The sites of action of 5-hydroxytryptamine in nerve-muscle preparations from the guinea-pig small intestine and colon.

1 The sites of action of 5-hydroxytryptamine (5-HT) were examined in isolated segments of guinea-pig intestine. Mechanical records were taken from the longitudinal muscle of the ileum and proximal colon and from the circular muscle of the ileum and distal colon.2 In order to examine direct actions of 5-HT, nerve-mediated responses were blocked with tetrodotoxin (0.2 mug/ml). There was a gradient in the responsiveness of the longitudinal muscle of the ileum; in the proximal ileum it was usually unresponsive, whereas in the distal ileum about 30% of the amplitude of contraction was caused by a direct effect on the muscle. In the circular muscle from all parts of the ileum, direct effects on the muscle were weak or absent. In the distal colon, the circular muscle was almost always unresponsive to direct effects of 5-HT even when concentrations of 5-HT as great as 100 mug/ml were used. All direct actions of 5-HT on intestinal muscle were blocked by methysergide (1 mug/ml), which itself did not affect nerve-mediated responses.3 Excitatory cholinergic nerves and excitatory and inhibitory nerves which released unidentified substances were all stimulated by 5-HT. The contractions mediated through cholinergic nerves were blocked by hyoscine (0.6 mug/ml).4 Tachyphylaxis to the action of 5-HT occurred both for effects mediated through nerves and for direct effects on the muscle. Responses returned promptly after 5-HT was washed from the organ bath.5 While 5-HT blocked its own action on neural receptors, it did not antagonize the stimulation of nicotinic receptors on cholinergic neurones by 1-1 dimethyl-4-phenylpiperazinium iodide (DMPP). Moreover, pentolinium markedly reduced contractions caused by DMPP without significantly affecting responses to 5-HT. In contrast, (+)-tubocurarine, another nicotinic receptor antagonist, was effective in reducing contractions caused by 5-HT.6 Phenyldiguanide, which has been reported to antagonize the stimulant action of 5-HT on cholinergic neurones in the mouse small intestine, did not cause any significant reduction in the action of 5-HT on cholinergic neurones in the guinea-pig ileum unless a concentration of 1 mg/ml was used. However, contractions elicited by carbachol and DMPP were antagonized to a similar extent by phenyldiguanide at this concentration. Antagonism of the action of 5-HT at neural receptors by bromolysergic acid and by tryptamine was found but it was not specific, these drugs causing comparable decreases in responses to 5-HT, carbachol and DMPP.7 The present results, which show that 5-HT has little or no direct effect on the circular muscle of the ileum and colon, imply that, if 5-HT is a transmitter in enteric reflexes, it must be released from interneurones.