Noise and single channels activated by excitatory amino acids in rat cerebellar granule neurones

Activation and desensitization of hippocampal kainate receptors

We have used whole-cell recordings and rapid agonist applications to characterize the physiological properties of kainate receptors expressed by rat hippocampal neurons in dissociated cell culture. Activation of NMDA and AMPA receptors was prevented by inclusion of the noncompetitive antagonists MK-801 (2 microM) and GYKI 53655 (100 microM), respectively. In the presence of these inhibitors, both kainate (EC50 = 23 microM) and glutamate (EC50 = 310 microM) evoked desensitizing currents. Maximal peak currents for kainate with GYKI 53655 were 15 +/- 3% as large as in control solutions without GYKI. In contrast to currents mediated by AMPA receptors, kainate currents recorded in GYKI were blocked potently by lanthanum (IC50 = 2 microM) and were desensitized by 1 microM 2S,4R-4-methylglutamate (SYM 2081). Coapplication of either 5 microM AMPA or 500 microM aspartate had little effect on responses to kainate, although AMPA alone elicited current at 1 mM. In most cells, the currents evoked by kainate, glutamate, and SYM 2081 varied linearly with membrane potential and reversed near 0 mV. Kainate elicited substantial current at steady state (approximately 30% of peak), whereas responses to glutamate and SYM 2081 desensitized almost completely within 0.2-2 sec. Inhibition produced by a 10 sec desensitizing prepulse was half-maximal at 0.22 microM for SYM 2081 and 13 microM for glutamate. Recovery from desensitization to kainate and glutamate was >80% complete within 60 sec but was three- to fourfold slower after exposure to SYM 2081. Exposure to Concanavalin A blocked desensitization of the currents but also reduced the peak current amplitudes. Collectively, these results confirm that kainate-preferring receptors underlie the currents evoked by kainate, glutamate, or SYM-2081 in the presence of GYKI 53655; they are not mediated by electrogenic transport or by AMPA-preferring receptors that are insensitive to GYKI. In contrast to previous work on embryonic hippocampal neurons, our results show that the properties of kainate receptors expressed by cells from older animals are distinct from those displayed by homomeric assemblies of the GluR6 subunit.

Activation and desensitization of hippocampal kainate receptors

We have used whole-cell recordings and rapid agonist applications to characterize the physiological properties of kainate receptors expressed by rat hippocampal neurons in dissociated cell culture. Activation of NMDA and AMPA receptors was prevented by inclusion of the noncompetitive antagonists MK-801 (2 microM) and GYKI 53655 (100 microM), respectively. In the presence of these inhibitors, both kainate (EC50 = 23 microM) and glutamate (EC50 = 310 microM) evoked desensitizing currents. Maximal peak currents for kainate with GYKI 53655 were 15 +/- 3% as large as in control solutions without GYKI. In contrast to currents mediated by AMPA receptors, kainate currents recorded in GYKI were blocked potently by lanthanum (IC50 = 2 microM) and were desensitized by 1 microM 2S,4R-4-methylglutamate (SYM 2081). Coapplication of either 5 microM AMPA or 500 microM aspartate had little effect on responses to kainate, although AMPA alone elicited current at 1 mM. In most cells, the currents evoked by kainate, glutamate, and SYM 2081 varied linearly with membrane potential and reversed near 0 mV. Kainate elicited substantial current at steady state (approximately 30% of peak), whereas responses to glutamate and SYM 2081 desensitized almost completely within 0.2-2 sec. Inhibition produced by a 10 sec desensitizing prepulse was half-maximal at 0.22 microM for SYM 2081 and 13 microM for glutamate. Recovery from desensitization to kainate and glutamate was >80% complete within 60 sec but was three- to fourfold slower after exposure to SYM 2081. Exposure to Concanavalin A blocked desensitization of the currents but also reduced the peak current amplitudes. Collectively, these results confirm that kainate-preferring receptors underlie the currents evoked by kainate, glutamate, or SYM-2081 in the presence of GYKI 53655; they are not mediated by electrogenic transport or by AMPA-preferring receptors that are insensitive to GYKI. In contrast to previous work on embryonic hippocampal neurons, our results show that the properties of kainate receptors expressed by cells from older animals are distinct from those displayed by homomeric assemblies of the GluR6 subunit.

Insulin-like growth factor and potassium depolarization maintain neuronal survival by distinct pathways: possible involvement of PI 3-kinase in IGF-1 signaling

Cultured cerebellar granule neurons die by apoptosis when switched from a medium containing an elevated level of potassium (K+) to one with lower K+ (5 mM). Death resulting from the lowering of K+ can be prevented by insulin-like growth factor (IGF-1). To understand how IGF-1 inhibits apoptosis and maintains neuronal survival, we examined the role of phosphoinositide 3-kinase (PI 3-kinase). Activation of PI 3-kinase has been shown previously to be required for NGF-mediated survival in the PC12 pheochromocytoma cell line. We find that in primary neurons, IGF-1 treatment leads to a robust activation of PI 3-kinase, as judged by lipid kinase assays and Western blot analysis. Activation of PI 3-kinase is likely to occur via tyrosine phosphorylation of the insulin receptor substrate protein. Treatment with two chemically distinct inhibitors of PI 3-kinase, wortmannin and LY294002, reduces PI 3-kinase activation by IGF-1 and inhibits its survival-promoting activity, suggesting that PI 3-kinase is necessary for IGF-1-mediated survival. Death resulting from PI 3-kinase blockade is accompanied by DNA fragmentation, a hallmark of apoptosis. Furthermore, neurons subjected to PI 3-kinase blockade can be rescued by transcriptional and translation inhibitors, suggesting that IGF-1-mediated activation of PI 3-kinase leads to a suppression of "killer gene" expression. In sharp contrast to IGF-1, elevated K+ does not activate PI 3-kinase and can maintain neuronal survival in the presence of PI 3-kinase inhibitors. Therefore, survival of granule neurons can be maintained by PI 3-kinase dependent (IGF-1-activated) and independent (elevated K+-activated) pathways.

Insulin-like growth factor and potassium depolarization maintain neuronal survival by distinct pathways: possible involvement of PI 3-kinase in IGF-1 signaling

Cultured cerebellar granule neurons die by apoptosis when switched from a medium containing an elevated level of potassium (K+) to one with lower K+ (5 mM). Death resulting from the lowering of K+ can be prevented by insulin-like growth factor (IGF-1). To understand how IGF-1 inhibits apoptosis and maintains neuronal survival, we examined the role of phosphoinositide 3-kinase (PI 3-kinase). Activation of PI 3-kinase has been shown previously to be required for NGF-mediated survival in the PC12 pheochromocytoma cell line. We find that in primary neurons, IGF-1 treatment leads to a robust activation of PI 3-kinase, as judged by lipid kinase assays and Western blot analysis. Activation of PI 3-kinase is likely to occur via tyrosine phosphorylation of the insulin receptor substrate protein. Treatment with two chemically distinct inhibitors of PI 3-kinase, wortmannin and LY294002, reduces PI 3-kinase activation by IGF-1 and inhibits its survival-promoting activity, suggesting that PI 3-kinase is necessary for IGF-1-mediated survival. Death resulting from PI 3-kinase blockade is accompanied by DNA fragmentation, a hallmark of apoptosis. Furthermore, neurons subjected to PI 3-kinase blockade can be rescued by transcriptional and translation inhibitors, suggesting that IGF-1-mediated activation of PI 3-kinase leads to a suppression of "killer gene" expression. In sharp contrast to IGF-1, elevated K+ does not activate PI 3-kinase and can maintain neuronal survival in the presence of PI 3-kinase inhibitors. Therefore, survival of granule neurons can be maintained by PI 3-kinase dependent (IGF-1-activated) and independent (elevated K+-activated) pathways.

Single-channel properties of recombinant AMPA receptors depend on RNA editing, splice variation, and subunit composition

Non-NMDA glutamate receptor subunits of the AMPA-preferring subfamily combine to form ion channels with heterogeneous functional properties. We have investigated the effects of RNA editing at the Q/R site, splice variation of the "flip/flop" cassette, and multimeric subunit assembly on the single-channel conductance and kinetic properties of the recombinant AMPA receptors formed from GluR2 and GluR4 expressed in HEK 293 cells. We found that AMPA receptor single-channel conductance was dependent on the Q/R site editing state of the subunits comprising the channel. Calcium-permeable (unedited) channels had resolvable single-channel events with main conductance states of 7-8 pS, whereas fully edited GluR2 channels had very low conductances of approximately 300 fS (estimated from noise analysis). Additionally, the flip splice variant of GluR4 conferred agonist-dependent conductance properties reminiscent of those found for a subset of AMPA receptors in cultured cerebellar granule cells. These results provide a description of the single-channel properties of certain recombinant AMPA receptors and suggest that the single-channel conductance may be determined by the expression of edited GluR2 subunits in neurons.

Single-channel properties of recombinant AMPA receptors depend on RNA editing, splice variation, and subunit composition

Non-NMDA glutamate receptor subunits of the AMPA-preferring subfamily combine to form ion channels with heterogeneous functional properties. We have investigated the effects of RNA editing at the Q/R site, splice variation of the "flip/flop" cassette, and multimeric subunit assembly on the single-channel conductance and kinetic properties of the recombinant AMPA receptors formed from GluR2 and GluR4 expressed in HEK 293 cells. We found that AMPA receptor single-channel conductance was dependent on the Q/R site editing state of the subunits comprising the channel. Calcium-permeable (unedited) channels had resolvable single-channel events with main conductance states of 7-8 pS, whereas fully edited GluR2 channels had very low conductances of approximately 300 fS (estimated from noise analysis). Additionally, the flip splice variant of GluR4 conferred agonist-dependent conductance properties reminiscent of those found for a subset of AMPA receptors in cultured cerebellar granule cells. These results provide a description of the single-channel properties of certain recombinant AMPA receptors and suggest that the single-channel conductance may be determined by the expression of edited GluR2 subunits in neurons.

Embryonic and postnatal expression of four gamma-aminobutyric acid transporter mRNAs in the mouse brain and leptomeninges

The distribution of gamma-aminobutyric acid (GABA) transporter mRNAs (mGATs) was studied in mouse brain during embryonic and postnatal development using in situ hybridization with radiolabeled oligonucleotide probes. Mouse GATs 1 and 4 were present in the ventricular and subventricular zones of the lateral ventricle from gestational day 13. During postnatal development, mGAT1 mRNA was distributed diffusely throughout the brain and spinal cord, with the highest expression present in the olfactory bulbs, hippocampus, and cerebellar cortex. The mGAT4 message was densely distributed throughout the central nervous system during postnatal week 1; however, the hybridization signal in the cerebral cortex and hippocampus decreased during postnatal weeks 2 and 3, and in adults, mGAT4 labeling was restricted largely to the olfactory bulbs, midbrain, deep cerebellar nuclei, medulla, and spinal cord. Mouse GAT2 mRNA was expressed only in proliferating and migrating cerebellar granule cells, whereas mGAT3 mRNA was absent from the brain and spinal cord throughout development. Each of the four mGATs was present to some degree in the leptomeninges. The expression of mGATs 2 and 3 was almost entirely restricted to the pia-arachnoid, whereas mGATs 1 and 4 were present only in specific regions of the membrane. Although mGATs 1 and 4 may subserve the classical purpose of terminating inhibitory GABAergic transmission through neuronal and glial uptake mechanisms, GABA transporters in the pia-arachnoid may help to regulate the amount of GABA available to proliferating and migrating neurons at the sub-pial surface during perinatal development.

Proton inhibition of the NMDA-gated channel in isolated catfish cone horizontal cells

The effect of H+ on the N-methyl-D-aspartate-induced (NMDA) membrane current in enzymatically isolated catfish cone horizontal cells was investigated. Extracellular acidification to pH 5.5 blocked nearly completely the NMDA-induced current and reduced desensitization. The pK for the H+ effect was 6.5, near that for the free amino acid histidine. Protons did not alter the receptor affinity for NMDA and the inhibition was insensitive to the membrane potential and surface charge screening. However, extracellular H+ increased the IC50 for Zn2+. These results indicate that protons can modulate the NMDA-induced membrane current by a mechanism that may include interaction with histidine residues.

Deactivation and desensitization of non-NMDA receptors in patches and the time course of EPSCs in rat cerebellar granule cells

1. Spontaneous and evoked non-NMDA receptor-mediated EPSCs were recorded from cerebellar granule cells in slices at approximately 24 and approximately 34 degrees C. The EPSC decay was fitted with the sum of two exponential functions. 2. The time courses of non-NMDA receptor deactivation and desensitization were determined with fast concentration jumps of glutamate onto patches from cultured granule cells. Deactivation (decay time constant tau = 0.6 ms at 24 degrees C) was substantially faster than desensitization (tau = 4 ms). Both processes were fitted by single exponential functions. 3. The decay of the fast component of the spontaneous EPSC (tau EPSCfast = 0.9 ms at 23 degrees C) was marginally slower than deactivation but too fast to be determined by desensitization. Our results suggest that the decay of this component is set by both the rate of decline of transmitter concentration and channel deactivation. 4. A simple diffusion model predicts that the time course of transmitter in the cleft declines slowly during the later stages of its action. The slow phase of transmitter removal could account for the time course of the slow component of the spontaneous EPSC (tau EPSCslow = 8 ms at 23 degrees C).

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.

Reopening of single L-type Ca2+ channels in mouse cerebellar granule cells: dependence on voltage and ion concentration

1. We recorded the activity of single L-type Ca2+ channels from cell-attached patches on mouse cerebellar granule cells. The experiments investigated the mechanism of channel reopening at negative membrane potentials following a strong depolarization. 2. L-type channels that reopened following a strong depolarization showed a wide distribution of single-channel conductances, which ranged from 16 to 28 pS in the presence of 90 mM Ba2+. 3. The distribution of the latencies before reopening was fitted as the sum of two exponential components with time constants tau f approximately 1 and tau s approximately 12 ms at -70 mV. Hyperpolarization reduced the time constant of the slower component approximately e-fold per 43 mV, but had no effect on the faster component. 4. Raising the concentration of external Ba2+ reduced the time constant of the slower component of the reopening latency without altering the fast component. The time constant of the slow component was approximately 27 ms in 10 mM Ba2+ and decreased to 12 ms in 90 mM Ba2+ at -70 mV. The relation between the time constant and external Ba2+ saturated with an apparent KD of approximately 20 mM. 5. The distribution of reopening times was best fitted as the sum of two exponential components with time constants tau f approximately 0.5 ms and tau s approximately 4.5 ms at -70 mV. The conditional latencies before reopening into either the short or long open state were indistinguishable. 6. The results are consistent with the idea that a positively charged blocker occludes the pore during depolarization and channels reopen as the blocker dissociates following repolarization to negative potentials.

Mechanisms for synchronous calcium oscillations in cultured rat cerebellar neurons

Removal of Mg2+ caused oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential in cultured cerebellar granule neurons. Oscillations of [Ca2+]i were synchronous in all the cells, and were restricted to the neurons (immunocytochemically identified) that responded to exogenous N-methyl-D-aspartate (NMDA). Oscillations were blocked by Ca2+ removal, nickel, NMDA receptor antagonists, omega-agatoxin IVA, tetrodotoxin, sodium removal and gamma-aminobutyric acid, but not by dihydropyridines, omega-conotoxin M VIIA or by emptying the intracellular Ca2+ stores with thapsigargin or ionomycin. The upstroke of the [Ca2+]i oscillations coincided in time with an increase in manganese permeability of the plasma membrane. Propagation of the [Ca2+]i wave followed more than one pathway and the spatiotemporal pattern changed with time. Membrane potential oscillations consisted of transient slow depolarizations of approximately 20 mV with faster phasic activity superimposed. We propose that the synchronous [Ca2+]i oscillations are the expression of irradiation of random excitation through a neuronal network requiring generation of action potentials and functional glutamatergic synapses. Oscillations of -Ca2+-i are due to cyclic Ca2+ entry through NMDA receptor channels activated by synaptic release of glutamate, which requires Ca2+ entry through P-type Ca2+ channels activated by action potentials at the presynaptic terminal.

The Physiological Society proceedings of the scientific meeting held at University College London, 16-18 April 1996. Abstracts

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Pentobarbitone modulation of NMDA receptors in neurones isolated from the rat olfactory brain

1. The action of pentobarbitone on the N-methyl-D-aspartate (NMDA) receptors of neurones freshly dissociated from the olfactory bulb and olfactory tubercle has been studied using patch-clamp techniques. 2. Pentobarbitone produced a concentration-dependent depression of the currents evoked by NMDA with an IC50 value of c. 250 microM. 3. Analysis of the NMDA-evoked noise produced power spectra that could be fitted by the sum of two Lorentzians with corner frequencies of 17 and 82 Hz. Pentobarbitone increased the corner frequency of the high frequency component but did not alter the apparent single channel conductance estimated from the noise. 4. Single channel recordings in either the cell-attached or outside-out patch configurations revealed that NMDA (20 or 50 microM) opened channels with a main conductance level around 55 pS and a principal subconductance around 44 pS. The uncorrected mean open time of the channels was 3.4 ms and mean burst length was 6.0 ms. Mean cluster length was about 12 ms. 5. Pentobarbitone produced a concentration-dependent reduction in both mean open time and burst length. Mean cluster length was much less affected. Pentobarbitone did not decrease unitary current amplitude or bias the open-state current amplitude distribution in favour of a particular substate. 6. From these data it appears that pentobarbitone depresses the inward current evoked by NMDA by reducing the probability of channel opening and this results from a shortening of the lifetime of the channel open state and by decreasing burst length.

Low nanomolar serotonin inhibits the glutamate receptor/nitric oxide/cyclic GMP pathway in slices from adult rat cerebellum

The function of serotonin afferents to the cerebellum has been investigated by monitoring the effects of serotoninergic drugs on the production of cyclic GMP elicited in cerebellar slices by activation of ionotropic glutamate receptors. Exposure of adult rat cerebellar slices to N-methyl-D-aspartate (1 nM to 1 microM) or to (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 1 nM to 10 microM) elicited concentration-dependent and saturable rises in the levels of cyclic GMP. These responses were blocked by selective antagonists at the N-methyl-D-aspartate or AMPA receptors and by inhibiting nitric oxide synthase, but were insensitive to tetrodotoxin. When tested between 0.1 and 10 nM, serotonin, the serotonin1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin and the serotonin2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane inhibited, concentration-dependently, the cyclic GMP responses evoked by near-maximal (0.1 microM) concentrations of N-methyl-D-aspartate or AMPA. The EC50 values (concentrations causing half-maximal effect) ranged between 0.7 and 2.1 nM. The actions of serotonin were totally abolished by methiothepin, a mixed-type serotonin receptor antagonist. Thus, the serotonergic cerebellar afferents may exert a potent inhibitory control on the excitatory transmission mediated by N-methyl-D-aspartate and AMPA receptors; the inhibition occurs through both serotonin1A and serotonin2 receptors. As the glutamate receptor-dependent cyclic GMP responses involve production of nitric oxide, a diffusible activator of guanylate cyclase, the above inhibitory serotonin receptors may have multiple localization.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular spermine confers rectification on rat calcium-permeable AMPA and kainate receptors

1. Whole-cell recordings were made from cerebellar granule cells cultured in high-K+ medium to induce expression of Ca(2+)-permeable AMPA receptors. Current-voltage (I-V) plots of agonist-evoked responses showed varying degrees of inward rectification, but became linear within 5-10 min. 2. Recombinant Ca(2+)-permeable kainate receptors, composed of GluR6(Q)/KA-2 subunits, exhibited rectifying whole-cell I-V plots that became linear in outside-out patches. 3. Loss of rectification in granule cells was prevented by including 100 microM spermine in the pipette; the degree of rectification was then correlated with Ca2+ permeability. 4. Spermine also prevented loss of rectification in patches containing GluR6(Q)/KA-2 receptors (IC50, 1.7 microM). 5. We suggest that spermine, or a similar cellular constituent, may act as a cytoplasmic factor conferring inward rectification on Ca(2+)-permeable non-NMDA receptors, and that 'washout' of this factor underlies the observed loss of rectification.

Effects of protein kinase C modulation on NMDA receptor mediated regulation of neurotransmitter enzyme and c-fos protein in cultured neurons

The role of protein kinase C (PKC) in N-methyl-D-aspartate (NMDA) receptor-mediated biochemical differentiation and c-fos protein expression was investigated in cultured cerebellar granule neurons. The biochemical differentiation of glutamatergic granule cells was studied in terms of the specific activity of phosphate-activated glutaminase, an enzyme treatment in the synthesis of the putative neurotransmitter pool of glutamate. When the partially depolarized cells were treated with NMDA for the last 1 to 3 days (between 2 and 5 days in vitro), it elevated the specific activity of glutaminase. In contrast, NMDA had little effect on the activity of aspartate aminotransferase or of lactate dehydrogenase. Treatment of 10-day old granule neurons with NMDA also resulted in a marked increase in the immunocytochemically measured expression of c-fos protein. The increases in both the activity of glutaminase and the steady state level of c-fos protein were specific to the activation of NMDA receptors, as they were completely blocked by D,L-2-amino-5-phosphonovaleric acid. The specific stimulation of NMDA receptors in PKC-depleted granule neurons or in the presence of reasonably specific PKC inhibitors also produced significant elevation in the activity of glutaminase and the expression of c-fos protein. These increases were similar in magnitude to those observed in the granule neurons of the respective control groups. Our findings demonstrate that PKC is not directly involved in the NMDA receptor-mediated signal transduction processes associated with biochemical differentiation and c-fos induction in cerebellar granule neurons.

An electrophysiological investigation of the properties of a murine recombinant 5-HT3 receptor stably expressed in HEK 293 cells

1. The pharmacological and biophysical properties of a recombinant 5-HT3 receptor have been studied by use of patch-clamp techniques applied to HEK 293 cells stably transfected with the murine 5-HT3 R-A cDNA. 2. At a holding potential of -60 mV, 77% of cells investigated responded to ionophoretically applied 5-HT with an inward current. Such currents were unaffected by methysergide (1 microM), or ketanserin (1 microM), but were antagonized in a concentration-dependent and reversible manner by the selective 5-HT3 receptor antagonist, ondansetron (IC50 = 440 pM) and the non-selective antagonists (+)-tubocurarine (IC50 = 1.8 nM) and metoclopramide (IC50 50 nM). 3. The 5-HT-induced current reversed in sign (E5-HT) at approximately -2mV and exhibited inward rectification. The influence of extra- and intracellular ion substitutions upon E5-HT indicates the 5-HT-evoked current to be mainly mediated by a mixed monovalent cation conductance. 4. Calcium and magnesium (0.1-10 nM) produced a concentration-dependent, voltage-independent, inhibition of the 5-HT-induced response. Zinc (0.3-300 microM) exerted a biphasic effect with low concentrations enhancing, and high concentrations depressing, the 5-HT-evoked current. 5. Fluctuation analysis of inward currents evoked by a low (1 microM) concentration of 5-HT suggests the current to be mediated by the opening of channels with a conductance of 420 fS. 6. The pharmacological and biophysical properties of the 5-HT3 R-A are similar to those previously described for 5-HT3 receptors native to murine neuroblastoma cell lines, with the exception that the function of the recombinant receptor was enhanced by low concentrations of zinc. This observation suggests that the properties of the native receptor are not completely represented by the 5-HT3 R-A subunit alone.

Dimensions of the narrow portion of a recombinant NMDA receptor channel

Glutamate-activated single-channel and ensemble currents were recorded from Xenopus laevis oocytes and HEK 293 cells expressing a recombinant NMDA receptor, assembled from NR1 and NR2A subunits. Cesium was the main charge carrier, and organic cations were used to determine the presence of vestibules of this channel and to estimate its pore diameter. The large organic cations tris-(hydroxymethyl)-aminomethane (Tris), N-methyl-glucamine (NMG), arginine (NMG), arginine (Arg), choline, and tetramethylammonium (TMA), when added in millimolar concentrations to the extracellular or cytoplasmic side, produced a voltage-dependent blockade of single-channel Cs+ currents. These molecules behaved as impermeant ions that only partially traverse the channel from either side. The smaller cations trimethylammonium (TriMA) and dimethylammonium (DMA) produced a small and nearly voltage-independent reduction in current amplitude, suggesting that they are permeant. In biionic experiments with Cs+ as the reference ion, the large blocking cations NMG, Arg, Tris, TMA, choline, hexamethonium (Hme), triethylammonium (TriEA), and tetraethylammonium (TEA) showed no measurable permeability. TriMA and smaller ammonium derivatives were permeant. Both the permeability and single-channel conductance of organic cations, relative to Cs+, decreased as the ion size increased. The results suggest that the NMDA receptor has extracellular and cytoplasmic mouths that can accommodate large cations up to 7.3 A in mean diameter. The narrow portion of the pore is estimated to have a mean diameter of 5.5 A.

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)

Chronic mild acidosis specifically reduces functional expression of N-methyl-D-aspartate receptors and increases long-term survival in primary cultures of cerebellar granule cells

Previous studies suggest that chronic depolarization by addition of 25 mM KCl or N-methyl-D-aspartate to primary cultures of cerebellar granule cells promotes expression of the N-methyl-D-aspartate subtype of glutamate receptor, as determined by electrophysiological responsiveness and susceptibility to excitotoxicity. Recent studies have demonstrated that acute mild acidosis reduces N-methyl-D-aspartate receptor channel activity by a non-competitive action of H+ on an extracellular site of the receptor channel complex. Since the level of N-methyl-D-aspartate receptor expression in granule cell cultures is activity-dependent, we examined whether chronic mildly acidotic culture conditions would selectively diminish the level of N-methyl-D-aspartate responsiveness in granule cells, in effect producing a functional level of expression more comparable to that observed in vivo. To test this, cerebellar granule cells from eight-day neonatal rats were grown in an HCO3-buffered medium containing elevated K+ (25 mM KCl) either under standard conditions (95% air/5% CO2, pH 7.4), or under chronic mildly acidotic conditions (90% air/10% CO2, estimated pH of 7.1). Glutamate receptor subtype expression was subsequently assessed using standard neurotoxicity assays, a quantitative immunoblotting assay for N-methyl-D-aspartate receptors and whole cell patch clamp recordings. Cells grown in the 10% CO2 environment exhibited a significant reduction in susceptibility to L-glutamate neurotoxicity (at least 10-fold), but not kainate-induced neurotoxicity, relative to cells grown in 5% CO2. In both culture conditions, L-glutamate- and kainate-induced toxicity were mediated by activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, respectively, as determined by the sensitivity of agonist-induced toxicity to specific receptor antagonists. Using polyclonal antibodies generated against a peptide sequence recognizing five of eight splice variants in the common "R1" subunit of N-methyl-D-aspartate receptors, a 31% reduction in the amount of immunoreactive protein was observed in membrane preparations from cells grown in 10% CO2, relative to the amount detected in cells grown in 5% CO2. Moreover, perfusion of cells with glutamate (50 microM) in a nominally Mg(2+)-free solution containing glycine (2 microM) elicited N-methyl-D-aspartate antagonist-sensitive inward currents in proportionately fewer cells cultured in 10% CO2, relative to cells cultured in 5% CO2. Long-term survival was also significantly enhanced in cells exposed chronically to mild acidotic culture conditions, relative to cells grown under standard pH conditions (22 days, 10% CO2 vs 16 days, 5% CO2).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Targeted disruption of NMDA receptor 1 gene abolishes NMDA response and results in neonatal death

In vitro studies have suggested that the NMDA receptor consists of an essential subunit, NR1, and various modulatory NR2 subunits. To test this hypothesis directly in vivo, we generated mice carrying a disrupted NR1 allele. NMDA-inducible increases in intracellular calcium and membrane currents were abolished in neurons from homozygous null mutants (NR1-/-). Thus, NR1 has a unique role, which cannot be substituted by any other subunit, in determining the activity of the endogenous NMDA receptor. A concomitant reduction in levels of NR2B but not NR2A occurred in NR1-/- mice, demonstrating that there is an interdependence of subunit expression. NR1-/- mice died 8-15 hr after birth, indicating a vital neonatal function for the NMDA receptor. Although the NMDA receptor has been implicated in several aspects of neurodevelopment, overall neuroanatomy of NR1-/- mice appeared normal. Pathological evidence suggested that respiratory failure was the ultimate cause of death.

Activation of glutamate receptors in response to membrane depolarization of hair cells isolated from chick cochlea

1. Experiments were performed to identify the neurotransmitter released from hair cells of chick cochlea. An isolated hair cell was closely apposed to a cultured granule cell of the rat cerebellum, and both cells were whole-cell voltage clamped by utilizing a nystatin perforated patch technique. 2. Depolarization of hair cells to potentials more positive than -20 mV induced currents in the granule cell in a 10 mM Ca2+ extracellular medium. Amplitudes of induced currents were dependent on the membrane potential of granule cells and showed an outward-going rectification. The induced current in granule cells was reversibly suppressed by a local application of 2-amino-5-phosphonovalerate (APV), which indicates that the current was generated through the activation of an NMDA subtype of the glutamate receptor expressed on the granule cell. 3. The current amplitude of the granule cell was dependent on the size of hair cell depolarization. The size of current induced in a granule cell held at +55 mV was progressively increased with hair cell depolarization from -20 to +10 mV. At more positive potentials, the current amplitude was decreased. This voltage dependence was similar to but did not exactly match that of Ca2+ currents in the hair cell. The granule cell current appeared at more positive membrane potentials than the Ca2+ current in hair cells. 4. When intracellular Ca2+ concentration was increased by UV irradiation of the hair cell loaded with a caged Ca2+ compound, nitr-5, the closely apposed granule cell generated an outward current when voltage clamped at +55 mV. 5. These observations (paragraphs 2-4) imply that the most likely neurotransmitter released from the hair cell at its synapse with the afferent nerve terminal is glutamate.

Patch-clamp analysis of direct steroidal modulation of glutamate receptor-channels

Steroid hormones regulate the neuroendocrine and behavioral functions of the brain by using a number of diverse cellular mechanisms. Many steroids exert rapid electrophysiological effects on neurons, involving specific interactions with membrane components, such as neurotransmitter receptors. Previous studies suggest that the steroids, estrogen and pregnenolone sulfate (PS), might directly modulate glutamate receptors. The present experiments utilized patch-clamp recording of glutamate receptor-channels in excised membrane patches to test for direct modulation by these steroids. Characteristic single-channel activity from N-methyl-D-aspartate (NMDA) receptors could be elicited in both inside-out and outside-out patches excised from acutely dissociated hippocampal neurons. PS, but not 17 beta-estradiol, increased the open probability of NMDA channel activity in inside-out and outside-out patches. The PS-induced increase in open probability could be attributed to an increase in both frequency of opening and mean open time of the NMDA receptor, though the effect on frequency of opening was more prominent. The non-NMDA agonist, kainate, induced continuous shifts and increased noise in the baseline current of outside-out patches, but rarely activated clearly resolvable single-channel openings. 17 beta-estradiol and PS had no apparent effect on the kainate-induced currents. These findings suggest that some steroids can directly modulate glutamate receptors, but other steroids may utilize indirect mechanisms for regulating glutamatergic synaptic transmission.

Arachidonic acid depresses non-NMDA receptor currents

Arachidonic acid has been proposed as an intercellular messenger in the nervous system. It is released when glutamate acts on postsynaptic receptors, potentiates NMDA receptor currents and depresses glutamate uptake. Here we report the effects of arachidonic acid on non-NMDA receptor currents, studied by whole-cell clamping isolated neurons and neurons in tissue slices. In cultured cerebellar granule cells and in freshly isolated hippocampal pyramidal cells arachidonic acid decreased the current produced by iontophoresed AMPA. This depression was not due to increased desensitization of the AMPA receptor. In cerebellar slices, arachidonic acid depressed the non-NMDA component of the synaptic current at the mossy fibre to granule cell and the parallel fibre to Purkinje cell synapses. However, this depression was not always seen, possibly because the lipophilic arachidonic acid is absorbed by superficial cells in the slice and does not reach the synapse being studied. Depression of non-NMDA receptor currents by arachidonic acid may reflect the presence of an arachidonic acid binding site on the non-NMDA receptor, but non-NMDA receptor subunits show much less sequence homology with fatty acid binding proteins than does the NMDA receptor.

Voltage-dependent kinetics of N-methyl-D-aspartate synaptic currents in rat cerebellar granule cells

Decay kinetics of N-methyl-D-aspartate excitatory postsynaptic currents (NMDA-EPSCs) have been voltage-dependent in some, but not all neurons studied so far, and almost no information has been available on the voltage-dependence of the rising phase. In this work we investigated the effect of membrane potential on rising and decay kinetics of the NMDA-EPSC in cerebellar granule cells using the tight-seal whole-cell recording technique. NMDA-EPSCs were evoked by electrical mossy fibre stimulation in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione, 1.2 mM Mg2+ and 5 microM glycine. The rate of rise of NMDA-EPSCs remained substantially unchanged when the cell was depolarized, indicating that the limiting step of channel opening was voltage-insensitive. The NMDA-EPSC, however, flattened around the peak and the time-to-peak increased. This observation was explained by the influence of decay. Decay was biphasic and slowed down with membrane depolarization. Moreover, the fast component of decay increased less than the slow component. This complex voltage-dependence may extend the integrative role of the NMDA current during synaptic transmission.

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)

Potentiation of N-methyl-D-aspartate-evoked elevation of intracellular Ca2+ concentrations by exogenous glycine in cerebellar granule cells

The effect of glycine on the intracellular free Ca2+ concentration ([Ca2+])i response to N-methyl-D-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca2+]i plateau, which was abolished by removal of extracellular Ca2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca2+]i response to NMDA by exogenous glycine.

Fibrous and protoplasmic astrocytes express GABAA receptors that differ in benzodiazepine pharmacology

Astrocytes cultured from spinal cord contain two morphologically distinguishable types of astrocytes: fibrous and protoplasmic cells. Both astrocyte subtypes, in culture, are able to express GABAA receptors, and their activation results in inward currents at the resting potential. Using patch-clamp electrophysiology we characterized their basic receptor pharmacology and compared it to spinal cord neurons that were also present in small numbers in these cultures. As in neuronal GABAA receptors, the local anesthetic pentobarbital effectively potentiated GABA-induced currents in both astrocyte subtypes. Similarly, the benzodiazepine diazepam, on average doubled GABA-induced currents in both astrocytes subtypes. In contrast to these effects that were similar in both astrocytes types and similar to spinal cord neurons, the response to the convulsant methyl-4-ethyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM), which is an inverse benzodiazepine agonist differs between astrocyte subtypes. DMCM reduced GABA-induced currents by about 50% in fibrous astrocytes as we also observed with spinal cord neurons. In contrast, DMCM increased GABA currents in protoplasmic astrocytes by up to 150%, an effect never observed in neurons. DMCM potentiations of GABA currents have recently been attributed to differences in receptor subunit composition. Our results thus indicate that subtypes of astrocytes express GABAA receptors that differ pharmacologically and likely differ also in subunit composition.

Induction of apoptosis in cerebellar granule neurons by low potassium: inhibition of death by insulin-like growth factor I and cAMP

High levels of extracellular K+ ensure proper development and prolong survival of cerebellar granule neurons in culture. We find that when switched from a culture medium containing high K+ (25 mM) to one containing a low but more physiological K+ concentration (5 mM), differentiated granule neurons degenerate and die. Death induced by low K+ is due to apoptosis (programmed cell death), a form of cell death observed extensively in the developing nervous system and believed to be necessary for proper neurogenesis. The death process is accompanied by cleavage of genomic DNA into internucleosome-sized fragments, a hallmark of apoptosis. Inhibitors of transcription and translation suppress apoptosis induced by low K+, suggesting the necessity for newly synthesized gene products for activation of the process. Death can be prevented by insulin-like growth factor I but not by several other growth/neurotrophic factors. cAMP but not the protein kinase C activator phorbol 12-myristate 13-acetate can also support survival in low K+. In view of the large numbers of granule neurons that can be homogeneously cultured, our results offer the prospect of an excellent model system to study the mechanisms underlying apoptosis in the central nervous system and the suppression of this process by survival factors such as insulin-like growth factor I.

Sodium, calcium and late potassium currents are reduced in cerebellar granule cells cultured in the presence of a protein complex conferring resistance to excitatory amino acids

Whole-cell, patch-clamp recordings were used to study voltage-gated currents generated by cerebellar granule cells that were cultured in medium containing either 10% fetal calf serum (hereafter termed S + granules) or neurite outgrowth and adhesion complex (NOAC, hereafter called NOAC granules). NOAC is a protein complex found in rabbit serum that renders granules resistant to the excitotoxic action of excitatory amino acids. During depolarizing commands both S+ and NOAC granules generated Na+ and Ca2+ inward currents and an early and a late K+ outward currents. However, Na+ and Ca2+ inward currents and late outward K+ currents recorded in NOAC granules were smaller than those seen in S+ granules. Furthermore, although of similar amplitude, early K+ currents displayed different kinetics in the two types of neurons. Thus, these data demonstrate that the electrophysiological properties of cerebellar granules, and probably of other neuronal populations, depend upon serum components and raise the possibility that an analogous modulation might be operative in vivo, and play a role in development, synaptic plasticity or neuropathological processes.

Preventing errors when estimating single channel properties from the analysis of current fluctuations

The conductance, number, and mean open time of ion channels can be estimated from fluctuations in membrane current. To examine potential errors associated with fluctuation analysis, we simulated ensemble currents and estimated single channel properties. The number (N) and amplitude (i) of the underlying single channels were estimated using nonstationary fluctuation analysis, while mean open time was estimated using covariance and spectral analysis. Both excessive filtering and the analysis of segments of current that were too brief led to underestimates of i and overestimates of N. Setting the low-pass cut-off frequency of the filter to greater than five times the inverse of the effective mean channel open time (burst duration) and analyzing segments of current that were at least 80 times the effective mean channel open time reduced the errors to < 2%. With excessive filtering, Butterworth filtering gave up to 10% less error in estimating i and N than Bessel filtering. Estimates of mean open time obtained from the time constant of decay of the covariance, tau obs, at low open probabilities (Po) were much less sensitive to filtering than estimates of i and N. Extrapolating plots of tau obs versus mean current to the ordinate provided a method to estimate mean open time from data obtained at higher Po, where tau obs no longer represents mean open time. Bessel filtering gave the least error when estimating tau obs from the decay of the covariance function, and Butterworth filtering gave the least error when estimating tau obs from spectral density functions.

A parametric modeling of ionic channel current fluctuations using third-order statistics and its application to estimation of the kinetic parameters of single ionic channels

A parametric modeling of stationary ionic-channel current fluctuations (SICF's) using third-order cumulants is presented and its application to estimation of the kinetic parameters of single ionic channels is discussed. We consider the case where third-order cumulants of SICF's are nonzero, and where SICF's are corrupted by an unobservable additive colored Gaussian noise that is independent of SICF's. First, we construct a virtual synthesizer that yields an output whose third-order cumulants are equivalent to those of SICF's on a specific slice. The synthesizer output is expressed by the sum of N5 - 1 first-order differential equation systems, where N8 denotes the number of states of single ionic channels. Next, discretizing the synthesizer output, we derive a discrete autoregressive (AR(N8 - 1)) process driven by the sum of N8 - 1 moving average (MA(N9 - 2)) processes. Then the AR coefficients are explicitly related to the kinetic parameters of single ionic channels, implying that the kinetic parameters can be estimated by identifying the ARMA coefficients using the third-order cumulants. In order to assess the validity of the proposed modeling and the accuracy of parameter estimates, Monte Carlo simulation is carried out in which the closed-open and closed-open-blocked schemes are treated as specific examples.

Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse

We have analyzed the variance associated with the decay of the non-NMDA receptor component of synaptic currents, recorded from mossy fiber-granule cell synapses in cerebellar slices, to obtain a conductance estimate for the synaptic channel. Current fluctuations arising from the random channel gating properties were separated from those arising from the fluctuations in the population of channels by subtracting the mean excitatory postsynaptic current (EPSC) waveform scaled to the EPSC peak amplitude. A weighted mean single-channel conductance of approximately 20 pS was determined from the relationship between the mean current and the variance around the mean during the decay of evoked and spontaneous synaptic currents. This result suggests that high conductance non-NMDA channels, such as the 10-30 pS glutamate receptor channel previously characterized in granule cells, carry the majority of the fast component of the EPSC at this synapse. In addition, our data are consistent with the activation of surprisingly few (approximately 10) non-NMDA channels by a single packet of transmitter.

Evidence for more than one type of non-NMDA receptor in outside-out patches from cerebellar granule cells of the rat

1. Application of non-NMDA (non-N-methyl-D-aspartate) receptor agonists onto outside-out patches of cerebellar granule cells gave two characteristic types of response (in different patches) which we have referred to as 'high conductance' and 'low conductance' responses. At a qualitative level these patches could be readily distinguished by the size of the noise increase accompanying their membrane currents. 2. In high conductance patches both AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate gave discrete single-channel conductances (10-30 pS), while in low conductance patches, AMPA produced small discrete events (6-10 pS), and kainate opened channels with conductances too small to be directly resolved. All patches examined contained NMDA receptor channels with characteristic 50 and 40 pS conductance levels. 3. Despite the marked differences in single-channel conductances, kainate dose-response curves constructed for high and low conductance patches had similar EC50 values of approximately 150 microM. 4. Spectral analysis of low conductance kainate responses gave an estimated channel conductance of approximately 1.5 pS. In these same low conductance patches AMPA produced discrete openings with two conductance levels; their mean conductances (and relative proportions) were 6 (87%) and 10 pS (13%). 5. In high conductance patches, glutamate (10-30 microM), AMPA (3-10 microM), and kainate (10-30 microM), each activated non-NMDA channels with three multiple conductance levels. The amplitudes of these conductance levels (approximately 10, 20 and 30 pS) were similar for each of the agonists, and their relative proportions (i.e. areas in the amplitude histograms) were constant for all three agonists. In addition, the relative proportion of levels was constant between patches, and all three levels were invariably present. These observations are all consistent with the idea that the three multiple conductances originate from a single receptor channel, activated by AMPA, kainate and glutamate. 6. Non-NMDA single-channel current-voltage (I-V) plots showed outward rectification in high conductance patches. For all three multiple conductance levels the ratio of outward to inward single-channel slope conductance was 1.8 +/- 0.1 and this rectification remained present in symmetrical Na+ solutions. 7. In high conductance patches, the events produced by a rapid application of 20-50 microM glutamate were compared with those activated during steady-state application.(ABSTRACT TRUNCATED AT 400 WORDS)

Different proportions of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor currents at the mossy fibre-granule cell synapse of developing rat cerebellum

The mossy fibre-granule cell synapse undergoes major developmental changes during the second and third weeks after birth. We investigated synaptic transmission during postnatal days 10-22 by means of whole-cell patch-clamp recordings from granule cells in situ. Parasagittal slices were cut from rat cerebellar vermis, and excitatory postsynaptic currents were evoked in granule cells by mossy fibre stimulation with 1.2 mM Mg++ in the extracellular solution. In the majority of granule cells recorded at postnatal days 16-22, excitatory currents were characterized by a fast initial peak followed by a slower component, while in many of the cells recorded at more immature stages, the fast peak was virtually absent. Pharmacological and kinetic data indicated that the fast and slow components were mediated by non-N-methyl-D-aspartate and N-methyl-D-aspartate receptor activation, respectively. The magnitude of the non-N-methyl-D-aspartate current increased with developmental age, while the magnitude of the NMDA current did not change markedly. The age-dependent change of the non-N-methyl-D-aspartate currents could not be accounted for by changes in recording conditions or granule cell electrotonic properties. Furthermore, from postnatal day 11 to 16 the extent of Mg++ block on the N-methyl-D-aspartate receptor did not change, and could not explain the increasing non-N-methyl-D-aspartate/N-methyl-D-aspartate current ratio. We concluded therefore that the age-dependent increase of the non-N-methyl-D-aspartate current was the main cause of the different postsynaptic current waveforms observed at different ages. The developmental change in the proportion of N-methyl-D-aspartate and non-N-methyl-D-aspartate currents may be relevant to the processes regulating granule cell maturation and excitability.

Single channel properties at the synaptic site

The properties of single channels activated during spontaneous postsynaptic currents in small cultured rat hippocampal neurons were investigated in low-noise whole cell recordings. The technique of nonstationary fluctuation analysis, which has previously been applied to sodium currents, was modified so that fluctuations were measured around the least-squares scaled fit of the ensemble average to individual synaptic currents. This had the effect of separating channel gating fluctuations from the quantal fluctuations of scale from event to event. Single channel amplitude was estimated from the variance--current distribution, and the kinetics of channel gating fluctuations were studied. Channels involved in the non-N-methyl-D-aspartate (non-NMDA) phase of the excitatory glutamatergic postsynaptic current showed a single channel amplitude of 1.5 pS, while those in the NMDA phase had a conductance of 42 pS. The single channel conductance estimated for the inhibitory chloride synaptic current was 14 pS. In addition, NMDA phase channel openings could be resolved directly in the whole cell current against the low noise level afforded by the small cells. Single channel lifetime and amplitude distributions of the channels activated during the postsynaptic current were measured, and confirmed the accuracy of the fluctuation method.

Effect of magnesium on calcium influx activated by glutamate and its agonists in cultured cerebellar granule cells

The effects of Mg2+ on the glutamate-, kainate-, N-methyl-D-aspartate- and quisqualate-induced influx of 45Ca2+ were studied in cultured cerebellar granule cells. The N-methyl-D-aspartate- and quisqualate-evoked influx was totally and the kainate- and glutamate-evoked influx partially blocked in 1.3 mM extracellular Mg2+. The increase in influx induced by kainate, quisqualate and glutamate was maximal at 0.1 mM Mg2+, whereas N-methyl-D-aspartate was most effective in totally Mg(2+)-free media. D-2-Amino-5-phosphonovalerate blocked partially and phencyclidine completely the enhancement of Ca2+ influx by 1 mM quisqualate in 0.1-mM Mg2+ medium. The effect of 10 microM quisqualate was also significantly inhibited by antagonists specific for different glutamate receptor subtypes, including N-methyl-D-aspartate, (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and metabotropic receptors. This evidences a heterogeneous action of quisqualate, mediated by different glutamate receptor subtypes in 0.1 mM Mg2+ medium. The efficacy of quisqualate in inducing influx of Ca2+ and the selectivity of antagonists for different receptors are also modified by extracellular Mg2+.

Regulation of neurotransmitter enzyme by quisqualate subtype glutamate receptors in cultured cerebellar and hippocampal neurons

The possible involvement of ionotropic and metabotropic quisqualate (QA) receptors in neuronal plasticity was studied in cultured glutamatergic cerebellar or hippocampal cells in terms of the specific activity of phosphate-activated glutaminase, an enzyme important in the synthesis of the putative neurotransmitter pool of glutamate. When cerebellar or hippocampal neurons were treated with QA, it elevated the specific activity of glutaminase in a dose-dependent manner. The half-maximal effect was obtained at about 0.1 microM, the maximum increase was at about 1 microM, but levels higher than 10 microM QA produced progressive reduction in glutaminase activity. In contrast, QA had little effects on the activities of lactate dehydrogenase and aspartate aminotransferase and the amount of protein, indicating that the increase in glutaminase was relatively specific. The QA-mediated increase in glutaminase was mimicked by the ionotropic QA receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; EC50, about 0.5 microM), but not by the metabotropic QA receptor agonist trans-(+-)-1-amino-cyclopentyl-1,3,dicarboxylate (t-ACPD; up to 0.5 mM). The specific ionotropic QA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited QA- and AMPA-mediated increases in glutaminase activity in a dose-dependent manner, whereas other glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate, gamma-D-glutamyl aminomethyl sulphonic acid and gamma-D-glutamyl diethyl ester were ineffective. The elevation of neurotransmitter enzyme was Ca(2+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.

Glutamate receptor channels in isolated patches from CA1 and CA3 pyramidal cells of rat hippocampal slices

1. Currents activated by glutamate receptor (GluR) agonists were recorded from outside-out patches isolated from the soma of visually identified pyramidal neurones of the CA3 and CA1 region of rat hippocampal slices. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), L-glutamate (L-Glu), and kainate (KA) were delivered either by bath application through perfusion of the recording chamber or by rapid application via a piezo-driven two-barrelled fast application system. 2. Bath application of each of the three agonists activated inward currents in all patches (n = 134) at holding potentials of -50 or -60 mV. The current amplitude increased in size between 3 to 30 microM-AMPA and 100 microM to 1 mM-KA. With this slow mode of bath application, the responses showed no apparent desensitization even at saturating concentrations of AMPA (30 microM) and KA (1 mM). 3. The ratio of currents activated by 30 microM-AMPA and 300 microM-KA showed a characteristic difference between CA3 and CA1 neurones. The ratio was 0.242 +/- 0.028 (mean +/- S.E.M., n = 16) for CA3 cell patches and 0.097 +/- 0.012 (n = 8) for CA1 cell patches indicating that GluRs in the two cell populations are different. 4. The steady-state current-voltage relations (I-Vs) for AMPA- and KA-activated currents showed pronounced outward rectification for both cell types (when the main cations are Na+ in the bath and Cs+ in the pipette solution). The current reversed close to 0 mV and the ratio of chord conductances 80 mV on either side of the reversal potential was 2.66 for KA-activated currents in CA3 cell patches and 2.60 in CA1 cell patches. AMPA-activated currents showed a time-dependent increase after steps to positive membrane potentials and a decrease after steps to negative voltages, indicating that a gating process is responsible for outward rectification of the steady-state I-V. 5. The permeability (P) of GluR channels was high for Na+ as compared to Cs+ for both cell types (PNa/PCs = 0.88 and 0.84). The permeability was low for N-methyl-D-glucamine+ (PNMG/PCs < or = 0.03) and Ca2+ (PCa/PCs < or = 0.05). 6. The current noise level increased during application of AMPA or KA. Apparent single-channel conductances obtained from fluctuation analysis were higher for AMPA than for KA, but similar for both cell types. In CA3 cell patches, AMPA activated channels with an apparent chord conductance of 7.2 pS, KA of 3.0 pS conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of glutamate agonist-induced influx of calcium into neurons by gamma-L-glutamyl and beta-L-aspartyl dipeptides

Gamma-L-Glutamate and beta-L-aspartate dipeptides, present in the mammalian brain with a yet unknown function, were shown to affect the influx of Ca2+ into cultured cerebellar granule cells. The most active peptides, gamma-L-glutamyl-L-aspartate, gamma-L-glutamyl-L-glutamate and gamma-L-glutamylglycine, enhanced the basal influx but inhibited the glutamate-activated influx of Ca2+ in a dose-dependent manner. Gamma-L-Glutamyl-L-aspartate, the strongest inhibitor of the glutamate-activated influx of Ca2+, exhibited selective Mg(2+)-dependent antagonism in the N-methyl-D-aspartate (NMDA)-activated influx of Ca2+. This finding may explain its previously shown deleterious effects on the long-term memory. On the other hand, gamma-L-glutamyl-L-aspartate enhanced alone the entry of Ca2+ into neurons. This effect was antagonized by the non-NMDA antagonists 6-nitro-7-cyanoquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX), suggesting a non-NMDA receptor-mediated action, that may also be involved in excitotoxicity in some neurodegenerative disorders.

Single-channel currents of NMDA type activated by L- and D-homocysteic acid in cerebellar granule cells in culture

The whole-cell and the outside-out configurations of the patch-clamp technique were used to study macroscopic and single-channel currents evoked by the enantiomers of homocysteic acid, the sulphur containing analogue of glutamate, in cerebellar granule cells in culture. L-Homocysteic acid (L-HC, 15 microM) and D-homocysteic acid (D-HC, 50 microM) induced whole-cell currents of comparable amplitude, that were abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (AP-5) (20 microM). AMPA (100 microM) induced whole-cell currents which were not modified by AP-5 (20 microM) but were blocked by CNQX. In the outside-out configuration, both L-HC and D-HC (15 microM) elicited single-channel currents of the same conductance, mean open time and reversal potential as the NMDA-induced single-channel events. In the presence of Mg2+ (2 mM), D- and L-HC-induced single-channel currents were voltage-dependent. These data suggest that in cerebellar granule cells in culture, both L-HC and D-HC activate the same NMDA receptor channel complex.

Noise of secretagogue-induced inward currents dependent on extracellular calcium in rat mast cells

We analyzed the noise of the inward currents induced by stimulation of rat peritoneal mast cells with compound 48/80 (48/80), a secretagogue, and examined the role of extracellular Ca2+ in generation of the large noise. In the presence of 2 mM Ca2+ in the external solution, the power density spectra of the 48/80-induced inward currents in most cells were fitted with the sum of two Lorentzian functions. The cut-off frequencies (fc) at -50 mV for the low and high frequency components were 16.3 +/- 7.3 (n = 10) and 180 +/- 95 (n = 9) Hz. Involvement of a cation-selective channel in the large noise was identified in some cells, but the single channel current amplitude estimated from parameters of the noise varied among cells (0.20-2.47 pA at -50 mV), thereby indicating that the currents were mediated by more than two classes of channel. The low frequency component of the 48/80-induced currents was suppressed by lowering the extracellular Ca2+ concentration to 1 microM with the addition of EGTA, without appreciable changes in the high frequency component. When the extracellular Ca2+ was reduced to 1 microM by EGTA 1 min prior to stimulation, 48/80 induced little or no currents in most cells and small currents in some cells. The power density spectra of the small currents were fitted mainly by a single Lorentzian curve with an fc of 150 +/- 5.8 Hz (n = 3). Re-admission of 1.3 mM Ca2+ produced a low frequency part of current noise with an fc of 18.8 (n = 2) Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of intracellular free calcium in normal and dystrophic mouse cerebellar neurons

We measured free intracellular calcium ([Ca2+]i) in cultured cerebellar granule cells from normal and mdx mice. Resting levels of ([Ca2+]i) were 24% higher in the dystrophic neurons (normal: 61.2 +/- 1.5 nM calcium, n = 104; dystrophic: 76.1 +/- 2.4 nM calcium, n = 136, P less than 0.01). Dystrophic neurons showed a significantly greater increase in ([Ca2+]i) in the presence of elevated (18 mM) extracellular calcium levels. Resting sodium levels ([Na+]i), however, were found to be similar in normal and dystrophic granule neurons. In addition, sodium influx rates after ouabain inhibition of the Na+/K+ ATPase were also identical. Therefore, the increased permeability of granule neurons was specific to calcium, and did not result from a non-selective cation-permeable conductance. Unlike granule cells, astrocytes do not express dystrophin. Glial cells from normal and dystrophic mice showed no difference in their resting free calcium levels or their response to a high calcium load. Thus, cerebellar granule neurons from mdx mice show a calcium-specific regulatory defect similar to that found in dystrophic muscle fibers, while cerebellar glial cells, which do not normally express dystrophin, have no such defect.

Potentiation of NMDA receptor currents by arachidonic acid

Arachidonic acid is released by phospholipase A2 when activation of N-methyl-D-aspartate (NMDA) receptors by neurotransmitter glutamate raises the calcium concentration in neurons, for example during the initiation of long-term potentiation and during brain anoxia. Here we investigate the effect of arachidonic acid on glutamate-gated ion channels by whole-cell clamping isolated cerebellar granule cells. Arachidonic acid potentiates, and makes more transient, the current through NMDA receptor channels, and slightly reduces the current through non-NMDA receptor channels. Potentiation of the NMDA receptor current results from an increase in channel open probability, with no change in open channel current. We observe potentiation even with saturating levels of agonist at the glutamate- and glycine-binding sites on these channels; it does not result from conversion of arachidonic acid to lipoxygenase or cyclooxygenase derivatives, or from activation of protein kinase C. Arachidonic acid may act by binding to a site on the NMDA receptor, or by modifying the receptor's lipid environment. Our results suggest that arachidonic acid released by activation of NMDA (or other) receptors will potentiate NMDA receptor currents, and thus amplify increases in intracellular calcium concentration caused by glutamate. This may explain why inhibition of phospholipase A2 blocks the induction of long-term potentiation.