Abstract

We investigated the effects of various selective 5-hydroxytryptamine (5-HT)₃ receptor antagonists, including GK-128 [2-[(2-methylimidazol-1-yl)methyl]benzo[f]thiochromen-1-one monohydrochloride hemihydrate], on colonic function. In conscious rats, 5-HT and a 5-HT₃ receptor agonist, 2-methyl-5-HT, dose-dependently increased fecal pellet output, but another 5-HT₃ receptor agonist,m-chlorophenylbiguanide, did not affect output. The selective 5-HT₃ receptor antagonists GK-128, granisetron, ramosetron, azasetron and ondansetron depressed the increase in fecal pellet output caused by 2-methyl-5-HT and by wrap-restraint stress. However, the rank order of potency of antagonists in the two defecation models was not consistent with that for the von Bezold-Jarisch reflex. Although granisetron and ramosetron dose-dependently reduced the spontaneous excretion of fecal pellets, GK-128 did not affect it. These results suggest that GK-128 may be used for the treatment of stress-induced gastrointestinal dysfunction. Furthermore, the present results suggest that the 5-HT₃ receptor involved in colonic motility may be different from the classically defined 5-HT₃ receptor and/or that the regulation of colonic motility mediated by 5-HT₃ receptors during stress may be different from normal physiological conditions.