BDNF and major depressive disorder: BDNF transcripts containing Bdnf exon-IV and -VI are particularly interesting as their translation is directly or indirectly regulated by changes in neuronal activity (Hong et al., 2008; West et al., 2014; Tuvikene et al., 2016) and their dysregulation is linked to various brain pathologies related to sleep, loss of fear memory (Hill et al., 2016), and depression (Marosi and Mattson, 2014).