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

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 eight-fold increase in the median antinociceptive dose of morphine was found. In small doses (0.09-0.17 mug per mouse), beta-endorphin suppressed abrupt withdrawal jumping. Met-enkephalin, even in high doses (200 mug per mouse), did not suppress abrupt withdrawal jumping.