BDNF and Alzheimer disease: Post‐mortem studies in AD revealed a severe reduction in the expression levels of BDNF (Phillips et al. 1991) and TrkA and B, upregulation of proNGF (due to decrease of proteolytic conversion) and sortilin (Fahnestock et al. 2001), and failure of retrograde transport of NGF signal in cholinergic neurons (Siegel and Chauhan 2000; Mufson et al. 1995).