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 timecourse of 17P-estradiol (E,) activation of female sexual behavior in gonadectomized male and female rats implanted with sribcutaneous E, Silastic capsules for 6 h, 24 h and 48 h. Both CPR levels and mating behavior increase monotonically with the time 0; E, 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,-induced CPR increase a t 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,-induced receptors were found in the following structures: VMN, 18 h after 6 h of E, treatment and after 24 h and 48 h of continuous E, exposure; PVPOA, only after 48 h of continuous E, exposure; ARC at 24 h and 48 h; and pituitary after all E, treatment. Mating behavior was tested under two conditions: E, alone (2 h after removal of E, capsules) and E,+ progesterone (2 h after a progesterone injection given 10 min after concluding the first test). Receptivity was first observed after 24 h E, exposure in female rats, whereas in male rats a small response appeared only after 48 h of E, exposure. After progesterone priming, the time of E, 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 in!iibitory neural input plays a role in determining the threshold level of E,-induced CPR, which is sufficient to trigger lordosis behavior.Thc sequential secretion of ovarian steroid 17B-estradiol (E,) and Progesterone (P) during the estrous cycle, interact synergistically controlling the onset, quality and duration of the female rat sexual behavior (1, 2). The gonadal steroid effect on this behavior expression can be mimicked in ovariectomized female rats by trei tment with physiological doses of estrogens followed by P priming (3, 4).In male rats the expression of female sexual behavior is not normally expressed. However, chronic administration of estradiol kilxoate (EB) in various dosages to castrated males resulted in a moderate increase of the lordosis behavior (5-7) which is poorly or not at all increased by P treatment (8,9). The sex differences in the response to E, and P treatment have been attributed to diff'crences in the est...

Estrogen Increases Spine Density in Ventromedial Hypothalamic Neurons of Peripubertal Rats

“…The fact that these sex differences are evident after the critical period of sex ual differentiation but before puberty strongly suggests that they are the product of the developmental actions of testosterone in males, rendering them less sensitive to estradiol. This has pre viously been observed for progestin receptor induction in the VMN [45][46][47] and shown to depend on the postnatal organiz ing effects of testosterone [48] in determining the neurotrophic response of the VMN neurons to estradiol.…”

Colocalization of Progesterone Receptor and Somatostatin Immunoreactivities in the Hypothalamus of the Male and Female Guinea Pig

“…The absence of dif ferences in the numbers of PR-IR cells in the Ar is in agreement with the measurements of PR made by Thorn ton et al [24], However, these authors have not deter mined whether there is any sex difference in the VL. On the other hand, in rats treated with estradiol benzoate, several studies have shown marked sex differences in the PR levels and the numbers of cells expressing PR mRNA in the ventrolateral aspect of the ventromedial nucleus and the Ar [25,26]. The discrepancies may arise from methodological differences or from differences in ana tomical definition of the regions studied.…”

Sex Differences in the Regulation of Oxytocin Receptors by Ovarian Steroids in the Ventromedial Hypothalamus of the Rat

“…Other sexually dimorphic membrane actions of steroids like the modula tion of dopamine release by E in the striatum, have been associated with sexually dimorphic sensorimotor func tions [34], However, a reduced effectiveness of E in induc ing intracellular receptors for P also contributes to the be havioral insensitivity of male rats to P [35,36] and the activation of protein synthesis by P bound to estrogen-in duced receptors is important for the facilitation of lor dosis behavior [37,38]. Thus, sexually dimorphic geno mic and membrane actions of steroid hormones may both contribute to the sexual dimorphism that affects repro ductive behavior.…”