BDNF and depressive disorder: Several studies have shown reduced serum levels of brain-derived neurotrophic factor (BDNF) in patients with MD compared to healthy individuals (Brunoni et al., 2008; Molendijk et al., 2014; Sen et al., 2008), and evidence also exists to support renormalization of BDNF levels upon successful anti-depression interventions (Brunoni et al., 2008; 2014).