Inhibition via muscarinic M-1 receptor stimulation of TRH-induced prolactin release in estrogen-primed rats.

The cholinergic mechanism involved in the regulation of prolactin secretion induced by thyrotropin-releasing hormone (TRH) in estrogen-primed male rats was investigated. Intraperitoneal injections of pharmacological doses of TRH (20 mg/kg), MK-771 (2 mg/kg) and DN-1417 (10 mg/kg) increased serum prolactin levels in estrogen-primed male rats, but not in nonprimed male rats. This increase was reduced by pilocarpine, a muscarinic M-1 receptor agonist, but not by carbachol, a M-2 receptor agonist. Pirenzepine, a peripheral M-1 receptor antagonist, did not affect TRH-induced prolactin release, but antagonized the inhibitory effect of pilocarpine. The results indicate that TRH and TRH analogues increase prolactin secretion in the estrogen-primed male rats and that a stimulation of muscarinic M-1 receptors exerts an inhibitory effect on the prolactin release induced by TRH in the estrogen primed male rats.