IFNG and major depressive disorder: Stressful conditions can affect the gut microbiota, which in turn induces the production of inflammatory mediators (mainly IL-6 and IFN-γ).256 Proinflammatory cytokines or toxic QUIN resulting from alterations in the microbiota may pass through the BBB and activate NMDARs.243 Under the dysfunction of liver, the level of ammonia is increased in the brain.257 The pathogenic factors of various organs at the body level and the pathological changes of glial cells at the cellular level should attract more attention to explain the pathogenesis of MDD.