| Summary: | Apelin, a kind of active polypeptide, has many biological functions, such as promoting food intake, enhancing immunity, and regulating energy balance. In mammals, studies have indicated that apelin is involved in regulating food intake. However, there are relatively few studies about the regulatory effect of apelin on fish feeding, and the specific mechanism is not clear. Therefore, the purpose of this study was to preliminarily investigate the regulatory effects of apelin on key genes of feeding and growth in common carp (<i>Cyprinus Carpio</i> L.) through in vitro and in vivo experiments. In the present study, after incubation with different concentrations of Pyr-apelin-13 (0, 10, 100, and 1000 nM) in hypothalamic fragments, the expressions of <i>Neuropeptide Y</i> (<i>NPY</i>) and <i>Agouti related peptide</i> (<i>AgRP</i>) mRNA were significantly up-regulated at 12 and 3 h, respectively, and the significant down-regulation of <i>Cocaine and amphetamine-related transcript</i> (<i>CART</i>) mRNA expression was observed at 1 and 3 h. In vivo, after Pyr-apelin-13 oral administration (0, 1, 10, and 100 pmol/g), the <i>orexin</i> mRNA level in the hypothalamus of common carp was significantly increased at 1, 6, and 12 h, while <i>CART/</i>(<i>Proopiomelanocortin</i>) <i>POMC</i> mRNA levels in the hypothalamus of common carp were significantly down-regulated. Following incubation with different concentrations of Pyr-apelin-13 (0, 10, 100, and 1000 nM) in primary hepatocytes, <i>GHR</i> (<i>Growth hormone receptor</i>), <i>IGF2</i> (<i>Insulin-like growth factor 2</i>), <i>IGFBP2</i> (<i>Insulin like growth factor binding protein 2</i>), and <i>IGFBP3</i> (<i>Insulin like growth factor binding protein 3</i>) mRNA levels were significantly increased at 3 h. In vivo, the levels of <i>IGF1</i> (<i>Insulin-like growth factor 1</i>), <i>IGF2</i>, <i>IGFBP2</i> (<i>Insulin like growth factor binding protein 2</i>), and <i>IGFBP3</i> mRNA were significantly increased after the oral administration of Pyr-apelin-13 in the hepatopancreas, in a time and dose-dependent manner. These results support the hypothesis that Pyr-apelin-13 might regulate the feeding and growth of common carp through mediating the expressions of appetite- and growth-related genes. Overall, apelin, which is an orexigenic peptide, improves food intake and is involved in the growth of common carp.
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