Interaction of progesterone and estrogens on the hypothalamic center controlling estrous behavior in sheep.

Behavioral estrus of castrated and with progesterone treated ewes, induced by implantation of 17-beta-estradiol into the area controlling the sexual behavior, was completely modified as compared with that of castrated but non treated with progesterone ones. The length of behavioral estrus in the non treated ewes was 24-30 hr and its symptoms were rather weakly expressed. On the contrary, the ewes treated with progesterone did not exhibit any behavioral estrus after implantation throughout the whole period of the treatment. They showed symptoms of behavioral estrus on the 2nd day after the injections of progesterone were stopped; in such cases the symptoms were strongly expressed and lasted for 3-6 days. Since the estradiol, implanted into the hypothalamic sexual behavior center, was acting locally, the described reaction of ewes might be thought of as the effect of interaction of progesterone and estradiol being displayed on the level of specific neuronal receptors of this center.