BDNF and major depressive disorder: The neurotrophic hypothesis of depression is based on the stress-induced downregulation and rapid-acting antidepressant-induced upregulation of BDNF and neuropeptide VGF (nonacronymic) expression in the brain (Adlard and Cotman, 2004; Thakker-Varia et al., 2010; Nasca et al., 2013; Lin et al., 2014).