Metabotropic glutamate receptor (mGluR) neurons express members of the mGluR family (GRM1-8), which are G protein-coupled receptors that modulate synaptic transmission, neuronal excitability, and plasticity. Unlike ionotropic glutamate receptors (NMDA, AMPA, kainate), mGluRs produce slower, modulatory effects via second messenger cascades. Dysregulation of mGluR signaling contributes to excitotoxicity in stroke, Alzheimer disease, Parkinson disease, ALS, and Huntington disease, making these receptors important therapeutic targets.
| Taxonomy | ID | Name / Label |
|---|---|---|
| Cell Ontology (CL) | CL:0000197 | sensory receptor cell |
Group I mGluRs are predominantly postsynaptic and mediate excitatory modulation[1]:
mGluR1 (GRM1):
mGluR5 (GRM5):
Group I mGluRs couple to Gq/11 proteins, activating phospholipase C-β (PLC-β), which hydrolyzes PIP2 to IP3 and DAG. This triggers calcium release from intracellular stores and protein kinase C activation.
Group II mGluRs are primarily presynaptic autoreceptors that inhibit glutamate release[2]:
mGluR2 (GRM2):
mGluR3 (GRM3):
Group II mGluRs couple to Gi/o proteins, inhibiting adenylyl cyclase and reducing cAMP levels. They also activate G protein-gated inwardly rectifying potassium (GIRK) channels.
Group III mGluRs are presynaptic receptors that inhibit neurotransmitter release[3]:
mGluR4 (GRM4):
mGluR6 (GRM6):
mGluR7 (GRM7):
mGluR8 (GRM8):
mGluRs are class C GPCRs with a unique architecture[4]:
Glutamate binding: Occurs in the VFT domain, which closes around the ligand like a Venus flytrap. This conformational change is transmitted through the cysteine-rich domain to the 7TM.
Dimerization: mGluRs function as obligate dimers. Ligand binding stabilizes the active dimeric conformation.
G protein coupling: Group I receptors couple to Gq/11; Group II and III receptors couple to Gi/o.
Allosteric modulation: Positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) bind to the 7TM domain and modulate receptor function without directly activating it.
GRK phosphorylation: G protein-coupled receptor kinases (GRKs) phosphorylate activated mGluRs, leading to β-arrestin recruitment and internalization.
PKC feedback: Protein kinase C phosphorylates Group I mGluRs, reducing their responsiveness (homologous desensitization).
Homer proteins: Scaffold proteins that link mGluR5 to intracellular calcium stores and regulate receptor trafficking.
Excessive mGluR activation contributes to excitotoxic neuronal death in multiple conditions[5]:
Mechanism: Group I mGluR activation can potentiate NMDA receptor currents and increase intracellular calcium, triggering apoptotic and necrotic cell death pathways.
| Disease | mGluR Pathology | Therapeutic Target |
|---|---|---|
| Alzheimer disease | mGluR5-Aβ interaction, calcium dysregulation | mGluR5 NAMs |
| Parkinson disease | mGluR5-mediated STN hyperactivity | mGluR5 NAMs (mavoglurant) |
| Huntington disease | Striatal mGluR5 upregulation, excitotoxicity | mGluR5 NAMs |
| ALS | Cortical mGluR dysfunction, motor neuron death | Group II agonists |
| Stroke | Peri-infarct mGluR5 activation | Group II agonists |
In PD, loss of dopaminergic input to the striatum leads to overactivity of the indirect pathway via D2 receptor loss. mGluR5 antagonists can reduce this overactivity[6]:
mGluR5 interacts with amyloid-β oligomers and contributes to their toxic effects[7]:
Group II mGluR agonists may protect motor neurons by reducing cortical glutamate release[8]:
Anxiety/depression: mGluR2/3 agonists (e.g., pomaglumetad) showed efficacy in preclinical models but mixed results in clinical trials
Schizophrenia: mGluR2/3 agonists may reduce psychosis via suppression of cortical glutamate release
Addiction: mGluR5 antagonists reduce reward-seeking behavior in preclinical models
Advantages over orthosteric ligands:
mGluR5 NAMs[9]:
mGluR4 PAMs:
Group II agonists:
mGluR6 is essential for ON bipolar cell signaling. In congenital stationary night blindness with GRM6 mutations, gene therapy approaches are under investigation[10].
](/diseases/parkinsons-disease-—-related-neurodegenerative-disease)## External Links
Niswender CM, Conn PJ. Metabotropic glutamate receptors: pharmacology and therapeutic opportunities. Annu Rev Pharmacol Toxicol. 2010. ↩︎
Schoepp DD. Unveiling the functions of presynaptic metabotropic glutamate receptors in the central nervous system. J Pharmacol Exp Ther. 2001. ↩︎
Flor PJ et al. Group III metabotropic glutamate receptors. Neuropharmacology. 2008. ↩︎
Pin JP et al. The activation mechanism of class-C GPCRs. Curr Opin Pharmacol. 2016. ↩︎
Wang R, Reddy PH. Role of glutamate and NMDA receptors in Alzheimer disease. J Alzheimers Dis. 2017. ↩︎
Johnston TH et al. mGluR5 negative allosteric modulators for Parkinson disease. Neuropharmacology. 2018. ↩︎
Hamilton A et al. mGluR5 and the amyloid cascade in Alzheimer disease. Neuropharmacology. 2017. ↩︎
Remke AM et al. Group II metabotropic glutamate receptors in ALS. Neurodegener Dis. 2016. ↩︎
Gregory KJ et al. Allosteric modulation of metabotropic glutamate receptors. Chem Rev. 2014. ↩︎
Zeitz C et al. GRM6 mutations in congenital stationary night blindness. Prog Retin Eye Res. 2014. ↩︎