– Aβ induces both depressive-like behavior and decreases brain serotonin levels;– Increased microglial activity of IDO might partially explain reduced serotonin levels;– Reduced tryptophan and increased quinolinic acid in plasma might drive depressive-like behavior in AD;– Alterations in the dopaminergic system, including reduced levels of dopamine and its receptors might contribute to hippocampus-dependent memory deficits and reward circuitry dysfunction. Here, IDO1 is linked to Alzheimer disease.