Sequential inhibitory effect of progesterone on lordotic responsiveness in rats: time course, estrogenic nullification, and actinomycin-D insensitivity.

Abstract

The sequential inhibitory effect of progesterone (P) may represent a model of how P acts to terminate sexual receptivity in the rodent. In the present studies, the sequential inhibitory effect of P on lordotic responsiveness was demonstrated in ovariectomized rats bearing Silastic capsules of estradiol (E2) for 48 h and injected with P 3 h after E2 removal and again 24 h later. In a time course study, inhibition of hordotic responsiveness was evident 18 h following continuous exposure to P, establishing that P requires a relatively long time-interval to develop its inhibitory effect on lordotic responsiveness. The RNA synthesis inhibitor actinomycin-D infused intraventricularly into E2-primed rats at the time of the first of two P injections did not prevent the appearance of the sequential inhibitory effect of P, suggesting that despite the long time-interval required for the development of P inhibition of sexual behavior, new RNA synthesis is not required. Results of the present studies also show that the continued presence of E2 for 11 h or more following the first of two P injections prevents the appearance of (nullifies) the sequential inhibitory effect of P, confirming and extending previous reports that estrogen can interfere with the development of the sequential inhibitory effect of P.