Facilitation of lordosis in ovariectomized rats by intracerebral progesterone implants

Abstract

Ovariectomized female rats were estrogen-primed by subcutaneous injection and two days later were tested for sexual receptivity 6 h following bilateral implants of crystalline progesterone or cholesterol combined with a subthreshold injection of progesterone administered systemically. Cannulae were located in either the medial basal hypothalamus (MBH) or the mesencephalic reticular formation (MRF) in different groups of females.Significant facilitation of estrous behavior was induced by brain progesterone in females whose cannulae were located in the MBH. Intracerebral progesterone was not effective in females with MRF cannulae. Within the MBH the ventromedial and arcuate nuclei appeared most responsive to the estrus-facilitating actions of progesterone. The possibility that the behavioral effects of implanted progesterone were mediated by entry of hormone into the ventricular system or general circulation is considered unlikely. It is suggested that progesterone facilitates lordosis behavior via a facilitatory rather than an inhibitory effect on MBH neurons.