A team of researchers headed by Fred Gage, PhD, of the Salk Institute, found that experience enhances the survival of new neurons in a brain area called the dentate gyrus, and that more of these new neurons were activated when exposed to the same experience later. This change in function may be a mechanism for long-term memory. The findings are published in the March 21 issue of The Journal of Neuroscience.

"The results identify a critical period for experience-induced enhancement of new neuron survival in the hippocampus," says Elizabeth Gould, PhD, of Princeton University, who was not affiliated with the study. The hippocampus contains the dentate gyrus.

After injecting mice with a chemical used to mark proliferating cells, the researchers exposed the animals to an "enriched cage" environment, containing tunnels, shelters, and a running wheel. After several weeks, the researchers again exposed the mice in the same enriched experience. They discovered that the enriched experience increased new neuron survival and that more new neurons were activated by re-exposure to the same environment.

To determine if the increase in neuronal activity was due to having the same experience or if any new experience was sufficient to achieve this effect, the researchers exposed mice to the enriched cage first and then a water maze task. While both cases promoted new neuron survival, more new neurons were activated in mice that had repeated the same experience but not in those that were exposed to the different experience (the water maze).

The researchers also compared the number of neurons at four weeks and four months after injections of the chemical marker. This comparison indicates that only young neurons are affected by the repeated experience; once new neurons survive the early period of development, their long-term survival and activity become stable. This suggests that the information on the experience is kept for a long time, says Gage.

"This finding indicates the way in which new neurons in the adult brain could contribute to learning and memory by storing the information of one's previous experiences," Gage says. Future research, he says, should focus on understanding when new neurons are active during memory-related behavior, how experience affects this activity, and what the consequences are for other brain regions.

This study was supported by the Mind Science Foundation, the Uehara Memorial Foundation, the JSPS Postdoctoral Fellowship for Research Abroad, the National Institute on Aging, the National Institute of Neurological Disorders and Stroke, the Defense Advanced Research Projects Agency, and the Lookout Fund.

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