Rapid action of estrogens on intracellular calcium oscillations in primate luteinizing hormone-releasing hormone-1 neurons

The electrophysiologic properties of gonadotropin-releasing hormone neurons

For about two decades, recordings of identified gonadotropin-releasing hormone (GnRH) neurons have provided a wealth of information on their properties. We describe areas of consensus and debate the intrinsic electrophysiologic properties of these cells, their response to fast synaptic and neuromodulatory input, Ca2+ imaging correlates of action potential firing, and signaling pathways regulating these aspects. How steroid feedback and development change these properties, functions of GnRH neuron subcompartments and local networks, as revealed by chemo- and optogenetic approaches, are also considered.

Physiological Characterization and Transcriptomic Properties of GnRH Neurons Derived From Human Stem Cells

Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus play a key role in the regulation of reproductive function. In this study, we sought an efficient method for generating GnRH neurons from human embryonic and induced pluripotent stem cells (hESC and hiPSC, respectively). First, we found that exposure of primitive neuroepithelial cells, rather than neuroprogenitor cells, to fibroblast growth factor 8 (FGF8), was more effective in generating GnRH neurons. Second, addition of kisspeptin to FGF8 further increased the efficiency rates of GnRH neurogeneration. Third, we generated a fluorescent marker mCherry labeled human embryonic GnRH cell line (mCh-hESC) using a CRISPR-Cas9 targeting approach. Fourth, we examined physiological characteristics of GnRH (mCh-hESC) neurons: similar to GnRH neurons in vivo, they released the GnRH peptide in a pulsatile manner at ~60 min intervals; GnRH release increased in response to high potassium, kisspeptin, estradiol, and neurokinin B challenges; and injection of depolarizing current induced action potentials. Finally, we characterized developmental changes in transcriptomes of GnRH neurons using hESC, hiPSC, and mCh-hESC. The developmental pattern of transcriptomes was remarkably similar among the 3 cell lines. Collectively, human stem cell-derived GnRH neurons will be an important tool for establishing disease models to understand diseases, such as idiopathic hypothalamic hypogonadism, and testing contraceptive drugs.

Mechanism of pulsatile GnRH release in primates: Unresolved questions

The pulsatility of GnRH release is essential for reproductive function. The key events in reproductive function, such as puberty onset and ovulatory cycles, are regulated by the frequency and amplitude modulation of pulsatile GnRH release. Abnormal patterns of GnRH pulsatility are seen in association with disease states, such as polycystic ovarian syndrome and anorexia nervosa. Recent studies with physiological, track-tracing, optogenetic and electrophysiological recording experiments indicate that a group of kisspeptin neurons in the arcuate nucleus (ARC) of the hypothalamus are responsible for pulsatile GnRH release. Thus, the kisspeptin neuron in the ARC has been called the "GnRH pulse-generator." However, a few pieces of evidence do not quite fit into this concept. This article reviews some old works and discusses unresolved issues on the mechanism of GnRH pulse generation.

Controlling cellular distribution of drugs with permeability modifying moieties

Phenotypic screening provides compounds with very limited target cellular localization data. In order to select the most appropriate target identification methods, determining if a compound acts at the cell-surface or intracellularly can be very valuable. In addition, controlling cell-permeability of targeted therapeutics such as antibody-drug conjugates (ADCs) and targeted nanoparticle formulations can reduce toxicity from extracellular release of drug in undesired tissues or direct activity in bystander cells. By incorporating highly polar, anionic moieties via short polyethylene glycol linkers into compounds with known intracellular, and cell-surface targets, we have been able to correlate the cellular activity of compounds with their subcellular site of action. For compounds with nuclear (Brd, PARP) or cytosolic (dasatinib, NAMPT) targets, addition of the permeability modifying group (small sulfonic acid, polycarboxylic acid, or a polysulfonated fluorescent dye) results in near complete loss of biological activity in cell-based assays. For cell-surface targets (H₃, 5HT_1A, β₂AR) significant activity was maintained for all conjugates, but the results were more nuanced in that the modifiers impacted binding/activity of the resulting conjugates. Taken together, these results demonstrate that small anionic compounds can be used to control cell-permeability independent of on-target activity and should find utility in guiding target deconvolution studies and controlling drug distribution of targeted therapeutics.

Membrane-initiated estrogen signaling via Gq-coupled GPCR in the central nervous system

The last few decades have revealed increasing complexity and depth to our knowledge of receptor-mediated estrogen signaling. Nuclear estrogen receptors (ERs) ERα and ERβ remain the fundamental dogma, but recent research targeting membrane-bound ERs urges for a more expanded view on ER signaling. ERα and ERβ are also involved in membrane-delineated signaling alongside membrane-specific G protein-coupled estrogen receptor 1 (GPER1), ER-X, and the Gq-coupled membrane ER (Gq-mER). Membrane ERs are responsible for eliciting rapid responses to estrogen signaling, and their importance has been increasingly indicated in central nervous system (CNS) regulation of such functions as reproduction, energy homeostasis, and stress. While the Gq-mER signaling pathway is well characterized, the receptor structure and gene remains uncharacterized, although it is not similar to the nuclear ERα/β. This review will describe the current knowledge of this putative membrane ER and its selective ligand, STX, from its initial characterization in hypothalamic melanocortin circuitry to recent research exploring its role in the CNS outside of the hypothalamus.

Neuroestradiol in regulation of GnRH release

Contribution to Special Issue on Fast effects of steroids. The concept that the positive feedback effect of ovarian estradiol (E₂) results in GnRH and gonadotropin surges is a well-established principle. However, a series of studies investigating the rapid action of E₂ in female rhesus monkeys has led to a new concept that neuroestradiol, synthesized and released in the hypothalamus, also contributes to regulation of the preovulatory GnRH surge. This unexpected finding started from our surprising observation that E₂ induces rapid stimulatory action in GnRH neurons in vitro. Subsequently, we confirmed that a similar rapid stimulatory action of E₂ occurs in vivo. Unlike subcutaneous injection of E₂ benzoate (EB), a brief (10-20 min), direct infusion of EB into the median eminence in ovariectomized (OVX) female monkeys rapidly stimulates release of GnRH and E₂ in a pulsatile manner, and the EB-induced GnRH and E₂ release is blocked by simultaneous infusion of the aromatase inhibitor, letrozole. This suggests that stimulated release of E₂ is of hypothalamic origin. To further determine the role of neuroestradiol we examined the effects of letrozole on EB-induced GnRH and LH surges in OVX females. Results indicate that letrozole treatment greatly attenuated the EB-induced GnRH and LH surges. Collectively, neuroestradiol released from the hypothalamus appears to be necessary for the positive feedback effect of E₂ on the GnRH/LH surge.

New insights into fetal mammary gland morphogenesis: differential effects of natural and environmental estrogens

An increased breast cancer risk during adulthood has been linked to estrogen exposure during fetal life. However, the impossibility of removing estrogens from the feto-maternal unit has hindered the testing of estrogen’s direct effect on mammary gland organogenesis. To overcome this limitation, we developed an ex vivo culture method of the mammary gland where the direct action of estrogens can be tested during embryonic days (E)14 to 19. Mouse mammary buds dissected at E14 and cultured for 5 days showed that estrogens directly altered fetal mammary gland development. Exposure to 0.1 pM, 10 pM, and 1 nM 17 β-estradiol (E2) resulted in monotonic inhibition of mammary buds ductal growth. In contrast, Bisphenol-A (BPA) elicited a non-monotonic response. At environmentally relevant doses (1 nM), BPA significantly increased ductal growth, as previously observed in vivo, while 1 μM BPA significantly inhibited ductal growth. Ductal branching followed the same pattern. This effect of BPA was blocked by Fulvestrant, a full estrogen antagonist, while the effect of estradiol was not. This method may be used to study the hormonal regulation of mammary gland development, and to test newly synthesized chemicals that are released into the environment without proper assessment of their hormonal action on critical targets like the mammary gland.

Dual Actions of Mammalian and Piscine Gonadotropin-Inhibitory Hormones, RFamide-Related Peptides and LPXRFamide Peptides, in the Hypothalamic-Pituitary-Gonadal Axis

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that decreases gonadotropin synthesis and release by directly acting on the gonadotrope or by decreasing the activity of gonadotropin-releasing hormone (GnRH) neurons. GnIH is also called RFamide-related peptide in mammals or LPXRFamide peptide in fishes due to its characteristic C-terminal structure. The primary receptor for GnIH is GPR147 that inhibits cAMP production in target cells. Although most of the studies in mammals, birds, and fish have shown the inhibitory action of GnIH in the hypothalamic-pituitary-gonadal (HPG) axis, several in vivo studies in mammals and many in vivo and in vitro studies in fish have shown its stimulatory action. In mouse, although the firing rate of the majority of GnRH neurons is decreased, a small population of GnRH neurons is stimulated by GnIH. In hamsters, GnIH inhibits luteinizing hormone (LH) release in the breeding season when their endogenous LH level is high but stimulates LH release in non-breeding season when their LH level is basal. Besides different effects of GnIH on the HPG axis depending on the reproductive stages in fish, higher concentration or longer duration of GnIH administration can stimulate their HPG axis. These results suggest that GnIH action in the HPG axis is modulated by sex-steroid concentration, the action of neuroestrogen synthesized by the activity of aromatase stimulated by GnIH, estrogen membrane receptor, heteromerization and internalization of GnIH, GnRH, and estrogen membrane receptors. The inhibitory and stimulatory action of GnIH in the HPG axis may have a physiological role to maintain reproductive homeostasis according to developmental and reproductive stages.

Estrogen Receptor Beta and 2-arachidonoylglycerol Mediate the Suppressive Effects of Estradiol on Frequency of Postsynaptic Currents in Gonadotropin-Releasing Hormone Neurons of Metestrous Mice: An Acute Slice Electrophysiological Study

Gonadotropin-releasing hormone (GnRH) neurons are controlled by 17β-estradiol (E2) contributing to the steroid feedback regulation of the reproductive axis. In rodents, E2 exerts a negative feedback effect upon GnRH neurons throughout the estrus-diestrus phase of the ovarian cycle. The present study was undertaken to reveal the role of estrogen receptor subtypes in the mediation of the E2 signal and elucidate the downstream molecular machinery of suppression. The effect of E2 administration at low physiological concentration (10 pM) on GnRH neurons in acute brain slices obtained from metestrous GnRH-green fluorescent protein (GFP) mice was studied under paradigms of blocking or activating estrogen receptor subtypes and interfering with retrograde 2-arachidonoylglycerol (2-AG) signaling. Whole-cell patch clamp recordings revealed that E2 significantly diminished the frequency of spontaneous postsynaptic currents (sPSCs) in GnRH neurons (49.62 ± 7.6%) which effect was abolished by application of the estrogen receptor (ER) α/β blocker Faslodex (1 μM). Pretreatment of the brain slices with cannabinoid receptor type 1 (CB1) inverse agonist AM251 (1 μM) and intracellularly applied endocannabinoid synthesis blocker THL (10 μM) significantly attenuated the effect of E2 on the sPSCs. E2 remained effective in the presence of tetrodotoxin (TTX) indicating a direct action of E2 on GnRH cells. The ERβ specific agonist DPN (10 pM) also significantly decreased the frequency of miniature postsynaptic currents (mPSCs) in GnRH neurons. In addition, the suppressive effect of E2 was completely blocked by the selective ERβ antagonist PHTPP (1 μM) indicating that ERβ is required for the observed rapid effect of the E2. In contrast, the ERα agonist PPT (10 pM) or the membrane-associated G protein-coupled estrogen receptor (GPR30) agonist G1 (10 pM) had no significant effect on the frequency of mPSCs in these neurons. AM251 and tetrahydrolipstatin (THL) significantly abolished the effect of E2 whereas AM251 eliminated the action of DPN on the mPSCs. These data suggest the involvement of the retrograde endocannabinoid mechanism in the rapid direct effect of E2. These results collectively indicate that estrogen receptor beta and 2-AG/CB1 signaling mechanisms are coupled and play an important role in the mediation of the negative estradiol feedback on GnRH neurons in acute slice preparation obtained from intact, metestrous mice.

Prolonged infusion of estradiol benzoate into the stalk median eminence stimulates release of GnRH and kisspeptin in ovariectomized female rhesus macaques

Our recent study indicates that a brief infusion (20 min) of estradiol (E2) benzoate (EB) into the stalk-median eminence (S-ME) stimulates GnRH release with a latency of approximately 10 minutes. In contrast to the effect induced by a brief infusion of EB, it has previously been shown that systemic EB administration suppresses release of GnRH, kisspeptin, and LH with a latency of several hours, which is known as the negative feedback action of E2. We speculated that the differential results by these 2 modes of EB administration are due to the length of E2 exposure. Therefore, in the present study, the effects of EB infusion for periods of 20 minutes, 4 hours, or 7 hours into the S-ME of ovariectomized female monkeys on the release of GnRH and kisspeptin were examined using a microdialysis method. To assess the effects of the EB infusion on LH release, serum samples were also collected. The results show that similar to the results with 20-minute infusion, both 4- and 7-hour infusions of EB consistently stimulated release of GnRH and kisspeptin from the S-ME accompanied by LH release in the general circulation. In contrast, sc injection of EB suppressed all 3 hormones (GnRH, kisspeptin, and LH) measured. It is concluded that regardless of the exposure period, direct E2 action on GnRH and kisspeptin neurons in the S-ME, where their neuroterminals are present, is stimulatory, and the E2-negative feedback effects do not occur at the S-ME level.

Activation of GPR30 attenuates chronic pain-related anxiety in ovariectomized mice

Estrogen regulates neuroendocrine and inflammatory processes that play critical roles in neuroinflammation, anxiety, and chronic pain. Patients suffering from chronic pain often complain of anxiety. However, limited information is available regarding the neural circuitry of chronic pain-related anxiety and the related function of estrogen. Hindpaw injection of complete Freund's adjuvant (CFA) and chronic constriction injury (CCI) of the sciatic nerve induced notable pain sensitization and anxiety-like behavior in ovariectomized (OVX) mice. We found that the level of G-protein-coupled receptor 30 (GPR30), a membrane estrogen receptor, was significantly increased in the basolateral amygdala (BLA) of ovariectomized (OVX) mice suffering from chronic inflammatory and neuropathic pain. Subcutaneous injection or BLA local infusion of the GPR30 agonist G1 significantly reduced anxiety-like behavior in CFA-injected and CCI-OVX mice; however, this treatment did not alter the nociceptive threshold. GPR30 knock down by shRNA in the BLA of OVX mice inhibited the anxiolytic effects of GPR30 activation. G1 administration reversed the upregulation of GluR1 subunit in AMPA and NR2A-containing NMDA receptors and the downregulation of GABAA receptors in the BLA of CFA-injected and CCI-OVX mice. Electrophysiological recording revealed that GPR30 activation could prevent imbalance between excitatory and inhibitory transmissions in the BLA synapses of CFA-injected OVX mice. In conclusion, GPR30 activation induced anxiolytic effects but did not affect the nociceptive threshold of mice under chronic pain. The anxiolytic effects of GPR30 were partially due to maintaining the balance between excitatory and inhibitory transmissions in the BLA.

A Multi-Oscillatory Circadian System Times Female Reproduction

Rhythms in female reproduction are critical to insure that timing of ovulation coincides with oocyte maturation and optimal sexual arousal. This fine tuning of female reproduction involves both the estradiol feedback as an indicator of oocyte maturation, and the master circadian clock of the suprachiasmatic nuclei (SCN) as an indicator of the time of the day. Herein, we are providing an overview of the state of knowledge regarding the differential inhibitory and stimulatory effects of estradiol at different stages of the reproductive axis, and the mechanisms through which the two main neurotransmitters of the SCN, arginine vasopressin, and vasoactive intestinal peptide, convey daily time cues to the reproductive axis. In addition, we will report the most recent findings on the putative functions of peripheral clocks located throughout the reproductive axis [kisspeptin (Kp) neurons, gonadotropin-releasing hormone neurons, gonadotropic cells, the ovary, and the uterus]. This review will point to the critical position of the Kp neurons of the anteroventral periventricular nucleus, which integrate both the stimulatory estradiol signal, and the daily arginine vasopressinergic signal, while displaying a circadian clock. Finally, given the critical role of the light/dark cycle in the synchronization of female reproduction, we will discuss the impact of circadian disruptions observed during shift-work conditions on female reproductive performance and fertility in both animal model and humans.

Kisspeptin and Gonadotropin-Releasing Hormone Neuronal Excitability: Molecular Mechanisms Driven by 17β-Estradiol

Kisspeptin is a neuropeptide that signals via a Gαq-coupled receptor, GPR54, in gonadotropin-releasing hormone (GnRH) neurons and is essential for pubertal maturation and fertility. Kisspeptin depolarizes and excites GnRH neurons primarily through the activation of canonical transient receptor potential (TRPC) channels and the inhibition of K+ channels. The gonadal steroid 17β-estradiol (E2) upregulates not only kisspeptin (Kiss1) mRNA but also increases the excitability of the rostral forebrain Kiss1 neurons. In addition, a primary postsynaptic action of E2 on GnRH neurons is to upregulate the expression of channel transcripts that orchestrate the downstream signaling of kisspeptin in GnRH neurons. These include not only TRPC4 channels but also low-voltage-activated T-type calcium channels and high-voltage-activated L-, N- and R-type calcium channel transcripts. Moreover, E2 has direct membrane-initiated actions to alter the excitability of GnRH neurons by enhancing ATP-sensitive potassium channel activity, which is critical for maintaining GnRH neurons in a hyperpolarized state for the recruitment of T-type calcium channels that are important for burst firing. Therefore, E2 modulates the excitability of GnRH neurons as well as of Kiss1 neurons by altering the expression and/or function of ion channels; moreover, kisspeptin provides critical excitatory input to GnRH neurons to facilitate burst firing activity and peptide release.

Neuroestradiol in the hypothalamus contributes to the regulation of gonadotropin releasing hormone release

Release of gonadotropin releasing hormone (GnRH) from the medial basal hypothalamus (MBH)/median eminence region (S-ME) is essential for normal reproductive function. GnRH release is profoundly regulated by the negative and positive feedback effects of ovarian estradiol (E2). Here we report that neuroestradiol, released in the S-ME, also directly influences GnRH release in ovariectomized female monkeys, in which the ovarian source of E2 is removed. We found that (1) brief infusion of E2 benzoate (EB) to the S-ME rapidly stimulated release of GnRH and E2 in the S-ME of ovariectomized monkeys, (2) electrical stimulation of the MBH resulted in GnRH release as well as E2 release, and (3) direct infusion of an aromatase inhibitor to the S-ME suppressed spontaneous GnRH release as well as the EB-induced release of GnRH and E2. These findings reveal the importance of neuroestradiol as a neurotransmitter in regulation of GnRH release. How circulating ovarian E2 interacts with hypothalamic neuroestrogens in the control of GnRH release remains to be investigated.

Role of estrogen receptors and g protein-coupled estrogen receptor in regulation of hypothalamus-pituitary-testis axis and spermatogenesis

Male reproductive function is under the control of both gonadotropins and androgens through a negative feedback loop that involves the hypothalamus, pituitary, and testis known as hypothalamus-pituitary-gonadal axis (HPG). Indeed, estrogens also play an important role in regulating HPG axis but the study on relative contribution to the inhibition of gonadotropins secretion exerted by the amount of estrogens produced within the hypothalamus and/or the pituitary or by the amount of circulating estrogens is still ongoing. Moreover, it is known that the maintenance of spermatogenesis is controlled by gonadotropins and testosterone, the effects of which are modulated by a complex network of locally produced factors, including estrogens. Physiological effects of estrogens are mediated by the classical nuclear estrogen receptor alpha and estrogen receptor beta, which mediate both genomic and rapid signaling events. In addition, estrogens induce rapid non-genomic responses through a membrane-associated G protein-coupled estrogen receptor (GPER). Ours and other studies reported that, in the testis, GPER is expressed in both normal germ cells and somatic cells and it is involved in mediating the estrogen action in spermatogenesis controlling proliferative and/or apoptotic events. Interestingly, GPER expression has been revealed also in the hypothalamus and pituitary. However, its role in mediating estrogen rapid actions in this context is under investigation. Recent studies indicate that GPER is involved in modulating gonadotropin-releasing hormone (GnRH) release as well as gonadotropins secretion. In this review, we will summarize the current knowledge concerning the role of estrogen/estrogen receptors molecular pathways in regulating GnRH, follicle-stimulating hormone, and luteinizing hormone release at the hypothalamic and pituitary levels in males as well as in controlling specific testicular functions such as spermatogenesis, focusing our attention mainly on estrogen signaling mediated by GPER.

GnRH neurons of young and aged female rhesus monkeys co-express GPER but are unaffected by long-term hormone replacement

Menopause is caused by changes in the function of the hypothalamic-pituitary-gonadal axis that controls reproduction. Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus orchestrate the activity of this axis and are regulated by hormonal feedback loops. The mechanisms by which GnRH responds to the primary regulatory sex steroid hormone, estradiol (E2), are still poorly understood in the context of menopause. Our goal was to determine whether the G protein-coupled estrogen receptor (GPER) is co-expressed in adult primate GnRH neurons and whether this changes with aging and/or E2 treatment. We used immunofluorescence double-labeling to characterize the co-expression of GPER in GnRH perikarya and terminals in the hypothalamus. Young and aged rhesus macaques were ovariectomized and given long-term (~2-year) hormone treatments (E2, E2 + progesterone, or vehicle) selected to mimic currently prescribed hormone replacement therapies used for the alleviation of menopausal symptoms in women. We found that about half of GnRH perikarya co-expressed GPER, while only about 12% of GnRH processes and terminals in the median eminence (ME) were double-labeled. Additionally, many GPER-labeled processes were in direct contact with GnRH neurons, often wrapped around the perikarya and processes and in close proximity in the ME. These results extend prior work by showing robust co-localization of GPER in GnRH in a clinically relevant model, and they support the possibility that GPER-mediated E2 regulation of GnRH occurs both in the soma and terminals in nonhuman primates.

Non-classical effects of estradiol on cAMP responsive element binding protein phosphorylation in gonadotropin-releasing hormone neurons: mechanisms and role

Gonadotropin-releasing hormone (GnRH) is produced by a heterogenous neuronal population in the hypothalamus to control pituitary gonadotropin production and reproductive function in all mammalian species. Estradiol is a critical component for the communication between the gonads and the central nervous system. Resolving the mechanisms by which estradiol modulates GnRH neurons is critical for the understanding of how fertility is regulated. Extensive studies during the past decades have provided compelling evidence that estradiol has the potential to alter the intracellular signal transduction mechanisms. The common target of many signaling pathways is the phosphorylation of a key transcription factor, the cAMP response element binding protein (CREB). This review first addresses the aspects of estradiol action on CREB phosphorylation (pCREB) in GnRH neurons. Secondly, this review considers the receptors and signaling network that regulates estradiol's action on pCREB within GnRH neurons and finally it summarizes the physiological significance of CREB to estrogen feedback.

Estrogen receptor GPR30 exerts anxiolytic effects by maintaining the balance between GABAergic and glutamatergic transmission in the basolateral amygdala of ovariectomized mice after stress

G-protein-coupled receptor 30 (GPR30)/G-protein-coupled estrogen receptor is a novel estrogen membrane receptor that localizes to the cell membrane and endoplasmic reticulum. GPR30 is widely distributed and has numerous physiological functions in the central nervous system. We found that GPR30 is highly expressed in the basolateral amygdala (BLA). Additionally, GPR30 expression in the amygdala of ovariectomized (OVX) mice significantly increased after acute stress and was accompanied by anxiety-like behaviors. These effects, however, were reversed by local infusion of the GPR30 agonist (G1) in the BLA. Protein assessments revealed that G1 attenuated the up-regulation of the GluR1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and NR2A-containing N-methyl-d-aspartate receptors (NMDARs) in the BLA of OVX mice using an acute stress paradigm. In the same model, we found that the agonist also blocked the down-regulation of γ-aminobutyric acid A (GABAA) receptors and NR2B-containing NMDARs. Electrophysiological recording showed that the activation of GPR30 increased the inhibitory synaptic transmission in the BLA. Overall, our results indicate that estradiol reduces anxiety-like behaviors induced by acute stress at least partially through GPR30 signaling, maintaining the balance between GABAergic and glutamatergic transmission in the BLA of OVX-stressed mice.

Activity-dependent modulation of gonadotrophin-releasing hormone neurone activity by acute oestradiol

Oestradiol (E₂) exerts potent feedback actions upon gonadotrophin-releasing hormone (GnRH) neurones and part of this feedback action may occur through the rapid action of E₂. Using a transgenic GnRH-Pericam mouse line that allows real-time intracellular calcium concentrations ([Ca²⁺](i)) to be monitored in adult GnRH neurones in a brain slice preparation, we examined the acute effects of 100 pM-100 nM E₂ on [Ca²⁺](i) transients in spontaneously active GnRH neurones. Approximately 30% of GnRH neurones exhibit spontaneous [Ca²⁺](i) transients at a frequency greater than two transients/15 min in adult female mice. In these cells, treatment with an incremental 1, 10, 100 nM E₂ protocol or 100 pM E₂ alone resulted in the suppression or complete cessation of [Ca²⁺](i) transients in 15 of 18 (83%) GnRH neurones. This effect was mimicked by E₂ bound to albumin, suggesting a membrane site of action, and was maintained in oestrogen receptor β knockout mice, indicating that this receptor is not essential for the rapid suppression of [Ca²⁺](i) transients. These findings contrast with those GnRH neurones exhibiting very few or no [Ca²⁺](i) transients (< 2 transients/15 min) that exhibit the opposite response of being activated by acute E₂. A series of dual calcium-cell-attached electrical recordings showed that [Ca²⁺](i) transients were associated with GnRH neurone burst firing and that E₂ suppression or activation of [Ca²⁺](i) transients was mirrored by a depression or initiation of burst firing. Taken together, these studies demonstrate that the acute actions of E₂ on GnRH neurones are critically dependent upon their pattern of burst firing.

Membrane-initiated actions of estradiol that regulate reproduction, energy balance and body temperature

It is well known that many of the actions of estrogens in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there now exists compelling evidence for membrane estrogen receptors in hypothalamic and other brain neurons. But, it is not well understood how estrogens signal 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 estrogens can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, estrogens 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 estrogens in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Neuroestrogen, rapid action of estradiol, and GnRH neurons

Estradiol plays a pivotal role in the control of GnRH neuronal function, hence female reproduction. A series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol directly modify GnRH neuronal activity in primate GnRH neurons through GPR30 and STX-sensitive receptors. Similar rapid direct actions of estradiol through estrogen receptor beta are also described in mouse GnRH neurons. In this review, we propose two novel hypotheses as a possible physiological role of estradiol in primates. First, while ovarian estradiol initiates the preovulatory GnRH surge through interneurons expressing estrogen receptor alpha, rapid direct membrane-initiated action of estradiol may play a role in sustaining GnRH surge release for many hours. Second, locally produced neuroestrogens may contribute to pulsatile GnRH release. Either way, estradiol synthesized in interneurons in the hypothalamus may play a significant role in the control of the GnRH surge and/or pulsatility of GnRH release.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

Membrane estrogen receptor regulation of hypothalamic function

Over the decades, our understanding of estrogen receptor (ER) function has evolved. Today we are confronted by at least two nuclear ERs, ERα and ERβ, and a number of putative membrane ERs, including ERα, ERβ, ER-X, GPR30 and Gq-mER. These receptors all bind estrogens or at least estrogenic compounds and activate intracellular signaling pathways. In some cases, a well-defined pharmacology and physiology has been discovered. In other cases, the identity or the function remains to be elucidated. This mini-review attempts to synthesize our understanding of 17β-estradiol membrane signaling within hypothalamic circuits involved in homeostatic functions, focusing on reproduction and energy balance.

Estradiol acts directly and indirectly on multiple signaling pathways to phosphorylate cAMP-response element binding protein in GnRH neurons

Rapid, nonclassical 17β-estradiol (E2) actions are thought to play an important role in the modulation of neuronal function. The present study addresses the intracellular signaling cascades involved in the rapid E2-induced phosphorylation of cAMP response element binding protein (CREB) in GnRH neurons. Administration of E2 to adult female mice resulted in the activation of ERK1/2 in GnRH neurons within 15 min. In vitro studies using pharmacological antagonists showed that ERK1/2 was essential for E2-induced CREB phosphorylation in GnRH neurons. Upstream to this, protein kinase A and calcium/calmodulin-dependent protein kinase type II, but not protein kinase C, were found to be necessary for E2-induced phosphorylation of ERK1/2. This rapid E2 signaling cascade in GnRH neurons was found to require both direct and indirect E2 actions. E2 failed to phosphorylate ERK1/2 and CREB in GnRH neuron-specific estrogen receptor β knockout mice in vivo. Equally, however, a cocktail of tetrodotoxin and γ-aminobutyric acid(A)/glutamate receptor antagonists also blocked E2-induced ERK1/2 phosphorylation in GnRH neurons in wild-type mice in vitro. Together, these observations indicate that E2 acts through calcium/calmodulin-dependent protein kinase type II and protein kinase A to rapidly phosphorylate ERK1/2, which then acts to phosphorylate CREB in adult female GnRH neurons. Intriguingly, these effects of E2 are dependent upon both direct ERβ mechanisms as well as indirect actions mediated by afferent inputs to GnRH neurons.

G-protein-coupled receptor 30 mediates rapid neuroprotective effects of estrogen via depression of NR2B-containing NMDA receptors

17-β-estradiol (E2) is a steroid hormone involved in neuroprotection against excitotoxicity and other forms of brain injury. Through genomic and nongenomic mechanisms, E2 modulates neuronal excitability and signal transmission by regulating NMDA and non-NMDA receptors. However, the mechanisms and identity of the receptors involved remain unclear, even though studies have suggested that estrogen G-protein-coupled receptor 30 (GPR30) is linked to protection against ischemic injury. In the culture cortical neurons, treatment with E2 and the GPR30 agonist G1 for 45 min attenuated the excitotoxicity induced by NMDA exposure. The acute neuroprotection mediated by GPR30 is dependent on G-protein-coupled signals and ERK1/2 activation, but independent on transcription or translation. Knockdown of GPR30 using short hairpin RNAs (shRNAs) significantly reduced the E2-induced rapid neuroprotection. Patch-clamp recordings revealed that GPR30 activation depressed exogenous NMDA-elicited currents. Short-term GPR30 activation did not affect the expression of either NR2A- or NR2B-containing NMDARs; however, it depressed NR2B subunit phosphorylation at Ser-1303 by inhibiting the dephosphorylation of death-associated protein kinase 1 (DAPK1). DAPK1 knockdown using shRNAs significantly blocked NR2B subunit phosphorylation at Ser-1303 and abolished the GPR30-mediated depression of exogenous NMDA-elicited currents. Lateral ventricle injection of the GPR30 agonist G1 (0.2 μg) provided significant neuroprotection in the ovariectomized female mice subjected to middle cerebral artery occlusion. These findings provide direct evidence that fast neuroprotection by estradiol is partially mediated by GPR30 and the subsequent downregulation of NR2B-containing NMDARs. The modulation of DAPK1 activity by GPR30 may be an important mediator of estradiol-dependent neuroprotection.

Estradiol directly attenuates sodium currents and depolarizing afterpotentials in isolated gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neuron is the pivotal control center in a tightly regulated reproductive axis. The release of GnRH controls estradiol production by the ovary, and estradiol acts at the hypothalamus to regulate GnRH release. However, the mechanisms of estradiol feedback are just beginning to be understood. We have previously shown that estradiol administered to the female mouse modulates sodium currents in fluorescently-labeled GnRH neurons. In the current studies, estradiol (1 nM) was applied directly, for 16-24h, to hypothalamic cultures from young or aged female ovariectomized mice. The direct application of estradiol modulated a tetrodotoxin-sensitive sodium current in isolated GnRH neurons from both young and aged animals. Estradiol, and the specific estrogen receptor-β agonist DPN, decreased current amplitude measured in the morning (AM), but had no effect on afternoon currents. These compounds also decreased the rise and decay slope of the current response, increased the width of the current, and increased action potential width in AM recordings. In addition, estradiol decreased the amplitude of the depolarizing afterpotential (DAP); this effect was not time-of-day dependent. The ER-β agonist DPN did not mimic the effect of estradiol on DAPs, and the modulation of DAPs by estradiol was no longer present in cells from postreproductive animals. These results indicate that estradiol can affect the physiology of GnRH neurons via multiple pathways that are differentially regulated during the transition to reproductive senescence, suggesting that estradiol regulation of GnRH neuronal output is modulated during the aging process.

Rapid direct action of estradiol in GnRH neurons: findings and implications

Estradiol plays a pivotal role in the control of gonadotropin-releasing hormone (GnRH) neuronal function and female reproduction. While positive and negative feedback actions of estradiol that enhance and suppress release of GnRH and LH are primarily mediated through estrogen receptor alpha located in interneurons, a series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol also directly modify GnRH neuronal activity. We observed this phenomenon in cultured primate GnRH neurons, but similar rapid direct actions of estradiol are also described in cultured GnRH neurons and green fluorescent protein-labeled GnRH neurons of mice. Importantly, rapid direct action of estradiol in GnRH neurons is mediated through membrane or membrane associated receptors, such as GPR30, STX-sensitive receptors, and ERβ. In this review, possible implications of this rapid estradiol action in GnRH neurons are discussed.

Estrogenic regulation of the GnRH neuron

Reproductive function is regulated by the secretion of luteinizing hormone (LH) and follicle-stimulating hormone from the pituitary and the steroid hormones from the gonads. The dynamic changes in the levels of the reproductive hormones regulate secondary sex characteristics, gametogenesis, cellular function, and behavior. Hypothalamic GnRH neurons, with cell bodies located in the basal hypothalamus, represent the final common pathway for neuronally derived signals to the pituitary. As such, they serve as integrators of a dizzying array of signals including sensory inputs mediating information about circadian, seasonal, behavioral, pheromonal, and emotional cues. Additionally, information about peripheral physiological function may also be included in the integrative signal to the GnRH neuron. These signals may communicate information about metabolic status, disease, or infection. Gonadal steroid hormones arguably exert the most important effects on GnRH neuronal function. In both males and females, the gonadal steroid hormones exert negative feedback regulation on axis activity at both the level of the pituitary and the hypothalamus. These negative feedback loops regulate homeostasis of steroid hormone levels. In females, a cyclic reversal of estrogen feedback produces a positive feedback loop at both the hypothalamic and pituitary levels. Central positive feedback results in a dramatic increase in GnRH secretion (Moenter et al., 1992; Xia et al., 1992; Clarke, 1993; Sisk et al., 2001). This is coupled with an increase in pituitary sensitivity to GnRH (Savoy-Moore et al., 1980; Turzillo et al., 1995), which produces the massive surge in secretion of LH that triggers ovulation. While feedback regulation of the axis in males is in part mediated by estrogen receptors (ER), there is not a clear consensus as to the relative role of ER versus AR signaling in males (Lindzey et al., 1998; Wersinger et al., 1999). Therefore, this review will focus on estrogenic signaling in the female.

Rapid nongenomic effects of oestradiol on gonadotrophin-releasing hormone neurones

That oestradiol can have both negative- and positive-feedback actions upon the release of gonadotrophin-releasing hormone (GnRH) has been understood for decades. The vast majority of studies have investigated the effects of in vivo oestrogen administration. In the past decade, evidence has accumulated in many neuronal and non-neuronal systems indicating that, in addition to traditional genomic action via transcription factor receptors, steroids can also initiate effects rapidly via signalling cascades typically associated with the cell membrane. Here, we review work examining the rapid actions of oestradiol on GnRH neurones, addressing the questions of dose dependence, receptor subtypes, signalling cascades and intrinsic and synaptic properties that are rapidly modulated by this steroid.

17β-oestradiol regulation of gonadotrophin-releasing hormone neuronal excitability

17β-Oestradiol (E(2)) is essential for cyclical gonadotrophin-releasing hormone (GnRH) neuronal activity and secretion. In particular, E(2) increases the excitability of GnRH neurones during the afternoon of pro-oestrus in the rodent, which is associated with increased synthesis and secretion of GnRH. It is well established that E(2) regulates the activity of GnRH neurones through both presynaptic and postsynaptic mechanisms. E(2) significantly modulates the mRNA expression of numerous ion channels in GnRH neurones and alters the associated endogenous conductances, including potassium (K(ATP) , A-type) currents and low-voltage T-type and high-voltage L-type calcium currents. Notably, K(ATP) channels are critical for maintaining GnRH neurones in a hyperpolarised state for recruiting the T-type calcium channels, which are important for burst firing in GnRH neurones. In addition, there are other critical channels contributing to burst firing pattern, including the small conductance Ca(2+) -activated K(+) channels that may be modulated by E(2) . Despite these advances, the cellular mechanisms underlying the cyclical GnRH neuronal activity and GnRH release are largely unknown. Ultimately, the ensemble of both pre- and postsynaptic targets of the actions of E(2) will dictate the excitability and activity pattern of GnRH neurones.

STX, a novel nonsteroidal estrogenic compound, induces rapid action in primate GnRH neuronal calcium dynamics and peptide release

Previously, we reported that 1 nM 17ß-estradiol (E(2)) induces a rapid action, which is, in part, mediated through the G protein-coupled receptor GPR30 in primate GnRH neurons. Because it has been reported that the diphenylacrylamide compound, STX, causes estrogenic action in the mouse and guinea pig hypothalamus, the present study examined effects of STX in primate GnRH neurons and whether there is an action independent of GPR30. Results are summarized as follows. STX (10 nM) exposure increased 1) the oscillation frequency of intracellular calcium concentration ([Ca(2+)](i)), 2) the percentage of cells stimulated, and 3) the synchronization frequency of [Ca(2+)](i) oscillations. STX (10-100 nM) also stimulated GnRH release. The effects of STX on both [Ca(2+)](i) oscillations and GnRH release were similar to those caused by E(2) (1 nM), although with less magnitude. STX (10 nM)-induced changes in [Ca(2+)](i) oscillations were not altered by GPR30 small interfering RNA transfection, indicating that STX-sensitive receptors differ from GPR30. Finally, a higher dose of E(2) (10 nM) induced a larger change in [Ca(2+)](i) oscillations than that with a smaller dose of E(2) (1 nM), and the effects of 10 nM E(2) were reduced but not completely blocked by GPR30 small interfering RNA transfection, indicating that the effects of 10 nM E(2) in primate GnRH neurons are mediated by multiple membrane receptors, including GPR30 and STX-sensitive receptors. Collectively, the rapid action of E(2) mediated through GPR30 differs from that mediated through STX-sensitive receptors. The molecular structure of the STX-sensitive receptor remains to be identified.

Rapid action of estradiol in primate GnRH neurons: the role of estrogen receptor alpha and estrogen receptor beta

Estrogens play a pivotal role in the control of female reproductive function. Recent studies using primate GnRH neurons derived from embryonic nasal placode indicate that 17β-estradiol (E(2)) causes a rapid stimulatory action. E(2) (1nM) stimulates firing activity and intracellular calcium ([Ca(2+)](i)) oscillations of primate GnRH neurons within a few min. E(2) also stimulates GnRH release within 10min. However, the classical estrogen receptors, ERα and ERβ, do not appear to play a role in E(2)-induced [Ca(2+)](i) oscillations or GnRH release, as the estrogen receptor antagonist, ICI 182,780, failed to block these responses. Rather, this rapid E(2) action is, at least in part, mediated by a G-protein coupled receptor GPR30. In the present study we further investigate the role of ERα and ERβ in the rapid action of E(2) by knocking down cellular ERα and ERβ by transfection of GnRH neurons with specific siRNA for rhesus monkey ERα and ERβ. Results indicate that cellular knockdown of ERα and ERβ failed to block the E(2)-induced changes in [Ca(2+)](i) oscillations. It is concluded that neither ERα nor ERβ is required for the rapid action of E(2) in primate GnRH neurons.

Voltage-gated potassium currents are targets of diurnal changes in estradiol feedback regulation and kisspeptin action on gonadotropin-releasing hormone neurons in mice

Estradiol has both negative and positive feedback actions upon gonadotropin-releasing hormone (GnRH) release; the latter actions trigger the preovulatory GnRH surge. Although neurobiological mechanisms of the transitions between feedback modes are becoming better understood, the roles of voltage-gated potassium currents, major contributors to neuronal excitability, are unknown. Estradiol alters two components of potassium currents in these cells: a transient current, I(A), and a sustained current, I(K). Kisspeptin is a potential mediator between estradiol and GnRH neurons and can act directly on GnRH neurons. We examined how estradiol, time of day, and kisspeptin interact to regulate these conductances in a mouse model exhibiting daily switches between estradiol negative (morning) and positive feedback (evening). Whole-cell voltage clamp recordings were made from GnRH neurons in brain slices from ovariectomized (OVX) mice and from OVX mice treated with estradiol (OVX+E). There were no diurnal changes in either I(A) or I(K) in GnRH neurons from OVX mice. In contrast, in GnRH neurons from OVX+E mice, I(A) and I(K) were greater during the morning when GnRH neuron activity is low and smaller in the evening when GnRH neuron activity is high. Estradiol increased I(A) in the morning and decreased it in the evening, relative to that in cells from OVX mice. Exogenously applied kisspeptin reduced I(A) regardless of time of day or estradiol status. Estradiol, interacting with time of day, and kisspeptin both depolarized I(A) activation. These findings extend our understanding of both the neurobiological mechanisms of estradiol negative vs. positive regulation of GnRH neurons and of kisspeptin action on these cells.

Physiology of the gonadotrophin-releasing hormone (GnRH) neurone: studies from embryonic GnRH neurones

Gonadotrophin-releasing hormone (GnRH)-secreting neurones are the final output of the central nervous system driving fertility in all mammals. Although it has been known for decades that the efficiency of communication between the hypothalamus and the pituitary depends on the pulsatile profile of GnRH secretion, how GnRH neuronal activity is patterned to generate pulses at the median eminence is unknown. To date, the scattered distribution of the GnRH cell bodies remains the main limitation to assessing the cellular events that could lead to pulsatile GnRH secretion. Taking advantage of the unique developmental feature of GnRH neurones, the nasal explant model allows primary GnRH neurones to be maintained within a micro-network where pulsatile secretion is preserved and where individual cellular activity can be monitored simultaneously across the cell population. This review summarises the data obtained from work using this in vitro model, and brings some insights into GnRH cellular physiology.

17α-Ethinylestradiol and nonylphenol affect the development of forebrain GnRH neurons through an estrogen receptors-dependent pathway

There is growing evidence that neuroendocrine circuits controlling development and reproduction are targeted by EDCs. We have previously demonstrated that low concentrations of 17α-ethinylestradiol (EE2) disrupt the development of forebrain GnRH neurons during zebrafish development. The objectives of the present study were to determine whether the weak estrogenic compound, nonylphenol (NP), could elicit similar effects to EE2 and to what extent the estrogen receptors are involved in mediating these effects. Using immunohistochemistry, we confirmed that EE2 exposure induces an increase in the number of GnRH-ir neurons and we demonstrated that NP is able to produce similar effects in a concentration-dependent manner. The effects of both NP and EE2 were shown to be blocked by the estrogen receptors (ERs) antagonist ICI 182-780, demonstrating the involvement of functional ERs in mediating their effects. Altogether, these results highlight the need to consider neuroendocrine networks as critical endpoints in the field of endocrine disruption.

Physiological consequences of membrane-initiated estrogen signaling in the brain

Many of the actions of 17beta-estradiol (E2) in the central nervous system (CNS) are mediated via the classical nuclear steroid receptors, ER(alpha) and ERbeta, which interact with the estrogen response element to modulate gene expression. In addition to the nuclear-initiated estrogen signaling, E2 signaling in the brain can occur rapidly within minutes prior to any sufficient effects on transcription of relevant genes. These rapid, membrane-initiated E2 signaling mechanisms have now been characterized in many brain regions, most importantly in neurons of the hypothalamus and hippocampus. Furthermore, our understanding of the physiological effects of membrane-initiated pathways is now a major field of interest in the hypothalamic control of reproduction, energy balance, thermoregulation and other homeostatic functions as well as the effects of E2 on physiological and pathophysiological functions of the hippocampus. Membrane signaling pathways impact neuronal excitability, signal transduction, cell death, neurotransmitter release and gene expression. This review will summarize recent findings on membrane-initiated E2 signaling in the hypothalamus and hippocampus and its contribution to the control of physiological and behavioral functions.

Regulation of endogenous conductances in GnRH neurons by estrogens

17β-estradiol (E2) regulates the activity of the gonadotropin-releasing hormone (GnRH) neurons through both presynaptic and postsynaptic mechanisms, and this ovarian steroid hormone is essential for cyclical GnRH neuronal activity and secretion. E2 has significant actions to modulate the mRNA expression of numerous ion channels in GnRH neurons and/or to enhance (suppress) endogenous conductances (currents) including potassium (K(ATP), A-type) and calcium low voltage T-type and high voltage L-type currents. Also, it is well documented that E2 can alter the excitability of GnRH neurons via direct action, but the intracellular signaling cascades mediating these actions are not well understood. As an example, K(ATP) channels are critical ion channels needed for maintaining GnRH neurons in a hyperpolarized state for recruiting T-type calcium channels that are important for burst firing in GnRH neurons. E2 modulates the activity of K(ATP) channels via a membrane-initiated signaling pathway in GnRH neurons. Obviously there are other channels, including the small conductance activated K(+) (SK) channels, that maybe modulated by this signaling pathway, but the ensemble of mER-, ERα-, and ERβ-mediated effects both pre- and post-synaptic will ultimately dictate the excitability of GnRH neurons.

Identified GnRH neuron electrophysiology: a decade of study

Over the past decade, the existence of transgenic mouse models in which reporter genes are expressed under the control of the gonadotropin-releasing hormone (GnRH) promoter has made possible the electrophysiological study of these cells. Here, we review the intrinsic and synaptic properties of these cells that have been revealed by these approaches, with a particular regard to burst generation. Advances in our understanding of neuromodulation of GnRH neurons and synchronization of this network are also discussed.

Epigenetic changes coincide with in vitro primate GnRH neuronal maturation

Cellular and molecular mechanisms underlying pulsatile GnRH release are not well understood. In the present study, we examined the developmental changes in intracellular calcium dynamics, peptide release, gene expression, and DNA methylation in cultured GnRH neurons derived from the nasal placode of rhesus monkeys. We found that GnRH neurons were functionally immature, exhibiting little fluctuation in intracellular calcium ([Ca(2+)](i)) and sparse pulses of GnRH peptide release in the first 12 d in vitro (div). By 14-18 div, GnRH neurons exhibited periodic [Ca(2+)](i) oscillations, synchronizing at approximately 60-min intervals and GnRH pulses occurred at approximately 60-min intervals. Interestingly, the total GnRH peptide release further increased after 18 div. Measurement of GnRH mRNA and gene CpG methylation status at 0, 14, and 20 div indicated that mRNA levels significantly (P < 0.05) increased between 14 and 20 div, just as maximal decapeptide release was observed. By bisulfite sequencing across a 5' CpG island of the GnRH gene, we further found that methylation at eight of 14 CpG sites significantly (P < 0.05) decreased between 0 and 20 div. These data indicate that epigenetic differentiation occurs during GnRH neuronal development and suggest that increased GnRH gene expression and decreased CpG methylation status are molecular phenotypes of mature GnRH neurons. To our knowledge, this is the first report that developmental DNA demethylation occurs in postmitotic neurons toward a stable neuronal phenotype.

17 β-estradiol rapidly increases ATP-sensitive potassium channel activity in gonadotropin-releasing hormone neurons [corrected] via a protein kinase signaling pathway

17Beta-estradiol (E2) both inhibits and excites GnRH neurons via presynaptic as well as postsynaptic mechanisms. Although it has been demonstrated that E2 can alter the excitability of GnRH neurons via direct actions, the intracellular signaling cascades mediating these actions are not well understood. Previously we have shown that the activity of one of the critical ion channels needed for maintaining GnRH neurons in a hyperpolarized state, the ATP-sensitive potassium channel (K(ATP)) channel, is augmented by E2 in ovariectomized females. However, the mRNA expression of the K(ATP) channel subunits Kir6.2 and SUR1 are unchanged with in vivo E2 treatment. Therefore, to elucidate the cellular signaling mechanism(s) modulating the channel activity, we did whole-cell patch-clamp recording of enhanced green fluorescent protein-GnRH neurons from ovariectomized female mice to study the acute effects of E2. E2 dose-dependently (EC(50) = 0.6 nM) enhanced the diazoxide (channel opener)-activated K(ATP) channel currents by 1.2- to 2.0-fold, which was antagonized by ICI 182,780. E2-BSA was equally as effective as E2, whereas 17 alpha-estradiol [corrected] had no effect. The protein kinase A (PKA) activator forskolin mimicked the effects of E2, whereas the PKA inhibitor H89 and the protein kinase C (PKC) inhibitor bisindolylmaleimide I blocked the effects of E2. Similar to E2, STX, a membrane estrogen receptor (ER) agonist that does not bind to ERalpha or ERbeta, also potentiated the diazoxide-induced K(ATP) channel current by 1.5-fold. Therefore, E2 can potentiate K(ATP) channel activity in GnRH neurons through a membrane ER-activated PKC-PKA signaling pathway.

Estrogen receptor beta-selective agonists stimulate calcium oscillations in human and mouse embryonic stem cell-derived neurons

Estrogens are used extensively to treat hot flashes in menopausal women. Some of the beneficial effects of estrogens in hormone therapy on the brain might be due to nongenomic effects in neurons such as the rapid stimulation of calcium oscillations. Most studies have examined the nongenomic effects of estrogen receptors (ER) in primary neurons or brain slices from the rodent brain. However, these cells can not be maintained continuously in culture because neurons are post-mitotic. Neurons derived from embryonic stem cells could be a potential continuous, cell-based model to study nongenomic actions of estrogens in neurons if they are responsive to estrogens after differentiation. In this study ER-subtype specific estrogens were used to examine the role of ERα and ERβ on calcium oscillations in neurons derived from human (hES) and mouse embryonic stem cells. Unlike the undifferentiated hES cells the differentiated cells expressed neuronal markers, ERβ, but not ERα. The non-selective ER agonist 17β-estradiol (E₂) rapidly increased [Ca2+]i oscillations and synchronizations within a few minutes. No change in calcium oscillations was observed with the selective ERα agonist 4,4′,4″-(4-Propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT). In contrast, the selective ERβ agonists, 2,3-bis(4-Hydroxyphenyl)-propionitrile (DPN), MF101, and 2-(3-fluoro-4-hydroxyphenyl)-7-vinyl-1,3 benzoxazol-5-ol (ERB-041; WAY-202041) stimulated calcium oscillations similar to E₂. The ERβ agonists also increased calcium oscillations and phosphorylated PKC, AKT and ERK1/2 in neurons derived from mouse ES cells, which was inhibited by nifedipine demonstrating that ERβ activates L-type voltage gated calcium channels to regulate neuronal activity. Our results demonstrate that ERβ signaling regulates nongenomic pathways in neurons derived from ES cells, and suggest that these cells might be useful to study the nongenomic mechanisms of estrogenic compounds.

Calcium dynamics in gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons represent the key output cells of the neuronal network controlling fertility. Intracellular calcium ion concentration ([Ca(2+)](i)) is likely to be a key signaling tool used by GnRH neurons to regulate and co-ordinate multiple cell processes. This review examines the dynamics and control of [Ca(2+)](i) in GT1 cells, embryonic GnRH neurons in the nasal placode culture, and adult GnRH neurons in the acute brain slice preparation. GnRH neurons at all stages of development display spontaneous [Ca(2+)](i) transients driven, primarily, by their burst firing. However, the intracellular mechanisms generating [Ca(2+)](i) transients, and the control of [Ca(2+)](i) by neurotransmitters, varies markedly across the different developmental stages. The functional roles of [Ca(2+)](i) transients are beginning to be unraveled with one key action being that of regulating the dynamics of GnRH neuron burst firing.

Recent discoveries on the control of gonadotrophin-releasing hormone neurones in nonhuman primates

Since Ernst Knobil proposed the concept of the gonadotrophin-releasing hormone (GnRH) pulse-generator in the monkey hypothalamus three decades ago, we have made significant progress in this research area with cellular and molecular approaches. First, an increase in pulsatile GnRH release triggers the onset of puberty. However, the question of what triggers the pubertal increase in GnRH is still unclear. GnRH neurones are already mature before puberty but GnRH release is suppressed by a tonic GABA inhibition. Our recent work indicates that blocking endogenous GABA inhibition with the GABA(A) receptor blocker, bicuculline, dramatically increases kisspeptin release, which plays an important role in the pubertal increase in GnRH release. Thus, an interplay between the GABA, kisspeptin, and GnRH neuronal systems appears to trigger puberty. Second, cultured GnRH neurones derived from the olfactory placode of monkey embryos exhibit synchronised intracellular calcium, [Ca(2+)](i), oscillations and release GnRH in pulses at approximately 60-min intervals after 14 days in vitro (div). During the first 14 div, GnRH neurones undergo maturational changes from no [Ca(2+)](i) oscillations and little GnRH release to the fully functional state. Recent work also shows GnRH mRNA expression increases during in vitro maturation. This mRNA increase coincides with significant demethylation of a CpG island in the GnRH 5'-promoter region. This suggests that epigenetic differentiation occurs during GnRH neuronal maturation. Third, oestradiol causes rapid, direct, excitatory action in GnRH neurones and this action of oestradiol appears to be mediated through a membrane receptor, such as G-protein coupled receptor 30.

Altered GABAA receptor-mediated synaptic transmission disrupts the firing of gonadotropin-releasing hormone neurons in male mice under conditions that mimic steroid abuse

Gonadotropin-releasing hormone (GnRH) neurons are the central regulators of reproduction. GABAergic transmission plays a critical role in pubertal activation of pulsatile GnRH secretion. Self-administration of excessive doses of anabolic androgenic steroids (AAS) disrupts reproductive function and may have critical repercussions for pubertal onset in adolescent users. Here, we demonstrate that chronic treatment of adolescent male mice with the AAS 17alpha-methyltestosterone significantly decreased action potential frequency in GnRH neurons, reduced the serum gonadotropin levels, and decreased testes mass. AAS treatment did not induce significant changes in GABAA receptor subunit mRNA levels or alter the amplitude or decay kinetics of GABAA receptor-mediated spontaneous postsynaptic currents (sPSCs) or tonic currents in GnRH neurons. However, AAS treatment significantly increased action potential frequency in neighboring medial preoptic area (mPOA) neurons and GABAA receptor-mediated sPSC frequency in GnRH neurons. In addition, physical isolation of the more lateral aspects of the mPOA from the medially localized GnRH neurons abrogated the AAS-induced increase in GABAA receptor-mediated sPSC frequency and the decrease in action potential firing in the GnRH cells. Our results indicate that AAS act predominantly on steroid-sensitive presynaptic neurons within the mPOA to impart significant increases in GABAA receptor-mediated inhibitory tone onto downstream GnRH neurons, resulting in diminished activity of these pivotal mediators of reproductive function. These AAS-induced changes in central GABAergic circuits of the forebrain may significantly contribute to the disruptive actions of these drugs on pubertal maturation and the development of reproductive competence in male steroid abusers.

Diurnal in vivo and rapid in vitro effects of estradiol on voltage-gated calcium channels in gonadotropin-releasing hormone neurons

A robust surge of gonadotropin-releasing hormone (GnRH) release triggers the luteinizing hormone surge that induces ovulation. The GnRH surge is attributable to estradiol feedback, but the mechanisms are incompletely understood. Voltage-gated calcium channels (VGCCs) regulate hormone release and neuronal excitability, and may be part of the surge-generating mechanism. We examined VGCCs of GnRH neurons in brain slices from a model exhibiting daily luteinizing hormone surges. Mice were ovariectomized (OVX), and a subset was treated with estradiol implants (OVX+E). OVX+E mice exhibit negative feedback in the A.M. and positive feedback in the P.M. GnRH neurons express prominent high-voltage-activated (HVA) and small low-voltage-activated (LVA) macroscopic (whole-cell) Ca currents (I(Ca)). LVA-mediated currents were not altered by estradiol or time of day. In contrast, in OVX+E mice, HVA-mediated currents varied with time of day; HVA currents in cells from OVX+E mice were lower than those in cells from OVX mice in the A.M. but were higher in the P.M. These changes were attributable to diurnal alternations in L- and N-type components. There were no diurnal changes in any aspect of HVA-mediated I(Ca) in OVX mice. Acute in vitro treatment of cells from OVX and OVX+E mice with estradiol rapidly increased HVA currents primarily through L- and R-type VGCCs by activating estrogen receptor beta and GPR30, respectively. These results suggest multiple mechanisms contribute to the overall feedback regulation of HVA-mediated I(Ca) by estradiol. In combination with changes in synaptic inputs to GnRH neurons, these intrinsic changes in GnRH neurons may play critical roles in estradiol feedback.

Corticosteroids: way upstream

Studies into the mechanisms of corticosteroid action continue to be a rich bed of research, spanning the fields of neuroscience and endocrinology through to immunology and metabolism. However, the vast literature generated, in particular with respect to corticosteroid actions in the brain, tends to be contentious, with some aspects suffering from loose definitions, poorly-defined models, and appropriate dissection kits. Here, rather than presenting a comprehensive review of the subject, we aim to present a critique of key concepts that have emerged over the years so as to stimulate new thoughts in the field by identifying apparent shortcomings. This article will draw on experience and knowledge derived from studies of the neural actions of other steroid hormones, in particular estrogens, not only because there are many parallels but also because 'learning from differences' can be a fruitful approach. The core purpose of this review is to consider the mechanisms through which corticosteroids might act rapidly to alter neural signaling.

Control of CNS neuronal excitability by estrogens via membrane-initiated signaling

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system (CNS) are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane-associated steroid receptors for E2 in hypothalamic and other brain neurons. Indeed, we are just beginning to understand how E2 signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. We know 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. This review will concentrate on rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus and hippocampus, the nature of receptors involved and how they contribute to CNS functions.