Biochemical and Immunocytochemical Assessment of Neural Progestin Receptors Following Estradiol Treatments that Eliminate the Sex Difference in Progesterone‐Facilitated Lordosis in Guinea‐Pigs

Abstract

A single injection of estradiol benzoate (10 pg), while highly effective in ovariectomized female guinea-pigs, does not prime castrated males to display progesterone-facilitated lordosis. In contrast, adult males and females exhibit the same, high degree of progesterone-facilitated lordosis when primed with two, 2.0 pg injections of free estradiol-17B (pulse regimen). We compared neural progestin receptor induction after these different estradiol treatments by in vitro radioligand binding assays and imrnunocytochemistry. Binding assays confirmed previous observations of lower concentrations of cytosol progestin receptors in the rnediobasal hypothalamus-preoptic area in estradiol benzoate-treated males than in females. No such sex difference was observed in animals that had been exposed to the behaviorally effective estradiol pulse regimen; rather, hypothalamic-preoptic area progestin receptor concentrations in these animals did not differ from the low levels observed in males treated with the behaviorally ineffective estradiol benzoate regimen. lmmunocytochemical analysis revealed progestin receptor-immunoreactivity in fewer cells in the medial preoptic nucleus-anterior hypothalamic nucleus, periventricular preoptic area and arcuate nucleus of estradiol pulse-as compared to estradiol benzoate-treated males and females. Estradiol benzoate treatment induced progestin receptor-immunoreactivity in more cells in the medial preoptic area and ventrolateral hypothalamus than estradiol pulses in males, but not in females. Surprisingly, in these regions estradiol benzoate-treated males had significantly more progestin receptor-immunoreactive cells than females.These experiments yield two major findings: First, as has been shown in rats, the display of progesterone-facilitated lordosis is not inflexibly differentiated according to sex in guinea-pigs. Furthermore, reduced concentrations of estradiol-induced progestin receptors in the hypothalamus and preoptic area cannot account for the lack of progesterone-facilitated lordosis that is observed following priming with estradiol benzoate in males. Secondly, in the medial preoptic area and ventrolateral hypothalamus of female guinea-pigs, estradiol pulses are as effective as estradiol benzoate in inducing progestin receptors. These observations, taken together with the finding of equal behavioral efficacy of the two estradiol treatments, are consistent with the hypothesis that estradiol-induced progestin receptors in these regions of the brain are involved in progesterone-facilitated lordosis in female guinea-pigs.The display of feminine sexual behavior in many rodents, including guinea-pigs, is believed to be sexually differentiated. Exposure of the inherently female brain to gonadal steroid hormones (testosterone and/or estradiol) during a pennatal 'critical period' permanently masculinizes and defeminizes the neural mechanisms underlying the expression of sexual behavior and estrous cyclicityThe sexually receptive posture, lordosis, can be induced in ovariectomized ...

Estradiol pulses induce progestin receptors selectively in substance P‐immunoreactive neurons in the ventrolateral hypothalamus of female guinea pigs

“…This idea is also supported by the strong correlation of VLH PR-IR and behavioral sensitivity to progesterone following estradiol pulses (Olster and Blaustein, 1990a) and during progesterone-induced desensitization (Blaustein and Turcotte, 1990). This region may be analogous to the rat ventrolateral subdivision of the ventromedial nucleus of the hypothalamus (VL-VMN) , which is also behaviorally responsive to estradiol and progesterone (Rubin and Barfield, 1983a,b), and which contains a dense population of estrogen- (Stumpf, 1968;Pfaff and Keiner, 1973) and progestin-concentrating cells ( Warembourg, 1978;Sar, 1988;Fox, Harlan, Shivers, and Pfaff, 1990).…”

“…We and others have found that treatment of OVX guinea pigs with low doses of estradiol-17p, administered as pulses, is as effective as higher doses of the hormone (estradiol-I7p in Silastic capsules, or single injections of estradiol benzoate), in priming for behavioral responsiveness to progesterone ( Wilcox, Barclay, and Feder, 1984;Olster and Blaustein, 1990a). Furthermore, the induction of progestin receptors in the VLH is similar following injections of estradiol pulses and estradiol benzoate (Olster and Blaustein, 1990a), suggesting extraordinary sensitivity of some neurons in this particular region to the progestin receptor-inducing actions of behaviorally effective estradiol pulses.…”

Sex differences in the physiology of eating

“…Because reproductive hormone levels cycle in female rats, however, constant-release pellets cannot be considered physiological. Indeed, daily or continuous peripheral administration of low estradiol doses or even single high doses of long-lasting estrogens such as estradiol valerate can disrupt ovarian cycling, induce pseudopregnancy, elicit progressive, aphysiological changes in several brain neurochemical receptor systems and in behavior, and greatly accelerate the degeneration of the arcuate nucleus (83,179,347,467,468,528,529,647). In contrast, a weekly cyclic estradiol injection regimen in which 10 g estradiol benzoate was injected on Tuesdays and Wednesdays and progesterone priming and sexual-receptivity tests were done on Fridays led to stable, normal levels of progestin and oxytocin receptors and in sexual receptivity (647).…”

Progestin receptors in substance P‐immunoreactive neurons in the hypothalamus of male guinea pigs after behaviorally effective estradiol pulse treatment

“…A p value < 0.05 was the criterion for statistical significance. (Olster and Blaustein, 1990), estradiol treatment of ORCH guinea pigs resulted in the induction of PR-IR in many cells of the VLH (Figs. 1, 2).…”

“…It was hypothesized that lower concentrations of estradiol-induced progestin receptors in the mediobasal hypothalamus and preoptic area in orchidectomized (ORCH) males contributed to the lack of behavioral responsiveness to progesterone ( Blaustein, Ryer, and Feder, 1980). However, we have found that administration of estradiol in the form of discrete pulses [in contrast to a single injection of the longer-acting estradiol benzoate ( EB)] effectively primes ORCH males to display progesterone-facilitated lordosis ( Olster and Blaustein, 1990). Furthermore, the concentrations of progestin receptors (measured by in vitro radioligand binding assay) in combined mediobasal hypothalamus and preoptic area are not different in EB-and estradiol pulse-injected males; moreover, the number of progestin receptor-immunoreactive ( PR-IR) cells in several regions within the hypothalamus and preoptic area (i.e., VLH, medial preoptic area) is actually higher following injections of behaviorally ineffective EB, as compared to behaviorally effective estradiol pulses (Olster and Blaustein, 1990).…”