Researchers in Germany and the United States collaborated to show that older mice living in an enriched environment generate---and keep---higher numbers of neurons linked to memory functions.

"Our study suggests that, in mice, we can reduce the effects of aging on the brain with a sustained active and challenging life, even if this stimulation is only begun in middle age," said lead author Gerd Kemperman, M.D.

Kemperman advises caution in extrapolating animal data to humans but is hopeful that these results are relevant for people.

"Activity will certainly do no harm and most likely benefit people if they use our results as a motivation to be more active. They might even do something good for the nerve cells that are involved in learning and memory processes," said Kemperman.

The report links two separate lines of research. The first is a host of studies in recent years showing that people who are more active in middle and later years, either intellectually or physically, are not as susceptible to cognitive decline or neurodegenerative disorders such as Alzheimer's disease.

The second line of research has revolved around the hippocampus, one of the few regions of the brain that continually generates new neurons in adults. This regenerative process has led scientists to speculate that the negative effects of normal aging---and possibly those of Alzheimer's and other disorders-can be countered by stimulating this activity.

Previous studies have shown that exposing mice to an "enriched" environment increases production of hippocampal neurons. This effect occurs in old mice as well as young. The current study sought to determine whether this production can be sustained over a long period in middle and later life, and whether there are any corresponding effects on mental abilities.

Kemperman and colleagues housed mice in one of two ways: in a standard small, bare cage shared with a few other mice, or in an "enriched" environment consisting of a large cage with plastic tunnels, running wheel, and other objects, shared with many other mice. The enriched environment was also rearranged periodically.

The mice lived this way between the ages of 10 and 20 months, roughly the equivalent of middle to old age with the average mouse lifespan of 24 months.

Remarkably, at the end of this ten-month period, the old mice in the enriched environment were generating five times as many new hippocampal neurons as the control mice in the standard, unchanging environment. This finding shows that the activity can have a sustained effect, even on older animals.

The animals in the enriched environment also showed significant improvements over the controls on various behavioral tests. When placed in a new environment, they explored and then adapted to the environment more quickly than control mice. In a standard learning test, they also outperformed the controls.

"It is not clear how far the amazing regeneration of the hippocampus can be used to treat disorders that have already done structural damage. It is likely that there is some capacity for this, but nobody knows how this might be achieved," said Kemperman.