Naloxone-induced contracture of ileum from stressed guinea pigs

The in vitro preparation of guinea-pig ileum taken from animals which had been stressed by swimming for 5 min in water at 20°C responded by a dose-dependent contracture to the narcotic antagonist naloxone. This response was antagonised by atropine (8 × 10 −8 g/ml). Ileum preparations from non-stressed guinea pigs did not respond to naloxone in concentrations up to 1 × 10 −5 g/ml. The possibility that this response is a consequence of the stress-induced release of endogenous opiate peptides is discussed.