Abstract
The neurotransmitter dopamine regulates the brain processes that control movement, emotional response and the ability to experience pleasure and pain. It plays a major role in addiction: cocaine, alcohol and other drugs alter dopamine function, promoting feelings of satisfaction and pleasure. A Brookhaven National Laboratory study [Lancet (2001) 357, 354–357] now indicates that obese individuals have fewer dopamine receptors, which might explain the need to eat more to stimulate these pleasure pathways, analogous to the increasing cravings of drug addicts. ‘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. Dopamine-receptor levels were tested in ten severely obese individuals and ten normal-weight control individuals by injection of a radiotracer that was tagged to bind to dopamine receptors. Positron emission tomography (PET) scanning then indicated receptor intensity by the strength of the signal. Obese individuals had fewer dopamine receptors than normal-weight individuals. Furthermore, the number of dopamine receptors decreased as the obesity of the individual increased. It is not conclusive whether dopamine receptor levels are endogenously low in obese individuals, thereby triggering over-eating, or whether the decrease in the number of receptors is a result of the brain's effort to compensate for chronically high dopamine levels brought about by over-eating. ‘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. Drugs altering dopamine function are often themselves addictive and are therefore of limited use. Exercise, however, is known to increase dopamine release and raise the number of dopamine receptors – a most beneficial effect, in view of these recent results. S. de B.
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