Progesterone in the ventromedial hypothalamus of ovariectomized, estrogen-primed rats inhibits subsequent facilitation of estrous behavior by systemic progesterone.

The putative neural target sites of progesterone's (P) inhibitory influence on estrous behavior were re-examined utilizing intracranial hormone implants. Subjects were estrogen-primed ovariectomized, Long-Evans rats, and all were outfitted with permanent indwelling guide cannulae aimed for the ventromedial hypothalamus (VMH), preoptic area (POA), or midbrain. In a series of 4 weekly testing paradigms, the ability of blank or P-filled implants to both facilitate estrous responsiveness and to interfere with an animal's ability to exhibit estrous behavior following a systemic injection of P 24 hours later was assessed. P-filled implants placed bilaterally into the VMH inhibited the subsequent facilitation of estrous behavior by systemic P administration. Neither P-filled implants in other brain regions nor blank implants lowered into the VMH had this same effect. Seven of the females that exhibited progesterone-induced behavioral refractoriness 24 h after P stimulation of the VMH exhibited facilitation of estrous responsiveness in behavioral tests given 4.5 h after cannulae placement suggesting that progesterone may exert its facilitative and inhibitory actions on estrous responsiveness at the same neuroanatomical locus.