Aging does not affect the proportion of dorsal medial entorhinal cortex cells active during track running behavior

Juliana Liang , James P. Lister, Carol A. Barnes

Aging does not affect the proportion of dorsal medial entorhinal cortex cells active during track running behavior

Previous work has shown that hippocampal place cells in aged rats show changes in their population dynamics: in CA1 there are errors in retrieving the correct map for a known location (e.g., Barnes et al., 1997), and in CA3 they exhibit difficulty in establishing a new map for a novel location (Wilson et al., 2005). The entorhinal cortex provides input to the hippocampus and could contribute to these observed age-related changes in the population code for space. The medial entorhinal cortex (MEC) contains ?grid cells? that fire in a highly spatially regular fashion (e.g., Hafting et al., 2005). These cells fire in repeating patterns across an entire environment and are believed to contribute significantly to the spatial signal within downstream hippocampal regions. The current experiment investigated whether aging affects population activity in MEC during track running, by quantifying expression of the immediate early gene Arc using the ?catFISH? method (Guzowski et al., 1999). Young and old rats were trained to run in alternating clockwise and counterclockwise laps on a circular track in a constant environment, an activity known to activate Arc expression. The rigid kinetics of Arc localization to distinct cellular compartments allows this gene to mark neuronal activity during discrete behavioral epochs. Arc expression was visualized with fluorescence in situ hybridization, and neurons were quantified as nuclear positive (Nuc+), cytoplasmic positive (Cyto+), and double-labeled (Dbl+). Repeated measures ANOVA revealed a significant (p<0.01) effect of the Arc-compartment factor ? there was a significantly higher proportion of Dbl+ neurons than Nuc+ or Cyto+ in both age groups. The age factor was not significant, indicating that the ability of MEC to reactivate the same populations during the two sessions of track running in the same environment is not affected by age. This suggests that at least the cellular composition of the path integration input to the hippocampus is preserved during aging. This work was supported by McKnight Brain Research Foundation; NIH Grant AG033460; NIA Grant AG036053, and NIA Grant AG003376.

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