Changes of plasma levels of neuroactive amino acids, and nitric oxide (NO, calculated as citruline/arginine) in melancholic major depressive disorder. Boxplot showing the median, 25th-75th percentiles, and maximum and minimum value range. (A): Asp, aspartic acid; (B): Gly, glycine; (C): GABA, gamma-aminobutyric acid; (D): NO, nitric oxide. CTR, control; MDD, major depressive disorder. **P <0.01, ***P <0.001.