Leucine sensing in rainbow trout hypothalamus is direct but separate from mTOR signalling in the regulation of food intake

Abstract

In prior studies, we evidenced the presence in fish hypothalamus of different systems sensing changes in leucine levels to relate them to food intake control. To assess whether or not those effects are due to a direct action of leucine, we evaluated in the present in vitro study if rainbow trout (Oncorhynchus mykiss) hypothalamus respond to changes in leucine levels in a way comparable to that observed in vivo. Thus, we incubated hypothalamus in vitro with increased leucine concentrations (1, 5 and 25 mM). Then, we evaluated parameters related to mechanisms of amino acid sensing as well as transcription factors, and neuropeptides. Amino acid sensing systems related to metabolism of BCAA and glutamine and GCN2 kinase are activated by the presence of leucine, in a way comparable to that observed in vivo thus supporting a direct action of leucine on them. However, the activation of these systems did not trigger mTOR signalling pathway including downstream protein targets of mTOR, transcription factors, and mRNA abundance of neuropeptides involved in food intake the regulation of food intake. Therefore, the activation of hypothalamic mTOR observed after leucine administration in vivo is not a direct response to leucine but indirect. The absence in vitro of connections between hypothalamus and other brain areas and/or the lack of changes in levels of hormones, particularly insulin, could be responsible of this differential response.