Antagonism of estrogen-induced prolactin release by progesterone.

Previous work from our laboratory has shown that during the process of nuclear occupancy of the progesterone receptor complex (1-2 h), nuclear estradiol receptors of the anterior pituitary are depleted. The purpose of this study was to determine whether the depletion of nuclear estradiol receptors by progesterone had functional biological significance. The ovariectomized (26 days of age) immature rat was used as the model for analysis of this question. The ability of estradiol to release prolactin from the anterior pituitary was the function chosen to determine the biological significance of the progesterone and estradiol interactions. In response to estradiol exposure (2 JIg/rat), prolactin release reached peak values from 8 h to 12 h and returned to control levels by 24 h. A second injection of estradiol 13 h after the initial injection stimulated a second increase in serum prolactin at 25 h. This model of two injections of estradiol 13 h apart served to provide adequate levels of anterior pituitary progesterone receptors and elevated serum prolactin levels upon which superimposed progestin modulation could be examined. A single injection of progesterone (0.8 mg/kg BW) 1 h before the second estradiol injection blocked the increase in serum prolactin. This action was a receptor-mediated event because progesterone had no effect without estrogen priming or when the progesterone antagonist RU486 was used. Finally, when the interval between the progesterone and second estradiol injection was extended to 4 h, a time period when progesterone does not deplete pituitary nuclear estrogen receptors, the estrogen-induced increase in serum prolactin was not blocked. In conclusion, progesterone inhibition of estrogen action by depletion of nuclear estrogen receptors from the anterior pituitary could prove to be an important mechanism of progesterone action within the pituitary. Other actions of estrogen and progesterone on prolactin secretion could be via the modulation of hypothalamic dopamine and or endorphins but these were not investigated in this study.