Variations in levels of estrogen receptor mRNA were investigated in the medial preoptic nucleus, arcuate nucleus, and ventromedial nucleus of the hypothalamus throughout the phases of the female estrous cycle and compared with those in ovariectomized female and intact male rats. Female Wistar rats were killed during estrus, metes- trus, diestrus, or proestrus or 72 h after ovariectomy as were a group of intact male rats. Brains were removed and frozen, and 20-μm cryostat sections were thaw-mounted onto slides and hybridized with a35S-labeled antisense estrogen receptor probe. Section-mounted slides were processed, apposed to x-ray film, then dipped in liquid emulsion, and quantified. After exposure, estrogen receptor mRNA was detected in several brain regions, including the medial preoptic nucleus, arcuate nucleus, and ventromedial nucleus of the hypothalamus. Estrogen receptor mRNA levels in the medial preoptic nucleus were highest during estrus and metestrus, attenuated at diestrus, and low during proestrus. In contrast, the hybridization signal in the arcuate and ventromedial nuclei was low during estrus and then gradually increased throughout the cycle until it peaked during proestrus. Ovariectomized females exhibited an elevated level of estrogen receptor mRNA in all brain regions investigated. Hybridization signal in male medial preoptic nucleus and ventromedial nucleus was reduced compared with those in both intact and ovariectomized females. Estrogen receptor mRNA levels in the arcuate nucleus were similar to those in intact females, but less than those in ovariectomized animals. The results of these studies demonstrate that estrogen receptor mRNA levels are sexually dimorphic, vary during the estrous cycle, and increase after ovariectomy. Furthermore, these results indicate that the magnitude and direction of change observed during the estrous cycle are region specific and suggest that factors other than endogenous estrogen levels differentially modulate estrogen receptor mRNA expression in the hypothalamus.
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