IDO1 and schizophrenia: Tryptophan/kynurenine metabolism mediates glutamatergic neurotransmission (NMDA receptor-dependent, N-methyl-D-aspartic acid) through a pro-inflammatory immune response associated with activation of the enzyme indoleamine 2.3-dioxygenase (IDO), resulting in increased production of kynurenic acid in the brain and an imbalance in glutamatergic neurotransmission, leading to NMDA antagonism in schizophrenia [123].