Distribution of occupied and unoccupied estrogen receptors in the rat brain: effects of physiological gonadal steroid exposure.

In vitro autoradiographic methods have been developed for selective measurement of occupied and unoccupied estrogen receptors (ERs) in brain tissue sections. Addition of protamine sulfate traps unoccupied ERs in the tissue sections, allowing them to be detected after a short period of incubation with labeled estrogen. Occupied ERs are assessed, after washing in buffer without protamine to eliminate unoccupied receptor, by incubating the sections for 2 h at 37 C to exchange isotopically labeled steroid for the endogenous unlabeled ligand. Total ER binding capacity is estimated by summing the values for occupied and unoccupied ER. In all brain regions of normal females, ER occupation is low at estrus, reflecting the very low levels of circulating estradiol present at this stage of the estrous cycle, rising to approximately 50% of binding capacity at proestrus. By contrast, in intact males ER occupation varies considerably between brain regions, from a high of 55% of binding capacity in the bed nucleus of the stria terminalis to a low of 21% in the hypothalamic arcuate nucleus. Gonadectomy or treatment of intact males with the aromatase inhibitor 4-hydroxy androstenedione greatly reduces or eliminates ER occupation, depending on the brain region. In both sexes, changes in levels of endogenous gonadal steroids have little effect on total (occupied plus unoccupied) ER concentrations, with the exception of the hypothalamic ventromedial nucleus of the female, in which total ER concentration declines at estrus. These results are consistent with the hypothesis that local aromatization may be the primary determinant of regional ER occupation in the brain of the male rat, in contrast to the female, in which high levels of ER occupation are found only during the preovulatory estrogen surge. Although physiological changes in circulating estradiol and aromatizable androgen concentrations induce large changes in ER occupation, they have little effect on total ER content in most regions of the brain, suggesting that previous reports of changes in ER messenger RNA levels under different conditions of gonadal steroid exposure may not be directly reflected in steady state levels of the cognate receptor site.