IDO1 and hippocampal atrophy: It is noteworthy that IDO1 is a proinflammatory enzyme that converts l-tryptophan (the precursor of serotonin; 5-HT) to kynurenine and its neurotoxic metabolites that are able to produce oxidative stress by increasing the production of reactive oxygen species (ROS) or to overstimulate hippocampal N-methyl-d-aspartate (NMDA) receptors and lead to apoptosis and hippocampal atrophy [33].