Actions of progestins on estrous behaviour in female rats

The present study evaluated the facilitative actions of progesterone and the synthetic progestin R 5020 on estrous responsiveness in ovariectomized, estrogen-primed female rats. The dose-response and time-response characteristics of the behavior facilitating actions of both progesterone and R 5020 were measured. The threshold doses for the facilitation of estrous behavior in estrogen-primed female rats were 1 microgram of R 5020 and 100 micrograms of progesterone. These doses of progestins facilitated estrous responsiveness with a similar time course that approached maximum at one hour. To examine the possible mechanism(s) of action of each progestin the synthetic progestin antagonist RU 38486 was used. The inhibitory effects of RU 38486 on estrous behavior facilitated by a threshold dose of progesterone or R 5020 were found to be almost identical. RU 38486 (5 mg) administered 1 hr prior to progesterone or R 5020 suppressed lordosis behavior by 44% and 47% respectively. These results suggest that progesterone and R 5020 facilitate estrous responsiveness through the same mechanism.