Molecular Imaging of Opioid System in Idiopathic Parkinson's Disease

Opioid receptors are localized throughout peripheral and central nervous system and interact with endogenous opioid peptides and drugs including heroin, synthetic opioids, and pain relievers (codeine, morphine). If several opioid PET tracers exist for preclinical studies, only a few have been used in human. Some tracers are selective for one subtype of opioid receptors (e.g., [¹¹C]CAF (carfentanil) for μ receptor) while others are not ([¹¹C]DPN (diprenorphine)). As shown by imaging studies, the opioid system is involved in pain processing, but also in addiction, neuropsychiatric manifestations (harm avoidance, sadness, novelty seeking behavior), feeding and food disorders and, finally, movement disorders and levodopa-induced dyskinesias. However, no imaging study has analyzed the potential dysfunction of opioid system in pain manifestations in Parkinson's disease. In addition, the involvement of opioid system in impulse control disorders and neuropsychiatric manifestations has never been studied in Parkinson's disease. Thus, there is an urgent need to understand the impact of opioid system dysfunctions in Parkinson's disease.