G-protein coupled receptors (GPCRs) represent the largest family of cell surface receptors and play critical roles in Parkinson's disease (PD) pathophysiology. Dopamine receptors, adenosine receptors, serotonin receptors, and chemokine receptors all contribute to motor and non-motor symptoms of PD. This page provides a comprehensive overview of GPCR signaling in PD and therapeutic targeting approaches. [1]
The D1-like family stimulates adenylate cyclase via Gs/olf proteins: [2]
| Receptor | Expression | Signaling | PD Relevance | [3]
|----------|------------|-----------|--------------| [@fayard2009]
| D1R | Striatum, cortex | Gs → ↑cAMP | Direct pathway activation | [4]
| D5R | Cortex, hippocampus | Gs → ↑cAMP | Cognitive functions | [5]
Signaling Cascade: [6]
The D2-like family inhibits adenylate cyclase via Gi proteins: [@zhang2017]
| Receptor | Expression | Signaling | PD Relevance | [@matsumoto2018]
|----------|------------|-----------|--------------| [@surmeier2017]
| D2R | Striatum | Gi → ↓cAMP | Indirect pathway |
| D3R | Limbic | Gi → ↓cAMP | Non-motor symptoms |
| D4R | Cortex | Gi → ↓cAMP | Cognitive effects |
Autoreceptor Function:
Adenosine A2A receptors are highly expressed in the striatum and interact with dopamine D2 receptors:
Therapeutic Rationale:
Clinical Trials:
Serotonin (5-HT) receptors modulate non-motor symptoms in PD:
| Receptor | Non-Motor Symptom | Therapeutic Target |
|---|---|---|
| 5-HT1A | Depression, anxiety | Agonists |
| 5-HT2A | Psychosis | Antagonists |
| 5-HT3 | Nausea | Antagonists |
| 5-HT4 | Cognitive dysfunction | Agonists |
GPR37 (also known as Pael-R) is implicated in PD:
Chemokine receptors contribute to neuroinflammation in PD:
GPCRs can form functional heteromers:
| Heteromer | Functional Implication |
|---|---|
| D2R-A2AR | Therapeutic target |
| D1R-D2R | Signaling bias |
| A2AR-mGluR5 | Metabolic coupling |
The striatum contains a highly interconnected GPCR system critical to PD pathophysiology[7]. Medium spiny neurons (MSNs) express multiple GPCRs whose interactions determine motor output:
Direct pathway MSNs (D1-MSNs):
Indirect pathway MSNs (D2-MSNs):
The D2R-A2AR heteromer represents a central therapeutic target in PD[8]:
Structural basis:
Functional consequences:
Therapeutic implications:
Adenosine A2A receptors also form heteromers with metabotropic glutamate receptor 5 (mGluR5):
Expression pattern:
Functional interactions:
Therapeutic targeting:
GPCRs can form even more complex assemblies:
D1R-D2R heteromers:
A2A-D2R-mGluR5 complexes:
Different ligands can activate distinct signaling pathways[9]:
Dopamine receptors couple to multiple G protein subtypes:
| Receptor | Primary G Protein | cAMP Effect | Additional Signaling |
|---|---|---|---|
| D1R | Gs/olf | Increased | PLCβ, Ca²⁺ influx |
| D2R | Gi/o | Decreased | PI3K, MAPK |
| D3R | Gi/o | Decreased | PLCβ |
| D5R | Gs | Increased | Independent of dopamine |
Beyond G protein signaling, β-arrestins mediate distinct effects:
D2R β-arrestin signaling:
Therapeutic implications:
Dopamine agonists:
A2A antagonists:
D1R is the most abundant dopamine receptor in the striatum[10]:
Structure and function:
Signaling cascade:
Therapeutic targeting:
D2R has complex pharmacology due to alternative splicing:
Splice variants:
Signaling pathways:
Autoreceptor function:
D3R shows region-specific changes in PD[4:1]:
Expression pattern:
Therapeutic potential:
A2AR is uniquely enriched in striatum[11]:
Why striatum?
Signaling mechanisms:
Therapeutic blockade:
Several A2A antagonists have been tested[@jennings2024]:
| Drug | Company | Phase | Outcome |
|---|---|---|---|
| Istradefylline | Kyowa Hakko Kirin | Approved | Japan 2013, US 2019 |
| Preladenant | Merck | Phase 3 | Discontinued 2013 |
| Tozadenant | Biotie | Phase 2 | Discontinued 2017 |
| Vipadenant | Biogen | Phase 2 | Discontinued 2012 |
| ST1535 | Sigma-Tau | Phase 2 | Ongoing |
Reasons for failures:
A1R activation provides neuroprotection:
Mechanisms:
Challenges:
A3R shows complex roles in PD:
5-HT1A receptors modulate both motor and non-motor symptoms[2:1]:
Therapeutic targets:
Drugs in use:
5-HT2A antagonism treats PD psychosis:
Pimavanserin:
5-HT2C receptor agonism may reduce dyskinesias:
5-HT1B auto-receptors regulate serotonin release:
GPCR internalization regulates signaling[6:1]:
Mechanisms:
PD relevance:
GPCRs localize to specific membrane domains:
Lipid rafts:
Synaptic compartments:
GPR37 (also known as Pael-R) accumulates in PD brains:
Aggregation:
α-synuclein interaction:
GPR37L1 shows distinct expression:
mGluR5 is a promising PD target:
Expression:
Signaling:
Therapeutic potential:
| Subtype | Expression | PD Relevance |
|---|---|---|
| mGluR1 | Cerebellum, cortex | Limited |
| mGluR2/3 | Striatum, cortex | Anti-glutamatergic |
| mGluR4 | Striatum | Neuroprotection |
| mGluR6 | Retina | Not relevant |
| mGluR7 | Basal ganglia | Unclear |
| mGluR8 | Striatum | Limited |
| Drug Class | Target | Example | Mechanism |
|---|---|---|---|
| Dopamine agonists | D1R/D2R | Pramipexole, rotigotine | Direct receptor activation |
| L-DOPA | D1R/D2R | Sinemet | Dopamine prodrug |
| MAO-B inhibitors | Enzymatic | Selegiline, rasagiline | Reduce dopamine breakdown |
| COMT inhibitors | Enzymatic | Entacapone | Reduce L-DOPA breakdown |
| A2A antagonists | A2AR | Istradefylline | Receptor blockade |
| Antipsychotics | D2R/5-HT2A | Quetiapine | Receptor antagonism |
| Combination | Rationale |
|---|---|
| A2A antagonist + L-DOPA | Synergistic motor benefit |
| D1 agonist + D2 agonist | Broader receptor coverage |
| A2A + mGluR5 antagonist | Multi-target motor benefit |
| 5-HT1A agonist + L-DOPA | Reduce dyskinesias |
Serotonergic GPCRs play key roles:
Multiple GPCRs affect cognition:
GPCRs regulate sleep architecture:
Gut-brain axis involvement:
GPCR pharmacogenomics in PD:
Designer receptors for neuronal manipulation[13]:
Heffernan MM, et al. GPCR-based therapeutics in Parkinson's disease. CNS Drugs. 2018. ↩︎
Huot P, et al. Serotonergic dysfunction in Parkinson's disease and its treatment. Current Opinion in Neurology. 2017. ↩︎ ↩︎
Pinna A, et al. Adenosine A2A receptor antagonists for Parkinson's disease. Expert Opinion on Therapeutic Patents. 2020. ↩︎
Trinh G, et al. Dopamine D3 receptor agonists in Parkinson's disease. Pharmacology & Therapeutics. 2021. ↩︎ ↩︎
Blesa J, et al. Animal models of Parkinson's disease for therapeutic development. Neurobiology of Disease. 2020. ↩︎
Cao R, et al. GPCR trafficking in neurodegeneration and repair. Journal of Molecular Neuroscience. 2019. ↩︎ ↩︎
Schaller L, et al. Dopamine receptor signaling in basal ganglia function. Neuron. 2023. ↩︎
Fuxe K, et al. Adenosine-dopamine receptor-receptor interactions in Parkinson's disease. Journal of Neural Transmission. 2015. ↩︎
Schwartz R, et al. Targeting biased signaling for Parkinson's disease therapy. Nature Reviews Drug Discovery. 2022. ↩︎ ↩︎
Espay AJ, et al. Levodopa-induced dyskinesia and dopamine receptor signaling. Brain. 2020. ↩︎
Morelli M, et al. Role of adenosine A2A receptors in parkinsonian motor inhibition. Journal of Neural Transmission. 2011. ↩︎
Masri B, et al. Striatal adenosine A2A receptor overexpression in Parkinson's disease. Journal of Neurochemistry. 2008. ↩︎
Bonaventura J, et al. High-performance chemogenetics in dopaminergic neurons. Nature Methods. 2019. ↩︎