Enriched mice show adult neuron boost.

If you want to liven up the cages of laboratory mice, try giving them paper and plastic tubes, nesting material, a fiber tunnel with several openings, and a running wheel from an early age. Not only do the rodents revel in this type of enriched environment, but so does a part of their brain involved in learning and memory, a new study finds.

Compared to adult mice living in cages containing simply food and water, grown mice living in the above conditions-as well as receiving treats such as cheese and popcorn-exhibit many more nerve cells in one portion of the hippocampus, contend neuroscientist Fred H. Gage of the Salk Institute for Biological Studies in La Jolla, Calif., and his coworkers.

That enrichment translates into an average of 40,000 more neurons in tissue that serves as an entryway for information from the brain's outer layer into the hippocampus and its adjoining structures, Gage's team reports in the April 3 Nature. Neuron production typically continues in this part of the rodent hippocampus throughout adulthood.

"We were overwhelmed by the magnitude of the increase, which represents a gain of 15 percent in the number of these nerve cells," Gage remarks.

Researchers have already noted that adult rodents in enriched surroundings perform better on tests of learning and memory. Attempts to explain this effect have focused on possible increases in the number and strength of connections between neurons, whereas the new data also implicate cell proliferation in at least one brain region.

The scientists randomly assigned 24 mice to a standard or an enriched cage at the age of weaning, 21 days, when the animals are ready to live independently. Forty days later, the enriched group performed better on a maze task.

At that time, microscopic analyses of the brains of five animals from each group showed no differences in number of hippocampal cells. Four weeks later, however, the remaining enriched mice had more of these cells than the other mice.

An enriched environment may foster the survival of new hippocampal cells rather than simply generating more of them, Gage theorizes.

"This study is a breakthrough in our knowledge of what can go on in adult animal brains," says neuroscientist William T. Greenough of the University of Illinois at Urbana-Champaign.

However, he adds, the significance of the findings for humans is unclear. Some investigators suspect that in primates, the hippocampus generates no new neurons after birth.