There is a report [60] indicating that the synaptic numerical
density per unit area remains constant in the CA1 stratum
radiatum of aged Sprague–Dawley and Fischer 344 rats
relative to young adults. This work, however, used random
single sections and was conducted at a time when unbiased
stereological techniques of synapse sampling and counting
were not yet available. Most importantly, aged animals were
not behaviorally characterized in that study. The aged groups
examined might include a high proportion of rats with unimpaired
spatial learning. If it were the case, no age-related
change in the synaptic numerical density would have been
determined because the results reported here demonstrate
that a decreased density of synapses is characteristic only of
aged learning-impaired rats and not of learning-unimpaired
ones