Pharmacological effects of the gastroprokinetic agent mosapride citrate]

Mosapride citrate (mosapride) is a novel gastroprokinetic agent that enhances the gastrointestinal motility by stimulating the 5-hydroxytryptamine4 (5-HT4) receptor. Mosapride dose-dependently enhanced the gastric emptying of a liquid or solid meal in rats with a potency equal to that of cisapride and more potent than that of metoclopramide. In rats, mosapride improved the gastric emptying delayed by gastroduodenal surgical intervention. In the conscious dogs with force transducers implanted chronically, mosapride stimulated antral and duodenal motility with a potency equal to those of cisapride. In isolated guinea-pig ileal longitudinal muscle preparations, mosapride enhanced the electrically stimulated contractions, and the enhancing effect of mosapride was antagonized by a high dose of tropisetron, a 5-HT4-receptor antagonist. In addition, mosapride inhibited [3H]-GR-113808 binding to 5-HT4 receptor sites of guinea-pig ileum and striatum. Mosapride had no affinity for dopamine D2 receptor, whereas metoclopramide and cisapride had a high affinity for dopamine D2 receptor. In isolated guinea-pig papillary muscles, mosapride did not prolong the duration of action potentials, whereas cisapride concentration-relatedly prolonged it. In conclusion, mosapride is a selective and potent 5-HT4 receptor agonist and improves gastrointestinal symptoms in patients with non-ulcer dyspepsia without causing the extrapyraminal syndrome associated with dopamine-D2-receptor blockage and adverse cardiovascular effects such as torsadoes de points.     

The in vivo gastrointestinal activity of TD-5108, a selective 5-HT4 receptor agonist with high intrinsic activity

The in vivo preclinical pharmacodynamic profile of TD-5108, a selective 5-HT(4) receptor agonist with high intrinsic activity, was compared to that of the clinically studied gastrointestinal pro-kinetic agents, tegaserod, cisapride and mosapride. The activity of TD-5108 was evaluated in guinea pig colonic transit, rat oesophageal relaxation and dog gastrointestinal smooth muscle contractility models. Subcutaneous administration of TD-5108, tegaserod, cisapride and mosapride increased guinea pig colonic transit (rank order of potencies: TD-5108 > tegaserod > cisapride > mosapride). Following intravenous and intraduodenal dosing, TD-5108, tegaserod, cisapride and mosapride produced dose-dependent relaxation of the rat oesophagus. On a molar basis, TD-5108 was approximately twofold less potent than tegaserod following intravenous dosing but 6- or 86-fold more potent than cisapride or mosapride, respectively, and 9- or 18-fold more potent than tegaserod or cisapride, respectively, after intraduodenal administration. Orally dosed TD-5108 increased the contractility of the canine antrum, duodenum and jejunum with higher potency than tegaserod. The selective 5-HT(4) receptor agonist, TD-5108, demonstrates robust in vivo activity in the guinea pig, rat and dog gastrointestinal tracts.     

Effects of mosapride citrate, a 5-HT4-receptor agonist, on gastric distension-induced visceromotor response in conscious rats

Mosapride citrate (mosapride), a prokinetic agent with 5-HT(4)-receptor agonistic activity, is known to enhance gastric emptying and alleviate symptoms in patients with functional dyspepsia (FD). As hyperalgesia and delayed gastric emptying play an important role in the pathogenesis of FD, we used in this study balloon gastric distension to enable abdominal muscle contractions and characterized the visceromotor response (VMR) to such distension in conscious rats. We also investigated the effects of mosapride on gastric distension-induced VMR in the same model. Mosapride (3-10 mg/kg, p.o.) dose-dependently inhibited gastric distension-induced VMR in rats. However, itopride even at 100 mg/kg failed to inhibit gastric distension-induced VMR in rats. Additionally, a major metabolite M1 of mosapride, which possesses 5-HT(3)-receptor antagonistic activity, inhibited gastric distension-induced VMR. The inhibitory effect of mosapride on gastric distension-induced visceral pain was partially, but significantly inhibited by SB-207266, a selective 5-HT(4)-receptor antagonist. This study shows that mosapride inhibits gastric distension-induced VMR in conscious rats. The inhibitory effect of mosapride is mediated via activation of 5-HT(4) receptors and blockage of 5-HT(3) receptors by a mosapride metabolite. This finding indicates that mosapride may be useful in alleviating FD-associated gastrointestinal symptoms via increase in pain threshold.     

The gastrointestinal motor effect of benzamide derivatives is unrelated to 5-HT3 receptor blockade

The prokinetic properties of a number of 5-HT3 antagonists containing the benzamide moiety (metoclopramide, cisapride, BRL 24924, zacopride) were compared with those of the chemically unrelated antagonist, ICS 205-930. Their 5-HT3 antagonistic potency was evaluated using the Bezold-Jarisch test. All compounds accelerated gastric emptying of beads in the rat, with potencies comparable to those found for inhibiting the Bezold-Jarisch reflex. Metoclopramide, cisapride and zacopride potentiated the twitch contraction of guinea pig ileum and contracted the isolated guinea pig colon in a concentration-dependent manner. Furthermore, they increased the contractility of the gastric Heidenhain pouch in the conscious dog. In contrast, ICS 205-930 was devoid of agonist or antagonist activities in all models except gastric emptying in the rat. Two findings, (1) that benzamide derivatives showed high efficacy in all models of gastrointestinal motility in contrast to ICS 205-930 that was active only to increase gastric emptying, and (2) the different potency order of benzamides in the Bezold-Jarisch test as compared to the in vitro motility tests, indicate that 5-HT3 receptors are involved in gastric emptying, whereas a different receptor operates in the other models used.     

Loperamide inhibits tachykinin NK3-receptor-triggered serotonin release without affecting NK2-receptor-triggered serotonin release from guinea pig colonic mucosa

The effect of loperamide on tachykinin NK(2)- and NK(3)-receptor-mediated 5-HT outflow from guinea pig colonic mucosa was investigated in vitro. The selective tachykinin NK(2)-receptor agonist [beta-Ala(8)]-neurokinin A(4-10) (betaAla-NKA) or the selective NK(3)-receptor agonist senktide elicited an increase in 5-HT outflow from whole colonic strips, but not from mucosa-free muscle layer preparations. The enhancing effect of betaAla-NKA and senktide was prevented by the selective NK(2)-receptor antagonist GR94800 or the selective NK(3)-receptor antagonist SB222200. Loperamide concentration-dependently suppressed the senktide-evoked 5-HT outflow, but failed to affect the betaAla-NKA-evoked 5-HT outflow. The kappa-opioid receptor antagonist nor-binaltorphimine or the delta-opioid receptor antagonist naltrindole displaced the concentration-response curve for the suppressant action of loperamide to the right without significant depression of the maximum. However, the mu-opioid receptor antagonist CTOP did not affect the suppressant effect of loperamide. We concluded that the NK(3) receptor-triggered 5-HT release from colonic mucosa is suppressed by loperamide-sensitive mechanisms, whereas the NK(2)-receptor-triggered 5-HT release is loperamide-insensitive. Our data also suggest that the suppressant effect of loperamide is probably mediated by the activation of kappa- and delta-opioid receptors located on intrinsic neurons.     

The in vitro pharmacological profile of TD-5108, a selective 5-HT4 receptor agonist with high intrinsic activity

The in vitro pharmacological profile of TD-5108, a novel, selective 5-HT(4) receptor agonist, was compared to that of clinically efficacious gastroprokinetic 5-HT(4) receptor agonists. TD-5108 produced an elevation of cyclic adenosine monophosphate in human embryonic kidney 293 cells expressing the human recombinant 5-HT(4(c)) (h5-HT(4(c))) receptor (pEC(50) = 8.3) and 5-HT(4) receptor-mediated relaxation of the rat esophagus (pEC(50) = 7.9) and contraction of the guinea pig colon (pEC(50) = 7.9). In all in vitro assays, TD-5108 was a high intrinsic activity agonist, unlike tegaserod, mosapride, and cisapride which, in the majority of test systems, had lower intrinsic activity. TD-5108 had high affinity (pK (i) = 7.7) and selectivity (>/=25-fold) for h5-HT(4(c)) receptors over other biogenic amine receptors. TD-5108 was >500-fold selective over other 5-HT receptors (including h5-HT(2B) and h5-HT(3A)) and, at 3 muM, had no effect on human ether-à-go-go-related gene K(+) channels. In conclusion, TD-5108 is a selective 5-HT(4) receptor agonist in vitro. The high intrinsic activity and preferential binding of TD-5108 to 5-HT(4) over other 5-HT receptors may result in an improved clinical profile for the treatment of gastrointestinal disorders of reduced motility.     

Facilitation of acetylcholine release by SK-951, a benzofuran derivative, via the 5-hydroxytryptamine4 receptor in guinea pig stomach

Facilitation of acetylcholine (ACh) release by SK-951 ((-)4-amino-N-[2-(1-azabicyclo[3.3.0] octan-5-yl)ethyl]-5-chloro-2,3-dihydro-2-methylbenzo[b]furan-7-carboxami de hemifumarate), a benzofuran derivative, via the 5-hydroxytryptamine (5-HT)4 receptor in guinea pig stomach was examined by in vitro receptor autoradiography and functional studies. [125I]SB207710 binding was detected in the myenteric plexus of the gastric corpus. High densities of binding sites were observed in the myenteric plexus and a moderate density in the muscle layer. SK-951 inhibited the binding of [125I]SB207710, a specific 5-HT4-receptor ligand, as in the case of SB204070, a specific 5-HT4-receptor antagonist, thus indicating the presence of 5-HT4 receptors in guinea pig stomach. SK-951 as well as 5-HT enhanced the electrically stimulated twitch contractions of gastric corpus strips, which were sensitive to tetrodotoxin and atropine, and enhanced electrically stimulated release of ACh from corporal strips, which was tetrodotoxin-sensitive and Ca2+-dependent. The enhancements of twitch contractions and ACh release by SK-951 were antagonized by GR113808, a selective 5-HT4-receptor antagonist. Thus, SK-951 binds to 5-HT4 receptors of the guinea pig gastric corpus and may accelerate gastric motility due to facilitation of ACh release.     

Ability of mosapride to bind to 5-HT4 receptor in the human stomach.

Ability of mosapride, a gastrokinetic agent, to bind to 5-HT4 receptor was examined in the stomach of human and guinea pig by in vitro receptor autoradiography. [125I]SB207710 binding sites were detected in the muscle layer including the myenteric plexus of the stomach from both humans and guinea pigs, although the binding was observed more clearly and densely in the stomach of guinea pigs than humans. Mosapride as well as SB204070 inhibited the binding of [125I]SB207710. Thus, mosapride possesses the ability to bind to 5-HT4 receptors of human stomach and may modulate the motility, as in the case of guinea pig stomach.     

Investigation into the 5-hydroxytryptophan-evoked luminal 5-hydroxytryptamine release from the guinea pig colon

The effect of an endogenous 5-hydroxytryptamine (5-HT) precursor, 5-hydroxytryptophan (5-HTP), on the luminal outflow of 5-HT was examined using the luminally perfused isolated colon of the guinea pig, a model that would facilitate the pharmacological analysis of luminal 5-HT release from enterochromaffin cells (EC cells). 5-HTP (1-10 microM) concentration-dependently caused an increase of the luminal outflow of 5-HT. Either tetrodotoxin (0.3 microM) or atropine (0.2 microM) did not affect the 5-HTP-evoked increase in luminal 5-HT outflow, while the L-type calcium channel blocker, nicardipine (1 microM) or diltiazem (1 microM) reduced the 5-HTP-evoked 5-HT outflow by 47% and 61%, respectively. SB203186 (1 microM), a 5-HT4-receptor antagonist, enhanced the 5-HTP-evoked 5-HT outflow, while ramosetron (1 microM), a 5-HT3-receptor antagonist reduced the stimulating effect of 5-HTP by 66%. Ketanserin (0.1 microM), a 5-HT2A-receptor antagonist did not modify the stimulatory effect of 5-HTP. It is concluded that in the guinea pig colon, 5-HTP facilitates the luminal 5-HT release from EC cells, with no involvement of neuronal mechanisms and a non-neuronal cholinergic system. Furthermore, non-neuronal 5-HT3 and 5-HT4 receptors appear to contribute to the regulation of the luminal 5-HT release evoked by 5-HTP. This new bioassay of the guinea pig colon allows the pharmacological characterization of uncomplicated luminal 5-HT release from EC cells.     

A role for endogenous peptide YY in tachykinin NK2 receptor-triggered 5-HT release from guinea pig isolated colonic mucosa

BACKGROUND AND PURPOSE

The colon-derived peptide hormone, peptide YY (PYY), regulates colonic motility, secretion and postprandial satiety; but little is known about the influence of endogenous PYY on 5-HT release from colonic mucosa. Tachykinin NK₂ receptor-selective agonist, βAla-NKA-(4-10) induces 5-HT release from guinea pig colonic mucosa via NK₂ receptors on the mucosal layer. The present study was designed to determine the influence of endogenous PYY on 5-HT release from guinea pig colonic mucosa, evoked by the NK₂ receptor agonist, βAla-NKA-(4-10).

EXPERIMENTAL APPROACH

Muscle layer-free mucosal preparations of guinea pig colon were incubated in vitro and the outflow of PYY or 5-HT and its metabolite, 5-HIAA, from these preparations were determined by enzyme immunoassays or HPLC with electrochemical detection respectively.

KEY RESULTS

βAla-NKA-(4-10) produced a tetrodotoxin-resistant sustained increase in the outflow of PYY and 5-HT from the mucosal preparations. The βAla-NKA-(4-10)-evoked 5-HT outflow was partially inhibited by Y₁ receptor antagonist, BIBO3304, and Y₂ receptor antagonist, BIIE0246, but with less potency. Exogenously-applied PYY also produced a sustained increase in the outflow of 5-HT that was inhibited by Y₁ blockade but not Y₂ blockade.

CONCLUSION AND IMPLICATIONS

Our findings support the view that the NK₂ receptor-selective agonist, βAla-NKA-(4-10) produces a long-lasting PYY release from guinea pig colonic mucosa via NK₂ receptors on L cells and βAla-NKA-(4-10)-evoked 5-HT release is in part mediated by endogenously released PYY, acting mainly on Y₁ receptors on EC cells. The PYY-containing L cells appear to play a role in controlling the release of 5-HT from colonic EC cells.     

5-HT(1B/D) receptor agonist, SKF99101H, induces locomotor hyperactivity in the guinea pig

Previous studies in guinea pigs have shown that while a serotonin 5-HT(1B/D) receptor agonist, GR46611, does not induce locomotor activation when given alone, it markedly enhances the locomotor response to selective 5-HT(1A) receptor agonists, 8-OH-DPAT and buspirone. In these studies, we found that another 5-HT(1B/D) agonist, 3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate (SKF99101H), significantly elevated locomotor activity in guinea pigs when given alone. We assessed the relative contribution of 5-HT1(1A) and 5-HT(1B/D) receptors in the mediation of this effect.Activity was measured by photobeam interrupts in opaque Perspex cylinders linked to a computer. SKF99101H (20 mg/kg s. c.) significantly increased the locomotor activity in guinea pigs. The locomotor stimulant effect of SKF99101H (20 mg/kg s.c) was reversed by the selective 5-HT(1B/D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl -1,2,4-oxadiazol-3-yl)[1,1biphenyl]4-carboxamide (GR127935; 0.06-0. 25 mg/kg s.c.). The 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY100635; 0.05-0.25 mg/kg s.c.), slightly but significantly attenuated the hyperactivity induced by SKF99101H. These findings suggest that 5-HT(1B/D) receptor agonists may require concomitant activation of 5-HT(1A) receptors to induce locomotor activity in guinea pigs. The 5-HT(2A) receptor antagonist 6[2-[4-[bis(4-fluorophenyl)methylene]-1-piperidinyl]-ethyl]-7-methyl- 5H-thiazol[3,2-a]pyrimidin-5-one (ritanserin) had no effect on SKF99101H-induced hyperactivity, suggesting that these receptors are not involved in the mediation of SKF99101H-induced hyperactivity. SKF99101H-induced hyperactivity was significantly attenuated by the D(1) dopamine receptor antagonist SCH 23390 (0.005-025 mg/kg), but not by the D(2) dopamine receptor antagonist raclopride (0.25-1.0 mg/kg), possibly suggesting the selective involvement of D(1) dopaminergic receptors in the mediation of the stimulant actions of the 5-HT(1B/D) agonist.     

Do 5-HT4 Receptors Mediate the Intestinal Secretory Response to 5-HT in Rat In-vivo?

The involvement of the recently characterized 5-HT₄ receptor in the actions of 5-hydroxytryptamine (5-HT) on jejunal, ileal and colonic electrogenic ion secretion was investigated in the rat in-vivo. 5-HT and the 5-HT₁-, 5-HT₂- and 5-HT₄-receptor agonist 5-methoxytryptamine (5-MeOT), induced a rise in transintestinal PD in all regions of the gut. However, the 5-HT₄-receptor agonists renzapride and cisapride had no effect. Furthermore, the 5-HT₄-receptor antagonists SDZ 205–557 (2-diethylaminoethyl-[2-methoxy-4-amino-5-chloro] benzoate), tropisetron and SB 204070 ([1-butyl-4-piperidinylmethyl]-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate hydrochloride) did not affect the secretory response to either 5-HT or 5-MeOT in the jejunum, but did cause a small inhibition in the ileum and colon. It is concluded that 5-HT₄ receptors do not make a contribution to the electrically monitored 5-HT intestinal secretory response in the rat jejunum in-vivo, but may play a small role in the ileum and colon.     

Melatonin inhibits tachykinin NK₂ receptor-triggered 5-HT release from guinea pig isolated colonic mucosa

BACKGROUND AND PURPOSE

Melatonin is involved in the regulation of colonic motility, and sensation, but little is known about the influence of melatonin on 5-hydroxytryptamine (5-HT) release from colonic mucosa. A tachykinin NK₂ receptor-selective agonist, [β-Ala⁸]-neurokinin A_4-10[βAla-NKA-(4-10)] can induce 5-HT release from guinea pig colonic mucosa via NK₂ receptors on the mucosal layer. The present study was designed to determine the influence of melatonin on 5-HT release from guinea pig colonic mucosa, evoked by the NK₂ receptor agonist, βAla-NKA-(4-10).

EXPERIMENTAL APPROACH

The effect of melatonin was investigated on the outflow of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) from muscle layer-free mucosal preparations of guinea pig colon, using high-performance liquid chromatography with electrochemical detection.

KEY RESULTS

Melatonin caused a sustained decline in the βAla-NKA-(4-10)-evoked 5-HT outflow from the muscle layer-free mucosal preparations, but failed to affect its metabolite 5-HIAA outflow. The specific MT₃ receptor agonist, 5-methoxycarbonylamino-N-acetyltryptamine mimicked the inhibitory effect of melatonin on βAla-NKA-(4-10)-evoked 5-HT outflow. A MT₃ receptor antagonist prazosin shifted the concentration-response curve of melatonin to the right in a concentration-dependent manner and depressed the maximum effect, but neither a combined MT₁/MT₂ receptor antagonist luzindole, nor a MT₂ receptor antagonist N-pentanoyl-2-benzyltryptamine modified the concentration–response curve to melatonin.

CONCLUSIONS AND IMPLICATIONS

Melatonin inhibits NK₂ receptor-triggered 5-HT release from guinea pig colonic mucosa by acting at a MT₃ melatonin receptor located directly on the mucosal layer, without affecting 5-HT degradation processes. Possible contributions of MT₁/MT₂ melatonin receptors to the inhibitory effect of melatonin appear to be negligible. Melatonin may act as a modulator of excess 5-HT release from colonic mucosa.     

Prokinetic benzamides stimulate peristaltic activity in the isolated guinea pig ileum by activation of 5-HT4 receptors.

Substituted benzamides such as metoclopramide, cisapride, zacopride, renzapride or BRL 20627, stimulate intestinal motility in various species. As they are antagonists at 5-HT3 and agonists at 5-HT4 receptors and as both mechanisms could potentially contribute to their gastrointestinal prokinetic effect, the underlying mechanism is unclear. To clarify this, the effect of some substituted benzamides on gut motility was investigated in the isolated guinea pig ileum using the Trendelenburg technique, in which pressure-induced peristaltic contractions are measured. All benzamides stimulated the peristaltic reflex with the rank order of potency: renzapride greater than cisapride greater than BRL 20627 greater than (+/-)-zacopride greater than metoclopramide. ICS 205-930, granisetron and 2-methyl-5-HT did not change the peristaltic response. 5-HT and 5-methoxytryptamine potently mimicked the effect of the benzamides. The effect of 5-HT was not blocked by ICS 205-930 (10(-7) M). These results indicate that the Trendelenburg preparation is suitable for the investigation of intestinal prokinetic effects of the substituted benzamides. Furthermore, the results suggest that the intestinal effect of benzamides results from activation of 5-HT4 receptors rather than from blockade of 5-HT3 receptors.     

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.     

Identification of SK-951, a novel benzofuran derivative, as an agonist to 5-HT4 receptors

The pharmacological profile of SK-951 ((-)4-amino-N-[2-(1-azabicyclo[3.3.0]octan-5-yl) ethyl]-5-chloro-2,3-dihydro-2-methylbenzo[b]furan-7-carboxamide hemifumarate) was identified in relation to serotonin 5-HT3 and 5-HT4 receptors by the receptor binding assay and functional studies. The receptor binding assay showed that SK-951 bound to the 5-HT3 receptor with a high affinity, to the 5-HT4 receptor with relatively higher affinity and to the muscarinic M2 receptor with a low affinity, but not to dopamine D1 and D2 and serotonin 5-HT1 and 5-HT2 and muscarinic M1 and M3 receptors. SK-951 caused relaxations of tunica muscularis mucosae preparations from rat esophagus which were precontracted with carbachol, and the effects were antagonized by GR113808, a selective 5-HT4 antagonist. In the longitudinal muscle with myenteric plexus (LMMP) preparations from guinea pig ileum, SK-951 enhanced the electrically-stimulated contraction of preparations in which the 5-HT1, 5-HT2 and 5-HT3 receptors were blocked, and it enhanced the electrically-stimulated release of [3H]acetylcholine (ACh). These effects of SK-951 were antagonized by GR113808. SK-951 inhibited the 5-HT3 receptor-mediated contractions. These results indicate that SK-951 possesses properties of an agonist for the 5-HT4 receptor and an antagonist for the 5-HT3 receptor. Thus, SK-951 is a new and potent 5-HT4-receptor agonist and causes contractions of guinea pig ileum mediated by enhancement of ACh release via the 5-HT4 receptor.     

The 5-HT4 receptor agonist, tegaserod, is a potent 5-HT2B receptor antagonist in vitro and in vivo

1 Tegaserod (Zelnorm) is a potent 5-hydroxytryptamine4 (5-HT4) receptor agonist with clinical efficacy in disorders associated with reduced gastrointestinal motility and transit. The present study investigated the interaction of tegaserod with 5-HT2 receptors, and compared its potency in this respect to its 5-HT4 receptor agonist activity. 2 Tegaserod had significant binding affinity for human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi=7.5, 8.4 and 7.0, respectively). The 5-HT2B receptor-binding affinity of tegaserod was identical to that at human recombinant 5-HT4(c) receptors (mean pKi=8.4) in human embryonic kidney-293 (HEK-293) cells stably transfected with the human 5-HT4(c) receptor. 3 Tegaserod (0.1-3 microm) inhibited 5-HT-mediated contraction of the rat isolated stomach fundus potently (pA2=8.3), consistent with 5-HT(2B) receptor antagonist activity. Tegaserod produced, with similar potency, an elevation of adenosine 3',5' cyclic monophosphate in HEK-293 cells stably transfected with the human 5-HT4(c) receptor (mean pEC50=8.6), as well as 5-HT4) receptor-mediated relaxation of the rat isolated oesophagus (mean pEC50=8.2) and contraction of the guinea-pig isolated colon (mean pEC50=8.3). 4 Following subcutaneous administration, tegaserod (0.3 or 1 mg kg(-1)) inhibited contractions of the stomach fundus in anaesthetized rats in response to intravenous dosing of alpha-methyl 5-HT (0.03 mg kg(-1)) and BW 723C86 (0.3 mg kg(-1)), selective 5-HT2B receptor agonists. At similar doses, tegaserod (1 and 3 mg kg(-1) subcutaneously) evoked a 5-HT4 receptor-mediated increase in colonic transit in conscious guinea-pigs. 5 The data from this study indicate that tegaserod antagonizes 5-HT2B receptors at concentrations similar to those that activate 5-HT4 receptors. It remains to be determined whether this 5-HT2B receptor antagonist activity of tegaserod contributes to its clinical profile.     

KDR-5169, a new gastrointestinal prokinetic agent, enhances gastric contractile and emptying activities in dogs and rats.

KDR-5169, 4-amino-5-chloro-N-[1-(3-fluoro-4-methoxybenzyl)piperidin-4-yl]-2-(2-hydroxyethoxy)benzamide hydrochloride dihydrate, is a new prokinetic with a dual action, i.e., stimulation of the 5-HT4 receptor and antagonism of the dopamine D2 receptor. In this study, we determined in vitro activities of KDR-5169 towards both receptors and demonstrated the effect of the compound on gastrointestinal motor activity in conscious dogs and rats. In dogs, intravenous KDR-5169 stimulated upper gastrointestinal motility in the fasting state and also eliminated the depressive effect of 3,4-dihydroxyphenylalanine (L-DOPA) on this motility in the postprandial state. The effect of KDR-5169 on gastric emptying was further characterized by the use of three rat gastroparesis models (dopamine D2 receptor agonist (quinpirol)-, abdominal surgery-, or combined-situation-induced). Domperidone (a dopamine D2 receptor antagonist) was effective in the quinpirol-delay and combination-delay models, and cisapride and mosapride (5-HT4 receptor agonists) were effective in the surgery-delay model. Only KDR-5169 eliminated the delay of gastric emptying in all three models. In addition, KDR-5169 accelerated emptying to above the normal level in the combination-delay model. These results suggest that KDR-5169 would be effective in various types of gastric ileus caused by different mechanisms.     

GR46611 potentiates 5-HT1A receptor-mediated locomotor activity in the guinea pig.

5-HT(1B/D) receptor agonists such as GR46611 (3-[3-(2-Dimethylaminoethyl)-H-indol-5-yl]-N-(4-methoxybenzyl)acrylamide ) are known to lower body temperature in guinea pigs. Although stimulation of their functional analogs in rats, the 5-HT1B receptor induces hyperlocomotion, this effect has yet to be demonstrated with 5-HT(1B/D) receptor agonists in the guinea pig. Previous studies have shown that 5-HT1A agonists increase locomotor activity in guinea pigs. The current study set out to examine the effects of 5-HT(1B/D) receptor stimulation on locomotor activity in the guinea pig and to examine the interaction between 5-HT1A and 5-HT(1B/D) receptor stimulation on locomotor activity in that species. The full agonist at 5-HT1A receptors, 8-OH-DPAT (R(+)-8-Hydroxy-dipropylaminotetralin HBr) dose-dependently increased locomotor activity in guinea pigs (0.3-1.25 mg kg(-1) s.c.), as to a lesser extent, did the partial agonist, buspirone (8-[4-[4-(2-Pyramidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5 ]decane-7,9-dione HCl) (5.0-20.0 mg kg(-1) s.c.). The 5-HT(1B/D) receptor agonist GR46611 had no effect on locomotor activity in guinea pigs at doses up to 40 mg kg(-1) s.c. 8-OH-DPAT-induced behavioural activation was reversed by the selective 5-HT1A receptor antagonist WAY100635 (N-[-2-[4-(-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyrinidyl) cyclo hexanocarboxamide trihydro-chloride), with a minimum effective dose of 0.006 mg kg(-1), but not by the 5-HT(1B/D) receptor antagonist GR127935 (2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyli c acid [4-methoxy-3-(4-methyl-piperazin-1-yl)phenyl]-amide) (0.25-1.0 mg kg(-1)). GR46611, at doses that were without effect given alone (0.5-2.5 mg kg(-1)), significantly enhanced the locomotor response to subthreshold doses of 8-OH-DPAT (0.5 mg kg(-1)) and buspirone (10 mg kg(-1)). The effect of GR46611 on 8-OH-DPAT-induced hyperactivity was reversed by pretreatment with GR127935 and with WAY 100635 indicating that activation of both receptors was required for the expression of locomotor hyperactivity. These findings suggest that activation of 5-HT(1B/D) receptors alone may not stimulate locomotor activity but it does potentiate the locomotion induced by 5-HT1A receptor stimulation in guinea pigs.     

Pharmacological characterization of a novel 5-HT4 receptor agonist, TS-951, in vitro.

The pharmacological effect of a novel selective 5-HT4 receptor agonist, TS-951 (N-[endo-8-(3-hydroxypropyl)-8-azabicyclo[3.2.1]oct-3-yl]-1-isopropyl-2-oxo-1,2-dihydro-3-quinolinecarboxamide) was investigated in vitro. TS-951 potently inhibited specific [3H]GR113808 binding both in guinea-pig striatum and in mouse brain. The affinity of TS-951 for the 5-HT4 receptor was higher than those of other agonists, 5-HT, cisapride, mosapride and renzapride. On the longitudinal muscle of the guinea-pig ileum, TS-951 caused a concentration-dependent increase in the amplitude of electrically induced submaximal twitch contractions. On the longitudinal muscle of the guinea-pig distal colon, TS-951 also caused concentration-dependent contractions. TS-951 is a high-affinity, selective and potent 5-HT4 receptor agonist. This compound therefore can be considered as a useful pharmacological tool for investigating 5-HT4 receptor-mediated events.