Characterization and distribution of estrogen receptors in the diencephalon of the gray short-tailed opossum

The Brazilian gray short-tailed opossum is a pouchless marsupial whose young are born sexually undifferentiated making this animal ideal for developmental studies. Previously, Etgen and Fadem (Dev. Brain Res., 49 (1989) 131-133; Gen. Comp. Endocrinol., 66 (1987) 441-446) detected estrogen receptor (ER) in the hypothalamus-preoptic area, and compared males and females in the adult and during development. In this study we characterized the ER and determined its distribution in specific diencephalic regions in the brains of adult male and female opossums. ER were measured by the in vitro binding of [³H]estradiol to cytosol of microdissected brain nuclear regions. Radioinert moxestrol (R2858) was used to define non-specific binding. Saturation analysis showed a single high-affinity binding site. Binding was displaced by estradiol (E₂), diethylstilbestrol (DES) and R2858, but not by non-estrogenic steroids. Ligand bound receptor adhered to DNA-cellulose and was eluted as a single peak with 0.2-0.3 M NaCl. High levels of ER were found in the medial preoptic-periventricular area. Intermediate levels were seen in the ventromedial hypothalamic nucleus, medial amygdala and arcuate nucleus. No sex differences were observed. The presence of a neural ER and its similarity of distribution to that of the laboratory rat support the use of this animal model in studies examining steroid dependent organization of the hypothalamus.