BDNF and Alzheimer disease: Moreover, alterations in BDNF expression and BDNF/TrkB signaling pathway might induce synapse loss and the consequent cognitive dysfunction (Song et al., 2015), while early downregulation of BDNF in AD was associated with the severity of cognitive impairments (Peng et al., 2005; Garzon and Fahnestock, 2007).