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Non-Linear Decline in Neuronal Density within the Cerebral Cortices and Hippocampus in very Aged Rats: Implications for Causality

Lukasz M Karbowksi, Robert M Lafrenie and Michael A Persinger

A clear change in the slope of the declining numbers of neurons in the cerebral cortices and hippocampus could reflect the “critical” mass, where qualitative shifts occur for correlative behaviors. In the present study, numbers of neurons within the layers of different regions of the cerebral cortices and the hippocampus were counted for rats, between the ages of 30 and 940 days. A conspicuous sudden inflection in the gradual decline of neurons occurred for both regions, around 700 days of age. Partial correlation analyses indicated that the cortical decline was dependent upon loss of neurons in the hippocampus. Neurons within layers 5 and 6 of the prefrontal cortices were particularly affected. These results are consistent with human brain data and the clinical course of dementias, and suggest that a subset of less than about 25% of original neurons may drive the geriatric process. Identification of these neurons by biomolecular indicators could lead to effective interventions.