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Upton, NY -- Dopamine, a brain chemical associated with addiction to cocaine, alcohol, and other drugs, may also play an important role in obesity. According to a study by scientists at the U.S. Department of Energy's Brookhaven National Laboratory, obese people have fewer receptors for dopamine, a neurotransmitter that helps produce feelings of satisfaction and pleasure. The findings, which will appear in the February 3, 2001 issue of The Lancet, imply that obese people may eat more to try to stimulate the dopamine "pleasure" circuits in their brains, just as addicts do by taking drugs.
"The results from this study suggest that strategies aimed at improving dopamine function might be beneficial in the treatment of obese individuals," says physician Gene-Jack Wang, the lead scientist on the study.
Brookhaven scientists have done extensive research showing that dopamine plays an important role in drug addiction. Among other things, they've found that addictive drugs increase the level of dopamine in the brain, and that addicts have fewer dopamine receptors than normal subjects.
"Since eating, like the use of addictive drugs, is a highly reinforcing behavior, inducing feelings of gratification and pleasure, we suspected that obese people might have abnormalities in brain dopamine activity as well," says psychiatrist Nora Volkow, who was also involved in the study.
To test this hypothesis, the scientists measured the number of dopamine receptors in the brains of ten severely obese individuals and ten normal controls. Their method consisted of giving each volunteer subject an injection containing a radiotracer, a radioactive chemical "tag" designed to bind to dopamine receptors in the brain. Then, the researchers scanned the subjects' brains using a positron emission tomography (PET ) camera. The PET camera picks up the radioactive signal of the tracer and shows where it is bound to dopamine receptors in the brain. The strength of the signal indicates the number of receptors.
Obese individuals, the scientists found, had fewer dopamine receptors than normal-weight subjects. And within the obese group, the number of dopamine receptors decreased as the subjects' body mass index, an indicator of obesity, increased. That is, the more obese the individual, the lower the number of receptors.
"It's possible that obese people have fewer dopamine receptors because their brains are trying to compensate for having chronically high dopamine levels, which are triggered by chronic overeating," says Wang. "However, it's also possible that these people have low numbers of dopamine receptors to begin with, making them more vulnerable to addictive behaviors including compulsive food intake."
The researchers note that, based on this study alone, they cannot conclude whether the brain changes they've detected are a consequence or a cause of obesity. They also acknowledge that the regulation of body weight is extremely complex, involving many physiological mechanisms and neurotransmitters. But they do suggest that addressing the dopamine receptor deficiency or finding other ways to regulate dopamine in obese people might help reduce their tendency to overeat.
Unfortunately, many of the drugs that have been shown to alter dopamine levels are highly addictive. But exercise, which has other obvious benefits in weight control, is another way obese subjects might be able to stimulate their dopamine pleasure and satisfaction circuits, the researchers suggest. "In animal studies conducted elsewhere, exercise has been found to increase dopamine release and to raise the number of dopamine receptors," Volkow says. This suggests that obese people might be able to boost their dopamine response through exercise instead of eating -- just one more reason to exercise if you're trying to lose weight.
This study was funded by the U.S. Department of Energy.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
Note to local editors: Gene-Jack Wang lives in Port Jefferson and Nora Volkow lives in Shoreham, both in New York.