BDNF and major depressive disorder: Recently, BDNF has been implicated as a possible biomarker for psychiatric diseases such as schizophrenia, bipolar disorder and major depression, given that BDNF levels are lower in these patients than in healthy subjects (Castren & Kojima, 2017; Eisen et al., 2015; Grah et al., 2014; Hashimoto, 2010; Kim et al., 2007; Lee & Kim, 2009; Priya et al., 2016).