BDNF and major depressive disorder: The evidence of the correlation between neurodegeneration and depression can be mainly derived from the following aspects: (1) Hippocampal and the pre-frontal cortex (PFC) volume has been observed to be consistently reduced in the depressed patients (134–136); (2) Decreased levels of BDNF, dendritic atrophy, decreased neurogenesis, and increased neuronal death in patients with depression (137, 138); (3) Depression and other neurodegenerative diseases can exhibit a high co-morbidity (139).