Antagonism by SB 204070 of 5-HT-evoked contractions in the dog stomach: an in-vivo model of 5-HT4 receptor function

[Current Status of Translational Research on Constipation]

Constipation is one of the most common gastrointestinal disorders with a prevalence up to 16.5% in the general population. It is frequently multifactorial and the pathophysiologic mechanism of constipation is not fully understood. Many preclinical studies of constipation have used animal models. Translational research using these animal models is essential to the investigation of neurogenic and myogenic mechanisms of colon, and to the estimation of the clinical efficacy of new drugs. In this review, we discuss some of the current translational research projects on constipation using animal models.

GSK962040: a small molecule motilin receptor agonist which increases gastrointestinal motility in conscious dogs

BACKGROUND

GSK962040, a small molecule motilin receptor agonist, was identified to address the need for a safe, efficacious gastric prokinetic agent. However, as laboratory rodents lack a functional motilin system, studies in vivo have been limited to a single dose, which increased defecation in rabbits. Motilin agonists do not usually increase human colonic motility, so gastric prokinetic activity needs to be demonstrated.

METHODS

The effect of intravenous GSK962040 on gastro-duodenal motility was assessed in fasted dogs implanted with strain gauges. Activity was correlated with blood plasma concentrations of GSK962040 (measured by HPLC-MS/MS) and potency of GSK962040 at the dog recombinant receptor [using a Fluorometric Imaging Plate Reader (Molecular Devices, Wokingham, UK) after expression in HEK293 cells].

KEY RESULTS

GSK962040 activated the dog motilin receptor (pEC(50) 5.79; intrinsic activity 0.72, compared with [Nle(13) ]-motilin). In vivo, GSK962040 induced phasic contractions, the duration of which was dose-related (48 and 173 min for 3 and 6 mg kg(-1) ), driven by mean plasma concentrations >1.14 μmol L(-1) . After the effects of GSK962040 faded, migrating motor complex (MMC) activity returned. Migrating motor complex restoration was unaffected by 3 mg kg(-1) GSK962040 but at 6 mg kg(-1) , MMCs returned 253 min after dosing, compared with 101 min after saline (n=5 each).

CONCLUSIONS & INFERENCES

The results are consistent with lower potency for agonists at the dog motilin receptor, compared with humans. They also define the doses of GSK962040 which stimulate gastric motility. Correlation of in vivo and in vitro data in the same species, together with plasma concentrations, guides further studies and translation to other species.

In vitro and in vivo pharmacological characterization of PF-01354082, a novel partial agonist selective for the 5-HT(4) receptor

The pharmacological profile of PF-01354082, a selective 5-HT(4) receptor partial agonist, was investigated. PF-01354082 displayed high affinity for human 5-HT(4d) and dog 5-HT(4h) receptors in binding studies, having Ki values of 2.0 nM and 4.2 nM, respectively. By contrast, PF-01354082 did not show significant affinity for several other 5-HT receptors (5-HT(1A), 5-HT(1B), 5-HT(1D), 5-HT(2A), 5-HT(2B), 5-HT(2C), 5-HT(3A), and 5-HT(7)) or the dopamine D(2long) receptor. Functional assays using either cells expressing human recombinant 5-HT(4d) receptors or rat tunica muscularis mucosae demonstrated that PF-01354082 exhibited partial agonist activity at the 5-HT(4) receptor. The effects of PF-01354082 on in vitro receptor binding, ion channel activity, and sites of uptake were further investigated. PF-01354082 did not show biologically relevant binding activity at concentrations up to 10 microM except for binding to the 5-HT(4e) receptor. Furthermore, PF-01354082 decreased I(HERG) current by only 11% at a concentration of 300 microM, indicating that the compound had greater than 150,000-fold selectivity for the human 5-HT(4d) receptor over hERG channels. An in vivo study using a gastric motility model in conscious dogs demonstrated that oral administration of PF-01354082 resulted in marked and sustained stimulation of gastric motility in a dose-dependent manner. These results indicate that PF-01354082 is an orally active, highly selective, partial agonist of the human 5-HT(4) receptor that is expected to exert a favorable effect on gastrointestinal motor disorders with reduced adverse effects mediated by other related receptors.

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.

Pharmacological characterization of the 5-HT receptors mediating contraction and relaxation of canine isolated proximal stomach smooth muscle

1. We aimed to characterize 5-HT receptors mediating contraction and relaxation to 5-HT in dog proximal stomach longitudinal muscle (LM) strips. 2. Of the tryptamine analogues tested, 5-HT was the most potent contractile agent at basal length, while 5-CT was the most potent relaxant of PGF(2alpha)-induced contraction. Neither the contractions to 5-HT, nor the relaxations to 5-CT were influenced by tetrodotoxin, illustrating that action potential propagation is not involved. 3. The 5-HT-induced contraction was antagonized by mesulergine (0.03 to 0.3 microM) and ketanserin (2 - 20 nM), but the antagonism was not of a simple competitive nature, indicating multiple receptor involvement. Ketanserin (3 to 30 nM) and mesulergine (30 nM) competitively antagonized the alpha-Me-5-HT-induced contraction (pK(B): 8.83+/-0.09 and pA(2): 8.25+/-0.06 respectively). These affinity values are in line with literature affinities of ketanserin and mesulergine at 5-HT(2A) receptors in various bioassays. 4. The 5-CT-induced inhibition of PGF(2alpha)-induced contraction was competitively antagonized by mesulergine (pK(B) estimate: 8.52+/-0.12) and by the selective 5-HT(7) receptor antagonist SB-269970 (pK(B) estimate: 9.36+/-0.14). Both pK(B) estimates are in line with literature affinities of these compounds for 5-HT(7) receptors. Mesulergine (30 nM) and SB-269970 (10 nM) shifted the relaxant curve to 5-HT parallel to the right in the presence of ketanserin (0.3 microM) (pA(2) estimates of 8.08+/-0.10 and 8.75+/-0.14 respectively), indicative of 5-HT(7) receptor involvement. 5. It is concluded that 5-HT induces dog proximal stomach (LM) contraction via smooth muscle 5-HT(2A) receptors and relaxation via smooth muscle 5-HT(7) receptors.

Characterization of the receptors involved in the 5-HT-induced excitation of canine antral longitudinal muscle

1. We aimed to characterize the 5-HT receptors involved in the 5-HT-induced effect on electrically induced contractions of dog antrum longitudinal muscle in vitro. 2. In the presence of L-NOARG (0.1 mM), electrical field stimulation (EFS) induced atropine- and tetrodotoxin-sensitive contractions. Tetrodotoxin or atropine left any agonist tested ineffective. These EFS-induced contractions were on average enhanced by 5-HT (0.3 microM), however, pronounced variation in the response to 5-HT was observed. There were non-significant trends of the selective 5-HT3 receptor antagonist granisetron (1 microM), and methysergide (1 microM; preventing interactions of 5-HT with 5-HT1, 5-HT2, 5-ht5, 5-HT6 and 5-HT7 receptors) to increase the response to 5-HT. The selective 5-HT4 receptor antagonist GR 113808 (0.1 microM) displayed a non-significant trend to inhibit the 5-HT-induced increase. 3. Combination experiments with methysergide (1 microM), granisetron (1 microM) and GR 113808 (0.1 microM) revealed that the 5-HT (0.3 microM)-induced response consisted of (1) an excitatory component blocked by GR 113808, (2) excitatory and inhibitory components both blocked by methysergide. 4. The selective 5-HT4 receptor agonist prucalopride (0.3 microM) increased EFS-induced contractions, an effect prevented by GR 113808 (0.1 microM). 5. The increase of EFS-induced contractions by the preferential 5-HT2 receptor agonist alpha-Me-5-HT (0.3 microM) was antagonized by 5-HT2B receptor antagonists. 6. The 5-HT1/5-HT7 receptor agonist 5-carboxamidotryptamine (5-CT; 0.3 microM) inhibited EFS-induced contractions. This was prevented by methysergide (1 microM), the 5-HT7 receptor antagonist mesulergine (0.3 microM) and the selective 5-HT7 receptor antagonist SB-269970 (0.3 microM). 7. In the presence of GR 113808 (0.1 microM), alpha-Me-5-HT (1 microM) increased EFS-induced contractions. The 5-HT (0.3 microM)-induced inhibition of the stimulation by alpha-Me-5-HT was prevented by SB-269970 (0.3 microM). 8. In conclusion, dog antral longitudinal muscle is endowed with (1) excitatory neuronal 5-HT4 receptors and 5-HT2B receptors and (2) inhibitory smooth muscle 5-HT7 receptors.

5-HT(4) receptors mediating enhancement of contractility in canine stomach; an in vitro and in vivo study

We aimed to study 5-HT(4) receptors in canine stomach contractility both in vivo and in vitro. In anaesthetized Beagle dogs, the selective 5-HT(4) receptor agonist prucalopride (i.v.) induced dose-dependent tonic stomach contractions under isobaric conditions, an effect that was antagonized by the selective 5-HT(4) receptor antagonist GR 125487 (10 microg kg(-1), i.v.). Electrical field stimulation (EFS) of corpus longitudinal muscle strips resulted in atropine- and tetrodotoxin-sensitive contractions (L-NOARG (0.1 mM) present in all organ bath solutions). Prucalopride increased these contractions (maximal response after single-dose addition (0.3 microM): 165% of initial value, or after cumulative addition: 188%). In the presence of methysergide (3 microM), 5-HT also increased EFS-contractions (after single-dose addition (0.3 microM): increase to 192%, after cumulative addition: 148%). The selective 5-HT(4) receptor antagonists GR 113808 (0.1 microM) or GR 125487 (10 nM) antagonized the prucalopride (0.3 microM)-induced contraction increments. When EFS-induced contractions were blocked by atropine or tetrodotoxin, prucalopride was ineffective. In the presence of methysergide (3 microM), the contraction increases to 5-HT (0.3 microM) were prevented by GR 113808 (0.1 microM). The prucalopride curve (pEC(50) 7.9) was shifted in parallel to the right by GR 113808 3 nM (pA(2) 9.4). In the presence of methysergide (3 microM), the curve to 5-HT (pEC(50) 8.1) was competitively antagonized by GR 113808, yielding a Schild slope of 0.8+/-0.2 (pK(B) of 9.1 with unit Schild slope). In corpus circular muscle strips, the prucalopride (0.3 microM)-induced augmentation of EFS-contractions (258%) was also prevented by GR 113808 (0.1 microM) (124%). In conclusion, the effects of 5-HT(4) receptor agonists on proximal stomach motor activity in vivo can be explained by an effect on 5-HT(4) receptors on cholinergic nerves within the gastric muscle wall.

Pharmacological profile of YM-31636, a novel 5-HT3 receptor agonist, in vitro

We investigated the in vitro pharmacological profile of YM-31636 (2-(1H-imidazol-4-ylmethyl)-8H-indeno[1,2-d]thiazole monofumarate). In cloned human 5-HT3A receptors, YM-31636 had a pKi value of 9.67 vs. ramosetron and pKi values for other 5-HT3 receptor agonists were less than 7. YM-31636 showed very low affinities for other receptors. YM-31636 induced contraction of isolated guinea pig distal colon. The intrinsic activity was approximately 0.90 compared with 5-hydroxytryptamine's (5-HT) 1.0, and the potency was 26 times greater than that of 5-HT. YM-31636 increased short-circuit current (Isc) in the isolated guinea pig distal colon. In this case, the relative intrinsic activity was approximately 0.19. In isolated guinea pig right atrium, YM-31636 induced tachycardia with the relative intrinsic activity of approximately 0.23. All these effects of YM-31636 were antagonized by ramosetron, a selective 5-HT3 receptor antagonist. These results suggest that YM-31636 is a potent and selective 5-HT3 receptor agonist, preferentially acting on the contraction of the colon.

5-HT(4) receptors on cholinergic nerves involved in contractility of canine and human large intestine longitudinal muscle

5-HT(4) receptors mediate circular muscle relaxation in both human and canine large intestine, but this phenomenon alone can not explain the improvement in colonic motility induced by selective 5-HT(4) receptor agonists in vivo. We set out to characterize 5-HT(4) receptor-mediated effects in longitudinal muscle strips of canine and human large intestine. Electrical field stimulation (EFS) was applied providing submaximal isotonic contractions. L-NOARG (0.1 mM) was continuously present in the organ bath to preclude nitric oxide-induced relaxation to EFS. The selective 5-HT(4) receptor agonist prucalopride (0.3 microM) enhanced EFS-evoked contractions, that were antagonized in both preparations by the selective 5-HT(4) receptor antagonist GR 113808 (0.1 microM). The prucalopride-induced increase was present in canine ascending and descending colon, but absent in rectum. Regional differences in response to prucalopride were not observed in human ascending and sigmoid colon and rectum. Incubation with atropine (1 microM) or tetrodotoxin (0.3 microM) inhibited EFS-induced contractions, which were then unaffected by prucalopride (0.3 microM) in both tissues. In the presence of methysergide (3 microM; both tissues) and granisetron (0.3 microM; only human tissues), 5-HT (0.3 microM) enhanced EFS-induced contractions, an effect that was antagonized by GR 113808 (0.1 microM). In the presence of atropine or tetrodotoxin, EFS-induced contractions were inhibited, leaving 5-HT (0.3 microM) ineffective in both preparations. This study demonstrates for the first time that in human and canine large intestine, 5-HT(4) receptors are located on cholinergic neurones, presumably mediating facilitating release of acetylcholine, resulting in enhanced longitudinal muscle contractility. This study and previous circular muscle strip studies suggest that 5-HT(4) receptor agonism facilitates colonic propulsion via a coordinated combination of inhibition of circumferential resistance and enhancement of longitudinal muscle contractility.

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.

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.

Pharmacological characterization of 5-HT4 receptors mediating relaxation of canine isolated rectum circular smooth muscle

This study aimed to characterize for the first time in vitro 5-HT4 receptors in the canine gastrointestinal tract. For this purpose, we used circular muscle strips of the canine isolated rectum. In the presence of methysergide (60 microM), 5-HT induced relaxation of methacholine (1 microM)-precontracted muscle strips, yielding a monophasic sigmoidal concentration-relaxation curve (pEC50 7.2+/-0.07). Tetrodotoxin (0.3 microM) did not affect the curve to 5-HT, suggesting the inhibitory 5-HT receptor is located on the smooth muscle. Granisetron (0.3 microM) did also not affect the curve to 5-HT, which excludes the 5-HT3 receptor mediating the relaxation to 5-HT. The presence of methysergide rules out the involvement of 5-HT1, 5-HT2 or 5-HT7 receptors. 5-HT, the selective 5-HT4 receptor agonists R076186, prucalopride (R093877) and SDZ HTF-919 and the 5-HT4 receptor agonists cisapride and 5-MeOT relaxed the muscle strips with a rank order of potency R076186 = 5-HT > cisapride > prucalopride > or = SDZ HTF-919 > 5-MeOT. The selective 5-HT4 receptor antagonists GR 125487, RS 39604 and GR 113808 competitively antagonized the relaxations to 5-HT, yielding pK(B) estimates of 9.7, 7.9 and 9.1, respectively. The selective 5-HT4 receptor antagonist SB 204070 shifted the curve to 5-HT rightward and depressed the maximal response (apparent pA2 10.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5-HT4 receptor agonists R076186 (pA2 8.8). It is concluded that 5-HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5-HT4 receptors. For the first time, a nonhuman species was shown to exhibit relaxant 5-HT4 receptors in the large intestine.

5-HT4 receptor antagonism does not affect motor and reward mechanisms in the rat

5-HT4 receptors are concentrated in areas of the brain which are rich in dopamine neuronal markers, which may suggest that they influence motor and reward processes. We tested this hypothesis by examining the effects of a 5-HT4 receptor antagonist, 8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1,4-dioxan-5-car boxylate hydrochloride (SB-204070-A) on amphetamine- and nicotine-induced locomotor stimulation in intact rats. In rats with unilateral 6-hydroxydopamine-induced lesions of the ascending nigrostriatal dopaminergic projection, SB-204070-A was tested for its effects on amphetamine-induced rotation. SB-204070-A was also tested for its effects on rewarded behaviour maintained by intracranial self-stimulation. SB-204070-A did not alter behaviour under any of these conditions, suggesting a lack of involvement of the 5-HT4 receptor in motor and reward processes.

Effect of TKS159, a novel 5-hydroxytryptamine4 agonist, on gastric contractile activity in conscious dogs

A novel 5-hydroxytryptamine (5-HT)4 receptor agonist, TKS159, ¿4-amino-5-chloro-2-methoxy-N-[(2S,4S)-1-ethyl-2- hydroxymethyl-4-pyrrolidinyl] benzamide), has recently been developed as a gastroprokinetic drug. Cisapride is already used clinically to increase gastric contractions. The stimulatory effects of TKS159 and cisapride on gastric contractions were examined using force transducers chronically implanted on the vagally denervated pouch (Heidenhain pouch) and the vagally innervated main stomach in conscious dogs. Contractile activity was analysed by computer and expressed as a motor index. Intravenous administration of TKS159 or cisapride significantly increased the motor index in both the main stomach and the Heidenhain pouch during the fed and fasted states. Pharmacological characterization in the fasted state revealed that the contraction-stimulating activity of TKS159 and cisapride on the stomach was significantly inhibited by atropine, hexamethonium and a 5-HT4 receptor antagonist, SDZ 205-557. Granisetron (a 5-HT3 receptor antagonist) significantly inhibited cisapride-induced, but not TKS159-induced gastric contractions. The plasma motilin concentration was significantly increased after cisapride, but not after TKS159 injection. In conclusion, TKS159 has a contractile-stimulating effect on both the innervated and the denervated stomach. It is likely that a cholinergic pathway and 5-HT4 receptors are involved in producing the contractions, although other mechanisms cannot be excluded. Cisapride has almost the same characteristics, but the present findings suggest the involvement of motilin and 5-HT3 receptors in the effects of cisapride.

Synthesis and structure-activity relationships of potent and orally active 5-HT4 receptor antagonists: indazole and benzimidazolone derivatives

A series of indole-3-carboxamides, indazole-3-carboxamides, and benzimidazolone-3-carboxamides was synthesized and evaluated for antagonist affinity at the 5-HT4 receptor in the rat esophagus. The endo-3-tropanamine derivatives in the indazole and benzimidazolone series possessed greater 5-HT4 receptor affinity than the corresponding indole analogues. 5-HT4 receptor antagonist affinity was further increased by alkylation at N-1 of the aromatic heterocycle. In a series of 1-isopropylindazole-3-carboxamides, replacement of the bicyclic tropane ring system with the monocyclic piperidine ring system or an acyclic aminoalkylene chain led to potent 5-HT4 receptor antagonists. In particular, those systems in which the basic amine was substituted with groups capable of forming hydrogen bonds showed increased 5-HT4 receptor antagonist activity. While some of these compounds displayed high affinity for other neurotransmitter receptors (in particular, 5-HT3, alpha1, and 5-HT2A receptors), as the conformational flexibility of the amine moiety increased, the selectivity for the 5-HT4 receptor also increased. From this series of compounds, we identified LY353433 (1-(1-methylethyl)-N-[2-[4-[(tricyclo[3.3.1.1(3, 7)]dec-1-ylcarbonyl)amino]-1-piperidinyl]ethyl]-1H-indazole-3- carboxamide) as a potent and selective 5-HT4 receptor antagonist with clinically suitable pharmacodynamics.

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.

Selective and functional 5-hydroxytryptamine4 receptor antagonism by SB 207266

1. The pharmacology of a novel 5-HT4 receptor antagonist, SB 207266 has been evaluated in vitro in the guinea-pig distal colon longitudinal muscle myenteric plexus (LMMP) and in vivo in the dog Heidenhain pouch. 2. SB 207266 is a highly potent antagonist of 5-HT-evoked, cholinergically-mediated contractions in the guinea-pig distal colon. Low concentrations (0.1-10 nM) produced a parallel shift to the right of the concentration-effect curve (apparent pA2 10.6 +/- 0.1) with no significant effect on the maximum response. With higher concentrations of SB 207266 (30 nM and above) the maximum response to 5-HT was reduced. 3. The antagonism seen with SB 207266 cannot be attributed to a non-selective effect since high concentrations (1 microM) had no effect on cholinergically-mediated contractions evoked by the nicotinic receptor agonist DMPP in the same preparation. 4. SB 207266 is not an irreversible antagonist since the effects of the compound were reversible upon washing of the tissue. 5. In the dog Heidenhain pouch, oral (0.1-100 micrograms kg-1) and intravenous (0.1-100 micrograms kg-1) administration of SB 207266 produced a dose-dependent antagonism of the contractions evoked by a bolus intravenous injection of 5-HT. An ID50 for SB 207266 of 1.3 micrograms kg-1 was obtained following i.v. administration and 9.6 micrograms kg-1 following oral administration. 6. The antagonistic effects of SB 207266 (0.1-100 micrograms kg-1) in the dog Heidenhain pouch were long lasting since, following oral administration, the response to 5-HT was reduced for at least 135 min. 7. SB 207266 is a highly potent, highly selective and orally active 5-HT4 receptor antagonist. This compound is the first orally active amide to be identified in this class of antagonists and as such is an important new tool in the evaluation of 5-HT4 receptor function both in vitro and in vivo.