CHARACTERIZATION OF 5-HT RECEPTORS IN RAT PROXIMAL COLON

1. In view of its multiple sites of action, we investigated the activi ty of 5-hydroxytryptamine (5-HT, serotonin) on various potential recep tors in the isolated proximal colon of rats. 2. 5-HT induced concentra tion dependent contractions of colonic strips (pEC(50)=7.54+/-0.12), 3 . The 5-HT1 receptor agonist, 5-carboxamidotryptamine, induced concent ration dependent con tractions (pEC(50)=5.93+/-0.27); however, neither the 5-HT3 receptor-agonist, phenylbiguanide, nor the 5-HT4 receptor-a gonist, renzapride, caused contractions at concentrations as high as 1 0(-4) M. 4. The 5-HT1/2 receptor antagonist, methiothepin, caused conc entraction-dependent nonsurmountable antagonism. The 5-HT3 receptor an tagonist, tropisetron, inhibited the contractions to a concentration o f 5-HT greater than or equal to 10(-6) M. Ketanserin had no effect on responses to 5-HT. 5. Tetrodotoxin and atropine had no effect on respo nses to 5-HT. 6. We conclude that contractions to 5-HT are mediated by 5-HT1-like and probably 5-HT3 receptors that activate tetrodotoxin in sensitive mechanisms. (C) 1998 Elsevier Science Inc.