Identification and characterization of serotonin 5-HT4 receptor binding sites in human brain: comparison with other mammalian species

Usmarapride (SUVN-D4010), a 5-HT4 receptor partial agonist for the potential treatment of Alzheimer's disease: Behavioural, neurochemical and pharmacological profiling

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which affects cognitive functions with negative impact on day to day activities and an ultimate loss of independent living. Current standard of care (SOC) for AD, viz. donepezil, rivastigmine, galantamine, memantine etc. either alone or in combination show modest efficacy without changing the course of the disease. On prolonged treatment, side effects are more common with an eventual loss of efficacy. Aducanumab, a monoclonal antibody is a disease modifying therapeutic agent targeting the toxic amyloid beta (Aβ) proteins for its clearance. However, it is found to have only modest efficacy in AD patients and its approval by FDA is controversial. Alternate, effective and safe therapeutics are need of the hour, as AD cases are expected to be doubled by 2050. Recently, 5-HT₄ receptors have been envisioned as target for alleviating AD associated cognitive impairment with potential disease modifying ability impacting disease progression. Usmarapride is a 5-HT₄ receptor partial agonist, being developed for the possible treatment of AD with symptomatic and disease modifying potential. Usmarapride demonstrated promising effects in ameliorating cognitive deficits in diverse animal models of episodic, working, social, and emotional memories. Usmarapride produced elevation in cortical acetylcholine in rats. Furthermore, usmarapride increased levels of soluble amyloid precursor protein alpha, a potential mechanism to reverse toxic Aβ peptide pathology. Usmarapride also potentiated the pharmacological effects of donepezil in animal models. To conclude, usmarapride may be a promising intervention for alleviating the cognitive dysfunction in AD patients with disease modifying potential.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Therapeutics of Neurotransmitters in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterized by the loss of memory, multiple cognitive impairments and changes in the personality and behavior. Several decades of intense research have revealed that multiple cellular changes are involved in disease process, including synaptic damage, mitochondrial abnormalities and inflammatory responses, in addition to formation and accumulation of amyloid-β (Aβ) and phosphorylated tau. Although tremendous progress has been made in understanding the impact of neurotransmitters in the progression and pathogenesis of AD, we still do not have a drug molecule associated with neurotransmitter(s) that can delay disease process in elderly individuals and/or restore cognitive functions in AD patients. The purpose of our article is to assess the latest developments in neurotransmitters research using cell and mouse models of AD. We also updated the current status of clinical trials using neurotransmitters' agonists/antagonists in AD.

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.

In-vivo rat striatal 5-HT4 receptor occupancy using non-radiolabelled SB207145

Objectives

The objective of the current investigation was to develop a simple, rapid method for determining in-vivo 5-hydroxytryptamine type 4 receptor (5-HT4R) occupancy in rat brain using non-radiolabelled SB207145 as a tracer for accelerating the drug discovery process.

Methods

In-vivo tracer optimization studies for tracer dose, survival intervals and brain distribution profile were carried out in rats. The tracer was pharmacologically validated using potent well-characterized 5-HT4R ligands. The brain regional concentrations of tracer (SB207145); plasma and brain concentrations of 5-HT4R ligands were quantified using high-performance liquid chromatography coupled with a tandem mass spectrometric detector (LC-MS/MS).

Key findings

SB207145 showed a higher specific binding in striatum (1.96 ng/g) and lower binding in cerebellum (0.66 ng/g), which is consistent with findings of other published 5-HT4R expression studies. Pretreatment with potent 5-HT4 ligands dose-dependently reduced striatal SB207145 concentration and the effective dose to achieve 50% receptor occupancy (ED50) values were 4.8, 2.0, 7.4, 9.9, 3.8 and 0.02 mg/kg for GR113808, piboserod, prucalopride, RS67333, TD8954 and PF04995274, respectively.

Conclusions

Results from the mass spectrometry approach to determine 5-HT4R occupancy in rat brain are comparable with those reported using radiolabelled scintillation spectroscopy methods. In conclusion, the LC-MS/MS characterization permits use of tracer at a preclinical stage in high-throughput fashion as well as characterization of target expression.

5-HT4 receptors constitutively promote the non-amyloidogenic pathway of APP cleavage and interact with ADAM10

In addition to the amyloidogenic pathway, amyloid precursor protein (APP) can be cleaved by α-secretases, producing soluble and neuroprotective APP alpha (sAPPα) (nonamyloidogenic pathway) and thus preventing the generation of pathogenic amyloid-β. However, the mechanisms regulating APP cleavage by α-secretases remain poorly understood. Here, we showed that expression of serotonin type 4 receptors (5-HT(4)Rs) constitutively (without agonist stimulation) induced APP cleavage by the α-secretase ADAM10 and the release of neuroprotective sAPPα in HEK-293 cells and cortical neurons. This effect was independent of cAMP production. Interestingly, we demonstrated that 5-HT(4) receptors physically interacted with the mature form of ADAM10. Stimulation of 5-HT(4) receptors by an agonist further increased sAPPα secretion, and this effect was mediated by cAMP/Epac signaling. These findings describe a new mechanism whereby a GPCR constitutively stimulates the cleavage of APP by α-secretase and promotes the nonamyloidogenic pathway of APP processing.

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.

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.

Regulation of amygdalar PKA by beta-arrestin-2/phosphodiesterase-4 complex is critical for fear conditioning

Beta-arrestins, key regulators of receptor signaling, are highly expressed in the central nervous system, but their roles in brain physiology are largely unknown. Here we show that beta-arrestin-2 is critically involved in the formation of associative fear memory and amygdalar synaptic plasticity. In response to fear conditioning, beta-arrestin-2 translocates to amygdalar membrane where it interacts with PDE-4, a cAMP-degrading enzyme, to inhibit PKA activation. Arrb2(-/-) mice exhibit impaired conditioned fear memory and long-term potentiation at the lateral amygdalar synapses. Moreover, expression of the beta-arrestin-2 in the lateral amygdala of Arrb2(-/-) mice, but not its mutant form that is incapable of binding PDE-4, restores basal PKA activity and rescues conditioned fear memory. Taken together, our data demonstrate that the feedback regulation of amygdalar PKA activation by beta-arrestin-2 and PDE-4 complex is critical for the formation of conditioned fear memory.

Selective desensitization of the 5-HT4 receptor-mediated response in pig atrium but not in stomach

BACKGROUND AND PURPOSE

The time dependency of the effect of 5-HT(4) receptor agonists depends on many specific regulatory mechanisms, which vary between tissues. This has important implications with regard to the effects of endogenous 5-HT, as well as to the clinical use of 5-HT(4) receptor agonists, and might contribute to tissue selectivity of agonists.

EXPERIMENTAL APPROACH

The progression and desensitization of 5-HT(4) receptor-mediated responses were evaluated in an organ bath set-up using two, clinically relevant, porcine in vitro models: gastric cholinergic neurotransmission and atrial contractility.

KEY RESULTS

Exposure of gastric tissue to 5-HT or to the selective 5-HT(4) receptor agonists prucalopride and M0003 results in a sustained non-transient effect during exposure; after washout, the response to a subsequent challenge with 5-HT shows no clear desensitization. Incubation of left atrial tissue with 5-HT resulted in a transient response, leading after washout to a marked desensitization of the subsequent response to 5-HT. The selective 5-HT(4) receptor agonists prucalopride and M0003 induce only very weak atrial responses whereas they are very effective in desensitizing the atrial response to 5-HT. The observations also suggest that the properties of prucalopride and M0003 to bind to and/or activate the 5-HT(4) receptor differ from those of 5-HT. This difference might have contributed to the observed desensitization.

CONCLUSIONS AND IMPLICATIONS

The high potency of prucalopride and M0003 in desensitizing the response to 5-HT together with their low efficacy in the atrium emphasizes the cardiac safety of this class of 5-HT(4) receptor agonists.

Modifications of 5-HT4 receptor expression in rat brain during memory consolidation

Pharmacological evidence indicates a specific role of 5-HT(4) receptors on memory function. These receptors are members of G-protein-coupled 7-transmembrane domain receptor superfamily, are positively coupled to adenylyl cyclase, and are heterogeneously located in some structures important for memory, such as the hippocampus and cortical regions. To further clarify 5-HT(4) receptors' role in memory, the expression of these receptors in passive (P3) untrained and autoshaping (A3) trained (3 sessions) adult (3 months) and old (P9 or A9; 9 months) male rats was determined by autoradiography. Adult trained (A3) rats showed a better memory respect to old trained (A9). Using [(3)H] GR113808 as ligand (0.2 nM specific activity 81 Ci/mmol) for 5-HT(4) receptor expression, 29 brain areas were analyzed, 16 areas of A3 and 17 of A9 animals displayed significant changes. The medial mammillary nucleus of A3 group showed diminished 5-HT(4) receptor expression, and in other 15 brain areas of A3 or 10 of A9 animals, 5-HT(4) receptors were increased. Thus, for A3 rats, 5-HT(4) receptors were augmented in olfactory lobule, caudate putamen, fundus striatum, CA2, retrosplenial, frontal, temporal, occipital, and cingulate cortex. Also, 5-HT(4) receptors were increased in olfactory tubercule, hippocampal CA1, parietal, piriform, and cingulate cortex of A9. However, hippocampal CA2 and CA3 areas, and frontal, parietal, and temporal cortex of A9 rats, expressed less 5-HT(4) receptors. These findings suggest that serotonergic activity, via 5-HT(4) receptors in hippocampal, striatum, and cortical areas, mediates memory function and provides further evidence for a complex and regionally specific regulation over 5-HT receptor expression during memory formation.

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.

Altered 5-HT2A and 5-HT4 postsynaptic receptors and their intracellular signalling systems IP3 and cAMP in brains from depressed violent suicide victims

Serotonin 5-HT2A and 5-HT4 binding parameters and their second messengers 1,4,5-inositol triphosphate (IP3) and cyclic adenosyl monophosphate (cAMP) were studied in the frontal cortex, hippocampus, caudate nucleus and amygdala of 19 control subjects and 19 antidepressant-free, violent suicide victims. A significantly higher number of 5-HT4 receptors and higher second messenger cAMP concentrations were found in the frontal cortex and caudate nucleus of the depressed suicide victims as compared with the control group. Furthermore, significantly increased 5-HT2A binding sites and IP3 concentrations were noted in the caudate nucleus of the suicide victims, together with a significantly reduced number of 5-HT2A binding sites, higher binding affinity and increased IP3 concentrations in the hippocampus. No significant alterations in 5-HT4 and cAMP or in 5-HT2A and IP3 concentrations were observed in the amygdala. The caudate nucleus of depressed suicide victims seems to be the brain region with the highest alteration of the serotonergic system, and hence with the most diagnostic sensitivity. Further studies on suicidality and depression should focus on the functionality of the caudate nucleus.

Noradrenaline and 5-hydroxytryptamine in cerebellar nuclei of the rat: functional effects on neuronal firing

The firing rate of single cerebellar nuclear neurons was studied during microiontophoretic application of noradrenaline (NA), 5-hydroxytryptamine (5-HT) and their agonists in deeply anesthetized rats. NA application depressed the neuronal firing rate more in the medial nucleus (MN) than in the interpositus (IN) and in the lateral nucleus (LN). These responses were mimicked by alpha(2) and, to a lesser extent, beta receptor agonists. 5-HT evoked inhibition in MN and various effects (inhibitory, excitatory, biphasic) in IN and LN. Excitatory responses were more numerous in the posterior than in the anterior zone of IN. Agonists at 5-HT(1A) and 5-HT(2) receptors mimicked inhibition only. In conclusion, NA and 5-HT exerted a similar action on MN neurons; in contrast, the effects of 5-HT on IN and LN were more differentiated than those exerted by NA.

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.

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.

Regulation of GABAergic inhibition by serotonin signaling in prefrontal cortex: molecular mechanisms and functional implications

Serotonergic neurotransmission in prefrontal cortex (PFC) plays a key role in regulating emotion and cognition under normal and pathological conditios. Increasing evidence suggests that serotonin receptors are involved in the complex regulation of GABAergic inhibitory transmission in PFC. Activation of postsynaptic 5-HT2 receptors in PFC pyramidal neurons inhibits GABAA-receptor currents via phosphorylation of GABAA receptor gamma2 subunits by RACK1-anchored PKC. In contrast, activation of postsynaptic 5-HT4 receptors produces an activity-dependent bi-directional regulation of GABA-evoked currents in PFC pyramidal neurons, which is mediated through phosphorylation of GABAA-receptor beta subunits by anchored PKA. On the presynaptic side, GABAergic inhibition is regulated by 5-HT through the activation of 5-HT2, 5-HT1, and 5-HT3 receptors on GABAergic intereneurons. These data provide a molecular and cellular mechanism for serotonin to dynamically regulate synaptic transmission and neuronal excitability in the PFC network, which may underlie the actions of many antidepressant and antipsychotic drugs.

Different distributions of the 5-HT reuptake complex and the postsynaptic 5-HT(2A) receptors in Brodmann areas and brain hemispheres

The aim of the present study was to determine the distribution of the presynaptic 5-HT reuptake complex and the 5-HT(2A) receptors through Brodmann areas from two control subjects, together with the possible existence of laterality between both brain hemispheres. A left laterality was observed in the postsynaptic 5-HT(2A) binding sites, with significantly higher B(max) values in the left frontal and cingulate cortex. In frontal cortex, [3H]imipramine and [3H]paroxetine binding showed the highest B(max) values in areas 25, 10 and 11. In cingulate cortex, the highest [3H]imipramine and [3H]paroxetine B(max) values were noted in Brodmann area 33 followed by area 24, while postsynaptic 5-HT(2A) receptors were mainly distributed through Brodmann areas 23 and 29. In temporal cortex, the highest [3H]imipramine and [3H]paroxetine B(max) was noted in Brodmann areas 28 and 34, followed by areas 35 and 38. All Brodmann areas from parietal cortex (1, 2, 3, 4, 5, 6, 7, 39, 40 and 43) showed similar presynaptic and postsynaptic binding values. In occipital cortex no differences were observed with regard to the brain hemisphere or to the Brodmann area (17, 18 and 19). These results suggest the need to carefully define the brain hemisphere and the Brodmann areas studied, as well to avoid comparisons between studies including different Brodmann areas or brain hemispheres.

Activity-dependent bidirectional regulation of GABA(A) receptor channels by the 5-HT(4) receptor-mediated signalling in rat prefrontal cortical pyramidal neurons

Emerging evidence has implicated a potential role for 5-HT(4) receptors in cognition and anxiolysis. One of the main target structures of 5-HT(4) receptors on 'cognitive and emotional' pathways is the prefrontal cortex (PFC). As GABAergic signalling plays a key role in regulating PFC functions, we examined the effect of 5-HT(4) receptors on GABA(A) receptor channels in PFC pyramidal neurons. Application of 5-HT(4) receptor agonists produced either an enhancement or a reduction of GABA-evoked currents in PFC neurons, which are both mediated by anchored protein kinase A (PKA). Although PKA phosphorylation of GABA(A) receptor beta3 or beta1 subunits leads to current enhancement or reduction respectively in heterologous expression systems, we found that beta3 and beta1 subunits are co-expressed in PFC pyramidal neurons. Interestingly, altering PKA activation levels can change the direction of the dual effect, switching enhancement to reduction and vice versa. In addition, increased neuronal activity in PFC slices elevated the PKA activation level, changing the enhancing effect of 5-HT(4) receptors on the amplitude of GABAergic inhibitory postsynaptic currents (IPSCs) to a reduction. These results suggest that 5-HT(4) receptors can modulate GABAergic signalling bidirectionally, depending on the basal PKA activation levels that are determined by neuronal activity. This modulation provides a unique and flexible mechanism for 5-HT(4) receptors to dynamically regulate synaptic transmission and neuronal excitability in the PFC network.

Differences in signal transduction of two 5-HT4 receptor splice variants: compound specificity and dual coupling with Galphas- and Galphai/o-proteins

This study documents differences in ligand binding and signal transduction properties between the human (h) 5-hydroxytryptamine (5-HT)4a and h5-HT4b receptor splice variants stably expressed in human embryonic kidney 293 cells. The fraction of the [3H]5-HT high-affinity site relative to the whole receptor population measured with [3H]GR113808 was higher for the h5-HT4a isoform (around 0.4) than for the 5-HT4b isoform (around 0.2) and was independent of the level of expression. The potency and efficacy of reference compounds tested for the cAMP response differed slightly but significantly between both variants. Most remarkably, 5-methoxytryptamine and prucalopride were found more potent on the 5-HT4b variant, whereas SDZ-HTF 919 and SB204070 were more potent on the 5-HT(4a) variant. Guanosine-5'-O-(3-[35S]thio)triphosphate binding on membranes and cAMP assays in whole cells revealed that only the h5-HT4b isoform coupled to Galphai/o-proteins in addition to its well-documented Galphas coupling. In contrast, the h5-HT4a receptor coupled only to Galphas-proteins, however, was able to trigger an increase in the intracellular calcium concentration ([Ca(2+)]i). The observed [Ca(2+)]i increase did not occur through inositol phosphate formation and was not sensitive to Bordetella pertussis toxin, forskolin, or 3-isobutyl-1-methylxanthine (pre)treatment but was due to Ca(2+) influx from the extracellular environment. Interestingly, the Ca(2+) pathway was dependent on high receptor expression levels and was compound-specific, because benzamide-like compounds triggered two to three times higher responses than indoleamines. Taken together, these data provide the first evidence for fine functional differences between C-terminal splice variants of the h5-HT4 receptor, which may contribute to a better understanding of the functional diversity of this receptor class.

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.

Pharmacological characterization of almotriptan: an indolic 5-HT receptor agonist for the treatment of migraine

Almotriptan (3-[2-(dimethylamino)ethyl]-5-(pyrrolidin-1-ylsulfonylmethyl )-1H-indo le) has been studied in several models predictive of activity and selectivity at 5-HT receptors. Almotriptan showed low nanomolar affinity for the 5-HT(1B) and 5-HT(1D) receptors in several species, including the human, while affinity for 5-HT receptors other than 5-HT(1B/1D) was clearly less. Affinity for 5-HT(7) and 5-HT(1A) receptors was approximately 40 and 60 times lower than that for 5-HT(1B/1D) receptors, respectively. Almotriptan did not exhibit significant affinity for several non-5-HT receptors studied up to 100 microM. Almotriptan inhibited forskolin-stimulated cyclic AMP accumulation in HeLa cells transfected with 5-HT(1B) or 5-HT(1D) human receptors. In this model, almotriptan had the same efficacy as serotonin and an affinity in the low nanomolar range. It induced vasoconstriction in several vessels in which it was compared with sumatriptan. In isolated dog saphenous veins, almotriptan elicited concentration-dependent contractions with an EC(50) of 394 nM. In both these systems, almotriptan behaved as a full agonist. Infusion of almotriptan into the porcine meningeal vasculature induced vasoconstriction. In contrast, in the pig renal and rabbit mesenteric arteries, it had a very low maximal efficacy even at 100 microM, with similar results obtained in the rabbit renal artery. The results suggest that almotriptan is a potent and selective 5-HT(1B/1D) receptor agonist, with selectivity for the cranial vasculature as compared with peripheral vessels.

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.

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.

No change in the density of the serotonin1A receptor, the serotonin4 receptor or the serotonin transporter in the dorsolateral prefrontal cortex from subjects with schizophrenia

Changes in serotonin receptors and the serotonin transporter have been reported in the dorsolateral prefrontal cortex from subjects with schizophrenia, an area of the brain thought to be important in the pathology of the illness. To further our understanding on how such changes could play a role in the pathology of the illness, in situ radioligand binding with autoradiography was used to measure the density of the serotonin1A receptor, the serotonin4 receptor and the serotonin transporter in the dorsolateral prefrontal cortex, obtained at autopsy, from 10 schizophrenic and 10 control subjects. The binding of [3H]8-OH-DPAT to serotonin1A receptor, [3H]GR113808 to the 5HT4 receptor and [3H]citalopram to serotonin transporter was not altered in subjects with schizophrenia. significantly, only in tissue from the control subjects was there a relationship between age and the density of the serotonin4 receptor in Brodmann's areas 8 (r = 0.71, P = 0.02) and 10 (r = -0.67, P = 0.03). Importantly, this confounding factor did not influence the comparison of the density of serotonin4 receptor in the tissue from the schizophrenic and control subjects. This study has failed to show a difference in the density of serotonin1A receptor, the serotonin4 receptor or the serotonin transporter in the dorsolateral prefrontal cortex (Brodmann's areas 8, 9 and 10) from subjects with schizophrenia. These data suggest that not all serotonergic markers are altered in the dorsolateral prefrontal cortex from schizophrenic subjects.

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.

A serotonin-4 receptor-like pseudogene in humans

During a search for new G-protein-linked receptors for dopamine and serotonin, we found a serotonin-4 receptor-like pseudogene. This receptor-like pseudogene is intronless, contains an in-frame stop codon following transmembrane-3, and has two one-nucleotide insertions between transmembrane-5 and -6 regions which alter the reading frame. The predicted amino acid sequence of the human pseudogene is about 35% identical with that of the rat serotonin-4 receptor.

Characterization of 5-HT receptors on human pulmonary artery and vein: functional and binding studies

1. This study aimed to investigate the 5-hydroxytryptamine (5-HT) receptors mediating contraction of ring preparations isolated from human pulmonary arteries and veins. In functional studies, the responses to 5-HT, sumatriptan, ergotamine, serotonin-O-carboxymethyl-glycyl-tyrosinamide (SCMGT), alpha-methyl 5-HT (alpha-Me) and 2-methyl 5-HT (2-Me) were studied with WAY100635, GR127935, ritanserin, zacopride and SB204070 as antagonists. 2. All agonists produced concentration-dependent contractions of human pulmonary artery and vein preparations. The order of potency (-log ECS0 values) was ergotamine (6.88) > 5-HT (6.41) > or = SCMGT (6.20) = sumatriptan (6.19) > or = alpha-Me (6.04) in the artery, and ergotamine (7.84) > 5-HT (6.96) > sumatriptan (6.60) = alpha-Me (6.56) > SCMGT (6.09) in the vein. The potency of each agonist, except for SCMGT, was greater in vein than in artery preparations. Contractile responses to 5-HT were similar in intact and endothelium-denuded preparations but responses to sumatriptan were enhanced in artery rings without endothelium. 3. GR127935 (1 nM to 0.5 microM) produced an unsurmountable antagonism of the response to 5-HT, sumatriptan, ergotamine and SCMGT. Ritanserin (1 nM to 1 microM) also reduced the maximum contractile responses to 5-HT, ergotamine and alpha-Me in artery and vein preparations without affecting those to sumatriptan and SCMGT. In endothelium-denuded preparations, surmountable antagonism of sumatriptan by GR127935 (in the presence of ritanserin) and of alpha-Me by ritanserin (in the presence of GR127935) allowed for the calculation of the apparent pK(B) values of GR127935 (9.17+/-0.11 in artery and 9.11+/-0.05 in vein) and ritanserin (8.82+/-0.09 in artery and 8.98+/-0.12 in vein). 4. WAY100635 (1 nM to 1 microM), zacopride (1 nM to 1 microM), or SB204070 (1 nM) did not significantly alter the concentration-response curves for 5-HT, sumatriptan, ergotamine, SCMGT or 2-Me in human pulmonary artery or vein thus indicating that 5-HT1A, 5-HT3 and 5-HT4 receptors are presumably not involved in the contractile response to these agonists. 5. Binding studies using selective radioligands for different 5-HT receptors could not detect the presence of 5-HT1A receptor binding in human pulmonary blood vessels whereas the 5-HT(1B/1D) radioligand [3H]-5CT significantly labelled a population of specific binding sites in both vessel types. The presence of 5-HT2A receptors could also be inferred from the level of binding of [3H]-ketanserin to membranes obtained from human pulmonary vessels, although significance could not be reached for arteries. 5-HT4 specific receptor binding was scarce in veins and absent in the case of arteries. 6. These findings indicate that the human pulmonary artery and vein have a mixed functional population of 5-HT(1B/1D) and 5-HT2A receptors mediating the contractile response to 5-HT which is consistent with results of the binding studies.

Arylcarbamate derivatives of 1-piperidineethanol as potent ligands for 5-HT4 receptors

A series of carbamate derivatives (7) of 2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoates, which have been described as potent agonists and antagonists of 5-HT4 receptors, were synthesized. They were evaluated using radioligand binding assays with [3H]GR 113808, a 5-HT4 receptor selective ligand, in the rat striatum and the electrically stimulated myenteric plexus longitudinal muscle of the guinea pig. In contrast to the previously described ester derivatives, a drop in the affinity for 5-HT4 receptors was observed and the compounds were inactive as agonists in the guinea pig ileum preparation. Unexpectedly, the ortho-substituted carbamates 8b,c (R' = H, RO = MeO or EtO, R" = H) had nanomolar affinity for 5-HT4 receptors (Ki = 8.9 +/- 0.5 and 2.6 +/- 0.4 nM, respectively). As reported previously, the cis- or trans-3,5-dimethyl substitution of piperidine (8n,o) was particularly favorable (Ki = 1.1 +/- 0.6 nM for both isomers). 8c is an antagonist equipotent to the 5-HT4 receptor antagonist SDZ 205-557 (1).

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.

New esters of 4-amino-5-chloro-2-methoxybenzoic acid as potent agonists and antagonists for 5-HT4 receptors

A number of benzoates derived from 4-amino-5-chloro-2-methoxybenzoic acid and substituted 1-piperidineethanol were synthesized and found to be potent 5-HT4 receptor agonists in the electrically-stimulated myenteric plexus and longitudinal muscle of the guinea pig ileum and the rat esophagus muscle. Monosubstitution of the piperidine ring with Me, OH, NH-Ac, or CONH2 groups gave compounds equipotent to 7a (ML 10302), a 5-HT4 receptor agonist previously reported to have nanomolar affinity. 7a,k were as potent as serotonin (5-HT) but had maximal responses which were only 60-80% of that of 5-HT, suggesting a partial agonist profile for these compounds. Binding assays were performed with [3H]GR 113808 in the rat striatum, and several of these compounds were found to have nanomolar affinity for 5-HT4 receptors (7a, Ki = 1.07 +/- 0.5 nM; 7k, Ki = 1.0 +/- 0.3 nM). The introduction of two methyl groups on the piperidine ring brought about a dramatic change in the pharmacological profile of 2-[(cis- and trans-3,5-dimethylpiperidinyl)ethyl]-4-amino-5-chloro-2- methoxybenzoate, 7g,h. 7g (Ki = 0.26 +/- 0.06 nM) inhibited the relaxant action of 5-HT in the rat esophagus muscle with a pA2 value of 8.6. The advantage of the ester function was demonstrated by comparing the activity of several such compounds at 5-HT4 receptors with those of the corresponding amidic derivatives. This difference was less marked when the basic moiety was sterically constrained as in the quinuclidine and tropane moieties. Structural analyses of 7a,g were performed by determining their X-ray crystal structures and by molecular modeling (SYBYL). A relatively limited number of minimum energy conformers was found for both compounds. They were characterized by the cis folded conformation of the ethyl chain and by the orientation of the lone pair of the nitrogen atom pointing out of the molecule as seen in conformationally-constrained benzamides such as zacopride and renzapride. A hypothetical model for the 5-HT4 receptor with two sites for the binding of agonist and antagonist molecules was proposed.

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.

Effects of 5-HT4 receptor antagonists on rat behaviour in the elevated plus-maze test

The anxiolytic-like effects of a variety of 5-HT receptor agonists and antagonists have been intensively studied in animal models. However, no direct effects of agents modulating 5-HT4 receptors have been reported, in spite of their suggestive location in the brain. The objective of the present study was the determination of the effects of two selective 5-HT4 receptor antagonists, SB 204070 [1-butyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate] and GR 113808 [[1-[2-methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl 1-methyl-1 H-indole-3-carboxylate], in the elevated plus-maze test in rats. Results have shown that both 5-HT4 receptor antagonists exhibit an anxiolytic-like profile, although only at the dose of 1.0 mg/kg (s.c.). At this dose, both compounds significantly increased the percentage of time spent in open arms exploration, while other variables evaluated remained unaffected at the dose range tested. Results suggest that 5-HT4 receptor antagonists could have some anxiolytic-like properties, although their effects seem more limited and less consistent than those presented by classic anxiolytics, such as diazepam. However, they are similar to those exhibited by granisetron [endo-1-methyl-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-1 H-indazole-3-carboxamide], a 5-HT3 receptor antagonist.

Comparison of the densities of 5-HT4 receptors, beta 1- and beta 2-adrenoceptors in human atrium: functional implications

We measured in human atrium the density of 5-HT4 receptors, labelled with [125I]-SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1, 4-benzodioxan-5-carboxylate), and compared it with the density of beta1- and beta2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. [125I]-SB 207710 (5-1200 pmol/l) labelled a small population of saturable binding sites (Bmax approximately 4 fmol/mg protein) with a pK(D) of 9.7 and with 5-HT4 receptor characteristics, as assessed with competing ligands. The density of atrial binding sites with 5-HT4 receptor characteristics was 10 and 5 times lower, respectively, than the density of beta 1- and beta 2-adrenoceptors. We suggest that the small 5-HT4 receptor population may in part explain why the positive inotropic effects of 5-HT are smaller than those of catecholamines mediated through beta 1- and beta 2-adrenoceptors.

Pharmacological comparison between [3H]GR 113808 binding sites and functional 5-HT4 receptors in neurons

5-HT4 receptors positively coupled to adenylyl cyclase and possessing unique pharmacological properties were first described in mouse colliculi neurons using functional studies. The recent introduction of a radiolabeled 5-HT4 receptor antagonist, [3H]GR 113808 [1-[2-(methylsulphonylamino)ethyl]4-piperidinyl]methyl-1-methyl-in dole-3 carboxylate] having high specificity and affinity allowed the pharmacological comparison between the specific binding sites identified with this compound and the functional 5-HT4 receptors in the same preparation, the colliculi neurons. We show here that [3H]GR 113808 binding is saturable in this preparation and reveals a homogeneous population of sites with a pKd value of 9.5 +/- 0.2 and a Bmax of 75 +/- 23 fmol/mg protein. Seventeen agonists and six antagonists with molecules structurally related either to indoles, benzamides or benzimidazolones and previously known as 5-HT4 receptor ligands, were tested for their ability to compete with [3H]GR 113808 binding sites and to stimulate or inhibit 5-HT-stimulated adenylyl cyclase activity. Highly significant correlations were obtained between the affinities of either agonists or antagonists for [3H]GR 113808 binding sites and their potencies for functional 5-HT4 receptors (r = 0.87 and 0.99, respectively). In addition, we also found good correlations between the Kd of several 5-HT4 receptor ligands determined in cell membranes of mouse colliculi neurons and their Kd determined in previous studies in guinea-pig striatum (0.95) and in human caudate (0.97). [3H]GR 113808 binding studies demonstrated that the 50% decrease in 5-HT-stimulated cAMP accumulation which followed a 5 min exposure period with 5-HT (10 microM) was not accompanied by any significant decrease in the number of binding sites. Longer exposure periods with 5-HT resulted in a decrease in [3H]GR 113808 binding sites which started to be significant after 30 min.

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

Rats were injected unilaterally with 6-hydroxydopamine either in the medial forebrain bundle or in the dorsolateral substantia nigra. Another group was injected unilaterally with kainate in the striatum. The loss of neurons was assessed by a reduction in tyrosine hydroxylase-like immunoreactivity for dopaminergic neurons, and choline acetyltransferase-like and glutamate decarboxylase-like immunoreactivities for cholinergic and GABAergic neurons, respectively. Brain sections also were analysed by autoradiography on 20 micron sections with the radio-iodinated serotonin-4 receptor antagonist [125I]SB 207710 [Brown A. M. et al. (1993) Br. J. Pharmac. 110, 10P]. Kainate injections in the striatum resulted in loss of choline acetyltransferase- and glutamate decarboxylase-like immunoreactive cell bodies in this area. There was also a decrease in glutamate decarboxylase-like immunoreactivity on the ipsilateral side in the substantia nigra and entopeduncular nucleus. These changes were accompanied by substantial (> 50%) decreases in [125I]SB 207710 binding in both the ipsilateral striatum (confined to the lesioned area) and substantia nigra, with no change in either the nucleus accumbens or the globus pallidus. There was also significant loss of [125I]SB 207710 binding in the ipsilateral entopeduncular nucleus. 6-Hydroxydopamine lesions placed either in the medial forebrain bundle or in the substantia nigra failed to decrease [125I]SB 207710 binding in any of these areas, although there was total loss of tyrosine hydroxylase-like immunoreactive terminals in the striatum and cell bodies in the nigra. We conclude that serotonin-4 receptors are present on projection neurons, both on their perikarya in the striatum and terminals in the nigra and entopeduncular nucleus. It is likely that these receptors are located on the GABAergic projection neurons and possibly on cholinergic and GABAergic interneurons. However, serotonin-4 receptors are not located on dopaminergic neurons, either on their cell bodies in the substantia nigra or terminals in the striatum.

Characterization of [3H]GR 113808 binding to 5-HT4 receptors in brain tissues from patients with neurodegenerative disorders

[3H]GR 113808 binding studies in guinea-pig brain tissue revealed a temperature dependency and pharmacological specificity consistent with labelling of a 5-HT4 receptor. Detailed comparison of competition data between brain regions from human and guinea-pig suggest minor differences in the rank order of affinity. Studies in brain tissue from patients with Alzheimer's Huntington's and Parkinson's disease have revealed specific deficits in 5-HT4 receptor densities.

New players in the 5-HT receptor field: genes and knockouts

In recent years, cloning has revealed the existence of no less than 14 mammalian 5-HT receptors, as well as evidence that isoforms of the 5-HT4 and 5-HT7 receptors are generated by alternative splicing. In addition, molecular biology techniques have provided new tools with which to study the function of 5-HT receptors. For example, new technologies are emerging that will allow the generation of either inducible or tissue-specific knockouts (or both). In this review José Lucas and René Hen focus on the characteristics of the most recently cloned receptors and the contribution of molecular biology to the understanding of 5-HT receptor function.

Labelling with [125I]-SB 207710 of a small 5-HT4 receptor population in piglet right atrium: functional relevance

1. We investigated the affinity of SB 207710 for sinoatrial 5-HT4 receptors and the density of right atrial 5-HT4 receptors with [125I]-SB 207710 in right atria of new-born piglets. 2. SB 207710 (1-100 nM) antagonized the 5-HT-evoked tachycardia surmountably with a pKB of 9.8. 3. [125I]-SB 207710 (5-1500 pM) labelled a small population of saturable binding sites with a pKD of 10.1 and with 5-HT4 receptor characteristics. The density of atrial binding sites with 5-HT4 receptor characteristics was 174 and 22 times lower respectively than those of atrial beta 1- and beta 2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. 4. We suggest that the small 5-HT4 receptor population may in part explain why the maximal tachycardia caused by 5-HT is smaller than that caused by catecholamines.

5-Hydroxytryptamine (5-HT)4 receptors in post mortem human brain tissue: distribution, pharmacology and effects of neurodegenerative diseases

1. The distribution, pharmacology and effects of neurodegenerative diseases on 5-HT4 receptors in human brain have been characterized in vitro. 2. The 5-HT4 receptor in post mortem human brain tissue was specifically labelled with [3H]-GR 113808. In human putamen, this ligand labelled a homogeneous population of sites, with an apparent affinity (-log Kd) of 10.1 and a density (Bmax) of 5.73 fmol mg-1 tissue. The pharmacology of this site was characterized by use of a series of displacing ligands, and the following rank order of apparent affinities (with mean +/- s.d. -log Ki values in parentheses) was generated: GR113808 (10.05 +/- 0.04) > SDZ 205,557 (8.65 +/- 0.08) > DAU 6285 (7.95 +/- 0.04) > BIMU-1 (7.81 +/- 0.06) > DAU 6215 (7.42 +/- 0.23) > tropisetron (7.39 +/- 0.23) > 5-HT (7.32 +/- 1.00) > BIMU-8 (7.25 +/- 0.04) > (R)-zacopride (5.82 +/- 0.04). The Hill coefficients were not significantly different from unity, consistent with an interaction at a single site. A comparison of the affinities of these compounds with those obtained from guinea-pig striatum indicated no evidence of species differences. 3. The regional distribution of 5-HT4 receptors was assessed by determining the density of binding sites for [3H]-GR 113808. The distribution were as follows (with mean +/- s.d. Bmax values, fmol mg-1 tissue, in parentheses): caudate nucleus (8.7 +/- 1.5), lateral pallidum (8.6 +/- 5.5), putamen (5.7 +/- 3.0), medial pallidum (3.8 +/- 0.9), temporal cortex (2.6 +/- 0.6), hippocampus (2.4 +/- 0.8), amygdala (2.3 +/-1.1), frontal cortex (1.7 +/- 0.5), cerebellar cortex (<1.0). In these studies, the affinities of GR 113808 were not significantly different.4. The density of 5-HT4 receptors selected from regions of post mortem brains of patients with Parkinson's disease, Huntington's disease and Alzheimer's disease were compared to age-matched controls. In Parkinson's disease, there was no significant difference between control or patient values(mean +/- s.d. Bmax values, fmol mg-1 tissue; putamen, control 4.74 +/- 0.07, patient 5.86 +/- 1.48; substantia nigra, control 4.21 +/- 2.56, patient 5.57 +/- 0.10). In Huntington's disease, there was a significant decrease in putamen (control 5.33 +/- 1.08, patient 2.68 +/- 1.08), while in Alzheimer's disease, there was a marked loss of receptors in hippocampus (control 2.34 +/- 0.62, patient 0.78 +/- 0.61), in frontal cortex (control,1.76 +/- 0.19, patient 1.30 +/- 0.22). Receptor density in temporal cortex showed a decrease, but did not achieve statistical significance (control 2.06 +/- 0.21, patient 1.44 +/- 0.64).5. These data suggest a heterogeneous distribution of 5-HT4 receptors in human brain, with high to moderate densities in basal ganglia and limbic structures. These receptors may not be principally co-localized on dopaminergic cell bodies or terminals, given the lack of change observed in Parkinson's disease. The loss of 5-HT4 receptors in the putamen in Huntington's disease raises the possibility of their presence on intrinsic striatal GABAergic or cholinergic neurones. The marked loss of receptors in hippocampal and cortical regions in the brains from patients with Alzheimer's disease is consistent with a role for the 5-HT4 receptor in cognitive processing.

Identification of serotonin 5-HT4 recognition sites in the porcine caudate nucleus by radioligand binding

Specific binding for the serotonin 5-HT4 receptor (5-HT4R) radioligand [3H]GR 113808 was identified in pig caudate nucleus and characterized by serotonin subtype selective drugs. Binding was inhibited by serotonin and by synthetic indoles, benzamides and benzimidazolones known to characterize the 5-HT4R in functional tests. Rank order of potency of 5-HT4R antagonists was: GR 125487 (Ki, 0.19 nM) > GR 113808 >> SC 53606 > SDZ 205,557 > RS 235971/190 > DAU 6285 > tropisetron > DAU 6215. GR 125487 and GR 113808 were highly selective with respect to the 5-HT3 receptor (5-HT3R). Rank order of potency of 5-HT4R agonists was: SC 53116 (Ki, 21 nM) > BIMU 1 > cisapride > BIMU 8 > serotonin > renzapride > S-zacopride > metoclopramide > R-zacopride > 5-methoxytryptamine >> 5-carboxamidotryptamine. BIMU 8, renzapride, metoclopramide and the zacopride enantiomers gave shallow competition curves. The agonists were substantially less selective than the antagonists with respect to the 5-HT3R. With only two exceptions, SCH 23390 and metergoline, which bound with sub-microM affinity to the 5-HT4R, binding was not inhibited by compounds selective for other G-protein-coupled or channel-gated receptors. Highly significant correlations in affinities of compounds for 5-HT4R in caudata of pigs, guinea pigs and humans were found suggesting no difference among mammalian species.