BDNF and major depressive disorder: Levels of γ -GABA (Cryan and Kaupmann, 2005; Kumar et al., 2013; Olivier et al., 2013), serotonin (Wikoff et al., 2009; Hillemacher et al., 2018), norepinephrine (NE) (Diaz Heijtz et al., 2011; Clarke et al., 2013), and BDNF (aan het Rot et al., 2009; Diaz Heijtz et al., 2011; Gareau et al., 2011; Bus et al., 2015) are decreased in patients with depression and anxiety.