Synaptic NMDA Receptors Research Articles

Sensitivity of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials and synaptic plasticity to TCN 201 and TCN 213 in rat hippocampal slices.

Whereas ifenprodil has been used as a selective GluN1/GluN2B (NR1/NR2B, B-type) receptor antagonist to distinguish between GluN2B (NR2B) and GluN2A (NR2A)-containing N-methyl-d-aspartate receptors (NMDARs), TCN 201 (3-chloro-4-fluoro-N-[4-[[2-(phenylcarbonyl)hydrazino]carbonyl]benzyl]benzenesulphonamide) and TCN 213 [N-(cyclohexylmethyl)-2-[{5-[(phenylmethyl)amino]-1,3,4-thiadiazol-2-yl}thio]acetamide] have been found to be selective GluN1/GluN2A (NR1/NR2A, A-type) antagonists. Based on the premise that A- and B-types are major synaptic NMDARs, we examined whether inhibition of NMDAR excitatory postsynaptic potentials (EPSPs) by the TCN compounds and ifenprodil are complementary. Contrary to this prediction, inhibition of NMDAR EPSPs by the TCN compounds and ifenprodil were largely overlapping in the CA1 region of hippocampal slices from 30-day-old rats. After partial inhibition by ifenprodil, TCN compounds produced little further suppression of NMDAR EPSPs. Similarly, after partial inhibition by TCN compounds ifenprodil failed to further suppress NMDAR EPSPs. However, low micromolar d-2-amino-5-phosphonovalerate, a competitive NMDAR antagonist, which alone only partially inhibits NMDAR EPSPs, markedly suppresses residual NMDAR responses in the presence of ifenprodil or the TCNs, suggesting that low 2-amino-5-phosphonovalerate antagonizes both ifenprodil- and TCN-insensitive synaptic NMDARs. These observations can be most readily interpreted if ifenprodil and TCNs act on a similar population of synaptic NMDARs. Recent lines of evidence suggest that the majority of hippocampal synaptic NMDARs are triheteromers. If so, modulation of GluN2A, and not just GluN2B NMDARs, could dampen long-term depression (LTD). Indeed, both TCNs, like ifenprodil, blocked LTD, suggesting the involvement of ifenprodil- and TCN-sensitive NMDARs in LTD induction. However, the TCNs plus ifenprodil failed to inhibit long-term potentiation (LTP), suggesting that neither ifenprodil- nor TCN-sensitive NMDARs are essential for LTP induction.

NMDA receptor activity determines neuronal fate: location or number?

Abstract It is widely believed that the proper activation of N-methyl-D-aspartate (NMDA) receptors (NMDARs) promotes neuronal survival, whereas an excessive activation of NMDARs leads to neuronal damage. NMDARs are found at both synaptic and extrasynaptic sites. One current prevailing theory proposes the dichotomy of NMDAR activity. The role of the two population receptors is mutual antagonism. The activation of synaptic NMDARs, such as synaptic activity at physiological levels, promotes neuronal survival. However, the activation of extrasynaptic NMDARs occurring during stroke, brain injury, and chronic neurological diseases contributes to neuronal death. Thus, the location of NMDARs determines the neuronal fate. However, the theory is greatly challenged. Several studies suggested that synaptic NMDARs are involved in neuronal death. Recently, our work further showed that the coactivation of synaptic and extrasynaptic NMDARs contributes to neuronal death under neuronal insults. Therefore, we propose that the magnitude and duration of NMDAR activation determines the neuronal fate. More interestingly, there appears to be some subtle differences in the affinity between synaptic and extrasynaptic NMDARs, shedding light on the development of selective drugs to block extrasynaptic NMDARs.

Scribble1/AP2 Complex Coordinates NMDA Receptor Endocytic Recycling

The appropriate trafficking of glutamate receptors tosynapses is crucial for basic synaptic function and synaptic plasticity. It is now accepted that NMDA receptors (NMDARs) internalize and are recycled at the plasma membrane but also exchange between synaptic and extrasynaptic pools; these NMDAR properties are also key to governing synaptic plasticity. Scribble1 is a large PDZ protein required for synaptogenesis and synaptic plasticity. Herein, we show that the level of Scribble1 is regulated in an activity-dependent manner and that Scribble1 controls the number of NMDARs at the plasma membrane. Notably, Scribble1 prevents GluN2A subunits from undergoing lysosomal trafficking and degradation by increasing their recycling to the plasma membrane following NMDAR activation. Finally, we show that a specific YxxR motif on Scribble1 controls these mechanisms through a direct interaction with AP2. Altogether, our findings define a molecular mechanism to control the levels of synaptic NMDARs via Scribble1 complex signaling.

Diurnal inhibition of NMDA-EPSCs at rat hippocampal mossy fibre synapses through orexin-2 receptors.

Diurnal release of the orexin neuropeptides orexin-A (Ox-A, hypocretin-1) and orexin-B (Ox-B, hypocretin-2) stabilises arousal, regulates energy homeostasis and contributes to cognition and learning. However, whether cellular correlates of brain plasticity are regulated through orexins, and whether they do so in a time-of-day-dependent manner, has never been assessed. Immunohistochemically we found sparse but widespread innervation of hippocampal subfields through Ox-A- and Ox-B-containing fibres in young adult rats. The actions of Ox-A were studied on NMDA receptor (NMDAR)-mediated excitatory synaptic transmission in acute hippocampal slices prepared around the trough (Zeitgeber time (ZT) 4-8, corresponding to 4-8 h into the resting phase) and peak (ZT 23) of intracerebroventricular orexin levels. At ZT 4-8, exogenous Ox-A (100 nm in bath) inhibited NMDA receptor-mediated excitatory postsynaptic currents (NMDA-EPSCs) at mossy fibre (MF)-CA3 (to 55.6 ± 6.8% of control, P = 0.0003) and at Schaffer collateral-CA1 synapses (70.8 ± 6.3%, P = 0.013), whereas it remained ineffective at non-MF excitatory synapses in CA3. Ox-A actions were mediated postsynaptically and blocked by the orexin-2 receptor (OX2R) antagonist JNJ10397049 (1 μm), but not by orexin-1 receptor inhibition (SB334867, 1 μm) or by adrenergic and cholinergic antagonists. At ZT 23, inhibitory effects of exogenous Ox-A were absent (97.6 ± 2.9%, P = 0.42), but reinstated (87.2 ± 3.3%, P = 0.002) when endogenous orexin signalling was attenuated for 5 h through i.p. injections of almorexant (100 mg kg(-1)), a dual orexin receptor antagonist. In conclusion, endogenous orexins modulate hippocampal NMDAR function in a time-of-day-dependent manner, suggesting that they may influence cellular plasticity and consequent variations in memory performance across the sleep-wake cycle.

Co-activation of synaptic and extrasynaptic NMDA receptors by neuronal insults determines cell death in acute brain slice

Overactivation of NMDA receptors is linked to cell death during neuronal insults. However the precise role of synaptic and extrasynaptic NMDA receptors remains to be further determined. In this study, we used the acute brain slice to examine the contributions of synaptic and extrasynaptic NMDA receptors to neuronal death. By activation of synaptic NMDA receptors with bath application of 100μM bicuculline in acute brain slices, we observed a significant up-regulation in activation of neuronal survival-related signaling (p-CREB, p-ERK1/2 and p-AKT), without an obvious increase of LDH release and neuronal death. Interestingly, activation of extrasynaptic NMDA receptors alone by high dose of glutamate (200μM) following blockade of synaptic NMDA receptors with co-application of 20μM MK801 and 100μM bicuculline, we failed to observe inhibition of neuronal survival signaling and neuronal damage. In contrast, co-activation of synaptic and extrasynaptic NMDA receptors by applying 200μM glutamate or oxygen–glucose deprivation (OGD) to acute brain slices for 30min, we observed a significant inhibition of CREB, ERK1/2 and AKT activation, an increase of LDH release and neuronal condensation. Together, co-activation of synaptic and extrasynaptic NMDA receptors by neuronal insults contributes to cell death in acute brain slice.

Hippocampal NMDA receptors and the previous experience effect on memory

N-methyl-D-aspartate receptors (NMDAR) are thought to be responsible for switching synaptic activity specific patterns into long-term changes in synaptic function and structure, which would support learning and memory. Hippocampal NMDAR blockade impairs memory consolidation in rodents, while NMDAR stimulation improves it.Adult rats that explored twice an open field (OF) before a weak though overthreshold training in inhibitory avoidance (IA), expressed IA long-term memory in spite of the hippocampal administration of MK-801, which currently leads to amnesia.Those processes would involve different NMDARs. The selective blockade of hippocampal GluN2B-containing NMDAR with ifenprodil after training promoted memory in an IA task when the training was weak, suggesting that this receptor negatively modulates consolidation.In vivo, after 1h of an OF exposure-with habituation to the environment-, there was an increase in GluN1 and GluN2A subunits in the rat hippocampus, without significant changes in GluN2B. Coincidentally, in vitro, in both rat hippocampal slices and neuron cultures there was an increase in GluN2A-NMDARs surface expression at 30min; an increase in GluN1 and GluN2A levels at about 1h after LTP induction was also shown.We hypothesize that those changes in NMDAR composition could be involved in the “anti-amnesic effect” of the previous OF. Along certain time interval, an increase in GluN1 and GluN2A would lead to an increase in synaptic NMDARs, facilitating synaptic plasticity and memory; while then, an increase in GluN2A/GluN2B ratio could protect the synapse and the already established plasticity, perhaps saving the specific trace.

Synaptic GluN2A and GluN2B containing NMDA receptors within the superficial dorsal horn activated following primary afferent stimulation.

NMDA receptors are important elements in pain signaling in the spinal cord dorsal horn. They are heterotetramers, typically composed of two GluN1 and two of four GluN2 subunits: GluN2A-2D. Mice lacking some of the GluN2 subunits show deficits in pain transmission yet functional synaptic localization of these receptor subtypes in the dorsal horn has not been fully resolved. In this study, we have investigated the composition of synaptic NMDA receptors expressed in monosynaptic and polysynaptic pathways from peripheral sensory fibers to lamina I neurons in rats. We focused on substance P receptor-expressing (NK1R+) projection neurons, critical for expression of hyperalgesia and allodynia. EAB-318 and (R)-CPP, GluN2A/B antagonists, blocked both monosynaptic and polysynaptic NMDA EPSCs initiated by primary afferent activation by ∼90%. Physiological measurements exploiting the voltage dependence of monosynaptic EPSCs similarly indicated dominant expression of GluN2A/B types of synaptic NMDA receptors. In addition, at synapses between C fibers and NK1R+ neurons, NMDA receptor activation initiated a secondary, depolarizing current. Ifenprodil, a GluN2B antagonist, caused modest suppression of monosynaptic NMDA EPSC amplitudes, but had a widely variable, sometimes powerful, effect on polysynaptic responses following primary afferent stimulation when inhibitory inputs were blocked to mimic neuropathic pain. We conclude that GluN2B subunits are moderately expressed at primary afferent synapses on lamina I NK1R+ neurons, but play more important roles for polysynaptic NMDA EPSCs driven by primary afferents following disinhibition, supporting the view that the analgesic effect of the GluN2B antagonist on neuropathic pain is at least in part, within the spinal cord.

The glutamate receptor GluN2 subunit regulates synaptic trafficking of AMPA receptors in the neonatal mouse brain.

The N-methyl-D-aspartate receptor (NMDAR) plays various physiological and pathological roles in neural development, synaptic plasticity and neuronal cell death. It is composed of two GluN1 and two GluN2 subunits and, in the neonatal hippocampus, most synaptic NMDARs are GluN2B-containing receptors, which are gradually replaced with GluN2A-containing receptors during development. Here, we examined whether GluN2A could be substituted for GluN2B in neural development and functions by analysing knock-in (KI) mice in which GluN2B is replaced with GluN2A. The KI mutation was neonatally lethal, although GluN2A-containing receptors were transported to the postsynaptic membrane even without GluN2B and functional at synapses of acute hippocampal slices of postnatal day 0, indicating that GluN2A-containing NMDARs could not be substituted for GluN2B-containing NMDARs. Importantly, the synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) subunit GluA1 was increased, and the transmembrane AMPAR regulatory protein, which is involved in AMPAR synaptic trafficking, was increased in KI mice. Although the regulation of AMPARs by GluN2B has been reported in cultured neurons, we showed here that AMPAR-mediated synaptic responses were increased in acute KI slices, suggesting differential roles of GluN2A and GluN2B in AMPAR expression and trafficking in vivo. Taken together, our results suggest that GluN2B is essential for the survival of animals, and that the GluN2B-GluN2A switching plays a critical role in synaptic integration of AMPARs through regulation of GluA1 in the whole animal.

Increasing small conductance Ca2+-activated potassium channel activity reverses ischemia-induced impairment of long-term potentiation.

Global cerebral ischemia following cardiac arrest and cardiopulmonary resuscitation (CA/CPR) causes injury to hippocampal CA1 pyramidal neurons and impairs cognition. Small conductance Ca(2+)-activated potassium channels type 2 (SK2), expressed in CA1 pyramidal neurons, have been implicated as potential protective targets. Here we showed that, in mice, hippocampal long-term potentiation (LTP) was impaired as early as 3 h after recovery from CA/CPR and LTP remained impaired for at least 30 days. Treatment with the SK2 channel agonist 1-Ethyl-2-benzimidazolinone (1-EBIO) at 30 min after CA provided sustained protection from plasticity deficits, with LTP being maintained at control levels at 30 days after recovery from CA/CPR. Minimal changes in glutamate release probability were observed at delayed times after CA/CPR, implicating post-synaptic mechanisms. Real-time quantitative reverse transcriptase-polymerase chain reaction indicated that CA/CPR did not cause a loss of N-methyl-D-aspartate (NMDA) receptor mRNA at 7 or 30 days after CA/CPR. Similarly, no change in synaptic NMDA receptor protein levels was observed at 7 or 30 days after CA/CPR. Further, patch-clamp experiments demonstrated no change in functional synaptic NMDA receptors at 7 or 30 days after CA/CPR. Electrophysiology recordings showed that synaptic SK channel activity was reduced for the duration of experiments performed (up to 30 days) and that, surprisingly, treatment with 1-EBIO did not prevent the CA/CPR-induced loss of synaptic SK channel function. We concluded that CA/CPR caused alterations in post-synaptic signaling that were prevented by treatment with the SK2 agonist 1-EBIO, indicating that activators of SK2 channels may be useful therapeutic agents to prevent ischemic injury and cognitive impairments.

Cell-type specific mechanisms of D-serine uptake and release in the brain.

Accumulating evidence during the last decade established that D-serine is a key signaling molecule utilized by neurons and astroglia in the mammalian central nervous system. D-serine is increasingly appreciated as the main physiological endogenous coagonist for synaptic NMDA receptors at central excitatory synapses; it is mandatory for long-term changes in synaptic strength, memory, learning, and social interactions. Alterations in the extracellular levels of D-serine leading to disrupted cell-cell signaling are a trademark of many chronic or acute neurological (i.e., Alzheimer disease, epilepsy, stroke) and psychiatric (i.e., schizophrenia) disorders, and are associated with addictive behavior (i.e., cocaine addiction). Indeed, fine tuning of the extracellular levels of D-serine, achieved by various molecular machineries and signaling pathways, is necessary for maintenance of accurate NMDA receptor functions. Here, we review the experimental data supporting the notion that astroglia and neurons use different pathways to regulate levels of extracellular D-serine.

Rapid Hebbian axonal remodeling mediated by visual stimulation.

We examined how correlated firing controls axon remodeling, using in vivo time-lapse imaging and electrophysiological analysis of individual retinal ganglion cell (RGC) axons that were visually stimulated either synchronously or asynchronously relative to neighboring inputs in the Xenopus laevis optic tectum. RGCs stimulated out of synchrony rapidly lost the ability to drive tectal postsynaptic partners while their axons grew and added many new branches. In contrast, synchronously activated RGCs produced fewer new branches, but these were more stable. The effects of synchronous activation were prevented by the inhibition of neurotransmitter release and N-methyl-D-aspartate receptor (NMDAR) blockade, which is consistent with a role for synaptic NMDAR activation in the stabilization of axonal branches and suppression of further exploratory branch addition.

Antidepressants that inhibit both serotonin and norepinephrine reuptake impair long-term potentiation in hippocampus.

Monoamine reuptake inhibitors can stimulate expression of brain-derived neurotrophic factor (BDNF) and alter long-term potentiation (LTP), a widely used model for the synaptic mechanisms that underlie memory formation. BDNF expression is upregulated during LTP, and BDNF in turn positively modulates LTP. Previously, we found that treatment with venlafaxine, a serotonin and norepinephrine reuptake inhibitor (SNRI), but not citalopram, a selective serotonin reuptake inhibitor (SSRI), reduced LTP in hippocampal area CA1 without changing hippocampal BDNF protein expression. We tested the hypothesis that combined serotonin and norepinephrine reuptake inhibition is necessary for LTP impairment, and we reexamined the potential role of BDNF by testing for region-specific changes in areas CA1, CA3, and dentate gyrus. We also tested whether early events in the LTP signaling pathway were altered to impair LTP. Animals were treated for 21 days with venlafaxine, imipramine, fluoxetine, or maprotiline. In vitro hippocampal slices were used for electrophysiological measurements. Protein expression was measured by enzyme-linked immunosorbent assay (ELISA) and Western blotting. LTP was impaired only following treatment with combined serotonin and norepinephrine reuptake inhibitors (venlafaxine, imipramine) but not with selective serotonin (fluoxetine) or norepinephrine (maprotiline) reuptake inhibitors. BDNF protein expression was not altered by venlafaxine or imipramine treatment, nor were postsynaptic depolarization during LTP inducing stimulation or synaptic membrane NMDA receptor subunit expression affected. LTP is impaired by chronic treatment with antidepressant that inhibit both serotonin and norepinephrine reuptake; this impairment results from changes that are downstream of postsynaptic depolarization and calcium influx.

Distinct modifications of convergent excitatory and inhibitory inputs in developing olfactory circuits

The interaction between excitatory and inhibitory inputs is critical to neuronal signal processing. However, little is known about this fundamental property, largely due to the inability to clearly isolate the respective inputs. Here we took advantage of the characteristic stereotypical architecture of synaptic connections in the main olfactory bulb, which enabled us to entirely separate excitatory and inhibitory inputs. Using paired stimulation of two glomeruli located apart at different intensities, we separately elicited excitatory and inhibitory inputs and mimicked stimulation of competing mitral cells (MCs) with different odorants. We performed dual whole-cell patch recording of evoked excitatory postsynaptic responses (EPSPs) and inhibitory postsynaptic responses (IPSPs) in current-clamp mode from two competitive MCs that are connected to the two stimulated glomeruli in slices of the main olfactory bulb in 2–3-week-old rats. We deliberately held the recorded cells at a relative hyperpolarized potential. This manipulation not only suppressed action potential generation but also excluded the possible contamination of inhibitory components in excitatory inputs. We found that in weakly activated MCs repetitive EPSP–IPSP interactions (5Hz, 180 times) induced long-term potentiation (LTP) and long-term depression (LTD) in convergent excitatory and inhibitory inputs, respectively. Unexpectedly, these forms of plasticity depend on activity of somatic (mainly non-synaptic) NMDA receptors (NMDARs). In contrast, the same repetitive stimulation induced the LTP of excitatory inputs in strongly activated MCs (MC2) that require activity of synaptic NMDARs. These distinct forms of plasticity in the developing olfactory circuit may represent a novel rule of modification in convergent inputs that leads to decorrelation of inputs and facilitates odor discrimination.

Differential effects of synaptic and extrasynaptic NMDA receptors on Aβ-induced nitric oxide production in cerebrocortical neurons.

Oligomerized amyloid-β (Aβ) peptide is thought to contribute to synaptic damage, resulting in dysfunctional neuronal networks in patients with Alzheimer's disease. It has been previously suggested that Aβ may be detrimental to neuronal health, at least in part, by triggering oxidative/nitrosative stress. However, the mechanisms underlying this process remain to be elucidated. Here, using rat primary cerebrocortical cultures, we demonstrate that Aβ1-42 oligomers trigger a dramatic increase in intracellular nitric oxide (NO) concentration via a process mediated by activation of NMDA-type glutamate receptors (NMDARs). Considering that synaptic NMDARs and extrasynaptic NMDARs (eNMDARs) can have opposite effects on neuronal viability, we explored their respective roles in Aβ-induced increases in NO levels. Surprisingly, after pharmacological isolation of eNMDARs, we discovered that eNMDARs are primarily responsible for the increase in neuronal NO triggered by Aβ oligomers. Moreover, we found that the eNMDAR-mediated increase in NO can produce S-nitrosylation of Drp1 (dynamin-related protein 1) and Cdk5 (cyclin-dependent kinase 5), targets known to contribute to Aβ-induced synaptic damage. These results suggest that pharmacological intervention specifically aimed at eNMDARs may decrease Aβ-induced nitrosative stress and thus ameliorate neurotoxic damage to synapses.

14-3-3 Proteins Are Required for Hippocampal Long-Term Potentiation and Associative Learning and Memory

14-3-3 is a family of regulatory proteins highly expressed in the brain. Previous invertebrate studies have demonstrated the importance of 14-3-3 in the regulation of synaptic functions and learning and memory. However, the in vivo role of 14-3-3 in these processes has not been determined using mammalian animal models. Here, we report the behavioral and electrophysiological characterization of a new animal model of 14-3-3 proteins. These transgenic mice, considered to be a 14-3-3 functional knock-out, express a known 14-3-3 inhibitor in various brain regions of different founder lines. We identify a founder-specific impairment in hippocampal-dependent learning and memory tasks, as well as a correlated suppression in long-term synaptic plasticity of the hippocampal synapses. Moreover, hippocampal synaptic NMDA receptor levels are selectively reduced in the transgenic founder line that exhibits both behavioral and synaptic plasticity deficits. Collectively, our findings provide evidence that 14-3-3 is a positive regulator of associative learning and memory at both the behavioral and cellular level.

Simvastatin Treatment Enhances NMDAR-Mediated Synaptic Transmission by Upregulating the Surface Distribution of the GluN2B Subunit

The ramifications of statins on plasma cholesterol and coronary heart disease have been well documented. However, there is increasing evidence that inhibition of the mevalonate pathway may provide independent neuroprotective and procognitive pleiotropic effects, most likely via inhibition of isoprenoids, mainly farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP). FPP and GGPP are the major donors of prenyl groups for protein prenylation. Modulation of isoprenoid availability impacts a slew of cellular processes including synaptic plasticity in the hippocampus. Our previous work has demonstrated that simvastatin (SV) administration improves hippocampus-dependent spatial memory, rescuing memory deficits in a mouse model of Alzheimer's disease. Treatment of hippocampal slices with SV enhances long-term potentiation (LTP), and this effect is dependent on the activation of Akt (protein kinase B). Further studies showed that SV-induced enhancement of hippocampal LTP is driven by depletion of FPP and inhibition of farnesylation. In the present study, we report the functional consequences of exposure to SV at cellular/synaptic and molecular levels. While application of SV has no effect on intrinsic membrane properties of CA1 pyramidal neurons, including hyperpolarization-activated cyclic-nucleotide channel-mediated sag potentials, the afterhyperpolarization (AHP), and excitability, SV application potentiates the N-methyl D-aspartate receptor (NMDAR)-mediated contribution to synaptic transmission. In mouse hippocampal slices and human neuronal cells, SV treatment increases the surface distribution of the GluN2B subunit of the NMDAR without affecting cellular cholesterol content. We conclude that SV-induced enhancement of synaptic plasticity in the hippocampus is likely mediated by augmentation of synaptic NMDAR components that are largely responsible for driving synaptic plasticity in the CA1 region.

(301) CK2 mediates increased synaptic NMDA receptor activity and pain hypersensitivity induced by calcineurin inhibitor

(301) CK2 mediates increased synaptic NMDA receptor activity and pain hypersensitivity induced by calcineurin inhibitor

Differential Regulation of GABAB Receptor Trafficking by Different Modes of N-methyl-d-aspartate (NMDA) Receptor Signaling

Inhibitory GABAB receptors (GABABRs) can down-regulate most excitatory synapses in the CNS by reducing postsynaptic excitability. Functional GABABRs are heterodimers of GABAB1 and GABAB2 subunits and here we show that the trafficking and surface expression of GABABRs is differentially regulated by synaptic or pathophysiological activation of NMDA receptors (NMDARs). Activation of synaptic NMDARs using a chemLTP protocol increases GABABR recycling and surface expression. In contrast, excitotoxic global activation of synaptic and extrasynaptic NMDARs by bath application of NMDA causes the loss of surface GABABRs. Intriguingly, exposing neurons to extreme metabolic stress using oxygen/glucose deprivation (OGD) increases GABAB1 but decreases GABAB2 surface expression. The increase in surface GABAB1 involves enhanced recycling and is blocked by the NMDAR antagonist AP5. The decrease in surface GABAB2 is also blocked by AP5 and by inhibiting degradation pathways. These results indicate that NMDAR activity is critical in GABABR trafficking and function and that the individual subunits can be separately controlled to regulate neuronal responsiveness and survival.

Synaptically Induced Long-Term Modulation of Electrical Coupling in the Inferior Olive

SummaryElectrical coupling mediated by gap junctions is widespread in the mammalian CNS, and the interplay between chemical and electrical synapses on the millisecond timescale is crucial for determining patterns of synchrony in many neural circuits. Here we show that activation of glutamatergic synapses drives long-term depression of electrical coupling between neurons of the inferior olive. We demonstrate that this plasticity is not triggered by postsynaptic spiking alone and that it requires calcium entry following synaptic NMDA receptor activation. These results reveal that glutamatergic synapses can instruct plasticity at electrical synapses, providing a means for excitatory inputs to homeostatically regulate the long-term dynamics of microzones in olivocerebellar circuits.

GluN2B and GluN2D NMDARs dominate synaptic responses in the adult spinal cord

The composition of the postsynaptic ionotropic receptors that receive presynaptically released transmitter is critical not only for transducing and integrating electrical signals but also for coordinating downstream biochemical signaling pathways. At glutamatergic synapses in the adult CNS an overwhelming body of evidence indicates that the NMDA receptor (NMDAR) component of synaptic responses is dominated by NMDARs containing the GluN2A subunit, while NMDARs containing GluN2B, GluN2C, or GluN2D play minor roles in synaptic transmission. Here, we discovered NMDAR-mediated synaptic responses with characteristics not described elsewhere in the adult CNS. We found that GluN2A-containing receptors contribute little to synaptic NMDAR responses while GluN2B dominates at synapses of lamina I neurons in the adult spinal cord. In addition, we provide evidence for a GluN2D-mediated synaptic NMDAR component in adult lamina I neurons. Strikingly, the charge transfer mediated by GluN2D far exceeds that of GluN2A and is comparable to that of GluN2B. Lamina I forms a distinct output pathway from the spinal pain processing network to the pain networks in the brain. The GluN2D-mediated synaptic responses we have discovered in lamina I neurons provide the molecular underpinning for slow, prolonged and feedforward amplification that is a fundamental characteristic of pain.