Activation of Estrous Behavior in Ovariectomized Rats by Intracerebral Implants of Estradiol Benzoate

There is a lack of agreement among several studies on the site of action of estradiol in the regulation of estrous behavior in mammals in general, and in rats in particular. Two experiments were performed in an attempt to clarify this issue. In the first experiment, 29 ovariectomized rats received unilateral 27 gauge implants of estradiol benzoate (EB) in various areas of the brain. Animals were tested over a three week period for estrous responses to an intact male rat. The greatest response resulted from implants in the ventromedial hypothalamus (VMH). Implants in the preopticanterior hypothalamic region (POA-AH) and other areas of the brain were significantly less effective, but when progesterone (P) was given almost all animals responded, and the magnitude of responses did not differ among the various groups. In the second experiment 30 gauge tubing with EB in the lumen was employed in order to restrict the effective dose of hormone delivered to the brain. Implants were placed in the VMH, POA-AH, or in other areas of the brain in 119 ovariectomized females. Twentythree of 30 animals with implants in the VMH exhibited estrous behavior while only 8 out of 28 females with implants in the POA-AH showed positive responses. Implants in other areas of the brain resulted in little if any such behavior. Among the females which exhibited positive responses, those with VMH implants had significantly higher estrous behavior scores than did those with POA-AH implants. Following treatment with P the differential proportion of responders remained; however, under this condition the degree of response did not vary between females with VMH implants and those with implants in the POA-AH. These results indicate that estradiol may act at both the VMH and the POA-AH in the activation of female sexual behavior, but that the predominant effect is on the VMH.