A large amount of evidence has highlighted a decreased level of BDNF as well as mRNA related to BDNF signalling in the hippocampus of patients with severe AD [137,138] but also in other brain regions such as the entorhinal cortex, frontal cortex, temporal cortex, and parietal cortex [137,139,140,141,142,143,144,145,146]. Here, BDNF is linked to Alzheimer disease.