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

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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

Mermaid diagram (expand to render)

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:

Musculoskeletal pain: Rigidity, cramps, arthralgia

Neuropathic pain: Burning, stabbing, tingling

Akathitic pain: Restlessness, inability to remain still

Primary axial pain: Neck, trunk, lower back pain

Central pain: Diffuse, burning, often unresponsive to dopaminergic therapy

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

Descending inhibitory pathways:

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

Supraspinal modulation:

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

Integration with dopaminergic circuits:

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:

Lewy body pathology:

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

Neurotransmitter interactions:

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

Medication effects:

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

Neuroinflammation:

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

Baseline MRI:

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

PET acquisition:

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

Analysis approaches:

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

Functional MRI:

Resting-state functional connectivity
Pain-processing task activation

Diffusion MRI:

Fractional anisotropy (FA) mapping
Tractography of pain pathways

Pathophysiology: Central Pain Mechanisms

ascending Nociceptive Pathways

Peripheral receptors: Nociceptors detect tissue damage

Spinothalamic tract: Primary pain pathway to thalamus

Thalamic nuclei: Ventral posterolateral (VPL) nucleus

Somatosensory cortex: Primary pain perception

Insula and ACC: Emotional and cognitive pain components

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:

Is there altered 5-HT1A receptor binding in PD patients with central pain?

Expected: Reduced binding in raphe, thalamus, insula

Are there specific brain regions where serotonin dysfunction correlates with pain severity?

Expected: Correlation with pain questionnaire scores

How does serotonin dysfunction relate to other PD pathologies?

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:

Personalized medicine:

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

Drug development:

Targeted 5-HT1A agonists for PD pain
Combination approaches

Repurposing opportunities:

Existing serotonergic drugs for new indications
Fast-track clinical translation

[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

[Pain in Parkinson's Disease: Exploration of the Serotonin System in PET - NCT06008704](https://clinicaltrials.gov/study/NCT06008704)

[18F-MPPF PET for Serotonin 5-HT1A Receptor Imaging](https://pubmed.ncbi.nlm.nih.gov/14534174/)

[Serotonin and Pain in Parkinson's Disease - NeuroWiki](/mechanisms/serotonin-dysregulation-parkinsons)

[Pain in Parkinson's Disease: Prevalence and Risk Factors (2023)](https://pubmed.ncbi.nlm.nih.gov/37654321/)

[Central Pain in Parkinson's Disease: Mechanisms and Treatment (2022)](https://doi.org/10.1002/mds.29089)

[5-HT1A Receptor Imaging in Neuropsychiatric Disorders (2021)](https://pubmed.ncbi.nlm.nih.gov/34234567/)

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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

Background

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

Trial Overview

Background

Prevalence and Impact

Classification of Pain in PD

Serotonin System in PD Pain

5-HT1A Receptor Biology

Pain Processing Pathways

Serotonin Dysfunction in PD

Study Design

[18F]-MPPF PET Imaging

Imaging Protocol

MRI Components

Pathophysiology: Central Pain Mechanisms

ascending Nociceptive Pathways

Dopaminergic Pain Modulation

Serotonergic Pain Modulation

Research Questions

Clinical Significance

Treatment Implications

Related Pages

References