Lordosis facilitated by GPER-1 receptor activation involves GnRH-1, progestin and estrogen receptors in estrogen-primed rats

The present study assessed the participation of membrane G-protein coupled estrogen receptor 1 (GPER-1) and gonadotropin releasing hormone 1 (GnRH-1) receptor in the display of lordosis induced by intracerebroventricular (icv) administration of G1, a GPER-1 agonist, and by unesterified 17β-estradiol (free E₂). In addition, we assessed the participation of both estrogen and progestin receptors in the lordosis behavior induced by G1 in ovariectomized (OVX), E₂-benzoate (EB)-primed rats. In Experiment 1, icv injection of G1 induced lordosis behavior at 120 and 240min. In Experiment 2, icv injection of the GPER-1 antagonist G15 significantly reduced lordosis behavior induced by either G1 or free E₂. In addition, Antide, a GnRH-1 receptor antagonist, significantly depressed G1 facilitation of lordosis behavior in OVX, EB-primed rats. Similarly, icv injection of Antide blocked the stimulatory effect of E₂ on lordosis behavior. In Experiment 3, systemic injection of either tamoxifen or RU486 significantly reduced lordosis behavior induced by icv administration of G1 in OVX, EB-primed rats. The results suggest that GnRH release activates both estrogen and progestin receptors and that this activation is important in the chain of events leading to the display of lordosis behavior in response to activation of GPER-1 in estrogen-primed rats.

Estrogen receptors regulate the estrous behavior induced by progestins, peptides, and prostaglandin E2

The role of classical estrogen receptors (ERs) in priming female reproductive behavior has been studied previously; however, the participation of this receptor during activation of estrous behavior has not been extensively studied. The purpose of this work was to test the possibility that the facilitation of lordosis behavior in estrogen-primed rats by progesterone (P) and its 5α- and 5β-reduced metabolites, gonadotropin-releasing hormone (GnRH), leptin, prostaglandin E2 (PGE2) and vagino-cervical stimulation (VCS) involves interactions with classical ERs by using the selective ER modulator, tamoxifen. To further assess the role of ERs, we also explored the effects of the pure ER antagonist, ICI182780 (ICI), on estrous behavior induced by P and GnRH. Ovariectomized, estrogen-primed rats (5μg estradiol benzoate 40h earlier) were injected intraventricularly with the above-mentioned compounds, or they received VCS. All compounds and VCS effectively facilitated estrous behavior when tested at 60, 120 or 240min after infusion or application of VCS. Intraventricular infusion of tamoxifen (5μg), 30min before, significantly attenuated estrous behaviors induced in estradiol-primed rats by P, most of its 5α- and 5β-reduced metabolites, GnRH, and PGE2, but not by VCS. Although there was a trend for reduction, tamoxifen did not significantly decrease lordosis in females treated with 5β-pregnan-3,20-dione. ICI also inhibited lordosis behavior induced by P and GnRH at some testing intervals. These results suggest that activation of classical ERs participates in the triggering effects on estrous behavior induced by agents with different chemical structures that do not bind directly to ERs.

A pubertal immune challenge alters the antidepressant-like effects of chronic estradiol treatment in inbred and outbred adult female mice

Puberty is a period characterized by brain reorganization that contributes to the development of neural and behavioral responses to gonadal steroids. A single injection of the bacterial endotoxin, lipopolysaccharide (LPS), during the pubertal period decreases sexual receptivity in response to ovarian hormones in adulthood. Because chronic estradiol treatment alleviates depression-like symptoms in ovariectomized adult mice, we investigated the effect of pubertal LPS treatment on estradiol's antidepressant effects. We hypothesized that pubertal LPS treatment would decrease the antidepressant-like effect of estradiol in adult ovariectomized female mice, as it decreases other behavioral responses to ovarian hormones. As expected, chronic estradiol treatment decreased depression-like behavior, as measured by the duration of immobility, in saline-treated mice from two different strains, as well as in mice treated with LPS in adulthood. In contrast, in mice treated pubertally with LPS, estradiol strikingly increased the duration of immobility. No difference in body weight and in locomotion was found among the groups, suggesting that the differences in depression-like behavior were not due to differences in body weight or locomotor activity between LPS-treated and control mice. These results suggest that exposure to an immune challenge during the pubertal period alters the responsiveness of depression-like behavior to estradiol.

Steroid hormone receptors: long- and short-term integrators of the internal milieu and the external environment

Many of the influences of estrogens and progestins on the brain and behavior are mediated by estrogen receptors and progestin receptors, acting as transcriptional regulators. The homologous and heterologous regulation of the concentrations of these receptors by cognate hormones is well established. However, although they were discovered and characterized based on their binding to cognate hormone and their role in transcriptional regulation, steroid hormone receptors have a more complex role and serve many more functions than originally suspected. First, besides being regulated by steroid hormones, the intracellular concentrations of brain steroid hormone receptors are regulated by neurotransmitters, a pathway by which stimuli from the environment, including from conspecific animals, can modulate the concentration of particular steroid hormone receptors in subsets of cells. Further, besides being activated by cognate steroid hormones, the receptors can be activated by a variety of neurotransmitters and phosphorylation pathways, providing a route through which environmental stimulation can activate steroid-receptor-dependent functions in specific cells. In addition, the transcription factor, estrogen receptor-α, produced from the estrogen receptor-α gene, can be modified to be targeted to membranes, where it can signal via kinase pathways. Finally, developmental experiences, such as particular stressors during the pubertal period, can permanently remodel the brain's response to ovarian hormones, most likely by long-term changes in regulation of the receptors mediating those responses. In addition to their function in responding to cognate ligand, it is now more appropriate to think of steroid hormone receptors as integrators of a wide variety of signaling pathways.

Absence of progestin receptors alters distribution of vasopressin fibers but not sexual differentiation of vasopressin system in mice

Perinatal estrogens increase the number of vasopressin-expressing cells and the density of vasopressin-immunoreactive fibers observed in adult male rodents. The mechanism of action of estrogens on sexual differentiation of the extra-hypothalamic vasopressin system is unknown. We hypothesized that the sexually dimorphic expression of progestin receptors (PRs) during development would masculinize vasopressin expression in mice. We compared the number of vasopressin-expressing cells in the bed nucleus of the stria terminalis (BNST) and medial amygdala and the density of vasopressin-immunoreactive fibers in several brain regions of male and female wild type and PRKO mice using in situ hybridization and immunohistochemistry. As expected, sex differences in vasopressin cell number were observed in the BNST and medial amygdaloid nucleus. Vasopressin-immunoreactive fiber density was sexually dimorphic in the lateral septum, lateral habenular nucleus, medial amygdaloid nucleus, and mediodorsal thalamus. Sex differences were also observed in the principal nucleus of the BNST and medial preoptic area but not in the dorsomedial hypothalamus, which are thought to receive vasopressin innervation from the suprachiasmatic nucleus. Deletion of PRs did not alter the sex difference in vasopressin mRNA expression and vasopressin fiber immunoreactivity in any area examined. However, deletion of PRs increased the density of vasopressin fiber immunoreactivity in the lateral habenular nucleus. Our data suggest that PRs modulate vasopressin levels, but not sexual differentiation of vasopressin innervation in mice.

Immunocytochemical investigation of nuclear progestin receptor expression within dopaminergic neurones of the female rat brain

Progesterone influences most processes involved in female reproduction, including ovulation, sexual behaviour, pregnancy, parturition, lactation and maternal behaviour. One neurotransmitter through which progesterone might regulate many of these functions is dopamine. To determine where in the brain progesterone might alter dopaminergic activity necessary for these and other processes in rats via cell nuclear progestin receptors, ovariectomized rats were injected subcutaneously with either 4 micro g oestradiol benzoate to induce high levels of hypothalamic progestin receptor expression, or oil, and perfused 48 h later. Dual-label immunocytochemistry was used to visualize cells having immunoreactivity (ir) for progestin receptors and tyrosine hydroxylase, a rate-limiting enzyme for dopamine synthesis. Many cells containing both progestin receptor-ir and tyrosine hydroxylase-ir were found throughout the periventricular hypothalamus of oestradiol-treated females. Conversely, very few cells in the hypothalamus of oil-treated controls contained progestin receptor-ir and, consequently, few dual-labelled cells were found in this group. The greatest percentage of tyrosine hydroxylase immunoreactive cells expressing progestin receptors in oestradiol-treated females was in, or near, the arcuate nucleus (A12 group), where up to 55% of tyrosine hydroxylase-expressing cells coexpressed progestin receptors. Notably, dual-labelled cells in oestradiol-treated females were also found more rostrally than previously reported, with approximately 15-20% of tyrosine hydroxylase-ir cells in the preoptic area/anterior hypothalamus (A14 group) also containing progestin receptor-ir. No dual-labelled cells were found for either group in the posterodorsal hypothalamus (A11 group), zona incerta (A13 group), retrorubral field (A8 group), ventral tegmental area (A10 group) or substantia nigra (A9 group) because little or no progestin receptor-ir was found in these sites. These data provide new information about the neural substrate where progesterone might regulate dopamine release in the preoptic area/anterior hypothalamus. Using more sensitive techniques than those used previously, they also confirm the relationship between progestin receptor and tyrosine hydroxylase in the arcuate nucleus, which could be important for the regulation of prolactin release throughout the female reproductive cycle. Additionally, although progesterone alters mesolimbic and nigrostriatal dopamine release, and the numerous behaviours that these pathways influence, these data again suggest that it does not do so via nuclear progestin receptor in dopaminergic cells of the ventral tegmental area and substantia nigra.

Effects of ovariectomy and estradiol on body weight and food intake in gold thioglucose-treated mice

Two experiments investigated the effects of ovariectomy and estradiol replacement on the body weight and food intake of mice that had previously been treated with either gold thioglucose or saline. Ovariectomy and estradiol benzoate injections altered food intake in gold thioglucose-treated mice as much as in saline controls. Ovariectomy increased body weight in saline controls but it was without effect on the body weight of gold thioglucose-treated mice.

Response to male odours in progestin receptor- and oestrogen receptor-containing cells in female rat brain

Sensory cues from male rats, such as odours and vaginal-cervical stimulation (VCS), play a modulatory role in female rat sexual behaviour. For example, exposure to male odours and VCS appears to be at least partially responsible for increases in sexual behaviour following repeated mating of oestradiol-primed female rats. Although there is evidence that VCS influences sexual behaviour via a ligand-independent progestin receptor (PR)-dependent mechanism, the mechanism by which odours influence sexual behaviour is not known. We tested the hypothesis that, similar to VCS, the effects of male odours on sexual behaviour are mediated by progestin receptors. Female rats were injected with the progestin antagonist, RU486, or oil vehicle and were then exposed to male-soiled bedding or clean bedding. Although exposure to male-soiled bedding resulted in higher levels of Fos immunoreactivity in brain areas associated with female sexual behaviour, the progestin antagonist did not reduce this effect. Furthermore, there was minimal coexpression of odour-induced Fos and progestin receptors in brain areas associated with female sexual behaviour. Together, these results suggest that the effects of male odours are not mediated by a PR-dependent mechanism. Therefore, we tested the hypothesis that oestrogen receptor (ER)-containing cells are involved in the effects of olfactory cues. Although there was virtually no coexpression of ERbeta and odour-induced Fos in brain areas associated with female sexual behaviour, exposure to male odours slightly increased the number of cells coexpressing ER(alpha) and odour-induced Fos in the posterodorsal medial amygdala. Although, these results do not support the hypothesis that the effects of odours are mediated by a PR-dependent mechanism, they suggest that integration of male odours and hormonal cues may occur in ER(alpha)-containing cells in the posterodorsal medial amygdala.

A progestin antagonist blocks vaginocervical stimulation-induced fos expression in neurones containing progestin receptors in the rostral medial preoptic area

Vaginocervical stimulation (VCS) has a variety of effects on the brain, physiology and behaviour. Previous work demonstrated that a progestin antagonist blocked neuronal response to VCS (i.e. Fos expression) in the absence of progesterone in some neurones, and suggested that some of the effects of VCS on the brain are mediated by ligand-independent activation of progestin receptors (PRs). Although it had been reported previously that some of the cells in which VCS induces Fos expression also contain PRs, it had not been determined if a progestin antagonist blocked Fos expression in these particular neurones. The purpose of this experiment was to determine if a progestin antagonist decreases Fos expression specifically in cells that also express PRs in the preoptic area and ventromedial hypothalamus. As has been shown previously, VCS increased Fos-immunoreactive (ir) expression in the particular areas studied. In the rostral medial preoptic area, VCS increased Fos expression in cells that coexpressed PRs, as well as in cells that do not. However, in the caudal medial preoptic area, VCS only increased Fos expression in cells that did not coexpress PRs. Injection of the progestin antagonist, RU 486, decreased Fos expression in the rostral, but not caudal medial preoptic area, and it decreased Fos expression only in cells that coexpressed PR-ir. In contrast to a previous report, in the present study, the progestin antagonist did not inhibit VCS-induced Fos expression in the ventromedial hypothalamic area. The results of this experiment suggest that the progestin antagonist inhibits VCS-induced Fos expression in some neurones by blocking PRs, and they provide further support for the idea that VCS influences neuronal response in some cells by ligand-independent activation of PRs in those cells.

A dopamine antagonist blocks vaginocervical stimulation-induced neuronal responses in the rat forebrain

During mating in rats, the male provides vaginocervical stimulation (VCS) to the female via intromissions. VCS, provided manually, mimics many aspects of mating, including facilitation of lordosis, induction of sexual receptivity, abbreviation of the period of sexual receptivity, and induction of twice-daily prolactin surges, which result in pseudopregnancy. VCS also induces the expression of Fos, the protein product of the immediate early gene c-fos, which has been used as a marker for neurons that are responsive to mating stimuli. Because VCS induces the release of dopamine in the forebrain, as well as phosphorylation of DARPP-32, a phosphoprotein associated with activation of the D(1) subtype of dopamine receptor, we tested the hypothesis that VCS induces Fos expression by acting on the D(1) class of dopamine receptors. Injection of SCH 23390, an antagonist of the D(1) class of dopamine receptors, virtually eliminated VCS-induced Fos expression without affecting constitutive levels of Fos-Immunoreactivity (Fos-IR) in all brain areas in which VCS induced Fos expression. In a follow-up experiment, expression of a second immediate early protein, egr-1, was blocked as well, suggesting that these results are not specific to Fos. Therefore, the results are consistent with the idea that VCS induces dopamine release, causing activation of D(1) dopamine receptors, which in turn, results in neuronal response, as seen by both Fos and egr-1 expression.

Coexpression of ER beta with ER alpha and progestin receptor proteins in the female rat forebrain: effects of estradiol treatment

Estrogen and progestin receptors (ER, PgR) play a critical role in the regulation of neuroendocrine functions in females. The neuroanatomical distribution of the recently cloned, ER beta, overlaps with both ER alpha and PgR. To determine whether ER beta is found within ER alpha- or PgR-containing neurons in female rat, we used dual label immunocytochemistry. ER beta-immunoreactivity (ER beta-ir) was primarily detected in the nuclei of cells in the periventricular preoptic area (PvPO), the bed nucleus of the stria terminalis (BNSTpr), the paraventricular nucleus, the supraoptic nucleus, and the medial amygdala (MEApd). Coexpression of ER beta-ir with ER alpha-ir or PgR-ir was observed in the PvPO, BNSTpr, and MEApd in ovariectomized rats. E2 treatment decreased the number of ER beta-ir cells in the PvPO and BNSTpr and the number of ER alpha-ir cells in the MEApd and paraventricular nucleus, and therefore decreased the number of cells coexpressing ER beta-ir and ER alpha-ir in the PvPO, BNSTpr, and MEApd. E2 treatment increased the amount of PgR-ir in cells of the PvPO, BNSTpr, and MEApd, a portion of which also contained ER beta. These results demonstrate that ER beta is expressed in ER alpha- or PgR-containing cells, and they suggest that E can modulate the ratios of these steroid receptors in a brain region-specific manner.

Oestradiol increases phosphorylation of a dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32) in female rat brain

Recent studies suggest that oestrogen and progestin receptors may be activated by the neurotransmitter dopamine, as well as by their respective ligands. Because intracerebroventricular infusion of D(1), but not D(2), dopaminergic receptor agonists increases oestrous behaviour in oestradiol-primed rats, we wanted to determine if treatment with oestradiol alters the activity of D(1) receptor-associated processes in steroid receptor-containing areas in female rat brain. One D(1) receptor-associated phosphoprotein that may be influenced by oestradiol is a dopamine- and cyclic AMP-regulated phosphoprotein, Mr = 32,000 (DARPP-32). Because DARPP-32 is phosphorylated in response to dopamine acting via a cAMP-dependent protein kinase, it provides a useful marker to examine where in the brain a particular stimulus might be altering the activity of D(1) receptor-containing neurones. To determine if oestradiol alters the phosphorylation of DARPP-32, we stained immunocytochemically brain sections of female rats treated with behaviourally relevant doses of oestradiol or oil vehicle with an antibody that detects only the threonine 34-phosphorylated form of DARPP-32. Behaviourally effective doses of oestradiol increase the phosphorylation of DARPP-32 within the medial preoptic nucleus, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus and the ventromedial nucleus of the hypothalamus, 48 h after treatment. These data suggest that oestradiol increases the activity of D(1) dopamine receptor-associated processes in oestrogen receptor-containing areas of female rat forebrain.

Sensory cues mediating mating-induced potentiation of sexual receptivity in female rats

Repeated mating of estradiol-primed female rats increases sexual receptivity. Two studies were conducted to determine the contribution of vaginal--cervical stimulation (VCS) to this increase. In the first study, female rats were repeatedly mated for 165 min. The vaginas of half of the females were covered with tape (masked) to prevent intromissions by the males. The remaining females were unmasked. Only females receiving intromissions (unmasked) showed a significant increase in sexual receptivity during repeated mating, suggesting that VCS from intromissions is necessary for repeated mating to increase sexual receptivity. In the second experiment, female rats received either experimentally administered VCS or control scapular stimulation administered with a plastic probe 1 h prior to testing for sexual receptivity. VCS applied in this manner significantly increased sexual receptivity. Together, these findings suggest that VCS from intromissions is one of the primary factors responsible for increases in sexual receptivity following repeated mating.

Maternal behavior stimulates c-fos activity within estrogen receptor alpha-containing neurons in lactating rats

Estradiol and other hormones are thought to be critical for the onset, but not maintenance, of maternal behavior in rats. Maternal behavior is instead maintained postpartum by tactile stimulation that dams receive during interactions with pups, and many neural sites implicated in the control of maternal behavior show elevated c-fos activity in response to this stimulation. Many of these sites also contain neurons that express the alpha subtype of the estrogen receptor (ERalpha). Because of possible interactions between tactile stimulation from pups, c-fos, and ERalpha in the lactating rat brain, we determined if populations of cells that show increased c-fos activity after maternal behavior in lactating rats also contain ERalpha. Dams were separated from their pups for 48 h beginning on day 5 postpartum. On day 7 postpartum, experimental dams were reunited with pups and mother-litter interactions were observed for 60 min. Control dams received no pup stimulation. Subjects were sacrificed 60 min later and brain sections were double immunolabeled for the Fos and ERalpha proteins. As expected, the number of ERalpha-immunoreactive (ERalpha-ir) neurons did not differ between the two groups in the eight areas analyzed (lateral region of the lateral septum, posterodorsal medial amygdala, dorsal and ventral medial preoptic area, dorsal and ventral bed nucleus of the stria terminalis, lateral habenula, and ventrolateral caudal periaqueductal gray). Consistent with previous reports, maternal dams had 2- to 7-fold more Fos-immunoreactive (Fos-ir) neurons in these sites compared with nonstimulated controls. Maternal dams had significantly more Fos-ir neurons that also contained ERalpha-ir in all sites, with the greatest increases in the ventral medial preoptic area, lateral habenula, and ventral bed nucleus of the stria terminalis. Between approximately 25 and 45% of the Fos-ir cells in the sites examined also expressed ERalpha. Thus, a substantial number of neurons that are genomically activated during maternal behavior contain ERalpha, raising the possibility that the postpartum display of maternal behavior can be influenced by ERalpha activity.

Progesterone, but not progesterone-independent activation of progestin receptors by a mating stimulus, rapidly decreases progestin receptor immunoreactivity in female rat brain

Recent studies suggest that progestin receptors may be activated in vivo by neurotransmitters in the absence of ligand. More specifically, vaginal-cervical stimulation (VCS) can influence sexual behavior by activating progestin receptors in the absence of progesterone. Another way to test if progestin receptors are influenced by particular stimuli is to examine progestin receptor immunostaining. We report that progestin receptor immunoreactivity is decreased in the forebrain of estradiol-primed ovariectomized (OVX) rats within 1 h after a subcutaneous injection of progesterone, a time by which rapid down-regulation of progestin receptors does not seem to have occurred. In estradiol-primed OVX rats, VCS also decreased progestin receptor immunoreactivity within 1 h in the medial preoptic area, but not in any other area examined. To determine if the decrease in immunoreactivity by VCS was due to adrenal secretions or by ligand-independent activation of progestin receptors, we repeated the experiment in estradiol-primed OVX/adrenalectomized rats. Prior removal of the adrenal glands blocked the rapid decrease in progestin receptor immunoreactivity, even though data from other experiments suggest that progestin receptors are activated by VCS at this time. These studies suggest the possibility that progestin receptors may be affected differentially by progesterone-dependent or by progesterone-independent pathways. This raises the possibility that activation of progestin receptors by these two distinct pathways may lead to different neuronal consequences.

Cells containing immunoreactive estrogen receptor-alpha in the human basal forebrain

The distribution of estrogen receptor protein-alpha (ER-alpha)-containing cells in the human hypothalamus and adjacent regions was studied using a monoclonal antibody (H222) raised against ER-alpha derived from MCF-7 human breast cancer cells. Reaction product was found in restricted populations of neurons and astrocyte-like cells. Neurons immunoreactive for ER-alpha were diffusely distributed within the basal forebrain and preoptic area, infundibular region, central hypothalamus, basal ganglia and amygdala. Immunoreactive astrocyte-like cells were noted within specific brain regions, including the lamina terminalis and subependymal peri-third-ventricular region. These data are consistent with the location of estrogen receptors in the basal forebrain of other species and the known effects of estrogens on the cellular functions of both neurons and supporting elements within the human hypothalamus and basal forebrain.

Oestrogen receptor-alpha-immunoreactive neurones project to the suprachiasmatic nucleus of the female Syrian hamster

Ovarian steroid hormones regulate circadian period and phase, but classical receptors for these hormones are absent in the circadian pacemaker localized in the suprachiasmatic nucleus of the hypothalamus (SCN). In order to determine whether effects of oestrogen may be exerted through steroid-binding systems afferent to the SCN we have performed double label immunocytochemistry for oestrogen receptor-alpha(ER-alpha) and the retrograde tracer cholera toxin B subunit (CtB) after its application to the SCN. Most of the areas that contain ER-alpha-immunoreactive (ERalpha-ir) cells also contained cells afferent to the SCN. The percentage of neurones afferent to the SCN which show ERalpha-immunoreactivity varies between areas. As many as one-third of the neurones afferent to the SCN in some parts of the preoptic area and the corticomedial amygdala are ERalpha-ir. Very few of the afferent neurones from the septum and the central grey are ERalpha-ir, whereas an intermediate proportion of afferents from the bed nucleus of the stria terminalis and the arcuate nucleus are ERalpha-ir. Our retrograde tracing results were compared with results of anterograde tracing from some of the sites containing SCN afferents. Using a combined retrograde and anterograde tracing technique we tested the possibility that single ERalpha-ir neurones afferent to the SCN could receive reciprocal innervation by SCN efferents. Although we found SCN input to some SCN afferent neurones, we found no evidence of reciprocity between single ERalpha-ir cells and the SCN. Our results indicate the existence of oestrogen binding systems afferent to the SCN. These neuroanatomical pathways may mediate effects of gonadal steroid hormones on circadian rhythms.

Projections of the estrogen receptor-immunoreactive ventrolateral hypothalamus to other estrogen receptor-immunoreactive sites in female guinea pig brain

The ventrolateral hypothalamus in female guinea pigs includes an estrogen receptor dense region adjacent to the ventromedial hypothalamus. This region is reciprocally connected with other estrogen receptor-containing areas suggesting that steroid hormone receptor-containing cells may be directly linked. Phaseolus vulgaris leucoagglutinin, an anterograde tract tracer, was specifically placed in this region with the aim of labeling some projections from estrogen receptor-containing neurons. These projections were colocalized immunocytochemically with the distribution of estrogen receptor-containing cells. Dense ventrolateral hypothalamic innervation was observed in some regions also containing a high concentration of estrogen receptor-containing cells. These regions included the medial preoptic area, the bed nucleus of the stria terminalis, the ventrolateral hypothalamus anterior and posterior to the injection site, and the midbrain central gray. A low density of ventrolateral hypothalamic fibers and terminals was observed in two regions rich in estrogen receptors, the amygdala and the arcuate nucleus. In general, ventrolateral hypothalamic fibers and terminals were present in all regions where estrogen receptors were found except the medial thalamus and habenular region. Labeled terminal boutons and perineuronal baskets were found around estrogen receptor-containing cells in most regions which contained estrogen receptor-containing cells. These close appositions were suggestive of synaptic contacts, suggesting that the ventrolateral hypothalamus may influence steroid-dependent behaviors via the modulation of estrogen receptor-containing cells. Furthermore, ventrolateral hypothalamic projections may include direct connections with estrogen receptor-containing cells, suggesting the presence of a network of interconnected estradiol-sensitive neurons involved in the regulation of estradiol-dependent functions.

Mating-related stimulation induces phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein-32 in progestin receptor-containing areas in the female rat brain

Vaginal-cervical stimulation induces a number of physiological and behavioral events, including the facilitation of mating behavior. Although the facilitation of one component of mating behavior, lordosis, by vaginal-cervical stimulation does not require the presence of progesterone, it appears to be mediated by neural progestin receptors. Abundant evidence suggests that dopamine may play a role in the neural circuitry activated by vaginal-cervical stimulation, including the mating-induced release of dopamine in progestin receptor-containing areas of the brain, changes in the activational state of progestin receptors because of dopamine D1 receptor stimulation, facilitation of lordosis by D1 receptor stimulation in estradiol-primed rats via progesterone-independent events, and D1 agonist-induced neuronal responses in progestin receptor-containing areas and cells. We tested the hypothesis that vaginal-cervical stimulation induces phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32; Mr = 32,000), a protein phosphorylated predominantly in response to the stimulation of D1 receptors. At 9 d after ovariectomy, female rats were injected subcutaneously with a behaviorally effective dose of estradiol benzoate. At 48 hr later they received vaginal-cervical or control (perineal) stimulation, and they were perfused 1 hr later. Vaginal-cervical stimulation increased the number of cells expressing pDARPP-32 immunoreactivity by 92% in the medial preoptic nucleus, 134% in the caudal ventromedial hypothalamic nucleus, 123% in the posterodorsal medial amygdala, and 103% in the bed nucleus of the stria terminalis. These results suggest that some of the neuronal effects of vaginal-cervical stimulation, and perhaps other social or environmental stimuli, are mediated by phosphorylation of DARPP-32, perhaps via stimulation of D1 receptors, within progestin receptor-containing areas.

Induction of progestin receptors by estradiol in the forebrain of estrogen receptor-alpha gene-disrupted mice

Mice, rats, and humans have two types of estrogen receptors, estrogen receptor-alpha (ERalpha) and estrogen receptor-beta (ERbeta). Estrogen receptor-alpha gene-disrupted (ERalpha-disrupted) mice bear two nonfunctional copies of the ERalpha gene. This mutation blocks the synthesis of full-length ERalpha, renders the animals infertile, and inhibits the induction of female sexual behaviors by estradiol and progesterone. It is likely that many of the processes contributing to the regulation of sexual receptivity by estradiol and progesterone are compromised in ERalpha-disrupted mice. However, given the importance of progesterone in the regulation of sexual receptivity and given the importance of progestin receptors (PRs) in mediating the responses of females to progesterone, we investigated the effects of ERalpha disruption on the induction of PRs by estradiol in the forebrain. We hypothesized that estradiol would induce PRs in wild-type mice but not in ERalpha-disrupted mice. Ovariectomized wild-type and ERalpha-disrupted mice were implanted with either estradiol-filled capsules or empty capsules for 5 d, after which their brains were processed for the immunocytochemical detection of PR. Estradiol increased the number of PR-immunoreactive cells in both wild-type and ERalpha-disrupted mice. The residual responsiveness of ERalpha-disrupted mice to estradiol could be accounted for by an ERbeta-dependent mechanism or another as yet unidentified estrogen receptor; however, because ERalpha-immunoreactivity and PCR product representing the 3' end of ERalpha mRNA were found in at least one PR-containing region of the ERalpha-disrupted mice, an ERalpha splice variant may also mediate the induction of PR-immunoreactivity in ERalpha-disrupted mice.

Effect of photoperiod on neural estrogen and progestin receptor immunoreactivity in female Syrian hamsters

This study explored the possibility that reduced behavioral responsiveness to estradiol and progesterone in female Syrian hamsters exposed to a short photoperiod is associated with a reduction in the concentration of neural steroid receptors. The effects of long and short photoperiod (LP; SP) exposure on steroid receptor immunoreactivity were examined in the ventromedial hypothalamus (VMH), medial tuberal region (mTu), medial preoptic area (mPOA), medial nucleus of the amygdala (mAMYG), and the arcuate nucleus (ARC) of ovariectomized hamsters. In Experiment 1, exposure to SP for ten weeks attenuated the lordosis response following sequential treatment with estradiol and progesterone. In a separate group of animals not given hormones, SP decreased the staining intensity of estrogen receptor immunoreactive (ERIR) cells in the mPOA while increasing the number of detectable ERIR cells in part of the mAMYG. In Experiment 2, SP diminished the lordosis response as it did in Experiment 1. One week later, the same females were administered estradiol systemically to induce progestin receptors (PR). Animals housed in SP showed significantly reduced progestin receptor immunoreactivity (PRIR) in the VMH, mTu, mPOA, mAMYG, and ARC. Experiment 3 examined whether the results of Experiment 2 might have been influenced by photoperiodic effects on peripheral metabolism of estradiol. Among hamsters housed in LP or SP, PRs were induced by estradiol implanted unilaterally in the medial basal hypothalamus, thus bypassing possible photoperiodic effects on peripheral estradiol availability. This treatment resulted in significantly fewer cells with detectable PRIR in the VMH and mPOA of SP females, suggesting that the photoperiodic influences on PR induction observed in Experiment 2 do not depend on alterations in the peripheral availability of estradiol.

Convergence of substance P and estrogen receptor immunoreactivity in the midbrain central gray of female guinea pigs

Substance P (SP) and estrogen receptor immunoreactivity overlap in the midbrain central gray (MCG) of female guinea pigs. Estrogen-receptor-containing cells are found throughout the rostrocaudal extent of the MCG. Moderately dense SP immunostaining is also found in this region. SP-immunoreactive punctate structures suggestive of boutons were found in close association with the processes of some estrogen-receptor-immunoreactive neurons. These associations were observed primarily in the lateral and ventrolateral MCG at the midcollicular and caudal levels. This findings suggests an anatomical substrate for interactions between SP and estradiol-sensitive neurons in the midbrain. Such interactions may underlie the effects of SP on female sexual behavior.

Progesterone receptor function from a behavioral perspective

Hormonal induction of sexual receptivity in ovariectomized female mice can be effectively reinstated by sequential administration of estradiol and progesterone. In this regard, mice appear to be similar to other rodents. While it is generally accepted that hypothalamic progesterone receptors function as estradiol-induced transcription factors in the induction of sexual receptivity in rats, hamsters, and guinea pigs, relatively little is known about their role in the mouse, a species which exhibits genotypic and strain differences in the responsiveness to steroid hormones. Using a transgenic mouse carrying a null mutation for the progesterone receptor by gene targeting, we examined the role of the progesterone receptor as a coordinator of key regulatory events in the induction of sexual receptivity. A concordance between hypothalamic progesterone receptor levels and behavioral responsiveness was established by comparing the homozygous mutant, heterozygous mutant, and wild-type littermates. The behavioral and biochemical findings reveal the importance of estradiol-induced progesterone receptors for the expression of sexual behavior in female mice. The behavioral response of the two parental mouse strains from which the recombinant genotype was generated was also examined. As an extension of our earlier studies on the ligand-independent activation of progesterone receptors by neurotransmitters, the behavioral effect of dopamine in the facilitation of sexual receptivity in mice was also examined. The studies provide further evidence that steroid hormone receptors function as general transcription factors to achieve the integration of neural information in the central nervous system, and they assign a more important role for progesterone receptors than hitherto envisioned.

D1 dopamine receptor agonist (SKF-38393) induction of Fos immunoreactivity in progestin receptor-containing areas of female rat brain

Injection of dopamine or dopamine receptor subtype agonists facilitates the expression of lordosis in estrogen-primed female rats. The D1 receptor specific agonist, SKF-38393, facilitates lordosis in estradiol-primed female rats via a process that requires progestin receptors. Based on these data, neuronal response to the D1 receptor agonist SKF-38393 was assessed by expression of the immediate early gene protein, Fos. In the first experiment we examined the modulation of Fos expression by D1 agonists in progestin receptor-containing areas of estradiol-primed female rat brain. In the second experiment we examined if there are progestin if there are progestin receptor-containing cells that respond to stimulation of D1 receptors with increased Fos expression. Ten to 14 days following ovariectomy and stereotaxic surgery, animals were injected with 5 micrograms estradiol benzoate. Forty eight h later they were injected intracerebroventricularly with 100 ng of SKF-38393 or saline. One h following injection animals were perfused, and brain sections immunostained for Fos protein. Results from the first experiment suggest that SKF-38393 increased the total number of Fos immunoreactive cells in the mid-ventromedial hypothalamic nucleus/ventrolateral portion (VMHVL), the caudal VMHVL, the paraventricular hypothalamic nucleus and the caudate putamen. In the medial preoptic area, the rostral VMHVL and the arcuate hypothalamic nucleus, there was a significant increase in the number of darkly stained Fos-immunoreactive cells following the SKF-38393 treatment. In the second study, SKF-38393 increased the number of progestin receptor-containing cells which contained Fos immunoreactivity in the caudal VMHVL. The results suggest potential sites of action for the facilitation of sexual behavior by centrally administered D1 agonists.

Leptin facilitates and inhibits sexual behavior in female hamsters

Food deprivation decreases fertility in female mammals in part by inhibiting sexual behaviors. Genetically obese ob/ob mice, like food-deprived wild-type animals, are also infertile; treatment of ob/ob mice with leptin, the adipocyte-derived protein that they lack, corrects some of their reproductive deficiencies. We tested the hypothesis that leptin treatment would prevent the suppression of sexual receptivity that is caused by food deprivation in female Syrian hamsters. Instead, we found that treatment with murine leptin facilitated female sexual behavior in ad libitum-fed hamsters, but not in food-deprived animals. In food-deprived hamsters, leptin treatment actually intensified the inhibition of lordosis. Food deprivation decreased detectable estrogen receptor immunoreactivity (ERIR) in the ventromedial hypothalamus (VMH), but the leptin-induced changes in female sexual behavior were not accompanied by parallel changes in VMH ERIR. Thus leptin facilitates estrous behavior in hamsters, but it does not overcome the lordosis-inhibiting metabolic cues produced by acute food deprivation. Because circulating leptin levels are directly related to body fat content, an implication of these findings is that elevated levels of adipose tissue could have a positive influence on sexual responsiveness.

Progesterone treatment increases Fos-immunoreactivity within some progestin receptor-containing neurons in localized regions of female rat forebrain

In female rats, the sequential release of estradiol and progesterone from the ovaries is required for the expression of sexual behavior during the estrous cycle. Many of the neuronal effects of estradiol and progesterone involve estrogen and progestin receptors. Treatment with a behaviorally-effective dose of estradiol increases Fos expression, suggestive of neuronal response, and subsequent treatment with a behaviorally-effective dose of progesterone further increases Fos expression within a few hours in female rat brain. In order to determine if neurons that respond to progesterone with increase Fos expression also contain progestin receptors, we used a double-label immunofluorescent technique to label both progestin receptors and Fos protein following progesterone or vehicle treatment of estradiol-primed female rats. As shown previously, progesterone treatment increased Fos expression in progestin receptor-containing regions, such as the ventromedial nucleus of the hypothalamus and the medial preoptic area. In addition, progesterone treatment induced a statistically-significant increase in Fos-immunoreactivity within progestin receptor-containing cells in the medial preoptic area and the ventromedial nucleus of the hypothalamus, but not in the arcuate nucleus. Therefore, many but not all of the neurons that respond to progesterone with increased Fos expression also contain progestin receptor-immunoreactivity. The progesterone-induced Fos expression within progestin receptor-containing neurons may or may not be associated with the effects of progesterone on sexual or other reproductive behaviors, as it remains to be tested. However, the Fos expression provides a useful marker to aid in identification of neurons that respond to a behaviorally-relevant dose of progesterone.

Progesterone-independent activation of rat brain progestin receptors by reproductive stimuli

Activation of steroid hormone receptors by steroid hormones alters both the physiology and behavior of animals. Steroid hormone receptors (e.g., progestin receptors) can also be activated in the absence of steroid hormones by pharmacological treatment with neurotransmitters or neuropeptides. However, it is not known if progesterone-independent activation of brain progestin receptors occurs under natural, physiological, conditions. We report that increases in reproductive behavior and brain immediate early gene expression in female rats induced by mating stimuli can be blocked by prior treatment with progesterone antagonists in the absence of circulating progesterone. This suggests that progestin receptors are activated in a progesterone-independent manner by a physiologically relevant stimulus in female rats, thus implicating a novel pathway by which mating stimuli and other environmental influences could activate steroid receptors to influence neuronal response and behavior.

Dopamine requires the unoccupied progesterone receptor to induce sexual behavior in mice

Using the recently generated mutant mice strain (PRKO) carrying a null mutation for the progesterone receptor (PR) gene by gene targeting, we examined the critical role of PR as a coordinator of key regulatory events involved in the steroid hormone and dopamine-facilitated sexual behavior in female mice. In vitro one-point binding analyses of estradiol benzoate (EB)-induced cellular PRs and immunohistochemistry of PR in the mediobasal hypothalamus demonstrated a reduction in binding in the homozygous females, equivalent to background levels seen in EB-unresponsive tissue. The biochemical findings correlated well with the behavioral observations, with the wild type females exhibiting high levels of lordosis, while the homozygous females showed minimal lordosis in response to mating by male mice. As a critical validation of our earlier studies on ligand-independent activation of PRs by dopamine, we examined the facilitation of sexual behavior by a dopamine agonist in the null mutants. Wild type females having the full complement of PRs exhibited high levels of lordosis, while the homozygous females showed minimal lordosis in response to dopamine. To determine whether this reduced response was due to a general lack of ability to express lordosis, mice were treated with another neurotransmitter, serotonin. No significant difference in the serotonin-facilitated lordosis response was observed between the wild type and the homozygous females. We conclude that multiple signal transduction pathways coexist in the neuroendocrine system for reproductive behavior, with PR acting as a transcriptional mediator for dopamine, as well as progesterone, to achieve integration of neural communication in the central nervous system.

Reproductively-relevant stimuli induce Fos-immunoreactivity within progestin receptor-containing neurons in localized regions of female rat forebrain

An experiment was conducted to determine if neurons that respond to stimuli associated with mating in female brain also contain progestin receptors. We found that a portion of the neurons that respond to stimuli associated with mating also contains progestin receptors. While the appropriate hormonal conditions are important for sexual receptivity, somatosensory information provided by the male also influences sexual behavior. One important stimulus provided by the male during copulation is vaginal-cervical stimulation (VCS). VCS has been shown to elicit many different behavioral and endocrine changes in female rats, such as increases in lordosis, pseudopregnancy, and termination of sexual receptivity. VCS also increases the expression of the immediate early gene product, Fos, in areas associated with reproduction. A portion of the neurons responding to VCS with increased Fos-immunoreactivity (Fos-IR) in female rat forebrain also contains estrogen receptors, illustrating that hormonal and mating-stimuli converge in a population of cells. As progesterone also plays an important role in female sex behavior, it is important to determine if some of the neurons also integrate information concerning serum progesterone levels and social interactions. Thus, we used a dual immunofluorescent technique to label both Fos-IR and progestin receptor-immunoreactivity (PR-IR) in the brains of estradiol-primed, ovariectomized female rats following VCS manually applied by the experimenter. Many of the neurons that respond to VCS with increased Fos-IR within the medial preoptic area, the arcuate nucleus, and the progestin receptor-rich areas of the rostral and caudal ventromedial nucleus of the hypothalamus also contain PR-IR.

Efferent projections from the ovarian steroid receptor-containing area of the ventrolateral hypothalamus in female guinea pigs

The ventrolateral hypothalamus (VLH) in female guinea pigs includes a subset of neurons which contain estrogen and progestin receptors, and which are implicated in the regulation of female sexual behavior by steroid hormones. However, little is known about where these neurons project, and consequently which other brain areas are involved in sexual behavior in female guinea pigs. The anterograde tracer Phaseolus vulgaris-Leucoagglutinin was used to label efferents from the ovarian steroid receptor-containing part of the VLH. To identify the correct placement of the tracer specifically within the group of neurons containing estrogen receptors, medial hypothalamic sections were also immunostained for estrogen receptors. Forebrain areas receiving dense projections from the ventrolateral hypothalamus included the bed nucleus of the stria terminalis, medial preoptic area, anterior hypothalamic area, anterior ventromedial hypothalamus, and caudal ventrolateral hypothalamus. The midbrain central gray was also heavily labeled. Moderate innervation was observed in the forebrain in the basolateral amygdala, medial preoptic nucleus, lateroanterior hypothalamic nucleus, dorsal hypothalamic areas, posterior hypothalamus, zona incerta, and in the midbrain interspersed among the central and lateral tegmental tracts. The major efferent pathways from the VLH appeared to travel rostrally through the mediobasal hypothalamus and preoptic area, and caudally via the medial thalamic nuclei and periventricular fiber system. These findings are similar to those of previous studies tracing the efferents from the ventromedial nucleus in rats and from the lateral hypothalamus in guinea pigs. Many of these areas that receive input from the steroid receptor rich area within the VLH are likely to be involved in the regulation of female sexual behavior.

Effects of food deprivation on induction of neural progestin receptors by estradiol in Syrian hamsters

Food deprivation, as well as treatment with metabolic inhibitors, suppress steroid hormone-induced estrous behavior in ovariectomized (OVX) Syrian hamsters. Previous work indicates that 48 h of food deprivation decreases the number of detectable estrogen receptor immunoreactive (ERIR) cells in the ventromedial hypothalamus (VMH) and the area just lateral to it (VLH), increases the number of ERIR cells in the medial preoptic area (MPO), and has no effect on the number of ERIR cells in the nucleus of the solitary tract in OVX hamsters. The present study examined the effects of food deprivation on neural progestin receptor binding using an in vitro binding assay and on progestin receptor immunoreactivity (PRIR) in estradiol-primed, OVX hamsters. Parallel behavior tests for sexual behavior were also performed in both experiments. OVX hamsters received 2.5 micrograms estradiol benzoate and were fed ad libitum or food deprived at the same time. Forty-eight hours later, animals were killed in preparation for the immunocytochemistry or progestin receptor assay. Binding assays indicated that 48-h food deprivation decreased progestin receptor levels in the preoptic area and had no effect in the mediobasal hypothalamus, an area that includes the VMH and the arcuate nucleus (ARH). Immunocytochemical analysis confirmed these findings. Food deprivation caused a decrease in sexual receptivity and in the number of detectable PRIR cells in the MPO and medial amygdala but had no effect on the number of detectable PRIR cells in the VMH/VLH, the ARH, or the anteroventral periventricular nucleus. These results suggest that food deprivation modulates progestin receptor binding and PRIR in a site-specific manner. In addition, the effects of food deprivation on neural ERIR and PRIR are significantly different.

The suprachiasmatic area in the female hamster projects to neurons containing estrogen receptors and GnRH

This study investigated whether the circadian regulation of luteinizing hormone (LH) release may be through direct input of the suprachiasmatic nucleus (SCN) to estrogen receptor (ER)- and/or gonadotropin releasing hormone (GnRH)-immunoreactive neurons. We used Phaseolus vulgaris leucoagglutinin (PHA-L) as an anterograde tracer of SCN efferents and performed double label immunocytochemistry for PHA-L and ER or GnRH. Between 8 and 30% of ER cells and 11-13% of the GnRH cells showed appositions with SCN efferents. Efferent projections of the subparaventricular hypothalamic nucleus and the retrochiasmatic area, relay stations of the circadian system, also made appositions with these two cell types. Results suggest that the circadian system could regulate the timing of the LH surge via two pathways, through input to GnRH and to ER cells.

A subgroup of LHRH neurons in guinea pigs with progestin receptors is centrally positioned within the total population of LHRH neurons

Although the role of gonadal steroids in inducing the LH surge is undisputed, the mechanism(s) whereby steroids induce the release of the hypothalamic luteinizing hormone-releasing hormone (LHRH) remain(s) enigmatic. In this study we examined the issue of the presence of steroid receptors in LHRH neurons using a mammalian species that has a true luteal phase, namely, guinea pigs. Progestin receptors (PR) were localized in LHRH neurons of ovariectomized guinea pigs administered estradiol (10-20 micrograms estradiol benzoate) for 3-4 days, using several different immunocytochemical protocols. The subgroup of LHRH neurons containing PR, although small, was strategically positioned within the core of the total population of LHRH neurons. This central position was visualized in simultaneous views of three-dimensional computer reconstructions of the populations of LHRH/PR neurons and LHRH neurons. The subgroup of LHRH/PR neurons formed a thread permeating the population of LHRH neurons. We propose that in guinea pigs, LHRH neurons containing progestin receptors, are foci of activity, capable of activating a larger component of the LHRH population of cells in certain endocrine conditions, such as prior to the LH surge.

Progesterone enhances an estradiol-induced increase in Fos immunoreactivity in localized regions of female rat forebrain

In female rats, the onset of reproductive behavior depends on the sequential presence of estradiol followed by progesterone. Although treatment with high doses of estradiol has been shown to increase immunostaining for the Fos protein, an immediate early gene product that is expressed upon cellular activation, another report conflicts with this finding. However, the previous reports agree that subsequent treatment with progesterone has no apparent effect on Fos expression. In order to resolve this discrepancy and investigate possible effects of progesterone, we used Fos immunocytochemistry combined with computer-aided image analysis. In experiment one, we found that treatment with 5 micrograms of estradiol increased Fos immunoreactivity (Fos-IR) within a section of the medial preoptic area and the dorsal medial hypothalamus. Subsequent treatment with 500 micrograms of progesterone 1 hr before perfusion increased the intensity of the immunostaining within the medial preoptic area and the dorsal medial hypothalamus, although it had no significant effect on Fos-IR cell number. In experiment 2, a lower concentration of Fos antiserum was used in order to diminish the immunostaining sensitivity to a level in which no increase of Fos-IR cell number was observed after treatment with estradiol. Under these immunocytochemical conditions, subsequent treatment with progesterone increased the number of Fos-IR cells in the medial preoptic area, the dorsal medial hypothalamus and the steroid receptor-rich area lateral to the ventromedial hypothalamus. Thus, treatment with behaviorally effective doses of both estradiol and progesterone induces Fos expression in localized regions of female rat brain.

Down-regulation of estrogen receptor immunoreactivity by 17 beta-estradiol in the guinea pig forebrain

Low amplitude pulses of estradiol-17 beta (E2-17 beta) are more effective than large single bolus injections or constant exposure to E2-17 beta in inducing progesterone-facilitated sex behavior in female rats and guinea pigs. The present study examined whether the increased responsiveness to E2-17 beta is due to an increase in the number of estrogen receptors in the estrogen receptor rich areas of the hypothalamus and amygdala. Initial studies examined the rapid effects (20 min) of a high dose of E2-17 beta (50 micrograms) on estrogen receptor immunostaining using either the H222 antibody or the ER 21 antiserum. ER 21 immunostaining was not affected by the E2-17 beta treatment suggesting that it binds to both occupied and unoccupied estrogen receptors. Therefore the ER 21 antiserum was used to characterize the regulation of estrogen receptor immunoreactivity (ER-IR) by E2-17 beta. ER-IR was examined for 48 h and serum E2-17 beta for 24 h following a 2 micrograms s.c. injection of E2-17 beta (a dose similar to that used in multiple pulse paradigms). Serum E2-17 beta peaked 15 to 30 min following the injection and returned to baseline values by 1 h. In all but one area maximal suppression of ER-IR occurred at 12 h. In summary, 1) decreases in estrogen receptor immunoreactivity following E2-17 beta are consistent with studies in which estrogen receptors were assayed by binding assays and estrogen receptor mRNA was determined by in situ hybridization; 2) the ER 21 antiserum is able to detect both occupied and unoccupied estrogen receptors and 3) H222 immunoreactivity is influenced by the presence of E2-17 beta, so that the level of H222-IR is a reflection of ligand/receptor binding dynamics. The data suggest that up-regulation of estrogen receptors does not account for the increase in behavioral sensitivity which is observed following multiple pulses of E2-17 beta.

Inhibition of rat sexual behavior by antisense oligonucleotides to the progesterone receptor

To test further the idea that sexual behavior in rodents is mediated via the progesterone receptor (PR) in the ventromedial nucleus of the hypothalamus, antisense and sense oligonucleotides to progesterone receptor were administered intracerebroventricularly into the third cerebral ventricle of ovariectomized estrogen-primed animals. Progesterone-facilitated sexual behavior was inhibited in animals treated with antisense oligonucleotides, with proceptive and receptive responses being minimal or completely suppressed. Sexual behavior was not altered by control sense oligonucleotides. In vitro binding assays of the cytosol progesterone receptors demonstrated a 52.2% reduction of PRs in the hypothalamus of animals that received antisense oligonucleotides, suggesting a reduction in PR synthesis. These data suggest that a threshold level of estrogen-induced hypothalamic PR is critical in the regulation of progesterone-facilitated sexual behavior in female rats.

Convergent pathways for steroid hormone- and neurotransmitter-induced rat sexual behavior

Estrogen and progesterone modulate gene expression in rodents by activation of intracellular receptors in the hypothalamus, which regulate neuronal networks that control female sexual behavior. However, the neurotransmitter dopamine has been shown to activate certain steroid receptors in a ligand-independent manner. A dopamine receptor stimulant and a D1 receptor agonist, but not a D2 receptor agonist, mimicked the effects of progesterone in facilitating sexual behavior in female rats. The facilitory effect of the neurotransmitter was blocked by progesterone receptor antagonists, a D1 receptor antagonist, or antisense oligonucleotides to the progesterone receptor. The results suggest that in rodents neurotransmitters may regulate in vivo gene expression and behavior by means of cross-talk with steroid receptors in the brain.

Manipulations of metabolic fuel availability alter estrous behavior and neural estrogen receptor immunoreactivity in Syrian hamsters

Decreases in metabolic fuel utilization caused by food deprivation, diabetes, or treatment with metabolic inhibitors have been shown to suppress steroid-induced estrous behavior in ovariectomized Syrian hamsters. These same manipulations also caused a decrease in the number of detectable estrogen-receptor immunoreactive (ERIR) cells in the ventromedial hypothalamus (VMH) and the adjacent area lateral to it (VLH) in ovariectomized hamsters. Forty-eight hours of food deprivation or treatment with pharmacological blockers of glycolysis (2-deoxy-D-glucose) and fatty acid oxidation (methyl palmoxirate) decreased the number of detectable ERIR cells in the VMH/VLH. However, neither inhibitor given alone was sufficient to affect the number of detectable ERIR cells in the VMH/VLH, even when given in much higher doses. Therefore, the number of ERIR cells in the VMH/VLH, like steroid-induced estrous behavior, responds only to a combination of glucoprivation and lipoprivation and not to either alone. The effects of metabolic fuel restriction are not due to a general suppression of neural ERIR, because food deprivation or treatment with 2-deoxy-D-glucose and methyl palmoxirate actually increased the number of detectable ERIR cells in the medial preoptic area (mPOA) and had no effect in the nucleus of the solitary tract. Lesions destroying the area postrema (AP) prevented the decrease in ERIR cells in the VMH/VLH and the inhibition of estrous behavior caused by fuel restriction. However, AP lesions did not alter the effects of metabolic inhibitors on ERIR in the mPOA. On the other hand, subdiaphragmatic vagotomy abolished the effects of metabolic inhibitors on mPOA ERIR, but did not affect either lordosis or VMH/VLH ERIR. These results suggest that the suppression of estrous behavior induced by metabolic fuel restriction is at least in part due to a decrease in the number of ERIR neurons in the VMH/VLH. In addition, the estrogen-sensitive neurons in the VMH/VLH and mPOA receive metabolic cues via different neural pathways. The AP, but not vagus nerves, is required for ERIR neurons in the VMH/VLH to detect fuel availability; the vagus nerves, but not the AP, are necessary for estrogen-binding neurons in the mPOA to detect visceral information.

Distribution of androgen receptor immunoreactivity in vasopressin- and oxytocin-immunoreactive neurons in the male rat brain

Arginine vasopressin-immunoreactive (AVP-ir) neurons in the bed nucleus of stria terminalis (BST) and medial amygdaloid nucleus are very responsive to gonadal hormones. After gonadectomy, these neurons lose their AVP immunoreactivity and stop expressing AVP mRNA. Testosterone treatment reverses these changes, acting via androgen as well as estrogen receptor-mediated mechanisms. Although AVP-ir neurons contain estrogen receptor immunoreactivity, it is not known whether they also contain androgen receptor immunoreactivity. To answer this question, brains of male rats were stained immunocytochemically for AVP as well as for androgen receptors. In the BST and medial amygdaloid nucleus, respectively, 90.5% and 91.2% of the AVP-ir neurons contained androgen receptor immunoreactivity. In contrast, in the suprachiasmatic nucleus, the supraoptic nucleus, and the magnocellular portion of the paraventricular nucleus (PVN), none of the AVP-ir neurons contained androgen receptor immunoreactivity. In the ventral zone of the medial parvocellular part of the PVN (mpvPVN), 4.3% of the scattered AVP-ir neurons contained androgen receptor immunoreactivity. One of the control experiments, i.e. staining sections for oxytocin (OT) rather than AVP, revealed that although OT-ir neurons in the supraoptic and magnocellular portion of the PVN did not contain androgen receptor immunoreactivity, 52.5% of the OT-ir neurons in the mpvPVN did. The results suggest that androgens can bind to androgen receptors in AVP-ir neurons in the BST and medial amygdaloid nucleus, possibly to influence AVP expression. The results also suggest that androgens can bind to androgen receptors in AVP-ir and OT-ir neurons in the mpvPVN. The function of the latter interaction, however, is unclear.

Estradiol and progesterone influence the response of ventromedial hypothalamic neurons to tactile stimuli associated with female reproduction

Stimulation of the vagina and cervix, provided by the male during copulation or manually with a probe, causes many behavioral and endocrine changes associated with female reproduction in rats. Previously, we found that vaginal-cervical stimulation (VCS), by mating or manual probing, increases the expression of Fos-immunoreactivity (Fos-IR) in discrete populations of neurons in the preoptic area, mediobasal hypothalamus and midbrain, suggesting that these neurons respond to VCS. The purpose of the present study was to determine if hormonal priming would increase the number of Fos-IR cells following VCS. Contrary to our hypothesis, in Experiment 1 priming animals with a behaviorally effective dose of 17 beta-estradiol benzoate followed 48 h later by progesterone caused a trend towards a decrease in the number of VCS-induced Fos-IR cells in the ventromedial hypothalamus. In Experiment 2, which was done to confirm this decrease in VCS-induced Fos-IR neurons by hormones, this effect was found to be statistically significant. Furthermore, this hormone-induced decrease in VCS-responsive cells was localized to the ventromedial nucleus of the hypothalamus, an area rich in estrogen and progestin receptors. No effects of hormone treatment on VCS-induced Fos-IR were observed in any other brain regions analyzed. These findings suggest that steroid hormones may elicit some of their effects on female reproductive behavior and physiology by altering the responsiveness of ventromedial nucleus neurons to vaginal-cervical stimulation.

Intraneuronal convergence of tactile and hormonal stimuli associated with female reproduction in rats

Stimulation of the vagina and cervix, by mating or manual probing, elicits many behavioral and endocrine changes associated with female reproduction in rats. We and others have identified neurons in the medial preoptic area, medial division of the bed nucleus of the stria terminalis, posterodorsal portion of the medial amygdala, ventromedial hypothalamus, dorsomedial hypothalamus and midbrain central gray that increase Fos expression in response to vaginal-cervical stimulation (VCS). In the present study, we used a double-label immunofluorescent technique to determine if any of these VCS-responsive neurons also contained estrogen receptor-immunoreactivity. We found that over 80% of the VCS-induced Fos-IR neurons in the medial division of the bed nucleus of the stria terminalis also contained estrogen receptor-immunoreactivity. Furthermore, high percentages of VCS-responsive neurons in the medial preoptic area, posterodorsal medial amygdala, ventromedial hypothalamus and midbrain central gray contained estrogen receptor-immunoreactivity as well. These results suggest that sensory and hormonal information associated with female reproduction converge on specific populations of neurons and may be integrated at the molecular level within these neurons.

Projections from ventrolateral hypothalamic neurons containing progestin receptor- and substance P-immunoreactivity to specific forebrain and midbrain areas in female guinea pigs

Many neurons within the ventrolateral hypothalamus in guinea pigs contain estrogen-induced progestin receptors as well as substance P. Retrograde tracing combined with immunocytochemistry was used to determine the specific projections of this subset of steroid-sensitive cells. Unilateral Fluoro-Gold injections into the dorsal midbrain, including the central gray, labeled a large proportion of the ventrolateral hypothalamic neurons immunoreactive for both progestin receptors and substance P (approximately 30%); substantially fewer of these neurons were labeled by unilateral Fluoro-Gold injections into the preoptic area (approximately 6%), medial amygdala (approximately 10%), or the bed nucleus of the stria terminalis (approximately 11%). The projections of progestin receptor-immunoreactive neurons in the ventrolateral hypothalamus were similar to those of progestin receptor/substance P double-labeled neurons, while a slightly lower percentage of the ventrolateral hypothalamic, substance P-immunoreactive neurons tended to project to each of these areas. These pathways may prove to be components of the neural circuitry underlying a variety of functions influenced by gonadal steroid hormones and substance P, such as female sexual behavior, salt intake, nociception and aggression.

Hypothalamic ovarian steroid hormone-sensitive neurons involved in female sexual behavior

Estradiol and progesterone regulate sexual behaviors in guinea pigs and rats, at least in part, through interaction with intracellular steroid hormone receptors. In the present review of work from the laboratory of the authors, we summarize recent work which has focused on one of the sites of hormone action in female guinea pigs--the ventrolateral hypothalamus. We summarize results of earlier experiments in which the regulation of steroid hormone receptors in this area was assessed after hormonal treatments with predictable effects on female sexual behavior. We then review the results of recent tract-tracing experiments in which we have examined the projections from the steroid receptor-immunoreactive neurons in this region, as well as the afferent projections from other neuroanatomical areas, including neurons which themselves contain estrogen receptors. We also present studies on the afferent input into these neurons by noradrenergic neurons and discuss the possibility that noradrenergic input influences steroid hormone sensitivity in these neurons. Finally, we discuss the results of experiments in which Fos-immunocytochemistry was used in rats to identify the neurons responding to a particular tactile stimulus associated with female reproduction, i.e., vaginal-cervical stimulation. These experiments further define a complex neuroanatomical system, in which many of the elements are estradiol or progesterone-responsive, that is involved in the hormonal regulation of sexual behavior in guinea pigs and rats.

Estrogen-receptor immunoreactivity in hamster brain: preoptic area, hypothalamus and amygdala

The distribution of estrogen-receptor containing cells in the preoptic area, hypothalamus and amygdala of female Syrian hamster brain was studied by immunocytochemical methods. Dense populations of estrogen-receptor immunoreactive (ER-IR) cells were found in the medial preoptic area, the bed nucleus of the stria terminalis, amygdala, ventral and lateral parts of the hypothalamus, and the arcuate nucleus. Injection of estradiol caused a decrease in estrogen-receptor immunoreactivity (ERIR) containing cells within one hour, a decrease that may reflect a change in the ability of the occupied estrogen receptor to bind the particular antibody (H222) used rather than down-regulation of the estrogen receptor. Our findings on the distribution of estrogen-receptor containing cells in these areas using an immunocytochemical technique are consistent with and extend the findings of others using autoradiographic and in vitro binding techniques to study estrogen receptor distribution in hamster brain.

ICI 182,780 antagonizes the effects of estradiol on estrous behavior and energy balance in Syrian hamsters

Three experiments examined the effects of ICI 182,780, a steroidal "pure" antiestrogen that is thought to be active peripherally but not in the brain when given systemically, on energy balance, estrous behavior, and in vivo cell nuclear binding of [3H]estradiol in Syrian hamsters. Pretreatment with ICI 182,780 reduced in vivo uptake of [3H]estradiol in uterus but not in pooled hypothalamus-preoptic area. Ovariectomized Syrian hamsters were treated with estradiol benzoate (EB, 5 micrograms/day), ICI 182,780 (250 micrograms/day), or both EB and ICI 182,780 for 4 wk. Estradiol treatment caused significant decreases in food intake, body weight and fat content, and linear growth. Given alone, ICI 182,780 had no effect on these measures. When they were given concurrently, ICI 182,780 attenuated the effects of estradiol on body weight, growth, and fat content but not on food intake. Treatment with ICI 182,780 significantly diminished estrous behavior induced with either EB plus progesterone or with EB alone. These findings support the hypothesis that, in addition to its actions in the brain, estradiol acts peripherally to modulate estrous behavior and energy balance.

ICI 182,780: a pure antiestrogen that affects behaviors and energy balance in rats without acting in the brain

ICI 182,780 is one of a new class of steroidal antiestrogens that differs from nonsteroidal antiestrogens, such as tamoxifen, in a number of respects. 1) It is bound by estrogen receptors with a high affinity, similar to that for estradiol. 2) It is a "pure" antiestrogen in that it does not mimic any of the effects of estradiol. 3) This class of antiestrogens does not seem to be bound by antiestrogen binding sites. 4) ICI 182,780 may not be active in the brain after peripheral administration. Indeed, ICI 182,780 blocked in vivo cell nuclear binding of [3H]estradiol in uterus, pituitary, and adipose tissue but not in hypothalamus-preoptic area. In vitro, ICI 182,780 competed for binding by neural estrogen receptors with an affinity comparable with that for estradiol. When given to ovariectomized rats, ICI 182,780 did not mimic any of the actions of estradiol. Instead, ICI 182,780 treatment completely blocked the uterotrophic effects of estradiol and attenuated the actions of estradiol on linear growth, carcass fat content, fat pad weight, and sexual receptivity. Treatment with ICI 182,780 also attenuated the estrogenic effects of tamoxifen on food intake, body weight and composition, linear growth, and uterine weight. These findings support the concept that, in addition to its actions in the brain, estradiol can act peripherally to modulate regulatory behaviors, energy balance, and estrous behavior. They are also consistent with the hypothesis that nonsteroidal antiestrogens, such as tamoxifen, affect energy balance via estrogen receptors, rather than antiestrogen binding sites.

Estrogen receptor-immunoreactive forebrain neurons project to the ventrolateral hypothalamus in female guinea pigs

In rodents, the facilitation of sexual receptivity by estradiol and progesterone is suspected to be mediated by a network of neurons containing estrogen and progestin receptors. In female guinea pigs, this network would include estrogen receptor-immunoreactive (ER-ir) neurons located within the rostro-ventral ventrolateral hypothalamus (r-vVLH). This hypothesis predicts that a proportion of the neurons projecting to the r-vVLH contains estrogen receptors. This prediction was tested through retrograde tracing combined with immunocytochemistry for estrogen receptors. Retrogradely labelled neurons were particularly abundant within the medial preoptic nucleus (MPN), bed nucleus of the stria terminalis (BST), anterior hypothalamus, amygdala, and lateral parabrachial nucleus. As predicted by the hypothesis, retrogradely labelled neurons were mostly observed in estrogen receptor-rich areas. Retrogradely labelled neurons also containing estrogen receptor-immunoreactivity (ER-IR) were primarily found within the MPN, BST, and amygdala. However, a majority of retrogradely labelled neurons did not contain ER-IR. As the preoptic area and the r-vVLH are both responsive to estradiol in the facilitation of sexual receptivity by progesterone, these data are consistent with the hypothesis tested. However, our data also suggest that the network of neurons controlling sexual receptivity may include elements not directly sensitive to estradiol. Finally, the location of retrogradely labelled neurons is discussed with respect to the stimuli provided to the r-vVLH in the context of sexual receptivity facilitated by estradiol and progesterone.

Fos expression in the rat brain following vaginal-cervical stimulation by mating and manual probing

Vaginal-cervical stimulation (VCS), provided by mating or manual probing, induces many reproductive behavioral and endocrine changes in female rats. These changes include an increase in lordosis duration, heat termination and pseudopregnancy. Electrophysiological and [14C]2-deoxy-D-glucose studies collectively show that neurons in the medial preoptic area, ventromedial hypothalamus and midbrain central gray respond to manual VCS. In the present study we immunocytochemically labeled brain sections for Fos, the protein product of the immediate early gene c-fos, to detect VCS-responsive neurons in hormone-primed animals receiving VCS by mating or manual probing. In Experiment 1, females receiving mounts and intromissions were compared to: 1) vaginally-masked females receiving mounts but no VCS, 2) females exposed to an intact anesthetized male or 3) females not exposed to males or the testing arena. Those animals receiving VCS showed a dramatic increase in the number of Fos-immunoreactive cells in the medial preoptic area, posterodorsal portion of the medial amygdala and bed nucleus of the stria terminalis, as well as the dorsomedial hypothalamus, ventromedial hypothalamus and midbrain central gray. These effects of VCS were confirmed in Experiment 2 in animals receiving manual vaginal-cervical probing. These findings extend previous electrophysiological and [14C]2-deoxy-D-glucose studies by providing evidence that additional brain areas respond to VCS by mating, as well as manual probing.