IDO activation results in diverting TRP from synthesis of 5-hydroxytryptophan (5-HTP) and 5-hydroxytryptamine (5-HT) to the generation of TRP metabolites such as quinolinic acid, which is known to be neurotoxic and thereby may lead to depression-like behaviors through both serotonin and glutamate pathways [41,55]. This evidence concerns the gene IDO1 and major depressive disorder.