BDNF and major depressive disorder: Brain‐derived neurotrophic factor (BDNF), one of the neurotrophic factors, does main favour in neurogenesis, neural survival and growth.40, 41 BDNF binds to its tropomyosin receptor kinase B (TrkB) to activate downstream signalling pathways and then completes phosphorylation and activation of the cAMP response element‐binding protein (CREB).42 The BDNF reduction in both hippocampus and PFC is closely associated with depression.