NGF and Alzheimer disease: Studies have proved that the expression of TrkA, but not p75NTR, is downregulated in the BFCNs and the cortex of AD patients [74,75], while the NGF-immunoreactive protein (which is now thought to be proNGF, the NGF precursor protein) level is elevated in the cortex and hippocampus and degraded in the basal forebrain [37,76,77].