Physical activity might do more than just build muscle mass; it also may help protect against the brain-cell loss that occurs with Parkinson's disease, Alzheimer's disease, a stroke or even normal aging, research suggests.
At a plenary talk at APA's 2005 Annual Convention, University of Texas at Austin psychology professor Tim Schallert, PhD, highlighted his and others' work in this area.
Recent research has shown that people who are more physically active are less likely to develop Parkinson's or Alzheimer's disease later in life, Schallert said. But, he explained, these human studies only demonstrate correlation--they don't resolve the question of whether exercise actually protects against brain cell loss.
So Schallert and his colleagues are simulating Parkinson's disease in rats to investigate that question more directly. Parkinson's disease's trademark symptom--a loss of motor control--is caused by the progressive death of dopamine-producing nerve cells in the brain. Schallert and his research team simulate the disease in rats by giving the animals a neurotoxin that slowly kills those same cells.
Ordinarily, administering the neurotoxin in the left side of the brain would cause the rat to lose control of its right forelimb. But in a 2001 study published in The Journal of Neuroscience (Vol. 21, No. 12, pages 4,427-4,435) Schallert and his colleagues found that if they restricted the animal's left forelimb--forcing it to exercise the right one--before and after administering the neurotoxin, the rat's right forelimb remained usable and the brain's dopamine neurons didn't degenerate as much. In contrast, restraining the right limb in a sling caused the dopamine cells to degenerate even more quickly.
Schallert and others have also investigated animal models of traumatic brain injury and stroke. In general, exercise before and sometime after the injury helps protect brain cells, but intense physical activity immediately after the injury can actually increase cell death--possibly because injured cells near the damage site are more vulnerable to stressors for a few days.
Researchers don't yet know why physical exercise is generally associated with reduced brain cell loss. But in a series of studies, Schallert and his colleagues have found that motor enrichment activates proteins that are known to enhance cell survival and growth.
Also, exposing rats to mild levels of a neurotoxin activates all sorts of brain repair mechanisms that can protect the rat against a later, stronger dose of the toxin. Physical exercise, Schallert explained, may stress the brain in the same way as a mild toxin, and thus protect it against a later, more extreme stress.
"It's the no-pain, no-gain approach--what doesn't kill a cell makes it stronger," Schallert said.