BDNF and major depressive disorder: Despite some studies suggesting that patients with major depression have alterations in cortisol and BDNF levels (Leuchter et al., 2010; Tadić et al., 2011; Herbert, 2013), there is no consensus (Pariante et al., 2004; Molendijk et al., 2011; Elfving et al., 2012; Papakostas et al., 2013).