Modulation of peripheral energy metabolism through central leucine administration in rainbow trout (<i>Oncorhynchus mykiss</i>)

We aimed to evaluate the role of central leucine administration in the modulation of peripheral energy metabolism in fish. For this, rainbow trout (<i>Oncorhynchus mykiss</i>) were administered via intracerebroventricular 1 μL·100 g^-1 body mass of saline solution alone (Control) or containing 10 μg·μL^-1 of leucine. Samples of plasma, liver, adipose tissue, white muscle, and red muscle were collected 1- and 3-h postinjection. Firstly, metabolite levels were assessed in plasma and liver and a decrease in liver triglyceride at 1 h and an increase in plasma fatty acid at 3 h were observed. Metabolites levels were also assessed in white muscle, revealing decreased levels of α-amino acids and glycogen at 1 h. In addition, liver enzymatic activity and mRNA levels related to glucose, fatty acid, and amino acid metabolism showed no relevant changes. Then, energy metabolism in adipose tissue and muscle was assessed by examining the mRNA abundance of genes related to metabolism and oxidative capacity, thermogenesis, mitochondrial dynamics (mitochondrial fusion and fission), and other metabolic regulatory factors. Mitochondrial fusion was significantly influenced at 1-h postinjection in white muscle (upregulation of <i>mfn1</i>, <i>mfn2</i>, <i>tfam</i>, and <i>opa1</i>) and to a lesser extent in red muscle (upregulation of <i>tfam</i>). These findings differ from studies in mammals with leucine and in fish with other nutrients, in which liver metabolism is modulated. This also highlights the importance of leucine and its relationship with muscle and mitochondrial dynamics in controlling energy homeostasis in fish.<b>NEW & NOTEWORTHY</b> We studied how central leucine administration affects peripheral energy metabolism in rainbow trout. No significant changes were found in liver metabolism, differing from mammalian comparable studies and from changes in hypothalamic energy status elicited by other nutrients in fish. Mitochondrial fusion was notably influenced in white muscle and to a lesser extent in red muscle. These findings highlight the unique role of leucine in the maintenance of energy homeostasis in fish.