IDO1 and major depressive disorder: To investigate whether ICV-STZ-induced depression-like behaviors are related to the IDO-mediated kynurenine pathway, we examined the levels of kynurenine metabolites in several brain regions associated with mood disorders, including the anterior cingulate cortex (ACC), PrL, IL, lateral habenula (LHb), dorsal raphe nucleus (DRN), hippocampus (Hip), and locus coeruleus (LC).