Lack of NMDA receptor subtype selectivity for hippocampal long-term potentiation

Differential regulation of AMPA receptor GluA1 phosphorylation at serine 831 and 845 associated with activation of NMDA receptor subpopulations

AMPA receptors and NMDA receptors are the main subtypes of ionotropic glutamate receptors in the vertebrate central nervous system. Accumulating evidence demonstrates that two serine sites, S831 and S845, on the AMPA receptor GluA1 subunit, are phosphorylation-regulated and profoundly involved in NMDA receptor-dependent synaptic plasticity. On the other hand, recent studies have revealed distinct functional consequences of activating synaptic or extrasynaptic NMDA receptors, or of activating GluN2A- or GluN2B-containing NMDA receptors. Therefore, it is essential to determine how phosphorylation of the GluA1 at S831 and S845 is regulated by NMDA receptor subpopulations. In this study, we demonstrated transiently increased phosphorylation of GluA1 at S831 and persistently decreased phosphorylation of GluA1 at S845 by bath application of NMDA to hippocampal slices from rats. Interestingly, we also found a differential regulation of phosphorylation of GluA1 at S831 and S845 by activation of NMDA receptor subpopulations: the synaptic and/or the GluN2A-containing NMDA receptors were more likely to mediate up-regulation of GluA1 phosphorylation at S831 and down-regulation of GluA1 phosphorylation at S845, while the extrasynaptic NMDA receptors down-regulated GluA1 phosphorylation at S831. Taken together, our results suggest the NMDA receptor subpopulations differentially regulate GluA1 phosphorylation, which may contribute to NMDA receptor-dependent synaptic plasticity.

Cyclophosphamide induces NR2B phosphorylation-dependent facilitation on spinal reflex potentiation

It is well-established that cyclophosphamide (CYP) can sensitize the pelvic afferent nerve arising from the urinary bladder and therefore induce suprapubic pain. To test the possibility that CYP might mediate the development of visceral hypereflexia/hyperalgesia by facilitating spinal activity-dependent neural plasticity, we compared the pelvic-urethra reflex activity and spinal N-methyl-d-aspartate receptor NR2B subunit (NR2B) phosphorylation in rats treated with vehicle solution and CYP. Compared with vehicle solution, when accompanied by upregulation of phosphorylated NR2B expression in the lumbosacral (L6–S2) dorsal horn, CYP increased the evoked spikes in spinal reflex potentiation induced by repetitive stimulation (1 stimulation/1 s). Moreover, intraperitoneal pretreatments with NG-nitro-l-arginine methyl ester and roscovitine, nitric oxide synthase and cyclin-dependent protein kinase 5 (Cdk5) antagonists, respectively, overwrote CYP-enhanced reflex potentiation and NR2B phosphorylation. When compared with the untreated group, the treatment with small-interfering RNA of NR2B, which decreased the expression of NR2B expression, abolished CYP-dependent reflex facilitation and spinal NR2B phosphorylation. These results suggested that CYP might facilitate spinal reflex potentiation mediated by N-methyl-d-aspartate receptors and participate in the development of visceral hypereflexia/hyperalgesia through nitric oxide- and Cdk5-dependent NR2B phosphorylation at the lumbosacral dorsal horn.

Kv4.2 block of long-term potentiation is partially dependent on synaptic NMDA receptor remodeling

Proper expression of synaptic NMDA receptors (NMDARs) is necessary to regulate synaptic Ca²(+) influx and the induction the long-term potentiation (LTP) in the mammalian hippocampus. Previously we reported that expressing the A-type K(+) channel subunit Kv4.2 in CA1 neurons of organotypic slice cultures reduced synaptic NR2B-containing NMDAR expression and completely blocked LTP induced by a pairing protocol. As pretreatment with an NMDAR antagonist (APV) overnight blocked the reduction of NR2B-containing receptors in neurons expressing EGFP-labeled Kv4.2 (Kv4.2g), we hypothesized that LTP would be rescued in Kv4.2g neurons by overnight treatment with APV. We report here that the overnight APV pretreatment in Kv4.2g-expressing neurons only partially restored potentiation. This partial potentiation was completely blocked by inhibition of the CAMKII kinase. These results indicate that A-type K(+) channels must regulate synaptic integration and plasticity through another mechanism in addition to their regulation of synaptic NR2 subunit composition. We suggest that dendritic excitability, which is regulated by Kv4.2 expression, also contributes to synaptic plasticity.

Immunogold electron microscopic evidence of differential regulation of GluN1, GluN2A, and GluN2B, NMDA-type glutamate receptor subunits in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Benzodiazepine withdrawal-anxiety is associated with enhanced α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)-mediated glutamatergic transmission in rat hippocampal CA1 synapses due to enhanced synaptic insertion and phosphorylation of GluA1 homomers. Interestingly, attenuation of withdrawal-anxiety is associated with a reduction in N-methyl-D-aspartate receptor (NMDAR)-mediated currents and subunit expression, secondary to AMPA receptor potentiation. Therefore, in this study ultrastructural evidence for possible reductions in NMDAR GluN1, GluN2A, and GluN2B subunits was sought at CA1 stratum radiatum synapses in proximal dendrites using postembedding immunogold labeling of tissues from rats withdrawn for 2 days from 1-week daily oral administration of the benzodiazepine, flurazepam (FZP). GluN1-immunogold density and the percentage of immunopositive synapses were significantly decreased in tissues from FZP-withdrawn rats. Similar decreases were observed for GluN2B subunits; however, the relative lateral distribution of GluN2B-immunolabeling within the postsynaptic density did not change after BZ withdrawal. In contrast to the GluN2B subunit, the percentage of synapses labeled with the GluN2A subunit antibody and the density of immunogold labeling for this subunit was unchanged. The spatial localization of immunogold particles associated with each NMDAR subunit was consistent with a predominantly postsynaptic localization. The data therefore provide direct evidence for reduced synaptic GluN1/GluN2B receptors and preservation of GluN1/GluN2A receptors in the CA1 stratum radiatum region during BZ withdrawal. Based on collective findings in this benzodiazepine withdrawal-anxiety model, we propose a functional model illustrating the changes in glutamate receptor populations at excitatory synapses during benzodiazepine withdrawal.

The balance between excitation and inhibition and functional sensory processing in the somatosensory cortex

The balance between excitation and inhibition (E/I balance) is tightly regulated in adult cortices to maintain proper nervous system function. Disturbed E/I balance is associated with numerous neuropsychological disorders, such as autism, epilepsy and schizophrenia. The present review will discuss aspects of Hebbian and homeostatic mechanisms regulating excitatory and inhibitory balance related to sensory processing in somatosensory cortex of rodents. Additionally, changes in the E/I balance during sensory manipulation will be discussed.

Hippocampal synaptic metaplasticity requires the activation of NR2B-containing NMDA receptors

The potential to exhibit synaptic plasticity itself is modulated by previous synaptic activity, which has been termed as metaplasticity. In this paper, we demonstrated that the activation of N-methyl-d-aspartate (NMDA) receptor 2B (NR2B) subunit in NNDA receptors was required for hippocampal metaplasticity at Schaffer collateral-commissural fiber-CA1 synapses. Brief 5 Hz priming stimulation did not cause long-term synaptic plasticity; however, it could result in the inhibition of subsequently evoked long-term potentiation (LTP). Meanwhile, the application of selective antagonists for NR2B subunit of NMDA receptors after delivering priming stimulation could block the metaplasticity. In contrast, LTP induction was not affected by NR2B antagonists in slices without pre-treatment of priming stimulation. These results indicated that the activation of NR2B-containing NMDA receptors was required for metaplasticity.

Synaptic neurotransmission depression in ventral tegmental dopamine neurons and cannabinoid-associated addictive learning

Drug addiction is an association of compulsive drug use with long-term associative learning/memory. Multiple forms of learning/memory are primarily subserved by activity- or experience-dependent synaptic long-term potentiation (LTP) and long-term depression (LTD). Recent studies suggest LTP expression in locally activated glutamate synapses onto dopamine neurons (local Glu-DA synapses) of the midbrain ventral tegmental area (VTA) following a single or chronic exposure to many drugs of abuse, whereas a single exposure to cannabinoid did not significantly affect synaptic plasticity at these synapses. It is unknown whether chronic exposure of cannabis (marijuana or cannabinoids), the most commonly used illicit drug worldwide, induce LTP or LTD at these synapses. More importantly, whether such alterations in VTA synaptic plasticity causatively contribute to drug addictive behavior has not previously been addressed. Here we show in rats that chronic cannabinoid exposure activates VTA cannabinoid CB1 receptors to induce transient neurotransmission depression at VTA local Glu-DA synapses through activation of NMDA receptors and subsequent endocytosis of AMPA receptor GluR2 subunits. A GluR2-derived peptide blocks cannabinoid-induced VTA synaptic depression and conditioned place preference, i.e., learning to associate drug exposure with environmental cues. These data not only provide the first evidence, to our knowledge, that NMDA receptor-dependent synaptic depression at VTA dopamine circuitry requires GluR2 endocytosis, but also suggest an essential contribution of such synaptic depression to cannabinoid-associated addictive learning, in addition to pointing to novel pharmacological strategies for the treatment of cannabis addiction.

EphrinB2 induces pelvic-urethra reflex potentiation via Src kinase-dependent tyrosine phosphorylation of NR2B

Recently, the role of EphB receptor (EphBR) tyrosine kinase and their ephrinB ligands in pain-related neural plasticity at the spinal cord level have been identified. To test whether Src-family tyrosine kinase-dependent glutamatergic N-methyl-d-aspartate receptor NR2B subunit phosphorylation underlies lumbosacral spinal EphBR activation to mediate pelvic-urethra reflex potentiation, we recorded external urethra sphincter electromyogram reflex activity and analyzed protein expression in the lumbosacral (L(6)-S(2)) dorsal horn in response to intrathecal ephrinB2 injections. When compared with vehicle solution, exogenous ephrinB2 (5 μg/rat it)-induced reflex potentiation, in associated with phosphorylation of EphB1/2, Src-family kinase, NR2B Y1336 and Y1472 tyrosine residues. Both intrathecal EphB1 and EphB2 immunoglobulin fusion protein (both 10 μg/rat it) prevented ephrinB2-dependent reflex potentiation, as well as protein phosphorylation. Pretreatment with PP2 (50 μM, 10 μl it), an Src-family kinase antagonist, reversed the reflex potentiation, as well as Src kinase and NR2B phosphorylation. Together, these results suggest the ephrinB2-dependent EphBR activation, which subsequently provokes Src kinase-mediated N-methyl-d-aspartate receptor NR2B phosphorylation in the lumbosacral dorsal horn, is crucial for the induction of spinal reflex potentiation contributing to the development of visceral pain and/or hyperalgesia in the pelvic area.

Role of the NR2A/2B subunits of the N-methyl-D-aspartate receptor in glutamate-induced glutamic acid decarboxylase alteration in cortical GABAergic neurons in vitro

The vulnerability of brain neuronal cell subpopulations to neurologic insults varies greatly. Among cells that survive a pathological insult, for example ischemia or brain trauma, some may undergo morphological and/or biochemical changes that may compromise brain function. The present study is a follow-up of our previous studies that investigated the effect of glutamate-induced excitotoxicity on the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67)'s expression in surviving DIV 11 cortical GABAergic neurons in vitro [Monnerie and Le Roux, (2007) Exp Neurol 205:367-382, (2008) Exp Neurol 213:145-153]. An N-methyl-D-aspartate receptor (NMDAR)-mediated decrease in GAD expression was found following glutamate exposure. Here we examined which NMDAR subtype(s) mediated the glutamate-induced change in GAD protein levels. Western blotting techniques on cortical neuron cultures showed that glutamate's effect on GAD proteins was not altered by NR2B-containing diheteromeric (NR1/NR2B) receptor blockade. By contrast, blockade of triheteromeric (NR1/NR2A/NR2B) receptors fully protected against a decrease in GAD protein levels following glutamate exposure. When receptor location on the postsynaptic membrane was examined, extrasynaptic NMDAR stimulation was observed to be sufficient to decrease GAD protein levels similar to that observed after glutamate bath application. Blocking diheteromeric receptors prevented glutamate's effect on GAD proteins after extrasynaptic NMDAR stimulation. Finally, NR2B subunit examination with site-specific antibodies demonstrated a glutamate-induced, calpain-mediated alteration in NR2B expression. These results suggest that glutamate-induced excitotoxic NMDAR stimulation in cultured GABAergic cortical neurons depends upon subunit composition and receptor location (synaptic vs. extrasynaptic) on the neuronal membrane. Biochemical alterations in surviving cortical GABAergic neurons in various disease states may contribute to the altered balance between excitation and inhibition that is often observed after injury.

In vivo chronic intermittent ethanol exposure reverses the polarity of synaptic plasticity in the nucleus accumbens shell

Glutamatergic synaptic plasticity in the nucleus accumbens (NAc) is implicated in response to sensitization to psychomotor-stimulating agents, yet ethanol effects here are undefined. We studied the acute in vitro and in vivo effects of ethanol in medium spiny neurons from the shell NAc subregion of slices of C57BL/6 mice by using whole-cell voltage-clamp recordings of α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excitatory postsynaptic current (EPSCs). Synaptic conditioning (low-frequency stimulation with concurrent postsynaptic depolarization) reliably depressed AMPA EPSCs by nearly 30%; this accumbal long-term depression (LTD) was blocked by a nonselective N-methyl-D-aspartate (NMDA) receptor antagonist (DL-2-amino-5-phosphonovaleric acid) and a selective NMDA receptor 2B antagonist [R-(R*,S*)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidine propanol]. Acute ethanol exposure inhibited the depression of AMPA EPSCs differentially with increasing concentrations, but this inhibitory action of ethanol was occluded by a D1-selective dopamine receptor agonist. Ethanol dependence was elicited in C57BL/6 mice by two separate 4-day bouts of chronic intermittent ethanol (CIE) vapor exposure. When assessed 24 h after a single bout of in vivo CIE vapor exposure, NAc LTD was absent, and instead NMDA receptor-dependent synaptic potentiation [long-term potentiation (LTP)] was reliably observed. It is noteworthy that both LTP and LTD were completely absent after an extended withdrawal (72 h) after a single 3-day CIE vapor bout. These observations demonstrate that 1) accumbal synaptic depression is mediated by NR2B receptors, 2) accumbal synaptic depression is highly sensitive to both acute and chronic ethanol exposure, and 3) alterations in this synaptic process may constitute a neural adaptation that contributes to the induction and/or expression of ethanol dependence.

Synaptic versus extrasynaptic NMDA receptor signalling: implications for neurodegenerative disorders

There is a long-standing paradox that NMDA (N-methyl-D-aspartate) receptors (NMDARs) can both promote neuronal health and kill neurons. Recent studies show that NMDAR-induced responses depend on the receptor location: stimulation of synaptic NMDARs, acting primarily through nuclear Ca(2+) signalling, leads to the build-up of a neuroprotective 'shield', whereas stimulation of extrasynaptic NMDARs promotes cell death. These differences result from the activation of distinct genomic programmes and from opposing actions on intracellular signalling pathways. Perturbations in the balance between synaptic and extrasynaptic NMDAR activity contribute to neuronal dysfunction in acute ischaemia and Huntington's disease, and could be a common theme in the aetiology of neurodegenerative diseases. Neuroprotective therapies should aim to both enhance the effect of synaptic activity and disrupt extrasynaptic NMDAR-dependent death signalling.

Hippocampal long-term depression is required for the consolidation of spatial memory

Although NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) and long-term depression (LTD) of glutamatergic transmission are candidate mechanisms for long-term spatial memory, the precise contributions of LTP and LTD remain poorly understood. Here, we report that LTP and LTD in the hippocampal CA1 region of freely moving adult rats were prevented by NMDAR 2A (GluN2A) and 2B subunit (GluN2B) preferential antagonists, respectively. These results strongly suggest that NMDAR subtype preferential antagonists are appropriate tools to probe the roles of LTP and LTD in spatial memory. Using a Morris water maze task, the LTP-blocking GluN2A antagonist had no significant effect on any aspect of performance, whereas the LTD-blocking GluN2B antagonist impaired spatial memory consolidation. Moreover, similar spatial memory deficits were induced by inhibiting the expression of LTD with intrahippocampal infusion of a short peptide that specifically interferes with AMPA receptor endocytosis. Taken together, our findings support a functional requirement of hippocampal CA1 LTD in the consolidation of long-term spatial memory.

Cocaine-induced homeostatic regulation and dysregulation of nucleus accumbens neurons

Homeostatic response is an endowed self-correcting/maintaining property for living units, ranging from subcellular domains, single cells, and organs to the whole organism. Homeostatic responses maintain stable function through the ever-changing internal and external environments. In central neurons, several forms of homeostatic regulation have been identified, all of which tend to stabilize the functional output of neurons toward their prior "set-point." Medium spiny neurons (MSNs) within the forebrain region the nucleus accumbens (NAc) play a central role in gating/regulating emotional and motivational behaviors including craving and seeking drugs of abuse. Exposure to highly salient stimuli such as cocaine administration not only acutely activates a certain population of NAc MSNs, but also induces long-lasting changes in these neurons. It is these long-lasting cellular alterations that are speculated to mediate the increasingly strong cocaine-craving and cocaine-seeking behaviors. Why do the potentially powerful homeostatic mechanisms fail to correct or compensate for these drug-induced maladaptations in neurons? Based on recent experimental results, this review proposes a hypothesis of homeostatic dysregulation induced by exposure to cocaine. Specifically, we hypothesize that exposure to cocaine generates false molecular signals which misleads the homeostatic regulation process, resulting in maladaptive changes in NAc MSNs. Thus, many molecular and cellular alterations observed in the addicted brain may indeed result from homeostatic dysregulation. This review is among the first to introduce the concept of homeostatic neuroplasticity to understanding the molecular and cellular maladaptations following exposure to drugs of abuse.

Specific involvement of postsynaptic GluN2B-containing NMDA receptors in the developmental elimination of corticospinal synapses

The GluN2B (GluRepsilon2/NR2B) and GluN2A (GluRepsilon1/NR2A) NMDA receptor (NMDAR) subtypes have been differentially implicated in activity-dependent synaptic plasticity. However, little is known about the respective contributions made by these two subtypes to developmental plasticity, in part because studies of GluN2B KO [Grin2b(-/-) (2b(-/-))] mice are hampered by early neonatal mortality. We previously used in vitro slice cocultures of rodent cerebral cortex (Cx) and spinal cord (SpC) to show that corticospinal (CS) synapses, once present throughout the SpC, are eliminated from the ventral side during development in an NMDAR-dependent manner. To study subtype specificity of NMDAR in this developmental plasticity, we cocultured Cx and SpC slices derived from postnatal day 0 (P0) animals with different genotypes [2b(-/-), Grin2a(-/-) (2a(-/-)), or WT mice]. The distribution of CS synapses was studied electrophysiologically and with a voltage-sensitive dye. Synapse elimination on the ventral side was blocked in WT(Cx)-2b(-/-)(SpC) pairs but not in WT(Cx)-2a(-/-)(SpC) or 2b(-/-)(Cx)-WT(SpC) pairs. CS axonal regression was also observed through live imaging of CS axons labeled with enhanced yellow fluorescent protein (EYFP) through exo utero electroporation. These findings suggest that postsynaptic GluN2B is selectively involved in CS synapse elimination. In addition, the elimination was not blocked in 2a(-/-) SpC slices, where Ca(2+) entry through GluN2B-mediated CS synaptic currents was reduced to the same level as in 2b(-/-) slices, suggesting that the differential effect of GluN2B and GluN2A in CS synapse elimination might not be explained based solely on greater Ca(2+) entry through GluN2B-containing channels.

Long-term potentiation in the CA1 hippocampus induced by NR2A subunit-containing NMDA glutamate receptors is mediated by Ras-GRF2/Erk map kinase signaling

Background

NMDA-type glutamate receptors (NMDARs) are major contributors to long-term potentiation (LTP), a form of synaptic plasticity implicated in the process of learning and memory. These receptors consist of calcium-permeating NR1 and multiple regulatory NR2 subunits. A majority of studies show that both NR2A and NR2B-containing NMDARs can contribute to LTP, but their unique contributions to this form of synaptic plasticity remain poorly understood.

Methodology/Principal Findings

In this study, we show that NR2A and NR2B-containing receptors promote LTP differently in the CA1 hippocampus of 1-month old mice, with the NR2A receptors functioning through Ras-GRF2 and its downstream effector, Erk Map kinase, and NR2B receptors functioning independently of these signaling molecules.

Conclusions/Significance

This study demonstrates that NR2A-, but not NR2B, containing NMDA receptors induce LTP in pyramidal neurons of the CA1 hippocamus from 1 month old mice through Ras-GRF2 and Erk. This difference add new significance to the observation that the relative levels of these NMDAR subtypes is regulated in neurons, such that NR2A-containing receptors become more prominent late in postnatal development, after sensory experience and synaptic activity.

Metabotropic glutamate receptors in the trafficking of ionotropic glutamate and GABA(A) receptors at central synapses

The trafficking of ionotropic glutamate (AMPA, NMDA and kainate) and GABA(A) receptors in and out of, or laterally along, the postsynaptic membrane has recently emerged as an important mechanism in the regulation of synaptic function, both under physiological and pathological conditions, such as information processing, learning and memory formation, neuronal development, and neurodegenerative diseases. Non-ionotropic glutamate receptors, primarily group I metabotropic glutamate receptors (mGluRs), co-exist with the postsynaptic ionotropic glutamate and GABA(A) receptors. The ability of mGluRs to regulate postsynaptic phosphorylation and Ca(2+) concentration, as well as their interactions with postsynaptic scaffolding/signaling proteins, makes them well suited to influence the trafficking of ionotropic glutamate and GABA(A) receptors. Recent studies have provided insights into how mGluRs may impose such an influence at central synapses, and thus how they may affect synaptic signaling and the maintenance of long-term synaptic plasticity. In this review we will discuss some of the recent progress in this area: i) long-term synaptic plasticity and the involvement of mGluRs; ii) ionotropic glutamate receptor trafficking and long-term synaptic plasticity; iii) the involvement of postsynaptic group I mGluRs in regulating ionotropic glutamate receptor trafficking; iv) involvement of postsynaptic group I mGluRs in regulating GABA(A) receptor trafficking; v) and the trafficking of postsynaptic group I mGluRs themselves.

Plasticity of synaptic GluN receptors is required for the Src-dependent induction of long-term potentiation at CA3-CA1 synapses

The induction of long-term potentiation (LTP) of CA3-CA1 synapses requires activation of postsynaptic N-methyl-D-aspartate receptors (GluNRs). At resting potential, the contribution of GluNRs is limited by their voltage-dependent block by extracellular Mg(2+). High-frequency afferent stimulation is required to cause sufficient summation of excitatory synaptic potentials (EPSPs) to relieve this block and to permit an influx of Ca(2+). It has been assumed that this relief of Mg(2+) block is sufficient for induction. We postulated that the induction of LTP also requires a Src-dependent plasticity of GluNRs. Using whole-cell recordings, LTP (GluARs) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors-EPSCS was induced by pairing postsynaptic depolarization with presynaptic stimulation. This LTP was both GluNR and Src-dependent, being sensitive to AP-5, a GluNR selective antagonist, or to SU6656, a Src-selective inhibitor. When CNQX was used to block all GluARs, we observed a long-lasting potentiation of GluNR-mediated EPSCs. This plasticity was prevented by transiently blocking GluNRs during the induction protocol or by chelating intracellular Ca(2+). GluNRs plasticity was also prevented by bath applications of SU6656 or intracellular applications of the Src-selective inhibitory peptide, Src(40-58). It was also blocked by preventing activation of protein kinase C, a kinase that is upstream of Src-kinase-dependent regulation of GluNRs. Both GluN2A and GluN2B receptors were found to contribute to the plasticity of GluNRs. The contribution of GluNRs and, in particular, their plasticity to the maintenance of LTP was explored using AP5 and SU6656, respectively. When applied >20 min after induction neither drug influenced the magnitude of LTP. However, when applied immediately after induction, treatment with either drug caused the initial magnitude of LTP to progressively decrease to a sustained phase of reduced amplitude. Collectively, our findings suggest that GluNR plasticity, although not strictly required for induction, is necessary for the maintenance of a nondecrementing component of LTP.

NR2B-NMDA receptor-mediated increases in intracellular Ca2+ concentration regulate the tyrosine phosphatase, STEP, and ERK MAP kinase signaling

NMDA receptors regulate both the activation and inactivation of the extracellular signal-regulated kinase (ERK) signaling cascade, a key pathway involved in neuronal plasticity and survival. This bi-directional regulation of ERK activity by NMDA receptors has been attributed to opposing actions of NR2A- versus NR2B-containing NMDA receptors, but how this is implemented is not understood. Here, we show that glutamate-mediated intracellular Ca(2+) increases occur in two phases, a rapid initial increase followed by a delayed larger increase. Both phases of the Ca(2+) increase were blocked by MK-801, a non-selective NMDA receptor inhibitor. On the other hand, selective inhibition of NR2B-NMDA receptors by Ifenprodil or Ro 25-6981 blocked the delayed larger phase but had only a small effect on the rapid initial increase. The rapid initial increase in Ca(2+), presumably because of NR2A-NMDAR activation, was sufficient to activate ERK, whereas the large delayed increases in Ca(2+) mediated by NR2B-NMDARs were necessary for dephosphorylation and subsequent activation of striatal-enriched phosphatase, a neuron-specific tyrosine phosphatase that in turn mediates the dephosphorylation and inactivation of ERK. We conclude that the magnitude of Ca(2+) increases mediated through NR2B-NMDA receptors plays a critical role in the regulation of the serine/threonine and tyrosine kinases and phosphatases that are involved in the regulation of ERK activity.

Oligomerized Abeta25-35 induces increased tyrosine phosphorylation of NMDA receptor subunit 2A in rat hippocampal CA1 subfield

Amyloid-beta peptide (Abeta) plays a causal role in the pathogenesis of Alzheimer's disease (AD). To elucidate the mechanisms underlying the over-activation of NMDA receptors in AD, we investigated the alteration of NR2A tyrosine phosphorylation after intracerebroventricular infusion of Abeta25-35 oligomers. Abeta25-35 treatment resulted in the elevated tyrosine phosphorylation of NR2A in rat hippocampal CA1 subfield and facilitated the interactions of NR2A or PSD-95 with Src kinases. PP2, a specific inhibitor of Src family protein tyrosine kinases (SrcPTKs), not only attenuated the Abeta25-35-induced increases in the tyrosine phosphorylation of NR2A and in the associations among Src, NR2A, and PSD-95, but also protected against neuronal loss in the CA1 region. Preapplication of a noncompetitive NMDA receptor antagonist amantadine, an NR2A-selective NMDA receptor antagonist NVP-AAM077, or an NR2B-selective NMDA receptor antagonist Ro25-6981 inhibited the increased tyrosine phosphorylation of NR2A and prevented the associations among Src, NR2A, and PSD-95, but Ro25-6981 had less contribution. These results suggest that the activation of NMDA receptors after Abeta treatment promotes the formation of NR2A-PSD-95-Src complex and thus increases the tyrosine phosphorylation of NR2A by Src kinases, which up-regulates the function of NMDA receptors. Such positive feedback mediates the Abeta-induced over-activation of NMDA receptors and is involved in neuronal impairment.

Burst-induced anti-Hebbian depression acts through short-term synaptic dynamics to cancel redundant sensory signals

Weakly electric fish can enhance the detection and localization of important signals such as those of prey in part by cancellation of redundant spatially diffuse electric signals due to, e.g., their tail bending. The cancellation mechanism is based on descending input, conveyed by parallel fibers emanating from cerebellar granule cells, that produces a negative image of the global low-frequency signals in pyramidal cells within the first-order electrosensory region, the electrosensory lateral line lobe (ELL). Here we demonstrate that the parallel fiber synaptic input to ELL pyramidal cell undergoes long-term depression (LTD) whenever both parallel fiber afferents and their target cells are stimulated to produce paired burst discharges. Paired large bursts (4-4) induce robust LTD over pre-post delays of up to +/-50 ms, whereas smaller bursts (2-2) induce weaker LTD. Single spikes (either presynaptic or postsynaptic) paired with bursts did not induce LTD. Tetanic presynaptic stimulation was also ineffective in inducing LTD. Thus, we have demonstrated a form of anti-Hebbian LTD that depends on the temporal correlation of burst discharge. We then demonstrated that the burst-induced LTD is postsynaptic and requires the NR2B subunit of the NMDA receptor, elevation of postsynaptic Ca(2+), and activation of CaMKIIbeta. A model incorporating local inhibitory circuitry and previously identified short-term presynaptic potentiation of the parallel fiber synapses further suggests that the combination of burst-induced LTD, presynaptic potentiation, and local inhibition may be sufficient to explain the generation of the negative image and cancellation of redundant sensory input by ELL pyramidal cells.

Loss of GluN2B-containing NMDA receptors in CA1 hippocampus and cortex impairs long-term depression, reduces dendritic spine density, and disrupts learning

NMDA receptors (NMDARs) are key mediators of certain forms of synaptic plasticity and learning. NMDAR complexes are heteromers composed of an obligatory GluN1 subunit and one or more GluN2 (GluN2A-GluN2D) subunits. Different subunits confer distinct physiological and molecular properties to NMDARs, but their contribution to synaptic plasticity and learning in the adult brain remains uncertain. Here, we generated mice lacking GluN2B in pyramidal neurons of cortex and CA1 subregion of hippocampus. We found that hippocampal principal neurons of adult GluN2B mutants had faster decaying NMDAR-mediated EPSCs than nonmutant controls and were insensitive to GluN2B but not NMDAR antagonism. A subsaturating form of hippocampal long-term potentiation (LTP) was impaired in the mutants, whereas a saturating form of LTP was intact. An NMDAR-dependent form of long-term depression (LTD) produced by low-frequency stimulation combined with glutamate transporter inhibition was abolished in the mutants. Additionally, mutants exhibited decreased dendritic spine density in CA1 hippocampal neurons compared with controls. On multiple assays for corticohippocampal-mediated learning and memory (hidden platform Morris water maze, T-maze spontaneous alternation, and pavlovian trace fear conditioning), mutants were impaired. These data further demonstrate the importance of GluN2B for synaptic plasticity in the adult hippocampus and suggest a particularly critical role in LTD, at least the form studied here. The finding that loss of GluN2B was sufficient to cause learning deficits illustrates the contribution of GluN2B-mediated forms of plasticity to memory formation, with implications for elucidating NMDAR-related dysfunction in disease-related cognitive impairment.

Distinct roles of NR2A and NR2B cytoplasmic tails in long-term potentiation

NMDA receptors (NMDARs) are critical mediators of activity-dependent synaptic plasticity, but the differential roles of NR2A- versus NR2B-containing NMDARs have been controversial. Here, we investigate the roles of NR2A and NR2B in long-term potentiation (LTP) in organotypic hippocampal slice cultures using RNA interference (RNAi) and overexpression, to complement pharmacological approaches. In young slices, when NR2B is the predominant subunit expressed, LTP is blocked by the NR2B-selective antagonist Ro25-6981 [R-(R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol]. As slices mature and NR2A expression rises, activation of NR2B receptors became no longer necessary for LTP induction. LTP was blocked, however, by RNAi knockdown of NR2B, and this was rescued by coexpression of an RNAi-resistant NR2B (NR2B*) cDNA. Interestingly, a chimeric NR2B subunit in which the C-terminal cytoplasmic tail was replaced by that of NR2A failed to rescue LTP, whereas the reverse chimera, NR2A channel with NR2B tail, was able to restore LTP. Thus, expression of NR2B with its intact cytoplasmic tail is required for LTP induction, at an age when channel activity of NR2B-NMDARs is not required for LTP. Overexpression of wild-type NR2A failed to rescue LTP in neurons transfected with the NR2B-RNAi construct, despite restoring NMDA-EPSC amplitude to a similar level as NR2B*. Surprisingly, an NR2A construct lacking its entire C-terminal cytoplasmic tail regained its ability to restore LTP. Together, these data suggest that the NR2B subunit plays a critical role for LTP, presumably by recruiting relevant molecules important for LTP via its cytoplasmic tail. In contrast, NR2A is not essential for LTP, and its cytoplasmic tail seems to carry inhibitory factors for LTP.

Downregulation of Kv4.2 channels mediated by NR2B-containing NMDA receptors in cultured hippocampal neurons

Somatodendritic Kv4.2 channels mediate transient A-type potassium currents (I(A)), and play critical roles in controlling neuronal excitability and modulating synaptic plasticity. Our studies have shown an NMDA receptor-dependent downregulation of Kv4.2 and I(A). NMDA receptors are heteromeric complexes of NR1 combined with NR2A-NR2D, mainly NR2A and NR2B. Here, we investigate NR2B receptor-mediated modulation of Kv4.2 and I(A) in cultured hippocampal neurons. Application of glutamate caused a reduction in total Kv4.2 protein levels and Kv4.2 clusters, and produced a hyperpolarized shift in the inactivation curve of I(A). The effects of glutamate on Kv4.2 and I(A) were inhibited by pretreatment of NR2B-selective antagonists. NR2B-containing NMDA receptors are believed to be located predominantly extrasynaptically. Like application of glutamate, selective activation of extrasynaptic NMDA receptors caused a reduction in total Kv4.2 protein levels and Kv4.2 clusters, which was also blocked by NR2B-selective antagonists. In contrast, specific stimulation of synaptic NMDA receptors had no effect on Kv4.2. In addition, the influx of Ca(2+) was essential for extrasynaptic modulation of Kv4.2. Calpain inhibitors prevented the reduction of total Kv4.2 protein levels following activation of extrasynaptic NMDA receptors. These results demonstrate that the glutamate-induced downregulation of Kv4.2 and I(A) is mediated by NR2B-containing NMDA receptors and is linked to proteolysis by calpain, which might contribute to the development of neuronal hyperexcitability and neurodegenerative diseases.

Bidirectional synaptic plasticity in response to single or paired pulse activation of NMDA receptors

It is still incompletely known how NMDA receptors (NMDA-R) regulate bidirectional synaptic plasticity. We examined this issue by an experimental protocol in which paired pulse stimulation (PPS) with 50ms interstimulus interval and basal frequency of 0.1Hz was applied to CA1 area of rat hippocampal slices during low Mg(2+) perfusion. Under blockade of NMDA-Rs by AP5, PPS for 12-60min led to only a minor depression. In contrast, when PPS was applied in the absence of AP5, there was a prominent short-term potentiation (STP), mainly of AMPA-R mediated responses, with peak at 1min and lasting 10-15min. The STP was followed by a slowly developing long-term depression (LTD). Applying AP5 during the STP, converted it to a stable increase relative to the control pathway. Following peak STP, plasticity was controlled in a composite manner. Whereas the initial decay was counteracted by NMDA-R activation, the following LTD was dependent on such activation. Our data suggest that synaptic changes do not only depend on the instantaneous, NMDA-dependent Ca(2+) concentration in the dendritic spine, but are also influenced by prior induction events. In addition to NMDA-R driven processes, passive relaxation contributes to the synaptic plasticity and in some cases outbalances the active control.

Conditional knockout of NMDA receptors in dopamine neurons prevents nicotine-conditioned place preference

Nicotine from smoking tobacco produces one of the most common forms of addictive behavior and has major societal and health consequences. It is known that nicotine triggers tobacco addiction by activating nicotine acetylcholine receptors (nAChRs) in the midbrain dopaminergic reward system, primarily via the ventral tegmental area. Heterogeneity of cell populations in the region has made it difficult for pharmacology-based analyses to precisely assess the functional significance of glutamatergic inputs to dopamine neurons in nicotine addiction. By generating dopamine neuron-specific NR1 knockout mice using cre/loxP-mediated method, we demonstrate that genetic inactivation of the NMDA receptors in ventral tegmental area dopamine neurons selectively prevents nicotine-conditioned place preference. Interestingly, the mutant mice exhibit normal performances in the conditioned place aversion induced by aversive air puffs. Therefore, this selective effect on addictive drug-induced reinforcement behavior suggests that NMDA receptors in the dopamine neurons are critical for the development of nicotine addiction.

Fractionating spatial memory with glutamate receptor subunit-knockout mice

In recent years, the contribution that different glutamate receptor subtypes and subunits make to spatial learning and memory has been studied extensively using genetically modified mice in which key proteins are knocked out. This has revealed dissociations between different aspects of spatial memory that were not previously apparent from lesion studies. For example, studies with GluA1 AMPAR [AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor] subunit-knockout mice have revealed the presence of a GluA1-dependent, non-associative short-term memory mechanism that is important for performance on spatial working memory tasks, and a GluA1-independent, long-term associative memory mechanism which underlies performance on spatial reference memory tasks. Within this framework we have also studied the contributions of different GluN2-containing NMDARs [NMDA (N-methyl-D-aspartate) receptors] to spatial memory. Studies with GluN2 NMDAR mutants have revealed different contributions from GluN2A- and GluN2B-containing NMDARs to spatial learning. Furthermore, comparison of forebrain- and hippocampus-specific GluN2B-knockout mice has demonstrated that both hippocampal and extra-hippocampal NMDARs make important contributions to spatial memory performance.

Neuronal transporters regulate glutamate clearance, NMDA receptor activation, and synaptic plasticity in the hippocampus

In the mammalian brain, the specificity of excitatory synaptic transmission depends on rapid diffusion of glutamate away from active synapses and the powerful uptake capacity of glutamate transporters in astrocytes. The extent to which neuronal glutamate transporters influence the lifetime of glutamate in the extracellular space remains unclear. Here we show that EAAC1, the predominant neuronal glutamate transporter at excitatory synapses in hippocampal area CA1, buffers glutamate released during synaptic events and prolongs the time course of its clearance by astrocytes. EAAC1 does not significantly alter activation of receptors in the synaptic cleft. Instead, it reduces recruitment of perisynaptic/extrasynaptic NR2B-containing NMDARs, thereby facilitating induction of long-term potentiation by short bursts of high-frequency stimulation. We describe novel roles of EAAC1 in regulating glutamate diffusion and propose that NMDARs at different subsynaptic locations can make distinct contributions to the regulation of synaptic strength.

Synaptic NR2A- but not NR2B-Containing NMDA Receptors Increase with Blockade of Ionotropic Glutamate Receptors

NMDA receptors (NMDAR) are key molecules involved in physiological and pathophysiological brain processes such as plasticity and excitotoxicity. Neuronal activity regulates NMDA receptor levels in the cell membrane. However, little is known on which time scale this regulation occurs and whether the two main diheteromeric NMDA receptor subtypes in forebrain, NR1/NR2A and NR1/NR2B, are regulated in a similar fashion. As these differ considerably in their electrophysiological properties, the NR2A/NR2B ratio affects the neurons’ reaction to NMDA receptor activation. Here we provide evidence that the basal turnover rate in the cell membrane of NR2A- and NR2B-containing receptors is comparable. However, the level of the NR2A subtype in the cell membrane is highly regulated by NMDA receptor activity, resulting in a several-fold increased insertion of new receptors after blocking NMDAR for 8 h. Blocking AMPA receptors also increases the delivery of NR2A-containing receptors to the cell membrane. In contrast, the amount of NR2B-containing receptors in the cell membrane is not affected by ionotropic glutamate receptor block. Moreover, electrophysiological analysis of synaptic currents in hippocampal cultures and CA1 neurons of hippocampal slices revealed that after 8 h of NMDA receptor blockade the NMDA EPSCs increase as a result of augmented NMDA receptor-mediated currents. In conclusion, synaptic NR2A- but not NR2B-containing receptors are dynamically regulated, enabling neurons to change their NR2A/NR2B ratio within a time scale of hours.

Genetic enhancement of memory and long-term potentiation but not CA1 long-term depression in NR2B transgenic rats

One major theory in learning and memory posits that the NR2B gene is a universal genetic factor that acts as rate-limiting molecule in controlling the optimal NMDA receptor's coincidence-detection property and subsequent learning and memory function across multiple animal species. If so, can memory function be enhanced via transgenic overexpression of NR2B in another species other than the previously reported mouse species? To examine these crucial issues, we generated transgenic rats in which NR2B is overexpressed in the cortex and hippocampus and investigated the role of NR2B gene in NMDA receptor-mediated synaptic plasticity and memory functions by combining electrophysiological technique with behavioral measurements. We found that overexpression of the NR2B subunit had no effect on CA1-LTD, but rather resulted in enhanced CA1-LTP and improved memory performances in novel object recognition test, spatial water maze, and delayed-to-nonmatch working memory test. Our slices recordings using NR2A- and NR2B-selective antagonists further demonstrate that the larger LTP in transgenic hippocampal slices was due to contribution from the increased NR2B-containing NMDARs. Therefore, our genetic experiments suggest that NR2B at CA1 synapses is not designated as a rate-limiting factor for the induction of long-term synaptic depression, but rather plays a crucial role in initiating the synaptic potentiation. Moreover, our studies provide strong evidence that the NR2B subunit represents a universal rate-limiting molecule for gating NMDA receptor's optimal coincidence-detection property and for enhancing memory function in adulthood across multiple mammalian species.

Reduced spatial learning in mice treated with NVP-AAM077 through down-regulating neurogenesis

NR2A-containing N-methyl-D-aspartate (NMDA) receptors have important roles in influencing the long-term potentiation and spatial memory. Here using Morris water maze, we found that inhibition of NR2A-containing NMDA receptors by [(R)-[(S)-1-(4-bromophenyl)-ethylamino]-(2, 3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) hindered the formation of spatial memory. An increasing number of reports suggest that adult hippocampal neurogenesis is involved in hippocampal-mediated learning. To explore the possible mechanisms understanding the reduced spatial memory by NVP-AAM077, we investigated the effects of NVP-AAM077 on neurogenesis. We found that NVP-AAM077 inhibited progenitor cells proliferation in the subventricular zone and dentate gyrus and reduced the survival of newborn cells in the dentate gyrus in the adult mice. In null mutant mice lacking neuronal nitric oxide synthase (nNOS) gene (nNOS(-/-)), the effects of NVP-AAM077 on neurogenesis disappeared. In addition, NVP-AAM077 increased nNOS enzymatic activity. Our findings suggest that NVP-AAM077 reduced spatial learning through down-regulating neurogenesis in the adult hippocampus.

Metaplastic regulation of long-term potentiation/long-term depression threshold by activity-dependent changes of NR2A/NR2B ratio

In vivo experience induces changes in synaptic NMDA receptor (NMDAR) subunit components, which are correlated with subsequent modifications of synaptic plasticity. However, little is known about how these subunit changes regulate the induction threshold of subsequent plasticity. At hippocampal Schaffer collateral-CA1 synapses, we first examined whether a recent history of neuronal activity could affect subsequent synaptic plasticity through its actions on NMDAR subunit components. We found that prior activity history produced by priming stimulations (PSs) across a wide range of frequencies (1-100 Hz) could induce bidirectional changes in the NR2A/NR2B ratio, which governs the threshold for subsequent long-term potentiation/long-term depression (LTP/LTD). Manipulating the NR2A/NR2B ratio through partial NR2 subunit blockade mimicked the PS regulation of the LTP/LTD threshold. Our results demonstrate that activity-dependent changes in the NR2A/NR2B ratio can be critical factors in metaplastic regulation of the LTP/LTD threshold.

NR2A at CA1 synapses is obligatory for the susceptibility of hippocampal plasticity to sleep loss

A loss in the necessary amount of sleep alters expression of genes and proteins implicated in brain plasticity, but key proteins that render neuronal circuits sensitive to sleep disturbance are unknown. We show that mild (4-6 h) sleep deprivation (SD) selectively augmented the number of NR2A subunits of NMDA receptors on postsynaptic densities of adult mouse CA1 synapses. The greater synaptic NR2A content facilitated induction of CA3-CA1 long-term depression in the theta frequency stimulation range and augmented the synaptic modification threshold. NR2A-knock-out mice maintained behavioral response to SD, including compensatory increase in post-deprivation resting time, but hippocampal synaptic plasticity was insensitive to sleep loss. After SD, the balance between synaptically activated and slowly recruited NMDA receptor pools during temporal summation was disrupted. Together, these results indicate that NR2A is obligatory for the consequences of sleep loss on hippocampal synaptic plasticity. These findings could advance pharmacological strategies aiming to sustain hippocampal function during sleep restriction.

NR2A/B-containing NMDA receptors mediate cocaine-induced synaptic plasticity in the VTA and cocaine psychomotor sensitization

Cocaine-induced modifications of glutamatergic synaptic transmission in the mesolimbic system play a key role in adaptations that promote addictive behaviors. In particular, the activation of ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR) in the ventral tegmental area (VTA) is critical for both cocaine-induced synaptic plasticity induced by a single cocaine injection and for the initiation of cocaine psychomotor sensitization. In this study, we set to determine whether the NR2 subunits of the NMDAR play a specific role in triggering cocaine-induced alterations in synaptic plasticity and the development of psychomotor sensitization. We found that inhibition of NR2A-containing NMDARs by NVP-AAM077, or NR2B-containing receptors by ifenprodil, blocked cocaine-induced increase in the AMPAR/NMDAR currents ratio, a measure of long-term potentiation (LTP) in vivo, in VTA neurons 24h following a single cocaine injection. Furthermore, inhibition of the NR2A subunit during the development of psychomotor sensitization attenuated the enhanced locomotor activity following repeated cocaine injections. Together, these results suggest that NR2-containing NMDA receptors play an important role in the machinery that triggers synaptic and behavioral adaptations to drugs of abuse such as cocaine.

Induction and expression of GluA1 (GluR-A)-independent LTP in the hippocampus

Long-term potentiation (LTP) at hippocampal CA3-CA1 synapses is thought to be mediated, at least in part, by an increase in the postsynaptic surface expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors induced by N-methyl-d-aspartate (NMDA) receptor activation. While this process was originally attributed to the regulated synaptic insertion of GluA1 (GluR-A) subunit-containing AMPA receptors, recent evidence suggests that regulated synaptic trafficking of GluA2 subunits might also contribute to one or several phases of potentiation. However, it has so far been difficult to separate these two mechanisms experimentally. Here we used genetically modified mice lacking the GluA1 subunit (Gria1(-/-) mice) to investigate GluA1-independent mechanisms of LTP at CA3-CA1 synapses in transverse hippocampal slices. An extracellular, paired theta-burst stimulation paradigm induced a robust GluA1-independent form of LTP lacking the early, rapidly decaying component characteristic of LTP in wild-type mice. This GluA1-independent form of LTP was attenuated by inhibitors of neuronal nitric oxide synthase and protein kinase C (PKC), two enzymes known to regulate GluA2 surface expression. Furthermore, the induction of GluA1-independent potentiation required the activation of GluN2B (NR2B) subunit-containing NMDA receptors. Our findings support and extend the evidence that LTP at hippocampal CA3-CA1 synapses comprises a rapidly decaying, GluA1-dependent component and a more sustained, GluA1-independent component, induced and expressed via a separate mechanism involving GluN2B-containing NMDA receptors, neuronal nitric oxide synthase and PKC.

NR2A-containing NMDA receptors are required for L-LTP induction and depotentiation in CA1 region of hippocampal slices

Long-term potentiation (LTP) is a well-characterized form of synaptic plasticity that fulfills many of the criteria for the neural correlate of memory. LTP reversal (or depotentiation, DP) is thought to correlate with prevention or elimination of memory storage. LTP during and immediately after induction can be easily reversed by afferent stimulation, when applied within the optimal time window. The aim of the present study was to determine whether later-phase LTP (L-LTP) could be reversed by special patterned stimulation applied at 2 h after LTP induction, as well as to characterize the receptor mechanisms underlying this reversal. Field excitatory postsynaptic potentials evoked by Schaffer collateral stimulation were recorded from the CA1 subfield of adult rat hippocampal slices. Results demonstrated that stable LTP, which was induced by six theta-burst stimulations, was mediated by NR2A-containing N-methyl-d-aspartate receptors (NMDARs). This L-LTP was partially reversed by high-intensity paired-pulse low-frequency stimulation (HI-PP-LFS) and was inhibited by Zn(2+) (30 nm), a voltage-independent NR2A-NMDAR antagonist. However, NR2B-NMDAR antagonists (Ro 25-6981, 1 mum) displayed no effect on L-LTP reversal. L-LTP partial reversal was also induced by HI-PP-LFS, when the protein synthesis inhibitors anisomycin (25 microm) and cycloheximide (60 microm) were applied following LTP induction. These results suggested that NR2A-containing NMDARs are required for L-LTP induction and DP in the hippocampal CA1 area of adult rats. Moreover, HI-PP-LFS was an effective stimulation pattern to induce DP.

Double dissociation of spike timing-dependent potentiation and depression by subunit-preferring NMDA receptor antagonists in mouse barrel cortex

Spike timing–dependent plasticity (STDP) is a strong candidate for an N-methyl-D-aspartate (NMDA) receptor-dependent form of synaptic plasticity that could underlie the development of receptive field properties in sensory neocortices. Whilst induction of timing-dependent long-term potentiation (t-LTP) requires postsynaptic NMDA receptors, timing-dependent long-term depression (t-LTD) requires the activation of presynaptic NMDA receptors at layer 4-to-layer 2/3 synapses in barrel cortex. Here we investigated the developmental profile of t-LTD at layer 4-to-layer 2/3 synapses of mouse barrel cortex and studied their NMDA receptor subunit dependence. Timing-dependent LTD emerged in the first postnatal week, was present during the second week and disappeared in the adult, whereas t-LTP persisted in adulthood. An antagonist at GluN2C/D subunit–containing NMDA receptors blocked t-LTD but not t-LTP. Conversely, a GluN2A subunit–preferring antagonist blocked t-LTP but not t-LTD. The GluN2C/D subunit requirement for t-LTD appears to be synapse specific, as GluN2C/D antagonists did not block t-LTD at horizontal cross-columnar layer 2/3-to-layer 2/3 synapses, which was blocked by a GluN2B antagonist instead. These data demonstrate an NMDA receptor subunit-dependent double dissociation of t-LTD and t-LTP mechanisms at layer 4-to-layer 2/3 synapses, and suggest that t-LTD is mediated by distinct molecular mechanisms at different synapses on the same postsynaptic neuron.

The effects of amyloid precursor protein on postsynaptic composition and activity

The amyloid precursor protein (APP) is cleaved to produce the Alzheimer disease-associated peptide Abeta, but the normal functions of uncleaved APP in the brain are unknown. We found that APP was present in the postsynaptic density of central excitatory synapses and coimmunoprecipitated with N-methyl-d-aspartate receptors (NMDARs). The presence of APP in the postsynaptic density was supported by the observation that NMDARs regulated trafficking and processing of APP; overexpression of the NR1 subunit increased surface levels of APP, whereas activation of NMDARs decreased surface APP and promoted production of Abeta. We transfected APP or APP RNA interference into primary neurons and used electrophysiological techniques to explore the effects of APP on postsynaptic function. Reduction of APP decreased (and overexpression of APP increased) NMDAR whole cell current density and peak amplitude of spontaneous miniature excitatory postsynaptic currents. The increase in NMDAR current by APP was due to specific recruitment of additional NR2B-containing receptors. Consistent with these findings, immunohistochemical experiments demonstrated that APP increased the surface levels and decreased internalization of NR2B subunits. These results demonstrate a novel physiological role of postsynaptic APP in enhancing NMDAR function.

The ratio of NR2A/B NMDA receptor subunits determines the qualities of ocular dominance plasticity in visual cortex

Bidirectional synaptic plasticity during development ensures that appropriate synapses in the brain are strengthened and maintained while inappropriate connections are weakened and eliminated. This plasticity is well illustrated in mouse visual cortex, where monocular deprivation during early postnatal development leads to a rapid depression of inputs from the deprived eye and a delayed strengthening of inputs from the non-deprived eye. The mechanisms that control these bidirectional synaptic modifications remain controversial. Here we demonstrate, both in vitro and in vivo, that genetic deletion or reduction of the NR2A NMDA receptor subunit impairs activity-dependent weakening of synapses and enhances the strengthening of synapses. Although brief monocular deprivation in juvenile WT mice normally causes a profound depression of the deprived-eye response without a change in the non-deprived eye response, NR2A-knockout mice fail to exhibit deprivation-induced depression and instead exhibit precocious potentiation of the non-deprived eye inputs. These data support the hypothesis that a reduction in the NR2A/B ratio during monocular deprivation is permissive for the compensatory potentiation of non-deprived inputs.

Colon mustard oil instillation induced cross-organ reflex sensitization on the pelvic-urethra reflex activity in rats

We investigated the participation of cyclin-dependent kinase-5 (Cdk5)-mediated N-methyl-D-aspartate receptor (NMDAR) NR2B subunit phosphorylation in cross-organ reflex sensitization caused by colon irritation. The external urethral sphincter electromyogram (EUSE) reflex activity evoked by the pelvic afferent nerve test stimulation (TS, 1 stimulation/30s) and protein expression in the spinal cord and dorsal root ganglion tissue (T13-L2 and L6-S2 ipsilateral to the stimulation) in response to colon mustard oil (MO) instillation were tested in anesthetized rats. When compared with a baseline reflex activity with a single action potential evoked by the TS before the administration of test agents, MO instillation into the descending colon sensitized the evoked activity characterized by elongated firing in the reflex activity in association with increased protein levels of Cdk5, PSD95, and phosphorylated NR2B (pNR2B) but not of total NR2B (tNR2B) in the spinal cord tissue. Both cross-organ reflex sensitization and increments in protein expression were reversed by intra-colonic pretreatments with ruthenium red (a non-selective transient receptor potential vanilloid, TRPV, antagonist), capsaizepine (a TRPV1-selective antagonist), lidocaine (a nerve conduction blocker) as well as by the intra-thecal pretreatment with APV (a NRMDR antagonist) Co-101244 (a NR2B-selective antagonist) and roscovitine (a Cdk5 antagonist). Moreover, compared with the control group, both the increase in pNR2B and the cross-organ reflex sensitization were attenuated in the si-RNA of NR2B rats. All these results suggested that Cdk-dependent NMDAR NR2B subunit phosphorylation mediates the development of cross-organ pelvic-urethra reflex sensitization caused by acute colon irritation which could possibly underlie the high concurrence of pelvic pain syndrome with irritable bowel syndrome.

Rapid, bidirectional remodeling of synaptic NMDA receptor subunit composition by A-type K+ channel activity in hippocampal CA1 pyramidal neurons

The transient, A-type K+ current (IA) controls the excitability of CA1 pyramidal neuron dendrites by regulating the back-propagation of action potentials and by shaping synaptic input. Dendritic A-type K+ channels are targeted for modulation during long-term potentiation (LTP) and we have recently shown that activity-dependent internalization of the A-type channel subunit Kv4.2 enhances synaptic currents. However, the effect of changes in IA on the ability to induce subsequent synaptic plasticity (metaplasticity) has not been investigated. Here, we show that altering functional Kv4.2 expression level leads to a rapid, bidirectional remodeling of CA1 synapses. Neurons exhibiting enhanced IA showed a decrease in relative synaptic NR2B/NR2A subunit composition and did not exhibit LTP. Conversely, reducing IA by expression of a Kv4.2 dominant-negative or through genomic knockout of Kv4.2 led to an increased fraction of synaptic NR2B/NR2A and enhanced LTP. Bidirectional synaptic remodeling was mimicked in experiments manipulating intracellular Ca2+ and dependent on spontaneous activation of NMDA receptors and CaMKII activity. Our data suggest that A-type K+ channels are an integral part of a synaptic complex that regulates Ca2+ signaling through spontaneous NMDAR activation to control synaptic NMDAR expression and plasticity.

The effects of NR2 subunit-dependent NMDA receptor kinetics on synaptic transmission and CaMKII activation

N-Methyl-D-aspartic acid (NMDA) receptors are widely expressed in the brain and are critical for many forms of synaptic plasticity. Subtypes of the NMDA receptor NR2 subunit are differentially expressed during development; in the forebrain, the NR2B receptor is dominant early in development, and later both NR2A and NR2B are expressed. In heterologous expression systems, NR2A-containing receptors open more reliably and show much faster opening and closing kinetics than do NR2B-containing receptors. However, conflicting data, showing similar open probabilities, exist for receptors expressed in neurons. Similarly, studies of synaptic plasticity have produced divergent results, with some showing that only NR2A-containing receptors can drive long-term potentiation and others showing that either subtype is capable of driving potentiation. In order to address these conflicting results as well as open questions about the number and location of functional receptors in the synapse, we constructed a Monte Carlo model of glutamate release, diffusion, and binding to NMDA receptors and of receptor opening and closing as well as a model of the activation of calcium-calmodulin kinase II, an enzyme critical for induction of synaptic plasticity, by NMDA receptor-mediated calcium influx. Our results suggest that the conflicting data concerning receptor open probabilities can be resolved, with NR2A- and NR2B-containing receptors having very different opening probabilities. They also support the conclusion that receptors containing either subtype can drive long-term potentiation. We also are able to estimate the number of functional receptors at a synapse from experimental data. Finally, in our models, the opening of NR2B-containing receptors is highly dependent on the location of the receptor relative to the site of glutamate release whereas the opening of NR2A-containing receptors is not. These results help to clarify the previous findings and suggest future experiments to address open questions concerning NMDA receptor function.

The role of NMDA receptor subtypes in short-term plasticity in the rat entorhinal cortex

We have previously shown that spontaneous release of glutamate in the entorhinal cortex (EC) is tonically facilitated via activation of presynaptic NMDA receptors (NMDAr) containing the NR2B subunit. Here we show that the same receptors mediate short-term plasticity manifested by frequency-dependent facilitation of evoked glutamate release at these synapses. Whole-cell patch-clamp recordings were made from layer V pyramidal neurones in rat EC slices. Evoked excitatory postsynaptic currents showed strong facilitation at relatively low frequencies (3 Hz) of activation. Facilitation was abolished by an NR2B-selective blocker (Ro 25-6981), but unaffected by NR2A-selective antagonists (Zn(2+), NVP-AAM077). In contrast, postsynaptic NMDAr-mediated responses could be reduced by subunit-selective concentrations of all three antagonists. The data suggest that NMDAr involved in presynaptic plasticity in layer V are exclusively NR1/NR2B diheteromers, whilst postsynaptically they are probably a mixture of NR1/NR2A, NR1/NR2B diheteromers and NR1/NR2A/NR2B triheteromeric receptors.