TPH2 and major depressive disorder: Due to its long-lasting antidepressant effects, tryptophan hydroxylase 2 (TPH2), which is the rate-limiting enzyme for brain serotonin (5-hydroxytryptamine, 5-HT) biosynthesis, has been shown to be a promising target for the therapeutic treatment of neuropsychiatric disorders including depression and anxiety due to the close relationship between 5-HT system dysfunction and depression [23–26].