Stress Effects on the Mechanisms Regulating Appetite in Teleost Fish

Corticosterone treatment results in fat deposition and body mass maintenance without effects on feeding behaviour or immunity in female lizards (Tropidurus catalanensis)

Throughout life, animals must maintain homeostasis while coping with challenging events. The period after reproduction can be challenging for oviparous females to maintain homeostasis since they direct most of their energy stores to vitellogenesis, possibly increasing the vulnerability to stressors. Changes in glucocorticoids' (GC) secretion promote various behavioural and physiological adjustments daily and to restore balance after facing stressors. However, when GC are elevated for extended periods, which usually occurs in response to chronic exposure to stressors, they can affect feeding behaviour and suppress the immune function. We aim to elucidate the effects of chronic corticosterone (CORT) exposure on feeding behaviour, body condition and immune function in female lizards, Tropidurus catalanensis, in the post-reproductive period. Thirty animals were divided into three groups: 1. Control (no experimental procedure performed); 2. Empty Implant (animals implanted with empty silastic tube); and 3. CORT Implant (animals implanted with silastic tube filled with CORT, with a chronic continuous release for at least a week). CORT plasma levels feeding behaviour, body condition (body index [BI] and fat index [FI]), leukocyte count, and several immune function variables (bacterial killing ability [BKA], hemagglutination titer, phytohemagglutinin [PHA] immune challenge and leukocyte count) were evaluated. After implantation, CORT treated animals maintained stable body mass through the experiment, while Control and Empty Implant groups displayed weight loss. In the CORT treated animals, there was also a positive relation between BI and FI, and higher FI when compared to groups 1 and 2. No effects of CORT were observed on feeding behaviour nor on the immune function.

Chronic social isolation affects feeding behavior of juvenile zebrafish (Danio rerio)

Many organisms exhibit social behaviors and are part of some scheme of social structure. Zebrafish are highly social, shoaling fish and therefore, social isolation may have notable impacts on their physiology and behavior. The objective of this study was to evaluate the effects of social isolation on feed intake, monoaminergic system related gene expression, and intestinal health of juvenile zebrafish fed a high-inclusion soybean meal based diet. At 20 days post-fertilization zebrafish were randomly assigned to chronic isolation (1 fish per 1.5 L tank) or social housing (6 fish per 9 L tank) with 18 tanks per treatment group (n = 18). Dividers were placed between all tanks to prevent visual cues between fish. Zebrafish were fed a commercial fishmeal based diet until 35 days post-fertilization and then fed the experimental high-inclusion soybean meal based diet until 50 days post-fertilization. At the end of the experiment (51 days post-fertilization), the mean total length, weight, and weight gain were not significantly different between treatment groups. Feed intake and feed conversion ratio were significantly higher in chronic isolation fish than in social housing fish. Expression of monoaminergic and appetite-related genes were not significantly different between groups. The chronic isolation group showed higher expression of the inflammatory gene il-1b, however, average intestinal villi width was significantly smaller and average length-to-width ratio was significantly higher in chronic isolation fish, suggesting morphological signs of inflammation were not present at the time of sampling. These results indicate that chronic isolation positively affects feed intake of juvenile zebrafish and suggest that isolation may be useful in promoting feed intake of less-palatable diets such as those based on soybean meal.

Response of CRH system in brain and gill of marine medaka to seawater acidification

Corticotropin-releasing hormone (CRH) is mainly secreted by the hypothalamus to regulate stress when environmental factors change. Gills contact with water directly and may also secrete CRH to maintain local homeostasis. Ocean acidification changes water chemical parameters and is becoming an important environmental stressor for marine fish. The response of brain and gill CRH systems to ocean acidification remains unclear. In this study, marine medaka were exposed to CO2-acidified seawater (440 ppm, 1000 ppm, and 1800 ppm CO2) for 2 h, 4 h, 24 h, and 7 d, respectively. At 2 h and 4 h, the expression of crh mRNA in gills increased with increasing CO2 concentration. Crh protein is expressed mainly in the lamellae cells. crhbp and crhr1 expression also increased significantly. However, at 2 h and 4 h, acidification caused little changes in these genes and Crh protein expression in the brain. At 7 d, Crh-positive cells were detected in the hypothalamus; moreover, Crh protein expression in the whole brain increased. It is suggested that CRH autocrine secretion in gills is responsible for local acid-base regulation rather than systemic mobilization after short-term acidification stress, which may help the rapid regulation of body damage caused by environmental stress.

A non-microcystin-producing Microcystis wesenbergii strain alters fish food intake by disturbing neuro-endocrine appetite regulation

Cyanobacterial harmful algal blooms (cHABs) are pervasive sources of stress resulting in neurotoxicity in fish. A member of the widely distributed Microcystis genus of bloom-forming cyanobacteria, Microcystis wesenbergii can be found in many freshwater lakes, including Dianchi Lake (China), where it has become one of the dominant contributors to the lake's recurrent blooms. However, unlike its more well-known counterpart M. aeruginosa, the effects of dense non-microcystin-containing M. wesenbergii blooms are seldom studied. The disturbance of appetite regulation and feeding behaviour can have downstream effects on the growth of teleost fish, posing a significant challenge to aquaculture and conservation efforts. Here we examined the effects of M. wesenbergii blooms on the food intake of Acrossocheilus yunnanensis, a native cyprinid in southern China. This fish species has disappeared in Dianchi Lake, and its reintroduction might be negatively affected by the presence of this newly-dominant Microcystis species. We co-cultured juvenile A. yunnanensis with a non-microcystin-producing strain of M. wesenbergii at initial densities between 5 × 104 and 1 × 106 cells/mL and monitored fish feeding behaviour and changes in neurotransmitter and hormone protein levels. High-density M. wesenbergii cultures increased the feeding rate of co-cultured fish, elevating concentrations of appetite-stimulating signalling molecules (Agouti-related protein and γ-aminobutyric acid), while decreasing inhibitory ones (POMC). These changes coincided with histopathological alterations and reduced somatic indices in brain and intestinal tissues. Given this potential for detrimental effects and dysregulation of food intake, further studies are necessary to determine the impacts of chronic exposure of M. wesenbergii in wild fish.

Chronic cortisol stimulation enhances hypothalamus-specific enrichment of metabolites in the rainbow trout brain

The hypothalamus is a key integrating center that is involved in the initiation of the corticosteroid stress response, and in regulating nutrient homeostasis. Although cortisol, the principal glucocorticoid in humans and teleosts, plays a central role in feeding regulation, the mechanisms are far from clear. We tested the hypothesis that the metabolic changes to cortisol exposure signal an energy excess in the hypothalamus, leading to feeding suppression during stress in fish. Rainbow trout (Oncorhynchus mykiss) were administered a slow-release cortisol implant for 3 days, and the metabolite profiles in the plasma, hypothalamus, and the rest of the brain were assessed. Also, U-13C-glucose was injected into the hypothalamus by intracerebroventricular (ICV) route, and the metabolic fate of this energy substrate was followed in the brain regions by metabolomics. Chronic cortisol treatment reduced feed intake, and this corresponded with a downregulation of the orexigenic gene agrp, and an upregulation of the anorexigenic gene cart in the hypothalamus. The U-13C-glucose-mediated metabolite profiling indicated an enhancement of glycolytic flux and tricarboxylic acid intermediates in the rest of the brain compared with the hypothalamus. There was no effect of cortisol treatment on the phosphorylation status of AMPK or mechanistic target of rapamycin in the brain, whereas several endogenous metabolites, including leucine, citrate, and lactate were enriched in the hypothalamus, suggesting a tissue-specific metabolic shift in response to cortisol stimulation. Altogether, our results suggest that the hypothalamus-specific enrichment of leucine and the metabolic fate of this amino acid, including the generation of lipid intermediates, contribute to cortisol-mediated feeding suppression in fish.NEW & NOTEWORTHY Elevated cortisol levels during stress suppress feed intake in animals. We tested whether the feed suppression is associated with cortisol-mediated alteration in hypothalamus metabolism. The brain metabolome revealed a hypothalamus-specific metabolite profile suggesting nutrient excess. Specifically, we noted the enrichment of leucine and citrate in the hypothalamus, and the upregulation of pathways involved in leucine metabolism and fatty acid synthesis. This cortisol-mediated energy substrate repartitioning may modulate the feeding/satiety centers leading to the feeding suppression.

Melatonin as an anti-stress signal: effects on an acute stress model and direct actions on interrenal tissue in goldfish

Background

Melatonin is a key hormone in regulation of circadian rhythms, and involved in many rhythmic functions, such as feeding and locomotor activity. Melatonin reportedly counteracts stress responses in many vertebrates, including fish. However, targets for this action of melatonin and underlying mechanisms remain unknown.

Results

This study reports potential anti-stress properties of melatonin in goldfish (Carassius auratus), with a focus on its effect on plasma cortisol, food intake, and locomotor activity, all of them involved in the responses to stress exposure. Indeed, acute injection of melatonin counteracted stress-induced hypercortisolinemia and reduced food intake. The reduced locomotor activity following melatonin treatment suggests a possible sedative role in fish. To assess whether this anti-stress effects of melatonin involve direct actions on interrenal tissue, in vitro cultures of head kidney (containing the interrenal cortisol-producing tissue) were carried out in presence of ACTH, melatonin, and luzindole, an antagonist of melatonin receptors. Melatonin in vitro reduced ACTH-stimulated cortisol release, an effect attenuated by luzindole; this suggests the presence of specific melatonin receptors in interrenal tissue.

Conclusions

Our data support a role for melatonin as an anti-stress signal in goldfish, and suggest that the interrenal tissue of teleosts may be a plausible target for melatonin action decreasing cortisol production.

Feeding-Regime-Dependent Intestinal Response of Rainbow Trout after Administration of a Novel Probiotic Feed

Intensive fish farming is associated with a high level of stress, causing immunosuppression. Immunomodulators of natural origin, such as probiotics or phytoadditives, represent a promising alternative for increasing the immune function of fish. In this study, we tested the autochthonous trout probiotic strain L. plantarum R2 in a newly developed, low-cost application form ensuring the rapid revitalization of bacteria. We tested continuous and cyclic feeding regimes with regard to their effect on the intestinal immune response and microbiota of rainbow trout. We found that during the continuous application of probiotic feed, the immune system adapts to the immunomodulator and there is no substantial stimulation of the intestinal immune response. During the cyclic treatment, after a 3-week break in probiotic feeding and the reintroduction of probiotics, there was a significant stimulation of the gene expression of molecules associated with both cellular and humoral immunity (CD8, TGF-β, IL8, TLR9), without affecting the gene expression for IL1 and TNF-α. We can conclude that, in aquaculture, this probiotic feed can be used with a continuous application, which does not cause excessive immunostimulation, or with a cyclic application, which provides the opportunity to stimulate the immunity of trout, for example, in periods of stress.

Interactive Effect of Dietary Gamma-Aminobutyric Acid (GABA) and Water Temperature on Growth Performance, Blood Plasma Indices, Heat Shock Proteins and GABAergic Gene Expression in Juvenile Olive Flounder Paralichthys olivaceus

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the central nervous system of living organisms and has the ability to reduce the magnitude of stress in humans and animals. In this study, we evaluated the supplemental effects of GABA on normal and high water temperature based on growth, blood plasma composition as well as heat shock proteins and GABA-related gene expression in juvenile olive flounder. For this, a 2 × 2 factorial design of experiment was employed to investigate the dietary effects of GABA at 0 mg/kg of diet (GABA0 diet) and 200 mg/kg of diet (GABA200 diet) in water temperatures of 20 ± 1 °C (normal temperature) and 27 ± 1 °C (high temperature) for 28 days. A total of 180 fish with an average initial weight of 40.1 ± 0.4 g (mean ± SD) were distributed into 12 tanks, of which, each tank contained 15 fish based on the 4 dietary treatment groups in triplicate. At the end of the feeding trial, the results demonstrated that both temperature and GABA had significant effects on the growth performance of the fish. However, fish fed the GABA200 diet had a significantly higher final body weight, weight gain and specific growth rate as well as a significantly lower feed conversion ratio than the fish fed the GABA0 diet at the high water temperature. A significant interactive effect of water temperature and GABA was observed on the growth performance of olive flounder based on the two-way analysis of variance. The plasma GABA levels in fish were increased in a dose-dependent manner at normal or high water temperatures, whereas cortisol and glucose levels were decreased in fish fed GABA-supplemented diets under temperature stress. The GABA-related mRNA expression in the brains of the fish such as GABA type A receptor-associated protein (Gabarap), GABA type B receptor 1 (Gabbr1) and glutamate decarboxylase 1 (Gad1) were not significantly affected by GABA-supplemented diets under normal or temperature stressed conditions. On the other hand, the mRNA expression of heat shock proteins (hsp) in the livers of the fish, such as hsp70 and hsp90, were unchanged in fish fed the GABA diets compared to the control diet at the high water temperature. Collectively, the present study showed that dietary supplementation with GABA could enhance growth performance, and improve the feed utilization, plasma biochemical parameters and heat shock proteins and GABA-related gene expression under the stress of high water temperatures in juvenile olive flounder.

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.

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Tissue distribution of appetite regulation genes and their expression in the Amazon fish Colossoma macropomum exposed to climate change scenario

Climate change leads to an increase in water acidification and temperature, two environmental factors that can change fish appetite and metabolism, affecting fish population in both wild and aquaculture facilities. Therefore, our study tested if climate change affects gene expression levels of two appetite-regulating peptides - Neuropeptide Y (NPY) and Cholecystokinin (CCK) - in the brain of tambaqui, Colossoma macropomum. Additionally, we show the distribution of these genes throughout the body. Amino acid sequences of CCK and NPY of tambaqui showed high similarity with other Characiformes, with the closely related order Cypriniformes, and even with the more distantly related order Salmoniformes. High apparent levels of both peptides were expressed in all brain areas, while expression levels varied for peripheral tissues. NPY and CCK mRNA were detected in all peripheral tissues but cephalic kidney for CCK. As for the effects of climate change, we found that fish exposed to extreme climate scenario (800 ppm CO2 and 4.5 °C above current climate scenario) had higher expression levels of NPY and lower expression levels of CCK in the telencephalon. The extreme climate scenario also increased food intake, weight gain, and body length. These results suggest that the telencephalon is probably responsible for sensing the metabolic status of the organism and controlling feeding behavior through NPY, likely an orexigenic hormone, and CCK, which may act as an anorexigenic hormone. To our knowledge, this is the first study showing the effects of climate change on the endocrine regulation of appetite in an endemic and economically important fish from the Amazon. Our results can help us predict the impact of climate change on both wild and farmed fish populations, thus contributing to the elaboration of future policies regarding their conservation and sustainable use.

Too stressed to eat: Investigating factors associated with appetite loss in subordinate rainbow trout

Juvenile rainbow trout (Oncorhynchus mykiss) form dominance hierarchies in which subordinates experience chronic social stress and suppression of food intake. Here we tested the hypothesis that inhibition of food intake reflects increased expression of anorexigenic (appetite inhibiting) signals and decreased expression of orexigenic (appetite stimulating) signals. Trout were confined in pairs for 1 or 4 days, or were confined in pairs for 4 days and then allowed to recover from social interactions for 2 or 4 days; sham fish were handled identically but held alone. Subordinates did not feed during social interaction and had lower food intake than dominants or shams during recovery. In parallel, plasma cortisol (∼18-26x) and liver leptin (lep-a1) transcript abundance (∼10-14x) were elevated in subordinates during social interaction but not recovery, suggesting that these factors contributed to the suppression of food intake. Fish deemed likely to become subordinate based on inhibition of food intake in response to a mild stressor also showed elevated liver lep-a1 transcript abundance (∼5x). The moderate response in these fish coupled with a correlation between liver lep-a1 and cortisol suggest that stress-induced elevation of cortisol increased liver lep-a1 transcript abundance in subordinate trout, contributing to stress-induced suppression of food intake.

Effects of stress-associated odor on ventilation rate and feeding performance in Nile tilapia

In this study, we described immediate/acute reactions to stress-related chemical cues (SC - Stress Cue) in fish. Specifically, we evaluated the effects of SC on ventilation rate (VR) and feeding behavior in Nile tilapia (Oreochromis niloticus), a highly relevant species for world aquaculture, therefore, to understand the diversity of stressful contexts and stress responses in this species have important practical applications (stress reduction). Stress cue was obtained from conspecifics exposed to a handling stressor. Stress was confirmed by measuring plasma cortisol levels. The responses to SC were contrasted to chemical control cues: a cue originated from non-stressed conspecifics and pure water (vehicle control). We observed that Nile tilapia exposed to SC had an increase in VR, but without effects on feeding behavior (feeding latency and ingestion). Thus, the SC is a stressor that induces readily stress response (VR increase), suggesting sympathetic activation, but did not change feeding performance. In practical terms, it is positive because although social propagation of stress via SC elicits a stress response, it did not harm appetite.

Cross Effects of Diets and Rearing Temperatures on Gastrointestinal Evacuation and Growth Performance in Adult Sabah Groupers (Epinephelus fuscoguttatus × E. lanceolatus)

This study explores the gastrointestinal evacuation time (GET) and gastrointestinal evacuation rate (GER) of the popular Sabah grouper (Epinephelus fuscoguttatus × E. lanceolatus) adults using two established methods (X-radiography and serial slaughter) and square root modeling using different temperatures: 28 °C, 30 °C, 32 °C, and 34 °C and different diets: pellet (ash: 11.4 ± 0.08; moisture: 29.0 ± 0.01; protein 37.5 ± 0.80; lipid 15.0 ± 0.13) and trash fish: Sardinella sp. (ash: 2.3 ± 0.15; moisture: 78.5 ± 0.33; protein 55.4 ± 0.62; lipid 7.3 ± 0.25) and the impact on growth indices. The results indicate that the GET shortened as temperature increased from 28 °C to 30 °C; however, it was prolonged when it surged to 32 °C and 34 °C. The groupers fed with trash fish at a temperature of 30 °C had the shortest GER (0.41 ± 0.10 g hr^-1) whereas groupers fed with pellet at 34 °C had the longest GER (0.95 ± 0.02 g hr^-1). Likewise, the highest SGR (16.25 ± 2.11% day^-1) was observed at 30 °C for groupers fed with a trash fish diet. The condition (K) value was lowest at 34 °C for groupers fed with a pellet diet (1.01 ± 0.04) and highest at 30 °C for groupers fed with trash fish (1.45 ± 0.04). Our results suggest that temperature and diet influence growth indices and GE of adult Sabah groupers. Incorporation of this information will allow better management of this commercially important grouper species when reared in a controlled aquaculture environment.

Endocannabinoid receptors are involved in enhancing food intake in rainbow trout

The mechanisms involved in hedonic regulation of food intake, including endocannabinoid system (ECs) are scarcely known in fish. We recently demonstrate in rainbow trout the presence of a rewarding response mediated by ECs in hypothalamus and telencephalon when fish fed a lipid-enriched diet, and that central administration of main agonists of ECs namely AEA or 2-AG exert a bimodal effect on feed intake in fish with low doses inducing an increase that disappears with the high dose of both endocannabinoids (EC). To assess the precise involvement of the different receptors of the ECs (CNR1, TRPV1, and GPR55) in this response we injected intracerebroventricularly AEA or 2-AG in the absence/presence of specific receptor antagonists (AM251, capsazepine, and ML193; respectively). The presence of antagonists clearly counteracts the effect of EC supporting the specificity of EC action inducing changes not only in ECs but also in GABA and glutamate metabolism ultimately leading to the increase observed in food intake response.

Glucocorticoid receptor activation reduces food intake independent of hyperglycemia in zebrafish

Chronic cortisol exposure suppresses food intake in fish, but the central mechanism(s) involved in appetite regulation are unclear. Stress and the associated increase in cortisol levels increase hepatic gluconeogenesis, leading to hyperglycemia. As hyperglycemia causes a reduction in food intake, we tested the hypothesis that cortisol-induced hyperglycemia suppresses feeding in zebrafish (Danio rerio). We first established that stress-independent hyperglycemia suppressed food intake, and this corresponded with a reduction in the phosphorylation of the nutrient sensor, AMP-activated protein kinase (AMPK) in the brain. Chronic cortisol exposure also led to hyperglycemia and reduced food intake, but the mechanisms were distinct. In cortisol-exposed fish, there were no changes in brain glucose uptake or AMPK phosphorylation. Also, the phosphorylation of Akt and mTOR was reduced along with an increase in redd1, suggesting an enhanced capacity for proteolysis. Loss of the glucocorticoid receptor did not rescue cortisol-mediated feeding suppression but did increase glucose uptake and abolished the changes seen in mTOR phosphorylation and redd1 transcript abundance. Taken together, our results indicate that GR activation enhances brain proteolysis, and the associated amino acids levels, and not hyperglycemia, maybe a key mediator of the feeding suppression in response to chronic cortisol stimulation in zebrafish.

Magnetic nanoparticles-induced neurotoxicity and oxidative stress in brain of rainbow trout: Mitigation by ulexite through modulation of antioxidant, anti-inflammatory, and antiapoptotic activities

The prevalent exposition of metallic nanoparticles (MNPs) to the aquatic medium and their negative influence on human life is one of the major concerns global. Stress mechanization, as a non-specific and pervasive response, involves all physiological systems, particularly the closely interconnected neuroendocrine and immune systems. In this study, which was designed to obtain more data on the biological effects of ulexit, which prevents oxidative DNA damage by protecting against toxicity damage and offers new antioxidant roles. The concomitant use of ulexite (UX, as 18.75 mg/l) as a natural therapeutic agent against exposure to magnetic nanoparticles (Fe₃O₄-MNPs/0.013 ml/l) on Oncorhynchus mykiss was investigated for 96 h. The brain tissues were taken at the 48th and 96th hours of the trial period, the effects on neurotoxic, pro-inflammatory cytokine genes, antioxidant immune system, DNA and apoptosis mechanisms were analyzed. In the present study, it was determined that AChE activity and BDNF level in the brain tissue decreased over time in the Fe₃O₄-MNPs group compared to the control, and UX tried to depress this inhibition. While inhibition was determined in antioxidant system biomarkers (SOD, CAT, GPx, and GSH values), an induction was observed in lipid peroxidation indicators (MDA and MPO values) in Fe₃O₄-MNPs applied group. The same group data showed that TNF-α, IL-6, 8-OHdG and caspase-3 levels were increased, but Nrf-2 levels were decreased. The alterations in all biomarkers were found to be significant at the p < 0.05 level. In general, it was determined that Fe₃O₄-MNPs caused stress in O. mykiss and UX exhibited a positive effect on this stress management.

Exposure to salinity induces oxidative damage and changes in the expression of genes related to appetite regulation in Nile tilapia (Oreochromis niloticus)

Variations in water salinity and other extrinsic factors have been shown to induce changes in feeding rhythms and growth in fish. However, it is unknown whether appetite-related hormones mediate these changes in Nile tilapia (Oreochromis niloticus), an important species for aquaculture in several countries. This study aimed to evaluate the expression of genes responsible for appetite regulation and genes related to metabolic and physiological changes in tilapia exposed to different salinities. Moreover, the study proposed to sequence and to characterize the cart, cck, and pyy genes, and to quantify their expression in the brain and intestine of the fish by quantitative polymerase chain reaction (qPCR). The animals were exposed to three salinities: 0, 6, and 12 parts per thousand (ppt) of salt for 21 days. Furthermore, lipid peroxidation, reactive oxygen species, DNA damage, and membrane fluidity in blood cells were quantified by flow cytometry. The results indicated an increased expression of cart, pyy, and cck and a decreased expression of npy in the brain, and the same with cck and npy in the intestine of fish treated with 12 ppt. This modulation and other adaptive responses may have contributed to the decrease in weight gain, specific growth rate, and final weight. In addition, we showed oxidative damage in blood cells resulting from increasing salinity. These results provide essential data on O. niloticus when exposed to high salinities that have never been described before and generate knowledge necessary for developing biotechnologies that may help improve the production of economically important farmed fish.

Gene expression and latitudinal variation in the stress response in Fundulus heteroclitus

Atlantic killifish, Fundulus heteroclitus, are intertidal marsh fish found along the east coast of North America. Associated with the thermal gradient along this coast, northern and southern killifish populations are known to differ in morphology, behavior, and physiology, including in their cortisol stress response. Our goal was to explore population differences in the stress response and identify underlying molecular mechanisms. We measured responses to both acute and repeated stress in plasma cortisol, stress axis mRNA expression, and body condition in northern and southern killifish. Following an acute stressor, the southern population had higher cortisol levels than the northern population but there was no difference between populations following repeated stress. In the brain, both corticotropin releasing factor and its binding protein had higher expression in the southern than the northern population, but the northern population showed more changes in mRNA levels following a stressor. In the head kidney, Melanocortin 2 Receptor and steroidogenic acute regulatory protein mRNA levels were higher in the southern population suggesting a larger capacity for cortisol synthesis than in the northern fish. Lastly, the glucocorticoid receptor GR1 mRNA levels were greater in the liver of southern fish, suggesting a greater capacity to respond to cortisol, and GR2 had differential expression in the head kidney, suggesting an interpopulation difference in stress axis negative feedback loops. Southern, but not northern, fish were able to maintain body condition following stress, suggesting that these differences in the stress response may be important for adaptation across latitudes.

Effect of Fish Stock Density on Hormone Genes Expression from Brain and Gastrointestinal Tract of Salmo salar

Simple Summary

Various long-term stress conditions may exist in fish cultivation, damaging the physiological responses that regulate the fish growth and feed. Different signalers connect the brain with the gastrointestinal tract, including the perception of stress factors for the regulation of physiological responses. Here, we evaluated the effect of varying culture densities of Salmo salar post-smolt on the gene expression of some brain and gastrointestinal hormone signalers. We found that high stock densities could promote the levels of molecules associated with feed inhibition, which could be related to the stress pathway regulated by corticoids. Thus, the expression of these peptide hormones could be used as biological markers to improve production practices in fish aquaculture.

Abstract

A variety of long-term stress conditions may exist in fish cultivation, some of which are so severe that fish can no longer reestablish homeostasis. In teleost fish, the brain and gastrointestinal tract integrate signals that include the perception of stress factors regulating physiological responses, such as social stress by fish population density, where peripheral and central signals, such as peptide hormones, are the main regulators. Therefore, we proposed in this study to analyze the effect of different stock densities (SD) in the gene expression of brain neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP), together with the gastrointestinal peptide hormones leptin (Lep), vasointestinal peptide (VIP), and protachykinin-1 (Prk-1) in Salmo salar post-smolt. The coding sequence of S. salar VIP and Prk-1 precursors were firstly cloned and characterized. Then, the mRNA expression of these genes, together with the NPY, Lep, and CGRP genes, were evaluated in post-smolts kept at 11 Kg/m³, 20 Kg/m³, and 40 Kg/m³. At 14 days of culture, the brain CGRP and liver leptin mRNA levels increased three and tenfold in the post-smolt salmons kept at the highest SD, respectively. The high levels of leptin were kept during all the fish culture experiments. In addition, the highest expression of intestine VIP mRNA was obtained on Day 21 in the group of 40 Kg/m³ returning to baseline on Day 40. In terms of stress biochemical parameters, cortisol levels were increased in the 20 Kg/m³ and 40 Kg/m³ groups on Day 40 and were the highest in the 20 Kg/m³ group on Day 14. This study provides new insight into the gastrointestinal signals that could be affected by chronic stress induced by high stock density in fish farming. Thus, the expression of these peptide hormones could be used as molecular markers to improve production practices in fish aquaculture.

Effects of Cadmium and Lead on Muscle and Liver Glycogen Levels of Climbing Perch (Anabas testudineus)

The aim of this study was to assess the glycogen content in the muscle and liver tissues of the climbing perch (Anabas testudineus) exposed to sublethal concentrations of Cd and Pb over 28 days of exposure and 14 days of depuration. Muscle and liver glycogen levels in A. testudineus after Pb or Cd treatment were significantly lower (p < 0.05) than that of A. testudineus in the control group during the exposure phase. In the recovery phase, muscle, and liver glycogen levels in A. testudineus increased in all Pb treatment groups, whereas they continuously decreased in all Cd treatment groups. Fish affected by Cd had obvious difficulties recovering from the stress response. It was concluded that exposure to the tested concentrations of Pb and Cd could be a potent endocrine activity disruptor, which may lead to adverse impacts on the health of A. testudineus.

Immunomodulation and skin microbiota perturbations during an episode of chronic stress in gilthead seabream

Fish stress is a major concern in the aquaculture industry. Many stressors coming from routine practices can predispose fish to compromised growth, immunity and overall health. This study focuses on the characterization of the skin microbiota using next generation sequencing (NGS) platform by targeting a genomic marker 16S and to determine growth performance and immune status of gilthead seabream (Sparus aurata) during an episode of chronic stress. Two groups were established: control group and chronically stressed group. Stressed fish were subjected to 1 min air exposure twice a week for 4 weeks. Results showed that stress negatively affected fish growth performance. Cellular and humoral systemic immunity remained unaffected while local immunity in skin was positively stimulated (total IgM and peroxidase). Skin mucus microbial composition showed significant differences especially after 14 days. Stressed fish showed a decrease in the abundance of the genera Acinetobacter, NS3a_marine_group and Pseudomonas, while Pseudoalteromonas and Marinagarivorans increased significantly. In conclusion, air exposure stress was associated with alterations in skin mucosal immunity and microbial composition that may have been beneficial to the host favoring adaptation to stress.

Domestication of farmed fish via the attenuation of stress responses mediated by the hypothalamus-pituitary-inter-renal endocrine axis

Human-directed domestication of terrestrial animals traditionally requires thousands of years for breeding. The most prominent behavioral features of domesticated animals include reduced aggression and enhanced tameness relative to their wild forebears, and such behaviors improve the social tolerance of domestic animals toward both humans and crowds of their own species. These behavioral responses are primarily mediated by the hypothalamic-pituitary-adrenal (inter-renal in fish) (HPA/I) endocrine axis, which is involved in the rapid conversion of neuronal-derived perceptual information into hormonal signals. Over recent decades, growing evidence implicating the attenuation of the HPA/I axis during the domestication of animals have been identified through comprehensive genomic analyses of the paleogenomic datasets of wild progenitors and their domestic congeners. Compared with that of terrestrial animals, domestication of most farmed fish species remains at early stages. The present review focuses on the application of HPI signaling attenuation to accelerate the domestication and genetic breeding of farmed fish. We anticipate that deeper understanding of HPI signaling and its implementation in the domestication of farmed fish will benefit genetic breeding to meet the global demands of the aquaculture industry.

Rank- and sex-specific differences in the neuroendocrine regulation of glucocorticoids in a wild group-living fish

Individuals that live in groups experience different challenges based on their social rank and sex. Glucocorticoids have a well-established role in coordinating responses to challenges and glucocorticoid levels often vary between ranks and sexes. However, the neuroendocrine mechanisms regulating glucocorticoid dynamics in wild groups are poorly understood, making it difficult to determine the functional consequences of differences in glucocorticoid levels. Therefore, we observed wild social groups of a cooperatively breeding fish (Neolamprologus pulcher) and evaluated how scale cortisol content (an emerging method to evaluate cortisol dynamics in fishes) and expression of glucocorticoid-related genes varied across group members. Scale cortisol was detectable in ~50% of dominant males (7/17) and females (7/15)-but not in any subordinates (0/16)-suggesting that glucocorticoid levels were higher in dominants. However, the apparent behavioural and neuroendocrine factors regulating cortisol levels varied between dominant sexes. In dominant females, higher cortisol was associated with greater rates of territory defense and increased expression of corticotropin-releasing factor in the preoptic and hypothalamic regions of the brain, but these patterns were not observed in dominant males. Additionally, transcriptional differences in the liver suggest that dominant sexes may use different mechanisms to cope with elevated cortisol levels. While dominant females appeared to reduce the relative sensitivity of their liver to cortisol (fewer corticosteroid receptor transcripts), dominant males appeared to increase hepatic cortisol breakdown (more catabolic enzyme transcripts). Overall, our results offer valuable insights on the mechanisms regulating rank- and sex-based glucocorticoid dynamics, as well as the potential functional outcomes of these differences.

Viral Infection Drives the Regulation of Feeding Behavior Related Genes in Salmo salar

The feeding behavior in fish is a complex activity that relies on the ability of the brain to integrate multiple signals to produce appropriate responses in terms of food intake, energy expenditure, and metabolic activity. Upon stress cues including viral infection or mediators such as the proinflammatory cytokines, prostaglandins, and cortisol, both Pomc and Npy/Agrp neurons from the hypothalamus are stimulated, thus triggering a response that controls both energy storage and expenditure. However, how appetite modulators or neuro-immune cues link pathogenesis and energy homeostasis in fish remains poorly understood. Here, we provide the first evidence of a molecular linkage between inflammation and food intake in Salmon salar. We show that in vivo viral challenge with infectious pancreatic necrosis virus (IPNV) impacts food consumption by activating anorexic genes such as mc4r, crf, and pomcb and 5-HT in the brain of S. salar. At the molecular level, viral infection induces an overall reduction in lipid content in the liver, favoring the production of AA and EPA associated with the increment of elovl2 gene. In addition, infection upregulates leptin signaling and inhibits insulin signaling. These changes are accompanied by a robust inflammatory response represented by the increment of Il-1b, Il-6, Tnfa, and Pge2 as well as an increased cortisol level in vivo. Thus, we propose a model in which hypothalamic neurons respond to inflammatory cytokines and stress-related molecules and interact with appetite induction/inhibition. These findings provide evidence of crosstalk between pathogenesis-driven inflammation and hypothalamic-pituitary-adrenocortical axes in stress-induced food intake behavior in fish.

Physiological response and survival of Atlantic mackerel exposed to simulated purse seine crowding and release

Understanding how animals physiologically respond to capture and release from wild capture fishing is fundamental for developing practices that enhance their welfare and survival. As part of purse seine fishing for small pelagic fish in northern European waters, excess and/or unwanted catches are routinely released from the net in a process called slipping. Due to excessive crowding in the net prior to release, post-slipping mortality rates can be unacceptably high. Atlantic mackerel (Scomber scombrus) support large and economically important purse seine fisheries but are known to be particularly vulnerable to such crowding-induced mortality. Developing management advice to promote post-slipping survival for this species is currently challenging, due to a lack of understanding of how crowding influences their physiology. Here we examine the physiological response, recovery and survival of wild caught mackerel exposed to various degrees and durations of simulated crowding stress in a series of sea cage trials. The magnitude of the physiological response and its time to recovery was positively correlated with crowding density and duration and was characterized by cortisol elevation, energy mobilization and anaerobic metabolite accumulation. There were also indications of osmoregulatory disturbance. Skin injury and mortality rates showed a similar positive relationship to crowding density. The physiological disturbance was recoverable for most fish. Instead, the rate at which mortalities developed and the physiological profile of moribund fish indicated that skin injury, likely arising from abrasive contact with netting and other fish during crowding, was the probable cause of mortality. Injured fish also exhibited a loss of allometric condition relative to non-injured survivors. Crowding treatments were potentially confounded by differences in ambient oxygen reduction, water temperature and pre-treatment fish condition between trials, and densities were replicated only once. These results contribute to the development of welfare conscious fishing practices that aim to reduce post-slipping mortality.

Uncoupling Thermotolerance and Growth Performance in Chinook Salmon: Blood Biochemistry and Immune Capacity

Ocean warming and extreme sea surface temperature anomalies are threatening wild and domesticated fish stocks in various regions. Understanding mechanisms for thermotolerance and processes associated with divergent growth performance is key to the future success of aquaculture and fisheries management. Herein, we exposed Chinook salmon (Oncorhynchus tshawytscha) to environmentally relevant water temperatures (19–20 °C) approaching their upper physiological limit for three months and sought to identify blood biomarkers associated with thermal stress and resilience. In parallel, blood biochemical associations with growth performance were also investigated. Temperature stress-activated leukocyte apoptosis induced a minor immune response, and influenced blood ion profiles indicative of osmoregulatory perturbation, regardless of how well fish grew. Conversely, fish displaying poor growth performance irrespective of temperature exhibited numerous biomarker shifts including haematology indices, cellular-based enzyme activities, and blood clinical chemistries associated with malnutrition and disturbances in energy metabolism, endocrine functioning, immunocompetence, redox status, and osmoregulation. Findings provide insight into mechanisms of stress tolerance and compromised growth potential. Biochemical phenotypes associated with growth performance and health can potentially be used to improve selective breeding strategies.

Trajectory of leptin and leptin receptor in vertebrates: Structure, function and their regulation

The present review provides a comparative insight into structure, function and control of leptin system in fishes, herptiles, birds and mammals. In general, leptin acts as an anorexigenic hormone since its administration results in decrease of food intake in vertebrates. Nonetheless, functional paradox arises in fishes from contradictory observations on level of leptin during fasting and re-feeding. In addition, leptin is shown to modulate metabolic functions in fishes, reptiles, birds and mammals. Leptin also regulates reproductive and immune functions though more studies are warranted in non-mammalian vertebrates. The expression of leptin and its receptor is influenced by numerous factors including sex steroids, stress and stress-induced catecholamines and glucocorticoids though their effect in non-mammalian vertebrates is hard to be generalized due to limited studies.

Leptin signalling in teleost fish with emphasis in food intake regulation

Leptin, the product of the obese (ob or Lep) gene, was first cloned in teleost fish in 2005, more than a decade after its identification in mammals. This was because bony fish and mammalian leptins share a very low amino acid sequence identity, which suggests different functionality of the leptin system in fish compared to that of mammals. Indeed, major differences are evident between the mammalian and fish leptin system. Thus, for instance, mammalian leptin is synthesized and released by the adipose tissue in response to the amount of fat depots, while several tissues (mainly the liver) are the main sources of leptin in fish, whose determining factors of production are still unclear. In mammals, the main physiological role for leptin is its involvement in the maintenance of energy balance by decreasing food intake and increasing energy expenditure, although a wide variety of actions have been attributed to this hormone (e.g., regulation of lipid and carbohydrate metabolism, reproduction and immune functions). In fish, available literature also points towards a multifunctional nature for leptin, although knowledge on its functions is limited. In this review, we offer an overview of teleostean leptin structure and mechanism of action, and discuss the available knowledge on the role of this hormone in food intake regulation in teleost fish, aiming to provide a comparative overview between the functioning of the teleostean and mammalian leptin systems.

Food intake, growth, and expression of neuropeptides regulating appetite in clown anemonefish (Amphiprion ocellaris) exposed to predicted climate changes

The clown anemonefish (Amphiprion ocellaris) is a common model species in studies assessing the impact of climate changes on tropical coral fish physiology, metabolism, growth, and stress. However, the basic endocrine principles for the control of food intake and energy homeostasis, under normal and elevated sea temperatures, in this species remain unknown. In this work, we studied food intake and growth in clown anemonefish reared at different temperatures and with different food availability. We also analyzed expression of genes in the melanocortin system, which is believed to be involved in the control of appetite and feeding behavior. These were two paralogues of pomc: pomca and pomcb; two paralogs of agrp: agrp1 and agrp2; and one mc4r-like. Groups of juvenile clown anemonefish were exposed to four experimental treatments combining (orthogonal design) two rearing temperatures: 28 °C (T28; normal) and 32 °C (T32; high) and two feeding regimes: one (1 M; 08:00) or three (3 M; 08:00, 12:00, 15:00) meals per day, fed to satiety by hand. The results showed that high temperature (T32) did not affect the average growth rate but induced a stronger asymmetrical individual body weight of the fish within the population (tank). Lower feeding frequency (1 M) resulted in lower growth rates at both rearing temperatures. Fish reared at high temperature had higher total daily food intake, which correlated with a lower expression of pomca, supporting an anorexigenic role of this gene. High temperature combined with restricted feeding induced higher agrp1 levels and resulted in a higher food intake in the morning meal compared to the control. This supports an orexigenic role for agrp1. mRNA levels of agrp2 responded differently from agrp1, supporting different roles for the paralogues. Levels of mc4r-like inversely correlated with fish body weight, indicating a possible size/stage dependence of gene expression. In conclusion, our results indicate that the melanocortin system is involved in adjusting appetite and food intake of clown anemonefish in response to elevated temperature and low food availability.

Negative effect of ectoparasite burdens on the condition factor from farmed tilapia Oreochromis niloticus in the Yucatan, Mexico

Nile tilapia (Oreochromis niloticus Linnaeus, 1758) is one of the most important aquaculture species in the world, and, when introduced, the ectoparasites of Nile tilapia have followed. Currently, farmers worldwide consider these ectoparasites harmless, but intensities can reach up to 1000 individuals per fish in tropical regions. In this cross-sectional study, we used the condition factor to estimate the potential effects of low (45 ± 31 ectoparasites per fish) and high (295 ± 191) ectoparasitic burdens across 28 tilapia farms and included the analysis of the effects of 44 management and environmental variable from the farms. A stepwise procedure in a multiple linear regression analysis retained the variables that explained the most variance, which was the ectoparasitic burden (57 %). We found significantly higher values of the condition factor in Nile tilapia with low ectoparasitic burden than in those with high ectoparasitic burden. Additionally, Nile tilapia with a high ectoparasitic burden weighed less than half than those with a low burden (102 ± 105 g versus 230 ± 128 g, respectively). We also found a significant non-linear, negative relationship between the ectoparasitic burden and the relative condition factor values per fish, which was most likely due to an increase in gill mucus caused by the ectoparasitic burden that depleted energy in the Nile tilapia. The economic consequence of the ectoparasitic burden translated roughly into a loss up to 12-15 % in profit margin per ton of fish, based on the price of Nile tilapia in the Mexican market.

Distress Regulates Different Pathways in the Brain of Common Carp: A Preliminary Study

In this study, a stress trial was conducted with common carp, one of the most important species in aquaculture worldwide, to identify relevant gene regulation pathways in different areas of the brain. Acute distress due to exposure to air significantly activated the expression of the immediate early gene c-fos in the telencephalon. In addition, evidence for regulation of the two corticotropin-releasing factor (crf) genes in relation to their binding protein (corticotropin-releasing hormone-binding protein, crh-bp) is presented in this preliminary study. Inferences on the effects of due to exposure to air were obtained by using point estimation, which allows the prediction of a single value. This constitutes the best description to date of the previously generally unknown effects of stress in different brain regions in carp. Furthermore, principal component analyses were performed to reveal possible regulation patterns in the different regions of the fish brain. In conclusion, these preliminary studies on gene regulation in the carp brain that has been influenced by exposure to a stressor reveal that a number of genes may be successfully used as markers for exposure to unfavourable conditions.

Phenol-rich fulvic acid as a water additive enhances growth, reduces stress, and stimulates the immune system of fish in aquaculture

Aquaculture has become imperative to cover the demands for dietary animal protein. Simultaneously, it has to overcome prejudices from excessive use of antibiotics and environmental impacts. Natural supplements are traditionally applied orally. In this study, we demonstrated another pathway: the gills. Humic substances are immunostimulants and a natural part of every aquatic ecosystem, making them ideal to be used as bath stimulants. Five and 50 mg C/L of a fulvic acid-rich humic substance was added for 28 days to the water of juvenile rainbow trout (Oncorhynchus mykiss). This fulvic acid is characterized by a high content of phenolic moieties with persistent free radicals and a high electron exchange capacity. The high concentration of the fulvic acid significantly increased growth and reduced the food conversion ratio and the response to a handling-stressor. Phagocytosis and potential killing activity of head kidney leukocytes were increased, as well as the total oxyradical scavenging capacity (TOSC) and lysozyme activity in the gills. In conclusion, immunostimulation via gills is possible with our fulvic acid, and the high phenolic content improved overall health and stress resistance of fish.

Fish Feed Intake, Feeding Behavior, and the Physiological Response of Apelin to Fasting and Refeeding

Feed is one of the most important external signals in fish that stimulates its feeding behavior and growth. The intake of feed is the main factor determining efficiency and cost, maximizing production efficiency in a fish farming firm. The physiological mechanism regulating food intake lies between an intricate connection linking central and peripheral signals that are unified in the hypothalamus consequently responding to the release of appetite-regulating genes that eventually induce or hinder appetite, such as apelin; a recently discovered peptide produced by several tissues with diverse physiological actions mediated by its receptor, such as feed regulation. Extrinsic factors have a great influence on food intake and feeding behavior in fish. Under these factors, feeding in fish is decontrolled and the appetite indicators in the brain do not function appropriately thus, in controlling conditions which result in the fluctuations in the expression of these appetite-relating genes, which in turn decrease food consumption. Here, we examine the research advancements in fish feeding behavior regarding dietary selection and preference and identify some key external influences on feed intake and feeding behavior. Also, we present summaries of the results of research findings on apelin as an appetite-regulating hormone in fish. We also identified gaps in knowledge and directions for future research to fully ascertain the functional importance of apelin in fish.

Cortisol regulates insulin-like growth-factor binding protein (igfbp) gene expression in Atlantic salmon parr

The growth hormone (Gh)/insulin-like growth-factor (Igf)/Igf binding protein (Igfbp) system regulates growth and osmoregulation in salmonid fishes, but how this system interacts with other endocrine systems is largely unknown. Given the well-documented consequences of mounting a glucocorticoid stress response on growth, we hypothesized that cortisol inhibits anabolic processes by modulating the expression of hepatic igfbp mRNAs. Atlantic salmon (Salmo salar) parr were implanted intraperitoneally with cortisol implants (0, 10, and 40 μg g^-1 body weight) and sampled after 3 or 14 days. Cortisol elicited a dose-dependent reduction in specific growth rate (SGR) after 14 days. While plasma Gh and Igf1 levels were unchanged, hepatic igf1 mRNA was diminished and hepatic igfbp1b1 and -1b2 were stimulated by the high cortisol dose. Plasma Igf1 was positively correlated with SGR at 14 days. Hepatic gh receptor (ghr), igfbp1a, -2a, -2b1, and -2b2 levels were not impacted by cortisol. Muscle igf2, but not igf1 or ghr, levels were stimulated at 3 days by the high cortisol dose. As both cortisol and the Gh/Igf axis promote seawater (SW) tolerance, and particular igfbps respond to SW exposure, we also assessed whether cortisol coordinates the expression of branchial igfbps and genes associated with ion transport. Cortisol stimulated branchial igfbp5b2 levels in parallel with Na⁺/K⁺-ATPase (NKA) activity and nka-α1b, Na⁺/K⁺/2Cl^--cotransporter 1 (nkcc1), and cystic fibrosis transmembrane regulator 1 (cftr1) mRNA levels. The collective results indicate that cortisol modulates the growth of juvenile salmon via the regulation of hepatic igfbp1s whereas no clear links between cortisol and branchial igfbps previously shown to be salinity-responsive could be established.

Metabolic mechanisms of Coilia nasus in the natural food intake state during migration

As a prominent member of freshwater and coastal fish faunas, Coilia nasus migrates annually from the sea up the Yangtze River in China to spawn. It is traditionally believed that C. nasus generally do not feed during their spawning migration. However, we recently documented the occurrence of food intake phenomenon in C. nasus following voluntary fasting. The purpose of the current study is to explore the metabolic mechanisms on C. nasus in response to food intake during migration. A total of 23,159 differentially expressed mRNA molecules and 204 metabolites were identified in transcriptome and metabolome analyses. Our results provide insights into the activation of energy consumption and reinforcement of energy storage during migration, and also identify key genes involved in food intake regulation. Our findings will be useful for future research on population recruitment and energy utilization in wild C. nasus.

Neuropeptide Y deficiency induces anxiety-like behaviours in zebrafish (Danio rerio)

Neuropeptide Y (NPY) controls energy homeostasis including orexigenic actions in mammalians and non-mammalians. Recently, NPY has attracted attention as a mediator of emotional behaviour and psychosomatic diseases. However, its functions are not fully understood. We established npy gene-deficient (NPY-KO) zebrafish (Danio rerio) to assess the relationship between NPY and emotional behaviours. The NPY-KO zebrafish exhibited similar growth, but pomc and avp mRNA levels in the brain were higher as compared to wild-type fish. NPY-KO zebrafish exhibited several anxiety-like behaviours, such as a decrease in social interaction in mirror test and decreased locomotion in black-white test. The acute cold stress-treated NPY-KO zebrafish exhibited anxiety-like behaviours such as remaining stationary and swimming along the side of the tank in the mirror test. Moreover, expression levels of anxiety-associated genes (orx and cck) and catecholamine production (gr, mr, th1 and th2) were significantly higher in NPY-KO zebrafish than in wild-type fish. We demonstrated that NPY-KO zebrafish have an anxiety phenotype and a stress-vulnerability like NPY-KO mice, whereby orx and/or catecholamine signalling may be involved in the mechanism actions.

Effects of Chattonella antiqua on the swimming behavior and brain monoamine metabolism of juvenile yellowtail (Seriola quinqueradiata)

Being the precursor of serotonin and melatonin, dietary supplementation with tryptophan (TRP) may modulates behavior, stress responses, and antioxidant capacity in fish. In this study, effects of Chattonella exposure on the swimming behavior and brain monoamine metabolism of yellowtail fed a commercial diet (control diet) or that enriched by 1.5% L-TRP (TRP + diet) were investigated. A 7-day dietary TRP supplementation elevated spontaneous swimming speed of yellowtail and mitigated their behavioral response to Chattonella (250 cells/mL) exposure. A 30-day dietary TRP supplementation elevated growth of juvenile yellowtail. Lethal exposure to Chattonella (1000 cells/mL) significantly elevated the turnover rates of serotonin, dopamine, and norepinephrine metabolism in fish fed control diet, but did not alter the serotonin turnover rate in fish fed TRP + diet. Our results suggested that dietary supplementation with TRP had potential to mitigate the stress response in yellowtail to Chattonella, partly via mediating their brain monoamine metabolism.

Effects of short-term exercise on food intake and the expression of appetite-regulating factors in goldfish

In mammals, growing evidence indicates that exercise affects food intake, metabolism and the expression and blood levels of appetite regulators. In this study, we examined the effects of short-term (30 min, at low and high water flow) exercise on food intake, glucose levels and the expressions of appetite regulators in goldfish hypothalamus (irisin, orexin, CART, leptin), intestine (CCK, PYY, proglucagon/GLP-1), muscle (irisin) and liver (leptin), of brain-derived neurotrophic factor (BDNF) in brain, interleukin-6 (IL6) in muscle and hypothalamus, and major metabolic enzymes, the glycolytic enzyme glucokinase (GCK) and its regulatory protein (GCKR) in liver, the lipolytic enzyme lipoprotein lipase in intestine and muscle, and trypsin in intestine. Fish submitted to high flow exercise had a lower post-exercise food intake compared to control fish but no differences were seen in glucose levels between groups. Exercise induced an increase in hypothalamic expression levels of CART, IL6 and BDNF, but not orexin, irisin, CRF, leptin and NPY. High flow exercise induced an increase in intestine CCK, PYY and GLP-1, and muscle irisin and IL-6 expression levels. Exercise had no effects on expression levels of hepatic leptin or any of the metabolic enzymes examined. Our results suggest that, in goldfish, short-term exercise might decrease feeding in part by affecting the expressions of myokines and peripheral, but not central appetite regulators or metabolic enzyme/hormones.

Modulation of Pituitary Response by Dietary Lipids and Throughout a Temperature Fluctuation Challenge in Gilthead Sea Bream

Low temperatures provoke drastic reductions in gilthead sea bream (Sparus aurata) activity and nourishment, leading to growth arrest and a halt in production. However, scarce data exist concerning the implications of central core control during the cold season. The aim of this work was to study the effects of low temperature and recovery from such exposure on the pituitary activity of sea bream juveniles fed 18% or 14% dietary lipid. A controlled indoor trial was performed to simulate natural temperature fluctuation (22 °C to 14 °C to 22 °C). Meanwhile, we determined the regulatory role of the pituitary by analyzing the gene expression of some pituitary hormones and hormone receptors via qPCR, as well as plasma levels of thyroidal hormones. In response to higher dietary lipids, hormone pituitary expressions were up-regulated. Induced low temperatures and lower ingesta modulated pituitary function up-regulating GH and TSH and thyroid and glucocorticoid receptors. All these findings demonstrate the capacity of the pituitary to recognize both external conditions and to modulate its response accordingly. However, growth, peripheral tissues and metabolism were not linked or connected to pituitary function at low temperatures, which opens an interesting field of study to interpret the hypothalamus–pituitary–target axis during temperature fluctuations in fish.

Suppression of leptin-AI/AII transcripts by insulin in goldfish liver: A fish specific response of leptin under food deprivation

Leptin is primarily considered a peripheral satiety hormone and is also found to perform important roles in energy homeostasis in vertebrates ranging from fish to mammals. The liver is a major source of leptin production in teleost fish. Using goldfish as a model, a previous report by our group illustrated the positive regulation of leptin mRNA levels by treatment with the hyperglycemic hormone glucagon, and our present study provided evidence for the negative regulation of hepatic leptin-AI and leptin-AII transcripts through the administration of the hypoglycemic hormone insulin. This study is the first to demonstrate changes in the hepatopancreatic insulin, glucagon, leptin-AI and leptin-AII mRNA levels in goldfish during fasting and refeeding. Insulin was found to be effective in suppressing leptin-AI and leptin-AII transcript levels in goldfish liver via both in vivo intraperitoneal injection and in vitro cell incubation approaches. Only the insulin receptor, not the IGF-I receptor, was involved in insulin-inhibited leptin mRNA level. The suppression of leptin levels by insulin was caused by the activation of MKK_3/6/p³⁸MAPK and MEK_1/2/Erk_1/2 cascades. Insulin treatment could eliminate the stimulation of glucagon on leptin mRNA level. Our study describes the regulation and signal transduction mechanism of insulin on leptin mRNA levels in the goldfish liver, suggesting that the leptin function in fish is speculated to be not only an anorexigenic factor but also a metabolic mediator. This also supports the hypothesis that the poikilothermal fish use a passive survival strategy during the periods of food deprivation, which is mediated by the fish-specifically high leptin levels induced by the cooperation of insulin and glucagon.

Fish as models for understanding the vertebrate endocrine regulation of feeding and weight

The frequencies of eating disorders and obesity have increased worldwide in recent years. Their pathophysiologies are still unclear, but recent evidence suggests that they might be related to changes in endocrine and neural factors that regulate feeding and energy homeostasis. In order to develop efficient therapeutic drugs, a more thorough knowledge of the neuronal circuits and mechanisms involved is needed. Although to date, rodents have mostly been used models in the area of neuroscience and neuroendocrinology, an increasing number of studies use non-mammalian vertebrates, in particular fish, as model systems. Fish present several advantages over mammalian models and they share genetic and physiological homology to mammals with close similarities in the mechanisms involved in the neural and endocrine regulation of appetite. This review briefly describes the regulation of feeding in two model species, goldfish and zebrafish, how this regulation compares to that in mammals, and how these fish could be used for studies on endocrine regulation of eating and weight and its dysregulations.

Changes in gene expressions of hypothalamic neuropeptides controlling feeding behaviors in bilateral nephrectomized rats

Anorexia is one of the most widespread eating disorders that appears to contribute to malnutrition in patients with advanced kidney dysfunction. The changes of neuropeptides controlling feeding behaviors synthesized in the hypothalamus under several physiological condition could induce anorexia. While several mechanisms underlying uremic anorexia have been proposed, the changes of hypothalamic neuropeptides controlling feeding behaviors of uremic patients are poorly understood. The gene expressions of hypothalamic neuropeptides controlling feeding behaviors were evaluated after bilateral nephrectomy, which is a model of acute kidney dysfunction, by in situ hybridization histochemistry. Food consumption decreased markedly in bilateral nephrectomized rats. The mRNA levels of corticotrophin-releasing hormone, proopiomelanocortin, cocaine- and amphetamine-regulated transcript, which suppress feeding behavior, were significantly higher in bilateral nephrectomized rats than in sham-operated rats. On the other hand, the mRNA levels of Agouti-related peptide, neuropeptide Y, melanin-concentrating hormone, and orexin, which promote feeding behavior, were significantly lower in bilateral nephrectomized rats than in sham-operated rats. In addition, the plasma level of leptin, which has an anorexic effect, increased after bilateral nephrectomy. The results suggest that hypothalamic neuropeptides controlling feeding behaviors may be involved in the development of anorexia in bilateral nephrectomized rats. This report is the first step to elucidating the physiological mechanisms of anorexia in patients with kidney dysfunction.