Synchronization of Ca(2+) oscillations among primate LHRH neurons and nonneuronal cells in vitro

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.

The polygamous GnRH neuron: Astrocytic and tanycytic communication with a neuroendocrine neuronal population

To ensure the survival of the species, hypothalamic neuroendocrine circuits controlling fertility, which converge onto neurons producing gonadotropin-releasing hormone (GnRH), must respond to fluctuating physiological conditions by undergoing rapid and reversible structural and functional changes. However, GnRH neurons do not act alone, but through reciprocal interactions with multiple hypothalamic cell populations, including several glial and endothelial cell types. For instance, it has long been known that in the hypothalamic median eminence, where GnRH axons terminate and release their neurohormone into the pituitary portal blood circulation, morphological plasticity displayed by distal processes of tanycytes modifies their relationship with adjacent neurons as well as the spatial properties of the neurohemal junction. These alterations not only regulate the capacity of GnRH neurons to release their neurohormone, but also the activation of discrete non-neuronal pathways that mediate feedback by peripheral hormones onto the hypothalamus. Additionally, a recent breakthrough has demonstrated that GnRH neurons themselves orchestrate the establishment of their neuroendocrine circuitry during postnatal development by recruiting an entourage of newborn astrocytes that escort them into adulthood and, via signalling through gliotransmitters such as prostaglandin E2, modulate their activity and GnRH release. Intriguingly, several environmental and behavioural toxins perturb these neuron-glia interactions and consequently, reproductive maturation and fertility. Deciphering the communication between GnRH neurons and other neural cell types constituting hypothalamic neuroendocrine circuits is thus critical both to understanding physiological processes such as puberty, oestrous cyclicity and aging, and to developing novel therapeutic strategies for dysfunctions of these processes, including the effects of endocrine disruptors.

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

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

Neuroestradiol in regulation of GnRH release

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

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.

GnRH Episodic Secretion Is Altered by Pharmacological Blockade of Gap Junctions: Possible Involvement of Glial Cells

Episodic release of GnRH is essential for reproductive function. In vitro studies have established that this episodic release is an endogenous property of GnRH neurons and that GnRH secretory pulses are associated with synchronization of GnRH neuron activity. The cellular mechanisms by which GnRH neurons synchronize remain largely unknown. There is no clear evidence of physical coupling of GnRH neurons through gap junctions to explain episodic synchronization. However, coupling of glial cells through gap junctions has been shown to regulate neuron activity in their microenvironment. The present study investigated whether glial cell communication through gap junctions plays a role in GnRH neuron activity and secretion in the mouse. Our findings show that Glial Fibrillary Acidic Protein-expressing glial cells located in the median eminence in close vicinity to GnRH fibers expressed Gja1 encoding connexin-43. To study the impact of glial-gap junction coupling on GnRH neuron activity, an in vitro model of primary cultures from mouse embryo nasal placodes was used. In this model, GnRH neurons possess a glial microenvironment and were able to release GnRH in an episodic manner. Our findings show that in vitro glial cells forming the microenvironment of GnRH neurons expressed connexin-43 and displayed functional gap junctions. Pharmacological blockade of the gap junctions with 50 μM 18-α-glycyrrhetinic acid decreased GnRH secretion by reducing pulse frequency and amplitude, suppressed neuronal synchronization and drastically reduced spontaneous electrical activity, all these effects were reversed upon 18-α-glycyrrhetinic acid washout.

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.

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

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

Kisspeptin and GnRH pulse generation

The reproductive neuropeptide gonadotropin-releasing hormone (GnRH) has two modes of secretion. Besides the surge mode, which induces ovulation in females, the pulse mode of GnRH release is essential to cause various reproductive events in both sexes, such as spermatogenesis, follicular development, and sex steroid synthesis. Some environmental cues control gonadal activities through modulating GnRH pulse frequency. Researchers have looked for the anatomical location of the mechanism generating GnRH pulses, the GnRH pulse generator, in the brain, because an artificial manipulation of GnRH pulse frequency is of therapeutic importance to stimulate or suppress gonadal activity. Discoveries of kisspeptin and, consequently, KNDy (kisspeptin/neurokinin B/dynorphin) neurons in the hypothalamus have provided a clue to the possible location of the GnRH pulse generator. Our analyses of hypothalamic multiple-unit activity revealed that KNDy neurons located in the hypothalamic arcuate nucleus might play a central role in the generation of GnRH pulses in goats, and perhaps other mammalian species. This chapter further discusses the possible mechanisms for GnRH pulse generation.

A network model of the periodic synchronization process in the dynamics of calcium concentration in GnRH neurons

Mathematical neuroendocrinology is a branch of mathematical neurosciences that is specifically interested in endocrine neurons, which have the uncommon ability of secreting neurohormones into the blood. One of the most striking features of neuroendocrine networks is their ability to exhibit very slow rhythms of neurosecretion, on the order of one or several hours. A prototypical instance is that of the pulsatile secretion pattern of GnRH (gonadotropin releasing hormone), the master hormone controlling the reproductive function, whose origin remains a puzzle issue since its discovery in the seventies. In this paper, we investigate the question of GnRH neuron synchronization on a mesoscopic scale, and study how synchronized events in calcium dynamics can arise from the average electric activity of individual neurons. We use as reference seminal experiments performed on embryonic GnRH neurons from rhesus monkeys, where calcium imaging series were recorded simultaneously in tens of neurons, and which have clearly shown the occurrence of synchronized calcium peaks associated with GnRH pulses, superposed on asynchronous, yet oscillatory individual background dynamics. We design a network model by coupling 3D individual dynamics of FitzHugh-Nagumo type. Using phase-plane analysis, we constrain the model behavior so that it meets qualitative and quantitative specifications derived from the experiments, including the precise control of the frequency of the synchronization episodes. In particular, we show how the time scales of the model can be tuned to fit the individual and synchronized time scales of the experiments. Finally, we illustrate the ability of the model to reproduce additional experimental observations, such as partial recruitment of cells within the synchronization process or the occurrence of doublets of synchronization.

Neuroestrogen, rapid action of estradiol, and GnRH neurons

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

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

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

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

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

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.

Epigenetic changes coincide with in vitro primate GnRH neuronal maturation

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

Calcium oscillations in the olfactory nonsensory cells of the goldfish, Carassius auratus

BACKGROUND

The olfactory nonsensory cells contribute to the maintenance of normal functions of the olfactory epithelium (OE). Specifically, the ciliated nonsensory cells of teleosts play important roles in the odorant detection by OE in aqueous environment. Their cilia show strong beating activities and cause water flow at the OE surface, making the detection of odorants by OE more efficient. Because intracellular Ca2+ level has been reported to play an important role in ciliary beating, the ciliary beating activity may be regulated by intracellular Ca2+ dynamics of these ciliated nonsensory cells.

METHODS

We performed Ca2+ imaging experiments to analyze the Ca2+ dynamics in acutely dissociated OE cells of the goldfish. Furthermore, we examined the contribution of the Ca2+ dynamics to the ciliary beating frequency (CBF) at the surface of the intact OE.

RESULTS

Olfactory nonsensory cells showed both spontaneous intracellular Ca2+ oscillations and propagating intercellular Ca2+ waves. Application of 2-aminoethoxydiphenylborate (2-APB), which antagonizes IP3-induced Ca2+ release from intracellular stores suppressed these Ca2+ oscillations. Furthermore, 2-APB application to the intact OE lamellae resulted in the decrease of CBF at the surface of the OE.

CONCLUSIONS

These results indicate that spontaneous intracellular calcium oscillations persistently up-regulate the ciliary beating at the surface of the OE in teleosts.

GENERAL SIGNIFICANCE

Ciliary beating activity at the surface of OE can be regulated by the Ca2+ dynamics of olfactory nonsensory cells. Because this ciliary movement causes inflow of external fluid into the nostril, this regulation is suggested to influence the efficiency of odorant detection by OE.

A biological role for the gonadotrophin-releasing hormone (GnRH) metabolite, GnRH-(1-5)

Gonadotrophin-releasing hormone (GnRH) was first isolated in the mammal and shown to be the primary regulator of the reproductive system through its initiation of pituitary gonadotrophin release. Subsequent to its discovery, this form of GnRH has been shown to be one of many structural variants found in the brain and peripheral tissues. Accordingly, the original form first discovered and cloned in the mammal is commonly referred to as GnRH-I. In addition to the complex regulation of GnRH-I synthesis, release and function, further evidence suggests that the processing of GnRH-I produces yet another layer of complexity in its activity. GnRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15), which cleaves the hormone at the covalent bond between the fifth and sixth residue of the decapeptide (Tyr(5)-Gly(6)) to form GnRH-(1-5). It was previously thought that the cleavage of GnRH-I by EP24.15 represents the initiation of its degradation. Here, we review the evidence for the involvement of GnRH-(1-5), the metabolite of GnRH-I, in the regulation of GnRH-I synthesis, secretion and facilitation of reproductive behaviour.

Rapid action of oestrogen in luteinising hormone-releasing hormone neurones: the role of GPR30

Previously, we have shown that 17beta-oestradiol (E(2)) induces an increase in firing activity and modifies the pattern of intracellular calcium ([Ca(2+)](i)) oscillations with a latency < 1 min in primate luteinising hormone-releasing hormone (LHRH) neurones. A recent study also indicates that E(2), the nuclear membrane impermeable oestrogen, oestrogen-dendrimer conjugate, and the plasma membrane impermeable oestrogen, E(2)-BSA conjugate, all similarly stimulated LHRH release within 10 min of exposure in primate LHRH neurones, indicating that the rapid action of E(2) is caused by membrane signalling. The results from a series of studies further suggest that the rapid action of E(2) in primate LHRH neurones appears to be mediated by GPR30. Although the oestrogen receptor antagonist, ICI 182, 780, neither blocked the E(2)-induced LHRH release nor the E(2)-induced changes in [Ca(2+)](i) oscillations, E(2) application to cells treated with pertussis toxin failed to result in these changes in primate LHRH neurones. Moreover, knockdown of GPR30 in primate LHRH neurones by transfection with human small interference RNA for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not. Finally, the GPR30 agonist, G1, resulted in changes in [Ca(2+)](i) oscillations similar to those observed with E(2). In this review, we discuss the possible role of G-protein coupled receptors in the rapid action of oestrogen in neuronal cells.

Involvement of G protein-coupled receptor 30 (GPR30) in rapid action of estrogen in primate LHRH neurons

Previously, we have reported that 17beta-estradiol (E(2)) induces an increase in firing activity of primate LH-releasing hormone (LHRH) neurons. The present study investigates whether E(2) alters LHRH release as well as the pattern of intracellular calcium ([Ca(2+)](i)) oscillations and whether G protein-coupled receptor 30 (GPR30) plays a role in mediating the rapid E(2) action in primate LHRH neurons. Results are summarized: 1) E(2), the nuclear membrane-impermeable estrogen, estrogen-dendrimer conjugate, and the plasma membrane-impermeable estrogen, E(2)-BSA conjugate, all stimulated LHRH release within 10 min of exposure; 2) whereas the estrogen receptor antagonist, ICI 182,780, did not block the E(2)-induced LHRH release, E(2) application to cells treated with pertussis toxin failed to induce LHRH release; 3) GPR30 mRNA was expressed in olfactory placode cultures, and GPR30 protein was expressed in a subset of LHRH neurons; 4) pertussis toxin treatment blocked the E(2)-induced increase in [Ca(2+)](i) oscillations; 5) knockdown of GPR30 in primate LHRH neurons by transfection with small interfering RNA (siRNA) for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not; 6) the estrogen-dendrimer conjugate-induced increase in [Ca(2+)](i) oscillations also did not occur in LHRH neurons transfected with GPR30 siRNA; and 7) G1, a GPR30 agonist, resulted in changes in [Ca(2+)](i) oscillations, similar to those observed with E(2). Collectively, E(2) induces a rapid excitatory effect on primate LHRH neurons, and this rapid action of E(2) appears to be mediated, in part, through GPR30.

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

Feedback controls of estrogen in LHRH-1 neurons play a pivotal role in reproductive function. However, the mechanism of estrogen action in LHRH-1 neurons is still unclear. In the present study, the effect of estrogens on intracellular calcium ([Ca(2+)](i)) oscillations in primate LHRH-1 neurons was examined. Application of 17beta-estradiol (E(2), 1 nm) for 10 min increased the frequency of [Ca(2+)](i) oscillations within a few minutes. E(2) also increased the frequency of [Ca(2+)](i) synchronization among LHRH-1 neurons. Similar E(2) effects on the frequency of [Ca(2+)](i) oscillations were observed under the presence of tetrodotoxin, indicating that estrogen appears to cause direct action on LHRH-1 neurons. Moreover, application of a nuclear membrane-impermeable estrogen dendrimer conjugate, not control dendrimer, resulted in a robust increase in the frequencies of [Ca(2+)](i) oscillations and synchronizations, indicating that effects estrogens on [Ca(2+)](i) oscillations and their synchronizations do not require their entry into the cell nucleus. Exposure of cells to E(2) in the presence of the estrogen receptor antagonist ICI 182,780 did not change the E(2)-induced increase in the frequency of [Ca(2+)](i) oscillations or the E(2)-induced increase in the synchronization frequency. Collectively, estrogens induce rapid, direct stimulatory actions through receptors located in the cell membrane/cytoplasm of primate LHRH-1 neurons, and this action of estrogens is mediated by an ICI 182,780-insensitive mechanism yet to be identified.

LHRH-(1-5): a bioactive peptide regulating reproduction

Luteinizing hormone-releasing hormone-I (LHRH-I) was isolated from the mammalian hypothalamus and shown to be the primary regulator of reproduction through its initiation of pituitary gonadotropin release. Subsequently, it has also been shown to have non-pituitary actions. Although the regulation of LHRH-I synthesis and release has been extensively studied, there is additional evidence to suggest that processing of the peptide represents another layer of regulation. The focus of this review will be on evidence for the action of LHRH-(1-5), the pentapeptide metabolite of LHRH-I, in regulating LHRH-I synthesis, secretion and reproductive behavior. The involvement of LHRH-(1-5) in the control of aspects of reproduction might represent yet another level of regulatory complexity through neuropeptide processing.

An increase in in vivo release of LHRH and precocious puberty by posterior hypothalamic lesions in female rhesus monkeys (Macaca mulatta)

We have previously shown that a decrease in gamma-aminobutyric acid (GABA) tone and a subsequent increase in glutamatergic tone occur in association with the pubertal increase in luteinizing hormone releasing hormone (LHRH) release in primates. To further determine the causal relationship between developmental changes in GABA and glutamate levels and the pubertal increase in LHRH release, we examined monkeys with precocious puberty induced by lesions in the posterior hypothalamus (PH). Six prepubertal female rhesus monkeys (17.4 +/- 0.1 mo of age) received lesions in the PH, three prepubertal females (17.5 +/- 0.1 mo) received sham lesions, and two females received no treatments. LHRH, GABA, and glutamate levels in the stalk-median eminence before and after lesions were assessed over two 6-h periods (0600-1200 and 1800-2400) using push-pull perfusion. Monkeys with PH lesions exhibited external signs of precocious puberty, including significantly earlier menarche in PH lesion animals (18.8 +/- 0.2 mo) than in sham/controls (25.5 +/- 0.9 mo, P<0.001). Moreover, PH lesion animals had elevated LHRH levels and higher evening glutamate levels after lesions, whereas LHRH changes did not occur in sham/controls until later. Changes in GABA release were not discernible, since evening GABA levels already deceased at 18-20 mo of age in both groups and morning levels remained at the prepubertal levels. The age of first ovulation in both groups did not differ. Collectively, PH lesions may not be a good tool to investigate the mechanism of puberty, and, taking into account the recent findings on the role of kisspeptins, the mechanism of the puberty onset in primates is more complex than we initially anticipated.

Label-free detection of neuron–drug interactions using acoustic and Kelvin vibrational fields

Kelvin and acoustic fields of high-frequency have been employed in the non-invasive investigation of immortalized hypothalamic neurons, in order to assess their response to different concentrations of specific drugs, toxins, a stress-reducing hormone and neurotrophic factors. In an analytical systems biology approach, this work constitutes a first study of living neuron cultures by scanning Kelvin nanoprobe (SKN) and thickness shear mode (TSM) acoustic wave techniques. N-38 hypothalamic mouse neurons were immobilized on the gold electrode of 9 MHz TSM acoustic wave devices and gold-coated slides for study by SKN. The neurons were exposed to the neurochemicals betaseron, forskolin, TCAP, and cerebrolysin. Signals were collected with the TSM in real-time mode, and with the SKN in scanning and real-time modes, as the drugs were applied at biologically significant concentrations. With the TSM, for all drugs, some frequency and resistance shifts were in the same direction, contrary to normal functioning for this type of instrument. Possible mechanisms are presented to explain this behaviour. An oscillatory signal with periodicity of approximately 2 min was observed for some neuron-coated surfaces, where the amplitude of these oscillations was altered upon application of certain neurotrophic factors. These two new techniques present novel and non-invasive electrodeless methods for detecting changes at the cellular level caused by a variety of neuroactive compounds, without killing or destroying the neurons.

Firing pattern and rapid modulation of activity by estrogen in primate luteinizing hormone releasing hormone-1 neurons

We have shown previously that cultured LHRH-1 neurons, derived from monkey olfactory placode region, exhibit pulsatile LHRH-1 release at hourly intervals and spontaneous intracellular calcium oscillations, which synchronize at a frequency similar to LHRH-1 release. Brief application of estrogen induced a rapid increase in the frequency of intracellular calcium oscillations and the frequency of synchronizations. The estrogen-induced frequency of intracellular calcium oscillations was mediated by estrogen receptors (ER), whereas the frequency of synchronizations was not mediated by ER. In the present study, we further examined the rapid action of estrogen using patch-clamp recording in primate LHRH-1 neurons. Cell-attached patch-clamp recording showed that LHRH-1 neurons exhibited monophasic or biphasic action currents that were sensitive to an increase in extracellular K+ and the sodium channel blocker tetrodotoxin. The majority (90%) of LHRH-1 neurons showed irregular firing patterns composed of bursts and irregular beatings of action currents, which further formed a "cluster" firing pattern. Brief application of 17beta-estradiol (1 nM) increased the firing frequency and burst duration of LHRH-1 neurons with a latency of 60-120 sec for up to 25 min. ICI182,780, an ER antagonist, blocked the 17beta-estradiol-induced increase in the firing activity of LHRH-1 neurons. These results suggest that 1) primate LHRH-1 neurons exhibit complex firing patterns composed of activities with different time domains, 2) estrogen causes rapid stimulatory action of firing activity, and 3) this estrogen action is mediated by ER in primate LHRH-1 neurons.

Possible role of 5'-adenosine triphosphate in synchronization of Ca2+ oscillations in primate luteinizing hormone-releasing hormone neurons

LHRH neurons derived from the olfactory placode region of monkey embryos exhibit spontaneous intracellular Ca2+ ([Ca2+]i) oscillations that synchronize among LHRH neurons and nonneuronal cells at a frequency similar to pulsatile LHRH release. To understand the mechanism of intercellular communication between LHRH neurons and nonneuronal cells, which leads to synchronization, we examined the possible role of ATP. 1) ATP, not ADP or AMP, stimulated both LHRH release and [Ca2+]i concentration, whereas the ATP-induced [Ca2+]i response was abolished by infusion of apyrase, which hydrolyzes ATP; 2) the ATP-induced [Ca2+]i response occurred in normal (but not low) extracellular Ca2+ and was blocked by the voltage-dependent L-type Ca2+ channel blocker, nifedipine; 3) pharmacological experiments with purinergic receptor agonists and antagonists indicated that the ATP-induced [Ca2+]i response in LHRH neurons was mediated through P2X, but not P2Y, receptors; 4) cloning and sequencing studies suggested that P2X2 and P2X4 transcripts were present in olfactory placode cultures; and 5) P2X2 receptors and P2X4 were expressed in LHRH neurons. The results suggest that ATP may play a role in intercellular communication when LHRH neurons synchronize, and raise the possibility that nonneuronal cells, such as glia, may be a crucial component of the in vivo LHRH neurosecretory system.

Sexually dimorphic hormonal regulation of the gap junction protein, CX43, in rats and altered female reproductive function in CX43+/- mice

Astrocytic gap junctional communication is important in steroid hormone regulation of reproductive processes at the level of the hypothalamus, including estrous cyclicity and sexual behavior. We examined the effects of estradiol and progesterone on the abundance of the gap junctional protein, connexin 43 (CX43), which is highly expressed in astrocytes. Gonadectomized rats received hormone treatments that induce maximal sexual behavior and gonadotropin surges in females (estrogen for 48 h followed by progesterone, estrogen alone or progesterone alone). Control animals received vehicle (oil) injections. In the female rat preoptic area (POA), containing the gonadotropin-releasing hormone (GnRH) cell bodies, treatment with estrogen, progesterone or estrogen + progesterone significantly increased CX43 protein levels in immunoblots. In contrast, estrogen + progesterone significantly decreased CX43 levels in the male rat POA. This sexually dimorphic hormonal regulation of CX43 was not evident in the hypothalamus, which contains primarily GnRH nerve terminals. Treatment with estrogen + progesterone significantly decreased CX43 levels in both the male and female hypothalamus. To examine the role of CX43 in female reproductive function, we studied heterozygous female CX43 (CX43+/-) mice. Most mutant mice did not show normal estrous cycles. In addition, when compared to wild type females, CX43+/- mice had reduced lordosis behavior. These data suggest that hypothalamic CX43 expression is regulated by steroid hormones in a brain-region-specific and sexually dimorphic manner. Therefore, gap junctional communication in the POA and hypothalamus may be a factor regulating the estrous cycle and sexual behavior in female rodents.

Local dissipation and coupling properties of cellular oscillators: a case study on calcium oscillations

Synchronised signal transduction between cells is crucial, since it assures fast and immutable information processing, which is vital for flawless functioning of living organisms. The question arises how to recognise the ability of a cell to be easily coupled with other cells. In the present paper, we investigate the system properties that determine best coupling abilities and assure the most efficient signal transduction between cells. A case study is done for intercellular calcium oscillations. For a particular diffusion-like coupled system of cellular oscillators, we determined the minimal gap-junctional permeability that is necessary for synchronisation of initially asynchronous oscillators. Our results show that dissipation is a crucial system property that determines the coupling ability of cellular oscillators. We found that low dissipation assures synchronisation of coupled cells already at very low gap-junctional permeability, whereas highly dissipative oscillators require much higher gap-junctional permeability in order to synchronise. The results are discussed in the sense of their biological importance for systems where the synchronous responses of cells were recognised to be indispensable for appropriate physiological functioning of the tissue.

Nonhuman primates contribute unique understanding to anovulatory infertility in women

Anovulatory infertility affects a large proportion of reproductive-aged women. Major improvements in successful clinical treatment of this prevalent disorder in women's health have been made possible because of biomedical research employing nonhuman primates. Experiments on female rhesus monkeys were the first to demonstrate that the key hypothalamic neurotransmitter, gonadotropin-releasing hormone, involved in stimulating pituitary gonadotropin synthesis, storage, and release was bioactive only when released in approximately hourly bursts. This breakthrough in understanding gonadotropin regulation enabled identification of hypogonadotropic, apparently normogonadotropic, and hypergonadotropic forms of anovulatory infertility, and development of appropriate stimulatory or inhibitory gonadotropin therapies. Treatments to overcome anovulatory infertility represent one of the major advances in clinical reproductive endocrinology during the last 25 yr. The future promise of nonhuman primate models for human ovulatory dysfunction, however, may be based on an increased understanding of molecular and physiological mechanisms responsible for fetal programming of adult metabolic and reproductive defects and for obesity-related, hyperinsulinemic impairment of oocyte development.

In VitroParadigms for the Study of GnRH Neuron Function and Estrogen Effects

The elaboration of in vitro paradigms has enabled direct study of GnRH secretion and the regulation of this process. Common findings using different models are the pulsatile nature and calcium-dependency of GnRH secretion, the excitatory effect of glutamate, and the inhibitory or excitatory effect of GABA. Among the different paradigms, the fetal olfactory placode cultures exhibit the unique property of migration in vitro and may retain the capacity to undergo maturational changes in vitro. The short-term incubation of hypothalamic explants obtained at different ages enables one to study developmental changes as well. Estrogens may have important roles in the regulation of GnRH function and can act indirectly via the neighboring neuronal/glial apparatus and directly on GnRH neurons at the cell body and terminal levels. A direct effect is supported by the recent localization of ERalpha and ERbeta transcripts in GnRH neurons using most paradigms. Discrepant effects of estrogens on GnRH neurons were observed since GnRH biosynthesis is inhibited while GnRH secretion can be either stimulated, unaffected, or reduced. It is likely that the regulatory role of sex steroids including estradiol is very complex since it could involve direct and indirect effects on GnRH neurons through genomic and/or non-genomic mechanisms.

Mechanisms underlying episodic gonadotropin-releasing hormone secretion

The episodic secretion of gonadotropin-releasing hormone (GnRH) is crucial for fertility, but the cellular mechanisms and network properties generating GnRH pulses are not well understood. We will explore three primary aspects of this intermittent hormonal signal: the source of rhythm(s), the possible mechanisms comprising oscillator(s), and how GnRH neurons are synchronized to produce a pulse of hormone release into the pituitary portal blood. Current knowledge will be reviewed, and hypotheses and working models proposed for future studies.