IDO1 and major depressive disorder: Increased activation of IDO-1 can lead to decreased serotonin, thereby depleting the mood modulatory neurotransmitter [see serotonin hypothesis in review (13)], while increasing the N-methyl-D-aspartate (NMDA) agonist, QA, to exert an imbalance in (hypo)serotonergic and (hyper)glutamatergic neurotransmissions in depression.