BDNF and major depressive disorder: According to our findings, BDNF binds to tropomyosin receptor kinase B (TrkB), the high affinity receptor for BDNF, and BDNF-TrkB activates (phosphorylates) ERK1/2 via the Ras-Raf-MEK-ERK cascade [37], which mediates survival, growth, differentiation, plasticity, and anti-depression [13,38].