Intracellular blockade of GABAA receptors in the rat hippocampal neurons

The intracellular blockade of GABA_A-receptor-mediated currents is a useful approach to suppress the GABAergic conductance in a single cell and to isolate the glutamatergic component of network-driven activities. Previously an approach has been described allowing intracellular blockade of GABA_A receptors by means of intracellular dialysis of a neuron with the pipette-filling solution, in which fluoride ions that hardly pass through the GABA_A receptor channels substitute for Cl^? and in which Mg²⁺ and ATP are omitted to induce rundown of the GABA_A receptors during whole-cell patch-clamp recordings. However, the kinetics of suppression of GABAergic conductance and the effect on the currents mediated by glutamate receptors remain unknown. Here, using whole-cell recordings with fluoride-based, Mg²⁺- and ATP-free solution on CA3 hippocampal neurons of neonatal rats, we show that after 1 h of such dialysis, both spontaneous and evoked GABA_A-receptor-mediated synaptic currents and responses induced by the GABA_A receptor agonist isoguvacine were completely suppressed. Inward GABAergic postsynaptic currents were suppressed prior to outward currents. Synaptic responses mediated by AMPA receptors were not affected by the dialysis, whereas the NMDA-receptor-mediated postsynaptic currents were reduced by approximately 20%. Dialysis with fluoride-based Mg²⁺, ATP-free solution either fully blocked giant depolarizing potentials (GDPs) in CA3 pyramidal cells (n = 2) or reduced the charge crossing the membrane during GDPs and shifted the GDP reversal potential to more positive values (n = 5). The dialysis-resistant component of GDPs was mediated by glutamate receptors, since: (i) it reversed around 0 mV; (ii) it demonstrated a negative slope conductance at negative membrane voltages, which is characteristic of NMDA receptor-mediated responses; (iii) kinetics of the individual events composing the dialysis-resistant component of GDPs at negative voltages were very similar to those of AMPA receptor-mediated synaptic currents. Thus, this procedure can be useful to isolate the glutamate receptor-mediated component of neuronal network-driven activities.     

Mg2+/Ca2+-ATPase Activity Is Not Enriched in Synaptic Vesicles Isolated from Rat Cerebral Cortex

Neuronal ATPases comprise a wide variety of enzymes which are not uniformly distributed in different membrane preparations. Since purified vesicle fractions have Mg²⁺/Ca²⁺-ATPase, the purpose of the present study was to know whether such enzyme activities have a preferential concentration in a synaptic vesicle fraction in order to be used as markers for these organelles. Resorting to a procedure developed in this Institute, we fractionated the rat cerebral cortex by differential centrifugation following osmotic shock of a crude mitochondrial fraction and separated a purified synaptic vesicle fraction over discontinuous sucrose gradients. Mg²⁺/Ca²⁺-ATPase activities and ultrastructural studies of isolated fractions were carried out. It was observed that similar specific activities for Mg²⁺/Ca²⁺-ATPases were found in all fractions studied which contain synaptic vesicles and/or membranes. Although the present results confirm the presence of Mg²⁺ and Ca²⁺-ATPase activities in synaptic vesicles preparations, they do not favor the contention that Mg²⁺/Ca²⁺-ATPase is a good marker for synaptic vesicles.     

Depolarization gates spine calcium transients and spike-timing-dependent potentiation

Timing-dependent long-term potentiation (t-LTP) is induced when synaptic activity is immediately followed by one or more back-propagating action potentials (bAPs) in the postsynaptic cell. As a mechanistic explanation, it has been proposed that the bAP removes the Mg2+ block of synaptic NMDA receptors, allowing for rapid Ca2+ entry at the active synapse. Recent experimental studies suggest that this model is incomplete: NMDA receptor-based coincidence detection requires strong postsynaptic depolarization, usually provided by AMPA receptor currents. Apparently, the brief AMPA-EPSP does not only enable t-LTP, it is also responsible for the very narrow time window for t-LTP induction. The emerging consensus puts the spine in the center of coincidence detection, as active conductances on the spine together with the electrical resistance of the spine neck regulate the depolarization of the spine head and thus Ca2+ influx during pairing. A focus on postsynaptic voltage during synaptic activation not only encompasses spike-timing-dependent plasticity (STDP), but explains also the cooperativity and frequency-dependence of plasticity.     

Calcineurin Mediates Synaptic Scaling Via Synaptic Trafficking of Ca2+-Permeable AMPA Receptors

Homeostatic synaptic plasticity is a negative-feedback mechanism for compensating excessive excitation or inhibition of neuronal activity. When neuronal activity is chronically suppressed, neurons increase synaptic strength across all affected synapses via synaptic scaling. One mechanism for this change is alteration of synaptic AMPA receptor (AMPAR) accumulation. Although decreased intracellular Ca²⁺ levels caused by chronic inhibition of neuronal activity are believed to be an important trigger of synaptic scaling, the mechanism of Ca²⁺-mediated AMPAR-dependent synaptic scaling is not yet understood. Here, we use dissociated mouse cortical neurons and employ Ca²⁺ imaging, electrophysiological, cell biological, and biochemical approaches to describe a novel mechanism in which homeostasis of Ca²⁺ signaling modulates activity deprivation-induced synaptic scaling by three steps: (1) suppression of neuronal activity decreases somatic Ca²⁺ signals; (2) reduced activity of calcineurin, a Ca²⁺-dependent serine/threonine phosphatase, increases synaptic expression of Ca²⁺-permeable AMPARs (CPARs) by stabilizing GluA1 phosphorylation; and (3) Ca²⁺ influx via CPARs restores CREB phosphorylation as a homeostatic response by Ca²⁺-induced Ca²⁺ release from the ER. Therefore, we suggest that synaptic scaling not only maintains neuronal stability by increasing postsynaptic strength but also maintains nuclear Ca²⁺ signaling by synaptic expression of CPARs and ER Ca²⁺ propagation.     

Modulation by Ionotropic Excitatory Amino Acids and Potassium of (±)-1-Aminocyclopentane-trans-1,3-Dicarboxylic Acid-Stimulated Phosphoinositide Hydrolysis in Mouse Cerebellar Granule Cells

Abstract: The effect of ionotropic excitatory amino acids and potassium on the formation of inositol phosphates elicited by the metabotropic glutamate receptor agonist (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) was studied in mouse cerebellar granule cells. In Mg²⁺-containing buffers, NMDA (50–100 µM), α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 10–1,000 µM), and high potassium (10–30 mM) enhanced synergistically the response to a maximally effective concentration of 500 µMtrans-ACPD. Potentiation of the trans-ACPD response was blocked by higher concentrations of NMDA (>500 µM) and potassium (>35 mM) but not by AMPA (up to 1 mM). The potentiation by NMDA of the trans-ACPD-stimulated phosphoinositide hydrolysis was blocked by d,l -2-amino-5-phosphonopentanoic acid (APV), a competitive NMDA-receptor antagonist. Under Mg²⁺-free conditions, the accumulation of inositol phosphates in the presence of trans-ACPD alone was equal to that attained by trans-ACPD in Mg²⁺-containing buffers when costimulated with maximally enhancing concentrations of NMDA (50 µM). trans-ACPD potentiated synergistically the NMDA-evoked increases in cytosolic free-Ca²⁺ levels in Mg²⁺-containing but not in Mg²⁺-free solutions, and moreover did not enhance the AMPA-evoked increases in cytosolic free-Ca²⁺ levels. The calcium ionophore A23187 caused a dose-dependent increase in inositol phosphate accumulation but did not enhance the response stimulated by trans-ACPD alone. These results demonstrate the existence of cross talk between metabotropic and ionotropic glutamate receptors in cerebellar granule cells. The exact mechanism remains unclear but appears to involve interplay of G protein-coupled phospholipase C activation and regulated elevation of cytosolic free-Ca²⁺ levels. This study may provide a framework for future investigations at the cellular and molecular level that clarify the functional relevance and molecular mechanisms that are described.     

The mechanism of neuroprotection by positive modulation of Ca<Superscript>2+</Superscript>-activated K<Superscript>+</Superscript> channels of cerebellar neurons in primary culture

Here we show that positive modulators (CyPPA and NS309) of Ca²⁺-activated K⁺ channels of small (SK) and intermediate (IK) conductances in cerebellar neurons decrease glutamate-evoked Ca²⁺ entry into neurons independently on the presence of Mg²⁺ in extracellular media. An analysis of neuronal viability after long-term (240 min) glutamate treatments demonstrated neuroprotective action of CyPPA and NS309. Extracellular Mg²⁺ did not protect neurons from apoptosis during prolonged treatment with glutamate. Activation of SK and IK channels results in local membrane hyperpolarization, which enhances Mg²⁺ block of NMDA receptors and reduces activation of voltage-dependent Ca²⁺ channels, which can explain neuroprotection caused by CyPPA or NS309. The obtained results reveal an important role Ca²⁺-activated K⁺ channels of small and intermediate conductance in the regulation of Ca²⁺ entry into cerebellar neurons via NMDA receptors and voltage-gated Ca²⁺ channels.     

A hemicord locomotor network of excitatory interneurons: a simulation study

Locomotor burst generation is simulated using a full-scale network model of the unilateral excitatory interneuronal population. Earlier small-scale models predicted that a population of excitatory neurons would be sufficient to produce burst activity, and this has recently been experimentally confirmed. Here we simulate the hemicord activity induced under various experimental conditions, including pharmacological activation by NMDA and AMPA as well as electrical stimulation. The model network comprises a realistic number of cells and synaptic connectivity patterns. Using similar distributions of cellular and synaptic parameters, as have been estimated experimentally, a large variation in dynamic characteristics like firing rates, burst, and cycle durations were seen in single cells. On the network level an overall rhythm was generated because the synaptic interactions cause partial synchronization within the population. This network rhythm not only emerged despite the distributed cellular parameters but relied on this variability, in particular, in reproducing variations of the activity during the cycle and showing recruitment in interneuronal populations. A slow rhythm (0.4–2 Hz) can be induced by tonic activation of NMDA-sensitive channels, which are voltage dependent and generate depolarizing plateaus. The rhythm emerges through a synchronization of bursts of the individual neurons. A fast rhythm (4–12 Hz), induced by AMPA, relies on spike synchronization within the population, and each burst is composed of single spikes produced by different neurons. The dynamic range of the fast rhythm is limited by the ability of the network to synchronize oscillations and depends on the strength of synaptic connections and the duration of the slow after hyperpolarization. The model network also produces prolonged bouts of rhythmic activity in response to brief electrical activations, as seen experimentally. The mutual excitation can sustain long-lasting activity for a realistic set of synaptic parameters. The bout duration depends on the strength of excitatory synaptic connections, the level of persistent depolarization, and the influx of Ca²⁺ ions and activation of Ca²⁺-dependent K⁺ current.     

Catalytically Dead αCaMKII K42M Mutant Acts as a Dominant Negative in the Control of Synaptic Strength

During long-term potentiation (LTP) of excitatory synapses, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is activated by Ca²⁺ influx through NMDA receptors that potentiate AMPA receptor currents by insertion of additional GluR1-containing receptors at the synapse and by increasing AMPA channel conductance, as well as by stimulating structural changes. CaMKII is also involved in the maintenance of LTP and contributes to maintenance of behavioral sensitization by cocaine or amphetamine. Recent studies show that transient expression of catalytically dead αCaMKII K42M mutant after exposure to amphetamine persistently reverses the behavioral effects of the addiction. A suggested interpretation is that this mutant acts as a dominant negative in the control of synaptic strength, but this interpretation has not been physiologically tested. Here we investigate the effect of αCaMKII K42M mutant expressed in single CA1 pyramidal neurons on basal excitatory neurotransmission in cultured rat hippocampal organotypic slices. The mutant caused nearly 50% reduction in the basal CA3–CA1 transmission, while overexpression of the wild-type αCaMKII had no effect. This result is consistent with the dominant negative hypothesis, but there are complexities. We found that the decrease in basal transmission did not occur when activity in the slices was suppressed after transfection by TTX or when NMDA receptors were blocked by APV. Thus, the dominant negative effect requires neural activity for its expression.     

Ionotropic glutamate receptor subunit distribution on hypoglossal motoneuronal pools in the rat

The expression of ionotropic glutamate receptor subunits in the motoneuronal pools of the hypoglossal nucleus was studied using specific antibodies against subunits of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes. The highest numbers of intensely immunolabelled motoneurons were found in the dorsal tier and caudoventromedial part of the hypoglossal nucleus with all antibodies except that against the GluR1 AMPA subunit. Labelling for the GluR1 subunit was weak except for caudally located groups of motoneurons which innervate tongue muscles related to respiratory activity. By contrast, most motoneurons were intensely immunostained with antibodies against GluR2/3 and GluR4 subunits of the AMPA subtype. The low staining observed using an antibody specific for the GluR2 subunit (which prevents Ca²⁺-entry through AMPA channels) strongly suggests that AMPA receptors in hypoglossal motoneurons are Ca²⁺-permeable. Immunolabelling for the GluR5/6/7 kainate receptor subunits was found in many motoneuronal somata as well as in thin axon-like profiles and puncta that resembled synaptic boutons. Most motoneurons were intensely immunostained for the NMDA receptor subunit NR1. These results show that the hypoglossal nucleus contains five heterogeneous pools of motoneurons which innervate functionally defined groups of tongue muscles. The uneven expression of the different receptor subunits analysed here could reflect diverse phenotypic properties of hypoglossal motoneurons which might be expected to generate different patterns of motor responses under different physiological or pathological conditions.     

Extracellular Ca2+ ions reduce NMDA receptor conductance and gating

Brief intracellular Ca²⁺ transients initiate signaling routines that direct cellular activities. Consequently, activation of Ca²⁺-permeable neurotransmitter-gated channels can both depolarize and initiate remodeling of the postsynaptic cell. In particular, the Ca²⁺ transient produced by NMDA receptors is essential to normal synaptic physiology, drives the development and plasticity of excitatory central synapses, and also mediates glutamate excitotoxicity. The amplitude and time course of the Ca²⁺ signal depends on the receptor’s conductance and gating kinetics; these properties are themselves influenced both directly and indirectly by fluctuations in the extracellular Ca²⁺ concentration. Here, we used electrophysiology and kinetic modeling to delineate the direct effects of extracellular Ca²⁺ on recombinant GluN1/GluN2A receptor conductance and gating. We report that, in addition to decreasing unitary conductance, Ca²⁺ also decreased channel open probability primarily by lengthening closed-channel periods. Using one-channel current recordings, we derive a kinetic model for GluN1/GluN2A receptors in physiological Ca²⁺ concentrations that accurately describes macroscopic channel behaviors. This model represents a practical instrument to probe the mechanisms that control the Ca²⁺ transients produced by NMDA receptors during both normal and aberrant synaptic signaling.     

NMDA and Non-NMDA Receptor-Mediated Release of [3H]GABA from Granule Cell Dendrites of Rat Olfactory Bulb

Abstract: We have studied the effect of glutamate and the glutamatergic agonists N-methyl-d -aspartate (NMDA), kainate, and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) on [³H]GABA release from the external plexiform layer of the olfactory bulb. The GABA uptake blocker nipecotic acid significantly increased the basal [³H]GABA release and the release evoked by a high K⁺ concentration, glutamate, and kainate. The glutamate uptake blocker pyrrolidine-2,4-dicarboxylate (2,4-PDC) inhibited by 50% the glutamate-induced [³H]GABA release with no change in the basal GABA release. The glutamatergic agonists NMDA, kainate, and AMPA also induced a significant [³H]GABA release. The presence of glycine and the absence of Mg²⁺ have no potentiating effect on NMDA-stimulated release; however, when the tissue was previously depolarized with a high K⁺ concentration, a significant increase in the NMDA response was observed that was potentiated by glycine and inhibited by the NMDA receptor antagonist 2-amino-5-phosphonoheptanoic acid (AP-7). The kainate and AMPA effects were antagonized by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not by AP-7. The glutamate effect was also inhibited by CNQX but not by the NMDA antagonist 2-amino-5-phosphonopentanoic acid (AP-5); nevertheless, in the presence of glycine, [³H]GABA release evoked by glutamate was potentiated, and this response was significantly antagonized by AP-5. Tetrodotoxin inhibited glutamate- and kainate-stimulated [³H]GABA release but not the NMDA-stimulated release. The present results show that in the external plexiform layer of the olfactory bulb, glutamate is stimulating GABA release through a presynaptic, receptor-mediated mechanism as a mixed agonist on NMDA and non-NMDA receptors; glutamate is apparently also able to induce GABA release through heteroexchange.     

Effect of In Vivo Administration of Naloxone on ATP-ase's Enzyme Systems of Synaptic Plasma Membranes from Rat Cerebral Cortex

Naloxone is a specific competitive antagonist of morphine, acting on opiate receptors, located on neuronal membranes. The effects of in vivo administration of naloxone on energy-consuming non-mitochondrial ATP-ases were studied in two different types of synaptic plasma membranes from rat cerebral cortex, known to contain a high density of opiate receptors. The enzyme activities of Na⁺, K⁺-ATP-ase, Ca²⁺, Mg²⁺-ATP-ase and Mg²⁺-ATP-ase and of acetylcholinesterase (AChE) were evaluated on synaptic plasma membranes obtained from control and treated animals with effective dose of naloxone (12g · kg^–1 i.m. 30 minutes). In control (vehicle-treated) animals specific enzyme activities assayed on these two types of synaptic plasma membranes are different, being higher on synaptic plasma membranes of II type than of I type, because the first fraction is more enriched in synaptic plasma membranes. The acute treatment with naloxone produced a significant decrease in Ca²⁺,Mg²⁺-ATP-ase activity and an increase in AChE activity, only in synaptic plasma membranes of II type. The decrease of Ca²⁺,Mg²⁺-ATP-ase enzymatic activity and the increased AChE activity are related to the interference of the drug on Ca²⁺ homeostasis in synaptosoplasm, that leads to the activation of calcium-dependent processes, i.e. the extrusion of neurotransmitter. These findings give further evidence that pharmacodynamic characteristics of naloxone are also related to increase [Ca²⁺] _i , interfering with enzyme systems (Ca²⁺,Mg²⁺-ATP-ase) and that this drug increases acetylcholine catabolism in synaptic plasma membranes of cerebral cortex.     

Pharmacological characterization of metabotropic glutamate receptors in cultured cerebellar granule cells

A detailed pharmacological characterization of metabotropic glutamate receptors (mGluR) was performed in primary cultures of cerebellar granule cells at 6 days in vitro (DIV). The rank order of agonists induced polyphosphoinositide (PPI) hydrolysis (after correcting for the ionotropic component in the response) was as follows: in terms of efficiency, Glu>quisqualate (quis)=ibotenate (ibo)>(1_S,3_R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD)>-methyl-amino-l-alanine (BMAA) and in terms of potency, quis>ACPD>Glu>ibo=BMAA. Ionotropic excitatory amino acid (EAA) receptor agonists, such as -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) were relatively inactive (in the presence of Mg²⁺). Quis and ACPD-induced PPI hydrolysis was unaffected by ionotropic Glu receptor antagonists, but was inhibited, in part by L-2-amino-3-phosphonopropionate (AP3). In contrast, Glu-or ibo- induced PPI hydrolysis was reduced, in part, by both AP3 and NMDA receptor antagonists. Characteristic interactions involving different transmitter receptors were noted. PPI hydrolysis evoked by quis and 1_S,3_R-ACPD was not additive. In contrast, PPI hydrolysis stimulated by quis/ACPD and carbamylcholine was additive (indicating different receptors/transduction pathways). In the presence of Mg²⁺, the metabotropic response to quis/AMPA and NMDA was synergistic (this being consistent with AMPA receptor-induced depolarization activating NMDA receptor). On the other hand, in Mg²⁺-free buffer the effects of quis and NMDA, at concentrations causing maximal PPI hydrolysis, were additive (indicating that PPI hydrolysis was effected by two different mechanisms). Thus, in cerebellar granule cells EAAs elicit PPI hydrolysis by acting at two distinct receptor types: (i) metabotropic Glu receptors (mGluR), with pharmacological characteristics suggesting the expression of a unique mGluR receptor that shows certain similarities to those observed for the mGluR1 subtype (Aramori and Nakanishi, 1992) and (ii) NMDA receptors. The physiological agonist, Glu, is able to stimulate both receptor classes.Abbreviations ACPD (1_S,3_R)-1-amino-cyclopentane-1,3-dicarboxylic acid - AMPA -amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid - AP₃ L-2-amino-3-phosphono-propionate - AP₅ D-2-amino-5-phosphonopentenoate - BMAA -methyl-amino-L-alanine - DIV days in vitro - DNOX 6,7-dinitroouinoxoline-2,3-dione - EAA excitatory amino acids - Glu glutamate - InsP inositol monophosphate - mGluR metabotropic glutamate receptors - MK-801 (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohept-5,10-imine hydrogen maleate - NMDA N-methyl-D-aspartate - PPI polyphosphoinositide - quis quisqualate     

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.     

Voltage-dependent and Src-mediated regulation of NMDA receptor single channel outward currents in cortical neurons

A voltage-dependent but Ca²⁺-independent regulation of N-methyl-D-aspartate (NMDA) receptor outward activity was studied at the single channel level using outside-out patches of cultured mouse cortical neurons. Unlike the inward activity associated with Ca²⁺ and Na⁺ influx, the NMDA receptor outward K⁺ conductance was unaffected by changes in Ca²⁺ concentration. Following a depolarizing pre-pulse, the single channel open probability (NP _o), amplitude, and open duration of the NMDA inward current decreased, whereas the same pre-depolarization increased those parameters of the NMDA outward current (pre-pulse facilitation). The outward NP _o was increased by the pre-pulse facilitation, disregarding Ca²⁺ changes. The voltage–current relationships of the inward and outward currents were shifted by the pre-depolarization toward opposite directions. The Src family kinase inhibitor, PP1, and the Src kinase antibody, but not the anti-Fyn antibody, blocked the pre-pulse facilitation of the NMDA outward activity. On the other hand, a hyperpolarizing pre-pulse showed no effect on NMDA inward currents but inhibited outward currents (pre-pulse depression). Application of Src kinase, but not Fyn kinase, prevented the pre-pulse depression. We additionally showed that a depolarization pre-pulse potentiated miniature excitatory synaptic currents (mEPSCs). The effect was blocked by application of the NMDA receptor antagonist AP-5 during depolarization. These data suggest a voltage-sensitive regulation of NMDA receptor channels mediated by Src kinase. The selective changes in the NMDA receptor-mediated K⁺ efflux may represent a physiological and pathophysiological plasticity at the receptor level in response to dynamic changes in the membrane potential of central neurons.     

Amplification of calcium signals at dendritic spines provides a method for CNS quantal analysis

It has been proposed that the small volume of a dendritic spine can amplify Ca2+ signals during synaptic transmission. Accordingly, we have performed calculations to determine whether the activation of N-methyl-D-aspartate (NMDA) type glutamate receptors during synaptic transmission results in significant elevation in intracellular Ca2+ levels, permitting optical detection of synaptic signals within a single spine. Simple calculations suggest that the opening of even a single NMDA receptor would result in the influx of approximately 310 000 Ca2+ ions into the small volume of a spine, producing changes in Ca2+ levels that are readily detectable using high affinity Ca2+ indicators such as fura-2 or fluo-3. Using fluorescent Ca2+ indicators, we have imaged local Ca2+ transients mediated by NMDA receptors in spines and dendritic shafts attributed to spontaneous miniature synaptic activity. Detailed analysis of these quantal events suggests that the current triggering these transients is attributed to the activation of <10 NMDA receptors. The frequency of these miniature synaptic Ca2+ transients is not randomly distributed across synapses, as some synapses can display a >10-fold higher frequency of transients than others. As expected for events mediated by NMDA receptors, miniature synaptic Ca2+ transients were suppressed by extracellular Mg2+ at negative membrane potentials; however, the Mg2+ block could be removed by depolarization.     

AMPA/NMDA cooperativity and integration during a single synaptic event

Coexistence of AMPA and NMDA receptors in glutamatergic synapses leads to a cooperative effect that can be very complex. This effect is dependent on many parameters including the relative and absolute number of the two types of receptors and biophysical parameters that can vary among synapses of the same cell. Herein we simulate the AMPA/NMDA cooperativity by using different number of the two types of receptors and considering the effect of the spine resistance on the EPSC production. Our results show that the relative number of NMDA with respect to AMPA produces a different degree of cooperation which depends also on the spine resistance.     

Molecular mechanisms of glutamate receptor-mediated excitotoxic neuronal cell death

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

Nicotine induces dendritic spine remodeling in cultured hippocampal neurons

Cholinergic neurons in the CNS are involved in synaptic plasticity and cognition. Both muscarinic and nicotinic acetylcholine receptors (nAChRs) influence plasticity and cognitive function. The mechanism underlying nAChR‐induced plasticity, however, has remained elusive. Here, we demonstrate morphological changes in dendritic spines following activation of α4β2* nAChRs, which are expressed on glutamatergic pre‐synaptic termini of cultured hippocampal neurons. Exposure of the neurons to nicotine resulted in a lateral enlargement of spine heads. This was abolished by dihydro‐β‐erythroidine, an antagonist of α4β2* nAChRs, but not by α‐bungarotoxin, an antagonist of α7 nAChRs. Tetanus toxin or a mixture of 2‐amino‐5‐phosphonovaleric acid and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione, antagonists of NMDA‐ and AMPA‐type glutamate receptors, blocked the nicotine‐induced spine remodeling. In addition, nicotine exerted full spine‐enlarging response in the post‐synaptic neuron whose β2 nAChR expression was knocked down. Finally, pre‐treatment with nicotine enhanced the Ca²⁺‐response of the neurons to glutamate. These data suggest that nicotine influences the activity of glutamatergic neurotransmission through the activation of pre‐synaptic α4β2 nAChRs, resulting in the modulation of spinal architecture and responsiveness. The present findings may represent one of the cellular mechanisms underlying cholinergic tuning of brain function.

     

Endogenous γ-l-glutamylglutamate is a partial agonist at the N-methyl-d-aspartate receptors in cultured cerebellar granule cells

-l-Glutamylglutamate (LGG), an endogenous constituent of the brain, reduced the glutamateevoked increase in intracellular Ca²⁺ in cultured cerebellar granule cells. The extent and properties of this inhibition were different at different Mg²⁺ concentrations. The intracellular Ca²⁺ response to NMDA was slightly enhanced by 0.1 mM LGG in normal (1.3 mM) Mg²⁺ medium, but in Mg²⁺-free medium LGG was stimulatory at low (0.1–1 M) NMDA and inhibitory at high (0.1–1 mM) NMDA concentrations. In the absence of Mg²⁺, LGG alone increased cytosolic free Ca²⁺ and depolarized the cells. These effects were potentiated by glycine and blocked by extracellular Mg²⁺, 2-amino-5-phosphonopentanoate (APV), 7-chlorokynurenate, 3-amino-1-hydroxypyrrolidin-2-one (HA-966) and 5,7-dinitroquinoxaline-2,3-dione (MNQX). The results indicate that LGG is a partial NMDA agonist. On the other hand, the non-NMDA antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) also inhibited the effects of LGG. This indicates an involvement of non-NMDA receptors in the actions of LGG. The consequent depolarization may also contribute to the activation of NMDA receptor-governed ionophores.