TAC1 and Alzheimer disease: The level of substance P decreases in the cortex, hippocampus and striatum in Alzheimer's disease patients and animal models (Bouras et al., 1990; Quigley and Kowall, 1991; Nag et al., 1999; Ahmed et al., 2004), but increases in the pallidum and substantia nigra (Bouras et al., 1990).