BDNF and depressive disorder: Meanwhile, gut microbiota has a profound influence on the progression of AD and animal studies have shown that fecal microbiota transplantation from AD into germ-free mice interferes with recipient cognitive function, induces anxiety/depression behavior decreases expression of brain-derived neurotrophic factor (BDNF), and the function of mGluR1/PKC ε pathway in the brain (Zhao et al., 2020).