Inhibition of in vivo pituitary release of luteinizing hormone in lactating rats by exogenous prolactin.

The mechanism whereby the reciprocal relationship between the plasma levels of prolactin and HL is maintained in lactating rats under different degrees of suckling stimulus has been investigated in the present study. Plasma levels of luteinizing hormone (LH) in lactating rats suckling two pups could be reduced significantly by injecting prolactin (PRL). This reduction was also evident in ovariectomized and ovariectomized-adrenalectomized lactating rats, thus excluding mediation of the inhibitory effect by steroids from end-organs. The in vivo response of the pituitary to exogenous LHRH was lower in rats suckling eight pups than those suckling two pups. Prolactin administered prior to LHRH caused an inhibition of the response of the pituitary to exogenous LHRH in rats suckling two pups. These data provide evidence for the hypothesis that in the lactating rat, in the presence of minimal suckling stimulus, the causative factor for reducing serum LH levels is prolactin, which acts by altering the pituitary responsiveness to LHRH.