BDNF and depressive disorder: By combining the evidence from neurophysiological studies with evidence from studies of morphological alterations caused by stress in animal brains,13, 14, 15 and by associating the morphological alteration to the decreased volume of the hippocampus in patients with MDD,16, 17 Duman et al.18 proposed the neurotrophin hypothesis, in which BDNF is involved in the pathophysiology of depression.