Estradiol plus progesterone treatment and precopulatory behavior in ovariectomized female rats

Abstract

The effects of progesterone in ovariectomized female Wistar rats, chronically injected with estradiol plus progesterone, were determined utilizing a scale of sexual responsiveness (SR) based on the hierarchical organization of copulatory (lordosis posture) and precopulatory (presenting posture, hopping, darting) behaviors. Standardized mating tests were performed 48 hours after estradiol dipropionate application (E) and 4 hours after progesterone (P) application. Experiment 1: Starting with the 5th day following ovariectomy animals in two groups were injected SC with 15 μg E in regular weekly intervals (for 8 weeks), the animals in one of the groups obtaining further SC injections of 1.0 mg P 44 hours later. The SR intensity induced by E and P application gradually increased and was eventually higher than in the E application. Experiment 2: Animals primed with 6 μg E were divided into four groups that received different P doses: 0.0, 0.4, 1.0 or 2.2 mg. This treatment lasted 11 weeks. Increased SR occurred only with the 1.0 and 2.2 mg P doses. However, these doses were equally effective. Experiment 3: Animals in five groups obtained 3, 5, 10, 15 or 30 μg E for 7 weeks. Every female was also given weekly injections of P that ranged from 0.0 to 2.4 mg. With 3, 5 and 10 μg E there were only small differences in the SR induced by small and large doses of P. On the other hand, animals receiving 15 μg E were differentially affected by P injections. Higher injected doses of P increased the SR induced by 15 μg E. The 30μg E dose combined with injections of P around 0.6 mg induced higher SR than when combined with injections of P over 1.0 mg. Experiment 4: Animals utilized in Experiment 3 which had received 30 μg E were subjected to further experimentation. The deletion of P caused a rapid decrease in SR. A predicted increase in SR was observed after reintroducing the 0.6 mg P administration. Finally, after the E and P administrations were stopped, precopulatory behavior disappeared but the females still exhibited the lordosis posture two weeks later. Based on these results the general conclusion is made that the dose-response relationship between progesterone and proceptive (precopulatory) behavior is dependent on the quantity of estradiol which is jointly administered. By certain combinations of dosage levels of estradiol and progesterone, the highest level of sexual responsiveness can be induced in ovariectomized rats.