NTRK2 and major depressive disorder: Multiple lines of evidence link brain-derived neurotrophic factor (BDNF), its specific receptor tropomyosin-receptor-kinase B (TrkB), and subsequent mammalian target of rapamycin complex 1 (mTORC1) signaling to the pathophysiology of MDD, as well as the therapeutic mechanisms of antidepressants (Nestler et al., 2002; Hashimoto et al., 2004; Duman and Monteggia 2006; Martinowich et al., 2007; Hashimoto, 2010; Hashimoto, 2011, 2013a; Duman et al., 2012; Castrén, 2014; Lindholm and Castrén, 2014).