The estrous cycle of the rat: effects on the accumulation of estrogenic metabolites in brain and peripheral tissues

Abstract

Summary Female rats exhibiting regular 4 day viginal estrous cycles were injected with tritiated estradiol and tested for sexual receptivity 1 h and sacrificed 2 h later. Upon sacrifice wet weights of uterus and pituitary were taken. Samples from the anterior and posterior hypothalamus, anterior mesencephalon, cerebral cortex, liver, and blood plasma were assayed for retained estrogenic metabolites. Samples from the cortex, uterus, pituitary, liver, and blood plasma were assayed for total retained radioactivity. Results suggest an anterior > posterior gradient in basal brain for the retention of estradiol and an opposite pattern for the retention of estrone. During the estrous cycle the highest levels of retained estradiol in the hypothalamus were found during the periods of presumed low endogenous estrogen titers (diestrus and estrus), while the lowest levels were found during periods of presumed high endogenous estrogen titers (early and late proestrus). Retention of neither estradiol nor estrone showed any other significant fluctuations in the tissues in which this was examined. Total radioactivity levels retained in the uterus and pituitary were also highest during diestrus and lowest during proestrus. Total retained radioactivity did not fluctuate the other tissues examined. Females were found to be sexually receptive only during the evening of proestrus, at a time when uterine and pituitary wet weights were also found to be at their maxima. Results were discussed in terms of the possible specialized role of regionally localized estrogenic metabolites in the control of reproductive behavior and physiology in the female rat.