Abstract
The action of 5-hydroxytryptamine (5-HT) in the rat ileum was analysed pharmacologically. In the isolated rat ileum, either mono or biphasic contraction was induced depending on the concentrations of 5-HT. The former was induced by lower concentrations of 5-HT (<6.25 × 10-7M) and the latter by higher concentrations of 5-HT (>2.5 × 10-6M). The monophasic contraction and the tonic contraction in response to 5-HT were blocked by methysergide (MTG). The phasic contraction in response to 5-HT after inhibition of muscular receptor with MTG was potentiated by physostigmine and blocked by mecamylamine (MC) or tetrodotoxin (TTX). The response to 5-HT changed from a contraction to a relaxation by preincubation with MTG plus hyoscine. The relaxation was inhibited by TTX but not by MC and such was provoked even in the ileum from rats treated with 6-hydroxydopamine. Therefore, the inhibitory neuronal receptor for 5-HT does not appear to belong to the adrenergic nervous system. At lower concentrations of 5-HT, relaxation was induced with MTG which was only partially inhibited by TTX. These results indicate that the stimulatory responses to 5-HT were caused by activations of muscular receptors and of intramural parasympathetic ganglion cells while inhibitory responses were caused by activations of non-adrenergic inhibitory nerve terminals and of non-neuronal elements.
