Catecholamines and the actions of morphine on the guinea-pig ileum.

The effects of catecholamines in modifying the response to some analgesic drugs of the guinea-pig ileum, electrically stimulated in vitro, were studied. Pre-treatment with reserpine reduced the potency of morphine in depressing twitches of the preparation. Noradrenaline and especially dopamine mimicked with stricking similarity the effects of morphine in producing 'chronic tolerance' and stimulating twitch height at high concentrations, and reversed 'acute tolerance' produced by repeated administration of 10 ng/ml of morphine. No phase of the response of the preparation to morphine was affected by phentolamine and propranolol, haloperidol or droperidol. The efflux of catecholamines from the preparations was depressed by morphine but output in the various phases of the response did not differ significantly from each other.