Heightened D3 dopamine receptor levels in cocaine dependence and contributions to the addiction behavioral phenotype: a positron emission tomography study with [11C]-+-PHNO

The dopamine system is a primary treatment target for cocaine dependence (CD), but research on dopaminergic abnormalities (eg, D2 receptor system deficiencies) has so far failed to translate into effective treatment strategies. The D3 receptor system has recently attracted considerable clinical interest, and D3 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 D3 receptor system in CD, using the positron emission tomography (PET) radiotracer [(11)C]-(+)-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]-(+)-PHNO to assess D3 receptor binding (BPND; calculated regionally using the simplified reference tissue model), and for comparison, a second scan with [(11)C]raclopride to assess D2/3 binding. CD subjects also completed a behavioral battery to characterize the addiction behavioral phenotype. CD subjects showed higher [(11)C]-(+)-PHNO BPND than HC in the substantia nigra, which correlated with behavioral impulsiveness and risky decision making. In contrast, [(11)C]raclopride BPND was lower across the striatum in CD, consistent with previous literature in 2 week abstinence. The data suggest that in contrast to a D2 deficiency, CD individuals may have heightened D3 receptor levels, which could contribute to addiction-relevant traits. D3 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.

Dopamine response to psychosocial stress in chronic cannabis users: a PET study with [11C]-+-PHNO

A number of addictions have been linked with decreased striatal dopamine (DA) receptor availability and DA release. Stress has a key role in cannabis craving, as well as in modulation of dopaminergic signaling. The present study aimed to assess DA release in response to a laboratory stress task with [(11)C]-(+)-PHNO positron emission tomography in cannabis users (CU). Thirteen healthy CU and 12 healthy volunteers (HV) were scanned during a sensorimotor control task (SMCT) and under a stress condition using the validated Montreal imaging stress task (MIST). The simplified reference tissue model (SRTM) was used to obtain binding potential (BP(ND)) in striatal subdivisions: limbic striatum (LST), associative striatum (AST), and sensorimotor striatum (SMST). Stress-induced DA release (indexed as a percentage of reduction in [(11)C]-(+)-PHNO BP (ND)) between CU and HV was tested with analysis of variance. SMCT BP(ND) was significantly higher in CU compared with HV in the AST (F=10.38, p=0.003), LST (F=4.95, p=0.036), SMST (F=4.33, p=0.048), and whole striatum (F=9.02, p=0.006). Percentage of displacement (change in BP(ND) between SMCT and MIST PET scans) was not significantly different across groups in any brain region, except in the GP (-5.03±14.6 in CU, compared with 6.15±12.1 in HV; F=4.39, p=0.049). Duration of cannabis use was significantly associated with stress-induced [(11)C]-(+)-PHNO displacement by endogenous DA in the LST (r=0.566, p=0.044), with no effect in any other brain region. In conclusion, despite an increase in striatal BP(ND) observed during the control task, chronic cannabis use is not associated with alterations in stress-induced DA release.