Turnover analysis of glutamate receptors identifies a rapidly degraded pool of the N-methyl-D-aspartate receptor subunit, NR1, in cultured cerebellar granule cells

Mobile NMDA receptors at hippocampal synapses

Glutamate receptors are concentrated in the postsynaptic complex of central synapses. This implies a highly organized and stable postsynaptic membrane with tightly anchored receptors. Recent reports of rapid AMPA receptor insertion and removal at synapses have challenged this view. We examined the stability of synaptic NMDA receptors on cultured hippocampal neurons using the open-channel blockers (+)-MK-801 and ketamine to tag synaptic NMDA receptors. NMDA receptor-mediated EPSCs showed an anomalous recovery following "irreversible" MK-801 block. The recovery could not be attributed to MK-801 unbinding or insertion of new receptors, suggesting that membrane receptors had moved laterally into the synapse. At least 65% of synaptic NMDA receptors were mobile. Our results indicate that NMDA receptors can move laterally between synaptic and extrasynaptic pools, providing evidence for a dynamic organization of synaptic NMDA receptors in the postsynaptic complex.

Antisense oligonucleotides to N-methyl-D-aspartate receptor subunits attenuate formalin-induced nociception in the rat

Noxious peripheral stimuli increase the sensitivity of central nociceptive neurons to subsequent noxious stimuli. This occurs in part through activation of spinal N-methyl-D-aspartate (NMDA) receptors. These receptors are heteromeric complexes of NMDA-R1 and NMDA-R2 A--D subunits. NMDA-R1 is necessary for the formation of functional NMDA receptors whereas the R2 subunits (A-D) modify the properties of the receptor. However, the role of the various receptor subtypes in nociception has not been established. In this study, we used intrathecally administered phosphodiester antisense oligonucleotides (ODEs) to examine the role of the NMDA-R1, NMDA-R2C and NMDA-R2D subunits in the mediation of formalin-induced nociception in the rat. The antisense ODEs against the NMDA-R1 and NMDAR-2C subunits reduced nociceptive behaviors whereas the corresponding sense ODEs had no effect. In contrast, nociception was unaffected by the antisense ODE to NMDAR-2D. Using an RNase protection assay, we also found that each antisense ODE selectively decreased the level of the corresponding mRNA in the lumbar spinal cord but that the sense ODEs had no such effect. Accordingly, these data provide evidence that the R1 and R2C subunits, but not R2D, of the NMDA receptor participate in the development of formalin-induced nociception.

N-methyl-D-aspartate receptors: transient loss of NR1/NR2A/NR2B subunits after traumatic brain injury in a rodent model

Hippocampal N-methyl-D-aspartate (NMDA) receptor subunits, by virtue of their involvement in excitotoxic injury as well as memory association, may play an important role in the pathophysiologic mechanisms of traumatic brain injury (TBI). In this study, temporal changes in NMDA receptor subunit (NR1, NR2A, and NR2B) levels in rat hippocampus after TBI were investigated by Western blot and mRNA expression levels by RT-PCR methods. Sprague-Dawley rats (250-350 g) were employed, and a controlled cortical impact injury device was used to produce the TBI in rodents. At different postinjury time points (2, 6, 12, 24, and 48 hr), the rat hippocampi were dissected out for protein and RNA preparation. Western blot analysis revealed significant decreases of NR1, NR2A, and NR2B subunit proteins at 6 and 12 hr postinjury in rat hippocampus. Complete recovery of NR1, NR2A, and NR2B subunit protein to the levels of sham controls was observed at 24 hr postinjury. However, RT-PCR analysis did not show any significant change in the mRNA levels at 2, 6, and 12 hr postinjury in comparison with sham controls, suggesting nontranscriptional change in the levels of these subunits. Thus, TBI can produce transient degradation of NMDA receptor subunits in the hippocampus, which might contribute to temporary memory impairment after injury.

Role of RyRs and IP3 receptors after traumatic injury to spinal cord white matter

Calcium influx and elevation of intracellular free calcium (Ca2+i), with subsequent activation of degenerative enzymes is hypothesized to cause cell injury and death after trauma. We examined the effects of traumatic compressive injury on (Ca2+)i dynamics in spinal cord white matter. We conducted electrophysiological studies with ryanodine and inositol (1,4,5)-triphosphate (IP3) receptor agonists and antagonists in an in vitro model of spinal cord injury (SCI). A 25-30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (37 degrees C) and injured with a modified clip (2-g closing force) for 15 sec. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes. The CAP decreased to 55.2+/-6.8% of control (p < 0.05) after spinal cord injury (SCI). Chelation of Ca2+i with BAPTA-AM (a high-affinity calcium chelator) promoted significantly greater recovery of CAP amplitude (83.2+/-4.2% of control; p < 0.05) after injury. Infusion of caffeine (1 and 10 mM) exacerbated CAP amplitude decline (45.1+/-5.9% of control; p < 0.05; 44.6+/-3.1% of control; p < 0.05) postinjury. Blockade of Ca2+i release through ryanodine-sensitive receptors (RyRs) with dantrolene (10 microM) and ryanodine (50 microM), conferred significant (p < 0.05) improvement in CAP amplitude after injury. On the other hand, blockade of Ca2+i with inositol (1,4,5)-triphosphate receptor (IP3Rs) blocker 2APB (10 microM) also conferred significant improvement in CAP amplitude after injury (82.9+/-7.9%; p < 0.05). In conclusion, the injurious effects of Ca2+i in traumatic central nervous system (CNS) white matter injury appear to be mediated both by RyRs and through IP3Rs calcium-induced calcium release receptors (CICRs).

NMDA receptors and PSD-95 are found in attachment plaques in cerebellar granular layer glomeruli

N-methyl-D-aspartate (NMDA) receptors mediate long-term changes in excitatory synapses in response to glutamate release. In the cerebellar granular layer, most glutamatergic synapses are formed between mossy terminals and granule cell dendrites, which together with some other components, make up complex glomerular structures. Glomeruli contain numerous attachment plaques (or puncta adherentia), which are sites of adhesion between cells. These structures are found mainly between granule cell dendrites, and probably help maintain the integrity of glomeruli. Attachment plaques contain adhesive proteins such as cadherins. In this study, we show that NMDA receptors are common at these attachment plaques, in addition to being found at synapses. We used four different antibodies to the NMDA receptor subunit, NR1, and another to NR2A/B. In contrast, labelling for an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptor antibody was seen only in a few attachment plaques, although AMPA receptors were seen frequently at glomerular synapses. We also show that substantial levels of the NMDA receptor-associated protein, PSD-95, are found in both synapses and attachment plaques. One way that NMDA receptors mediate changes in synapses is through effects on synaptic cadherins, which change their adhesive properties in response to NMDA receptor activation and consequently may alter synaptic function. The presence of NMDA receptors in attachment plaques suggests that these receptors mediate changes in the adhesive properties of these plaques, similar to this function in synapses.

Strychnine-blocked glycine receptor is removed from synapses by a shift in insertion/degradation equilibrium

The long-term inhibition by strychnine of glycine receptor activity in neurons provokes the receptor's selective intracellular accumulation and disappearance from synapses. This could result either from a disruption of the postsynaptic anchoring of the receptor or from an arrest of its exocytic transport. In this study we combined biochemical and fluorescence microscopy analyses to determine on a short time scale the fate of the strychnine-inactivated glycine receptor. Quantification of the cellular content of receptor showed that the rapid accumulation depends on protein synthesis. Cell surface biotinylation of neurons demonstrated that strychnine did not accelerate the turnover rate of the receptor. Labeling of endosomes indicated that, in strychnine-treated cells, the accumulated receptor is not blocked in the endosomal transport pathway. Taken together, these results indicate that strychnine does not destabilize the postsynaptic receptor but triggers its disappearance from synapses by a nondegradative sequestration of newly synthesized molecules in a nonendocytic compartment.

Turnover analysis of N-methyl-D-aspartate receptor subunit NR1 protein in PC12 cells

The post-translational fate of N-methyl-D-aspartate receptor (NMDAR) subunit NR1 was characterized in PC12 cells using pulse-chase labeling, block of protein synthesis by cyclohexamide and deglycosylation by endoglycosidase H. Metabolic labeling of NR1 protein indicated a biphasic degradation of NR1 protein with half-lives of 1.6 and 16.1 h for a rapidly (78%) and a slowly (22%) degrading population. Immunoprecipitation of NR1 following the block of protein synthesis by cyclohexamide revealed that the rapidly and slowly degrading pools mainly consisted of the NR1 splice variants NR1-4a and NR1-2a. Sensitivity of NR1 protein to deglycosylation by endoglycosidase H indicated the presence of an immature form of NR1 that was retained in the endoplasmic reticulum. PC12 cells serve as a useful model for the elucidation of translational and post-translational mechanisms of NMDAR expression.

The N-methyl-D-aspartate receptor splice variant NR1-4 C-terminal domain. Deletion analysis and role in subcellular distribution

The intracellular C-terminal domain of the N-methyl-d-aspartate receptor (NMDAR) subunits 1 (NR1) and 2 (NR2) are important, if not essential, to the process of NMDAR clustering and anchoring at the plasma membrane and the synapse. Eight NR1 splice variants exist, four of which arise from alternative splicing of the C-terminal exon cassettes. Alternative splice variants may display a differential ability to interact with synaptic anchoring proteins, and splicing of C-terminal exon cassettes may alter the mechanism(s) of subcellular localization and targeting. The NR1-4 isoform has a significantly different C-terminal composition than the prototypic NR1-1 isoform. Whereas the NR1-1 C terminus is composed of C0, C1, and C2 exon cassettes, the NR1-4 C terminus is composed of the C0 and C2' cassettes. In the present study, we address the importance of the NR1-4 C-terminal exon cassettes (C0C2') in subcellular localization in differentiated pheochromocytoma (PC12) cells, in organotypic cultures of dorsal root ganglia, and also in heterologous cells. NR1-4-green fluorescent protein chimeras were created with deletion of either C0, C2', or both cassettes to address their importance in subcellular distribution and cell surface expression of the NR1-4 subunit. These experiments demonstrate that the NR1-4 splice variant found predominantly in the spinal cord uses the C0 cassette, to a large degree, to organize the subcellular distribution of this receptor subunit. Although the role of the C2' subunit is less clear, it may be involved in subunit clustering. However, this clustering is not always as efficient as that attributed to C0 alone or to the natural combination of C0C2'. Finally, although an intact C-terminal domain is neither necessary for interaction with the NR2A subunit nor surface expression of the NR1-4 subunit, the C-terminal domain fragment alone blocks surface expression of native NR1-4, in a dominant negative fashion, when the two are coexpressed.

Subcellular distribution of high-voltage-activated calcium channel subtypes in rat globus pallidus neurons

Globus pallidus (GP) neurons receive dense inhibitory synaptic inputs interspersed with sparse excitatory inputs distributed across the entire extent of their somata and dendrites. Yet, despite this predominance of inhibitory influence, GP neurons fire at a high tonic rate, suggesting that intrinsic properties play an important role in determining the physiological characteristics of these neurons. High-voltage-activated (HVA) calcium channels represent an important class of conductances that plays roles in controlling neurotransmitter release, postsynaptic excitability, and intracellular calcium signaling. To better understand the intrinsic properties of GP neurons, we examined the subcellular localization of HVA calcium channels by using immunocytochemistry at the electron microscopic level. Peroxidase labeling with antibodies against P/Q-, N-, and R-type HVA calcium channels demonstrated the presence of these channels in both proximal and distal dendrites of GP neurons. P/Q-, N-, and R-type channels were also found in presynaptic terminals, whereas L-type channels were found exclusively postsynaptically in neuronal elements. Immunogold labeling demonstrated that, although the density of intracellular L-type calcium channel labeling remains constant throughout the proximal-distal extent of the dendritic tree of GP neurons, the density of plasma membrane-bound channels is greater in distal dendrites. The finding of HVA calcium channels distributed throughout the whole dendritic tree of GP neurons indicates that these channels may interact with synaptic inputs to allow rich processing possibilities for GP neuron dendrites. Furthermore, the finding of a greater density of plasma membrane-bound L-type channels in distal dendrites expands the view that L-type channels are important only in somatic and proximal locations.

The expression of dominant-negative subunits selectively suppresses neuronal AMPA and kainate receptors

Glutamate-gated ion channels are widely expressed in neurons where they serve a host of cellular functions. An appealing, but yet unexplored, way to delineate the functions of particular glutamate receptor subtypes is to direct the expression of dominant-negative and gain-of-function mutant subunits. We tested the ability of two dominant-negative subunits, an alpha-amino-3-hydroxy-5-methyl-isoxazolproprionic acid receptor subunit and a kainate receptor subunit, to silence recombinant and neuronal glutamate receptors. Co-expression studies in non-neuronal cells indicated that the inclusion of a single mutant subunit was sufficient to silence the receptor. When expressed in cerebellar granule cells, the dominant-negative subunits silenced native channels in a subtype-specific fashion. Immunocytochemical staining of control and transfected neurons, as well as studies with a gain-of-function glutamate receptor-1 mutant, indicated that the mutant subunits were expressed at levels roughly equal to the total abundance of related native subunits, and both dominant-negatives suppressed native channel expression 60-65% when tested 24 h post-transfection. If co-assembly of the mutant subunits with related native subunits is combinatorial, this level of suppression gives receptor half-lives of approximately 20 h.

Chronic ethanol exposure increases gene transcription of subunits of an N-methyl-D-aspartate receptor-like complex in cortical neurons in culture

Chronic exposure to ethanol leads to adaptive responses in neurons, including enhanced activity of N-methyl-D-aspartate (NMDA) receptors. Chronic treatment of neurons with ethanol also leads to increases in the protein levels of three subunits of an NMDA receptor-like complex. In the present study, we explored whether the increases in subunit protein levels are the result of either increased transcription or diminished protein turnover. We found that a 72-h exposure of cortical neurons in culture to 100 mM ethanol caused enhanced transcription of the genes for the glutamate-binding (GBP) and glycine-binding protein subunits of the receptor-like complex. This treatment had no effect on protein turnover of either GBP or of the NMDA receptor subunit NR1, suggesting that the increases in protein expression are the result of increased DNA transcription.

Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intracellular pool of AMPA receptors in rat brain. By velocity gradient centrifugation of microsomal light membranes from rat brain, we identified a membrane fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA receptors. This membrane compartment sedimented more slowly than synaptosomes but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 and syntaxin-13. Morphological examination of this fraction revealed round and tubular vesicles ranging from approximately 50 to 300 nm in diameter. Immunocytochemistry of cultured hippocampal neurons showed that a significant portion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associated with tubulovesicular recycling endosomes) and with transferrin receptors. Taken together, these results suggest that a pool of intracellular GluR2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA receptors.

Synapse-associated protein 97 selectively associates with a subset of AMPA receptors early in their biosynthetic pathway

The regulation of AMPA receptors at the postsynaptic membrane is a fundamental component of synaptic plasticity. In the hippocampus, the induction of long-term potentiation increases the delivery of GluR1, a major AMPA receptor subunit in hippocampal pyramidal neurons, to the synaptic plasma membrane through a mechanism that requires the PDZ binding domain of GluR1. Synapse-associated protein 97 (SAP97), a member of the membrane-associated guanylate kinase family, is believed to associate with AMPA receptors (AMPARs) containing the GluR1 subunit, but the functional significance of these interactions is unclear. We investigated the interaction of GluR1 with SAP97, the only PDZ protein known to interact with GluR1. We find that interactions involving SAP97 and GluR1 occur early in the secretory pathway, while the receptors are in the endoplasmic reticulum or cis-Golgi. In contrast, few synaptic receptors associate with SAP97, suggesting that SAP97 dissociates from the receptor complex at the plasma membrane. We also show that internalization of GluR1, as triggered by NMDAR activation, does not require SAP97. These results implicate GluR1-SAP97 interactions in mechanisms underlying AMPA receptor targeting.

Synapse-associated protein 97 selectively associates with a subset of AMPA receptors early in their biosynthetic pathway

The regulation of AMPA receptors at the postsynaptic membrane is a fundamental component of synaptic plasticity. In the hippocampus, the induction of long-term potentiation increases the delivery of GluR1, a major AMPA receptor subunit in hippocampal pyramidal neurons, to the synaptic plasma membrane through a mechanism that requires the PDZ binding domain of GluR1. Synapse-associated protein 97 (SAP97), a member of the membrane-associated guanylate kinase family, is believed to associate with AMPA receptors (AMPARs) containing the GluR1 subunit, but the functional significance of these interactions is unclear. We investigated the interaction of GluR1 with SAP97, the only PDZ protein known to interact with GluR1. We find that interactions involving SAP97 and GluR1 occur early in the secretory pathway, while the receptors are in the endoplasmic reticulum or cis-Golgi. In contrast, few synaptic receptors associate with SAP97, suggesting that SAP97 dissociates from the receptor complex at the plasma membrane. We also show that internalization of GluR1, as triggered by NMDAR activation, does not require SAP97. These results implicate GluR1-SAP97 interactions in mechanisms underlying AMPA receptor targeting.

Activation of metabotropic glutamate receptor 1 accelerates NMDA receptor trafficking

Regulation of neuronal NMDA receptors (NMDARs) by group I metabotropic glutamate receptors (mGluRs) is known to play a critical role in synaptic transmission. The molecular mechanisms underlying mGluR1-mediated potentiation of NMDARs are as yet unclear. The present study shows that in Xenopus oocytes expressing recombinant receptors, activation of mGluR1 potentiates NMDA channel activity by recruitment of new channels to the plasma membrane via regulated exocytosis. Activation of mGluR1alpha induced (1) an increase in channel number times channel open probability, with no change in mean open time, unitary conductance, or reversal potential; (2) an increase in charge transfer in the presence of NMDA and the open channel blocker MK-801, indicating an increased number of functional NMDARs in the cell membrane; and (3) increased NR1 surface expression, as indicated by cell surface Western blots and immunofluorescence. Botulinum neurotoxin A or expression of a dominant negative mutant of synaptosomal associated protein of 25 kDa molelcular mass (SNAP-25) greatly reduced mGluR1alpha-mediated potentiation, indicating that receptor trafficking occurs via a SNAP-25-mediated form of soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor-dependent exocytosis. Because group I mGluRs are localized to the perisynaptic region in juxtaposition to synaptic NMDARs at glutamatergic synapses in the hippocampus, mGluR-mediated insertion of NMDARs may play a role in synaptic transmission and plasticity, including long-term potentiation.

In vivo control of NMDA receptor transcript level in motoneurons by viral transduction of a short antisense gene

Glutamate receptors play critical roles in normal and pathological processes. We developed an antisense gene delivery strategy to modulate the NMDA type of glutamate receptor. Using transient transfection in vitro and viral mediated gene transfer in vitro and in vivo, the effect of expression of an antisense gene fragment (60 bp) of the NR1 subunit was tested. Immunoblot analysis showed an antisense-concentration-dependent reduction in the NR1 subunit upon transient co-transfection of a plasmid expressing a sense NR1 gene and a plasmid expressing the antisense fragment into COS-7 cells. After recombination into an adenoviral vector, this antisense fragment reduced the amount of endogenous NR1 protein in PC12 cells. Finally, direct intraparenchymal injection of the viral vector into rat spinal cord resulted in diminished NR1 in motor neurons. Our results demonstrate the efficacy of this approach, which combines antisense with viral gene delivery to control the expression of specific genes in vivo. This approach may also be useful in reducing excitatory neurotransmission in vivo, with implications for the treatment of spinal disorders such as amyotrophic lateral sclerosis or chronic pain.

Effect of gestational ethanol exposure on the NMDA receptor complex in rat forebrain: from gene transcription to cell surface

Effects of gestational ethanol exposure on the trafficking of the NMDA receptor complex were investigated. Studies focused on three distinct processes in NMDA receptor translocation: (1) the level of gene transcription (2) nascent NMDA receptor subunits (NR) associated with the endoplasmic reticulum bound chaperone protein calnexin and (3) NMDA receptors associated with the cell surface anchoring protein PSD-95. Forebrain mRNA and membrane proteins were isolated from postnatal day 1 rat pups from prenatally ethanol exposed, pair-fed and ad libitum experimental groups. Ribonuclease protection assays were carried out to determine the levels of NR2A, NR2B, and NR2C mRNA within the treatment groups determined. Results indicated that gestational ethanol exposure did not affect the gene transcription of the NR2 subunits. Immunoprecipitation experiments were conducted with an anti-calnexin antibody or an anti-PSD-95 antibody and the immunoprecipitates probed for NR1 and NR2 subunits. Within the anti-calnexin immunoprecipitates, no NR2A, NR2B or NR2C subunits were detectable, but a significant pool of NR1 subunits was identified. These findings suggest that NR1 subunits but not NR2 subunits are associated with calnexin within the endoplasmic reticulum. Further, gestational ethanol exposure significantly increased the NR1 polypeptide levels in the anti-calnexin immunoprecipitate. Anti-PSD-95 immunoprecipitates revealed an abundance of NR1 and NR2B subunits, and these complexes were unaffected by gestational ethanol exposure. No NR2A or NR2C subunits were detected. These results suggest that gestational ethanol exposure significantly affects the assembly and transport of NMDA receptors. Gestational ethanol exposure may not alter the composition of the PSD-95 associated NMDA receptor complex.

The NR1 subunit of the N-methyl-D-aspartate receptor can be efficiently expressed alone in the cell surface of mammalian cells and is required for the transport of the NR2A subunit

We have used a heterologous system of expression of N-methyl-D-aspartate (NMDA) receptors based on the use of vaccinia virus to analyse the maturation, transport, assembly and differential expression of the NR1 and NR2A subunits of the receptors. We have demonstrated that the NR1 subunit is efficiently transported to the plasma membrane in cells expressing NR1 alone, similarly to cells producing NR1 and NR2A together. In contrast, NR2A requires NR1 expression to be located at the cell surface. The stability of both receptor subunits expressed alone is similar to that obtained in cells producing NR1 and NR2A. In pulse-chase experiments, the NR1 subunit displays a biphasic decay, with a fraction of the protein having a half-life of only 1 h and the remaining presenting a turnover longer than 24 h, similar to values obtained for the NR2A subunit. Our results also show a maturation process affecting the carbohydrate moiety in the NR1 subunit, such that immature NR1 has a much shorter half-life than the mature form or the NR2A subunit. Finally, we show that only a fraction of mature NR1 interacts with NR2A to form multimeric functional complexes.

An NMDA receptor ER retention signal regulated by phosphorylation and alternative splicing

Formation of mature excitatory synapses requires the assembly and delivery of NMDA receptors to the neuronal plasma membrane. A key step in the trafficking of NMDA receptors to synapses is the exit of newly assembled receptors from the endoplasmic reticulum (ER). Here we report the identification of an RXR-type ER retention/retrieval motif in the C-terminal tail of the NMDA receptor subunit NR1 that regulates receptor surface expression in heterologous cells and in neurons. In addition, we show that PKC phosphorylation and an alternatively spliced consensus type I PDZ-binding domain suppress ER retention. These results demonstrate a novel quality control function for alternatively spliced C-terminal domains of NR1 and implicate both phosphorylation and potential PDZ-mediated interactions in the trafficking of NMDA receptors through early stages of the secretory pathway.

Schedule of NMDA receptor subunit expression and functional channel formation in the course of in vitro-induced neurogenesis

NE-7C2 neuroectodermal cells derived from forebrain vesicles of p53-deficient mouse embryos (E9) produce neurons and astrocytes in vitro if induced by all-trans retinoic acid. The reproducible morphological stages of neurogenesis were correlated with the expression of various NMDA receptor subunits. RT-PCR studies revealed that GluRepsilon1 and GluRepsilon4 subunit mRNAs were transcribed by both non-induced and neuronally differentiated cells. GluRepsilon3 subunit mRNAs were not synthesized by NE-7C2 cells and increased numbers of messages from the GluRepsilon2 gene were detected only after neural network formation. The presence of the GluRzeta1 protein was detected throughout neural induction, whereas retinoic acid-induced neuron formation elevated the amount of exon 21 (C1)- and exon 22 (C2)-containing GluRzeta1 mRNAs and resulted in the appearance of exon 5 (N1)-containing transcripts. NMDA-elicited Ca(2+)-signals were detected only in cells displaying neuronal morphology, but preceding the appearance of synapsin-I immunoreactivity. Our findings demonstrated that, in spite of the presence of subunits necessary for channel formation, functional channels were formed by NE-7C2 cells no sooner than the time of neurite maturation. The data show that the cell line provides a suitable model to analyse the mechanisms involved in NMDA receptor gene expression before the appearance of synaptic communication.

Changes of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors in layer V of epileptogenic, chronically isolated rat neocortex

In vivo chronic partial isolation of neocortical islands results in epileptogenesis that involves pyramidal neurons of layer V. To test whether an alteration in glutamate receptors might contribute to the epileptiform activity, we analysed the time-course of light microscopic changes in expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors using subunit-specific antibodies. The isolation caused a rapid down-regulation of immunoreactivity for GluR1 and GluR2/3 subunits in deep layer V pyramidal neurons within the neocortical island which was evident 24h post-lesion, and within three days was reduced to about 40-60% of the control level. Many pyramidal cells in deep layer V completely lacked GluR2. Between one and four weeks of survival, down-regulation of GluR2/3 and GluR2 involved the majority of pyramidal layer V neurons, except for cells in the upper part of layer V, and those within narrow areas of all sub-laminae of layer V ("micro-islands"). Initial down-regulation was also observed one to three days post-lesion for subunits 1 and 2 of the N-methyl-D-aspartate receptor, but in contrast to GluR2/3 immunoreactivity, NMDAR2A/B immunoreactivity was enhanced three weeks post-lesion. The present data provide evidence for plastic changes in glutamate receptors in neurons of partially isolated neocortical island. A sub-population of layer V neurons remains relatively unaffected, and would presumably be capable of generating fast glutamatergic synaptic potentials necessary for the development of synchronous epileptiform activity.

Low-threshold N-methyl-D-aspartate receptor function correlates negatively with learning

The intermediate, medial hyperstriatum ventrale (IMHV) is an area of the forebrain of the domestic chick which exhibits great plasticity. Moreover, there is a strong link between plasticity in the IMHV and specific changes in behaviour. The IMHV in vitro is still plastic, and many of its physiological properties are age-dependent, peaking in slices taken from 3- or 4-day-old birds. This 'window' coincides with an important transitional period in a chick's normal behavioural development. It has also been claimed that reversal training is at its most effective in 3- and 4-day-old birds - a proposition which was confirmed by the experiments reported here. A combination of in vivo training followed by in vitro electrophysiology also revealed that the function of low-threshold N-methyl-D-aspartate receptors (one of the age-related variables) is negatively related to the effectiveness of reversal training, when age is held constant.

Intrauterine hypoxia-ischemia alters expression of the NMDA receptor in the young rat brain

Effects of intrauterine hypoxia-ischemia (HI) on expression of the NMDA receptor subunits as well as on [3H]MK-801 binding of the NMDA receptor were studied in 1-day to 30-day old rat brain. Intrauterine HI conditions were achieved on gestation day 17 by clamping the uterine vasculature for 30 min followed by removal of the clamps to permit reperfusion. As determined by reverse-transcriptase polymerase chain reaction, prenatal HI significantly reduced mRNA expression of the NRI subunit of the NMDA receptor in the hippocampus of 4, 8, and 30-day old rat brains. NR2A and NR2B subunit mRNAs were expressed in the hippocampus and the cortex of both the control and the prenatal HI rat brains. Intrauterine HI did not significantly affect expression of either the NR2A or NR2B subunit mRNA. Consistent with the RT-PCR data, protein expression of the NRI subunit in the hippocampus, but not the cortex, of 21-day old prenatal HI rat brains was significantly decreased as compared to the control rat brain. Intrauterine HI also significantly reduced binding affinity, but not the number of binding sites, of the NMDA receptor to [3H]MK-801, a noncompetitive antagonist of the NMDA receptor, in the hippocampus of 21-day old rat brain. The overall results suggest that prenatal HI-induced reduction of NRI expression and the altered binding ability of the NMDA receptor in the young rat brain may contribute to other long-lasting effects of intrauterine HI that we reported previously.

Molecular identification of NMDA glutamate receptors expressed in bone cells

The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor has recently been identified in bone, but the molecular composition of this receptor expressed by bone cells is unknown. NMDA receptor (NMDAR) is a hetero-oligomeric protein composed of two classes of subunits, the essential subunit NR1 and NR2A to D subunits that do not by themselves produce functional channels but potentiate NR1 activity and confer functional variability to the receptor. These subunits coassemble in different combinations to form functionally distinct NMDAR. In this study, we have investigated the molecular composition of NMDAR expressed by osteoblasts and osteoclasts in culture, using RT-PCR analysis, in situ hybridization and immunocytochemistry. Specific probes were designed for the different subunits of the NMDAR, and we showed by RT-PCR analysis that mammalian osteoclasts expressed NR2B and NR2D subunits mRNAs but not NR2A and NR2C mRNAs. Rat calvaria and MG63 osteoblastic cells also expressed several NR2 subunits mRNAs, namely NR2A, NR2B, and NR2D. In situ hybridization on isolated rabbit osteoclasts and MG63 cells has confirmed the localization of NR1, NR2B, and NR2D transcripts in osteoclasts and NR1, NR2A, NR2B, and NR2D transcripts in MG63 cells. The expression of NR2D protein by bone cells was shown by immunofluorescence. These results demonstrate for the first time that osteoblasts and osteoclasts express several NR2 subunits, suggesting a molecular diversity of NMDAR channels similar to what was shown for brain. The presence of distinct functional NMDAR on bone cells may be associated with various states of bone cell differentiation and function.

Protein kinase C modulates NMDA receptor trafficking and gating

Regulation of neuronal N-methyl-D-aspartate receptors (NMDARs) by protein kinases is critical in synaptic transmission. However, the molecular mechanisms underlying protein kinase C (PKC) potentiation of NMDARs are uncertain. Here we demonstrate that PKC increases NMDA channel opening rate and delivers new NMDA channels to the plasma membrane through regulated exocytosis. PKC induced a rapid delivery of functional NMDARs to the cell surface and increased surface NR1 immunofluorescence in Xenopus oocytes expressing NMDARs. PKC potentiation was inhibited by botulinum neurotoxin A and a dominant negative mutant of soluble NSF-associated protein (SNAP-25), suggesting that receptor trafficking occurs via SNARE-dependent exocytosis. In neurons, PKC induced a rapid delivery of functional NMDARs, assessed by electrophysiology, and an increase in NMDAR clusters on the surface of dendrites and dendritic spines, as indicated by immunofluorescence. Thus, PKC regulates NMDAR channel gating and trafficking in recombinant systems and in neurons, mechanisms that may be relevant to synaptic plasticity.

Relationship of altered glutamate receptor subunit mRNA expression to acute cell loss after spinal cord contusion

Alterations in the expression of ionotropic glutamate receptors (GluR) contribute to neuronal loss after brain ischemia and epilepsy. In order to determine whether altered expression of GluR subunits might contribute to cell loss after spinal cord injury (SCI), we performed a time course study of subunit mRNA expression using quantitative in situ hybridization. Expression was studied in ventral horn motor neurons (VMN) and glia in adjacent ventral white matter at 15 min and 4, 8, and 24 h after SCI in tissue sections 4 mm rostral and caudal to the injury epicenter. We found that the AMPA subunit GluR2 was significantly down-regulated in VMN at 24 h, but not at the earlier times examined, although half the loss of VMN in these locations occurs by 8 h after injury. No changes in the normal expression of GluR2 or GluR4 were found in white matter where glial loss occurs after SCI. NMDA subunits NR1 and NR2A were significantly and rapidly up-regulated in VMN after SCI, but only caudal to the lesion site, while VMN loss is similar rostral and caudal to the epicenter. Thus, the temporal pattern of AMPA and the spatial pattern of NMDA subunit expression changes were distinct from the pattern of VMN loss after SCI. We conclude that altered GluR subunit expression after SCI is unlikely to be involved in secondary cell loss and instead may be involved with plasticity and reorganization of the injured spinal cord.

Insulin promotes rapid delivery of N-methyl-D- aspartate receptors to the cell surface by exocytosis

Insulin potentiates N-methyl-d-aspartate receptors (NMDARs) in neurons and Xenopus oocytes expressing recombinant NMDARs. The present study shows that insulin induced (i) an increase in channel number times open probability (nP(o)) in outside-out patches excised from Xenopus oocytes, with no change in mean open time, unitary conductance, or reversal potential, indicating an increase in n and/or P(o); (ii) an increase in charge transfer during block of NMDA-elicited currents by the open channel blocker MK-801, indicating increased number of functional NMDARs in the cell membrane with no change in P(o); and (iii) increased NR1 surface expression, as indicated by Western blot analysis of surface proteins. Botulinum neurotoxin A greatly reduced insulin potentiation, indicating that insertion of new receptors occurs via SNARE-dependent exocytosis. Thus, insulin potentiation occurs via delivery of new channels to the plasma membrane. NMDARs assembled from mutant subunits lacking all known sites of tyrosine and serine/threonine phosphorylation in their carboxyl-terminal tails exhibited robust insulin potentiation, suggesting that insulin potentiation does not require direct phosphorylation of NMDAR subunits. Because insulin and insulin receptors are localized to glutamatergic synapses in the hippocampus, insulin-regulated trafficking of NMDARs may play a role in synaptic transmission and plasticity, including long-term potentiation.

Developmental changes in synaptic AMPA and NMDA receptor distribution and AMPA receptor subunit composition in living hippocampal neurons

AMPA and NMDA receptors mediate most excitatory synaptic transmission in the CNS. We have developed antibodies that recognize all AMPA or all NMDA receptor variants on the surface of living neurons. AMPA receptor variants were identified with a polyclonal antibody recognizing the conserved extracellular loop region of all four AMPA receptor subunits (GluR1-4, both flip and flop), whereas NMDA receptors were immunolabeled with a polyclonal antibody that binds to an extracellular N-terminal epitope of the NR1 subunit, common to all splice variants. In non-fixed brain sections these antibodies gave labeling patterns similar to autoradiographic distributions with particularly high levels in the hippocampus. Using these antibodies, in conjunction with GluR2-specific and synaptophysin antibodies, we have directly localized and quantified surface-expressed native AMPA and NMDA receptors on cultured living hippocampal neurons during development. Using a quantitative cell ELISA, a dramatic increase was observed in the surface expression of AMPA receptors, but not NMDA receptors, between 3 and 10 d in culture. Immunocytochemical analysis of hippocampal neurons between 3 and 20 d in vitro shows no change in the proportion of synapses expressing NMDA receptors (approximately 60%) but a dramatic increase (approximately 50%) in the proportion of them that also express AMPA receptors. Furthermore, over this period the proportion of AMPA receptor-positive synapses expressing the GluR2 subunit increased from approximately 67 to approximately 96%. These changes will dramatically alter the functional properties of hippocampal synapses.

Suramin prevents cerebellar granule cell-death induced by dequalinium

In this study, we demonstrated that an anticancer drug, dequalinium, a bisquaternary ammonium compound, is a potent neurotoxicant with IC(50) of 0.46 microM on the cultured cerebellar granule neurons. Its selective neurotoxicity revealed by 100-fold more toxic than the other two analogs, pancuronium and vecuronium. The mechanisms underlying dequalinium (DQ)-induced neurotoxicity were explored and found to be associated with decreased mitochondrial membrane potential, increased free radical production and ATP depletion. Suramin (a nonselective purinergic P(2) receptor antagonist and an anticancer drug) but not the glutamate receptor antagonists, MK-801, NBQX (1,2,3,4 tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium), and DNQX (6,7-dinitroquinoxaline-2,3-dione) significantly prevents the DQ-induced neurotoxicity. By means of microfluorometric image-processing technique using the fluorescent probes, fluorescein diacetate/propidium iodide and Hoechst 33258, respectively, we showed that 1 microM DQ for 24 h induced about 53.5% of apoptosis and 37.5% of necrosis. All of these effects of DQ can be completely prevented by suramin. From these results, we conclude that DQ-induced neurotoxicity was not mediated by glutamate receptor, but by increasing free radical productions and cell energy depletion. Suramin with its beneficial antagonistic effects on DQ-induced neurotoxicity may provide an effective approach for neurodegeneration.

PDZ domain suppression of an ER retention signal in NMDA receptor NR1 splice variants

The NMDA receptor NR1 subunit has four splice variants that differ in their C-terminal, cytoplasmic domain. We investigated the contribution of the C-terminal cassettes, C0, C1, C2, and C2', to trafficking of NR1 in heterologous cells and neurons. We identified an ER retention signal (RRR) in the C1 cassette of NR1, which is similar to the RXR motif in ATP-sensitive K(+) channels (Zerangue et al., 1999). We found that surface expression of NR1-3, which contains C1, is due to a site on the C2' cassette, which includes the terminal 4 amino acid PDZ-interacting domain. This site suppresses ER retention of the C1 cassette and leads to surface expression. These findings suggest a role for PDZ proteins in facilitating the transition of receptors from an intracellular pool to the surface of the neuron.

Remodeling of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit composition in hippocampal neurons after global ischemia

Transient global ischemia induces selective delayed cell death, primarily of principal neurons in the hippocampal CA1. However, the molecular mechanisms underlying ischemia-induced cell death are as yet unclear. The present study shows that global ischemia triggers a pronounced and cell-specific reduction in GluR2 [the subunit that limits Ca(2+) permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors] in vulnerable CA1 neurons, as evidenced by immunofluorescence of brain sections and Western blot analysis of microdissected hippocampal subfields. At 72 h after ischemia (a time before cell death), virtually all CA1 pyramidal neurons exhibited greatly reduced GluR2 immunolabeling throughout their somata and dendritic processes. GluR2 immunolabeling was unchanged in pyramidal cells of the CA3 and granule cells of the dentate gyrus, regions resistant to ischemia-induced damage. Immunolabeling of the AMPA receptor subunit GluR1 was unchanged in CA1, CA3, and dentate gyrus. Western analysis indicated that GluR2 subunit abundance was markedly reduced in CA1 at 60 and 72 h after the ischemic insult; GluR1 abundance was unchanged in all subfields at all times examined. These findings, together with the previous observation of enhanced AMPA-elicited Ca(2+) influx in postischemic CA1 neurons, show that functional GluR2-lacking, Ca(2+)-permeable AMPA receptors are expressed in vulnerable neurons before cell death. Thus, the present study provides an important link in the postulated causal chain between global ischemia and delayed death of CA1 pyramidal neurons.

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Looking at Receptors: What Have Fluorescent Receptors and Channels Told Us?

The receptors and channels that reside on the surface of a neuron enable it to respond to and integrate a wide variety of signals. Electrophysiology has made it possible to study the behavior of these channels in remarkable detail. For instance, patch-clamp recording has made it possible in many instances to actually resolve the opening and closing of individual channels. Similarly, immuncytochemistry has provided us with static images of where these proteins are in a neuron. Nevertheless, we know remarkably little about how these proteins are actually used by living cells. Fundamental questions concerning how long they are at the surface, how localized they are, how quickly they are internalized in response to activation, or how free they are to move about on the surface remain to be addressed. One way to answer such questions is to fluorescently label these proteins and image them in living cells. The discovery of the jellyfish green fluorescent protein has recently made this feasible, and the new views it is providing are the topic of this review.

Postnatal developmental changes in AMPA and NMDA receptors in the rat vestibular nuclei

Changes in the expression of the AMPA receptor subunits GluR1-4 and of the NMDA receptor subunits NR1, NR2A-D were investigated in the developing rat medial and lateral vestibular nuclei. Analyses were performed using nonradioactive in situ hybridization and immunoblotting with subunit-specific antibodies. During the postnatal development, glutamatergic receptor subunits were differentially expressed in the vestibular nuclei. The level of expression of GluR1, GluR4 and NR1 subunits was higher in the developing brain as compared to the adult. We observed a gradual increase in GluR2/3, NR2A, NR2B and NR2C levels of expression in the medial and lateral vestibular nuclei during the first 3 weeks of postnatal development. In situ hybridization results were consistent with immunoblot analyses. The differential expression of AMPA and NMDA receptor subunits in immature vestibular neurons is consistent with changes in glutamate receptor properties. This may be related to the postsynaptic regulation of receptor subunits associated with the synaptic plasticity of the vestibular neuron connections during specific sequences of postnatal development.

Nucleus-specific expression of ionotropic glutamate receptor subunit mRNAs and binding sites in primate thalamus

Thalamic afferents and efferents utilize glutamate as their primary neurotransmitter. There are four families of glutamate receptors that can transduce this activity, as well as regulate glutamate release from thalamic relay neurons. The three ionotropic subtypes are of particular importance, because subunit composition confers variability in functional properties of each subtype. We have quantified the expression of NMDA, AMPA and kainate receptors in the thalamus of the macaque using receptor autoradiography and in situ hybridization. NMDA receptors are multimeric associations of NR1 and NR2A-NR2D subunits that form ligand-gated ion channels. Particular subunits are associated with modulatory binding sites that affect receptor activity. NR1 was the most abundant subunit mRNA; NR2A, NR2B, and NR2D subunit mRNAs were also present, but were expressed in nucleus-specific patterns. Very high levels of [3H]ifenprodil binding to the polyamine site of the NMDA complex were detected in a fairly homogeneous distribution. Binding of the ion channel ligand [3H]MK-801 was also abundant, and limbic nuclei expressed higher levels than motor nuclei or the reticular nucleus. [3H]CGP39653 binding to the glutamate site of the NMDA receptor was the least abundant of the NMDA receptor binding sites. There was variability in the stoichiometric relationships of binding sites across nuclei, suggesting that there is heterogeneity in the pharmacological properties of NMDA receptors expressed in the thalamus. AMPA and kainate are also multimeric associations of specific subunits that form ligand-gated ion channels. These subunits are encoded by specific genes: gluR1-gluR4 for AMPA receptors, and gluR5-gluR7 and KA1-KA2 for kainate receptors. GluR4 and gluR6 mRNAs were, respectively the most abundant of the AMPA and kainate receptor subunit transcripts. Both AMPA and kainate receptor subunit transcripts were expressed in a nucleus-specific pattern. The binding of [3H]kainate was higher than that of [3H]AMPA throughout the thalamus, but AMPA subunit mRNA levels were three to five orders of magnitude higher than those encoding the kainate receptor subunits. The mismatch between the levels of expression of kainate receptor subunit transcripts and binding sites is suggestive of a presynaptic localization of kainate receptors on thalamic afferents. These results suggest that ionotropic glutamate receptors are heterogeneously expressed in the thalamus of the primate, and that their differential expression is both subunit- and nucleus-specific.

Caloric restriction prevents age-related deficits in LTP and in NMDA receptor expression

A major focus of aging research has been the search for treatments that will prevent or ameliorate the memory deficits associated with aging. One paradigm, lifelong caloric restriction, has been reported to reduce some of the effects of aging. In the current report, we examined the effects of this treatment on age-related deficits in LTP, a putative cellular building block for memory formation. We report here that lifelong caloric restriction completely prevents the age-related deficit in LTP. In addition, we report that there is a dramatic decrease in the expression of the NMDA receptor subunit NR1 in aged rats and this age-related defect is also prevented by caloric restriction. These data provide a molecular and cellular mechanism by which life long caloric restriction may ameliorate some of the cognitive deficits associated with the aging process.

Status epilepticus decreases glutamate receptor 2 mRNA and protein expression in hippocampal pyramidal cells before neuronal death

Kainic acid (KA)-induced status epilepticus in adult rats leads to delayed, selective death of pyramidal neurons in the hippocampal CA1 and CA3. Death is preceded by down-regulation of glutamate receptor 2 (GluR2) mRNA and protein [the subunit that limits Ca(2+) permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] in CA1 and CA3, as indicated by in situ hybridization, immunolabeling, and quantitative Western blotting. GluR1 mRNA and protein are unchanged or slightly increased before cell death. These changes could lead to formation of GluR2-lacking, Ca(2+)-permeable AMPA receptors and increased toxicity of endogenous glutamate. GluR2 immunolabeling is unchanged in granule cells of the dentate gyrus, which are resistant to seizure-induced death. Thus, formation of Ca(2+)-permeable AMPA receptors may be a critical mediator of delayed neurodegeneration after status epilepticus.

Status epilepticus decreases glutamate receptor 2 mRNA and protein expression in hippocampal pyramidal cells before neuronal death

Kainic acid (KA)-induced status epilepticus in adult rats leads to delayed, selective death of pyramidal neurons in the hippocampal CA1 and CA3. Death is preceded by down-regulation of glutamate receptor 2 (GluR2) mRNA and protein [the subunit that limits Ca(2+) permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] in CA1 and CA3, as indicated by in situ hybridization, immunolabeling, and quantitative Western blotting. GluR1 mRNA and protein are unchanged or slightly increased before cell death. These changes could lead to formation of GluR2-lacking, Ca(2+)-permeable AMPA receptors and increased toxicity of endogenous glutamate. GluR2 immunolabeling is unchanged in granule cells of the dentate gyrus, which are resistant to seizure-induced death. Thus, formation of Ca(2+)-permeable AMPA receptors may be a critical mediator of delayed neurodegeneration after status epilepticus.

Exon 5 and spermine regulate deactivation of NMDA receptor subtypes

Deactivation of N-methyl-D-aspartate (NMDA) channels after brief agonist exposure determines the duration of their synaptic activation during excitatory neurotransmission. We performed patch-clamp recordings of L-glutamate responses from human embryonic kidney tumoral cells (HEK293) expressing NR1 subunit variants lacking exon 5 together with the NR2B subunit. These responses had deactivation components that lasted several seconds. The presence of exon 5 or spermine greatly accelerated deactivation of L-glutamate responses through alterations in desensitization. These effects were also observed at positive holding potentials and in the presence of physiological Mg(2+). Thus NR1 splicing and polyamines may have profound effects on the kinetics of NMDA receptor-mediated synaptic transmission.

Opposing effects of excitatory amino acids on chick embryo spinal cord motoneurons: excitotoxic degeneration or prevention of programmed cell death

Acute administration of a single dose of NMDA on embryonic day (E) 7 or later induces a marked excitotoxic injury in the chick spinal cord, including massive necrotic motoneuron (MN) death. When the same treatment was performed before E7, little, if any, excitotoxic response was observed. Chronic treatment with NMDA starting on E5 prevents the excitotoxic response produced by a later "acute" administration of NMDA. Additionally, chronic NMDA treatment also prevents the later excitotoxic injury induced by non-NMDA glutamate receptor agonists, such as kainate or AMPA. Chronic NMDA treatment also reduces normal MN death when treatment is maintained during the period of naturally occurring programmed cell death (PCD) of MNs and rescues MNs from PCD induced by early peripheral target deprivation. The trophic action of chronic NMDA treatment appears to involve a downregulation of glutamate receptors as shown by both a reduction in the obligatory NR1 subunit protein of the NMDA receptor and a decrease in the kainate-induced Co(2+) uptake in MNs. Both tolerance to excitotoxicity and trophic effects of chronic NMDA treatment are prevented by the NMDA receptor antagonist MK-801. Additionally, administration of MK-801 alone results in an increase in MN PCD. These data indicate for the first time that early activation of NMDA receptors in developing avian MNs in vivo has a trophic, survival-promoting effect, inhibiting PCD by a target-independent mechanism that involves NMDA receptor downregulation.

NMDA-receptors 1 and 2A/B coassembly increased in human epileptic focal cortical dysplasia

PURPOSE

This study was designed to quantify the relation between expressions of NMDA receptor (NMDAR) subunits (1 and 2A/B) and the epileptogenicity in human focal cortical dysplasia.

METHODS

Immunoblotting and immunoprecipitation were used to quantify these receptor subunits in tissue resected from EEG-verified epileptic and distal nonepileptic frontal cortical areas in each of three patients as determined by chronic subdural electrode recordings. In each patient, adjacent sections were immunostained to verify that the numbers of dysplastic neurons were greater in epileptic than in nonepileptic cortex.

RESULTS

In all patients, NMDAR2A/B expressions and their coassemblies with NMDAR1 were increased in epileptic dysplastic cortex compared with the relatively normal appearing nonepileptic cortex. For all three patients, there were no significant differences in NMDAR1 protein expressions between the two EEG groups.

CONCLUSIONS

These results suggest that increased NMDAR1-NMDAR2A/B coassembly contributes to hyperexcitability in dysplastic cortical neurons and focal seizure onsets.

Bidirectional, experience-dependent regulation of N-methyl-D-aspartate receptor subunit composition in the rat visual cortex during postnatal development

In the visual cortex, as elsewhere, N-methyl-D-aspartate receptors (NMDARs) play a critical role in triggering long-term, experience-dependent synaptic plasticity. Modifications of NMDAR subunit composition alter receptor function, and could have a large impact on the properties of synaptic plasticity. We have used immunoblot analysis to investigate the effects of age and visual experience on the expression of different NMDAR subunits in synaptoneurosomes prepared from rat visual cortices. NMDARs at birth are comprised of NR2B and NR1 subunits, and, over the first 5 postnatal weeks, there is a progressive inclusion of the NR2A subunit. Dark rearing from birth attenuates the developmental increase in NR2A. Levels of NR2A increase rapidly (in <2 hr) when dark-reared animals are exposed to light, and decrease gradually over the course of 3 to 4 days when animals are deprived of light. These data reveal that NMDAR subunit composition in the visual cortex is remarkably dynamic and bidirectionally regulated by sensory experience. We propose that NMDAR subunit regulation is a mechanism for experience-dependent modulation of synaptic plasticity in the visual cortex, and serves to maintain synaptic strength within an optimal dynamic range.

Shaping excitation at glutamatergic synapses

Glutamatergic synapses vary, exhibiting EPSCs of widely different magnitudes and timecourses. The main contributors to this variability are: presynaptic factors, including release probability, quantal content and vesicle composition; factors that modulate the concentration and longevity of glutamate in the cleft, including diffusion and the actions of glutamate transporters; and postsynaptic factors, including the types and locations of ionotropic glutamate receptors, their numbers, and the nature and locations of associated intracellular signalling systems.

Differential distribution of intracellular glutamate receptors in dendrites

Glutamate receptors are synthesized in the cell body and transported in intracellular compartments to the target synapse. The objective of the present study was to analyze the intracellular pool of glutamate receptors and determine whether the intracellular pool was related to the synaptic distribution of the receptors. As a model system, we chose the fusiform cell of the dorsal cochlear nucleus for which we have previously demonstrated that receptors are selectively targeted to synapses on apical and basal dendrites. A combination of retrograde tracing and postembedding immunogold labeling was used to quantify intracellular receptors in segments of apical and basal dendrites. Immunolabeling for GluR4 and mGluR1alpha is present at synapses on basal dendrites but not on apical dendrites, whereas immunolabeling for GluR2/3 is present at both populations of synapses. In the analysis of intracellular pools, we find that GluR2/3 is equally distributed in apical and basal dendrites, whereas GluR4 and mGluR1alpha are more concentrated in basal dendrites than in apical dendrites. These findings indicate that the distribution of intracellular receptors is related to that of synaptic receptors and suggest that a mechanism exists in neurons to target proteins to dendritic domains soon after synthesis. We found no evidence for the existence of a pool of intracellular receptors, which could represent a receptor reserve, near the postsynaptic density. Receptors were often found in clusters associated with tubulovesicular membranes of the endoplasmic reticulum, identified with immunoglobulin binding protein (BIP) or calnexin, suggesting that this organelle is involved in receptor transport in dendrites.

Surface expression of AMPA receptors in hippocampal neurons is regulated by an NSF-dependent mechanism

Here, we show that disruption of N-ethylmaleimide-sensitive fusion protein- (NSF-) GluR2 interaction by infusion into cultured hippocampal neurons of a blocking peptide (pep2m) caused a rapid decrease in the frequency but no change in the amplitude of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs). N-methyl-D-aspartate (NMDA) receptor-mediated mEPSCs were not changed. Viral expression of pep2m reduced the surface expression of alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptors, whereas NMDA receptor surface expression in the same living cells was unchanged. In permeabilized neurons, the total amount of GluR2 immunoreactivity was unchanged, and a punctate distribution of GluR2 was observed throughout the dendritic tree. These data suggest that the NSF-GluR2 interaction is required for the surface expression of GluR2-containing AMPA receptors and that disruption of the interaction leads to the functional elimination of AMPA receptors at synapses.