BDNF and major depressive disorder: A key tenet of this hypothesis is that the hippocampal expression of BDNF is negatively modulated by stressors and positively modulated by chronic antidepressant treatments [4]; in addition, more recent studies have shown that BDNF/trkB signaling and function in the mesocorticolimbic dopamine (DA) circuits also play a key role in the etiopathology of depression [6].