Direct inhibitory ovarian effects of prolactin in the process of ovulation

The present study was designed to determine the effects of prolactin (PRL) on the process of follicle rupture and oocyte maturation using an in vitro perfused rabbit ovary model. In the first experiment ovaries were perfused for 12 hours with or without PRL at 10(2) or 10(3) ng/ml. Ovulation did not occur in any ovaries in the absence of gonadotropin. The majority of follicular oocytes did not progress beyond the germinal vesicle stage following treatment with PRL. The percentage of follicular oocytes which showed evidence of degeneration was also comparable in both groups. The concentrations of progesterone in the perfusate did not differ significantly between PRL-treated and control ovaries. In the second experiment, ovaries were perfused with or without PRL at 10, 10(2), or 10(3) ng/ml. Thirty minutes later 50 IU of human chorionic gonadotropin (hCG) was added to the perfusate of all ovaries. The addition of PRL to the perfusate inhibited hCG-induced ovulation in a dose-related fashion. The degree of ovum maturity and degeneration was comparable in the two groups. PRL did not affect hCG-stimulated progesterone production by the perfused rabbit ovaries. The present study demonstrates that PRL acts directly on the ovary to influence the process of ovulation, resulting in the inhibition of hCG-induced follicle rupture. These data suggest that PRL inhibits ovulation by mechanism(s) independent of ovarian progesterone synthesis.