Nitric oxide (NO) may be a neurotransmitter related to major depressive disorder (MDD) because the selective neuronal NO synthase (NOS) inhibitor, 7-nitroindazole, induces dose-dependent antidepressant-like effects. However, its role in MDD is not yet known. The purpose of our study was to determine if antidepressants improve depression via the NO pathway using an acute depressive rat model induced by L-arginine (AR). Three types of antidepressants were examined, fluoxetine (FLX, 10 mg/kg), milnacipran (MIL, 30 mg/kg), and mirtazapine (MIR, 10 mg/kg), in a depressive model that used AR (750 mg/kg) pretreatment. mRNA expression levels of three NOS subtypes were analyzed by real-time PCR, as well as serum NO levels. Significant increases in iNOS mRNA expression levels were found in brain regions after AR treatment, although the eNOS gene tended to decrease with AR injection. After antidepressant treatment, there were no mRNA expression changes in either nNOS or iNOS. However, eNOS mRNA expression significantly increased with FLX (cerebellum, P = 0.011; hippocampus, P = 0.011; midbrain, P = 0.011; pons, P = 0.013; striatum, P = 0.011; and thalamus, P < 0.001). There was a statistically significant increase in serum NO levels with MIL treatment (P = 0.011). We conclude that changes in eNOS mRNA levels in the brain with FLX treatment, and amount of serum NO with MIL treatment may be related to antidepressant effects of both agents, but further experiments are needed to confirm involvement of the NO system in MDD. (C) 2015 Elsevier Ireland Ltd. All rights reserved.