BDNF and major depressive disorder: The investigations in animal models of stress and depression showed a decrease in BDNF mRNA and protein levels in the hippocampus, prefrontal cortex, frontal cortex and amygdala [20–26], and an increase in the nucleus accumbens and ventral tegmental area (VTA) [27,28], which could indicate a distinct role of BDNF in different brain structures.