Tyrosine (Y), a non-essential amino acid, is synthesized de novo from phenylalanine and converted into L-DOPA in mammals. In the brain, ROS and RNS are increased both in acute and chronic stress conditions, leading peroxynitrite (PN) production. PN can attack Y residues of functional proteins and turn off their normal functions by nitration, called Y-nitration. Under chronic immobilization stress (CIS), glutamine synthetase (GS) activity is decreased without expression change and the decrement of glutamine (Gln) and glutamate (Glu) is found in the prefrontal cortex (PFC), which results in low-active glutamatergic signaling. We found that Y-nitration of GS was increased by CIS. Thus, we hypothesized something that can prevent nitration or facilitate denitration of GS under CIS condition, it would be candidates for antidepressants. At first, we tested whether Y could prevent nitration or facilitate denitration using human recombinant GS and mouse brain lysate by GS activity assay system. After confirming the effect, we examined whether Y has anti-depressive and anti-anxiolytic effects on CIS-induced mouse depression model using Y-supplemented diet. Diet-supplemented Y showed anti-depressive effect and significantly increased the GS activity without expression change in the PFC. It was interesting that Y-nitration of GS was decreased, leading the increments of Glu and Gln levels in the PFC. Finally, hypoactive glutamatergic signaling in stressed mouse PFC had changed into control level. These results suggested that appropriate exogenous Y could be a new antidepressant candidate with an innovative mechanism, GS activation through Y denitration.