| Metabotropic glutamate receptor 5 (mGluR5) is a group I class C G protein‑coupled glutamate receptor that belongs to the GRM1/GRM5 subfamily and couples primarily to Gq/11 to regulate excitatory synaptic transmission and plasticity in the central nervous system. The receptor contains a large bilobed extracellular Venus flytrap domain that binds glutamate, a cysteine‑rich linker, a seven‑transmembrane helical core, and an intracellular C‑terminal tail that associates with scaffold and signaling proteins at postsynaptic densities and at intracellular membranes. Activation by glutamate induces conformational changes that promote Gq/11‑dependent stimulation of phospholipase C, generation of inositol 1,4,5‑trisphosphate and diacylglycerol, release of Ca²⁺ from intracellular stores, and activation of protein kinase C, with additional engagement of mitogen‑activated protein kinase cascades and protein kinase D phosphorylation that extends the temporal range of mGluR5 signaling. The receptor localizes at perisynaptic regions of excitatory synapses where it interacts functionally with NMDA receptor signaling to modulate synaptic strength, dendritic excitability, and long‑term forms of synaptic plasticity, and intracellular pools of mGluR5 on endomembranes and nuclear compartments couple to distinct Gq/11‑linked pathways that generate localized Ca²⁺ signals and transcriptional responses associated with sustained synaptic activity. mGluR5 is expressed at high levels in cortical, hippocampal, and striatal neurons and is also detectable in astrocytes and microglia, where receptor activation influences glutamate handling, neuroglial signaling, and the balance between pro‑ and anti‑inflammatory outputs, placing this receptor at the interface of neuronal and glial regulation of circuit excitability. The C‑terminal domain forms complexes with PSD‑95, Homer, and SHANK family proteins that organize receptor positioning in perisynaptic nanodomains and couple mGluR5 activation to downstream effectors controlling spine morphology, receptor trafficking, and gene expression programs linked to learning and memory and to stress‑related behavioral responses. Dysregulated mGluR5 signaling associates with fragile X‑related synaptic dysfunction, addiction, anxiety, mood disorders, neurodegeneration, and neuropathic pain, where altered receptor expression or signaling bias contributes to changes in synaptic plasticity, nociceptive processing, or emotional behavior. |
