Opioids modulate intestinal peristalsis at a site of action additional to that modulating acetylcholine release

The nature of the interaction between cholinergic and opioid mechanisms controlling peristalsis in the intact segment of the guinea-pig ileum was examined. The induction of rhythmic peristaltic activity by acetylcholine was dose dependently inhibited and facilitated by preapplication of, respectively, normorphine and naloxone. The action of intestinal opioids occurs, at least partially, at a site of action differing from that modulating the release of acetylcholine from the myenteric plexus. Interruption of the peristaltic reflex by hexamethonium, atropine, tetrodotoxin or desensitization of the segments to serotonin also strongly impaired or abolished the initiation of circular muscle contractions produced by naloxone. Therefore, no indication could be found for a direct action of naloxone upon the circular muscle. Rather, intestinal opioids appear to be involved in regulating the activity of the reflex arc.