Hormonal control of sexual receptivity during the estrous cycle of the rat

The effects of ovarian hormones on the display of sexual receptivity were investigated in female rats showing 4-day estrous cycles. Acute ovariectomies were performed on the day of expected behavioral estrus, timed so that the influence of ovarian estrogen alone, or of ovarian estrogen plus progesterone could be evaluated. High levels of receptivity can be elicited only following the combined action of estrogen and progesterone. Estrogen alone is not sufficient; supplementary estradiol benzoate fails to induce estrous behavior if the facilitative action of progesterone is prevented. Biphasic effects of progesterone were not detected under the physiological conditions employed and most likely are of minimal importance in regulating the duration of behavioral estrus. Estrus duration is controlled primarily by the levels of estrogen which condition the neural substrate for lordosis during the 24–36 hr period preceding the onset of receptivity. Exogenous estradiol benzoate added to the estrogen already secreted by the ovaries prolongs the period during which lordosis behavior can be elicited by progesterone injection. Possible contributing effects of adrenal progesterone to the control of estrous behavior were examined.