Brain-derived neurotrophic factor is released as a mixture of pro and mature BDNF in an activity-dependent manner, in which the former binds with pan neurotrophin receptor (p75 NTR) to induce apoptosis and facilitate long-term depression (37), whereas mature BDNF binds to TrkB to promote cell survival and increase neuronal spine complexity (38). Here, BDNF is linked to major depressive disorder.