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Description
The ability to detect novelty in a familiar environment is a critical process across the lifespan as well as across species. The detection of change in a familiar environment requires a comparison between the sensory information from the current environment and an internal representation of that information stored in memory. The hippocampus has been suggested to be a critical substrate to support this match-mismatch process. Studies have shown that the hippocampus, in both rodents and humans, is particularly vulnerable to the effects of aging; therefore, the match-mismatch process may be affected by aging. In the present study, 6 mo and 25 mo old rats were tested on two tasks designed to assess age-related changes in novelty detection for object, odors, object-place associations, and odor-place associations. Subjects in each group were divided randomly and tested on either an (1) object-based or (2) odor-based exploratory task. Testing on each task involved seven 6-min exploratory sessions. During the first session, each rat was allowed to explore an open field environment. During sessions 2-4, each rat was allowed to explore five stimuli (either objects or odor cups) arranged in a geometric configuration on a platform. During sessions 5-6, two objects or odor cups were spatially displaced prior to exploration; one to a position previously occupied by another object or odor cup, and the second to a novel location outside of the original geometric configuration. During session 7, the novel geometric configuration (from sessions 5-6) was maintained and a novel object/odor cup replaced a previously explored object/odor cup in the configuration. The results of the present study demonstrated no age-related differences in habituation to the objects or odor cups during Sessions 2-4. Twenty-five mo old rats showed significant deficits compared to 6 mo old rats in detecting spatial displacement of objects during Sessions 5-6. However, neither 6 mo nor 25 mo rats showed increased exploration of the spatially displaced odor cups during sessions 5-6 on the odor task. There were no significant age-related differences in novel object detection during Session 7; however, 25 mo rats showed significantly poorer novel odor detection compared to 6 mo rats. These data indicate that object-place and odor novelty detection may be impaired in aged rats. Although the data suggest that 25 mo rats show significant deficits in the detection of odor novelty, the findings indicate that olfactory stimuli may not be well suited to measure odor-place memory using the present paradigm.
