Whole-cell recordings from preoptic/hypothalamic slices reveal burst firing in gonadotropin-releasing hormone neurons identified with green fluorescent protein in transgenic mice

Population bursts in a modular neural network as a mechanism for synchronized activity in KNDy neurons

The pulsatile activity of gonadotropin-releasing hormone neurons (GnRH neurons) is a key factor in the regulation of reproductive hormones. This pulsatility is orchestrated by a network of neurons that release the neurotransmitters kisspeptin, neurokinin B, and dynorphin (KNDy neurons), and produce episodic bursts of activity driving the GnRH neurons. We show in this computational study that the features of coordinated KNDy neuron activity can be explained by a neural network in which connectivity among neurons is modular. That is, a network structure consisting of clusters of highly-connected neurons with sparse coupling among the clusters. This modular structure, with distinct parameters for intracluster and intercluster coupling, also yields predictions for the differential effects on synchronization of changes in the coupling strength within clusters versus between clusters.

Electrophysiological characterization of medial preoptic neurons in lactating rats and its modulation by hypocretin-1

The medial preoptic area (mPOA) undergoes through neuroanatomical changes across the postpartum period, during which its neurons play a critical role in the regulation of maternal behavior. In addition, this area is also crucial for sleep-wake regulation. We have previously shown that hypocretins (HCRT) within the mPOA facilitate active maternal behaviors in postpartum rats, while the blockade of endogenous HCRT in this area promotes nursing and sleep. To explore the mechanisms behind these HCRT actions, we aimed to evaluate the effects of juxta-cellular HCRT-1 administration on mPOA neurons in urethane-anesthetized postpartum and virgin female rats. We recorded mPOA single units and the electroencephalogram (EEG) and applied HCRT-1 juxta-cellular by pressure pulses. Our main results show that the electrophysiological characteristics of the mPOA neurons and their relationship with the EEG of postpartum rats did not differ from virgin rats. Additionally, neurons that respond to HCRT-1 had a slower firing rate than those that did not. In addition, administration of HCRT increased the activity in one group of neurons while decreasing it in another, both in postpartum and virgin rats. The mechanisms by which HCRT modulate functions controlled by the mPOA involve different cell populations.

Gonadal Cycle-Dependent Expression of Genes Encoding Peptide-, Growth Factor-, and Orphan G-Protein-Coupled Receptors in Gonadotropin- Releasing Hormone Neurons of Mice

Rising serum estradiol triggers the surge release of gonadotropin-releasing hormone (GnRH) at late proestrus leading to ovulation. We hypothesized that proestrus evokes alterations in peptidergic signaling onto GnRH neurons inducing a differential expression of neuropeptide-, growth factor-, and orphan G-protein-coupled receptor (GPCR) genes. Thus, we analyzed the transcriptome of GnRH neurons collected from intact, proestrous and metestrous GnRH-green fluorescent protein (GnRH-GFP) transgenic mice using Affymetrix microarray technique. Proestrus resulted in a differential expression of genes coding for peptide/neuropeptide receptors including Adipor1, Prokr1, Ednrb, Rtn4r, Nmbr, Acvr2b, Sctr, Npr3, Nmur1, Mc3r, Cckbr, and Amhr2. In this gene cluster, Adipor1 mRNA expression was upregulated and the others were downregulated. Expression of growth factor receptors and their related proteins was also altered showing upregulation of Fgfr1, Igf1r, Grb2, Grb10, and Ngfrap1 and downregulation of Egfr and Tgfbr2 genes. Gpr107, an orphan GPCR, was upregulated during proestrus, while others were significantly downregulated (Gpr1, Gpr87, Gpr18, Gpr62, Gpr125, Gpr183, Gpr4, and Gpr88). Further affected receptors included vomeronasal receptors (Vmn1r172, Vmn2r-ps54, and Vmn1r148) and platelet-activating factor receptor (Ptafr), all with marked downregulation. Patch-clamp recordings from mouse GnRH-GFP neurons carried out at metestrus confirmed that the differentially expressed IGF-1, secretin, and GPR107 receptors were operational, as their activation by specific ligands evoked an increase in the frequency of miniature postsynaptic currents (mPSCs). These findings show the contribution of certain novel peptides, growth factors, and ligands of orphan GPCRs to regulation of GnRH neurons and their preparation for the surge release.

Networking of glucagon-like peptide-1 axons with GnRH neurons in the basal forebrain of male mice revealed by 3DISCO-based immunocytochemistry and optogenetics

Glucagon-like peptide-1 (GLP-1) regulates reproduction centrally, although, the neuroanatomical basis of the process is unknown. Therefore, the putative networking of the central GLP-1 and gonadotropin-releasing hormone (GnRH) systems was addressed in male mice using whole mount immunocytochemistry and optogenetics. Enhanced antibody penetration and optical clearing procedures applied to 500–1000 µm thick basal forebrain slices allowed the simultaneous visualization of the two distinct systems in the basal forebrain. Beaded GLP-1-IR axons innervated about a quarter of GnRH neurons (23.2 ± 1.4%) forming either single or multiple contacts. GnRH dendrites received a more intense GLP-1 innervation (64.6 ± 0.03%) than perikarya (35.4 ± 0.03%). The physiological significance of the innervation was examined by optogenetic activation of channelrhodopsin-2 (ChR2)-expressing axons of preproglucagon (GCG) neurons upon the firing of GnRH neurons by patch clamp electrophysiology in acute brain slices of triple transgenic mice (Gcg-cre/ChR2/GFP-GnRH). High-frequency laser beam stimulation (20 Hz, 10 ms pulse width, 3 mW laser power) of ChR2-expressing GCG axons in the mPOA increased the firing rate of GnRH neurons (by 75 ± 17.3%, p = 0.0007). Application of the GLP-1 receptor antagonist, Exendin-3-(9-39) (1 μM), prior to the photo-stimulation, abolished the facilitatory effect. In contrast, low-frequency trains of laser pulses (0.2 Hz, 60 pulses) had no effect on the spontaneous postsynaptic currents of GnRH neurons. The findings indicate a direct wiring of GLP-1 neurons with GnRH cells which route is excitatory for the GnRH system. The pathway may relay metabolic signals to GnRH neurons and synchronize metabolism with reproduction.

Chemogenetic Suppression of GnRH Neurons during Pubertal Development Can Alter Adult GnRH Neuron Firing Rate and Reproductive Parameters in Female Mice

Gonadotropin-releasing hormone (GnRH) neurons control anterior pituitary, and thereby gonadal, function. GnRH neurons are active before outward indicators of puberty appear. Prenatal androgen (PNA) exposure mimics reproductive dysfunction of the common fertility disorder polycystic ovary syndrome (PCOS) and reduces prepubertal GnRH neuron activity. Early neuron activity can play a critical role in establishing circuitry and adult function. We tested the hypothesis that changing prepubertal GnRH neuron activity programs adult GnRH neuron activity and reproduction independent of androgen exposure in female mice. Activating (3Dq) or inhibitory (4Di) designer receptors exclusively activated by designer drugs (DREADDs) were targeted to GnRH neurons using Cre-lox technology. In control studies, the DREADD ligand clozapine n-oxide (CNO) produced the expected changes in GnRH neuron activity in vitro and luteinizing hormone (LH) release in vivo. CNO was administered to control or PNA mice between two and three weeks of age, when GnRH neuron firing rate is reduced in PNA mice. In controls, reducing prepubertal GnRH neuron activity with 4Di increased adult GnRH neuron firing rate and days in diestrus but did not change puberty onset or GABA transmission to these cells. In contrast, activating GnRH neurons had no effect on reproductive parameters or firing rate and did not rescue reproductive phenotypes in PNA mice. These studies support the hypothesis that prepubertal neuronal activity sculpts elements of the adult reproductive neuroendocrine axis and cyclicity but indicate that other PNA-induced programming actions are required for full reproductive phenotypes and/or that compensatory mechanisms overcome activity-mediated changes to mitigate reproductive changes in adults.

High-Frequency Firing Activity of GnRH1 Neurons in Female Medaka Induces the Release of GnRH1 Peptide From Their Nerve Terminals in the Pituitary

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons play an important role in promoting secretion of pituitary luteinizing hormone (LH) and ovulation by releasing GnRH peptide. The release of GnRH peptide is generally assumed to be mainly modulated according to the firing activity of GnRH neurons. However, the relationship between the firing activity and the release of GnRH peptide has been elusive. We analyzed the relationship using two lines of transgenic medaka (gnrh1:enhanced green fluorescent protein and lhb:inverse-pericam) for the combined electrophysiological and Ca2+ imaging analyses. We show that a high-frequency firing activity induced by an excitatory neurotransmitter, glutamate, strongly increases [Ca2+]i in the cell bodies of GnRH1 neurons, which should lead to stimulation of GnRH release. We examined whether this high-frequency firing actually leads to the release of endogenous GnRH1 peptide from the nerve terminals projecting to the pituitary LH cells using a whole brain-pituitary preparation of a fish generated by crossing the two types of transgenic fish. Ca2+ imaging analyses showed that local glutamate activation of GnRH1 cell bodies, but not their nerve terminals in the pituitary, induced a substantial Ca2+ response in LH cells that was abolished in the presence of a GnRH receptor antagonist, Analog M. These results suggest that such an evoked high-frequency firing activity of GnRH1 cell body stimulates the release of endogenous GnRH1 peptide from the axon terminals to the pituitary LH cells. Thus, the findings of the present study have clearly demonstrated the relationship between the firing activity of hypothalamic GnRH neurons and the release of GnRH peptide.

Spike and Neuropeptide-Dependent Mechanisms Control GnRH Neuron Nerve Terminal Ca2+ over Diverse Time Scales

Fast cell-to-cell communication in the brain is achieved by action potential-dependent synaptic release of neurotransmitters. The fast kinetics of transmitter release are determined by transient Ca²⁺ elevations in presynaptic nerve terminals. Neuromodulators have previously been shown to regulate transmitter release by inhibiting presynaptic Ca²⁺ influx. Few studies to date have demonstrated the opposite, that is, neuromodulators directly driving presynaptic Ca²⁺ rises and increases in nerve terminal excitability. Here we use GCaMP Ca²⁺ imaging in brain slices from mice to address how nerve terminal Ca²⁺ is controlled in gonadotropin-releasing hormone (GnRH) neurons via action potentials and neuromodulators. Single spikes and bursts of action potentials evoked fast, voltage-gated Ca²⁺ channel-dependent Ca²⁺ elevations. In contrast, brief exposure to the neuropeptide kisspeptin-evoked long-lasting Ca²⁺ plateaus that persisted for tens of minutes. Neuropeptide-mediated Ca²⁺ elevations were independent of action potentials, requiring Ca²⁺ entry via voltage-gated Ca²⁺ channels and transient receptor potential channels in addition to release from intracellular store mechanisms. Together, these data reveal that neuromodulators can exert powerful and long-lasting regulation of nerve terminal Ca²⁺ independently from actions at the soma. Thus, GnRH nerve terminal function is controlled over disparate timescales via both classical spike-dependent and nonclassical neuropeptide-dependent mechanisms.SIGNIFICANCE STATEMENT Nerve terminals are highly specialized regions of a neuron where neurotransmitters and neurohormones are released. Many neuroendocrine neurons release neurohormones in long-duration bursts of secretion. To understand how this is achieved, we have performed live Ca²⁺ imaging in the nerve terminals of gonadotropin-releasing hormone neurons. We find that bursts of action potentials and local neuropeptide signals are both capable of evoking large increases in nerve terminal Ca²⁺ Increases in Ca²⁺ driven by spike bursts last seconds; however, the increases in nerve terminal Ca²⁺ driven by neuropeptides can persist for tens of minutes. These findings reveal new mechanisms by which neuroendocrine nerve terminal Ca²⁺ can be controlled in the brain.

Progress and Challenges in the Search for the Mechanisms of Pulsatile Gonadotropin-Releasing Hormone Secretion

Fertility relies on the proper functioning of the hypothalamic-pituitary-gonadal axis. The hormonal cascade begins with hypothalamic neurons secreting gonadotropin-releasing hormone (GnRH) into the hypophyseal portal system. In turn, the GnRH-activated gonadotrophs in the anterior pituitary release gonadotropins, which then act on the gonads to regulate gametogenesis and sex steroidogenesis. Finally, sex steroids close this axis by feeding back to the hypothalamus. Despite this seeming straightforwardness, the axis is orchestrated by a complex neuronal network in the central nervous system. For reproductive success, GnRH neurons, the final output of this network, must integrate and translate a wide range of cues, both environmental and physiological, to the gonadotrophs via pulsatile GnRH secretion. This secretory profile is critical for gonadotropic function, yet the mechanisms underlying these pulses remain unknown. Literature supports both intrinsically and extrinsically driven GnRH neuronal activity. However, the caveat of the techniques supporting either one of the two hypotheses is the gap between events recorded at a single-cell level and GnRH secretion measured at the population level. This review aims to compile data about GnRH neuronal activity focusing on the physiological output, GnRH secretion.

Dissecting Autocrine Effects on Pulsatile Release of Gonadotropin-Releasing Hormone in Cultured Rat Hypothalamic Tissue

The control of reproductive function is manifested centrally through the control of hypothalamic release of gonadotropin-releasing hormone (GnRH) in episodic events or pulses. For GnRH release to occur in pulses, GnRH neurons must coordinate release events periodically to elicit a bolus of GnRH. We used a perifusion culture system to examine the release of GnRH from both intact hypothalami and enzymatically dispersed hypothalamic cells after challenge with GnRH analogs to evaluate the role of anatomical neuronal connections on autocrine/paracrine signals by GnRH on GnRH neurons. The potent GnRH agonist des-Gly10-D-Ala6-GnRH N-ethylamide, potent GnRH antagonists D-Phe2-D-Ala6-GnRH and D-Phe2,6-Pro3-GnRH or vehicle were infused, whereas GnRH release from hypothalamic tissue and cells were measured. PULSAR analysis of GnRH release profiles was conducted to evaluate parameters of pulsatile GnRH release. Infusion of the GnRH agonist resulted in a decrease in mean GnRH (P < 0.001), pulse nadir (P < 0.01), and pulse frequency (P < 0.05) but no effect on pulse amplitude. Infusion of GnRH antagonists resulted in an increase in mean GnRH (P < 0.001), pulse nadir (P < 0.05), and pulse frequency (P < 0.05) and in GnRH pulse amplitude only in dispersed cells (P < 0.05). These results are consistent with the hypothesis that GnRH inhibits endogenous GnRH release by an ultrashort-loop feedback mechanism and that treatment of hypothalamic tissue or cells with GnRH agonist inhibits ultrashort-loop feedback, whereas treatment with antagonists disrupts normal feedback to GnRH neurons and elicits an increased GnRH signal.

Electrical synapses connect a network of gonadotropin releasing hormone neurons in a cichlid fish

Initiating and regulating vertebrate reproduction requires pulsatile release of gonadotropin-releasing hormone (GnRH1) from the hypothalamus. Coordinated GnRH1 release, not simply elevated absolute levels, effects the release of pituitary gonadotropins that drive steroid production in the gonads. However, the mechanisms underlying synchronization of GnRH1 neurons are unknown. Control of synchronicity by gap junctions between GnRH1 neurons has been proposed but not previously found. We recorded simultaneously from pairs of transgenically labeled GnRH1 neurons in adult male Astatotilapia burtoni cichlid fish. We report that GnRH1 neurons are strongly and uniformly interconnected by electrical synapses that can drive spiking in connected cells and can be reversibly blocked by meclofenamic acid. Our results suggest that electrical synapses could promote coordinated spike firing in a cellular assemblage of GnRH1 neurons to produce the pulsatile output necessary for activation of the pituitary and reproduction.

Optogenetic activation of GnRH neurons reveals minimal requirements for pulsatile luteinizing hormone secretion

The mechanisms responsible for generating the pulsatile release of gonadotropins from the pituitary gland are unknown. We develop here a methodology in mice for controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons in vivo to establish the minimal parameters of activation required to evoke a pulse of luteinizing hormone (LH) secretion. Injections of Cre-dependent channelrhodopsin (ChR2)-bearing adeno-associated virus into the median eminence of adult GnRH-Cre mice resulted in the selective expression of ChR2 in hypophysiotropic GnRH neurons. Acute brain slice experiments demonstrated that ChR2-expressing GnRH neurons could be driven to fire with high spike fidelity with blue-light stimulation frequencies up to 40 Hz for periods of seconds and up to 10 Hz for minutes. Anesthetized, ovariectomized mice had optical fibers implanted in the vicinity of GnRH neurons within the rostral preoptic area. Optogenetic activation of GnRH neurons for 30-s to 5-min time periods over a range of different frequencies revealed that 10 Hz stimulation for 2 min was the minimum required to generate a pulse-like increment of LH. The same result was found for optical activation of GnRH projections in the median eminence. Increases in LH secretion were compared with endogenous LH pulse parameters measured from ovariectomized mice. Driving GnRH neurons to exhibit simultaneous burst firing was ineffective at altering LH secretion. These observations provide an insight into how GnRH neurons generate pulsatile LH secretion in vivo.

Activation of hypothalamic gono-like neurons in female rats during estrus

In mammals, gonadal function is controlled by the activity of hypothalamic gonadotropin-releasing hormone neurons, which control the secretion of adenohypophyseal and gonadal hormones. However, there are a number of unanswered questions in relation to gonadal function. It is currently unknown how erotogenic stimulation of the genitals influences the subpopulation of hypothalamic medial preoptic area neurons, antidromically identified as projecting to the median eminence at different periods of the estrous cycle. Additionally, the distinctiveness of hypothalamic medial preoptic area neurons, with respect to methods of feedback control by exogenous hormones, is also unknown. In this study, spontaneous discharges from individual neurons encountered within the medial preoptic area, gono-like neurons, were recorded extracellularly using glass microelectrodes. To confirm the cellular and histochemical properties of the recording units, antidromic stimulation was performed using a side-by-side bipolar stimulating electrode placed into the median eminence, alongside microiontophoretic injections of the conventional tracer, horseradish peroxidase. In addition, further immunohistochemical analyses were performed. Results showed that elevated gono-neuron activity was accompanied by increased background activity and greater responses to erotogenic stimuli during estrus. Application of clitoral traction stimulation resulted in increased activation of the gono-like neurons. This neuronal activity was noticeably inhibited by β-estradiol administration. Immunohistochemical analyses revealed the presence of gonadotropin-releasing hormone-reactive protein in hypothalamic cells in which electrophysiological recordings were taken. Thus, medial preoptic area neurons represent the subset of hypothalamic gonadotropin-releasing hormone neurons described from brain slices in vitro, and might serve as a useful physiological model to form the basis of future in vivo studies.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Kisspeptins modulate the biology of multiple populations of gonadotropin-releasing hormone neurons during embryogenesis and adulthood in zebrafish (Danio rerio)

Kisspeptin1 (product of the Kiss1 gene) is the key neuropeptide that gates puberty and maintains fertility by regulating the gonadotropin-releasing hormone (GnRH) neuronal system in mammals. Inactivating mutations in Kiss1 and the kisspeptin receptor (GPR54/Kiss1r) are associated with pubertal failure and infertility. Kiss2, a paralogous gene for kiss1, has been recently identified in several vertebrates including zebrafish. Using our transgenic zebrafish model system in which the GnRH3 promoter drives expression of emerald green fluorescent protein, we investigated the effects of kisspeptins on development of the GnRH neuronal system during embryogenesis and on electrical activity during adulthood. Quantitative PCR showed detectable levels of kiss1 and kiss2 mRNA by 1 day post fertilization, increasing throughout embryonic and larval development. Early treatment with Kiss1 or Kiss2 showed that both kisspeptins stimulated proliferation of trigeminal GnRH3 neurons located in the peripheral nervous system. However, only Kiss1, but not Kiss2, stimulated proliferation of terminal nerve and hypothalamic populations of GnRH3 neurons in the central nervous system. Immunohistochemical analysis of synaptic vesicle protein 2 suggested that Kiss1, but not Kiss2, increased synaptic contacts on the cell body and along the terminal nerve-GnRH3 neuronal processes during embryogenesis. In intact brain of adult zebrafish, whole-cell patch clamp recordings of GnRH3 neurons from the preoptic area and hypothalamus revealed opposite effects of Kiss1 and Kiss2 on spontaneous action potential firing frequency and membrane potential. Kiss1 increased spike frequency and depolarized membrane potential, whereas Kiss2 suppressed spike frequency and hyperpolarized membrane potential. We conclude that in zebrafish, Kiss1 is the primary stimulator of GnRH3 neuronal development in the embryo and an activator of stimulating hypophysiotropic neuron activities in the adult, while Kiss2 plays an additional role in stimulating embryonic development of the trigeminal neuronal population, but is an RFamide that inhibits electrical activity of hypophysiotropic GnRH3 neurons in the adult.

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.

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.

Regulation of electrical bursting in a spatiotemporal model of a GnRH neuron

Gonadotropin-releasing hormone (GnRH) neurons are hypothalamic neurons that control the pulsatile release of GnRH that governs fertility and reproduction in mammals. The mechanisms underlying the pulsatile release of GnRH are not well understood. Some mathematical models have been developed previously to explain different aspects of these activities, such as the properties of burst action potential firing and their associated Ca(2+) transients. These previous studies were based on experimental recordings taken from the soma of GnRH neurons. However, some research groups have shown that the dendrites of GnRH neurons play very important roles. In particular, it is now known that the site of action potential initiation in these neurons is often in the dendrite, over 100 μm from the soma. This raises an important question. Since some of the mechanisms for controlling the burst length and interburst interval are located in the soma, how can electrical bursting be controlled when initiated at a site located some distance from these controlling mechanisms? In order to answer this question, we construct a spatio-temporal mathematical model that includes both the soma and the dendrite. Our model shows that the diffusion coefficient for the spread of electrical potentials in the dendrite is large enough to coordinate burst firing of action potentials when the initiation site is located at some distance from the soma.

Two types of burst firing in gonadotrophin-releasing hormone neurones

Gonadotrophin-releasing hormone (GnRH) neurones fire spontaneous bursts of action potentials, although little is understood about the underlying mechanisms. In the present study, we report evidence for two types of bursting/oscillation driven by different mechanisms. Properties of these different types are clarified using mathematical modelling and a recently developed active-phase/silent-phase correlation technique. The first type of GnRH neurone (1-2%) exhibits slow (∼0.05 Hz) spontaneous oscillations in membrane potential. Action potential bursts are often observed during oscillation depolarisation, although some oscillations were entirely subthreshold. Oscillations persist after blockade of fast sodium channels with tetrodotoxin (TTX) and blocking receptors for ionotropic fast synaptic transmission, indicating that they are intrinsically generated. In the second type of GnRH neurone, bursts were irregular and TTX caused a stable membrane potential. The two types of bursting cells exhibited distinct active-phase/silent-phase correlation patterns, which is suggestive of distinct mechanisms underlying the rhythms. Further studies of type 1 oscillating cells revealed that the oscillation period was not affected by current or voltage steps, although amplitude was sometimes damped. Oestradiol, an important feedback regulator of GnRH neuronal activity, acutely and markedly altered oscillations, specifically depolarising the oscillation nadir and initiating or increasing firing. Blocking calcium-activated potassium channels, which are rapidly reduced by oestradiol, had a similar effect on oscillations. Kisspeptin, a potent activator of GnRH neurones, translated the oscillation to more depolarised potentials, without altering period or amplitude. These data show that there are at least two distinct types of GnRH neurone bursting patterns with different underlying mechanisms.

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

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

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

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.

Regulation of endogenous conductances in GnRH neurons by estrogens

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

Reading between the spikes of the hypothalamic neural code

Reading the spike coding of hypothalamic neurones presents a considerable challenge because they exhibit highly irregular firing patterns. Electrophysiologists working in the motor and sensory systems, in which neurones fire more regularly, have devised satisfactory methods to describe the firing of cells, although the statistical assumptions that underlie the methods do not apply to hypothalamic neurones. Measurement of neural activity is nevertheless vital to characterise the activity of neuroendocrine cells. It has thus become necessary to develop methods suitable for the analysis of the highly irregular spike discharge patterns of both spontaneous and stimulus-evoked firing of hypothalamic neurones. We review techniques used to meet this challenge and demonstrate their considerable capacity to address important physiological questions. We also introduce a novel approach for valid statistical estimation of the information conveyed by the response of a single neurone to a periodic stimulus. The approach demonstrated significant diurnal rhythms of synaptic connectivity between hypothalamic nuclei.

Identified GnRH neuron electrophysiology: a decade of study

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

Expression of kv4.3 voltage-gated potassium channels in rat gonadotrophin-releasing hormone (GnRH) neurons during the estrous cycle

Regular and timely electrical activity of gonadotrophin-releasing hormone (GnRH) neurons accompanies the pulsatile release of GnRH that plays a central role in regulating fertility. Although transient outward A-type currents (I(A)) have been electrophysiologically identified in GnRH neurons, the molecular identity of the channels that underlie these currents are unknown. Several families of voltage-gated potassium channels can underlie I(A). However, the biophysical properties of I(A) described in previous electrophysiological studies are strongly characteristic of members of the Kv4 family of voltage-gated channels. We, therefore, sought to determine the presence of Kv4 channels in GnRH neurons. We used reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis to determine whether Kv4 messenger RNA (mRNA) and protein are present in the rat medial preoptic area (MPOA) and median eminence (ME). We used double-label immunohistochemistry to determine whether Kv4 colocalized with GnRH cell bodies in the MPOA and GnRH axons in the ME. Kv4.3 channels co-localized with GnRH in the MPOA but not in the ME. Neither Kv4.2 nor Kv4.1 co-localized with GnRH in either the MPOA or the ME. The electrical activity of GnRH neurons changes dramatically during the estrous cycle. We, therefore, studied the change in Kv4.3 expression in GnRH neurons during the estrous cycle. In the estrus phase, 58.05% of GnRH neurons expressed Kv4.3 compared to 74.48% in diestrus-proestrus rats (P < .05). Our data suggest that Kv4.3 is the major molecular component of I(A) in GnRH neurons, and furthermore that the expression of Kv4.3 changes significantly during the rat estrous cycle.

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.

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.

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

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

Voltage-gated Ca2+ channel mRNAs and T-type Ca2+ currents in rat gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in the neuroendocrine regulation of reproduction. We have previously reported that rat GnRH neurons exhibit voltage-gated Ca(2+) currents. In this study, oligo-cell RT-PCR was carried out to identify subtypes of the alpha(1) subunit of voltage-gated Ca(2+) channels in adult rat GnRH neurons. GnRH neurons expressed mRNAs for all five types of voltage-gated Ca(2+) channels. For T-type Ca(2+) channels, alpha(1H) was dominantly expressed in GnRH neurons. Electrophysiological analysis in acute slice preparations revealed that GnRH neurons from adult rats exhibited T-type Ca(2+) currents with fast inactivation kinetics (~20 ms at -30 mV) and a time constant of recovery from inactivation of ~0.6 s. These results indicate that rat GnRH neurons express subtypes of the alpha(1) subunit for all five types of voltage-gated Ca(2+) channel, and that alpha(1H) was the dominant subtype in T-type Ca(2+) channels.

Regular pacemaker activity characterizes gonadotropin-releasing hormone 2 neurons recorded from green fluorescent protein-transgenic medaka

GnRH2 is a molecule conserved from fish to humans, suggesting its important functions. However, recent studies have shown that GnRH2 neurons project widely in the brain but not to the pituitary, which suggests their functions other than stimulation of gonadotropin secretion. In contrast to the wealth of knowledge in GnRH1 and GnRH3 neuronal systems, the GnRH2 neuronal system remains to be studied, and there has been no single cell approach so far, partly because of the lack of GnRH2 system in rodents. Here, we generated GnRH2-green fluorescent protein (GFP) transgenic medaka for the first single cell electrophysiological recording from GnRH2 neurons in vertebrates. Whole-cell and on-cell patch clamp analyses revealed their regular pacemaker activities that are intrinsic to the GnRH2 neurons. Pacemaker activities of GnRH2 neurons were not peculiar to medaka because dwarf gourami GnRH2 neurons also showed similar pacemaker activities. By comparing with spontaneous action currents from GFP-expressing GnRH1 and GnRH3 neurons in the adult transgenic medaka, which were already in our hands, we have demonstrated that GnRH2 neurons show pacemaker activity similar to nonhypophysiotropic GnRH3 neurons but not to hypophysiotropic GnRH1 neurons. Thus, by taking advantage of medaka brain, which has all three GnRH neuronal systems with different axonal projection patterns and thus different functions, we have gained insights into the close relationship between the pattern of spontaneous electrical activity and the functions of the three. Moreover, the three types of GnRH-GFP transgenic medaka will provide useful models for studying multifunctional GnRH systems in future.

Pulsatile GnRH secretion: roles of G protein-coupled receptors, second messengers and ion channels

The pulsatile secretion of GnRH from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to multiple G proteins. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca(2+)- and cAMP-dependent signaling and GnRH neuronal firing. The firing of individual and/or bursts of action potentials (APs) in spontaneously active GnRH neurons is followed by afterhyperpolarization (AHP) that lasts from several milliseconds to several seconds. GnRH-induced activation of GnRH neurons causes a significant increase in medium AHP that is partially sensitive to apamin. GnRH-induced modulation of Ca(2+) influx and the consequent changes in AHP current suggest that the GnRH receptors expressed in hypothalamic GnRH neurons are important modulators of their neuronal excitability. The coexistence of multiple regulatory mechanisms could provide a high degree of redundancy in the maintenance of this crucial component of the reproductive process. It is also conceivable that this multifactorial system could reflect the gradation from simple to more complex neuroendocrine control systems for regulating hypothalamo-pituitary function and gonadal activity during the evolution of the GnRH pulse generator.

Three-dimensional properties of GnRH neuroterminals in the median eminence of young and old rats

The decapeptide gonadotropin-releasing hormone (GnRH), which regulates reproduction in all vertebrates, is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 months) and old (22-24 months) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.

Kisspeptin signalling in the hypothalamic arcuate nucleus regulates GnRH pulse generator frequency in the rat

BACKGROUND

Kisspeptin and its G protein-coupled receptor (GPR) 54 are essential for activation of the hypothalamo-pituitary-gonadal axis. In the rat, the kisspeptin neurons critical for gonadotropin secretion are located in the hypothalamic arcuate (ARC) and anteroventral periventricular (AVPV) nuclei. As the ARC is known to be the site of the gonadotropin-releasing hormone (GnRH) pulse generator we explored whether kisspeptin-GPR54 signalling in the ARC regulates GnRH pulses.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the effects of kisspeptin-10 or a selective kisspeptin antagonist administration intra-ARC or intra-medial preoptic area (mPOA), (which includes the AVPV), on pulsatile luteinizing hormone (LH) secretion in the rat. Ovariectomized rats with subcutaneous 17beta-estradiol capsules were chronically implanted with bilateral intra-ARC or intra-mPOA cannulae, or intra-cerebroventricular (icv) cannulae and intravenous catheters. Blood samples were collected every 5 min for 5-8 h for LH measurement. After 2 h of control blood sampling, kisspeptin-10 or kisspeptin antagonist was administered via pre-implanted cannulae. Intranuclear administration of kisspeptin-10 resulted in a dose-dependent increase in circulating levels of LH lasting approximately 1 h, before recovering to a normal pulsatile pattern of circulating LH. Both icv and intra-ARC administration of kisspeptin antagonist suppressed LH pulse frequency profoundly. However, intra-mPOA administration of kisspeptin antagonist did not affect pulsatile LH secretion.

CONCLUSIONS/SIGNIFICANCE

These data are the first to identify the arcuate nucleus as a key site for kisspeptin modulation of LH pulse frequency, supporting the notion that kisspeptin-GPR54 signalling in this region of the mediobasal hypothalamus is a critical neural component of the hypothalamic GnRH pulse generator.

17Beta-estradiol regulation of T-type calcium channels in gonadotropin-releasing hormone neurons

T-type calcium channels are responsible for generating low-threshold spikes that facilitate burst firing and neurotransmitter release in neurons. Gonadotropin-releasing hormone (GnRH) neurons exhibit burst firing, but the underlying conductances are not known. Previously, we found that 17beta-estradiol (E2) increases T-type channel expression and excitability of hypothalamic arcuate nucleus neurons. Therefore, we used ovariectomized oil- or E2-treated EGFP (enhanced green fluorescent protein)-GnRH mice to explore the expression and E2 regulation of T-type channels in GnRH neurons. Based on single-cell reverse transcriptase-PCR and real-time PCR quantification of the T-type channel alpha(1) subunits, we found that all three subunits were expressed in GnRH neurons, with expression levels as follows: Cav3.3 > or = Cav3.2 > Cav3.1. The mRNA expression of the three subunits was increased with surge-inducing levels of E2 during the morning. During the afternoon, Cav3.3 mRNA expression remained elevated, whereas Cav3.1 and Cav3.2 were decreased. The membrane estrogen receptor agonist STX increased the expression of Cav3.3 but not Cav3.2 in GnRH neurons. Whole-cell patch recordings in GnRH neurons revealed that E2 treatment significantly augmented T-type current density at both time points and increased the rebound excitation during the afternoon. Although E2 regulated the mRNA expression of all three subunits in GnRH neurons, the increased expression combined with the slower inactivation kinetics of the T-type current indicates that Cav3.3 may be the most important for bursting activity associated with the GnRH/LH (luteinizing hormone) surge. The E2-induced increase in mRNA expression, which depends in part on membrane-initiated signaling, leads to increased channel function and neuronal excitability and could be a mechanism by which E2 facilitates burst firing and cyclic GnRH neurosecretion.

An integrated model of electrical spiking, bursting, and calcium oscillations in GnRH neurons

The plasma membrane electrical activities of neurons that secrete gonadotropin-releasing hormone (GnRH) have been studied extensively. A couple of mathematical models have been developed previously to explain different aspects of these activities. The goal of this article is to develop a single model that accounts for the previously modeled experimental results and some more recent results that have not been accounted for. The latter includes two types of membrane potential bursting mechanisms and their associated cytosolic calcium oscillations. One bursting mechanism has not been reported in experiments and is thus regarded as a model prediction. Although the model is mainly based on data collected in immortalized GnRH cell lines, it is capable of explaining some properties of GnRH neurons observed in several other preparations including mature GnRH neurons in hypothalamic slices. We present a spatial model that incorporates a detailed description of calcium dynamics in a three-dimensional cell body with the ion channels evenly distributed on the cell surface. A phenomenological reduction of the spatial model into a simplified form is also presented. The simplified model will facilitate the study of the roles of plasma membrane electrical activities in the pulsatile release of GnRH.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

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

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

Robust synchrony and rhythmogenesis in endocrine neurons via autocrine regulations in vitro and in vivo

Episodic pulses of gonadotropin-releasing hormone (GnRH) are essential for maintaining reproductive functions in mammals. An explanation for the origin of this rhythm remains an ultimate goal for researchers in this field. Some plausible mechanisms have been proposed among which the autocrine-regulation mechanism has been implicated by numerous experiments. GnRH binding to its receptors in cultured GnRH neurons activates three types of G-proteins that selectively promote or inhibit GnRH secretion (Krsmanovic et al. in Proc. Natl. Acad. Sci. 100:2969-2974, 2003). This mechanism appears to be consistent with most data collected so far from both in vitro and in vivo experiments. Based on this mechanism, a mathematical model has been developed (Khadra and Li in Biophys. J. 91:74-83, 2006) in which GnRH in the extracellular space plays the roles of a feedback regulator and a synchronizing agent. In the present study, we show that synchrony between different neurons through sharing a common pool of GnRH is extremely robust. In a diversely heterogeneous population of neurons, the pulsatile rhythm is often maintained when only a small fraction of the neurons are active oscillators (AOs). These AOs are capable of recruiting nonoscillatory neurons into a group of recruited oscillators while forcing the nonrecruitable neurons to oscillate along. By pointing out the existence of the key elements of this model in vivo, we predict that the same mechanism revealed by experiments in vitro may also operate in vivo. This model provides one plausible explanation for the apparently controversial conclusions based on experiments on the effects of the ultra-short feedback loop of GnRH on its own release in vivo.

Gonadotropin-releasing hormone-1 neuronal activity is independent of hyperpolarization-activated cyclic nucleotide-modulated channels but is sensitive to protein kinase a-dependent phosphorylation

Pulsatile release of GnRH-1 stimulates the anterior pituitary and induces secretion of gonadotropin hormones. GnRH-1 release is modulated by many neurotransmitters that act via G protein-coupled membrane receptors. cAMP is the most ubiquitous effector for these receptors. GnRH-1 neurons express hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel protein in vivo. HCN channels are involved in neuronal pacemaking and can integrate cAMP signals. cAMP-dependent protein kinase (PKA) is also activated by cAMP signals, and PKA-dependent phosphorylation modulates voltage-activated channels. In this report, these two pathways were examined in GnRH-1 neurons as integrators of forskolin (FSK)-induced stimulation. The HCN3 isoform was detected in GnRH-1 neurons obtained from mouse nasal explants. ZD7288, a HCN channel blocker, significantly reduced the efficiency of FSK to stimulate GnRH-1 neurons, whereas blockade of PKA with Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium did not attenuate the FSK-induced stimulation. To ensure that disruption of HCN channels on GnRH-1 neurons was responsible for reduction of FSK stimulation, experiments were performed removing gamma-aminobutyric acid (GABA), the major excitatory input to GnRH-1 neurons in nasal explants. Under these conditions, Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium, but not ZD7288, altered the FSK-induced response of GnRH-1 neurons. These studies indicate that PKA-dependent phosphorylation is involved in the FSK-induced stimulation of GnRH-1 neurons rather than HCN channels, and HCN channels integrate the FSK-induced stimulation on GABAergic neurons. In addition, blockade of HCN channels did not modify basal GnRH-1 neuronal activity when GABAergic input was intact or removed, negating a role for these channels in basal GABAergic or GnRH-1 neuronal activity.

Dendrites determine the contribution of after depolarization potentials (ADPs) to generation of repetitive action potentials in hypothalamic gonadotropin releasing-hormone (GnRH) neurons

The impact of structure in modulating synaptic signals originating in dendrites is widely recognized. In this study, we focused on the impact of dendrite morphology on a local spike generating mechanism which has been implicated in hormone secretion, the after depolarization potential (ADP). Using multi-compartmental models of hypothalamic GnRH neurons, we systematically truncated dendrite length and determined the consequence on ADP amplitude and repetitive firing. Decreasing the length of the dendrite significantly increased the amplitude of the ADP and increased repetitive firing. These effects were observed in dendrites both with and without active conductances suggesting they largely reflect passive characteristics of the dendrite. In order to test the findings of the model, we performed whole-cell recordings in GnRH neurons and elicited ADPs using current injection. During recordings, neurons were filled with biocytin so that we could determine dendritic and total projection (dendrite plus axon) length. Neurons exhibited ADPs and increasing ADP amplitude was associated with decreasing dendrite length, in keeping with the predictions of the models. Thus, despite the relatively simple morphology of the GnRH neuron's dendrite, it can still exert a substantial impact on the final neuronal output.

Dominant role of betagamma subunits of G-proteins in oxytocin-evoked burst firing

Pulsatile neuropeptide secretion is associated with burst firing patterns; however, intracellular signaling cascades leading to bursts remain unclear. We explored mechanisms underlying burst firing in oxytocin (OT) neurons in the supraoptic nucleus in brain slices from lactating rats. Application of 10 pm OT for 30 min or progressively rising OT concentrations from 1 to 100 pm induced burst firing in OT neurons in patch-clamp recordings. Burst generation was blocked by OT antagonist and ionotropic glutamate receptor blockers or tetanus toxin. Blocking G-protein activation with suramin or intracellular GDP-beta-S, but not intracellularly administered antibody against the OT-receptor (OTR) C terminus, blocked bursts. Moreover, pretreatment of slices with pertussis toxin, an inhibitor of G(i/o)-proteins, did not block OT-evoked bursts, suggesting that G(i)/G(o) activation is unnecessary for burst generation. Thus, we further examined G alpha(q/11)-associated signaling pathways in OT-evoked bursts. Inhibition of phospholipase C or RhoA/Rho kinase did not block bursts. Activation of G betagamma subunits using myristoylated G betagamma-binding peptide (mSIRK) caused bursts, whereas intracellularly loaded antibody against G beta subunit blocked OT-evoked bursts. Blocking Src family kinase, but not phosphatidylinositol 3-kinase, occluded OT-evoked bursts. Similar to the effects of OT on EPSCs, mSIRK inhibited tonic EPSCs and elicited EPSC clustering. Finally, suckling caused dissociation of OTRs and G beta subunits from G alpha(q/11) subunits shown by coimmunoprecipitation and immunocytochemistry, supporting crucial roles for OTRs and G betagamma subunits in the milk-ejection reflex. We conclude that G betagamma subunits play a dominant role in burst firing evoked by applied OT or by suckling.

Metastin/kisspeptin and control of estrous cycle in rats

Estrous cyclicity is controlled by a cascade of neuroendocrine events, involving the activation of the hypothalamo-pituitary-gonadal axis. Two modes of gonadotropin-releasing hormone (GnRH) are well established to regulate the estrous cycle: one is a tonic or pulse mode of secretion which is responsible for the stimulation of follicular development and steroidogenesis; the other is a surge mode, which is solely responsible for the induction of luteinizing hormone (LH) surges, eventually leading to ovulation. Metastin/kisspeptin-GPR54 signaling has been suggested to control ovarian cyclicity through regulating the two modes of GnRH release. A population of metastin/kisspeptin neurons located in the anteroventral periventricular nucleus (AVPV) is considered to trigger GnRH surge and thus to mediate the estrogen positive feedback action on GnRH release. The other hypothalamic population of metastin/kisspeptin neurons is located in the arcuate nucleus (ARC) and could be involved in generating GnRH pulses and mediating negative feedback action of estrogen on GnRH release. GnRH neurons express mRNA for GPR54, a metastin/kisspeptin receptor, and have a close association with metastin/kisspeptin neurons at the cell body and terminal level, but the precise mechanism by which this peptide regulates the two modes of GnRH release needs to be determined. Metastin/kisspeptin, therefore, is a key hypothalamic neuropeptide, which is placed immediately upstream of GnRH neurons and relays the peripheral steroidal information to GnRH neurons to control estrous cyclicity.

Physiologic regulation of a tetrodotoxin-sensitive sodium influx that mediates a slow afterdepolarization potential in gonadotropin-releasing hormone neurons: possible implications for the central regulation of fertility

The brain controls fertility through release of gonadotropin-releasing hormone (GnRH), but the mechanisms underlying action potential patterning and GnRH release are not understood. We investigated whether GnRH neurons exhibit afterdepolarizing potentials (ADPs) and whether these are modified by reproductive state. Whole-cell current-clamp recordings of GnRH neurons in brain slices from ovariectomized mice revealed a slow ADP (sADP) after action potentials generated by brief current injection. Generating two or four spikes enhanced sADP amplitude and duration. sADP amplitude was not affected by blocking selected neurotransmitter/neuromodulator receptors, delayed-rectifier potassium channels, calcium-dependent cation channels, or hyperpolarization-activated cation channels but was halved by the calcium channel blocker cadmium and abolished by tetrodotoxin. Cadmium also reduced peak latency. Intrinsic mechanisms underlying the sADP were investigated using voltage-clamp protocols simulating action potential waveforms. A single action potential produced an inward current, which increased after double and quadruple stimulation. Cadmium did not affect current amplitude but reduced peak latency. Pretreatment with blockers of calcium-activated potassium currents (I(KCa)) reproduced this shift and blocked subsequent cadmium-induced changes, suggesting cadmium changes latency indirectly by blocking I(KCa). Tetrodotoxin abolished the inward current, suggesting that it is carried by sodium. In contrast, I(KCa) blockers increased the inward current, indicating that I(KCa) may oppose generation of the sADP. Strong sADPs were suprathreshold, generating repetitive spontaneous firing. I(ADP), sADP, and excitability were enhanced by in vivo estradiol, which triggers a preovulatory surge of GnRH release. Physiological feedback modification of this inward current and resulting sADP may modulate action potential firing and subsequent GnRH release.

Hyperpolarization-activated cation channels are expressed in rat hypothalamic gonadotropin-releasing hormone (GnRH) neurons and immortalized GnRH neurons

OBJECTIVES

The current research was conducted to determine whether hyperpolarization-activated cyclic nucleotide-gated (HCN1-4) channels are expressed in gonadotropin-releasing hormone (GnRH) neurons in the female rat hypothalamus and immortalized GnRH neurons (GT1-7 cells).

METHODS

Double-label fluorescence immunohistochemistry was used to colocalize HCN1-4 channels and GnRH in GnRH neurons in the female rat hypothalamus. Reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting, and immunocytochemistry were used to analyze HCN channel gene expression in GT1-7 cells.

RESULTS

Double-label fluorescence immunohistochemistry showed that 43% of hypothalamic GnRH neurons immunostained for HCN2 and 90% of GnRH neurons immunostained for HCN3. RT-PCR and Western blot showed expression of all four HCN channel subunits in GT1-7 cells. Double-label immunocytochemistry showed cytoplasmic immunostaining of HCN2 and HCN3 in GT1-7 cells.

CONCLUSIONS

This study demonstrates for the first time that HCN channels are expressed in GnRH neurons in the rat hypothalamus and GT1-7 cells. Our research supports the hypothesis that HCN channels may be involved in electrical bursting activity and pulsatile GnRH secretion in endogenous GnRH neurons and GT1-7 cells.

The SK channel blocker apamin inhibits slow afterhyperpolarization currents in rat gonadotropin-releasing hormone neurones

Gonadotropin-releasing hormone (GnRH) neurones play an essential role in the hypothalamo-pituitary-gonadal axis. As for other neurones, the discharge pattern of action potentials is important for GnRH neurones to properly function. In the case of a luteinizing hormone (LH) surge, for example, GnRH neurones are likely to continuously fire for more than an hour. For this type of firing, GnRH neurones must have a certain intrinsic property. To address this issue, we investigated the voltage-gated Ca(2+) currents and Ca(2+)-activated voltage-independent K(+) currents underlying afterhyperpolarization, because they affect cell excitability. Dispersed GnRH neurones from adult GnRH-EGFP (enhanced green fluorescent protein) transgenic rats were cultured overnight and then used for an electrophysiological experiment involving the perforated patch-clamp configuration. The GnRH neurones showed five subtypes of voltage-gated Ca(2+) currents, i.e. the T-, L-, N-, P/Q- and R-types. The GnRH neurones also showed a slow afterhyperpolarization current (I(sAHP)), but not a medium one. It is reported that the SK channel blocker apamin (10 pm-100 nm) blocks a medium afterhyperpolarization current but not an I(sAHP). In contrast to previous reports, the I(sAHP) observed in rat GnRH neurones was potently blocked by apamin. In addition, the GnRH neurones expressed transcripts for SK1-3 channels. The results indicate that rat GnRH neurones express all five subtypes of voltage-gated Ca(2+) channels and exhibit an apamin-sensitive I(sAHP), which may allow continuous firing in response to a relatively strong depolarizing input.

A model for the pulsatile secretion of gonadotropin-releasing hormone from synchronized hypothalamic neurons

Cultured gonadotropin-releasing hormone (GnRH) neurons have been shown to express GnRH receptors. GnRH binding to its receptors activates three types of G-proteins at increasing doses. These G-proteins selectively activate or inhibit GnRH secretion by regulating the intracellular levels of Ca2+ and cAMP. Based on these recent observations, we build a model in which GnRH plays the roles of a feedback regulator and a diffusible synchronizing agent. We show that this GnRH-regulated GnRH-release mechanism is sufficient for generating pulsatile GnRH release. The model reproduces the observed effects of some key drugs that disturb the GnRH pulse generator in specific ways. Simulations of 100 heterogeneous neurons revealed that the synchronization mediated by a common pool of diffusible GnRH is robust. The population can generate synchronized pulsatile signals even when all the individual GnRH neurons oscillate at different amplitudes and peak at different times. These results suggest that the positive and negative effects of the autocrine regulation by GnRH on GnRH neurons are sufficient and robust in generating GnRH pulses.

Endogenous activation of metabotropic glutamate receptors modulates GABAergic transmission to gonadotropin-releasing hormone neurons and alters their firing rate: a possible local feedback circuit

Gonadotropin-releasing hormone (GnRH) neurons are the primary central regulators of fertility, receiving input from GABAergic afferents via GABA(A) receptors. We tested whether metabotropic glutamate receptors (mGluRs) regulate GABA transmission to GnRH neurons and GnRH neuronal firing pattern. Whole-cell recordings were performed under conditions isolating ionotropic GABA postsynaptic currents (PSCs) in brain slices. The broad-spectrum mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) decreased the frequency of GABA(A)-mediated spontaneous PSCs in a reversible manner. Amplitude and kinetics were not altered, suggesting that afferent GABA neurons are the primary targets. TTX eliminated the effects of ACPD in most tested neurons. Group II [2-(2,3-dicarboxycyclopropyl) glycine] and III (L-AP-4) mGluR agonists mediated this response; a group I agonist (3,5-dihydroxyphenylglycine) was not effective. The broad-spectrum antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) and/or (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) (group III antagonist) enhanced spontaneous PSC frequency, particularly when initial frequency was low, suggesting that endogenous activation of mGluRs regulates GABA transmission to GnRH neurons. Extracellular recordings were used to evaluate GnRH neuron firing rate within the network. ACPD reduced firing rate, and MCPG plus CPPG had an opposite effect, indicating that mGluRs help control excitability of the GnRH network. GnRH neurons express vesicular glutamate transporters, suggesting they may corelease this transmitter. Simulation of firing activity in a GnRH neuron decreased PSC frequency in that cell, an effect blocked by antagonism of mGluRs but not GnRH receptors. These results demonstrate an inhibition of GABAergic inputs to GnRH neurons by mGluRs via a presynaptic mechanism. Through this mechanism, local glutamate milieu, possibly contributed by GnRH neurons themselves, plays an important role in modulating GnRH release and the central regulation of fertility.