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

Lack of effect of a 5-HT3 antagonist, pancopride, on lower oesophageal sphincter pressure in volunteers

Effects of pancopride (5 and 10 mg, intravenously), on lower oesophageal sphincter pressure (LOESP), were assessed in healthy volunteers by means of oesophageal manometry. After pancopride 10 mg, the LOESP was higher than placebo and 5 mg pancopride but there were no differences among the three treatments (P = 0.42). The areas under the curve were similar without differences, neither with absolute measurements (P = 0.53) nor after a baseline correction (P = 0.16). In conclusion, pancopride has no clinically relevant effect on lower oesophageal sphincter pressure.

Serotonin receptors: subtypes, functional responses and therapeutic relevance

Recent, rapid progress in the molecular biology of serotonin (5-HT) receptors requires conceptual re-thinking with respect to receptor classification. Thus, based on operational criteria (agonist and antagonist rank order), as well as transduction mechanisms involved and the structure of the receptor protein, the Nomenclature Committee of the Serotonin Club has proposed the following classification and nomenclature: the main receptor types 5-HT1 to 5-HT4, recombinant receptors (e.g. 5-ht5 to 5-ht7) and 'orphan' receptors. The aim of the present review is to discuss the events leading to this classification, the criteria for and functional responses mediated by various 5-HT receptors, as well as the therapeutic possibilities with 5-HT ligands.

Review article: functional dyspepsia--should treatment be targeted on disturbed physiology?

In patients who present with chronic unexplained upper abdominal pain or discomfort (functional dyspepsia), therapy should ideally be targeted on correcting the individual's disturbed pathophysiology. Here, putative mechanisms implicated in functional dyspepsia and potential approaches to therapy are critically reviewed in order to determine if targeting treatment is of value. Pharmacological therapies reviewed include those that aim to correct disordered gastric emptying (e.g. cisapride, dopaminergic receptor antagonists, macrolides), reduce visceral hypersensitivity (e.g. somatostatin analogues, cholecystokinin antagonists, opioid agonists, serotonin type 3 receptor antagonists), reduce gastric acid secretion (e.g. H2-blockers, acid pump inhibitors), cure Helicobacter pylori infection, enhance muscosal defence (e.g. sucralfate, bismuth) or modify central nervous system processes. It is concluded that the imperfectly understood pathophysiology of functional dyspepsia contributes to the paucity of established efficacious therapies.

Visceral perception in irritable bowel syndrome. Rectal and gastric responses to distension and serotonin type 3 antagonism

We wished to determine if visceral perception in the rectum and stomach is altered in patients with irritable bowel syndrome and to evaluate the effects on visceral sensation of 5-HT3 receptor blockade. Twelve community patients with diarrhea-predominant irritable bowel syndrome and 10 healthy controls were studied in a double-blind, randomized, placebo-controlled study. Using two barostats, the stomach and rectum were distended, with pressure increments of 4 mm Hg, from 10 to 26 mm Hg; visceral perception was measured on an ordinal scale of 0-10. Personality traits were measured using standard psychological methods, and somatic pain was evaluated by immersion of the nondominant hand in cold water. The effect of 5-HT3 antagonism was tested with a single intravenous dose of ondansetron at 0.15 mg/kg. Gastric perception was higher in irritable bowel syndrome, but rectal distension was perceived similarly in irritable bowel syndrome and controls. Pain tolerance to cold water was also similar in irritable bowel syndrome and controls. Ondansetron induced rectal relaxation and increased rectal compliance but did not significantly alter gastric compliance or visceral perception. Psychological test scores were similar in patients and controls. We conclude that in this group of psychologically normal patients with irritable bowel syndrome, who were not chronic health-care seekers, visceral perception was normal. Ondansetron did not alter gut perception in health or in irritable bowel syndrome.

Effect of different coffees on esophageal acid contact time and symptoms in coffee-sensitive subjects

The purpose of this study was to correlate the effects of different coffees on esophageal acid contact, heartburn, and regurgitation in patients with coffee-sensitivity. Twenty volunteers with coffee-sensitivity were studied in a double-blind, 3 period, crossover study examining the effect of three regular (caffeinated) coffees (a coffee from the USA--"A"; a "treated" coffee from Europe--"B"; and an "untreated" coffee from Europe--"C") before and after a high-fat test meal. The median acid contact times for coffees A, B, and C were 6.5%, 9%, and 10.5%, respectively (A vs. C, p = 0.005). Significantly fewer patients reported any symptoms with coffee A compared with coffee C (p < 0.05). Symptoms were usually more frequent and severe after the test meal. There was a trend toward fewer and less severe symptoms with the treated coffee (B) compared with its untreated counterpart (C). Our conclusions are as follows: (a) Different coffees induce variations in gastroesophageal reflux in coffee-sensitive individuals. (b) Coffee can be treated in a manner which decreases heartburn symptoms by 75% while decreasing acid contact by only 14%. (c) Gastroesophageal reflux and symptoms of coffee sensitivity increase with the concomitant ingestion of food. (d) Symptoms of dyspepsia appear to be influenced by variations in both the coffee itself and characteristics of susceptible individuals. (e) Although gastroesophageal reflux is important in the genesis of coffee-sensitivity, there must be other factors which act in concert with reflux to produce symptoms of coffee-sensitivity.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastrointestinal motility disorders in scleroderma

After the skin, the gastrointestinal tract is the second most common target of systemic sclerosis. The major clinical manifestations include gastroesophageal reflux, small bowel bacterial overgrowth, malnutrition, and intestinal pseudoobstruction. Treatment is symptomatic and supportive. Gastroesophageal reflux can usually be adequately managed with prokinetic drugs, omeprazole, and judicious use of antireflux surgery. If Barrett's esophagus is present, periodic endoscopic monitoring for development of dysplastic changes or adenocarcinoma is indicated. Bacterial overgrowth usually responds to rotating antibiotics and prokinetic drugs. Malnutrition and intestinal pseudoobstruction remain the major problems and often home total parenteral nutrition is required. Intestinal pseudoobstruction occurs in two phases: an early, neuropathic phase may respond to prokinetic drugs (metoclopramide, cisapride, octreotide, and erythromycin) and dietary modification (low-residue diets, vitamin supplementation). In the late myopathic phase, therapy is usually ineffective. Treatment consists of nutritional support. Careful manometric and radiographic localization of affected segments of stomach and small and large intestines may allow judicious surgical resection or venting procedures to reduce symptoms in this unfortunate group of patients.

Inhibition of 5-hydroxytryptamine re-uptake impairs human gall-bladder emptying

BACKGROUND

5-Hydroxytryptamine (5-HT) is an important neurotransmitter in the enteric nervous system. The intrinsic neural plexus of the gall-bladder resembles the enteric nervous system and similarly contains 5-HT neurones. The action of 5-HT on gallbladder motility has been investigated in animals but its effect on the human gall-bladder in vivo is unknown.

METHODS

The effect of indirect 5-HT agonism using paroxetine, a specific inhibitor of neuronal 5-HT reuptake, on gall-bladder motility was investigated in 12 healthy volunteers. In a randomized double-blind crossover design, gall-bladder motility was assessed after administration of 30 mg paroxetine daily for two days and after placebo. Ultrasonography was used to determine gall-bladder volumes while fasting and at 5 min intervals following a 250 kcal mixed liquid meal.

RESULTS

Fasting gall-bladder volumes of 21.8 +/- 3.2 ml on placebo and 28.0 +/- 3.5 ml on paroxetine were similar. Paroxetine impaired postprandial gall-bladder emptying. Residual gall-bladder volume was 10.2 +/- 2.7 ml with placebo and 17.1 +/- 2.7 ml with paroxetine (P < 0.05). Ejection fraction was 57.3 +/- 7.7% on placebo and 40.9 +/- 4.7% on paroxetine (P < 0.05).

CONCLUSION

5-HT pathways may participate in the regulation of biliary motility, and this study demonstrates an inhibitory role of 5-HT in the control of human gall-bladder emptying.

Alosetron does not affect the visceral perception of gastric distension in healthy subjects

BACKGROUND

The effect of alosetron, a new specific 5-HT3 receptor antagonist, on the visceral perception in response to gastric distension was assessed in 12 healthy male subjects in a randomized, double-blind, placebo controlled crossover trial.

METHODS

Each subject was given orally either alosetron 1 mg b.d. or placebo b.d. for 6 days (wash-out period 7-28 days). At the end of each dosing period, both isobarometric and isovolumetric gastric distensions were performed using an electronic barostat.

RESULTS

Alosetron did not modify the gastric wall compliance (pressure-volume relationship). Alosetron had an effect similar to placebo on the visceral perception scores in both isobarometric and isovolumetric distensions. The mean (+/- SEM) thresholds for abdominal discomfort were, respectively, 16.8 +/- 0.7 mmHg and 825 +/- 61 mL with alosetron, 16.7 +/- 0.6 mmHg and 883 +/- 45 mL with Placebo (P = NS).

CONCLUSIONS

5-HT3 receptors do not appear to be involved in the visceral perception of gastric distension in healthy subjects.

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.

Prokinetic agents for lower gastrointestinal motility disorders

Prokinetic agents are currently being investigated as potential therapies for motility disorders of the lower gastrointestinal tract. Cholinergic agonists such as bethanechol are known to improve postoperative ileus but are limited because of side effects. Dopamine antagonists such as domperidone appear to have maximal prokinetic effect in the proximal gastrointestinal tract and are effective for such conditions as gastroparesis and gastroesophageal reflux, but they appear to have little physiologic effect in the colon or in colonic motility disorders. Naloxone, an opioid antagonist, appears to hold promise in patients with irritable bowel syndrome, small intestinal pseudo-obstruction, and constipation. Erythromycin exerts its prokinetic effect by acting as a motilin agonist; it has been used in the treatment of diabetic gastroparesis and appears to improve symptoms of colonic pseudo-obstruction and postoperative ileus. Metoclopramide, a combined cholinergic agonist and dopamine antagonist, is currently used exclusively for proximal motility dysfunction. Cisapride appears to hold the most promise for patients with colonic motility disorders. In patients with postoperative ileus, cisapride is associated with an increased return of bowel function compared with placebo. In patients with chronic constipation, cisapride increases stool frequency and decreases laxative abuse in both adults and children. Hopefully, as an understanding of gastrointestinal motility increases, effective prokinetic agents will be developed that will improve symptoms of patients with large bowel motility disorders and may also help to predict those patients who benefit from surgical management for constipation.

Effect of metoclopramide, ondansetron and granisetron on aldosterone secretion in man

The plasma aldosterone response following the administration of drugs with antagonist and agonist activity at Serotonin 3 and 4 (5-HT3&4) receptors has been examined in 9 healthy male volunteers receiving the following four treatments i.v. in a randomised, cross-over sequence: ondansetron 8 mg, granisetron 3 mg, metoclopramide 20 mg, and saline 20 ml. Metoclopramide significantly increased the mean plasma aldosterone level to 196% of basal level at 5 min. It rose to 234% at 15 min and remained at more than 185% of basal level for the duration of the experiment. The response to ondansetron and granisetron did not differ significantly from placebo. If dopamine antagonism is discounted, the results suggest that metoclopramide-induced aldosterone secretion results from its agonist activity at 5-HT4 receptors, although slow neuronal depolarization via an unidentified receptor remains a possibility. Antagonism at the 5-HT3 receptor plays no role, as the selective antagonist, granisetron, did not elicit a significant response. It seems unlikely that the 5-HT4 receptor is the second, low affinity binding site of ondansetron, unless it had no agonist activity at this receptor.

Gastrointestinal effects of 5-hydroxytryptamine and related drugs

This paper deals with the effects of 5-hydroxytryptamine (5-HT; serotonin) and related drugs on the gastrointestinal tract (GIT). The nomenclature and classification of 5-HT receptors, as well as their putative role in the GIT are updated in this review. Besides its effects on the cardiovascular system, which have been extensively described, several lines of evidence suggest a role for 5-HT in regulating gastrointestinal functions. 5-HT is present in the gastrointestinal tissues, and can elicit contraction or relaxation by activation of a wide variety of mechanisms and receptors. At least four main types of receptors (5-HT1, 5-HT2, 5-HT3 and 5-HT4) have been described and all the four types seem to influence the GIT. In this respect, the 5-HT2, and in some cases the 5-HT1 receptors, appear to be present on the gastrointestinal smooth muscle, while 5-HT3 and 5-HT4 are mainly neuronal.

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)