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Objective: It has been assumed that some behavioral changes associated with repeated exposure to dopaminergic psychostimulant drugs might be explained by changes in activity of dopamine receptors, including the dopamine [D.sub.1] receptor, which is linked by a stimulatory G protein to the effector enzyme adenylyl cyclase. To establish whether dopamine [D.sub.1] receptor function might be altered in human methamphetamine users, the authors measured dopamine-stimulated adenylyl cyclase activity in the brain of chronic human users of the drug. Method: Adenylyl cyclase activity stimulated by dopamine and by guanylyl-imidodiphosphate (to assess G protein and adenylyl cyclase coupling) was determined in the postmortem brain tissue of 16 methamphetamine users who had used the drug both recently and chronically (i.e., at least 1 year) as well as 21 matched comparison subjects. Results: A 25%-30% decrease in the maximal extent of dopamine stimulation of adenylyl cyclase activity was seen in the striatum (nucleus accumbens, caudate, and putamen) of the methamphetamine users. No changes were found in basal or guanylyl-imidodiphosphate-stimulated enzyme activity. Conclusions: These data suggest that dopamine receptor function linked to adenylyl cyclase is partially desensitized in the striatum of human methamphetamine users. Decreased dopamine [D.sub.1] receptor function might underlie part of the known (drug withdrawal syndrome) or expected (drug tolerance) consequences of methamphetamine exposure in humans.