BDNF and major depressive disorder: The excessive secretion of glucocorticoids and inflammatory cytokines suppresses the expression of brain-derived neurotrophic factor (BDNF), which is closely associated with the occurrence of psychiatric disorders including depression through the modulation of neuroplasticity in the brain [8], in the hippocampus and activates the innate and adaptive immune responses in the gastrointestinal tract, resulting in the outbreak of anxiety, depression, gut inflammation, and gut dysbiosis [9–11].