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Exploring exercise and anorexia

The running rats

(September 21, 2000, Gazette)


(L-R) Drs. Virginia Grant and Bow Revusky of the Psychology department look on as graduate
student Jennifer Smith gives one of the rats a chance to run on the wheel in the laboratory.

 

 

Photo by Chris Hammond

By Andris Petersens
SPARK correspondent

A pair of psychologists at Memorial are using rats and running wheels to study the relationship between intense physical activity and the development of anorexia nervosa. Dr. Bow Revusky and Dr. Virginia Grant have received funding from NSERC for a project called Activity-Induced Suppression of Eating and Related Phenomena. This project stems from an experimental model called “activity anorexia”. Activity anorexia is experimentally induced by allowing rats to eat for 60 minutes each day and leaving them to spend the rest of the day in a running wheel. The rats do not have to run in the wheel but some spend almost all day running.

“Some of them cover huge distances – 12 kilometres or so,” Dr. Grant said. Rats tend to increase their running over days and their eating is inadequate. It is not just that they don’t eat enough to compensate for their exercise. They actually eat less than other rats that have the same feeding schedule but are not allowed to run in the wheel. The rats that are allowed to run in the wheel lose a lot of weight.

“If you let it go on too long – meaning two weeks at most – rats could actually starve to death. This is actually an experimental model of anorexia nervosa,” Dr. Revusky explained.

People in certain kinds of occupations like athletes, gymnasts, dancers, and models tend to be excessively concerned about their weight. When they go on diets, they are especially vulnerable to anorexia nervosa because they engage in a high level of physical activity.

“Diet and exercise can be a deadly combination,” said Dr. Grant. “You have to be careful. Once it gets to a certain level, a vicious circle comes into effect. Exercise tends to decreases eating, resulting in weight loss, leading to increased exercise that tends to decrease eating, and so on.”

“We started to work on this model about five years ago. In particular, we are concerned with the rewarding or motivational processes activated by wheel running,” said Dr. Revusky.

“Rats run spontaneously in the wheel, even more when they are hungry. Rats will also learn to press a lever to gain the opportunity to run in the wheel, and they press more vigorously when they are hungry. These findings indicate that wheel running is rewarding to a rat and that its reward value is increased when the rat is hungry.”

That wheel running is rewarding implies that it activates a reward system in the brain. Lots of things, such as eating tasty food or having sex, are presumed to do so. Wheel running, at least for rats, is just one of these things. Drs. Grant and Revusky hypothesized that a high level of continuing activation of the reward system is responsible for the suppression of eating.

Therefore, an important aspect of their hypothesis is the notion that the reward system in the animal’s brain activated by wheel running continues to be active for some amount of time after wheel running stops.

“Whatever they experience after wheel running is pleasant,” Dr. Grant said. When the animal is put in the feeding cage to eat after wheel running, it tends to be less inclined to eat due to the continuing activation of the reward system.

To test that idea, Drs. Grant and Revusky did the following experiment. On some occasions, the rats were allowed to run for a lengthy period of time in the activity wheels to activate the reward system or produce pleasant feelings. Then, the rats were removed from the wheels and were each placed in a distinctive chamber. This should result in an association between the distinctive chamber and the pleasant feelings present after wheel running.

On other occasions, these rats were put in a different chamber without prior wheel running. No association should occur between the second chamber and the pleasant feelings. Later, the rats were given a test during which they could choose between the two chambers. Consistent with the notion that the activation of the reward system continues after wheel stops, the rats showed a preference for the distinctive chamber that was associated with the pleasant feelings present after wheel running.