Tonic activation of NMDA receptors by ambient glutamate enhances excitability of neurons

Extracellular diffusivity determines contribution of high-versus low-affinity receptors to neural signaling

Diffusion-weighted magnetic resonance imaging detects physiological changes in the human brain by highlighting alterations in local diffusivity. However, the causal link between brain tissue diffusivity and neural activity is poorly understood. Synaptic physiology studies in vitro coupled with biophysical modeling have suggested that extracellular diffusion affects the spatial profile of receptor activation during synaptic discharges. Here, we attempt to address this issue more directly, by recording synaptic currents from individual cells in acute brain slices while reducing the bath medium diffusivity by 25-30% (measured with two-photon microscopy) using inert dextran molecules. We find that retarding extracellular diffusion increases the activation of high-affinity NMDA, but not low-affinity AMPA, receptors in response to remote, spontaneous or evoked, synaptic releases of the common excitatory neurotransmitter glutamate. The results suggest that variations in extracellular diffusivity could reflect an altered contribution of higher- versus lower-affinity receptor types to the network activity of synaptic circuits.

Presynaptic NMDA autoreceptors facilitate axon excitability: a new molecular target for the anticonvulsant gabapentin

Gabapentin is a drug with anticonvulsant and analgesic properties causing the reduction of neurotransmitter release. We show that one of the mechanisms implicated in this effect of gabapentin is the reduction of the axon excitability measured as an amplitude change of the presynaptic fibre volley (FV) in the CA1 area of rat hippocampal slices. Interestingly, we found that gabapentin-induced depression of FV is mimicked and occluded by NMDA receptor (NMDA-R) antagonists, indicating that these receptors are located presynaptically and are activated by ambient levels of glutamate. Conversely, NMDA application (20 microM, 10 min) elicits a reversible FV potentiation which is reduced by gabapentin. Both NMDA- and gabapentin-induced FV changes are partially explained by modifications in the firing threshold of individual fibres. Increasing [K(+)](o) does not mimic or occlude (at a concentration of 6.5 mM) the effect of NMDA on FV amplitude, which makes it unlikely that a rise in [K(+)](o) induced by NMDA receptor activation could indirectly participate in the potentiation of the FV. The NMDA-induced FV potentiation is independent of extracellular calcium presence but is completely inhibited in a low-Na(+) solution (50% reduction) or under NMDA channel block (high Mg(2+) or MK 801). These findings suggest that sodium entry through presynaptic NMDA-R channels facilitates axon excitability. The interaction of gabapentin with this newly described mechanism might contribute to its therapeutic benefits.

Tonic excitation and inhibition of neurons: ambient transmitter sources and computational consequences

Tonic activation of excitatory and inhibitory receptors, by the ambient concentration of neurotransmitters in the extracellular space of the brain, has been suggested to underlie phenomena as diverse as relapse to cocaine use by reward pathways in the striatum, sparse coding of motor information in the cerebellum, and control of the development of the cerebral and cerebellar cortices. Here we assess the mechanisms which may determine the ambient levels of excitatory and inhibitory neurotransmitters, and consider their likely effect on information processing.

Memantine Blocks α7* Nicotinic Acetylcholine Receptors More Potently Than N-Methyl-D-aspartate Receptors in Rat Hippocampal Neurons

The N-methyl-d-aspartate (NMDA) receptor antagonist memantine is an approved drug for treatment of Alzheimer's disease (AD). Other such treatments are cholinesterase inhibitors and nicotinic acetylcholine receptor (nAChR)-sensitizing agents such as galantamine. The present study was designed to test whether memantine exerts any effect on the cholinergic system, in particular the Ca(2+)-conducting alpha7(*) nAChR, in cultured hippocampal neurons. Memantine caused a concentration-dependent reduction of the amplitudes of whole-cell currents evoked by the alpha7(*) nAChR-selective agonist choline (10 mM) or by N-methyl-d-aspartate (NMDA) (50 muM) plus glycine (10 muM). It also inhibited tonically activated NMDA receptors. Memantine was more potent in inhibiting alpha7(*) nAChRs than NMDA receptors; at -60 mV, the IC(50) values for memantine were 0.34 and 5.1 muM, respectively. Consistent with an open-channel blocking mechanism, memantine-induced NMDA receptor inhibition was voltage and use-dependent; the Hill coefficient (n(H)) was approximately 1. Memantine-induced alpha7(*) nAChR inhibition had an n(H) < 1 and showed a variable voltage dependence; the effect was voltage-independent at 0.1 muM, becoming voltage-dependent at >/=1 muM. Thus, memantine interacts with more than one class of sites on the alpha7(*) nAChRs. One is voltage-sensitive and therefore likely to be within the receptor channel. The other is voltage-insensitive and therefore likely to be in the extracellular domain of the receptor. It is suggested that blockade of alpha7(*) nAChRs by memantine could decrease its effectiveness for treatment of AD, particularly at early stages when the degrees of nAChR dysfunction and of cognitive decline correlate well.

Stimulation of N-methyl-d-aspartate receptors, AMPA receptors or metabotropic glutamate receptors leads to rapid internalization of AMPA receptors in cultured nucleus accumbens neurons

In hippocampus and other regions, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are inserted into synapses during long-term potentiation and removed during long-term depression. However, little is known about regulation of AMPA receptor trafficking in the nucleus accumbens (NAc), despite growing evidence that glutamate-dependent forms of plasticity in the NAc contribute to drug addiction. Using postnatal rat NAc cultures and an immunocytochemical method that selectively detects newly internalized GluR1, we studied the regulation of AMPA receptor internalization in NAc neurons by glutamate agonists. Newly internalized GluR1 was detected during 15 or 30 min of incubation at room temperature, indicating a basal rate of GluR1 turnover. The rate of GluR1 internalization was increased by glutamate (50 microM) within 5 min of its addition. Glutamate-induced GluR1 internalization was partially blocked by either an AMPA receptor antagonist (CNQX; 20 microM) or an N-methyl-D-aspartate (NMDA) receptor antagonist (APV; 50 microM). Both NMDA (50 microM) and AMPA (50 microM) increased GluR1 internalization in a Ca(2+)-dependent manner. The NMDA effect was blocked by APV while the AMPA effect was blocked by APV or CNQX. We interpret these findings to suggest that NMDA and AMPA ultimately trigger GluR1 internalization through the same NMDA receptor-dependent pathway. The effect of glutamate was also partially blocked by the group 1 metabotropic glutamate receptor antagonist N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC; 50 microM), while the group 1 agonist 3,5-dihydroxyphenylglycine (DHPG; 50 microM) stimulated GluR1 internalization. These data suggest that AMPA receptors on NAc neurons may be subject to rapid regulation of their surface expression in response to changes in the activity of glutamate inputs from cortical and limbic regions.

Contribution of astrocytes to synaptic transmission in the rat supraoptic nucleus

Astrocytes, besides supporting metabolic and scaffolding functions, play a prominent role in the modulation of neuronal communication. In particular, they are responsible for clearing synaptically-released glutamate via highly specific transporters located on their plasma membrane. Since glutamate is the main excitatory neurotransmitter in the central nervous system (CNS), astrocytes are likely to play a central role in the regulation of synaptic processing and overall cellular excitability. We recently investigated the influence of astrocytes on glutamatergic and GABAergic transmission in the rat supraoptic nucleus (SON) of the hypothalamus. This nucleus is part of the hypothalamus-neurohypophysial system (HNS), which constitutes a conspicuous example of activity-dependent neuroglial plasticity, in which certains physiological conditions, such as parturition, lactation, and dehydration are accompanied by a structural remodeling of the neurones, their synaptic inputs and their surrounding glia. The use of pharmacological inhibitors of glutamate transporters on this model, in which a physiological change in the astrocyte environment occurs, has brought new insights on the contribution of astrocytes to both excitatory and inhibitory neurotransmissions. The astrocytic environment of neurons appears to control glutamate uptake and diffusion in the extracellular space. This has direct repercussions on the tonic level of activation of presynaptic glutamate receptors and, as a consequence, on the release of neurotransmitter. This short review summarizes data obtained so far, which clearly support the view that astrocytes are indeed a third partner in synaptic transmission, and which show that the supraoptic nucleus represents a remarkable model to study dynamic physiological interactions between astrocytes and neurons.

Excitation-Neurogenesis Coupling in Adult Neural Stem/Progenitor Cells

A wide variety of in vivo manipulations influence neurogenesis in the adult hippocampus. It is not known, however, if adult neural stem/progenitor cells (NPCs) can intrinsically sense excitatory neural activity and thereby implement a direct coupling between excitation and neurogenesis. Moreover, the theoretical significance of activity-dependent neurogenesis in hippocampal-type memory processing networks has not been explored. Here we demonstrate that excitatory stimuli act directly on adult hippocampal NPCs to favor neuron production. The excitation is sensed via Ca(v)1.2/1.3 (L-type) Ca(2+) channels and NMDA receptors on the proliferating precursors. Excitation through this pathway acts to inhibit expression of the glial fate genes Hes1 and Id2 and increase expression of NeuroD, a positive regulator of neuronal differentiation. These activity-sensing properties of the adult NPCs, when applied as an "excitation-neurogenesis coupling rule" within a Hebbian neural network, predict significant advantages for both the temporary storage and the clearance of memories.

Role of glial amino acid transporters in synaptic transmission and brain energetics

This article reviews how the uptake of neurotransmitter by glial amino acid transporters limits the spatial spread of transmitter to preserve the independent operation of nearby synapses, temporally shapes postsynaptic currents, and regulates the effects of tonic transmitter release. We demonstrate the importance of amino acid uptake and recycling mechanisms for preventing the loss of energetically costly neurotransmitter from the brain, and also examine the suggestion that glutamate uptake into glia plays a key role in regulating the energy production of the brain. Finally, we assess the role of glial amino acid transporters in transmitter recycling pathways.

Distribution and properties of functional postsynaptic kainate receptors on neocortical layer V pyramidal neurons

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic transmission and signal processing. In the present study we investigated the location and properties of functional kainate receptors (KARs) on the somatodendritic membrane of rat neocortical layer V pyramidal neurons. Infrared-guided laser stimulation was used to apply glutamate photolytically to the soma and various sites along the apical dendrite. Electrical currents, resulting from the activation of pharmacologically isolated KARs, were measured by whole-cell patch-clamp recording. In addition, KARs on somatic and dendritic outside-out patches were activated while still within the brain tissue. We found that functional KARs are located on the entire somatodendritic membrane that was examined. Fast kinetics, a linear I-V relationship, and a relatively high single-channel conductance are characteristic features of these receptors. We provide evidence that the unitary properties of somatic and dendritic KARs are identical. Regarding the subcellular distribution of KARs, our results indicate that the density of these receptors increases toward the distal dendrite. They are located mainly at extrasynaptic sites but also mediate fast synaptic signaling triggered by afferent stimulation. The differential distribution speaks in favor of a selective targeting of KARs on central neurons and may reflect a mechanism for a location-dependent regulation of synaptic efficacy. Furthermore, it is feasible to assume that extrasynaptic KARs could be activated by a "spillover" of synaptically released glutamate, ambient glutamate in the CSF, or glutamate released from adjacent astrocytes.

NMDA and AMPA receptors contribute to the nicotinic cholinergic excitation of CA1 interneurons in the rat hippocampus

In the hippocampus, glutamatergic inputs to pyramidal neurons and interneurons are modulated by alpha7* and alpha3beta4* nicotinic acetylcholine receptors (nAChRs), respectively, present in glutamatergic neurons. This study examines how nicotinic AMPA, and NMDA receptor nAChR activities are integrated to regulate the excitability of CA1 stratum radiatum (SR) interneurons in rat hippocampal slices. At resting membrane potentials and in the presence of extracellular Mg2+ (1 mM), nicotinic agonists triggered in SR interneurons excitatory postsynaptic currents (EPSCs) that had two components: one mediated by AMPA receptors, and the other by NMDA receptors. As previously shown, nicotinic agonist-triggered EPSCs resulted from glutamate released by activation of alpha3beta4* nAChRs in glutamatergic neurons/fibers synapsing directly onto the neurons under study. The finding that CNQX caused more inhibition of nicotinic agonist-triggered EPSCs than expected from the blockade of postsynaptic AMPA receptors indicated that this nicotinic response also depended on the AMPA receptor activity in the glutamatergic neurons synapsing onto the interneuron under study. Nicotinic agonists always triggered action potentials in CA1 SR interneurons. In most interneurons, these action potentials resulted from activation of somatodendritic AMPA receptors and alpha7* nAChRs. In interneurons expressing somatodendritic alpha4beta2* nAChRs, activation of these receptors caused sufficient membrane depolarization to remove the Mg2+-induced block of somatodendritic NMDA receptors; in these neurons, nicotinic agonist-triggered action potentials were partially dependent on NMDA receptor activation. Removing extracellular Mg2+ or clamping the neuron at positive membrane potentials revealed the existence of a tonic NMDA current in SR interneurons that was unaffected by nAChR activation or inhibition. Thus integration of the activities of nAChRs, NMDA, and AMPA receptors in different compartments of CA1 neurons contributes to the excitability of CA1 SR interneurons.

Ethanol and brain plasticity: receptors and molecular networks of the postsynaptic density as targets of ethanol

Brain plasticity refers to the ability of the brain to undergo structural and functional changes. It is a necessary process that allows us to adapt and learn from our environment and is fundamental to our survival. However, under certain conditions, these neuroadaptive responses can have adverse consequences. In particular, increasing evidence indicates that plastic processes are coopted by drugs of abuse, leading to addiction and associated drug-seeking behaviors. An extensive and diverse group of studies ranging from the molecular to the behavioral level has also strongly implicated glutamatergic neurotransmission as a critical mediator of experience-dependent synaptic plasticity. Thus, it is vital to understand how drugs of abuse interact and potentially alter glutamatergic neurotransmission and associated signal transduction processes. This review will focus on the cellular and molecular neuroscience of alcoholism, with emphasis on events at the glutamatergic postsynaptic density (PSD) and dendritic spine dynamics that appear to underlie much of the structural and functional plasticity of the CNS.

Glutamate-mediated [Ca2+]c dynamics in spontaneously firing dopamine neurons of the rat substantia nigra pars compacta

The mechanism by which glutamate regulates the cytosolic free Ca2+ concentration ([Ca2+]c) in spontaneously firing dopamine neurons is not clear. Thus we have investigated the glutamate-mediated [Ca2+]c dynamics in the acutely isolated dopamine neurons from the rat substantia nigra pars compacta by measuring [Ca2+]c and spontaneously occurring action potentials (SAPs). The freshly isolated dopamine neurons showed tetrodotoxin (TTX)-sensitive spontaneous firing of 2-3 Hz and the resting [Ca2+]c decreased with abolition of the SAPs. The level of [Ca2+]c was affected by the spontaneous firing rate. In the presence of the Na+ channel antagonist, TTX (0.5 microM), glutamate increased [Ca2+]c by activating different glutamate receptors depending on the glutamate concentration used. Addition of glutamate at low concentrations (<3 microM) raised [Ca2+]c mainly by activating metabotropic glutamate receptors (mGluR), whereas at high concentrations (>10 microM) it raised [Ca2+]c mainly by activating AMPA/kainate receptors. The contribution of NMDA receptors to the glutamate-mediated [Ca2+]c rises was largest at intermediate concentrations of glutamate. Activation of mGluR elicited a Ca2+ release from intracellular Ca2+ stores and continuous Ca2+ influx out of the cell. The spontaneous firing activities were highly enhanced by submicromolar levels of glutamate and abolished at levels above 10 microM. From these results, we conclude that at low glutamate concentrations the [Ca2+]c in the dopamine neurons is mainly governed by mGluR and the firing activities, whose rate is regulated at submicromolar glutamate concentrations, but at higher glutamate concentrations [Ca2+]c is dominantly affected by AMPA/kainate receptors.

Bioactivity of a peptide derived from acetylcholinesterase: electrophysiological characterization in guinea-pig hippocampus

Acetylcholinesterase is well known to have noncholinergic functions. Only recently, however, has the salient part been identified of the molecule responsible for these nonclassical actions, a peptide of 14 amino acids towards the C-terminus of acetylcholinesterase. The aim of this study was to test the bioactivity of this 'acetylcholinesterase-peptide' using intracellular recordings in guinea-pig hippocampal slices. In the presence of tetrodotoxin, acetylcholinesterase-peptide alone affected neither the membrane potential nor the input resistance of CA1 neurons; however, a modulatory action was observed, as a concentration-dependent decrease of N-methyl-d-aspartic acid-induced depolarization. When calcium potentials were elicited by a depolarizing current pulse, application of acetylcholinesterase-peptide increased or reduced the degree of calcium spike firing in, respectively, the presence or absence of the N-methyl-d-aspartic acid antagonist d(-)-2-amino-5-phosphonopentanoic acid. In contrast, a peptide derived from the equivalent region of butyrylcholinesterase, which also hydrolyses acetylcholine, had no effect. In conclusion, acetylcholinesterase-peptide has a selective bioactivity in the hippocampus; it could thus offer new ways of targeting mechanisms of calcium-induced neurotoxicity.

5-hydroxyindole causes convulsions and increases transmitter release in the CA1 region of the rat hippocampus

1 5-hydroxyindole (5-OHi) is a proposed tryptophan metabolite able to cause convulsions when systemically injected into rodents. We studied its effects using microdialysis in vivo and electrophysiological approaches in vitro. 2 Local administration of 5-OHi into the CA1 region of the rat hippocampus, via a microdialysis probe, significantly increased glutamate concentrations in the dialysates. 3 In rat hippocampal slices, using extracellular recordings in the CA1 region, 5-OHi (30-300 microM) increased the amplitude of population spikes and fEPSPs. 4 In the same preparation, using intracellular recordings in CA1 pyramidal neurons, 5-OHi reduced the latency of firing induced by direct depolarization and increased both evoked excitatory and slow inhibitory postsynaptic potential amplitudes, without affecting the resting membrane potential, the after-hyperpolarization or the neuronal input resistance. It also altered GABA(A)-mediated neurotransmission by increasing the frequency and the amplitude of pharmacologically isolated spontaneous inhibitory postsynaptic currents (sIPSC). 5 In separate experiments, performed by measuring AMPA or NMDA-induced depolarization in cortical wedges, 5-OHi did not modify glutamate receptor agonist responses. 6 Our results show that 5-OHi causes convulsions, modifies the properties and the function of the hippocampal circuitry, and facilitates the output of both excitatory and inhibitory transmitters.

Localization and Synaptic Release of N-acetylaspartylglutamate in the Chick Retina and Optic Tectum

The neuropeptide, N-acetylaspartylglutamate (NAAG), was identified in the chick retina (1.4 nmol/retina) by HPLC, radioimmunoassay and immunohistochemistry. This acidic dipeptide was found within retinal ganglion cell bodies and their neurites in the optic fibre layer of the retina. Substantial, but less intense, immunoreactivity was detected in many amacrine-like cells in the inner nuclear layer and in multiple bands within the inner plexiform layer. In addition, NAAG immunoreactivity was observed in the optic fibre layer and in the neuropil of the superficial layers of the optic tectum, as well as in many cell bodies in the tectum. Using a newly developed, specific and highly sensitive (3 fmol/50 microl) radioimmunoassay for NAAG, peptide release was detected in isolated retinas upon depolarization with 55 mM extracellular potassium. This assay also permitted detection of peptide release from the optic tectum following stimulation of action potentials in retinal ganglion cell axons of the optic tract. Both of these release processes required the presence of extracellular calcium. Electrically stimulated release from the tectum was reversibly blocked by extracellular cadmium. These findings suggest that NAAG serves an extracellular function following depolarization-induced release from retinal amacrine neurons and from ganglion cell axon endings in the chick optic tectum. These data support the hypothesis that NAAG functions in synaptic communication between neurons in the visual system.

Modulation of synaptic transmission by astrocytes in the rat supraoptic nucleus

One of the functions of astroglial cells in the central nervous system is to clear synaptically-released glutamate from the extracellular space. This is performed thanks to specific transporters of the excitatory amino acid expressed on their surface. The way by which astrocytic glutamate uptake contributes to synaptic transmission has been investigated via numerous experimental approaches but has never been addressed under conditions where neuroglial interactions are physiologically modified. Recently, we took advantage of the neuroglial plastic properties of the hypothalamo-neurohypophysial system to examine the consequences of a physiological reduction in the astrocytic coverage of neurons on glutamatergic synaptic transmission. This experimental model has brought some insights on the physiological interactions between glial cells and neurons at the level of the synapse. In particular, it has revealed that the degree of glial coverage of neurons influences glutamate concentration at the vicinity of excitatory synapses and, as a consequence, affects the level of activation of presynaptic glutamate receptors. Astrocytes, therefore, appear to contribute to the regulation of neuronal excitability by modulating synaptic efficacy at glutamatergic nerve terminals.

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.

Cooperative endocannabinoid production by neuronal depolarization and group I metabotropic glutamate receptor activation

Endocannabinoids are retrograde messengers that are released from central neurons by depolarization-induced elevation of intracellular Ca2+ concentration [Ca2+]I or by activation of a group I metabotropic glutamate receptor (mGluR). We studied the interaction between these two pathways for endocannabinoid production in rat hippocampal neurons. We made a paired whole-cell recording from cultured hippocampal neurons with inhibitory synaptic connections. Activation of group I mGluRs, mainly mGluR5, by the specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), suppressed inhibitory postsynaptic currents (IPSCs) in about half of the neuron pairs. A cannabinoid agonist, WIN55,212-2, suppressed IPSCs in all DHPG-sensitive pairs but not in most of DHPG-insensitive pairs. The effects of both DHPG and WIN55,212-2 were abolished by the cannabinoid antagonists, AM281 and SR141716A, indicating that activation of group I mGluR releases endocannabinoids and suppress inhibitory neurotransmitter release through activation of presynaptic cannabinoid receptors. Depolarization of the postsynaptic neurons caused a transient suppression of IPSCs, a phemomenon termed depolarization-induced suppression of inhibition (DSI) that was also abolished by cannabinoid antagonists. Importantly, DSI was enhanced significantly when group I mGluRs were activated simultaneously by DHPG. This enhancement was much more prominent than expected from the simple summation of depolarization-induced and group I mGluR-induced endocannabinoid release. DHPG caused no change in depolarization-induced Ca2+ transients, indicating that the enhanced DSI by DHPG was not due to the augmentation of Ca2+ influx. Enhancement of DSI by DHPG was also observed in hippocampal slices. These results suggest that two pathways work in a cooperative manner to release endocannabinoids via a common intracellular cascade.

Tonic and spillover inhibition of granule cells control information flow through cerebellar cortex

We show that information flow through the adult cerebellar cortex, from the mossy fiber input to the Purkinje cell output, is controlled by furosemide-sensitive, diazepam- and neurosteroid-insensitive GABA(A) receptors on granule cells, which are activated both tonically and by GABA spillover from synaptic release sites. Tonic activation of these receptors contributes a 3-fold larger mean inhibitory conductance than GABA released synaptically by high-frequency stimulation. Tonic and spillover inhibition reduce the fraction of granule cells activated by mossy fiber input, generating an increase of coding sparseness, which is predicted to improve the information storage capacity of the cerebellum.

Synergistic enhancement of glutamate-mediated responses by serotonin and forskolin in adult mouse spinal dorsal horn neurons

Glutamate is the major excitatory amino acid neurotransmitter in the CNS, including the neocortex, hippocampus, and spinal cord. Normal synaptic transmission is mainly mediated by glutamate AMPA and/or kainate receptors. Glutamate N-methyl-D-aspartate (NMDA) receptors are normally inactive and only activated when a sufficient postsynaptic depolarization is induced by the activity. Here we show that in sensory synapses of adult mouse, some synaptic responses (26.3% of a total of 38 experiments) between primary afferent fibers and dorsal horn neurons are almost completely mediated by NMDA receptors. Dorsal root stimulation did not elicit any detectable AMPA/kainate receptor-mediated responses in these synapses. Unlike young spinal cord, serotonin alone did not produce any long-lasting synaptic enhancement in adult spinal dorsal horn neurons. However, co-application of the adenylyl cyclase activator forskolin and serotonin (5-HT) produced long-lasting enhancement, including the recruitment of functional AMPA receptor-mediated responses. Calcium-sensitive, calmodulin-regulated adenylyl cyclases (AC1, AC8) are required for the enhancement. Furthermore the thresholds for generating action potential responses were decreased, and, in many cases, co-application of forskolin and 5-HT led to the generation of action potentials by previously subthreshold stimulation of primary afferent fibers in the presence of the NMDA receptor blocker 2-amino-5-phosphonovaleric acid. Our results suggest that pure NMDA synapses exist on sensory neurons in adult spinal cord and that they may contribute to functional sensory transmission. The synergistic recruitment of functional AMPA responses by 5-HT and forskolin provides a new cellular mechanism for glutamatergic synapses in mammalian spinal cord.

Long-term depression in the nucleus accumbens: a neural correlate of behavioral sensitization to cocaine

A compelling model of experience-dependent plasticity is the long-lasting sensitization to the locomotor stimulatory effects of drugs of abuse. Adaptations in the nucleus accumbens (NAc), a component of the mesolimbic dopamine system, are thought to contribute to this behavioral change. Here we examine excitatory synaptic transmission in NAc slices prepared from animals displaying sensitization 10-14 days after repeated in vivo cocaine exposure. The ratio of AMPA (alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid) receptor- to NMDA (N-methyl-d-aspartate) receptor-mediated excitatory postsynaptic currents (EPSCs) was decreased at synapses made by prefrontal cortical afferents onto medium spiny neurons in the shell of the NAc. The amplitude of miniature EPSCs at these synapses also was decreased, as was the magnitude of long-term depression. These data suggest that chronic in vivo administration of cocaine elicits a long-lasting depression of excitatory synaptic transmission in the NAc, a change that may contribute to behavioral sensitization and addiction.

Distinguishing between GABA(A) receptors responsible for tonic and phasic conductances

Cell-to-cell communication in the mammalian nervous system does not solely involve direct synaptic transmission. There is considerable evidence for a type of communication between neurons through chemical means that lies somewhere between the rapid synaptic information transfer and the relatively non-specific neuroendocrine secretion. Here I review some of the experimental evidence accumulated for the GABA system indicating that GABA(A) receptor-gated Cl-channels localized at synapses differ significantly from those found extrasynaptically. These two types of GABA(A) receptor are involved in generating distinctly different conductances. Thus, the development and search for pharmacological agents specifically aimed at selectively altering synaptic and extrasynaptic GABA(A) conductances is within reach, and is expected to provide novel insights into the regulation of neuronal excitability.

The modification of NMDA receptors by visual experience in the rat retina is age dependent

Extensive studies have shown that the activation of N-methyl-D-aspartate receptors (NMDARs) and the subsequent rise in the levels of postsynaptic calcium are critical events in the initiation of synaptic plasticity. Modification of the amount, or of the subunit composition of NMDARs, alters receptor function thereby affecting the development and/or efficacy of synaptic transmission. In the present study, a Western blot analysis was employed to investigate the effects of visual experience and age on the differential expression of NMDARs in the rat retina. A crude synaptic membrane fraction (SPM) was prepared and assayed with antibodies specific for either the NR1, NR2A or NR2B subunits. Relative to control animals raised in a diurnal light-dark cycle, a period of 1 week of dark-rearing caused an increase in the relative amount of NR1, a decrease in the level of NR2A, and no change in the level of NR2B subunit expression in postnatal day 12 rats. At 2 months of age, 1 week of dark-rearing had less effect, and at 6 months of age there was no difference between dark-reared and control animals. The effect of light exposure on dark-reared animals was tested for the 2-month-old animals. Light exposure for long periods (days), but not short periods (h), could reverse the dark-rearing effects. These data provide evidence for a developmentally regulated plasticity of NMDAR subunits in the retina.

Neuroprotective effects of extracellular glutamate are absent in hippocampal organotypic cultures treated with the amyloid peptide Abeta(25-35)

Hippocampal cells are particularly vulnerable in Alzheimer's disease but the cause of cell death is unknown. Amyloid toxicity has been implicated in hippocampal cell death, but its specific mechanisms are poorly understood. We used confocal microscopy to examine the effects of the amyloid peptide fragment 25-35 (Abeta(25-35)) on cell death in organotypic hippocampal slice cultures. Addition of glutamate to the culture medium significantly improved nerve cell survival in cultures subjected to consecutive medium exchanges. This effect was lost if cultures were treated with the amyloid peptide fragment Abeta(25-35) but not the inactive peptide 35-25. These data suggest that one of the mechanisms responsible for amyloid toxicity may be inhibition of the survival promoting effects of extracellular glutamate.

Enhanced NMDA receptor activity in retinal inputs to the rat suprachiasmatic nucleus during the subjective night

Circadian oscillator mechanisms in the suprachiasmatic nucleus (SCN) can be reset by photic input, which is mediated by glutamatergic afferents originating in the retina. A key question is why light can only induce phase shifts of the biological clock during a restricted period of the circadian cycle, namely the subjective night. One of several possible mechanisms holds that glutamatergic transmission at retinosuprachiasmatic synapses would be altered, in particular the contribution of glutamate receptor subtypes to the postsynaptic response. By studying the contributions of NMDA and non-NMDA glutamate receptors to the retinal input to SCN in whole-cell patch-clamp recordings in acutely prepared slices, we tested the hypothesis that NMDA receptor current evoked by optic nerve activity is potentiated during the subjective night. During the day the NMDA component of the EPSC evoked by optic nerve stimulation was found less frequently and was significantly smaller in magnitude than during the night. In contrast, the non-NMDA component did not show a significant day-night difference. When the magnitude of the NMDA component was normalized to that of the non-NMDA component, the day-night difference was maintained, suggesting a selective potentiation of NMDA receptor conductance. In addition to contributing to electrically evoked EPSCs, the NMDA receptor was found to sustain a small, tonically active inward current during the night phase. No significant tonic contribution by NMDA receptors was detected during the day. These results suggest, first, a dual mode of NMDA receptor function in the SCN and, second, a clock-controlled type of receptor plasticity, which may gate the transfer of photic input to phase-shifting mechanisms operating at the level of molecular autoregulatory feedback loops.

Dendritic glutamate autoreceptors modulate signal processing in rat mitral cells

It has been shown recently that in mitral cells of the rat olfactory bulb, N-methyl-D-aspartate (NMDA) autoreceptors are activated during mitral cell firing. Here we consider in more details the mechanisms of mitral cell self-excitation and its physiological relevance. We show that both ionotropic NMDA and non-NMDA autoreceptors are activated by glutamate released from primary and secondary dendrites. In contrast to non-NMDA autoreceptors, NMDA autoreceptors are almost exclusively located on secondary dendrites and their activation generates a large and sustained self-excitation. Both intracellularly evoked and miniature NMDA-R mediated synaptic potentials are blocked by intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) and result from a calcium-dependent release of glutamate. Self-excitation can be produced by a single spike, and trains of spikes result in frequency facilitation. Thus activation of excitatory autoreceptors is a major function of action potentials backpropagating in mitral cell dendrites, which results in an immediate positive feedback counteracting recurrent inhibition and increasing the signal-to-noise ratio of olfactory inputs.

Effects of deafferentation on the electrophysiology of ventral cochlear nucleus neurons

When cochlear pathology impairs the afferent innervation of the ventral cochlear nucleus (VCN), electrical responses of the auditory brainstem are altered and changes in cell and synaptic morphology are observed. However, the impact of deafferentation on the electrical properties of cells in the VCN is unknown. We examined the electrical properties of single neurons in the anterior and posterior VCN following bilateral cochlear removal in young rats. In control animals, two populations of cells were distinguished: those with a linear subthreshold current-voltage relationship and repetitive firing of action potentials with regular interspike intervals (type I), and those with rectifying subthreshold current-voltage relationships and phasic firing of 1-3 action potentials (type II). Measures of action potential shape further distinguished these two groups. Two weeks following cochlear removal, both electrical response patterns were still seen. Type I cells showed a higher input resistance. Deafferented single-spiking type II cells were slightly more depolarized, had smaller action potentials, smaller afterhyperpolarizations and shorter membrane time constants, whereas multiple-spiking type II cells were apparently unaffected. These changes in the electrical properties of VCN neurons following cochlear injury may adversely affect central processing of sounds presented acoustically or electrically by prostheses.

Dendrosomatic voltage and charge transfer in rat neocortical pyramidal cells in vitro

Most excitatory synapses on neocortical pyramidal cells are located on dendrites, which are endowed with a variety of active conductances. The main origin for action potentials is thought to be at the initial segment of the axon, although local regenerative activity can be initiated in the dendrites. The transfer characteristics of synaptic voltage and charge along the dendrite to the soma remains largely unknown, although this is an essential determinant of neural input-output transformations. Here we perform dual whole-cell recordings from layer V pyramidal cells in slices from somatosensory cortex of juvenile rats. Steady-state and sinusoidal current injections are applied to characterize the voltage transfer characteristics of the apical dendrite under resting conditions. Furthermore, dendrosomatic charge and voltage transfer are determined by mimicking synapses via dynamic current-clamping. We find that around rest, the dendrite behaves like a linear cable. The cutoff frequency for somatopetal current transfer is around 4 Hz, i.e., synaptic inputs are heavily low-pass filtered. In agreement with linearity, transfer resistances are reciprocal in opposite directions, and the centroids of the synaptic time course are on the order of the membrane time constant. Transfer of excitatory postsynaptic potential (EPSP) charge, but not peak amplitude, is positively correlated with membrane potential. We conclude that the integrative properties of dendrites in infragranular neocortical pyramidal cells appear to be linear near resting membrane potential. However, at polarized potentials charge transferred is voltage-dependent with a loss of charge at hyperpolarized and a gain of charge at depolarized potentials.

Neural activity and survival in the developing nervous system

Recent evidence suggests that blockade of normal excitation in the immature nervous system may have profound effects on neuronal survival during the period of natural cell death. Cell loss following depression of electrical activity in the central nervous system (CNS) may explain the neuropsychiatric deficits in humans exposed to alcohol or other CNS depressants during development. Thus, understanding the role of electrical activity in the survival of young neurons is an important goal of modern basic and clinical neuroscience. Here we review the evidence from in vivo and in vitro model systems that electrical activity participates in promoting neuronal survival. We discuss the potential role of moderate elevations of intracellular calcium in promoting survival, and we address the possible ways in which activity and conventional trophic factors may interact.

N-Acetylaspartylglutamate: the most abundant peptide neurotransmitter in the mammalian central nervous system

In the progress of science, as in life, timing is important. The acidic dipeptide, N-acetylaspartylglutamate (NAAG), was discovered in the mammalian nervous system in 1965, but initially was not considered to be a neurotransmitter candidate. In the mid-1980s, a few laboratories revisited the question of NAAG's role in the nervous system and pursued hypotheses regarding its function that ranged from a precursor for the transmitter pool of glutamate to a direct role as a peptide transmitter. Since that time, NAAG has been tested against nearly all of the established criteria for identification of a neurotransmitter. It successfully meets each of these tests, including a concentrated presence in neurons and synaptic vesicles, release from axon endings in a calcium-dependent manner following initiation of action potentials, and extracellular hydrolysis by membrane-bound peptidase activity. NAAG is the most prevalent and widely distributed neuropeptide in the mammalian nervous system. NAAG activates NMDA receptors with a low potency that may vary among receptor subtypes, and it is a highly selective agonist at the type 3 metabotropic glutamate receptor (mGluR3). Acting through this receptor, NAAG reduces cyclic AMP levels, decreases voltage-dependent calcium conductance, suppresses excitotoxicity, influences long-term potentiation and depression, regulates GABA(A) receptor subunit expression, and inhibits synaptic release of GABA from cortical neurons. Cloning of peptidase activities against NAAG provides opportunities to study the cellular and molecular mechanisms by which synaptic NAAG peptidase activity is controlled. Given the codistribution of this peptide with a spectrum of traditional transmitters and its ability to activate mGluR3, we speculate that one role for NAAG following synaptic release is the activation of metabotropic autoreceptors that inhibit subsequent transmitter release. A second role is the production of extracellular glutamate following NAAG hydrolysis.

Potentiation of NMDA receptor function by the serine protease thrombin

Although serine proteases and their receptors are best known for their role in blood coagulation and fibrinolysis, the CNS expresses many components of an extracellular protease signaling system including the protease-activated receptor-1 (PAR1), for which thrombin is the most effective activator. In this report we show that activation of PAR1 potentiates hippocampal NMDA receptor responses in CA1 pyramidal cells by 2.07 +/- 0.27-fold (mean +/- SEM). Potentiation of neuronal NMDA receptor responses by thrombin can be blocked by thrombin and a protein kinase inhibitor, and the effects of thrombin can be mimicked by a peptide agonist (SFLLRN) that activates PAR1. Potentiation of the NMDA receptor by thrombin in hippocampal neurons is significantly attenuated in mice lacking PAR1. Although high concentrations of thrombin can directly cleave both native and recombinant NR1 subunits, the thrombin-induced potentiation we observe is independent of NMDA receptor cleavage. Activation of recombinant PAR1 also potentiates recombinant NR1/NR2A (1.7 +/- 0.06-fold) and NR1/NR2B (1.41 +/- 0.11-fold) receptor function but not NR1/NR2C or NR1/NR2D receptor responses. PAR1-mediated potentiation of recombinant NR1/NR2A receptors occurred after activation with as little as 300 pm thrombin. These data raise the intriguing possibility that potentiation of neuronal NMDA receptor function after entry of thrombin or other serine proteases into brain parenchyma during intracerebral hemorrhage or extravasation of plasma proteins during blood-brain barrier breakdown may exacerbate glutamate-mediated cell death and possibly participate in post-traumatic seizure. Furthermore, the ability of neuronal protease signaling to control NMDA receptor function may also have roles in normal brain development.

Light-evoked excitatory synaptic currents of X-type retinal ganglion cells

The excitatory amino acid receptor (EAAR) types involved in the generation of light-evoked excitatory postsynaptic currents (EPSCs) were examined in X-type retinal ganglion cells. Using isolated and sliced preparations of cat and ferret retina, the light-evoked EPSCs of X cells were isolated by adding picrotoxin and strychnine to the bath to remove synaptic inhibition. N-methyl-D-aspartate (NMDA) receptors contribute significantly to the light-evoked EPSCs of ON- and OFF-X cells at many different holding potentials. An NMDA receptor contribution to the EPSCs was observable when retinal synaptic inhibition was either normally present or pharmacologically blocked. NMDA receptors formed 80% of the peak light-evoked EPSC at a holding potential of -40 mV; however, even at -80 mV, 20% of the light-evoked EPSC was NMDA-mediated. An alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor-mediated component to the light-evoked EPSCs predominated at a holding potential of -80 mV. The light-evoked EPSC was blocked by the AMPA receptor-selective antagonist GYKI52466 (50-100 microM). The AMPA receptor-mediated EPSC component had a linear current-voltage relation. AMPA receptors form the main non-NMDA EAAR current on both ON- and OFF- X ganglion cell dendrites. When synaptic transmission was blocked by the addition of Cd(2+) to the Ringer, application of kainate directly to ganglion cells evoked excitatory currents that were strongly blocked by GYKI52466. Experiments using selective EAAR modulators showed the AMPA receptor-selective modulator cyclothiazide potentiated glutamate-evoked currents on X cells, while the kainate receptor-selective modulator concanavalin A (ConA) had no effect on kainate-evoked currents. Whereas the present study confirms the general notion that AMPA EAAR-mediated currents are transient and NMDA receptor-mediated currents are sustained, current-voltage relations of the light-evoked EPSC at different time points showed the contributions of these two receptor types significantly overlap. Both NMDA and AMPA EAARs can transmit transient and sustained visual signals in X ganglion cells, suggesting that much signal shaping occurs presynaptically in bipolar cells.

Cyclic GMP-dependent feedback inhibition of AMPA receptors is independent of PKG

In central neurons, the second messenger cGMP is believed to induce long-term changes in efficacy at glutamatergic synapses through activation of protein kinase G (PKG). Stimulating nitric oxide synthase, activating soluble guanylyl cyclase or elevating concentrations of intracellular cGMP depressed excitatory synaptic transmission in CA1 hippocampal neurons. Unexpectedly, intracellular cGMP depressed responses of AMPA receptors and inhibited excitatory postsynaptic currents in hippocampal neurons independently of phosphorylation. Our findings demonstrate that cGMP's modulation of excitatory transmission may involve a coupling of AMPA channel activity to levels of cGMP.

Acute decrease in net glutamate uptake during energy deprivation

The extracellular glutamate concentration ([glu](o)) rises during cerebral ischemia, reaching levels capable of inducing delayed neuronal death. The mechanisms underlying this glutamate accumulation remain controversial. We used N-methyl-D-aspartate receptors on CA3 pyramidal neurons as a real-time, on-site, glutamate sensor to identify the source of glutamate release in an in vitro model of ischemia. Using glutamate and L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) as substrates and DL-threo-beta-benzyloxyaspartate (TBOA) as an inhibitor of glutamate transporters, we demonstrate that energy deprivation decreases net glutamate uptake within 2-3 min and later promotes reverse glutamate transport. This process accounts for up to 50% of the glutamate accumulation during energy deprivation. Enhanced action potential-independent vesicular release also contributes to the increase in [glu](o), by approximately 50%, but only once glutamate uptake is inhibited. These results indicate that a significant rise in [glu](o) already occurs during the first minutes of energy deprivation and is the consequence of reduced uptake and increased vesicular and nonvesicular release of glutamate.

The Lurcher mutation of an alpha-amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid receptor subunit enhances potency of glutamate and converts an antagonist to an agonist

A point mutation of the GluRdelta2 (A654T) glutamate receptor subunit converts it into a functional channel, and a spontaneous mutation at this site is thought to be responsible for the neurodegeneration of neurons in the Lurcher mouse. This mutation is located in a hydrophobic region of the M3 domain of this subunit, and this alanine is conserved throughout many of the glutamate receptors. We show here that site-directed mutagenesis of the homologous alanine (A636T; GluR1-L(c)) in the GluR1 AMPA receptor subunit alters its channel properties. The apparent potencies of both kainate and glutamate were increased 85- and 2000-fold, respectively. Furthermore, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)was converted from a competitive antagonist into a potent agonist. Our results demonstrate that a single amino acid within or near the putative second transmembrane region of the GluR1 subunit is critical for the binding/gating properties of this AMPA receptor.

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.

Inhibition of uptake unmasks rapid extracellular turnover of glutamate of nonvesicular origin

Maintaining glutamate at low extracellular concentrations in the central nervous system is necessary to protect neurons from excitotoxic injury and to ensure a high signal-to-noise ratio for glutamatergic synaptic transmission. We have used DL-threo-beta-benzyloxyaspartate (TBOA), an inhibitor of glutamate uptake, to determine the role of glutamate transporters in the regulation of extracellular glutamate concentration. By using the N-methyl-D-aspartate receptors of patched CA3 hippocampal neurons as "glutamate sensors," we observed that application of TBOA onto organotypic hippocampal slices led to a rapid increase in extracellular glutamate concentration. This increase was Ca(2+)-independent and was observed in the presence of tetrodotoxin. Moreover, prevention of vesicular glutamate release with clostridial toxins did not affect the accumulation of glutamate when uptake was inhibited. Inhibition of glutamine synthase, however, increased the rate of accumulation of extracellular glutamate, indicating that glial glutamate stores can serve as a source in this process. TBOA blocked synaptically evoked transporter currents in astrocytes without inducing a current mediated by the glutamate transporter. This indicates that this inhibitor is not transportable and does not release glutamate by heteroexchange. These results show that under basal conditions, the activity of glutamate transporters compensates for the continuous, nonvesicular release of glutamate from the intracellular compartment. As a consequence, acute disruption of transporter activity immediately results in significant accumulation of extracellular glutamate.

Disruption of temporo-entorhinal connections abolishes the facilitatory effect of angiotensins on memory in rats

It has been found in our laboratory that the positive influence of angiotensin II (AII) and its 3-7 fragment [AII(3-7)] on learning and memory processes is mediated by the excitatory amino acids, since it was abolished by NMDA receptor antagonists. The purpose of the present study was to investigate whether bilateral disruption of glutamatergic temporo- entorhinal connections may have an influence on the facilitatory effect of both angiotensin peptides on memory motivated affectively. The bilateral transections of temporo-entorhinal connections were made in 27 male rats 10 days before testing the influence of intracerebroventricular AII and AII(3-7) injection on retrieval of a passive avoidance response. Twenty-seven additional rats served as sham-operated controls. Twenty-five lesioned and 25 sham-operated animals were accepted to the final analysis. AII and its 3-7 fragment significantly improved the retrieval process in sham-operated groups of rats. Bilateral disruption of temporo-entorhinal connections totally abolished the facilitatory effect of both angiotensins on recall of information in a passive avoidance situation. Moreover, bilateral disruption of temporo-entorhinal connections markedly but not significantly attenuated crossings of squares, evaluated in an open field test, without an influence on rearings and bar approaches. These results may suggest that in the facilitatory effect of AII and AII(3-7) on memory motivated affectively involves reciprocal glutamatergic connection between lateral entorhinal cortex and temporal cortex.

NMDA receptor-mediated changes of spontaneous activity patterns in thalamocortical slice cultures

Spontaneous activity is a hallmark of the thalamocortical system in vivo. Up until now, in vitro preparations of this system have been shown to be spontaneously active only when inhibition was reduced or N-methyl-D-aspartate (NMDA) receptor-mediated currents were facilitated via low extracellular magnesium levels. This study investigated the dependence of spontaneous thalamocortical activity patterns on NMDA receptor function via variation of extracellular magnesium levels (0-1 mM) and by the application of the specific NMDA receptor-antagonist D-2-amino-5-phosphonovalerate (AP5) in the absence of magnesium. We used cocultures of rat neocortical and thalamic slices which have been shown to develop reciprocal synaptic connections similar to those in vivo. Multi-site extracellular recordings revealed that the cultures were spontaneously active at all concentrations of magnesium and AP5, albeit with a high variability among cultures. Activity consisted of burst-like events which were largely synchronized within as well as among the neural tissues, and thalamic background activity during periods of neocortical quiescence. Each tissue was capable of triggering activity in the other, indicating that both thalamocortical and corticothalamic synaptic connections were functional. With increasing magnesium concentration, activity rates declined in both tissues and the site of origin of the synchronous, burst-like events shifted from neocortex to thalamus. AP5 in magnesium-free perfusion solution had qualitatively similar effects. We conclude that thalamic activity is not as dependent on the facilitation of NMDA receptor-mediated currents as neocortical activity and consequently, that the thalamus is the pacemaker of thalamocortical synchronized activity in physiological in vitro conditions.

Calcium influx through NMDA receptors, chronic receptor inhibition by ethanol and 2-amino-5-phosponopentanoic acid, and receptor protein expression

Chronic treatment of neurons with either ethanol or competitive and noncompetitive antagonists of NMDA receptors leads to enhanced expression of NMDA receptor density and function in neurons. The signal transduction pathways for such receptor up-regulation are not known. The focus of the present study was on the role of Ca2+ entry into neurons, either through receptor or voltage-gated channels, in the expression of the NMDA receptor subunit NR1 and the 71-kDa glutamate-binding protein (GBP) of a glutamate/NMDA receptor-like complex. Chronic inhibition of NMDA receptors in cortical neurons in primary cultures by either 100 mM ethanol or 100 microM 2-amino-5-phosphonopentanoic acid (2-AP5) increased the expression of NR1 and GBP. The effect of 2-AP5 on the expression of the two proteins was not additive with that of ethanol when neuronal cultures were treated with both agents at the same time. However, the effects of ethanol on NR1 and GBP expression were blocked by the simultaneous treatment with NMDA (50 microM). Activation or inhibition of other glutamate ionotropic receptors had no effect on the expression of NR1 and GBP. The inhibition of L- or N-type voltage-sensitive Ca2+ channels and voltage-gated Na+ channels also had little effect on the expression of either protein; neither did exposure of neurons to elevated extracellular Ca2+ concentrations (3 or 5 mM). On the other hand, treatment of neurons for 48 h with the intracellular Ca2+ chelator BAPTA-AM as well as partial chelation of extracellular Ca2+ with EGTA caused an up-regulation in NR1 and GBP expression. The enhanced expression of NR1 in neurons treated for 48 h with either ethanol or EGTA was correlated with increases in the activity of NMDA receptors demonstrated as a doubling of the NMDA-stimulated rise in intracellular free Ca2+ concentration. The effects of chronic administration of EGTA on both NR1 expression as well as NMDA receptor function were probably related to an acute inhibition by EGTA of NMDA-induced Ca2+ influx into neurons. It appears that the expression of both the NR1 subunit of NMDA receptors and the GBP of a receptor-like complex is regulated by intracellular Ca2+, especially that entering through NMDA receptor ion channels.

Low resting potential and postnatal upregulation of NMDA receptors may cause Cajal-Retzius cell death

Using in situ patch-clamp techniques in rat telencephalic slices, we have followed resting potential (RP) properties and the functional expression of NMDA receptors in neocortical Cajal-Retzius (CR) cells from embryonic day 18 to postnatal day 13, the time around which these cells normally disappear. We find that throughout their lives CR cells have a relatively depolarized RP (approximately -50 mV), which can be made more hyperpolarized (approximately -70 mV) by stimulation of the Na/K pump with intracellular ATP. The NMDA receptors of CR cells are subjected to intense postnatal upregulation, but their similar properties (EC50, Hill number, sensitivity to antagonists, conductance, and kinetics) throughout development suggest that their subunit composition remains relatively homogeneous. The low RP of CR cells is within a range that allows for the relief of NMDA channels from Mg2+ blockade. Our findings are consistent with the hypothesis that CR cells may degenerate and die subsequent to uncontrolled overload of intracellular Ca2+ via NMDA receptor activation by ambient glutamate. In support of this hypothesis we have obtained evidence showing the protection of CR cells via in vivo blockade of NMDA receptors with dizocilpine.

Adenylyl cyclase activation modulates activity-dependent changes in synaptic strength and Ca2+/calmodulin-dependent kinase II autophosphorylation

Activation of the Ca2+- and calmodulin-dependent protein kinase II (CaMKII) and its conversion into a persistently activated form by autophosphorylation are thought to be crucial events underlying the induction of long-term potentiation (LTP) by increases in postsynaptic Ca2+. Because increases in Ca2+ can also activate protein phosphatases that oppose persistent CaMKII activation, LTP induction may also require activation of signaling pathways that suppress protein phosphatase activation. Because the adenylyl cyclase (AC)-protein kinase A signaling pathway may provide a mechanism for suppressing protein phosphatase activation, we investigated the effects of AC activators on activity-dependent changes in synaptic strength and on levels of autophosphorylated alphaCaMKII (Thr286). In the CA1 region of hippocampal slices, briefly elevating extracellular Ca2+ induced an activity-dependent, transient potentiation of synaptic transmission that could be converted into a persistent potentiation by the addition of phosphatase inhibitors or AC activators. To examine activity-dependent changes in alphaCaMKII autophosphorylation, we replaced electrical presynaptic fiber stimulation with an increase in extracellular K+ to achieve a more global synaptic activation during perfusion of high Ca2+ solutions. In the presence of the AC activator forskolin or the protein phosphatase inhibitor calyculin A, this treatment induced a LTP-like synaptic potentiation and a persistent increase in autophosphorylated alphaCaMKII levels. In the absence of forskolin or calyculin A, it had no lasting effect on synaptic strength and induced a persistent decrease in autophosphorylated alphaCaMKII levels. Our results suggest that AC activation facilitates LTP induction by suppressing protein phosphatases and enabling a persistent increase in the levels of autophosphorylated CaMKII.

Excitatory synaptic inputs to pyramidal neurons of the lateral amygdala

Whole-cell patch clamp recordings were made from pyramidal neurons in the rat lateral amygdala (LA). Synaptic currents were evoked by stimulating in either the external capsule (ec), internal capsule (ic) or basolateral nucleus (BLA). Stimulation of either the ic, ec or BLA evoked a glutamatergic excitatory synaptic current (EPSC) which was mediated by both non-NMDA and NMDA (N-methyl-d-aspartic acid) receptors. The ratio of the amplitude of the NMDA receptor-mediated component measured at +40 mV to the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component measured at -60 mV was similar regardless of whether EPSCs were evoked in the ec, ic or BLA. At resting membrane potentials, excitatory synaptic potentials evoked from either the ec or putative thalamic inputs were unaffected by application of the NMDA receptor antagonist APV. Spontaneous glutamatergic currents had two components to their decay phase. The slow component was selectively blocked by the NMDA receptor antagonist D-APV, indicating that AMPA and NMDA receptors are colocalized in spiny neurons. We conclude that pyramidal cells of the LA receive convergent inputs from the cortex, thalamus and basal nuclei. At all inputs, both AMPA/kainate and NMDA-type receptors are active and colocalized in the postsynaptic density.

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.

Rat hippocampal neurons maintain their own GABAergic synaptic transmission in culture

The whole-cell patch-clamp technique was used to record monosynaptic inhibitory postsynaptic currents (IPSCs) from pairs of hippocampal neurons cultured for 2-3 weeks. The application of fresh physiological solution for 2-3 min reversibly reduced the amplitude of evoked GABAergic IPSCs to 72.5% of control value. The amplitude and frequency of spontaneous IPSCs decreased too. The depression of evoked IPSCs was significantly smaller or absent if conditioned solution was applied (physiological solution which had been previously in contact with neurons for 30 min). Currents evoked by exogenously applied GABA were unaffected by fresh solution. These results suggest that hippocampal neurons release some endogenous substance(s), by which they up regulate presynaptically their own inhibitory synaptic transmission.

Modulation of LTP induction by NMDA receptor activation and nitric oxide release

In the CA1 hippocampal region, the induction of long-term potentiation (LTP) requires activation of N-methyl-D-aspartate receptors (NMDARs). However, untimely NMDAR activation either immediately prior to or following tetanic stimulation inhibits LTP generation. This NMDAR-mediated LTP inhibition is overcome by inhibitors of nitric oxide synthase (NOS) and hemoglobin, suggesting the involvement of NO. Additionally, NO inhibitors can promote the ability of weak tetanic stimuli to produce LTP under basal conditions in hippocampal slices. Recent experiments indicate that untimely NMDAR activation contributes to the failure of LTP induction during periods of low glucose exposure and hypoxia. Following hypoxia there is also a delayed form of LTP inhibition that is reversed by NMDAR antagonists and NO inhibitors. These results suggest that there are physiological and pathological conditions during which NMDAR activation and NO release modulate the induction of synaptic plasticity.

Monocular deprivation induces homosynaptic long-term depression in visual cortex

Brief monocular deprivation during early postnatal development can lead to a depression of synaptic transmission that renders visual cortical neurons unresponsive to subsequent visual stimulation through the deprived eye. The Bienenstock-Cooper-Munro (BCM) theory proposes that homosynaptic mechanisms of long-term depression (LTD) account for the deprivation effects. Homosynaptic depression, by definition, occurs only at active synapses. Thus, in contrast to the commonly held view that the synaptic depression caused by monocular deprivation is simply a result of retinal inactivity, this theoretical framework indicates that the synaptic depression may actually be driven by the residual activity in the visually deprived retina. Here we examine the validity of this idea by comparing the consequences of brief monocular deprivation by lid suture with those of monocular inactivation by intra-ocular treatment with tetrodotoxin. Lid suture leaves the retina spontaneously active, whereas tetrodotoxin eliminates all activity. In agreement with the BCM theory, our results show that monocular lid suture causes a significantly greater depression of deprived-eye responses in kitten visual cortex than does treatment with tetrodotoxin. These findings have important implications for mechanisms of experience-dependent plasticity in the neocortex.

Dopamine decreases the excitability of layer V pyramidal cells in the rat prefrontal cortex

In both primates and rodents, the prefrontal cortex (PFC) is highly innervated by dopaminergic fibers originating from the ventral tegmental area, and activation of this mesocortical dopaminergic system decreases spontaneous and evoked activity in the PFC in vivo. We have examined the effects of dopamine (DA), over a range of concentrations, on the passive and active membrane properties of layer V pyramidal cells from the rat medial PFC (mPFC). Whole-cell and perforated-patch recordings were made from neurons in rat mPFC. As a measure of cell excitability, trains of action potentials were evoked with 1-sec-long depolarizing current steps. Bath application of DA (0.05-30 microM) produced a reversible decrease in the number of action potentials evoked by a given current step. In addition, DA reversibly decreased the input resistance (RN) of these cells. In a subset of experiments, a transient increase in excitability was observed after the washout of DA. Control experiments suggest that these results are not attributable to changes in spontaneous synaptic activity, age-dependent processes, or strain-specific differences in dopaminergic innervation and physiology. Pharmacological analyses, using D1 agonists (SKF 38393 and SKF 81297), a D1 antagonist (SCH 23390), a D2 receptor agonist (quinpirole), and a D2 antagonist (sulpiride) suggest that decreases in spiking and RN are mediated by D2 receptor activation. Together, these results demonstrate that DA, over a range of concentrations, has an inhibitory effect on layer V pyramidal neurons in the rat mPFC, possibly through D2 receptor activation.

NMDA receptor mechanisms contribute to neuroplasticity induced in caudalis nociceptive neurons by tooth pulp stimulation

We recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant and prolonged increases in jaw muscle electromyographic (EMG) activity that are suggestive of central neuroplasticity. Because small-fiber afferents, including pulp afferents, access nociceptive neurons in trigeminal (V) subnucleus caudalis, this study examined whether pulpal application of MO induces neuroplastic changes in caudalis nociceptive neurons (wide dynamic range and nociceptive specific) and whether central N-methyl--aspartate (NMDA) receptor mechanisms are involved in these MO-induced neuroplastic changes. After pretreatment with vehicle (saline, 10 microliter i.t.) to the surface of the medulla, the pulpal application of MO to the maxillary molar tooth pulp produced a significant increase in neuronal spontaneous activity, a significant expansion of the pinch and/or tactile mechanoreceptive field (RF), a significant decrease in mechanical threshold, and significant increases in neuronal responses to graded pinch stimuli. Compared with vehicle-treated rats, pretreatment with the NMDA receptor antagonist MK-801 (10 microgram/10 microliter i.t.) followed by MO application to the pulp in another group of rats significantly reduced or abolished these MO-induced neuroplastic changes in nociceptive neurons. In another group of rats pretreated with saline (intrathecally), mineral oil application to the pulp did not show any significant changes in spontaneous activity or RF properties over the 40-min observation period. The pulpal application of MO in other rats (pretreated with saline, intrathecally) did not produce any significant neuroplastic changes in caudalis low-threshold mechanoreceptive neurons. These results indicate that the MO-induced activation of molar pulpal afferents can produce profound NMDA receptor-related neuroplastic changes in caudalis nociceptive neurons. Such neuroplastic changes may contribute to the hyperalgesia and spread of pain that can be associated with pulpal inflammation.

Persistent depolarizing action of GABA in rat Cajal-Retzius cells

1. To characterize membrane properties that might be relevant to the function and fate of Cajal-Retzius (CR) cells, the pharmacological and physiological effects of GABA acting at GABAA receptors were studied in CR cells from embryonic (E18) and postnatal (P11-13) slices of rat neocortex. 2. From the embryonic to the postnatal stage, GABA-induced maximum current almost tripled, the EC50 increased from 38 to 74 microM, and the Hill number increased from 1.4 to 1.9. Muscimol-elicited currents were qualitatively and quantitatively similar to those produced by GABA. 3. GABA-induced changes in the amplitude of large-conductance Ca2+-activated K+ channel current recorded on-cell from E18 CR cells were consistent with depolarization. 4. GABA-mediated depolarization of embryonic and postnatal CR cells was studied directly with perforated-patch recording techniques. Ten micromolar and 1 mM GABA, respectively, depolarized E18 CR cells to -27 +/- 1 and -25 +/- 3 mV. These same concentrations of GABA depolarized P11 CR cells to -36 +/- 3 and -23 +/- 3 mV. 5. In postnatal cortex, GABA (100 microM) increased the firing rate of CR cells 7.3-fold. By contrast, the firing of hippocampal pyramidal cells from slices of the same age (P12) was totally and reversibly blocked by GABA. 6. These experiments suggest that contrary to its postnatal inhibitory shift observed in other cells, the depolarizing effect of GABA remains in CR cells from E18 until their virtual disappearance.