Increased dendritic extent in hippocampal CA1 neurons from aged F344 rats

Age-related dendritic alterations were evaluated in F344 rats following a water maze assessment of spatial memory. Based on the probe trial times, 39% of the aged animals were designated impaired. CA1 pyramidal neurons were labeled intracellularly with neurobiotin in brain slices prepared from these animals. Neurons (aged: n = 15; young: n = 11) were reconstructed using a microscope-based three-dimensional system. Increased dendritic length was observed in the aged neurons both for basal dendrites (aged = 4.54 mm and YOUNG = 3.33 mm) and the entire neurons (aged = 14.8 mm and YOUNG = 10.8 mm). However, dendritic length values did not correlate with the individual animalʹs probe trial time. Sholl analysis revealed a diffuse increase in dendritic branch intersections in the cells from aged rats, which on branch order analysis was noted to be due to an increased number of distal branches. Mean electronic distance to dendritic terminals, a functional assessment of synaptic efficacy, was longer in the aged neurons (aged = 0.67 λ and YOUNG = 0.55 λ). These results suggest a lengthening and increased complexity of CA1 pyramidal neurons with successful aging, which may represent either an intrinsic response to aging or a reactive partial denervation response to a loss of afferent inputs.