BDNF and Alzheimer disease: It is already known that an impaired BDNF/TrkB-FL system plays an important role in the pathogenesis of AD (Phillips et al., 1991; Connor et al., 1997; Ferrer et al., 1999; Arancibia et al., 2008; Zuccato and Cattaneo, 2009; Kemppainen et al., 2012; Nagahara et al., 2013; Jerónimo-Santos et al., 2015).