BDNF and major depressive disorder: In one study BDNF gene expression was assessed in mice after 30 days of exercise (Sleiman et al., 2016) and in another study on rats where anti-depression treatment effects were studied on BDNF gene the expected time for expression of the same gene was taken as 2 weeks, thus giving enough time for the gene expression to change (Russo-Neustadt et al., 2000).