BDNF and major depressive disorder: The imbalanced activation of neuron-endocrine and hormonal function can give rise to affective disorders such as depression by suppressing brain-derived neurotrophic factor (BDNF), dopamine, and serotonin levels in the brain and gut inflammation and microbiota dysbiosis by inducing inflammatory immune responses in the gastrointestinal tract [[5], [6], [7]].