BDNF and major depressive disorder: Clinical studies have indicated that lower levels of fibroblast growth factor-2 (FGF-2), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF) in the postmortem brain or blood from patients with major depressive disorder (MDD) were attenuated by antidepressant medications [5]–[8].