Age-related deterioration in conditional avoidance task in the SAM-P/10 mouse, an animal model of spontaneous brain atrophy.

A novel inbred strain of mouse 'SAM-P/10' (Senescence Accelerated Mouse) is a model of age-related brain atrophy characterized by age-related loss and shrinkage of neurons in the cerebral neocortex. Age-related changes in learning and memory skills of SAM-P/10 mice were investigated using a newly developed conditional avoidance task in a T-maze. Comparisons were made with findings in the SAM-R/1 strain which shows a little loss and no shrinkage of neocortical neurons. Four-month-old SAM-R/1 and SAM-P/10 performed well during a 10-day training schedule of the conditional avoidance task. SAM-R/1 mice over 17 months of age were slower learners than younger SAM-R/1 mice but reached nearly the same high percentage avoidance as seen in the 4-month-old mice during the last 4 days of the schedule. Performance of the SAM-P/10 mice gradually worsened with aging and 10- to 12-month-old SAM-P/10 mice could not reach the percentage avoidance seen with the 4-month-old mice, even after the 10-day training. When the mean percentage of successful avoidance or escape behavior on every training day was plotted, the curves were much the same for both SAM-R/1 and SAM-P/10 mice, of any age. These results show that aged SAM-P/10 mice retained the left-right turning discrimination in the T-maze and lost the ability to predict the forthcoming aversive shock by associating conditioned stimulus and unconditioned stimulus.