Effects of fasting on the gene expression of appetite regulators in three Characiformes with different feeding habits (Gymnocorymbus ternetzi, Metynnis argenteus and Exodon paradoxus).

In order to assess potential interspecific differences in the endocrine mechanisms regulating feeding in Characiformes, we used three fish species with different feeding habits: two Characidae, the omnivore black widow tetra and the carnivore bucktooth tetra, and one Serrasalmidae, the herbivore silver dollar, as models. cDNAs encoding for appetite-regulating peptides (orexin, cocaine and amphetamine regulated transcript CART, cholecystokinin CCK and leptin) were isolated and their tissue distribution examined. The protein sequences of the three species showed most similarities with those of other Characiformes, followed by Cypriniformes and Siluriformes. mRNAs of all four peptides were expressed in the brain. Orexin, CCK and leptin mRNAs were widely distributed in peripheral tissues of all species. CART mRNA displayed a wide peripheral distribution in bucktooth but was predominant in brain in black widow tetra and silver dollar. In order to assess possible interspecific differences in the response to fasting, we compared the expression of these peptides in fed and fasted fish. Fasting induced increases in orexin expression in all species, but decreased brain CART and leptin expressions in silver dollar only. In the intestine, fasting induced a decrease in CCK expression in silver dollar and black widow, and a decrease in leptin expression in bucktooth. Our results suggest that, in Characiformes, different responses of appetite-regulating peptides to fasting are related to both feeding habits and family. The results of this comparative study provide new insights on the regulation of feeding of economically important Characiforme species, which might be valuable for their management and farming.

Irisin in goldfish (Carassius auratus): Effects of irisin injections on feeding behavior and expression of appetite regulators, uncoupling proteins and lipoprotein lipase, and fasting-induced changes in FNDC5 expression.

Irisin is a peptide cleaved from the fibronectin type III domain containing protein 5 (FNDC5) gene that is secreted predominantly by muscle cells but also by other tissues including brain and intestine. In mammals, irisin has been shown to have thermogenic actions via the modulation of uncoupling proteins (UCPs) and to affect feeding and energy homeostasis via actions in brain, adipose tissue, liver, muscle and gastrointestinal tract. To examine the role of irisin on feeding and metabolism in fish, the effects of peripheral (intraperitoneal) injections of irisin on feeding behavior, glucose levels and the mRNA expressions of appetite regulators (cocaine and amphetamine regulated transcript CART, agouti related protein AgRP, orexin), UCPs and lipoprotein lipase LPL and brain factors (brain-derived neurotrophic factor , BDNF and tyrosine hydroxylase TH) were assessed in brain, white muscle and intestine. Irisin injections (100ng/g) induced a decrease in food intake and increases in brain orexin, CART1 and CART2, UCP2, BDNF, muscle UCP2 and intestine LPL mRNA expressions but did not affect blood glucose levels, brain AgRP, TH, UCP1, UCP3 and LPL or muscle UCP1, UCP3 and LPL expressions. A partial goldfish FNDC5 cDNA was isolated and the expressions of FDNC5, UCPs, LPL and BDNF were also compared between fed and fasted fish. Fasting induced decreases FNDC5 mRNA expression in the brain and intestine, but not in muscle. Fasting also induced increases in brain BDNF and LPL expressions and increases in UCP1, UCP2, UCP3 and LPL expressions in muscle. Our result suggest that irisin is an anorexigenic factor in fish and its actions might be in part mediated by appetite-regulating factors such as CART and orexins as well as UCP2 and brain factors such as BDNF.