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Role of the Dopamine System in Addiction

NCJ Number
N D Volkow
Date Published
5 pages
This paper reports on cocaine and nicotine addiction studies that examined the role of the dopamine system in addiction.
Positron emission tomography (PET) -- which can be used to obtain molecular images and measurements of receptors, transporters, and enzymes in the living human brain -- was used to investigate the dopamine system in the addictive state. The first studies involved the investigation of dopamine D2 receptors in cocaine abusers. Three different studies that measured D2 receptors in cocaine abusers all showed the same results. Compared with normal controls, there was a significant reduction in D2 receptors in cocaine abusers, even after the subject had been through a protracted withdrawal. Because there are no measurements on the subjects before they were cocaine abusers, researchers do not know the extent to which these data show the effects of chronic cocaine use, or whether they indicate some variable that may predispose individuals to the use of cocaine. Animal experiments must be done in parallel to control the involved variables. The studies also observed that the brain regions for which the correlations with D2 receptors were significant predominantly corresponded to cortical projections of the dopamine system. Further, cortical mechanisms were found to be important in addictive behavior, perhaps because they are less active when they should be more active. Disruption of cortical mechanisms under cocaine use may be related to dysfunction of the dopamine system. In a study of nicotine effects, it was found that nicotine has a substance (currently unknown) that inhibits MAO B. Because MAO B destroys dopamine, its inhibitions would result in a higher concentration of dopamine. Researchers thus postulate that the inhibition of MAO B by cigarettes may act synergistically with nicotine to increase dopamine concentration. 10 figures