The Relevance of Hypothalamic and Hyphophyseal Progestin Receptor Regulation in the Induction and Inhibition of Sexual Behavior in the Female Rat

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Abstract
Modifications in the concentration of progestinbinding sites (PR) in rat hypothalamus and pituitary, measured with [3H]R5020 (promegestone, 17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) have been related to the facilitative and inhibitory actions of progesterone (P) oh female sexual behavior. In the ovariectomized rat, induction of PR followed the same pattern as induction of lordosis. Exposure of the brain to estrogens for less than 12 h is ineffective in bringing about lordosis and did not increase PR. The optimal interval between estrogen and P administration for lordosis is 24–48 h; the maximum increase in PR took place 24–48 h after estrogen priming. This induction was estrogen dose dependent; the dose of estradiol benzoate (EB) required for lordosis (2-5 μg/rat) produced a 2-fold increase in PR after 40 h. In males, the increase in PR after EB priming was lower and in accord with the much higher EB doses required for a lordosis response. When a large dose of P was injected simultaneously with EB (concurrent inhibition) or 24 h after EB (sequential inhibition), the induction of PR was considerably inhibited as was the induction of lordosis; R5020 was 50–100 times more inhibitory than P. Thus, the dual action of P on lordosis seems to be related to an initial estrogen-dependent increase in PR in the brain, followed by an action of P resulting in the reduction of its own binding sites.