IDO1 and depressive disorder: After a series of enzymatic reactions in the kynurenine pathway, the final product, quinolinic acid, also participates in neuropathic pain modulation.479 For the depression comorbidity, pain induces upregulation of IDO expression both in the hippocampus and liver, accompanied by alterations in metabolites in the kynurenine pathway and increased expression of KMO.