Redistribution of spatial representation in the hippocampus of aged rats performing a spatial memory task.

Abstract

Young and old rats performed on a maze according to a forced-choice and then a spatial memory procedure either in the same or a different environment.Aged rats were slower to learn the spatial memory task when tested in the same, but not in a different, room.One interpretation of this pattern of results is that although old rats learn new rules as quickly as young rats, they show less flexibility with old rules and familiar spatial information.Impaired choice accuracy during asymptote performance suggests poor processing of trial-unique information by old rats.Spatial correlates of hippocampal CA1 and hilar cells varied with task demand: CA1 cells of aged rats showed more spatially selective place fields, whereas hilar cells showed more diffuse location coding during spatial memory, and not forced-choice, tests.Such representational reorganization may reflect a compensatory response to age-related neurobiological changes in hippocampus.Extensive literature indicates that normal aging is accompanied by a decline in the ability to acquire new information.Importantly, this impairment appears selective to certain types of information.For example, aged rats perform relatively poorly on complex visuospatial memory tasks (for a review, see Barnes, 1990).Past studies have ruled out the possibility that the impaired performance is due to changes in motor system function, motivation level, or visual function.The selectivity of the learning impairment has been demonstrated by many laboratories that found that aged rats perform as well as young rats on visually dependent, nonspatial maze tasks (Barnes, Green, Baldwin, & Johnson, 1987;Lowry, Ingram, Olton, Waller, Reynolds, & London, 1985;Rapp, Rosenberg, & Gallagher, 1987;Winocur, 1988).Given that hippocampal lesions often produce deficits in tasks requiring animals to learn trial-unique data, the spatial learning impairment of old rats may be mediated by age-associated neurobiological changes in hippocampus.One could also argue that old rats have difficulty learning the rules associated with particular spatial tasks.The latter interpretation suggests the involvement of nonhippocampal brain structures (e.g., frontal cortex; Winocur & Moscovitch, 1990).Therefore, we evaluated the relative contribution of rule-based and item-and event-based learning systems to the spatial navigation deficit shown by old rats.Also, given the extensive evidence indicating at least hippocampal involvement in the spatial decline of old animals (e.g., Barnes, 1979;Gallagher, Bostock, & King, 1985), we investigated possible age changes in spatial coding by hippocampal neurons.