Accordingly, several studies focused on BDNF levels in blood as potential biomarker for major depression disorder (MDD) and reported lower BDNF blood level in both serum and plasma of drug-free patients compared to healthy controls (Bocchio-Chiavetto et al., 2010; Karege et al., 2005; Lee & Kim, 2008; Sen, Duman & Sanacora, 2008; Shimizu et al., 2003). Here, BDNF is linked to major depressive disorder.