Quisqualate- and kainate-activated channels in mouse central neurones in culture

Major dorsoventral differences in the modulation of the local CA1 hippocampal network by NMDA, mGlu5, adenosine A2A and cannabinoid CB1 receptors

Recent research points to diversification in the local neuronal circuitry between dorsal (DH) and ventral (VH) hippocampus that may be involved in the large-scale functional segregation along the long axis of the hippocampus. Here, using CA1 field recordings from rat hippocampal slices, we show that activation of N-methyl-d-aspartate receptors (NMDARs) reduced excitatory transmission more in VH than in DH, with an adenosine A1 receptor-independent mechanism, and reduced inhibition and enhanced postsynaptic excitability only in DH. Strikingly, co-activation of metabotropic glutamate receptor-5 (mGluR5) with NMDAR, by CHPG and NMDA respectively, strongly potentiated the effects of NMDAR in DH but had not any potentiating effect in VH. Furthermore, the synergistic actions in DH were occluded by blockade of adenosine A2A receptors (A2ARs) by their antagonist ZM 241385 demonstrating a tonic action of these receptors in DH. Exogenous activation of A2ARs by 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680) did not change the effects of mGluR5-NMDAR co-activation in either hippocampal pole. Importantly, blockade of cannabinoid CB1 receptors (CB1Rs) by their antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281) restricted the synergistic actions of mGluR5-NMDARs on excitatory synaptic transmission and postsynaptic excitability and abolished their effect on inhibition. Furthermore, AM 281 increased the excitatory transmission only in DH indicating that CB1Rs were tonically active in DH but not VH. Removing the magnesium ions from the perfusion medium neither stimulated the interaction between mGluR5 and NMDAR in VH nor augmented the synergy of the two receptors in DH. These findings show that the NMDAR-dependent modulation of fundamental parameters of the local neuronal network, by mGluR5, A2AR and CB1R, markedly differs between DH and VH. We propose that the higher modulatory role of A2AR and mGluR5, in combination with the role of CB1Rs, provide DH with higher functional flexibility of its NMDARs, compared with VH.

Retour aux sources: defining the structural basis of glutamate receptor activation

Ionotropic glutamate receptors (iGluRs) are the major excitatory neurotransmitter receptor in the vertebrate CNS and, as a result, their activation properties lie at the heart of much of the neuronal network activity observed in the developing and adult brain. iGluRs have also been implicated in many nervous system disorders associated with postnatal development (e.g. autism, schizophrenia), cerebral insult (e.g. stroke, epilepsy), and disorders of the ageing brain (e.g. Alzheimer's disease, Parkinsonism). In view of this, an emphasis has been placed on understanding how iGluRs activate and desensitize in functional and structural terms. Early structural models of iGluRs suggested that the strength of the agonist response was primarily governed by the degree of closure induced in the ligand-binding domain (LBD). However, recent studies have suggested a more nuanced role for the LBD with current evidence identifying the iGluR LBD interface as a "hotspot" regulating agonist behaviour. Such ideas remain to be consolidated with recently solved structures of full-length iGluRs to account for the global changes that underlie channel activation and desensitization.

Probing TARP modulation of AMPA receptor conductance with polyamine toxins

The properties of synaptic AMPA receptors (AMPARs) depend on their subunit composition and association with transmembrane AMPAR regulatory proteins (TARPs). Although both GluA2 incorporation and TARP association have been shown to influence AMPAR channel conductance, the manner in which different TARPs modulate the mean channel conductance of GluA2-containing AMPARs is unknown. Using ultrafast agonist application and nonstationary fluctuation analysis, we found that TARP subtypes differentially increase the mean channel conductance, but not the peak open probability, of recombinant GluA2-containing AMPARs. TARP γ-8, in particular, enhances mean channel conductance to a greater degree than γ-2, γ-3, or γ-4. We then examined the action of a use-dependent antagonist of GluA2-containing AMPARs, philanthotoxin-74 (PhTx-74), on recombinant AMPARs and on GluA2-containing AMPARs in cerebellar granule neurons from stargazer mice transfected with TARPs. We found that the rate and extent of channel block varies with TARP subtype, in a manner that correlates linearly with mean channel conductance. Furthermore, block of GluA2-containing AMPARs by polyamine toxins varied depending on whether channels were activated by the full agonist glutamate or the partial agonist kainate, consistent with conductance state-dependent block. Block of GluA2-lacking AMPARs by PhTx-433 is also modulated by TARP association and is a function of agonist efficacy. Our data indicate that channel block by polyamine toxins is sensitive to the mean channel conductance of AMPARs, which varies with TARP subtype and agonist efficacy. Furthermore, our results illustrate the utility of polyamine toxins as sensitive probes of AMPAR channel conductance and suggest the possibility that TARPs may influence their channel properties by selectively stabilizing specific channel conformations, rather than altering the pore structure.

The expanding social network of ionotropic glutamate receptors: TARPs and other transmembrane auxiliary subunits

Ionotropic glutamate receptors (iGluRs) underlie rapid, excitatory synaptic signaling throughout the CNS. After years of intense research, our picture of iGluRs has evolved from them being companionless in the postsynaptic membrane to them being the hub of dynamic supramolecular signaling complexes, interacting with an ever-expanding litany of other proteins that regulate their trafficking, scaffolding, stability, signaling, and turnover. In particular, the discovery that transmembrane AMPA receptor regulatory proteins (TARPs) are AMPA receptor auxiliary subunits that are critical determinants of their trafficking, gating, and pharmacology has changed the way we think about iGluR function. Recently, a number of novel transmembrane proteins have been uncovered that may also serve as iGluR auxiliary proteins. Here we review pivotal developments in our understanding of the role of TARPs in AMPA receptor trafficking and gating, and provide an overview of how newly discovered transmembrane proteins expand our view of iGluR function in the CNS.

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

Underlying mechanism for NMDA receptor antagonism by the anti-inflammatory drug, sulfasalazine, in mouse cortical neurons

Sulfasalazine (SULFA), of anti-inflammatory drugs, shows a protective action against NMDA-induced neuronal toxicity. Here, we used an electrophysiological study of the pharmacological effects of SULFA on NMDA receptors to examine the molecular mechanisms underlying the neuroprotective role of SULFA. The drug acted as a typical noncompetitive inhibitor with neither agonist- nor use-dependency, and antagonized NMDA-evoked responses in a voltage-independent manner, suggesting that SULFA is not an open channel blocker. Noise and single channel analyses showed that SULFA-blocked NMDA responses by reducing the number of NMDA channels available for activation, and also reduced the channel open probability without changing single channel conductance. Moreover, SULFA accelerated NMDA desensitization without affecting the affinity of the receptor for NMDA or glutamate. Taken together, these data indicate that SULFA blocks the NMDA response by reducing the number of NMDA channels available for activation. This appears to occur via a SULFA-induced decrease in the channel open probability, and a concomitant acceleration of the desensitization response, which is likely associated with a reduced affinity for glycine. SULFA indeed decreased the glycine-potentiated NMDA response without binding directly to the glycine site. Our results suggest that SULFA acts as a noncompetitive NMDA receptor antagonist with an allosteric glycine modulation.

Ionotropic Glutamate Receptor Recognition and Activation

Ionotropic glutamate receptors are the major excitatory neurotransmitters in mammalian brain but are found throughout the animal kingdom as well as in plants and bacteria. A great deal of progress in understanding the structure of these essential neurotransmitter receptors has been made since the first examples were cloned and sequenced in 1989. The atomic structure of the ligand-binding domain of several ionotropic glutamate receptors has been determined, and a great deal of progress has been made in relating the structural properties of the binding site to the function of the intact receptor. In addition, the identification of glutamate receptors from a wide variety of organisms ranging from several types of bacteria to Arabidopsis to a range of animal species has made glutamate receptors a molecular laboratory for studying the evolution of proteins. The fact that glutamate receptors are a particularly ancient intercellular signaling molecule suggests a potential role in the transition from single celled to multicellular organisms. This review focuses on the structure and dynamics of ionotropic glutamate receptors and their relation to the function and evolution of these proteins.

Flash photolysis reveals a diversity of ionotropic glutamate receptors on the mitral cell somatodendritic membrane

It is widely held that the soma and basal dendrites of olfactory bulb mitral cells receive exclusively inhibitory synaptic input from local interneurons. However, the mitral somatodendritic membrane exhibits immunoreactivity for a variety of glutamate receptors, and blocking GABA receptors unmasks mitral cell self-excitation. This excitation is proposed to be mediated either by diffuse spillover of the mitral cells' own released glutamate, or by punctate transmission from glutamate-releasing granule cells. This study examined the pharmacology and kinetics of glutamate sensitivity of mitral cells by flash photolysis of nitroindoline caged glutamates, which facilitate reliable activation of receptors in the synaptic cleft. Wide-field laser uncaging (3.5-ms flash) of approximately 0.5-1 mM glutamate onto the soma activated large currents with fast (3.4-ms rise, 7.5-ms decay) and slow (64-ms rise, >10-s decay) components. In 100 microM APV, slow currents were reduced to 53% of control (257-ms rise, 2-s decay), displayed outward rectification in 1.3 mM Mg2+, and blocked by 15 microM 5,7-dichlorokynurenate. Responses to less, similar 100 microM glutamate were fully antagonized by 100 microM APV, consistent with competitive inhibition at high-affinity NMDA receptors. An APV-resistant NMDA receptor was not observed, refuting the punctate transmission model. Fast currents were blocked by 10 microM NBQX, boosted 3.28-fold by 100 microM cyclothiazide, and resolved into AMPA (40%) and kainate (60%) receptor components by 100 microM SYM2206. The results suggest that self-excitation depends on AMPA, kainite, and conventional NMDA autoreceptors on the mitral cell.

Both MK801 and NBQX reduce the neuronal damage after impact-acceleration brain injury

To understand the pathogenesis of diffuse axonal injury, we investigated the temporal and spatial profiles of neuronal degeneration in impact-acceleration injury in rats using Fluoro-Jade (FJ) staining. Impact-acceleration injury was produced in Wistar rats by the method described by Marmarou et al. with some modifications. Animals were sacrificed 1, 2, 7, 14, or 28 days after injury. Paraffin-embedded coronal sections were stained with HE or FJ, or analyzed immunohistochemically for GFAP or amyloid precursor protein (APP). FJ-positive degenerative neurons were found primarily in the dorsal brainstem and thalamus from 1 to 2 days following injury and these were associated with GFAP expression. However, FJ-positive cells were rarely found after 7 days. In all rats, significant expression of APP was observed primarily in the cingulum, cerebral peduncle and pontomedullary junction. FJ also stained these injured axons. Intrathecal administration of both NMDA and AMPA/kinate glutamate receptor antagonists MK-801 and NBQX, respectively, reduced the neuronal injury. NBQX showed more significant effects on axonal injury than MK-801. These observations indicate that not only axonal damage, but also primary neuronal damage occurs in this impact-acceleration injury model. It is also suggested that NBQX can act both directly on neuronal cells and white matter and that NMDA could have a significant protective effect against not only neuronal, but also axonal injury.

Cross-desensitization Reveals Pharmacological Specificity of Excitatory Amino Acid Receptors in Isolated Hippocampal Neurons

Ionic currents elicited by excitatory amino acids were studied, using the concentration clamp method, in enzymatically isolated rat hippocampal neurons. Cross-desensitization between the responses to various agonists was applied to separate the activity of two types of receptors, N-methyl-d-aspartate (NMDA) and non-NMDA. NMDA receptors were selectively activated by NMDA, l- and d-aspartate, d-glutamate and quinolinate. Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate appeared to be selective, and quisqualate relatively less selective non-NMDA agonists, acting on the same receptor type. l-Glutamate, l- and d-homocysteate activated both receptor types. It is supposed that two receptor sites, activation site and desensitization site, control the action of agonists at the non-NMDA receptor. When examined in the cross-desensitization experiments, NMDA and non-NMDA receptors appear to be represented by the two homogeneous and independent receptor populations operating different ionic channels.

Collaboration of fibronectin matrix and neurotrophin in regulating spontaneous transmitter release at developing neuromuscular synapses in Xenopus cell cultures

Integrins mediate cell-extracellular matrix connection and are particularly important during neuronal development. We here investigated the regulation of fibronectin (FN) matrix and neurotrophins on the embryonic synaptic transmission. Spontaneous synaptic currents (SSCs) were recorded from innervated myocytes of 1-day-old Xenopus cultures by whole-cell recordings. The SSC increasing action of alpha,beta-methylene adenosine triphosphate was enhanced in neurons grown on FN substratum, which was further potentiated by chronic treatment with brain-derived neurotrophic factor (BDNF). The SSC increasing action of thapsigargin, carbonyl cyanide m-chlorophenylhydrazone and N-methyl-D-aspartate was also markedly potentiated in neurons grown on FN-coated glass coverslips and chronically treated with BDNF. FN matrix or BDNF alone only exerts slight potentiation on the SSC increasing action of these three drugs. Our results suggest that FN matrix can collaborate with neurotrophin in regulating synaptic transmission at developing motoneurons, which may play an important role in the maturation of embryonic neuromuscular junction.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

Intracellular survival pathways against glutamate receptor agonist excitotoxicity in cultured neurons. Intracellular calcium responses

Cultured rat cerebellar granule cells are resistant to the excitotoxic effects of N-methyl-D-aspartate (NMDA) and non-NMDA receptor agonists under three conditions: 1) prior to day seven in vitro when cultured in depolarizing concentrations of potassium [25 mM]; 2) at any time in vitro when cultured in non-depolarizing concentrations of potassium 5 mM[; and 3) when neurons, cultured in depolarizing concentrations of potassium 25 mM[ for eight days in vitro, are pretreated with a subtoxic concentration of NMDA. The focus of this paper is to determine: a) whether the resistance to excitotoxicity by NMDA and non-NMDA receptor agonists is due to a decreased intracellular calcium Ca++[i response to glutamate receptor agonists in cultured rat cerebellar granule cells; or b) whether Ca++[i levels induced by the agonists are similar to those observed under excitotoxic conditions. Granule cells, matured in non-depolarizing growth medium, treated with glutamate resulted in an increase in Ca++[i followed by a plateau that remained above baseline in virtually all neurons that responded to glutamate. The response was rapid in onset (< 10 sec) and the pattern of response heterogeneous in that cells responsive to glutamate increased their Ca++[i to different extents; some cells did not respond to glutamate. Kainate also produced significant elevations in Ca++[i. The Ca++[i response to glutamate in neurons matured in depolarizing (25 mM K+) growth medium for three days was rapid, transient and heterogeneous, which reached a plateau that was elevated above baseline levels; removing the glutamate markedly reduced the Ca++[i concentration. Activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors by kainic acid produced similar changes in Ca++[i responses. At a time when cultured cerebellar granule cells become susceptible to the excitotoxic effects of glutamate acting at NMDA receptors (day in vitro (DIV) 8) in depolarizing growth medium, glutamate elicited Ca++[i responses similar to those observed at a culture time when the neurons are not susceptible to the excitotoxic effects of glutamate (DIV 3). Pretreatment of the cultured neurons with a subtoxic concentration of NMDA, which protects all neurons against the excitotoxic effects of glutamate, did not alter the maximal Ca++[i elicited by an excitotoxic concentration of glutamate.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

Mobilisation of intracellular Ca2+ by mGluR5 metabotropic glutamate receptor activation in neonatal rat cultured dorsal root ganglia neurones

The ability of metabotropic glutamate receptor activation to mobilise intracellular calcium was investigated in cultured dorsal root ganglion (DRG) neurones from neonatal rats using the calcium sensitive fluorescent dye Fura-2. L-glutamate (10 microM) caused sustained and oscillatory increases in intracellular calcium concentration ([Ca2+]i) in a subpopulation of cultured DRG neurones. The oscillatory responses were not blocked by combined application of the ionotropic glutamate receptor antagonists MK 801 (2 microM) and CNQX (20 microM). Oscillations in [Ca2+]i were also observed following application of the nonselective metabotropic glutamate receptor (mGluR) agonist, trans-(1S,3R)-1-aminocyclopentane-1S, 3R-dicarboxylic acid (1S,3R)-ACPD, 20 microM) and the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 microM). These responses were blocked by the selective Group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (100 microM) and Ca2+ release channel inhibitors ryanodine (100 microM) and dantrolene (10 microM). The predominantly Group II agonist (2S,2'R,3'R)-2-(2'3'-dicarboxy-cyclopropyl)glycine (DCG-IV, 100 microM) failed to produce Ca2+ transients alone but suppressed responses to CHPG. Reverse transcriptase PCR techniques, using primers specific to Group I mGluRs, revealed the presence of mGluR5 but not mGluR1 mRNA in these cells. Therefore, glutamate can cause a slowly activating and reversible mobilisation of [Ca2+]i in sensory neurones by activation of ionotropic receptors, and can induce oscillatory calcium transients by selectively activating metabotropic glutamate receptors that are likely to be of the mGluR5 subtype.

Functional changes of N-methyl-D-aspartic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate channels in gerbil hippocampal CA1, in relation to postischemic enhancement of glutamate receptor-mediated responses

Glutamate receptor-mediated responses have been reported to be enhanced in the postischemic CA1 pyramidal neurons before the appearance of delayed neuronal death, and the enhancement has been thought to be one of crucial factors leading postischemic CA1 pyramidal neurons to irreversible neuronal injury. In the present study, we examined what changes in functional properties of N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) channels are responsible for the enhanced postischemic glutamate receptor-mediated responses. Gerbils were subjected to 5-min ischemia to induce the enhancement of glutamate receptor-mediated responses and the hippocampal slices were prepared 3 h after ischemia. Single channel activities evoked by NMDA and AMPA were recorded from outside-out patches excised from the postischemic CA1 pyramidal neurons. The main conductance levels of NMDA and AMPA channels in the postischemic CA1 pyramidal neurons were not significantly different from those in control CA1 pyramidal neurons. The mean open time and the open-state probability of NMDA and AMPA channels significantly increased in the postischemic CA1 pyramidal neurons (NMDA channels: mean open time, 1.4-fold increase; open-state probability, 1.5-fold increase) (AMPA channels: mean open time, 1.3-fold increase; open-state probability, 1.8-fold increase). These findings indicate that the increases in the mean open time and the open-state probability of NMDA and AMPA channels are responsible for the enhancement of postischemic NMDA and non-NMDA receptor-mediated responses.

Passive avoidance training results in increased responsiveness of voltage- and ligand-gated calcium channels in chick brain synaptoneurosomes

A temporal cascade of events has been described from a number of biochemical investigations of passive avoidance training in day-old chicks. Among these, within minutes of training, there is a transient, enhanced release of glutamate and increased agonist and antagonist binding to N-methyl-D-aspartate-sensitive glutamate receptors in the intermediate medial hyperstriatum ventrale of the forebrain. Some 6.5 h later, alpha-amino-3-hydroxy-5-methyl-4-isoxazo lepropionate binding to glutamate receptors is also increased in the same region. These processes might be predicted to affect the uptake of calcium via voltage-sensitive calcium channels or glutamate receptor-associated channels, thereby changing the intracellular calcium concentration. To test this possibility, we have measured the calcium concentration in synaptoneurosomes, containing both pre- and postsynaptic elements, prepared from left and right intermediate medial hyperstriatum ventrale at various times following training, using Fura 2-AM as the indicator of intracellular calcium concentration. Synaptoneurosomes, prepared immediately and 5 min after training, were stimulated with 70 mM potassium chloride in the presence of 2 mM calcium, resulting in a significantly enhanced increase in calcium concentration in synaptoneurosomes from the left hemisphere of trained chicks. This effect was absent in samples obtained at later times after training. N-Methyl-D-aspartate (0.5 mM) induced a significant enhancement in the increase in calcium concentration in intermediate medial hyperstriatum ventrale from both left and right hemispheres 10 min and 30 min after training. At 3 h and 6 h after training, alpha-amino-3-hydroxy-5-methyl-4-isoxazo lepropionate (0.5 mM) induced a significantly enhanced increase in calcium concentration in samples from either hemisphere. These results suggest that immediately after training there is an engagement of both pre- and postsynaptic voltage-sensitive calcium channels, followed by an increased reponse to N-methyl-D-aspartate receptor stimulation, and coinciding with the enhanced calcium-dependent glutamate release and an increase in N-methyl-D-aspartate-sensitive glutamate receptor binding that has been reported previously. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-sensitive mechanisms are activated at a later stage of memory formation, when increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding to glutamate receptors has been reported. Thus, responsiveness of calcium channels to agonist stimulation is implicated in temporally diverse stages in the cascade of events involved in memory formation following passive avoidance training in the chick.

Glutamate receptors in the mammalian central nervous system

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

Neuronal excitatory properties of human immunodeficiency virus type 1 Tat protein

Neuronal dysfunction and cell death in patients with human immunodeficiency virus type-1 (HIV-1) infection may be mediated by HIV-1 proteins and products released from infected cells. Two HIV-1 proteins, the envelope glycoprotein gp120 and nonstructural protein Tat, are neurotoxic. We have determined the neuroexcitatory properties of HIV-1 tat protein using patch-clamp recording techniques. When fmoles of Tat were applied extracellularly, it elicited dose-dependent depolarizations of human fetal neurons in culture and rat CA1 neurons in slices, both in the absence and presence of tetrodotoxin. These responses were voltage-dependent, reversed at approximately 0 mV, and were significantly increased by repetitive applications with no evidence of desensitization. That these responses to Tat were due to direct actions on neurons was supported by observations that Tat dose-dependently depolarized outside-out patches excised from cultured human neurons. Removal of extracellular Ca2+ decreased the responses both in neurons and membrane patches. This is the first demonstration that an HIV-1 protein can, in the absence of accessory cells, directly excite neurons and leads us to speculate that Tat may be a causative agent in HIV-1 neurotoxicity.

Expression of glutamate receptor subunits in alpha-motoneurons

Whole-cell recordings from 6.5 day embryonic chick alpha-motoneurons indicated the presence of AMPA, kainate, and NMDA glutamate receptor subtypes in each motoneuron tested. AMPA consistently evoked a desensitizing response, while kainate could evoke either a desensitizing or non-desensitizing whole-cell response. In excised membrane patches, desensitizing AMPA responses appeared to be colocalized with non-desensitizing kainate responses. Desensitizing kainate responses were seen in some patches which were not responsive to AMPA, suggesting that kainate selective subunits and AMPA selective subunits localize separately on the motoneuron membrane. To determine which of the known glutamate receptor subunits might underlie these responses, we used RT-PCR amplification to detect subunits present in mRNA isolated from adult rat spinal cord and from a highly enriched motoneuron population from embryonic chick. Sequencing of the amplified cDNA was used to verify the identity of the products and of the alternative splice variants of GluR1-4. In rat spinal cord, all subunits that we attempted to detect, including AMPA selective subunits GluR1-4, kainate selective subunits GluR5-7 and KA1-2, and NMDA subunit NR1 were present. The isolated motoneurons also contained AMPA subunits GluR1, 2, and 4, and kainate subunits GluR6 and 7. The GluR2 and 4 subunits were specifically processed by splicing, present primarily as the flip splice form.

Glutamatergically induced pattern of Ca2+ driving potential as a mechanism of postsynaptic plasticity

Simulation studies were performed in a model of neuronal dendrite with Na+ and K+ channels and with ionotropic and metabotropic glutamate receptors. The ionotropic receptors were either N-methyl-D-aspartate (NMDA)-sensitive, voltage-dependent, and permeable to Ca2+, Na+, and K+, or non-NMDA-sensitive, voltage-independent, and permeable to Na+ and K+. The metabotropic receptors provided a catalytic effect on Ca2+-induced Ca2+ release from intracellular stores. Local intracellular concentration [Ca2+]i in the cytoplasm was changed because of exchange with the stores, axial diffusion, and transmembrane inward passive and outward pump fluxes. Tonic activation of ionotropic and metabotropic receptors in a particular range of intensities triggered the formation of spatially periodic [Ca2+]i hot and cold bands arising from an initial uniform state. The period and width of the bands were smaller at higher levels of tonic NMDA activation and higher metabotropically controlled rates of Ca2+-induced Ca2+ release. The bandwidths also depended on the dendrite diameter, the specific membrane, and cytoplasm resistivity. This activity-induced pattern led to long-term, spatially inhomogeneous change in local excitatory postsynaptic potentials (EPSPs) of NMDA synapses phasically activated with the same presynaptic intensity. The phasic EPSPs were potentiated if the synapse occurred in the hot band.

Electrophysiological demonstration of N-methyl-D-aspartate receptors at the afferent synapse of catfish electroreceptor organs

An excitatory amino acid, most probably L-glutamate, acts as a neurotransmitter at the receptor cell--afferent fibre synapses in the ampullary electroreceptor organs of the freshwater catfish Ictalurus nebulosus. In the present study, we have used an electrophysiological approach to investigate the presence of N-methyl-D-aspartate receptors at this level. N-Methyl-D-aspartate, dissolved in an Mg(2+)-containing (normal) solution, had no effect on afferent activity, not even at 5 mM. However, addition of 5 mM N-methyl-D-aspartate to an Mg(2+)-free solution evoked an enduring increase in firing rate. The application of N-methyl-D-aspartate combined with electrical sine wave stimulation produced a firing increase in the primary afferents, even in the presence of Mg2+ (1.5 mM). Glycine (0.01-0.001 mM) significantly potentiated the N-methyl-D-aspartate responses. Addition of antagonists of the actions of N-methyl-D-aspartate, 7-chlorokynurenic acid, DL-2-amino-5-phosphonovaleric acid and ketamine in concentrations of 0.5-2.0 mM led to a decrease in resting and stimulus-evoked activity. 7-Chlorokynurenic acid also blocked the responses to application of N-methyl-D-aspartate. The glycine agonist D-serine (0.01 mM) prevented the 7-chlorokynurenic inhibitory effect. These results suggest the involvement of N-methyl-D-aspartate receptors in mediating the actions of L-glutamate at the afferent synapses of the electroreceptor organs of the catfish.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Digital processing of the current noise evoked by kainate in cerebellar granule cells

Current induced in cultured cerebellar granule cells by the bath application of kainate (500 microM) was measured using the conventional patch-clamp technique. Two different kinds of responses were observed after the agonist perfusion. Some cells exhibited small inward whole-cell currents: 116 +/- 40 pA (7 cells) at a clamp potential of -60 mV; in other cells, the agonist induced significantly larger currents: 420 +/- 35 pA (6 cells) at a clamp potential of -60 mV. The current flowing in the agonist-activated ionic channels was indirectly estimated by processing the fluctuations of whole-cell current by means of an original parametric method. Mean conductance of the underlying channels was then determined from the single-channel current estimated at different clamp potentials. In the cells exhibiting small inward currents, the mean conductance was equal to 0.5 +/- 0.2 pS (7 cells), whereas in the cells with large inward currents it was 3 +/- 0.4 pS (6 cells). This result gives a coherent explanation of the different kinds of responses observed at macroscopic level in the whole-cell current and confirms that kainate-activated channels can exhibit different levels of conductance.

Functional organization of cone bipolar cells in the rat retina

The responses of cone bipolar cells in slices of rat retina to ionotropic glutamate receptor agonists were recorded with the whole cell voltage-clamp technique in the presence of 5 mM Co2+ and nominally 0 mM Ca2+ extracellularly. Application of the non-N-methyl-D-aspartate (non-NMDA) receptor agonists kainate and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate had a series of effects on cone bipolar cells (types 1-9), and the different cell types could be classified as ON- or OFF-type cells according to which type(s) of responses they displayed. First, direct responses were observed in cell types 1-4 as short-latency inward currents at -70 mV with reversal potentials (E(rev)s) close to 0 mV, characteristic of nonselective cation channels. Second, some cells, among types 5-9, did not display short-latency inward currents to kainate at -70 mV. Other type 5-8 cells displayed short-latency kainate responses, but the currents could not be reversed (E(rev) of +40 mV or greater). I suggest that these responses are conveyed to the cone bipolar cells through gap junctions, most likely with AII amacrine cells. The lack of reversal is likely due to a substantial voltage drop across the gap junctions resulting in an inadequate voltage control of AII amacrine cells when the recording pipette is on the cone bipolar cell. Kainate responses recorded directly from AII amacrine cells had E(rev) approximately 0 mV. Third, long-latency indirect responses selective for chloride ions (E(rev) approximately chloride equilibrium potential) were observed in many cone bipolar cells during longer-lasting application of kainate. The long-latency response component was suppressed by coapplication of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist picrotoxin and the GABA(C) receptor antagonist 3-aminopropyl(methyl)phosphinic acid. This long-latency component was absent in axotomized bipolar cells, suggesting that it was due to external Ca2+-independent release of GABA onto the axon terminals of the cone bipolar cells. All kainate-evoked response components were blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Application of NMDA evoked no response in cone bipolar cells. These results suggest that cone bipolar cells types 1-4 are OFF cone bipolar cells, whereas cone bipolar cells types 5-9 are ON cone bipolar cells.

Role of NMDA and non-NMDA ionotropic glutamate receptors in traumatic spinal cord axonal injury

We examined the role of glutamatergic mechanisms in acute injury to rat spinal cord white matter. Compound action potentials (CAPs) were recorded from isolated dorsal column segments in vitro. Under control conditions (Ringer's solution), the CAPs decreased to 71.4 +/- 2.0% of preinjury values after compression injury with a clip exerting a closing force of 2 g. The combination of the NMDA receptor blocker APV (50 microM) and the AMPA/kainate (KA) receptor blocker CNQX (10 microM) resulted in significantly improved recovery of CAP amplitude postinjury; however, the NMDA receptor antagonist APV alone did not enhance postinjury recovery, and infusion of NMDA (10 microM) did not affect recovery of the CAPs. In contrast, the AMPA/KA receptor blockers NBQX (10 microM) or CNQX (10 microM) significantly enhanced the recovery of CAP amplitude postinjury. The agonists AMPA (100 microM) or KA (100 microM) resulted in significant attenuation of CAP amplitude postinjury. Coapplication of AMPA/KA plus NBQX and CNQX was also associated with improved functional recovery. After incubation with AMPA and KA, Co(2+)-positive glia were visualized in spinal cord white matter. Similar results were seen after compressive injury but not in control cords. Immunohistochemistry and Western blot analysis demonstrated AMPA (GluR4)- and KA (GluR6/7 and KA2)-positive astrocytes in spinal cord white matter. In summary, non-NMDA ionotropic glutamate receptors seem to be involved in the pathophysiology of traumatic spinal cord injury. The presence of AMPA (GluR4) and KA (GluR6/7 and KA2) receptors on periaxonal astrocytes suggests a role for these cells in glutamatergic white matter injury.

DNA fragmentation and Prolonged expression of c-fos, c-jun, and hsp70 in kainic acid-induced neuronal cell death in transgenic mice overexpressing human CuZn-superoxide dismutase

Kainic acid (KA) neurotoxicity was examined in transgenic (Tg) mice overexpressing human CuZn-superoxide dismutase (SOD-1). The doses of KA required to produce seizures, the severity of the seizures, and the regions damaged were similar in SOD-1 Tg and non-transgenic wild-type mice. Intraperitoneal KA injection induced seizure-related neuronal damage in the CA3 and CA1 regions of the hippocampus and in other regions of the brain in both SOD-1 Tg and wild-type mice. These damaged neurons were labeled with the terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL) technique up to 72 h, although no significant difference in the number of TUNEL-positive neurons was observed between SOD-1 Tg and wild-type mice. In situ hybridization showed that c-fos, c-jun, and hsp70 genes were expressed in the hippocampus, cortex, and other regions of the brain after KA treatment. The expression of these genes was maximal 1 to 4 h following KA treatment but persisted longer in the hippocampus and other regions in SOD-1 Tg compared with wild-type mice; however, cell death in the hippocampus, assessed using cresyl violet staining, was similar in SOD-1 Tg and wild-type mice. The data show that superoxide radicals modulate both immediate early gene and heat shock gene expression after KA-induced seizures. The prolonged expression of c-fos, c-jun, and hsp70 in SOD-1 Tg compared with wild-type mice may indicate that hippocampal neurons survive longer in SOD-1 Tg than in wild-type animals; however, cell death as well as the seizure threshold, seizure severity and the pattern of regional vulnerability were not affected substantially by increased levels of SOD in the brain.

Role of NMDA and non-NMDA ionotropic glutamate receptors in traumatic spinal cord axonal injury

We examined the role of glutamatergic mechanisms in acute injury to rat spinal cord white matter. Compound action potentials (CAPs) were recorded from isolated dorsal column segments in vitro. Under control conditions (Ringer's solution), the CAPs decreased to 71.4 +/- 2.0% of preinjury values after compression injury with a clip exerting a closing force of 2 g. The combination of the NMDA receptor blocker APV (50 microM) and the AMPA/kainate (KA) receptor blocker CNQX (10 microM) resulted in significantly improved recovery of CAP amplitude postinjury; however, the NMDA receptor antagonist APV alone did not enhance postinjury recovery, and infusion of NMDA (10 microM) did not affect recovery of the CAPs. In contrast, the AMPA/KA receptor blockers NBQX (10 microM) or CNQX (10 microM) significantly enhanced the recovery of CAP amplitude postinjury. The agonists AMPA (100 microM) or KA (100 microM) resulted in significant attenuation of CAP amplitude postinjury. Coapplication of AMPA/KA plus NBQX and CNQX was also associated with improved functional recovery. After incubation with AMPA and KA, Co(2+)-positive glia were visualized in spinal cord white matter. Similar results were seen after compressive injury but not in control cords. Immunohistochemistry and Western blot analysis demonstrated AMPA (GluR4)- and KA (GluR6/7 and KA2)-positive astrocytes in spinal cord white matter. In summary, non-NMDA ionotropic glutamate receptors seem to be involved in the pathophysiology of traumatic spinal cord injury. The presence of AMPA (GluR4) and KA (GluR6/7 and KA2) receptors on periaxonal astrocytes suggests a role for these cells in glutamatergic white matter injury.

Effect of RNA editing and subunit co-assembly single-channel properties of recombinant kainate receptors

1. Patch-clamp methods have been used to examine single-channel properties of recombinant GluR5 and GluR6 kainate-preferring glutamate receptors which differ in a single amino acid residue as a result of RNA editing at the Q/R (glutamine/arginine) site. Subunits were expressed alone or in combination with the high-affinity kainate receptor subunit KA - 2 in transfected human embryonic kidney (HEK-293) cells. 2. In outside-out patches, unedited homomeric GluR6(Q) receptors exhibited directly resolved domoate-activated single-channel conductances of 8, 15 and 25 pS. Variance analysis of GluR6(Q) responses gave a mean conductance of 5.4 pS, while the edited isoform GluR6(R) had an unusually low channel conductance (225 fS). 3. Homomeric channels composed of GluR5(Q) subunits exhibited three conductance states of 5, 9 and 14 pS characterized by prolonged burst activations in the presence of domoate. In contrast, the GluR5(R) subunit, which has not previously been reported to form functional homomeric receptors, had an extremely low conductance (< 200 fS). 4. Heteromeric GluR6(Q)/KA-2 kainate receptors gave single-channel events indistinguishible from homomeric GluR6(Q) channels. Conversely, openings produced by GluR5(Q)KA-2 and GluR5(Q) receptors differed from each other in their kinetic properties. The primary effect of co-expression of KA-2 with GluR5(Q) was a dramatic shortening in channel burst length. 5. Spectral and variance analyses were used to estimate mean single-channel conductances of heteromeric edited receptor-channels; channel conductances were 950 fS for GluR5(R)KA-2 receptors and 700 fS for GluR6(R)/KA-2 receptors. Both receptor types had significantly higher conductances than the respective homomeric channels, GluR5(R) and GluR6(R). 6. We conclude that Q/R site editing dramatically reduces single-channel conductance. Furthermore, we find similarity between the kainate receptor-channels described in sensory neurones and the recombinant GluR5(Q) homomeric channel. Characterization of recombinant single-channel properties could therefore aid identification of the native kainate receptors.

Potentiation of neurotransmitter release by activation of presynaptic glutamate receptors at developing neuromuscular synapses of Xenopus

1. Glutamate receptors play important roles in synaptic plasticity and neural development. Here we report that, at the developing neuromuscular synapses in Xenopus cultures, the activation of presynaptic glutamate receptors at motor nerve terminals potentiates spontaneous acetylcholine (ACh) release. 2. Co-cultures of spinal neurons and myotomal muscle cells were prepared from 1-day-old Xenopus embryos. Spontaneous synaptic currents (SSCs) were recorded from innervated myocytes using whole-cell recording. Bath application of glutamate (10 microM) markedly increased the frequency of SSCs, and the action of glutamate was reversible. 3. Pretreatment with 0.3 microM tetrodotoxin, which blocks Na+ channels and the conduction of action potentials, only slightly inhibited the potentiating action of glutamate on SSCs. Furthermore, the enhancement of ACh secretion was much more prominent when glutamate was applied locally to the synaptic region. 4. Three types of glutamate receptor agonists, kainate, quisqualate, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and NMDA (N-methyl-D-aspartate), were effective in inducing the potentiating effect. The ranking order was: glutamate > kainate > NMDA > AMPA > quisqualate. Glycine potentiated the effects induced by NMDA. Metabotropic receptors were not involved in the potentiating action of glutamate. 5. The potentiating effect of glutamate depended on the influx of Ca2+ through both L-type Ca2+ channels and NMDA-gated channels. 6. Since glutamate is known to be co-released with ACh at some cholinergic nerve terminals, the released glutamate may serve as a positive feedback regulation of ACh secretion at developing neuromuscular junctions via its action on presynaptic glutamate receptors.

Innervation of burst firing spiny interneurons by pyramidal cells in deep layers of rat somatomotor cortex: paired intracellular recordings with biocytin filling

Intracellular recordings were obtained from a class of neuron defined electrophysiologically as burst firing interneurons in layers V and VI in slices of adult rat somatomotor cortex. Four of these cells were recovered histologically. These four cells had resting membrane potentials between -68 and -80 mV, a mean input resistance of 77 +/- 16.2 M omega (measured from the voltage deflection produced by a 100 ms, 0.5 nA hyperpolarizing pulse delivered from a membrane potential of -80 mV) and responded to injections of depolarizing current from membrane potentials negative of -70 to -75 mV with an initial burst of action potentials followed by a complex afterhyperpolarization. In response to injection of larger (0.5-1.5 nA) hyperpolarizing current pulses from membrane potentials between -60 and -70 mV, 15 of 20 burst firing cells (three of four recovered histologically) that were tested displayed delayed inward rectification, and in all 20 cells of this type, responses to large negative current pulses were followed by a rebound depolarization that could initiate action potentials. Filling of four of these cells with biocytin and subsequent histological processing revealed that they were bitufted with sparsely to medium spiny dendrites and extensive local axon ramifications. These neurons are similar to low threshold spiking cells [Kawaguchi (1993) J. Neurophysiol. 69, 416-431]. Ultrastructural examination of the axons of three cells revealed that of 53 labelled terminals studied, the majority formed synaptic contacts with dendritic shafts. Filling neurons with biocytin during paired intracellular recordings resulted in three well labelled interneurons, each of which was postsynaptic to a simultaneously recorded pyramidal neuron. In these pairs both cells were identified, but the presynaptic axon was poorly labelled in one. In one of the two pairs in which the pre- and postsynaptic neurons were fully recovered, light microscopic assessment indicated that the axon of the presynaptic pyramid formed 12 close appositions with dendrites of the postsynaptic interneuron. Six of these appositions were examined at the electron microscopic level and were identified as possible synaptic contacts. In the other pair three of six close appositions observed at the light level were verified as possible synaptic connections at the ultrastructural level. These correlated electrophysiological and anatomical studies provide the first evidence for connections from pyramid to burst firing interneurons in the neocortex and indicate that these connections can be mediated by multiple synaptic contacts. The accompanying paper describes the functional properties of these connections.

The effects of (-)- and (+)-beta-cyclazocine on NMDA-evoked responses and NMDA-mediated cell damage in cultured rat hippocampal neurons

Microspectrofluorimetric measurements of excitatory amino acid-evoked rises in intracellular free calcium concentration ([Ca2+]i), electrophysiological measurements of currents through single NMDA receptor-operated ion channels and estimates of cellular viability following NMDA challenge were employed to examine the interactions of (-)- and (+)-beta-cyclazocine with the NMDA receptor-channel complex in cultured rat hippocampal neurons. Rises in [Ca2+]i evoked by NMDA, but not those evoked by kainate, AMPA or 50 mM K+, were reduced by (-)-beta-cyclazocine in a concentration- and use-dependent manner with an estimated IC50 value of 272 nM. In outside-out patches, (-)-beta-cyclazocine did not change the magnitudes of unitary NMDA-evoked currents but diminished both the frequency of channel openings and their mean open time. The IC50 for (-)-beta-cyclazocine against NMDA channel open state probability was estimated at 84 nM. The actions of (-)-beta-cyclazocine were consistent with a voltage-dependent open channel block of the NMDA channel with a blocking rate constant of 7.03.10(7) M-1.s-1 at -40 mV. Neurons exposed to a high concentration of NMDA in vitro were protected from death by 1 and 10 microM (-)-beta-cyclazocine. In all of the above assays, (+)-beta-cyclazocine was considerably less potent an NMDA antagonist and neuroprotective agent than (-)-beta-cyclazocine; the IC50 for (+)-beta-cyclazocine against channel open state probability was estimated at 14 microM. The results demonstrate that (-)-beta-cyclazocine is a potent and selective inhibitor of NMDA-evoked responses in cultured rat hippocampal neurons and an effective neuroprotective agent in vitro.

Hyperosmotic activation of transmitter release from presynaptic terminals onto retinal ganglion cells

A method for evoking neurotransmitter release without light stimulation has been developed and applied to a retinal slice preparation of the tiger salamander (Ambystoma Tigrum). This method utilizes a micropipette containing hyperosmotic levels of sucrose in Ringer, positioned within the inner plexiform layer (IPL) under visual control. Intermittent pressure (between 0.1 and 2 bars) applied to the pipette evoked release of neurotransmitters which were evaluated with whole-cell recording (WCR) technique applied to cells in the ganglion cell layer. Pharmacological studies were used to characterize the properties of the hyperosmotic sucrose-evoked response (HSER) and in some cases, we compared the HSER with synaptic currents evoked by light stimulation. The HSER typically consisted of both inhibitory and excitatory components with a reversal potential in between that for chloride (approximately -60 mV) and non-specific cation channels (approximately 0 mV). Relatively pure inhibition or excitation could be revealed through pharmacological techniques by blocking the inhibition with picrotoxin/strychnine or by blocking the glutamatergic neurotransmission with D-AP7 (D-2-amino-7-phosphonoheptanoate) and NBQX (2,3-dihydroxy-6-nitro-sulfamoyl -benzo (F) quinoxaline). A comparison of light-evoked responses (LER) and the HSER suggested that they activate the same pool of releasable neurotransmitter.

Modulation of adenosine-induced cAMP accumulation via metabotropic glutamate receptors in chick optic tectum

Changes on cyclic adenosine monophosphate (cAMP) levels in response to adenosine and glutamate and the subtype of glutamate receptors involved in this interaction were studied in slices of optic tectum from 3-day-old chicks. cAMP accumulation mediated by adenosine (100 microM) was abolished by 8-phenyltheophylline (15 microM). Glutamate and the glutamatergic agonists kainate or trans-D, L-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) did not evoke cAMP accumulation. Glutamate blocked the adenosine response in a dose-dependent manner. At 100 microM, glutamate did not inhibit the effect of adenosine. The 1 mM and 10 mM doses of glutamate inhibited adenosine-induced cAMP accumulation by 55% and 100%, respectively. When glutamatergic antagonists were used, this inhibitory effect was not affected by 200 microM 6,7-dihydroxy-2,3,dinitroquinoxaline (DNQX), an ionotropic antagonist, and was partially antagonized by 1 mM (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)M-CPG], a metabotropic antagonist, while 1 mM L-2-amino-3-phosphonopropionate (L-AP3) alone, another metabotropic antagonist, presented the same inhibitory effect of glutamate. Kainate (10 mM) and trans-ACPD (100 microM and 1 mM) partially blocked the adenosine response. This study indicates the involvement of metabotropic glutamate receptors in adenylate cyclase inhibition induced by glutamate and its agonists trans-ACPD and kainate.

Evidence for calcium/calmodulin dependence of spinule retraction in retinal horizontal cells

Horizontal cells of the carp retina alter their synaptic connections with cones during dark and light adaptation. At light onset, dendrites of horizontal cells, which are positioned laterally at the ribbon synapse, form "spinules," little processes with membrane densities. Spinules are retracted again during dark adaptation. Spinule retraction is also elicited upon glutamate application to the retina. In the present study, we address the question whether calcium/calmodulin-dependent pathways are involved in dark- and glutamate-evoked spinule retraction. Light-adapted retinas were isolated and subsequently dark adapted during incubation in media of different calcium concentrations. Spinule retraction was clearly blocked in low-calcium solutions (5 microM and 50 nM CaCl2). Incubation in medium containing cobalt chloride (2 mM) had the same effect. Both treatments blocked the glutamate-induced spinule retraction as well. These results indicate that spinule retraction is induced by a calcium influx into horizontal cells. To investigate whether calmodulin, the primary calcium receptor in eukaryotic cells, is present at the site of spinule formation, light- and dark-adapted retinas, embedded in LR White resin, were labelled with an antibody against calmodulin and gold-conjugated secondary antibodies. Horizontal cell dendrites at the ribbon synapse revealed strong calmodulin immunoreactivity, which was more than twice as high in light- as in dark-adapted retinas. The incubation of isolated retinas with the calmodulin antagonists W5 and W13 inhibited spinule retraction. In summary, these results suggest that spinule retraction may be regulated by calcium influx into horizontal cells and subsequent calcium/calmodulin-dependent pathways.

Selective stimulation of excitatory amino acid receptor subtypes and the survival of granule cells in culture: effect of quisqualate and AMPA

Differentiating granule cells develop survival requirements in vitro which can be met by treatment with high K+ or excitatory amino acids. Promotion of cell survival by N-methyl-D-aspartate (NMDA) or kainate has already been established and here we report that treatment of the cells with alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) or quisqualate (QA) also leads to cell rescue. In comparison with the effect of NMDA, the influence of AMPA/QA is small, resulting in a 20-30% increase in cell survival, with a peak at a very narrow concentration range (0.5-2.0 microM QA and 5-10 microM AMPA). The effect is exclusive to AMPA receptor stimulation, since stimulation of metabotropic glutamate receptors with (1S3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) has no effect. Furthermore, AMPA/QA rescue of cells is blocked by ionotropic non-NMDA receptor antagonists, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzoquinoxaline (NBQX). In addition, both nifedipine and dizolcipline (MK-801) interfered with the cell survival promoting effect of AMPA, suggesting that the influence of AMPA is mediated via calcium influx involving both depolarization-activated voltage sensitive calcium channels and NMDA receptors stimulated as a result of AMPA-induced release of glutamate. Possible reasons for the small cell survival promoting effect of AMPA/QA compared with the influence of high K+ or NMDA are discussed.

Involvement of the N-methyl-D-aspartate receptors of the vestibular nucleus in the gaze-holding system of the cat

Eye movements were recorded in alert cats after injections into one of the medial vestibular nuclei (MVN) either of a N-methyl-D-aspartate (NMDA) antagonist or of a non-NMDA antagonist. A gaze-holding failure was caused by the NMDA antagonist when it was injected into the central part of the MVN but not when it was injected into the rostral part of that nucleus. By contrast, injections of the non-NMDA-receptor antagonist into the MVN did not cause any sign of gaze-holding failure. We conclude that the NMDA receptors located in the central part of the MVN are involved in the gaze-holding system.

Motor performance, histologic damage, and calcium influx in rats treated with NBQX after focal ischemia

2,3-Dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline (NBQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist, has been reported to provide neuronal protection after global ischemia. The objectives of this study were to evaluate the neuroprotective effects of NBQX initiated after focal cortical ischemia and to validate a method for measuring functional outcome in this model. Male spontaneously hypertensive rats (SHRs) were exposed to various durations of transient or permanent tandem middle cerebral artery (MCA) occlusion. Studies compared motor performance using balance beam and prehensile-traction tests, calcium-calmodulin (Ca-CaM) binding by immunohistochemistry, and infarct volume between NBQX-treated animals [intravenous (i.v.) 5 mg/kg/h x 6 h or intraperitoneal (i.p.) 30 mg/kg q 30 min x 3 begun postischemia] and controls. All ischemic groups performed less well than sham-operated controls on the motor performance tasks in proportion to the severity of ischemia. No significant improvement in motor performance was noted in the NBQX-treated versus the control animals after 1 h or permanent MCA/CCA occlusion. Treatment with NBQX (i.v. or i.p. dosing) did not reduce Ca-CaM binding after 1 h of occlusion with 1 h of reperfusion or after 2 h of occlusion. Similarly, there was no reduction in infarct size between NBQX-treated and control animals after 24 h of permanent MCA/CCA occlusion (74.6 +/- 7.1 vs. 80.1 +/- 6.0 ml; ns) or after 1 h of occlusion with 23 h of reperfusion (55.1 +/- 4.4 vs. 47.4 +/- 6.2 ml; ns).(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of non-NMDA receptor channels in type-2 astrocytes and granule cells from rat cerebellum

1. Patch-clamp recording methods have been used to compare the pharmacological properties and single-channel characteristics of non-NMDA receptor channels in cerebellar type-2 astrocytes and granule cells. 2. In type-2 astrocytes whole-cell concentration-response curves for glutamate, quisqualate, AMPA and kainate gave EC50 values of 5.8, 3.8, 7.6 and 160 microM and Hill slopes of 1.65, 1.18, 1.64 and 1.65, respectively, resembling estimates for granule cell receptors. 3. The non-NMDA receptor antagonists CNQX and diCl-HQC (see Methods) inhibited whole-cell kainate currents in both cell types. The IC50 for CNQX antagonism of the kainate response was 536 nM in type-2 astrocytes, and 500 nM in granule cells. The IC50 for diCl-HQC was 3.5 microM in astrocytes and 3.7 microM in granule cells. 4. CNQX acted as a competitive antagonist of whole-cell kainate responses in type-2 astrocytes and granule cells giving Schild plots with a slope near 1. The equilibrium constant, K, for CNQX binding was 524 nM in astrocytes and 489 nM in granule cells. 5. Quisqualate and AMPA responses showed rapid desensitization in type-2 astrocytes with a ratio of steady-state to peak response of 0.09. Concanavalin A reduced this desensitization. 6. Non-NMDA channels in type-2 astrocytes and granule cells showed a low permeability to Ca2+ ions with a reversal potential, for kainate-activated whole-cell currents in isotonic Ca2+, of approximately -25 mV for astrocytes and -45 mV for granule cells. 7. Outside-out patches from type-2 astrocytes exhibited a range of single-channel conductances that were superficially similar to the glutamate-activated conductances in granule cells. However, the type-2 astrocytes were devoid of NMDA receptors, hence all of these conductances originated from non-NMDA channels. Their slope conductances were approximately 11, 21, 32, 42 and 52 pS. Amplitudes were verified with mean low-variance plots and single-channel current-voltage curves, which were linear. 8. There was also evidence of lower conductance kainate-activated channels in astrocyte patches. From noise analysis their estimated mean conductance was 1.9 pS, as described for the 'low-conductance' type kainate responses in cerebellar neurones. 9. Apparent open times, shut times and burst lengths of AMPA-activated (3-10 microM) channels were examined in patches from type-2 astrocytes, and kinetic properties of the 40 and 50 pS levels were compared with the lower levels. 10. Our results indicate some marked pharmacological similarities between non-NMDA receptor channels in type-2 astrocytes and granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of glutamate binding sites in cultured cerebellar granule cells and cortical astrocytes

Membranes prepared from cerebellar granule cells and cortical astrocytes exhibited specific, saturable binding of L-[3H]glutamate. The apparent binding constant KD was 135 nM and 393 nM and the maximal binding capacity Bmax 42 and 34 mumol/kg in granule cells and astrocytes, respectively. In granule cells the binding was strongly inhibited by the glutamate receptor agonists kainate, quisqualate, N-methyl-D-aspartate (NMDA), L-homocysteate and ibotenate, and the antagonist DL-5-aminophosphonovalerate. In astrocytes, only quisqualate among these was effective. L-Aspartate, L-cysteate, L-cysteinesulphinate and gamma-D-glutamylglycine were inhibitors in both cell types. The binding was totally displaced in both cell types by L-cysteinesulphinate with IC50 in the micromolar range. In astrocytes the binding was also totally displaced by quisqualate, but in granule cells only partially by NMDA, kainate and quisqualate in turn. It is concluded from the relative potencies of agonists and antagonists in [3H]glutamate binding that cerebellar granule cells express the NMDA, kainate and quisqualate types of the glutamate receptor, while only the quisqualate-sensitive binding seems to be present in cortical astrocytes.

Asynchrony of mossy fibre inputs and excitatory postsynaptic currents in rat hippocampus

1. Excitatory postsynaptic currents (EPSCs) were studied by whole-cell voltage-clamp recording (WCR) from pyramidal cells in the CA3 field of rat hippocampal slices. Input from mossy fibres was evoked by stimuli applied to stratum granulosum ('dentate gyrus stimulation'). This often resulted in complex, multi-component EPSCs with rise times as long as 5.0 ms (mean = 2.5 ms). In contrast, individual EPSC components typically had rise times between 0.3 and 1.0 ms. 2. To isolate monosynaptic, mossy fibre-driven EPSC components, slices were exposed to 'suppressing' media that reduced response amplitudes by 64-88%. In five out of six cases, long EPSC rising phases (> 3 ms) retained the same shape during suppression. This implied that EPSCs were driven by asynchronously active mossy fibre inputs. 3. From latencies of antidromically driven granule cell population spikes (GCPSs) a mean conduction velocity of 0.67 m/s was inferred. Conduction distance had practically no correlation with GCPS duration, implying that velocity dispersion was small and did not desynchronize mossy fibre impulses. EPSC components exhibited 'surplus' latency; they occurred 0.9-4.8 ms after latencies expected on the basis of direct conduction distances. 4. Mossy fibre volleys (MFVs) were evoked by dentate gyrus stimulation and studied with neurotransmission disabled. MFV negative phases lasted from 2.5 to 4.5 ms and had multiple components. By comparison, negative phases of Schaffer collateral fibre volleys (SCFVs) were always simple in shape and lasted 1.5 ms or less. MFV components had surplus latencies similar to those of EPSC components. Late MFV components did not require high stimulus intensities. 5. Widespread activation of granule cells occurred when stimuli were applied to single loci in the stratum granulosum. This implies that such stimuli elicit antidromic impulses in hilar collaterals of mossy fibres, which could result in activation of orthodromic impulses in mossy fibre trunks that had not been stimulated directly. After anti-, then orthodromic conduction, impulses would arrive in the CA3 subfield with 'surplus' latency. 6. When cuts were made in the hilus to prevent anti-/orthodromic conduction, MFV durations were reduced, but only to a small extent. This implies that surplus latency and asynchrony arise in part by anti-/orthodromic conduction, and partly by a mechanism that is intrinsic to mossy fibres or their 'giant' boutons. 7. Because of desynchronization of mossy fibre inputs, there probably are significant differences between kinetic properties of averaged, compound mossy fibre EPSCs and those of unitary mossy fibre EPSCs (i.e. currents driven by input from single presynaptic axons).(ABSTRACT TRUNCATED AT 400 WORDS)

The non-psychotropic cannabinoid (+)-(3S,4S)-7-hydroxy-delta 6- tetrahydrocannabinol 1,1-dimethylheptyl (HU-211) attenuates N-methyl-D-aspartate receptor-mediated neurotoxicity in primary cultures of rat forebrain

The non-psychotropic cannabinoid (+)-(3S,4S)-7-hydroxy-delta 6- tetrahydrocannabinol 1,1-dimethylheptyl (HU-211), a stereoselective inhibitor of the N-methyl-D-aspartate (NMDA) receptor, protects primary cultures of rat forebrain against NMDA receptor-mediated neurotoxicity. Cell mortality produced by exposure for 10 min to NMDA or glutamate was reduced to approximately 18 or 27%, respectively, by application of 50 microM HU-211 for 10-15 min during or after exposure of cultures to excitatory amino acid. This effect of HU-211 was dependent on its concentration (EC50 = 8.7 +/- 4 microM). HU-211 also reduced the toxicity induced by brief exposure (10 min) to kinase or quisqualate, though less effectively. HU-211 may therefore prove useful as a non-psychoactive drug that protects against neurotoxicity mediated by the NMDA receptor.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

The TINS/TiPS Lecture. The molecular biology of mammalian glutamate receptor channels

In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

Waveform analysis suggests that LTP alters the kinetics of synaptic receptor channels

The waveform of an isolated excitatory monosynaptic response reflects the kinetics of transmitter release, the kinetics of synaptic receptor channels and the filtering properties of neurons. Results reported here indicate that long-term potentiation (LTP) causes correlated decreases in the rise time and decay time constant of synaptic potentials recorded in hippocampal slices in which inhibitory currents and post-synaptic spiking were suppressed. Statistical comparisons of waveforms revealed that the distortions introduced by LTP could be corrected by stretching the time-scale of potentiated responses according to the percent change in the decay time constant. The LTP associated decrease in the decay time constant also obtained in slices from immature hippocampus which contain spines and dendrites greatly simplified from those of the adult. Hence, filtering properties of spines are not likely involved in the effect. Paired-pulse facilitation (PPF), a transient increase in transmitter release, did not reproduce the waveform effects of LTP but did cause a slight leftward shift of the response. These results suggest that LTP modifies the kinetics of receptor channels, and that PPF accelerates release.