AGING SELECTIVELY DECREASES THE PROPORTION OF DENTATE GYRUS PUTATIVE GRANULE CELLS WITH PLACE FIELDS

P.A. Letts *, J. Meltzer, S.R. VanRhoads, C.A. Barnes

NSMA , Univ Arizona, Tucson, AZ, USA

The dentate gyrus (DG) has been identified as the hippocampal subregion most vulnerable to the effects of normal aging (Small et al., 2004). In the present study, young and memory-impaired old Fischer-344 rats were implanted with a hyperdrive. Hippocampal cell firing from CA1, CA3, and the DG were simultaneously recorded with tetrodes as the rats ran around a circular track. Tetrode locations during recording sessions were verified by histological reconstruction. Cells in the dentate gyrus were classified on the basis of firing rate as either putative granule cells (pGCs) or interneurons (Jung & McNaughton, 1993). The percentage of pyramidal cells with place fields was either similar between the aged and young rats (CA1) or slightly increased in the old rats (CA3). In the DG, the percentage of pGC with place fields was significantly reduced in aged animals. While the majority of pGCs in young animals either exhibited a place field or spiked at extremely low frequencies during behavior, the majority of pGCs in aged animals spiked randomly in all areas of the apparatus. Previous studies have demonstrated that aged, memory-impaired rats often retrieve incorrect hippocampal spatial maps when placed in a familiar environment (pattern completion error, Barnes et al., 1997) and often fail to generate a new map following environmental changes (pattern separation error, Tanila et al., 1997). Memory models have implicated CA3 for pattern completion and the DG for pattern separation (e.g., McNaughton & Morris, 1987; Marr, 1971). The observed trend of impaired selectivity of pGC firing within the DG may result in increased network noise and the dysfunctional pattern separation observed in aged, memory-impaired animals.

Support Contributed By: AG012609 & AG007434

Key words: memory impairment, hippocampus, behavior, rat