Figure 2: Interrelationships between indoleamine 2,3-dioxygenase (IDO) and nitric oxide synthase (NOS) in macrophages or glial cells, and the potential interactions with neurons by means of N-methyl-D-aspartate (NMDA)-receptor-induced nitric-oxide (NO) formation. Arg, arginine; 3-HAA, 3-hydroxyanthranilic acid; 3-HK, 3-hydroxykynurenine; IFN-γ, interferon-γ; IL, interleukin; Kyn, kynurenine; KynA, kynurenic acid; LPS, lipopolysaccharide; mRNA, messenger RNA; iNOS, inducible nitric-oxide synthase; TGF-β, transforming growth factor-β; TNF-α, tumor necrosis factor-α; Trp, tryptophan; xA, xanthurenic acid. The broken lines represent possible reactions [200].