Evidence for the presence in rainbow trout brain of amino acid-sensing systems involved in the control of food intake.

To assess the hypothesis of central amino acid-sensing systems involved in the control of food intake in fish, we carried out two experiments in rainbow trout. In the first one, we injected intracerebroventricularly two different branched-chain amino acids (BCAAs), leucine and valine, and assessed food intake up to 48 h later. Leucine decreased and valine increased food intake. In a second experiment, 6 h after similar intracerebroventricular treatment we determined changes in parameters related to putative amino acid-sensing systems. Different areas of rainbow trout brain present amino acid-sensing systems responding to leucine (hypothalamus and telencephalon) and valine (telencephalon), while other areas (midbrain and hindbrain) do not respond to these treatments. The decreased food intake observed in fish treated intracerebroventricularly with leucine could relate to changes in mRNA abundance of hypothalamic neuropeptides [proopiomelanocortin (POMC), cocaine- and amphetamine-related transcript (CART), neuropeptide Y (NPY), and agouti-related peptide (AgRP)]. These in turn could relate to amino acid-sensing systems present in the same area, related to BCAA and glutamine metabolism, as well as mechanistic target of rapamycin (mTOR), taste receptors, and general control nonderepressible 2 (GCN2) kinase signaling. The treatment with valine did not affect amino acid-sensing parameters in the hypothalamus. These responses are comparable to those characterized in mammals. However, clear differences arise when comparing rainbow trout and mammals, in particular with respect to the clear orexigenic effect of valine, which could relate to the finding that valine partially stimulated two amino acid-sensing systems in the telencephalon. Another novel result is the clear effect of leucine on telencephalon, in which amino acid-sensing systems, but not neuropeptides, were activated as in the hypothalamus.