Lower level of endogenous dopamine in patients with cocaine dependence: Findings from PET imaging of D₂/D₃ receptors following acute dopamine depletion

Objective: Previous positron emission tomography (PET) imaging studies have demonstrated that cocaine dependence is associated with a decrease in dopamine type 2 and 3 (D₂/D₃) receptor binding in cocaine-dependent individuals relative to healthy comparison subjects. However, given the nature of PET imaging, it is possible that the measured decrease in radiotracer binding results from an increase in baseline dopamine levels. The purpose of this study was to measure D₂/D₃ receptors following acute dopamine depletion in cocaine-dependent volunteers relative to healthy comparison subjects. Method: Cocaine-dependent volunteers (N=15) and healthy matched comparison subjects (N=15) were scanned using PET, with the dopamine receptor radiotracer [¹¹C]raclopride, at baseline and again following acute depletion of endogenous dopamine via alpha-methyl-para-tyrosine (AMPT) administration. Changes in radiotracer binding were measured in the subdivisions of the striatum (caudate, putamen, and ventral striatum) in addition to the striatum as a whole. Results: Findings revealed that cocaine-dependent volunteers exhibited lower levels of endogenous dopamine relative to comparison subjects, which was measured as an increase in [¹¹C]raclopride binding following AMPT administration. The increase in [¹¹C] raclopride binding in the striatum was 11.1% (SD=4.4%) in healthy comparison subjects and 5.7% (SD=5.9%) in cocaine-dependent volunteers. Similar differences were seen in the subdivisions of the striatum. Conclusions: The decrease in striatal D₂/D₃ receptors associated with cocaine dependence cannot be attributed to higher levels of endogenous dopamine.