Prolactin inhibits pituitary luteinizing hormone-releasing hormone receptors in the

Pituitary LHRH receptors were studied in intact immature and mature female rats as well as in adult castrated animals of both sexes under experimental conditions leading to marked alterations of plasma PRL levels. Implantation of three anterior pituitary glands under the kidney capsule on the morning of estrus prevented the increase in pituitary LHRH receptor content measured 3 days later on the morning of expected proestrus. Pituitary implants slightly decreased pituitary LHRH receptor levels in ovariectomized adult rats while simultaneous treatment with 17β-estradiol (E2; 1 μg daily) markedly potentiated the inhibitory effect of pituitary implants and led to much higher plasma PRL concentrations. Although pituitary implants alone had no effect on plasma gonadotropin levels, they potentiated the inhibitory effect of E2 on both plasma LH and FSH on both plasma LH and FSH on both plasma LH and FSH levels. Treatment with the dopaminergic agonist 2α-bromoergocryptine (CB-154) completely reversed the inhibitory effect of pituitary implants on pituitary LHRH receptor levels and plasma gonadotropin concentration. A marked loss of pituitary LHRH receptors was also found after treatment with a high dose (20 μg daily) of E2 in castrated male rats, this effect also being reversible by simultaneous treatment with CB-154. Hyperprolactinemia induced by treatment with ovine PRL or sulpiride in immature female rats decreased pituitary LHRH receptor content and reversed the stimulatory effect of treatment with an LHRH agonist on the same parameter. The present data clearly show that hyperprolactinemia exerts inhibitory effects on pituitary LHRH receptor levels and basal gonadotropin secretion. Although a pituitary site of action of PRL is not excluded, it is tempting to suggest that the above described inhibitory effects on pituitary LHRH receptors as well as on gonadotropin secretion are mediated by PRL-induced changes of pulsatile hypothalamic LHRH secretion.