BDNF and major depressive disorder: A preponderance of clinical and experimental data indicates an essential role for BDNF in the pathophysiology of depression, leading to the “Neurotrophin Hypothesis of Depression” (Chen et al., 2001; Dwivedi et al., 2003; Duman and Monteggia, 2006; Sen et al., 2008; Dunham et al., 2009; Dwivedi, 2009; Tripp et al., 2012).