Inhibition by cholinergic agonists of the prolactin release induced by morphine.

The cholinergic agonists, pilocarpine, physostigmine and nicotine, inhibited the prolactin release induced by morphine in male rats in vivo. Pilocarpine also inhibited the release of prolactin induced by beta-endorphin or metoclopramide without affecting the basal and haloperidol-stimulated serum prolactin levels. The inhibitory effect of pilocarpine on the morphine-stimulated release of prolactin was antagonized by concurrent administration of atropine but not by atropine methylnitrate or by mecamylamine, while the inhibition by nicotine was antagonized by mecanylamine but not by atropine. The stimulation of prolactin release by morphine and its reversal by pilocarpine were observed after the administration of haloperidol or alpha-methyltyrosine. These results suggest that the central cholinergic system exertes an inhibitory influence on the prolactin release induced by morphine or beta-endorphin and the cholinergic inhibition is not mediated via catecholaminergic neurons.