BDNF and major depressive disorder: As to how BDNF correlates with depression, currently it is widely accepted that changes in BDNF expression reverses or exacerbates depression-associated dysfunction in neurogenesis, neuro-survival, and synaptic plasticity, since BDNF promotes the survival of existing neurons and encourages the growth and differentiation of new neurons and synapses (Dean and Keshavan, 2017)