SEX-DIFFERENCES IN ESTROGEN-RECEPTOR AND PROGESTIN RECEPTOR INDUCTIONIN THE GUINEA-PIG HYPOTHALAMUS AND PREOPTIC AREA

Quantitative in vitro autoradiography was used to determine if regiona l sex differences in estrogen receptor (ER) content and/or estrogen re sponsiveness, as indicated by an increase in progestin receptor (PR), are present in the adult guinea pig brain. Adult male and female guine a pigs were gonadectomized 1 week before subcutaneous injection of 25 mu g estradiol benzoate (EB)/kg body wt or the sesame oil vehicle. Ani mals were killed by decapitation 44 h after injection. Unoccupied PRs, and unoccupied and occupied ERs, were measured in discrete brain regi ons by quantitative in vitro autoradiography using [H-3]R5020 and [H-3 ]estradiol as ligands, respectively. In vehicle-injected controls, a h igher level of ER was found in the arcuate nucleus (ARC), dorsal media l nucleus (DMN) and ventrolateral nucleus (VLN) of females as compared to males. At 44 h after EB injection, 32-55% of the ERs were occupied ; however, EB treatment caused a marked down-regulation of total recep tor (calculated as occupied + unoccupied receptor) in most of the brai n regions examined, including the periventricular preoptic area (PVP), medial preoptic area (MPO), bed nucleus of the stria terminalis, para ventricular nucleus, ARC, ventrolateral hypothalamus (VLH), VLN, and D MN. In EB-treated animals, PR binding was detectable in the PVP, MPO, ARC, VLH, and VLN, with higher levels of binding observed in the PVP, MPG, and VLN of the female as compared to the male. No PR binding was observed in oil-injected control animals. These results demonstrate re gion-specific sex differences in ER as well as estrogen induced regula tion of progestin and ERs in the guinea pig brain. The discordance bet ween the regional distributions of sex differences in ER and estro en- induced PR implies that sex differences in ER and estrogen-induced PR implies that sex differences in estrogen response may not be clearly l inked to a sex difference in receptor number. Instead, sex differences in response may involve differences in receptor number within specifi c subpopulations of estrogen target cells or may involve differences i n ER dynamics.