Mechanisms of the contractile responses to morphine of the mouse colon in vitro.

1. Morphine induced a contractile response in the mouse colon which consisted of two phases or components. 2. The first component was dose-related and was inhibited by tetrodotoxin, atropine and naloxone, but was insensible to hexamethonium, propranolol, phentolamine, diphenhydramine and methysergide. 3. The second component of the contractile effect was not modified by atropine, hexamethonium, propranolol, phentolamine, guanethidine or diphenhydramine, but was antagonized by naloxone, tetrodotoxin and serotonin antagonists. 4. Tachyphylaxis was observed only for the first component of the morphine induced contractile response of the mouse colon.