IDO1 and major depressive disorder: Inflammation has been shown to be involved in depression-related pathophysiological mechanisms such as monoaminergic neurotransmission alterations, activation of the indoleamine-pyrrole 2,3-dioxygenase (IDO) enzyme degrading tryptophan towards neurotoxic metabolites, glutamate-related neurotoxicity, and Hypothalamic-Pituitary-Adrenal (HPA)-axis dysregulation (Felger and Lotrich, 2013; Miller and Raison, 2016; Obici et al., 2002; Simmons et al., 2020)