Sex dimorphism of glucosensing parameters and appetite-regulating peptides in the hypothalamus of rainbow trout broodstocks.

Rainbow trout (Oncorhynchus mykiss) is traditionally considered as a poor user of digestible carbohydrates harbouring persistent postprandial hyperglycaemia and decreased growth performances when fed a diet containing more than 20% of digestible carbohydrates. While this glucose-intolerant phenotype is well-described in juveniles, evidence points to a particular regulation of glucose metabolism in rainbow trout broodstrocks. By detecting changes in glucose levels and triggering a specific metabolic response, the hypothalamus plays a key role in the regulation of peripheral glucose metabolism. Therefore, our objective was to assess, for the first time in fish, the short-term consequences in hypothalamus, the glucose sensing and feed intake regulation of feeding mature female and male, and neomale rainbow trout with a diet containing either no or a 33% carbohydrate. The hypothalamic glucosensing capacity was assessed through mRNA levels of glucosensing related-genes and feed intake regulation through appetite-regulating peptides. Our data indicate that a brief period of carbohydrate intake (5 meals at 8 °C) did not induce specific changes in glucosensing capacity and appetite-regulating peptides in the hypothalamus of rainbow trout broodstock. Our results did however demonstrate, for the first time in fish, the existence of sex dimorphism of glucosensing-related genes and appetite-regulating peptides.

Effects of dietary oxidized fish oil supplementation on oxidative stress and antioxidant defense system in juvenile rainbow trout (Oncorhynchus mykiss).

The objective of the study was to characterize the response of the antioxidant defense system against dietary prooxidant conditions in rainbow trout juveniles. Fish (initial mean weight: 62 ±â€¯1 g) were fed three fishmeal and plant-derived protein-based diets supplemented with 15% fresh fish oil (CTL diet), 15% fresh fish oil from tuna by-products (BYP diet) or 15% autooxidized fish oil (OX diet) over a 12-week growth trial at 17.5 ±â€¯0.5 °C. No significant differences in growth performance were recorded between dietary groups. Muscle lipid content was reduced and n-6 PUFA levels were increased in rainbow trout fed diets BYP and OX compared to CTL. After 12 weeks of feeding, the level of lipid peroxidation products in muscle was not affected whereas the 8-isoprostane content in liver was increased in fish fed diet OX as well as plasma total and oxidized glutathione contents. The hepatic and muscle contents for α-tocopherol were decreased in fish fed BYP and OX. Hepatic antioxidant enzyme activities and mRNA levels were not affected after 12 weeks of feeding, except for catalase and glutathione peroxidase 1b2 mRNA levels that were decreased in trout fed diet OX. Fish fed diet OX and BYP displayed also reduced cytosolic Nrf2 and both cytosolic and nuclear NF-κB protein levels in liver. The present work indicates that feeding rainbow trout juveniles with fresh fish oil from by-products or moderately oxidized lipid appears not to be detrimental to the growth performance of fish. The mechanisms beyond the control of the antioxidant defense system by moderately oxidized lipid require further investigations in rainbow trout juveniles.

Feeding rainbow trout with a lipid-enriched diet: effects on fatty acid sensing, regulation of food intake and cellular signaling pathways.

Using rainbow trout fed with low-fat or high-fat diets, we aimed to determine whether the response of food intake, mRNA abundance of hypothalamic neuropeptides involved in the metabolic regulation of food intake and fatty acid sensing systems in the hypothalamus and liver are similar to results previously observed when levels of specific fatty acids were raised by injection. Moreover, we also aimed to determine if the phosphorylation state of intracellular energy sensor 5'-AMP-activated protein kinase (AMPK), and proteins involved in cellular signaling such as protein kinase B (Akt) and target of rapamycin (mTOR) display changes that could be related to fatty acid sensing and the control of food intake. The increased levels of fatty acids in the hypothalamus and liver of rainbow trout fed with a high-fat diet only partially activated fatty acid sensing systems and did not elicit changes in food intake, suggesting that the fatty acid sensing response in fish is more dependent on the presence of specific fatty acids, such as oleate or octanoate, rather than to the global increase in fatty acids. We also obtained, for the first time in fish, evidence for the presence and function of energy sensors such as AMPK and proteins involved in cellular signaling, like mTOR and Akt, in the hypothalamus. These proteins in the hypothalamus and liver were generally activated in fish fed the high-fat versus low-fat diet, suggesting that cellular signaling pathways are activated in response to the increased availability of fatty acids.