Characterization of a pineal-independent diurnal rhythm in neural estrogen receptors and its possible behavioral consequences.

Abstract

We have previously reported a diurnal variation in brain cytoplasmic estrogen receptors in female rats in the absence of circulating estrogens. In this report we characterize this diurnal rhythm. Levels of cytoplasmic estrogen receptors are greater during light periods than during dark periods in the hypothalamus and in the preoptic area of ovariectomized (OVX) rats. A similar diurnal variation in cytoplasmic receptor content is also found in the brains of female hamsters, but no light-dark differences are seen in the brains of castrated male rats. β-Adrenergic blockade, which inhibits pineal function, had a marginal effect on receptor concentration during the dark. However, pinealectomy did not alter light-dark differences in receptor concentration. The behavioral consequences of the diurnal fluctuations in levels of cytoplasmic estrogen receptors were investigated. Seven different estrogen regimens were administered to OVX female rats at light and dark time points; progesterone was injected 5 h before behavioral testing. Six estrogen paradigms providing comparable hormonal stimuli in the light and dark failed to induce diurnal differences in sexual receptivity. One estrogen regimen consisting of a Silastic capsule of unesterified estradiol in oil followed 24 h later by a pulse of estradiol induced higher levels of sexual receptivity when treatment began during dark periods than during light periods. In parallel experiments nuclear estrogen receptor levels were measured by exchange assay 4 h after Silastic capsules were implanted. Higher levels of nuclear receptors were found during dark periods compared to light periods. Light-dark differences in nuclear translocation of receptors and behavioral sensitivity to estrogen are probably a consequence of light-dark differences in release of estradiol from Silastics resulting in light-dark differences in serum estradiol. Since these serum levels of hormone are quite low (less than 3 pg/ml), the effectiveness of the Silastic-pulse paradigm suggests the importance of small early changes of estradiol levels during the estrous cycle and supports a two-phase mechanism of estrogen action. Saturating doses of estradiol produce similar levels of brain nuclear estrogen receptors whether administered at light or dark time points. Thus, cytoplasmic estrogen receptor content is not indicative of the functional effectiveness of the receptors.