The Temporal Relationship between Estrogen-Inducible Progestin Receptors in the Female Rat Brain and the Time Course of Estrogen Activation of Mating Behavior*

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Abstract
17β-Estradiol has been shown to induce progestin receptor synthesis in those areas of the brain which mediate sexual behavior, the mediobasal hypothalamus (MBH) and the preoptic area (POA). In this study, we explored the temporal relationship between inducible progestin receptors in both MBH-POA and pituitary and the time course of estrogen activation of mating behavior. Both mating behavior and cytosol [3H]R5020 binding in brain and pituitary increase monotonically when Silastic capsules containing 17β-estradiol are implanted and decrease monotonically when the capsules are removed. After implantation of estradiol, increases in cytosol [3H]R5020 binding are seen at 12, 24, 36, and 48 h. Receptivity is first observed at 18 h; subsequent increases in the lordosis quotient score are seen at 24, 36, and 48 h. After 48 h, inducible progestin receptors in both MBH-POA and pituitary are maximal, and sexual receptivity is approaching maximal levels. After removal of estradiol, decreases in cytosol [3H]R5020 binding and in mating behavior are observed at 12, 24, 36, and 48 h. After 48 h, inducible progestin receptors in both the MBH-POA and pituitary approach the levels seen in unstimulated animals, and sexual receptivity is not observed. The time course of decay of estradiol-inducible progestin receptor is a first order process with a half life of 24.7 h in the MBH-POA and 26.6 h in the pituitary. Because sexual receptivity is first observed when inducible progestin receptors in the brain are 26% of the maximal level and is last observed when progestin receptors in brain are 34% of the maximal level, the level of inducible progestin receptors in the MBH-POA in the female rat brain appears to be a correlate of the competence to express feminine sexual behavior.