The distribution of splice variants of the NMDAR1 subunit mRNA in adult rat brain

Src dependency of the regulation of LTP by alternative splicing of GRIN1 exon 5

Alternative splicing of Grin1 exon 5 regulates induction of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses: LTP in mice lacking the GluN1 exon 5-encoded N1 cassette (GluN1a mice) is significantly increased compared with that in mice compulsorily expressing this exon (GluN1b mice). The mechanism underlying this difference is unknown. Here, we report that blocking the non-receptor tyrosine kinase Src prevents induction of LTP in GluN1a mice but not in GluN1b. We find that activating Src enhances pharmacologically isolated synaptic N-methyl-d-aspartate receptor (NMDAR) currents in GluN1a mice but not in GluN1b. Moreover, we observe that Src activation increases the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor component of Schaffer collateral-evoked excitatory post-synaptic potentials in GluN1a mice, but this increase is prevented by blocking NMDARs. We conclude that at these synapses, NMDARs in GluN1a mice are subject to upregulation by Src that mediates induction of LTP, whereas NMDARs in GluN1b mice are not regulated by Src, leading to Src-resistance of LTP. Thus, we have uncovered that a key regulatory mechanism for synaptic potentiation is gated by differential splicing of exon 5 of Grin1. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

NR1 Splicing Variant NR1a in Cerebellar Granule Neurons Constitutes a Better Motor Learning in the Mouse

As an excitatory neuron in the cerebellum, the granule cells play a crucial role in motor learning. The assembly of NMDAR in these neurons varies in developmental stages, while the significance of this variety is still not clear. In this study, we found that motor training could specially upregulate the expression level of NR1a, a splicing form of NR1 subunit. Interestingly, overexpression of this splicing variant in a cerebellar granule cell-specific manner dramatically elevated the NMDAR binding activity. Furthermore, the NR1a transgenic mice did not only show an enhanced motor learning, but also exhibit a higher efficacy for motor training in motor learning. Our results suggested that as a "junior" receptor, NR1a facilitates NMDAR activity as well as motor skill learning.

Alcohol exposure alters pre-mRNA splicing of antiapoptotic Mcl-1L isoform and induces apoptosis in neural progenitors and immature neurons

Alternative splicing and expression of splice variants of genes in the brain may lead to the modulation of protein functions, which may ultimately influence behaviors associated with alcohol dependence and neurotoxicity. We recently showed that ethanol exposure can lead to pre-mRNA missplicing of Mcl-1, a pro-survival member of the Bcl-2 family, by downregulating the expression levels of serine/arginine rich splicing factor 1 (SRSF1). Little is known about the physiological expression of these isoforms in neuronal cells and their role in toxicity induced by alcohol exposure during the developmental period. In order to investigate the impact of alcohol exposure on alternative splicing of Mcl-1 pre-mRNA and its role in neurotoxicity, we developed a unique primary human neuronal culture model where neurospheres (hNSPs), neural progenitors (hNPCs), immature neurons, and mature neurons were cultured from the matching donor fetal brain tissues. Our data suggest that neural progenitors and immature neurons are highly sensitive to the toxic effects of ethanol, while mature neuron cultures showed resistance to ethanol exposure. Further analysis of Mcl-1 pre-mRNA alternative splicing by semi-quantitative and quantitative analysis revealed that ethanol exposure causes a significant decrease in Mcl-1L/Mcl-1S ratio in a dose and time dependent manner in neural progenitors. Interestingly, ectopic expression of Mcl-1L isoform in neural progenitors was able to recover the viability loss and apoptosis induced by alcohol exposure. Altogether, these observations suggest that alternative splicing of Mcl-1 may play a crucial role in neurotoxicity associated with alcohol exposure in the developing fetal brain.

Alcohol-Mediated Missplicing of Mcl-1 Pre-mRNA is Involved in Neurotoxicity

BACKGROUND

Heavy and chronic ethanol (EtOH) exposure can cause significant structural and functional damage to the adult brain. The most devastating consequence of EtOH exposure is the neurotoxicity associated with the depletion of neurons. Regulation of splice variants in the brain can modulate protein functions, which may ultimately affect behaviors associated with alcohol dependence and EtOH-mediated neurotoxicity. As alcohol consumption is associated with neurotoxicity, it is possible that altered splicing of survival and pro-survival factors during the development of alcoholism may contribute to the neurotoxicity.

METHODS

Primary human neurons and a neuroblastoma cell line were exposed to different concentrations of EtOH for various time periods. Cell viability and neuronal marker expression were analyzed by MTT assay and immunoblotting, respectively. Effect of EtOH exposure on splicing regulatory protein expression and alternative splicing of candidate genes was analyzed by a biochemical approach. Transcriptional activity of serine/arginine-rich splicing factor 1 (SRSF1) gene was determined by reporter gene analysis.

RESULTS

Our results suggest that EtOH exposure to neuronal cells at 25 mM and higher concentrations are detrimental. In addition, EtOH exposure caused a dramatic reduction in SRSF1 expression levels. Furthermore, EtOH exposure led to pre-mRNA missplicing of Mcl-1, a pro-survival member of the Bcl-2 family, by down-regulating the expression levels of SRSF1. Moreover, ectopic expression of both SRSF1 and Mcl-1L isoform was able to recover EtOH-mediated neurotoxicity.

CONCLUSIONS

Our results suggest that EtOH exposure can lead to pre-mRNA missplicing of Mcl-1 in neuronal cells. Our results indicate that EtOH exposure of neurons leads to a decrease in the ratio of Mcl-1L/Mcl-1S by favoring pro-apoptotic Mcl-1S splicing over anti-apoptotic Mcl-1L isoform suggesting that Mcl-1S may play a crucial role in neurotoxicity associated with alcohol consumption.

Molecular and cellular dissection of NMDA receptor subtypes as antidepressant targets

A growing body of evidence supports the idea that drugs targeting the glutamate system may represent a valuable therapeutic alternative in major depressive disorders (MDD). The rapid and prolonged mood elevating effect of the NMDA receptor (NMDAR) antagonist ketamine has been studied intensely. However, its clinical use is hampered by deleterious side-effects, such as psychosis. Therefore, a better understanding of the mechanisms of the psychotropic effects after NMDAR blockade is necessary to develop glutamatergic antidepressants with improved therapeutic profile. Here we review recent experimental data that addressed molecular/cellular determinants of the antidepressant effect mediated by inactivating NMDAR subtypes. We refer to results obtained both in pharmacological and genetic animal models, ranging from global to conditional NMDAR manipulation. Our main focus is on the contribution of different NMDAR subtypes to the psychoactive effects induced by NMDAR ablation/blockade. We review data analyzing the effect of NMDAR subtype deletions limited to specific neuronal populations/brain areas in the regulation of mood. Altogether, these studies suggest effective and putative specific NMDAR drug targets for MDD treatment.

Chronic-Stress-Induced Behavioral Changes Associated with Subregion-Selective Serotonin Cell Death in the Dorsal Raphe

The current study examined the neurochemical mechanisms and neuroanatomical changes underlying coexisting behavioral effects associated with chronic-stress-induced alterations in serotonin (5HT) neurons. Chronic unpredictable stress (CUS) to adult male rats produced depression-like changes with cognitive dysfunction and selective cell death in the interfascicular nucleus of the dorsal raphe (DRif), resulting in decreased 5HTergic innervation of medial prefrontal cortex (mPFC). Twenty-one days of CUS decreased basal plasma levels of corticosterone and produced a shorter latency to immobility and longer durations of immobility in the force-swim test that persisted for 1 month after CUS. Deficits in acquisition, recall, perseveration, and reversal learning were evident 1 month after CUS. MK801 treatment during CUS blocked the changes in the forced-swim test and deficits in memory recall. These behavioral changes were associated with terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive soma and the eventual loss of 5HT neurons in the DRif and its projections to the mPFC as evidenced by fewer labeled cells in the DRif after retrograde tracer injections into the mPFC of stressed rats. Similar to the effects of MK801 on behavior, MK801 pretreatment during stress blocked the CUS-induced decreases in 5HT soma within the DRif and its projections to the mPFC. Finally, the depression-like behaviors were blocked by acute injection of the 5HT2A/C agonist (-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride into the mPFC before forced-swim testing. These results identify a cause and mechanism of 5HTergic dysfunction of the mPFC and associated mood and cognitive behaviors.SIGNIFICANCE STATEMENT Chronic stress causes persistent mood and cognitive changes typically associated with dysregulated serotonin (5HT) transmission in the medial prefrontal cortex (mPFC), but the cause of this dysregulation is unknown. Prior studies have focused on 5HTergic terminals in this region, but this study shows that chronic stress causes NMDA-receptor-dependent and subregion-specific cell death of 5HT neurons in the dorsal raphe. The consequent decreased 5HT innervation of the mPFC was associated with mood and cognitive changes that persisted long after the termination of stress. These findings identify a mechanism of subregion-selective death of 5HT neurons in the dorsal raphe, a defined neuroanatomical pathway, and a behavioral phenotype that mirror stress-associated diseases such as major depressive disorder.

Residues in the GluN1 C-terminal domain control kinetics and pharmacology of GluN1/GluN3A N-methyl-d-aspartate receptors

N-methyl-d-aspartate (NMDA) receptors assembled from GluN1 and GluN3 subunits are unique in that they form glycine-gated excitatory channels that are insensitive to glutamate and NMDA. Alternative splicing of the GluN1 subunit mRNA results in eight variants with regulated expression patterns and post-translational modifications. Here we investigate the role of residues in the GluN1 C-terminal alternatively spliced cassettes in receptor gating and modulation. We measured whole-cell currents from recombinant GluN1/GluN3A receptors expressed in HEK293 cells that differed in the sequence of their GluN1 C-terminal tail. We found that these residues controlled the level of steady-state activity and the degree to which activity was facilitated by zinc and protons. Further, we found that the phosphorylation status of sites specific to certain variants can also modulate channel activity. Based on these results we suggest that GluN1 C-terminal domain splicing may confer cell-specific and activity-dependent regulation onto the level and pharmacologic sensitivity of GluN1/GluN3A currents.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

Structural determinants and mechanism of action of a GluN2C-selective NMDA receptor positive allosteric modulator

NMDA receptors are tetrameric complexes of GluN1, GluN2A-D, and GluN3A-B subunits and are involved in normal brain function and neurologic disorders. We identified a novel class of stereoselective pyrrolidinone (PYD) positive allosteric modulators for GluN2C-containing NMDA receptors, exemplified by methyl 4-(3-acetyl-4-hydroxy-1-[2-(2-methyl-1H-indol-3-yl)ethyl]-5-oxo-2,5-dihydro-1H-pyrrol-2-yl)benzoate. Here we explore the site and mechanism of action of a prototypical analog, PYD-106, which at 30 μM does not alter responses of NMDA receptors containing GluN2A, GluN2B, and GluN2D and has no effect on AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid] and kainate receptors. Coapplication of 50 μM PYD-106 with a maximally effective concentration of glutamate and glycine increases the response of GluN1/GluN2C NMDA receptors in HEK-293 cells to 221% of that obtained in the absence of PYD (taken as 100%). Evaluation of the concentration dependence of this enhancement revealed an EC50 value for PYD of 13 μM. PYD-106 increased opening frequency and open time of single channel currents activated by maximally effective concentrations of agonist but only had modest effects on glutamate and glycine EC50. PYD-106 selectively enhanced the responses of diheteromeric GluN1/GluN2C receptors but not triheteromeric GluN1/GluN2A/GluN2C receptors. Inclusion of residues encoded by GluN1-exon 5 attenuated the effects of PYD. Three GluN2C residues (Arg194, Ser470, Lys470), at which mutagenesis virtually eliminated PYD function, line a cavity at the interface of the ligand binding and the amino terminal domains in a homology model of GluN1/GluN2C built from crystallographic data on GluN1/GluN2B. We propose that this domain interface constitutes a new allosteric modulatory site on the NMDA receptor.

Probing NMDA receptor GluN2A and GluN2B subunit expression and distribution in cortical neurons

The spatial distribution of N-methyl-d-aspartate receptor (NMDAR) subunits in layer 5 (L5) neurons of the medial prefrontal cortex (mPFC) is important for integrating input-output signals involved in cognitive functions and motor behavior. In this study, focal laser scanning photostimulation of caged glutamate, slice electrophysiology, and small peptide pharmacology, were used to map the distribution of functional GluN2A and GluN2B subunits of the NMDAR from L5 neurons of wild-type (WT) and GluN2A(-/-) mice. Focal uncaging of glutamate evoked spatially-restricted glutamatergic responses on various dendritic locations of pyramidal neurons in the mPFC. Analyses of the spatial arrangements of the GluN2A and GluN2B subunits were performed by comparing inhibition of glutamatergic responses in the presence of the GluN2A-selective pharmacological antagonist, NVP-AAM077 (NVP), and the GluN2B-selective peptidic antagonist, conantokin-G (con-G). We found that apical and basal expression and distribution of GluN2A and GluN2B were similar in L5 mPFC neurons of WT mice. However, the inhibition of glutamatergic responses by NVP in brain slices of GluN2A(-/-) mice were dramatically decreased, while con-G inhibition remained similar to that observed in WT brain slices. The data obtained show that expression and spatial arrangement of GluN2B subunits is independent of GluN2A in L5 neurons of the mPFC. These findings have important ramifications for NMDAR organization and function in L5 pyramidal neurons of the mPFC, and show that specific populations of NMDARs can be antagonized, while sparing other subgroups of NMDARs, thus preserving selective NMDAR functions, an important therapeutic advantage.

Anti-NMDA receptor encephalitis antibody binding is dependent on amino acid identity of a small region within the GluN1 amino terminal domain

Anti-NMDA receptor (NMDAR) encephalitis is a newly identified autoimmune disorder that targets NMDARs, causing severe neurological symptoms including hallucinations, psychosis, and seizures, and may result in death (Dalmau et al., 2008). However, the exact epitope to which these antibodies bind is unknown. A clearly defined antigenic region could provide more precise testing, allow for comparison of immunogenicity between patients to explore potential clinically relevant variations, elucidate the functional effects of antibodies, and make patients' antibodies a more effective tool with which to study NMDAR function. Here, we use human CSF to explore the antigenic region of the NMDAR. We created a series of mutants within the amino terminal domain of GluN1 that change patient antibody binding in transfected cells in stereotyped ways. These mutants demonstrate that the N368/G369 region of GluN1 is crucial for the creation of immunoreactivity. Mass spectrometry experiments show that N368 is glycosylated in transfected cells and rat brain regions; however, this glycosylation is not directly required for epitope formation. Mutations of residues N368/G369 change the closed time of the receptor in single channel recordings; more frequent channel openings correlates with the degree of antibody staining, and acute antibody exposure prolongs open time of the receptor. The staining pattern of mutant receptors is similar across subgroups of patients, indicating consistent immunogenicity, although we have identified one region that has a variable role in epitope formation. These findings provide tools for detailed comparison of antibodies across patients and suggest an interaction between antibody binding and channel function.

HIV-1gp120 induces neuronal apoptosis through enhancement of 4-aminopyridine-senstive outward K+ currents

Human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) usually occurs late in the course of HIV-1 infection and the mechanisms underlying HAD pathogenesis are not well understood. Accumulating evidence indicates that neuronal voltage-gated potassium (Kv) channels play an important role in memory processes and acquired neuronal channelopathies in HAD. To examine whether Kv channels are involved in HIV-1-associated neuronal injury, we studied the effects of HIV-1 glycoprotein 120 (gp120) on outward K+ currents in rat cortical neuronal cultures using whole-cell patch techniques. Exposure of cortical neurons to gp120 produced a dose-dependent enhancement of A-type transient outward K+ currents (IA). The gp120-induced increase of IA was attenuated by T140, a specific antagonist for chemokine receptor CXCR4, suggesting gp120 enhancement of neuronal IA via CXCR4. Pretreatment of neuronal cultures with a protein kinase C (PKC) inhibitor, GF109203X, inhibited the gp120-induced increase of IA. Biological significance of gp120 enhancement of IA was demonstrated by experimental results showing that gp120-induced neuronal apoptosis, as detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and caspase-3 staining, was attenuated by either an IA blocker 4-aminopyridine or a specific CXCR4 antagonist T140. Taken together, these results suggest that gp120 may induce caspase-3 dependent neuronal apoptosis by enhancing IA via CXCR4-PKC signaling.

Protective effect of gan mai da zao decoction in unpredictable chronic mild stress-induced behavioral and biochemical alterations

AIM

Growing evidence indicates that the glutamatergic system, especially the abnormalities of glutamate and N-methyl-D-aspartate (NMDA) receptors contribute to the pathophysiology and possibly the pathogenesis of major depressive disorders. This study is to evaluate the effect of gan mai da zao (GMDZ) decoction on glutamate and NMDA receptor in unpredictable chronic mild stress (UCMS) rats.

MATERIALS AND METHODS

Sucrose preference test and open field test were used to estimate the depressive-like behaviors of UCMS rats. Glutamate levels and NMDA receptor subunits (NR1, NR2A and NR2B) in the frontal cortex and hippocampus were determined by HPLC-FLD and by western-blot respectively.

RESULTS

32 days UCMS induced depressive-like behaviors, increased glutamate concentration and decreased NMDA receptor subunits NR2A and NR2B in the frontal cortex and hippocampus of rats. However, NR1 expression remained constant in stressed rats compared with normal. The GMDZ decoction alleviated the depressive-like behavior, decreased glutamate level, and increased expression of NMDA receptor subunit NR2A and NR2B in the frontal cortex and hippocampus of stressed rats.

CONCLUSIONS

These results suggest that GMDZ treatment reversed chronic unpredictable stress-induced depressive-like behaviors in UCMS rats, possibly via reducing glutamate levels and increasing the NMDA receptor subunits NR2A and NR2B in frontal cortex and hippocampus.

Phosphorylation of spinal N-methyl-d-aspartate receptor NR1 subunits by extracellular signal-regulated kinase in dorsal horn neurons and microglia contributes to diabetes-induced painful neuropathy

The N-methyl-d-aspartate receptor (NMDAR) contributes to central sensitization in the spinal cord, a phenomenon which comprises various pathophysiological mechanisms responsible for neuropathic pain-like signs in animal models. NMDAR function is modulated by post-translational modifications including phosphorylation, and this is proposed to underlie its involvement in the production of pain hypersensitivity. As in diabetic patients, streptozotocin-induced diabetic rats exhibit or not somatic mechanical hyperalgesia; these rats were named DH and DNH respectively. At three weeks of diabetes, we present evidence that somatic mechanical hyperalgesia was correlated with an enhanced phosphorylation of the NMDAR NR1 subunit (pNR1) in the rat spinal cord. This increase was not found in normal and DNH rats, suggesting that this regulation was specific to hyperalgesia. Double immunofluorescence studies revealed that the numbers of pNR1-immunoreactive neurons and microglial cells were significantly increased in all laminae (I-II and III-VI) of the dorsal horn from DH animals. Western-blots analysis showed no change in NR1 protein levels, whatever the behavioural and glycemic status of the animals. Chronic intrathecal treatment (5μg/rat/day for 7days) by U0126 and MK801, which blocked MEK (an upstream kinase of extracellular signal-regulated protein kinase: ERK) and the NMDAR respectively, simultaneously suppressed somatic mechanical hyperalgesia developed by diabetic rats and decreased pNR1. These results indicate for the first time that increased expression of pNR1 is regulated by ERK and the NMDAR via a feedforward mechanism in spinal neurons and microglia and represents one mechanism involved in central sensitization and somatic mechanical hyperalgesia after diabetes.

N-methyl-d-aspartic acid receptors are altered by stress and alcohol in Wistar-Kyoto rat brain

Previous studies have shown that the Wistar-Kyoto (WKY) rat strain is more sensitive to stressors and consumes significant quantities of alcohol under basal as well as stressful conditions when compared to other strains. Given that the glutamate neurotransmitter system has been implicated in depression and addiction, the goals of the present study were to investigate the effects of stress and stress-alcohol interactions on N-methyl-d-aspartate (NMDA) receptors in the rat brain. Thus this study measured the binding of [(3)H] MK-801 to NMDA receptors in the prefrontal cortex (PFC), caudate putamen (CPu), nucleus accumbens (NAc), hippocampus (HIP) and basolateral amygdala (BLA) in WKY rats in comparison to the Wistar (WIS) rat strain. Our results suggested that while voluntary alcohol consumption did not alter NMDA receptors in the PFC, CPu or NAc in either rat strain, it increased NMDA receptors in the HIP and BLA in both strains. In contrast, chronic stress increased NMDA receptors in the PFC, CPu, NAc in WKY rats but not in WIS rats. Chronic stress also decreased NMDA receptors in the HIP and increased NMDA receptors in the BLA in both strains. Alcohol co-treatment with stress increased NMDA receptors in the PFC, CPu and NAc in WKY rats but not in WIS rats. Interestingly, while alcohol co-treatment did not reverse stress induced decreases in NMDA receptors in the HIP, it reduced the binding of NMDA receptors in the BLA to control levels in both strains. Thus it appears that NMDA receptors in the PFC, CPu and NAc may be more sensitive to the effects of stress and could be implicated in the stress-induced alcohol consumption behavior seen in WKY rats. In contrast, NMDA receptors in the HIP and BLA may reflect an adaptive response and may not be responsible for the stress susceptible phenotype of the WKY rat strain.

Alcoholism and alternative splicing of candidate genes

Gene expression studies have shown that expression patterns of several genes have changed during the development of alcoholism. Gene expression is regulated not only at the level of transcription but also through alternative splicing of pre-mRNA. In this review, we discuss some of the evidence suggesting that alternative splicing of candidate genes such as DRD2 (encoding dopamine D2 receptor) may form the basis of the mechanisms underlying the pathophysiology of alcoholism. These reports suggest that aberrant expression of splice variants affects alcohol sensitivities, and alcohol consumption also regulates alternative splicing. Thus, investigations of alternative splicing are essential for understanding the molecular events underlying the development of alcoholism.

Circadian and developmental regulation of N-methyl-d-aspartate-receptor 1 mRNA splice variants and N-methyl-d-aspartate-receptor 3 subunit expression within the rat suprachiasmatic nucleus

The circadian rhythms of mammals are generated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Its intrinsic period is entrained to a 24 h cycle by external cues, mainly by light. Light impinging on the SCN at night causes either advancing or delaying phase shifts of the circadian clock. N-methyl-d-aspartate receptors (NMDAR) are the main glutamate receptors mediating the effect of light on the molecular clockwork in the SCN. They are composed of multiple subunits, each with specific characteristics whose mutual interactions strongly determine properties of the receptor. In the brain, the distribution of NMDAR subunits depends on the region and developmental stage. Here, we report the circadian expression of the NMDAR1 subunit in the adult rat SCN and depict its splice variants that may constitute the functional receptor channel in the SCN. During ontogenesis, expression of two of the NMDAR1 subunit splice variants, as well as the NMDAR3A and 3B subunits, exhibits developmental loss around the time of eye opening. Moreover, we demonstrate the spatial and developmental characteristics of the expression of the truncated splice form of NMDAR1 subunit NR1-E in the brain. Our data suggest that specific properties of the NMDAR subunits we describe within the SCN likely influence the photic transduction pathways mediating the clock entrainment. Furthermore, the developmental changes in NMDAR composition may contribute to the gradual postnatal maturation of the entrainment pathways.

Differentiated P19 cells express N-methyl-D-aspartate receptor 1 mRNA binding trans-acting proteins and four N-methyl-D-aspartate receptor 1 splice variants comparable to those in cultured fetal cortical neurons

Differentiated P19 cells naturally express N-methyl-D-aspartate (NMDA) receptors and serve as a good in vitro model system with which to study NMDA receptor regulation. Here we examined expression of NR1 mRNA binding trans-acting proteins and NR1 splice variants in P19 cells. After exposure to retinoic acid, P19 cells were differentiated for 2, 4, 6, and 8 days in vitro (DIV). Total RNA and protein extracts from differentiated P19 cells were utilized to examine NR1 and NR2B expression. A steady increase in NR1 and NR2B mRNA and protein levels was observed with respect to days of differentiation. NR2B mRNA was detected within 2 DIV. However, NR2B protein appeared only at 4 DIV. By contrast, minimal expression of NR1 mRNA could be detected in undifferentiated P19 cells, whereas NR1 protein was detected at 4 DIV. RT-PCR analysis identified expression of four of eight full-length NR1 splice variants, similar to the expression pattern seen in fetal cortical neurons (FCN). These data were confirmed by ribonuclease protection assays. RNA gel shift assays and Northwestern analysis revealed the expression of NR1 mRNA binding trans-acting proteins in P19 neurons comparable to those expressed in FCN. RNA super gel shift assays confirmed the presence of the NR1 mRNA binding trans-acting protein GIIbeta in the NR1-3'UTR-P19 protein complex. Levels of GIIbeta polypeptide increased with increase in days of differentiation. Taken together, our data demonstrate that differentiated P19 cells are comparable to FCN and hence provide an excellent in vitro model for studying NR1 mRNA regulation at the posttranscriptional level.

Effects of NR1 splicing on NR1/NR3B-type excitatory glycine receptors

Background

N-methyl-D-aspartate receptors (NMDARs) are the most complex of ionotropic glutamate receptors (iGluRs). Subunits of this subfamily assemble into heteromers, which – depending on the subunit combination – may display very different pharmacological and electrophysiological properties. The least studied members of the NMDAR family, the NR3 subunits, have been reported to assemble with NR1 to form excitatory glycine receptors in heterologous expression systems. The heterogeneity of NMDARs in vivo is in part conferred to the receptors by splicing of the NR1 subunit, especially with regard to proton sensitivity.

Results

Here, we have investigated whether the NR3B subunit is capable of assembly with each of the eight functional NR1 splice variants, and whether the resulting receptors share the unique functional properties described for NR1-1a/NR3. We provide evidence that functional excitatory glycine receptors formed regardless of the NR1 isoform, and their pharmacological profile matched the one reported for NR1-1a/NR3: glycine alone fully activated the receptors, which were insensitive to glutamate and block by Mg²⁺. Surprisingly, amplitudes of agonist-induced currents showed little dependency on the C-terminally spliced NR1 variants in NR1/NR3B diheteromers. Even more strikingly, NR3B conferred proton sensitivity also to receptors containing NR1b variants – possibly via disturbing the "proton shield" of NR1b splice variants.

Conclusion

While functional assembly could be demonstrated for all combinations, not all of the specific interactions seen for NR1 isoforms with coexpressed NR2 subunits could be corroborated for NR1 assembly with NR3. Rather, NR3 abates trafficking effects mediated by the NR1 C terminus as well as the N-terminally mediated proton insensitivity. Thus, this study establishes that NR3B overrides important NR1 splice variant-specific receptor properties in NR1/NR3B excitatory glycine receptors.

Contribution of NMDA receptor-mediated component to the EPSP in mouse Schaffer collateral synapses under single pulse stimulation protocol

The degree to which NMDA receptors contribute to hippocampal CA(1) stratum radiatum excitatory postsynaptic potentials (EPSP) is a matter of debate. This experiment was designed to resolve the issue by documenting and positively identifying the elements of the NMDA dependent component in the extracellularly recorded stratum radiatum CA(1) field potential under low stimulation conditions and in the presence of physiologic levels of Mg(2+). We show that EPSP generation consists of activation of both AMPA and NMDA receptor channels, which mediate distinct components of the recorded field potential. We propose that the EPSP is a combination of two waves rather than one, which sometimes has been attributed to the exclusive activation of AMPA channels. Our data suggest that the three recorded peaks signify different events. The first peak reflects the presynaptic volley while the other two represent the actual EPSP. The first peak of the EPSP is determined mainly by flow of ions through AMPA channels. The second peak most likely is determined by the concurrence of two phenomena: ionic flow through NMDA channels and the source corresponding to the sink generated at the cell bodies in the pyramidal layer. The NMDA dependent component was recorded when Mg(2+) was present in physiological concentrations. The presynaptic volley and second peak do not saturate over a 10-fold increase of the stimulation charge and their amplitudes are highly correlated. The first peak amplitude rapidly saturates. The sensitivity of the recorded signals is different, the first peak being the most sensitive (1.25-0.26 mV/nC). Isolation of NMDA dependent components under physiological conditions when using a single pulse low stimulation protocol would allow more precise investigations of the NMDA dependent forms of synaptic plasticity.

Effects of exercise on NMDA receptor subunit contributions to bidirectional synaptic plasticity in the mouse dentate gyrus

We examined synaptic plasticity in the dentate gyrus (DG) of the hippocampus in vitro in juvenile C57Bl6 mice (28-40 days of age), housed in control conditions with minimal enrichment (Controls) or with access to an exercise wheel (Runners). LTP expression was significantly greater in slices from Runners than in those from Controls, but could be blocked by APV in both groups. LTP was significantly reduced by NR2B subunit antagonists in both groups. NVP-AAM077, an antagonist with a higher preference for NR2A subunits over NR2B subunits, blocked LTP in slices from Runners and produced a slight depression in Control animals. LTD in the DG was also blocked by APV, but not by either of the NR2B specific antagonists. Strikingly, NVP-AAM077 prevented LTD in Runners, but not in Control animals, suggesting an increased involvement of NR2A subunits in LTD in animals that exercise. NVP-AAM077 did not block LTD in NR2A Knock Out (KO) animals that exercised, as expected. In an attempt to discern whether NMDA receptors located at extrasynaptic sites could play a role in the induction of LTD, DL-TBOA was used to block excitatory amino acid transport and increase extracellular glutamate levels. Under these conditions, LTD was not blocked by the co-application of a specific NR2B subunit antagonist in either group, but NVP-AAM077 again blocked LTD selectively in Runners. These results indicate that NR2A and NR2B subunits play a significant role in LTP in the DG, and that exercise can significantly alter the contribution of NMDA NR2A subunits to LTD.

Relationship between mRNA expression of splice forms of the zeta1 subunit of the N-methyl-D-aspartate receptor and spatial memory in aged mice

Age-related changes in the protein and mRNA expression of some of the splice forms of the zeta1 (NR1) subunit of the NMDA receptor have been seen in mice and rats. The present study was designed to determine whether individual splice forms of the zeta1 subunit of the NMDA receptor within prefrontal/frontal cortical regions contribute to memory deficits during aging and whether experience in learning tasks can influence the expression of the splice forms. mRNA expression of 4 splice forms (zeta1-1, zeta1-3, zeta1-a and zeta1-b) and mRNA for all known splice forms (zeta1-pan) were examined by in situ hybridization. mRNA for C-terminal splice forms, zeta1-1 (+ C1 and + C2 cassettes) and zeta1-3 (+ C1 and + C2'), showed significant declines during aging in several brain regions even though overall zeta1-pan mRNA expression was not significantly affected by aging. This suggests that these splice forms are more influenced by aging than the subunit as a whole. There was an increase in the expression of zeta1-a (-N1 cassette) splice form in the behaviorally-experienced old mice relative to the younger groups. Old mice with high levels of mRNA expression for the zeta1-a splice form in orbital cortex showed the best performances in the working memory task, but the poorest performances in the cued, associative learning task. These results suggest that there is a complex interaction between zeta1 splice form expression and performance of memory tasks during aging.

NMDA receptor NR2 subunit dependence of the slow component of magnesium unblock

NMDA receptor activity is important for many physiological functions, including synapse formation and alterations in synaptic strength. NMDA receptors are composed most commonly of NR1 and NR2 subunits. There are four NR2 subunits (NR2A-NR2D). NR2 subunit expression varies across both brain regions and developmental stages. The identity of the NR2 subunit within a functional NMDA receptor helps to determine many pharmacological and biophysical receptor properties, including strength of block by external Mg2+ (Mg(o)2+). Mg(o)2+ block confers strong voltage dependence to NMDA receptor-mediated responses and is critically important for many of the functions that the NMDA receptor plays within the CNS. Here we describe the NR2 subunit dependence of the kinetics of Mg(o)2+ unblock after rapid depolarizations. We find that Mg(o)2+ unblocks from NR1/2A and NR1/2B receptors with a prominent slow component similar to that previously described in native hippocampal and cortical NMDA receptors. Strikingly, this slow component of Mg(o)2+ unblock is completely absent from NR1/2C and NR1/2D receptors. Thus currents from NR1/2C and NR1/2D receptors respond more rapidly to fast depolarizations than currents from NR1/2A and NR1/2B receptors. In addition, the slow component of Mg(o)2+ unblock from NR1/2B receptors is consistently slower than from NR1/2A receptors. This makes rapid depolarizations, such as action potential waveforms, more efficacious at stimulating Mg(o)2+ unblock from NR1/2A than from NR1/2B receptors. These NR2 subunit differences in the kinetics of Mg(o)2+ unblock are likely to help determine the contribution of each NMDA receptor subtype to current flow during synaptic activity.

Modulation of inhibitory and excitatory synaptic transmission in rat inferior colliculus after unilateral cochleectomy: an in situ and immunofluorescence study

We investigated whether inhibitory synaptic transmission mediated through glycinergic receptor, GABAA receptors, glutamic acid decarboxylase, the enzyme synthesizing GABA, and excitatory synaptic transmission through alpha-amino-3-hydroxi-5-methylisoxazole-4-propionic acid receptors and N-methyl-D-aspartate receptors are affected in the inferior colliculus by unilateral surgical cochleectomy. In situ hybridization and immunohistofluorescence studies were performed in normal and lesioned adult rats at various times following the lesion (1-150 days). Unilateral auditory deprivation decreased glycine receptor alpha1 and glutamic acid decarboxylase 67 expression in the contralateral central nucleus of the inferior colliculus. This decrease began one day after cochleectomy, and continued until day 8; thereafter expression was consistently low until day 150. The glycine receptor alpha1 subunit decrease did not occur if a second contralateral cochleectomy was performed either on day 8 or 150 after the first cochleectomy. Bilateral cochleectomy caused also a bilateral inferior colliculus diminution of glutamic acid decarboxylase 67 mRNA at post-lesion day 8 but there were no changes in glycine receptor alpha1 compared with controls. In contrast, the abundance of other alpha2-3, and beta glycine receptor, gephyrin, the anchoring protein of glycine receptor, the alpha1, beta2 and gamma2 subunits of GABAA receptors, GluR2, R3 subunits of alpha-amino-3-hydroxi-5-methylisoxazole-4-propionic acid receptors, and NR1 and NR2A transcripts of N-methyl-D-aspartate receptors was unaffected during the first week following the lesion. Thus, unilateral cochlear removal resulted in a selective and long-term decrease in the amount of the glycine receptor alpha1 subunit and of glutamic acid decarboxylase 67 in the contralateral central nucleus of the inferior colliculus. These changes most probably result from the induced asymmetry of excitatory auditory inputs into the central nucleus of the inferior colliculus and may be one of the mechanisms involved in the tinnitus frequently encountered in patients suffering from a sudden hearing loss.

Contribution of NR2A and NR2B NMDA subunits to bidirectional synaptic plasticity in the hippocampus in vivo

It has recently been proposed that activation of the NR2A subunit results in Long-term potentiation (LTP) induction, whereas activation of the NR2B subunit results in long-term depression (LTD) induction. The present study undertakes to replicate these findings in vivo to determine if a role for specific subunits in synaptic plasticity can be shown in the intact brain. Field recordings were made from the CA1 subfield of the hippocampus using Schaffer collateral stimulation in anesthetized male Sprague-Dawley rats. Antagonists of the N-methyl-D-aspartate receptors NR2A and NR2B subunits were administered by either intraperitoneal (i.p.) or intrahippocampal (i.h.) injections to assess their involvement in LTP (100 Hz stimuli) and LTD (200 Paired-burst stimuli). i.h. injection of Ro25-6981 (100 microM) significantly attenuated hippocampal LTP expression and completely blocked LTD expression. When administered i.p., Ro25-6981 (6 mg/kg) again blocked LTD, but did not significantly diminish the expression of LTP. When NVP-AAM077 was administered i.h. (80 microM) both LTP and LTD were completely abolished. The administration of this compound i.p. (1.2 mg/kg) also significantly attenuated LTP, but did not affect LTD. These data suggest that both NR2A and NR2B subunits can play roles in LTP and LTD in the hippocampus in vivo.

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

The effects of aging on different C-terminal splice forms of the zeta1(NR1) subunit of the N-methyl-d-aspartate receptor in mice

Changes in NMDA receptors in the prefrontal/frontal cortex and hippocampus of C57BL/6 mice during aging show a relationship to declines in spatial learning. The present study was designed to determine whether aging influences the mRNA expression of different splice forms of the zeta1 subunit of the NMDA receptor. We examined the mRNA of 4 C-terminal splice forms with the use of in situ hybridization. The zeta1-1 splice form (+C1 and +C2 cassettes) overall showed a maintenance of mRNA density from 3 to 10 months of age, followed by a significant decline by 26 months of age. In contrast, the mRNA for the zeta1-3 splice form [+C1 and +C2'(-C2)] showed significant declines between 3- and 10-month-old mice. These declines were maintained in the old mice. The zeta1-2 splice form (-C1 and +C2) showed a near-significant decrease in expression during aging across all brain regions. The zeta1-4 subunit mRNA [-C1 and +C2' (-C2)] showed no significant changes with increased age. These results indicate that there is a differential effect of aging on different splice variants of the zeta1 subunit of the NMDA receptor and those that are affected show a different temporal pattern of aging. This heterogeneity has implications for producing imbalances in the modulation of the remaining receptors.

Enhancement of NMDA receptor phosphorylation of the spinal dorsal horn and nucleus gracilis neurons in neuropathic rats

NR1 is an essential component of functional NMDA receptors and can be activated by phosphorylation. It is suggested that phosphorylation of NR1 (pNR1) contributes to central sensitization after intradermal capsaicin injection. The present study investigates whether increases of spinal pNR1 are correlated to central sensitization and thus pain behaviors in neuropathic pain. Neuropathic rats were produced by L5 spinal nerve ligation, mechanical thresholds of the paw were measured, and then the L4/5 spinal cords and the nucleus gracilis (NG) were removed and immunostained for pNR1. The results showed that the number of pNR1-immunoreactive neurons was significantly increased in the ipsilateral cord, at 3, 7, and 28 days after nerve ligation and these increases coincide with mechanical allodynia. The increase of pNR1-immunoreactive neurons in the NG was observed only at 28 days after the nerve ligation. Western blot analyses confirmed the significant increase of pNR1 protein in spinal dorsal horn after nerve ligation. A protein kinase A inhibitor, H89, moderately reversed mechanical allodynia in 7 day neuropathic rats. Many pNR1-immunoreactive neurons were identified as projection neurons by retrograde tracer. The data suggest that PKA mediated NMDA receptor phosphorylation plays an important role in spinal nerve ligation induced neuropathic pain.

Neonatal toluene exposure alters N-methyl-D-aspartate receptor subunit expression in the hippocampus and cerebellum in juvenile rats

Recent evidence indicates that toluene is a non-competitive inhibitor of N-methyl-d-aspartate (NMDA) receptor-mediated synaptic currents. The NMDA receptor plays a major role in neuronal development and differentiation. The present study characterized the long-term effects of toluene exposure during synaptogenesis on the expression of NMDA receptor subunits (NR1, NR2A and NR2B). Neonatal rats were administered toluene (500 mg/kg, ip) daily over postnatal days (PN) 4-9. The expression of NMDA receptor subunits in rat brain was measured on PN 30. Western blot analysis demonstrated that toluene exposure significantly increased NR2A expression in the hippocampus and cerebellum. Immunohistochemical results indicated that the increased NR2A expression is mainly in hippocampal CA1-stratum oriens, CA1-stratum radiatum, CA1-lacunosm molecular, CA2- stratum oriens, and dentate gyrus-molecular layer and the cerebellar Purkinje cell layer, respectively. In contrast, the levels of NR2B in the toluene-exposed rats were decreased in the molecular layer. These results suggest that the region-specific changes in the expression of NMDA receptor subunits may be related to the neurobehavioral dysfunction following toluene exposure during synaptogenesis.

NMDA receptor GluRepsilon/NR2 subunits are essential for postsynaptic localization and protein stability of GluRzeta1/NR1 subunit

In NMDA receptors, GluRepsilon/NR2 subunits strictly require the GluRzeta1/NR1 subunit to exit from endoplasmic reticulum (ER) to the cell surface in vitro and to the postsynapse in vivo, whereas C terminus-dependent self-surface delivery has been demonstrated for the GluRzeta1 subunit in vitro. To test whether this leads to C terminus-dependent self-postsynaptic expression in neurons in vivo, we investigated the GluRzeta1 subunit in cerebellar granule cells lacking two major GluRepsilon subunits, GluRepsilon1/NR2A and GluRepsilon3/NR2C. In the mutant cerebellum, synaptic labeling for the GluRzeta1 subunit containing the C2 (GluRzeta1-C2) or C2' (GluRzeta1-C2') cassette was reduced at mossy fiber-granule cell synapses to the extrasynaptic level. The loss was not accompanied by decreased transcription and translation levels, increased extrasynaptic labeling, or ER accumulation. Quantitative immunoblot revealed substantial reductions in the mutant cerebellum of GluRzeta1-C2 and GluRzeta1-C2'. The most severe deficit was observed in the postsynaptic density (PSD) fraction: mutant levels relative to the wild-type level were 12.3 +/- 3.3% for GluRzeta1-C2 and 17.0 +/- 4.6% for GluRzeta1-C2'. The GluRzeta1 subunit carrying the C1 cassette (GluRzeta1-C1) was, although low in cerebellar content, also reduced to 12.7 +/- 3.5% in the mutant PSD fraction. Considering a trace amount of other GluRepsilon subunits in the mutant cerebellum, the severe reductions thus represent that the GluRzeta1 subunit, by itself, is virtually unable to accumulate at postsynaptic sites, regardless of C-terminal forms. By protein turnover analysis, the degradation of the GluRzeta1 subunit was accelerated in the mutant cerebellum, being particularly rapid for that carrying the C2 cassette. Therefore, accompanying expression of GluRepsilon subunits is essential for postsynaptic localization and protein stability of the GluRzeta1 subunit.

Developmental expression of N-methyl-D-aspartate glutamate receptor 1 splice variants in the chick retina

Glutamate is the major excitatory neurotransmitter in the vertebrate retina. The N-methyl-D-aspartate glutamate receptor (NMDAR) is assembled as a tetramer containing NR1 and NR2, and possibly NR3 subunits, NR1 being essential for the formation of the ion channel. The NMDAR1 (NR1) gene encodes for mRNAs that generate at least eight functional variants by alternative splicing of exon 5 (cassette N1), 21 (cassette C1), or 22 (cassettes C2 or C2'). NR1 splice variants were identified in the mature chick retina, and their variation during embryonic development (ED) was analyzed. NR1 was shown to lack N1 in early ED, shifting to N1-containing variants in the mature retina, which could contribute to explaining the distinct biochemical properties of retinal NMDARs compared with the CNS. Sequence analysis of C-terminal variants containing C1 and C2 cassettes suggests a membrane-targeting mechanism for avian NMDARs distinct from that in mammals. An NR1 variant containing a novel alternative C-terminal splice exon named C3 was found, which encodes six amino acids containing a predicted casein kinase II phosphorylation site. This new variant is expressed in the retina during a restricted period of ED, coincident with the generation of spontaneous calcium activity waves, which precedes synapse formation in the retina, suggesting its participation in this process.

Ionotropic glutamate receptor subunit immunoreactivity of vagal preganglionic neurones projecting to the rat heart

The ionotropic glutamate receptor subunits expressed by vagal preganglionic neurones in the rat medulla oblongata were examined by using fluorescence immunolabelling combined with retrograde neuronal tracing. The general population of these neurones in the medulla was identified by intraperitoneal injections of Fluorogold and also with choline acetyltransferase antibodies. Cardiac projecting neurones were specifically identified by applying the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine (DiI) to the heart or by injecting cholera toxin B-subunit into the pericardium. Both tracers labelled populations of neurones lying in the dorsal vagal nucleus, intermediate reticular formation and nucleus ambiguus, and when both tracers were applied simultaneously, approximately 50% of cells were dual-labelled. Control experiments established that the labelling was specific for neurones projecting to the heart. Most vagal preganglionic neurones, including those projecting to the heart, irrespective of their location in the medulla, had a similar profile of glutamate receptor immunoreactivity. Labelling of somata for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) subunit GluR1 was weak or absent, while labelling with antibodies directed to GluR2, a common sequence of GluR2 and GluR3, and GluR4 was moderate or intense. All neurones studied appeared to express the N-methyl-D-aspartate (NMDA) receptor subunit NR1, and while antibodies recognising the NR2A and NR2B splice variants gave strong labelling, immunoreactivity with a NR2B specific antibody was weaker. Weak to moderate labelling was seen in some neurones using antibodies to the kainate receptor subunits KA2 and GluR5-7. These results are consistent with neurophysiological data indicating the presence of AMPA, NMDA and kainate responses in cardiac vagal preganglionic neurones, and suggest that these neurones are similar to other vagal parasympathetic preganglionic neurones in expressing mainly AMPA receptor subunits GluR2-4.

Role of protein kinase A in phosphorylation of NMDA receptor 1 subunits in dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

Protein phosphorylation is a major mechanism for regulation of N-methyl-D-aspartate (NMDA) receptor function. The NMDA receptor 1 subunit (NR1) is phosphorylated by protein kinase A (PKA) on serine 890 and 897. We have recently reported that there is enhanced phosphorylation of NR1 on serine 897 in dorsal horn and spinothalamic tract (STT) neurons after intradermal injection of capsaicin (CAP) in rats [Zou et al. (2000) J. Neurosci. 20, 6989-6997]. Whether or not this phosphorylation, which develops during central sensitization following CAP injection, is mediated by PKA remains to be determined. In this study, western blots and immunofluorescence staining were employed to observe if pretreatment with a PKA inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, HCl (H89), blocks the enhanced phosphorylation of NR1 on serine 897 following injection of CAP into the glabrous skin of one hind paw of anesthetized rats. Western blots showed that pretreatment with H89 caused a decrease in CAP-induced phosphorylation of NR1 protein in spinal cord segments L(4)-S(1). In experiments using immunofluorescence staining, the numbers of phospho-NR1-like immunoreactive (p-NR1-LI) neurons seen after CAP injection were significantly decreased in the dorsal horn of the L(4)-L(5) segments on the side ipsilateral to the injection after PKA was inhibited. When STT cells were labeled by microinjection of the retrograde tracer, fluorogold, we found that the proportion of p-NR1-LI STT cells on the side ipsilateral to the injection in the superficial laminae of spinal cord segments L(4)-L(5) was markedly reduced when H89 was administered intrathecally before CAP injection. However, the proportion of p-NR1-LI STT cells in deep laminae was unchanged unless the PKC inhibitor, chelerythrine chloride, was co-administered with H89. Combined with our previous findings, the present results indicate that NR1 in spinal dorsal horn neurons, including the superficial dorsal horn STT cells, is phosphorylated following CAP injection and that this phosphorylation is due to the action of PKA. However, the phosphorylation of deep STT cells involves both PKA and PKC.

Two N-methyl-D-aspartate receptors in rat dorsal root ganglia with different subunit composition and localization

N-methyl-D-aspartate (NMDA) receptors in sensory afferents participate in chronic pain by mediating peripheral and central sensitization. We studied the presence of NMDA receptor subunits in different types of primary afferents. Western blots indicated that rat dorsal root ganglia (DRG) contain NR1, NR2B, NR2C, and NR2D but not NR2A. Real-time RT-PCR showed that NR2B and NR2D were expressed at higher levels than NR2A and NR2C in DRG. Immunofluorescence with an antibody that recognized NR1 and another that recognized NR2A and NR2B showed that NR1 and NR2B colocalized in 90% of DRG neurons, including most A-fibers (identified by the presence of neurofilament 200 kDa). In contrast, an antibody recognizing NR2C and NR2D labeled only neurofilament-negative DRG profiles. This antibody stained practically all DRG cells that contained calcitonin gene-related peptide and neurokinins and those that bound isolectin B4. The percentage of cells immunoreactive for NR1, NR2A/NR2B, and NR2C/NR2D were the same in the T9, T12, L4, and L6 DRG. The intracellular distribution of the NR2 subunits was strikingly different: Whereas NR2A/NR2B immunoreactivity was found in the Golgi apparatus and occasionally at the plasma membrane, NR2C/NR2D immunoreactivity was found in the cytoplasm but not in the Golgi. The NR1 subunit was present throughout the cytoplasm and was more intense in the Golgi. These findings indicate that DRG neurons have two different NMDA receptors, one containing the NR1, NR2D, and possibly the NR2C subunits, found only in C-fibers, and the diheteromer NR1/NR2B, present in the Golgi apparatus of both A- and C-fibers.

Traumatic brain injury: developmental differences in glutamate receptor response and the impact on treatment

Perinatal brain injury following trauma, hypoxia, and/or ischemia represents a substantial cause of pediatric disabilities including mental retardation. Such injuries lead to neuronal cell death through either necrosis or apoptosis. Numerous in vivo and in vitro studies implicate ionotropic (iGluRs) and metabotropic (mGluRs) glutamate receptors in the modulation of such cell death. Expression of glutamate receptors changes as a function of developmental age, with substantial implications for understanding mechanisms of post-injury cell death and its potential treatment. Recent findings suggest that the developing brain is more susceptible to apoptosis after injury and that such caspase mediated cell death may be exacerbated by treatment with N-methyl-D-aspartate receptor antagonists. Moreover, group I metabotropic glutamate receptors appear to have opposite effects on necrotic and apoptotic cell death. Understanding the relative roles of glutamate receptors in post-traumatic or post-ischemic cell death as a function of developmental age may lead to novel targeted approaches to the treatment of pediatric brain injury.

Differential effects of chronic ethanol treatment on N-methyl-D-aspartate R1 splice variants in fetal cortical neurons

Functional N-methyl-D-aspartate receptors consisting of NR1 and NR2 subunits are an important site of action of ethanol. Chronic ethanol treatment increases the NR1 polypeptide levels in vivo and in vitro. Chronic ethanol treatment in vitro does not significantly alter the NR1 mRNA levels, even though under similar culture conditions ethanol (50 mm, 5 days) enhances the half-life of NR1 mRNA in fetal cortical neurons. To address this phenomenon, we determined by reverse transcription-polymerase chain reaction and Western blotting whether ethanol (50 mm, 5 days) has a splice variant-specific effect on the expression of the NR1 subunit in mouse fetal cortical neurons. This report analyzes for the first time the distribution of all NR1 splice variants in these neurons. Our data indicate the presence of NR1-3a,b and NR1-4a,b splice variants in cortical neurons. Chronic ethanol treatment significantly decreased the mRNA levels of exon 5-containing NR1 splice variants (NR1-3b and NR1-4b) (-E5/+E5 = 4.6 in untreated neurons and 6.1 in ethanol-treated neurons) and had no effect on the mRNA levels of NR1-3 (+E21/-E22) and NR1-4 (-E21/-E22) splice variants. At the polypeptide level, chronic ethanol treatment significantly reduced exon 5-containing splice variants (NR1-3b and NR1-4b). However, ethanol (50 mm, 5 days) induced a significant increase in polypeptide levels of NR1-4 (-E21/-E22), without any effect on NR1-3 (+E21/-E22) polypeptide levels. These results demonstrate that chronic ethanol treatment has a selective effect on the expression of NR1 splice variants at both the mRNA and polypeptide levels in mouse fetal cortical neurons.

Molecular mechanisms of glutamate receptor-mediated excitotoxic neuronal cell death

Excitotoxicity is one of the most extensively studied processes of neuronal cell death, and plays an important role in many central nervous system (CNS) diseases, including CNS ischemia, trauma, and neurodegenerative disorders. First described by Olney, excitotoxicity was later characterized as an excessive synaptic release of glutamate, which in turn activates postsynaptic glutamate receptors. While almost every glutamate receptor subtype has been implicated in mediating excitotoxic cell death, it is generally accepted that the N-methyl-D-aspartate (NMDA) subtypes play a major role, mainly owing to their high calcium (Ca2+) permeability. However, other glutamate receptor subtypes such as 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionate (AMPA) or kainate receptors have also been attributed a critical role in mediating excitotoxic neuronal cell death. Although the molecular basis of glutamate toxicity is uncertain, there is general agreement that it is in large part Ca(2+)-dependent. The present review is aimed at summarizing the molecular mechanisms of NMDA receptor and AMPA/kainate receptor-mediated excitotoxic neuronal cell death.

Glutamate(NMDA) receptor NR1 subunit mRNA expression in Alzheimer's disease

We analyzed the expression profile of two NMDAR1 mRNA isoform subsets, NR1(0XX) and NR1(1XX), in discrete regions of human cerebral cortex. The subsets are characterized by the absence or presence of a 21-amino acid N-terminal cassette. Reverse transcription polymerase chain reaction for NR1 isoforms was performed on total RNA preparations from spared and susceptible regions from 10 pathologically confirmed Alzheimer's disease (AD) cases and 10 matched controls. Primers spanning the splice insert yielded two bands, 342 bp (NR1(0XX)) and 405 bp (NR1(1XX)), on agarose gel electrophoresis. The bands were visualized with ethidium and quantified by densitometry. NR1(1XX) transcript expression was calculated as a proportion of the NR1(1XX) + NR1(0XX) total. Values were significantly lower in AD cases than in controls in mid-cingulate cortex, p < 0.01, superior temporal cortex, p < 0.01 and hippocampus, p approximately 0.05. Cortical proportionate NR1(1XX) transcript expression was invariant over the range of ages and areas of controls tested, at approximately 50%. This was also true for AD motor and occipital cortex. Proportionate NR1(1XX) expression in AD cingulate and temporal cortex was lower at younger ages and increased with age: this regression was significantly different from that in the homotropic areas of controls. Variations in NR1 N-terminal cassette expression may underlie the local vulnerability to excitotoxic damage of some areas in the AD brain. Alternatively, changes in NR1 mRNA expression may arise as a consequence of the AD disease process.

NMDA receptor subunits GluRepsilon1, GluRepsilon3 and GluRzeta1 are enriched at the mossy fibre-granule cell synapse in the adult mouse cerebellum

Cerebellar N-methyl-D-aspartate (NMDA) receptors are concentrated in the granular layer and are involved in motor coordination and the induction of long-term potentiation at mossy fibre-granule cell synapses. In the present study, we used immunohistochemistry to examine the distribution of NMDA receptor subunits in the adult mouse cerebellum. We found that appropriate pepsin pretreatment of sections greatly enhanced the sensitivity and specificity of immunohistochemical detection. As a result, intense immunolabelling for GluRepsilon1 (NR2A), GluRepsilon3 (NR2C), and GluRzeta1 (NR1) all appeared in synaptic glomeruli of the granular layer. Double immunofluorescence showed that these subunits were colocalized in individual synaptic glomeruli. Within the glomerulus, NMDA receptor subunits were located between centrally-located huge mossy fibre terminals and peripherally-located tiny Golgi axon terminals. By immunoelectron microscopy, all three subunits were detected at the postsynaptic junction in granule cell dendrites, forming synapses with mossy fibre terminals. Consistent with the known functional localization, GluRepsilon1, GluRepsilon3, and GluRzeta1 are, thus, anatomically concentrated at the mossy fibre-granule cell synapse. By contrast, immunohistochemical signals were very low in Purkinje cell somata and dendrites in the molecular layer. The lack of GluRzeta1 immunolabelling in Purkinje cells was unexpected because the cells express GluRzeta1 mRNA at high levels and high levels of GluRzeta1 protein in the molecular layer were revealed by immunoblot. As Purkinje cells are exceptionally lacking GluRepsilon expression, the discrepant result may provide in vivo evidence suggesting the importance of accompanying GluRepsilon subunits in synaptic localization of GluRzeta1.

The anti-craving compound acamprosate acts as a weak NMDA-receptor antagonist, but modulates NMDA-receptor subunit expression similar to memantine and MK-801

NMDA-receptor-mediated mechanisms may be crucial in addictive states, e.g. alcoholism, and provide a target for the novel anti-craving compound acamprosate. Here, the pharmacological effects of acamprosate on NMDA-receptors were studied using electrophysiological techniques in different cell lines in vitro. Additionally, a possible modulation of brain NMDA-receptor subunit expression was examined in vivo in rats, and compared to two effective non-competitive NMDA-receptor antagonists, memantine and MK-801. Electrophysiology in cultured hippocampal neurons (IC(50) approx. 5.5mM) and Xenopus oocytes (NR1-1a/NR2A assemblies: IC(50) approx. 350 microM, NR1-1a/NR2B: IC(50) approx. 250 microM) consistently revealed only a weak antagonism of acamprosate on native or recombinant NMDA-receptors. In HEK-293 cells, acamprosate showed almost no effect on NR1-1a/NR2A or NR1-1a/NR2B recombinants (IC(50)s not calculated). Protein blotting demonstrated an up-regulation of NMDA-receptor subunits after acamprosate as well as after memantine or MK-801, in comparison to controls. After acamprosate, protein levels were increased in the cortex (NR1-3/1-4: 190+/-11% of controls) and hippocampus (NR1-1/1-2: 163+/-11%). The up-regulations observed after memantine (cortex, NR2B: 172+/-17%; hippocampus, NR1-1/1-2: 156+/-8%) or MK-801 (cortex, NR2B: 174+/-22%; hippocampus, NR1-1/1-2: 140+/-3%) were almost identical. No changes were detected in the brainstem. The present data indicate an extremely weak antagonism of NMDA-receptors by acamprosate. However, its ability to modulate the expression of NMDA-receptor subunits in specific brain regions - shared with the well established NMDA-antagonists memantine and MK-801 - may be of relevance for its therapeutic profile, especially considering the growing importance of NMDA-receptor plasticity in the research of ethanol addiction.

Differential expression of alternatively spliced isoforms of neuronal nitric oxide synthase (nNOS) and N-methyl-D-aspartate receptors (NMDAR) in knockout mice deficient in nNOS alpha (nNOS alpha(Delta/Delta) mice)

Recent data suggest that the neuronal isoform of nitric oxide synthase (nNOS) and glutamate receptors of the N-methyl-D-aspartate (NMDA) type are physically coupled and, hence, functionally interrelated. Several alternatively spliced isoforms of the N-methyl-D-aspartate receptor 1 (NMDAR1) subunit and the neuronal nitric oxide synthase (nNOS) are known, and recent studies have shown that a spliced C-terminal may be responsible for the coupling of NMDAR's to nNOS via its PDZ domain and the postsynaptic density protein PSD95. However, little is known about whether and to what extent changes in nNOS expression influence NMDA receptor density or function. We have therefore compared the localization of nNOS alpha, beta and gamma with that of two relevant NMDAR1 splice variants in wild-type mice versus knockout mice deficient in nNOS alpha, generated by homologous recombination with a targeted deletion of exon 2, containing one PDZ domain (nNOS alpha(Delta/Delta) mice). Whereas nNOS alpha was completely absent in nNOS alpha(Delta/Delta) mice, nNOS beta and gamma were expressed in both wild-type and knockout animals. nNOS gamma mRNA, though, was hardly detectable, if at all, mainly within the olfactory bulb, the cerebellum and mesencephalic nuclei of knockout animals. The expression of the NMDAR1-1 splice variant (without any short carboxy-terminal amino acid motif, recognized by PDZ domains) was remarkably decreased in striatal, cortical, hippocampal and cerebellar tissue in nNOS alpha(Delta/Delta) animals, but no changes in NMDAR1-4 (with an alternatively spliced C-terminal and thus with a PDZ binding motif) mRNA and protein levels were observed. While NMDAR1-4 may be related to receptor targeting and clustering to PSD95 and to nNOS, our data suggest that differences in nNOS expression obviously do not directly influence gene expression of this particular NMDAR splice variant. Otherwise, the observed diminution of NMDAR1-1 splice variant mRNA and protein levels may, at least partially, explain the decreased vulnerability of nNOS alpha(Delta/Delta) mice to glutamate-mediated neurotoxicity.

Glutamate-mediated transmission, alcohol, and alcoholism

Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.

Developmental differences in alternative splicing of the NR1 protein in rat cortex and cerebellum

Expression of the C-terminal cassettes of the NR1 protein was examined using a quantitative Western blot method with cassette-specific antibodies. Measurements were made of the percent of total NR1 protein that contained a specific cassette in both the cerebellum and cortex over development. In the cortex, the C1 cassette was shown to be present in about half of total NR1 protein with no change over development. While about half of total NR1 in the cerebellum at postnatal day 42 (P42) contained the C1 cassette, little NR1 protein with this cassette was seen at young ages. In both the cerebellum and cortex, the C2 and C2' cassettes showed opposite developmental patterns, with the C2 cassette decreasing and the C2' cassette increasing over age. Together with previous data on the expression of the N1 cassette, this study describes the alternatively spliced forms of NR1 protein that are dominant at different ages. In the young cerebellum, the NR1(001) form appeared dominant, while in the young cortex there appeared to be a mix of NR1(001) and NR1(011). The most common splice forms of NR1 protein in the adult cerebellum appeared to be NR1(111) and NR1(100). In the adult cortex, there appeared to be a mix of NR1(001) and NR1(011). These data on the expression of the alternatively spliced forms of NR1 allow predictions on the possible characteristics of NMDA receptors in different regions at specific ages.

Enhanced phosphorylation of NMDA receptor 1 subunits in spinal cord dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

The functional enhancement of NMDA receptors after peripheral tissue injury is proposed to contribute to the sensitization of spinothalamic tract (STT) cells and hyperalgesia. Protein phosphorylation is a major mechanism for the regulation of NMDA receptor function. In this study, Western blots, immunofluorescence double labeling, and the retrograde tracing method were used to examine whether phosphorylation of NMDA receptor 1 (NR1) subunits increases in spinal cord tissue and spinal dorsal horn neurons, especially in STT cells, after injection of capsaicin (CAP) into the glabrous skin of one hindpaw of anesthetized rats. Western blots showed that phosphorylated NR1 protein in spinal cord tissue was increased 30 min after CAP injection. Immunofluorescence double-labeling staining showed no significant difference in the number of the NR1-like immunoreactive neurons in laminae I-VII in the lumbosacral segments (L(4)-S(1)) on the ipsilateral and the contralateral sides 30 min after CAP or vehicle injection. However, the numbers of phospho-NR1-like immunoreactive neurons were significantly increased on the ipsilateral side compared with the vehicle injection group. STT cells were labeled by bilateral microinjections of the retrograde tracer fluorogold into the lateral thalamus, including the ventral-posterior lateral nucleus. Immunofluorescence staining was performed at 30, 60, and 120 min after CAP injection or at 30 min after vehicle injection. There was a significant increase in the proportion of STT cells with phosphorylated NR1 subunits compared either with the contralateral side 30 and 60 min after CAP injection or either side of animals after intradermal injection of vehicle. These results provide direct evidence that NMDA receptors in STT cells are phosphorylated after CAP injection.