Changes in dopamine D2 receptors and 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine uptake in the brain of 6-hydroxydopamine-lesioned rats.

We studied tracer distributions in positron emission tomography of ligands for dopamine D1 receptors ([11C]SCH23390) and D2 receptors ([11C]raclopride) and the dopamine precursor analog 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA), as a measurement of presynaptic dopaminergic function, in the brain after 6-hydroxydopamine lesioning of the medial forebrain bundle in rats. The unilateral lesions were confirmed behaviorally by methamphetamine-induced rotation 2 weeks after lesioning, and the brains were analyzed by tissue dissection following an intravenous bolus of each tracer 3 weeks after lesioning. [11C]Raclopride, but not [11C]SCH23390, showed a higher accumulation in the striatum on the lesion side compared with that on the non-lesioned (intact) side. On the other hand, a lower accumulation of [18F]FDOPA was found in the striatum and cerebral cortex on the lesion side. Our studies demonstrate upregulation of dopamine D2 receptors in the striatum and a decrease in FDOPA uptake in both the striatum and cerebral cortex ipsilateral to the 6-hydroxydopamine lesions. Therefore, the combination of a D2 antagonist and FDOPA may provide a potentially useful method for assessing the effects of dopamine depletion in Parkinson's disease.