Comparison of the effects of estradiol-17β and the synthetic estrogen, RU-2858, on lordosis behavior in adult female rats and guinea pigs

Abstract

Female sexual behavior (as assessed by lordosis responses) in adult ovariectomized guinea pigs and rats was measured after a single subcutaneous injection of either the synthetic estrogen RU-2858 or estradiol-17β (E) and a subsequent injection of progesterone (P). Thresholds for responsiveness to each estrogen were determined over a range of doses from 1 to 640 μg/kg of body weight. RU-2858 was able to stimulate lordosis at doses several times lower than E in both guinea pigs and rats. In general, guinea pigs were less sensitive to both estrogenic compounds than rats and were particularly insensitive to E. Approximately 40% of guinea pigs given RU-2858 displayed lordosis prior to P injection. Subsequent P injection did not greatly enhance the intensity of lordosis responses in these animals. These guinea pigs were also more sensitive than the remaining 60% of RU-2858-injected guinea pigs on several indexes of lordosis intensity (e.g., lordosis threshold, duration of heat, duration of individual lordosis responses). Rats did not respond behaviorally to RU-2858 without a subsequent P injection nearly as frequently as did guinea pigs. At the doses used in this study, E did not facilitate lordosis behavior in rats or guinea pigs in the absence of a subsequent P injection.