Progestin receptor induction and sexual behavior by estradiol treatment in male and female rats.

Abstract Sex differences in the induction of cytosolic progestin receptors (CPR) by estrogen priming were correlated with the sex differences in behavioral responses. We evaluated the temporal relationship between CPR in several brain regions and pituitary and the time-course of 17beta-estradiol (E(2)) activation of female sexual behavior in gonadectomized male and female rats implanted with subcutaneous E(2) Silastic capsules for 6 h, 24 h and 48 h. Both CPR levels and mating behavior increase monotonically with the time of E(2) exposure. Induction of CPR was observed in the periventricular region of the preoptic area (PVPOA), arcuate nucleus (ARC), ventromedial nuclei (VMN) and pituitary in both sexes. A small induction of CPR was found in parietal cortex. The VMN in female rats showed a significant E(2)-induced CPR increase at all times of exposure, while in male rats this induction was only significant after 24 h. Significant sex differences in absolute CPR levels and E(2)-induced receptors were found in the following structures: VMN, 18 h after 6 h of E(2) treatment and after 24 h and 48 h of continuous E(2) exposure; PVPOA, only after 48 h of continuous E(2) exposure; ARC at 24 h and 48 h; and pituitary after all E(2) treatment. Mating behavior was tested under two conditions: E(2) alone (2 h after removal of E(2) capsules) and E(2)+progesterone (2 h after a progesterone injection given 10 min after concluding the first test). Receptivity was first observed after 24 h E(2) exposure in female rats, whereas in male rats a small response appeared only after 48 h of E(2) exposure. After progesterone priming, the time of E(2) exposure necessary for expression of female sexual behavior was reduced to 6 h in females and 24 h in males. The appearance of mating behavior appears to follow that of inducible CPR in the VMN in both sexes. In addition, the CPR levels associated with the first receptivity either by male rats (16.6 fmol/mg protein) or female rats (15.3 fmol/mg protein) are very similar suggesting the presence of a threshold level controlling the expression of feminine sexual behavior. It is likely that inhibitory neural input plays a role in determining the threshold level of E(2)-induced CPR, which is sufficient to trigger lordosis behavior.