Progesterone in the ventromedial hypothalamus facilitates estrous behavior in ovariectomized, estrogen-primed rats

The putative neural target sites of progesterone (P) action for the facilitation of estrous behavior in female rats were reexamined using the hormone implant technique. Subjects were ovariectomized, estrogen-primed Long-Evans females. All were outfitted with bilateral double barreled cannulae assemblies consisting of 23-gauge guide cannulae and 28-gauge inserts. Subjects in Exp 1 received sc injections of individually determined threshold priming doses of estradiol benzoate. Estrogenic priming for subjects in Exp 2 was provided by sc placed Silastic capsules (5 mm) filled with 17 beta-estradiol-cholesterol (1:20). Each subject was tested for estrous behavior with a male after P-filled cannulae and after blank inserts were lowered into the brain. Behavioral tests were conducted 1 and 4.5 h postcannulae placement. Thirty-five of 40 females with P in the ventromedial hypothalamus exhibited high levels of lordosis behavior during the 4.5-h test. Moreover, 29 of the 35 exhibited solicitation behavior as well. Estrous behavior was not exhibited by these animals during the 1-h test. Implants in other regions of the brain (i.e. mesencephalic reticular formation, preoptic area, central grey, caudate putamen, and hippocampus; n = 80) did not consistently facilitate estrous responsiveness. In Exp 3, levels of estrous responsiveness were similar before and after adrenalectomy (n = 10); thus, facilitation of estrous behavior by P-filled implants in the ventromedial hypothalamus was not dependent upon activation of the hypothalamic-pituitary-adrenal axis.