Pain in Parkinson's Disease: Exploration of the Serotonin System in PET

Pain in Parkinson's Disease: Exploration of the Serotonin System in PET

Trial Overview

| Field | Value |
|-------|-------|
| NCT Number | [NCT06008704](https://clinicaltrials.gov/study/NCT06008704) |
| Status | Recruiting |
| Sponsor | University research (France/Switzerland) |
| Phase | Observational |
| Intervention | [18F]-MPPF PET scan + MRI |

Background

Pain in Parkinson's Disease: Exploration of the Serotonin System in PET

Trial Overview

| Field | Value |
|-------|-------|
| NCT Number | [NCT06008704](https://clinicaltrials.gov/study/NCT06008704) |
| Status | Recruiting |
| Sponsor | University research (France/Switzerland) |
| Phase | Observational |
| Intervention | [18F]-MPPF PET scan + MRI |

Background

Pain is a common non-motor symptom in Parkinson's disease, affecting up to 60% of patients. While dopaminergic mechanisms have been extensively studied, the serotonin system may play a key role in PD-related central pain.

Prevalence and Impact

Pain in PD is a significant contributor to disability and reduced quality of life:

• 60% of PD patients experience pain during disease progression
• Pain often precedes motor symptoms by years
• Pain is underreported and undertreated in PD
• Pain scores correlate with depression and anxiety

Classification of Pain in PD

The King's College Hospital classification divides PD pain into five categories:

Central pain is the focus of this trial and is characterized by:

• Spontaneous burning or aching sensation
• Often bilateral and symmetric
• Not relieved by dopaminergic medications
• Associated with sensory abnormalities

Serotonin System in PD Pain

The [serotonin system](/mechanisms/serotonin-dysregulation-parkinsons) is implicated in pain processing through multiple mechanisms:

5-HT1A Receptor Biology

The 5-HT1A receptor is a G-protein coupled receptor (GPCR) that:

• Couples to Gi/o proteins, inhibiting adenylate cyclase
• Is abundantly expressed in:
• Dorsal raphe nucleus (primary serotonergic nucleus)
• Hippocampus (CA1, CA3 regions)
• Amygdala
• Cortex (layer V pyramidal neurons)
• Spinal cord dorsal horn

Pain Processing Pathways

• 5-HT1A receptors in the rostral ventromedial medulla (RVM)
• Activation inhibits nociceptive transmission in dorsal horn
• Dysregulation leads to pain amplification

• 5-HT1A in thalamus and cortex modulates pain perception
• Altered receptor binding correlates with pain severity

• Serotonin-dopamine interactions in basal ganglia
• 5-HT1A receptors modulate striatal dopamine release
• Combined dysfunction may explain treatment-resistant pain

Serotonin Dysfunction in PD

Multiple factors contribute to serotonin system impairment in PD:

• Serotonergic neurons in dorsal raphe are susceptible
• Loss of 5-HT neurons correlates with disease stage

• Dopamine loss disrupts serotonergic feedback loops
• Reduced 5-HT1A autoreceptor function

• Dopaminergic medications may affect serotonin transmission
• SSRIs and other serotonergic drugs commonly used

• Activated microglia in pain processing regions
• Cytokines alter serotonin receptor function

Study Design

• Type: Observational (biomarker study)
• Imaging:
• [18F]-MPPF PET: Visualizes 5-HT1A serotonin receptors
• Multimodal MRI: Morphometric and functional connectivity analysis
• Population:
• PD patients with central pain
• PD patients without central pain (controls)

[18F]-MPPF PET Imaging

[18F]-MPPF (4-(2'-methoxyphenyl)-1-[2'-(N-2''-pyridinyl)-p-fluorobenzamido]ethyl-piperazine) is a PET radiotracer with ideal properties for 5-HT1A imaging:

| Property | Value |
|----------|-------|
| Half-life | 109.8 minutes |
| Specific activity | >37 GBq/μmol |
| Kd for 5-HT1A | 0.6-1.3 nM |
| Selectivity | >100x over other 5-HT receptors |

Imaging Protocol

• T1-weighted MPRAGE for anatomical reference
• T2-weighted FLAIR for pathology assessment
• Diffusion tensor imaging (DTI) for white matter integrity

• 60-minute dynamic acquisition after injection
• Reconstruction with attenuation correction
• Regional binding potential (BPND) calculation

• Region-of-interest (ROI) analysis
• Voxel-based analysis (SPM)
• Parametric imaging

MRI Components

Structural MRI:

• Voxel-based morphometry (VGM) for gray matter volume
• Cortical thickness analysis

• Resting-state functional connectivity
• Pain-processing task activation

• Fractional anisotropy (FA) mapping
• Tractography of pain pathways

Pathophysiology: Central Pain Mechanisms

ascending Nociceptive Pathways

Dopaminergic Pain Modulation

The basal ganglia play a crucial role in pain processing:

• Striatum: Receives pain-related input, modulates perception
• Substantia nigra: Dopaminergic pain inhibition
• Globus pallidus: Output to thalamus

Serotonergic Pain Modulation

5-HT1A receptors modulate pain at multiple levels:

| Level | Mechanism | Effect |
|-------|-----------|--------|
| Spinal cord | Presynaptic inhibition of primary afferents | Reduced nociceptive input |
| Brainstem | RVM modulation of descending pathways | Enhanced inhibition |
| Thalamus | Thalamic relay modulation | Altered sensory gating |
| Cortex | Cortical processing integration | Pain perception modulation |

Research Questions

This biomarker study addresses critical questions:

• Expected: Reduced binding in raphe, thalamus, insula

• Expected: Correlation with pain questionnaire scores

• Expected: Association with disease duration, motor severity

Clinical Significance

This biomarker study may:

• Identify new therapeutic targets: Serotonergic drugs for PD pain
• 5-HT1A agonists (e.g., tandospirone, buspirone)
• SSRI/SNRI augmentation strategies
• Improve pain diagnosis: Biomarkers for central vs peripheral pain
• Differentiate pain subtypes
• Guide treatment selection
• Elucidate pain mechanisms: Understanding non-dopaminergic contributions
• Explain treatment resistance
• Identify novel pathways

Treatment Implications

Understanding serotonin dysfunction could lead to:

• PET imaging to select patients for serotonergic therapy
• Response prediction based on receptor status

• Targeted 5-HT1A agonists for PD pain
• Combination approaches

• Existing serotonergic drugs for new indications
• Fast-track clinical translation

Related Pages

• [Serotonin Dysregulation in Parkinson's Disease](/mechanisms/serotonin-dysregulation-parkinsons)
• [Non-Motor Symptoms in PD](/diseases/parkinsons-disease)
• [Neuroimaging Biomarkers for PD](/diagnostics/neuroimaging-biomarkers-parkinsons)
• [Pain in Parkinson's Disease](/therapeutics/pain-management-parkinsons)
• [Dorsal Raphe Nucleus](/anatomy/dorsal-rapme-nucleus) - location of serotonergic neurons
• [5-HT1A Receptor](/proteins/5ht1a-receptor) - target of [18F]-MPPF
• [Parkinson's Disease Pain Classification](/mechanisms/parkinsons-pain-classification)

References