BDNF and depressive symptom measurement: In line with this perspective, reduced serum levels and mRNA expression of BDNF were reported in drug-free depressed patients compared to those treated with antidepressants or healthy controls (Karege et al., 2002; Shimizu et al., 2003; Molendijk et al., 2011; Cattaneo et al., 2013), and low BDNF levels were negatively correlated with symptom severity (Shimizu et al., 2003) although it has also been found that BDNF levels did not parallel core clinical features of depression, including severity of symptoms (Molendijk et al., 2011).