Ionic requirements for neurotoxic effects of excitatory amino acid analogues in rat cerebellar slices

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

Altered calcium signaling following traumatic brain injury

Cell death and dysfunction after traumatic brain injury (TBI) is caused by a primary phase, related to direct mechanical disruption of the brain, and a secondary phase which consists of delayed events initiated at the time of the physical insult. Arguably, the calcium ion contributes greatly to the delayed cell damage and death after TBI. A large, sustained influx of calcium into cells can initiate cell death signaling cascades, through activation of several degradative enzymes, such as proteases and endonucleases. However, a sustained level of intracellular free calcium is not necessarily lethal, but the specific route of calcium entry may couple calcium directly to cell death pathways. Other sources of calcium, such as intracellular calcium stores, can also contribute to cell damage. In addition, calcium-mediated signal transduction pathways in neurons may be perturbed following injury. These latter types of alterations may contribute to abnormal physiology in neurons that do not necessarily die after a traumatic episode. This review provides an overview of experimental evidence that has led to our current understanding of the role of calcium signaling in death and dysfunction following TBI.

Bone marrow derived mesenchymal stem cell transplantation in cerebellar degeneration: a behavioral study

In addition to its key role in complex motor function, the cerebellum is increasingly recognized to have a role in cognition. Thus, motor and cognitive deficits can be associated with cerebellar degeneration. After unilateral lesion in cerebellum (folia VI) was caused by Quinolinic acid, CM-DiI labeled mesenchymal stem cells (MSCs), which were isolated and purified from bone marrow, were transplanted into the damaged folium. Motor function was assessed using the cylinder test, rotarod, hanging wire and beam balance during 6 weeks after transplantation. Cognitive function was assessed using the Morris water maze learning paradigm in 3 weeks after transplantation. Six weeks after transplantation surviving MSCs were detectable in QA-treated animals. The MSC-transplanted group showed markedly improved functional performance in spatial memory, motor learning, locomotor asymmetry, dysmetria, abnormality in neuromuscular strength and equilibrium 2-6 weeks compared with the controls. We found that cerebellar lesions produced deficits (folia VI) in motor and cognitive aspects of a spatial task. The results indicate that transplantation of MSCs can significantly reduce the behavioral abnormalities of these animals during six weeks after engraftment. According to results of this assay, cell therapy by means of bone marrow derived adult stem cells promises for treatment of cerebellar diseases.

Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

The neurodegenerative disease Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in early stages of the disease.

Enhanced vulnerability to NMDA toxicity in sublethal traumatic neuronal injury in vitro

Traumatic brain injury causes neuronal disruption and triggers secondary events leading to additional neuronal death. To study injuries triggered by secondary events, we exposed cultured cortical neurons to sublethal mechanical stretch, thus eliminating confounding death from primary trauma. Sublethally stretched neurons maintained cell membrane integrity, viability, and electrophysiological function. However, stretching induced in the cells a heightened vulnerability to subsequent challenges with L-glutamate or NMDA. This heightened vulnerability was specifically mediated by NMDA receptors (NMDARs), as stretched neurons did not become more vulnerable to either kainate toxicity or to that induced by the Ca(2+) ionophore A23187. Stretch-enhanced vulnerability to NMDA occurred independently of endogenous glutamate release, but required Ca(2+) and Na(+) influx through NMDARs. Stretch did not affect the electrophysiological properties of NMDARs nor excitatory synaptic activity, indicating that specificity of enhanced vulnerability to NMDA involves postsynaptic mechanisms downstream from NMDARs. To test whether this specificity requires physical interactions between NMDARs and cytoskeletal elements, we perturbed actin filaments and microtubules, both of which are linked to NMDARs. This had no effect on the stretch-induced vulnerability to NMDA, suggesting that sublethal stretch does not affect cell survival through the cytoskeleton. Our data illustrate that sublethal in vitro stretch injury triggers distinct signaling pathways that lead to secondary injury, rather than causing a generalized increase in vulnerability to secondary insults.

Caffeine impairs short-term neurological outcome after concussive head injury in rats

OBJECTIVE

Adenosine is an endogenous neuroprotective agent that is released during ischemia, hypoxia, epilepsy, and ischemic brain injury. Caffeine is a receptor antagonist for adenosine that might interfere with the neuroprotective effect of adenosine in ischemic-hypoxic conditions. An investigation was undertaken to study the effect of caffeine on neurological function, edema formation, and blood-brain barrier permeability after experimental head injury in rats.

METHODS

Adult female Wistar rats classified into different groups received caffeine intraperitoneally at doses of 0, 50, 100, and 150 mg/kg body weight. Thirty minutes after the caffeine treatment, the animals were subjected to concussive head injury (CHI) administered by a controlled cortical impact device. Neurological severity score was recorded in each rat at 2 hours after CHI. Specific gravity, water content (as an indicator of edema), and blood-brain barrier impairment were analyzed in the cortical tissue surrounding the injury site. The levels of myeloperoxidase and malondialdehyde in the cortical region were measured as indicators of neutrophil infiltration and lipid peroxidation, respectively.

RESULTS

A significant increase in righting latency and neurological deficiency after CHI was observed in caffeine-treated rats as compared with untreated animals. Although no deaths occurred in the rats exposed to CHI after pretreatment with saline, pretreatment with caffeine caused significant mortality of animals after trauma in a dose-dependent manner. Caffeine also exacerbated neutrophil infiltration, edema, and disruption of blood-brain barrier in the traumatic cortex. Light microscopy of brain revealed more severe hemorrhage and neuronal degeneration in the injured hemisphere of caffeine-treated rats as compared with rats in the injury-alone group. A significant increase in malondialdehyde in the brain of injured rats treated with caffeine before CHI clearly indicated the role of oxidative stress.

CONCLUSION

Caffeine adversely affects outcome after CHI, possibly as a result of blockade of adenosine receptors. The findings also point toward the involvement of free radical-mediated neuronal damage in caffeine-induced exacerbation of neurotrauma.

Inducible expression and pharmacology of recombinant NMDA receptors, composed of rat NR1a/NR2B subunits

An ecdysone-inducible mammalian expression system was used to study expression of recombinant N-methyl-D-aspartate (NMDA) receptors. Human embryonic kidney (HEK) 293 cells expressing the regulatory vector pVgRXR (EcR 293 cells) were transfected with rat NR1a and NR2B cDNAs using the inducible vector pIND (Invitrogen). Inducible expression of the NR2B subunit in cell clone designated EcR/rNR1a2B was investigated using quantitative RT-PCR and flow cytometry based immunocytochemical methods. The mRNA level of the NR2B subunits in EcR/rNRa2B cells was dependent on the concentration of the ecdysone analogue inducing agent, muristerone A (MuA). Similarly, NR2B subunit protein expression was higher in cells pre-treated with the inducing agent. Functionally active NMDA receptors were also detected in EcR/rNR1a2B cells after MuA induction. In presence of the inducing factor, NMDA-evoked ion currents as well as increase in cytoplasmic calcium-concentrations were measured using whole-cell patch clamp and fluorometric calcium measuring techniques. The pharmacological profile of the expressed NMDA receptors was characterised by comparing the inhibitory activity of several NR2B subunit selective NMDA antagonists in EcR/rNR1a2B cells with that observed in primary cultures of rat cortical neurones. Whereas the efficacies of the NR2B subunit selective NMDA antagonists were similar in EcR/rNR1a2B cells and in neurones, their maximal inhibitory effects were significantly higher in cells expressing NR1a/NR2B recombinant receptors. This study demonstrates that recombinant NMDA receptors can be expressed in an inducible way in non-neuronal cell lines using the ecdysone-inducible mammalian expression system. Such cell lines can be suitable tools in high throughput functional screening for potential subtype selective modulators of the NMDA receptor.

Mechanisms of AMPA Neurotoxicity in Rat Brain Slices

The mechanisms underlying the neurodegenerative effects of the glutamate receptor agonist, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate), were studied using brain slice preparations of young rat (8 - 9 days old) cerebellum and hippocampus. Rapid AMPA toxicity (exerted on some cerebellar interneurons) was inhibited by including the appropriate receptor blocker, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione, 10 microM), in the exposing solution. The degeneration of other neurons, including Purkinje cells and hippocampal pyramidal neurons, persisted. It could, however, be largely prevented if CNQX was included for 1.5 h during the post-incubation period, suggesting that an enduring 'rebound' AMPA receptor activation was responsible for this delayed type of degeneration, not the exposure itself. In cerebellar slices, independent evidence for the occurrence, postexposure, of persisting AMPA receptor stimulation was obtained electrophysiologically. Omission of Ca2+ during the exposure period (and for 10 min beforehand) markedly reduced rapid AMPA toxicity but was ineffective in protecting most of the Purkinje cells. However, if the slices were previously starved of Ca2+ for 1 h, then most of these neurons survived, even if the ion was reinstated during the recovery period. Slow AMPA toxicity, which takes place during long (2 h) exposures, could be inhibited either by CNQX or by omission of Ca2+ (30 min preincubation). The results indicate that the rapid oedematous necrosis induced by AMPA, like that caused by N-methyl-d-aspartate and kainate, is likely to involve excessive influx of Ca2+. In contrast, the induction of the delayed mechanisms, as well as its 'expression' during the postincubation period, probably depends on intracellular Ca2+, rather than Ca2+ influx.

12-hydroxyeicosatetrenoate (12-HETE) attenuates AMPA receptor-mediated neurotoxicity: evidence for a G-protein-coupled HETE receptor

12-hydroxyeicosatetraenoic acid (12-HETE) is a neuromodulator that is synthesized during ischemia. Its neuronal effects include attenuation of calcium influx and glutamate release as well as inhibition of AMPA receptor (AMPA-R) activation. Because 12-HETE reduces ischemic injury in the heart, we examined whether it can also reduce neuronal excitotoxicity. When treated with 12-(S)HETE, cortical neuron cultures subjected to AMPA-R-mediated glutamate toxicity suffered up to 40% less damage than untreated cultures. The protective effect of 12-(S)HETE was concentration-dependent (EC50 = 88 nm) and stereostructurally selective. Maximal protection was conferred by 300 nm 12-(S)HETE; 300 nm 15-(S)HETE was similarly protective, but 300 nm 5-(S)HETE was less effective. The chiral isomer 12-(R)HETE offered no protection; neither did arachidonic acid or 12-(S)hydroperoxyeicosatetraenoic acid. Excitotoxicity was calcium-dependent, and 12-(S)HETE was demonstrated to protect by inactivating N and L (but not P) calcium channels via a pertussis toxin-sensitive mechanism. Calcium imaging demonstrated that 12-(S)HETE also attenuates glutamate-induced calcium influx into neurons via a pertussis toxin-sensitive mechanism, suggesting that it acts via a G-protein-coupled receptor. In addition, 12-(S)HETE stimulates GTPgammaS binding (indicating G-protein activation) and inhibits adenylate cyclase in forskolin-stimulated cultures over the same concentration range as it exerts its anti-excitotoxic and calcium-influx attenuating effects. These studies demonstrate that 12-(S)HETE can protect neurons from excitotoxicity by activating a G(i/o)-protein-coupled receptor, which limits calcium influx through voltage-gated channels.

Interleukin-10 prevents glutamate-mediated cerebellar granule cell death by blocking caspase-3-like activity

Interleukin-10 (IL-10) has been shown to reduce neuronal degeneration after CNS injury. However, the molecular mechanisms underlying the neuroprotective properties of this cytokine are still under investigation. Glutamate exacerbates secondary injury caused by trauma. Thus, we examined whether IL-10 prevents glutamate-mediated cell death. We used rat cerebellar granule cells in culture because these neurons undergo apoptosis upon exposure to toxic concentrations of glutamate (100-500 microm) or NMDA (300 microm). Pretreatment of cerebellar granule cells with IL-10 (1-50 ng/ml) elicited a dose- and time-dependent reduction of glutamate-induced excitotoxicity. Most importantly, IL-10 reduced the number of apoptotic cells when added to the cultures together or 1 hr after glutamate. Using patch-clamping and fluorescence Ca(2+) imaging techniques, we examined whether IL-10 prevents glutamate toxicity by blocking the function of NMDA channel. IL-10 failed to affect NMDA channel properties and to reduce NMDA-mediated rise in intracellular Ca(2+). Thus, this cytokine appears to prevent glutamate toxicity by a mechanism unrelated to a blockade of NMDA receptor function. Various proteases, such as caspase-3, and transcription factors, such as nuclear factor kappaB (NF-kappaB), have been proposed to participate in glutamate-mediated apoptosis. Thus, we examined whether IL-10 modulates the activity of these apoptotic markers. IL-10 blocked both the glutamate-mediated induction of caspase-3 as well as NF-kappaB DNA binding activity, suggesting that the neuroprotective properties of IL-10 may rely on its ability to block the activity of proapoptotic proteins.

Cannabinoid receptor activation and elevated cyclic AMP reduce glutamate neurotoxicity

Cannabinoid receptor activation in vivo reduces ischemic injury, a phenomenon that has not been successfully reproduced in vitro. Because cyclic adenosine monophosphate (cAMP) levels are radically elevated during ischemic reperfusion, but cannabinoid receptor activation reduces cAMP levels, we hypothesized that cannabinoids might prevent in vitro glutamate toxicity if reperfusion was simulated by cAMP supplementation after glutamate removal. Although neuronal cultures were unaffected by the single addition of either cannabinoid or dibutyryl cAMP (dbcAMP), glutamate toxicity was reduced by 20% when cannabinoid was present during glutamate exposure and either dbcAMP or forskolin was added after glutamate removal. Further studies revealed that cannabinoid receptor activation reduces glutamate toxicity by attenuating calcium influx through N- and P/Q-type calcium channels. The effect of glutamate exposure on neuronal cAMP levels was also examined. Glutamate exposure significantly reduced neuronal cAMP levels, although suppression was even greater when cannabinoid was present. Because neurological outcome after ischemia is poor when cAMP levels during reperfusion are low, it is hypothesized that cAMP elevation after glutamate exposure may offset excitotoxic and/or cannabinoid receptor-induced cAMP depletion. Cannabinoids protect against ischemic injury in vivo, but only reduce toxicity in vitro when cAMP levels are elevated, possibly suggesting that cAMP elevation during reperfusion reduces brain injury by off-setting the effect of Gi/o protein-coupled systems on adenylate cyclase.

Neurochemical changes and laser Doppler flowmetry in the endothelin-1 rat model for focal cerebral ischemia

Generalized neurotransmitter overflow into the extracellular space, after cerebral ischemia, has been suggested to contribute to subsequent neuronal death. This study aims to investigate the striatal release of the neurotransmitters dopamine (DA), glutamate (Glu) and gamma-aminobutyric acid (GABA) by means of microdialysis, in a rat model for focal transient cerebral ischemia. Ischemia was induced by the application of 120 pmol endothelin-1 (Et-1), adjacent to the middle cerebral artery (MCA) in freely moving rats. Ischemia produced a large increase in extracellular striatal DA concentrations (2400%), Glu (5500%) and GABA (800%) concentrations. Laser Doppler flowmetry in anaesthetized rats, indicated that the blood flow within the striatum decreased by 75+/-11%. The period of sustained drop of blood flow, was dose-dependently related to the concentration Et-1 injected. Histological analysis of brain slices, taken from anaesthetized and conscious animals, indicated a 500 pmol dose of Et-1 was required to produce a similar infarct in anaesthetized rats to a 120 pmol dose of Et-1 in freely moving rats. The immediate drop in striatal blood flow, and the prompt increase of extracellular DA, after the micro-application of Et-1, were quite striking. This suggests that the DA release, rather than the Glu overflow may be the primary event initiating the cascade of processes ultimately leading to cell death and neurological deficits.

Na(+) dependence and the role of glutamate receptors and Na(+) channels in ion fluxes during hypoxia of rat hippocampal slices

Spreading depression (SD) as well as hypoxia-induced SD-like depolarization in forebrain gray matter are characterized by near complete depolarization of neurons. The biophysical mechanism of the depolarization is not known. Earlier we reported that simultaneous pharmacological blockade of all known major Na(+) and Ca(2+) channels prevents hypoxic SD. We now recorded extracellular voltage, Na(+), and K(+) concentrations and the intracellular potential of individual CA1 pyramidal neurons during hypoxia of rat hippocampal tissue slices after substituting Na(+) in the bath by an impermeant cation, or in the presence of channel blocking drugs applied individually and in combination. Reducing extracellular Na(+) concentration [Na(+)](o) to 90 mM postponed the hypoxia-induced extracellular DC-potential deflection (DeltaV(o)) and reduced its amplitude, and it also postponed the SD-like depolarization of neurons. After lowering [Na(+)](o) to 25 mM, SD-like DeltaV(o) became very small, indicating that an influx of Na(+) is required for SD; influx of Ca(2+) ions alone is not sufficient. We then asked whether the SD-related Na(+) current flows through glutamate-controlled and/or through voltage-gated Na(+) channels. Administration of either the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) postponed the hypoxic DeltaV(o) and depressed its amplitude but the effect of the combined administration of these two drugs was not greater than that of either alone. During the early phase of hypoxia, before SD onset, [K(+)](o) increased faster and reached a much higher level in the presence of glutamate antagonists than in their absence. The [K(+)](o) level reached at the height of hypoxic SD was, however, not affected. When TTX was added to DNQX and CPP, SD was prevented in half the trials. When SD did occur, it was greatly delayed, yet eventually neurons depolarized to the same extent as in normal solution. The SD-related sudden drop in [Na(+)](o) was depressed by only 19% in the presence of the three drugs, indicating that Na(+) can flow into cells through pathways other than ionotropic glutamate receptors and TTX-sensitive Na(+) channels. We conclude that, when they are functional, glutamate-receptor-mediated and voltage-gated Na(+) currents are the major generators of the self-regenerative rapid depolarization, but in their absence other pathways can sometimes take their place. The final level of SD-like depolarization is determined by positive feedback and not by the number of channels available. A schematic flow chart of the events generating hypoxic SD is discussed.

Attenuation of brain edema, blood-brain barrier breakdown, and injury volume by ifenprodil, a polyamine-site N-methyl-D-aspartate receptor antagonist, after experimental traumatic brain injury in rats

OBJECTIVE

Traumatic brain injury (TBI) has been shown to induce a significant change in polyamine metabolism. Polyamines and polyamine-dependent calcium influx play an important role in mediating the effects of excitotoxic amino acids at the N-methyl-D-aspartate (NMDA) receptor site. We studied the effects of ifenprodil, known as a noncompetitive inhibitor of polyamine sites at the NMDA receptor, on brain edema formation, blood-brain barrier breakdown, and volume of injury after TBI.

METHODS

Experimental TBI was induced in Sprague-Dawley rats by a controlled cortical impact device, functioning at a velocity of 3 m/s to produce a 2-mm deformation. Ifenprodil or saline (10 mg/kg) was injected intraperitoneally immediately after the cortical impact injury and then every 90 minutes until 6 hours after TBI. Blood-brain barrier breakdown was evaluated quantitatively 6 hours after injury by fluorometric assay of Evans blue extravasation. Brain water content, an indicator of brain edema, was measured with the wet-dry method 24 hours after TBI. Injury volume was quantitated from the brain slices stained with 2% cresyl violet solution 7 days after TBI.

RESULTS

Blood-brain barrier breakdown was significantly lower in the traumatic cortex of the ifenprodil-treated group than in the saline-treated group (84.4 +/- 26.8 microg/g versus 161.8 +/- 27 microg/g, respectively, P < 0.05). Brain edema was significantly reduced in the cortex of the ifenprodil-treated group relative to that in the saline-treated group (80.9 +/- 0.5% versus 82.4 +/- 0.6% respectively, P < 0.05). Ifenprodil treatment reduced injury volume significantly (14.9 +/- 8.1 mm3 versus 24.4 +/- 6.7 mm3, P < 0.05).

CONCLUSION

The polyamine-site NMDA receptor antagonist ifenprodil affords significant neuroprotection in a controlled cortical impact brain injury model and may hold promise for the discovery and treatment of the mechanism of delayed neurological deficits after TBI.

The pharmacological profile of L-glutamate transport in human NT2 neurones is consistent with excitatory amino acid transporter 2

The human teratocarcinoma cell line NTera2/D1 can be differentiated to produce post-mitotic neurones (NT2-N cells) by prolonged (> 3 week) exposure to retinoic acid. In this study, we describe the characterisation of high-affinity Na+-dependent L-glutamate transport activity in post-mitotic differentiated NT2-N cells. NT2-N cells, but not the undifferentiated precursor cells, transported L-glutamate in a Na+-dependent manner, as determined by equimolar replacement of Na+ with choline. L-glutamate uptake was saturable and Eadie-Hofstee transformation of the saturation data revealed a Km of 10.6+/-0.8 microM, and a maximum transport capacity (Vmax) of 100.3+/-12.3 pmol min(-1) mg(-1) protein. Pharmacological characterisation of the transport activity in NT2-N cells produced a rank order of inhibitory activity which was identical to that determined for the human excitatory amino acid transporter 2 which we have analysed in a stable mammalian cell line (Madin Darby Canine Kidney (MDCK) cells). Of particular note, L-glutamate transport by NT2-N cells was sensitive to both dihydrokainate and kainate. The expression of human excitatory amino acid transporter mRNAs was studied using reverse transcriptase polymerase chain reaction. NT2-N cells expressed transcripts for excitatory amino acid transporters 2 and 3, but not for the subtypes 1, 4 and 5. We conclude that although the mRNA expression studies suggest the presence of transcripts for both excitatory amino acid transporter 2 and 3 in NT2-N cells, the sensitivity to dihydrokainate and kainate determined in the pharmacological analysis indicates that, of the known transporter subtypes, excitatory amino acid transporter 2 contributes to the bulk of the L-glutamate transport activity present in these cells.

Brain-derived neurotrophic factor and basic fibroblast growth factor downregulate NMDA receptor function in cerebellar granule cells

Evidence has accumulated to suggest that the NMDA glutamate receptor subtype plays an important role in neuronal degeneration evoked by hypoxia, ischemia, or trauma. Cerebellar granule cells in culture are vulnerable to NMDA-induced neuronal excitotoxicity. In these cells, brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (FGF2) prevent the excitotoxic effect of NMDA. However, little is known about the molecular mechanisms underlying the protective properties of these trophic factors. Using cultured rat cerebellar granule cells, we investigated whether BDNF and FGF2 prevent NMDA toxicity by downregulating NMDA receptor function. Western blot and RNase protection analyses were used to determine the expression of the various NMDA receptor subunits (NR1, NR2A, NR2B, and NR2C) after BDNF or FGF2 treatment. FGF2 and BDNF elicited a time-dependent decrease in the expression of NR2A and NR2C subunits. Because NMDA receptor activation leads to increased intracellular Ca2+ concentration ([Ca2+]i), we studied the effect of the BDNF- and FGF2-induced reduction in NR2A and NR2C synthesis on the NMDA-evoked Ca2+ responses by single-cell fura-2 fluorescence ratio imaging. BDNF and FGF2 reduced the NMDA-mediated [Ca2+]i increase with a time dependency that correlates with their ability to decrease NR2A and NR2C subunit expression, suggesting that these trophic factors also induce a functional downregulation of the NMDA receptor. Because sustained [Ca2+]i is believed to be causally related to neuronal injury, we suggest that BDNF and FGF2 may protect cerebellar granule cells against excitotoxicity by altering the NMDA receptor-Ca2+ signaling via a downregulation of NMDA receptor subunit expression.

The intact isolated (ex vivo) retina as a model system for the study of excitotoxicity

Excitotoxicity is defined as a mode of neural cell death triggered by overactivation of receptors for the amino acid transmitter glutamate. There is considerable evidence that excitotoxicity is responsible for cell death in several neuropathological states, including some retinal diseases. The isolated retina, particularly from chick embryos, has been used extensively as an experimental system to characterize this process. This paper summarizes the use of isolated retina as a model system for studies of excitotoxicity from a theoretical and methodological point of view, and reviews results obtained from studies utilizing this system.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

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.

Sensory changes and pain after abdominal hysterectomy: a comparison of anesthetic supplementation with fentanyl versus magnesium or ketamine

Drugs interacting with opioid or N-methyl-D-aspartate (NMDA) receptors may have differing effects on post-surgical sensory changes, such as central inhibition or spinal excitation. We compared the effect of supplementing isoflurane/N2O/O2 anesthesia with an opioid agonist (fentanyl [n = 15]) or two drugs inhibiting the NMDA system differently (magnesium, ketamine [n = 15 in each group]) on sensory changes after abdominal hysterectomy. Electric sensation, pain detection, and pain tolerance thresholds were determined (preoperatively and 1, 4, 24 h, and 5 days postoperatively) in arm, thoracic, incision, and leg dermatomes together with pain scores and cumulative morphine consumption. Thresholds relative to the arm were derived to unmask segmental sensory changes hidden by generalized changes. Absolute thresholds were increased 1-24 h, returning to baseline on Day 5, without overall differences among drugs. Fentanyl thresholds were lower 1 h and higher 5 days postoperatively compared with magnesium and ketamine; thresholds were lower at 24 h for magnesium versus ketamine. Relative thresholds increased compared with baseline only with fentanyl (1-4 h); none decreased. Pain scores and morphine consumption were similar. Thus, all adjuvants suppressed spinal sensitization after surgery. Fentanyl showed the most, and magnesium the least, central sensory inhibition up to 5 days postoperatively, with different patterns of inhibition directly postsurgery versus later. Differences in sensory processing were not reflected in clinical measures.

IMPLICATIONS

We studied the effects on postsurgical sensory processing of general anesthesia supplemented by drugs affecting opioid or N-methyl-D-aspartate receptors using sensory thresholds. Generalized central sensory inhibition, differently affected by the drugs, predominated after surgery. All drugs suppressed spinal excitation. Clinical pain measures did not reflect sensory change.

The olivocerebellar projection mediates ibogaine-induced degeneration of Purkinje cells: a model of indirect, trans-synaptic excitotoxicity

Ibogaine, an indole alkaloid that causes hallucinations, tremor, and ataxia, produces cerebellar neurotoxicity in rats, manifested by degeneration of Purkinje cells aligned in narrow parasagittal bands that are coextensive with activated glial cells. Harmaline, a closely related alkaloid that excites inferior olivary neurons, causes the same pattern of Purkinje cell degeneration, providing a clue to the mechanism of toxicity. We have proposed that ibogaine, like harmaline, excites neurons in the inferior olive, leading to sustained release of glutamate at climbing fiber synapses on Purkinje cells. The objective of this study was to test the hypothesis that increased climbing fiber activity induced by ibogaine mediates excitotoxic Purkinje cell degeneration. The inferior olive was pharmacologically ablated in rats by a neurotoxic drug regimen using 3-acetylpyridine, and cerebellar damage attributed to subsequent administration of ibogaine was analyzed using immunocytochemical markers for neurons and glial cells. The results show that ibogaine administered after inferior olive ablation produced little or no Purkinje cell degeneration or glial activation. That a lesion of the inferior olive almost completely prevents the neurotoxicity demonstrates that ibogaine is not directly toxic to Purkinje cells, but that the toxicity is indirect and dependent on integrity of the olivocerebellar projection. We postulate that ibogaine-induced activation of inferior olivary neurons leads to release of glutamate simultaneously at hundreds of climbing fiber terminals distributed widely over the surface of each Purkinje cell. The unique circuitry of the olivocerebellar projection provides this system with maximum synaptic security, a feature that confers on Purkinje cells a high degree of vulnerability to excitotoxic injury.

Synthesis and in vivo evaluation of 7-chloro-5-[123I]iodo-4-oxo-1,4-dihydroquinoline-2-carboxylic acid

As a possible radioligand for SPECT visualization of the NMDA receptors in the central nervous system, 7-chloro-5-[123I]iodokynurenic acid was prepared. This paper presents the synthesis of both the radioactive and the non-radioactive product, starting from 5-bromo-7-chlorokynurenic acid and using a non-isotopic nucleophilic halogen exchange reaction in the presence of iodide (Na123I or KI). Under the best labelling conditions, the radiochemical yield was 85%. The specific activity based on UV detection was found to be higher than 1 Ci/mumol (= 37 GBq/mumol) and the chemical and radiochemical (> 95%) purity of the tracer was checked by RP-HPLC.

L-2-chloropropionic acid-induced neurotoxicity in the rat: a valuable model for studying selective neuronal cell death in vivo

1. L-2-Chloropropionic acid (L-CPA) is neurotoxic when administered orally as a neutral sodium salt in high doses to rats, resulting in a selective destruction of cerebellar granule cells with the result that animals develop marked difficulties in maintaining normal locomotion. 2. Cerebellar granule cell destruction is accompanied by a reduction in cerebellar glutamate and aspartate concentrations, reductions in the density of glutamate receptors located in the cerebellar granule cell layer and development of cerebellar oedema. No other cell type in the brain, nor other organ, is affected by L-CPA. 3. The neuronal cell death is necrotic in type, involving the activation of N-methyl-D-aspartate (NMDA) type glutamate receptors and there is some evidence for a partial role of nitric oxide in the development of the neurotoxicity. 4. Contrary to work performed on NMDA mediated cell death using neuronal cell culture approaches, L-CPA-induced granule cell death does not appear to arise from the production of excess quantities of cytotoxic free radicals, but may involve selective calcium-activated proteases, such as the calpains. 5. Tentative evidence suggests that L-CPA may interfere with voltage-dependent calcium channels in the cerebellum leading to activation of the cell death and resulting in the destruction of the granule cells. 6. In conclusion, L-CPA-induced neurotoxicity may provide valuable information on the neurochemical pathways involved in neuronal cell death that is associated with many neurological diseases.

Neurotoxic effects of kainic acid on substantia nigra neurons in rat brain slices

Excitatory amino acids (EAAs) have been implicated as mediators of cell death in neurodegenerative diseases involving catecholamine neurons. Few studies, however, have examined the toxic effects of EAAs on identified catecholamine neurons in vitro. We have investigated the neurotoxic effects of kainic acid in a rat brain substantia nigra (SN) slice preparation. Rats (60-80 g) were anesthetised with halothane and killed by cervical dislocation. SN slices, 300 microm thick, were incubated at 35 degrees C in a modified Krebs solution in the presence or absence of kainic acid and then fixed and processed for either immunohistochemistry (IHC) or electron microscopy (EM). In IHC experiments, SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. In control slices, the antibody labeled not only the cell body but also the prolific dendritic arbor of SN neurons. Treatment with 50 microM kainic acid for 15 min or 2 h resulted in loss of TH staining and apparent fragmentation of the dendrites. EM provided ultrastructural evidence for kainic acid-induced degeneration of the dendritic arbor of SN neurons. Typically, the dendritic membrane was broken, or diffuse and collapsed. Ultrastructural damage, including clumping and marginalization of chromatin and vacuolation of the cytoplasm, was also observed in cell bodies. Damage to the dendritic arbor may occur early in the neurotoxic events leading to cell death, preceding the loss of the cell body. Our observations are consistent with the postulated role of EAAs as mediators of catecholamine neuron death.

A novel NMDA receptor antagonist protects against N-methyl-D-aspartate- and glutamate-induced neurotoxicity in the goldfish retina

4(R)-(3-Phenylpropyl)-2(S)-glutamic acid, C(3), is a synthetic analogue of L-glutamate. This analogue reversibly inhibits the membrane depolarization of neurons in the CA1 region of rat hippocampal slices evoked by N-methyl-D-aspartate (NMDA), with an EC50 value of 3.6 microM, whereas the depolarization of these neurons evoked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid is not inhibited by C(3). Analyses of the inhibitory effect of C(3) on NMDA-evoked currents of dissociated rat hippocampal neurons further revealed that C(3) acts as a competitive antagonist of NMDA receptors and that the inhibitory action of C(3) is not use-dependent. Using goldfish retina as a model, we found that the neuronal damage produced by glutamate or by NMDA was effectively prevented by C(3). Incubation of retinas with high concentrations of C(3), up to 1 mM, did not induce pathomorphological changes in retinal neurons. These results suggest that C(3) is a useful neuroprotectant against excitotoxic damage of neurons.

N-methyl-D-aspartate receptors expressed in a nonneuronal cell line mediate subunit-specific increases in free intracellular calcium

N-methyl-D-aspartate (NMDA) receptors can mediate cell death in neurons and in non-neuronal cells that express recombinant NMDA receptors. In neurons, increases in intracellular calcium correlate with NMDA receptor-mediated death, supporting a key role for loss of cellular calcium homeostasis in excitotoxic cell death. In the present study, free intracellular calcium concentrations were examined in response to activation of recombinant NMDA receptors expressed in human embryonic kidney 293 cells. Intracellular calcium was measured in transfected cell populations by cotransfection with the calcium-sensitive, bioluminescent protein aequorin and by single cell imaging with the fluorescent calcium indicator fluo-3. Agonist application to NR1/2A or NR1/2B-transfected cells elicited robust rises in intracellular calcium. NR1/2A responses were inhibited by the noncompetitive antagonists MK-801 and dextromethorphan and were dependent on extracellular calcium but not on intracellular calcium stores. In contrast, no detectable intracellular calcium responses were observed in NR1/2C-transfected cells. These findings indicate that NMDA receptors in the absence of other neuron-specific factors can mediate increases in intracellular calcium with subunit specificity and extracellular calcium dependence.

Failure of glycine site NMDA receptor antagonists to protect against L-2-chloropropionic acid-induced neurotoxicity highlights the uniqueness of cerebellar NMDA receptors

Cultured cerebellar granule cells and cerebellar slices from neonatal rats have been widely used to examine the biochemistry of excitatory amino acid-induced cell death mediated in part by the activation of NMDA receptors. However, the NMDA subunit stoichiometry, producing functional NMDA receptors is different in cultured granule cells, neonatal and adult rat cerebellum as compared to the NMDA receptors in forebrain regions. We have used the L-2-chloropropionic acid (L-CPA) (750 mg/kg) model of NMDA-mediated selective cerebellar granule cell necrosis in vivo to examine the role of the glycine binding site and possible effect of the NR2C subunit (which is largely expressed only in the cerebellum) on granule cell necrosis. The abilities of various NMDA receptor antagonists were examined in vivo to determine the relative contribution of both glutamate and glycine sites involved in the L-CPA-induced neurotoxicity. The potent neuroprotective, non-competitive NMDA receptor antagonist dizocilpine (MK-801) was compared with glutamate and glycine site NMDA antagonists. We have examined a number of markers for the L-CPA-induced granule cell necrosis. The L-CPA-induced reduction in cerebellar aspartate and glutamate concentrations were used as markers of granule cell necrosis. We also measured the cerebellar water content and sodium concentrations as measures of the L-CPA-induced cerebellar edema that accompanies the granule cell necrosis. Finally the ability of the NMDA antagonists to attenuate the L-CPA-induced reductions in body weight gain and the prevention of the loss in hindlimb function using a behavioral measure of hindlimb retraction were examined. The potent glutamate antagonists, CPP and CGP40116 and dizocilpine prevented the L-CPA-induced locomotor dysfunction and granule cell necrosis as measured by their ability to prevent L-CPA-induced reduction in aspartate and glutamate concentrations. CPP, CGP40116 and dizocilpine also prevented the appearance of cerebellar edema following L-CPA administration. In addition, dizocilpine, CPP and CGP40116 were able to partially prevent the L-CPA-induced loss in body weight over the 48 h experimental period. In contrast, none of the glycine partial agonists or antagonists, namely (+/-)HA-966, D-cycloserine, MDL-29,951, DPCQ, MNQX or L-701 252 were able to prevent the L-CPA-induced loss in body weight, L-CPA-induced granule cell necrosis and behavioral disturbances when administered to rats. None of the NMDA antagonists had any effect on the cerebellar neurochemistry when injected alone or had any effect on animal behavior except for dizocilpine, CPP, CGP40116 and (+/-)HA-966 which resulted in a transient sedation for between three and five hours immediately following their administration. In conclusion, we demonstrated that NMDA open channel blockade and glutamate antagonists can provide full neuroprotection against the L-CPA-induced granule cell necrosis. The failure of the glycine partial agonist and antagonists to provide any neuroprotection against L-CPA-induced neurotoxicity in the cerebellum contrast with their neuroprotective efficacy in other animal models of excitatory amino acid-induced cell death in forebrain regions in vivo. We therefore suggest that the glycine site plays a lesser role in modulating NMDA receptor function in the cerebellum and may explain why cells expressing NMDA receptors composed of NR1/NR2C subunits are particularly resistant to excitatory amino acid-induced neurotoxicity.

Spinal anaesthesia induced by intrathecal magnesium sulphate

We have demonstrated in a rat model that the intrathecal injection of 0.02 ml of 6.3% magnesium sulphate, a concentration iso-osmolar with rat plasma, will produce a state of spinal anaesthesia and general sedation, lasting approximately 1 h. These effects reversed completely after 6 h, without evidence of neurotoxicity, immediately or during the period 1 week following the injection. The accompanying changes in haemodynamic and respiratory functions were minimal throughout the period of anaesthesia and compare favourably with those induced by an intrathecal bolus of 0.04 ml of 2% lignocaine.

The calcium response to the excitotoxin kainate is amplified by subsequent reduction of extracellular sodium

The relation between intracellular and extracellular [Na+] and [Ca2+] and membrane potential during stimulation of non-N-methyl-D-aspartate glutamate receptors has been studied in cerebellar granule cells using the fluorescent indicators SBFI, fura-2 and the bisoxonol membrane potential probe DiBaC4(3). Kainate increased both [Ca2+]i (intracellular [Ca2+]) and [Na+]i (intracellular [Na+]) and depolarized the membrane. This elevation of [Ca2+]i was only partially dependent on the presence of extracellular Na+ at the time of kainate addition. Removal of extracellular Na+ itself had a very minor effect on the [Ca2+]i or membrane potential of unstimulated cells. If extracellular Na+ was removed (in order to reverse the [Na+] gradient) or its concentration reduced during stimulation with kainate, the membrane depolarization recovered as expected. However, the intracellular level of sodium recovered only very slowly and the [Ca2+]i rose sharply, rather than recovering as might be expected on repolarization of depolarized cells possessing voltage sensitive calcium channels. This effect of extracellular [Na+] reduction on [Ca2+]i was mimicked by ouabain, another agent that causes accumulation of [Na+] in cells. These results suggest that Na+/Ca2+ exchange may play a major role in calcium homeostasis in stimulated cells, and that the levels of Na+ inside and outside the cell are critical in determining the effect of receptor stimulation on the intracellular [Ca2+].

AMPA receptors in cerebellar granule cells during development in culture

The survival and maturation of differentiating cerebellar granule cells in culture are known to be promoted by excitatory amino acids (EAAs) which, however, compromise the survival of mature cells. In contrast to the trophic effect, the toxic effect of alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropiate (AMPA) could only be elicited when the desensitisation of AMPA receptors was blocked, cyclothiazide being used in this study. Nevertheless, even under these conditions, toxicity induced by AMPA in contrast to kainate was, at 9 DIV, only half of the maximal toxicity attained by 13-16 DIV. Since cellular responses to AMPA depend so dramatically on the maturational stage of granule cells, we examined here whether this characteristic is related to developmental changes in AMPA receptor properties, which may result from changes in the subunit composition of the receptor. In contrast to toxicity, AMPA-induced 45Ca2+ influx (determined in the presence of cyclothiazide and the NMDA receptor blocker MK-801) reached a maximum already at 9 DIV. This also applied to a fraction of the 45Ca2+ uptake which persisted either after Cd2+ application or under Na(+)-free conditions and therefore presumably was mediated directly through AMPA receptor channels. Quantitative analysis of Western blots showed that the amounts of GluR4 and to a lesser extent GluR2/3/4c are substantial already at 2 DIV, remaining fairly constant until 9 DIV, followed by an increase by 16 DIV. However GluR1, which is hardly detectable in granule cells in vivo and is also low early in vitro, increased almost linearly with cultivation time.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of primary neuronal culture as a model for soman-induced neurotoxicity and effectiveness of memantine as a neuroprotective drug

An in vitro mammalian model neuronal system to evaluate the intrinsic toxicity of soman and other neurotoxicants as well as the efficacy of potential countermeasures was investigated. The link between soman toxicity, glutamate hyperactivity and neuronal death in the central nervous system was investigated in primary dissociated cell cultures from rat hippocampus and cerebral neocortex. Exposure of cortical or hippocampal neurons to glutamate for 30 min produced neuronal death in almost 80% of the cells examined at 24 h. Hippocampal neurons exposed to soman for 15-120 min at 0.1 microM concentration caused almost complete inhibition (> or = 90%) of acetylcholinesterase but failed to show any evidence of effects on cell viability, indicating a lack of direct cytotoxicity by this agent. Acetylcholine (ACh, 0.1 mM), alone or in combination with soman, did not potentiate glutamate toxicity in hippocampal neurons. Memantine, a drug used for the therapy of Parkinson's disease, spasticity and other brain disorders, significantly protected hippocampal and cortical neurons in culture against glutamate and N-methyl-D-aspartate (NMDA) excitotoxicity. In rats a single dose of memantine (18 mg/kg) administered 1 h prior to a s.c. injection of a 0.9 LD50 dose of soman reduced the severity of convulsions and increased survival. Survival, however, was accompanied by neuronal loss in the frontal cortex, piriform cortex and hippocampus.

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.

[125I]endothelin binding in rat cerebellum is increased following L-2-chloropropionic-acid-induced granule cell necrosis

The systemic administration of L-2-chloropropionic acid (L-CPA) to rats produced a marked depletion of cerebellar granule cells (> 80% of the total) when administered in a single oral dose of 750 or 250 mg/kg/day for 3 days. The nature of the L-CPA-induced neurotoxicity is currently unknown but it exhibits a number of features in common with excitatory amino acid-induced neuronal cell death. We observed an increase in [125I]endothelin-1 (ET-1) binding in the cerebellar cortex, as measured by quantitative receptor autoradiography, which occurs at 48 h, but not 24 h, following the 750 mg/kg L-CPA dosing regimen. The increase in [125I]ET-1 binding did not parallel the cellular damage and resultant astrocyte proliferation, as measured by GFAP immunoreactivity, which was primarily confined to the granular layer of the cerebellar cortex. The increased [125I]ET-1 binding occurred in the molecular layer of the cerebellar cortex (controls 2.03 +/- 0.26 fmol/mg tissue; L-CPA-treated 6.69 +/- 0.45 fmol/mg tissue, n = 6; p < 0.01, Student's t-test) which appeared to contain astrocytic processes originating from the large increase in astrocyte number situated in the granular layer. Pretreatment of the rats with the irreversible NMDA receptor antagonist, MK-801, protected the cerebellar granule cells against the L-CPA neurotoxicity and also prevented the increase in [125I]ET-1 binding in the cerebellar cortex. The increased [125I]ET-1 binding in rat cerebellum appears to be linked to the reactive gliosis that occurs in association with neuronal cell injury.

Calcium dynamics in the central nervous system

Calcium ions are critically important in many functions of the nervous system from neurotransmitter release to intracellular signal transduction. The large difference between intracellular and extracellular calcium ion concentration ([Ca2+]) highlights the importance of the mechanisms controlling influx and efflux of this ion. Loss of the regulatory ability of these mechanisms and the subsequent increased intracellular calcium levels may be involved in pathological events of brain trauma, stroke, epilepsy and other diseases. Ca2+ dynamics in the CNS ranging from 'waves' to 'spirals' are being studied because of the availability of fluorescent indicators of Ca2+ combined with confocal microscopy. Cellular mechanisms of Ca2+ signal transduction have been extensively reviewed (Tsien and Tsein, 1990; Carafoli, 1992; Berridge, 1993; Berridge and Dupont, 1994; Pozzan et al., 1994; Clapham, 1995; Ghosh and Greenberg, 1995). The aim of this review is to present the types of Ca2+ dynamics observed in the CNS thus far, both in normal brain function as well as in response after injury.

Glutamate-induced intracellular calcium changes and neurotoxicity in cortical neurons in vitro: effect of chemical ischemia

To study the role of calcium in neuronal death during ischemia, we examined the characteristics of intracellular Ca2+ ([Ca2+]i) changes in single rat forebrain neurons exposed for 5 min to glutamate (3 microM + 1 microM glycine), NMDA (30 microM + 1 microM glycine), kainate (100 microM) or high K+ (50 mM), under both normal and ischemic conditions. The parameters of [Ca2+]i change measured included peak [Ca2+]i level, plateau [Ca2+]i level, area under the [Ca2+]i response curve and time taken by [Ca2+]i to recover to 10% of the peak response. Under normal conditions, all the agonists studied produced similar [Ca2+]i changes. Chemical ischemia simulated by application of 5 mM KCN in glucose-free buffer had no effect on the basal level of [Ca2+]i, but significantly enhanced and prolonged the [Ca2+]i changes produced by all the agonists. However, in toxicity studies, chemical ischemia significantly potentiated the toxicity of only glutamate and N-methyl-D-aspartate. In correlation studies, all the neurons which died displayed an irreversible secondary [Ca2+]i load prior to loss of viability. These studies suggest that while Ca2+ entry may play a critical role in neuronal death, the magnitude of initial [Ca2+]i change does not predict the toxicity of an agonist in cortical neurons.

Lack of Purkinje cell loss in adult rat cerebellum following protracted axotomy: degenerative changes and regenerative attempts of the severed axons

The cerebellar Purkinje cells, due to their geometrical disposition and their high calbindin content, offer an optimal system in which to test the adequacy of current opinions on axotomy effects. We have, therefore, analyzed with calbindin immunostaining the morphological changes of Purkinje cells from 1 day to 6 months after axonal section in the cerebellar white matter. This method allows us to study the morphological changes in their dendrites, cell bodies, and axons. We have also searched for simultaneous changes in glial cells and vascularization by using cell type-specific markers. In addition, an ultrastructural study of Purkinje cells, 7 days after large electrolytic lesions affecting the white matter and the overlying granular layer, was carried out to determine whether amputation of the recurrent collateral system provokes a fast neuronal death. Neither the Purkinje cells axotomized close to their cell bodies (electrolytic lesions) nor those axotomized in the white matter (cerebellar transection) degenerated. Thus, this study demonstrates that Purkinje cells are extremely resistant to axotomy; those severed in the white matter at distances varying from 100 microns to 3 mm remain alive for as long as 6 months. At all survival times studied, axotomized Purkinje cells exhibited few changes in their somata and dendrites, as well as in their glial microenvironment. The major changes occurred in the axonal compartment. Axonal alterations, namely the presence of torpedoes and hypertrophy of the recurrent collateral system, were early events already noticeable 24 hours after the lesion, although they later differed in their time course and spatial distribution. It is remarkable that the distal segments of the central stumps of the cut axons survived in large numbers without any apparent retraction, with their terminal varicosities apposed to the wall of the wound cavity even 6 months after the lesion. Nevertheless, these segments were thinner than normal Purkinje cell axons (axonal atrophy). Despite this apparent immutability, some regenerative attempts did occur in the severed axons, such as axonal sprouts penetrating the deeper region of the granular layer in zones close to the lesion, presence of arciform axons, and hypertrophy of the recurrent collateral system. However, the Purkinje cell axons did not regenerate, and these neurons remained separated from their targets by a cavity in virtually all cases.

Rectification properties and Ca2+ permeability of glutamate receptor channels in hippocampal cells

Excitatory amino acids exert a depolarizing action on central nervous system cells through an increase in cationic conductances. Non-NMDA receptors have been considered to be selectively permeable to Na+ and K+, while Ca2+ influx has been thought to occur through the NMDA receptor subtype. Recently, however, the expression of cloned non-NMDA receptor subunits has shown that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are permeable to Ca2+ whenever the receptor lacks a particular subunit (edited GluR-B). The behaviour of recombinant glutamate receptor channels predicts that Ca2+ would only permeate through receptors that show strong inward rectification and vice versa, i.e. AMPA receptors with linear current-voltage relationships would be impermeable to Ca2+. Using the whole-cell configuration of the patch-clamp technique, we have studied the Ca2+ permeability and the rectifying properties of AMPA receptors, when activated by kainate, in hippocampal neurons kept in culture or acutely dissociated from differentiated hippocampus. Cells were classified according to whether they showed outward rectifying (type I), inward rectifying (type II) or almost linear (type III) current-voltage relationships for kainate-activated responses. AMPA receptors of type I cells (52.2%) were mostly Ca(2+)-impermeable (PCa/PCs = 0.1), while type II cells (6.5%) expressed Ca(2+)-permeable receptors (PCa/PCs = 0.9). Type III cells (41.3%) showed responses with low but not negligible Ca2+ permeability (PCa/PCs = 0.18). The degree of Ca2+ permeability and inward rectification were well correlated in cultured cells, i.e. more inward rectification corresponded to higher Ca2+ permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Exposure to kainic acid mimics the effects of axotomy in cerebellar Purkinje cells of the adult rat

We have investigated the long-term structural changes which affect Purkinje cells exposed to a single dose of kainic acid. Following intraparenchymal injection of the excitotoxin in the cerebellar cortex (1 microliter of a 1 mg/ml solution), Purkinje cells which survived within the lesioned area or close to its edges showed remarkable axonal abnormalities, involving the formation of torpedoes, hypertrophy of recurrent collaterals and atrophy of the corticofugal portion of the axon. In addition, their dendritic trees were often affected by conspicuous regressive alterations. The climbing fibres contacting these Purkinje cells were characterized by thick perisomatic plexuses, whereas their peridendritic branches were atrophic. The dendrites innervated by such atrophic olivary arbours were studded with huge numbers of newly formed spines. These alterations were already present a few days after kainic acid administration and persisted for the total period of observation of 6 months after the lesion. The remarkable similarity between the abnormalities of Purkinje cells exposed to kainic acid and those observed after axotomy indicates that in these two conditions common mechanisms determine analogous long-lasting modifications in the affected neurons. It is proposed that kainic acid-induced intracellular calcium overload disrupts cytoskeletal components and impairs axonal transport, thus depriving the affected Purkinje cells of retrograde trophic influences from their target neurons. As a consequence the affected neurons undergo long-lasting regressive modifications and compensatory remodelling phenomena.

Nitric oxide: a downstream mediator of calcium toxicity in the ischemic cascade

Loss of cellular calcium homeostasis or the production of nitric oxide (NO) have been cited as possible mechanisms that may contribute to neuronal degeneration during ischemia. We therefore examined whether cellular calcium blockade, using the agent HA1077, was protective during anoxia in hippocampal neuronal cell cultures, and whether the in vitro effects of this drug were linked to the NO pathway. Administration of the agent during anoxia was neuroprotective in neuronal cell culture. In contrast, HA1077 did not protect hippocampal neurons during NO exposure. In addition, inhibition of NO synthesis in conjunction with HA1077 application during anoxia did not significantly increase survival beyond the maximum protection afforded by HA1077 alone. These results suggest that calcium may be an initial messenger in the ischemic cascade, but that subsequent neuronal degeneration is dependent upon the NO pathway.