The role of the uterus in regulation of heat duration in cycling rats.

These experiments examined the effects of hysterectomy on heat duration and on the reinduction of estrous behavior by progesterone (P) following the termination of spontaneous heat in 4-day cycling rats. Hysterectomy did not affect the onset of estrus but prolonged heat duration. The average duration of sexual receptivity for hysterectomized (H) and sham-hysterectomized (SH) rats was 18.2 and 13.0 hr, respectively. Furthermore, H animals injected with either 0.5 mg P within 2 hr, or 4.0 mg P 24 hr following the termination of natural estrus showed significantly higher lordosis and solicitation responses than SH rats similarly treated. These behavioral findings were correlated with the level of hypothalamic progestin receptors. That is, H animals had a significantly higher concentration of progestin receptors than SH rats immediately following the termination of spontaneous heat and also 24 hr later. Both in estrous-cycling rats and in gonadectomized animals treated with estradiol benzoate (EB), hysterectomy resulted in higher serum estradiol (E2) levels. The results of these experiments suggest that prolongation of the period of sexual receptivity and the facilitated behavioral responses to P following the cessation of estrus in hysterectomized animals may be due to a lowered clearance rate of circulating estradiol which presumably enhances the estrogen conditioning of the neural substrates.