Effect of selective 5HT3 antagonist (GR 38032F) on small intestinal transit and release of gastrointestinal peptides

Contribution of Serotonin 3A Receptor to Motor Function and Its Expression in the Gastrointestinal Tract

INTRODUCTION

The serotonin 3A receptor (5-HT3AR) is involved in vomiting and gastrointestinal motility. However, it is not well understood the expression pattern of 5-HT3AR in the gut immunohistochemically and how much contribution of 5-HT3AR to upper or lower intestinal motility.

OBJECTIVES

We investigated the contribution of 5-HT3AR to gastrointestinal motor function by using 5-HT3AR KO mice and sought to identify 5-HT3AR-expressing cells via immunohistochemical staining using 5-HT3AR-GFP reporter mice.

METHODS

The expression of 5-HT3AR was measured in each section of the gut through real-time PCR. The motor function of the stomach and colon was assessed via the 13C-octanoic acid breath test and colonic bead expulsion test, respectively, using 5-HT3AR KO mice. 5-HT3AR-expressing cells in the muscle layer of the gut were identified by immunohistochemical staining using 5-HT3AR-GFP reporter mice.

RESULTS

5-HT3AR was expressed throughout the digestive tract, and 5-HT3AR expression in the stomach and lower digestive tract was higher than that in the other sections. Motor function in the stomach and colon was lower in 5-HT3AR KO mice than in WT mice. As a result of immunohistochemical staining using GFP reporter mice, cholinergic neurons and PDGFRα+ cells were shown to express 5-HT3AR. In contrast, 5-HT3AR indicated by GFP fluorescence was rarely detected in ICC and smooth muscle cells.

CONCLUSIONS

These results show that 5-HT3AR is highly expressed in the stomach and large intestine and that the activation of 5-HT3AR accelerates gastric emptying and large intestine transit. Additionally, 5-HT3AR is highly expressed in cholinergic neurons and some interstitial cells, such as PDGFRα+ cells.

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.

Effects of ramosetron on gastrointestinal transit of Guinea pig

BACKGROUND/AIMS

A selective 5-hydroxytryptamine (5-HT) type 3 receptor antagonist, ramosetron, inhibits stress-induced abnormal defecation in animals and is currently used as a therapeutic drug for irritable bowel syndrome with diarrhea. The aim of this study is to investigate the effect of ramosetron on altered gastrointestinal (GI) transit.

METHODS

Male guinea pigs weighing approximately 300 g were used. The effect of ramosetron was investigated on altered GI transit induced by thyrotropin-releasing hormone (TRH), 5-HT, or mustard oil (MO). GI transit was evaluated by the migration of charcoal mixture from the pylorus to the most distal point, and expressed as a percentage (%) of charcoal migration (cm) of the total length of total small intestine (cm).

RESULTS

The average charcoal transit was 51.3 ± 20.1% in the control (vehicle) group, whereas in the ramosetron group charcoal moved 56.6 ± 21.9%, 46.9 ± 9.14% and 8.4 ± 5.6% of the total small intestine at the concentrations of 10, 30 and 100 µg/kg, respectively. GI transit after administration of TRH (100 µg/kg), 5-HT (10 mg/kg) or MO (10 mg/kg) was accelerated compared to vehicle (5-HT, 94.9 ± 9.22%; TRH, 73.4 ± 14.7%; MO, 81.0 ± 13.7%). Ramosetron inhibited GI transit altered by 5-HT, TRH or MO.

CONCLUSIONS

Ramosetron modulated GI transit. We suggest that ramosetron may be therapeutically useful for those with accelerated upper GI transit.

Functional diarrhea

Chronic diarrhea is a frequent and challenging problem in clinical medicine. In a considerable subgroup of these, no underlying cause is identified and this is referred to as functional diarrhea. A consensus definition for functional diarrhea is based on loose stool consistency and chronicity and absence of coexisting irritable bowel syndrome. Underlying pathophysiology includes rapid intestinal transit, which may be worsened by stress or be triggered by a preceding infectious gastroenteritis. Diagnostic work-up aims at exclusion of underlying organic disease. Treatment starts with dietary adjustments, aiming at decreasing nutrients that enhance transit and stool and at identifying precipitating food items.

Role of 5-hydroxytryptamine mechanisms in mediating the effects of small intestinal glucose on blood pressure and antropyloroduodenal motility in older subjects

Postprandial hypotension is an important clinical problem, particularly in the elderly. 5-Hydroxytryptamine3 (5-HT3) mechanisms may be important in the regulation of splanchnic blood flow and blood pressure (BP), and in mediating the effects of small intestinal nutrients on gastrointestinal motility. The aims of this study were to evaluate the effects of the 5-HT3 antagonist granisetron on the BP, heart rate (HR), and antropyloroduodenal (APD) motility responses to intraduodenal glucose in healthy older subjects. Ten subjects (5 male, 5 female, aged 65-76 yr) received an intraduodenal glucose infusion (3 kcal/min) for 60 min (t = 0-60 min), followed by intraduodenal saline for a further 60 min (t = 60-120 min) on 2 days. Granisetron (10 microg/kg) or control (saline) was given intravenously at t = -25 min. BP (systolic and diastolic), HR, and APD pressures were measured. Pressure waves in the duodenal channel closest ("local") to the infusion site were quantified separately. During intraduodenal glucose, there were falls in systolic and diastolic BP and a rise in HR (P < 0.0001 for all); granisetron had no effect on these responses. Granisetron suppressed the number and amplitude (P < 0.05 for both) of local duodenal pressures during intraduodenal glucose. Otherwise, the effects of intraduodenal glucose on APD motility did not differ between study days. We conclude that in healthy older subjects, 5-HT3 mechanisms modulate the local duodenal motor effects of, but not the cardiovascular responses to, small intestinal glucose.

Influence of acute serotonin reuptake inhibition on colonic sensorimotor function in man

BACKGROUND

It is unclear whether decreased serotonin transporter function contributes to sensorimotor abnormalities in irritable bowel syndrome.

AIM

To study the influence of acute serotonin transporter inhibition on colonic sensorimotor function in man.

METHODS

Ten healthy subjects (five men, aged 20-29 years) underwent a combined manometry/barostat study of the descending colon on two occasions. Stepwise distentions by 2 mmHg increments were performed until discomfort. Subsequently, placebo or citalopram 20 mg were administered i.v. over 20 min and distentions were repeated. Afterwards, isobaric tone measurements were performed 30 min before and 90 min after ingestion of a meal. High-amplitude propagated contractions, colonic motility index, colonic compliance, sensitivity and colonic response to a meal after placebo or citalopram were compared by t-test and two-way ANOVA.

RESULTS

Citalopram induced a significant increase in colonic motility index (5.6 +/- 0.9 to 0.8 +/- 1.9 mL*min, P < 0.005) and high-amplitude propagated contractions (32 after citalopram vs. 2 after placebo, P < 0.05), which were associated with abdominal cramping. Administration of citalopram increased colonic compliance (10.3 +/- 1.5 vs. 14.5 +/- 2.2 mL/mmHg, P < 0.01) and inhibited colonic response to a meal (volume decrease 48 +/- 12 vs. 16 +/- 12 mL, P < 0.01).

CONCLUSIONS

Acute serotonin transporter inhibition in man increases colonic phasic contractility and the occurrence of high-amplitude propagated contractions, increases colonic compliance and suppresses the colonic tonic response to a meal. These data suggest that both release and elimination of 5-hydroxytryptamine by serotonin transporter are involved in the control of colonic motility in man.

Intraduodenal guar attenuates the fall in blood pressure induced by glucose in healthy older adults

OBJECTIVES

Postprandial hypotension occurs frequently in older people and may result in syncope and falls. It has recently been established that the magnitude of the fall in blood pressure is related to the rate at which glucose enters the small intestine. We addressed the hypothesis that the fall in blood pressure induced by an intraduodenal glucose infusion is influenced by the interaction of glucose with the small intestinal absorptive epithelium.

METHODS

Eight healthy older participants (four male, four female, age 70.3 +/- 3.4 years) were studied on two separate occasions, in single-blind, randomized order. Participants received an intraduodenal glucose infusion (3 kcal/min) with or without guar gum (4 g) for 60 minutes (0-60 minutes), followed by 0.9% saline intraduodenally for a further 60 minutes (60-120 minutes). Blood pressure and heart rate were measured every 3 minutes. Levels of blood glucose, plasma insulin, glucagon-like peptide-1 (GLP-1), and glucose-dependant insulinotropic-polypeptide (GIP) were also determined.

RESULTS

Between t = 0 and t = 30 minutes, the magnitude of the fall in systolic blood pressure (p =.03) and increase in heart rate (p =.027) were lower after guar. The blood glucose (p =.009), plasma insulin (p =.027), plasma GLP-1 (p =.018), and GIP (p <.001) responses to intraduodenal glucose were attenuated by guar.

CONCLUSIONS

In healthy older participants, the magnitude of the fall in systolic blood pressure and increase in heart rate induced by intraduodenal glucose are attenuated when the exposure of glucose to the small intestinal mucosa and subsequent glucose absorption is slowed by guar.

How to improve drug development for functional disorders

Although several studies and publications have been devoted to the evaluation of novel pharmacotherapeutic approaches in functional bowel disorders, the number of successful drug development programs has been disappointingly small. The difficulties of adequate drug development are related to the heterogeneity of the functional disorders, the lack of understanding of the underlying pathophysiology, and the lack of short-term mechanistic studies that predict clinical outcome. When drugs address a single pathophysiological mechanism, the heterogeneity has a negative impact on the possible margin when patients are not selected on the basis of the specific pathophysiological disorder. Although mechanistic studies may provide a rationale for drug development, they do not predict symptomatic success and do not identify the most appropriate dose for clinical trials. Further mechanistic studies are badly needed to improve our understanding of the pathophysiological basis of functional disorders. Future studies might aim at the development of non-invasive test that identify relevant pathophysiological mechanisms, at refining assessment of symptom pattern and at the development of short-term therapeutic trials.

Effect of a novel 5-HT3 receptor agonist MKC-733 on upper gastrointestinal motility in humans

BACKGROUND

Although 5-HT3 antagonists have been used to treat chemotherapy-induced emesis and diarrhoea-predominant irritable bowel syndrome, the effects of 5-HT3 agonists in humans are unknown.

AIM

To determine the effect of MKC-733, a selective 5-HT3 receptor agonist, on upper gastrointestinal motility.

METHODS

Oral MKC-733 (0.2, 1 and 4 mg) was compared with placebo in three randomized, double-blind, cross-over studies in healthy males. Antroduodenal manometry was recorded for 8 h during fasting and 3 h post-prandially (n = 12). Gastric emptying and small intestinal transit were determined by gamma-scintigraphy (n = 16). Gastric emptying, accommodation and antral motility were determined by echoplanar magnetic resonance imaging (n = 12).

RESULTS

MKC-733 (4 mg) increased the number of migrating motor complexes recorded in the antrum and duodenum (P < 0.001), but had no effect on post-prandial motility. MKC-733 delayed scintigraphically assessed liquid gastric emptying (P = 0.005) and accelerated small intestinal transit (P = 0.038). Echoplanar magnetic resonance imaging confirmed the delayed gastric emptying (P < 0.001) and demonstrated a significant increase in cross-sectional area of the proximal stomach (P < 0.01).

CONCLUSIONS

MKC-733 delays liquid gastric emptying in association with relaxation of the proximal stomach, stimulates fasting antroduodenal migrating motor complex activity and accelerates small intestinal transit.

Benefits and risks of newer treatments for chemotherapy-induced and postoperative nausea and vomiting

Nausea and vomiting are common adverse effects of chemotherapy, radiation therapy, anaesthesia and surgery. The incidence of chemotherapy-induced nausea and vomiting (CINV) is estimated to vary from 30 to 90%, depending on the type of chemotherapeutic agent used. Radiation-induced emesis varies with anatomical site radiated but is estimated to have an overall incidence of approximately 40%. The incidence of postoperative nausea and vomiting (PONV) depends on the type of anaesthesia and surgery, but overall is estimated to be 20-30%. Evidence-based medicine and meta-analysis have been used to direct medical therapy to help determine equivalence, optimal dose, timing, safety and efficacy of antiemetic medications. Concepts such as the number needed to treat and number needed to harm are helpful to guide the clinician regarding the benefits and risks of a particular treatment. The serotonin 5-HT(3) receptor antagonists ondansetron, granisetron, tropisetron and dolasetron have been important additions to the antiemetic armamentarium. The 5-HT(3) receptor antagonists are similar in chemical structure, efficacy and adverse effect profile. They appear to have no important differences among themselves in clinical outcomes for CINV and PONV. Headache, dizziness, constipation and diarrhoea are their most common adverse effects, and when they occur they are usually mild and easily managed. Haemodynamic changes and extrapyramidal adverse effects are uncommon. ECG changes such as prolonged corrected QT (QTc) interval are infrequent, dose-related and overall judged to be clinically insignificant. As most studies with the 5-HT(3) antagonists have been conducted on relatively healthy patients, caution should be exercised when these drugs are used in susceptible patients with co-morbidities. The clinician must weigh the benefit of administering an antiemetic for CINV or PONV against the risk of occurrence of an adverse event.

Pharmacologic therapy for the irritable bowel syndrome

The management of the irritable bowel syndrome (IBS) remains unsatisfactory. For abdominal pain, antispasmodics are, at best, of only modest efficacy. Tricyclic antidepressants in low dose are useful (with the number needed to treat being three), but side effects and patient concerns regarding use of a centrally acting agent for depression remain limitations. Selective serotonin reuptake inhibitors are of uncertain efficacy in IBS. Opioid agonists, especially loperamide, are useful for diarrhea but not for pain in IBS; rebound constipation also remains a problem. Bile salt sequestering agents are not of established value in IBS but seem to be useful clinically in a small group of IBS patients with diarrhea. Aloestron, a 5HT(3) antagonist, should be reserved, if available, for women with severe diarrhea predominant IBS who have failed to respond to conventional therapy, and started at a low dose. Fiber and bulking agents may help constipation in some trials, but the evidence that they are efficacious in IBS is equivocal; they are frequently prescribed as first-line drugs for IBS regardless of the primary bowel disturbance but often increase bloating, gas, and pain. Laxatives are not of established value in IBS but are often taken by patients with constipation predominant IBS. Tegaserod, a partial 5HT(4) agonist, is now available in the United States and other countries for use in women with IBS whose primary bowel symptom is constipation; its efficacy in men and in those with alternating bowel habits is unknown. Probiotics are of uncertain efficacy. Chinese herbal medicine data are insufficient. Other new drugs in development include the cholecystokinin antagonists and novel visceral analgesics. Both current and potential therapies for IBS are reviewed in this article.

Does intravenous ondansetron affect gastric emptying of a solid meal, gastric electrical activity or blood hormone levels in healthy volunteers?

BACKGROUND

In previous studies, tropisetron has been shown to accelerate gastric emptying of a solid meal. However, it is uncertain whether other specific 5-hydroxytryptamine-3 receptor antagonists, such as ondansetron, also have a gastroprokinetic effect in humans.

AIM

To evaluate the effect of ondansetron on gastric half-emptying time (T1/2) of a solid meal, gastric myoelectrical activity and hormone levels in 14 healthy volunteers.

METHODS

In a placebo-controlled, randomized, crossover study, we investigated the effects of ondansetron (8 mg intravenously) on the gastric emptying of solids (by scintigraphy), gastric myoelectrical activity (by electrogastrography) and the post-prandial release of cholecystokinin, gastrin, human pancreatic polypeptide, gastric inhibitory polypeptide, vasoactive intestinal polypeptide, motilin, substance P and galanin.

RESULTS

The average T1/2 values were 86 min and 85.5 min without lag time (P=0.082) and 92 min and 93 min with lag time (P=0.158) for the placebo and ondansetron treatments, respectively. The average T1/2 of female volunteers was significantly longer than that of male volunteers. The dominant gastric electrical frequency and hormone plasma concentrations were not altered by ondansetron.

CONCLUSIONS

Ondansetron did not affect the gastric emptying of solids, the dominant gastric electrical frequency or the plasma concentrations of the analysed gastrointestinal peptides.

Serotoninergic neuroenteric modulators

Irritable bowel syndrome (IBS) is common and can be disabling. Several drugs that modulate serotonin (5HT) and other neurotransmitters in the gut (neuroenteric modulators) have either become available or are in development, but progress has been slowed by toxicity. Blockade of 5HT(3) receptors slows colonic transit, increases fluid absorption and increases left colon compliance. Alosetron, a potent 5HT(3) receptor antagonist, has, in women but not in men, a clinically significant but modest therapeutic gain over placebo in the relief of abdominal pain and discomfort and bowel-habit disturbance (but not bloating) in diarrhoea-predominant IBS. However, the drug unexpectedly was associated with ischaemic colitis and, very rarely, severe constipation-induced complications, and alosetron has been withdrawn. Cilansetron may have similar efficacy in men and women. 5HT(4) receptor stimulation results in accelerated colonic transit, and tegaserod, a partial 5HT(4) receptor agonist, has modest but clinically significant advantage over placebo in constipation-predominant IBS; the benefit seems to be confined to females. Long-term published data are lacking and safety concerns have been raised. Prucalopride, a full 5HT(4) agonist that has been promising in idiopathic chronic constipation, may also be limited by toxicity. Other 5HT receptor antagonists and agonists are under development for IBS. However, for modulators of single receptors to achieve a substantial therapeutic gain, and to do so safely, drug targets based on the pathophysiology of IBS need to be better defined.

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.

Involvement of 5-HT3 and 5-HT4 receptors in the motor activity of isolated vascularly perfused rat duodenum

The involvement of serotonin (5-HT) receptor subtypes in motor activity of the ex vivo vascularly perfused rat duodenum was investigated. Clusters of phasic contractions (CPCs), migrating in an oral to anal direction, were obtained without any stimulation. Drug effects were evaluated by changes in different components of the pressure waves, such as motor index (MI), frequency, amplitude and duration of the CPC. The effect of 5-HT depletion on motor activity was examined in animals treated with p-chlorophenylalanine (PCPA). The MI, frequency and duration of CPC were decreased by PCPA, but the amplitude was not affected, suggesting that endogenous 5-HT may play an important role in regulation of the motor activity of the rat intestine. The importance of the 5-HT receptor subtypes in the regulation of motor activity was examined. Neither the nonselective 5-HT1 and 5-HT2 receptor antagonist, methysergide, nor the 5-HT2 receptor antagonist, ketanserin, affected motor activity. However, the 5-HT3 receptor antagonists, granisetron and azasetron, decreased percentage MI, frequency, percentage amplitude and percentage duration of CPC. The 5-HT4 receptor antagonist, SB204070, exerted both excitatory and inhibitory actions, with a higher dose (10 nM) stimulating percentage MI, frequency, percentage amplitude and percentage duration, and a lower dose (0.1 nM or 1 nM) decreasing percentage MI and percentage duration of CPC. These results suggest that endogenous 5-HT regulates the motor activity of the rat duodenum through 5-HT3 and 5-HT4 receptors, with the former mediating the stimulatory influence and the latter mediating both stimulatory and inhibitory influences.

The pharmacological properties of the novel selective 5-HT3 receptor antagonist, alosetron, and its effects on normal and perturbed small intestinal transit in the fasted rat

The purpose of this study was to investigate the pharmacological properties of the novel, selective 5-HT3 receptor antagonist, alosetron, and its effects on transit time in both the normal and perturbed small intestine of the rat. Alosetron concentration-dependently inhibited radioligand binding in membranes containing rat and human 5-HT3 receptors with estimated pKi values of 9.8 (n = 3) and 9.4 (n = 6), respectively. In selectivity studies alosetron had little or no significant affinity for any of the many other receptors and ion channels studied. Alosetron potently antagonized the depolarization produced by 5-HT in the rat vagus nerve (estimated pKB value of 9.8, n = 25). In anaesthetized rats, i. v. administration of alosetron inhibited 2-methyl-5-HT induced bradycardia (Bezold Jarisch index) at 1 and 3 microg kg-1, with an agonist dose ratio of approximately 3.0 at 1.0 microg kg-1, = 3-5). Alosetron administered via the duodenum also inhibited this reflex, with duration of action that was significantly longer than that seen with ondansetron (120-60 min, respectively, n = 6). Alosetron had no significant effect on normal small intestinal propulsion in the rat, but fully reversed the increase in intestinal propulsion (96%, n = 3) produced by egg albumin challenge. Alosetron is a highly selective 5-HT3 antagonist which normalizes perturbed small intestinal propulsion. Previous clinical data in IBS patients together with the transit data provide a good rationale for further studies with alosetron in IBS patients.

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.

Effects of ondansetron on electrically evoked contraction in rat stomach fundus: possible involvement of 5-HT2B receptors

This study examined the effects of ondansetron, an antagonist of the 5-hydroxytryptamine (5-HT3) receptor without 5-HT4 receptor agonistic activity. on electrically evoked contractions and acetylcholine release in rat stomach fundus strips. Ondansetron (10(-8)-10(-4) M) produced a concentration-dependent increase in the magnitude of the electrically evoked contraction, while it inhibited the release of acetylcholine induced by electrical field stimulation. The stimulatory effect of ondansetron (10(-6) M) on electrically evoked contractions was antagonized by yohimbine, a 5-HT2B receptor antagonist, but not by SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-[diethylamino] ethyl ester), a 5-HT4 receptor antagonist. In the presence of tetrodotoxin, ondansetron (10(-7)-10(-5) M) enhanced the contractions induced by acetylcholine. This stimulatory effect of ondansetron on acetylcholine-induced contractions was antagonized by yohimbine. These data suggest that ondansetron potentiates the contractile response to acetylcholine in the rat stomach fundus through the activation of 5-HT2B receptors.

Constipation evoked by 5-HT3-receptor antagonism: evidence for heterogeneous efficacy among different antagonists in guinea-pigs

The abilities of selective 5-HT3-receptor antagonists to evoke constipation were examined in conscious guinea-pigs and in preparations of guinea-pig isolated colon. Compared with vehicle-treated guinea-pigs, acute doses of granisetron (0.1, 1 and 10 mg kg-1, i.p.) and tropisetron (10 mg kg-1, i.p., but not 1 and 0.1 mg kg-1, i.p.) significantly (P < 0.05) reduced the total number of faecal pellets excreted during a 12-h observation period. By contrast, BRL 46470 (0.1-10 mg kg-1, i.p.) had no significant effect on the incidence of defecation. Mid-to-distal lengths of guinea-pig isolated colon spontaneously expelled faecal pellets. Granisetron (0.1 and 1 microM) and tropisetron (1 microM) reduced or prevented the rate at which they were spontaneously expelled. Morphine (0.1 microM) and clonidine (10 nM) also showed faecal pellet transit time. Naloxone (0.1 microM) had no effects alone, but reversed the actions of granisetron, morphine and clonidine. BRL 46470 (1 microM) had no significant effect on the transit of faecal pellets in guinea-pig isolated colon. In segments of guinea-pig isolated colon which did not contain faecal pellets, granisetron, tropisetron and BRL 46470 antagonized the ability of 5-HT to evoke cholinergically-mediated contractions of the longitudinal muscle. The respective pA2 values and slopes of the Schild plots were 8.5 +/- 0.05, slope 1.06 +/- 0.03; 8.5 +/- 0.1, slope 0.91 +/- 0.04; and 7.9 +/- 0.1, slope 0.93 +/- 0.05. Our experiments suggest that not all 5-HT3-receptor antagonists are the same.(ABSTRACT TRUNCATED AT 250 WORDS)

Drugs affecting serotonin receptors

A greater understanding of the various serotonin receptor subtypes has led to a clearer appreciation of the role of serotonin in gastrointestinal motility, sensation and secretion. Serotonin is definitely involved in the aetiopathogenesis of cisplatin-induced emesis and carcinoid diarrhoea. The application of serotonergic drugs in clinical therapeutics for gut disturbances is presently dominated by the use of 5-HT3 antagonists for acute chemotherapy-induced nausea and vomiting, and the use of substituted benzamides which are 5-HT4 agonists stimulating gut motor function through 5-HT4 neuronal receptors. The best-studied 5-HT4 agonist is cisapride, which has been shown to stimulate motility at several levels of the gut. Cisapride is approved for healing and maintenance treatment of reflux oesophagitis and is used in several countries for the alleviation of symptoms consistent with regional stasis, from dyspepsia to constipation. Carcinoid diarrhoea is a prototypic disease associated with deranged serotonin metabolism, and a rationale for using 5-HT3 or 5-HT4 antagonists is based on the recent appreciation of the important role of impaired gut motor function in carcinoid diarrhoea. In the future, greater understanding of the serotonin receptor subtypes and their role in gut disorders may lead to novel approaches to alleviate increased visceral perception of functional gastrointestinal disorders, to correct changes in colonic capacitance, or to alter gastrointestinal motility that contributes to diarrhoea or constipation. However, at the present time, it must be stressed that these uses are still at an experimental stage and that careful validation and proper controlled studies are still required.

The importance of 5-hydroxytryptamine receptors in the gut

Most of the 5-hydroxytryptamine (5-HT) present in the adult human body is located in the gastrointestinal tract. The vast majority is contained in enteroendocrine cells, the rest exists mainly in myenteric interneurons separated from the mucosa by an intraenteric barrier. Physiological studies suggest that 5-HT plays a vital role in mediating both sensory and reflex responses to gastrointestinal stimuli and, thus, this transmitter is closely implicated in gut reactions. This review outlines some of the evidence for different 5-HT receptors, summarizes the role of 5-HT in mediating gut sensitivity and motor activity, secretion and more complex activities, such as emesis and diarrhoea and identifies the clinical role of drugs acting on 5-HT receptors in the treatment of emesis, diarrhoea, the control of abdominal pain and discomfort and the rectification of gastrointestinal motility.

5-Hydroxytryptamine and human small intestinal motility: effect of inhibiting 5-hydroxytryptamine reuptake

Parenteral 5-hydroxytryptamine stimulates small intestinal motility, but the effect of continuous stimulation with 5-hydroxytryptamine on the human migrating motor complex is unknown. Using a selective 5-hydroxytryptamine reuptake inhibitor, paroxetine, this study investigated the effect of indirect 5-hydroxytryptamine agonism on fasting small intestinal motility and transit. Eight healthy subjects were studied while receiving paroxetine 30 mg daily for five days and while receiving no treatment, in random order. Ambulant small intestinal motility was recorded from five sensors positioned from the duodenojejunal flexure to the ileum for 16-18 hours. Paroxetine reduced the migrating motor complex periodicity mean (SEM) from 81 (6) min to 67 (4) min (p < 0.05), and increased the propagation velocity of phase III from 3.1 to 4.7 cm/min in the proximal jejunum (p < 0.01), and from 1.6 to 3.4 cm/min distally (p < 0.001). Orocaecal transit time measured by lactulose hydrogen breath test was reduced by paroxetine from 70 (9) min to 48 (7) min (p < 0.05). These data suggest that 5-hydroxytryptamine participates in the control of migrating motor complexes in humans, and that selective 5-hydroxytryptamine reuptake inhibitors have a prokinetic action in the human small intestine.

A possible interaction between serotonin and cholecystokinin-8 in the ileo-colonic motor responses to feeding in rats

The effects of cholecystokinin (CCK)-A, CCK-B, and 5-hydroxytryptamine-2 (5-HT2) receptor antagonists on the postprandial myoelectric activity of the ileum and the proximal and distal colon in rats were studied. After L364,718, a CCK-A receptor antagonist, or ritanserin, 5-HT2 receptor antagonist, the duration of the ileal postprandial motor pattern was reduced by 60%. In contrast, L365,260, a CCK-B receptor antagonist, had no effect. In the distal colon, the postprandial response was inhibited by CCK-A, CCK-B, and 5-HT2 receptor antagonists, whereas they were inactive in the proximal colon. CCK-8 administered at the end of the meal suppressed the inhibitory effect of ritanserin on the ileum and the distal colon. These results suggest that, in rats, CCK-8 is involved in the control of the ileal motor response to feeding through CCK-A receptors and in that of the distal colon through both CCK-A and CCK-B receptors. Serotonin seems to play a role in CCK-8 control through 5-HT2 receptors.

Ondansetron. An update of its therapeutic use in chemotherapy-induced and postoperative nausea and vomiting

Ondansetron is a selective 5-HT3 receptor antagonist which has previously been reported in the Journal to be a promising new agent for use as prophylaxis against nausea and vomiting caused by chemotherapy and radiotherapy. Since the publication of this original review, further studies have been published that show ondansetron to be an effective antiemetic agent in patients receiving chemotherapy and radiotherapy. Several studies have shown ondansetron to be a more effective antiemetic agent than high-dose metoclopramide in patients with emesis induced by high- and low-dose cisplatin treatment, and noncisplatin chemotherapy-induced emesis. The drug as mono-therapy does not appear to offer any advantage over alternative therapies against delayed high-dose cisplatin-induced nausea and vomiting; however, extremely limited data suggest that ondansetron plus dexamethasone may be useful in this indication. Trials have shown combination therapy with ondansetron and dexamethasone to be significantly more effective than both ondansetron monotherapy and a standard antiemetic regimen comprising metoclopramide, dexamethasone and diphenhydramine against acute high-dose cisplatin-induced emesis. Results from a number of small scale trials suggest that ondansetron may be an effective treatment for chemotherapy-induced emesis refractory to conventional antiemetic therapy. Ondansetron also appears to be more effective against refractory emesis induced by noncisplatin chemotherapy than that induced by cisplatin chemotherapy. Several trials have shown ondansetron to be more effective than placebo as prophylaxis against postoperative nausea and vomiting; a further trial has shown single-dose ondansetron to be significantly more effective than single-dose droperidol or metoclopramide in this indication. In addition, several trials have shown ondansetron to be more effective than placebo as treatment for nausea and vomiting that has commenced postoperatively. The overall incidence of adverse events in ondansetron recipients during chemotherapy-induced emesis studies was 36%. Headache and constipation are the most common adverse events during ondansetron therapy. Thus, recent data affirms the efficacy of ondansetron in the treatment of acute chemotherapy-induced nausea and vomiting and shows it to be especially efficacious when combined with dexamethasone. It appears that the drug will also have a substantial role in the prophylaxis and treatment of postoperative nausea and vomiting.

Selective 5-hydroxytryptamine type 3 receptor antagonism with ondansetron as treatment for diarrhea-predominant irritable bowel syndrome: a pilot study

Serotoninergic innervation may contribute to the control of colonic motility and to visceral sensation from the large bowel. Indeed, ondansetron hydrochloride, a selective 5-hydroxytryptamine type 3 receptor antagonist, has been shown to slow colonic transit in healthy volunteers. Thus, we wished to determine whether 5-hydroxytryptamine type 3 receptor blockade slows colonic and small bowel transit in patients with diarrhea-predominant irritable bowel syndrome (IBS) and whether symptoms would be ameliorated with drug therapy. Of 14 patients with well-established IBS who entered a randomized, double-blind, placebo-controlled crossover pilot trial of 4 weeks of treatment with ondansetron, 16 mg three times daily, 11 completed the study. A minimal "washout period" of 4 weeks (median, 7 weeks) separated the two phases of the trial because patients were required to have similar symptoms before both periods of the study. Colonic transit tended to be longer during drug therapy than during the placebo trial, but this difference was not significant. Small intestinal transit and orocecal transit were unchanged by the drug. The integrated and peak postprandial increases in neurotensin, peptide YY, and human pancreatic polypeptide in serum were not significantly different in the drug and placebo periods. After treatment with ondansetron, stool consistency improved significantly; however, stool frequency, stool weight, abdominal pain, and the symptom criteria for IBS were not significantly altered by the drug. The results of this pilot study suggest that the motor effects expected with 5-hydroxytryptamine type 3 receptor blockade (namely, slowed colonic transit) may be diminished in some patients with IBS. The subjective improvement in stool consistency may reflect changes in the perception of defecation.(ABSTRACT TRUNCATED AT 250 WORDS)

Review article: 5-hydroxytryptamine agonists and antagonists in the modulation of gastrointestinal motility and sensation: clinical implications

Serotonin (5-hydroxytryptamine; 5-HT) is found in the enteric nervous system where it has been implicated in controlling gastrointestinal motor function. A number of receptor or recognition sites have been identified in the gut, but recently most attention has focused on the 5-HT3 and 5-HT4 receptors. The functional role of the 5-HT3 receptor remains incompletely understood, but it is probably involved in the modulation of colonic motility and visceral pain in the gut. A number of selective 5-HT3 antagonists have been developed including ondansetron, granisetron, tropisetron renzapride and zacopride. While the substituted benzamide prokinetics (for example, metoclopramide, cisapride) also block 5-HT3 receptors in high concentrations, their prokinetic action is believed to be on the basis of their agonist effects on the putative 5-HT4 receptor. Some 5-HT3 antagonists have 5-HT4 agonist activity (for example, renzapride, zacopride) and others do not (for example, ondansetron, granisetron), while tropisetron in high concentrations is a 5-HT4 antagonist. Based on the pharmacological data, it has been suggested that specific 5-HT antagonists and agonists may prove to be beneficial in a number of gastrointestinal disorders including the irritable bowel syndrome, functional dyspepsia, non-cardiac chest pain, gastrooesophageal reflux and refractory nausea. In this review, the rationale for the use of these compounds is discussed, and the available experimental evidence is summarized.

Ondansetron. Therapeutic use as an antiemetic

Ondansetron (GR 38032F) is a highly selective 5-HT3 receptor antagonist, one of a new class of compounds which may have several therapeutic applications. Animal and clinical studies show that ondansetron reduces the 24-hour incidence and severity of nausea and vomiting induced by cytotoxic drugs, including cisplatin, and by single exposure, high dose radiation. Ondansetron is more effective than high dose metoclopramide in the 24 hours following chemotherapy, and preliminary clinical evidence suggests that it is equally effective in the following 4 days. It is also more effective than the 'moderate' doses of metoclopramide used to suppress emesis following radiotherapy. The antiemetic efficacy of ondansetron is enhanced by dexamethasone in cisplatin-treated patients. Importantly, extrapyramidal effects have not been reported with ondansetron. Further comparisons are required with standard combination antiemetic therapy to complement the data presently available. Thus, ondansetron is a promising new agent for prophylaxis against nausea and vomiting in chemotherapy and radiotherapy. It may be particularly useful in young and elderly patients who are more susceptible to extrapyramidal symptoms induced by high dose metoclopramide. With its improved tolerability and clinical response profiles, ondansetron represents an important advance in a difficult area of therapeutics.

5-Hydroxytryptamine and 5-hydroxytryptamine-3 receptor antagonists

5-Hydroxytryptamine (5-HT) is present throughout the gastrointestinal tract, which acts as the major reservoir of this substance in the body. Its physiologic role has not been clearly established, although it seems likely that 5-HT is involved in the regulation of aspects of intestinal motility such as peristalsis and the migrating motor complex. In disease states the contribution of 5-HT is perhaps more clearly established, particularly its role in chemotherapy-induced emesis, in the carcinoid syndrome, and, possibly, in mediating the effect of some intestinal secretagogues, notably cholera toxin. Many of the functions of 5-HT in the gut have been elucidated as a result of the development of antagonists to 5-HT receptors. However, some of these compounds have 5-HT agonist activity as well as 5-HT receptor blocking activity, making interpretation of their effects in health and disease difficult. Nevertheless, 5-HT receptor antagonists are finding an important place in the management of the carcinoid syndrome and in chemotherapy-induced emesis and may well evolve as important agents for modulating gut motility and for inhibiting secretory states in the small and large intestine. The suggestion that 5-HT3 receptor antagonists might also modulate visceral sensation in the gut is of great interest because of their potential to relieve symptoms of functional bowel disorders such as pain, urgency, and bowel frequency.

Closing remarks. Ondansetron: effects on gastrointestinal motility

Ondansetron (GR 38032F), a 5-hydroxytryptamine-3 (5-HT3) receptor antagonist, is a highly effective and safe drug for the prophylaxis and treatment of emesis induced by various chemotherapy regimens in cancer patients. Recent studies have shown that ondansetron is also effective in post-anaesthesia and radiation-induced nausea and vomiting. When compared with high-dose metoclopramide, ondansetron appeared to be superior. Furthermore, ondansetron has been shown to improve stool consistency and to reduce stool frequency in patients with diarrhoea-predominant irritable bowel syndrome.

GR 38032F (ondansetron), a selective 5HT3 receptor antagonist, slows colonic transit in healthy man

The newly recognized class of 5-hydroxytryptamine receptors (5HT3) may be involved in the induction of nausea, since their pharmacological antagonists are effective against emesis induced by chemotherapy. 5HT3 receptors are present on enteric neurons, and 5HT3 blockers may produce mild constipation; we thus hypothesized that 5HT3 receptors would modulate colonic motility. To determine if GR 38032F, a selective 5HT3 antagonist known to have antiemetic effects, influences colonic transit in health, a randomized, double-blind, placebo-controlled crossover study was performed. Using a radiopaque marker technique, colonic transit was quantified in 39 healthy volunteers (19 men, 20 nonpregnant women) 18-70 years of age. On a standard 25-g fiber diet, 16 mg of GR 38032F was given orally thrice daily. Gastrointestinal peptides (peptide YY, human pancreatic polypeptide, neurotensin, motilin, gastrin-cholecystokinin, substance P) were also measured in plasma fasting and postprandially. Mean total colonic transit time on placebo was 27.8 hr, while on GR 38032F it was 39.1 hr (P less than 0.0005). Transit times through the left colon (P less than 0.0005) and rectosigmoid (P less than 0.05) were prolonged by the drug, but right colonic transit was not significantly altered. Transit times did not correlate with age or gender, but subjects with shorter transit times were significantly more affected than were those with longer transit times. The peak release of peptide YY was minimally decreased following GR 38032F (P less than 0.01), but the peak and integrated postprandial responses of human pancreatic polypeptide, neurotensin, motilin, gastrin-cholecystokinin, and substance P were not significantly altered by the drug.(ABSTRACT TRUNCATED AT 250 WORDS)