The melanocortin system is one of the most complex hormonal systems in vertebrates. Atypically, the signaling of melanocortin receptors is regulated by the binding of endogenous antagonists, named agouti-signaling protein (ASIP) and agouti-related protein (AGRP). Teleost specific genome duplication (TSGD) rendered new gene copies in teleost fish and up to four different genes of the agouti family of peptides have been characterized. In this paper, molecular cloning was used to characterize mRNA of the agouti family of peptides in sea bass. Four different genes were identified: AGRP1, ASIP1, AGRP2 and ASIP2. The AGRP1 gene is mainly expressed in the brain whereas ASIP1 is mainly expressed in the ventral skin. Both ASIP2 and AGRP2 are expressed in the brain and the pineal gland but also in some peripheral tissues. Immunocytochemical studies demonstrated that AGRP1 is exclusively expressed within the lateral tuberal nucleus, the homologue of the mammalian arcuate nucleus in fish. Long-term fasting (8-29 days) increased the hypothalamic expression of AGRP1 but depressed AGRP2 expression (15-29 days). In contrast, the hypothalamic expression of ASIP2 was upregulated during short-term fasting suggesting that this peptide could be involved in the short term regulation of food intake in the sea bass.

An experiment was conducted to investigate the effects of fasting and re-feeding on hypothalamic 5'-AMP-activated protein kinase (AMPK) levels and (an)orexigenic neuropeptides. Male Arbor Acres chicks (7-day-old, n=160) were allocated to four equal treatment groups: control chicks (fed ad libitum for 48 h, C48), chicks that were fasted for 48 h (F48), chicks that were first fasted for 48 h and then re-fed for 24h (F48C24), and chicks that were fed ad libitum for 72h (C72). Fasting for 48 h significantly (P<0.05) increased the ratio of phosphorylated AMPKα to total AMPKα and phosphorylated LKB1 to total LKB1, whereas re-feeding for 24h reduced these ratios to that of the ad libitum fed C72 chicks. The gene expressions of agouti-related peptide (AgRP), neuropeptide Y (NPY), melanocortin receptor 4, melanin-concentrating hormone, prepro-orexins and carnitine palmitoyltransferase-1 were significantly (P<0.05) increased in the fasted chicks relative to the ad libitum fed C48 group. The gene expression of pro-opiomelanocortin (POMC), as well as cocaine- and amphetamine-regulated transcript (CART) was not affected by the nutritional status. Fasting significantly (P<0.05) decreased the mRNA levels of fatty acid synthase (FAS) and sterol regulatory element binding protein-1 (SREBP-1). The results suggest that the LKB1/AMPK signal pathway is involved in the energy homeostasis of fasted chicks, and its possible role in feed intake regulation might be mediated by the AgRP/NPY rather than the POMC/CART pathway.

Unlike mammals, the role of adipokines and more particularly of chemerin in the regulation of food intake is totally unknown in avian species. Here we investigated the effect of chemerin on the food and water consumption and on the body weight in chicken. We studied the effects on the plasma glucose and insulin concentrations and the hypothalamic neuropeptides and AMPK signaling pathway. Female broiler chickens were intraperitoneally injected, daily for 13 days with either vehicle (saline; n = 25) or chemerin (8 μg/kg; n = 25 and 16 μg/kg; n = 25). Food and water intakes were recorded 24 h after each administration. Overnight fasted animals were sacrificed at day 13 (D13), 24 h after the last injection and hypothalamus and left cerebral hemispheres were collected. Chemerin and its receptors protein levels were determined by western-blot. Gene expression of neuropeptide Y (Npy), agouti-related peptide (Agrp), corticotrophin releasing hormone (Crh), pro-opiomelanocortin (Pomc), cocaine and amphetamine-regulated transcript (Cart) and Taste 1 Receptor Member 1 (Tas1r1) were evaluated by RT-qPCR. In chicken, we found that the protein amount of chemerin, CCRL2 and GPR1 was similar in left cerebral hemisphere and hypothalamus whereas CMKLR1 was higher in hypothalamus. Chemerin administration (8 and 16 μg/kg) decreased both food intake and body weight compared to vehicle without affecting water intake and the size or volume of different brain subdivisions as determined by magnetic resonance imaging. It also increased plasma insulin levels whereas glucose levels were decreased. These data were associated with an increase in Npy and Agrp expressions and a decrease in Crh, Tas1r1 mRNA expression within the hypothalamus. Furthermore, chemerin decreased hypothalamic CMKLR1 protein expression and AMPK activation. Taken together, these results support that chemerin could be a peripheral appetite-regulating signal through modulation of hypothalamic peptides expression in chicken.

The first studies that identified leptin and its receptor (LepR) in mammals were based on mutant animals that displayed dramatic changes in body-weight and regulation of energy homeostasis. Subsequent studies have shown that a deficiency of leptin or LepR in homoeothermic mammals results in hyperphagia, obesity, infertility and a number of other abnormalities. The physiological roles of leptin-mediated signaling in ectothermic teleosts are still being explored. Here, we produced medaka with homozygous LepR gene mutation using the targeting induced local lesions in a genome method. This knockout mutant had a point mutation of cysteine for stop codon at the 357th amino acid just before the leptin-binding domain. The evidence for loss of function of leptin-mediated signaling in the mutant is based on a lack of response to feeding in the expression of key appetite-related neuropeptides in the diencephalon. The mutant lepr−/− medaka expressed constant up-regulated levels of mRNA for the orexigenic neuropeptide Ya and agouti-related protein and a suppressed level of anorexigenic proopiomelanocortin 1 in the diencephalon independent of feeding, which suggests that the mutant did not possess functional LepR. Phenotypes of the LepR-mutant medaka were analyzed in order to understand the effects on food intake, growth, and fat accumulation in the tissues. The food intake of the mutant medaka was higher in post-juveniles and adult stages than that of wild-type (WT) fish. The hyperphagia led to a high growth rate at the post-juvenile stage, but did not to significant alterations in final adult body size. There was no additional deposition of fat in the liver and muscle in the post-juvenile and adult mutants, or in the blood plasma in the adult mutant. However, adult LepR mutants possessed large deposits of visceral fat, unlike in the WT fish, in which there were none. Our analysis confirms that LepR in medaka exert a powerful influence on the control on food intake. Further analyses using the mutant will contribute to a better understanding of the role of leptin in fish. This is the first study to produce fish with leptin receptor deficiency.

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.