IL-6 may increase indoleamine-2,3-dioxygenase (IDO) activity, leading to activation of the kynurenine pathway, which triggers the production of the neurotoxic n-methyl-d-aspartate glutamate agonists quinolinic acid and 3-hydroxykynurenine, causing neurodegeneration and serious depression (44). This evidence concerns the gene IL6 and major depressive disorder.