CHAT and Alzheimer disease: The activity of choline acetyltransferase in the AD brains was greatly reduced in the amygdala, hippocampus, and cortex, in which the concentration of acetylcholine was decreased at synapses.50–52 The activity of glutamic acid decarboxylase, tyrosine hydroxylase, aromatic amino acid decarboxylase, dopamine-β-hydroxylase, and monoamine oxidase in all the areas of the AD brains studied appeared to be well within the normal range.