Cytokines including interleukin (IL)-1, IL-6, tumor necrosis (TNF)-alpha and interferon (IFN)-gamma induce indoleamine 2,3 dioxygenase (IDO), the enzyme responsible for degrading tryptophan, the primary precursor of serotonin (5-HT), into kynurenine, which is eventually metabolized into quinolinic acid (QUIN), a potent NMDA agonist and stimulator of glutamate (Glu) release.