Exercise fuels the brain's stress buffers.

Exercise may improve mental health by helping the brain cope better with stress, according to research into the effect of exercise on neurochemicals involved in the body's stress response.

Preliminary evidence suggests that physically active people have lower rates of anxiety and depression than sedentary people. But little work has focused on why that should be. So to determine how exercise might bring about its mental health benefits, some researchers are looking at possible links between exercise and brain chemicals associated with stress, anxiety and depression.

So far there's little evidence for the popular theory that exercise causes a rush of endorphins. Rather, one line of research points to the less familiar neuromodulator, norepinephrine, which might help the brain deal with stress more efficiently.

Subduing stress

Work in animals since the late 1980s finds that exercise increases brain concentrations of norepinephrine in brain regions involved in the body's stress response, said Rod K. Dishman, PhD, of the University of Georgia, who has conducted some experiments.

Norepinephrine is particularly interesting to researchers because 50 percent of the brain's supply is produced in the locus coeruleus, a brain area that connects most of the brain regions involved in emotional and stress responses. The chemical is thought to play a major role in modulating the action of other, more prevalent, neurotransmitters that play a direct role in the stress response. And although researchers are unsure of exactly how most antidepressants work, they know that some increase brain concentrations of norepinephrine.

But Mark Sothmann, PhD, of Indiana University School of Medicine, doesn't think it's a simple matter of more norepinephrine equals less stress and anxiety and therefore less depression. Instead, he and others think exercise thwarts depression and anxiety by enhancing the body's ability to respond to stress.

"Long-term exercise training potentially adjusts the responsiveness of the [stress reaction] system and makes it more efficient" said Sothmann.

Is all exercise equal?

The type of exercise might also determine its benefits. In a study in rats comparing exercise to the antidepressant drug imipramine, Dishman found that unforced exercise provided more benefits than either the drug or forced exercise.

He induced a depression-like condition in rats using the drug clomipramine; they showed sever al behavioral signs of depression, including an impaired sex drive. He then gave one group of rats 24-hour access to a running-wheel for 12 weeks. Another group ran on a treadmill for an hour a day, six days a week for 12 weeks. A third group received imipramine for the last six days of the 12-week experimental period. And a fourth group remained sedentary and received no treatment for the 12 weeks.

The rats given imipramine showed the hallmark response to the drug: an increase in brain concentrations of norepinephrine, an increase in serotonin metabolism-another neurotransmitter associated with depression-and a decrease in the density of beta receptors-the brain-cell receptors that norepinephrine attaches to. Both exercise groups also showed these changes. But only the wheel-running rats saw increased sexual activity, the behavioral measure Dishman used to rate depression.

These results imply that "prescribing" exercise may not improve mental fitness, said Dishman. In fact, it probably isn't "fitness" per se, that provokes the neurochemical benefits seen by researchers. For example, Dishman's treadmill-trained rats exercised more and became more physically fit than wheel-running rats. However, the wheel runners received the biggest behavioral boost as measured by increased sexual activity. And research in humans finds the same trend, said Sothmann. The ultimate benefits probably result from a combination of biological and social factors, said Dishman.

Biologically, exercise seems to give the body a chance to practice dealing with stress. It forces the body's physiological systems-all of which are involved in the stress response-to communicate much more closely than usual: The cardiovascular system communicates with the renal system, which communicates with the muscular system.

This article was provided by the American Psychological Association.