| Summary: | Previous studies reveal decreases in hippocampal interneuron cell densities during normal aging. However, considerable variation in results exists within the literature. Overall, interneuron populations show either decreases or conservation in cell numbers expressing calcium binding proteins parvalbumin (PV) and calbindin (CB), and neuropeptides somatostatin (SOM) and neuropeptite Y (NPY) in hippocampal subregions of CA1, CA3, dentate granule cell layer, and dentate hilus. Notably, only few of the past aging studies showed correlations in cell loss with behavioral impairments in aged animals. This issue was addressed in the present study using male, young and old Fischer 344 rats, that were behaviorally characterized on four tasks before immunohistochemical staining and cell type quantification. Rats performed the Morris Watermaze, W-Track Continuous Spatial Alternation Task, Spontaneous Object Recognition (SOR) task, and Temporal Object Recognition (TOR) task. The old rats showed age-related deficits only in hippocampal-dependent memory tasks. Immunofluorescent imaging revealed an increase in SOM-immunoreactive interneurons in the dentate granule cell layer, as well as an increase in NPY expression in the dentate hilus. All other regions in which neurons were quantified showed no changes in any of the selected interneuron types examined. Contrary to previous findings, we found no decreases in interneuron populations anywhere in the hippocampus.
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