A comparative study of functional 5-HT4 receptors in human colon, rat oesophagus and rat ileum

Protective actions of a luminally acting 5-HT4 receptor agonist in mouse models of colitis

BACKGROUND

5-hydroxytryptamine 4 receptors (5-HT4 Rs) are expressed in the colonic epithelium, and previous studies have demonstrated that luminal administration of agonists enhances motility, suppresses nociception, and is protective in models of inflammation. We investigated whether stimulation with a luminally acting 5-HT4 R agonist is comparable to previously tested absorbable compounds.

METHODS

The dextran sodium sulfate (DSS), trinitrobenzene sulfonic acid (TNBS), and interleukin 10 knockout (IL-10KO) models of colitis were used to test the protective effects of the luminally acting 5-HT4 R agonist, 5HT4-LA1, in the absence and presence of a 5-HT4 R antagonist. The compounds were delivered by enema to mice either before (prevention) or after (recovery) the onset of active colitis. Outcome measure included disease activity index (DAI) and histological evaluation of colon tissue, and effects on wound healing and fecal water content were also assessed.

KEY RESULTS

Daily enema of 5HT4-LA1 attenuated the development of, and accelerated recovery from, active colitis. Enema administration of 5HT4-LA1 did not attenuate the development of colitis in 5-HT4 R knockout mice. Stimulation of 5-HT4 Rs with 5HT4-LA1 increased Caco-2 cell migration (accelerated wound healing). Daily administration of 5HT4-LA1 did not increase fecal water content in active colitis.

CONCLUSIONS AND INFERENCES

Luminally restricted 5-HT4 R agonists are comparable to absorbable compounds in attenuating and accelerating recovery from active colitis. Luminally acting 5-HT4 R agonists may be useful as an adjuvant to current inflammatory bowel disease (IBD) treatments to enhance epithelial healing.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Localization of the 5-hydroxytryptamine 4 receptor in equine enteric neurons and extrinsic sensory fibers

BACKGROUND

Serotonin plays a pivotal role in regulating gut motility, visceral sensitivity, and fluid secretion via specific receptors. Among these receptors, 5-HT₄ exerts a prominent control on gut motor function. Although the prokinetic effect exerted by 5-HT₄ agonists is well known, the cellular sites of 5-HT₄ expression remain poorly understood in large mammals, e.g., horses. In this study, we evaluated the distribution of 5-HT₄ in the horse intestine and in foals with enteric aganglionosis, reminiscent of human Hirschsprung's disease.

METHODS

The intestine and spinal ganglia were obtained from three healthy horses and two foals with hereditary ileocolonic aganglionosis. Tissues were processed for immunohistochemistry using a specific antibody to 5-HT₄ and a variety of neuronal markers. Myenteric and submucosal plexus 5-HT₄ -immunoreactive (IR) neurons were quantified as relative percentage (mean±SD) to the total number of neurons counted. Furthermore, the density of 5-HT₄ -IR nerve fibers was evaluated in the mucosa and tunica muscularis.

KEY RESULTS

The 5-HT₄ immunoreactivity was localized to large percentages of myenteric neurons ranging from 28±9% (descending colon) to 63±19% (ileum), and submucosal neurons ranging from 54±6% (ileum) to 68±14% (duodenum). The 5-HT₄ -immunoreactivity was co-expressed by some substance P-IR (SP-IR) spinal ganglion neurons and extrinsic sensory fibers of aganglionic foals.

CONCLUSIONS & INFERENCES

The presence of 5-HT₄ in many enteric and extrinsic sensory neurons and nerve fibers provides solid morphological evidence of the cellular sites of 5-HT₄ expression in horses. The evidence of SP-IR sensory neurons positive for 5-HT₄ suggests its role in visceral sensitivity.

[Current Status of Translational Research on Constipation]

Constipation is one of the most common gastrointestinal disorders with a prevalence up to 16.5% in the general population. It is frequently multifactorial and the pathophysiologic mechanism of constipation is not fully understood. Many preclinical studies of constipation have used animal models. Translational research using these animal models is essential to the investigation of neurogenic and myogenic mechanisms of colon, and to the estimation of the clinical efficacy of new drugs. In this review, we discuss some of the current translational research projects on constipation using animal models.

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.

Effects of 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists on gastrointestinal motor activity in dogs

AIM: To study the effects of 5-hydroxytryptamine (5-HT) receptor antagonists on normal colonic motor activity in conscious dogs.

METHODS: Colonic motor activity was recorded using a strain gauge force transducer in 5 dogs before and after 5-HT2B, 5-HT3 and 5-HT4 receptor antagonist administration. The force transducers were implanted on the serosal surfaces of the gastric antrum, terminal ileum, ileocecal sphincter and colon. Test materials or vehicle alone was administered as an intravenous bolus injection during a quiescent period of the whole colon in the interdigestive state. The effects of these receptor antagonists on normal gastrointestinal motor activity were analyzed.

RESULTS: 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists had no contractile effect on the fasting canine terminal ileum. The 5-HT3 and 5-HT4 receptor antagonists inhibited phase III of the interdigestive motor complex of the antrum and significantly inhibited colonic motor activity. In the proximal colon, the inhibitory effect was dose dependent. Dose dependency, however, was not observed in the distal colon. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

CONCLUSION: The 5-HT3 and 5-HT4 receptor antagonists inhibited normal colonic motor activity. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

Assessment of the pharmacological properties of 5-methoxyindole derivatives at 5-HT4 receptors

Objectives

The aim was to examine the biological activity of 5-methoxytryptamine derivatives at the 5-hydroxytryptamine (5-HT)4 receptor to explore the effect of substitution on the aliphatic amine of the 5-methoxyamine scaffold.

Methods

Three compounds were tested for affinity at the 5-HT4 receptor by radioligand binding and functional activity using guinea-pig ileum and human colon circular muscle preparations and also in the mouse whole gut transit test.

Key findings

The three compounds all had agonist properties at the 5-HT4 receptor but their efficacy differed in the different functional tests. Compound 3 had the highest affinity for the 5-HT4 receptor and was a full agonist at relaxing human colon circular muscle with efficacy closest to 5-HT. Compounds 1 and 2 were partial agonists in this assay with lower efficacies; compound 2 was a full agonist in the guinea-pig ileum assay whereas compound 3 was a partial agonist. Compounds 1 and 2 also showed activity in the mouse gut transit assay while compound 3 had no activity.

Conclusions

Of the compounds tested, compound 3 was the most promising 5-HT4 receptor agonist and the results highlight the value of using human tissue in functional tests when assessing compounds for potential activity.

Investigation of neurogenic excitatory and inhibitory motor responses and their control by 5-HT(4) receptors in circular smooth muscle of pig descending colon

The aim of this study was to investigate whether the pig colon descendens might be a good model for the responses mediated via the different locations of human colonic 5-HT(4) receptors. The intrinsic excitatory and inhibitory motor neurotransmission in pig colon descendens was therefore first characterized. In circular smooth muscle strips, electrical field stimulation (EFS) at basal tone induced only in the combined presence of the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and the SK channel blocker apamin voltage-dependent on-contractions. These on-contractions were largely reduced by the neuronal conductance blocker tetrodotoxin (TTX) and by the muscarinic receptor antagonist atropine, illustrating activation of cholinergic neurons. The 5-HT(4) receptor agonist prucalopride facilitated submaximal EFS-evoked cholinergic contractions and this effect was prevented by the 5-HT(4) receptor antagonist GR113808, supporting the presence of facilitating 5-HT(4) receptors on the cholinergic nerve endings innervating circular muscle in pig colon descendens. Relaxations were induced by EFS in strips pre-contracted with substance P in the presence of atropine. The responses at lower stimulation voltages were abolished by TTX. L-NAME or apamin alone did not influence or only moderately reduced the relaxations, but L-NAME plus apamin abolished the relaxations at lower stimulation voltages, suggesting that NO and ATP act as inhibitory neurotransmitters in a redundant way. Prucalopride did not influence the EFS-induced relaxations at lower stimulation voltage, nor did it per se relax contracted circular muscle strips. No evidence for relaxing 5-HT(4) receptors, either on inhibitory neurons or on the muscle cells was thus obtained in pig colon descendens circular muscle.

Constipation, IBs and the 5-HT4 Receptor: What Role for Prucalopride?

After the problems associated with the non-selective 5-HT4 receptor agonists cisapride and tegaserod, the 5-HT4 receptor is now beginning to come in from the cold. Thus, prucalopride is now the first of a new class of drug defined by selectivity and high intrinsic activity at the 5-HT4 receptor. Prucalopride has been developed for treatment of chronic constipation rather than constipation-predominant irritable bowel syndrome (IBS). This follows the trend of first evaluating new gastrointestinal (GI) prokinetic drugs in disorders where disrupted GI motility is known to exist, rather than in a functional bowel disorder where changes in motility are uncertain. If prucalopride is not progressed towards the IBS indication, it has at least shown the way for other selective 5-HT4 receptor agonists. Most notable among these is TD-5108 (velusetrag), also characterized by good selectivity at the 5-HT4 receptor, high intrinsic activity and efficacy in patients with chronic constipation.

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.

Distribution of serotonin receptors and interacting proteins in the human sigmoid colon

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT(2B) receptor and all 5-HT(7) receptor splice variants were expressed in all tissues. 5-HT(4) a,b,c and n splice variants were also expressed in all tissues and 5-HT(4d), 5-HT(4g) and 5-HT(4i) were only detected in some samples. The 5-HT(2A) receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT(3A) subunit was expressed in all tissues, whereas the 5-HT(3E) subunit was mainly found in the mucosa layer while the 5-HT(3B) subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT(3) receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function.

Comparison of 5-HT4 and 5-HT7 receptor expression and function in the circular muscle of the human colon

Serotonin receptors are potential targets for treating functional bowel disorders. This study investigated the functional roles and expression of the 5-HT4 and the 5-HT7 receptor, which coexist in human colon circular smooth muscle. 5-HT3 receptor expression was also investigated. Part of the relaxant response to 5-HT was due to activation of 5-HT4 receptors as the apparent pKB value of the selective 5-HT4 antagonist, GR 113808, was 9.36. 5-HT4 mRNA levels were low in five tissues and undetectable in four others, but all responded to 5-HT with an EC50 value of 102.54+/-19.32 nM. The contribution of 5-HT7 receptors to the response was not readily demonstrated using the selective 5-HT7 antagonist, SB-269970, as its apparent pKB value of 7.19 (5-HT4 block with 1 microM GR 113808) was lower than the value obtained using the 5-HT7 guinea pig ileum assay (8.62). Nevertheless, the 5-HT7 receptor was expressed more consistently than the 5-HT4, but at similar levels. The 5-HT(3Ashort) and 5-HT(3B) subunits were co-expressed at similar levels, but the 5-HT(3Along) subunit was detected in only five of the nine samples tested. The findings show that 5-HT4-induced relaxation occurs at low to undetectable levels of tissue mRNA, as measured by qPCR. Although 5-HT7 receptor mRNA is detected at low, but consistent levels, the functional activity of this receptor is not readily identified given the currently available drugs.

5-HT4 receptor agonists enhance both cholinergic and nitrergic activities in human isolated colon circular muscle

Previous studies have demonstrated mixed inhibitory and facilitatory effects of 5-hydroxytryptamine-4 (5-HT(4)) receptor agonists on electrical field stimulation (EFS)-induced responses in human isolated colon. Here we report three types of responses to EFS in human isolated colon circular muscle: monophasic cholinergic contraction during EFS, biphasic response (nitrergic relaxation during EFS followed by cholinergic contraction after termination of EFS) and triphasic response (cholinergic contraction followed by nitrergic relaxation during EFS and a tachykininergic contraction after EFS). The effects of two 5-HT(4) receptor agonists, prucalopride and tegaserod were then investigated on monophasic responses only. Each compound inhibited contractions during EFS in a concentration-dependent manner. In the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) however, prucalopride and tegaserod enhanced the contractions in a concentration-dependent manner. In strips where the tone was elevated with substance-P and treated with scopolamine, EFS-induced relaxations were enhanced by the two agonists. The above observed effects by the two agonists were abolished by 5-HT(4) receptor antagonist SB-204070. The two agonists did not alter the tone raised by substance-P in the presence of scopolamine and l-NAME and did not affect carbachol-induced contractions in the presence of tetrodotoxin. These results suggest that in the circular muscle of human colon, 5-HT(4) receptor agonists simultaneously facilitate the activity of neurones which release the inhibitory and excitatory neurotransmitters, nitric oxide and acetylcholine respectively.

5-HT4 receptors contribute to the motor stimulating effect of levosulpiride in the guinea-pig gastrointestinal tract

BACKGROUND

The dopamine D2 receptor antagonist levosulpiride is a substituted benzamide derivative, whose gastrokinetic properties are exploited clinically for the management of functional dyspepsia. However, for other benzamide derivatives, such as cisapride and mosapride, agonism towards serotonin 5-HT4 receptors is considered the main mechanism leading to gastrointestinal prokinesia.

AIMS

To assess whether levosulpiride is able to activate 5-HT4 receptors in the guinea-pig isolated gastrointestinal tract.

MATERIALS AND METHODS

Circular muscle strips from gastric antrum, and colonic longitudinal muscle strips were used to detect electrically stimulated neurogenic contractions. The effect of levosulpiride was assessed in the absence and presence of GR125487, a selective 5-HT4 receptor antagonist. Furthermore, potential interaction of levosulpiride with 5-HT3 receptors and tissue cholinesterases was assessed in unstimulated ileal longitudinal muscle-myenteric plexus preparations.

RESULTS

Antral and colonic strip contractions were cholinergic/tachykinergic in nature. Micromolar concentrations of levosulpiride potentiated submaximal responses, through a mechanism competitively antagonized by GR125487 (pKB=9.4). In LMMPs, levosulpiride slightly affected contractions caused by the 5-HT, receptor agonist 2-methyl-5-HT, and had no effect on contractions to exogenous acetylcholine.

CONCLUSIONS

Our results indicate that levosulpiride acts as a moderate agonist at the 5-HT4 receptor. This property, together with antagonism at D2 receptors, may contribute to its gastrointestinal prokinetic effect.

Effects of mosapride citrate, a 5-HT4 receptor agonist, on colonic motility in conscious guinea pigs

It is known that 5-HT(4) receptors in the colon of guinea pigs show a distribution similar to that in humans. Thus, we examined the effects of mosapride citrate (mosapride) and cisapride, two 5-HT(4)-receptor agonists, on colonic motility in conscious guinea pigs implanted with force transducers. Mosapride and cisapride administered intragastrically at doses of 3 - 30 mg/kg significantly enhanced the colonic motility. The enhancing effect of mosapride was antagonized by atropine or GR113808, a 5-HT(4)-receptor antagonist, but not by methysergide, a 5-HT(1)- and 5-HT(2)-receptor antagonist; ondansetron, a 5-HT(3)-receptor antagonist; or CP-99994, a tachykinin NK(1)-receptor antagonist. In vitro receptor autoradiography showed that mosapride and cisapride inhibit the specific binding of [(125)I]-SB207710, a selective radioligand of 5-HT(4) receptors, in the colon of guinea pigs. These results suggest that mosapride enhances colonic motility through the 5-HT(4)-receptor activation in guinea pigs and may be useful for treating constipation in patients with colonic motility dysfunction.

Benzimidazole derivatives. 3. 3D-QSAR/CoMFA model and computational simulation for the recognition of 5-HT(4) receptor antagonists

A three-dimensional quantitative structure-affinity relationship study (3D-QSAR), using the comparative molecular field analysis (CoMFA) method, and subsequent computational simulation of ligand recognition have been successfully applied to explain the binding affinities for the 5-HT(4) receptor (5-HT(4)R) of a series of benzimidazole-4-carboxamides and carboxylates derivatives 1-24. The K(i) values of these compounds are in the range from 0.11 to 10 000 nM. The derived 3D-QSAR model shows high predictive ability (q(2) = 0.789 and r(2) = 0.997). Steric (contribution of 43.5%) and electrostatic (50.3%) fields and solvation energy (6.1%) of this novel class of 5-HT(4)R antagonists are relevant descriptors for structure-activity relationships. Computational simulation of the complexes between the benzimidazole-4-carboxamide UCM-21195 (5) and the carboxylate UCM-26995 (21) and a 3D model of the transmembrane domain of the 5-HT(4)R, constructed using the reported crystal structure of rhodopsin, have allowed us to define the molecular details of the ligand-receptor interaction that includes (i) the ionic interaction between the NH group of the protonated piperidine of the ligand and the carboxylate group of Asp(3.32), (ii) the hydrogen bond between the carbonyl oxygen of the ligand and the hydroxyl group of Ser(5.43), (iii) the hydrogen bond between the NH group of Asn(6.55) and the aromatic ring of carboxamides or the ether oxygen of carboxylates, (iv) the interaction of the electron-rich clouds of the aromatic ring of Phe(6.51) and the electron-poor hydrogens of the carbon atoms adjacent to the protonated piperidine nitrogen of the ligand, and (v) the pi-sigma stacking interaction between the benzimidazole system of the ligand and the benzene ring of Tyr(5.38). Moreover, the noticeable increase in potency at the 5-HT(4)R sites, by the introduction of a chloro or bromo atom at the 6-position of the aromatic ring, is attributed to the additional electrostatic and van der Waals interaction of the halogen atom in a small cavity located between transmembrane domains 5 and 6.

In vivo assessment of acceleration of motor activity associated with acetylcholine release via 5-hydroxytryptamine4 receptor in dog intestine

Effect of mosapride, a benzamide, on the motor activity associated with the release of endogenous acetylcholine (ACh) from enteric neurons was examined in the ileum of anesthetized dogs using an in vivo microdialysis method and compared with the effect of 5-hydroxytryptamine (5-HT). Intraarterial administration of 5-HT accelerated intestinal motor activity and increased the concentration of dialysate ACh, and the responses were inhibited by SB204070, a specific 5-HT4-receptor antagonist, but were apparently not affected by methiothepin, ketanserin and granisetron. Intraarterial administration of mosapride, a prokinetic benzamide, accelerated intestinal motor activity and the concentration of dialysate ACh increased. The effects of mosapride were antagonized by SB204070. Specific [125I]SB207710 binding was observed in the myenteric and submucosal plexuses and muscle layers of dog ileum by in vitro receptor autoradiography. High densities of [125I]SB207710 binding sites were detected in the myenteric and submucosal plexuses. Mosapride as well as SB204070 inhibited [125I]SB207710 binding. Thus, in the whole body of dogs, 5-HT and mosapride accelerated the intestinal motor activity due to the increases in ACh release mediated by stimulation of the 5-HT4 receptor.

5-HT4 receptor enhances the propulsive power of the peristaltic reflex in the rat distal colon

The present study was designed to investigate the nature of the 5-HT4 receptors affecting the peristaltic reflex in the isolated rat distal colon. A single peristaltic reflex was evoked by infusing Tyrode solution into the lumen of the isolated segment using our modified Trendelenburg's method under an isovolumic condition. A 5-HT4 receptor agonist, mosapride (10 nM), did not affect the threshold pressure and the propagation distance of the reflex contraction, but significantly increased the developed pressure (maximum pressure - threshold pressure) from 19.1 +/- 11.8 cm H2O in the control to 26.8 +/- 13.0 cm H2O (n = 8, P<0.05) without affecting the resting muscle tone in the distal colon. The present result suggests that stimulation of 5-HT4 receptor enhances the propulsive power to move the luminal contents in the rat distal colon.

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.

5-HT inhibits spontaneous contractility of isolated sheep mesenteric lymphatics via activation of 5-HT(4) receptors

Spontaneous isometric contractions were measured in rings of sheep mesenteric lymphatic vessels in vitro. 5-Hydroxytryptamine (5-HT) produced a concentration-dependent decrease in spontaneous contraction frequency and force which was not antagonised by either the nonspecific 5-HT(1)/5-HT(2) receptor antagonist methysergide (1 microM) or the 5-HT(3) receptor antagonist ondansetron (1 microM). The 5-HT(4) receptor agonist BIMU-8 mimicked the inhibitory effect of 5-HT and its effects were abolished by the 5-HT(4) receptor antagonist DAU 6285 (1 microM). DAU-6285 also abolished the inhibitory effect of 5-HT and unmasked a weak excitatory response, which was mimicked by the 5-HT(2) receptor agonist alpha-methyl-5-hydroxytryptamine maleate. This excitatory response was, in turn, blocked by the 5-HT(2) receptor antagonist pirenperone (1 microM). The results of this study suggest that sheep mesenteric lymphatics possess both 5-HT(4) receptors and 5-HT(2) receptors. The inhibitory 5-HT(4) receptor appeared to be the predominant subtype since the excitatory response to 5-HT could only be observed in the presence of the 5-HT(4) receptor antagonist DAU 6285.

5-HT(4) receptors on cholinergic nerves involved in contractility of canine and human large intestine longitudinal muscle

5-HT(4) receptors mediate circular muscle relaxation in both human and canine large intestine, but this phenomenon alone can not explain the improvement in colonic motility induced by selective 5-HT(4) receptor agonists in vivo. We set out to characterize 5-HT(4) receptor-mediated effects in longitudinal muscle strips of canine and human large intestine. Electrical field stimulation (EFS) was applied providing submaximal isotonic contractions. L-NOARG (0.1 mM) was continuously present in the organ bath to preclude nitric oxide-induced relaxation to EFS. The selective 5-HT(4) receptor agonist prucalopride (0.3 microM) enhanced EFS-evoked contractions, that were antagonized in both preparations by the selective 5-HT(4) receptor antagonist GR 113808 (0.1 microM). The prucalopride-induced increase was present in canine ascending and descending colon, but absent in rectum. Regional differences in response to prucalopride were not observed in human ascending and sigmoid colon and rectum. Incubation with atropine (1 microM) or tetrodotoxin (0.3 microM) inhibited EFS-induced contractions, which were then unaffected by prucalopride (0.3 microM) in both tissues. In the presence of methysergide (3 microM; both tissues) and granisetron (0.3 microM; only human tissues), 5-HT (0.3 microM) enhanced EFS-induced contractions, an effect that was antagonized by GR 113808 (0.1 microM). In the presence of atropine or tetrodotoxin, EFS-induced contractions were inhibited, leaving 5-HT (0.3 microM) ineffective in both preparations. This study demonstrates for the first time that in human and canine large intestine, 5-HT(4) receptors are located on cholinergic neurones, presumably mediating facilitating release of acetylcholine, resulting in enhanced longitudinal muscle contractility. This study and previous circular muscle strip studies suggest that 5-HT(4) receptor agonism facilitates colonic propulsion via a coordinated combination of inhibition of circumferential resistance and enhancement of longitudinal muscle contractility.

Effects of a serotonin 5-HT(4) receptor antagonist SB-207266 on gastrointestinal motor and sensory function in humans

BACKGROUND

Serotonin 5-HT(4) receptors are located on enteric cholinergic neurones and may regulate peristalsis. 5-HT(4) receptors on primary afferent neurones have been postulated to modulate visceral sensation. While 5-HT(4) agonists are used as prokinetic agents, the physiological role of 5-HT(4) receptors in the human gut is unknown.

AIMS

Our aim was to characterise the role of 5-HT(4) receptors in regulating gastrointestinal motor and sensory function in healthy subjects under baseline and stimulated conditions with a 5-HT(4) receptor antagonist.

METHODS

Part A compared the effects of placebo to four doses of a 5-HT(4) receptor antagonist (SB-207266) on the cisapride mediated increase in plasma aldosterone (a 5-HT(4) mediated response) and orocaecal transit in 18 subjects. In part B, 52 healthy subjects received placebo, or 0.05, 0.5, or 5 mg of SB-207266 for 10-12 days; gastric, small bowel, and colonic transit were measured by scintigraphy on days 7-9, and fasting and postprandial colonic motor function, compliance, and sensation during distensions were assessed on day 12.

RESULTS

Part A: 0.5, 5, and 20 mg doses of SB-207266 had significant and quantitatively similar effects, antagonising the cisapride mediated increase in plasma aldosterone and acceleration of orocaecal transit. Part B: SB-207266 tended to delay colonic transit (geometric centre of isotope at 24 (p=0.06) and 48 hours (p=0.08)), but did not have dose related effects on transit, fasting or postprandial colonic motor activity, compliance, or sensation.

CONCLUSION

5-HT(4) receptors are involved in the regulation of cisapride stimulated orocaecal transit; SB 207266 tends to modulate colonic transit but not sensory functions or compliance in healthy human subjects.

An improved in vitro bioassay for the study of 5-HT(4) receptors in the human isolated large intestinal circular muscle

Recently, it was demonstrated that 5-HT induces relaxation of human colon circular muscle through activation of 5-HT(4) receptors and 5-HT(7) receptors. The aim of the current study was to develop a new in vitro bioassay of human colon that would facilitate the pharmacological analysis of 5-HT responses mediated solely by 5-HT(4) receptors. Contracting circular muscle strips with KCl (80 mM) yielded a stable contractile tension and, in contrast to muscarinic cholinoceptor agonists and histamine, a profound reduction of spontaneous contractility. This allowed the establishment of reproducible, fully-defined, agonist concentration-response curves by cumulative dosing. Under these conditions, 5-HT induced a concentration-dependent relaxation (pEC(50) 7.31, Hill slope 0.91). Neither methysergide (10 microM) nor granisetron (1 microM) affected the 5-HT-induced relaxation, suggesting that 5-HT(1), 5-HT(2), 5-HT(3), 5-ht(5), 5-HT(6) or 5-HT(7) receptors are not involved. The lack of effect of tetrodotoxin (0.3 microM) indicated a direct effect of 5-HT on the smooth muscle. The selective 5-HT(4) receptor antagonists GR 113808, GR 125487 and RS 39604 competitively antagonized the 5-HT-induced relaxation (pK(B) 9.43, 10.12 and 8.53, respectively). SB 204070 (1 nM) produced a rightward shift (pA(2) 10.34) and depression of the 5-HT curve. These affinity estimates are similar to those previously reported for 5-HT(4) receptors. The selective 5-HT(4) receptor agonists, prucalopride and R076186, induced relaxations (pEC(50) 7.50 and 7.57, respectively), that were blocked by GR 113808 (3 nM), yielding pA(2) estimates of 9.31 and 9.21, respectively. To summarise, in KCl (80 mM)-contracted muscle strips, 5-HT induces relaxation through activation of a homogeneous smooth muscle 5-HT(4) receptor population. This new bioassay allows the focused, pharmacological characterization of human colonic 5-HT(4) receptors in vitro.

Structure of the human serotonin 5-HT4 receptor gene and cloning of a novel 5-HT4 splice variant

Several variants of the serotonin 5-HT4 receptor are known to be produced by alternative splicing. To survey the existence and usage of exons in humans, we cloned the human 5-HT4 gene. Based on sequence analysis seven C-terminal variants (a-g) and one internal splice variant (h) were found. We concentrated in this study on the functional characterization of the novel splice variant h, which leads to the insertion of 14 amino acids into the second extracellular loop of the receptor. The h variant was cloned as a splice combination with the C-terminal b variant; therefore, we call this receptor 5-HT4(hb). This novel receptor variant was expressed transiently in COS-7 cells, and its pharmacological profile was compared with those of the previously cloned 5-HT4(a) and 5-HT4(b) isoforms, with the latter being the primary reference for the h variant. In competition binding experiments using reference 5-HT4 ligands, no significant differences were detected. However, the broadly used 5-HT4 antagonist GR113808 discriminated functionally among the receptor variants investigated. As expected, it was an antagonist on the 5-HT4(a) and 5-HT4(b) variant but showed partial agonistic activity on the 5-HT4(hb) variant. These data emphasize the importance of variations introduced by splicing for receptor pharmacology and may help in the understanding of conflicting results seen with 5-HT4 ligands in different model systems.

Benzimidazole derivatives. Part 1: Synthesis and structure-activity relationships of new benzimidazole-4-carboxamides and carboxylates as potent and selective 5-HT4 receptor antagonists

New benzimidazole-4-carboxamides 1-16 and -carboxylates 17-26 were synthesized and evaluated for binding affinity at serotonergic 5-HT4 and 5-HT3 receptors in the CNS. Most of the synthesized compounds exhibited moderate-to-very high affinity (in many cases subnanomolar) for the 5-HT4 binding site and no significant affinity for the 5-HT3 receptor. SAR observations and structural analyses (molecular modeling, INSIGHT II) indicated that the presence of a voluminous substituent in the basic nitrogen atom of the amino moiety and a distance of ca. 8.0 A from this nitrogen to the aromatic ring are of great importance for high affinity and selectivity for 5-HT4 receptors. These results confirm our recently proposed model for recognition by the 5-HT4 binding site. Amides 12-15 and esters 24 and 25 bound at central 5-HT4 sites with very high affinity (Ki = 0.11-2.9 nM) and excellent selectivity over serotonin 5-HT3, 5-HT2A, and 5-HT1A receptors (Ki > 1000-10,000 nM). Analogues 12 (Ki(5-HT4) = 0.32 nM), 13 (Ki(5-HT4) = 0.11 nM), 14 (Ki(5-HT4) = 0.29 nM) and 15 (Ki(5-HT4) = 0.54 nM) were pharmacologically characterized as selective 5-HT4 antagonists in the isolated guinea pig ileum (pA2 = 7.6, 7.9, 8.2 and 7.9, respectively), with a potency comparable to the 5-HT4 receptor antagonist RS 39604 (pA2 = 8.2). The benzimidazole-4-carboxylic acid derivatives described in this paper represent a novel class of potent and selective 5-HT4 receptor antagonists. In particular, compounds 12-15 could be interesting pharmacological tools for the understanding of the role of 5-HT4 receptors.

Evidence for 5-HT7 receptors mediating relaxation of human colonic circular smooth muscle

5-HT4 receptors mediate relaxation of human colon circular muscle. However, after 5-HT4 receptor blockade (SB 204070 10 nM), 5-HT still induced a relaxation (pEC50 6.3). 5-HT4 receptors were sufficiently blocked, as the curves to 5-HT obtained in the presence of 10 and 100 nM SB 204070 were indistinguishable. This 5-HT-induced relaxation was tetrodotoxin-insensitive, indicative of a smooth muscle relaxant 5-HT receptor. This, and the rank order of potency (5-CT=5-MeOT=5-HT) suggested involvement of 5-HT1 or 5-HT7 receptors. Mesulergine, a 5-HT7 receptor antagonist at nanomolar concentrations, and a 5-HT1 receptor antagonist at micromolar concentrations, competitively antagonized the 5-HT-induced relaxation (pKB 8.3) and antagonized the relaxation to 5-CT. Methysergide antagonized the 5-HT-induced relaxation (pA2 7.6). It is concluded that the profile of the smooth muscle inhibitory 5-HT receptor resembles that of the 5-HT7 receptor. These data provide the first evidence for functional human 5-HT7 receptors.

Pharmacological characterization of 5-HT4 receptors mediating relaxation of canine isolated rectum circular smooth muscle

This study aimed to characterize for the first time in vitro 5-HT4 receptors in the canine gastrointestinal tract. For this purpose, we used circular muscle strips of the canine isolated rectum. In the presence of methysergide (60 microM), 5-HT induced relaxation of methacholine (1 microM)-precontracted muscle strips, yielding a monophasic sigmoidal concentration-relaxation curve (pEC50 7.2+/-0.07). Tetrodotoxin (0.3 microM) did not affect the curve to 5-HT, suggesting the inhibitory 5-HT receptor is located on the smooth muscle. Granisetron (0.3 microM) did also not affect the curve to 5-HT, which excludes the 5-HT3 receptor mediating the relaxation to 5-HT. The presence of methysergide rules out the involvement of 5-HT1, 5-HT2 or 5-HT7 receptors. 5-HT, the selective 5-HT4 receptor agonists R076186, prucalopride (R093877) and SDZ HTF-919 and the 5-HT4 receptor agonists cisapride and 5-MeOT relaxed the muscle strips with a rank order of potency R076186 = 5-HT > cisapride > prucalopride > or = SDZ HTF-919 > 5-MeOT. The selective 5-HT4 receptor antagonists GR 125487, RS 39604 and GR 113808 competitively antagonized the relaxations to 5-HT, yielding pK(B) estimates of 9.7, 7.9 and 9.1, respectively. The selective 5-HT4 receptor antagonist SB 204070 shifted the curve to 5-HT rightward and depressed the maximal response (apparent pA2 10.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5-HT4 receptor agonists R076186 (pA2 8.8). It is concluded that 5-HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5-HT4 receptors. For the first time, a nonhuman species was shown to exhibit relaxant 5-HT4 receptors in the large intestine.

Regional difference in correlation of 5-HT4 receptor distribution with cholinergic transmission in the guinea pig stomach

Localization and function of 5-HT4 receptors in the stomach were examined in mucosa-free preparations of antrum, corpus and fundus from guinea pig stomach by determination of acetylcholine release and in vitro receptor autoradiography. Specific [125I]SB207710, (1-n-butyl-4-piperidinyl) methyl-8-amino-7-iodo-1,4-benzodioxane-5-carboxylate, binding sites were detected in 3 regions of the stomach. High densities of binding were observed in the myenteric plexus of antrum and corpus, but not fundus. In mucosa-free preparations treated with 5-HT1, 5-HT2 and 5-HT3 receptor antagonists, 5-HT (10(-8)-10(-6) M) potentiated the electrically stimulated (0.5 Hz, 1 ms) outflow of [3H]acetylcholine from antrum and corpus strips preloaded with [3H]choline, but not from fundus strips, and the potentiation was antagonized by SB204070, (1-n-butyl-4-piperidinyl) methyl-8-amino-7-chloro-1,4-benzodioxane-5-carboxylate. Thus, 5-HT4 receptors are located on myenteric cholinergic neurons in the antrum and corpus of guinea pig stomach and their activation evokes the release of acetylcholine.

Effects of propranolol and L-NAME on beta-adrenoceptor-mediated relaxation in rat carotid artery

1. The properties of beta-adrenoceptors mediating vascular relaxation in rat isolated carotid artery were investigated. Ring segments of arteries were preconstricted with the thromboxane A2 receptor agonist U-46619 and relaxation to beta-adrenoceptor agonists determined. 2. Isoprenaline produced a concentration-dependent relaxation of U-44619-constricted arteries. The concentration-response curve (CRC) to isoprenaline was shifted to the right by propranolol (1 microM) although the shift was less (105 fold; pA2, 8.02) than would be expected for an effect of isoprenaline at classical beta-adrenoceptors (300-1000 fold; pA2, 8.5-9). L-NAME (100 microM) significantly reduced responses to isoprenaline, lowering the slope of the CRC and reducing the maximum response. 3. The selective beta3-adrenoceptor agonists, BRL 37344 and ZD2079, also produced concentration-dependent relaxation of the arteries. L-NAME (100 microM) shifted the BRL 37344 CRC to the right 15 fold with no reduction in the slope or maximum response. L-NAME (100 microM) had no significant effect on the ZD2079 CRC. 4. In conclusion, relaxation to isoprenaline in rat carotid artery is inhibited by propranolol in a manner suggesting a mixed population of classical (beta1-/beta2-) and atypical (beta3-) adrenoceptors. The presence of beta3-adrenoceptors was confirmed by the relaxant effects of the selective beta3-adrenoceptor agonists BRL 37344 and ZD2079. L-NAME attenuated responses to both isoprenaline and the beta3-adrenoceptor agonist BRL 37344, suggesting a role for endothelial release of nitric oxide in beta-adrenoceptor mediated relaxation. However, the relaxant effect of BRL 37344 was attenuated by L-NAME to a lesser extent than that of isoprenaline. In addition, L-NAME had no effect on relaxation induced by ZD2079. These results suggest that there may be a differential contribution of endothelium to classical beta-and beta3-adrenoceptor-mediated effects, with endothelium contributing less to beta3-adrenoceptor-mediated relaxation.

The action of 5-hydroxytryptamine on normal and cystic fibrosis mouse colon: effects on secretion and intracellular calcium

The ability of mouse colon to generate a secretory response to stimulation by 5-hydroxytryptamine (5-HT) was investigated in intact colonic sheets mounted in Ussing chambers. A preparation of intact isolated crypts was used to determine whether 5-HT action was associated with an elevation of cytosolic calcium levels, measured using the calcium-sensitive fluorescent dye, fura-2. 5-HT increased the short-circuit current, an effect that was inhibited by 55% in the absence of chloride and by 83% in the presence of serosal frusemide, consistent with the stimulation of electrogenic chloride secretion. This was confirmed by the observation that colonic tissue from transgenic cystic fibrosis mice (n = 4) failed to respond to 5-HT, although wild-type tissues generated an increased short-circuit current of 52.4+/-1.1 microAcm(-2) (n = 9). The electrical response to 5-HT was calcium-dependent. 5-HT action was unaffected by tetrodotoxin and was not mimicked by the 5-HT3 agonist 1-phenylbiguanide, indicating that neural mechanisms are not involved. The cyclooxygenase inhibitor indomethacin, however, reduced the 5-HT-induced rise in short-circuit current by 73%, suggesting that prostaglandin production contributes to the response. Stimulation of crypts with acetylcholine elicited an increase in cytosolic calcium levels, but no such response was detected on application of 5-HT (10(-6) to 10(-4) M), suggesting that 5-HT does not directly modulate intracellular calcium in colonic crypt cells. It is concluded that mouse colon responds to 5-HT challenge with a stimulation of electrogenic chloride secretion and that this effect is mediated by indirect mechanisms that might involve immune elements within the colonic wall.

Ability of mosapride to bind to 5-HT4 receptor in the human stomach

Ability of mosapride, a gastrokinetic agent, to bind to 5-HT4 receptor was examined in the stomach of human and guinea pig by in vitro receptor autoradiography. [125I]SB207710 binding sites were detected in the muscle layer including the myenteric plexus of the stomach from both humans and guinea pigs, although the binding was observed more clearly and densely in the stomach of guinea pigs than humans. Mosapride as well as SB204070 inhibited the binding of [125I]SB207710. Thus, mosapride possesses the ability to bind to 5-HT4 receptors of human stomach and may modulate the motility, as in the case of guinea pig stomach.

Identification of SK-951, a novel benzofuran derivative, as an agonist to 5-HT4 receptors

The pharmacological profile of SK-951 ((-)4-amino-N-[2-(1-azabicyclo[3.3.0]octan-5-yl) ethyl]-5-chloro-2,3-dihydro-2-methylbenzo[b]furan-7-carboxamide hemifumarate) was identified in relation to serotonin 5-HT3 and 5-HT4 receptors by the receptor binding assay and functional studies. The receptor binding assay showed that SK-951 bound to the 5-HT3 receptor with a high affinity, to the 5-HT4 receptor with relatively higher affinity and to the muscarinic M2 receptor with a low affinity, but not to dopamine D1 and D2 and serotonin 5-HT1 and 5-HT2 and muscarinic M1 and M3 receptors. SK-951 caused relaxations of tunica muscularis mucosae preparations from rat esophagus which were precontracted with carbachol, and the effects were antagonized by GR113808, a selective 5-HT4 antagonist. In the longitudinal muscle with myenteric plexus (LMMP) preparations from guinea pig ileum, SK-951 enhanced the electrically-stimulated contraction of preparations in which the 5-HT1, 5-HT2 and 5-HT3 receptors were blocked, and it enhanced the electrically-stimulated release of [3H]acetylcholine (ACh). These effects of SK-951 were antagonized by GR113808. SK-951 inhibited the 5-HT3 receptor-mediated contractions. These results indicate that SK-951 possesses properties of an agonist for the 5-HT4 receptor and an antagonist for the 5-HT3 receptor. Thus, SK-951 is a new and potent 5-HT4-receptor agonist and causes contractions of guinea pig ileum mediated by enhancement of ACh release via the 5-HT4 receptor.

Nitrogen-containing heteroalicycles with serotonin receptor binding affinity: development of gastroprokinetic and antiemetic agents

To obtain gastroprokinetic agents with more potent and selective activity than metoclopramide and cisapride, a series of N-(4-benzyl-2-morpholinylmethyl)benzamides were designed and prepared. Their synthesis and structure-activity relationships were described. As a result, mosapride was selected as a promising candidate for potent gastroprokinetic activity with selective 5-HT4 receptor agonistic activity. As an extension to this project, the novel benzamide and the carboxamide derivatives having 1-benzyl-4-methylhexahydro-1,4-diazepine ring in the amine moiety were prepared and evaluated for 5-HT3 receptor antagonistic activity. DAT-582 was identified as an antiemetic agent in cancer chemotherapy. The asymmetric synthesis of DAT-582 and the SAR studies were briefly reviewed. In further modifications of the N-(1-benzyl-4-methylhexahydro-1,4-diazepin-6-yl)benzamides, the novel nicotinamides with 1-ethyl-4-methylhexahydro-1,4-diazepin ring were found to have potent 5-HT3 and dopamine D2 and D3 receptor antagonistic activities and to show weak central nervous system depression and extrapyramidal syndrome. After extensive SARs, AS-8112 was selected as a broad antiemetic agent.

Functional gut disorders: from motility to sensitivity disorders. A review of current and investigational drugs for their management

Functional gut disorders include several clinical entities defined on the basis of symptom patterns (e.g., functional dyspepsia, irritable bowel syndrome, functional abdominal pain, functional abdominal bloating), for which there is no established pathophysiological mechanism. Because there is no well-defined pathophysiological target, treatment should be aimed at symptom improvement. Prokinetics and antispasmodics have been widely used in the treatment of functional gut disorders on the assumption that disordered motility is the underlying cause of symptoms, and symptom improvement is indeed achievable with these compounds in some, but not all, patients with features of hypo- or hypermotility, respectively. In the first part of this review, we cover the basic pharmacology and discuss the rationale for the clinical use of prokinetics and antispasmodics. On the other hand, in the past few years, the explosive growth in the research focusing on visceral sensitivity and visceral reflexes has suggested that at least some patients with functional gut disorders have altered visceral perception. Thus, the second part of the review covers these developments and focuses on studies addressing the issue of drugs modulating visceral sensitivity.

Characterization of 5-HT receptors in rat proximal colon

1. In view of its multiple sites of action, we investigated the activity of 5-hydroxytryptamine (5-HT, serotonin) on various potential receptors in the isolated proximal colon of rats. 2. 5-HT induced concentration-dependent contractions of colonic strips (pEC50=7.54+/-0.12). 3. The 5-HT1 receptor agonist, 5-carboxamidotryptamine, induced concentration-dependent contractions (pEC50=5.93+/-0.27); however, neither the 5-HT3 receptor-agonist, phenylbiguanide, nor the 5-HT4 receptor-agonist, renzapride, caused contractions at concentrations as high as 10(-4) M. 4. The 5-HT 1/2 receptor antagonist, methiothepin, caused concentration-dependent nonsurmountable antagonism. The 5-HT3 receptor antagonist, tropisetron, inhibited the contractions to a concentration of 5-HT> or =10(-6) M. Ketanserin had no effect on responses to 5-HT. 5. Tetrodotoxin and atropine had no effect on responses to 5-HT. 6. We conclude that contractions to 5-HT are mediated by 5-HT1-like and probably 5-HT3 receptors that activate tetrodotoxin insensitive mechanisms.

Investigation into the effect of 5-hydroxytryptamine on fluid transport in the rat small intestine

1. 5-Hydroxytryptamine (5-HT) has been shown to cause a consistent secretory effect in the rat small intestine only when administered luminally or by close intraarterial infusion. Intraluminal 5-HT-induced secretion is possibly mediated by 5-HT4 receptors. Therefore, it was decided to investigate the effect of 5-HT and selective 5-HT4 receptor agonists (SC 53116 and DAU 6236) on intestinal fluid transport in rat jejunum and ileum. The study also investigated the effect of a selective 5-HT4 receptor antagonist (GR 113808) against the intraluminally administered 5-HT. 2. 5-HT receptor agonists and antagonists were administered intraluminally in pentobarbitone-anesthetized rats. Changes in intestinal fluid transport across the intestinal wall were measured by a single pass technique. 3. Intraluminal 5-HT produced significant antiabsorptive effects is both the jejunum and ileum. The 5-HT-induced responses were blocked by intraluminal administration of the 5-HT4 receptor antagonist GR 113808. The 5-HT4 agonist SC 53116 induced antiabsorptive effects in both regions of the small intestine, but DAU 6236 did not affect the rates of fluid transport. 4. The results indicate that a 5-HT4 receptor has a role in the luminal 5-HT-induced antiabsorptive effect on intestinal fluid transport in the rat.

Characterization of adenosine receptors on rat ileum, ileal longitudinal muscle and muscularis mucosae

Adenosine receptors were studied in isolated rat ileum, ileal longitudinal muscle and muscularis mucosae, using a range of agonists and an antagonist. In the rat ileal longitudinal muscle adenosine receptor agonists relaxed the tissues. N6-cyclopentyladenosine (CPA) was more potent than 5'-N-ethylcarboxamidoadenosine (NECA) or adenosine and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (1 nM) gave a 5-fold parallel shift to the right of the concentration-response curves to both CPA and NECA corresponding to an apparent pA2 value of 9.6 suggesting that the agonists relax via adenosine A1 receptors. In the intact ileum adenosine receptor agonists also relaxed the tissue but NECA and CPA were equipotent. DPCPX (3 nM) however inhibited responses to both CPA and NECA with dose-ratios of 8 and 15.6, corresponding to pA2 values of 9.3 and 9.7, respectively. DPCPX (300 nM) gave a much greater shift to the right of the concentration-response curve to NECA with a dose-ratio of 769, corresponding to an apparent pA2 of 9.4. This suggests that the agonists are acting at adenosine A1 receptors to cause relaxation of the whole tissue. Adenosine receptor agonists contracted rat ileal muscularis mucosae with a potency order indicative of an A adenosine receptor. DPCPX (3-100 nM) antagonized responses to CPA giving a linear Schild plot with a slope close to unity and a pA2 of 8.4 suggesting an action on adenosine A1 receptors.

Structural organization and neuropeptide distribution in the mammalian enteric nervous system, with special attention to those components involved in mucosal reflexes

Gastrointestinal events such as peristalsis and secretion/absorption processes are influenced by the enteric nervous system, which is capable of acting largely independently from other parts of the nervous system. Several approaches have been used to further our understanding of the underlying mechanisms of specific enteric microcircuits. Apart from pharmacological and physiological studies, the deciphering of the chemical coding of distinct morphological and functional enteric neuron classes, together with a detailed analysis of their projections by the application of immunocytochemistry, of tracing, and of denervation techniques, have substantially contributed to our knowledge. In view of existing interspecies and regional differences, it is of major importance to expand our knowledge of the enteric nervous system in mammals other than the guinea-pig, the most commonly used experimental animal in this research area. This will increase our chances of finding a valid model, from which well-founded extrapolations can be made regarding the precise function of distinct enteric neuron types regulating motility and ion transport in the human gastrointestinal tract.

Signal transduction pathways for serotonin as an intestinal secretagogue

This review presents a signal transduction pathways for serotonin (5-hydroxytryptamine, 5-HT) as an intestinal secretagogue and some recently published related findings. 5-HT is a secretagogue in the small and large intestine of all studied species including pig and man. 5-HT mediates intestinal secretion through activation of at least the epithelial 5-HT2, and neuronal 5-HT3, and 5-HT4 receptors in the submucosal plexus, including a reflex arc. 5-HT activates both a cholinergic and a non-cholinergic pathway in its secretory response. Intracellular mediators include at least eicosanoids (prostaglandin E2), calcium, phosphoinositols (1,4,5-inositol trisphosphate) and maybe nitric oxide and cyclic nucleotides. Pig small intestine appears to be an appropriate model for the human small intestine with respect to the signal transduction pathways for 5-HT as an intestinal secretagogue. Species and segmental differences in the signal transduction pathways for 5-HT as an intestinal secretagogues are discussed together with related news on 5-HT receptors, 5-HT antagonists in clinical use, the enteric nervous system, and intracellular mediators.

Post-synaptic 5-HT4 receptor modulation of tachykinergic excitation of rat oesophageal tunica muscularis mucosae

Interaction between the 5-HT4 receptor and cholinergic-dependent and -independent contraction of the rat oesophageal muscularis mucosae was determined. Substance P- (in the presence of atropine) and carbachol-precontracted tissue was relaxed by tryptamines and the substituted benzamides with the following rank order of potency: 5-HT > 5-methoxytryptamine > cisapride > (R)-zacopride > lintopride > metoclopramide, consistent with 5-HT4 receptor activation. The response to 5-HT was not antagonized by tetrodotoxin, methysergide or ondansetron; but was shifted to the right by GR113808 ([1-[2-[methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl-1-H- indole-3-carboxylate) in substance P- and carbachol-precontracted tissue, confirming 5-HT4-mediated relaxation. This study shows for the first time that although 5-HT4 receptors are involved in the modulation of cholinergic neurotransmission they can also act independently of this system modulating tachykinergic responsiveness.

VB20B7, a novel 5-HT-ergic agent with gastrokinetic activity. I. Interaction with 5-HT3 and 5-HT4 receptors

This study describes the in-vitro interaction of the gastrokinetic agent 2[1-(4-piperonyl)piperazinyl]benzothiazole (VB20B7) with the 5-hydroxytryptamine 5-HT3 and 5-HT4 receptor subtypes, using functional as well as radioligand binding studies. The benzamide derivative cisapride was used as a comparison. In radioligand binding assays VB20B7 showed, like cisapride, a weak affinity at 5-HT3 receptors from rat cerebral cortex. The new compound lacked any affinity at other 5-HT receptors or at dopaminergic D2 receptors, whereas cisapride showed high affinity for the 5-HT4 receptors from guinea-pig hippocampus and moderate affinity at dopaminergic D2 receptors. In the non-stimulated guinea-pig ileum, the concentration-response curves to the specific 5-HT3 agonist 2-Me-5-HT and to 5-HT were shifted to the right by VB20B7. In the rat oesophagus tunica muscularis mucosae preparation (TMM), VB20B7 was evaluated for its activity at 5-HT4 receptors. VB20B7 behaved as a 5-HT4 receptor agonist, inducing a concentration-dependent relaxation of the preparation precontracted with carbachol. In this preparation, VB20B7 and cisapride were able to stimulate adenylate cyclase activity, an effect probably mediated through activation of 5-HT4 receptors, as can be inferred from the blockade by the 5-HT4 antagonist, tropisetron, of the enhanced cAMP formation. However, consistent with the lack of affinity at central 5-HT4 receptors, VB20B7 did not stimulate cAMP formation in guinea-pig hippocampal slices. VB20B7 also caused an increase in the twitch response of the transmurally stimulated guinea-pig ileum, although at a concentration higher than cisapride. This effect was blocked by desensitization of the 5-HT4 receptor with 5-MeOT and also by the 5-HT4 receptor antagonist tropisetron. Both VB20B7 and cisapride increased the K(+)-evoked acetylcholine release in this preparation. The results show that VB20B7 possesses affinity for 5-HT4 receptors located in the rat TMM and guinea-pig ileum preparations, but is devoid of affinity at central 5-HT4 receptors. In addition, VB20B7 shows low to moderate affinity at both central and peripheral (enteric) 5-HT3 receptors. The interaction of VB20B7 with the peripheral 5-HT4 and 5-HT3 receptors may be relevant for the gastrokinetic effects of the new compound.

5-Hydroxytryptamine and functional bowel disorders

The possibility that 5-hydroxytryptamine (5-HT) acts as a key sensitising agent in the aetiology of irritable bowel syndrome (IBS) is reviewed. The strategic locations of 5-HT and its receptors are described, the most dominant being the 5-HT3 and 5-HT4 type. 5-HT, acting mostly at 5-HT3 or 5-HT3-like receptors, enhances the sensitivity of visceral neurones projecting between the gut and the central nervous systems. 5-HT, acting at 5-HT4 receptors promotes the sensitivity of enteric neurones that react to luminal stimuli. 5-HT4 and 5-HT3 receptors also mediate, respectively, sensitising and physiological actions of 5-HT on gastro-intestinal motor and secretory functions. This distribution implies that some 5-HT3 receptor antagonists might reduce certain symptoms of IBS, such as pain, by reducing the reactivity of the visceral afferent neurones linking the gut with the brain and spinal cord. However, such antagonists are not likely to find widespread clinical acceptance because they can also affect normal lower bowel function and promote constipation. 5-HT4 receptor antagonists, by contrast, reduce 5-HT-induced enteric nerve hypersensitivity without notably affecting the function of the normal bowel. Accordingly, these agents may reduce the symptoms of IBS directly, by reducing the incidence of defecation and diarrhoea and indirectly, by reducing both 'rebound' constipation and the post-prandial discomfort and pain associated with gastrointestinal hyper-reactivity.

Changes in sensitivity of 5-HT receptor mediated functional responses in the rat oesophagus, fundus and jejunum following chronic infusion with 5-hydroxytryptamine

The effects of chronic infusion with 5-hydroxytryptamine (5-HT, 75 micrograms/kg per hour) for 5 or 10 days in vivo on the responses of rat oesophagus, fundus and jejunum to 5-HT and partial 5-HT receptor agonists in vitro were investigated. In the rat oesophagus, chronic treatment produced rightward shifts of the 5-HT4 receptor-mediated concentration-effect curves to 5-HT (dose-ratio = 3.8, day 5 and 2.8, day 10) and SC 53116 (1-S,8-S)-4-amino-5-chloro-N-[(hexahydro-1H-pyrrolizin-1-yl) methyl]-2-methoxy-benzamide hydrochloride, dose-ratio = 7.1, day 5 and 8.9, day 10) as compared to control tissues. The maximum effect of 5-HT and SC 53116 in rat oesophagus was reduced following the 10 day treatment. The 5-HT2B receptor-mediated contractile effect of 5-HT on rat fundus from treated animals responded with a significantly reduced potency (dose-ratio = 4.1, 5-day; 4.2, 10-day) compared to control tissues. The maximum response to 5-HT was reduced in tissues from animals treated for 10 days. The concentration-effect curve to the partial agonist 1-(3-chlorophenyl)-piperazine dihydrochloride (mCPP) was shifted to the right in fundic tissue from treated animals (dose-ratio = 2.5, 5-day; 2.8, 10-day) compared to control tissues. The maximum response to mCPP was also reduced in tissues from 5-HT treated animals. It has recently been shown in a preliminary characterisation that in rat jejunum, 5-HT produces a biphasic concentration-effect curve which is mediated by a putative 5-ht7 (first phase) and 5-HT3 (second phase) receptor mechanism. In the present study 5-HT produced a control biphasic concentration-effect curve in rat jejunum with a pEC50-1 value of 8.5 +/- 3.5 and a pEC50-2 value of 5.9 +/- 0.3 and maximum response values of 43.8 (32.5-55.0)% (Emax1) and 65.3 (41.7-88.9)% (Emax 2) of the response to acetylcholine. In jejunal tissues from treated animals, 5-HT produced its contractile effect in vitro with a 3.7 and 2.8 fold (5-day) and a 1.3 and 1.4 fold (10-day) rightward shift of the first and second phase respectively of the control concentration-effect curve to 5-HT. The maximum response produced by 5-HT in the first phase in jejunal tissues from animals treated for 10 days was significantly reduced by 49.1%. These findings represent the first report that chronic infusion of 5-HT produces a residual desensitisation of the 5-HT4, 5-HT2B, and putative 5-ht7/5-HT3 receptor-mediated responses in rat oesophagus, fundus and jejunum respectively when measured in vitro.

Characterisation of a postjunctional 5-ht7-like and a prejunctional 5-HT3 receptor mediating contraction of rat isolated jejunum

The 5-HT (5-hydroxytryptamine)-induced contractile biphasic concentration-effect curve in rat isolated jejunum was investigated. The pEC50 values for the first and second phases were 8.0 and 6.1, respectively. The responses were insensitive to atropine (0.1 microM), ketanserin (2 microM), (-)-pindolol (5 microM), yohimbine (0.1 microM) and GR 113808 ({1-[2-(methyl-sulphonylamino)ethyl]-4-piperidinyl}methyl 1-methyl-1 H-indole-3-carboxylate, 1 microM) but susceptible to cocaine (10 microM). The low affinity phase was blocked by tetrodotoxin (1 microM), ondansetron (1 microM) and SR48968 (S)-N-methyl-N-[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl]benzamide, 0.1 microM). The high affinity phase was antagonised non-surmountably by fluoxetine (1 microM) methysergide (0.1 microM), spiperone (0.1 microM) and methiothepin (0.1 microM). Ritanserin (0.01-0.1 microM) and mesulergine (0.01-0.1 microM) acted as surmountable, competitive antagonists with pA2 values of 8.0 and 8.1, respectively. Clozapine (0.1 microM) was a surmountable antagonist with an apparent pA2 value of 8.0. The rank potency order of the 5-HT receptor agonists was 5-CT (5-carboxyamidotryptamine) > or = 5-HT = 5-methoxytryptamine > or = alpha-methyl-5-HT > > 8-OH-DPAT ((+/-)-2-dipropyl-amino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene) > dipropyl-5-CT > renzapride = sumatriptan. The responses to 5-HT and 5-CT were not potentiated by pargyline (10 and 100 microM). It is suggested that rat jejunum contains a neuronal 5-HT3 receptor facilitating neurokinin release and a contractile smooth muscle 5-HT receptor with a pharmacological operational profile similar to the cloned 5-ht7 receptor.

Comparison of 5-HT4 receptors in guinea-pig colon and rat oesophagus: effects of novel agonists and antagonists

5-HT4 receptors in isolated distal colon myenteric plexus of guinea-pig, mediating contraction of longitudinal smooth muscle, have been further characterized by selective agonists and antagonists. The indole agonists, 5-HT and 5-methoxytryptamine (5-MeOT), were full agonists (relative to 5-HT) with potency values (pEC50) of 8.0 +/- 0.1 (n = 50) and 7.8 +/- 0.1 (n = 12), respectively. 5-HT4 receptor agonists of other structural classes, including benzimidazolones (BIMU 1 and BIMU 8), and benzamides ((S)-zacopride, (R)-zacopride, renzapride, SC 49518) were partial agonists with intrinsic activities less than that of 5-HT. In general, the potencies for these compounds at 5-HT4 receptors in guinea-pig colon were similar to the potencies seen in the rat isolated oesophagus, where 5-HT4 receptors mediate relaxation. GR 113808 ¿[1-[2-[(methylsulfonyl)amino]ethyl]-4-piperidinyl] methyl1-methyl-1H-indole-3-carboxylate¿, RS 39604 ¿1-[4-amino-5-chloro-2-(3, 5-dimethoxybenzyloxy)phenyl]-3[1-[2-[(methylsulfonyl)amino] ethyl]-4-piperidinyl]-1-propanone hydrochloride and SB 204070 ¿(1-n-butyl-4-piperidinyl)methyl 8-amino-7-chloro-1, 4-benzodioxane-5-carboxylate¿ antagonized 5-HT responses with pA2 values of 9.1 +/- 0.1, 9.0 +/- 0.2 and 11.0 +/- 0.1, respectively. These affinity values were similar to those obtained at 5-HT4 receptors in isolated rat oesophagus (9.0+/- 0.4, 9.3 +/- 0.1 and 10.6 +/- 0.1 respectively). Despite these operational similarities between 5-HT4 receptors in guinea-pig colon and rat oesophagus, several novel compounds have revealed important differences between 5-HT4 receptors in the two tissues. For example, the substituted benzoate, RS 23597 ¿3-(piperidine-1-yl) propyl-4-amino-5-chloro-2-methoxybenzoate hydrochloride, acted as a partial agonist (intrinsic activity 0.5) in guinea-pig colon with a potency of 7.6 +/-0.1 (n = 16). In isolated rat oesophagus, however, this compound was a surmountable antagonist (pA2 = 7.8 +/- 0.1) with no intrinsic activity. In contrast, the substituted naphthalimide (S)RS 56532 ¿(S)-6-amino-5-chloro-2-(1-azabicyclo[2, 2, 2]octan-3-yl) 2,3-dihydro-1H-benz[de] isoquinoline-1,3-dione hydrochloride¿, was a potent (pEC50 = 7.9 +/- 0.1), efficacious partial agonist (intrinsic activity = 0.8) in the rat oesophagus. However, in guinea-pig colon, it was a surmountable antagonist with an affinity (pKB) of 9.4 +/- 0.1. Furthermore, several novel, selective, 5-HT4 compounds also showed opposing patterns of intrinsic activities similar to those described for RS 23597 and (S)RS 56532. It is concluded that these differences are inconsistent with differences in 5-HT4 receptor reserves, and may suggest that 5-HT4 receptors in the guinea-pig colon and the rat oesophagus can be operationally distinguished.

Further investigation into the signal transduction mechanism of the 5-HT4-like receptor in the circular smooth muscle of human colon

1. The nature of the receptor coupling mechanism of the 5-hydroxytryptamine4 (5-HT4) receptor in the circular smooth muscle of the human colon has been further investigated. 2. 5-HT stimulated cyclic AMP generation and caused a relaxation in a concentration-dependent fashion, with EC50 values of 175.5 and 274.9 nM respectively. DAU 6236 increased cyclic AMP formation and caused a relaxant effect but was a partial agonist relative to 5-HT. 3. The 5-HT4 receptor antagonist, GR 113808, inhibited cyclic AMP formation and relaxation induced by 5-HT with -log Ki values of 9.1 (cyclic AMP) and 8.9 (relaxation) and apparent pA2 values of 9.2 (cyclic AMP) and 9.5 (relaxation). 4. Ondansetron and methysergide failed to inhibit cyclic AMP formation or the relaxation induced by 5-HT. 5. The phosphodiesterase inhibitor, IBMX, produced a concentration-dependent relaxation (EC50 = 30 microM) and at 1 microM it enhanced the 5-HT-induced relaxation producing a leftward shift of the 5-HT concentration-effect curve with a concentration-ratio of 4.1. Rolipram caused a concentration-dependent relaxation (EC50 = 564.8 nM) and at 200 nm caused a leftward shift of the concentration-effect curve to 5-HT with a concentration-ratio of 5.5. 6. Application of the adenylyl cyclase inhibitor, SQ 22536 (0.1 mM), and the protein kinase inhibitors, H7 (100 nM) and H89 (200 nM), inhibited the relaxant effect of 5-HT inducing a rightward shift of the concentration-effect curve with concentration-ratios of 10.1, 2.7 and 4.2 respectively. 7. Forskolin stimulated cyclic AMP production and caused a relaxation. The maximum relaxant effect of forskolin (6 microM, 13.8 +/- 1.9 cm.s) was not significantly different from the maximum relaxant effect of 5-HT (10 microM, 12.7 +/- 4.9 cm.s). However, the cyclic AMP levels stimulated by forskolin (6 microM, 49.3 +/- 6.6 pmol mg-1) were markedly greater than those stimulated by 5-HT (10 microM, 7.6 +/- 2.0 pmol mg-1). 8. In conclusion, these results indicate that the 5-HT4 receptors of the circular smooth muscle of human colon mediate relaxation and inhibition of spontaneous contractions via activation of adenylyl cyclase, formation of cyclic AMP and activation of protein kinase A.

Stimulation of cyclic AMP formation in the circular smooth muscle of human colon by activation of 5-HT4-like receptors

5-HT stimulated cyclic AMP generation in human colonic circular smooth muscle in a concentration-dependent fashion (EC50 = 229.1 nM). DAU 6236 also increased cyclic AMP formation and was a partial agonist relative to 5-HT. GR 113808 inhibited the cyclic AMP formation induced by 5-HT with a -log Ki value of 9.1 and an apparent pA2 value of 9.2. Ondansetron and methysergide failed to inhibit cyclic AMP formation induced by 5-HT. These results indicate that the 5-HT4 receptors of human colonic circular muscle mediate relaxation and inhibition of spontaneous contractions via formation of cyclic AMP.