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

Low doses of estradiol, administered as pulses, are as effective as higher doses for priming ovariectomized (OVX) guinea pigs to display progesterone-facilitated lordosis. High doses of estradiol, administered by constant-release implants, induce progestin receptors in many substance P-immunoreactive (SP-IR) neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes OVX guinea pigs to respond behaviorally to progesterone. To test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH, OVX females received estradiol implants 1 week prior to perfusion, or two pulses of estradiol- 17β, injected 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. No significant differences were observed in the total number of progestin receptor-immunoreactive (PR-IR) or substance P-immunoreactive cells in the VLH and VLH/ventromedial hypothalamus (VMH), respectively, of females receiving the two estradiol treatments. However, the percentage of PR-IR cells in the VLH also immunoreactive for SP was significantly higher in the estradiol pulse-treated (53%), than in the estradiol capsule-implanted animals (36%). These data suggest that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH and are consistent with the hypothesis that substance P is involved in progesterone-facilitated lordosis in guinea pigs.     

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

Pulsatile administration of estradiol effectively primes orchidectomized (ORCH) male guinea pigs to display progesterone-facilitated lordosis. In contrast, a single injection of estradiol benzoate (EB) is not behaviorally effective. In ovariectomized female guinea pigs, estradiol pulses induce progestin receptors selectively in substance P neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes females to respond behaviorally to progesterone. To test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P neurons in the VLH in males, ORCH animals received a single injection of EB 40 h before, or two pulses of estradiol-17β, 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. The only difference found between the two estradiol treatment groups was a higher number of progestin receptorimmunoreactive (PR-IR) cells in the rostral VLH of estradiol pulse-treated males. There were no significant differences in the number of PR-IR cells in the mid- or caudal VLH, nor in the number of substance P-immunoreactive (SP-IR) neurons in the VLH/ventromedial hypothalamus (VMH) of animals receiving the two estradiol treatments. Furthermore, the percentage of PR-IR cells in the VLH also immunoreactive for SP did not differ between the estradiol pulse- (22%–25%) and the EB-injected animals (22%–32%). These data do not support the hypothesis that administration of behaviorally effective estradiol pulses, as compared to behaviorally ineffective EB injections, induce progestin receptors selectively in substance P neurons in the VLH of male guinea pigs.     

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

Pulsatile administration of estradiol effectively primes orchidectomized (ORCH) male guinea pigs to display progesterone-facilitated lordosis. In contrast, a single injection of estradiol benzoate (EB) is not behaviorally effective. In ovariectomized female guinea pigs, estradiol pulses induce progestin receptors selectively in substance P neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes females to respond behaviorally to progesterone. To test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P neurons in the VLH in males, ORCH animals received a single injection of EB 40 h before, or two pulses of estradiol-17 beta, 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. The only difference found between the two estradiol treatment groups was a higher number of progestin receptor-immunoreactive (PR-IR) cells in the rostral VLH of estradiol pulse-treated males. There were no significant differences in the number of PR-IR cells in the mid- or caudal VLH, nor in the number of substance P-immunoreactive (SP-IR) neurons in the VLH/ventromedial hypothalamus (VMH) of animals receiving the two estradiol treatments. Furthermore, the percentage of PR-IR cells in the VLH also immunoreactive for SP did not differ between the estradiol pulse- (22%-25%) and the EB-injected animals (22%-32%). These data do not support the hypothesis that administration of behaviorally effective estradiol pulses, as compared to behaviorally ineffective EB injections, induce progestin receptors selectively in substance P neurons in the VLH of male guinea pigs.     

Development of estradiol-induced progestin receptor immunoreactivity in the hypothalamus of female guinea pigs

The inability of young female guinea pigs to display progesterone-facilitated lordosis has been attributed, in part, to a deficiency in the concentration of hypothalamic estradiol-induced progestin receptors, as measured by in vitro binding assays. An immunocytochemical technique was used to ascertain where, within the mediobasal hypothalamus, estradiol-induced progestin receptor levels are lower in immature than in adult females. Adult (greater than 7 weeks) and juvenile (3 weeks) ovariectomized females received 10 micrograms estradiol benzoate, a dose that primes adult, but not immature females to respond behaviorally to progesterone. Progestin receptor-immunoreactive (PR-IR) cells were counted in the arcuate nucleus (ARC) and ventrolateral hypothalamus (VLH), the two regions containing the densest populations of estradiol-induced progestin receptors in the mediobasal hypothalamus. There was no age difference in the number of PR-IR cells in the rostral or caudal VLH, but immunostaining was darker in the rostral VLH of juveniles as compared to adults. We found similar numbers of PR-IR cells in the rostral and mid-ARC, but 35% fewer immunostained cells in the caudal ARC of immature, as compared to adult females. Furthermore, staining intensity was weaker in the mid- and caudal ARC of the juvenile females. These data suggest that the ARC, not the VLH, is a site of fewer estradiol-induced progestin receptors in immature females.     

Development of steroid-induced lordosis in female guinea pigs: effects of different estradiol and progesterone treatments, clonidine, and early weaning

The ability of developing ovariectomized (OVX) guinea pigs to display lordosis following a variety of steroid treatments which are behaviorally effective in adults was examined. Females OVX at 11 days of age did not display lordosis at 20 days of age, following treatment with several dose combinations of estradiol benzoate (EB, 10-50 micrograms) and progesterone (0.5-5 mg). By 30 days of age, 25% of the animals responded to EB plus progesterone, and by 40 days of age, adult-typical responses were observed. The developmental profile of responsiveness to steroids was not altered by varying the age at OVX, or by allowing pups to remain with a lactating mother. OVX females given estradiol (E2) implants did not exhibit progesterone-facilitated lordosis earlier than those treated with EB: however, the former group did show an unusually high incidence of progesterone-independent lordosis at 40 days of age. Twenty-day-old females also did not respond behaviorally to discrete pulses of E2 followed by progesterone, a treatment which was very effective in adults. Finally, lordosis was not facilitated in EB-primed, 20-day-old females by the alpha-noradrenergic agonist, clonidine, a treatment which was effective in adult females. These data illustrate a variety of conditions under which juvenile female guinea pigs do not exhibit steroid-induced lordosis. Since 20-day-old, EB-treated females also did not exhibit clonidine-facilitated lordosis, incomplete development of the central steroid-responsive and/or noradrenergic systems may contribute to the inability to display steroid-induced estrous behavior at this age.     

Immunocytochemical colocalization of progestin receptors and β-endorphin or enkephalin in the hypothalamus of female guinea pigs

Double-label immunocytochemistry was used to determine whether estradiol-induced progestin receptors and either β-endorphin or leucine-enkephalin are colocalized in female guinea pig brain. Ovariectomized, adult guinea pigs were implanted with capsules containing estradiol-17β to induce high levels of progestin receptors, and injected intracerebroventricularly with co chicine to improve visualization of the opiate peptides. Sections through the hypothalamus and preoptic area were processed for progestin receptor, followed by β-endorphin or leucine-enkephalin immunocytochemistry. As reported previously, high concentrations of progestin receptor-immunoreactive (PR-IR) cells were found in the preoptic area (medial and periventricular portions, medial preoptic nucleus) and hypothalamus (anterior hypothalamic and arcuate nuclei, ventrolateral area). Many β-endorphin-IR cells contained PR-IR in the arcuate nucleus and its surroundings (33%) and in the dorsomedial area of the hypothalamus (64%). Scattered enkephalin-IR cells were found in the septal nucleus, medial and lateral preoptic area, bed nucleus of the stria terminalis, and the arcuate nucleus. The ventromedial nucleus of the hypothalamus and dorsolateral magnocellular nucleus, respectively, contained moderate and heavy concentrations of enkephalin-IR cells. Although some of these areas also contained PR-IR, enkephalin-IR was colocalized consistently with PR-IR only in a small number of cells in the arcuate nucleus and ventromedial/ventrolateral area of the hypothalamus. These data, taken together with earlier observations that virtually all cells containing estradiol-induced PR-IR also contain estrogen receptor-IR, provide neuroanatomical evidence that hypothalamic actions of progesterone and estradiol may be mediated by β-endorphin and/or enkephalin.     

Down-regulation of progestin receptors in guinea pig brain: new findings using an immunocytochemical technique

Progesterone injection in estradiol-primed, ovariectomized guinea pigs results in down-regulation of hypothalamic progestin receptors determined by in vitro binding assays. In order to determine if progesterone also decreases immunostaining of progestin receptors and if progestin receptors are down-regulated preferentially in particular neuroanatomical areas, ovariectomized guinea pigs were injected with doses of estradiol benzoate (10 micrograms at 42 h before progesterone injection) and progesterone (500 micrograms at 4, 12, or 24 h before perfusion) that reliably induce the expression of lordosis and subsequent behavioral refractoriness to progesterone. Progestin receptor-immunoreactive cells were counted in sections from discrete parts of the preoptic area and hypothalamus. As expected, estradiol dramatically increased cell nuclear, and, to a lesser extent, cytoplasmic, immunostaining in defined regions of the preoptic area and hypothalamus. By 12 h after progesterone injection, the number of progestin receptor-immunoreactive cells had decreased in some areas, but not others. The rostral and caudal aspects of the ventrolateral hypothalamus were particularly responsive showing a substantial decrease in progestin receptor-immunoreactivity by 12 h after injection. No decreases in the progestin receptor-immunoreactive cell number were observed in any of the preoptic regions examined, although obvious decreases in immunostaining intensity were seen. The results of these immunocytochemical experiments extend earlier findings from in vitro progestin binding experiments and demonstrate that as with progestin binding, progestin receptor-immunoreactivity decreases when progesterone is injected in a behavioral desensitization procedure. Furthermore, they point to the ventrolateral hypothalamus as one site in which the down-regulation of progestin receptors may be particularly responsive to progesterone.     

Immunocytochemical colocalization of progestin receptors and beta-endorphin or enkephalin in the hypothalamus of female guinea pigs

Double-label immunocytochemistry was used to determine whether estradiol-induced progestin receptors and either beta-endorphin or leucine-enkephalin are colocalized in female guinea pig brain. Ovariectomized, adult guinea pigs were implanted with capsules containing estradiol-17 beta to induce high levels of progestin receptors, and injected intracerebroventricularly with colchicine to improve visualization of the opiate peptides. Sections through the hypothalamus and preoptic area were processed for progestin receptor, followed by beta-endorphin or leucine-enkephalin immunocytochemistry. As reported previously, high concentrations of progestin receptor-immunoreactive (PR-IR) cells were found in the preoptic area (medial and periventricular portions, medial preoptic nucleus) and hypothalamus (anterior hypothalamic and arcuate nuclei, ventrolateral area). Many beta-endorphin-IR cells contained PR-IR in the arcuate nucleus and its surroundings (33%) and in the dorsomedial area of the hypothalamus (64%). Scattered enkephalin-IR cells were found in the septal nucleus, medial and lateral preoptic area, bed nucleus of the stria terminalis, and the arcuate nucleus. The ventromedial nucleus of the hypothalamus and dorsolateral magnocellular nucleus, respectively, contained moderate and heavy concentrations of enkephalin-IR cells. Although some of these areas also contained PR-IR, enkephalin-IR was colocalized consistently with PR-IR only in a small number of cells in the arcuate nucleus and ventromedial/ventrolateral area of the hypothalamus. These data, taken together with earlier observations that virtually all cells containing estradiol-induced PR-IR also contain estrogen receptor-IR, provide neuroanatomical evidence that hypothalamic actions of progesterone and estradiol may be mediated by beta-endorphin and/or enkephalin.     

Facilitation of lordosis in guinea pigs by an alpha-noradrenergic agonist is independent of progestin receptor stimulation

Because manipulations of the noradrenergic system affect both lordosis behavior and progestin receptor levels in female guinea pigs, the present study attempted to determine if the noradrenergic (NE) system affects lordosis solely because of its impact on progestin receptors. Although the progestin receptor antagonist RU486 significantly reduced progesterone-facilitated lordosis, it had no effect on lordosis induced by the alpha-NE agonist clonidine in estrogen-primed female guinea pigs. This indicates that although progesterone may facilitate lordosis in female guinea pigs via activation of progestin receptors, the alpha-noradrenergic agonist clonidine does not mediate lordosis through the same mechanism.     

Progesterone-Facilitated Lordosis in Medial Preoptic Area-Lesioned, Juvenile Guinea Pigs

Juvenile female guinea pigs rarely display lordosis in response to estradiol and progesterone treatments that elicit sexual receptivity in adults. To test the hypothesis that the medial preoptic area (MPOA) tonically inhibits the display of steroid-induced lordosis in juveniles, 11-day-old guinea pigs were ovariectomized (OVX) and received bilateral, sham, or electrolytic lesions aimed at the MPOA 3–4 days later. At 20–22 days of age, these females were tested for the expression of sexual receptivity following injections of estradiol benzoate (EB, 10 μg sc) and progesterone (0.5 mg sc, 40 h after EB). The lesions damaged portions of the MPOA, the nucleus of the diagonal band of Broca, the lateral aspect of the medial preoptic nucleus, the medial part of the preventricular portion of the periventricular nucleus, and the anterior commissure. The lesions did not alter the display of estradiol-induced lordosis. However, after treatment with EB plus progesterone, 20% of the sham-lesioned females displayed lordosis, as compared to 80% of the MPOA-lesioned animals. These data are consistent with the hypothesis that neurons originating in and/or traversing the MPOA tonically suppress the display of progesterone-facilitated lordosis in juvenile guinea pigs. Removal of this inhibitory input allows prepubertal females to respond behaviorally to estradiol and progesterone in an adult-typical fashion.     

Anti-estrogenic suppression of the lordosis response in female rats

The anti-estrogen, CI 628, was used to suppress the lordosis response induced by sequential injections of estrogen and progesterone in ovariectomized (OVX) female rats. Appropriate doses of CI 628 completely abolished sexual receptivity in females administered estradiol benzoate (EB) in sesame oil. This behavioral effect could be attenuated by providing increased quantities of EB or decreased quantities of CI 628. Anti-estrogenic effects on lordosis induced by free estradiol in saline (E) were assessed after first establishing behaviorally equivalent doses of EB and E. This was accomplished by determining thresholds for E-induced lordosis. OVX females were approximately seven times less sensitive to E than to EB. CI 628 had no significant effects on E-induced lordosis, in contrast to the complete abolition of lordosis in females treated with behaviorally equivalent EB doses. A possible mechanism to explain this differential responsiveness of EB- and E-treated females is discussed.     

Progesterone facilitation of lordosis in male and female Sprague-Dawley rats following priming with estradiol pulses

Adult male Sprague-Dawley rats rarely exhibit progesterone-facilitated lordosis following steroid treatments which are effective in females. In contrast, progesterone-facilitated lordosis has been observed following priming with estradiol pulses in another strain. The aim of this study was to compare progesterone-facilitated feminine sexual behavior in adult male and female Sprague-Dawley rats following priming with estradiol benzoate (EB) or estradiol pulses. Female sexual behavior was measured in adult, gonadectomized males and females treated as follows: Two pulses of estradiol followed by progesterone or oil the next day; EB (two doses) for 3 days, and progesterone or oil the next day. These protocols were repeated at 4- or 6-day intervals, respectively. Progesterone-facilitated lordosis was observed consistently in both sexes treated with estradiol pulses. By the fifth test, lordosis quotients did not differ between the sexes, but the lordosis ratings in progesterone-treated males remained lower than those observed in females. Proceptivity (hop-darting) was facilitated by progesterone in females, but was never observed in males. Lordosis was induced in both sexes by 15 micrograms EB, but was not reliably facilitated by progesterone. Treatment with the lower dose of EB (1.5 micrograms) induced high levels of receptivity in females (occasionally facilitated by progesterone), but not in males regardless of subsequent treatment (i.e, progesterone or oil). These data suggest that progesterone-facilitated lordosis can be induced in male Sprague-Dawley rats, if a regimen of estradiol pulses is used. Thus, the brain of the adult male is not inflexibly differentiated with regard to progesterone facilitation of feminine receptive behavior.     

Progesterone in high doses may overcome progesterone's desensitization effect on lordosis by translocation of hypothalamic progestin receptors

The cellular mechanism by which progesterone desensitizes the central nervous sytem to further facilitation of lordosis by progesterone was studied. In the first experiment guinea pigs were injected with doses of estradiol benzoate and progesterone that result in refractoriness to further stimulation of lordosis by moderate doses of progesterone. Confirming previous reports, a large dose of progesterone was effective in overcoming this desensitization, resulting in lordosis responses. A lower dose of progesterone was ineffective in this regard. In subsequent experiments, cytosol and nuclear progestin receptors were measured in the hypothalamus-preoptic area and cerebral cortex with an in vitro exchange assay using [³H]R 5020 as the ligand. A high, effective dose of progesterone resulted in 46–48% higher levels of nuclear progestin receptors assayed in the hypothalamus-preoptic area than did a lower, ineffective dose. These results are consistent with the idea that progesterone injection in estradiol-primed rodents causes a subsequent desensitization to itself by decreasing the concentration of cytoplasmic progestin receptors in hypothalamus-preoptic area. A subsequent progesterone injection then results in a lower level of nuclear progestin receptors. Large doses of progesterone may overcome this desensitization by causing accumulation of levels of nuclear progestin receptors sufficient to facilitate lordosis.     

Effects of serotonin agonists on lordosis,myoclonus, and cytoplasmic progestin receptors in guinea pigs

Peripheral treatment with the serotonin releaser fenfluramine or the serotonin agonist quipazine abolished lordosis behavior in ovariectomized estradiol and progesterone-primed female guinea pigs. Quipazine was also effective when administered into a lateral cerebroventricle. The lowest dose of fenfluramine that induced myoclonus (10 mg/kg) was higher than the dose needed to inhibit lordosis (5 mg/kg). Therefore, it appears that myoclonus and lordosis are differentially sensitive to serotonin agonists. The effects of quipazine on lordosis were time dependent. Quipazine had no effect on lordosis when given prior to the onset of sexual receptivity. These data suggest that serotonin agonists might be effective only when progesterone has had sufficient time to induce sexual receptivity. Quipazine did not affect cytoplasmic progestin receptors in brain areas involved in steroid hormone effects on lordosis. This finding, and the finding that quipazine had no effect on lordosis when given prior to the onset of sexual receptivity, suggest increased serotonin transmission does not interfere with estrogen priming or sensitivity of hypothalamic cells to progesterone.     

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH–ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH–ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol + progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol + progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol + progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.     

Different doses of estradiol benzoate induce conditioned place preference after paced mating

The ovarian hormones estrogen and progesterone are required for the complete display of sexual behavior in female rats. Paced mating produces a reward state in intact cycling and ovariectomized (OVX), hormonally primed females as evaluated by the conditioned place preference (CPP) paradigm. Most of the studies that have evaluated CPP induced by paced mating in OVX females have used relatively high doses of estradiol benzoate (EB). In the present study we determined if different doses of EB, combined with progesterone (P), could induce CPP after paced mating. For this purpose OVX female rats were divided in five groups that received one of different doses of estradiol benzoate (5, 2.5, 1.25 or 0.625 μg estradiol + 0.5 mg of progesterone) before being allowed to pace the sexual interaction and conditioned in a CPP paradigm. We found that the lowest dose of EB used (0.625 μg) significantly reduced the lordosis quotient and the lordosis coefficient. Even though these females paced the sexual interaction, they didn't change its original preference, suggesting that sexual interaction did not induce a positive affective, reward state. Females allowed to pace the sexual interaction with higher doses of EB developed CPP after paced mating. These results indicate that a threshold of estradiol is required for paced mating to induce CPP.     

金黄地鼠视觉中枢发育过程中P物质神经元数量及分布的变化

本实验用免疫细胞化学技术观察了不同年龄金黄地鼠视皮层和上丘中P物质(SP)阳性神经元数量和分布的变化,同时观察了不同年龄金黄地鼠视皮层SP阳性神经元的形态和类型。结果表明,出生后10天小鼠视皮层SP阳性神经元为36％,Ⅱ—Ⅳ层密度最大,约占40％。上丘中SP阳性神经元约为37％。出生后20天,视皮层及上丘中SP阳性神经元分别减少到23％和16％。视皮层Ⅱ—Ⅳ层减少最明显,Ⅴ层和Ⅵ层变化不大。成年鼠视皮层及上丘中偶见SP神经元,但出现一些SP阳性纤维。出生10天及20天鼠视皮层中SP阳性神经元的形态及类型没有差别。     

Tamoxifen and ICI 182,780 activate hypothalamic G protein-coupled estrogen receptor 1 to rapidly facilitate lordosis in female rats

In the female rat, sexual receptivity (lordosis) can be facilitated by sequential activation of estrogen receptor (ER) α and G protein-coupled estrogen receptor 1 (GPER) by estradiol. In the estradiol benzoate (EB) primed ovariectomized (OVX) rat, EB initially binds to ERα in the plasma membrane that complexes with and transactivates metabotropic glutamate receptor 1a to activate β-endorphin neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). This activates MPN μ-opioid receptors (MOP), inhibiting lordosis. Infusion of non-esterified 17β-estradiol into the ARH rapidly reduces MPN MOP activation and facilitates lordosis via GPER. Tamoxifen (TAM) and ICI 182,780 (ICI) are selective estrogen receptor modulators that activate GPER. Therefore, we tested the hypothesis that TAM and ICI rapidly facilitate lordosis via activation of GPER in the ARH. Our first experiment demonstrated that injection of TAM intraperitoneal, or ICI into the lateral ventricle, deactivated MPN MOP and facilitated lordosis in EB-primed rats. We then tested whether TAM and ICI were acting rapidly through a GPER dependent pathway in the ARH. In EB-primed rats, ARH infusion of either TAM or ICI facilitated lordosis and reduced MPN MOP activation within 30 min compared to controls. These effects were blocked by pretreatment with the GPER antagonist, G15. Our findings demonstrate that TAM and ICI deactivate MPN MOP and facilitate lordosis in a GPER dependent manner. Thus, TAM and ICI may activate GPER in the CNS to produce estrogenic actions in neural circuits that modulate physiology and behavior.     

Ovarian steroids and hypothalamic norepinephrine release: studies using in vivo brain microdialysis

This study employed microdialysis in urethane-anesthetized female rats to monitor ovarian steroid-dependent changes in KCl-evoked levels of extracellular norepinephrine (NE) in the ventromedial hypothalamus. An initial KCl stimulus (Sl) increased NE from low or undetectable levels in all animals. A second KCl stimulus (S2) given several hr later evoked 40% less NE release than did Sl in ovariectomized (OVX) females or OVX females given only estrogen or progestin. In contrast, the two KCl-evoked NE releases were equivalent in OVX females administered both estrogen and progestin. These results suggest that ovarian steroids may act as presynaptic modulators of NE release in the ventromedial hypothalamus.     

Induction of Cytosolic Progestin Binding Sites by Catecholestrogens in Rat Pituitary Gland and Uterus: Different Potencies of 2- and 4-Hydroxyestradiol

The ability of catecholestrogens to induce cytosolic progestin binding sites in the hypothalamus, pituitary gland, and uterus of ovariectomised-adrenalectomised rats was demonstrated by the increase in high-affinity [3H]promegestone binding sites (KD 1.39, 0.50, and 0.54 nM, respectively) following a single subcutaneous injection (26.4 micrograms/animal) of the 3.4-dibenzoate ester of 4-hydroxyestradiol. The affinity and the time course of induction of these binding sites were very similar to those after a single injection of an equivalent dose (20 micrograms/animal) of estradiol 3-benzoate, exhibiting maximal receptor levels after 44 h. Widely differing efficacies in the induction of progestin binding sites were observed between the dibenzoate esters of 2- and 4-hydroxyestradiol. 2-Hydroxyestradiol 2,3-dibenzoate was ineffective in the pituitary gland up to a dose of 132 micrograms/animal, whereas 4-hydroxyestradiol dibenzoate was equipotent to estradiol benzoate, showing a maximal induction of progestin binding sites at single doses in the range of 13.2-26.4 micrograms/animal (equivalent to 10-20 micrograms of estradiol benzoate). As compared to the pituitary gland, the uterus was much more sensitive to the systemic administration of estrogen benzoates. At single doses in the range of 1.32-6.6 micrograms/animal (equivalent to 1-5 micrograms of estradiol benzoate), 4-hydroxyestradiol dibenzoate induced maximal levels of progestin receptors, and even 2-hydroxyestradiol dibenzoate, when given at a high dose (132.4 micrograms/animal, equivalent to 100 micrograms of estradiol benzoate), produced a slight increase in progestin binding sites.