Social isolation stress impairs passive avoidance learning in senescence-accelerated mouse (SAM)

Despite cumulative evidence showing the detrimental effect of psychosocial stress on the learning/memory functions in dementia diseases, the precise neurobiological mechanisms behind such an effect remain unclear. Mice of the senescence-accelerated mice prone 10 (SAMP10) strain, a neurodegenerative dementia model, were chronically exposed to social isolation stress from the age of 5 weeks. At the age of 12 weeks, conditioning memory and spatial memory were evaluated by one-trial passive avoidance and Y-maze tests, respectively. Chronic social isolation stress significantly reduced conditioning memory but did not affect spatial memory. Although further behavioral tasks using an elevated plus maze and a pain threshold test exhibited stress-induced analgesia, an analysis of covariance excluded the possibility that such analgesia might contribute to the stress-induced impairment of conditioning memory. In addition, endocrinological and immunohistochemical analysis revealed that isolation stress elevated the serum corticosterone levels and inhibited the increase in c-Fos expression in the central amygdaloidal nucleus (CeA) that is required for conditioning memory during passive avoidance learning. In conclusion, chronic social isolation stress exacerbated conditioning memory in SAM mice, probably through a glucocorticoid-mediated decrease in neural activation in the CeA.