MAPT and Alzheimer disease: Additionally, a couple of research reveal that when 5-HT level is decreased that cause increased tau expression and cellular aggregation (John et al., 1991) which have been associated with the Alzheimer’s disease (AD) (Yang and Schmitt, 2001; Doraiswamy, 2003; Gudelsky and Yamamoto, 2003; Kovacs et al., 2003).