Studies on hypothalamic estradiol binding during estrous cycle and after ovariectomy

Specific binding of 3H-estradiol (1 microgram kg-1 injected s.c. 1 h before sacrifice) in the hypothalamus, anterior pituitary and uterus of cycling and ovariectomized female rats was investigated. Preoptic-anterior hypothalamic area (POA-AH) and median eminence-basal hypothalamus (ME-BH) were separated by gross dissection from the bulk of hypothalamus. Pieces of parietal cortex served as controls. The lowest amount of 3H-estradiol specifically bound to cytoplasmic and nuclear receptors in POA-AH and ME-BH was found in proestrus, while in estrus-metestrus it was the highest. In contrast to hypothalamic estrogen sensitive areas, the lowest uterine binding was found in estrus and the highest in diestrus-proestrus. Changes in estradiol binding in the anterior pituitary were similar to that in the uterus. Ovariectomy made 48 h before 3H-estradiol injection markedly increased the nuclear binding in POA-AH and ME-BH compared to that observed in cycling animals. However, it decreased nuclear binding in the uterus. Cytoplasmic binding seems to be decreased in the hypothalamus as well as in the uterus of ovariectomized animals as compared to the highest level observed during estrous cycle in intact ones.