Estradiol induces diurnal shifts in GABA transmission to gonadotropin-releasing hormone neurons to provide a neural signal for ovulation

Early-Life Resource Scarcity in Mice Does Not Alter Adult Corticosterone or Preovulatory Luteinizing Hormone Surge Responses to Acute Psychosocial Stress

Early-life stressors can affect reproductive development and change responses to adult stress. We tested if resource scarcity in the form of limited bedding and nesting (LBN) from postnatal days (PND) 4 to 11 delayed sexual maturation in male and female mice and/or altered the response to an acute, layered, psychosocial stress (ALPS) in adulthood. Contrary to the hypotheses, age and mass at puberty were unaffected by the present application of LBN. Under basal conditions and after ALPS, corticosterone concentrations in males, diestrous females, and proestrous females reared in standard (STD) or LBN environments were similar. ALPS disrupts the luteinizing hormone (LH) surge in most mice when applied on the morning of proestrus; this effect was not changed by resource scarcity. In this study, the paucity of effects in the offspring may relate to a milder response of CBA dams to the paradigm. While LBN dams exited the nest more often and their offspring were smaller than STD-reared offspring on PND11, dam corticosterone concentrations were similar on PND11. To test if ALPS disrupts the LH surge by blunting the increase in excitatory GABAergic input to gonadotropin-releasing hormone (GnRH) neurons on the afternoon of proestrus, we conducted whole-cell voltage-clamp recordings. The frequency of GABAergic postsynaptic currents in GnRH neurons was not altered by LBN, ALPS, or their interaction. It remains possible that ALPS acts at afferents of GnRH neurons, changes response of GnRH neurons to input, and/or alters pituitary responsiveness to GnRH and that a more pronounced resource scarcity would affect the parameters studied.

Reversible Sterilization of Channel Catfish via Overexpression of Glutamic Acid Decarboxylase Gene

The confinement of transgenic fish is essential to prevent their escape and reproduction in natural ecosystems. Reversible transgenic sterilization is a promising approach to control the reproduction of transgenic fish. Therefore, the present study was conducted to develop a reversibly sterile channel catfish (Ictalurus punctatus) via the transgenic overexpression of the goldfish (Carassius auratus) glutamic acid decarboxylase (GAD) gene driven by the common carp (Cyprinus carpio) β-actin promoter to disrupt normal gamma-aminobutyric acid (GABA) regulation. Three generations of GAD-transgenic fish were produced. All studied generations showed repressed reproductive performance; however, this was not always statistically significant. In F1, 5.4% of the transgenic fish showed a sexual maturity score ≥ 4 (maximum = 5) at five years of age, which was lower (p = 0.07) than that of the control group (16.8%). In the spawning experiments conducted on F1 transgenic fish at six and nine years of age, 45.5% and 20.0% of fish spawned naturally, representing lower values (p = 0.09 and 0.12, respectively) than the percentages in the sibling control fish of the same age (83.3% and 66.7%, respectively). Four of six pairs of the putative infertile six-year-old fish spawned successfully after luteinizing hormone-releasing hormone analog (LHRHa) therapy. Similar outcomes were noted in the three-year-old F2 fish, with a lower spawning percentage in transgenic fish (20.0%) than in the control (66.7%). In one-year-old F2-generation transgenic fish, the observed mean serum gonadotropin-releasing hormone (GnRH) levels were 9.23 ± 2.49 and 8.14 ± 2.21 ng/mL for the females and males, respectively. In the control fish, the mean levels of GnRH were 11.04 ± 4.06 and 9.03 ± 2.36 ng/mL for the females and males, respectively, which did not differ significantly from the control (p = 0.15 and 0.27 for females and males, respectively). There was no significant difference in the estradiol levels of the female transgenic and non-transgenic fish in the one- and four-year-old F2-generation fish. The four-year-old F2-generation male transgenic fish exhibited significantly (p < 0.05) lower levels of GnRH and testosterone than the control fish. In conclusion, while overexpressing GAD repressed the reproductive abilities of channel catfish, it did not completely sterilize transgenic fish. The sterilization rate might be improved through selection in future generations.

Cholinergic Control of GnRH Neuron Physiology and Luteinizing Hormone Secretion in Male Mice: Involvement of ACh/GABA Cotransmission

Gonadotropin-releasing hormone (GnRH)-synthesizing neurons orchestrate reproduction centrally. Early studies have proposed the contribution of acetylcholine (ACh) to hypothalamic control of reproduction, although the causal mechanisms have not been clarified. Here, we report that in vivo pharmacogenetic activation of the cholinergic system increased the secretion of luteinizing hormone (LH) in orchidectomized mice. 3DISCO immunocytochemistry and electron microscopy revealed the innervation of GnRH neurons by cholinergic axons. Retrograde viral labeling initiated from GnRH-Cre neurons identified the medial septum and the diagonal band of Broca as exclusive sites of origin for cholinergic afferents of GnRH neurons. In acute brain slices, ACh and carbachol evoked a biphasic effect on the firing rate in GnRH neurons, first increasing and then diminishing it. In the presence of tetrodotoxin, carbachol induced an inward current, followed by a decline in the frequency of miniature postsynaptic currents (mPSCs), indicating a direct influence on GnRH cells. RT-PCR and whole-cell patch-clamp studies revealed that GnRH neurons expressed both nicotinic (α4β2, α3β4, and α7) and muscarinic (M1-M5) AChRs. The nicotinic AChRs contributed to the nicotine-elicited inward current and the rise in firing rate. Muscarine via M1 and M3 receptors increased, while via M2 and M4 reduced the frequency of both mPSCs and firing. Optogenetic activation of channelrhodopsin-2-tagged cholinergic axons modified GnRH neuronal activity and evoked cotransmission of ACh and GABA from a subpopulation of boutons. These findings confirm that the central cholinergic system regulates GnRH neurons and activates the pituitary-gonadal axis via ACh and ACh/GABA neurotransmissions in male mice.

CRISPR-Cas9 knockdown of ESR1 in preoptic GABA-kisspeptin neurons suppresses the preovulatory surge and estrous cycles in female mice

Evidence suggests that estradiol-sensing preoptic area GABA neurons are involved in the preovulatory surge mechanism necessary for ovulation. In vivo CRISPR-Cas9 editing was used to achieve a 60-70% knockdown in estrogen receptor alpha (ESR1) expression by GABA neurons located within the regions of the rostral periventricular area of the third ventricle (RP3V) and medial preoptic nuclei (MPN) in adult female mice. Mice exhibited variable reproductive phenotypes with the only significant finding being mice with bilateral ESR1 deletion in RP3V GABA neurons having reduced cFos expression in gonadotropin-releasing hormone (GnRH) neurons at the time of the surge. One sub-population of RP3V GABA neurons expresses kisspeptin. Re-grouping ESR1-edited mice on the basis of their RP3V kisspeptin expression revealed a highly consistent phenotype; mice with a near-complete loss of kisspeptin immunoreactivity displayed constant estrus and failed to exhibit surge activation but retained pulsatile luteinizing hormone (LH) secretion. These observations demonstrate that ESR1-expressing GABA-kisspeptin neurons in the RP3V are essential for the murine preovulatory LH surge mechanism.

Circadian and kisspeptin regulation of the preovulatory surge

Fertility in mammals is ultimately controlled by a small population of neurons - the gonadotropin-releasing hormone (GnRH) neurons - located in the ventral forebrain. GnRH neurons control gonadal function through the release of GnRH, which in turn stimulates the secretion of the anterior pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In spontaneous ovulators, ovarian follicle maturation eventually stimulates, via sex steroid feedback, the mid-cycle surge in GnRH and LH secretion that causes ovulation. The GnRH/LH surge is initiated in many species just before the onset of activity through processes controlled by the central circadian clock, ensuring that the neuroendocrine control of ovulation and sex behavior are coordinated. This review aims to give an overview of anatomical and functional studies that collectively reveal some of the mechanisms through which the central circadian clock regulates GnRH neurons and their afferent circuits to drive the preovulatory surge.

Increased GABA transmission to GnRH neurons after intrahippocampal kainic acid injection in mice is sex-specific and associated with estrous cycle disruption

Patients with epilepsy develop reproductive endocrine comorbidities at a rate higher than that of the general population. Clinical studies have identified disrupted luteinizing hormone (LH) release patterns in patients of both sexes, suggesting potential epilepsy-associated changes in hypothalamic gonadotropin-releasing hormone (GnRH) neuron function. In previous work, we found that GnRH neuron firing is increased in diestrous females and males in the intrahippocampal kainic acid (IHKA) mouse model of temporal lobe epilepsy. Notably, GABA_A receptor activation is depolarizing in adult GnRH neurons. Therefore, here we tested the hypothesis that increased GnRH neuron firing in IHKA mice is associated with increased GABAergic drive to GnRH neurons. When ionotropic glutamate receptors (iGluRs) were blocked to isolate GABAergic postsynaptic currents (PSCs), no differences in PSC frequency were seen between GnRH neurons from control and IHKA diestrous females. In the absence of iGluR blockade, however, GABA PSC frequency was increased in GnRH neurons from IHKA females with disrupted estrous cycles, but not saline-injected controls nor IHKA females without estrous cycle disruption. GABA PSC amplitude was also increased in IHKA females with disrupted estrous cycles. These findings suggest the presence of an iGluR-dependent increase in feed-forward GABAergic transmission to GnRH neurons specific to IHKA females with comorbid cycle disruption. In males, GABA PSC frequency and amplitude were unchanged but PSC duration was reduced. Together, these findings suggest that increased GABA transmission helps drive elevated firing in IHKA females on diestrus and indicate the presence of a sex-specific hypothalamic mechanism underlying reproductive endocrine dysfunction in IHKA mice.

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.

Neuroendocrine interactions of the stress and reproductive axes

Reproduction is controlled by a sequential regulation of the hypothalamo-pituitary-gonadal (HPG) axis. The HPG axis integrates multiple inputs to maintain proper reproductive functions. It has long been demonstrated that stress alters fertility. Nonetheless, the central mechanisms of how stress interacts with the reproductive system are not fully understood. One of the major pathways that is activated during the stress response is the hypothalamo-pituitary-adrenal (HPA) axis. In this review, we discuss several aspects of the interactions between these two neuroendocrine systems to offer insights to mechanisms of how the HPA and HPG axes interact. We have also included discussions of other systems, for example GABA-producing neurons, where they are informative to the overall picture of stress effects on reproduction.

Morphological assessment of GABA and glutamate inputs to GnRH neurons in intact female mice using expansion microscopy

The roles GABAergic and glutamatergic inputs in regulating the activity of the gonadotrophin-releasing hormone (GnRH) neurons at the time of the preovulatory surge remain unclear. We used expansion microscopy to compare the density of GABAergic and glutamatergic synapses on the GnRH neuron cell body and proximal dendrite in dioestrous and pro-oestrous female mice. An evaluation of all synapses immunoreactive for synaptophysin revealed that the highest density of inputs to rostral preoptic area GnRH neurons occurred within the first 45 µm of the primary dendrite (approximately 0.19 synapses µm-1 ) with relatively few synapses on the GnRH neuron soma or beyond 45 µm of the dendrite (0.05-0.08 synapses µm-1 ). Triple immunofluorescence labelling demonstrated a predominance of glutamatergic signalling with twice as many vesicular glutamate transporter 2 synapses detected compared to vesicular GABA transporter. Co-labelling with the GABAA receptor scaffold protein gephyrin and the glutamate receptor postsynaptic density marker Homer1 confirmed these observations, as well as the different spatial distribution of GABA and glutamate inputs along the dendrite. Quantitative assessments revealed no differences in synaptophysin, GABA or glutamate synapses at the proximal dendrite and soma of GnRH neurons between dioestrous and pro-oestrous mice. Taken together, these studies demonstrate that the GnRH neuron receives twice as many glutamatergic synapses compared to GABAergic synapses and that these inputs preferentially target the first 45 µm of the GnRH neuron proximal dendrite. These inputs appear to be structurally stable before the onset of pro-oestrous GnRH surge.

Gonadotropin-Releasing Hormone (GnRH) Neuron Potassium Currents and Excitability in Both Sexes Exhibit Minimal Changes upon Removal of Negative Feedback

Gonadotropin-releasing hormone (GnRH) drives pituitary secretion of luteinizing hormone and follicle-stimulating hormone, which in turn regulate gonadal functions including steroidogenesis. The pattern of GnRH release and thus fertility depend on gonadal steroid feedback. Under homeostatic (negative) feedback conditions, removal of the gonads from either females or males increases the amplitude and frequency of GnRH release and alters the long-term firing pattern of these neurons in brain slices. The neurobiological mechanisms intrinsic to GnRH neurons that are altered by homeostatic feedback are not well studied and have not been compared between sexes. During estradiol-positive feedback, which is unique to females, there are correlated changes in voltage-gated potassium currents and neuronal excitability. We thus hypothesized that these same mechanisms would be engaged in homeostatic negative feedback. Voltage-gated potassium channels play a direct role in setting excitability and action potential properties. Whole-cell patch-clamp recordings of GFP-identified GnRH neurons in brain slices from sham-operated and castrated adult female and male mice were made to assess fast and slow inactivating potassium currents as well as action potential properties. Surprisingly, no changes were observed among groups in most potassium current properties, input resistance, or capacitance, and this was reflected in a lack of differences in excitability and specific action potential properties. These results support the concept that, in contrast to positive feedback, steroid-negative feedback regulation of GnRH neurons in both sexes is likely conveyed to GnRH neurons via mechanisms that do not induce major changes in the biophysical properties of these cells.

Functions of the GABAergic system on serum LH concentrations in pre-pubertal Nellore heifers

This study was conducted to evaluate the luteinizing hormone (LH) secretion pattern after gamma-aminobutyric acid (GABA_A) antagonist to determine the effects of the GABAergic system on LH secretion during reproductive maturation in pre-pubertal Nellore heifers. Nellore heifers (n = 10) were administered a picrotoxin injection of 0.18 mg/kg, i.v. Blood samples were collected every 15 min for 3 h at different developmental stages (8, 10, 14 and 17 mo of age). Plasma concentrations of LH were quantified using an RIA (sensitivity of 0.04 ng/mL and CV of 15 %). There was an interaction between treatment and age (P = 0.034). Picrotoxin-treated heifers had lesser (P ≤  0.05) LH mean concentrations during a 3 h period at 10 and 17 mo of age compared to control heifers (P ≤  0.05). Comparing the period before and after Picrotoxin injection in the same animals, there was a 33 % decrease in LH concentration at 8 mo of age (P = 0.0165). These results indicate the GABAergic system has a stimulatory function in inducing LH secretion in pre-pubertal Nellore heifers. These findings corroborate previous results that GABA increases GnRH/LH secretion in other species during the pre-pubertal period.

Insulin-Like Growth Factor 1 Increases GABAergic Neurotransmission to GnRH Neurons via Suppressing the Retrograde Tonic Endocannabinoid Signaling Pathway in Mice

INTRODUCTION

Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons orchestrate various physiological events that control the onset of puberty. Previous studies showed that insulin-like growth factor 1 (IGF-1) induces the secretion of GnRH and accelerates the onset of puberty, suggesting a regulatory role of this hormone upon GnRH neurons.

METHODS

To reveal responsiveness of GnRH neurons to IGF-1 and elucidate molecular pathways acting downstream to the IGF-1 receptor (IGF-1R), in vitro electrophysiological experiments were carried out on GnRH-GFP neurons in acute brain slices from prepubertal (23-29 days) and pubertal (50 days) male mice.

RESULTS

Administration of IGF-1 (13 nM) significantly increased the firing rate and frequency of spontaneous postsynaptic currents and that of excitatory GABAergic miniature postsynaptic currents (mPSCs). No GABAergic mPSCs were induced by IGF-1 in the presence of the GABAA-R blocker picrotoxin. The increase in the mPSC frequency was prevented by the use of the IGF-1R antagonist, JB1 (1 µM), or the intracellularly applied PI3K blocker (LY294002, 50 µM), showing involvement of IGF-1R and PI3K in the mechanism. Blockade of the transient receptor potential vanilloid 1, an element of the tonic retrograde endocannabinoid machinery, by AMG9810 (10 µM) or antagonizing the cannabinoid receptor type-1 by AM251 (1 µM) abolished the effect.

DISCUSSION/CONCLUSION

These findings indicate that IGF-1 arrests the tonic retrograde endocannabinoid pathway in GnRH neurons, and this disinhibition increases the release of GABA from presynaptic terminals that, in turn, activates GnRH neurons leading to the fine-tuning of the hypothalamo-pituitary-gonadal axis.

The role of non-neuronal cells in hypogonadotropic hypogonadism

The hypothalamic-pituitary-gonadal axis is controlled by gonadotropin-releasing hormone (GnRH) released by the hypothalamus. Disruption of this system leads to impaired reproductive maturation and function, a condition known as hypogonadotropic hypogonadism (HH). Most studies to date have focused on genetic causes of HH that impact neuronal development and function. However, variants may also impact the functioning of non-neuronal cells known as glia. Glial cells make up 50% of brain cells of humans, primates, and rodents. They include radial glial cells, microglia, astrocytes, tanycytes, oligodendrocytes, and oligodendrocyte precursor cells. Many of these cells influence the hypothalamic neuroendocrine system controlling fertility. Indeed, glia regulate GnRH neuronal activity and secretion, acting both at their cell bodies and their nerve endings. Recent work has also made clear that these interactions are an essential aspect of how the HPG axis integrates endocrine, metabolic, and environmental signals to control fertility. Recognition of the clinical importance of interactions between glia and the GnRH network may pave the way for the development of new treatment strategies for dysfunctions of puberty and adult fertility.

A CRH Receptor Type 1 Agonist Increases GABA Transmission to GnRH Neurons in a Circulating-Estradiol-Dependent Manner

GnRH neurons are central regulators of reproduction and respond to factors affecting fertility, such as stress. Corticotropin-releasing hormone (CRH) is released during stress response. In brain slices from unstressed controls, CRH has opposite, estradiol-dependent effects on GnRH neuron firing depending on the CRH receptor activated; activating CRHR-1 stimulates whereas activating CRHR-2 suppresses activity. We investigated possible direct and indirect mechanisms. Mice were ovariectomized and either not treated further (OVX) or given a capsule producing high positive feedback (OVX + E) or low negative feedback (OVX + low E) physiologic circulating estradiol levels. We tested possible direct effects on GnRH neurons by altering voltage-gated potassium currents. Two types of voltage-gated potassium currents (transient IA and sustained IK) were measured; neither CRHR-1 nor CRHR-2 agonists altered potassium current density in GnRH neurons from OVX + E mice. Further, neither CRH nor receptor-specific agonists altered action potential generation in response to current injection in GnRH neurons from OVX + E mice. To test the possible indirect actions, GABAergic postsynaptic currents were monitored. A CRHR-1 agonist increased GABAergic transmission frequency to GnRH neurons from OVX + E, but not OVX, mice, whereas a CRHR-2 agonist had no effect. Finally, we tested if CRH alters the firing rate of arcuate kisspeptin neurons, which provide an important excitatory neuromodulatory input to GnRH neurons. CRH did not acutely alter firing activity of these neurons from OVX, OVX + E or OVX + low E mice. These results suggest CRH increases GnRH neuron activity in an estradiol-dependent manner in part by activating GABAergic afferents. Mechanisms underlying inhibitory effects of CRH remain unknown.

Central aspects of systemic oestradiol negative- and positive-feedback on the reproductive neuroendocrine system

The central nervous system regulates fertility via the release of gonadotrophin-releasing hormone (GnRH). This control revolves around the hypothalamic-pituitary-gonadal axis, which operates under traditional homeostatic feedback by sex steroids from the gonads in males and most of the time in females. An exception is the late follicular phase in females, when homeostatic feedback is suspended and a positive-feedback response to oestradiol initiates the preovulatory surges of GnRH and luteinising hormone. Here, we briefly review the history of how mechanisms underlying central control of ovulation by circulating steroids have been studied, discuss the relative merit of different model systems and integrate some of the more recent findings in this area into an overall picture of how this phenomenon occurs.

Differential Roles of Hypothalamic AVPV and Arcuate Kisspeptin Neurons in Estradiol Feedback Regulation of Female Reproduction

Mammalian reproductive function includes puberty onset and completion, reproductive cyclicity, steroidogenesis, gametogenesis, fertilization, pregnancy, and lactation; all are indispensable to perpetuate species. Reproductive cycles are critical for providing the hormonal milieu needed for follicular development and maturation of eggs, but cycles, in and of themselves, do not guarantee ovulation will occur. Here, we review the roles in female reproductive neuroendocrine function of two hypothalamic populations that produce the neuropeptide kisspeptin, demonstrating distinct roles in maintaining cycles and ovulation.

Estradiol Enhances the Depolarizing Response to GABA and AMPA Synaptic Conductances in Arcuate Kisspeptin Neurons by Diminishing Voltage-Gated Potassium Currents

Synaptic and intrinsic properties interact to sculpt neuronal output. Kisspeptin neurons in the hypothalamic arcuate nucleus help convey homeostatic estradiol feedback to central systems controlling fertility. Estradiol increases membrane depolarization induced by GABA_A receptor activation in these neurons. We hypothesized that the mechanisms underlying estradiol-induced alterations in postsynaptic response to GABA, and also AMPA, receptor activation include regulation of voltage-gated potassium currents. Whole-cell recordings of arcuate kisspeptin neurons in brain slices from ovariectomized (OVX) and OVX+estradiol (OVX+E) female mice during estradiol negative feedback revealed that estradiol reduced capacitance, reduced transient and sustained potassium currents, and altered voltage dependence and kinetics of transient currents. Consistent with these observations, estradiol reduced rheobase and action potential latency. To study more directly interactions between synaptic and active intrinsic estradiol feedback targets, dynamic clamp was used to simulate GABA and AMPA conductances. Both GABA and AMPA dynamic clamp-induced postsynaptic potentials (PSPs) were smaller in neurons from OVX than OVX+E mice; blocking transient potassium currents eliminated this difference. To interrogate the role of the estradiol-induced changes in passive intrinsic properties, different Markov model structures based on the properties of the transient potassium current in cells from OVX or OVX+E mice were combined in silico with passive properties reflecting these two endocrine conditions. Some of tested models reproduced the effect on PSPs in silico, revealing that AMPA PSPs were more sensitive to changes in capacitance. These observations support the hypothesis that PSPs in arcuate kisspeptin neurons are regulated by estradiol-sensitive mechanisms including potassium conductances and membrane properties.SIGNIFICANCE STATEMENT Kisspeptin neurons relay estradiol feedback to gonadotropin-releasing hormone neurons, which regulate the reproductive system. The fast synaptic neurotransmitters GABA and glutamate rapidly depolarize arcuate kisspeptin neurons and estradiol increases this depolarization. Estradiol reduced both potassium current in the membrane potential range typically achieved during response to fast synaptic inputs and membrane capacitance. Using simulated GABA and glutamate synaptic inputs, we showed changes in both the passive and active intrinsic properties induced by in vivo estradiol treatment affect the response to synaptic inputs, with capacitance having a greater effect on response to glutamate. The suppression of both passive and active intrinsic properties by estradiol feedback thus renders arcuate kisspeptin neurons more sensitive to fast synaptic inputs.

Changes in Both Neuron Intrinsic Properties and Neurotransmission Are Needed to Drive the Increase in GnRH Neuron Firing Rate during Estradiol-Positive Feedback

Central output of gonadotropin-releasing hormone (GnRH) neurons controls fertility and is sculpted by sex-steroid feedback. A switch of estradiol action from negative to positive feedback initiates a surge of GnRH release, culminating in ovulation. In ovariectomized mice bearing constant-release estradiol implants (OVX+E), GnRH neuron firing is suppressed in the morning (AM) by negative feedback and activated in the afternoon (PM) by positive feedback; no time-of-day-dependent changes occur in OVX mice. In this daily surge model, GnRH neuron intrinsic properties are shifted to favor increased firing during positive feedback. It is unclear whether this shift and the observed concomitant increase in GABAergic transmission, which typically excites GnRH neurons, are independently sufficient for increasing GnRH neuron firing rate during positive feedback or whether both are needed. To test this, we used dynamic clamp to inject selected previously recorded trains of GABAergic postsynaptic conductances (PSgs) collected during the different feedback states of the daily surge model into GnRH neurons from OVX, OVX+E AM, and OVX+E PM mice. PSg trains mimicking positive feedback initiated more action potentials in cells from OVX+E PM mice than negative feedback or OVX (open feedback loop) trains in all three animal models, but the positive-feedback train was most effective when applied to cells during positive feedback. In silico studies of model GnRH neurons in which >1000 PSg trains were tested exhibited the same results. These observations support the hypothesis that GnRH neurons integrate fast-synaptic and intrinsic changes to increase firing rates during positive feedback.SIGNIFICANCE STATEMENT Infertility affects 15%-20% of couples; failure to ovulate is a common cause. Understanding how the brain controls ovulation is critical for new developments in both infertility treatment and contraception. Ovarian estradiol alters both the intrinsic properties of gonadotropin-releasing hormone (GnRH) neurons and synaptic inputs to these cells coincident with production of sustained GnRH release that ultimately triggers ovulation. We demonstrate here using dynamic clamp and mathematical modeling that estradiol-induced shifts in synaptic transmission alone can increase firing output, but that the intrinsic properties of GnRH neurons during positive feedback further poise these cells for increased response to higher frequency synaptic transmission. These data suggest that GnRH neurons integrate fast-synaptic and intrinsic changes to increase firing rates during the preovulatory GnRH surge.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Modulation of Gonadotropin-Releasing Hormone Neuron Activity and Secretion in Mice by Non-peptide Neurotransmitters, Gasotransmitters, and Gliotransmitters

Gonadotropin-releasing hormone (GnRH) neuron activity and GnRH secretion are essential for fertility in mammals. Here, I review findings from mouse studies on the direct modulation of GnRH neuron activity and GnRH secretion by non-peptide neurotransmitters (GABA, glutamate, dopamine, serotonin, norepinephrine, epinephrine, histamine, ATP, adenosine, and acetylcholine), gasotransmitters (nitric oxide and carbon monoxide), and gliotransmitters (prostaglandin E2 and possibly GABA, glutamate, and ATP). These neurotransmitters, gasotransmitters, and gliotransmitters have been shown to directly modulate activity and/or GnRH secretion in GnRH neurons in vivo or ex vivo (brain slices), from postnatal through adult mice, or in embryonic or immortalized mouse GnRH neurons. However, except for GABA, nitric oxide, and prostaglandin E2, which appear to be essential for normal GnRH neuron activity, GnRH secretion, and fertility in males and/or females, the biological significance of their direct modulation of GnRH neuron activity and/or GnRH secretion in the central regulation of reproduction remains largely unknown and requires further exploration.

Mapping GABA and glutamate inputs to gonadotrophin-releasing hormone neurones in male and female mice

Gonadotrophin-releasing hormone (GnRH) neurone function is dependent upon gonadal steroid hormone feedback, which is communicated in large part through an afferent neuronal network. The classical neurotransmitters GABA and glutamate are important regulators of GnRH neurone activity and are implicated in mediating feedback signals. In the present study, we aimed to determine whether GABAergic or glutamatergic input to GnRH neurones differs between males and females and/or exhibits morphological plasticity in response to steroid hormone feedback in females. Tissue collected from GnRH-green fluorescent protein (GFP) male and female mice in dioestrus underwent immunofluorescence labelling of GFP and either the vesicular GABA transporter (VGAT) or the vesicular glutamate transporter 2 (VGLUT2). No differences in the densities or absolute numbers of VGAT-immunoreactive (-IR) or VGLUT2-IR puncta apposed to GnRH neurones were identified between males and females. The most significant input from either neurotransmitter was to the proximal dendritic region and 80% of VGAT-IR puncta apposed to GnRH neurones co-localised with synaptophysin. Putative inputs were also assessed in ovariectomised (OVX) female mice treated with negative (OVX+E) or positive (OVX+E+E) feedback levels of oestrogen, and OVX+E+E mice were killed during the expected GnRH/luteinising hormone surge. No differences in VGLUT2-IR contacts to GnRH neurones were identified between animals under the negative-feedback influence of oestrogen (OVX+E) or the positive influence of oestrogen (OVX+E+E), regardless of cFos activation status. By contrast, a significant elevation in putative GABAergic inputs to GnRH neurones at the time of the preovulatory surge was found in the cFos-negative subset of GnRH neurones, both at the level of the soma and at the proximal dendrite. Taken together, these data suggest that, although GABAergic and glutamatergic innervation of GnRH neurones is not sexually differentiated, cyclic fluctuations in steroid hormone feedback over the female oestrous cycle result in plastic changes in GABAergic inputs to a subpopulation of GnRH neurones.

Changes in GABAergic Transmission to and Intrinsic Excitability of Gonadotropin-Releasing Hormone (GnRH) Neurons during the Estrous Cycle in Mice

Gonadotropin-releasing hormone (GnRH) neurons form the final common central output pathway controlling fertility and are regulated by steroid feedback. In females, estradiol feedback action varies between negative and positive; negative feedback typically regulates episodic GnRH release whereas positive feedback initiates a surge of GnRH, and subsequently luteinizing hormone (LH) release ultimately triggering ovulation. During the estrous cycle, changes between estradiol negative and positive feedback occur with cycle stage and time of day, with positive feedback in the late afternoon of proestrus in nocturnal species. To test the hypotheses that synaptic and intrinsic properties of GnRH neurons are regulated by cycle stage and time of day, we performed whole-cell patch-clamp studies of GnRH neurons in brain slices from mice at two times considered negative feedback (diestrous PM and proestrous AM) and during positive feedback (proestrous PM). GABAergic transmission can excite GnRH neurons and was higher in cells from proestrous PM mice than cells from proestrous AM mice and approached traditional significance levels relative to cells from diestrous PM mice. Action potential response to current injection was also greater in cells from proestrous PM mice than the other two groups. Interestingly, the hormonal milieu of proestrous AM provided stronger negative feedback on both GnRH neuron excitability and GABAergic postsynaptic current (PSC) amplitude than diestrous PM. These observations demonstrate elements of both synaptic and intrinsic properties of GnRH neurons are regulated in a cycle-dependent manner and provide insight into the neurobiological mechanisms underlying cyclic changes in neuroendocrine function among states of estradiol negative and positive feedback.

Dynamic and Sex-Specific Changes in Gonadotropin-Releasing Hormone Neuron Activity and Excitability in a Mouse Model of Temporal Lobe Epilepsy

Reproductive endocrine disorders are prominent comorbidities of temporal lobe epilepsy (TLE) in both men and women. The neural mechanisms underlying these comorbidities remain unclear, but hypothalamic gonadotropin-releasing hormone (GnRH) neurons may be involved. Here, we report the first direct demonstrations of aberrant GnRH neuron function in an animal model of epilepsy. Recordings of GnRH neuron firing and excitability were made in acute mouse brain slices prepared two months after intrahippocampal injection of kainate (KA) or control saline, a well-established TLE model in which most females develop comorbid estrous cycle disruption. GnRH neurons from control females showed elevated firing and excitability on estrus compared with diestrus. By contrast, cells from KA-injected females that developed prolonged, disrupted estrous cycles (KA-long) showed the reverse pattern. Firing rates of cells from KA-injected females that maintained regular cycles (KA-regular) were not different from controls on diestrus, but were reduced on estrus. In KA-injected males, only GnRH neurons in the medial septum displayed elevated firing. In contrast to the diestrus versus estrus and sex-specific changes in firing, GnRH neuron intrinsic excitability was elevated in all KA-injected groups, indicating a role for afferent synaptic and neuromodulatory inputs in shaping overall changes in firing activity. Furthermore, KA-injected females showed cycle-stage-specific changes in circulating sex steroids on diestrus and estrus that also differed between KA-long and KA-regular groups. Together, these findings reveal that the effects of epilepsy on the neural control of reproduction are dynamic across the estrous cycle, distinct in association with comorbid estrous cycle disruption severity, and sex-specific.

Estradiol Increases Glutamate and GABA Neurotransmission into GnRH Neurons via Retrograde NO-Signaling in Proestrous Mice during the Positive Estradiol Feedback Period

Surge release of gonadotropin-releasing hormone (GnRH) is essential in the activation of pituitary gonadal unit at proestrus afternoon preceded by the rise of serum 17β-estradiol (E2) level during positive feedback period. Here, we describe a mechanism of positive estradiol feedback regulation acting directly on GnRH-green fluorescent protein (GFP) neurons of mice. Whole-cell clamp and loose patch recordings revealed that a high physiological dose of estradiol (200 pM), significantly increased firing rate at proestrus afternoon. The mPSC frequency at proestrus afternoon also increased, whereas it decreased at metestrus afternoon and had no effect at proestrus morning. Inhibition of the estrogen receptor β (ERβ), intracellular blockade of the Src kinase and phosphatidylinositol 3 kinase (PI3K) and scavenge of nitric oxide (NO) inside GnRH neurons prevented the facilitatory estradiol effect indicating involvement of the ERβ/Src/PI3K/Akt/nNOS pathway in this fast, direct stimulatory effect. Immunohistochemistry localized soluble guanylate cyclase, the main NO receptor, in both glutamatergic and GABAergic terminals innervating GnRH neurons. Accordingly, estradiol facilitated neurotransmissions to GnRH neurons via both GABA_A-R and glutamate/AMPA/kainate-R. These results indicate that estradiol acts directly on GnRH neurons via the ERβ/Akt/nNOS pathway at proestrus afternoon generating NO that retrogradely accelerates GABA and glutamate release from the presynaptic terminals contacting GnRH neurons. The newly explored mechanism might contribute to the regulation of the GnRH surge, a fundamental prerequisite of the ovulation.

Dominant Neuropeptide Cotransmission in Kisspeptin-GABA Regulation of GnRH Neuron Firing Driving Ovulation

A population of kisspeptin-GABA coexpressing neurons located in the rostral periventricular area of the third ventricle (RP3V) is believed to activate gonadotropin-releasing hormone (GnRH) neurons to generate the luteinizing hormone (LH) surge triggering ovulation. Selective optogenetic activation of RP3V kisspeptin (RP3V^KISS) neurons in female mice for >30 s and ≥10 Hz in either a continuous or bursting mode was found to reliably generate a delayed and long-lasting activation of GnRH neuron firing in brain slices. Optogenetic activation of RP3V^KISS neurons in vivo at 10 Hz generated substantial increments in LH secretion of similar amplitude to the endogenous LH surge. Studies using GABA_A receptor antagonists and optogenetic activation of RP3V GABA (RP3V^GABA) neurons in vitro revealed that low-frequency (2 Hz) stimulation generated immediate and transient GABA_A receptor-mediated increases in GnRH neuron firing, whereas higher frequencies (10 Hz) recruited the long-lasting activation observed following RP3V^KISS neuron stimulation. In vivo, 2 Hz activation of RP3V^GABA neurons did not alter LH secretion, whereas 10 Hz stimulation evoked a sustained large increase in LH identical to RP3V^KISS neuron activation. Optogenetic activation of RP3V^KISS neurons in which kisspeptin had been deleted did not alter LH secretion. These studies demonstrate the presence of parallel transmission streams from RP3V neurons to GnRH neurons that are frequency dependent and temporally distinct. This comprises a rapid and transient GABA_A receptor-mediated activation and a slower onset kisspeptin-mediated stimulation of long duration. At the time of the LH surge, GABA release appears to be functionally redundant with the neuropeptide kisspeptin being the dominant cotransmitter influencing GnRH neuron output.SIGNIFICANCE STATEMENT Miscommunication between the brain and ovaries is thought to represent a major cause of infertility in humans. Studies in rodents suggest that a population of neurons located in the rostral periventricular area of the third ventricle (RP3V) are critical for activating the gonadotropin-releasing hormone (GnRH) neurons that trigger ovulation. The present study provides evidence that an RP3V neuron population coexpressing kisspeptin and GABA provides a functionally important excitatory input to GnRH neurons at the time of ovulation. This neural input releases GABA and/or kisspeptin in the classical frequency dependent and temporally distinct nature of amino acid-neuropeptide cotransmission. Unusually, however, the neuropeptide stream is found to be functionally dominant in activating GnRH neurons at the time of ovulation.

Morphological Analysis of the Axonal Projections of EGFP-Labeled Esr1-Expressing Neurons in Transgenic Female Medaka

Some hypothalamic neurons expressing estrogen receptor α (Esr1) are thought to transmit a gonadal estrogen feedback signal to gonadotropin-releasing hormone 1 (GnRH1) neurons, which is the final common pathway for feedback regulation of reproductive functions. Moreover, estrogen-sensitive neurons are suggested to control sexual behaviors in coordination with reproduction. In mammals, hypothalamic estrogen-sensitive neurons release the peptide kisspeptin and regulate GnRH1 neurons. However, a growing body of evidence in nonmammalian species casts doubt on the regulation of GnRH1 neurons by kisspeptin neurons. As a step toward understanding how estrogen regulates neuronal circuits for reproduction and sex behavior in vertebrates in general, we generated a transgenic (Tg) medaka that expresses enhanced green fluorescent protein (EGFP) specifically in esr1-expressing neurons (esr1 neurons) and analyzed their axonal projections. We found that esr1 neurons in the preoptic area (POA) project to the gnrh1 neurons. We also demonstrated by transcriptome and histological analyses that these esr1 neurons are glutamatergic or γ-aminobutyric acidergic (GABAergic) but not kisspeptinergic. We therefore suggest that glutamatergic and GABAergic esr1 neurons in the POA regulate gnrh1 neurons. This hypothesis is consistent with previous studies in mice that found that glutamatergic and GABAergic transmission is critical for estrogen-dependent changes in GnRH1 neuron firing. Thus, we propose that this neuronal circuit may provide an evolutionarily conserved mechanism for regulation of reproduction. In addition, we showed that telencephalic esr1 neurons project to medulla, which may control sexual behavior. Moreover, we found that some POA-esr1 neurons coexpress progesterone receptors. These neurons may form the neuronal circuits that regulate reproduction and sex behavior in response to the serum estrogen/progesterone.

Gonadotropin-Releasing Hormone (GnRH) Neuron Excitability Is Regulated by Estradiol Feedback and Kisspeptin

Gonadotropin-releasing hormone (GnRH) neurons produce the central output controlling fertility and are regulated by steroid feedback. A switch from estradiol negative to positive feedback initiates the GnRH surge, ultimately triggering ovulation. This occurs on a daily basis in ovariectomized, estradiol-treated (OVX+E) mice; GnRH neurons are suppressed in the morning and activated in the afternoon. To test the hypotheses that estradiol and time of day signals alter GnRH neuron responsiveness to stimuli, GFP-identified GnRH neurons in brain slices from OVX+E or OVX female mice were recorded during the morning or afternoon. No differences were observed in baseline membrane potential. Current-clamp revealed GnRH neurons fired more action potentials in response to current injection during positive feedback relative to all other groups, which were not different from each other despite reports of differing ionic conductances. Kisspeptin increased GnRH neuron response in cells from OVX and OVX+E mice in the morning but not afternoon. Paradoxically, excitability in kisspeptin knock-out mice was similar to the maximum observed in control mice but was unchanged by time of day or estradiol. A mathematical model applying a Markov Chain Monte Carlo method to estimate probability distributions for estradiol- and time of day-dependent parameters was used to predict intrinsic properties underlying excitability changes. A single identifiable distribution of solutions accounted for similar GnRH neuron excitability in all groups other than positive feedback despite different underlying conductance properties; this was attributable to interdependence of voltage-gated potassium channel properties. In contrast, redundant solutions may explain positive feedback, perhaps indicative of the importance of this state for species survival.SIGNIFICANCE STATEMENT Infertility affects 15%-20% of couples; failure to ovulate is a common cause. Understanding how the brain controls ovulation is critical for new developments in both infertility treatment and contraception. Gonadotropin-releasing hormone (GnRH) neurons are the final common pathway for central neural control of ovulation. We studied how estradiol feedback regulates GnRH excitability, a key determinant of neural firing rate using laboratory and computational approaches. GnRH excitability is upregulated during positive feedback, perhaps driving increased neural firing rate at this time. Kisspeptin increased GnRH excitability and was essential for estradiol regulation of excitability. Modeling predicts that multiple combinations of changes to GnRH intrinsic conductances can produce the firing response in positive feedback, suggesting the brain has many ways to induce ovulation.

Prepubertal Development of GABAergic Transmission to Gonadotropin-Releasing Hormone (GnRH) Neurons and Postsynaptic Response Are Altered by Prenatal Androgenization

Gonadotropin-releasing hormone (GnRH) neurons regulate reproduction through pulsatile GnRH release. Women with polycystic ovary syndrome (PCOS) have persistently elevated luteinizing hormone release frequency, reflecting GnRH release; this exacerbates hyperandrogenemia and disrupted reproductive cycles that are characteristic of this disorder. Clinical evidence suggests that neuroendocrine features of PCOS may manifest peripubertally. Adult mice prenatally exposed to androgens (PNA) mimic several reproductive features of PCOS. GnRH neurons from these mice have increased firing activity and receive increased GABAergic transmission, which is excitatory. When changes emerge during development is unknown. To study the typical postnatal development of GABAergic transmission and the effects of PNA treatment and sex, whole-cell voltage-clamp recordings were made of GABAergic postsynaptic currents (PSCs) in GnRH neurons in brain slices from prepubertal through adult control and PNA female and male mice. GABAergic transmission was present by 1 week of age in females and males and increased in frequency, reaching adult levels at 3 and 4 weeks, respectively. GABAergic PSC frequency was elevated in 3-week-old PNA versus control females. PSC frequency in both controls and PNA mice was activity independent, suggesting that PNA induces changes in synapse organization. PNA also alters the functional response of GnRH neurons to GABA. GABA induced firing in fewer neurons from 3-week-old PNA than control females; membrane potential depolarization induced by GABA was also reduced in cells from PNA mice at this age. PNA thus induces changes during development in the presynaptic organization of the GABAergic network afferent to GnRH neurons as well as the postsynaptic GnRH neuron response, both of which may contribute to adult reproductive dysfunction.SIGNIFICANCE STATEMENT The central neuronal network that regulates reproduction is overactive in polycystic ovary syndrome (PCOS), a leading cause of infertility. Recent evidence of neuroendocrine dysfunction in midpubertal girls suggests that the pathophysiological mechanisms underlying PCOS may arise before pubertal maturation. Prenatal exposure to androgens (PNA) in mice mimics several neuroendocrine features of PCOS. GABAergic transmission to gonadotropin-releasing hormone (GnRH) neurons is important for reproduction and is increased in adult PNA mice. The typical development of this network and when changes with PNA and sex arise relative to puberty are unknown. These studies provide evidence that PNA alters prepubertal development of the GABAergic network afferent to GnRH neurons, including both the presynaptic organization and postsynaptic response. These changes may contribute to reproductive dysfunction in adults.

Estradiol-Dependent Stimulation and Suppression of Gonadotropin-Releasing Hormone Neuron Firing Activity by Corticotropin-Releasing Hormone in Female Mice

Gonadotropin-releasing hormone (GnRH) neurons are the final central regulators of reproduction, integrating various inputs that modulate fertility. Stress typically inhibits reproduction but can be stimulatory; stress effects can also be modulated by steroid milieu. Corticotropin-releasing hormone (CRH) released during the stress response may suppress reproduction independent of downstream glucocorticoids. We hypothesized CRH suppresses fertility by decreasing GnRH neuron firing activity. To test this, mice were ovariectomized (OVX) and either implanted with an estradiol capsule (OVX+E) or not treated further to examine the influence of estradiol on GnRH neuron response to CRH. Targeted extracellular recordings were used to record firing activity from green fluorescent protein-identified GnRH neurons in brain slices before and during CRH treatment; recordings were done in the afternoon when estradiol has a positive feedback effect to increase GnRH neuron firing. In OVX mice, CRH did not affect the firing rate of GnRH neurons. In contrast, CRH exhibited dose-dependent stimulatory (30 nM) or inhibitory (100 nM) effects on GnRH neuron firing activity in OVX+E mice; both effects were reversible. The dose-dependent effects of CRH appear to result from activation of different receptor populations; a CRH receptor type-1 agonist increased firing activity in GnRH neurons, whereas a CRH receptor type-2 agonist decreased firing activity. CRH and specific agonists also differentially regulated short-term burst frequency and burst properties, including burst duration, spikes/burst, and/or intraburst interval. These results indicate that CRH alters GnRH neuron activity and that estradiol is required for CRH to exert both stimulatory and inhibitory effects on GnRH neurons.

Dynamics of GnRH Neuron Ionotropic GABA and Glutamate Synaptic Receptors Are Unchanged during Estrogen Positive and Negative Feedback in Female Mice

Inputs from GABAergic and glutamatergic neurons are suspected to play an important role in regulating the activity of the gonadotropin-releasing hormone (GnRH) neurons. The GnRH neurons exhibit marked plasticity to control the ovarian cycle with circulating estradiol concentrations having profound "feedback" effects on their activity. This includes "negative feedback" responsible for suppressing GnRH neuron activity and "positive feedback" that occurs at mid-cycle to activate the GnRH neurons to generate the preovulatory luteinizing hormone surge. In the present study, we employed brain slice electrophysiology to question whether synaptic ionotropic GABA and glutamate receptor signaling at the GnRH neuron changed at times of negative and positive feedback. We used a well characterized estradiol (E)-treated ovariectomized (OVX) mouse model to replicate negative and positive feedback. Miniature and spontaneous postsynaptic currents (mPSCs and sPSCs) attributable to GABA_A and glutamatergic receptor signaling were recorded from GnRH neurons obtained from intact diestrous, OVX, OVX + E (negative feedback), and OVX + E+E (positive feedback) female mice. Approximately 90% of GnRH neurons exhibited spontaneous GABA_A-mPSCs in all groups but no significant differences in the frequency or kinetics of mPSCs were found at the times of negative or positive feedback. Approximately 50% of GnRH neurons exhibited spontaneous glutamate mPSCs but again no differences were detected. The same was true for spontaneous PSCs in all cases. These observations indicate that the kinetics of ionotropic GABA and glutamate receptor synaptic transmission to GnRH neurons remain stable across the different estrogen feedback states.

The physiology of functional hypothalamic amenorrhea associated with energy deficiency in exercising women and in women with anorexia nervosa

An energy deficiency is the result of inadequate energy intake relative to high energy expenditure. Often observed with the development of an energy deficiency is a high drive for thinness, dietary restraint, and weight and shape concerns in association with eating behaviors. At a basic physiologic level, a chronic energy deficiency promotes compensatory mechanisms to conserve fuel for vital physiologic function. Alterations have been documented in resting energy expenditure (REE) and metabolic hormones. Observed metabolic alterations include nutritionally acquired growth hormone resistance and reduced insulin-like growth factor-1 (IGF-1) concentrations; hypercortisolemia; increased ghrelin, peptide YY, and adiponectin; and decreased leptin, triiodothyronine, and kisspeptin. The cumulative effect of the energetic and metabolic alterations is a suppression of the hypothalamic-pituitary-ovarian axis. Gonadotropin releasing hormone secretion is decreased with consequent suppression of luteinizing hormone and follicle stimulating hormone release. Alterations in hypothalamic-pituitary secretion alters the production of estrogen and progesterone resulting in subclinical or clinical menstrual dysfunction.

Glucagon-Like Peptide-1 Excites Firing and Increases GABAergic Miniature Postsynaptic Currents (mPSCs) in Gonadotropin-Releasing Hormone (GnRH) Neurons of the Male Mice via Activation of Nitric Oxide (NO) and Suppression of Endocannabinoid Signaling Pathways

Glucagon-like peptide-1 (GLP-1), a metabolic signal molecule, regulates reproduction, although, the involved molecular mechanisms have not been elucidated, yet. Therefore, responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the GLP-1 analog Exendin-4 and elucidation of molecular pathways acting downstream to the GLP-1 receptor (GLP-1R) have been challenged. Loose patch-clamp recordings revealed that Exendin-4 (100 nM-5 μM) elevated firing rate in hypothalamic GnRH-GFP neurons of male mice via activation of GLP-1R. Whole-cell patch-clamp measurements demonstrated increased excitatory GABAergic miniature postsynaptic currents (mPSCs) frequency after Exendin-4 administration, which was eliminated by the GLP-1R antagonist Exendin-3(9-39) (1 μM). Intracellular application of the G-protein inhibitor GDP-β-S (2 mM) impeded action of Exendin-4 on mPSCs, suggesting direct excitatory action of GLP-1 on GnRH neurons. Blockade of nitric-oxide (NO) synthesis by Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME; 100 μM) or N(5)-[Imino(propylamino)methyl]-L-ornithine hydrochloride (NPLA; 1 μM) or intracellular scavenging of NO by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO; 1 mM) partially attenuated the excitatory effect of Exendin-4. Similar partial inhibition was achieved by hindering endocannabinoid pathway using cannabinoid receptor type-1 (CB1) inverse-agonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-(1-piperidyl) pyrazole-3-carboxamide (AM251; 1 μM). Simultaneous blockade of NO and endocannabinoid signaling mechanisms eliminated action of Exendin-4 suggesting involvement of both retrograde machineries. Intracellular application of the transient receptor potential vanilloid 1 (TRPV1)-antagonist 2E-N-(2, 3-Dihydro-1,4-benzodioxin-6-yl)-3-[4-(1, 1-dimethylethyl)phenyl]-2-Propenamide (AMG9810; 10 μM) or the fatty acid amide hydrolase (FAAH)-inhibitor PF3845 (5 μM) impeded the GLP-1-triggered endocannabinoid pathway indicating an anandamide-TRPV1-sensitive control of 2-arachidonoylglycerol (2-AG) production. Furthermore, GLP-1 immunoreactive (IR) axons innervated GnRH neurons in the hypothalamus suggesting that GLP-1 of both peripheral and neuronal sources can modulate GnRH neurons. RT-qPCR study confirmed the expression of GLP-1R and neuronal NO synthase (nNOS) mRNAs in GnRH-GFP neurons. Immuno-electron microscopic analysis revealed the presence of nNOS protein in GnRH neurons. These results indicate that GLP-1 exerts direct facilitatory actions via GLP-1R on GnRH neurons and modulates NO and 2-AG retrograde signaling mechanisms that control the presynaptic excitatory GABAergic inputs to GnRH neurons.

The neuroendocrine genesis of polycystic ovary syndrome: A role for arcuate nucleus GABA neurons

Polycystic ovary syndrome (PCOS) is a prevalent and distressing endocrine disorder lacking a clearly identified aetiology. Despite its name, PCOS may result from impaired neuronal circuits in the brain that regulate steroid hormone feedback to the hypothalamo-pituitary-gonadal axis. Ovarian function in all mammals is controlled by the gonadotropin-releasing hormone (GnRH) neurons, a small group of neurons that reside in the pre-optic area of the hypothalamus. GnRH neurons drive the secretion of the gonadotropins from the pituitary gland that subsequently control ovarian function, including the production of gonadal steroid hormones. These hormones, in turn, provide important feedback signals to GnRH neurons via a hormone sensitive neuronal network in the brain. In many women with PCOS this feedback pathway is impaired, resulting in the downstream consequences of the syndrome. This review will explore what is currently known from clinical and animal studies about the identity, relative contribution and significance of the individual neuronal components within the GnRH neuronal network that contribute to the pathophysiology of PCOS. We review evidence for the specific neuronal pathways hypothesised to mediate progesterone negative feedback to GnRH neurons, and discuss the potential mechanisms by which androgens may evoke disruptions in these circuits at different developmental time points. Finally, this review discusses data providing compelling support for disordered progesterone-sensitive GABAergic input to GnRH neurons, originating specifically within the arcuate nucleus in prenatal androgen induced forms of PCOS.

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.

Expression of ESR1 in Glutamatergic and GABAergic Neurons Is Essential for Normal Puberty Onset, Estrogen Feedback, and Fertility in Female Mice

Circulating estradiol exerts a profound influence on the activity of the gonadotropin-releasing hormone (GnRH) neuronal network controlling fertility. Using genetic strategies enabling neuron-specific deletion of estrogen receptor α (Esr1), we examine here whether estradiol-modulated GABA and glutamate transmission are critical for the functioning of the GnRH neuron network in the female mouse. Using Vgat- and Vglut2-ires-Cre knock-in mice and ESR1 immunohistochemistry, we demonstrate that subpopulations of GABA and glutamate neurons throughout the limbic forebrain express ESR1, with ESR1-GABAergic neurons being more widespread and numerous than ESR1-glutamatergic neurons. We crossed Vgat- and Vglut2-ires-Cre mice with an Esr1(lox/lox) line to generate animals with GABA-neuron-specific or glutamate-neuron-specific deletion of Esr1. Vgat-ires-Cre;Esr1(lox/lox) mice were infertile, with abnormal estrous cycles, and exhibited a complete failure of the estrogen positive feedback mechanism responsible for the preovulatory GnRH surge. However, puberty onset and estrogen negative feedback were normal. Vglut2-ires-Cre;Esr1(lox/lox) mice were also infertile but displayed a wider range of deficits, including advanced puberty onset, abnormal negative feedback, and abolished positive feedback. Whereas <25% of preoptic kisspeptin neurons expressed Cre in Vgat- and Vglut2-ires-Cre lines, ∼70% of arcuate kisspeptin neurons were targeted in Vglut2-ires-Cre;Esr1(lox/lox) mice, possibly contributing to their advanced puberty phenotype. These observations show that, unexpectedly, ESR1-GABA neurons are only essential for the positive feedback mechanism. In contrast, we reveal the key importance of ESR1 in glutamatergic neurons for multiple estrogen feedback loops within the GnRH neuronal network required for fertility in the female mouse.

Hypothalamic gonadotropin-releasing hormone (GnRH) receptor neurons fire in synchrony with the female reproductive cycle

Gonadotropin-releasing hormone (GnRH) controls mammalian reproduction via the hypothalamic-pituitary-gonadal (hpg) axis, acting on gonadotrope cells in the pituitary gland that express the GnRH receptor (GnRHR). Cells expressing the GnRHR have also been identified in the brain. However, the mechanism by which GnRH acts on these potential target cells remains poorly understood due to the difficulty of visualizing and identifying living GnRHR neurons in the central nervous system. We have developed a mouse strain in which GnRHR neurons express a fluorescent marker, enabling the reliable identification of these cells independent of the hormonal status of the animal. In this study, we analyze the GnRHR neurons of the periventricular hypothalamic nucleus in acute brain slices prepared from adult female mice. Strikingly, we find that the action potential firing pattern of these neurons alternates in synchrony with the estrous cycle, with pronounced burst firing during the preovulatory period. We demonstrate that GnRH stimulation is sufficient to trigger the conversion from tonic to burst firing in GnRHR neurons. Furthermore, we show that this switch in the firing pattern is reversed by a potent GnRHR antagonist. These data suggest that endogenous GnRH acts on GnRHR neurons and triggers burst firing in these cells during late proestrus and estrus. Our data have important clinical implications in that they indicate a novel mode of action for GnRHR agonists and antagonists in neurons of the central nervous system that are not part of the classical hpg axis.

Positive, but not negative feedback actions of estradiol in adult female mice require estrogen receptor α in kisspeptin neurons

Hypothalamic kisspeptin (Kiss1) neurons express estrogen receptor α (ERα) and exert control over GnRH/LH secretion in female rodents. It has been proposed that estradiol (E2) activation of ERα in kisspeptin neurons in the arcuate nucleus (ARC) suppresses GnRH/LH secretion (negative feedback), whereas E2 activation of ERα in kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) mediates the release of preovulatory GnRH/LH surges (positive feedback). To test these hypotheses, we generated mice bearing kisspeptin cell-specific deletion of ERα (KERαKO) and treated them with E2 regimens that evoke either negative or positive feedback actions on GnRH/LH secretion. Using negative feedback regimens, as expected, E2 effectively suppressed LH levels in ovariectomized (OVX) wild-type (WT) mice to the levels seen in ovary-intact mice. Surprisingly, however, despite the fact that E2 regulation of Kiss1 mRNA expression was abrogated in both the ARC and AVPV of KERαKO mice, E2 also effectively decreased LH levels in OVX KERαKO mice to the levels seen in ovary-intact mice. Conversely, using a positive feedback regimen, E2 stimulated LH surges in WT mice, but had no effect in KERαKO mice. These experiments clearly demonstrate that ERα in kisspeptin neurons is required for the positive, but not negative feedback actions of E2 on GnRH/LH secretion in adult female mice. It remains to be determined whether the failure of KERαKO mice to exhibit GnRH/LH surges reflects the role of ERα in the development of kisspeptin neurons, in the active signaling processes leading to the release of GnRH/LH surges, or both.

GABAergic transmission to kisspeptin neurons is differentially regulated by time of day and estradiol in female mice

Gonadotropin-releasing hormone (GnRH) secretion is regulated by estradiol feedback. This feedback switches from negative to positive in females; this switch depends on time of day in many species. Estradiol feedback is likely conveyed via afferents. Kisspeptin neurons of the arcuate nucleus and anteroventral-periventricular region (AVPV) may differentially regulate GnRH neurons during negative and positive feedback, respectively. We tested estradiol and time of day regulation of GABAergic transmission and postsynaptic response to GABA in these two populations using transgenic mice with GFP-identified kisspeptin neurons. Ovariectomized (OVX) mice treated or not with estradiol (E) were studied in the AM (negative feedback) or PM (positive feedback). GABAA receptor reversal potential was unaffected by time of day or estradiol. GABA depolarized the membrane potential of arcuate neurons from OVX+E mice; this response was blunted in cells from OVX mice. GABA hyperpolarized AVPV kisspeptin neurons, except in the OVX PM group in which GABA did not alter membrane potential attributable to a PM hyperpolarization of baseline membrane potential. In both kisspeptin neuron populations from OVX mice, the frequency of GABAergic spontaneous postsynaptic currents was increased in the PM; this increase was blunted by estradiol. In arcuate, but not AVPV, kisspeptin neurons, estradiol reduced miniature postsynaptic current amplitude independent of time of day. Using nonstationary fluctuation analysis and diazepam to manipulate GABAA receptor apparent affinity, the decrease in arcuate miniature postsynaptic current amplitude was attributed to decreased number of receptors bound by GABA. Time of day and estradiol feedback thus both target presynaptic and postsynaptic mechanisms to differentially regulate kisspeptin neurons via GABAergic transmission.

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.

The role of GABA in the regulation of GnRH neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. Gamma-amino butyric acid (GABA) has long been implicated as one of the major players in the regulation of GnRH neurons. Although GABA is typically an inhibitory neurotransmitter in the mature adult central nervous system, most mature GnRH neurons show the unusual characteristic of being excited by GABA. While many reports have provided much insight into the contribution of GABA to the activity of GnRH neurons, the precise physiological role of the excitatory action of GABA on GnRH neurons remains elusive. This brief review presents the current knowledge of the role of GABA signaling in GnRH neuronal activity. We also discuss the modulation of GABA signaling by neurotransmitters and neuromodulators and the functional consequence of GABAergic inputs to GnRH neurons in both the physiology and pathology of reproduction.

Increased use-dependent plasticity in chronic insomnia

STUDY OBJECTIVES

During normal sleep several neuroplasticity changes occur, some of which are considered to be fundamental to strengthen memories. Given the evidence linking sleep to neuroplasticity, it is conceivable that individuals with chronic sleep disruption, such as patients with chronic insomnia (CI), would experience abnormalities in neuroplastic processes during daytime. Protocols testing use-dependent plasticity (UDP), one of the mechanisms underlying formation of motor memories traces, provide a sensitive measure to assess neuroplasticity in the context of motor training.

DESIGN AND PARTICIPANTS

A well-established transcranial magnetic stimulation (TMS) paradigm was used to evaluate the ability of patients with CI and age-matched good sleeper controls to undergo UDP. We also investigated the effect of insomnia on intracortical motor excitability measures reflecting GABAergic and glutamatergic mechanisms.

SETTING

Human Brain Physiology Laboratory, Johns Hopkins Medical Institutions.

MEASUREMENTS AND RESULTS

We found that patients with CI experienced increased UDP changes relative to controls. This effect was not due to differences in motor training. In addition, patients with CI showed enhanced intracortical facilitation relative to controls, in the absence of changes in intracortical inhibitory measures.

CONCLUSION

This study provides the first evidence that patients with chronic insomnia have an increased plasticity response to physical exercise, possibly due to larger activation of glutamatergic mechanisms. This suggests a heightened state of neuroplasticity, which may reflect a form of maladaptive plasticity, similar to what has been described in dystonia patients and chronic phantom pain after amputation. These results could lead to development of novel treatments for chronic insomnia.

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.

Neurobiological study of fish brains gives insights into the nature of gonadotropin-releasing hormone 1-3 neurons

Accumulating evidence suggests that up to three different molecular species of GnRH peptides encoded by different paralogs of gnrh genes are expressed by anatomically distinct groups of GnRH neurons in the brain of one vertebrate species. They are called gnrh1, gnrh2, and gnrh3. Recent evidence from molecular, anatomical, and physiological experiments strongly suggests that each GnRH system functions differently. Here, we review recent advancement in the functional studies of the three different GnRH neuron systems, mainly focusing on the electrophysiological analysis of the GnRH-green fluorescent protein (GFP) transgenic animals. The introduction of GFP-transgenic animals for the electrophysiological analysis of GnRH neurons greatly advanced our knowledge on their anatomy and electrophysiology, especially of gnrh1 neurons, which has long defied detailed electrophysiological analysis of single neurons because of their small size and scattered distribution. Based on the results of recent studies, we propose that different electrophysiological properties, especially the spontaneous patterns of electrical activities and their time-dependent changes, and the axonal projections characterize the different functions of GnRH1-3 neurons; GnRH1 neurons act as hypophysiotropic neuroendocrine regulators, and GnRH2 and GnRH3 neurons act as neuromodulators in wide areas of the brain.

Ghrelin decreases firing activity of gonadotropin-releasing hormone (GnRH) neurons in an estrous cycle and endocannabinoid signaling dependent manner

The orexigenic peptide, ghrelin is known to influence function of GnRH neurons, however, the direct effects of the hormone upon these neurons have not been explored, yet. The present study was undertaken to reveal expression of growth hormone secretagogue receptor (GHS-R) in GnRH neurons and elucidate the mechanisms of ghrelin actions upon them. Ca²⁺-imaging revealed a ghrelin-triggered increase of the Ca²⁺-content in GT1-7 neurons kept in a steroid-free medium, which was abolished by GHS-R-antagonist JMV2959 (10µM) suggesting direct action of ghrelin. Estradiol (1nM) eliminated the ghrelin-evoked rise of Ca²⁺-content, indicating the estradiol dependency of the process. Expression of GHS-R mRNA was then confirmed in GnRH-GFP neurons of transgenic mice by single cell RT-PCR. Firing rate and burst frequency of GnRH-GFP neurons were lower in metestrous than proestrous mice. Ghrelin (40nM-4μM) administration resulted in a decreased firing rate and burst frequency of GnRH neurons in metestrous, but not in proestrous mice. Ghrelin also decreased the firing rate of GnRH neurons in males. The ghrelin-evoked alterations of the firing parameters were prevented by JMV2959, supporting the receptor-specific actions of ghrelin on GnRH neurons. In metestrous mice, ghrelin decreased the frequency of GABAergic mPSCs in GnRH neurons. Effects of ghrelin were abolished by the cannabinoid receptor type-1 (CB1) antagonist AM251 (1µM) and the intracellularly applied DAG-lipase inhibitor THL (10µM), indicating the involvement of retrograde endocannabinoid signaling. These findings demonstrate that ghrelin exerts direct regulatory effects on GnRH neurons via GHS-R, and modulates the firing of GnRH neurons in an ovarian-cycle and endocannabinoid dependent manner.

Shift in Kiss1 cell activity requires estrogen receptor α

Reproductive function requires timely secretion of gonadotropin-releasing hormone, which is controlled by a complex excitatory/inhibitory network influenced by sex steroids. Kiss1 neurons are fundamental players in this network, but it is currently unclear whether different conditions of circulating sex steroids directly alter Kiss1 neuronal activity. Here, we show that Kiss1 neurons in the anteroventral periventricular and anterior periventricular nuclei (AVPV/PeN) of males and females exhibit a bimodal resting membrane potential (RMP) influenced by K(ATP) channels, suggesting the presence of two neuronal populations defined as type I (irregular firing patterns) and type II (quiescent). Kiss1 neurons in the arcuate nucleus (Arc) are also composed of firing and quiescent cells, but unlike AVPV/PeN neurons, the range of RMPs did not follow a bimodal distribution. Moreover, Kiss1 neuronal activity in the AVPV/PeN, but not in the Arc, is sexually dimorphic. In females, estradiol shifts the firing pattern of AVPV/PeN Kiss1 neurons and alters cell capacitance and spontaneous IPSCs amplitude of AVPV/PeN and Arc Kiss1 populations in an opposite manner. Notably, mice with selective deletion of estrogen receptor α (ERα) from Kiss1 neurons show cellular activity similar to that observed in ovariectomized females, suggesting that estradiol-induced changes in Kiss1 cellular properties require ERα. We also show that female prepubertal Kiss1 neurons are under higher inhibitory influence and all recorded AVPV/PeN Kiss1 neurons were spontaneously active. Collectively, our findings indicate that changes in cellular activity may underlie Kiss1 action in pubertal initiation and female reproduction.

mRNA expression of ion channels in GnRH neurons: subtype-specific regulation by 17β-estradiol

Burst firing of neurons optimizes neurotransmitter release. GnRH neurons exhibit burst firing activity and T-type calcium channels, which are vital for burst firing activity, are regulated by 17β-estradiol (E2) in GnRH neurons. To further elucidate ion channel expression and E2 regulation during positive and negative feedback on GnRH neurosecretion, we used single cell RT-PCR and real-time qPCR to quantify channel mRNA expression in GnRH neurons. GFP-GnRH neurons expressed numerous ion channels important for burst firing activity. E2-treatment sufficient to induce an LH surge increased mRNA expression of HCN1 channels, which underlie the pacemaker current, the calcium-permeable Ca(V)1.3, Ca(V)2.2, Ca(V)2.3 channels, and TRPC4 channels, which mediate the kisspeptin excitatory response. E2 also decreased mRNA expression of SK3 channels underlying the medium AHP current. Therefore, E2 exerts fundamental changes in ion channel expression in GnRH neurons, to prime them to respond to incoming stimuli with increased excitability at the time of the surge.