Localization of serotonin-4 receptors in the striatonigral pathway in rat brain

The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents

BORLAND, J.M., The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents, NEUROSCI BIOBEH REV 21(1) XXX-XXX, 2024.-Sociality shapes an organisms' life. The nucleus accumbens is a critical brain region for mental health. In the following review, the effects of different types of social interactions on the physiology of neurons in the nucleus accumbens is synthesized. More specifically, the effects of sex behavior, aggression, social defeat, pair-bonding, play behavior, affiliative interactions, parental behaviors, the isolation from social interactions and maternal separation on measures of excitatory synaptic transmission, intracellular signaling and factors of transcription and translation in neurons in the nucleus accumbens in rodent models are reviewed. Similarities and differences in effects depending on the type of social interaction is then discussed. This review improves the understanding of the molecular and synaptic mechanisms of sociality.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Synthesis and structure-activity relationship studies in serotonin 5-HT4 receptor ligands based on a benzo[de][2,6]naphthridine scaffold

A small series of serotonin 5-HT4 receptor ligands has been designed from flexible 2-methoxyquinoline compounds 7a,b by applying the conformational constraint approach. Ligands 7a,b and the corresponding conformationally constrained analogues 8a-g were synthesized and their interactions with the 5-HT4 receptor were examined by measuring both binding affinity and the ability to promote or inhibit receptor-G protein coupling. Ester derivative 7a and conformationally constrained compound 8b were demonstrated to be the most interesting compounds showing a nanomolar 5-HT4R affinity similar to that shown by reference ligands cisapride (1) and RS-23,597-190 (4). The result was rationalized by docking studies in term of high similarity in the binding modalities of flexible 7a and conformationally constrained 8b. The intrinsic efficacy of some selected ligands was determined by evaluating the receptor-G protein coupling and the results obtained demonstrated that the nature and the position of substituents play a critical role in the interaction of these ligands with their receptor.

Visceral analgesic effect of 5-HT(4) receptor agonist in rats involves the rostroventral medulla (RVM)

The 5-HT(4) receptor agonist tegaserod (TEG) has been reported to modulate visceral pain. However, the underlying mechanism remains unknown. The objective of the present study was to examine the analgesic mechanism and site of action of TEG. In male rats, visceral pain was assessed by measuring visceromotor response (VMR) to colorectal distension (CRD). Inflammation was induced by intracolonic injection of tri-nitrobenzene sulfonic acid (TNBS). The effect of TEG on the VMR was tested by injecting intraperitoneal (i.p.), intrathecal (i.t.), intracerebroventricular (i.c.v) or in the rostroventral medulla (RVM). The effect of the drug was also tested on responses of CRD-sensitive pelvic nerve afferents (PNA) and lumbo-sacral (LS) spinal neurons. Systemic injection of TEG attenuated VMR in naive and TNBS-treated rats. Similarly, supraspinal, but not spinal, injection of TEG attenuated the VMR. While GR113808, (selective 5-HT(4) antagonist) blocked the effect, naloxone (NLX) an opioid receptor antagonist reversed the effect of TEG. Although i.t. NLX did not block the inhibitory effect of TEG in VMR study, i.t. injection of α2-adrenergic receptor antagonist yohimbine blocked the effect of TEG when given systemically. While TEG had no effect on the responses of CRD-sensitive PNA, it inhibited the responses of CRD-sensitive LS neurons in spinal intact condition. This inhibition was blocked by GR113808, NLX and β-funaltrexamine (β-FNA) when injected into the RVM. Results indicate that TEG produces analgesia via activation of supraspinal 5-HT(4) receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

Presynaptic control of serotonin on striatal dopamine function

RATIONALE

The influences of the serotonergic system on dopamine (DA) neuron activity have received considerable attention during the last three decades due to the real opportunity to improve disorders related to central DA neuron dysfunctions such as Parkinson's disease, schizophrenia, or drug abuse with serotonergic drugs. Numerous biochemical and behavioral data indicate that serotonin (5-HT) affects dopaminergic terminal function in the striatum.

OBJECTIVE

The authors propose a thorough examination of data showing controversial effects induced by striatal 5-HT on dopaminergic activity.

RESULTS

Inhibitory and excitatory effects of exogenous 5-HT have been reported on DA release and synthesis, involving various striatal 5-HT receptors. 5-HT also promotes an efflux of DA through reversal of the direction of DA transport. By analogy with the mechanism of action described for amphetamine, the consequences of 5-HT entering DA terminals might explain both the excitatory and inhibitory effects of 5-HT on presynaptic DA terminal activity, but the physiological relevance of this mechanism is far from clear. The recent data suggest that the endogenous 5-HT system affects striatal DA release in a state-dependent manner associated with the conditional involvement of various 5-HT receptors such as 5-HT(2A), 5-HT(2C), 5-HT(3), and 5-HT(4) receptors.

CONCLUSION

Methodological and pharmacological issues have prevented a comprehensive overview of the influence of 5-HT on striatal DA activity. The distribution of striatal 5-HT receptors and their restricted influence on DA neuron activity suggest that the endogenous 5-HT system exerts multiple and subtle influences on DA-mediated behaviors.

Modulation of midbrain dopamine neurotransmission by serotonin, a versatile interaction between neurotransmitters and significance for antipsychotic drug action

Schizophrenia has been associated with a dysfunction of brain dopamine (DA). This, so called, DA hypothesis has been refined as new insights into the pathophysiology of schizophrenia have emerged. Currently, dysfunction of prefrontocortical glutamatergic and GABAergic projections and dysfunction of serotonin (5-HT) systems are also thought to play a role in the pathophysiology of schizophrenia. Refinements of the DA hypothesis have lead to the emergence of new pharmacological targets for antipsychotic drug development. It was shown that effective antipsychotic drugs with a low liability for inducing extra-pyramidal side-effects have affinities for a range of neurotransmitter receptors in addition to DA receptors, suggesting that a combination of neurotransmitter receptor affinities may be favorable for treatment outcome.This review focuses on the interaction between DA and 5-HT, as most antipsychotics display affinity for 5-HT receptors. We will discuss DA/5-HT interactions at the level of receptors and G protein-coupled potassium channels and consequences for induction of depolarization blockade with specific attention to DA neurons in the ventral tegmental area (VTA) and the substantia nigra zona compacta (SN), neurons implicated in treatment efficacy and the side-effects of schizophrenia, respectively. Moreover, it has been reported that electrophysiological interactions between DA and 5-HT show subtle, but important, differences between the SN and the VTA which could explain (in part) the effectiveness and lower propensity to induce side-effects of the newer atypical antipsychotic drugs. In that respect the functional implications of DA/5-HT interactions for schizophrenia will be discussed.

Hyperfunction of muscarinic receptor maintains long-term memory in 5-HT4 receptor knock-out mice

Patients suffering from dementia of Alzheimer's type express less serotonin 4 receptors (5-HTR₄), but whether an absence of these receptors modifies learning and memory is unexplored. In the spatial version of the Morris water maze, we show that 5-HTR₄ knock-out (KO) and wild-type (WT) mice performed similarly for spatial learning, short- and long-term retention. Since 5-HTR₄ control mnesic abilities, we tested whether cholinergic system had circumvented the absence of 5-HTR₄. Inactivating muscarinic receptor with scopolamine, at an ineffective dose (0.8 mg/kg) to alter memory in WT mice, decreased long-term but not short-term memory of 5-HTR₄ KO mice. Other changes included decreases in the activity of choline acetyltransferase (ChAT), the required enzyme for acetylcholine synthesis, in the septum and the dorsal hippocampus in 5-HTR₄ KO under baseline conditions. Training- and scopolamine-induced increase and decrease, respectively in ChAT activity in the septum in WT mice were not detected in the 5-HTR₄ KO animals. Findings suggest that adaptive changes in cholinergic systems may circumvent the absence of 5-HTR₄ to maintain long-term memory under baseline conditions. In contrast, despite adaptive mechanisms, the absence of 5-HTR₄ aggravates scopolamine-induced memory impairments. The mechanisms whereby 5-HTR₄ mediate a tonic influence on ChAT activity and muscarinic receptors remain to be determined.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Endogenous serotonin excites striatal cholinergic interneurons via the activation of 5-HT 2C, 5-HT6, and 5-HT7 serotonin receptors: implications for extrapyramidal side effects of serotonin reuptake inhibitors

The striatum is richly innervated by serotonergic afferents from the raphe nucleus. We explored the effects of this input on striatal cholinergic interneurons from rat brain slices, by means of both conventional intracellular and whole-cell patch-clamp recordings. Bath-applied serotonin (5-HT, 3-300 microM), induced a dose-dependent membrane depolarization and increased the rate of spiking. This effect was mimicked by the 5-HT reuptake blockers citalopram and fluvoxamine. In voltage-clamped neurons, 5-HT induced an inward current, whose reversal potential was close to the K(+) equilibrium potential. Accordingly, the involvement of K(+) channels was confirmed either by increasing extracellular K(+) concentration and by blockade of K(+) channels with barium. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) profiling demonstrated the presence of 5-HT2C, 5-HT6, and 5-HT7 receptor mRNAs in identified cholinergic interneurons. The depolarization/inward current induced by 5-HT was partially mimicked by the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine and antagonized by both ketanserin and the selective 5-HT2C antagonist RS102221, whereas the selective 5-HT3 and 5-HT4 receptor antagonists tropisetron and RS23597-190 had no effect. The depolarizing response to 5-HT was also reduced by the selective 5-HT6 and 5-HT7 receptor antagonists SB258585 and SB269970, respectively, and mimicked by the 5-HT7 agonist, 5-CT. Accordingly, activation of either 5-HT6 or 5-HT7 receptor induced an inward current. The 5-HT response was attenuated by U73122, blocker of phospholipase C, and by SQ22,536, an inhibitor of adenylyl cyclase. These results suggest that 5-HT released by serotonergic fibers originating in the raphe nuclei has a potent excitatory effect on striatal cholinergic interneurons.

The anatomy of co-morbid neuropsychiatric disorders based on cortico-limbic synaptic interactions

Many brain disorders appear to involve dysfunctions of aminergic systems. Alterations in dopamine activity may underpin both schizophrenia and the establishment and maintenance of drug dependence while disruption of serotonergic signalling may be crucial in both depression and schizophrenia. The co-existence of nicotine and alcohol abuse with depression and schizophrenia is well-documented as is the particular vulnerability of adolescents. At the same time, a common group of brain structures is increasingly implicated in neuropathological studies. For example, depression may involve a lack of serotonin signalling, particularly in the prefrontal cortex, while in schizophrenia there is evidence for reduced dopamine signalling in the same brain region, co-existing with hyperactivity in the mesolimbic dopamine pathway. Increased dopamine release from the mesolimbic dopamine pathway is also a common factor of drugs of abuse. Furthermore, the control of motivational behaviour and dopamine release is apparently modified by hippocampal and amygdala activity, both brain regions showing pathological changes in schizophrenia and depression. Our work has focused on the intricate synaptic interactions of aminergic terminals and cortical and subcortical neurons in order to unravel the anatomical basis for these disorders and their treatments. We show convergence of dopamine and cortical inputs onto single neurons in the nucleus accumbens, and between different cortical inputs to individual neurons, providing a basis for the gating mechanisms attributed to these interactions. We have also examined local and extrinsic connections in the prefrontal cortex and the basis for regulation of both cortical neurons and midbrain dopamine neurons by serotonin from the raph é nucleus. Together with data concerning subcellular receptor distributions, this information provides a detailed synaptic framework for interpreting behavioural, pharmacological and physiological data and enhances our understanding of possible circuitry underlying comorbidity of disorders such as schizophrenia and depression with drug abuse, information invaluable in the introduction of enhanced therapies.

Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission

The neurotransmitter dopamine (DA) has a long association with normal functions such as motor control, cognition, and reward, as well as a number of syndromes including drug abuse, schizophrenia, and Parkinson's disease. Studies show that serotonin (5-HT) acts through several 5-HT receptors in the brain to modulate DA neurons in all 3 major dopaminergic pathways. There are at least fourteen 5-HT receptor subtypes, many of which have been shown to play some role in mediating 5-HT/DA interactions. Several subtypes, including the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT3 and 5-HT4 receptors, act to facilitate DA release, while the 5-HT2C receptor mediates an inhibitory effect of 5-HT on DA release. Most 5-HT receptor subtypes only modulate DA release when 5-HT and/or DA neurons are stimulated, but the 5-HT2C receptor, characterized by high levels of constitutive activity, inhibits tonic as well as evoked DA release. This review summarizes the anatomical evidence for the presence of each 5-HT receptor subtype in dopaminergic regions of the brain and the neuropharmacological evidence demonstrating regulation of each DA pathway. The relevance of 5-HT receptor modulation of DA systems to the development of therapeutics used to treat schizophrenia, depression, and drug abuse is discussed. Lastly, areas are highlighted in which future research would be maximally beneficial to the treatment of these disorders.

Adaptive changes in serotonin neurons of the raphe nuclei in 5-HT4receptor knock-out mouse

Decreased serotonin (5-HT) transmission is thought to underlie several mental diseases, including depression and feeding disorders. However, whether deficits in genes encoding G protein-coupled receptors may down-regulate the activity of 5-HT neurons is unknown currently. Based on recent evidence that stress-induced anorexia may involve 5-HT(4)receptors (5-HT(4)R), we measured various aspects of 5-HT function in 5-HT(4)R knock-out (KO) mice. When compared to dorsal raphe nucleus (DRN) 5-HT neurons from wild-type mice, those from 5-HT(4)R KO mice exhibited reduced spontaneous electrical activity. This reduced activity was associated with diminished tissue levels of 5-HT and its main metabolite, 5-hydroxyindole acetic acid (5-HIAA). Cumulative, systemic doses of the 5-HT uptake blocker citalopram, that reduced 5-HT cell firing by 30% in wild-type animals, completely inhibited 5-HT neuron firing in the KO mice. This effect was reversed by administration of the 5-HT(1A) receptor (5-HT(1A)R) antagonist, WAY100635, in mice of both genotypes. Other changes in DRN of the KO mice included increases in the levels of 5-HT plasma membrane transporter sites and mRNA, as well as a decrease in the density of 5-HT(1A)R sites without any change in 5-HT(1A) mRNA content. With the exception of increased 5-HT turnover index in the hypothalamus and nucleus accumbens and a decreased density of 5-HT(1A)R sites in the dorsal hippocampus (CA1) and septum, no major changes were detected in 5-HT territories of projection, suggesting region-specific adaptive changes. The mechanisms whereby 5-HT(4)R mediate a tonic positive influence on the firing activity of DRN 5-HT neurons and 5-HT content remain to be determined.

Serotonin 5-HT4receptors and their mRNAs in rat and guinea pig brain: Distribution and effects of neurotoxic lesions

Serotonin 5-HT4 receptors are widely distributed in the periphery and in brain, where they modulate the release of various neurotransmitters and have been implicated in learning and memory. Nine C-terminal splice variants of this receptor have been cloned in mammalian species. In the rat, three such variants have been described: 5-HT4(a), 5-HT4(b), and 5-HT4(e). In the present study, we have examined several aspects of the distribution of these receptors in brain. First, we provide, in rat and guinea pig, a detailed comparison of the distribution of 5-HT4 receptors labeled by the antagonist [125I]-SB 207710 with the distribution of their encoding mRNA visualized by in situ hybridization histochemistry (ISHH). The results suggest that, in several projection systems (striato-nigral and striato-pallidal pathways, projection from dentate granule cells to field CA3, habenulo-interpeduncular pathway), 5-HT4 receptors are located both somatodendritically and axonally. Second, we have analyzed the distribution of mRNA for the three known rat splice variants by reverse transcription-polymerase chain reaction (RT-PCR) and by ISHH. RT-PCR indicates that all three variants are widely distributed, with 5-HT4(b) mRNA being present in all regions examined (olfactory tubercle, striatum, hippocampus, inferior colliculus, substantia nigra, parietal cortex) and 5-HT4(a) and 5-HT4(e) showing a somewhat more restricted distribution. In other regions (periaqueductal gray, reticular formation, medial septum, diagonal band), faint ISHH signals are observed for 5-HT4(a)+4(e) mRNAs, whereas 5-HT4(b) mRNA signals are almost undetectable. Finally, neurotoxic lesions of basal ganglia components in guinea pig also indicate a location of these receptors on terminals of striatal projection neurons.

Autoradiographic distribution of serotonin transporters and receptor subtypes in human brain

Several neurochemical in vitro and in vivo imaging studies have been aimed at characterizing the localization of serotonin receptors and transporters in the human brain. In this study, a detailed comparison of the distribution of a number of 5-HT receptor subtypes and the 5-HT transporter was carried out in vitro using human postmortem brain tissue. Anatomically adjacent whole hemisphere sections were incubated with specific radioligands for the 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(4) receptors and the 5-HT transporter. The autoradiograms revealed different laminar and regional distribution patterns in the isocortex, where 5-HT(1A) and 5-HT(4) receptor binding showed highest densities in superficial layers and 5-HT(2A) receptor binding was most abundant in middle layers. In cortical regions, 5-HT transporters were concentrated to several limbic lobe structures (posterior uncus, entorhinal, cingulate, insular and temporal polar regions). 5-HT(1A) receptor densities were also high in limbic cortical regions (hippocampus, posterior entorhinal cortex, and subcallosal area) compared to the isocortex. Subregionally different distribution patterns were observed in the basal ganglia with a trend toward higher levels in ventral striatal (5-HT(1B) receptors) and pallidal (5-HT transporters and 5-HT(1B) receptors) regions. The localization in regions belonging to limbic cortico-striato-pallido-thalamic circuits is in line with the documented role of 5-HT in modulation of mood and emotion, and the suggested involvement of this system in pathophysiology of various psychiatric disorders. The qualitative and quantitative information reported in this study might provide important complements to in vivo neuroimaging studies of the 5-HT system.

Distribution of 5-HT4 receptors in the postmortem human brain--an autoradiographic study using [125I]SB 207710

The autoradiographic distribution of the 5-HT4 receptor was described using human postmortem brain sections and the selective radioligand [125I]SB 207710 [(1-n-butyl-4-piperidinyl)methyl-8-amino-7-[125I]iodo-1,4-benzodioxane-5-carboxylate]. The specific binding was highest in regions of the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus and substantia nigra) and the hippocampal formation (CA1 and subiculum). In the neocortex, the binding showed a distinct lamination pattern with high levels in superficial layers and a band displaying lower levels in deep cortical layers. The results confirm previous studies on the distribution of 5-HT4 receptors in the human brain in vitro and provide high-resolution correlates for in vivo imaging studies using the radioligand recently developed for single photon emission tomography (SPET), [123I]SB 207710.

Central serotonin4 receptors selectively regulate the impulse-dependent exocytosis of dopamine in the rat striatum: in vivo studies with morphine, amphetamine and cocaine

In vivo microdialysis and single-cell extracellular recordings were used to assess the involvement of serotonin(4) (5-HT(4)) receptors in the effects induced by morphine, amphetamine and cocaine on nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. The increase in striatal DA release induced by morphine (2.5 mg/kg, s.c.) was significantly reduced by the selective 5-HT(4) antagonists GR 125487 (0.1 and 1 mg/kg, i.p.) or SB 204070 (1 mg/kg, i.p.), and potentiated by the 5-HT(4) agonist prucalopride (5 mg/kg, i.p.). Neither of these compounds affected morphine-stimulated DA release in the nucleus accumbens. In both regions, amphetamine (2 mg/kg, i.p.) and cocaine (15 mg/kg, i.p.) induced DA release was affected neither by GR 125487 nor by prucalopride. None of the 5-HT agents used modified basal DA release in either brain region. Finally, GR 125487 (445 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra pars compacta nor the ventral tegmental area, significantly reduced morphine (0.1-10 mg/kg, i.v.) stimulated firing of nigrostriatal DA neurons only. These results confirm that 5-HT(4) receptors exert a state-dependent facilitatory control restricted to the nigrostriatal DA pathway, and indicate that 5-HT(4) receptors selectively modulate DA exocytosis associated with increased DA neuron firing rate.

The distribution of 5-HT(6) receptors in rat brain: an autoradiographic binding study using the radiolabelled 5-HT(6) receptor antagonist [(125)I]SB-258585

We used the highly selective 5-HT(6) receptor radioligand [(125)I]SB-258585 (4-iodo-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]benzene-sulfonamide) to perform autoradiographic binding studies on the rat brain. High levels of specific binding occurred in the corpus striatum, nucleus accumbens, Islands of Calleja and the olfactory tubercle. A high level of binding also appeared in the choroid plexus. Moderate levels occurred in several regions of the hippocampal formation and in certain regions of the cerebral cortex, thalamus, hypothalamus, and substantia nigra; and very low levels in the globus pallidus, cerebellum, other mesencephalic regions, and the rhombencephalon. Displacement of total binding with 10 microM unlabelled SB-214111 (4-bromo-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]benzene-sulfonamide), another selective 5-HT(6) receptor antagonist, or 10 microM unlabelled methiothepin, reduced binding to barely discernible levels. Some animals received unilateral injections of 6-hydroxydopamine (6-OHDA) into the median forebrain bundle to lesion the nigro-striatal pathway before autoradiographic examination. Effectiveness of the 6-OHDA lesions in the substantia nigra and striatum was confirmed with tyrosine hydroxylase immunohistochemistry. Such lesions resulted in no significant changes in [(125)I]SB-SB258585 binding in any brain region examined, suggesting that 5-HT(6) receptors in the striatum are not located on dendritic, somatic or terminal elements of dopaminergic neurones. Thus, the striatal binding sites seen in this study may be on intrinsic GABAergic or cholinergic neurones, or on terminals of projection neurones from the thalamus or cerebral cortex. The 5-HT(6) receptor ligand binding seen here in the striatum, accumbens, olfactory tubercle, Islands of Calleja, cerebral cortex and hippocampus are in concordance with previous immunohistochemical studies, and suggest a possible involvement of 5-HT(6) receptors in locomotor control, cognition, memory, and control of affect. The high levels of binding observed in the choroid plexus in this study have not been reported before. This finding suggests that 5-HT(6) receptors could play a role in the control of cerebrospinal fluid dynamics.

Central 5-HT(4) receptors and dopamine-dependent motor behaviors: searching for a functional role

In this study, we evaluated the role of central 5-HT(4) receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT(4) receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT(4) antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 microl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT(4) antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT(4) receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission.

Cloning and characterization of a novel human 5-HT4 receptor variant that lacks the alternatively spliced carboxy terminal exon. RT-PCR distribution in human brain and periphery of multiple 5-HT4 receptor variants

We have cloned a novel C-terminal splice variant of serotonin 5-HT4 receptors from human hippocampus. The deduced protein extends only one aminoacid past the splicing point. We propose to call the novel variant h5-HT4(n) since it contains none of the C-terminal exons alternatively spliced in other variants. The pharmacological profile of h5-HT4(n) stably expressed in HeLa cells is in agreement with other reported variants. Stably transfected cells showed increased basal levels of intracellular cAMP in absence of agonist, indicating constitutive activity of the expressed receptors. 5-HT induced robust increases of intracellular cAMP. The 5-HT4 receptor antagonist GR 113808 blocked the effects of 5-HT and brought intracellular cAMP below basal constitutive levels, indicating inverse agonism of this compound in this system. The RT-PCR distribution of all known human C-terminal splice variants in human brain regions and periphery showed complex patterns of variant expression, with the novel variant h5-HT4(n) being widely and abundantly expressed.

Regulation of striatal neuropeptide mRNAs: effects of the 5-HT(2) antagonist SR46349B in adult rats with a neonatal 6-hydroxydopamine lesion

The intrastriatal injection of 6-hydroxydopamine (6-OHDA) in newborn rats produces a marked striatal dopamine (DA) depletion, accompanied by a serotonin (5-HT) hyperinnervation and an up-regulation of 5-HT receptors. The aim of the present study was to investigate whether the increase in 5-HT(2) receptors could compensate for some of the DA lesion-induced effects, such as the increase in striatal preproenkephalin (PPE) and the decrease in preprotachykinin A (PPT-A) mRNA levels. Three months after the DA lesion, the effect of the selective 5-HT(2) antagonist SR46349B was investigated by a subacute treatment (10 mg/kg, IP, twice per day for 3.5 days). In sham-operated rats, the blockade of 5-HT(2) receptors decreased PPE mRNA levels in the striatum and, by contrast, had no effect on PPT-A mRNA levels. In rats with a unilateral neonatal DA lesion, SR46349B had no more effect on PPE mRNA levels in the intact striatum and was unable to modify the lesion induced-increase in PPE mRNA. The decrease in PPT-A mRNA levels induced by the neonatal DA lesion was not changed after SR46349B treatment in the posterior part of the lesioned striatum. Our results suggest that SR46349B indirectly decreases PPE mRNA levels in striatopallidal neurons in intact animals through a desinhibition of DA neuron activity. This is further evidenced by the lack of PPE mRNA changes in the DA lesioned striatum despite the up-regulation of 5-HT(2) receptor transmission induced in this model. Finally, the absence of any effect of 5-HT(2) antagonist on the expression of PPT-A mRNA in intact animals is discussed. The precise role of 5-HT(2) receptor on PPT-A mRNA biosynthesis after a neonatal lesion should be clarified by further experiments using 5-HT(2) agonists.

NMDA receptor antagonism modifies the synergistic regulation of striatal tachykinin gene expression induced by dopamine D(1) and serotonin(2) receptor stimulation following neonatal dopamine depletion

Co-application of SKF-38393 (dopamine D(1) agonist; 1 mg/kg) and DOI (serotonin(2) agonist; 1 mg/kg) induced a synergistic increase in striatal preprotachykinin (PPT) mRNA levels in adult rats 60 days after neonatal intracerebroventricular injection of 6-hydroxydopamine. This magnitude of response was not observed in intact (vehicle-injected) rats and was restricted to the dorsomedial (DM, 333+/-25% of lesion) subregion of the anterior striatum, with smaller increases observed in the dorsolateral striatum (DL, 206+/-26% of lesion). A single i.p. injection of MK-801 (NMDA antagonist; 0.1 mg/kg) administered prior to dopamine D(1) (D(1)) and serotonin(2) (5-HT(2)) receptor co-stimulation suppressed the synergistic regulation of PPT mRNA expression in the DM striatum, but also produced a large increase in PPT message levels within the DL striatum (321+/-17% of lesion). These data suggest that the synergistic regulation of PPT mRNA within the DM striatum induced by D(1)/5-HT(2) receptor co-stimulation in the dopamine lesioned rat is dependent on NMDA receptor activity. However, MK-801 may simultaneously potentiate striatal PPT mRNA expression by a separate mechanism due to the changed environment of the dopamine-depleted basal ganglia.

Neurochemical and electrophysiological evidence that 5-HT4 receptors exert a state-dependent facilitatory control in vivo on nigrostriatal, but not mesoaccumbal, dopaminergic function

In this study we investigated, using in vivo microdialysis and single unit recordings, the role of serotonin4 (5-HT4) receptors in the control of nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. In freely moving rats, the 5-HT4 antagonist GR 125487 (1 mg/kg, i.p.), without effect on its own, significantly reduced the enhancement of striatal DA outflow induced by 0.01 (-35%) and 0.1 (-66%), but not 1 mg/kg, s.c. haloperidol (HAL). Intrastriatal infusion of GR 125487 (1 microM) had no influence on basal DA outflow, but attenuated (-49%) the effect of 0.01 mg/kg HAL. Systemic administration of GR 125487 modified neither basal nor 0.01 mg/kg HAL-stimulated accumbal DA outflow. In halothane-anaesthetized rats, 1 or 10 mg/kg GR 125487, without effect by itself, failed to modify the changes in accumbal and striatal DA outflow elicited by electrical stimulation (300 microA, 1 ms, 20 Hz, 15 min) of the dorsal raphe nucleus. Finally, GR 125487 (444 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra or the ventral tegmental area, reduced HAL-stimulated (1--300 microg/kg, i.v.) impulse flow of nigrostriatal DA neurons only. These results indicate that 5-HT4 receptors exert a facilitatory control on both striatal DA release and nigral DA neuron impulse flow only when nigrostriatal DA transmission is under activated conditions. Furthermore, they indicate that the striatum constitutes a major site for the expression of the control exerted by 5-HT4 receptors on DA release. In contrast, 5-HT4 receptors have no influence on mesoaccumbal DA activity in either basal or activated conditions.

Serotonin-mediated striatal dopamine release involves the dopamine uptake site and the serotonin receptor

Modulation of striatal dopamine (DA) release by serotonin (5HT) and its antagonists was studied utilizing in vitro perfusion techniques. In isolated striatal tissue, 5HT (10 microM) increased the fractional basal release of labeled DA. The 5HT(2/1c) antagonist ketanserin (5 microM) also stimulated the basal release. These two effects were mediated by different mechanisms as cocaine (10 microM) greatly inhibited the 5HT-mediated response, but slightly increased the ketanserin-mediated response. 6-Nitroquipazine maleate (10 microM, 5HT uptake inhibitor) partially inhibited both responses. Inhibition by GBR 12909 (DA uptake inhibitor) at 1 microM of the 5HT-mediated DA release was similar to that of cocaine, but at 10 microM it increased release before addition of 5HT, and maintained elevated DA release while present in the incubation medium. At 1 microM GBR 12909, ketanserin-mediated DA release was stimulated and a much greater release was seen at 10 microM, but the prolonged release was not observed as after 5HT-mediated release. Among other antagonists methiothepin (5HT(1,2,6) antagonist) also enhanced DA release, whereas oxymetazoline (5HT(1A,1B,1D) agonist) had no effect. RS2359-190 (5HT(4) antagonist) had a small effect (slight stimulation) on 5HT-mediated DA release, and no effect on ketanserin-mediated DA release. CGS 12066A (5HT(1B) agonist) inhibited 5HT-mediated DA release. The glutamate antagonist MK-801 and the GABA(A) antagonist bicuculline had no affect on either response. These results indicate that 5HT-mediated DA release occurs via reversal of the DA transporter and that inhibitory presynaptic 5HT heteroreceptors and both inhibitory and stimulatory somato-dendritic 5HT receptors regulate release. In addition to the reversal of the transporter, an inhibitory 5HT(2) component was identified.

A review of the role of serotonin receptors in psychiatric disorders

Serotonin (5-hydroxytryptamine, 5-HT) mediates a wide variety of physiological functions by activating multiple receptors, and abnormalities of these receptor systems has been implicated in many psychiatric disorders including anxiety, depression, psychosis, migraine, disorders of sexual functioning, sleep, cognition, and feeding. Many of the currently used treatments for these disorders act by affecting the serotonergic system. Observation of serotonin receptor alterations, before and following effective treatments, may yield important insights into the aetiology of these psychiatric disorders and may ultimately lead to more selective and effective therapies. Copyright 2000 John Wiley & Sons, Ltd.

Mapping of serotonin 5-HT(4) receptor mRNA and ligand binding sites in the post-mortem human brain

The anatomical localization of 5-HT(4) receptor mRNA and 5-HT(4) receptor protein was examined in sections of post-mortem human brain by in situ hybridization histochemistry and radioligand receptor autoradiography. In the in situ hybridization study, the highest levels of 5-HT(4) receptor mRNA were found in caudate nucleus, putamen, nucleus accumbens, and in the hippocampal formation. No 5-HT(4) receptor mRNA was detected in globus pallidus and substantia nigra. For receptor autoradiography, two new and highly selective radioligands were compared: [(3)H]prucalopride, which preferentially labels the G-protein coupled fraction of receptors, and [(3)H]R116712, which labels the entire receptor population at subnanomolar concentrations. [(3)H]Prucalopride and [(3)H]R116712 binding was performed on human brain hemisphere sections. The highest densities for both radioligands were found in the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus, substantia nigra). Moderate to low densities were detected in the hippocampal formation and in the cortical mantle. Mismatches between 5-HT(4) receptor mRNA and binding sites in the globus pallidus and the substantia nigra suggested that the binding sites may be localized on axonal projections originating from the striatum. To compare densities of binding sites, concentration binding curves with [(3)H]prucalopride, [(3)H]R116712 and [(3)H]GR113808 were performed on membranes from homogenates of several human brain regions. Comparison of B(max)-values obtained with [(3)H]prucalopride and [(3)H]R116712 indicated that the G-protein coupled fraction of 5-HT(4) receptors in the substantia nigra was exceptionally high (54%) in comparison with percentages (16-27%) found in the frontal cortex, the striatum and the hippocampus. Such a high percentage (40%) of [(3)H]prucalopride vs. [(3)H]R116712 binding was also observed in the substantia nigra in the receptor autoradiography experiments. The [(3)H]prucalopride binding was GppNHp-sensitive, whereas [(3)H]R116712 and [(3)H]GR113808 was not. These data indicate that in the substantia nigra 5-HT(4) receptors are more strongly coupled to their signal transduction pathway than in other brain regions.

Serotoninergic neurons and serotonin receptors: gains from cytochemical approaches

Serotonergic systems, their phylogeny and ontogeny have been thoroughly described up to the ultrastructural level, thanks to the multiplicity of methodological approaches. They have often been referred to as a 'Rosetta stone', as several features first described for serotonin neurons or paraneurons have been then extended to other neurotransmitter systems: coexistence with neuropeptides or even a canonical neurotransmitter (GABA), volume transmission, regrowth after lesioning, and characterization of multiple receptor subtypes. This review deals with the contributions of neuroanatomical approaches for studying serotoninergic systems, and focuses on recent advances concerning the topological relationships between serotonergic innervation, receptors and target cells. This aspect is particularly important with regard to the possibility for serotonin to act through classical synaptic transmission and/or non-junctional transmission. Serotonin then can selectively regulate different neuronal systems through the activation of distinct receptor subtypes, which in turn can be linked to different transduction pathways. Neurocytochemical approaches constitute unique tools to analyse both anatomical and functional characteristics of complex neuronal systems.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Functional-anatomical implications of the nucleus accumbens core and shell subterritories

The nucleus accumbens, a major part of the ventral striatum, comprises numerous subterritories and compartments, of which the core and shell appear to be dominant. Shell exhibits greater chemical neuroanatomical diversity than core and is rather directly connected to it by a robust, feed-forward, striatopallido-thalamocortico-striatal pathway. Shell and extended amygdala share afferents, but the two are distinguished by their outputs, strongly toward cortex for shell and descendent toward brain stem effector sites for extended amygdala. Shell responds independently to stimulation by excitatory amino acids and dopamine, which are more mutually permissive in the core. Accordingly, the shell responds to a broad variety of physiological and pharmacological stimuli, including psychomotor and opioid drugs. Whereas locomotion and oro-facial movements are elicitable from the shell, lesions and blockade of EAA transmission in the core reduce locomotion. It is hypothesized that core-shell has a feed-forward functional organization.

5-HT4 receptor antagonism does not affect motor and reward mechanisms in the rat

5-HT4 receptors are concentrated in areas of the brain which are rich in dopamine neuronal markers, which may suggest that they influence motor and reward processes. We tested this hypothesis by examining the effects of a 5-HT4 receptor antagonist, 8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1,4-dioxan-5-car boxylate hydrochloride (SB-204070-A) on amphetamine- and nicotine-induced locomotor stimulation in intact rats. In rats with unilateral 6-hydroxydopamine-induced lesions of the ascending nigrostriatal dopaminergic projection, SB-204070-A was tested for its effects on amphetamine-induced rotation. SB-204070-A was also tested for its effects on rewarded behaviour maintained by intracranial self-stimulation. SB-204070-A did not alter behaviour under any of these conditions, suggesting a lack of involvement of the 5-HT4 receptor in motor and reward processes.

5-HT4 receptor involvement in the serotonin-enhanced dopamine efflux from the substantia nigra of the freely moving rat: a microdialysis study

The functional regulation by serotonin (5-HT) receptors of the 5-HT-enhanced dopamine (DA) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. Exogenously administered or extracellularly enhanced 5-HT (by means of intranigral citalopram perfusion) (both 1 microM for 1 h) significantly increased nigral DA efflux to 165% and 145%, respectively. Intranigral administration of pindolol (10 microM, 3 h), a 5-HT1A/1B receptor antagonist which is clinically used in order to block 5-HT1A/1B autoreceptors, did not affect DA levels but significantly increased nigral 5-HT levels to 135%. Co-perfusion of this antagonist with 5-HT (1 microM, 1 h) did not abolish the 5-HT-induced DA release from the SN as DA was increased to 166%. Local application of the 5-HT1A/1B receptor agonist, CP 93129 (1 microM, 1 h), increased DA release from the SN to 4770% whereas 5-HT release was significantly decreased to 75%. Co-perfusion of the 5-HT1A/1B receptor antagonist, pindolol, with this agonist only partly abolished the CP 93129-induced DA release whereas the CP 93129-induced decrease in nigral 5-HT release was completely abolished. Administration of the 5-HT2A/2C receptor antagonist, ketanserin (50 microM, 3 h), significantly increased DA to 143% and 5-HT release to 363%. Co-perfusion of this antagonist with 5-HT still caused an increase in nigral DA release to 214%. Intranigral perfusion of the 5-HT4 receptor antagonist, RS 39604 (10 microM, 3 h), did not affect DA levels but significantly decreased nigral 5-HT levels to 74%. Co-perfusion of this antagonist with 5-HT was able to prevent the 5-HT-enhanced DA efflux from the SN. From this study it can be concluded that the 5-HT-enhanced (and possibly the citalopram-induced) nigral DA release is 5-HT4 receptor mediated.

Anxiolytic-like actions of the selective 5-HT4 receptor antagonists SB 204070A and SB 207266A in rats

The highly selective 5-HT4 receptor antagonists, SB 204070A (0.001-0.1 mg/kg s.c., 30 min pretest) and SB 207266A (0.01, 1 and 10 mg/kg p.o., 1 hr pre-test), increased time spent in social interaction without affecting locomotor activity, in a rat 15 min social interaction test under high light, unfamiliar conditions. At 1 and 10 mg/kg s.c., SB 204070A was no longer active. These results are consistent with the profile expected of anxiolytic treatments in this procedure. In a rat 5 min elevated x-maze test, SB 204070A (0.01 and 1 mg/kg s.c., 30 min pre-test) significantly increased the percentage of time spent on the open arms. SB 204070A (0.01 mg/kg s.c.) and SB 207266A (1 mg/kg p.o., 1 hr pre-test) also increased percentage entries to the open arms. Neither compound affected locomotion at any dose tested in the procedure. The effects of both compounds in this procedure are also consistent with anxiolysis. Neither SB 204070A (0.1 or 1 mg/kg s.c., 30 min pre-test) nor SB 207266A (0.1 or 1 mg/kg p.o., 1 hr pre-test) affected either unpunished or punished responding, in a rat Geller-Seifter conflict model of anxiety. The maximal efficacy of both SB 204070A and SB 207266A in the rat social interaction test was similar to that of the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg s.c. or p.o.) used as a positive control, but was considerably less in the elevated x-maze procedure. The results suggest that 5-HT4 receptor antagonists may have modest anxiolytic-like actions in rats.

Differential actions of serotonin, mediated by 5-HT1B and 5-HT2C receptors, on GABA-mediated synaptic input to rat substantia nigra pars reticulata neurons in vitro

The ability of serotonin to modulate GABA-mediated synaptic input to substantia nigra pars reticulata (SNr) neurons was investigated with the use of whole-cell patch-clamp recording from slices of rat midbrain. Fast evoked GABA(A) receptor-mediated synaptic currents (IPSCs) were attenuated reversibly approximately 60% by serotonin, which also caused an inward current with reversal potential of -25 mV. This inward current was blocked by the 5-HT2 receptor antagonist ritanserin, whereas the IPSC depression was blocked by the 5-HT1B receptor antagonist pindolol. The amplitude ratio of IPSC pairs (50 msec interpulse interval) was enhanced by serotonin (in ritanserin) and also by the GABA(B) receptor agonist baclofen (which also depressed the IPSC), consistent with a presynaptic site of action in both cases. In contrast, spontaneous tetrodotoxin-sensitive GABA(A) synaptic currents (sIPSCs) were increased in frequency by serotonin (an action that was sensitive to ritanserin, but not pindolol) but reduced in frequency by baclofen. SNr neurons therefore receive inhibitory synaptic input mediated by GABA(A) receptors from at least two distinct sources. One, probably originating from the striatum, may be depressed via presynaptic 5-HT1B and GABA(B) receptors. The second is likely to arise from axon collaterals of SNr neurons themselves and is facilitated by an increase in firing via postsynaptic, somatodendritic 5-HT2C receptor activation, but it is depressed by GABA(B) receptor activation. Thus, serotonin can both depolarize and disinhibit SNr neurons via 5-HT2C and 5-HT1B receptors, respectively, but excitation may be limited by GABA released from axon collaterals.

Localization of 5-HT4 receptor mRNA in rat brain by in situ hybridization histochemistry

Oligonucleotide probes that recognize two cloned splice variants (5-HT4S and 5-HT4L) of 5-HT4 receptors were used to study by in situ hybridization the localization in rat brain of mRNA encoding these receptors. A probe common to both variants reveals high levels of transcripts in olfactory tubercle, some components of the basal ganglia (caudate putamen, ventral striatum), medial habenula and hippocampal formation. Similar patterns of distribution are obtained with probes that recognize each splice variant individually, suggesting that no dramatic differences exist in their respective regional distribution. Comparison of mRNA distribution with receptor distribution as visualized with [125I]SB 207710 indicates that 5-HT4 receptors are localized both somatodendritically in e.g. caudate putamen and on axon terminals in e.g. substantia nigra and globus pallidus.

Lesion study of the distribution of serotonin 5-HT4 receptors in rat basal ganglia and hippocampus

The regional distribution of 5-hydroxytryptamine (5-HT4) receptors labelled with [3H]GR113808 was examined in rat basal ganglia and hippocampus after specific lesions. Lesion of serotonin neurons induced by injections of 5,7-dihydroxytryptamine into the dorsal and medial raphe nuclei resulted in increased 5-HT4 receptor binding in most regions examined, compared with controls. More precisely, there was a 78% increase in the rostral but no change in the caudal part of caudate-putamen, and 83% and 54% increases in the shell and core of the nucleus accumbens respectively. In the substantia nigra, the increase in 5-HT4 binding was larger (72%) than that in the globus pallidus (32%). In the hippocampus, 63%, 30% and 28% increases were measured in CA2, CA1 and CA3 respectively. Following lesion of dopamine neurons by intranigral injection of 6-hydroxydopamine, increased 5-HT4 receptor binding was observed in the caudal (59%), but not the rostral part of caudate-putamen, as well as in the globus pallidus (93%). Since no decreases in 5-HT4 receptor density were detected after the dopamine lesion, it was concluded that these receptors are not expressed in dopamine neurons. Kainic acid lesions of the caudate-putamen were associated with dramatic local decreases in 5-HT4 receptor binding on the injected side (-89%), which suggested that striatal neurons express 5-HT4 receptors. Corresponding decreases of 72 and 20% in receptor density were detected in globus pallidus and substantia nigra, consistent with a presumed localization of 5-HT4 receptors on striatal GABA neurons projecting to these regions. In the substantia nigra, the decrease in [3H]GR113808 binding was localized to the pars lateralis, indicating that striatal neurons belonging to the cortico-striato-nigro-tectal pathway, and containing GABA and dynorphin, express 5-HT4 receptors.