TNF and depressive disorder: The inflammatory hypothesis of depression proposes that inflammatory cytokines, such as interferon (IFN)-α, IFN-γ, and tumor necrosis factor-α (TNF-α), increase the activity of the enzyme indoleamine 2,3-dioxygenase (IDO), leading to an activation of the catabolism of the serotonin precursor tryptophan to kynurenine [5–9].