BDNF and major depressive disorder: The change of BDNF level in both peripheral blood and CNS and the imbalance or insufficient of pro-BDNF transformation into mBDNF have been found to be involved in the pathogenesis of multiple diseases such as depression, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, multiple sclerosis, and cerebral ischemic stroke (Eyileten et al., 2021; Wang M. et al., 2021).