Sex differences in estrogen receptor and progestin receptor induction in the guinea pig hypothalamus and preoptic area.

Quantitative in vitro autoradiography was used to determine if regional sex differences in estrogen receptor (ER) content and/or estrogen responsiveness, as indicated by an increase in progestin receptor (PR), are present in the adult guinea pig brain. Adult male and female guinea pigs were gonadectomized 1 week before subcutaneous injection of 25 micrograms estradiol benzoate (EB)/kg body wt or the sesame oil vehicle. Animals were killed by decapitation 44 h after injection. Unoccupied PRs, and unoccupied and occupied ERs, were measured in discrete brain regions by quantitative in vitro autoradiography using [3H]R5020 and [3H]estradiol as ligands, respectively. In vehicle-injected controls, a higher level of ER was found in the arcuate nucleus (ARC), dorsal medial 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 receptor (calculated as occupied+ unoccupied receptor) in most of the brain regions examined, including the periventricular preoptic area (PVP), medial preoptic area (MPO), bed nucleus of the stria terminalis, paraventricular nucleus, ARC, ventrolateral hypothalamus (VLH), VLN, and DMN. 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, MPO, and VLN of the female as compared to the male. No PR binding was observed in oil-injected control animals. These results demonstrate region-specific sex differences in ER as well as estrogen-induced regulation of progestin and ERs in the guinea pig brain. The discordance between the regional distributions of sex differences in ER and estrogen-induced PR implies that sex differences in ER and estrogen-induced PR implies that sex differences in estrogen response may not be clearly linked to a sex difference in receptor number. Instead, sex differences in response may involve differences in receptor number within specific subpopulations of estrogen target cells or may involve differences in ER dynamics.