Early development of forebrain gonadotrophin-releasing hormone (GnRH) neurones and the role of GnRH as an autocrine migration factor

Hedgehog-PKA signaling and gnrh3 regulate the development of zebrafish gnrh3 neurons

GnRH neurons secrete GnRH that controls the development of the reproduction system. Despite many studies, the signals controlling the development of GnRH neurons from its progenitors have not been fully established. To understand the development of GnRH neurons, we examined the development of gnrh3-expressing cells using a transgenic zebrafish line that expresses green fluorescent protein (GFP) and LacZ driven by the gnrh3 promoter. GFP and LacZ expression recapitulated that of gnrh3 in the olfactory region, olfactory bulb and telencephalon. Depletion of gnrh3 by morpholinos led to a reduction of GFP- and gnrh3-expressing cells, while over-expression of gnrh3 mRNA increased the number of these cells. This result indicates a positive feed-forward regulation of gnrh3 cells by gnrh3. The gnrh3 cells were absent in embryos that lack Hedgehog signaling, but their numbers were increased in embryos overexpressing shhb. We manipulated the amounts of kinase that antagonizes the Hedgehog signaling pathway, protein kinase A (PKA), by treating embryos with PKA activator forskolin or by injecting mRNAs encoding its constitutively active catalytic subunit (PKA*) and dominant negative regulatory subunit (PKI) into zebrafish embryos. PKA* misexpression or forskolin treatment decreased GFP cell numbers, while PKI misexpression led to ectopic production of GFP cells. Our data indicate that the Hedgehog-PKA pathway participates in the development of gnrh3-expressing neurons during embryogenesis.

Comprehensive analysis of GnRH2 neuronal projections in zebrafish

The presence and conservation of GnRH2 across vertebrate species suggest important biological roles. However, the function of GnRH2 remains unclear. A good research model for GnRH2 functional studies is still lacking largely due to the absence of GnRH2 in the widely used mouse model. Hence, we used the zebrafish, for which powerful genetic tools are available, and developed a transgenic (Tg) line expressing enhanced green fluorescence protein (eGFP). The high sensitivity of eGFP, which can diffuse throughout the neuron, enables us to document the complete projectome of GnRH2 neurons at different developmental stages. Fine projection structures were observed without sacrificing the fish. Crossed with the GnRH3:tdTomato Tg line, the GnRH2:eGFP Tg line provides us with an opportunity to visualize the entire GnRH system simultaneously in one organism. This work will provide a framework to understand the function of the highly-conserved GnRH2 system.

A novel model for development, organization, and function of gonadotropes in fish pituitary

The gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are key regulators of the reproductive axis in vertebrates. Despite the high popularity of zebrafish as a model organism for studying reproductive functions, to date no transgenic zebrafish with labeled gonadotropes have been introduced. Using gonadotropin regulatory elements from tilapia, we generated two transgenic zebrafish lines with labeled gonadotropes. The tilapia and zebrafish regulatory sequences were highly divergent but several conserved elements allowed the tilapia promoters to correctly drive the transgenes in zebrafish pituitaries. FSH cells reacted to stimulation with gonadotropin releasing hormone by proliferating and showing increased transgene fluorescence, whereas estrogen exposure caused a decrease in cell number and transgene fluorescence. Transgene fluorescence reflected the expression pattern of the endogenous fshb gene. Ontogenetic expression of the transgenes followed typical patterns, with FSH cells appearing early in development, and LH cells appearing later and increasing dramatically in number with the onset of puberty. Our transgenic lines provide a powerful tool for investigating the development, anatomy, and function of the reproductive axis in lower vertebrates.

Early development of the gonadotropin-releasing hormone neuronal network in transgenic zebrafish

Understanding development of gonadotropin-releasing hormone (GnRH) neuronal circuits is fundamental to our understanding of reproduction, but not yet well understood. Most studies have been focused on GnRH neurons located in the hypothalamus and preoptic area (POA), which directly regulate the pituitary-gonadal axis. In zebrafish (Danio rerio), two forms of GnRH have been identified: GnRH2 and GnRH3. GnRH3 neurons in this species plays two roles: hypophysiotropic and neuromodulatory, depending on their location. GnRH3 neurons in the ventral telencephalon, POA, and hypothalamus control pituitary-gonadal function; in other areas (e.g., terminal nerve), they are neuromodulatory and without direct action on reproduction. To investigate the biology of GnRH neurons, a stable line of transgenic zebrafish was generated in which the GnRH3 promoter drives expression of a bright variant of green fluorescent protein (Emerald GFP, or EMD). This provides unprecedented sensitivity in detecting and imaging GnRH3 neurons during early embryogenesis in the transparent embryo. Using timelapse confocal imaging to monitor the time course of GnRH3:EMD expression in the live embryo, we describe the emergence and development of GnRH3 neurons in the olfactory region, hypothalamus, POA, and trigeminal ganglion. By 50 h post fertilization, these diverse groups of GnRH3 neurons project broadly in the central and peripheral nervous systems and make anatomical connections with each other. Immunohistochemistry of synaptic vesicle protein 2 (a marker of synaptic transmission) in this transgenic model suggests synaptic formation is occurring during early development of the GnRH3 neural network. Electrophysiology reveals early emergence of responsiveness to the stimulatory effects of kisspeptin in terminal nerve GnRH3 neurons. Overall, our findings reveal that the GnRH3 neuronal system is comprised of multiple populations of neurons as a complicated network.

Profiling, Bioinformatic, and Functional Data on the Developing Olfactory/GnRH System Reveal Cellular and Molecular Pathways Essential for This Process and Potentially Relevant for the Kallmann Syndrome

During embryonic development, immature neurons in the olfactory epithelium (OE) extend axons through the nasal mesenchyme, to contact projection neurons in the olfactory bulb. Axon navigation is accompanied by migration of the GnRH+ neurons, which enter the anterior forebrain and home in the septo-hypothalamic area. This process can be interrupted at various points and lead to the onset of the Kallmann syndrome (KS), a disorder characterized by anosmia and central hypogonadotropic hypogonadism. Several genes has been identified in human and mice that cause KS or a KS-like phenotype. In mice a set of transcription factors appears to be required for olfactory connectivity and GnRH neuron migration; thus we explored the transcriptional network underlying this developmental process by profiling the OE and the adjacent mesenchyme at three embryonic ages. We also profiled the OE from embryos null for Dlx5, a homeogene that causes a KS-like phenotype when deleted. We identified 20 interesting genes belonging to the following categories: (1) transmembrane adhesion/receptor, (2) axon-glia interaction, (3) scaffold/adapter for signaling, (4) synaptic proteins. We tested some of them in zebrafish embryos: the depletion of five (of six) Dlx5 targets affected axonal extension and targeting, while three (of three) affected GnRH neuron position and neurite organization. Thus, we confirmed the importance of cell-cell and cell-matrix interactions and identified new molecules needed for olfactory connection and GnRH neuron migration. Using available and newly generated data, we predicted/prioritized putative KS-disease genes, by building conserved co-expression networks with all known disease genes in human and mouse. The results show the overall validity of approaches based on high-throughput data and predictive bioinformatics to identify genes potentially relevant for the molecular pathogenesis of KS. A number of candidate will be discussed, that should be tested in future mutation screens.

Differential regulation of GnRH ligand and receptor genes in the brain and pituitary of Atlantic cod exposed to different photoperiod

The onset of puberty and reproduction are tightly controlled by extrinsic and intrinsic inputs combined with genetically determined biological blueprints. Environmental inputs are then mediated by the brain-pituitary-gonad endocrine axis resulting in a unified output. In fish, one of the primary factors controlling the timing of sexual maturation is light, although how these signals are mediated in the brain and pituitary is not well understood. We therefore aimed to elucidate the molecular basis of the control of reproduction during the first spawning season in two year old female Atlantic cod. To this end, we measured GnRH and GnRH-R variant gene expression in brains and pituitaries collected from cod kept under four different photoperiod regimes: natural light (NL), continuous light (LL) and combined treatment of NL-LL and LL-NL. LL inhibited sexual development and spawning and LL-NL delayed sexual development and spawning. LL inhibited the spawning-related increase in brain GnRH3 and pituitary GnRH-R2a gene expression found under NL conditions, and the expression of these genes were delayed in concert with spawning of LL-NL cod. This study indicates that regulation of brain GnRH3 and pituitary GnRH-R2a genes likely mediates photoperiod induced changes in cod gonadal maturation.

Lactobacillus rhamnosus accelerates zebrafish backbone calcification and gonadal differentiation through effects on the GnRH and IGF systems

Endogenous microbiota play essential roles in the host’s immune system, physiology, reproduction and nutrient metabolism. We hypothesized that a continuous administration of an exogenous probiotic might also influence the host’s development. Thus, we treated zebrafish from birth to sexual maturation (2-months treatment) with Lactobacillus rhamnosus, a probiotic species intended for human use. We monitored for the presence of L. rhamnosus during the entire treatment. Zebrafish at 6 days post fertilization (dpf) exhibited elevated gene expression levels for Insulin-like growth factors -I and -II, Peroxisome proliferator activated receptors -α and -β, VDR-α and RAR-γ when compared to untreated-10 days old zebrafish. Using a gonadotropin-releasing hormone 3 GFP transgenic zebrafish (GnRH3-GFP), higher GnRH3 expression was found at 6, 8 and 10 dpf upon L. rhamnosus treatment. The same larvae exhibited earlier backbone calcification and gonad maturation. Noteworthy in the gonad development was the presence of first testes differentiation at 3 weeks post fertilization in the treated zebrafish population -which normally occurs at 8 weeks- and a dramatic sex ratio modulation (93% females, 7% males in control vs. 55% females, 45% males in the treated group). We infer that administration of L. rhamnosus stimulated the IGF system, leading to a faster backbone calcification. Moreover we hypothesize a role for administration of L. rhamnosus on GnRH3 modulation during early larval development, which in turn affects gonadal development and sex differentiation. These findings suggest a significant role of the microbiota composition on the host organism development profile and open new perspectives in the study of probiotics usage and application.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

A conserved non-reproductive GnRH system in chordates

Gonadotropin-releasing hormone (GnRH) is a neuroendocrine peptide that plays a central role in the vertebrate hypothalamo-pituitary axis. The roles of GnRH in the control of vertebrate reproductive functions have been established, while its non-reproductive function has been suggested but less well understood. Here we show that the tunicate Ciona intestinalis has in its non-reproductive larval stage a prominent GnRH system spanning the entire length of the nervous system. Tunicate GnRH receptors are phylogenetically closest to vertebrate GnRH receptors, yet functional analysis of the receptors revealed that these simple chordates have evolved a unique GnRH system with multiple ligands and receptor heterodimerization enabling complex regulation. One of the gnrh genes is conspicuously expressed in the motor ganglion and nerve cord, which are homologous structures to the hindbrain and spinal cord of vertebrates. Correspondingly, GnRH receptor genes were found to be expressed in the tail muscle and notochord of embryos, both of which are phylotypic axial structures along the nerve cord. Our findings suggest a novel non-reproductive role of GnRH in tunicates. Furthermore, we present evidence that GnRH-producing cells are present in the hindbrain and spinal cord of the medaka, Oryzias latipes, thereby suggesting the deep evolutionary origin of a non-reproductive GnRH system in chordates.

Regulation of temporal and spatial organization of newborn GnRH neurons by IGF signaling in zebrafish

When and how newborn neurons are organized to form a functional network in the developing brain remains poorly understood. An attractive model is the gonadotropin-releasing hormone (GnRH) neuron system, master regulator of the reproductive axis. Here we show that blockage of IGF signaling, a central growth-promoting signaling pathway, by the induced expression of a dominant-negative form of IGF1 receptor (IGF1R) or specific IGF1R inhibitors delayed the emergence of GnRH2 neurons in the midbrain and GnRH3 neurons in the olfactory bulb region. Blockage of IGF signaling also resulted in an abnormal appearance of GnRH3 neurons outside of the olfactory bulb region, although it did not change the locations of other olfactory neurons, GnRH2 neurons, or brain patterning. This IGF action is developmental stage-dependent because the blockade of IGF signaling in advanced embryos had no such effect. An application of phosphatidylinositol 3-kinase (PI3K) inhibitors phenocopied the IGF signaling deficient embryos, whereas the MAPK inhibitors had no effect, suggesting that this IGF action is mediated through the PI3K pathway. Real-time in vivo imaging studies revealed that the ectopic GnRH3 neurons emerged at the same time as the normal GnRH3 neurons in IGF-deficient embryos. Further experiments suggest that IGF signaling affects the spatial distribution of newborn GnRH3 neurons by influencing neural crest cell migration and/or differentiation. These results suggest that the IGF-IGF1R-PI3K pathway regulates the precise temporal and spatial organization of GnRH neurons in zebrafish and provides new insights into the regulation of GnRH neuron development.

Characterization of GFP-tagged GnRH-containing terminalis neurons in transgenic zebrafish

The terminalis nerve (TN) has been described in all vertebrate species, in which it plays important roles in animal behavior and physiology. In teleost fish, the TN is located in the olfactory bulb and in its nerve tract. Here, we report a study on the characterization of the TN cell development, axon projection and physiology in zebrafish (Danio rerio). We have generated several lines of transgenic zebrafish [Tg (GnRH-3::GFP)] that express GFP in the TN cells. The transgenes are expressed under the transcriptional control of the zebrafish GnRH-3 promoter. During development, the first GFP-positive TN cell was identified at approximately 34 h post-fertilization (hpf). By 38 hpf, several clusters of TN cells were identified in the olfactory bulb and olfactory nerve tract. In the olfactory bulb, the TN cells projected axons caudally. In the forebrain, some of the TN axons extended caudally, but most crossed the midline of the brain at the commissural anterior. In the midbrain, some of the TN axons extended dorsally towards the tectum, whereas other axons extended caudally, or extended ventrally to the optic nerve where they entered the neural retina. We also examined the cell membrane property of the TN cells. Using patch-clamp techniques, we recorded spontaneous and evoked action potentials from isolated TN cells. We examined the expression of glutamate receptors in the TN cells. The data shed light on the mechanisms of TN function in the nervous system and in the regulation of animal physiology.

Characterization of voltage-activated ionic currents in the GnRH-containing terminalis nerve in transgenic zebrafish

The terminalis nerve (TN) is in a class of cranial nerves that plays important roles in animal development, physiology and behavior. Here, we report a study on the characterization of voltage-activated ionic currents in GnRH-containing TN cells in zebrafish. The experiments were performed using acutely dissociated TN cells from the transgenic zebrafish Tg (GnRH-3::GFP). In the transgenic zebrafish, the TN cells express GFP under the transcriptional control of the zebrafish GnRH-3 promoter. In all of the GnRH-containing TN cells examined, we recorded both low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium current (I(Ca)). The characteristics of the I(Ca) were similar to those described in other zebrafish cell types. However, the distribution patterns of the currents in the GnRH-containing TN cells were different in comparison to the distribution of the currents in other cell types. In addition, we characterized TTX-sensitive sodium current (I(Na)) and 4AP-sensitive and TEA-resistant potassium current (I(K)). The characteristics of voltage-activated I(Na) and I(K) in the GnRH-containing TN cells were similar to those described in other zebrafish cell types. Together, the data from this study revealed the electrophysiological properties of the GnRH-containing TN cells, thereby providing insight on the regulatory mechanisms of TN-signaling in animal physiology.

17alpha-ethinylestradiol disrupts the ontogeny of the forebrain GnRH system and the expression of brain aromatase during early development of zebrafish

Until now, studies dedicated to the actions of endocrine disrupting chemicals (EDCs) on the reproductive axis have been concerned with their effects at the gonadal level leaving their actions on neuroendocrine circuits controlling reproduction virtually unexplored. In vertebrates, gonadotropin-releasing hormone (GnRH) is the key factor controlling the activity of the reproductive axis. The development and functioning of GnRH neurons are finely tuned by a series of factors, notably sex steroids, making these neurons potential targets of EDCs, notably in aquatic species. By means of immunohistochemistry, we examined the effects of low levels of ethinylestradiol (EE2 0.02 nM, 0.1 nM, 0.5 nM), a potent synthetic estrogen, on early development (at 5, 10, 20, 30 days post-fertilization) of the forebrain GnRH neurons in a model fish species, the zebrafish (Danio rerio). In parallel, the ER-regulated expression of cytochrome P450 aromatase B (AroB) protein, which is encoded by the cyp19a1b gene, was precisely mapped at the brain and pituitary levels in developing control and EE2-exposed zebrafish. We show that EE2 disrupts the ontogeny of GnRH system by inducing an increase in the number of GnRH-ir neurons and GnRH fibers based on their immunoreactivity as well as a decrease in the size of the GnRH-ir soma and a modification of the migration profile of GnRH-ir neurons. Furthermore, we report a spectacular dose and time-dependent induction of AroB expression in radial glial cells of the developing brain further illustrating the extreme sensitivity of AroB to xenoestrogen and the relevance of AroB as biomarker of xenoestrogen effects on the central nervous system. Collectively, these original and relevant observations highlight the sensitivity of GnRH and AroB to a synthetic estrogen during embryogenesis. These data reinforce the need to further study the mechanisms underlying EDC effects on key neuroendocrine circuits involved in reproduction and brain development of vertebrates.

Acquisition of spontaneous electrical activity during embryonic development of gonadotropin-releasing hormone-3 neurons located in the terminal nerve of transgenic zebrafish (Danio rerio)

There are multiple populations of gonadotropin-releasing hormone (GnRH) neurons that have distinct physiological and behavioral functions. Teleost fish have a population of GnRH3 neurons located in the terminal nerve (TN) associated with the olfactory bulb that is thought to play a neuromodulatory role in multiple physiological systems, including olfactory, visual, and reproductive. We used transgenic zebrafish in which the GnRH3 promoter drives expression of a green fluorescent protein to identify GnRH3 neurons during development in live embryos. Unlike with hypophysiotropic GnRH neurons of zebrafish, TN-GnRH3 neurons are of neural crest origin and are one of the first populations of GnRH neurons to develop in the early embryo. Using a combination of optical imaging and electrophysiology, we showed that during the first 3 days post-fertilization, TN-GnRH3 neurons increase in number, extend neural projections, move in association with tissue expansion, and acquire an adult-pattern of spontaneous action potential firing. Early during development, about half of the neurons were quiescent/non-firing. Later, at 3 days post-fertilization, there was an increase in the proportion of neurons showing action potential firing and an increase in the number of neurons that showed an adult-like tonic or beating pattern of action potential firing with a firing frequency similar to that seen in adult TN-GnRH3 neurons. This study represents the first neurophysiological investigation of developing GnRH neurons in live embryos--an important advancement in understanding their potential non-reproductive roles during embryogenesis.

Embryonic gonadotropin-releasing hormone signaling is necessary for maturation of the male reproductive axis

Gonadotropin-releasing hormone (GnRH) signaling regulates reproductive physiology in mammals. GnRH is released by a subset of hypothalamic neurons and binds to GnRH receptor (GnRHR) on gonadotropes in the anterior pituitary gland to control production and secretion of gonadotropins that in turn regulate the activity of the gonads. Central control of reproduction is well understood in adult animals, but GnRH signaling has also been implicated in the development of the reproductive axis. To investigate the role of GnRH signaling during development, we selectively ablated GnRHR-expressing cells in mice. This genetic strategy permitted us to identify an essential stage in male reproductive axis development, which depends on embryonic GnRH signaling. Our experiments revealed a striking dichotomy in the gonadotrope population of the fetal anterior pituitary gland. We show that luteinizing hormone-expressing gonadotropes, but not follicle-stimulating hormone-expressing gonadotropes, express the GnRHR at embryonic day 16.75. Furthermore, we demonstrate that an embryonic increase in luteinizing hormone secretion is needed to promote development of follicle-stimulating hormone-expressing gonadotropes, which might be mediated by paracrine interactions within the pituitary. Moreover, migration of GnRH neurons into the hypothalamus appeared normal with appropriate axonal connections to the median eminence, providing genetic evidence against autocrine regulation of GnRH neurons. Surprisingly, genetic ablation of GnRHR expressing cells significantly increased the number of GnRH neurons in the anterior hypothalamus, suggesting an unexpected role of GnRH signaling in establishing the size of the GnRH neuronal population. Our experiments define a functional role of embryonic GnRH signaling.

From nose to brain: development of gonadotrophin-releasing hormone-1 neurones

Gonadotrophin-releasing hormone-1 (GnRH-1) is essential for mammalian reproduction, controlling release of gonadotrophins from the anterior pituitary. GnRH-1 neurones migrate from the nasal placode into the forebrain during development. Although first located within the nasal placode, the embryonic origin/lineage of GnRH-1 neurones is still unclear. The migration of GnRH-1 cells is the best characterised example of neurophilic/axophilic migration, with the cells using a subset of olfactory-derived vomeronasal axons as their pathway and numerous molecules to guide their movement into the forebrain. Exciting work in this area is beginning to identify intersecting pathways that orchestrate the movement of these critical neuroendocrine cells into the central nervous system, both spatially and temporally, through a diverse and changing terrain. Once within the forebrain, little is known about how the axons target the median eminence and ultimately secrete GnRH-1 in a pulsatile fashion.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Cxcl12a-Cxcr4b signaling is important for proper development of the forebrain GnRH system in zebrafish

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons control pituitary gonadotropin secretion and gametogenesis. In the course of development, these neurons migrate from the olfactory placode to the hypothalamus. The precise molecular mechanism of this neuronal migration is unclear. Here, we investigated whether the chemokine receptor, Cxcr4b, and its cognate ligand, Cxcl12a, are required for proper migration of GnRH3 neurons in zebrafish. Deviated GnRH3 axonal projections and neuronal migration were detected in larvae that carry a homozygote cxcr4b mutation. Similarly, knockdown of Cxcr4b or Cxcl12a led to the appearance of abnormal GnRH3 axonal projections and cell migration, including absence of the characteristic lateral crossing of GnRH3 axons at the anterior commissure and optic chiasm. Double-labeling analysis has shown that cxcr4b and cxcl12a are expressed along the GnRH3 migration pathway (i.e. olfactory placode, terminal nerve and the optic chiasm). The results of this study suggest that the Cxcl12a-Cxcr4b ligand-receptor pair are involved in the migration of GnRH3 neurons in zebrafish, and are therefore crucial for the development of this system.

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

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

Targeted gonadotropin-releasing hormone-3 neuron ablation in zebrafish: effects on neurogenesis, neuronal migration, and reproduction

Hypophysiotropic GnRH neurons are located in the preoptic area and ventral hypothalamus of sexually mature vertebrates. In several species, the embryonic origin of hypophysiotropic GnRH neurons remains unclear. Using the Tg(GnRH3:EGFP) zebrafish line, in which GnRH3 neurons express EGFP, GnRH3 neurons in the olfactory region were specifically and individually ablated during early development using laser pulses. After ablation, the olfactory region maintained the capacity to regenerate GnRH3 neurons. However, this capacity was time-limited. When ablation of GnRH3 cells was conducted at 2 d after fertilization, high regeneration rates were observed, but regeneration capacity significantly decreased when ablation was performed at 4 or 6 d after fertilization. Unilateral GnRH3 neuron ablation results in unilateral soma presence. These unilateral somata are capable of projecting fiber extensions bilaterally. Successful bilateral GnRH3 soma ablation during development resulted in complete lack of olfactory, terminal nerve, preoptic area, and hypothalamic GnRH3 neurons and fibers in 12-wk-old animals. Mature animals lacking GnRH3 neurons exhibited arrested oocyte development and reduced average oocyte diameter. Animals in which GnRH3 neurons were partially ablated exhibited normal oocyte development; however, their fecundity was significantly reduced. These findings demonstrate that the hypophysiotropic GnRH3 populations in zebrafish consist of neurons that originate in the olfactory region during early development. The presence of GnRH3 neurons of olfactory region origin in reproductively mature zebrafish is a prerequisite for normal oocyte development and reproduction.

Endocrine/paracrine control of zebrafish ovarian development

Ovarian differentiation and the processes of follicle development, oocyte maturation and ovulation are complex events, requiring the coordinated action of regulatory molecules. In zebrafish, ovarian development is initiated at 10 days after hatching and fish become sexually mature at 3 months. Adult zebrafish have asynchronous ovaries, which contain follicles of all stages of development. Eggs are spawned daily under proper environmental conditions in a population of zebrafish, with individual females spawning irregularly every 4-7 days in mixed sex conditions. Maximal embryo viability is achieved when sexually isolated females are bred in 10-day intervals [Niimi, A.J., LaHam, Q.N., 1974. Influence of breeding time interval on egg number, mortality, and hatching of the zebra fish Brachydanio verio. Can. J. Zool. 52, 515-517]. Similar to other vertebrates, hormones from the hypothalamus-pituitary-gonadal axis play important roles in regulating follicle development. Follicle stimulating hormone (FSH) stimulates estradiol production, which in turn, promotes viteollogenesis. Luteinizing hormone (LH) stimulates the production of 17,20beta-dihydroxy-4-pregnen-3-one (17,20betaP) or maturation inducing hormone (MIH) which acts through membrane progestin receptors to activate maturation promoting factor, leading to oocyte maturation. Recent studies in zebrafish have also provided novel insights into the functions of ovary-derived growth factors in follicle development and oocyte maturation. The present review summarizes the current knowledge on how endocrine and paracrine factors regulate ovarian development in zebrafish. Special emphasis is placed on how follicle development and oocyte maturation in adult females is regulated by gonadotropins, ovarian steroids and growth factors produced by the ovary.

The zebrafish as a model system for forebrain GnRH neuronal development

Development and function of the forebrain gonadotropin-releasing hormone (GnRH) neuronal system has long been the focus of study in various vertebrate species. This system is crucial for reproduction and an important model for studying tangential neuronal migration. In addition, the finding that multiple forms of GnRH exist in the CNS as well as in non-CNS tissues, coupled with the fact that GnRH fibers project to many CNS regions, implies that GnRH has a variety of functions in addition to its classic reproductive role. The study of the GnRH system and its functions is, however, limited by available model systems and methodologies. The transgenic (Tg) GnRH3:EGFP zebrafish line, in which GnRH3 neurons express EGFP, allows in vivo study of the GnRH3 system in the context of the entire animal. Coupling the use of this line with the attributes and molecular tools available in zebrafish has expanded our ability to study the forebrain GnRH system. Herein, we discuss the use of the Tg(GnRH3:EGFP) zebrafish line as a model for studying forebrain GnRH neurons, both in developing larvae and in sexually mature animals. We also discuss the potential use of this line to study regulation of GnRH3 system development.

Neurotrophic effect of gonadotropin-releasing hormone on neurite extension and neuronal migration of embryonic gonadotropin-releasing hormone neurons in chick olfactory nerve bundle culture

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in regulating the reproductive function of vertebrates. These neurons are known to originate in the olfactory placode and migrate along olfactory-related axons to reach the forebrain during embryonic development. Although GnRH is suggested to be secreted during such migration, its physiological significance is unknown. This point is difficult to explore in vivo because recent studies suggest that GnRH is an important factor for normal brain development and that modification of the embryonic GnRH system by exogenous GnRH analogue or genetic methods would result in dysgenesis of the brain. Therefore, to study the role of GnRH in the migratory process of GnRH neurons, we established an in vitro chick embryonic olfactory nerve bundle explant model. Embryonic day 7.5-8 olfactory nerve bundles were cultured in a mixture of Matrigel and collagen gel. At day 3 of culture, GnRH neurons extended their unbranched neurites and migrated out from both edges of the explant. The nature of neurite extension and migratory behavior of GnRH neurons was well maintained in the gel containing 25% Matrigel and 50% collagen. With this culture system, we examined the effect of GnRH on the migrating GnRH neurons. Cetrorelix, a GnRH antagonist, was found to inhibit significantly neurite growth and neuronal migration of GnRH neurons, the effects of which were repressed by the addition of chicken GnRH-I. These results suggest that GnRH functions as one of the regulating factors of GnRH neuronal development by promoting neurite extension and neuronal migration.

The gonadotropin-releasing hormone (GnRH) neuronal population is normal in size and distribution in GnRH-deficient and GnRH receptor-mutant hypogonadal mice

Hypothalamic GnRH neurons are essential for initiation and regulation of reproductive function. In addition to pituitary gonadotrope stimulation, activity of GnRH through its receptor (GnRHR) has been suggested to include autocrine regulation of the GnRH neuron. Two hypogonadal mouse strains, the Gnrh1 mutant (hpg) mice and Gnrhr mutant mice were used to investigate the potential role of GnRH signaling in the proper development and maintenance of GnRH neurons. Immunocytochemical analysis of heterozygous hpg mice revealed a GnRH neuron population that was normal in size and distribution, indicating no effect from reduced Gnrh1 gene dosage on the neurons themselves. To visualize GnRH neurons in homozygous GnRH-deficient hpg mice, heterozygous hpg mice were crossed with GnRH-green fluorescent protein (GFP) transgenic mice with targeted expression of the GFP reporter gene in GnRH neurons. Analysis of forebrains of homozygous hpg/GFP-positive mice immunostained for GFP revealed a normal population size and appropriate distribution of GnRH neurons in hpg mice, with immunoreactive neuronal processes present at the median eminence. Similarly, adult mice deficient in functional GnRHR possessed a full complement of GnRH neurons in the basal forebrain that was indistinguishable from the distribution of GnRH neurons in their wild-type counterparts. Moreover, hpg/GFP neurons retained the ability to generate spontaneous bursts of action potential firing activity, suggesting that GnRH peptide is not required for this function. These data establish that autocrine-paracrine GnRH-signaling is not a prerequisite for the developmental migration of GnRH neurons into the brain or for the projection of GnRH neurosecretory axons.