beta-Endorphin: cross tolerance to and cross physical dependence on morphine.

Abstract
The effects of .beta.-endorphin on the antinociceptive responses and abrupt withdrawal jumping in morphine-dependent mice were studied. Mice were rendered morphine dependent by implantation of a morphine pellet (75 mg base) for 3 days. The analgesic response to .beta.-endorphin decreased after morphine pellet implantation, as evidenced by an 8-fold increase in the median antinociceptive dose. A similar increase in the median antinociceptive dose of morphine was found. In small doses (0.09-.17 .mu.g/mouse), .beta.-endorphin suppressed abrupt withdrawal jumping. Met[methionine]-enkephalin, even in high doses (200 .mu.g/mouse), did not suppress abrupt withdrawal jumping.