BDNF and depressive disorder: Even though most studies suggest that the serum BDNF levels of patients with depression are significantly lower than healthy controls (Gersner et al., 2014; Huang et al., 2017; Alves and Rocha, 2018; Wagner et al., 2018), this finding is not unanimous (Hellweg et al., 2008; Jevtović et al., 2011; Molendijk et al., 2011; Papakostas et al., 2011; Elfving et al., 2012).