TDO2 and major depressive disorder: Excessive pro-inflammatory cytokines produced in depression over activate the enzymes indoleamine 2,3-dioxygnease (IDO) and tryptophan 2,3-dioxygenase (TDO), promoting the kynurenine pathway and consequently reducing the activation of the 5-HT pathway and decreasing the production of 5-HT (Miura et al., 2008).