High density and food deprivation affect arginine vasotocin, isotocin and melatonin in gilthead sea bream (Sparus auratus)

Early starvation in European seabass (Dicentrarchus labrax) larvae has no drastic effect on hepatic intermediary metabolism in juveniles

The present study aims to investigate nutritional programming through early starvation in the European seabass (Dicentrarchus labrax). European seabass larvae were fasted at three different developmental periods for three durations from 60 to 65 dph (F1), 81 to 87 dph (F2), and 123 to 133 dph (F3). Immediate effects were investigated by studying gene expression of npy (neuropeptide Y) and avt (Arginine vasotocin) in the head, while potential long-term effects (i.e., programming) were evaluated on intermediary metabolism later in life (in juveniles). Our findings indicate a direct effect regarding gene expression in the head only for F1, with higher avt mRNA level in fasted larved compared to controls. The early starvation periods had no long-term effect on growth performance (body weight and body length). Regarding intermediary metabolism, we analyzed related key plasma metabolites which reflect the intermediary metabolism: no differences for glucose, triglycerides, and free fatty acids in the plasma were observed in juveniles irrespective of the three early starvation stimuli. As programming is mainly linked to molecular mechanisms, we then studied hepatic mRNA levels for 23 key actors of glucose, lipid, amino acid, and energy metabolism. For many of the metabolic genes, there was no impact of early starvation in juveniles, except for three genes involved in glucose metabolism (glut2-glucose transporter and pk-pyruvate kinase) and lipid metabolism (acly-ATP citrate lyase) which were higher in F2 compared to control. Together, these results highlight that starvation between 81 to 87 dph may have more long-term impact, suggesting the existence of a developmental window for programming by starvation. In conclusion, European seabass appeared to be resilient to early starvation during larvae stages without drastic impacts on intermediary metabolism later in life.

Effects of different acute stressors on the regulation of appetite genes in the carp (Cyprinus carpio L.) brain

Our understanding of the timing of stress responses and specific roles of different regulatory pathways that drive stress responses is incomplete. In particular, the regulation of appetite genes as a consequence of exposure to different stressors has not been studied in sufficient detail in fish. Therefore, a stress trial was conducted with koi carp, aiming at identifying typical effects of stress on regulation of appetite genes. The stressors tank manipulation, air exposure and feed rewarding were chosen. The responses to these stressors were evaluated 10, 30 and 60 min after the stressors were applied. Orexigenic and anorexigenic genes were investigated in four different brain regions (telencephalon, hypothalamus, optic tectum and rhombencephalon). The results show that, apart from the typical appetite regulation in the hypothalamus, the different brain regions also display pronounced responses of appetite genes to the different stressors. In addition, several genes in the serotonergic, dopaminergic and gaba-related pathways were investigated. These genes revealed that rearing in pairs of two and opening of the tank lid affected anorexigenic genes, such as cart and cck, which were not changed by air exposure or feed rewarding. Moreover, distress and eustress led to limited, but distinguishable gene expression pattern changes in the investigated brain regions.

Oxidative damages and antioxidant defences after feeding a single meal in rainbow trout

Feeding and digestion are metabolically demanding causing a rise on metabolic rate called Specific Dynamic Action (SDA). Although SDA has been vastly reported in fish, its potential consequences on the oxidative-antioxidant balance has not been evaluated to date in fish, a model with a long alkaline tide associated with feeding as well. Using rainbow trout (Oncorhynchus mykiss) as a model species, the aims of the present study were to: (1) assess potential oxidative damages and changes in oxidative defences after feeding on a single meal, and (2) identify the timescale of such changes over a 96 h post-feeding period. Oxidative damage in proteins and lipids and the activities of four enzymatic antioxidant defences: superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST) were measured in gill, stomach, intestine and liver. DNA damage was measured in red blood cells. Fish were sampled before and after 1.5, 6, 24, 48, 72 and 96 h of ingestion of a 3% body mass ration. Trends of post-prandial damage were present in all tissues, but only protein oxidation varied significatively during digestion in the stomach. The intestine and stomach presented the highest enzymatic activities, likely due to the high metabolic action that these tissues have during digestion, with peaks during post-feeding: at 24 h of SOD in stomach and at 48 h of CAT in intestine. Observed GPx peaks during post-feeding in gills are likely due to the exacerbated demands for ion fluxes and/or oxygen during feeding. The differential response of the antioxidant system observed in tissues of rainbow trout during digestion indicates a coordinated and tissue-specific antioxidant defence.

High Stocking Density and Food Deprivation Increase Brain Monoaminergic Activity in Gilthead Sea Bream (Sparus aurata)

In teleosts, brain monoamines (dopamine and serotonin) participate in the early response to different acute stressors. However, little is known regarding their role during chronic stress. In a 2 × 2 factorial design, the influence of a high stocking density (HSD) and/or food deprivation (FD) on the brain monoaminergic activity in gilthead sea bream (Sparus aurata) was evaluated. Following a 21-day experimental design, samples from the plasma and brain regions (telencephalon, hypothalamus, and optic tectum) were collected. The dopamine (DA), serotonin (5HT), and their main metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5 hydroxyindoleacetic acid (5HIAA), contents were HPLC-assessed in brain tissues, and the ratios DOPAC/DA and 5HIAA/5HT were calculated as indicators of enhanced monoaminergic activity. The plasma levels of cortisol and catecholamine were also evaluated. The cortisol levels increased in fish exposed to HSD and normally fed but, also, in all FD groups, whereas the NA levels decreased in LSD-FD animals. Within the brain, the dopaminergic and serotonergic activities in telencephalon and hypothalamus increased in fish subjected to HSD and in the telencephalon of LSD-FD fish. While DA (hypothalamus) and 5HT (telencephalon) increased in the animals submitted to a HSD, food-deprived fish did not show such an increase. Taken together, our results supported the hypothesis of brain monoaminergic activity participating in maintaining and orchestrating the endocrine response to chronic stress in fish.

Aflatoxicosis Dysregulates the Physiological Responses to Crowding Densities in the Marine Teleost Gilthead Seabream (Sparus aurata)

Several studies in fish have shown that aflatoxin B1 (AFB1) causes a disparity of species-dependent physiological disorders without compromising survival. We studied the effect of dietary administration of AFB1 (2 mg AFB1 kg^-1 diet) in gilthead seabream (Sparus aurata) juveniles in combination with a challenge by stocking density (4 vs. 40 g L^-1). The experimental period duration was ten days, and the diet with AFB1 was administered to the fish for 85 days prior to the stocking density challenge. Our results indicated an alteration in the carbohydrate and lipid metabolites mobilization in the AFB1 fed group, which was intensified at high stocking density (HSD). The CT group at HSD increased plasma cortisol levels, as expected, whereas the AFB1-HSD group did not. The star mRNA expression, an enzyme involved in cortisol synthesis in the head kidney, presented a ninefold increase in the AFB1 group at low stocking density (LSD) compared to the CT-LSD group. Adenohypophyseal gh mRNA expression increased in the AFB1-HSD but not in the CT-HSD group. Overall, these results confirmed that chronic AFB1 dietary exposure alters the adequate endocrinological physiological cascade response in S. aurata, compromising the expected stress response to an additional stressor, such as overcrowding.

Endocrine-Disrupting Compounds in Fish Physiology, with Emphasis on their Effects on the Arginine Vasotocin/Isotocin System

The purposes of this review are to promote better use of existing knowledge of marine pollutants especially endocrine-disrupting compounds (EDCs) and to draw attention to the slow progression of the research on the influence of those compounds on arginine vasotocin/isotocin system (AVT/IT) in fish. EDCs are leading to the degradation of fish habitats, reducing their spawning potential and possibly their population parameters (e.g. growth, maturation), by preventing fish from breeding and rebuilding their populations. Therefore, searching for new welfare indicators such as AVT and IT and developing research procedures mimicking environmental conditions using a versatile fish model is extremely important. Fish species such as Zebrafish (Daniorerio) and round goby (Neogobiusmelanostomus) can be recommended as very suitable modelsfor studying estrogenic EDCs on the AVT/IT system and other hormones involved in the neuroendocrine regulation of physiological processes in fish.These studies would not only improve our understanding of the effects of EDCs on vertebrates but could also help safeguard the well-being of aquatic and terrestrial organisms from the harmful effects of these compounds.

Probiotic Shewanella putrefaciens (SpPdp11) as a Fish Health Modulator: A Review

Aquaculture is considered one of the largest food production sectors in the world. Probiotics have long been considered as a beneficial tool in this industry since these microorganisms improve the welfare of different fish species by modulating several physiological functions, such as metabolism, digestion, immune response, stress tolerance, and disease resistance, among others. SpPdp11, a probiotic isolated from the skin of healthy gilthead seabream, has been the center of attention in a good number of studies since its discovery. The purpose of this paper is to summarize, comment, and discuss the current knowledge related to the effects of SpPdp11 in two commercially important fish species in aquaculture (gilthead seabream and Senegalese sole). Furthermore, some considerations for future studies are also indicated.

Social context affects aggression and brain vasotocin and isotocin level in the round goby

On the wild spawning grounds, the round gobies Neogobius melanostomus are subjected to different social cues, such as sex-separation and high fish density. We designed an experiment to stimulate natural social stress when fish are separated from opposite sex individuals and exposed to close proximity of same-sex conspecifics. We examined the effects of different sex compositions on aggressiveness and brain concentrations of arginine vasotocin (AVT) and isotocin (IT), as AVT and IT are known to be involved in aggressive interactions during reproduction. The round gobies were kept in three experimental groups: same-sex groups broken down into male-only and female-only groups and mixed-sex groups. In this study, males and females from same-sex groups showed overt aggression and competition. Separation stress stimulated aggressive responses in both sexes, but the link between brain AVT and IT concentration and aggressive behavior was evident only in male-only group. In the male-only group, AVT and IT levels were the highest. This study shows that sex composition of the social environment can affect aggressive behavior as well as AVT and IT concentration in the whole brain of the round goby.

Intermediary metabolic response and gene transcription modulation on the Sub-Antarctic notothenioid Eleginops maclovinus (Valenciennes, 1930) injected with two strains of Piscirickettsia salmonis

Pathogen interactions with cultured fish populations are well studied, but their effects on native fishes have not been characterized. In Chile, the disease caused by bacterial species Piscirickettsia salmonis represents one of the main issues and is considered to be one of the important pathogens in the field of aquaculture. They have been found to infect native fish. Therefore, it is necessary to understand the impact of P. salmonis on native species of local commercial value, as well as the potential impact associated with the emergence of antibiotic-resistant strains of P. salmonis. Due to this purpose, the native fish Eleginops maclovinus was used in our study. Fish were randomly distributed in tanks and intraperitoneally inoculated with two strains of P. salmonis. No mortality was recorded during the experiment. Cortisol, glucose and total α-amino acid levels increased in fish injected with AUSTRAL-005 strain compared to sham-injected and LF-89-inoculated fish. Moreover, results showed an increase in the activity of carbohydrates and lipids metabolism in liver; and an increase in the carbohydrates, lipids and total α-amino acid metabolism in muscle after injection with AUSTRAL-005. Our results suggest that P. salmonis modulates the physiology of E. maclovinus and the physiological impact increase in the presence of the antibiotic-resistant strain AUSTRAL-005.

Linking stocking densities and feeding strategies with social and individual stress responses on gilthead seabream (Sparus aurata)

Intensive aquaculture and poor management practices can cause stress and compromise welfare of farmed fish. This study aimed to assess the potential links between stocking densities and feeding methods with social and individual stress responses on juvenile seabream (Sparus aurata) through risk-taking and hypoxia tests. Seabream was first experimentally reared under two different densities: high (HD: 11-65 kg m^-3) and low (LD: 3-15 kg m^-3). After 120 days under these conditions, increment in fish weight was not affected by different stocking densities. HD seemed to induce a stronger schooling behavior on seabream juveniles seeking for the group safety during the risk test; while LD increased the mean number of movements per fish recorded and the time of first response. Additionally, HD conditions delayed the time of first response of proactive fish during hypoxia tests. Glucose levels were higher in reactive fish compared to proactive ones, being highly significant in fish reared at HD. In parallel, juvenile seabream was also experimentally reared for 106 days under two different feeding strategies: hand-feeding (HF) and self-demanding feeding (DF), which influenced fish growth and foraging behavior at group and individual level. HF method induced a positive effect on fish weight compared to DF systems. Time of first response during both hypoxia and risk-taking tests was shorter in HF fish than DF fish, and the mean number of movements per fish during risk-taking behavior tests was lower for DF fish compared to HF fish. No differences were found in glucose and cortisol concentrations between behavioral traits (proactive/reactive) and feeding strategies. Triggering actions of seabream in DF systems were also assessed, which seemed to be highly dependent on particular individuals and not related to proactive individuals. DF systems however reinforce the social hierarchy within the fish group, which might lead to a higher competitiveness for resources among fishes, increasing the social hierarchy, and therefore, the stress. The findings of this study provide valuable information to the industry for the management of fish stress and welfare under production conditions at social and individual level.

Behavioral and physiological responses to stocking density in sea bream (Sparus aurata): Do coping styles matter?

Stocking density is considered a stress factor for fish and is therefore one of the numerous concerns about fish welfare in an aquaculture context. Stress coping styles (SCS) are defined as a coherent set of individual physiological and behavioral differences in stress responses that are consistent across time and context and appear to be promising for improving fish welfare in aquaculture. The aim of the present study was to describe the physiological and zootechnical performances of gilthead sea bream (Sparus aurata) at different stocking densities (low density, LD: 15 kg/m³ and high density, HD: 30 kg/m³), depending on individual SCS. To do so, the fish SCS were first screened by measuring boldness (prior to the experiment). Three consecutive samplings were performed over the experiment to measure several blood parameters, including hematocrit (Hct), red blood cell count (RBCC), hemoglobin (Hb), cortisol, adrenalin, noradrenalin, glucose, lactate, and lysozyme, to infer the consequence of the SCS profile on the welfare condition in response to stocking density. Finally, swimming activity was recorded in a subsample of individuals (9 BOLD and 9 SHY individuals per density), and BOLD individuals displayed higher swimming activity than SHY ones at HD, while the opposite pattern was observed at LD. According to principal component analysis, physiological parameters are linked to the SCS profile, mostly at the beginning of the experiment, while density effects on physiology remain during the entire experiment duration. In conclusion, regarding all the variables observed, fish SCS appeared to be promising criteria to select the most adaptive individuals relating to rearing conditions and therefore improve welfare.

Evaluation of the effects of different stocking densities on growth and stress responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus in recirculating aquaculture systems

This study was aimed at evaluating the physiological and metabolic responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus to stocking density. Hybrid grouper juveniles (mean ± SE = 25.43 ± 2.36 g live mass) were stocked for 22 weeks in a recirculating aquaculture system (RAS) under four different densities: low stocking density (LD; 1.03 kg m^-3 ), medium stocking density (MD; 2.06 kg m^-3 ), high stocking density (HD; 3.09 kg m^-3 ) and extra-high stocking density (EHD; 4.11 kg m^-3 ). Biometric variables were recorded and plasma, liver, intestine and stomach samples were taken for biochemical analysis at the end of the experimental period. Final stocking densities were 6.27, 16.04, 23.77 and 28.32 kg m^-3 , respectively, with significant differences in growth performance. Our results showed that the best growth rates and feed utilisation occurred in the MD group. Higher plasma cortisol and glucose levels and lower triglyceride levels reflected the stress responses in the EHD group. Moreover, the activity of aspartate and alanine transaminases was elevated in the HD and EHD groups due to enhanced gluconeogenesis. The activity of the digestive enzyme pepsin significantly increased in the MD group. We found that 2.06-3.09 kg m^-3 is the most suitable starting density for culturing juvenile hybrid grouper in recirculating aquaculture systems.

Arginine Vasotocin and Cortisol Co-regulate Vasotocinergic, Isotocinergic, Stress, and Thyroid Pathways in the Gilthead Sea Bream (Sparus aurata)

In teleosts, a complex interaction between several endocrine axes modulates physiological functions related to metabolism, stress, and osmoregulation. Although many studies in fish underline the interconnection between the hypothalamic–pituitary–interrenal (HPI) and hypothalamic–pituitary–thyroid (HPT) endocrine axes, their relationship with the vasotocinergic and isotocinergic systems remains unknown. The aim of the present study is therefore to shed light on the potential cross-regulations between HPT, HPI, and the vasotocinergic and isotocinergic axes in gilthead sea bream (Sparus aurata) at hypothalamic, hypophyseal, and plasma levels. Sea breams were administered with intraperitoneal slow-release implants containing different doses of vasotocin (the active peptide in vasotocinergic system) or cortisol (the last component of HPI axis). Plasma osmolality was higher in active neuropeptides vasotocin (Avt)-treated fish, indicating an osmoregulatory function of this hormone. Low concentrations of Avt increased hypothalamic arginine vasotocin precursor (avt) mRNA levels and increased Avt storage in the pituitary. Avt treatment down-regulated hypothalamic arginine vasotocin receptor v1a-type (avtrv1a), suggesting a negative paracrine co-regulation of the HPI axis due to the close location of avtrv1a and adrenocorticotropin hormone (Acth) cells in the anterior pituitary. Furthermore, the up-regulation observed in arginine vasotocin receptor v2-type (avtrv2) suggests their involvement in metabolic and cortisol-related pathways in the hypothalamus. The decrease in isotocin (It) pituitary storage and the up-regulation of it receptor, observed in the Avt-treated group, reinforce the idea of an interconnection between the vasotocinergic and isotocinergic systems. Cortisol and Avt administration each inhibited the HPI axis, down-regulating crh gene expression in the absence of variations in corticotropin releasing hormone binding protein (crhbp). Finally, both hormonal treatments activated the HPT axis via up-regulation of trh and down-regulation of thrb. Our results provide evidence for strong interactions among the Avt/It, HPI, and HPT axes of marine teleosts, particularly at the hypothalamic level.

Physio-metabolic response of rainbow trout during prolonged food deprivation before slaughter

Fish normally undergo periods of food deprivation that are longer than non-hibernating mammals. In aquacultured rainbow trout (Oncorhynchus mykiss), it is unclear how fasting may affect their physiological adaptative response, especially when they are normally fed daily. In addition, that response may vary with temperature, making it necessary to express fasting duration in terms of degree days. In the current study, trout were fasted for 5, 10, and 20 days (55, 107, and 200 degree days (°C d), respectively). To assess the physiological response of fish to fasting, different biometric, blood, plasma, and metabolic parameters were measured, as well as liver fatty acid composition. The fish weight, condition factor, and the hepato-somatic index of 5-day fasted trout were not significantly different from those of control fish. Gastric pH increased as fasting progressed while plasma concentrations of glucose, triglycerides, and total proteins decreased significantly after 10 days of fasting, while the percentage of non-esterified fatty acids increased. There were no significant differences in plasma ions (sodium, potassium, and calcium), except for chloride ion which decreased after 5 days of fasting. Liver glycogen decreased after 5 days of fasting while glycogen concentration in muscle did not decrease until 20 days of fasting. Liver color presented a higher chroma after 5 days of fasting, suggesting a mobilization of reserves. Finally, acetylcholinesterase activity in the brain was not affected by food deprivation but increased after 10 days of fasting in liver and muscle, suggesting the mobilization of body reserves, but without severely affecting basal metabolism.

What can we learn from glucocorticoid administration in fish? Effects of cortisol and dexamethasone on intermediary metabolism of gilthead seabream (Sparus aurata L.)

In aquaculture facilities fish welfare could be compromised due to stressors. Fish deal with stress, inter alia, through the activation of the hypothalamic-pituitary-interrenal endocrine axis and, as a result, corticosteroids are released into the blood. Recent studies have described that corticosteroids actions depend on the specific affinities to their receptors, and the subsequent differentiated responses. Cortisol is the main corticosteroid hormone in teleost fish, being its actions dependent on the intensity and time of exposure to stressors. Short-term effects of corticosteroids are well described, but long-term effects, including changes in the energy management directly affecting growth and survival, are less understood in fish. Here we show the effects of chronic oral administration of cortisol and the synthetic glucocorticoid dexamethasone (DXM) on the intermediary metabolism of the gilthead seabream (Sparus aurata). We described a higher energy expenditure associated to both corticosteroids resulting in lower growth rates of fish. Moreover, the effects of these compounds were tissue-dependant, with differences between both hormones. Thus, cortisol-fed animals accumulated triglycerides in the liver, while DXM treatment led to glycogen storage. Cortisol and DXM stimulated amino acids catabolism and gluconeogenic pathways in muscle and gills, but the effects were significantly enhanced in DXM-fed fish. The described effects highlighted differentiated mechanisms of action associated to both corticosteroids under chronic stress conditions. Further studies should aim at describing those pathways in detail, with special attention to the functionality of glucocorticoid receptor isoforms. The effects described here for S. aurata juveniles, may serve as a basis to assess long-term stress in future comparative studies with other aquaculture species.

Effects of long-term feed deprivation on body weight loss, muscle composition, plasma metabolites, and intermediate metabolism of meagre (Argyrosomus regius) under different water temperatures

The effect of feed deprivation at four water temperatures (17, 20, 23, 26 °C) was investigated in meagre (Argyrosomus regius) of initial mean weight ± SD, 116.16 ± 4.74 g, in triplicate groups. Fish were deprived of feed for a period of 60 days and sampled on days 0, 14, 41, and 60, during which body weight, specific growth rate, somatic indices, muscle proximate composition, plasma metabolite levels (total lipids, proteins, cholesterol, triglycerides, glucose), and liver and muscle enzymatic activities [L-lactate dehydrogenase (L-LDH), citrate synthase (CS), malate dehydrogenase (MDH)] were evaluated. Long-term feed deprivation resulted in a significant decrease in body weight, condition factor (CF), hepatosomatic index (HSI), muscle lipids, and plasma metabolites (all except proteins) and increase in muscle moisture. Plasma glucose concentration decreased with time and became significantly lower at 41 and 60 days. Glucose concentration and weight loss expressed a different response in the short term (14 days) than in the long term (14 and 60 days) of feed deprivation, suggesting a change in glucose metabolic profile. After 60 days of feed deprivation, there was an increase in the L-LDH activity in the liver at all temperature levels, which reflects a rising glycolytic potential by activating the carbohydrate metabolism and an ATP-dependent demand. MDH activity increased in the liver and muscle, except at 17 °C in the muscle, which indicates aerobic glycolysis and lipolysis. CS activity in the liver increased after the 60 days, whereas that in the muscle decreased, indicating the muscle is less dependent on aerobic oxidation for energy reserves.

Impact of Air Exposure on Vasotocinergic and Isotocinergic Systems in Gilthead Sea Bream (Sparus aurata): New Insights on Fish Stress Response

The hypothalamus-pituitary-interrenal (HPI) and hypothalamus-sympathetic-chromaffin cell (HSC) axes are involved in the regulation of the stress response in teleost. In this regard, the activation of a complex network of endocrine players is needed, including corticotrophin-releasing hormone (Crh), Crh binding protein (Crhbp), proopiomelanocortin (Pomc), thyrotropin-releasing hormone (Trh), arginine vasotocin (Avt), and isotocin (It) to finally produce pleiotropic functions. We aimed to investigate, using the gilthead sea bream (Sparus aurata) as a biological model, the transcriptomic response of different endocrine factors (crh, crhbp, pomcs, trh), neuropeptides (avt and it), and their specific receptors (avtrv1a, avtrv2, and itr) in four important target tissues (hypothalamus, pituitary, kidney and liver), after an acute stress situation. We also investigated several stress hormones (catecholamines and cortisol). The stress condition was induced by air exposure for 3 min, and hormonal, metabolic and transcriptomic parameters were analyzed in a time course response (15 and 30 min, and 1, 2, 4, and 8 h post-stress) in a total of 64 fish (n = 8 fish per experimental group; p = 0.05; statistical power = 95%). Our results showed that plasma noradrenaline, adrenaline and cortisol values increased few minutes after stress exposure. At hypothalamic and hypophyseal levels, acute stress affected mRNA expression of all measured precursors and hormonal factors, as well as their receptors (avtrs and itr), showing the activation, at central level, of HPI, HSC, and Avt/It axes in the acute stress response. In addition, stress response also affected mRNA levels of avtrs and itr in the head kidney, as well as the steroidogenic acute regulatory protein (star) and tyrosine hydroxylase (th) expression, suggesting their participation in the HPI and HSC axes activation. Moreover, the pattern of changes in hepatic avtrs and itr gene expression also highlights an important role of vasotocinergic and isotocinergic pathways in liver metabolic organization after acute stress events. Our results demonstrate, both at transcriptional and circulating levels of several hormones, the existence of a complex activation of different endocrine pathways in S. aurata related to the stress pathways, where vasotocinergic and isotocinergic systems can also be considered key players of the acute stress response orchestration.

Cortisol and melatonin in the cutaneous stress response system of fish

The stress hormone cortisol, together with antioxidants, melatonin (Mel) and its biologically active metabolites, 5-methoxykynuramines, including AFMK, set up a local stress response system in mammalian skin. Our in vitro study of the European flounder (Platichthys flesus) was designed to examine whether Mel and AFMK would respond to cortisol while a glucocorticoid is added to the incubation medium. The concentrations of cortisol in the incubation medium mimic plasma cortisol levels seen in fish exposed to different types of stresses such as handling, confinement, high density, food-deprivation or air-exposure. We measured Mel and AFMK in skin explants and culture media using high-performance liquid chromatography (HPLC) with fluorescence detection. We also analysed melanosome response (dispersion/aggregation) in the explants subjected to the different treatments. Cortisol stimulated the release of Mel and AFMK from skin explants in a dose-dependent manner. Melanosome dispersion and a darkening of the skin explants were observed after incubation with cortisol. This study is the first to demonstrate the interrelationship between cortisol and Mel/AFMK in fish skin. Our data strongly suggest that the cutaneous stress response system (CSRS) is present in fish. The question remains whether Mel, AFMK or cortisol are synthetized locally in fish skin and/or transported by the bloodstream. The presence of the CSRS should be taken into account during elaboration of new indicators of fish welfare both in aquaculture and in the wild.

Osmoregulatory role of vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata L)

Gilthead sea bream, Sparus aurata L., is an important fish species for the Mediterranean aquaculture and is considered a good model for studying the osmoregulatory process, due to its capacity to cope with great changes in environmental salinity (5-60‰). Our group studied the osmoregulatory role of different endocrine systems in this species, focusing on the vasotocinergic and isotocinergic systems over several years. For this purpose, the cDNAs coding for pro-vasotocin (pro-vt), pro-isotocin (pro-it), two arginine vasotocin (AVT) receptors (avtr v1a2- and v2-types) and one IT receptor (itr) were cloned. Acclimation to different environmental salinities induced a direct lineal relationship between plasma AVT levels and salinity, with no changes in plasma IT values. In addition, higher values in vasotocinergic, isotocinergic and stress pathways (pro-vt and pro-it gene expression, AVT and IT storage and plasma cortisol levels) in both hypo- and/or hyper-osmotic transfers, suggest an interaction between cortisol and AVT/IT pathways. Moreover, gene expression of specific receptors, as well as the use of different in vitro techniques, demonstrated an important osmoregulatory orchestration in different organs. In addition, individuals intraperitoneally injected with AVT and transferred to different environmental salinities enhanced plasma cortisol levels and/or gill Na⁺, K⁺-ATPase activity. These effects could be related to the energy repartitioning process occurring during osmotic adaptation of S. aurata to extreme environmental salinities, which could be mediated not only by plasma cortisol but also by AVT. Finally, our results indicated a very important role of the vasotocinergic and/or isotocinergic systems in both osmoregulatory and non-osmoregulatory organs.

The nonapeptide isotocin in goldfish: Evidence for serotonergic regulation and functional roles in the control of food intake and pituitary hormone release

Nonapeptides are a highly conserved family of peptides synthesized in the neuroendocrine brain and acting on central and peripheral receptors to regulate physiological functions in vertebrates. While the evolution of the two gene families of oxytocin-like and vasopressin-like nonapeptides and their receptors, as well as the neuroanatomy of their independent neuronal circuits have been well-characterized across vertebrate species, comparative studies on the physiological roles across vertebrates are lagging behind. In the current study, we focused on the comparative neuroendocrine functions and regulation of isotocin, the teleost homologue of mammalian oxytocin. Specifically, we address the hypothesis that isotocin exerts opposing effects on food intake and reproduction, which are well-established effects of its homologue oxytocin in mammalian species. Using goldfish, a well-characterized model of neuroendocrine regulation of both food intake and reproduction, we here showed that isotocin acts as an anorexigenic factor while exerting stimulatory effects on pituitary luteinizing hormone and growth hormone release. Given the dual inhibitory and stimulatory roles of serotonin on food intake and pituitary release of reproductive hormone in goldfish, we also investigated the potential crosstalk between both systems using immunohistochemistry and pharmacological approaches. Results provide neuroanatomical and pharmacological evidence for serotonergic regulation of magnocellular isotocinergic neurons in the preoptic area and pituitary. Together, these findings firstly provide the basis to investigate neuroendocrine cross-talk between serotonergic and nonapeptidergic systems in the regulation of both food intake and reproduction in goldfish, and secondly point to a conserved function of oxytocin-like peptides in the differential neuroendocrine control of both physiological processes in vertebrates.

Unraveling vasotocinergic, isotocinergic and stress pathways after food deprivation and high stocking density in the gilthead sea bream

The influence of chronic stress, induced by food deprivation (FD) and/or high stocking density (HSD), was assessed on stress, vasotocinergic and isotocinergic pathways of the gilthead sea bream (Sparus aurata). Fish were randomly assigned to one of the following treatments: (1) fed at low stocking density (LSD-F; 5kg·m^-3); (2) fed at high stocking density (HSD-F, 40kg·m^-3); (3) food-deprived at LSD (LSD-FD); and (4) food-deprived at HSD (HSD-FD). After 21days, samples from plasma, liver, hypothalamus, pituitary and head-kidney were collected. Both stressors (FD and HSD) induced a chronic stress situation, as indicated by the elevated cortisol levels, the enhancement in corticotrophin releasing hormone (crh) expression and the down-regulation in corticotrophin releasing hormone binding protein (crhbp) expression. Changes in plasma and liver metabolites confirmed a metabolic adjustment to cope with energy demand imposed by stressors. Changes in avt and it gene expression, as well as in their specific receptors (avtrv1a, avtrv2 and itr) at central (hypothalamus and pituitary) and peripheral (liver and head-kidney) levels, showed that vasotocinergic and isotocinergic pathways are involved in physiological changes induced by FD or HSD, suggesting that different stressors are handled through different stress pathways in S. aurata.

Neurohypophysial Hormones Regulate Amphibious Behaviour in the Mudskipper Goby

The neurohypophysial hormones, arginine vasotocin and isotocin, regulate both hydromineral balance and social behaviors in fish. In the amphibious mudskipper, Periophthalmus modestus, we previously found arginine-vasotocin-specific regulation of aggressive behavior, including migration of the submissive subordinate into water. This migration also implies the need for adaptation to dehydration. Here, we examined the effects of arginine vasotocin and isotocin administration on the amphibious behavior of individual mudskippers in vivo. The mudskippers remained in the water for an increased period of time after 1-8 h of intracerebroventricular (ICV) injection with 500 pg/g arginine vasotocin or isotocin. The 'frequency of migration' was decreased after ICV injection of arginine vasotocin or isotocin, reflecting a tendency to remain in the water. ICV injections of isotocin receptor antagonist with arginine vasotocin or isotocin inhibited all of these hormonal effects. In animals kept out of water, mRNA expression of brain arginine vasotocin and isotocin precursors increased 3- and 1.5-fold, respectively. Given the relatively wide distribution of arginine vasotocin fibres throughout the mudskipper brain, induction of arginine vasotocin and isotocin under terrestrial conditions may be involved also in the preference for an aquatic habitat as ligands for brain isotocin receptors.

Cortisol stimulates arginine vasotocin and isotocin release from the hypothalamo-pituitary complex of round goby (Neogobius melanostomus): Probable mechanisms of action

There were two aims of this in vitro perfusion study. Firstly, to determine which class of receptors, glucocorticoid (GRs) or mineralocorticoid (MRs), are involved in cortisol regulation of arginine vasotocin (AVT) and isotocin (IT) release from the hypothalamo-pituitary (H-P) complex of round goby (Neogobius melanostomus). Secondly, to determine which pathways, genomic or non-genomic, are involved in the aformentioned process.The H-P explants were perfused with cortisol (1.4 × 10(-) (7)  M, 2.8 × 10(-7) M, 0.4 × 10(-6) M); only the highest dose significantly increased a release of both nonapeptides. In the perfusion of H-P explants, we used cortisol (0.4 × 10(-6) M) in combination with GRs antagonist RU486 (0.3 × 10(-6)  M) or MRs antagonist C03DA01 (0.36 × 10(-6)  M) or transcription inhibitor Actinomycin D (1 × 10(-7)  M). All inhibitors were also tested seperately. The contents of AVT and IT in the perfusion media was determined by high-performance liquid chromatography (HPLC) with UV detection. This study suggested that different mechanisms were involved in the regulation of AVT and IT release from H-P complex in round goby. Apparently it was GRs but not MRs that were involved in cortisol regulation of AVT and IT release. In the case of AVT, our data points to both genomic and non-genomic pathways mediating the effect of cortisol; in the case of IT, it is only the non-genomic pathway. This study presents the first feasible mechanisms of cortisol action on AVT and IT release from the H-P complex in round goby.

Vasotocin and isotocin regulate aquaporin 1 function in the sea bream

Aquaporins (AQPs) are specific transmembrane water channels with an important function in water homeostasis. In terrestrial vertebrates, AQP2 function is regulated by vasopressin (AVP) to accomplish key functions in osmoregulation. The endocrine control of aquaporin function in teleosts remains little studied. Therefore, in this study we investigated the regulatory role of vasotocin (AVTR) and isotocin (ITR) receptors in Aqp1 paralog gene function in the teleost gilthead sea bream (Sparus aurata). The complete coding regions of Aqp1a, Aqp1b, AVTR V1a2-type, AVTR V2-type and ITR from sea bream were isolated. A Xenopus oocyte-swelling assay was used to functionally characterize AQP1 function and regulation by AVT and IT through their cognate receptors. Microinjection of oocytes with Aqp1b mRNA revealed regulation of water transport via PKA (IBMX+forskolin sensitive), whereas Aqp1a mRNA injection had the same effect via PKC signaling (PDBU sensitive). In the absence of expressed receptors, AVT and IT (10−8 mol l−1) were unable to modify AQP1 function. AVT regulated AQP1a and AQP1b function only when the AVTR V2-type was co-expressed. IT regulated AQP1a function, but not AQP1b, only when ITR was present. Considering that Aqp1a and Aqp1b gene expression in the sea bream intestine is highly salinity dependent in vivo, our results in ovo demonstrate a regulatory role for AVT and IT in AQP1 function in the sea bream in the processing of intestinal fluid to achieve osmoregulation.

Cortisol modulates vasotocinergic and isotocinergic pathways in the gilthead sea bream

In the present study, we assessed the responses of the vasotocinergic and isotocinergic systems to chronic stress induced by cortisol administration in the gilthead sea bream (Sparus aurata). Pituitary and plasma arginine vasotocin (AVT) and isotocin (IT) levels, as well as hypothalamic pro-vasotocin (pro-VT) and pro-isotocin (pro-IT) mRNA expression levels, were analysed. In addition, the mRNA levels of three receptors, AVTR type V1a2, AVTR type V2 and ITR, were analysed in several target organs associated with the following physiological processes: (i) integration and control (hypothalamus), (ii) metabolism and its control (liver and hypothalamus), (iii) osmoregulation (gills) and (iv) stress response (head kidney). Specimens were injected intraperitoneally with slow-release implants (5 μL g(-1) body mass) containing coconut oil alone (control group) or with cortisol (50 μg g(-1) body mass; cortisol group). Both AVT and IT synthesis and release were correlated with plasma cortisol values, suggesting a potential interaction between both hormonal systems and cortisol administration. Our results suggest that the activation of hepatic metabolism as well as the hypothalamic control of metabolic processes provide the energy necessary to overcome stress, which could be partly mediated by AVTRs and ITR. Upregulation of branchial AVT and IT receptor expression following cortisol treatment suggests an involvement of the vasotocinergic and isotocinergic systems in the regulation of ion channels/transporters during stressful situations. Finally, changes in AVT and IT receptor mRNA expression in the head kidney suggest these nonapeptides participate in feedback mechanisms that regulate the synthesis/release of cortisol. Our results indicate a relationship between cortisol and both the vasotocinergic and isotocinergic systems during simulated chronic stress in S. aurata.

Stocking density and Piscirickettsia salmonis infection effect on Patagonian blennie (Eleginops maclovinus, Cuvier 1830) skeletal muscle intermediate metabolism

The need to expand aquaculture production has led to other fish to be considered as potential species for culture, such as the sub-Antarctic notothenioid Eleginops maclovinus (Valenciennes, 1830). The aim of this study was to determine the cumulative effect of density and pathogen infection by protein extract of Piscirickettsia salmonis on skeletal muscle metabolism. In a first experiment, specimens were submitted to three different stocking densities: (1) 3.1 kg m(-3), (2) 15 kg m(-3) and (3) 60 kg m(-3), for a period of 10 days. In a second experiment, metabolic changes caused by an infection of P. salmonis protein extract (a single injection of 0.5 μL P. salmonis protein extract g body weight(-1) was inoculated in the fish) and its combined effect with stocking density was assessed during a period of 10 days. This study concludes that stress caused by high stocking density led to the reorganization of some metabolic routes to fulfill skeletal muscle energy needs. Furthermore, infection response by pathogen P. salmonis differed when stocking density increased, suggesting an increase of energy needs with density in skeletal muscle of infected fish.

Combined effects of high stocking density and Piscirickettsia salmonis treatment on the immune system, metabolism and osmoregulatory responses of the Sub-Antarctic Notothenioid fish Eleginops maclovinus

The aim of this study was to evaluate immunological, metabolic and osmoregulatory secondary stress responses in Eleginops maclovinus specimens submitted to three different stocking densities: i) low (3.1 kg m(-3)), medium (15 kg m(-3)) and high (60 kg m(-3)) during 10 days, alone or in combination with a previous treatment of a protein extract of the pathogen Piscirickettsia salmonis (0.5 μg g weight body(-1)). Plasma, liver, gill and kidney samples were obtained at the end of both experiments. Plasma cortisol and amino acid levels increased, while plasma glucose, triglyceride and lactate levels decreased at higher stocking densities. However, no effects were observed on serum Immunoglobulin type M (IgM anti P. salmonis level) values. Gill Na(+), K(+)-ATPase activity enhanced under these experimental conditions, suggesting an osmotic imbalance. Energy metabolism changes, assessed by metabolite concentrations and enzyme activities, indicated a reallocation of energetic substrates at higher stocking densities. Specimens inoculated with a protein extract of P. salmonis and maintained at different stocking densities showed primary stress response, as all groups enhanced plasma cortisol concentrations. Serum IgM levels increased after treatment with P. salmonis extract but a negative influence of high stocking density on IgM production was observed when immune system was activated. Furthermore, treatment with P. salmonis protein extract evoked deep changes in the metabolite stores in all tissues tested, indicating a mobilization of energy substrates in response to infection. The results show that stocking density induced immunological, metabolic and osmoregulatory secondary stress responses in E. maclovinus specimens and that previous treatment with P. salmonis compromise these changes.

Urotensin II inhibits arginine vasotocin and stimulates isotocin release from nerve endings in the pituitary of gilthead sea bream (Sparus aurata)

The aim of this in vitro study was to determine whether arginine vasotocin (AVT) and isotocin (IT) release from nerve endings is affected by urotensin II (UII) in gilthead sea bream pituitary. Primary cultures of pituitary cells were exposed to 10(-12), 10(-10), and 10(-8) M UII for 6, 24, and 48 hr. AVT and IT contents were determined in the culture media by high performance liquid chromatography (HPLC). UII at all doses decreased AVT release after 6, 24, and 48 hr of incubation. IT release was increased by UII only after 24 hr of incubation. This study, for the first time, indicates that UII affects AVT and IT release from nerve endings in the pituitary of Sparus aurata. It is presumed that UII together with AVT and IT may control response to different salinities in fishes.

In vitro effect of cortisol and urotensin I on arginine vasotocin and isotocin secretion from pituitary cells of gilthead sea bream Sparus aurata

This study aimed at determining whether in vitro secretion of two neuropeptides, arginine vasotocin (AVT) and isotocin (IT), from pituitary cells of gilthead sea bream Sparus aurata was affected by cortisol and urotensin (UI). Pituitary cells were exposed to 1·4 × 10(-8) , 1·4 × 10(-7) and 0·4 × 10(-6)  M cortisol and 10(-12) , 10(-10) and 10(-8)  M UI for 6, 24 and 48 h, respectively. AVT and IT contents were determined in the culture media by high-performance liquid chromatography (HPLC). An increase in AVT secretion and a decrease in IT secretion were observed at all cortisol doses. UI increased AVT secretion after 6 h of incubation at all doses. After 24 h, however, only the highest dose of UI still displayed an effect. IT secretion was not influenced by UI. It was thus demonstrated that cortisol does influence AVT and IT secretion from S. aurata pituitary cells, while UI regulates AVT secretion, as a component of hypothalamic-pituitary-interrenal (HPI) axis in this species.

Arginine vasotocin treatment induces a stress response and exerts a potent anorexigenic effect in rainbow trout, Oncorhynchus mykiss

The peptide arginine vasotocin (AVT), homologous to mammalian arginine vasopressin, is involved in many aspects of fish physiology, such as osmoregulation, regulation of biological rhythms, reproduction, metabolism or responses to stress, and the modulation of social behaviours. Because a decrease in appetite is a general response to stress in fish and other vertebrates, we investigated the role of AVT as a possible food intake regulator in fish. We used i.c.v. injections for central administration of AVT to rainbow trout (Oncorhynchus mykiss). In a first experiment, we evaluated the temporal response of food intake after AVT treatment. In a second experiment, we investigated the effects of central AVT administration on the response of typical stress markers (plasma cortisol, glucose and lactate), as well as brain serotonergic, noradrenergic and dopaminergic activity. In addition, the mRNA levels of genes involved in food intake regulation [neuropetide Y, pro-opiomelanocortin (POMC), cocaine- and amphetamine-regulated transcript (CART) and corticotrophin-releasing factor (CRF)] and in CRF- (CRF-binding protein) and AVT-signalling (pro-VT and AVT receptor), were also assessed after AVT treatment. Our results showed that AVT is a potent anorexigenic factor in fish. Increases of plasma cortisol and glucose after AVT treatment strongly suggest that AVT administration induced a stress response and that AVT action was mediated by hypothalamic-pituitary-interrenal axis activation, which was also supported by the increase of the serotonergic activity in trout telencephalon and hypothalamus. The increased hypothalamic levels of POMC and CART suggest that these peptides might have a role in the anorexigenic action of AVT, whereas the involvement of CRF signalling is unclear.

Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol

Cortisol has been suggested to mediate the effect of stress on pineal melatonin synthesis in fish. Therefore, we aimed to determine how pineal melatonin synthesis is affected by exposing rainbow trout to different stressors, such as hypoxia, chasing and high stocking density. In addition, to test the hypothesis that cortisol is a mediator of such stress-induced effects, a set of animals were intraperitoneally implanted with coconut oil alone or containing cortisol (50 mg kg(-1) body mass) and sampled 5 or 48 h post-injection at midday and midnight. The specificity of such effect was also assessed in cultured pineal organs exposed to cortisol alone or with the general glucocorticoid receptor antagonist, mifepristone (RU486). Stress (in particular chasing and high stocking density) affected the patterns of plasma and pineal organ melatonin content during both day and night, with the greatest reduction occurring at night. The decrease in nocturnal melatonin levels in the pineal organ of stressed fish was accompanied by increased serotonin content and decreased AANAT2 enzymatic activity and mRNA abundance. Similar effects on pineal melatonin synthesis to those elicited by stress were observed in trout implanted with cortisol for either 5 or 48 h. These data indicate that stress negatively influences the synthesis of melatonin in the pineal organ, thus attenuating the day-night variations of circulating melatonin. The effect might be mediated by increased cortisol, which binds to trout pineal organ-specific glucocorticoid receptors to modulate melatonin rhythms. Our results in cultured pineal organs support this. Considering the role of melatonin in the synchronization of daily and annual rhythms, the results suggest that stress-induced alterations in melatonin synthesis could affect the availability of fish to integrate rhythmic environmental information.

Potential roles of arginine-vasotocin in the regulation of aggressive behavior in the mudskipper (Periophthalmus modestus)

The hypothalamic hormones, arginine-vasotocin (VT) and isotocin (IT), play central roles in osmoregulation and in the regulation of social behaviors including aggressive behavior in many vertebrates including fish. Here, we examined whether these hormones are associated with aggressive behavior in the mudskipper (Periophthalmus modestus). The mudskipper is an amphibious fish, which lives in the brackish water of river mouths and displays unique aggressive behavior. Upon introduction to each other in an experimental tank with aquatic and terrestrial areas, a pair of males can be classified as aggressive dominant or submissive subordinate based on the frequency of their aggressive acts, which is significantly higher in dominant male. Additionally, the length of stay in terrestrial area of dominant was longer than that of the subordinate. The latter remained in aquatic area almost throughout the period of behavioral observation. The expression of brain VT mRNA was significantly higher in subordinate than in dominant, whereas neither IT mRNA expression nor plasma cortisol level differed between subordinate and dominant male. On the other hand, an intracerebroventricular injection of VT increased aggressive behaviors in mudskippers. In addition to known roles of VT in mediation of aggressive behavior, these results may shed light on the role of endogenous VT toward water migration in submissive mudskippers. The amphibious fish is a valuable experimental model to observe the relationship between effects of central VT on the osmoregulation and social behavioral regulation in vertebrates.

AVT is involved in the regulation of ion transport in the intestine of the sea bream (Sparus aurata)

The intestine of marine fish plays a crucial role in ion homeostasis by selective processing of ingested fluid. Although arginine vasotocin (AVT) is suggested to play a role in ion regulation in fish, its action in the intestine has not been demonstrated. Thus, the present study investigated in vitro the putative role of AVT in intestinal ion transport in the sea bream (Sparus aurata). A cDNA encoding part of an AVT receptor was isolated and phylogenetic analysis revealed it clustered with the V1a2-type receptor clade. V1a2 transcripts were expressed throughout the gastrointestinal tract, from esophagus to rectum, and were most abundant in the rectum regardless of long-term exposure to external salinities of 12, 35 or 55p.p.t. Basolateral addition of AVT (10(-6)M) to the anterior intestine and rectum of sea bream adapted to 12, 35 or 55p.p.t. mounted in Ussing chambers produced rapid salinity and region dependent responses in short circuit current (Isc), always in the absorptive direction. In addition, AVT stimulation of absorptive Isc conformed to a dose-response curve, with significant effects achieved at 10(-8)M, which corresponds to physiological values of plasma AVT for this species. The effect of AVT on intestinal Isc was insensitive to the CFTR selective inhibitor NPPB (200μM) applied apically, but was completely abolished in the presence of apical bumetanide (200μM). We propose a role for AVT in the regulation of ion absorption in the intestine of the sea bream mediated by an absorptive bumetanide-sensitive mechanism, likely NKCC2.

Vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata): an osmoregulatory story

To investigate the physiological roles of arginine vasotocin (AVT) and isotocin (IT) in osmoregulatory process in gilthead sea bream (Sparus aurata), a time course study (0, 12h, and 1, 3, 7 and 14 days) has been performed in specimens submitted to hypoosmotic (from 40‰ salinity to 5‰ salinity) or hyperosmotic (from 40‰ salinity to 55‰ salinity) challenges. Plasma and liver osmoregulatory and metabolic parameters, as well as AVT and IT pituitary contents were determined concomitantly with hypothalamic pro-vasotocin (pro-VT) and pro-isotocin (pro-IT) mRNA expression levels. Previously, sequences coding for pro-VT and pro-IT cDNAs were cloned. Two osmoregulatory periods related to plasma osmolality and metabolic parameter variations could be distinguished: i) an adaptative period, from 12h to 3 days after transfer, and ii) a chronic regulatory period, starting at day 3 after transfer. Higher values in hypothalamic pro-VT and pro-IT mRNA expression as well as in pituitary AVT and IT storage levels in both hypo- and/or hyper-osmotic transfers have been distinguished. These increase correlated with changes in plasma cortisol levels, suggesting an interaction between this hormone and pro-VT expression. Furthermore, pro-IT expression enhancement also suggests a role of the isotocinergic system as a modulator in the acute stress response induced by hyper-osmotic challenge in S. aurata.

Stocking density influences brain arginine vasotocin (AVT) and isotocin (IT) levels in males and females of three-spined stickleback (Gasterosteus aculeatus)

Arginine vasotocin (AVT) and isotocin (IT) are fish neurohormones produced in separate parvocellular and magnocellular preoptic neurons of Teleostei. Apart from well-established peripheral action as hormones they are important neurotransmitters in central nervous system in fish. In the present study, we examined an influence of stocking density on whole brain AVT and IT concentrations in males and females of three-spined stickleback (Gasterosteus aculeatus). In males, the highest AVT levels have been found at stocking densities of 10 and 30 individuals per 30-l tank. On the other hand, in females, AVT concentration was significantly higher in those kept alone. Brain IT concentrations significantly increased along with stocking density only in females and did not change in males. The sex-dependent responses indicate a different stimulation of AVT and IT neurons in males and females. Consequently, roles of the neurohormones in males and females exposed to stress of overcrowding must be different.

Melatonin treatment alters glucosensing capacity and mRNA expression levels of peptides related to food intake control in rainbow trout hypothalamus

As demonstrated in previous studies, the functioning of brain glucosensing systems in rainbow trout is altered under stress conditions in a way that they are unable to respond properly to changes in glucose levels. Melatonin has been postulated as necessary for homeostatic control of energy metabolism in several vertebrate groups, and in fish it has been suggested as an anti-stress molecule. To evaluate the possible effects of melatonin on glucosensing, we have incubated hypothalamus and hindbrains of rainbow trout at different glucose concentrations in the presence of increased doses (0.01, 1, and 100nM) of melatonin assessing whether or not the responses to changes in glucose levels of parameters related to glucosensing (glucose, glycogen and glucose 6-phosphate levels, activities of GK, GSase and PK, and mRNA content of GK, GLUT2, Kir6.x-like, and SUR-like) are modified in the presence of melatonin. While no effects of melatonin were observed in hindbrain, in hypothalamus melatonin treatment up-regulated glucosensing parameters, especially under hypo- and normo-glycaemic conditions. The effects of melatonin in hypothalamus occurred apparently through MT(1) receptors since most effects were counteracted by the presence of luzindole but not by the presence of 4-P-PDOT. Moreover, melatonin treatment induced in hypothalamus increased mRNA expression levels of NPY and decreased mRNA levels of POMC, CART, and CRF. A role of the hormone in daily re-adjustment of hypothalamic glucosensor machinery is discussed.

Behavioural indicators of welfare in farmed fish

Behaviour represents a reaction to the environment as fish perceive it and is therefore a key element of fish welfare. This review summarises the main findings on how behavioural changes have been used to assess welfare in farmed fish, using both functional and feeling-based approaches. Changes in foraging behaviour, ventilatory activity, aggression, individual and group swimming behaviour, stereotypic and abnormal behaviour have been linked with acute and chronic stressors in aquaculture and can therefore be regarded as likely indicators of poor welfare. On the contrary, measurements of exploratory behaviour, feed anticipatory activity and reward-related operant behaviour are beginning to be considered as indicators of positive emotions and welfare in fish. Despite the lack of scientific agreement about the existence of sentience in fish, the possibility that they are capable of both positive and negative emotions may contribute to the development of new strategies (e.g. environmental enrichment) to promote good welfare. Numerous studies that use behavioural indicators of welfare show that behavioural changes can be interpreted as either good or poor welfare depending on the fish species. It is therefore essential to understand the species-specific biology before drawing any conclusions in relation to welfare. In addition, different individuals within the same species may exhibit divergent coping strategies towards stressors, and what is tolerated by some individuals may be detrimental to others. Therefore, the assessment of welfare in a few individuals may not represent the average welfare of a group and vice versa. This underlines the need to develop on-farm, operational behavioural welfare indicators that can be easily used to assess not only the individual welfare but also the welfare of the whole group (e.g. spatial distribution). With the ongoing development of video technology and image processing, the on-farm surveillance of behaviour may in the near future represent a low-cost, noninvasive tool to assess the welfare of farmed fish.

Different environmental temperatures affect amino acid metabolism in the eurytherm teleost Senegalese sole (Solea senegalensis Kaup, 1858) as indicated by changes in plasma metabolites

Senegalese sole (Solea senegalensis) is a eurytherm teleost that under natural conditions can be exposed to annual water temperature fluctuations between 12 and 26°C. This study assessed the effects of temperature on sole metabolic status, in particular in what concerns plasma free amino acid changes during thermal acclimation. Senegalese sole maintained at 18°C were acclimated to either cold (12°C) or warm (26°C) environmental temperatures for 21 days. Fish maintained at 18°C served as control. Plasma concentrations of cortisol, glucose, lactate, triglycerides, proteins, and free amino acids were assessed. Cold acclimation influenced interrenal responses of sole by increasing cortisol release. Moreover, plasma glucose and lactate concentrations increased linearly with temperature, presumably reflecting a higher metabolic activity of sole acclimated to 26°C. Acclimation temperature affected more drastically plasma concentrations of dispensable than that of indispensable amino acids, and different acclimation temperatures induced different responses. Asparagine, glutamine and ornithine seem to be of particular importance for ammonia detoxification mechanisms, synthesis of triglycerides that may be used during homeoviscous adaptation and, to a lesser extent, as energetic substrates in specimens acclimated to 12°C. When sole is acclimated to 26°C taurine, glutamate, GABA and glycine increased, which may suggest important roles as antioxidant defences, in osmoregulatory processes and/or for energetic purposes at this thermal regimen. In conclusion, acclimation to different environmental temperatures induces several metabolic changes in Senegalese sole, suggesting that amino acids may be important for thermal acclimation.

Feed deprivation in Senegalese sole (Solea senegalensis Kaup, 1858) juveniles: effects on blood plasma metabolites and free amino acid levels

The effects of prolonged feed deprivation were assessed on blood plasma metabolites and free amino acid levels in Solea senegalensis. Juvenile specimens were maintained at two experimental conditions (24 h fasted and 21 days feed-deprived). In feed-deprived fish, relative growth rate and hepatosomatic index as well as plasma triglycerides and proteins levels were significantly lower. However, plasma cortisol levels were higher in feed-deprived fish, while plasma glucose and lactate values were not significantly different between treatments. Furthermore, feed-deprived fish showed higher levels of total plasma free amino acids than 24 h fasted fish. In 21 days feed-deprived sole, an increase in plasma cortisol levels may suggest a functional role in mobilizing energy due to the catabolic action of this hormone in teleosts. Higher levels of glutamine, arginine and ornithine in 21 days feed-deprived fish may be indicative of a dual role for these amino acids: ammonia detoxification and carbon source for gluconeogenesis. The increased plasma glucogenic and branched-chain amino acid levels, together with the maintenance of plasma glucose in 21 days feed-deprived sole, suggest active liver gluconeogenic processes supported by tissue proteolysis.

A novel approach to AVT and IT studies in fish brain and pituitary: in vitro perfusion technique

The study was designed to develop a new procedure for perfusion of brain and pituitary explants collected from three-spined stickleback (Gasterosteus aculeatus) and round goby (Neogobius melanostomus). The procedure was elaborated for studies of arginine vasotocin (AVT) and isotocin (IT) release from explants of both species. AVT and IT, analogs of mammalian vasopressin and oxytocin, are neurohormones produced in hypothalamus and released in neurohypophysis of Teleostei. Both nonapeptides are used as biomarkers of fish well being. Three perfusion sets were applied to test the method of medium transport into gradient container, without or with aeration. Medium supply to the gradient container from the top, without aeration is recommended only for short-term studies. Aeration of the medium with a mixture of 95% O(2) and 5% CO(2) at a pressure of 127.51 mm Hg is necessary for a long-term research. Transport of one or two media in the gradient container from the top and the bottom, simultaneously, requires aeration with a mixture of 95% O(2) and 5% CO(2) at a pressure of 315.03 mm Hg. Although the presented procedure has been elaborated for studies of AVT and IT in fish explants, after only minor modification, if any, it can serve many other purposes.

Chronic and acute stress responses in Senegalese sole (Solea senegalensis): the involvement of cortisol, CRH and CRH-BP

The hypothalamus-pituitary-interrenal (HPI) axis is pivotal in the endocrine stress response of fish. Hypothalamic corticotropin-releasing hormone (CRH) initiates the endocrine stress response and stimulates the release of adrenocorticotropic hormone (ACTH) from the pituitary pars distalis, which in turn activates cortisol production and release by the interrenal cells of the head kidney. CRH activity depends on the levels of a specific CRH binding protein (CRH-BP). We have characterized the cDNAs coding for CRH and CRH-BP in Senegalese sole (Solea senegalensis) and investigated their mRNA expression in juveniles that were submitted to a protocol that involved exposure to a chronic stressor (viz. increased cultivation densities) followed by an acute stressor (viz. transfer to increased ambient salinity). Juveniles were cultivated at three densities (1.9, 4.7 and 9.8 kg/m(2)) for 33 days, and then exposed to an osmotic challenge that involved transfer from seawater (39‰ salinity, SW) to hypersaline seawater (55‰, HSW). The highest density imposed stress as indicated by elevated cortisol levels and CRH mRNA expression compared to fish stocked at low density. Fish kept at high density differentially responded to a posterior transfer to HSW; no cortisol or CRH response was seen, but osmoregulatory and metabolic parameters were affected. No differences in CRH-BP mRNA expression levels were found at different stocking densities; transfer to HSW enhanced expression in both low and high density stocked animals, suggesting that CRH-BP acts as a modulator of the acute stress response, not so of the chronic stress response. We conclude that stocking of Senegalese sole at high density is a stressful condition that may compromise the capacity to cope with subsequent stressors.

Changes in plasma melatonin levels and pineal organ melatonin synthesis following acclimation of rainbow trout (Oncorhynchus mykiss) to different water salinities

Melatonin has been suggested to play a role in fish osmoregulation, and in salmonids has been related to the timing of adaptive mechanisms during smolting. It has been described that acclimation to different environmental salinities alters levels of circulating melatonin in a number of fish species, including rainbow trout. However, nothing is known regarding salinity effects on melatonin synthesis in the pineal organ, which is the main source of rhythmically produced and secreted melatonin in blood. In the present study we have evaluated, in rainbow trout, the effects of acclimation to different salinities on day and night plasma melatonin values and pineal organ melatonin synthesis. Groups of freshwater (FW)-adapted rainbow trout were placed in tanks with four different levels of water salinity (FW, 6, 12, 18 p.p.t.; parts per thousand) and maintained for 6 h or 5 days. Melatonin content in plasma and pineal organs, as well as the pineal content of serotonin (5-HT) and its main oxidative metabolite (5-hydroxyindole-3-acetic acid; 5-HIAA) were measured by high performance liquid chromatography. In addition, day–night changes in pineal organ arylalkylamine N-acetyltransferase (AANAT2) activity and aanat2 gene expression were studied. Plasma osmolalities were found to be higher in rainbow trout exposed to all salinity levels compared with the control FW groups. A salinity-dependent increase in melatonin content was found in both plasma and pineal organs. This effect was observed during the night, and was related to an increase in aanat2 mRNA abundance and AANAT2 enzyme activity, both of which also occurred during the day. Also, the levels of indoles (5-HT, 5-HIAA) in the pineal organ were negatively affected by increasing water salinity, which seems to be related to the higher recruitment of 5-HT as a substrate for the increased melatonin synthesis. A stimulatory effect of salinity on pineal aanat2 mRNA expression was also identified. These results indicate that increased external salinity promotes melatonin synthesis in the pineal organ of rainbow trout by enhancing synthesis of AANAT protein independently of its regulation by light. The possibility that pineal melatonin is a target for hormones involved in the response of fish to osmotic challenge is discussed, as well as the potential role of melatonin in the timing of osmoregulatory processes.

Elevated whole brain arginine vasotocin with Aroclor 1254 exposure in two Syngnathus pipefishes

The neurohypophysial hormone arginine vasotocin (AVT) and the neuroendocrine system strongly interact with the rest of the teleostean endocrine system. The aim of this study is to investigate the effects of exposure to an endocrine disruptor on whole brain AVT concentrations for the pipefishes Syngnathus floridae and S. fuscus. Following treatment over the entire brood period, AVT concentrations were significantly higher for Aroclor 1254-exposed, post-brooding males compared to controls for both species. Considering both previously documented seventeen-fold increases in AVT for brooding males with embryos in some developmental stages and changes in parental nutrient concentrations after Aroclor 1254 exposure, these data begin to address potential physiological mechanisms that may underlie paternal activities in syngnathid males.