Age-related shrinkage of cortically projecting cholinergic neurons: a selective effect.

The number and size of basal forebrain neurons that provide the cholinergic innervation for the cerebral cortex, amygdala, and hippocampus were studied in young and aged mice. The results showed that these neurons became substantially smaller with increasing age. This effect was relatively selective, since the immediately adjacent cholinergic neurons in the striatum did not show a change of similar magnitude. The shrinkage of these basal forebrain neurons may account for the decline of cholinergic innervation that occurs with age. In the material that we examined, aging did not influence the number of cholinergic neurons in the basal forebrain, only their size. It seems, therefore, that the age-related changes in cholinergic function (and their putative behavioral consequences) are not associated with a substantial component of irreversible cell death.