Seasonal effect of gonadotrophin inhibitory hormone on gonadotrophin-releasing hormone-induced gonadotroph functions in the goldfish pituitary

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.

Dual effects of gonadotropin-inhibitory hormone on testicular development in prepubertal Minxinan Black rabbits (Oryctolagus cuniculus)

Gonadotropin-inhibitory hormone (GnIH) is a neurohormone that not only suppresses reproduction at the brain level but also regulates steroidogenesis and gametogenesis at the gonad level. However, its function in gonadal physiology has received little attention in rabbits. The main objective of this study was to evaluate the effects of GnIH on testicular development and function in prepubertal Minxinan Black rabbits (Oryctolagus cuniculus). In the present study, we investigated the serum reproductive hormone concentration, testicular parameters, morphology of seminiferous tubules, apoptosis of testicular cells, and expression of reproductive-related genes in male prepubertal Minxinan Black rabbits intraperitoneally administered with 0, 0.5, 5, or 50 μg quail GnIH-related peptides (qGnIH) for 10 days. Compared with the vehicle, administration with 5 μg of qGnIH downregulated the serum testosterone concentration and mRNA levels of spermatogenic genes (PCNA, FSHR, INHβA, HSF1, and AR) and upregulated the apoptosis rate of testicular cells; administration with 50 μg of qGnIH decreased the serum testosterone concentration and hypothalamic GnIH gene mRNA level and increased the serum LH concentration, pituitary LHβ gene mRNA level, testicular weight, gonadosomatic index (GSI), and spermatogenic cell layer thickness. It is concluded that GnIH could exert dual actions on testicular development depending on the male prepubertal rabbits receiving different intraperitoneal doses.

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.

Thyroid Hormones Deficiency Impairs Male Germ Cell Development: A Cross Talk Between Hypothalamic-Pituitary-Thyroid, and—Gonadal Axes in Zebrafish

In vertebrates, thyroid hormones are critical players in controlling different physiological processes such as development, growth, metabolism among others. There is evidence in mammals that thyroid hormones are also an important component of the hormonal system that controls reproduction, although studies in fish remain poorly investigated. Here, we tested this hypothesis by investigating the effects of methimazole-induced hypothyroidism on the testicular function in adult zebrafish. Treatment of fish with methimazole, in vivo, significantly altered zebrafish spermatogenesis by inhibiting cell differentiation and meiosis, as well as decreasing the relative number of spermatozoa. The observed impairment of spermatogenesis by methimazole was correlated with significant changes in transcript levels for several genes implicated in the control of reproduction. Using an in vitro approach, we also demonstrated that in addition to affecting the components of the brain-pituitary-peripheral axis, T3 (triiodothyronine) also exerts direct action on the testis. These results reinforce the hypothesis that thyroid hormones are an essential element of multifactorial control of reproduction and testicular function in zebrafish and possibly other vertebrate species.

Comparative insights of the neuroanatomical distribution of the gonadotropin-inhibitory hormone (GnIH) in fish and amphibians

This paper intends to apprise the reader regarding the existing knowledge on the neuroanatomical distribution of GnIH-like peptides in in fish and amphibians in both the adult stage and during ontogenesis. The neuroanatomical distribution of GnIH-like neuropeptides appears quite different in the studied species, irrespective of the evolutionary closeness. The topology of the olfactory bulbs can affect the distribution of neurons producing the GnIH-like peptides, with a tendency to show a more extended distribution into the brains with pedunculate olfactory bulbs. Therefore, the variability of the GnIH-like system could also reflect specific adaptations rather than evolutionary patterns. The onset of GnIH expression was detected very early during development suggesting its precocious roles, and the neuroanatomical distribution of GnIH-like elements showed a generally increasing trend. This review highlights some critical technical aspects and the need to increase the number of species to be studied to obtain a complete neuroanatomical picture of the GnIH-like system.

Metabolic Changes During Growth and Reproductive Phases in the Liver of Female Goldfish (Carassius auratus)

Hormones of the brain-pituitary-peripheral axis regulate metabolism, gonadal maturation, and growth in vertebrates. In fish, reproduction requires a significant energy investment to metabolically support the production of hundreds of eggs and billions of sperms in females and males, respectively. This study used an LC-MS-based metabolomics approach to investigate seasonally-related changes in metabolic profile and energy allocation patterns in female goldfish liver. We measured basal metabolic profile in female goldfish at three phases of the reproductive cycle, including 1) Maximum growth period in postovulatory regressed phase, 2) mid recrudescence in fish with developing follicles, and 3) late recrudescence when the ovary contains mature ovulatory follicles. We also investigated changes in the liver metabolism following acute treatments with GnRH and GnIH, known to be involved in controlling reproduction and growth in goldfish. Chemometrics combined with pathway-driven bioinformatics revealed significant changes in the basal and GnRH/GnIH-induced hepatic metabolic profile, indicating that metabolic energy allocation is regulated to support gonadal development and growth at different reproductive cycles. Overall, the findings support the hypothesis that hormonal control of reproduction involves accompanying metabolic changes to energetically support gonadotropic and somatotropic activities in goldfish and other oviparous vertebrates.

Advancing reproductive neuroendocrinology through research on the regulation of GnIH and on its diverse actions on reproductive physiology and behavior

The discovery of gonadotropin-inhibitory hormone (GnIH) in 2000 has led to a new research era of reproductive neuroendocrinology because, for a long time, researchers believed that only gonadotropin-releasing hormone (GnRH) regulated reproduction as a neurohormone. Later studies on GnIH demonstrated that it acts as a new key neurohormone inhibiting reproduction in vertebrates. GnIH reduces gonadotropin release andsynthesis via the GnIH receptor GPR147 on gonadotropes and GnRH neurons. Furthermore, GnIH inhibits reproductive behavior, in addition to reproductive neuroendocrine function. The modification of the synthesis of GnIH and its release by the neuroendocrine integration of environmental and internal factors has also been demonstrated. Thus, the discovery of GnIH has facilitated advances in reproductive neuroendocrinology. Here, we describe the advances in reproductive neuroendocrinology driven by the discovery of GnIH, research on the effects of GnIH on reproductive physiology and behavior, and the regulatory mechanisms underlying GnIH synthesis and release.

Gonadotropin inhibitory hormone receptors (GnIHRs): Molecular characterization and synergistic effect of different drugs in Indian major carp, Labeo catla

After the discovery of Gonadotropin-inhibitory hormone (GnIH) in birds in 2000, it showed different roles in different vertebrate classes and even in different species of same classes. In birds and mammals, GnIH inhibits the expression of gonadotropins during reproduction, while in fishes it exerts both inhibitory and stimulatory effect on reproduction. The current study evaluates the role of GnIH during reproduction in Labeo catla. The partial cDNA sequence of GnIHR1 and GnIHR3 receptor genes was identified by degenerate PCR. The mRNA expression analysis of GnIHRs during different reproductive phases showed that the expression of all three GnIH receptor genes is highest during spawning phase. The expression of GnIH receptors is detected in both brain and gonads except for GnIHR3 which only expressed in gonads. The in vivo experiments with GnIH antagonist, RF313 drastically reduced the expression level of reproduction related genes like LH, FSH, and GnRH at 1 h post-injection. In another experiment the surge induced by cGnIH-III peptide on gonadotropins gene expression is further increased when co-injected with LHRHa. However, co-injection of melatonin along with cGnIH-III peptide had opposite effects. These results showed that the GnIH/GnIHRs system has positive effect on reproduction in L. catla.

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Distribution of the Novel RFRP-3/receptors system in the epididymis of the seasonal desert rodent, Gerbillus tarabuli, during sexual activity

RFamide-related peptide (RFRP-3), the Mammalian ortholog of the Avian gonadotropin-inhibitory hormone (GnIH), is a novel neuropeptide known for its inhibitory regulatory effect on reproduction in various mammalian species. However, a stimulatory action has been reported. This paper aims to: i) study the histology of the epididymis (caput) of Gerbillus tarabuli during the breeding period; and ii) to determine the distribution of the "RFRP-3/receptors system" in the epididymis (caput) of this desert rodent during the active season, and thus, to inspect its potential local interfering in sperm maturation. For that, immunohistochemistry was performed to detect the epididymal immunolocalizations of the three molecules, RFRP-3, GPR147, and GPR74. This is the first report of the epididymis histology in Gerbillus tarabuli, as it is the first evidence of the existence of the RFRP-3/Receptor system in the same organ of the same species. During the breeding season, moderate immunostaining of the RFRP-3/receptors system was present in the caput epididymis' epithelial parts (basal and principal cells) and spermatozoa. In contrast, these three molecules were absent in the peritubular and muscle coat's myoid cells and of the interstitial part of the caput epididymis. The results suggest that the epididymis is a potential source of RFRP-3 in the desert Rodent, Gerbillus tarabuli, which may function as a paracrine and/or autocrine factor affecting the main epididymis' function: sperm maturation.

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.

Interaction between thyroid hormones and gonadotropin inhibitory hormone in ex vivo culture of zebrafish testis: An approach to study multifactorial control of spermatogenesis

Reproduction is under multifactorial control of neurohormones, pituitary gonadotropins, as well as of local gonadal signaling systems including sex steroids, growth factors and non-coding RNAs. Among the factors, gonadotropin-inhibitory hormone (Gnih) is a novel RFamide neuropeptide which directly modulates gonadotropin synthesis and release from pituitary, and in the gonads, Gnih mediated inhibitory actions on gonadotropin response of zebrafish spermatogenesis. Thyroid hormones are peripheral hormones which are also known to interact with reproductive axis, in particular, regulating testicular development and function. This study investigated the interaction between Gnih and thyroid hormones in zebrafish spermatogenesis using in vivo and ex vivo approaches. Three experimental groups were established: "control" (non-treated fish), "methimazole" and "methimazole + T4". Fish were exposed to goitrogen methimazole for 3 weeks; T4 (100 μg/L) was added in the water from the second week only in the "reversal treatment" group. After exposure, testes were dissected out and immediately incubated in Leibovitz's L-15 culture medium containing hCG, Gnih or hCG + Gnih for 7 days. Germ cell cysts and haploid cell population were evaluated by histomorphometry and flow cytometry, respectively. Our results showed that hypothyroidism affected germ cell development in basal and gonadotropin-induced spermatogenesis, in particular, meiosis and spermiogenesis. Hypothyroid testes showed lower amount of spermatozoa, and decreased potency of hCG. We also showed that goitrogen treatment nullified the inhibitory actions of Gnih on the gonadotropin-induced spermatogenesis. This study provided evidences that thyroid hormones are important regulatory factors for hCG- and Gnih-mediated functions in zebrafish spermatogenesis.

Seasonally related metabolic changes and energy allocation associated with growth and reproductive phases in the liver of male goldfish (Carassius auratus)

Reproduction and growth follow a seasonal pattern in many fish species involving changes in gonadal development, growth, and metabolism. Significant metabolic energy is needed during gametogenesis in both female and male to produce hundreds of eggs and billions of sperms. Seasonal variations are controlled by the hormones of brain-pituitary-peripheral axis and are accompanied by significant metabolic changes. There is evidence that GnRH and GnIH are among the key neurohormones that regulate the reciprocal control of growth and reproduction. The objective of this study was to investigate changes in metabolic profile and energy allocation patterns at different stages of reproduction, using goldfish as a model organism and LC-MS as analytical platform for metabolic analysis. Goldfish undergoes a clear seasonal cycle of growth and reproduction. In vivo experiments were conducted at three different time point of the annual cycle: regressed gonadal phase (peak growth phase), mid gametogenesis and late gametogenesis. Emphasis is placed on changes in liver metabolic pathways to energetically sustain the physiological processes related to growth and reproduction. Moreover, we tested the hypothesis that GnRH and GnIH may play a role in the regulation of metabolism by investigating acute effects of these peptides at different stages of reproductive cycle. SIGNIFICANCE: The findings in this paper provide novel information on the seasonal changes in basal metabolism during different stages of reproductive cycle, and evidence for differential allocation of energy during reciprocal control of reproduction and growth in goldfish. Chemometrics combined with pathway-driven bioinformatics elucidated a shift in the metabolic profile, indicating distinct patterns of energy allocation in the reproductive and growth seasons. Furthermore, to our knowledge this is the first study to provide evidence for a possible regulatory role of GnRH and GnIH in liver metabolism and energy allocation patterns associated with growth and reproductive processes. Together our findings present a framework for better understanding of the hormonally induced changes in metabolism to energetically sustain growth and reproduction in fish and other oviparous species undergoing seasonal cycle.

Gonadotropin-inhibitory hormone (GnIH): A new key neurohormone controlling reproductive physiology and behavior

The discovery of novel neurohormones is important for the advancement of neuroendocrinology. In early 1970s, gonadotropin-releasing hormone (GnRH), a hypothalamic neuropeptide that promotes gonadotropin release, was identified to be an endogenous neurohormone in mammals. In 2000, thirty years later, another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), that inhibits gonadotropin release, was found in quail. GnIH acts via GPR147 and inhibits gonadotropin release and synthesis and reproductive function in birds through actions on GnRH neurons in the hypothalamus and pituitary gonadotrophs. Later, GnIH was found in other vertebrates including humans. GnIH studies have advanced the progress of reproductive neuroendocrinology. Furthermore, recent GnIH studies have indicated that abnormal changes in GnIH expression may cause pubertal disorder and reproductive dysfunction. Here, we describe GnIH discovery and its impact on the progress of reproductive neuroendocrinology. This review also highlights advancement and perspective of GnIH studies on drug development for pubertal disorder and reproductive dysfunction. (149/150).

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.

Discovery of gonadotropin-inhibitory hormone (GnIH), progress in GnIH research on reproductive physiology and behavior and perspective of GnIH research on neuroendocrine regulation of reproduction

Based on extensive studies on gonadotropin-releasing hormone (GnRH) it was assumed that GnRH is the only hypothalamic neurohormone regulating gonadotropin release in vertebrates. In 2000, however, Tsutsui's group discovered gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide that inhibits gonadotropin release, in quail. Subsequent studies by Tsutsui's group demonstrated that GnIH is conserved among vertebrates, acting as a new key neurohormone regulating reproduction. GnIH inhibits gonadotropin synthesis and release through actions on gonadotropes and GnRH neurons via GnIH receptor, GPR147. Thus, GnRH is not the sole hypothalamic neurohormone controlling vertebrate reproduction. The following studies by Tsutsui's group have further demonstrated that GnIH has several important functions in addition to the control of reproduction. Accordingly, GnIH has drastically changed our understanding about reproductive neuroendocrinology. This review summarizes the discovery of GnIH, progress in GnIH research on reproductive physiology and behavior and perspective of GnIH research on neuroendocrine regulation of reproduction.

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.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Gonadotropin-inhibitory hormone in teleosts: New insights from a basal representative, the eel

Since its discovery in birds, gonadotropin-inhibitory hormone (GnIH) has triggered investigation in the other groups of vertebrates. In the present study, we have identified a single gnih gene in the European eel (Anguilla anguilla), a representative species of a basal group of teleosts (Elopomorphs). We have also retrieved a single gnih gene in Osteoglossomorphs, as well as in more recently emerged teleosts, Clupeocephala. Phylogeny and synteny analyses allowed us to infer that one of the two gnih paralogs emerged from the teleost-specific whole genome duplication (TWGD or 3R), would have been lost shortly after the 3R, before the emergence of the basal groups of teleosts. This led to the presence of a single gnih in extant teleosts as in other vertebrates. Two gnih paralogs were still found in some teleost species, such as in salmonids, but resulting from the additional whole genome duplication that specifically occurred in this lineage (4R). Eel gnih was mostly expressed in the diencephalon part of the brain, as analyzed by quantitative real-time PCR. Cloning of eel gnih cDNA confirmed that the sequence of the GnIH precursor encoded three putative mature GnIH peptides (aaGnIH-1, aaGnIH-2 and aaGnIH-3), which were synthesized and tested for their direct effects on eel pituitary cells in vitro. Eel GnIH peptides inhibited the expression of gonadotropin subunits (lhβ, fshβ, and common a-subunit) as well as of GnRH receptor (gnrh-r2), with no effect on tshβ and gh expression. The inhibitory effect of GnIH peptides on gonadotropic function in a basal teleost is in agreement with an ancestral inhibitory role of GnIH in the neuroendocrine control of reproduction in vertebrates.

Multifactorial control of reproductive and growth axis in male goldfish: Influences of GnRH, GnIH and thyroid hormone

Reproduction and growth are under multifactorial control of neurohormones and peripheral hormones. This study investigated seasonally related effects of GnIH, GnRH, and T3 on the reproductive and growth axis in male goldfish at three stages of gonadal recrudescence. The effects of injection treatments with GnRH, GnIH and/or T3 were examined by measuring serum LH and GH levels, as well as peripheral transcript levels, using a factorial design. As expected, GnRH elevated serum LH and GH levels in a seasonally dependant manner, with maximal elevations of LH in late stages of gonadal recrudescence (Spring) and maximal increases in GH in the regressed gonadal stage (Summer). GnIH injection increased serum LH and GH levels only in fish at the regressed stage but exerted both stimulatory and inhibitory effects on GnRH-induced LH responses depending on season. T3 treatment mainly had stimulatory effects on circulating LH levels and inhibitory effects on serum GH concentrations. In the liver and testes, we observed seasonal differences in thyroid receptors, estrogen receptors, vitellogenin, follicle-stimulating hormone receptor, aromatase and IGF-I transcript levels that were tissue- and sex-specific. Generally, there were no clear correlation between circulating LH and GH levels and peripheral transcript levels, presumably due to time-related response and possible direct interaction of GnRH and GnIH at the level of liver and testis. The results support the hypothesis that GnRH and GnIH are important components of multifactorial mechanisms that work in concert with T3 to regulate reciprocal control of reproduction and growth in goldfish.

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.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Knockout of the Gnrh genes in zebrafish: effects on reproduction and potential compensation by reproductive and feeding-related neuropeptides

Gonadotropin-releasing hormone (GNRH) is known as a pivotal upstream regulator of reproduction in vertebrates. However, reproduction is not compromised in the hypophysiotropic Gnrh3 knockout line in zebrafish (gnrh3-/-). In order to determine if Gnrh2, the only other Gnrh isoform in zebrafish brains, is compensating for the loss of Gnrh3, we generated a double Gnrh knockout zebrafish line. Surprisingly, the loss of both Gnrh isoforms resulted in no major impact on reproduction, indicating that a compensatory response, outside of the Gnrh system, was evoked. A plethora of factors acting along the reproductive hypothalamus-pituitary axis were evaluated as possible compensators based on neuroanatomical and differential gene expression studies. In addition, we also examined the involvement of feeding factors in the brain as potential compensators for Gnrh2, which has known anorexigenic effects. We found that the double knockout fish exhibited upregulation of several genes in the brain, specifically gonadotropin-inhibitory hormone (gnih), secretogranin 2 (scg2), tachykinin 3a (tac3a), and pituitary adenylate cyclase-activating peptide 1 (pacap1), and downregulation of agouti-related peptide 1 (agrp1), indicating the compensation occurs outside of Gnrh cells and therefore is a noncell autonomous response to the loss of Gnrh. While the differential expression of gnih and agrp1 in the double knockout line was confined to the periventricular nucleus and hypothalamus, respectively, the upregulation of scg2 corresponded with a broader neuronal redistribution in the lateral hypothalamus and hindbrain. In conclusion, our results demonstrate the existence of a redundant reproductive regulatory system that comes into play when Gnrh2 and Gnrh3 are lost.

The gonadotropin-inhibitory hormone system of fish: The case of sea bass (Dicentrarchus labrax)

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide belonging to the RFamide peptide family that was first discovered in quail by Tsutsui and co-workers in the year 2000. Since then, different GnIH orthologues have been identified in all vertebrate groups, from agnathans to mammals. These GnIH genes synthesize peptide precursors that encompass two to four C-terminal LPXRFamide peptides. Functional and behavioral studies carried out in birds and mammals have demonstrated a clear inhibitory role of GnIH on GnRH and gonadotropin synthesis and secretion as well as on aggressive and sexual behavior. However, the effects of Gnih orthologues in reproduction remain controversial in fish with both stimulatory and inhibitory actions being reported. In this paper, we will review the main findings obtained in our laboratory on the Gnih system of the European sea bass, Dicentrarchus labrax. The sea bass gnih gene encodes two putative Gnih peptides (sbGnih1 and sbGnih2), and is expressed in the olfactory bulbs/telencephalon, diencephalon, midbrain tegmentum, rostral rhombencephalon, retina and testis. The immunohistochemical study performed using specific antibodies developed in our laboratory revealed Gnih-immunoreactive (ir) perikarya in the same central areas and Gnih-ir fibers that profusely innervated the brain and pituitary of sea bass. Moreover, in vivo studies revealed the inhibitory role of centrally- and peripherally-administered Gnih in the reproductive axis of male sea bass, by acting at the brain (on gnrh and kisspeptin expression), pituitary (on gnrh receptors and gonadotropin synthesis and release) and gonadal (on androgen secretion and gametogenesis) levels. Our results have revealed the existence of a functional Gnih system in sea bass, and have provided evidence of the differential actions of the two Gnih peptides on the reproductive axis of this species, the main inhibitory role in the brain and pituitary being exerted by the sbGnih2 peptide. Recent studies developed in our laboratory also suggest that Gnih might be involved in the transduction of photoperiod and temperature information to the reproductive axis, as well as in the modulation of daily and seasonal rhythmic processes in sea bass.

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.

Effects of GnRH and the dual regulatory actions of GnIH in the pituitary explants and brain slices of Astyanax altiparanae males

The pituitary gonadotropins, Fsh (follicle-stimulating hormone) and Lh (luteinizing hormone), regulate testicular development and functions in all vertebrates. At the pituitary, different signaling systems regulate the synthesis and secretion of the gonadotropins, such as the hypothalamic neuropeptides GnRH (gonadotropin-releasing hormone) and GnIH (gonadotropin-inhibitory hormone). While GnRH exerts stimulatory roles, the actions of GnIH remain controversial for many teleost species. Therefore, the aim of this study was to evaluate the in vitro effects of chicken GnRH2 (cGnRH2) and zebrafish GnIH-3 (zGnIH-3) on the male gonadotropin and GnRH system expression using pituitary explants and brain slices from a neotropical species with economical and ecological relevance, Astyanax altiparanae. Our results showed that in males, cGnRH2 increased fshb and lhb mRNA levels in the pituitary explants. Interestingly, zGnIH-3 has no effect on basal gonadotropin expression, however zGnIH-3 decreased the cGnRH2-induced fshb and lhb transcripts in male pituitary explants. In the male brain slices, zGnIH-3 showed stimulatory effects, increasing gnrh2 mRNA levels. Overall, our results suggested that GnIH seems to have dual regulatory actions on gonadotropin and GnRH2 expression of A. altiparanae males. This study provided basic information on endocrine regulation of A. altiparanae reproduction, and the obtained results will expand our knowledge, improving the reproductive management of this economically important freshwater species.

In vitro effects of tongue sole LPXRFa and kisspeptin on relative abundance of pituitary hormone mRNA and inhibitory action of LPXRFa on kisspeptin activation in the PKC pathway

Results of previous studies indicated the existence of LPXRFa, the piscine ortholog of gonadotropin-inhibitory hormone (GnIH), and kisspeptin (Kiss2) in tongue sole (Cynoglossus semilaevis), and that LPXRFa exerts an inhibitory effect on Kiss2 activation in the protein kinase A (PKA) pathway. The functions in the control of reproduction and whether LPXRFa antagonizes the action of Kiss2 by inhibiting the protein kinase C (PKC) pathway, however, are still unknown. In the present study, there was an initial investigation of the direct effects of LPXRFa and Kiss2 on relative abundance of pituitary hormone mRNA transcripts using a whole pituitary culture system. Results indicated that LPXRFa-1 specifically functioned to increase relative abundance of lhβ mRNA when there were comparisons with the control, without any effect on relative abundance of gh, gthα and fshβ mRNA. Treatment with LPXRFa-2 resulted in a reduction in relative abundance of gthα and lhβ mRNA, and did not alter relative abundance of fshβ mRNA. Treatment of LPXRFa-2 resulted in a greater relative abundance of gh mRNA. Treatment with Kiss2, however, resulted in an increase in relative abundance of gthα and fshβ mRNA transcripts, without altering relative abundances of gh and lhβ mRNA. Subsequently, there was valuation of the potential interaction between LPXRFa and kisspeptin in COS-7 cells transfected with the cognate receptors. Both LPXRFa-1 and LPXRFa-2 suppressed serum responsive element-dependent luciferase (SRE-luc) activity when compared to stimulation with Kiss2 alone, indicating an inhibitory effect of LPXRFa on kisspeptin activation on the PKC pathway. Overall, data from the present study provide novel evidence for differential actions of LPXRFa and kisspeptin on pituitary hormone synthesis as well as for the interaction between LPXRFa and kisspeptin systems in teleosts.

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.

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.

How to Contribute to the Progress of Neuroendocrinology: Discovery of GnIH and Progress of GnIH Research

It is essential to discover novel neuropeptides that regulate the functions of pituitary, brain and peripheral secretory glands for the progress of neuroendocrinology. Gonadotropin-releasing hormone (GnRH), a hypothalamic neuropeptide stimulating gonadotropin release was isolated and its structure was determined by Schally's and Guillemin's groups at the beginning of the 1970s. It was subsequently shown that GnRH is highly conserved among vertebrates. GnRH was assumed the sole hypothalamic neuropeptide that regulates gonadotropin release in vertebrates based on extensive studies of GnRH over the following three decades. However, in 2000, Tsutsui's group isolated and determined the structure of a novel hypothalamic neuropeptide, which inhibits gonadotropin release, in quail, an avian species, and named it gonadotropin-inhibitory hormone (GnIH). Following studies by Tsutsui's group demonstrated that GnIH is highly conserved among vertebrates, from humans to agnathans, and acts as a key neuropeptide inhibiting reproduction. Intensive research on GnIH demonstrated that GnIH inhibits gonadotropin synthesis and release by acting on gonadotropes and GnRH neurons via GPR147 in birds and mammals. Fish GnIH also regulates gonadotropin release according to its reproductive condition, indicating the conserved role of GnIH in the regulation of the hypothalamic-pituitary-gonadal (HPG) axis in vertebrates. Therefore, we can now say that GnRH is not the only hypothalamic neuropeptide controlling vertebrate reproduction. In addition, recent studies by Tsutsui's group demonstrated that GnIH acts in the brain to regulate behaviors, including reproductive behavior. The 18 years of GnIH research with leading laboratories in the world have significantly advanced our knowledge of the neuroendocrine control mechanism of reproductive physiology and behavior as well as interactions of the HPG, hypothalamic-pituitary-adrenal and hypothalamic-pituitary-thyroid axes. This review describes how GnIH was discovered and GnIH research progressed in this new research era of reproductive neuroendocrinology.

Comparative and Evolutionary Aspects of Gonadotropin-Inhibitory Hormone and FMRFamide-Like Peptide Systems

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was found in the brain of Japanese quail when investigating the existence of RFamide peptides in birds. GnIH was named because it decreased gonadotropin release from cultured anterior pituitary, which was located in the hypothalamo-hypophysial system. GnIH and GnIH precursor gene related peptides have a characteristic C-terminal LPXRFamide (X = L or Q) motif that is conserved in jawed vertebrates. Orthologous peptides to GnIH are also named RFamide related peptide or LPXRFamide peptide from their structure. A G-protein coupled receptor GPR147 is the primary receptor for GnIH. Similarity-based clustering of neuropeptide precursors in metazoan species indicates that GnIH precursor of vertebrates is evolutionarily related to FMRFamide precursor of mollusk and nematode. FMRFamide peptide is the first RFamide peptide that was identified from the ganglia of the venus clam. In order to infer the evolutionary history of the GnIH-GnIH receptor system we investigate the structural similarities between GnIH and its receptor and well-studied nematode Caenorhabditis elegans (C. elegans) FMRFamide-like peptides (FLPs) and their receptors. We also compare the functions of FLPs of nematode with GnIH of chordates. A multiple sequence alignment and phylogenetic analyses of GnIH, neuropeptide FF (NPFF), a paralogous peptide of GnIH, and FLP precursors have shown that GnIH and NPFF precursors belong to different clades and some FLP precursors have structural similarities to either precursor. The peptide coding regions of FLP precursors in the same clade align well with those of GnIH or NPFF precursors. Alignment of GnIH (LPXRFa) peptides of chordates and FLPs of C. elegans grouped the peptides into five groups according to the last C-terminal amino acid sequences, which were MRFa, LRFa, VRFa, IRFa, and PQRFa. Phylogenetic analysis of receptors suggested that GPR147 has evolutionary relationships with FLP receptors, which regulate reproduction, aggression, locomotion, and feeding. GnIH and some FLPs mediate the effect of stress on reproduction and behavior, which may also be a conserved property of these peptide systems. Future studies are needed to investigate the mechanism of how neuropeptide precursor genes are mutated to evolve new neuropeptides and their inheritance.

Review: Structure, function and evolution of GnIH

Neuropeptides that possess the Arg-Phe-NH₂ motif at their C-termini (i.e., RFamide peptides) have been characterized in the nervous system of both invertebrates and vertebrates. In vertebrates, RFamide peptides make a family and consist of the groups of gonadotropin-inhibitory hormone (GnIH), neuropeptide FF (NPFF), prolactin-releasing peptide (PrRP), kisspeptin (kiss1 and kiss2), and pyroglutamylated RFamide peptide/26RFamide peptide (QRFP/26RFa). It now appears that these vertebrate RFamide peptides exert important neuroendocrine, behavioral, sensory, and autonomic functions. In 2000, GnIH was discovered as a novel hypothalamic RFamide peptide inhibiting gonadotropin release in quail. Subsequent studies have demonstrated that GnIH acts on the brain and pituitary to modulate reproductive physiology and behavior across vertebrates. To clarify the origin and evolution of GnIH, the existence of GnIH was investigated in agnathans, the most ancient lineage of vertebrates, and basal chordates, such as tunicates and cephalochordates (represented by amphioxus). This review first summarizes the structure and function of GnIH and other RFamide peptides, in particular NPFF having a similar C-terminal structure of GnIH, in vertebrates. Then, this review describes the evolutionary origin of GnIH based on the studies in agnathans and basal chordates.

The gonadotropin-inhibitory hormone (Lpxrfa) system's regulation of reproduction in the brain-pituitary axis of the zebrafish (Danio rerio)

Gonadotropin-inhibitory hormone (GNIH) was discovered in quail with the ability to reduce gonadotropin expression/secretion in the pituitary. There have been few studies on GNIH orthologs in teleosts (LPXRFamide (Lpxrfa) peptides), which have provided inconsistent results. Therefore, the goal of this study was to determine the roles and modes of action by which Lpxrfa exerts its functions in the brain-pituitary axis of zebrafish (Danio rerio). We localized Lpxrfa soma to the ventral hypothalamus, with fibers extending throughout the brain and to the pituitary. In the preoptic area, Lpxrfa fibers interact with gonadotropin-releasing hormone 3 (Gnrh3) soma. In pituitary explants, zebrafish peptide Lpxrfa-3 downregulated luteinizing hormone beta subunit and common alpha subunit expression. In addition, Lpxrfa-3 reduced gnrh3 expression in brain slices, offering another pathway for Lpxrfa to exert its effects on reproduction. Receptor activation studies, in a heterologous cell-based system, revealed that all three zebrafish Lpxrfa peptides activate Lpxrf-R2 and Lpxrf-R3 via the PKA/cAMP pathway. Receptor activation studies demonstrated that, in addition to activating Lpxrf receptors, zebrafish Lpxrfa-2 and Lpxrfa-3 antagonize Kisspeptin-2 (Kiss2) activation of Kisspeptin receptor-1a (Kiss1ra). The fact that kiss1ra-expressing neurons in the preoptic area are innervated by Lpxrfa-ir fibers suggests an additional pathway for Lpxrfa action. Therefore, our results suggest that Lpxrfa may act as a reproductive inhibitory neuropeptide in the zebrafish that interacts with Gnrh3 neurons in the brain and with gonadotropes in the pituitary, while also potentially utilizing the Kiss2/Kiss1ra pathway.

Molecular characterization and expression profiles of LPXRFa at the brain-pituitary-gonad axis of half-smooth tongue sole (Cynoglossus semilaevis) during ovarian maturation

Gonadotropin-inhibitory hormone (GnIH) has been characterized by its ability to inhibit either basal or gonadotropin-releasing hormone (GnRH)-induced gonadotropin synthesis and release in birds and mammals. However, the physiological role of GnIH on the reproductive axis in fish remains inconclusive, with most studies focusing on the orders Cypriniformes and Perciformes. To gain insight into the role of GnIH in the regulation of reproduction in the order Pleuronectiformes, we first cloned the LPXRFa gene, the piscine ortholog of GnIH, in the half-smooth tongue sole. The full-length cDNA of LPXRFa was 918bp in size with an open reading frame (ORF) of 585bp that encoded a 194 amino acids preprohormone with a calculated molecular mass and isoelectric point of 21.73kDa and 6.52, respectively. The LPXRFa precursor encoded two putative peptide sequences that included -MPMRF or -MPQRF motifs at the C-terminal. Tissue distribution analysis showed that LPXRFa transcripts could be detected at high levels in the brains of both sexes and to a lesser extent in the ovary, heart and stomach of females, while a noteworthy expression was observed in the kidney and muscle of males. Furthermore, the expression patterns of LPXRFa mRNA during ovarian maturation were also investigated. In the brain, the mRNA expression of LPXRFa increased significantly at stage III, declined at stage V and reached a maximum at stage VI. In the pituitary, the levels of LPXRFa mRNA remained stable during ovarian maturation and increased significantly to the top level at stage V and then declined back to basal levels. In contrast, the ovarian LPXRFa mRNA levels declined sharply at stage III and remained depressed over the course of ovarian maturation. Taken together, our results provide further evidence for the existence of LPXRFa in the order Pleuronectiformes and suggest its possible involvement in the regulation of reproduction in the female tongue sole.

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.

Response to gonadotropin-releasing hormone challenge: Seasonal variation in steroid production in a viviparous lizard, Tiliqua nigrolutea

The hypothalamic-pituitary-gonadal axis plays a central role in the regulation of gamete maturation, sex steroid production and the stimulation of reproductive behaviours in vertebrates. In seasonal breeders, the timely activation and deactivation of this control system is important to ensure successful reproduction: this process is not well understood in species which breed irregularly. Males of the viviparous blotched blue-tongued lizard, Tiliqua nigrolutea, breed annually, while females display a multiennial cycle. We investigated seasonal variation in hypothalamic-pituitary-gonadal axis responsiveness in both sexes of T. nigrolutea. We measured changes in plasma concentrations of testosterone and estrogen in response to a single intraperitoneal injection of a GnRH agonist, chicken-II LH-RH, at three reproductively distinct times of year. Plasma testosterone concentrations in males were significantly increased during gonadal quiescence, but not initial or final spermatogenesis. There was no estrogen response in males at any time of year. Conversely, in females, there was an increase in plasma testosterone, but not estrogen, concentration, in reproductively quiescent females several months in advance of a successful pregnancy. These results indicate clear variation in HPG axis activity with sex, season and reproductive condition in this seasonally breeding viviparous lizard. This study opens the way for further investigation into the mechanisms by which internal (body condition) and external seasonal cues (temperature and photoperiod) are coordinated to regulate reproduction in irregularly-breeding reptiles.

Effects of pinealectomy on the neuroendocrine reproductive system and locomotor activity in male European sea bass, Dicentrarchus labrax

The seasonally changing photoperiod controls the timing of reproduction in most fish species, however, the transduction of this photoperiodic information to the reproductive axis is still unclear. This study explored the potential role of two candidate neuropeptide systems, gonadotropin-inhibitory hormone (Gnih) and kisspeptin, as mediators between the pineal organ (a principle transducer of photoperiodic information) and reproductive axis in male European sea bass, Dicentrarchus labrax. Two seven-day experiments of pinealectomy (Px) were performed, in March (end of reproductive season) and August (resting season). Effects of Px and season on the brain expression of gnih (sbgnih) and its receptor (sbgnihr), kisspeptins (kiss1, kiss2) and their receptors (kissr2, kissr3) and gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) and the main brain receptor (gnrhr-II-2b) genes, plasma melatonin levels and locomotor activity rhythms were examined. Results showed that Px reduced night-time plasma melatonin levels. Gene expression analyses demonstrated a sensitivity of the Gnih system to Px in March, with a reduction in sbgnih in the mid-hindbrain, a region with bilateral connections to the pineal organ. In August, kiss2 levels increased in Px animals but not in controls. Significant differences in expression were observed for diencephalic sbgnih, sbgnihr, kissr3 and tegmental gnrh2 between seasons. Recordings of locomotor activity following surgery revealed a change from light-synchronised to free-running rhythmic behavior. Altogether, the Gnih and Kiss2 sensitivity to Px and seasonal differences observed for Gnih and its receptor, Gnrh2, and the receptor for Kiss2 (Kissr3), suggested they could be mediators involved in the relay between environment and seasonal reproduction.

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.

Expression and actions of GnIH and its orthologs in vertebrates: Current status and advanced knowledge

The physiology of reproduction is very complex and is regulated by multiple factors, including a number of hypothalamic neuropeptides. In last few decades, various neuropeptides have been discovered to be involved in stimulation or inhibition of reproduction. In 2000, Tsutsui and colleagues uncovered gonadotropin-inhibitory hormone (GnIH), a neuropeptide generating inhibitory drive to the reproductive axis, in the brain of Coturnix quail. Afterward, GnIH orthologs were discovered in other vertebrates from fish to mammals including human. In these vertebrates, all the discovered GnIH and its ortholgs have LPXRFamide (X=L or Q) sequence at C-terminus. GnIH orthologs of mammals and primates are also termed as RFamide-related peptide (RFRP)-1 and -3 that too have an LPXRFamide (X=L or Q) motif at their C-terminus. GnIH and its orthologs form a member of the RFamide peptide family. GnIH signals via its canonical G protein coupled receptor 147 (GPR147). Both GnIH and GPR147 are expressed in hypothalamus and other brain regions. Besides actions through the hypothalamic GnRH and kisspeptinergic neurons, GnIH-GPR147 signaling exerts inhibitory effect on the reproductive axis via pituitary gonadotropes and directly at gonadal level. Various factors including availability and quality of food, photoperiod, temperature, social interaction, various stresses and some diseases modulate GnIH-GPR147 signaling. In this review, we have discussed expression and actions of GnIH and its orthologs in vertebrates. Special emphasis is given on the role of GnIH-GPR147 signaling pathway in the regulation of reproduction. We have also reviewed and discussed currently available literature on the participation of GnIH-GPR147 signaling pathway in the stress modulation of reproduction.

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.