Sexual dimorphism of steroidogenesis regulated by GnIH in the goldfish, Carassius auratus

Gonadotropin-inhibitory hormone and its receptors in teleosts: Physiological roles and mechanisms of actions

Gonadotropin-inhibitory hormone (GnIH) was the first reported hypothalamic neuropeptide inhibiting reproduction in vertebrates. Since its discovery in the quail brain, its orthologs have been identified in a variety of vertebrate species and even protochordates. Depending on the species, the GnIH precursor polypeptides comprise two, three or four mature peptides of the RFamide family. It has been well documented that GnIH inhibits reproduction at the brain-pituitary-gonadal levels and participates in metabolism, stress response, and social behaviors in birds and mammals. However, most studies in fish have mainly been focused on the physiological roles of GnIH in the control of reproduction and results obtained are in some cases conflicting, leaving aside its potential roles in the regulation of other functions. In this manuscript we summarize the information available in fish with respect to the structural diversity of GnIH peptides and functional roles of GnIH in reproduction and other physiological processes. We also highlight the molecular mechanisms of GnIH actions on target cells and possible interactions with other neuroendocrine factors.

Molecular characterization and ontogenetic expression profiles of LPXRFa and its receptor in Japanese flounder (Paralichthys olivaceus)

Investigations concerning the LPXRFa system are rarely conducted in flatfish species. Here, we first identified and characterized lpxrfa and its cognate receptor lpxrfa-r genes in the Japanese flounder (Paralichthys olivaceus). The coding DNA sequence of lpxrfa was 579 bp in length, wich encoded a 192-aa preprohormone that can produce three mature LPXRFa peptides. The open reading frame (ORF) of lpxrfa-r was 1446 bp in size, and encoded a 481-aa LPXRFa-R protein that encompassed seven hydrophobic transmembrane domains. Subsequently, tissue distribution expression profiles of lpxrfa and lpxrfa-r transcripts were assayed by quantitative real-time PCR. The results indicated that expressions of lpxrfa transcripts were detected at the highest levels in the brain of both females and males, however, lpxrfa-r transcripts were remarkablely expressed in the brain tissue of female fish and in the testis tissue of male fish. Furthermore, transcript levels of lpxrfa and lpxrfa-r genes were investigated during early ontogenetic development, with the maximum expression levels at 30 days post-hatching. Overall, these data contribute to providing preliminary proof for the existence and structure of the LPXRFa system in Japanese flounder, and the study is just the foundation for researching physiological function of LPXRFa system in this species.

LPXRFa and its receptor in yellowtail kingfish (Seriola lalandi): Molecular cloning, ontogenetic expression profiles, and stimulatory effects on growth hormone and gonadotropin gene expression

Despite its functional significance in mammals and birds, the biological role of gonadotropin-inhibitory hormone (GnIH) in reproduction is still far from being fully understood in teleosts. In the current study, we have identified LPXRFa, the piscine ortholog of GnIH, and its cognate receptor (LPXRFa-R) in yellowtail kingfish (YTK), which is considered as a promising species for aquaculture industry worldwide. The YTK cDNA sequence of lpxrfa was 534 base pair (bp) in length and encoded a 178-amino acids (aa) preprohormone. The LPXRFa precursor comprised three putative peptide sequences that included -MPMRF, -MPQRF, or -LPERL motifs at the C-termini, respectively. The YTK lpxrfa-r cDNA sequence was composed of 1265 bp that gave rise to a LPXRFa-R of 420 aa, encompassing the characteristic seven hydrophobic transmembrane domains. In males, both lpxrfa and lpxrfa-r transcripts could be detected at high levels in the brain and testis. In females, a noteworthy expression of lpxrfa was observed in the brain and ovary, while the expression of lpxrfa-r was especially evident only in the brain. To study the ontogeny of LPXRFa system, transcript levels were also investigated during early life stages. Variable expression of the LPXRFa system was observed during all stages of YTK embryogenesis. The highest expression of lpxrfa and lpxrfa-r were noticed at 7 dph and 15 dph, respectively. Furthermore, LPXRFa peptides stimulated growth hormone (gh), luteinizing hormone (lhβ) and follicle-stimulating hormone (fshβ) gene expression from the pituitary. Taken together, our results provide initial evidence for the existence of the LPXRFa system in yellowtail kingfish and suggest its possible involvement at early development and reproductive functions.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

Bile acid production is life-stage and sex-dependent and affected by primer pheromones in the sea lamprey

Pheromonal bile salts are important for sea lampreys (Petromyzon marinus Linnaeus) to complete their life cycle. The synthesis and release of a releaser/primer pheromone 3-keto petromyzonol sulfate (3kPZS) by spermiating males have been well characterized. 3kPZS evokes sexual behaviors in ovulatory females, induces immediate 3kPZS release in spermiating males, and elicits neuroendocrine responses in prespawning adults. Another primer pheromone released by spermiating males, 3-keto allocholic acid (3kACA), antagonizes the neuroendocrine effects of 3kPZS in prespermiating males. However, the effects of 3kACA and 3kPZS on pheromone production in prespawning adults is unclear. To understand the foundation of pheromone production, we examined sea lamprey bile salt levels at different life stages. To investigate the priming effects of 3kACA and 3kPZS, we exposed prespawning adults with vehicle or synthetic 3kACA or 3kPZS. We hypothesized that endogenous bile salt levels were life-stage and sex-dependent, and differentially affected by 3kACA and 3kPZS in prespawning adults. Using ultra-performance liquid chromatography tandem mass spectrometry, we found that sea lampreys contained distinct mixtures of bile salts in the liver and plasma at different life stages. Males usually contained higher amounts of bile salts than females. Petromyzonamine disulfate was the most abundant C27 bile salt and petromyzonol sulfate was the most abundant C24 bile salt. Waterborne 3kACA and 3kPZS exerted differential effects on bile salt production in the liver and gill, their circulation and clearance in the plasma, and their release into water. We conclude that bile salt levels are life-stage and sex-dependent and differentially affected by primer pheromones.

The reproductive regulation of LPXRFa and its receptor in the hypothalamo-pituitary-gonadal axis of the spotted scat (Scatophagus argus)

Gonadotropin-inhibitory hormone (GnIH) plays a critical role in regulating gonadotropin-releasing hormone (GnRH), gonadotropin hormone (GtH), and steroidogenesis. The Lpxrfa (the piscine ortholog of GnIH) system has been found to regulate fish reproduction. To gain insight into the role of Lpxrfa in the regulation of spotted scat (Scatophagus argus) reproduction, spotted scat Lpxrfa (ssLpxrfa), and its receptor (ssLpxrfa-r) were cloned and analyzed. Tissue distribution and expression patterns at the hypothalamo-pituitary-gonadal axis (HPG axis) of sslpxrfa and sslpxrfa-r mRNA were also investigated during gonadal development of spotted scat. The open reading frame (ORF) of the sslpxrfa was 606 bp encoding 201 amino acids and includes a putative signal peptide and two mature ssLpxrfa peptides with LPXRFamide motif at their C-terminus. The sslpxrfa-r ORF was 1449 bp encoding 482 amino acids and contracted a seven-hydrophobic transmembrane (TM) domain structure. The tissue distribution showe d that the sslpxrfa was highly expressed in hypothalami, gill, and the gonads. In addition, sslpxrfa-r was highly expressed in hypothalami, pituitaries, and the gonads. Quantitative real-time polymerase chain reaction (qPCR) revealed that sslpxrfa had the highest expression in the hypothalami and pituitaries, and the lowest expression in the gonads in stage V. During gonadal development, the expression of sslpxrfa-r was gradually increased in the hypothalami but reduced in the gonads. However, no obvious trend was observed in the pituitaries. The expression of sslpxrfa and sslpxrfa-r decreased significantly after injection with 17β-estradiol (E₂). However, the expression of both sslpxrfa and sslpxrfa-r was not changed after injection with 17α-methyltestosterone(17α-MT) in the hypothalami. In addition, no changes were observed in the expression of fshβ and lhβ in the pituitaries after injecting ssLpxrfa-1. However, ssLpxrfa-2 could downregulate the expression of sbgnrh and fshβ in the hypothalami and pituitaries, respectively. Taken together, these findings suggested that ssLpxrfa may participate in E₂ feedback in reproduction and regulate the reproductive axis of spotted scat.

Effects of gonadotropin-inhibitory hormone on early and late stages of spermatogenesis in ex-vivo culture of zebrafish testis

Gonadotropin-inhibitory hormone (Gnih) is known to play a role in the regulation of reproduction in vertebrates by influencing gonadotropin release and synthesis. While the endocrine actions of Gnih have been identified in several species, its paracrine/autocrine effects in the control of spermatogenesis are less defined. We have used ex vivo culture of zebrafish testis to investigate the role of gonadal zebrafish Gnih (zGnih) in the regulation of the spermatogenic process. We used FACScan cell cycle analysis, morphometric quantifications, BrdU incorporation and caspase-3 activity assays as well as measuring 11-Ketotestosterone (11-KT) level in the culture media. FACScan analysis and morphometric quantification results demonstrated direct action of zGnih on basal and gonadotropin (Lh and Fsh)-induced spermatogenesis. Treatment with zGnih (10 nM) significantly decreased the number of G0/G1 cells after 7-days of culture while no significant changes were found in the proportion area of spermatogonia cell types. Investigation of DNA synthesis using BrdU (5-Bromo-2'-Deoxyuridine) labeling showed that treatment with zGnih (10 nM) significantly decreased proliferative activity of type A spermatogonia, while increased the mitotic activity of type B spermatogonia. We also showed that treatment with zGnih (100 nM) completely eliminated 11-KT release induced by 100 ng/ml Fsh. Treatment with zGnih (10 and 100 nM) also inhibited both hCG and Fsh-induced spermatogenesis. These results, plus our previous findings, demonstrate that zGnih produced locally in the testis is a component of a complex multifactorial system that regulates testicular function in zebrafish.

Gonadotropin Inhibitory Hormone and Its Receptor: Potential Key to the Integration and Coordination of Metabolic Status and Reproduction

Since its discovery as a novel gonadotropin inhibitory peptide in 2000, the central and peripheral roles played by gonadotropin-inhibiting hormone (GnIH) have been significantly expanded. This is highlighted by the wide distribution of its receptor (GnIH-R) within the brain and throughout multiple peripheral organs and tissues. Furthermore, as GnIH is part of the wider RF-amide peptides family, many orthologues have been characterized across vertebrate species, and due to the promiscuity between ligands and receptors within this family, confusion over the nomenclature and function has arisen. In this review, we intend to first clarify the nomenclature, prevalence, and distribution of the GnIH-Rs, and by reviewing specific localization and ligand availability, we propose an integrative role for GnIH in the coordination of reproductive and metabolic processes. Specifically, we propose that GnIH participates in the central regulation of feed intake while modulating the impact of thyroid hormones and the stress axis to allow active reproduction to proceed depending on the availability of resources. Furthermore, beyond the central nervous system, we also propose a peripheral role for GnIH in the control of glucose and lipid metabolism at the level of the liver, pancreas, and adipose tissue. Taken together, evidence from the literature strongly suggests that, in fact, the inhibitory effect of GnIH on the reproductive axis is based on the integration of environmental cues and internal metabolic status.

Arg-Vasotocin Directly Activates Isotocin Receptors and Induces COX2 Expression in Ovoviviparous Guppies

Oxytocin (OT) is a crucial regulator of reproductive behaviors, including parturition in mammals. Arg-vasopressin (AVP) is a nonapeptide homologous to Arg-vasotocin (AVT) in teleosts that has comparable affinity for the OT receptor. In the present study, ovoviviparous guppies (Poecilia reticulata) were used to study the effect of AVT on delivery mediated by the activation of prostaglandin (PG) biosynthesis via isotocin (IT) receptors (ITRs). One copy each of it and avt and two copies of itrs were identified in guppies. The results of the affinity assay showed that various concentrations of AVT and IT (10^-6, 10^-7, and 10^-8 mol/L) significantly activated itr1 (P < 0.05). In vitro experiments revealed significant upregulation (P < 0.05) of cyclooxygenase 2 (cox2), which is the rate-limiting enzyme involved in PG biosynthesis, and itr1 by AVT and IT. Furthermore, dual in situ hybridization detected positive signals for itr1 and cox2 at the same site, implying that ITR1 may regulate cox2 gene expression. Measurement of prostaglandin F_2a (PGF_2a) concentrations showed that AVT induced PGF_2a synthesis (P < 0.05) and that the effect of IT was not significant. Finally, intraperitoneal administration of PGF_2a significantly induced premature parturition of guppies. This study is the first to identify and characterize AVT and ITRs in guppies. The findings suggest that AVT promotes PG biosynthesis via ITR and that PGF_2a induces delivery behavior in ovoviviparous guppies.

Nesfatin-1-like peptide suppresses hypothalamo-pituitary-gonadal mRNAs, gonadal steroidogenesis, and oocyte maturation in fish†

Nucleobindin (Nucb)-1 and Nucb2 are DNA and Ca2+ binding proteins with multiple functions in vertebrates. Prohormone convertase-mediated processing of Nucb2 results in the production of biologically active nesfatin-1. Nesfatin-1 is involved in the regulation of reproduction in many vertebrates, including fish. Our lab originally reported a nesfatin-1-like peptide (Nlp) encoded in Nucb1 that exhibits nesfatin-1-like metabolic effects. We hypothesized that Nlp has a suppressive role in the reproductive physiology of fish. In this research, whether Nlp regulates reproductive hormones and oocyte maturation in fish were determined. Single intraperitoneal (IP) injection of goldfish Nlp (50 ng/g body weight) suppressed salmon and chicken gonadotropin-releasing hormone (sgnrh and cgnrh2), gonadotropin-inhibiting hormone (gnih) and its receptor (gnihr), and kisspeptin and brain aromatase mRNA expression in the hypothalamus of both male and female goldfish. In the pituitary, Nlp decreased mRNAs encoding lhb, fshb and kisspeptin and its receptor, while a significant increase in gnih and gnihr was observed. In the gonads, lh (only in male fish) and fsh receptor mRNAs were also significantly downregulated in Nlp-injected fish. Sex-specific modulation of gnih, gnihr, and kisspeptin system in the gonads was also observed. Nlp decreased sex steroidogenic enzyme encoding mRNAs and circulating levels of testosterone and estradiol. In addition, incubation of zebrafish ovarian follicles with Nlp resulted in a reduction in oocyte maturation. These results provide evidence for a robust role for Nlp in regulating reproductive hormones in goldfish and oocyte maturation in zebrafish, and these effects resemble that of nesfatin-1.

Hypothalamic- and pituitary-derived growth and reproductive hormones and the control of energy balance in fish

Most endocrine systems in the body are influenced by the hypothalamic-pituitary axis. Within this axis, the hypothalamus delivers precise signals to the pituitary gland, which in turn releases hormones that directly affect target tissues including the liver, thyroid gland, adrenal glands and gonads. This action modulates the release of additional hormones from the sites of action, regulating key physiological processes, including growth, metabolism, stress and reproduction. Pituitary hormones are released by five distinct hormone-producing cell types: somatotropes (which produce growth hormone), thyrotropes (thyrotropin), corticotropes (adrenocorticotropin), lactotropes (prolactin) and gonadotropes (follicle stimulating hormone and luteinizing hormone), each modulated by specific hypothalamic signals. This careful and distinct organization of the hypothalamo-pituitary axis has been classically associated with the existence of many lineal axes (e.g., the hypothalamic-pituitary-gonadal axis) in charge of the control of the different physiological processes. While this traditional concept is valid, it is becoming apparent that hormones produced by the hypothalamo-pituitary axis have diverse effects. For instance, gonadotropin-releasing hormone II has been associated with a suppressive effect on food intake in fish. Likewise, growth hormone has been shown to influence appetite, swimming activity and aggressive behavior in fish. This review will focus on the hypothalamic and pituitary hormones classically involved in regulating growth and reproduction, and will attempt to provide a general overview of the current knowledge on their actions on energy balance and appetite in fish. It will also give a brief perspective of the role of some of these peptides in integrating feeding, metabolism, growth and reproduction.

Seasonal Related Multifactorial Control of Pituitary Gonadotropin and Growth Hormone in Female Goldfish: Influences of Neuropeptides and Thyroid Hormone

Female reproduction is under multifactorial control of brain-pituitary-peripheral origin. The present study provides information on seasonal changes in circulating LH and GH concentrations, as well as transcript levels for a number of genes involved in the regulation of reproduction and growth in female goldfish. We also provide information on the effects of treatments with GnRH and/or GnIH, and their interaction with T3, at three stages of gonadal recrudescence. Maximum basal concentration of LH was observed at late recrudescence (Spring) while no seasonal changes in basal serum GH levels was detected. Serum LH and GH levels were stimulated by GnRH as expected, depending on the season. GnIH stimulated basal GH concentrations in gonadally regressed fish. GnIH inhibitory action on GnRH-induced LH response was observed in late, but not in mid recrudescence. T3 actions on basal and GnRH- or GnIH-induced GH secretion were generally inhibitory, depending on season. Administration of T3 attenuated GnRH-induced LH responses in mid and late stages of gonadal recrudescence, and the presence of GnIH abolished inhibitory actions of T3 in fish at mid recrudescence. Our results also demonstrated seasonal patterns in basal and GnRH- and/or GnIH-induced transcript levels for ERα, ERβI, FSHR, aromatase, TRαI, TRβ, IGF-I, and Vtg in the liver and ovary. However, there were no clear correlations between changes in transcript levels and circulating levels of LH and GH. The results support the hypothesis that GnRH, GnIH, and T3 are contributing factors in complex reciprocal control of reproduction and growth in goldfish.

RFRP3 influences basal lamina degradation, cellular death, and progesterone secretion in cultured preantral ovarian follicles from the domestic cat

The hypothalamic neuropeptide RFRP3 can suppress hypothalamic GnRH neuron activation and inhibit gonadotropin release from the anterior pituitary. RFRP3 is also produced locally in the ovary and can inhibit steroidogenesis and follicle development in many vertebrates. However, almost nothing is known about the presence and regulatory action of RFRP3 in gonads of any carnivore species. Such knowledge is important for developing captive breeding programs for endangered carnivores and for inhibiting reproduction in feral species. Using the domestic cat as a model, our objectives were to (1) demonstrate the expression of feline RFRP3 (fRFRP3) and its receptor in the cat ovary and (2) assess the influence of fRFRP3 on ovarian follicle integrity, survival, and steroidogenesis in vitro. We first confirmed that fRFRP3 and its receptors (NPFFR1 and NPFFR2) were expressed in cat ovaries by sequencing PCR products from ovarian RNA. We then isolated and cultured preantral ovarian follicles in the presence of 10 or 1 µM fRFRP3 + FSH (1 µg/mL). We recorded the percentage of morphologically viable follicles (basal lamina integrity) over 8 days and calculated percentage survival of follicles on Day 8 (using fluorescent markers for cell survival and death). Last, we quantified progesterone accumulation in media. 10 µM fRFRP3 had no observable effect on viability, survival, or steroid production compared to follicles exposed to only FSH. However, 1 µM fRFRP3 decreased the percentage of morphologically viable follicles and the percentage of surviving follicles on Day 8. At the same time, 1 µM fRFRP3 increased the accumulation of progesterone in media. Our study shows, for the first time, direct action of RFRP3 on the follicle as a functional unit, and it is the first in a carnivore species. More broadly, our results support a conserved, inhibitory action of RFRP3 on ovarian follicle development and underscore the importance of comparative functional studies.

Recent studies of LPXRFa receptor signaling in fish and other vertebrates

The hypothalamo-pituitary-gonadal (HPG) axis plays a major role in coordinating the reproduction of fish and other vertebrates. Gonadotropin-releasing hormone (GnRH) is the primary stimulatory factor responsible for the hypothalamic control of gonadotropin secretion. In 2000, a previously unidentified hypothalamic neuropeptide was isolated from the brain of Japanese quail and termed gonadotropin-inhibitory hormone (GnIH) based on its ability to directly inhibit gonadotropin release from the cultured quail anterior pituitary gland. One year later, the cDNA sequence that encodes the quail GnIH precursor polypeptide was cloned and was found to encompass two further peptides (GnIH-related peptide (RP)-1 and GnIH-RP-2) besides GnIH. To date, GnIH orthologous have been detected in a variety of vertebrates from fish to humans. These peptides possess a characteristic-LPXRFa (X = L or Q) motif at the C-terminus and are designated as LPXRFa peptides. It is generally accepted that LPXRFa peptides act on GnRH neurons in the hypothalamus to inhibit gonadotropin synthesis and release in addition to affecting the pituitary function in birds and mammals. However, the exact physiological role of LPXRFa is still uncertain in fish and dual actions of LPXRFa on the HPG axis have been observed. Research aiming to elucidate the detailed signaling pathways mediating the actions of LPXRFa on target cells may contribute to understanding the functional divergence of the LPXRFa system in teleosts. Accordingly, this review will discuss the recent advances in LPXRFa receptor signaling, as well as the potential interactions on cell signaling induced by other factors, such as GnRH and kisspeptin.

Natural sex change in fish

Sexual fate can no longer be considered an irreversible deterministic process that once established during early embryonic development, plays out unchanged across an organism's life. Rather, it appears to be a dynamic process, with sexual phenotype determined through an ongoing battle for supremacy between antagonistic male and female developmental pathways. That sexual fate is not final and is actively regulated via the suppression or activation of opposing genetic networks creates the potential for flexibility in sexual phenotype in adulthood. Such flexibility is seen in many fish, where sex change is a usual and adaptive part of the life cycle. Many fish are sequential hermaphrodites, beginning life as one sex and changing sometime later to the other. Sequential hermaphrodites include species capable of female-to-male (protogynous), male-to-female (protandrous), or bidirectional (serial) sex change. These natural forms of sex change involve coordinated transformations across multiple biological systems, including behavioral, anatomical, neuroendocrine and molecular axes. Here we review the biological processes underlying this amazing transformation, focusing particularly on the molecular aspects, where new genomic technologies are beginning to help us understand how sex change is initiated and regulated at the molecular level.

Melanocortin-4 Receptor in Spotted Sea Bass, Lateolabrax maculatus: Cloning, Tissue Distribution, Physiology, and Pharmacology

Melanocortin-4 receptor (MC4R) plays important roles in regulation of multiple physiological processes including energy homeostasis, reproduction, sexual function, and other functions in mammals. Recent studies suggested that teleost MC4Rs have different physiological functions and pharmacological characteristics when compared to mammalian MC4Rs. In this study, we investigated spotted sea bass (Lateolabrax maculatus) MC4R (LmMC4R) physiology and pharmacology. Spotted sea bass mc4r consisted of a 984 bp open reading frame encoding a protein of 327 amino acids. LmMC4R was homologous to those of several teleost MC4Rs and human MC4R (hMC4R). qRT-PCR and in situ hybridization revealed that mc4r transcripts were highly expressed in the brain, followed by pituitary and liver. Brain mc4r transcripts were down-regulated in long-term and short-term fasting challenges. LmMC4R was a functional receptor with lower maximal binding and higher basal activity than hMC4R. THIQ was not able to displace ¹²⁵I-NDP-MSH but could affect intracellular cAMP accumulation, suggesting that it was an allosteric ligand for LmMC4R. In vitro studies with spotted sea bass brain cells indicated that mRNA levels of neuropeptide Y and Agouti-related peptide were down-regulated by α-MSH. In summary, we cloned spotted sea bass MC4R, and showed that it had different pharmacological properties compared to hMC4R, and potentially different functions.

The Gonadotropin-Inhibitory Hormone: What We Know and What We Still Have to Learn From Fish

Gonadotropin-inhibitory hormone, GnIH, is named because of its function in birds and mammals; however, in other vertebrates this function is not yet clearly established. More than half of the vertebrate species are teleosts. This group is characterized by the 3R whole genome duplication, a fact that could have been responsible for the great phenotypic complexity and great variability in reproductive strategies and sexual behavior. In this context, we revise GnIH cell bodies and fibers distribution in adult brains of teleosts, discuss its relationship with GnRH variants and summarize the few reports available about the ontogeny of the GnIH system. Considering all the information presented in this review, we propose that in teleosts, GnIH could have other functions beyond reproduction or act as an integrative signal in the reproductive process. However, further studies are required in order to clarify the role of GnIH in this group including its involvement in development, a key stage that strongly impacts on adult life.

Molecular identification of the Dyn/Kor system and its potential role in the reproductive axis of goldfish

To ascertain the significance of the dynorphin/kappa-opioid receptor (Dyn/Kor) system in fish reproduction, prodynorphin (pdyn) cDNA was cloned from goldfish. Two Dyn peptides (DynA and DynB) are present in the goldfish prodynorphin precursor. Both DynA and DynB are biologically active as they are able to functionally interact with the goldfish Kor expressed in cultured eukaryotic cells to suppress forskolin-induced CRE promoter activity. RT-PCR analysis showed that pdyn is widely expressed in brain regions, with the highest expression in hypothalamus. During ovarian development, hypothalamic pdyn and kor mRNA levels are lower in the early vitellogenic stage. Then the biological effects of Dyn peptides on salmon gonadotropin releasing hormone (sgnrh), luteinizing hormone beta (lhb) and follicle stimulating hormone beta (fshb) mRNA synthesis were further investigated in goldfish. Intraperitoneal injections of DynA and DynB significantly reduced hypothalamic sgnrh and pituitary lhb and fshb mRNA levels in male goldfish, but these two peptides only down-regulated sgnrh and lhb mRNA expression in female goldfish. In vitro studies revealed that DynA also decreased lhb mRNA levels in primary cultures of pituitary cells, indicating that this peptide can exert its actions at the pituitary level. Our findings suggest that the Dyn/Kor system plays a negative role in regulating the reproductive axis in goldfish.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Characterization of Gnrh/Gnih elements in the olfacto-retinal system and ovary during zebrafish ovarian maturation

Gonadotropin releasing hormone (GnRH) is one of the key players of brain-pituitary-gonad axis, exerting overall control over vertebrate reproduction. In zebrafish, two variants were characterized and named as Gnrh2 and Gnrh3. In this species, Gnrh3, the hypohysiotropic form, is expressed by neurons of the olfactory-retinal system, where it is related with food detection, intra/interspecific recognition, visual acuity and retinal processing modulation. Previous studies have reported the presence of Gnrh receptors in the zebrafish retina, but not yet in the zebrafish olfactory epithelium. The current study analyzed the presence of gnrh2 and gnrh3, their receptors (gnrhr 1,2,3 and 4) and gnih (gonadotropin inhibitory hormone) transcripts, as well as the Gnrh3 protein in the olfactory epithelium (OE), olfactory bulb (OB), retina and ovary during zebrafish ovarian maturation. We found an increase of gnrh receptors transcripts in the OE at the final stages of ovarian maturation. In the OE, Gnrh3 protein was detected in the olfactory receptor neurons cilia and in the olfactory nerve fibers. Interestingly, in the OB, we found an inverse expression pattern between gnih and gnrh3. In the retina, gnrhr4 mRNA was found in the nuclei of amacrine, bipolar, and ganglion cells next to Gnrh3 positive fibers. In the ovary, gnrh3, gnrhr2 and gnrhr4 transcripts were found in perinucleolar oocytes, while gnih in oocytes at the cortical alveolus stage. Our results suggested that Gnrh/Gnih elements are involved in the neuromodulation of the sensorial system particularly at the final stages of maturation, playing also a paracrine role in the ovary.

Gonadotropin-Inhibitory Hormone, the Piscine Ortholog of LPXRFa, Participates in 17β-Estradiol Feedback in Female Goldfish Reproduction

Gonadotropin-inhibitory hormone (GnIH) plays a critical role in regulating gonadotropin-releasing hormone, gonadotropin hormone, and steroidogenesis in teleosts. In the present study, we sought to determine whether 17β-estradiol (E2) acts directly on GnIH neurons to regulate reproduction in goldfish, a seasonal breeder, and we investigated the role of estrogen receptors (ERs) in mediating this process. We found that GnIH neurons coexpress three types of ERs. Ovariectomy and letrozole implantation into female goldfish at the vitellogenic stage elicited a substantial decrease in the expression of GnIH messenger RNA (mRNA), and E2 supplementation abolished this effect. In primary cultured hypothalamus cells, E2 increased GnIH mRNA levels; surprisingly, selective ERα and ERβ agonists showed opposite effects in regulating GnIH mRNA levels. Using genome walking, we isolated a 2329-bp section of the GnIH promoter sequence, and 7 half-estrogen response elements (EREs) were found in the promoter region. Luciferase assays and electrophoretic mobility shift assay results show that the half-ERE element at -2203 is the key site for competitive binding between ERα and ERβ. Ovariectomy and letrozole implantation into female goldfish in the maturating stage did not change the GnIH mRNA expression levels. Taken together, these findings suggest that E2 binds to multiple types of ERs, which competitively bind to the same half-ERE binding site of the GnIH promoter to achieve both positive and negative feedback in response to estrogen to regulate goldfish reproduction at different stages of ovarian development.

Molecular identification of StAR and 3βHSD1 and characterization in response to GnIH stimulation in protogynous hermaphroditic grouper (Epinephelus coioides)

Gonadal steroids are critical factors in reproduction and sex reverse process. StAR (steroidogenic acute regulatory protein), transferring the cholesterol from the outer mitochondrial membrane to the inner membrane, is the rate-limiting factor of steroidogenesis. 3βHSD (3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase), converting Δ5-steroids into Δ4-steroids, is an important oxidoreductase in steroidogenesis. In the present study, StAR and 3βHSD1 were cloned and characterized from protogynous orange-spotted grouper. StAR cDNA contains an 861bp open reading frame (ORF), encoding a predicted protein of 286 amino acids, and the ORF of 3βHSD1 was 1125bp, encoding a predicted protein of 374 amino acids. The transcript of StAR was mainly expressed in gonad, while 3βHSD1 mRNA was predominantly detected in brain and gonad. In the previous study, we found the expression of GnIH mRNA level in male, as well as in 17 alpha-methyltestosterone (MT)-induced male fish was significantly higher than in female fish, this indicating that GnIH/GnIHR signaling might be involved in the regulation of sex reversal and male maintenance. In order to figure out the function of GnIH in steroidogenesis, the expression of StAR and 3βHSD1 regulated by GnIH was examined. In vitro study showed that treatment of cultured ovary fragments with gGnIH peptides significantly stimulated the expression of StAR and 3βHSD1. In addition, the mRNA levels of StAR and 3βHSD1 were significantly increased after intraperitoneal injection (i.p.) with gGnIH peptides. Moreover, during MT-induced sex change from female to male, the levels of StAR mRNA significantly increased by 5.2, 24.8 and 353.5 folds, and that of 3βHSD1 mRNA by 3.5, 32.5 and 55.4 folds at the 2nd, 4th and 6th week after MT implantation, respectively. Collectively, our results indicate that GnIH may be involved in the regulation of sex reversal or male maintenance by stimulating the expression of StAR and 3βHSD1 in protogynous grouper.

Dual Actions of Mammalian and Piscine Gonadotropin-Inhibitory Hormones, RFamide-Related Peptides and LPXRFamide Peptides, in the Hypothalamic-Pituitary-Gonadal Axis

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that decreases gonadotropin synthesis and release by directly acting on the gonadotrope or by decreasing the activity of gonadotropin-releasing hormone (GnRH) neurons. GnIH is also called RFamide-related peptide in mammals or LPXRFamide peptide in fishes due to its characteristic C-terminal structure. The primary receptor for GnIH is GPR147 that inhibits cAMP production in target cells. Although most of the studies in mammals, birds, and fish have shown the inhibitory action of GnIH in the hypothalamic-pituitary-gonadal (HPG) axis, several in vivo studies in mammals and many in vivo and in vitro studies in fish have shown its stimulatory action. In mouse, although the firing rate of the majority of GnRH neurons is decreased, a small population of GnRH neurons is stimulated by GnIH. In hamsters, GnIH inhibits luteinizing hormone (LH) release in the breeding season when their endogenous LH level is high but stimulates LH release in non-breeding season when their LH level is basal. Besides different effects of GnIH on the HPG axis depending on the reproductive stages in fish, higher concentration or longer duration of GnIH administration can stimulate their HPG axis. These results suggest that GnIH action in the HPG axis is modulated by sex-steroid concentration, the action of neuroestrogen synthesized by the activity of aromatase stimulated by GnIH, estrogen membrane receptor, heteromerization and internalization of GnIH, GnRH, and estrogen membrane receptors. The inhibitory and stimulatory action of GnIH in the HPG axis may have a physiological role to maintain reproductive homeostasis according to developmental and reproductive stages.

A Journey through the Gonadotropin-Inhibitory Hormone System of Fish

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that belongs to the RFamide peptide family and was first identified in the quail brain. From the discovery of avian GnIH, orthologous GnIH peptides have been reported in a variety of vertebrates, including mammals, amphibians, teleosts and agnathans, but also in protochordates. It has been clearly established that GnIH suppresses reproduction in avian and mammalian species through its inhibitory actions on brain GnRH and pituitary gonadotropins. In addition, GnIH also appears to be involved in the regulation of feeding, growth, stress response, heart function and social behavior. These actions are mediated via G protein-coupled GnIH receptors (GnIH-Rs), of which two different subtypes, GPR147 and GPR74, have been described to date. With around 30,000 species, fish represent more than one-half of the total number of recognized living vertebrate species. In addition to this impressive biological diversity, fish are relevant because they include model species with scientific and clinical interest as well as many exploited species with economic importance. In spite of this, the study of GnIH and its physiological effects on reproduction and other physiological processes has only been approached in a few fish species, and results obtained are in some cases conflicting. In this review, we summarize the information available in the literature on GnIH sequences identified in fish, the distribution of GnIH and GnIH-Rs in central and peripheral tissues, the physiological actions of GnIH on the reproductive brain-pituitary-gonadal axis, as well as other reported effects of this neuropeptide, and existing knowledge on the regulatory mechanisms of GnIH in fish.

Testicular Steroidogenesis and Locomotor Activity Are Regulated by Gonadotropin-Inhibitory Hormone in Male European Sea Bass

Gonadotropin-inhibitory hormone (GnIH) is a neurohormone that suppresses reproduction by acting at both the brain and pituitary levels. In addition to the brain, GnIH may also be produced in gonads and can regulate steroidogenesis and gametogenesis. However, the function of GnIH in gonadal physiology has received little attention in fish. The main objective of this study was to evaluate the effects of peripheral sbGnih-1 and sbGnih-2 implants on gonadal development and steroidogenesis during the reproductive cycle of male sea bass (Dicentrarchus labrax). Both Gnihs decreased testosterone (T) and 11-ketotestosterone (11-KT) plasma levels in November and December (early- and mid-spermatogenesis) but did not affect plasma levels of the progestin 17,20β-dihydroxy-4-pregnen-3-one (DHP). In February (spermiation), fish treated with sbGnih-1 and sbGnih-2 exhibited testicles with abundant type A spermatogonia and partial spermatogenesis. In addition, we determined the effects of peripheral Gnih implants on plasma follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) levels, as well as on brain and pituitary expression of the main reproductive hormone genes and their receptors during the spermiation period (February). Treatment with sbGnih-2 increased brain gnrh2, gnih, kiss1r and gnihr transcript levels. Whereas, both Gnihs decreased lhbeta expression and plasma Lh levels, and sbGnih-1 reduced plasmatic Fsh. Finally, through behavioral recording we showed that Gnih implanted animals exhibited a significant increase in diurnal activity from late spermatogenic to early spermiogenic stages. Our results indicate that Gnih may regulate the reproductive axis of sea bass acting not only on brain and pituitary hormones but also on gonadal physiology and behavior.

Sexual plasticity: A fishy tale

Teleost fish exhibit remarkably diverse and plastic patterns of sexual development. One of the most fascinating modes of plasticity is functional sex change, which is widespread in marine fish including species of commercial importance; however, the regulatory mechanisms remain elusive. In this review, we explore such sexual plasticity in fish, using the bluehead wrasse (Thalassoma bifasciatum) as the primary model. Synthesizing current knowledge, we propose that cortisol and key neurochemicals modulate gonadotropin releasing hormone and luteinizing hormone signaling to promote socially controlled sex change in protogynous fish. Future large-scale genomic analyses and systematic comparisons among species, combined with manipulation studies, will likely uncover the common and unique pathways governing this astonishing transformation. Revealing the molecular and neuroendocrine mechanisms underlying sex change in fish will greatly enhance our understanding of vertebrate sex determination and differentiation as well as phenotypic plasticity in response to environmental influences. Mol. Reprod. Dev. 84: 171-194, 2017. © 2016 Wiley Periodicals, Inc.

Molecular, cellular, morphological, physiological and behavioral aspects of gonadotropin-inhibitory hormone

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was isolated from the brains of Japanese quail in 2000, which inhibited luteinizing hormone release from the anterior pituitary gland. Here, we summarize the following fifteen years of researches that investigated on the mechanism of GnIH actions at molecular, cellular, morphological, physiological, and behavioral levels. The unique molecular structure of GnIH peptide is in its LPXRFamide (X=L or Q) motif at its C-terminal. The primary receptor for GnIH is GPR147. The cell signaling pathway triggered by GnIH is initiated by inhibiting adenylate cyclase and decreasing cAMP production in the target cell. GnIH neurons regulate not only gonadotropin synthesis and release in the pituitary, but also regulate various neurons in the brain, such as GnRH1, GnRH2, dopamine, POMC, NPY, orexin, MCH, CRH, oxytocin, and kisspeptin neurons. GnIH and GPR147 are also expressed in gonads and they may regulate steroidogenesis and germ cell maturation in an autocrine/paracrine manner. GnIH regulates reproductive development and activity. In female mammals, GnIH may regulate estrous or menstrual cycle. GnIH is also involved in the regulation of seasonal reproduction, but GnIH may finely tune reproductive activities in the breeding seasons. It is involved in stress responses not only in the brain but also in gonads. GnIH may inhibit male socio-sexual behavior by stimulating the activity of cytochrome P450 aromatase in the brain and stimulates feeding behavior by modulating the activities of hypothalamic and central amygdala neurons.

Goldfish neurokinin B: Cloning, tissue distribution, and potential role in regulating reproduction

Neurokinin B (NKB) is a member of the tackykinin (TAC) family known to play a critical role in the neuroendocrine regulation of reproduction in mammals. However, its biological functions in teleosts are less clear. The aim of this study was to determine the role of NKB in fish reproduction using goldfish as a model. Two transcripts, TAC3a and TAC3b, which encode several NKBs, including NKBa-13, NKBa-10, NKBb-13, and NKBb-11, were cloned. Phylogenetic analysis revealed that NKBa-10 and NKBb-11 are closely related to mammalian NKB, while NKB-13s are more conserved in teleosts. Quantitative real-time PCR analyses in various tissues showed that TAC3a and TAC3b mRNAs were mainly expressed in the brain. In situ hybridization further detected TAC3a and TAC3b mRNAs in several regions of the brain known to be involved in the regulation of reproduction and metabolism, as well as in the neurohypophysis of the pituitary. To investigate the potential role of NKBs in reproduction, goldfish were injected intraperitoneally with synthetic NKBa-13, -10, NKBb-13, or -11 peptides and the mRNA levels of hypothalamic gonadotropin-releasing hormone (GnRH) and pituitary gonadotropin subunits were measured. NKBa-13, -10, or NKBb-13, but not -11, significantly increased hypothalamic salmon GnRH and pituitary FSHβ and LHβ mRNA levels in both female and male goldfish. Finally, ovariectomy increased, while estradiol replacement reduced, TAC3a mRNA levels without affecting TAC3b expression in the hypothalamus. These data suggest that NKBa-13, -10, and NKBb-13 play a role in mediating the estrogen negative feedback regulation of gonadotropins.

Molecular identification of GnIH/GnIHR signal and its reproductive function in protogynous hermaphroditic orange-spotted grouper (Epinephelus coioides)

Gonadotropin-inhibitory hormone (GnIH) and its receptor (GnIHR) play an important role in reproduction regulation in birds, mammals and some teleost species. In protogynous hermaphroditic orange-spotted grouper (Epinephelus coioides), the GnIH/GnIHR signaling pathway and its reproductive function have not been addressed yet. In this study, GnIH and GnIHR in orange-spotted grouper were characterized. gGnIH possessed three putative peptides (gGnIH-I, -II, -III), while gGnIHR showed the characteristics of G protein-coupled receptor and was clustered with GPR147. Functional assays demonstrated that three synthetic gGnIH peptides significantly decreased the forskolin-induced CRE promoter activity, but only gGnIH-I could significantly decrease SRE promoter activity in COS-7 cells transfected with gGnIHR. During the process of ovarian differentiation and development, gGnIH mRNA level in hypothalamus was low at the gonadal primordium stage with gonia, then increased significantly at the early differentiated gonad with primary growth oocytes, while decreased significantly at the developing gonads with cortical-alveolus and vitellogenic stage oocytes. During MT-induced sex reversal, gGnIH mRNA level in hypothalamus increased significantly when the fish completely reversed from female to male. However, gGnIHR mRNA level in pituitary decreased significantly in intersex and completely reversed male fish. Intraperitoneal injection (i.p.) of three gGnIH peptides significantly decreased GnRH1 mRNA levels in hypothalamus, and gGnIH-II significantly inhibited synthesis of LHβ in pituitary. In summary, we firstly identified the GnIH/GnIHR signal in protogynous orange-spotted grouper, which might be involved in the regulation of the reproductive function of sex differentiation, gonadal development and sex reversal via regulating the synthesis of both GnRH and GtH.

Structural and functional divergence of gonadotropin-inhibitory hormone from jawless fish to mammals

Gonadotropin-inhibitory hormone (GnIH) was discovered as a novel hypothalamic peptide that inhibits gonadotropin release in the quail. The presence of GnIH-homologous peptides and its receptors (GnIHRs) have been demonstrated in various vertebrate species including teleosts, suggesting that the GnIH-GnIHR family is evolutionarily conserved. In avian and mammalian brain, GnIH neurons are localized in the hypothalamic nuclei and their neural projections are widely distributed. GnIH acts on the pituitary and gonadotropin-releasing hormone neurons to inhibit reproductive functions by decreasing gonadotropin release and synthesis. In addition, GnIH-GnIHR signaling is regulated by various factors, such as environmental cues and stress. However, the function of fish GnIH orthologs remains inconclusive because the physiological properties of fish GnIH peptides are debatable. This review summarizes the current research progress in GnIH-GnIHR signaling and their physiological functions in vertebrates with special emphasis on non-mammalian vertebrate species.

Central pathways integrating metabolism and reproduction in teleosts

Energy balance plays an important role in the control of reproduction. However, the cellular and molecular mechanisms connecting the two systems are not well understood especially in teleosts. The hypothalamus plays a crucial role in the regulation of both energy balance and reproduction, and contains a number of neuropeptides, including gonadotropin-releasing hormone (GnRH), orexin, neuropeptide-Y, ghrelin, pituitary adenylate cyclase-activating polypeptide, α-melanocyte stimulating hormone, melanin-concentrating hormone, cholecystokinin, 26RFamide, nesfatin, kisspeptin, and gonadotropin-inhibitory hormone. These neuropeptides are involved in the control of energy balance and reproduction either directly or indirectly. On the other hand, synthesis and release of these hypothalamic neuropeptides are regulated by metabolic signals from the gut and the adipose tissue. Furthermore, neurons producing these neuropeptides interact with each other, providing neuronal basis of the link between energy balance and reproduction. This review summarizes the advances made in our understanding of the physiological roles of the hypothalamic neuropeptides in energy balance and reproduction in teleosts, and discusses how they interact with GnRH, kisspeptin, and pituitary gonadotropins to control reproduction in teleosts.