Inhibition by cholinergic agonists of the prolactin release induced by morphine

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Abstract
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 β-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 α-methyltyrosine. These results suggest that the central cholinergic system exerts an inhibitory influence on the prolactin release induced by morphine or β-endorphin and the cholinergic inhibition is not mediated via catecholaminergic neurons.