Synaptic transmission at the Schaffer-CA1 synapse is blocked by 6,7-dinitro-quinoxaline-2,3-dione. An in vivo brain dialysis study in the rat

Mechanisms of photoswitch conjugation and light activation of an ionotropic glutamate receptor

The analysis of cell signaling requires the rapid and selective manipulation of protein function. We have synthesized photoswitches that covalently modify target proteins and reversibly present and withdraw a ligand from its binding site due to photoisomerization of an azobenzene linker. We describe here the properties of a glutamate photoswitch that controls an ion channel in cells. Affinity labeling and geometric constraints ensure that the photoswitch controls only the targeted channel, and enables spatial patterns of light to favor labeling in one location over another. Photoswitching to the activating state places a tethered glutamate at a high (millimolar) effective local concentration near the binding site. The fraction of active channels can be set in an analog manner by altering the photostationary state with different wavelengths. The bistable photoswitch can be turned on with millisecond-long pulses at one wavelength, remain on in the dark for minutes, and turned off with millisecond long pulses at the other wavelength, yielding sustained activation with minimal irradiation. The system provides rapid, reversible remote control of protein function that is selective without orthogonal chemistry.

A steady-state model of spreading depression predicts the importance of an unknown conductance in specific dendritic domains

Spreading depression (SD) is a pathological wave of transient neuronal inactivation. We recently reported that the characteristic sustained complete depolarization is restricted to specific cell domains where the input resistance (R(in)) first becomes negligible before achieving partial recovery, whereas in adjacent, more polarized membranes it drops by much less. The experimental study of the participating membrane channels is hindered by their mixed contribution and heterogeneous distribution. Therefore, we derived a biophysical model to analyze the conductances that replicate the subcellular profile of R(in) during SD. Systematic variation of conductance densities far beyond the ranges reported failed to fit the experimental values. Besides standard potassium, sodium, and Glu-mediated conductances, the initial opening and gradual closing of an as yet undetermined large conductance is required to account for the evolution of R(in). Potassium conductances follow in the relative contribution and their closing during the late phase is also predicted. Large intracellular potential gradients from zero to rest are readily sustained between shunted and adjacent SD-spared membranes, which remain electroregenerative. The gradients are achieved by a combination of high-conductance subcellular domains and transmembrane ion redistribution in extended but discrete dendritic domains. We conclude that the heterogeneous subcellular behavior is due to local membrane properties, some of which may be specifically activated under extreme SD conditions.

Computational analysis of action potential initiation in mitral cell soma and dendrites based on dual patch recordings

In olfactory mitral cells, dual patch recordings show that the site of action potential initiation can shift between soma and distal primary dendrite and that the shift is dependent on the location and strength of electrode current injection. We have analyzed the mechanisms underlying this shift, using a model of the mitral cell that takes advantage of the constraints available from the two recording sites. Starting with homogeneous Hodgkin-Huxley-like Na(+)-K(+) channel distribution in the soma-dendritic region and much higher sodium channel density in the axonal region, the model's channel kinetics and density were adjusted by a fitting algorithm so that the model response was virtually identical to the experimental data. The combination of loading effects and much higher sodium channel density in the axon relative to the soma-dendritic region results in significantly lower "voltage threshold" for action potential initiation in the axon; the axon therefore fires first unless the voltage gradient in the primary dendrite is steep enough for it to reach its higher threshold. The results thus provide a quantitative explanation for the stimulus strength and position dependence of the site of action potential initiation in the mitral cell.

Hydroxylamine blocks pre- but not postsynaptic adenosine A(1) receptor-mediated actions in rat hippocampus

The commonly used nitric oxide donor, hydroxylamine (NH(2)OH), can block or reverse the inhibition of glutamatergic transmission by adenosine or an adenosine A(1) agonist in rat hippocampal slice. In these experiments, hydroxylamine did not affect the adenosine A(1) receptor-mediated depression of postsynaptic excitability. We conclude that hydroxylamine acts presynaptically to counter adenosine A(1) receptor-mediated inhibition of synaptic transmission.

Presynaptic and postsynaptic actions of halothane at glutamatergic synapses in the mouse hippocampus

Whole-cell patch-clamp recordings in adult mouse hippocampal slices were used to test the mechanism by which the volatile anesthetic halothane inhibits glutamate receptor-mediated synaptic transmission. Non-N-methyl-D-aspartate (nonNMDA) and NMDA receptor-mediated currents in CA1 pyramidal cells were pharmacologically isolated by bath application of D,L-2-amino-5-phosphonovaleric acid (APV; 100 microM) or 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 5 microM), respectively. Halothane blocked both nonNMDA and NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to a similar extent (IC50 values of 0.66 and 0.57 mM, respectively). Partial blockade of the EPSCs by lowering the extracellular concentration of calcium ([Ca2+]o), but not by application of CNQX (1 microM), was accompanied by an increase in paired-pulse facilitation (PPF). Halothane-induced blockade of the EPSCs also was associated with an increase in PPF. The effects of halothane on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and NMDA receptor-mediated currents induced by agonist iontophoresis, were compared. AMPA-induced currents were blocked with an IC50 of 1.7 mM. NMDA-induced currents were significantly less sensitive to halothane (IC50 of 5.9 mM). The effect of halothane on iontophoretic AMPA dose-response curves was tested. Halothane suppressed the maximal response to AMPA without affecting its EC50, suggesting a noncompetitive mechanism of inhibition. All effects of halothane were reversible upon termination of the exposure to the drug. These data suggest that halothane blocks central glutamatergic synaptic transmission by presynaptically inhibiting glutamate release and postsynaptically blocking the AMPA subtype of glutamate receptors.

Effects of the gliotoxin fluorocitrate on spreading depression and glial membrane potential in rat brain in situ

DC extracellular potential shifts (deltaVo) associated with spreading depression (SD) reflect massive cell depolarization, but their cellular generators remain obscure. We have recently reported that the glial specific metabolic poison fluorocitrate (FC) delivered by microdialysis in situ caused a rapid impairment of glial function followed some hours later by loss of neuronal electrogenic activity and neuron death. We have used the time windows for selective decay of cell types so created to study the relative participation of glia and neurons in SD, and we report a detailed analysis of the effects of FC on evoked SD waves and glial membrane potential (Vm). Extracellular potential (Vo), interstitial potassium concentration ([K+]o), evoked potentials, and transmembrane glial potentials were monitored in the CA1 area before, during, and after administration of FC with or without elevated K+ concentration in the dialysate. SD waves propagated faster and lasted longer during FC treatment. DeltaVo in stratum pyramidale, which normally are much shorter and of smaller amplitude than those in stratum radiatum, expanded during FC treatment to match those in stratum radiatum. The coalescing SD waves that develop late during prolonged high-K+ dialysis and are typically limited to stratum radiatum, also expanded into stratum pyramidale under the influence of FC. SD provoked in neocortex normally does not spread to the CA1, but during FC treatment it readily reached CA1 via entorhinal cortex. Once neuronal function began to deteriorate, SD waves became smaller and slower, and eventually failed to enter the region around the FC source. Slow, moderately negative deltaVo that mirrored [K+]o increments could still be recorded well after neuronal function and SD-associated Vo had disappeared. Glial cell Vm gradually depolarized during FC administration, beginning much before depression of neuronal antidromic action potentials. Calculations based on the results predict a large decrease in glial potassium content during FC treatment. The results are compatible with neurons being the major generator of the deltaVo associated with SD. We conclude that energy shortage in glial cells makes brain tissue more susceptible to SD and therefore it may increase the risk of neuron damage.

Polyhydroxylated C60, fullerenol, a novel free-radical trapper, prevented hydrogen peroxide- and cumene hydroperoxide-elicited changes in rat hippocampus in-vitro

The role of polyhydroxylated C60 (fullerenol), a novel free-radical trapper, in prevention of hydrogen peroxide- and cumene hydroperoxide-elicited damage was studied in hippocampal slices from the rat in-vitro. The interactions of polyhydroxylated C60, adenosine and 6,7-dinitroquinoxaline-2,3-dione (DNQX) were also compared. Hydrogen peroxide (0.006-0.02%) and cumene hydroperoxide (0.5-1.0 mM) both reversibly reduced the amplitudes of CA1-evoked population spikes in the hippocampal slices. Deferoxamine (1 mM) had little effect on the population spikes. Deferoxamine (1 mM) significantly prevented the hydrogen peroxide (0.006%) elicited inhibition of the population spikes. Polyhydroxylated C60 (0.1 mM) significantly prevented hydrogen peroxide- or cumene hydroperoxide-elicited reduction of the population spikes and also prevented the effects of hydrogen peroxide and cumene hydroperoxide on paired-pulse facilitation in the hippocampal slice. Adenosine reduced the amplitude of population spikes and promoted paired-pulse facilitation in the CA1 region of the hippocampus. Polyhydroxylated C60 did not alter either of the effects of adenosine on the population spikes. DNQX reduced the amplitude of the population spikes in the CA1 region but did not affect the ratio of paired-pulse facilitation. Fullerenol did not alter either effect of DNQX on the population spikes. These results suggested that polyhydroxylated C60 prevented hydrogen peroxide- and cumene hydroperoxide-elicited damage in the hippocampuss slices. These effects might be associated with the free-radical scavenging activity of polyhydroxylated C60.

Unchanged balance between levels of mRNA encoding AMPA glutamate receptor subtypes following global cerebral ischemia in the rat

Transient global ischemia leads to glutamate mediated delayed neuronal death in the CA1 but not in the CA3 region of the rat hippocampus, and changes in AMPA receptor subunit composition has been proposed to cause a difference in excitatory input to the CA1 and CA3 regions. In situ hybridization with riboprobes for AMPA receptor subtype GluR1-4 mRNA was performed on sections from the brain of sham operated and ischemic rats in two models (neck cuff and 4-vessel occlusion combined with hypotension) with identical results: the content of the GluR1-3 mRNA species was down regulated in the hippocampal regions CA1 and CA3 but only weak changes were observed in the dentate gyrus. The down regulation observed in CA1 was non-selective among GluR1-3, i.e. all GluR mRNA species showed approximately the same degree of down regulation. A change in calcium permeability of the AMPA channels mediated by a shift in channel sub-unit composition and corroborating an increased calcium influx is thus not supported by these findings.

Differential effects of NBQX on the distal and local toxicity of glutamate agonists administered intra-hippocampally

The ability of the non-NMDA glutamate antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline) to protect the brain against the neuronal death caused by glutamate agonists was examined. Glutamate agonists and NBQX were co-injected into the dorsal region of the rat hippocampus and 4 days later the brain was examined histochemically for the loss of neurons. 95 nmol NBQX prevented the toxicity of glutamate agonists acting on the AMPA receptor (quisqualate and AMPA [L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate]), except for the higher dose of AMPA where toxicity was only partially reduced. This dose of NBQX also prevented about 50% of the toxicity of kainate, but produced a slight increase in the size of the lesions caused by NMDA (N-methyl-D-aspartate). With 190 nmol NBQX, a variable degree of non-specific damage resulted, but was mainly confined to the dentate region. Allowing for this damage, almost complete protection against the toxicity of non-NMDA glutamate agonists was obtained, with a partial protection against NMDA toxicity. Kainate, and a high dose of AMPA (2 nmol), consistently caused neuronal death in other limbic regions of the brain in addition to the hippocampal damage. About 50% of rats treated with 15 nmol quisqualate also showed damage to limbic regions. Both doses of NBQX prevented this distal damage caused by quisqualate, but not that caused by kainate. With AMPA, only the high dose of NBQX blocked the distal toxicity. Diazepam also blocked the distal toxicity of AMPA, but had only a minor effect on the hippocampal damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Kainic acid-induced seizures and brain damage in the rat: different effects of NMDA- and AMPA receptor antagonists

We have studied the effect of two glutamate receptor antagonists on seizures and hippocampal neurone loss in the rat after systemic kainic acid administration. Intraperitoneal injection of the novel AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolproprionic acid) receptor antagonist NBQX (6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione) (30 mg/kg x 3 and 15 mg/kg x 3) administered 30 and 15 min. before and simultaneously with injection of kainic acid (5 mg/kg) intraperitoneally, dramatically enhanced the toxicity of kainic acid leading to death of all animals. When the NBQX dose was reduced to 8 mg/kg x 3, all animals survived and neurone damage in the hippocampus did not differ from control animals. When NBQX (30 mg/kg x 3) was administered 30- or 60 min after injection of kainic acid (8 mg/kg) intraperitoneally, no changes were observed concerning survival rates, seizure generation and neurone loss. Post-kainic acid treatment with the non-competitive NMDA receptor antagonist MK-801 (0.5 mg/kg and 1.0 mg/kg), 30 and 60 min. after intraperitoneally injection of kainic acid 8 mg/kg, abolished seizures in all animals and the neurone damage in the hippocampus was completely prevented. The results emphasize the importance of the NMDA-receptor activation for seizure generation and subsequent brain damage after intraperitoneally kainic acid. The paradoxical, unexpected effects of NBQX contrast to the protective effect of this compound after cerebral ischaemia and hypoglycaemia, conditions which are also characterized by glutamate-mediated damage. One possible explanation of the lowered seizure threshold to kainic acid after NBQX could be that NBQX is blocking AMPA receptors on interneurones more efficiently than on pyramidal cells.

Taurine release evoked by NMDA receptor activation is largely dependent on calcium mobilization from intracellular stores

It is known that the activation of N-methyl-D-aspartate (NMDA) receptors leads to an increase in extracellular taurine concentration in different brain regions. The mechanism that mediates this effect is not totally understood. In this study, rat hippocampal slices were used to determine the dependence of NMDA-induced taurine release on extracellular calcium and/or on calcium mobilization from intracellular stores. NMDA was administered through a microdialysis probe inserted into the slice, at the level of CA1 stratum radiatum, which was also used to collect amino acids from the extracellular space. Field potentials evoked by stimulation of the Schaffer collaterals and recorded in the stratum pyramidale of CA1 were used as a control of NMDA receptor activation. NMDA induced a marked increase in extracellular taurine levels and a decrease in field potential amplitude, and both effects were suppressed in the presence of MK-801, a blocker of the NMDA receptor-linked channel. Dantrolene, an inhibitor of calcium release from intracellular stores, partially inhibited the extracellular taurine increase, while 2-nitro-4-carboxyphenyl-N,N-diphenyl carbamate (NCDC), an inhibitor of phosphatidylinositol-specific phospholipase C activation, had no effect. Removal of extracellular calcium diminished, but did not abolish, the extracellular taurine increase caused by NMDA. The remaining taurine response was totally suppressed by dantrolene, and also by NCDC. These results demonstrate that the release of taurine induced by NMDA receptor activation is triggered by the increase in cytoplasmic calcium concentration. We suggest that, under physiological conditions, calcium influx provides the signal for NMDA-induced taurine release, which is amplified by calcium-dependent calcium mobilization from intracellular stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prolonged elevation of potassium on hippocampus of anesthetized rats

We tested the after-effects of prolonged depolarization on neurons in situ in the mammalian brain and examined the site of blockade responsible for failure of synaptic transmission. The CA1 region of the hippocampus of anesthetized rats was exposed to solutions containing elevated concentration of K+ (100-125 mmol/l), administered either by microdialysis in intact brain or by irrigation of the exposed surface of the hippocampus. Recovery was observed for 5-6 1/2 h. When K+ was administered by microdialysis, evoked potentials were recorded from points near (up to 0.2 mm) and far (0.7-1.0 mm) from the dialysis probe. High K+ dialysis induced recurrent waves of spreading depression and, in about half of the preparations, a prolonged unstable depressed state. In the intervals between SD waves orthodromic but not antidromic population spikes remained severely depressed at the 'far' recording site. Following high K+ dialysis orthodromic population spikes recovered in a triphasic cycle: partial recovery with hyper-transmission was followed by secondary depression and finally by slow partial or complete recovery. Final recovery was less complete in cases that have experienced prolonged spreading depression. Current source density analysis revealed that during secondary depression transmission was blocked due to failure of dendritic action potentials. When the exposed hippocampus was irrigated with high K+ solution ortho- and antidromic evoked potentials recovered completely following high K+ exposure of less than 30 min, incompletely after 45 min and failed entirely after 60 min. We conclude that prolonged steady depolarization of hippocampal CA1 pyramidal neurons causes lasting loss of function. Dendritic function is especially prone to depolarization-induced injury. CA1 neurons are less vulnerable in situ than they are in vitro.

Blockade of antidromic invasion of CA1 pyramidal cells during synaptic activation of NMDA receptors

We have studied in situ the excitability state of the axon-soma membrane of CA1 pyramidal cells in the rat during synaptic activation of N-methyl-D-aspartate (NMDA) receptors. Repetitive activation (3-5 Hz) of Schaffer collaterals provoked a NMDA receptor-mediated component in the field excitatory postsynaptic potential (fEPSP) within 15 s. The generation of this component follows a characteristic self-limiting cycle, vanishing after 6-10 s. When alvear shocks were paired to the orthodromic volleys, the antidromic population spike (PS) was completely abolished only if the NMDA receptor-mediated fEPSP had occurred. This blockade of antidromic invasion lasted for 120-150 ms after each orthodromic shock. A reduction in the safety factor for axon-soma transmission is presumed during NMDA receptor synaptic activation.

Propagation of spreading depression among dendrites and somata of the same cell population

The propagation of sustained potential shifts associated with spreading depression (SD) was studied by microelectrodes placed in diverse layers at different locations in hippocampus of anesthetized rats. SD was induced by raising interstitial potassium concentration ([K+]0) focally in the CA1 region of the dorsal hippocampus either by microdialysis or by microinjection. Recurrent waves of SD propagated from the dialysis site throughout the hippocampus. Potential shifts (delta V0) associated with SD usually began earlier and were always of larger amplitude and longer duration in stratum (st.) radiatum (layer of apical dendrites) than in st. pyramidale (layer of pyramidal cell somata). The velocity of propagation in the two layers differed and varied independently one from the other. When SD was provoked by orthodromic train stimuli, the apparent direction of propagation in st. pyramidale was opposite that in st. radiatum. Microinjection of high K+ solution was more likely to provoke SD when placed in the st. radiatum, 50-100 microns ventral to st. pyramidale, than in other cytoarchitectonic layers. In about half the trials after 30 to 90 min of high K+ dialysis, a prolonged depressed state developed during which the potential in st. radiatum shifted at irregular intervals between near-rest level and a strongly negative level, while delta V0 shifts in st. pyramidale were smaller and more irregular in amplitude. This state is termed prolonged unstable SD". When the NMDA receptor antagonist CPP was dialyzed together with high K+, the onset of SD was postponed and delta V0 waves propagated in st. pyramidale without corresponding waves in st. radiatum; less frequently the other way around.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemia as an excitotoxic lesion: protection against hippocampal nerve cell loss by denervation

There are several indications for an involvement of neuroexcitatory mechanisms in ischemic neuron damage. Since we forwarded the hypothesis in 1982 that the transmitter glutamate is playing a key role, several lines of evidence have substantiated this: there is a pronounced transmitter release induced by ischemia and there is uptake of Ca++ via NMDA-operated calcium channels. Under certain circumstances postischemic neuron death can be impaired by administration of either NMDA-antagonists or calcium blockers. Further proof for the induction of harmful excitatory mechanisms by ischemia has been obtained by preischemic denervation of the vulnerable nerve cells. After transient cerebral ischemia in rats or gerbils, there are signs of irreversible damage (eosinophilia) of neurons in the dentate hilus (somatostatin-positive cells) after 2-3 hours and of hippocampal pyramidal neurons after 2-3 days (delayed neuron death). In the first case, removal of the (main) input to hilus cells by degranulation (colchicine selectively eliminates granule cells) protects these. In the case of pyramidal neurons removal of Schaffer collaterals/commisurals or input from the entorhinal cortex have a protective effect. Recently, we have measured glutamate and calcium in CA1 of denervated rats during 10 min of ischemia, and it turns out that there is almost no extracellular glutamate release or lowering of calcium in contrast to ischemic animals with intact innervation. Also in the postischemic period there are indications of a continuation of the damaging processes induced by ischemia. Besides the well known postischemic hypoperfusion, a prolonged release of glutamate has been reported, as well as burst firing in some models.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterizationin vivo of the NMDA receptor-mediated component of dentate granule cell population synaptic responses to perforant path input

The NMDA receptor-mediated component of the hippocampal granule cell population excitatory postsynaptic potential response to low frequency (< 0.2 Hz) stimulation of the medial perforant path was characterized in vivo. Extracellular recordings were obtained from the dentate molecular layer in anesthetized rabbits, and glutamatergic and GABAergic antagonists were applied locally by pressure ejection. To measure the NMDA-mediated component, the NMDA receptor antagonist D-5-aminophosphonovalerate (APV) was applied during the constant ejection of physiological saline, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and/or bicuculline methiodide. In general agreement with the results of attempts by other investigators to identify NMDA responses in vivo, APV did not significantly reduce the response to a single stimulus impulse in the presence of saline. However, an NMDA-mediated response was revealed when alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate receptor-mediated current flow was eliminated by applying the non-NMDA receptor antagonist CNQX. The NMDA component was negative-going as predicted, but its duration was considerably less than indicated in other studies of the dentate in vitro. The relative magnitudes of the NMDA and non-NMDA components of the EPSP were found to vary as a function of stimulus intensity or frequency. The NMDA receptor-mediated component represented 12% of the control response and increased to over 25% in response to higher stimulus intensities. A brief, high-frequency burst of impulses evoked a larger NMDA component in the presence of CNQX and was able to evoke an NMDA component in the presence of saline. Surprisingly, short trains of stimulation at lower frequencies typically produced suppression of the NMDA component. In a final series of experiments, it was found that many characteristics of the NMDA component were substantially altered by GABAergic inhibition. In the presence of the GABAA antagonist bicuculline, the magnitude of NMDA receptor-mediated responses was increased and their duration was greatly extended. Additionally, in the presence of bicuculline, the NMDA component facilitated markedly in response to frequencies of stimulus input > 20 Hz. These results indicate in vivo that the initiation and duration of NMDA current flow depend strongly upon the intensity and frequency of perforant path stimulation. In addition, the NMDA response to a single impulse appears to be reduced and truncated by input from GABAA receptor-mediated feedback and/or feedforward inhibition, and this inhibition affects temporal summation of NMDA receptor-mediated responses over a wide range of input frequencies. It is suggested that such inhibition results from the activation of GABAA receptors located on granule cell dendritic shafts.(ABSTRACT TRUNCATED AT 400 WORDS)

Protection against ischemic hippocampal CA1 damage in the rat with a new non-NMDA antagonist, NBQX

Two glutamate antagonists were tested in a rat model of complete, transient cerebral ischemia. Six days after 10 min ischemia the mean loss of hippocampal CA1 pyramidal neurones was 73%. Administration of the AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) antagonist NBQX (2,3-dihydro-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) reduced the pyramidal neurone loss to 1%, 11% and 15%, when given before, immediately after or 1 h after ischemia, respectively. MK-801 (dizocilpine), a competitive NMDA antagonist gave no protection in this model. We suggest that the AMPA receptor transduction mechanisms are sensitized by ischemia and that the postischemic blockade of the main glutamatergic input to the CA1 cells with NBQX impairs the deleterious effect of "normal" postischemic excitatory transmission.

Regional cerebral protein synthesis after transient ischemia in the rat: effect of the AMPA antagonist NBQX

Normothermic rats with 12 min, complete cerebral ischemia were treated with the AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F) quinoxalinedione (NBQX) [10], which prevents CA1 pyramidal neuron loss. Twenty hours after ischemia, cerebral protein synthesis rate (CPSR) was measured autoradiographically using [35S]methionine. Ischemia caused a 38% decrease of CPSR in CA1, and postischemic treatment with NBQX caused a 66% decrease in this region. Also treatment with NBQX alone resulted in a decrease (22% in CA1) of the CPSR. Since some evidence exists that the neuroprotective effect of NBQX is related to blockade of the fast AMPA-mediated transmission, the further decrease of the postischemic CPSR in CA1 could be a mere side effect.

Blockade of the AMPA receptor prevents CA1 hippocampal injury following severe but transient forebrain ischemia in adult rats

The cytoprotective effect of NBQX, a selective AMPA receptor antagonist, was tested following 10 min of severe forebrain ischemia using the 4-vessel occlusion model. Immediately, and at 15 and 30 min following reperfusion, adult Wistar rats received intraperitoneal injections of either saline (n = 5), 1 mg lithium chloride (n = 17) or 30 mg/kg of the lithium salt of NBQX (n = 18). In saline-treated animals 82 +/- 12% of CA1 hippocampal neurons were lost. Of those treated with lithium 70 +/- 23% were injured, while those given NBQX sustained only 40 +/- 34% CA1 necrosis (P less than 0.01). Twelve of 18 NBQX-treated animals had less than 30% CA1 injury as compared with 1 of 17 lithium-treated animals. The AMPA receptor may play a more important role than the NMDA receptor in selective ischemic necrosis of hippocampal neurons.

Receptor sub-types involved in responses of Purkinje cell to exogenous excitatory amino acids and local electrical stimulation in cerebellar slices in the rat

The effects of the NMDA receptor antagonist, 2-amino-5-phosphonovalerate (APV) and non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on responses of Purkinje cells to exogenous excitatory amino acids and to electrical stimulation of the parallel fibres, were investigated in slices of the cerebellum of the rat. Glutamate, aspartate, kainate and quisqualate all induced excitation of Purkinje cells. Responses to kainate and quisqualate were blocked by CNQX (10 microM) but not by APV (10 microM). N-Methyl-D-aspartate induced biphasic excitatory-inhibitory responses, both components of which were blocked by APV but not by CNQX. The inhibitory component was less sensitive to blockade by APV but was totally blocked by bicuculline, the GABAA receptor antagonist. Parallel fibre stimulation most commonly induced inhibition of Purkinje cells, with or without preceding excitation. This inhibition was blocked by APV and excitatory responses were often revealed. A less commonly-observed predominantly excitatory response was blocked by CNQX but not by APV and inhibition tended to be revealed. These data suggest that parallel fibre-Purkinje cell synapses possess non-NMDA postsynaptic receptors, while the parallel fibre-inhibitory interneuron synapses possess functional NMDA receptors.

Osmolality-induced changes in extracellular volume alter epileptiform bursts independent of chemical synapses in the rat: importance of non-synaptic mechanisms in hippocampal epileptogenesis

The contribution of non-synaptic mechanisms to the seizure susceptibility of rat CA1 hippocampal pyramidal cells was examined in vitro by testing the effects of osmolality on synchronous neuronal activity, using solutions which blocked chemical synaptic transmission both pre- and post-synaptically. Decreases in osmolality, which shrink the extracellular volume, caused or enhanced epileptiform bursting. Increases in osmolality with membrane-impermeant solutes, which expand the extracellular volume, blocked or greatly reduced epileptiform discharges. Reductions in the extracellular volume, therefore, can enhance synchronization among CA1 hippocampal neurons through non-synaptic mechanisms. Since similar osmotic treatments are known to modify epileptiform discharges in several models of epilepsy, non-synaptic mechanisms are probably more important in hippocampal epileptogenesis than previously realized and may contribute to the high susceptibility of this brain region to epileptic seizures in animals and humans. These data also provide a possible explanation for the observation in humans that decreased plasma osmolality, which can be associated with a wide range of clinical syndromes, leads to seizures.

Role of endogenous taurine on the glutamate analogue-induced neurotoxicity in the rat hippocampus in vivo

The glutamate analogues N-methyl-D-aspartate (NMDA), kainic acid (KA), and quisqualic acid (QA), prepared in different hypertonic media, were perfused in vivo in the hippocampal CA1 field of rats using a microdialysis technique. Extracellular taurine levels, estimated after analysis of the taurine content of dialysates, increased during perfusion of all three agonists but varied according to the osmolarity of the medium. The NMDA-induced increase in extracellular taurine content was only slightly inhibited by perfusion of 150 and 300 mM sucrose. The KA-evoked increase was partially dependent on extracellular osmolarity, because addition of 50 and 150 mM sucrose caused a dose-dependent inhibition that was not augmented using higher sucrose concentrations. QA caused a taurine increase that was totally abolished by addition of 50 mM sucrose. These results indicate that the rise in extracellular taurine level elicited by QA and part of the increase elicited by KA are probably due to a release caused by the cellular swelling that these substances evoke, a finding substantiating the previously proposed osmoregulatory role of taurine. However, almost all the increase in extracellular taurine content caused by NMDA and all the osmotically insensitive part of the KA-evoked rise cannot be explained as release triggered by cell swelling and may reflect a function of taurine other than osmoregulation.

Extracellular taurine increase in rat hippocampus evoked by specific glutamate receptor activation is related to the excitatory potency of glutamate agonists

Taurine increases in brain extracellular space due to glutamate agonists were studied in vivo in the rat hippocampus using a dialysis technique, both in the absence and in the presence of glutamate receptor antagonists. Extracellular taurine levels increased during perfusions of agonists, listed in descending order of potency: kainate (KA), N-methyl-D-aspartate (NMDA), and quisqualate (QA). While taurine increases due to KA or QA perfusions were inhibited by 6,7-dinitro-quinoxaline-2,3-dione (DNQX), those induced by NMDA were abolished in the presence of 3-(carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). These results indicate that increases in extracellular taurine levels evoked by NMDA, KA or QA in the rat hippocampus are caused by activation of their specific receptors. Field potentials, concomitantly recorded, were quickly abolished during NMDA or KA perfusions (0.1 mM), while QA (0.25 mM) induced the appearance of bicuculline-like evoked responses. Since taurine has been proposed as an osmoregulatory substance in the rat brain, and cell swelling is known to be an early component of glutamate agonists neurotoxicity, the increases in extracellular taurine reported here could be due to taurine released through an osmoregulatory process, counteracting the neurotoxic cellular oedema induced by glutamate agonists.