Fish melanocortin system

Thermal modulation of energy allocation during sex determination in the European sea bass (Dicentrarchus labrax)

Water temperature governs physiological functions such as growth, energy allocation, and sex determination in ectothermic species. The European sea bass (Dicentrarchus labrax) is a major species in European aquaculture, exhibiting early dimorphic growth favoring females. The species has a polygenic sex determination system that interacts with water temperature to determine an individual's sex, with two periods during development that are sensitive to temperature. The current study investigated the influence of water temperature on energy allocation and sex-biased genes during sex determination and differentiation periods. RNA-Sequencing and qPCR analyses were conducted in two separate experiments, of either constant water temperatures typical of aquaculture conditions or natural seasonal thermal regimes, respectively. We focused on eight key genes associated with energy allocation, growth regulation, and sex determination and differentiation. In Experiment 1, cold and warm temperature treatments favored female and male proportions, respectively. The RNA-seq analysis highlighted sex-dependent energy allocation transcripts, with higher levels of nucb1 and pomc1 in future females, and increased levels of egfra and spry1 in future males. In Experiment 2, a warm thermal regime favored females, while a cold regime favored males. qPCR analysis in Experiment 2 revealed that ghrelin and nucb1 were down-regulated by warm temperatures. A significant sex-temperature interaction was observed for pank1a with higher and lower expression for males in the cold and warm regimes respectively, compared to females. Notably, spry1 displayed increased expression in future males at the all-fins stage and in males undergoing molecular sex differentiation in both experimental conditions, indicating that it provides a novel, robust, and consistent marker for masculinization. Overall, our findings emphasize the complex interplay of genes involved in feeding, energy allocation, growth, and sex determination in response to temperature variations in the European sea bass.

Intraperitoneal administration of α-melanocyte stimulating hormone (α-MSH) suppresses food intake and induces anxiety-like behavior via the brain MC4 receptor-signaling pathway in goldfish

α-Melanocyte stimulating hormone (α-MSH) is a peptide hormone released from the intermediate lobe of the pituitary which regulates body pigmentation. In addition to the pituitary, α-MSH is also produced in the midbrain, and exerts both anorexigenic and an anxiogenic actions. Acyl ghrelin and cholecystokinin are peripheral hormones derived from the digestive tract which affect the brain to control food intake and feeding behavior in vertebrates. In the present study, hypothesizing that plasma α-MSH may also stimulate the brain and exert central effects, we examined whether peripherally administered α-MSH affects food intake and psychomotor activity using a goldfish model. Intraperitoneal (IP) administration of α-MSH at 100 pmol g-1 body weight (BW) reduced food consumption and enhanced thigmotaxis. These α-MSH-induced actions were blocked by intracerebroventricular administration of HS024, an antagonist of the melanocortin 4 receptor (MC4R), at 50 pmol g-1 BW, whereas these actions were not attenuated by pretreatment with an IP-injected excess amount of capsaicin, a neurotoxin that destroys primary sensory (vagal and splanchnic) afferents, at 160 nmol g-1 BW. Transcripts for the MC4R showed higher expression in the diencephalon in other regions of the brain. These results suggest that, in goldfish, IP administered α-MSH is taken up by the brain, and also acts as anorexigenic and anxiogenic factor via the MC4R signaling pathway.

Agouti-Induced Anxiety-Like Behavior Is Mediated by Central Serotonergic Pathways in Zebrafish

Overexpression of the agouti-signaling protein (asip1), an endogenous melanocortin antagonist, under the control of a constitutive promoter in zebrafish [Tg(Xla.Eef1a1:Cau.Asip1]iim4] (asip1-Tg) increases food intake by reducing sensitivity of the central satiety systems and abolish circadian activity rhythms. The phenotype also shows increased linear growth and body weight, yet no enhanced aggressiveness in dyadic fights is observed. In fact, asip1-Tg animals choose to flee to safer areas rather than face a potential threat, thus suggesting a potential anxiety-like behavior (ALB). Standard behavioral tests, i.e., the open field test (OFT), the novel object test (NOT), and the novel tank dive test (NTDT), were used to investigate thigmotaxis and ALB in male and female zebrafish. Results showed that the asip1-Tg strain exhibited severe ALB in every test, mainly characterized by pronounced freezing behavior and increased linear and angular swimming velocities. asip1-Tg animals exhibited low central serotonin (5-HT) and dopamine (DA) levels and high turnover rates, thus suggesting that central monoaminergic pathways might mediate melanocortin antagonist-induced ALB. Accordingly, the treatment of asip1-Tg animals with fluoxetine, a selective serotonin reuptake inhibitor (SSRI), reversed the ALB phenotype in NTDT as well as 5-HT turnover. Genomic and anatomical data further supported neuronal interaction between melanocortinergic and serotonergic systems. These results suggest that inhibition of the melanocortin system by ubiquitous overexpression of endogenous antagonist has an anxiogenic effect mediated by serotonergic transmission.

Functional relation of agouti signaling proteins (ASIPs) to pigmentation and color change in the starry flounder, Platichthys stellatus

We investigated the involvement of agouti-signaling proteins (ASIPs) in morphological pigmentation and physiological color change in flatfishes. We isolated ASIP1 and 2 mRNAs from the skin of starry flounder (Platichthys stellatus), and compared their amino acid (aa) structures to those of other animals. Then, we examined the mRNA expression levels of two ASIPs (Sf-ASIPs) in the pigmented ocular body and in the unpigmented blind body, as well as in the ordinary skin and in albino skin, in flatfishes. To investigate the role of Sf-ASIPs in physiological color change (color camouflage), we compared the expression of the two genes in two background colors (dark-green and white). Sf-ASIP1 cDNA had a 375-bp open reading frame (ORF) that encoded a protein consisting of 125 aa residues, and Sf-ASIP2 cDNA had a 402-bp ORF that encoded a protein consisting of 132 aa residues. RT-PCR revealed that the strongest Sf-ASIP1 and Sf-ASIP2 expression levels were observed in the eye and blind-skin, respectively. In Sf-ASIP1, the gene expression did not differ between the ocular-side skin and blind-side skin, nor between ordinary skin and abnormal skin of the fish. However, in Sf-ASIP2, the expression level was significantly higher in blind-side skin, compared to ocular-side skin, suggesting that the ASIP2 gene is related to the countershading body pigment pattern of the fish. In addition, the Sf-ASIP2 gene expression level was lower in the pigmented spot regions than in the unpigmented spot regions of the malpigmented pseudo-albino skins on the ocular side, implying that ASIP2 is responsible for the ocular-side pseudo-albino. Additionally, ASIP2 gene expression in the blind-side skin of ordinary fish was enhanced by a white tank, implying that a bright background color could inhibit hypermelanosis in the blind-side skin of cultured flounder by increasing the activity of the Sf-ASIP2 gene. However, we did not find any relationship of ASIPs with camouflage color changes. In conclusion, the ASIP2 gene is related to the morphological pigmentation (countershading and malpigmentation) of the skin in starry flounder, but not with physiological color changes (color camouflage) in the ocular-side skin.

Role of melanocortin system in the locomotor activity rhythms and melatonin secretion as revealed by agouti-signalling protein (asip1) overexpression in zebrafish

Temporal signals such as light and temperature cycles profoundly modulate animal physiology and behaviour. Via endogenous timing mechanisms which are regulated by these signals, organisms can anticipate cyclic environmental changes and thereby enhance their fitness. The pineal gland in fish, through the secretion of melatonin, appears to play a critical role in the circadian system, most likely acting as an element of the circadian clock system. An important output of this circadian clock is the locomotor activity circadian rhythm which is adapted to the photoperiod and thus determines whether animals are diurnal or nocturnal. By using a genetically modified zebrafish strain known as Tg (Xla.Eef1a1:Cau.asip1)iim04, which expresses a higher level of the agouti signalling protein 1 (Asip1), an endogenous antagonist of the melanocortin system, we observed a complete disruption of locomotor activity patterns, which correlates with the ablation of the melatonin daily rhythm. Consistent with this, in vitro experiments also demonstrated that Asip1 inhibits melatonin secretion from the zebrafish pineal gland, most likely through the melanocortin receptors expressed in this gland. Asip1 overexpression also disrupted the expression of core clock genes, including per1a and clock1a, thus blunting circadian oscillation. Collectively, these results implicate the melanocortin system as playing an important role in modulating pineal physiology and, therefore, circadian organisation in zebrafish.

Visual Perception of Density and Density-Dependent Growth in Medaka (Oryzias latipes): A Suitable Model for Studying Density Effects in Fish

High stocking densities have negative effects on fish. However, the mechanism mediating density perception and growth inhibition is still unknown. This study was conducted to confirm the occurrence of growth inhibition and evaluate changes in growth-related factors in fish reared under high-stocking-density conditions and to determine the role of vision in density perception of medaka. In the graduated-stocking experiment, growth inhibition was clearly observed in fish reared at higher densities, although environmental factors, such as water quality, dissolved oxygen, and feeding conditions, were the same in each experimental group. Differences in growth were observed between the 6-fish and 8-fish groups, indicating that medaka have a superior sense that allows them to accurately perceive the number of individuals in their surroundings. In the pseudo-high stocking experiment, the inner 2-L tank in both groups contained six fish; however, the outer 3-L tank in the pseudo group contained several fish, while that of the control group contained only water. Growth inhibition was observed among the fish in the inner tank of the pseudo group despite having similar spatial density with the control group. These findings suggest that vision is important for density perception. The gene expression of growth-related and metabolic-regulatory hormones decreased in the high-density group. Furthermore, neuropeptide Y expression increased, while pro-opiomelanocortin expression decreased in the high-density group. This study is the first to report that fish can visually perceive density and the resulting growth inhibition, and concluded that medaka is a suitable model for studying density effects and perception in fish.

Unraveling the periprandial changes in brain serotonergic activity and its correlation with food intake-related neuropeptides in rainbow trout Oncorhynchus mykiss

This study explored changes in brain serotonin content and activity together with hypothalamic neuropeptide mRNA abundance around feeding time in rainbow trout, as well as the effect of one-day fasting. Groups of trout fed at two (ZT2) and six (ZT6) hours after lights on were sampled from 90 minutes before to 240 minutes after feeding, while additional groups of non-fed trout were also included in the study. Changes in brain amine and metabolite contents were measured in hindbrain, diencephalon and telencephalon, while in the diencephalon the mRNA abundance of tryptophan hydroxylase (tph1, tph2), serotonin receptors (5htr1a, 5htr1b and 5htr2c) and several neuropeptides (npy, agrp1, cartpt, pomca1, crfb) involved in the control of food intake were also assessed. The results showed changes in the hypothalamic neuropeptides that were consistent with the expected role for each in the regulation of food intake in rainbow trout. Serotonergic activity increased rapidly at the time of food intake in the diencephalon and hindbrain and remained high for much of the postprandial period. This increase in serotonin abundance was concomitant with elevated levels of pomca1 mRNA in the diencephalon, suggesting that serotonin might act on brain neuropeptides to promote a satiety profile. Furthermore, serotonin synthesis and neuronal activity appear to increase already before the time of feeding, suggesting additional functions for this amine before and during food intake. Exploration of serotonin receptors in the diencephalon revealed only small changes for gene expression of 5htr1b and 5htr2c receptors during the postprandial phase. Therefore, the results suggest that serotonin may play a relevant role in the regulation of feeding behavior in rainbow trout during periprandial time, but a better understanding of its interaction with brain centers involved in receiving and processing food-related signals is still needed.

LC-MS/MS based characterisation and differential expression of proteins in Himalayan snow trout, Schizothorax labiatus using LFQ technique

Molecular characterization of fish muscle proteins are nowadays considered as a key component to understand the role of specific proteins involved in various physiological and metabolic processes including their up and down regulation in the organisms. Coldwater fish specimens including snow trouts hold different types of proteins which help them to survive in highly diversified temperatures fluctuating from 0 to 20 °C. So, in current study, the liquid chromatography mass spectrometry using label free quantification technique has been used to investigate the muscle proteome profile of Schizothorax labiatus. For proteomic study, two weight groups of S. labiatus were taken from river Sindh. The proteomic analysis of group 1 revealed that a total of 235 proteins in male and 238 in female fish were recorded. However, when male and female S. labiatus were compared with each other on the basis of spectral count and abundance of peptides by ProteinLynx Global Server software, a total of 14 down-regulated and 22 up-regulated proteins were noted in this group. The highly down-regulated ones included homeodomain protein HoxA2b, retinol-binding protein 4, MHC class II beta chain and proopiomelanocortin while as the highly expressed up-regulated proteins comprised of gonadotropin I beta subunit, NADH dehydrogenase subunit 4, manganese superoxide dismutase, recombinase-activating protein 2, glycosyltransferase, chymotrypsin and cytochrome b. On the other hand, the proteomic characterisation of group 2 of S. labiatus revealed that a total of 227 proteins in male and 194 in female fish were recorded. When male and female S. labiatus were compared with each other by label free quantification, a total of 20 down-regulated and 18 up-regulated proteins were recorded. The down-regulated protein expression of group 2 comprised hepatic lipase, allograft inflammatory factor-1, NADH dehydrogenase subunit 4 and myostatin 1 while the highly expressed up-regulated proteins included glycogen synthase kinase-3 beta variant 2, glycogen synthase kinase-3 beta variant 5, cholecystokinin, glycogen synthase kinase-3 beta variant 3 and cytochrome b. Significant (P < 0.05) difference in the expression of down-regulated and up-regulated proteins was also noted between the two sexes of S. labiatus in each group. According to MS analysis, the proteins primarily concerned with the growth, skeletal muscle development and metabolism were down-regulated in river Sindh, which indicates that growth of fish during the season of collection i.e., winter was slow owing to less food availability, gonad development and low metabolic activity. While, the proteins related to immune response of fish were also noted to be down-regulated thereby signifying that the ecosystem has less pollution loads, microbial, pathogenic and anthropogenic activities. It was also found that the proteins involved in glycogen metabolism, reproductive and metabolic processes, particularly lipid metabolism were up-regulated in S. labiatus. The significant expression of these proteins may be connected to pre-spawning, gonad development and use of stored food as source of energy. The information generated in this study can be applied to future research aimed at enhancing food traceability, food safety, risk management and authenticity analysis.

Agouti-Signalling Protein Overexpression Reduces Aggressiveness in Zebrafish

Feeding motivation plays a crucial role in food intake and growth. It closely depends on hunger and satiation, which are controlled by the melanocortin system. Overexpression of the inverse agonist agouti-signalling protein (ASIP) and agouti-related protein (AGRP) leads to enhanced food intake, linear growth, and weight. In zebrafish, overexpression of Agrp leads to the development of obesity, in contrast to the phenotype observed in transgenic zebrafish that overexpress asip1 under the control of a constitutive promoter (asip1-Tg). Previous studies have demonstrated that asip1-Tg zebrafish exhibit larger sizes but do not become obese. These fish display increased feeding motivation, resulting in a higher feeding rate, yet a higher food ration is not essential in order to grow larger than wild-type (WT) fish. This is most likely attributed to their improved intestinal permeability to amino acids and enhanced locomotor activity. A relationship between high feeding motivation and aggression has been previously reported in some other transgenic species showing enhanced growth. This study aims to elucidate whether the hunger observed in asip1-Tg is linked to aggressive behaviour. Dominance and aggressiveness were quantified using dyadic fights and mirror-stimulus tests, in addition to the analysis of basal cortisol levels. The results indicate that asip1-Tg are less aggressive than WT zebrafish in both dyadic fights and mirror-stimulus tests.

Reproductive phase-dependent and sexually dimorphic expression of leptin and its receptor in different parts of brain of spotted snakehead Channa punctata

The reproductive phase-wise leptin (lep) and its receptor (lepr) expression in different parts of the brain of adult male and female spotted snakehead Channa punctata reveals sexual dimorphism in the brain leptin system. In anterior, middle and posterior parts of the brain of males, a maximum lep was observed in resting, spawning and postspawning reproductive phases, respectively. In females, a high level of lep was seen during the preparatory phase in the anterior brain, preparatory and postspawning phases in the middle brain and resting and postspawning phases in the posterior brain. Nonetheless, the transcript level of lepr was recorded highest during the spawning phase, irrespective of sex and region of the brain. Regardless of the reproductive state of fishes, lep and lepr were seen considerably high in middle and posterior parts of male brain than that of female, implying the involvement of factors other than sex steroids for sex-related variation in the leptin system in these regions of the brain. Nonetheless, no sex difference was evidenced in the expression of either ligand or its receptor in the anterior brain. In summary, the presence of lep and lepr in different regions of the brain and variation in their expression depending on sex and reproductive phases raise the possibility of pivotal actions of leptin in influencing neuronal circuitry and thereby reproductive functions.

Loss-of-function mutations in melanocortin-1 receptor modulate immune response in teleost fishes

The melanocortin system is an ancient neuroendocrine system conserved from teleosts to mammals. The melanocortin system is a set of complex neuroendocrine signaling pathways involved in numerous physiological processes, and particularly associated with the hypothalamic-pituitary-interrenal (HPI) axis response. The melanocortin 1 receptor (MC1R) is the central melanocortin receptor involved in pigmentation in vertebrates, including fish. In order to assess the immune role of MC1R, this study used a homozygous Mc1r knockout zebrafish. Hence, skin cortisol levels, variations in the blood leucocyte population, as well as the expression levels of immune genes in various tissues of wild-type TU strain (Tübingen, Nüsslein-Volhard Lab) (WT) and homozygous mc1r knockout zebrafish (mc1r^K.O.) stimulated with LPS was carried out. Results show that the mc1r^K.O. mutant fish produce lower levels of cortisol in mucus and fewer macrophages in blood after exposure to LPS compared to control fish. Regarding the expression of immune genes, mutant fish show a significant increase in the expression of the anti-inflammatory interleukin il10. These results suggest that the mc1r^K.O. mutant fish may follow an alternative mechanism among the immune responses, where macrophages seem to have an anti-inflammatory function, attenuating nitric oxide (NO) production and providing an advantage through the mitigation of excessive or strong inflammatory reactions. Nonetheless, a lower number of this cell type could imply a reduced phagocytic potential in the face of an infection. At the same time, lower cortisol levels in the mc1r^K.O. mutant fish could be an advantage as for the lower susceptibility to stress and the physiological and metabolic consequences of high cortisol levels.

Role of the melanocortin system in zebrafish skin physiology

The agouti-signaling protein (ASIP) acts as both a competitive antagonist and inverse agonist of melanocortin receptors which regulate dorsal-ventral pigmentation patterns in fish. However, the potential role of ASIP in the regulation of additional physiological pathways in the skin is unknown. The skin plays a crucial role in the immune function, acting as a physical limitation against infestation and also as a chemical barrier due to its ability to synthesize and secrete mucus and many immune effector proteins. In this study, the putative role of ASIP in regulating the immune system of skin has been explored using a transgenic zebrafish model overexpressing the asip1 gene (ASIPzf). Initially, the structural changes in skin induced by asip1 overexpression were studied, revealing that the ventral skin of ASIPzf was thinner than that of wild type (WT) animals. A moderate hypertrophy of mucous cells was also found in ASIPzf. Histochemical studies showed that transgenic animals appear to compensate for the lower number of cell layers by modifying the mucus composition and increasing lectin affinity and mucin content in order to maintain or improve protection against microorganism adhesion. ASIPzf also exhibit higher protein concentration under crowding conditions suggesting an increased mucus production under stressful conditions. Exposure to bacterial lipopolysaccharide (LPS) showed that ASIPzf exhibit a faster pro-inflammatory response and increased mucin expression yet severe skin injures and a slight increase in mortality was observed. Electrophysiological measurements show that the ASIP1 genotype exhibits reduced epithelial resistance, an indicator of reduced tissue integrity and barrier function. Overall, not only are ASIP1 animals more prone to infiltration and subsequent infections due to reduced skin epithelial integrity, but also display an increased inflammatory response that can lead to increased skin sensitivity to external infections.

Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Mutations in melanocortin-4 receptor: From fish to men

Melanocortin-4 receptor (MC4R), expressed abundantly in the hypothalamus, is a critical regulator of energy homeostasis, including both food intake and energy expenditure. Shortly after the publication in 1997 of the Mc4r knockout phenotypes in mice, including increased food intake and severe obesity, the first mutations in MC4R were reported in humans in 1998. Studies in the subsequent two decades have established MC4R mutation as the most common monogenic form of obesity, especially in early-onset severe obesity. Studies in animals, from fish to mammals, have established the conserved physiological roles of MC4R in all vertebrates in regulating energy balance. Drug targeting MC4R has been recently approved for treating morbid genetic obesity. How the MC4R can be exploited for animal production is highly worthy of active investigation.

Atlantic mackerel (Scomber scombrus) change skin colour in response to crowding stress

Wild capture can be stressful for fish. Stress has the potential to induce mortality in released unwanted catches or negative flesh quality consequences in retained ones. Such effects compromise sustainable natural resource management and industry profitability. Mitigating stress during capture is therefore desirable. Biological indicators of stress can objectively inform fishers as to the functional welfare status of catches during fishing operations. If they are to be of practical use in mitigating stress during wild capture events, such indicators must be quantifiable, respond rapidly, reflect the level of induced stress and be easily observable. Atlantic mackerel (Scomber scombrus) are extensively targeted by purse seine fisheries in European waters but are particularly vulnerable to stress. Excessive crowding in the net is thought to be the principal stress mechanism. There is therefore a need to develop indicators of crowding stress for this species so that catch welfare can be improved. Here, we demonstrate that S. scombrus exhibit a skin colour change from predominately green to predominately blue when exposed to crowding stress. In sea cage trials, we induced various degrees of stress in groups of wild-caught S. scombrus by manipulating crowding density and its duration. Skin colour was quantified in air using digital photography. The colour change occurred rapidly (within the typical duration of crowding events in the fishery), and its magnitude was correlated to the severity and duration of crowding. Bluer fish were also associated with higher levels of plasma lactate. No appreciable colour change was observed in uncrowded (control) groups during the treatment period. Nonetheless, unstressed S. scombrus did turn blue <1 h after death. Together, these results indicate that skin colour change has the potential to be a useful real-time indicator of crowding stress for S. scombrus and could therefore be used to improve welfare during wild capture fishing.

Intracerebroventricular administration of α-melanocyte-stimulating hormone (α-MSH) enhances thigmotaxis and induces anxiety-like behavior in the goldfish Carassius auratus

α-Melanocyte-stimulating hormone (α-MSH) is a body pigmentation-regulating hormone secreted from the intermediate lobe of the pituitary in vertebrates. It is also produced in the brain, and acts as an anorexigenic neuropeptide involved in feeding regulation. In rodents, intracerebroventricular (ICV) administration of α-MSH has been shown to affect not only feeding behavior, but also psychomotor activity. However, there is still no information regarding the psychophysiological effects of α-MSH on behavior in fish. Therefore, we examined the effect of synthetic α-MSH on psychomotor activity in goldfish. Since this species prefers the edge to the central area of a tank, we used this as a preference test for assessing psychomotor activity. When α-MSH was administered ICV at 1 and 10 pmol g^-1 body weight (BW), the time spent in the edge area of a tank was prolonged at 10 pmol g^-1 BW. However, α-MSH at these doses did not affect locomotor activity. The action of α-MSH mimicked those of FG-7142 (a central-type benzodiazepine receptor (CBR) inverse agonist with an anxiogenic effect) at 10 pmol g^-1 BW and melanotan II (a melanocortin 4 receptor (MC4R) agonist) at 50 pmol g^-1 BW, whereas ICV administration of tofisopam (a CBR agonist with an anxiolytic effect) at 10 pmol g^-1 BW prolonged the time spent in the central area. The anxiogenic-like effect of α-MSH was abolished by treatment with the MC4R antagonist HS024 at 50 pmol g^-1 BW. These data indicate that α-MSH affects psychomotor activity in goldfish, and exerts an anxiogenic-like effect via the MC4R-signaling pathway.

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.

Probiotics Improve Eating Disorders in Mandarin Fish (Siniperca chuatsi) Induced by a Pellet Feed Diet via Stimulating Immunity and Regulating Gut Microbiota

Eating disorders are directly or indirectly influenced by gut microbiota and innate immunity. Probiotics have been shown to regulate gut microbiota and stimulate immunity in a variety of species. In this study, three kinds of probiotics, namely, Lactobacillus plantarum, Lactobacillus rhamnosus and Clostridium butyricum, were selected for the experiment. The results showed that the addition of three probiotics at a concentration of 10⁸ colony forming unit/mL to the culture water significantly increased the ratio of the pellet feed recipients and survival rate of mandarin fish (Siniperca chuatsi) under pellet-feed feeding. In addition, the three kinds of probiotics reversed the decrease in serum lysozyme and immunoglobulin M content, the decrease in the activity of antioxidant enzymes glutathione and catalase and the decrease in the expression of the appetite-stimulating regulator agouti gene-related protein of mandarin fish caused by pellet-feed feeding. In terms of intestinal health, the three probiotics reduced the abundance of pathogenic bacteria Aeromonas in the gut microbiota and increased the height of intestinal villi and the thickness of foregut basement membrane of mandarin fish under pellet-feed feeding. In general, the addition of the three probiotics can significantly improve eating disorders of mandarin fish caused by pellet feeding.

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.

Central serotonin participates in the anorexigenic effect of GLP-1 in rainbow trout (Oncorhynchus mykiss)

The incretin, glucagon-like peptide-1 (GLP-1) is a major player in the gut-brain axis regulation of energy balance and in fish it seems to exert a negative influence on food intake. In this study, we investigated the role of the brain serotonergic system in the effects promoted by a peripheral GLP-1 injection on food intake in rainbow trout (Oncorhynchus mykiss). For this, in a first experiment the incretin was intraperitoneally injected (100 ng/g body weight) alone or in combination with a 5HT_2C receptor antagonist (SB 242084, 1 µg/g body weight) and food intake was measured 30, 90, and 180 min later. In a second experiment, we studied the effect of these treatments on mRNA abundance of hypothalamic neuropeptides that control food intake. In addition, the effect of GLP-1 on serotonin metabolism was assessed in hindbrain and hypothalamus. Our results show that GLP-1 induced a significant food intake inhibition, which agreed with the increased expression of anorexigenic neuropeptides pomc and cart in the hypothalamus. Furthermore, GLP-1 stimulated the synthesis of serotonin in the hypothalamus, which might be indicative of a higher use of the neurotransmitter. The effects of GLP-1 on food intake were partially reversed when a serotonin receptor antagonist, SB 242084, was previously administered to trout. This antagonist also reversed the stimulatory effect of the hormone in hypothalamic pomca1 mRNA abundance. We conclude that hypothalamic serotonergic pathways are essential for mediating the effects of GLP-1 on food intake in rainbow trout. In addition, the 5HT2C receptor subtype seems to have a prominent role in the inhibition of food intake induced by GLP-1 in this species.

Growth Performance After Agouti-Signaling Protein 1 (Asip1) Overexpression in Transgenic Zebrafish

The melanocortin system is a key structure in the regulation of energy balance. Overexpression of inverse agonists, agouti-signaling protein (ASIP), and agouti-related protein (AGRP) results in increased food intake, linear growth, and body weight. ASIP regulates dorsal-ventral pigment polarity through melanocortin 1 receptor (MC1R) and overexpression induces obesity in mice by binding to central MC4R. Asip1 overexpression in transgenic zebrafish (asip1-Tg) enhances growth, yet experiments show fish overexpressing Asip1 do not develop obesity even under severe feeding regimes. Asip1-Tg fish do not need to eat more to grow larger and faster; thus, increased food efficiency can be observed. In addition, asip1-Tg fish reared at high density are able to grow far more than wild-type (WT) fish reared at low density, although asip1-Tg fish seem to be more sensitive to crowding stress than WT fish, thus making the melanocortin system a target for sustainable aquaculture, especially as the U.S. Food and Drug Association has recently approved transgenic fish trading.

Biased signaling in fish melanocortin-4 receptors (MC4Rs): Divergent pharmacology of four ligands on spotted scat (Scatophagus argus) and grass carp (Ctenopharyngodon idella) MC4Rs

The melanocortin-4 receptor (MC4R) plays a critical role in the regulation of energy homeostasis in both mammals and fish. Several fish MC4Rs recently characterized have high constitutive activities, potentially associated with food intake and growth rate. In the present study, we systematically investigated the effects of four human MC4R (hMC4R) antagonists, including agouti-related peptide (AgRP), Ipsen 5i, ML00253764, and MCL0020, on the cAMP and ERK1/2 signaling of two fish MC4Rs: spotted scat (Scatophagus argus) MC4R (saMC4R) and grass carp (Ctenopharyngodon idella) MC4R (ciMC4R), with hMC4R as a control. We showed that both saMC4R and ciMC4R were constitutively active with significantly increased basal cAMP levels. AgRP acted as an inverse agonist in cAMP signaling pathway in both fish MC4Rs whereas MCL0020 functioned as an inverse agonist for ciMC4R but a weak neutral antagonist for saMC4R. Ipsen 5i and MCL0020 behaved as neutral allosteric modulators in the cAMP signaling of fish MC4Rs. The saMC4R and ciMC4R had similar basal pERK1/2 levels as hMC4R and the pERK1/2 levels of the two fish MC4Rs were significantly increased upon stimulation with all four ligands. In summary, our studies demonstrated the existence of biased signaling in fish MC4R. We also showed dramatic pharmacological differences of human and fish MC4Rs with synthetic ligands. Our data provided novel insights and led to a better understanding of fish MC4R pharmacology.

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Experimental manipulation of perceived predation risk and cortisol generates contrasting trait trajectories in plastic crucian carp

Most animals constitute potential prey and must respond appropriately to predator-mediated stress in order to survive. Numerous prey also adaptively tailor their response to the prevailing level of risk and stress imposed by their natural enemies, i.e. they adopt an inducible defence strategy. Predator exposure may activate the stress axis, and drive the expression of anti-predator traits that facilitate survival in a high-risk environment (the predation-stress hypothesis). Here, we quantified two key morphological anti-predator traits, body morphology and coloration, in crucian carp reared in the presence or absence of a predator (pike) in addition to experimental manipulation of physiological stress via implants containing either cortisol or a cortisol inhibitor. We found that predator-exposed fish expressed a deeper-bodied phenotype and darker body coloration as compared with non-exposed individuals. Skin analyses revealed that an increase in the amount of melanophores caused the dramatic colour change in predator-exposed fish. Increased melanization is costly, and the darker body coloration may act as an inducible defence against predation, via a conspicuous signal of the morphological defence or by crypsis towards dark environments and a nocturnal lifestyle. By contrast, the phenotype of individuals carrying cortisol implants did not mirror the phenotype of predator-exposed fish but instead exhibited opposite trajectories of trait change: a shallow-bodied morphology with a lighter body coloration as compared with sham-treated fish. The cortisol inhibitor did not influence the phenotype of fish i.e. neither body depth nor body coloration differed between this group and predator-exposed fish with a sham implant. However, our results illuminate a potential link between stress physiology and morphological defence expression.

Diet-Induced Growth Is Regulated via Acquired Leptin Resistance and Engages a Pomc-Somatostatin-Growth Hormone Circuit

Anorexigenic pro-opiomelanocortin (Pomc)/alpha-melanocyte stimulating hormone (αMSH) neurons of the hypothalamic melanocortin system function as key regulators of energy homeostasis, also controlling somatic growth across different species. However, the mechanisms of melanocortin-dependent growth control still remain ill-defined. Here, we reveal a thus-far-unrecognized structural and functional connection between Pomc neurons and the somatotropic hypothalamo-pituitary axis. Excessive feeding of larval zebrafish causes leptin resistance and reduced levels of the hypothalamic satiety mediator pomca. In turn, this leads to reduced activation of hypophysiotropic somatostatin (Sst)-neurons that express the melanocortin receptor Mc4r, elevated growth hormone (GH) expression in the pituitary, and enhanced somatic growth. Mc4r expression and αMSH responsiveness are conserved in Sst-expressing hypothalamic neurons of mice. Thus, acquired leptin resistance and attenuation of pomca transcription in response to excessive caloric intake may represent an ancient mechanism to promote somatic growth when food resources are plentiful.

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.

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.

Pharmacological modulation of melanocortin-4 receptor by melanocortin receptor accessory protein 2 in Nile tilapia

The melanocortin-4 receptor (MC4R) acts as a member of G-protein coupled receptors and participate in food intake and energy expenditure. Melanocortin 2 receptor accessory protein 2 (MRAP2) plays a critical role in regulating MC4R signaling in mammals and zebrafish. However, evidence on their interaction in other teleost species remains elusive. Here, we cloned and assessed the evolutionary aspect and pharmacological modulation of MRAP2 on MC4R signaling in Nile tilapia (Oreochromis niloticus). Tissue distribution analysis of tmc4r and tmrap2 confirmed their co-expression in the brain region. tMRAP2 protein could form antiparallel homo-dimer and directly interacted with tMC4R in vitro and presence of tMRAP2 led to the reduction of agonist response and surface expression of tMC4R. Overall, our findings provide a comparative overview on the evolutionary conservation, genomic distribution, tissue-specific expression and pharmacological profile of the MC4R and MRAP2 in another non-mammalian teleost.

Analyzing the signaling properties of gar (Lepisosteus oculatus) melanocortin receptors: Evaluating interactions with MRAP1 and MRAP2

RT-PCR analysis of gar pituitary and brain indicated that different combinations of gar melanocortin receptor mRNAs are present in the same tissues with mRNAs for gar mrap1 and gar mrap2. Against this background, an objective of this study was to determine whether the ligand sensitivity for either ACTH or α-MSH was affected when gar (g) melanocortin receptors (Mcrs) were co-expressed with either of the accessory proteins gMrap1 or gMrap2 in Chinese Hamster Ovary cells. The results indicated that gMc2r has an obligatory requirement for co-expression with gMrap1 in order for the receptor to be activated by hACTH(1-24). In addition, activation of gMc2r did not occur when the receptor was expressed alone or co-expressed with gMrap2. Furthermore, co-expression of gMc2r with gMrap1 followed by stimulation with NDP-MSH resulted in a low level of activation (only at 10-7 M and 10-6 M). However, gMc1r, gMc3r, gMc4r, and gMc5r responded to stimulation by NDP-MSH in a more robust manner. Co-expression of gMc1r, gMc3r, gMc4r, and gMc5r with gMRAP1 had no effect on sensitivity to stimulation by NDP-MSH or hACTH(1-24). Co-expression with gMRAP2 had no negative or positive effect on ligand sensitivity for gMc1r, gMc3r, and gMc5r, however this treatment did increase the activation of CHO cells transfected with gMc4r following stimulation with both hACTH(1-24) (p < 0.001), and NDP-MSH (p < 0.001). Co-expression of gMC5R with either gMRAP1 or gMRAP2 increased trafficking of gMC5R to the plasma membrane. These pharmacological observations are compared to the response of melanocortin receptors from other neopterygian fishes, cartilaginous fishes, and tetrapods to stimulation by ACTH(1-24) and forms of α-MSH.

Evolution of proopiomelanocortin

Proopiomelanocortin (POMC) belongs to the opioid/orphanin gene family whose peptide precursors include either opioid (YGGF) or the orphanin/nociceptin core sequences (FGGF). In addition to POMC the family includes the proenkephalin (PENK), prodynorphin (PDYN), and nociceptin/proorphanin (PNOC) precursors. The opioid core sequence in POMC is incorporated by the β-endorphin that occupies the C-terminal region but this propeptide also exhibits at least two "alien" melanocortin core sequences (HFRW). An ACTH/MSH fragment merged into the opioid fragment not earlier than the two tetraploidizations of the vertebrate genome. Therefore, POMC exhibit a complex "evolutionary life" since the gene has coevolved together with two different receptor systems, i.e., opioid and melanocortin following a horse trading system. In this article, we summarize the different evolutionary hypotheses proposed for POMC evolution.

Melanocortin Receptor Accessory Protein 2-Induced Adrenocorticotropic Hormone Response of Human Melanocortin 4 Receptor

Melanocortin 4 receptor (MC4R), a canonical melanocyte-stimulating hormone receptor, is the main responsible for monogenic obesity in humans. Previous studies in fish and avian species showed that MC4R becomes an ACTH receptor after interaction with the melanocortin receptor accessory protein 2 (MRAP2). We show that human MC4R behaves in a similar way through its interaction with MRAP2. This evolutionary conservation of MRAP2-induced ligand selectivity supports a physiological role for the interaction with MC4R. Both proteins are coexpressed in the same hypothalamic neurons, providing an anatomical substrate and molecular mechanism for the central therapeutic actions of ACTH in the treatment of infantile spasms. These neurons may link the effects of stress on the energy balance independently of glucocorticoid secretion. The complex MC4R-MRAP2 throws light on the action of ACTH and, by extension, on the relay of stress-related information to additional biological systems.

Molecular cloning, tissue distribution, and pharmacological characterization of blunt snout bream (Megalobrama amblycephala) melanocortin-5 receptor

The melanocortin-5 receptor (MC5R) plays an important role in the regulation of exocrine secretion in mammals. Its function in fish is not well established. In this study, we reported the molecular cloning, tissue expression, and pharmacological characterization of Megalobrama amblycephala MC5R (MamMC5R), as well as the effect of catching stress on its expression. The full-length cDNA of Mammc5r gene was 1237 bp, consisted of a 990-bp open reading frame encoding 329 amino acids. Sequence analyses revealed that the nucleotide and amino acid sequences of Mammc5r were highly homologous (> 90%) with MC5Rs of zebrafish, common carp, and goldfish. Tissue expression profile analysis showed that Mammc5r was widely expressed in both central and peripheral tissues, with the highest expression in the brain. Catching stress significantly changed the expression of Mammc5r in the skin, brain, and eye. In the skin, the expression level of Mammc5r was significantly reduced at 1 h and 4 h and increased at 24 h after catching stress. The Mammc5r expression level was rapidly upregulated in the brain and eye at 1 h and then decreased to the level before stress at 4 h and 24 h. With human MC5R (HsaMC5R) as a control, several agonists including α-melanocyte-stimulating hormone (α-MSH), and β-MSH in addition to an analogue [Nle⁴, D-Phe⁷]-α-MSH (NDP-MSH), were used to investigate the binding and signaling properties of MamMC5R. The results revealed that MamMC5R had the highest affinity for NDP-MSH, followed by α- and β-MSH. Taken together, the data suggested that MamMC5R might play a role in stress response in M. amblycephala.

Sensing Glucose in the Central Melanocortin Circuits of Rainbow Trout: A Morphological Study

In mammals, glucosensing markers reside in brain areas known to play an important role in the control of food intake. The best characterized glucosensing mechanism is that dependent on glucokinase (GK) whose activation by increased levels of glucose leads in specific hypothalamic neurons to decreased or increased activity, ultimately leading to decreased food intake. In fish, evidence obtained in recent years suggested the presence of GK-like immunoreactive cells in different brain areas related to food intake control. However, it has not been established yet whether or not those neuronal populations having glucosensing capacity are the same that express the neuropeptides involved in the metabolic control of food intake. Therefore, we assessed through dual fluorescent in situ hybridization the possible expression of GK in the melanocortinergic neurons expressing proopiomelanocortin (POMC) or agouti-related protein (AGRP). POMC and AGRP expression localized exclusively in the rostral hypothalamus, in the ventral pole of the lateral tuberal nucleus, the homolog of the mammalian arcuate nucleus. Hypothalamic GK expression confined to the ependymal cells coating the ventral pole of the third ventricle but some expression level occurred in the AGRP neurons. GK expression seems to be absent in the hypothalamic POMC neurons. These results suggest that AGRP neurons might sense glucose directly through a mechanism involving GK. In contrast, POMC neurons would not directly respond to glucose through GK and would require presynaptic inputs to sense glucose. Ependymal cells could play a critical role relying glucose metabolic information to the central circuitry regulating food intake in fish, especially in POMC neurons.

Effects of Lactogen 13, a New Probiotic Preparation, on Gut Microbiota and Endocrine Signals Controlling Growth and Appetite of Oreochromis niloticus Juveniles

In the present study, Nile tilapia Oreochromis niloticus was used as experimental model to study the molecular effects of a new probiotic preparation, Lactogen 13 (Lactobacillus rhamnosus IMC 501® encapsulated with vegetable fat matrices by spray chilling and further indicated as probiotic microgranules), on growth and appetite during larval development. Probiotic microgranules were administered for 30 days to tilapia larvae starting from first feeding. Molecular analysis using high-throughput sequencing revealed that the probiotic could populate the gastrointestinal tract and modulate the microbial communities by significantly increasing the proportion of Lactobacillus as well as reducing the proportion of potential pathogens such as members of the Family Microbacteriaceae, Legionellaceae, and Weeksellaceae. Morphometric analysis evidenced that body weight and total length significantly increased after probiotic treatment. This increase coincided with the modulation of genes belonging to the insulin-like growth factors (igfs) system and genes involved on myogenesis, such as myogenin, and myogenic differentiation (myod). Alongside the improvement of growth, an increase of feed intake was evidenced at 40 days post-fertilization (dpf) in treated larvae. Gene codifying for signals belonging to the most prominent systems involved in appetite regulation, such as neuropeptide y (npy), agouti-related protein (agrp), leptin, and ghrelin were significantly modulated. These results support the hypothesis that gastrointestinal (GI) microbiota changes due to probiotic administration modulate growth and appetite control, activating the endocrine system of tilapia larvae.

Melatonin treatment suppresses appetite genes and improves adipose tissue plasticity in diet-induced obese zebrafish

PURPOSE

Overweight and obesity are important risk factors for diabetes, cardiovascular diseases, and premature death in modern society. Recently, numerous natural and synthetic compounds have been tested in diet-induced obese animal models, to counteract obesity. Melatonin is a circadian hormone, produced by pineal gland and extra-pineal sources, involved in processes which have in common a rhythmic expression. In teleost, it can control energy balance by activating or inhibiting appetite-related peptides. The study aims at testing effects of melatonin administration to control-fed and overfed zebrafish, in terms of expression levels of orexigenic (Ghrelin, orexin, NPY) and anorexigenic (leptin, POMC) genes expression and morphometry of visceral and subcutaneous fat depots.

METHODS

Adult male zebrafish (n = 56) were divided into four dietary groups: control, overfed, control + melatonin, overfed + melatonin. The treatment lasted 5 weeks and BMI levels of every fish were measured each week. After this period fishes were sacrificed; morphological and morphometric studies have been carried out on histological sections of adipose tissue and adipocytes. Moreover, whole zebrafish brain and intestine were used for qRT-PCR.

RESULTS

Our results demonstrate that melatonin supplementation may have an effect in mobilizing fat stores, in increasing basal metabolism and thus in preventing further excess fat accumulation. Melatonin stimulates the anorexigenic and inhibit the orexigenic signals.

CONCLUSIONS

It seems that adequate melatonin treatment exerts anti-obesity protective effects, also in a diet-induced obesity zebrafish model, that might be the result of the restoration of many factors: the final endpoint reached is weight loss and stabilization of weight gain.

Expression of genes for melanotropic peptides and their receptors for morphological color change in goldfish Carassius auratus

To evaluate the association of the melanotropic peptides and their receptors for morphological color change, we investigated the effects of changes in background color, between white and black, on xanthophore density in the scales and expression levels of genes for hormonal peptides and corresponding receptors (MCH-R2, MC1R, and MC5R) in goldfish (Carassius auratus). The xanthophore density in both dorsal and ventral scales increased after transfer from a white to black background. However, xanthophore density in dorsal scales increased after transfer from a black to white background, and that of ventral scales decreased after transfer from a black to black background, which served as the control. In the white-reared fish, melanin-concentrating hormone (mch) mRNA content in the brain was higher than that in black-reared fish, whereas proopiomelanocortin a (pomc-a) mRNA content in the pituitary was lower than that in the black-reared fish. Agouti-signaling protein (asp) mRNA was detected in the ventral skin but not in the dorsal skin. No difference was observed in the asp mRNA content between fish reared in white or black background, suggesting that ASP might not be associated with background color adaptation. In situ hybridization revealed that both mc1r and mc5r were expressed in the xanthophores in scales. The mRNA content of mc1r in scales did not always follow the background color change, whereas those of mc5r decreased in the white background and increased in the black background, suggesting that mc5r might be a major factor reinforcing the function of MSH in morphological color changes. White backgrounds increased mch mRNA content in the brain, but decreased mch-r2 mRNA content in the scales. These altered expression levels of melanotropin receptors might affect reactivity to melanotropins through long-term adaptation to background color.

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.

Neuropeptidomic Analysis of Zebrafish Brain

A wide variety of bioactive peptides are present in all metazoan species where they govern diverse functions as small messenger molecules. In the last 15 years, mass spectrometry-based methods have identified endogenous peptides in diverse species. Mass spectrometry enables the precise peptide sequences to be determined, including the potential existence of truncated versions or the presence of post-translational modifications. Because small modifications can have a large effect on biological activity, knowledge of the actual peptide sequences paves the way for further functional studies such as analysis of neuropeptidergic signaling cascades. Zebrafish (Danio rerio) is an important animal model that is commonly used in a wide range of studies. Here we provide a detailed description of the peptide extraction procedure and peptidomics workflow for zebrafish.

Developmental evolution of the forebrain in cavefish, from natural variations in neuropeptides to behavior

The fish Astyanax mexicanus comes in two forms: the normal surface-dwelling and the blind depigmented cave-adapted morphs. Comparing the development of their basal forebrain, we found quantitative differences in numbers of cells in specific clusters for six out of nine studied neuropeptidergic cell types. Investigating the origins of these differences, we showed that early Shh and Fgf signaling impact on the development of NPY and Hypocretin clusters, via effect on Lhx7 and Lhx9 transcription factors, respectively. Finally, we demonstrated that such neurodevelopmental evolution underlies behavioral evolution, linking a higher number of Hypocretin cells with hyperactivity in cavefish. Early embryonic modifications in signaling/patterning at neural plate stage therefore impact neuronal development and later larval behavior, bridging developmental evolution of a neuronal system and the adaptive behavior it governs. This work uncovers novel variations underlying the evolution and adaptation of cavefish to their extreme environment.

The melanocortin system regulates body pigmentation and social behaviour in a colour polymorphic cichlid fish

The melanocortin system is a neuroendocrine system that regulates a range of physiological and behavioural processes. We examined the extent to which the melanocortin system simultaneously regulates colour and behaviour in the cichlid fish Astatotilapia burtoni We found that yellow males are more aggressive than blue males, in line with previous studies. We then found that exogenous α-melanocyte-stimulating hormone (α-MSH) increases yellowness of the body and dispersal of xanthophore pigments in both morphs. However, α-MSH had a morph-specific effect on aggression, with only blue males showing an increase in the rate of aggression. Exogenous agouti signalling peptide (ASIP), a melanocortin antagonist, did not affect coloration but reduced the rate of aggression in both colour morphs. Blue males had higher cortisol levels than yellow males. Neural gene expression of melanocortin receptors (mcr) and ligands was not differentially regulated between colour morphs. In the skin, however, mc1r and pro-opiomelanocortin (pomc) β were upregulated in blue males, while asip 1 was upregulated in yellow males. The effects of α-MSH on behaviour and body coloration, combined with morph-specific regulation of the stress response and the melanocortin system, suggest that the melanocortin system contributes to the polymorphism in behaviour and coloration in A. burtoni.

Stress Effects on the Mechanisms Regulating Appetite in Teleost Fish

The homeostatic regulation of food intake relies on a complex network involving peripheral and central signals that are integrated in the hypothalamus which in turn responds with the release of orexigenic or anorexigenic neuropeptides that eventually promote or inhibit appetite. Under stress conditions, the mechanisms that control food intake in fish are deregulated and the appetite signals in the brain do not operate as in control conditions resulting in changes in the expression of the appetite-related neuropeptides and usually a decreased food intake. The effect of stress on the mechanisms that regulate food intake in fish seems to be mediated in part by the corticotropin-releasing factor (CRF), an anorexigenic neuropeptide involved in the activation of the HPI axis during the physiological stress response. Furthermore, the melanocortin system is also involved in the connection between the HPI axis and the central control of appetite. The dopaminergic and serotonergic systems are activated during the stress response and they have also been related to the control of food intake. In addition, the central and peripheral mechanisms that mediate nutrient sensing capacity and hence implicated in the metabolic control of appetite are inhibited in fish under stress conditions. Finally, stress also affects peripheral endocrine signals such as leptin. In the present minireview, we summarize the knowledge achieved in recent years regarding the interaction of stress with the different mechanisms that regulate food intake in fish.

The interaction of MC3R and MC4R with MRAP2, ACTH, α-MSH and AgRP in chickens

The interaction of melanocortin-4 (MC4R) and melanocortin-3 (MC3R) receptors with proopiomelanocortin (POMC)-derived peptides (e.g. α-MSH), agouti-related protein (AgRP) and melanocortin-2 receptor accessory protein 2 (MRAP2) is suggested to play critical roles in energy balance of vertebrates. However, evidence on their interaction in birds remains scarce. Our study aims to reveal their interaction in chickens and the results showed that (1) chicken (c-)MC3R and cMC4R expressed in Chinese hamster ovary (CHO) cells can be activated by α-MSH and ACTH_1-39 equipotently, monitored by a pGL3-CRE-luciferase reporter system; (2) cMC3R and cMC4R, when co-expressed with cMRAP2 (or cMRAP, a cMRAP2 homolog), show increased sensitivity to ACTH treatment and thus likely act as ACTH-preferring receptors, and the interaction between cMC3R/cMC4R and cMRAP2 was demonstrated by co-immunoprecipitation assay; (3) both cMC3R and cMC4R display constitutive activity when expressed in CHO cells, as monitored by dual-luciferase reporter assay, and cMRAP2 (and cMRAP) can modulate their constitutive activity; (4) AgRP inhibits the constitutive activity of cMC3R/cMC4R, and it also antagonizes ACTH/α-MSH action on cMC4R/cMC3R, indicating that AgRP functions as the inverse agonist and antagonist for both receptors. These findings, together with the co-expression of cMC4R, cMC3R, cMRAP2, cAgRP and cPOMC in chicken hypothalamus detected by quantitative real-time PCR, suggest that within the hypothalamus, α-MSH/ACTH, AgRP and MRAP2 may interact at the MC4R(/MC3R) interface to control energy balance. Furthermore, our data provide novel proof for the involvement of MRAP2 (and MRAP) in fine-tuning the constitutive activity and ligand sensitivity and selectivity of both MC3R and MC4R in vertebrates.

Distribution and female reproductive state differences in orexigenic and anorexigenic neurons in the brain of the mouth brooding African cichlid fish, Astatotilapia burtoni

Integration of reproduction and metabolism is necessary for species survival. While the neural circuits controlling energy homeostasis are well-characterized, the signals controlling the relay of nutritional information to the reproductive axis are less understood. The cichlid fish Astatotilapia burtoni is ideal for studying the neural regulation of feeding and reproduction because females cycle between a feeding gravid state and a period of forced starvation while they brood developing young inside their mouths. To test the hypothesis that candidate neuropeptide-containing neurons known to be involved in feeding and energy homeostasis in mammals show conserved distribution patterns, we performed immunohistochemistry and in situ hybridization to localize appetite-stimulating (neuropeptide Y, NPY; agouti-related protein, AGRP) and appetite-inhibiting (cocaine and amphetamine-regulated transcript, CART; pro-opiomelanocortin, pomc1a) neurons in the brain. NPY, AGRP, CART, and pomc1a somata showed distribution patterns similar to other teleosts, which included localization to the lateral tuberal nucleus (NLT), the putative homolog of the mammalian arcuate nucleus. Gravid females also had larger NPY and AGRP neurons in the NLT compared to brooding females, but brooding females had larger pomc1a neurons compared to gravid females. Hypothalamic agrp mRNA levels were also higher in gravid compared to brooding females. Thus, larger appetite-stimulating neurons (NPY, AGRP) likely promote feeding while females are gravid, while larger pomc1a neurons may act as a signal to inhibit food intake during mouth brooding. Collectively, our data suggest a potential role for NPY, AGRP, POMC, and CART in regulating energetic status in A. burtoni females during varying metabolic and reproductive demands.

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.

α-MSH and melanocortin receptors at early ontogeny in European sea bass (Dicentrarchus labrax, L.)

Temporal patterns of whole-body α-MSH concentrations and of transcripts of melanocortin receptors during early development as well as the endocrine response (α-MSH, cortisol, MCR mRNAs) to stress at the end of the larval period were characterized in Dicentrarchus labrax. Immunohistochemistry showed α-MSH positive cells in the pituitary pars intermedia in all stages examined. As development proceeds, α-MSH content gradually increases; mRNA levels of mc2r and mc4r remain low until first feeding where peak values are observed. Mc1r expression was constant during development, pomc mRNA levels remain low until the stage of flexion after which a significant increase is observed. At the stage of the formation of all fins, whole-body cortisol and α-MSH concentrations responded with peak values at 2 h post stress. Additionally, the stress challenge resulted in elevated transcript levels of pomc, mc2r and mc4r but not in mc1r, with a pattern characterized by peak values at 1 h post stress and a strong correlation with whole body α-MSH concentrations was found. Our data provide for the first time a view on the importance of the α-MSH stress response in early development of European sea bass, an additional and relatively poorly understood signal involved in the stress response in teleosts.

Molecular cloning, tissue distribution, and pharmacological characterization of melanocortin-4 receptor in grass carp (Ctenopharyngodon idella)

Melanocortin-4 receptor (MC4R) plays a pivotal role in the mediation of leptin action on food intake and energy expenditure in mammals. The MC4R has also been identified in several teleosts, and its importance in the regulation of fish energy homeostasis is emerging. We herein reported on the molecular cloning, tissue distribution, and pharmacological characterization of MC4R in grass carp (Ctenopharyngodon idella), an economically and ecologically important fish. We showed that grass carp MC4R (ciMC4R) consisted of a 981 bp open reading frame encoding a protein of 326 amino acids, highly homologous (>95%) to several teleost MC4Rs. Phylogenetic and synteny analysis further indicated ciMC4R was closely related to piscine MC4Rs. Using reverse transcription PCR, we found that mc4r messenger RNA was expressed in the brain as well as various peripheral tissues in grass carp. The pharmacological properties of ciMC4R were investigated using 4 agonists, including α-melanocyte stimulating hormone (α-MSH), β-MSH, [Nle⁴, D-Phe⁷]-MSH (NDP-MSH), and adrenocorticotropic hormone (ACTH). We showed that all 4 ligands could bind to ciMC4R and initiate dose-dependent intracellular cyclic adenosine monophosphate (cAMP) accumulation. Grass carp MC4R had the highest affinity for NDP-MSH. Both NDP-MSH and ACTH (1-24) exhibited higher potencies compared to the other 2 endogenous agonists. The ciMC4R was constitutively active, with significantly increased basal cAMP level compared with that of human MC4R (P < 0.01). The availability of ciMC4R and its pharmacologic characteristics provide a basis for future investigation of its functional roles in regulating diverse physiological processes and novel insights into understanding the mechanism of food habit transition in grass carp.