GnIH plays a negative role in regulating GtH expression in the common carp, Cyprinus carpio L

Effects of dopamine and melatonin treatment on the expression of the genes associated with artificially induced sexual maturation in Japanese eel, Anguilla japonica

Japanese eel (Anguilla japonica) is a commercially important fish species in Asia. Understanding factors like photoperiod, temperature, and lunar cycles is crucial for successful aquaculture and managing its reproduction. Melatonin and dopamine (DA) are essential for regulating reproduction in vertebrates, including fish. This study investigated the effects of melatonin and DA on the reproductive system of mature male Japanese eels to better understand reproductive regulation in fish. To clarify the effects of these hormones on sexual maturation in eels, a critical stage in the reproductive process, sexual maturation was induced by injecting human chorionic gonadotropin, which stimulates the production of sex hormones. To check the effect of melatonin and DA on sexual maturation, DA, melatonin, and DA + domperidone were intraperitoneally injected into fish from each group (six per treatment) at a dose of 1 mg/kg body weight. The fish were then examined using quantitative RT-PCR by comparing the messenger RNA level of reproduction-related genes (gonadotropin releasing hormone 1; gnrh1, gonadotropin releasing hormone 2; gnrh2, follicle stimulating hormone; fshβ, luteinizing hormone; lhβ and DA receptor 2b; d2b), involved in the gonadotropic axis in eels, to those that received a control injection. The results indicate significant differences in the expression levels of gnrh1, gnrh2 and d2b in the brain and d2b, fshβ, lhβ in the pituitary at different stages of sexual maturation. Melatonin appears to enhance the production of sex gonadotropins, whereas DA inhibits them. These findings suggest an interaction between melatonin and DA in regulating reproduction in Japanese eels.

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.

Molecular cloning and bioinformatic characterization of Gonadotropin Inhibitory Hormone (GnIH) and its receptors in the freshwater murrel, Channa punctatus (Bloch, 1793)

Gonadotropin inhibitory hormone belonging to the RFamide peptide family, a hypothalamic neuropeptide, regulates Hypothalamus-pituitary-gonadal (HPG) axis and inhibits gonadal development. GnIH polypeptide precursor has an Arg-Phe-NH2 (RFamide) motif at the C-terminal, which has LPXRF (X = Q or L) domain. The actions of GnIH are mediated through G-protein coupled receptors and upto three receptors have been characterized in many teleosts. GnIH exerts its inhibitory effect on the HPG axis through direct interaction with GnRH and Kisspeptin neurons in the brain and acts directly on the pituitary gonadotrophs. To decipher the role of GnIH in Indian freshwater murrel, Channa punctatus, we sequenced the cDNA encoding GnIH and its two receptors. The identified GnIH mRNA encodes three RFamide peptides having -MPMRF, -MPQRF, and -LPQRFamide motifs. In silico analysis of the amino acid sequence of GnIH exhibits its molecular and functional properties and the protein-protein interaction with significant factors regulating the HPG axis. The 3-D structure of GnIH and its receptors, provides more relevant information about the active residues of these proteins which might be involved in their functioning and interaction with other proteins. Molecular dynamic simulation of GnIH protein has provided more insight into its dynamic behavior. The expression of GnIH and its receptors, shows an inverse correlation with gonadal development during the annual reproductive cycle.

Variation in Hypothalamic GnIH Expression and Its Association with GnRH and Kiss1 during Pubertal Progression in Male Rhesus Monkeys (Macaca mulatta)

Modulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion across postnatal development in higher primates is not fully understood. While gonadotropin-inhibitory hormone (GnIH) is reported to suppress reproductive axis activity in birds and rodents, little is known about the developmental trajectory of GnIH expression in rhesus monkeys throughout the pubertal transition. This study was aimed at examining the variation in GnIH immunoreactivity (-ir) and associated changes among GnIH, GnRH, and Kiss1 mRNA expression in the hypothalamus of infant, juvenile, prepubertal, and adult male rhesus monkeys. The brains from rhesus macaques were collected from infancy until adulthood and were examined using immunofluorescence and RT-qPCR. The mean GnIH-ir was found to be significantly higher in prepubertal animals (p < 0.01) compared to infants, and significantly reduced in adults (p < 0.001). Significantly higher (p < 0.001) GnRH and Kiss1 mRNA expression was noted in adults while GnIH mRNA expression was the highest at the prepubertal stage (p < 0.001). Significant negative correlations were seen between GnIH-GnRH (p < 0.01) and GnIH-Kiss1 (p < 0.001) expression. Our findings suggest a role for GnIH in the prepubertal suppression of the reproductive axis, with disinhibition of the adult reproductive axis occurring through decreases in GnIH. This pattern of expression suggests that GnIH may be a viable target for the development of novel therapeutics and contraceptives for humans.

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.

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.

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.

Effects of LPXRFamide peptides on chub mackerel gonadotropin secretion

Gonadotropin-inhibitory hormone (GnIH), a neuropeptide, suppresses gonadotropin (GTH) secretion in birds and mammals. In fish, the GnIH homolog LPXRFamide (LPXRFa) produces mature peptides with species-dependent effects on sexual reproduction. Here, we investigated the effects of LPXRFa on GTH secretion in the chub mackerel (cm; Scomber japonicus). We cloned cmlpxrfa (603 bp) and cmlpxrfa-r (1,416 bp). Additionally, we isolated lpxrfa from the bluefin tuna (Thunnus orientalis) to confirm the conservation of the LPXRFa mature sequence. Phylogenetic analysis showed that the LPXRFa precursor protein produces three mature peptides, LPXRFa-1, -2, and - 3, in both species. Reverse transcription-quantitative PCR revealed that cmlpxrfa is expressed in the hypothalamus and thalamus and midbrain (T.MB), and sexual differences were observed. Receptor expression was observed in the pre-optic area, hypothalamus, T.MB, and pituitary. Female hypothalamic lpxrfa expression did not change during puberty. Reporter gene assay showed that LPXRFa induced receptor activation via the CRE and SRE signaling pathways. However, in the presence of forskolin, an intracellular cyclic AMP enhancer, none of the LPXRFa could suppress receptor activity. The in vitro bioassay results showed that gonadotropin-releasing hormone-1 (GnRH1) had no effect on follicle-stimulating hormone (FSH) secretion, whereas the three LPXRFa significantly increased FSH secretion in pituitary cells from male chub mackerel. Contrarily, GnRH1 and three LPXRFa significantly increased luteinizing hormone (LH) secretion. The in vivo administration of LPXRFa had no effect on fshb and lhb expression in pre-pubertal and mature male chub mackerel. Overall, cmLPXRFa lacks the ability to suppress GTH secretion but can promote GTH secretion.

Organization of the gonadotropin-inhibitory hormone (Lpxrfa) system in the brain of zebrafish (Danio rerio)

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that inhibits gonadotropin secretion in birds and mammals. However, the role of GnIH (Lpxrfa) in teleosts is unknown. In this study, a transgenic zebrafish (Danio rerio) line Tg(gnih:mCherry) was developed to determine the organization of GnIH neurons in the brain. Another transgenic line, Tg(gnih:mCherry; gnrh3:eGFP), was established to determine the positional relationships between GnIH and GnRH3 neurons. In these transgenic lines, the mCherry protein was specifically expressed in GnIH neurons, and eGFP was expressed exclusively in GnRH3 neurons. We found that GnIH cell somata were restricted to the posterior periventricular nucleus (NPPv). Most GnIH neuronal processes projected to the hypothalamus, but a few extended to the posterior tuberculum, telencephalon, and olfactory bulb. GnIH neuronal processes were in close apposition with GnRH3 cell somata and processes in the preoptic-hypothalamic area but were seldom in direct contact. However, in the olfactory bulb, GnIH neuronal processes were in proximity to the terminal nerve GnRH3 cell somata. Neither GnIH cell soma nor neuronal processes were detected in the pituitary, although GnIH receptor mRNAs (npffr1l1, npffr1l2, and npffr1l3) were detected. Intraperitoneal administration of GnIH-3 peptides promoted the transcription of brain gnrh3 as well as pituitary fshβ but not lhβ. Thus, GnIH cell somata were specifically distributed in the NPPv, and their fibers extended to the hypothalamus and advanced to the telencephalon and olfactory bulb. We conclude that GnIH may directly stimulate terminal nerve GnRH3 neurons in the zebrafish brain.

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.

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.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

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.

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.

Daily variation of D2 dopamine receptor transcription in the brain of the Japanese eel Anguilla japonica and its regulation with dopamine and melatonin

Dopamine plays a crucial role in controlling reproduction in eels, and its action is mediated through D2-type dopamine receptors. D2A and D2B receptors in the Japanese eel Anguilla japonica were cloned and characterized in the present study. Attention (daily expression patterns in the brain and endogenous regulation) was paid to D2B receptor because it is considered to play a crucial role in eel reproduction. The cDNAs of D2A and D2B receptors had open reading frames comprising 456 and 454 amino acid residues, respectively, which were phylogenetically clustered with those of other teleost species. Both receptors were highly expressed in the brain. D2B receptor transcript levels exhibited high day/low night variation in the midbrain and pituitary, suggesting that its transcription in these tissues is regulated in a daily manner, possibly under influence of melatonin. Intraperitoneal injection of dopamine downregulated D2B receptor transcription significantly in the midbrain and moderately in the pituitary within 1 h, but upregulated its transcription in the forebrain. Co-injection of dopamine with its antagonist (domperidone) reversed the effect of dopamine in the pituitary and forebrain, but not in the midbrain, suggesting that the effect of dopamine on D2B receptor transcription differs among brain regions. The same treatment with melatonin resulted in decreased D2B receptor transcription in the midbrain. These findings indicate that dopamine and melatonin have key roles in the daily variation in D2B receptor transcription in the brain of Japanese eel, and that they are related to a daily base secretion of hormones in the hypothalamic-pituitary-gonadal axis in this species.

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.

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.

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.

New techniques, applications and perspectives in neuropeptide research

Neuropeptides are one of the most diverse classes of signaling molecules and have attracted great interest over the years owing to their roles in regulation of a wide range of physiological processes. However, there are unique challenges associated with neuropeptide studies stemming from the highly variable molecular sizes of the peptides, low in vivo concentrations, high degree of structural diversity and large number of isoforms. As a result, much effort has been focused on developing new techniques for studying neuropeptides, as well as novel applications directed towards learning more about these endogenous peptides. The areas of importance for neuropeptide studies include structure, localization within tissues, interaction with their receptors, including ion channels, and physiological function. Here, we discuss these aspects and the associated techniques, focusing on technologies that have demonstrated potential in advancing the field in recent years. Most identification and structural information has been gained by mass spectrometry, either alone or with confirmations from other techniques, such as nuclear magnetic resonance spectroscopy and other spectroscopic tools. While mass spectrometry and bioinformatic tools have proven to be the most powerful for large-scale analyses, they still rely heavily on complementary methods for confirmation. Localization within tissues, for example, can be probed by mass spectrometry imaging, immunohistochemistry and radioimmunoassays. Functional information has been gained primarily from behavioral studies coupled with tissue-specific assays, electrophysiology, mass spectrometry and optogenetic tools. Concerning the receptors for neuropeptides, the discovery of ion channels that are directly gated by neuropeptides opens up the possibility of developing a new generation of tools for neuroscience, which could be used to monitor neuropeptide release or to specifically change the membrane potential of neurons. It is expected that future neuropeptide research will involve the integration of complementary bioanalytical technologies and functional assays.

Growth Hormone Overexpression Disrupts Reproductive Status Through Actions on Leptin

Growth and reproduction are closely related. Growth hormone (GH)-transgenic common carp exhibit accelerated growth and delayed reproductive development, which provides an amenable model to study hormone cross talk between the growth and reproductive axes. We analyzed the energy status and reproductive development in GH-transgenic common carp by using multi-tissue RNA sequencing, real-time-PCR, Western blotting, ELISA, immunofluorescence, and in vitro incubation. The expression of gys (glycogen synthase) and igfbp1 (insulin-like growth factor binding protein) as well as blood glucose concentrations are lower in GH-transgenic carp. Agrp1 (agouti-related protein 1) and sla (somatolactin a), which are related to appetite and lipid catabolism, are significantly higher in GH-transgenic carp. Low glucose content and increased appetite indicate disrupted metabolic and energy deprivation status in GH-transgenic carp. Meanwhile, the expression of genes, such as gnrhr2 (gonadotropin-releasing hormone receptor 2), gthα (gonadotropin hormone, alpha polypeptide), fshβ (follicle stimulating hormone, beta polypeptide), lhβ [luteinizing hormone, beta polypeptide] in the pituitary, cyp19a1a (aromatase A) in the gonad, and cyp19a1b (aromatase B) in the hypothalamus, are decreased in GH-transgenic carp. In contrast, pituitary gnih (gonadotropin inhibitory hormone), drd1 (dopamine receptor D1), drd3 (dopamine receptor D3), and drd4 (dopamine receptor D4) exhibit increased expression, which were associated with the retarded reproductive development. Leptin receptor mRNA was detected by fluorescence in situ hybridization in the pituitary including the pars intermedia and proximal pars distalis, suggesting a direct effect of leptin on LH. Recombinant carp Leptin protein was shown to stimulate pituitary gthα, fshβ, lhβ expression, and ovarian germinal vesicle breakdown in vitro. In addition to neuroendocrine factors, we suggest that reduced hepatic leptin signaling to the pituitary might be part of the response to overexpression of GH and the resulting delay in puberty onset.

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.