Karine Rousseau

Patrick Prunet

Sylvie Dufour
email: sylvie.dufour@mnhn.fr

Rousseau Karine

Dufour Sylvie

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Keywords: stress, reproduction, teleosts, gonadotropic axis, corticotropic axis, evolution

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Neuroendocrine mechanisms of the interactions between corticotropic and gonadotropic axes in teleosts

3.1. Regulatory effects of actors of the corticotropic axis on the gonadotropic axis 3.1.1. Roles of corticosteroids and their receptors 3.1.1.1. Expression and role along the gonadotropic axis 3.1.1.2. In vivo effects of corticosteroids 3.1.1.3. In vitro effects of corticosteroids 3.1.2. Roles POMC-derived peptides 3.1.3. Roles of CRH 3.2. Regulatory effects of actors of the gonadotropic axis on the corticotropic axis 3.2.1. Roles of sex steroids 3.

Roles of other actors Conclusions Introduction

The first definition of stress by [START_REF] Selye | The evolution of the stress concept[END_REF] as "… the non-specific response of the body to any demand made upon it", refers to a general adaptation syndrome as a response to a stressor, which is similar irrespective of its nature [START_REF] Selye | The evolution of the stress concept[END_REF]. Since then, various definitions have appeared in the literature but the most complete one proposed by Schreck defines stress as "the physiological cascade of events that occurs when an organism is attempting to resist death or re-establish homeostatic norms in face of an insult" [START_REF] Schreck | Accumulation and long-term effects of stress in fish[END_REF]. In response to a stressful situation and the recognition of a threat by the central nervous system, an adaptive compensatory non-specific response takes place with an initial adrenergic response and a subsequent synthesis and secretion of cortisol due to activation of the corticotropic axis, hypothalamus-pituitary-adrenal (HPA) in mammals, birds and reptiles, and hypothalamus-pituitary-interrenal gland/head kidney (HPI) in amphibians and teleosts.

Activation of these endocrine pathways constitutes the primary responses, which lead to make energy available for systems involved in the stress responses. Thus, activation of the cardiovascular and respiratory responses accompanied by osmoregulatory disfunctions favour oxygen and energy substrates distribution and constitute the secondary responses. Tertiary responses refer to aspects of whole-animal performance and generally are maladaptative; they include changes in growth, swimming capacity, and modified behavioral patterns (feeding, aggression) [for reviews: [START_REF] Barton | Stress in fishes: A diversity of responses with particular reference to changes in circulating corticosteroids[END_REF][START_REF] Gorissen | The endocrinology of the stress response in fish : an adaptationphysiological view[END_REF][START_REF] Schreck | The concept of stress in fish[END_REF][START_REF] Wendelaar Bonga | The stress response in fish[END_REF]].

Stress is, in most cases, reported to affect reproduction in a deleterious way, but may also induce positive outcomes and a substantial body of research has been dedicated to decipher the mechanisms underlying the complex relationships between stress and reproduction [for reviews: [START_REF] Fuzzen | Stress and Reproduction[END_REF][START_REF] Leatherland | Stressors, glucocorticoids and ovarian function in teleosts[END_REF][START_REF] Milla | Corticosteroids: Friends or foes of teleost fish reproduction?[END_REF][START_REF] Pankhurst | Reproduction and development[END_REF][START_REF] Schreck | Stress and fish reproduction: The roles of allostasis and hormesis[END_REF]]. Since the first paper by [START_REF] Selye | Effect of adaptation to various damaging agents on the female sex organs in the rat[END_REF] [START_REF] Selye | Effect of adaptation to various damaging agents on the female sex organs in the rat[END_REF] which proposed that activation of the hypothalamo-pituitary-adrenal (HPA) axis inhibits the hypothalamopituitary-gonads (HPG) axis, this paradigm has been established mainly in mammalian species [START_REF] Chand | Stress and reproduction: Controversies and challenges[END_REF] but also in fish for which numerous reports emphasizing effects of corticosteroids on reproduction have been produced [START_REF] Fuzzen | Stress and Reproduction[END_REF][START_REF] Leatherland | Stressors, glucocorticoids and ovarian function in teleosts[END_REF][START_REF] Milla | Corticosteroids: Friends or foes of teleost fish reproduction?[END_REF][START_REF] Pankhurst | Reproduction and development[END_REF][START_REF] Schreck | Stress and fish reproduction: The roles of allostasis and hormesis[END_REF] . In this paper, we will focus our interest on the relationships between stress and reproductive neuroendocrine axes, trying to decipher direct stress effects and separate them from systemic effects on other biological functions. Thanks to the recent progress brought by genomic studies, we will be able to integrate the most recent informations provided by molecular phylogeny analyses and describe effects of new actors, which appeared via different rounds of whole genome duplications. This review will focus on teleost fish but with an evolutionary perspective requiring comparison with other vertebrate species.

Various interactions between stress, cortisol and reproduction in teleosts

Many relationships between stress and reproduction, with the influence of environmental conditions, have been documented in natural situation and in aquaculture. In this context, we first consider in this section the effects of stressors on reproduction, which are frequently associated with high cortisol levels.

Negative interactions between stress and reproduction

Early works in many teleost species reported the deleterious effects of stress and stress-related increase of cortisol on gonadotropic axis and reproductive performance. Chronically-induced stress such as by confinement or captivity and acutely-induced stress such as by predators or by aquaculture practices (handling, frequent netting, tank draining, crowding, noise) result in reduced plasma androgen and oestrogen levels [brown trout Salmo trutta: [START_REF] Pickering | The effects of acute and chronic stress on the levels of reproductive hormones in the plasma of mature male brown trout, Salmo trutta[END_REF]; wild spotted seatrout Cynoscion nebulosus: [START_REF] Safford | Effects of capture and handling on circulating levels of gonadal steroids and cortisol in the spotted seatrout, Cynoscion nebulosus[END_REF]; rainbow trout [START_REF] Clearwater | The response to capture and confinement stress of plasma cortisol, plasma sex steroids and vitellogenic oocytes in the marine teleost, red gurnard[END_REF]; striped trumpeter Latris lineata: [START_REF] Morehead | Effect of capture, confinement and repeated sampling on plasma steroid concentrations and oocyte size in female striped trumpeter Latris lineata (Latrididae)[END_REF]; roach Rutilus rutilus: [START_REF] Pottinger | Plasma cortisol and 17 -oestradiol levels in roach exposed to acute and chronic stress[END_REF]; black bream Acanthopagrus butcheri: [START_REF] Haddy | Stress-induced changes in concentrations of plasma sex steroids in black bream[END_REF]; sockeye salmon Oncorhynchus nerka: [START_REF] Kubokawa | Effects of acute stress on plasma cortisol, sex steroid hormone and glucose levels in male and female sockeye salmon during the breeding season[END_REF]; snapper Pagrus auratus: [START_REF] Carragher | Stress and reproduction in a commercially important marine fish, Pagrus auratus (Sparidae)[END_REF][START_REF] Cleary | The Effect of Capture and Handling Stress on Plasma Steroid Levels and Gonadal Condition in Wild and Farmed Snapper Pagrus auratus (Sparidae)[END_REF]; spiny damselfish Acanthochromis polyacanthus: [START_REF] Pankhurst | Stress inhibition of reproductive endocrine processes in a natural population of the spiny damselfish Acanthochromis polyacanthus[END_REF]; striped bass Morone saxatilis: [START_REF] Castranova | The effects of stress on androgen production, spermiation response and sperm quality in high and low cortisol responsive domesticated male striped bass[END_REF]; jundia Rhamdia quelen: [START_REF] Soso | The effects of stressful broodstock handling on hormonal profiles and reproductive performance of rhamdia quelen (Quoy & Gaimard) females[END_REF]; tilapia Oreochromis mossambicus: [START_REF] Chabbi | Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus[END_REF]]. Stress is also associated with reduced plasma vitellogenin (Vg) levels [brown and rainbow trout: [START_REF] Campbell | Preliminary evidence that chronic confinement stress reduces the quality of gametes produced by brown and rainbow trout[END_REF]]. It also decreases plasma luteinizing hormone (LH) levels [white sucker Catostomus commersoni: [START_REF] Van Der Kraak | Exposure to bleached kraft pulp mill effluent disrupts the pituitary-gonadal axis of white sucker at multiple sites[END_REF]],

suppresses LH secreting cell activity [tilapia: [START_REF] Chabbi | Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus[END_REF]], as well as decreases hypothalamic gonadotropin-releasing hormone 1 (GnRH1) mRNA levels [jack mackerel Trachurus japonicus: [START_REF] Imanaga | Characterization of gonadotropin-releasing hormone and gonadotropin in jack mackerel (Trachurus japonicus): Comparative gene expression analysis with respect to reproductive dysfunction in captive and wild fish[END_REF]]. After applied stress, impacts on final stages of reproduction are also observed such as delayed ovulation [rainbow trout: [START_REF] Campbell | Stress Reduces the Quality of Gametes Produced by Rainbow Trout[END_REF][START_REF] Contreras-Sanchez | Effects of stress on the reproductive performance of rainbow trout (Oncorhynchus mykiss)[END_REF]], reduced egg size in females [brown and rainbow trout: [START_REF] Campbell | Preliminary evidence that chronic confinement stress reduces the quality of gametes produced by brown and rainbow trout[END_REF][START_REF] Campbell | Stress Reduces the Quality of Gametes Produced by Rainbow Trout[END_REF] ; Neolamprologus pulcher: [START_REF] Mileva | Effects of maternal stress on egg characteristics in a cooperatively breeding fish[END_REF] and reduced sperm counts in males [brown and rainbow trout: [START_REF] Campbell | Preliminary evidence that chronic confinement stress reduces the quality of gametes produced by brown and rainbow trout[END_REF]],

increased gonadal atresia [(pike Esox lucius: (De Montalembert et al., 1978); red gurnard: [START_REF] Clearwater | The response to capture and confinement stress of plasma cortisol, plasma sex steroids and vitellogenic oocytes in the marine teleost, red gurnard[END_REF]; snapper: [START_REF] Cleary | The Effect of Capture and Handling Stress on Plasma Steroid Levels and Gonadal Condition in Wild and Farmed Snapper Pagrus auratus (Sparidae)[END_REF]; Atlantic bluefin tuna Thunnus thynnus: [START_REF] Corriero | Evidence that severe acute stress and starvation induce rapid atresia of ovarian vitellogenic follicles in Atlantic bluefin tuna, Thunnus thynnus (L.) (Osteichthyes: Scombridae)[END_REF]], and reduced fecundity [rainbow trout : [START_REF] Contreras-Sanchez | Effects of stress on the reproductive performance of rainbow trout (Oncorhynchus mykiss)[END_REF]]. Furthermore, impacts on progeny also occur such as reduced length [tropical damselfish Pomacentrus amboinensis: [START_REF] Mccormick | Indirect effects of heterospecific interactions on progeny size through maternal stress[END_REF][START_REF] Mccormick | Mothers matter: Crowding leads to stressed mothers and smaller offspring in marine fish[END_REF][START_REF] Mccormick | Experimental test of the effect of maternal hormones on larval quality of a coral reef fish[END_REF][START_REF] Mccormick | Behaviorally induced maternal stress in a fish influences progeny quality by a hormonal mechanism[END_REF]],

decreased survival [brown and rainbow trout: [START_REF] Campbell | Preliminary evidence that chronic confinement stress reduces the quality of gametes produced by brown and rainbow trout[END_REF][START_REF] Campbell | Stress Reduces the Quality of Gametes Produced by Rainbow Trout[END_REF]] or occurrence of abnormalities [Atlantic cod Gadus morhua: [START_REF] Morgan | The effect of stress on reproduction in Atlantic cod[END_REF]] [for reviews: [START_REF] Billard | Stress, environment and reproduction in teleost fish[END_REF][START_REF] Pankhurst | Effects of stress on reproduction and growth of fish. In Fish stress and health in aquaculture[END_REF][START_REF] Pankhurst | Reproduction and development[END_REF][START_REF] Pickering | Environmental stress and the survival of brown trout, Salmo trutta[END_REF]].

7

Socially mediated stress response and downregulated reproductive function are observed in some teleosts, such as in cichlids. For example, subordinate (non-territorial) males of Astatotilapia (Haplochromis) burtoni display high expression of hypothalamic and pituitary corticotropin-releasing hormone (CRH) receptor as well as high plasma cortisol levels, but low mRNA levels of hypothalamic GnRH1 and of pituitary GnRH receptor, LH and FSH, as well as low plasma levels of LH and FSH, and androgens, compared to dominant (territorial) males [for review: [START_REF] Maruska | Social regulation of reproduction in male cichlid fishes[END_REF]].

Positive interactions between cortisol and reproduction related to peculiar life cycles

Cortisol and environmentally-related spawning activity in some teleosts

The catfish, Heteropneustes fossilis, awaits the onset of the monsoon rainfall for spawning in order to get favorable environmental conditions. Interestingly, in this species, plasma cortisol levels exhibit a peak at monsoon [START_REF] Lamba | Circannual and circadian variations in plasma levels of steroids (cortisol, estradiol-17β estrone, and testosterone) correlated with the annual gonadal cycle in the catfish, Heteropneustes fossilis (Bloch)[END_REF]Sundararaj andGoswami, 1966a, 1966b), and glucocorticoids are effective both in vivo and in vitro in inducing ovulation [START_REF] Sundararaj | Hormonal regulation of in vivo and in vitro oocyte maturation in the catfish, Heteropneustes fossilis (bloch)[END_REF]. In vivo administration of an ovulatory dose of LH in this species induces an increase in plasma levels of cortisol followed by an increase in sex steroid (T and E2) levels, while ACTH injection only stimulates cortisol levels. The authors suggest that gonadotropin acts at two loci, the interrenal and the ovary [START_REF] Goswami | Gonadotrophin-induced oocyte maturation in the catfish, Heteropneustes fossilis (Bloch), requires steroidogenesis in both interrenal and ovary[END_REF]. All these data indicate a potential positive role of cortisol in the induction of spawning in the catfish. Another striking example is the killifish Fundulus heteroclitus which exhibits synchronized spawning with the semilunar cycle of spring tides and a peak of plasma cortisol level coincides with spawning [START_REF] Borowsky | Semilunar Changes in Estradiol and Cortisol Coincident with Gonadal Maturation and Spawning in the Killifish Fundulus heteroclitus[END_REF].

Cortisol and upstream reproductive migration in salmonids

Upstream migration and spawning require high energy reserve mobilization; in Atlantic salmon Salmo salar, for example, between 60% and 70% of the body reserves are spent during upstream migration and spawning [START_REF] Jonsson | Changes in Proximate Composition and Estimates of Energetic Costs During Upstream Migration and Spawning in Atlantic Salmon Salmo salar[END_REF]. High plasma cortisol levels and interrenal hyperplasia have long been reported during the later stages of the spawning migration and at the time of spawning in various salmonids [Pacific sockeye salmon: [START_REF] Donaldson | Corticosteroid dynamics in Pacific salmon[END_REF][START_REF] Hane | Changes in Plasma 17-Hydroxycorticosteroids Accompanying Sexual Maturation and Spawning of the Pacific Salmon (Oncorhynchus Tschawytscha) and Rainbow Trout (Salmo Gairdnerii)[END_REF][START_REF] Idler | Biochemical studies on sockeye salmon during spawning migration. VII. Steroid hormones in plasma[END_REF]; migratory (steelhead) rainbow trout: [START_REF] Robertson | Hyperadrenocorticism in Spawning migratory and Nonmigratory Rainbow Trout (Salmo gairdnerii ) ; Comparison with Pacific Salmon (Genus Oncorhynchus)[END_REF]; Atlantic salmon: [START_REF] Schmidt | Steroid hormones in the plasma of salmon at various states of maturation[END_REF]; kokanee salmon, landlocked form of sockeye salmon: [START_REF] Carruth | Elevation of plasma cortisol during the spawning migration of landlocked kokanee 57 salmon (Oncorhynchus nerka kennerlyi)[END_REF]]. However, one study in sockeye salmon reports that sexual maturation is not necessarily accompanied by elevated plasma cortisol levels [START_REF] Fagerlund | Plasma cortisol concentration in relation to stress in adult sockeye salmon during the freshwater stage of their life cycle[END_REF]. Increased cortisol level is also described at spawning in non-migratory rainbow trout [START_REF] Bry | Plasma cortisol levels of female rainbow trout (Salmo gairdneri) at the end of the reproductive cycle: Relationship with oocyte stages[END_REF][START_REF] Robertson | Hyperadrenocorticism in Spawning migratory and Nonmigratory Rainbow Trout (Salmo gairdnerii ) ; Comparison with Pacific Salmon (Genus Oncorhynchus)[END_REF]. Altogether these studies indicate that high cortisol levels in salmonids are, at least, not deleterious to sexual maturation and spawning.

Cortisol and oceanic reproductive migration in eels

The downstream and oceanic migrations of eels (Anguilla species), as well as their whole gonadal development from prepubertal stage to full sexual maturation, require both metabolic energy and metabolites that come exclusively from body stores [for review: [START_REF] Palstra | Swimming physiology of European silver eels (Anguilla anguilla L.): Energetic costs and effects on sexual maturation and reproduction[END_REF]]. Clevestam and collaborators report that 45% of European eel (Anguilla Anguilla) from Baltic sea would be within 90% of complete energy depletion after migration and reproduction and 20% would have completey exhausted their initial fat reserves [START_REF] Clevestam | Too short to spawn? Implications of small body size and swimming distance on successful migration and maturation of the European eel Anguilla anguilla[END_REF]. Eel, at the silver prepubertal, downstream migratory, stage and during their subsequent reproductive oceanic migration and sexual migration, are fasting and all their metabolic stores, accumulated during the juvenile growth (yellow stage) phase, will be mobilized. In silver eels, an elevation of plasma cortisol levels is observed prior to downstream migration [START_REF] Van Ginneken | Silvering of European eel (Anguilla anguilla L.): Seasonal changes of morphological and metabolic parameters[END_REF]. Early studies of hypophysectomy and adrenalectomy report the need of an intact pituitary-interrenal axis for maintenance of liver glycogen levels in eels [Anguilla anguilla: [START_REF] Hatey | La fonction glycogénique dur foie de l'anguille (Anguilla anguilla L.) après hypophysectomie[END_REF]; American eel Anguilla rostrata: [START_REF] Butler | Hormonal control of gluconeogenesis in the North American eel (Anguilla rostrata)[END_REF]]. Cortisol induces mobilization of lipid and protein stores and stimulates hepatic neoglucogenesis [for review: [START_REF] Butler | Structure and function of the adrenal gland of fishes[END_REF]; Japanese eel Anguilla japonica: [START_REF] Chan | Effect of cortisol on the metabolism of the eel, Anguilla japonica[END_REF]; Anguilla anguilla: [START_REF] Dave | Effects of cortisol on the fatty acid composition of the total blood plasma lipids in the European eel, Anguilla anguilla[END_REF]; Anguilla rostrata: [START_REF] Butler | Hormonal control of gluconeogenesis in the North American eel (Anguilla rostrata)[END_REF][START_REF] Foster | Cortisol and liver metabolism of immature American eels, Anguilla rostrata (LeSueur)[END_REF]]. Cortisol also induces eel vertebral demineralization, by promoting both osteocytic osteolysis and osteoclastic resorption, allowing mobilization of phosphocalcic stores necessary for vitellogenin synthesis [START_REF] Sbaihi | Cortisol mobilizes mineral stores from vertebral skeleton in the European eel: An ancestral origin for glucocorticoid-induced osteoporosis?[END_REF]. Besides these actions on metabolism necessary for the eel migratory and reproductive processes, cortisol may also directly stimulate the gonadotropic axis, as shown by its positive effect on pituitary lhβ mRNA and LH protein content in vivo and in vitro [Anguilla anguilla: [START_REF] Huang | Cortisol selectively stimulates pituitary gonadotropin β-subunit in a primitive teleost, Anguilla anguilla[END_REF]].

Thus, in the eel, cortisol may both coordinate storage mobilization and participate in the induction of sexual maturation during reproductive migration.

Cortisol and gonadal sex differentiation

In some teleost fish species, gonadal sex determination and/or sex change are under the regulation of environmental factors. An increasing number of data proposes cortisol as a key factor integrating environmental cues (such as temperature or social status) to induce male sex determination/sex change [for reviews: [START_REF] Fernandino | Environmental stress-induced testis differentiation: Androgen as a by-product of cortisol inactivation[END_REF][START_REF] Fernandino | Sex determination in Neotropical fish: Implications ranging from aquaculture technology to ecological assessment[END_REF][START_REF] Goikoetxea | Stress and sex: Does cortisol mediate sex change in fish?[END_REF][START_REF] Liu | Sexual plasticity: A fishy tale[END_REF][START_REF] Perry | A model for social control of sex change: Interactions of behavior, neuropeptides, glucocorticoids, and sex steroids[END_REF][START_REF] Solomon-Lane | Stress and serial adult metamorphosis: Multiple roles for the stress axis in socially regulated sex change[END_REF][START_REF] Todd | Bending Genders: The Biology of Natural Sex Change in Fish[END_REF]]. Cortisol administration or stress-induced cortisol release (including high temperature-induced effects) promotes masculinization in a number of teleosts [rainbow trout: (van den Hurk and van Oordt, 1985); pejerrey Odontesthes bonariensis: [START_REF] Hattori | Cortisol-induced masculinization: Does thermal stress affect gonadal fate in pejerrey, a teleost fish with temperature[END_REF]; Japanese flounder Paralichthys olivaceus: [START_REF] Yamaguchi | Cortisol is involved in temperature-dependent sex determination in the Japanese flounder[END_REF]; Southern flounder Paralichthys lethostigma: [START_REF] Mankiewicz | Masculinizing effect of background color and cortisol in a flatfish with environmental sex-determination[END_REF]; three-spot wrasse Halichoeres trimaculatus: [START_REF] Nozu | Cortisol administration induces sex change from ovary to testis in the protogynous wrasse, Halichoeres trimaculatus[END_REF]; black sea bass Centropristis striata: [START_REF] Miller | The effects of stress, cortisol administration and cortisol inhibition on black sea bass (Centropristis striata) sex differentiation[END_REF]; medaka Oryzias latipes [START_REF] Hayashi | High temperature causes masculinization of genetically female medaka by elevation of cortisol[END_REF]; orange spotted grouper Epinephelus coioides [START_REF] Chen | The Administration of Cortisol Induces Female-to-Male Sex Change in the Protogynous Orange-Spotted Grouper, Epinephelus coioides[END_REF]]. Several potential pathways are described for cortisol triggering maleness: cross-talk between glucocorticoid and androgen metabolism (11β-hydroxysteroid dehydrogenase, 11βHSD, and 11β-hydroxylase, Cyp11b, enzymes) promoting the synthesis of 11-ketotestosterone (11KT); inhibition of aromatase (cyp19a1a) expression, leading to a shift in steroidogenesis from estrogens to androgens; and upregulation of anti-Müllerian homone (amh) expression [for reviews: [START_REF] Fernandino | Environmental stress-induced testis differentiation: Androgen as a by-product of cortisol inactivation[END_REF][START_REF] Fernandino | Sex determination in Neotropical fish: Implications ranging from aquaculture technology to ecological assessment[END_REF][START_REF] Goikoetxea | Stress and sex: Does cortisol mediate sex change in fish?[END_REF][START_REF] Liu | Sexual plasticity: A fishy tale[END_REF][START_REF] Perry | A model for social control of sex change: Interactions of behavior, neuropeptides, glucocorticoids, and sex steroids[END_REF][START_REF] Solomon-Lane | Stress and serial adult metamorphosis: Multiple roles for the stress axis in socially regulated sex change[END_REF]]. In the pejerrey, cortisol increases the expression of hsd11b2, the gene for 11βHSD, an enzyme which deactivates cortisol to cortisone and catalyzes the final step in 11-KT synthesis [START_REF] Fernandino | The cortisol and androgen pathways cross talk in high temperature-induced masculinization: The 11β-hydroxysteroid dehydrogenase as a key enzyme[END_REF]. Typical DNA binding sites of glucocorticoid receptor (GR) i.e. glucocorticoid response elements (GRE) are identified in the cyp19a1a promoter in the goby Gobiodon histrio [START_REF] Gardner | Sex change strategy and the aromatase genes[END_REF] and in the black sea bass [START_REF] Miller | The effects of stress, cortisol administration and cortisol inhibition on black sea bass (Centropristis striata) sex differentiation[END_REF].

Interestingly, GR is also shown to directly interact with cAMP-responsive element (CRE) on cyp19a1a promoter of Japanese flounder, down-regulating the expression of aromatase [START_REF] Yamaguchi | Cortisol is involved in temperature-dependent sex determination in the Japanese flounder[END_REF]. Temperature-dependent sex determination (TSD) is shown to involve regulation of DNA methylation of the cyp19a1a promoter [European sea bass Dicentrarchus labrax: [START_REF] Navarro-Martín | DNA methylation of the gonadal aromatase (cyp19a) promoter is involved in temperature-dependent sex ratio shifts in the European sea bass[END_REF]], and such epigenetic modifications may also be a mechanism by which cortisol regulates aromatase expression.

All these studies highlight the role of cortisol as a mediator between environmental conditions, especially temperature, and male sex determination in various teleost species. This confers to cortisol a key-role in the potential impact of global climatic change on the modification of sex ratio in some teleost species and its consequences on population sustainability.

Animal welfare, environment, stress and reproduction

Concerns about fish welfare have increased considerably during the recent years owing to the growing public interest for the negative impacts of intensification in aquaculture and for scientific debate about pain in fish. Many authors agree on a definition for which welfare is not only based on physical health but also lack of mental suffering and presence of positive feelings [START_REF] Huntingford | Current issues in fish welfare[END_REF][START_REF] Segner | Health of farmed fish: Its relation to fish welfare and its utility as welfare indicator[END_REF][START_REF] Sneddon | Stress management and welfare[END_REF][START_REF] Stevens | Stress and welfare in ornamental fishes: what can be learned from aquaculture?[END_REF][START_REF] Toni | Review: Assessing fish welfare in research and aquaculture, with a focus on European directives[END_REF]. In this context, most of the welfare issues are related to stress responses, experience of pains, growth problems, incidence of disease, abnormal behaviors and less frequently to degradated breeding with negative impact on male or female reproduction. These effects observed in aquaculture environment are not only the consequence of exposure of adult fish to stressors (e.g. confinement, handling, hypoxia or poor water quality) but also associated with abnormal behavior or impossibility to develop a normal behavior created by captive environment [START_REF] Sneddon | Stress management and welfare[END_REF]). Thus, one potential consequence of confinement is irregular spawners which suffer from a low fertilisation rate and a broken ovulatory rhythm [START_REF] Kjesbu | The spawning activity of cod, Gadus morhua L[END_REF][START_REF] Patterson | The effect of exercise and captivity on energy partitioning, reproductive maturation and fertilization success in adult sockeye salmon[END_REF]. Intense swimming activity during chase and capture can also be sufficient to compromise reproduction [START_REF] Pankhurst | Effects of stress on reproduction and growth of fish. In Fish stress and health in aquaculture[END_REF]. Repeated acute confinement stress during the spawning season as well as poor body condition or food deprivation have also negative effects on fecundity in cod, all effects which have been suggested to be related to altered energy allocation [START_REF] Bogevik | The effect of dietary lipid content and stress on egg quality in farmed Atlantic cod Gadus morhua[END_REF][START_REF] Kjesbu | Fecundity, atresia, and egg size of captive Atlantic cod (Gadus mrhua) in relation to proximate body composition[END_REF][START_REF] Kjesbu | The spawning activity of cod, Gadus morhua L[END_REF][START_REF] Lambert | Energetic consequences of reproduction in Atlantic cod (Gadus morhua) in relation to spawning level of somatic energy reserves[END_REF].

Behavioral needs can also be a welfare issue, not only by preventing maltreatment but also by providing adult fish resources to perform natural behavioural repertoire necessary for reproduction, accommodations which are species specific [START_REF] Sneddon | Stress management and welfare[END_REF]. Sometimes also, search for optimal performance in aquaculture can lead to welfare issue in relation to reproduction. In salmon aquaculture, early maturation can be a significant welfare issue. The maturation process is energetically expensive which is reflected in early maturing salmon by decreased growth rate and increased mortality through susceptibility to pathogens [START_REF] Gjerde | Response to individual selection for age at sexual maturity in Atlantic salmon[END_REF][START_REF] Thorpe | Reproductive strategies in Atlantic salmon, Salmo salar[END_REF]. Selection for both fast growth and late sexual maturation has been considered problematic as it has been suggested that there is a correlation between the phenotypes fast growth and early sexual maturation [START_REF] Thorpe | Inheritance of developmental rates in Atlantic salmon, Salmo salar[END_REF]. The importance of energy cost for reproduction also appears when considering exercised female sockeye salmon compared to non-exercised fish: they displayed lower lipid content, delayed maturity, lower egg deposition rate and higher egg mortality prior to ovulation [START_REF] Chellappa | Depletion of energy reserves during reproductive aggression in male three-spinned stickleback, Gasterosteus aculeatusL[END_REF][START_REF] Hansen | Discovery of a novel insect neuropeptide signaling system closely related to the insect adipokinetic hormone and corazonin hormonal systems[END_REF]. Methods used in aquaculture for egg collection from female is also a crucial procedure for fish reproduction in salmonid aquaculture: comparison between various methods (massage of the fish abdomen versus pressure of gas) in rainbow trout indicated that air stripping led to better quality of eggs and better juvenile survival rate [START_REF] Kowalski | Effectiveness of the air stripping in two salmonid fish, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta morpha fario)[END_REF]. In recreational fisheries, studies on angling and release of nesting species during the spawning season reported decreased progenity survival and paternal nest abandonment and impaired care [START_REF] Cooke | Locomotory impairment of nesting male largemouth bass following catch-and-release angling[END_REF][START_REF] Hanson | Effects of different angling practices on post-release behaviour of nest-guarding male black bass, Micropterus spp[END_REF][START_REF] Philipp | The Impact of Catch-and-Release Angling on the Reproductive Success of Smallmouth Bass and Largemouth Bass[END_REF].

However, specific investigations on gonadal development of final maturation find very few adverse effects [START_REF] Booth | Effects of late-season catch and release angling on anaerobic metabolism, acid-base status, survival, and gamete viability in wild Atlantic salmon (Salmo salar)[END_REF][START_REF] Hall | Effects of angling on postrelease mortality, gonadal development and somatic condition of Australian bass Macquaria novemaculeata[END_REF][START_REF] Lowerre-Barbieri | Emerging issues and methodological advances in fisheries reproductive biology[END_REF].

Moreover, when testing effects of mild angling and release on golden perch Macquaria ambigua, normal gonadal development is observed in angled fish, a result attributed to the flexible reproductive strategy of that species and benigness of the mouth hooking [START_REF] Hall | Sublethal effects of angling and release on golden perch Macquaria ambigua: implications for reproduction and fish health[END_REF]. These results illustrate the complexity of welfare questions related to reproduction, which certainly need to take into account the specificity of the fish species and/or the characterisitics of the environment and stressors.

Overview of neuroendocrine stress and reproductive axes in teleosts

Teleost specific traits compared to other vertebrates

In mammals, brain-pituitary-peripheral glands axes are a classical schema when describing the endocrine regulation of major physiological functions, such as reproduction with the gonadotropic axis or stress with the corticotropic axis. Similar structures are observed in teleost fish which also present specificities which need to be considered for understanding stress/reproduction relationship in these species.

Anatomo-functional specificities

Pituitary regionalisation and direct innervation

The pituitary gland is a vertebrate innovation [for reviews: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF][START_REF] Sower | Landmark discoveries in elucidating the origins of the hypothalamicpituitary system from the perspective of a basal vertebrate, sea lamprey[END_REF]]. In all vertebrates, the pituitary consists of the adenohypophysis and the neurohypophysis. The adenohypophysis has been subdivided by early studies into the pars distalis and the pars intermedia, with in tetrapods, an additional pars tuberalis [for reviews: [START_REF] De Beer | The Evolution of the Pituitary[END_REF][START_REF] Dores | The evolution of the pituitary[END_REF]]. In contrast to tetrapods in which cells of the pars distalis seem widely distributed, in teleosts, the different types of cells of the pars distalis are regionalised [Poecilia formosa and latipinna [START_REF] Olivereau | Contribution à l'histophysiologie de l'hypophyse des téléostéens, en particulier de celle de Poecilia species[END_REF]; European eel [START_REF] Olivereau | Observations sur l'hypophyse de l'Anguille femelle, en particulier lors de la maturation sexuelle[END_REF]; Atlantic halibut Hippoglossus hippoglossus [START_REF] Weltzien | The brain-pituitary-gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)[END_REF]; zebrafish Danio rerio [START_REF] Pogoda | Molecular genetics of pituitary development in zebrafish[END_REF]; for reviews: [START_REF] Schreibman | Functional morphology of the teleost pituitary gland[END_REF][START_REF] Trudeau | Multimodal hypothalamo-hypophysial communication in the vertebrates[END_REF][START_REF] Zohar | Neuroendocrinology of reproduction in teleost fish[END_REF]]. This allowed an anatomical subdivision of the pars distalis (PD), in teleosts, into an anterior region, the rostral pars distalis (RPD) composed of lactotropes (prolactin cells) and corticotropes (corticotropin, ACTH cells), and a posterior region, the proximal (or caudal) pars distalis (PPD) composed of somatotropes (growth hormone cells), thyreotropes (thyrotropin TSH cells) and gonadotropes (luteinizing hormone LH and follicle stimulating hormone FSH cells). Interestingly, in teleosts, the two gonadotropins, LH and FSH, are expressed in different pituitary cells, in contrast to the situation observed in mammals which produce LH and FSH in the same pituitary cell [for review: [START_REF] Kanda | Evolution of the regulatory mechanisms for the hypothalamic-pituitarygonadal axis in vertebrates-hypothesis from a comparative view[END_REF]]. This feature allowed Golan and collaborators to investigate in zebrafish the differential organization of LH and FSH cells and they reported that LH cells are coupled via strong gap-junction, while FSH cells show long cytoplasmic extension to maintain contact, possibly explaining their differential release patterns [START_REF] Golan | Anatomical and functional gonadotrope networks in the teleost pituitary[END_REF]. In teleosts as in tetrapods, the pars intermedia of the pituitary contains melanotrope cells which produce MSH (melanocyte-stimulating hormone) from the same precusor as ACTH.

In tetrapods, brain hypophysiotropic neurons project to the median eminence at the basis of the hypothalamus and release their neurohormones into the hypophyseal portal vascular system, which carries them to the pars distalis. Differently, in teleosts, the axonal endings of the brain hypophysiotropic neurons terminate in close vicinity to the cells of the adenohypophysis providing a direct innervation [for review: [START_REF] Trudeau | Multimodal hypothalamo-hypophysial communication in the vertebrates[END_REF][START_REF] Zohar | Neuroendocrinology of reproduction in teleost fish[END_REF]].

Caudal neurosecretory system

The caudal neurosecretory system (CNSS) is unique to some non-mammalian vertebrates such as teleosts (Osteichthyes) and elasmobranchs (Chondrichthyes). It is situated in the posterior region of the spinal cord and it comprises large neurosecretory neurons named after their discoverer, the Dahlgren cells [START_REF] Dahlgren | The electric motor nerve centers in the skate (Rajidae)[END_REF]. These neurons project their axons to a neurohaemal organ, the urophysis, where neuropeptides are released into the renal portal system via the caudal vein to the head kidney [for reviews: [START_REF] Bern | The caudal neurosecretory of fishes[END_REF][START_REF] Mccrohan | Fish caudal neurosecretory system: A model for the study of neuroendocrine secretion[END_REF][START_REF] Winter | The caudal neurosecretory system: Control and function of a novel neuroendocrine system in fish[END_REF]]. Two of these neuropeptides were first isolated from CNSS and named urotensins: urotensin I in white sucker Catostomus commersoni [START_REF] Lederis | Complete amino acid sequence of urotensin I, a hypotensive and corticotropin-releasing neuropeptide from Catostomus[END_REF] and common carp Cyprinus carpio [START_REF] Ichikawa | Isolation and amino acid sequence of urotensin I, a vasoactive and ACTH-releasing neuropeptide, from the carp (Cyprinus carpio) urophysis[END_REF] and urotensin II in the goby Gillichthys mirabilis [START_REF] Pearson | Urotensin II: a somatostatin--like peptide in the caudal neurosecretory system of fishes[END_REF]. Urotensin I is related to CRH [for review: [START_REF] Lovejoy | Molecular evolution of GPCRs: CRH/CRH receptors[END_REF]] and urotensin II to somatostatin [for review: [START_REF] Tostivint | Molecular evolution of GPCRs: Somatostatin/urotensin II receptors[END_REF]]. Apart from these urotensins, the CNSS produces two other neuropeptides, CRH and parathyroid hormone-related protein, as well as a neurotransmitter, acethylcholine [for review [START_REF] Mccrohan | Fish caudal neurosecretory system: A model for the study of neuroendocrine secretion[END_REF]]. In teleosts, together with the preoptic area of the brain, the CNSS is the major source of CRH and urotensins [European flounder Platichthys flesus: [START_REF] Lu | Coexpression of corticotropin-releasing hormone and urotensin I precursor genes in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus): A possible shared role in peripheral regulation[END_REF]; rainbow trout: [START_REF] Bernier | Heads or tails? Stressor-specific expression of corticotropin-releasing factor and urotensin I in the preoptic area and caudal neurosecretory system of rainbow trout[END_REF][START_REF] Craig | Differential increase in forebrain and caudal neurosecretory system CRF and urotensin I gene expression associated with seawater transfer in rainbow trout[END_REF]; zebrafish: [START_REF] Alderman | Ontogeny of the corticotropin-releasing factor system in zebrafish[END_REF]], two neuropeptides which are involved in the stress axis (cf § 2.2.2.1).

Teleost specific whole genome duplication

Two whole genome duplications (WGD) likely occurred in ancestral vertebrates and are named 1R and 2R for first and second round of WGD, respectively [for review: [START_REF] Dehal | Two rounds of whole genome duplication in the ancestral vertebrate[END_REF]]. An additional WGD specifically occurred at the basis of the teleost lineage, referred to as teleost specific WGD (TWGD) or as 3R for third round of WGD [for review: [START_REF] Meyer | From 2R to 3R: Evidence for a fish-specific genome duplication (FSGD)[END_REF]]. WGD events led to the expansion of gene numbers. This additional WGD in teleosts would have favored the evolutionary success and the remarkable biological diversity of teleosts, the largest vertebrate group. Concerning the neuroendocrine axes, the 3R is at the origin of additional paralogs for a number of actors of both gonadotropic and corticotropic axes in teleosts. Further additional WGD occurred more recently independently in some teleost groups, such as in salmonids [START_REF] Lien | The Atlantic salmon genome provides insights into rediploidization[END_REF][START_REF] Robertson | Lineage-specific rediploidization is a mechanism to explain time-lags between genome duplication and evolutionary diversification[END_REF] and in carps [START_REF] Larhammar | Molecular genetic aspects of tetraploidy in the common carp cyprinus carpio[END_REF][START_REF] Wang | Transcriptome analysis reveals the time of the fourth round of genome duplication in common carp (Cyprinus carpio)[END_REF] and are referred to as 4R for fourth round of WGD. Conservation of duplicated paralogs may be related to amplification of function, sharing of multiple preexisting functions (subfunctionalization) or acquisition of new function (neofunctionalization) [for review: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF]].

The corticotropic axis

Main actors of the corticotropic axis in vertebrates

In all vertebrates, stress response is regulated by the corticotropic axis (HPA in mammals and sauropsids and HPI in amphibians and teleosts) [for review: [START_REF] Gorissen | The endocrinology of the stress response in fish : an adaptationphysiological view[END_REF]]. The neurohormone, corticotropin-releasing hormone (CRH), as its name indicates, controls the production and release of corticotropin (also named adrenocorticotropic hormone, ACTH), at the pituitary level. ACTH is the result of tissue-specific post-translational processing of proopiomelanocortin (POMC), together with melanocyte stimulating hormone (MSH), and βendorphin (β-END). ACTH and MSH, called melanocortins (MC) act via MC receptors (MCR), while β-END acts via opioid receptors. ACTH from the corticotrophs of the pars distalis of the pituitary controls via type 2 MCR (melanocortin-2 receptor, MC2R), glucocorticoid production and release from adrenal cortex cells in amniotes or interrenal cells in amphibians and teleosts. Major glucocorticoids are cortisol in most mammals and ray finned fish and corticosterone in most birds, amphibians and reptiles [for review: [START_REF] Aerts | Quantification of a glucocorticoid profile in non-pooled samples is pivotal in stress research across vertebrates[END_REF]]. MC2R, in teleosts as in tetrapods, is ligand selective as it can only be activated by ACTH and not by MSH, and requires coexpression with an accessory protein, melanocortin-2 receptor accessory protein (MRAP) for trafficking to the cell surface [for reviews: [START_REF] Dores | Hypothesis and Theory: Revisiting Views on the Co-evolution of the Melanocortin Receptors and the Accessory Proteins, MRAP1 and MRAP2[END_REF][START_REF] Dores | Melanocortin receptors: Evolution of ligand selectivity for melanocortin peptides[END_REF]]. Glucocorticoids negatively feedback on the brain (hypothalamic CRH) / pituitary (ACTH) corticotropic axis [for reviews: [START_REF] Bernier | Regulation And Contribution Of Corticotropic, Melanotropic And Thyrotropic Axes To The Stress Response In Fishes[END_REF][START_REF] Faught | The Molecular Stress Response[END_REF][START_REF] Gorissen | The endocrinology of the stress response in fish : an adaptationphysiological view[END_REF]]; this negative feedback regulation of the HPI axis involves glucocorticoid receptor (GR) signaling pathway.

Specific features of the corticotropic axis in teleosts

The hypothalamic regulation of corticotropes and melanotropes in teleosts has already been extensively reviewed by Bernier and collaborators [START_REF] Bernier | Regulation And Contribution Of Corticotropic, Melanotropic And Thyrotropic Axes To The Stress Response In Fishes[END_REF].

CRH and related peptides and their receptors

CRH, first isolated from sheep hypothalamus, together with urotensin I (UI) isolated from the CNSS of the white sucker [START_REF] Lederis | Complete amino acid sequence of urotensin I, a hypotensive and corticotropin-releasing neuropeptide from Catostomus[END_REF] and the common carp [START_REF] Ichikawa | Isolation and amino acid sequence of urotensin I, a vasoactive and ACTH-releasing neuropeptide, from the carp (Cyprinus carpio) urophysis[END_REF], and with sauvagine (SVG) isolated from the skin of the amphibian Phylomedusa sauvagei [START_REF] Montecucchi | Isolation and amino ai-cid composition of sauvagine[END_REF], form a large family of peptides. Later on, urocortins were identified in mammals and also found to be related to CRH: urocortin 1 (Ucn1), ortholog of UI and SVG [START_REF] Vaughan | Urocortin, a mammalian neuropeptide related to fish urotensin I and to corticotropinreleasing factor[END_REF]; urocortin 2 [Ucn2: [START_REF] Reyes | Urocortin II: A member of the corticotropin-releasing factor (CRF) neuropeptide family that is selectively bound by type 2 CRF receptors[END_REF]] and urocortin 3 [Ucn3: [START_REF] Lewis | Identification of urocortin III, an additional member of the corticotropin-releasing factor (CRF) family with high affinity for the CRF2 receptor[END_REF]] [for review: [START_REF] Lovejoy | Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates[END_REF]]. All these peptides, CRH and urocortins in mammals and CRH and UI/SVG in teleosts and amphibians, represent the CRH/urocortin family.

The complex evolutionary scenario of this family was recently clarified by Cardoso and collaborators [START_REF] Cardoso | Corticotropin-releasing hormone family evolution: Five ancestral genes remain in some lineages[END_REF]. As previously suggested by Hwang and collaborators, two ancestral crh/ucn1 and ucn2/ucn3 genes likely arose by specific gene duplication before vertebrate WGD events (Hwang et al., 2013). Both ancestral genes were duplicated twice in ancestral vertebrates via 1R and 2R, followed by some paralog losses, leading to up to 5 genes (crh1, crh2, ucn1 issued from ancestral crh/ucn1; ucn2, ucn3, issued from ancestral ucn2/ucn3) in extant representative species of some vertebrate lineages such as chondrichthyans, holosteans and actinistians [START_REF] Cardoso | Corticotropin-releasing hormone family evolution: Five ancestral genes remain in some lineages[END_REF]. Teleost specific 3R resulted in the duplication of crh1 into two paralogs crh1a and crh1b conserved in many species [for review: [START_REF] Cardoso | Corticotropin-releasing hormone family evolution: Five ancestral genes remain in some lineages[END_REF]]. Crh2 may have been lost in recent teleosts [START_REF] Cardoso | Corticotropin-releasing hormone family evolution: Five ancestral genes remain in some lineages[END_REF], while one 3R-crh2 paralog has been conserved in basal groups of teleosts [ [START_REF] Maugars | Evolution of the corticotropinreleasing hormone paralogs in teleosts[END_REF] and Maugars et al. unpublished data].

The involvement of CRH in the stress response in teleosts is well-documented [for reviews: [START_REF] Flik | CRF and stress in fish[END_REF][START_REF] Gorissen | The endocrinology of the stress response in fish : an adaptationphysiological view[END_REF]]. Teleost crhb (crh1b) paralog is widely expressed in the brain of various teleost species [goldfish Carassius auratus: [START_REF] Bernier | Differential expression of corticotropin-releasing factor (CRF) and urotensin I precursor genes, and evidence of CRF gene expression regulated by cortisol in goldfish brain[END_REF]; flounder Platichthys flesus: [START_REF] Lu | Coexpression of corticotropin-releasing hormone and urotensin I precursor genes in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus): A possible shared role in peripheral regulation[END_REF]; zebrafish: [START_REF] Alderman | Localization of corticotropin-releasing factor, urotensin I, and CRF-binding protein gene expression in the brain of the zebrafish, Danio rerio[END_REF]; Japanese eel: [START_REF] Amano | Cloning of corticotropin-releasing hormone (CRH) precursor cDNA and immunohistochemical detection of CRH peptide in the brain of the Japanese eel, paying special attention to gonadotropin-releasing hormone[END_REF]; Astatotilapia burtoni: [START_REF] Carpenter | Social opportunity rapidly regulates expression of CRF and CRF receptors in the brain during social ascent of a teleost fish, Astatotilapia burtoni[END_REF]; Schizothorax prenanti: [START_REF] Wang | Schizothorax prenanti corticotropin-releasing hormone (CRH): Molecular cloning, tissue expression, and the function of feeding regulation[END_REF]]. To our knowledge, the expression of the crha (crh1a) paralog was only investigated in Astatotilapia burtoni and zebrafish: in the zebrafish, crha expression is restricted to the lateral tuberal nucleus of the ventral hypothalamus, while in A. burtoni, no crha expression is detected in the brain [START_REF] Grone | Divergent evolution of two corticotropin-releasing hormone (CRH) genes in teleost fishes[END_REF]. The authors also show that, while both forms are expressed in the retina of Astatotilapia burtoni, only crhb expression is observed in the retina of zebrafish [START_REF] Grone | Divergent evolution of two corticotropin-releasing hormone (CRH) genes in teleost fishes[END_REF]. These first data suggest species-specific variations in the respective roles of 3R-duplicated crh1 paralogs in teleosts.

In mammals, CRH binds to G-protein coupled receptors, CRHR1 and CRHR2, which belong to the class 2 subfamily B1 of secretin-like receptor superfamily. Crhr1 was duplicated via teleost-3R into two paralogs (crhr1a and crhr1b) which were conserved in many extant teleosts, while one of 3R-duplicated crhr2 paralogs would have been lost [for review: [START_REF] Cardoso | New insights into the evolution of vertebrate CRH (corticotropin-releasing hormone) and invertebrate DH44 (diuretic hormone 44) receptors in metazoans[END_REF]]. CRHR1s are thought to mediate CRH action along the corticotropic/stress axis, as, like mammalian CRHR1, teleost CRHR1s have similar affinity for CRH and UI/Ucn1 while CRHR2 has higher affinity for UI and urocortins (2 and 3) than for CRH [catfish Ameirus nebulosus: [START_REF] Arai | Characterization of Three Corticotropin-Releasing Factor Receptors in Catfish: A Novel Third Receptor Is Predominantly Expressed in Pituitary and Urophysis[END_REF]; chum salmon Oncorhynchus keta: [START_REF] Pohl | Cloning and functional pharmacology of two corticotropin-releasing factor receptors from a teleost fish[END_REF]; common carp: [START_REF] Manuel | Corticotropin-releasing factor-binding protein (CRF-BP) inhibits CRF-and urotensin-I-mediated activation of CRF receptor-1 and -2 in common carp[END_REF]].

The ACTH-releasing action of CRH has been demonstrated in vitro in many teleost species [goldfish : [START_REF] Fryer | Cortisol inhibits the ACTH-releasing activity of urotensin I, CRF and sauvagine observed with superfused goldfish pituitary cells[END_REF]; rainbow trout: [START_REF] Baker | In the trout, CRH and AVT synergize to stimulate ACTH release[END_REF][START_REF] Pierson | A V1-type receptor for mediating the neurohypophysial hormone-induced ACTH release in trout pituitary[END_REF]; gilthead sea bream Sparus aurata: [START_REF] Rotllant | Pituitary and interrenal function in gilthead sea bream (Sparus aurata L., Teleostei) after handling and confinement stress[END_REF][START_REF] Rotllant | Pituitary proopiomelanocortin-derived peptides and hypothalamus-pituitaryinterrenal axis activity in gilthead sea bream (sparus aurata) during prolonged crowding stress: Differential regulation of adrenocorticotropin hormone and α-melanocytestimulating[END_REF]; Mozambique tilapia Oreochromis mossambicus: [START_REF] Van Enckevort | Oreochromis mossambicus (tilapia) corticotropinreleasing hormone: cDNA sequence and bioactivity[END_REF]; common carp: [START_REF] Metz | Localization, expression and control of adrenocorticotropic hormone in the nucleus preopticus and pituitary gland of common carp (Cyprinus carpio L.)[END_REF]], as in mammals (Rivier et al., 1983;[START_REF] Rivier | Mediation by Corticotropin Releasing Factor (CRF) of Adenohypophysial Hormone Secretion[END_REF], birds [START_REF] Carsia | Corticotropin-releasing factor stimulates the release of adrenocorticotropin from domestic fowl pituitary cells[END_REF] and amphibians [START_REF] Tonon | Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro[END_REF]. In the common carp, the release of ACTH is stimulated by CRH but only when ACTH cells are submitted to DA inhibition [START_REF] Metz | Localization, expression and control of adrenocorticotropic hormone in the nucleus preopticus and pituitary gland of common carp (Cyprinus carpio L.)[END_REF].

CRH stimulates not only ACTH release, but also α-MSH release, in various teleosts such as in Mozambique tilapia [START_REF] Lamers | A specific role for TRH in release of diacetyl alpha-MSH in tilapia stressed by acid water[END_REF][START_REF] Van Enckevort | Oreochromis mossambicus (tilapia) corticotropinreleasing hormone: cDNA sequence and bioactivity[END_REF], gilthead sea bream [START_REF] Rotllant | Pituitary and interrenal function in gilthead sea bream (Sparus aurata L., Teleostei) after handling and confinement stress[END_REF], red porgy Pagrus pagrus [START_REF] Van Der Salm | Differential release of α-melanophore stimulating hormone isoforms by the pituitary gland of red porgy, Pagrus pagrus[END_REF] and common carp [START_REF] Van Den Burg | Plasma α-MSH and acetylated β-endorphin levels following stress vary according to CRH sensitivity of the pituitary melanotropes in common carp, Cyprinus carpio[END_REF]. This α-MSH-releasing effect of CRH is also reported in mammals such as rat [START_REF] Meunier | CRF stimulates alpha-MSH secretion and cyclic AMP accumulation in rat pars intermedia cells[END_REF][START_REF] Pottinger | The effect of cortisol administration on hepatic and plasma estradiol-binding capacity in immature female rainbow trout (Oncorhynchus mykiss)[END_REF]. In contrast, in an amphibian, the frog Rana ridibunda, while CRH is a potent stimulator of ACTH release by anterior pituitary cells, it is ineffective on α-MSH release by neurointermediate lobes in vitro [START_REF] Tonon | Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro[END_REF].

Other CRH-related peptides such as UI and sauvagine can induce in goldfish the release in vitro of ACTH [START_REF] Fryer | Cortisol inhibits the ACTH-releasing activity of urotensin I, CRF and sauvagine observed with superfused goldfish pituitary cells[END_REF][START_REF] Fryer | Urotensin I, a CRF-like neuropeptide stimulates ACTH release from the teleost pituitary[END_REF], as well as of α-MSH [START_REF] Tran | CRF, Urotensin I, and sauvagine stimulate the release of POMC-derived peptides from goldfish neurointermediate lobe cells[END_REF].

Interestingly, UI and SVG are equipotent to CRH in stimulating ACTH release by rat anterior pituitary cells (Rivier et al., 1983), while in the frog Rana ridibunda, they are unable to stimulate ACTH release [START_REF] Tonon | Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro[END_REF]. Concerning α-MSH, UI and SVG induce its release by the neurointermediate lobe in vitro in Xenopus laevis (Verburg-Van Kemenade et al., 1987), but not in Rana ridibunda [START_REF] Tonon | Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro[END_REF]. In addition to its action on the pituitary, UI is able to directly stimulate cortisol release, and to potentiate ACTH-stimulatory effect, on interrenals in vitro in rainbow trout [START_REF] Arnold-Reed | Peptide hormones influence in vitro interrenal secretion of cortisol in the trout, Oncorhynchus mykiss[END_REF] and European flounder [START_REF] Kelsall | Native urotensins influence cortisol secretion and plasma cortisol concentration in the euryhaline flounder, Platichthys flesus[END_REF]. Finally, in maturing masou salmon Oncorhynchus masou, UI rather than CRH could be involved in the control of hypercortisolemia, as hypothalamic uI expression rises in correlation with enhanced cortisol secretion, while crh expression does not [START_REF] Westring | Seasonal changes in CRF-I and urotensin I transcript levels in masu salmon: Correlation with cortisol secretion during spawning[END_REF].

In the rainbow trout, both crh and uI expressions in the brain (hypothalamus and preoptic area) increase in response to various stressors: hyperosmotic challenge [START_REF] Craig | Differential increase in forebrain and caudal neurosecretory system CRF and urotensin I gene expression associated with seawater transfer in rainbow trout[END_REF], hyperammonemia, isolation [START_REF] Bernier | Heads or tails? Stressor-specific expression of corticotropin-releasing factor and urotensin I in the preoptic area and caudal neurosecretory system of rainbow trout[END_REF] as well as hypoxia [START_REF] Bernier | Heads or tails? Stressor-specific expression of corticotropin-releasing factor and urotensin I in the preoptic area and caudal neurosecretory system of rainbow trout[END_REF][START_REF] Bernier | CRF-related peptides contribute to stress response and regulation of appetite in hypoxic rainbow trout[END_REF]. Notably, crh and uI expressions increase also in the caudal neurosecretory system (CNSS) in response to various stressors in the rainbow trout [hyperosmotic challenge: [START_REF] Craig | Differential increase in forebrain and caudal neurosecretory system CRF and urotensin I gene expression associated with seawater transfer in rainbow trout[END_REF]; hyperammonemia: [START_REF] Bernier | Heads or tails? Stressor-specific expression of corticotropin-releasing factor and urotensin I in the preoptic area and caudal neurosecretory system of rainbow trout[END_REF]] and in the olive Japanese flounder, Paralichthys olivaceus [acute hypothermal stress: [START_REF] Yuan | Dynamic Responses of the Caudal Neurosecretory System (CNSS) Under Thermal Stress in Olive Flounder (Paralichthys olivaceus)[END_REF]]. In the European flounder, net restraint induces an increase in crh expression in the CNSS, but not in the hypothalamus, as well as an increase in CNSS cortisol receptors in addition to an increase in plasma cortisol levels [START_REF] Lu | Coexpression of corticotropin-releasing hormone and urotensin I precursor genes in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus): A possible shared role in peripheral regulation[END_REF]. In this species, glucocorticoid receptors have been colocalized with UI in the CNSS [START_REF] Bond | In teleost fish the caudal neurosecretory system (CNSS) affords pituitary-independent control of cortisol secretion[END_REF]. All these data suggest the possible involvement of the CNSS in the stress-specific regulation of cortisol production in teleosts and the existence of an extra-pituitary feedback system on CNSS for the suppression of cortisol secretion. Earlier in vivo studies already reported the possible involvement of CNSS in the regulation of the corticotropic axis: injection of urophysis extracts or synthetic Catostomus commersoni UI in flounder induces an elevation of cortisol [START_REF] Arnold-Reed | Steroidogenic role of the caudal neurosecretory system in the flounder, Platichthys flesus[END_REF]; in goldfish, urophysectomy produces marked elevation of hypothalamic UI-like activity, pituitary ACTH and plasma cortisol, which can not be observed in urophysectomized fish receiving dexamethasone [START_REF] Woo | Activation of hypothalamohypophysial-interrenal system by urophysectomy in goldfish[END_REF]. More functional investigations are needed to assess the possibility of a major contribution of CNSS CRH-related peptides to the regulation of cortisol secretion and stress responses.

Dopamine and other neurohormones

*Dopamine

In mammals, different data have been reported concerning the effects of dopamine (DA) on ACTH release between in vitro and in vivo studies. Treatment of rat anterior pituitary glands [START_REF] Van Loon | Effect of dopamine on the biological activity and in vitro release of ACTH and FSH[END_REF] or human corticotroph adenoma cells [START_REF] Ishibashi | Direct effects of thyrotropin-releasing hormone, cyproheptadine, and dopamine on adrenocorticotropin secretion from human corticotroph adenoma cells in vitro[END_REF] with DA lowers ACTH release. In contrast, administration of DA agonists to rats elevates plasma ACTH concentrations and subsequently plasma corticosterone levels, and this elevation is prevented by pretreatment with DA antagonists [START_REF] Borowsky | Semilunar Changes in Estradiol and Cortisol Coincident with Gonadal Maturation and Spawning in the Killifish Fundulus heteroclitus[END_REF][START_REF] Ježová | Increase in plasma ACTH after dopaminergic stimulation in rats[END_REF]. An α-MSH-release inhibitory effect of DA is observed in amphibians [Rana pipiens: [START_REF] Saland | Interaction of β-endorphin, naloxone and dopamine: effects on melanocyte-stimulating hormone secretion of amphibian pituitaries in vitro[END_REF]; Rana ridibunda : [START_REF] Jenks | Regulation of biosynthesis and release of pars intermedia peptides in Rana ridibunda: Dopamine affects both acetylation and release of α-MSH[END_REF]; Xenopus laevis : (Verbug-Van [START_REF] Verbug-Van Kemenade | GABA and Dopamine Act Directly on Melanotropes of Xenopus to inhibit MSH secretion[END_REF]].

Possible involvement of DA in the hypothalamic regulation of teleost corticotropes and melanotropes has been suggested by Metz and colleagues. As basal ACTH release increases slowly and steadily over time when pituitary glands of common carp are incubated in vitro, they conclude that ACTH release is under a predominant inhibitory control in vivo (Metz et al., 2004). They also show that DA inhibits ACTH release and is necessary for CRHstimulation of ACTH release [START_REF] Metz | Localization, expression and control of adrenocorticotropic hormone in the nucleus preopticus and pituitary gland of common carp (Cyprinus carpio L.)[END_REF]. It was previously reported that DA was able to inhibit in vitro α-MSH release by goldfish [START_REF] Omeljaniuk | Dopamine inhibition of gonadotropin and α-melanocyte-stimulating hormone release in vitro from the pituitary of the goldfish (Carassius auratus)[END_REF] and red porgy [START_REF] Van Der Salm | Differential release of α-melanophore stimulating hormone isoforms by the pituitary gland of red porgy, Pagrus pagrus[END_REF] pituitaries as well as by Mozambique tilapia neurointermediate lobes [START_REF] Lamers | Regulation of differential release of alpha-melanocyte-stimulating hormone forms from the pituitary pf a teleost fish, Oreochromis mossambicus[END_REF]. In vitro treatment with DA D2 receptor agonists inhibits α-MSH release, while DA D1 receptor agonists have a stimulatory effect on α-MSH release, in Mozambique tilapia exposed to water with low pH (acid stress) [START_REF] Lamers | Acid stress induces a D1like dopamine receptor in pituitary MSH cells of Oreochromis mossambicus[END_REF]. Overall, these results support an involvement of DA in the regulation of ACTH and α-MSH release probably interdependently with CRH pathway [for review: [START_REF] Gorissen | The endocrinology of the stress response in fish : an adaptationphysiological view[END_REF]].

*Serotonin

In mammals, serotonin (5-hydroxytryptamine, 5-HT), a monoamine neurotransmitter, is known to stimulate the HPA axis [for review: [START_REF] Dinan | Serotonin and the regulation of hypothalamic-pituitary-adrenal axis function[END_REF]] and to control stress coping [for review: (Puglisi-Allegra and Andolina, 2015)]. 5-HT mainly acts on the brain, but it can also act directly on the adrenal and pituitary glands [for review: [START_REF] Dinan | Serotonin and the regulation of hypothalamic-pituitary-adrenal axis function[END_REF]]. The brain network involves CRH, GABA and glutamate neurons [for review: (Puglisi-Allegra and Andolina,

2015)].

Similar results have been reported in teleost fish. Injection of a selective agonist for serotonin receptor 5-HT 1A (8-OH-DPAT) results in increased plasma cortisol concentrations in some teleosts [rainbow trout: [START_REF] Winberg | Serotonin as a regulator of hypothalamic-pituitary-interrenal activity in teleost fish[END_REF]; Gulf toadfish Opsanus beta: [START_REF] Medeiros | The serotonin subtype 1A receptor regulates cortisol secretion in the Gulf toadfish, Opsanus beta[END_REF]; goldfish: [START_REF] Lim | Serotonin directly stimulates cortisol secretion from the interrenals in goldfish[END_REF]]. In the Arctic charr Salvelinus alpinus, comparison between stressed and unstressed fish show that this selective agonist has a stimulatory effect on the HPI axis in unstressed fish, while having a suppressive effect on the stress-induced activation of the HPI axis in fish stressed by handling and ip injections [START_REF] Höglund | Stimulatory and inhibitory effects of 5-HT1A receptors on adrenocorticotropic hormone and cortisol secretion in a teleost fish, the Arctic charr (Salvelinus alpinus)[END_REF].

In the Gulf toadfish, injection of 8-OH-DPAT results in increased hypothalamic crh mRNA levels and ACTH release from the pituitary and these effects are inhibited by crowding stress [START_REF] Medeiros | Crowding stress inhibits serotonin 1A receptor-mediated increases in corticotropin-releasing factor mRNA expression and adrenocorticotropin hormone secretion in the Gulf toadfish[END_REF]. However in goldfish, injection of this agonist has no effect at the brain level but rather acts at interrenal tissue to stimulate cortisol secretion [START_REF] Lim | Serotonin directly stimulates cortisol secretion from the interrenals in goldfish[END_REF].

In vitro, 5-HT stimulates cortisol release by interrenals in Gulf toadfish [kidney pieces: [START_REF] Medeiros | Elevated cortisol inhibits adrenocorticotropic hormone-and serotonin-stimulated cortisol secretion from the interrenal cells of the gulf toadfish (Opsanus beta)[END_REF]] and in goldfish [superfused head kidney tissue: [START_REF] Lim | Serotonin directly stimulates cortisol secretion from the interrenals in goldfish[END_REF]]. In goldfish, Lim and colleagues demonstrate the involvement of multiple 5-HT receptor subtypes (5-HT 1A and 5-HT 4 ) in the interrenal paracrine effect of 5-HT [START_REF] Lim | Serotonin directly stimulates cortisol secretion from the interrenals in goldfish[END_REF], while in Gulf toadfish, Medeiros and McDonald show that 5-HT 4 does mediate 5-HT action but 5-HT 1A does not [START_REF] Medeiros | Elevated cortisol inhibits adrenocorticotropic hormone-and serotonin-stimulated cortisol secretion from the interrenal cells of the gulf toadfish (Opsanus beta)[END_REF].

* Thyrotropin-releasing hormone

In mammals and in adult amphibians, thyrotropin-releasing hormone (TRH) is the main neurohormone controlling the thyrotropic axis i.e. the synthesis and release of thyrotropin (TSH) at the pituitary level, which stimulates the production of thyroid hormones at the thyroid level. This TSH-releasing role seems to be taken on by CRH in larval amphibians, reptiles and birds [for reviews: [START_REF] De Groef | Role of corticotropinreleasing hormone as a thyrotropin-releasing factor in non-mammalian vertebrates[END_REF][START_REF] Galas | TRH acts as a multifunctional hypophysiotropic factor in vertebrates[END_REF]]. In teleosts, investigations on the in vitro effect of TRH on pituitary TSH production show either no effect [common carp: synthesis, [START_REF] Kagabu | Effects of thyrotropin-releasing hormone and its metabolites, cyclo(His-Pro) and TRH-OH, on growth hormone and prolactin synthesis in primary cultured pituitary cells of the common carp, Cyprinus carpio[END_REF]; mRNA levels, [START_REF] Geven | Central and peripheral integration of interrenal and thyroid axes signals in common carp (Cyprinus carpio L.)[END_REF]; coho salmon Oncorhynchus kisutch, release: [START_REF] Larsen | In vitro thyrotropin-releasing activity of corticotropin-releasing hormone-family peptides in coho salmon, Oncorhynchus kisutch[END_REF]] or a stimulatory effect [bighead carp Aristichthys nobilis, mRNA levels: [START_REF] Chatterjee | Molecular cloning of cDNA encoding thyroid stimulating hormone β subunit of bighead carp Aristichthys nobilis and regulation of its gene expression[END_REF][START_REF] Chowdhury | In vitro effects of mammalian leptin, neuropeptide-Y, β-endorphin and galanin on transcript levels of thyrotropin β and common α subunit mRNAs in the pituitary of bighead carp (aristichthys nobilis)[END_REF]; Japanese eel, mRNA levels: [START_REF] Han | Cloning of the cDNA for thyroid stimulating hormone β subunit and changes in activity of the pituitary -Thyroid axis during silvering of the Japanese eel, Anguilla japonica[END_REF]]. CRH acts as a TSH-releasing factor on coho salmon pituitary cells [START_REF] Larsen | In vitro thyrotropin-releasing activity of corticotropin-releasing hormone-family peptides in coho salmon, Oncorhynchus kisutch[END_REF], but has no effect on tshβ mRNA levels in common carp pituitary glands in vitro [START_REF] Geven | Central and peripheral integration of interrenal and thyroid axes signals in common carp (Cyprinus carpio L.)[END_REF].

Conversely to the effect of CRH on TSH, TRH can exert an ACTH-releasing effect in teleosts as shown by in vitro studies [goldfish pituitary dispersed cells: [START_REF] Tran | TRH stimulates the release of POMC-derived peptides from goldfish melanotropes[END_REF]; gilthead sea bream pituitary: [START_REF] Rotllant | Pituitary proopiomelanocortin-derived peptides and hypothalamus-pituitaryinterrenal axis activity in gilthead sea bream (sparus aurata) during prolonged crowding stress: Differential regulation of adrenocorticotropin hormone and α-melanocytestimulating[END_REF]]. TRH is also a potent stimulator of α-MSH release by the pars intermedia in many teleost species [goldfish: [START_REF] Tran | TRH stimulates the release of POMC-derived peptides from goldfish melanotropes[END_REF]; tilapia: [START_REF] Lamers | Regulation of differential release of alpha-melanocyte-stimulating hormone forms from the pituitary pf a teleost fish, Oreochromis mossambicus[END_REF]; rainbow trout: [START_REF] Schwartzentruber | Structural requirements for stimulation by thyrotropin-releasing hormone of alpha-MSH release from rainbow trout (Oncorhynchus mykiss) pituitary fragments in vitro[END_REF]; gilthead sea bream: [START_REF] Rotllant | Pituitary proopiomelanocortin-derived peptides and hypothalamus-pituitaryinterrenal axis activity in gilthead sea bream (sparus aurata) during prolonged crowding stress: Differential regulation of adrenocorticotropin hormone and α-melanocytestimulating[END_REF]; red porgy: [START_REF] Van Der Salm | Differential release of α-melanophore stimulating hormone isoforms by the pituitary gland of red porgy, Pagrus pagrus[END_REF]]. This α-MSH-releasing effect of TRH is also observed in amphibians [Rana ridibunda: [START_REF] Tonon | Thyrotropin-releasing hormone stimulates the release of melanotropin from frog neurointermediate lobes in vitro[END_REF]; Xenopus laevis: (B. Verburg-Van Kemenade et al., 1987)].

After a stress (prolonged crowding), in the gilthead sea bream, the pituitary presents a differential response of ACTH and α-MSH to TRH and CRH treatments: the stimulation of ACTH by CRH is attenuated, but not the stimulation of ACTH by TRH, while both CRH and TRH stimulation of α-MSH are enhanced [START_REF] Rotllant | Pituitary proopiomelanocortin-derived peptides and hypothalamus-pituitaryinterrenal axis activity in gilthead sea bream (sparus aurata) during prolonged crowding stress: Differential regulation of adrenocorticotropin hormone and α-melanocytestimulating[END_REF]. In Mozambique tilapia, a dual organization of the stress axis has been demonstrated, as after exposure to low-pH water, the CRH/ACTH axis is replaced by a TRH/di-acetylated α-MSH axis for the production of cortisol [START_REF] Lamers | A specific role for TRH in release of diacetyl alpha-MSH in tilapia stressed by acid water[END_REF]. Overall, these results suggest the involvement of TRH in the regulation of corticotropes and melanotropes, notably during exposure to stress.

*Arginine vasotocin, isotocin and arginine vasopressin

Arginine vasotocin (AVT), isotocin (IST) and arginine vasopressin (AVP) are all peptides produced by neurons of the preoptic nucleus and released by the neurohypophysis. AVT and IST are present in non-mammalian vertebrates and closely related to mammalian AVP.

As in mammals, these neuropeptides are able to stimulate the release of ACTH in vitro [goldfish: [START_REF] Fryer | ACTH-releasing activity of urotensin I and ovine CRF: Interactions with arginine vasotocin, isotocin and arginine vasopressin[END_REF]; trout: [START_REF] Baker | In the trout, CRH and AVT synergize to stimulate ACTH release[END_REF][START_REF] Bond | Effect of acute restraint on hypothalamic pro-vasotocin mRNA expression in flounder, Platichthys flesus[END_REF][START_REF] Pierson | A V1-type receptor for mediating the neurohypophysial hormone-induced ACTH release in trout pituitary[END_REF]] and thus increase plasma cortisol levels in teleosts in vivo [goldfish: [START_REF] Fryer | Neurohypophysial hormonal control of cortisol secretion in the teleost Carassius auratus[END_REF]; trout: [START_REF] Baker | In the trout, CRH and AVT synergize to stimulate ACTH release[END_REF][START_REF] Bond | Effect of acute restraint on hypothalamic pro-vasotocin mRNA expression in flounder, Platichthys flesus[END_REF][START_REF] Pierson | A V1-type receptor for mediating the neurohypophysial hormone-induced ACTH release in trout pituitary[END_REF]]. In addition, in mammals and birds [for review: [START_REF] Cornett | A possible mechanism contributing to the synergistic action of vasotocin (VT) and corticotropin-releasing hormone (CRH) receptors on corticosterone release in birds[END_REF]], they potentiate the stimulatory action of CRH on ACTH in vivo and in vitro [START_REF] Gillies | Corticotropin-releasing activity of the new CRF is potentiated several times by vasopressin[END_REF]Rivier and Vale, 1983;[START_REF] Turkelson | In vitro potentiation of the activity of synthetic ovine corticotropin-releasing factor by arginine vasopressin[END_REF]. In teleosts, contradictory data are available concerning a possible synergy with CRH on ACTH release in vitro: they do synergize in trout [START_REF] Baker | In the trout, CRH and AVT synergize to stimulate ACTH release[END_REF], while they do not in goldfish [START_REF] Fryer | ACTH-releasing activity of urotensin I and ovine CRF: Interactions with arginine vasotocin, isotocin and arginine vasopressin[END_REF]]. In the frog Rana ridibunda, no potentiation of CRHinduced ACTH release is observed when anterior pituitary cells are incubated with a combination of AVP and CRH [START_REF] Tonon | Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro[END_REF].

In rainbow trout, a study shows that AVT mRNA levels in neurons of the preoptic nucleus are elevated during acute stress but not chronic stress [START_REF] Gilchriest | The effects of acute and chronic stresses on vasotocin gene transcripts in the brain of the rainbow trout (Oncorhynchus mykiss)[END_REF].

POMC-derived peptides and their receptors

*POMC-derived peptides

Pro-opiomelanocortin (POMC), together with proenkephalin (PENK), prodynorphin (PDNY) and proorphanin (PNOC), form the opioid/orphanin gene family [ [START_REF] Sundström | Concomitant duplications of opioid peptide and receptor genes before the origin of jawed vertebrates[END_REF]; for review: [START_REF] Dores | Analyzing the evolution of the opioid/orphanin gene family[END_REF]]. POMC is post-translationally processed and some of the peptides obtained undergo further modifications such as C-terminal amidation and Nacetylation [for reviews: [START_REF] Dores | Evolution of POMC: Origin, phylogeny, posttranslational processing, and the melanocortins[END_REF][START_REF] Takahashi | Posttranslational Modifications of Proopiomelanocortin in Vertebrates and Their Biological Significance[END_REF]]. POMC organizational plan varies among vertebrates [for reviews: [START_REF] Dores | Evolution of POMC: Origin, phylogeny, posttranslational processing, and the melanocortins[END_REF][START_REF] Dores | Trends in the evolution of the proopiomelanocortin gene[END_REF]]. POMC is the precursor for ACTH, melanotropins (α-, β-and γ-MSH), corticotropin-like intermediate peptide (CLIP), lipotropins (β-and γ-LPH) and β-endorphin, in tetrapods and lungfish (Amemiya et al., 1999a;[START_REF] Nakanishi | Nucleotide sequence of cloned cDNA for bovine corticotropin-βlipotropin precursor[END_REF]. Chondrichthyans have an additional melanotropin, δ-MSH (Amemiya et al., 1999b). Teleosts lack γ-MSH [START_REF] Kitahara | Absence of a gamma-melanocyte-stimulating hormone sequence in proopiomelanocortin mRNA of chum salmon Oncorhynchus keta[END_REF][START_REF] Lee | Cloning of a neoteleost (Oreochromis mossambicus) pro-opiomelanocortin (POMC) cDNA reveals a deletion of the γ-melanotropin region and most of the joining peptide region: Implications for POMC processing[END_REF], but pomc-β of cichlids and pomacentrids encodes a novel melanocortin peptide, ε-MSH, the result of a tandem duplication of the segment encoding ACTH [START_REF] Harris | Complex structural and regulatory evolution of the pro-opiomelanocortin gene family[END_REF]. In all these vertebrates, the adenohypophysis possesses two cell populations that express POMC: in the pars distalis (PD), the corticotropes in which POMC is processed to ACTH (and β-lipotropin), and in the pars intermedia (PI), the melanotropes in which POMC is processed to ACTH, which is further cleaved to give α-MSH, corticotropin-like intermediate peptide and β-endorphin [for review: [START_REF] Takahashi | Posttranslational Modifications of Proopiomelanocortin in Vertebrates and Their Biological Significance[END_REF]]. Differently, agnathans possess two 'POMC' genes: pro-opiocortin (POC), which encodes ACTH and a different β-endorphin, is expressed in the pars distalis; proopiomelanotropin (POM), which encodes α-, and β-MSH, and β-endorphin, is expressed in the pars intermedia [START_REF] Takahashi | The pituitary-interrenal axis of fish: A review focusing on the lamprey and flounder[END_REF][START_REF] Takahashi | Melanotropin and corticotropin are encoded on two distinct genes in the lamprey, the earliest evolved extant vertebrate[END_REF].

Teleost 3R gave rise to pomc gene duplicates: pomc-α (a or A) and pomc-β (b or B), with pomc-β having lost a functional β-endorphin [START_REF] De Souza | Subfunctionalization of expression and peptide domains following the ancient duplication of the proopiomelanocortin gene in teleost fishes[END_REF]. Further independent gene duplications during teleost evolution resulted in duplicates of pomc-α such as in halibut and carp [pomc-I and -II: (De Souza et al., 2005)], sea bream [pomc-α1 and -α2: [START_REF] Cardoso | Divergence of duplicate POMC genes in gilthead sea bream Sparus auratus[END_REF]] and Astatotilapia burtoni [pomc-α1 and -α2: [START_REF] Harris | Complex structural and regulatory evolution of the pro-opiomelanocortin gene family[END_REF]]. In barfin flounder Verasper moseri, a third pomc gene, named pomc-C, was also identified [START_REF] Takahashi | Expression of three proopiomelanocortin subtype genes and mass spectrometric identification of POMC-derived peptides in pars distalis and pars intermedia of barfin flounder pituitary[END_REF]; pomc-C belongs to the pomc-β clade while pomc-A and-B belong to the pomc-α clade and thus should be renamed -β, -α1 and -α2, respectively, according to Cardoso and collaborators [START_REF] Cardoso | Divergence of duplicate POMC genes in gilthead sea bream Sparus auratus[END_REF]. In rainbow trout, apart from pomc-B, pomc-A1 and -A2 are likely the result of the salmonid genome duplication (4R) [START_REF] Leder | The pro-opiomelanocortin genes in rainbow trout (Oncorhynchus mykiss): Duplications, splice variants, and differential expression[END_REF].

As in other vertebrates [e.g. in rodents: [START_REF] Kraicer | Pars intermedia and pars distalis: two sites of ACTH production in the rat hypophysis[END_REF]], ACTH is produced in teleosts from POMC by both the pars distalis (PD; corticotropes) and the pars intermedia (PI; melanotropes) of the pituitary, but ACTH of the pars intermedia is further cleaved to produce smaller peptides and cortisol feedbacks only on the pars distalis ACTH [for review: [START_REF] Fryer | Control of Corticotropin Secretion in Teleost Fishes[END_REF]]. In tetraodon Tetraodon nigroviridis, pomc-α and not pomc-β is expressed in the PD, while both are expressed in the PI [START_REF] De Souza | Subfunctionalization of expression and peptide domains following the ancient duplication of the proopiomelanocortin gene in teleost fishes[END_REF]. In the sea bream, pomc-α2 and not pomc-α1 is expressed in the PD, while only pomc-α1 is expressed in the PI [START_REF] Cardoso | Divergence of duplicate POMC genes in gilthead sea bream Sparus auratus[END_REF]. In the pituitary of barfin flounder, all three pomc genes present in this species are expressed in the PD [START_REF] Takahashi | Expression of three proopiomelanocortin subtype genes and mass spectrometric identification of POMC-derived peptides in pars distalis and pars intermedia of barfin flounder pituitary[END_REF]. In Astatotilapia burtoni [START_REF] Harris | Complex structural and regulatory evolution of the pro-opiomelanocortin gene family[END_REF] and rainbow trout [START_REF] Leder | The pro-opiomelanocortin genes in rainbow trout (Oncorhynchus mykiss): Duplications, splice variants, and differential expression[END_REF], all three pomc are expressed in the pituitary, but no indication of the region is reported. This indicates speciesspecific variation in the expression of the various pomc paralogs by the PD corticotropic cells through teleost radiation.

Species-specific variations among teleosts may also concern the pomc paralog linked to stress response. In the gilthead sea bream, an enhanced pituitary expression of pomc-α2 (pomc-b in the paper) and a decreased one of pomc-α1 (pomc-a in the paper) are observed after acute stress due to air exposure [START_REF] Skrzynska | Impact of air exposure on vasotocinergic and isotocinergic systems in gilthead sea bream (Sparus aurata): New insights on fish stress response[END_REF]], leading the authors to link pomc-α2 and not pomc-α1 to stress response in sea bream. However, in the same species, no change in the expression of both paralogs was previously reported after exposure to other acute stress, chasing and persecution [START_REF] Toni | Sedative effect of 2-phenoxyethanol and essential oil of Lippia alba on stress response in gilthead sea bream (Sparus aurata)[END_REF]. In Rhamdia quelen, both pituitary pomc-α2 (pomcb in the paper) and pomc-α1 (pomca in the paper) expressions are unchanged after stressful situation (transport) [START_REF] Saccol | Myrcia sylvatica essential oil mitigates molecular, biochemical and physiological alterations in Rhamdia quelen under different stress events associated to transport[END_REF]. In contrast, in the Senegalese sole, Solea senegalensis, pomc-α1 (pomc-a in the paper) expression in the pituitary is down-regulated in juveniles chronically stressed by high stocking density, whereas pomc-α2 (pomc-b in the paper) expression levels remain unaffected [START_REF] Wunderink | Subfunctionalization of pomc paralogues in senegalese sole (solea senegalensis)[END_REF]. All these data in three different species do not lead to clear conclusions concerning a preferential involvement of one or another pomc paralog in stress response among teleosts.

An in vitro corticotropic action (i.e. induction of cortisol release from head kidney) of α-MSH (desacetyl and diacetyl forms) has been reported in rainbow trout [START_REF] Rance | The in vitro response of the trout interrenal to various fragments of ACTH[END_REF], Mozambique tilapia [START_REF] Lamers | A role for di-acetyl-alpha-melanocytestimulating hormone in the control of cortisol release in the teleost Oreochromis mossambicus[END_REF] and barfin flounder [START_REF] Kobayashi | Melanocortin receptor subtypes in interrenal cells and corticotropic activity of α-melanocytestimulating hormones in barfin flounder, Verasper moseri[END_REF], but not in common carp [START_REF] Metz | ACTH, α-MSH, and control of cortisol release: Cloning, sequencing, and functional expression of the melanocortin-2 and melanocortin-5 receptor in Cyprinus carpio[END_REF].

*Melanocortin receptors

In tetrapods, five MCRs have been identified. In teleost fish, the number of receptors increases up to six in zebrafish, which has two MC5R paralogs (mc5ra and mc5rb) [START_REF] Västermark | The early origin of melanocortin receptors, agoutirelated peptide, agouti signalling peptide, and melanocortin receptor-accessory proteins, with emphasis on pufferfishes, elephant shark, lampreys, and amphioxus[END_REF], while pufferfish Fugu has only four, with no melanocortin mc3r and only one copy of melanocortin mc5r [START_REF] Logan | The structure and evolution of the melanocortin and MCH receptors in fish and mammals[END_REF]. Concerning the ligand selectivity of MCRs, all of the paralogous MCRs can be activated by both ACTH and α-MSH in extant cartilaginous fishes, while in extant teleosts and tetrapods, MC2R can be activated only by ACTH. The appearance of MRAP1 paralleled the emergence of this MC2R ligand selectivity [for reviews: [START_REF] Dores | Hypothesis and Theory: Revisiting Views on the Co-evolution of the Melanocortin Receptors and the Accessory Proteins, MRAP1 and MRAP2[END_REF][START_REF] Dores | Melanocortin receptors: Evolution of ligand selectivity for melanocortin peptides[END_REF]].

In mammals, the MCRs have distinct expression site and functions [for reviews: [START_REF] Cone | Studies on the physiological functions of the melanocortin system[END_REF][START_REF] Dores | Molecular evolution of GPCRs: Melanocortin/melanocortin receptors[END_REF]]: MC1R, expressed in melanocytes, is involved in skin and hair pigmentation; MC2R, expressed in adrenal cortex, is involved in adrenal steroidogenesis and stress response; MC3R and MC4R, expressed in the brain, are involved in the control of energy homeostasis; MC5R, expressed in a variety of exocrine glands, such as sebaceous, lacrimal and preputial glands, is involved in exocrine gland secretion. These features can be also found in teleosts but some peculiarities can be noted. Of particuliar interest, mc5r is coexpressed with mc2r in the interrenal of several teleosts [rainbow trout: [START_REF] Aluru | Molecular characterization, tissue-specific expression, and regulation of melanocortin 2 receptor in rainbow trout[END_REF][START_REF] Haitina | Cloning, tissue distribution, pharmacology and threedimensional modelling of melanocortin receptors 4 and 5 in rainbow trout suggest close evolutionary relationship of these subtypes[END_REF]; common carp Cyprinus carpio: [START_REF] Metz | ACTH, α-MSH, and control of cortisol release: Cloning, sequencing, and functional expression of the melanocortin-2 and melanocortin-5 receptor in Cyprinus carpio[END_REF]; barfin flounder: [START_REF] Kobayashi | Melanocortin receptor subtypes in interrenal cells and corticotropic activity of α-melanocytestimulating hormones in barfin flounder, Verasper moseri[END_REF]], as in the chicken adrenal [START_REF] Takeuchi | Melanocortin receptor genes in the chicken -Tissue distributions[END_REF] and Xenopus tropicalis interrenal/kidney [START_REF] Dores | Views on the co-evolution of the melanocortin-2 receptor, MRAPs, and the hypothalamus/pituitary/adrenal-interrenal axis[END_REF], suggesting a possible role of MC5R in the regulation of HPI/HPA axis in these non-mammalian vertebrates.

Corticosteroids and their receptors

*Glucocorticoids

In fish, corticosteroids are synthetized by the interrenal tissue, a tissue embedded inside the anterior part of the kidney and homologous to the adrenal cortex in mammals. Corticosteroids are steroid hormones divided into glucocorticoids and mineralocorticoids. Even if cortisol is the major corticosteroid in teleosts [for review: [START_REF] Mommsen | Cortisol in teleosts: Dynamics, mechanisms of action, and metabolic regulation[END_REF]], others such as 11deoxycortisol (17, 21 dihydroxy-4-pregnene-3,20 dione), 11-deoxycorticosterone (DOC) and corticosterone are also detected in plasma of teleosts [for review: [START_REF] Butler | Structure and function of the adrenal gland of fishes[END_REF]; winter flounder Pseudopleuronectes americanus: [START_REF] Campbell | Steroids in the plasma of the winter flounder (Pseudopleuronectes americanus walbaum). A seasonal study and investigation of steroid involvement in oocyte maturation[END_REF]; rainbow trout Salmo gairdneri: [START_REF] Campbell | Identification and quantification of steroids in the serum of rainbow trout during spermiation and oocyte maturation[END_REF]]. All teleosts so far studied lack aldosterone, which is the principal mineralocorticoid in mammals [START_REF] Gilmour | Mineralocorticoid receptors and hormones: Fishing for answers[END_REF], and it is generally accepted that cortisol exerts both glucocorticoid and mineralocorticoid actions in teleosts [START_REF] Mccormick | Endocrine control of osmoregulation in teleost fish[END_REF][START_REF] Mccormick | Are we missing a mineralocorticoid in teleost fish? Effects of cortisol, deoxycorticosterone and aldosterone on osmoregulation, gill Na+,K+-ATPase activity and isoform mRNA levels in Atlantic salmon[END_REF]. DOC is shown to be a potent agonist of mineralocorticoid receptor (MR) [START_REF] Sturm | 11-Deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor[END_REF]. Study in rainbow trout has shown a sustained upregulation of plasma DOC levels during a confinement stress time-course. However, the low DOC levels compared to cortisol measured in the plasma do not favour an activity of DOC through MR receptors [START_REF] Kiilerich | Regulation of the corticosteroid signalling system in rainbow trout HPI axis during confinement stress[END_REF] and physiological relevance of this hormone action is still unclear in fish [START_REF] Prunet | Multiple corticosteroid receptors in fish: From old ideas to new concepts[END_REF].

*Glucocorticoid and mineralocorticoid receptors

Glucocorticoid and mineralocorticoid receptors (GR and MR respectively) are corticosteroid receptors which belong to the nuclear receptor superfamily [for review: [START_REF] Bury | The evolution, structure and function of the ray finned fish (Actinopterygii) glucocorticoid receptors[END_REF]]. In teleosts, 3R gave rise to duplicated gr (gr1 and gr2) with one receptor (GR1) retaining a 9 aminoacid insert which does not exist in GR2 [for review: [START_REF] Bury | The evolution, structure and function of the ray finned fish (Actinopterygii) glucocorticoid receptors[END_REF]]. Zebrafish is an exception as this species conserved only one of the two gr paralogs [START_REF] Schaaf | Discovery of a functional glucocorticoid receptor β-isoform in zebrafish[END_REF]. In some teleosts, GR1 has two splice variants, GR1a and GR1b [rainbow trout: [START_REF] Takeo | Fish glucocorticoid receptor with splicing variants in the DNA binding domain[END_REF]; Haplochromis burtoni: [START_REF] Greenwood | Multiple corticosteroid receptors in a teleost fish: Distinct sequences, expression patterns, and transcriptional activities[END_REF]; Tetraodon and Takifugu species: [START_REF] Stolte | Evolution of glucocorticoid receptors with different glucocorticoid sensitivity[END_REF]; marine medaka Oryzias dancena: (M. Kim et al., 2011)]. 3R gave also rise to duplicated mr (mr1 and mr2) which have been both conserved in a basal teleost, the European eel [ [START_REF] Lafont | Characterization and regulation during reproduction of four nuclear corticosteroid receptors in the European eel, Anguilla anguilla., in: 10th International Symposium on Reproductive Physiology of Fish. membrane estrogen receptors in basal teleosts, Anguilla sp.: Identification, evolutionary history and differential expression regulation[END_REF] and Lafont et al. unpublished data], while only the mr1 paralog would have been conserved in extant teleosts so far studied [for review: [START_REF] Baker | Evolution of the Mineralocorticoid Receptor[END_REF]]. As they are issued from the 3R, these two mr paralogs in the eel should rather be named mra and mrb, according to the commonly used nomenclature for teleost 3Rparalogs. However, they must be distinguished from the two mr forms previously found in the rainbow trout, named rtmra and rtmrb [START_REF] Sturm | 11-Deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor[END_REF], which may represent allelic variants or paralogs issued from salmonid-4R of mr1.

GR1 and GR2 present differential affinities (sensitivities) and show distinct functionality with GR2 having a higher hormonal transcriptional activity at equimolar concentration and higher sensitivity [START_REF] Bury | Evidence for two distinct functional glucocorticoid receptors in teleost fish[END_REF]. This difference in sensitivity is not restricted to the salmoniformes and have been also observed with the two GRs in Pantodon buchholzi (Li et al., 2012), carp [START_REF] Stolte | Corticosteroid receptors involved in stress regulation in common carp , Cyprinus carpio[END_REF], marine medaka (Oryzias dacena) (M. Kim et al., 2011), and the Japanese medaka (Oryzias latipes) [START_REF] Miyagawa | Characterization of Oryzias latipes glucocorticoid receptors and their unique response to progestins[END_REF]. From such in vitro differences in functionality, one can hypothetize that the two teleost GRs may have different roles: the hypersensitive GR2 could play a prominent role during basal circulatory cortisol concentrations (unstressed) whereas the less sensitive GR1 may become prominent during stressful situations when cortisol levels are high [ [START_REF] Bury | Evidence for two distinct functional glucocorticoid receptors in teleost fish[END_REF]; for review: [START_REF] Bury | The evolution, structure and function of the ray finned fish (Actinopterygii) glucocorticoid receptors[END_REF]]. So far, such functionally distinct roles have not been established in fish. However, a recent study of stress effects in salmonids shows differential contribution of the corticosteroid receptors in the regulation of HPI axis activity [START_REF] Kiilerich | Regulation of the corticosteroid signalling system in rainbow trout HPI axis during confinement stress[END_REF] and suggests a negative feedback regulation of cortisol release at the pituitary level via MR, while a short loop regulation occurs at the interrenal level via GR in rainbow trout exposed to 7 dayconfinement. The authors also observe the presence of the 2 GR and the MR in hypophysial ACTH cells confirming the regulation of the HPI axis at the pituitary level by both GRs and MR. Similar conclusions have been also suggested when studying in the Atlantic salmon effects of unpredictable chronic stress or to repeated chasing stress [START_REF] Madaro | A comparative study of the response to repeated chasing stress in Atlantic salmon (Salmo salar L.) parr and post-smolts[END_REF][START_REF] Madaro | Stress in Atlantic salmon: Response to unpredictable chronic stress[END_REF]. In the common carp, Stolte and collaborators report that, in fish exposed to prolonged and strong stressors, mRNA levels of all three corticosteroid receptors (gr1, gr2 and mr) are down-regulated in some brain regions, but not in CRH neurons or pituitary ACTH cells [START_REF] Stolte | Corticosteroid receptors involved in stress regulation in common carp , Cyprinus carpio[END_REF], suggesting a role of all three receptors in stress regulation in this species.

In zebrafish [START_REF] Schaaf | Discovery of a functional glucocorticoid receptor β-isoform in zebrafish[END_REF], the single gr (gr2) gene has two splice variants, grα and grβ [START_REF] Hollenberg | Primary structure and expression of a functionakl human glucocorticoid receptor cDNA[END_REF], with a variant lacking the C-terminal portion of the GR as described in human [START_REF] Hollenberg | Primary structure and expression of a functionakl human glucocorticoid receptor cDNA[END_REF]. In human, grβ may act as a dominant-repressor of the wildtype variant [START_REF] Bamberger | Glucocorticoid receptor β, a potential endogenous inhibitor of glucocorticoid action in humans[END_REF]. These splicing variants are co-expressed in a number of zebrafish tissues (brain, spleen, liver, intestine, heart, gills and muscle), grα mRNA levels being significantly higher than those of grβ [START_REF] Schaaf | Discovery of a functional glucocorticoid receptor β-isoform in zebrafish[END_REF]. Despite initial data suggesting that grβ has a key role in the negative regulation of grα [START_REF] Chatzopoulou | Transcriptional and metabolic effects of glucocorticoid receptor α and β signaling in zebrafish[END_REF], a recent study by the same group using transgenic zebrafish with inducible expression of grβ suggests that this isoform does not have a functional role in transcription regulation [START_REF] Chatzopoulou | Functional analysis reveals no transcriptional role for the glucocorticoid receptor β-isoform in zebrafish[END_REF]. Functional roles of corticosteroid receptors during stress have been studied in this fish species using mutant fish. Thus, GR knockout zebrafish show an inability to cope with stressor such as placement into an unfamiliar environment, even after repeated exposure [START_REF] Ziv | An affective disorder in zebrafish with mutation of the glucocorticoid receptor[END_REF]. These fish also show hypercortisolemia and fail to exihibit a cortisol stress response, while KO zebrafish for MR have a delayed but sustained cortisol response [START_REF] Faught | The mineralocorticoid receptor is essential for stress axis regulation in zebrafish larvae[END_REF]. Both KO mutants do not show the hyperactivity in response to light observed in wild type zebrafish [START_REF] Faught | The mineralocorticoid receptor is essential for stress axis regulation in zebrafish larvae[END_REF]. These recent data suggest distinct but complementary roles for GR and MR in the development and regulation of the stress axis in zebrafish: MR represses HPI axis during development while GR regulates basal cortisol levels in the context of negative feedback regulation during stress.

Both receptors are also involved in the control of stress-related behaviour in zebrafish [START_REF] Faught | The mineralocorticoid receptor is essential for stress axis regulation in zebrafish larvae[END_REF]. Such a role in the regulation of stress-related behaviour is suggested in medaka as well [START_REF] Sakamoto | Principal function of mineralocorticoid signaling suggested by constitutive knockout of the mineralocorticoid receptor in medaka fish[END_REF].

The gonadotropic axis

Main actors of the gonadotropic axis in vertebrates

The brain-pituitary-gonadal axis is responsible for the control of the reproductive function in all vertebrates. It regulates gamete production, sexual secondary characters and behaviors, as well as key-steps of the life cycle such as puberty, seasonal reproduction or sex change in some hermaphrodic species including teleost representatives.

The brain integrates various environmental and social cues, such as temperature, photoperiod, habitat conditions, presence of partners, as well as internal cues such as developmental stage, size, nutritional status and energy stores, via various neuronal networks which converge into the positive or negative regulation of the activity of gonadotropin-releasing hormone (GnRH) hypophysiotropic neurons considered as the master control of reproduction [START_REF] Gore | GnRH: The Master Molecule of Reproduction[END_REF].

GnRH binds to its cognate membrane receptor (GnRH-R) expressed by the pituitary gonadotrope cells, and which belongs to the rhodopsin-like, class A, G-protein coupled receptor (GPCR) superfamily [for reviews: [START_REF] Flanagan | Gonadotropin-releasing hormone (GnRH) receptor structure and GnRH binding[END_REF][START_REF] Millar | Gonadotropin-Releasing Hormone Receptors[END_REF]]. GnRH stimulates the synthesis and release of the gonadotropins, LH and FSH.

The two pituitary gonadotropins, LH and FSH are glycoprotein hormones, composed of two subunits alpha and beta. The alpha subunit, named glycoprotein hormone alpha subunit (Gpα), is common to both gonadotropins, LH, FSH and to the thyrotropin, TSH, while the beta subunit is specific to each hormone [for reviews: [START_REF] Cahoreau | Structure-function relationships of glycoprotein hormones and their subunits' ancestors[END_REF][START_REF] Pierce | Glycoprotein Hormones: Structure and Function[END_REF]]. LH and FSH bind to their respective GPCR membrane receptors LHR and FSHR expressed by the gonads. The gonadotropins activate gonadal functions, gametogenesis and steroidogenesis. The enzymatic pathway of the biosynthesis of sex steroids is largely conserved among vertebrates. Sex steroids, androgens, estrogens, progestagens, bind to their respective nuclear receptors, AR, ER, PR, belonging to the nuclear receptor superfamily [for review: [START_REF] Mangelsdorf | The nuclear receptor superfamily: The second decade[END_REF]]. As more recently discovered, steroid may also act via binding to GPCR membrane receptors, such as estradiol-membrane receptor and progesterone membrane receptor [for review: [START_REF] Thomas | Rapid steroid hormone actions initiated at the cell surface and the receptors that mediate them with an emphasis on recent progress in fish models[END_REF]]. Sex steroids act on multiple targets, including the gonads themselves, a variety of peripheral tissues as well as the brain and pituitary, where they exert positive and negative feedback controls on the gonadotropic axis.

Specific features of the gonadotropic axis in teleosts

GnRHs and their receptors

Up to three gnrh genes are present in extant teleosts, a larger number than in mammals (up to two genes), which had been initially attributed to the 3R. However, subsequent comparative studies revealed the presence of three gnrh genes also in basal vertebrate representatives such as a chondrichthyan, the catshark, Scyliorhynus canicula and a basal sarcopterygian, the coelacanth, Latimeria chalumnae. Phylogeny and synteny studies support the 1R/2R origin of the three vertebrate gnrh and their inheritance by the teleost lineage. In contrast, 3R had no impact on the gnrh gene number in extant teleosts, indicating an early loss of 3R-paralogs in this lineage [for reviews: (D. K. Kim et al., 2011;[START_REF] Roch | Evolution of GnRH: Diving deeper[END_REF][START_REF] Tostivint | Evolution of the gonadotropin-releasing hormone (GnRH) gene family in relation to vertebrate tetraploidizations[END_REF]]. A maximum of three gnrh (1, 2, 3) genes are thus present in teleosts, such as in medaka, while additional, independent and species-specific losses led to the presence of only two gnrh genes in some species, e.g. gnrh1 and 2 in the eel, or gnrh2 and 3 in the zebrafish. The situation is more complex concerning GnRH receptors. Recent studies indicate that a large number of GnRH receptors resulted from 1R/2R as well as from local gene duplications in ancestral vertebrates; subsequently, several independent gene loss events led to a variable number of gnrhr genes throughout vertebrate evolution. In teleosts, 3R-duplicated paralogs as well as additional duplications in some lineages increased the number of GnRH-R [up to 5 in acanthopterygians and 6 in salmonids: [START_REF] Ciani | Gnrh receptor gnrhr2bbα is expressed exclusively in lhb-expressing cells in Atlantic salmon male parr[END_REF]] as compared to other actinopterygians, with species-specific variations among teleosts due to independent gene losses [ [START_REF] Ciani | Gnrh receptor gnrhr2bbα is expressed exclusively in lhb-expressing cells in Atlantic salmon male parr[END_REF]; for reviews: [START_REF] Roch | GnRH receptors and peptides: Skating backward[END_REF][START_REF] Sefideh | Local duplication of gonadotropin-releasing hormone (GnRH) receptor before two rounds of whole genome duplication and origin of the mammalian GnRH receptor[END_REF][START_REF] Williams | Dynamic evolution of the GnRH receptor gene family in vertebrates[END_REF]].

Dopamine and other neurohormones

*Dopamine

A crucial control is exerted by dopaminergic neurons which directly inhibit the gonadotrope cell activity and counteract the effect of GnRH, as first discovered in cyprinids by Peter and collaborators and retrieved in many other teleosts species including basal representatives such as the eel [for reviews: [START_REF] Dufour | Neuroendocrine control by dopamine of teleost reproduction[END_REF][START_REF] Dufour | Dopaminergic inhibition of reproduction in teleost fishes: Ecophysiological and evolutionary implications[END_REF]]. In relation with environmental and internal cues, and in a species-specifc manner, this dopaminergic control of gonadotropic cells plays key roles in various reproductive steps in teleosts such as puberty, final oocyte maturation and ovulation, spermiation, or sex change [for reviews: [START_REF] Dufour | Neuroendocrine control by dopamine of teleost reproduction[END_REF][START_REF] Dufour | Dopaminergic inhibition of reproduction in teleost fishes: Ecophysiological and evolutionary implications[END_REF]]. A major inhibitory role of DA in regulating reproduction is also reported in amphibians, especially in the control of spawning [for review: [START_REF] Vu | Neuroendocrine control of spawning in amphibians and its practical applications[END_REF]].

Beside GnRH and DA, a variety of other neurohomones can also directly regulate the activity of teleost gonadotrophs, with species-specific variations, such as kisspeptin (Kiss), gonadotropin-inhibitory hormone (GnIH), neurokinin, gamma-aminobutyric acid (GABA), noradrenaline, serotonin etc [for reviews: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF][START_REF] Zohar | Neuroendocrinology of reproduction in teleost fish[END_REF]]. As proposed by Trudeau, these multiple independent controls of gonadotropic cells may provide compensatory regulatory mechanisms, possibly explaining the low or no impact of KO of major genes such as kiss or gnrh observed in recent studies [START_REF] Tran | CRF, Urotensin I, and sauvagine stimulate the release of POMC-derived peptides from goldfish neurointermediate lobe cells[END_REF]. Species-specific variations in the multiple controls of gonadotrophs highlight the evolutionary plasticity of the brain-pituitary anatomo-functional relationships in the teleost lineage, which may have favored the remarkable diversity of their reproductive cycles.

*GnIH

Gonadotropin-inhibitory hormone (GnIH or RFRP-3 in mammals), a RFamide peptide, was discovered from quail brain by Tsutsui and collaborators and found to be involved in the direct inhibitory pituitary regulation of LH [START_REF] Tsutsui | A novel avian hypothalamic peptide inhibiting gonadotropin release[END_REF]. Shortly after, the GnIH precursor was shown to encode two other peptides [START_REF] Satake | Characterization of a cDNA encoding a novel avian hypothalamic neuropeptide exerting an inhibitory effect on gonadotropin release[END_REF]. Since then, gnih gene homologs, encoding multiple peptides, have been identified in other vertebrates [for reviews: [START_REF] Muñoz-Cueto | A journey through the gonadotropin-inhibitory hormone system of fish[END_REF][START_REF] Tsutsui | Review: Structure, function and evolution of GnIH[END_REF]]. These peptides, in mammals, also possess an inhibitory action on gonadotropins, while in amphibians they have GH-releasing effects [for review: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF]]. Recently, it was demonstrated that only a single gnih gene was present in representative species of elopomorphs (Anguilla species), the most basal group of teleosts, as well as in the other teleosts [START_REF] Maugars | Gonadotropin-inhibitory hormone in teleosts : New insights from a basal representative , the eel[END_REF]. This result indicates a loss of one of the two 3R-duplicated gnih paralogs shortly after the 3R [START_REF] Maugars | Gonadotropin-inhibitory hormone in teleosts : New insights from a basal representative , the eel[END_REF].

Depending on teleost species, inhibitory [e.g. European eel [START_REF] Pasquier | Eel Kisspeptins: Identification, functional activity, and inhibition on both pituitary LH and GnRH receptor expression[END_REF]; common carp [START_REF] Peng | GnIH plays a negative role in regulating GtH expression in the common carp, Cyprinus carpio[END_REF]; zebrafish [START_REF] Spicer | The gonadotropin-inhibitory hormone (Lpxrfa) system's regulation of reproduction in the brain-pituitary axis of the zebrafish (Danio rerio)[END_REF]], stimulatory [e.g. sockeye salmon [START_REF] Amano | Novel fish hypothalamic neuropeptides stimulate the release of gonadotrophins and growth hormone from the pituitary of sockeye salmon[END_REF]; grass puffer Takifugu niphobles [START_REF] Shahjahan | Synchronised expressions of LPXRFamide peptide and its receptor genes: Seasonal, diurnal and circadian changes during spawning period in grass puffer[END_REF]; tilapia Oreochromis niloticus [START_REF] Biran | LPXRFa, the piscine ortholog of GnIH, and LPXRF receptor positively regulate gonadotropin secretion in tilapia (Oreochromis niloticus)[END_REF]; catla Catla catla [START_REF] Kumar | Molecular characterization of gonadotropin-inhibitory hormone (GnIH) gene and effect of intramuscular injection of GnIH peptide on the reproductive axis in Catla catla[END_REF]; half-smooth tongue sole Cynoglossus semilaevis [START_REF] Wang | In vitro effects of tongue sole LPXRFa and kisspeptin on relative abundance of pituitary hormone mRNA and inhibitory action of LPXRFa on kisspeptin activation in the PKC pathway[END_REF]] or a lack of effect [e.g. goldfish [START_REF] Qi | Evidences for the regulation of GnRH and GTH expression by GnIH in the goldfish, Carassius auratus[END_REF]; Astyanax altiparanae [START_REF] Branco | Effects of GnRH and the dual regulatory actions of GnIH in the pituitary explants and brain slices of Astyanax altiparanae males[END_REF]] of GnIH have been described for gonadotropins in vitro [for review: [START_REF] Muñoz-Cueto | A journey through the gonadotropin-inhibitory hormone system of fish[END_REF][START_REF] Tsutsui | Review: Structure, function and evolution of GnIH[END_REF][START_REF] Ubuka | Dual actions of mammalian and piscine gonadotropin-inhibitory hormones, RFamide-related peptides and LPXRFamide peptides, in the hypothalamicpituitary-gonadal axis[END_REF]], suggesting a large variability of GnIH function in reproduction across teleosts.

* Kisspeptin

Kisspeptin, another RF-amide peptide, is encoded by the Kiss-1 gene, a metastasis suppressor gene, isolated in 1996 from melanoma cells [START_REF] Lee | KiSS-1 , a Novel Human Malignant Melanoma[END_REF]. This 54 amino-acid peptide, also called metastin, is processed into shorter peptides which all bind to kisspeptin receptor, KissR (or GPR54) [START_REF] Ohtaki | Metastasis suppressor gene KiSS-1 encodes peptide ligand of a G-proteincoupled receptor[END_REF]. In 2003, three research groups discovered the key role of the kisspeptin system in the activation of gonadotropic axis and the control of reproduction in human and mice [START_REF] De Roux | Hypogonadotropic hypogonadism due to loss of function of the KiSS1-derived peptide receptor GPR54[END_REF][START_REF] Funes | The KiSS-1 receptor GPR54 is essential for the development of the murine reproductive system[END_REF][START_REF] Seminara | The GPR54 Gene as a Regulator of Puberty[END_REF]. In vivo and in vitro studies further demonstrate the stimulatory role of kisspeptin at different levels of the gonadotropic axis: hypothalamus (GnRH neurons), pituitary (LH cells) and gonads [for reviews: [START_REF] Pinilla | Kisspeptins and reproduction: Physiological roles and regulatory mechanisms[END_REF][START_REF] Putteeraj | A "timed" kiss is essential for reproduction: Lessons from mammalian studies[END_REF]]. The few available studies in amphibians and reptiles also suggest a stimulatory role of kisspeptin on the HPG axis, while this system might have been lost in some birds [for review: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF]].

However, data in teleosts report stimulatory, no or inhibitory effects of kisspeptin peptides on GnRH or gonadotropins, suggesting a large variabilty of the role of Kiss system in eleost reproduction according to species [for reviews: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF][START_REF] Tena-Sempere | Comparative insights of the kisspeptin/kisspeptin receptor system: Lessons from non-mammalian vertebrates[END_REF]]. Up to four paralogs of Kiss and KissR resulted from 1R and 2R in early vertebrates, but subsequent gene losses led to the presence of only a single Kiss gene and a single KissR gene in human. In teleosts, due to paralog losses, no impact of 3R on the number of both Kiss and KissR genes has been found, leading to a maximum of two Kiss and three KissR genes, which is no more than in a non-teleost actinopterygian such as the gar, Lepisosteus oculatus [for reviews: [START_REF] Pasquier | Molecular evolution of GPCRs: Kisspeptin/kisspeptin receptors[END_REF][START_REF] Pasquier | Comparative evolutionary histories of kisspeptins and kisspeptin receptors in vertebrates reveal both parallel and divergent features[END_REF]].

Gonadotropins and their receptors

As in mammals, only two gonadotropins, LH and FSH, are present in all extant teleosts, including basal representative species such as the eel (elopomorphs) [START_REF] Yoshiura | Duality of gonadotropin in a primitive teleost, Japanese eel (Anguilla japonica)[END_REF], indicating an early loss after 3R of the putative additional 3R-paralogs [for review: [START_REF] Dufour | Origin and evolution of the neuroendocrine control of reproduction in vertebrates, with special focus on genome and gene duplications[END_REF]]. In tetrapods, LH and FSH bind to their respective receptors, LHR and FSHR. As recently shown, an additional lhr paralog arose from a local gene duplication in ancestral actinopterygians, and the two lhr (lhr1 and lhr2) where inherited by the teleost lineage [START_REF] Maugars | Demonstration of the coexistence of duplicated LH receptors in teleosts, and their origin in ancestral actinopterygians[END_REF]. Due to gene losses after 3R, the number of gonadotropin receptors did not further increase in teleosts with a single fshr and up to two lhr. Additional independent and species-specific gene losses led to the presence of a single lhr in some teleosts, such as only lhr1 in medaka and lhr2 in zebrafish [START_REF] Maugars | Demonstration of the coexistence of duplicated LH receptors in teleosts, and their origin in ancestral actinopterygians[END_REF].

Sex steroids and their receptors

teleosts, 3R duplication of a key-enzyme, the aromatase (Cyp19a1a and b), has been largely documented [for review: [START_REF] Zhang | Genes encoding aromatases in teleosts: Evolution and expression regulation[END_REF]].

Sex steroid nuclear receptors have been duplicated by 3R and the conservation of most of the paralogs led to the presence of a larger number of nuclear receptors in teleosts as compared to mammals [for review: [START_REF] Ogino | Functional distinctions associated with the diversity of sex steroid hormone receptors ESR and AR[END_REF]]. This is also the case for the steroid membrane receptors. Concerning estradiol receptors, tetrapods possess two nuclear receptors (ESR1 also named ERα, and ESR2 also named ERß) and a single membrane receptor (GPER) while teleosts, as a result of 3R, possess three nuclear receptors (esr1 and duplicated esr2a and esr2b) and two membrane receptors (gpera and gperb) (Lafont et al., 2016). One of the duplicated esr1 paralog would have been lost shortly after 3R. A species-specific gene loss occurred in zebrafish, which possesses a single gper paralog, while both gpera and b paralogs have been conserved in the other teleosts investigated so far (Lafont et al., 2016). For androgen receptors, 3R also generated two ar paralogs (ara and arb) in teleosts [START_REF] Ogino | Evolutionary history and functional characterization of androgen receptor genes in jawed vertebrates[END_REF]. Recently, Morini and colleagues revealed the presence of duplicated progesterone nuclear receptors (pgra or pgr1, and pgrb or pgr2) in the eel, which originated from 3R. In other teleosts however only a single pgr (orthologous to eel pgrb/pgr2) has been found, and synteny analysis indicates the loss of pgra (pgr1) paralog in the teleost lineage after the emergence of the basal teleost group of elopomorphs [START_REF] Morini | Nuclear and membrane progestin receptors in the European eel: Characterization and expression in vivo through spermatogenesis[END_REF]. Two pgr paralogs are reported in the goldfish, likely resulting from 4R. The evolutionary history of membrane progestin receptors is even more complex as five isoforms have been characterized in vertebrates [START_REF] Morini | Nuclear and membrane progestin receptors in the European eel: Characterization and expression in vivo through spermatogenesis[END_REF]. The conservation of multiple steroid receptor genes in teleosts likely reflects subfonctionalization and neofunctionalization processes, which may have contributed to species-specific diversification of sexual chararacters, regulatory processes and reproductive cycles throughout teleost radiation.

Neuroendocrine mechanisms of the interactions between corticotropic and gonadotropic axes in teleosts

3.1. Regulatory effects of actors of the corticotropic axis on the gonadotropic axis 3.1.1. Roles of corticosteroids and their receptors

Expression and role along the gonadotropic axis

In the rainbow trout, GR are expressed in GnRH-and dopaminergic neurons as well as on gonadotrope cells [ [START_REF] Teitsma | Glucocorticoid Receptor Immunoreactivity in Neurons and Pituitary Cells Implicated in Reproductive Functions in Rainbow Trout: A Double Immunohistochemical Study[END_REF]; for review: [START_REF] Teitsma | Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout[END_REF]]. In addition, GR and ER colocalize in the same neurons and pituitary cells [for review: [START_REF] Teitsma | Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout[END_REF]].

In Astatotilapia burtoni, both GR1 and GR2 are expressed in GnRH1 neurons in the preoptic area [START_REF] Korzan | Social regulation of cortisol receptor gene expression[END_REF]. A direct action of cortisol on GnIH neurons and targets is suggested in teleosts by the presence of several GR responsive elements (GRE) in the promoters of zebrafish gnih and gnih receptor [START_REF] Ogawa | Structural and functional divergence of gonadotropin-inhibitory hormone from jawless fish to mammals[END_REF]. GRs are also expressed in gonads in various teleosts [midshipman Porichthys notatus: [START_REF] Arterbery | Corticosteroid receptor expression in a teleost fish that displays alternative male reproductive tactics[END_REF] ; Astatotilapia burtoni: [START_REF] Maruska | Plasticity of the reproductive axis caused by social status change in an african cichlid fish: II. Testicular gene expression and spermatogenesis[END_REF]].

MR is expressed in brain, especially in GnRH1 neurons [Astatotilapia burtoni: [START_REF] Korzan | Social regulation of cortisol receptor gene expression[END_REF]], and in gonads of various teleosts [Haplochromis burtoni: [START_REF] Greenwood | Multiple corticosteroid receptors in a teleost fish: Distinct sequences, expression patterns, and transcriptional activities[END_REF]; rainbow trout: [START_REF] Milla | Plasma 11-deoxycorticosterone (DOC) and mineralocorticoid receptor testicular expression during rainbow trout Oncorhynchus mykiss spermiation: Implication with 17alpha, 20beta-dihydroxyprogesterone on the milt fluidity?[END_REF][START_REF] Sturm | 11-Deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor[END_REF]; midshipman Porichthys notatus: [START_REF] Arterbery | Corticosteroid receptor expression in a teleost fish that displays alternative male reproductive tactics[END_REF]; Astatotilapia burtoni: [START_REF] Maruska | Plasticity of the reproductive axis caused by social status change in an african cichlid fish: II. Testicular gene expression and spermatogenesis[END_REF]; for review: [START_REF] Takahashi | The role of "mineralocorticoids" in teleost fish: Relative importance of glucocorticoid signaling in the osmoregulation and "central" actions of mineralocorticoid receptor[END_REF]].

These expression patterns of GR and MR along the gonadotropic axis suggest potential major actions of glucocorticoids on reproductive functions at different levels, involving endocrine loops and/or paracrine/autocrine regulations.

Corticosteroids themselves are produced by gonads in teleosts [for review: [START_REF] Kime | Classical and non-clssical reproductive steroids in fish[END_REF]].

Cortisol is present in sperm and seminal fluid [Pacific herring Clupea harengus and North sea plaice Pleuronectes platessa: (Scott et al., 1991b(Scott et al., , 1991a))]. 11-deoxycortisol is produced by the ovary [Gillichthys mirabilis and Microgadus proximus: [START_REF] Colombo | Biosynthesis of 11deoxycorticosteroids by teleost ovaries and discussion of their possible role in oocyte maturation and ovulation[END_REF]; sea bass: [START_REF] Colombo | Emergence of Ovarian 11-Deoxycorticosteroid Biosynthesis at ovulation time in the sea bass, Dicentrarchus labrax L[END_REF]; Jenynsia lineata: [START_REF] Tesone | Steroid biosynthesis in the gonads of the teleost fish Jenynsia lineata[END_REF]; goldfish: [START_REF] Kime | In vitro biosynthesis of steroids, including 11-deoxycortisol and 5α-pregnane-3β,7α,17,20β-tetrol, by ovaries of the goldfish Carassius auratus during the stage of oocyte final maturation[END_REF]] and testis [Jenynsia lineata: [START_REF] Tesone | Steroid biosynthesis in the gonads of the teleost fish Jenynsia lineata[END_REF]]. Like 11-deoxycortisol, 11deoxycorticosterone (DOC) is an important product of ovarian [Leptocottus armatus, Gillichthys mirabilis and Microgadus proximus: [START_REF] Colombo | Biosynthesis of 11deoxycorticosteroids by teleost ovaries and discussion of their possible role in oocyte maturation and ovulation[END_REF]; sea bass: [START_REF] Colombo | Emergence of Ovarian 11-Deoxycorticosteroid Biosynthesis at ovulation time in the sea bass, Dicentrarchus labrax L[END_REF]] and/or testicular [Jenynsia lineata: [START_REF] Tesone | Steroid biosynthesis in the gonads of the teleost fish Jenynsia lineata[END_REF] steroidogenesis. Future studies should investigate whether ACTH, gonadotropins or both, associated or not to other factors are regulating such corticosteroid production by gonads in fish.

In vivo effects of corticosteroids

Pioneer works in loach Misgurnus fossilis [START_REF] Kirshenblatt | The action of steroid hormones in female vy'un[END_REF], conger Conger conger [START_REF] Mousset | Sur la mturation sexuelle du congre[END_REF] and European eel [START_REF] Fontaine | Sur la maturation des organes génitaux de l'Anguille femelle (Anguilla anguilla L.) et l'émission spontanée des oeufs en aquarium[END_REF] show that injection of glucocorticoids (desoxycorticosterone acetate) has positive effects on reproduction as it is able to induce oocyte maturation and spawning. In the catfish Heteropneustes fossilis also, adrenal corticosteroids (cortisol, cortisone and desoxycorticosterone acetate) are shown to be effective in inducing ovulation and spawning in hypophysectomized gravid fish, suggesting a direct positive action on the ovary independently of the pituitary (Sundararaj and Goswami, 1966a). Use of metopiron, an adrenocortical inhibitor which can induce « chemical interrenalectomy », allows to show that LH-induced effect on ovulation and spawning in catfish was interrenal-dependent (Sundararaj and Goswami, 1966b). Treatment with cortisol increases pituitary gonadotropin in juvenile rainbow trout [implant: [START_REF] Crim | Onset of gonadotropic hormone accumulation in the immature trout pituitary gland in response to estrogen or aromatizable androgen steroid hormones[END_REF]] and prepubertal European eel [injection: [START_REF] Dufour | Effects of steroid hormones on pituitary immunoreactive gonadotropin in European freshwater eel, Anguilla anguilla L[END_REF][START_REF] Huang | Cortisol selectively stimulates pituitary gonadotropin β-subunit in a primitive teleost, Anguilla anguilla[END_REF]]. Altogether, these data suggest positive interactions of glucocorticoids on the HPG axis at both juvenile and adult stages in some teleost species.

In contrast, a lot of studies report negative effects of glucocorticoids on the reproductive axis.

Most of them are conducted in salmonids. In the brown trout, implantation of cortisol to sexually maturing male and female fishes result in smaller gonads, lower plasma levels of sex steroids and pituitary gonadotropin content [START_REF] Carragher | The deleterious effects of cortisol implantation on reproductive function in two species of trout, Salmo trutta L. and Salmo gairdneri Richardson[END_REF]. Differently, in the maturing male rainbow trout, a cortisol implant decreases plasma gonadotropin levels while having no effects on other parameters (plasma sex steroids, pituitary gonadotropin content and gonad size) [START_REF] Carragher | The deleterious effects of cortisol implantation on reproductive function in two species of trout, Salmo trutta L. and Salmo gairdneri Richardson[END_REF]. In the rainbow trout, intraperitoneal implant of cortisol decreases hepatic E2-binding site concentration, but increases plasma E2-binding capacity in immature females [START_REF] Pottinger | The effect of cortisol administration on hepatic and plasma estradiol-binding capacity in immature female rainbow trout (Oncorhynchus mykiss)[END_REF] and decreases ER and Vg expression in maturating females [START_REF] Lethimonier | Transcriptional Interference Between Glucocorticoid Receptor and Estradiol Receptor Mediates the Inhibitory Effect of Cortisol on Fish Vitellogenesis1[END_REF]. Cortisol injection depresses plasma E2 and T levels, without affecting plasma gonadotropin levels in vitellogenic rainbow trout [START_REF] Pankhurst | Evidence that acute stress inhibits ovarian steroidogenesis in rainbow trout in vivo, through the action of cortisol[END_REF]. In the male rainbow trout, testicular mr expression and plasma DOC increase around the time of spermiation, and the coadministration of DOC with 17α, 20β-dihydroprogesterone decreases the spermatocrit value and therefore increases the sperm fluidity [START_REF] Milla | Plasma 11-deoxycorticosterone (DOC) and mineralocorticoid receptor testicular expression during rainbow trout Oncorhynchus mykiss spermiation: Implication with 17alpha, 20beta-dihydroxyprogesterone on the milt fluidity?[END_REF]. In Atlantic salmon, Eriksen and collaborators report impact on progeny of maternal exposure to cortisol (implant), including increased mortality and prevalence of deformities, reduced development and yolk-sac volume [START_REF] Eriksen | Long-term effects of maternal cortisol exposure and mild hyperthermia during embryogeny on survival, growth and morphological anomalies in farmed Atlantic salmon Salmo salar offspring[END_REF][START_REF] Eriksen | Prespawning stress 65 in farmed Atlantic salmon Salmo salar: Maternal cortisol exposure and hyperthermia during embryonic development affect offspring survival, growth and incidence of malformations[END_REF]. During a field study on spawning grounds, use in female pink Pacific salmon Oncorhynchus gorbuscha of cortisol implant which further increases endogenous cortisol levels, impairs reproductive success (fewer eggs) and survival [START_REF] Mcconnachie | Consequences of acute stress and cortisol manipulation on the physiology, behavior, and reproductive outcome of female Pacific salmon on spawning grounds[END_REF]. The use of metyrapone, which prevents the synthesis of cortisol from 11-deoxycortisol by inhibiting 11β-hydroxylase, does not change reproductive success [START_REF] Mcconnachie | Consequences of acute stress and cortisol manipulation on the physiology, behavior, and reproductive outcome of female Pacific salmon on spawning grounds[END_REF]. Altogether, these studies indicate that endogenous physiological cortisol levels, which raise at the time of upstream migration and spawning in salmonids (cf §1.2.2), do not impair reproduction, although higher levels may have a negative effect [START_REF] Mcconnachie | Consequences of acute stress and cortisol manipulation on the physiology, behavior, and reproductive outcome of female Pacific salmon on spawning grounds[END_REF].

Deleterious reproductive in vivo effects of corticosteroids have also been shown in other teleost species. In immature male common carp fed with cortisol-containing food pellets over pubertal period, brain sGnRH content, pituitary lh and fsh mRNA levels, plasma LH levels and testicular androgen (T and 11KT) secretion are decreased (Consten et al., 2001a).

Testicular development in this species is retarded by this long-term cortisol treatment (Consten et al., 2001a(Consten et al., , 2001b)). In contrast, dietary administered cortisol to channel catfish Ictalurus punctatus does not modify reproduction [START_REF] Small | Effect of dietary cortisol administration on growth and reproductive success of channel catfish[END_REF]. In the Arctic char, coexposure to cortisol and E2 results in reduced plasma Vg levels and no change in Vg mRNA levels, suggesting a post-transcriptional action of cortisol [START_REF] Berg | 17beta-estradiol induced vitellogenesis is inhibited by cortisol at the post-transcriptional level in Arctic char (Salvelinus alpinus)[END_REF]. Implantation of cortisol causes retardation of the ovarian growth and depression of plasma sex steroid levels in tilapia Oreochromis mossambicus [female: (Foo and Lam, 1993a); male: (Foo and Lam, 1993b)]. Later, Gennotte and collaborators show that cortisol treatment induces both positive (reduction of the time before ovulation) and negative (reduction of fecundity) effects on the reproduction in other tilapia species, Oreochromis niloticus [START_REF] Gennotte | Cortisol is responsible for positive and negative effects in the ovarian maturation induced by the exposure to acute stressors in Nile tilapia, Oreochromis niloticus[END_REF].

In the cinnamon clownfish, Amphiprion melanopus, injection of cortisol increases gnih mRNA, and reduces sbGnRH mRNA and plasma levels of LH and FSH [START_REF] Choi | Effect of cortisol on gonadotropin inhibitory hormone (GnIH) in the cinnamon clownfish, Amphiprion melanopus[END_REF].

As, in this teleost species, GnIH inhibits gonadotropin expression [START_REF] Choi | Effects of gonadotropin inhibitory hormone or gonadotropin-releasing hormone on reproduction-related genes in the protandrous cinnamon clownfish, Amphiprion melanopus[END_REF], cortisol treatment leads to a dual negative effect on gonadotropin via an increase in GnIH and decrease in GnRH. A similar upregulation of hypothalamic gnih (rfrp3) mRNA levels is observed in rats after acute or chronic immobilization/restraint stress [START_REF] Kirby | Stress increases putative gonadotropin inhibitory hormone and decreases luteinizing hormone in male rats[END_REF][START_REF] Yang | Acute psychosocial stress inhibits lh pulsatility and kiss1 neuronal activation in female mice[END_REF], upregulation which is abolished by adrenalectomy [START_REF] Kirby | Stress increases putative gonadotropin inhibitory hormone and decreases luteinizing hormone in male rats[END_REF] .

Concerning the effect of stress or corticosteroid treatment on the kisspeptin system, all the available data have been obtained in rodents [for reviews: [START_REF] Acevedo-Rodriguez | Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling[END_REF][START_REF] Iwasa | The roles of kisspeptin and gonadotropin inhibitory hormone in stress-induced reproductive disorders[END_REF]] and show a reduction of hypothalamic Kiss1 and KissR1 mRNA levels [rat, restraint stress or subcutaneous injection of corticosterone: [START_REF] Kime | In vitro biosynthesis of steroids, including 11-deoxycortisol and 5α-pregnane-3β,7α,17,20β-tetrol, by ovaries of the goldfish Carassius auratus during the stage of oocyte final maturation[END_REF]; mice, implant containing corticosterone: [START_REF] Luo | Corticosterone blocks ovarian cyclicity and the LH surge via decreased kisspeptin neurons activation in female mice[END_REF]; mice, restraint stress: [START_REF] Yang | Acute psychosocial stress inhibits lh pulsatility and kiss1 neuronal activation in female mice[END_REF]]. It would be relevant to investigate the regulation of the kisspeptin system by stress and corticosteroids in teleosts, which show different effects of kisspeptin on the control of gonadotropins according to the species.

In vitro effects of corticosteroids

In vitro investigations pin-point the direct actions of cortisol at the different levels of the HPG axis. In the rat hypothalamic rfrp-expressing cell line (rHypoE-23) [START_REF] Gingerich | The generation of an array of clonal, immortalized cell models from the rat hypothalamus: analysis of melatonin effects on kisspeptin and gonadotropin-inhibitory hormone neurons[END_REF], treatment with corticosterone increases gnih(rfrp) mRNA levels and the application of a GR antagonist blocks this increase [START_REF] Gojska | Glucocorticoid receptor-mediated regulation of Rfrp (GnIH) and Gpr147 (GnIH-R) synthesis in immortalized hypothalamic neurons[END_REF][START_REF] Son | Molecular basis for the activation of gonadotropin-inhibitory hormone gene transcription by corticosterone[END_REF]. In vitro investigations should be developed in teleosts, using hypothalamic neurons, to study possible direct effects of corticosteroids on the various neuronal populations (GnRH, DA, GnIH and kisspeptin) involved in the gonadotropic axis.

At the pituitary level, cortisol affects reproduction-related gene expression in Atlantic cod pituitary cultures [START_REF] Von Krogh | Cortisol differentially affects cell viability and reproduction-related gene expression in Atlantic cod pituitary cultures dependent on stage of sexual maturation[END_REF]. In this species, cortisol has dual effects on fshβ expression, stimulating expression in cells from mature fish, while inhibiting expression in cells from spent fish [START_REF] Von Krogh | Cortisol differentially affects cell viability and reproduction-related gene expression in Atlantic cod pituitary cultures dependent on stage of sexual maturation[END_REF]. In contrast, cortisol has no direct effect on lhβ expression [START_REF] Von Krogh | Cortisol differentially affects cell viability and reproduction-related gene expression in Atlantic cod pituitary cultures dependent on stage of sexual maturation[END_REF]. While gnrhr2a transcript levels largely increase following cortisol treatment, gnrhr1b expression decreases in cells from spent fish and was unaffected at other maturity stages [START_REF] Von Krogh | Cortisol differentially affects cell viability and reproduction-related gene expression in Atlantic cod pituitary cultures dependent on stage of sexual maturation[END_REF]. In the prepubertal European eel, cortisol increases LH cellular content and lhβ mRNA levels by pituitary cells in culture [START_REF] Huang | Cortisol selectively stimulates pituitary gonadotropin β-subunit in a primitive teleost, Anguilla anguilla[END_REF]. Future studies should investigate the effects of corticosteroids on the levels of other pituitary targets such as GnIH and kisspeptin receptors in different teleost species, considering that, in teleosts, GnIH and kisspeptin possess direct inhibitory or stimulatory effects on the pituitary depending on the species.

At the liver level, dexamethasone inhibits both basal and E2-stimulated ER expression and subsequent Vg expression in trout hepatocyte aggregates [START_REF] Lethimonier | Transcriptional Interference Between Glucocorticoid Receptor and Estradiol Receptor Mediates the Inhibitory Effect of Cortisol on Fish Vitellogenesis1[END_REF]. This is achieved via an inhibition by GR of the transcriptional factor CCAAT/enhancer-binding protein (C/EPBβ) binding to the ER promoter region [START_REF] Lethimonier | The glucocorticoid receptor represses the positive autoregulation of the trout estrogen receptor gene by preventing the enhancer effect of a C/EBPβ-like protein[END_REF].

At the gonadal level, different data are available on the direct effect of cortisol on female gonadal steroidogenesis depending on species [ [START_REF] Pankhurst | Further evidence of the equivocal effects of cortisol on in vitro steroidogenesis by ovarian follicles of rainbow trout Oncorhynchus mykiss[END_REF]; for review: [START_REF] Pankhurst | Reproduction and development[END_REF]]. A decrease in E2 and T release by cultured ovarian follicles after cortisol treatment has been observed in rainbow trout [START_REF] Carragher | The effect of cortisol on the secretion of sex steroids from cultured ovarian follicles of rainbow trout[END_REF], while no such effect has been reported in goldfish, common carp and Pagrus auratus [START_REF] Pankhurst | Evidence that the inhibitory effects of stress on reproduction in teleost fish are not mediated by the action of cortisol on ovarian steroidogenesis[END_REF]. In the rainbow trout, cortisol is shown to have suppressive effects on basal and LH-or cAMPstimulated T and E2 production and secretion, but no effect on basal or LH-stimulated 17,20β-dihydroxy-4-pregnene-3-one (17,20β-P) [START_REF] Barkataki | Cortisol inhibition of 17β-estradiol secretion by rainbow trout ovarian follicles involves modulation of star and P450scc gene expression[END_REF][START_REF] Reddy | Effects of cortisol and triiodo-L-thyronine on the steroidogenic capacity of rainbow trout ovarian follicles at two stages of oocyte maturation[END_REF].

In the same species, cortisol is reported to enhance the efficiency of gonadotropin to induce intrafollicular oocyte maturation [START_REF] Jalabert | Modulation par différents stéroïdes non maturants de l'efficacité de la 17βhydroxy-20 β-dihydroprogestérone ou d'un extrait gonadotrope sur la maturation intrafolliculaire in vitro des ovocytes de la Truite arc-en-ciel Salmo gairdnerii[END_REF] and to positively regulate the secretion of follicular 17α-hydroxy-20β-dihydroprogesterone (DHP) induced by gonadotropin in vitro [START_REF] Jalabert | The modulatory effect in vitro of oestradiol17 beta, testosterone or cortisol on the output of 17alpha;-hydroxy-20beta;-dihydroprogesterone by rainbow trout (Salmo gairdneri) ovarian follicles stimulated by the maturational gonadotropin s-GtH[END_REF]. All these data suggest that, in the rainbow trout, cortisol has a positive (or no) effect on the synthesis of progestagen (Maturation Inducing Steroid, MIS) during final oocyte maturation, while being inhibitor at earlier stages of oogenesis, on the production of T and E2 during vitellogenesis. In the zebrafish, cortisol does not inhibit hCGstimulated E2 production, while ACTH did [START_REF] Alsop | Adrenocorticotropic hormone suppresses gonadotropin-stimulated estradiol release from zebrafish ovarian follicles[END_REF]. In other teleost species, glucocorticoids are also able to act on final oocyte maturation. Glucocorticoids have been shown to induce in vitro oocyte maturation in the goldfish [desoxycorticosterone: [START_REF] Jalabert | Étude Comparée de l'action des hormones hypophysaires et stéroïdes sur la maturation in vitro des ovocytes de la truite et du carssin (poissons téléostéens[END_REF]] and ovulation in the medaka [cortisol: [START_REF] Hirose | Endocrine control of ovulation in medaka (Oryzias latipes) and Ayu (Plecoglossus altivelis)[END_REF]]. In the amago salmon (Oncorhynchus rhodurus), DOC can enhance oocyte maturation induced by gonadotropin or by DHP in vitro [START_REF] Young | Oocyte maturation in the Amago salmon (Oncorhynchus rhodurus ): In Vitro Effects o f Salmon Gonadotropin , Steroids , and cyanoketone (an inhibitor of 3p-Hydroxy-A5-Steroid Dehydrogenase)[END_REF]. Cortisol treatment induces DNA damage on zebrafish stage I and II follicles, and increases mc2r expression in stage II follicles in vitro [START_REF] Sousa | Morphological and molecular effects of cortisol and ACTH on zebrafish stage I and II Follicles[END_REF].

Data on male gonadal steroidogenesis, even if fewer, seem more consistent than those on female steroidogenesis. In the common carp, treatment of male testes in vitro with dexamethasone reduces the induction of androgen secretion by carp pituitary extract [START_REF] Consten | Corticosteroids Affect the Testicular Androgen Production in Male Common Carp (Cyprinus carpio L.)[END_REF]. Consten and collaborators have previously reported a decreased in vitro production of testicular androgen during early stages of puberty in the common carp fed with cortisol [START_REF] Consten | Inhibitory effects of cortisol on in vivo and in vitro androgen secretion in male common carp, Cyprinus carpio[END_REF]. In male rainbow trout, DOC possesses an inhibitory effect on the testicular production of DHP in vitro [START_REF] Milla | Plasma 11-deoxycorticosterone (DOC) and mineralocorticoid receptor testicular expression during rainbow trout Oncorhynchus mykiss spermiation: Implication with 17alpha, 20beta-dihydroxyprogesterone on the milt fluidity?[END_REF]. In the Japanese eel, in vitro cortisol treatment induces DNA replication in spermatogonia and enhances the spermatogonial proliferation stimulated by 11KT, revealing a stimulatory effect of cortisol on spermatogenesis [START_REF] Ozaki | Roles of 11β-hydroxysteroid dehydrogenase in fish spermatogenesis[END_REF].

Roles of POMC-derived peptides

Other stress-related endocrine factors have also been studied for their involvement in the regulation of the HPG axis.

Pomca-deficient zebrafish, obtained with a TALENs approach, have enhanced somatic growth, reduced anxiety-related behavioral responses, and exhibit hypocortisolism associated with hyperandrogenism [START_REF] Shi | Hyperandrogenism in POMCa-deficient zebrafish enhances somatic growth without increasing adiposity[END_REF]. The authors suggest that the hypocortisolemia and hyperandrogenism are probably due to the changes in expression of interrenal steroidogenic enzymes involved in the synthesis of cortisol (downregulation of hsd3 expression) and androgens (upregulation of cyp17a1a and cyp19a1a expression), respectively [START_REF] Shi | Hyperandrogenism in POMCa-deficient zebrafish enhances somatic growth without increasing adiposity[END_REF].

In vitro experiments show a direct role of ACTH on zebrafish ovaries: ACTH treatment suppresses hCG-stimulated E2 release from ovarian follicles [START_REF] Alsop | Adrenocorticotropic hormone suppresses gonadotropin-stimulated estradiol release from zebrafish ovarian follicles[END_REF], and induces a high level of oocyte vacuolization, as well as DNA damage on stage I and II follicles [START_REF] Sousa | Morphological and molecular effects of cortisol and ACTH on zebrafish stage I and II Follicles[END_REF]. In agreement with the direct action of ACTH on the gonads, the ACTH receptor MC2R is highly expressed in gonads of various teleosts [rainbow trout: [START_REF] Aluru | Molecular characterization, tissue-specific expression, and regulation of melanocortin 2 receptor in rainbow trout[END_REF]; zebrafish: [START_REF] Agulleiro | Role of melanocortin receptor accessory proteins in the function of zebrafish melanocortin receptor type 2[END_REF][START_REF] Alsop | Adrenocorticotropic hormone suppresses gonadotropin-stimulated estradiol release from zebrafish ovarian follicles[END_REF]; barfin flounder: [START_REF] Kobayashi | Melanocortin receptor subtypes in interrenal cells and corticotropic activity of α-melanocytestimulating hormones in barfin flounder, Verasper moseri[END_REF]; sea bass: [START_REF] Agulleiro | Molecular Characterization and Functional Regulation of Melanocortin 2 Receptor (MC2R) in the Sea Bass. A Putative Role in the Adaptation to Stress[END_REF]].

Another POMC-related peptides, the opioid peptide β-endorphin, is also produced and released in response to stressors in teleosts [gilthead sea bream: [START_REF] Arends | The stress response of the gilthead sea bream (Sparus aurata L.) to air exposure and confinement[END_REF][START_REF] Mosconi | Acetyl salmon endorphin-like and interrenal stress response in male gilthead sea bream, Sparus aurata[END_REF]; tilapia: [START_REF] Chabbi | Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus[END_REF]]. This peptide may be involved in the reproductive response to stress as a suppression of LH secretion, an inhibition of vitellogenic follicular growth and a reduction of gonado-somatic index (GSI) are observed in tilapia injected with β-endorphin [START_REF] Chabbi | β-Endorphin-induced Inhibition of Vitellogenic Follicular Growth in the Fish Oreochromis mossambicus: Evidence for Opioidergic Mediation of Ovarian Stress Response[END_REF][START_REF] Ganesh | Naltrexone attenuates stress-induced suppression of LH secretion in the pituitary gland in the Cichlid fish Oreochromis mossambicus: Evidence for the opioidergic mediation of reproductive stress response[END_REF]. Treatment with naltrexone, an opioid receptor antagonist, attenuates these inhibitory effects of βendorphin in tilapia [START_REF] Chabbi | β-Endorphin-induced Inhibition of Vitellogenic Follicular Growth in the Fish Oreochromis mossambicus: Evidence for Opioidergic Mediation of Ovarian Stress Response[END_REF][START_REF] Ganesh | Naltrexone attenuates stress-induced suppression of LH secretion in the pituitary gland in the Cichlid fish Oreochromis mossambicus: Evidence for the opioidergic mediation of reproductive stress response[END_REF]. In common carp, naltrexone stimulates the in vitro LH release from whole pituitary gland as well as from dispersed pituitary cells, revealing direct paracrine pituitary effects of opioids [START_REF] Socha | Naltrexone Stimulates LH Secretion in Vitro From the common carop (Cyprinus carpio L.) pituitary gland[END_REF].

Roles of CRH

The direct and indirect effects of CRH on the gonadotropic axis have been well-documented in mammals [for reviews: [START_REF] Chand | Stress and reproduction: Controversies and challenges[END_REF][START_REF] Kageyama | Regulation of gonadotropins by corticotropin-releasing factor and urocortin[END_REF]]. CRH, injected centrally [ovariectomized female rat: [START_REF] Ono | Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat[END_REF][START_REF] Rivier | Influence of corticotropin-releasing factor on reproductive functions in the rat[END_REF]; intact and castrated male rat: [START_REF] Frias | Effect of corticotropin releasing factor injected into the 718[END_REF]], but not peripherally [human: [START_REF] D'agata | Effect of CRF on the release of anterior pituitary hormones in normal subjects and patients with Cushing's disease[END_REF]; rat: [START_REF] Ono | Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat[END_REF][START_REF] Rivier | Influence of corticotropin-releasing factor on reproductive functions in the rat[END_REF]], inhibits LH release. Centrally administered CRH also down-regulates Kiss1 and Kiss1r mRNA levels in ovariectomized E2 replaced female rats [START_REF] Kime | In vitro biosynthesis of steroids, including 11-deoxycortisol and 5α-pregnane-3β,7α,17,20β-tetrol, by ovaries of the goldfish Carassius auratus during the stage of oocyte final maturation[END_REF]. Central administration of α-helical CRH, a nonselective CRH-R antagonist, reverses the inhibitory effect of stress on pulsatile release of LH in castrated male rat (Rivier et al., 1986). In vitro, CRH down-regulates GnRH release from rat hypothalamic slices and this inhibition is suppressed by α-helical CRH [START_REF] Nikolarakis | Corticotropin-releasing factor (CRF) inhibits gonadotropin-releasing hormone (GnRH) release from superfused rat hypothalami in vitro[END_REF]. In parallel to its central inhibitory effect, CRH is also able to act at the pituitary level through CRH-R2 to inhibit LH release in mice [START_REF] Raftogianni | Deciphering the Contributions of CRH Receptors in the Brain and Pituitary to Stress-Induced Inhibition of the Reproductive Axis[END_REF]. Previous data in rat failed to show any effect of CRH on LH release in vitro [START_REF] Ono | Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat[END_REF].

In teleosts, there are very few investigations regarding CRH action on the gonadotropic axis.

Injection of CRH to stripped tilapia Oreochromis mossambicus resulted in diminished immunoreactive LH content in the pituitary, lower GSI and absence of vitellogenic follicles in the ovary [START_REF] Chabbi | Glucocorticoid synthesis inhibitor metyrapone blocks stressinduced suppression along luteinizing hormone secreting cells-ovary axis in the fish Oreochromis mossambicus[END_REF]. The glucocorticoid synthesis inhibitor, metyrapone, abolishes the inhibitory effects of CRH treatment on LH secretion and follicle growth, demonstrating an action via cortisol [START_REF] Chabbi | Glucocorticoid synthesis inhibitor metyrapone blocks stressinduced suppression along luteinizing hormone secreting cells-ovary axis in the fish Oreochromis mossambicus[END_REF]. Direct pituitary or gonadal effect of CRH needs to be investigated through in vitro studies. CRH-R mRNAs have already been detected in the pituitary [catfish Ameiurus nebulosus: [START_REF] Arai | Characterization of Three Corticotropin-Releasing Factor Receptors in Catfish: A Novel Third Receptor Is Predominantly Expressed in Pituitary and Urophysis[END_REF]; common carp: [START_REF] Huising | Structural characterisation of a cyprinid (Cyprinus carpio L.) CRH, CRH-BP and CRH-R1, and the role of these proteins in the acute stress response[END_REF]; Astatotilapia burtoni: [START_REF] Chen | Sequences, expression patterns and regulation of the corticotropin-releasing factor system in a teleost[END_REF]; tilapia Oreochromis mossambicus: [START_REF] Aruna | The acute salinity changes activate the dual pathways of endocrine responses in the brain and pituitary of tilapia[END_REF][START_REF] Aruna | Involvement of Corticotrophin-Releasing Hormone and Corticosteroid Receptors in the Brain-Pituitary-Gill of Tilapia During the Course of Seawater Acclimation[END_REF]], but future investigations should aim at determining whether these receptors are expressed in other cell types than corticotrophs, notably in gonadotrophs. CRH-R are also expressed in the ovary [Astatotilapia burtoni: [START_REF] Chen | Sequences, expression patterns and regulation of the corticotropin-releasing factor system in a teleost[END_REF]; olive flounder Paralichthys olivaceus: [START_REF] Zhou | Dynamic Expression and Regulation of Urotensin I and Corticotropin-Releasing Hormone Receptors in Ovary of Olive Flounder Paralichthys olivaceus[END_REF]], which suggests a direct action of CRH on the gonads.

Regulatory effects of actors of the gonadotropic axis on the corticotropic axis

Conversely, some studies reveal a role of actors of the gonadotropic axis on the corticotropic axis in teleosts.

Roles of sex steroids

In the sockeye salmon, gonadectomy blocks the cortisol prespawning rise [START_REF] Donaldson | Effect of sexual maturation and gonadectomy at sexual maturity on cortisol secretion rate in sockeye salmon (Oncorhynchus nerka)[END_REF][START_REF] Robertson | Prolongation of the Life Span of Kokanee Salmon (Oncorhynchus Nerka Kennerlyi) By Castration Before Beginning of Gonad Development[END_REF] and sex steroids, estrogens and androgens, can induce interrenal hyperplasia as well as increase in plasma cortisol levels in both intact and gonadectomized fish [ [START_REF] Donaldson | Cortisol secretion rate in gonadectomized female sockeye (Oncorhrynchus nerka): effects of estrogen and cortisol treatment[END_REF][START_REF] Fagerlund | The effect of androgens on the distribution and secretion of cortisol in gonadectomized male sockeye salmon (Oncorhynchus nerka)[END_REF]; for review: [START_REF] Dickhoff | Salmonids and annual fishes: death after sex[END_REF]]. The situation is different in the rainbow and brown trout in which implantation of T and 11-KT attenuates ACTH and cortisol-elevation after confinement stress, while E2 implantation increases stress-induced ACTH and cortisol levels [START_REF] Pottinger | Testosterone, 11ketotestosterone, and estradiol-17β modify baseline and stress-induced interrenal and corticotropic activity in trout[END_REF]. In the rainbow trout, in vivo treatment with 11KT suppresses interrenal responsiveness to ACTH in vitro [START_REF] Young | 11-Ketotestosterone suppresses interrenal activity in rainbow trout (Oncorhynchus mykiss)[END_REF], while DHP stimulates cortisol production by interrenal tissue cultured in vitro [START_REF] Barry | Effects of 17α,20β-dihydroxy-4pregnen-3-one on cortisol production by rainbow trout interrenal tissue in vitro[END_REF]. McQuillan and collaborators compared the in vitro effect of E2 on cortisol synthesis by interrenals of the chinook salmon, Oncorhynchus tshawytscha and rainbow trout: they show that E2 suppresses the ability of juvenile and mature chinook salmon interrenals to produce cortisol, whereas it has no effect in the rainbow trout [START_REF] Mcquillan | Effects of sex steroids, sex, and sexual maturity on cortisol production: An in vitro comparison of chinook salmon and rainbow trout interrenals[END_REF]. These data reveal potential differences among species and reproductive stage concerning the role of sexual steroids in the induction of hypercortisolemia in salmonids.

Interestingly, the interrenal in fish has been reported to produce androgen [rainbow trout: [START_REF] Arai | In Vitro Transformation of Steroids by the Head the Body Kidney , and the Corpuscles of Stannius of the Rainbow Trout (Salmo gairdneri)[END_REF]; coho salmon: [START_REF] Schreck | Regulation of the interrenal of fishes: non-classical control mechanisms[END_REF]; tilapia Oreochromis mossambicus: [START_REF] Balm | Corticosteroid biosynthesis in the interrenal cells of the teleost fish, Oreochromis mossambicus[END_REF]; African catfish, Clarias gariepinus: [START_REF] Vermeulen | Adrenal tissue in the male African catfish, Clarias gariepinus: localization and steroid hormone secretion[END_REF]], as in mammals [for review: [START_REF] Rainey | Regulation of the adrenal androgen biosynthesis[END_REF]]. In human, the production and secretion of adrenal androgens (dehydroepiandrosterone, DHEA and its sulfate ester, DHEAS) in the midchildhood, termed adrenarche, has been involved in the timing of puberty [START_REF] Remer | Prepubertal adrenarchal androgens and animal protein intake independently and differentially influence pubertal timing[END_REF]. Such a role of interrenal androgens could be also envisioned in teleosts.

A potential remarkable interaction between reproductive and stress actors is illustrated by the recent finding of the ability of progesterone to bind to the mineralocorticoid receptor MR.

Recent in vitro studies indicate that progesterone can activate MR in the elephant shark Callorhinchus milii, in ray-finned fishes including trout [START_REF] Sturm | 11-Deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor[END_REF] and zebrafish [START_REF] Fuller | Molecular evolution of the switch for progesterone and spironolactone from mineralocorticoid receptor agonist to antagonist[END_REF][START_REF] Katsu | Progesterone activation of zebrafish mineralocorticoid receptor may influence growth of some transplanted tumors[END_REF] and in chickens, with an EC50 of less than 1 nM for these MRs, which makes this steroid a potential physiological activator of these MRs [for review: [START_REF] Baker | Progesterone: An enigmatic ligand for the mineralocorticoid receptor[END_REF]]. Moreover, in the elephant shark and ray-finned fish, MR are expressed in several tissues, including ovaries and testis [START_REF] Katsu | Transcriptional activation of elephant shark mineralocorticoid receptor by corticosteroids, progesterone, and spironolactone[END_REF][START_REF] Milla | Plasma 11-deoxycorticosterone (DOC) and mineralocorticoid receptor testicular expression during rainbow trout Oncorhynchus mykiss spermiation: Implication with 17alpha, 20beta-dihydroxyprogesterone on the milt fluidity?[END_REF][START_REF] Sturm | 11-Deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor[END_REF]. This led Baker and Katsu to suggest that progesterone may have multifaceted physiological roles through MR in these species [for review: [START_REF] Baker | Progesterone: An enigmatic ligand for the mineralocorticoid receptor[END_REF]].

Roles of other actors

Injection of salmon gonadotropin to sockeye salmon has no impact on plasma cortisol or cortisone levels nor on interrenal activity [START_REF] Donaldson | Effect of ACTH and salmon gonadotropin on interrenal and thyroid activity of gonadectomized adult sockeye salmon (Oncorhynchus nerka)[END_REF]. However, in catfish Heteropneustes fossilis, in vitro studies suggest that gonadotropin (LH) stimulates the interrenal to produce corticosteroids which in turn act on the oocytes to induce maturation [START_REF] Sundararaj | Hormonal regulation of in vivo and in vitro oocyte maturation in the catfish, Heteropneustes fossilis (bloch)[END_REF]. Such studies in Heteropneustes fossilis highlight the complex synergy between gonadotropic and corticotropic axes in the induction of oocyte final maturation, ovulation and spawning in response to monsoon-related environmental cues (cf §1.2.1).

Other actors of the gonadotropic axis, such as GnRH, might act on the corticotropic axis in teleosts as evidenced in mammals. GnRH is shown to stimulate ACTH release from the rat pituitary [START_REF] Gambacciani | GnRH stimulates ACTH and immunoreactive beta-endorphin release from the rat pituitary in vitro[END_REF] and also to regulate the expression of pituitary corticotropin-releasing hormone binding protein (CRH-BP) by gonadotrope-like cell line [START_REF] Westphal | Gonadotropin-releasing hormone (GnRH) positively regulates corticotropin-releasing hormone-binding protein expression via multiple intracellular signaling pathways and a multipartite GnRH response element in αT3-1 cells[END_REF], highlighting the potential importance of GnRH/CRH interactions at the pituitary level in the interface between stress and reproductive axes. To our knowledge such investigations have not yet been performed in teleosts.

Recent cyto-anatomical studies suggest a potential role of GnIH not only in the gonadotropic axis but also in the corticotropic axis in teleosts. In tilapia, GnIH (LPXRFa) axons are closely associated with various pituitary cell types including gonadotrophs, corticotrophs and melanotrophs, and GnIH receptors (LPXRFa-Receptor) are expressed in LH, ACTH and alpha MSH cells [ [START_REF] Ogawa | Distribution of LPXRFa, a gonadotropin-inhibitory hormone ortholog peptide, and LPXRFa receptor in the brain and pituitary of the tilapia[END_REF]; for review: [START_REF] Muñoz-Cueto | A journey through the gonadotropin-inhibitory hormone system of fish[END_REF]]. It would be very interesting now to clarify whether GnIH is involved in the regulation of these cell types and in the production of ACTH and MSH.

Conclusion and perspectives

Stress impact on reproduction has long been studied in teleosts, especially in the frame of aquaculture. However, most of the accumulated data come from studies on the possible action of peripheral hormone, cortisol, on sexual maturation. Emerging data in mammals show that actors of higher levels (especially brain) should catch researcher attention. The teleost specific whole genome duplication (3R) had made such studies perhaps more difficult, multiplicating the number of genes, but thanks to the recent publications of an increasing number of teleost genomes and the development of molecular and genome editing technology tools, it is now possible to investigate their respective functions across teleost species diversity. Thus future directions should aim, for example, at dissecting mechanisms of action of the different paralogs of genes involved in the corticotropic axis and in the caudal neurosecretory system in the regulation of various levels of the gonadotropic axis. Such studies are all the more important that the present review gives multiple examples of species-specific regulatory mechanisms which probably reflect the wide species-specific plasticity of fish reproductive responses when fish are exposed to stressful situations.

Figure 1 :

 1 Figure 1: Stress and reproduction neuroendocrine axes in teleosts.

Figure 2 :

 2 Figure 2: Effects of corticotropic axis on gonadotropic axis in vertebrates.

Introduction 1. Various interactions between stress, cortisol and reproduction in teleosts

	1.1. Negative interactions between stress and reproduction
	1.2. Positive interactions between cortisol and reproduction related to peculiar life
	cycles
	1.2.1. Cortisol and environmentally-related spawning activity in some teleosts
	1.2.2. Cortisol and upstream reproductive migration in salmonids
	1.2.3. Cortisol and oceanic reproductive migration in eels
	1.3. Cortisol and gonadal sex differentiation
	1.4. Animal welfare, environment, stress and reproduction
	2. Overview of neuroendocrine stress and reproductive axes in teleosts
	2.1. Teleost specific traits compared to other vertebrates
	2.1.1. Anatomo-functional specificities
	2.1.1.1. Pituitary regionalisation and direct innervation
	2.1.1.2. Caudal neurosecretory system
	2.1.2. Teleost specific whole genome duplication
	2.2. The corticotropic axis
	2.2.1. Main actors of the corticotropic axis in vertebrates
	2.2.2. Specific features of the corticotropic axis in teleosts
	2.2.2.1. CRH and related peptides and their receptors
	2.2.2.2. Dopamine and other neurohormones
	2.2.2.3. POMC-derived peptides and their receptors
	2.2.2.4. Corticosteroids and their receptors
	2.3. The gonadotropic axis
	2.3.1. Main actors of the gonadotropic axis in vertebrates
	2.3.2. Specific features of the gonadotropic axis in teleosts
	2.3.2.1. GnRHs and their receptors
	2.3.2.2. Dopamine and other neurohormones
	2.3.2.3. Gonadotropins and their receptors
	2.3.2.4. Sex steroids and their receptors

Table 2 : Major actors of the gonadotropic axis in human and teleosts.

 2 This table underlines the larger gene numbers in teleosts due to teleost-specific whole genome duplication (3R) but also to the conservation of paralogs resulting from early vertebrate whole genome duplications (1R and 2R), or from local gene duplication (see the text for details). In teleosts, some of the paralogs have been lost in some species. Supplemental paralogs issued from additional whole genome duplication (4R, which occurred in salmonids and carps) are not indicated. Abbreviations: GnRH, gonadotropin-releasing hormone; GnRH-R, gonadotropin-releasing hormone receptor; GnIH, gonadotropin inhibitory hormone; kiss,

kisspeptin; kiss-R, kisspeptin receptor; LHβ, luteinising hormone; FSHβ, follicle stimulating hormone; LH-R, luteinising hormone receptor; FSH-R, follicle stimulating hormone receptor; ER, estrogen receptor; esr, nuclear estrogen receptor; gper, membrane estrogen receptor; AR, androgen receptor; PR, progestin receptor; pgr, nuclear progestin receptor; mpgr, membrane progestin receptor.

Table 1 : Major actors of the corticotropic axis in human and teleosts.

 1

	Protein	Gene	Human	Teleosts	References / Review articles
	CRH	crh	crh1	crh1a	Lovejoy et al 2014; Cardoso et al 2016
				crh1b	Maugars et al 2016
			crh2	crh2
	CRH-R	crhr	crhr1	crhr1a	Cardoso et al 2014; Lovejoy et al 2014
				crhr1b
			crhr2	crhr2
	POMC	pomc	pomc	pomc-α1	Dores and Lecaude 2005; Cardoso et al
				pomc-α2	2011; Dores and Baron 2011
				pomc-β
	MC1R	mc1r	mc1r	mc1r	Vastermark and Schioth 2011; Dores et
	MC2R	mc2r	mc2r	mc2r	al 2014, 2016
	MC3R	mc3r	mc3r	mc3r
	MC4R	mc4r	mc4r	mc4r
	MC5R	mc5r	mc5r	mc5ra
				mc5rb
	GR	gr	gr	gr1	Bury 2017
				gr2
	MR	mr	mr	mr1	Lafont et al 2014; Baker and Katsu 2019
				mr2
	UI	uI		uI	Lovejoy et al 2014

Table 2 : Major actors of the gonadotropic axis in human and teleosts.

 2

	Protein	Gene	Human	Teleosts	References / Review articles
	GnRH	gnrh	gnrh1	gnrh1	Kim et al 2011; Roch et al 2011;
			gnrh2	gnrh2	Tostivint 2011
				gnrh3
	GnRH-R	gnrhr	gnrhr-I	Up to 5 gnrhr	Roch et al 2014; Sefideh et al
					2014; William et al 2014; Ciani
					et al 2020
	GnIH	gnih	gnih/rfrp	gnih	Tsutsui et al 2018; Maugars et al
					2020
	Kiss	kiss	kiss1	kiss1	Pasquier et al 2012, 2014
				kiss2
	Kiss-R	kissR	kiss1R	kiss1R	Pasquier et al 2012, 2014
				kiss2R
				kiss3R
	LHβ	lhβ	lhβ	lhβ	Dufour et al 2020
	FSHβ	fshβ	fshβ	fshβ	Dufour et al 2020
	LH-R	lhr	lhcgr	lhr1	Maugars and Dufour 2015
				lhr2
	FSH-R	fshr	fshr	fshr	Maugars and Dufour 2015
	ER	esr	esr1	esr1	Lafont et al 2016; Ogino et al
			esr2	esr2a	2018
				esr2b
		gper	gper	gpera	Lafont et al 2016
				gperb
	AR	ar	ar	ara	Ogino et al 2009
				arb
	PR	pgr	pgr	pgra	Morini et al 2019
				pgrb
		mpgr	multiple	multiple	Morini et al 2019

liver axis (brain-pituitary-gonad-liver). Abbreviations: ACTH, adrenocorticotropin; β-end, βendorphin; CRH, corticotropin-releasing hormone; E2, estradiol; FSH, follicle stimulating hormone; GnIH, gonadotropin inhibitory hormone; GnRH, gonadotropin-releasing hormone; GR, glucocorticoid receptor; kiss, kisspeptin; LH, luteinising hormone; MC2R, melanocortin receptor 2; MR, mineralocorticoid receptor; T, testosterone; 11-KT, 11-ketotestosterone;. This table underlines the expansion of gene numbers in teleosts due to teleost-specific whole genome duplication (3R). In teleosts, some of the paralogs have been lost in some species.