Age-related dendritic alterations were evaluated in F344 rats followin
g a water maze assessment of spatial memory. Based on the probe trial
times, 39% of the aged animals were designated impaired. CAI pyramidal
neurons were labeled intracellularly with neurobiotin in brain slices
prepared from these animals. Neurons (aged: n = 15; young: n = ii) we
re reconstructed using a microscope-based three-dimensional system. In
creased dendritic length was observed in the aged neurons both for bas
al dendrites (aged = 4.54 mm and young = 3.33 mm) and the entire neuro
ns (aged = 14.8 mm and young = 10.8 mm). However, dendritic length val
ues did not correlate with the individual animal's probe trial time. S
hell analysis revealed a diffuse increase in dendritic branch intersec
tions in the cells from aged rats, which on branch order analysis was
noted to be due to an increased number of distal branches. Mean electr
otonic distance to dendritic terminals, a functional assessment of syn
aptic efficacy, was longer in the aged neurons (aged = 0.67 lambda and
young = 0.55 lambda). These results suggest a lengthening and increas
ed complexity of CA1 pyramidal neurons with successful aging, which ma
y represent either an intrinsic response to aging or a reactive partia
l denervation response to a loss of afferent inputs.