Ovarian Responsiveness to Exogenous Gonadotropins During Lactation in the Rat and Monkey

Abstract

During lactation, the normal function of the ovary in the rat and monkey is generally suppressed. Ovarian refractoriness to gonadotropins as a possible causative factor in lactational amenorrhea was tested in the present study. The increase in serum and ovarian steroid levels and the induction of ovulation following pregnant mare serum gonadotropin (PMSG) and ovine luteinizing hormone (LH) supplementation were used for assessing ovarian function. In the rat, the response of the ovary to PMSG was shown to be dependent upon the intensity of the suckling stimulus—the greater the intensity, the lower the responsiveness. Rats suckling a minimal number of pups also showed lowered responsiveness if ovine prolactin were given along with PMSG. Following PMSG and ovine LH treatment, lactating monkeys showed increases in serum estrogen and progesterone levels. The injection of ovine prolactin appeared to reduce ovarian responsiveness to PMSG in terms of estrogen production. In contrast, ovine prolactin given with ovine LH (when administered as an ovulation trigger) appeared to facilitate progesterone production from the primate corpus luteum. These results suggest that during lactation the ovary continues to respond to gonadotropins, although the response is reduced.