Aging, Alzheimer's disease, and the cholinergic system of the basal forebrain.

All giant neurons of the medial basal forebrain stained for choline acetyltransferase (ChAT). Cell numbers declined from 400,000 to 475,000 in young controls to approximately 140,000 in elderly controls. Five senile dementia cases had counts ranging from 45,000 to 100,000 cells. ChAT levels in control frontal cortex decreased from 1.2 mumol/hr/100 mg protein at age 40 to 0.5 at age 95. Five senile dementia cases had levels ranging from 0.04 to 0.30. When the cholinergic cell count in the basal forebrain drops below about 100,000 cells, the level of cortical ChAT may be so low that clinical dementia appears.