Ghrelin increases food intake, swimming activity and growth in juvenile brown trout (Salmo trutta)

Side-Stream Based Marine Solubles From Atlantic Cod (Gadus morhua) Modulate Appetite and Dietary Nutrient Utilization in Atlantic Salmon (Salmo salar L.) and can Replace Fish Meal

Whitefish fisheries' side-stream biomass is an abundant underutilized resource that can be valorized to benefit future aquaculture sustainability. Four novel ingredients based on side-streams from Atlantic cod (Gadus morhua) fileting were produced. FM-hb, a fish meal (FM), and FPH-hb, a fish protein hydrolysate based on heads (h) and backbones (b); FM-hbg, a FM based on heads, backbones, and viscera/guts (g); and FPC-g, a fish protein concentrate based on viscera preserved in formic acid. Four diets were prepared containing one of the ingredients replacing 50% of the dietary FM protein, in addition to a positive (FM10) and a negative (FM5) control. The six diets were fed to triplicate tanks with Atlantic salmon (Salmo salar L.; 113 ± 1 g) over 8 weeks. Besides general performance, gut and brain gene expression for selected hormones and key neuropeptides involved in the control of appetite and digestive processes were studied during feeding and postprandial, and possible reference levels for Atlantic salmon were established. All side-stream-added diets performed well, with no significant differences in performance and biometrics between the treatments. Some gene expression differences were observed, but no well-defined patterns emerged supporting clear dietary effects related to digestive performance or appetite. However, in the brain, a short-time upregulation of agouti-related protein-1 (agrp1), corresponded to higher cumulative feed intake (FI) for the FM10 diet supporting notions that this may be a candidate biomarker for appetite in salmon. Expression of stomach ghrelin-1 (ghrl1) was higher than ghrelin-2 (ghrl2) and membrane-bound O-acyltransferase domain-containing 4 (mboat4), and midgut peptide YYa-2 (pyya2) and glucagon-a (gcga) were higher than peptide YYb-1 (pyyb1). A comparison showed that midgut peptide YYa-1 (pyya1), pyya2, and gcga expressions were higher than in the hindgut, which is opposite of what is found in mammals. In conclusion, this study shows that sustainable side-stream raw materials with different characteristics can partly replace high-quality commercial FMs giving similar performance.

The influence of ghrelin agonist ipamorelin acetate on the hypothalamic-pituitary-testicular axis in a cichlid fish, Oreochromis mossambicus

Ghrelin, a peptide found in the brain and gut, is predicted to play a significant role in the control of various physiological systems in fish. The objective of this study was to examine the impact of ipamorelin acetate (IPA), a ghrelin agonist, on the reproductive axis of the tilapia Oreochromis mossambicus. The administration of either 5 or 30 µg of IPA for 21 days led to a significant and dose-dependent rise in food intake concomitant with a significant increase in the numbers of primary spermatocytes, secondary spermatocytes, and early spermatids compared to the control group. There was a significant rise in the number of late spermatids, as well as the areas of the lobule and lumen, in fish treated with 30 µg of IPA, compared to the control group. Moreover, there was no significant difference in the percentage of gonadotropin-releasing hormone (GnRH)-immunoreactive fibres in the hypothalamus and anterior pituitary gland across different groups. However, a significant elevation in the expression of androgen receptor protein was observed in fish treated with 30 µg of IPA. Furthermore, the concentrations of luteinizing hormone (LH) and 11-ketotestosterone (11-KT) in the serum of fish treated with either 5 or 30 µg of IPA were significantly elevated in comparison to the control group. Collectively, these findings suggest that the administration of ghrelin enhances the development of germ cells during the meiosis-I phase and that this effect might be mediated via the stimulation of 11-KT and androgen receptors at the testicular level and LH at the pituitary level in the tilapia.

Characterizing Growth-Retarded Japanese Eels (Anguilla japonica): Insights into Metabolic and Appetite Regulation

During field surveys and culture procedures, large growth disparities in Anguilla japonica have been observed. However, the potential causes are unknown. This study explored differences in digestive ability, metabolic levels, and transcriptomic profiles of appetite-related genes between growth-retarded eel (GRE) and normal-growing eel (NGE) under the same rearing conditions. The results showed that growth hormone (gh) mRNA expression in GREs was considerably lower than NGEs. The levels of total protein (TP), total cholesterol (T-CHO), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), blood ammonia (BA), blood urea nitrogen (BUN), and alkaline phosphatase (ALP) in GREs were significantly lower than in NGEs. Conversely, levels of glucose (GLU), alanine aminotransferase (ALT), and aspartate transaminase (AST) were higher in GREs. The activities of SOD, CAT, and T-AOC levels were also significantly lower in GREs, as were the activities of glucose-related enzymes including hexokinase (HK), pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6PASE). Additionally, orexigenic genes (npy and ghrelin) were dramatically downregulated, whereas anorexigenic genes (crh and pyy) were significantly upregulated in GREs. These findings suggested that variances in growth hormone, metabolic activities, and appetite level could be associated with the different growth rates of A. japonica. The present research not only revealed the characteristics of the growth, metabolism, and appetite of GREs but also offered new perspectives into the substantial growth discrepancies in A. japonica, providing novel ideas for enhancing fish growth.

Hepatopancreas Transcriptome Analysis of Spinibarbus sinensis to Reveal Different Growth-Related Genes

Spinibarbus sinensis, also known as Qingbo, is an important economic fish in China. However, the detailed mechanisms underlying its growth are still unknown. To excavate the genes and signaling pathways related to its growth, we compared the transcriptome profiles of the hepatopancreas tissues of S. sinensis, with two groups of growth rate for evaluation. An average of 66,304,909 and 68,739,585 clean reads were obtained in the fast growth (FG) and slow growth (SG) group, respectively. The differential gene expression analysis results showed that 272 differentially expressed genes (DEGs) were screened between the FG and SG groups, including 101 up-regulated genes and 171 down-regulated genes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis results showed that GO terms related to metabolic process, organic substance metabolic process, and catalytic activity were enriched, pathway signals related to steroid biosynthesis and protein digestion and absorption were also detected. Meanwhile, the potential key regulatory genes sst2, fndc4, and cckra related to the growth of S. sinensis were screened. Reverse transcript fluorescence quantitative PCR (RT-qPCR) validation of 18 DEGs associated with growth differences showed that the RT-qPCR results were consistent with RNA-seq analysis, and nine genes, stk31, gpr149, angptl1, fstl1, sik1, ror2, nlrc3, pdlim2, and nav2 were significantly expressed in the FG group. bmp1, stc1, gpatch8, sstrt2, s100a1, ktf6, cckar6, sync1, bhlha15, a total of nine genes were significantly expressed in the SG group. This study provides basic information for improving the growth characteristics of S. sinensis and the functional research of candidate genes.

The Interplay between Central and Peripheral Systems in Feed Intake Regulation in European Seabass (Dicentrarchus labrax) Juveniles

The present study aimed to evaluate the effects of feeding or feed deprivation on the orexigenic and anorexigenic responses at the central (whole brain) and peripheral (anterior and posterior intestine, stomach, and liver) system levels in European seabass. For this purpose, a group of fish (208 g) was fed a single meal daily for 8 days (fed group) and another group was feed-deprived for 8 days (unfed group). Compared to the fed group, in the whole brain, feed deprivation did not induce changes in npy, agrp1, and cart2 expression, but increased agrp2 and pomc1 expression. In the anterior intestine, feed deprivation increased cck expression, while in the posterior intestine, the npy expression increased and pyyb decreased. In the stomach, the ghr expression decreased regardless of the feeding status. The hepatic lep expression increased in the unfed fish. The present results suggest a feed intake regulation mechanism in European seabass similar to that observed in other teleosts.

The movement ecology of fishes

Movement of fishes in the aquatic realm is fundamental to their ecology and survival. Movement can be driven by a variety of biological, physiological and environmental factors occurring across all spatial and temporal scales. The intrinsic capacity of movement to impact fish individually (e.g., foraging) with potential knock-on effects throughout the ecosystem (e.g., food web dynamics) has garnered considerable interest in the field of movement ecology. The advancement of technology in recent decades, in combination with ever-growing threats to freshwater and marine systems, has further spurred empirical research and theoretical considerations. Given the rapid expansion within the field of movement ecology and its significant role in informing management and conservation efforts, a contemporary and multidisciplinary review about the various components influencing movement is outstanding. Using an established conceptual framework for movement ecology as a guide (i.e., Nathan et al., 2008: 19052), we synthesized the environmental and individual factors that affect the movement of fishes. Specifically, internal (e.g., energy acquisition, endocrinology, and homeostasis) and external (biotic and abiotic) environmental elements are discussed, as well as the different processes that influence individual-level (or population) decisions, such as navigation cues, motion capacity, propagation characteristics and group behaviours. In addition to environmental drivers and individual movement factors, we also explored how associated strategies help survival by optimizing physiological and other biological states. Next, we identified how movement ecology is increasingly being incorporated into management and conservation by highlighting the inherent benefits that spatio-temporal fish behaviour imbues into policy, regulatory, and remediation planning. Finally, we considered the future of movement ecology by evaluating ongoing technological innovations and both the challenges and opportunities that these advancements create for scientists and managers. As aquatic ecosystems continue to face alarming climate (and other human-driven) issues that impact animal movements, the comprehensive and multidisciplinary assessment of movement ecology will be instrumental in developing plans to guide research and promote sustainability measures for aquatic resources.

Ghrelin-Ghrelin receptor (GSHR) pathway via endocannabinoid signal affects the expression of NPY to promote the food intake of Siberian sturgeon (Acipenser baerii)

Previous data suggested that activation of endocannabinoid receptor 1 (CB1) was necessary for the orexigenic effect of Ghrelin in rodents, but the information is limited in teleosts. To investigate the feeding regulation pathway of Ghrelin and CB1 in Siberian sturgeon (Acipenser baerii), this study first identified the Ghrelin (345 bp, complete coding sequence) and Ghrelin receptor (GHSR, 500 bp, partial coding sequence) sequences, and then detected their tissue distribution patterns, which showed that Ghrelin is mainly distribution in peripheral tissues, while GSHR is mainly in different brain divisions. Besides, the qPCR before and after feeding showed that the mRNA expressions of Ghrelin and GHSR were inhibited after feeding in telencephalon, diencephalon and mesencephalon. Subsequently, the food intake and appetite factor expressions were measured by i.c.v. co-injection of Ghrelin and GSHR antagonist. The results showed that Ghrelin promoted the food intake of Siberian sturgeon, which was reversed by its receptor antagonist. Besides, i.c.v. injection of Ghrelin decreased telencephalon CART expression while increased NPY expression in the three brain regions. In addition, to further explore the relationship of Ghrelin and CB1 signal regulating feeding, the co-injection of Ghrelin and CB1 antagonists was performed. The results showed that AM6545 (CB1 peripheral restricted antagonist) failed to affect the orexigenic effect of Ghrelin and the expression pattern of NPY mRNA in the telencephalon. While in the diencephalon, the increase of food intake and NPY mRNA expression induced by Ghrelin was completely reversed by Rimonabant (CB1 global antagonist). These results indicate Ghrelin-GSHR pathway promotes the food intake of Siberian sturgeon by inducing the expression of NPY in the diencephalon, and the stimulating effect will be reversed by cannabinoid receptor antagonism. This study provides a foundation for understanding the pathways Ghrelin and CB1 signals in appetite regulation of the teleost.

Feeding frequency and dietary protein/carbohydrate ratio affect feed intake and appetite regulation-related genes expression in gilthead seabream (Sparus aurata)

To evaluate the effects of feeding frequency (FF) and dietary protein/carbohydrate (P/CH) ratios on appetite regulation of gilthead seabream, two practical diets were formulated to include high protein and low carbohydrate (P50/CH10 diet) or low protein and high carbohydrate (P40/CH20 diet) content and each diet was fed to triplicate groups of fish until visual satiation each meal at a FF of 1, 2, or 3 meals per day. Feed intake and feed conversion ratio were higher in fish fed 2 or 3 meals than 1 meal per day and in fish fed the P40/CH20 than the P50/CH10 diet. The specific growth rate was only affected by FF, being higher in fish fed 2 or 3 meals per day than 1 meal per day. Expression of the cocaine-amphetamine-related transcript, corticotropin-releasing hormone, ghrelin receptor-a (ghsr-a), leptin, and neuropeptide y in the brain, cholecystokinin (cck) in the intestine, and leptin and ghrelin in the stomach was not affected by FF or dietary P/CH ratio. This is the first time that ghrelin cells were immune-located in the stomach of gilthead seabream. Fish fed 3 meals per day presented lower cck expression in the brain than those fed twice per day and higher hepatic ghsr-b expression than those fed once per day. Fish fed P40/CH20 diet presented higher hepatic leptin expression than those fed P50/CH10 diet. In conclusion, present results indicate that feeding a P40/CH20 diet at 3 meals a day seems to decrease the satiation feeling of gilthead seabream compared to fish fed higher P/CH ratio diets or fed 1 or 2 meals a day.

Coordinate regulation of feeding, metabolism, and growth: Perspectives from studies in fish

This paper develops a model for coordinate regulation of feeding, metabolism, and growth based on studies in fish. Many factors involved with the control of feeding [e.g., cholecystokinin (CCK) and ghrelin (GRLN)], energy metabolism [e.g., insulin (INS), glucagon (GLU), glucagon-like peptide (GLP), and somatostatins (SS), produced in the endocrine pancreas; and leptin (LEP) produced broadly], and growth [e.g., GRLN, growth hormone (GH), insulin-like growth factors (IGFs), GH receptors (GHR), IGF receptors (IGFR)] interact at various levels. Many such interactions serve to coordinate these systems to favor anabolic processes (i.e., lipid and protein synthesis, glycogenesis) and growth, including GH promotion of feeding and stimulation of INS production/secretion and the upregulation of GHR and IGFR by GRLN. As nutrient and stored energy status change, various feedbacks serve to curtail feeding and transition the animal from an anabolic/growth state to a catabolic state. Many factors, including LEP and IGF, promote satiety, whereas SS downregulates INS signaling as well as IGF production and GHR and IGFR abundance. As INS and IGF levels fall, GH becomes disconnected from growth as a result of altered linkage of GHR to cell signaling pathways. As a result, the catabolic actions of GH, GLU, GLP, LEP, and SS prevail, mobilizing stored energy reserves. Coordinate regulation involves relative abundances of blood-borne hormones as well as the ability to adjust responsiveness to hormones (via receptor and post-receptor events) in a cell-/tissue-specific manner that results from genetic and epigenetic programming and modulation by the local milieu of hormones, nutrients, and autocrine/paracrine interactions. The proposed model of coordinate regulation demonstrates how feeding, metabolism, and growth are integrated with each other and with other processes, such as reproduction, and how adaptive adjustments can be made to energy allocation during an animal's life history and/or in response to changes in environmental conditions.

Gilthead seabream (Sparus aurata) in vitro adipogenesis and its endocrine regulation by leptin, ghrelin, and insulin

Leptin, ghrelin, and insulin influence lipid metabolism and thus can directly affect adipose tissue characteristics, modulating the organoleptic quality of aquaculture fish. The present study explored gilthead seabream (Sparus aurata) cultured preadipocytes development, and the regulation of adipogenesis by those three hormones. Preadipocytes presented a fibroblast-like phenotype during the proliferation phase that changed to round-shaped with an enlarged cytoplasm filled with lipid droplets after complete differentiation, confirming the characteristics of mature adipocytes. peroxisome proliferator-activated receptor-γ (pparγ) expression was higher at the beginning of the culture, while fatty acid synthase and 3-hydroxyacyl-CoA dehydrogenase gradually increased with cell maturation. The expression of lipoprotein lipase-like, lysosomal acid lipase (lipa), fatty acid translocase/cluster of differentiation-36 (cd36), and leptin receptor (lepr) were not affected during cell culture development; and undetectable expression levels were observed for leptin. Concerning regulation, leptin inhibited lipid accumulation significantly reducing pparγ and cd36 gene expression, both in early differentiating and mature adipocytes, while ghrelin decreased the expression of pparγ in the early differentiating phase but did not reduce intracellular lipid content significantly. Additional insulin past the onset of adipogenesis did not affect lipid accumulation either. In conclusion, at present culture conditions leptin has an anti-adipogenic function in differentiating preadipocytes of gilthead seabream and continues exerting this role in mature adipocytes, while ghrelin and insulin do not seem to influence adipogenesis progression. A better understanding of leptin, ghrelin, and insulin impact on the adipogenic process could help in the prevention of fat accumulation, improving aquaculture fish production and quality.

Expression of ghrelin and leptin in the chemosensory system of adult zebrafish

Numerous data show that the chemosensory system seems to be modulated by changes in the circulating levels of different molecules such as ghrelin, orexin, leptin, NPY, CCK. The chemosensory system of the zebrafish is represented by the taste buds (skin, oral and oropharyngeal), the olfactory rosette and the solitary chemosensorial cells (SCCs). The purpose of our study was to analyze the distribution of two peripheral hormones such as ghrelin and leptin in the chemosensory organs of the zebrafish. Our results demonstrated the presence of immunoreaction for all antibodies used in the zebrafish chemosensory organs even if with different distribution. In particular, IR was observed for ghrelin in the olfactory rosette while IR for leptin was found in the olfactory rosette, in the skin and oropharyngeal taste buds and in the gills. Both these hormones were detected in the intestine, used as a control.

Progress in understanding the roles of Urocortin3 (UCN3) in the control of appetite from studies using animal models

Urocortin3 (UCN3), the newest member of corticotrophin releasing hormone (CRH) family polypeptides, is an anorexic factor discovered in 2001, which has a strong inhibitory effect on animal appetite regulation. UCN3 is widely distributed in various tissues of animals and has many biological functions. Based on the research progress of UCN3 on mammals and non-mammals, this paper summarized the discovery, tissue distribution, appetite regulation and mechanism of UCN3 in animals, in order to provide a reference for feeding regulation and growth in mammals and fish in further research and production.

Effects of CCK-8 and GLP-1 on fatty acid sensing and food intake regulation in trout

We hypothesize that cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1) are involved in the modulation of metabolic regulation of food intake by fatty acids in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with 1 ng/g of CCK-8 and with 2 ng/g of GLP-1 on food intake, expression of neuropeptides involved in food intake control and the activity of fatty acid-sensing systems in hypothalamus and hindbrain. Food intake decreased up to 24 h post-treatment to 49.8-72.3% and 3.1-17.8% for CCK-8 and GLP-1, respectively. These anorectic responses are associated with changes in fatty acid metabolism and an activation of fatty acid-sensing mechanisms in the hypothalamus and hindbrain. These changes occurred in parallel with those in the expression of anorexigenic and orexigenic peptides. Moreover, we observed that the activation of fatty acid sensing and the enhanced anorectic potential elicited by CCK-8 and GLP-1 treatments occurred in parallel with the activation of mTOR and FoxO1 and the inhibition of AMPKα, BSX and CREB. The results are discussed in the context of metabolic regulation of food intake in fish.

Fasting Upregulates npy, agrp, and ghsr Without Increasing Ghrelin Levels in Zebrafish (Danio rerio) Larvae

Food intake in fish and mammals is orchestrated by hypothalamic crosstalk between orexigenic (food intake stimulation) and anorexigenic (food intake inhibition) signals. Some of these signals are released by peripheral tissues that are associated with energy homeostasis or nutrient availability. During the fish larva stage, orexigenic stimulation plays a critical role in individual viability. The goal of this study was to assess the mRNA levels of the main neuropeptides involved in food intake regulation (npy, agrp, carppt, and pomc), in concert with the mRNA levels and peptide levels of ghrelin, under a fasting intervention at the larval stage in zebrafish (Danio rerio). Prior to the fasting intervention, the zebrafish larva cohort was reared for 20 days post fertilization (dpf) and then randomly divided into two groups of 20 individuals. One group was subjected to a fasting intervention for 5 days (fasted group), and the other group was fed normally (fed group); this experimental protocol was performed twice independently. At the end of the fasting period, individuals from each experimental group were divided into different analysis groups, for evaluations such as relative gene expression, immunohistochemistry, and liquid chromatography coupled to nano high-resolution mass spectrometry (nLC-HRMS) analyses. The relative expression levels of the following genes were assessed: neuropeptide Y (npy), agouti-related peptide (agrp), proopiomelanocortin (pomc), cocaine and amphetamine-regulated transcript (cartpt), ghrelin (ghrl), ghrelin O-acyltransferase (mboat4), growth hormone secretagogue receptor (ghsr), and glucokinase (gck). In the fasted group, significant upregulation of orexigenic peptides (npy - agrp) and ghsr was observed, which was associated with significant downregulation of gck. The anorexigenic peptides (pomc and cartpt) did not show any significant modulation between the groups, similar to mboat4. Contrary to what was expected, the relative mRNA upregulation of the orexigenic peptides observed in the fasted experimental group could not be associated with significant ghrelin modulation as assessed by three different approaches: qPCR (relative gene expression of ghrelin), nLC-HRMS (des-acyl-ghrelin levels), and immunohistochemistry (integrated optical density of prepropeptides in intestinal and hepatopancreas tissues). Our results demonstrate that zebrafish larvae at 25 dpf exhibit suitable modulation of the relative mRNA levels of orexigenic peptides (npy and agrp) in response to fasting intervention; nevertheless, ghrelin was not coregulated by fasting. Therefore, it can be suggested that ghrelin is not an essential peptide for an increase in appetite in the zebrafish larva stage. These results give rise to new questions about food intake regulation factors in the early stages of fish.

The anorectic effect of central PYY1-36 treatment in rainbow trout (Oncorhynchus mykiss) is associated with changes in mRNAs encoding neuropeptides and parameters related to fatty acid sensing and metabolism

We hypothesized that peptide YY (PYY) is involved in the metabolic regulation of food intake in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with 10 ng/g PYY_1-36 on food intake, expression of neuropeptides involved in food intake control, and the activity of fatty acid-sensing systems. The administration of PYY_1-36 caused a significant reduction in food intake up to 24 h post-treatment. This anorectic action was associated with changes 2 h after treatment in mRNA abundance of neuropeptides involved in metabolic regulation of food intake in hypothalamus (decreased NPY and raised CART values) and hindbrain (increased POMCa1 values). We also observed that PYY_1-36 treatment induced changes in mRNA abundance of parameters related to fatty acid sensing and metabolism in hypothalamus (decreased values of ACLY, PPARγ, and SREBP1c) and hindbrain (increased values of LPL, FAT/CD36, PPARα, PPARγ, and SREBP1c and decreased values of UCP2a). PYY_1-36 treatment also increased mRNA abundance of mTOR. In general, it seems that mRNAs encoding some components of the machinery required for fatty acid sensing and metabolism are activated by PYY_1-36. The response observed was higher in the hindbrain than in the hypothalamus, supporting the greater importance of this brain area in mediating the modulatory effects of gastrointestinal hormones on feeding regulation.

Brain morphometric profiles and their seasonal modulation in fish (Liza aurata) inhabiting a mercury contaminated estuary

Mercury (Hg) is a potent neurotoxicant known to induce important adverse effects on fish, but a deeper understanding is lacking regarding how environmental exposure affects the brain morphology and neural plasticity of specific brain regions in wild specimens. In this work, it was evaluated the relative volume and cell density of the lateral pallium, hypothalamus, optic tectum and molecular layer of the cerebellum on wild Liza aurata captured in Hg-contaminated (LAR) and non-contaminated (SJ) sites of a coastal system (Ria de Aveiro, Portugal). Given the season-related variations in the environment that fish are naturally exposed, this assessment was performed in the winter and summer. Hg triggered a deficit in cell density of hypothalamus during the winter that could lead to hormonal dysfunctions, while in the summer Hg promoted larger volumes of the optic tectum and cerebellum, indicating the warm period as the most critical for the manifestation of putative changes in visual acuity and motor-dependent tasks. Moreover, in fish from the SJ site, the lateral pallium relative volume and the cell density of the hypothalamus and optic tectum were higher in the winter than in summer. Thus, season-related stimuli strongly influence the size and/or cell density of specific brain regions in the non-contaminated area, pointing out the ability of fish to adapt to environmental and physiological demands. Conversely, fish from the Hg-contaminated site showed a distinct seasonal profile of brain morphology, presenting a larger optic tectum in the summer, as well as a larger molecular layer of the cerebellum with higher cell density. Moreover, Hg exposure impaired the winter-summer variation of the lateral pallium relative size (as observed at SJ). Altogether, seasonal variations in fish neural morphology and physiology should be considered when performing ecotoxicological studies in order to better discriminate the Hg neurotoxicity.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

Ghrelin and Its Receptors in Gilthead Sea Bream: Nutritional Regulation

Ghrelin is involved in the regulation of growth in vertebrates through controlling different functions, such as feed intake, metabolism, intestinal activity or growth hormone (Gh) secretion. The aim of this work was to identify the sequences of preproghrelin and Ghrelin receptors (ghsrs), and to study their responses to different nutritional conditions in gilthead sea bream (Sparus aurata) juveniles. The structure and phylogeny of S. aurata preproghrelin was analyzed, and a tissue screening was performed. The effects of 21 days of fasting and 2, 5, 24 h, and 7 days of refeeding on plasma levels of Ghrelin, Gh and Igf-1, and the gene expression of preproghrelin, ghsrs and members of the Gh/Igf-1 system were determined in key tissues. preproghrelin and the receptors are well conserved, being expressed mainly in stomach, and in the pituitary and brain, respectively. Twenty-one days of fasting resulted in a decrease in growth while Ghrelin plasma levels were elevated to decrease at 5 h post-prandial when pituitary ghsrs expression was minimum. Gh in plasma increased during fasting and slowly felt upon refeeding, while plasma Igf-1 showed an inverse profile. Pituitary gh expression augmented during fasting reaching maximum levels at 1 day post-feeding while liver igf-1 expression and that of its splice variants decreased to lowest levels. Liver Gh receptors expression was down-regulated during fasting and recovered after refeeding. This study demonstrates the important role of Ghrelin during fasting, its acute down-regulation in the post-prandial stage and its interaction with pituitary Ghsrs and Gh/Igf-1 axis.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Appetite-Controlling Endocrine Systems in Teleosts

Mammalian studies have shaped our understanding of the endocrine control of appetite and body weight in vertebrates and provided the basic vertebrate model that involves central (brain) and peripheral signaling pathways as well as environmental cues. The hypothalamus has a crucial function in the control of food intake, but other parts of the brain are also involved. The description of a range of key neuropeptides and hormones as well as more details of their specific roles in appetite control continues to be in progress. Endocrine signals are based on hormones that can be divided into two groups: those that induce (orexigenic), and those that inhibit (anorexigenic) appetite and food consumption. Peripheral signals originate in the gastrointestinal tract, liver, adipose tissue, and other tissues and reach the hypothalamus through both endocrine and neuroendocrine actions. While many mammalian-like endocrine appetite-controlling networks and mechanisms have been described for some key model teleosts, mainly zebrafish and goldfish, very little knowledge exists on these systems in fishes as a group. Fishes represent over 30,000 species, and there is a large variability in their ecological niches and habitats as well as life history adaptations, transitions between life stages and feeding behaviors. In the context of food intake and appetite control, common adaptations to extended periods of starvation or periods of abundant food availability are of particular interest. This review summarizes the recent findings on endocrine appetite-controlling systems in fish, highlights their impact on growth and survival, and discusses the perspectives in this research field to shed light on the intriguing adaptations that exist in fish and their underlying mechanisms.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.

Unveiling the neurotoxicity of methylmercury in fish (Diplodus sargus) through a regional morphometric analysis of brain and swimming behavior assessment

The current study aims to shed light on the neurotoxicity of MeHg in fish (white seabream - Diplodus sargus) by the combined assessment of: (i) MeHg toxicokinetics in the brain, (ii) brain morphometry (volume and number of neurons plus glial cells in specific brain regions) and (iii) fish swimming behavior (endpoints associated with the motor performance and the fear/anxiety-like status). Fish were surveyed for all the components after 7 (E7) and 14 (E14) days of dietary exposure to MeHg (8.7μgg^-1), as well as after a post-exposure period of 28days (PE28). MeHg was accumulated in the brain of D. sargus after a short time (E7) and reached a maximum at the end of the exposure period (E14), suggesting an efficient transport of this toxicant into fish brain. Divalent inorganic Hg was also detected in fish brain along the experiment (indicating demethylation reactions), although levels were 100-200 times lower than MeHg, which pinpoints the organic counterpart as the great liable for the recorded effects. In this regard, a decreased number of cells in medial pallium and optic tectum, as well as an increased hypothalamic volume, occurred at E7. Such morphometric alterations were followed by an impairment of fish motor condition as evidenced by a decrease in the total swimming time, while the fear/anxiety-like status was not altered. Moreover, at E14 fish swam a greater distance, although no morphometric alterations were found in any of the brain areas, probably due to compensatory mechanisms. Additionally, although MeHg decreased almost two-fold in the brain during post-exposure, the levels were still high and led to a loss of cells in the optic tectum at PE28. This is an interesting result that highlights the optic tectum as particularly vulnerable to MeHg exposure in fish. Despite the morphometric alterations reported in the optic tectum at PE28, no significant changes were found in fish behavior. Globally, the effects of MeHg followed a multiphasic profile, where homeostatic mechanisms prevented circumstantially morphometric alterations in the brain and behavioral shifts. Although it has become clear the complexity of matching brain morphometric changes and behavioral shifts, motor-related alterations induced by MeHg seem to depend on a combination of disruptions in different brain regions.

Is really endogenous ghrelin a hunger signal in chickens? Association of GHSR SNPs with increase appetite, growth traits, expression and serum level of GHRL, and GH

Chicken growth hormone secretagogue receptor (GHSR) is a receptor for ghrelin (GHRL), a peptide hormone produced by chicken proventriculus, which stimulates growth hormone (GH) release and food intake. The purpose of this study was to search for single nucleotide polymorphisms (SNPs) in exon 2 of GHSR gene and to analyze their effect on the appetite, growth traits and expression levels of GHSR, GHRL, and GH genes as well as serum levels of GH and GHRL in Mandara chicken. Two adjacent SNPs, A239G and G244A, were detected in exon 2 of GHSR gene. G244A SNP was non-synonymous mutation and led to replacement of lysine amino acid (aa) by arginine aa, while A239G SNP was synonymous mutation. The combined genotypes of A239G and G244A SNPs produced three haplotypes; GG/GG, GG/AG, AG/AG, which associated significantly (P<0.05) with growth traits (body weight, average daily gain, shank length, keel length, chest circumference) at age from >4 to 16w. Chickens with the homozygous GG/GG haplotype showed higher growth performance than other chickens. The two SNPs were also correlated with mRNA levels of GHSR and GH (in pituitary gland), and GHRL (in proventriculus and hypothalamus) as well as with serum level of GH and GHRL. Also, chickens with GG/GG haplotype showed higher mRNA and serum levels. This is the first study to demonstrate that SNPs in GHSR can increase appetite, growth traits, expression and level of GHRL, suggesting a hunger signal role for endogenous GHRL.

Intracerebroventricular ghrelin treatment affects lipid metabolism in liver of rainbow trout (Oncorhynchus mykiss)

We aimed to elucidate in rainbow trout (Oncorhynchus mykiss) the effects of central ghrelin (GHRL) treatment on the regulation of liver lipid metabolism, and the possible modulatory effect of central GHRL treatment on the simultaneous effects of raised levels of oleate. Thus, we injected intracerebroventricularly (ICV) rainbow trout GHRL in the presence or absence of oleate and evaluated in liver variables related to lipid metabolism. Oleate treatment elicited in liver of rainbow trout decreased lipogenesis and increased oxidative capacity in agreement with previous studies. Moreover, as demonstrated for the first time in fish in the present study, GHRL also acts centrally modulating lipid metabolism in liver, resulting in increased potential for lipogenesis and decreased potential for fatty acid oxidation, i.e. the converse effects to those elicited by central oleate treatment. The simultaneous treatment of GHRL and oleate confirmed these counteractive effects. Thus, the nutrient sensing mechanisms present in hypothalamus, particularly those involved in sensing of fatty acid, are involved in the control of liver energy metabolism in fish, and this control is modulated by the central action of GHRL. These results give support to the notion of hypothalamus as an integrative place for the regulation of peripheral energy metabolism in fish.

Ghrelin modulates hypothalamic fatty acid-sensing and control of food intake in rainbow trout

There is no information available on fish as far as the possible effects of ghrelin on hypothalamic fatty acid metabolism and the response of fatty acid-sensing systems, which are involved in the control of food intake. Therefore, we assessed in rainbow trout the response of food intake, hypothalamic fatty acid-sensing mechanisms and expression of neuropeptides involved in the control of food intake to the central treatment of ghrelin in the presence or absence of a long-chain fatty acid such as oleate. We observed that the orexigenic actions of ghrelin in rainbow trout are associated with changes in fatty acid metabolism in the hypothalamus and an inhibition of fatty acid-sensing mechanisms, which ultimately lead to changes in the expression of anorexigenic and orexigenic peptides resulting in increased orexigenic potential and food intake. Moreover, the response to increased levels of oleate of hypothalamic fatty acid-sensing systems (activation), expression of neuropeptides (enhanced anorexigenic potential) and food intake (decrease) were counteracted by the simultaneous treatment with ghrelin. These changes provide evidence for the first time in fish of a possible modulatory role of ghrelin on the metabolic regulation by fatty acid of food intake occurring in the hypothalamus.

Sex biased expression of ghrelin and GHSR associated with sexual size dimorphism in yellow catfish

Sexual size dimorphism has been observed in many cultivable fish species including yellow catfish, in which male fish grow much faster than female fish. Ghrelin is a potent stimulator of pituitary growth hormone (GH) release and known to potentially promote food intake and body weight gain. In order to investigate the molecular mechanism of sexual size dimorphism in yellow catfish (Pelteobagrus fulvidraco), ghrelin and its functional receptor, growth hormone secretagogue receptor (GHSR) cDNAs were cloned. Real-time PCR indicated that both ghrelin and GHSR were more highly expressed in hypothalamus and gut of male fish than female. During normal larval development, expression of ghrelin and GHSR genes was significantly higher in males than in females. 17a-Methyltestosterone (MT) treatment enhanced the expression of ghrelin in female larval fish and GHSR in both sexes, whereas the expression of ghrelin in male larval fish increased in the beginning, then decreased as the treatment time prolonged. Furthermore, the expression of ghrelin and GHSR in male juvenile was significantly increased compared with female juvenile, in short and long term fasting periods, suggesting that male fish may have a better appetite than female during fasting. Our results demonstrate that sex difference in the expression of ghrelin and GHSR may be involved in sexual size dimorphism by regulating feeding and GH/IGF signaling in yellow catfish.

Cloning, tissue distribution and effects of fasting on mRNA expression levels of leptin and ghrelin in red-bellied piranha (Pygocentrus nattereri)

cDNAs encoding the appetite regulating peptides leptin and ghrelin were isolated in red-bellied piranha (Characiforme, Serrasalmidae) and their mRNA tissue and brain distributions examined. When compared to other fish, the sequences obtained for all peptides were most similar to that of other Characiforme fish and Siluriformes. All peptides were widely expressed within the brain and in several peripheral tissues, including gastrointestinal tract. In order to better understand the role of these peptides in the regulation of feeding of red-bellied piranha, the mRNA expression levels of leptin and ghrelin were examined in both brain and intestine, in fed and 7-day fasted fish. No significant differences in expression were seen in whole brain for either peptide. Within the intestine, there was a decrease in leptin mRNA expression and an increase in ghrelin mRNA expression in fasted fish, compared to fed fish. The results suggest that leptin and ghrelin might play a major role in the regulation of feeding and energy homeostasis of red-bellied piranha and this role might be more prominent in the intestine than in the brain.

Comparative analysis reveals loss of the appetite-regulating peptide hormone ghrelin in falcons

Ghrelin and leptin are key peripherally secreted appetite-regulating hormones in vertebrates. Here we consider the ghrelin gene (GHRL) of birds (class Aves), where it has been reported that ghrelin inhibits rather than augments feeding. Thirty-one bird species were compared, revealing that most species harbour a functional copy of GHRL and the coding region for its derived peptides ghrelin and obestatin. We provide evidence for loss of GHRL in saker and peregrine falcons, and this is likely to result from the insertion of an ERVK retrotransposon in intron 0. We hypothesise that the loss of anorexigenic ghrelin is a predatory adaptation that results in increased food-seeking behaviour and feeding in falcons.

Contribution of glucose- and fatty acid sensing systems to the regulation of food intake in fish. A review

Food intake in fish is a complex process regulated through many different factors including abundance of energy and nutrients. In recent years, evidence have been obtained in several fishes, mainly in rainbow trout, regarding the presence and functioning in brain areas of metabolic sensors informing about changes in the levels of nutrients like glucose and fatty acids. The activity of these sensors relate to the control of food intake through changes in the expression of anorexigenic and orexigenic neuropeptides. The present review will provide a picture of the main results obtained to date in these studies, as well as perspectives for future research in the field.

Crosstalking between the "gut-brain" hormone ghrelin and the circadian system in the goldfish. Effects on clock gene expression and food anticipatory activity

Ghrelin is a potent orexigenic signal mainly synthesized in the stomach and foregut of vertebrates. Recent studies in rodents point out that ghrelin could also act as an input for the circadian system and/or as an output of peripheral food-entrainable oscillators, being involved in the food anticipatory activity (FAA). In this study we pursue the possible interaction of ghrelin with the circadian system in a teleost, the goldfish (Carassius auratus). First, we analyzed if ghrelin is able to modulate the core clock functioning by regulating clock gene expression in fish under a light/dark cycle 12L:12D and fed at 10 am. As expected the acute intraperitoneal (IP) injection of goldfish ghrelin (gGRL[1-19], 44 pmol/g bw) induced the expression of hypothalamic orexin. Moreover, ghrelin also induced (∼ 2-fold) some Per clock genes in hypothalamus and liver. This effect was partially counteracted in liver by the ghrelin antagonist ([D-Lys(3)]-GHRP-6, 100 pmol/g bw). Second, we investigated if ghrelin is involved in daily FAA rhythms. With this aim locomotor activity was studied in response to IP injections (5-10 days) of gGRL[1-19] and [D-Lys(3)]-GHRP-6 at the doses above indicated. Ghrelin and saline injected fish showed similar 24h activity patterns. However, ghrelin antagonist treatment abolished the FAA in schedule fed fish under 24h light, suggesting the involvement of the endogenous ghrelin system in this pre-feeding activity. Altogether these results suggest that ghrelin could be acting as an input for the entrainment of the food-entrainable oscillators in the circadian organization of goldfish.