The Melanocortin System in Atlantic Salmon (Salmo salar L.) and Its Role in Appetite Control

Identification of Three POMCa Genotypes in Largemouth Bass (Micropterus salmoides) and Their Differential Physiological Responses to Feed Domestication

Diverse feeding habits in teleosts involve a wide range of appetite-regulating factors. As an appetite-suppressing gene, the polymorphisms of POMCa in largemouth bass (Micropterus salmoides) were validated via sequencing and high-resolution melting (HRM). The frequency distribution of different POMCa genotypes were analyzed in two populations, and physiological responses of different POMCa genotypes to feed domestication were investigated. The indel of an 18 bp AU-rich element (ARE) in the 3' UTR and four interlocked SNP loci in the ORF of 1828 bp of POMCa cDNA sequence were identified in largemouth bass and constituted three genotypes of POMC-A I, II, and III, respectively. POMC-A I and Allele I had increased frequencies in the selection population than in the non-selection population (p < 0.01), 63.55% vs. 43.33% and 0.7850 vs. 0.6778, respectively. POMC-A I possessed the lowest value of POMCa mRNA during fasting (p < 0.05) and exhibited growth and physiological advantages under food deprivation and refeeding according to the levels of body mass and four physiological indicators, i.e., cortisol (Cor), growth hormone (GH), insulin-like growth factor-1 (IGF-1), and glucose (Glu). The identification of three POMCa genotypes, alongside their varying physiological responses during feed domestication, suggests a selective advantage that could be leveraged in molecular marker-assisted breeding of largemouth bass that are adapted to feeding on formula diet.

Enhancing growth, antioxidant capacity, and immune response in tilapia (Oreochromis niloticus) through curcumin supplementation across varied stocking density paradigms

The present study evaluated the effects of curcumin on growth, immune and antioxidant response in tilapia (Oreochromis niloticus). An optimum dose of curcumin was investigated by feeding four different levels of this compound in combination with three different regimes of stocking density (12 treatments). Fish were reared at three densities; low density (LD = 1.50 kg/m3), medium density (MD = 3.00 kg/m3), and high density (HD = 4.50 kg/m3). Each treatment was fed with four different levels of dietary supplementation of curcumin (C0 = 0 mg/kg, C1 = 50 mg/kg, C2 = 100 mg/kg, and C3 = 150 mg/kg) for 60 days. Each treatment has three replicates (n = 50/replicate in LD, 100/replicate in MD, 150/ replicate in HD). Although better growth was observed in MD, however treatments at all densities fed with C1 diet showed improved growth as compared to other diets. Chemical composition of fish and activity of amylase, lipase and protease in all treatments were noted to be similar. Levels of antioxidant enzymes (catalase, superoxide dismutase and glutathione peroxidase) and cortisol in MD and HD treatments were similar to those in LD treatment. However, fish fed with C1 diet in each density treatment showed the lowest values of antioxidant enzymes. Similarly, the levels of malondialdehyde were noted to be similar in MD and HD treatments as compared to that in LD. Its levels were lower in fish fed with C1 and C3 diets in all density treatments. Expression of pro-opiomelanocortin-α (POMC-α), Somatostatins-1 (SST-1) and Interleukin 1-β (IL-1β) did not increase in MD and HD treatments in response to high stocking density when compared with LD treatment. The lowest levels of these genes were noted in fish fed with C2 and C3 diets in all treatments. In conclusion, supplementation of curcumin in diet of tilapia improved growth and antioxidant response in tilapia. optimum dose of curcumin for tilapia culture is 50 mg/kg at the density of 3.00 kg/m3which might be further investigated for intensive culture.

Assessing the effects of N-acetyl cysteine on growth, antioxidant and immune response in tilapia (Oreochromis niloticus) under different regimes of stocking densities

The study investigated the impact of N-acetyl cysteine on growth, immune response, and antioxidant activity in tilapia (Oreochromis niloticus). Fish were reared at three densities (1.50, 3.00, and 4.50 kg/m3) with four levels of N-acetyl cysteine supplementation (0, 2, 4, and 6 mg/kg) over 60 days. Better growth was observed at low density, but at all densities, fish fed the highest N-acetyl cysteine level (6 mg/kg) showed improved growth. Chemical composition of fish and activity of amylase, lipase and protease in all treatments were noted to be insignificant. The levels of antioxidant enzymes (catalase, superoxide dismutase and glutathione peroxidase) and cortisol in HD treatments were high as compared to LD and MD treatment. However, fish fed with N3 diet in each density treatment showed the lowest level of antioxidant enzymes as well as cortisol. Similarly, the levels of malondialdehyde were noted to be high at HD treatments as compared to that in LD and MD. Its levels were lower in fish fed with N3 diets in all density treatments. Expression of somatostatins-1 did not increase in MD and HD treatments in response to high stocking density when compared with LD treatment. However, pro-opiomelanocortin-α level was reduced after N3 diet in HD treatment and interleukin 1-β expression increased after N3 supplement in HD treatment. In conclusion, N-acetyl cysteine supplementation improved growth and antioxidant response in tilapia. The most optimum dose of N-acetyl cysteine was noted to be 6 mg/kg at high stocking, suggesting the potential role of this nutraceutical in tilapia intensive culture.

Long-term stress induced cortisol downregulation, growth reduction and cardiac remodeling in Atlantic salmon

Stress and elevated plasma cortisol in salmonids have been linked with pathological remodeling of the heart and deterioration of fitness and welfare. However, these associations were based on biomarkers that fail to provide a retrospective view of stress. This study is the first whereby the association of long-term stress, using scale cortisol as a chronic stress biomarker, with cardiac morphology and growth performance of wild Atlantic salmon (Salmo salar) is made. Growth, heart morphology, plasma and scale cortisol levels, and expression of genes involved in cortisol regulation of the hypothalamic-pituitary-interrenal axis of undisturbed fish (control) were compared with those of fish exposed daily to stress for 8 weeks. Though scale cortisol levels showed a time-dependent accumulation in both groups, plasma and scale cortisol levels of stress group fish were 29.1% and 25.0% lower than those of control fish, respectively. These results correlated with the overall upregulation of stress-axis genes involved in the systemic negative feedback of cortisol, and local feedback via 11β-hydroxysteroid dehydrogenases, glucocorticoid and mineralocorticoid receptors in the stress treatment at the hypothalamus and pituitary level. These lower cortisol levels were, however, counterintuitive in terms of the growth performance as stress group fish grew 33.7% slower than control fish, which probably influenced the 8.4% increase in relative ventricle mass in the stress group. Though compact myocardium area between the treatments was comparable, these parameters showed significant linear correlations with scale cortisol levels, indicating the involvement of chronic stress in cardiac remodeling. These findings underscore the importance of scale cortisol as biomarker when associating chronic stress with long-term processes including cardiac remodeling.

Divergent Pharmacology and Biased Signaling of the Four Melanocortin-4 Receptor Isoforms in Rainbow Trout (Oncorhynchus mykiss)

The melanocortin-4 receptor (MC4R) is essential for the modulation of energy balance and reproduction in both fish and mammals. Rainbow trout (Oncorhynchus mykiss) has been extensively studied in various fields and provides a unique opportunity to investigate divergent physiological roles of paralogues. Herein we identified four trout mc4r (mc4ra1, mc4ra2, mc4rb1, and mc4rb2) genes. Four trout Mc4rs (omMc4rs) were homologous to those of teleost and mammalian MC4Rs. Multiple sequence alignments, a phylogenetic tree, chromosomal synteny analyses, and pharmacological studies showed that trout mc4r genes may have undergone different evolutionary processes. All four trout Mc4rs bound to two peptide agonists and elevated intracellular cAMP levels dose-dependently. High basal cAMP levels were observed at two omMc4rs, which were decreased by Agouti-related peptide. Only omMc4rb2 was constitutively active in the ERK1/2 signaling pathway. Ipsen 5i, ML00253764, and MCL0020 were biased allosteric modulators of omMc4rb1 with selective activation upon ERK1/2 signaling. ML00253764 behaved as an allosteric agonist in Gs-cAMP signaling of omMc4rb2. This study will lay the foundation for future physiological studies of various mc4r paralogs and reveal the evolution of MC4R in vertebrates.

Stress response of Rhamdia quelen to the interaction stocking density - Feeding regimen

This study aimed to verify the effect of different feeding and stocking conditions during 14 days on the gene expression of several hormones and enzymes related to the stress cascade and metabolic parameters in silver catfish Rhamdia quelen under the following experimental conditions: 1) fed at low stocking density (2.5 kg m^-3, LSD-F); 2) fed at high stocking density (32 kg m^-3, HSD-F); 3) food-deprived at LSD (LSD-FD); and 4) food-deprived at HSD (HSD-FD). Fish from LSD-F and HSD-F groups were fed daily (1 % of their body mass), while fish from food-deprived groups (LSD-FD and HSD-FD) were not fed during the experimental time. Plasma metabolic parameters (glucose, lactate, triglycerides, and proteins) and hepatosomatic index (HSI) were evaluated. In addition, mRNA expression of genes related to the stress axis (crh, pomca, pomcb, nr3c2, star, hsd11b2 and hsd20b), heat shock protein family (hsp90 and hspa12a), sodium-dependent noradrenaline transporter (slc6a2), and growth axis (gh and igf1) were also assessed. Specific growth rate and HSI decreased in food-deprived fish regardless of stocking density. The HSD-FD group showed weight loss compared to the HSD-F, LSD-F, and LSD-FD groups. Plasma glucose and triglycerides were reduced in food-deprived groups, while lactate and protein levels did not change. The expression of key players of the stress response (crh, pomca, pomcb, hsd11b2, nr3c2, and hsp90b) and growth (gh and igf1) pathways were differently regulated depending on the experimental condition, whereas no statistical difference between treatments was found for hsd20b, scl6a2, hspa12a, and star mRNAs expression. This study suggests that LSD acts as a stressor affecting negatively the physiological status of fed fish, as demonstrated by the reduction in growth rates, altered metabolic orchestration, and a higher crh mRNA expression. In addition, food deprivation also increased mRNA expression of other assessed genes (nr3c2, hsp90b, pomca, and pomcb) in fish from the HSD group, indicating higher responsiveness to stress in this stocking density when combined with food deprivation.

Melanocortin receptor 3 and 4 mRNA expression in the adult female Syrian hamster brain

Melanocortin 3 receptors (MC3R) and melanocortin 4 receptors (MC4R) are vital in regulating a variety of functions across many species. For example, the dysregulation of these receptors results in obesity and dysfunction in sexual behaviors. Only a handful of studies have mapped the expression of MC3R and MC4R mRNA across the central nervous system, with the primary focus on mice and rats. Because Syrian hamsters are valuable models for functions regulated by melanocortin receptors, our current study maps the distribution of MC3R and MC4R mRNA in the Syrian hamster telencephalon, diencephalon, and midbrain using RNAscope. We found that the expression of MC3R mRNA was lowest in the telencephalon and greatest in the diencephalon, whereas the expression of MC4R mRNA was greatest in the midbrain. A comparison of these findings to previous studies found that MC3R and MC4R expression is similar in some brain regions across species and divergent in others. In addition, our study identifies novel brain regions for the expression of MC3Rs and MC4Rs, and identifies cells that co-express bothMC3 and MC4 receptors within certain brain regions.

Light conditions during Atlantic salmon embryogenesis affect key neuropeptides in the melanocortin system during transition from endogenous to exogenous feeding

During the first feeding period, fish will adapt to exogenous feeding as their endogenous source of nutrients is depleted. This requires the development of a functional physiological system to control active search for food, appetite, and food intake. The Atlantic salmon (Salmo salar) melanocortin system, a key player in appetite control, includes neuronal circuits expressing neuropeptide y (npya), agouti-related peptide (agrp1), cocaine- and amphetamine-regulated transcript (cart), and proopiomelanocortin (pomca). Little is known about the ontogeny and function of the melanocortin system during early developmental stages. Atlantic salmon [0-730 day degrees (dd)] were reared under three different light conditions (DD, continuous darkness; LD, 14:10 Light: Dark; LL, continuous light) before the light was switched to LD and the fish fed twice a day. We examined the effects of different light conditions (DD _LD , LD _LD , and LL _LD ) on salmon growth, yolk utilization, and periprandial responses of the neuropeptides npya1, npya2, agrp1, cart2a, cart2b, cart4, pomca1, and pomca2. Fish were collected 1 week (alevins, 830 dd, still containing yolk sac) and 3 weeks (fry, 991 dd, yolk sac fully consumed) into the first feeding period and sampled before (-1 h) and after (0.5, 1.5, 3, and 6 h) the first meal of the day. Atlantic salmon reared under DD _LD , LD _LD , and LL _LD had similar standard lengths and myotome heights at the onset of first feeding. However, salmon kept under a constant light condition during endogenous feeding (DD _LD and LL _LD ) had less yolk at first feeding. At 830 dd none of the neuropeptides analyzed displayed a periprandial response. But 2 weeks later, and with no yolk remaining, significant periprandial changes were observed for npya1, pomca1, and pomca2, but only in the LD _LD fish. This suggests that these key neuropeptides serve an important role in controlling feeding once Atlantic salmon need to rely entirely on active search and ingestion of exogenous food. Moreover, light conditions during early development did not affect the size of salmon at first feeding but did affect the mRNA levels of npya1, pomca1, and pomca2 in the brain indicating that mimicking natural light conditions (LD _LD ) better stimulates appetite control.

Mapping key neuropeptides involved in the melanocortin system in Atlantic salmon (Salmo salar) brain

The melanocortin system is a key regulator of appetite and food intake in vertebrates. This system includes the neuropeptides neuropeptide y (NPY), agouti-related peptide (AGRP), cocaine- and amphetamine-regulated transcript (CART), and pro-opiomelanocortin (POMC). An important center for appetite control in mammals is the hypothalamic arcuate nucleus, with neurons that coexpress either the orexigenic NPY/AGRP or the anorexigenic CART/POMC neuropeptides. In ray-finned fishes, such a center is less characterized. The Atlantic salmon (Salmo salar) has multiple genes of these neuropeptides due to whole-genome duplication events. To better understand the potential involvement of the melanocortin system in appetite and food intake control, we have mapped the mRNA expression of npy, agrp, cart, and pomc in the brain of Atlantic salmon parr using in situ hybridization. After identifying hypothalamic mRNA expression, we investigated the possible intracellular coexpression of npy/agrp and cart/pomc in the tuberal hypothalamus by fluorescent in situ hybridization. The results showed that the neuropeptides were widely distributed, especially in sensory and neuroendocrine brain regions. In the hypothalamic lateral tuberal nucleus, the putative homolog to the mammalian arcuate nucleus, npya, agrp1, cart2b, and pomca were predominantly localized in distinct neurons; however, some neurons coexpressed cart2b/pomca. This is the first demonstration of coexpression of cart2b/pomca in the tuberal hypothalamus of a teleost. Collectively, our data suggest that the lateral tuberal nucleus is the center for appetite control in salmon, similar to that of mammals. Extrahypothalamic brain regions might also be involved in regulating food intake, including the olfactory bulb, telencephalon, midbrain, and hindbrain.

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.

Neuropeptide Y and melanocortin receptors in fish: regulators of energy homeostasis

Energy homeostasis, which refers to the physiological processes that the energy intake is exquisitely coordinated with energy expenditure, is critical for survival. Therefore, multiple and complex mechanisms have been involved in the regulation of energy homeostasis. The central melanocortin system plays an important role in modulating energy homeostasis. This system includes the orexigenic neurons, expressing neuropeptide Y/Agouti-related protein (NPY/AgRP), and the anorexigenic neurons expressing proopiomelanocortin (POMC). The downstream receptors of NPY, AgRP and post-translational products of POMC are G protein-coupled receptors (GPCRs). This review summarizes the compelling evidence demonstrating that NPY and melanocortin receptors are involved in energy homeostasis. Subsequently, the comparative studies on physiology and pharmacology of NPY and melanocortin receptors in humans, rodents and teleosts are summarized. Also, we provide a strategy demonstrating the potential application of the new ligands and/or specific variants of melanocortin system in aquaculture.

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.

Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar)

Food intake is a vital process that supplies necessary energy and essential nutrients to the body. Information regarding luminal composition in the gastrointestinal tract (GIT) collected through mechanical and nutrient sensing mechanisms are generally conveyed, in both mammals and fish, to the hypothalamic neurocircuits. In this context, ghrelin, the only known hormone with an orexigenic action, and the intestinal peptide transporters 1 and 2, involved in absorption of dietary di- and tripeptides, exert important and also integrated roles for the nutrient uptake. Together, both are potentially involved in signaling pathways that control food intake originating from different segments of the GIT. However, little is known about the role of different paralogs and their response to fasting. Therefore, after 3 weeks of acclimatization, 12 Atlantic salmon (Salmo salar) post-smolt were fasted for 4 days to explore the gastrointestinal response in comparison with fed control (n = 12). The analysis covered morphometric (weight, length, condition factor, and wet content/weight fish %), molecular (gene expression variations), and correlation analyses. Such short-term fasting is a common and recommended practice used prior to any handling in commercial culture of the species. There were no statistical differences in length and weight but a significant lower condition factor in the fasted group. Transcriptional analysis along the gastrointestinal segments revealed a tendency of downregulation for both paralogous genes slc15a1a and slc15a1b and with significant lowered levels in the pyloric ceca for slc15a1a and in the pyloric ceca and midgut for slc15a1b. No differences were found for slc15a2a and slc15a2b (except a higher expression of the fasted group in the anterior midgut), supporting different roles for slc15 paralogs. This represents the first report on the effects of fasting on slc15a2 expressed in GIT in teleosts. Transcriptional analysis of ghrelin splicing variants (ghrl-1 and ghrl-2) showed no difference between treatments. However, correlation analysis showed that the mRNA expression for all genes (restricted to segment with the highest levels) were affected by the residual luminal content. Overall, the results show minimal effects of 4 days of induced fasting in Atlantic salmon, suggesting that more time is needed to initiate a large GIT response.

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

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

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.