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Cell nuclear estrogen receptors and cytosol progestin receptors were measured in the pituitary gland, preoptic area and hypothalamus throughout the estrous cycle of the rat. Cell nuclear estrogen receptor levels paralleled changes in serum estradiol cencentrations with highest values on proestrus and lowest on diestrus. Proestrous values were 50–60% of capacity for each tissue. Cytosol progestin receptor number in these tissues was also highest on proestrus and lowest on diestrus. With these data as a guide, Silastic capsules filled with estradiol-cholesterol mixtures were used to generate physiologic levels of estrogen receptor occupation within the brain-pituitary complex of ovariectomized rats and to examine the kinetics of estradiol stimulation of lordosis behavior and cyclic gonadotropin release. Our results indicate that the effectiveness of an estradiol stimulus to elicit lordosis or luteinizing hormone release depends on at least three factors: the magnitude of the increment in serum estradiol and brain and pituitary cell nuclear estradiol receptor levels; the duration over which these increments are maintained; and the interval from previous exposure to estrogen.

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