Dopamine Release in Chronic Cannabis Users: A [11C]Raclopride Positron Emission Tomography Study

Background: Low striatal dopamine 2/3 receptor (D-2/3) availability and low ventrostriatal dopamine (DA) release have been observed in alcoholism and cocaine and heroin dependence. Less is known about the dopaminergic system in cannabis dependence. We assessed D-2/3 availability and DA release in abstinent cannabis users compared with control subjects and explored relationships to cannabis use history using [C-11]raclopride positron emission tomography and an amphetamine challenge paradigm. Methods: Sixteen recently abstinent, psychiatrically healthy cannabis-using participants (27.3 +/- 6.1 years, 1 woman, 15 men) and 16 matched control subjects (28.1 +/- 6.7 years, 2 women, 14 men) completed two positron emission tomography scans, before and after injection of intravenous d-amphetamine (.3 mg/kg). Percent change in [C-11]raclopride binding after amphetamine (change in nondisplaceable binding potential, Delta BPND) in subregions of the striatum was compared between groups. Correlations with clinical parameters were examined. Results: Cannabis users had an average consumption of 517 +/- 465 estimated puffs per month, indicating mild to moderate cannabis dependence. Neither baseline BPND nor Delta BPND differed from control subjects in any region of interest, including ventral striatum. In cannabis-dependent subjects, earlier age of onset of use correlated with lower [Delta BPND] in the associative striatum when controlling for current age. Conclusions: Unlike other addictions, cannabis dependence of mild to moderate severity is not associated with striatal DA alterations. However, earlier or longer duration of use is related to lower DA release in the associative striatum. These observations suggest a more harmful effect of use during adolescence; more research is needed to distinguish effects of chronicity versus onset.