Endogenous NMDA-receptor activation regulates glutamate release in cultured spinal neurons

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

The effect of GABA-B receptors in the basolateral amygdala on passive avoidance memory impairment induced by MK-801 in rats

MK-801 (dizocilpine) is a potent non-competitive N-methyl-[D]-aspartate (NMDA) receptor antagonist that affects cognitive function, learning, and memory. As we know, NMDA receptors are significantly involved in memory function, as well as GABA (Gamma-Aminobutyric acid) receptors. In this study, we aimed to discover the effect of GABA-B receptors in the basolateral amygdala (BLA) on MK-801-induced memory impairment. We used 160 male Wistar rats. The shuttle box was used to evaluate passive avoidance memory and locomotion apparatus was used to evaluate locomotor activity. MK-801 (0.125, 0.25, and 0.5 μg/rat), baclofen (GABA-B agonist, 0.0001, 0.001, and 0.01 μg/rat) and phaclofen (GABA-B antagonist, 0.0001, 0.001, and 0.01 μg/rat) were injected intra-BLA, after the training. The results showed that MK-801 at the dose of 0.5 μg/rat, baclofen at the doses of 0.001 and 0.01 μg/rat, and phaclofen at the doses of 0.001 and 0.01 μg/rat, impaired passive avoidance memory. Locomotor activity did not alter in all groups. Furthermore, the subthreshold dose of both baclofen (0.0001 μg/rat) and phaclofen (0.0001 μg/rat) restored the impairment effect of MK-801 (0.5 μg/rat) on memory. Also, both baclofen (0.0001 μg/rat) potentiated the impairment effect of MK-801 (0.125 μg/rat) and phaclofen (0.0001 μg/rat) potentiated the impairment effect of MK-801 (0.125 and 0.25 μg/rat) on passive avoidance memory. In conclusion, our results indicated that BLA GABA-B receptors can alter the effect of NMDA inactivation on passive avoidance memory.

Resveratrol derivative excited postsynaptic potentiation specifically via PKCβ-NMDA receptor mediation

Several decades have passed since resveratrol (RSV) was first identified in red wine. Researchers have reported the pleiotropic anti-oxidant, anti-inflammatory, anti-cancer, anti-aging, and neuronal protective effects of resveratrol and its glycosylated derivative. However, few studies have distinguished the minute differences in the properties between resveratrol and its glycosylated derivative in terms of synaptic plasticity. As an abundant natural product of glycosylated resveratrol, the derivative 2,3,4',5-tetrahydroxystilbene-2-O-β-d-glucoside (TSG) has been determined to be a better option for long-term potentiation (LTP) in the hippocampus under physiological and pathological conditions than resveratrol. TSG, as well as its parent molecule RSV, could elicit early-LTP and recover fast excitatory postsynaptic potentials (EPSPs) in the hippocampus. Using various modalities, including pre- and post-whole-cell patch clamping techniques in the calyx of Held, pharmacological inhibition of the N-methyl-d-aspartic acid receptor (NMDAr) and the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAr) as well as protein kinase C (PKC) activation, we demonstrated that TSG, unlike RSV, could merely promote NMDA-mediated EPSC via PKCβ cascade. Our results provide new knowledge that glycosylation of resveratrol could significantly improve its specificity in promoting sole NMDAr mediation of EPSPs, in addition to improving solubility and resistance against oxidation in vivo. These observations could contribute to further exploration of pharmaceutical evaluation of glycosylated stilbene in the future.

A quantitative model of cortical spreading depression due to purinergic and gap-junction transmission in astrocyte networks

Spreading depression (SD), a propagating wave of electrical silence in the cortex and archicortex, involves depolarization of neurons and astrocytes for approximately 1 min, due principally to a large increase in extracellular K+. SD is accompanied by large increases in extracellular ATP and is blocked by glutamate N-methyl-D-aspartate receptor antagonists. As a principal means of transmission between astrocytes is through their release of ATP, we have investigated if a model in which SD is driven by the effects of astrocyte waves of ATP interacting with waves of glutamate release from neurons and astrocytes can give a quantitative account of experimental observations on SD. We show that the characteristics of SD and the accompanying extracellular ionic changes can be accommodated by such a model-whether astrocyte transmission is principally through the release of ATP, as in archicortex (hippocampus) and spinal cord, or via gap junctions, as in the neocortex. Furthermore, these models give quantitative accounts of the effects on the characteristics of SD of agents toxic for astrocytes and of gap-junction blockers. Finally, an additional series of critical tests of the model is suggested.

The role of NMDA receptor subtypes in short-term plasticity in the rat entorhinal cortex

We have previously shown that spontaneous release of glutamate in the entorhinal cortex (EC) is tonically facilitated via activation of presynaptic NMDA receptors (NMDAr) containing the NR2B subunit. Here we show that the same receptors mediate short-term plasticity manifested by frequency-dependent facilitation of evoked glutamate release at these synapses. Whole-cell patch-clamp recordings were made from layer V pyramidal neurones in rat EC slices. Evoked excitatory postsynaptic currents showed strong facilitation at relatively low frequencies (3 Hz) of activation. Facilitation was abolished by an NR2B-selective blocker (Ro 25-6981), but unaffected by NR2A-selective antagonists (Zn(2+), NVP-AAM077). In contrast, postsynaptic NMDAr-mediated responses could be reduced by subunit-selective concentrations of all three antagonists. The data suggest that NMDAr involved in presynaptic plasticity in layer V are exclusively NR1/NR2B diheteromers, whilst postsynaptically they are probably a mixture of NR1/NR2A, NR1/NR2B diheteromers and NR1/NR2A/NR2B triheteromeric receptors.

Stimulation of chemokine CXC receptor 4 induces synaptic depression of evoked parallel fibers inputs onto Purkinje neurons in mouse cerebellum

In the present work, we studied the effects of the stimulation of the chemokine CXC receptor 4 (CXCR4) by the stromal-derived cell growth factor-1alpha (SDF-1alpha) on the evoked excitatory postsynaptic current. This was generated in Purkinje neurons (PN) from mouse cerebellar slices by the stimulation of parallel fibers. It was found that the amplitude of EPSC was reversibly reduced by SDF-1alpha application. This effect was dose-dependent (IC(50)=0.34 nM) and was abolished by the anti-CXCR4 monoclonal antibody (mAb) 12G5. This SDF-1alpha-induced synaptic depression was caused by a decrease of evoked glutamate release, rather than a decrease in the postsynaptic glutamate receptor (GluR) sensitivity, as the mean amplitude of the spontaneous EPSCs was not influenced by chemokine application. Moreover, NMDA receptors (NMDARs) are involved in EPSC depression being inhibited by the NMDAR blocker 2-amino-5-phosphonopentanoic acid (AP-5). The mechanisms by which SDF-1alpha modulates neurotransmission in the cerebellar cortex are discussed.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

NR2B-containing NMDA autoreceptors at synapses on entorhinal cortical neurons

We have previously shown that presynaptic N-methyl-D-aspartate receptors (NMDARs) can facilitate glutamate release onto principal neurons in the entorhinal cortex (EC). In the present study, we have investigated the subunit composition of these presynaptic NMDARs. We recorded miniature alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs), from visually identified neurons in layers II and V of the EC in vitro. In both layers, bath application of the NR2A/B subunit-selective agonist, homoquinolinic acid (HQA), resulted in a marked facilitation of mEPSC frequency. Blockade of presynaptic Ca(2+) entry through either NMDARs or voltage-gated Ca(2+) channels with Co(2+) prevented the effects of HQA, confirming that Ca(2+) entry to the terminal was required for facilitation. When the NR2B-selective antagonist, ifenprodil, was applied prior to HQA, the increase in mEPSC frequency was greatly reduced. In addition, we found that an NMDAR antagonist blocked frequency-dependent facilitation of evoked release and reduced mEPSC frequency in layer V. Thus we have demonstrated that NMDA autoreceptors in layer V of the EC bear the NR2B subunit, and that NMDARs are also present at terminals onto superficial neurons.

The effects of forelimb deafferentation on the post-natal development of synaptic plasticity in the hippocampus

The effects of partial deafferentation of the forelimb on the development of long-term potentiation in the hippocampus of rats aged 13-18 days were studied. Long-term potentiation in hippocampus field CA1 was of greater amplitude and duration in control rats at 16-18 days of post-natal ontogenesis than in adult animals. Partial deafferentation by section of the median nerve in the forelimb on the 13th day of life led to the disappearance of this excess at 16-18 days. The peak in synaptic plasticity occurred later in operated animals--on day 17--and was much less marked than in controls. The decreases in the amplitude and duration of long-term potentiation in hippocampal field CA1 in operated animals provides evidence for a decrease in the sensitivity and/or number of NMDA receptors. This suggests that partial deafferentation of one limb may lead not to a decrease but to an increase in spike and synaptic activity in the hippocampus, which in normal conditions may affect the maturation of the plastic properties of synaptic transmission associated with the expression and positions of NMDA receptors. The level of long-term potentiation in sham-operated rats was significantly greater than in controls of the same age. This significant increase in NMDA-dependent long-term potentiation may be explained by a decrease in the level of activation due to anesthesia. It is suggested that the decrease in the spike activity of cells receiving signals from the median nerve may be compensated for by activation of other specific and non-specific inputs.

Presynaptic N-methyl-D-aspartate receptor activation inhibits neurotransmitter release through nitric oxide formation in rat hippocampal nerve terminals

In brain synapses, nitric oxide synthase activation is coupled to N-methyl-D-aspartate-mediated calcium entry at postsynaptic densities through regulatory protein complexes, however a presynaptic equivalent to this signaling mechanism has not yet been identified. Novel evidence indicates that N-methyl-D-aspartate glutamate receptors may play a presynaptic role in synaptic plasticity. Thus, we investigated whether ionotropic glutamate receptor activation in isolated nerve terminals regulates neurotransmitter release, through nitric oxide formation. N-Methyl-D-aspartate dose-dependently inhibited the release of glutamate evoked by 4-aminopyridine (IC(50)=155 microM), and this effect was reversed by the N-methyl-D-aspartate receptor antagonist D-(-)-2-amino-5-phosphopentanoic acid and by the nitric oxide synthase inhibitor, L-nitroarginine, in synaptosomes isolated from whole hippocampus, CA3 and CA1 areas, but not from the dentate gyrus. In contrast, the 4-aminopyridine-evoked release of glutamate was reduced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate by a nitric oxide-independent mechanism, since it was not blocked by L-nitroarginine, and N-methyl-D-aspartate, but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate, significantly increased cGMP formation. Presynaptic N-methyl-D-aspartate receptors are probably involved since removing extracellular nitric oxide with the scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide did not block the depression of glutamate release by N-methyl-D-aspartate. The mechanism underlying this depression involves the inhibition of synaptic vesicle exocytosis since N-methyl-D-aspartate/nitric oxide inhibited the release of [3H]glutamate and [14C]GABA evoked by hypertonic sucrose. The results also suggest that presynaptic N-methyl-D-aspartate receptors may function as auto- and heteroreceptors.

NT-3 evokes an LTP-like facilitation of AMPA/kainate receptor-mediated synaptic transmission in the neonatal rat spinal cord

Neurotrophin-3 (NT-3) is a neurotrophic factor required for survival of muscle spindle afferents during prenatal development. It also acts postsynaptically to enhance the monosynaptic excitatory postsynaptic potential (EPSP) produced by these fibers in motoneurons when applied over a period of weeks to the axotomized muscle nerve in adult cats. Similar increases in the amplitude of the monosynaptic EPSP in motoneurons are observed after periodic systemic treatment of neonatal rats with NT-3. Here we show an acute action of NT-3 in enhancing the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptor-mediated fast monosynaptic EPSP elicited in motoneurons by dorsal root (DR) stimulation in the in vitro hemisected neonatal rat spinal cord. The receptor tyrosine kinase inhibitor K252a blocks this action of NT-3 as does the calcium chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) injected into the motoneuron. The effect of NT-3 resembles long-term potentiation (LTP) in that transient bath application of NT-3 to the isolated spinal cord produces a long-lasting increase in the amplitude of the monosynaptic EPSP. An additional similarity is that activation of N-methyl-D-aspartate (NMDA) receptors is required to initiate this increase but not to maintain it. The NMDA receptor blocker MK-801, introduced into the motoneuron through the recording microelectrode, blocks the effect of NT-3, indicating that NMDA receptors in the motoneuron membrane are crucial. The effect of NT-3 on motoneuron NMDA receptors is demonstrated by its enhancement of the depolarizing response of the motoneuron to bath-applied NMDA in the presence of tetrodotoxin (TTX). The potentiating effects of NT-3 do not persist beyond the first postnatal week. In addition, EPSPs with similar properties evoked in the same motoneurons by stimulation of descending fibers in the ventrolateral funiculus (VLF) are not modifiable by NT-3 even in the initial postnatal week. Thus, NT-3 produces synapse-specific and age-dependent LTP-like enhancement of AMPA/kainate receptor-mediated synaptic transmission in the spinal cord, and this action requires the availability of functional NMDA receptors in the motoneuron.

Analysis of relations between NMDA receptors and GABA release at olfactory bulb reciprocal synapses

In the mammalian olfactory bulb, signal processing is mediated by synaptic interactions between dendrites. Glutamate released from mitral cell dendrites excites dendritic spines of granule cells, which in turn release GABA back onto the mitral cell dendrites, forming a reciprocal synaptic pair. This feedback synaptic circuit was shown to be mediated predominantly by NMDA receptors. We further utilized caged Ca2+ compounds to obtain insight into the mechanism that couples NMDA receptor activation to GABA release. Feedback inhibition elicited by photo-release of caged Ca2+ in mitral cell secondary dendrites persisted when voltage-gated Ca2+ channels were blocked by cadmium (Cd2+) and nickel (Ni2+). These results indicate that Ca2+ influx through NMDA receptors can directly trigger presynaptic GABA release for local dendrodendritic feedback inhibition.

Development of glutamatergic synaptic activity in cultured spinal neurons

The development of glutamatergic synapses involves a sequence of events that are still not well understood. We have studied the time course of the development of glutamatergic synapses in cultured spinal neurons by characterizing spontaneous synaptic currents recorded from cells maintained in vitro for different times. At short times in culture (2 days in vitro; DIV2), spontaneous synaptic activity consisted almost solely of N-methyl-D-aspartate (NMDA) receptor (NMDAR) openings. In contrast, older neurons (DIV5 to DIV8) displayed clear alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR)-mediated synaptic currents, while the NMDAR-mediated activity remained small. Between 8 and 14 days in vitro there was a large increase in the density of synaptically activated NMDARs, although there was no significant increase in the density of the NMDAR-mediated current activated by exogenous glutamate. The results indicate that there is a switch in NMDAR targeting from somatic to synaptic regions during the course of the second in vitro week. Finally, our results support the conclusion that the spontaneous synaptic activity displayed in culture depends on ongoing NMDAR-mediated activity, even when the expression of synaptic NMDARs is low.

Properties of miniature glutamatergic EPSCs in neurons of the locomotor regions of the developing zebrafish

As a first step in understanding the development of synaptic activation in the locomotor network of the zebrafish, we examined the properties of spontaneous, glutamatergic miniature excitatory postsynaptic currents (mEPSCs). Whole cell patch-clamp recordings were obtained from visually identified hindbrain reticulospinal neurons and spinal motoneurons of curarized zebrafish 1-5 days postfertilization (larvae hatch after the 2nd day of embryogenesis). In the presence of tetrodotoxin (TTX) and blockers of inhibitory receptors (strychnine and picrotoxin), we detected fast glutamatergic mEPSCs that were blocked by the AMPA/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). At positive voltages or in the absence of Mg(2+), a second, slower component of the mEPSCs was revealed that the N-methyl-D-aspartate (NMDA) receptor-selective antagonist DL-2-amino-5-phosphonovalerate (AP-5) abolished. In the presence of both CNQX and AP-5, all mEPSCs were eliminated. The NMDA component of reticulospinal mEPSCs had a large single-channel conductance estimated to be 48 pS. Larval AMPA/kainate and NMDA components of the mEPSCs decayed with biexponential time courses that changed little during development. At all stages examined, approximately one-half of synapses had only NMDA responses (lacking AMPA/kainate receptors), whereas the remainder of the synapses were composed of a mixture of AMPA/kainate and NMDA receptors. There was an overall increase in the frequency and amplitude of mEPSCs with an NMDA component in reticulospinal (but not motoneurons) during development. These results indicate that glutamate is a prominent excitatory transmitter in the locomotor regions of the developing zebrafish and that it activates either NMDA receptors alone at functionally silent synapses or together with AMPA/kainate receptors.

Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture

We used brainstem motoneurons recorded in organotypic slice co-cultures maintained for more than 18 days in vitro, together with multibarrel ionophoretic applications of glutamate receptor agonists and bath applications of specific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) iontophoresis, and express NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-amino-5-phosphonovaleric acid (APV)- and (-)1-(4-aminophenyl)-3-methyl-carbamoyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine [GYKI 53784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials with latencies distributed in three regularly spaced groups. Excitatory postsynaptic potentials (EPSPs) in the earliest group have a similar latency and time course and correspond to monosynaptic activation. EPSPs in later groups have more scattered latencies and time courses and correspond to polysynaptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-competitive AMPA receptor antagonist GYKI 53784 (LY303070). Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the frequency of spontaneous EPSPs without modifying their shape or amplitude. We conclude that excitatory synapses on brainstem motoneurons in vitro are mainly activated through AMPA receptors (AMPA-Rs). NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic transmission within the premotor interneuronal network.

NMDA receptor-mediated control of presynaptic calcium and neurotransmitter release

Before action potential-evoked Ca2+ transients, basal presynaptic Ca2+ concentration may profoundly affect the amplitude of subsequent neurotransmitter release. Reticulospinal axons of the lamprey spinal cord receive glutamatergic synaptic input. We have investigated the effect of this input on presynaptic Ca2+ concentrations and evoked release of neurotransmitter. Paired recordings were made between reticulospinal axons and the neurons that make axo-axonic synapses onto those axons. Both excitatory and inhibitory paired-cell responses were recorded in the axons. Excitatory synaptic inputs were blocked by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and by the NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP-5; 50 microM). Application of NMDA evoked an increase in presynaptic Ca2+ in reticulospinal axons. Extracellular stimulation evoked Ca2+ transients in axons when applied either directly over the axon or lateral to the axons. Transients evoked by the two types of stimulation differed in magnitude and sensitivity to AP-5. Simultaneous microelectrode recordings from the axons during Ca2+ imaging revealed that stimulation of synaptic inputs directed to the axons evoked Ca2+ entry. By the use of paired-cell recordings between reticulospinal axons and their postsynaptic targets, NMDA receptor activation was shown to enhance evoked release of transmitter from the axons that received axoaxonic inputs. When the synaptic input to the axon was stimulated before eliciting an action potential in the axon, transmitter release from the axon was enhanced. We conclude that NMDA receptor-mediated input to reticulospinal axons increases basal Ca2+ within the axons and that this Ca2+ is sufficient to enhance release from the axons.