Relative contributions of oestradiol and progesterone to the maintenance of sexual receptivity in mated female hamsters.

Sexual receptivity was measured as total lordosis duration (TLD) per 10 min test period with mating tests standardized across 24 h by giving the first test at the time when the lights were switched off (0 h) during a schedule of 14 h light : 10 h hardness and re-testing at intervals of 2 h. Long-term (14 days), high-level oestrogen treatment was necessary to achieve TLD scores similar to those found in female hamsters treated with oestrogen plus progesterone for 3 days. Mating of female animals treated with oestrogen only always resulted in a significant decrease in TLD scores at later tests (0 v. 2 h). If progesterone was given after the test at 0 h, a significant increase in TLD score was found at the test at 2 h and this increase was maintained across several tests. By 8 h TLD scores had decreased significantly in progesterone-treated mated female hamsters. Progesterone appeared to have, therefore, at least two roles in the regulation of sexual responsiveness in the female hamster: (a) under normal conditions of oestrogen exposure (1--3 days) progesterone was necessary for the induction of sexual receptivity and (b) progesterone was necessary for the maintenance of sexual responsiveness following short-term (10 min) mating in female hamsters treated with oestrogen only. In the oestrogen- and progesterone-treated female animal which is no longer receptive as a result of previous exposure to progesterone plus having been mated, sexual receptivity can be re-induced by treatment with a larger dose of progesterone. These findings have indicated that progesterone is the primary hormonal agent which regulates sexual receptivity in the mated female hamster. Furthermore, the eventual inhibitory effect of progesterone on sexual receptivity was not absolute even in the mated female animal but rather represent some form of habituation to that particular dosage of progesterone to which the animal had been previously exposed.