A splice variant of the N-methyl-D-aspartate (NMDAR1) receptor

Transient down-regulation of NMDA receptor subunit gene expression in the rat retina following NMDA-induced neurotoxicity is attenuated in the presence of the non-competitive NMDA receptor antagonist MK-801

Excitotoxic challenge has been thought to directly target NMDA-receptive neurons to undergo cell death. Recent evidence suggests that NMDA induced cell death is a selective process and that the specificity may be determined by the subunit composition of the NMDA receptor. Using a rat retinal model, we examined the effects of NMDA induced neurotoxicity on the regulation of NMDA receptor subunit gene and protein expression levels to determine if excitotoxic challenge preferentially regulates one or more of the NMDA receptor subunits. Following NMDA insult, the mRNA levels for NR1(com ), NR2A, NR2B and, to a lesser extent, the NR2C subunit were substantially reduced within 24 hr post-treatment (PT), and remained depressed for up to 48 hr. Levels for NR2D, although initially suppressed as early as 6 hr-PT, were least affected by NMDA insult and showed almost full recovery by 48 hr. By 10 days, the levels of gene expression for all five subunits recovered to levels that were indistinguishable from sham treated and untreated retinas. Co-administration of MK-801 with NMDA suppressed the effects of NMDA-induced down-regulation of all five genes. Protein levels for NR1(com ), NR2A and NR2B were also monitored at select time points following NMDA-insult. By 2 days-PT, protein levels for the three subunits were dramatically reduced. By day 10, the levels of protein expression for NR1(com)and NR2B remained suppressed despite the rise in gene expression for these two subunits, whereas protein for NR2A showed a substantial rise in expression. Of the five genes assayed, NR2A and NR2B showed the greatest reduction in expression following NMDA treatment, suggesting that one or both of these subunit may signal events leading to neuronal cell death in the retina. Conversely, gene expression of the NR2D subunit was least affected by NMDA exposure. In view of the evidence that the NR2D subunit is expressed by rod bipolar cells in the rat and that these neurons do not die following NMDA insult, it appears that inclusion of this subunit into functional receptors may provide protection against NMDA-induced cell death. Although the significance of the transient down-regulation of four out of the five NMDA receptor subunits is still not fully understood, the recovery of expression of these genes by day 10-PT indicates that not all of the NMDA receptive neurons are susceptible to NMDA-induced cell death. The preferential down-regulation of the NR2A and NR2B receptor subunits may implicate these subunits as key players in mediating the excitotoxic signal in the retina and possibly elsewhere in the brain.

NMDAR1 isoforms in the rat superior olivary complex and changes after unilateral cochlear ablation

Normal expression and deafness related changes in expression of NMDAR1 isoforms were examined in the rat superior olivary complex (SOC) using in situ hybridization with S35 labeled oligoprobes. Expression was assessed in three SOC nuclei, the lateral and medial superior olives (LSO, MSO) and the medial nucleus of the trapezoid body (MNTB). Silver grain labeling over principal cells of each region was assessed using METAMORPH image analysis system. Counts were made in ipsi- and contralateral sides after unilateral cochlear ablation and in treated and untreated animals. In the normal SOC, NMDAR1a expression was higher than 1b and 1-2 expression was followed by 1-4 and 1-1, with 1-3 below the level for detection. The levels and ratio were comparable in LSO, MSO and MNTB. Five days after cochlear ablation 1a, 1-1, 1-2 and 1-4 showed significant decreases in the ipsilateral LSO and 1-a and 1-2 showed significant decreases in the contralateral MNTB, with no significant changes in the MSO. At 20 days after deafening, no significant changes were seen for any isoform in any nucleus. The transient deafness-induced decreases in expression of NMDAR1 isoforms correlate with loss of excitation.

Selective coexpression of NMDAR2A/B and NMDAR1 subunit proteins in dysplastic neurons of human epileptic cortex

NR1 and NR2 are the two gene families for the NMDA receptor. In vitro studies show that while NR2 alone is nonfunctional, NR1 alone produces weak currents to glutamate or NMDA. We previously showed by immunocytochemistry (ICC) that in normal appearing, nonepileptic human cortical neurons, only NR1 and not NR2 proteins were expressed, in contrast to the presence of both NR1 and NR2 in normal rat cortical neurons. We also showed, in dysplastic epileptic cortex, that both NR1 and NR2 were highly expressed using ICC on adjacent 30-microm sections. However, the relative coexpressions of NR1 and NR2 proteins in single neurons in single sections of human epileptic cortex were unknown. In this study, we used double-labeled immunofluorescence and confocal microscopy to examine the distribution and coexpression of subunit proteins for NR1 and NR2A/B in both nondysplastic (control comparison) and dysplastic regions of human brain resected for the treatment of intractable epilepsy (11 patients). In nondysplastic regions, cortical neurons did not have immunoreactivity (ir) for NR2A/B, whereas NR1-ir was abundant. By contrast, dysplastic neurons in the regions with epileptic cortical dysplasia showed intense NR2A/B-ir in the somata and their dendritic processes. These same NR2A/B-ir dysplastic neurons were colabeled by NR1. These results demonstrate directly that dysplastic neurons express both NR2A/B and NR1 proteins, whereas nondysplastic cortical neurons express only NR1 proteins. Selective coexpression of NR2A/B and NR1 in dysplastic neurons suggests that NR2A/B may form heteromeric NR1-NR2 coassemblies and hyperexcitability in dysplastic neurons that could contribute to focal seizure onset.

NMDA receptor subunit gene expression in the rat brain: a quantitative analysis of endogenous mRNA levels of NR1Com, NR2A, NR2B, NR2C, NR2D and NR3A

Electrophysiological recordings have shown NMDA receptors to be heterogenous structures capable of responding to selected antagonists and agonists in multiple ways. This diversity in functional response has led investigators to conclude that these channels are comprised of unique combinations of receptor subunits which determine a cell's functional NMDA-signature [H. Meguro, H. Mori, K. Araki, E. Kushiya, T. Kutsuwada, M. Yamazaki, T. Kumanishi, M. Arakawa, K. Sakimura, M. Mishina, Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs, Nature (London) 357 (1992) 70-74; T. Ishii, K. Moriyoshi, H. Sugihara, K. Sakurada, H. Kadotani, M. Yokoi, C. Akazawa, R. Shigemoto, N. Mizuno, S. Nakanishi, Molecular characterization of the family of the N-methyl-d-aspartate receptor subunits, J. Biol. Chem. 268 (1993) 2836-2843; K.A. Wafford, C.J. Bain, B. Le Bourdelles, P.J. Whiting, J.A. Kemp, Preferential co-assembly of recombinant NMDA receptors composed of three different subunits, NeuroReport 4 (1993) 1347-1349; T. Priestley, P. Laughton, J. Myers, B. Le Bourdelles, J. Kerby, P.J. Whiting, Pharmacological properties of recombinant human N-methyl-d-aspartate receptors comprising NR1a/NR2A and NR1a/NR2B subunit assemblies expressed in permanently transfected mouse fiberblast cells, Mol. Pharmacol. 48 (1995) 841-848; P.H. Seeburg, N. Burnashev, G. Kohr, T. Kuner, R. Sprengel, H. Monyer, The NMDA receptor channel: molecular design of a coincidence detector, Recent Prog. Horm. Res. 50 (1995) 19-34; A.L. Buller, D.T. Monagahan, Pharmacological heterogeneity of NMDA receptors: characterization of NR1a/NR2D heteromers expressed in Xenopus oocytes, Eur. J. Pharmacol. 320 (1997) 87-94]. In situ hybridization and immunocytochemical studies have shown that there is a spatio-temporal level of expression throughout the brain for each of the receptor subunits with some regions showing a strong preference for a particular subunit. Although these studies collectively show that there are regional differences with respect to NMDA receptor subunit expression in the brain, it has not been determined at what level(s) these genes are expressed or whether each region displays a unique NMDA-subunit signature. The present study was undertaken to examine the level of gene expression for the NR1, NR2A, NR2B, NR2C, NR2D and NR3A receptor subunits in isolated regions of rat brain using the nuclease protection assay. Results show that each of the brain regions examined expresses all six NMDA receptor subunits. The level of message expression for NR1 greatly exceeded that of the other subunits combined, with values ranging from 67-88% of the total subunit gene expression. The relative proportions of the other subunits (NR2A-D and NR3A) varied widely, suggesting that NMDA receptor composition is unique to each region of the brain.

Differential distribution of N-methyl-D-aspartate receptor-2 subunit messenger RNA in the rat superior olivary complex

The distribution of N-methyl-D-aspartate receptor subunit messenger RNA was examined in the superior olivary complex of the rat using in situ hybridization methods. Non-radioactive methods showed N-methyl-D-aspartate receptor-1 and N-methyl-D-aspartate receptor-2A, -2B and -2C subunit expression to be abundant in principal cells of all major superior olivary complex regions, with N-methyl-D-aspartate receptor-1 expression higher than N-methyl-D-aspartate receptor-2. After auto-radiographic in situ hybridization, counting silver grains over cells was used to quantitate and compare levels of expression of the N-methyl-D-aspartate receptor-2 subunits. N-Methyl-D-aspartate receptor-2A, -2B and -2C expression was detected over cells in the lateral and medial superior olive, the medial, lateral and ventral nucleus of the trapezoid body, and the superior paraolivary nucleus. N-Methyl-D-aspartate receptor-2D expression was only observed in the lateral superior olive, lateral nucleus of the trapezoid body and ventral nucleus of the trapezoid body. Three distinct patterns of labeling were observed. The lateral superior olive and ventral nucleus of the trapezoid body showed higher expression of N-methyl-D-aspartate receptor-2A and -2C than other subunits; the medial nucleus of the trapezoid body showed higher N-methyl-D-aspartate receptor-2B expression than other subunits; and the medial superior olive, superior paraolivary nucleus and ventral nucleus of the trapezoid body showed relatively equivalent expression of N-methyl-D-aspartate receptor-2A, -2B and -2C subunits. N-Methyl-D-aspartate receptor-2D had the lowest expression, with levels greater than background in only the lateral superior olive, ventral nucleus of the trapezoid body and lateral nucleus of the trapezoid body. Immunocytochemistry using antibodies to N-methyl-D-aspartate receptor-2A, -2B and -2C showed immunolabeling consistent with in situ hybridization results. These results show diversity in expression of N-methyl-D-aspartate receptor-2 subunits in different superior olivary complex regions and would predict pharmacological differences.

Identification of a long variant of mRNA encoding the NR3 subunit of the NMDA receptor: its regional distribution and developmental expression in the rat brain

A longer variant of rat mRNA encoding the NR3 subunit of the NMDA receptor has been identified. It contains a 60-bp insertion at the nucleotide position 3007 in the intracellular domain of the C-terminal of the previously cloned variant. Therefore, the NR3 mRNA exists in at least two variants--with the insert (NR3-long; NR3-l) and without the insert (NR3-short; NR3-s). The NR3-l variant is expressed throughout the adult rat brain. Moreover, this variant predominates in the occipital and entorhinal cortices, thalamus and cerebellum. Analysis of NR3-l development indicates that it is regulated in a region-specific manner.

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A-C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A-C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed.

The pharmacology of native N-methyl-D-aspartate receptor subtypes: different receptors control the release of different striatal and spinal transmitters

1. N-methyl-D-aspartate (NMDA) increases the release of radiolabelled dopamine, GABA, acetylcholine and spermidine from rat striatal slices and of noradrenaline from the dorsal cervical spinal cord. 2. These five responses show differing sensitivities to NMDA and also to a variety of competitive antagonists, NMDA channel blockers, glycine antagonists and polyamine site antagonists. 3. Inhibitory activity profiles for 20 different antagonists are presented. All compounds tested showed some degree of selectivity with regard to the different responses and each response showed particular characteristics that suggested mediation by a particular native NMDA receptor subtype. 4. Receptors controlling dopamine, GABA and noradrenaline release were generally more sensitive to most antagonists compared to those controlling acetylcholine and spermidine release. 5. Receptors controlling spermidine release were furthermore insensitive to magnesium, argiotoxin, ifenprodil and eliprodil and displayed low sensitivity to memantine, dextrorphan and dextromethorphan. 6. Receptors controlling noradrenaline release could be further discriminated from those controlling dopamine and GABA release by very high sensitivity to magnesium and MK-801 and to the glycine antagonist L-689,560 but not to other glycine antagonists (CNQX, DNQX, 7-Chlorokynurenate, HA-966). 7. Many other individual drug or receptor differences were noted. The different profiles observed suggest a wide diversity of native NMDA receptors with different properties and an unexpectedly rich pharmacopeia of subtype selective antagonists of native NMDA receptors. 8. Matching subtype selectivity to particular behavioural effects may be possible and the design of subtype selective NMDA antagonists for particular clinical applications while avoiding side effect generation seems to be feasible.

Nerve growth factor up-regulates the N-methyl-D-aspartate receptor subunit 1 promoter in PC12 cells

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays important roles in synaptic plasticity, the induction of long term potentiation, and excitotoxicity. Mechanisms governing the regulation of expression of its subunit genes remain largely unknown. The promoter of the essential subunit of the NMDA receptor heteromer, NMDAR1, contains DNA binding elements recognized by the nerve growth factor-inducible/early growth reaction factor (NGFI/Egr) family of transcription factors that are rapidly induced by neurotrophins, such as nerve growth factor (NGF). This study examined the effect of NGF on the activity of the N-methyl-D-aspartate receptor subunit 1 (NMDAR1) promoter/luciferase reporter constructs in PC12 cells, which contain the high affinity TrkA receptor for NGF and the low affinity p75(NTR) receptor for neurotrophins. NGF up-regulated the activity of the NMDAR1 promoter by 3-4-fold in a time- and dose-dependent manner. 5' deletional analysis of the promoter indicated that the responsive element(s) resides in the proximal region containing GSG and Sp1 sites. Mutational analysis of these sites revealed that both were important for NGF regulation. Transient expression of Egr-1 increased activity of the wild type promoter but failed to increase activity of a GSG mutant promoter. Other neurotrophins did not activate the promoter, while K-252a inhibited the action of NGF. These results suggest that the NGF effect is mediated by the high affinity NGF receptor, Trk A and that neurotrophin binding to the low affinity neurotrophin receptor, p75(NTR), alone does not affect the promoter activity. Our results suggest that NGF is able to up-regulate the activity of the NMDAR1 promoter and may play a role in controlling the expression levels of NMDA receptors.

Evidence for native NMDA receptor subtype pharmacology as revealed by differential effects on the NMDA-evoked release of striatal neuromodulators: eliprodil, ifenprodil and other native NMDA receptor subtype selective compounds

NMDA increases the release of [14C]acetylcholine and [3H]spermidine or of [14C]GABA and [3H]dopamine from rat striatal slices. The pharmacology of these responses suggests that release of dopamine and GABA, acetylcholine, and spermidine is mediated, respectively, by three distinct NMDA receptor subtypes. IC50 values of compounds for the inhibition of dopamine and GABA release were closely matched, suggesting mediation by the same subtype. This receptor was generally more sensitive to all NMDA antagonists tested relative to that controlling acetylcholine or spermidine release (channel blockers, glycine antagonists, competitive antagonists and polyamine antagonists). The receptors controlling acetylcholine and spermidine release were characterised by lower antagonist sensitivity in general, and that controlling spermidine release was further defined by a marked insensitivity to ifenprodil, eliprodil, magnesium, dextromethorphan, dextrorphan, memantine, desipramine and polyamine spider toxins. In binding studies in which the displacement of 2 nM [3H]MK801 was studied in membranes prepared from a number of brain regions (in the presence of saturating concentrations of glutamate, glycine and spermidine) small regional differences in IC50 values were observed for a number of channel blockers, but no compound generated biphasic displacement curves that would allow masking of a particular subtype and it was not possible to detect binding components that were insensitive to memantine, dextrorphan dextromethorphan or desipramine. Ifenprodil produced biphasic displacement curves in the 1-day-old rat cortex and midbrain (with IC50 values of approximately 2 and 70 microM) and both ifenprodil and eliprodil displaced a small proportion (18%) of [3H]MK-801 with high affinity in the adult rat spinal cord. Displacement of [3H]MK801 by these compounds in all other adult brain regions (cortex, striatum, hippocampus, thalamus, pons, medulla, cerebellum) was monophasic and of low affinity. In general the subtype selectivity suggested by the release studies was not mirrored in the binding experiments, probably because of excessive heterogeneity of sites in the membrane preparations and to the subtype selectivity of [3H]MK801 itself.

Origin of aspartate-induced responses in rat cerebellar Purkinje cells

The effects of glutamate, aspartate and N-methyl-D-aspartate (NMDA) on Purkinje cells and interneurons were investigated in cerebellar slice cultures using the whole-cell configuration of the patch-clamp technique. L-Glutamate and L-aspartate induced inward currents in Purkinje cells voltage-clamped at -60 mV. In standard external solution, the amplitude of the responses induced by the two amino-acids was a linear function of the membrane potential. L-Aspartate-induced currents were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective antagonist of non-NMDA receptors. NMDA, a selective agonist of NMDA receptors, had no effect of its own on the excitability of Purkinje cells, but was effective in blocking the responses induced by aspartate in Purkinje cells in a voltage-independent manner. In contrast, D-(-)-2-amino-5-phosphonovaleric acid (D-APV), a selective antagonist of NMDA receptors, had no effect on aspartate-induced responses. D-Aspartate also induced responses in Purkinje cells, and the amplitude of these responses was a linear function of the membrane potential. Currents induced by L- and D-aspartate were inhibited by dihydrokainate, a glutamate uptake blocker. In sodium-free external solution, glutamate still induced outward currents in Purkinje cells, whereas L- and D-aspartate no longer evoked any current. When sodium was replaced by lithium in the external medium, no change in the holding current could be detected in Purkinje cells maintained at -60 mV; moreover, in this bathing medium L-aspartate no longer evoked any current whereas glutamate-induced responses were still present. In contrast, interneurons were sensitive to both NMDA and aspartate applications, and these responses were antagonized by D-APV. In addition, aspartate still induced an outward current in sodium-free external solution. This study presents rather direct evidence in favour of L-aspartate as being a very selective NMDA receptor agonist in the cerebellum. L-Aspartate-induced currents in Purkinje cells are not due to activation of mixed NMDA/non-NMDA receptors, but are probably due to the release of L-glutamate induced by aspartate through glutamate uptake.

Expression of N-methyl-D-aspartate receptor subunit mRNAs in the rat pheochromocytoma cell line PC12

The expression of the N-methyl-D-aspartate (NMDA) receptor subunit mRNAs NMDAR1 and NMDAR2A-D was characterized in undifferentiated and nerve growth factor (NGF)-differentiated PC12 cells using Northern blotting, RNase protection assays (RPA) and polymerase chain reaction (PCR). PC12 cells expressed predominately the splice variant NMDAR1-4a and smaller amounts of NMDAR1-1a, NMDAR1-2a and NMDAR1-3a. No splice isoforms containing exon 5 were detected. The NMDAR2C subunit was detected in PC12 cells by Northern blotting and trace amounts of NMDAR2A, B and D were detected by PCR. PC12 cells may be a useful model system for the study of the transcriptional and post-transcriptional regulation of expression of the NMDA receptor subunit genes, including the alternative splicing of NMDAR1 pre-mRNAs.

Localization of NMDA receptor subunit mRNAs in the rat locus coeruleus

N-Methyl-D-aspartate (NMDA)-activated ionotropic glutamate receptors in the CNS are thought to play a crucial role in cognitive processes, neurological disorders as well as in progressive neurodegenerative diseases. In spite of the overwhelming evidence for the existence of structurally different subunits of NMDA receptors in the CNS, the functional relevance of this heterogeneity is still poorly understood. A first step in this direction is to demonstrate the receptor composition in well-characterized transmitter-specific neuronal populations, such as the noradrenergic neurons of the rat locus coeruleus (LC). LC neurons may play a key role in the regulation of vigilance, attention, learning and memory, as well as anxiety and are affected in neurodegenerative disorders. In this study we examined, by means of in situ hybridization with 35S-labelled oligodeoxynucleotide probes, the distribution of mRNAs encoding the splice variants of the NMDAR1 subunit as well as four NMDAR2 subunits (A-D) in the rat LC. Identified neurons express mRNAs encoding several NMDAR1 subunit isoforms (4a, 2a > 2b, 4b) as well as NMDAR2 subunits (2B > 2D), whereas other transcripts (1a,1b,3a,3b,2A,2C) were not detected. These findings suggest that NMDA receptors in the LC are composed of unique combination(s) of subunits, e.g. 4a-2B, of as yet unknown stoichiometry. Whether the identification of this potential drug target can be exploited, e.g. in the development of new anxiolytics, antidepressants, or neuroprotective agents, awaits further investigations.

Human N-methyl-D-aspartate receptor modulatory subunit hNR3: cloning and sequencing of the cDNA and primary structure of the protein

Several cDNA clones encoding the human N-methyl-D-aspartate receptor modulatory subunit hNR3, were isolated from a human fetal brain library. The hNR3 cDNA demonstrated a 91.3-91.5% nucleotide (nt) identity with the rat NR2B and mouse epsilon 2 cDNAs. The nt sequence of hNR3 would encode a 1484 amino acid (aa) protein that has a 98.4-98.5% identity with the mouse epsilon 2 and rat NR2B subunits.

Expression patterns of a glutamate-binding protein in the rat central nervous system: comparison with N-methyl-D-aspartate receptor subunit 1 in rat

Using radioactive in situ hybridization histochemistry, we examined the topographical patterns of expression of the messenger RNA encoding a glutamate-binding protein (N-methyl-D-aspartate receptor glutamate-binding protein in rat; NMDARgbs) in the central nervous system of the rat. Expression patterns of N-methyl-D-aspartate receptor glutamate-binding protein were compared with those of N-methyl-D-aspartate receptor subunit 1 (NMDAR1) of the N-methyl-D-aspartate receptor on adjacent sections. N-methyl-D-aspartate receptor glutamate-binding protein is not expressed in glial cells. The expression of both N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor subunit 1 was observed in virtually all neurons throughout the central nervous system. The mean level of N-methyl-D-aspartate receptor subunit 1 expression was higher than that of N-methyl-D-aspartate receptor glutamate-binding protein. Similar topographical patterns of expression of N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor were observed in most regions, except in discrete thalamic, hypothalamic and brainstem nuclei. Concomitantly for N-methyl-D-aspartate receptor glutamate-binding protein and N-methyl-D-aspartate receptor subunit 1, the highest expression levels were distributed in the mitral layer of main and accessory olfactory bulbs, granule cell layer of the dentate gyrus, polymorphic and pyramidal layers of CA1-3 fields of Ammon's horn. A slightly less prominent expression was observed in the glomerular and granule cell layers of main and accessory olfactory bulbs, anterior olfactory nucleus, layer 2 of piriform cortex, olfactory tubercle and taenia tecta. In the cerebellum, the prominent level of N-methyl-D-aspartate receptor glutamate-binding protein expression was slightly higher in the Purkinje cell layer than in the granule cell layer, an opposite pattern being observed for N-methyl-D-aspartate receptor subunit 1. A moderately high expression level of both messenger RNAs was observed in the medial septal nucleus, nucleus of the diagonal band of Broca, dorsal part of the endopiriform nucleus, and in the anteroventral and anterolateral parts of the bed nucleus of the stria terminalis. In the neocortex, the mean expression level of N-methyl-D-aspartate receptor glutamate-binding protein is moderate, while the mean level of N-methyl-D-aspartate receptor subunit 1 expression is high. With both probes, layer IV is slightly less labeled than the other layers.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning and sequence analysis of additional splice variants encoding human N-methyl-D-aspartate receptor (hNR1) subunits

Two cDNA clones representing previously unidentified human N-methyl-D-aspartate receptor (hNR1) subunit polypeptides were isolated and sequenced. Clone hNR1-4 was isolated from a human hippocampus cDNA library and was presumably generated by alternative RNA splicing in the 3' amino acid (aa) coding regions. The hNR1-4 cDNA demonstrated an 85.7% nucleotide (nt) identity to the corresponding rat NR1 (rNR1) cDNA. The nt sequence of hNR1-4 would encode a protein that has a 99.8% identity with the corresponding rNR1 subunit. Clone hNR1N was isolated by polymerase chain reaction (PCR)-mediated amplification of a 0.6-kb DNA fragment from human cerebellum cDNA. The nt sequence of this DNA fragment was identical to previously isolated hNR1 cDNA clones, except for the presence of a 63-bp DNA insertion that would encode an additional 21 aa. This DNA insertion occurs in the 5' aa coding regions of hNR1 and presumably represents an exon that is subject to alternative splicing. The nt and aa sequences of this exon are identical between human and rat.

Human N-methyl-D-aspartate receptor modulatory subunit hNR2A: cloning and sequencing of the cDNA and primary structure of the protein

Several cDNA clones encoding the human N-methyl-D-aspartate receptor modulatory subunit hNR2A, were isolated from human hippocampus and fetal brain libraries. DNA sequence analysis revealed overlapping clones permitting the reconstruction of full-length hNR2A cDNA. The hNR2A cDNA demonstrated an 88-89% nucleotide (nt) identity with the corresponding rodent cDNAs. The nt sequence of hNR2A would encode a 1464-aa protein that has a 95.2% identity with the rodent NR2A subunits.

Cellular localization and laminar distribution of NMDAR1 mRNA in the rat cerebral cortex

N-methyl-D-aspartate (NMDA) receptors, which play a critical role in many cortical functions, are composed of a heteromeric assembly of different subunits: of these, the NMDA receptor subunit 1 (NMDAR1) is a constant component of, and thus an excellent marker for, NMDA receptors. In this study, we have investigated the cellular localization and laminar distribution of NMDAR1 mRNA in the cerebral cortex of adult rats by in situ hybridization histochemistry with a 35S-labeled cRNA probe. Specificity and background levels were determined in adjacent sections incubated with a 35S-labeled sense RNA. In sections incubated with the antisense RNA probe, specific hybridization signal was observed in a large number of cells. Some cells, however, did not appear to contain NMDAR1 mRNA. The vast majority of these unlabeled cells were small, suggesting that they are astrocytes or other small nonneuronal cells. Double-labeling studies with in situ hybridization histochemistry and immunocytochemistry with antibodies to glial fibrillary acidic protein (GFAP) showed that about 95.7% of the GFAP-positive cells did not express NMDAR1 mRNA, indicating that virtually all astrocytes do not contain this transcript. A semiquantitative evaluation of cortical neurons, defined as those cells larger than the GFAP-positive astrocytes, revealed that about 80% were associated with silver grains. The number of silver grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and all labeled neurons were grouped into five groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), D (31-40 grains), and E (> 40 grains). The number of neurons belonging to each group was then evaluated according to their occurrence in each cortical layer. In layer I all labeled neurons were in group A, whereas in layers II-III and V-VI positive neurons were in group A-E. In layer IV most neurons were in groups A and B, whereas only a few were in group E. These observations indicate that 1) virtually all cortical cells containing NMDAR1 mRNA in adult rats are neurons; 2) about 80% of all cortical neurons express NMDAR1 mRNA; and 3) labeled neurons can be divided into several groups on the basis of NMDAR1 mRNA levels expressed, which presumably reflect the number of NMDA receptors. The existence of neurons with a different number of receptors may be a critical factor for determining the physiological effect of NMDA receptor activation.(ABSTRACT TRUNCATED AT 400 WORDS)