Estrogen receptor protein is differentially regulated in the preoptic area of the brain and in the uterus during the rat estrous cycle

A previous in situ hybridization study in our laboratory has shown that the level of estrogen receptor mRNA in the preoptic area of the brain varied during the rat estrous cycle and differed between the sexes. While these observations suggest that the receptor protein levels may also change, a direct measurement of receptor protein is required to substantiate this regulation. The present study utilized a quantitative Western blot method to evaluate changes in the total content of estrogen receptor protein in the cycling female and intact male preoptic area and to compare changes in the brain with those which occur in the uterus. Tissue samples from brain and uterus were homogenized, separated by electrophoresis, and transferred to membranes. Estrogen receptor was then detected with two different antibodies, ER-715 and H-222, using a chemiluminescent Western blot assay to generate film autoradiographic images. A 66-kD band in tissue samples from brain and uterus was evaluated. Densitometric analysis of autoradiograms detected significant alterations in estrogen receptor content over the estrous cycle, with brain levels being highest during metestrus, attenuated on diestrus, and lowest during proestrus and estrus. A comparison of the male and female preoptic area revealed that the level of estrogen receptor in the cycling female was significantly higher than in the male. In contrast, the uterine estrogen receptor content was low during metestrus, diestrus, and estrus and elevated on proestrus. These results suggest that expression of the estrogen receptor in the preoptic area changes during the estrous cycle and is greater in the female than the male brain. Hypothalamic estrogen receptor content changes were not in phase with those of the uterus, suggesting that the regulation of estrogen receptor protein is tissue specific.