Facilitation of lordosis by estradiol in the mesencephalic central gray

Ovariectomized female hamsters received unilateral implants of estradiol aimed at the mesencephalic central gray (MCG). Postoperative levels of lordosis and ultrasonic vocalization were used to evaluate implant effects on sexual receptivity and proceptivity. Females with large MCG implants showed higher levels of receptivity than females with similar implants at control sites. In addition, females with small MCG implants showed little receptivity while the implant served as the sole source of estrogen, but diverged from controls by showing elevated receptivity during supplemental treatment with low systemic doses of estradiol. These results suggest that estradiol implants in the MCG facilitate receptivity if, and only if, accompanied by exposure of other brain areas to low estrogen levels. In turn, this suggests that the MCG, though not a self-sufficient site for the hormonal priming of lordosis, does modulate sexual motivation by mediating facilitatory effects of estrogen on receptivity and mating.