For many people, increasing forgetfulness is an unwelcome side effect of growing old. Just how the human brain reacts to aging, independent of specific diseases such as Alzheimer¿s, has proved difficult to discern. Now a report published online by the Proceedings of the National Academy of Sciences has identified a specific section of the brain that is most vulnerable during the twilight years.

Using human subjects to study age-related changes in the brain is problematic because it is hard to exclude patients who may be suffering from early-stage Alzheimer¿s but have not yet been diagnosed. Scott A. Small of Columbia University and his colleagues thus turned to rhesus monkeys and rats--animals that experience brain changes as they age but are free of Alzheimer¿s-type diseases--instead. Using magnetic resonance imaging (MRI), the team measured blood flow to the brain and found that older monkeys displayed a significant decline in blood volume in a section of the hippocampus known as the dentate gyrus.

The researchers employed a different approach to analyze brain activity in rats. They monitored activity of a gene associated with learning, ARC, in the hippocampus and found that older animals had lower levels of expression in the dentate gyrus compared with their younger counterparts. Together, the results suggest that the dentate gyrus is the brain region most sensitive to aging in a number of species. Alzheimer¿s also affects the hippocampus, the authors note, so the new findings will be helpful in differentiating between brain changes attributable to normal aging processes and those arising from disease.