Gamma-aminobutyric acid and glutamate differentially regulate intracellular calcium concentrations in mouse gonadotropin-releasing hormone neurons

Targeted Glutamate Supply Boosts Insulin Concentrations, Ovarian Activity, and Ovulation Rate in Yearling Goats during the Anestrous Season

The neuroendocrine regulation of the seasonal reproductive axis requires the integration of internal and external signals to ensure synchronized physiological and behavioral responses. Seasonal reproductive changes contribute to intermittent production, which poses challenges for optimizing goat product yields. Consequently, a significant objective in seasonal reproduction research is to attain continuous reproduction and enhance profitability in goat farming. Glutamate plays a crucial role as a modulator in several reproductive and metabolic processes. Hence, the aim of this study was to evaluate the potential impact of exogenous glutamate administration on serum insulin concentration and ovarian function during the out-of-season period in yearling goats. During the anestrous season, animals were randomly located in individual pens to form two experimental groups: (1) glutamate (n = 10, live weight (LW) = 29.1 ± 1.02 kg, body condition score (BCS) = 3.4 ± 0.2 units) and (2) control (n = 10; LW = 29.2 ± 1.07 kg, BCS = 3.5 ± 0.2), with no differences (p < 0.05) regarding LW and BCS. Then, goats were estrus-synchronized, and blood sampling was carried out for insulin quantification. Ovaries were ultrasonographically scanned to assess ovulation rate (OR), number of antral follicles (AFs), and total ovarian activity (TOA = OR + AF). The research outcomes support our working hypothesis. Certainly, our study confirms that those yearling goats treated with exogenous glutamate displayed the largest (p < 0.05) insulin concentrations across time as well as an augmented (p < 0.05) out-of-season ovarian activity.

Gamma-aminobutyric acid (GABA) suppresses hemocyte phagocytosis by binding to GABA receptors and modulating corresponding downstream pathways in blood clam, Tegillarca granosa

Although accumulating data demonstrated that gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, plays an important regulatory role in immunity of vertebrates, its immunomodulatory function and mechanisms of action remain poorly understood in invertebrates such as bivalve mollusks. In this study, the effect of GABA on phagocytic activity of hemocytes was evaluated in a commercial bivalve species, Tegillarca granosa. Furthermore, the potential regulatory mechanism underpinning was investigated by assessing potential downstream targets. Data obtained demonstrated that in vitro GABA incubation significantly constrained the phagocytic activity of hemocytes. In addition, the GABA-induced suppression of phagocytosis was markedly relieved by blocking of GABA_A and GABA_B receptors using corresponding antagonists. Hemocytes incubated with lipopolysaccharides (LPS) and GABA had significant higher K⁺-Cl^- cotransporter 2 (KCC2) content compared to the control. In addition, GABA treatment led to an elevation in intracellular Cl^-, which was shown to be leveled down to normal by blocking the ionotropic GABA_A receptor. Treatment with GABA_A receptor antagonist also rescued the suppression of GABA_A receptor-associated protein (GABARAP), KCC, TNF receptor associated factor 6 (TRAF6), inhibitor of nuclear factor kappa-B kinase subunit alpha (IKKα), and nuclear factor kappa B subunit 1 (NFκB) caused by GABA incubation. Furthermore, incubation of hemocytes with GABA resulted in a decrease in cAMP content, an increase in intracellular Ca²⁺, and downregulation of cAMP-dependent protein kinase (PKA), calmodulin kinase II (CAMK2), calmodulin (CaM), calcineurin (CaN), TRAF6, IKKα, and NFκB. All these above-mentioned changes were found to be evidently relieved by blocking the metabotropic G-protein-coupled GABA_B receptor. Our results suggest GABA may play an inhibitory role on phagocytosis through binding to both GABA_A and GABA_B receptors, and subsequently regulating corresponding downstream pathways in bivalve invertebrates.

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.

Puberty, A Sensitive Window of Hypothalamic Development and Plasticity

Puberty is a developmental period characterized by a broad range of physiologic changes necessary for the acquisition of adult sexual and reproductive maturity. These changes mirror complex modifications within the central nervous system, including within the hypothalamus. These modifications result in the maturation of a fully active hypothalamic-pituitary-gonadal (HPG) axis, the neuroendocrine cascade ensuring gonadal activation, sex steroid secretion, and gametogenesis. A complex and finely regulated neural network overseeing the HPG axis, particularly the pubertal reactivation of gonadotropin-releasing hormone (GnRH) secretion, has been progressively unveiled in the last 3 decades. This network includes kisspeptin, neurokinin B, GABAergic, and glutamatergic neurons as well as glial cells. In addition to substantial modifications in the expression of key targets, several changes in neuronal morphology, neural connections, and synapse organization occur to establish mature and coordinated neurohormonal secretion, leading to puberty initiation. The aim of this review is to outline the current knowledge of the major changes that neurons secreting GnRH and their neuronal and glial partners undergo before and after puberty. Emerging mediators upstream of GnRH, uncovered in recent years, are also addressed herein. In addition, the effects of sex steroids, particularly estradiol, on changes in hypothalamic neurodevelopment and plasticity are discussed.

Ca2+ involved in GABA signal transduction for phenolics accumulation in germinated hulless barley under NaCl stress

In this study, in order to investigate the role of Ca2+ in GABA signal transduction involved in phenolics accumulation in barley seedlings under NaCl stress, the seedlings were treated with exogenous GABA and its synthesis inhibitor, 3-mercaplopropionic acid (3-MP), as well as Ca2+ channel blockers La3+, Ca2+ chelator EGTA, and Ca2+ release channel inhibitor 2-aminoethoxydiphenyl borate (2-APB). The results showed that GABA significantly enhanced phenolics, calcium and calmodulin content. It also induced Ca2+ influx in barley root tips cells, and altered the distribution of Ca2+, making calcium precipitates more uniform and intensive. While, 3-MP treatment led to opposite changes, which suggested that GABA was essential for calcium content increase. In addition, accumulation of phenolics was inhibited by LaCl3, EGTA and 2-APB treatments, and this inhibition could be alleviated partly by exogenous GABA. Taken together, Ca2+ was involved in GABA signal transduction for phenolics accumulation in barley seedlings under NaCl stress.

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.

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.

Impact of Precooling and Controlled-Atmosphere Storage on γ-Aminobutyric Acid (GABA) Accumulation in Longan (Dimocarpus longan Lour.) Fruit

Longan (Dimocarpus longan Lour.) fruit cultivars 'Chuliang' and 'Shixia' were analyzed for γ-aminobutyric acid (GABA) accumulation after precooling and in controlled-atmosphere storage. Fruit were exposed to 5% O2 plus 3%, 5%, or 10% CO2 at 4 °C, and GABA and associated enzymes, aril firmness, and pericarp color were measured. Aril softening and pericarp browning were delayed by 5% CO2 + 5% O2. GABA concentrations and glutamate decarboxylase (GAD; EC 4.1.1.15) activities declined during storage at the higher-CO2 treatments. However, GABA aminotransferase (GABA-T; EC 2.6.1.19) activities in elevated CO2-treated fruit fluctuated during storage. GABA concentrations increased after precooling treatments. GAD activity and GABA-T activity were different between cultivars after precooling. GABA concentrations in fruit increased after 3 days of 10% CO2 + 5% O2 treatment and then declined as storage time increased. GABA accumulation was associated with stimulation of GAD activity rather than inhibition of GABA-T activity.

Estrogen permits vasopressin signaling in preoptic kisspeptin neurons in the female mouse

The cellular mechanisms governing the impact of the central circadian clock on neuronal networks are incompletely understood. We examine here the influence of the suprachiasmatic nucleus output neuropeptide arginine-vasopressin (AVP) on the activity of preoptic area kisspeptin neurons. These cells integrate circadian and hormonal signals within the neuronal network that regulates fertility in females. Electrophysiological recordings in brain slices from kisspeptin-GFP mice showed that AVP dose-dependently increased the firing rate of most kisspeptin neurons. These actions were mediated directly at the kisspeptin neuron. Experiments in mice expressing the calcium indicator GCaMP3 in kisspeptin neurons enabled simultaneous monitoring of intracellular calcium concentrations ([Ca(2+)]i) in multiple cells and revealed that AVP increased [Ca(2+)]i in >80% of diestrous kisspeptin neurons via a mechanism involving voltage-gated calcium channels. We next examined whether AVP signaling in kisspeptin neurons was time and ovarian cycle dependent. AVP exerted the same effects on diestrous and proestrous days of the ovarian cycle, whether hours before [zeitgeber time 4 (ZT4)-ZT6] or just before (ZT10) the expected time of the proestrous preovulatory luteinizing hormone surge. Remarkably, however, AVP signaling was critically dependent on circulating ovarian steroids as AVP no longer excited preoptic kisspeptin neurons in ovariectomized mice, an effect that was fully restored by estradiol treatment. Together, these studies show that AVP exerts a potent and direct stimulatory influence upon the electrical activity and [Ca(2+)]i of most preoptic kisspeptin neurons. Unexpectedly, estrogen is found to permit circadian AVP signaling at preoptic kisspeptin neurons rather than dynamically modulate its activity throughout the estrous cycle.

Chloride Accumulators NKCC1 and AE2 in Mouse GnRH Neurons: Implications for GABAA Mediated Excitation

A developmental “switch” in chloride transporters occurs in most neurons resulting in GABA_A mediated hyperpolarization in the adult. However, several neuronal cell subtypes maintain primarily depolarizing responses to GABA_A receptor activation. Among this group are gonadotropin-releasing hormone-1 (GnRH) neurons, which control puberty and reproduction. NKCC1 is the primary chloride accumulator in neurons, expressed at high levels early in development and contributes to depolarization after GABA_A receptor activation. In contrast, KCC2 is the primary chloride extruder in neurons, expressed at high levels in the adult and contributes to hyperpolarization after GABA_A receptor activation. Anion exchangers (AEs) are also potential modulators of responses to GABA_A activation since they accumulate chloride and extrude bicarbonate. To evaluate the mechanism(s) underlying GABA_A mediated depolarization, GnRH neurons were analyzed for 1) expression of chloride transporters and AEs in embryonic, pre-pubertal, and adult mice 2) responses to GABA_A receptor activation in NKCC1^-/- mice and 3) function of AEs in these responses. At all ages, GnRH neurons were immunopositive for NKCC1 and AE2 but not KCC2 or AE3. Using explants, calcium imaging and gramicidin perforated patch clamp techniques we found that GnRH neurons from NKCC1^-/- mice retained relatively normal responses to the GABA_A agonist muscimol. However, acute pharmacological inhibition of NKCC1 with bumetanide eliminated the depolarization/calcium response to muscimol in 40% of GnRH neurons from WT mice. In the remaining GnRH neurons, HCO₃^- mediated mechanisms accounted for the remaining calcium responses to muscimol. Collectively these data reveal mechanisms responsible for maintaining depolarizing GABA_A mediated transmission in GnRH neurons.

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.

Gas release-based prescreening combined with reversed-phase HPLC quantitation for efficient selection of high-γ-aminobutyric acid (GABA)-producing lactic acid bacteria

High γ-aminobutyric acid (GABA)-producing lactobacilli are promising for the manufacture of GABA-rich foods and to synthesize GRAS (generally recognized as safe)-grade GABA. However, common chromatography-based screening is time-consuming and inefficient. In the present study, Korean kimchi was used as a model of lactic acid-based fermented foods, and a gas release-based prescreening of potential GABA producers was developed. The ability to produce GABA by potential GABA producers in de Man, Rogosa, and Sharpe medium supplemented with or without monosodium glutamate was further determined by HPLC. Based on the results, 9 isolates were regarded as high GABA producers, and were further genetically identified as Lactobacillus brevis based on the sequences of 16S rRNA gene. Gas release-based prescreening combined with reversed-phase HPLC confirmation was an efficient and cost-effective method to identify high-GABA-producing LAB, which could be good candidates for probiotics. The GABA that is naturally produced by these high-GABA-producing LAB could be used as a food additive.

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.

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.

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 and seasonality of reproduction

Wild and domesticated species display seasonality in reproductive function, controlled predominantly by photoperiod. Seasonal alterations in breeding status are caused by changes in the secretion of gonadotropin-releasing hormone (GnRH) that are mediated by upstream neuronal afferents that regulate the GnRH cells. In particular, kisspeptin appears to play a major role in seasonality of reproduction, transducing the feedback effect of gonadal steroids as well as having an independent (nonsteroid dependent) circannual rhythm. A substantial body of data on this issue has been obtained from studies in sheep and hamsters and this is reviewed here in detail. Kisspeptin function is upregulated during the breeding season in sheep, stimulating reproductive function, but contradictory data are found in Siberian and Syrian hamsters. The relative quiescence of kisspeptin cells in the nonbreeding season can be counteracted by administration of the peptide, leading to activation of reproductive function. Although there is a major role for melatonin in the transduction of photoperiod to the reproductive system, kisspeptin cells do not appear to express the melatonin receptor, so the means by which seasonality changes the level of kisspeptin activity remains unknown.

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

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

Simulated GABA synaptic input and L-type calcium channels form functional microdomains in hypothalamic gonadotropin-releasing hormone neurons

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons integrate the multiple internal and external cues that regulate sexual reproduction. In contrast to other neurons that exhibit extensive dendritic arbors, GnRH neurons usually have a single dendrite with relatively little branching. This largely precludes the integration strategy in which a single dendritic branch serves as a unit of integration. In the present study, we identify a gradient in L-type calcium channels in dendrites of mouse GnRH neurons and its interaction with GABAergic and glutamatergic inputs. Higher levels of L-type calcium channels are in somata/proximal dendrites (i.e., 0-26 μm) and distal dendrites (∼130 μm dendrite length), but intervening midlengths of dendrite (∼27-130 μm) have reduced L-type calcium channels. Using uncaging of GABA, there is a decreasing GABAergic influence along the dendrite and the impact of GABA(A) receptors is dependent on activation of L-type calcium channels. This results in amplification of proximal GABAergic signals and attenuation of distal dendritic signals. Most interestingly, the intervening dendritic regions create a filter through which only relatively high-amplitude, low-frequency GABAergic signaling to dendrites elicits action potentials. The findings of the present study suggest that GnRH dendrites adopt an integration strategy whereby segments of single nonbranching GnRH dendrites create functional microdomains and thus serve as units of integration.

GnRH neuron firing and response to GABA in vitro depend on acute brain slice thickness and orientation

The GnRH neurons exhibit long dendrites and project to the median eminence. The aim of the present study was to generate an acute brain slice preparation that enabled recordings to be undertaken from GnRH neurons maintaining the full extent of their dendrites or axons. A thick, horizontal brain slice was developed, in which it was possible to record from the horizontally oriented GnRH neurons located in the anterior hypothalamic area (AHA). In vivo studies showed that the majority of AHA GnRH neurons projected outside the blood-brain barrier and expressed c-Fos at the time of the GnRH surge. On-cell recordings compared AHA GnRH neurons in the horizontal slice (AHAh) with AHA and preoptic area (POA) GnRH neurons in coronal slices [POA coronal (POAc) and AHA coronal (AHAc), respectively]. AHAh GnRH neurons exhibited tighter burst firing compared with other slice orientations. Although α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) excited GnRH neurons in all preparations, γ-aminobutyric acid (GABA) was excitatory in AHAc and POAc but inhibitory in AHAh slices. GABA(A) receptor postsynaptic currents were the same in AHAh and AHAc slices. Intriguingly, direct activation of GABA(A) or GABA(B) receptors respectively stimulated and inhibited GnRH neurons regardless of slice orientation. Subsequent experiments indicated that net GABA effects were determined by differences in the ratio of GABA(A) and GABA(B) receptor-mediated effects in "long" and "short" dendrites of GnRH neurons in the different slice orientations. These studies document a new brain slice preparation for recording from GnRH neurons with their extensive dendrites/axons and highlight the importance of GnRH neuron orientation relative to the angle of brain slicing in studying these neurons in vitro.

Initiation and propagation of action potentials in gonadotropin-releasing hormone neuron dendrites

Gonadotropin-releasing hormone (GnRH) neurons are the final output neurons in a complex neuronal network that regulates fertility. The morphology of GnRH neuron dendrites is very different to other central neurons in that they are very long, thin, and unbranched. To study the function of these dendrites, we have used Na(+) and Ca(2+) imaging in combination with dual soma and dendrite electrical recordings in brain slices from GnRH-GFP mice. Here, we show that GnRH neurons actively propagate Na(+) spikes throughout their dendrites. Multisite dendritic recordings confirmed that these spikes were observed in one of the dendrites before the soma in the great majority of neurons tested. Na(+) imaging experiments revealed that the initial 150 μm of dendrite has a higher density of functional Na(+) channels than more distal regions, suggesting that this region of dendrite is highly excitable and may be the site of spike initiation. Finally, we show that the depolarization from dendritic spikes opens voltage-gated Ca(2+) channels giving rise to dendritic Ca(2+) transients. Together, these findings suggest that the proximal dendrites of GnRH neurons are highly excitable and are likely to be the site of action potential initiation in these neurons.

Understanding calcium homeostasis in postnatal gonadotropin-releasing hormone neurons using cell-specific Pericam transgenics

The gonadotropin-releasing hormone (GnRH) neurons are the key output cells of a complex neuronal network controlling fertility in mammals. To examine calcium homeostasis in postnatal GnRH neurons, we generated a transgenic mouse line in which the genetically encodable calcium indicator ratiometric Pericam (rPericam) was targeted to the GnRH neurons. This mouse model enabled real-time imaging of calcium concentrations in GnRH neurons in the acute brain slice preparation. Investigations in GnRH-rPericam mice revealed that GnRH neurons exhibited spontaneous, long-duration (~8s) calcium transients. Dual electrical-calcium recordings revealed that the calcium transients were correlated perfectly with burst firing in GnRH neurons and that calcium transients in GnRH neurons regulated two calcium-activated potassium channels that, in turn, determined burst firing dynamics in these cells. Curiously, the occurrence of calcium transients in GnRH neurons across puberty or through the estrous cycle did not correlate well with the assumption that GnRH neuron burst firing was contributory to changing patterns of pulsatile GnRH release at these times. The GnRH-rPericam mouse was also valuable in determining differential mechanisms of GABA and glutamate control of calcium levels in GnRH neurons as well as effects of G-protein-coupled receptors for GnRH and kisspeptin. The simultaneous measurement of calcium levels in multiple GnRH neurons was hampered by variable rPericam fluorescence in different GnRH neurons. Nevertheless, in the multiple recordings that were achieved no evidence was found for synchronous calcium transients. Together, these observations show the great utility of transgenic targeting strategies for investigating the roles of calcium with specified neuronal cell types.

Control of GnRH secretion: one step back

The reproductive system is controlled by gonadotropin releasing hormone (GnRH) secretion from the brain, which is finely modulated by a number of factors including gonadal sex steroids. GnRH cells do not express estrogen receptor α, but feedback is transmitted by neurons that are at least 'one step back' from the GnRH cells. Modulation by season, stress and nutrition are effected by neuronal pathways that converge on the GnRH cells. Kisspeptin and gonadotropin inhibitory hormone (GnIH) neurons are regulators of GnRH secretion, the former being a major conduit for transmission of sex steroid feedback. GnIH cells project to GnRH cells and may play a role in the seasonal changes in reproductive activity in sheep. GnIH also modulates the action of GnRH at the level of the pituitary gonadotrope. This review focuses on the role that kisspeptin and GnIH neurons play, as modulators that are 'one step back' from GnRH neurons.

Depolarising and hyperpolarising actions of GABA(A) receptor activation on gonadotrophin-releasing hormone neurones: towards an emerging consensus

The gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones of a complex neuronal network that controls fertility. It is now appreciated that GABAergic neurones within this network provide an important regulatory influence on GnRH neurones. However, the consequences of direct GABA(A) receptor activation on adult GnRH neurones have been controversial for nearly a decade now, with both hyperpolarising and depolarising effects being reported. This review provides: (i) an overview of GABA(A) receptor function and its investigation using electrophysiological approaches and (ii) re-examines the past and present results relating to GABAergic regulation of the GnRH neurone, with a focus on mouse brain slice data. Although it remains difficult to reconcile the results of the early studies, there is a growing consensus that GABA can act through the GABA(A) receptor to exert both depolarising and hyperpolarising effects on GnRH neurones. The most recent studies examining the effects of endogenous GABA release on GnRH neurones indicate that the predominant action is that of excitation. However, we are still far from a complete understanding of the effects of GABA(A) receptor activation upon GnRH neurones. We argue that this will require not only a better understanding of chloride ion homeostasis in individual GnRH neurones, and within subcellular compartments of the GnRH neurone, but also a more integrative view of how multiple neurotransmitters, neuromodulators and intrinsic conductances act together to regulate the activity of these important cells.

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

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

Frequency-dependent recruitment of fast amino acid and slow neuropeptide neurotransmitter release controls gonadotropin-releasing hormone neuron excitability

The anteroventral periventricular nucleus (AVPV) is thought to play a key role in regulating the excitability of gonadotropin-releasing hormone (GnRH) neurons that control fertility. Using an angled, parahorizontal brain slice preparation we have undertaken a series of electrophysiological experiments to examine how the AVPV controls GnRH neurons in adult male and female mice. More than half (59%) of GnRH neurons located in the rostral preoptic area were found to receive monosynaptic inputs from the AVPV in a sex-dependent manner. AVPV stimulation frequencies <1 Hz generated short-latency action potentials in GnRH neurons with GABA and glutamate mediating >90% of the evoked fast synaptic currents. The AVPV GABA input was dominant and found to excite or inhibit GnRH neurons in a cell-dependent manner. Increasing the AVPV stimulation frequency to 5-10 Hz resulted in the appearance of additional poststimulus inhibitory as well as delayed excitatory responses in GnRH neurons that were independent of ionotropic amino acid receptors. The inhibition observed immediately following the end of the stimulation period was mediated partly by GABA(B) receptors, while the delayed activation was mediated by the neuropeptide kisspeptin. The latter response was essentially absent in Gpr54 knock-out mice and abolished by a Gpr54 antagonist. Together, these studies show that AVPV neurons provide direct amino acid and neuropeptidergic inputs to GnRH neurons. Low-frequency activation generates predominant GABA/glutamate release with higher frequency activation recruiting release of kisspeptin. This frequency-dependent release of amino acid and neuropeptide neurotransmitters greatly expands the range of AVPV control of GnRH neuron excitability.

Tonic extrasynaptic GABA(A) receptor currents control gonadotropin-releasing hormone neuron excitability in the mouse

It is well established that the GABA(A) receptor plays an important role in regulating the electrical excitability of GnRH neurons. Two different modes of GABA(A) receptor signaling exist: one mediated by synaptic receptors generating fast (phasic) postsynaptic currents and the other mediated by extrasynaptic receptors generating a persistent (tonic) current. Using GABA(A) receptor antagonists picrotoxin, bicuculline methiodide, and gabazine, which differentiate between phasic and tonic signaling, we found that ∼50% of GnRH neurons exhibit an approximately 15-pA tonic GABA(A) receptor current in the acute brain slice preparation. The blockade of either neuronal (NO711) or glial (SNAP-5114) GABA transporter activity within the brain slice revealed the presence of tonic GABA signaling in ∼90% of GnRH neurons. The GABA(A) receptor δ subunit is only found in extrasynaptic GABA(A) receptors. Using single-cell RT-PCR, GABA(A) receptor δ subunit mRNA was identified in GnRH neurons and the δ subunit-specific agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol was found to activate inward currents in GnRH neurons. Perforated-patch clamp studies showed that 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol exerted the same depolarizing or hyperpolarizing effects as GABA on juvenile and adult GnRH neurons and that tonic GABA(A) receptor signaling regulates resting membrane potential. Together, these studies reveal the presence of a tonic GABA(A) receptor current in GnRH neurons that controls their excitability. The level of tonic current is dependent, in part, on neuronal and glial GABA transporter activity and mediated by extrasynaptic δ subunit-containing GABA(A) receptors.

Phenotypic and molecular characterization of proliferating and differentiated GnRH-expressing GnV-3 cells

GnRH neurons provide the primary driving force upon the neuroendocrine reproductive axis. Here we used GnV-3 cells, a model of conditionally immortalized GnRH-expressing neurons, to perform an analysis of cell cycle and compare the gene expression profile of proliferating cells with differentiated cells. In the proliferation medium, 45 ± 1.5% of GnV-3 cells are in S-phase by FACS analysis. In the differentiation medium, only 9 ± 0.9% of them are in S-phase, and they acquire the characteristic bipolar shape displayed by preoptic GnRH neurons in vivo. In addition, GnV-3 cells in the differentiated state exhibit electrophysiological properties characteristic of neurons. Transcriptomic analysis identified up-regulation of 1931 genes and down-regulation of 1270 genes in cells grown in the differentiation medium compared to cells in the proliferation medium. Subsequent gene ontology study indicated that genes over-expressed in proliferating GnV-3 cells were mainly involved in cell cycle regulations, whereas genes over-expressed in differentiated cells were mainly involved in processes of differentiation, neurogenesis and neuronal morphogenesis. Taken together, these data demonstrate the occurrence of morphological and physiological changes in GnV-3 cells between the proliferating and the differentiated state. Moreover, the genes differentially regulated between these two different states are providing novel pathways potentially important for a better understanding of the physiology of mature GnRH neurons.

Knockdown of GABA(A) receptor signaling in GnRH neurons has minimal effects upon fertility

The amino acid gamma-aminobutyric acid (GABA) is thought to play a key role in shaping the activity of the GnRH neurons throughout embryonic and postnatal life. However, the physiological roles of direct GABA inputs to GnRH neurons remain unknown. Using a Cre-LoxP strategy, we generated a targeted mouse line, in which all (98 +/- 1%) GnRH neurons had the gamma2-subunit of the GABA(A) receptor deleted. Electrophysiological recordings of GABA(A)-mediated postsynaptic currents from green fluorescent protein-tagged GnRH neurons with the gamma2-subunit knocked out (GnRH gamma2 KO) showed that the amplitude and frequency of GABA(A) postsynaptic currents were reduced by 70% (P < 0.01) and 77% (P < 0.05), respectively, and that the response to exogenous GABA was reduced by 90% (P < 0.01). Evaluation of male and female GnRH gamma2 KO mice revealed completely normal fecundity, estrous cycles, and puberty onset. Further investigation with gonadectomy and different steroid replacement regimens showed normal basal levels of LH in both sexes, and a normal estradiol-evoked positive feedback mechanism in females. However, the increment in LH after gonadectomy in GnRH gamma2 KO female mice was double that of controls (P < 0.05) and also more potently suppressed by 17-beta-estradiol (P < 0.05). A similar but nonsignificant trend was observed in GnRH gamma2 KO male mice. Together, these findings show that 70-90% reductions in the normal levels of GABA(A) receptor activity at the GnRH neuron appear to impact upon the estrogen negative feedback mechanism but are, nevertheless, compatible with normal fertility in mice.

Energy balance and puberty onset: emerging role of central mTOR signaling

The onset of puberty is gated by body energy reserves and nutritional cues. The adipose hormone leptin is an essential signal for the metabolic control of puberty, through mechanisms that are yet to be fully characterized. Mammalian target of rapamycin (mTOR), an energetic cell sensor, operates at specific hypothalamic nuclei as a transducer for leptin effects on feeding and energy homeostasis. This review summarizes recent experimental evidence supporting a role for central mTOR signaling in puberty onset. These findings are discussed in the context of topical developments in the field, such as recognition of the roles of the cAMP responsive element-binding protein regulated transcription coactivator-1 (Crtc1) and kisspeptins in the metabolic control of reproduction, thus highlighting novel mechanisms responsible for coupling puberty and energy homeostasis.

Glutamate regulation of GnRH neuron excitability

The gonadotropin-releasing hormone (GnRH) neuronal network is the master controller of the reproductive axis. It is widely accepted that the amino acid transmitters GABA and glutamate play important roles in controlling GnRH neuron excitability. However, remarkably few studies have examined the functional role of direct glutamate regulation of GnRH neurons. Dual-labeling investigations have shown that GnRH neurons express receptor subunits required for AMPA, NMDA and kainate signaling in a heterogeneous manner. Electrophysiological and calcium imaging studies have confirmed this heterogeneity and shown that while the majority of adult GnRH neurons express AMPA/kainate receptors, only small sub-populations have functional NMDA or metabotropic glutamate receptors. Accumulating evidence suggests that one important role of direct glutamate signaling at GnRH neurons is for their activation at the time of puberty. Whereas in vivo studies have indicated the importance of NMDA signaling within the whole of the GnRH neuronal network, including afferent neurons and glia, investigations at the level of the GnRH neuron suggest that peripubertal changes in AMPA receptor expression may be dominant in the mouse. The sources of glutamatergic inputs to the GnRH neurons are only just beginning to be examined and include the anteroventral periventricular nucleus as well as the possibility that GnRH neurons may use glutamate as a neurotransmitter in recurrent collateral innervation. It is expected that a full understanding of the glutamatergic regulation of GnRH neurons will provide significant insight into the mechanisms underlying their control of reproductive function.

Corticotrophin-releasing factor and stress-induced inhibition of the gonadotrophin-releasing hormone pulse generator in the female

It is well established that stress activates the hypothalamo-pituitary-adrenal (HPA) axis and suppresses the hypothalamo-pituitary-gonadal (HPG) axis. A large literature dealing with various stressors that regulate gonadotrophin-releasing hormone (GnRH) secretion in a variety of species (including nonhuman primates, sheep, and rats) provides evidence that stress modulates GnRH secretion by activating the corticotrophin-releasing factor (CRF) system and sympathoadrenal pathways, as well as the limbic brain. Different stressors may suppress the HPG axis by activating or inhibiting various pathways in the CNS. In addition to CRF being the principal hypophysiotropic factor driving the HPA axis, it is a potent inhibitor of the GnRH pulse generator. The suppression of the GnRH pulse generator by a variety of stressful stimuli can be blocked by CRF antagonists, suggesting a pivotal role for endogenous CRF. The differential roles for CRF receptor type 1 (CRF-R1) and CRF-R2 in stress-induced suppression of the GnRH pulse generator add to the complexity of CRF regulation of the HPG axis. Although the precise sites and mechanisms of action remain to be elucidated, noradrenergic and gamma-amino-butyric acid (GABA) neurones are implicated in the system's regulation, and opioids and kisspeptin in the medial preoptic area (mPOA) and hypothalamic arcuate nucleus (ARC) may operate downstream of the CRF neuronal system.

The calcium oscillator of GnRH-1 neurons is developmentally regulated

Oscillations in intracellular calcium levels have been described in GnRH-1 neurons in both prenatal and adult cells. However, differences have been reported in the mechanisms underlying these [Ca(2+)](i) oscillations, dependent on the model used. The goal of this study was to address whether these changes depend on the maturation status of GnRH-1 neurons by assaying prenatal GnRH-1 cells maintained in explants, at two different developmental stages. This report documents an increase in the frequency of [Ca(2+)](i) oscillations between 1 and 3 wk of in vitro maturation. During the early stage, [Ca(2+)](i) oscillations are blocked by tetrodotoxin and are mainly triggered by excitatory neurotransmitters, gamma-aminobutyric acid (GABA), and glutamate. In contrast, in the later stage, some cells exhibit residual tetrodotoxin-insensitive [Ca(2+)](i) oscillations, which are sustained by action potential-independent GABA and glutamate release. The strength of these two excitatory inputs remained relatively constant during the maturation process, and the increase in frequency of [Ca(2+)](i) oscillations observed at the later stage is due to a novel excitatory input carried by cholecystokinin. Together, these data indicate developmentally regulated release and interactions of neurotransmitters (known regulators of GnRH-1 cells in adults) and point to extrinsic factors regulating GnRH-1 cellular physiology.

The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges

Ovarian steroids normally exert homeostatic negative feedback on GnRH release. During sustained exposure to elevated estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of estradiol switches to positive, inducing a surge of GnRH release from the brain, which signals the pituitary LH surge that triggers ovulation. In rodents, this switch appears dependent on a circadian signal that times the surge to a specific time of day (e.g., late afternoon in nocturnal species). Although the precise nature of this daily signal and the mechanism of the switch from negative to positive feedback have remained elusive, work in the past decade has provided much insight into the role of circadian/diurnal and estradiol-dependent signals in GnRH/LH surge regulation and timing. Here we review the current knowledge of the neurobiology of the GnRH surge, in particular the actions of estradiol on GnRH neurons and their synaptic afferents, the regulation of GnRH neurons by fast synaptic transmission mediated by the neurotransmitters gamma-aminobutyric acid and glutamate, and the host of excitatory and inhibitory neuromodulators including kisspeptin, vasoactive intestinal polypeptide, catecholamines, neurokinin B, and RFamide-related peptides, that appear essential for GnRH surge regulation, and ultimately ovulation and fertility.

Calcium dynamics in gonadotropin-releasing hormone neurons

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

Electrophysiological characteristics of gonadotrophin-releasing hormone 1-3 neurones: insights from a study of fish brains

The vertebrate gonadotrophin-releasing hormone (GnRH) neurones are considered to consist of one group of hypothalamic neuroendocrine and two groups of extrahypothalamic neuromodulatory GnRH neurones, and each group of neurones expresses different molecular species of GnRH peptide. Different GnRH peptides are produced by one of the three paralogous GnRH genes, gnrh1, gnrh2 and gnrh3, which are considered to have originated from gene duplications. All three GnRH systems are well developed in teleost brains. By taking advantage of this, and especially the use of GnRH-green fluorescent protein transgenic fish, the anatomical and electrophysiological properties of all three types of GnRH neurones can now be studied. The hypophysiotropic GnRH1 neurones in the preoptic area show episodic spontaneous electrical activities, whereas the extrahypothalamic GnRH2 neurones in the midbrain and GnRH3 neurones in the terminal nerve show regular intrinsic pacemaker activities. It is suggested that these different electrophysiological properties are related to their different functions (i.e. GnRH1 neurones act as hypophysiotropic neuroendocrine regulators and GnRH2 and GnRH3 neurones act as neuromodulators). The present review focuses on recent electrophysiological analyses of GnRH3 neurones, which have revealed the excitatory GABAergic and the inhibitory FMRFamide-like peptidergic regulations acting upon them, as well as gap junctional electrotonic coupling.

Excitatory action of GABA in the terminal nerve gonadotropin-releasing hormone neurons

The terminal nerve (TN)-gonadotropin-releasing hormone (GnRH) neurons have been suggested to function as a neuromodulatory system that regulates the motivational and arousal state of the animal and have served as a model system for the study of GnRH neuron physiology. To investigate the synaptic control of the TN-GnRH neurons, we analyzed electrophysiologically the effect of GABA on the TN-GnRH neurons. GABA generally hyperpolarizes most of the neurons in the adult brain by activating GABA(A) receptors while the activation of GABA(A) receptors depolarizes some specific neurons in the mature brain. Here we examined the GABA(A) receptor-mediated responses in the TN-GnRH neurons of adult teleost fish, the dwarf gourami, by means of gramicidin-perforated patch-clamp and cell-attached patch-clamp recordings. The reversal potential for the currents through GABA(A) receptors under the voltage clamp was depolarized relative to the resting membrane potential. GABA(A) receptor activation depolarized TN-GnRH neurons under the current clamp and had excitatory effect on their electrical activity, whereas the stronger GABA(A) receptor activation had bidirectional effect (excitatory-inhibitory). This excitatory effect is suggested to arise from high [Cl(-)](i) and was shown to be suppressed by bumetanide, the blocker of Cl(-)-accumulating sodium-potassium-2-chloride co-transporter (NKCC). The present results demonstrate that GABA(A) receptor activation induces excitation in TN-GnRH neurons, which may facilitate their neuromodulatory functions by increasing their spontaneous firing frequencies. The excitatory actions of GABA in the adult brain have recently been attracting much attention, and the easily accessible large TN-GnRH neurons should be a nice model system to analyze their physiological functions.