TNF and depressive disorder: Exposure to stressors such as immobilization stress (IS) and social defeat stimulates the secretion of adrenaline and glucocorticoids in the adrenal gland via the hypothalamic–pituitary–adrenal (HPA) axis and tumor necrosis factor (TNF)-α and interleukin (IL)-6 in immune cells, leading to the outbreak of anxiety/depression and gut inflammation3,4.