The aim of the present study was to evaluate in hypothalamus and hindbrain of rainbow trout in vitro the effect of leptin treatment on glucosensing capacity and the expression of orexigenic and anorexigenic peptides involved in the control of food intake. In a first experiment, the response of parameters involved in glucosensing (GK, PK and GSase activities; GK expression and glucose; glycogen and DHAP levels) and the expression of orexigenic (NPY) and anorexigenic (POMC, CART, CRF) peptides was assessed in hypothalami and hindbrain incubated for 1h with 2, 4 or 8mM d-glucose alone (controls) or with 10nM leptin, or with 10nM leptin plus inhibitors of leptin signaling pathways (50nM wortmannin and 500nM AG490). Leptin treatment increased levels in parameters involved in glucosensing. Leptin treatment decreased NPY mRNA levels in hypothalamus without affecting the expression of the other peptides assessed. Leptin effects were reverted in the presence of inhibitors for all parameters assessed suggesting the involvement of JAK/STAT and IRS-PI(3)K pathways. In a second experiment, we observed time-dependent (1-3h) and dose (10, 20 and 50nM)- effects of leptin treatment in decreasing NPY mRNA levels without affecting expression of the other peptides assessed. Considering the orexigenic action of NPY in fish, it seems that the anorexic effect of leptin can be mediated by reduced expression of NPY occurring in hypothalamus, and that change can be related to the activation of the glucosensing system occurring simultaneously.