Neuronal number and size are preserved in the nucleus basalis of aged rhesus monkeys.

Neurons in the nucleus basalis of Meynert (NBM) were analyzed morphometrically in 21 rhesus monkeys ranging in age from 9 to 33 years. Numbers of cholinergic neurons were similar across all ages at several NBM levels in either Nissl-stained paraffin sections or sections processed immunocytochemically for nerve growth factor receptor (p75LNGFr). Size of NBM neurons was larger in aged monkeys than young monkeys at all NBM levels, particularly in the most posterior subdivision. A subset of monkeys were behaviorally characterized shortly before death, and partial correlation analyses indicated that increased age was associated with declines in recognition memory, visuospatial orientation, and reaction time. Controlling for age, spatial memory and concurrent discrimination abilities were associated with lower cell number in intermediate NBM. Numbers of neurons in anterior NBM did not correlate with any behavioral measure. These observations indicate that numbers of NBM cholinergic neurons are stable with age, that NBM neurons become hypertrophic in older animals, and that morphometric indices of cholinergic neurons are associated with cognitive function.