Do 5-HT4 receptors mediate the intestinal secretory response to 5-HT in rat in-vivo?

Pharmacological analysis of components of the change in transmural potential difference evoked by distension of rat proximal small intestine in vivo

The reflex response to distension of the small intestine in vivo is complex and not well understood. The aim of this study was to characterize the neural mechanisms contributing to the complex time course of the intestinal secretory response to distension. Transmucosal potential difference (PD) was used as a marker for mucosal chloride secretion, which reflects the activity of the secretomotor neurons. Graded distensions (5, 10, and 20 mmHg) of distal rat duodenum with saline for 5 min induced a biphasic PD response with an initial peak (rapid response) followed by a plateau (sustained response). The rapid response was significantly reduced by the neural blockers tetrodotoxin and lidocaine (given serosally) and by intravenous (iv) administration of the ganglionic blocker hexamethonium and the NK(1) receptor antagonist SR-140333. Serosal TTX and iv SR-140333 significantly reduced the sustained response, which was also reduced by the NK(3) receptor antagonist talnetant and by the vasoactive intestinal polypeptide (VPAC) receptor antagonist [4Cl-d-Phe(6), Leu(17)]-VIP. Serosal lidocaine and iv hexamethonium had no significant effect on this component. Inhibition of nitric oxide synthase had no effect on any of the components of the PD response to distension. The PD response to distension thus seems to consist of two components, a rapidly activating and adapting component operating via nicotinic transmission and NK(1) receptors, and a slow component operating via VIP-ergic transmission and involving both NK(1) and NK(3) receptors.

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

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

Plasma levels of 5-HT and 5-HIAA increased after intestinal ischemia/reperfusion in rats

Intestinal ischemia/reperfusion (I/R) causes serious systemic injury, mainly from a variety of bioactive substances released from the injured intestine. To assess the possible roles of serotonin (5-hydroxytryptamine, 5-HT), a bioactive amine mainly stored in the intestine, in I/R injury, we assayed the levels of tryptophan, 5-HT, and 5-hydroxyindole acetic acid (5-HIAA) in the blood and intestine in a rat I/R model. Plasma 5-HT increased significantly over time after reperfusion; the plateau level was obtained 4 h after reperfusion and was associated with an increase in 5-HIAA. Plasma tryptophan levels declined gradually after reperfusion. The ratio of 5-HIAA/5-HT was significantly higher in I/R rats than in control rats, suggesting that elevated 5-HT was quickly metabolized in the systemic circulation. In the intestine, 5-HT decreased dramatically, whereas tryptophan increased. This phenomenon was prominent in the severely damaged intestine. These findings suggest that the injured intestine released large amounts of 5-HT, whereas its synthesis in the injured intestine was suppressed. An increase in 5-HT in the circulation may be related to various circulatory disturbances observed in humans after intestinal ischemia.

SB-207266: 5-HT4 receptor antagonism in human isolated gut and prevention of 5-HT-evoked sensitization of peristalsis and increased defaecation in animal models

SB-207266 is a new 5-HT4 receptor antagonist which in a pilot study reduced the symptoms of irritable bowel syndrome. To help validate this and further studies, we examined the ability of SB-207266 to antagonize at the human 5-HT4 receptor (human isolated intestine) and to affect the mechanisms of peristalsis (guinea-pig isolated ileum) and defaecation (conscious, fed mice). In the human intestine, the potency of 5-HT4 receptor antagonism (pKB 9.98) was similar to that previously demonstrated using a guinea-pig model of the receptor, validating the use of SB-207266 in clinical trials. In each of the animal models, SB-207266 did not affect normal patterns of intestinal motility measured in the absence of exogenous 5-HT. However, SB-207266 10-1000 pM concentration-dependently antagonized the ability of 5-HT (0.1 microM) to sensitize the peristaltic reflex and lower the distension threshold at which peristalsis was evoked. In mice, oral or subcutaneous (s.c.) doses of SB-207266 dose-dependently prevented the ability of the 5-HT precursor, 5-hydroxytryptophan (5-HTP, 10 mg kg-1 s.c.) to increase both the rate of defaecation of formed faecal pellets and their fluid content. SB-207266 was maximally active at 10 micrograms kg-1 s.c. and 1000 micrograms kg-1 p.o. SB-207266 may therefore represent a new class of therapeutic agent, capable of preventing the actions of an important sensitizer of gut function.

The response of rat colonic mucosa to 5-hydroxytryptamine in health and following restraint stress

The present study characterized the rat colonic secretory response to 5-hydroxytryptamine (5-HT) and determined alterations in this response following stress. 5-HT stimulated rat colonic short-circuit current in a concentration-dependent fashion (pD2 = 5.19). This response was subject to desensitization and was mimicked by the indolealkylamines with a rank order potency of 5-HT approximately alpha-methyl-5-HT > 5-carboxytryptamine approximately 5-methoxytryptamine. 2-Methyl-5-HT was a partial agonist. The colonic response to 5-HT was unaltered by methysergide (10 microM), ritanserin (0.1 microM), ondansetron (1 microM) or clozapine (10 microM), but was antagonized by the 5-HT4 receptor antagonists SB204070 (pD'2 = 9.32), GR113808 (pKb = 8.56), DAU6285 (pKb = 6.07) and SDZ205557 (pKb = 6.80). The response of colonic epithelial and oesophageal tunica muscularis mucosae to 5-HT is therefore mediated by a similar 5-HT4 receptor. Following wrap restraint stress, the colonic response to 5-HT became bimodal. Half of the preparations were hyper-responsive, while the rest were hypo-responsive to 5-HT. This 5-HT4 receptor may therefore be involved in stress related changes in fluid transport.

5-Hydroxytryptamine-induced secretion by rat jejunum in-vitro involves several 5-hydroxytryptamine receptor subtypes

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced anion secretion by rat jejunum have been investigated by testing the effects of selective agonists and antagonists in-vitro using both intact and stripped intestinal sheets. In both intact and stripped jejunum 5-HT and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, induced concentration-dependent increases in the short-circuit current (SCC), although 5-methoxytryptamine induced a smaller maximum response. In intact sheets 1-phenylbiguanide, a selective 5-HT3 agonist, induced a response that was similar in magnitude to that of 5-methoxytryptamine, but in stripped preparations it had little effect. Tetrodotoxin inhibited the response of intact jejunum to 5-HT (by 86%) and 5-methoxytryptamine (by 85%) and abolished the response to 1-phenylbiguanide. In stripped sheets inhibition of the 5-HT response by tetrodotoxin was reduced to 27%. Desensitization to 1-phenylbiguanide reduced the response to 5-HT in intact but not stripped sheets whereas, in contrast, desensitization to 5-methoxytryptamine inhibited the 5-HT response in stripped sheets but was without effect in intact sheets. Mianserin, a 5-HT1, 5-HT2 and 5-HT3 antagonist, and renzapride, a 5-HT1 and 5-HT3 antagonist, both reduced the maximum response to 5-HT, but 5-HTP-DP, a 5-HT1 antagonist, was without effect. The 5-HT3 antagonist granisetron reduced the response to 5-HT in intact, but not in stripped sheets. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine in both preparations, but did not alter the response to 5-HT. It is concluded that 5-HT-induced jejunal secretion involves more than one 5-HT receptor subtype, with both neural and non-neural mechanisms contributing to the response.

Neural and non-neural activation of electrogenic secretion by 5-hydroxytryptamine in the rat ileum in vitro

5-hydroxytryptamine (5-HT) stimulates electrogenic Cl- secretion in rat ileum stripped of its outer smooth musculature and myenteric plexus. The myenteric plexus, however, is a site of 5-HT synthesis in the gut, and the plexus mediates electrogenic ion secretion activated by luminal enterotoxin STa and taurocholate. Thus, we investigated the role of the myenteric plexus in 5-HT-induced electrogenic secretion in vitro by measuring short-circuit current (Isc, microamps) with voltage-clamp apparatus as an index of electrogenic Cl-secretion in rat ileum which was either stripped of the myenteric plexus or was left intact. Serosally added 5-HT stimulated electrogenic Cl- secretion in muscle-stripped and intact ileum in a concentration-dependent manner. Pre-treatment of stripped ileum with atropine (1 micron), hexamethonium (100 microns), tetrodotoxin (1.25 microns) and capsaicin (1 micron) for 15 min did not effect the maximum Isc induced by 5-HT which would implicate a direct action on the enterocyte. In intact ilea, however, tetrodotoxin (TTX) and capsaicin reduced significantly the maximum values of Isc stimulated by 5-HT, and the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) caused a significant decrease in the maximum response to 5-HT. These results suggest that electrogenic secretion induced by 5-HT in rat ileum in vitro occurs partly by activation of a non-neural pathway probably involving a direct interaction with the enterocyte, and partly via a nitrinergic-myenteric secretory reflex activated by sensory afferent fibres. These data highlight the danger of characterising intestinal secretory activity from in vitro experiments by using muscle-stripped tissue only.

Several receptor subtypes contribute to 5-hydroxytryptamine-induced secretion by rat ileum in-vitro

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced secretion by rat ileum were investigated in-vitro using selective agonists and antagonists. 5-HT induced a dose-dependent increase in the short-circuit current (SCC) generated by both intact and stripped sheets of rat ileum. 1-Phenylbiguanide, a selective 5-HT3 agonist, and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, also increased the SCC. In intact sheets 5-HT was more effective than either 1-phenylbiguanide or 5-methoxytryptamine, whereas in stripped sheets 5-HT and 5-methoxytryptamine were equipotent, with 1-phenylbiguanide having little effect. Tetrodotoxin abolished the response of intact sheets to 1-phenylbiguanide but only reduced the responses to 5-HT and 5-methoxytryptamine by 57% and 54%, respectively. This inhibition was reduced to 25% in stripped sheets. The 5-HT3 antagonist granisetron abolished the response to 1-phenylbiguanide, but did not alter the effects of 5-HT. Ketanserin, a 5-HT2 antagonist, had a small effect on the actions of 5-methoxytryptamine in intact, but not stripped, sheets and no effect on the response to 5-HT in either preparation. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine, but had less effect on the response to 5-HT. Desensitization to 1-phenylbiguanide inhibited the response to 5-HT in intact, but not stripped sheets, whereas desensitization to 5-methoxytryptamine abolished the 5-HT response in stripped sheets, but induced only 42% inhibition in intact sheets. Previous exposure to a combination of both 1-phenylbiguanide and 5-methoxytryptamine abolished the 5-HT-induced increase in SCC in both preparations. The findings suggest that 5-HT-induced ileal secretion involves more than one 5-HT receptor subtype and that both neural and non-neural mechanisms contribute to the response.

Increased defaecation caused by 5-HT4 receptor activation in the mouse

The precursor to 5-hydroxytryptamine (5-HT), 5-hydroxytryptophan, (5-HTP, 5-50 mg.kg-1) administered subcutaneously (s.c.) to conscious, fed mice caused a dose dependent increase in faecal pellet and fluid output. To avoid provoking watery diarrhoea, all experiments were performed using 5-HTP at 10 mg.kg-1. This dose caused maximal increases in the fluid content (471 +/- 41%) and number of formed faecal pellets defaecated (328 +/- 13% n = 25), 10 and 20 min respectively after administration, when compared to saline-treated mice. In both saline- and 5-HTP-treated mice methiothepin, ketanserin, mianserin and granisetron reduced defaecation at high s.c. doses (100 micrograms.kg-1 or 1000 micrograms.kg-1). The 5-HT4 receptor antagonists, DAU 6285 (endo-6-methoxy-8-methyl-8-azabicyclo[3.2.1]oct-3-yl-2,3-dihydro-2-oxo-1 H-benzimidazole-1-carboxylate hydrochloride), SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester) and SB 204070 ([1-butyl-4-piperidinylmethyl]-8-amino-7-chloro-1,4-benzodioxan -5- carboxylate), had no effects when administered s.c. to saline-treated mice, but dose-dependently inhibited the 5-HTP-evoked responses. Only SB 204070 at 1000 micrograms.kg-1 completely inhibited the responses to 5-HTP returning them to normal levels. We conclude that SB 204070 is a potent antagonist for the investigation of 5-HT4 receptor function in both normal and disturbed gastrointestinal activity.

The effects of cisapride on elemental content in neonatal mouse small intestine in vivo

The effects of cisapride on elemental content of enterocytes, smooth muscle cells and lamina propria in neonatal mouse jejunum were studied using electron probe X-ray microanalysis. One hour after injection with cisapride (1.7 or 0.017 mg/kg body weight i.p.), Cl content was significantly reduced in villus base, crypt and smooth muscle cells and Na content decreased in muscle cells. No changes were observed in Na or Cl within villus tip cells. Total Ca content did not change significantly in any cell type following treatment with cisapride. These results confirm that cisapride induces net Cl- secretion in neonatal mouse jejunum.

Evidence that the secretory response of rat intestine to 5-hydroxytryptamine in-vivo involves more than one 5-hydroxytryptamine-receptor subtype

The transintestinal potential difference (PD) across rat mid-small intestine and proximal colon was measured in-vivo. The 5-hydroxytryptamine (5-HT)-induced increase in PD, which reflects a stimulation of electrogenic C1 secretion, was mimicked by both 2-methyl-5-hydroxytryptamine (2-CH3-5-HT), an agonist at 5-HT3 receptors, and 5-methoxytryptamine (5-MT), an agonist that lacks affinity for 5-HT3 receptors. The 5-HT3 antagonist granisetron caused a marked inhibition of the response to 2-CH3-5-HT in both regions, but only produced a small inhibition of the small intestinal response to 5-HT, with a more pronounced effect in the colon. The failure of granisetron to produce a marked antagonism of the 5-HT-induced rise in the transintestinal PD, coupled with the ability of 5-MT to induce a secretory response, indicates that 5-HT3 receptors are not the only ones involved in the stimulation of C1 secretion. The 5-HT2 antagonist ketanserin failed to influence the response to 5-HT in either the small intestine or the colon, but it did inhibit the action of 5-MT, having a much greater effect in the small intestine. In the presence of granisetron however, ketanserin also inhibited the small intestinal response to 5-HT, having only a minimal effect in the colon. This suggests that 5-HT2 receptors can also play a role in the activation of C1 secretion. These observations suggest that both 5-HT2 and 5-HT3 receptors contribute to the stimulation of electrogenic C1 secretion by 5-HT, with 5-HT2 receptors playing a more prominent role in the small intestine and 5-HT3 receptors being more important in the colon.