Therefore, IDO-1-mediated degradation of Trp through the KP can be associated with depression via two potential mechanisms: i) decreasing the bioavailability of circulating Trp as an essential amino acid for the synthesis of serotonin and ii) production of neurotoxic metabolites downstream of IDO-1 especially QUIN [160, 161]. Here, IDO1 is linked to major depressive disorder.