Fast NMDA receptor-mediated synaptic currents in neurons from mice lacking the epsilon2 (NR2B) subunit

Depression of Synaptic N-methyl-D-Aspartate Responses by Xenon and Nitrous Oxide

In "synapse bouton preparation" of rat hippocampal CA3 neurons, we examined how Xe and N₂O modulate N-methyl-D-aspartate (NMDA) receptor-mediated spontaneous and evoked excitatory post-synaptic currents (sEPSC_NMDA and eEPSC_NMDA). This preparation is a mechanically isolated single neuron attached with nerve endings (boutons) preserving normal physiologic function and promoting the exact evaluation of sEPSC_NMDA and eEPSC_NMDA responses without influence of extrasynaptic, glial, and other neuronal tonic currents. These sEPSCs and eEPSCs are elicited by spontaneous glutamate release from many homologous glutamatergic boutons and by focal paired-pulse electric stimulation of a single bouton, respectively. The s/eEPSC_AMPA/KA and s/eEPSC_NMDA were isolated pharmacologically by their specific antagonists. Thus, independent contributions of pre- and postsynaptic responses could also be quantified. All kinetic properties of s/eEPSC_AMPA/KA and s/eEPSC_NMDA were detected clearly. The s/eEPSC_NMDA showed smaller amplitude and slower rise and 1/e decay time constant (τ _Decay) than s/eEPSC_AMPA/KA Xe (70%) and N₂O (70%) significantly decreased the frequency and amplitude without altering the τ _Decay of sEPSC_NMDA They also decreased the amplitude but increased the Rf and PPR without altering the τ _Decay of the eEPSC_NMDA These data show clearly that "synapse bouton preparation" can be an accurate model for evaluating s/eEPSC_NMDA Such inhibitory effects of gas anesthetics are primarily due to presynaptic mechanisms. Present results may explain partially the powerful analgesic effects of Xe and N₂O. SIGNIFICANCE STATEMENT: We could record pharmacologically isolated NMDA receptor-mediated spontaneous and (action potential-evoked) excitatory postsynaptic currents (sEPSC_NMDA and eEPSC_NMDA) and clearly detect all kinetic parameters of sEPSC_NMDA and eEPSC_NMDA at synaptic levels by using "synapse bouton preparation" of rat hippocampal CA3 neurons. We found that Xe and N₂O clearly suppressed both sEPSC_NMDA and eEPSC_NMDA. Different from previous studies, present results suggest that Xe and N₂O predominantly inhibit the NMDA responses by presynaptic mechanisms.

N-terminal alternative splicing of GluN1 regulates the maturation of excitatory synapses and seizure susceptibility

The majority of NMDA receptors (NMDARs) in the brain are composed of 2 GluN1 and 2 GluN2 subunits. The inclusion or exclusion of 1 N-terminal and 2 C-terminal domains of GluN1 results in 8 splicing variants that exhibit distinct temporal and spatial patterns of expression and functional properties. However, previous functional analyses of Grin1 variants have been done using heterologous expression and the in vivo function of Grin1 splicing is unknown. Here we show that N-terminal splicing of GluN1 has important functions in the maturation of excitatory synapses. The inclusion of exon 5 of Grin1 is up-regulated in several brain regions such as the thalamus and neocortex. We find that deletion of Grin1 exon 5 disrupts the developmental remodeling of NMDARs in thalamic neurons and the effect is distinct from that of Grin2a (GluN2A) deletion. Deletion of Grin2a or exon 5 of Grin1 alone partially attenuates the shortening of NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSCs) during early life, whereas deletion of both Grin2a and exon 5 of Grin1 completely abolishes the developmental change in NMDAR-EPSC decay time. Deletion of exon 5 of Grin1 leads to an overproduction of excitatory synapses in layer 5 pyramidal neurons in the cortex and increases seizure susceptibility in adult mice. Our findings demonstrate that N-terminal splicing of GluN1 has important functions in synaptic maturation and neuronal network excitability.

GluN2B-Containing NMDA Receptors Regulate AMPA Receptor Traffic through Anchoring of the Synaptic Proteasome

NMDA receptors play a central role in shaping the strength of synaptic connections throughout development and in mediating synaptic plasticity mechanisms that underlie some forms of learning and memory formation in the CNS. In the hippocampus and the neocortex, GluN1 is combined primarily with GluN2A and GluN2B, which are differentially expressed during development and confer distinct molecular and physiological properties to NMDA receptors. The contribution of each subunit to the synaptic traffic of NMDA receptors and therefore to their role during development and in synaptic plasticity is still controversial. We report a critical role for the GluN2B subunit in regulating NMDA receptor synaptic targeting. In the absence of GluN2B, the synaptic levels of AMPA receptors are increased and accompanied by decreased constitutive endocytosis of GluA1-AMPA receptor. We used quantitative proteomic analysis to identify changes in the composition of postsynaptic densities from GluN2B(-/-) mouse primary neuronal cultures and found altered levels of several ubiquitin proteasome system components, in particular decreased levels of proteasome subunits. Enhancing the proteasome activity with a novel proteasome activator restored the synaptic levels of AMPA receptors in GluN2B(-/-) neurons and their endocytosis, revealing that GluN2B-mediated anchoring of the synaptic proteasome is responsible for fine tuning AMPA receptor synaptic levels under basal conditions.

Prioritizing the development of mouse models for childhood brain disorders

Mutations in hundreds of genes contribute to cognitive and behavioral dysfunction associated with developmental brain disorders (DBDs). Due to the sheer number of risk factors available for study combined with the cost of developing new animal models, it remains an open question how genes should be prioritized for in-depth neurobiological investigations. Recent reviews have argued that priority should be given to frequently mutated genes commonly found in sporadic DBD patients. Intrigued by this idea, we explored to what extent "high priority" risk factors have been studied in animals in an effort to assess their potential for generating valuable preclinical models capable of advancing the neurobiological understanding of DBDs. We found that in-depth whole animal studies are lacking for many high priority genes, with relatively few neurobiological studies performed in construct valid animal models aimed at understanding the pathological substrates associated with disease phenotypes. However, some high priority risk factors have been extensively studied in animal models and they have generated novel insights into DBD patho-neurobiology while also advancing early pre-clinical therapeutic treatment strategies. We suggest that prioritizing model development toward genes frequently mutated in non-specific DBD populations will accelerate the understanding of DBD patho-neurobiology and drive novel therapeutic strategies. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Calsyntenin-1 regulates targeting of dendritic NMDA receptors and dendritic spine maturation in CA1 hippocampal pyramidal cells during postnatal development

Calsyntenin-1 is a transmembrane cargo-docking protein important for kinesin-1-mediated fast transport of membrane-bound organelles that exhibits peak expression levels at postnatal day 7. However, its neuronal function during postnatal development remains unknown. We generated a knock-out mouse to characterize calsyntenin-1 function in juvenile mice. In the absence of calsyntenin-1, synaptic transmission was depressed. To address the mechanism, evoked EPSPs were analyzed revealing a greater proportion of synaptic GluN2B subunit-containing receptors typical for less mature synapses. This imbalance was due to a disruption in calsyntenin-1-mediated dendritic transport of NMDA receptor subunits. As a consequence of increased expression of GluN2B subunits, NMDA receptor-dependent LTP was enhanced at Schaffer collateral-CA1 pyramidal cell synapses. Interestingly, these defects were accompanied by a decrease in dendritic arborization and increased proportions of immature filopodia-like dendritic protrusions at the expense of thin-type dendritic spines in CA1 pyramidal cells. Thus, these results highlight a key role for calsyntenin-1 in the transport of NMDA receptors to synaptic targets, which is necessary for the maturation of neuronal circuits during early development.

Triheteromeric NMDA receptors at hippocampal synapses

NMDA receptors are composed of two GluN1 (N1) and two GluN2 (N2) subunits. Constituent N2 subunits control the pharmacological and kinetic characteristics of the receptor. NMDA receptors in hippocampal or cortical neurons are often thought of as diheteromeric, meaning that they contain only one type of N2 subunit. However, triheteromeric receptors with more than one type of N2 subunit also have been reported, and the relative contribution of diheteromeric and triheteromeric NMDA receptors at synapses has been difficult to assess. Because wild-type hippocampal principal neurons express N1, N2A, and N2B, we used cultured hippocampal principal neurons from N2A and N2B knock-out mice as templates for diheteromeric synaptic receptors. However, summation of N1/N2B and N1/N2A EPSCs could not account for the deactivation kinetics of wild-type EPSCs. To make a quantitative estimate of NMDA receptor subtypes at wild-type synapses, we used the deactivation kinetics and the effects of the competitive antagonist NVP-AAM077. Our results indicate that three types of NMDA receptors contribute to wild-type EPSCs, with at least two-thirds being triheteromeric receptors. Functional isolation of synaptic triheteromeric receptors revealed deactivation kinetics and pharmacology that were distinct from either diheteromeric receptor subtype. Because of differences in open probability, synaptic triheteromeric receptors outnumbered N1/N2A receptors by 5.8 to 1 and N1/N2B receptors by 3.2 to 1. Our results suggest that triheteromeric NMDA receptors must either be preferentially assembled or preferentially localized at synapses.

Synaptic NMDA receptors in basolateral amygdala principal neurons are triheteromeric proteins: physiological role of GluN2B subunits

N-methyl-(D)-aspartate (NMDA) receptors are heteromultimeric ion channels that contain an essential GluN1 subunit and two or more GluN2 (GluN2A-GluN2D) subunits. The biophysical properties and physiological roles of synaptic NMDA receptors are dependent on their subunit composition. In the basolateral amygdala (BLA), it has been suggested that the plasticity that underlies fear learning requires activation of heterodimeric receptors composed of GluN1/GluN2B subunits. In this study, we investigated the subunit composition of NMDA receptors present at synapses on principal neurons in the BLA. Purification of the synaptic fraction showed that both GluN2A and GluN2B subunits are present at synapses, and co-immunoprecipitation revealed the presence of receptors containing both GluN2A and GluN2B subunits. The kinetics of NMDA receptor-mediated synaptic currents and pharmacological blockade indicate that heterodimeric GluN1/GluN2B receptors are unlikely to be present at glutamatergic synapses on BLA principal neurons. Selective RNA interference-mediated knockdown of GluN2A subunits converted synaptic receptors to a GluN1/GluN2B phenotype, whereas knockdown of GluN2B subunits had no effect on the kinetics of the synaptically evoked NMDA current. Blockade of GluN1/GluN2B heterodimers with ifenprodil had no effect, but knockdown of GluN2B disrupted the induction of CaMKII-dependent long-term potentiation at these synapses. These results suggest that, on BLA principal neurons, GluN2B subunits are only present as GluN1/GluN2A/GluN2B heterotrimeric NMDA receptors. The GluN2B subunit has little impact on the kinetics of the receptor, but is essential for the recruitment of signaling molecules essential for synaptic plasticity.

Early mechanisms of pathobiology are revealed by transcriptional temporal dynamics in hippocampal CA1 neurons of prion infected mice

Prion diseases typically have long pre-clinical incubation periods during which time the infectious prion particle and infectivity steadily propagate in the brain. Abnormal neuritic sprouting and synaptic deficits are apparent during pre-clinical disease, however, gross neuronal loss is not detected until the onset of the clinical phase. The molecular events that accompany early neuronal damage and ultimately conclude with neuronal death remain obscure. In this study, we used laser capture microdissection to isolate hippocampal CA1 neurons and determined their pre-clinical transcriptional response during infection. We found that gene expression within these neurons is dynamic and characterized by distinct phases of activity. We found that a major cluster of genes is altered during pre-clinical disease after which expression either returns to basal levels, or alternatively undergoes a direct reversal during clinical disease. Strikingly, we show that this cluster contains a signature highly reminiscent of synaptic N-methyl-D-aspartic acid (NMDA) receptor signaling and the activation of neuroprotective pathways. Additionally, genes involved in neuronal projection and dendrite development were also altered throughout the disease, culminating in a general decline of gene expression for synaptic proteins. Similarly, deregulated miRNAs such as miR-132-3p, miR-124a-3p, miR-16-5p, miR-26a-5p, miR-29a-3p and miR-140-5p follow concomitant patterns of expression. This is the first in depth genomic study describing the pre-clinical response of hippocampal neurons to early prion replication. Our findings suggest that prion replication results in the persistent stimulation of a programmed response that is mediated, at least in part, by synaptic NMDA receptor activity that initially promotes cell survival and neurite remodelling. However, this response is terminated prior to the onset of clinical symptoms in the infected hippocampus, seemingly pointing to a critical juncture in the disease. Manipulation of these early neuroprotective pathways may redress the balance between degeneration and survival, providing a potential inroad for treatment.

Neurodegenerative diseases affect an ever-increasing proportion of the population; therefore, there is an urgent need to develop treatments. Prion disorders belong to this group of diseases and although rare and uniquely transmissible, share many features on a sub-cellular level. Central to disease is progressive synaptic impairment that invariably leads to the irreversible loss of neurons. Understanding this process is undoubtedly essential for rational drug discovery. In this study we looked at neurons very early in disease, when prions are barely detectable and there are no clinical symptoms observed. Specifically, we performed a comprehensive analysis of transcriptional changes within a particularly dense area of neurons, the CA1 hippocampus region, from prion-infected and control mice. In this way we were able to enrich our data for molecular changes unique to neurons and minimize those changes characteristic of support cells such as astrocytes and microglia. We detected the activation of a transcriptional program indicative of a protective mechanism within these neurons early in disease. This mechanism diminished as disease progressed and was lost altogether, concurrently with the onset of clinical symptoms. These findings demonstrate the ability of neurons to mount an initial neuroprotective response to prions that could be exploited for therapy development.

Differential stimulus-dependent synaptic recruitment of CaMKIIα by intracellular determinants of GluN2B

The calcium-calmodulin activated kinase CaMKII mediates many forms of learning and memory. Activity-regulated translocation of CaMKII to synapses is important for its functions in synaptic plasticity. Here, we tested the role of the NMDA receptor subunit GluN2B in recruiting CaMKIIα to synapses with different paradigms: global bath stimulation of NMDA receptors, a chemical long term potentiation (cLTP) protocol that selectively activates synaptic NMDA receptors, or local stimulation of NMDA receptors at a contiguous set of ~10-30 synapses that triggers a propagating synaptic accumulation of CaMKII. Global or cLTP-induced synaptic accumulation of CaMKIIα occurred in wild-type but not sister GluN2B -/- cultured mouse hippocampal neurons. Expression of YFP-GluN2B, but not a similar level of YFP-GluN2A, rescued global and cLTP-induced CaMKIIα translocation. Using chimeric constructs, the pore-forming extracellular and membrane domains of GluN2A combined with the cytoplasmic tail of GluN2B were sufficient to rescue CaMKIIα translocation, whereas the reverse chimera was ineffective. Furthermore, the dual point mutation R1300Q,S1303D in GluN2B that blocks interaction of this high affinity site with CaMKII abolished rescue. Thus, CaMKII binding to GluN2B is required for global and cLTP-induced synaptic accumulation of CaMKIIα. However, surprisingly, locally induced propagating synaptic accumulation of CaMKIIα occurred normally in GluN2B -/- neurons, indistinguishably from wild-type. Thus, synaptic trapping of CaMKII during locally induced propagating translocation occurs by different mechanisms and molecular partners compared with global stimulation and cLTP paradigms. These findings underscore the complex regulatory properties and molecular interactions of CaMKIIα, a key player in synaptic plasticity.

Amino-terminal ligands prolong NMDA Receptor-mediated EPSCs

The amino-terminal domains of NMDA receptor subunits are important for receptor assembly and desensitization, and incorporate the high-affinity binding sites for zinc and ifenprodil. These amino-terminal ligands are thought of as subunit-specific receptor inhibitors. However, multiple NMDA receptor subtypes contribute to EPSCs at wild-type hippocampal synapses. To understand the action of amino-terminal ligands, we first used cultured hippocampal neurons from N2A and N2B knock-out mice. EPSCs from these neurons have properties that are consistent with N1/N2B and N1/N2A diheteromeric receptors, respectively. As expected, zinc reduced the EPSC peak amplitude from N2B KO neurons, but surprisingly also prolonged the deactivation, resulting in a marked redistribution of charge. Consistent with prolongation of the EPSC, zinc produced a longer latency to first opening of glutamate-bound receptors, which resulted in a decrease in the number of receptors that opened by the peak. Ifenprodil had similar effects on EPSCs from N2A KO neurons. In neurons from wild-type mice, zinc or ifenprodil reduced the EPSC peak, but only zinc caused significant charge redistribution, consistent with a small contribution of N1/N2B diheteromers in these neurons. Our results indicate that ligand binding to amino-terminal domains can alter the behavior of synaptic NMDA receptors under the nonequilibrium conditions of glutamate release during synaptic transmission. By prolonging EPSCs, amino-terminal ligands could markedly affect the computational properties of NMDA receptors and could potentially be exploited for therapeutic purposes.

Triheteromeric N-methyl-D-aspartate receptors differentiate synaptic inputs onto pyramidal neurons in somatosensory cortex: involvement of the GluN3A subunit

N-methyl-D-aspartate receptors (NMDARs) are glutamatergic by virtue of glutamate-binding GluN2 subunits and glycinergic by virtue of glycine-binding GluN1 and GluN3 subunits. The existence, location, and functional-significance of NMDARs containing both GluN2 and GluN3 subunits have as yet remained unelucidated. Here we report on the discovery and characterization of a novel type of NMDARs, found at layer (L)1/primary whisker-motor-cortex inputs onto L5 pyramidal neurons in somatosensory cortex, that are distinct in structure and function from conventional GluN2A-containing NMDARs at thalamic/striatal (Str) inputs onto the same neurons. These receptors had a threshold-like activation at hyperpolarized holding-potentials with strong outward rectification of their current-voltage relationships unlike any known GluN1/GluN2-containing NMDARs. Pharmacology revealed a triheteromeric-receptor with features common to glutamate-activated GluN1/GluN2-containing and glycine-activated GluN1/GluN3-containing diheteromeric NMDARs. However, unlike GluN1/GluN3 receptors, NMDARs at L1 inputs were activated by glutamate and blocked by d-AP5, Ca(2+)-permeable, and more efficient at integrating and potentiating EPSPs selectively over Str inputs during high-frequency stimulation while obviating the need for AMPAR-mediated depolarization.

Short-term field stimulation mimics synaptic maturation of hippocampal synapses

Many aspects of synaptic transmission are modified during development, reflecting not only the consequence of developmental programmes of gene expression, but also the effects of ongoing neural activity. We investigated the role of synaptic activity in the maturation of Schaffer collateral (SC)-CA1 synapses using sustained low frequency field stimulation of acute brain slices. Between postnatal days 4-6 and 14-16, mouse SC-CA1 synapses in naïve slices showed a developmental decrease in the probability of transmitter release (P(r)) and an increase in the contribution of GluN2A (NR2A) subunits to the NMDA receptor-mediated excitatory postsynaptic current (EPSC). Surprisingly, these developmental changes could be mimicked by short term (4 h) in vitro synaptic activity in slices taken from postnatal days (PND) 4-6 mice. However, different activity levels were required to alter release probability compared to the NMDA receptor subunit composition. Spontaneous synaptic activity was sufficient to alter the NMDA receptor subunit composition, but sustained low-frequency field stimulation of the brain slice (0.1 Hz, 4 h) was necessary to reduce release probability, as assessed 1 h following the cessation of stimulation. The protein synthesis inhibitor anisomycin blocked the effect of field stimulation on release probability. These results indicate that features of mature excitatory synapses can be rapidly induced in immature neurons. The activity dependence of the P(r) and NMDA receptor subunit composition serves as a sensitive indicator of prior neural activity, and provides dual mechanisms for homeostatic control of excitatory synaptic efficacy.

A critical role for GluN2B-containing NMDA receptors in cortical development and function

The subunit composition of N-methyl D-aspartate receptors (NMDARs) is tightly regulated during cortical development. NMDARs are initially dominated by GluN2B (NR2B), whereas GluN2A (NR2A) incorporation increases after birth. The function of GluN2B-containing NMDARs during development, however, is incompletely understood. We generated a mouse in which we genetically replaced GluN2B with GluN2A (2B→2A). Although this manipulation restored NMDAR-mediated currents at glutamatergic synapses, it did not rescue GluN2B loss of function. Protein translation-dependent homeostatic synaptic plasticity is occluded in the absence of GluN2B, and AMPA receptor contribution is enriched at excitatory cortical synapses. Our experiments indicate that specificity of GluN2B-mediated signaling is due to its unique interaction with the protein effector alpha calcium-calmodulin kinase II and the regulation of the mTOR pathway. Homozygous 2B→2A mice exhibited high rates of lethality, suppressed feeding, and depressed social exploratory behavior. These experiments indicate that GluN2B-containing NMDARs activate unique cellular processes that cannot be rescued by replacement with GluN2A.

Dissecting the contribution of individual receptor subunits to the enhancement of N-methyl-d-aspartate currents by dopamine D1 receptor activation in striatum

Dopamine, via activation of D1 receptors, enhances N-methyl-d-aspartate (NMDA) receptor-mediated responses in striatal medium-sized spiny neurons. However, the role of specific NMDA receptor subunits in this enhancement remains unknown. Here we used genetic and pharmacological tools to dissect the contribution of NR1 and NR2A/B subunits to NMDA responses and their modulation by dopamine receptors. We demonstrate that D1 enhancement of NMDA responses does not occur or is significantly reduced in mice with genetic knock-down of NR1 subunits, indicating a critical role of these subunits. Interestingly, spontaneous and evoked α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid (AMPA) receptor-mediated responses were significantly enhanced in NR1 knock-down animals, probably as a compensatory mechanism for the marked reduction in NMDA receptor function. The NMDA receptor subunits NR2A and NR2B played differential roles in D1 modulation. Whereas genetic deletion or pharmacological blockade of NR2A subunits enhanced D1 potentiation of NMDA responses, blockade of NR2B subunits reduced this potentiation, suggesting that these regulatory subunits of the NMDA receptor counterbalance their respective functions. In addition, using D1 and D2 receptor EGFP-expressing mice, we demonstrate that NR2A subunits contribute more to NMDA responses in D1-MSSNs, whereas NR2B subunits contribute more to NMDA responses in D2 cells. The differential contribution of discrete receptor subunits to NMDA responses and dopamine modulation in the striatum has important implications for synaptic plasticity and selective neuronal vulnerability in disease states.

A specific class of interneuron mediates inhibitory plasticity in the lateral amygdala

The lateral amygdala (LA) plays a key role in emotional learning and is the main site for sensory input into the amygdala. Within the LA, pyramidal neurons comprise the major cell population with plasticity of inputs to these neurons thought to underlie fear learning. Pyramidal neuron activity is tightly controlled by local interneurons, and GABAergic modulation strongly influences amygdala-dependent learning. Synaptic inputs to some interneurons in the LA can also undergo synaptic plasticity, but the identity of these cells and the mechanisms that underlie this plasticity are not known. Here we show that long-term potentiation (LTP) in LA interneurons is restricted to a specific type of interneuron that is defined by the lack of expression of synaptic NR2B subunits. We find that LTP is only present at cortical inputs to these cells and is initiated by calcium influx via calcium-permeable AMPA receptors. LTP is maintained by trafficking of GluR2-lacking AMPA receptors that require an interaction with SAP97 and the actin cytoskeleton. Our results define a novel population of interneurons in the LA that control principal neuron excitability by feed-forward inhibition of cortical origin. This selective enhanced inhibition may contribute to reducing the activity of principal neurons engaged during extinction of conditioned fear.

Setdb1 histone methyltransferase regulates mood-related behaviors and expression of the NMDA receptor subunit NR2B

Histone methyltransferases specific for the histone H3-lysine 9 residue, including Setdb1 (Set domain, bifurcated 1)/Eset/Kmt1e are associated with repressive chromatin remodeling and expressed in adult brain, but potential effects on neuronal function and behavior remain unexplored. Here, we report that transgenic mice with increased Setdb1 expression in adult forebrain neurons show antidepressant-like phenotypes in behavioral paradigms for anhedonia, despair, and learned helplessness. Chromatin immunoprecipitation in conjunction with DNA tiling arrays (ChIP-chip) revealed that genomic occupancies of neuronal Setdb1 are limited to <1% of annotated genes, which include the NMDA receptor subunit NR2B/Grin2B and other ionotropic glutamate receptor genes. Chromatin conformation capture and Setdb1-ChIP revealed a loop formation tethering the NR2B/Grin2b promoter to the Setdb1 target site positioned 30 kb downstream of the transcription start site. In hippocampus and ventral striatum, two key structures in the neuronal circuitry regulating mood-related behaviors, Setdb1-mediated repressive histone methylation at NR2B/Grin2b was associated with decreased NR2B expression and EPSP insensitivity to pharmacological blockade of NR2B, and accelerated NMDA receptor desensitization consistent with a shift in NR2A/B subunit ratios. In wild-type mice, systemic treatment with the NR2B antagonist, Ro25-6981 [R-(R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol], and hippocampal small interfering RNA-mediated NR2B/Grin2b knockdown resulted in behavioral changes similar to those elicited by the Setdb1 transgene. Together, these findings point to a role for neuronal Setdb1 in the regulation of affective and motivational behaviors through repressive chromatin remodeling at a select set of target genes, resulting in altered NMDA receptor subunit composition and other molecular adaptations.

NMDAR-mediated EPSCs are maintained and accelerate in time course during maturation of mouse and rat auditory brainstem in vitro

NMDA receptors (NMDARs) mediate a slow EPSC at excitatory glutamatergic synapses throughout the brain. In many areas the magnitude of the NMDAR-mediated EPSC declines with development and is associated with changes in subunit composition, but the mature channel composition is often unknown. We have employed the calyx of Held terminal with its target, the principal neuron of the medial nucleus of the trapezoid body (MNTB), to examine the NMDAR-mediated EPSC during synapse maturation from P10 to P40. Our data show that the calyx has reached a mature state by around P18. The NMDAR-mediated EPSC amplitude (and dominant decay ) fell from around 5 nA (: 40-50 ms) at P10/11 to 0.3-0.5 nA (: 10-15 ms) by P18. The mature NMDAR-EPSC showed no sensitivity to ifenprodil, indicating lack of NR2B subunits, and no block by submicromolar concentrations of zinc, consistent with NR1-1b subunit expression. Additionally, from P11 to P18 there was a reduction in voltage-dependent block and the apparent dissociation constant for [Mg(2+)](o) (K(o)) changed from 7.5 to 14 mm. Quantitative PCR showed that the relative expression of NR2A and NR2C increased, while immunohistochemistry confirmed the presence of NR2A, NR2B and NR2C protein. Although the mature NMDAR-EPSC is small, it is well coupled to NO signalling, as indicated by DAR-4M imaging. We conclude that native mature NMDAR channels at the calyx of Held have a fast time course and reduced block by [Mg(2+)](o), consistent with dominance of NR2C subunits and functional exclusion of NR2B subunits. The pharmacology suggests a single channel type and we postulate that the mature NMDARs consist of heterotrimers of NR1-1b-NR2A-NR2C.

NMDA receptor GluN2B (GluR epsilon 2/NR2B) subunit is crucial for channel function, postsynaptic macromolecular organization, and actin cytoskeleton at hippocampal CA3 synapses

GluN2B (GluRepsilon2/NR2B) subunit is involved in synapse development, synaptic plasticity, and cognitive function. However, its roles in synaptic expression and function of NMDA receptors (NMDARs) in the brain remain mostly unknown because of the neonatal lethality of global knock-out mice. To address this, we generated conditional knock-out mice, in which GluN2B was ablated exclusively in hippocampal CA3 pyramidal cells. By immunohistochemistry, GluN2B disappeared and GluN1 (GluRzeta1/NR1) was moderately reduced, whereas GluN2A (GluRepsilon1/NR2A) and postsynaptic density-95 (PSD-95) were unaltered in the mutant CA3. This was consistent with protein contents in the CA3 crude fraction: 9.6% of control level for GluN2B, 47.7% for GluN1, 90.6% for GluN2A, and 98.0% for PSD-95. Despite the remaining NMDARs, NMDAR-mediated currents and long-term potentiation were virtually lost at various CA3 synapses. Then, we compared synaptic NMDARs by postembedding immunogold electron microscopy and immunoblot using the PSD fraction. In the mutant CA3, GluN1 was severely reduced in both immunogold (20.6-23.6%) and immunoblot (24.6%), whereas GluN2A and PSD-95 were unchanged in immunogold but markedly reduced in the PSD fraction (51.4 and 36.5%, respectively), indicating increased detergent solubility of PSD molecules. No such increased solubility was observed for GluN2B in the CA3 of GluN2A-knock-out mice. Furthermore, significant decreases were found in the ratio of filamentous to globular actin (49.5%) and in the density of dendritic spines (76.2%). These findings suggest that GluN2B is critically involved in NMDAR channel function, organization of postsynaptic macromolecular complexes, formation or maintenance of dendritic spines, and regulation of the actin cytoskeleton.

Prenatal development of NMDA receptor composition and function in trigeminal neurons

The prenatal development of neural circuits for rhythmical oral-motor behaviors used for feeding is essential for the survival of the newborn mammal. The N-methyl-D-aspartate (NMDA) receptor plays a critical role in brainstem circuits underlying postnatal oral-motor behaviors. To understand a role for the NMDA receptor in the emergence of sucking behavior we conducted physiological and immunohistochemical experiments using fetal rats. Physiology experiments examined the development of the NMDA dose response of the brainstem circuit responsible for generating rhythmical trigeminal activity by recording trigeminal motor outputs using an in vitro preparation. The high dose of NMDA agonist bath application affected the mean cycle duration of rhythmical trigeminal activity (RTA) at both embryonic day (E) 18-19 and E20-21 in comparison with standard concentration of NMDA agonist. NMDA receptor immunohistochemistry studies, using antibodies directed against subunits NR1, NR2A, NR2B, NR3A and NR3B were performed to determine the prenatal regulation of NMDA subunits in trigeminal motoneurons (Mo5), and mesencephalic trigeminal neurons (Me5) between E17 to E20. In Mo5, NR1, NR2A, NR2B and NR3A immunoreactivity was observed throughout the time frame sampled. NR3B immunoreactivity was not observed in Mo5 or Me5. In Mo5, there was a significant decrease in the percentage of NR2B immunoreactive neurons between E17 and E20, and a concurrent increase in the NR2A/NR2B ratio between E17 and E20. In Me5, NR1, NR2A and NR3A immunoreactivity was observed throughout the time frame sampled; a significant decrease in the percentage of NR2A immunoreactive neurons between E17 and E20, and NR3A immunoreactive neurons between E17 and E18 occurred. The timing of subunit changes between E17 and E18 is coincident with the prenatal emergence of rhythmical jaw movements, and in vitro rhythmical trigeminal activity, shown in earlier studies. Our data suggest that NMDA receptor plays an important role in the development and function of prenatal oral-motor circuits.

A specialized NMDA receptor function in layer 5 recurrent microcircuitry of the adult rat prefrontal cortex

In the prefrontal cortex, NMDA receptors are important for normal prefrontal functions such as working memory, and their dysfunction plays a key role in the pathological processes of psychiatric disorders such as schizophrenia. Little is known, however, about the synaptic properties of NMDA receptors in the local circuits of recurrent excitation, a leading candidate mechanism underlying working memory. We investigated the NMDA receptor-mediated currents at monosynaptic connections between pairs of layer 5 pyramidal neurons. We found that NMDA receptor-mediated currents at prefrontal synapses in the adult, but not young, rats exhibit a twofold longer decay time-constant and temporally summate a train of stimuli more effectively, compared to those in the primary visual cortex. Experiments with pharmacological, immunocytochemical, and biochemical approaches further suggest that, in the adult animals, neurons express significantly more NR2B subunits in the prefrontal cortex than the visual cortex. The NR2B-rich synapses in the prefrontal circuitry may be critically implicated in online cognitive computations and plasticity in learning, as well as psychiatric disorders.

Amygdala infusions of an NR2B-selective or an NR2A-preferring NMDA receptor antagonist differentially influence fear conditioning and expression in the fear-potentiated startle test

Within the amygdala, most N-methyl-D-aspartic acid (NMDA) receptors consist of NR1 subunits in combination with either NR2A or NR2B subunits. Because the particular subunit composition greatly influences the receptors' properties, we investigated the contribution of both subtypes to fear conditioning and expression. To do so, we infused the NR1/NR2B receptor antagonist CP101,606 (0.5, 1.5, or 4.5 microg/amygdala) or the NR1/NR2A-preferring antagonist NVP-AAM077 (0.075, 0.25, 0.75, or 2.5 microg/amygdala) into the amygdala prior to either fear conditioning (i.e., light-shock pairings) or fear-potentiated startle testing. CP101,606 nonmonotonically disrupted fear conditioning but did not disrupt fear expression. NVP-AAM077 dose-dependently disrupted fear conditioning as well as fear expression. The results suggest that amygdala NR1/NR2B receptors play a special role in fear memory formation, whereas NR1/NR2A receptors participate more generally in synaptic transmission.

Regulation of AMPA receptor recruitment at developing synapses

Fast synaptic current at most excitatory synapses in the brain is carried by AMPA and NMDA subtypes of ionotropic glutamate receptors (AMPARs and NMDARs). During development there is an increase in the ratio of AMPAR- to NMDAR-mediated current at these synapses. Recent studies indicate that NMDAR signaling early in development negatively regulates AMPAR expression and function at multiple levels, which likely accounts for the small AMPAR current at developing synapses. This contrasts with the positive role of NMDAR signaling in recruiting AMPARs to synapses during long-term potentiation in the adult brain. Thus, NMDARs exert differential effects on the recruitment of AMPA receptors to synapses depending on the developmental state of the neural circuit.

NR2B signaling regulates the development of synaptic AMPA receptor current

The postnatal maturation of glutamatergic synapses involves a change in composition and functional contribution of postsynaptic receptors. Developing cortical synapses are dominated by NMDA receptors (NMDARs) containing NR2B subunits and are characterized by a low ratio of AMPA/NMDA receptor-mediated current. Synapse maturation is marked by the incorporation of NR2A-containing NMDA receptors and an increase in the AMPA/NMDA current ratio. We show here that NMDARs containing the NR2B subunit regulate glutamatergic transmission at developing synapses by negatively influencing the synaptic incorporation of AMPA receptors (AMPARs). Genetic removal of NR2B leads to increased surface expression and synaptic localization of AMPA receptor subunits and a corresponding increase in AMPAR-mediated synaptic current. Enrichment of synaptic AMPARs, in the absence of NR2B signaling, is associated with increased levels of transmembrane AMPAR regulatory protein (TARP) expression and is blocked by expression of a dominant-negative TARP construct (gamma-2deltaC). These observations suggest that NR2B signaling limits AMPA receptor incorporation at developing synapses by negatively regulating TARP expression and provide a mechanism to explain the maintenance of low AMPA/NMDA ratio at immature glutamatergic synapses.

Climbing-fibre activation of NMDA receptors in Purkinje cells of adult mice

Among principal neurons, adult Purkinje cells have long been considered unusual in lacking functional NMDA receptors. This view has emerged largely from studies on rats, where NMDA receptors are expressed in Purkinje cells of newborn animals, but are lost after 2 weeks. By contrast, immunolabelling data have shown that Purkinje cells from adult mice express multiple NMDA receptor subunits, suggesting a possible species difference. To investigate the presence of functional NMDA receptors in Purkinje cells of mice, and to explore the contribution of different receptor subunits, we made whole-cell and single-channel patch-clamp recordings from Purkinje cells of wild-type and NR2D-/- mice of different ages. Here we report that multiple NMDA receptor subtypes are indeed expressed in Purkinje cells of young and adult mice; in the adult, both NR2A- and NR2B-containing subtypes are present. Furthermore, we show that NMDA receptor-mediated EPSCs can be evoked by climbing fibre stimulation, and appear to be mediated mainly by NR2A-containing receptors.

NMDA receptor subunits have differential roles in mediating excitotoxic neuronal death both in vitro and in vivo

Well-documented experimental evidence from both in vitro and in vivo models of stroke strongly supports the critical involvement of NMDA receptor-mediated excitotoxicity in neuronal damage after stroke. Despite this, the results of clinical trials testing NMDA receptor antagonists as neuroprotectants after stroke and brain trauma have been discouraging. Here, we report that in mature cortical cultures, activation of either synaptic or extrasynaptic NR2B-containing NMDA receptors results in excitotoxicity, increasing neuronal apoptosis. In contrast, activation of either synaptic or extrasynaptic NR2A-containing NMDA receptors promotes neuronal survival and exerts a neuroprotective action against both NMDA receptor-mediated and non-NMDA receptor-mediated neuronal damage. A similar opposing action of NR2B and NR2A in mediating cell death and cell survival was also observed in an in vivo rat model of focal ischemic stroke. Moreover, we found that blocking NR2B-mediated cell death was effective in reducing infarct volume only when the receptor antagonist was given before the onset of stroke and not 4.5 h after stroke. In great contrast, activation of NR2A-mediated cell survival signaling with administration of either glycine alone or in the presence of NR2B antagonist significantly attenuated ischemic brain damage even when delivered 4.5 h after stroke onset. Together, the present work provides a molecular basis for the dual roles of NMDA receptors in promoting neuronal survival and mediating neuronal damage and suggests that selective enhancement of NR2A-containing NMDA receptor activation with glycine may constitute a promising therapy for stroke.

Effect of varied gestational stress on acquisition of spatial memory, hippocampal LTP and synaptic proteins in juvenile male rats

Some but not other forms of prenatal stress have been shown to impair spatial memory in adult male offspring. It is not clear if this is because of the intensity of the stress, age of rats, or the way in which learning is assessed. We examined the effect of daily varied prenatal stress consisting of 30 min restraint, saline injections and 15 min forced swim on day 17-21 of gestation on spatial learning, synaptic plasticity and the expression of key proteins of the post synaptic density (PSD) in the hippocampus of males aged 4-5 weeks. Prenatal stress impaired spatial learning in the Morris water maze and induced a significant decrease in long-term potentiation (LTP) in hippocampal slices. There was no change in the paired pulse facilitation ratio but there was a significant reduction in the expression of the NR2B subunit of the glutamate type NMDA receptor and the GluR1 subunit of the AMPA receptor, both of which are important modulators of LTP. These changes were accompanied by a remarkable increase in the scaffolding protein PSD95, which interacts with the intracellular carboxy terminal domains of the NR2 subunits. The high levels of PSD95 may have contributed to the impairment of LTP by disrupting the clustering of NMDA receptors in CA1 synapses. The alteration by prenatal stress in the relative amounts of scaffolding proteins and those which compose glutamate receptors could explain the depression of LTP and impairment in the acquisition of spatial learning.

Virally mediated knock-down of NR2 subunits ipsilateral to the deprived eye blocks ocular dominance plasticity

NMDA receptors (NMDARs) are important in developmental plasticity in the visual cortex. The NR2A and NR2B subunits of this receptor develop with different time courses, suggesting that they play different roles in plasticity. To understand the role of the NR2B subunit, we knocked-down NR2B gene expression in visual cortex by injecting a recombinant adenovirus containing an antisense NR2B oligonucleotide. To assess knock-down, we injected the recombinant adenovirus into the right visual cortex of rats (p22) or mice (p30). Eight days later we perfused the animals and processed the visual cortex for NMDAR subunit immunoreactivity (IR). NR2B-IR was depleted dramatically in the neuropil near the injection. Depletion was more modest in the neuronal somata. Surprisingly, NR2A-IR was also reduced, but NR1-IR was not reduced. To assess the functional effects of depletion, we measured ocular dominance plasticity with monocular deprivation (MD). We compared mice receiving the NR2B antisense virus with mice receiving virus containing only the GFP sequence and mice receiving no injection. All injections were between p26 and p29 in the right cortex and bilateral recordings were performed 6-8 days later. Animals receiving the antisense virus lost plasticity if the right eye was deprived. If the left eye was deprived, the cortex was normally plastic bilaterally. Injection of control virus had no effect on plasticity. The data indicate that ocular dominance plasticity requires normal NMDARs in the hemisphere ipsilateral to the deprived eye but not in the hemisphere contralateral to the deprived eye.

Effect of Conantokin G on NMDA receptor-mediated spontaneous EPSCs in cultured cortical neurons

Conantokin G (Con G), derived from the venom of Conus geographus, is the most characterized natural peptide antagonist targeted to N-methyl-D-aspartate (NMDA) receptors. Although Con G is known to bind to the glutamate binding site on the NR2 subunit of the receptor, it is unclear whether it can allosterically modulate the function of the receptor through the glycine binding site on the NR1 subunit. This study was designed to evaluate the action of Con G on NMDA receptor-mediated spontaneous excitatory postsynaptic currents (sEPSCs) and its modulation by glycine in cultured cortical neurons (13-19 days in vitro) using the whole cell patch-clamp technique. Con G inhibited NMDA receptor-mediated sEPSCs in a concentration-dependent manner. Also, the potency of Con G decreased as a function of time in culture. The inhibition of EPSCs observed after application of Con G in the presence of high (10 microM) and nominal (no added) concentrations of glycine was not different at 13 days in vitro (DIV). Furthermore, similar results were obtained with experiments on Con G-induced inhibition of NMDA-evoked whole cell currents. These results indicate that glycine concentrations do not have a direct effect on Con G-induced inhibition of NMDA currents. In addition, age dependency in the action of Con G on cortical neurons in vitro suggests that this model system would be useful in examining the effects of different agonists/antagonists on native synaptic NMDA receptors.

N-methyl-D-aspartate receptor NR2B subunit gene GRIN2B in schizophrenia and bipolar disorder: Polymorphisms and mRNA levels

The NR2B protein is a critical structural and functional subunit of the NMDA glutamate receptor. The glutamate neurotransmitter system has been implicated in psychosis and schizophrenia, and so we looked for genetic association and measured gene expression in human DNA and brain samples, respectively, of the GRIN2B gene that codes for the NR2B protein. We tested three genetic polymorphisms: G-200T (5'UTR), A5806C and T5988C (both 3'UTR) in 180 matched schizophrenia case-control pairs, 86 schizophrenia nuclear family trios, and 318 bipolar disorder trios (of which 158 probands had psychotic symptoms). We measured brain GRIN2B mRNA levels in schizophrenia, bipolar disorder and unaffected controls (n = 35 each). We detected genetic association between the G-200T marker and schizophrenia (p = 0.002), between T5988C and bipolar disorder (p = 0.02), and between A5806C and bipolar disorder with psychotic symptoms (p = 0.0038). The T-C-C haplotype was transmitted more frequently with bipolar disorder, but less often with schizophrenia, while the G-C-T haplotype was transmitted more often in schizophrenia. Significant differences were found in overall haplotype frequencies between schizophrenia cases and controls (p = 0.005). GRIN2B expression levels in schizophrenia, bipolar disorder and controls were not significantly different. The genetic findings suggest a role for GRIN2B in schizophrenia and bipolar disorder.

Maternal stress produces learning deficits associated with impairment of NMDA receptor-mediated synaptic plasticity

Stress in adulthood can have a profound effect on physiology and behavior, but the extent to which prolonged maternal stress affects the brain function of offspring when they are adult remains primarily unknown. In the present work, chronic immobilization stress to pregnant mice affected fetal growth and development. When pups born from stressed mice were reared to adulthood in an environment identical to that of nonstressed controls, several physiological parameters were essentially unaltered. However, spatial learning and memory was significantly impaired in the maternally stressed offspring in adulthood. Furthermore, electrophysiological examination revealed a significant reduction in NMDA receptor-mediated long-term potentiation in the CA1 area of hippocampal slices. Subsequent biochemical analysis demonstrated a substantial decrease in NR1 and NR2B subunits of the NMDA receptor in synapses of the hippocampus, and the interaction between these two subunits appeared to be reduced. These results suggest that prolonged maternal stress leads to long-lasting malfunction of the hippocampus, which extends to and is manifested in adulthood.

Synaptic and Extrasynaptic NMDA Receptor NR2 Subunits in Cultured Hippocampal Neurons

Early in development, neurons only express NR1/NR2B-containing N-methyl-d-aspartate (NMDA) receptors. Later, NR2A subunits are upregulated during a period of rapid synapse formation. This pattern is often interpreted to indicate that NR2A-containing receptors are synaptic and that NR2B-containing receptors are extrasynaptic. We re-examined this issue using whole cell recordings in cultured hippocampal neurons. As expected, the inhibition of whole cell currents by the NR2B-specific antagonist, ifenprodil, progressively decreased from 69.5 ± 2.4% [6 days in vitro (DIV)] to 54.9 ± 2.6% (8 DIV), before reaching a plateau in the second week (42.5 ± 2%, 12–19 DIV). In NR2A−/− neurons, which express only NR1/NR2B-containing NMDA receptors, autaptic excitatory postsynaptic currents (EPSCs; ≥12 DIV) were more sensitive to ifenprodil and decayed more slowly than EPSCs in wild-type neurons. Thus NR2B-containing receptors were not excluded from synapses. We blocked synaptic NMDA receptors with MK-801 during evoked transmitter release, thus allowing us to isolate extrasynaptic receptors. Ifenprodil inhibition of this extrasynaptic population was highly variable in different neurons. Furthermore, extrasynaptic receptors in autaptic cultures were only partially blocked by ifenprodil, indicating that NR2A-containing receptors are not exclusively confined to the synapse. Extrasynaptic NR2A-containing receptors were also detected in NR2A−/− neurons transfected with full-length NR2A. Truncation of the NR2A C terminus did not eliminate synaptic expression of NR2A-containing receptors. Our results indicate that NR2A- and NR2B-containing receptors can be located in either synaptic or extrasynaptic compartments.

Nickel differentially affects NMDA receptor channels in developing cultured rat neurons

Nickel (Ni(2+)) is a transition metal that exerts multiple and complex effects on N-methyl-d-aspartate (NMDA) channels. In both HEK293 cells and Xenopus laevis oocytes expressing recombinant NMDA receptors, Ni(2+) (<100 microM) caused a potentiation of NR2B-containing channels but a voltage-independent inhibition in those containing NR2A. We took advantage of this different response to investigate the developmental switch between NR2B and NR2A subunits in neonatal rat cerebellar granule cells up to 16 days in vitro (DIV) and in rat embryo cortical neurons up to 35 DIV. In both cultures, the effect of Ni(2+) on the NMDA current gradually changed from potentiating to inhibitory with progressing DIV, and the decline of potentiation correlated well with the decrease in sensitivity for the NR2B specific antagonist ifenprodil. Dose-dependent experiments confirmed that Ni(2+) has a different effect in younger cultures with respect to older ones, in agreement with an increase of the percentage of NR2A-containing receptors. The developmental switch occurred within the first 5 DIV in cerebellar granule cells and after 20 DIV in cortical neurons. All these data indicate that Ni(2+) is a suitable marker for the identification of NR2A and NR2B native channel subunits and can be used to trace the development of NMDA receptor composition.

Zinc and copper: pharmacological probes and endogenous modulators of neuronal excitability

As well as being key structural components of many proteins, increasing evidence suggests that zinc and copper ions function as signaling molecules in the nervous system and are released from the synaptic terminals of certain neurons. In this review, we consider the actions of these two ions on proteins that regulate neuronal excitability. In addition to the established actions of zinc, and to a lesser degree copper, on excitatory and inhibitory ligand-gated ion channels, we show that both ions have a number of actions on selected members of the voltage-gated-like ion channel superfamily. For example, zinc is a much more effective blocker of one subtype of tetrodotoxin (TTX)-insensitive sodium (Na+) channel (NaV1.5) than other Na+ channels, whereas a certain T-type calcium (Ca2+) channel subunit (CaV3.2) is particularly sensitive to zinc. For potassium (K+) channels, zinc can have profound effects on the gating of certain KV channels whereas zinc and copper have distinct actions on closely related members of the 2 pore domain potassium channel (K2P) channel family. In addition to direct actions on these proteins, zinc is able to permeate a number of membrane proteins such as (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors, Ca2+ channels and some transient receptor potential (trp) channels. There are a number of important physiological and pathophysiological consequences of these many actions of zinc and copper on membrane proteins, in terms of regulation of neuronal excitability and neurotoxicity. Furthermore, the concentration of free zinc and copper either in the synaptic cleft or neuronal cytoplasm may contribute to the etiology of certain disease states such as Alzheimer's disease (AD) and epilepsy.

Enhancement of synaptic plasticity through chronically reduced Ca2+ flux during uncorrelated activity

The plasticity of synapses within neural circuits is regulated by activity, but the underlying mechanisms remain elusive. Using the dye FM1-43 to directly image presynaptic function, we found that large numbers of presynaptic terminals in hippocampal cultures have a low release probability. While these terminals were not readily modifiable, a transient but not permanent long-term reduction of network activity or Ca2+ influx could increase their modifiability. This modulation of plasticity was mediated by Ca2+ flux through NMDA and voltage-gated calcium channels and was lost within 48 hr. A more permanent enhancement of synaptic plasticity was achieved by selectively reducing the Ca2+ flux associated with uncorrelated activity via adjustment of the voltage-dependent Mg2+ block of the NMDAR. Upregulation of NR2B-containing NMDARs induced by this treatment is an important but not sole contributor to the enhancement of plasticity. Thus, quantity and quality of activity have differential effects on the intrinsic plasticity of neurons.

Extrasynaptic NR2B and NR2D subunits of NMDA receptors shape 'superslow' afterburst EPSC in rat hippocampus

In conditions of facilitated synaptic release, CA3/CA1 synapses generate anomalously slow NMDA receptor-mediated EPSCs (EPSC(NMDA)). Such a time course has been attributed to the cooperation of synapses through glutamate spillover. Imitating a natural pattern of activity, we have applied short bursts (2-7 stimuli) of high-frequency stimulation and observed a spike-to-spike slow-down of the EPSC(NMDA) kinetics, which accompanied synaptic facilitation. It was found that the early component of the EPSC(NMDA) and the burst-induced late component of the EPSC(NMDA) have distinct pharmacological properties. The competitive NMDA antagonist R-(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (D-CPP), which has higher affinity to NR2A than to NR2B subunits and lowest affinity at NR2D subunits, significantly slowed down the decay rate of the afterburst EPSC while leaving the kinetics of the control current unaffected. In contrast, ifenprodil, a highly selective NR2B antagonist, and [+/-]-cis-1-[phenanthren-2yl-carbonyl]piperazine-2,3-dicarboxylic acid (PPDA), a competitive antagonist that is moderately selective for NR2D subunits, more strongly inhibited the late component of the afterburst EPSC(NMDA). The receptors formed by NR2B and (especially) NR2D subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors. Furthermore, NR2B is preferentially and NR2D is exclusively located on extrasynaptic membranes. As the density of active synapses increases, the confluence of released glutamate makes EPSC decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC(NMDA.)

Pathway-specific differences in subunit composition of synaptic NMDA receptors on pyramidal neurons in neocortex

Heterogeneity of synaptic inputs onto neocortical layer 5 pyramidal neurons could result from differences in the underlying receptors, yet previous work has shown that functional attributes of AMPA receptors are uniform among synaptic connections onto these neurons. To determine whether NMDA receptors (NMDARs) would be similarly uniform, we compared in the same pyramidal neurons pharmacologically isolated NMDAR-mediated EPSCs evoked by stimulation of two anatomically distinguishable pathways, callosal or intracortical. Based on differences in voltage dependence, decay kinetics, apparent Mg2+sensitivity, and subunit-specific (NR2A, NR2B, and NR2C/D) pharmacology, we found NMDARs at these inputs to be distinct. Furthermore, NMDARs activated by the intracortical pathway were more efficient at integrating EPSPs and bringing the neuron closer to the spike-firing threshold than the callosal pathway. These results suggest that pyramidal neurons encode information differentially depending on the origin of their neocortical inputs and that NMDAR-dependent synaptic plasticity may be pathway specific.

Trafficking of NMDA receptors

The NMDA receptor (NMDAR) plays a central role in the function of excitatory synapses. Recent studies have provided interesting insights into several aspects of the trafficking of this receptor in neurons. The NMDAR is not a static resident of the synapse. Rather, the number and composition of synaptic NMDARs can be modulated by several factors. The interaction of PDZ proteins, generally thought to occur at the synapse, appears to occur early in the secretory pathway; this interaction may play a role in the assembly of the receptor complex and its exit from the endoplasmic reticulum. This review addresses recent advances in our understanding of NMDAR trafficking and its synaptic delivery and maintenance.

Development and subunit composition of synaptic NMDA receptors in the amygdala: NR2B synapses in the adult central amygdala

NMDA receptors are well known to play an important role in synaptic development and plasticity. Functional NMDA receptors are heteromultimers thought to contain two NR1 subunits and two or three NR2 subunits. In central neurons, NMDA receptors at immature glutamatergic synapses contain NR2B subunits and are largely replaced by NR2A subunits with development. At mature synapses, NMDA receptors are thought to be multimers that contain either NR1/NR2A or NR1/NR2A/NR2B subunits, whereas receptors that contain only NR1/NR2B subunits are extrasynaptic. Here, we have studied the properties of NMDA receptors at glutamatergic synapses in the lateral and central amygdala. We find that NMDA receptor-mediated synaptic currents in the central amygdala in both immature and mature synapses have slow kinetics and are substantially blocked by the NR2B-selective antagonists (1S, 2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propano and ifenprodil, indicating that there is no developmental change in subunit composition. In contrast, at synapses on pyramidal neurons in the lateral amygdala, whereas NMDA EPSCs at immature synapses are slow and blocked by NR2B-selective antagonists, at mature synapses their kinetics are faster and markedly less sensitive to NR2B-selective antagonists, consistent with a change from NR2B to NR2A subunits. Using real-time PCR and Western blotting, we show that in adults the ratio of levels of NR2B to NR2A subunits is greater in the central amygdala than in the lateral amygdala. These results show that the subunit composition synaptic NMDA receptors in the lateral and central amygdala undergo distinct developmental changes.

Excitatory amino acid receptors of the electrosensory system: the NR1/NR2B N-methyl-D-aspartate receptor

The amino acid sequence of the N-methyl-D-aspartate (NMDA) receptor subunit NR2B from the brown ghost knife fish Apteronotus leptorhynchus has been determined and compared with the sequence of the murine NR2B. This comparison revealed high levels of sequence conservation throughout the ligand binding and membrane spanning segments. The functional properties of the NR1 and NR2B receptor complex were examined by coexpression in HEK cells. The recombinant AptNR1/NR2B receptors produced robust currents after stimulation with glutamate or NMDA in the presence of glycine. Measurements of the concentration dependencies for these agonists indicated that the agonist binding sites on the apteronotid receptor are highly conserved, with nearly identical agonist affinities to those of the murine NR1/NR2B receptor. The kinetic responses of the fish receptor were also highly conserved, with deactivation rates for the AptNR2B receptor matching those of the murine NR2B containing receptor. Evidently, most of the unique functional properties that reside in the NR2B receptor subunit have been well conserved in teleost NMDA receptors. On the other hand, the apteronitid receptor displayed a lowered sensitivity to voltage-dependent Mg(2+) block and a reduced affinity for the NR2B-specific noncompetitive antagonist ifenprodil. We conclude that the functional properties that result from the incorporation of the NR2B receptor in the NMDA receptor complex have been maintained since the evolutionary divergence of teleost and mammalian organisms.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

Relationship between availability of NMDA receptor subunits and their expression at the synapse

The effect of increasing the expression of NMDA subunits in cerebellar granule cells (CGCs) by transfection was studied to determine how the availability of various NMDA subunits controls both the total pool of functional receptors and the synaptic pool. Overexpression of either NR2A or NR2B, but not splice variants of NR1, by transfection caused a significant increase in the total number of functional NMDA receptors and in surface NR1 subunit cluster density in CGCs in primary culture. These data solidify the central role of NR2 subunit availability in determining the number of cell surface receptors. Overexpression of either NR2A or NR2B significantly altered the deactivation kinetics of NMDA-mediated miniature EPSCs (NMDA-mEPSCs). However, there was no significant effect of NR2 subunit overexpression on the mEPSC amplitude or single-channel conductance. NR2 subunit overexpression did not change the rate of block by MK-801 of NMDA-mediated currents in excised patches from CGCs, indicating that subunit composition does not regulate peak open probability of the channel in CGCs. With the overexpression of a mutant of NR2B lacking the PDZ binding domain, there was an increase in the total number of NMDA receptors without a change in mEPSC kinetics. Therefore, the entry of NMDA receptors into the synapse requires a PDZ binding domain and is limited by means other than receptor subunit availability.

Association of NR3A with the N-methyl-D-aspartate receptor NR1 and NR2 subunits

The NR3A subunit of the N-methyl-D-aspartate receptor has been shown to form glutamatergic receptor complexes with NR1 and NR2 subunits and excitatory glycinergic receptor complexes with NR1 alone. We developed an antibody to NR3A and, using quantitative immunoblotting techniques, determined the degree of association between the NR3A subunit and the NR1 and NR2 subunits as well as changes in these associations during development. NR3A expression peaks between postnatal days 7 and 10 in the cortex, midbrain, and hippocampus and reaches higher maximal expression levels in these areas than in the olfactory bulb and cerebellum. Immunoprecipitation experiments with an anti-NR1 antibody demonstrated that the majority of NR3A is associated with NR1 in postnatal day 10 rat cortex (80 +/- 8%), decreasing by half (38 +/- 4%) in the adult rat cortex. Using the anti-NR3A antibody in immunoprecipitation studies, we find that 9.7 +/- 0.8% of NR1, 8.7 +/- 1.8% of NR2A, and 5.0 +/- 0.6% of NR2B are associated with NR3A at postnatal day 10. These values decrease by about half in adult rat cortex. The results of this study demonstrate that NR3A is expressed, distributed, and associated with other subunits in a manner that supports its role in synaptic transmission throughout the rat brain, perhaps playing different roles during development.