Progressive decline in spatial learning and integrity of forebrain cholinergic neurons in rats during aging

Rats distributed over five different age groups, 3, 12, 18, 24 and 30 months of age, were screened for their spatial learning and memory ability in the Morris water maze, and the degree of place navigational impairments was correlated with morphological changes in the four major forebrain cholinergic cell groups (medial septum, MS; vertical limb of the diagonal band of Broca, VDB; nucleus basalis magnocellularis, NBM; and striatum) using choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) histochemistry. Impaired place navigation developed progressively with age, such that 8% of the 12-month-old rats, 45% of the 18-month-old, 53% of the 24-month-old, and over 90% of the 30-month-old rats were behaviorally impaired. Significant reductions in the number of ChAT/NGFr-positive cell bodies, amounting to between 19 and 45%, were observed in all four cell groups, and the remaining cells were reduced in size (6-24% reduction in cross-sectional area in the oldest age groups). Although the morphological changes were less severe and tended to develop later than the behavioral impairments, there was overall a significant correlation between water maze performance and ChAT/NGFr-positive cell counts, and to a lesser degree also cell size in all four cell groups. These changes were also highly correlated with age. The highest correlations were seen in MS, VDB and NBM, which are known to play a role in spatial memory performance in young rats. The results indicate that degenerative and/or atrophic changes in the forebrain cholinergic system and decline in spatial learning ability are parallel processes during aging. Although the magnitude of the morphological changes does not appear to be substantial enough, by itself, to explain the severe spatial learning impairments that develop in the oldest animals, the present data are consistent with the view that impaired function in the forebrain cholinergic system can contribute to age-dependent cognitive decline in rodents.