A possible direct effect of prolactin on follicular activity

To examine whether high serum prolactin levels inhibit follicular maturation, prolactin was injected during diestrus of intact female rats or endogenous prolactin levels were raised by applying a suckling stimulus. Injections of rat prolactin (100 micrograms per injection) given 2 and 1 days before proestrus resulted in a lower estradiol production by proestrous follicles during a 4-h incubation period than follicles isolated from control rats. In 4 out of 7 animals this occurred without a change in serum progesterone and luteinizing hormone (LH) concentrations. In the 3 remaining animals corpus luteum function was activated. In these animals serum LH concentrations were decreased and follicular estradiol production was further suppressed. To study follicular development in the presence of suckling-induced hyperprolactinemia, the following experiment was performed. Removal of a 5-pup litter at Day 13 (0900 h) of lactation (Day 1 = day of parturition) resulted in ovulation at Day 16. Replacement of a new litter 24 h after litter removal did not interfere with ovulation on Day 16. This procedure allowed the study of follicular development between Days 14 and 15 in the presence of raised serum prolactin levels. It appeared that this treatment did not affect follicular growth, but in vitro estradiol production by preovulatory follicles isolated at Day 15 was lower than in follicles isolated from nonlactating animals. In 3 out of 13 animals corpus luteum function was reactivated. In these animals LH levels and follicular estradiol production were significantly suppressed. Treatment with bromocriptine (1 mg per injection) on Days 13 and 14, in addition to litter replacement, restored the high estradiol production at Day 15 without affecting serum LH concentrations. The results of this study demonstrate that in the presence of high prolactin levels, follicular estradiol production is low. The inverse relation between prolactin and follicular estradiol production in the presence of unchanged serum LH levels suggests that prolactin can have a direct action on estrogen biosynthesis of follicle cells.