REACTIVITY TO NOVELTY IN COGNITIVELY-IMPAIRED AND COGNITIVELY-UNIMPAIRED AGED RATS AND YOUNG-RATS

Two distinct populations of aged, Long-Evans rats can be identified on the basis of performance in the Morris water maze task. Aged (24 mont h) unimpaired rats perform similarly to young (six month) animals. Age d, impaired rats display latencies to find the submerged platform grea ter than two standard deviations from the mean of the young animals. A hallmark of efficient cognitive processing is the ability to cope wit h environmental change. Consequently, the present studies were conduct ed to assess if aged, impaired animals display differential reactivity to repeated exposure to novel stimuli. Reactivity was assessed by exa mining the degree of (i) consumption of a novel gustatory/olfactory st imulus (sweetened milk), (ii) pain inhibition induced by exposure to a novel hot-plate (48.5 degrees C) apparatus and (iii) exploratory beha viour in an elevated plus maze and a novel open field. Aged, impaired rats exhibited lower milk consumption on day one and protracted reacti vity (lower consumption over days two to eight) in comparison to aged, unimpaired and young animals. Aged, impaired rats were more reactive to novelty on the hot plate test (as indicated by longer paw lick late ncies); this novelty-induced pain inhibition did not habituate in aged , impaired rats following repeated plate exposures. The degree of expl oratory behaviour in both the plus maze and the open field was reduced in aged, impaired rats. This effect was not entirely a consequence of deficient affective mechanisms, as measures of anxiety (e.g., time in open arms, time in inner squares) were not different among aged impai red, aged unimpaired and young animals. These results are the first to demonstrate that behavioural deficits observed in aged, impaired anim als extend beyond the impairments observed in the water maze. This beh avioural profile is attributed. in part, to heightened anxiety. In add ition, the impairments observed in aged, impaired animals may also ref lect a reduced sensitivity to the positive incentive properties of nov el stimuli. (C) 1998 IBRO. Published by Elsevier Science Ltd.