Mg2+ and memantine block of rat recombinant NMDA receptors containing chimeric NR2A/2D subunits expressed in Xenopus laevis oocytes

Memantine: Updating a rare success story in pro-cognitive therapeutics

The great potential for NMDA receptor modulators as druggable targets in neurodegenerative disorders has been met with limited success. Considered one of the rare exceptions, memantine has consistently demonstrated restorative and prophylactic properties in many AD models. In clinical trials memantine slows the decline in cognitive performance associated with AD. Here, we provide an overview of the basic properties including pharmacological targets, toxicology and cellular effects of memantine. Evidence demonstrating reductions in molecular, physiological and behavioural indices of AD-like impairments associated with memantine treatment are also discussed. This represents both an extension and homage to Dr. Chris Parson's considerable contributions to our fundamental understanding of a success story in the AD treatment landscape.

A Frameshift Variant of GluN2A Identified in an Epilepsy Patient Results in NMDA Receptor Mistargeting

N-methyl-D-aspartate receptors (NMDARs) are crucial for neuronal development and synaptic plasticity. Dysfunction of NMDARs is associated with multiple neurodevelopmental disorders, including epilepsy, autism spectrum disorder, and intellectual disability. Understanding the impact of genetic variants of NMDAR subunits can shed light on the mechanisms of disease. Here, we characterized the functional implications of a de novo mutation of the GluN2A subunit (P1199Rfs*32) resulting in the truncation of the C-terminal domain. The variant was identified in a male patient with epileptic encephalopathy, multiple seizure types, severe aphasia, and neurobehavioral changes. Given the known role of the CTD in NMDAR trafficking, we examined changes in receptor localization and abundance at the postsynaptic membrane using a combination of molecular assays in heterologous cells and rat primary neuronal cultures. We observed that the GluN2A P1199Rfs*32-containing receptors traffic efficiently to the postsynaptic membrane but have increased extra-synaptic expression relative to WT GluN2A-containing NMDARs. Using in silico predictions, we hypothesized that the mutant would lose all PDZ interactions, except for the recycling protein Scribble1. Indeed, we observed impaired binding to the scaffolding protein postsynaptic protein-95 (PSD-95); however, we found the mutant interacts with Scribble1, which facilitates the recycling of both the mutant and the WT GluN2A. Finally, we found that neurons expressing GluN2A P1199Rfs*32 have fewer synapses and decreased spine density, indicating compromised synaptic transmission in these neurons. Overall, our data show that GluN2A P1199Rfs*32 is a loss-of-function variant with altered membrane localization in neurons and provide mechanistic insight into disease etiology.

NMDA receptor function in inhibitory neurons

N-methyl-d-aspartate receptors (NMDARs) are present in the majority of brain circuits and play a key role in synaptic information transfer and synaptic plasticity. A key element of many brain circuits are inhibitory GABAergic interneurons that in themselves show diverse and cell-type-specific NMDAR expression and function. Indeed, NMDARs located on interneurons control cellular excitation in a synapse-type specific manner which leads to divergent dendritic integration properties amongst the plethora of interneuron subtypes known to exist. In this review, we explore the documented diversity of NMDAR subunit expression in identified subpopulations of interneurons and assess the NMDAR subtype-specific control of their function. We also highlight where knowledge still needs to be obtained, if a full appreciation is to be gained of roles played by NMDARs in controlling GABAergic modulation of synaptic and circuit function. This article is part of the 'Special Issue on Glutamate Receptors - NMDA receptors'.

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

Acute high-fat diet (aHFD) exposure induces a brief period of hyperphagia before caloric balance is restored. Previous studies have demonstrated that this period of regulation is associated with activation of synaptic N-methyl-D-aspartate (NMDA) receptors on dorsal motor nucleus of the vagus (DMV) neurons, which increases vagal control of gastric functions. Our aim was to test the hypothesis that activation of DMV synaptic NMDA receptors occurs subsequent to activation of extrasynaptic NMDA receptors. Sprague-Dawley rats were fed a control or high-fat diet for 3-5 days prior to experimentation. Whole-cell patch-clamp recordings from gastric-projecting DMV neurons; in vivo recordings of gastric motility, tone, compliance, and emptying; and food intake studies were used to assess the effects of NMDA receptor antagonism on caloric regulation. After aHFD exposure, inhibition of extrasynaptic NMDA receptors prevented the synaptic NMDA receptor-mediated increase in glutamatergic transmission to DMV neurons, as well as the increase in gastric tone and motility, while chronic extrasynaptic NMDA receptor inhibition attenuated the regulation of caloric intake. After aHFD exposure, the regulation of food intake involved synaptic NMDA receptor-mediated currents, which occurred in response to extrasynaptic NMDA receptor activation. Understanding these events may provide a mechanistic basis for hyperphagia and may identify novel therapeutic targets for the treatment of obesity.

Potential of Medicinal Plants as Neuroprotective and Therapeutic Properties Against Amyloid-β-Related Toxicity, and Glutamate-Induced Excitotoxicity in Human Neural Cells

Alzheimer's disease (AD) and Parkinson's disease (PD) are notorious neurodegenerative diseases amongst the general population. Being age-associated diseases, the prevalence of AD and PD is forecasted to rapidly escalate with the progressive aging population of the world. These diseases are complex and multifactorial. Among different events, amyloid β peptide (Aβ) induced toxicity is a well-established pathway of neuronal cell death, which plays a vital function in AD. Glutamate, the major excitatory transmitter, acts as a neurotoxin when present in excess at the synapses; this latter mechanism is termed excitotoxicity. It is hypothesised that glutamate-induced excitotoxicity contributes to the pathogenesis of AD and PD. No cure for AD and PD is currently available and the currently approved drugs available to treat these diseases have limited effectiveness and pose adverse effects. Indeed, plants have been a major source for the discovery of novel pharmacologically active compounds for distinct pathological conditions. Diverse plant species employed for brain-related disorders in traditional medicine are being explored to determine the scientific rationale behind their uses. Herein, we present a comprehensive review of plants and their constituents that have shown promise in reversing the (i) amyloid-β -related toxicity in AD models and (ii) glutamate-induced excitotoxicity in AD and PD models. This review summarizes information regarding the phytochemistry, biological and cellular activities, and clinical trials of several plant species in view to provide adequate scientific baseline information that could be used in the drug development process, thereby providing effective leads for AD and PD.

Leptin modulates pancreatic β-cell membrane potential through Src kinase-mediated phosphorylation of NMDA receptors

The adipocyte-derived hormone leptin increases trafficking of KATP and Kv2.1 channels to the pancreatic β-cell surface, resulting in membrane hyperpolarization and suppression of insulin secretion. We have previously shown that this effect of leptin is mediated by the NMDA subtype of glutamate receptors (NMDARs). It does so by potentiating NMDAR activity, thus enhancing Ca2+ influx and the ensuing downstream signaling events that drive channel trafficking to the cell surface. However, the molecular mechanism by which leptin potentiates NMDARs in β-cells remains unknown. Here, we report that leptin augments NMDAR function via Src kinase-mediated phosphorylation of the GluN2A subunit. Leptin-induced membrane hyperpolarization diminished upon pharmacological inhibition of GluN2A but not GluN2B, indicating involvement of GluN2A-containing NMDARs. GluN2A harbors tyrosine residues that, when phosphorylated by Src family kinases, potentiate NMDAR activity. We found that leptin increases phosphorylation of Tyr-418 in Src, an indicator of kinase activation. Pharmacological inhibition of Src or overexpression of a kinase-dead Src mutant prevented the effect of leptin, whereas a Src kinase activator peptide mimicked it. Using mutant GluN2A overexpression, we show that Tyr-1292 and Tyr-1387 but not Tyr-1325 are responsible for the effect of leptin. Importantly, β-cells from db/db mice, a type 2 diabetes mouse model lacking functional leptin receptors, or from obese diabetic human donors failed to respond to leptin but hyperpolarized in response to NMDA. Our study reveals a signaling pathway wherein leptin modulates NMDARs via Src to regulate β-cell excitability and suggests NMDARs as a potential target to overcome leptin resistance.

N-Methyl-D-Aspartate Receptors in Hematopoietic Cells: What Have We Learned?

The N-methyl-D-aspartate receptor (NMDAR) provides a pathway for glutamate-mediated inter-cellular communication, best known for its role in the brain but with multiple examples of functionality in non-neuronal cells. Data previously published by others and us provided ex vivo evidence that NMDARs regulate platelet and red blood cell (RBC) production. Here, we summarize what is known about these hematopoietic roles of the NMDAR. Types of NMDAR subunits expressed in megakaryocytes (platelet precursors) and erythroid cells are more commonly found in the developing rather than adult brain, suggesting trophic functions. Nevertheless, similar to their neuronal counterparts, hematopoietic NMDARs function as ion channels, and are permeable to calcium ions (Ca2+). Inhibitors that block open NMDAR (memantine and MK-801) interfere with megakaryocytic maturation and proplatelet formation in primary culture. The effect on proplatelet formation appears to involve Ca2+ influx-dependent regulation of the cytoskeletal remodeling. In contrast to normal megakaryocytes, NMDAR effects in leukemic Meg-01 cells are diverted away from differentiation to increase proliferation. NMDAR hypofunction triggers differentiation of Meg-01 cells with the bias toward erythropoiesis. The underlying mechanism involves changes in the intracellular Ca2+ homeostasis, cell stress pathways, and hematopoietic transcription factors that upon NMDAR inhibition shift from the predominance of megakaryocytic toward erythroid regulators. This ability of NMDAR to balance both megakaryocytic and erythroid cell fates suggests receptor involvement at the level of a bipotential megakaryocyte-erythroid progenitor. In human erythroid precursors and circulating RBCs, NMDAR regulates intracellular Ca2+ homeostasis. NMDAR activity supports survival of early proerythroblasts, and in mature RBCs NMDARs impact cellular hydration state, hemoglobin oxygen affinity, and nitric oxide synthase activity. Overexcitation of NMDAR in mature RBCs leads to Ca2+ overload, K+ loss, RBC dehydration, and oxidative stress, which may contribute to the pathogenesis of sickle cell disease. In summary, there is growing evidence that glutamate-NMDAR signaling regulates megakaryocytic and erythroid cells at different stages of maturation, with some intriguing differences emerging in NMDAR expression and function between normal and diseased cells. NMDAR signaling may provide new therapeutic opportunities in hematological disease, but in vivo applicability needs to be confirmed.

Humanin rescues cultured rat cortical neurons from NMDA-induced toxicity through the alleviation of mitochondrial dysfunction

N-methyl-D-aspartate (NDMA) receptor-mediated excitotoxicity has been implicated in a variety of pathological situations such as Alzheimer’s disease (AD) and Parkinson’s disease. However, no effective treatments for the same have been developed so far. Humanin (HN) is a 24-amino acid peptide originally cloned from the brain of patients with AD and it prevents stress-induced cell death in many cells/tissues. In our previous study, HN was found to effectively rescue rat cortical neurons. It is still not clear whether HN protects the neurons through the attenuation of mitochondrial dysfunction. In this study, excitatory toxicity was induced by NMDA, which binds the NMDA receptor in primarily cultured rat cortical neurons. We found that NMDA (100 μmol/L) dramatically induced the decrease of cell viability and caused mitochondrial dysfunction. Pretreatment of the neurons with HN (1 μmol/L) led to significant increases of mitochondrial succinate dehydrogenase (SDH) activity and membrane potential. In addition, HN pretreatment significantly reduced the excessive production of both reactive oxygen species (ROS) and nitric oxide (NO). Thus, HN could attenuate the excitotoxicity caused by the overactivation of the NMDA receptor through the alleviation of mitochondrial dysfunction.

Tonically active NMDA receptors--a signalling mechanism critical for interneuronal excitability in the CA1 stratum radiatum

In contrast to tonic extrasynaptic γ-aminobutyric acid (GABA)A receptor-mediated signalling, the physiological significance of tonic extrasynaptic N-methyl-D-aspartate (NMDA) receptor (NMDAR)-mediated signalling remains uncertain. In this study, reversible open-channel blockers of NMDARs, memantine and phencyclidine (PCP) were used as tools to examine tonic NMDAR-mediated signalling in rat hippocampal slices. Memantine in concentrations up to 10 μM had no effect on synaptically evoked NMDAR-mediated responses in pyramidal neurons or GABAergic interneurons. On the other hand, 10 μM memantine reduced tonic NMDAR-mediated currents in GABAergic interneurons by approximately 50%. These tonic NMDAR-mediated currents in interneurons contributed significantly to the excitability of the interneurons as 10 μM memantine reduced the disynaptic inhibitory postsynaptic current in pyramidal cells by about 50%. Moreover, 10 μM memantine, but also PCP in concentrations ≤ 1 μM, increased the magnitude of the population spike, likely because of disinhibition. The relatively higher impact of tonic NMDAR-mediated signalling in interneurons was at least partly explained by the expression of GluN2D-containing NMDARs, which was not observed in mature pyramidal cells. The current results are consistent with the idea that low doses of readily reversible NMDAR open-channel blockers preferentially inhibit tonically active extrasynaptic NMDARs, and they suggest that tonically active NMDARs contribute more prominently to the intrinsic excitation in GABAergic interneurons than in pyramidal cells. It is proposed that this specific difference between interneurons and pyramidal cells can explain the disinhibition caused by the Alzheimer's disease medication memantine.

Memantine selectively blocks extrasynaptic NMDA receptors in rat substantia nigra dopamine neurons

Recent studies suggest that selective block of extrasynaptic N-methyl-d-aspartate (NMDA) receptors might protect against neurodegeneration. We recorded whole-cell currents with patch pipettes to characterize the ability of memantine, a low-affinity NMDA channel blocker, to block synaptic and extrasynaptic NMDA receptors in substantia nigra zona compacta (SNC) dopamine neurons in slices of rat brain. Pharmacologically isolated NMDA receptor-mediated EPSCs were evoked by electrical stimulation, whereas synaptic and extrasynaptic receptors were activated by superfusing the slice with NMDA (10 µM). Memantine was 15-fold more potent for blocking currents evoked by bath-applied NMDA compared to synaptic NMDA receptors. Increased potency for blocking bath-applied NMDA currents was shared by the GluN2C/GluN2D noncompetitive antagonist DQP-1105 but not by the high-affinity channel blocker MK-801. Our data suggest that memantine causes a selective block of extrasynaptic NMDA receptors that are likely to contain GluN2C/2D subunits. Our results justify further investigations on the use of memantine as a neuroprotective agent in Parkinson's disease.

Molecular bases of NMDA receptor subtype-dependent properties

NMDA receptors (NMDARs) are a class of ionotropic glutamate receptors (iGluRs) that are essential for neuronal development, synaptic plasticity, learning and cell survival. Several features distinguish NMDARs from other iGluRs and underlie the crucial roles NMDARs play in nervous system physiology. NMDARs display slow deactivation kinetics, are highly Ca(2+) permeable, and require depolarization to relieve channel block by external Mg(2+) ions, thereby making them effective coincidence detectors. These properties and others differ among NMDAR subtypes, which are defined by the subunits that compose the receptor. NMDARs, which are heterotetrameric, commonly are composed of two GluN1 subunits and two GluN2 subunits, of which there are four types, GluN2A-D. 'Diheteromeric' NMDARs contain two identical GluN2 subunits. Gating and ligand-binding properties (e.g. deactivation kinetics) and channel properties (e.g. channel block by Mg(2+)) depend strongly on the GluN2 subunit contained in diheteromeric NMDARs. Recent work shows that two distinct regions of GluN2 subunits control most diheteromeric NMDAR subtype-dependent properties: the N-terminal domain is responsible for most subtype dependence of gating and ligand-binding properties; a single residue difference between GluN2 subunits at a site termed the GluN2 S/L site is responsible for most subtype dependence of channel properties. Thus, two structurally and functionally distinct regions underlie the majority of subtype dependence of NMDAR properties. This topical review highlights recent studies of recombinant diheteromeric NMDARs that uncovered the involvement of the N-terminal domain and of the GluN2 S/L site in the subtype dependence of NMDAR properties.

Ionotropic GABA and glycine receptor subunit composition in human pluripotent stem cell-derived excitatory cortical neurones

We have assessed, using whole-cell patch-clamp recording and RNA-sequencing (RNA-seq), the properties and composition of GABA_A receptors (GABA_ARs) and strychnine-sensitive glycine receptors (GlyRs) expressed by excitatory cortical neurons derived from human embryonic stem cells (hECNs). The agonists GABA and muscimol gave EC₅₀ values of 278 μm and 182 μm, respectively, and the presence of a GABA_AR population displaying low agonist potencies is supported by strong RNA-seq signals for α2 and α3 subunits. GABA_AR-mediated currents, evoked by EC₅₀ concentrations of GABA, were blocked by bicuculline and picrotoxin with IC₅₀ values of 2.7 and 5.1 μm, respectively. hECN GABA_ARs are predominantly γ subunit-containing as assessed by the sensitivity of GABA-evoked currents to diazepam and insensitivity to Zn²⁺, together with the weak direct agonist action of gaboxadol; RNA-seq indicated a predominant expression of the γ2 subunit. Potentiation of GABA-evoked currents by propofol and etomidate and the lack of inhibition of currents by salicylidine salycylhydrazide (SCS) indicate expression of the β2 or β3 subunit, with RNA-seq analysis indicating strong expression of β3 in hECN GABA_ARs. Taken together our data support the notion that hECN GABA_ARs have an α2/3β3γ2 subunit composition – a composition that also predominates in immature rodent cortex. GlyRs expressed by hECNs were activated by glycine with an EC₅₀ of 167 μm. Glycine-evoked (500 μm) currents were blocked by strychnine (IC₅₀ = 630 nm) and picrotoxin (IC₅₀ = 197 μm), where the latter is suggestive of a population of heteromeric receptors. RNA-seq indicates GlyRs are likely to be composed of α2 and β subunits.

Glutamate receptor pores

Glutamate receptors are ligand-gated ion channels that mediate fast excitatory synaptic transmission throughout the central nervous system. Functional receptors are homo- or heteromeric tetramers with each subunit contributing a re-entrant pore loop that dips into the membrane from the cytoplasmic side. The pore loops form a narrow constriction near their apex with a wide vestibule toward the cytoplasm and an aqueous central cavity facing the extracellular solution. This article focuses on the pore region, reviewing how structural differences among glutamate receptor subtypes determine their distinct functional properties.

Mg2+ block properties of triheteromeric GluN1-GluN2B-GluN2D NMDA receptors on neonatal rat substantia nigra pars compacta dopaminergic neurones

Native NMDA receptors (NMDARs) are tetrameric channels formed by two GluN1 and two GluN2 subunits. So far, seven NMDARs subunits have been identified and they can form diheteromeric or triheteromeric NMDARs (more than one type of GluN2 subunit). Extracellular Mg²⁺ is an important regulator of NMDARs, and particularly the voltage dependence of Mg²⁺ block is crucial to the roles of NMDARs in synaptic plasticity and the integration of synaptic activity with neuronal activity. Although the Mg²⁺ block properties of diheteromeric NMDARs are fully investigated, properties of triheteromeric NMDARs are still not clear. Our previous data suggested that dopaminergic neurones expressed triheteromeric GluN1–GluN2B–GluN2D NMDARs. Here, using NMDARs in dopaminergic neurones from postnatal day 7 (P7) rats as a model system, we characterize the voltage-dependent Mg²⁺ block properties of triheteromeric NMDARs. In control conditions, external Mg²⁺ significantly inhibits the whole cell NMDA-evoked current in a voltage-dependent manner with IC₅₀ values of 20.9 μm, 53.3 μm and 173 μm at −90 mV, −70 mV and −50 mV, respectively. When the GluN2B-selective antagonist ifenprodil was applied, the Mg²⁺ sensitivity of the residual NMDA-mediated currents (which is mainly carried by GluN1–GluN2B–GluN2D NMDARs) is reduced to IC₅₀ values of 45.9 μm (−90 mV), 104 μm (−70 mV) and 276 μm (−50 mV), suggesting that triheteromeric GluN1–GluN2B–GluN2D NMDARs have less affinity for external Mg²⁺ than GluN1–GluN2B receptors. In addition, fitting I_NMDA–V curves with a trapping Mg²⁺ block model shows the triheteromeric GluN1–GluN2B–GluN2D NMDARs have weaker voltage-dependent Mg²⁺ block (δ = 0.56) than GluN1–GluN2B NMDARs. Finally, our concentration jump and single channel recordings suggest that GluN1–GluN2B–GluN2D rather than GluN1–GluN2D NMDARs are present. These data provide information relevant to Mg²⁺ block characteristics of triheteromeric NMDARs and may help to better understand synaptic plasticity, which is dependent on these triheteromeric NMDARs.

The preventive effect of NR2B and NR2D-containing NMDAR antagonists on Aβ-induced LTP disruption in the dentate gyrus of rats

Amyloid β-protein (Aβ) in the brain of Alzheimer's disease (AD) potently inhibits the synaptic plasticity subsequently causing the cognitive deficits. Long-term potentiation (LTP) of synaptic transmission is thought to be an important cellular mechanism underlying memory formation. Different NR2 subunits are involved in NMDA receptor-dependent LTP. In the present study, we investigated the roles of NR2B and NR2D-containing NMDAR on Aβ(1-42)-induced LTP deficits in the hippocampal slices of rats by using selective NMDAR antagonists. First, we found that Aβ(1-42) significantly inhibited the LTP in the dentate gyrus of slices as reported before. Following that the Aβ(1-42)-induced LTP inhibition was prevented by the pre-perfusion of the specific NR2B-containing NMDAR antagonists ifenprodil (approximately >200-fold selectivity for NR2B) and Ro25-6981 (>3,000-fold selectivity for NR2B), as well as PPDA, a specific NR2D receptor antagonist. Meanwhile, the antagonists on their own had no or only partial effects on the normal LTP in the same dose condition. These findings not only support the effects of NR2B and NR2D subunits on Aβ(1-42)-induced LTP deficits, but also imply that preferentially targeting NR2B- and NR2D-containing NMDARs may provide an effective means to prevent cognitive deficits in the early AD.

Direct pharmacological monitoring of the developmental switch in NMDA receptor subunit composition using TCN 213, a GluN2A-selective, glycine-dependent antagonist

BACKGROUND AND PURPOSE

Developmental switches in NMDA receptor subunit expression have been inferred from studies of GluN2 expression levels, changes in kinetics of glutamatergic synaptic currents and sensitivity of NMDA receptor-mediated currents to selective GluN2B antagonists. Here we use TCN 213, a novel GluN2A-selective antagonist to identify the presence of this subunit in functional NMDA receptors in developing cortical neurones.

EXPERIMENTAL APPROACH

Two-electrode voltage-clamp (TEVC) recordings were made from Xenopus laevis oocytes to determine the pharmacological activity of TCN 213 at recombinant NMDA receptors. TCN 213 antagonism was studied in cultures of primary cortical neurones, assessing the NMDA receptor dependency of NMDA-induced excitotoxicity and monitoring developmental switches in NMDA receptor subunit composition.

KEY RESULTS

TCN 213 antagonism of GluN1/GluN2A NMDA receptors was dependent on glycine but independent of glutamate concentrations in external recording solutions. Antagonism by TCN 213 was surmountable and gave a Schild plot with unity slope. TCN 213 block of GluN1/GluN2B NMDA receptor-mediated currents was negligible. In cortical neurones, at a early developmental stage predominantly expressing GluN2B-containing NMDA receptors, TCN 213 failed to antagonize NMDA receptor-mediated currents or to prevent GluN2B-dependent, NMDA-induced excitoxicity. In older cultures (DIV 14) or in neurones transfected with GluN2A subunits, TCN 213 antagonized NMDA-evoked currents. Block by TCN 213 of NMDA currents inversely correlated with block by ifenprodil, a selective GluN2B antagonist.

CONCLUSIONS AND IMPLICATIONS

TCN 213 selectively blocked GluN1/GluN2A over GluN1/GluN2B NMDA receptors allowing direct dissection of functional NMDA receptors and pharmacological profiling of developmental changes in native NMDA receptor subunit composition.

TCN 201 selectively blocks GluN2A-containing NMDARs in a GluN1 co-agonist dependent but non-competitive manner

Antagonists that are sufficiently selective to preferentially block GluN2A-containing N-methyl-d-aspartate receptors (NMDARs) over GluN2B-containing NMDARs are few in number. In this study we describe a pharmacological characterization of 3-chloro-4-fluoro-N-[4-[[2-(phenylcarbonyl)hydrazino]carbonyl]benzyl]benzenesulphonamide (TCN 201), a sulphonamide derivative, that was recently identified from a high-throughput screen as a potential GluN2A-selective antagonist. Using two-electrode voltage-clamp (TEVC) recordings of NMDAR currents from Xenopus laevis oocytes expressing either GluN1/GluN2A or GluN1/GluN2B NMDARs we demonstrate the selective antagonism by TCN 201 of GluN2A-containing NMDARs. The degree of inhibition produced by TCN 201 is dependent on the concentration of the GluN1-site co-agonist, glycine (or d-serine), and is independent of the glutamate concentration. This GluN1 agonist-dependency is similar to that observed for a related GluN2A-selective antagonist, N-(cyclohexylmethyl)-2-[{5-[(phenylmethyl)amino]-1,3,4-thiadiazol-2-yl}thio]acetamide (TCN 213). Schild analysis of TCN 201 antagonism indicates that it acts in a non-competitive manner but its equilibrium constant at GluN1/GluN2A NMDARs indicates TCN 201 is around 30-times more potent than TCN 213. In cortical neurones TCN 201 shows only modest antagonism of NMDAR-mediated currents recorded from young (DIV 9–10) neurones where GluN2B expression predominates. In older cultures (DIV 15–18) or in cultures where GluN2A subunits have been over-expressed TCN 201 gives a strong block that is negatively correlated with the degree of block produced by the GluN2B-selective antagonist, ifenprodil. Nevertheless, while TCN 201 is a potent antagonist it must be borne in mind that its ability to block GluN2A-containing NMDARs is dependent on the GluN1-agonist concentration and is limited by its low solubility.

Open-channel blockade is less effective on GluN3B than GluN3A subunit-containing NMDA receptors

The GluN3 subunits of the N-methyl-d-aspartate (NMDA) receptor are known to reduce its Ca²⁺ permeability and Mg²⁺ sensitivity, however, little is known about their effects on other channel blockers. cRNAs for rat NMDA receptor subunits were injected into Xenopus oocytes and responses to NMDA and glycine were recorded using two electrode voltage clamp. Channel block of receptors containing GluN1-1a/2A, GluN1-1a/2A/3A or GluN1-1a/2A/3B subunits was characterised using Mg²⁺, memantine, MK-801, philanthotoxin-343 and methoctramine. IC₅₀ values for Mg²⁺ and memantine increased when receptors contained GluN3A subunits and were further increased when they contained GluN3B, e.g. IC₅₀s at − 75 mV for block of GluN1-1a/2A, GluN1-1a/2A/3A and GluN1-1a/2A/3B receptors respectively were 4.2, 22.4 and 40.1 μM for Mg²⁺, and 2.5, 7.5 and 17.5 μM for memantine. Blocking activity was found to be fully or partially restored when G or R (at the N and N + 1 sites respectively) were mutated to N in GluN3A. Thus, the changes cannot be attributed to the loss of the N or N + 1 sites alone, but rather involve both sites or residues elsewhere. Block by MK-801 and philanthotoxin-343 was also reduced by GluN3A, most strongly at − 100 mV but not at − 50 mV, and by GluN3B at all V_h. Methoctramine was the least sensitive to introduction of GluN3 subunits suggesting a minimal interaction with the N and N + 1 sites. We conclude that GluN3B-containing receptors provide increased resistance to channel block compared to GluN3A-containing receptors and this must be due to differences outside the deep pore region (N site and deeper).

Negative modulation of NMDA receptor channel function by DREAM/calsenilin/KChIP3 provides neuroprotection?

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels highly permeable to calcium and essential to excitatory neurotransmission. The NMDARs have attracted much attention because of their role in synaptic plasticity and excitotoxicity. Evidence has recently accumulated that NMDARs are negatively regulated by intracellular calcium binding proteins. The calcium-dependent suppression of NMDAR function serves as a feedback mechanism capable of regulating subsequent Ca²⁺ entry into the postsynaptic cell, and may offer an alternative approach to treating NMDAR-mediated excitotoxic injury. This short review summarizes the recent progress made in understanding the negative modulation of NMDAR function by DREAM/calsenilin/KChIP3, a neuronal calcium sensor (NCS) protein.

Distinct pharmacological and functional properties of NMDA receptors in mouse cortical astrocytes

BACKGROUND AND PURPOSE

Astrocytes of the mouse neocortex express functional NMDA receptors, which are not blocked by Mg(2+) ions. However, the pharmacological profile of glial NMDA receptors and their subunit composition is far from complete.

EXPERIMENTAL APPROACH

We tested the sensitivity of NMDA receptor-mediated currents to the novel GluN2C/D subunit-selective antagonist UBP141 in mouse cortical astrocytes and neurons. We also examined the effect of memantine, an antagonist that has substantially different affinities for GluN2A/B and GluN2C/d-containing receptors in physiological concentrations of extracellular Mg(2+).

KEY RESULTS

UBP141 had a strong inhibitory action on NMDA receptor-mediated transmembrane currents in the cortical layer II/III astrocytes with an IC(50) of 2.29 µM and a modest inhibitory action on NMDA-responses in the pyramidal neurons with IC(50) of 19.8 µM. Astroglial and neuronal NMDA receptors exhibited different sensitivities to memantine with IC(50) values of 2.19 and 10.8 µM, respectively. Consistent with pharmacological differences between astroglial and neuronal NMDA receptors, NMDA receptors in astrocytes showed lower Ca(2+) permeability than neuronal receptors with P(Ca) /P(Na) ratio of 3.4.

CONCLUSIONS AND IMPLICATIONS

The biophysical and pharmacological properties of the astrocytic NMDA receptors strongly suggest that they have a tri-heteromeric structure composed of GluN1, GluN2C/D and GluN3 subunits. The substantial difference between astroglial and neuronal NMDA receptors in their sensitivity to UBP141 and memantine may enable selective modulation of astrocytic signalling that could be very helpful for elucidating the mechanisms of neuron-glia communications. Our results may also provide the basis for the development of novel therapeutic agents specifically targeting glial signalling.

A single GluN2 subunit residue controls NMDA receptor channel properties via intersubunit interaction

NMDA receptors (NMDARs) are glutamate-gated ion channels that are present at most excitatory mammalian synapses. The four GluN2 subunits (GluN2A-D) contribute to four diheteromeric NMDAR subtypes that have divergent physiological and pathological roles. Channel properties that are fundamental to NMDAR function vary among subtypes. We investigated the amino acid residues responsible for variations in channel properties by creating and examining NMDARs containing mutant GluN2 subunits. We found that the NMDAR subtype specificity of three crucial channel properties, Mg(2+) block, selective permeability to Ca(2+) and single-channel conductance, were all controlled primarily by the residue at a single GluN2 site in the M3 transmembrane region. Mutant cycle analysis guided by molecular modeling revealed that a GluN2-GluN1 subunit interaction mediates the site's effects. We conclude that a single GluN2 subunit residue couples with the pore-forming loop of the GluN1 subunit to create naturally occurring variations in NMDAR properties that are critical to synaptic plasticity and learning.

Protection from glutamate-induced excitotoxicity by memantine

This study investigates whether the uncompetitive N-methyl-D-aspartic acid receptor antagonist, memantine, is able to protect dissociated cortical neurons from glutamate-induced excitotoxicity (GIE). Treatment with glutamate resulted in a significant loss of synchronization of neuronal activity as well as a significant increase in the duration of synchronized bursting events (SBEs). By administering memantine at the same time as glutamate, we were able to completely prevent these changes to the neuronal activity. Pretreatment with memantine was somewhat effective in preventing changes to the culture synchronization but was unable to fully protect the synchronization of electrical activity between neurons that showed high levels of synchronization prior to injury. Additionally, memantine pretreatment was unable to prevent the increase in the duration of SBEs caused by GIE. Thus, the timing of memantine treatment is important for conferring neuroprotection against glutamate-induced neurotoxicity. Finally, we found that GIE leads to a significant increase in the burst duration. Our data suggest that this may be due to an alteration in the inhibitory function of the neurons.

Selective inhibition of GluN2D-containing N-methyl-D-aspartate receptors prevents tissue plasminogen activator-promoted neurotoxicity both in vitro and in vivo

Background

Tissue plasminogen activator (tPA) exerts multiple functions in the central nervous system, depending on the partner with which it interacts. In particular, tPA acts as a positive neuromodulator of N-methyl-D-aspartate glutamatergic receptors (NMDAR). At the molecular level, it has been proposed that the pro-neurotoxicity mediated by tPA might occur through extrasynaptic NMDAR containing the GluN2D subunit. Thus, selective antagonists targeting tPA/GluN2D-containing NMDAR signaling would be of interest to prevent noxious effects of tPA.

Results

Here, we compared three putative antagonists of GluN2D-containing NMDAR and we showed that the new compound UBP145 ((2R*,3S*)-1-(9-bromophenan-threne-3-carbonyl)piperazine-2,3-dicarboxylic acid) is far more selective for GluN2D subunits than memantine and PPDA (phenanthrene derivative (2S*, 3R*)-1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid). Indeed, in vitro, in contrast to the two other compounds, UBP145 prevented NMDA toxicity only in neurons expressing GluN2D (ie, in cortical but not hippocampal neurons). Furthermore, in cultured cortical neurons, UBP145 fully prevented the pro-excitotoxic effect of tPA. In vivo, we showed that UBP145 potently prevented the noxious action of exogenous tPA on excitotoxic damages. Moreover, in a thrombotic stroke model in mice, administration of UBP145 prevented the deleterious effect of late thrombolysis by tPA.

Conclusions

In conclusion, tPA exerts noxious effects on neurons by acting on GluN2D-containing NMDAR and pharmacological antagonists of GluN2D-containing NMDAR could be used to prevent the ability of tPA to promote neurotoxicity.

Quantification of the Mg2+-induced potency shift of amantadine and memantine voltage-dependent block in human recombinant GluN1/GluN2A NMDARs

Clinically, amantadine and memantine are drugs whose therapeutic utility is linked to their ability to block N-methyl-D-aspartate receptors (NMDARs) in a voltage-dependent manner. Nevertheless many studies that have characterized the pharmacological actions of amantadine and memantine have done so in the absence of physiological levels of Mg(2+) ions. This study quantifies the extent to which Mg(2+) alters the potency of the block produced by both amantadine and memantine at human recombinant GluN1/GluN2A NMDARs. Human recombinant GluN1/GluN2A NMDARs were expressed in Xenopus laevis oocytes and two-electrode voltage-clamp recordings were made at -80, -60 and -40 mV to quantify amantadine and memantine block in the absence and presence of Mg(2+). Amantadine and memantine blocked human GluN1/GluN2A NMDARs in a voltage-dependent manner with IC(50) values (at -80 mV) of 49 ± 6 μM (n = 7) and 1.0 ± 0.3 μM (n = 7), respectively. In the presence of Mg(2+) (1mM) the equivalent IC(50) values were 165 ± 10 μM (n=6) and 6.6 ± 0.3 μM (n = 5). Similarly in the presence of amantadine or memantine the potency of Mg(2+) in blocking GluN1/GluN2A NMDARs was reduced. The decrease in the potencies of both amantadine and memantine in the presence of physiological concentrations of Mg(2+) indicates that other targets (e.g. α7-nicotinic acetylcholine receptors and 5-HT(3) receptors) in addition to NMDARs may well be sites of the therapeutic action of these channel blockers.

Memantine preferentially blocks extrasynaptic over synaptic NMDA receptor currents in hippocampal autapses

Glutamate is the major excitatory neurotransmitter in the brain. The NMDA subtype of glutamate receptors (NMDAR) is known to mediate many physiological neural functions. However, excessive activation of NMDARs contributes to neuronal damage in various acute and chronic neurological disorders. To avoid unwanted adverse side effects, blockade of excessive NMDAR activity must therefore be achieved without affecting its physiological function. Memantine, an adamantane derivative, has been used for the treatment of Alzheimer's disease with an excellent clinical safety profile. We previously showed that memantine preferentially blocked neurotoxicity mediated by excessive NMDAR activity while relatively sparing normal neurotransmission, in part because of its uncompetitive antagonism with a fast off-rate. Here, using rat autaptic hippocampal microcultures, we show that memantine at therapeutic concentrations (1-10 microM) preferentially blocks extrasynaptic rather than synaptic currents mediated by NMDARs in the same neuron. We found that memantine blocks extrasynaptic NMDAR-mediated currents induced by bath application of 100 microM NMDA/10 microM glycine with a twofold higher potency than its blockade of the NMDAR component of evoked EPSCs (EPSCs(NMDAR)); this effect persists under conditions of pathological depolarization in the presence of 1 mm extracellular Mg(2+). Thus, our findings provide the first unequivocal evidence to explain the tolerability of memantine based on differential extrasynaptic/synaptic receptor blockade. At therapeutic concentrations, memantine effectively blocks excessive extrasynaptic NMDAR-mediated currents, while relatively sparing normal synaptic activity.

Atomoxetine acts as an NMDA receptor blocker in clinically relevant concentrations

BACKGROUND AND PURPOSE

There is increasing evidence that not only the monoaminergic but also the glutamatergic system is involved in the pathophysiology of attention-deficit hyperactivity disorder (ADHD). Hyperactivity of glutamate metabolism might be causally related to a hypoactive state in the dopaminergic system. Atomoxetine, a selective noradrenaline reuptake inhibitor, is the first non-stimulant approved for the treatment of this disorder. Here we have evaluated the effects of atomoxetine on glutamate receptors in vitro.

EXPERIMENTAL APPROACH

The whole-cell configuration of the patch-clamp technique was used to analyse the effect of atomoxetine on N-methyl-d-aspartate (NMDA) receptors in cultured rodent cortical and hippocampal neurons as well as on NMDA receptors heterologously expressed in human TsA cells.

KEY RESULTS

Atomoxetine blocked NMDA-induced membrane currents. Half-maximal inhibition emerged at about 3 microM which is in the range of clinically relevant concentrations found in plasma of patients treated with this drug. The inhibition was voltage-dependent, indicating an open-channel blocking mechanism. Furthermore, the inhibitory potency of atomoxetine did not vary when measured on NMDA receptors from different brain regions or with different subunit compositions.

CONCLUSIONS AND IMPLICATIONS

The effective NMDA receptor antagonism by atomoxetine at low micromolar concentrations may be relevant to its clinical effects in the treatment of ADHD. Our data provide further evidence that altered glutamatergic transmission might play a role in ADHD pathophysiology.

Neuronal glutamate and GABAA receptor function in health and disease

Glutamate and GABA (gamma-aminobutyric acid) are the predominant excitatory and inhibitory neurotransmitters in the mammalian CNS (central nervous system) respectively, and as such have undergone intense investigation. Given their predominance, it is no wonder that the reciprocal receptors for these neurotransmitters have attracted so much attention as potential targets for the promotion of health and the treatment of disease. Indeed, dysfunction of these receptors underlies a number of well-characterized neuropathological conditions such as anxiety, epilepsy and neurodegenerative diseases. Although intrinsically linked, the glutamatergic and GABAergic systems have, by and large, been investigated independently, with researchers falling into the 'excitatory' or 'inhibitory' camps. Around 70 delegates gathered at the University of St Andrews for this Biochemical Society Focused Meeting aimed at bringing excitation and inhibition together. With sessions on behaviour, receptor structure and function, receptor trafficking, activity-dependent changes in gene expression and excitation/inhibition in disease, the meeting was the ideal occasion for delegates from both backgrounds to interact. This issue of Biochemical Society Transactions contains papers written by those who gave oral presentations at the meeting. In this brief introductory review, I put into context and give a brief overview of these contributions.

Single-channel properties of N-methyl-D-aspartate receptors containing chimaeric GluN2A/GluN2D subunits

Subtypes of NMDARs (N-methyl-D-aspartate receptors) display differences in their pharmacological and biophysical properties. The differences are, to a large extent, determined by the identities of the GluN2 (glutamate-binding) NMDAR subunits that are co-expressed with GluN1 (glycine-binding) subunits, which form the final tetrameric NMDAR assembly. Of the four GluN2 subunits that exist (termed A-D), NMDARs composed of GluN1/GluN2A and GluN1/GluN2D subunits display the greatest differences in their sensitivities to a variety of agonists, antagonists and channel blockers as well as showing marked differences in their single-channel conductances and deactivation kinetics. Here, we describe a series of experiments where we have generated and studied two chimaeric GluN2A/GluN2D subunits. The first chimaera, referred to as GluN2A(2D-M1M2M3), replaces the membrane-associated regions M1, M2 and M3 of the GluN2A subunit with the corresponding regions found in the GluN2D subunit. The second chimaera, GluN2A(2D-S1M1M2M3S2), replaces the same three membrane-associated regions of the GluN2A subunit plus the LBD (ligand-binding domain) with the corresponding regions of the GluN2D subunit. Our results show that the identity of the GluN2 LBD not only controls glutamate potency, but also influences the potency of the NMDAR co-agonist glycine, whereas the single-channel conductance and the duration of single activations of ion channels can be predicted by the identities of the M1-M3 regions and the LBD.

Inhibition of recombinant GluN1/GluN2A and GluN1/GluN2B N-methyl-D-aspartate receptors by single malt whiskies

NMDARs (N-methyl-D-aspartate receptors) are considered to be a target for the inhibitory actions of ethanol. While profound inhibition of both native and recombinant NMDARs can be observed following the application of high concentrations of ethanol the levels of inhibition seen with lower concentrations of ethanol are more modest. Here, we report the effects of inhibiting NMDAR-mediated responses with ethanol concentrations that are experienced during the social consumption of alcohol comparing levels of inhibition seen with 'pure' ethanol with those produced by ethanol contained in three popular single malt whiskies.

GluN2B subunit-containing NMDA receptor antagonists prevent Abeta-mediated synaptic plasticity disruption in vivo

Currently, treatment with the relatively low-affinity NMDA receptor antagonist memantine provides limited benefit in Alzheimer's disease (AD). One probable dose-limiting factor in the use of memantine is the inhibition of NMDA receptor-dependent synaptic plasticity mechanisms believed to underlie certain forms of memory. Moreover, amyloid-beta protein (Abeta) oligomers that are implicated in causing the cognitive deficits of AD potently inhibit this form of plasticity. Here we examined if subtype-preferring NMDA receptor antagonists could preferentially protect against the inhibition of NMDA receptor-dependent plasticity of excitatory synaptic transmission by Abeta in the hippocampus in vivo. Using doses that did not affect control plasticity, antagonists selective for NMDA receptors containing GluN2B but not other GluN2 subunits prevented Abeta(1-42) -mediated inhibition of plasticity. Evidence that the proinflammatory cytokine TNFalpha mediates this deleterious action of Ass was provided by the ability of TNFalpha antagonists to prevent Abeta(1-42) inhibition of plasticity and the abrogation of a similar disruptive effect of TNFalpha using a GluN2B-selective antagonist. Moreover, at nearby synapses that were resistant to the inhibitory effect of TNFalpha, Abeta(1-42) did not significantly affect plasticity. These findings suggest that preferentially targeting GluN2B subunit-containing NMDARs may provide an effective means of preventing cognitive deficits in early Alzheimer's disease.

Alzheimer's disease amyloid beta-protein and synaptic function

Alzheimer's disease (AD) is characterized neuropathologically by the deposition of different forms of amyloid beta-protein (A beta) including variable amounts of soluble species that correlate with severity of dementia. The extent of synaptic loss in the brain provides the best morphological correlate of cognitive impairment in clinical AD. Animal research on the pathophysiology of AD has therefore focussed on how soluble A beta disrupts synaptic mechanisms in vulnerable brain regions such as the hippocampus. Synaptic plasticity in the form of persistent activity-dependent increases or decreases in synaptic strength provide a neurophysiological substrate for hippocampal-dependent learning and memory. Acute treatment with human-derived or chemically prepared soluble A beta that contains certain oligomeric assemblies, potently and selectively disrupts synaptic plasticity causing inhibition of long-term potentiation (LTP) and enhancement of long-term depression (LTD) of glutamatergic transmission. Over time these and related actions of A beta have been implicated in reducing synaptic integrity. This review addresses the involvement of neurotransmitter intercellular signaling in mediating or modulating the synaptic plasticity disrupting actions of soluble A beta, with particular emphasis on the different roles of glutamatergic and cholinergic mechanisms. There is growing evidence to support the view that NMDA and possibly nicotinic receptors are critically involved in mediating the disruptive effect of A beta and that targeting muscarinic receptors can indirectly modulate A beta's actions. Such studies should help inform ongoing and future clinical trials of drugs acting through the glutamatergic and cholinergic systems.

Pharmacological characterization of recombinant NR1/NR2A NMDA receptors with truncated and deleted carboxy termini expressed in Xenopus laevis oocytes

BACKGROUND AND PURPOSE

The carboxy terminal domain (CTD) of NR2 N-methyl-d-aspartate receptor (NMDAR) subunits interacts with numerous scaffolding and signal transduction proteins. Mutations of this region affect trafficking and downstream signalling of NMDARs. This study determines to what extent characteristic pharmacological properties of NR2A-containing NMDARs are influenced by this key functional domain.

EXPERIMENTAL APPROACH

Using recombinant receptor expression in Xenopus laevis oocytes and two electrode voltage clamp recordings we characterized pharmacological properties of rat NR1/NR2A NMDARs with altered CTDs. We assessed the effects of truncating [at residue Iso1098; NR2A(trunC)] and deleting [from residue Phe822; NR2A(delC)] the CTD of NR2A NMDAR subunits on agonist potencies, channel block by Mg(2+) and memantine and potentiation of NMDAR-mediated responses by chelating contaminating divalent cations.

KEY RESULTS

Truncation or deletion of the CTD of NR2A NMDAR subunits did not affect glutamate potency [EC(50) = 2.2 micromol.L(-1), NR2A(trunC); 2.7 micromol.L(-1), NR2A(delC) compared with 3.3 micromol.L(-1), NR2A(WT)] but did significantly increase glycine potency [EC(50) = 500 nmol.L(-1), NR2A(trunC); 900 nmol.L(-1), NR2A(delC) compared with 1.3 micromol.L(-1), NR2A(WT)]. Voltage-dependent Mg(2+) block of NR2A(WT)- and NR2A(trunC)-containing NMDARs was similar but low concentrations of Mg(2+) (1 micromol.L(-1)) potentiated NR1/NR2A(delC) NMDARs. Memantine block was not affected by changes to the structure of the NR2A CTD. EDTA-induced potentiation was similar at each of the three NMDAR constructs.

CONCLUSIONS AND IMPLICATIONS

Of the parameters studied only minor influences of the CTD were observed; these are unlikely to compromise interpretation of studies that make use of CTD-mutated recombinant receptors or transgenic mice in investigations of the role of the CTD in NMDAR signalling.

In developing hippocampal neurons, NR2B-containing N-methyl-D-aspartate receptors (NMDARs) can mediate signaling to neuronal survival and synaptic potentiation, as well as neuronal death

It has been suggested that NR2B-containing N-methyl-d-aspartate (NMDA) receptors have a selective tendency to promote pro-death signaling and synaptic depression, compared with the survival promoting, synapse potentiating properties of NR2A-containing NMDA receptors. A preferential localization of NR2A-containing NMDA receptors at the synapse in maturing neurons could thus explain differences in synaptic vs. extrasynaptic NMDA receptor signaling. We have investigated whether NMDA receptors can mediate signaling to survival, death, and synaptic potentiation, in dissociated rat neuronal cultures at a developmental stage prior to significant NR2A expression and subunit-specific differences between synaptic and extrasynaptic NMDA receptors. We show that in developing hippocampal neurons, the progressive reduction in sensitivity of NMDA receptor currents to the NR2B antagonist ifenprodil applies to both synaptic and extrasynaptic locations. However, the reduction is less acute in extrasynaptic currents, indicating that NR2A does partition preferentially, but not exclusively, into synaptic locations at DIV>12. We then studied NMDA receptor signaling at DIV10, when both synaptic and extrasynaptic NMDA receptors are both overwhelmingly and equally NR2B-dominated. To analyze pro-survival signaling we studied the influence of synaptic NMDA receptor activity on staurosporine-induced apoptosis. Blockade of spontaneous NMDAR activity with MK-801, or ifenprodil exacerbated the apoptotic insult. Furthermore, MK-801 and ifenprodil both antagonized neuroprotection promoted by enhancing synaptic activity. Pro-death signaling induced by a toxic dose of NMDA is also blocked by NR2B-specific antagonists. Using a cell culture model of synaptic NMDA receptor-dependent synaptic potentiation, we find that this is mediated exclusively by NR2B-containing N-methyl-D-aspartate receptors, as implicated by NR2B-specific antagonists and the use of selective vs. non-selective doses of the NR2A-preferring antagonist NVP-AAM077. Therefore, within a single neuron, NR2B-NMDA receptors are able to mediate both survival and death signaling, as well as model of NMDA receptor-dependent synaptic potentiation. In this instance, subunit differences cannot account for the dichotomous nature of NMDA receptor signaling.

Modulation of glycine potency in rat recombinant NMDA receptors containing chimeric NR2A/2D subunits expressed in Xenopus laevis oocytes

Heteromeric NMDARs are composed of coagonist glycine-binding NR1 subunits and glutamate-binding NR2 subunits. The majority of functional NMDARs in the mammalian central nervous system (CNS) contain two NR1 subunits and two NR2 subunits of which there are four types (A-D). We show that the potency of a variety of endogenous and synthetic glycine-site coagonists varies between recombinant NMDARs such that the highest potency is seen at NR2D-containing and the lowest at NR2A-containing NMDARs. This heterogeneity is specified by the particular NR2 subunit within the NMDAR complex since the glycine-binding NR1 subunit is common to all NMDARs investigated. To identify the molecular determinants responsible for this heterogeneity, we generated chimeric NR2A/2D subunits where we exchanged the S1 and S2 regions that form the ligand-binding domains and coexpressed these with NR1 subunits in Xenopus laevis oocytes. Glycine concentration-response curves for NMDARs containing NR2A subunits including the NR2D S1 region gave mean glycine EC(50) values similar to NR2A(WT)-containing NMDARs. However, receptors containing NR2A subunits including the NR2D S2 region or both NR2D S1 and S2 regions gave glycine potencies similar to those seen in NR2D(WT)-containing NMDARs. In particular, two residues in the S2 region of the NR2A subunit (Lys719 and Tyr735) when mutated to the corresponding residues found in the NR2D subunit influence glycine potency. We conclude that the variation in glycine potency is caused by interactions between the NR1 and NR2 ligand-binding domains that occur following agonist binding and which may be involved in the initial conformation changes that determine channel gating.