PRESERVED NEURON NUMBER IN THE HIPPOCAMPUS OF AGED RATS WITH SPATIAL-LEARNING DEFICITS

Hippocampal neuron loss is widely viewed as a hallmark of normal aging . Moreover, neuronal degeneration is thought to contribute directly to age-related deficits in learning and memory supported by the hippocam pus. By taking advantage of improved methods for quantifying neuron nu mber, the present study reports evidence challenging these long-standi ng concepts. The status of hippocampal-dependent spatial learning was evaluated in young and aged Long-Evans rats using the Morris water maz e, and the total number of neurons in the principal cell layers of the dentate gyrus and hippocampus was quantified according to the optical fractionator technique. For each of the hippocampal fields, neuron nu mber was preserved in the aged subjects as a group and in aged individ uals with documented learning and memory deficits indicative of hippoc ampal dysfunction. The findings demonstrate that hippocampal neuronal degeneration is not an inevitable consequence of normal aging and that a loss of principal neurons in the hippocampus fails to account for a ge-related learning and memory impairment. The observed preservation o f neuron number represents an essential foundation for identifying the neurobiological effects of hippocampal aging that account for cogniti ve decline.