Interaction of myenteric neurons and extrinsic nerves in the intestinal inhibitory response induced by mesenteric nerve stimulation.

Effects of the mesenteric nerve stimulation (MNS) on the twitch contraction induced by field stimulation were investigated regarding the relationship between myenteric neurons and extrinsic cholinergic nerves in the guinea-pig mesenteric nerve-ileal preparation. The twitch contraction was inhibited after MNS. The inhibition of the twitch contraction after MNS was induced twice, just after MNS (1st inhibition) and 2-3 min later (2nd inhibition) (type I), or once, just after MNS (1st inhibition) (type II), in recovery course of twitch contraction for 6-8 min. The 1st inhibition was slightly decreased by guanethidine and hexamethonium. The inhibitory response (1st inhibition) in both types I and II was recovered to the control level by pretreatment with naloxone (recovered twitch contraction), but the late inhibitory response (2nd inhibition) was markedly observed after 2-3 min in types I and II. Either the 1st or the 2nd inhibition was not altered by capsaicin, desensitization to calcitonin gene-related polypeptide (CGRP), vasoactive intestinal polypeptide (VIP), somatostatin, or galanin. The recovered twitch contraction in types I and II was decreased by CGRP-desensitization, or capsaicin. These results suggest that the first inhibitory response was induced by enteric opioid neurons connected with extrinsic cholinergic nerves, but the 2nd inhibition was induced by unknown substances other than CGRP, VIP, somatostatin, and galanin. The twitch contraction may partly be induced by endogenous neurokinin-like substances. And, some CGRP containing neurons, which connect with extrinsic cholinergic nerves, probably activate the intrinsic excitatory neurons.