Melatonin stimulation of prolactin secretion in male rats: Influence of pinealectomy, castration or gonadal steroid pretreatment

Intravenous administration of melatonin stimulates plasma prolactin (PRL) secretion in untreated or estrogen-progesterone, urethane-anesthetized adult male rats. Melatonin failed to elicit the plasma PRL response either in rats which had been previously primed for 3 days with either estrogen or progesterone alone or in castrated rats. Replacement treatment with sc injections of testosterone propionate in castrated rats restored the PRL response to melatonin injections. Since PRL levels in pinealectomized rats rose after administration of melatonin, it is thought that the PRL-releasing effect of the indole is not immediately and directly via an action of melatonin on the pineal.