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

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

A Role for Serotonin in Modulating Opposing Drive and Brake Circuits of Impulsivity

Impulsivity generally refers to a deficit in inhibition, with a focus on understanding the neural circuits which constitute the “brake” on actions and gratification. It is likely that increased impulsivity can arise not only from reduced inhibition, but also from a heightened or exaggerated excitatory “drive.” For example, an action which has more vigor, or is fueled by either increased incentive salience or a stronger action-outcome association, may be harder to inhibit. From this perspective, this review focuses on impulse control as a competition over behavioral output between an initially learned response-reward outcome association, and a subsequently acquired opposing inhibitory association. Our goal is to present a synthesis of research from humans and animal models that supports this dual-systems approach to understanding the behavioral and neural substrates that contribute to impulsivity, with a focus on the neuromodulatory role of serotonin. We review evidence for the role of serotonin signaling in mediating the balance of the “drive” and “brake” circuits. Additionally, we consider parallels of these competing instrumental systems in impulsivity within classical conditioning processes (e.g., extinction) in order to point us to potential behavioral and neural mechanisms that may modulate the competing instrumental associations. Finally, we consider how the balance of these competing associations might contribute to, or be extracted from, our experimental assessments of impulsivity. A careful understanding of the underlying behavioral and circuit level contributions to impulsivity is important for understanding the pathogenesis of increased impulsivity present in a number of psychiatric disorders. Pathological levels of impulsivity in such disorders are likely subserved by deficits in the balance of motivational and inhibitory processes.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

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.

Serotonin 5-HT4 receptors in the nucleus accumbens are specifically involved in the appetite suppressant and not locomotor stimulant effects of MDMA ('ecstasy')

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA) abuse is a substantial problem in young adults. Due to a high focus on body image in this population, two main factors that may encourage MDMA use are the appetite suppressant and locomotor stimulant effects of this drug. The nucleus accumbens (NAc) is a brain region associated with the regulation of motivated and locomotor behaviours, and recent evidence suggests that NAc 5-HT4 receptors are likely to be involved in the appetite suppressant effect of MDMA. It has not yet been shown whether 5-HT4 receptors of the NAc are involved in the locomotor stimulant effects of MDMA, which may also contribute to a reduction in food intake.

OBJECTIVES

This study aimed to investigate the effect of local antagonism of serotonin 5-HT4 receptors in the NAc in the appetite suppressant and locomotor stimulant effects of MDMA.

METHODS

Male hooded Wistar rats underwent surgery for the implantation of bilateral NAc microinjection cannulae under isofluorane anesthesia. Following 5-7 days of recovery, the rats received bilateral microinjections of the 5-HT4 antagonist RS39604 into the NAc immediately prior to either saline or MDMA administration. Food intake, water intake, body weight and locomotor activity were measured.

RESULTS

RS39604 significantly increased food intake and increased weight loss in MDMA-treated but not saline-treated rats. Measures of MDMA-induced water intake or locomotor activity were not altered by antagonist administration.

CONCLUSIONS

These results demonstrate that 5-HT4 receptors in the NAc specifically regulate the appetite suppressant effects of MDMA but not MDMA-induced water intake or locomotor activity.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Preclinical investigations into the antipsychotic potential of the novel histamine H3 receptor antagonist GSK207040

OBJECTIVES

To test the novel nonimidazole histamine H3 receptor antagonist 5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazapin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide (GSK207040) in a series of behavioral and neurochemical paradigms designed to evaluate its antipsychotic potential.

MATERIALS AND METHODS

Acute orally administered GSK207040 was investigated for its capacity to reverse a 24-h-induced deficit in novel object recognition memory, deficits in prepulse inhibition (PPI) induced by isolation rearing, and hyperlocomotor activity induced by amphetamine. The acute neurochemical effects of GSK207040 were explored by analyzing rat anterior cingulate cortex microdialysates for levels of dopamine, noradrenaline, and acetylcholine and by c-fos immunohistochemistry. The potential for interaction with the antipsychotic dopamine D2 receptor antagonist haloperidol was explored behaviorally (spontaneous locomotor activity and catalepsy), biochemically (plasma prolactin), and via ex vivo receptor occupancy determinations.

RESULTS

GSK207040 significantly enhanced object recognition memory (3 mg/kg) and attenuated isolation rearing-induced deficits in PPI (1.0 and 3.2 mg/kg) but did not reverse amphetamine-induced increases in locomotor activity. There was no evidence of an interaction of GSK207040 with haloperidol. GSK207040 (3.2 mg/kg) raised extracellular concentrations of dopamine, noradrenaline, and acetylcholine in the anterior cingulate cortex and c-fos expression in the core of the nucleus accumbens was increased at doses of 3.2 and 10.0 mg/kg.

CONCLUSIONS

The behavioral and neurochemical profile of GSK207040 supports the potential of histamine H3 receptor antagonism to treat the cognitive and sensory gating deficits of schizophrenia. However, the failure of GSK207040 to reverse amphetamine-induced locomotor hyperactivity suggests that the therapeutic utility of histamine H(3) receptor antagonism versus positive symptoms is less likely, at least following acute administration.

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors, citalopram and fluvoxamine, in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

RATIONALE

Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment.

OBJECTIVES

This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA.

METHODS

The most selective antagonists currently available were used.

RESULTS

Citalopram-induced LA was dose-dependently attenuated by the 5-HT1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT1B antagonist, SB224,289, but unaffected by the selective 5-HT1A antagonist, WAY100,635. The selective antagonists at 5-HT2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT2B antagonist, SB204,741, and the 5-HT2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT3 (ondansetron), 5-HT4 (GR125,487), 5-HT6 (SB271,046) and 5-HT7 (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine.

CONCLUSIONS

The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT1B and 5-HT2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.

Involvement of 5-hydroxytryptamine4 receptor in the exacerbation of neuronal loss by psychological stress in the hippocampus of SHRSP with a transient ischemia

A transient forebrain ischemia produced a delayed neuronal death of the hippocampus pyramidal cells in stroke-prone spontaneously hypertensive rats (SHRSP). Long term exposure of rats to stress has been reported to induce deleterious effects on the brain including morphological neuronal degeneration in the hippocampus. The present study was designed to examine the effects of psychological and physical stress on the ischemia-related neuronal death and the effects of 5-hydroxytryptamine(4) (5-HT(4)) receptor antagonist. SHRSP were exposed to the psychological or physical stress for 60 min in the communication box once or repeatedly for 3 days and occluded. SB204070, a 5-HT(4) receptor antagonist was injected before the occlusion. Seven days after the occlusion, the number of the neurons damaged morphologically was examined. A transient bilateral carotid occlusion produced a neuronal death of the CA1 subfield of the hippocampus in a time-dependent manner between 3 and 10 min. A 4 min occlusion induced very little morphological damage and a 5 min one produced a significant neuronal death. Exposure of rats to the psychological stress during 60 min for 3 days before the ischemic insults damaged the pyramidal cells by 4 min ischemia much more than without stress. Physical stress daily for 3 times also increased the damaged neurons. Pretreatment of SB204070 0.1 mg/kg after the stress exposure for 3 days significantly decreased the neuronal damage exacerbated by the stress exposure; however, it did not alter the damage induced by 4 or 10 min occlusion without stress. These results suggest that the repeated exposure of animals to the stress dramatically exacerbates the neuronal death by a transient ischemia and the 5-HT(4) receptor may be involved in the stress-induced exacerbating mechanism of the neuronal damage.

5-HT4 receptors in the hippocampus modulate rat locomotor activity

In the present study, the ability of 5-hydroxytryptamine-4 (5-HT4) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion via microdialysis probes. The local infusion of 5-HT bilaterally into the striatum did not alter rat motor activity. The local infusion of 1.0 mM 5-HT into the bilateral hippocampus, but not lower doses, significantly increased motor activity as compared with the baseline values or the control rats. During the day hours (0700-1900, light on), the local infusion of either 5-HT4 agonist, 5-MeOT (100 microM) or mosapride (10 microM), but not in their lower concentrations, into the bilateral hippocampus significantly increased motor activity as compared with the baseline values or the control rats. Almost all increased motor activity was normal forward locomotion. This 5-MeOT-induced hyperlocomotion was completely reversed by the combined infusion of a 5-HT4 antagonist, either GR125487D (100 microM), SB204070 (100 microM) or RS23597-190 (100 microM). During the night hours (1900-0700, light off), the local infusion of either SB204070 (100 microM) or RS23597-190 (100 microM), but not in their lower concentrations, into the bilateral hippocampus significantly decreased rat motor activity and inhibited rat nocturnal hyperactivity. These hypoactivities during the night hours induced by 5-HT4 antagonist were reversed by the combined infusion of a 5-HT4 agonist, 5-MeOT (100 microM). The present study demonstrates that the serotonergic neurons projecting to the hippocampus, but not to the striatum, modulate rat locomotor activity by stimulating 5-HT4 receptors in the hippocampus.

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.

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.

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

SB-243213; a selective 5-HT2C receptor inverse agonist with improved anxiolytic profile: lack of tolerance and withdrawal anxiety

SB-243213 (5-methyl-1-[[-2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-6-trifluoromethylindoline hydrochloride) is a new, selective 5-hydroxytryptamine (5-HT)2C receptor inverse agonist. SB-243213 has high affinity for the human 5-HT2C receptor (pK(i) 9.37) and greater than a 100-fold selectivity over a wide range of neurotransmitter receptors, enzymes and ion channels. In in vitro functional studies, SB-243213 acted as an inverse agonist at the human 5-HT2C receptor with a pK(b) of 9.8. In in vivo studies, SB-243213 was a potent inhibitor of central 5-HT2C receptor-mediated function in rats, blocking meta-chlorophenylpiperazine-induced hypolocomotion with an ID50 of 1.1 mg/kg p.o. and a long duration of action (>8 h). In rats, SB-243213 exhibited anxiolytic-like activity in both the social interaction and Geller-Seifter conflict tests. Importantly, unlike diazepam, chronic administration of SB-243213 did not result in the development of either tolerance to the anxiolytic-like effects or withdrawal anxiogenesis. Furthermore, in rodents, SB-243213 did not affect seizure threshold, did not increase body weight or induce catalepsy, but attenuated the haloperidol-induced catalepsy. SB-243213 did not affect amphetamine-, MK-801- or phencyclidine-induced hyperactivity. In conclusion, SB-243213 may possess an improved anxiolytic profile compared to benzodiazepines. SB-243213 also modulates dopaminergic transmission, lacks pro-psychotic properties and may have utility in the treatment of schizophrenia and motor disorders.

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.

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.