Cytoplasmic progestin receptors in female Guinea Pig brain and their relationship to refractoriness in expression of female sexual behavior

Abstract

Summary Cytoplasmic progestin receptors were assayed in the hypothalamus-preoptic area-septum and midbrain of ovariectomized guinea pigs given sequential treatment with estradiol benzoate and progesterone. As little as 50 μg of progesterone injected 40 h after estradiol benzoate caused a failure to display lordosis in response to a second progesterone injection 24 h after the first. Both 50 μg and 500 μg of progesterone decreased the concentration of cytoplasmic progestin receptors 24 h later in hypothalamus-preoptic area-septum and midbrain in estrogen-primed, ovariectomized guinea pigs. Therefore, progesterone injections that caused behavioral refractoriness to further stimulation by progesterone also decreased the concentration of availabke cytoplasmic progestin receptors. When progesterone was administered concurrently with 1.6 μg of estradiol benzoate, 500 μg, but not 100 μg suppressed behavioral responsiveness to a second progesterone injection 40 h later. The high dose, but not the low dose, also decreased the concentration of available cytoplasmic progestin receptors 40 h later in both the hypothalamus-preoptic area-septum and midbrain. These findings are consistent with the notion that a decrease in brain progestin receptor concentration is one of the subcellular mechanisms involved in failure of a second dose of progesterone to facilitate lordosis behavior in estrogen-primed animals exposed to a first dose of progesterone 24–40 h earlier.