The inhibition of female rabbit sexual behavior by progesterone: progesterone receptor-dependent and-independent effects

In the pregnant domestic rabbit, scent marking (“chinning”) and sexual behavior are inhibited by ovarian-derived progesterone (P). In order to distinguish behavioral effects of P that are PR-dependent from those mediated by its ring A reduced metabolites, we administered P, P+RU486 (PR antagonist), chlormadinone acetate (CA, synthetic progestin that does not form ring A reduced metabolites), or vehicle to ovariectomized (ovx) estradiol-benzoate (EB)-treated female rabbits, via sc injection, on experimental day 0. Chinning was quantified daily, and mating tests were done on days -1, 1, 3, 5, and 7. On day 1, chinning was significantly decreased, and the latency to be mounted by the male was significantly increased (indicating decreased sexual attractivity of the female) in P-treated females. The effect of P on chinning, but not its effect on sexual attractivity, was completely blocked by RU486 and replicated by CA. Although CA had no effect on attractivity on day 1, it decreased both sexual receptivity and attractivity on day 3. In a preference test in which the male could interact with either an ovx EB-treated female or an ovx female that had received one of the above hormone treatments 24 h earlier, P decreased sexual attractivity and increased aggression. The effect of P on aggression, but not its effect on attractivity, was blocked by RU486 and replicated by CA. These results indicate that both PR-dependent and PR-independent mechanisms decrease sexual attractivity, whereas PR activation is necessary for the inhibition of chinning and sexual receptivity, and for the stimulation of aggression.