Importance of Progesterone to Induction of Sexual Receptivity in Spayed Female Rats.∗

Young and his co-workers1-3 have reported that although estrogen may induce heat behavior in spayed female rats, the administration of progesterone to estrogen-treated animals markedly increases sexual receptivity. Ball 4 , 5 was unable to confirm these results. She was of the opinion that her failure may have been due to peculiarities in the strain of rats used. In all of the studies listed the behavior of females has been recorded as nonreceptive, slightly receptive or receptive. In the course of a longer experiment we have collected some data which tend to resolve some of the differences between the findings of Ball and those of Young and his co-workers. Further, our results include complete and sensitive records of different degrees of receptivity in treated females. Procedure. Virgin female rats were raised in segregation and ovariectomized at 3 or 4 months of age. Hormone injections and mating tests were initiated approximately 3 weeks after castration. The effects of 3 types of injections upon sexual receptivity were tested. Seven spayed females were injected with 500 R.U. of estrogen∗ and tested for sexual receptivity 66-68 hr later. Eight days after the first estrogen treatment the same 7 animals were injected with 500 R.U. of estrogen followed 48 hr later by 0.5 mg of progesterone.† Sex tests were conducted 16-18 hr after the progesterone treatment. The second injection schedule was repeated 3 times at weekintervals, and mating tests were carried out in each instance. Six spayed females were injected with 100 R.U. of estrogen followed 48 hr later by 0.5 mg of progesterone, Sex tests were conducted 16-18 hr after the progesterone injection. jection. Two weeks later these same 6 females were injected with 500 R.U. of estrogen followed 48 hr later by 1 mg of progesterone, and mating tests occurred 16-18 hr after the injection of progesterone. After