The effects of environmental changes on the endocrine regulation of feeding in fishes.

Fishes are exposed to natural and anthropogenic changes in their environment, which can have major effects on their behaviour and their physiology, including feeding behaviour, food intake and digestive processes. These alterations are owing to the direct action of environmental physico-chemical parameters (i.e. temperature, pH, turbidity) on feeding physiology but can also be a consequence of variations in food availability. Food intake is ultimately regulated by feeding centres of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues such as the gastrointestinal tract. These endocrine signals stimulate or inhibit food intake, and interact with each other to maintain energy homeostasis. Changes in environmental conditions might change feeding habits and rates, thus affecting levels of energy stores, and the expression of endocrine appetite regulators. This review provides an overview of how environmental changes and food availability could affect feeding and these endocrine networks in fishes. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Effects of temperature on food intake and the expression of appetite regulators in three Characidae fish: The black-skirted tetra (Gymnocorymbus ternetzi), neon tetra (Paracheirodon innesi) and Mexican cavefish (Astyanax mexicanus).

The Characidae family of fish is composed of commercially important species for which little is known about the regulation of feeding. Fish are ectotherms so that their body temperature fluctuates with the temperature of the surrounding water. Changes in water temperature can thus have major effects on the physiology of fish, in particular their feeding. The mechanisms by which appetite is influenced by changes in temperatures in fish remain unclear. In this study, we examined the effects of temperature on feeding behavior, food intake and the expression of appetite regulators in three characid fish (black tetra, neon tetra and cavefish) by submitting them to four different temperatures for 2 weeks (20°C, 24°C, 28°C, 32°C). In all species, food intake increased with increasing temperature. In neon and black tetras, increasing temperatures decreased expressions of orexin and leptin and increased that of cocaine and amphetamine regulated transcript (CART). In cavefish, temperature had no effect on brain orexin, leptin or CART. In all three species, higher temperatures induced increases in intestine expression of cholecystokinin (CCK), but no effects were seen for intestine ghrelin and peptide YY expressions. Our results show that temperature affects feeding in Characidae fish and induces species-specific changes in the expression of appetite regulators.

The role of visfatin/ NAMPT in the regulation of feeding in goldfish (Carassius auratus).

The protein NAMPT (nicotinamide phosphoribosyltransferase, encoded by the NAPMT gene) is present in two forms. The intracellular form of NAMPT (iNAMPT) is the rate-limiting enzyme in a major nicotinamide adenine dinucleotide (NAD) biosynthetic pathway and regulates cellular metabolism. NAMPT is also secreted by cells in the extracellular milieu, and referred to as extracellular NAMPT (eNAMPT or visfatin). In mammals, visfatin has been linked to various metabolic disorders. However, the role of visfatin in regulating energy homeostasis in fish is not known. In this study, we assessed the effects of nutritional status on NAMPT mRNA expression and the effects of visfatin peripheral injections on food intake and the expression of appetite regulators in goldfish. Our results show that NAMPT is widely expressed in peripheral tissues and brain. Fasting induced increases in NAMPT expression in liver but had no effect on either brain or intestine NAMPT expression levels. Intraperitoneal injections of visfatin (400 ng/g) induced an increase in food intake and in expression levels of hepatic leptin and sirtuin1. Visfatin injections decreased intestine CCK and PYY, and telencephalon (but not hypothalamic) orexin and NPY expression levels. Visfatin did not affect plasma glucose levels, intestine ghrelin or brain CART, POMC and AgRP expressions. These data suggest that visfatin/NAMPT might be involved in the regulation of feeding and energy homeostasis in goldfish.

Shining light on the transcriptome: Molecular regulatory networks leading to a fast-growth phenotype by continuous light in an environmentally sensitive teleost (Atherinopsidae).

Photoperiod can profoundly affect the physiology of teleost fish, including accelerated growth here defined as "fast growth phenotypes". However, molecular regulatory networks (MRNs) and biological processes being affected by continuous illumination and which allow some teleost species evident plasticity to thrive under this condition are not yet clear. Therefore, to provide a broad perspective of such mechanisms, Chirostoma estor fish were raised and sampled for growth under a simulated control (LD) 12 h Light: 12 h Dark or a continuous illumination (LL) 24 h Light: 0 h Dark since fertilization. The experiment lasted 12 weeks after hatching (wah), the time at which fish were sampled for growth, length, and whole-body cortisol levels. Additionally, 3 heads of fish from each treatment were used to perform a de novo transcriptome analysis using Next-Generation Sequencing. Fish in LL developed the fast growth phenotype with significant differences visible at 4 wah and gained 66% more mass by 12 wah than LD fish. Cortisol levels under LL were below basal levels at all times compared to fish in LD, suggesting circadian dysregulation effects. A strong effect of LL was observed in samples with a generalized down-regulation of genes except for Reactive Oxygen Species responses, genome stability, and growth biological processes. To our knowledge, this work is the first study using a transcriptomic approach to understand environmentally sensitive MRNs that mediate phenotypic plasticity in fish submitted to continuous illumination. This study gives new insights into the plasticity mechanisms of teleost fish under constant illumination.

Feed intake and gene expression of appetite-regulating hormones in Salminus brasiliensis fed diets containing soy protein concentrate.

Dourado (Salminus brasiliensis) is a large carnivorous fish with high commercial value for which sustainable aquaculture relies on the substitution of expensive dietary animal protein sources in aquafeeds, in particular fish meal (FM), by cheaper plant protein, such as soy protein concentrate (SPC). This study aimed at evaluating feed intake and gene expression of appetite- regulating hormones [orexin, cocaine and amphetamine regulated transcript (CART), leptin, cholecystokinin (CCK) and peptide YY (PYY)] in the intestine, pyloric caeca and hypothalamus of juvenile dourado fed diets containing graded levels of SPC and FM as dietary protein sources for a period of three weeks. Increasing dietary plant protein contents reduced daily feed consumption and the expressions of the anorexigenic hormone CCK in the anterior intestine and in pyloric caeca and PYY in pyloric caeca. No changes were detected in the hypothalamic expression of appetite-regulating hormones, suggesting that gastrointestinal hormones are more involved in the decrease in feeding induced by plant protein diets than central appetite-regulating systems.

Possible role of transcription factors (BSX, NKX2.1, IRX3 and SIRT1) in the regulation of appetite in goldfish (Carassius auratus).

The homeobox genes play important roles in the embryonic development of animals. Recent evidence suggests they might also regulate feeding and act as transcription factors of appetite regulators. Examples of these genes are a brain-specific homeobox transcription factor (BSX), NK2 homeobox 1 (NKX2.1) and the Iroquois homeobox 3 (IRX3). Sirtuin1 (SIRT1) acts as a transcription factor for nutrient (e.g. lipid, glucose) homeostasis and responds to stress and nutrient availability, and has been shown to interact with appetite regulators. Very little is known about the role of these genes in the regulation of feeding and nutrient homeostasis in fish. In this study, we assessed the roles of BSX, NKX2.1, IRX3 and SIRT1 in the central regulation of feeding in goldfish by examining their mRNA brain distribution, assessing the effects of fasting on their brain expression and assessing the effects of peripheral injections of cholecystokinin (CCK, a brain-gut peptide), on their brain expression. All genes showed a widespread distribution in the brain, with high levels in the hypothalamus. In both hypothalamus and telencephalon, fasting induced increases in BSX, IRX3 and NKX2.1 expressions but had no effect on SIRT1 expression levels. CCK injections increased hypothalamic expression levels of IRX3 and SIRT1, and telencephalic expression levels of NKX2.1 and SIRT1, with no effect on either hypothalamic BSX or NKX2.1 expression levels or telencephalon BSX or IRX3 expression levels. Our results suggest that, in goldfish as in mammals, central BSX, NKX2.1, IRX3 and SIRT1 are present in regions of the brain regulating feeding, are sensitive to nutrient status and interact with appetite-regulating peptides.

Effects of thyroxine and propylthiouracil on feeding behavior and the expression of hypothalamic appetite-regulating peptides and thyroid function in goldfish (Carassius auratus).

There is poor evidence for an association between thyroidal state, feeding and appetite regulation in fish. We assessed how an altered thyroid state influences feeding behavior, food intake and expression of hypothalamic appetite-regulating peptides (Klotho-α and Klotho-ß; orexin, OX; cholecystokinin, CCK; agouti-related peptide, AgRP; cannabinoid receptor 1, CB1) in goldfish. We also measured the expressions of hypothalamic, pituitary and liver transcripts that regulate the thyroid [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptor (TRH-R) type 1, thyroid stimulating hormone beta (TSHß), deiodinases (DIO2, DIO3), UDP-glucuronosyltransferase (UGT1A1), thyroid receptor alpha and beta (TRα, TRß)], and circulating levels of total thyroxine (tT4) and total triiodothyronine (tT3). Goldfish were implanted with propylthiouracil (PTU) or T4 osmotic pumps for 12 days. T4- treatment increased feeding behavior but not food intake, increased central TSHß and DIO2, and hepatic DIO2 transcript expression and increased central DIO3 mRNA. Under hyperthyroid conditions, hypothalamic Klotho and CCK expressions were downregulated, suggesting an increased metabolic state and a hypothalamic response to regulate energy balance. AgRP, OX and CB1 were not affected by T4 treatment. PTU had no effect on any of the parameters examined, suggesting it is not a sensitive thyroid inhibitor in fish. Overall, we show that unlike in mammals, hyperthyroid conditions in goldfish do not lead to an increased desire or need to consume food, furthering evidence for a weak link between the thyroid and appetite.

Response of the thyroid axis and appetite-regulating peptides to fasting and overfeeding in goldfish (Carassius auratus).

The thyroid axis is a major regulator of metabolism and energy homeostasis in vertebrates. There is conclusive evidence in mammals for the involvement of the thyroid axis in the regulation of food intake, but in fish, this link is unclear. In order to assess the effects of nutritional status on the thyroid axis in goldfish, Carassius auratus, we examined brain and peripheral transcripts of genes associated with the thyroid axis [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptors (TRH-R type 1 and 2), thyroid stimulating hormone beta (TSHß), deiodinase enzymes (DIO2, DIO3) and UDP-glucoronsyltransferase (UGT)] and appetite regulators [neuropeptide Y (NPY), proopiomelanocortin (POMC), agouti-related peptide (AgRP) and cholecystokinin (CCK)] in fasted and overfed fish for 7 and 14 day periods. We show that the thyroid axis responds to overfeeding, with an increase of brain TRH and TSHß mRNA expression after 14 days, suggesting that overfeeding might activate the thyroid axis. In fasted fish, hepatic DIO3 and UGT transcripts were downregulated from 7 to 14 days, suggesting a time-dependent inhibition of thyroid hormone degradation pathways. Nutritional status had no effect on circulating levels of thyroid hormone. Central appetite-regulating peptides exhibited temporal changes in mRNA expression, with decreased expression of the appetite-inhibiting peptide POMC from 7 to 14 days for both fasted and overfed fish, with no change in central NPY or AgRP, or intestinal CCK transcript expression. Compared to control fish, fasting increased AgRP mRNA expression at both 7 and 14 days, and POMC expression was higher than controls only at 7 days. Our results indicate that nutritional status time-dependently affects the thyroid axis and appetite regulators, although no clear correlation between thyroid physiology and appetite regulators could be established. Our study helps to fill a knowledge gap in current fish endocrinological research on the effects of energy balance on thyroid metabolism and function.

The Role of the Thyroid Axis in Fish.

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Effects of temperature on feeding and digestive processes in fish.

As most fish are ectotherms, their physiology is strongly affected by temperature. Temperature affects their metabolic rate and thus their energy balance and behavior, including locomotor and feeding behavior. Temperature influences the ability/desire of the fish to obtain food, and how they process food through digestion, absorb nutrients within the gastrointestinal tract, and store excess energy. As fish display a large variability in habitats, feeding habits, and anatomical and physiological features, the effects of temperature are complex and species-specific. The effects of temperature depend on the timing, intensity, and duration of exposure as well as the speed at which temperature changes occur. Whereas acute short-term variations of temperature might have drastic, often detrimental, effects on fish physiology, long-term gradual variations might lead to acclimation, e.g. variations in metabolic and digestive enzyme profiles. The goal of this review is to summarize our current knowledge on the effects of temperature on energy homeostasis, with specific focus on metabolism, feeding, digestion, and how fish are often able to "adapt" to changing environments through phenotypic and physiological changes.

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.

The Piranha Genome Provides Molecular Insight Associated to Its Unique Feeding Behavior.

The piranha enjoys notoriety due to its infamous predatory behavior but much is still not understood about its evolutionary origins and the underlying molecular mechanisms for its unusual feeding biology. We sequenced and assembled the red-bellied piranha (Pygocentrus nattereri) genome to aid future phenotypic and genetic investigations. The assembled draft genome is similar to other related fishes in repeat composition and gene count. Our evaluation of genes under positive selection suggests candidates for adaptations of piranhas' feeding behavior in neural functions, behavior, and regulation of energy metabolism. In the fasted brain, we find genes differentially expressed that are involved in lipid metabolism and appetite regulation as well as genes that may control the aggression/boldness behavior of hungry piranhas. Our first analysis of the piranha genome offers new insight and resources for the study of piranha biology and for feeding motivation and starvation in other organisms.

Effects of fasting on the central expression of appetite-regulating and reproductive hormones in wild-type and Casper zebrafish (Danio rerio).

Appetite and reproduction are closely related functions that are both regulated by brain hormones. Appetite stimulators include orexin and neuropeptide Y (NPY), and reproductive hormones include gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin, and neurokinin B (NKB). GnRH stimulates the secretion of pituitary gonadotropes, and kisspeptin and GnIH modulate this action. Kisspeptin secretion is further controlled by neurokinin B (NKB) and dynorphin A (Dyn). To better understand the mechanisms regulating appetite and reproduction in fish, we examined the effects of fasting, reproductive stage, gender, and strain on the brain mRNA expression of appetite (orexin and NPY) and reproductive (GnRH, kisspeptin, GnIH, and NKB) hormones in zebrafish. In order to compare strains, we used both wild-type and transparent Casper zebrafish. In female wild-type zebrafish, fasting increased the expression of all hormones investigated, with the exception of Kiss2. Only NPY and Kiss2 were increased in male wild-type zebrafish during fasting. In Casper zebrafish, only GnIH and NKB in males were affected by fasting, suggesting that Casper fish may be more resistant to fasting than wild fish. Fasting increased expressions of orexin, GnRH2, Kiss1, GnIH and NKB in wild-type females with more eggs or larger eggs relative to body weight, compared to those with fewer or smaller eggs, suggesting that more mature females are more affected by fasting. No significant interactions of fasting and reproductive stage were noted in female Casper fish. To investigate whether differences between Casper and wild-type fish were due to genes involved in pigmentation, we compared the brain mRNA expressions of enzymes involved in melanin synthesis (tyrosinase and tyrosine hydroxylase - TH), melanocortin receptors (MC3R and MC4R), and the melanocortin precursor (proopiomelanocortin - POMC) between the two strains. Casper zebrafish had lower levels of MC3R, tyrosinase, TH1, TH2, and POMC than wild-type fish. Overall, our results suggest the existence of gender- and reproductive stage-specific, as well as strain-specific variations in the mechanisms regulating feeding and reproduction in zebrafish, and that the melanocortin system and melanin pathways may be in part responsible for these differences between strains.

Effects of potential climate change -induced environmental modifications on food intake and the expression of appetite regulators in goldfish.

Climate changes due to global warming result in part from the release of gases such as carbon dioxide (CO2) and methane into the atmosphere and results in warming and acidification of water bodies, and changes precipitation and wind patterns, which might in turn affect water currents, turbulence and turbidity. These changes might affect feeding and its endocrine control. Feeding is regulated by central and peripheral hormones that either stimulate (e.g. orexin, ghrelin) or inhibit (e.g. irisin, cocaine and amphetamine regulated transcript - CART, cholecystokinin - CCK and peptide YY -PYY) food intake. In this study we examined the effects of four climate change-related environmental factors (i.e. temperature, pH, turbulence and turbidity) on food intake and the hypothalamic and intestinal expressions of appetite regulators in fish, using goldfish as a model. High temperatures increased food intake and the brain expression of orexin, and decrease brain CART 1 and intestinal CCK, PYY and ghrelin. Low pHs decreased feeding and increased the expressions of CART1 and CART2 in the hypothalamus and CCK and PYY in the intestine. Turbulence (waves) induced an increase in food intake and a decrease in mRNA expression levels of both CART1 and CART2 in the hypothalamus and both CCK and PYY in the intestine. Turbidity (low visibility) did not affect food intake but increased locomotion and the time taken to reach satiation, while increasing brain orexin and intestinal PYY expression levels and lowering CART1 hypothalamic expression. The results of this study suggest that environmental stress affects feeding physiology of goldfish and bring new insights on how fish might respond to climate changes.

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.

Gut Microbiota and Energy Homeostasis in Fish.

The microorganisms within the intestinal tract (termed gut microbiota) have been shown to interact with the gut-brain axis, a bidirectional communication system between the gut and the brain mediated by hormonal, immune, and neural signals. Through these interactions, the microbiota might affect behaviors, including feeding behavior, digestive/absorptive processes (e.g., by modulating intestinal motility and the intestinal barrier), metabolism, as well as the immune response, with repercussions on the energy homeostasis and health of the host. To date, research in this field has mostly focused on mammals. Studies on non-mammalian models such as fish may provide novel insights into the specific mechanisms involved in the microbiota-brain-gut axis. This review describes our current knowledge on the possible effects of microbiota on feeding, digestive processes, growth, and energy homeostasis in fish, with emphasis on the influence of brain and gut hormones, environmental factors, and inter-specific differences.

Cloning and effects of fasting on the brain expression levels of appetite-regulators and reproductive hormones in glass catfish (Kryptopterus vitreolus).

The regulation of feeding is a complex process that involves coordination between various signals. Feeding hormones can be described as orexigenic (stimulate food intake, e.g. orexin and neuropeptide Y - NPY) or anorexigenic (inhibit food intake, e.g. cocaine and amphetamine regulated transcript - CART). Reproduction and energy homeostasis are closely linked, as factors that affect appetite have also been shown to influence reproductive hormones and behaviors. Gonadotropin-releasing hormone (GnRH) is one of the most influential factors controlling reproduction. Although our understanding of the endocrine regulation of feeding and reproduction in fish is progressing, many gaps still remain, particularly in catfish. Glass catfish (Kryptopterus vitreolus) are freshwater fish known for their natural transparency. In this study, we isolated cDNA encoding reproductive hormones (GnRH1, GnRH2) and appetite regulators (orexin, NPY, and CART) from glass catfish and examined their distribution in various tissues. All peptides had wide distributions across various brain and peripheral tissues, except CART, which was only present in brain. In order to assess whether limited energy supply affects these peptides, we examined the effects of fasting on their brain mRNA expression levels. Fasting increased the expression of both the orexigenic (i.e. orexin and NPY) and anorexigenic (i.e. CART) hormones, and decreased expression levels of GnRH1, but did not affect GnRH2. Overall, our results suggest that fasting affects the expression of peptides involved in both feeding and reproduction, and provides new insights on the endocrine mechanisms that regulate feeding and reproduction in catfish.

Effects of fasting on the gene expression of appetite regulators in three Characiformes with different feeding habits (Gymnocorymbus ternetzi, Metynnis argenteus and Exodon paradoxus).

In order to assess potential interspecific differences in the endocrine mechanisms regulating feeding in Characiformes, we used three fish species with different feeding habits: two Characidae, the omnivore black widow tetra and the carnivore bucktooth tetra, and one Serrasalmidae, the herbivore silver dollar, as models. cDNAs encoding for appetite-regulating peptides (orexin, cocaine and amphetamine regulated transcript CART, cholecystokinin CCK and leptin) were isolated and their tissue distribution examined. The protein sequences of the three species showed most similarities with those of other Characiformes, followed by Cypriniformes and Siluriformes. mRNAs of all four peptides were expressed in the brain. Orexin, CCK and leptin mRNAs were widely distributed in peripheral tissues of all species. CART mRNA displayed a wide peripheral distribution in bucktooth but was predominant in brain in black widow tetra and silver dollar. In order to assess possible interspecific differences in the response to fasting, we compared the expression of these peptides in fed and fasted fish. Fasting induced increases in orexin expression in all species, but decreased brain CART and leptin expressions in silver dollar only. In the intestine, fasting induced a decrease in CCK expression in silver dollar and black widow, and a decrease in leptin expression in bucktooth. Our results suggest that, in Characiformes, different responses of appetite-regulating peptides to fasting are related to both feeding habits and family. The results of this comparative study provide new insights on the regulation of feeding of economically important Characiforme species, which might be valuable for their management and farming.

The effects of fasting and appetite regulators on catecholamine and serotonin synthesis pathways in goldfish (Carassius auratus).

Monoamine neurotransmitters such as catecholamines [dopamine (DA), norepinephrine (NE) and epinephrine (E)] and serotonin have been shown to influence feeding in vertebrates. In order to better understand the role of monoamine neurotransmitters in the regulation of feeding in fish, we examined the effects of fasting on the brain and intestine gene expression of enzymes involved in their synthesis pathways (SPR: sepiapterin reductase; DHPR: dihydropteridine reductase; TH: tyrosine hydroxylase; TPH: tryptophan hydroxylase; AADC: aromatic l-amino acid decarboxylase; DBH: dopamine ß-hydroxylase) in goldfish. In order possible interactions between the monoaminergic pathways and appetite-regulating hormones, we examined the effects of intraperitoneal injections of orexin, CCK and irisin on the brain and intestine gene expression of these enzymes. Fasting increased the expressions of SPR, TH, DBH, TPH1 and DHPR in the brain but did not affect the intestinal expressions of any of the enzymes examined, suggesting that nutritional status might affect the synthesis of monoamines in the central nervous system. CCK injections decreased feeding and increased SPR, TH, and TPH expressions in both brain and intestine. Orexin injections increased feeding and SPR and AADC expressions in the brain but did not affect the expressions of any of the enzymes in the intestine. Irisin injections decreased feeding and increased TPH2 and AADC brain expressions and TH and SPR intestinal expressions, and decreased TPH1 brain expression and AADC intestinal expression. Our results suggest that feeding/fasting and appetite-regulating hormones modulate in part the catecholamine and serotonin synthesis pathways in goldfish.

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.