BDNF and major depressive disorder: Stress decreases BDNF concentrations in hippocampus and PFC of animal models of depression (Smith et al., 1995; Duman and Monteggia, 2006; Filho et al., 2015), in line with the reduced expression of this neurotrophic factor observed at cortical, hippocampal, and peripheral level of depressed patients (Thompson Ray et al., 2011; Reinhart et al., 2015).