BDNF and major depressive disorder: Recently, the neurotrophin hypothesis has received significant attention, and brain-derived neurotrophic factor (BDNF) is one of the most important neurotrophic factors that is widely measured.3, 4, 5, 6 The hypothesis proposes that decreased neurotrophic signaling is a major factor underlying the pathophysiology of depression, and its restoration underlie the actions of antidepressant treatment.