BDNF and major depressive disorder: Decreased levels of BDNF-TrkB signaling pathway were often reported in depressed subjects, and up-regulation of BDNF-TrkB via pharmacological agents in the prefrontal cortex (PFC) and hippocampus can confer resilience to inescapable stress and alleviate depression symptom (Ren et al., 2016; Kato et al., 2018).