NGF and major depressive disorder: Among these neurotrophic factors, NGF is one of the key modulators for neurogenesis, neurite outgrowth, nerve plasticity, and axonal outgrowth during neuronal development, and indeed many neurodegenerative diseases are closely associated with insufficiency of NGF in the brain, e.g., depression and Alzheimer’s disease (Xu et al., 2013; Xu et al., 2016).