Motor response secondary to the release of endogenous serotonin by tryptamine in guinea-pig ileal longitudinal muscle with adherent myenteric plexus.

The mechanism of the contractile action of tryptamine on the longitudinal smooth muscle with adherent myenteric plexus in the guinea-pig ileum was reexamined in view of new evidence that tryptamine releases the enteric neural serotonin (5-HT) and noradrenaline. The desensitization to 5-HT abolished the contractile response to tryptamine (5 and 10 microM). The contractile response to tryptamine (1-5 microM) was progressively decreased by repeated application. The progressive decrease by tryptamine was more pronounced than that by exogenous 5-HT. Tetrodotoxin and atropine significantly decreased the contraction upon the 2nd trial of tryptamine (5 microM) by about 40% and 45% of the control, respectively. However, guanethidine or cocaine did not change the contraction by tryptamine (5 microM). A 5-HT uptake inhibitor, zimelidine, which had been demonstrated to block the release of preloaded [3H]5-HT by tryptamine (Takaki et al., 1985), intensively reduced the contractile response to tryptamine to the same extent as atropine. These results are consistent with the previous finding that tryptamine releases the physiologically active 5-HT from myenteric neurons (Takaki et al., 1985). The 5-HT released by tryptamine, at least in part, acts on myenteric cholinergic neurons, resulting in the muscular contraction.