IDO1 and schizophrenia: This links to wider bodies of data showing increases in pro-inflammatory cytokines and dysregulated HPA axis, leading to increased indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), driving the raised levels of kynurenine and especially kynurenic acid evident in schizophrenia [331,332].