Definition of estrogen receptor pathway critical for estrogen positive feedback to gonadotropin-releasing hormone neurons and fertility

Differential regulation of gonadotropin-releasing hormone neuron activity and membrane properties by acutely applied estradiol: dependence on dose and estrogen receptor subtype

Gonadotropin-releasing hormone (GnRH) neurons are critical to controlling fertility. In vivo, estradiol can inhibit or stimulate GnRH release depending on concentration and physiological state. We examined rapid, nongenomic effects of estradiol. Whole-cell recordings were made of GnRH neurons in brain slices from ovariectomized mice with ionotropic GABA and glutamate receptors blocked. Estradiol was bath applied and measurements completed within 15 min. Estradiol from high physiological (preovulatory) concentrations (100 pm) to 100 nm enhanced action potential firing, reduced afterhyperpolarizing potential (AHP) and increased slow afterdepolarization amplitudes (ADP), and reduced I(AHP) and enhanced I(ADP). The reduction of I(AHP) was occluded by previous blockade of calcium-activated potassium channels. These effects were mimicked by an estrogen receptor (ER) beta-specific agonist and were blocked by the classical receptor antagonist ICI182780. ERalpha or GPR30 agonists had no effect. The acute stimulatory effect of high physiological estradiol on firing rate was dependent on signaling via protein kinase A. In contrast, low physiological levels of estradiol (10 pm) did not affect intrinsic properties. Without blockade of ionotropic GABA and glutamate receptors, however, 10 pm estradiol reduced firing of GnRH neurons; this was mimicked by an ERalpha agonist. ERalpha agonists reduced the frequency of GABA transmission to GnRH neurons; GABA can excite to these cells. In contrast, ERbeta agonists increased GABA transmission and postsynaptic response. These data suggest rapid intrinsic and network modulation of GnRH neurons by estradiol is dependent on both dose and receptor subtype. In cooperation with genomic actions, nongenomic effects may play a role in feedback regulation of GnRH secretion.

Endocrine-disrupting chemicals: an Endocrine Society scientific statement

There is growing interest in the possible health threat posed by endocrine-disrupting chemicals (EDCs), which are substances in our environment, food, and consumer products that interfere with hormone biosynthesis, metabolism, or action resulting in a deviation from normal homeostatic control or reproduction. In this first Scientific Statement of The Endocrine Society, we present the evidence that endocrine disruptors have effects on male and female reproduction, breast development and cancer, prostate cancer, neuroendocrinology, thyroid, metabolism and obesity, and cardiovascular endocrinology. Results from animal models, human clinical observations, and epidemiological studies converge to implicate EDCs as a significant concern to public health. The mechanisms of EDCs involve divergent pathways including (but not limited to) estrogenic, antiandrogenic, thyroid, peroxisome proliferator-activated receptor gamma, retinoid, and actions through other nuclear receptors; steroidogenic enzymes; neurotransmitter receptors and systems; and many other pathways that are highly conserved in wildlife and humans, and which can be modeled in laboratory in vitro and in vivo models. Furthermore, EDCs represent a broad class of molecules such as organochlorinated pesticides and industrial chemicals, plastics and plasticizers, fuels, and many other chemicals that are present in the environment or are in widespread use. We make a number of recommendations to increase understanding of effects of EDCs, including enhancing increased basic and clinical research, invoking the precautionary principle, and advocating involvement of individual and scientific society stakeholders in communicating and implementing changes in public policy and awareness.

Estrogens regulate posttranslational modification of neural cell adhesion molecule during the estrogen-induced gonadotropin surge

Estrogen-induced synaptic plasticity (EISP) in the periventricular area (PVA) of the hypothalamus is necessary for the preovulatory gonadotropin surge. Because in situ enzymatic desialization of hypothalamic polysialylated (PSA) neural cell adhesion molecule (NCAM) blocked EISP, we examined the presence and amount of NCAM isotopes, PSA-NCAM, and sialylation enzymes in microdissected mouse hypothalamus tissues from proestrous afternoon [peak of estrogens and nadir of arcuate nucleus (AN) synapses] and metestrous morning (nadir of estrogens and highest AN synapses). Immunohistochemistry confirmed immunoreactive (ir) PSA-NCAM staining in the perineural spaces of the PVA. The extent of staining was cycle dependent, with more dense and complete profiles of individual neurons limned by the ir-PSA-NCAM staining on proestrus and less on metestrus. Western blots showed that high levels of ir-PSA-NCAM on proestrus are accompanied by diminished ir-NCAM-140 and -180 but not ir-NCAM-120 and the reverse on metestrus (P < 0.05). To evaluate the increase of sialylated NCAM at the expense of desialylated protein, expression of the responsible polysialyltransferase enzymes polysialyltransferase (ST8Sia IV) and sialyltransferase (ST8Sia II) mRNA levels were measured using RT-PCR. Both polysialyltransferase and sialyltransferase mRNA are more abundant on proestrus than metestrus (P < 0.05), indicating that these enzymes are regulated by estrogens. These results support estrogen-regulated formation and extrusion of hydrophilic PSA-NCAM into perineural spaces in the PVA as part of the mechanism of EISP.

Leptin indirectly regulates gonadotropin-releasing hormone neuronal function

The adipose-derived hormone leptin communicates information about metabolic status to the hypothalamic GnRH neuronal system. It is unclear whether leptin can act directly on GnRH neurons. To examine this, we used three approaches. First, the presence of leptin-induced signal transducer and activator of transcription-3 activation was examined in GnRH neurons in male and female rats. Intracerebroventricular treatment with 4 mug leptin-induced robust signal transducer and activator of transcription-3 expression within the anteroventral periventricular nucleus but not in GnRH neurons. Second, fertility was assessed in male and female CRE-loxP transgenic mice with conditional leptin receptor (Lepr) deletion from either all forebrain neurons or GnRH neurons only. Forebrain neuron LEPR deletion prevented the onset of puberty resulting in infertility in males and females and blocked estradiol-induced LH surge. However, mice with GnRH neuron-selective Lepr deletion exhibited normal fertility apart from a slight puberty delay in males. Lastly, the highly sensitive technique of single-cell nested PCR was used to test for Lepr transcript presence in individual GnRH neurons, identified in situ using GnRH-green fluorescent protein transgenics. Whereas 75% of positive control (proopiomelanocortin) neurons contained Lepr mRNA, no (none of 18) GnRH neurons were Lepr mRNA positive. Collectively, these results show that leptin does not act directly on GnRH neurons in rats and mice. Leptin appears to regulate GnRH function via forebrain neurons that are afferent to GnRH because forebrain neuronal LEPR deletion caused infertility. The location and phenotype of these leptin-responsive neurons remains to be elucidated.

The ventral premammillary nucleus links fasting-induced changes in leptin levels and coordinated luteinizing hormone secretion

Physiological conditions of low leptin levels like those observed during negative energy balance are usually characterized by the suppression of luteinizing hormone (LH) secretion and fertility. Leptin administration restores LH levels and reproductive function. Leptin action on LH secretion is thought to be mediated by the brain. However, the neuronal population that mediates this effect is still undefined. The hypothalamic ventral premammillary nucleus (PMV) neurons express a dense concentration of leptin receptors and project to brain areas related to reproductive control. Therefore, we hypothesized that the PMV is well located to mediate leptin action on LH secretion. To test our hypothesis, we performed bilateral excitotoxic lesions of the PMV in adult female rats. PMV-lesioned animals displayed a clear disruption of the estrous cycle, remaining in anestrus for 15-20 d. After apparent recovery of cyclicity, animals perfused in the afternoon of proestrus showed decreased Fos immunoreactivity in the anteroventral periventricular nucleus and in gonadotropin releasing hormone neurons. PMV-lesioned animals also displayed decreased estrogen and LH secretion on proestrus. Lesions caused no changes in mean food intake and body weight up to 7 weeks after surgery. We further tested the ability of leptin to induce LH secretion in PMV-lesioned fasted animals. We found that complete lesions of the PMV precluded leptin stimulation of LH secretion on fasting. Our findings demonstrate that the PMV is a key site linking changing levels of leptin and coordinated control of reproduction.

Impaired estrogen feedback and infertility in female mice with pituitary-specific deletion of estrogen receptor alpha (ESR1)

Mice lacking estrogen receptor alpha in the pituitary gonadotroph (PitEsr1KO) were generated to determine the physiologic role of pituitary estrogen signaling in the reproductive axis. PitEsr1KO female mice are subfertile or infertile and have elevated levels of serum luteinizing hormone (LH) and LH beta subunit gene expression, reflecting a lack of estrogen negative feedback effect on the gonadotroph. While serum LH values are elevated in PitEsr1KO mice, the degree of elevation is much less than that observed in ESR1-null mice, indicating that the hypothalamus must also have an important role in estrogen negative feedback. PitEsr1KO mice also demonstrate a defect in estrogen positive feedback, as surge LH values and estrous cyclicity are absent in these mice. Although sex steroid feedback in the reproductive axis is thought to involve discrete anatomic regions that mediate either a positive or negative estrogen effect, PitEsr1KO mice demonstrate novel evidence that localizes both estrogen positive feedback and estrogen negative feedback to the gonadotroph, which suggests that they may be mechanistically related.

Estrogen regulation of gene expression in GnRH neurons

Estrogen plays an essential role in the regulation of the female reproductive hormone axis, and specifically is a major regulator of GnRH neuronal function in the female brain. GnRH neuronal cell lines were used to explore the direct effects of estradiol on gene expression in GnRH neurons. The presence of estrogen receptor (ER) binding sites was established by a receptor-binding assay, and estrogen receptor alpha and beta mRNA were identified in GN11 cells and ERbeta in GT1-7 cells using RT-PCR analysis of mRNA. ERalpha was more abundantly expressed in GN11 cells than ERbeta as assessed by real-time PCR. Additionally, GN11 cells expressed significantly more of both ERalpha and beta than GT1-7 cells. Functional studies in GN11 and GT1-7 demonstrated estrogen down regulation of endogenous mouse GnRH mRNA levels using quantitative real-time PCR (qRT-PCR). Correspondingly, estradiol also reduced secretion of GnRH from both the GN11 and GT1-7 cell lines. Since estradiol has been shown to regulate progesterone receptor (PR) expression; similar studies were performed demonstrating an estradiol mediated increase in PR in both cell lines. Estradiol regulation of ER expression was also explored and these studies indicated that estradiol decreased ERalpha and ERbeta mRNA levels in a dose-dependent manner in GN11 and GT1-7 cells. These effects were blocked by the addition of the estrogen receptor antagonist ICI 182,780. Both PPT, a specific ERalpha agonist, and DPN, a specific ERbeta agonist, inhibited GnRH gene expression in GN11 cells, but only DPN inhibited GnRH gene expression in GT1-7 cells, consistent with their undetectable levels of ERalpha expression. These studies characterize a direct inhibitory effect of estradiol on GnRH in GnRH neurons, and a direct stimulatory effect of estradiol on PR gene expression. In addition, the agonist studies indicate that there is a functional overlap of ERalpha and ERbeta regulation in GnRH neurons. These studies may give insight into the molecular regulation of estrogen negative feedback in the central reproductive axis.

Age- and hormone-regulation of N-methyl-D-aspartate receptor subunit NR2b in the anteroventral periventricular nucleus of the female rat: implications for reproductive senescence

Glutamate, acting through its N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the hypothalamus, regulates reproductive neuroendocrine functions via direct and indirect actions upon gonadotrophin-releasing hormone (GnRH) neurones. Previous studies indicate that the NMDA receptor subunit NR2b undergoes changes in protein and gene expression in the hypothalamus in general, and on GnRH neurones in particular, during reproductive ageing. In the present study, we examined whether the NR2b-expressing cell population, both alone and in association with the NR1 subunit (i.e. the latter subunit is necessary for a functional NMDA receptor), is altered as a function of age and ⁄ or steroid hormone treatment. Studies focused on the anteroventral periventricular (AVPV) nucleus of the hypothalamus, a region critically involved in the control of reproduction. Young (3-5 months), middle-aged (9-12 months), and aged (approximately 22 months) female rats were ovariectomised and, 1 month later, they were treated sequentially with oestradiol plus progesterone, oestradiol plus vehicle, or vehicle plus vehicle, then perfused. Quantitative stereologic analysis of NR2b-immunoreactive cell numbers in the AVPV showed an age-associated decrease in the density of NR2b-immunoreactive cells, but no effect of hormone treatment. In a second study, immunofluorescent double labelling of NR2b and NR1 was analysed by confocal microscopy of fraction volume, a semi-quantitative measure of fluorescence intensity. No effect of ageing was detected for immunofluorescent NR1 or NR2b alone, whereas the NR2b fraction volume increased in the oestradiol plus vehicle group. With ageing, the fraction volume of the NR2b/NR1-colocalised subunits increased. Together with the stereology results, this suggests that, although fewer cells express the NR2b subunit in the ageing AVPV, a greater percentage of these subunits are co-expressed with NR1. Our results suggest that the subunit composition of NMDA receptors in the AVPV undergo both age- and hormonal-regulation, which may be related to previous observations of changes in functional responses of reproductive neuroendocrine systems to NMDA receptor modulators with ageing.

The organizational-activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues

The 1959 publication of the paper by Phoenix et al. was a major turning point in the study of sexual differentiation of the brain. That study showed that sex differences in behavior, and by extension in the brain, were permanently sexually differentiated by testosterone, a testicular secretion, during an early critical period of development. The study placed the brain together in a class with other major sexually dimorphic tissues (external genitalia and genital tracts), and proposed an integrated hormonal theory of sexual differentiation for all of these non-gonadal tissues. Since 1959, the organizational-activational theory has been amended but survives as a central concept that explains many sex differences in phenotype, in diverse tissues and at all levels of analysis from the molecular to the behavioral. In the last two decades, however, sex differences have been found that are not explained by such gonadal hormonal effects, but rather because of the primary action of genes encoded on the sex chromosomes. To integrate the classic organizational and activational effects with the more recently discovered sex chromosome effects, we propose a unified theory of sexual differentiation that applies to all mammalian tissues.

Developmental profiles of neuroendocrine gene expression in the preoptic area of male rats

Reproductive function is controlled by GnRH cells and their steroid-sensitive regulatory inputs. The proper maturation of this system is critical to sexual development and maintenance of adult function. However, the molecular mechanisms underlying these developmental changes, and the potential roles of gonadal hormones in sculpting these processes, have not been fully explored. We performed a developmental profile from postnatal day (P) 1 through P60 of a network of five genes in the preoptic area (POA) that are critical to reproduction in male Sprague Dawley rats. GnRH, estrogen receptors-alpha, and -beta, androgen receptor (AR), and progesterone receptor (PR) mRNAs in the POA were assayed, and serum hormones were measured, in developing male rats. We also used a Taqman low-density array to identify candidate genes that may be important in development. Of the five targeted genes, only AR and PR changed robustly (7- and 3- to 4-fold increases, respectively) during development. All of the gonadal serum hormones changed markedly and with very different patterns from their receptor mRNAs: testosterone decreased from P1 to P30 and then increased to P60; progesterone peaked on P30; and estradiol decreased from P1 to P30. Using the Taqman low-density array, we identified several genes that changed dramatically in the POA with development, particularly G protein-coupled receptor 30, IGF-I, vitamin D receptor, estrogen-related receptor-alpha, and thyroid receptor-alpha. Our data demonstrate developmental stage-specific changes in neuroendocrine genes, particularly AR and PR. Moreover, the relationships between hormones and their corresponding receptors undergo dynamic changes across development in male rats.

p21-Activated kinase mediates rapid estradiol-negative feedback actions in the reproductive axis

Nonclassical estrogen receptor alpha (ERalpha) signaling can mediate E(2) negative feedback actions in the reproductive axis; however, downstream pathways conveying these effects remain unclear. These studies tested the hypothesis that p21-activated kinase 1 (PAK1), a serine/threonine kinase rapidly activated by E(2) in nonneural cells, functions as a downstream node for E(2) signaling pathways in cells of the preoptic area, and it may thereby mediate E(2) negative feedback effects. Treatment of ovariectomized (OVX) rats with estradiol benzoate (EB) caused rapid and transient induction of phosphorylated PAK1 immunoreactivity in the medial preoptic nucleus (MPN) but not the arcuate nucleus. To determine whether rapid induction of PAK phosphorylation by E(2) is mediated by nonclassical [estrogen response element (ERE)-independent] ERalpha signaling, we used female ERalpha null (ERalpha(-/-)) mice possessing an ER knock-in mutation (E207A/G208A; AA), in which the mutant ERalpha is incapable of binding DNA and can signal only through membrane-initiated or ERE-independent genotropic pathways (ERalpha(-/AA) mice). After 1-h EB treatment, the number of pPAK1-immunoreactive cells in the MPN was increased in both wild-type (ERalpha(+/+)) and ERalpha(-/AA) mice but was unchanged in ERalpha(-/-) mice. Serum luteinizing hormone (LH) was likewise suppressed within 1 h after EB treatment in ERalpha(+/+) and ERalpha(-/AA) but not ERalpha(-/ -) mice. In OVX rats, 5-min intracerebroventricular infusion of a PAK inhibitor peptide but not control peptide blocked rapid EB suppression of LH secretion. Taken together, our findings implicate PAK1 activation subsequent to nonclassical ERalpha signaling as an important component of the negative feedback actions of E(2) in the brain.

Central and peripheral effects of RFamide-related peptide-3 on luteinizing hormone and prolactin secretion in rats

Hypothalamic RFamide-related peptide-3 (RFRP-3) neurons inhibit LH secretion via a central action. A direct hypophysiotropic action on the gonadotropes has also been suggested. To assess central RFRP-3 effects on the GnRH/LH surge that induces ovulation, ovariectomized rats were subjected to an estradiol plus progesterone surge-induction protocol. Chronic infusion of RFRP-3 (2.5 or 25 ng/h, intracerebroventricularly) caused a dose-dependent 50-60% inhibition of GnRH neuronal activation (assessed by colocalization with the immediate early gene c-Fos) at the surge peak compared with vehicle-treated controls. RFRP-3 also suppressed neuronal activation in the anteroventral periventricular region, which provides stimulatory input to GnRH neurons, by 50-80% compared with control values. To test whether centrally administered RFRP-3 inhibits pulsatile GnRH/LH secretion, chronically ovariectomized, low-level estradiol-treated rats without surge induction were blood sampled every 10 min for 4 h. Bolus injection of RFRP-3 (0, 2.5, or 25 microg, intracerebroventricularly) after 1.5 h did not affect subsequent LH pulse frequency, pulse amplitude, or the mean concentrations of LH or prolactin. RFRP-3 treatment of isolated anterior pituitary cells at moderate doses of up to 10(-7) m did not significantly inhibit LH release, either with or without GnRH cotreatment. These data reveal a central inhibitory effect of RFRP-3 on the hypothalamo-pituitary gonadal axis specifically during the estradiol-induced GnRH/LH surge. This effect may include actions of RFRP-3 on GnRH neurons and/or their anteroventral periventricular afferent inputs but is unlikely to involve direct inhibition of LH secretion at the level of the gonadotrope.

Developmental programming: contribution of prenatal androgen and estrogen to estradiol feedback systems and periovulatory hormonal dynamics in sheep

Prenatal testosterone excess leads to neuroendocrine and periovulatory disruptions in the offspring culminating in progressive loss of cyclicity. It is unknown whether the mediary of these disruptions is androgen or estrogen, because testosterone can be aromatized to estrogen. Taking a reproductive life span approach of studying control, prenatal testosterone, and dihydrotestosterone-treated offspring, this study tested the hypothesis that disruptions in estradiol-negative but not -positive feedback effects are programmed by androgenic actions of testosterone and that these disruptions in turn will have an impact on the periovulatory hormonal dynamics. The approach was to test estradiol-negative and -positive feedback responses of all three groups of ovary-intact females during prepubertal age and then compare the periovulatory dynamics of luteinizing hormone, follicle-stimulating hormone, estradiol, and progesterone during the first breeding season. The findings show that estradiol-negative but not estradiol-positive feedback disruptions in prenatal testosterone-treated females are programmed by androgenic actions of prenatal testosterone excess and that follicular phase estradiol and gonadotropins surge disruptions during reproductive life are consistent with estrogenic programming. Additional studies carried out testing estradiol-positive feedback response over time found progressive deterioration of estradiol-positive feedback in prenatal testosterone-treated sheep until the time of puberty. Together, these findings provide insight into the mechanisms by which prenatal testosterone disrupts the reproductive axis. The findings may be of translational relevance since daughters of mothers with hyperandrogenism are at risk of increased exposure to androgens.

Sagittalis nucleus: a novel hypothalamic nucleus

The sagittalis nucleus (SGN) of the hypothalamus is a newly-identified nucleus that is located in the interstitial area between the arcuate and ventromedial nuclei of the rat hypothalamus and for which the long axis of the nucleus is oriented sagittally. Interestingly, the SGN exhibits structural and physiological sex differences, as defined by Nissl staining and oestrogen receptor (ER)alpha immunoreactivity (-ir), being larger in males than females. The structural sex difference is established by sex steroid action in neonates because the treatment of female pups with testosterone propionate masculinised the SGN. The phenotypical sex difference in ERalpha-ir is mediated hormonally in adulthood. Ovariectomy of female rats caused a significant increase in ERalpha-ir in the SGN, and eliminated the physiological sex difference, but with recovery to the level of gonad-intact females when given oestradiol replacement. Adult females have oestrous cycle-related variations in ERalpha-ir in the SGN, with levels at a nadir during the evening of pro-oestrous. The discovery of the SGN, a target of sex steroid action, provides a new opportunity for explaining hormonal regulation of sexually-differentiated behavioural and endocrine functions.

Oestrogen, kisspeptin, GPR54 and the pre-ovulatory luteinising hormone surge

Ovulation is central to mammalian fertility, yet the precise mechanism through which oestrogen triggers the gonadotrophin-releasing hormone (GnRH) surge that generates the pre-ovulatory luteinising hormone (LH) surge has remained elusive. The recent discovery that kisspeptin-GPR54 signalling is an essential regulator of the neuroendocrine axis at puberty has led investigators to evaluate the role of kisspeptin in the pre-ovulatory GnRH surge mechanism. Kisspeptin neurones are known to express oestrogen and progesterone receptors and have their cell bodies located in brain regions implicated in the positive-feedback mechanism in several mammalian species. In rodents, kisspeptin neurones located in the rostral periventricular area of the third ventricle (RP3V) are positively regulated by oestrogen and most likely are activated by oestrogen at the time of positive feedback. A similar scenario appears to exist for a sub-population of kisspeptin neurones located in the mediobasal hypothalamus of sheep and primates. The majority of GnRH neurones express GPR54, and kisspeptin causes an intense electrical activation of these cells. In concordance with this, kisspeptin administration in vivo results in an abrupt and prolonged release of LH in all mammalian species examined to date. Functional evidence from immunoneutralisation and knockout studies suggests that RP3V kisspeptin neurones projecting to GnRH neurones are an essential component of the surge mechanism in rodents. Taken together, the studies undertaken to date provide substantial evidence in support of a key role of kisspeptin-GPR54 signalling in the generation of the oestrogen-induced pre-ovulatory surge mechanism in mammals.

Rapid action of oestrogen in luteinising hormone-releasing hormone neurones: the role of GPR30

Previously, we have shown that 17beta-oestradiol (E(2)) induces an increase in firing activity and modifies the pattern of intracellular calcium ([Ca(2+)](i)) oscillations with a latency < 1 min in primate luteinising hormone-releasing hormone (LHRH) neurones. A recent study also indicates that E(2), the nuclear membrane impermeable oestrogen, oestrogen-dendrimer conjugate, and the plasma membrane impermeable oestrogen, E(2)-BSA conjugate, all similarly stimulated LHRH release within 10 min of exposure in primate LHRH neurones, indicating that the rapid action of E(2) is caused by membrane signalling. The results from a series of studies further suggest that the rapid action of E(2) in primate LHRH neurones appears to be mediated by GPR30. Although the oestrogen receptor antagonist, ICI 182, 780, neither blocked the E(2)-induced LHRH release nor the E(2)-induced changes in [Ca(2+)](i) oscillations, E(2) application to cells treated with pertussis toxin failed to result in these changes in primate LHRH neurones. Moreover, knockdown of GPR30 in primate LHRH neurones by transfection with human small interference RNA for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not. Finally, the GPR30 agonist, G1, resulted in changes in [Ca(2+)](i) oscillations similar to those observed with E(2). In this review, we discuss the possible role of G-protein coupled receptors in the rapid action of oestrogen in neuronal cells.

Involvement of G protein-coupled receptor 30 (GPR30) in rapid action of estrogen in primate LHRH neurons

Previously, we have reported that 17beta-estradiol (E(2)) induces an increase in firing activity of primate LH-releasing hormone (LHRH) neurons. The present study investigates whether E(2) alters LHRH release as well as the pattern of intracellular calcium ([Ca(2+)](i)) oscillations and whether G protein-coupled receptor 30 (GPR30) plays a role in mediating the rapid E(2) action in primate LHRH neurons. Results are summarized: 1) E(2), the nuclear membrane-impermeable estrogen, estrogen-dendrimer conjugate, and the plasma membrane-impermeable estrogen, E(2)-BSA conjugate, all stimulated LHRH release within 10 min of exposure; 2) whereas the estrogen receptor antagonist, ICI 182,780, did not block the E(2)-induced LHRH release, E(2) application to cells treated with pertussis toxin failed to induce LHRH release; 3) GPR30 mRNA was expressed in olfactory placode cultures, and GPR30 protein was expressed in a subset of LHRH neurons; 4) pertussis toxin treatment blocked the E(2)-induced increase in [Ca(2+)](i) oscillations; 5) knockdown of GPR30 in primate LHRH neurons by transfection with small interfering RNA (siRNA) for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not; 6) the estrogen-dendrimer conjugate-induced increase in [Ca(2+)](i) oscillations also did not occur in LHRH neurons transfected with GPR30 siRNA; and 7) G1, a GPR30 agonist, resulted in changes in [Ca(2+)](i) oscillations, similar to those observed with E(2). Collectively, E(2) induces a rapid excitatory effect on primate LHRH neurons, and this rapid action of E(2) appears to be mediated, in part, through GPR30.

Neurobiological mechanisms underlying oestradiol negative and positive feedback regulation of gonadotrophin-releasing hormone neurones

The feedback actions of ovarian oestradiol during the female reproductive cycle are among the most unique in physiology. During most of the cycle, oestradiol exerts homeostatic, negative feedback upon the release of gonadotrophin-releasing hormone (GnRH). Upon exposure to sustained elevated oestradiol levels, however, there is a switch in the feedback effects of this hormone to positive, resulting in induction of a surge in the release of GnRH that serves as a neuroendocrine signal to initiate the ovulatory cascade. We review recent developments stemming from studies in an animal model exhibiting daily switches between positive and negative feedback that have probed the neurobiological mechanisms, including changes in neural networks and intrinsic properties of GnRH neurones, underlying this switch in oestradiol action.

Kisspeptin/metastin: a key molecule controlling two modes of gonadotrophin-releasing hormone/luteinising hormone release in female rats

Kisspeptin (also known as metastin), a hypothalamic peptide, has attracted attention as a key molecule in the release of gonadotrophin-releasing hormone (GnRH) in various mammalian species, such as rodents, sheep and primates. Two populations of kisspeptin neurones in the brain may control two modes of GnRH release to time the onset of puberty and regulate oestrous cyclicity in rats and mice. One population of kisspeptin neurones, located in the anteroventral periventricular nucleus, appears to be responsible for the induction of the GnRH surge that leads to the luteinising hormone surge and ovulation. The other, located in the hypothalamic arcuate nucleus, appears to be involved in generating GnRH pulses, resulting in luteinising hormone pulses followed by follicular development and steroidogenesis in the ovary. The present review focuses on the physiological role of the two populations of kisspeptin neurones in controlling gonadal functions by generating the two modes of GnRH release in a female rat model.

Estradiol suppresses glutamatergic transmission to gonadotropin-releasing hormone neurons in a model of negative feedback in mice

A surge of gonadotropin-releasing hormone (GnRH) release from the brain triggers the luteinizing hormone (LH) surge that causes ovulation. The GnRH surge is initiated by a switch in estradiol action from negative to positive feedback. Estradiol signals critical for the surge are likely transmitted to GnRH neurons at least in part via estradiol-sensitive afferents. Using an ovariectomized estradiol-treated (OVX+E) mouse model that exhibits daily LH surges, we examined changes in glutamate transmission to GnRH neurons during negative feedback and positive feedback. Spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptors (AMPA/KA Rs) or N-methyl-D-aspartate receptors (NMDARs) were recorded in GnRH neurons from OVX+E and OVX mice. There were no diurnal changes in the percentage of GnRH neurons from OVX mice exhibiting EPSCs. In cells from OVX+E mice, the profile of AMPA/KA R-mediated and NMDAR-mediated EPSCs showed changes dependent on time of day. Comparison of AMPA/KA R-mediated EPSC frequency in OVX+E and OVX cells showed that estradiol suppressed transmission during negative feedback but had no effect during positive feedback. Tetrodotoxin treatment to block action potential firing did not affect AMPA/KA R-mediated EPSC frequency in OVX cells during negative feedback or in OVX+E cells during positive feedback, suggesting that estradiol-induced suppression of glutamate transmission may be primarily due to activity-independent changes. The diurnal removal of estradiol-induced suppression of AMPA/KA R-mediated glutamate transmission to GnRH neurons during positive feedback suggests that the primary role for estradiol-induced changes in glutamate transmission may be in mediating negative feedback.

17Beta-estradiol regulation of the mRNA expression of T-type calcium channel subunits: role of estrogen receptor alpha and estrogen receptor beta

Low-voltage-activated (T-type) calcium channels are responsible for burst firing and transmitter release in neurons and are important for exocytosis and hormone secretion in pituitary cells. T-type channels contain an alpha1 subunit, of which there are three subtypes, Cav3.1, -3.2, and -3.3, and each subtype has distinct kinetic characteristics. Although 17beta-estradiol (E2) modulates T-type calcium channel expression and function, little is known about the molecular mechanisms involved. We used real-time PCR quantification of RNA extracted from hypothalamic nuclei and pituitary in vehicle and E2-treated C57BL/6 mice to elucidate E2-mediated regulation of Cav3.1, -3.2, and -3.3 subunits. The three subunits were expressed in both the hypothalamus and the pituitary. E2 treatment increased the mRNA expression of Cav3.1 and -3.2, but not Cav3.3, in the medial preoptic area and the arcuate nucleus. In the pituitary, Cav3.1 was increased with E2 treatment, and Cav3.2 and -3.3 were decreased. To examine whether the classical estrogen receptors (ERs) were involved in the regulation, we used ERalpha- and ERbeta-deficient C57BL/6 mice and explored the effects of E2 on T-type channel subtypes. Indeed, we found that the E2-induced increase in Cav3.1 in the hypothalamus was dependent on ERalpha, whereas the E2 effect on Cav3.2 was dependent on both ERalpha and ERbeta. However, the E2-induced effects in the pituitary were dependent on only the expression of ERalpha. The robust E2 regulation of T-type calcium channels could be an important mechanism by which E2 increases the excitability of hypothalamic neurons and modulates pituitary secretion.

Chapter 2: hypothalamic neural systems controlling the female reproductive life cycle gonadotropin-releasing hormone, glutamate, and GABA

The hypothalamic-pituitary-gonadal (HPG) axis undergoes a number of changes throughout the reproductive life cycle that are responsible for the development, puberty, adulthood, and senescence of reproductive systems. This natural progression is dictated by the neural network controlling the hypothalamus including the cells that synthesize and release gonadotropin-releasing hormone (GnRH) and their regulatory neurotransmitters. Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters in the central nervous system, and as such contribute a great deal to modulating this axis throughout the lifetime via their actions on receptors in the hypothalamus, both directly on GnRH neurons as well as indirectly through other hypothalamic neural networks. Interactions among GnRH neurons, glutamate, and GABA, including the regulation of GnRH gene and protein expression, hormone release, and modulation by estrogen, are critical to age-appropriate changes in reproductive function. Here, we present evidence for the modulation of GnRH neurosecretory cells by the balance of glutamate and GABA in the hypothalamus, and the functional consequences of these interactions on reproductive physiology across the life cycle.

Kisspeptins and the control of gonadotropin secretion in male and female rodents

Kisspeptins, the products of KiSS-1 gene acting via G protein-coupled receptor 54 (GPR54), have recently emerged as fundamental gatekeepers of gonadal function by virtue of their ability to stimulate gonadotropin secretion. Indeed, since the original disclosure of the reproductive facet of the KiSS-1/GPR54 system, an ever-growing number of studies have substantiated the extraordinary potency of kisspeptins to elicit gonadotropin secretion in different mammalian species, under different physiologic and experimental conditions, and through different routes of administration. In this context, studies conducted in laboratory rodents have been enormously instrumental to characterize: (i) the primary mechanisms of action of kisspeptins in the control of gonadotropin secretion; (ii) the pharmacological consequences of acute vs. continuous activation of GPR54; (iii) the roles of specific populations of kisspeptin-producing neurons at the hypothalamus in mediating the feedback effects of sex steroids; (v) the function of kisspeptins in the generation of the pre-ovulatory surge of gonadotropins; and (iv) the influence of sex steroids on GnRH/gonadotropin responsiveness to kisspeptins. While some of those aspects of kisspeptin function will be covered elsewhere in this Special Issue, we summarize herein the most salient data, obtained in laboratory rodents, that have helped to define the physiologic roles and putative pharmacological implications of kisspeptins in the control of male and female gonadotropic axis.

Sex steroid control of hypothalamic Kiss1 expression in sheep and rodents: comparative aspects

In recent years, the Kiss1 gene has been cast into the reproductive spotlight. In the short period since the discovered link between kisspeptins, the encoded peptides of Kiss1, and fertility, these peptides are now known to be critical for the neuroendocrine control of reproduction. Kisspeptin producing cells in the hypothalamus are poised to become the 'missing link' in the sex steroid feedback control of GnRH secretion. These cells contain all the necessary components to relay information of the sex steroid environment to GnRH neurons, which possess the kisspeptin receptor, GPR54. Sex steroids regulate Kiss1 mRNA, and kisspeptin expression in the hypothalamus, in a manner consistent with both negative and positive feedback control of GnRH. The precise nature of sex steroid effects, in particular those of estrogen, on Kiss1 expression have been extensively studied in the female rodent and ewe. In the arcuate nucleus (ARC) of both species, kisspeptin cells appear to forward signals pertinent to negative feedback regulation of GnRH, although in the ewe it appears this population of Kiss1 cell is also responsible for positive feedback regulation of GnRH at the time of the preovulatory GnRH/LH surge. In rodents, these positive feedback signals appear to be mediated by kisspeptin cells exclusively within the anteroventral periventricular nucleus (AVPV). There are no Kiss1 cells in the ovine AVPV, but there is a population in the preoptic area. The role these preoptic area cells play in the sex steroid feedback regulation of GnRH secretion, if any, is yet to be revealed.

The neuroanatomy of the kisspeptin system in the mammalian brain

The kisspeptin precursor is the protein transcribed from the Kiss-1 gene and the kisspeptins are the peptides that are posttranslationally processed from the precursor. The kisspeptins activate the G-protein coupled receptor GPR54 and are strongly implicated in puberty onset and in regulation of the hypothalamo-pituitary gonadal axis in mammals. Physiological studies have indicated that these effects occur via a direct activation of the GnRH neurons, and at an unknown site in the median eminence or directly on the gonadotropes. Paradoxically, while the function of kisspeptin is relatively well understood, little data are available about the localization of kisspeptin neurons in the brain, and in particular the projection patterns of kisspeptin containing axons implicated in regulation of the hypothalamo-pituitary gonadal axis. This review covers the current information about the localization of kisspeptin neurons in the mammalian brain and discusses the facts and artifacts of the methods of their detection. The available data suggest that kisspeptins are synthesized in neurons in the anteroventral periventricular nucleus and the arcuate nucleus. Both populations are considered to be involved in control of gonadotropes. In addition, kisspeptin nerve terminals and receptors are found in other hypothalamic area suggesting that kisspeptins are involved in regulation of other yet unknown homeostatic or neuroendocrine functions.

Kisspeptin and the regulation of the hypothalamic-pituitary-gonadal axis in the rhesus monkey (Macaca mulatta)

The present article reviews recent studies of monkeys and, in some cases, humans that have been conducted to examine the role of kisspeptin-GPR54 signaling in the regulation of the hypothalamic-pituitary-gonadal axis in higher primates. This area of peptide biology was initiated in 2003 by the discovery that loss of function mutations of GPR54 in man were associated with hypogonadotropic hypogonadism and absent or delayed puberty. Puberty in the monkey, an experimental model commonly used to study this fundamental developmental stage, is first described. This is followed by a review of the role of kisspeptin in the regulation of the postnatal ontogeny of GnRH pulsatility. The roles of kisspeptin in GnRH pulse generation and in the feedback loops governing gonadotropin secretion in primates are then discussed. A brief section on kisspeptin-GPR54 signaling at the pituitary and gonadal levels is also included. The review concludes with a discussion of the phenomenon of GPR54 downregulation by continuous exposure to kisspeptin and its therapeutic implications.

Down-regulation of hypothalamic kisspeptin and its receptor, Kiss1r, mRNA expression is associated with stress-induced suppression of luteinising hormone secretion in the female rat

Identification of kisspeptin (Kiss1) and its G protein-coupled receptor 54 (Kiss1r) as an essential component of the hypothalamic-pituitary-gonadal (HPG) axis controlling gonadotrophin secretion raises the possibility that kisspeptin-Kiss1r signalling may play a critical role in the transduction of stress-induced suppression of reproduction. We examined the effects of: (i) three different stressors, known to suppress pulsatile luteinising hormone (LH) secretion; (ii) corticotrophin-releasing factor (CRF); and (iii) corticosterone on Kiss1 and Kiss1r expression in key hypothalamic sites regulating gonadotrophin secretion: the medial preoptic area (mPOA) and arcuate nucleus (ARC). Ovariectomised oestrogen-replaced rats were implanted with i.v., subcutaneous or i.c.v. cannulae. Blood samples were collected at 5-min intervals for 5-6 h for detection of LH. Quantitative reverse transcriptase-polymerase chain reaction was used to determine Kiss1 and Kiss1r mRNA levels in brain punches of the mPOA and ARC collected 6 h after restraint, insulin-induced hypoglycaemia or lipopolysaccharide stress, or after i.c.v. administration of CRF, or acute or chronic subcutaneous administration of corticosterone. We observed down-regulation of at least one component of the kisspeptin-Kiss1r signalling system by each of the stress paradigms within the mPOA and ARC. CRF decreased Kiss1 and Kiss1r expression in both the mPOA and ARC. Both acute and chronic stress levels of corticosterone resulted in a concomitant decrease in Kiss1 and an increase in kiss1r mRNA expression in the mPOA and ARC. This differential regulation of Kiss1 and Kiss1r might account for the lack of effect corticosterone has on pulsatile LH secretion. Considering the pivotal role for kisspeptin-Kiss1r signalling in the control of the HPG axis, these results suggest that the reduced Kiss1-Kiss1r expression may be a contributing factor in stress-related suppression of LH secretion.

Physiological role of metastin/kisspeptin in regulating gonadotropin-releasing hormone (GnRH) secretion in female rats

Various studies have attempted to unravel the physiological role of metastin/kisspeptin in the control of gonadotropin-releasing hormone (GnRH) release. A number of evidences suggested that the population of metastin/kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) is involved in generating a GnRH surge to induce ovulation in rodents, and thus the target of estrogen positive feedback. Females have an obvious metastin/kisspeptin neuronal population in the AVPV, but males have only a few cell bodies in the nucleus, suggesting that the absence of the surge-generating mechanism or positive feedback action in males is due to the limited AVPV metastin/kisspeptin neuronal population. On the other hand, the arcuate nucleus (ARC) metastin/kisspeptin neuronal population is considered to be involved in the regulation of tonic GnRH release. The ARC metastin/kisspeptin neurons show no sex difference in their expression, which is suppressed by gonadal steroids in both sexes. Thus, the ARC population of metastin/kisspeptin neurons is a target of estrogen negative feedback action on tonic GnRH release. The lactating rat model provided further evidence indicating that ARC metastin/kisspeptin neurons are involved in GnRH pulse generation, because pulsatile release of luteinizing hormone (LH) is profoundly suppressed by suckling stimulus and the LH pulse suppression is well associated with the suppression of ARC metastin/kisspeptin and KiSS-1 gene expression in lactating rats.

Daily changes in GT1-7 cell sensitivity to GnRH secretagogues that trigger ovulation

Circadian rhythms in behavior and physiology are orchestrated by a master biological clock located in the suprachiasmatic nucleus (SCN). Circadian oscillations are a cellular property, with 'clock' genes and their protein products forming transcription-translation feedback loops that maintain 24-hour rhythmicity. Although the expression of clock genes is thought to be ubiquitous, the function of local, extra-SCN timing mechanisms remains elusive. We hypothesized that extra-SCN clock genes control local temporal sensitivity to upstream modulatory signals, allowing system-specific processes to be carried out during individual, optimal times of day. To test this possibility, we examined changes in the sensitivity of immortalized GnRH neurons, GT1-7 cells, to timed stimulation by two key neuropeptides thought to trigger ovulation on the afternoon of proestrus, kisspeptin and vasoactive intestinal polypeptide (VIP). We noted a prominent daily rhythm of clock gene expression in this cell line. GT1-7 cells also exhibited daily changes in cellular peptide expression and GnRH secretion in response to kisspeptin and VIP stimulation. These responses occurred without changes in GnRH transcription. These findings are consistent with the notion that GnRH cells are capable of intrinsic circadian cycles that may be fundamental for coordinating daily changes in sensitivity to signals impacting the reproductive axis.

Sexual differentiation and the Kiss1 system: hormonal and developmental considerations

The nervous system (both central and peripheral) is anatomically and physiologically differentiated between the sexes, ranging from gender-based differences in the cerebral cortex to motoneuron number in the spinal cord. Although genetic factors may play a role in the development of some sexually differentiated traits, most identified sex differences in the brain and behavior are produced under the influence of perinatal sex steroid signaling. In many species, the ability to display an estrogen-induced luteinizing hormone (LH) surge is sexually differentiated, yet the specific neural population(s) that allows females but not males to display such estrogen-mediated "positive feedback" has remained elusive. Recently, the Kiss1/kisspeptin system has been implicated in generating the sexually dimorphic circuitry underlying the LH surge. Specifically, Kiss1 gene expression and kisspeptin protein levels in the anteroventral periventricular (AVPV) nucleus of the hypothalamus are sexually differentiated, with females displaying higher levels than males, even under identical hormonal conditions as adults. These findings, in conjunction with accumulating evidence implicating kisspeptins as potent secretagogues of gonadotropin-releasing hormone (GnRH), suggest that the sex-specific display of the LH surge (positive feedback) reflects sexual differentiation of AVPV Kiss1 neurons. In addition, developmental kisspeptin signaling via its receptor GPR54 appears to be critical in males for the proper sexual differentiation of a variety of sexually dimorphic traits, ranging from complex social behavior to specific forebrain and spinal cord neuronal populations. This review discusses the recent data, and their implications, regarding the bi-directional relationship between the Kiss1 system and the process of sexual differentiation.

Emerging methodologies for the study of hypothalamic gonadotropin-releasing-hormone (GnRH) neurons

Gonadotropin-releasing-hormone (GnRH) neurons form part of a central neural oscillator that controls sexual reproduction through intermittent release of the GnRH peptide. Activity of GnRH neurons, and by extension release of GnRH, has been proposed to reflect intrinsic properties and synaptic input of GnRH neurons. To study GnRH neurons, we used traditional electrophysiology and computational methods. These emerging methodologies enhance the elucidation of processing in GnRH neurons. We used dynamic current-clamping to understand how living GnRH somata process input from glutamate and GABA, two key neurotransmitters in the neuroendocrine hypothalamus. In order to study the impact of synaptic integration in dendrites and neuronal morphology, we have developed full-morphology models of GnRH neurons. Using dynamic clamping, we have demonstrated that small-amplitude glutamatergic currents can drive repetitive firing in GnRH neurons. Furthermore, application of simulated GABAergic synapses with a depolarized reversal potential have revealed two functional subpopulations of GnRH neurons: one population in which GABA chronically depolarizes membrane potential (without inducing action potentials) and a second population in which GABAergic excitation results in slow spiking. Finally, when AMPA-type and GABA-type simulated inputs are applied together, action potentials occur when the AMPA-type conductance occurs during the descending phase of GABAergic excitation and at the nadir of GABAergic inhibition. Compartmental computer models have shown that excitatory synapses at >300 microns from somtata are unable to drive spiking with purely passive dendrites. In models with active dendrites, distal synapses are more efficient at driving spiking than somatic inputs. We then used our models to extend the results from dynamic current clamping at GnRH somata to distribute synaptic inputs along the dendrite. We show that propagation delays for dendritic synapses alter synaptic integration in GnRH neurons by widening the temporal window of interaction for the generation of action potentials. Finally, we have shown that changes in dendrite morphology can modulate the output of GnRH neurons by altering the efficacy of action potential generation in response to after-depolarization potentials (ADPs). Taken together, the methodologies of dynamic current clamping and multi-compartmental modeling can make major contributions to the study of synaptic integration and structure-function relationships in hypothalamic GnRH neurons. Use of these methodological approaches will continue to provide keen insights leading to conceptual advances in our understanding of reproductive hormone secretion in normal and pathological physiology and open the door to understanding whether the mechanisms of pulsatile GnRH release are conserved across species.

Classical estrogen receptor alpha signaling mediates negative and positive feedback on gonadotropin-releasing hormone neuron firing

During the female reproductive cycle, the neuroendocrine action of estradiol switches from negative feedback to positive feedback to initiate the preovulatory GnRH and subsequent LH surges. Estrogen receptor-alpha (ERalpha) is required for both estradiol negative and positive feedback regulation of LH. ERalpha may signal through estrogen response elements (EREs) in DNA and/or via ERE-independent pathways. Previously, a knock-in mutant allele (ERalpha-/AA) that selectively restores ERE-independent signaling onto the ERalpha-/- background was shown to confer partial negative but not positive estradiol feedback on serum LH. The current study investigated the roles of the ERE-dependent and ERE-independent ERalpha pathways for estradiol feedback at the level of GnRH neuron firing activity. The above ERalpha genetic models were crossed with GnRH-green fluorescent protein mice to enable identification of GnRH neurons in brain slices. Targeted extracellular recordings were used to monitor GnRH neuron firing activity using an ovariectomized, estradiol-treated mouse model that exhibits diurnal switches between negative and positive feedback. In wild-type mice, GnRH neuron firing decreased in response to estradiol during negative feedback and increased during positive feedback. In contrast, both positive and negative responses to estradiol were absent in GnRH neurons from ERalpha-/- and ERalpha-/AA mice. ERE-dependent signaling is thus required to increase GnRH neuron firing to generate a GnRH/LH surge. Furthermore, ERE-dependent and -independent ERalpha signaling pathways both appear necessary to mediate estradiol negative feedback on serum LH levels, suggesting central and pituitary estradiol feedback may use different combinations of ERalpha signaling pathways.

Nonclassical estrogen modulation of presynaptic GABA terminals modulates calcium dynamics in gonadotropin-releasing hormone neurons

There is increasing recognition that estrogen exerts multifaceted regulatory effects on GnRH neurons. The acute effects of estrogen on calcium dynamics in these cells were examined using a transgenic mouse line that allows real-time measurement of intracellular calcium concentration ([Ca2+]i) in GnRH neurons in the acute brain slice preparation. 17-beta-Estradiol (E2) at 100 pm-100 nm was found to activate [Ca2+]i transients in approximately 40% of GnRH neurons with an approximate 15-min latency. This effect was not replicated by E2-BSA, which limits E2 action to the membrane, 17-alpha-estradiol, the inactive isomer at classical estrogen receptors (ERs), or G-1 the GPR30 agonist. E2 continued to activate [Ca2+]i transients when transcription was blocked. An ER alpha-selective agonist was equally potent in activating [Ca2+]i transients, and E2 remained effective in ERbeta knockout x GnRH-Pericam mice. E2's activation of [Ca2+]i transients continued in the presence of tetrodotoxin, which blocks action potential-dependent transmission, but was abolished completely by the further addition of a gamma-aminobutyric acid (GABA)A receptor antagonist. Exogenous GABA was found to initiate [Ca2+]i transients in GnRH neurons. Whole cell, voltage-clamp recordings of GnRH-green fluorescence protein neurons revealed that E2 generated discrete bursts of miniature inhibitory postsynaptic currents with a latency of approximately 15 min. These observations provide evidence for a new mechanism of nonclassical estrogen action within the brain. Estrogen interacts with the classical ERalpha at the level of the GABAergic nerve terminal to regulate action potential-independent GABA release that, in turn, controls postsynaptic calcium dynamics.

Critical roles for fast synaptic transmission in mediating estradiol negative and positive feedback in the neural control of ovulation

A switch in the balance of estradiol feedback actions from negative to positive initiates the GnRH surge, triggering the LH surge that causes ovulation. Using an ovariectomized, estradiol-treated (OVX+E) mouse model that exhibits daily switches between negative in the morning and positive feedback in the evening, we investigated the roles of fast synaptic transmission in regulating GnRH neuron firing during negative and positive feedback. Targeted extracellular recordings were used to monitor activity of GnRH neurons from OVX+E and OVX mice in control solution or solution with antagonists to both ionotropic glutamate and gamma-aminobutyric acid receptors (blockade). Blockade had no effect on activity of OVX cells. In contrast, in OVX+E cells in the morning, blockade increased activity compared with control cells, whereas in the evening, blockade decreased activity. In vivo barbiturate sedation of OVX+E mice that blocked LH surge induction prevented the in vitro evening changes in firing rate and response to blockade. These observations suggest at least partial inversion of the negative-to-positive switch in estradiol feedback action and indicate that changes in fast synaptic transmission to GnRH neurons and within the network of cells presynaptic to GnRH neurons are critical for mediating estradiol negative and positive feedback actions on GnRH neurons. Fast synaptic transmission may also affect GnRH neuron activity indirectly through altering release of excitatory and inhibitory neuromodulators onto GnRH neurons at specific times of day. Fast synaptic transmission is thus critical for proper generation and timing of the GnRH surge.

Follicle-stimulating hormone responses to kisspeptin in the female rat at the preovulatory period: modulation by estrogen and progesterone receptors

Ovulation is triggered by the preovulatory surge of gonadotropins that, in rodents, is defined by the concomitant rise in circulating LH and FSH at the afternoon of proestrus (primary surge), followed by persistently elevated FSH levels at early estrus (secondary surge). In recent years, kisspeptins, products of the KiSS-1 gene that act via G protein-coupled receptor 54, have emerged as an essential hypothalamic conduit for the generation of the preovulatory LH surge by conveying positive feedback effects of estradiol onto GnRH neurons, an event that involves not only estradiol-induced transcription of the KiSS-1 gene at the anteroventral periventricular nucleus but also its ability to modulate GnRH/LH responses to kisspeptin. However, little is known about the potential modulation of FSH responsiveness to kisspeptin by sex steroids in the cyclic female. We report herein analyses on the consequences of selective blockade of estrogen receptors (ER)-alpha and -beta, as well as progesterone receptor (PR), on the ovulatory surges of FSH and their modulation by kisspeptin. Antagonism of ERalpha or PR equally blunted the primary and secondary surges of FSH and nullified FSH responses to kisspeptin at the preovulatory period. Conversely, selective blockade of ERbeta failed to induce major changes in terms of endogenous FSH surges, yet it decreased FSH responses to exogenous kisspeptin. In contrast, FSH responses to GnRH were fully conserved after ERbeta blockade and partially preserved after inhibition of ERalpha and PR signaling. Finally, secondary FSH secretion was rescued by kisspeptin in females with selective blockade of ERalpha but not PR. In sum, our results substantiate a concurrent, indispensable role of ERalpha and PR in the generation of FSH surges and the stimulation of FSH responses to kisspeptin at the ovulatory period. In addition, our data suggest that ERbeta might operate as a subtle, positive modulator of the preovulatory FSH responses to kisspeptin, a role that is opposite to its putative inhibitory action on kisspeptin-induced LH secretion and might contribute to the dissociation of gonadotropin secretion at the ovulatory phase in the cyclic female rat.

Kisspeptin-GPR54 signaling is essential for preovulatory gonadotropin-releasing hormone neuron activation and the luteinizing hormone surge

Kisspeptin and its receptor GPR54 have recently been identified as key signaling partners in the neural control of fertility in animal models and humans. The gonadotropin-releasing hormone (GnRH) neurons represent the final output neurons of the neural network controlling fertility and are suspected to be the primary locus of kisspeptin-GPR54 signaling. Using mouse models, the present study addressed whether kisspeptin and GPR54 have a key role in the activation of GnRH neurons to generate the luteinizing hormone (LH) surge responsible for ovulation. Dual-label immunocytochemistry experiments showed that 40-60% of kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3V) expressed estrogen receptor alpha and progesterone receptors. Using an ovariectomized, gonadal steroid-replacement regimen, which reliably generates an LH surge, approximately 30% of RP3V kisspeptin neurons were found to express c-FOS in surging mice compared with 0% in nonsurging controls. A strong correlation was found between the percentage of c-FOS-positive kisspeptin neurons and the percentage of c-FOS-positive GnRH neurons. To evaluate whether kisspeptin and/or GPR54 were essential for GnRH neuron activation and the LH surge, Gpr54- and Kiss1-null mice were examined. Whereas wild-type littermates all exhibited LH surges and c-FOS in approximately 50% of their GnRH neurons, none of the mutant mice from either line showed an LH surge or any GnRH neurons with c-FOS. These observations provide the first evidence that kisspeptin-GPR54 signaling is essential for GnRH neuron activation that initiates ovulation. This broadens considerably the potential roles and therapeutic possibilities for kisspeptin and GPR54 in fertility regulation.

Membrane-initiated estrogen signaling in hypothalamic neurons

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. But it is not well understood how estrogen signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Estradiol regulation of progesterone synthesis in the brain

Steroidogenesis is now recognized as a global phenomenon in the brain, but how it is regulated and its relationship to circulating steroids of peripheral origin have remained more elusive issues. Neurosteroids, steroids synthesized de novo in nervous tissue, have a large range of actions in the brain, but it is only recently that the role of neuroprogesterone in the regulation of arguably the quintessential steroid-dependent neural activity, regulation of the reproduction has been appreciated. Circuits involved in controlling the LH surge and sexual behaviors were thought to be influenced by estradiol and progesterone synthesized in the ovary and perhaps the adrenal. It is now apparent that estradiol of ovarian origin regulates the synthesis of neuroprogesterone, and it is the locally produced neuroprogesterone that is involved in the initiation of the LH surge and subsequent ovulation. In this model, estradiol induces the transcription of progesterone receptors while stimulating synthesis of neuroprogesterone. Although the complete signaling cascade has not been elucidated, many of the features have been characterized. The synthesis of neuroprogesterone occurs primarily in astrocytes and requires the interaction of membrane-associated estrogen receptor-alpha with metabotropic glutamate receptor-1a. This G protein-coupled receptor activates a phospholipase C that in turn increases inositol trisphosphate (IP3) levels mediating the release of intracellular stores of Ca2+ via an IP3 receptor gated Ca2+ channel. The large increase in free cytoplasmic Ca2+ ([Ca2+]i) stimulates the synthesis of progesterone, which can then diffuse out of the astrocyte and activate estradiol-induced progesterone receptors in local neurons to trigger the neural cascade to produce the LH surge. Thus, it is a cooperative action of astrocytes and neurons that is needed for estrogen positive feedback and stimulation of the LH surge.

New insights into the classical and non-classical actions of estrogen: evidence from estrogen receptor knock-out and knock-in mice

Estrogen receptor alpha (ERalpha) mediates estrogen (E2) actions in the brain and is critical for normal reproductive function and behavior. In the classical pathway, ERalpha binds to estrogen response elements (EREs) to regulate gene transcription. ERalpha can also participate in several non-classical pathways, including ERE-independent gene transcription via protein-protein interactions with transcription factors and rapid, non-genotropic pathways. To distinguish between ERE-dependent and ERE-independent mechanisms of E2 action in vivo, we have created ERalpha null mice that possess an ER knock-in mutation (E207A/G208A; "AA"), in which the mutant ERalpha cannot bind to DNA but retains activity in ERE-independent pathways (ERalpha(-/AA) mice). Understanding the molecular mechanisms of ERalpha action will be helpful in developing pharmacological therapies that differentiate between ERE-dependent and ERE-independent processes. This review focuses on how the ERalpha(-/AA) model has contributed to our knowledge of ERalpha signaling mechanisms in estrogen regulation of the reproductive axis and sexual behavior.

Converse regulatory functions of estrogen receptor-alpha and -beta subtypes expressed in hypothalamic gonadotropin-releasing hormone neurons

Estradiol (E(2)) acts as a potent feedback molecule between the ovary and hypothalamic GnRH neurons, and exerts both positive and negative regulatory actions on GnRH synthesis and secretion. However, the extent to which these actions are mediated by estrogen receptors (ERs) expressed in GnRH neurons has been controversial. In this study, Single-cell RT-PCR revealed the expression of both ERalpha and ERbeta isoforms in cultured fetal and adult rat hypothalamic GnRH neurons. Both ERalpha and ERbeta or individual ERs were expressed in 94% of cultured fetal GnRH neurons. In adult female rats at diestrus, 68% of GnRH neurons expressed ERs, followed by 54% in estrus and 19% in proestrus. Expression of individual ERs was found in 24% of adult male GnRH neurons. ERalpha exerted marked G(i)-mediated inhibitory effects on spontaneous action potential (AP) firing, cAMP production, and pulsatile GnRH secretion, indicating its capacity for negative regulation of GnRH neuronal function. In contrast, increased E(2) concentration and ERbeta agonists increase the rate of AP firing, GnRH secretion, and cAMP production, consistent with ERbeta-dependent positive regulation of GnRH secretion. Consonant with the coupling of ERalpha to pertussis toxin-sensitive G(i/o) proteins, E(2) also activates G protein-activated inwardly rectifying potassium channels, decreasing membrane excitability and slowing the firing of spontaneous APs in hypothalamic GnRH neurons. These findings demonstrate that the dual actions of E(2) on GnRH neuronal membrane excitability, cAMP production, and GnRH secretion are mediated by the dose-dependent activation of ERalpha and ERbeta expressed in hypothalamic GnRH neurons.

Disrupted female reproductive physiology following neonatal exposure to phytoestrogens or estrogen specific ligands is associated with decreased GnRH activation and kisspeptin fiber density in the hypothalamus

It is well established that estrogen administration during neonatal development can advance pubertal onset and prevent the maintenance of regular estrous cycles in female rats. This treatment paradigm also eliminates the preovulatory rise of gonadotropin releasing hormone (GnRH). It remains unclear, however, through which of the two primary forms of the estrogen receptor (ERalpha or ERbeta) this effect is mediated. It is also unclear whether endocrine disrupting compounds (EDCs) can produce similar effects. Here we compared the effect of neonatal exposure to estradiol benzoate (EB), the ERalpha specific agonist 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT), the ERbeta specific agonist diarylpropionitrile (DPN) and the naturally occurring EDCs genistein (GEN) and equol (EQ) on pubertal onset, estrous cyclicity, GnRH activation, and kisspeptin content in the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei. Vaginal opening was significantly advanced by EB and GEN. By 10 weeks post-puberty, irregular estrous cycles were observed in all groups except the control group. GnRH activation, as measured by the percentage of immunopositive GnRH neurons that were also immunopositive for Fos, was significantly lower in all treatment groups except the DPN group compared to the control group. GnRH activation was absent in the PPT group. These data suggest that neonatal exposure to EDCs can suppress GnRH activity in adulthood, and that ERalpha plays a pivotal role in this process. Kisspeptins (KISS) have recently been characterized to be potent stimulators of GnRH secretion. Therefore we quantified the density of KISS immunolabeled fibers in the AVPV and ARC. In the AVPV, KISS fiber density was significantly lower in the EB and GEN groups compared to the control group but only in the EB and PPT groups in the ARC. The data suggest that decreased stimulation of GnRH neurons by KISS could be a mechanism by which EDCs can impair female reproductive function.

Temporal and spatial regulation of CRE recombinase expression in gonadotrophin-releasing hormone neurones in the mouse

Gonadotrophin-releasing hormone (GnRH) neurones located within the brain are the final neuroendocrine output regulating the reproductive hormone axis. Their small number and scattered distribution in the hypothalamus make them particularly difficult to study in vivo. The Cre/loxP system is a valuable tool to delete genes in specific cells and tissues. We report the production of two mouse lines that express the CRE bacteriophage recombinase in a GnRH-specific manner. The first line, the GnRH-CRE mouse, contains a transgene in which CRE is under the control of the murine GnRH promoter and targets CRE expression specifically to GnRH neurones in the hypothalamus. The second line, the GnRH-CRETeR mouse, uses the same murine GnRH promoter to target CRE expression to GnRH neurones, but is modified to be constitutively repressed by a tetracycline repressor (TetR) expressed from a downstream tetracycline repressor gene engineered within the transgene. GnRH neurone-specific CRE expression can therefore be induced by treatment with doxycycline which relieves repression by TetR. These GnRH-CRE and GnRH-CRETeR mice can be used to study the function of genes expressed specifically in GnRH neurones. The GnRH-CRETeR mouse can be used to study genes that may have distinct roles in reproductive physiology during the various developmental stages.

Roles of oestrogen receptors alpha and beta in behavioural neuroendocrinology: beyond Yin/Yang

Oestrogen receptor beta (ERbeta) was discovered more than 10 years ago. It is widely distributed in the brain. In some areas, such as the entorhinal cortex, it is present as the only ER, whereas in other regions, such as the bed nucleus of the stria terminalis and preoptic area, it can be found co-expressed with ERalpha, often within the same neurones. These ERs share ligands, and there are several complex relationships between the two receptors. Initially, the relationship between them was labelled as 'yin/yang', meaning that the actions of each complemented those of the other, but now, years later, other relationships have been described. Based on evidence from neuroendocrine and behavioural studies, three types of interactions between the two oestrogen receptors are described in this review. The first relationship is antagonistic; this is evident from studies on the role of oestrogen in spatial learning. When oestradiol is given in a high, chronic dose, spatial learning is impaired. This action of oestradiol requires ERalpha, and when ERbeta is not functional, lower doses of oestradiol have this negative effect on behaviour. The second relationship between the two receptors is one that is synergistic, and this is illustrated in the combined effects of the two receptors on the production of the neuropeptide oxytocin and its receptor. The third relationship is sequential; separate actions of the two receptors are postulated in activation and organisation of sexually dimorphic reproductive behaviours. Future studies on all of these topics will inform us about how ER selective ligands might affect oestrogen functions at the organismal level.

Vasoactive intestinal polypeptide can excite gonadotropin-releasing hormone neurons in a manner dependent on estradiol and gated by time of day

A surge of GnRH release signals the LH surge that triggers ovulation. The GnRH surge is dependent on a switch in estradiol feedback from negative to positive and, in rodents, a daily neural signal, likely from the suprachiasmatic nuclei. Vasoactive intestinal polypeptide (VIP) may be involved in suprachiasmatic nuclei-GnRH neuron communication. Here we assessed the effects of acute VIP (5 min treatment) on GnRH neuron function using targeted extracellular recordings of firing activity of GnRH neurons in brain slices. We examined the effect of VIP on firing rate at different times of day using an established ovariectomized, estradiol-treated (OVX+E) mouse model that exhibits daily LH surges timed to the late afternoon. Cells from OVX animals (no estradiol) did not respond to VIP, regardless of time of day. With estradiol, the effect of VIP on GnRH neurons was dependent on the time of recording. During negative feedback, OVX+E cells did not respond. VIP increased firing in cells recorded during surge onset, but this excitatory response was reduced at surge peak. Acute treatment of OVX+E cells during surge peak with a VIP receptor antagonist decreased GnRH neuron firing. This suggests endogenous VIP may both increase GnRH neuron firing during the surge and occlude response to exogenous VIP. These data provide functional evidence for VIP effects on GnRH neurons and indicate that both estradiol and time of day gate the GnRH neuron response to this peptide. VIP may provide an excitatory signal from the circadian clock that helps time the GnRH surge.

Maternal influences on the sexual behavior and reproductive success of the female rat

In many species, including humans, there is evidence for parental effects on within-sex variations in reproductive behavior. In the present studies we found that variations in postnatal maternal care were associated with individual differences in female sexual behavior in the rat. Females born to and reared by dams that showed enhanced pup licking/grooming (i.e., High LG mothers) over the first week postpartum showed significantly reduced sexual receptivity and alterations in the pacing of male mounting (i.e., longer inter-intromission intervals) observed in a paced mating test. There were minimal effects on the sexual behavior of the male offspring. The female offspring of High LG mothers showed a reduced lordosis rating, a decreased mount:intromission ratio, received fewer ejaculations and were less likely to achieve pregnancy following mating in the paced mating context. The data suggest maternal influences on the sexual development of the female rat that are functionally relevant for reproductive success. Together with previous studies these findings imply that maternal care can 'program' reproductive strategies in the female rat.

The role of kisspeptins and GPR54 in the neuroendocrine regulation of reproduction

Neurons that produce gonadotropin-releasing hormone (GnRH) reside in the basal forebrain and drive reproductive function in mammals. Understanding the circuitry that regulates GnRH neurons is fundamental to comprehending the neuroendocrine control of puberty and reproduction in the adult. This review focuses on a family of neuropeptides encoded by the Kiss1 gene, the kisspeptins, and their cognate receptor, GPR54, which have been implicated in the regulation of GnRH secretion. Kisspeptins are potent secretagogues for GnRH, and the Kiss1 gene is a target for regulation by gonadal steroids (e.g., estradiol and testosterone), metabolic factors (e.g., leptin), photoperiod, and season. Kiss1 neurons in the arcuate nucleus may regulate the negative feedback effect of gonadal steroids on GnRH and gonadotropin secretion in both sexes. The expression of Kiss1 in the anteroventral periventricular nucleus (AVPV) is sexually dimorphic, and Kiss1 neurons in the AVPV may participate in the generation of the preovulatory GnRH/luteinizing hormone (LH) surge in the female rodent. Kiss1 neurons have emerged as primary transducers of internal and environmental cues to regulate the neuroendocrine reproductive axis.

Synaptic integration in hypothalamic gonadotropin releasing hormone (GnRH) neurons

The impact of the A-type GABA (GABA-A) receptor in gonadotropin releasing hormone (GnRH) neurons is controversial. In adult GnRH neurons, the GABA-A receptor conductance has been reported to either hyperpolarize or depolarize GnRH neurons. Regardless of whether GABA is inhibitory or excitatory in GnRH neurons, GABAergic input would be integrated with post-synaptic potentials generated by other synaptic inputs. We used dynamic current clamping and compartmental computer modeling to examine the integration of AMPA-type glutamatergic input and GABA-mediated input in both the hyperpolarizing (inhibitory) and depolarizing (excitatory) modes in GnRH neurons from transgenic mice (Mus Musculus) generated on a C57BL6 background. In both living and model neurons, action potentials were most likely a few ms after a maximum in AMPA conductance coincided with a minimum in inhibitory GABA. Excitatory GABA interacted differently with AMPA, with spikes most likely, in both dynamic clamping of living neurons and in model neurons, when a maximum in AMPA coincided with the decay from peak of a maximum in GABA. Distributing synapses along the dendrite maximized the temporal relationship between AMPA and GABA conductances and therefore, the potential for spiking. Thus, these two dominant neurotransmitters could interact in multiple frames to generate action potentials in GnRH neurons.