IDO1 and major depressive disorder: Regarding the abovementioned causes, we speculate that alcohol may lead to bacterial translocation and induce systemic inflammation, which is subsequently attributed to the activation of IDO-proinflammatory cytokines that decrease 5-HT and increase Kyn levels, while the activation of IDO is associated with the upregulation of Kyn and the downregulation of 5-HT in relation to depression (Miura et al., 2008; Yabut et al., 2019).