...
Anxiety disorders are the most prevalent class of mental illness, yet currently available treatments are often ineffective or inadequate, leaving many patients with lingering symptoms. The serotonin 1A receptor (1AR) has been implicated in the etiology of these disorders, which often show comorbidity with cognitive dysfunction. Mice with the 1AR genetically deleted or "knocked out" (1AKO) during a critical period in development (postnatal days 13-21) exhibit anxiety-like behavior and learning and memory deficits, and may therefore represent a useful genetic model in studying the neurobiological effects of this receptor. The hippocampus has been shown to highly express the 1AR and to be a key mediator in memory and the regulation of emotion. The experiments in this thesis focus on the structural and functional hippocampal changes in the 1AKO mouse compared to wild-type mice in order to elucidate the cellular mechanisms behind the alterations in behavior. Electrophysiology was used in the CA1 region of the hippocampus to show that pyramidal neurons in the 1AKO mouse receive less glutamatergic input than control mice during the critical period, resulting in decreased AMPA-mediated excitation and LTP in the adult. Interestingly, morphological analyses demonstrated a significant enhancement in proximal dendritic branching in both the juvenile and adult 1AKO mouse that may be the result of the developmental effects of increased serotonergic efflux. Additional experiments focused on the role of corticotropin-releasing factor (CRF) in the 1AKO mouse, based on the fact that peak hippocampal levels of this neuropeptide coincide in time with the critical period of development when 1AR deletion has pronounced effects. We found that adult 1AKO mice showed increased numbers of CRF-containing interneurons and that CRF1 receptor antagonism restored CA1 LTP to control levels. Taken together, these results reveal a complex interplay of decreased synaptogenesis and number of AMPA receptors, and excessive activation of CRF1 receptors that may underlie the cognitive deficits and anxiety-like behavior of the 1AKO mouse. The experiments support the continued research into the neurobiological mechanisms of human anxiety disorders.
... Los autorreceptores de serotonina en los mamíferos son todos miembros de la familia de receptores 5-HT 1 . Estos receptores tienen una alta afinidad por la serotonina y están acoplados a proteínas G i / o , 57 que a su vez activan canales de potasio, 58 con lo cual se produce una hiperpolarización que disminuye la frecuencia de disparo de las neuronas. En invertebrados, los autorreceptores de serotonina activan sobre todo canales de cloro. ...
Serotonin is fundamental for the modulation of social behavior, emotions and a wide variety of physiological functions. The functions of serotonergic systems have been highly conserved along the evolutionary scale and in general small numbers of neurons innervate virtually all the nervous system, and exert multiple effects depending on the site of release. Synaptic pools produce fast and local effects, while extrasynaptic pools in the soma, dendrites, axons and the periphery of synapses produce diffuse effects, characteristic of mood modulation. Serotonin release from synaptic terminals is produced by exocytosis of small clear vesicles and is activated by single or low-frequency impulses, while increases in the stimulation frequency produce synaptic facilitation and depression. In contrast, release from the soma is produced by exocytosis of dense-cored vesicles and requires stimulation at high frequencies, the activation of L-type calcium channels and calcium-induced calcium release from intracellular stores. Serotonin released from the presynaptic terminals immediately activates autoreceptors in the same terminals, locally decreasing the subsequent excitability, firing frequency and release. Differential regulation of serotonin release in different cell compartments allows the same neuron to produce different types of effects depending on the firing rate.
... The 5-HT 1A receptor is located on the soma and dendrites of 5-HT neurons, which is considered to be an autoreceptor. Stimulation of the 5-HT 1A autoreceptor inhibits the firing rate of serotonergic neurons, whereas activation of the postsynaptic receptor induces inhibitory responses on target structures [26][27]. 5-HT 1A receptors have a significant presence in the cerebral cortex and the hippocampus where they are predominantly found in pyramidal cells [28]. In addition, prefrontal cortex 5-HT 1A receptors are highly co-localized with 5-HT 2A receptors [29][30]. ...
Serotonin most often acts as an inhibitory neurotransmitter which also modulates post-synaptic activity within other neurotransmitter systems. Serotonin is synthesized from the essential amino acid tryptophan in Raphe complex neurons which innervate extensive brain regions including the cerebral cortex, hippocampus, basal ganglia and cerebellum. Cognitive information is processed through synaptic connections with other regions of the brain, forming neural circuits as well as sub-serving systems that influence behavior. Previously investigations have concluded that serotonin has a modulatory role in neural systems, causing functional activity in the organization of various processes that relate to cognitive activity such as learning, memory, maintaining attention and behavioral switching. Serotonergic modulatory mechanisms include plastic changes at different biological organizational levels ranging from the molecular level, to adaptive behavior. Several psychopathological entities are associated with abnormal variations in brain serotonin levels. However, these involve other neurotransmitter systems such as dopamine, norepinephrine or acetylcholine. Therefore, an experimental study of the physiological interaction between serotonin and other neurotransmitter systems in normal and atypical cognitive abilities would be crucial for the improvement of therapeutic strategies applied to patients showing psychopathologies associated with dysfunctional serotonin neurotransmission.
... It is well documented that the activity of raphe 5-HT neurons is under regulatory control by 5-HT 1A autoreceptors and K + channels. Early electrophysiological studies in vivo [1] and in midbrain slices [2] suggested that stimulation of 5-HT receptors hyperpolarize dorsal raphe 5-HT neurons by an increase in K + conductance. Using intracellular recordings, Williams et al. [3] showed that stimulation of 5-HT or GABA B receptors on dorsal raphe 5-HT neurons activates an inwardly rectifying K + conductance via a pertussis toxin sensitive G-protein. ...
G protein-activated inwardly rectifying potassium (GIRK) channels in 5-HT neurons are assumed to be principal effectors of 5-hydroxytryptamine 1A (5-HT1A) autoreceptors, but their pharmacology, subunit composition and the role in regulation of 5-HT neuron activity have not been fully elucidated. We sought for a pharmacological tool for assessing the functional role of GIRK channels in 5-HT neurons by characterizing the effects of drugs known to block GIRK channels in the submicromolar range of concentrations. Whole-cell voltage-clamp recording in brainstem slices were used to determine concentration-response relationships for the selected GIRK channel blockers on 5-HT1A autoreceptor-activated inwardly rectifying K+ conductance in rat dorsal raphe 5-HT neurons. 5-HT1A autoreceptor-activated GIRK conductance was completely blocked by the nonselective inwardly rectifying potassium channels blocker Ba2+ (EC50 = 9.4 μM, full block with 100 μM) and by SCH23390 (EC50 = 1.95 μM, full block with 30 μM). GIRK-specific blocker tertiapin-Q blocked 5-HT1A autoreceptor-activated GIRK conductance with high potency (EC50 = 33.6 nM), but incompletely, i.e. ~16% of total conductance resulted to be tertiapin-Q-resistant. U73343 and SCH28080, reported to block GIRK channels with submicromolar EC50s, were essentially ineffective in 5-HT neurons. Our data show that inwardly rectifying K+ channels coupled to 5-HT1A autoreceptors display pharmacological properties generally expected for neuronal GIRK channels, but different from GIRK1-GIRK2 heteromers, the predominant form of brain GIRK channels. Distinct pharmacological properties of GIRK channels in 5-HT neurons should be explored for the development of new therapeutic agents for mood disorders.
... In anaesthetized rats the 5-HT cells of DRN were given precise electrophysiological characterization by Aghajanian and colleagues. The discharge pattern of these neurons is characterized by the regular, slow (0.5-2.5 spikes per second) firing rate and the long duration of bi-triphasic action potential (2-5ms) conductance (Aghajanian, Lakoski 1984;Burlhis, Aghajanian 1987). ...
... The Gi protein inhibits adenylyl cyclase, the enzyme that catalyzes synthesis of the second messenger cyclic adenosine monophosphate (cAMP). It is, however, worth noting that the inhibitory action of 5-HT and 5-HT 1A agonist on the firing rate of raphe 5-HT neurons is mediated by opening the K + channels, thereby increasing conductance to potassium (Aghajanian and Lakoski, 1984). ...
Among several serotonin receptors, the 5-HT2B is one of the less studied in the brain. Although this receptor is only weakly expressed in the nervous system, it appears to be involved in several biological mechanisms linked to neuropsychiatric dysfunctions and/or action of several drugs. Indeed, the selective serotonin reuptake inhibitors (SSRIs) act by blocking the serotonin transporter, but their acute and chronic effects are specifically blunted when the 5-HT2B receptor is pharmacologically blocked or when its gene is knocked-out. This lack of effect is also observed in mice submitted to chronic stress paradigms that induce depressive-like states. On the contrary, response to non-serotonergic antidepressants are conserved in mice lacking a functional 5-HT2B receptor. Recent studies have demonstrated that 5-HT2B receptors are expressed by serotonergic neurons, and that they can act as positive autoreceptors, counterbalancing the actions of the negative 5-HT1A autoreceptors. In addition, elegant studies confirm that it is the 5-HT2B autoreceptor that is required for SSRIs effects. All in all, the specific lack of responses to SSRIs exhibited by mice knocked-out for the 5-HT2B receptor make these animals an ideal model for the study of resistance to SSRIs antidepressants, a condition highly reported in clinics. Besides, the impaired response to serotonergic antidepressants, mice constitutively lacking the 5-HT2B receptor have increased basal levels of BDNF in the hippocampus. The “antidepressant-like phenotype” exhibited by these mice offers an interesting tool for the study of new molecules potentially acting as antidepressants.
The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.
5-HT neurotransmission system is targeted by drugs useful in behavioural disorders, including anxiety, depression, psychosis and eating disorders. 5-HT1A autoreceptors, located on 5-HT neurones of the midbrain raphe nuclei, are coupled to K channels through a pertusis toxin-sensitive G-protein. 5-HT1A receptor agonists inhibit adenylyl cyclase, while 5-HT2C receptor agonists activate two signal transduction pathways coupled with these receptors. 5-HT1A and 5-HT2C receptors have lots potential in treating the disorders with less or no side effects. Keywords: 5-HT1A, 5-HT2C, Receptor.
The topic of this chapter has been growing by leaps and bounds in the last decade. An enormous amount of information has been generated which cannot be reviewed comprehensively in a relatively short space. (A recent survey of transmitters in only a small region of the brain, the olfactory bulb (Halasz and Shepherd, 1983), covers 40 pages and lists 320 references!) The aim therefore will be to summarise the main points of importance concerning the probable transmitters and their modes of action. No attempt will be made at a systematic historical survey: for greater details of the earlier history, readers are referred to some comprehensive reviews that appeared 12 years ago (Curtis and Johnston, 1974; Kmjević, 1974a; Tebecis, 1974).
The physiological processes of the cerebral cortex change with shifts in states of vigilance. The matter of this chapter is the fluctuation in the electrical activity of the cerebrum during various behavioral conditions, as reflected in EEG waves, evoked potentials, and activities of different types of thalamic and cortical neurons. I shall discuss the alterations in field potentials and cellular events as they appear during various stages of the waking—sleep cycle. The state of quiet or resting sleep with high-amplitude and synchronous EEG waves (EEG-synchronized sleep) will be treated as opposed to both EEG-desynchronized states of wakefulness and rapid-eye-movement (REM) sleep when dreaming episodes occur.
It is beyond doubt that major depression can be treated by selectively manipulating the function of different brain neurotransmitters (1,2). Yet, of the many brain neuronal systems, the serotonin (5-hydroxytryptamine, 5-HT) system is the most common neurobiological target for such treatments. Tricyclic antidepressants (TCAs) act on 5-HT and noradrenergic (NE) neurons by inhibiting, with different potencies, transmitter reuptake (3,4), and monoamine (MAO) oxidase inhibitors (MAOIs) increase 5-HT and NE transmission by preventing their metabolism. Yet, it was not until the advent of the selective serotonin reuptake inhibitors (SSRIs) that the antidepressant potential of 5-HT transporter blockade was fully appreciated (5,6). The clinically useful SSRIs are chemically dissimilar, but share the property of selectively inhibiting the 5-HT reuptake process (Fig. 1) Unlike TCAs, the SSRIs display little affinity for aminergic receptors (7) and therefore lack the severe side effects associated with the use of the former agents. This results in both an improved quality of life for the patients and greater treatment compliance, which is compromised in some instances by the use of TCAs. It is generally recognized that the antidepressant efficacy of SSRIs is comparable to that of TCAs, although several studies have shown that the latter are more effective in severely depressed inpatients (5,6,8,9).
The past decade has seen a remarkable growth in our understanding of the pharmacology and physiology of the various receptors for serotonin (5-hydroxytryptamine, 5-HT). Since 1979, when radioligand binding techniques were used to distinguish subtypes of 5-HT binding sites (Peroutka and Snyder, 1979), no less than ten subtypes of 5-HT receptors have been characterized (Richardson and Engel, 1986; Bradley et al., 1986; Mawe et al., 1986; Heuring and Peroutka, 1987; Dumuis et al., 1988b; Leonhardt et al., 1989; Conner and Monsour, 1990). However, the biochemical characterization of these receptors has been hampered by the lack of selective radioligands and/or cell lines expressing single well-characterized receptor subtypes.
Within the past decade, molecular cloning techniques have confirmed that 5-hydroxytryptamine (5-HT) receptor subtypes, originally predicted from radioligand binding and functional studies (e.g., 5-HT1, 5-HT2, 5-HT3, 5-HT4), represent separate and distinct gene products. This knowledge has had a crucial impact on electrophysiological approaches to the 5-HT system in two important ways: (1) studies on previously recognized 5-HT receptors are now being directed, through the use of in situ mRNA hybridization and immunocytochemical maps, more precisely toward neurons that express these specific 5-HT receptor subtypes and (2) the functional role of previously unrecognized receptors (e.g., 5-HT5, 5-HT6, 5-HT7) can now be explored. Depending on the expression pattern for each type of neuron, the various 5-HT receptor subtypes can interact with their own set of G proteins, second messengers, and ion channels, to give rise to the wide range of electrophysiological actions produced by 5-HT throughout the brain and spinal cord. In addition, it is becoming evident that more than one 5-HT receptor sub-type may be expressed by the same neuron or by different neurons within the same region. Thus, while the following review is organized primarily according to individual 5-HT receptor subtypes, interactions between different receptor subtypes within a single neuron or region are also discussed where appropriate.
Full appreciation of CNS drug actions is dependent on understanding the basic physiology of their targeted neurotransmitter system(s). For the past 20 years, we have been studying the factors that regulate the functional activity of the brain serotonin system in behaving animals. This chapter describes our research efforts in three subject areas:
1.
Studies examining the effects of physiological, environmental, and behavioral manipulations on serotoninergic neuronal activity in behaving cats.
2.
A complementary program to this electrophysiology, which employs similar manipulations, but uses in vivo microdialysis to measure changes in extracellular levels of serotonin in various “postsynaptic” brain areas in behaving rats or cats.
3.
Experiments that examine drug effects on serotoninergic neuronal activity in behaving cats in order to elucidate the pharmacology and physiology of the somatodendritic 5-HT1A autoreceptor.
Overall, this research has provided us with a unique perspective for drug action on the CNS serotoninergic system and, more broadly, on the basic role of serotonin in physiology and behavior.
the loss of wakefulness, or the onset of sleep, causes significant changes in the central regulation of autonomic function. Because of the global nature of these state-dependent changes in neural control, the output of many organ systems is altered during sleep. The fact that multiple organ systems may be simultaneously affected by changes in states of consciousness underscores the potential clinical relevance of these sleep-dependent changes in physiology. Recognition of this relevance has spawned the development of a new clinical subspeciality (31, 50, 53) referred to by some as sleep-disorders medicine (19). In addition to sleep disorders per se (52), considerable evidence documents cardiovascular (16, 57; see chapters 3 and 5 in this volume) and respiratory (10; see chapters 7 and 8 in this volume) disorders that occur during sleep. The chapters comprising this volume illustrate both the widespread nature of sleep-dependent changes in physiology and the potential relevance of these physiological changes for clinical medicine. This chapter briefly outlines some of the conceptual and historical factors that have contributed to the cellular study of sleep-dependent changes in central autonomic control. The purpose of the present chapter is to introduce this volume by illustrating an investigative approach that seeks to unify cellular level analyses with integrative physiology.
Subtypes of brain serotonin (5-HT) receptors mediate distinct electrophysiological effects via different effector mechanisms: 1) 5-HT1 receptors induce a slow hyperpolarization via pertussis toxin-sensitive G proteins coupled to the opening of K+ channels; 2) 5-HT2 receptors induce a slow depolarization, partly by a closing of K+ channels negatively modulated by a stimulation of phosphoinositide (PI) hydrolysis; 3) 5-HT3 receptors induce a response distinct from the others, a fast, rapidly desensitizing excitation.
Serotonin (5-hydroxytryptamine; 5-HT), along with dopamine and norepinephrine, is one of the biogenic amines. But unlike dopamine and norepinephrine, which are catecholamines, 5-HT is derived from an indole nucleus and is therefore classified as an indoleamine. Although the major focus of current research on serotonin concerns its role in the nervous system, historically the amine became known by viture of its presence in the blood and gut (see Cooper et al., 1978; Douglas, 1980).
The possibility that multiple serotonin (5-HT) receptors exist in the CNS was first suggested by microiontophoretic studies in the cerebral cortex (Roberts and Straughan, 1967) and subcortical regions (Haigler and Aghajanian, 1974). In these early experiments, putative 5-HT antagonists, such as methysergide and cinanserin, blocked excitatory but not inhibitory effects of 5-HT, indicating that at least two types of 5-HT receptors may be present in the brain, one for excitation and one for inhibition. Subsequently, radioligand-binding techniques disclosed the presence of multiple 5-HT binding sites in the brain. As reviewed elsewhere in this volume, this conclusion derives from the finding that the neuroleptic agent [3H]spiperone labels 5-HT binding sites in the frontal cortex (Creese and Snyder, 1978; Leysen and Laduron, 1977; Leysen et al., 1978) that exhibit features distinct from those sites labeled by [3H]5-HT itself (Peroutka and Snyder, 1979). At the [3H] spiperone-labeled site, putative 5-HT antagonists display nanomolar affinities, whereas 5-HT and 5-HT agonists bind in the micromolar concentration range (Peroutka and Snyder, 1983). Conversely, 5-HT and certain 5-HT agonists display more potent binding affinities than antagonists at the [3H]5-HT-labeled site.
Antidepressant drugs mainly target plasma membrane transporters for monoamines with various selectivities for serotonin, noradrenaline and dopamine, and inhibit reuptake of these amines after their release, thus increasing their extracellular levels. In addition, antidepressant drugs share the property of being able to block the serotonin-gated cation channel (5-HT3 receptor). Drugs targeting receptors for substance P or corticotrophin-releasing factor have also been proposed as antidepressants. Chronic treatments with antidepressants induce changes in the activity of serotonin and dopamine neurons, notably by desensitizing inhibitory autoreceptors. After chronic antidepressant treatment, all these primary interactions seem to result in convergent adaptive changes mediated through the cyclic AMP/ CREB/BDNF cascade. These changes are responsible for enhancement of the responsiveness to the mesolimbic dopamine system mediating motivation and reward, and for an increase of neurogenesis in the hippocampus, which is involved in the control of the hypothalamus-pituitary-adrenal axis mediating stress responses. This rather coherent chain of events accounts for most of the observations made in animals receiving chronic antidepressant treatments, and has received some, as yet limited, support from clinical studies.
Soon after the development of binding assays using [3H]5-hydroxytryptamine ([3H]5-HT) to label the specific recognition sites for serotonin on membranebound receptors in the central nervous system (CNS), differences from one brain area to another were noted which suggested the existence of several distinct classes of binding sites. In particular, comparison of the fate of [3H]5- HT high-affinity binding sites in the rat brain after the selective degeneration of serotoninergic neurones due to the intra-raphe infusion of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) showed that both the hippocampal and striatal sites are located on postsynaptic targets of serotoninergic projections (Nelson et al. 1978), where they are subjected, however, to differential adaptive changes after the lesion. Thus a significant increase in the density of [3H]5-HT high-affinity binding sites was noted in the hippocampus but not in the striatum (Nelson et al. 1978). Studies of the pharmacological properties of [3H]5-HT high-affinity binding also revealed that the radioactive indoleamine probably recognises several distinct high-affinity sites in brain membranes. Indeed, inhibition of [3H]5-HT high-affinity binding by drugs such as methiothepin and quipazine yielded (apparent) Hill coefficients of less than 1.0, as expected of the heterogeneity of corresponding binding sites (Nelson et al. 1978).
This chapter discusses the recent literature of chemistry and pharmacology of potassium (K) channel openers (PCO's). Electrophysiologic techniques have shown that K channels are modulated via three general avenues: by ligands, voltage, and G proteins. Finally, a variety of chemical signals activate specific receptors that in turn modulate K channels via G protein-coupled mechanisms. A wide variety of synthetic compounds are also capable of nonspecifically blocking K channels, and recently compounds have become available that appear more selective for specific subtypes of K channels. Several drugs that had been previously described as “nonspecific vasodilators” have been found to exert their effects, by increasing K conductance. Smooth muscles, in general, contain a variety of K channel subtypes with high conductance (250-300 pS) Ca-activated K channels predominating. The specific K channel subtype(s) mediating the vasodilator effects of PCOs is under active investigation. PCO's have important pharmacological effects on the uterus, bronchial smooth muscle, urogenital tract, and on gastrointestinal smooth muscle. K channels have been extensively studied also in cardiac muscle. An adenosine triphosphate (ATP)-inhibited K channel is prominent in pancreatic β-cells. This channel is normally open and is responsible for the relatively high resting membrane potential of these cells. Because of the prevalence and extreme diversity of K channels, PCO's could have a variety of potentially useful clinical effects. Because of their ability to relax the vascular smooth muscle, these agents may be useful in the treatment of asthma, in the treatment of bladder hyperreflexia, in ischemic bowel syndrome, and in the treatment of alopecia areata.
An animal model is a representation of some aspects of a human disease. Modeling mental disease is problematic in that the symptoms may be poorly defined and the underlying pathophysiology poorly understood. This is especially true of depression, which is the name given to a heterogeneous group of disorders having in common the disturbance of mood (Cronholm, 1984; Klerman, 1984; Baldessarini, 1985). Models of depression are made primarily for two reasons: to allow experimental manipulation of behavioral and biochemical variables that might provide insight into the etiology and underlying pathophysiology of the disease and to allow prediction of how a variable such as an antidepressant drug might affect the disease (Everitt and Keverne, 1979).
It has been well known for several years that an important inhibitory mechanism in mammalian nervous tissue is an increase in the conductance of the membrane to chloride ions (Coombs et al. 1955). The two major substances involved in this mechanism are GABA and glycine and it is now known that their action is exerted through membrane receptors which incorporate the chloride ion channel (Grenningloh et al. 1987; Schofield et al. 1987). The role of potassium conductance increase in neuronal inhibition has been less clearly demonstrated except for some notable work in the invertebrate nervous system (Kehoe, 1972) and, particularly, in the mechanism of vagal inhibition of the heart (Trautwein & Dudel, 1958). Whereas the effects of agents that increase chloride conductance may be either excitatory or inhibitory, depending on the absolute magnitude of the chloride equilibrium potential, an increase in the cell conductance to potassium ions is universally inhibitory if it is sufficiently strong. This chapter describes some examples of synaptic inhibition mediated by potassium conductance increase in mammalian neurones, and points out that this mechanism of neuronal inhibition seems likely to be common to the action of agonists at an increasing number of neurotransmitter receptors.
Based on electrophysiological, neurochemical, and neuropharmacological approaches, it is currently accepted that serotonin (5-HT) functions to promote waking (W) and to inhibit (permissive role) REM sleep (REMS). Serotonergic neurons of the dorsal raphe nucleus (DRN) fire at a steady rate during W, decrease their firing during slow-wave sleep (SWS), and virtually cease activity during REMS. Serotonin released during W activates 5-HT1A somatodendritic receptors and 5-HT2A/2C receptors expressed by GABAergic interneurons, and induces a decrease of the firing rate of 5-HT cells characteristic of SWS. In addition to local inhibitory circuits, GABAergic neurons of the ventrolateral preoptic nucleus play a role in the deactivation of the 5-HT and all other arousal systems, which results in the occurrence of REMS. Studies on the effects on REMS of direct administration of selective 5-HT1A (8-OH-DPAT, flesinoxan), and 5-HT2A/2C (DOI) receptor agonists into the DRN tend to indicate that quite different mechanisms are involved in their effects. Direct infusion of 8-OH-DPAT or flesinoxan into the DRN significantly enhances REMS, and this effect is prevented by local infusion of the selective 5-HT1A receptor antagonist WAY 100635. In agreement with the reciprocal interaction hypothesis of REMS generation, inhibition of DRN serotonergic neurons following somatodendritic 5-HT 1A receptor stimulation suppressed 5-HT inhibition of mesopontine cholinergic neurons and increased REMS. Infusion of DOI into the DRN induces a significant reduction of REMS in the rat. Pretreatment with selective 5-HT 2A and 5-HT2C receptor antagonists prevents the DOIinduced suppression of REMS. Serotonin-containing neurons of the DRN do not express 5-HT2A or 5-HT2C receptors. The 5-HT2A and 5-HT2C receptor-containing neurons are predominantly GABAergic interneurons and projection neurons. Since DOI inhibits the firing of serotonergic neurons in the DRN and reduces the extracellular concentration of 5-HT, it can be proposed that the DOI activation of long-projection GABAergic neurons that express 5-HT2A/2C receptors would be responsible for the inhibition of cholinergic cells in the laterodorsal tegmental and pedunculopontine tegmental nuclei (LDT/PPT) and the suppression of REMS. Microinjection of 8-OH-DPAT or flesinoxan into the LDT/PPT induces an inhibitory response on target neurons and the suppression of REMS. Moreover, infusion of DOI into the LDT/PPT selectively decreases REMS. In this respect, activation of 5-HT2A/2C receptors expressed by GABAergic interneurons in the LDT/PPT would produce the local release of GABA and the reduction of the behavioral state.
The involvement of the 5-HT1A and the 5-HT1B receptor in the regulation of sleep and waking is complex due to a multitude of presynaptic and/or postsynaptic actions also involving other neurotransmitter systems. Both receptors produce an important inhibitory feed back to the serotonergic raphe neurons. Overall, most studies support the possibility that stimulation of postsynaptic 5-HT1A receptors, e.g., via systemic administration of a high dose of agonists increases wakefulness and decreases sleep. Local administration of agonists in dorsal raphe nucleus mainly produces a response similar to the “low-dose” systemic administration, decreasing wakefulness and increasing rapid eye movement (REM) sleep via disinhibition of mesopontine REM sleep promoting neurons. Systemic administration of 5-HT1B receptors agonists consistently increases wakefulness and decreases REM sleep, as do the 5-HT1A agonists. The mechanism by which 5-HT1B receptors affect state modulation remain elusive. The general arousing effects of 5-HT1A and 5-HT1B agonists should also be considered in relation to the multiple, largely redundant, neurotransmitter systems that maintain arousal. Finally, 5-HT1A and 5-HT1B receptor are important modulators of the circadian rhythm largely by affecting the response of the suprachiasmatic nucleus to light and the secretion of melatonin from the pineal gland. The development of more selective ligands seems crucial to further explore the role of these receptors in state modulation.
The purpose of this study was to ascertain the identity of presumed noradrenergic or serotonergic neurons recorded by single cell techniques in the mammalian brain. A double labeling method was developed in which intracellular injections of a red fluorescing dye (ethidium bromide) could be co-localized with the formaldehyde-induced green fluorescence of norepinephrine or yellow fluorescence of serotonin. By this method, neurons of the rat locus coeruleus that display a characteristic activation-inhibition response to noxious stimuli were confirmed to be noradrenergic; the slow, rhythmically firing neurons of the dorsal raphe nucleus were confirmed to be serotonergic.
[3H] Serotonin (5 HT) binds to membrane preparations of rat brain in a saturable fashion and with substrate specificity and regional variations consistent with its binding to the postsynaptic serotonin receptor. The dissociation constant for [3H]5 HT binding is about 8 nM, and the total number of 5 HT binding sites in the brain is 16 pmoles/g of tissue, wet wt. There is considerable structural specificity in the affinity of various tryptamines for the [3H]5 HT binding sites, with a crucial role played by the 5 hydroxy substituent. d [3H]Lysergic acid diethylamide (LSD) binding sites have substrate specificity requirements similar to the [3H]5 HT binding sites, but the 5 hydroxy substituent is less critical. 5 HT and related agonists have about 100 times more affinity for 5 HT than LSD binding sites, while classical 5 HT antagonists have 4 to 100 times greater affinity for LSD binding sites. LSD itself has a similar affinity for 5 HT and LSD binding sites. Raphe lesions which result in degeneration of 5 HT neurons do not lower [3H]5 HT binding, indicating that binding does not take place to presynaptic 5 HT neurons. Regional variations in serotonin and LSD binding are fairly similar. Highest binding occurs in the corpus striatum, hippocampus, and cerebral cortex, with lowest binding in the cerebellum. The ontogeny of 5 HT and LSD binding sites is nearly identical and does not appear to depend on functionally intact presynaptic 5 HT neuronal input.
[3H]5-hydroxytryptamine (5-HT), [3H]lysergic acid diethylamide (LSD) and [3H]spiroperidol bind to membranes from the rat frontal cerebral cortex in a manner indicating a selective interaction with serotonin receptors. Differential drug potencies in competing for [3H]5-HT and [3H]spiroperidol binding sites suggest that these two [3H]ligands respectively label two distinct populations of receptors, while [3H]LSD labels both the [3H]5-HT and [3H]spiroperidol sites. After incubation of brain membranes with 30 nM spiroperidol, drug specificity of the residual [3H]LSD binding resembles that of receptors labeled by [3H]5-HT. Conversely, drug effects on [3H]LSD binding in the presence of 300 nM 5-HT resembles effects with [3H]spiroperidol. We propose that [3H]5-HT and [3H]spiroperidol label distinct populations of serotonin receptors in rat brain, designated 5-HT1 and 5-HT2 receptors, respectively. [3H]LSD appears to bind to both receptors to a similar extent.
A novel perfusion chamber is described for studying the physiology and pharmacology of brain slices maintained in vitro. The chamber is easy to make and allows stable, high quality intracellular recording and rapid exchange of the perfusion fluid.
Lysergic acid diethylamide (LSD) and serotonin (5HT) were applied directly by microiontophoresis to 5HT-containing neurons in the midbrain raphe nuclei. The firing of these neurons was markedly inhibited by both LSD and 5HT. The effects of LSD given systemically and microiontophoretically were similar. This suggests that the inhibition of raphe neurons which occurs after the systemic administration of LSD could be due to a direct action of the drug.
A variety of drugs reported to antagonize serotonin were found to affect spinal cord potentials evoked by electrical stimulation of the caudal raphe nuclei of the cat. These brain stem-evoked dorsal root potentials (DRPs) consisted of a short latency depolarization (DRP-1), which was evoked by stimulation of a wide variety of sites in the medial brain stem and a long latency potential (DRP-2), which was elicited only when stimuli were applied near the raphe. The ability of serotonergic antagonists to increase or decrease these DRPs was dependent on the dose of the drug administered. High doses of lysergic acid diethylamide tartrate (LSD), 2-bromo-D-lysergic acid diethylamide bitartrate (BOL), methysergide and cinanserin each produced an immediate inhibition of DRP-2 and a simultaneous enhancement of DRP-1, both of which recovered by approximately 30 min. Each of the drugs produced a dose-related inhibition of DRP-2 at high doses, with LSD being the most potent and cinanserin the least potent. In contrast, low doses of LSD, BOL and methysergide elicited little or no immediate change in either DRP-2 or DRP-1, but produced an enhancement of DRP-2 which developed slowly over a period of 60 to 90 min. This increase in DRP-2 was most dramatic after administration of LSD and was not accompanied by changes in DRP-1. The inhibition of DRP-2 by high doses of LSD, BOL, methysergide and cinanserin may result primarily from inhibition of postsynaptic serotonergic receptors located on the primary afferent terminals. The increase in DRP-2 produced by low doses of LSD, BOL and methysergide is postulated to result from an interaction with receptors distinct from those which produced the inhibition of DRP-2 at higher doses.
Intracellular recordings were obtained from facial motoneurons in anesthetized rats. The effects of iontophoretically applied serotonin were compared to those of the excitatory amino acids glutamate and DL-homocysteic acid (DLH), and the inhibitory amino acids, glycine, GABA and muscimol, under various conditions of membrane polarization and intracellular chloride concentration. Iontophortically applied serotonin caused a depolarization of facial motoneurons which was accompanied by increased input resistance and increased neuronal excitability. Experiments comparing the response to serotonin with those of glycine, GABA, and muscimol demonstrated that the serotonin effect does not involve changes in membrane conductance to chloride. Comparisons of serotonin with glutamate and DLH at varying levels of membrane hyperpolarization indicated that the serotonin-induced depolarization is not caused by increased conductance to sodium or calcium, and differs in its underlying ionic mechanism from depolarizations induced by glutamate and DLH. Results were consistent with the hypothesis that serotonin causes depolarization, increased input resistance, and increased excitability in rat facial motoneurons by decreasing resting membrane conductance to potassium ions. Such changes in motoneurons in the brain stem and spinal cord probably account for some of the physiological and behavioral effects observed during pharmacological activation of serotonin receptors.
Extracellular and intracellular recordings were made from dorsal raphe (DR) neurons in frontal rat brain slices maintained in vitro. A population of neurons was found which displayed electrophysiological and pharmacological characteristics of serotonin-containing DR neurons recorded in vivo. Recorded extracellularly, these neurons displayed biphasic or triphasic action potentials of 1.5-3.0 ms duration, and discharged with a slow and steady rhythm. Recorded intracellularly these neurons displayed action potentials of about 1.8 ms duration, which were followed by large (10-20 mV) after hyperpolarizations which normally lasted 200-800 ms. These presumed serotonergic DR neurons were inhibited by LSD and serotonin. They were excited by norepinephrine, or the alpha-agonist phenylephrine, and these activations could be reduced or blocked by alpha-adrenoreceptor antagonists including the selective alpha 1-antagonist, prazosin. The major difference between the in vitro recordings and previous in vivo recordings from anesthetized animals was a reduction in the number of spontaneously firing DR neurons. This was probably due, at least in part, to a disfacilitation of serotonergic DR neurons in the slice caused by the functional removal of a tonic noradrenergic input.
1. Intracellular activity was recorded from neurones in the CA1 pyramidal layer of slices of rat hippocampus maintained in vitro. 2. Application of 5-HT in a droplet or via ionophoresis produced a 3-5 mV hyperpolarization associated with a 30% decrease in input resistance. 3. The response to 5-HT was minimal with a drop concentration of 1 microM and maximal with 100 microM. The responses appeared to be blocked by methysergide applied in the superfusion medium. 4. The responses to 5-HT were minimal when the drug was applied in the apical dendritic region and maximal when it was applied near the soma. 5. 5-HT produced no substantial changes in e.p.s.p.s evoked by stimulation of the Schaffer collateral-commissural system or in i.p.s.p.s which were occasionally encountered following stimuli to the stratum radiatum. 6. The responses to 5-HT are true post-synaptic responses and are not indirect effects since they are present in a Ca2+-deficient Mg2+-enriched medium which blocks synaptic transmission. 7. The responses to 5-HT were not dependent on extracellular Cl- concentration. 8. These experiments indicate that 5-HT produces its effects in the rat hippocampus by activating K+ channels.
Intracellular recordings in vivo from serotonergic dorsal raphe neurons of the rat brain reveal that these cells undergo a pronounced postspike hyperpolarization followed by a gradual interspike depolarization leading to the succeeding spike. Such repetitive cycles of interspike hyperpolarization and depolarization, which can be termed "pacemaker potentials', can account for the automaticity of these cells. When serotonergic neuronal firing is inhibited by LSD, such pacemaker potentials no longer occur and the cells remain in an hyperpolarized state.
A slice preparation of the rat mesencephalon containing the dorsal and medial raphe nucleus, the periaqueductal grey, the superior colliculi and the reticular formation is described. Intracellular recordings showed marked differences in the passive membrane properties of neurones of the dorsal raphe. Serotonin-containing neurons were characterized by a high membrane input resistance, a very long time constant and by the presence of membrane rectification only in a very hyperpolarized (less than -120 mV) region of their voltage-current relationship. In most of the neurones in and around the dorsal raphe area a brief pulse of depolarizing current was followed by a pronounced after-hyperpolarization, which appeared to be mediated by the activation of a Ca2+ dependent-K+ conductance.
Neurones from the supraoptic nucleus in the rat hypothalamic slice preparation in vitro have been recorded from using conventional extracellular techniques. Neurones which fired in a slow continuous manner (putative oxytocinergic cells) increased their firing pattern in response to dopamine applied in the perfusate or pressure ejected onto the slice. Of these neurones, 85% were inhibited by spiperone, haloperidol or cis-flupenthixol applied in the bath. A small proportion (approximately 15%) of neurones excited by dopamine (DA) were not affected by these DA antagonists but were inhibited strongly by (+/-)-sulpiride. Similar results were also obtained on a small number of cells recorded from an 'island-slice' preparation, utilized to isolate the observed responses to an area immediately adjacent to the supraoptic nucleus. These results may be relevant to earlier work on in vivo anaesthetized preparations using intracerebroventricular injections of DA and antagonists, and indicate that such substances may act directly on a synapse(s) within the magnocellular nucleus in addition to other possible actions within the neurohypophyseal tract. The differential response to the dopamine antagonists indicate that the majority of continuously firing neurones in the supraoptic nucleus are of the DI receptor class, but the presence of possible D2 receptors on a small proportion of these neurones suggests that a second type of continuously firing neurone may exist in this nucleus.
The present investigation was designed to determine the effect of hallucinogens on the facilitating action of serotonin (5-HT) and norepinephrine (NE) in the facial nucleus. Intravenous administration of d-lysergic acid diethylamide (LSD, 5–10 μg/kg), mescaline (0.5–1.0 mg/kg), or psilocin (0.5–1.0 mg/kg) had no effect by themselves on the glutamate-induced excitation of facial motoneurons. In contrast, the facilitation of facial neuron excitation by iontophoretically applied 5-HT and NE was enhanced 6–10 fold by these hallucinogens. The LSD-enhanced responses to 5-HT and NE continued for at least 4 hours after administration of the hallucinogen. Iontophoretic application of LSD or mescaline (low currents) also markedly potentiated the facilitating effect of 5-HT and NE. Higher currents of LSD (15–40 nA) temporarily antagonized the response to 5-HT. The nonhallucinogen ergot derivatives lisuride and methysergide failed to potentiate the facilitating effects of 5-HT or NE. These observations suggest that hallucinogens potentiate the effect of monoamines on facial motoneurons by increasing the sensitivity of 5-HT and NE receptors. A novel mechanism regarding the psychedelic effects of hallucinogens is discussed.
The effects of serotonin (5-HT) on extracellular potassium concentration ([K+]0) were measured with ion-selective microelectrodes in rat hippocampal slices. Electrical stimulation of an excitatory afferent system, the Schaffer collateral commissural pathway, caused a 2–4 mM rise in [K+]0 in the stratum pyramidale of area CA1. 5-HT caused a 0.6–1.1 mM rise in [K+]0. This rise was associated with hyperpolarization of neurons and cessation of their spontaneous spike discharge. Methysergide, a 5-HT antagonist, reduced the 5-HT effect. The change in [K+]0 was highest in stratum moleculare and lowest in stratum pyramidale, the opposite gradient to that found with excitatory electrical stimulation. The 5-HT-induced [K+]0 changes were maximal in CA1 stratum moleculare, intermediate in the dentate stratum granulare and almost non-existent in the CA3 stratum pyramidale.GABA, but not norepinephrine, produced a small (up to 0.5 mM) rise in [K+]0 in stratum pyramidale. Extracellular calcium concentration measured with a Ca2+-sensitive microelectrode was reduced by electrical stimulation but unchanged by 5-HT or norepinephrine. It is suggested that 5-HT hyperpolarizes hippocampal cells by activation of sodium- and calcium-independent potassium channels, which cause a rise in [K+]0.
The application of serotonin to certain myenteric plexus neurones in the guinea pig small intestine causes a slow depolarization of membrane potential, accompanied by increased neuronal excitability and input resistance. On the other hand, microiontophoretic application of large amounts of serotonin onto mammalian spinal motoneurones is reported to cause membrane hyperpolarization and decreased excitability. However, on the basis of recording spinal reflex activity, serotonin has been reported to enhance net motoneurone activity. Moreover, studies using extracellular single-cell recording techniques indicate that serotonin in small amounts facilitates synaptically or glutamate-induced excitation of mammalian motoneurones in the facial nucleus and spinal cord. It was suggested that these facilitatory actions were modulatory in nature, as serotonin did not induce motoneurone spiking in the absence of extrinsic excitatory input. The study reported here investigated the membrane mechanisms underlying these modulatory effects by obtaining intracellular recordings from rat facial motoneurones during extracellular microiontophoretic application of serotonin, methysergide (a serotonin antagonist) and noradrenaline. Serotonin caused a slow depolarization of membrane potential of about 5 mV which remained sub-threshold, accompanied by an increase in electrical excitability of the neurone, and an increase in input resistance. Noradrenaline caused the same changes. Methysergide antagonized the effects of serotonin, but not noradrenaline, indicating that these actions of serotonin are selective and receptor mediated.