FGFR3 and Alzheimer disease: Moreover, Aβ oligomers are considered a primary cause of impairment of the cholinergic system in the brains of AD patients, which show multiple aspects of cholinergic system dysfunction such as decreases in Ach levels, low activity of choline synthetase, and a reduction in the number of cholinergic receptors (Bao et al., 2012; Yang et al., 2014).