Glutamate Receptor Channels Research Articles

Excitatory amino acid-induced currents in rat septal cholinergic neurons in culture.

Whole-cell voltage-clamp recordings were used to study excitatory amino acid-induced currents in neurons isolated from the septum of fetal rat brains. The neurons were cultured for more than four weeks on a feeder layer composed of glial cells obtained from the septal region. Septal neurons were either fusiform, triangular or multipolar and 83% of cells showed acetylcholinesterase activity. L-Glutamate, kainate, quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) applied by local perfusion produced inward currents (Iglu, Ikai, Iquis and IAMPA, respectively) at -44mV which increased in amplitude with increasing concentration of agonist; they desensitized when induced at higher concentrations except for the Ikai. The EC50s for the peak Ikai and sustained Iglu, Iquis and IAMPA were 55, 13, 0.39 and 3.5 microM, respectively. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) depressed Ikai and IAMPA evoked at a concentration of 10 microM (IC50s: 0.58 and 0.84 microM, respectively). Schild analysis for the CNQX action on Ikai gave a dissociation constant of 0.27 microM for CNQX. n-Methyl-D-aspartate (NMDA) (with glycine, 3 microM) produced an inward current (INMDA) at -44 mV whose peak amplitude enhanced with increased concentrations (EC50 = 32 microM). INMDA was potentiated by glycine (EC50 = 0.15 microM) and inhibited by D-2-amino-5-phosphovalerate (IC50 = 9.9 microM for INMDA evoked at a concentration of 50 microM). MK-801 (0.1-10 microM) inhibited INMDA in a dose- and use-dependent manner. INMDA was (0.1-10 microM) inhibited INMDA in a dose- and use-dependent manner. INMDA was potentiated by spermine (EC50 = 247 microM; 91% increase at 1mM) in a manner independent of holding potential (VH). INMDA was inhibited by Mg2+ and Zn2+ (IC50 = 673 and 39 microM, respectively, at -44 mV) in a manner dependent on VH; the magnitudes of a depolarization required for an e-fold increase in their IC50s in a range of -64 to -24 mV were 16 and 22 mV, respectively. The action of Zn2+ was independent of VH > -24 mV. Current-voltage relations for Ikai, Iquis and IAMPA exhibited outward rectification, while that of INMDA showed a region of negative conductance at VH < -30 mV, which disappeared in a Mg(2+)-free solution. Reversal potentials for Ikai, Iquis, IAMPA and INMDA were close to 0 mV, indicating the involvement of non-specific cation channels. Increasing extracellular Ca2+ concentration from 2.4 to 30 mM did not affect the Ikai and Iquis, reversal potential showing negligible Ca2+ component, but shifted INMDA reversal potential to a more positive potential, yielding a ratio of Ca2+ permeability to that of monovalent cation to be 13. Cholinergic septal neurons in culture express non-NMDA-(AMPA/kainate-) and NMDA-type of glutamate receptor channels. Their properties were quantitatively similar to those of glutamate receptor channels on other types of neurons in the brain except for the actions of endogenous neuromodulators (Mg2+, Zn2+ and spermine) on NMDA receptor channels. It is suggested that NMDA receptor channels on different types of neurons may play a distinct role depending on a difference in the actions of these neuromodulators.

Molecular mechanisms controlling calcium entry through AMPA-type glutamate receptor channels

Molecular mechanisms controlling calcium entry through AMPA-type glutamate receptor channels

Evidence for glutamate‐mediated activation of hippocampal neurons by glial calcium waves

Communication from astrocytes to neurons has recently been reported by two laboratories, but different mechanisms were though to underlie glial calcium wave activation of associated neurons. Neuronal calcium elevation by glia observed in the present report is similar to that reported previously, where an increase in neuronal calcium was demonstrated in response to glial stimulation. In the present study hippocampal neurons plated on a confluent glial monolayer displayed a transient increase in intracellular calcium following a short delay after the passage of a wave of increased calcium in underlying glia. Activated cells displayed action potentials in response to glial waves and showed antineurofilament immunoreactivity. Finally, the N-methyl-D-aspartate glutamate receptor antagonist DL-2-amino-5-phosphonovaleric acid and the non-NMDA glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione significantly reduced the responsiveness of neurons to glial calcium waves. Our results indicate that hippocampal neurons growing on hippocampal or cortical astrocytes respond to glial calcium waves with elevations in calcium and increased electrical activity. Furthermore, we show that in most cases this communication appears to be mediated by ionotropic glutamate receptor channels.

The double-ticker: an improved fast drug-application system reveals desensitization of the glutamate channel from a closed state.

The present study describes a modification of the fast drug-application technique (ticker) which combines two fast-application systems, 'the double-ticker'. With the double-ticker, drugs can be applied to excised patches from either one of the tickers permitting switching among three different solutions in the sub-millisecond range. We made use of this advantageous feature of the double-ticker to study two aspects of the glutamate receptor channel in crayfish muscle. The first concerns revealing the number of glutamate binding sites from measurements of a dose-response relation (2-3 sites). The other relates to the state from which the receptor undergoes desensitization. For the quisqualate-sensitive glutamate receptor desensitization occurs from a closed state. This is in addition to desensitization from an open state.

Glial cells in the mouse hippocampus express AMPA receptors with an intermediate Ca2+ permeability.

Recently, we could demonstrate the 'complex' glial cells in mouse hippocampal slices express glutamate receptor changes of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate subtypes. In the present study, we further characterized this glial receptor. Since voltage-clamp control is imperfect and diffusion barriers hinders the quantitative analysis of the receptor currents in situ, the patch-clamp technique was applied to glial cells acutely isolated from the mouse hippocampal CA1 stratum radiatum subregion. A concentration-clamp technique was used which enabled very fast exchange of the extracellular solutions. Thus, it was possible to characterize the transient receptor currents with high time resolution. Application of L-glutamate, AMPA and L-homocysteate induced rapidly activating and fast desensitizing receptor currents in the suspended glial cells. In contrast, kainate induced non-desensitizing currents. The corresponding dose-response curve revealed a half-maximum of current activation at 350 microM. The current/voltage relationship of the kainate-evoked response was linear with a reversal potential at approximately 9 mV. Analysis of the reversal potential in solutions containing high concentrations of CaCl2 confirmed earlier in situ data by demonstrating significant Ca2+ permeability of the glial glutamate receptor channels in the hippocampus. The kainate-induced receptor currents were markedly increased by cyclothiazide, a substance which selectively potentiates glutamate receptors of the AMPA subtype. We conclude that glial cells of the juvenile hippocampus mainly express heteromeric high-affinity AMPA receptors. Most probably, the receptor channels are assembled from the low Ca(2+)-permeable glutamate receptor-2 subunit together with Ca(2+)-permeable AMPA-preferring subunits.

Fractional calcium currents through recombinant GluR channels of the NMDA, AMPA and kainate receptor subtypes.

1. Simultaneous fluorescence and whole-cell current measurements using the calcium indicator dye fura-2 were made in HEK 293 cells expressing recombinant glutamate receptor (GluR) channels, and fractional Ca2+ currents (the proportion of whole-cell current carried by Ca2+; Pf) were determined. 2. Cells expressing N-methyl-D-aspartate receptor (NMDAR) channels showed glutamate activated Ca2+ inflow in the voltage range -60 to 40 mV in normal extracellular solution. Ca2+ inflow decreased in a voltage-dependent manner at membrane potentials more negative than -30 mV due to Mg2+ block. Voltage dependence of block at negative potentials was stronger in cells expressing the NR1-NR2A as compared with cells expressing NR1-NR2C subunits. 3. Fractional Ca2+ currents through NMDARs were independent of extracellular Mg2+ and varied between 8.2% (NR1-NR2C subunits) and 11% (NR1-NR2A subunits) in normal extracellular solution (1.8 mM Ca2+) at -60 mV membrane potential. Pf values increased with increasing [Ca2+]o in the range of 0.5-10mM [Ca2+]o in a saturating fashion. 4. In cells expressing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) subunits which were unedited at the Q/R site of the putative transmembrane segment TM2 (Q-form), or in cells coexpressing unedited and edited subunits (R-form), the glutamate-evoked Ca2+ inflow increased from 20 to -80 mV in an almost linear way. 5. Fractional Ca2+ currents through AMPAR channels depended on subunit composition. Pf values of Q-form homomeric channels at -60 mV and 1.8 mM [Ca2+]o were between 3.2 and 3.9%. They were slightly voltage dependent and increased with [Ca2+]o in the range 1.8-10mM. Pf values in cells co-expressing Q- and R-form subunits were almost one order of magnitude smaller (0.54%). 6. Relative concentrations of Q-form and R-form GluR-B subunit-specific cDNAs used for cell transfection determined the expression of functionally different heteromeric AMPARS. Pf decreased with increasing relative concentration of R-form encoding CDNAs from 3.4 to 1.4%, demonstrating that editing of the Q/R site of GluR-B subunits decreases Ca2+ inflow through heteromeric AMPARs. 7. Cells expressing the GluR-6 subunit of the kainate receptor (KAR) family were characterized by Pf values which depended on the editing in the TM1 and TM2 segments. Pf values were largest for the Q-form (1.55-2.0%) and lowest for R-form channels (< 0.2%), suggesting that Q/R site editing also decreases Ca2+ inflow through KAR channels. Cells co-expressing both subunit forms showed an intermediate value (0.58%).(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-permeable AMPA and NMDA receptor channels in basket cells of rat hippocampal dentate gyrus.

1. Glutamate receptor (GluR) channels were studied in basket cells in the dentate gyrus of rat hippocampal slices. Basket cells were identified by their location, dendritic morphology and high frequency of action potentials generated during sustained current injection. 2. Dual-component currents were activated by fast application of glutamate to outside-out membrane patches isolated from basket cell somata (10 microM glycine, no external Mg2+). The fast component was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the slow component by D-2-amino-5-phosphonopentanoic acid (D-AP5). This suggests that the two components were mediated by alpha-amino-3- hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR)/kainate receptor and N-methyl-D-aspartate receptor (NMDAR) channels, respectively. The mean ratio of the peak current of the NMDAR component to that of the AMPAR/kainate receptor component was 0.22 (1 ms pulses of 10 mM glutamate). 3. The AMPAR/kainate receptor component, which was studied in isolation in the presence of D-AP5, was identified as AMPAR mediated on the basis of the preferential activation by AMPA as compared with kainate, the weak desensitization of kainate-activated currents, the cross-desensitization between AMPA and kainate, and the reduction of desensitization by cyclothiazide. 4. Deactivation of basket cell AMPARs following 1 ms pulses of glutamate occurred with a time constant (tau) of 1.2 +/- 0.1 ms (mean +/- S.E.M.). During 100 ms glutamate pulses AMPARs desensitized with a tau of 3.7 +/- 0.2ms. 5. The peak current-voltage (I-V) relation of AMPAR-mediated currents in Na(+)-rich extracellular solution showed a reversal potential of -4.0 +/- 2.6 mV and was characterized by a a doubly rectifying shape. The conductance of single AMPAR channels was estimated as 22.6 +/- 1.6 pS using non-stationary fluctuation analysis. AMPARs expressed in hippocampal basket cells were highly Ca2+ permeable (PCa/PK = 1.79). 6. NMDARs in hippocampal basket cells were studied in isolation in the presence of CNQX. Deactivation of NMDARs activated by glutamate pulses occurred bi-exponentially with mean tau values of 266 +/- 23 ms (76%) and 2620 +/- 383 ms (24%). 7. The peak I-V relation of the NMDAR-mediated component in Na(+)-rich extracellular solution showed a reversal potential of 1.5 +/- 0.6 mV and a region of negative slope at negative membrane potentials in the presence of external Mg2+, due to voltage-dependent block by these ions. The conductance of single NMDAR channels in the main open state was 50.2 +/- 1.8 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular Cloning and Differential Expression Patterns of Avian Glutamate Receptor mRNAs

We have identified and studied potential ionotropic glutamate receptor genes in pigeon brain. Three cDNA clones exhibit significant amino acid sequence identity to members of a rodent ligand-gated ion channel family. One of them, GluP-II, encodes a full-length AMPA-sensitive glutamate receptor GluR2 (GluR-B) homologue, whereas the other two partial clones, designated as GluP-III and -IV, are nearly identical to rodent GluR3 (GluR-C) and GluR4 (GluR-D) receptor subunits. Northern analysis demonstrated that the avian genes are widely expressed in the brain. Within the brain regions analyzed by in situ hybridization histochemistry, the three avian GluR subunits showed distinct and regionally specific mRNA expression patterns in the adult. Most of the differences in their expression were observed in cell types of the telencephalon, certain thalamic nuclei, the optic tectum, and the cerebellar cortex. A particularly striking finding was the expression of GluP-II in Golgi epithelial/Bergmann glial cells. In contrast, Bergmann glial cells in rat cerebellum do not express GluR2 (GluR-B) subunit genes. Immunoreactivity for a monoclonal sequence-specific antipeptide antibody was widespread and most prominent in Purkinje cell perikarya and their dendrites, neuronal cell bodies of the ectostriatum, and the deep optic tectum. These results demonstrate the existence of multiple subunits of the ionotropic glutamate receptor channel family in avians. Excitatory amino acid receptor genes appear to be highly conserved during evolution.

125I]epidepride binding in post mortem brain of schizophrenic subjects

Exciting new milestones in glutamate receptor (GluR) channel research include the following: the cloning of N-methyl-d-aspartate (NMDA) receptors; delineation of molecular determinants for ion flow through glutamate-gated channels; the discovery that Ca2+ permeability of non-NMDA receptor channels is determined by RNA editing; the construction of antibodies and their use in immunocytochemical localizations of α-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor subunits in the rat brain; and the return to prominence of the high-affinity kainate site with the publication of cDNA sequences for subunits (GluR-5, -6, -7; KA-1, -2) constituting subtypes of this site. Major unresolved issues comprise the transmembrane topology and subunit stoichiometries of native receptor channels.

Impairment of motor coordination, Purkinje cell synapse formation, and cerebellar long-term depression in GluRδ2 mutant mice

Of the six glutamate receptor (GIuR) channel subunit families identified by molecular cloning, five have been shown to constitute either the AMPA, kainate, or NMDA receptor channel, whereas the function of the δ subunit family remains unknown. The selective localization of the δ2 subunit of the GluRδ subfamily in cerebellar Purkinje cells prompted us to examine its possible physiological roles by the gene targeting technique. Analyses of the GluRδ mutant mice reveal that the δ2 subunit plays important roles in motor coordination, formation of parallel fiber-Purkinje cell synapses and climbing fiber-Purkinje cell synapses, and long-term depression of parallel fiber-Purkinje cell synaptic transmission. These results suggest a close relationship between synaptic plasticity and synapse formation in the cerebellum.

Editing of α-Amino-3-hydroxy-5-methylisoxazole-4-propionic Acid Receptor GluR-B Pre-mRNA in Vitro Reveals Site-selective Adenosine to Inosine Conversion

In neurons of the mammalian brain primary transcripts of genes encoding subunits of glutamate receptor channels can undergo RNA editing, leading to altered properties of the transmitter-activated channel. Editing of these transcripts is a nuclear process that targets specific adenosines and requires a double-stranded RNA structure configured from complementary exonic and intronic sequences. We show here that the two independent editing sites in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR-B pre-mRNA are edited with positional accuracy by nuclear extract from HeLa cells. Nucleotide analysis by thin layer chromatography of the edited RNA sequences revealed selective adenosine to inosine conversion, most likely reflecting the participation of double-stranded RNA adenosine deaminase. Our results predict the presence of inosine-containing codons in other mammalian mRNAs.

Polyamine regulation of N-methyl-D-aspartate receptor channels.

Endogenous polyamines such as spermine and spermidine have multiple effects in the central nervous system and have been suggested to be neurotransmitters or neuromodulators. One effect of the polyamines is to regulate the activity of the N-methyl-D-aspartate receptor (NMDAR) channel subtype of glutamate receptor channels. The effects of polyamines on NMDAR currents are complex, suggesting the presence of one or more polyamine-binding sites on the receptor channel. Electrophysiological studies have shown that polyamines enhance NMDAR currents by increasing channel opening frequency and by increasing the affinity of the receptor for glycine. Polyamines have been shown to reduce NMDAR currents by producing voltage-dependent reduction of single-channel amplitudes and/or by producing an open channel block. Recent molecular biological studies have shown that the polyamine effects on NMDAR channels involve interactions with multiple NMDAR subunits and are characterizing the structural basis for the polyamine regulation of NMDAT receptor channels.

N‐Linked Glycosylation of the α‐Amino‐3‐Hydroxy‐5‐Methylisoxazole‐4‐Propionate(AMPA)‐Selective Glutamate Receptor Channel α2 Subunit Is Essential for the Acquisition of Ligand‐Binding Activity

The N-linked glycosylation of the alpha 2 subunit of the mouse alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-selective glutamate receptor (GluR) channel was characterized. The receptor subunit protein has five putative N-glycosylation sites. The recombinant receptor proteins were identified by [35S]methionine/[35S]cysteine metabolic labeling, western blot analysis, immunocytochemical detection, and [3H]AMPA binding experiments when expressed in insect Spodoptera frugiperda cells using a baculovirus system. The effect of tunicamycin on the metabolic labeling and immunoblots suggested that the two products, a major protein species of approximately 102 kDa and a minor species of approximately 98 kDa, correspond to glycosylated and unglycosylated forms, respectively, which was also supported by the enzymic deglycosylation experiments. Immunofluorescence staining of tunicamycin-treated cells expressing only the unglycosylated form differed little from that of tunicamycin-nontreated cells expressing both glycosylated and unglycosylated forms. The lack of AMPA-binding activity of the unglycosylated form expressed in the presence of tunicamycin suggested that N-glycosylation is required, directly or indirectly, for functional expression in insect cells for ligand binding. These results demonstrate that occupancy of at least one N-glycosylation site is required for the formation and maintenance of the GluR alpha 2 subunit protein in an active conformation for ligand binding. Possible roles of N-glycosylation of GluR alpha 2 subunit protein are discussed.

Effects of propofol on various AMPA-, kainate- and NMDA-selective glutamate receptor channels expressed in Xenopus oocytes

Effects of a general intravenous anesthetic 2,6-diisopropylphenol (propofol) on various glutamate receptor (GluR) channels were examined on the α1 and α1 α2 GluR channels selective for α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), the β2 γ2 GluR channels selective for kainate, and the ε2 χ1 and ε3 χ1 and ϵ3 χ1 N-methyl- d-aspartate (NMDA) receptor channels expressed in Xenopus oocytes. Propofol suppressed the current responses of the α1 α2 , β2 γ2 , ε2 χ1 and ε3 χ1 channels in a dose-dependent manner, whereas it enhanced the current responses of the α1 channel. The extents of inhibition were in the order ε2/χ > ε3/χ1 > β2/γ2 > α1/α2 channels. During perfusion of 500 μM propofol, the α1 α2 , β2 γ2 , ϵ2 χ1 and ε3 χ1 channels were progressively suppressed. Furthermore, 10 min perfusion of 20 μM propofol inhibited the ε2 χ1 channel by 24%. These results suggest that clinical concentrations (∼35 μM) of propofol suppress the NMDA receptor channels slightly.

Light- and electron-microscopic localization of the glutamate receptor channel 62 subunit in the mouse Purkinje cell

The localization of the glutamate receptor channel δ2 subunit was investigated by immunohistochemistry. The δ2-immunoreactivity was observed exclusively in the molecular layer of the cerebellar cortex. The electron microscopic analysis showed that the δ2 subunit was localized in the dendritic spines of the Purkinje cells. The immunopositive spines often formed synaptic contacts with parallel fiber varicosities. As for the Purkinje cells ectopically localized in the cerebellar nuclei and brain stem, the dendritic shafts and cell bodies were strongly labeled. These ectopic Purkinje cells also formed asymmetrical synapses at the δ2-immunopositive dendritic spines. Considering the specific localization of the δ2 subunit in the postsynaptic site of the Purkinje cells, the subunit is suggested to be involved in the excitatory synaptic transmission in the cells, as a component of the glutamate receptor channel.

Inhibition by suramin and reactive blue 2 of GABA and glutamate receptor channels in rat hippocampal neurons

Effects of suramin and reactive blue 2 (RB2), compounds known as antagonists at P2-purinoceptors, on ionic currents mediated through GABA and glutamate receptor channels were investigated in rat hippocampal neurons. Under whole-cell voltage-clamp, suramin (30 to 300 microM) and RB2 (10 to 100 microM) inhibited a current activated by 10 microM GABA in a concentration-dependent manner. Suramin (100 and 300 microM) and RB2 (10 and 30 microM) also inhibited an inward current activated by kainic acid (100 microM), an agonist at non-NMDA type glutamate receptor channels, and an inward current activated by N-methyl-D-aspartate (NMDA; 100 microM), an agonist at NMDA type glutamate receptor channels. The inhibition by suramin or RB2 did not exhibit voltage-dependence between -30 and -90 mV in the case of the GABA- or the kainate-evoked current. In contrast, the inhibition by these compounds of the NMDA-evoked current exhibited voltage-dependence and was enhanced by hyperpolarization. When the concentration of agonists was increased by 5- or 10-fold, the magnitude of the inhibition by suramin of the kainate-evoked current and the magnitude of the inhibition by RB2 of the NMDA-evoked current were attenuated. alpha,beta-Methylene ATP (100 microM) did not affect the GABA-, kainate- or NMDA-activated current. The results suggest that suramin and RB2 inhibit GABA receptor channels and glutamate receptor channels. The blockade of these channels must be taken into account when these compounds are used as pharmacological tools to examine an involvement of P2-purinoceptors, especially in preparations where GABAergic or glutamatergic neurotransmission is expected.

Single glutamate channels in CA1 pyramidal neurones after transient ischaemia.

Patch clamp recordings were made from CA1 pyramidal neurones to study changes in the glutamate receptor subtypes in the gerbil hippocampus after transient ischaemia. In whole-cell recordings, the maximum chord conductances of AMPA currents in ischaemic neurones were increased over those of control neurones but NMDA-induced currents in the ischaemic neurones were smaller than the control. In AMPA-activated single channel currents, an open time histogram of the control neurones was well fitted by a single exponential function whereas in the ischaemic patches it was fitted by a double exponential function, indicating that currents consisted of at least two kinetically different types. These functional changes of the glutamate receptor channels may contribute to the abnormalities of the excitatory synaptic currents recorded in post-ischaemic CA1 neurones.

Suppression of LTD in cultured Purkinje cells deficient in the glutamate receptor delta 2 subunit.

Involvement of the glutamate receptor channel delta 2 subunit in cerebellar long-term depression (LTD) was studied in cultures prepared from wild-type and mutant mice deficient in the delta 2 subunit. LTD of the glutamate response was induced by pairing glutamate applications and depolarization of a Purkinje cell in wild-type culture. However, in cultured Purkinje cells prepared from mutant mice, the same conditioning failed to induce LTD. Immunocytological staining showed that mutant Purkinje cells develop and express calbindin (a marker protein for Purkinje cells) as do wild-type cells, but they express no delta 2 subunit protein. The results indicate that the glutamate receptor channel delta 2 subunit is involved in the postsynaptic down-regulation of glutamate sensitivity, presumably during cerebellar LTD.

Dendritic glutamate receptor channels in rat hippocampal CA3 and CA1 pyramidal neurons.

1. Properties of dendritic glutamate receptor (GluR) channels were investigated using fast application of glutamate to outside-out membrane patches isolated from the apical dendrites of CA3 and CA1 pyramidal neurons in rat hippocampal slices. CA3 patches were formed (15-76 microns from the soma) in the region of mossy fibre (MF) synapses, and CA1 patches (25-174 microns from the soma) in the region of Schaffer collateral (SC) innervation. 2. Dual-component responses consisting of a rapidly rising and decaying component followed by a second, substantially slower, component were elicited by 1 ms pulses of 1 mM glutamate in the presence of 10 microM glycine and absence of external Mg2+. The fast component was selectively blocked by 2-5 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the slow component by 30 microM D-2-amino-5-phosphonopentanoic acid (D-AP5), suggesting that the fast and slow components were mediated by the GluR channels of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and NMDA type, respectively. The peak amplitude ratio of the NMDA to AMPA receptor-mediated components varied between 0.03 and 0.62 in patches from both CA3 and CA1 dendrites. Patches lacking either component were rarely observed. 3. The peak current-voltage (I-V) relationship of the fast component was almost linear, whereas the I-V relationship of the slow component showed a region of negative slope in the presence of 1 mM external Mg2+. The reversal potential for both components was close to 0 mV. 4. Kainate-preferring GluR channels did not contribute appreciably to the response to glutamate. The responses to 100 ms pulses of 1 mM glutamate were mimicked by application of 1 mM AMPA, whereas 1 mM kainate produced much smaller, weakly desensitizing currents. This suggests that the fast component is primarily mediated by the action of glutamate on AMPA-preferring receptors. 5. The mean elementary conductance of AMPA receptor channels was about 10 pS, as estimated by non-stationary fluctuation analysis. The permeability of these channels to Ca2+ was low (approximately 5% of the permeability to Cs+). 6. The elementary conductance of NMDA receptor channels was larger, with a main conductance state of about 45 pS. These channels were 3.6 times more permeable to Ca2+ than to Cs+.(ABSTRACT TRUNCATED AT 400 WORDS)

P2-33 - Suramin and reactive blue 2, P2-purinoceptor antagonists, block GABA and glutamate receptor channels in rat hippocampal neurons

P2-33 - Suramin and reactive blue 2, P2-purinoceptor antagonists, block GABA and glutamate receptor channels in rat hippocampal neurons