BDNF and depressive disorder: A common single-nucleotide substitution in the human bdnf gene results in a Val66Met substitution in the BDNF pro-peptide sequence and has been shown to affect activity-dependent BDNF secretion and to be associated with a decrease in hippocampal volume, impairment of episodic memory, and increase in depression and anxiety disorders (Chen et al. 2006; Egan et al. 2003; Soliman et al. 2010; Verhagen et al. 2010).