Isolation of the serotoninergic 5-HT4(e) receptor from human heart and comparative analysis of its pharmacological profile in C6-glial and CHO cell lines

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Luminal 5-HT4 receptors-A successful target for prokinetic actions

The prokinetic effects of 5-HT₄ receptor (5-HT₄ R) agonists have been utilized clinically for almost three decades to relieve symptoms of constipation. Surprisingly, the mechanism(s) of action of these compounds is still being debated. Recent studies highlight luminal 5-HT₄ Rs as an alternative and effective target for these prokinetic agents. These include the study by Shokrollahi et al (2019, Neurogastroenterol Motil, e13598) published in the current issue of Neurogastroenterology and Motility, who found that activation of mucosal 5-HT₄ Rs by intraluminal prucalopride, significantly enhanced propulsive motor patterns in rabbit colon. The authors highlight the idea that development of agonists targeting luminal 5-HT₄ Rs in the colonic mucosa might be more effective and safer in achieving prokinetic effects on intestinal motility. The purpose of this mini-review is to discuss the evidence for luminal 5-HT₄ Rs as an emerging target for prokinetic agents in facilitating propulsive motor patterns in the colon.

Novel multitarget-directed ligands targeting acetylcholinesterase and σ1 receptors as lead compounds for treatment of Alzheimer's disease: Synthesis, evaluation, and structural characterization of their complexes with acetylcholinesterase

Pleiotropic intervention may be a requirement for effective limitation of the progression of multifactorial diseases such as Alzheimer's Disease. One approach to such intervention is to design a single chemical entity capable of acting on two or more targets of interest, which are accordingly known as Multi-Target Directed Ligands (MTDLs). We recently described donecopride, the first MTDL able to simultaneously inhibit acetylcholinesterase and act as an agonist of the 5-HT₄ receptor, which displays promising activities in vivo. Pharmacomodulation of donecopride allowed us to develop a novel series of indole derivatives possessing interesting in vitro activities toward AChE and the σ₁ receptor. The crystal structures of complexes of the most promising compounds with Torpedo californica AChE were solved in order to further understand their mode of inhibition.

Synthesis and SAR of Imidazo[1,5-a]pyridine derivatives as 5-HT4 receptor partial agonists for the treatment of cognitive disorders associated with Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease which has a higher prevalence and incidence in older people. The need for improved AD therapies is unmet. The 5-hydroxytryptamine4 receptor (5-HT4R) partial agonists may be of benefit for both the symptomatic and disease-modifying treatment of cognitive disorders associated with AD. Herein, we report the design, synthesis and SAR of imidazo[1,5-a] pyridine derivatives as 5-HT4R partial agonists. The focused SAR, optimization of ADME properties resulted the discovery of compound 5a as potent, selective, brain penetrant 5-HT4 partial agonist as a lead compound with good ADME properties and efficacy in both symptomatic and disease modifying animal models of cognition.

Distribution of 5-HT3, 5-HT4, and 5-HT7 Receptors Along the Human Colon

Background/Aims

Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT₃ and 5-HT₄ receptors are clinically relevant. The aim was to examine the distribution of 5-HT₃, 5-HT₄, and 5-HT₇ receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine.

Methods

Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction.

Results

The 5-HT₃ receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT₃ receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT₄ and 5-HT₇ receptors were expressed throughout the colon although the 5-HT₄ receptor d, g, and i variants were expressed less often.

Conclusions

The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT₃ receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.

5-HT4-receptors modulate induction of long-term depression but not potentiation at hippocampal output synapses in acute rat brain slices

The subiculum is the principal target of CA1 pyramidal cells and mediates hippocampal output to various cortical and subcortical regions of the brain. The majority of subicular pyramidal cells are burst-spiking neurons. Previous studies indicated that high frequency stimulation in subicular burst-spiking cells causes presynaptic NMDA-receptor dependent long-term potentiation (LTP) whereas low frequency stimulation induces postsynaptic NMDA-receptor-dependent long-term depression (LTD). In the present study, we investigate the effect of 5-hydroxytryptamine type 4 (5-HT4) receptor activation and blockade on both forms of synaptic plasticity in burst-spiking cells. We demonstrate that neither activation nor block of 5-HT4 receptors modulate the induction or expression of LTP. In contrast, activation of 5-HT4 receptors facilitates expression of LTD, and block of the 5-HT4 receptor prevents induction of short-term depression and LTD. As 5-HT4 receptors are positively coupled to adenylate cyclase 1 (AC1), 5-HT4 receptors might modulate PKA activity through AC1. Since LTD is blocked in the presence of 5-HT4 receptor antagonists, our data are consistent with 5-HT4 receptor activation by ambient serotonin or intrinsically active 5-HT4 receptors. Our findings provide new insight into aminergic modulation of hippocampal output.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Association of 5-hydroxytryptamine (serotonin) receptor 4 (5-HTR4) gene polymorphisms with asthma

BACKGROUND AND OBJECTIVE

The neurotransmitter, 5-hydroxytryptamine, acts as an immunomodulator by stimulating the release of inflammatory cytokines and regulating the function of dendritic cells and monocytes. The 5-hydroxytryptamine receptor 4 (HTR4) gene is located in a region previously linked to an increased risk of asthma and atopy. The aim of this study was to investigate the association between HTR4 and asthma.

METHODS

Thirty-two single nucleotide polymorphisms (SNP) in HTR4 were investigated by direct sequencing of 24 DNA samples from unrelated Korean subjects.

RESULTS

The 32 genetic variants comprised 22 intronic SNP, two SNP in the 3'-untranslated region (exon 7) and eight SNP in the 3'-downstream region. Logistic regression analysis showed that two intronic polymorphisms were significantly associated with the risk of asthma. Two minor HTR4 alleles, +142828G>A and +122769G>A, occurred at significantly higher frequencies in the asthmatic group than in the healthy control group (49.59% vs 42.29%, P=0.003, and 47.99% vs 40.35%, P=0.008, respectively), and these differences remained significant after correction for multiple testing (P=0.05, dominant mode of inheritance; and P=0.03, dominant mode, respectively). Haplotype analysis revealed three haplotype blocks. The frequency of haplotype 1 in block 2 was significantly higher in asthmatics (P=0.003, dominant mode), whereas the frequency of haplotype 4 in block 3 was significantly lower in asthmatics (P=0.0009, dominant mode).

CONCLUSIONS

SNP and haplotypes of the HTR4 gene were associated with the asthma phenotype and genetic variation of HTR4 may affect susceptibility to the development of asthma.

Serotonin Receptors – From Molecular Biology to Clinical Applications

Serotonin (5-hydroxytryptamine) is an ubiquitary monoamine acting as one of the neurotransmitters at synapses of nerve cells. Serotonin acts through several receptor types and subtypes. The profusion of 5-HT receptors should eventually allow a better understanding of the different and complex processes in which serotonin is involved. Its role is expected in the etiology of several diseases, including depression, schizophrenia, anxiety and panic disorders, migraine, hypertension, pulmonary hypertension, eating disorders, vomiting and irritable bowel syndromes. In the past 20 years, seven distinct families of 5-HT receptors have been identified and various subpopulations have been described for several of them. Increasing number of 5-HT receptors has made it difficult to unravel the role of 5-HT receptor subpopulations due to the lack of suitable selective agents. The present review describes the different populations and nomenclature of recently discovered 5-HT receptors and their pharmacological relevance.

Human 5-HT(4) and 5-HT(7) receptor splice variants: are they important?

G-protein-coupled receptors (GPCRs), which are encoded by >300 genes in the human genome, are by far the largest class of targets for modern drugs. These macromolecules display inherent adaptability of function, which is partly due to the production of different forms of the receptor protein. These are commonly called 'isoforms' or 'splice variants' denoting the molecular process of their production/assembly. Not all GPCRs are expressed as splice variants, but certain subclasses of 5-HT receptors are for example, the 5-HT(4) and 5-HT(7) receptors. There are at least 11 human 5-HT(4) and three h5-HT(7) receptor splice variants. This review describestheir discoveries, nomenclature and structures. The discovery that particular splice variants are tissue specific (or prominent) has highlighted their potential as future drug targets. In particular, this review examines the functional relevance of different 5-HT(4) and 5-HT(7) receptor splice variants. Examples are given to illustrate that splice variants have differential modulatory influences on signalling processes. Differences in agonist potency and efficacies and also differences in desensitisation rates to 5-HT occur with both 5-HT(4) and 5-HT(7) receptor splice variants. The known and candidate signalling systems that allow for splice variant specific responses include GPCR interacting proteins (GIPs) and GPCR receptor kinases (GRKs) which are examined.Finally, the relevance of 5-HT receptor splice variants to clinical medicine and to the pharmaceutical industry is discussed.

Interactions of Magnolia and Ziziphus extracts with selected central nervous system receptors

ETHNOPHARMACOLOGICAL RELEVANCE

Magnolia officinalis Rehder and Wilson [Magnoliaceae] bark and Ziziphus spinosa (Buhge) Hu ex. Chen. [Fam. Rhamnaceae] seed have a history of use in traditional Asian medicine for mild anxiety, nervousness and sleep-related problems.

AIM OF THE STUDY

To identify pharmacological targets, extracts of Magnolia officinalis (ME), Ziziphus spinosa (ZE), and a proprietary fixed combination (MZE) were tested for affinity with central nervous system receptors associated with relaxation and sleep.

METHODS

In vitro radioligand binding and cellular functional assays were conducted on: adenosine A(1), dopamine (transporter, D(1), D(2S), D(3), D(4.4) and D(5)), serotonin (transporter, 5-HT(1A), 5-HT(1B), 5-HT(4e), 5-HT(6) and 5-HT(7)) and the GABA benzodiazepine receptor.

RESULTS

Interactions were demonstrated with the adenosine A(1) receptor, dopamine transporter and dopamine D(5) receptor (antagonist activity), serotonin receptors (5-HT(1B) and 5-HT(6) antagonist activity) and the GABA benzodiazepine receptor at a concentration of 100 microg/ml or lower. ME had an affinity with adenosine A(1) (K(i) of 9.2+/-1.1 microg/ml) and potentiated the GABA activated chloride current at the benzodiazepine subunits of the GABA receptor (maximum effect at 50 microg/ml). ME had a modest antagonist action with 5-HT(6) and ZE with the 5-HT(1B) receptor.

CONCLUSION

The interactions in the receptor binding models are consistent with the traditional anxiolytic and sleep-inducing activities of Magnolia officinalis bark and Ziziphus spinosa seed.

Design, synthesis, and biological evaluation of new 5-HT4 receptor agonists: application as amyloid cascade modulators and potential therapeutic utility in Alzheimer's disease

Serotonin 5-HT(4) receptor (5-HT(4)R) agonists are of particular interest for the treatment of Alzheimer's disease because of their ability to ameliorate cognitive deficits and to modulate production of amyloid beta-protein (Abeta). However, despite the range of 5-HT(4)R agonists synthesized to date, potent and selective 5-HT(4)R agonists are still lacking. In the present study, two libraries of molecules based on the scaffold of ML10302, a highly specific and partial 5-HT(4)R agonist, were efficiently prepared by parallel supported synthesis and their binding affinities and agonist activities evaluated. Furthermore, we showed that, in vivo, the two best candidates exhibited neuroprotective activity by increasing the level of the soluble form of the amyloid precursor protein (sAPPalpha) in the cortex and hippocampus of mice. Interestingly, one of these compounds could also inhibit Abeta fibril formation in vitro.

Contribution of active and inactive states of the human 5-HT4d receptor to the functional activities of 5-HT4-receptor agonists

In the present study, binding affinities of 5-hydroxytryptamine-4 (5-HT(4)) ligands for the human 5-HT(4d) receptor were determined using the agonist [(3)H]5-HT and the selective 5-HT(4) antagonist [(3)H]GR113,808. We also compared the affinity differences between [(3)H]5-HT binding (K(H)) and [(3)H]GR113,808 binding (K(L)) with their activities as 5-HT(4) ligands. Binding studies using [(3)H]5-HT revealed that the human 5-HT(4d) receptor has two binding sites, whereas [(3)H]GR113,808 yielded a single binding site. Additionally, the number of [(3)H]5-HT binding sites decreased in the presence of guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS), but the number of [(3)H]GR113,808 sites did not change. In competitive binding assays, full agonists such as 5-methoxytryptamine and tegaserod showed 2- to 8-fold higher affinities for [(3)H]5-HT binding (K(H)) than for [(3)H]GR113,808 binding (K(L)) (K(H)<K(L)). Conversely, antagonists showed lower affinities for [(3)H]5-HT binding than for [(3)H]GR113,808 binding (K(H)>K(L)). Finally, partial agonists displayed similar binding affinities for both radioligands (K(H) = K(L)). These findings suggest that the equilibrium between active and inactive states of the human 5-HT(4d) receptor relies on the functional activities of 5-HT(4) ligands, and these states affect the affinities of 5-HT(4) ligands in the competitive binding assay.

Pharmacology and metabolism of renzapride : a novel therapeutic agent for the potential treatment of irritable bowel syndrome

BACKGROUND AND OBJECTIVE

Renzapride (ATL-1251), a novel benzamide, is currently under clinical development for the treatment of irritable bowel syndrome (IBS). Previous in vitro and in vivo experimental studies have characterized renzapride as a full serotonin 5-HT(4) receptor agonist on the gut and a 5-HT(3) receptor antagonist. Clinical studies have confirmed the therapeutic efficacy, tolerability and safety of renzapride in patients with constipation-predominant IBS. This study set out to characterize the pharmacological profile of renzapride and its potential metabolic products at both 5-HT and other monoamine receptors in the gut.

METHODS

The affinity of renzapride, its (+) and (-) enantiomers, and its primary metabolite, renzapride N-oxide and its enantiomers, for serotonin receptors was assessed by means of in vitro radioligand binding inhibition studies. After membranes prepared from animal tissue or membranes of cell lines transfected with cloned human receptors had been incubated with radiolabelled ligand with high affinity for a specific receptor, renzapride was added to competitively inhibit this binding. Levels of bound radioligand were measured by filtration and counting of the bound radioactivity. In instances where >50% inhibition of radioligand binding had occurred, the inhibition constant (K(i)) was calculated. Metabolism of renzapride by liver microsomes was assessed by incubating 10 micromol/L renzapride with human liver microsome samples for 60 minutes at 37 degrees C. After the reaction was stopped, the samples were centrifuged and the supernatant analysed for metabolites by high-pressure liquid chromatography (HPLC). The potential inhibitory effects of renzapride on cytochrome P450 (CYP) enzymes were assessed by incubating renzapride at various concentrations over a 1-500 micromol/L concentration range with microsomes genetically engineered to express a single CYP.

RESULTS

Renzapride was selective for serotonergic receptors and, in particular, had high affinity for human 5-HT(3) and guinea-pig 5-HT(4) receptors (K(i) 17 and 477 nm, respectively). Inhibitory properties at 5-HT(2B) receptors were also identified for renzapride, as well as some affinity for 5-HT(2A) and 5-HT(2C) receptors. Renzapride N-oxide and its enantiomers demonstrated much lower affinity for all 5-HT receptors compared with renzapride. Renzapride was metabolized by liver microsomes to a limited extent and there was no significant non-microsomal metabolism of renzapride. Renzapride did not inhibit the major CYP drug-metabolizing enzymes CYP2C9, CYP2D6, CYP1A2, CYP2A6, CYP2C19, CYP2E1 or CYP3A4 at concentrations consistent with use in a clinical setting.

CONCLUSIONS

These results confirm and extend earlier studies in animal and human receptors that show renzapride is a potent and generally full 5-HT(4) receptor agonist and 5-HT(3) receptor antagonist. The results reported in the present study indicate that the metabolites of renzapride are minor and are unlikely to contribute to its therapeutic profile or lead to interaction of renzapride with other drugs that inhibit the major drug-metabolizing enzymes in the liver at therapeutic doses. These data contribute to the understanding of the pharmacological actions and metabolic fate of renzapride in vivo.

The in vitro pharmacological profile of TD-5108, a selective 5-HT(4) receptor agonist with high intrinsic activity

The in vitro pharmacological profile of TD-5108, a novel, selective 5-HT(4) receptor agonist, was compared to that of clinically efficacious gastroprokinetic 5-HT(4) receptor agonists. TD-5108 produced an elevation of cyclic adenosine monophosphate in human embryonic kidney 293 cells expressing the human recombinant 5-HT(4(c)) (h5-HT(4(c))) receptor (pEC(50) = 8.3) and 5-HT(4) receptor-mediated relaxation of the rat esophagus (pEC(50) = 7.9) and contraction of the guinea pig colon (pEC(50) = 7.9). In all in vitro assays, TD-5108 was a high intrinsic activity agonist, unlike tegaserod, mosapride, and cisapride which, in the majority of test systems, had lower intrinsic activity. TD-5108 had high affinity (pK (i) = 7.7) and selectivity (> or =25-fold) for h5-HT(4(c)) receptors over other biogenic amine receptors. TD-5108 was >500-fold selective over other 5-HT receptors (including h5-HT(2B) and h5-HT(3A)) and, at 3 microM, had no effect on human ether-à-go-go-related gene K+ channels. In conclusion, TD-5108 is a selective 5-HT(4) receptor agonist in vitro. The high intrinsic activity and preferential binding of TD-5108 to 5-HT4 over other 5-HT receptors may result in an improved clinical profile for the treatment of gastrointestinal disorders of reduced motility.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

The cardiovascular safety profile of renzapride, a novel treatment for irritable bowel syndrome

The cardiac safety of renzapride, a novel benzamide currently under clinical development for the treatment of irritable bowel syndrome, was investigated in a four-way randomized crossover electrocardiographic clinical study in healthy human subjects and also in an in vitro cardiac conductivity study in sheep isolated Purkinje fibres. The primary endpoint in the clinical study was prolongation of the individually corrected QT interval (QTci). No clinically or statistically significant prolongation of QTci after 4 or 20 mg renzapride compared with placebo was observed. The relative effects of renzapride and cisapride in the in vitro study showed that the cardiac action potential duration was unaltered by 0.2 and 2 microM renzapride, shortened by 20 microM renzapride, and prolonged by 1 microM cisapride. Cisparide was also a 1000-fold more potent inhibitor of human ether-a-go-go related gene (hERG) channels in HEK293 cells than renzapride. These studies indicate that therapeutic doses of renzapride are unlikely to prolong cardiac action potentials and, therefore, are also unlikely to cause cardiac arrhythmias in clinical use.

Synthesis of specific bivalent probes that functionally interact with 5-HT(4) receptor dimers

G-protein-coupled receptor dimerization directs the design of new drugs that specifically bind to receptor dimers. Here, we generated a targeted series of homobivalent ligands for serotonin 5-HT(4) receptor (5-HT(4)R) dimers composed of two 5-HT(4)R-specific ML10302 units linked by a spacer. The design of spacers was assisted by molecular modeling using our previously described 5-HT(4)R dimer model. Their syntheses were based on Sonogashira-Linstrumelle coupling methods. All compounds retained high-affinity binding to 5-HT(4)R but lost the agonistic character of the monomeric ML10302 compound. Direct evidence for the functional interaction of both pharmacophores of bivalent ligands with the 5-HT(4)R was obtained using a bioluminescence resonance energy transfer (BRET) based assay that monitors conformational changes within 5-HT(4) dimers. Whereas the monovalent ML10302 was inactive in this assay, several bivalent derivatives dose-dependently increased the BRET signal, indicating that both pharmacophores functionally interact with the 5-HT(4) dimer. These bivalent ligands may serve as a new basis for the synthesis of potential drugs for 5-HT(4)-associated disorders.

Immunomodulation by maternal autoantibodies of the fetal serotoninergic 5-HT4 receptor and its consequences in early BALB/c mouse embryonic development

Background

The presence of functional 5-HT₄ receptors in human and its involvement in neonatal lupus erythematosus (NLE) have prompted us to study the receptor expression and role during embryogenesis. Earlier we managed to demonstrate that female BALB/c mice immunized against the second extracellular loop (SEL) of the 5-HT₄ receptor gave birth to pups with heart block. To explain this phenomenon we investigated the expression of 5-HT₄ receptors during mouse embryogenesis. At the same time we looked whether the consequence of 5-HT₄ receptor immunomodulation observed earlier is in relation to receptor expression.

We studied the expression of 5-HT₄ receptor at the mRNA level and its two isoforms 5-HT_4(a) and 5-HT_4(d) at the protein level in embryos from BALB/c mice, at 8^th, 12^th, 18^th gestation days (GD) and 1 day post natal (DPN). Simultaneously the receptor activity was inhibited by rising antibodies, in female mice against SEL of the receptor. The mice were mated and embryos were collected at 8^th, 12^th, 18^th GD and 1 DPN.

Results

5-HT₄ receptor mRNA increased in brain from 12^th GD to 1 DPN. Its expression gradually decreased in heart and disappeared at birth. This was consistent with expression of the receptor isoforms 5-HT_{4(a) and (d)}. Abnormalities like decreased number of embryos, growth delay, spina bifida and sinus arrhythmia from 12^th GD were documented in pups of mice showing anti-5-HT₄ receptor antibodies.

Conclusion

serotoninergic 5-HT₄ receptor plays an important role in mouse foetal development. In BALB/c mice there is a direct relation between the expression of receptor and the deleterious effect of maternal anti-5-HT₄ receptor autoantibodies in early embryogenesis.

Conformational state of human cardiac 5-HT(4(g)) receptors influences the functional effects of polyclonal anti-5-HT(4) receptor antibodies

The functional effects of the anti-G21V antibody directed against the second extracellular loop of human heart 5-HT(4) receptors can differ when the receptors are expressed in different cell lines. Here, we extend these studies to show variation in the responses of 5-HT(4(g)) receptors to the antibody within the same expression system. In a previous report no effect of the anti-G21V antibodies had been shown upon 5-HT(4(g)) receptors expressed in CHO cells. Here the same antibodies alone or when added before 5-HT had a functional "inverse-agonist like" effect upon 5-HT(4(g)) receptors expressed in a separate line of CHO cells. Although these CHO cells showed a lower efficacy of cAMP production evoked by 5-HT than the previous report they express a similar h5-HT(4(g)) receptor density. Inhibition of either phosphodiesterases or Gi proteins had no effect upon the action of the antibody. Conformational states of the 5-HT(4) receptor and/or equilibrium between different states of receptors may then determine the functional effect of antibodies against this receptor.

A comparison of the pharmacological properties of guinea-pig and human recombinant 5-HT4 receptors

BACKGROUND AND PURPOSE

5-HT(4) receptor agonists are used therapeutically to treat disorders of reduced gastrointestinal motility. Since such compounds are evaluated in guinea-pigs, we cloned, expressed and pharmacologically characterized the guinea-pig 5-HT(4) and human 5-HT(4(b)) splice variant, which share 95% homology. The functional properties of guinea-pig 5-HT(4(b)) receptors were compared with native receptors in guinea-pig colon.

EXPERIMENTAL APPROACH

Membrane radioligand binding and whole cell cAMP accumulation assays were used to determine the affinities, potencies and intrinsic activities (IA). Contraction of the guinea-pig distal colon longitudinal muscle myenteric plexus preparation (LMMP) was monitored to evaluate functional activity.

KEY RESULTS

pK(i) values for guinea-pig and human recombinant receptors, and guinea-pig striatum 5-HT(4) receptors, were in agreement, as were the potency and IA values for guinea-pig and human 5-HT(4) receptors expressed at a similar density ( approximately 0.2 pmol mg(-1) protein). Tegaserod was a potent (pEC(50)=8.4 and 8.7, respectively), full agonist at both guinea-pig and human 5-HT(4) receptors. In contrast, in the LMMP preparation, tegaserod was a potent, partial agonist (pEC(50)=8.2; IA=66%).

CONCLUSIONS AND IMPLICATIONS

Close agreement between the pharmacological properties of guinea-pig and human 5-HT(4) receptors support the use of guinea-pig model systems for the identification of 5-HT(4) receptor therapeutics. However, the mechanisms underlying the different agonist properties of tegaserod in recombinant and isolated tissue preparations, and the extent to which these impact the clinical efficacy of tegaserod as a prokinetic agent, remain to be determined.

Porcine left atrial and sinoatrial 5-HT(4) receptor-induced responses: fading of the response and influence of development

1.--In this study, we aimed to characterize in vitro the effects of the benzofuran 5-HT(4) receptor agonists prucalopride, R149402 and R199715 and the indolic agents tegaserod and 5-HT in the atria of young pigs (10-11 weeks) and newborn piglets. 2.--In the paced left atrium of young pigs, only 5-HT results in positive inotropic responses when administered cumulatively (maximal effect relative to isoprenaline=53%, pEC(50)=6.8); however, all agonists showed lusitropic effects. Noncumulative administration results in greater positive inotropic responses for 5-HT and induces moderate positive inotropic responses for the other agonists; these responses fade. 3.--Phosphodiesterase (PDE) enzyme inhibition with 3-isobutyl-1-methylxanthine (IBMX; 20 microM) enhances the responses to cumulatively administered 5-HT (maximal effect=89%, pEC(50)=7.7) and reveals clear positive inotropic effects for prucalopride, tegaserod, R149402 and R199715; fading is abolished. The maximal effect of the benzofurans is less pronounced than that of the indoles. 4.--In the spontaneously beating right atrium of young pigs, all agonists show chronotropic activity when administered cumulatively in the absence of IBMX, without fade. Benzofurans behaved as partial agonists compared to 5-HT (maximal effect=54%, pEC(50)=6.5). 5.--In newborns, the inotropic activity of the agonists in the IBMX-treated left atrium was less pronounced than in the young pig; the same applied for the chronotropic response in the right atrium, except for 5-HT. 6.--In conclusion, the atrial responses to 5-HT(4) receptor activation increase in the first months of life; the inotropic response is regulated by PDEs. Prucalopride, R149402 and R199715 are partial agonists compared to 5-HT.

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

Expression and function of 5-HT4 receptors in the mouse enteric nervous system

The aim of the current study was to identify enteric 5-HT(4) splice variants, locate enteric 5-HT(4) receptors, determine the relationship, if any, of the 5-HT(4) receptor to 5-HT(1P) activity, and to ascertain the function of 5-HT(4) receptors in enteric neurophysiology. 5-HT(4a), 5-HT(4b), 5-HT(4e), and 5-HT(4f) isoforms were found in mouse brain and gut. The ratio of 5-HT(4) expression to that of the neural marker, synaptophysin, was higher in gut than in brain but was similar in small and large intestines. Submucosal 5-HT(4) expression was higher than myenteric. Although transcripts encoding 5-HT(4a) and 5-HT(4b) isoforms were more abundant, those encoding 5-HT(4e) and 5-HT(4f) were myenteric plexus specific. In situ hybridization revealed the presence of transcripts encoding 5-HT(4) receptors in subsets of enteric neurons, interstitial cells of Cajal, and smooth muscle cells. IgY antibodies to mouse 5-HT(4) receptors were raised, affinity purified, and characterized. Nerve fibers in the circular muscle and the neuropil in ganglia of both plexuses were highly 5-HT(4) immunoreactive, although only a small subset of neurons contained 5-HT(4) immunoreactivity. No 5-HT(4)-immunoreactive nerves were detected in the mucosa. 5-HT and 5-HT(1P) agonists evoked a G protein-mediated long-lasting inward current that was neither mimicked by 5-HT(4) agonists nor blocked by 5-HT(4) antagonists. In contrast, the 5-HT(4) agonists renzapride and tegaserod increased the amplitudes of nicotinic evoked excitatory postsynaptic currents. Enteric neuronal 5-HT(4) receptors thus are presynaptic and probably exert their prokinetic effects by strengthening excitatory neurotransmission.

A high-affinity monoclonal antibody with functional activity against the 5-hydroxytryptaminergic (5-HT4) receptor

Splenocytes from a BALB/c mouse immunised with a synthetic peptide corresponding to the second extracellular loop of the 5-HT4 receptor were fused with SP2/O myeloma cells to produce a monoclonal antibody. The monoclonal antibody was of the IgG2b isotype. The antibody recognised the human 5-HT4(g) (h5-HT4(g)) receptor by immunoblots and by immunofluorescence on chinese hamster ovary (CHO) cells expressing this 5-HT4 receptor isoform. Epitope mapping of the antibody suggested the recognition of a conformational epitope, encompassing the N- and C-terminal fragments of the second extracellular loop. Kinetic experiments using surface plasmon resonance showed that the antibody had a picomolar affinity for its cognate peptide. Inhibition experiments using the same methodology confirmed the specificity of the interaction. The antibody at a concentration of 500 pM competitively inhibited inverse agonist GR113808 binding and showed an inverse agonist effect on the basal activity of CHO cells expressing the 5-HT4(g) receptor. The antibody decreased the effect of 5-HT at 500 and 50 pM concentrations but it increased 5-HT-induced cAMP levels at 5 pM. The dual effect of the monoclonal antibody could be ascribed to mono- or bivalent recognition of the receptor. The antibody described here is the first example of a high-affinity modulator of the 5-HT4 receptor.

Constitutive dimerization of human serotonin 5-HT4 receptors in living cells

Serotonin 5-HT4 receptor isoforms are G protein-coupled receptors (GPCRs) with distinct pharmacological properties and may represent a valuable target for the treatment of many human disorders. Here, we have explored the process of dimerization of human 5-HT4 receptor (h5-HT4R) by means of co-immunoprecipitation and bioluminescence resonance energy transfer (BRET). Constitutive h5-HT4(d)R dimer was observed in living cells and membrane preparation of CHO and HEK293 cells. 5-HT4R ligands did not influence the constitutive energy transfer of the h5-HT4(d)R splice variant in intact cells and isolated plasma membranes. In addition, we found that h5-HT4(d)R and h5-HT4(g)R which structurally differ in the length of their C-terminal tails were able to form constitutive heterodimers independently of their activation state. Finally, we found that coexpression of h5-HT4R and beta2-adrenergic receptor (beta2AR) led to their heterodimerization. Given the large number of h5-HT4R isoforms which are coexpressed in a same tissue, our results points out the complexity by which this 5-HTR sub-type mediates its biological effects.

Functional studies of the 5'-untranslated region of human 5-HT4 receptor mRNA

The serotonin 5-HT4 receptor (where 5-HT stands for 5-hydroxy-tryptamine) is a member of the seven transmembrane-spanning G-protein-coupled family of receptors and mediates many cellular functions both in the central nervous system and at the periphery. In the present study, we isolated and characterized the 5'-flanking region of the h5-HT4 (human 5-HT4) receptor. We demonstrate the existence of a novel exon that corresponds to the 5'-untranslated region of the h5-HT4 receptor gene. RNase protection analysis and reverse transcriptase-PCR experiments performed on human atrial RNA demonstrated that the major transcription start site of the h5-HT4 receptor gene is located at -3185 bp relative to the first ATG codon. In addition, a 1.2 kb promoter fragment which drives the transcription of the 5-HT4 receptor was characterized. The promoter region lacks TATA and CAAT canonical motifs in the appropriate location, but contains putative binding sites for several transcription factors. Transient transfection assays revealed that the (-3299/-3050) gene fragment possesses the ability to promote the expression of the luciferase reporter gene in human cell lines. In contrast, the promoter was silent in monkey COS-7 cells, indicating the requirement of specific factors to initiate transcription in human cells. In addition to the promoter element, enhancer activity was found in a region (-220/-61) located in the long 5'-untranslated region. Mutational analysis, gel shift and transfection assays identified an Nkx2.5 (NK2-transcription-factor-related 5)-like binding site as a regulatory sequence of this enhancer. Our results suggest a complex regulation of the h5-HT4 receptor gene expression involving distinct promoters and non-coding exons.

Palmitoylation of the 5-hydroxytryptamine4a receptor regulates receptor phosphorylation, desensitization, and beta-arrestin-mediated endocytosis

The mouse 5-hydroxytryptamine4a (5-HT4a) receptor is an unusual member of the G protein-coupled receptor superfamily because it possesses two separate carboxyl-terminal palmitoylation sites, which may allow the receptor to adopt different conformations in an agonist-dependent manner (J Biol Chem 277:2534-2546, 2002). By targeted mutation of the proximal (Cys-328/329) or distal (Cys-386) palmitoylation sites, or a combination of both, we generated 5-HT4a receptor variants with distinct functional characteristics. In this study, we showed that upon 5-HT stimulation, the 5-HT4a receptor undergoes rapid (t(1/2) approximately 2 min) and dose-dependent (EC50 approximately 180 nM) phosphorylation on serine residues by a staurosporine-insensitive receptor kinase. Overexpression of GRK2 significantly reduced the receptor-promoted cAMP formation. The Cys328/329-Ser mutant, which is constitutively active in the absence of ligand, exhibited enhanced receptor phosphorylation under both basal and agonist-stimulated conditions and was more effectively desensitized and internalized via a beta-arrestin-2 mediated pathway compared with the wild-type 5-HT4a. In contrast, G protein activation, phosphorylation, desensitization, and internalization of the other palmitoylation-deficient receptor mutants were affected differently. These findings suggest that palmitoylation plays an important role in modulating 5-HT4a receptor functions and that G protein activation, phosphorylation, desensitization, and internalization depend on the different receptor conformations.

In vitro binding experiments with a Valerian, hops and their fixed combination extract (Ze91019) to selected central nervous system receptors

The fixed valerian-hops extract combination Ze91019 is used as a sleep aid. Although its exact mechanism of action is not well understood, earlier studies indicate that the CNS effect of valerian might occur through interaction with the GABA, melatonin and/or the adenosine systems in the brain. The use of hops in sleep remedies, however, is mainly based on traditional use and scarce scientific information. In this report, the binding of Ze91019, and the component valerian and hops extracts within, was tested on 14 subtypes of five classes of central receptors (dopamine, serotonin, melatonin, MCH and neuropeptide-Y). Binding affinities could be demonstrated at some of the screened melatonin (ML1 and ML2) and serotonin (5-HT4e, 5-HT6 and 5-HT7) receptor subtypes.

Functional serotonin 5-HT4 receptors in porcine and human ventricular myocardium with increased 5-HT4 mRNA in heart failure

Serotonin (5-hydroxytryptamine, 5-HT) increases contractile force and elicits arrhythmias through 5-HT(4) receptors in porcine and human atrium, but its ventricular effects are unknown. We now report functional 5-HT(4) receptors in porcine and human ventricle. 5-HT(4) mRNA levels were determined in porcine and human ventricles and contractility studied in ventricular trabeculae. Cyclic AMP-dependent protein kinase (PKA) activity was measured in porcine ventricle. Porcine and human ventricles expressed 5-HT(4) receptor mRNA. Ventricular 5-HT(4(b)) mRNA was increased by four times in 20 failing human hearts compared with five donor hearts. 5-HT increased contractile force maximally by 16% (EC(50)=890 nM) and PKA activity by 20% of the effects of (-)-isoproterenol (200 microM) in ventricular trabeculae from new-born piglets in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine. In ventricular trabeculae from adult pigs (3-isobutyl-1-methylxanthine present) 5-HT increased force by 32% (EC(50)=60 nM) and PKA activity by 39% of (-)-isoproterenol. In right and left ventricular trabeculae from failing hearts, exposed to modified Krebs solution, 5-HT produced variable increases in contractile force in right ventricular trabeculae from 4 out of 6 hearts and in left ventricular trabeculae from 3 out of 3 hearts- range 1-39% of (-)-isoproterenol, average 8%. In 11 left ventricular trabeculae from the failing hearts of four beta-blocker-treated patients, pre-exposed to a relaxant solution with 0.5 mM Ca(2+) and 1.2 mM Mg(2+) followed by a switch to 2.5 mM Ca(2+) and 1 mM Mg(2+), 5-HT (1-100 microM, 3-isobutyl-1-methylxanthine present) consistently increased contractile force and hastened relaxation by 46% and 25% of (-)-isoproterenol respectively. 5-HT caused arrhythmias in three trabeculae from 3 out of 11 patients. In the absence of phosphodiesterase inhibitor, 5-HT increased force in two trabeculae, but not in another six trabeculae from 4 patients. All 5-HT responses were blocked by 5-HT(4) receptor antagonists. We conclude that phosphodiesterase inhibition uncovers functional ventricular 5-HT(4) receptors, coupled to a PKA pathway, through which 5-HT enhances contractility, hastens relaxation and can potentially cause arrhythmias.

New insights into the human 5-HT4 receptor binding site: exploration of a hydrophobic pocket

A body of evidences suggests that a hydrophobic pocket of the human 5-HT(4) receptor contributes to the high affinity of some bulky 5-HT(4) ligands. A thorough study of this pocket was performed using mutagenesis and molecular modeling. Ligand binding or competition studies with selected bulky ligands (RS39604, RS100235, [(3)H]GR113808 and ML11411) and small ligands (5-HT and ML10375) were carried out on wild-type and mutant receptors (W7.40A/F, Y7.43F, R3.28L) transiently transfected in COS-7 cells. The functional activity of the mutated receptors was evaluated by measuring the ability of 5-HT to stimulate adenylyl cyclase. For W7.40F mutation, no changes in the affinity of studied ligands and in the functional activity of the mutant receptor were observed, in contrary to W7.40A mutation, which abolished both binding of ligands and 5-HT-induced cAMP production. Mutation R3.28L revealed a totally silent receptor with a basal level of cAMP production similar to the mock control despite its ability to product cAMP in the presence of 5-HT. Moreover, a one order loss of affinity of RS39604 and a 45-fold increase of ML11411 affinity were observed. Mutation Y7.43F modified the affinity of GR113808, which displays a 13-fold lower affinity for the mutant than for the wild-type receptor. In conclusion, in the hydrophobic pocket, two polar amino acids are able to interact through hydrogen bonds with bulky ligands depending on their chemical properties. Moreover, these experimental data may validate the proposed new three-dimensional model of the human 5-HT(4) receptor.

Pharmacological aspects of anticancer drug-induced emesis with emphasis on serotonin release and vagal nerve activity

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somatoautonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs such as cisplatin is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestine and the brainstem. It is proposed that cytotoxic drugs evoke 5-HT release from the enterochromaffin (EC) cells in the intestinal mucosa and that the released 5-HT stimulates the 5-HT receptors on the adjacent vagal afferent nerves. The depolarization of the vagal afferent nerves stimulates the vomiting center in the brainstem and eventually induces a vomiting reflex. 5-HT released from EC cells seems to mediate the cisplatin-induced emesis sensitive to 5-HT(3) receptor antagonists. The release of 5-HT from the EC cells, however, is regulated by polymodal mechanisms on autoreceptors or heteroreceptors. The precise role of 5-HT on the occurrence of vomiting has not been fully elucidated. The present review aims to describe the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagus nerve activity. Various methods for predicting emesis are also evaluated.

Crosstalk between Rap1 and Rac regulates secretion of sAPPalpha

Cyclic AMP (cAMP) is produced by activation of Gs protein-coupled receptors and regulates many physiological processes through activation of protein kinase A (PKA). However, a large body of evidence indicates that cAMP also regulates specific cellular functions through PKA-independent pathways. Here, we show that a small GTPase of the Rho family, Rac, is regulated by cAMP in a PKA-independent manner. We also show that Rac activation results from activation of Rap1 through the cAMP guanine nucleotide-exchange factor (GEF) Epac1. Activation of the Gs-coupled serotonin 5-HT(4) receptor initiates this signalling cascade in various cell types. Furthermore, we demonstrate that crosstalk between the Ras and Rho GTPase families is involved in cAMP-dependent processing of amyloid precursor protein (APP), a key protein in Alzheimer's disease. Indeed, Epac1 regulates secretion of the non-amyloidogenic soluble form of APP (sAPPalpha) through Rap1 and Rac. Our data identify an unsuspected connection between two families of small GTPases and imply that Rac can function downstream of cAMP/Epac1/Rap1 in a novel signal transduction secretory pathway.

Synthesis and characterization of the first fluorescent antagonists for human 5-HT4 receptors

Fluorescent antagonists for human 5-HT(4) receptors were synthesized based on ML10302 1, a potent 5-HT(4) receptor agonist and on piperazine analogue 2. These molecules were derived with three fluorescent moieties, dansyl, naphthalimide, and NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl), through alkyl chains. The synthesized molecules were evaluated in binding assays on the recently cloned human 5-HT(4(e)) receptor isoform stably expressed in C6 glial cells with [(3)H]GR113808 as the radioligand. The affinity values depended upon the basal structure together with the alkyl chain length. The derivatives based on ML10302 were more potent ligands than the derivatives based on piperazine analogue. For ML10302-based ligands, dansyl and NBD derivatives attached through a chain length of one carbon atom 17a and 32, respectively, led to affinities close to the affinity of ML10302. The most potent compounds 17a, 28, and 32 produced an inhibition of the 5-HT stimulated cyclic AMP synthesis in the same cellular system with nanomolar K(b) values. Fluorescent properties of 17a, 28, and 32 were more particularly studied. Interactions of the fluorescent ligand 28 with the h5-HT(4(e)) receptor were indicated using h5-HT(4(e)) receptor transfected C6 glial cell membranes and entire cells. Ligand 28 was also used in fluorescence microscopy experiments in order to label h5-HT(4(e)) receptor transfected C6 glial cells, and subcellular localization of these receptors was more precisely determined using confocal microscopy.

Characterization of human 5-HT4(d) receptor desensitization in CHO cells

1 Serotonin 5-HT(4) receptor isoforms differ in their C-terminal tail and yet little is known about their regulation. In this study, we investigated the desensitization of two human 5-HT(4) receptors stably expressed in CHO cells, with a special emphasis on the h5-HT(4(d)) isoform. 2 Exposure of h5-HT(4(d)) and h5-HT(4(e)) receptors to 1 micro M 5-HT induced a rapid desensitization of the adenylyl cyclase response. The h5-HT(4(d)) receptor desensitized with a faster rate (t(1/2)<5 min) than the h5-HT(4(e)) receptor (t(1/2)=15 min) and after 10 min 5-HT treatment cAMP production was reduced by approximately 70%. 3 5-HT-induced h5-HT(4(d)) receptor desensitization was mimicked by 8-Bromo-cAMP, a cAMP analogue, and was inhibited by [n-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide, 2HCl] (H-89), an inhibitor of cAMP-dependent protein kinase (PKA). Inhibitors of endocytosis (sucrose, 0.45 M and concanavaline A, 0.25 mg ml(-1)) partially reversed the h5-HT(4(d)) receptor desensitization process. 4 Given the prominent role of PKA in agonist-induced desensitization, we mutated the four putative PKA phosphorylation sites present in the third intracellular loop (Ser242, Thr253, Thr255) and the C terminal tail (Ser338) of the h5-HT(4(d)) receptor. Surprisingly, mutated receptors in which either one or all four putative phosphorylation sites were substituted to alanine did not impair receptor desensitization suggesting that PKA might act on nonconsensus sites. 5 Altogether, our data demonstrate that the C-terminal tail of h5-HT(4) receptors may influence the rate of agonist-induced desensitization and we provide evidence for a major role of PKA in h5-HT(4(d)) receptor desensitization.

SL65.0155, a novel 5-hydroxytryptamine(4) receptor partial agonist with potent cognition-enhancing properties

SL65.0155 [5-(8-amino-7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-3-[1-(2-phenyl ethyl)-4-piperidinyl]-1,3,4-oxadiazol-2(3H)-one monohydrochloride] is a novel benzodioxanoxadiazolone compound with high affinity for human 5-hydroxytryptamine (5-HT)(4) receptors (K(i) of 0.6 nM) and good selectivity (greater than 100-fold for all other receptors tested). In cells expressing the 5-HT(4(b)) and 5-HT(4(e)) splice variants, SL65.0155 acted as a partial agonist, stimulating cAMP production with a maximal effect of 40 to 50% of serotonin. However, in the rat esophagus preparation, SL65.0155 acted as a 5-HT(4) antagonist with a pK(b) of 8.81. In addition, SL65.0155 potently improved performance in several tests of learning and memory. In the object recognition task, it improved retention at 24 h when administered i.p. or p.o. (0.001-0.1 mg/kg). This effect was antagonized by the 5-HT(4) antagonist SDZ 205,557, itself without effect, demonstrating that the promnesic effects of SL65.0155 are mediated by 5-HT(4) agonism. SL65.0155 also reversed the cognitive deficits of aged rats in the linear maze task and the scopolamine-induced deficit of mice in the water maze task. Furthermore, the combined administration of an inactive dose of SL65.0155 with the cholinesterase inhibitor rivastigmine resulted in a significant promnesic effect, suggesting a synergistic interaction. SL65.0155 was devoid of unwanted cardiovascular, gastrointestinal, or central nervous system effects with doses up to more than 100-fold higher than those active in the cognitive tests. These results characterize SL65.0155 as a novel promnesic agent acting via 5-HT(4) receptors, with an excellent preclinical profile. Its broad range of activity in cognitive tests and synergism with cholinesterase inhibitors suggest that SL65.0155 represents a promising new agent for the treatment of dementia.

Molecular, pharmacological and functional diversity of 5-HT receptors

Serotonin (5-hydroxytryptamine, 5-HT) is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT(1) to 5-HT(7)) largely on the basis of their structural and operational characteristics. Whilst this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity continues to emerge. The challenge for modern 5-HT research has therefore been to define more precisely the properties of the systems that make this incredible diversity possible. Much progress in this regard has been made during the last decade with the realisation that serotonin is possibly the least conservative monoamine transmitter and the cloning of its many receptors. Coupled with the actions of an extremely avid and efficient reuptake system, this array of receptor subtypes provides almost limitless signalling capabilities to the extent that one might even question the need for other transmitter systems. However, the complexity of the system appears endless, since posttranslational modifications, such as alternate splicing and RNA editing, increase the number of proteins, oligomerisation and heteromerisation increase the number of complexes, and multiple G-protein suggest receptor trafficking, allowing phenotypic switching and crosstalk within and possibly between receptor families. Whether all these possibilities are used in vivo under physiological or pathological conditions remains to be firmly established, but in essence, such variety will keep the 5-HT community busy for quite some time. Those who may have predicted that molecular biology would largely simplify the life of pharmacologists have missed the point for 5-HT research in particular and, most probably, for many other transmitters. This chapter is an attempt to summarise very briefly 5-HT receptor diversity. The reward for unravelling this complex array of serotonin receptor--effector systems may be substantial, the ultimate prize being the development of important new drugs in a range of disease areas.

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.

The human serotonin 5-HT4 receptor regulates secretion of non-amyloidogenic precursor protein

The serotonin 5-HT(4) receptor has recently gained a lot of attention for its functional roles in central processes such as memory and cognition. In this study, we show that activation of the human 5-HT(4) (h5-HT(4)) receptor stimulates the secretion of the non-amyloidogenic soluble form of the amyloid precursor protein (sAPPalpha). 5-HT enhanced the level of secreted sAPPalpha in a time- and dose-dependent manner in Chinese hamster ovary cells stably expressing the h5-HT(4(e)) receptor isoform. The increase was inhibited by the selective 5-HT(4) receptor antagonist, GR113808. The 5-HT(4) selective agonists, prucalopride and renzapride, also increased secreted sAPPalpha in IMR32 human neuroblastoma cells. The stimulatory effect of 5-HT was mimicked by forskolin, a direct activator of adenylyl cyclase, and 8-bromo-cAMP, a membrane-permeant cAMP analogue. On the contrary, inhibition of protein kinase A (PKA) by H89 potentiated the 5-HT-induced increase in both secreted and cellular sAPPalpha. This phenomenon involves a novel PKA-independent stimulatory process that overcomes a PKA-dependent inhibitory one. Finally, activation of the h5-HT(4(e)) receptor did not modify extracellular amyloid beta-protein in Chinese hamster ovary cells transfected with the human APP695. Given the neuroprotective and enhancing memory effects of sAPPalpha, our results may open a new avenue for the treatment of Alzheimer's disease.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Quantitative mRNA analysis of five C-terminal splice variants of the human 5-HT4 receptor in the central nervous system by TaqMan real time RT-PCR

5-HT4 receptors mediate several physiological effects of 5-HT, particularly in the central nervous system (CNS), heart and gut. Recently, several C-terminal splice variants of the human 5-HT4 (h5-HT4) receptor have been described, namely h5-HT4(a), h5-HT4(b), h5-HT4(c), h5-HT4(d) and h5-HT4(g). Previous tissue distribution data suggest some degree of specificity in the mRNA expression patterns of the different h5-HT4 receptor splice variants. However, comparison of the mRNA expression profiles of these splice variants is difficult due to the non-quantitative methods used, and in addition, there is very limited data on the expression of each splice variant in human CNS subregions. In the present study we used a single technique, TaqMan real time quantitative RT-PCR, to investigate the mRNA distribution of 5-HT4 receptor C-terminal splice variants in multiple human CNS and peripheral tissues. Using a primer/probe set that amplified all 5-HT4 splice variants (5-HT4pan), the highest CNS expression of 5-HT4 receptor mRNA was observed in basal ganglia, amygdala and hippocampus, consistent with previous studies. h5-HT4(a), h5-HT4(b), h5-HT4(c) and h5-HT4(g) were predominantly expressed in various CNS tissues, compared to most peripheral tissues, but there were differences in expression levels and distribution patterns of each variant. The distribution profile and expression levels observed for the 5-HT4(b) splice variant were virtually identical to that obtained with the 5-HT4pan primer/probe set, whilst the other splice variants were expressed at much lower levels and with different expression patterns obtained with both 5-HT4(b) and 5-HT4pan primer/probe sets. Highest levels of 5-HT4(g) were observed in the hypothalamus and cortex, whilst the 5-HT4(a) variant was highest in the amygdala. 5-HT4(c) expression was highest in the pituitary gland whilst 5-HT4(d) mRNA was only detected in the small intestine at very low levels and not in the CNS. In conclusion, we have shown quantitative differences in the mRNA distribution profiles of the 5-HT4 receptor C-terminal splice variants in human CNS subregions as well as peripheral tissues. In addition, our data suggests that the h5-HT4(b) variant is the most predominant form of the 5-HT4 receptor in humans.

Inhibitory activity of antibodies against the human atrial 5-HT(4)receptor

Antibodies directed against the second extracellular loop of G protein-coupled receptors have been shown to exert "agonist-like" activities. In order to test the hypothesis that this is a general phenomenon, antibodies were raised in rabbits against a synthetic peptide corresponding to the second extracellular loop of the newly sequenced human cardiac 5-HT(4)receptor. The antibodies were affinity-purified and shown to recognize the 5-HT(4)receptor in immunoblots of Chinese hamster ovary (CHO) cells expressing the receptor. The antibodies had no intrinsic effect but could depress the activation of L -type calcium channel induced by serotonin in human atrial cells. This antagonist-like effect was exerted both by intact IgG and by Fab fragments. These results are physiologically important since it has been shown that the 5-HT(4)receptor could be a target for autoantibodies in mothers at risk of giving birth to children with neonatal atrio-ventricular block.

Induction of neonatal lupus in pups of mice immunized with synthetic peptides derived from amino acid sequences of the serotoninergic 5-HT4 receptor

We have previously suggested that the recognition of a cross-reactive epitope on the 5-HT4 receptor and the 52-kDa SSA/Ro protein by serotonin-antagonizing autoantibodies could explain the electrophysiological symptoms of congenital heart block in neonatal lupus. To confirm this hypothesis, we immunized female mice with four synthetic peptides corresponding to the recognized epitopes. All mice developed anti-peptide antibodies, which cross-reacted with the Ro52 and 5-HT4 receptor peptides and recognized both cognate proteins. Peptide-immune mice were mated. The pups from mice immunized with the Ro52 peptides had no symptoms of neonatal lupus apart from bradycardia. However, pups from mice immunized with the 5-HT4 receptor peptides and bradycardia, atrioventricular block of type I or II, longer QT intervals, skin rashes and neuromotor problems. The 5-HT4 receptor was detectable in the different fetal tissues affected (heart, skin and brain) by immunohistochemistry. Hearts from diseased pups were less developed and showed disorganized myocardial hyperplasia, compared to the normal littermates. These results demonstrate that the serotoninergic 5-HT4 receptor is the antigenic target of physiopathological autoantibodies in neonatal lupus.

New arylpiperazine derivatives as antagonists of the human cloned 5-HT(4) receptor isoforms

New derivatives of arylpiperazine 9 were designed from ML 10302, a potent 5-HT(4) receptor agonist in the gastrointestinal system. Compounds were synthesized by condensation of a number of available arylpiperazines or heteroarylpiperazines with 2-bromoethyl 4-amino-5-chloro-2-methoxybenzoate. They were evaluated in binding assays on the recently cloned human 5-HT(4(e)) isoform stably expressed in C6 glial cells with [(3)H]GR 113808 as the radioligand. The affinity values (K(i)) depended upon the substituent on the aromatic ring. A chlorine atom produced a marked drop in activity (K(i) > 100 nM), while a m-methoxy group gave a compound with nanomolar affinity (K(i) = 3 nM). The most potent compounds were the heterocyclic derivatives with pyrimidine, pyrazine, pyridazine, or pyridine moieties (compounds 9r, 9t, 9u, 9x, respectively). K(i) values for 9a and 9r were determined for the 5-HT(4(a)), 5-HT(4(b)), 5-HT(4(c)), and 5-HT(4(d)) receptor isoforms transiently expressed in COS cells. The results indicated that the compounds were not selective. They produced an inhibition of the 5-HT-stimulated cyclic AMP synthesis in the C6 glial cells stably expressing the 5-HT(4(e)) receptor and shifted the 5-HT concentration-effect curve on adenylyl cyclase activity with pK(D) values of 7.44 and 8.47, respectively. In isolated human atrial myocytes, 9r antagonized the stimulatory effect of 5-HT on the L-type calcium current (I(Ca)) with a K(D) value of 0.7 nM.

Pharmacological characterization of the human 5-HT(4(d)) receptor splice variant stably expressed in Chinese hamster ovary cells

The recently identified C-terminal splice variant of the human 5-HT(4) receptor, the h5-HT(4(d)) receptor, was stably expressed in a CHO cell line at 493+/-25 fmol mg(-1) protein. We analysed its pharmacological properties by measuring binding affinities and 5-HT(4) ligand-induced cyclic AMP production. The pharmacological binding profile determined in competition studies with the specific antagonist [(3)H]-GR113808 revealed a rank order of affinity of 5-HT(4) ligands for the h5-HT(4(d)) receptor that was consistent with those previously reported for other 5-HT(4) receptor isoforms. In adenylyl cyclase functional assays, the h5-HT(4(d)) receptor displayed equipotent coupling for all 5-HT(4) agonists tested (EC(50) in the range of 1 - 6 nM). EC(50) values were lower than those previously obtained with the 5-HT(4(e)) receptor stably expressed in CHO cells indicating that the 5-HT(4(d)) receptor was more efficiently coupled to its effector than the 5-HT(4(e)) receptor isoform. Moreover, in terms of agonist efficacy (E(max)), the benzamide derivative, renzapride displayed full agonist properties at the h5-HT(4(d)) receptor (same E(max) as 5-HT) whereas it was previously shown to be a partial agonist at the h5-HT(4(e)) receptor. A constitutive activity of the h5-HT(4(d)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand. Surprisingly, two 5-HT(4) ligands, SB204070 and RS39604 which are described as highly potent antagonists in various biological models, revealed partial agonist properties at the h5-HT(4(d)) receptor. We conclude that C-terminal tails of 5-HT(4) receptor isoforms may directly influence their functional properties.