OPIOID PATHWAYS EXERT A TONIC RESTRAINT IN THE GUINEA-PIG ISOLATED COLON - CHANGES AFTER CHRONIC SYMPATHETIC DENERVATION

We have studied the effects of naloxone on acetylcholine and noradrena line release in the guinea-pig isolated distal colon, and have assesse d the effect of naloxone on electrically-induced contraction of the lo ngitudinal muscle and non-adrenergic, non-cholinergic (NANC) relaxatio ns of the circular muscle coat. Naloxone dose-dependently increased re sting and electrically-evoked acetylcholine release and electrically-e voked noradrenaline release. Naloxone was more potent in increasing re sting acetylcholine release in colonic specimens obtained after chroni c sympathetic denervation. Naloxone (1 mum) did not affect electricall y-induced contractions of the longitudinal muscle, while it enhanced N ANC relaxations of the circular muscle. The effects observed with nalo xone in the present experiments suggest that opioid pathways exert a t onic restraint on neurotransmission in the guinea-pig colon. After sup pression of the adrenergic inhibitory tone, the functional relevance o f opioid pathways seems to be increased.