Long workouts or tricky memory tasks can boost the number of brain cells in adult rodents, according to two studies in the March issue of Nature Neuroscience. But it's not clear whether huffing around the block will make couch potatoes any smarter.
Rodents possess a remarkable capacity for generating new nerve cells throughout adulthood, a feat not deemed possible in humans until last fall (ScienceNOW, 29 October 1998 ). The rejuvenation is mainly confined to the hippocampus, a brain region involved in learning and memory. In 1997, a team led by neurobiologist Fred Gage of the Salk Institute of Biological Studies in La Jolla, California, reported that adult mice grew more nerve cells if their bleak cages were outfitted like a playground, including toys and buddies. But Gage couldn't tell which activity was providing the crucial stimulation.
In the new study, Gage limited the animals to one main activity, such as learning, running, or swimming. To identify new brain cells, Gage injected the mice with a marker molecule that slips into the replicated DNA of dividing cells. Then he simply counted the labeled cells under the microscope. Mice that ran several miles each day on an exercise wheel, he found, produced almost twice as many new cells as groups that had to swim or find a submerged platform in a water basin. Gage says an increased level of growth factors in the brain, triggered by heavy exercise, may boost cell proliferation. This finding was a surprise, Gage admits, because he'd expected learning tasks to provide the trigger.
Learning did turn out to be important in a study run by neurobiologist Elizabeth Gould at Princeton University. Water mazes and other learning tasks didn't change the number of new brain cells in adult rats, but more than doubled the number of new cells that survived past 2 weeks. Gould speculates that young neurons may require critically timed input signals from connecting cells--maybe in the form of learning--to survive.
The two findings may dovetail, says Neal Cohen, a cognitive neuroscientist at the University of Illinois, Urbana-Champaign. Extended exercise might primarily spur cell proliferation, whereas learning might boost survival. The next step, Gage adds, is to see whether the new cells make running mice any smarter. As for humans, who get more exercise and stimulation than lab mice, "it's an open question whether extra running does anything" for our brains, Gage says.