BDNF and depressive disorder: Both depression and the Met allele of the Val66Met polymorphism on the gene coding for BDNF may be accompanied by reductions in structural and functional connectivity of the hippocampus and prefrontal cortex (Frodl et al., 2007; Molendijk et al., 2012; Kim et al., 2013; Ziegler et al., 2013; Tatham et al., 2016), and BDNF concentrations in serum have been linked to depressive disorders (Shimizu et al., 2003; Wolkowitz et al., 2011; Bus et al., 2015).