BDNF and major depressive disorder: There are many hypotheses for depression, such as the monoamine neurotransmitter hypotheses, neurotrophic factor hypotheses (mainly brain-derived neurotrophic factor (BDNF) expression and functional downregulation), neurocircuitry hypotheses, neuroendocrine hypotheses, neuroinflammation hypotheses, gut microbiota hypotheses, neuroplasticity hypotheses, hypothalamic-pituitary-adrenal (HPA) axis hypothesis, etc. [21–26].