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updated 8/26/2009 7:59:49 PM ET 2009-08-26T23:59:49

Just as there is the endorphin rush of a "runner's high," there can also be the valley of despair when something prevents avid runners from getting their daily fix of miles.

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Now, researchers at Tufts University may have confirmed this addiction by showing that an intense running regimen in rats can release brain chemicals that mimic the same sense of euphoria as opiate use. They propose that moderate exercise could be a "substitute drug" for human heroin and morphine addicts.

Given all of the benefits of exercise, many people commit to an active running routine. Somewhere during a longer, more intense run when stored glycogen is depleted, the pituitary gland and the hypothalamus release endorphins that can provide that "second wind" that keeps a runner going.

This sense of being able to run all day is similar to the pain-relieving state that opiates provide, scientists have known. So a team led by Robin Kanarek, professor of psychology at Tufts University, wondered whether they could also produce similar withdrawal symptoms, which would indicate that intense running and opiate abuse have a similar biochemical effect.

Running rodents
The team divided 44 male rats and 40 female rats into four groups. One group was housed inside an exercise wheel, and another group had none. Each group was divided again, either allowing access to food for only one hour per day or for 24 hours per day. Though tests on humans would be needed to confirm this research, rodents are typically good analogues to illuminate how the human body works.

The rodents existed in these environments for several weeks. Finally, all groups were given Naloxone, a drug used to counteract an opiate overdose and produce immediate withdrawal symptoms.

The active rats displayed a significantly higher level of withdrawal symptoms than the inactive rats. Also, the active rats that were only allowed food for one hour per day exercised the most and showed the most intense reaction to Naloxone. This scenario mimics the actions of humans suffering from anorexia athletica, also known as hypergymnasia, that causes an obsession not only with weight but also with continuous exercise to lose weight.

"Exercise, like drugs of abuse, leads to the release of neurotransmitters such as endorphins and dopamine, which are involved with a sense of reward," Kanarek said. "As with food intake and other parts of life, moderation seems to be the key. Exercise, as long as it doesn't interfere with other aspects of one's life, is a good thing with respect to both physical and mental health."

The study appears in the August issue of Behavioral Neuroscience, published by the American Psychological Association.

Treatment ideas
Kanarek hopes to use these results to design treatment programs for heroin and morphine addicts that substitute the all-natural high of exercise in place of the drugs.

"These findings, in conjunction with results of studies demonstrating that intake of drugs of abuse and running activates the endogenous opioid and dopamine reward systems, suggest that it might be possible to substitute drug-taking behavior with naturally rewarding behavior," she writes.

She also wants to do further research on understanding the neurophysiology of extreme eating and exercise disorders. "The high comorbidity of drug abuse and eating disorders provides further evidence of a common neurobiological basis for these disorders," Kanarek concludes.

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