IDO1 and mood disorder: Mechanistically, such immune activation can upregulate the enzyme indoleamine 2,3-dioxygenase 1 (IDO1), diverting tryptophan from serotonin synthesis toward neurotoxic kynurenine metabolites (e.g., 3-hydroxykynurenine, quinolinic acid) that impair synaptic plasticity and drive excitotoxicity, a process observed in both human and rodent models of mood disorders [18,19,20,21].