Gonadotropin-releasing hormone neurons express K(ATP) channels that are regulated by estrogen and responsive to glucose and metabolic inhibition

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.

Cycling matters: Sex hormone regulation of vascular potassium channels

Sex hormones and the reproductive cycle (estrus in rodents and menstrual in humans) have a known impact on arterial function. In spite of this, sex hormones and the estrus/menstrual cycle are often neglected experimental factors in vascular basic preclinical scientific research. Recent research by our own laboratory indicates that cyclical changes in serum concentrations of sex -hormones across the rat estrus cycle, primary estradiol, have significant consequences for the subcellular trafficking and function of KV. Vascular potassium channels, including KV, are essential components of vascular reactivity. Our study represents a small part of a growing field of literature aimed at determining the role of sex hormones in regulating arterial ion channel function. This review covers key findings describing the current understanding of sex hormone regulation of vascular potassium channels, with a focus on KV channels. Further, we highlight areas of research where the estrus cycle should be considered in future studies to determine the consequences of physiological oscillations in concentrations of sex hormones on vascular potassium channel function.

Central and peripheral mechanisms involved in the control of GnRH neuronal function by metabolic factors

Gonadotropin-releasing hormone (GnRH) neurons are the final output pathway for the brain control of reproduction. The activity of this neuronal population, mainly located at the preoptic area of the hypothalamus, is controlled by a plethora of metabolic signals. However, it has been documented that most of these signal impact on GnRH neurons through indirect neuronal circuits, Kiss1, proopiomelanocortin, and neuropeptide Y/agouti-related peptide neurons being some of the most prominent mediators. In this context, compelling evidence has been gathered in recent years on the role of a large range of neuropeptides and energy sensors in the regulation of GnRH neuronal activity through both direct and indirect mechanisms. The present review summarizes some of the most prominent recent advances in our understanding of the peripheral factors and central mechanisms involved in the metabolic control of GnRH neurons.

Sustained remission of type 2 diabetes in rodents by centrally administered fibroblast growth factor 4

Type 2 diabetes (T2D) is a major health and economic burden worldwide. Despite the availability of multiple drugs for short-term management, sustained remission of T2D is currently not achievable pharmacologically. Intracerebroventricular administration of fibroblast growth factor 1 (icvFGF1) induces sustained remission in T2D rodents, propelling intense research efforts to understand its mechanism of action. Whether other FGFs possess similar therapeutic benefits is currently unknown. Here, we show that icvFGF4 also elicits a sustained antidiabetic effect in both male db/db mice and diet-induced obese mice by activating FGF receptor 1 (FGFR1) expressed in glucose-sensing neurons within the mediobasal hypothalamus. Specifically, FGF4 excites glucose-excited (GE) neurons while inhibiting glucose-inhibited (GI) neurons. Moreover, icvFGF4 restores the percentage of GI neurons in db/db mice. Importantly, intranasal delivery of FGF4 alleviates hyperglycemia in db/db mice, paving the way for non-invasive therapy. We conclude that icvFGF4 holds significant therapeutic potential for achieving sustained remission of T2D.

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.

The modulation of potassium channels by estrogens facilitates neuroprotection

Estrogens, the sex hormones, have the potential to govern multiple cellular functions, such as proliferation, apoptosis, differentiation, and homeostasis, and to exert numerous beneficial influences for the cardiovascular system, nervous system, and bones in genomic and/or non-genomic ways. Converging evidence indicates that estrogens serve a crucial role in counteracting neurodegeneration and ischemic injury; they are thereby being considered as a potent neuroprotectant for preventing neurological diseases such as Alzheimer's disease and stroke. The underlying mechanism of neuroprotective effects conferred by estrogens is thought to be complex and multifactorial, and it remains obscure. It is well established that the K+ channels broadly expressed in a variety of neural subtypes determine the essential physiological features of neuronal excitability, and dysfunction of these channels is closely associated with diverse brain deficits, such as ataxia and epilepsy. A growing body of evidence supports a neuroprotective role of K+ channels in malfunctions of nervous tissues, with the channels even being a therapeutic target in clinical trials. As multitarget steroid hormones, estrogens also regulate the activity of distinct K+ channels to generate varying biological actions, and accumulated data delineate that some aspects of estrogen-mediated neuroprotection may arise from the impact on multiple K+ channels, including Kv, BK, KATP, and K2P channels. The response of these K+ channels after acute or chronic exposure to estrogens may oppose pathological abnormality in nervous cells, which serves to extend our understanding of these phenomena.

Epilepsy-associated increase in gonadotropin-releasing hormone neuron firing in diestrous female mice is independent of chronic seizure burden severity

Reproductive endocrine disorders are common comorbidities of temporal lobe epilepsy (TLE). Our previous studies using the intrahippocampal kainic acid (IHKA) mouse model of TLE demonstrated that many females show prolonged estrous cycles and hypothalamic gonadotropin-releasing hormone (GnRH) neurons exhibit elevated firing during diestrus. However, it is unknown whether the degree of change in GnRH neuron activity is dependent on epilepsy severity. Here, we used 24/7 in vivo electroencephalography (EEG) and in vitro electrophysiological recordings in acute brain slices to assess GnRH neuron firing in relation to chronic seizure burden in diestrous female mice at two months after IHKA injection. We found that percentage of time in seizure activity in the 24 h prior to slice preparation is an accurate proxy of overall seizure burden. Firing rates of GnRH neurons from EEG-recorded IHKA mice were increased in comparison to controls, but no relationships were found between GnRH neuron firing and seizure burden measured in vivo. The independence of GnRH neuron firing rate in relation to seizure burden was unaffected by GnRH neuron soma location or estrous cycle length. Furthermore, GnRH neuron firing rates were not yet different from control values when measured 1 month after injection, when epileptogenesis is already complete in IHKA mice. These findings indicate that the severity of epilepsy and the degree of downstream disruption to GnRH neuron activity are independent, suggesting that susceptibility to reproductive endocrine comorbidities is driven by other risk factors.

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.

Hydrogen peroxide suppresses excitability of gonadotropin-releasing hormone neurons in adult mouse

It has been reported that reactive oxygen species (ROS) derived from oxygen molecule reduction can interfere with the cross-talk between the hypothalamic-pituitary-gonadal (HPG) axis and other endocrine axes, thus affecting fertility. Furthermore, ROS have been linked to GnRH receptor signaling in gonadotropes involved in gonadotropin release. There has been evidence that ROS can interfere with the HPG axis and gonadotropin release at various levels. However, the direct effect of ROS on gonadotropin-releasing hormone (GnRH) neuron remains unclear. Thus, the objective of this study was to determine the effect of hydrogen peroxide (H₂O₂), an ROS source, on GnRH neuronal excitabilities in transgenic GnRH-green fluorescent protein-tagged mice using the whole-cell patch-clamp electrophysiology. In adults, H₂O₂ at high concentrations (mM level) hyperpolarized most GnRH neurons tested, whereas low concentrations (pM to μM) caused slight depolarization. In immature GnRH neurons, H₂O₂ exposure induced excitation. The sensitivity of GnRH neurons to H₂O₂ was increased with postnatal development. The effect of H₂O₂ on adult female GnRH neurons was found to be estrous cycle-dependent. Hyperpolarization mediated by H₂O₂ persisted in the presence of tetrodotoxin, a voltage-gated Na⁺ channel blocker, and amino-acids receptor blocking cocktail containing blockers for the ionotropic glutamate receptors, glycine receptors, and GABA_A receptors, indicating that H₂O₂ could act on GnRH neurons directly. Furthermore, glibenclamide, an ATP-sensitive K⁺ (K_ATP) channel blocker, completely blocked H₂O₂-mediated hyperpolarization. Increasing endogenous H₂O₂ by inhibiting glutathione peroxidase decreased spontaneous activities of most GnRH neurons. We conclude that ROS can act as signaling molecules for regulating GnRH neuron's excitability and that adult GnRH neurons are sensitive to increased ROS concentration. Results of this study demonstrate that ROS have direct modulatory effects on the HPG axis at the hypothalamic level to regulate GnRH neuron's excitabilities.

Neuromodulation or energy failure? Metabolic limitations silence network output in the hypoxic amphibian brainstem

Hypoxia tolerance in the vertebrate brain often involves chemical modulators that arrest neuronal activity to conserve energy. However, in intact networks, it can be difficult to determine whether hypoxia triggers modulators to stop activity in a protective manner or whether activity stops because rates of ATP synthesis are insufficient to support network function. Here, we assessed the extent to which neuromodulation or metabolic limitations arrest activity in the respiratory network of bullfrogs-a circuit that survives moderate periods of oxygen deprivation, presumably, by activating an inhibitory noradrenergic pathway. We confirmed that hypoxia and norepinephrine (NE) reduce network output, consistent with the view that hypoxia may cause the release of NE to inhibit activity. However, these responses differed qualitatively; hypoxia, but not NE, elicited a large motor burst and silenced the network. The stereotyped response to hypoxia persisted in the presence of both NE and an adrenergic receptor blocker that eliminates sensitivity to NE, indicating that noradrenergic signaling does not cause the arrest. Pharmacological inhibition of glycolysis and mitochondrial respiration recapitulated all features of hypoxia on network activity, implying that reduced ATP synthesis underlies the effects of hypoxia. Finally, activating modulatory mechanisms that dampen neuronal excitability when ATP levels fall, KATP channels and AMP-dependent protein kinase, did not resemble the hypoxic response. These results suggest that energy failure-rather than inhibitory modulation-silences the respiratory network during hypoxia and emphasize the need to account for metabolic limitations before concluding that modulators arrest activity as an adaptation for energy conservation in the nervous system.

Role of insulin in the neuroendocrine control of reproduction

Infertility associated with insulin resistance is characterised by abnormal hormone secretion by the hypothalamus, pituitary gland and gonads. These endocrine tissues can maintain insulin sensitivity even when tissues such as the muscle and liver become insulin-resistant, resulting in excessive insulin stimulation as hyperinsulinaemia develops. Experiments conducted to determine the role of neuronal insulin signalling in fertility were unable to recapitulate early findings of hypogonadotrophic hypogonadism in mice lacking insulin receptors throughout the brain. Rather, it was eventually shown that astrocytes critically mediate the effects of insulin on puberty timing and adult reproductive function. However, specific roles for neurones and gonadotrophs have been revealed under conditions of hyperinsulinaemia and by ablation of insulin and leptin receptors. The collective picture is one of multiple insulin-responsive inputs to gonadotrophin releasing hormone neurones, with astrocytes being the most important player.

A closer look at the role of insulin for the regulation of male reproductive function

While insulin demonstrates to have a considerable influence on the reproductive system, there are various unanswered questions regarding its precise sites, mechanisms of action, and roles for the developing and functioning of the adult male reproductive system. Apart from its effects on glucose level, insulin has an important role in the reproductive system directly by binding on insulin and IGF receptors in the brain and testis. To date, however, the effect of insulin or its alterations on blood-testis-barrier, as an important regulator of normal spermatogenesis and fertility, has not yet been studied. This review aimed to focus on the experimental and clinical studies to describe mechanisms by which insulin affects the hypothalamic-pituitary-gonadal (HPG) axis, testicular cells, spermatozoa, and sexual behavior. Moreover, we discussed the mechanism and impact of insulin changes in type 1 (insulin deficiency along with persisted or even increased sensitivity) and 2 (insulin resistance along with increased insulin level at the early stages of disease) diabetes and obesity on the male reproductive tract.

Role of potassium channels in female reproductive system

Potassium channels are widely expressed in most types of cells in living organisms and regulate the functions of a variety of organs, including kidneys, neurons, cardiovascular organs, and pancreas among others. However, the functional roles of potassium channels in the reproductive system is less understood. This mini-review provides information about the localization and functions of potassium channels in the female reproductive system. Five types of potassium channels, which include inward-rectifying (Kir), voltage-gated (Kv), calcium-activated (K_Ca), 2-pore domain (K_2P), and rapidly-gating sodium-activated (Slo) potassium channels are expressed in the hypothalamus, ovaries, and uterus. Their functions include the regulation of hormone release and feedback by Kir6.1 and Kir6.2, which are expressed in the luteal granulosa cells and gonadotropin-releasing hormone neurons respectively, and regulate the functioning of the hypothalamus–pituitary–ovarian axis and the production of progesterone. Both channels are regulated by subtypes of the sulfonylurea receptor (SUR), Kir6.1/SUR2B and Kir6.2/SUR1. K_v and Slo2.1 affect the transition from uterine quiescence in late pregnancy to the state of strong myometrial contractions in labor. Intermediate- and small-conductance K_Ca modulate the vasodilatation of the placental chorionic plate resistance arteries via the secretion of nitric oxide and endothelium-derived hyperpolarizing factors. Treatment with specific channel activators and inhibitors provides information relevant for clinical use that could help alter the functions of the female reproductive system.

Insulin-induced hypoglycaemia suppresses pulsatile luteinising hormone secretion and arcuate Kiss1 cell activation in female mice

Stress suppresses pulsatile luteinising hormone (LH) secretion in a variety of species, although the mechanism underlying this inhibition of reproductive function remains unclear. Metabolic stress, particularly hypoglycaemia, is a clinically-relevant stress type that is modelled with bolus insulin injection (insulin-induced hypoglycaemia). The present study utilised ovariectomised C57BL/6 mice to test the hypothesis that acute hypoglycaemia suppresses pulsatile LH secretion via central mechanisms. Pulsatile LH secretion was measured in 90-minute sampling periods immediately prior to and following i.p. injection of saline or insulin. The secretion of LH was not altered over time in fed animals or acutely fasted (5 hours) animals following an i.p. saline injection. By contrast, insulin elicited a robust suppression of pulsatile LH secretion in fasted animals, preventing LH pulses in five of six mice. To identify the neuroendocrine site of impairment, a kisspeptin challenge was performed in saline or insulin pre-treated animals in a cross-over design. LH secretion in response to exogenous kisspeptin was not different between animals pre-treated with saline or insulin, indicating normal gonadotrophin-releasing hormone cell and pituitary responses during acute hypoglycaemia. Based on this finding, the effect of insulin-induced hypoglycaemia on arcuate kisspeptin (Kiss1) cell function was determined using c-Fos as a marker of neuronal activation. Insulin caused a significant suppression in the percentage of Kiss1 cells in the arcuate nucleus that contained c-Fos compared to saline-injected controls. Taken together, these data support the hypothesis that insulin-induced hypoglycaemia suppresses pulsatile LH secretion in the female mouse via predominantly central mechanisms, which culminates in the suppression of the arcuate Kiss1 population.

Proestrus Differentially Regulates Expression of Ion Channel and Calcium Homeostasis Genes in GnRH Neurons of Mice

In proestrus, the changing gonadal hormone milieu alters the physiological properties of GnRH neurons and contributes to the development of the GnRH surge. We hypothesized that proestrus also influences the expression of different ion channel genes in mouse GnRH neurons. Therefore, we performed gene expression profiling of GnRH neurons collected from intact, proestrous and metestrous GnRH-GFP transgenic mice, respectively. Proestrus changed the expression of 37 ion channel and 8 calcium homeostasis-regulating genes. Voltage-gated sodium channels responded with upregulation of three alpha subunits (Scn2a1, Scn3a, and Scn9a). Within the voltage-gated potassium channel class, Kcna1, Kcnd3, Kcnh3, and Kcnq2 were upregulated, while others (Kcna4, Kcnc3, Kcnd2, and Kcng1) underwent downregulation. Proestrus also had impact on inwardly rectifying potassium channel subunits manifested in enhanced expression of Kcnj9 and Kcnj10 genes, whereas Kcnj1, Kcnj11, and Kcnj12 subunit genes were downregulated. The two-pore domain potassium channels also showed differential expression with upregulation of Kcnk1 and reduced expression of three subunit genes (Kcnk7, Kcnk12, and Kcnk16). Changes in expression of chloride channels involved both the voltage-gated (Clcn3 and Clcn6) and the intracellular (Clic1) subtypes. Regarding the pore-forming alpha-1 subunits of voltage-gated calcium channels, two (Cacna1b and Cacna1h) were upregulated, while Cacna1g showed downregulation. The ancillary subunits were also differentially regulated (Cacna2d1, Cacna2d2, Cacnb1, Cacnb3, Cacnb4, Cacng5, Cacng6, and Cacng8). In addition, ryanodine receptor 1 (Ryr1) gene was downregulated, while a transient receptor potential cation channel (Trpm3) gene showed enhanced expression. Genes encoding proteins regulating the intracellular calcium homeostasis were also influenced (Calb1, Hpca, Hpcal1, Hpcal4, Cabp7, Cab 39l, and Cib2). The differential expression of genes coding for ion channel proteins in GnRH neurons at late proestrus indicates that the altering hormone milieu contributes to remodeling of different kinds of ion channels of GnRH neurons, which might be a prerequisite of enhanced cellular activity of GnRH neurons and the subsequent surge release of the neurohormone.

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.

Discovery and Characterization of VU0529331, a Synthetic Small-Molecule Activator of Homomeric G Protein-Gated, Inwardly Rectifying, Potassium (GIRK) Channels

G protein-gated, inwardly rectifying, potassium (GIRK) channels are important regulators of cellular excitability throughout the body. GIRK channels are heterotetrameric and homotetrameric combinations of the Kir3.1-4 (GIRK1-4) subunits. Different subunit combinations are expressed throughout the central nervous system (CNS) and the periphery, and most of these combinations contain a GIRK1 subunit. For example, the predominance of GIRK channels in the CNS are composed of GIRK1 and GIRK2 subunits, while the GIRK channels in cardiac atrial myocytes are made up mostly of GIRK1 and GIRK4 subunits. Although the vast majority of GIRK channels contain a GIRK1 subunit, discrete populations of cells that express non-GIRK1-containing GIRK (non-GIRK1/X) channels do exist. For instance, dopaminergic neurons in the ventral tegmental area of the brain, associated with addiction and reward, do not express the GIRK1 subunit. Targeting these non-GIRK1/X channels with subunit-selective pharmacological probes could lead to important insights into how GIRK channels are involved in reward and addiction. Such insights may, in turn, reveal therapeutic opportunities for the treatment or prevention of addiction. Previously, our laboratory discovered small molecules that can specifically modulate the activity of GIRK1-containing GIRK channels. However, efforts to generate compounds active on non-GIRK1/X channels from these scaffolds have been unsuccessful. Recently, ivermectin was shown to modulate non-GIRK1/X channels, and historically, ivermectin is known to modulate a wide variety of neuronal channels and receptors. Further, ivermectin is a complex natural product, which makes it a challenging starting point for development of more selective, effective, and potent compounds. Thus, while ivermectin provides proof-of-concept as a non-GIRK1/X channel activator, it is of limited utility. Therefore, we sought to discover a synthetic small molecule that would serve as a starting point for the development of non-GIRK1/X channel modulators. To accomplish this, we used a high-throughput thallium flux assay to screen a 100 000-compound library in search of activators of homomeric GIRK2 channels. Using this approach, we discovered VU0529331, the first synthetic small molecule reported to activate non-GIRK1/X channels, to our knowledge. This discovery represents the first step toward developing potent and selective non-GIRK1/X channel probes. Such molecules will help elucidate the role of GIRK channels in addiction, potentially establishing a foundation for future development of therapies utilizing targeted GIRK channel modulation.

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.

Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling

Reproduction and fertility are regulated via hormones of the hypothalamic-pituitary-gonadal (HPG) axis. Control of this reproductive axis occurs at all levels, including the brain and pituitary, and allows for the promotion or inhibition of gonadal sex steroid secretion and function. In addition to guiding proper gonadal development and function, gonadal sex steroids also act in negative- and positive-feedback loops to regulate reproductive circuitry in the brain, including kisspeptin neurones, thereby modulating overall HPG axis status. Additional regulation is also provided by sex steroids made within the brain, including neuroprogestins. Furthermore, because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signalling, including cortiscosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. This review covers recent data related to the neural, hormonal and stress regulation of the HPG axis and emerging interactions between the HPG and HPA axes, focusing on actions at the level of the brain and pituitary.

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.

Increased cerebrospinal fluid levels of GABA, testosterone and estradiol in women with polycystic ovary syndrome

STUDY QUESTION

Do cerebrospinal fluid (CSF) concentrations of gamma-aminobutyric acid (GABA), testosterone (T) and estradiol (E2) differ in women with polycystic ovary syndrome (PCOS) as compared to eumenorrheic, ovulatory women (EW)?

SUMMARY ANSWER

Women with PCOS displayed higher CSF levels of GABA and E2, and possibly T, than EW.

WHAT IS KNOWN ALREADY

The chronic anovulation characteristic of PCOS has been attributed to increased central GnRH drive and resulting gonadotropin aberrations. Androgens are thought to regulate GABA, which in turn regulates the neural cascade that modulates GnRH drive.

STUDY DESIGN, SIZE, DURATION

This cross-sectional observational study included 15 EW and 12 non-obese women with PCOS who consented to a lumbar puncture in addition to 24 h of serum blood collection at 15-min intervals.

PARTICIPANTS/MATERIALS, SETTING, METHODS

In total, 27 women were studied at a the General Clinical Research Center (GCRC) at the University of Pittsburgh. Serum analytes included T, E2 and androstenedione. CSF analytes included GABA, glutamate, glucose, T and E2.

MAIN RESULTS AND THE ROLE OF CHANCE

Women with PCOS had higher CSF GABA as compared to EW (9.04 versus 7.04 μmol/L, P < 0.05). CSF glucose and glutamate concentrations were similar between the two groups. CSF T was 52% higher (P = 0.1) and CSF E2 was 30% higher (P < 0.01) in women with PCOS compared to EW. Circulating T was 122% higher (P < 0.01) and circulating E2 was 75% higher (P < 0.01) in women with PCOS than in EW.

LIMITATIONS REASONS FOR CAUTION

The study is limited by its small sample size and the technical limitations of measuring CSF analytes that are pulsatile and have short half-lives.

WIDER IMPLICATIONS OF THE FINDINGS

Women with PCOS displayed significantly higher circulating levels of T and E2, significantly higher CSF levels of E2, and higher levels of CSF testosterone, although the latter was not statistically significant. A better understanding of the central milieu informs our understanding of the mechanisms mediating increased the GnRH drive in PCOS and lends a new perspective for understanding the presentation, pathogenesis and potential health consequences of PCOS, including gender identity issues.

STUDY FUNDING/COMPETING INTEREST(S)

No conflicts of interest. The study was funded by NIH grants to SLB (RO1-MH50748, U54-HD08610) and NIH RR-00056 to the General Clinical Research Center of the University of Pittsburgh.

TRIAL REGISTRATION NUMBER

NCT01674426.

Neuroendocrine integration of nutritional signals on reproduction

Reproductive function in mammals is energetically costly and therefore tightly regulated by nutritional status. To enable this integration of metabolic and reproductive function, information regarding peripheral nutritional status must be relayed centrally to the gonadotropin-releasing hormone (GNRH) neurons that drive reproductive function. The metabolically relevant hormones leptin, insulin and ghrelin have been identified as key mediators of this 'metabolic control of fertility'. However, the neural circuitry through which they act to exert their control over GNRH drive remains incompletely understood. With the advent of Cre-LoxP technology, it has become possible to perform targeted gene-deletion and gene-rescue experiments and thus test the functional requirement and sufficiency, respectively, of discrete hormone-neuron signaling pathways in the metabolic control of reproductive function. This review discusses the findings from these investigations, and attempts to put them in context with what is known from clinical situations and wild-type animal models. What emerges from this discussion is clear evidence that the integration of nutritional signals on reproduction is complex and highly redundant, and therefore, surprisingly difficult to perturb. Consequently, the deletion of individual hormone-neuron signaling pathways often fails to cause reproductive phenotypes, despite strong evidence that the targeted pathway plays a role under normal physiological conditions. Although transgenic studies rarely reveal a critical role for discrete signaling pathways, they nevertheless prove to be a good strategy for identifying whether a targeted pathway is absolutely required, critically involved, sufficient or dispensable in the metabolic control of fertility.

Oleate induces KATP channel-dependent hyperpolarization in mouse hypothalamic glucose-excited neurons without altering cellular energy charge

•

Oleate and low glucose hyperpolarize and inhibit GT1-7 and mouse GE neurons by activation of K_ATP.

•

Oleate inhibition of GT1-7 neuron activity is not mediated by AMPK or fatty acid oxidation.

•

Activation of K_ATP by oleate requires ATP hydrolysis but does not reduce the levels ATP or the ATP:ADP ratio.

•

GT1-7 hyperpolarization by oleate is not dependent on UCP2.

•

Oleate and low glucose depolarize a subpopulation of hypothalamic GI neurons.

The unsaturated fatty acid, oleate exhibits anorexigenic properties reducing food intake and hepatic glucose output. However, its mechanism of action in the hypothalamus has not been fully determined. This study investigated the effects of oleate and glucose on GT1-7 mouse hypothalamic cells (a model of glucose-excited (GE) neurons) and mouse arcuate nucleus (ARC) neurons. Whole-cell and perforated patch-clamp recordings, immunoblotting and cell energy status measures were used to investigate oleate- and glucose-sensing properties of mouse hypothalamic neurons. Oleate or lowered glucose concentration caused hyperpolarization and inhibition of firing of GT1-7 cells by the activation of ATP-sensitive K⁺ channels (K_ATP). This effect of oleate was not dependent on fatty acid oxidation or raised AMP-activated protein kinase activity or prevented by the presence of the UCP2 inhibitor genipin. Oleate did not alter intracellular calcium, indicating that CD36/fatty acid translocase may not play a role. However, oleate activation of K_ATP may require ATP metabolism. The short-chain fatty acid octanoate was unable to replicate the actions of oleate on GT1-7 cells. Although oleate decreased GT1-7 cell mitochondrial membrane potential there was no change in total cellular ATP or ATP/ADP ratios. Perforated patch and whole-cell recordings from mouse hypothalamic slices demonstrated that oleate hyperpolarized a subpopulation of ARC GE neurons by K_ATP activation. Additionally, in a separate small population of ARC neurons, oleate application or lowered glucose concentration caused membrane depolarization. In conclusion, oleate induces K_ATP-dependent hyperpolarization and inhibition of firing of a subgroup of GE hypothalamic neurons without altering cellular energy charge.

Dopaminergic inhibition of gonadotropin-releasing hormone neurons in the cichlid fish Astatotilapia burtoni

Dopamine regulates reproduction in part by modulating neuronal activity within the hypothalamic-pituitary-gonadal (HPG) axis. Previous studies suggested numerous mechanisms by which dopamine exerts inhibitory control over the HPG axis, ultimately changing the levels of sex steroids that regulate reproductive behaviors. However, it is not known whether these mechanisms are conserved across vertebrate species. In particular, it is unknown whether mechanisms underlying dopaminergic control of reproduction are shared between mammals and teleost fish. In mammals, dopamine directly inhibits gonadotropin-releasing hormone (GnRH1) hypothalamic neurons, the gatekeepers for activation of the HPG axis. Here, we demonstrate, for the first time in teleost fish, dopaminergic control of GnRH1 neurons via direct dopamine type-2-like receptor (D2R)-mediated inhibition within the hypothalamus. These results suggest that direct dopaminergic control of GnRH1 neurons via interactions in the hypothalamus is not exclusive to tetrapod reproductive control, but is likely conserved across vertebrate species.

Sexual Motivation in the Female and Its Opposition by Stress

A well worked-out motivational system in laboratory animals produces estrogen-dependent female sex behavior. Here, we review (a) the logical definition of sexual motivation and (b) the basic neuronal and molecular mechanisms that allow the behavior to occur. Importantly, reproductive mechanisms in the female can be inhibited by stress. This is interesting because, in terms of the specificity of neuroendocrine dynamics in space and time, the two families of phenomena, sex and stress, are the opposite of each other. We cover papers that document stress effects on the underlying processes of reproductive endocrinology in the female. Not all of the mechanisms for such inhibition have been clearly laid out. Finally, as a current topic of investigation, this system offers several avenues for new investigation which we briefly characterize.

Hindbrain lactate regulates preoptic gonadotropin-releasing hormone (GnRH) neuron GnRH-I protein but not AMPK responses to hypoglycemia in the steroid-primed ovariectomized female rat

Steroid positive-feedback activation of the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) neuroendocrine axis propagates the pre ovulatory LH surge, a crucial component of female reproduction. Our work shows that this key event is restrained by inhibitory metabolic input from hindbrain A2 noradrenergic neurons. GnRH neurons express the ultra-sensitive energy sensor adenosine 5'-monophosphate-activated protein kinase (AMPK); here, we investigated the hypothesis that GnRH nerve cell AMPK and peptide neurotransmitter responses to insulin-induced hypoglycemia are controlled by hindbrain lack of the oxidizable glycolytic end-product L-lactate. Data show that hypoglycemic inhibition of LH release in steroid-primed ovariectomized female rats was reversed by coincident caudal hindbrain lactate infusion. Western blot analyses of laser-microdissected A2 neurons demonstrate hypoglycemic augmentation [Fos, estrogen receptor-beta (ER-β), phosphoAMPK (pAMPK)] and inhibition (dopamine-beta-hydroxylase, GLUT3, MCT2) of protein expression in these cells, responses that were normalized by insulin plus lactate treatment. Hypoglycemia diminished rostral preoptic GnRH nerve cell GnRH-I protein and pAMPK content; the former, but not the latter response was reversed by lactate. Results implicate caudal hindbrain lactoprivic signaling in hypoglycemia-induced suppression of the LH surge, demonstrating that lactate repletion of that site reverses decrements in A2 catecholamine biosynthetic enzyme and GnRH neuropeptide precursor protein expression. Lack of effect of lactate on hypoglycemic patterns of GnRH AMPK activity suggests that this sensor is uninvolved in metabolic-inhibition of positive-feedback-stimulated hypophysiotropic signaling to pituitary gonadotropes.

17β-Estradiol increases persistent Na(+) current and excitability of AVPV/PeN Kiss1 neurons in female mice

In vitro slice studies have revealed that there are significant differences in the spontaneous firing activity between anteroventral periventricular/periventricular preoptic nucleus (AVPV/PeN) and arcuate nucleus (ARC) kisspeptin (Kiss1) neurons in females. Although both populations express similar endogenous conductances, we have discovered that AVPV/PeN Kiss1 neurons express a subthreshold, persistent sodium current (INaP) that dramatically alters their firing activity. Based on whole-cell recording of Kiss1-Cre-green fluorescent protein (GFP) neurons, INaP was 4-fold greater in AVPV/PeN vs ARC Kiss1 neurons. An LH surge-producing dose of 17β-estradiol (E2) that increased Kiss1 mRNA expression in the AVPV/PeN, also augmented INaP in AVPV/PeN neurons by 2-fold. Because the activation threshold for INaP was close to the resting membrane potential (RMP) of AVPV/PeN Kiss1 neurons (-54 mV), it rendered them much more excitable and spontaneously active vs ARC Kiss1 neurons (RMP = -66 mV). Single-cell RT-PCR revealed that AVPV/PeN Kiss1 neurons expressed the requisite sodium channel α-subunit transcripts, NaV1.1, NaV1.2, and NaV1.6 and β subunits, β2 and β4. Importantly, NaV1.1α and -β2 transcripts in AVPV/PeN, but not ARC, were up-regulated 2- to 3-fold by a surge-producing dose of E2, similar to the transient calcium current channel subunit Cav3.1. The transient calcium current collaborates with INaP to generate burst firing, and selective blockade of INaP by riluzole significantly attenuated rebound burst firing and spontaneous activity. Therefore, INaP appears to play a prominent role in AVPV/PeN Kiss1 neurons to generate spontaneous, repetitive burst firing, which is required for the high-frequency-stimulated release of kisspeptin for exciting GnRH neurons and potentially generating the GnRH surge.

Differential Gene Expression in Gonadotropin-Releasing Hormone Neurons of Male and Metestrous Female Mice

BACKGROUND

Gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in the regulation of the hypothalamic-pituitary gonadal axis in a sex-specific manner. We hypothesized that the differences seen in reproductive functions of males and females are associated with a sexually dimorphic gene expression profile of GnRH neurons.

METHODS AND RESULTS

We compared the transcriptome of GnRH neurons obtained from intact metestrous female and male GnRH-green fluorescent protein transgenic mice. About 1,500 individual GnRH neurons from each sex were sampled with laser capture microdissection followed by whole-transcriptome amplification for gene expression profiling. Under stringent selection criteria (fold change >1.6, adjusted p value 0.01), Affymetrix Mouse Genome 430 PM array analysis identified 543 differentially expressed genes. Sexual dimorphism was most apparent in gene clusters associated with synaptic communication, signal transduction, cell adhesion, vesicular transport and cell metabolism. To validate microarray results, 57 genes were selected, and 91% of their differential expression was confirmed by real-time PCR. Similarly, 88% of microarray results were confirmed with PCR from independent samples obtained by patch pipette harvesting and pooling of 30 GnRH neurons from each sex. We found significant differences in the expression of genes involved in vesicle priming and docking (Syt1, Cplx1), GABAergic (Gabra3, Gabrb3, Gabrg2) and glutamatergic (Gria1, Grin1, Slc17a6) neurotransmission, peptide signaling (Sstr3, Npr2, Cxcr4) and the regulation of intracellular ion homeostasis (Cacna1, Cacnb1, Cacng5, Kcnq2, Kcnc1).

CONCLUSION

The striking sexual dimorphism of the GnRH neuron transcriptome we report here contributes to a better understanding of the differences in cellular mechanisms of GnRH neurons in the two sexes.

Dehydroepiandrosterone, its metabolites and ion channels

This review is focused on the physiological and pathophysiological relevance of steroids influencing the activities of the central and peripheral nervous systems with regard to their concentrations in body fluids and tissues in various stages of human life like the fetal development or pregnancy. The data summarized in this review shows that DHEA and its unconjugated and sulfated metabolites are physiologically and pathophysiologically relevant in modulating numerous ion channels and participate in vital functions of the human organism. DHEA and its unconjugated and sulfated metabolites including 5α/β-reduced androstane steroids participate in various physiological and pathophysiological processes like the management of GnRH cyclic release, regulation of glandular and neurotransmitter secretions, maintenance of glucose homeostasis on one hand and insulin insensitivity on the other hand, control of skeletal muscle and smooth muscle activities including vasoregulation, promotion of tolerance to ischemia and other neuroprotective effects. In respect of prevalence of steroid sulfates over unconjugated steroids in the periphery and the opposite situation in the CNS, the sulfated androgens and androgen metabolites reach relevance in peripheral organs. The unconjugated androgens and estrogens are relevant in periphery and so much the more in the CNS due to higher concentrations of most unconjugated steroids in the CNS tissues than in circulation and peripheral organs. This article is part of a Special Issue entitled "Essential role of DHEA".

Electrical properties of kisspeptin neurons and their regulation of GnRH neurons

Kisspeptin neurons are critical components of the neuronal network controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons. A variety of genetically-manipulated mouse models have recently facilitated the study of the electrical activity of the two principal kisspeptin neuron populations located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARN) in acute brain slices. We discuss here the mechanisms and pathways through which kisspeptin neurons regulate GnRH neuron activity. We then examine the different kisspeptin-green fluorescent protein mouse models being used for kisspeptin electrophysiology and the data obtained to date for RP3V and ARN kisspeptin neurons. In light of these new observations on the spontaneous firing rates, intrinsic membrane properties, and neurotransmitter regulation of kisspeptin neurons, we speculate on the physiological roles of the different kisspeptin populations.

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.

Connecting metabolism and reproduction: roles of central energy sensors and key molecular mediators

It is well established that pubertal activation of the reproductive axis and maintenance of fertility are critically dependent on the magnitude of body energy reserves and the metabolic state of the organism. Hence, conditions of impaired energy homeostasis often result in deregulation of puberty and reproduction, whereas gonadal dysfunction can be associated with the worsening of the metabolic profile and, eventually, changes in body weight. While much progress has taken place in our knowledge about the neuroendocrine mechanisms linking metabolism and reproduction, our understanding of how such dynamic interplay happens is still incomplete. As paradigmatic example, much has been learned in the last two decades on the reproductive roles of key metabolic hormones (such as leptin, insulin and ghrelin), their brain targets and the major transmitters and neuropeptides involved. Yet, the molecular mechanisms whereby metabolic information is translated and engages into the reproductive circuits remain largely unsolved. In this work, we will summarize recent developments in the characterization of the putative central roles of key cellular energy sensors, such as mTOR, in this phenomenon, and will relate these with other molecular mechanisms likely contributing to the brain coupling of energy balance and fertility. In doing so, we aim to provide an updated view of an area that, despite still underdeveloped, may be critically important to fully understand how reproduction and metabolism are tightly connected in health and disease.

The regulation of reproductive neuroendocrine function by insulin and insulin-like growth factor-1 (IGF-1)

The mammalian reproductive hormone axis regulates gonadal steroid hormone levels and gonadal function essential for reproduction. The neuroendocrine control of the axis integrates signals from a wide array of inputs. The regulatory pathways important for mediating these inputs have been the subject of numerous studies. One class of proteins that have been shown to mediate metabolic and growth signals to the CNS includes Insulin and IGF-1. These proteins are structurally related and can exert endocrine and growth factor like action via related receptor tyrosine kinases. The role that insulin and IGF-1 play in controlling the hypothalamus and pituitary and their role in regulating puberty and nutritional control of reproduction has been studied extensively. This review summarizes the in vitro and in vivo models that have been used to study these neuroendocrine structures and the influence of these growth factors on neuroendocrine control of reproduction.

Reproductive neuroendocrine dysfunction in polycystic ovary syndrome: insight from animal models

Polycystic ovary syndrome (PCOS) is a common endocrinopathy with elusive origins. A clinically heterogeneous disorder, PCOS is likely to have multiple etiologies comprised of both genetic and environmental factors. Reproductive neuroendocrine dysfunction involving increased frequency and amplitude of gonadotropin-releasing hormone (GnRH) release, as reflected by pulsatile luteinizing hormone (LH) secretion, is an important pathophysiologic component in PCOS. Whether this defect is primary or secondary to other changes in PCOS is unclear, but it contributes significantly to ongoing reproductive dysfunction. This review highlights recent work in animal models, with a particular emphasis on the mouse, demonstrating the ability of pre- and postnatal steroidal and metabolic factors to drive changes in GnRH/LH pulsatility and GnRH neuron function consistent with the observed abnormalities in PCOS. This work has begun to elucidate how a complex interplay of ovarian, metabolic, and neuroendocrine factors culminates in this syndrome.

Role of dorsal vagal complex A2 noradrenergic neurons in hindbrain glucoprivic inhibition of the luteinizing hormone surge in the steroid-primed ovariectomized female rat: effects of 5-thioglucose on A2 functional biomarker and AMPK activity

Neuro-glucostasis is required for normal expression of the steroid positive-feedback-induced preovulatory pituitary luteinizing hormone (LH) surge, a critical element of female reproduction. Glucoprivic signals from the caudal hindbrain restrain this surge, but the cellular source of this stimulus is unclear. Norepinephrine (NE) exerts well-defined stimulatory effects on the reproductive neuroendocrine axis. Our studies show that medullary A2 noradrenergic neurons are both estrogen- and glucoprivic-sensitive. Here, we investigated the premise that the LH surge is inhibited by A2 cell reactivity to hindbrain glucopenia and diminished preoptic NE neurotransmission. Estradiol- and progesterone-primed ovariectomized (OVX) female rats were injected into the caudal fourth ventricle (CV4) with the glucose anti-metabolite, 5-thioglucose (5TG) or saline (SAL) prior to onset of the LH surge. Pretreatment by intra-CV4 delivery of the selective catecholamine neurotoxin, 6-OHDA, attenuated LH output, but prevented inhibition by 5TG. 5TG modified patterns of steroid feedback-associated Fos staining of A2, but not other medullary catecholamine cell groups. Intra-preoptic administration of the alpha₁-adrenergic receptor agonist, methoxamine, elicited site-specific reversal of hindbrain glucoprivic suppression of gonadotropin-releasing hormone (GnRH) neuron Fos labeling and LH release. Western blotting of laser-microdissected A2 neurons revealed glucoprivic stimulation of Fos, but inhibition of the catecholamine synthetic enzyme, dopamine-β-hydroxylase; 5TG also diminished A2 estrogen receptor (ER)-α and progesterone receptor profiles, but augmented ER-β protein. Intriguingly, A2 AMPK activity was decreased in 5TG-treated rats, despite down-regulation of GLUT3 and no change in MCT2 protein expression. Rostral preoptic GnRH neurons also exhibited decreased AMPK activation simultaneous with apparent reduction of neuropeptide signaling to the pituitary. The present studies demonstrate that hindbrain glucoprivation inhibits the LH surge, in part, by reducing preoptic noradrenergic input, and furthermore implicate A2 neurons as a source of this altered signal. Results also suggest that AMPK sensor deactivation does not supersede the impact of pharmacological inhibition of glucose catabolism on A2 cell function nor afferent signaling of hindbrain glucopenia on GnRH neurons. Further studies are needed to determine if decreased AMPK activation in these cell populations reflect compensatory gain in positive energy balance and/or direct effects of estrogen on AMPK.

Cross-talk between reproduction and energy homeostasis: central impact of estrogens, leptin and kisspeptin signaling

The central nervous system receives hormonal cues (e.g., estrogens and leptin, among others) that influence reproduction and energy homeostasis. 17β-estradiol (E2) is known to regulate gonadotropin-releasing hormone (GnRH) secretion via classical steroid signaling and rapid non-classical membrane-initiated signaling. Because GnRH neurons are void of leptin receptors, the actions of leptin on these neurons must be indirect. Although it is clear that the arcuate nucleus of the hypothalamus is the primary site of overlap between these two systems, it is still unclear which neural network(s) participate in the cross-talk of E2 and leptin, two hormones essential for reproductive function and metabolism. Herein we review the progress made in understanding the interactions between reproduction and energy homeostasis by focusing on the advances made to understand the cellular signaling of E2 and leptin on three neural networks: kisspeptin, pro-opiomelanocortin (POMC) and neuropeptide Y (NPY). Although critical in mediating the actions of E2 and leptin, considerable work still remains to uncover how these neural networks interact in vivo.

Molecular mechanisms that drive estradiol-dependent burst firing of Kiss1 neurons in the rostral periventricular preoptic area

Kisspeptin (Kiss1) neurons in the rostral periventricular area of the third ventricle (RP3V) provide excitatory drive to gonadotropin-releasing hormone (GnRH) neurons to control fertility. Using whole cell patch clamp recording and single-cell (sc)RT-PCR techniques targeting Kiss1-CreGFP or tyrosine hydroxylase (TH)-EGFP neurons, we characterized the biophysical properties of these neurons and identified the critical intrinsic properties required for burst firing in 17β-estradiol (E2)-treated, ovariectomized female mice. One-fourth of the RP3V Kiss1 neurons exhibited spontaneous burst firing. RP3V Kiss1 neurons expressed a hyperpolarization-activated h-current (Ih) and a T-type calcium current (IT), which supported hyperpolarization-induced rebound burst firing. Under voltage clamp conditions, all Kiss1 neurons expressed a kinetically fast Ih that was augmented 3.4-fold by high (LH surge-producing)-E2 treatment. scPCR analysis of Kiss1 neurons revealed abundant expression of the HCN1 channel transcripts. Kiss1 neurons also expressed a Ni(2+)- and TTA-P2-sensitive IT that was augmented sixfold with high-E2 treatment. CaV3.1 mRNA was also highly expressed in these cells. Current clamp analysis revealed that rebound burst firing was induced in RP3V Kiss1 neurons in high-E2-treated animals, and the majority of Kiss1 neurons had a hyperpolarization threshold of -84.7 mV, which corresponded to the V&frac12; for IT de-inactivation. Finally, Kiss1 neurons in the RP3V were hyperpolarized by μ- and κ-opioid and GABAB receptor agonists, suggesting that these pathways also contribute to rebound burst firing. Therefore, Kiss1 neurons in the RP3V express the critical channels and receptors that permit E2-dependent rebound burst firing and provide the biophysical substrate that drives the preovulatory surge of GnRH.

Pacemaking kisspeptin neurons

Kisspeptin (Kiss1) neurons are vital for reproduction. Gonatotrophin-releasing hormone (GnRH) neurons express the kisspeptin receptor (GPR54), and kisspeptins potently stimulate the release of GnRH by depolarizing and inducing sustained action potential firing in GnRH neurons. As such, Kiss1 neurons may be the presynaptic pacemaker neurons in the hypothalamic circuitry that controls reproduction. There are at least two different populations of Kiss1 neurons; one in the rostral periventricular area (RP3V) that is stimulated by oestrogens and the other in the arcuate nucleus that is inhibited by oestrogens. How each of these Kiss1 neuronal populations participates in the regulation of the reproductive cycle is currently under intense investigation. Based on electrophysiological studies in the guinea-pig and mouse, Kiss1 neurons in general are capable of generating burst-firing behaviour. Essentially, all Kiss1 neurons, which have been studied thus far in the arcuate nucleus, express the ion channels necessary for burst firing, which include hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels and the T-type calcium (Cav3.1) channels. In voltage-clamp conditions, these channels produce distinct currents that can generate burst-firing behaviour in current-clamp conditions. The future challenge is to identify other key channels and synaptic inputs involved in the regulation of the firing properties of Kiss1 neurons and the physiological regulation of the expression of these channels and receptors by oestrogens and other hormones. The ultimate goal is to understand how Kiss1 neurons control the different phases of GnRH neurosecretion, hence reproduction.

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.

Glucose responsiveness in a novel adult-derived GnRH cell line, mHypoA-GnRH/GFP: involvement of AMP-activated protein kinase

Glucose regulates energy homeostasis and reproductive function within the hypothalamus. The underlying mechanisms responsible for glucose regulation of GnRH gene transcription were investigated using a novel murine immortalized, adult-derived hypothalamic cell line, mHypoA-GnRH/GFP. Analysis of GnRH mRNA synthesis and secretion following agonist treatment demonstrated that the mHypoA-GnRH/GFP cell line is a representative model of in vivo GnRH neurons. c-fos mRNA levels, following glucose exposure, indicated that these neurons were responsive to low (0.5mM) and high (5mM) glucose, and high glucose stimulated GnRH mRNA transcription in a metabolism-dependent manner. Glucose inhibited AMPK activity, and was linked to the downstream stimulation of GnRH mRNA levels. The effect was confirmed with an AMPK antagonist, Compound C. Collectively, these findings demonstrate that glucose can directly regulate GnRH transcription, while implicating the AMPK pathway as an essential mediator of nutritional signaling in a novel GnRH neuronal cell model.

The membrane estrogen receptor ligand STX rapidly enhances GABAergic signaling in NPY/AgRP neurons: role in mediating the anorexigenic effects of 17β-estradiol

Besides its quintessential role in reproduction, 17β-estradiol (E2) is a potent anorexigenic hormone. E2 and the selective Gq-coupled membrane estrogen receptor (Gq-mER) ligand STX rapidly increase membrane excitability in proopiomelanocortin (POMC) neurons by desensitizing the coupling of GABAB receptors to G protein-coupled inwardly rectifying K(+) channels (GIRKs), which upon activation elicit a hyperpolarizing outward current. However, it is unknown whether E2 and STX can modulate GABAB signaling in neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons. We used single-cell RT-PCR and whole cell patch clamping with selective pharmacological reagents to show that NPY/AgRP cells of mice express the GABAB-R1 and -R2 receptors and are hyperpolarized by the GABAB agonist baclofen in an E2-dependent manner. In males, E2 rapidly attenuated the coupling of GABAB receptors to GIRKs, which was blocked by the general PI3K inhibitors wortmannin and LY-294002 or the selective p110β subunit inhibitor TGX-221. The ERα-selective agonist propyl pyrazole triol mimicked the effects of E2. STX, in contrast, enhanced the GABAB response in males, which was abrogated by the estrogen receptor (ER) antagonist ICI 182,780. In gonadectomized mice of both sexes, E2 enhanced or attenuated the GABAB response in different NPY/AgRP cells. Coperfusing wortmannin with E2 or simply applying STX always enhanced the GABAB response. Thus, in NPY/AgRP neurons, activation of the Gq-mER by E2 or STX enhances the GABAergic postsynaptic response, whereas activation of ERα by E2 attenuates it. These findings demonstrate a clear functional dichotomy of rapid E2 membrane-initiated signaling via ERα vs. Gq-mER in a CNS neuron vital for regulating energy homeostasis.

Kisspeptin activation of TRPC4 channels in female GnRH neurons requires PIP2 depletion and cSrc kinase activation

Kisspeptin signaling via its Gαq-coupled receptor GPR54 plays a crucial role in modulating GnRH neuronal excitability, which controls pituitary gonadotropins secretion and ultimately reproduction. Kisspeptin potently depolarizes GnRH neurons primarily through the activation of canonical transient receptor potential (TRPC) channels, but the intracellular signaling cascade has not been elucidated. Presently, we have established that kisspeptin activation of TRPC channels requires multiple membrane and intracellular signaling molecules. First, phosphatidylinositol-4,5-bisphosphate (PIP(2)) hydrolysis by phospholipase Cβ is required because whole-cell dialysis of Dioctanoylglycerol-PIP(2) (DiC8-PIP(2)) inhibited the kisspeptin activation of TRPC channels, and the phosphatidylinositol 4-kinase inhibitor wortmannin, which attenuates PIP(2) synthesis, prolonged TRPC channel activation. Using single cell RT-PCR, we identified that the mRNA for the PIP(2)-interacting TRPC channel subunit, TRPC4α, is expressed in GnRH neurons. Depletion of intracellular Ca(2+) stores by thapsigargin and inositol 1,4,5-trisphosphate had no effect, indicating that the TRPC channels are not store-operated. Neither removing extracellular Ca(2+) nor buffering intracellular Ca(2+) with EGTA or BAPTA had any effect on the kisspeptin activation of the TRPC channels. However, the Ca(2+) channel blocker Ni(2+) inhibited the kisspeptin-induced inward current. Moreover, inhibition of protein kinase C by bisindolylmaleimide-I or calphostin C had no effect, but activation of protein kinase C by phorbol 12,13-dibutyrate occluded the kisspeptin-activated current. Finally, inhibition of the cytoplasmic tyrosine kinase cSrc by genistein or the pyrazolo-pyrimidine PP2 blocked the activation of TRPC channels by kisspeptin. Therefore, TRPC channels in GnRH neurons are receptor-operated, and kisspeptin activates TRPC channels through PIP(2) depletion and cSrc tyrosine kinase activation, which is a novel signaling pathway for peptidergic excitation of GnRH neurons.

Kisspeptin inhibits a slow afterhyperpolarization current via protein kinase C and reduces spike frequency adaptation in GnRH neurons

Kisspeptin signaling via its cognate receptor G protein-coupled receptor 54 (GPR54) in gonadotropin-releasing hormone (GnRH) neurons plays a critical role in regulating pituitary secretion of luteinizing hormone and thus reproductive function. GPR54 is G(q)-coupled to activation of phospholipase C and multiple second messenger signaling pathways. Previous studies have shown that kisspeptin potently depolarizes GnRH neurons through the activation of canonical transient receptor potential channels and inhibition of inwardly rectifying K(+) channels to generate sustained firing. Since the initial studies showing that kisspeptin has prolonged effects, the question has been why is there very little spike frequency adaption during sustained firing? Presently, we have discovered that kisspeptin reduces spike frequency adaptation and prolongs firing via the inhibition of a calcium-activated slow afterhyperpolarization current (I(sAHP)). GnRH neurons expressed two distinct I(sAHP), a kisspeptin-sensitive and an apamin-sensitive I(sAHP). Essentially, kisspeptin inhibited 50% of the I(sAHP) and apamin inhibited the other 50% of the current. Furthermore, the kisspeptin-mediated inhibition of I(sAHP) was abrogated by the protein kinase C (PKC) inhibitor calphostin C, and the PKC activator phorbol 12,13-dibutyrate mimicked and occluded any further effects of kisspeptin on I(sAHP). The protein kinase A (PKA) inhibitors H-89 and the Rp diastereomer of adenosine 3',5'-cyclic monophosphorothioate had no effect on the kisspeptin-mediated inhibition but were able to abrogate the inhibitory effects of forskolin on the I(sAHP), suggesting that PKA is not involved. Therefore, in addition to increasing the firing rate through an overt depolarization, kisspeptin can also facilitate sustained firing through inhibiting an apamin-insensitive I(sAHP) in GnRH neurons via a PKC.

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.

Ion channels and information processing in GnRH neuron dendrites

Recent findings indicate that a majority of action potentials originate from dendrites of GnRH neurons. This localization of the dendrite as the principle site of action potential initiation has sparked considerable interest in the nature of ionic channels throughout GnRH neurons. This paper will review the ionic conductances described within GnRH neurons and their implications for physiological output, such as sensitivity to steroids and diurnal state. To date, a majority of information regarding ionic conductances in GnRH neurons pertains to somata and the first 50–100 µm of dendrite length. Thus, unraveling the tapestry created by the nature and distribution of dendritic conductances in GnRH neurons lies at the forefront of understanding the control of reproductive hormone secretion.

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.

Kisspeptin excitation of GnRH neurons

Kisspeptin binding to its cognate G protein-coupled receptor (GPR54, aka Kiss1R) in gonadotropin-releasing hormone (GnRH) neurons stimulates peptide release and activation of the reproductive axis in mammals. Kisspeptin has pronounced pre- and postsynaptic effects, with the latter dominating the excitability of GnRH neurons. Presynaptically, kisspeptin increases the excitatory drive (both GABA-A and glutamate) to GnRH neurons and postsynaptically, kisspeptin inhibits an A-type and inwardly rectifying K(+) (Kir 6.2 and GIRK) currents and activates nonselective cation (TRPC) currents to cause long-lasting depolarization and increased action potential firing. The signaling cascades and the multiple intracellular targets of kisspeptin actions in native GnRH neurons are continuing to be elucidated. This review summarizes our current state of knowledge about kisspeptin signaling in GnRH neurons.

Negative effects of high glucose exposure in human gonadotropin-releasing hormone neurons

Metabolic disorders are often associated with male hypogonadotropic hypogonadism, suggesting that hypothalamic defects involving GnRH neurons may impair the reproductive function. Among metabolic factors hyperglycemia has been implicated in the control of the reproductive axis at central level, both in humans and in animal models. To date, little is known about the direct effects of pathological high glucose concentrations on human GnRH neurons. In this study, we investigated the high glucose effects in the human GnRH-secreting FNC-B4 cells. Gene expression profiling by qRT-PCR, confirmed that FNC-B4 cells express GnRH and several genes relevant for GnRH neuron function (KISS1R, KISS1, sex steroid and leptin receptors, FGFR1, neuropilin 2, and semaphorins), along with glucose transporters (GLUT1, GLUT3, and GLUT4). High glucose exposure (22 mM; 40 mM) significantly reduced gene and protein expression of GnRH, KISS1R, KISS1, and leptin receptor, as compared to normal glucose (5 mM). Consistent with previous studies, leptin treatment significantly induced GnRH mRNA expression at 5 mM glucose, but not in the presence of high glucose concentrations. In conclusion, our findings demonstrate a deleterious direct contribution of high glucose on human GnRH neurons, thus providing new insights into pathogenic mechanisms linking metabolic disorders to reproductive dysfunctions.