5-hydroxytryptamine-3 receptors are involved in the initiation of gastric phase-3 motor activity in humans

Motilin Comparative Study: Structure, Distribution, Receptors, and Gastrointestinal Motility

Motilin, produced in endocrine cells in the mucosa of the upper intestine, is an important regulator of gastrointestinal (GI) motility and mediates the phase III of interdigestive migrating motor complex (MMC) in the stomach of humans, dogs and house musk shrews through the specific motilin receptor (MLN-R). Motilin-induced MMC contributes to the maintenance of normal GI functions and transmits a hunger signal from the stomach to the brain. Motilin has been identified in various mammals, but the physiological roles of motilin in regulating GI motility in these mammals are well not understood due to inconsistencies between studies conducted on different species using a range of experimental conditions. Motilin orthologs have been identified in non-mammalian vertebrates, and the sequence of avian motilin is relatively close to that of mammals, but reptile, amphibian and fish motilins show distinctive different sequences. The MLN-R has also been identified in mammals and non-mammalian vertebrates, and can be divided into two main groups: mammal/bird/reptile/amphibian clade and fish clade. Almost 50 years have passed since discovery of motilin, here we reviewed the structure, distribution, receptor and the GI motility regulatory function of motilin in vertebrates from fish to mammals.

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.

The Role of GI Peptides in Functional Dyspepsia and Gastroparesis: A Systematic Review

Functional dyspepsia (FD) and gastroparesis (GP) are common disorders of the upper gastrointestinal tract. The pathophysiology of these conditions is likely to be heterogenous, and factors such as altered motility, sensitivity and response to nutrition have been identified as putative underlying mechanisms. Motility, sensitivity as well as responses to nutrition can be influenced or mediated by peptide hormones and serotonin released from the gastrointestinal mucosa. This review summarizes the role of GI peptides in functional dyspepsia and gastroparesis. In most studies, the levels of somatostatin, ghrelin, and motilin did not differ between healthy volunteers and FD or GP patients, but higher symptom burden was often correlated with higher peptide levels. Ghrelin and motilin receptor agonists showed promising results in improvement of the gastric emptying, but the link with improvement of symptoms is less predictable. Serotonin agonists have a potential to improve symptoms in both FD and idiopathic gastroparesis. Drugs acting on the GLP-1 and on the PYY receptors deserve further investigation. There is a need for systematic large scale studies.

Importance of the enteric nervous system in the control of the migrating motility complex

The migrating motility complex (MMC), a cyclical phenomenon, represents rudimentary motility pattern in the gastrointestinal tract. The MMC is observed mostly in the stomach and gut of man and numerous animal species. It contains three or four phases, while its phase III is the most characteristic. The mechanisms controlling the pattern are unclear in part, although the neural control of the MMC seems crucial. The main goal of this article was to discuss the importance of intrinsic innervation of the gastrointestinal tract in MMC initiation, migration, and cessation to emphasize that various MMC-controlling mechanisms act through the enteric nervous system. Two main neural regions, central and peripheral, are able to initiate the MMC. However, central regulation of the MMC may require cooperation with the enteric nervous system. When central mechanisms are not active, the MMC can be initiated peripherally in any region of the small bowel. The enteric nervous system affects the MMC in response to the luminal stimuli which can contribute to the initiation and cessation of the cycle, and it may evoke irregular phasic contractions within the pattern. The hormonal regulators released from the endocrine cells may exert a modulatory effect upon the MMC mostly through the enteric nervous system. Their central action could also be considered. It can be concluded that the enteric nervous system is involved in the great majority of the MMC-controlling mechanisms.

Nitric oxide regulation of migrating motor complex: randomized trial of N(G)-monomethyl-L-arginine effects in relation to muscarinic and serotonergic receptor blockade

AIM

The migrating motor complex (MMC) propels contents through the gastrointestinal tract during fasting. Nitric oxide (NO) is an inhibitory neurotransmitter in the gastrointestinal tract. Little is known about how NO regulates the MMC. In this study, the aim was to examine nitrergic inhibition of the MMC in man using N(G)-monomethyl-L-arginine (L-NMMA) in combination with muscarinic receptor antagonist atropine and 5-HT3 receptor antagonist ondansetron.

METHODS

Twenty-six healthy volunteers underwent antroduodenojejunal manometry for 8 h with saline or NO synthase (NOS) inhibitor L-NMMA randomly injected I.V. at 4 h with or without atropine or ondansetron. Plasma ghrelin, motilin and somatostatin were measured by ELISA. Intestinal muscle strip contractions were investigated for NO-dependent mechanisms using L-NMMA and tetrodotoxin. NOS expression was localized by immunohistochemistry.

RESULTS

L-NMMA elicited premature duodenojejunal phase III in all subjects but one, irrespective of atropine or ondansetron. L-NMMA shortened MMC cycle length, suppressed phase I and shifted motility towards phase II. Pre-treatment with atropine extended phase II, while ondansetron had no effect. L-NMMA did not change circulating ghrelin, motilin or somatostatin. Intestinal contractions were stimulated by L-NMMA, insensitive to tetrodotoxin. NOS immunoreactivity was detected in the myenteric plexus but not in smooth muscle cells.

CONCLUSION

Nitric oxide suppresses phase III of MMC independent of muscarinic and 5-HT3 receptors as shown by nitrergic blockade, and acts through a neurocrine disinhibition step resulting in stimulated phase III of MMC independent of cholinergic or 5-HT3 -ergic mechanisms. Furthermore, phase II of MMC is governed by inhibitory nitrergic and excitatory cholinergic, but not 5-HT3 -ergic mechanisms.

Interdigestive migrating motor complex -its mechanism and clinical importance

Migrating motor complex (MMC) is well characterized by the appearance of gastrointestinal (GI) contractions in the interdigestive state. The physiological importance of gastric MMC is a mechanical and chemical cleansing of the empty stomach in preparation for the next meal. MMC cycle is mediated via the interaction between motilin and 5-hydroxytryptamine (5-HT) by the positive feedback mechanism in conscious dogs. Luminal administration of 5-HT initiates duodenal phase II and phase III with a concomitant increase of plasma motilin release. Duodenal 5-HT concentration is increased during gastric phase II and phase III. Intravenous infusion of motilin increases luminal 5-HT content and induces phase III. 5-HT₄ antagonists significantly inhibit both of gastric and intestinal phase III, while 5-HT₃ antagonists inhibit only gastric phase III. These suggest that gastric MMC is regulated via vagus, 5-HT_3/4 receptors and motilin, while intestinal MMC is regulated via intrinsic primary afferent neurons (IPAN) and 5-HT₄ receptors. We propose the possibility that maximally released motilin by a positive feedback depletes 5-HT granules in the duodenal EC cells, resulting in no more contractions. Stress is highly associated with the pathogenesis of functional dyspepsia (FD). Acoustic stress attenuates gastric phase III without affecting intestinal phase III in conscious dogs, via reduced vagal activity. Subset of FD patients shows reduced vagal activity and impaired gastric phase III. The impaired gastric MMC may aggravate dyspeptic symptoms following a food ingestion. Maintaining MMC cycle in the interdigestive state is an important factor to prevent the postprandial dyspeptic symptoms.

Motilin: towards a new understanding of the gastrointestinal neuropharmacology and therapeutic use of motilin receptor agonists

The gastrointestinal hormone motilin has been known about for >40 years, but after identification of its receptor and subsequent development of new tools and methods, a reappraisal of its actions is required. Firstly, it is important to note that motilin and ghrelin receptors are members of the same family (similar genomic organization, gastrointestinal distribution and abilities to stimulate gastrointestinal motility), yet each fails to recognize the ligand of the other; and whereas ghrelin and ghrelin receptors are widespread outside the gastrointestinal tract, motilin and its receptors are largely restricted to the gastrointestinal tract. Secondly, although some studies suggest motilin has activity in rodents, most do not, and receptor pseudogenes exist in rodents. Thirdly, motilin preferentially operates by facilitating enteric cholinergic activity rather than directly contracting the muscle, despite the relatively high expression of receptor immunoreactivity in muscle. This activity is ligand-dependent, with short-lasting actions of motilin contrasting with longer-lasting actions of the non-selective and selective motilin receptor agonists erythromycin and GSK962040. Finally, the use of erythromycin (also an antibiotic drug) to treat patients requiring acceleration of gastric emptying has led to concerns over safety and potential exacerbation of antibiotic resistance. Replacement motilin receptor agonists derived from erythromycin (motilides) have been unsuccessful. New, non-motilide, small molecule receptor agonists, designed to minimize self-desensitization, are now entering clinical trials for treating patients undergoing enteral feeding or with diabetic gastroparesis. Thus, for the translational pharmacologist, the study of motilin illustrates the need to avoid overreliance on artificial systems, on structural information and on animal studies.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

Mechanism of interdigestive migrating motor complex

Migrating motor complex (MMC) is well characterized by the appearance of gastrointestinal contractions in the interdigestive state. This review article discussed the mechanism of gastrointestinal MMC. Luminal administration of 5-hydroxytryptamine (5-HT) initiates duodenal phase II followed by gastrointestinal phase III with a concomitant increase of plasma motilin release in conscious dogs. Duodenal 5-HT concentration is increased during gastric phase II and phase III. Intravenous infusion of motilin increases luminal 5-HT content and induces gastrointestinal phase III. 5-HT(4) antagonists significantly inhibits both of gastric and intestinal phase III, while 5-HT(3) antagonists inhibited only gastric phase III. These suggest that gastrointestinal MMC cycle is mediated via the interaction between motilin and 5-HT by the positive feedback mechanism. Gastric MMC is regulated via vagus, 5-HT(3/4) receptors and motilin, while intestinal MMC is regulated via intrinsic primary afferent neurons and 5-HT(4) receptors. Stress is highly associated with the pathogenesis of functional dyspepsia. Acoustic stress attenuates gastric phase III without affecting intestinal phase III in conscious dogs, via reduced vagal activity and increased sympathetic activity. It has been shown that subset of functional dyspepsia patients show reduced vagal activity and impaired gastric phase III. The physiological importance of gastric MMC is a mechanical and chemical cleansing of the empty stomach in preparation for the next meal. The impaired gastric MMC may aggravate dyspeptic symptoms following a food ingestion. Thus, maintaining gastric MMC in the interdigestive state is an important factor to prevent the postprandial dyspeptic symptoms.

The migrating motor complex: control mechanisms and its role in health and disease

The migrating motor complex (MMC) is a cyclic, recurring motility pattern that occurs in the stomach and small bowel during fasting; it is interrupted by feeding. The MMC is present in the gastrointestinal tract of many species, including humans. The complex can be subdivided into four phases, of which phase III is the most active, with a burst of contractions originating from the antrum or duodenum and migrating distally. Control of the MMC is complex. Phase III of the MMC with an antral origin can be induced in humans through intravenous administration of motilin, erythromycin or ghrelin, whereas administration of serotonin or somatostatin induces phase III activity with duodenal origin. The role of the vagus nerve in control of the MMC seems to be restricted to the stomach, as vagotomy abolishes the motor activity in the stomach, but leaves the periodic activity in the small bowel intact. The physiological role of the MMC is incompletely understood, but its absence has been associated with gastroparesis, intestinal pseudo-obstruction and small intestinal bacterial overgrowth. Measuring the motility of the gastrointestinal tract can be important for the diagnosis of gastrointestinal disorders. In this Review we summarize current knowledge of the MMC, especially its role in health and disease.

Influence of the 5-HT3 receptor antagonist ondansetron on gastric sensorimotor function and nutrient tolerance in healthy volunteers

BACKGROUND

Serotonin is believed to be involved in the regulation of the gastric accommodation reflex in man however which receptor subtype(s) are involved remains to be elucidated.

METHODS

Eleven healthy subjects (nine men, age 19-30) underwent a gastric barostat and a drinking test after treatment with either placebo or ondansetron (8 mg intravenously). During the barostat protocol an intragastric flaccid bag was stepwise distended (2 mmHg increments 2 min) to determine gastric compliance and sensitivity to distention. Subsequently, the pressure level was set at intra-abdominal pressure +2 mmHg while volume was followed before and after administration of a liquid meal (200 mL; 300 kcal). During the drink test volunteers drank at a rate of 15 mL min(-1) until maximal satiation. Results (mean ± SEM) were compared using t-tests and mixed model analysis.

KEY RESULTS

Gastric compliance was not significantly altered by ondansetron (51.5 ± 5.6 vs 49.2 ± 5.2 mL mmHg(-1)), neither were the pressure thresholds for first perception or discomfort. Ondansetron treatment did not affect basal gastric tone (173 ± 14 vs 156 ± 12 mL), neither did it affect the amplitude of the meal-induced relaxation (160 ± 52 vs 131 ± 43 mL) or the maximum volume increase after the meal (264 ± 54 mL vs 234 ± 51 mL). During the drinking test the amount of liquid meal ingested at maximum satiation was significantly increased by ondansetron (784 ± 74 vs 907 ± 64 mL, P < 0.05).

CONCLUSIONS & INFERENCES

These data suggest that 5-HT acting at 5-HT(3) receptors is not involved in the control of gastric sensorimotor function, but contributes to the regulation of hunger and satiation in man.

The hungry stomach: physiology, disease, and drug development opportunities

During hunger, a series of high-amplitude contractions of the stomach and small intestine (phase III), which form part of a cycle of quiescence and contractions (known as the migrating motor complex, MMC), play a "housekeeping" role prior to the next meal, and may contribute toward the development of hunger. Several gastrointestinal (GI) hormones are associated with phase III MMC activity, but currently the most prominent is motilin, thought to at least partly mediate phase III contractions of the gastric MMC. Additional GI endocrine and neuronal systems play even more powerful roles in the development of hunger. In particular, the ghrelin-precursor gene is proving to have a complex physiology, giving rise to three different products: ghrelin itself, which is formed from a post-translational modification of des-acyl-ghrelin, and obestatin. The receptors acted on by des-acyl-ghrelin and by obestatin are currently unknown but both these peptides seem able to exert actions which oppose that of ghrelin, either indirectly or directly. An increased understanding of the actions of these peptides is helping to unravel a number of different eating disorders and providing opportunities for the discovery of new drugs to regulate dysfunctional gastric behaviors and appetite. To date, ghrelin and motilin receptor agonists and antagonists have been described. The most advanced are compounds which activate the ghrelin and motilin receptors which are being progressed for disorders associated with gastric hypomotility.

The influence of citalopram on interdigestive gastrointestinal motility in man

BACKGROUND

Administration of 5-hydroxytryptamine (5HT) and selective 5HT receptor ligands modifies interdigestive motility in animals and in man.

AIM

To study the effect of citalopram, a selective 5-HT reuptake inhibitor, on interdigestive motility in man.

METHODS

In 20 healthy subjects, antroduodenojejunal motor activity was studied manometrically. Basal interdigestive motor activity was recorded until the passage of two activity fronts. Ten minutes after the second activity front, placebo or 20 mg of citalopram was administered intravenously in a double-blind randomized fashion. Recording continued until the passage of two more activity fronts had occurred.

RESULTS

Administration of citalopram induced a premature small intestinal phase 3 after 35 +/- 6.4 min, compared to 120 +/- 17 min after placebo P < 0.01. Citalopram shortened MMC cycle length at the expense of phase 1 and phase 2 and significantly increased the motility index during phase 2 in the antrum and the small intestine.

CONCLUSIONS

In the interdigestive state in man, intravenous administration of the selective 5-HT reuptake inhibitor citalopram induces a premature intestinal phase 3 and suppresses gastric activity fronts. Phase 2 motility is stimulated both in the stomach and in the small bowel after citalopram. These data suggest that 5HT is involved in the control of interdigestive motility.

Mechanism of interdigestive migrating motor complex in conscious dogs

BACKGROUND

The migrating motor complex (MMC) is well characterized by the appearance of gastrointestinal contractions in the interdigestive state. This study was designed to clarify the mechanisms of gastric MMC (G-MMC) and intestinal MMC (I-MMC) in conscious dogs.

METHODS

Five strain gauge transducers were implanted on the stomach and intestine. To investigate the correlation between luminal 5-HT and phase III contractions, gastric and duodenal juices were collected during the MMC cycle. The 5-HT concentrations in gastric and duodenal juice were measured by HPLC. To investigate whether luminal 5-HT initiates MMC, 5-HT (10(-8)-10(-6) M, 10 ml) was administered into the duodenum 20 min after gastric phase III. To investigate the involvement of 5-HT(3) or 5-HT(4) receptors in mediating G-MMC and I-MMC, 5-HT(3) antagonists (ondansetron) or 5-HT(4) antagonists (GR 125,487) were infused for 120 min.

RESULTS

Luminal administration of 5-HT (10(-6) M) initiated duodenal phase II followed by G-MMC and I-MMC with a concomitant increased release of plasma motilin. The duodenal 5-HT concentration was significantly increased during phase II (59 +/- 9 ng/ml) and phase III (251 +/- 21 ng/ml) compared to that of phase I (29 +/- 5 ng/ml). On the other hand, the 5-HT content in the stomach was not significantly changed throughout the MMC cycle. Intravenous infusion of motilin (0.3 microg/kg/h) increased the luminal 5-HT content and induced G-MMC and I-MMC. 5-HT(4) antagonists significantly inhibited both G-MMC and I-MMC, while 5-HT(3) antagonists inhibited only G-MMC.

CONCLUSION

It is suggested that the MMC cycle is mediated by a positive feedback mechanism via the interaction between motilin and 5-HT.

Prolonged duodenal acid perfusion and dyspeptic symptom occurrence in healthy volunteers

The pathophysiology of functional dyspepsia (FD) is unknown and several mechanisms associated with specific symptom patterns have been recently proposed. Increased duodenal acid exposure has been supposed to be associated with nausea, but recently an increase of severity of several dyspeptic symptoms was noted in a subset of dyspeptic patients. As its pathogenetic role is still unclear, we evaluated an involvement of duodenal acid exposure in symptom generation by inducing a hyperacidity status of the duodenum. Twelve young adult healthy volunteers in a randomized, double-blind protocol, underwent duodenal acid (0.2 N, 5 mL min(-1)) or saline perfusion, antropyloroduodenal manometry and duodenal pH monitoring both during fasting and postprandially. Every 15 min, severity of discomfort, fullness, bloating, belching, nausea, heartburn, epigastric burning, satiety and pain were evaluated by visual analogue scale. During acid perfusion, symptom scores for discomfort, bloating, nausea, epigastric burning were significantly higher (P < 0.01) compared to saline. Postprandial antral motility index was lower (2.96 +/- 1.8 vs 3.62 +/- 1.8, P = 0.01) and jejunal motility index higher (4.87 +/- 1.0 vs 4.37 +/- 1.4, P = 0.01) during acid perfusion. Occurrence and duration of phases III of the migrating motor complex showed no difference. Duodenal acid perfusion causes a sensitization to dyspeptic symptoms and induces antral hypomotility and jejunal hypercontractility. Through these mechanisms, increased duodenal acid exposure may play a role in the pathophysiology of FD symptoms.

Effects of ondansetron on gastric tone and motility changes induced by a prolonged intraduodenal infusion of nutrients. Results of a placebo-controlled study

Our aim was to study the 5-HT3 antagonist, ondansetron, on gastric motility changes induced by duodenal infusion of nutrients. First, the effects of a 2-hr intraduodenal infusion (IDI) of a caloric diet on antral motility were assessed. Second, a crossed-over placebo-controlled study assessed 3-day oral intake of ondansetron (8 mg bid) on gastric motility changes induced by the IDI. During the IDI, antral numbers of waves (NW) as area under curve (AUC) were lower than fasting values. After infusion, antral NW and AUC increased to return to basal values. The antral area increased slightly shortly after the start of the IDI, then remained stable. When subjects received ondansetron, the only significant motor effect was a higher antral NW and AUC during the first 30 min of the IDI (P < 0.05). After the end of the IDI, the AUC and NW increased in both the distal and the proximal antrum. The increase was lower by ondansetron in the proximal antrum. Proximal stomach relaxation induced was not influenced by ondansetron. In conclusion, an IDI of nutrients decreased antral motility, increased the antral area, and promoted fundic relaxation. This inhibitory effect was rapidly reversible. Ondansetron induced only minor motility changes in the antrum and had no effect on fundic relaxation promoted by IDI.

Pharmacological characterization of 5-Hydroxytryptamine-receptor subtypes in circular muscle from the rat stomach

5-Hydroxytryptamine (5-HT) modulates gastric motility and gastric emptying via a variety of 5-HT receptor subtypes. However, regional and functional differences among 5-HT receptor subtypes in the rat stomach are not fully investigated. Thus, we aimed to characterize 5-HT receptor subtypes involved in the 5-HT-induced contractions in the isolated antral, corporal and fundic circular muscles of the rat stomach by measuring the contractile force. 5-HT induced concentration-dependent contractions in the antrum, corpus and fundus. 5-HT-induced antral contractions were partly blocked by atropine and enhanced by tetrodotoxin (TTX). Neither atropine nor TTX affected the corporal or the fundic contractions to 5-HT. In the antrum, 5-HT-induced contractions were inhibited by methysergide, tended to be inhibited by ketanserin, enhanced by SB-203186, but were not affected by WAY-100635, GR127935, RS-127445, ondansetron, or SB-269970. In the corpus, 5-HT-induced contractions were inhibited by ketanserin or methysergide. In the fundus, 5-HT-induced contractions were blocked by methysergide or RS-127445, but were enhanced by cinanserin or SB-203186. It is thus concluded that contractile responses to 5-HT in the antrum are mediated by 5-HT receptors on both smooth muscle and neurons whilst in the corpus and fundus responses are mainly mediated by 5-HT receptors on smooth muscle. Moreover, the antrum presents the contractile 5-HT2A and 5-HT2B receptors and the relaxant 5-HT4 receptors. The corpus presents the contractile 5-HT2A receptors, and the fundus presents the contractile 5-HT2B receptors and the relaxant 5-HT2A and 5-HT4 receptors.

Effects of ghrelin on feeding regulation and interdigestive migrating complex in rats

OBJECTIVE

Ghrelin is an orexigenic peptide with remarkable prokinetic effects. However, its mechanisms in regulating feeding and gastrointestinal migrating myoelectrical complex (MMC) are not fully understood. The aim of this study was to examine the effects of ghrelin on feeding regulation and duodenal motility in rats.

MATERIAL AND METHODS

Feeding regulation was investigated at different times after intravenous injection of ghrelin and its receptor antagonist (D-Lys3)GHRP-6 in fasted rats. Rats were supplied with a pair of silver bipolar electrodes embedded in the duodenal serosa for electromyography. Ghrelin was injected intravenously into the rats during the fed state or fasted state. Some rats were pretreated with atropine, phentolamine, propranolol, L-arginine, the 5-hydroxytryptamine3 receptor antagonist ondansetron, and (D-Lys3)GHRP-6.

RESULTS

Ghrelin had little effect on food intake in the first 30 min after administration, but was found to increase feeding during the subsequent hours. (D-Lys3)GHRP-6 decreased feeding and antagonized the effect of ghrelin. Ghrelin induced duodenal MMC after administration in the fed state, and shortened the duodenal MMC cycle length and the duration of phase III during fasting. The amplitude and frequency of phase III were increased without affecting the percentage of phase III in the MMC cycle. Pretreatment with atropine, L-arginine, ondansetron, and (D-Lys3)GHRP-6 inhibited the effects of ghrelin. Propranolol and phentolamine had little influence on these effects.

CONCLUSIONS

Ghrelin appears to be closely related to feeding and intestinal motility. The excitatory effects of ghrelin are dependent on the cholinergic pathway, and it has a close relationship with the NOS-NO or 5-HT pathway. The ghrelin receptor is involved in its activities.

The serotonin signaling system: from basic understanding to drug development for functional GI disorders

Serotonin is an important gastrointestinal signaling molecule. It is a paracrine messenger utilized by enterochromaffin (EC) cells, which function as sensory transducers. Serotonin activates intrinsic and extrinsic primary afferent neurons to, respectively, initiate peristaltic and secretory reflexes and to transmit information to the central nervous system. Serotonin is also a neurotransmitter utilized by a system of long descending myenteric interneurons. Serotonin is synthesized through the actions of 2 different tryptophan hydroxylases, TpH1 and TpH2, which are found, respectively, in EC cells and neurons. Serotonin is inactivated by the serotonin reuptake transporter (SERT)-mediated uptake into enterocytes or neurons. The presence of many serotonin receptor subtypes enables selective drugs to be designed to therapeutically modulate gastrointestinal motility, secretion, and sensation. Current examples include tegaserod, a 5-HT(4) partial agonist, which has been approved for treatment of irritable bowel syndrome (IBS) with constipation in women and for chronic constipation in men and women. The 5-HT(3) antagonists, granisetron and ondansetron, are useful in combating the nausea associated with cancer chemotherapy, and alosetron is employed in the treatment of IBS with diarrhea. Serotonergic signaling abnormalities have also been putatively implicated in the pathogenesis of functional bowel diseases. Other compounds, for which efficacy has not been rigorously established, but which may have value, include tricyclic antidepressants and serotonin selective reuptake inhibitors to combat IBS, and 5-HT(1) agonists, which enhance gastric accommodation, to treat functional dyspepsia. The initial success encountered with serotonergic agents holds promise for newer and more potent insights and therapies of brain-gut disorders.

Synthesis and SAR of 1,3-disubstituted cyclohexylmethyl urea and amide derivatives as non-peptidic motilin receptor antagonists

A series of 1,3-disubstituted cyclohexylmethyl urea and amide derivatives were synthesized as motilin receptor antagonists. Starting from known motilin antagonists, 1a and 1b, the cyclopentene scaffold was replaced and the four recognition elements optimized to arrive at a potent novel series.

Triptans and gastric accommodation: pharmacological and therapeutic aspects

In the past decade, several studies have reported a significant delay of gastric emptying induced by the anti-migraine agent sumatriptan (a 5-hydroxytryptamine (5-HT)1B/D receptor agonist) in healthy human beings. In patients with functional dyspepsia, sumatriptan improves gastric accommodation after food consumption and reduce perception of gastric distension, hence relieving epigastric symptoms. Recent studies have established that impaired accommodation after food consumption is a major patho-physiological mechanism in functional dyspepsia and restoration of accommodation is considered to be a potential therapeutic target. The precise site of action of sumatriptan in humans is at present unknown, although recent studies carried out using a canine model indicate that sumatriptan exerts its action on gastric accommodation through 5-HT1B receptors, since both GR127935 and SB216641 (respectively, non selective 5-HT1B/D and selective 5-HT1B receptor antagonists) fully antagonised the effects of sumatriptan. Gastric relaxation and enhanced accommodation to a distending stimulus seem to be a class effect of triptans, since it occurs not only with sumatriptan, but also with second-generation triptans (rizatriptan and naratriptan), at least in a canine model. In dyspeptic patients, administration of triptans would be able to restore gastric accommodation after a meal and to improve symptoms of early satiety, confirming the therapeutic potential of 5-HT1B/D receptor agonists in functional dyspepsia.

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.

Effect of the 5-HT1 agonist sumatriptan on oesophageal motor pattern in patients with ineffective oesophageal motility

The 5-HT1 agonist sumatriptan (SUM) elicits an increase in amplitude of oesophageal motor waves and of lower oesophageal sphincter (LOS) tone in healthy subjects. The aim of the study was to evaluate whether such an effect occurs also in patients with ineffective oesophageal motility (IOM). 16 patients (nine males and seven females, age range 34-55 years) with chest pain and mild to moderate dysphagia were studied; all had undergone previous cardiologic, radiologic and upper gastrointestinal endoscopic exams that were normal. An oesophageal manometry was performed using an electronic probe to record swallows, oesophageal, LOS and gastric motility. The patients whose motor pattern were compatible with IOM (>30% of motor waves with amplitude <30 mmHg and/or non-transmitted) received SUM or placebo 6 mg s.c., injected in the morning and in the afternoon in a random order. The data analysis was limited to 1 h before and 1 h after the drug injections. Ten out of the 16 patients showed an IOM motor pattern. The administration of SUM caused a significant increase in the number of swallows (SUM 99.5 +/- 15.4 vs 78.6 +/- 16.1 basal, P = 0.03) and of primary oesophageal motor waves (SUM 89.6 +/- 13.4 vs 67.2 +/- 12.9 basal, P = 0.04) with no significant changes in the percentage of swallows associated with propagation. Placebo was not associated with increase in the number of swallows (80.3 +/- 14.6, P = 0.9) or of primary oesophageal motor waves (70.1 +/- 12.3, P = 0.7). The amplitude and the percentage of propagated oesophageal motor waves as well as the mean basal LOS tone were unaltered by SUM. There was no change in the symptoms reported after SUM. Although effective in healthy subjects, SUM 6 mg s.c. improves only the numbers but not the amplitude or propagation of oesophageal motility of patients with IOM. The 5-HT1 pathway and its acute stimulation seem to play only a minor role in the pathogenesis of such a disease.

Binding of radiolabeled porcine motilin and erythromycin lactobionate to smooth muscle membranes in various segments of the equine gastrointestinal tract

OBJECTIVE

To identify and characterize motilin receptors in equine duodenum, jejunum, cecum, and large colon and to determine whether erythromycin lactobionate competes with porcine motilin for binding to these receptors.

SAMPLE POPULATION

Specimens of various segments of the intestinal tracts of 4 adult horses euthanatized for reasons unrelated to gastrointestinal tract disease.

PROCEDURE

Cellular membranes were prepared from smooth muscle tissues of the duodenum, jejunum, pelvic flexure, and cecum. Affinity and distribution of motilin binding on membrane preparations were determined by use of 125I-labeled synthetic porcine motilin. Displacement studies were used to investigate competition between 125I-labeled synthetic porcine motilin and erythromycin lactobionate for binding to motilin receptors in various segments of bowel.

RESULTS

Affinity of 125I-labeled synthetic porcine motilin for the equine motilin receptor was estimated to be 6.1nM. A significantly higher number of motilin receptors was found in the duodenum than in the pelvic flexure and cecum. The jejunum had a significantly higher number of motilin receptors than the cecum. Erythromycin lactobionate displacement of 125I-labeled porcine motilin from the equine motilin receptor did not differ significantly among various segments of bowel.

CONCLUSIONS AND CLINICAL RELEVANCE

Motilin receptors were found in the duodenum, jejunum, pelvic flexure, and cecum of horses. The highest number of motilin receptors was in the duodenum, and it decreased in more distal segments of bowel. Erythromycin lactobionate competed with motilin binding in the equine gastrointestinal tract. This suggests that 1 of the prokinetic actions of erythromycin in horses is likely to be secondary to binding on motilin receptors.

Motilin regulates interdigestive gastric blood flow in dogs

BACKGROUND & AIMS

Gastric blood flow exhibits cyclical increases in phase with the interdigestive contractions and secretion of the stomach in dogs. The aim of this study is to clarify the regulatory role of motilin in interdigestive gastric blood flow in dogs.

METHODS

Blood flow of the left gastric (LGA) and superior mesenteric (SMA) arteries were measured by ultrasound transit-time blood-flow meters in 5 conscious dogs. Motilin was infused intravenously with or without Phe-cyclo[Lys-Tyr(3-tBu)-betaAla-]. trifluoroacetate (GM-109; motilin antagonist), granisetron (5-HT3 antagonist), atropine, hexamethonium (C6), phenoxybenzamine, propranolol, or cimetidine.

RESULTS

Motilin (12.5, 25, 50, and 100 pmol x kg(-1) x h(-1)) induced LGA blood-flow responses, consisting of a sustained increase and a rapid phasic change coupled with a contraction, without affecting the blood pressure, heart rate, and SMA blood flow. GM-109 completely abolished the LGA, motility, and secretory responses to motilin (100 pmol x kg(-1) x h(-1)). Atropine abolished motilin-induced gastric contractions, secretion, and phasic changes of LGA blood flow but failed to affect the sustained flow increase. However, atropine partially inhibited the LGA responses to lower doses of motilin. The LGA flow responses to motilin were not inhibited by granisetron, C6, alpha-adrenergic, beta-adrenergic, or H2 blockers. Motilin induced significantly larger gastric vasodilatation than the equivalent doses of VIP.

CONCLUSIONS

Motilin has a potent and selective gastric vasodilator effect, which appears to be mediated by both cholinergic and noncholinergic mechanisms. Motilin plays an important role in the regulation of interdigestive gastric blood flow in dogs.

Serotonergic modulating drugs for functional gastrointestinal diseases

After many years of basic research we have now begun to learn how to manipulate the serotonergic mechanisms within the gut. This has lead to a number of significant advances including 5HT3 antagonists for the treatment of functional diarrhoea, 5HT4 agonists for the treatment of constipation and 5HT1 agonists for the treatment of impaired fundal relaxation. Initial enthusiasm has been somewhat dented by the withdrawal of alosetron because of ischaemic colitis, but it remains to be seen whether this adverse event will be seen with other 5HT3 antagonists. Finally it should be recognized that, in a substantial proportion of patients attending clinics complaining of functional symptoms, anxiety is a major component. The drugs so far described are by and large devoid of CNS effects. It remains possible therefore that a drug which combines both peripheral and central effects would likely to be beneficial.

Effects of motilin on human interdigestive gastrointestinal and gallbladder motility, and involvement of 5HT3 receptors

A plasma motilin peak and a partial gallbladder emptying precede the antral phase III of the migrating motor complex (MMC). To clarify the causal relationship between these factors, we aimed to study the role of motilin in interdigestive gastrointestinal and gallbladder motility simultaneously. In addition, involvement of 5HT3 receptors in the action of motilin was studied. Eight fasting, healthy male volunteers received 13Leu-motilin or 0.9% NaCl i.v. for 30 min, in randomized order on two separate occasions, from 30 min after phase III. Seven of the eight subjects also received the 5HT3 receptor antagonist ondansetron in addition to motilin, on a third occasion. Antroduodenal motility, gallbladder volumes and plasma motilin were measured. The interval between the start of infusion and phase III was 95.0 (57.6-155.7) min for saline, 28.7 (21.0-33.2) min for motilin, and 39.3 (30.7-100.5) min for motilin + ondansetron (P < 0.05). Gallbladder volume decreased by one-third from 10 min after both motilin and motilin + ondansetron infusion (P < 0.05), and returned to baseline with duodenal passage of phase III. In two of the seven subjects phase III was absent after motilin + ondansetron, although gallbladder volume decreased and only refilled during a later spontaneous phase III. We conclude that motilin induces both partial gallbladder emptying and antral phase III. Indeed, although gallbladder emptying clearly precedes antral phase III, ondansetron only prevented phase III in some cases and had no effect on gallbladder emptying. Passage of phase III in the duodenum makes an important contribution to gallbladder refilling.

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.

Mechanisms in regulating the release of serotonin from the perfused rat stomach

The mechanisms regulating the release of serotonin into the portal circulation as well as into the gastric lumen were studied in the isolated vascularly and luminally perfused rat stomach. Immunohistochemical study of the rat stomach showed that serotonin-containing enterochromaffin (EC) cells were densely packed in the antral mucosa, sparsely scattered in the corpus, and not found in the fundus. Such morphological findings suggest that serotonin detected in this study may have originated from antral EC cells. Luminal acidification stimulated the vascular release of serotonin but did not affect the luminal release of serotonin. The basal release of serotonin into the vasculature was 10 times higher than that into the gastric lumen at intragastric pH 2. The vascular release of serotonin is regulated by stimulation from cholinergic nicotinic mechanisms, whereas inhibitory neurotransmitters such as vasoactive intestinal peptide and NO are probably not involved. Somatostatin and peptide YY originating from endocrine cells may exert direct inhibitory effects, possibly via somatostatin and peptide YY receptors on the EC cells, and a cholinergic muscarinic mechanism may exert indirect effects on the vascular release of serotonin via the muscarinic receptor on the endocrine cells.

The role of 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in the regulation of gut motility in the ferret

The role of 5-hydroxytryptamine (5-HT)3 and 5-HT4 receptors in the regulation of gut motility in the ferret was investigated. The selective 5-HT3 receptor antagonist ramosetron (1 - 10 microg/kg s.c.) prolonged the interval of gastric antral migrating motor complex, but had only slight effect on small intestinal and colonic motility in unfed animals. The selective 5-HT4 receptor antagonist SB 204070 did not affect motility throughout gut in unfed animals. Neither ramosetron nor SB 204070 affected the motility throughout gut in fed animals. In conclusion, neither 5-HT3 nor 5-HT4 receptors tonically regulate ferret gut motility except that 5-HT3 receptors have a key role in the occurrence of migrating motor complex specifically in the stomach. The role of 5-HT3 and 5-HT4 receptor system in the regulation of gut motility in ferrets is similar to that in other mammalian species studied, including humans. This similarity suggests that the ferret is a suitable model animal to study gut motor functions in humans.

Serotonin stimulates migrating myoelectric complex via 5-HT3-receptors dependent on cholinergic pathways in rat small intestine

We have investigated the effect of 5-hydroxytryptamine (5-HT) and different 5-HT-receptor antagonists and atropine on the migrating myoelectric complex in the rat small intestine. Infusion of 5-HT dose-dependently shortened the interval between phase III of the migrating myoelectric complex (MMC). In untreated animals the interval in upper jejunum was 19.1 (16.0-22.1) min. At doses of 10 and 20 nmol kg-1 min-1, the interval decreased to 15.2 (12.0-18.4) and 10.2 (9.4-11.0) min, respectively. The 5-HT3-receptor antagonist ondansetron (0.5 mg kg-1) alone increased the MMC interval from 20.8 (15.1-26.5) to 33.9 (19.4-48.4) min. Neither methiothepin (0.5 mg kg-1) nor ketanserin (0.5 mg kg-1), selective for 5-HT1/5-HT2- and 5-HT2-receptors, respectively, changed the MMC interval. The 5-HT4-receptor antagonist GR 113808 (0.5 mg kg-1) disrupted the MMC and induced irregular spiking activity. Ondansetron and atropine antagonized the 5-HT-induced shortening of the MMC interval. Neither methiothepin nor ketanserin affected the response to 5-HT. GR 113808 did not block the response to 5-HT in half of the animals; however, in the remaining ones MMC was disrupted and irregular spiking induced. In conclusion, these results show that 5-HT dose-dependently stimulates the cycling of the MMC in the small intestine via 5-HT3-receptors and a cholinergic final pathway. Our findings encourage further studies on the role of the 5-HT3-receptor in the control of gastrointestinal motility.

Effect of a selective 5-HT3 receptor agonist on gastric motility in fasted and fed dogs

The effect of m-chlorophenylbiguanide, a selective 5-HT3 receptor agonist, on gastric antral motility was investigated in conscious dogs with a force transducer implanted chronically. m-Chlorophenylbiguanide (0.1-1 mg/kg i.v.) dose dependently enhanced antral motility in the fasted state, and the amplitude of m-chlorophenylbiguanide (1 mg/kg i.v.)-induced antral contractions reached the level of natural phase III contractions. In contrast, m-chlorophenylbiguanide reduced the amplitude of antral contractions in the fed state. A selective 5-HT3 receptor antagonist, ramosetron (0.0003-0.03 mg/kg i.v.), inhibited both effects of m-chlorophenylbiguanide. m-Chlorophenylbiguanide (1 mg/kg i.v.)-induced contractions were inhibited by atropine (0.03 or 0.1 mg/kg i.v.). These results indicate that pharmacological activation of 5-HT3 receptors has opposite effects on canine gastric antral motility in the fasted and in the fed state, being stimulatory and inhibitory, respectively. The stimulatory effect seems to be mediated mainly via the release of acetylcholine.

Evidence for the involvement of 5-HT4 receptors in the 5-hydroxytryptamine-induced pattern of migrating myoelectric complex in sheep

1. The effects induced by 5-hydroxytryptamine (5-HT) on gastrointestinal myoelectric activity in conscious sheep were recorded through electrodes chronically implanted and analysed by computer. The 5-HT receptors and the cholinergic neuronal pathways involved in these actions were investigated. 2. The intravenous (i.v.) administration of 5-HT (2, 4 and 8 micrograms kg-1 min-1, 5 min) induced an antral inhibition concomitant with a duodenal activity front that migrated to the jejunum, followed by a period of intestinal inactivity. This myoelectric pattern closely resembled that observed in the phases III and I of the migrating myoelectric complex (MMC) in sheep. The 0.5 microgram kg-1 min-1 dose evoked the same pattern in only two out of the six animals used. Likewise, the 1 microgram kg-1 min-1 dose similarly affected four of the six animals. In addition, a transient stimulation was observed in the antrum and jejunum when the two highest doses were used. 3. The 5-HT1 antagonist, methiothepin (0.1 mg kg-1), the 5-HT2 antagonists, ritanserin (0.1 mg kg-1) and ketanserin (0.3 mg kg-1), the 5-HT3 antagonists, granisetron (0.2 mg kg-1) and ondansetron (0.5 mg kg-1), as well as the 5-HT4 antagonist, GR113808 (0.2 mg kg-1), did not modify the spontaneous gastrointestinal myoelectric activity. However, the cholinoceptor antagonists, atropine (0.2 mg kg-1) and hexamethonium (2 mg kg-1), inhibited gastrointestinal activity. 4. When these antagonists were injected i.v. 10 min before 5-HT (2 or 4 micrograms kg-1 min-1, 5 min), only GR113808, atropine and hexamethonium were able to modify the 5-HT-induced actions, all of them being completely blocked by the three antagonists. 5. Our data show that 5-HT initiates a MMC-like pattern in the gastrointestinal area in sheep through 5-HT4 receptors. Furthermore, these actions are mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, our results do not indicate a role for either 5-HT1, 5-HT2 or 5-HT3 receptors in the 5-HT-induced effects.

Motilin and clinical application

Motilin is a regulatory polypeptide of 22 amino acid residues and orginates in motilin cells scattered in the duodenal epithelium of most mammals and chickens. Motilin is released into the general circulation at about 100-min intervals during the interdigestive state and is the most important factor in controlling the interdigestive migrating contractions. Recent studies have revealed that motilin stimulates endogenous release of the endocrine pancreas. Clinical application of motilin as a prokinetic has become possible since erythromycin and its derivatives were proved to be nonpeptide motilin agonists.

Effects of 5-hydroxytryptamine agonists on myoelectric activity of the forestomach and antroduodenal area in sheep

To increase knowledge of the role of 5-hydroxytryptamine (5-HT) receptors in the regulation of reticuloruminal, omasal and antroduodenal myoelectric activity in sheep, the effects of 5-HT agonists on forestomach and antroduodenal myoelectric activity have been investigated in conscious sheep. 5-Carboxamidotryptamine, methysergide, alpha-methyl-5-HT, 2-methyl-5-HT, cisapride, zacopride or metoclopramide were infused intravenously for 5 min and myoelectric recordings were obtained from electrodes chronically implanted in the reticulum, rumen (dorsal sac), omasal body, abomasal antrum and duodenal bulb. The integrated activity of the reticular and ruminal spike bursts was modified only by the highest doses of alpha-methyl-5-HT, 2-methyl-5-HT, metoclopramide and cisapride. A phase III-like activity pattern was recorded in the antroduodenal area with all 5-HT-ergic agents and a dose-dependent inhibition of myoelectric activity was recorded in both reticulorumen and omasum at the same time as the antroduodenal effects. In the forestomach, methysergide alone induced inhibition of ruminal secondary contractions; 5-HT, alpha-methyl-5-HT, cisapride and metoclopramide, moreover, evoked an initial dose-dependent increase in antral activity. These results suggest that 5-HT1, 5-HT2, 5-HT3 and 5-HT4 receptors are involved in the regulation of the migrating myoelectric complex in sheep and in the genesis of forestomach hypomotility that is occasionally recorded concomitantly with the spontaneous duodenal phase III in sheep. 5-HT4 receptors also have a prokinetic action in the antral area.

Compounds possessing 5-HT3 receptor antagonistic activity inhibit intestinal propulsion in mice

The role of 5-HT3 receptors in the control of intestinal propulsive activity was investigated in mice by a simple method in which the time taken for excretion of the head of an orally administered non-absorbable marker (whole gut transit time) was measured. Selective 5-HT3 receptor antagonists ramosetron (YM060) at 0.01-0.3 mg/kg s. c. and ondansetron at 0.1-1 mg/kg s.c. dose-dependently prolonged the whole gut transit time. Prokinetic benzamides, such as renzapride (0.3-10 mg/kg s.c.), zacopride (0.01-0.3 mg/kg s.c.) and cisapride (0.1-3 mg/kg s.c.), which have been reported to possess 5-HT3 receptor blocking properties, also dose-dependently prolonged it. These results indicate that activation of 5-HT3 receptors seems to be one factor that underlies the physiological control of intestinal propulsive activity in mice. In contrast to their beneficial therapeutic effects on gastroduodenal dysmotility, prokinetic benzamides, at least those which have 5-HT3 receptor antagonistic activity, may be unsuitable in the treatment of impaired lower intestinal propulsive activity.

Involvement of somatostatin, bombesin and serotonin in the origin of the migrating myoelectric complex in sheep

Antroduodenal myoelectric activity was recorded in conscious sheep by electrodes chronically implanted in the muscular wall. Furthermore, plasma immunoreactive (i.r.) motilin, somatostatin and bombesin concentrations were determined by RIA. The intravenous infusion of somatostatin (20 ng/kg/min), bombesin (10 ng/kg/min) or serotonin (5-HT, 4 micrograms/kg/min) for 5 min, induced a duodenal myoelectric activity front followed by a period of quiescence. These duodenal events were concomitant with an antral inhibition. This pattern resembled that observed in a spontaneous migrating myoelectric complex (MMC) in sheep. Bombesin and 5-HT evoked an additional and transient increase in antral activity simultaneously with the duodenal activity front. On the other hand, plasma i.r. motilin levels did not show any fluctuations during spontaneous MMC cycles or after somatostatin, bombesin or 5-HT infusions. Likewise, plasma i.r. bombesin levels remained unchanged during spontaneous MMC or after administration of somatostatin. However, 5-HT -induced duodenal activity fronts were closely associated with a sharp peak in plasma i.r. bombesin. Finally, plasma i.r. somatostatin concentrations rose at the end of spontaneous phase III and peaked in phase I. A similar pattern of somatostatin release in plasma was found while the duodenal activity front and quiescence period developed after either 5-HT or bombesin infusions. These results do not indicate a role for motilin in the control of MMCs in sheep, although a definitive conclusion cannot be drawn until synthetic sheep motilin is available. However, our data suggest that somatostatin and bombesin-like peptides as well as 5-HT, acting in a coordinated manner, could be involved in the regulation of cyclical antroduodenal motor events in sheep.

5-HT3 receptor involvement in descending reflex relaxation in the rabbit isolated distal colon

In whole segments of rabbit distal colon with mucosa removed, descending reflex relaxations of the circular muscle (descending inhibition) elicited by inflating (0.1-1 ml) an intraluminal balloon, were partially antagonized by 100 microM hexamethonium and the 5-HT3 receptor antagonist, ondansetron (3 microM), and abolished by 1 microM tetrodotoxin. The inhibitory effects of hexamethonium and ondansetron were additive. Conversely, hexamethonium (100 microM) and ondansetron (3 microM) failed to reduce electrically induced non-adrenergic non-cholinergic (NANC) relaxations of colonic circular muscle. It is concluded that interneuronally released acetylcholine and 5-hydroxytryptamine (5-HT) activate descending inhibitory pathways supplying the circular coat, via nicotinic and 5-HT3 receptors, respectively. This evidence suggests a functional involvement of 5-hydroxytryptaminergic transmission in the descending inhibition of rabbit colon.

Stimulatory mechanism of EM523-induced contractions in postprandial stomach of conscious dogs

BACKGROUND & AIMS

EM523, a motilin agonist, is intended to be used as a gastroprokinetic during the postprandial period, but the mechanism(s) by which EM523 stimulates postprandial contractions in the stomach has not been studied before. The aim of this study was to examine the mechanism of contraction-stimulating activity by EM523 in fed dogs.

METHODS

Contractile activity in the gastric antrum of 5 dogs was monitored using a long-term implanted force transducer and measured by integrating the area under the curve. Test materials were continuously infused or injected intravenously.

RESULTS

EM523 (1-30 micrograms/kg) induced a dose-dependent increase in fed-type contractions. EM523-induced contractile activity was partially inhibited by atropine, hexamethonium, dopamine, 5-hydroxytryptamine 3 (5-HT3) receptor antagonist, and substance P antagonist. Atropine-resistant and EM523-induced contractions were further inhibited by 5-HT3 receptor antagonist and substance P antagonist, and the combined use of the two antagonists completely eliminated the atropine-resistant and EM523-induced contractions.

CONCLUSIONS

EM523-induced contractions in the fed stomach are quite different from phase III contractions in the fasted state and are mediated partially through the cholinergic pathway. The noncholinergic pathway involves 5-HT3 and neurokinin 1 receptors.

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.