L-glutamate serves as the neurotransmitter at the majority of excitatory synapses in the mammalian central nervous system (CNS). The existence of neuromodulatory glutamate receptors, called metabotropic glutamate receptors (mGluRs), provides a mechanism by which glutamate can modulate cell excitability and synaptic transmission via second messenger signaling pathways. The widespread distribution of mGluR proteins suggests that these neuromodulatory receptors have the ability to participate in numerous functions throughout the CNS and may represent ideal targets for therapeutic intervention in a wide variety of CNS disorders. mGluRs are members of the G-protein-coupled receptor (GPCR) superfamily, the most abundant receptor gene family in the human genome. GPCRs are membrane-bound proteins that are activated by extracellular ligands such as light, peptides, and neurotransmitters, and transduce intracellular signals via interactions with G proteins. 
The resulting change in conformation of the GPCR induced by ligand binding activates the G protein, which is composed of a heterotrimeric complex of α, β, and γ subunits. In their inactive state, G proteins are bound to guanosine 5/ - diphosphate (GDP); activation of the G protein causes the exchange of guanosine 5/-triphosphate (GTP) for GDP within the α subunit. Activated G protein subunits then modulate the function of various effector molecules such as enzymes, ion channels, and transcription factors. Inactivation of the G protein occurs when the bound GTP is hydrolyzed to GDP, resulting in reassembly of the heterotrimer. The GPCR family contains several subgroupings, and the majority of classical neurotransmitter GPCRs belong to family A. These receptors are often termed the rhodopsin-like GPCRs and are structurally similar in that they consist of an extracellular N-terminal domain, seven transmembrane-spanning domains, and an intracellular C-terminus.  
Depending on the cell type or neuronal population, group I mGluRs can activate a range of downstream effectors, includ- ing phospholipase D, protein kinase pathways such as casein kinase 1, cyclin-dependent protein kinase 5, Jun kinase, components of the mitogen-activated protein kinase/extracellular receptor kinase (MAPK/ERK) pathway, and the mammalian target of rapamycin (MTOR)/p70 S6 kinase pathway (22–25). The latter pathways, MAPK/ERK and MTOR/p70 S6 kinase, are thought to be particularly important for the regulation of synaptic plasticity by group I mGluRs. In contrast to group I mGluRs, group II and III mGluRs are coupled predominantly to Gi/o proteins. Gi/o linked receptors are classically coupled to the inhibition of adenylyl cyclase and directly regulate ion channels and other downstream signaling partners via liberation of Gβγ subunits. As with group I mGluRs, it is becoming increasingly appreciated that group II and group III mGluRs also couple to other signaling pathways, including activation of MAPK and phosphatidyl inositol 3-kinase PI3 kinase pathways, providing further complexity regarding the mechanisms by which these receptors can regulate synaptic transmission.

References

1.Niswender CM, Conn PJ. Annu Rev Pharmacol Toxicol. 2010;50:295–322.