Premature Aging Phenotype in Mice Lacking High-Affinity Nicotinic Receptors: Region-Specific Changes in Layer V Pyramidal Cell Morphology

The mechanisms by which aging leads to alterations in brain struc- ture and cognitive deficits are unclear. A deficient cholinergic system has been implicated as one of the main factors that could confer a heightened vulnerability to the aging process, and mice lacking high- affinity nicotinic receptors (β2 �/� ) have been proposed as an animal model of accelerated cognitive aging. To date, however, age-related changes in neuronal microanatomy have not been studied in these mice. In the present study, we examine the neuronal structure of yellow fluorescent protein (YFP + ) layer V neurons in 2 cytoarchitec- tonically distinct cortical regions in wild-type (WT) and β2 �/� animals. We find that (1) substantial morphological differences exist between YFP + cells of the anterior cingulate cortex (ACC) and primary visual cortex (V1), in both genotypes; (2) in WTanimals, ACC cells are more susceptible to aging compared with cells in V1; and (3) β2 deletion is associated with a regionally and temporally specific increase in vulnerability to aging. ACC cells exhibit a prema- turely aged phenotype already at 4-6 months, whereas V1 cells are spared in adulthood but strongly affected in old animals. Collectively, our data reveal region-specific synergistic effects of aging and geno- type and suggest distinct vulnerabilities in V1 and ACC neurons.