Temporal and spatial expression of gonadotropin releasing hormone (GnRH) in the brain of developing zebrafish (Danio rerio)

The Olfactory Tract: Basis for Future Evolution in Response to Rapidly Changing Ecological Niches

Within the forebrain the olfactory sensory system is unique from other sensory systems both in the projections of the olfactory tract and the ongoing neurogenic potential, characteristics conserved across vertebrates. Olfaction plays a crucial role in behaviors such as mate choice, food selection, homing, escape from predators, among others. The olfactory forebrain is intimately associated with the limbic system, the region of the brain involved in learning, memory, and emotions through interactions with the endocrine system and the autonomic nervous system. Previously thought to lack a limbic system, we now know that teleost fishes process emotions, have exceptional memories, and readily learn, behaviors that are often associated with olfactory cues. The association of neuromodulatory hormones, and more recently, the immune system, with odor cues underlies behaviors essential for maintenance and adaptation within natural ecological niches. Increasingly anthropogenic perturbations affecting ecosystems are impacting teleost fishes worldwide. Here we examine the role of the olfactory tract as the neural basis for the integration of environmental cues and resulting behaviors necessary for the regulation of biotic interactions that allow for future adaptation as the climate spins out of control.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

phoenixin(smim20), a gene coding for a novel reproductive ligand, is expressed in the brain of adult zebrafish

Gonadotropin-releasing hormone (GnRH) is a highly conserved neuroendocrine decapeptide that is essential for the onset of puberty and the maintenance of the reproductive state. In addition to its role as hypothalamic releasing hormone, GnRH has multiple functions including modulator of neural activity within the nervous system and of resulting behaviors. These multiple functions are reflected by the existence of multiple isoforms. Despite its importance as a critical hypothalamic releasing hormone, the gnrh1 gene has been lost in zebrafish, and its reproductive function is not compensated for by other GnRH isoforms (GnRH2 and GnRH3), suggesting that, surprisingly, zebrafish do not use any of the GnRH peptides to control reproduction and fertility. Previously we proposed that Phoenixin/SMIM20, a novel peptide identified in mammals and the ligand for the orphan GPR173, is a potential candidate to control the initiation of sexual development and fertility in the zebrafish. Here we confirm the sequence of the zebrafish phoenixin/smim20 gene and by RT-PCR show that it is expressed early in development through adulthood. Subsequently we show that phoenixin/smim20 is expressed in the adult brain including the regions of the hypothalamus important in the control of fertility and reproduction.

Sexual Dimorphic Distribution of Hypothalamic Tachykinin1 Cells and Their Innervations to GnRH Neurons in the Zebrafish

Substance P (SP) and neurokinin A (NKA), encoded by TAC1/Tac1 gene are members of the tachykinin family, which exert their neuromodulatory roles in vertebrate reproduction. In mammals, SP and NKA have been shown to regulate gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion via kisspeptin neurons. On the other hand, the role of SP/NKA in the regulation of reproduction in non-mammalian vertebrates is not well known. In the present study, we first localized expression of tac1 mRNA in the brain of male and female zebrafish, Danio rerio. Next, using an antibody against zebrafish tachykinin1 (Tac1), we examined the neural association of SP/NKA neural processes with GnRH3 neurons, and with kisspeptin (kiss2) neurons, in the brains of male and female zebrafish. In situ hybridization showed an apparent male-dominant tac1 expression in the ventral telencephalic area, the anterior and posterior parts of the parvocellular preoptic nucleus, and the suprachiasmatic nucleus. On the other hand, there was female-dominant tac1 expression in the ventral periventricular hypothalamus. Confocal images of double-labeled zebrafish Tac1 and GnRH3 showed associations between Tac1-immunoreactive processes and GnRH3 neurons in the ventral telencephalic area. In contrast, there was no apparent proximity of Tac1 processes to kiss2 mRNA-expressing neurons in the hypothalamus. Lastly, to elucidate possible direct action of SP/NKA on GnRH3 or Kiss2 neurons, expression of SP/NKA receptor, tacr1a mRNA was examined in regions containing GnRH3 or Kiss2 neurons by in situ hybridization. Expression of tacr1a mRNA was seen in several brain regions including the olfactory bulb, preoptic area and hypothalamus, where GnRH3 and Kiss2 cells are present. These results suggest that unlike in mammals, Tac1 may be involved in male reproductive functions via direct action on GnRH3 neurons but independent of kisspeptin in the zebrafish.

Distribution of Kiss2 receptor in the brain and its localization in neuroendocrine cells in the zebrafish

Kisspeptin is a hypothalamic neuropeptide, which acts directly on gonadotropin-releasing hormone (GnRH)-secreting neurons via its cognate receptor (GPR54 or Kiss-R) to stimulate GnRH secretion in mammals. In non-mammalian vertebrates, there are multiple kisspeptins (Kiss1 and Kiss2) and Kiss-R types. Recent gene knockout studies have demonstrated that fish kisspeptin systems are not essential in the regulation of reproduction. Studying the detailed distribution of kisspeptin receptor in the brain and pituitary is important for understanding the multiple action sites and potential functions of the kisspeptin system. In the present study, we generated a specific antibody against zebrafish Kiss2-R (=Kiss1Ra/GPR54-1/Kiss-R2/KissR3) and examined its distribution in the brain and pituitary. Kiss2-R-immunoreactive cell bodies are widely distributed in the brain including in the dorsal telencephalon, preoptic area, hypothalamus, optic tectum, and in the hindbrain regions. Double-labeling showed that not all but a subset of preoptic GnRH3 neurons expresses Kiss2-R, while Kiss2-R is expressed in most of the olfactory GnRH3 neurons. In the posterior preoptic region, Kiss2-R immunoreactivity was seen in vasotocin cells. In the pituitary, Kiss2-R immunoreactivity was seen in corticotropes, but not in gonadotropes. The results in this study suggest that Kiss2 and Kiss2-R signaling directly serve non-reproductive functions and indirectly subserve reproductive functions in teleosts.

Neuroendocrinology of reproduction: Is gonadotropin-releasing hormone (GnRH) dispensable?

Gonadotropin releasing hormone (GnRH) is a highly conserved neuroendocrine decapeptide that is essential for the onset of puberty and the maintenance of the reproductive state. First identified in mammals, the GnRH signaling pathway is found in all classes of vertebrates; homologues of GnRH have also been identified in invertebrates. In addition to its role as a hypothalamic releasing hormone, GnRH has multiple functions including modulating neural activity within specific regions of the brain. These various functions are mediated by multiple isoforms, which are expressed at diverse locations within the central nervous system. Here we discuss the GnRH signaling pathways in light of new reports that reveal that some vertebrate genomes lack GnRH1. Not only do other isoforms of GnRH not compensate for this gene loss, but elements upstream of GnRH1, including kisspeptins, appear to also be dispensable. We discuss routes that may compensate for the loss of the GnRH1 pathway.

Cell-type heterogeneity in the early zebrafish olfactory epithelium is generated from progenitors within preplacodal ectoderm

The zebrafish olfactory epithelium comprises a variety of neuronal populations, which are thought to have distinct embryonic origins. For instance, while ciliated sensory neurons arise from preplacodal ectoderm (PPE), previous lineage tracing studies suggest that both Gonadotropin releasing hormone 3 (Gnrh3) and microvillous sensory neurons derive from cranial neural crest (CNC). We find that the expression of Islet1/2 is restricted to Gnrh3 neurons associated with the olfactory epithelium. Unexpectedly, however, we find no change in Islet1/2+ cell numbers in sox10 mutant embryos, calling into question their CNC origin. Lineage reconstruction based on backtracking in time-lapse confocal datasets, and confirmed by photoconversion experiments, reveals that Gnrh3 neurons derive from the anterior PPE. Similarly, all of the microvillous sensory neurons we have traced arise from preplacodal progenitors. Our results suggest that rather than originating from separate ectodermal populations, cell-type heterogeneity is generated from overlapping pools of progenitors within the preplacodal ectoderm.

GnRH-mediated olfactory and visual inputs promote mating-like behaviors in male zebrafish

The engagement of sexual behaviors is regulated by a number of factors which include gene expression, hormone circulation, and multi-sensory information integration. In zebrafish, when a male and a female are placed in the same container, they show mating-like behaviors regardless of whether they are kept together or separated by a net. No mating-like behaviors are observed when same-sex animals are put together. Through the olfacto-visual centrifugal pathway, activation of the terminalis nerve in the olfactory bulb increases GnRH signaling in the brain and triggers mating-like behaviors between males. In zebrafish mutants or wild-type fish in which the olfacto-visual centrifugal pathway is impaired or chemically ablated, in response to odor stimulation the mating-like behaviors between males are no longer evident. Together, the data suggest that the combination of olfactory and visual signals alter male zebrafish's mating-like behaviors via GnRH signaling.

In silico predicted reproductive endocrine transcriptional regulatory networks during zebrafish (Danio rerio) development

The interconnected topology of transcriptional regulatory networks (TRNs) readily lends to mathematical (or in silico) representation and analysis as a stoichiometric matrix. Such a matrix can be 'solved' using the mathematical method of extreme pathway (ExPa) analysis, which identifies uniquely activated genes subject to transcription factor (TF) availability. In this manuscript, in silico multi-tissue TRN models of brain, liver and gonad were used to study reproductive endocrine developmental programming in zebrafish (Danio rerio) from 0.25h post fertilization (hpf; zygote) to 90 days post fertilization (dpf; adult life stage). First, properties of TRN models were studied by sequentially activating all genes in multi-tissue models. This analysis showed the brain to exhibit lowest proportion of co-regulated genes (19%) relative to liver (23%) and gonad (32%). This was surprising given that the brain comprised 75% and 25% more TFs than liver and gonad respectively. Such 'hierarchy' of co-regulatory capability (brain<liver<gonad) indicated presence of highly gene-specific TRNs in the brain, alluding to its role as 'master controller' of endocrine function. Second, TRN models were constrained with varying TF availabilities during zebrafish development. Normalized numbers of genes active during development showed concomitant activations between brain and gonad from 10 to 12 hpf (embryonic life stage) up to 30-90 dpf (adult life stage). This indicated a putative 'syncing' between the brain and gonad, and initiation of an early reproductive endocrine developmental program. Finally, comparison of in vivo active genes with those predicted in silico showed relatively good agreement for brain (49%), liver (27%) and gonad (32%). The multi-tissue TRN models presented can lend diagnostic insights into the effects of changing environmental and/or genetic constraints on reproductive endocrine function.

Zebrafish: A Versatile Animal Model for Fertility Research

The utilization of zebrafish in biomedical research is very common in the research world nowadays. Today, it has emerged as a favored vertebrate organism for the research in science of reproduction. There is a significant growth in amount numbers of scientific literature pertaining to research discoveries in reproductive sciences in zebrafish. It has implied the importance of zebrafish in this particular field of research. In essence, the current available literature has covered from the very specific brain region or neurons of zebrafish, which are responsible for reproductive regulation, until the gonadal level of the animal. The discoveries and findings have proven that this small animal is sharing a very close/similar reproductive system with mammals. More interestingly, the behavioral characteristics and along with the establishment of animal courtship behavior categorization in zebrafish have laid an even stronger foundation and firmer reason on the suitability of zebrafish utilization in research of reproductive sciences. In view of the immense importance of this small animal for the development of reproductive sciences, this review aimed at compiling and describing the proximate close similarity of reproductive regulation on zebrafish and human along with factors contributing to the infertility, showing its versatility and its potential usage for fertility research.

Effects of tris(2-butoxyethyl) phosphate exposure on endocrine systems and reproduction of zebrafish (Danio rerio)

Tris(2-butoxyethyl) phosphate (TBEOP), a widely used organophosphate flame retardant, has frequently been detected both in the environment and the biota. However, limited information is available on the effects of TBEOP on the endocrine system and its underlying mechanisms. We exposed adult zebrafish pairs to TBEOP at concentrations of 0, 2.1, 11, and 118 μg/L for 21 d, and investigated the effects on gene transcription and hormone production related to the hypothalamic-pituitary-gonadal (HPG) axis, and on reproduction. The adverse effects on the F1 generation were further examined. In male fish, plasma concentrations of 17β-estradiol were significantly increased along with up-regulation of cyp19a. Exposure to TBEOP at 118 μg/L led to a significant decrease in average egg production. Exposure of the F0 generation to TBEOP delayed hatching and lowered hatching rates in the F1 generation. The results demonstrate that exposure to TBEOP at environmentally relevant concentration levels could affect the sex hormone balance by altering regulatory circuits of the HPG axis, eventually leading to disruption of reproductive performance and the development of offspring.

Early-life stress changes expression of GnRH and kisspeptin genes and DNA methylation of GnRH3 promoter in the adult zebrafish brain

Early-life stress can cause long-term effects in the adulthood such as alterations in behaviour, brain functions and reproduction. DNA methylation is a mechanism of epigenetic change caused by early-life stress. Dexamethasone (DEX) was administered to zebrafish larvae to study its effect on reproductive dysfunction. The level of GnRH2, GnRH3, Kiss1 and Kiss2 mRNAs were measured between different doses of DEX treatment groups in adult zebrafish. Kiss1 and GnRH2 expression were increased in the 200mg/L DEX treated while Kiss2 and GnRH3 mRNA levels were up-regulated in the 2mg/L DEX-treated zebrafish. The up-regulation may be related to programming effect of DEX in the zebrafish larvae, causing overcompensation mechanism to increase the mRNA levels. Furthermore, DEX treatment caused negative impact on the development and maturation of the testes, in particular spermatogenesis. Therefore, immature gonadal development may cause positive feedback by increasing GnRH and Kiss. This indicates that DEX can alter the regulation of GnRH2, GnRH3, Kiss1 and Kiss2 in adult zebrafish, which affects maturation of gonads. Computer analysis of 1.5 kb region upstream of the 5' UTR of Kiss1, Kiss2, GnRH2 and GnRH3 promoter showed that there are putative binding sites of glucocorticoid response element and transcription factors involved in stress response. GnRH3 promoter analysed from pre-optic area, ventral telencephalon and ventral olfactory bulb showed higher methylation at CpG residues located on -1410, -1377 and -1355 between control and 2mg/L DEX-treated groups. Hence, early-life DEX treatment can alter methylation of GnRH3 gene promoter, which subsequently affects gene regulation and reproductive functions.

Zebrafish Olfacto-Retinal Centrifugal Axon Projection and Distribution: Effects of Gonadotropin-Releasing Hormone and Dopaminergic Signaling

The terminalis neurons (TNs) have been described in teleost species. In zebrafish, the TNs are located in the olfactory bulb. The TNs synthesize and release gonadotropin-releasing hormone (GnRH) as one of the major neurotransmitters. The TNs project axons to many brain areas, which include the neural retina. In the retina, the TN axons synapse with dopaminergic interplexiform cells (DA-IPCs) and retinal ganglion cells (RGCs). In this research, we examine the role of GnRH and dopaminergic signaling in TN axon projection to the retina using the transgenic zebrafish Tg(GnRH-3::GFP). While the TNs developed at 34 h postfertilization (hpf), the first TN axons were not detected in the retina until 48-50 hpf, when the first DA-IPCs were differentiated. In developing embryos, inhibition of retinal GnRH signaling pathways severely interrupted the projection of TN axons to the retina. However, inhibition of retinal dopaminergic signaling produced little effect on TN axon projection. In adult retinas, inactivation of GnRH receptors disrupted the patterns of TN axon distribution, and depletion of DA-IPCs abolished the TN axons. When DA-IPCs regenerated, the TN axons reappeared. Together, the data suggest that in developing zebrafish retinas GnRH signaling is required for TN axon projection, whereas in adult retinas activation of GnRH and dopaminergic signaling transduction is required for normal distribution of the TN axons.

Expression changes of mRNAs encoding kisspeptins and their receptors and gonadotropin-releasing hormones during early development and gonadal sex differentiation periods in the brain of chub mackerel (Scomber japonicus)

In recent years, brain kisspeptin system has been shown to be involved in diverse reproductive function, including sexual differentiation in vertebrates. Our previous reports demonstrated that the chub mackerel (Scomber japonicus) brain expresses two kisspeptin (kiss1, kiss2), two kisspeptin receptor (kissr1, kissr2) and three gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) genes. In the present study, using quantitative real-time PCR (qRT-PCR) assays, we analysed expression changes of these genes during early development (0-30dphs) and gonadal sex differentiation periods (37-60dphs). Absolute expression level of kiss-kissr-gnrh in the whole head was higher between 0 and 15dphs, in comparison to later developmental periods. Histological analyses revealed presence of sexually differentiated males and females with testicular and ovarian features at 37, 45, and 60dphs. In both males and females, kiss2, kissr1, and kissr2 levels were higher at 37dph, in comparison to 45 and 60dphs, with kiss1 showing no significant differences. Levels of all three gnrh mRNAs were higher at 45dph, in comparison to 60dph. Changes in the expression level of kiss-kissr-gnrh mRNAs in different brain regions of sexually differentiated males and females indicated differences in their regional distribution. These results suggest possible involvement of Kiss-KissR-GnRH systems during early development and gonadal sex differentiation in the chub mackerel.

Zebrafish adult-derived hypothalamic neurospheres generate gonadotropin-releasing hormone (GnRH) neurons

Gonadotropin-releasing hormone (GnRH) is a hypothalamic decapeptide essential for fertility in vertebrates. Human male patients lacking GnRH and treated with hormone therapy can remain fertile after cessation of treatment suggesting that new GnRH neurons can be generated during adult life. We used zebrafish to investigate the neurogenic potential of the adult hypothalamus. Previously we have characterized the development of GnRH cells in the zebrafish linking genetic pathways to the differentiation of neuromodulatory and endocrine GnRH cells in specific regions of the brain. Here, we developed a new method to obtain neural progenitors from the adult hypothalamus in vitro. Using this system, we show that neurospheres derived from the adult hypothalamus can be maintained in culture and subsequently differentiate glia and neurons. Importantly, the adult derived progenitors differentiate into neurons containing GnRH and the number of cells is increased through exposure to either testosterone or GnRH, hormones used in therapeutic treatment in humans. Finally, we show in vivo that a neurogenic niche in the hypothalamus contains GnRH positive neurons. Thus, we demonstrated for the first time that neurospheres can be derived from the hypothalamus of the adult zebrafish and that these neural progenitors are capable of producing GnRH containing neurons.

Early embryonic androgen exposure induces transgenerational epigenetic and metabolic changes

Androgen excess is a central feature of polycystic ovary syndrome (PCOS), which affects 6% to 10% of young women. Mammals exposed to elevated androgens in utero develop PCOS-like phenotypes in adulthood, suggesting fetal origins of PCOS. We hypothesize that excess androgen exposure during early embryonic development may disturb the epigenome and disrupt metabolism in exposed and unexposed subsequent generations. Zebrafish were used to study the underlying mechanism of fetal origins. Embryos were exposed to androgens (testosterone and dihydrotestosterone) early at 26 to 56 hours post fertilization or late at 21 to 28 days post fertilization. Exposed zebrafish (F0) were grown to adults and crossed to generate unexposed offspring (F1). For both generations, global DNA methylation levels were examined in ovaries using a luminometric methylation assay, and fasting and postprandial blood glucose levels were measured. We found that early but not late androgen exposure induced changes in global methylation and glucose homeostasis in both generations. In general, F0 adult zebrafish exhibited altered global methylation levels in the ovary; F1 zebrafish had global hypomethylation. Fasting blood glucose levels were decreased in F0 but increased in F1; postprandial glucose levels were elevated in both F0 and F1. This androgenized zebrafish study suggests that transient excess androgen exposure during early development can result in transgenerational alterations in the ovarian epigenome and glucose homeostasis. Current data cannot establish a causal relationship between epigenetic changes and altered glucose homeostasis. Whether transgenerational epigenetic alteration induced by prenatal androgen exposure plays a role in the development of PCOS in humans deserves study.

Sex and the migraine brain

The brain responds differently to environmental and internal signals that relate to the stage of development of neural systems. While genetic and epigenetic factors contribute to a premorbid state, hormonal fluctuations in women may alter the set point of migraine. The cyclic surges of gonadal hormones may directly alter neuronal, glial and astrocyte function throughout the brain. Estrogen is mainly excitatory and progesterone inhibitory on brain neuronal systems. These changes contribute to the allostatic load of the migraine condition that most notably starts at puberty in girls.

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.

Evolutionary Insights into the Steroid Sensitive kiss1 and kiss2 Neurons in the Vertebrate Brain

Kisspeptin was originally found as a peptide product of Kiss1 gene and is now supposed to be an essential central regulator of reproduction in mammals. However, there is now a growing body of evidence to suggest that kiss2, the paralogous gene for kiss1, evolved in parallel during vertebrate lineage, and the kiss2 product also activates the GPR54 (kisspeptin receptor) signaling pathways. Therefore, it is now widely accepted that both kiss1 and kiss2 are the kisspeptin genes. Interestingly, either kiss1 or kiss2 or both have been lost during evolution in many vertebrate species, and the functional significance of kiss1 or kiss2 for the central regulation of reproduction is suggested to vary according to the species. Here, we argue that the steroid sensitivity of the kiss1 or kiss2 neurons has been well conserved during evolution among tetrapods and teleosts, and thus it may be the key to understanding the functional homologies of certain populations of kisspeptin (kiss1 or kiss2) neurons among different species of vertebrates. In the present review, we will first introduce recent advances in the study of steroid sensitive kiss1 and kiss2 systems in vertebrates and effects of peptide administrations in vivo. By comparing the similarities and differences between kiss1 and kiss2 of neuronal localization and sensitivity to gonadal steroids in various tetrapods and teleosts, we discuss the evolution of kisspeptin neuronal systems after gene duplication of ancestral kisspeptin genes to give rise to kiss1 and kiss2.

Evolution of the vertebrate pth2 (tip39) gene family and the regulation of PTH type 2 receptor (pth2r) and its endogenous ligand pth2 by hedgehog signaling in zebrafish development

In mammals, parathyroid hormone (PTH), secreted by parathyroid glands, increases calcium levels in the blood from reservoirs in bone. While mammals have two PTH receptor genes, PTH1R and PTH2R, zebrafish has three receptors, pth1r, pth2r, and pth3r. PTH can activate all three zebrafish Pthrs while PTH2 (alias tuberoinfundibular peptide 39, TIP39) preferentially activates zebrafish and mammalian PTH2Rs. We know little about the roles of the PTH2/PTH2R system in the development of any animal. To determine the roles of PTH2 and PTH2R during vertebrate development, we evaluated their expression patterns in developing zebrafish, observed their phylogenetic and conserved synteny relationships with humans, and described the genomic organization of pth2, pth2r, and pth2r splice variants. Expression studies showed that pth2 is expressed in cells adjacent to the ventral part of the posterior tuberculum in the diencephalon, whereas pth2r is robustly expressed throughout the central nervous system. Otic vesicles express both pth2 and pth2r, but heart expresses only pth2. Analysis of mutants showed that hedgehog (Hh) signaling regulates the expression of pth2 transcripts more than that of nearby gnrh2-expressing cells. Genomic analysis showed that a lizard, chicken, and zebra finch lack a PTH2 gene, which is associated with an inversion breakpoint. Likewise, chickens lack PTH2R, while humans lack PTH3R, a case of reciprocally missing ohnologs (paralogs derived from a genome duplication). The considerable evolutionary conservation in genomic structure, synteny relationships, and expression of zebrafish pth2 and pth2r provides a foundation for exploring the endocrine roles of this system in developing vertebrate embryos.

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.

A whole life cycle assessment on effects of waterborne PBDEs on gene expression profile along the brain-pituitary-gonad axis and in the liver of zebrafish

Polybrominated diphenyl ethers (PBDEs) are now found ubiquitously in the aquatic environment and biota, and there is a growing concern that PBDEs may disrupt endocrine systems, leading to reproductive impairments of aquatic animals. In our study, zebrafish (Danio rerio) were exposed to the 5 ng/L, 1 μg/L and 50 μg/L of DE-71 for the duration of the whole life cycle (120 days, from eggs to adults). The expression of selected genes along the brain-pituitary-gonadal (BPG) axis and liver, and the levels of plasma sex hormones were examined. In male fish, up-regulation of GnRH in brain, FSHβ and LHβ in pituitary, FSH-receptor, LH-receptor, and CYP19a in testis was clearly evident, while down-regulation of CYP11a and 3β-HSD was found in testis. In female fish, a 2.4-fold up-regulation of 3β-HSD was found in ovary upon exposure to 50 μg/L of DE-71. GnRH in brain, FSHβ and LHβ in pituitary were also up-regulated, while ERβ, TH and TPH in brain and GnRH-receptor in pituitary were significantly down-regulated. Hepatic ERα, AR and VTG in males were all down-regulated, while hepatic ERα and AR in female were up-regulated. Serum estradiol (E2) was reduced in both male and female upon exposure to DE-71, while significant increases in serum testosterone (T) and 11-keto-testosterone (11-KT) were only found in male but not female fish. The ratio of T/E2 as well as the ratio of 11-KT/E2 in male fish increased in a dose-dependent manner upon exposure to DE-71. Our overall results showed that whole life exposure of DE-71 altered the expression of regulatory genes and receptors at all three levels of the BPG axis in zebrafish, and the responses are sex dependent. The observed disruption of GnRH and GtHs can be further related to the subsequent disruption in both levels and balance sex steroid hormones.

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.

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.

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.

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.

Molecular biomarkers of endocrine disruption in small model fish

A wide range of environmental contaminants can interfere with hormonal regulation in vertebrates. These endocrine disrupting chemicals (EDCs) are of high relevance for human and wildlife health, since endocrine signalling controls many essential physiological processes which impact on the individual's health, such as growth and development, stress response, and ultimately reproduction and population development. Small fish represent a cost-effective model for testing potential EDCs allowing the possibility to integrate from molecular to phenotypic and functional effects. We have comprehensively reviewed exposure-effect data from four different small model fish: zebrafish, medaka, fathead minnow, and the three-spined stickleback. The majority of available data refer to EDCs interfering with reproductive hormones. However, we have also included interactions with other hormone systems, particularly the thyroid hormones. We demonstrate that the available data clearly indicates the predictive potential of molecular biomarkers, supporting the development and regulatory application of simple molecular-based screening assays using small model fish for EDC testing.

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

Normal migration of the gonadotrophin-releasing hormone (GnRH) neurones during early development, from the olfactory region to the hypothalamus, is crucial for reproductive development in all vertebrates. The establishment of the GnRH system includes tangential migration of GnRH perikarya as well as extension of GnRH fibres to various areas of the central nervous system (CNS). The exact spatio-temporal nature of this process, as well as the factors governing it, are not fully understood. We studied the development of the GnRH system and the effects of GnRH knockdown using a newly developed GnRH3:EGFP transgenic zebrafish line. We found that enhanced green fluorescent protein is specifically and robustly expressed in GnRH3 neurones and fibres. GnRH3 fibres in zebrafish began to extend as early as 26 h post-fertilisation and by 4-5 days post-fertilisation had developed into an extensive network reaching the optic tract, telencephalon, hypothalamus, midbrain tegmentum and hindbrain. GnRH3 fibres also innervated the retina and projected into the trunk via the spinal cord. GnRH3 perikarya were observed migrating along their own fibres from the olfactory region to the preoptic area (POA) via the terminal nerve ganglion and the ventral telencephalon. GnRH3 cells were also observed in the trigeminal ganglion. The establishment of the GnRH3 fibre network was disrupted by morpholino-modified antisense oligonucleotides directed against GnRH3 causing abnormal fibre development and pathfinding, as well as anomalous GnRH3 perikarya localisation. These findings support the hypothesis that GnRH3 neurones migrate from the olfactory region to the POA and caudal hypothalamus. Novel data regarding the early development of the GnRH3 fibre network in the CNS and beyond are described. Moreover we show, in vivo, that GnRH3 is an important factor regulating GnRH3 fibre pathfinding and neurone localisation in an autocrine fashion.

A role for GnRH in early brain regionalization and eye development in zebrafish

Gonadotropin-releasing hormone (GnRH) is a highly conserved peptide that is expressed early in brain development in vertebrates. In zebrafish, we detected GnRH mRNA within 2h post fertilization by RT-PCR. To determine if GnRH is involved in development, we used gene knockdown techniques to block translation of gnrh2 or gnrh3 mRNA after which the expression patterns for gene markers were examined at 24h post fertilization with in situ hybridization. First, loss of either GnRH2 or GnRH3 affected regionalization of the brain as shown by a change in expression of fgf8 or pax2.1 genes in the midbrain-hindbrain boundary or diencephalon-midbrain boundary. Second, lack of GnRH2 and/or GnRH3 altered gene markers expressed in the formation of the eye cup (pax2.1, pax6.1, mab21l2 and meis1.1) or eye stalk (fgf8 and pax2.1). Third, knockdown of GnRH2 affected the size and shape of the midbrain and expression of gene markers therein. Results from assays with the TUNEL method and caspase-3 and -9 activity showed the brain and eye changes were unlikely to result from secondary apoptotic cell death before 24h post fertilization. These experiments suggest that GnRH loss-of-function affects early brain and eye formation during development.

Development of GnRH cells: Setting the stage for puberty

Cells containing gonadotropin-releasing hormone (GnRH) are essential not only for reproduction but also for neuromodulatory functions in the adult animal. A variety of studies have hinted at multiple origins for GnRH-containing cells in the developing embryo. We have shown, using zebrafish as a model system, that GnRH cells originate from precursors lying outside the olfactory placode: the region of the anterior pituitary gives rise to hypothalamic GnRH cells and the cranial neural crest gives rise to the GnRH cells of the terminal nerve and midbrain. Cells of both the forming anterior pituitary and cranial neural crest are closely apposed to the precursors of the olfactory epithelium during early development. Disruption of kallmann gene function results in loss of the hypothalamic but not the terminal nerve GnRH cells during early development. The GnRH proteins are expressed early in development and this expression is mirrored by the onset of GnRH receptor (GnRH-R) expression during early development. Thus the signaling of the GnRH neuronal circuitry is set up early in development laying the foundation for the GnRH network that is activated at puberty leading to reproductive function in the mature animal.

A role forfoxd3andsox10in the differentiation of gonadotropin-releasing hormone (GnRH) cells in the zebrafishDanio rerio

Gonadotropin-releasing hormone (GnRH) is found in a wide range of vertebrate tissues, including the nervous system. In general, GnRH has two functions: endocrine, acting as a releasing hormone; and neuromodulatory,affecting neural activity in the peripheral and central nervous system. The best understood population of GnRH cells is that of the hypothalamus, which is essential for reproduction. Less well understood are the populations of GnRH cells found in the terminal nerve and midbrain, which appear to be neuromodulatory in function. The GnRH-containing cells of the midbrain are proposed to arise from the mesencephalic region of the neural tube. Previously, we showed that neuromodulatory GnRH cells of the terminal nerve arise from cranial neural crest. To test the hypothesis that neuromodulatory GnRH cells of the midbrain also arise from neural crest, we used gene knockdown experiments in zebrafish to disrupt neural crest development. We demonstrate that decrement of the function of foxd3 and/or sox10, two genes important for the development and specification of neural crest, resulted in a reduction and/or loss of GnRH cells of the midbrain, as well as a reduction in the number of terminal nerve GnRH cells. Therefore, our data support a neural crest origin for midbrain GnRH cells. Additionally, we demonstrate that knockdown of kallmann gene function resulted in the loss of endocrine GnRH cells of the hypothalamus, but not of neuromodulatory GnRH cells of the midbrain and terminal nerve, thus providing additional evidence for separate pathways controlling the development of neuromodulatory and endocrine GnRH cells.

Chromosomal Organization, Evolutionary Relationship, and Expression of Zebrafish GnRH Family Members

Multiple forms of gonadotropin-releasing hormone (GnRH) are found in different vertebrates. In this study, we have cloned cDNA encoding the full-length gnrh3 and gnrh2 from zebrafish brain and characterized their structure and expression patterns. We performed phylogenetic analysis and compared conserved syntenies in the region surrounding the GnRH genes from human, chicken, pufferfish, and zebrafish genores. The gnrh3 and gnrh2 genes were mapped to LG17 and LG21, respectively. The zebrafish genome appears to lack an ortholog to human GNRH1, and the human genome appears to lack an ortholog of gnrh3. Expression of gnrh3 began in the olfactory pit at 24-26 h postfertilization and expanded to the olfactory bulb during early larval stage. Expression of gnrh2 is always in the midbrain. In addition, GnRH is also expressed in boundary cells surrounding seminiferous cysts of the testis. Thus, this detailed phylogenetic, chromosomal comparison, and expression study defines the identity and the evolutionary relationship of two zebrafish gnrh genes. We propose a model describing the evolution of gnrh genes involving ancestral duplication of the genes followed by selective loss of one gene in some teleosts.

Developmental role of GnRH and PACAP in a zebrafish model

GnRH is expressed early in development long before reproduction begins. To determine whether GnRH has a role in development, gene knockdown with morpholinos was used in one-cell zebrafish embryos to block translation of gnrh mRNA into protein. Gene knockdown of gnrh2, gnrh3 or both at the one-cell stage resulted in a high percentage of embryos at 24-48 h with a defective mid-hindbrain boundary and underdeveloped eyes; a small percentage of embryos at 72 h had a defective heart. In similar studies on GHRH-PACAP, gene knockdown resulted in a smaller brain and eyes, but a normal-appearing heart. The evidence supports a role for the three neuropeptides in early development.

Origin and development of GnRH neurons

Gonadotropin-releasing hormone (GnRH) is an essential decapeptide, with both endocrine and neuromodulatory functions in vertebrates. GnRH-containing cells of the forebrain were thought to originate in the olfactory placode and migrate to their central nervous system destinations, and those of the midbrain to arise locally from the neural tube. Here, the embryonic origins of GnRH cells are re-examined in light of recent data suggesting that forebrain GnRH cells arise from the anterior pituitary placode and cranial neural crest, from where they migrate to their final destinations. The emerging picture suggests that GnRH cells do not originate from the olfactory placodes, but arise from multiple embryonic origins, and transiently associate with the developing olfactory system as they migrate to ventral forebrain locations.

Development of the nervus terminalis: origin and migration

The origin of the nervus terminalis is one of the least well understood developmental events involved in generating the cranial ganglia of the forebrain in vertebrate animals. This cranial nerve forms at the formidable interface of the anteriormost limits of migrating cranial neural crest cells, the terminal end of the neural tube and the differentiating olfactory and adenohypophyseal placodes. The complex cellular interactions that give rise to the various structures associated with the sensory placode (olfactory) and endocrine placode (adenohypophysis) surround and engulf this enigmatic cranial nerve. The tortured history of nervus terminalis development (see von Bartheld, this issue, pages 13-24) reflects the lack of consensus on the origin (or origins), as well as the experimental difficulties in uncovering the origin, of the nervus terminalis. Recent technical advances have allowed us to make headway in understanding the origin(s) of this nerve. The emergence of the externally fertilized zebrafish embryo as a model system for developmental biology and genetics has shed new light on this century-old problem. Coupled with new developmental models are techniques that allow us to trace lineage, visualize gene expression, and genetically ablate cells, adding to our experimental tools with which to follow up on studies provided by our scientific predecessors. Through these techniques, a picture is emerging in which the origin of at least a subset of the nervus terminalis cells lies in the cranial neural crest. In this review, the data surrounding this finding will be discussed in light of recent findings on neural crest and placode origins.

Molecular polymorphism of native gonadotropin-releasing hormone (GnRH) is restricted to mammalian GnRH and [hydroxyproline9] GnRH in the developing rat brain

Although chicken gonadotropin-releasing hormone (GnRH)-II is thought to occur in most animal species, its presence and that of two other variants (lamprey GnRH-III, salmon GnRH) is questionable in rodents. Here we report on the GnRH peptides present in the hypothalamus and the remaining brain of rat of both sexes during development. No immunoreactivity was detected in the elution zone of either native or hydroxylated forms of the above three variants in any of brain extracts chromatographed. The main peptides detected were mammalian GnRH (mGnRH) and m[hydroxyproline9]GnRH (mHypGnRH). In the hypothalamus, these peptides were associated with their free acid and precursor forms. N-terminal fragments from both native decapeptides (GnRH) and mGnRH (GnRH) were observed only in the hypothalamus. C-terminal fragments were detected in both tissues. The relative proportions of mGnRH and mHypGnRH showed no developmental changes in the remaining brain. The hypothalamic proportions of mHypGnRH were high on day 5, and decreased from day 15 onwards. The [Gly11]-precursor to mHypGnRH molar ratio was twofold lower than with the non-hydroxylated peptides. The mGnRH to GnRH molar ratio increased in males but decreased in females during development. No sex-related differences were observed in the native decapeptide to GnRH molar ratio. It was concluded that (1) chicken GnRH-II is not present in all mammals, (2) mGnRH and mHypGnRH are the main GnRH isoforms present in the rat brain, (3) the processing of [Gly11]-precursor into mHypGnRH occurs at a higher rate than that of mGnRH, and (4) the catabolism does not interfere with the developmental changes undergone by the mGnRH and mHypGnRH brain contents.