Properties of non-NMDA excitatory amino acid-activated channels in isolated retinal horizontal cells.

The excitatory amino acid glutamate is believed to be the neurotransmitter used by some photoreceptors in the teleost retina. Past studies have shown that exogenous glutamate, and its analogs, are capable of affecting second-order retinal neurons in a manner consistent with the action of a photoreceptor transmitter. In an effort to characterize the properties of retinal glutamate channels on second-order neurons, non-NMDA excitatory amino acid-activated channels were studied in single horizontal cells isolated from the retina of the white bass. Using patch-clamp techniques single glutamate, kainate, and quisqualate channels were recorded. Two categories of channels were observed. The first was labeled slow-channels. Single-channel conductances and open times for this channel showed a range of values, but the average for channels activated by glutamate was 12 pS and 5.6 msec; quisqualate, 8.5 pS and 8.8 msec; and kainate, 8.5 pS and 4.5 msec. Openings of slow-channels elicited by the agonists tended to occur in bursts with a mean burst length of 38 msec. The bursts were punctuated by numerous, brief closings. The second channel category was termed fast channels. The agents glutamate, quisqualate, and kainate all activated channels in this category with open times of 1-2 msec and 2 prominent conductances in the range of about 10 and 20-30 pS. Activity of the fast channels tended to be noisy and no bursting behavior was observed.     

Kainic acid responses and toxicity show pronounced Ca2+ dependence

Responses of pyramidal neurons to ionophoretic kainate, quisqualate and N-methyl aspartate were studied in a submerged rat piriform cortex slice as a function of Ca2+ and Mg2+ concentrations. The results suggest that the channel activated by kainate is unusually influenced by Ca2+, excitotoxicity is Ca2+-dependent and a function of Ca2+ concentration, and the excitotoxic actions of various amino acid agonists are correlated with the Ca2+ dependence of their responses.     

Neurotoxic effects of excitatory amino acids in the mouse spinal cord: quisqualate and kainate but not N-methyl-D-aspartate induce permanent neural damage

Despite extensive evidence for the neurotoxic effects of excitatory amino acids (EAA) in the brain little is known about their neurotoxic action in the spinal cord. In this study we attempted to produce differential lesions of spinal neurons by pretreating mice, intrathecally, with high concentrations of the EAA: N-methyl-D-aspartate (NMDA), quisqualate and kainate. Pharmacological, behavioral and histological consequences were examined 1, 3, 7 and, in some cases, 30 days after pretreatment. A single, intrathecal, injection of high concentrations of quisqualate and kainate but not NMDA, resulted in damage to spinal cord neurons. The highest concentrations of these agonists produced, in some animals, a massive, non-selective destruction of neurons within the lumbar spinal cord, accompanied by complete paralysis of the hindlimbs. Pretreatment with lower concentrations of intrathecal kainate or quisqualate produced damage to spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal interneurons, as well as more limited damage to motor neurons. No detectable motor deficit could be detected but a decrease in responsiveness to noxious stimuli was observed. Such damage also manifest as a permanent decrease in the sensitivity of the spinal cord to EAA, as seen from the decrease in biting behavior elicited by intrathecal EAA. The neurotoxic effects of quisqualate were completely blocked by the quisqualate/kainate receptor antagonist glutamylaminomethylsulphonate (GAMS), but not the NMDA antagonist 2-amino-5-phosphovalerate. GAMS attenuated the effects of kainate only partially.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of quisqualate receptor desensitization in cultured postnatal rat hippocampal neurons.

The quisqualate class of glutamate receptors is thought to play an important role in excitatory synaptic transmission, synaptic plasticity, and neuronal death. Since desensitization is a prominent feature of the responses mediated by this class of receptors, we have characterized the rapidly desensitizing quisqualate response in cultured postnatal rat hippocampal neurons using the whole-cell patch-clamp technique. Quisqualate and its structural analogs elicit a peak current that rapidly decays to a steady-state level. In contrast, currents induced by kainate, NMDA, and their structural analogs exhibit either no decay or a much slower decay. The biophysical and pharmacological properties of the peak and steady-state quisqualate currents indicate that both are mediated by an ionotropic quisqualate receptor. Quisqualate currents desensitized monoexponentially by approximately 70% with a time constant near 80 msec. Both the rate and percentage of desensitization showed slight voltage dependence and were concentration dependent, reaching maximal values at saturation. Additionally, the overlap of the dose-response curves for activation of the steady-state current and desensitization of the peak current by a conditioning dose suggests that the two processes are related. Furthermore, desensitizing quisqualate currents were observed when Ca2+, Mg2+, Na+, K+, and Cl- were removed from the extracellular solution or their concentrations greatly reduced. These results suggest that the decline in the response is not caused by a simple open channel block mechanism. Despite the lack of desensitization by kainate, our observations are consistent with the hypothesis that quisqualate and kainate act at a single receptor-channel complex. Kainate and quisqualate appeared to interact competitively when applied simultaneously and noncompetitively when quisqualate was applied first. In addition, saturating doses of quisqualate and kainate gave steady-state currents of equal amplitude in neurons treated with the lectin WGA, an inhibitor of quisqualate receptor desensitization.     

Electrophysiological evidence for the presence of ionotropic and metabotropic excitatory amino acid receptors on dopaminergic neurons of the rat mesencephalon: an in vitro study.

The actions of the ionotropic and metabotropic excitatory amino acid agonists AMPA, quisqualate, kainate, NMDA and trans-ACDP were studied by means of intracellular electrophysiological recordings from dopaminergic neurons of rat mesencephalon in brain slices. It was observed that all these agents evoked an inward current in cells which were voltage-clamped near the resting potential (-50, -60 mV). The membrane responses produced by AMPA, kainate and quisqualate were associated with an increase of the apparent input conductance while the responses induced by NMDA and trans-ACDP were associated with a decrease in the apparent input conductance. Therefore, stimulation of ionotropic and metabotropic amino acid receptors activates inward currents in the dopaminergic cells by different mechanisms.     

Kainic acid responses and toxicity show pronounced Ca dependence

Responses of pyramidal neurons to ionophoretic kainate, quisqualate and N-methyl aspartate were studied in a submerged rat piriform cortex slice as a function of Ca²⁺ and Mg²⁺ concentrations. The results suggest that (a) the channel activated by kainate is unusually influenced by Ca²⁺, (b) excitotoxicity is Ca²⁺-dependent and a function of Ca²⁺ concentration, and (c) the excitotoxic actions of various amino acid agonists are correlated with the Ca²⁺ dependence of their responses.     

N-methyl-D-aspartate (NMDA) receptors are inactivated by trypsin

Acute isolation of hippocampal CA3 pyramidal cells using trypsin produces neurons which respond to kainate and quisqualate but not N-methyl-D-aspartate (NMDA). Incubation of 6- to 12-day-old cultured hippocampal neurons or slices of pyriform cortex with trypsin irreversibly removes the NMDA responses normally present without significant effect on responses to kainate or quisqualate. These data indicate that the NMDA receptor has a trypsin-sensitive component which is necessary for agonist recognition or ion channel activation.     

Pharmacological characterization of the glutamate receptor in cultured astrocytes

Cultured astrocytes from neonatal rat cerebral hemispheres are depolarized by the excitatory neurotransmitter glutamate. In this study we have used selective agonists of different neuronal glutamate receptor subtypes, namely, the N-methyl-D-aspartate (NMDA), kainate, and quisqualate type, to characterize pharmacologically the glutamate receptor in astrocytes. The agonists of the neuronal quisqualate receptor, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA) and quisqualate, depolarized the membrane. Kainate, an agonist of the neuronal kainate receptor, depolarized astrocytes more effectively than quisqualate. Combined application of kainate and quisqualate depolarized astrocytes to a level which was intermediate to that evoked by quisqualate and kainate individually. Agonists activating the neuronal NMDA receptor, namely NMDA and quinolinate, were ineffective. Application of NMDA did not alter the membrane potential even in combination with glycine or in Mg2+-free solution, conditions under which neuronal NMDA receptor activation is facilitated. The nonselective agonists L-cysteate, L-homocysteate, and beta-N-oxalylamino-L-alanine (BOAA) mimicked the effect of glutamate. Dihydrokainate, a blocker of glutamate uptake, did not, and several antagonists of neuronal glutamate receptors only slightly affect the glutamate response. These findings suggest that astrocytes express one type of glutamate receptor which is activated by both kainate and quisqualate, lending further support to the notion that cultured astrocytes express excitatory amino acid receptors which have some pharmacological similarities to their neuronal counterparts.     

Pharmacological properties of isolated horizontal and bipolar cells from the skate retina

Retinal neurons were enzymatically and mechanically dissociated from adult skate retinas and maintained in cell culture for up to 14 days. Intracellular recordings were made from isolated horizontal and bipolar cells while neurotransmitters were applied via pressure ejection. L-Glutamate, quisqualate, kainate, and gamma-amino-butyric acid (GABA), when applied to horizontal cells, produced large (60 to 70 mV), long-lasting depolarizations. These responses appear to consist of at least two components: a graded depolarization and a Ca++-dependent regenerative component. As regards bipolar cells, L-glutamate and its analogues depolarized about 30% of the cells tested, while GABA hyperpolarized most of these neurons. Both agents acted on bipolar cells by increasing conductance. Repeated applications of L-glutamate, quisqualate, kainate, and GABA to horizontal cells produced no desensitization, but in these circumstances the glutamate analogues, kainate and quisqualate, induced certain morphological changes, most notably a retraction of cell processes and the appearance of blebs on the cell surface.     

Glutamate Receptors of a Quisqualate-Kainate Subtype are Involved in the Presynaptic Regulation of Dopamine Release in the Cat Caudate Nucleus in vivo

Experiments were conducted with halothane-anesthetized cats implanted with a push-pull cannula in the caudate nucleus in order to estimate the effects of glutamate (GLU) agonists on the release of 3H-dopamine continuously synthesized from 3H-tyrosine. In the presence of tetrodotoxin (TTX), glutamate (10-8 M, 10-4 M) and kainate (KAI) (10-5 M) stimulated the release of 3H-dopamine while quisqualate (10-5 M) and N-methyl-D-aspartate (NMDA) (10-5 M) were without effect. The stimulatory effect of kainate (10-5 M) on 3H-dopamine release did not seem to be mediated by glutamate released from corticostriatal fibers, as not only kainate, but also quisqualate (QUI) and N-methyl-D-aspartate enhanced the efflux of glutamate through a tetrodotoxin-resistant process. Riluzole (10-5 M), gamma-D-glutamyl-glycine (GDGG) (10-5 M) and glutamine-diethyl-ester (10-5 M) prevented the stimulatory effect of kainate (10-5 M) while 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) (10-5 M), kynurenate (10-5 M) and 2-amino-5-phosphonovalerate (APV) (10-5 M) were without effect. In the presence of concanavalin A (CONA) (10-7 M), a lectin which is known to prevent the quisqualate-evoked desensitization of glutamate receptors, quisqualate (10-5 M) stimulated the release of 3H-dopamine. In addition, in the absence of concanavalin A, quisqualate (10-5 M) blocked the stimulatory effects of kainate (10-5 M) or glutamate (10-4 M) on 3H-dopamine release. These results suggest the involvement of receptors of the quisqualate/kainate subtype in the direct glutamate-induced presynaptic facilitation of dopamine release. In contrast to what was observed in the presence of tetrodotoxin, in the absence of the neurotoxin, high concentrations of glutamate (10-4 M) and kainate (10-5 M) reduced rather than stimulated the release of 3H-dopamine. A weak inhibitory effect was also observed with quisqualate (10-5 M) while N-methyl-D-aspartate (10-5 M) was without effect. In the light of previous studies, these latter observations suggest that glutamate can also exert an indirect inhibitory presynaptic influence on the release of dopamine from nerve terminals of the nigrostriatal dopaminergic neurons by acting on receptors of the quisqualate/kainate subtype located on striatal GABAergic neurons.     

Structure-activity relationships for amino acid transmitter candidates acting at N-methyl-D-aspartate and quisqualate receptors

Dose-response curves for activation of excitatory amino acid receptors on mouse embryonic hippocampal neurons in culture were recorded for 15 excitatory amino acids, including the L-isomers of glutamate, aspartate, and a family of endogenous sulfur amino acids. In the presence of 3 microM glycine, with no extracellular Mg, micromolar concentrations of 11 of these amino acids produced selective activation of N-methyl-D-aspartate (NMDA) receptors. L-Glutamate was the most potent NMDA agonist (EC50 2.3 microM) and quinolinic acid the least potent (EC50 2.3 mM). Dose-response curves were well fit by the logistic equation, or by a model with 2 independent agonist binding sites. The mean limiting slope of log-log plots of NMDA receptor current versus agonist concentration (1.93) suggests that a 2-site model is appropriate. There was excellent correlation between agonist EC50S determined in voltage clamp experiments and KdS determined for NMDA receptor binding (Olverman et al., 1988). With no added glycine, and 1 mM extracellular Mg, responses to NMDA were completely blocked; responses to kainate and quisqualate were unchanged. Under these conditions, glutamate and the sulfur amino acids activated a rapidly desensitizing response, similar to that evoked by micromolar concentrations of quisqualate and AMPA, but mM concentrations of L-aspartate, homoquinolinic acid, and quinolinic acid failed to elicit a non-NMDA receptor-mediated response. Except for L-glutamate (EC50 480 microM), the low potency of the sulfur amino acids prevented the study of complete dose-response curves for the rapidly desensitizing response at quisqualate receptors. Small-amplitude nondesensitizing quisqualate receptor responses were activated by much lower concentrations of all quisqualate receptor agonists. Full dose-response curves for the nondesensitizing response were obtained for 9 amino acids; L-glutamate was the most potent endogenous agonist (EC50 19 microM). Domoate (EC50 13 microM) and kainate (EC50 143 microM) activated large-amplitude, nondesensitizing responses.     

AMPA, kainate, and quisqualate activate a common receptor-channel complex on embryonic chick motoneurons

The actions of the putative quisqualate-selective agonist DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were examined in identified embryonic chick motoneurons using gigaseal recording techniques and compared with properties of the selective non-NMDA excitatory amino acid agonists kainate and quisqualate. Pressure application of AMPA induces an inward going current when neurons are voltage-clamped at negative membrane potentials. The current-voltage relationship for this response is linear with reversal near 0 mV. Over the range of 1 microM-10 mM, the AMPA-induced current is dose-dependent with an ED50 of 40 microM. AMPA currents are insensitive to the selective NMDA receptor antagonist, 2-amino-5-phosphonovalerate, and the putative quisqualate selective blocker, glutamate diethyl ester, but are partially inhibited by kynurenic acid. In competition experiments, applications of saturating concentrations of AMPA and either kainate or quisqualate produce responses intermediate between the response to either agonist alone, indicating commonality in the mechanism of these agents. Applications of AMPA with the NMDA-selective agonist aspartate give an additive response. Analysis of current fluctuations indicates that AMPA, quisqualate, and kainate gate a channel with a primary conductance near 20 pS. Differences in maximal macroscopic current evoked by saturating concentrations of AMPA, kainate, and quisqualate cannot be explained by differences in mean channel open time as the most efficacious agonist, kainate, has the shortest channel open time (AMPA = 5.9 +/- 0.4 msec, kainate = 2.7 +/- 0.1 msec, quisqualate = 5.0 +/- 0.5 msec). Rather, kainate induces a greater frequency of channel opening. This finding contrasts with results obtained at the nicotinic ACh receptor, where the most efficacious agonists have the longest mean channel open time. Our results suggest that AMPA acts at the same receptor-channel complex as kainate and quisqualate on chick motoneurons and support the hypothesis that only 2 classes of excitatory amino acid receptor complexes exist in this preparation.     

CNQX and DNQX block non-NMDA synaptic transmission but not NMDA-evoked locomotion in lamprey spinal cord

The motor pattern underlying locomotion in the lamprey is activated and maintained by excitatory amino acid neurotransmission. The quinoxalinediones 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) are potent and selective antagonists of non-N-methyl-D-aspartate (NMDA) receptors in the mammalian central nervous system. In the lamprey, these compounds are now shown to block fast excitatory synaptic potentials elicited in neurones of the spinal ventral horn. They selectively antagonise responses to the application of selective kainate and quisqualate receptor agonists (kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxalone (AMPA)) but do not influence NMDA receptor-mediated responses. Additionally, it is shown that the activation of NMDA receptors is sufficient to elicit and maintain fictive locomotion after blockade of non-NMDA receptors with either DNQX or CNQX. Conversely, activation of quisqualate receptors with AMPA, but not quisqualate leads to fictive locomotion with properties much like that activated by kainate.     

Serotonin Differentially Modulates Responses Mediated by Specific Excitatory Amino Acid Receptors in the Rat Locus Coeruleus

Microiontophoretic application of selective agonists for the three major excitatory amino acid receptors, N -methyl- d -aspartate (NMDA), quisqualate and kainate, increased the discharge rate of noradrenergic locus coeruleus (LC) neurons in vivo. NMDA activation was selectively attenuated by iontophoretic application of 2-amino-5-phosphonopentanoate (AP5), an antagonist at NMDA receptors, whereas kainate- and quisqualate-evoked responses were attenuated by both NMDA and non-NMDA antagonists iontophoresis. NMDA- and quisqualate-evoked responses were significantly decreased by co-iontophoresis of serotonin (5-HT). When the NMDA receptor-mediated component of the response to kainate was blocked with AP5 iontophoresis, 5-HT increased the response of LC neurons to kainate. These results revealed that 5-HT differentially modulates the responsiveness of LC neurons to excitatory amino acids, depending on the receptor subtypes responsible for the neuronal activation.     

Crossed forelimb extension produced in thalamic cats by injection of putative transmitter substances into the paralemniscal pontine reticular formation

To analyze the descending pathways of the paralemniscal pontine reticular formation (PLRF), a technique was used for the selective activation of cell bodies by localized injection of putative neurotransmitters in the PLRF. When a small amount (less than 0.1 microliter) of 0.1 M glutamate was injected into the PLRF unilaterally in thalamic cats, the forelimb contralateral (c-forelimb) to the injection was extended, and occasionally the ipsilateral forelimb was flexed. These responses were similar to those obtained by electrical stimulation of the PLRF, but were relatively weaker. Unit spikes of PLRF neurons were increased in frequency following administration of glutamate. The latent periods and durations of increases in spike frequency varied depending on the concentration and quantity of the glutamate solution, and were roughly similar to those of the extensor EMG in the c-forelimb. Since the firing of PLRF neurons preceded the EMG with 11 ms latency, the unit spike of PLRF neurons could be used as a triggering signal to observe a spike triggered averaged EMG response in the extensor muscle of the c-forelimb. Results similar to those with glutamate were observed upon administration of quisqualate, kainate and aspartate. The most effective compound was quisqualate. Application to the PLRF of 1-naphthylacetyl spermine (1-NA-Spm), an analogue of the natural spider toxin JSTX-3 and an antagonist of glutamate, suppressed both the PLRF neuron activity and the extensor EMG of the c-forelimb. These observations suggest that extensor muscles of the forelimb are excited by the contralateral PLRF, perhaps via the crossed reticulospinal tract from the PLRF. PLRF neurons may be activated by glutamate (quisqualate) receptors.     

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. I. Comparison of the effects of various excitotoxins, with particular reference to the loss of immunohistochemically identified cholinergic neurons.

The pedunculopontine tegmental nucleus (PPTg) has been shown to have cholinergic connections with the thalamus and basal ganglia. The ability of various doses of the excitotoxins (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA), folate, ibotenate, kainate, N-methyl-D-aspartate (NMDA), quinolinate and quisqualate to make lesions in the PPTg was examined, with particular reference to their ability to destroy cholinergic neurons identified using choline acetyltransferase (ChAT) immunohistochemistry. All of the toxins induced convulsive activity on recovery from surgical anesthesia and all except folate made lesions in the PPTg and surrounding structures. The size of the lesions was computed following examination of Cresyl violet stained sections. The largest lesions were made by kainate = AMPA greater than NMDA = ibotenate greater than quisqualate = quinolinate. All of the toxins destroyed cholinergic neurons, higher doses producing greater loss than lower. The ratio of cholinergic cell loss to general neuronal loss (assessed by Cresyl violet staining) was also computed, revealing marked differences between the toxins. Statistical analysis showed that there were significant differences between excitotoxins in terms of this ratio, but these were accounted for by the low dose of quinolinate (24 nmol) producing a significantly greater ratio of damage (12.18:1) than every other toxin. (Next highest ratio: quisqualate 60 nmol, 6.22:1.) Between the other toxins (kainate, AMPA, ibotenate, quisqualate, NMDA and the high dose of quinolinate) there were no statistically significant differences. Intense calcium deposits (stained by Alizarin red) were found frequently and often defined the borders of the lesion. Tyrosine hydroxylase immunohistochemistry revealed axons running below and into the area of lesioned tissue suggesting strongly that fibers were undamaged by the lesions. We conclude that in the PPTg, different excitotoxins make discriminably different lesions, both quantitatively and qualitatively. Unlike excitotoxic lesions in the basal forebrain quinolinate, not quisqualate, made the most selective lesions of cholinergic neurons and, unlike excitotoxic lesions in the septal nuclei, non-myelinated fibers were spared by ibotenate. The implications of these data for research into brainstem mechanisms of Parkinson's disease are discussed.     

Barbiturates, alcohols and the CNS excitatory neurotransmission: Specific effects on the kainate and quisqualate receptors

The effects of barbiturates and straight-chain aliphatic alcohols on the responses of rat striatal neurons to excitatory amino acids have been investigated. The responses to N-methyl-D-aspartate, quisqualate, kainate, L-glutamate and L-aspartate were measured by the increase in 22Na+ efflux rate that they produce in brain slices. The responses to quisqualate and kainate, measured in the 22Na+ efflux assay, were found to be partially blocked by barbiturates whereas the responses to N-methyl-D-aspartate, glutamate and aspartate were not. The kainate and quisqualate-induced increases in 22Na+ efflux rate were much more readily blocked by the presence of aliphatic alcohols than were the responses to N-methyl-D-aspartate, glutamate and aspartate. These results strengthen the idea of the existence of 4 distinct receptors for excitatory amino acids in the rat striatum. They are consistent with the presence on the kainate and quisqualate receptors, but not on the N-methyl-D-aspartate and glutamate/aspartate receptors of a hydrophobic domain which would provide a site of interaction for barbiturates and alcohols. They suggest that receptors for excitatory amino acids can be targets for the actions of barbiturates and alcohols on the central nervous system, and may mediate some of the anesthetic and hypnotic effects of these drugs.     

Non-NMDA receptor-mediated neurotoxicity in cortical culture

The neurotoxicity of 3 non-NMDA glutamate receptor agonists--kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), and quisqualate--was investigated quantitatively in dissociated murine cortical cultures. Five minute exposure to 500 microM kainate, but not AMPA, produced widespread acute neuronal swelling. Kainate-induced swelling was resistant to 2-amino-5-phosphonovalerate (APV) or replacement of extracellular sodium with choline but attenuated by either kynurenate or low concentrations of quisqualate. Unlike NMDA agonists, kainate or AMPA did not produce much late neuronal loss after a 5 min exposure. In contrast, 5 min exposure to 500 microM quisqualate produced both acute neuronal swelling and widespread late neuronal degeneration. This acute swelling was blocked by APV or by replacement of extracellular sodium by choline, consistent with mediation by NMDA receptors; we speculate that high concentrations of quisqualate may directly activate NMDA receptors or induce the release of endogenous glutamate. Quisqualate-induced late neuronal degeneration may be due to another unexpected process: cellular quisqualate uptake and delayed release, converting brief addition into prolonged exposure. Hours after thorough washout of exogenously added quisqualate, micromolar concentrations could be detected in the bathing medium by high performance liquid chromatography. With lengthy exposure (20-24 hr), all 3 non-NMDA agonists were potent neurotoxins, able to destroy neurons with EC50's of about 20 microM for kainate, 4 microM for AMPA, and 1 microM for quisqualate. Kynurenate and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but not APV or L-glutamate diethyl ester, were effective in attenuating the neuronal degeneration induced by these agonists. CNQX was about 3 times more selective than kynurenate against kainate-induced neuronal injury, but CNQX was still nearly equipotent with APV against NMDA-induced injury. Gamma-D-glutamylaminomethyl sulfonate exhibited partial antagonist specificity for AMPA-induced toxicity.     

Regulation of gene expression in astrocytes by excitatory amino acids.

We have studied the effect of excitatory amino acids on the expression of mRNA for the immediate early genes c-fos, c-jun, jun-B, and NGF-1A in isolated cortical astrocytes. The expression of the different genes was induced by 100 microM kainate, quisqualate, AMPA and high concentrations of K+ (140 mM). NMDA did not induce the expression of any of the genes studied. The effect of quisqualate stimulation was not inhibited by the antagonist CNQX or by withdrawal of external Ca2+. In contrast the kainate effect was abolished by CNQX but not by the removal of external Ca2+. However, elevated K+ induced c-fos only when calcium was present in the external medium. These findings suggest that type-1 astrocytes lack NMDA receptors and that the induction of genes by quisqualate and kainate is in part independent of the presence of calcium in the external medium and may be mediated through second messenger pathways.     

Some properties of membrane current fluctuations induced by kainate, quisqualate, and NMDA in cultured septal neurons of rat

Ionic currents induced by glutamate, kainate, quisqualate, and (NMDA) in cultured septal neurons were analyzed by fluctuation analysis. The power spectrum (PWS) of NMDA currentt fluctuations always fitted a single Lorentzian. PWSs of the other agonists fitted the sum of two Lorentzians; however, the slopes of PWSs became larger and the PWSs became closer to single Lorentzians as the number of drug application increased. This may be explained in such a way that, in multiple conductance channels activated by these agonists, the high frequency component decreases the gating activity in later recordings, whereas the low frequency component keeps its gating kinetics.