Investigation of the 5-HT receptor mediating relaxation in guinea-pig proximal colon

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Abstract
5-Hydroxytryptamine (5-HT) induced concentration-related relaxations (EC50 = 9·1 μm) of guinea-pig proximal colon pretreated with ketanserin (1 μm) and ondansetron (10 μm). This 5-HT-induced effect was neuronally mediated since it was blocked by tetrodotoxin (0·3 μm). 5-Carboxamidotryptamine was a full agonist and ten times more potent than 5-HT. α-Methyl-5-HT was a partial agonist. 2-Methyl-5-HT was without effect. Methysergide and metergoline were antagonists of 5-HT producing parallel shifts at 0·1 μm but unsurmountable antagonism at higher concentrations. pKB values of 8·0 and 7·3 were calculated for methysergide and metergoline, respectively. This study has identified a 5-HT-induced relaxation of guinea-pig proximal colon which is mediated via a neuronal 5-HT1-like receptor. However, the subtype has yet to be established.