BDNF and major depressive disorder: The neurodegenerative hypothesis of MDD and some studies in this field show low blood levels of BDNF in patients with MDD (Mannari et al., 2008; Monteleone et al., 2008; Lima-Ojeda et al., 2018), although this finding is not unanimous (Almeida et al., 2019; Verduijn et al., 2015; Vu and Student, 2009).