5-HT4 receptor antagonism in irritable bowel syndrome: effect of SB-207266-A on rectal sensitivity and small bowel transit

Probiotics Modulate Mouse Gut Microbiota and Influence Intestinal Immune and Serotonergic Gene Expression in a Site-Specific Fashion

Probiotic microorganisms may benefit the host by influencing diverse physiological processes, whose nature and underlying mechanisms are still largely unexplored. Animal models are a unique tool to understand the complexity of the interactions between probiotic microorganisms, the intestinal microbiota, and the host. In this regard, in this pilot study, we compared the effects of 5-day administration of three different probiotic bacterial strains (Bifidobacterium bifidum MIMBb23sg, Lactobacillus helveticus MIMLh5, and Lacticaseibacillus paracasei DG) on three distinct murine intestinal sites (ileum, cecum, and colon). All probiotics preferentially colonized the cecum and colon. In addition, probiotics reduced in the ileum and increased in the cecum and colon the relative abundance of numerous bacterial taxonomic units. MIMBb23sg and DG increased the inducible nitric oxide synthase (iNOS) in the ileum, which is involved in epithelial homeostasis. In addition, MIMBb23sg upregulated cytokine IL-10 in the ileum and downregulated the cyclooxygenase COX-2 in the colon, suggesting an anti-inflammatory/regulatory activity. MIMBb23sg significantly affected the expression of the main gene involved in serotonin synthesis (TPH1) and the gene coding for the serotonin reuptake protein (SERT) in the ileum and colon, suggesting a potential propulsive effect toward the distal part of the gut, whereas the impact of MIMLh5 and DG on serotonergic genes suggested an effect toward motility control. The three probiotics decreased the expression of the permeability marker zonulin in gut distal sites. This preliminary in vivo study demonstrated the safety of the tested probiotic strains and their common ability to modulate the intestinal microbiota. The probiotics affected host gene expression in a strain-specific manner. Notably, the observed effects in the gut were site dependent. This study provides a rationale for investigating the effects of probiotics on the serotonergic system, which is a topic still widely unexplored.

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.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Advances in understanding relationship between 5-hydroxytryptamine and its receptors and intestinal dysmotility in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common clinical functional gastrointestinal disease, which seriously affects the quality of life of patients. The pathogenesis of this disorder is unclear and may be related to the changes of visceral sensitivity, gastrointestinal motility, and the function of the brain-gut axis. 5-hydroxytryptamine (5-HT) is an important neurotransmitter, which exhibits a variety of biological effects including gastrointestinal secretion and motility regulation by binding to its receptors. The changes in the synthesis and release of 5-HT and in the expression and function of corresponding receptors are all involved in the pathophysiological process of IBS. In this paper, we will review the role of 5-HT and its receptors in intestinal dysmotility in IBS.

Updates on treatment of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a highly prevalent gastrointestinal disorder characterized by abdominal pain and discomfort in association with altered bowel habits. It is estimated to affect 10%-15% of the Western population, and has a large impact on quality of life and (in)direct healthcare costs. IBS is a multifactorial disorder involving dysregulation within the brain-gut axis, and it is frequently associated with gastrointestinal motor and sensory dysfunction, enteric and central nervous system irregularities, neuroimmune dysregulation, and post-infectious inflammation. As with other functional medical disorders, the treatment for IBS can be challenging. Conventional therapy for those with moderate to severe symptoms is largely unsatisfactory, and the development of new and effective drugs is made difficult by the complex pathogenesis, variety of symptoms, and lack of objective clinical findings that are the hallmark of this disorder. Fortunately, research advances over the past several decades have provided insight into potential mechanisms responsible for the pathogenesis of IBS, and have led to the development of several promising pharmaceutical agents. In recent years there has been much publicity over several of these new IBS medications (alosetron and tegaserod) because of their reported association with ischemic colitis and cardiovascular disease. While these agents remain available for use under restricted prescribing programs, this highlights the need for continued development of safe and effective medication for IBS. This article provides a physiologically-based overview of recently developed and frequently employed pharmaceutical agents used to treat IBS, and discusses some non-pharmaceutical options that may be beneficial in this disorder.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Effect of the NK(3) receptor antagonist, talnetant, on rectal sensory function and compliance in healthy humans

Visceral hypersensitivity is important in the pathophysiology of irritable bowel syndrome and thus a target for modulation in drug development. Neurokinin (NK) receptors, including NK(3) receptors, are expressed in the motor and sensory systems of the digestive tract. The aim of this study was to compare the effects of two different doses (25 and 100 mg) of the NK(3) receptor antagonist, talnetant (SB223412) with placebo on rectal sensory function and compliance in healthy volunteers studied at two centres. Rectal barostat tests were performed on 102 healthy volunteers, randomized to receive either oral talnetant 25 or 100 mg or placebo over 14-17 days. Studies were performed on three occasions: day 1 immediately prior to 1st dose, day 1 4 h postdose, and after 14- to17-day therapy. Compliance, and pressure thresholds for first sensation, urgency, discomfort and pain were measured using ascending method of limits, and sensory intensity ratings for gas, urgency, discomfort and pain determined during four random phasic distensions (12, 24, 36 and 48 mmHg). Talnetant had no effect on rectal compliance, sensory thresholds or intensity ratings compared with placebo. In general, the results obtained at the two centres differed minimally, with intensity scores at one centre consistently somewhat lower. At the doses tested, talnetant has no effect on rectal compliance or distension-induced rectal sensation in healthy participants.

Serotonin and its role in colonic function and in gastrointestinal disorders

Serotonin (5-HT) is most commonly thought of as a neurotransmitter in the central nervous system. However, the predominant site of serotonin synthesis, storage, and release is the enterochromaffin cells of the intestinal mucosa. Within the intestinal mucosa, serotonin released from EC cells activates neural reflexes associated with intestinal secretion, motility, and sensation. Two important receptors for serotonin that are located in the neural circuitry of the intestines are the 5-HT(3) and 5-HT(4) receptors; these are the targets of drugs designed to treat gastrointestinal disorders. 5-HT(3) receptor antagonists are used to treat nausea and emesis associated with chemotherapy and for functional disorders associated with diarrhea. 5-HT(4) receptor agonists are used as promotility agents to promote gastric emptying and to alleviate constipation. Because of the importance of serotonin in normal gut function and sensation, a number of studies have investigated potential changes in mucosal serotonin signaling in pathologic conditions. Despite the inconsistencies in the current literature, changes in serotonin signaling have now been demonstrated in inflammatory bowel disease, irritable bowel syndrome, postinfectious irritable bowel syndrome, and idiopathic constipation. Emerging evidence has led to many contradictory theories regarding serotonin signaling and its roles in the pathology of gut disorders. This review summarizes the current medications affecting serotonin signaling and provides an overview of our current knowledge of the changes in serotonin that occur in pathologic conditions.

Investigation of serotonin type 4 receptor expression in human and non-human primate gastrointestinal samples

BACKGROUND

The serotonin type 4 (5-HT4) receptor has been associated with functions of the gastrointestinal tract such as modulation of the peristaltic reflex, smooth muscle tone, intestinal secretion and visceral sensitivity. The activation of peripheral 5-HT4 receptors with agonists such as tegaserod has been shown to accelerate gastric emptying and improve symptoms of constipation in animals and humans. However, detailed data on the expression profile and on the localization of this receptor subtype are lacking so far.

OBJECTIVE

To study the pattern and expression levels of 5-HT4 receptor messenger RNA expression in the gut.

METHOD

Normal tissue samples were collected from the whole gastrointestinal tract of patients undergoing abdominal surgery and, in addition, of monkeys. We performed a comprehensive analysis of 5-HT4 receptor expression by quantitative reverse transcription-polymerase chain reaction, using human and non-human primate tissues from the oesophagus to the rectum. In addition, the brain and heart of non-human primates were analysed.

RESULTS

Significantly higher levels of 5-HT4 receptor mRNA were measured in the human stomach, duodenum, jejunum, ileum and caecum and also in the corresponding non-human primate gut segments, ranging from 2- to 12-fold compared with the liver. No differences were found between females and males of both human and non-human primates.

CONCLUSIONS

These results show 5-HT4 receptor mRNA expression throughout the gastrointestinal tract in humans and primates, and also support the preclinical and clinical findings of 5-HT4 receptors ligands exhibiting multiple effects throughout the gastrointestinal tract.

Modulation of intestinal gas dynamics in healthy human volunteers by the 5-HT receptor agonist tegaserod

OBJECTIVES

Bloating in irritable bowel syndrome (IBS) may result from impaired intestinal gas transit and is reduced by the 5-HT4 agonist tegaserod. Abnormal serotonergic function underlies many IBS symptoms, but the role of 5-HT4 pathways in regulating gas dynamics under healthy conditions is unexplored. We hypothesized that 5-HT4 activation by tegaserod stimulates gas transit in healthy individuals.

METHODS

Sixteen normal volunteers underwent jejunal perfusion of gas mixtures (88% N2, 5.5% O2, 6.5% CO2) at 11.2 mL/min x 3 h under control conditions and 3 h after oral tegaserod 6 mg on separate days. Gas collected from an intrarectal catheter was quantified using a barostat.

RESULTS

Under control conditions, gas evacuation after a lag period (1,959 +/- 428 s) was predominantly pulsatile with expulsion of 1,984 +/- 90 mL. A mean of 29 +/- 2 boluses with volumes of 72 +/- 5 mL were expelled. In 10 subjects with physiologic degrees of gas retention in control studies (248 +/- 73 mL), tegaserod increased expulsion from 1,768 +/- 73 to 1,973 +/- 37 mL and decreased retention to 43 +/- 37 mL (p < 0.05). Total volumes expelled as boluses were greater after tegaserod (1,708 +/- 73 vs 1,846 +/- 59 mL, p < 0.05) from increased bolus numbers in four subjects and increased bolus volumes in seven. Nonpulsatile continuous flow tended to increase with tegaserod (43 +/- 7 vs 126 +/- 43 mL, p= 0.10). Tegaserod did not increase evacuation in individuals without physiologic gas retention.

CONCLUSIONS

The 5-HT4 agonist tegaserod promotes evacuation of jejunally perfused gas mixtures in healthy humans. These findings provide the foundation for future investigations into use of 5-HT4 agonists in conditions of pathologic gas retention.

Identification of a 5-HT4 receptor antagonist clinical candidate through side-chain modification

Replacement of the N-butyl side-chain of lead 5-HT4 receptor antagonist 2 with propanesulfonylpiperidinyl, morpholinyl, and piperazinyl groups led to higher affinity analogs 4-6. In vitro drug metabolism screens and cassette pharmacokinetic studies in the dog led to identification of the N-methylpiperazinyl analog (6b), which displayed pharmacokinetic, selectivity, and safety parameters sufficient for advancement to the clinic for the treatment of urinary incontinence.

Meta-analysis: The treatment of irritable bowel syndrome

To evaluate therapies available for the treatment of irritable bowel syndrome, and provide consensus recommendations for their use, a total of 51 double-blind clinical trials using bulking agents, prokinetics, antispasmodics, alosetron, tegaserod and antidepressants were selected. The quality of studies was assessed using 5-point scale. Meta-analyses were performed on all studies, and on 'high-quality studies'. The efficacy of fibre in the global irritable bowel syndrome symptoms relief (OR: 1.9; 95% CI:1.5-2.4) was lost after exclusion of low-quality trials (OR: 1.4; 95% CI: 1.0-2.0, P = 0.06). When excluding the low-quality trials, an improvement of global irritable bowel syndrome symptoms with all antispasmodics (OR: 2.1; 95% CI:1.8-2.9) was maintained only for octylonium bromide, but on the basis of only two studies. Antidepressants were effective (OR: 2.6, 95% CI: 1.9-3.5), even after exclusion of low-quality studies (OR: 1.9, 95% CI: 1.3-2.7). Alosetron (OR: 2.2; 95% CI: 1.9-2.6) and tegaserod (OR: 1.4; 95% CI: 1.2-1.5) showed a significant effect in women. We recommend the use of tegaserod for women with irritable bowel syndrome with constipation and alosetron for women with severe irritable bowel syndrome with diarrhoea. Antidepressants can be beneficial for irritable bowel syndrome with diarrhoea patients with severe symptoms. Loperamide can be recommended in painless diarrhoea. Evidence is weak to recommend the use of bulking agents in the treatment of irritable bowel syndrome with constipation.

Review of the implications of dietary tryptophan intake in patients with irritable bowel syndrome and psychiatric disorders

In this review, we address the possible role of the essential amino acid L-tryptophan or its metabolic derivative 5-hydroxytryptophan in the modulation of serotonin (5-hydroxytryptamine) synthesis and thereby in affecting the pathophysiology of central and peripheral nervous system disorders, including depression and irritable bowel syndrome. L-Tryptophan may represent a link between apparently disparate functional disorders and is of interest for general gastroenterologists, neurogastroenterologists, and neurologists. On the basis of estimates showing that approximately 20% of patients with functional bowel disorders seeking care in referral centres have psychiatric comorbidity, we attempt to provide a conceptual framework for defining the possible role of L-tryptophan in this population.

Pharmacotherapy for intestinal motor and sensory disorders

Motor disorders of the gastrointestinal tract are characterized by definable impairments of gut contractile function. Other conditions exhibit specific disturbances of visceral afferent and efferent activity, which may underlie selected symptom complexes. Medications in several classes have been developed to treat these disorders of gastrointestinal function. Prokinetic agents are effective therapies for ailments with reduced motor function, whereas antispasmodic drugs reduce symptoms in conditions with exaggerated pressure wave activity. Recently, medications designed to blunt transmission in visceral sensory pathways have been proposed for use in the functional bowel disorders. Finally, some patients may benefit from initiation of nonspecific therapies, which have no appreciable effect on gut motor or sensory function.

The irritable bowel syndrome

Symptoms of IBS are very prevalent. One quarter of symptomatic individuals present for medical care because of symptom severity and psychologic factors. The pathogenesis of IBS is multifactorial, involving abnormalities in the gut, immune system, enteric sensory and motor nerves, and the CNS. IBS is diagnosed by symptomatology according to the Rome criteria and the absence of alarm findings suggestive of organic disease. Minimal testing is advocated to confirm the diagnosis in patients presenting with typical symptoms. Therapy is based on the dominant symptom (IBS subtype). Therapeutic options include dietary modifications, counseling, medications, and psychologic treatments. Novel therapies are being investigated to correct potentially pathogenetic peripheral and CNS abnormalities in IBS.

Irritable bowel syndrome neuropharmacology. A review of approved and investigational compounds

Anticholinergics and prokinetics are mainstays of therapy for Irritable Bowel Syndrome (IBS) patients despite their limited efficacy and troublesome side-effect profile. The clinical limitations of these drugs are a result of their relative broad and nonspecific pharmacologic interaction with various receptors. Recent advances in gut physiology have led to the identification of various receptor targets that may play a pivotal role in the pathogenesis of IBS. Medicinal chemists searching for safe and effective IBS therapies are now developing compounds targeting many of these specific receptors. The latest generation of anticholinergics, such as zamifenacin, darifenacin, and YM-905, provide selective antagonism of the muscarinic type-3 receptor. Tegaserod, a selective 5-HT4 partial agonist, tested in multiple clinical trials, is effective in reducing the symptoms of abdominal pain, bloating, and constipation. Ezlopitant and nepadudant, selective antagonists for neurokinin receptors type 1 and type 2, respectively, show promise in reducing gut motility and pain. Loperamide, a mu (mu) opioid receptor agonist, is safe and effective for IBS patients with diarrhea (IBS-D) as the predominant bowel syndrome. Fedotozine, a kappa (kappa) opioid receptor agonist, has been tried as a visccral analgesic in various clinical trials with conflicting results. Alosetron, a 5-HT3 receptor antagonist, has demonstrated efficacy in IBS-D patients but incidents of ischemic colitis seen in post-marketing follow-up resulted its removal from the market. Compounds that target cholecystokinin. A, N-methyl-D-aspartate, alpha 2-adrenergic, and corticotropin-releasing factor receptors are also examined in this review.

Alverine citrate fails to relieve the symptoms of irritable bowel syndrome: results of a double-blind, randomized, placebo-controlled trial

BACKGROUND

Alverine citrate has been used in the treatment of irritable bowel syndrome for many years.

AIMS

To compare the efficacy and safety of a new formulation of alverine citrate, a 120-mg capsule, with placebo given three times daily for 12 weeks.

METHODS

One hundred and seven patients with irritable bowel syndrome were entered into this three-centre, double-blind, randomized, placebo-controlled, parallel group trial. The primary end-point was relief of abdominal pain indicated by improvement in the scores for severity and frequency. Secondary efficacy variables included scores for other clinical symptoms and for overall well-being.

RESULTS

The severity and frequency of abdominal pain improved in 66% and 68% of patients treated with alverine citrate vs. 58% and 69% of the placebo group, but these differences were not significant. The mean percentage reduction in the scores for abdominal pain from baseline to the final assessment, although greater in the alverine citrate group (43.7%) compared with the placebo group (33.3%), was not statistically significant.

CONCLUSIONS

Alverine citrate is no better than placebo at relieving the symptoms of irritable bowel syndrome. Future trials should be designed to take into account the high and persistent placebo response seen in this condition.

The medical benefit of 5-HT research

5-HT research is now more than 50 years old, and it has generated a wealth of therapeutic agents, some of which have had a major impact on disease management. The 5-HT reuptake inhibitors (SSRIs) are among the most widely prescribed drugs for treating depression and a variety of other disorders including anxiety, social phobia and premenstrual dysphoria (PMD). The other major success stories of 5-HT research are the discovery of 5-HT1B/D receptor agonists for treating migraine and 5-HT3 receptor antagonists for chemotherapy and radiation-induced emesis. The role of 5-HT in the mechanism of action of antipsychotic agents remains a topic of intense research, which promises better treatments for schizophrenia in the future. Compounds interacting with 5-HT1F, 5-HT2C, 5-HT6 and 5-HT7 receptors are currently under investigation and may prove to have important therapeutic applications in the future.

Emerging treatments for irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional GI disorder that is associated with abdominal discomfort and altered bowel habits. It accounts for up to 28% of patients presenting to a gastroenterology practice and poses a significant personal, societal and economic burden internationally. The Manning, Rome I and Rome II criteria were developed to identify appropriate IBS patients for entry into IBS studies in a consistent manner. Refinements in the understanding of the physiology of the enteric nervous system (ENS), which controls motility, secretion and sensation, provided the basis for our comprehension of the pathophysiology of IBS. Visceral hypersensitivity and neurotransmitter imbalance currently receive the most attention as possible mechanisms of IBS. This article outlines conventional treatments and reviews the data on emerging and experimental therapies for IBS. Emerging therapies for IBS using 5-HT mediation include 5-HT(3) antagonists, such as ondasetron, granisetron and alosetron, as well as 5-HT(4) agonists such as tegaserod and prucalopride. In addition to opioid agonists (e.g. fedotozine) several other drugs that act on other ENS receptors are being studied. In spite of significant progress in IBS research, these emerging therapies require more studies before they can be utilised as clinical treatments.

Irritable bowel syndrome: new agents targeting serotonin receptor subtypes

Although the past few years have seen an exponential growth of compounds of potential interest for the treatment of functional gastrointestinal (GI) tract disorders, the gap that still exists between basic and clinical research is easily noticed if one considers the relative paucity of drugs that have received marketing authorisation for the treatment of irritable bowel syndrome (IBS). Traditional efficacy outcomes in drug development for IBS include the ability of the compound to affect GI tract motility (i.e. to exert a prokinetic or an antispasmodic effect), which is thought to be of importance if a motor disorder is the underlying pathophysiological mechanism. More recently, altered visceral sensitivity to a distending stimulus has been suggested to be a key pathophysiological feature, at least in some patients, and has become a target for therapeutic interventions. However, there is now growing consensus that the primary outcome measure in the treatment of functional disorders are those that reflect overall control of the patient's symptoms (pain, diarrhoea, constipation) in everyday situations such as the clinical global improvement scales. Although, in general, guidelines on the design of treatment trials for functional GI tract disorders advise against subcategorisation of patients according to the main symptom (because of symptom instability), subcategorisation indeed makes sense especially in IBS (constipation- or diarrhoea-predominant). Compounds with a specific indication for each subpopulation of patients are now emerging. The rationale for investigations on serotonin (5-hydroxytryptamine; 5-HT) receptor ligands in IBS rests mainly on the fact that serotonin, which may be released by enterochromaffin-like cells in the GI tract as well as from other sources, has a number of well documented motor effects on the GI tract and can produce hyperalgesia in several experimental models. Serotonin receptors belonging to the 5-HT3 and 5-HT4 subtype are the most extensively studied in gastroenterology, although hitherto 'orphan' receptor subtypes, such as the 5-HT7 and the 5-HT(1B/D) receptors, are now emerging. Among 5-HT3 receptor antagonists, alosetron was recently approved for the treatment of diarrhoea-predominant IBS and is an example of a compound that, at least theoretically, may act at multiple levels: by inhibiting visceral sensitivity, by increasing compliance, and by inhibiting excitatory 5-HT3 receptors located on both ascending and descending neuronal pathways involved in peristalsis. For this reason, 5-HT3 receptor antagonists may slow transit, hence the specific indication of alosetron in diarrhoea-predominant IBS. However, alosetron has been recently withdrawn by the manufacturer because of safety concerns. Hypomotility remains an attractive therapeutic target in IBS and the new generation of prokinetics includes several partial agonists at the 5-HT4 receptor, such as tegaserod (HTF-919) and prucalopride (R0-93877). In addition, preliminary evidence suggests that 5-HT4 receptors may also be involved in the modulation of visceral sensitivity. Second-generation 5-HT4 receptor agonists seem to be devoid of the QT-prolonging effects observed in some clinical circumstances with cisapride and may be more active at the colonic level. Piboserod (SB-207266A) is a 5-HT4 receptor antagonist under development for the treatment of diarrhoea-predominant IBS. Finally, interest in 5-HT7 and 5-HT(1B/D) receptor subtypes stems from the observation that the former receptors mediate smooth muscle relaxation (at least in the human colon), whereas sumatriptan (a 5-HT(1B/D) receptor agonist) can affect GI tract motility and visceral sensitivity.

Small-bowel motility

Advances in the field of small-bowel motility in the last few years include the delineation of the role of the intrinsic primary afferent neuron in eliciting the peristaltic reflex, the genetic control of the development of the enteric nervous system, and the potential role of the interstitial cells of Cajal as pacemakers of the intestine and colon. New promising molecules have been developed for therapeutic purposes, particularly in the modulators of serotonin receptors. Their effects on sensory and motor function has allowed a better recognition of the important role of 5-hydroxytryptamine in several disorders, especially in irritable bowel syndrome. In this brief review, we have elected to appraise the advances pertaining to human studies, with emphasis on novel methodologies, disease manifestations, and novel therapeutic strategies.

Gastrointestinal functional bowel disorders: new therapies

Present therapies for functional gastrointestinal disorders are symptomatic and mainly treat altered bowel habits. New therapies are focused on nerve-gut communication dysfunction: 5-HT3 antagonists and 5-HT4 agonists have demonstrated activity in clinical trials. Promising targets for upper gut dysmotility drugs are motilin and cholecystokinin A receptors. Tachykinins, calcitonin gene-related peptide or glutamate antagonists are the most relevant candidates for visceral pain.

Increased defecation during stress or after 5-hydroxytryptophan: selective inhibition by the 5-HT(4) receptor antagonist, SB-207266

5-HT(4) receptor antagonism prevents the ability of exogenous 5-HT or 5-HTP to sensitize the intestinal peristaltic reflex and increase the rate of defecation, generally without affecting non-stimulated intestinal function. In this study we confirmed the ability of the selective 5-HT(4) receptor antagonist SB-207266 1 - 1000 microg kg(-1) p.o., to prevent the increase in defecation evoked over a 60 min period by 5-HTP 10 mg kg(-1) s.c. in conscious mice, in the absence of an apparent constipating action. The role of endogenous 5-HT in the mechanisms of increased defecation and/or diarrhoea was then investigated in conscious, fed rats. This was evoked by 180 min exposure to restraint stress, which increased both the number and mean weight of formed, faecal pellets excreted over the entire time period. SB-207266 1 - 1000 microg kg(-1) p.o. (dosed 30 min before restraint) did not affect the increase in defecation evoked during the first 60 min of restraint stress, but significantly and dose-dependently reduced or prevented the increased defecation during the remaining 120 min of the experiment; this action occurred in the absence of an apparent constipating action of SB-207266. In fasted rats exposed to restraint stress, watery diarrhoea developed and although there was a tendency for SB-207266 1 - 1000 microg kg(-1) p.o. (dosed 30 min before restraint) to reduce the incidence of diarrhoea, this inhibition was not complete. We conclude that selective 5-HT(4) receptor antagonism prevents disruptions in defecation behaviours caused by exogenous or endogenous enteric 5-HT and that this activity is not accompanied by a concomitant suppression of activity (constipation-like) within the intestine itself.