However, at present, little information on the impact of intestinal fatty acid on fish appetite is available in the literature. Numerous studies in mammals have shown the connection between dietary FAs and appetite regulation. Oleate acid (OA), linoleic acid (LA), palmitic acid (PA), and α-linolenic acid (ALA) are common FAs of daily VO in the fish diet. Besides dietary lipid concentration, dietary fatty acid (FA) composition changes also influence fish appetite. Therefore, it is meaningful to investigate the mechanism of the influence of HFD on food intake. However, it is not clear how and whether dietary fat concentration affects the appetite of silver pomfret. In silver pomfret, excessive lipid intake can result in higher fat accretion and impaired growth performance. Previous studies have shown that HFD reduces food intake in rainbow trout, white seabass ( Atractoscion nobilis), grass carp, and blunt snout bream ( Megalobrama amblycephala). However, excessive fat in feed may suppress appetite, which will affect the metabolism and growth of fish and further limit the use of HFD in fish. High fat diets (HFD) and vegetable oils (VO) are extensively used in aquaculture. Previous studies in goldfish ( Carassius auratus), grass carp, Atlantic halibut ( Hippoglossus hippoglossus), and Nile tilapia ( Oreochromis niloticus) have demonstrated the appetite-inhibiting effect of PYYa and PYYb in teleost fish. The teleost PYY is comprised of two subtypes, PYYa, and PYYb. In mammals, PYY acts as an indicator and anorexigenic signal. Similarly, peptide YY (PYY), secreted by the intestine, is also an important appetite-regulating peptide. In teleost fish, GLP-1 could inhibit food intake in grass carp ( Ctenopharyngodon idella) and coho salmon ( Oncorhynchus kisutch). Glucagon-like peptide-1 (GLP-1), another peptide that is found in the intestine, also mediates appetite, promoting satiety and inhibiting food intake. Jönsson and coworkers demonstrated that ghrelin regulates appetite in Atlantic salmon ( Salmo salar) and rainbow trout ( Oncorhynchus mykiss). Previous studies have indicated that ghrelin regulates energy homeostasis and lipid metabolism in teleost. Ghrelin, originally reported to be present in the rat stomach, is the only known peripheral appetite stimulating hormone. The intestine is the largest hormone producing organ, containing at least 30 hormones, some of which play a crucial role in the regulation of appetite, such as ghrelin, glucagon-like peptide-1 (GLP-1), and peptide YY (PYY). Appetite and feed intake are affected by signals from the intestine, inducing satiety and hunger. The intestine, as a key player in maintaining systemic energy balance, does not only absorb nutrients, but also senses nutritional value and transmits metabolic information to other tissues. These results suggest the importance of ghrelin, glp-1, and pyy in coordinating food intake in response to fasting, dietary lipid concentration, and fatty acids in silver pomfret. The expression of pyyb was downregulated by LA, DHA, and EPA. OA decreased the expression of glp-1 and increased the expression of pyya. The ghrelin expression in the intestine was increased after palmitic acid, oleic acid (OA), linoleic acid (LA), α-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) incubation. The diet with 14% lipid promoted the expressions of glp-1, pyya, and pyyb, but inhibited the ghrelin expression. The pyya mRNA expression was increased after 72 h of fasting, while the pyyb expression remained stable during fasting. The glp- 1 mRNA expression was increased after 6 to 24 h of fasting. The ghrelin mRNA expression was relatively high an hour post-feeding and decreased after 3 to 72 h of fasting. Ghrelin, pyya, and pyyb mRNAs were highly expressed in the brain, while glp-1 was highly expressed in the gills and liver. PYYa and PYYb shared a low identity and were clustered to different PYY branches. A comparison analysis revealed that ghrelin and GLP-1 were relatively conserved in marine fish. However, limited information is available on these genes and their expression in response to nutrition in silver pomfret ( Pampus argenteus). Ghrelin, glucagon-like peptide-1 (GLP-1), and peptide YY (PYY) are potent hormones mediating food intake according to the nutritional status in fish.
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