IDO1 and major depressive disorder: Antidepressants can reduce serum levels of inflammatory cytokines and regulate cytokine actions by acting on intracellular adenosyl monophosphate (cAMP), serotonin metabolism, HPA axis, or neurogenesis.23 Another mechanism that has been considered for inflammation-induced depression is alteration of glutamatergic neurotransmission.TNF-α and interferon-γ stimulate activation of indoleamine 2, 3 dioxygenase (IDO).