Action of enkephalinergic neurons on the gastrointestinal motility

The actions of the enkephalinergic neurons in the myenteric plexus on the gastrointestinal motility were studied using the opiate antagonist naloxone in the guinea pig in vitro and in vivo. Naloxone increased or decreased spontaneous contractions of the isolated small intestine. Both responses were abolished by atropine. Naloxone potentiated or inhibited the twitch response to transmural stimulation at a frequency of 0.1 Hz and the contractile response of the ileum to the mesenteric nerve stimulation. Naloxone reversed an inhibition of the twitch response occurred after repetitive transmural stimulation (10 Hz) for 5 minutes in the isolated ileal segment. The peristaltic reflex response in the isolated jejunum induced by its distention was reduced by repeated distention. This reduction was reversed by naloxone. Naloxone also potentiated the contractile response of the stomach to efferent vagal stimulation in vivo. It is concluded from the present results that enkephalinergic neurons in the myenteric plexus may regulate the gastrointestinal motility by inhibiting release of acetylcholine from myenteric cholinergic neurons, and that naloxone has both opiate antagonistic and partial agonistic effects on the gastrointestinal motility of the guinea pig.