Effects of an Anti-estrogen on Neural Estradiol Binding and on Behaviors in Female Rats*

Abstract

The time course of changes in cytoplasmic estradiol-binding sites were determined along with in in vivo binding of [3H]estradiol by cell nuclei in the hypothalamus-preoptic area-septum of ovariectomized rats after a single injection of nafoxidine (4 mg i.p. in saline). These changes in neural estradiol binding were compared with changes in 2 estradiol-influenced behavioral responses-female sexual behavior and suppression of eating and body weight. Nafoxidine reduced brain cytoplasmic estradiol binding sites by 80% within 6 h, and these levels remained unchanged for 4 days. After nafoxidine (1 wk), cytoplasmic binding sites were 50% of controls and recovery was complete by day 15. Inhibition of [3H]estradiol binding in brain cell nuclei paralleled cytoplasmic binding site content. Nafoxidine was fully estrogenic with regard to eating and body weight. Eating was maximally suppressed within 24 h, and return to pretreatment levels closely paralleled cytoplasmic receptor levels. Estradiol + progesterone-induced sexual receptivity was totally inhibited 1 day after nafoxidine. Lordosis recovered from this inhibition more rapidly than would be predicted from the receptor data, suggesting that after an initial inhibitory period nafoxidine may be weakly estrogenic for lordosis. Nafoxidine apparently produces a prolonged inhibition of neural estradiol binding and alterations in estrogen-dependent behaviors. In the brain, estrogens probably have different molecular mechanisms of action on eating behavior and on female sexual behavior.