Teaching old rats new tricks: age-related impairments in olfactory reversal learning.

Recent work suggests that normal aging may be associated with decline in different brain systems. In the present study, young and aged Long-Evans rats were tested in a spatial version of the Morris water maze dependent on medial temporal lobe function and also on an odor discrimination reversal task previously used to investigate orbitofrontal function. Aged rats acquired the odor discrimination problems normally but were impaired in acquiring subsequent reversals of the problems. A subset of the aged rats also exhibited impaired spatial learning in the water maze. There was no correlation between reversal performance and spatial learning in the aged rats, indicating that the reversal learning impairment was not related to decline in medial temporal lobe function. Instead the performance of the aged rats on the odor discrimination task resembled that of young rats with neurotoxic lesions of orbitofrontal cortex. These data indicate that rats show independent decline of different brain systems during normal aging and suggest orbitofrontal cortex as one prefrontal area where changes may be localized for further study.