NMDA receptor

NMDA

Glutamic acid

Stylised depiction of an activated NMDAR. Glutamate is in the glutamate-binding site and glycine is in the glycine-binding site. Allosteric sites that would cause inhibition of the receptor are not occupied. NMDARs require the binding of two molecules of glutamate or aspartate and two of glycine.^[1]

The N-methyl-D-aspartate receptor (also known as the NMDA receptor or NMDAR), is a glutamate receptor and ion channel protein found in nerve cells. It is activated when glutamate and glycine (or D-serine) bind to it, and when activated it allows positively charged ions to flow through the cell membrane.^[2] The NMDA receptor is very important for controlling synaptic plasticity and memory function.^[3]

The NMDAR is a specific type of ionotropic glutamate receptor.^[4] The NMDA receptor is named this because the agonist molecule N-methyl-D-aspartate (NMDA) binds selectively to it, and not to other glutamate receptors. Activation of NMDA receptors results in the opening of an ion channel that is nonselective to cations, with a reversal potential near 0 mV. While the opening and closing of the ion channel is primarily gated by ligand binding, the current flow through the ion channel is voltage dependent. Extracellular magnesium (Mg²⁺) and zinc (Zn²⁺) ions can bind to specific sites on the receptor, blocking the passage of other cations through the open ion channel. Depolarization of the cell dislodges and repels the Mg²⁺ and Zn²⁺ ions from the pore, thus allowing a voltage-dependent flow of sodium (Na⁺) and small amounts of calcium (Ca²⁺) ions into the cell and potassium (K⁺) out of the cell.^[5][6][7][8]

Ca²⁺ flux through NMDARs is thought to be critical in synaptic plasticity, a cellular mechanism for learning and memory. The opening and closing (gating) of the NMDA receptor is complex. While it is primarily a ligand-gated channel, it does display weaker voltage-dependence modulation of the ligand-dependent gating. The ligand gating requires co-activation by two ligands: glutamate and either D-serine or glycine.^[9] The voltage-dependence of current through the channel is mainly due to binding of Mg²⁺ or Zn²⁺ ions to the protein as described above.

The activity of the NMDA receptor is affected by many psychoactive drugs such as phencyclidine (PCP), alcohol (ethanol) and dextromethorphan (DXM). The anaesthetic effects of the drugs ketamine and nitrous oxide are partially because of their effects on NMDA receptor activity.

Structure

The NMDA receptor forms a heterotetramer between two GluN1 and two GluN2 subunits (the subunits were previously denoted as NR1 and NR2), two obligatory NR1 subunits and two regionally localized NR2 subunits. A related gene family of NR3 A and B subunits have an inhibitory effect on receptor activity. Multiple receptor isoforms with distinct brain distributions and functional properties arise by selective splicing of the NR1 transcripts and differential expression of the NR2 subunits.

Each receptor subunit has modular design and each structural module also represents a functional unit:

• The extracellular domain contains two globular structures: a modulatory domain and a ligand-binding domain. NR1 subunits bind the co-agonist glycine and NR2 subunits bind the neurotransmitter glutamate.
• The agonist-binding module links to a membrane domain, which consists of three transmembrane segments and a re-entrant loop reminiscent of the selectivity filter of potassium channels.
• The membrane domain contributes residues to the channel pore and is responsible for the receptor's high-unitary conductance, high-calcium permeability, and voltage-dependent magnesium block.
• Each subunit has an extensive cytoplasmic domain, which contain residues that can be directly modified by a series of protein kinases and protein phosphatases, as well as residues that interact with a large number of structural, adaptor, and scaffolding proteins.

The glycine-binding modules of the NR1 and NR3 subunits and the glutamate-binding module of the NR2A subunit have been expressed as soluble proteins, and their three-dimensional structure has been solved at atomic resolution by x-ray crystallography. This has revealed a common fold with amino acid-binding bacterial proteins and with the glutamate-binding module of AMPA-receptors and kainate-receptors.

Variants

GluN1

There are eight variants of the NR1 subunit produced by alternative splicing of GRIN1:^[10]

• NR1-1a, NR1-1b; NR1-1a is the most abundantly expressed form.
• NR1-2a, NR1-2b;
• NR1-3a, NR1-3b;
• NR1-4a, NR1-4b;

GluN2

NR2 subunit in vertebrates (left) and invertebrates (right). Ryan et al., 2008

While a single NR2 subunit is found in invertebrate organisms, four distinct isoforms of the NR2 subunit are expressed in vertebrates and are referred to with the nomenclature NR2A through NR2D (encoded by GRIN2A, GRIN2B, GRIN2C, GRIN2D). Strong evidence shows that the genes encoding the NR2 subunits in vertebrates have undergone at least two rounds of gene duplication.^[11] They contain the binding-site for the neurotransmitter glutamate. More importantly, each NR2 subunit has a different intracellular C-terminal domain that can interact with different sets of signalling molecules.^[12] Unlike NR1 subunits, NR2 subunits are expressed differentially across various cell types and control the electrophysiological properties of the NMDA receptor. One particular subunit, NR2B, is mainly present in immature neurons and in extrasynaptic locations, and contains the binding-site for the selective inhibitor ifenprodil.

Whereas NR2B is predominant in the early postnatal brain, the number of NR2A subunits grows, and eventually NR2A subunits outnumber NR2B. This is called the NR2B-NR2A developmental switch, and is notable because of the different kinetics each NR2 subunit lends to the receptor.^[13] For instance, greater ratios of the NR2B subunit leads to NMDA receptors which remain open longer compared to those with more NR2A.^[14] This may in part account for greater memory abilities in the immediate postnatal period compared to late in life, which is the principle behind genetically altered 'doogie mice'.

There are three hypothetical models to describe this switch mechanism:

• Increase in synaptic NR2A along with decrease in NR2B
• Extrasynaptic displacement of NR2B away from the synapse with increase in NR2A
• Increase of NR2A diluting the number of NR2B without the decrease of the latter.

The NR2B and NR2A subunits also have differential roles in mediating excitotoxic neuronal death.^[15] The developmental switch in subunit composition is thought to explain the developmental changes in NMDA neurotoxicity.^[16] Disruption of the gene for NR2B in mice causes perinatal lethality, whereas the disruption of NR2A gene produces viable mice, although with impaired hippocampal plasticity.^[17] One study suggests that reelin may play a role in the NMDA receptor maturation by increasing the NR2B subunit mobility.^[18]

NR2B to NR2C switch

Granule cell precursors (GCPs) of the cerebellum, after undergoing symmetric cell division^[19] in the external granule-cell layer (EGL), migrate into the internal granule-cell layer (IGL) where they downregulate NR2B and activate NR2C, a process that is independent of neuregulin beta signaling through ErbB2 and ErbB4 receptors.^[20]

Ligands

Agonists

Activation of NMDA receptors requires binding of glutamate or aspartate (aspartate does not stimulate the receptors as strongly).^[21] In addition, NMDARs also require the binding of the co-agonist glycine for the efficient opening of the ion channel, which is a part of this receptor.

D-serine has also been found to co-agonize the NMDA receptor with even greater potency than glycine.^[22] D-Serine is produced by serine racemase, and is enriched in the same areas as NMDA receptors. Removal of D-serine can block NMDA-mediated excitatory neurotransmission in many areas. Recently, it has been shown that D-serine can be released both by neurons and astrocytes to regulate NMDA receptors.

NMDA receptor (NMDAR)-mediated currents are directly related to membrane depolarization. NMDA agonists therefore exhibit fast Mg²⁺ unbinding kinetics, increasing channel open probability with depolarization. This property is fundamental to the role of the NMDA receptor in memory and learning, and it has been suggested that this channel is a biochemical substrate of Hebbian learning, where it can act as a coincidence detector for membrane depolarization and synaptic transmission.

Some known NMDA receptor agonists include:

• Aminocyclopropanecarboxylic acid
• D-Cycloserine
• cis-2,3-Piperidinedicarboxylic acid
• Aspartic acid
• Glutamic acid
• Quinolinate
• Homocysteic acid
• D-Serine
• L-Serine
• D-Alanine
• L-Alanine
• ACPL
• Nebostinel (positive allosteric modulator of glycine site of NMDA receptor complex)
• Curcumin^[23][24]

Partial agonists

• N-Methyl-D-aspartic acid (NMDA)
• 3,5-Dibromo-L-phenylalanine^[25]
• Apimostinel (NRX-1074)
• Rapastinel (GLYX-13)

Glycine-site NMDA receptor partial agonists, such as rapastinel and apimostinel, are now viewed for the development of new drugs with antidepressant and analgesic effects without obvious psychotomimetic activities.^[26]

Antagonists

Antagonists of the NMDA receptor are used as anesthetics for animals and sometimes humans, and are often used as recreational drugs due to their hallucinogenic properties, in addition to their unique effects at elevated dosages such as dissociation. When certain NMDA receptor antagonists are given to rodents in large doses, they can cause a form of brain damage called Olney's lesions. NMDA receptor antagonists that have been shown to induce Olney's lesions include ketamine, phencyclidine, and dextrorphan (a metabolite of dextromethorphan), as well as some NMDA receptor antagonists used only in research environments. So far, the published research on Olney's lesions is inconclusive in its occurrence upon human or monkey brain tissues with respect to an increase in the presence of NMDA receptor antagonists.^[27]

Common agents in which NMDA receptor antagonism is the primary mechanism of action:

• AP5
• Conantokins
• Dextromethorphan
• Dexanabinol
• Diethyl ether
• Dizocilpine (MK-801)
• Ketamine
• Memantine
• Nitrous oxide
• Phencyclidine
• Xenon
• Methoxetamine
• Agmatine, Blocks NMDA receptors and other cation ligand-gated channels. Can also potentiate opioid analgesia.
• 4-Chlorokynurenine (AV-101), a prodrug of the potent and selective glycine(B)-site NMDA receptor antagonist 7-chlorokynurenic acid, is under development for the treatment of major depressive disorder.^[28][29]

Some common agents in which weak NMDA receptor antagonism is a secondary or additional action include:

• Amantadine^[30]
• Atomoxetine^[31]
• Dextropropoxyphene
• Ethanol
• Guaifenisen
• Huperzine A
• Ibogaine
• Ketobemidone
• Methadone – an opioid analgesic
• Tramadol – an atypical analgesic

Kynurenic acid is an endogenous NMDA receptor antagonist.

Modulators

The NMDA receptor is modulated by a number of endogenous and exogenous compounds:^[32]

• Aminoglycosides have been shown to have a similar effect to polyamines, and this may explain their neurotoxic effect.
• CDK5 regulates the amount of NR2B-containing NMDA receptors on the synaptic membrane, thus affecting synaptic plasticity.^[33][34]
• Polyamines do not directly activate NMDA receptors, but instead act to potentiate or inhibit glutamate-mediated responses.
• Reelin modulates NMDA function through Src family kinases and DAB1.^[35] significantly enhancing LTP in the hippocampus.
• Src kinase enhances NMDA receptor currents.^[36]
• Tianeptine, a rapid-acting antidepressant, is an allosteric modulator of NMDA receptor sites; both positive and negative, depending on cerebral location.
• Na⁺, K⁺ and Ca²⁺ not only pass through the NMDA receptor channel but also modulate the activity of NMDA receptors.
• Zn²⁺ and Cu²⁺ generally block NMDA current activity in a noncompetitive and a voltage-independent manner. However zinc may potentiate or inhibit the current depending on the neural activity.^[37]
• Pb^2+[38] is a potent NMDAR antagonist. Presynaptic deficits resulting from Pb²⁺ exposure during synaptogenesis are mediated by disruption of NMDAR-dependent BDNF signaling.
• Proteins of the major histocompatibility complex class I are endogenous negative regulators of NMDAR-mediated currents in the adult hippocampus,^[39] and are required for appropriate NMDAR-induced changes in AMPAR trafficking ^[39] and NMDAR-dependent synaptic plasticity and learning and memory.^[40][41]
• The activity of NMDA receptors is also strikingly sensitive to the changes in pH, and partially inhibited by the ambient concentration of H⁺ under physiological conditions.^[42] The level of inhibition by H⁺ is greatly reduced in receptors containing the NR1a subtype, which contains the positively charged insert Exon 5. The effect of this insert may be mimicked by positively charged polyamines and aminoglycosides, explaining their mode of action.
• NMDA receptor function is also strongly regulated by chemical reduction and oxidation, via the so-called "redox modulatory site."^[43] Through this site, reductants dramatically enhance NMDA channel activity, whereas oxidants either reverse the effects of reductants or depress native responses. It is generally believed that NMDA receptors are modulated by endogenous redox agents such as glutathione, lipoic acid, and the essential nutrient pyrroloquinoline quinone.

Receptor modulation

The NMDA receptor is a non-specific cation channel that can allow the passage of Ca²⁺ and Na⁺ into the cell and K⁺ out of the cell. The excitatory postsynaptic potential (EPSP) produced by activation of an NMDA receptor increases the concentration of Ca²⁺ in the cell. The Ca²⁺ can in turn function as a second messenger in various signaling pathways. However, the NMDA receptor cation channel is blocked by Mg²⁺ at resting membrane potential.^[44] Magnesium unblock is not instantaneous, to unblock all available channels, the postsynaptic cell must be depolarized for a sufficiently long period of time (in the scale of milliseconds).^[45]

Therefore, the NMDA receptor functions as a "molecular coincidence detector". Its ion channel opens only when the following two conditions are met: glutamate is bound to the receptor, and the postsynaptic cell is depolarized (which removes the Mg²⁺ blocking the channel). This property of the NMDA receptor explains many aspects of long-term potentiation (LTP) and synaptic plasticity.^[46]

NMDA receptors are modulated by a number of endogenous and exogenous compounds and play a key role in a wide range of physiological (e.g., memory) and pathological processes (e.g., excitotoxicity).

Clinical significance

Memantine is approved by the U.S. F.D.A and the European Medicines Agency for treatment of moderate-to-severe Alzheimer's disease,^[47] and has now received a limited recommendation by the UK's National Institute for Health and Care Excellence for patients who fail other treatment options.^[48]

Cochlear NMDARs are the target of intense research to find pharmacological solutions to treat tinnitus. Recently, NMDARs were associated with a rare autoimmune disease, anti-NMDAR encephalitis, that usually occurs due to cross reactivity of antibodies produced by the immune system against ectopic brain tissues, such as those found in teratoma.

NMDAR ligands, including ketamine, esketamine, rapastinel (GLYX-13), apimostinel (NRX-1074), 4-chlorokynurenine (AV-101), and CERC-301, are under development for the treatment of mood disorders, including major depressive disorder and treatment-resistant depression.^[28][29][49] In addition, ketamine is already employed for this purpose as an off-label therapy in some clinics.^[50][51]

Compared to dopaminergic stimulants, phencyclidine can produce a wider range of symptoms that resemble schizophrenia in healthy volunteers, in what has led to the glutamate hypothesis of schizophrenia.^[52] Experiments in which rodents are treated with NMDA receptor antagonist are today the most common model when it comes to testing of novel schizophrenia therapies or exploring the exact mechanism of drugs already approved for treatment of schizophrenia.

See also

• Long-term potentiation and depression
• Category: NMDA receptor antagonists
• NMDA
• AMPA
• AMPA receptor
• Calcium/calmodulin-dependent protein kinases
• Anti-glutamate receptor antibodies
• Anti-NMDA receptor encephalitis

External links

• Media related to NMDA receptor at Wikimedia Commons
• NMDA receptor pharmacology
• Motor Discoordination Results from Combined Gene Disruption of the NMDA Receptor NR2A and NR2C Subunits, But Not from Single Disruption of the NR2A or NR2C Subunit
• A schematic diagram summarizes three potential models for the switching of NR2A and NR2B subunits at developing synapses. - a figure from Liu et al., 2004^[53]
• Drosophila NMDA receptor 1 - The Interactive Fly

References