Metabolic fuels act on hypothalamic neurons to regulate feeding behavior and energy homeostasis, but the signaling mechanisms mediating these effects are not fully clear. Rats placed on a low protein diet (10% of calories) exhibited increased food intake (P < 0.05) and hypothalamic Agrp gene expression (P = 0.002). Direct intracerebroventricular (icv) injection of either an amino acid mixture (RPMI 1640) or leucine alone (1 ug) suppressed 24h food intake (P < 0.05), indicating that increasing amino acids concentrations within the brain is sufficient to suppress food intake. To define a cellular mechanism for these direct effects, GT1-7 hypothalamic cells were exposed to low amino acids for 16hrs. Decreasing amino acid availability increased Agrp mRNA levels in GT1-7 cells (P < 0.01), and this effect was attenuated by replacement of the amino acid leucine (P < 0.05). Acute exposure to elevated amino acid concentrations increased S6K phosphorylation via a rapamycin-sensitive mechanism, suggesting that amino acids directly stimulated mTOR signaling. To test whether mTOR signaling contributes to amino acid inhibition of Agrp gene expression, GT1-7 cells cultured in either low or high amino acids for 16hrs and were also treated with rapamcyin (50 nM). Rapamycin treatment increased Agrp mRNA levels in cells exposed to high amino acids (P = 0.01). Taken together, these observations indicate that amino acids can act within the brain to inhibit food intake, and that a direct, mTOR-dependent inhibition of Agrp gene expression may contribute this effect.