Evidence that 5-hydroxytryptamine does not mediate GABA-induced contractile responses in the guinea-pig proximal ileum.

Segments of the isolated proximal ileum of the guinea-pig were set up in the organ bath to record longitudinal muscle contractions. Both gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine induced neurally mediated cholinergic contractile responses; desensitization to 5-HT did not consistently depress contractile responses to GABA, whilst quipazine antagonized the neuronal responses induced by 5-HT without affecting responses to GABA. It is thus concluded that 5-HT does not mediate GABA-induced responses in the ileum.