Alzheimer's disease (AD) is a neurodegenerative disorder, characterized by a progressive impairment of cognitive and behavioral function. People with AD sometimes get lost in familiar places. Impairment of working and reference memory is thought to cause confusion with places, which can lead to wandering. However, previous animal studies have focused mainly on working memory, while reference memory deficit had not completely understood. We have demonstrated that β-amyloid (Aβ) infused rats caused impairment of reference memory using "residential maze task (RMT)". RMT is similar to natural feeding and drinking behavior for rats. Water cup is put in the start box, and powder-food is placed in the goal box, thus moderately hungry rats can learn the maze by coming and going between start and goal boxes in residence period. Rats can retain the acquired information in RMT over a month. Briefly, RMT is thought to be the most suitable task to evaluate reference memory in AD model rats.
It is known that social isolation in elderly people is a risk factor for dementia. However, in animal studies, much evidence has been obtained from experiment of early separation of mother and infant. The aim of this study was to examine whether social isolation (isolation rearing) in adulthood affects reference memory deficit in Aβ-infused rats.
Four-month-old male rats were divided into pair-reared group (PR) and isolation-reared (IR) group. Eighty days after the experimental rearing, rats were given surgery, and divided into one of the 4 experimental groups (PR-cont, PR-Aβ, IR-cont, IR-Aβ). Ten days after starting infusion of Aβ, rats were given the RMT. In the test trial conducted everyday before the residence period, all rats showed a significant decrease both of error response and running time to reach the goal box for 5 days. Two weeks after the end of learning period, rats were tested reference memory in RMT. IR-Aβ rats have increased the number of errors compared to other groups of rats.
These finding suggests that isolation rearing in adulthood hastens the onset of reference memory deficit in β-amyloid infused rats.