Positron emission tomography (PET) findings suggesting lower D2-type dopamine receptors and dopamine concentration in brains of stimulant users have prompted speculation that increasing dopamine signaling might help in drug-treatment. However, this strategy needs to consider the possibility, based on animal and postmortem human data, that dopaminergic activity at the related D3 receptor might, in contrast, be elevated, and thereby contribute to drug-taking behavior. We tested the hypothesis that D3 receptor binding is above-normal in methamphetamine (MA) polydrug users, using PET and the D3-preferring ligand [11C]-(+)-PHNO. Sixteen control subjects and 16 polydrug users reporting MA as their primary drug of abuse underwent PET scanning following [11C]-(+)-PHNO. Compared to control subjects, drug users had higher [11C]-(+)-PHNO binding in the D3-rich midbrain substantia nigra (SN, +46%, p<0.02) and in the globus pallidus (+9%, p=0.06) and ventral pallidum (+11%, p=0.1), whereas binding was slightly lower in the D2-rich dorsal striatum (~−4%, NS; −12% in heavy users, p=0.01) and related to drug-use severity. [11C]-(+)-PHNO binding ratio in D3-rich SN vs. D2-rich dorsal striatum was 55% higher in MA users (p=0.004), with heavy but not moderate users having ratios significantly different from controls. [11C]-(+)-PHNO binding in SN was related to self-reported “drug-wanting.” We conclude that the dopamine D3 receptor, unlike the D2 receptor, might be upregulated in brains of MA polydrug users although lower dopamine levels in MA users could have contributed to the finding. Pharmacological studies are needed to establish whether normalization of D3 receptor function could reduce vulnerability to relapse in stimulant abuse.

The dopamine system is a primary treatment target for cocaine dependence (CD), but research on dopaminergic abnormalities (eg, D 2 receptor system deficiencies) has so far failed to translate into effective treatment strategies. The D 3 receptor system has recently attracted considerable clinical interest, and D 3 antagonism is now under investigation as a novel avenue for addiction treatment. The objective here was to evaluate the status and behavioral relevance of the D 3 receptor system in CD, using the positron emission tomography (PET) radiotracer [ 11 C]-( þ )-PHNO. Fifteen CD subjects (many actively using, but all abstinent 7-240 days on scan day) and fifteen matched healthy control (HC) subjects completed two PET scans: one with [11 C]-( þ )-PHNO to assess D 3 receptor binding (BP ND ; calculated regionally using the simplified reference tissue model), and for comparison, a second scan with [ 11 C]raclopride to assess D 2/3 binding. CD subjects also completed a behavioral battery to characterize the addiction behavioral phenotype. CD subjects showed higher [ 11 C]-( þ )-PHNO BP ND than HC in the substantia nigra, which correlated with behavioral impulsiveness and risky decision making. In contrast, [11 C]raclopride BP ND was lower across the striatum in CD, consistent with previous literature in X2 week abstinence. The data suggest that in contrast to a D 2 deficiency, CD individuals may have heightened D 3 receptor levels, which could contribute to addiction-relevant traits. D 3 upregulation is emerging as a biomarker in preclinical models of addiction, and human PET studies of this receptor system can help guide novel pharmacological strategies for treatment.

Unlike with substance use disorder, there appear to be no marked differences in D2 /D3 levels between healthy subjects and pathological gamblers, suggesting that low receptor availability may not be a necessary feature of addiction. However, relationships between [11C]-(+)-PHNO binding and gambling severity/impulsiveness suggests involvement of the D3 receptor in impulsive/compulsive behaviours.

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