Partial agonistic action of morphine in the rat vas deferens

Effects of morphine on the force of contraction of rat vas deferens were investigated. Morphine and beta-endorphin decreased the electrically evoked twitch tension, in a dose dependent manner. The inhibitory effect of morphine, however, was much weaker than that of beta-endorphin. These effects of both morphine and beta-endorphin were completely antagonized by naloxone. In the presence of 30 microM morphine, the dose-response curve of beta-endorphin shifted to the right by about 10-fold. Moreover, morphine partly reversed the contraction depressed by 0.3 microM beta-endorphin, in a dose dependent manner. These findings suggest that morphine acts as a partial agonist on the rat vas deferens. Marked tolerance to beta-endorphin and change in the antagonist potency of morphine were not observed in the vas deferens isolated from morphine-dependent rats.