5-HT4 receptors are not involved in the control of small intestinal transit in the fasted conscious rat

Endogenous ghrelin and 5-HT regulate interdigestive gastrointestinal contractions in conscious rats

Endogenous ghrelin causes interdigestive contractions of the stomach in rats. In contrast, previous studies showed that 5-HT(3) and 5-HT(4) receptors were involved in regulating intestinal interdigestive contractions. We studied the possible role of endogenous ghrelin and 5-HT regulating interdigestive gastrointestinal (GI) contractions in rats. Four strain gauge transducers were implanted on the antrum, duodenum, and proximal and distal jejunum. After an overnight fast, GI contractions were recorded in freely moving conscious rats and ghrelin receptor antagonists [(d-lys3)GHRP6; 1 micromol/kg], 5-HT(3) antagonists (Ondansetron; 0.5 mg/kg) and 5-HT(4) antagonists (GR 125,487; 1 mg/kg) were administered (bolus iv). To evaluate the relationship between the luminal concentrations of 5-HT and phase III-like contractions of the duodenum, duodenal juice was collected via the intraduodenal catheter. 5-HT content of the duodenal juice was measured by HPLC. (d-lys3)GHRP6 significantly attenuated the occurrence and amplitude of phase III-like contractions of the antrum, but not the duodenum and jejunum. 5-HT(4) antagonists significantly reduced spontaneous phase III-like contractions of the jejunum, without affecting those of the antrum and duodenum. In contrast, 5-HT(3) antagonists did not affect phase III-like contractions in GI tract. Luminal concentration of 5-HT at the phase III-like contraction (36.0 +/- 13.3 ng/ml, n = 9) was significantly higher than that at the phase I-like contractions of the duodenum (4.9 +/- 1.6 ng/ml, n = 9, P < 0.05). It is suggested that released ghrelin from the gastric mucosa mediates gastric phase III-like contractions, whereas 5-HT released from enterochromaffin cells of the duodenal mucosa mediates intestinal phase III-like contractions via 5-HT(4) receptors.

Zebrafish based assays for the assessment of cardiac, visual and gut function--potential safety screens for early drug discovery

INTRODUCTION

Safety pharmacology is integral to the non-clinical safety assessment of new chemical entities prior to first administration to humans. The zebrafish is a well established model organism that has been shown to be relevant to the study of human diseases. The potential role of zebrafish in safety pharmacology was evaluated using reference compounds in three models assessing cardiac, visual and intestinal function.

METHODS

Compound toxicity was first established in zebrafish to determine the non toxic concentration of a blinded set of 16 compounds. In the cardiac assay, zebrafish larvae at 3 days post fertilisation (d.p.f.) were exposed to compounds for 3 h before measurement of the atrial and ventricular rates. To investigate visual function, the optomotor response was assessed in 8 d.p.f. larvae following a 5 day compound exposure. In the intestinal function assay, the number of gut contractions was measured in 7 d.p.f. larvae after a 1 h compound exposure. Finally, compound uptake was determined for 9 of the 16 compounds to measure the concentration of compound absorbed by the zebrafish larvae.

RESULTS

Seven compounds out of nine produced an expected effect that was statistically significant in the cardiac and visual functions assays. In the gut contraction assay, six out of ten compounds showed a statistically significant effect that was also the expected result whilst two displayed anticipated but non-significant effects. The compound uptake method was used to determine larval tissue concentrations and allowed the identification of false negatives when compound was poorly absorbed into the zebrafish.

DISCUSSION

Overall, results generated in three zebrafish larvae assays demonstrated a good correlation between the effects of compounds in zebrafish and the data available from other in vivo models or known clinical adverse effects. These results suggest that for the cardiac, intestinal and visual function, zebrafish assays have the potential to predict adverse drug effects and supports their possible role in early safety assessment of novel compounds.

Peptidergic regulation of gastrointestinal motility in rodents

Peptides involved in the endocrine and enteric nervous systems as well as in the central nervous system exert concerted action on gastrointestinal motility. Mechanical and chemical stimuli which induce peptide release from the epithelial endocrine cells are the earliest step in the initiation of peristaltic activities. Gut peptides exert hormonal effects, but peptide-containing stimulatory (Ach/substance P/tachykinin) and inhibitory (VIP/PACAP/NO) neurons are also involved in the induction of ascending contraction and descending relaxation, respectively. The dorsal vagal complex (DVC), located in the medulla of the brainstem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal motility. Several gut peptides act on the DVC to modify vagal cholinergic reflexes directly (PYY and PP) or indirectly via afferent fibers in the periphery (CCK and GLP-1). The DVC is also a primary site of action of many neuropeptides (such as TRH and NPY) in mediating gastrointestinal motor activities. The identification over the last few years of a number of neuropeptide systems has greatly changed the field of feeding and body weight regulation. By exploring the brain and gut systems that employ recently identified peptidergic molecules, it will be possible to elaborate on the central and peripheral pathways involved in the regulation of gastrointestinal motility.

Possible involvement of 5-HT and 5-HT2 receptors in acceleration of gastrointestinal transit by escin Ib in mice

We have reported previously that escin Ib accelerated gastrointestinal transit (GIT) in mice, and that its effect may be mediated by the release of endogenous prostaglandins (PGs) and nitric oxide (NO). In this study, the possible involvement of 5-HT and 5-HT receptors in the GIT acceleration of escin Ib was investigated in mice. The acceleration of GIT by escin Ib (25 or 50 mg/kg, p.o.) was attenuated by pretreatment with ritanserin (0.5-5 mg/kg, s.c., a 5-HT(2A/2C/2B) receptor antagonist), but not with MDL 72222 (1 and 5 mg/kg, s.c.) and metoclopramide (10 mg/kg, s.c.) (5-HT3 receptor antagonists) or tropisetron (1 and 10 mg/kg, s.c., a 5-HT(3/4) receptor antagonist). Furthermore, pretreatment with ketanserin (0.05-5 mg/kg, s.c.), haloperidol (1-5 mg/kg, s.c.) and spiperone (0.5-5 mg/kg, s.c.) (5-HT2A receptor antagonists), as well as a bolus of dl-p-chlorophenylalanine methyl ester (PCPA, 1000 mg/kg, p.o., 1, 6 or 24 h before administration of the sample) (an inhibitor of 5-HT synthesizing enzyme tryptophan hydroxylase) and reserpine (5 mg/kg, p.o.) (a 5-HT depletor), but not 6-hydroxydopamine (80 mg/kg, i.p., a dopamine depletor) or repeated PCPA (300 mg/kg x2, p.o., 72 and 48 h before administration of the sample), also attenuated the effects of escin Ib. It is postulated that escin Ib accelerates GIT, at least in part, by stimulating the synthesis of 5-HT to act through 5-HT2, possibly 5-HT2A receptors, which in turn causes the release of NO and PGs.

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.

Acceleration of gastrointestinal transit by momordin Ic in mice: possible involvement of 5-hydroxytryptamine, 5-HT(2) receptors and prostaglandins

Possible involvement of 5-hydroxytryptamine (5-HT), 5-HT receptors and prostaglandins in the acceleration of gastrointestinal transit by momordin Ic was investigated in mice. Accelerative effect of momordin Ic (25 mg/kg, p.o.) on gastrointestinal transit was attenuated by pretreatment with a bolus of DL-p-chlorophenylalanine methyl ester (an inhibitor of 5-HT synthesizing enzyme), but not repeated pretreatment with DL-p-chlorophenylalanine methyl ester. Furthermore, cyproheptadine (a nonselective 5-HT(2) receptor antagonist), ritanserin (a 5-HT(2A/2B/2C) receptor antagonist) and clozapine (a 5-HT(2A/2C) receptor antagonist) also attenuated the effect of momordin Ic, but methiothepin (a 5-HT(1) receptor antagonist), MDL 72222 (3-tropanyl-3,5-dichlorobenzoate) and metoclopramide (5-HT(3) receptor antagonists), tropisetron (a 5-HT(3/4) receptor antagonist), ketanserin and haloperidol (5-HT(2A) receptor antagonists) did not. These results suggested a possible involvement of endogenous 5-HT and 5-HT(2B/2C) over 5-HT(2A) receptors. Attenuation by pretreatment with indomethacin (an inhibitor of prostaglandins synthesis) suggested involvement of prostaglandins. It is postulated that momordin Ic accelerates gastrointestinal transit partially by stimulating synthesis of 5-HT to act through 5-HT(2), possibly 5-HT(2C) and/or 5-HT(2B) receptors, which, in turn, increases synthesis of prostaglandins.

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

BACKGROUND

Pre-clinical studies indicate that the 5-hydroxytryptamine (5-HT)4 receptor may be involved in the pathophysiology of irritable bowel syndrome and that antagonism of this receptor may be an effective therapeutic strategy.

AIM

To investigate the effects of SB-207266-A, a selective 5-HT4 receptor antagonist on rectal sensitivity and small bowel transit in patients with irritable bowel syndrome.

METHODS

Eighteen patients with diarrhoea-predominant irritable bowel syndrome and a history of increased rectal sensitivity were randomized to receive either SB-207266-A (20 mg) or placebo for 10 days. Following a washout period, patients were then crossed over to receive the alternative therapy for 10 days. Rectal sensitivity and orocaecal transit time were assessed on day 10 of each treatment period. In addition, patients were asked whether they had experienced any changes in their symptoms.

RESULTS

Fifteen patients completed the study. SB-207266-A significantly increased orocaecal transit time towards normal (placebo: 5.3 h (4.0-7.2 h), mean (IQR) vs. SB-207266-A: 6.5 h (4.8-8.0 h); P=0.027) and tended to decrease rectal sensitivity (volume to discomfort 89 mL (60-150 mL), geometric mean (IQR) vs. 107 mL (75-150 mL); P=0.134). Eleven out of 15 patients reported symptomatic improvements with SB-207266-A but none with placebo. SB-207266-A was well tolerated.

CONCLUSION

Our results support a role for the 5-HT4 receptor in the pathophysiology of irritable bowel syndrome and suggest that the selective 5-HT4 antagonist, SB-207266-A, is worthy of further evaluation in this disorder.

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

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

Functional gut disorders: from motility to sensitivity disorders. A review of current and investigational drugs for their management

Functional gut disorders include several clinical entities defined on the basis of symptom patterns (e.g., functional dyspepsia, irritable bowel syndrome, functional abdominal pain, functional abdominal bloating), for which there is no established pathophysiological mechanism. Because there is no well-defined pathophysiological target, treatment should be aimed at symptom improvement. Prokinetics and antispasmodics have been widely used in the treatment of functional gut disorders on the assumption that disordered motility is the underlying cause of symptoms, and symptom improvement is indeed achievable with these compounds in some, but not all, patients with features of hypo- or hypermotility, respectively. In the first part of this review, we cover the basic pharmacology and discuss the rationale for the clinical use of prokinetics and antispasmodics. On the other hand, in the past few years, the explosive growth in the research focusing on visceral sensitivity and visceral reflexes has suggested that at least some patients with functional gut disorders have altered visceral perception. Thus, the second part of the review covers these developments and focuses on studies addressing the issue of drugs modulating visceral sensitivity.

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.

Pharmacological characterization of the 5-hydroxytryptamine receptor mediating relaxation in the rat isolated ileum

1 The aim of the present study was to investigate a 5-HT4 receptor involvement in the mediation of a 5-HT-induced relaxation response in the rat isolated ileum in vitro. 2 Ileal segments were taken at regular intervals from the ileo-caecal junction to duodenum. 5-HT (1 microM) induced a relaxation or contraction response in segments taken from the terminal ileum: the relaxation decreased and finally disappeared as contractions dominated in the proximal tissues. The 5-HT-induced relaxations were enhanced in the terminal segments and the contractions attenuated in both terminal and proximal segments, in the presence of methysergide (1 microM) and atropine (0.1 microM). 3 In the presence of methysergide (1 microM) and atropine (0.1 microM), a cumulative addition of 5-HT (0.01-1 microM) induced a concentration-dependent relaxation in the terminal (1-20 cm from the ileo-ceacal junction) ileal segments which at higher concentrations of 5-HT (3-30 microM) reverted to contraction. 4 The rank order of potency of indole agonists in inducing a concentration-related relaxation response in tissues of the terminal ileum (pretreated with pargyline (100 microM) and in the presence of methysergide (1 or 100 microM) and atropine (0.1 microM) was 5-hydroxytryptamine (6.97 +/- 0.06), 5-methoxytryptamine (6.50 +/- 0.07), alpha-methyl-5-hydroxytryptamine (5.53 +/- 0.17), 5-carboxamidotryptamine (5.51 +/- 0.12) and 2-methyl-5-hydroxytryptamine (< 5), the pEC50 values (mean +/- s.e.mean) being shown in parentheses. 5 Pretreatment of tissues with pargyline (100 microM) selectively enhanced the potency of 5-methoxytryptamine by a factor of 19 but failed to modify the potency of the other indole agonists. 6 The 5-HT4 receptor antagonists, tropisetron, SDZ 205-557 and GR 113808 antagonized the relaxation response to 5-HT (in the presence of methysergide (1 or 10 microM) and atropine (0.1 microM)) with pKB values (95% CL) of 6.09 (5.94-6.24), 7.0 (6.9-7.09) and 8.95 (8.81-9.1) respectively. Apparent pKB values estimations for tropisetron (1 microM) and GR 113808 (10 nM) using the agonists 5-methoxytryptamine and 5-carboxamidotryptamine were 6.37 +/- 0.31, 5.91 +/- 0.38 and 8.83 +/- 0.11, 8.82 +/- 0.22 respectively. 7 Tropisetron (10 microM), SDZ 205-557 (3 microM) and GR 113808 (10-100 nM) caused an increase in basal tone of the rat terminal ileum when administered in the presence of methysergide and atropine. 8 The relaxation response to 5-HT in the rat terminal ileum was not antagonized by ritanserin (1 microM), ondansetron (1 microM) or N omega-nitro-L-arginine methyl ester (100 microM) and with only a twofold dextral shift of the concentration-response curve by tetrodotoxin (1 microM). 9 It is concluded that the relaxant response to 5-HT in the terminal region of the ileum is mediated directly at the smooth muscle; a ranked indole agonist potency and selective antagonism by 5-HT4 receptor antagonists tropisetron, SDZ 205-557 and GR 113808 indicate a 5-HT4 receptor involvement in the relaxation response.