BDNF and major depressive disorder: Neurotrophic factors (NF) such as Glial cell-derived neurotrophic factor (GDNF), beta-nerve growth factor (β-NGF), and brain-derived neurotrophic factor (BDNF), play key factors in promoting neuron growth, maturing, and repairing (Allen et al., 2013; Xiao and Le, 2016) and MDD patients have neurotrophic factor disorders (Shi et al., 2020).