Adenosine A(2A) receptors (A2ARs) are thought to interact negatively with the
dopamine D(2) receptor (D2R), so selective A2AR antagonists have attracted attention as novel treatments for
Parkinson's disease (PD). However, no information about the receptor in living patients with PD is available. The purpose of this study was to investigate the relationship between A2ARs and the dopaminergic system in the striata of
drug-naïve PD patients and PD patients with
dyskinesia, and alteration of these receptors after antiparkinsonian
therapy. We measured binding ability of striatal A2ARs using positron emission tomography (PET) with [7-methyl-(11)C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([(11)C]TMSX) in nine
drug-naïve patients with PD, seven PD patients with mild
dyskinesia and six elderly control subjects using PET. The patients and eight normal control subjects were also examined for binding ability of
dopamine transporters and D2Rs. Seven of the
drug-naïve patients underwent a second series of PET scans following
therapy. We found that the distribution volume ratio of A2ARs in the putamen were larger in the dyskinesic patients than in the control subjects (p<0.05, Tukey-Kramer post hoc test). In the
drug-naïve patients, the binding ability of the A2ARs in the putamen, but not in the head of caudate nucleus, was significantly lower on the more affected side than on the less affected side (p<0.05, paired t-test). In addition, the A2ARs were significantly increased after antiparkinsonian
therapy in the bilateral putamen of the
drug-naïve patients (p<0.05, paired t-test) but not in the bilateral head of caudate nucleus. Our study demonstrated that the A2ARs in the putamen were increased in the PD patients with
dyskinesia, and also suggest that the A2ARs in the putamen compensate for the asymmetrical decrease of
dopamine in
drug-naïve PD patients and that antiparkinsonian
therapy increases the A2ARs in the putamen. The A2ARs may play an important role in regulation of
parkinsonism in PD.