In vivo progesterone treatment enhances [3H]estradiol retention by neural tissue of the female rat.

[3H]Estradiol retention was examined for neural tissues of ovariectomized rats as a function of various progesterone pretreatments. Short-term progesterone pretreatment (6 or 24 h) with a 50 mg pellet of progesteron sc resulted in increased in vivo retention of [3H]estradiol when measured at 6 or 24 h following withdrawal of the progesterone source. This increase was greatest in arcuate-median eminence tissue. A less pronounced increase was seen in the preoptic-anterior hypothalamus and cerebral cortex, and no increase was seen in the amygdala or mammilary bodies. Following long term progesterone pretreatment increased [3H]estradiol retention was observed in vivo at 48, 72, 96 or 120 h in the continued presence of the progesterone source for arcuate-median eminence, preoptic-anterior hypothalamus, amygdala and cerebral cortex. Also, when progesterone pretreatment was 24 h, [3H]estradiol retention was still increased at 36, 48, 60 and 72 h following removal of the progesterone source for arcuate-median eminence, preoptic-anterior hypothalamus, amygdala and cerebral cortex. Our demonstration that progesterone pretreatment can significantly increase [3H]estradiol retention by neural tissue suggests a possible mechanism by which progesterone can regulate the timing of ovulation and sex behavior. Furthermore, our observations are in keeping with the finding that progesterone has little or no effect at the organismal or tissue level unless estrogen is present.