BDNF and major depressive disorder: In particular, decreased levels of the neurotrophins and their respective receptors TRKB and FLK1, have been reported in the PFC and hippocampus in post-mortem studies of subjects with depression (n = 10 vs. n = 24 controls; n = 14 vs. n = 14 controls) (Karege et al., 2005; Qi et al., 2015) as well as in rodent models of chronic stress (Heine et al., 2005; Elfving et al., 2010; Howell et al., 2011).