The relative effectiveness of progestins for facilitation and inhibition of sexual receptivity in hamsters.

In a series of experiments, we studied the biphasic actions of 4 progestins on sexual receptivity in female hamsters. Based on previous reports, we selected doses of progesterone (P), desoxycorticosterone acetate (DOCA), 5-alpha-dihydroprogesterone (DHP) and 20-alpha-hydroxyprogesterone (20-OHP) that were expected to have initial facilitative and/or subsequent inhibitory actions on sexual receptivity. Estrogen-primed ovariectomized female hamsters were given one of 4 doses of a progestin at hour 44 post EB, and were tested for facilitation (with tape over their vaginas to prevent intromissions) 4 hours later. At hour 68, all animals received 500 micrograms P and were tested for the inhibitory effects of the test progestin 4 hours later. In additional experiments, similar procedures were followed with an alternate vehicle (1% Tween-80 instead of sesame oil) and a longer initial progestin-to-facilitation test interval (8 rather than 4 hours) in attempts to improve bioavailability of the steroids. P and DOCA were clearly the most behaviorally effective progestins tested, and they were equally potent for facilitation and inhibition in an oil vehicle at the 4 hour interval. DHP given in oil caused some inhibition but no facilitation. 20-OHP had no behavioral effects at the doses tested. The results are discussed in light of previous results with these steroids in hamsters and other rodent species. Also, the differing patterns of effectiveness across conditions provide some, although qualified, support for our view that the biphasic actions of progestins may be controlled by separate mechanisms. However, an alternate explanation is also discussed.