Endocrine evidence that silvering, a secondary metamorphosis in the eel, is a pubertal rather than a metamorphic event

European eel (Anguilla anguilla) survival modeling based on a 22-year capture-mark-recapture survey of a Mediterranean subpopulation

Since the 1980s, the European eel (Anguilla anguilla) has declined by over 90% in recruitment across its European and North African distribution area. This diadromous fish spawns at sea and migrates into continental waters, where it grows for three to more than 30 years, depending on habitat conditions and location. During their growth, different habitat use tactics can locally influence the life-history traits of eels, including their survival rates. Thus, the spatio-temporal dimension of this species is crucial for management. Based on a rare Mediterranean long-term survey of more than 20 years (2001-2022) in an artificial drainage canal connected to a vast brackish lagoon (the Vaccarès lagoon), we aimed to study the dynamics of one subpopulation's life-history traits. We used Bayesian multistate capture-mark-recapture (CMR) models to assess the temporal variability in survival and abundance at both seasonal and inter-annual scales, considering life-stage structure. High survival rates and low detection probabilities were found for the undifferentiated and female yellow stages. In contrast, female silver eels exhibited lower survival rates and higher capture probabilities. Estimating detection probabilities and survival rates enabled accurate assessment of relative abundance across different life stages and time periods. Our findings indicated a substantial decrease in the abundance of undifferentiated and female yellow eels in the early 2000s, whereas the abundance of female silver eels remained consistently low yet stable throughout the study period. Considering the life stage seemed essential to study the dynamics of the eel during its continental growing period. The present results will provide key elements to propose and implement suitable sustainable environmental management strategies for eel conservation.

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

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

Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Simple Summary

Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail.

Abstract

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations.

TAC3/TACR3 System Function in the Catadromous Migration Teleost, Anguilla japonica

Neurokinin B (NKB), a member of the tachykinin (TAC) family, plays important roles in mammalian neuropeptide secretion in related to reproduction. However, its potential role in spawning migration teleost is less clear. In the present study, Japanese eel (Anguilla japonica) was employed to study the performance of NKB in regulating reproduction. Results showed that two tac3 and one tacr3 genes were identified in Japanese eel. Sequence analysis showed that two tac3 transcripts, tac3a and tac3b, encode four NKBs: NKBa-13, NKBa-10, NKBb-13, and NKBb-10. However, compared with other species, a mutation caused early termination of TACR3 protein was confirmed, leading to the loss of the 35 amino acid (aa) C-terminal of the receptor. Expression analysis in different tissues showed that both tac3a and tac3b mRNAs were highly expressed in the brain. In situ hybridization localized both tac3a and tac3b mRNAs to several brain regions, mainly in the telencephalon and hypothalamus. Because of the mutation in TACR3 of Japanese eel, we further analyzed whether it could activate the downstream signaling pathway. Luciferase assay results showed the negative regulation of cAMP Response Element (CRE) and Sterol Response Element (SRE) signal pathways by Japanese eel NKBs. Intraperitoneal injection of four different NKB mature peptides at 100 ng/g had negative effect on either gnrh or gth gene expression. However, the high concentration of NKBa-10 and NKBb-13 (1,000 ng/g) upregulated mgnrh and fshb or lhb expression level significantly, which may be mediated by other receptors. In general, the NKBs/NK3Rs system has important functions in regulating eel puberty onset.

Expression of transport proteins in the rete mirabile of european silver and yellow eel

BACKGROUND

In physoclist fishes filling of the swimbladder requires acid secretion of gas gland cells to switch on the Root effect and subsequent countercurrent concentration of the initial gas partial pressure increase by back-diffusion of gas molecules in the rete mirabile. It is generally assumed that the rete mirabile functions as a passive exchanger, but a detailed analysis of lactate and water movements in the rete mirabile of the eel revealed that lactate is diffusing back in the rete. In the present study we therefore test the hypothesis that expression of transport proteins in rete capillaries allows for back-diffusion of ions and metabolites, which would support the countercurrent concentrating capacity of the rete mirabile. It is also assumed that in silver eels, the migratory stage of the eel, the expression of transport proteins would be enhanced.

RESULTS

Analysis of the transcriptome and of the proteome of rete mirabile tissue of the European eel revealed the expression of a large number of membrane ion and metabolite transport proteins, including monocarboxylate and glucose transport proteins. In addition, ion channel proteins, Ca2+-ATPase, Na+/K+-ATPase and also F1F0-ATP synthase were detected. In contrast to our expectation in silver eels the expression of these transport proteins was not elevated as compared to yellow eels. A remarkable number of enzymes degrading reactive oxygen species (ROS) was detected in rete capillaries.

CONCLUSIONS

Our results reveal the expression of a large number of transport proteins in rete capillaries, so that the back diffusion of ions and metabolites, in particular lactate, may significantly enhance the countercurrent concentrating ability of the rete. Metabolic pathways allowing for aerobic generation of ATP supporting secondary active transport mechanisms are established. Rete tissue appears to be equipped with a high ROS defense capacity, preventing damage of the tissue due to the high oxygen partial pressures generated in the countercurrent system.

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues

Lipids, particularly fatty acids (FAs), are major sources of energy and nutrients in aquatic ecosystems and play key roles during vertebrate development. The European eel Anguilla anguilla goes through major biochemical and physiological changes throughout its lifecycle as it inhabits sea- (SW), and/or brackish- (BW) and/or freshwater (FW) habitats. With the ultimate goal being to understand the reasons for eels adopting a certain life history strategy (FW or SW residency vs. 'habitat shifting'), we explored differences in lipid content and FA composition of muscle, liver and eyes from eels collected across Norwegian SW, BW and FW habitats, and at different lifecycle stages (yellow to silver). FW and SW eels had a higher lipid content overall compared to BW eels, reflecting differences in food availability and life history strategies. SW eels had higher proportions of certain monounsaturated FAs (MUFAs; 18:1n-9, 20:1n-9), and of the essential polyunsaturated FAs 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid) than FW eels, reflecting a marine-based diet. In contrast, the muscle of FW eels had higher proportions of 18:3n-3, 18:2n-6 and 20:4n-6 (arachidonic acid), as is typical of FW organisms. MUFA proportions increased in later stage eels, consistent with the hypothesis that the eels accumulate energy stores prior to migration. In addition, the decrease of EPA with advancing stage may be associated with the critical role that this FA plays in eel sexual development. Lipid and FA information provided further understanding of the habitat use and overall ecology of this critically endangered species.

Unravelling the changes during induced vitellogenesis in female European eel through RNA-Seq: What happens to the liver?

The life cycle of European eel (Anguilla anguilla), a catadromous species, is complex and enigmatic. In nature, during the silvering process prior to their long spawning migration, reproductive development is arrested, and they cease feeding. In studies of reproduction using hormonal induction, eels are equivalently not feed. Therefore, in female eels that undergo vitellogenesis, the liver plays different, essential roles being involved both in vitellogenins synthesis and in reallocating resources for the maintenance of vital functions, performing the transoceanic reproductive migration and completing reproductive development. The present work aimed at unravelling the major transcriptomic changes that occur in the liver during induced vitellogenesis in female eels. mRNA-Seq data from 16 animals (eight before induced vitellogenesis and eight after nine weeks of hormonal treatment) were generated and differential expression analysis was performed comparing the two groups. This analysis detected 1,328 upregulated and 1,490 downregulated transcripts. Overrepresentation analysis of the upregulated genes included biological processes related to biosynthesis, response to estrogens, mitochondrial activity and localization, while downregulated genes were enriched in processes related to morphogenesis and development of several organs and tissues, including liver and immune system. Among key genes, the upregulated ones included vitellogenin genes (VTG1 and VTG2) that are central in vitellogenesis, together with ESR1 and two novel genes not previously investigated in European eel (LMAN1 and NUPR1), which have been linked with reproduction in other species. Moreover, several upregulated genes, such as CYC1, ELOVL5, KARS and ACSS1, are involved in the management of the effect of fasting and NOTCH, VEGFA and NCOR are linked with development, autophagy and liver maintenance in other species. These results increase the understanding of the molecular changes that occur in the liver during vitellogenesis in this complex and distinctive fish species, bringing new insights on European eel reproduction and broodstock management.

Reproductive physiological characteristics of tropical Celebes eels Anguilla celebesensis in relation to downstream migration and ovarian development

Downstream-migrating (n = 64) and non-migrating (n = 21) female Celebes eels Anguilla celebesensis were captured from the Poso Lake-River system on Sulawesi Island, Indonesia, and their reproductive physiological characteristics were examined. A histological observation of the ovaries revealed that most non-migrating eels were at the perinucleolus (43%) or oil-droplet (48%) stage, whereas most migrating eels were at the early vitellogenic (36%) or midvitellogenic (61%) stage. Transcript levels of gonadotropin genes (fshb, lhb) in the pituitary gland and concentrations of sex steroids [11-ketotestosterone (11-KT), testosterone, 17β-oestradiol (E₂ )] in blood plasma of migrating eels were significantly higher than those of non-migrating eels. The fshb messenger (m)RNA levels were lower in perinucleolus and oil-droplet stages and then significantly increased in the early vitellogenic stage. The lhb mRNA levels in vitellogenic-stage eels were significantly higher than those in perinucleolus- and oil-droplet-stage eels. The 11-KT levels of eels at the oil-droplet and vitellogenic stages were significantly higher than those of eels at the perinucleolus stage. The E₂ levels at the vitellogenic stage were significantly higher than those at the perinucleolus and oil-droplet stages. These dynamics of the reproductive hormones represented the physiological background of oogenesis in A. celebesensis that has remarkably well-developed oocytes just before downstream migration.

Growth potential can affect timing of maturity in a long-lived semelparous fish

Many diadromous fishes such as salmon and eels that move between freshwater and the ocean have evolved semelparous reproductive strategies, but both groups display considerable plasticity in characteristics. Factors such as population density and growth, predation risk or reproduction cost have been found to influence timing of maturation. We investigated the relationship between female size at maturity and individual growth trajectories of the long-lived semelparous European eel, Anguilla anguilla A Bayesian model was applied to 338 individual growth trajectories of maturing migration-stage female silver eels from France, Ireland, the Netherlands and Hungary. The results clearly showed that when growth rates declined, the onset of maturation was triggered, and the eels left their growth habitats and migrated to the spawning area. Therefore, female eels tended to attain larger body size when the growth conditions were good enough to risk spending extra time in their growth habitats. This flexible maturation strategy is likely related to the ability to use diverse habitats with widely ranging growth and survival potentials in the catadromous life-history across its wide species range.

A mechanistic model for studying the initiation of anguillid vitellogenesis by comparing the European eel (Anguilla anguilla) and the shortfinned eel (A. australis)

An inverse relation exists between the maturation stage at the start of the oceanic reproductive migration and the migration distance to the spawning grounds for the various eel species. The European eel Anguilla anguilla migrates up to 5-6000 km and leaves in a previtellogenic state. The shortfinned eel A. australis migrates 2-4000 km and leaves in an early vitellogenic state. In this study, we compared the early pubertal events in European silver eels with those in silver shortfinned eels to gain insights into the initiation of vitellogenesis. Immediately after being caught, yellow and silver eels of both species were measured and sampled for blood and tissues. Eye index (EI), gonadosomatic index (GSI) and hepatosomatic index (HSI) were calculated. Plasma 11-ketotestosterone (11-KT) and 17β-estradiol (E2) levels were measured by radioimmunoassay. Pituitary, liver and ovaries were dissected for quantitative real-time PCR analyses (pituitary dopamine 2b receptor d2br, gonadotropin-releasing hormone receptors 1 and 2 gnrhr1 and gnrhr2, growth hormone gh and follicle-stimulating hormone-β fshb; liver estrogen receptor 1 esr1; gonad follicle-stimulating hormone receptor fshr, androgen receptors α and β ara and arb, vitellogenin receptor vtgr and P450 aromatase cyp19). Silver eels of both species showed a drop in pituitary gh expression, progressing gonadal development (GSI of ∼1.5 in European eels and ∼3.0 in shortfinned eels) and steroid level increases. In shortfinned eels, but not European eels, expression of fshb, gnrhr1 and gnrhr2, and d2br in the pituitary was up-regulated in the silver-stage as compared to yellow-stage females, as was expression of fshr, ara and arb in the ovaries. Expression of esr1 in European eels remained low while esr1 expression was up-regulated over 100-fold in silver shortfinned eels. The mechanistic model for anguillid vitellogenesis that we present suggests a first step that involves a drop in Gh and a second step that involves Fsh increase when switching in the life history trade-off from growth to reproduction. The drop in Gh is associated with gonadal development and plasma steroid increase but precedes brain-pituitary-gonad axis (BPG) activation. The Fsh increase marks BPG activation and increased sensitivity of the liver to estrogenic stimulation, but also an increase in D2br-mediated dopaminergic signaling to the pituitary.

Expression of gonadotropin subunit and gonadotropin receptor genes in wild female New Zealand shortfinned eel (Anguilla australis) during yellow and silver stages

Despite tremendous importance of follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) as primary controllers of reproductive development, information on the expression profiles of the genes encoding gonadotropin subunits and gonadotropin receptors (Fshr and Lhr) in wild eels are essentially non-existent. This study investigated pituitary fshb and lhb mRNA levels and ovarian fshr and lhr mRNA levels of wild shortfinned eels, Anguilla australis at different stages of oogenesis. Protein expression of Fsh in the pituitary was also quantified and visualized using slot blot and immunohistochemistry. Pituitary fshb and lhb mRNA levels showed a differential expression pattern, fshb mRNA levels increasing significantly from the perinucleolus (PN) to the oil droplet stage (OD) before slightly decreasing (not significantly) in the early vitellogenic stage (EV). A similar trend was observed in relative Fsh protein levels analyzed by slot blot and immunohistochemistry, but this trend was not reflected in the plasma levels of sex steroids. In contrast, pituitary lhb mRNA levels increased significantly from the PN to EV stage. A higher expression of Fsh at both mRNA and protein levels in the pituitary of eels at the OD stage compared to other investigated stages suggests that synthesis of Fsh production in the pituitary may reach a peak at the OD stage. In the ovary, transcript abundances of fshr and lhr gradually increased during previtellogenic follicle growth, but markedly and significantly increased thereafter. Taken together, our data suggest i) that Fsh release may be very limited, or absent, prior to onset of puberty in shortfinned eels and ii) that Lh is not functionally important in this fish during the EV stage.

Characterization of a β2 adrenergic receptor protein precursor in the European eel (Anguilla anguilla) and its tissue distribution across silvering

This study provides the characterization and tissue distribution of a β2-AR in the female European eel during silvering, aiming to better understand the adrenergic system involvement in this critical maturation event. A putative β2-AR (ADRB2) mRNA was cloned and sequenced. Amino acid residues and motifs important for ligand binding are generally conserved across fish and between fish and mammals, although the occurrence of some sequence variabilities may explain the noted peculiarities of eel AR interaction with pharmacological ligands. The tissue distribution of the ADRB2 gene product was analyzed in five tissues of the eel at different silvering stages and compared with that of the ADRA1 mRNA encoding an α1-AR subtype. On the whole, data suggested that relative ADRA1/ADRB2 tissue expression across silvering is part of the preparatory (molecular) adjustments required to face changes in habitats and migration efforts.

The effects of migratory stage and 11-ketotestosterone on the expression of rod opsin genes in the shortfinned eel (Anguilla australis)

The androgen 11-ketotestosterone (11KT) can induce many of the changes associated with silvering, i.e., the transformation of a non-migrating 'yellow' eel into a migrating 'silver' eel. We posited that plasticity in spectral sensitivity of the eye, accompanied by expression of different opsins in the retina during silvering, is controlled by 11KT. To test this hypothesis, mRNA levels of freshwater (fwo) and seawater (swo) opsins and of the two androgen receptors (ara and arb) in retinas of wild-caught female shortfinned eels, Anguilla australis were compared. Swo expression was much higher (3-4 orders of magnitude) and fwo expression substantially lower in silver than in yellow eels, whereas mRNA levels of both ars did not differ between stages. Yellow eel retinas exposed to 11KT in vitro exhibited a robust dose-dependent increase in swo, but weak decreasing effects on fwo transcript abundance were inconsistent. Similarly, increased retinal swo expression was seen after in vivo treatment of yellow eels with 11KT implants, whereas expression of fwo remained unaffected. Lastly, co-treatment with 11KT and the androgen receptor blocker flutamide was undertaken to determine whether 11KT exerts its effects through nuclear androgen receptors. Flutamide did not block 11KT-affected expression of any target gene, neither in vivo nor in vitro. We conclude that 11KT greatly increases the abundance of swo, identifying the androgen as an important regulator of the opsin switch during silvering in freshwater eels.

Thyroid endocrine status of wild European eels (Anguilla anguilla) in the Loire (France). Relationships with organic contaminant body burdens

In teleost fish, thyroid function is involved in various critical physiological processes. Given the complexity of the hypothalamo-pituitary-thyroid (HPT) axis, a large number of genes and proteins can be the potential target of endocrine-disrupting compounds (EDCs). The aim of this study was to evaluate, in yellow and silver European eels (Anguilla anguilla), potential effects of EDCs on thyroid status by analyzing the associations between EDC body burdens and thyroid hormones (THs). In yellow individuals, greater free T3/T4 ratios (FT3/FT4) and lower plasma FT4 levels were associated with greater concentrations of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs), as highlighted by significant correlations with many congeners. Few positive relationships with alkylphenols were noticed. In contrast, silver eels usually exhibited less significant correlations between THs and contaminant loads. Expression of a series of genes involved in the HPT axis was also investigated in the silver individuals. Concerning mRNA expression in silver females, some main correlations were noticed: thyroid-stimulating hormone (TSHβ) gene expression was significantly correlated to numerous PCBs, and hepatic mRNA levels of deiodinase 2 (Deio 2) were negatively correlated to 2-hydroxyfluorene (2-OHF) and 2-naphtol (2-OHNa). Thyroid receptor (TRα and TRβ) mRNA levels exhibited weak negative correlations with some PBDEs in silver females and males. Hepatic vitellogenin (Vtg) mRNA levels were detected in all silver males but at lower levels than in silver females. In males, Vtg mRNA levels were positively associated to FT4/TT4. In silver females, strong positive correlations were found between congeners of PCBs, PBDEs and PFAS suggesting potential estrogenic effects. Overall, the observed results indicate that several organic contaminants, mainly dl-, ndl-PCBs and PBDEs, could be associated with changes in thyroid homeostasis in these fish, via direct or indirect interactions with peripheral deiodination, metabolism of T4 and mechanisms involved in TSHβ, Deio 2 and Vtg gene transcription.

Changes in ovarian gene expression profiles and plasma hormone levels in maturing European eel (Anguilla anguilla); Biomarkers for broodstock selection

Complete sexual maturation of European eels (Anguilla anguilla) in captivity can only be achieved via injections with gonadotropins. For female eels this procedure takes 4-6months and the response ranges from "unresponsive" to final maturation and ovulation. Reproductive success could be significantly increased via early selection of responders based on predictive markers and minimally invasive sampling methods. To get a better understanding of the genetic background of ovarian maturation of the European eel we performed a pilot deep-sequencing transcriptome analysis of ovarian tissue derived from a yellow eel, a prepubertal silver eel and a post-spawning matured eel. Two key players in steroidogenesis were strongly correlated with advanced sexual maturation, namely P450c17 and liver receptor homolog-1, suggesting that blood plasma steroids might qualify as minimally invasive markers for early detection of responders. Since the predictive value of plasma sex steroid levels for final maturation of the European eel had not yet been carefully examined, we performed an extensive artificial maturation trial. Farmed silver eels were treated with pituitary extracts and sampled at multiple time intervals. Expression of steroidogenesis-related genes in ovarian tissue of responding and non-responding eels after four weekly injections with pituitary extract was compared using a custom-built microarray and RNAseq. Increased expression of 17β-hsd1 was strongly linked to sexual maturation. Blood plasma levels of sex steroids were measured using ELISAs. We show that a 2.5-fold increase in blood-plasma estradiol level after 4 weekly pituitary extract injections is a strong predictor of final sexual maturation of female European eel.

Migratory Restlessness and the Role of Androgen for Increasing Behavioral Drive in the Spawning Migration of the Japanese eel

Migratory restlessness refers to a type of locomotor activity observed just before the onset of a migration. This behavior is primarily known in birds, where it is considered to be an indicator of the urge for migration. In contrast, little is known about migratory restlessness in fishes. To confirm migratory restlessness in a fish, we measured the locomotor activity of the Japanese eel, Anguilla japonica during its migration season. Migratory-phase silver eels showed higher locomotor activity in aquaria than yellow eels at the non-migratiory growth-phase. Silver eels stayed outside of their shelters for longer durations in dark periods than yellow eels and were active even in light periods when yellow eels were inactive in the shelters. Silver eels had higher levels of the androgen hormone 11-ketotestosterone at the end of experiment than yellow eels. Administration of 11-ketotesosterone to yellow eels induced higher levels of locomotor activity than that observed in non-treated controls. These findings suggest that anguillid eels exhibit migratory restlessness just before their spawning migration and that 11-ketotestosterone may be involved in the onset of this behavior.

Selection of best-performing reference gene products for investigating transcriptional regulation across silvering in the European eel (Anguilla anguilla)

The focus of the present study was to set a methodological approach for evaluating molecular mechanisms underlying silvering transformation in the European eel, Anguilla anguilla. Silvering is a tightly controlled process during which eels undergo significant morphological, physiological and behavioral changes, pre-adapting for the oceanic spawning migration. Female eels showing different silver indexes were caught in different seasons in the Comacchio Lagoon (North Adriatic Sea, Italy). Isolated hepatocytes from these eels were selected as the experimental model given the relevant role of these cells in metabolic functions potentially altered during silvering. Expression profiles of 7 candidate reference transcripts were analyzed seeking the most viable and robust strategies for accurate qPCR data normalization during silvering. Stability analysis and further statistical validation identified transcripts encoding the ribosomal proteins L13 and ARP as the appropriate reference genes in studies on A. anguilla through silvering. The identified reference transcripts were further used to evaluate expression profiles of target transcripts encoding the thyroid hormone receptor β (THRβ) and vitellogenin (vtg), known to be involved in silvering processes. To the best of our knowledge, this is the first study comparing THRβ expression in European eels across silvering.

Swimbladder function and the spawning migration of the European eel Anguilla anguilla

The spawning migration of the European eel is an extensive journey over 5000 to 7000 km from the European coast to the Sargasso Sea. Eels do not feed during this journey and on-board fuels must be sufficient to support the journey of 3.5 to 6 month, as well as sexual maturation and the spawning activity. Swimming of eels appears to be quite energy efficient compared to other fish species, and elevated hydrostatic pressure has been shown to even reduce the costs of transport. Recent studies revealed, however, that during traveling eels perform extensive diurnal migrations and swim at a depth of about 100-300 m at night time, but go down to 600-1000 m at day time. At a depth of 200 m eels are exposed to a hydrostatic pressure of 21 atmospheres (2.13 MPa), while at 800 m hydrostatic pressure increases to 81 atmospheres (8.21 MPa). Accordingly, without any compensation at a depth of 800 m swimbladder volume will be reduced to about 25% of the volume established with neutral buoyancy at 200 m. Consequently, these diurnal changes in depth must be taken into consideration for a calculation of the energy requirements of the spawning migration. Without compensation a compression of the swimbladder will result in a status of negative buoyancy, which makes swimming more costly. Trying to keep the status of neutral buoyancy during descent by gas secretion into the swimbladder in turn requires metabolic activity to enhance swimbladder perfusion and for acid production of the gas gland cells to stimulate gas secretion. During ascent gas is passively removed from the swimbladder in the resorbing section and in the blood transported to the gills, where it is lost into the water. Accordingly, the swimbladder appears to be a crucial organ for the spawning migration. It can be assumed that an impairment of swimbladder function for example due to an infection with the nematode Anguillicola crassus significantly threatens the success of the spawning migration.

Neuroendocrine gene expression reveals a decrease in dopamine D2B receptor with no changes in GnRH system during prepubertal metamorphosis of silvering in wild Japanese eel

Silvering is a prepubertal metamorphosis preparing the eel to the oceanic reproductive migration. A moderate gonad development occurs during this metamorphosis from the sedentary yellow stage to the migratory silver stage. The aim of this study was to elucidate the molecular aspects of various endocrine parameters of BPG axis at different ovarian developmental stages in wild yellow and silver female Japanese eels. The GSI of the sampled female eels ranged between 0.18 and 2.3%, corresponding to yellow, pre-silver and silver stages. Gonad histology showed changes from previtellogenic oocytes in yellow eels to early vitellogenic oocytes in silver eels. Both serum E2 and T concentrations significantly increased with ovarian development indicating a significant activation of steroidogenesis during silvering. In agreement with previous studies, significant increases in pituitary gonadotropin beta subunits FSH-β and LH-β transcripts were also measured by qPCR, supporting that the activation of pituitary gonadotropin expression is likely responsible for the significant ovarian development observed during silvering. We investigated for the first time the possible brain neuroendocrine mechanisms involved in the activation of the pituitary gonadotropic function during silvering. By analyzing the expression of genes representative of the stimulatory GnRH control and the inhibitory dopaminergic control. The transcript levels of mGnRH and the three GnRH receptors did not change in the brain and pituitary between yellow and silver stages, suggesting that gene expression of the GnRH system is not significantly activated during silvering. The brain transcript levels of tyrosine hydroxylase, limiting enzyme of DA synthesis did not change during silvering, indicating that the DA synthesis activity was maintained. In contrast, a significant decrease in DA-D2B receptor expression in the forebrain and pituitary was observed, with no changes in DA-D2A receptor. The decrease in the pituitary expression of DA-D2BR during silvering would allow a reduced inhibitory effect of DA. We may raise the hypothesis that this regulation of D2BR gene expression is one of the neuroendocrine mechanisms involved in the slight activation of the pituitary gonadotropin and gonadal activity that occur at silvering.

Metamorphosis in teleosts

Teleosts are the largest and most diverse group of vertebrates, and many species undergo morphological, physiological, and behavioral transitions, "metamorphoses," as they progress between morphologically divergent life stages. The larval metamorphosis that generally occurs as teleosts mature from larva to juvenile involves the loss of embryo-specific features, the development of new adult features, major remodeling of different organ systems, and changes in physical proportions and overall phenotype. Yet, in contrast to anuran amphibians, for example, teleost metamorphosis can entail morphological change that is either sudden and profound, or relatively gradual and subtle. Here, we review the definition of metamorphosis in teleosts, the diversity of teleost metamorphic strategies and the transitions they involve, and what is known of their underlying endocrine and genetic bases. We suggest that teleost metamorphosis offers an outstanding opportunity for integrating our understanding of endocrine mechanisms, cellular processes of morphogenesis and differentiation, and the evolution of diverse morphologies and life histories.

Profiles of mRNA expression for prolactin, growth hormone, and somatolactin in Japanese eels, Anguilla japonica: The effect of salinity, silvering and seasonal change

For understanding the functions of the growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family of hormones, we examined pituitary mRNA expression of these hormones in anguillid eels in relation to salinity difference, silvering, and seasonal change. Female Japanese eels (Anguilla japonica) were collected in the brackish Hamana Lake and its freshwater rivers from July to December. To clarify the effect of salinity, the habitat use history of the eels were determined using otolith microchemistry. Expression levels of mRNA of each hormone were determined using real time PCR. Although GH and PRL have been known to be osmoregulatory hormones, there were no consistent differences in expression levels of these hormones between different salinity habitats. In contrast, SL mRNA expression was higher in eels from freshwater rivers than from the brackish lake. GH mRNA expression clearly decreased during silvering, whereas PRL and SL mRNA expression did not change. We also showed that PRL mRNA and SL mRNA decreased in the brackish lake and PRL mRNA increased in freshwater rivers from autumn to early winter. These findings provide basic knowledge for a further understanding of the role of these hormones.

Quantitative bioassays for measuring biologically functional gonadotropins based on eel gonadotropic receptors

Significant declines in eel stocks have been noted in many parts of the world. Because eel aquaculture is dependent on wild-caught juveniles, there is a need to achieve artificial reproduction. Adult eel maturation is currently induced by repeated injections of purified gonadotropin (human chorionic gonadotropin [hCG]) or pituitary extract. Thus the determination of the biological efficacy and quantification of internal levels of gonadotropic hormones is important for optimizing artificial reproduction protocols. To quantify the plasma levels of biologically functional gonadotropic hormones, we developed a bioassay for luteinizing hormone (LH) and follicle-stimulating hormone (FSH) based on the stable expression of receptors in HEK293 cells of the Japanese eel Anguilla japonica LH (ajLHR) and the European eel Anguilla anguilla FSH (aaFSHR), respectively. Such cells also contain a firefly luciferase reporter gene driven by a cAMP-responsive element (CRE-Luc). We found that the obtained stable cells, with ajLHR, responded linearly to a more than 100,000-fold concentration range of hCG diluted in saline. The cells with aaFSHR showed a linear response to a 1000-fold concentration range of salmon pituitary extract mixed with saline. The biological functionality of the LH and FSH bioassays was validated using hCG, human FSH, and pituitary extracts from salmon, carp and eel. Since the toxins in eel plasma damaged the HEK293 cells, the protocol was adapted to selectively inactivate the toxins by heating at 37°C for 24h. This process successfully enabled the monitoring of hormone levels in blood plasma sampled from hCG-injected eels. In this paper, we describe the development of gonadotropin bioassays that will be useful for improving reproduction protocols in eel aquaculture.

The Anguilla spp. migration problem: 40 million years of evolution and two millennia of speculation

Anguillid eels Anguilla spp. evolved between 20 and 40 million years ago and possess a number of remarkable migratory traits that have fascinated scientists for millennia. Despite centuries of effort, the spawning areas and migrations are known only for a few species. Even for these species, information on migratory behaviour is remarkably sketchy. The latest knowledge on the requirements for successful migration and field data on the migrations of adults and larvae are presented, how experiments on swimming efficiency have progressed the understanding of migration are highlighted and the challenges of swimming at depth considered. The decline of Anguilla spp. across the world is an ongoing concern for fisheries and environmental managers. New developments in the knowledge of eel migration will, in addition to solving a centuries old mystery, probably help to identify how this decline might be halted or even reversed.

Androgen-specific regulation of FSH signalling in the previtellogenic ovary and pituitary of the New Zealand shortfinned eel, Anguilla australis

The evidence for androgens having a pivotal role in the functioning of the female reproductive axis--such as initiating puberty or vitellogenesis--is mounting. However, the use of aromatizable androgens and the tissue-specific focus of most studies often make it unclear if androgenic effects throughout the axis proceed via androgen or estrogen signalling mechanisms. In this study, we assessed the effects of 11-ketotestosterone (11KT, a non-aromatizable androgen) on the pituitary and ovary of previtellogenic (PV) freshwater eels Anguilla australis, comparing them with eels naturally undergoing early vitellogenesis (EV). We found that 11KT treatment produces molecular and morpho-physiological phenotypes that were generally intermediate between PV and EV. Most notably, we demonstrated that 11KT induces effects on follicle-stimulating hormone (FSH) signalling in the pituitary and ovaries that are in opposition to each other. Thus, 11KT significantly reduced fshβ subunit expression in the pituitary. At the same time, 11KT dramatically increased mRNA levels of ovarian FSH receptor and plasma levels of estradiol-17β, very likely sensitizing the previtellogenic follicle to the FSH signal. Androgens therefore may be important in facilitating puberty in the eel.

Pituitary gonadotropins FSH and LH are oppositely regulated by the activin/follistatin system in a basal teleost, the eel

European eels are blocked at a prepubertal silver stage due to a deficient production of pituitary gonadotropins. We investigated the potential role of activin/follistatin system in the control of eel gonadotropins. Through the development of qPCR assays for European eel activin β(B) and follistatin, we first analyzed the tissue distribution of the expression of these two genes. Both activin β(B) and follistatin are expressed in the brain, pituitary and gonads. In addition, a striking expression of both transcripts was also found in the retina and in adipose tissue. The effects of recombinant human activins and follistatin on eel gonadotropin gene expression were studied using primary cultures of eel pituitary cells. Activins A and B strongly stimulated FSHβ subunit expression in a time- and dose-dependent manner. In contrast, activin reduced LHβ expression, an inhibitory effect which was highlighted in the presence of testosterone, a known activator of eel LHβ expression. No effect of activin was observed on other pituitary hormones. Follistatin antagonized both the stimulatory and inhibitory effects of activin on FSHβ and LHβ expression, respectively. Activin is the first major stimulator of FSH expression evidenced in the eel. These results in a basal teleost further support the ancient origin and strong conservation of the activin/follistatin system in the control of FSH in vertebrates. In contrast, the opposite regulation of FSH and LH may have emerged in the teleost lineage.

Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have investigated the interplay between migration and maturation in the field during the downstream migration of female silver eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over 3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder parasites on their swimming performance.

Expression of androgen receptor mRNA in the ovary of Japanese eel, Anguilla japonica, during artificially induced ovarian development

In order to elucidate how androgens may mediate their effects on ovarian growth, we investigated the mRNA levels of two subtypes of androgen receptor (ara and arb) in the ovary of feminized Japanese eel (Anguilla japonica) during artificially induced ovarian development by quantitative real-time reverse transcriptase polymerase chain reaction and in situ hybridization. Ara mRNA levels were high from the late oil droplet stage to the late vitellogenic stage, whereas arb mRNA levels were high from the late oil droplet stage to the midvitellogenic stage. Both ar mRNAs were predominantly observed in the follicle cells and the epithelial cells of the ovigerous lamellae in all stages. In the oil droplet stage, oogonia exhibited intense signals for ar mRNAs. There was no obvious difference in localization pattern between ara and arb in all ovaries examined, irrespective of maturational stage. It was difficult to identify the follicle cell types that were positive for ar mRNA during ovarian development. Only in post-ovulatory follicles could theca and granulosa cells be clearly identified, and ar signals were observed in both layers. The predominant localization of ar mRNA in the follicle cells suggests that androgens play important roles in oocyte growth by acting on these cells in this species. We have shown the expression profile and localization of ar mRNA during ovarian development for the first time in an oviparous vertebrate.

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.

Developmental dynamics of cone photoreceptors in the eel

BACKGROUND

Many fish alter their expressed visual pigments during development. The number of retinal opsins expressed and their type is normally related to the environment in which they live. Eels are known to change the expression of their rod opsins as they mature, but might they also change the expression of their cone opsins?

RESULTS

The Rh2 and Sws2 opsin sequences from the European Eel were isolated, sequenced and expressed in vitro for an accurate measurement of their lambdamax values. In situ hybridisation revealed that glass eels express only rh2 opsin in their cone photoreceptors, while larger yellow eels continue to express rh2 opsin in the majority of their cones, but also have <5% of cones which express sws2 opsin. Silver eels showed the same expression pattern as the larger yellow eels. This observation was confirmed by qPCR (quantitative polymerase chain reaction).

CONCLUSIONS

Larger yellow and silver European eels express two different cone opsins, rh2 and sws2. This work demonstrates that only the Rh2 cone opsin is present in younger fish (smaller yellow and glass), the sws2 opsin being expressed additionally only by older fish and only in <5% of cone cells.

Cost of transport and optimal swimming speed in farmed and wild European silver eels (Anguilla anguilla)

A swimming speed of 0.4 meters per second (m s(-1)) is the minimal speed for European female silver eels to reach the spawning sites in the Sargasso Sea in time. As silver eels cease feeding when they start their oceanic migration, the cost of transport (COT) should be minimised and the swimming speed optimised to attain the highest energetic efficiency. In this study, we have investigated the optimal swimming speed (U(opt)) of silver eels since U(opt) may be higher than the minimal swimming speed and is more likely to resemble the actual cruise speed. A variety of swimming tests were performed to compare endurance swimming between farmed eels and wild eels, both in freshwater and in seawater. The swimming tests were run with 101 silver female eels (60-96 cm, 400-1500 g) in 22 Blazka-type swim tunnels in a climatised room at 18 degrees C with running freshwater or seawater. Tests were run at 0.5-1.0 m s(-1) with increments of 0.1 m s(-1), and either 2 h or 12 h intervals. Remarkably, both tests revealed no changes in oxygen consumption (M O2) and COT over time. U(opt) values ranged between 0.61 and 0.68 m s(-1) (0.74-1.02 BL s(-1)) for the different groups and were thus 53-70% higher than the minimal speed. At U(opt), the COT was 37-50 mg O2 kg(-1) km(-1). These relatively very low values confirm our earlier observations. COT values in seawater were about 20% higher than in freshwater. Assuming that migrating female silver eels cruise at their U(opt), they will be able to cover the distance to the Sargasso Sea in 3-4 months, leaving ample time for final maturation and finding mates.

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.

Differential expression and regulation of gonadotropins and their receptors in the Japanese eel, Anguilla japonica

Eel species have a striking life cycle with a blockade of puberty until the oceanic migration. We report the first molecular data on eel gonadotropin receptors. The partial sequences cloned covered two-third of the open reading frame and included most of the extracellular and transmembrane domains. Phylogenetic analysis partitioned the two eel gonadotropin receptors into the two teleost FSHR and LHR clusters, respectively. Real-time quantitative RT-PCR was used to quantify the expression of eel gonadotropins and their receptors. Similar levels of pituitary FSH-beta and LH-beta transcripts were found in the immature previtellogenic female eels. In contrast, ovarian FSHR mRNA level was at 100- to 185-fold higher than that of LHR. This revealed that FSHR rather LHR would mediate gonadotropin stimulation of the early stages of ovarian growth. Chronic treatment with fish pituitary homogenates, applied to induce eel sexual maturation, stimulated pituitary LH-beta but suppressed FSH-beta transcripts. In the ovaries, both FSHR and LHR mRNA were significantly increased in experimentally matured eels. Treatments with sexual steroids showed a stimulatory effect of estradiol-17beta (E(2)) on pituitary LH-beta mRNA levels, while FSH-beta transcripts were suppressed by E(2) or testosterone (T). In contrast, neither E(2) nor T-treatment had any significant effect on ovarian FSHR nor LHR transcripts. This suggests that steroid feedbacks may be responsible for the opposite regulation of pituitary gonadotropins in experimentally matured eels, but are not involved in the regulation of gonadotropin receptors. In conclusion, these are the first data on the sequence, expression and regulation of gonadotropin receptors in the eel. They provide new foundation for basic and applied research on eel reproduction.

Effects of high hydrostatic pressure on the pituitary-gonad axis in the European eel, Anguilla anguilla (L.)

European silver eels are thought to undergo sexual maturation during their oceanic reproductive migration from the European continent to their spawning area in the Sargasso Sea. Tracking data and various anatomical and physiological features suggest that silver eels migrate in deep sea, leading us to hypothesise that high hydrostatic pressure (HP) influences the induction of eel reproduction. We subjected female and male silver eels to 101ATA for 3 and 7 weeks, respectively, in a hyperbaric chamber equipped with a freshwater recirculation system. In comparison with control eels kept at 1 ATA, HP effects were tested against the messenger RNA levels of pituitary gonadotropins (LHbeta, FSHbeta) using quantitative real-time RT-PCR. The effects of HP on gonadal activity were estimated by measuring gonadosomatic index, oocyte diameter and plasma levels of vitellogenin (Vtg) and sex steroids (E(2), 11-KT). At the pituitary level, LHbeta expression tended to increase while FSHbeta expression decreased in both sex, leading to an increase in the LHbeta/FSHbeta ratio. This suggests a differential effect of HP on the expression of the two gonadotropins. In females submitted to HP, we observed a significant increase in oocyte diameter and plasma levels of 11-KT and E(2). A similar trend was observed for 11-KT plasma levels in males. In females, Vtg plasma levels also significantly increased, reflecting the stimulatory effect of sex steroids on hepatic vitellogenesis. Our results suggest that HP plays a specific and positive role in eel reproduction but additional environmental and internal factors are necessary to ensure complete sexual maturation.

Development of real-time RT-PCR assays for eel gonadotropins and their application to the comparison of in vivo and in vitro effects of sex steroids

Gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are key factors in the brain-pituitary-gonad axis and understanding their regulation remains essential for future management of eel reproduction. In this regard, we developed quantitative real-time RT-PCR (qrtRT-PCR) assays for the expression of European eel LHbeta, FSHbeta and GPalpha subunits, using the Light Cycler system. The qrtRT-PCR was adapted to permit detection of the three gonadotropin subunit mRNAs in individual pituitaries and in dispersed pituitary cells. The validated assays were applied to investigate the effects of sex steroids (estrogens and androgens) on gonadotropin subunit expression, in vivo in steroid-injected eels, and in vitro by steroid treatments of primary cultures of eel pituitary cells. In vivo, a stimulation of LHbeta mRNA was observed after estradiol (E2) treatments, while testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT) had no effect. Concerning FSHbeta expression, slight but non-significant decreases were observed after both E2 and androgen treatments. Different results were obtained in vitro: E2 induced an increase in FSHbeta mRNA levels but had no effect on LHbeta expression. In contrast, androgens (T and DHT) stimulated LHbeta expression while no significant variation was observed on FSHbeta mRNA levels following androgen treatment. Concerning the GPalpha mRNA, no significant effect of sexual steroids was observed in vivo or in vitro. This demonstrated specific direct actions of steroids on gonadotropin subunit expression. The differences observed between in vivo and in vitro experiments may be explained by the involvement of cerebral control, including GnRH and dopamine neurons, and their specific regulation by sex steroids. The data indicate that sex steroid feedbacks on gonadotropins are exerted via multiple pathways, indirectly at the brain level and directly on pituitary gonadotrope cells.