Excitatory amino acid-induced alterations of cytoplasmic free Ca2+ in individual cerebellar granule neurons: role in neurotoxicity

Elevated calcium after acute ischemic stroke: association with a poor short-term outcome and long-term mortality

BACKGROUND AND PURPOSE

An elevated intracellular calcium level is known to be a major initiator and activator of ischemic cell death pathway; however, in recent studies, elevated serum calcium levels have been associated with better clinical outcomes and smaller cerebral infarct volumes. The pathophysiological role played by calcium in ischemic stroke is largely unknown.

METHODS

Acute stroke patients from a prospective stroke registry, consecutively admitted during October 2002-September 2008, were included. Significant associations between the modified Rankin scale distribution at discharge and serum calcium or albumin-corrected calcium were identified using ordinal logistic regression analysis. Cox proportional hazard models were used for survival analysis.

RESULTS

Mean serum calcium and albumin-corrected calcium levels of the 1,915 participants on admission were 8.97±0.58 mg/dL and 9.07±0.49 mg/dL, respectively. Second [adjusted odds ratio 1.32 (95% confidence interval 1.07-1.61)] and third [1.24 (1.01-1.53)] tertiles of serum calcium level and the third [1.24 (1.01-1.53)] tertile of albumin-corrected calcium level were found to be independent risk factors for a poor discharge outcome. Significant relationships were observed with serum calcium [1.19 (1.03-1.38)] and albumin-corrected calcium [1.21(1.01-1.44)] as linear variables. However, only albumin-corrected calcium was associated with long-term mortality, third tertile [adjusted hazard ratio 1.40 (1.07-1.83)], and increase by 1 mg/dL [1.46 (1.16-1.84)].

CONCLUSIONS

Elevated albumin-corrected serum calcium levels are associated with a poorer short-term outcome and greater risk of long-term mortality after acute ischemic stroke.

Inhibition of voltage-gated Ca2+ channels by antazoline

We showed recently that imidazolines exert neuroprotection against hypoxia and NMDA toxicity in cerebellar and striatal neuronal cultures, through a voltage-dependent blockade of glutamatergic NMDA receptors. Here, we report that in striatal neuronal cultures from mouse embryos the imidazoline compound, antazoline, inhibits voltage-gated Ca2+ channels by acting at a phencyclidine-like site. This effect was fast, fully reversible, voltage-dependent and predominant on P/Q- and N-type Ca2+ channels. Taken together, these results suggest that imidazolines may elicit neuroprotective effects also by decreasing the release of glutamate through inhibition of presynaptic Ca2+ channels.

Strategies for neuroprotection against L-trans-2,4-pyrrolidine dicarboxylate-induced neuronal damage during energy impairment in vitro

Increased levels of extracellular excitatory amino acids and failure of energy metabolism are two conditions associated with brain ischemia. In the present study we have combined the simultaneous inhibition of glutamate uptake and mitochondrial electron transport chain to simulate neuronal damage associated with brain ischemia. Results show that cerebellar granule neurons are not vulnerable to transient glutamate uptake inhibition by L-trans-pyrrolidine-2,4-dicarboxylate (PDC) despite the increase in the extracellular concentration of glutamate, unless they are simultaneously exposed to the mitochondrial toxins 3-nitropropionic acid (3-NP) or sodium azide. Cell damage was assessed by light microscopy observation, by reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and by the fluorescent markers for live and dead cells, calcein and ethidium homodimer, respectively. The protective effect of alternative energy substrates, such as pyruvate, acetoacetate, and beta-hydroxybutyrate against PDC-induced neuronal death during 3-NP exposure was studied and compared to the effects of the antioxidant vitamin E, the spin trapper alpha-phenyl-N-tert-butylnitrone (PBN), voltage-dependent calcium channel antagonists, and glutamate receptor antagonists. Results show that neuronal damage can be efficiently prevented in the presence of pyruvate and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, whereas the non-NMDA receptor antagonist NBQX, acetoacetate, vitamin E, and PBN showed partial protection. In contrast, beta-hydroxybutyrate and voltage-dependent calcium channels blockers did not show any protective effect at the concentrations tested.

Possible role for the FosB/JunD AP-1 transcription factor complex in glutamate-mediated excitotoxicity in cultured cerebellar granule cells

The potent excitatory and neurotoxic actions of glutamate are known to influence the expression of a variety of genes, including those encoding the AP-1 transcription factor, which comprises proteins belonging to the Fos and Jun families. However, the precise role of Fos- and Jun-like transcription factors in these events remains elusive. Here we demonstrate, using primary cultures of mouse brain cerebellar granule cells as an in vitro model system, a possible involvement of the FosB/JunD heterodimer in excitotoxicity. Granule cells were grown for either 2 or 7 days in vitro (DIV) before exposure to varying concentrations (1-3000 microM) of the excitotoxin glutamate. In 7-DIV cells, glutamate induced a concentration-dependent neuronal death, whereas, in 2-DIV cells, no glutamate-induced neuronal damage was seen. We were particularly interested in comparing the protein composition of the AP-1 transcription factor complex in cells exposed to excitotoxic and to nontoxic conditions. AP-1 DNA binding activity was demonstrated by gel shift analysis in nuclear extracts derived from 7-DIV cells following exposure to either a nontoxic (10 microM) or an excitotoxic (250 microM) dose of glutamate and was similarly observed in extracts of 2-DIV cells exposed to the same levels of glutamate. Gel supershift analysis using antibodies against the different Fos and Jun family members allowed differentiation between AP-1 DNA binding in nuclear extracts as a function of both 1) viability status and 2) the stage of development. Of major significance was the finding that FosB could be detected as a component of AP-1 in 7-DIV cells only under excitotoxic conditions, whereas c-Fos, Fra-2, and JunD proteins were detectable under both excitotoxic and nontoxic conditions in cells of this age. In 2-DIV cells (in which glutamate is nontoxic), AP-1 comprised combinations of only Fra-1, Fra-2, c-Jun, and JunD. Because Fos family members are unable to form homodimers, this finding raises the possibility that the FosB/JunD heterodimer may have special significance in the mechanism of excitotoxic neuronal death.

The use of antioxidants to prevent glutamate-induced derangement of calcium ion metabolism in rat cerebral cortex synaptosomes

Glutamate is shown to induce increases in intracellular Ca2+ concentrations ([Ca2+]i), increases in 45Ca2+ influx, decreases in the activity of Na+,K+-ATPase activity, and activation of the Na+/Ca2+ exchanger in rat cerebral cortex synaptosomes. NMDA receptor antagonists virtually prevented these effects. Preincubation of synaptosomes with alpha-tocopherol, superoxide dismutase, and ganglioside GM1 normalized [Ca2+]i, 45Ca2+ influx, and Na+,K+-ATPase activity in rat cerebral cortex synaptosomes exposed to glutamate. Glutamate and GM1 activated the Na+/K+ exchanger, and their effects were additive. Calcium ions entering cerebral cortex nerve cells via NMDA receptors during exposure to high glutamate concentrations appeared to be only the trigger for the processes activating free-radical reactions. Activation of these reactions led to increases in Ca2+ influx into cells, decreases in Na+,K+-ATPase activity, and significant increases in [Ca2+]i, though this could be prevented by antioxidants and gangliosides.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Ca2+ store-dependent potentiation of Ca2+-activated non-selective cation channels in rat hippocampal neurones in vitro

1. Potentiation of calcium-activated non-selective cation (CAN) channels was studied in rat hippocampal neurones. CAN channels were activated by IP3-dependent Ca2+ release following metabotropic glutamate receptor (mGluR) stimulation either by Schaffer collateral input to CA1 neurones in brain slices in which ionotropic glutamate and GABAA receptors, K+ channels, and the Na+-Ca2+ exchanger were blocked or by application of the mGluR antagonist ACPD in cultured hippocampal neurones. 2. The CAN channel-dependent depolarization (DeltaVCAN) was potentiated when [Ca2+]i was increased in neurones impaled with Ca2+-containing microelectrodes. 3. Fura-2 measurements revealed a biphasic increase in [Ca2+]i when 200 microM ACPD was bath applied to cultured hippocampal neurones. This increase was greatly attenuated in the presence of Cd2+. 4. Thapsigargin (1 microM) caused marked potentiation of DeltaVCAN in CA1 neurones in the slices and of the CAN current (ICAN) measured in whole cell-clamped cultured hippocampal neurones. 5. Ryanodine (20 microM) also led to a potentiation of DeltaVCAN while neurones pretreated with 100 microM dantrolene failed to show potentiation of DeltaVCAN when impaled with Ca2+-containing microelectrodes. 6. The mitochondrial oxidative phosphorylation uncoupler carbonyl cyanide m-chlorophenyl hydrazone (2 microM) also caused a potentiation of DeltaVCAN. 7. CAN channels are subject to considerable potentiation following an increase in [Ca2+]i due to Ca2+ release from IP3-sensitive, Ca2+-sensitive, or mitochondrial Ca2+ stores. This ICAN potentiation may play a crucial role in the 'amplification' phase of excitotoxicity.

The difference in the effect of glutamate and NO synthase inhibitor on free calcium concentration and Na+, K+-ATPase activity in synaptosomes from various brain regions

The significant increase of free calcium concentration ([Ca2+]i) was found in rat cerebral cortex synaptosomes and hippocampal crude synaptosomal fraction after their exposure to glutamate. But no change of [Ca2+]i was revealed in cerebellar synaptosomes, the slight increase of [Ca2+]i in striatal synaptosomes was not significant. The presence of Ng-nitro-L-arginine methyl ester (L-NAME) in the incubation medium practically prevented the increase of [Ca2+]i initiated by glutamate in cerebral cortex synaptosomes, but not in hippocampal ones. The significant diminution of [Ca2+]i in the presence of this inhibitor was shown in striatal synaptosomes exposed to glutamate. Na+,K+-ATPase activity is significantly lower in cerebral cortex, striatal and hippocampal synaptosomes exposed to glutamate. L-NAME prevented the inactivation of this enzyme by glutamate. In cerebellar synaptosomes the tendency to the decrease of enzymatic activity in the presence of L-NAME was on the contrary noticed. Thus, the data obtained provide evidence of the protective effect of NO synthase inhibitor in brain cortex and striatal synaptosomes, but not in cerebellar synaptosomes. Synaptosomes appear to be an adequate model to study the regional differences in the mechanism of toxic effect of excitatory amino acids.

Assays using digital fluorescence: 1985-1998

Luminescence continues to provide comprehensive literature surveys which will be published in most issues. These are a continuation of the literature surveys begun in 1986 in the Journal of Bioluminescence and Chemiluminescence which, up until 1998, encompassed more than 6000 references cited by year or specialized topic. With this newly named journal these searches are expanding to reflect the journal's wider scope. In future we will cover all fundamental and applied aspects of biological and chemical luminescence and include not only bioluminescence and chemiluminescence but also fluorescence, time resolved fluorescence, electrochemiluminescence, phosphorescence, sonoluminescence, lyoluminescence and triboluminescence. The compilers would be pleased to receive any comments from the readership. Contact by e-mail: L.J. Kricka: larry_kricka@path1a.med.upenn.edu or P.E. Stanley: Stanley@LUMIWEB.COM Copyright 1999 John Wiley & Sons, Ltd.

Caspase-3 expression by cerebellar granule neurons is regulated by calcium and cyclic AMP

Caspase-3 enzyme activity is induced, and cell death follows, when cerebellar granule neurons (CGNs) from 8-day-old rats are transferred from an extracellular concentration of 25 mM K+ (25 mM [K+]e) to 5 mM [K+]e. Death of these neurons is diminished by an inhibitor of caspase-3 but not by an inhibitor of caspase-1. Actinomycin D and cycloheximide inhibit induction of caspase-3 and prevent death. Experiments in which CGN intracellular Ca2+ concentration ([Ca2+]i) was manipulated by either changing [K+]e or adding a voltage-gated Ca2+ channel antagonist or a Ca2+ ionophore to the medium showed that caspase-3 mRNA rises 2.5-fold when [Ca2+]i is diminished from 300 to 150 nM, with a corresponding rise in peak caspase enzyme activity. Whereas the caspase-3 mRNA level does not rise further with a still greater diminution in [Ca2+]i, peak caspase enzyme activity continues to increase, reaching sevenfold induction when [Ca2+]i is reduced to 55 nM. In CGNs in which [Ca2+]i is set at 55 nM by incubation in 5 mM [K+]e, treatment with forskolin or dibutyryl 3',5'-cyclic adenosine-5'-monophosphate delays caspase-3 induction and diminishes death but does not alter [Ca2+]i. We conclude that, in immature CGNs, both caspase-3 transcription and the subsequent processing of caspase-3 are induced by a fall in [Ca2+]i. Elevating cyclic AMP content delays caspase-3 induction by a mechanism that does not require an increase in [Ca2+]i.

Protection by imidazol(ine) drugs and agmatine of glutamate-induced neurotoxicity in cultured cerebellar granule cells through blockade of NMDA receptor

This study was designed to assess the potential neuroprotective effect of several imidazol(ine) drugs and agmatine on glutamate-induced necrosis and on apoptosis induced by low extracellular K+ in cultured cerebellar granule cells. Exposure (30 min) of energy deprived cells to L-glutamate (1-100 microM) caused a concentration-dependent neurotoxicity, as determined 24 h later by a decrease in the ability of the cells to metabolize 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) into a reduced formazan product. L-glutamate-induced neurotoxicity (EC50=5 microM) was blocked by the specific NMDA receptor antagonist MK-801 (dizocilpine). Imidazol(ine) drugs and agmatine fully prevented neurotoxicity induced by 20 microM (EC100) L-glutamate with the rank order (EC50 in microM): antazoline (13)>cirazoline (44)>LSL 61122 [2-styryl-2-imidazoline] (54)>LSL 60101 [2-(2-benzofuranyl) imidazole] (75)>idazoxan (90)>LSL 60129 [2-(1,4-benzodioxan-6-yl)-4,5-dihydroimidazole](101)>RX82 1002 (2-methoxy idazoxan) (106)>agmatine (196). No neuroprotective effect of these drugs was observed in a model of apoptotic neuronal cell death (reduction of extracellular K+) which does not involve stimulation of NMDA receptors. Imidazol(ine) drugs and agmatine fully inhibited [3H]-(+)-MK-801 binding to the phencyclidine site of NMDA receptors in rat brain. The profile of drug potency protecting against L-glutamate neurotoxicity correlated well (r=0.90) with the potency of the same compounds competing against [3H]-(+)-MK-801 binding. In HEK-293 cells transfected to express the NR1-1a and NR2C subunits of the NMDA receptor, antazoline and agmatine produced a voltage- and concentration-dependent block of glutamate-induced currents. Analysis of the voltage dependence of the block was consistent with the presence of a binding site for antazoline located within the NMDA channel pore with an IC50 of 10-12 microM at 0 mV. It is concluded that imidazol(ine) drugs and agmatine are neuroprotective against glutamate-induced necrotic neuronal cell death in vitro and that this effect is mediated through NMDA receptor blockade by interacting with a site located within the NMDA channel pore.

Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation

Following a hypoxic-ischemic insult, the collapse of ion gradients results in the inappropriate release of excitatory neurotransmitters. Although excitatory amino acids such as glutamate are the likely extracellular mediators of the ensuing neuronal cell death, the intracellular events occurring downstream of glutamate receptor activation are much less clear. The present review attempts to summarize how Ca2+ overload of neurons following a hypoxic-ischemic insult is neurotoxic. In particular, the interlocked relation between mitochondrial Ca2+ accumulation and subsequent neuronal cell death is examined.

Cerebral ischemia enhances polyamine oxidation: identification of enzymatically formed 3-aminopropanal as an endogenous mediator of neuronal and glial cell death

To elucidate endogenous mechanisms underlying cerebral damage during ischemia, brain polyamine oxidase activity was measured in rats subjected to permanent occlusion of the middle cerebral artery. Brain polyamine oxidase activity was increased significantly within 2 h after the onset of ischemia in brain homogenates (15.8 +/- 0.9 nmol/h/mg protein) as compared with homogenates prepared from the normally perfused contralateral side (7.4 +/- 0.5 nmol/h/mg protein) (P <0.05). The major catabolic products of polyamine oxidase are putrescine and 3-aminopropanal. Although 3-aminopropanal is a potent cytotoxin, essential information was previously lacking on whether 3-aminopropanal is produced during cerebral ischemia. We now report that 3-aminopropanal accumulates in the ischemic brain within 2 h after permanent forebrain ischemia in rats. Cytotoxic levels of 3-aminopropanal are achieved before the onset of significant cerebral cell damage, and increase in a time-dependent manner with spreading neuronal and glial cell death. Glial cell cultures exposed to 3-aminopropanal undergo apoptosis (LD50 = 160 microM), whereas neurons are killed by necrotic mechanisms (LD50 = 90 microM). The tetrapeptide caspase 1 inhibitor (Ac-YVAD-CMK) prevents 3-aminopropanal-mediated apoptosis in glial cells. Finally, treatment of rats with two structurally distinct inhibitors of polyamine oxidase (aminoguanidine and chloroquine) attenuates brain polyamine oxidase activity, prevents the production of 3-aminopropanal, and significantly protects against the development of ischemic brain damage in vivo. Considered together, these results indicate that polyamine oxidase-derived 3-aminopropanal is a mediator of the brain damaging sequelae of cerebral ischemia, which can be therapeutically modulated.

The role of the monosialoganglioside, GM1 as a neuroprotectant in an experimental model of cardiopulmonary bypass and hypothermic circulatory arrest

Twelve male dogs were placed on closed-chest cardiopulmonary bypass, subjected to 2 h of HCA at 18 degrees C, and rewarmed to 37 degrees C on closed-chest cardiopulmonary bypass. All animals were mechanically ventilated and monitored for 20 h before extubation and survived for 3 days. Group 1 dogs (n = 6) were pretreated with GM1, 30 mg/kg/24 h for 3 days before HCA, and received continuous infusion of GM1 during the procedure and 30 mg/kg/24 h for 3 days after HCA. Group 2 dogs (n = 6) received vehicle only. With a species-specific behavior scale that yielded a neurodeficit score ranging from 0% (normal) to 100% (brain dead), all animals were neurologically assessed every 12 h by two observers. After death at 72 h, brains were examined by glutamate receptor autoradiography and by histologic examination for patterns of selective neuronal necrosis and were scored blindly from 0 (normal) to 100 (severe injury). These results provide evidence of a role for GE in the development of HCA-induced brain injury and suggest that monosialogangliosides may have a neuroprotective effect in prolonged periods of HCA.

Excitatory amino acid-induced degeneration of dendrites of catecholamine neurons in rat substantia nigra

We have recently established a rat substantia nigra (SN) slice preparation in which a sensitive index of excitatory amino acid (EAA) toxicity was degeneration of the dendritic arbor of catecholamine neurons labelled by immunostaining for tyrosine hydroxylase (TH). The present study examined the pharmacological characteristics of EAA-induced neurotoxicity. Rats were anesthetised by halothane inhalation and killed, the brain was rapidly removed, and 400-microm-thick SN slices cut in the horizontal plane on a vibratome. Slices were incubated in saline buffer at 35 degreesC for 15 min to 6 h in the presence or absence or absence of kainic acid (KA) or N-methyl-D-aspartate (NMDA) in concentrations ranging from 10 to 500 microM. The slices were then fixed and resectioned into 40-microm sections that were coplanar with the parent slice. Dopaminergic SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. A feature of the immunostaining was that it labeled not only the cell body but also the prolific dendritic arborization of SN neurons. Dendritic damage was quantified by counting the proportion of neurons with intact dendrites after treatment with EAA. KA and NMDA caused loss of dendrites that was prevented by CNQX (20 microM) and MK-801 (20 microM), respectively, indicating that activation of either NMDA or non-NMDA receptors produces neurotoxicity. EAA-induced dendritic damage was observed within 2 h of treatment with a low concentration (10 microM) of KA and within 15 min if the concentration was increased to 500 microM. Thus the loss of dendrites occurs rapidly and precedes disintegration of the cell bodies. Furthermore, brief (15 min) exposure to EAA initiated damage in the dendrites which progressed after the EAA was removed from its receptor. The observations are consistent with the postulated role of EAAs in neurodegenerative diseases. Labeling the dendritic arbor provides a sensitive approach to investigating the cellular mechanisms of neurodegeneration of catecholamine neurons.

The role of calcium in apoptosis

One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid redistribution of ionised calcium throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Various physiological stimuli increase [Ca2+]i transiently and, thereby, induce cellular responses. However, under pathological conditions, changes of [Ca2+]i are generally more pronounced and sustained. Marked elevations of [Ca2+]i activate hydrolytic enzymes, lead to exaggerated energy expenditure, impair energy production, initiate cytoskeletal degradation, and ultimately result in cell death. Such Ca(2+)-induced cytotoxicity may play a major role in several diseases, including neuropathological conditions such as chronic neurodegenerative diseases and acute neuronal losses (e.g. in stroke).

Effect of L-glutamate on cholinergic neurotransmission in various brain regions and during the development of rats, when administered perinatally

Glutamate, as a monosodium salt (MSG) has neurotoxic effects on some brain regions when systemically given to young rats. Few studies have been conducted to establish the mechanisms involved in studying neurotoxicity resulting in neuronal death by glutamate (Glu) and its effects as related to different brain neuropathologies under in-vivo conditions and where the cholinergic system shows vulnerability. Thus, this paper aims to evaluate the binding kinetics of quinuclynidyl benzylate (QNB) to muscarinic receptors for acetylcholine and the activity of choline acetyltransferase (CAT) in rats treated with MSG (4 mg/g on days 1, 3, 5, and 7 after birth) during the rat development stages (days 14, 21, 30, and 60) in different brain regions. The results show that perinatal treatment with MSG significantly decreases the CAT activity and increases the affinity of [3H]-QNB and the number of receptors of the brain cortex during the ages studied. The striatum showed increased CAT activity and BMAX on days 30 and 60 after birth. Affinity and the number of receptors increased in the hippocampus only between days 21 through 60 after birth. NaCl given at MSG equimolar doses only modified the CAT activity but had no effect on the [3H]-QNB binding kinetics in any of the regions studied. The results show that MSG alters cholinergic neurotransmission in the central nervous system (CNS) and induces the development of compensating events suggesting an involvement in neuronal plasticity during the development of rat CNS.

Mastoparan-induced apoptosis of cultured cerebellar granule neurons is initiated by calcium release from intracellular stores

We have recently reported that mastoparan, a peptide toxin isolated from wasp venom, induces apoptosis in cultured cerebellar granule neurons that can be blocked by cholera toxin, an activator of Gs. Measurements of intracellular free calcium concentration ([Ca2+]i) reveal that mastoparan induces a dramatic elevation of [Ca2+]i that is frequently followed by enhanced leakage of fura-2 out of the neurons, suggesting that this rise in [Ca2+]i may be due to a more generalized change in membrane permeability. However, the mastoparan-induced initial elevation of [Ca2+]i is maintained in the absence of extracellular Ca2+, suggesting that the rise of [Ca2+]i is from intracellular stores. This conclusion is supported by the observation that depletion of [Ca2+]i stores by pretreatment with either caffeine or thapsigargin attenuates both the rise in [Ca2+]i and cell death induced by mastoparan. Phospholipase C (PLC) inhibitors, neomycin and U73122 block mastoparan-induced increases of [Ca2+]i and protect against neuronal death. Pretreatment with cholera toxin, but not pertussis toxin, reduced the mastoparan-induced rise in [Ca2+]i. Taken together, our data suggest that mastoparan initiates cell death in cerebellar granule neurons by inducing Ca2+ release from intracellular stores, probably via activation of PLC and IP3. A secondary or parallel process results in disruption of plasma membrane integrity and may be ultimately responsible for the death of these neurons by mastoparan.

Aluminum potentiates glutamate-induced calcium accumulation and iron-induced oxygen free radical formation in primary neuronal cultures

Aluminum is a neurotoxic metal that may be involved in the progression of neurodegenerative diseases, including Alzheimer disease and amyotrophic lateral sclerosis (ALS). Although the mechanism of action is not known, aluminum has been shown to alter Ca2+ flux and homeostasis, and facilitate peroxidation of membrane lipids. Since abnormal increases of intracellular Ca2+ and oxygen free radicals have both been implicated in pathways leading to neurodegeneration, we examined the effect of aluminum on these parameters in vitro using primary cultures of cerebellar granule cells. Exposure to glutamate (1-300 microM) caused a concentration-dependent uptake of 45Ca in granule cells to a maximum of 280% of basal. Pretreatment with AlCl3 (1-1000 microM) had no effect on 45Ca accumulation, but increased the uptake induced by glutamate. Similarly, AlCl3 had no effect on intracellular free Ca2+ levels measured using fluorescent probe fura-2, but potentiated the increase induced by glutamate. The production of reactive oxygen species (ROS) was examined using the fluorescent probe dichlorofluorescin. By itself, AlCl3 had little effect on ROS production. However, AlCl3 pretreatment potentiated the ROS production induced by 50 microM Fe2+. These results suggest that aluminum may facilitate increases in intracellular Ca2+ and ROS, and potentially contribute to neurotoxicity induced by other neurotoxicants.

Sodium channel modulators prevent oxygen and glucose deprivation injury and glutamate release in rat neocortical cultures

Neocortical cultures were deprived of oxygen and glucose to model ischemic neuronal injury. We used a graded series of periods of oxygen and glucose deprivation, providing graded insults. Cell death was measured by release of lactate dehydrogenase (LDH). One hundred and twenty to 240 min of deprivation caused graded increases in glutamate overflow, LDH release and 45Ca influx. Curves of LDH release with respect to deprivation time were shifted to longer intervals by treatment with tetrodotoxin (TTX; 3, 30 or 300 nM), phenytoin (10, 30 or 100 microM), lidocaine (10, 30 or 100 microM) or the N-methyl-D-aspartate antagonist CPP [3(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid, 3, 10, 30 or 100 microM]. Combined treatment with TTX and CPP caused pronounced rightward shifts of LDH deprivation curves. Our results indicate that Na+ channel blockade is neuroprotective in neocortex cultures. Our results also suggest that neuroprotection with Na+ channel blockers may be due to inhibition of glutamate release.

Media composition modulates excitatory amino acid-induced death of rat cerebellar granule cells

This study examined the effects of maintaining cells in different media and the role of serum in glutamate and NMDA-induced neurotoxicity in rat cerebellar granule cells. Glutamate stimulated a concentration-dependent cell death with similar potency in cerebellar granule cells grown in BME and Neurobasal media without serum. However, the maximal cell death to glutamate and N-methyl-D-aspartate (NMDA) varied in the different media compositions. In the presence of serum, glutamate and NMDA-induced excitotoxicity was abolished, suggesting a factor(s) in serum which influences glutamate-receptor mediated death. The protective effect of serum could be overcome by chronic stimulation with high doses of glutamate. The glutamate-stimulated increase in intracellular calcium load was attenuated in the presence of serum, resulting from an elevated basal calcium level, suggesting an association between raised basal calcium and neuroprotection.

Monosialoganglioside attenuates the excitatory and behavioural sensitization effects of ethanol

The effects of long-term monosialoganglioside GM1 treatment on the acute excitatory effects of ethanol and behavioural sensitization to this effect were studied, using locomotion frequency of mice observed in an open field as an experimental parameter. GM1 (30 mg/kg, once a day, for 21 days) did not modify mouse behaviour but decreased both the acute excitatory (1.8 g/kg) and the behavioural sensitization effects of ethanol (1.8 g/kg, once a day for 21 days, 30 min after GM1 injections). GM1 administered acutely 30 min or 24 h before ethanol did not modify the ethanol-induced increase in locomotion frequency. These results agree with previous reports in which ganglioside treatment modified both dopaminergic plasticity and other behavioural and biochemical effects of ethanol.

Determination of intracellular Ca2+ concentration can be a useful tool to predict neuronal damage and neuroprotective properties of drugs

The purpose of the present study was to examine the relationship between elevation in intracellular Ca2+ concentration ([Ca2+]i) and development of neuronal damage after cytotoxic hypoxia in vitro. Chick telencephalic neurons were exposed to NaCN 1 mM for up to 2 h. [Ca2+]i was assessed by means of fura-2 based microfluorometry and viability was measured by means of trypan blue exclusion on the same relocated cells for a period 24 h after initiation of hypoxia. Exposure to sodium cyanide resulted in an up to 10 fold increase in [Ca2+]i and led to subsequent neuronal damage. According to [Ca2+]i and viability neurons in four different stages could be revealed. The percentage of neurons showing elevated [Ca2+]i paralleled exactly the percentage of neuronal damage. The elevation in [Ca2+]i clearly preceded neuronal damage suggesting a time window for pharmacological intervention. The NMDA antagonists dizocilpine, memantine and amantadine were capable of reducing the percentage of neurons showing elevated [Ca2+]i and attenuated neuronal damage. Dizocilpine proved to be the most potent and amantadine to be the weakest antagonist whereas the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (NBQX) was ineffective. Under our experimental conditions, measurement of [Ca2+]i was able to predict the extent of neuronal damage as well as the neuroprotective potency of drugs.

Inability to restore resting intracellular calcium levels as an early indicator of delayed neuronal cell death

The hippocampus is especially vulnerable to excitotoxicity and delayed neuronal cell death. Chronic elevations in free intracellular calcium concentration ([Ca2+]i) following glutamate-induced excitotoxicity have been implicated in contributing to delayed neuronal cell death. However, no direct correlation between delayed cell death and prolonged increases in [Ca2+]i has been determined in mature hippocampal neurons in culture. This investigation was initiated to determine the statistical relationship between delayed neuronal cell death and prolonged increases in [Ca2+]i in mature hippocampal neurons in culture. Using indo-1 confocal fluorescence microscopy, we observed that glutamate induced a rapid increase in [Ca2+]i that persisted after the removal of glutamate. Following excitotoxic glutamate exposure, neurons exhibited prolonged increases in [Ca2+]i, and significant delayed neuronal cell death was observed. The N-methyl-D-aspartate (NMDA) channel antagonist MK-801 blocked the prolonged increases in [Ca2+]i and cell death. Depolarization of neurons with potassium chloride (KCl) resulted in increases in [Ca2+]i, but these increases were buffered immediately upon removal of the KCl, and no cell death occurred. Linear regression analysis revealed a strong correlation (R = 0.973) between glutamate-induced prolonged increases in [Ca2+]i and delayed cell death. These data suggest that excitotoxic glutamate exposure results in an NMDA-induced inability to restore resting [Ca2+]i (IRRC) that is a statistically significant indicator of delayed neuronal cell death.

Ethanol inhibits NMDA-induced toxicity and trophism in cultured cerebellar granule cells

In cerebellar granule cell cultures, glutamate and N-methyl-D-aspartate (NMDA) caused either neurotoxic or trophic effects, depending on the developmental stage of the neurones. Ethanol (100 mM) partly inhibited delayed neurotoxicity induced by the excitatory amino acids (25 microM glutamate for 15 min or 100 microM NMDA for 30 min) assessed 24 h after the incubations in mature cultures in the absence of Mg2+. Glycine (5 microM) potentiated the toxicity of glutamate and the ethanol inhibition, and was routinely added in these experiments. The viability of neurones in the presence of 25 mM K+ and 0.8 mM Mg2+ was not impaired when maintained in 40-50 mM ethanol for the whole culture period of 7 days. However, ethanol almost completely inhibited the trophic effects of NMDA on developing cultures in 12.5 mM K+/0.8 mM Mg2+ medium. Glutamate (25 microM) and NMDA (100 microM) potently induced 45Ca2+ uptake by granule cells from day 2 in vitro onward. Sixty-five per cent of the 15-min 45Ca2+ influx induced by glutamate and 80% of that induced by NMDA were inhibited by ethanol (100 mM). MK-801 (a non-competitive antagonist of NMDA receptors; 100 nM) completely inhibited the toxic and trophic actions of glutamate and NMDA, as well as the 45Ca2+ influx induced by NMDA, but only 80% of the 45Ca2+ influx induced by glutamate. These results show that the toxic and trophic actions of glutamate are mediated mainly by Ca2+ influx through NMDA receptors. Both of these actions and the underlying Ca2+ influx are significantly inhibited by ethanol at pharmacological concentrations (< or = 100 mM), although the mechanisms of inhibition still need further study.

Advances in the early management of patients with head injury

The last decade has seen continual improvement in our skills of visualizing and diagnosing the many types of human head injury. As we continue to unravel the complex biochemistry and molecular changes caused by trauma, we expect to find new methods and agents to enhance the extracellular milieu of injured but salvageable neurons and supporting cells, resulting in continued improvement in outcome for patients with severe head injury.

Ca(2+)- and Cl(-)-dependent, NMDA receptor-mediated neuronal death induced by depolarization in rat hippocampal organotypic cultures

The neurotoxicity induced by depolarization with high-K+ was investigated in rat hippocampal organotypic slice cultures. The exposure of cultures to 90 mM K+ solution for 30 min caused a severe neuronal injury in CA1 region while less damage was observed in CA3 and dentate gyrus over the following day. This neurotoxicity was prevented in a concentration dependent manner by NMDA antagonist MK-801 or CPP. Non-NMDA antagonist, DNQX, had no protective effect. Omission of Ca2+ from the exposure solution prevented the neurotoxicity. Voltage-dependent Ca2+ channel blockers, nifedipine and flunarizine, failed to prevent the neurotoxicity. These results suggest that the Ca2+ influx through the NMDA receptor is predominantly involved in this neurotoxicity. Apparent tissue swelling was observed immediately after the depolarization. This swelling was completely inhibited by omission of Cl- from the exposure solution, accompanied with complete protection against neurotoxicity. This suggests that Cl(-)-dependent tissue swelling also largely contributes to the neurotoxicity. Depolarization with application of MK-801 (10 microM) or omission of Ca2+ from the solution still caused apparent swelling, despite these treatment protected neuronal death. We hypothesize that Cl(-)-dependent tissue swelling may be involved in the release of the excitatory amino acid, which activates the NMDA receptor.

Prevention of apoptotic neuronal death by GM1 ganglioside. Involvement of Trk neurotrophin receptors

We have used serum-deprived cultures of wild type and genetically modified PC12 cells to investigate the molecular mechanisms by which monosialoganglioside (GM1) rescues neuronal cells from apoptotic death elicited by withdrawal of trophic support. Our findings indicate that GM1-promoted survival can be mediated in part by the Trk NGF receptor as well as by TrkB, and potentially by tyrosine kinase receptors for additional neurotrophic growth factors. Experiments employing K-252a, an inhibitor of Trk kinases, and PC12 cells overexpressing a dominant inhibitory form of Trk both indicate that a portion of the survival-promoting activity of GM1 is evoked by receptor dimerization and autophosphorylation. In consonance with this we find that GM1 stimulates Trk tyrosine autophosphorylation and Trk-associated protein kinase activity. These observations may provide a mechanism to account for the reported in vitro and in vivo trophic actions of GM1.

Complex correlation between excitatory amino acid-induced increase in the intracellular Ca2+ concentration and subsequent loss of neuronal function in individual neocortical neurons in culture

Primary cultures of cerebral cortical neurons and single-cell imaging of intracellular free Ca2+ concentration ([Ca2+]i) with the ratiometric dye fura-2 were used to assess excitatory amino acid (EAA)-induced neurotoxicity; the loss of neuronal function as defined by the ability of the cells to respond to K(+)-induced depolarization by a transient increase in Ca2+ influx was measured. The responsiveness of individual neurons was measured quantitatively as the [Ca2+]i values of the second KCl (2.KCl) stimulation divided by those of the first KCl (1.KCl) stimulation, giving the value of the ratio (2.KCl/1.KCl). Exposure to EAAs led to an increase in [Ca2+]i, but no simple correlation between the increase in [Ca2+]i and neuronal responsiveness could be demonstrated. Rather, below a threshold level of [Ca2+]i (ca. 1 microM), the neuronal responsiveness was largely independent of the glutamate receptor-agonist-induced increase in [Ca2+]i. However, when [Ca2+]i increased above this threshold level, the neurons almost invariably lost the ability to respond to a K(+)-induced depolarization, particularly after exposure to glutamate. Therefore, the cortical neurons were found to be exceptionally vulnerable to the glutamate-induced loss of function when compared with the effect induced by the glutamate receptor subtype-specific agonists, N-methyl-D-aspartate, quisqualate, and 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionate. The findings suggest that the loss of neuronal membrane polarization precedes plasma membrane disruption and is a sensitive marker of EAA-induced neurodegeneration observed at the single-cell level.

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)

Poly(ADP-ribose) polymerase: early involvement in glutamate-induced neurotoxicity in cultured cerebellar granule cells

Glutamate neurotoxicity is correlated with an increase of cytosolic free Ca2+. In some cell systems, activation of Ca2+ dependent endonucleases or formation of free radicals can damage DNA and activate the chromatin bound enzyme poly(ADP-ribose) polymerase (pADPRP). We have investigated whether pADPRP may be involved in glutamate neurotoxicity in vitro. Cerebellar granule cells at 12 days in culture when treated with a toxic dose of glutamate (100 microM) showed a rapid and transient increase of polyADP-ribose immunoreactivity. Cellular immunostaining was heterogeneous and returned to control levels after washout of glutamate. In the same cell preparations glutamate elicited a marked increase in enzyme protein immunoreactivity which persisted at later times. Non-toxic doses of glutamate did not affect immunostaining. In another set of experiments, pADPRP mRNA was increased 30 min after glutamate. In order to investigate the role of pADPRP in glutamate-mediated neurotoxicity, structurally different inhibitors of pADPRP (3-aminobenzamide, benzamide,3-aminophthalhydrazide) and their inactive analogues (benzoic acid and phthalimide) were tested in this model. Addition of the inhibitors to cultures 60 min before and during the 30 min of glutamate treatment prevented neuronal death by 60-100%, assessed 24 hr later. Glutamate-induced Ca2+ influx was not affected. Inactive analogues failed to afford neuroprotection. These data indicate that not only is pADPRP activated by the early, possibly Ca(2+)-mediated mechanisms initiated by glutamate, but that it might also actively contribute to the subsequent neuronal death.

Enhancement of NMDA toxicity and calcium responses by chronic exposure of cultured cortical neurons to ethanol

Exposure of rat cerebrocortical cultures to 100 mM ethanol for 3-4 days increased both the neurotoxic potency of N-methyl-D-aspartate (NMDA) and the maximal extent of NMDA-induced intracellular calcium (Ca2+i) elevations. In both control and ethanol-treated cultures, NMDA toxicity correlated closely with [Ca2+]i. Enhancement of NMDA responses may reflect neuronal adaptation to chronic ethanol exposure and could contribute to the pathogenesis of alcohol-related neurologic disorders.

Effects of gangliosides GM1 and GD1a on GAP-43 phosphorylation and dephosphorylation in isolated growth cones

Phosphorylation of the nervous system-specific protein GAP-43 in growth cones in vivo increases as the growth cones near their targets, at a time when the gangliosides GM1 and GD1a are being accumulated in the growth cone membrane, thus raising the possibility that the gangliosides could modulate GAP-43 behavior. We used a subcellular fraction of intact isolated growth cones to show that both GM1 and GD1a affected the calcium-dependent posttranslational regulation of GAP-43 in several similar ways. Both gangliosides induced rapid incorporation of phosphate into GAP-43; however, the induction was undetectable with our antibody 2G12 that is specific for kinase C-phosphorylated GAP-43. Furthermore, neither ganglioside stimulated kinase C activity in isolated growth cones, suggesting that the rapid phosphorylation may not be on Ser41, the kinase C site. However, both gangliosides did induce a slower accumulation of GAP-43 phosphorylated on Ser41, apparently by inhibiting a phosphatase. Finally, calcium-dependent proteolysis of GAP-43 was also stimulated by both GM1 and GD1a. In contrast, GD1a, but not GM1, caused the redistribution of GAP-43 into the isolated growth cone cytoskeleton. The results demonstrate that both gangliosides can modulate the calcium-dependent regulation of GAP-43.

Fluorimetric determination of electrically evoked increase in intracellular calcium in cultured cerebellar granule cells

A technique is described to measure the electrically evoked increase in intracellular calcium in cerebellar granule cells cultured on glass coverslips and preloaded with FURA-2. To minimize light scattering, the coverslip containing the granules was placed in the fluorimeter cuvette at a 30 degrees angle to the exciting light beam. The cuvette was provided with 2 platinum electrodes so as to stimulate the neurons with a tangential field. The [Ca2+]i transients were maximized by omitting Mg2+. The fluorescence peaks were directly related to the pulse (1 ms, 100 mA) frequency and to the train length. The responses were completely tetrodotoxin- and [Ca2+]o-dependent and could be replicated 5-6 times at 5-min intervals. At the stimulation rate of 20 Hz for 5 s, a condition ensuring submaximal peaks, the [Ca2+]i rose from the basal levels of 41 +/- 2.7 nmol/l to 89.6 +/- 5.8 nmol/l. The participation of various membrane channels in the electrically induced [Ca2+]i increase was demonstrated. 4-Aminopyridine (1 mM) increased the height of the peaks to 240%. Both nifedipine (10 microM) and omega-conotoxin (1 microM) reduced the transients by about 25%. The residual response (in the absence of Mg2+) depended mostly on the release of endogenous glutamate as it proved sensitive to NMDA, AMPA and t-ACPD receptor antagonists. Since a technique to measure the electrically evoked release of D-[3H]aspartate is presently available, the parallel determination of release and of [Ca2+]i in twin populations of cultured granule cells is possible.

Calcium indicators and excitotoxicity in cultured cortical neurons

Membrane-permeating, fluorescent Ca2+ indicators have been used to investigate the role of increased intracellular Ca2+ (Ca2+i) levels in excitotoxic neuronal injury, but their ability to chelate Ca2+i and their own toxic effects in some cells could obscure this relationship. N-Methyl-D-aspartate (NMDA)-stimulated Ca2+i responses and toxicity were measured in neuron-enriched rat cerebrocortical cultures loaded with either fluo-3 or fura-2. Ca2+i responses signaled by both indicators were similar in magnitude, and neither indicator reduced NMDA toxicity, measured by lactate dehydrogenase (LDH) release. Fluo-3 and fura-2 appear to be suitable for comparative studies of NMDA-induced Ca2+i responses and excitotoxicity.

The monosialoganglioside GM1 dose-dependently reduces regional cerebral metabolic rates for glucose in awake rats

Using the quantitative autoradiographic [14C]2-deoxyglucose technique, regional cerebral metabolic rates for glucose (rCMRglc) were measured in awake male Fischer-344 rats at 1, 2, 3, 4 and 6 h after administration of GM1 30 mg/kg and at 3 h after GM1 150 or 300 mg/kg. GM1 is a natural compound that is able to prevent neuron degeneration induced by exposure to excitatory amino acids in vitro and by ischemia or neurotoxins in vivo. GM1 30 mg/kg, a dose very effective in preventing excitatory amino acid-induced neurotoxicity, produced minimal rCMRglc change over a 6 h period. GM1 150 and 300 mg/kg reduced rCMRglc, in 14 (31%) and in 29 (64%) brain regions, respectively. Maximal metabolic effects occurred in hippocampal areas which possess, in specific subfields, the highest brain concentrations of different excitatory amino acid receptor subtypes. This finding suggests an effect by GM1 on postreceptor mechanisms common to different excitatory amino acids.

Desensitization of the NMDA receptor complex by glycinergic ligands in cerebellar granule cell cultures

Glutamate neurotoxicity was examined in cultured cerebellar granule neurons following both prolonged (20-24 h) and brief (45 min) exposure to compounds acting at strychnine-insensitive glycine receptors. Glutamate neurotoxicity was reduced in a concentration-dependent fashion by brief exposure to the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966) and the competitive antagonist, 7-chlorokynurenic acid (7-CK) with a rank order efficacy: 7-CK > HA-966 > ACPC. Neither D-cycloserine (D-CS) nor glycine affected neurotoxicity produced by maximum glutamate concentrations, while glycine but not D-CS augmented the effects of submaximum glutamate concentrations. Prolonged exposure of cultures to either full (glycine) or partial agonists (ACPC, D-CS, HA-966) abolished the neuroprotective effects of ACPC and significantly diminished the neuroprotective effects of HA-966. In contrast, the neuroprotective effects of 7-CK were only marginally reduced by prolonged exposure to glycinergic ligands, while the neuroprotection afforded by compounds acting at other loci on the NMDA receptor complex (e.g. 2-amino-5-phosphonopentanoate (APV) and dizocilpine (MK-801)) were unaltered. These effects may represent homologous desensitization of the NMDA receptor complex at its strychnine-insensitive glycine receptor induced by prolonged exposure to glycinergic agonists and partial agonists. Nonetheless, levels of the NMDA receptor subunit zeta 1 mRNA were unaffected by prolonged exposure to ACPC, indicating the apparent desensitization could involve a post-translational modification of the NMDA receptor complex.

Basic fibroblast growth factor modulates sensitivity of cultured hippocampal pyramidal neurons to glutamate cytotoxicity: interaction with ganglioside GM1

Basic fibroblast growth factor (bFGF), a polypeptide originally identified as a mitogen for a variety of cells including astroglial cells, also exhibits neurotrophic (survival) effects on a number of neuronal populations, among the latter being hippocampal pyramidal cells. The present study investigated the effects of bFGF on the sensitivity of pyramidal neurons to the excitatory neurotransmitter, glutamate, and possible modulation by monosialoganglioside GM1. Cultures were generated from embryonic day 18 rat hippocampus, and first treated with bFGF at 4-5 days in vitro. Twenty-four hours later, cells were exposed to glutamate (100 microM-1 mM) for a further 24 h in the continued presence of bFGF. The cytotoxic action caused by 200-500 microM glutamate, which normally is present at this culture stage, was reduced by bFGF in a concentration- and time-dependent manner. GM1 (100 microM), given alone 2 h prior to glutamate, also limited this neuronal loss by 50-80%. At lower concentrations, neither bFGF (0.3 ng/ml) nor GM1 (1-10 microM) alone for 24 h was effective in altering neuronal sensitivity to glutamate. However, given together for 24 h these levels of bFGF and GM1 were almost as efficacious as bFGF alone at 3-10 ng/ml. Similar results were obtained with more mature (12 day) cultures. The ability of GM1 to modulate trophic factor actions towards excitatory amino acids makes gangliosides useful tools in the study of central nervous system plasticity and repair processes.

35 mM K(+)-stimulated 45Ca2+ uptake in cerebellar granule cell cultures mainly results from NMDA receptor activation

In primary cultures of cerebellar granule cells, the Ca2+ influx resulting from K+ depolarization (35 mM) was equal to one-third of that observed with 100 microM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase (+2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and 45Ca2+ uptake measured after a 35 mM K+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K+. However, nifedipine (1 microM), which decreased by 60% the K(+)-induced 45Ca2+ uptake, did not reduce the 35 mM K(+)-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K(+)-stimulated 45Ca2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca2+ influx to occur through L-type voltage-operated Ca2+ channels.

Interaction of ganglioside GM1 with the B subunit of cholera toxin modulates intracellular free calcium in sensory neurons

The B subunit of cholera toxin, which binds specifically to GM1 ganglioside on cell surfaces, has previously been shown to modulate intracellular calcium levels and growth in several cell types. To explore a role for such changes in calcium in the growth regulatory function of cell-associated GM1 in neurons, dissociated neurons from chicken embryonic day 8 dorsal root ganglia were exposed to the B subunit. To enhance sensitivity to B subunit, some neurons were also enriched with added GM1 (100 microM) and then exposed to B subunit. Incubation of naive cultures with 1 microgram/ml of the B subunit was sufficient to produce modest increases in intracellular free calcium above basal levels in a minor percentage of cells for at least 5 min, as measured by fura-2 fluorescence imaging. Pretreatment of the cells with GM1 for 48 hr increased even further the elevations in intracellular free calcium and the percentage of responding neurons observed after B subunit exposure. These increases in intracellular calcium required the presence of external Ca2+, but were not inhibited by calcium channel blockers. Such changes in calcium were accompanied by fine alterations in morphology affecting mostly the branching of neurites and were more pronounced in the presence of GM1. However, the morphological changes did not result in altered neurofilament protein expression. Immunogold electron microscopy using anti-choleragenoid depicted extensive aggregations of immunoreactive gold particles on neuronal surfaces, which were more extensive in cells treated with GM1. The results demonstrate that cell incorporated GM1 may modulate calcium fluxes, perhaps accounting for the growth regulatory functions of GM1 in both neuronal and other cell types.

N-methyl-D-aspartate responses in rat cerebellar granule cells are modified by chronic depolarisation in culture

Following culture in high (25 mM) K+ conditions cerebellar granule cells only respond with a rise in cytosolic free calcium concentration ([Ca2+]i after removal of external Mg2+. When granule cells are grown in low (5 mM) K+ N-methyl-D-aspartate (NMDA) exerts a neurotrophic effect. We show that at the critical time for this effect NMDA will elicit a rise in [Ca2+]i in 5 mM K+ cultures even in the presence of Mg2+ and that growth in 25 mM K+ induces the rapid appearance of a Mg2+ block of NMDA receptors in granule cells. This suggests firstly, that a rise in [Ca2+]i could be involved in the neurotrophic effect of NMDA and secondly, that the characteristics of the NMDA responses in granule cells are modified as a result of growth under depolarising conditions.

Acetylcholine induces voltage-independent increase of cytosolic calcium in mouse myotubes

Electrophysiological, biochemical, and Ca2+ imaging studies of cultured mouse myotubes were used to investigate whether the neurotransmitter acetylcholine causes an increase in intracellular Ca2+ concentration ([Ca2+]i) through activation of a second messenger system. Bath applications of acetylcholine to myotubes (i) elicited a significant membrane current even in a Na(+)-free Ca2+ medium, when the current was carried mainly by calcium ions; (ii) caused a rapid and transient cytosolic accumulation of inositol 1,4,5-trisphosphate; (iii) evoked a conspicuous alpha-bungarotoxin-sensitive long-lasting [Ca2+]i enhancement even in the presence of Cd2+; and (iv) transiently increased [Ca2+]i when cells were equilibrated in a Ca(2+)-free atropine-containing medium. We propose that, in addition to opening ion channels, the nicotinic action of acetylcholine on the muscle cell membrane increases [Ca2+]i through activation of the inositol 1,4,5-trisphosphate second messenger system and mobilization of Ca2+ from intracellular stores.

Ganglioside GM1 prevents death induced by excessive excitatory neurotransmission in cultured hippocampal pyramidal neurons

Rat hippocampal pyramidal neurons in culture, exposed 30 min to Mg(2+)-free, glycine-supplemented medium undergo a selective (about 35%) degeneration over the next 24 h. This neuronal injury appeared to result from excitatory synaptic transmission and subsequent activation of N-methyl-D-aspartate (NMDA) receptors, as cell death could be blocked by tetrodotoxin and NMDA, but not non-NMDA, receptor antagonists. Ganglioside GM1, which has recently been described to protect against excitotoxin-induced damage, also prevented the death of hippocampal neurons associated with the above phenomenon. Gangliosides may be a novel therapeutic tool for brain injury associated with epileptic-like activity.