Memory impairment in aged primates is associated with focal death of cortical neurons and atrophy of subcortical neurons

Abstract

Mechanisms of cognitive decline with aging remain primarily unknown. We determined whether localized cell loss occurred in brain regions associated with age-related cognitive decline in primates. On a task requiring the prefrontal cortex, aged monkeys were impaired in maintaining representations in working memory. Stereological quantification in area 8A, a prefrontal region associated with working memory, demonstrated a significant 32 +/- 11% reduction in the number of Nissl-stained neurons compared with young monkeys. Furthermore, the number of immunolabeled cholinergic neurons projecting to this region of cortex from the nucleus basalis was also reduced by 50 +/- 6%. In contrast, neuronal number was strikingly preserved in an adjoining prefrontal cortical region also associated with working memory, area 46, and in the component of the nucleus basalis projecting to this region. These findings demonstrate extensive but highly localized loss of neocortical neurons in aged, cognitively impaired monkeys that likely contributes to cognitive decline. Cell degeneration, when present, extends transneuronally.

Figure 2.

Quantification of neuronal number and morphology in dorsolateral prefrontal cortex. A, Lateral surface of prefrontal cortex indicating area 46 (dark shading) and area 8A (light shading), in which neuronal number was quantified. Vertical lines indicate sterological boundaries. Lines marked r46 and c46 delineate rostral and caudal limits of area 46, and lines r8A and c8A denote rostral and caudal boundaries of area 8A. B, Thionin-stained coronal section of prefrontal cortex, showing grouping of layers II–IV and V–VI for quantification, on the basis of projection patterns. Scale bar, 150 μm. C, Higher magnification of thionin-stained sections in layers III–IV of area 8A in young adult; D, aged monkey, suggesting reduced cell density in aged monkey. Scale bar: C, D, 70 μm. E, Stereological quantification of neuronal number in area 46 and area 8A. In area 46, neuronal number was preserved in aged monkeys. In contrast, highly significant reduction in neuronal number was observed in layers II–IV (p < 0.0001) and V–VI in area 8A of aged monkeys (p < 0.05).

Figure 1.

Working memory performance on delayed response task. Performance of aged monkeys did not differ significantly from young adult monkeys at delays of 5 sec and less, but significant impairments were present at each delay period of 10 sec and longer.

Figure 3.

Reduction of AChE-stained axon density in dorsolateral prefrontal cortex in aged monkeys. AChE-staining of axons in area 8A of young (A) and aged (B) monkeys. Scale bar: A, B, 55 μm. C, Quantification of AChE-stained axon density shows significant reductions in layers I–IV of area 8A (p < 0.05), in which Nissl-stained neurons are also lost. In contrast, AChE-stained axons are preserved in area 46, in which the number of Nissl-stained neurons was also preserved. Reductions in cortical volume do not account for these changes (see Results).

Figure 4.

p75-labeled cholinergic neuronal numbers are selectively reduced in aged monkeys. Low-magnification views of representative sections from a young adult (A) and an aged (B) monkey demonstrate distribution of immunolabeled neurons in the intermediate division of the basal forebrain (Ch4i). ap, Ansa pdenuclaris; Ch4id, intermediodorsal component of Ch4 region; Ch4iv, intermedioventral component of Ch4 region. Scale bar: A, B, 270 μm. Higher-magnification views from a young adult (C) and an aged (D) monkey show a reduction in neurite density in old monkeys. Scale bar: C, D, 45 μm. E, Stereological quantification of the number of p75-labeled neurons revealed significant reductions in Ch4i (p < 0.0001), the division of the cholinergic basal forebrain that projects to the area of prefrontal cortex in which neuronal loss occurred (area 8A). Ch4a, which innervates the area of prefrontal cortex in which neuronal number was preserved, displayed no reduction (p > 0.05).