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.
D000818:Animals; D002369:Castration; D004343:Drug Implants; D004967:Estrogens / Q000494:pharmacology; D004971:Estrus / Q000187:drug effects*; D005260:Female; D007033:Hypothalamus, Middle / Q000187:drug effects*; D008636:Mesencephalon / Q000187:drug effects; D011247:Pregnancy; D011301:Preoptic Area / Q000187:drug effects; D011374:Progesterone / Q000494:pharmacology*; D051381:Rats; D012154:Reticular Formation / Q000187:drug effects
D004343:Drug Implants; D004967:Estrogens; D011374:Progesterone