Participation of progesterone receptors in facilitation and sequential inhibition of lordosis response induced by ring A-reduced progesterone metabolites in female mice.

Sexual receptivity in female rodents induced by the sequential injection of estrogen and progesterone is followed by a period in which females do not respond behaviorally to a second administration of progesterone (P); this is known as sequential inhibition. It has been proposed that the induction of sequential inhibition by progesterone in rats depends on down regulation of the progesterone receptor (PR) in brain areas involved in the expression of female sexual receptivity. P is rapidly metabolized to a variety of 5α- or 5β-ring A-reduced progestins (RPrg). These RPrg have little or no affinity for the PR. They stimulate sexual receptivity (lordosis) more potently than P itself in estrogen-primed rats and do not induce sequential inhibition. The purpose of the current study was to test the role of the PR in the facilitation of lordosis and sequential inhibition induced by P and the following RPrg: 5α-pregnandione (5α-DHP), 5α,3β-pregnanolone (5α,3β-Pgl), 5β-pregnanedione (5β-DHP), and 5β,3α-pregnanolone (5β,3α-Pgl) in ovariectomized (ovx) female mice primed with estradiol benzoate. The RPrg were tested in C57BL/6 mice and in a strain lacking the progesterone receptor expression (PRKO). Our results show that both facilitation and sequential inhibition of lordosis induced by progesterone require the presence of the progesterone receptor. Interestingly, some RPrg facilitate lordosis but do not induce sequential inhibition in female mice. Sexual receptivity induced by RPrg does not require the progesterone receptor. Thus, RPrg induce sexual receptivity, but they probably exert their effects through a different cellular mechanism that does not involve the progesterone receptor. (PsycINFO Database Record