The role of the uterus in reproductive behavior.

The uterus plays a major role in reproductive physiology. Numerous studies have shown that estradiol and progesterone exert their effects by binding to receptors within the cytoplasm of uterine cells and translocating these receptors to the nuclei where they presumably alter genomic activity. The extent of the steroid-receptor interaction in the cytoplasm and the nucleus is well correlated with uterine growth. Other physiological changes that take place in the uterus are also correlated with variations in the cytoplasmic and nuclear steroid receptor levels observed during different stages of reproductive cycle. In the rat, the uterus also exerts an inhibitory effect on the hormone-induced maternal and copulatory behaviors. Hysterectomy shortens the latency for the induction of maternal behavior and enhances sexual receptivity and proceptivity in ovariectomized, hormone treated rats. The mechanism of this inhibitory effect is not fully understood; however, it seems that the steroid binding capacity of the uterus mediates its behavioral influence. Thus when this capacity is reduced, as evidenced by lower levels of cytoplasmic estrogen receptors in long-term gonadectomized animals, the uterus loses its inhibitory effect on copulatory behavior.