New perspectives on neurochemical effects of amantadine in the brain of parkinsonian patients: a PET – [¹¹C]raclopride study

Summary.

Amantadine, is a non competitive NMDA receptors antagonist that has been proved beneficial in Parkinson's disease. However its mechanism of action at therapeutic doses is still under discussion. Aim of this study was to evaluate the effect of repeated administration of amantadine on striatal dopaminergic system by measuring [¹¹C]raclopride binding to striatal D₂ dopamine receptors, in patients with moderate idiopathic Parkinson's disease. Eight patients completed the study undergoing a PET scan, before and after 10–14 days treatment with Amantadine (200 mg/day). Patients were on treatment with L-DOPA, which was suspended 1 night before each PET scans, and free from dopaminergic agonists, anticholinergic and antidepressants. Amantadine treatment significantly increased [¹¹C-]Raclopride binding (caudate: 10% p = 0.04; putamen 11% p = 0.01). A slight reduction (−7.3%, p = 0.062) of UPDRS total scores was also observed. The increased availability of striatal D₂ receptors, is likely to be caused by drug induced modification of receptors expression. This hypothesis is consistent with previous experiments, indicating an increase in striatal D₂ receptors in rats treated with amantadine or other non competitive NMDA antagonists and suggests that the neo-synthesis of D₂ receptors may represent a reinforcing mechanism of drug efficacy.