Studying the brain’s serotonin system and pain

This project will explore the involvement of the serotonin system in the pathophysiology of PD-related central pain. Thus, the serotonin system will be evaluated in PD patients with and without central pain who will benefit from brain positron emission tomography (PET) allowing in vivo imaging of 5HT1A receptors and multimodal brain MRI including morphometric imaging and functional connectivity (resting state acquisition).

The prevalence of chronic pain in PD can be estimated at 60-80% from several epidemiological studies. Both semiological and pathophysiological classification proposes specific and non-specific pain in PD. While non-specific pain is not directly related to PD but may be aggravated by the disease, specific pain is a direct result of the disease with dystonic pain characterized by painful cramps in relation to motor symptoms and non-systematic central pain such as burning, paresthesia, compression (central parkinsonian pain). A case-control study reported that cramp-like pain and central pain were three times more frequent in parkinsonian patients than in the general population. Pathophysiologically, several studies suggest an abnormal nociceptive integration process in PD patients. Previous studies have indicated that the nociceptive signal is amplified along the pain transmission pathways. This could be related to increased facilitation through central sensitization of pain pathways or decreased inhibition (reduced activity of descending inhibitory control systems). Several recent studies suggest that the noradrenergic and/or serotonergic systems may be involved in the pathophysiology of PD-related pain. Therefore, this project will explore the involvement of the serotonergic system in the pathophysiology of pain using brain neuroimaging in PD patients with central pain. The present study hypothesize that the binding of the radiotracer \[18F\]-MPPF, allowing in vivo imaging of 5HT1A receptors, will be reduced in PD patients with central pain compared to non-painful PD patients at the level of the median raphe, but also at the level of several brain structures involved in the pain matrix such as the insula, the anterior and posterior cingulate cortex, the orbitofrontal cortex, etc. A correlation between the clinical parameters of pain and the brain structures in which MRP binding is decreased should make it possible to confirm the link between these serotonin binding anomalies and pain. Finally, the morphological and functional MRI study should make it possible to identify structural and functional abnormalities within the pain networks in painful Parkinson's patients.