The mGluRs perform a variety of functions in the central and peripheral nervous systems: for example, they are involved in learning, memory, anxiety, and the perception of pain. They are found in pre- and postsynaptic neurons in synapses of the hippocampus, cerebellum, and the cerebral cortex, as well as other parts of the brain and in peripheral tissues.
Metabotropic glutamate receptors can cause Ca2+ to be released from intracellular structures in which it is stored, such as the endoplasmic reticulum (ER). Activation of mGluRs causes the production of Inositol trisphosphate, which activates receptors on the ER that open Ca2+-permeable channels.
Eight different types of mGluRs, labeled mGluR1 to mGluR8 (GRM1 to GRM8), are divided into groups I, II, and III. Receptor types are grouped based on receptor structure and physiological activity. The mGluRs are further divided into subtypes, such as mGluR7a and mGluR7b.
The mGluRs in group I, including mGluR1 and mGluR5, are stimulated most strongly by the excitatory amino acid analog L-quisqualic acid. Stimulating the receptors causes an associated phospholipase C molecule to hydrolyze phosphoinositide phospholipids in the cell's plasma membrane.
These receptors are also associated with Na+ and K+ channels. Their action can be excitatory, increasing conductance, causing more glutamate to be released from the presynaptic cell, but they also increase inhibitory postsynaptic potentials, or IPSPs. They can also inhibit glutamate release and can modulate voltage-dependent calcium channels.
Group I mGluRs, but not other groups, are activated by 3,5-dihydroxyphenylglycine (DHPG), a fact which is useful to experimenters because it allows them to isolate and identify them.
Group II & Group III
The receptors in group II, including mGluRs 2 and 3, and group III, including mGluRs 4, 6, 7, and 8, (with some exceptions) prevent the formation of cyclic adenosine monophosphate, or cAMP, by activating a G protein that inhibits the enzyme adenylyl cyclase, which forms cAMP from ATP. These receptors are involved in presynaptic inhibition, and do not appear to affect postsynaptic membrane potential by themselves. Receptors in groups II and III reduce the activity of postsynaptic potentials, both excitatory and inhibitory, in the cortex.
The chemical 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) activates only group II mGluRs, and 2-amino-4-phosphonobutyrate (L-AP4) activates only group III mGluRs.
Different types of mGluRs are distributed differently in cells. For example, one study found that Group I mGluRs are mostly located on postsynaptic parts of cells while groups II and III are mostly located on presynaptic elements, though they have been found on both pre- and postsynaptic membranes.
Also, different mGluR subtypes are found predominantly in different parts of the body. For exaple, mGluR4 is located only in the brain, in locations such as the thalamus, hypothalamus and caudate nucleus. All mGluRs except mGluR6 are thought to exist in the hippocampus and entorhinal cortex.
It is thought that mGluRs play a role in a variety of different functions.
Modulation of other receptors
Metabotropic glutamate receptors are known to act as modulators of (affect the activity of) other receptors. For example, group I mGluRs are known to increase the activity of N-methyl-D-aspartate receptors, a type of ion channel-linked receptor that is central in a neurotoxic process called excitotoxicity. Proteins called PDZ proteins frequently anchor mGluRs near enough to NMDARs to modulate their activity. It has been suggested that mGluRs may act as regulators of neurons' vulnerability to excitotoxicity (a deadly neurochemical process involving glutamate receptor overactivation) through their modulation of NMDARs, the receptor most involved in that process. Excessive amounts of N-methyl-D-aspartate, an agonist for NMDARs, has been found to cause more damage to neurons in the presence of group I mGluR agonists.
On the other hand, agonists of group II and III mGluRs reduce NMDAR activity. Group II and III mGluRs tend to protect neurons from excitotoxicity, possibly by reducing the activity of NMDARs.
Metabotropic glutamate receptors are also thought to affect dopaminergic and adrenergic neurotransmission.
Role in plasticity
Like other glutamate receptors, mGluRs have been shown to be involved in synaptic plasticity. They participate in long term potentiation and long term depression, and they are removed from the synaptic membrane in response to agonist binding.
Roles in disease
Since metabotropic glutamate receptors are involved in a variety of functions, abnormalities in their expression can contribute to disease. For example, studies with mutant mice have suggested that mutations in expression of mGluR1 may be involved in the development of certain types of cancer.
In addition, manipulating mGluRs can be useful in treating some conditions. For example, clinical trial suggested that an mGlu2/3 agonist, LY354740, was effective in the treatment of generalized anxiety disorder. Also, some researchers have suggested that activation of mGluR4 could be used as a treatment for Parkinson's disease.
It was first suggested that mGluRs might exist in 1985, after it was noted that glutamate could stimulate phospholipase C through the activation of a receptor that did not belong to any of the ionotropic glutamate receptor families (NMDA, AMPA, or Kainate receptors. The suspicion that mGluRs existed was confirmed in 1987, and in 1991 the first mGluR was cloned.
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