Low calcium-induced release of glutamate results in autotoxicity of cerebellar granule cells

Cl(-)-dependent excitotoxicity is associated with 3H2O influx in chick embryonic retina

The aim of this study was to show that Cl(-)-dependent excitotoxicity, with its characteristic cell swelling, involves actual water influx into the intracellular compartment. Taking advantage of the Ca2+ omission paradigm of Cl(-)-dependent excitotoxicity, in the chick embryonic neural retina ex vivo, which is associated with toxicity levels (lactate dehydrogenase (LDH) release) considerably higher than those seen after simple exposure of the retinas to glutamate agonists, we have demonstrated that an intracellular water intake of 4.2 microl into retinal cells is associated with 13.3% total retinal LDH release. The fact that mannitol blocks both water inflow and LDH release appears to link both events from a pathogenic point of view.

Ca2+-dependent kainate excitotoxicity in the chick embryonic neural retina ex vivo

The chick embryonic neural retina ex vivo has been singled out as a unique example of Cl(-)-dependent/Ca2+-independent excitotoxicity. However, after continuous incubation with 100 microM kainate, we have demonstrated the susceptibility of the chick retina to Ca2+-mediated damage, which becomes apparent after 12 h of exposure to the agonist in the absence of Cl-. Of the 20.8% lactate dehydrogenase released after 24 h incubation with kainate, some 11% is Cl(-)-dependent and the rest (9.8%) is presumably Ca2+-dependent. Upon omission of both Cl- and Ca2+, a 5% residual toxicity can still be detected after 24 h. This can be overcome by inclusion of EGTA in the incubation medium to neutralize Ca2+ released during incubation. A Ca2+-dependent toxicity mechanism is then operative in the embryonic chick retina ex vivo.

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

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

Calcium channelopathies

Calcium is an important intracellular signaling molecule, and altered calcium channel function can cause widespread cellular changes. Genetic mutations in calcium channels that cause what appear to be trivial alterations of calcium currents in vitro can result in serious diseases in muscles and the nervous system. This article reviews calcium channelopathies in humans and mice.

Altered calcium channel currents in Purkinje cells of the neurological mutant mouse leaner

Mutations of the alpha1A calcium channel subunit have been shown to cause such human neurological diseases as familial hemiplegic migraine, episodic ataxia-2, and spinocerebellar ataxia 6 and also to cause the murine neurological phenotypes of tottering and leaner. The leaner phenotype is recessive and characterized by ataxia with cortical spike and wave discharges (similar to absence epilepsy in humans) and a gradual degeneration of cerebellar Purkinje and granule cells. The mutation responsible is a single-base substitution that produces truncation of the normal open reading frame beyond repeat IV and expression of a novel C-terminal sequence. Here, we have used whole-cell recordings to determine whether the leaner mutation alters calcium channel currents in cerebellar Purkinje cells, both because these cells are profoundly affected in leaner mice and because they normally express high levels of alpha1A. In Purkinje cells from normal mice, 82% of the whole-cell current was blocked by 100 nM omega-agatoxin-IVA. In Purkinje cells from homozygous leaner mice, this omega-agatoxin-IVA-sensitive current was 65% smaller than in control cells. Although attenuated, the omega-agatoxin-IVA-sensitive current in homozygous leaner cells had properties indistinguishable from that of normal Purkinje neurons. Additionally, the omega-agatoxin-IVA-insensitive current was unaffected in homozygous leaner mice. Thus, the leaner mutation selectively reduces P-type currents in Purkinje cells, and the alpha1A subunit and P-type current appear to be essential for normal cerebellar function.

Ca2+-independent excitotoxic neurodegeneration in isolated retina, an intact neural net: a role for Cl- and inhibitory transmitters

Rapidly triggered excitotoxic cell death is widely thought to be due to excessive influx of extracellular Ca2+, primarily through the N-methyl-D-aspartate subtype of glutamate receptor. By devising conditions that permit the maintenance of isolated retina in the absence of Ca2+, it has become technically feasible to test the dependence of excitotoxic neurodegeneration in this intact neural system on extracellular Ca2+. Using biochemical, Ca2+ imaging, and electrophysiological techniques, we found that (1) rapidly triggered excitotoxic cell death in this system occurs independently of both extracellular Ca2+ and increases in intracellular Ca2+; (2) this cell death is highly dependent on extracellular Cl-; and (3) lethal Cl- entry occurs by multiple paths, but a significant fraction occurs through pathologically activated gamma-aminobutyric acid and glycine receptors. These results emphasize the importance of Ca2+-independent mechanisms and the role that local transmitter circuitry plays in excitotoxic cell death.

Calcium ionophore-induced degradation of neurofilament and cell death in MSN neuroblastoma cells

Extensive necrotic death of MSN neuroblastoma cells could be induced after incubation with the calcium ionophore, A23187. The reaction was concentration-dependent and time course-dependent. Levels of the 66 kd/alpha-internexin neurofilament protein (NF-66) and the cognate heat shock protein 70 (Hsc 70) decreased during the Ca2+-activated cell death. Addition of the calcium chelator, ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) restored the normal level of NF-66 and partially that of the Hsc 70. Use of either calpain I or calpain II inhibitor could alleviate the reduction of 66 kd protein during the ionophore treatment whereas only calpain I inhibitor treatment was effective in restoring the normal level of the Hsc 70. Neither of these calpain inhibitors could block the ionophore triggered cell death. EGTA was toxic to cells in a wide range of concentration suggesting a calcium-independent activation of cell death mechanism.

Prenatal ethanol exposure enhances glutamate release stimulated by quisqualate in rat cerebellar granule cell cultures

Effects of prenatal ethanol exposure on extracellular glutamate accumulation stimulated by glutamate receptor agonists were studied in rat cerebellar granule cell cultures. The prenatal exposure to ethanol was achieved via maternal consumption of a Sustacal liquid diet containing either 5% ethanol or isocaloric sucrose (pair-fed) substituted for ethanol from gestation d 11 until the day of parturition. Neither the basal level of extracellular glutamate nor the increased accumulation of glutamate stimulated by KCl (40 mM) or by ionotropic glutamate receptor agonists, N-methyl-D-aspartate (NMDA) or kainate (KA) (100 microM each), in cells prepared from the ethanol-fed group was significantly different from that in cells prepared from the pair-fed group. Glutamate accumulation stimulated by quisqualate (QA, 100 microM) or by trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 250 microM) in the ethanol-fed group was higher than that in the pair-fed group by 116 and 36%, respectively. In the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 100 microM), an ionotropic QA receptor antagonist, the QA-induced accumulation of glutamate in the ethanol-fed group was still higher than that in the pair-fed group. In the presence of MK-801 (5 microM), an antagonist of the NMDA receptor, the enhanced accumulation of glutamate stimulated by either QA or t-ACPD was still observable in the ethanol-fed group as compared to the pair-fed group. Addition of (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG, 500 microM), a selective antagonist of the metabotropic glutamate receptor, abolished the enhanced accumulation of glutamate stimulated by either QA or t-ACPD in the ethanol-fed group. Although immunoblotting of mGluR1 and mGluR2/3 did not show apparent differences between the pair-fed and the ethanol-fed groups, the overall results suggest that the effect of prenatal ethanol exposure was selectively through a pathway mediated by the metabotropic glutamate receptor.

Effects of inhalation anesthetics of Kainate-induced glutamate release from cerebellar granule cells

The mechanisms of inhalation anesthesia may include inhibiting non NMDA excitatory amino acid neurotransmission. This possibility was addressed by measuring the effect of three anesthetics at clinically relevant concentration on kainate-induced glutamate release from cerebellar granule cells. Cerebellar granule cells were obtained from 8-day-old SD rats and maintained in vitro for 9-14 days. Medium glutamate concentrations were measured by HPLC after 90 minutes incubation with kainate or NMDA. Inhalation anesthetics were introduced by holding the cells under a continuous flow of air/anesthetic mixtures. All anesthetics tested did not effect NMDA-induced glutamate release. Halothane (1 MAC), isoflurane (1 MAC) inhibited kainate-induced glutamate release from the cultured cells whereas enflurane (1 MAC) had no effect on kainate-induced glutamate release. The difference between enflurane and the other anesthetics tested suggests that anesthesia is dependent on more than one process and the extent at which each function is perturbed is dependent on the specific anesthetic used. Halothane inhibition of kainate-induced glutamate release was not reversible by increasing kainate concentration, indicating halothane does not directly compete with kainate at its receptor.

Kainate produces concentration-dependent elevation of glutamate release but not cGMP levels in cultured neuron

1. Treatment of cultured cerebellar granule cells for 3 min with N-methyl-D-aspartate (NMDA) resulted in a concentration-dependent elevation of cyclic GMP. However, neither kainate (KA) nor NMDA produced a concentration-dependent elevation of this nucleotide after exposing cells to the agonist for 60 min. 2. Unlike the case for cGMP, both KA and NMDA produced concentration-dependent elevations of glutamate for 60 min incubation. 3. The NMDA-induced elevations of cGMP and glutamate were blocked by selective NMDA receptor antagonists. 4. The selective KA/alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist, 6,7-nitroquinoxaline-2,3-dione (DNQX), blocked the KA-induced elevations of cGMP with 3-min exposures, but it augmented the response with 60-min exposures. However, the KA-induced release of glutamate was prevented by DNQX. 5. The KA/AMPA receptor antagonist, GYKI 52466, blocked all KA-induced responses regardless of the incubation times.

Increased synthesis of specific proteins during glutamate-induced neuronal death in cerebellar culture

We have previously shown that glutamate-induced neurotoxicity is mediated by a sodium-chloride component and a calcium component in our cerebellar granule cell culture. In order to further characterize these two different components, the time course of neuronal death induced by glutamate (100 microM) in basal solution and in low sodium-chloride solution was studied by morphological and biochemical criteria. As shown by phase-contrast microscopy, cerebellar granule cells exhibited clear neuronal degeneration within 4 h after exposure to this excitotoxin. These morphological changes correlated [35S]methionine incorporation into proteins which rapidly declined during the first hour of treatment. Qualitative change in [35S]methionine incorporation into proteins was further investigated by two-dimensional gel electrophoresis performed after glutamate exposure in basal solution and in low sodium-chloride solution. Most of the proteins showed a decreased labelling after glutamate exposure as expected, but some polypeptides showed an increased labelling or appeared to be newly synthesized. Furthermore, a different pattern of protein synthesis was observed when glutamate exposure was performed in basal solution or in low sodium-chloride solution. The identification of these polypeptides and their implication in the neuronal death are discussed.

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)

Programmed cell death: its possible contribution to neurotoxicity mediated by calcium channel antagonists

Organic calcium channel antagonists attenuate ischemic or excitotoxic neuronal injury, probably by limiting Ca2+ influx through the voltage-gated calcium channels. However, the possibility that calcium channel antagonists may compromise neuronal survival with long-term exposure has not been systemically examined. In the present study, we report that cerebral cortical cultures exposed for 2 days to either nifedipine, verapamil, diltiazem, or flunarizine, undergo selective neuronal degradation in a concentration-dependent fashion. This degeneration could be attenuated by protein synthesis inhibitors cycloheximide and actinomycin-D. Cortical cultures incubated for 2 days in low calcium media also exhibit widespread neuronal damage, which is similarly blocked by cycloheximide. Although we cannot exclude other possibilities, these findings suggest that a decrease in intraneuronal calcium levels may trigger synthesis of proteins mediating neuronal cell death. Regardless of the exact toxic mechanisms involved, additional studies on neurotoxicity of calcium channel antagonists seem warranted since some of these compounds are currently being clinically used.

Amitriptyline prevents N-methyl-D-aspartate (NMDA)-induced toxicity, does not prevent NMDA-induced elevations of extracellular glutamate, but augments kainate-induced elevations of glutamate

The effect of amitriptyline on kainate- and N-methyl-D-aspartate (NMDA)-induced toxicity and release of amino acids from cerebellar granule neurons was studied. The ED50 for amitriptyline, imipramine, and nortriptyline protection against NMDA-induced toxicity was 6.9, 6.5, and 1.3 microM, respectively. None of these compounds protected against kainate-induced toxicity. Even though amitriptyline was protective against NMDA-induced toxicity, it had no effect on the NMDA-induced increase in extracellular levels of glutamate or aspartate from these cells, indicating a dissociation between NMDA receptor activation (as indicated by glutamate content elevations) and NMDA-induced toxicity. However, kainate and quisqualate treatment resulted in elevations of glutamate and taurine levels that were further augmented in the presence of 25 microM amitriptyline. These findings confirm the reports of others that tricyclic antidepressants have neuroprotective effects related to the NMDA receptor and expand on these reports by showing that even though there is protection against toxicity, the NMDA receptor is nevertheless activated, suggesting an involvement of these compounds at sites removed from the receptor. Furthermore, this is the first report showing an interaction of tricyclic antidepressants with the function of non-NMDA receptors.

Cyanide selectively augments kainate- but not NMDA-induced release of glutamate and taurine

The effect of cyanide on the kainate-, quisqualate- and N-methyl-D-aspartate (NMDA)-induced release of several amino acids from cerebellar granule neurons was studied. Cyanide, 100 microM, augmented the kainate- and quisqualate-induced release of glutamate and taurine in neurons but had no effect on the NMDA-induced release of these excitatory amino acids. In addition to the interaction with the above excitatory amino acids, cyanide had effects on several amino acids independent of excitatory amino acid stimulation; cyanide treatment resulted in a significant elevation over saline controls of arginine and taurine, but not alanine, aspartate+asparagine or glycine. With the exception of taurine, this pattern was not apparent in cells treated with any of the above excitatory amino acid.

Acute glutamate toxicity and its potentiation by serum albumin are determined by the Ca2+ concentration

Two different processes, mediated by the N-methyl-D-aspartate receptor, appear to cause acute cell death in cultured cerebellar granule cells. A Ca(2+)-independent process takes place at zero and very low concentration of the added cation. Under these conditions, the known destabilization of excitable membranes at low extracellular Ca2+ probably plays a major role. A Ca(2+)-dependent process becomes dominant as its concentration is increased above 1.0 mM. The remarkable potentiation of glutamate toxicity by serum albumin is a calcium-dependent reaction.

T-cell-mediated cytotoxicity

There are two competing theories to explain the mechanism(s) by which cytolytic T lymphocytes kill target cells: granule exocytosis of a pore-forming protein, and contact-induced internal disintegration. Accumulated evidence supports alternative pathways in lymphocytoxicity, possibly reflecting distinct effector functions expressed by different killer cells and cells at different stages of activation.