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

Developmental basis of SHH medulloblastoma heterogeneity

Many genes that drive normal cellular development also contribute to oncogenesis. Medulloblastoma (MB) tumors likely arise from neuronal progenitors in the cerebellum, and we hypothesized that the heterogeneity observed in MBs with sonic hedgehog (SHH) activation could be due to differences in developmental pathways. To investigate this question, here we perform single-nucleus RNA sequencing on highly differentiated SHH MBs with extensively nodular histology and observed malignant cells resembling each stage of canonical granule neuron development. Through innovative computational approaches, we connect these results to published datasets and find that some established molecular subtypes of SHH MB appear arrested at different developmental stages. Additionally, using multiplexed proteomic imaging and MALDI imaging mass spectrometry, we identify distinct histological and metabolic profiles for highly differentiated tumors. Our approaches are applicable to understanding the interplay between heterogeneity and differentiation in other cancers and can provide important insights for the design of targeted therapies.

Influence of bifenthrin exposure at different gestational stages on the neural development

Perinatal exposure to bifenthrin (BF) alters neurodevelopment. However, the most susceptible time period to BF exposure and the possible mechanisms are not clear. In the current study, pregnant female mice were treated with BF (0.5 mg/kg/d) at three different stages [gestational day (GD) 0-5, 6-15 and 16-birth (B)] and neurologic deficits were evaluated in offspring mice. BF exposure at GD 16-B significantly altered the locomotor activity and caused learning and memory impairments in 6-week-old offspring. Gestational BF exposure also caused neuronal loss in the region of cornu ammonis of hippocampi of 6-week-old offspring. Interestingly, neurobehavioral impairments and neuronal loss were not observed in offspring at 10-week-old. BF exposure at GD 16-B also decreased protein levels of VGluT1, NR1 and NR2A while increased the protein levels of NR2B and VGAT1, as well as the gene levels of Il-1β, Il-6 and Tnf-α in hippocampi of 6-week-old offspring. Collectively, these data demonstrate that gestational exposure to a low dose BF causes neurodevelopmental deficits that remit with the age and the late-stage of pregnancy is the most susceptible time window to BF exposure. Imbalance in excitatory/inhibitory neuronal transmission, altered expression levels of NMDA receptors and increased neural inflammation may be associated with BF prenatal exposure-triggered neurobehavioral impairments.

Chronic ethanol exposure during adolescence impairs simple spike activity of cerebellar Purkinje cells in vivo in mice

Cerebellar Purkinje cells (PCs) play critical roles in motor coordination and motor learning through their simple spike (SS) activity. Previous studies have shown that chronic ethanol exposure (CEE) in adolescents impairs learning, attention, and behavior, at least in part by impairing the activity of cerebellar PCs. In this study, we investigated the effect of CEE on the SS activity in urethane-anesthetized adolescent mice by in vivo electrophysiological recordings and pharmacological methods. Our results showed that the cerebellar PCs in CEE adolescent mice expressed a significant decrease in the frequency and an increase in the coefficient of variation (CV) of SS than control group. Blockade of ɤ-aminobutyric acid A (GABA_A) receptor did not change the frequency and CV of SS firing in control group but produced a significant increase in the frequency and a decrease in the CV of SS firing in CEE mice. The CEE-induced decrease in SS firing rate and increase in CV were abolished by application of an N-methyl-D-aspartate (NMDA) receptor blocker, D-APV, but not by anα-amino-3-hydroxy-5-methyl -4-isoxazolepropionic acid (AMPA) receptor antagonist, NBQX. Notably, the spontaneous spike rate of molecular layer interneurons (MLIs) in CEE mice was significantly higher than control group, which was also abolished by application of D-APV. These results indicate that adolescent CEE enhances the spontaneous spike firing rate of MLIs through activation of NMDA receptor, resulting in a depression in the SS activity of cerebellar PCs in vivo in mice.

GluN2D NMDA Receptors Gate Fear Extinction Learning and Interneuron Plasticity

The cerebellum is critically involved in the formation of associative fear memory and in subsequent extinction learning. Fear conditioning is associated with a long-term potentiation at both excitatory and inhibitory synapses onto Purkinje cells. We therefore tested whether fear conditioning unmasks novel forms of synaptic plasticity, which enable subsequent extinction learning to reset cerebellar circuitry. We found that fear learning enhanced GABA release from molecular layer interneurons and this was reversed after fear extinction learning. Importantly an extinction-like stimulation of parallel fibers after fear learning is sufficient to induce a lasting decrease in inhibitory transmission (I-LTD_stim) in the cerebellar cortex, a form of plasticity that is absent in naïve animals. While NMDA (N-methyl-D-aspartate) receptors are required for the formation and extinction of associative memory, the role of GluN2D, one of the four major NMDA receptor subunits, in learning and memory has not been determined. We found that fear conditioning elevates spontaneous GABA release in GluN2D KO as shown in WT mice. Deletion of GluN2D, however, abolished the I-LTD_stim induced by parallel fiber stimulation after learning. At the behavioral level, genetic deletion of GluN2D subunits did not affect associative learning and memory retention, but impaired subsequent fear extinction learning. D-cycloserine, a partial NMDA receptor (NMDAR) agonist, failed to rescue extinction learning in mutant mice. Our results identify GluN2D as a critical NMDAR subunit for extinction learning and reveal a form of GluN2D-dependent metaplasticity that is associated with extinction in the cerebellum.

Glutamate-Gated NMDA Receptors: Insights into the Function and Signaling in the Kidney

N-Methyl-d-aspartate receptor (NMDAR) is a glutamate-gated ionotropic receptor that intervenes in most of the excitatory synaptic transmission within the central nervous system (CNS). Aside from being broadly distributed in the CNS and having indispensable functions in the brain, NMDAR has predominant roles in many physiological and pathological processes in a wide range of non-neuronal cells and tissues. The present review outlines current knowledge and understanding of the physiological and pathophysiological functions of NMDAR in the kidney, an essential excretory and endocrine organ responsible for the whole-body homeostasis. The review also explores the recent findings regarding signaling pathways involved in NMDAR-mediated responses in the kidney. As established from diverse lines of research reviewed here, basal levels of receptor activation within the kidney are essential for the maintenance of healthy tubular and glomerular function, while a disproportionate activation can lead to a disruption of NMDAR's downstream signaling pathways and a myriad of pathophysiological consequences.

Loss of Ethanol Inhibition of N-Methyl-D-Aspartate Receptor-Mediated Currents and Plasticity of Cerebellar Synapses in Mice Expressing the GluN1(F639A) Subunit

BACKGROUND

Glutamatergic N-methyl-d-aspartate receptors (NMDARs) are well known for their sensitivity to ethanol (EtOH) inhibition. However, the specific manner in which EtOH inhibits channel activity and how such inhibition affects neurotransmission, and ultimately behavior, remains unclear. Replacement of phenylalanine 639 with alanine (F639A) in the GluN1 subunit reduces EtOH inhibition of recombinant NMDARs. Mice expressing this subunit show reduced EtOH-induced anxiolysis, blunted locomotor stimulation following low-dose EtOH administration, and faster recovery of motor function after moderate doses of EtOH, suggesting that cerebellar dysfunction may contribute to some of these behaviors. In the mature mouse cerebellum, NMDARs at the cerebellar climbing fiber (CF) to Purkinje cell (PC) synapse are inhibited by low concentrations of EtOH and the long-term depression (LTD) of parallel fiber (PF)-mediated currents induced by concurrent activation of PFs and CFs (PF-LTD) requires activation of EtOH-sensitive NMDARs. In this study, we examined cerebellar NMDA responses and NMDA-mediated synaptic plasticity in wild-type (WT) and GluN1(F639A) mice.

METHODS

Patch-clamp electrophysiological recordings were performed in acute cerebellar slices from adult WT and GluN1(F639A) mice. NMDAR-mediated currents at the CF-PC synapse and NMDAR-dependent PF-LTD induction were compared for genotype-dependent differences.

RESULTS

Stimulation of CFs evoked robust NMDA-mediated excitatory postsynaptic currents (EPSCs) in PCs that were similar in amplitude and kinetics between WT and GluN1(F639A) mice. NMDA-mediated CF-PC EPSCs in WT mice were significantly inhibited by EtOH (50 mM) while those in mutant mice were unaffected. Concurrent stimulation of CF and PF inputs induced synaptic depression of PF-PC EPSCs in both WT and mutant mice, and this depression was blocked by the NMDA antagonist DL-APV. The synaptic depression of PF-PC EPSCs in WT mice was also blocked by a low concentration of EtOH (10 mM) that had no effect on plasticity in GluN1(F639A) mice.

CONCLUSIONS

These results demonstrate that inhibition of cerebellar NMDARs may be a key mechanism by which EtOH affects cerebellar-dependent behaviors.

Spatiotemporal network coding of physiological mossy fiber inputs by the cerebellar granular layer

The granular layer, which mainly consists of granule and Golgi cells, is the first stage of the cerebellar cortex and processes spatiotemporal information transmitted by mossy fiber inputs with a wide variety of firing patterns. To study its dynamics at multiple time scales in response to inputs approximating real spatiotemporal patterns, we constructed a large-scale 3D network model of the granular layer. Patterned mossy fiber activity induces rhythmic Golgi cell activity that is synchronized by shared parallel fiber input and by gap junctions. This leads to long distance synchrony of Golgi cells along the transverse axis, powerfully regulating granule cell firing by imposing inhibition during a specific time window. The essential network mechanisms, including tunable Golgi cell oscillations, on-beam inhibition and NMDA receptors causing first winner keeps winning of granule cells, illustrate how fundamental properties of the granule layer operate in tandem to produce (1) well timed and spatially bound output, (2) a wide dynamic range of granule cell firing and (3) transient and coherent gating oscillations. These results substantially enrich our understanding of granule cell layer processing, which seems to promote spatial group selection of granule cell activity as a function of timing of mossy fiber input.

The cerebellum is an organ of peculiar geometrical properties, and has been attributed the function of applying spatiotemporal transforms to sensorimotor data since Eccles. In this work we have analyzed the spatiotemporal response properties of the first part of the cerebellar circuit, the granule layer. On the basis of a biophysically plausible and large-scale model of the cerebellum, constrained by a wealth of anatomical data, we study the network dynamics and firing properties of individual cell populations in response to 'realistic' input patterns. We make specific predictions about the spatiotemporal features of granule layer processing regarding the effects of the gap junction coupled network of Golgi cells on a spatially restricted input, in an effect we denominate first-takes-all. Furthermore, we calculate that the granule cell layer has a wide dynamic range, indicating that this is a system that can transmit large variations of input intensities.

Development of Glutamatergic Proteins in Human Visual Cortex across the Lifespan

Traditionally, human primary visual cortex (V1) has been thought to mature within the first few years of life, based on anatomical studies of synapse formation, and establishment of intracortical and intercortical connections. Human vision, however, develops well beyond the first few years. Previously, we found prolonged development of some GABAergic proteins in human V1 (Pinto et al., 2010). Yet as >80% of synapses in V1 are excitatory, it remains unanswered whether the majority of synapses regulating experience-dependent plasticity and receptive field properties develop late, like their inhibitory counterparts. To address this question, we used Western blotting of postmortem tissue from human V1 (12 female, 18 male) covering a range of ages. Then we quantified a set of postsynaptic glutamatergic proteins (PSD-95, GluA2, GluN1, GluN2A, GluN2B), calculated indices for functional pairs that are developmentally regulated (GluA2:GluN1; GluN2A:GluN2B), and determined interindividual variability. We found early loss of GluN1, prolonged development of PSD-95 and GluA2 into late childhood, protracted development of GluN2A until ∼40 years, and dramatic loss of GluN2A in aging. The GluA2:GluN1 index switched at ∼1 year, but the GluN2A:GluN2B index continued to shift until ∼40 year before changing back to GluN2B in aging. We also identified young childhood as a stage of heightened interindividual variability. The changes show that human V1 develops gradually through a series of five orchestrated stages, making it likely that V1 participates in visual development and plasticity across the lifespan.SIGNIFICANCE STATEMENT Anatomical structure of human V1 appears to mature early, but vision changes across the lifespan. This discrepancy has fostered two hypotheses: either other aspects of V1 continue changing, or later changes in visual perception depend on extrastriate areas. Previously, we showed that some GABAergic synaptic proteins change across the lifespan, but most synapses in V1 are excitatory leaving unanswered how they change. So we studied expression of glutamatergic proteins in human V1 to determine their development. Here we report prolonged maturation of glutamatergic proteins, with five stages that map onto life-long changes in human visual perception. Thus, the apparent discrepancy between development of structure and function may be explained by life-long synaptic changes in human V1.

NMDA receptors: linking physiological output to biophysical operation

NMDA receptors are preeminent neurotransmitter-gated channels in the CNS, which respond to glutamate in a manner that integrates multiple external and internal cues. They belong to the ionotropic glutamate receptor family and fulfil unique and crucial roles in neuronal development and function. These roles depend on characteristic response kinetics, which reflect the operation of the receptors. Here, we review biologically salient features of the NMDA receptor signal and its mechanistic origins. Knowledge of distinctive NMDA receptor biophysical properties, their structural determinants and physiological roles is necessary to understand the physiological and neurotoxic actions of glutamate and to design effective therapeutics.

N-methyl-D-Aspartate Receptors Contribute to Complex Spike Signaling in Cerebellar Purkinje Cells: An In vivo Study in Mice

N-methyl-D-aspartate receptors (NMDARs) are post-synaptically expressed at climbing fiber-Purkinje cell (CF-PC) synapses in cerebellar cortex in adult mice and contributed to CF-PC synaptic transmission under in vitro conditions. In this study, we investigated the role of NMDARs at CF-PC synapses during the spontaneous complex spike (CS) activity in cerebellar cortex in urethane-anesthetized mice, by in vivo whole-cell recording technique and pharmacological methods. Under current-clamp conditions, cerebellar surface application of NMDA (50 μM) induced an increase in the CS-evoked pause of simple spike (SS) firing accompanied with a decrease in the SS firing rate. Under voltage-clamp conditions, application of NMDA enhanced the waveform of CS-evoked inward currents, which expressed increases in the area under curve (AUC) and spikelet number of spontaneous CS. NMDA increased the AUC of spontaneous CS in a concentration-dependent manner. The EC₅₀ of NMDA for increasing AUC of spontaneous CS was 33.4 μM. Moreover, NMDA significantly increased the amplitude, half-width and decay time of CS-evoked after-hyperpolarization (AHP) currents. Blockade of NMDARs with D-(-)-2-amino-5-phosphonopentanoic acid (D-APV, 250 μM) decreased the AUC, spikelet number, and amplitude of AHP currents. In addition, the NMDA-induced enhancement of CS activity could not be observed after α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors were blocked. The results indicated that NMDARs of CF-PC synapses contributed to the spontaneous CS activity by enhancing CS-evoked inward currents and AHP currents.

GluN2D-containing NMDA receptors-mediate synaptic currents in hippocampal interneurons and pyramidal cells in juvenile mice

The differential regulation of the two major N-methyl-D-aspartate receptor (NMDAR) subunits GluN2A and GluN2B during development in forebrain pyramidal cells has been thoroughly investigated. In contrast, much less is known about the role of GluN2D, which is expressed at low levels and is downregulated following the second postnatal week. However, it appears that few cells, presumably interneurons, continue to express GluN2D also in juvenile mice. To investigate which hippocampal cell types express this subunit, we generated transgenic mice with EGFP-tagged GluN2D receptors. The expression of the transgene was confined to hippocampal interneurons, most of which were parvalbumin- and/or somatostatin-positive. Electrophysiological and morphological analyses showed that GluN2D was present mainly in fast spiking basket and axo-axonic cells. Based on pharmacological evidence and electrophysiological analysis of GluN2D knockout mice, we conclude that GluN2D-containing NMDARs mediate synaptic currents in hippocampal interneurons of young and juvenile mice and in CA1 pyramidal neurons of newborn mice.

The potential of targeting NMDA receptors outside the CNS

INTRODUCTION

NMDA receptor (NMDAR) is an ionotropic glutamate receptor with a high permeability to calcium and a unique feature of controlling numerous calcium-dependent processes. Apart from being widely distributed in the CNS, the presence of NMDAR and its potential significance in a variety of non-neuronal cells and tissues has become an interesting research topic.

AREAS COVERED

The current review summarizes prevailing knowledge on the role of NMDARs in the kidney, bone and parathyroid gland, three main organs responsible for calcium homeostasis, as well as in the heart, an organ whose function is highly dependable on balanced intracellular calcium concentrations. The review also examines studies that have advanced our understanding of the therapeutic potential of NMDAR agonists and antagonists in renal, cardiovascular and bone pathologies.

EXPERT OPINION

NMDARs have a preeminent role in many physiological and pathological processes outside the CNS. In certain organs and/or disease conditions, activating the NMDAR leads to beneficial effects for the target organ, whereas in other diseases cell signaling downstream of NMDAR activation can exacerbate their pathology. Therefore, targeting NMDARs therapeutically is rather intricate work, and surely requires more extensive investigation in order to properly tune up the diverse NMDAR's actions translating them into beneficial cellular responses.

N-methyl-d-aspartate inhibits cerebellar Purkinje cell activity via the excitation of molecular layer interneurons under in vivo conditions in mice

N-methyl-d-aspartate (NMDA) receptors play a key role in synaptic transmission, and are widely expressed on the membrane of granule cells, parallel fibers, and molecular layer interneurons (MLIs) in the cerebellar cortex of mammals. In cerebellar slices, activation of NMDA receptors increases inhibitory postsynaptic currents (IPSCs) of Purkinje cells (PCs). However, the effects of NMDA on the cerebellar network under in vivo conditions are currently unclear. In the present study, we examined the effects of NMDA on the spontaneous activity of PCs and MLIs in urethane-anesthetized mice by electrophysiological, pharmacological, and juxtacellular labeling methods. Our results revealed that cerebellar surface application of NMDA (5-200μM) reduced the PC simple spike (SS) firing rate in a dose-dependent manner. Application of GABAA receptor antagonist, SR95531 (20μM) abolished NMDA-induced inhibition of PCs spontaneous activity, and revealed NMDA-induced excitation of cerebellar PCs. NMDA receptor antagonist, DAP-V (250µM) did not affect the mean frequency of SS firing, but the SS firing rate of PCs became more regular than the control. In addition, NMDA increased the spike firing of both basket-type and stellate-type MLIs. Overall, these results indicated that NMDA-induced excitation of MLIs at the cerebellar surface may inhibit PC activity. Thus, NMDA receptors of MLIs may play a key role in regulating the spontaneous activity of PCs, and in information transmission and integration in cerebellar cortex.

N-methyl-D-aspartate receptors in human erythroid precursor cells and in circulating red blood cells contribute to the intracellular calcium regulation

The presence of N-methyl-d-aspartate receptor (NMDAR) was previously shown in rat red blood cells (RBCs) and in a UT-7/Epo human myeloid cell line differentiating into erythroid lineage. Here we have characterized the subunit composition of the NMDAR and monitored its function during human erythropoiesis and in circulating RBCs. Expression of the NMDARs subunits was assessed in erythroid progenitors during ex vivo erythropoiesis and in circulating human RBCs using quantitative PCR and flow cytometry. Receptor activity was monitored using a radiolabeled antagonist binding assay, live imaging of Ca(2+) uptake, patch clamp, and monitoring of cell volume changes. The receptor tetramers in erythroid precursor cells are composed of the NR1, NR2A, 2C, 2D, NR3A, and 3B subunits of which the glycine-binding NR3A and 3B and glutamate-binding NR2C and 2D subunits prevailed. Functional receptor is required for survival of erythroid precursors. Circulating RBCs retain a low number of the receptor copies that is higher in young cells compared with mature and senescent RBC populations. In circulating RBCs the receptor activity is controlled by plasma glutamate and glycine. Modulation of the NMDAR activity in RBCs by agonists or antagonists is associated with the alterations in whole cell ion currents. Activation of the receptor results in the transient Ca(2+) accumulation, cell shrinkage, and alteration in the intracellular pH, which is associated with the change in hemoglobin oxygen affinity. Thus functional NMDARs are present in erythroid precursor cells and in circulating RBCs. These receptors contribute to intracellular Ca(2+) homeostasis and modulate oxygen delivery to peripheral tissues.

Mini-review: synaptic integration in the cerebellar nuclei--perspectives from dynamic clamp and computer simulation studies

The cerebellar nuclei (CN) process inhibition from Purkinje cells (PC) and excitation from mossy and climbing fiber collaterals. CN neurons in slices show intrinsic pacemaking activity, which is easily modulated by synaptic inputs. Our work using dynamic clamping and computer modeling shows that synchronicity between PC inputs is an important factor in determining spike rate and spike timing of CN neurons and that brief pauses in PC inputs provide a potent stimulus to trigger CN spikes. Excitatory input can equally control spike rate, but, due to a large slow, NMDA component also amplifies responses to inhibitory inputs. Intrinsic properties of CN neurons are well suited to provide prolonged responses to strong input transients and could be involved in motor pattern generation. One such specific mechanism is given by fast and slow rebound bursting. Nevertheless, we are just beginning to unravel synaptic integration in the CN, and the outcome of the work to date is best characterized by the generation of new specific questions that lend themselves to a combined experimental and computer modeling approach in future studies.

A single GluN2 subunit residue controls NMDA receptor channel properties via intersubunit interaction

NMDA receptors (NMDARs) are glutamate-gated ion channels that are present at most excitatory mammalian synapses. The four GluN2 subunits (GluN2A-D) contribute to four diheteromeric NMDAR subtypes that have divergent physiological and pathological roles. Channel properties that are fundamental to NMDAR function vary among subtypes. We investigated the amino acid residues responsible for variations in channel properties by creating and examining NMDARs containing mutant GluN2 subunits. We found that the NMDAR subtype specificity of three crucial channel properties, Mg(2+) block, selective permeability to Ca(2+) and single-channel conductance, were all controlled primarily by the residue at a single GluN2 site in the M3 transmembrane region. Mutant cycle analysis guided by molecular modeling revealed that a GluN2-GluN1 subunit interaction mediates the site's effects. We conclude that a single GluN2 subunit residue couples with the pore-forming loop of the GluN1 subunit to create naturally occurring variations in NMDAR properties that are critical to synaptic plasticity and learning.

Acute neuregulin-1 signaling influences AMPA receptor mediated responses in cultured cerebellar granule neurons

Neuregulin-1 (NRG1) is a trophic and differentiation factor that signals through ErbB receptor tyrosine kinases to regulate nervous system development. Previous studies have demonstrated that NRG1 affects plasticity at glutamatergic synapses in principal glutamatergic neurons of the hippocampus and frontal cortex; however, immunohistochemical and genetic analyses strongly suggest these effects are indirect and mediated via ErbB4 receptors on GABAergic interneurons. Here, we used cultured cerebellar granule cells (CGCs) that express ErbB4 to analyze the cell-autonomous effects of NRG1 stimulation on glutamatergic function. These cultures have the advantage that they are relatively homogenous and consist primarily of granule neurons that express ErbB4. We show that acute NRG1 treatment does not affect whole-cell AMPA or NMDA receptor (NMDAR) mediated currents in CGCs at 10-12 days in vitro. NRG1 also does not affect the frequency or amplitude of spontaneous AMPAR or NMDAR mediated miniature excitatory post-synaptic currents (mEPSCs). To further investigate the effects of NRG1 on activity-dependent plasticity of glutamatergic synapses in CGCs, we characterized the effects of high-glyine/0 Mg(2+) (which activates synaptic NMDARs) on AMPAR-mEPSC frequency and amplitude. We show that high-glycine induces a form of chemical long-term potentiation (chemLTP) in CGCs characterized by an increase in AMPAR-mEPSC frequency but not amplitude. Moreover, NRG1 induces a decrease in AMPAR-mEPSC frequency following chemLTP, but does not affect AMPAR-mEPSC amplitude. CGCs in our cultures conditions express low levels of GluR1, in contrast to dissociated hippocampal cultures, but do express the long isoform of GluR4. This study provides first evidence that (1) high-glycine can induce plasticity at glutamatergic synapses in CGCs, and (2) that acute NRG1/ErbB-signaling can regulate glutamatergic plasticity in CGCs. Taken together with previous reports, our results suggest that, similar to Schaeffer collateral to CA1 synapses, NRG1 effects are activity dependent and mediated via modulation of synaptic AMPARs.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

Development of NMDA NR2 subunits and their roles in critical period maturation of neocortical GABAergic interneurons

The goals of this research are to (1) determine the changes in the composition of NMDA receptor (NMDAR) subunits in GABAergic interneurons during critical period (CP); and (2) test the effect of chronic blockage of specific NR2 subunits on the maturation of specific GABAergic interneurons. Our data demonstrate that: (1) The amplitude of NMDAR mediated EPSCs (EPSCs(NMDAR) ) was significantly larger in the postCP group. (2) The coefficient of variation (CV), τ(decay) and half-width of EPSCs(NMDAR) were significantly larger in the preCP group. (3) A leftward shift in the half-activation voltages in the postCP vs. preCP group. (4) Using subunit-specific antagonists, we found a postnatal shift in NR2 composition towards more NR2A mediated EPSCs(NMDAR) . These changes occurred within a two-day narrow window of CP and were similar between fast-spiking (FS) and regular spiking (RSNP) interneurons. (5) Chronic blockage of NR2A, but not NR2B, decreased the expression of parvalbumin (PV), but not other calcium binding proteins in layer 2/3 and 4 of barrel cortex. (6) Chronic blockage of NR2A selectively affected the maturation of IPSCs mediated by FS cells. In summary, we have reported, for the first time, developmental changes in the molecular composition of NMDA NR2 subunits in interneurons during CP, and the effects of chronic blockage of NR2A but not NR2B on PV expression and inhibitory synaptic transmission from FS cells. These results support an important role of NR2A subunits in developmental plasticity of fast-spiking GABAergic circuits during CP.

Stimulus-dependent state transition between synchronized oscillation and randomly repetitive burst in a model cerebellar granular layer

Information processing of the cerebellar granular layer composed of granule and Golgi cells is regarded as an important first step toward the cerebellar computation. Our previous theoretical studies have shown that granule cells can exhibit random alternation between burst and silent modes, which provides a basis of population representation of the passage-of-time (POT) from the onset of external input stimuli. On the other hand, another computational study has reported that granule cells can exhibit synchronized oscillation of activity, as consistent with observed oscillation in local field potential recorded from the granular layer while animals keep still. Here we have a question of whether an identical network model can explain these distinct dynamics. In the present study, we carried out computer simulations based on a spiking network model of the granular layer varying two parameters: the strength of a current injected to granule cells and the concentration of Mg²⁺ which controls the conductance of NMDA channels assumed on the Golgi cell dendrites. The simulations showed that cells in the granular layer can switch activity states between synchronized oscillation and random burst-silent alternation depending on the two parameters. For higher Mg²⁺ concentration and a weaker injected current, granule and Golgi cells elicited spikes synchronously (synchronized oscillation state). In contrast, for lower Mg²⁺ concentration and a stronger injected current, those cells showed the random burst-silent alternation (POT-representing state). It is suggested that NMDA channels on the Golgi cell dendrites play an important role for determining how the granular layer works in response to external input.

Intensive studies of Pavlovian delay eyelid conditioning suggest that the cerebellum can memorize a passage-of-time (POT) from the onset of an external stimulus. To account for possible mechanisms of such POT representation, some network models have been proposed to show that granule cells (grcs) in the cerebellar granular layer can exhibit random alternation of burst and silent modes under feedback inhibition from Golgi cells, resulting in non-recurrent generation of active granule cells populations. On the other hand, the oscillation of local field potential (LFP) has been observed in the cerebellar granular layer when animals stay at rest. Some network models have shown that grcs can elicit synchronous spikes in an oscillatory manner. These qualitatively different neural dynamics of the granular layer raises a question of how they can be accounted for by an identical network in the granular layer. Here we report that grc activities of a biologically plausible spiking network model undergo the state transition between synchronized oscillation and random burst-silent alternation, depending on the activation of NMDA channels on the Golgi cell dendrites and the strength of a current injected to grcs.

Oligodendrocytes as regulators of neuronal networks during early postnatal development

Oligodendrocytes are the glial cells responsible for myelin formation. Myelination occurs during the first postnatal weeks and, in rodents, is completed during the third week after birth. Myelin ensures the fast conduction of the nerve impulse; in the adult, myelin proteins have an inhibitory role on axon growth and regeneration after injury. During brain development, oligodendrocytes precursors originating in multiple locations along the antero-posterior axis actively proliferate and migrate to colonize the whole brain. Whether the initial interactions between oligodendrocytes and neurons might play a functional role before the onset of myelination is still not completely elucidated. In this article, we addressed this question by transgenically targeted ablation of proliferating oligodendrocytes during cerebellum development. Interestingly, we show that depletion of oligodendrocytes at postnatal day 1 (P1) profoundly affects the establishment of cerebellar circuitries. We observed an impressive deregulation in the expression of molecules involved in axon growth, guidance and synaptic plasticity. These effects were accompanied by an outstanding increase of neurofilament staining observed 4 hours after the beginning of the ablation protocol, likely dependent from sprouting of cerebellar fibers. Oligodendrocyte ablation modifies localization and function of ionotropic glutamate receptors in Purkinje neurons. These results show a novel oligodendrocyte function expressed during early postnatal brain development, where these cells participate in the formation of cerebellar circuitries, and influence its development.

The role of the cerebellum in schizophrenia: from cognition to molecular pathways

Beside its role in motor coordination, the cerebellum is involved in cognitive function such as attention, working memory, verbal learning, and sensory discrimination. In schizophrenia, a disturbed prefronto-thalamo-cerebellar circuit has been proposed to play a role in the pathophysiology. In addition, a deficit in the glutamatergic N-methyl-D-aspartate (NMDAf) receptor has been hypothesized. The risk gene neuregulin 1 may play a major role in this process. We demonstrated a higher expression of the NMDA receptor subunit 2D in the right cerebellar regions of schizophrenia patients, which may be a secondary upregulation due to a dysfunctional receptor. In contrast, the neuregulin 1 risk variant containing at least one C-allele was associated with decreased expression of NMDA receptor subunit 2C, leading to a dysfunction of the NMDA receptor, which in turn may lead to a dysfunction of the gamma amino butyric acid (GABA) system. Accordingly, from post-mortem studies, there is accumulating evidence that GABAergic signaling is decreased in the cerebellum of schizophrenia patients. As patients in these studies are treated with antipsychotics long term, we evaluated the effect of long-term haloperidol and clozapine treatment in an animal model. We showed that clozapine may be superior to haloperidol in restoring a deficit in NMDA receptor subunit 2C expression in the cerebellum. We discuss the molecular findings in the light of the role of the cerebellum in attention and cognitive deficits in schizophrenia.

High-Pass Filtering and Dynamic Gain Regulation Enhance Vertical Bursts Transmission along the Mossy Fiber Pathway of Cerebellum

Signal elaboration in the cerebellum mossy fiber input pathway presents controversial aspects, especially concerning gain regulation and the spot-like (rather than beam-like) appearance of granular to molecular layer transmission. By using voltage-sensitive dye imaging in rat cerebellar slices (Mapelli et al., 2010), we found that mossy fiber bursts optimally excited the granular layer above ∼50 Hz and the overlaying molecular layer above ∼100 Hz, thus generating a cascade of high-pass filters. NMDA receptors enhanced transmission in the granular, while GABA-A receptors depressed transmission in both the granular and molecular layer. Burst transmission gain was controlled through a dynamic frequency-dependent involvement of these receptors. Moreover, while high-frequency transmission was enhanced along vertical lines connecting the granular to molecular layer, no high-frequency enhancement was observed along the parallel fiber axis in the molecular layer. This was probably due to the stronger effect of Purkinje cell GABA-A receptor-mediated inhibition occurring along the parallel fibers than along the granule cell axon ascending branch. The consequent amplification of burst responses along vertical transmission lines could explain the spot-like activation of Purkinje cells observed following punctuate stimulation in vivo.

A realistic large-scale model of the cerebellum granular layer predicts circuit spatio-temporal filtering properties

The way the cerebellar granular layer transforms incoming mossy fiber signals into new spike patterns to be related to Purkinje cells is not yet clear. Here, a realistic computational model of the granular layer was developed and used to address four main functional hypotheses: center-surround organization, time-windowing, high-pass filtering in responses to spike bursts and coherent oscillations in response to diffuse random activity. The model network was activated using patterns inspired by those recorded in vivo. Burst stimulation of a small mossy fiber bundle resulted in granule cell bursts delimited in time (time windowing) and space (center-surround) by network inhibition. This burst–burst transmission showed marked frequency-dependence configuring a high-pass filter with cut-off frequency around 100 Hz. The contrast between center and surround properties was regulated by the excitatory–inhibitory balance. The stronger excitation made the center more responsive to 10–50 Hz input frequencies and enhanced the granule cell output (with spikes occurring earlier and with higher frequency and number) compared to the surround. Finally, over a certain level of mossy fiber background activity, the circuit generated coherent oscillations in the theta-frequency band. All these processes were fine-tuned by NMDA and GABA-A receptor activation and neurotransmitter vesicle cycling in the cerebellar glomeruli. This model shows that available knowledge on cellular mechanisms is sufficient to unify the main functional hypotheses on the cerebellum granular layer and suggests that this network can behave as an adaptable spatio-temporal filter coordinated by theta-frequency oscillations.

Gene expression of NMDA receptor subunits in the cerebellum of elderly patients with schizophrenia

To determine if NMDA receptor alterations are present in the cerebellum in schizophrenia, we measured NMDA receptor binding and gene expression of the NMDA receptor subunits in a post-mortem study of elderly patients with schizophrenia and non-affected subjects. Furthermore, we assessed influence of genetic variation in the candidate gene neuregulin-1 (NRG1) on the expression of the NMDA receptor in an exploratory study. Post-mortem samples from the cerebellar cortex of ten schizophrenic patients were compared with nine normal subjects. We investigated NMDA receptor binding by receptor autoradiography and gene expression of the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D by in situ hybridization. For the genetic study, we genotyped the NRG1 polymorphism rs35753505 (SNP8NRG221533). Additionally, we treated rats with the antipsychotics haloperidol or clozapine and assessed cerebellar NMDA receptor binding and gene expression of subunits to examine the effects of antipsychotic treatment. Gene expression of the NR2D subunit was increased in the right cerebellum of schizophrenic patients compared to controls. Individuals carrying at least one C allele of rs35753505 (SNP8NRG221533) showed decreased expression of the NR2C subunit in the right cerebellum, compared to individuals homozygous for the T allele. Correlation with medication parameters and the animal model revealed no treatment effects. In conclusion, increased NR2D expression results in a hyperexcitable NMDA receptor suggesting an adaptive effect due to receptor hypofunction. The decreased NR2C expression in NRG1 risk variant may cause a deficit in NMDA receptor function. This supports the hypothesis of an abnormal glutamatergic neurotransmission in the right cerebellum in the pathophysiology of schizophrenia.

Blockade of hindbrain NMDA receptors containing NR2 subunits increases sucrose intake

We have previously shown that blockade of N-methyl-d-aspartate (NMDA) receptors in the caudal brain stem delays satiation and increases food intake. NMDA receptors are heterodimers made up of distinct, but different, ion channel subunits. The NR2 subunits of the NMDA receptor contain the binding site for glutamate. About half of vagal afferents express immunoreactivity for NMDA NR2B subunit and about half of the NR2B expressing afferents also express NMDA NR2C or NR2D subunits. This suggests that increased food intake may be evoked by interference with glutamate binding to NMDA channels containing the NR2B subunit. To test this, we measured deprivation-induced intake of 15% sucrose solution following fourth ventricle and intra-nucleus of the solitary tract (intra-NTS) injections of Conantokin G (Con G; NR2B blocker), d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene; NR2B/2A blocker), and (+/-)-cis-1-(phenanthren-2yl-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA; NR2D/C blocker). Fourth ventricular administration of Con G (5, 20, 40, 80 ng), d-CPPene (3.0, 6.25, 12.5, 25, 50, 100 ng), and PPDA (300, 400 ng) increased sucrose intake significantly compared with control. Likewise, injections of Con G (10 ng), d-CPPene (5 ng, 10 ng), and PPDA (0.5, 1.0, 2.5, 5.0 ng) directly into the NTS significantly increased sucrose intake. These results show that hindbrain injection of competitive NMDA antagonists with selectivity or preference for the NMDA receptor NR2B or NR2C subunits increases food intake.

Ionotropic glutamate receptor mRNA expression in the human thalamus: absence of change in schizophrenia

Abnormalities in glutamate neurotransmission are thought to be among the major contributing factors to the pathophysiology of schizophrenia. Although schizophrenia has been regarded mostly as a disorder of higher cortical function, the cortex and thalamus work as a functional unit. Existing data regarding alterations of glutamate receptor subunit expression in the thalamus in schizophrenia remain equivocal. This postmortem study examined mRNA expression of ionotropic glutamate receptor (iGluR) subunits and PSD95 in 5 precisely defined and dissected thalamic subdivisions (medial and lateral sectors of the mediodorsal nucleus; and the ventral lateral posterior, ventral posterior, and centromedian nuclei) of persons with schizophrenia and matched controls using quantitative PCR with normalization to multiple endogenous controls. Among 15 genes examined (NR1 and NR2A-D subunits of the NMDA receptor; GluR1-4 subunits of the AMPA receptor; GluR5-7 and KA1-2 subunits of the kainate receptor; PSD95), all but two (GluR4 and KA1) were expressed at quantifiable levels. Differences in iGluR gene expression were seen between different thalamic nuclei but not between diagnostic groups. The relative abundance of transcripts was: NR1>>NR2A>NR2B>NR2D>NR2C for NMDA, GluR2>GluR1>GluR3 for AMPA, and KA2>GluR5>GluR7>GluR6 for kainate receptors. The expression of PSD95 correlated with the expression of NR1, NR2A, NR2B, NR2D and GluR6 in all nuclei. These results provide detailed and quantitative information on iGluR subunit expression in multiple nuclei of the human thalamus but suggest that alterations in their expression are not a prominent feature of schizophrenia.

NMDA receptor contribution to the climbing fiber response in the adult mouse Purkinje cell

Among integrative neurons displaying long-term synaptic plasticity, adult Purkinje cells seemed to be an exception by lacking functional NMDA receptors (NMDA-Rs). Although numerous anatomical studies have shown both NR1 and NR2 NMDA-R subunits in adult Purkinje cells, patch-clamp studies failed to detect any NMDA currents. Using more recent pharmacological and immunodetection tools, we demonstrate here that Purkinje cells from adult mice respond to exogenous NMDA application and that postsynaptic NMDA-Rs carry part of the climbing fiber-mediated EPSC (CF-EPSC), with undetectable contribution from presynaptic or polysynaptic NMDA currents. We also detect NR2-A/B subunits in adult Purkinje cells by immunohistochemistry. The NMDA-mediated CF-EPSC is barely detectable before 3 weeks postnatal. From the end of the third week, the number of cells displaying the NMDA-mediated CF-EPSC rapidly increases. Soon, this EPSC becomes detectable in all the Purkinje cells but is still very small. Its amplitude continues to increase until 12 weeks after birth. In mature Purkinje cells, we show that the NMDA-Rs contribute to the depolarizing plateau of complex spikes and increase their number of spikelets. Together, these observations demonstrate that mature Purkinje cells express functional NMDA receptors that become detectable in CF-EPSCs at approximately 21 d after birth and control the complex spike waveform.

NMDA receptor NR2B subunit over-expression increases cerebellar granule cell migratory activity

Glutamate acting on NMDA receptors (NMDARs) is known to influence cerebellar granule cell migration. Subunit composition of NMDARs in granule cells changes characteristically during development: NR2B subunit containing receptors are abundant during migration towards the internal granule cell layer but are gradually replaced by NR2A and/or NR2C subunits once the final position is reached. Cerebellar granule cell migration was investigated using mutant mouse lines either with a deletion of the NR2C gene (NR2C(-/-) mice) or expressing NR2B instead of the NR2C subunit (NR2C-2B mice). BrdU-labeling revealed that over-expression of NR2B increased granule cell translocation in vivo, while the lack of NR2C subunit did not have any detectable effects on cell migration. Cellular composition of wild-type and mutant dissociated cerebellar granule cell cultures isolated from 10-day-old cerebella were similar, but NR2C-2B cultures had elevated level of NR2B subunits and intracellular Ca2+ imaging revealed higher sensitivity towards the addition of NR2B-selective antagonist in vitro. Time-lapse videomicroscopic observations revealed that average migratory velocity and the proportion of translocating cell bodies were significantly higher in NR2C-2B than in wild-type cultures. Our results provide evidence that NR2B-containing NMDARs can have specialized roles during granule cell migration and can increase migratory speed.

Gating information by two-state membrane potential fluctuations

Two-state voltage fluctuations between a hyperpolarized down-state and a depolarized up-state have been observed experimentally in a wide variety of neurons across brain regions. Using a biophysical model, we show that synaptic input by NMDA receptors can cause such membrane potential fluctuations. In this model, when a neuron is driven by two input pathways with different AMPA/NMDA receptor content, the NMDA-rich input causes up-state transitions, whereas the AMPA-rich input generates spikes only in the up-state. Therefore the NMDA-rich pathway can gate input from an AMPA pathway in an all-or-none fashion by switching between different membrane potential states. Furthermore, once in the up-state, the NMDA-rich pathway multiplicatively increases the gain of a neuron responding to AMPA-rich input. This proposed mechanism for two-state fluctuations directly suggests specific computations, such as gating and gain modulation based on the distinct receptor composition of different neuronal pathways. The dynamic gating of input by up- and down-states may be an elementary operation for the selective routing of signals in neural circuits, which may explain the ubiquity of two-state fluctuations across brain regions.

Potentiation of mossy fiber EPSCs in the cerebellar nuclei by NMDA receptor activation followed by postinhibitory rebound current

Behavioral and computational studies predict that synaptic plasticity of excitatory mossy fiber inputs to cerebellar nuclear neurons is required for associative learning, but standard tetanization protocols fail to potentiate nuclear cell EPSCs in mouse cerebellar slices. Nuclear neurons fire action potentials spontaneously unless strongly inhibited by Purkinje neurons, raising the possibility that plasticity-triggering signals in these cells differ from those at classical Hebbian synapses. Based on predictions of neuronal activity during delay eyelid conditioning, we developed quasi-physiological induction protocols consisting of high-frequency mossy fiber stimulation and postsynaptic hyperpolarization. Robust, NMDA receptor-dependent potentiation of nuclear cell EPSCs occurred with protocols including a 150-250 ms hyperpolarization in which mossy fiber stimulation preceded a postinhibitory rebound depolarization. Mossy fiber stimulation potentiated EPSCs even when postsynaptic spiking was prevented by voltage-clamp, as long as rebound current was evoked. These data suggest that Purkinje cell inhibition guides the strengthening of excitatory synapses in the cerebellar nuclei.

N-methyl-D-aspartate (NMDA) and the regulation of mitogen-activated protein kinase (MAPK) signaling pathways: a revolving neurochemical axis for therapeutic intervention?

Excitatory synaptic transmission in the central nervous system (CNS) is mediated by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N-methyl-D-aspartate (NMDA) and non-NMDA (AMPA and KA) receptors. Extracellular signals control diverse neuronal functions and are responsible for mediating activity-dependent changes in synaptic strength and neuronal survival. Influx of extracellular calcium ([Ca(2+)](e)) through the NMDA receptor (NMDAR) is required for neuronal activity to change the strength of many synapses. At the molecular level, the NMDAR interacts with signaling modules, which, like the mitogen-activated protein kinase (MAPK) superfamily, transduce excitatory signals across neurons. Recent burgeoning evidence points to the fact that MAPKs play a crucial role in regulating the neurochemistry of NMDARs, their physiologic and biochemical/biophysical properties, and their potential role in pathophysiology. It is the purpose of this review to discuss: (i) the MAPKs and their role in a plethora of cellular functions; (ii) the role of MAPKs in regulating the biochemistry and physiology of NMDA receptors; (iii) the kinetics of MAPK-NMDA interactions and their biologic and neurochemical properties; (iv) how cellular signaling pathways, related cofactors and intracellular conditions affect NMDA-MAPK interactions and (v) the role of NMDA-MAPK pathways in pathophysiology and the evolution of disease conditions. Given the versatility of the NMDA-MAPK interactions, the NMDA-MAPK axis will likely form a neurochemical target for therapeutic interventions.

Changes in secondary glutamate release underlie the developmental regulation of excitotoxic neuronal cell death

Vulnerability to excitotoxicity increases during development in vivo and in vitro. To determine whether the mere presence of mature N-methyl-D-aspartate (NMDA) receptors coincides with the emergence of excitotoxicity or whether post-receptor signaling processes may also contribute, we examined the temporal relationship of NMDA receptor expression, function and toxicity using cortical cell cultures. Surface expression of all NMDA receptor subunits increased with time in culture. This correlated with NMDA receptor function, assessed both biochemically and electrophysiologically, but not with the appearance of excitotoxicity. Specifically, cells at day in vitro (DIV) 10 were less susceptible to NMDA receptor-induced neurotoxicity than those cultured for 14 days, even though receptor expression/function was identical. In addition, cell-attached single channel recordings revealed that NMDA receptor conductance, open probability, and frequency of channel openings were not significantly different between the two days. Intriguingly, depolarization-induced release of glutamate from cultures grown for 10 days was significantly lower than that released from cultures grown for 14 days. Further, exogenous addition of glutamate receptor agonists immediately after removal of NMDA rendered cultures at DIV 10 susceptible to excitotoxicity, while toxicity was significantly reduced by addition of an NMDA receptor antagonist immediately after exposure to NMDA at DIV 14. These data are the first to demonstrate that the subsequent, secondary release of glutamate plays an equal, if not more important, role than NMDA receptor development per se, in mediating the enhanced vulnerability of neurons to excitotoxicity that occurs with age.

The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective

The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.

Extrasynaptic NR2B and NR2D subunits of NMDA receptors shape 'superslow' afterburst EPSC in rat hippocampus

In conditions of facilitated synaptic release, CA3/CA1 synapses generate anomalously slow NMDA receptor-mediated EPSCs (EPSC(NMDA)). Such a time course has been attributed to the cooperation of synapses through glutamate spillover. Imitating a natural pattern of activity, we have applied short bursts (2-7 stimuli) of high-frequency stimulation and observed a spike-to-spike slow-down of the EPSC(NMDA) kinetics, which accompanied synaptic facilitation. It was found that the early component of the EPSC(NMDA) and the burst-induced late component of the EPSC(NMDA) have distinct pharmacological properties. The competitive NMDA antagonist R-(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (D-CPP), which has higher affinity to NR2A than to NR2B subunits and lowest affinity at NR2D subunits, significantly slowed down the decay rate of the afterburst EPSC while leaving the kinetics of the control current unaffected. In contrast, ifenprodil, a highly selective NR2B antagonist, and [+/-]-cis-1-[phenanthren-2yl-carbonyl]piperazine-2,3-dicarboxylic acid (PPDA), a competitive antagonist that is moderately selective for NR2D subunits, more strongly inhibited the late component of the afterburst EPSC(NMDA). The receptors formed by NR2B and (especially) NR2D subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors. Furthermore, NR2B is preferentially and NR2D is exclusively located on extrasynaptic membranes. As the density of active synapses increases, the confluence of released glutamate makes EPSC decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC(NMDA.)

Molecular biology and ontogeny of glutamate receptors in the mammalian central nervous system

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic terminals, glutamate binds to both ionotropic and metabotropic receptors to mediate fast, slow, and persistent effects on synaptic transmission and integrity. There are three types of ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA), and kainate receptors are principally activated by the agonist bearing its name and are permeable to cationic flux; hence, their activation results in membrane depolarization. All ionotropic glutamate receptors are believed to be composed of four distinct subunits, each of which is topologically arranged with three transmembrane-spanning and one pore-lining (hairpin loop) domain. In contrast, metabotropic glutamate receptors are G protein (guanine nucleotide-binding protein) -coupled receptors linked to second-messenger systems. Group I metabotropic glutamate receptors are linked to phospholipase C, which results in phosphoinositide hydrolysis and release of calcium from intracellular stores. Group II and group III metabotropic glutamate receptors are negatively linked to adenylate cyclase, which catalyzes the production of cyclic adenosine monophosphate. Each metabotropic glutamate receptor is composed of seven transmembrane-spanning domains, similar to other members of the superfamily of metabotropic receptors, which includes noradrenergic, muscarinic acetylcholinergic, dopaminergic, serotonergic (except type 3 receptors), and gamma-aminobutyric acid (GABA) type B receptors. This review summarizes the relevant molecular biology and ontogeny of glutamate receptors in the central nervous system and highlights some of the roles that they can play during brain development and in certain disease states.

Pathway-specific differences in subunit composition of synaptic NMDA receptors on pyramidal neurons in neocortex

Heterogeneity of synaptic inputs onto neocortical layer 5 pyramidal neurons could result from differences in the underlying receptors, yet previous work has shown that functional attributes of AMPA receptors are uniform among synaptic connections onto these neurons. To determine whether NMDA receptors (NMDARs) would be similarly uniform, we compared in the same pyramidal neurons pharmacologically isolated NMDAR-mediated EPSCs evoked by stimulation of two anatomically distinguishable pathways, callosal or intracortical. Based on differences in voltage dependence, decay kinetics, apparent Mg2+sensitivity, and subunit-specific (NR2A, NR2B, and NR2C/D) pharmacology, we found NMDARs at these inputs to be distinct. Furthermore, NMDARs activated by the intracortical pathway were more efficient at integrating EPSPs and bringing the neuron closer to the spike-firing threshold than the callosal pathway. These results suggest that pyramidal neurons encode information differentially depending on the origin of their neocortical inputs and that NMDAR-dependent synaptic plasticity may be pathway specific.

Synapses between parallel fibres and stellate cells express long-term changes in synaptic efficacy in rat cerebellum

Various forms of synaptic plasticity underlying motor learning have already been well characterized at cerebellar parallel fibre (PF)-Purkinje cell (PC) synapses. Inhibitory interneurones play an important role in controlling the excitability and synchronization of PCs. We have therefore tested the possibility that excitatory synapses between PFs and stellate cells (SCs) are also able to exhibit long-term changes in synaptic efficacy. In the present study, we show that long-term potentiation (LTP) and long-term depression (LTD) were induced at these synapses by a low frequency stimulation protocol (2 Hz for 60 s) and that pairing this low frequency stimulation protocol with postsynaptic depolarization induced a marked shift of synaptic plasticity in favour of LTP. This LTP was cAMP independent, but required nitric oxide (NO) production from pre- and/or postsynaptic elements, depending on the stimulation or pairing protocol used, respectively. In contrast, LTD was not dependent on NO production but it required activation of postsynaptic group II and possibly of group I metabotropic glutamate receptors. Finally, stimulation of PFs at 8 Hz for 15 s also induced LTP at PF-SC synapses. But in this case, LTP was cAMP dependent, as was also observed at PF-PC synapses for presynaptic LTP induced in the same conditions. Thus, long-term changes in synaptic efficacy can be accomplished by PF-SCs synapses as well as by PF-PC synapses, suggesting that both types of plasticity might co-operate during cerebellar motor learning.

The contribution of NMDA and AMPA conductances to the control of spiking in neurons of the deep cerebellar nuclei

We performed whole-cell patch-clamp recordings in vitro to investigate the integration of excitatory and inhibitory inputs in neurons of the deep cerebellar nuclei (DCN) by applying synthetic synaptic input patterns with dynamic clamping. We explored an input regime in which excitation and inhibition had an ongoing baseline rate because both input pathways show ongoing activity in vivo. We found that spiking was time-locked to transients in the inputs, consisting of brief decreases in inhibitory or increases in excitatory conductance. Such input transients were caused by synchronization among multiple inputs. However, we found that temporal synchrony in the inhibitory input pathway had preferential access to the control of DCN spiking, because the large NMDA component of the excitatory inputs smoothed out temporal transients in this pathway. Thus, synaptic integration in the DCN appears to be tuned to allow the cerebellar cortical output from Purkinje cells preferential access to the control of DCN spiking. The effect of temporal modulations in the inhibition was further enhanced by the voltage dependence of the NMDA inputs. Thus, the presence of a baseline of mossy and climbing fiber inputs boosted depolarizing responses caused by reduced inhibition by the voltage-dependent increase in inward NMDA current. Overall, our results show that correlated activity or pauses in populations of Purkinje cells are well suited to the dynamic control of DCN spiking. In addition, strong transients in excitation can directly drive DCN responses that bypass cerebellar cortical processing.

NR2B and NR2D subunits coassemble in cerebellar Golgi cells to form a distinct NMDA receptor subtype restricted to extrasynaptic sites

NMDA receptors (NMDARs) are thought to be tetrameric assemblies composed of NR1 and at least one type of NR2 subunit. The identity of the NR2 subunit (NR2A, -B, -C, -D) is critical in determining many of the functional properties of the receptor, such as channel conductance and deactivation time. Further diversity may arise from coassembly of more than one type of NR2 subunit, if the resulting triheteromeric assembly (NR1 plus two types of NR2) displays distinct functional properties. We have used gene-ablated mice (NR2D -/-) to examine the effects of the NR2D subunit on NMDAR channels and NMDAR EPSCs in cerebellar Golgi cells. These cells are thought to express both NR2B and NR2D subunits, a combination that occurs widely in the developing nervous system. Our experiments provide direct evidence that the low conductance NMDAR channels in Golgi cells arise from diheteromeric NR1/NR2D assemblies. To investigate whether a functionally distinct triheteromeric assembly was also expressed, we analyzed the kinetic and pharmacological properties of single-channel currents in isolated extrasynaptic patches. We found that after the loss of the NR2D subunit, the properties of the 50 pS NMDAR channels were altered. This result is consistent with the presence of a triheteromeric assembly (NR1/NR2B/NR2D) in cells from wild-type mice. However, we could find no difference in the properties of NMDAR-mediated EPSCs between wild-type and NR2D subunit ablated mice. Our experiments suggest that although both diheteromeric and triheteromeric NR2D-containing receptors are expressed in cerebellar Golgi cells, neither receptor type participates in parallel fiber to Golgi cell synaptic transmission. The presence of the NR2D subunit within an assembly may therefore result in its restriction to extrasynaptic sites.

Activation of NMDA receptors in rat dentate gyrus granule cells by spontaneous and evoked transmitter release

Activation of N-methyl-D-aspartate (NMDA) receptors by synaptically released glutamate in the nervous system is usually studied using evoked events mediated by a complex mixture of AMPA, kainate, and NMDA receptors. Here we have characterized pharmacologically isolated spontaneous NMDA receptor-mediated synaptic events and compared them to stimulus evoked excitatory postsynaptic currents (EPSCs) in the same cell to distinguish between various modes of activation of NMDA receptors. Spontaneous NMDA receptor-mediated EPSCs recorded at 34 degrees C in dentate gyrus granule cells (DGGC) have a frequency of 2.5 +/- 0.3 Hz and an average peak amplitude of 13.2 +/- 0.8 pA, a 10-90% rise time of 5.4 +/- 0.3 ms, and a decay time constant of 42.1 +/- 2.1 ms. The single-channel conductance estimated by nonstationary fluctuation analysis was 60 +/- 5 pS. The amplitudes (46.5 +/- 6.4 pA) and 10-90% rise times (18 +/- 2.3 ms) of EPSCs evoked from the entorhinal cortex/subiculum border are significantly larger than the same parameters for spontaneous events (paired t-test, P < 0.05, n = 17). Perfusion of 50 microM D(-)-2-amino-5-phosphonopentanoic acid blocked all spontaneous activity and caused a significant baseline current shift of 18.8 +/- 3.0 pA, thus identifying a tonic conductance mediated by NMDA receptors. The NR2B antagonist ifenprodil (10 microM) significantly reduced the frequency of spontaneous events but had no effect on their kinetics or on the baseline current or variance. At the same time, the peak current and charge of stimulus-evoked events were significantly diminished by ifenprodil. Thus spontaneous NMDA receptor-mediated events in DGGC are predominantly mediated by NR2A or possibly NR2A/NR2B receptors while the activation of NR2B receptors reduces the excitability of entorhinal afferents either directly or through an effect on the entorhinal cells.

Agmatine attenuates neuropathic pain in rats: possible mediation of nitric oxide and noradrenergic activity in the brainstem and cerebellum

Effect of agmatine (10-400 mg/kg) on neuropathic pain in a rat model produced by loose ligatures around the common sciatic nerve was studied. The involvement of possible alterations in nitric oxide (NO) levels [measured as its stable metabolites nitrate + nitrite] and in noradrenergic activity [measured as norepinephrine and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) levels] in this effect was also investigated biochemically in the brainstem and cerebellum. Agmatine increased the neuropathic pain threshold at 300 and 400 mg/kg. There was almost a twofold increase in nitrate + nitrite levels in the brainstem and cerebellum of the rats with neuropathic pain and agmatine decreased the high nitrate + nitrite levels only in the brainstem at 300 mg/kg and both in the brainstem and cerebellum at 400 mg/kg. Ligation of sciatic nerve resulted in almost twofold increase in norepinephrine and MHPG levels only in the brainstem of the rats. Agmatine decreased MHPG levels at 300 and 400 mg/kg, however it decreased norepinephrine levels only at the higher dose. These findings indicate that agmatine decreases neuropathic pain, an effect which may involve the reduction of NO levels and noradrenergic activity in the brain.

NMDA receptor 2 (NR2) C-terminal control of NR open probability regulates synaptic transmission and plasticity at a cerebellar synapse

The C-terminal domain of NMDA receptor 2 (NR2) subunits has been proposed to play a critical role in regulating NMDA receptor localization and function in postsynaptic densities. However, the mechanism of this regulation is not completely understood. In this paper we show that C-terminal truncation of NR2A and NR2C subunits in mice (NR2A/C(DeltaC/DeltaC)) impairs synaptic transmission and plasticity at the cerebellar mossy fiber-granule cell relay. Activation of synaptic NMDA receptors could be distinguished from that of extrasynaptic receptors by using the glutamate scavenger glutamate pyruvate transaminase and the open channel blocker MK801. NR2A/C(DeltaC/DeltaC) mice exhibited a specific reduction in synaptic NMDA receptor activation attributable to a severalfold decrease in channel open probability but not channel conductance. Immunodetection revealed normal developmental expression of NR subunit proteins. Quantitative immunogold analyses with an antibody to NR1 indicated that the reduction in receptor activation is not attributed to a reduced number of NR1-containing receptors in postsynaptic densities. Thus, NR2A/NR2C subunits and particularly their C termini regulate synaptic NMDA receptor activation and function by enhancing channel open probability, which is critical for long-term potentiation induction.

Activity-dependent recruitment of extrasynaptic NMDA receptor activation at an AMPA receptor-only synapse

We have identified an excitatory synapse in cerebellar molecular layer interneurons at which the level of presynaptic activity determines the receptor type involved in the postsynaptic response. When small numbers of parallel fibers are activated, EPSCs are mediated solely by AMPA receptors (AMPARs), despite our finding that NMDA receptors (NMDARs) are present in the dendrites of these cells. The EPSC kinetics are fast (tau decay = 0.82 +/- 0.05 msec at room temperature), consistent with the role these interneurons are thought to play in precisely timed inhibitory control of Purkinje cells. NMDARs are activated only when glutamate release is increased either by facilitation with brief high-frequency trains or by recruiting more presynaptic fibers with higher stimulus intensities. Under these conditions, EPSCs consist of a fast-rising AMPAR-mediated current followed by a slow component mediated by both NMDARs and AMPARs. Inhibitors of glutamate transport increase the amplitude and prolong the time course of the compound EPSCs. In contrast, the properties of fast AMPAR EPSCs resulting from the activation of few inputs remain unchanged when glutamate uptake is blocked. Our results suggest that, at these synapses, the postsynaptic density contains AMPARs alone. It is only when transmitter release is high enough for glutamate to diffuse to the extrasynaptic space and to reach concentrations sufficient to activate extrasynaptic receptors that NMDARs are involved in the postsynaptic response. We suggest that such a spatial separation of receptor types may provide a mechanism for rapid changes in EPSC properties, depending on the amount of synaptic activity.

Subtypes of NMDA receptors in new-born rat hippocampal granule cells

To investigate the properties of NMDA receptors expressed in new-born rat hippocampal granule cells, recordings were made of single-channel currents produced by application of glutamate or NMDA to outside-out membrane patches. Outside-out patches displayed two distinct patterns of single-channel activity. In some patches only high conductance single-channel activity composed of 42 and 50 pS currents was observed while in others both high (42 and 50 pS) and low (17 and 33 pS) conductance single-channel currents occurred. An absence of direct transitions connecting the smallest (17 pS) and largest (50 pS) conductance unitary currents, as well as an absence of direct transitions connecting 17, 42 and 50 pS currents in sequence, suggested that high and low conductance single-channel activity may have been produced as a result of the activation of two distinct NMDA receptor populations. The NR2B subunit-selective NMDA receptor antagonist, ifenprodil, blocked the high conductance currents suggesting that these receptors contain the NR2B subunit while a clear asymmetry in the frequency of direct transitions between 17 and 42 pS conductance levels indicates the presence of NMDA receptors containing NR2D subunits. In patches containing both high and low conductance-channel activity, evidence for negative coupling between NR2B- and NR2D-like channel activity was observed, suggesting receptors containing these subunits do not gate independently or that both NR2B and NR2D subunits may be part of a single receptor molecule. We conclude that NMDA receptors in P0 hippocampal granule cells are likely to be a mixture of NR1/NR2B diheteromers and receptors of novel molecular composition that may be triheteromeric receptors composed of NR1, NR2B and NR2D subunits.

Re-evaluation of phencyclidine low-affinity or "non-NMDA" binding sites

TCP and its derivative gacyclidine (+/- GK11) are high-affinity non-competitive antagonists of N-methyl-D-aspartate (NMDA) receptors (NMDARs) and as such exhibit significant neuroprotective properties. These compounds also bind with a low affinity to binding sites whose pharmacological profiles are different from that of NMDARs. With the intention to develop new strategies of neuroprotection, we found it mandatory to investigate whether 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP) and gacyclidine low-affinity sites are similar. The effects of several drugs selective for either NMDARs or the [(3)H]TCP low-affinity site (or PCP(3) site) on (+), (-)[(3)H]GK11 and [(3)H]TCP specific binding were investigated. Competition experiments on cerebellum homogenates revealed substantial differences between the pharmacological profiles of the PCP(3) site and that of gacyclidine's enantiomers low-affinity sites. Under experimental conditions preventing the interaction of the radioligands with NMDARs, the autoradiographic study showed, however, that the distributions of both [(3)H]TCP and (-)[(3)H]GK11 specific binding were similar. The specific labelling was low and uniform in telencephalic structures, whereas in the cerebellum it was higher in the molecular than in the granular layer. Finally, the analysis of competition experiments performed on tissues slices demonstrated that PCP(3) selective ligands were unable to prevent [(3)H]TCP or (-)[(3)H]GK11 binding to "non-NMDA" binding sites. As a whole, our data suggest that: (1) the different pharmacological profiles of [(3)H]TCP and [(3)H]gacyclidine enantiomers on low-affinity sites are due to their selectivity for specific NMDARs subpopulations; (2) the pharmacological isolation of TCP and gacyclidine "non-NMDA" binding sites is the most appropriate way to further study the low-affinity component of their specific binding. Obtaining reliable and specific pharmacological tools for those binding sites is of particular interest, since it is likely that they play a substantial role in the low neurotoxicity, and therefore tolerability, of gacyclidine, a new neuroprotective drug currently evaluated in clinical trials for the treatment of brain and spinal cord injuries.