We have previously obtained evidence on presence and functioning of central amino acid sensing systems in fish after intracerebroventricular injection of branched-chain amino acids (BCAA) L-leucine and L-valine. In hypothalamus these systems were activated by L-leucine but not by L-valine. The mechanisms involved are similar to those described in mammals: activation of BCAA metabolism, mTOR/AMPK signaling, taste receptors signaling, and mechanism dependent on detection of amino acid deficiency by GCN2 kinase. In order to establish whether or not the action of amino acids is direct, we carried out an intraperitoneal treatment with the same amino acids to assess changes in amino acid sensing systems. Thus, an intraperitoneal injection was carried out with three groups of rainbow trout: 0.5 mL/100 g body mass of saline solution alone (control) or containing 40 μmol/mL of leucine or 40 μmol/mL of valine. After 6 hours samples of hypothalamus were taken. Enzymatic activities, expression of mRNA and protein abundance of parameters related to amino acid sensing pathways, as well as expression of neuropeptides involved in the regulation of food intake NPY, AgRP, POMC-A1 and CART, were evaluated. In a second experiment the same solutions were injected to evaluate food intake 6 h, 24 h and 48 h post-treatment. Leucine and valine treatment decreased food intake, which is in agreement with changes in hypothalamic neuropeptides. These responses might relate to the activation of amino acid sensing systems. The results obtained after leucine treatment are basically the same than those observed after intracerebroventricular treatment whereas this was not the case for valine. Altogether, leucine seems to act directly in hypothalamus modulating the activity of amino acid sensing systems whereas valine action would be indirect.