Release of pituitary gonadotrophins GtH I and GtH II in the rainbow trout (Oncorhynchus mykiss): modulation by estradiol and catecholamines

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

Development of a giant grouper Luteinizing Hormone (LH) Enzyme-Linked Immunosorbent Assay (ELISA) and its use towards understanding sexual development in grouper

A recombinant giant grouper Luteinizing Hormone (LH) consisting of tethered beta and alpha subunits was produced in a yeast expression system. The giant grouper LH β-subunit was also produced and administered to rabbits for antibody development. The recombinant LH and its antibody were used to develop an Enzyme Linked Immunosorbent Assay (ELISA). This ELISA enabled detection of plasma LH levels in groupers at a sensitivity between 391 pg/ml and 200 ng/ml. Different species of grouper were assayed with this ELISA in conjunction with gonadal histology and body condition data to identify links between circulating LH levels and sexual development. We found that circulating levels of LH decreased when oocytes began to degenerate, and sex-transition gonadal characteristics were apparent when LH levels decreased further. When circulating LH levels were related to body condition (body weight/ body length), transitioning-stage fish had relatively high body condition but low plasma LH levels. This observation was similar across multiple grouper species and indicates that plasma LH levels combined with body condition may be a marker for early male identification in the protogynous hermaphrodite groupers.

Disruptive effects of nonylphenol on reproductive hormones, antioxidant enzymes, and histology of liver, kidney and gonads in Caspian trout smolts

The endocrine-disrupting effect of pollutants such as alkylphenols has been considered a major concern during recent years. Although the effects of nonylphenol on the reproductive hormones of fish have been investigated in several studies, the effect of this endocrine disruptor on reproductive hormones of immature fish and salmonid smolts has been less addressed. The present work studied the effects of 1, 10 and 100 μg/l concentrations of nonylphenol on the levels of plasma reproductive hormones and liver antioxidant enzymes as well as on histopathology of reproductive and non-reproductive organs of male and female Caspian brown trout (Salmo trutta caspius) smolts after 21 days of exposure. The results of the present study showed that environmentally relevant concentrations of nonylphenol affected plasma levels of sex steroids; gonadotropins, phosphorus, estradiol to testosterone ratio, and also caused histopathological lesions in liver, kidney and testis tissues of immature Caspian brown trout during smolting. Nonylphenol significantly increased the levels of estradiol in plasma of both male and female smolts exposed to nonylphenol compared with the control groups. Exposure to nonylphenol decreased testosterone and FSH levels in both genders. It has also increased plasma levels of LH in females but did not affect LH levels in male fish. Liver SOD and CAT content was decreased in nonylphenol-exposed smolts. Therefore, the release of this economically valuable and endangered species into the rivers contaminated with nonylphenol should be avoided as it can have significant effects on the development and reproductive function of smolts.

Ovarian aromatase loss-of-function mutant medaka undergo ovary degeneration and partial female-to-male sex reversal after puberty

Although estrogens have been generally considered to play a critical role in ovarian differentiation in non-mammalian vertebrates, the specific functions of estrogens during ovarian differentiation remain unclear. We isolated two mutants with premature stops in the ovarian aromatase (cyp19a1) gene from an N-ethyl-N-nitrosourea-based gene-driven mutagenesis library of the medaka, Oryzias latipes. In XX mutants, gonads first differentiated into normal ovaries containing many ovarian follicles that failed to accumulate yolk. Subsequently, ovarian tissues underwent extensive degeneration, followed by the appearance of testicular tissues on the dorsal side of ovaries. In the newly formed testicular tissue, strong expression of gsdf was detected in sox9a2-positive somatic cells surrounding germline stem cells suggesting that gsdf plays an important role in testicular differentiation during estrogen-depleted female-to-male sex reversal. We conclude that endogenous estrogens synthesized after fertilization are not essential for early ovarian differentiation but are critical for the maintenance of adult ovaries.

Actions of sex steroids on kisspeptin expression and other reproduction-related genes in the brain of the teleost fish European sea bass

Kisspeptins are well known as mediators of the coordinated communication between the brain-pituitary axis and the gonads in many vertebrates. To test the hypothesis that gonadal steroids regulate kiss1 and kiss2 mRNA expression in European sea bass (a teleost fish), we examined the brains of gonad-intact (control) and castrated animals, as well as castrated males (GDX) and ovariectomized females (OVX) that received testosterone (T) and estradiol (E₂) replacement, respectively, during recrudescence. In GDX males, low expression of kiss1 mRNA is observed by in situ hybridization in the caudal hypothalamus (CH) and the mediobasal hypothalamus (MBH), although hypothalamic changes in kiss1 mRNA levels were not statistically different among the groups, as revealed by real-time PCR. However, T strongly decreased kiss2 expression levels in the hypothalamus, which was documented in the MBH and the nucleus of the lateral recess (NRLd) in GDX T-treated sea bass males. Conversely, it appears that E₂ evokes low kiss1 mRNA in the CH, while there were cells expressing kiss2 in the MBH and NRLd in these OVX females. These results demonstrate that kisspeptin neurons are presumably sensitive to the feedback actions of sex steroids in the sea bass, suggesting that the MBH represents a major site for sex steroid actions on kisspeptins in this species. Also, recent data provide evidence that both positive and negative actions occur in key factors involved in sea bass reproductive function, including changes in the expression of gnrh-1/gonadotropin, cyp19b, er and ar genes and sex steroid and gonadotropin plasma levels in this teleost fish.

Wastewater treatment plant effluent alters pituitary gland gonadotropin mRNA levels in juvenile coho salmon (Oncorhynchus kisutch)

It is well known that endocrine disrupting compounds (EDCs) present in wastewater treatment plant (WWTP) effluents interfere with reproduction in fish, including altered gonad development and induction of vitellogenin (Vtg), a female-specific egg yolk protein precursor produced in the liver. As a result, studies have focused on the effects of EDC exposure on the gonad and liver. However, impacts of environmental EDC exposure at higher levels of the hypothalamic-pituitary-gonad axis are less well understood. The pituitary gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) are involved in all aspects of gonad development and are subject to feedback from gonadal steroids making them a likely target of endocrine disruption. In this study, the effects of WWTP effluent exposure on pituitary gonadotropin mRNA expression were investigated to assess the utility of Lh beta-subunit (lhb) as a biomarker of estrogen exposure in juvenile coho salmon (Oncorhynchus kisutch). First, a controlled 72-h exposure to 17α-ethynylestradiol (EE2) and 17β-trenbolone (TREN) was performed to evaluate the response of juvenile coho salmon to EDC exposure. Second, juvenile coho salmon were exposed to 0, 20 or 100% effluent from eight WWTPs from the Puget Sound, WA region for 72h. Juvenile coho salmon exposed to 2 and 10ng EE2L(-1) had 17-fold and 215-fold higher lhb mRNA levels relative to control fish. Hepatic vtg mRNA levels were dramatically increased 6670-fold, but only in response to 10ng EE2L(-1) and Fsh beta-subunit (fshb) mRNA levels were not altered by any of the treatments. In the WWTP effluent exposures, lhb mRNA levels were significantly elevated in fish exposed to five of the WWTP effluents. In contrast, transcript levels of vtg were not affected by any of the WWTP effluent exposures. Mean levels of natural and synthetic estrogens in fish bile were consistent with pituitary lhb expression, suggesting that the observed lhb induction may be due to estrogenic activity of the WWTP effluents. These results suggest that lhb gene expression may be a sensitive index of acute exposure to estrogenic chemicals in juvenile coho salmon. Further work is needed to determine the kinetics and specificity of lhb induction to evaluate its utility as a potential indicator of estrogen exposure in immature fish.

Functional Characterisation of Eel Dopamine D2 Receptors and Involvement in the Direct Inhibition of Pituitary Gonadotrophins

In various vertebrate species, dopamine (DA) exerts an inhibitory action on reproduction. In the European eel, DA plays a pivotal role in the inhibitory control of gonadotroph function and the blockade of puberty. In vivo studies have suggested that this effect is mediated by receptors pharmacologically related to the D2 family. In the European eel, two distinct D2 receptor (D2-R) paralogous genes have been identified (D2A-R and D2B-R) and both were shown to be expressed in the pituitary. We investigated the potential role of each paralogue in the control of gonadotroph function in this species. Eel recombinant D2A-R or D2B-R were expressed in HEK 293 cells, with a universal Gα subunit, and receptor activation was followed by inositol phosphate production. Recombinant D2-Rs exhibited a comparable affinity for DA, although they had differential affinities for mammalian D2-R agonists and antagonists, supporting subtle structure/activity differences. Furthermore, using eel pituitary cell primary cultures, the expression by gonadotroph cells of both native eel D2-R paralogues was examined by in situ hybridisation of D2A-R or D2B-R transcripts, coupled with immunofluorescence of luteinising hormone (LH)β or follicle-stimulating (FSH)β. LH and to a lesser extent, FSH cells expressed both D2-R transcripts but with a clear predominance of D2B-R. Notably, D2B-R transcripts were detected for the majority of LH cells. Accordingly, using these cultures, we showed that DA potently inhibited basal and testosterone-stimulated LHβ expression and less potently basal and activin-stimulated FSHβ expression. We also tested some D2-R antagonists, aiming to select the most adequate one to be used in innovative protocols for induction of eel sexual maturation. We identified eticlopride as the most potent inhibitor of DA action on basal and stimulated LH expression in vitro. Our data suggest a differential functionalisation of the duplicated receptor genes and demonstrate that mainly D2B-R is involved in the dopaminergic inhibitory control of eel gonadotroph function.

A Computational Model of the Rainbow Trout Hypothalamus-Pituitary-Ovary-Liver Axis

Reproduction in fishes and other vertebrates represents the timely coordination of many endocrine factors that culminate in the production of mature, viable gametes. In recent years there has been rapid growth in understanding fish reproductive biology, which has been motivated in part by recognition of the potential effects that climate change, habitat destruction and contaminant exposure can have on natural and cultured fish populations. New approaches to understanding the impacts of these stressors are being developed that require a systems biology approach with more biologically accurate and detailed mathematical models. We have developed a multi-scale mathematical model of the female rainbow trout hypothalamus-pituitary-ovary-liver axis to use as a tool to help understand the functioning of the system and for extrapolation of laboratory findings of stressor impacts on specific components of the axis. The model describes the essential endocrine components of the female rainbow trout reproductive axis. The model also describes the stage specific growth of maturing oocytes within the ovary and permits the presence of sub-populations of oocytes at different stages of development. Model formulation and parametrization was largely based on previously published in vivo and in vitro data in rainbow trout and new data on the synthesis of gonadotropins in the pituitary. Model predictions were validated against several previously published data sets for annual changes in gonadotropins and estradiol in rainbow trout. Estimates of select model parameters can be obtained from in vitro assays using either quantitative (direct estimation of rate constants) or qualitative (relative change from control values) approaches. This is an important aspect of mathematical models as in vitro, cell-based assays are expected to provide the bulk of experimental data for future risk assessments and will require quantitative physiological models to extrapolate across biological scales.

Reproduction in fishes and other vertebrates represents the timely coordination of many endocrine factors that culminate in the production of mature, viable gametes. Improving the ability to estimate reproductive performance in fish is important, due to the growth of the aquaculture industry and the need to maintain adequate broodstock and concerns over the effects of anthropogenic stressors on feral fish populations. We present here a quantitative, mathematical model of the female rainbow trout reproductive cycle. We show how the model is able to accurately describe experimentally measured data associated with pituitary, ovarian and liver reproductive performance. We also use the model to describe similar data sets collected in rainbow trout by other researchers. An important value of quantitative biological models is the ability to simulate various physiological conditions, real or hypothetical. We demonstrate this by predicting the effects of exposure to an endocrine disruptor on oocyte growth. The need to limit cost and animal usage will encourage future experimental studies to use in vitro methods. The model presented here can assist with the extrapolation of in vitro effects to the whole fish.

Effects of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment on ovarian development of the sapphire devil, Chrysiptera cyanea

In the neuroendocrine system controlling fish reproduction, dopamine (DA) acts as a gonadotropin inhibitory factor and plays a role in regulating gonadal development of certain species. The present study examined the effects of chemical destruction of dopaminergic neurons in the brain on DA production and ovarian development in the sapphire devil Chrysiptera cyanea, a reef-associated damselfish. The avidin-biotin-peroxidase complex method using an antibody against tyrosine hydroxylase (TH), a critical enzyme in the DA synthesis pathway, identified a population of dopaminergic neurons with somata in the anteroventral preoptic nucleus of the diencephalon and fibers terminating in the proximal pars distalis of the pituitary. Maintaining fish in seawater containing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at 0.02 and 0.2 µg/mL for 2 days resulted in decreases in DA, 3,4-dihydroxyphenylacetic acid (DOPAC; DA metabolite), and DA metabolic rate in the whole brain. The number of TH-positive neurons in the diencephalon decreased after 0.02 µg/mL MPTP treatment for 2 days. These results suggest that MPTP treatment destroys TH-positive neurons in the diencephalon, thereby decreasing the synthesis and release of DA from the brain. This treatment rescued ovarian development in fish with artificially retracted ovaries during the spawning season. The gonadosomatic index of MPTP-treated fish 5 and 7 days after treatment was significantly higher than that of control fish. Oocytes in the vitellogenic stages were observed in the ovaries of MPTP-treated fish, but not in control fish. These results suggest that DA in the brain drives ovarian development in the sapphire devil.

Hypothalamic-pituitary-gonadal endocrine system in the hagfish

The hypothalamic-pituitary system is considered to be a seminal event that emerged prior to or during the differentiation of the ancestral agnathans (jawless vertebrates). Hagfishes as one of the only two extant members of the class of agnathans are considered the most primitive vertebrates known, living or extinct. Accordingly, studies on their reproduction are important for understanding the evolution and phylogenetic aspects of the vertebrate reproductive endocrine system. In gnathostomes (jawed vertebrates), the hormones of the hypothalamus and pituitary have been extensively studied and shown to have well-defined roles in the control of reproduction. In hagfish, it was thought that they did not have the same neuroendocrine control of reproduction as gnathostomes, since it was not clear whether the hagfish pituitary gland contained tropic hormones of any kind. This review highlights the recent findings of the hypothalamic-pituitary-gonadal endocrine system in the hagfish. In contrast to gnathostomes that have two gonadotropins (GTH: luteinizing hormone and follicle-stimulating hormone), only one pituitary GTH has been identified in the hagfish. Immunohistochemical and functional studies confirmed that this hagfish GTH was significantly correlated with the developmental stages of the gonads and showed the presence of a steroid (estradiol) feedback system at the hypothalamic-pituitary levels. Moreover, while the identity of hypothalamic gonadotropin-releasing hormone (GnRH) has not been determined, immunoreactive (ir) GnRH has been shown in the hagfish brain including seasonal changes of ir-GnRH corresponding to gonadal reproductive stages. In addition, a hagfish PQRFamide peptide was identified and shown to stimulate the expression of hagfish GTHβ mRNA in the hagfish pituitary. These findings provide evidence that there are neuroendocrine-pituitary hormones that share common structure and functional features compared to later evolved vertebrates.

Effects of estradiol or testosterone treatment on expression of gonadotropin subunit mRNAs and proteins in the pituitary of juvenile brown hagfish, Paramyxine atami

A single functional gonadotropin (GTH) comprising two subunits, α and β, was recently identified in the pituitary of brown hagfish (Paramyxine atami). Little is known about the feedback mechanisms that regulate these GTH subunits by sex steroids in the hagfish. The present study was designed to examine feedback effects of estradiol and testosterone on mRNA expression and protein expression of both GTHα and GTHβ subunits in the pituitary of the juvenile P. atami. Intraperitoneal administration of estradiol over the course of 27days resulted in substantial accumulation of immunoreactive (ir)-GTHα and ir-GTHβ in the adenohypophysis, but testosterone treatments over 27days had no effects on ir-GTHα or ir-GTHβ. Estradiol treatment for 1, 2, 4 or 14days had no effect on GTHα mRNA levels. In contrast, after 2days of estradiol treatment, GTHβ mRNA levels had increased significantly from baseline, while these levels were not affected after treatment over 1, 4, or 14days. After 14days of testosterone treatment, both GTHα and GTHβ mRNA levels had decreased significantly from baseline levels. These results indicate that estradiol acted primarily to suppress the secretion of GTH, and hence resulted in the accumulations of ir-GTHα and ir-GTHβ in the pituitary. On the other hand, testosterone appeared to suppress both the synthesis and the secretion of GTH. Thus, estradiol and testosterone probably differ in their effects on the regulation of pituitary GTH synthesis and secretion in juvenile hagfish.

Molecular cloning and expression profile of sex-specific genes, Figla and Dmrt1, in the protogynous hermaphroditic fish, Halichoeres poecilopterus

The genes folliculogenesis specific basic helix-loop-helix (facor in the germline alpha, Figla) and doublesex and mab-3 related transcription factor 1 (Dmrt1) are female- and male-specific genes that play key roles in sex differentiation. To obtain a better understanding of the molecular mechanisms underlying female-to-male sex change, we cloned the cDNAs of these genes from an ovary and a testis of the protogynus wrasse, Halichoeres poecilopterus. This fish has two isoforms of Dmrt1, Dmrt1a and Dmrt1b, caused by an alternative splicing. The Dmrt1b has an insertion of three nucleotides (CAG) in the open reading frame. Figla and Dmrt1 displayed gonadal-specific expression and abundant in the ovaries and in the testes, respectively. In particular, levels of Figla expression in the ovaries were higher in the spawning season than in the non-spawning season. Once sex change began, Figla mRNA decreased and Dmrt1 mRNA increased with progression of oocyte degeneration and spermatogenesis. These expression levels were maintained until the completion of the sex change. Low Figla and high Dmrt1 were also observed in testes of primary males, which functioned as a gonochoristic male throughout its life span in this wrasse. The results of this study suggest that these genes may regulate the gonadal transition from ovary to testis by the same mechanism as that of formation and maintenance of the primary testis in H. poecilopterus.

Teleost maturation-inducing hormone, 17,20β-dihydroxypregn-4-en-3-one, peaks after spawning in Tinca tinca

During an eight month study of the reproductive cycle in two age groups, and in both sexes, of tench (Tinca tinca L.), it was found that plasma concentrations of the presumptive 'maturation inducing hormone (MIH)' 17,20β-dihydroxypregn-4-en-3-one (17,20β-P) did not reach a peak during the spawning season, but as much as two months after spawning had ceased. The cessation of the spawning season was confirmed by histological examination of the gonads and by measurement of 11-ketotestosterone and 17β-estradiol in the plasma of males and females, respectively. Measurements were also made of the 'alternative MIH' 17,20β,21-trihydroxypregn-4-en-3-one in the older fish. However, this steroid did not show the same pattern as 17,20β-P. An assessment was made of the prevalence of primary spermatocytes in the testes of post-spawned fish - to test an alternative hypothesis that 17,20β-P might be involved in the stimulation of meiosis. However, there was no evidence for any increase in testis differentiation post-spawning. In fact the testes became increasingly undifferentiated as the autumn progressed. The role, if any, of this 'unseasonal' peak of 17,20β-P production remains to be determined.

Endocrine modulation, inhibition of ovarian development and hepatic alterations in rainbow trout exposed to polluted river water

Under laboratory conditions, female rainbow trout were exposed to graded concentrations of water from the River Lambro, a polluted tributary of the River Po, and to the effluent of a large wastewater treatment plant which flows into the River Lambro. In field exposures, trout were held in cages in the River Po upstream and downstream from the confluence of the River Lambro. After 10-day (laboratory) and 30-day (laboratory and field) exposures, trout were examined for several chemical, biochemical and histological endpoints. The results indicated that exposure to complex mixtures of chemicals, including estrogen receptor agonists, aryl-hydrocarbon receptor agonists, and probably antiandrogens, had occurred. Exposure altered the plasma levels of 17β-estradiol and testosterone, and some treatments also enhanced the activity of hepatic ethoxyresorufin O-deethylase. Gonadal histology showed varying levels of degenerative processes characterised by oocyte atresia, haemorrhages, melano-macrophage centres (MMCs), and oogonia proliferation. Liver histology showed less severe effects.

Rapid dopaminergic modulation of the fish hypothalamic transcriptome and proteome

Background

Dopamine (DA) is a major neurotransmitter playing an important role in the regulation of vertebrate reproduction. We developed a novel method for the comparison of transcriptomic and proteomic data obtained from in vivo experiments designed to study the neuroendocrine actions of DA.

Methods and Findings

Female goldfish were injected (i.p.) with DA agonists (D1-specific; SKF 38393, or D2-specific; LY 171555) and sacrificed after 5 h. Serum LH levels were reduced by 57% and 75% by SKF 38393 and LY 171555, respectively, indicating that the treatments produced physiologically relevant responses in vivo. Bioinformatic strategies and a ray-finned fish database were established for microarray and iTRAQ proteomic analysis of the hypothalamus, revealing a total of 3088 mRNAs and 42 proteins as being differentially regulated by the treatments. Twenty one proteins and mRNAs corresponding to these proteins appeared on both lists. Many of the mRNAs and proteins affected by the treatments were grouped into the Gene Ontology categorizations of protein complex, signal transduction, response to stimulus, and regulation of cellular processes. There was a 57% and 14% directional agreement between the differentially-regulated mRNAs and proteins for SKF 38393 and LY 171555, respectively.

Conclusions

The results demonstrate the applicability of advanced high-throughput genomic and proteomic analyses in an amendable well-studied teleost model species whose genome has yet to be sequenced. We demonstrate that DA rapidly regulates multiple hypothalamic pathways and processes that are also known to be involved in pathologies of the central nervous system.

Dopaminergic activity in the brain of a tropical wrasse in response to changes in light and hydrostatic pressure

Many tropical wrasses show a daily pattern of spawning with gamete release typically near daytime high tide. The environmental cues the fish obtains from day-night and tidal cycles to ensure spawning synchrony and how those cues are transduced, however, are not fully understood. To gain insight into these issues, the involvement of monoamines in mediating endogenous day-night and tidal rhythms in the threespot wrasse, Halichoeres trimaculatus, were examined. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC, a metabolite of DA), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) in the brain of the fish were measured with high-performance liquid chromatography and electrochemical detection. DOPAC and the metabolic rate of DA activity (DOPAC/DA) were found to increase during the day and decrease during the night for fish held under a natural photoperiod. Fish acclimated to a 12:12 light-dark cycle and to constant dark conditions exhibited similar changes, whereas fish acclimated to constant light conditions exhibited little or no change. Intraperitoneal injection of melatonin resulted in a significant reduction in DOPAC/DA. Furthermore, DOPAC/DA was significantly lower in fish held at 3m compared to 0m depth, suggesting that hydrostatic pressure influences DA metabolic rate. These results indicate that light and hydrostatic pressure control dopaminergic turnover in the brain of threespot wrasse. Day-night and tidal changes in these two factors therefore may be the main environmental cues the fish uses to synchronize its spawning activity.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

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.

Neuroendocrine control by dopamine of teleost reproduction

While gonadotropin-releasing hormone (GnRH) is considered as the major hypothalamic factor controlling pituitary gonadotrophins in mammals and most other vertebrates, its stimulatory actions may be opposed by the potent inhibitory actions of dopamine (DA) in teleosts. This dual neuroendocrine control of reproduction by GnRH and DA has been demonstrated in various, but not all, adult teleosts, where DA participates in an inhibitory role in the neuroendocrine regulation of the last steps of gametogenesis (final oocyte maturation and ovulation in females and spermiation in males). This has major implications for inducing spawning in aquaculture. In addition, DA may also play an inhibitory role during the early steps of gametogenesis in some teleost species, and thus interact with GnRH in the control of puberty. Various neuroanatomical investigations have shown that DA neurones responsible for the inhibitory control of reproduction originate in a specific nucleus of the preoptic area (NPOav) and project directly to the region of the pituitary where gonadotrophic cells are located. Pharmacological studies showed that the inhibitory effects of DA on pituitary gonadotrophin production are mediated by DA-D2 type receptors. DA-D2 receptors have now been sequenced in several teleosts, and the coexistence of several DA-D2 subtypes has been demonstrated in a few species. Hypophysiotropic DA activity varies with development and reproductive cycle and probably is controlled by environmental cues as well as endogenous signals. Sex steroids have been shown to regulate dopaminergic systems in several teleost species, affecting both DA synthesis and DA-D2 receptor expression. This demonstrates that sex steroid feedbacks target DA hypophysiotropic system, as well as the other components of the brain-pituitary gonadotrophic axis, GnRH and gonadotrophins. Recent studies have revealed that melatonin modulates the activity of DA systems in some teleosts, making the melatonin-DA pathway a prominent relay between environmental cues and control of reproduction. The recruitment of DA neurons for the neuroendocrine control of reproduction provides an additional brain pathway for the integration of various internal and environmental cues. The plasticity of the DA neuroendocrine role observed in teleosts may have contributed to their large diversity of reproductive cycles.

External and internal controls of lunar-related reproductive rhythms in fishes

Reproductive activities of many fish species are, to some extent, entrained to cues from the moon. During the spawning season, synchronous spawning is repeated at intervals of c. 1 month (lunar spawning cycle) and 2 weeks (semi-lunar spawning cycle) or daily according to tidal changes (tidal spawning cycle). In species showing lunar-related spawning cycles, oocytes in the ovary develop towards and mature around a specific moon phase for lunar spawners, around spring tides for semi-lunar spawners and at daytime high tides for tidal spawners. The production of sex steroid hormones also changes in accordance with synchronous oocyte development. Since the production of the steroid hormones with lunar-related reproductive periodicity is regulated by gonadotropins, it is considered that the higher parts of the hypothalamus-pituitary-gonad axis play important roles in the perception and regulation of lunar-related periodicity. It is likely that fishes perceive cues from the moon by sensory organs; however, it is still unknown how lunar cues are transduced as an endogenous rhythm exerting lunar-related spawning rhythmicity. Recent research has revealed that melatonin fluctuated according to the brightness at night, magnetic fields and the tidal cycle. In addition, cyclic changes in hydrostatic pressure had an effect on monoamine contents in the brain. These factors may be indirectly related to the exertion of lunar-related periodicity. Molecular approaches have revealed that mRNA expressions of light-sensitive clock genes change with moonlight, suggesting that brightness at night plays a role in phase-shifting or resetting of biological clocks. Some species may have evolved biological clocks in relation to lunar cycles, although it is still not known how lunar periodicities are endogenously regulated in fishes. This review demonstrates that lunar-related periodicity is utilized and incorporated by ecological and physiological mechanisms governing the reproductive success of fishes.

Alterations in the brain monoaminergic neurotransmitters of rainbow trout related to naphthalene exposure at the beginning of vitellogenesis

The contents of dopamine (DA), noradrenaline (NA), serotonin (5HT), and some related metabolites were studied in different brain regions of rainbow trout at two different stages of sexual maturation (at the beginning of vitellogenesis), after naphthalene (NAP) administration. The effects of NAP varied according to duration of exposure, brain region and vitellogenesis stage of the trout, and were more significant during previtellogenesis. The changes observed in DA metabolism were generally stimulatory after exposure for 3 h, and either stimulatory or inhibitory (depending on the brain regions) after exposure for 3 days to NAP. NA levels were altered by NAP in various brain regions, but only during previtellogenesis. With respect to 5HT, treatment with NAP reduced levels of the amine and/or its main metabolite in most of the brain regions studied, particularly 3 h after treatment. The results suggest that NAP might interfere with the processes regulating brain monoamine metabolism, either locally or indirectly by altering steroid feedback to brain centres, and thus disrupt endocrine control of reproductive development through the brain-pituitary axis.

Melatonin activates brain dopaminergic systems in the eel with an inhibitory impact on reproductive function

In the eel, a deficit in gonadotrophin-releasing hormone (GnRH) and a strong dopaminergic (DA) inhibition are responsible for the blockade of gonad development if silver eels are prevented from their reproductive migration. Environmental factors that eels encounter during their oceanic reproductive migration are thought to play an important role in the stimulation of eel pubertal development. We investigated the potential role of melatonin, a known mediator of the effects of external factors on reproductive function in vertebrates. We demonstrated that a long-term melatonin treatment increased brain tyrosine hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA expression in a region-dependent way. Melatonin stimulated the dopaminergic system of the preoptic area, which is involved in the inhibitory control of gonadotrophin [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] synthesis and release. Moreover, we showed that the increased TH expression appeared to be consistent with melatonin binding site distribution as shown by 2[(125)I]-melatonin labelling studies. On the other hand, melatonin had no effects on the two eel native forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the pituitary-gonad axis, we showed that melatonin treatment decreased both gonadotrophin beta-subunit (LHbeta, FSHbeta) mRNA expression and reduced sexual steroid (11-ketotestosterone, oestradiol) plasma levels. This indicates that melatonin treatment had a negative effect on eel reproductive function. To our knowledge, the results of the present study provide the first evidence that melatonin enhances TH expression in specific brain regions in a non-mammalian species. By this mechanism melatonin could represent one pathway by which environmental factors could modulate reproductive function in the eel.

Molecular characterization and central distribution of the estradiol receptor alpha (ERalpha) in the sea bass (Dicentrarchus labrax)

Three different estrogen receptors (ERs) have been cloned and characterized in teleosts fish, i.e. ERalpha, ERbeta or ERbeta1 and ERgamma or ERbeta2. In order to study the sea bass ER subtype involved in the regulation of gonadotropin production, as well as to elucidate the possible involved neuronal pathways, we characterized the transactivation properties of the cloned sea bass ERalpha (sbERalpha) and studied its distribution in the brain and gonadotropic cells of the sea bass by in situ hybridization. The results revealed that sbERalpha transactivates promoters containing estradiol responsive elements (ERE) in a dose-response manner. The sbERalpha showed the highest affinity for 17-beta-estradiol. In situ hybridization studies demonstrated that ERalpha mRNA positive neurons are widely distributed within the sea bass brain, including the telencephalon, preoptic area, thalamus, hypothalamus, mesencephalic tectum and tegmentum and rhombencephalon. New estrogen dependent nuclei were described in all above areas. The sbERalpha was profusely expressed in the main neuroendocrine areas such as the preoptic area and hypothalamus, thus suggesting the steroidal modulation of the hypophysiotropic neurons. The presence of sbERalpha expression in the FSHbeta and LHbeta cells suggests a direct effect of estrogens in the control of gonadotropin hormone synthesis.

Bioindicators and reproductive effects of prolonged 17beta-oestradiol exposure in a marine fish, the sand goby (Pomatoschistus minutus)

The effects of 17beta-oestradiol (E2) on mortality, growth rates, sexual maturation, hepatic vitellogenin (VTG) mRNA expression and reproductive success were investigated during an 8-month, water-borne exposure of a marine fish, the sand goby (Pomatoschistus minutus). Indicators of oestrogenic exposure were investigated as predictors of population-level reproductive success. E2 exposure concentrations were <5 (below limit of detection), 16+/-3, 97+/-20 and 669+/-151 ng l(-1) (bootstrap means and standard errors). The carrier solvent (<20 microl l(-1) propan-2-ol) significantly reduced the rate of egg production compared to untreated fish, but did not significantly affect male VTG mRNA expression, brood size, or the other studied parameters. Fish exposed to 16 ng l(-1) E2 showed few adverse effects compared with solvent only-exposed fish. Exposure to 97 ng l(-1) E2 significantly inhibited male sexual maturation, induced male VTG mRNA expression and delayed spawning. The 97 ng l(-1) E2 exposed population also produced fertile eggs at a significantly slower rate than solvent controls; however, brood size, fertility and overall reproductive success were not significantly affected. Exposure to 669 ng l(-1) E2 significantly increased mortality, adversely affected haematological parameters and caused an almost total lack of reproductive activity, with both sexes failing to mature. Reproductive failure following exposure to 669 ng l(-1) E2 was evident in both sexes when crossed with untreated animals. This work indicates that marine fish are similarly as sensitive to oestrogenic exposure as freshwater fish, that exposure biomarkers such as VTG are more sensitive to exposure than are reproductive effects, and that the use of carrier solvents in long-term reproductive studies should be avoided.

Alteration of monoamine concentrations in the brain of medaka, Oryzias latipes, exposed to tributyltin

We measured the concentrations of monoamines in the brain of Japanese medaka, Oryzias latipes, exposed to tributyltin (TBT). Fish were exposed to 0, 1, 5, 25, or 125 microg g(-1) of TBT via the diet for 21 days. After the administration period, six males and six females in each treatment group were dissected and their brains were collected. The following monoamines were analyzed: dopamine (DA), norepinephrine (NE), epinephrine (E), and 5-hydroxytryptamine (5-HT). The metabolites of DA, 3,4-dihydroxyphenylacetic acid and homovanilic acid, and the metabolite of 5-HT, 5-hydroxyindolacetic acid were also analyzed. The concentration of DA in the brain of male medaka and the concentrations of 5-HT and NE in the female brains were significantly decreased by exposure to 125 microg TBT g(-1). The concentrations of 5-HT and NE in males and of DA in females were slightly decreased by 125 micrg g(-1) of TBT, although the differences were not statistically significant. The present study demonstrates that TBT alters monoamine concentrations in the brain of medaka.

Suppression of aromatase activity in populations of bream (Abramis brama) from the river Elbe, Germany

Aromatase activity was determined in brain and gonads of wild bream collected along the river Elbe, Germany, and correlated with other endocrine and reproductive endpoints such as plasma sex steroid concentrations, secondary sex characteristics (STI), plasma vitellogenin, gonad size (GSI), and maturation stages of germ cells (MS) that were reported for the same fish in a previous study. Furthermore, regional patterns of aromatase activity were correlated to a number of environmental factors such as exposure to environmental contaminants and parasitism. While aromatase activity was not detectable in the gonads of male and female fish with the assay used, fish of both genders revealed relatively great brain enzyme activities. As for most of the endocrine and reproductive parameters, with the exception of plasma testosterone (T), aromatase activities were significantly less in fish from a river stretch characterized by elevated exposures to organic contaminants and metals. Brain aromatase activity was positively and significantly correlated with plasma estradiol (E2) and MS in females, and showed a similar trend with plasma 11-ketotestosterone (11KT) and STI in males. No comparable trend occurred for T. This decrease of the reproductively relevant hormones 11KT and E2 may be indicative of a disruption of the last step in sex hormone synthesis, a hypothesis that was supported for E2 by the strong (R2=0.78, p<0.05) linear regression between aromatase activity and E2 in female bream. It is also hypothesized that the effects on brain aromatase activity were likely to be related to the disruption of other reproductive parameters including sexual maturity and expression of secondary sex characteristics. Although a number of factors such as exposure to pollutants and prevalence of the tapeworm Ligula intestinalis correlated with the suppression of aromatase activity, the exact causes for the regional decrease in brain aromatase activity remain unclear due to inconsistencies of these correlations between sampling events or gender.

Brain Expression of Tyrosine Hydroxylase and Its Regulation by Steroid Hormones in the European Eel Quantified by Real-Time PCR

In the eel, dopamine inhibits pubertal development. To investigate the regulatory mechanisms involved, we developed a quantitative real-time RT-PCR assay for measurement of brain expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of dopamine. TH expression was highest in the olfactory bulb, followed by the di-/mesencephalic areas and the telencephalon/preoptic area. TH expression in the optic lobes and hindbrain was low or below the detection limit. In vivo treatment with testosterone, but not estradiol, resulted in increased TH expression in the forebrain, except the optic tectum, but not in the hindbrain. The results were confirmed by in situ hybridization.

Acute and chronic effects of carrier solvents in aquatic organisms: a critical review

Recognising the scientific and regulatory need for testing relatively hydrophobic or 'difficult substances', the OECD currently recommends that selected organic solvents may be used in aquatic toxicity testing in order to help achieve more effective dispersion of the toxicant. The OECD recommends a maximum solvent concentration of 100 microl l(-1) (with specific gravity equivalents to 100 microl l(-1) in parentheses) for acetone (79 mg l(-1)), dimethylformamide (95 mg l(-1)), dimethylsulfoxide (1.10 mg l(-1)), ethanol (78.9 mg l(-1)), methanol (79.2 mg l(-1)) and triethylene glycol (1.12 mg l(-1)). While this recommendation is supported by historical data, we have recently observed evidence that some solvents may affect the reproduction of certain fish species, and also impact biomarkers of endocrine disruption. This review presents available data on the effects of solvents in aquatic organisms, supplemented by relevant information from mammalian studies (e.g. effects on liver enzyme induction potentially altering the metabolism of sex hormones). In conclusion, it is recommended that maximum effort should be given to avoiding the use of carrier solvents wherever possible, for example through the use of saturation columns or other physical methods (e.g. stirring or ultrasonification). Where solvent use is necessary, however, it is recommended that in reproduction studies with aquatic organisms, the maximum solvent concentration should not exceed 20 microl l(-1) of dilution water.

Molecular regulation of gonadotropin secretion by gonadotropin-releasing hormone in salmonid fishes

Gonadotropin-releasing hormone (GnRH) plays a central role in the control of reproductive function in vertebrates. In salmonids, salmon GnRH (sGnRH) secreted by preoptic GnRH neurons regulates gonadal maturation through stimulation of synthesis and release of pituitary gonadotropins (GTHs). In addition, several lines of our evidence indicate that sGnRH is involved in spawning behavior, and serves to integrate the gonadal maturation with the reproductive behavior. A growing number of studies show that the effects of GnRH are mediated by multiple subtypes of GnRH receptors, successive multiple signaling pathways, and finally multiple transcription factors which act cooperatively to stimulate transcription of GTH subunit genes. This complex regulatory system of the action of GnRH may serve as a molecular basis of divergent physiological strategies of reproductive success in various vertebrate species. In this article, recent data on the molecular mechanisms of action of GnRH are reviewed with special reference to the regulation of synthesis and release of GTHs in the pituitary of salmonids to elucidate the multifunctional action of GnRH.

Modeling vitellogenesis in female fish exposed to environmental stressors: predicting the effects of endocrine disturbance due to exposure to a PCB mixture and cadmium

A wide variety of chemical and physical environmental stressors have been shown to alter the reproductive processes in fish by interfering with endocrine function. Most endocrine indicators or biomarkers are static measures from dynamic hormonally-mediated processes, and often do not directly relate to reproductive endpoints of ecological significance. Adequate production of the yolk precursor protein, vitellogenin, is critical for the survival and normal development of the sensitive egg and yolk-sac larval fish life stages. We developed a model that simulates vitellogenesis in a mature female sciaenid fish. The model simulates the major biochemical reactions over a 6-month period from the secretion of gonadotropin (GtH) into the blood to the production of vitellogenin. We simulated the effects of two endocrine disrupting chemicals (EDCs) that have different actions on vitellogenin production: a PCB mixture and cadmium. Predicted changes in steroid concentrations and cumulative vitellogenin production compared favorably with changes reported in laboratory experiments. Simulations illustrate the potential utility of our model for interpreting reproductive endocrine biomarkers measured in fish collected from degraded environments.

Temporal changes in gene expression in the liver of male plaice (Pleuronectes platessa) in response to exposure to ethynyl oestradiol analysed by macroarray and Real-Time PCR

Suppression subtractive hybridisation (SSH) was used to generate cDNA libraries representing genes differentially-expressed in liver from male plaice (Pleuronectes platessa) exposed to ethynyl oestradiol (EE2). BLAST analysis and alignments of the clones with database sequence suggested at least three vitellogenin (VTG) genes and three zona radiata protein (ZRP) genes were represented. Clones with unique sequence (62 up-, 13 down-regulated) were arrayed as probes on nylon membranes to investigate temporal expression of oestrogen-responsive genes in experimental animals. Arrays were hybridised with radiolabelled cDNAs prepared from hepatic mRNA from animals treated with EE2 for various times upto 21 days and from treated animals transferred to clean water for upto a further 31 days. By day 21 of treatment 11 out of 17 probes from unidentified genes, 21/22 VTG, 13/14 ZRP, 2/2 liver aspartic proteinase (LAP) and 8/10 other gene sequences were induced by EE2 exposure. Of the down-regulated sequences, only three showed significant, decreased expression and these encode cytochrome b and two with cryptic functions. Based on the pattern of temporal response the up-regulated probes fell into two classes. Pattern A reached maximum expression by day 16 of exposure and then declined prior to removal of EE2 at 21 days. Pattern B genes reached maximal expression between day 16 and 22, declining only after removal of EE2. Independent investigation of the expression patterns of selected probes using quantitative Real-Time PCR reproduced the distinctive patterns. The results indicate a previously unrecognised mechanism for oestrogenic toxicity in which there is a selective down-regulation of some egg proteins, potentially diminishing the quality of eggs and this may contribute to reproductive failure described elsewhere.

Synergistic effects of salmon gonadotropin-releasing hormone and estradiol-17beta on gonadotropin subunit gene expression and release in masu salmon pituitary cells in vitro

Effects of salmon gonadotropin-releasing hormone (sGnRH) and estradiol-17beta (E2) on gene expression and release of gonadotropins (GTHs) were examined in masu salmon (Oncorhynchus masou) using primary pituitary cell cultures at three reproductive stages, initiation of sexual maturation in May, pre-spawning in July, and spawning in September. Amounts of GTH subunit mRNAs were determined by real-time polymerase chain reaction, and levels of GTH released in the medium were determined by RIA. In control cells, the amounts of three GTH subunit mRNAs (alpha2, FSHbeta, and LHbeta) peaked in July prior to spawning. FSH release spontaneously increased with gonadal maturation and peaked in September, whereas LH release remained low until July and extensively increased in September. Addition of E2 to the culture extensively increased the amounts of LHbeta mRNA in May and July in both sexes. It also increased the alpha2 mRNA in July in the females. In contrast, sGnRH alone did not have any significant effects on the amounts of three GTH subunit mRNAs at all stages, except for the elevation of alpha2 and FSHbeta mRNAs in July in the females. Nevertheless, synergistic effects by sGnRH and E2 were evident for all three GTH subunit mRNAs. In May, sGnRH in combination with E2 synergistically increased the amounts of LHbeta mRNA in the males and alpha2 mRNA in the females. However, in July the combination suppressed the amounts of alpha2 and FSHbeta mRNAs in the females. sGnRH alone stimulated LH release at all stages in both sexes, and the release was synergistically enhanced by E2. Synergistic stimulation of FSH release was also observed in May and July in both sexes. These results indicate that a functional interaction of sGnRH with E2 is differently involved in synthesis and release of GTH. The synergistic interaction modulates GTH synthesis differentially, depending on subunit, stage, and gender, whereas it potentiates the activity of GnRH to release GTH in any situation.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Altered sexual characteristics in guppies (Poecilia reticulata) exposed to 17beta-estradiol and 4-tert-octylphenol during sexual development

The effects of estrogenic compounds on the development of sexual characteristics in juvenile guppies (Poecilia reticulata) were examined. After exposure for 3 months, the sex ratio was female biased in guppies exposed to 0.5 microg/L 17beta-estradiol (E2) or higher but unaffected by exposure to 4-tert-octylphenol (OP). When exposed to 100 microg/L OP, the male guppy sexual behavior and sperm count were increased. In contrast, the male sperm count was markedly reduced after exposure to a high concentration of E2. The length of the gonopodium (copulatory organ) was increased when exposed to 100 microg/L OP and at all the tested concentrations of E2. Similar concentrations of OP and E2 reduced the gonad weight of the females. The results demonstrate that the development of sexual characteristics in guppies can be disturbed by environmentally realistic concentrations of E2 but higher concentrations of OP are needed to induce similar alterations.

Effects of progesterone and estradiol on the reproductive axis in immature diploid and triploid rainbow trout

In fish species, many studies demonstrated the crucial role of estradiol (E2) in the development of the reproductive axis, but progesterone (P) has been described mainly as a precursor steroid and no clear role by itself has been reported. Moreover, a cooperative effect of P (or another progestin) and E2 in fish has never been reported to our knowledge. In the present work, we investigated the effects of P, alone or in combination with E2, on the reproductive-axis of immature rainbow trout (Oncorhynchus mykiss). Liver vitellogenin and estradiol receptor (rtER) mRNA levels increased after E2 treatment, but were unchanged by P treatments as a reflection of peripheral action of steroids. In contrast, at the pituitary level, LH contents increased after E2 and/or P treatments. Focusing on the brain level, we confirmed a clear up regulation of rtER expression by E2 in sterile triploid females, and we also demonstrated a similar stimulating effect of P alone but no cooperative effect together with E2. In conclusion, our data demonstrate that in immature trout, prior to the beginning of the first reproductive cycle, unlike E2, P is able to stimulate the reproductive brain-pituitary axis without affecting vitellogenin synthesis in the liver.

Distribution of dopamine D2 receptor mRNAs in the brain and the pituitary of female rainbow trout: an in situ hybridization study

The distribution of D(2)R (dopamine D(2) receptor) mRNAs was studied in the forebrain of maturing female rainbow trout by means of in situ hybridization using a (35)S-labeled riboprobe (810 bp) spanning the third intracytoplasmic loop. A hybridization signal was consistently obtained in the olfactory epithelium, the internal cell layer of the olfactory bulbs, the ventral and dorsal subdivisions of the ventral telencephalon, and most preoptic subdivisions, with the notable exception of the magnocellular preoptic nucleus, and the periventricular regions of the mediobasal hypothalamus, including the posterior tuberculum. In the pituitary, the signal was higher in the pars intermedia than in the proximal and the rostral pars distalis, but no obvious correspondence with a given cell type could be assigned. Labeled cells were also located in the thalamic region, some pretectal nuclei, the optic tectum, and the torus semicircularis. These results provide a morphologic basis for a better understanding on the functions and evolution of the dopaminergic systems in lower vertebrates.

Differential regulation of tyrosine hydroxylase and estradiol receptor expression in the rainbow trout brain

In numerous fish species, dopamine has been found to strongly inhibit gonadotropin release. Among the enzymes that regulate dopamine turnover, tyrosine hydroxylase (TH), the rate-limiting anabolic enzyme, could be a target for endocrine feedback regulation. Since dopamine turnover is stimulated by estradiol in rainbow trout, we have investigated the effect of estradiol on TH and estradiol receptor expression. In situ hybridization was used to quantify mRNA levels in the brain of ovariectomized female rainbow trout implanted or not with estradiol pellets. We demonstrated that preoptic TH and estradiol receptor mRNA levels are greatly decreased by gonadectomy during vitellogenesis. For TH expression, this effect was reversed in part by estradiol supplementation. We have also confirmed the existence of an inhibitory gonadal feedback on FSH secretion, mediated by estradiol. The stimulating effect of estradiol on TH expression found in this study could be a pathway involved in gonadal feedback on gonadotropin release.

Localization of tyrosine hydroxylase and its messenger RNA in the brain of rainbow trout by immunocytochemistry and in situ hybridization

This report describes the distribution of tyrosine hydroxylase (TH)-expressing structures in the brain of rainbow trout (Oncorhynchus mykiss). TH neurons have been localized by the use of two complementary techniques, immunocytochemistry and in situ hybridization of TH messenger RNA. Results obtained from in situ hybridization and immunocytochemistry were in agreement. TH cells were observed in many areas of the brain, with a higher density at the level of the olfactory bulbs where TH-positive neurons are abundant in the internal cell layer. In the telencephalon, two populations of TH neurons can be distinguished: one group is located in the area ventralis telencephali pars dorsalis, and the other group is located in the area ventralis telencephali pars ventralis and extends laterally in the area ventralis telencephali pars lateralis. Many labeled neurons are also seen in the preoptic area as well as in the hypothalamus, where several clusters of TH-positive cells are observed. Some of these neurons located in the paraventricular organ grow a short cytoplasmic extension directed to the ventricular wall and are known to be cerebrospinal fluid-contacting cells. The most caudal TH neurons are observed at the level of the locus caeruleus. At the level of the pituitary, TH-positive fibers are observed in the neurohypophysis. The TH-immunoreactive innervation at the level of the pituitary provides a neuroanatomic basis for the effects of dopamine and/or norepinephrine on the release of pituitary hormones in fish.

Dopamine D2 receptors and secretion of FSH and LH: role of sexual steroids on the pituitary of the female rainbow trout

The role of sexual steroids in the modulation of a dopaminergic inhibitory tone on FSH and LH release was studied in the rainbow trout. The experiments were performed on previtellogenic trout, implanted or not with estradiol (E(2)), and vitellogenic trout. E(2) implant increased the circulating levels of LH and decreased the circulating levels of FSH in previtellogenic fish. The catecholamine inhibitor alphaMPT increased the circulating levels of LH, implanted or not with E(2). AlphaMPT increased circulating levels of LH in vitellogenic fish. This increase could be prevented by the dopaminergic agonist bromocryptine. The dopaminergic drugs had no effect on the circulating levels of FSH in all groups. E(2) decreased mRNA levels of sGnRH1 and sGnRH2 in the telencephalon of previtellogenic fish. The dopaminergic treatments had no effect on mRNA levels of both forms of sGnRH in previtellogenic and vitellogenic fish. Primary cultures of pituitary cells were primed for three days with steroids (E(2) or 17alpha-hydroxy, 20beta-dihydroprogesterone (17alpha20betaP)) before treatment with increasing doses of bromocryptine, associated or not with sGnRH. E(2), but not 17alpha20betaP, potentiated the sGnRH-induced release of LH. Bromocryptine induced a slight dose-dependent decrease of sGnRH-induced release of LH. This decrease was potentiated by 17alpha20betaP. E(2) and 17alpha20betaP had no effect on the release of FSH, but bromocryptine decreased the 10(-8)M sGnRH-induced release of FSH. In conclusion, the development of the dopaminergic inhibitory tone on gonadotropin release, at the onset of vitellogenesis, requires factors other than estradiol. E(2) should contribute in part to decrease the release of FSH. At the end of the reproductive cycle, 17alpha20betaP should reinforce the dopaminergic inhibitory tone.

Regulation of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression by gonadotropin-releasing hormone (GnRH) and sexual steroids in the Mediterranean Sea bass

The secretion of gonadotropins, the key reproductive hormones in vertebrates, is controlled from the brain by the gonadotropin-releasing hormone (GnRH), but also by complex steroid feedback mechanisms. In this study, after the recent cloning of the three gonadotropin subunits of sea bass (Dicentrarchus labrax), we aimed at investigating the effects of GnRH and sexual steroids on pituitary gonadotropin mRNA levels, in this valuable aquaculture fish species. Implantation of sea bass, in the period of sexual resting, for 12 days with estradiol (E2), testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT), almost suppressed basal expression of FSHbeta (four to 15-fold inhibition from control levels), while slightly increasing that of alpha (1.5-fold) and LHbeta (approx. twofold) subunits. Further injection with a GnRH analogue (15 microg/kg BW; [D-Ala6, Pro9-Net]-mGnRH), had no effect on FSHbeta mRNA levels, but stimulated (twofold) pituitary alpha and LHbeta mRNA levels in sham- and T-implanted fish, and slightly in E2- and DHT-implanted fish (approx. 1.5-fold). The GnRHa injection, as expected, elevated plasma LH levels with a parallel decrease on LH pituitary content, with no differences between implanted fish. In conclusion, high circulating steroid levels seems to exert different action on gonadotropin secretion, inhibiting FSH while stimulating LH synthesis. In these experimental conditions, the GnRHa stimulate LH synthesis and release, but have no effect on FSH synthesis.

Effect of genistein-enriched diets on the endocrine process of gametogenesis and on reproduction efficiency of the rainbow trout Oncorhynchus mykiss

Three practical diets were formulated to contain 0, 500, or 1000 ppm genistein. The three diets were distributed for 1 year to groups of rainbow trout undergoing their first gametogenesis and until spawning. Growth performance of rainbow trout was not affected by dietary treatments. Plasma cholesterol levels were equivalent between groups. In males, a slight but constant induction of vitellogenin (VTG) synthesis and a decrease in testosterone levels were observed. A slight decrease in plasma levels of betaFSH and betaLH was noticed at the end of spermatogenesis in the male fish fed a diet with 500 ppm (genistein) (from 2.16 +/- 0.39 to 1.47 +/- 0.23 for betaFSH and from 0.44 +/- 0.09 to 0.31 +/- 0.09 for betaLH). There was a significantly reduced 17alpha,20beta(OH)(2)-progesterone (from 10.93 +/- 0.88 in control to 5.46 +/- 0.92 in males and from 251.22 +/- 21.40 to 183.22 +/- 13.48 in females). Testicular development was accelerated in genistein-fed fish, and sperm motility and concentration were decreased in a dose-dependent manner at spawning. In females, a significant increase in plasma VTG occurred only at the beginning and at the end of oogenesis. Testosterone levels were decreased at the beginning of oogenesis. Both betaFSH and betaLH were decreased by genistein (from 6.38 +/- 1.55 to 3.44 +/- 0.82 for betaFSH and from 15.18 +/- 3.00 to 6.93 +/- 0.99 for betaLH in females), whereas spawning was delayed only in females fed the diet with 500 ppm of genistein. Gamete quality was impaired only in this group, as underlined by a lower percentage of ovulating females (from 100 to 79% at the end of the trial), a lower fertilization rate, and a lower viability of fry. These results may be explained by the agonistic/antagonistic effect of genistein on estrogen function related to the tissue ratio between endogenous estrogens/genistein.

Estradiol-17beta stimulates gonadotropin II expression and release in the protandrous male black porgy Acanthopagrus schlegeli Bleeker: a possible role in sex change

The objective of the present study was to investigate the in vivo effects of sex steroids (estradiol-17beta, E(2); testosterone, T) and the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT) on the levels of gonadotropin II (GTH II) in plasma and pituitary and on aromatase activity in 2-year-old male black porgy, Acanthopagrus schlegeli, during the prereproductive season. Black porgy GTH II and GTH II beta subunits were purified and anti-GTH II beta serum was induced. A specific radioimmunoassay for black porgy GTH II was developed. cDNA GTH II beta was also cloned from a black porgy pituitary cDNA library for use as a probe for Northern analysis. Male fish were divided into eight groups (n = 64): control; E(2) (3 doses, 2.4 ng, 72 ng, and 2.2 microg/g body weight); T (2 doses, 72 ng and 2.2 microg/g body weight); and DHT (2 doses, 72 ng and 2.2 microg/g body weight). Fish were injected with the respective vehicle or different doses of material on days 0, 8, and 16. Plasma was collected at 4-day intervals from days 4 to 20. Plasma GTH II concentrations were significantly increased (up to 45-fold) in the E(2) group from days 4 to 20 in a dose-dependent manner. In a further experiment during the late reproductive season, plasma GTH II levels increased at 4 h and on days 1 and 2 following a single injection of 1.0 microg E(2)/g body weight (on day 0). Androgens (T or DHT) had little or no effect on plasma GTH II. Pituitary GTH II contents on day 20 were significantly lower in the 72-ng E(2) and 2.2-microg E(2) groups but not in the 2.4-ng E(2) group compared with the control group. Pituitary GTH II beta mRNA levels were significantly stimulated in the 72-ng and 2.2-microg E(2) groups on day 20. Gonadal aromatase activity was not significantly changed in any of the treated or control groups. It is concluded that GTH II secretion in black porgy is regulated by an estrogen-specific effect. Increased plasma GTH II levels or other factors in addition to E(2) might be involved in the regulation of gonadal aromatase activity and sex change in protandrous black porgy.