Seasonal variation of the three native gonadotropin-releasing hormone messenger ribonucleic acids levels in the brain of female red seabream

Long-term exposure to aryl hydrocarbon receptor agonist neburon induces reproductive toxicity in male zebrafish (Danio rerio)

Neburon is a phenylurea herbicide that is widely used worldwide, but its toxicity is poorly studied. In our previous study, we found that neburon has strong aryl hydrocarbon receptor (AhR) agonist activity, but whether it causes reproductive toxicity is not clear. In the present study, zebrafish were conducted as a model organism to evaluate whether environmental concentrations of neburon (0.1, 1 and 10 µg/L) induce reproductive disorder in males. After exposure to neburon for 150 days from embryo to adult, that the average spawning egg number in high concentration group was 106.40, which was significantly lower than 193.00 in control group. This result was mainly due to the abnormal male reproductive behavior caused by abnormal transcription of genes associated with reproductive behavior in the brain, such as secretogranin-2a. The proportions of spermatozoa in the medium and high concentration groups were 82.40% and 83.84%, respectively, which were significantly lower than 89.45% in control group. This result was mainly caused by hormonal disturbances and an increased proportion of apoptotic cells. The hormonal disruption was due to the significant changes in the transcription levels of key genes in the hypothalamus-pituitary-gonadal axis following neburon treatment. Neburon treatment also significantly activated the AhR signaling pathway, causing oxidative stress damage and eventually leading to a significant increase in apoptosis in the exposed group. Together, these data filled the currently more vacant profile of neburon toxicity and might provide information to assess the ecotoxicity of neburon on male reproduction at environmentally relevant concentrations.

Long-Term Exposure to Neonicotinoid Insecticide Acetamiprid at Environmentally Relevant Concentrations Impairs Endocrine Functions in Zebrafish: Bioaccumulation, Feminization, and Transgenerational Effects

Neonicotinoid insecticides have attracted worldwide attention due to their ubiquitous occurrence and detrimental effects on aquatic organisms, yet their impacts on fish reproduction during long-term exposure remain unknown. Here, zebrafish (F0) were exposed to a neonicotinoid, acetamiprid, at 0.19-1637 μg/L for 154 d. Accumulation and biotransformation of acetamiprid were observed in adult fish, and the parent compound and its metabolite (acetamiprid-N-desmethyl) were transferred to their offspring. Acetamiprid caused slight survival reduction and significant feminization in F0 fish even at the lowest concentration. Hormone levels in F0 fish were remarkedly altered, that is, gonad 17β-estradiol (E2) significantly increased, while androstenedione decreased. The corresponding transcription of steroidogenic genes (ar, cyp19b, fshβ, gnrh2, gnrh3, and lhβ) were significantly upregulated in the brain and gonad of the females but downregulated in the males. The vtg1 gene expression in the liver of male fish was also upregulated. In addition to F0 fish, parental exposure to acetamiprid decreased hatchability and enhanced malformation of F1 embryos. Chronic exposure to acetamiprid at environmentally relevant concentrations altered hormone production and the related gene expression of the hypothalamic-pituitary-gonad (HPG) axis in a sex-dependent way, caused feminization and reproductive dysfunction in zebrafish, and impaired production and development of their offspring.

Effects of 3,4-dichloroaniline (3,4-DCA) and 4,4'-methylenedianiline (4,4'-MDA) on sex hormone regulation and reproduction of adult zebrafish (Danio rerio)

3,4-dichloroaniline (3,4-DCA) and 4,4'-methylenedianiline (4,4'-MDA) have been widely used in manufacture of many industrial and consumer products, and hence often detected in aquatic environment. Reproductive toxicity of aniline and its derivatives in aquatic organisms has been suggested, however, knowledge on the endocrine disruption potentials and toxicological consequences of both anilines are not well understood, especially in fish. In this study, we aimed to understand the effects of 3,4-DCA and 4,4'-MDA on sex hormone regulation and reproduction of adult zebrafish (Danio rerio). Following 21 d exposure, significant decreases of the reproduction were observed at 0.38 mg/L 3,4-DCA, and 4.6 mg/L 4,4'-MDA. Moreover, plasma concentrations of testosterone (T) and 17β-estradiol (E2) level were significantly decreased in both male and female fish following the exposure. The sex hormone changes could be explained by the regulatory changes of the genes along the hypothalamic-pituitary-gonadal (HPG) axis, including significant down-regulation of steroidogenic acute regulatory protein (star) and cytochrome P450 family 19 subfamily A (cyp19a) genes in the gonad. Moreover, inhibition of gonadotropin hormone signaling and prostaglandin-endoperoxide synthase 2 (ptgs2) gene expression were observed, suggesting potential disruption of oocyte maturation and ovulation by the exposure. Our observations indicate that 3,4-DCA and 4,4'-MDA can impair reproduction of zebrafish potentially through disruption of steroid hormone synthesis and ovulation.

Expression dynamics of the genes for the hypothalamo-pituitary-gonadal axis in tiger puffer (Takifugu rubripes) at different reproductive stages

Tiger puffer, Takifugu rubripes, a commercially important long-distance migratory fish, return to specific spawning grounds for reproduction. To clarify reproductive neuroendocrine system of the tiger puffer, the changes in the expression levels of the genes encoding three gonadotropin-releasing hormones (GnRHs), gonadotropin-inhibitory hormone (GnIH), GnIH receptor (GnIH-R), kisspeptin and kisspeptin receptor in the brain and gonadotropin (GTH) subunits, growth hormone (GH) and prolactin (PRL) in the pituitary were examined in the tiger puffer captured in the wild at different reproductive stages, namely immature and mature fish of both sexes, and post-ovulatory females that were obtained by hormonal treatment. The amounts of three gnrh mRNAs, gnih, gnih-r, fshb and lhb were substantially increased in the mature fish compared to the immature fish, especially in the females, and these augmented expressions were drastically decreased in the post-ovulatory females. gh expression showed a slight increase in the mature males. In contrast, kiss2, kiss2r and prl did not show significant changes in the males but significantly decreased in the post-ovulatory females. The present results demonstrate the expression dynamics of the hypothalamo-pituitary-gonadal axis genes associated with the reproductive conditions and the possible involvement of the GnRH/GnIH/GTH system in the regulation of the sexual maturation and spawning in the wild tiger puffer.

Molecular mechanism of reproductive toxicity induced by beta-cypermethrin in zebrafish

Beta-cypermethrin, a type II synthetic pyrethroid insecticide, is widely used in pest control. Several studies have demonstrated that beta-cypermethrin can affect the reproductive system of mammals. However, there is still a scarcity of information about the reproductive toxicity to fish induced by beta-cypermethrin and its molecular mechanism. Therefore, this study was conducted to address this scientific question, in which the adult zebrafish were exposed to 0 (blank control), 0 (acetone solvent control), 0.1, 0.5, and 2.5 μg/L of beta-cypermethrin for 21 days. A decrease in cumulative egg production of zebrafish was observed, indicating that beta-cypermethrin had a negative impact on reproductive capacity of zebrafish. Regarding the histomorphological analysis of gonads, the delay of gonadal development was observed after exposure for 21 days. In addition, significant changes in plasma 17β-estradiol (E2) and testosterone (T) were found in zebrafish. Further exploration showed that the transcription levels of hypothalamic-pituitary-gonadal (HPG) axis-related genes were remarkably changed, which corresponded well with the alterations of hormone levels. These results demonstrated that beta-cypermethrin might have an adverse effect on the reproduction system of zebrafish through delaying gonadal development, disturbing sex hormone secretion, and affecting HPG axis gene expression. This study suggests that beta-cypermethrin poses a potential threat to the reproduction of fish populations, and the toxicity assessment of beta-cypermethrin plays a vital role in the environmental risk assessment of pesticides.

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.

Underlying mechanisms of reproductive toxicity caused by multigenerational exposure of 2, bromo-4, 6-dinitroaniline (BDNA) to Zebrafish (Danio rerio) at environmental relevant levels

2-bromo-4, 6-dinitroaniline (BDNA) is a mutagenic aromatic amine involved in the production and degradation of Disperse blue 79, one of the most extensively used brominated azo dyes. In our previous study, a multigenerational exposure of BDNA (0.5, 5, 50 and 500 μg/L) to zebrafish from F₀ adult to F₂ larvae including a recovery group in F₂ larvae was conducted. The effects on apical points observed in individuals and the long-term effects predicted on population were all related to reproduction. In this study, we performed molecular analysis to elucidate the underlying mechanisms of the reproductive toxicity of BDNA. In F₁ generation, measurement of vitellogenin and transcription levels of genes associated with hypothalamus-pituitary-gland (HPG) axis, estrogen receptor (ER) and androgen receptor (AR) were conducted. There was a decrease in VTG level in the blood of F₁ female fish and transcription of genes related to ER was more affected than that of genes related to AR. These results were consistent with adverse effects that sexual differentiation was biased towards males and fecundity was impaired in a concentration-dependent manner in adults of F₁ generation after 150 days exposure. In F₂ generation, global gene transcriptions of F₂ larvae were investigated. It was uncovered that processes related to apoptosis, development and DNA damage were strongly affected. Alterations to these biological pathways accounted for the irreversible parental influence on a significant decrease in hatchability and increase in abnormality of F₂ larvae. All evidence suggested that the multigenerational exposure of BDNA posed lasting effects transmitted from parents to offspring that persisted after exposure ceased.

Exposure to perfluorooctane sulfonate based on circadian rhythm changes the fecundity and expression of certain genes on the hypothalamic-pituitary-gonadal-liver axis of female zebrafish

Exposure of a variety of experimental animals to perfluorooctane sulfonate (PFOS) has shown that it is a potent endocrine-disrupting chemical. However, its interaction with the circadian rhythm on responses along the hypothalamic - pituitary - gonadal - liver (HPGL) axis should be of significant value but has not been adequately investigated. In present study, the effects of PFOS on fecundity, levels of estradiol (E₂) and expression of certain genes on the HPGL axis at two time points (8:00 AM and 7:00 PM) were compared after female zebrafish were exposed to 0, 2, 20 and 200 μg/L PFOS for 21 days. In brain, expressions of gonadotropin-releasing hormone (GnRH), gonadotropin-releasing hormone receptor (GnRHr), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were significantly different after the exposure when sampled at 8:00 AM and at 7:00 PM (P < .05). In liver, significant down-regulation of vitellogenin1 (VTG1) and estrogenic receptor α (ERα) were observed at 7:00 PM compared with 8:00 AM (P < .05). In ovary, the level of CYP19 was significantly different at the two time points (P < .05). The increase of E₂ after exposure to 20 μg/L PFOS at 8:00 AM caused compensatory down-regulation of GnRHr and up-regulation of VTG1 and ERα, but not at 7:00 PM. Profiles of concentrations of E₂ and several gene expressions alongside the HPGL axis were different between two times points. The change of E₂ and gene expressions were more perturbed by PFOS at 8:00 AM than at 7:00 PM with circadian rhythm.

The Kiss2/GPR54 system stimulates the reproductive axis in male black porgy, Acanthopagrus schlegelii

Although it is well established that the Kiss1/GPR54 system stimulates the reproductive axis in mammals, its functional roles, especially in male reproduction of non-mammalian species, is less clear. In this study, we have isolated the full-length kiss2 and gpr54 cDNAs from black porgy (Acanthopagrus schlegelii). The Kiss2 precursor expressed from kiss2 comprises 124 amino acids and contains a highly conserved 10-amino acid sequence, Kiss2-10 (FNFNPFGLRF). GPR54 comprises 375 amino acid residues and contains distinct characteristics of G protein-coupled receptors. Real-time PCR analysis indicated that kiss2 and gpr54 were expressed highly in the brain regions. Moreover, intraperitoneal injection of porgy Kiss2-10 could stimulate genes expression of the gpr54, gnrh1, gnrh3, fshβ, lhβ, p450c17, star, and ar, and the serum testerone level in male black porgy. Our findings demonstrate that the Kisspeptin stimulates the male reproductive axis in black porgy.

Molecular cloning and characterization of a gonadotropin-releasing hormone 2 precursor cDNA in the catfish Heteropneustes fossilis: Expression profile and regulation by ovarian steroids

Gonadotropin-releasing hormone 2 (Gnrh2) is one of the three classes of Gnrh distributed in vertebrates and is highly conserved. In the present study, the cDNA encoding Gnrh2 was isolated and characterized in the ostariophysan catfish Heteropneustes fossilis (hf). The cDNA is 611 bp long with an open reading frame (ORF) of 261 bp that encodes a highly conserved protein of 86 amino acids. The deduced Gnrh2 precursor protein clustered with the vertebrate Gnrh2 type. The sequence identity of hfgnrh2 is 94% with African catfish (Clarias gariepinus) gnrh2 mRNA (accession no. X78047). The hfgnrh2 transcripts were expressed only in the brain and gonads with a higher expression in the female brain and ovary in both resting and prespawning phases. The expression was higher in the prespawning phase than the resting phase. The gnrh2 expression in the brain and ovary showed significant seasonal variations but with opposite patterns. In the brain, the expression was the highest in the preparatory phase, decreased progressively to low levels in the postspawning and resting phases. In the ovary, the transcript level was low in the resting and preparatory phases, increased sharply in the prespawning phase reaching the peak level in the spawning phase and declined sharply in the postspawning phase. The gnrh2 mRNA showed the highest expression in the hind brain-medulla oblongata and moderate to low expression in forebrain regions and pituitary. Ovariectomy resulted in a duration-dependent inhibition of hfgnrh2 mRNA levels in the resting and prespawning phases. Steroid (E₂, testosterone and progesterone) replacement treatments (0.5 μg/g body weight) in the 3- week ovariectomized fish restored the inhibition due to ovariectomy, elevated the expression over and above the sham level in the resting phase (E₂ group), and raised the levels almost to that of the sham group (testosterone and progesterone groups) in the prespawning phase. In the sham control groups, the steroid replacement resulted in a significant reduction in the mRNA levels. The expression of the gnrh2 mRNA in the brain-pituitary-gonadal axis and its regulation by gonadal steroids suggest that Gnrh2 may have a reproductive role in the catfish.

Neurotoxicity induced by microcystins and cylindrospermopsin: A review

Microcystins (MCs) and cylindrospermopsin (CYN) are among the most frequent toxins produced by cyanobacteria. These toxic secondary metabolites are classified as hepatotoxins and cytotoxin, respectively. Furthermore, both may present the ability to induce damage to the nervous system. In this sense, there are many studies manifesting the potential of MCs to cause neurotoxicity both in vitro and in vivo, due to their probable capacity to cross the blood-brain-barrier through organic anion transporting polypeptides. Moreover, the presence of MCs has been detected in brain of several experimental models. Among the neurological effects, histopathological brain changes, deregulation of biochemical parameters in brain (production of oxidative stress and inhibition of protein phosphatases) and behavioral alterations have been described. It is noteworthy that minority variants such as MC-LF and -LW have demonstrated to exert higher neurotoxic effects compared to the most studied congener, MC-LR. By contrast, the available studies concerning CYN-neurotoxic effects are very scarce, mostly showing inflammation and apoptosis in neural murine cell lines, oxidative stress, and alteration of the acetylcholinesterase activity in vivo. However, more studies are required in order to clarify the neurotoxic potential of both toxins, as well as their possible contribution to neurodegenerative diseases.

Copper caused reproductive endocrine disruption in zebrafish (Danio rerio)

Cu in surface waters has been demonstrated to affect aquatic animals at ecologically relevant concentrations. However, its effects on reproductive endocrine system and the underlying toxicological mechanisms are largely unknown. In this study, zebrafish (Danio rerio) were exposed to 0, 10, 20, 40 μg/L of Cu for 30 days. Growth, gonad histopathology, the hormone levels and the transcriptional profiles of genes in the hypothalamic-pituitary-gonadal (HPG) axis in both sexes were examined. The results indicated that body weight was significantly reduced, the gonadal development was affected, and the levels of E₂, T and 11-KT were remarkably disturbed in Cu-exposed fish. Moreover, the expression profiles of steroidogenesis-related genes in gonad (3βhsd, 17βhsd, cyp11a1, cyp17, cyp19a, lhr, fshr, hmgra and star) and in brains (ar, cyp19b, erα, er2β, lhβ, fshβ, gnrh2, gnrh3, gnrhr1, gnrh2 and gnrh4) displayed alterations after exposure to Cu. These results demonstrated that Cu could suppress the growth of zebrafish and significantly affect the reproductive biology in both sexes by damaging the structure of the gonads, altering the steroid hormone levels and the expressions of endocrine-related genes in HPG of zebrafish. This study suggests that Cu adversely affects the reproductive endocrine system in zebrafish and could pose a potential threat to fish populations inhabiting Cu-contaminated waters.

Biological response of zebrafish after short-term exposure to azoxystrobin

Azoxystrobin (AZ) is a broad-spectrum systemic fungicide that widely used in the world. The present study investigated the toxicity effects on zebrafish after short-term exposure of AZ. Results demonstrated that the larval stage was most susceptible to AZ in the multiple life stages of zebrafish, with 96 h-LC₅₀ value of 0.777 mg/L. Zebrafish larvae were exposed to different AZ concentrations (0, 0.1, 1, 10, 100 μg/L) and examined on 24, 48 and 72 h. It was found that AZ induced ROS accumulation, increased GST, GPX and POD activity and the transcriptions of antioxidant and stress response related genes, while the opposite trend occurred for SOD and CAT activity in 24-h or 48-h exposure period. The increased E₂ and VTG levels in zebrafish larvae, and altered transcription levels of regulatory and steroidogenic genes in the hypothalamus-pituitary-gonad (HPG) axis indicated the endocrine disruption capacity of AZ. The transcripts of mdm2, p53, ogg1, bcl2, bbc3, cas8 and cas9 involved in cell apoptosis, and the mRNA levels of cytokines and chemokines such as cxcl-c1c, ccl, il-1β, il-8, ifn, and tnfα were in accordance with the trends of the examined genes involved in oxidative stress and endocrine system. The results suggested that short-term exposure to AZ might impose ecotoxicological effects on zebrafish larvae, and the information presented here also provide molecular strategies and increase mechanistic understanding of AZ-induced toxic response, and help elucidate the environmental risks of AZ.

Synergistic potential of fenvalerate and triadimefon on endocrine disruption and oxidative stress during rare minnow embryo development

Pyrethroids have been reported to interact synergistically when co-exposed with azoles fungicides in different organisms. In the present study, we investigated the mixture toxicity of fenvalerate (FEN) and triadimefon (TDF) toward embryos of Gobiocypris rarus after 96 h exposure. Results demonstrated that TDF enhanced the acute toxicity of FEN. Exposure to binary mixtures of FEN and TDF resulted in synergistic responses of endocrine disruption by inducing the transcripts of several genes including vtg, erα, erβ1, erβ2, cyp19a, cyp1a, cyp4, cyp11a, gnrh3, gnrhr1a, star, and dmrt1. Furthermore, FEN and TDF mixture increased the VTG level and aromatase activity in rare minnow embryos. FEN and TDF co-exposure also regulated the mRNA of vezf, hsp70, p53, gadd45α, induced the synthesis of ROS and activity of GST, suggesting the synergistic potential of oxidative stress induced by FEN and TDF co-exposure. The results indicated that binary mixtures of FEN and TDF could simultaneously induce endocrine disruption and oxidative stress in a synergistic manner during rare minnow embryo development.

Effects of methomyl on steroidogenic gene transcription of the hypothalamic-pituitary-gonad-liver axis in male tilapia

Male tilapia were exposed to sub-lethal methomyl concentrations of 0, 0.2, 2, 20 or 200 μg/L for 30 d, and were subsequently cultured in methomyl-free water for 18 d. Relative transcript abundance of steroidogenic genes involved in the HPGL axis of male tilapia was examined at 30 d in the exposure test and at 18 d in the recovery test. The results revealed that low concentrations of methomyl (0.2 and 2 μg/L) did not cause significant changes in gene mRNA levels in the HPGL axis of male tilapia; thus, we considered 2 μg/L concentrations as the level that showed no apparent adverse endocrine disruption effects. However, higher concentrations of methomyl (20 and 200 μg/L) disrupted the endocrine system and caused significant increase in the levels of GnRH2, GnRH3, ERα, and ERβ genes in the hypothalamus, GnRHR and FSHβ genes in the pituitary, CYP19a, FSHR, and ERα genes in the testis, and VTG and ERα genes in the liver, and significantly decreased the levels of LHR, StAR, 3β-HSD, and ARα genes in the testis and LHβ gene in the pituitary, leading to changes in sex steroid hormone and vitellogenin levels in the serum and ultimately resulting in reproductive dysfunction in male tilapia. The recovery tests showed that the toxicity effect caused by 20 μg/L methomyl was reversible; however, the toxicity effect at 200 μg/L of methomyl was irreversible after 18 d. Therefore, we concluded that 200 μg/L was the threshold concentration for methomyl-induced irreversible endocrine disruption in male tilapia.

Development of a homologous radioimmunoassay for red seabream follicle stimulating hormone and regulation of gonadotropins by GnRH in red seabream, Pagrus major

Using a recombinant chimeric single-chain follicle stimulating hormone (FSH), we established a radioimmunoassay (RIA) for red seabream (Pagrus major) FSH (pmFSH) which became a powerful tool for studying reproductive physiology. We studied the profiles in plasma and pituitary concentrations of FSH and luteinizing hormone (LH) during sexual maturation. A pre-established RIA for red seabream LH was used for the LH measurements. The regulation of FSH and LH secretion from the pituitary was investigated using a gonadotropin-releasing hormone analog (GnRHa) in vivo and in vitro. Marked differences in plasma and pituitary FSH levels were observed between males and females; pituitary FSH content in males was much higher than that in females during all seasons, and plasma FSH levels in males were high during the spawning season, whereas those in females were unchanged. In contrast, plasma and pituitary levels of LH were elevated before and during the spawning season in males and females. Injecting or implanting (cholesterol pellet) a GnRHa into adult and juvenile red seabream resulted in significant increases in plasma LH concentrations; however, no significant change was observed in plasma FSH. Moreover, GnRHa stimulated only LH secretion in an in vitro experiment using dispersed pituitary cells. The discrete FSH and LH secretion profiles revealed suggest differential roles for the two gonadotropins during red seabream gametogenesis. In addition, the marked difference in pituitary FSH levels in males and females suggests the relative significance of FSH in male reproduction.

Microcystin-LR impairs zebrafish reproduction by affecting oogenesis and endocrine system

Previous studies have shown that microcystins (MCs) are able to exert negative effects on the reproductive system of fish. However, few data are actually available on the effects of MC-LR on the reproductive system of female fish. In the present study, female zebrafish were exposed to 2, 10, and 50 μg L(-1) of MC-LR for 21 d, and its effects on oogenesis, sex hormones, transcription of genes on the hypothalamic-pituitary-gonad (HPG) axis, and reproduction were investigated for the first time. It was observed that egg production significantly declined at ⩾ 10 μg L(-1) MC-LR. MC-LR exposure to zebrafish increased the concentrations of 17β-estradiol (E2) and vitellogenin (VTG) at 10 μg L(-1) level, whereas concentrations of E2, VTG and testosterone declined at 50 μg L(-1) MC-LR. The transcriptions of steroidogenic pathway gene (cyp19a, cyp19b, 17βhsd, cyp17 and hmgra) changed as well after the exposure and corresponded well with the alterations of hormone levels. A number of intra- and extra-ovarian factors, such as gnrh3, gnrhr1, fshβ, fshr, lhr, bmp15, mrpβ, ptgs2 and vtg1 which regulate oogenesis, were significantly changed with a different dose-related effect. Moreover, MC-LR exposure to female zebrafish resulted in decreased fertilization and hatching rates, and may suggest the possibility of trans-generational effects of MC-LR exposure. The results demonstrate that MC-LR could modulate endocrine function and oogenesis, eventually leading to disruption of reproductive performance in female zebrafish. These data suggest there is a risk for aquatic population living in MC polluted areas.

Changes in the gene expression profiles of the brains of male European eels (Anguilla anguilla) during sexual maturation

Background

The vertebrate brain plays a critical role in the regulation of sexual maturation and reproduction by integrating environmental information with developmental and endocrine status. The European eel Anguilla anguilla is an important species in which to better understand the neuroendocrine factors that control reproduction because it is an endangered species, has a complex life cycle that includes two extreme long distance migrations with both freshwater and seawater stages and because it occupies a key position within the teleost phylogeny. At present, mature eels have never been caught in the wild and little is known about most aspects of reproduction in A. anguilla. The goal of this study was to identify genes that may be involved in sexual maturation in experimentally matured eels. For this, we used microarrays to compare the gene expression profiles of sexually mature to immature males.

Results

Using a false discovery rate of 0.05, a total of 1,497 differentially expressed genes were identified. Of this set, 991 were expressed at higher levels in brains (forebrain and midbrain) of mature males while 506 were expressed at lower levels relative to brains of immature males. The set of up-regulated genes includes genes involved in neuroendocrine processes, cell-cell signaling, neurogenesis and development. Interestingly, while genes involved in immune system function were down-regulated in the brains of mature males, changes in the expression levels of several receptors and channels were observed suggesting that some rewiring is occurring in the brain at sexual maturity.

Conclusions

This study shows that the brains of eels undergo major changes at the molecular level at sexual maturity that may include re-organization at the cellular level. Here, we have defined a set of genes that help to understand the molecular mechanisms controlling reproduction in eels. Some of these genes have previously described functions while many others have roles that have yet to be characterized in a reproductive context. Since most of the genes examined here have orthologs in other vertebrates, the results of this study will contribute to the body of knowledge concerning reproduction in vertebrates as well as to an improved understanding of eel biology.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-799) contains supplementary material, which is available to authorized users.

Central pathways integrating metabolism and reproduction in teleosts

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

High water temperature impairs ovarian activity and gene expression in the brain-pituitary-gonadal axis in female red seabream during the spawning season

Temperature plays a pivotal role in the control of seasonal reproduction in temperate fish species. It is well known that temperatures that exceed a certain threshold impair gonadal development, maturation, and spawning. However, the endocrine mechanisms that underlie these effects are poorly understood. We evaluated the effect of high water temperature on the brain-pituitary-gonadal (B-P-G) axis of a perciform fish, red seabream, Pagrus (Chrysophrys) major during its spawning season (April-May). Fish were reared at either high (24 °C: H-group) or optimal/control (17 °C: C-group) temperatures for 5 or 10 d. After 5 d, the transcript abundance of gonadotropin-releasing hormone-1 (GnRH1) in the brain and GnRH receptor (GnRH-R) and FSH-β in the pituitary were significantly lower in H-group than in C-group. Conversely, there was no difference in pituitary LH-β mRNA levels, serum concentrations of estradiol-17β (E₂), or the gonadosomatic index (GSIs) between H- and C-groups on Day 5. After 10 d, the ovaries of all H-group fish had completely regressed and were filled with only perinucleolar stage primary oocytes and atretic oocytes. The brain GnRH1 expression, pituitary GnRH-R and pituitary LH-β expression, serum E₂ concentrations, and the GSI were significantly lower in the H-group on Day 10. Our results suggest that high water temperature is the proximate driver of the termination of the spawning season of female red seabream. The effect appears to be mediated by suppression of gene expression in the B-P-G axis.

Effects of non-steroidal anti-inflammatory drugs on hormones and genes of the hypothalamic-pituitary-gonad axis, and reproduction of zebrafish

This study was conducted in two experiments, to identify non-steroidal anti-inflammatory drugs (NSAIDs) with high endocrine disruption potentials, and to understand consequences of exposure to such NSAIDs in fish. In the first experiment, the effects of five NSAIDs on hormones and gene transcriptions of the hypothalamic-pituitary-gonad (HPG) axis were evaluated after 14 d exposure of adult zebrafish. Ibuprofen and mefenamic acids were identified to increase the concentrations of 17β-estradiol and testosterone in females significantly, while decreased those of testosterone among male fish. Significant up-regulation of fshβ, lhβ, fshr and lhr were observed in females, whereas down-regulation was observed in males exposed to each NSAID. In the second experiment, ibuprofen was chosen as a model chemical. Adult zebrafish pairs were exposed to ibuprofen for 21 d, and the effects on reproduction and development of offspring were examined. The egg production was significantly decreased at ≥1 μg/L ibuprofen, and parental exposure resulted in delayed hatching even when they were transferred to clean water for hatching. The results demonstrated that ibuprofen could modulate hormone production and related gene transcription of the HPG axis in a sex-dependent way, which could cause adverse effects on reproduction and the development of offspring.

Effects of TDCPP or TPP on gene transcriptions and hormones of HPG axis, and their consequences on reproduction in adult zebrafish (Danio rerio)

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and triphenyl phosphate (TPP) belong to the group of triester organophosphate flame retardants (OPFRs), which have been used in a wide range of consumer products. These chemicals have been frequently detected in effluents, surface water, and fish, and hence their potential adverse effects on aquatic ecosystem are of concern. The present study was conducted to investigate the reproduction-related effects and possible molecular mechanisms of TDCPP and TPP using a 21 day reproduction test employing adult zebrafish (Danio rerio). After 21 d of exposure to TDCPP or TPP, significant decrease in fecundity along with significant increases of plasma 17β-estradiol (E2) concentrations, vitellogenin (VTG) levels, and E2/testosterone (T) and E2/11-ketotestosterone (11-KT) ratios were observed. The transcriptional profiles of several genes of the hypothalamus-pituitary-gonad (HPG) axis changed as well after the exposure, but the trend was sex-dependent. In male fish, gonadotropin-releasing hormone2 (GnRH2), GnRHR3, cytochrome P450 (CYP) 19B, estrogen receptor α (ERα), ER2 β1, and follicle stimulating hormone β (FSHβ) were upregulated in the brain, while luteinizing hormone β (LHβ) and androgen receptor (AR) were downregulated. Corresponding to the upregulation of FSHβ and downregulation of LHβ in the brain, FSHR was upregulated and LHR was downregulated in the testis. Among the genes that regulate the steroidogenesis pathway, transcription of hydroxyl methyl glutaryl CoA reductase (HMGRA), steroidogenic acute regulatory protein (StAR), and 17β-hydroxysteroid dehydrogenase (17βHSD) decreased, while transcription of CYP11A, CYP17, and CYP19A increased. In female fish, transcription ofGnRH2 and GnRHR3 decreased, but FSHβ, LHβ, CYP19B, ERα, ER2β1, and AR transcription increased in the brain. In the ovary, FSHR and LHR were significantly upregulated, and most steroidogenic genes were significantly upregulated. The observed disruption of GnRH and GtHs could be further related to subsequent disruption in both sex steroid hormone balance and plasma VTG levels, as well as reproductive performance. Overall, our observation indicates that both TDCPP and TPP could disturb the sex hormone balance by altering regulatory mechanisms of the HPG axis, eventually leading to disruption of reproductive performance in fish.

Characterization of Pubertal Development Phases in Female Longtooth Grouper, Epinephelus bruneus via Classification of Bodyweight

Puberty is the developmental period which animals obtain the ability of reproducing sexually for the first time in life. In commercially important aquaculture fish species, the onset of puberty is a matter of major interest due to controlling of sexual maturation to improve broodstock management. To investigate pubertal characteristics of female longtooth grouper (Epinephelus bruneus), specimens were classified into three groups by the bodyweight, including 1, 2, and 3 kg group. Thereafter, we focused on ovarian development and level changes of endocrine regulation factors (GnRH, GTHs, steroid hormone). In the non-breeding season (April), the levels of endocrine regulation factors showed increasing trends in accordance with bodyweight gaining; nevertheless, the oocytes were growth phase belongs to almost peri-nucleous stages in all groups. In the breeding season (June), the levels of endocrine regulation factors were fluctuated that decreases in levels of sbGnRH and FSHβ mRNA expressions along with serum E₂ concentrations in 3 kg of group. However, LHβ mRNA expression levels sustained increasing trends by the bodyweight. Moreover, the oocytes developed that 2 kg and 3 kg groups obtained plentiful vitellogenic oocytes while 1 kg group was still composed with greater part of pre-vitellogenic oocytes. Especially, the oocytes of 3 kg group reached over 450 μm of diameters that indicating possibility to enter the final maturations. These results suggest that the progress of pubertal development in female E. bruneus could be classify into three phases via bodyweight, including pre-puberty (1 kg), early-puberty (2 kg) and puberty (3 kg).

Kisspeptin and seasonal control of reproduction in male European sea bass (Dicentrarchus labrax)

In the present study, we developed and validated real-time quantitative RT-PCR assays for a suite of genes involved in the brain-pituitary gonadal axis in fish including kisspeptin genes and its receptor (Kiss1, kiss2, kissr4) and gonadotropin-releasing hormone genes (sbGnRH, sGnRH, cGnRHII) in the brain, and gonadotropin genes (fshβ and lhβ) in the pituitary. Sex steroid profiles (T and 11-KT) and gonadal development were also studied over a full annual reproductive cycle in adult male sea bass. The cDNA partial sequence of sea bass kissr4 encoding 185 amino acids showed a high degree of conservation with other fish kissr4 subtype. Results clearly showed a seasonal profile for Kiss1, kiss2 and kissr4 mRNAs. Kissr4, fshβ and lhβ levels increased gradually and peaked during spermatogenesis (January) while Kiss1, kiss2, cGnRH-II as well as steroids showed peaks during early spawning (March). No significant seasonal changes were observed for sbGnRH and sGnRH expression. These results support the possible involvement of the kiss genes and their receptor (kissr4) in the seasonal control sea bass reproduction. However, a lack of correlation between kiss genes and sbGnRH expression and the mismatch between kisspeptin and the onset of gonadotropin surge contrast with previous findings.

Central distribution of kiss2 neurons and peri-pubertal changes in their expression in the brain of male and female red seabream Pagrus major

kisspeptins that are encoded by kiss1 gene are now considered the key regulator of reproduction from a number of studies in mammals. In most vertebrates, a paralogue of kiss1, called kiss2, is also present, and the functional significance of kisspeptins is not known precisely. In the present study, we have cloned kiss2 from a perciform teleost, the red seabream Pagrus major. The amino acid sequence deduced from the red seabream kiss2 contained a highly conserved 10-amino-acid residue, Kiss2(10) or kp-10. A kiss1-like transcript was also identified, but it appears to be non-functional due to the presence of a "premature" stop codon. Neurons that express kiss2 mRNA were distributed in the dorsal (NRLd) and ventral (NRLv) parts of nucleus recessi lateralis in the hypothalamus. In some fish a few kiss2-expressing neurons were detected in the preoptic area and nucleus ventralis tuberis. The number of kiss2-expressing neurons in the NRLd was larger during the first spawning season in both males and females compared with fish in the post-spawning periods. In males the number of kiss2 neurons in the NRLd of maturing fish was also larger than those in the post-spawning periods. In males the number of kiss2 neurons in the NRLv showed a similar pattern of changes to that of NRLd, while significant changes were not detected for females. The numbers of gonadotropin-releasing hormone 1 (GnRH1)-immunoreactive neurons in the preoptic area showed a similar pattern of change as those of kiss2 cells of the NRLd in both males and females (and also the NRLv in males). These results are in good agreement with the hypothesis that kiss2 neurons are involved in pubertal processes via regulatory influences on GnRH1 neurons in red seabream.

Mechanisms of oocyte development in European sea bass (Dicentrarchus labrax L.): investigations via application of unilateral ovariectomy

Unilateral ovariectomy (ULO) was performed in European sea bass (Dicentrarchus labrax L.) during late pre-vitellogenesis/early vitellogenesis. Plasma steroid levels and the expression of a suite of potential oogenesis-relevant genes in the ovary, brain, and pituitary were evaluated with the aim of understanding their involvement in the compensatory oocyte development occurring within the remaining ovarian lobe. After 69 days of surgery the remaining ovarian lobe in ULO fish was gravimetrically equivalent to an intact-paired ovary of sham operated, control fish. This compensatory ovarian growth was based on an increased number of early perinucleolar oocytes and mid-late stage vitellogenic follicles without an apparent recruitment of primary oocytes into the secondary growth phase. Plasma steroid levels were similar in ULO and control females at all time points analyzed, suggesting an increased steroid production of the remaining ovarian lobe in hemi-castrated females. Results of the gene expression survey conducted indicate that the signaling pathways mediated by Fsh and Gnrh1 constitute the central axes orchestrating the observed ovarian compensatory growth. In addition, steroid receptors, Star protein, Igfs, and members of the transforming growth factor beta superfamily including anti-Mullerian hormone and bone morphogenetic protein 4 were identified as potentially relevant players within this process, although their specific actions and interactions remain to be established. Our results demonstrate that ULO provides an excellent in vivo model for elucidating the interconnected endocrine and molecular mechanisms controlling oocyte development in European sea bass.

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

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

Differential expression of three types of gonadotropin-releasing hormone genes during the spawning season in grass puffer, Takifugu niphobles

Grass puffer, Takifugu niphobles, has unique spawning behavior; spawning occurs on beach only for several days around new moon and full moon from spring to early summer. To investigate the role of gonadotropin-releasing hormone (GnRH) in the reproductive function, genes encoding three types of GnRHs, namely seabream GnRH (sbGnRH), chicken GnRH-II (cGnRH-II) and salmon GnRH (sGnRH), were cloned and changes in their mRNA amounts were examined over the spawning season. In addition, changes in the pituitary gonadotropin subunit mRNAs and the plasma steroid hormones were examined over the spawning season. Fishes were assessed at four reproductive stages, i.e., in December (early maturation), in April (maturing), in May (spawning), and in July (post-spawning). Moreover, spawning fish just after releasing eggs and sperm were taken at a spawning bed. The amounts of sbGnRH mRNA were substantially elevated in May and the spawning fish in both sexes, concomitant with considerable elevations of follicle-stimulating hormone and luteinizing hormone beta subunit mRNAs and plasma estradiol-17beta (E(2)) and testosterone (T) levels. There were strong positive correlations between the sbGnRH mRNA and the plasma E(2) and T levels over the spawning season in both sexes. The amounts of cGnRH-II mRNA showed no noticeable changes except for an increase in the post-spawning females. The amounts of sGnRH mRNA in the males were significantly increased in May, but they were low in the spawning males. In the females, sGnRH mRNA increased from the maturing stage and reached a maximum in the post-spawning stage, in which a positive correlation with the plasma cortisol levels was observed. These specific changes suggest that the expression of three types of GnRH genes is differentially regulated during the spawning season, and sex steroids may be important for the differential expression of GnRH genes.

Immunological characterization and distribution of three GnRH forms in the brain and pituitary gland of chub mackerel (Scomber japonicus)

The presence of three gonadotropin-releasing hormone (GnRH) forms in the brain of the chub mackerel, Scomber japonicus, namely, salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and seabream GnRH (sbGnRH), was confirmed by combined high performance liquid chromatography (HPLC) and time-resolved fluoroimmunoassay (TR-FIA). Immunocytochemical localization of the three GnRH forms in the brain was Investigated by using specific antisera, to elucidate possible roles of each GnRH form in reproduction in this species, and double immunolabeling was used to localize GnRH-ir (immunoreactive) fibers Innervating the pituitary. sGnRH-ir neurons were localized in the ventral olfactory bulb and terminal nerve ganglion region. Further, sGnRH-ir fibers were found in different regions of the brain, with prominent fibers running in parallel in the preoptic area (POA) without entering the pituitary. cGnRH-II-ir cell bodies were observed only in the midbrain tegmentum region, with a wide distribution of fibers, which were dense in the midbrain tegmentum and spinal cord. SbGnRH-ir cell bodies were localized in the nucleus preopticus of the POA, with fibers in the olfactory bulb, POA, and hypothalamus. Among the three GnRH forms, only SbGnRH-ir fibers innervated the pituitary gland from the preoptic-hypothalamic region, targeting follicle stimulating hormone (FSH) and luteinizing hormone (LH)-producing cells in the proximal pars distalis, as demonstrated by double immunocytochemistry. The localization of the GnRH-ir system was similar in male and female fish. These results demonstrate that multiple GnRH forms exist in the brain of the chub mackerel and suggest that they serve different functions, with SbGnRH having a significant role in reproduction in stimulating FSH- and LH-producing cells, and sGnRH and cGnRH-II serving as neurotransmitters or neuromodulators.

Neuroendocrinology of reproduction in teleost fish

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

Influence of ethynylestradiol and methyltestosterone on the hypothalamo-hypophyseal-gonadal axis of adult air-breathing catfish, Clarias gariepinus

Adult male and female air-breathing catfish Clarias gariepinus were treated with ethynylestradiol (EE(2)) and methyltestosterone (MT) at concentrations of 1microg/L, respectively for 21 days. EE(2) treatment caused disappearance of spermatids/sperm from several testicular lumen/lobules in males while MT treatment to females led to precocious ovarian development. EE(2) caused significant fluid retention in all tissues including peritoneal cavity and seminal vesicles. Immunocytochemical localization of catfish GnRH (cfGnRH) and luteinizing hormone (LH) in preoptic area-hypothalamus (POA-H) and pituitary, respectively, revealed decreased immunoreactivity (ir-) following EE(2) treatment in males. MT treatment however caused no observable change in cfGnRH ir- and a significant increase in LH ir- in females. Semi-quantitative RT-PCR analysis indicated that cfGnRH transcripts in POA-H decreased significantly following EE(2) and MT treatment in males and females, respectively. Levels of POA-H dopamine (inhibitory monoamine for gonadotropin [GTH] synthesis and release) increased following EE(2) and MT treatment in males and females while levels of serotonin and norepinephrine (GTH-stimulatory monoamines) decreased significantly. The results demonstrate a direct in vivo effect of sex steroid analogs on cfGnRH-LH axis and monoaminergic system vis-à-vis on gonads in addition to probable direct action on gonads.

Defining global neuroendocrine gene expression patterns associated with reproductive seasonality in fish

BACKGROUND

Many vertebrates, including the goldfish, exhibit seasonal reproductive rhythms, which are a result of interactions between external environmental stimuli and internal endocrine systems in the hypothalamo-pituitary-gonadal axis. While it is long believed that differential expression of neuroendocrine genes contributes to establishing seasonal reproductive rhythms, no systems-level investigation has yet been conducted.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, by analyzing multiple female goldfish brain microarray datasets, we have characterized global gene expression patterns for a seasonal cycle. A core set of genes (873 genes) in the hypothalamus were identified to be differentially expressed between May, August and December, which correspond to physiologically distinct stages that are sexually mature (prespawning), sexual regression, and early gonadal redevelopment, respectively. Expression changes of these genes are also shared by another brain region, the telencephalon, as revealed by multivariate analysis. More importantly, by examining one dataset obtained from fish in October who were kept under long-daylength photoperiod (16 h) typical of the springtime breeding season (May), we observed that the expression of identified genes appears regulated by photoperiod, a major factor controlling vertebrate reproductive cyclicity. Gene ontology analysis revealed that hormone genes and genes functionally involved in G-protein coupled receptor signaling pathway and transmission of nerve impulses are significantly enriched in an expression pattern, whose transition is located between prespawning and sexually regressed stages. The existence of seasonal expression patterns was verified for several genes including isotocin, ependymin II, GABA(A) gamma2 receptor, calmodulin, and aromatase b by independent samplings of goldfish brains from six seasonal time points and real-time PCR assays.

CONCLUSIONS/SIGNIFICANCE

Using both theoretical and experimental strategies, we report for the first time global gene expression patterns throughout a breeding season which may account for dynamic neuroendocrine regulation of seasonal reproductive development.

mRNA expression of GnRH variants and receptors in the brain, pituitary and ovaries of pejerrey (Odontesthes bonariensis) in relation to the reproductive status

The present study examined the differential mRNA expression levels of three forms of GnRH (sGnRH, pjGnRH and cGnRH-II) and two forms of GnRH receptor (pjGnRH-R I and pjGnRH-R II) in the brain, pituitary, and ovaries of pejerrey in relation to the reproductive status. The analysis revealed the presence of significant amounts of mRNA of the three GnRH forms while the ovaries showed only two (sGnRH and pjGnRH). The GnRH receptor II was found ubiquitously in the brain, pituitary, and ovaries while the form I was detected only in the brain. The levels of pjGnRH mRNA in the brain and pjGnRH-R II in the pituitary gland varied in correlation with the ovarian condition. However, brain sGnRH and pjGnRH-R I mRNA levels reached a maximum during early stages of ovarian development. In contrast, the brain levels of cGnRH-II mRNA showed no variation. The present study also shows a good correlation of ovarian sGnRH and pjGnRH-R II mRNA levels with the reproductive condition, suggesting that these molecules are may be involved in the regulation of pejerrey ovarian function.

Effects of light and temperature conditions on the expression of GnRH and GtH genes and levels of plasma steroids in Odontesthes bonariensis females

In this study we examined the endocrine mediation between environmental factors (temperature and photoperiod) and the brain-pituitary-gonadal axis in females of pejerrey Odontesthes bonariensis. Changes in the expression of brain gonadotropin-releasing hormones (GnRHs) and gonadotropin (GtH) subunit [follicle stimulating-beta (FSH-beta), luteinizing hormone-beta (LH-beta), glycoprotein hormone-alpha (GPH-alpha)] genes, plasma gonadal steroids [estradiol (E(2)) and testosterone (T)], gonadal histology, and gonadosomatic index (GSI) in adult females exposed to combinations of short-day (8 h) or long-day (16 h) photoperiods and low (12 degrees C) or high (20 degrees C) temperatures after winter conditions (8 h light, 12 degrees C) were analyzed. Pejerrey females kept under the short photoperiod had low GSIs, and their ovaries contained only previtellogenic oocytes regardless of the experimental temperature. In contrast, females exposed to the long photoperiod had high GSIs and ovaries with vitellogenic oocytes at both temperatures. These fish also showed a significantly higher expression of sGnRH, pjGnRH, cGnRH-II (the three different GnRH variants found to date in the pejerrey brain), FSH-beta, LH-beta and GPH-alpha genes and plasma E(2 )levels than those at the shorter photoperiod. No significant changes were observed in plasma T levels. Based on these results, we concluded that the increase in day length but not that of temperature triggers the maturation of pejerrey females after the winter period of gonadal rest and that this occurs by an integrated stimulation of the various components of the brain-pituitary-gonad axis.

Sex and seasonal co-variation of arginine vasotocin (AVT) and gonadotropin-releasing hormone (GnRH) neurons in the brain of the halfspotted goby

Gonadotropin-releasing hormone (GnRH) and arginine vasotocin (AVT) are critical regulators of reproductive behaviors that exhibit tremendous plasticity, but co-variation in discrete GnRH and AVT neuron populations among sex and season are only partially described in fishes. We used immunocytochemistry to examine sexual and temporal variations in neuron number and size in three GnRH and AVT cell groups in relation to reproductive activities in the halfspotted goby (Asterropteryx semipunctata). GnRH-immunoreactive (-ir) somata occur in the terminal nerve, preoptic area, and midbrain tegmentum, and AVT-ir somata within parvocellular, magnocellular, and gigantocellular regions of the preoptic area. Sex differences were found among all GnRH and AVT cell groups, but were time-period dependent. Seasonal variations also occurred in all GnRH and AVT cell groups, with coincident elevations most prominent in females during the peak- and non-spawning periods. Sex and temporal variability in neuropeptide-containing neurons are correlated with the goby's seasonally-transient reproductive physiology, social interactions, territoriality and parental care. Morphological examination of GnRH and AVT neuron subgroups within a single time period provides detailed information on their activities among sexes, whereas seasonal comparisons provide a fine temporal sequence to interpret the proximate control of reproduction and the evolution of social behavior.

Identification of a gonadotropin-releasing hormone receptor orthologue in Caenorhabditis elegans

BACKGROUND

The Caenorhabditis elegans genome is known to code for at least 1149 G protein-coupled receptors (GPCRs), but the GPCR(s) critical to the regulation of reproduction in this nematode are not yet known. This study examined whether GPCRs orthologous to human gonadotropin-releasing hormone receptor (GnRHR) exist in C. elegans.

RESULTS

Our sequence analyses indicated the presence of two proteins in C. elegans, one of 401 amino acids [GenBank: NP_491453; WormBase: F54D7.3] and another of 379 amino acids [GenBank: NP_506566; WormBase: C15H11.2] with 46.9% and 44.7% nucleotide similarity to human GnRHR1 and GnRHR2, respectively. Like human GnRHR1, structural analysis of the C. elegans GnRHR1 orthologue (Ce-GnRHR) predicted a rhodopsin family member with 7 transmembrane domains, G protein coupling sites and phosphorylation sites for protein kinase C. Of the functionally important amino acids in human GnRHR1, 56% were conserved in the C. elegans orthologue. Ce-GnRHR was actively transcribed in adult worms and immunoanalyses using antibodies generated against both human and C. elegans GnRHR indicated the presence of a 46-kDa protein, the calculated molecular mass of the immature Ce-GnRHR. Ce-GnRHR staining was specifically localized to the germline, intestine and pharynx. In the germline and intestine, Ce-GnRHR was localized specifically to nuclei as revealed by colocalization with a DNA nuclear stain. However in the pharynx, Ce-GnRHR was localized to the myofilament lattice of the pharyngeal musculature, suggesting a functional role for Ce-GnRHR signaling in the coupling of food intake with reproduction. Phylogenetic analyses support an early evolutionary origin of GnRH-like receptors, as evidenced by the hypothesized grouping of Ce-GnRHR, vertebrate GnRHRs, a molluscan GnRHR, and the adipokinetic hormone receptors (AKHRs) and corazonin receptors of arthropods.

CONCLUSION

This is the first report of a GnRHR orthologue in C. elegans, which shares significant similarity with insect AKHRs. In vertebrates, GnRHRs are central components of the reproductive endocrine system, and the identification of a GnRHR orthologue in C. elegans suggests the potential use of C. elegans as a model system to study reproductive endocrinology.

Increased leptin may be involved in the short life span of ayu (Plecoglossus altivelis)

It is well known that ayu (Plecoglossus altivelis) die after spawning and have a life span of only 1 year. The determinants for such a short life span are probably connected with spawning and related changes in hormonal homeostases. One of these changes is that the ayu's feeding activity decreases both during and after spawning. We investigated the relationships among leptin, one of the regulators of food intake, and two other major hormones, 17 beta-estradiol and prolactin (PRL). Ir-leptin levels were significantly higher during spawning, and were associated with a decrease in appetite. Ir-leptin levels were also synchronized with levels of 17 beta-estradiol and PRL-like protein. Therefore, one possible explanation for the decrease in appetite during ayu spawning is that the elevation of 17 beta-estradiol homeostasis induced the secretion of Ir-leptin. The inability to decrease leptin to the basal levels because of high estrogen after spawning could be in part responsible for the short life span of ayu.

Changes in brain GnRH mRNA and pituitary GnRH peptide during testicular maturation in barfin flounder

The pleuronectid barfin flounder (Verasper moseri) expresses three forms of gonadotropin-releasing hormone (GnRH) in the brain. To clarify the physiological roles of the respective forms during testicular maturation, changes in brain GnRH mRNA levels and pituitary GnRH peptide levels were examined by real-time quantitative PCR and time-resolved fluoroimmunoassay, respectively. Fish hatched in April 2000. The gonadosomatic index remained low until October 2001 and then rapidly increased in January 2002. Fish continued to grow from hatching through testicular maturation. Fish spermiated in March 2002. The amount of seabream GnRH (sbGnRH) mRNA per brain significantly increased in January 2002 and remained at high levels in March 2002. The amounts of salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II) mRNA per brain did not show significant changes during the experimental periods. Pituitary sbGnRH peptide content significantly increased in March 2002. Pituitary sGnRH peptide and cGnRH-II peptide contents were extremely low compared to sbGnRH peptide levels and showed no significant changes during the experiment. These results indicate that sbGnRH is involved in the testicular maturation of barfin flounder.

Effects of gonadotropin-releasing hormone on pituitary-ovarian axis of one-year old pre-pubertal red seabream

To identify the pubertal development of the brain-pituitary-gonad (BPG) axis in female red seabream (Pagrus major), we investigated the effects of gonadotropin-releasing hormone agonist (GnRHa) on seabream (sb) GnRH mRNA levels in the brain, gonadotropin subunit mRNA levels in the pituitary, and serum concentrations of luteinizing hormone (LH), testosterone (T) and estradiol-17beta (E2) in pre-pubertal fish. Sexually immature 12-month-old fish were implanted with a cholesterol pellet containing GnRHa and maintained for 10-20 days. In the brain, GnRHa had no effect on sbGnRH mRNA levels. In the pituitary, although no marked changes were observed in follicle-stimulating hormone (FSH) beta subunit mRNA levels, the expression of glycoprotein (GP) alpha, and LHbeta subunit genes in the pituitary was drastically up-regulated (approximately 4- and 5-fold, respectively) and serum LH levels were also increased (approximately 3-fold) by GnRHa implantation. However, ovaries of GnRHa treated fish contained only oocytes at the peri-nucleolus stage, and oocyte development such as vitellogenesis and oocyte maturation did not occur throughout the experimental period. In these fish, even though LH was released, only slight increases in serum concentrations of T and E2 were observed. These results indicate that the pituitary gonadotropin cells of pre-pubertal 12-month-old fish were already receptive to GnRH stimulus, and acquired the ability to synthesize and release of LH as in the case of adult fish. Deficient factors for the onset of puberty by GnRHa treatment will be discussed.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

Effects of gonadotropin-releasing hormone agonist and dopamine antagonist on hypothalamus-pituitary-gonadal axis of pre-pubertal female red seabream (Pagrus major)

The effects of GnRH agonist (GnRHa) on the hypothalamus-pituitary-gonadal axis were studied in female pre-pubertal red seabream. Sexually immature 16-month-old fish were implanted intramuscularly with cholesterol pellets containing GnRHa or GnRHa in combination with domperidone, putative dopamine antagonist, and reared for 10-20 days. In both GnRHa and GnRHa+domperidone implanted groups, vitellogenesis was observed on Day 10 and ovulation was observed on Day 20, while ovarian development was not observed in the control fish throughout the experimental period. The levels of GnRH receptor mRNA were significantly higher in both GnRHa implanted groups than in the control. The expressions of all three gonadotropin subunit genes were up-regulated and serum luteinizing hormone levels were increased by the GnRHa implantation. Serum testosterone and estradiol-17beta levels were also increased on Day 10 and maintained high levels on Day 20. On the other hand, seabream (sb) GnRH mRNA levels in the brain were relatively low and unchanged in all experiment groups. The present study first shows that GnRH alone can induce precocious puberty in red seabream. These results indicate that the system of pituitary-gonadal axis has already been developed in 16-month-old fish and the commencement of sbGnRH secretion may be an important physiological event for the onset of puberty in the red seabream.