BDNF and major depressive disorder: MiR-124a is a direct regulator of BDNF expression, and as expected, BDNF is down-regulated in hippocampus of rats after social defeat stress, and over-expression of miR-124a in rat hippocampus exacerbates social defeat stress-induced depression-like behaviors as measured by novelty suppressed feeding, sucrose preference and force swim tests (Bahi et al., 2014).