Modification of NMDA receptor channels and synaptic transmission by targeted disruption of the NR2C gene

Roles of the glutamate receptor epsilon2 and delta2 subunits in the potentiation and prepulse inhibition of the acoustic startle reflex

We examined the regulation of the acoustic startle response in mutant mice of the N-methyl-D-aspartate (NMDA)- and delta-subtypes of the glutamate receptor (GluR) channel, which play important roles in neural plasticity in the forebrain and the cerebellum, respectively. Heterozygous mutant mice with reduced GluRepsilon2 subunits of the NMDA receptor showed strongly enhanced startle responses to acoustic stimuli. On the other hand, heterozygous and homozygous mutation of the other NMDA receptor GluRepsilon subunits exerted no, or only small effects on acoustic startle responses. The threshold of the auditory brainstem response of the GluRepsilon2-mutant mice was comparable to that of the wild-type littermates. The primary circuit of the acoustic startle response is a relatively simple oligosynaptic pathway located in the lower brainstem, whilst the expression of GluRepsilon2 is restricted to the forebrain. We thus suggest that the NMDA receptor GluRepsilon2 subunit plays a role in the regulation of the startle reflex. Ablation of the cerebellar Purkinje cell-specific delta2 subunit of the GluR channel exerted little effect on the acoustic startle response but resulted in the enhancement of prepulse inhibition of the reflex. Because inhibition of the acoustic startle response by a weak prepulse is a measure of sensorimotor gating, the process by which an organism filters sensory information, these observations indicate the involvement of the cerebellum in the modulation of sensorimotor gating.

Unipolar brush cells form a glutamatergic projection system within the mouse cerebellar cortex

Unipolar brush cells (UBCs) of the mammalian vestibulocerebellum receive mossy fiber projections primarily from the vestibular ganglion and vestibular nuclei. Recently, the axons of UBCs have been shown to generate an extensive system of cortex-intrinsic mossy fibers, which resemble traditional cerebellar mossy fiber afferents and synapse with granule cell dendrites and other UBCs. However, the neurotransmitter used by the UBC axon is still unknown. In this study, we used long-term organotypic slice cultures of the isolated nodulus (lobule X) from postnatal day 8 mouse cerebella to identify the neurotransmitter and receptors at synapses of the UBC axon terminals, relying on the notion that, in these cultures, all of the cortex-extrinsic fibers had degenerated during the first few days in vitro. Quantification of glutamate immunogold labeling showed that the UBC axon terminals have the same high gold-particle density as the glutamatergic parallel fiber varicosities. Furthermore, UBCs identified by calretinin immunoreactivity expressed the glutamate receptor subunits GluR2/3, NMDAR1, and mGluR2/3, like they do in the mature mouse cerebellum in situ. Evoked excitatory postsynaptic currents (EPSCs), spontaneous EPSCs, and burst discharges were demonstrated in UBCs and granule cells by patch-clamp recording. Both the evoked and spontaneous EPSCs were blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. We conclude that neurotransmission at the UBC axon terminals is glutamatergic. Thus, UBCs provide a powerful network of feedforward excitation within the granular layer, which may amplify vestibular signals and synchronize activity in clusters of functionally related granule cells which project vertically to patches of Purkinje cells.

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

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

NMDA receptor subunits: diversity, development and disease

N-methyl-D-aspartate receptors (NMDARs) are present at many excitatory glutamate synapses in the central nervous system and display unique properties that depend on their subunit composition. Biophysical, pharmacological and molecular methods have been used to determine the key features conferred by the various NMDAR subunits, and have helped to establish which NMDAR subtypes are present at particular synapses. Recent studies are beginning to address the functional significance of NMDAR diversity under normal and pathological conditions.

Cellular distribution of NMDA glutamate receptor subunit mRNAs in the human cerebellum

We have used a quantitative in situ hybridization method with human ribonucleotide probes to examine the regional and cellular distribution of N-methyl-D-aspartate receptor (NMDAR) subunit mRNAs in the human cerebellum. Purkinje cells showed very dense labeling for NMDAR1 mRNA, dense labeling for NMDAR2A mRNA, and moderate labeling for NMDAR2D mRNA, whereas labeling for NMDAR2C mRNA was low. Granule cells showed high hybridization signals for the NMDAR1 and NMDAR2C mRNAs and moderate signals for the NMDAR2A and NMDAR2D mRNAs. In addition intense labeling with the NMDAR2B probe was observed in medium-sized neurons with chromophilic cell bodies in the upper part of the granule cell layer, most likely representing Golgi cells. Neurons in the molecular layer, i.e., basket cells and stellate cells, showed moderate hybridization signals for NMDAR1 and NMDAR2D and low signal for NMDAR2C. Each type of cerebellar neuron analyzed displayed a distinct NMDAR2 subunit profile, suggesting that they are likely to have NMDA receptors with distinct properties.

Mutation analysis of the NMDAR2B (GRIN2B) gene in schizophrenia

NMDA receptor dysfunction may be involved in the pathophysiology of schizophrenia. Based on this hypothesis, we screened 48 Japanese patients with schizophrenia for mutations in the coding region of the NMDAR2B subunit gene (GRIN2B). An association study between the identified DNA sequence variants and schizophrenia was performed in 268 Japanese patients with schizophrenia and 337 Japanese control subjects. Eight single nucleotide polymorphisms were detected, all of which were synonymous. The association sample showed statistically significant excesses of homozygosity for the polymorphisms in the 3' region of the last exon in the patients with schizophrenia (P = 0.004) and higher frequency of the G allele of the 366C/G polymorphism (corrected P = 0.04) in the patients than in the controls. Although we did not detect NMDAR2B protein variants, our findings support the possibility that the GRIN2B gene or a locus in linkage disequilibrium with it may confer susceptibility to schizophrenia. Replication studies in independent samples are warranted.

Hyperfunction of dopaminergic and serotonergic neuronal systems in mice lacking the NMDA receptor epsilon1 subunit

NMDA receptors, an ionotropic subtype of glutamate receptors (GluRs) forming high Ca(2+)-permeable cation channels, are composed by assembly of the GluRzeta subunit (NR1) with any one of four GluRepsilon subunits (GluRepsilon1-4; NR2A-D). In the present study, we investigated neuronal functions in mice lacking the GluRepsilon1 subunit. GluRepsilon1 mutant mice exhibited a malfunction of NMDA receptors, as evidenced by alterations of [(3)H]MK-801 binding as well as (45)Ca(2+) uptake through the NMDA receptors. A postmortem brain analysis revealed that both dopamine and serotonin metabolism were increased in the frontal cortex and striatum of GluRepsilon1 mutant mice. The NMDA-stimulated [(3)H]dopamine release from the striatum was increased, whereas [(3)H]GABA release was markedly diminished in GluRepsilon1 mutant mice. When (+)bicuculline, a GABA(A) receptor antagonist, was added to the superfusion buffer, NMDA-stimulated [(3)H]dopamine release was significantly increased in wild-type, but not in the mutant mice. GluRepsilon1 mutant mice exhibited an increased spontaneous locomotor activity in a novel environment and an impairment of latent learning in a water-finding task. Hyperlocomotion in GluRepsilon1 mutant mice was attenuated by treatment with haloperidol and risperidone, both of which are clinically used antipsychotic drugs, at doses that had no effect in wild-type mice. These findings provide evidence that NMDA receptors are involved in the regulation of behavior through the modulation of dopaminergic and serotonergic neuronal systems. In addition, our findings suggest that GluRepsilon1 mutant mice are useful as an animal model of psychosis that is associated with NMDA receptor malfunction and hyperfunction of dopaminergic and serotonergic neuronal systems.

Mutation analysis of the N-methyl-D-aspartate receptor NR1 subunit gene (GRIN1) in schizophrenia

Dysfunction of N-methyl-D-aspartate (NMDA) type ionotropic glutamate receptors has been implicated in the etiology of schizophrenia based on psychotomimetic properties of the antagonist phencyclidine (PCP) and observation that mice expressing low levels of the N-methyl-D-aspartate receptor NR1 subunit exhibit behavioral alterations that may be ameliorated by neuroleptic drugs. Based on the hypothesis that some schizophrenic patients have functionally deficient mutation(s) of the gene encoding N-methyl-D-aspartate receptor NR1 subunit (GRIN1), we screened 48 Japanese patients with schizophrenia for mutations in the coding region of the GRIN1 gene. Four variants, IVS2-22T>C, IVS2-12G>A, IVS4-34C>T, and 1719G/A (Pro516Pro), were identified. No non-synonymous mutation was detected. No significant association was suggested by case-control comparisons. Results indicate that genomic variations of the GRIN1 gene are not likely to be involved substantially in the etiology of schizophrenia.

Fetal alcohol exposure alters neurosteroid modulation of hippocampal N-methyl-D-aspartate receptors

The actions of ethanol on brain ligand-gated ion channels have important roles in the pathophysiology of alcohol-related neurodevelopmental disorders and fetal alcohol syndrome. Studies have shown that N-methyl-d-aspartate (NMDA) receptors are among the ligand-gated ion channels affected by prenatal ethanol exposure. We exposed pregnant dams to an ethanol-containing liquid diet that results in blood ethanol levels near the legal intoxication limit in most states (0.08%). Primary cultures of hippocampal neurons were prepared from the neonatal offspring of these dams, and NMDA receptor function was assessed by patch clamp electrophysiological techniques after 6-7 days in culture in ethanol-free media. Unexpectedly, we did not detect any changes in hippocampal NMDA receptor function at either the whole-cell or single-channel levels. However, we determined that fetal alcohol exposure alters the actions of the neurosteroids pregnenolone sulfate and pregnenolone hemisuccinate, which potentiate NMDA receptor function. Western immunoblot analyses demonstrated that this alteration is not due to a change in the expression levels of NMDA receptor subunits. Importantly, in utero ethanol exposure did not affect the actions of neurosteroids that inhibit NMDA receptor function. Moreover, the actions of pregnenolone sulfate on type A gamma-aminobutyric acid and non-NMDA receptor function were unaltered by ethanol exposure in utero, which suggests that the alteration is specific to NMDA receptors. These findings are significant because they provide, at least in part, a plausible mechanistic explanation for the alterations in the behavioral responses to neurosteroids found in neonatal rats prenatally exposed to ethanol and to other forms of maternal stress (Zimmerberg, B., and McDonald, B. C. (1996) Pharmacol. Biochem. Behav. 55, 541-547).

Glutamate-mediated transmission, alcohol, and alcoholism

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

Roles of GABAergic inhibition and NMDA receptor subunits in the spatio-temporal integration in the cerebellar cortex of mice

The cerebellar cortex consists of relatively small numbers of identified neuronal types, which form simple and well-defined layers. However, a direct high-resolution demonstration of spatio-temporal pattern of information transmission there has been lacking. Using an optical recording technique with a membrane-potential sensitive dye, we studied the spatio-temporal pattern of excitation propagation induced by white matter stimulation in the slice preparations. We focused on physiological roles of inhibitory synapses and N-methyl-D-aspartate (NMDA) receptors. White matter stimulation induced postsynaptic long-lasting depolarization in the granular layer and transient depolarization in the molecular layer, respectively. Inhibitory synapses modestly suppressed the amplitude of slow depolarization in the granular layer, whereas they exerted powerful lateral inhibition in the molecular layer. Using mutant mice deficient in NMDA receptor subunits NR2A and/or NR2C, we also demonstrated that the NR2A and NR2C subunits expressed in granule neurons contribute to the early and late components of slow depolarization respectively, and that both subunits cooperatively support the temporal summation of depolarization. Taking into account the anatomical organization of the cerebellar cortex, these results might suggest that the granular layer is specialized more in the temporal integration of input signals and the molecular layer in the spatial integration.

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.

Developmental profile of the changing properties of NMDA receptors at cerebellar mossy fiber-granule cell synapses

During cerebellar development, granule cells display well characterized changes in the expression of NMDA receptor (NMDAR) NR2 subunits, switching from NR2B to NR2A and NR2C in mature cells. Although various studies, including experiments on mutant mice with one or more NR2 subunit types deleted, suggest that NR2A, NR2B, and NR2C subunits contribute to synaptic NMDARs, changes in the properties of the mossy fiber EPSC during development have not been fully evaluated. In particular, information on NMDAR EPSCs in mature animals is lacking. We have examined pharmacological and kinetic properties of NMDARs at mossy fiber-granule cell synapses from their formation to maturity [postnatal day 7 (P7)-P40 rats]. Significant changes were seen in the relative amplitudes of the non-NMDAR- and NMDAR-mediated components of the evoked EPSC and in the decay kinetics of the latter. The NMDA/non-NMDA ratio was similar at P7, P21, and P40, but showed a clear peak at P12. This change coincided with a speeding of the NMDAR EPSC decay, accompanied by a decrease in sensitivity to ifenprodil (selective NR2B-antagonist). By P21, sensitivity of the NMDAR EPSC to Mg(2+) was approximately threefold less than that at P12 (IC(50), 76 vs 28 microm), suggesting incorporation of the NR2C subunit. However, the predicted slowing of decay kinetics to a value more characteristic of NR2C deactivation, was not seen until P40. Our data are consistent with the known switch from NR2B to NR2A subunits during the first two postnatal weeks, but suggest a gradual incorporation of the NR2C subunit that modifies Mg(2+) sensitivity and only later influences EPSC kinetics.

Critical period for activity-dependent synapse elimination in developing cerebellum

Synapse elimination is considered to be the final step in neural circuit formation, by causing refinement of redundant connections formed at earlier developmental stages. The developmental loss of climbing fiber innervation from cerebellar Purkinje cells is an example of such synapse elimination. It has been suggested that NMDA receptors are involved in the elimination of climbing fiber synapses. In the present study, we probed the NMDA receptor-dependent period of climbing fiber synapse elimination by using daily intraperitoneal injections of the NMDA receptor antagonist MK-801. We found that blockade of NMDA receptors during postnatal day 15 (P15) and P16, but not before or after this period, resulted in a higher incidence of multiple climbing fiber innervation and caused a mild but persistent loss of motor coordination. Neither basic synaptic functions nor cerebellar morphology were affected by this manipulation. Chronic local application of MK-801 to the cerebellum during P15 and P16 also yielded a higher incidence of multiple climbing fiber innervation. During P15-P16, large NMDA receptor-mediated EPSCs were detected at the mossy fiber-granule cell synapse, but not at the parallel fiber-Purkinje cell or climbing fiber-Purkinje cell synapse. It is therefore likely that the NMDA receptors located at the mossy fiber-granule cell synapse mediate signals leading to the elimination of surplus climbing fibers. These results suggest that an NMDA receptor-dependent phase of climbing fiber synapse elimination lasts 2 d at most. During this phase, the final refinement of climbing fiber synapses occurs, and disruption of this process leads to permanent impairment of cerebellar function.

Changes in expression of NMDA receptor subunits in the rat lumbar spinal cord following neonatal nerve injury

The vulnerability of motoneurones to glutamate has been implicated in neurological disorders such as amyotrophic lateral sclerosis but it is not known whether specific receptor subtypes mediate this effect. In order to investigate this further, the expression of N-methyl-D-aspartate (NMDA) receptor subunits was studied during the first three post-natal weeks when motoneurones are differentially vulnerable to injury following neonatal nerve crush compared to the adult. Unilateral nerve crush was carried out at day 2 after birth (P2) which causes a decrease of 66% in motoneurone number by 14 days (P14). To study receptor expression in identified motoneurones, serial section analysis was carried out on retrogradely labelled common peroneal (CP) motoneurones by combined immunocytochemistry and in situ hybridization (ISH). mRNA levels were also quantified in homogenates from lumbar spinal cords in which the side ipsilateral to the crush was separated from the contralateral side. The NR1 subunit of the NMDA receptor was widely distributed in the spinal cord being expressed most strongly in motoneurone somata particularly during the neonatal period (P3-P7). The NR2 subunits were also expressed at higher levels in the somata and dendrites of neonatal motoneurones compared to older animals. NR2B mRNA was expressed at low to moderate levels throughout the studied period whereas NR2A mRNA levels were low until P21. Following unilateral nerve crush, an initial decrease in NR1 mRNA occurred at one day after nerve crush (P3) in labelled CP motoneurones ipsilateral to the crush which was followed by a significant increase in NR1 subunit expression at 5 days post-injury. This increase was bilateral although reaching greater significance ipsilateral to the crush compared with sham-operated animals. A significant increase in NR1 and NR2B mRNA post injury was also detected in spinal cord homogenates. In addition, the changes in levels of NR1 and NR2B mRNA were reflected by comparable bilateral changes at P7 in receptor protein determined by quantitative immunocytochemical analysis of NR1 and NR2 subunit expression in identified CP motoneurones indicating a co-ordinated regulation of receptor subunits in response to injury.

Molecular and functional properties of synaptically activated NMDA receptors in neonatal motoneurons in rat spinal cord slices

The functional properties of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSC) were studied in fluorescence-labelled motoneurons in thin spinal cord slices. The deactivation of NMDA receptor EPSCs in motoneurons voltage-clamped at +40 mV was independent of intensity or location of stimulation and of postnatal age [taufast = 28.5 +/- 4.6 ms (63.6 +/- 8.8%) and tauslow = 165.6 +/- 49.6 ms]. In the presence of 1 mM Mg2+ the amplitude of NMDA receptor EPSCs was voltage-dependent. Boltzmann analysis of the relationship between peak NMDA receptor EPSC amplitude and membrane potential suggested an apparent Kd of Mg2+ (at 0 mV) of 0.87 mM. Nonstationary variance analysis of NMDA receptor EPSCs gave an estimated single-channel conductance of 59 +/- 14 pS. Direct measurement of the NMDA receptor channel openings in outside-out patches isolated from motoneurons indicated the presence of single-channel conductance levels of 21.8 +/- 2.8 pS, 37. 1 +/- 3.2 pS, 49.6 +/- 5.1 pS and 69.6 +/- 4.2 pS. Single-cell RT-PCR analysis of mRNA revealed that NR1, NR2A-D and NR3A transcripts were expressed in motoneurons. These results suggest that specific assembly of NMDA receptor subunits in motoneurons determines the functional and pharmacological properties of the receptors in these cells.

Fast NMDA receptor-mediated synaptic currents in neurons from mice lacking the epsilon2 (NR2B) subunit

The N-methyl-D-aspartate (NMDA) receptor has been implicated in the formation of synaptic connections. To investigate the role of the epsilon2 (NR2B) NMDA receptor subunit, which is prominently expressed during early development, we used neurons from mice lacking this subunit. Although epsilon2(-/-) mice die soon after birth, we examined whether NMDA receptor targeting to the postsynaptic membrane was dependent on the epsilon2 subunit by rescuing hippocampal neurons from these mice and studying them in autaptic cultures. In voltage-clamp recordings, excitatory postsynaptic currents (EPSCs) from epsilon2(-/-) neurons expressed an NMDA receptor-mediated EPSC that was apparent as soon as synaptic activity developed. However, compared with wild-type neurons, NMDA receptor-mediated EPSC deactivation kinetics were much faster and were less sensitive to glycine, but were blocked by Mg(2+) or AP5. Whole cell currents from epsilon2(-/-) neurons were also more sensitive to block by low concentrations of Zn(2+) and much less sensitive to the epsilon2-specific antagonist ifenprodil than wild-type currents. The rapid NMDA receptor-mediated EPSC deactivation kinetics and the pharmacological profile from epsilon2(-/-) neurons are consistent with the expression of zeta1/epsilon1 diheteromeric receptors in excitatory hippocampal neurons from mice lacking the epsilon2 subunit. Thus epsilon1 can substitute for the epsilon2 subunit at synapses and epsilon2 is not required for targeting of NMDA receptors to the postsynaptic membrane.

Distinct synaptic and extrasynaptic NMDA receptors in developing cerebellar granule neurons

In rat cerebellar granule neurons, mRNA and protein levels of the NR2A and NR2C subunits of the NMDA receptor increase during the second postnatal week. At this time, mRNA and protein levels of the NR2B subunit begin to fall. To investigate targeting of NMDA receptor subunits, we performed whole-cell recordings from rat cerebellar granule neurons at different times during development and investigated the pharmacological and biophysical properties of mossy fiber-evoked NMDA EPSCs. Isolated NMDA EPSCs from newly formed synapses in the first postnatal week exhibited partial block by the NR2B subunit-specific antagonist (1S, 2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP 101,606). By the end of the second postnatal week, NMDA EPSCs were virtually unaffected by the NR2B antagonist. In parallel, NMDA EPSC decay times decreased over a similar developmental time course. We compared properties of synaptic NMDA receptors with extrasynaptic receptors that are present on the cell body with rapid application of glutamate to excised nucleated patches. Deactivation of patch responses accelerated with development and closely resembled evoked NMDA EPSCs in rats of the same age. However, patch responses were highly sensitive to CP 101,606 through the second postnatal week, and sensitivity was seen in some neurons up to the fourth postnatal week. Spermine potentiated peak NMDA patch responses from postnatal days 10-14 rats but had little effect on evoked NMDA EPSCs. Our data suggest selective targeting of a distinct NMDA receptor subtype to synaptic receptor populations in cerebellar granule neurons. Later in development, similar changes occur in the extrasynaptic receptor population.

Glutamate transporters contribute to the time course of synaptic transmission in cerebellar granule cells

Transporters are thought to assist in the termination of synaptic transmission at some synapses by removing neurotransmitter from the synapse. To investigate the role of glutamate transport in shaping the time course of excitatory transmission at the mossy fiber-granule cell synapse, the effects of transport impairment were studied using whole-cell voltage- and current-clamp recordings in slices of rat cerebellum. Impairment of transport by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) produced a prolongation of the decay of the AMPA receptor-mediated current after a repetitive stimulus, as well as prolongation of single stimulus-evoked EPSCs when AMPA receptor desensitization was blocked. PDC also produced a prolongation of both single and repetitive-evoked NMDA receptor-mediated EPSCs. Enzymatic degradation of extracellular glutamate did not reverse the PDC-induced prolongation of AMPA receptor-mediated current after a repetitive stimulus, suggesting that transporter binding sites participate in limiting glutamate spillover. In current-clamp recordings, PDC dramatically increased the total area of the EPSP and the burst duration evoked by single and repetitive stimuli. These data indicate that glutamate transporters play a significant role in sculpting the time course of synaptic transmission at granule cell synapses, most likely by limiting the extent of glutamate spillover. The contribution of transporters is particularly striking during repetitive stimulus trains at physiologically relevant frequencies. Hence, the structural arrangement of the glomerulus may enhance the contribution of transporters to information processing by limiting the extent of glutamate spillover between adjacent synapses.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Mice with reduced NMDA receptor expression display behaviors related to schizophrenia

N-methyl-D-aspartate receptors (NMDARs) represent a subclass of glutamate receptors that play a critical role in neuronal development and physiology. We report here the generation of mice expressing only 5% of normal levels of the essential NMDAR1 (NR1) subunit. Unlike NR1 null mice, these mice survive to adulthood and display behavioral abnormalities, including increased motor activity and stereotypy and deficits in social and sexual interactions. These behavioral alterations are similar to those observed in pharmacologically induced animal models of schizophrenia and can be ameliorated by treatment with haloperidol or clozapine, antipsychotic drugs that antagonize dopaminergic and serotonergic receptors. These findings support a model in which reduced NMDA receptor activity results in schizophrenic-like behavior and reveals how pharmacological manipulation of monoaminergic pathways can affect this phenotype.

Impairment of AMPA receptor function in cerebellar granule cells of ataxic mutant mouse stargazer

The spontaneous recessive mutant mouse stargazer (stg) begins to show ataxia around postnatal day 14 and display a severe impairment in the acquisition of classical eyeblink conditioning in adulthood. These abnormalities have been attributed to the specific reduction in brain-derived neurotrophic factor (BDNF) and the subsequent defect in TrkB receptor signaling in cerebellar granule cells (GCs). In the stg mutant cerebellum, we found that EPSCs at mossy fiber (MF) to GC synapses are devoid of the fast component mediated by AMPA-type glutamate receptors despite the normal slow component mediated by NMDA receptors. The sensitivity of stg mutant GCs to exogenously applied AMPA was greatly reduced, whereas that to NMDA was unchanged. Glutamate release from MF terminals during synaptic transmission to GCs appeared normal. By contrast, AMPA receptor-mediated EPSCs were normal in CA1 pyramidal cells of the stg mutant hippocampus. Thus, postsynaptic AMPA receptor function was selectively impaired in stg mutant GCs, although the transcription of four AMPA receptor subunit genes in the stg GC was comparable to the wild-type GC. We also examined the cerebellum of BDNF knockout mice and found that their MF-GC synapses had a normal AMPA receptor-mediated EPSC component. Thus, the impaired AMPA receptor function in the stg mutant GC is not likely to result from the reduced BDNF-TrkB signaling. These results suggest that the defect in MF to GC synaptic transmission is a major factor that causes the cerebellar dysfunction in the stg mutant mouse.

Reversal of glutamate excitotoxicity by activation of PKC-associated metabotropic glutamate receptors in cerebellar granule cells relies on NR2C subunit expression

Stimulation of metabotropic glutamate receptors (mGluRs) belonging to group I has been found to reduce N-methyl-D-aspartate (NMDA) receptor function in terms of both intracellular calcium concentration ([Ca2+]i) rise and neurotoxicity in cultured cerebellar granule cells. In the present study, we investigated whether the mGluR-elicited modulation of glutamate responses might rely on the heteromeric composition of NMDA receptor channel. NMDA receptors consist of two distinct groups of subunits: NR1, that is ubiquitously in the receptor complexes; and NR2A-D, that differentiate and potentiate NMDA receptor responses by assembling with NR1. Among NR2 subunits, only NR2A and NR2C mRNAs and relative proteins are detected in cerebellar granule cells at 10 days in vitro. To dissect the involvement of the two different subunits in making the NMDA receptor channel sensitive to modulation by group I mGluR agonists, expression of the NR2C subunit was prevented by treating the cells with specific antisense oligodeoxynucleotide (ODN). The capability of the mGluR agonists, trans-1-amino-cyclopentane-1,3-dicarboxylic acid (tACPD, 100 microM) or 3 hydroxyphenylglycine (3HPG, 100 microM), and the protein kinase C (PKC) activator, 4beta-phorbol-12,13-dibutyrate (PDBu, 1 microM), to inhibit the function of resultant NMDA receptors was then evaluated. We found that depletion of the NR2C subunit abolished the inhibitory effect of group I mGluR stimulation on glutamate-induced [Ca2+]i rise and neurotoxicity. The antisense ODN treatment also prevented the inhibitory effect of PDBu on glutamate responses. Conversely, in NR2C-lacking neurons, both group I mGluRs and PKC stimulation enhanced NMDA receptor-mediated effects. The present findings indicate that the capability of PKC-associated mGluRs to modulate native NMDA receptor function relies on the heteromeric configuration of the receptor-channel complex. Particularly, expression of the NR2C subunit is required to make the NMDA receptor sensitive to inhibitory modulation by mGluRs or PKC activation.

Genetic regulation of glutamate receptor ion channels

Transcriptional and translational regulation of glutamate receptor expression determines one of the key phenotypic features of neurons in the brain--the properties of their excitatory synaptic receptors. Up- and down-regulation of various glutamate receptor subunits occur throughout development, following ischemia, seizures, repetitive activation of afferents, or chronic administration of a variety of drugs. The promoters of the genes that encode the NR1, NR2B, NR2C, GluR1, GluR2, and KA2 subunits share several characteristics that include multiple transcriptional start sites within a CpG island, lack of TATA and CAAT boxes, and neuronal-selective expression. In most cases, the promoter regions include overlapping Sp1 and GSG motifs near the major initiation sites, and a silencer element, to guide expression in neurons. Manipulating the levels of glutamate receptors in vivo by generating transgenic and knockout mice has enhanced understanding of the role of specific glutamate receptor subunits in long-term potentiation and depression, learning, seizures, neural pattern formation, and survival. Neuron-specific glutamate receptor promoter fragments may be employed in the design of novel gene-targeting constructs to deliver future experimental transgene and therapeutic agents to selected neurons in the brain.

Mice with genetically modified NMDA and AMPA receptors

This manuscript summarizes mouse mutants for ionotropic glutamate receptors that were generated by different laboratories to analyze the function of the NMDA and AMPA receptors in the mouse. Thus, NMDA receptor mutant mice that were generated by the "knock-in" technology demonstrate that the NR1 and the NR2B subunits participate in the formation of NMDA receptors that are involved in vital functions like breathing and suckling of a newborn mouse. Mice that lack NR2A, -2C, and -2D subunits were described to be viable and have been used to study the role of NMDA receptors in adult mice. The depletion of the GluR-B subunit revealed an NMDA receptor-independent form of long-term potentiation (LTP). This AMPA receptor-mediated LTP at CA3/CA1 synapses was also observed in mice that carry an editing-deficient GluR-B allele even though these mice die prematurely after heavy epileptic seizures. In other mutants, the intracellular COOH-terminal domain of the NMDA receptor was truncated; and when compared to NMDA receptor "knock-out" mice, a functional knock-out of the NMDA receptor was observed. However, in the synapses of NR2AC/AC mutants, gatable NMDA receptors were synaptically activated, indicating that the knock-out phenotypes mediated by the COOH-terminally truncated NMDA receptors appear to reflect defective intracellular signaling.

Evidence for NMDA and mGlu receptor-dependent long-term potentiation of mossy fiber-granule cell transmission in rat cerebellum

Long-term potentiation (LTP) is a form of synaptic plasticity that can be revealed at numerous hippocampal and neocortical synapses following high-frequency activation of N-methyl--aspartate (NMDA) receptors. However, it was not known whether LTP could be induced at the mossy fiber-granule cell relay of cerebellum. This is a particularly interesting issue because theories of the cerebellum do not consider or even explicitly negate the existence of mossy fiber-granule cell synaptic plasticity. Here we show that high-frequency mossy fiber stimulation paired with granule cell membrane depolarization (-40 mV) leads to LTP of granule cell excitatory postsynaptic currents (EPSCs). Pairing with a relatively hyperpolarized potential (-60 mV) or in the presence of NMDA receptor blockers [5-amino--phosphonovaleric acid (APV) and 7-chloro-kynurenic acid (7-Cl-Kyn)] prevented LTP, suggesting that the induction process involves a voltage-dependent NMDA receptor activation. Metabotropic glutamate receptors were also involved because blocking them with (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG) prevented potentiation. At the cytoplasmic level, EPSC potentiation required a Ca2+ increase and protein kinase C (PKC) activation. Potentiation was expressed through an increase in both the NMDA and non-NMDA receptor-mediated current and by an NMDA current slowdown, suggesting that complex mechanisms control synaptic efficacy during LTP. LTP at the mossy fiber-granule cell synapse provides the cerebellar network with a large reservoir for memory storage, which may be needed to optimize pattern recognition and, ultimately, cerebellar learning and computation.

NMDA receptor diversity in the cerebellum: identification of subunits contributing to functional receptors

Recent studies of N-methyl-D-aspartate (NMDA) receptors have led to the suggestion that there are two distinct classes of native NMDA receptors, identifiable from their single-channel conductance properties. 'High-conductance' openings arise from NR2A- or NR2B-containing receptors, and 'low-conductance' openings arise from NR2C- or NR2D-containing receptors. In addition, the low-conductance channels show reduced sensitivity to block by Mg2+. The readily identified cell types and simple architecture of the cerebellum make it an ideal model system in which to determine the contribution of specific subunits to functional NMDA receptors. Furthermore, mRNA for all of these four NR2 subunits are represented in this brain region. We have examined NMDA channels in Purkinje cells, deep cerebellar nuclei (DCN) neurons and Golgi cells. First we find that NR2D-containing NMDA receptors give rise to low-conductance openings in cell-attached recordings from Purkinje cells. The characteristic conductance of these events cannot, therefore, be ascribed to patch excision. Second, patches from some DCN neurons exhibit mixed populations of high- and low-conductance openings. Third, Golgi cells also exhibit a mixed population of high- and low-conductance NMDA receptor openings. The features of these low-conductance openings are consistent with the presence of NR2D-containing NMDA receptors, as suggested by in situ hybridization data. On the other hand the existence of high-conductance channels, with properties typical of NR2B-containing receptors, was not expected. Our results provide new evidence about the subunit composition of NMDA receptors in identified cerebellar cells, and suggest that examination of single-channel properties is a potentially powerful approach for determining the possible subunit composition of native NMDA receptors.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Role of the carboxy-terminal region of the GluR epsilon2 subunit in synaptic localization of the NMDA receptor channel

The synaptic localization of the N-methyl-D-aspartate (NMDA) type glutamate receptor (GluR) channel is a prerequisite for synaptic plasticity in the brain. We generated mutant mice carrying the carboxy-terminal truncated GluR epsilon2 subunit of the NMDA receptor channel. The mutant mice died neonatally and failed to form barrelette structures in the brainstem. The mutation greatly decreased the NMDA receptor-mediated component of hippocampal excitatory postsynaptic potentials and punctate immunofluorescent labelings of GluR epsilon2 protein in the neuropil regions, while GluR epsilon2 protein expression was comparable. Immunostaining of cultured cerebral neurons showed the reduced punctate staining of the truncated GluR epsilon2 protein at synapses. These results suggest that the carboxy-terminal region of the GluRepsilon2 subunit is important for efficient clustering and synaptic localization of the NMDA receptor channel.

Increased thresholds for long-term potentiation and contextual learning in mice lacking the NMDA-type glutamate receptor epsilon1 subunit

The NMDA-type glutamate receptor (GluR) channel, composed of the GluRepsilon and GluRzeta subunits, plays a key role in synaptic plasticity in the CNS. The mutant mice lacking the GluRepsilon1 subunit exhibited a reduction in hippocampal long-term potentiation (LTP), but a stronger tetanic stimulation restored the impairment and the saturation level of LTP was unaltered. These results suggest an increase of threshold for LTP induction in the GluRepsilon1 mutant mice. After a series of backcrosses we established a GluRepsilon1 mutant mouse line with a 99.99% pure C57BL/6 genetic background. The performance of the mutant mice in tone- and context-dependent fear conditioning tests was comparable with that of the wild-type mice. However, a significant difference in the extent of contextual learning became apparent when the chamber exposure time before footshock was shortened. Furthermore, there was a significant difference in freezing responses immediately after footshock on the conditioning day between the wild-type and mutant mice, and the difference was not restored by longer chamber exposure in contrast to the contextual learning on the next day of the conditioning. These results suggest that the GluRepsilon1 subunit of the NMDA receptor channel is a determinant of thresholds for both hippocampal LTP and contextual learning and plays differential roles in two forms of contextual fear memories.

Nucleus- and cell-specific expression of NMDA and non-NMDA receptor subunits in monkey thalamus

Subcortical and corticothalamic inputs excite thalamic neurons via a diversity of glutamate receptor subtypes. Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) receptor subunits (GluR1-4; GluR5-7; NR1, NR2A-D) on a nucleus- and cell type-specific basis was examined by quantitative in situ hybridization histochemistry and by immunocytochemical staining for receptor subunits and colocalized gamma-aminobutyric acid (GABA) or calcium binding proteins. Levels of NMDA subunit expression, except NR2C, are higher than for the most highly expressed AMPA (GluR1,3,4) and kainate (GluR6) receptor subunits. Expression of NR2C, GluR2, GluR5, and GluR7 is extremely low. Major differences distinguish the reticular nucleus and the dorsal thalamus and, within the dorsal thalamus, the intralaminar and other nuclei. In the reticular nucleus, GluR4 is by far the most prominent, and NMDA receptors are at comparatively low levels. In the dorsal thalamus, NMDA receptors predominate. Anterior intralaminar nuclei are more enriched in GluR4 and GluR6 subunits than other nuclei, whereas posterior intralaminar nuclei are enriched in GluR1 and differ among themselves in relative NMDA receptor subunit expression. GABAergic intrinsic neurons of the dorsal thalamus express much higher levels of GluR1 and GluR6 receptor subunits than do parvalbumin- or calbindin-immunoreactive relay cells and low or absent NMDA receptors. Relay cells are dominated by NMDA receptors, along with GluR3 and GluR6 subunits not expressed by GABA cells. High levels of NR2B are found in astrocytes. Differences in NMDA and non-NMDA receptor profiles will affect functional properties of the thalamic GABAergic and relay cells.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.

Genetic analysis of synaptic development and plasticity: homeostatic regulation of synaptic efficacy

When experimentally challenged with perturbations in synaptic structure and function, neurons have the remarkable ability to regulate their synaptic efficacy back to the normal range. Recent genetic analysis has provided insights into the mechanisms controlling this form of synaptic homeostasis, with implications for our understanding of synaptic development and plasticity.

Selective scarcity of NMDA receptor channel subunits in the stratum lucidum (mossy fibre-recipient layer) of the mouse hippocampal CA3 subfield

Hippocampal synapses express two distinct forms of the long-term potentiation (LTP), i.e. NMDA receptor-dependent and -independent LTPs. To understand its molecular-anatomical basis, we produced affinity-purified antibodies against the GluRepsilon1 (NR2A), GluRepsilon2 (NR2B), and GluRzeta1 (NR1) subunits of the N-methyl-D-aspartate (NMDA) receptor channel, and determined their distributions in the mouse hippocampus. Using NMDA receptor subunit-deficient mice as the specificity controls, section pretreatment with proteases (pepsin and proteinase K) was found to be very effective to detect authentic NMDA receptor subunits. As the result of modified immunohistochemistry, all three subunits were detected at the highest level in the strata oriens and radiatum of the CA1 subfield, and high levels were also seen in most other neuropil layers of the CA1 and CA3 subfields and of the dentate gyrus. However, the stratum lucidum, a mossy fibre-recipient layer of the CA3 subfield, contained low levels of the GluRepsilon1 and GluRzeta1 subunits and almost excluded the GluRepsilon2 subunit. Double immunofluorescence with the AMPA receptor GluRalpha1 (GluR1 or GluR-A) subunit further demonstrated that the GluRepsilon1 subunit was colocalized in a subset, not all, of GluRalpha1-immunopositive structures in the stratum lucidum. Therefore, the selective scarcity of these NMDA receptor subunits in the stratum lucidum suggests that a different synaptic targeting mechanism exerts within a single CA3 pyramidal neurone in vivo, which would explain contrasting significance of the NMDA receptor channel in LTP induction mechanisms between the mossy fibre-CA3 synapse and other hippocampal synapses.

Functional and pharmacological differences between recombinant N-methyl-D-aspartate receptors

N-methyl-D-aspartic acid (NMDA) receptors transiently transfected into mammalian HEK-293 cells were characterized with subunit-specific antibodies and electrophysiological recordings. Deactivation time course recorded in response to fast-glutamate pulses were studied in isolated and lifted cells, as well as in outside-out membrane patches excised from cells expressing recombinant NR1 subunits in combination with the NR2A, NR2B, NR2C, or NR2D NMDA receptor subunits. Transfected cells were preidentified by the fluorescence emitted from the coexpressed Aequorea victoria jellyfish Green Lantern protein. Currents generated by NR1/NR2A channels displayed double exponential deactivation time course being faster than that in NR1/NR2B or NR1/NR2C channels. However, a large decay variability was observed within each cotransfection, suggesting that mechanisms additional to subunit composition may also regulate deactivation time course. NR1/NR2D channels displayed slowly deactivating currents. Channel deactivation was fast and comparable among receptors obtained by cotransfecting five distinct spliced variants of the NR1 subunit, each with the NR2A subunit. Additionally, recovery from desensitization was slower for NR1/NR2B than for NR1/NR2A channels. The average deactivation time course of responses to brief L-glutamate applications in cells where NR1/NR2A/NR2B cDNAs were cotransfected at variable ratio was intermediate between those of the NR1/NR2A and NR1/NR2B channels. Although immunocytochemical evidence indicates that the majority of cells are cotransfected by all plasmids in triple transfection, our experimental condition did not allow for a tight control of the expression of NMDA receptor subunits. This produced the result that many cells were characterized by deactivation time course and haloperidol sensitivities of separate NR1/NR2A and NR1/NR2B subunit heteromers. We also speculate on the possible formation of channels resulting from the coassembly in the same receptor of NR1/NR2A/NR2B subunits from a minority of cells that gave responses to brief application of L-glutamate characterized by slow deactivation time course and decreased haloperidol sensitivity.

Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo

NMDA receptors, a class of glutamate-gated cation channels with high Ca2+ conductance, mediate fast transmission and plasticity of central excitatory synapses. We show here that gene-targeted mice expressing NMDA receptors without the large intracellular C-terminal domain of any one of three NR2 subunits phenotypically resemble mice made deficient in that particular subunit. Mice expressing the NR2B subunit in a C-terminally truncated form (NR2B(deltaC/deltaC) mice) die perinatally. NR2A(deltaC/deltaC) mice are viable but exhibit impaired synaptic plasticity and contextual memory. These and NR2C(deltaC/deltaC) mice display deficits in motor coordination. C-terminal truncation of NR2 subunits does not interfere with the formation of gateable receptor channels that can be synaptically activated. Thus, the phenotypes of our mutants appear to reflect defective intracellular signaling.

Regulation of NMDA receptor subunit mRNA expression in the guinea pig vestibular nuclei following unilateral labyrinthectomy

The localization of neurons expressing mRNAs for the NR1 and NR2A-D subunits of the glutamatergic NMDA receptor was examined by non-radioactive in situ hybridization throughout the guinea pig vestibular nuclei. After deafferentation of the vestibular nuclei by unilateral labyrinthectomy, modifications of the mRNA distributions were followed for 30 days. A quantitative analysis was performed in the medial vestibular nucleus by comparison of the labelled neurons in the ipsi- and contra-lateral nuclei. In vestibular nuclei, the NR1 subunit mRNA was found in various populations of neurons. The NR2A and NR2C subunit mRNAs were less widely distributed, whereas little NR2D mRNA was detected and only rare cells contained NR2B mRNA. NR1 and NR2A-D mRNAs were colocalized in some but not other neuronal types. Twenty hours after the lesion, there was a transient ipsilateral increase of NR1 mRNA level in the medial vestibular nucleus, followed by a decrease 48 h after the lesion and, at 3 days, by recovery to the control level. An ipsilateral increase in the mRNA level of NR2C subunit was detected 20 h after lesion and maintained at 48 h. No significant changes were apparent in NR2A, NR2B and NR2D mRNA levels. The distributions and the differential signal intensities of NR2A-D mRNAs suggest various subunit organizations of the NMDA receptors in different neurons of the vestibular nuclei. Neuronal plasticity reorganizations in the vestibular nuclei following unilateral labyrinthectomy appear to include only changes in NR1 and NR2C mRNA levels modifying the functional diversity of the NMDA receptor in the ipsilateral medial vestibular nucleus neurons. The transient changes in NR1 and the NR2C subunit mRNA expressions in response to sensory deprivation are consistent with an active role for NMDA receptors in the appearance and development of the vestibular compensatory process.

The selectivity of the hair cell's mechanoelectrical-transduction channel promotes Ca2+ flux at low Ca2+ concentrations

The mechanoelectrical-transduction channel of the hair cell is permeable to both monovalent and divalent cations. Because Ca2+ entering through the transduction channel serves as a feedback signal in the adaptation process that sets the channel's open probability, an understanding of adaptation requires estimation of the magnitude of Ca2+ influx. To determine the Ca2+ current through the transduction channel, we measured extracellular receptor currents with transepithelial voltage-clamp recordings while the apical surface of a saccular macula was bathed with solutions containing various concentrations of K+, Na+, or Ca2+. For modest concentrations of a single permeant cation, Ca2+ carried much more receptor current than did either K+ or Na+. For higher cation concentrations, however, the flux of Na+ or K+ through the transduction channel exceeded that of Ca2+. For mixtures of Ca2+ and monovalent cations, the receptor current displayed an anomalous mole-fraction effect, which indicates that ions interact while traversing the channel's pore. These results demonstrate not only that the hair cell's transduction channel is selective for Ca2+ over monovalent cations but also that Ca2+ carries substantial current even at low Ca2+ concentrations. At physiological cation concentrations, Ca2+ flux through transduction channels can change the local Ca2+ concentration in stereocilia in a range relevant for the control of adaptation.

Prolonged physiological entrapment of glutamate in the synaptic cleft of cerebellar unipolar brush cells

The cellular mechanism underlying the genesis of the long-lasting alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor-mediated excitatory postsynaptic currents (EPSCs) at the mossy fiber (MF)-unipolar brush cell (UBC) synapse in rat vestibular cerebellum was examined with the use of whole cell and excised patch-clamp recording methods in thin cerebellar slices. Activation of MFs evokes an all-or-none biphasic AMPA-receptor-mediated synaptic current with a late component that peaks at 100-800 ms, which has been proposed to originate from an entrapment of glutamate in the MF-UBC synaptic cleft and is generated by the steady-state activation of AMPA receptors. Bath application of cyclothiazide, which blocks desensitization of AMPA receptors, produced a dose-dependent enhancement of the amplitude of the synaptic current (median effective dose 30 microM) and slowing of the rise time of the fast EPSC. N-methyl-D-aspartate-receptor-mediated EPSCs in UBCs were not potentiated in amplitude or time course by cyclothiazide (100 microM). The dose-response relations for the steady-state current evoked by glutamate acting at AMPA receptors in excised outside-out patches from UBC and granule somatic membranes was biphasic, peaking at 50 microM and declining to 50-70% of this value at 1 mM glutamate. When glutamate was slowly washed from patches to simulate the gradual decline of glutamate in the synapse, a late hump in the transmembrane current was observed in patches from both cell types. The delivery of a second MF stimulus at the peak of the slow EPSC evoked a fast EPSC of reduced amplitude followed by an undershoot of the subsequent slow current, consistent with the hypothesis that the peak of the slow EPSC reflects the peak of the biphasic steady-state dose-response curve. Estimates of receptor occupancy and glutamate concentration derived from the ratio of fast EPSC amplitudes, and the amplitude and polarity of the initial steady-state current in paired-pulse experiments, predict a slow decline of glutamate with a time constant of 800 ms, declining to ineffective concentrations at 5.4 s. Manipulation of cleft glutamate concentration by lowered extracellular calcium or delivery of brief stimulus trains abolished the slow EPSC and restored the undershoot to paired stimuli, respectively, in a manner consistent with a prolonged lifetime of glutamate in the cleft. The slow component of the EPSC was prolonged in duration by the glutamate reuptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate, suggesting that glutamate transport contributes to the time course of the synaptic current in UBCs. The data support the notion that the MF-UBC synapse represents an ultrastructural specialization to effectively entrap glutamate for unusually prolonged periods of time following release from MF terminals. The properties of the postsynaptic receptors and constraints on diffusional escape of glutamate imposed by synaptic ultrastructure and glutamate transporters act in concert to sculpt the time course of the resulting slow EPSC. This in turn drives a long-lasting train of action potentials in response to single presynaptic stimuli.

Alteration in NMDA receptor subunit mRNA expression in vulnerable and resistant regions of in vitro ischemic rat hippocampal slices

Brain insults, including cerebral ischemia, can alter glutamate receptor subunit expression in vulnerable neurons. Understanding these post-ischemic changes in glutamate receptors could enhance our ability to identify specific, novel neuroprotective compounds. Reverse transcription-polymerase chain reaction (RT-PCR) amplification was used to quantify the altered expression of the N-methyl-D-aspartate (NMDA) NR2A, NR2B and NR2C subunits relative to one another in rat hippocampal slices in resistant and vulnerable regions following in vitro oxygen-glucose deprivation. Ninety minutes after re-oxygenation and return to 10 mM glucose, there was a significant increase in the expression of NR2C relative to NR2B and NR2A in the slice as a whole, as well as in the selectively vulnerable CA1 region and the resistant CA3 and dentate gyrus regions.

Multiplex DNA typing of short-tandem-repeat loci on the Y chromosome

To facilitate evolutionary and forensic studies of DNA polymorphisms on the Y chromosome, we devised a multiplex amplification procedure for short-tandem-repeat (STR) loci. Four tetranucleotide STR loci (DYS19, DYS390, DYS391, and DYS393) were simultaneously amplified with FAM-labeled primers and genotypes were determined with an automated DNA sequencer. We typed 162 males from three U.S. populations (African-Americans, European-Americans and Hispanics) and found that the haplotype diversities range from 0.920 to 0.969. This quadruplex system provides a facile means of genotyping these Y chromosome STRs, and should be useful in population genetic and forensic applications.

NMDA receptor properties in rat supraoptic magnocellular neurons: characterization and postnatal development

Hypothalamo-neurohypophysial magnocellular neurons display specific electrical activities in relation to the mode of release of their hormonal content (vasopressin or oxytocin). These activities are under strong glutamatergic excitatory control. The implication of NMDA receptors in the control of vasopressinergic and oxytocinergic neurons is still a matter of debate. We here report the first detailed characterization of functional properties of NMDA receptors in voltage-clamped magnocellular neurons acutely dissociated from the supraoptic nucleus. All cells responded to NMDA with currents that reversed polarity around 0 mV and were inhibited by D-2-amino-5-phosphonovalerate (D-APV) and by 100 microM extracellular Mg2+ (at -80 mV). Sensitivity to the co-agonist glycine (EC50, 2 microM) was low compared with most other neuronal preparations. The receptors displayed low sensitivity to ifenprodil, were insensitive to glycine-independent potentiation by spermine, and had a unitary conductance of 50 pS. No evidence was found for two distinct cell populations, suggesting that oxytocinergic and vasopressinergic neurons express similar NMDA receptors. Characterization of NMDA receptors at different postnatal ages revealed a transient increase in density of NMDA currents during the second postnatal week. This was accompanied by a specific decrease in sensitivity to D-APV, with no change in NMDA sensitivity or any other properties studied. Supraoptic NMDA receptors thus present characteristics that strikingly resemble those of reconstituted receptors composed of NR1 and NR2A subunits. Understanding the functional significance of the development of NMDA receptors in the supraoptic nucleus will require further knowledge about the maturation of neuronal excitability, synaptic connections and neurohormone release mechanisms.

Involvement of three glutamate receptor epsilon subunits in the formation of N-methyl-D-aspartate receptors mediating excitotoxicity in primary cultures of mouse cerebellar granule cells

The N-methyl-D-aspartate receptors have been implicated in neuronal plasticity and their overactivation leads to neurotoxicity. Molecular cloning and co-expression of various glutamate receptor zeta and epsilon complementary DNAs support a heteromeric structural organization for N-methyl-D-aspartate receptors. In this study, we show that cerebellar granular neurons in primary culture of mouse express glutamate receptor zeta1 and at least three glutamate receptor epsilon (epsilon1, epsilon2, and epsilon3) protein subunits. In vitro, the temporal patterns of glutamate receptor epsilon1, epsilon2, and epsilon3 subunit expression depend on culture stages. By day 9, a somatic and neuritic immunolocalization for all N-methyl-D-aspartate subunits was clearly identified in most neuronal, but not glial cells. The role of particular subunits in N-methyl-D-aspartate-mediated excitotoxicity was probed by exposing the cerebellar granule cells to antisense oligodeoxynucleotides generated against specific N-methyl-D-aspartate receptor subunits. Antisense oligodeoxynucleotide treatments significantly down-regulated the amounts of the corresponding N-methyl-D-aspartate subunits. The decrease in N-methyl-D-aspartate subunit protein correlated with a reduction in N-methyl-D-aspartate-induced calcium influx and N-methyl-D-aspartate-mediated excitotoxicity in cerebellar cultures. In contrast, antisense oligodeoxynucleotide treatment failed to protect neurons from 1-methyl-4-phenylpyridinium-induced metabolic cell toxicity. Antisense oligodeoxynucleotide treatment targeted at N-methyl-D-aspartate glutamate receptor epsilon subunits demonstrate that glutamate receptor epsilon1, epsilon2, and epsilon3 proteins form N-methyl-D-aspartate receptors responsible for neurotoxic effects on cerebellar neurons. This study provides direct evidence for the existence of distinct N-methyl-D-aspartate receptor subunit proteins in cerebellar granule cells developing in vitro that may trigger N-methyl-D-aspartate-dependent excitotoxicity.

Absence of prostaglandin E2-induced hyperalgesia in NMDA receptor epsilon subunit knockout mice

1. We have previously found that intrathecal administration of prostaglandins E2 (PGE2) and D2 (PGD2) into conscious mice induced hyperalgesia by the hot plate test. The present study investigated the involvement of N-methyl-D-aspartate (NMDA) receptor in the prostaglandin-induced hyperalgesia by use of mice tacking NMDA receptor epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunits. 2. PGE2 induced hyperalgesia over a wide range of doses from 50 pg to 500 ng kg-1 in wild-type mice. But PGE2 could not induce hyperalgesia in epsilon 1, epsilon 4, or epsilon 1/epsilon 4 subunit knockout mice. 3. The NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (D-AP5), the non-NMDA receptor antagonist 7-D-glutamylaminomethyl sulphonic acid (GAMS), and the nitric oxide synthase inhibitor N epsilon-nitro-L-arginine methyl ester (L-NAME) inhibited the PGE2-induced hyperalgesia in wild-type mice. 4. PGD2 induced hyperalgesia at doses of 25 ng to 250 ng kg-1 in both wild-type and epsilon 1/epsilon 4 subunit knockout mice. The substance P receptor antagonist OP 96.345 blocked the PGD2-induced hyperalgesia in wild-type and epsilon 1/epsilon 4 subunit knockout mice. 5. These results demonstrate that the pathways leading to hyperalgesia are different between PGD2 and PGE2, and that both epsilon 1 and epsilon 4 subunits of the NMDA receptor are involved in the PGE2-induced hyperalgesia.

A direct comparison of the single-channel properties of synaptic and extrasynaptic NMDA receptors

The assumption that synaptic and extrasynaptic glutamate receptors are similar underpins many studies that have sought to relate the behavior of channels in excised patches to the macroscopic properties of the EPSC. We have examined this issue for NMDA receptors in cerebellar granule cells, the small size of which allows the opening of individual synaptic NMDA channels to be resolved directly. We have used whole-cell patch-clamp recordings to determine the conductance and open time of NMDA channels activated during the EPSC and used cell-attached and outside-out recordings to examine NMDA receptors in somatic membrane. Conductance and open time of synaptic channels were indistinguishable from those of extrasynaptic channels in cell-attached patches. However, the channel conductance in outside-out patches was 20% lower than in cell-attached recordings. This change was partially reduced by dantrolene and phalloidin, suggesting that it may involve depolymerization of actin following Ca2+ release from intracellular stores. Our results demonstrate that synaptic and extrasynaptic NMDA receptors have similar microscopic properties. However, NMDA channel conductance is reduced following the formation of an outside-out patch.