Reversible and irreversible neuronal damage caused by excitatory amino acid analogues in rat cerebellar slices

NO as a multimodal transmitter in the brain: discovery and current status

NO operates throughout the brain as an intercellular messenger, initiating its varied physiological effects by activating specialized GC-coupled receptors, resulting in the formation of cGMP. In line with the widespread expression of this pathway, NO participates in numerous different brain functions. This review gives an account of the discovery of NO as a signalling molecule in the brain, experiments that originated in the search for a mysterious cGMP-stimulating factor released from central neurones when their NMDA receptors were stimulated, and summarizes the subsequent key steps that helped establish its status as a central transmitter. Currently, various modes of operation are viewed to underlie its diverse behaviour, ranging from very local signalling between synaptic partners (in the orthograde or retrograde directions) to a volume-type transmission whereby NO synthesized by multiple synchronous sources summate spatially and temporally to influence intermingled neuronal or non-neuronal cells, irrespective of anatomical connectivity. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

How are necrotic cells recognized by their predators?

Necrosis is a type of cell death often caused by cell injury and is linked to human diseases including neuron degeneration, stroke, and cancer. Cells undergoing necrosis are engulfed and degraded by engulfing cells, their predators. The mechanisms by which necrotic cells are recognized and removed remain elusive. Here we comment on our recent findings that reveal new molecular mechanisms of necrotic-cell recognition. Through studying the C. elegans touch neurons undergoing excitotoxic necrosis, we identified a receptor/ligand pair that enables engulfing cells to recognize necrotic neurons. The phagocytic receptor CED-1 is activated through interaction with its ligand phosphatidylserine (PS), exposed on the surface of necrotic cells. Furthermore, against the common belief that necrotic cells have ruptured plasma membrane, we found that necrotic C. elegans touch neurons actively present PS on their outer surfaces while maintaining plasma membrane integrity. We further identified 2 mechanisms governing the presentation of PS, one of which is shared with cells undergoing apoptosis, a “cell suicide” event, whereas the other is unique to necrotic neurons. The influx of Ca²⁺, a key necrosis-triggering factor, is implicated in activating a neuronal PS-scramblase for PS exposure. We propose that the mechanisms controlling PS-exposure and necrotic-cell recognition by engulfing cells are likely conserved from worms to humans.

Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca(2+)-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common "eat me" signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Smooth endoplasmic reticulum dilation and degeneration in Purkinje neuron dendrites of aging ethanol‐fed female rats

The effects of chronic ethanol consumption on the extensive Purkinje neuron (PN) dendritic arbor of male rats include dilation of the smooth endoplasmic reticulum (SER) and dendritic regression. The purpose of the present study was to examine the molecular layer of female rats for the presence of ethanol-related SER dilation and evidence of degeneration within the PN dendritic arbor. Twenty-one 12-month-old Fischer 344 female rats (n = 7/treatment group) received a liquid ethanol, liquid control, or rat chow diet for a period of 40 weeks. Ethanol-fed rats received 35% of their dietary calories as ethanol. Pair-fed rats received a liquid control diet that was isocaloric to the ethanol diet. Chow-fed rats received standard laboratory rat chow ad libitum. At the end of treatment, tissues from the anterior and posterior lobes of the cerebellar vermis were viewed and photographed with the electron microscope. The diameters of SER profiles were measured and the density of degenerating bodies within the PN dendritic arbor was quantitated. In the posterior lobe, ethanol-related SER dilation was apparent. In the anterior lobe, the density of degenerating bodies within PN dendritic shafts was significantly increased but SER dilation in PN dendritic shafts was absent. These results confirm that SER dilation and dendritic degeneration in PN dendrites may precede and contribute to ethanol-related regression in female rats. In addition, comparison of these results with data obtained in male rats from a previous study suggests that PN dendrites in females may be more sensitive to the effects of ethanol.

Analyses of smooth endoplasmic reticulum of cerebellar parallel fibers in aging, ethanol-fed rats

The smooth endoplasmic reticulum (SER), a calcium storage organelle, is essential for normal neuronal function. Dilation of the SER is pathologic and a threat to neuronal calcium homeostasis. Dilation of the SER has been reported within the dendrites of cerebellar Purkinje neurons of aging rats after lengthy ethanol treatment. Ethanol-related alterations of parallel fiber SER have not been investigated despite the fact that such dilation may precede and contribute transsynaptically to SER dilation and degeneration in Purkinje neuron dendrites. Male Fischer 344 rats (n = 120; age = 12 months old) were randomly divided into three dietary groups (40 rats per group) and fed rat chow, the AIN-93M liquid control diet, or the AIN-93M liquid ethanol diet (without water) for 5, 10, 20, or 40 weeks (30 rats per time point). Sections from posterior vermal lobules were viewed with the electron microscope. Maximum and minimum diameters of parallel fiber SER profiles were measured. Ethanol-related dilation of parallel fiber SER was not found after 5, 10, 20, or 40 weeks of treatment. Age-related dilation of parallel fiber SER profiles did occur. These findings support the suggestions that (1) parallel fiber SER, unlike the SER in Purkinje neurons, is insensitive to ethanol and (2) the mechanisms by which ethanol and aging alter cerebellar function and structure are different.

Dorsal hippocampus and classical fear conditioning to tone and context in rats: effects of local NMDA-receptor blockade and stimulation

Consistent with the importance of the hippocampus in learning more complex stimulus relations, but not in simple associative learning, the dorsal hippocampus has commonly been implicated in classical fear conditioning to context, but not to discrete stimuli, such as a tone. In particular, a specific and central role in contextual fear conditioning has been attributed to mechanisms mediated by dorsal hippocampal N-methyl-D-aspartate (NMDA)-type glutamate receptors. The present study characterized the effects of blockade or tonic stimulation of dorsal hippocampal NMDA receptors by bilateral local infusion of the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine maleate; 6.25 microg/side) or of NMDA (0.7 microg/side), respectively, on classical fear conditioning to tone and context in Wistar rats. Freezing was used to measure conditioned fear. Regardless of whether conditioning was conducted with tone-shock pairings or unsignaled footshocks (background or foreground contextual conditioning), both NMDA and MK-801 infusion before conditioning resulted in reduced freezing during subsequent exposure to the conditioning context. Freezing during subsequent tone presentation in a new context, normally resulting from conditioning with tone-shock pairings, was not impaired by MK-801 but was strongly reduced by NMDA infusion before conditioning; this freezing was also reduced by NMDA infusion before tone presentation (in an experiment involving NMDA infusions before conditioning and subsequent tone presentation to assess the role of state-dependent learning). It was assessed whether unspecific infusion effects (altered sensorimotor functions, state dependency) or infusion-induced dorsal hippocampal damage contributed to the observed reductions in conditioned freezing. Our data suggest that formation of fear conditioning to context, but not tone, requires NMDA receptor-mediated mechanisms in the dorsal hippocampus. As indicated by the effects of NMDA, some dorsal hippocampal processes may also contribute to fear conditioning to tone. The role of the dorsal hippocampus and local NMDA receptor-mediated processes in fear conditioning to tone and context is discussed in comparison with ventral hippocampal processes.

Brain-derived neurotrophic factor can act as a pronecrotic factor through transcriptional and translational activation of NADPH oxidase

Several lines of evidence suggest that neurotrophins (NTs) potentiate or cause neuronal injury under various pathological conditions. Since NTs enhance survival and differentiation of cultured neurons in serum or defined media containing antioxidants, we set out experiments to delineate the patterns and underlying mechanisms of brain-derived neurotrophic factor (BDNF)-induced neuronal injury in mixed cortical cell cultures containing glia and neurons in serum-free media without antioxidants, where the three major routes of neuronal cell death, oxidative stress, excitotoxicity, and apoptosis, have been extensively studied. Rat cortical cell cultures, after prolonged exposure to NTs, underwent widespread neuronal necrosis. BDNF-induced neuronal necrosis was accompanied by reactive oxygen species (ROS) production and was dependent on the macromolecular synthesis. cDNA microarray analysis revealed that BDNF increased the expression of cytochrome b558, the plasma membrane-spanning subunit of NADPH oxidase. The expression and activation of NADPH oxidase were increased after exposure to BDNF. The selective inhibitors of NADPH oxidase prevented BDNF-induced ROS production and neuronal death without blocking antiapoptosis action of BDNF. The present study suggests that BDNF-induced expression and activation of NADPH oxidase cause oxidative neuronal necrosis and that the neurotrophic effects of NTs can be maximized under blockade of the pronecrotic action.

Prepulse inhibition in rats with temporary inhibition/inactivation of ventral or dorsal hippocampus

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensorimotor gating and is decreased in neuropsychiatric diseases, including schizophrenia. Hippocampal involvement in PPI has been the subject of several studies, in particular, as aberrant hippocampal activity has been associated with schizophrenia. In rats, chemical stimulation of the ventral hippocampus reduced PPI, while normal PPI was found following hippocampal lesions, suggesting that ventral hippocampal overactivity is detrimental for PPI, but that normal hippocampal activity does not contribute substantially to PPI. In the present study, we investigated the importance of hippocampal activity for PPI by examining PPI in Wistar rats with temporarily decreased hippocampal activity, aiming to avoid compensatory processes that may occur with permanent lesions. Bilateral ventral or dorsal hippocampal infusions of the gamma-aminobutyric acid A (GABA(A)) receptor agonist muscimol (1 microg/side) or the sodium-channel blocker tetrodotoxin (TTX, 10 ng/side) reduced PPI. This reduction is probably neuroleptic-resistant since haloperidol and clozapine did not antagonize the muscimol-induced decreases in PPI. PPI reduction by muscimol inhibition or TTX inactivation of the dorsal or ventral hippocampus indicates that hippocampal activity contributes to sensorimotor gating, suggesting intact PPI after permanent hippocampal lesions to reflect compensatory processes. The data are discussed with respect to hippocampal dysfunction in schizophrenia.

AMPA Neurotoxicity in Rat Cerebellar and Hippocampal Slices: Histological Evidence for Three Mechanisms

Excitatory amino acid-induced death of central neurons may be mediated by at least two receptor types, the so-called NMDA (N-methyl-d-aspartate) and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. We have studied the neurodegenerative mechanisms set in motion by AMPA receptor activation using incubated slices of 8-day-old rat cerebellum and hippocampus. In both preparations, AMPA induced a pattern of degeneration that differed markedly from the one previously shown to be elicited by NMDA. In cerebellar slices, AMPA induced the degeneration of most Purkinje cells together with a population of Golgi cells; in hippocampal slices the neurons were affected in the order CA3 > CA1 > dentate granule cells. Three mechanisms could be discerned: an acute one in which neurons (e.g. cerebellar Golgi cells) underwent a rapid degeneration; a delayed one in which the neurons (Purkinje cells and hippocampal neurons) appeared to be only mildly affected immediately after a 30 min exposure but then underwent a protracted degeneration during the postincubation period (1.5 - 3 h); and finally a slow toxicity, which took place during long (2 h) exposures to AMPA (3 - 30 microM). Although Purkinje cells were vulnerable in both cases, the efficacy of AMPA was higher for the delayed mechanism than for the slow one. The pathology displayed by the acutely destroyed Golgi neurons was a classical oedematous necrosis, whereas most neurons vulnerable to the delayed and slow mechanisms displayed a 'dark cell degeneration', whose cytological features bore a close resemblance to those of neurons irreversibly damaged by ischaemia, hypoglycaemia or status epilepticus in vivo.

Glutamate Toxicity: An Experimental and Theoretical Analysis

In slices of 8-day-old rat cerebellum, the lowest concentration of glutamate that induced toxicity (30 min exposure; 90 min recovery) was 100 microM, but the damage only occurred in the outermost regions. As the concentration was raised, the band of necrosis became progressively deeper until, at 3 mM, it was uniform across the slice thickness. At a test concentration of 300 microM, the width of the necrotic band did not change when either the exposure time or the recovery period was varied between 30 min and 3 h. These results are predicted by a theoretical model in which the diffusion of glutamate into brain tissue is countered by cellular uptake of the amino acid, and they argue against the idea that glutamate toxicity is inherently self-propagating. When slices were examined immediately after exposure (300 microM), a prominent swelling of glial cells was present at the slice surface. Swelling per se did not appear to compromise their uptake function, and the model predicts that cellular swelling, by reducing the rate of diffusion of glutamate, protects against glutamate toxicity. The damage produced by 3 mM glutamate, which was primarily exerted against granule cells, was prevented by N-methyl-d-aspartate (NMDA) receptor blockade, whereas antagonists acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors were ineffective. Under conditions of energy deprivation, the neurotoxic potency of glutamate was markedly enhanced and a normally non-toxic concentration (30 microM) became maximally toxic towards granule cells. Dark vacuolar degeneration of Purkinje cells was also present, and this could be inhibited by blocking AMPA receptors. The results and theoretical analysis suggest that intact brain tissue is remarkably resistant to glutamate toxicity, chiefly because of the formidable properties of the uptake system. However, under special circumstances, glutamate can become a potent neurotoxin and its toxicity can then involve both NMDA and AMPA receptors.

Fimbria-fornix transection and excitotoxicity produce similar neurodegeneration in the septum

Fimbria-fornix transection produces neuronal injury in the septum. This cellular pathology is characterized by somatodendritic vacuolar abnormalities in neurons. Because these cellular changes are reminiscent of some of the morphological abnormalities seen with glutamate receptor-mediated excitoxicity, we tested whether excitotoxic injury to the septal complex of adult rats mimics the degeneration observed within the dorsolateral septal nucleus and medial septal nucleus following fimbria-fornix transection. The septal complex was evaluated at various time-points (6 h to 14 days) by light and electron microscopy following unilateral injection of the N-methyl-D-aspartate receptor agonist quinolinate or the non-N-methyl-D-aspartate receptor agonist kainate, and the morphological changes observed were compared to those abnormalities in the medial septal nucleus and dorsolateral septal nucleus at three to 14 days after fimbria-fornix transection. The patterns of cytoplasmic abnormalities and vacuolar injury were morphologically similar in the somatodendritic compartment of neurons following excitotoxicity and axotomy paradigms. These similarities were most evident when comparing the persistently injured neurons in the penumbral regions of the excitotoxic lesions at one to 14 days recovery to neurons in the medial septal nucleus and dorsolateral septal nucleus at seven and 14 days after fimbria-fornix transection. Pretreatment with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate prior to unilateral fimbria-fornix transection attenuated the somatodentritic vacuolar damage found within the ipsilateral dorsolateral and medial septal nuclei at 14 days recovery. Because glutamate is the principal transmitter of hippocampal efferents within the fimbria-fornix, we conclude that postsynaptic glutamate receptor activation participates in the evolution of septal neuron injury following fimbria-fornix transection. Thus, excitotoxicity is a possible mechanism for transneuronal degeneration following central nervous system axotomy.

Micromolar L-glutamate induces extensive apoptosis in an apoptotic-necrotic continuum of insult-dependent, excitotoxic injury in cultured cortical neurones

Excitotoxicity induced by L-glutamate (Glu), when examined in a pure neuronal cortical culture, involved widespread apoptosis at concentrations of 1-10 microM as part of a continuum of injury, which at its most servere was purely necrotic. Cells, maintained in chemically defined neurobasal/B27 medium, were exposed at d7 for 2 h to Glu (1-500 microM), and cellular injury was analysed 2 and 24 h after insult using morphology (phase-contrast microscopy), a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) viability assay, nuclear staining with 4,6-diamidino-2-phenylindole (DAPI), terminal transferase-mediated dUTP nick end-labelling (TUNEL) and DNA fragmentation by gel electrophoresis. Glu-mediated neurotoxicity was prevented by MK-801 (5 microM), whilst CNQX (20 microM) attenuated injury by 20%. Exposure to intensive insults (100 and 500 microM Glu) induced necrosis characterized by rapid cell swelling (< 2 h) and lack of chromatin condensation, confirmed by DAPI nuclear staining. In contrast, mild insults (< 20 microM Glu) failed to produce acute neuronal swelling at < 2 h, but 24 h after injury resulted in a large number of apoptotic nuclei as confirmed by TUNEL and electrophoretic evidence of DNA fragmentation, which was attenuated by cycloheximide (0.1 microg/ml). Our findings indicate for the first time that physiological concentrations of Glu produce neuronal injury across a continuum involving apoptosis (< 20 microM) and increasingly necrosis(> 20 microM), dependent on the severity of the initial insult.

Antioxidant effects of 1,4-dihydropyridine and nitroso aryl derivatives on the Fe+3/ascorbate-stimulated lipid peroxidation in rat brain slices

1. Lipid peroxidation in rat brain slices was induced by Fe+3/ascorbate. 2. Brain lipid peroxidation, as measured by malondialdehyde formation, was inhibited by all the tested nitro aryl 1,4-dihydropyridine derivatives over a wide range of concentrations. The time-course antioxidant effects of the most representative agents were assessed. On the basis of both time-course and IC50 experiments the tentative order of antioxidant activity on rat brain slices could be: nicardipine>nisoldipine> (R,S/S,R)-furnidipine > (R,R/S,S)-furnidipine>nitrendipine>nimodipine> nifedipine. 3. 1,4-Dihydropyridine derivatives that lack of a nitro group in the molecule (isradipine, amlodipine) also inhibited lipid peroxidation in rat brain slices but at higher concentrations than that of nitro-substituted derivatives. 4. All the tested nitroso aryl derivatives [2,6-dimethyl-4-(2-nitrosophenyl)-3,5-pyridinedicar. boxylic acid dimethyl ester (NTP), nitrosotoluene, nitrosobenzene] were more potent inhibitors of lipid peroxidation than were the parent nitro compounds. In conclusion, on the basis of the IC50 values determined, the rank order of antioxidant potency for these derivatives can be established as: ortho-nitrosotoluene>NTP>nitrosobenzene.

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).

The effects of the tremorgenic mycotoxin penitrem A on the rat cerebellum

Within 10 minutes of intraperitoneal injection of penitrem A (3 mg/kg), rats develop severe generalized tremors and ataxia that persist for up to 48 hours. These are accompanied by a three- to fourfold increase in cerebellar cortical blood flow. Mitochondrial swelling occurs in cerebellar stellate and basket cells within 30 minutes of dosing and persists for more than 12 hours without leading to cell death. From 2 hours, Purkinje cell dendrites show early cytoplasmic condensation accompanied by fine vacuolation of smooth endoplasmic reticulum and enlargement of perikaryal mitochondria. From 6 hours, many Purkinje cells develop intense cytoplasmic condensation with eosinophilia that resembles "ischemic cell change," and from 12 hours, many other Purkinje cells show marked watery swelling. Astrocytes begin to swell from 0.5 hours after injection and show hypertrophy of organelles from 6 hours. Also from 6 hours onward, discrete foci of necrosis appear in the granule cell layer, while permeability of overlying meningeal vessels to horseradish peroxidase becomes evident at 8 hours. All changes are more severe in vermis and paravermis. Despite widespread loss of Purkinje cells, the animals' behavior becomes almost normal within a week. While tremor occurs with doses of 1.5 and 0.5 mg/kg, cellular damage is minimal. The tremor mechanism differs from that of harmaline since destruction of inferior olivary nuclei abolishes neither the tremor response to penitrem A nor the cellular damage. No morphological changes are found in other brain regions. The affinities of penitrem A for high-conductance calcium-dependent potassium channels and for gamma-aminobutyric acid receptors with the probability of resultant excitotoxity are considered to be important underlying factors for these changes.

Neuropathology of degenerative cell death in Caenorhabditis elegans

In Caenorhabditis elegans necrosis-like neuronal death is induced by gain-of-function (gf) mutations in two genes, mec-4 and deg-1, that encode proteins similar to subunits of the vertebrate amiloride-sensitive epithelial Na+ channel. We have determined the progress of cellular pathology in dying neurons via light and electron microscopy. The first detectable abnormality is an infolding of the plasma membrane and the production of small electron-dense whorls. Later, cytoplasmic vacuoles and larger membranous whorls form, and the cell swells. More slowly, chromatin aggregates and the nucleus invaginates. Mitochondria and Golgi are not dramatically affected until the final stages of cell death when organelles, and sometimes the cells themselves, lyse. Certain cells, including some muscle cells in deg-1 animals, express the abnormal gene products and display a few membrane abnormalities but do not die. These cells either express the mutant genes at lower levels, lack other proteins needed to form inappropriately functioning channels, or are better able to compensate for the toxic effects of the channels. Overall, the ultrastructural changes in these deaths suggest that enhanced membrane cycling precedes vacuolation and cell swelling. The pathology of mec-4(gf) and deg-1(gf) cells shares features with that of genetic disorders with alterations in channel subunits, such as hypokalemic periodic paralysis in humans and the weaver mouse, and with degenerative conditions, e.g., acute excitotoxic death. The initial pathology in all of these conditions may reflect attempts by affected cells to compensate for abnormal membrane proteins or functions.

Vicious cycle involving Na+ channels, glutamate release, and NMDA receptors mediates delayed neurodegeneration through nitric oxide formation

The mechanisms by which neurons die after cerebral ischemia and related conditions in vivo are unclear, but they are thought to involve voltage-dependent Na+ channels, glutamate receptors, and nitric oxide (NO) formation because selective inhibition of each provides neuroprotection. It is not known precisely what their roles are, nor whether they interact within a single cascade or in parallel pathways. These questions were investigated using an in vitro primary cell culture model in which striatal neurons undergo a gradual and delayed neurodegeneration after a brief (5 min) challenge with the glutamate receptor agonist NMDA. Unexpectedly, NO was generated continuously by the cultures for up to 16 hr after the NMDA exposure. Neuronal death followed the same general time course except that its start was delayed by approximately 4 hr. Application of the NO synthase inhibitor nitroarginine after, but not during, the NMDA exposure inhibited NO formation and protected against delayed neuronal death. Blockade of NMDA receptors or of voltage-sensitive Na+ channels [with tetrodotoxin (TTX)] during the postexposure period also inhibited both NO formation and cell death. The NMDA exposure resulted in a selective accumulation of glutamate in the culture medium during the period preceding cell death. This glutamate release could be inhibited by NMDA antagonism or by TTX, but not by nitroarginine. These data suggest that Na+ channels, glutamate receptors, and NO operate interdependently and sequentially to cause neurodegeneration. At the core of the mechanism is a vicious cycle in which NMDA receptor stimulation causes activation of TTX-sensitive Na+ channels, leading to glutamate release and further NMDA receptor stimulation. The output of the cycle is an enduring production of NO from neuronal sources, and this is responsible for delayed neuronal death. The same neurons, however, could be induced to undergo more rapid NMDA receptor-dependent death that required neither TTX-sensitive Na+ channels nor NO.

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.

Activation of multiple metabotropic glutamate receptor subtypes prevents NMDA-induced excitotoxicity in rat hippocampal slices

Metabotropic glutamate receptors (mGluRs) belong to a relative large receptor family consisting of multiple members with important roles in a number of brain functions. We report here that activation of mGluRs prevents the neurotoxic effect induced by N-methyl-D-aspartate (NMDA) in slices from the rat hippocampus. Neuroprotection was elicited when slices were simultaneously exposed to both the selective mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD) and NMDA. Persisting stimulation of mGluRs after the toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD was also evoked by its active isomer, (1S, 3R)-ACPD, and was prevented by the selective mGluR antagonist (+)-alpha-methyl-4-carboxyphenyl-glycine (500 microM), confirming that mGluR activation is involved in the mechanism of action of tACPD. The effect of 100 microM tACPD was reproduced by 100 microM quisqualate, an agonist of mGluR2 and mGluR3 subtypes. No neuroprotection was induced by L-2-amino-4-phosphonobutyrate, a selective agonist for mGluR4, mGluR6, mGluR7 and mGluR8, at 500 microM. Since the NMDA-mediated cell death in hippocampal slices is considered relevant to ischaemia-induced brain injury, these results indicate that mGluRs may be important safety devices used by neurons to decrease their sensitivity to excitotoxic stimuli and increase their chance of survival.

Knockout of glutamate transporters reveals a major role for astroglial transport in excitotoxicity and clearance of glutamate

Three glutamate transporters have been identified in rat, including astroglial transporters GLAST and GLT-1 and a neuronal transporter EAAC1. Here we demonstrate that inhibition of the synthesis of each glutamate transporter subtype using chronic antisense oligonucleotide administration, in vitro and in vivo, selectively and specifically reduced the protein expression and function of glutamate transporters. The loss of glial glutamate transporters GLAST or GLT-1 produced elevated extracellular glutamate levels, neurodegeneration characteristic of excitotoxicity, and a progressive paralysis. The loss of the neuronal glutamate transporter EAAC1 did not elevate extracellular glutamate in the striatum but did produce mild neurotoxicity and resulted in epilepsy. These studies suggest that glial glutamate transporters provide the majority of functional glutamate transport and are essential for maintaining low extracellular glutamate and for preventing chronic glutamate neurotoxicity.

Nitric oxide does not mediate acute glutamate neurotoxicity, nor is it neuroprotective, in rat brain slices

Nitric oxide (NO), generated upon glutamate receptor activation, elicits cyclic GMP accumulation through stimulation of guanylyl cyclase. NO is also a potential cytotoxin that has been suggested, on the basis of tissue culture experiments, to mediate neuronal damage associated with excessive activity of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. We have investigated the involvement of NO in the toxicity of glutamate receptor agonists in brain slice preparations. Slices of cerebellum and hippocampus from the developing rat exhibited neuronal necrosis following exposure (5-30 min) to NMDA (100 microM or 1 mM). When the exposures were carried out in the presence of NO synthase inhibitors, at concentrations suppressing NMDA-induced NO formation (as judged by measurements of cyclic GMP accumulation), the extent of injury was unaffected. To determine if exogenous NO is able to replicate NMDA toxicity, the slices were exposed to high concentrations of NO donating compounds for up to 2 hr. No damage was detectable. NO donors, moreover, neither reduced NMDA toxicity, nor potentiated the degeneration caused by just suprathreshold NMDA concentrations. The toxicities of non-NMDA agonists, or of glutamate itself, were also unaltered by NO synthase inhibitors or NO donors. Similar results were obtained using hippocampal slices from more mature animals. We conclude that the acute neurodegeneration mediated by NMDA or non-NMDA receptors in the slice preparations is not mediated by NO, nor is NO neuroprotective under these conditions.

Vulnerability of cerebellar granule cells to alcohol-induced cell death diminishes with time in culture

This study examined the effects of alcohol exposure on the viability of cerebellar granule cells in culture. Continuous alcohol exposure, starting 1 day after the cultures were established, significantly reduced granule cell numbers, even with a single day of exposure to an alcohol concentration as low as 100 mg/dl. The depletion of cerebellar granule cells by alcohol was concentration-dependent (greater loss of cells at higher alcohol concentrations) and duration-dependent (greater loss of cells at longer exposure durations). The loss of granule cells also depended on the number of days the granule cells were in culture before alcohol exposure. Alcohol was significantly more effective in reducing the cell numbers of newly established granule cell cultures (1 day in vitro) compared with older cultures (4 or 7 days in vitro). Cell cycle analysis established that the cerebellar granule cells did not proliferate in culture, indicating that alcohol exposure did not reduce cell numbers by interfering with cell proliferation in this system. Instead, alcohol-induced killing of the granule cells was the most likely mechanism to account for the depletion of granule cells in vitro. Granule cell cultures are a useful in vitro model system to study the cellular and molecular aspects of neuronal cell depletion associated with fetal alcohol exposure. The potential role of the N-methyl-D-aspartate receptor in this alcohol-induced neuronal cell death is discussed.

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

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

Ultrastructural evidence that mercuric chloride lowers the threshold for glutamate neurotoxicity in an organotypic culture of rat cerebellum

Separate exposure of organotypic cultures, derived from newborn rat cerebellum, to non-toxic concentration of either 100 microM glutamate (GLU) or 1 microM mercuric chloride (MC), for as long as 3 days, produced no distinct ultrastructural changes in neurons and glial cells. By contrast, simultaneous exposure to both agents resulted, as early as after 30 min, in microvacuolar degeneration of neurons and later on in postsynaptic abnormalities, typically accompanying excitotoxic lesions but not heavy metal-induced lesions. The results indicate that MC at low micromolar concentrations lowers the threshold for GLU neurotoxicity.

Use-dependent pentobarbital block of kainate and quisqualate currents

The effects of pentobarbital on whole-cell excitatory amino acid-induced currents were studies in cultured rat cortical neurons. Currents evoked by 40 microM kainate were reversibly inhibited by pentobarbital with an IC50 value of 50 microM. The block of the kainate response by pentobarbital was use dependent, requiring kainate stimulation. In the absence of kainate activation, 10 min perfusions of 100 microM pentobarbital inhibited kainate-induced currents less than 10%. Recovery from pentobarbital block also exhibited use dependence, reversing in 5-10 s with kainate stimulation, while persisting 10 min or more in the absence of agonist. Pentobarbital inhibition of the kainate response was not voltage dependent. Responses evoked by 10 microM quisqualate consisted of a peak current desensitizing to a smaller steady-state current. The co-application of 100 microM pentobarbital reduced the steady-state current by 49 +/- 5%. The peak current before desensitization, however, was inhibited less than 10%. Currents evoked by 25 microM N-methyl-D-aspartate were not significantly inhibited by co-application of 100 microM pentobarbital. The results suggest that the pentobarbital-induced inhibition of kainate responses involves open channel block and that the block of quisqualate currents primarily involve non-desensitizing receptor channels that generate steady-state currents.

Comparison of spontaneous motor pattern generation in non-hemisected and hemisected mouse spinal cord

Spontaneous electromyogram (EMG) patterns in the gastrocnemius (G) and tibialis anterior (TA) muscles of spinal cord-hindlimb explants from neonatal mice were investigated. Compared to non-hemisected explants, neither longitudinal hemisection of the spinal cord nor hemisection plus transection at L1 significantly altered the incidence of spontaneous motor rhythm. Therefore, not only does each half of the neonatal spinal cord contain sufficient circuitry to generate motor rhythm, but the more reduced preparations were just as likely to produce such activity. Hemisected preparations, however, exhibited slower rhythm, perhaps due to the loss of excitatory commissural connections. No correlation was found between the number of cycles in a rhythmic sequence and cycle period. In hemisected as well as non-hemisected explants, sequences of spontaneous EMG rhythm occurred in either the G or TA muscle, but not in both muscles simultaneously. Consequently, cycle-to-cycle alternation between rhythmic bursting in the G and TA muscles was not observed. The excitability in such preparations was apparently insufficient for maintained activations of both muscles (either for cycle-to-cycle alternation or for co-contraction).

Slices from the rat olfactory bulb maintained in vitro. Morphological aspects

Transverse, 400-microns-thick slices of 8-day-old rat olfactory bulb were incubated in Krebs-Henseleit medium with and without oxygenation. Following incubation, slices were fixed in aldehyde-osmium and embedded in resin for light and electron microscopy. After 2 h of incubation oxygenated preparations showed a structural preservation comparable to that of the freshly fixed olfactory bulb. Under hypoxic conditions mitral cells located on the medial side of the bulb were the most sensitive to the interruption of gassing, while ventricular cells and glomeruli were remarkably resistant as judged by morphological standards. The effects of short-term (up to 30 min) interruptions of gassing proved to be reversible. Our findings suggest that the incubated olfactory bulb slice may be a useful preparation for functional morphological studies.

Morphological response of endoplasmic reticulum in cerebellar Purkinje cells to calcium deprivation

Mobilisable intracellular Ca2+ stores are highly enriched in the cerebellum, particularly in Purkinje cells. We have detected, by light and electron microscopy, striking morphological changes in the presumed Ca2+ stores of Purkinje cells when slices of eight-day-old rat cerebellum were incubated in Ca(2+)-deficient media. After 30 min under these conditions, the endoplasmic reticulum became thinned and elongated. By 2 h, it was transformed into multilamellar, whorl-like inclusions with electron-dense cores. These changes were reversed on reintroduction of Ca2+. Analogous changes in other neurons were not observed. The results suggest that Ca2+ storage sites within Purkinje cells are capable of dramatic morphological change depending on the availability of Ca2+. The transformations may reflect, initially, depletion of Ca2+ from the stores and then homeostatic alterations in their capacity.

Mitogenic effects of excitatory amino acids in the adult rat retina

We studied the retinas of adult rats after the intravitreal injection of excitatory amino acids and ouabain. Kainic acid, domoic acid, N-methyl D-asparate and ouabain produced swelling and vacuolization of the outer plexiform, inner nuclear and inner plexiform layers and pyknosis. Mitoses were present in retinas treated with all agents other than N-methyl D-asparate. Rompun ketamine anesthesia blocked the mitogenic effects. Immunohistochemical labeling of both glial fibrillary acidic protein and S100 protein would indicate that the mitoses are occurring in glial cells. We suggest that the mitogenic effects are mediated through action on glial cationic channels, and might account for the reactive gliosis observed in some retinal lesions.

Appearance of NMDA receptors triggered by anoxia independent of voltage in vivo and in vitro

Using rat hippocampus we have studied the pattern of neuronal death, abnormal discharge and loss of electrical excitability in slices prepared from animals subjected to bilateral, four-vessel cerebral anoxia and in slices prepared from normal animals that are subjected to anoxia in the recording chamber. As others have reported, pyramidal neurons in area CA1 are lost first after anoxia, while CA3 neurons have an intermediate sensitivity, and those in dentate are relatively anoxia-resistant. After anoxic damage to the intact animal, neurons in both CA1 and CA3 show abnormal bursting discharges in response to synaptic activation for several days, and then the response in CA1 decreases in amplitude and finally the area become unexcitable. While antagonists for N-methyl-D-aspartate (NMDA) receptors have essentially no effect on synaptic responses in control animals, they reduce the bursting responses and greatly depress the small responses in CA1 as neurons are becoming unexcitable after anoxia. With intracellular recording CA1 neurons from animals made transiently anoxic, in contrast to controls, show prolonged synaptic responses, the later components of which are blocked by NMDA antagonists. When slices from normal animals are subjected to anoxia such that excitability is totally lost over a period of about 10 min, there is no significant membrane depolarization during the anoxic episode and recovery of excitability occurs with reoxygenation. However, a period of hyperexcitability and bursting follows and electrical excitability is lost in CA1 but not CA3 neurons after about 90 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate neurotoxicity and the inhibition of protein synthesis in the hippocampal slice

In some animal models of ischemia, neuronal degeneration can be prevented by the selective antagonism of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype, suggesting that glutamate released during ischemia causes injury by activating NMDA receptors. The rat hippocampal slice preparation was used as an in vitro model to study the pharmacology of glutamate toxicity and investigate why NMDA receptors are critical in ischemic injury. Acute toxicity was assessed by quantifying the inhibition of protein synthesis, which we confirmed by autoradiography to be primarily neuronal. The effect of NMDA was prevented by the specific antagonists MK-801 and ketamine, as well as by the less selective antagonist kynurenic acid. The less selective antagonists kynurenic acid and 6,7-dinitroquinoxaline-2,3-dione antagonized the effects of quisqualate and NMDA. In contrast to previous observations with dissociated neurons in tissue culture, the toxicity of glutamate was unaffected by antagonists, regardless of the glutamate concentration, the duration of exposure, or the presence of magnesium. The high concentration of glutamate required to inhibit protein synthesis and the inability of receptor antagonists to block the effect of glutamate suggest that either glutamate acts through a non-receptor-mediated mechanism, or that the receptor-mediated nature of glutamate effects are masked in the slice preparation, perhaps by the glial uptake of glutamate. The altered physiology induced by ischemia must potentiate the neurotoxicity of glutamate, because we observed with a brain slice preparation that only high concentrations of glutamate caused neurotoxicity in the presence of oxygen and glucose and that these effects were not reversed by glutamate receptor antagonists.

Neuronal degeneration in hippocampus and cerebellum of mutant spastic Han-Wistar rats

The neuropathology of the brain of mutant spastic Han-Wistar rats (Han-Wist SPA/SPA) was investigated using histological techniques. A surprising result was the detection of neuronal degeneration in the hippocampus and cerebellum of mutant spastic rat brains, whereas other regions, e.g. neocortex, isocortex, basal ganglia and thalamus, were overall normal. The CA3 sector in the septal third of the hippocampus including the cell band reaching into the hilus ('CA3c') showed a severe neuronal degeneration, whereas the granule cells of the dentate gyrus, several hilar neurons ('CA4') and the pyramidal cells in CA1 were found normal. In the cerebellum, a variable patchy degeneration of Purkinje cells was detected while the general layering was normal and granule cells and Golgi cells appeared preserved.

Excitatory amino acid neurotoxicity and neurodegenerative disease

The progress over the last 30 years in defining the role of excitatory amino acids in normal physiological function and in the abnormal neuronal activity of epilepsy has been reviewed in earlier articles in this series. In the last five years it has become clear that excitatory amino acids also play a role in a wide range of neurodegenerative processes. The evidence is clearest where the degenerative process is acute, but is more controversial for slow degenerative processes. In this article Brian Meldrum and John Garthwaite review in vivo and in vitro studies of the cytotoxicity of amino acids and summarize the contribution of such toxicity to acute and chronic neurodegenerative disorders.

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

Primary cultures of cerebellar rat granule neurons were grown for 18-22 days in vitro in the absence of antibiotics. When the cultures were placed in a low calcium (no EGTA) balanced salt solution at room temperature, rapid cell death occurred usually within 30 min of placing cells in the buffer. Changes in the cells were evident within 10 min and included an apparent cellular granulation with a partial loss of cell body birefringence at 10 x magnification which was complete by 30 min. This rapid death was prevented by (1) replacing chloride in the buffer with acetate; (2) increasing the osmolarity of the buffer by 30% with sucrose; (3) the addition of the selective excitatory amino acid (EAA) antagonist, 2-amino-7-phosphonoheptanoic acid (APH, 200 microM) but not by the selective kainate-quisqualate antagonist, glutamylaminomethylsulfonic acid (GAMS, 400 microM); or (4) the addition of one of the following calcium channel antagonists, verapamil (400 microM) diltiazem (150 microM) or lanthanum (5 microM). Placing cells in low calcium buffer resulted in a 3.7- and 3.2-fold increase in the non-selective secretion of aspartate and glutamate (as well as other amino acids) over baseline secretion (same buffer except containing 2.5 mM calcium). This increase was partially prevented by verapamil, but not by APH or chloride deletion. Verapamil only partially prevented the efflux of glutamate in buffer containing 1 mM EGTA. These results indicate that placing cells in low calcium buffer results in neurotoxicity secondary to both the influx of chloride and water in conjunction with the efflux of amino acids, some of which stimulate an excitatory amino acid receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective degeneration of cerebellar cortical neurons caused by cycad neurotoxin, L-beta-methylaminoalanine (L-BMAA), in rats

Both the racemate and the L-form of BMAA (beta-methylaminoalanine), when injected intraperitoneally into young rats, produced acute signs of cerebellar dysfunction and degeneration of cerebellar stellate, basket, Purkinje and Golgi cells, but not granule cells. Degenerative changes were also occasionally seen in cerebellar roof nuclei which may be secondary in nature. No other changes were found in the remainder of the central nervous system. The doses of the L-form of BMAA producing these changes were from 6 to 14 mumols/g body weight, i.e. the lower and upper levels of the dose range used by Vega and Bell (1967) and equivalent to 75 and 183 mg/rat. Doses of 1 to 4 mg/g body weight of the racemate were given to young rats less than 100 g in weight, but no changes were apparent after daily doses of the racemate of 0.5 mg/g body weight. Damage to cerebellar neurons is considered to be the result of excitotoxic activity. All cells showing degeneration are GABAergic, although not all are known to possess N-methyl-D-aspartate (NMDA) receptors. The present finding of selective cerebellar neuron damage may not conflict with the earlier findings of others, but our results suggest that L-BMAA has unusual glutamate receptor binding properties.

Glutamate- and aspartate-induced extracellular potassium and calcium shifts and their relation to those of kainate, quisqualate and N-methyl-D-aspartate in the isolated turtle cerebellum

Ion-selective microelectrodes can be used to evaluate the characteristics and laminar distribution of excitatory amino acid agonist-induced K+ and Ca2+ shifts in the extracellular environment of brain cells. This report describes the pattern of K+ increases and Ca2+ decreases elicited by glutamate and aspartate at 100 microns intervals in the isolated turtle cerebellum. These responses were compared to ion shifts evoked by kainate, quisqualate and N-methyl-D-aspartate. Glutamate and aspartate produced indistinguishable laminar patterns of ion shifts, with the greatest [K+]o and [Ca2+]o shifts in the granular layer. The average maximum granular and molecular layer increases in [K+]o were, respectively, 130% and 24% larger than the increase in the Purkinje cell layer. Kainate-induced increases in [K+]o also followed this granular greater than molecular greater than Purkinje cell layer pattern; however, the corresponding [Ca2+]o decreases were smaller and more variable. Quisqualate-evoked ion shifts in the molecular layer closely mimicked the shape of glutamate- and aspartate-induced responses. In the granular layer, however, quisqualate caused little ion change during iontophoresis followed by large [K+]o and [Ca+]o shifts after the end of the pulse. The minimal ion shifts induced during quisqualate application in the granular layer gave this agonist the distinction of being the only agent tested to have its greatest direct effect in the molecular layer. N-Methyl-D-aspartate caused large, two-phase [K+]o and [Ca2+]o shifts in the granular layer, only small [K+]o rises in the Purkinje cell and ventral molecular layers, and no response in the dorsal molecular layer. The lack of similarity between glutamate- and aspartate-induced ion shifts in the granular layer and those of any one agonist demonstrate the mixed agonist action of glutamate and aspartate in the cerebellum. These studies provide new information about the dynamics of excitatory amino acid receptor activation that is complementary to autoradiographic receptor mapping data and to single cell electrophysiological studies.

Glutamate dehydrogenase in cerebellar mutant mice: gene localization and enzyme activity in different tissues

Many similarities of both the inheritance pattern and the neuropathology can be observed between olivopontocerebellar atrophies, or so-called multiple system atrophies (MSAs), and murine cerebellar mutations like Purkinje cell degeneration, nervous, staggerer, weaver, and reeler. Our study aimed to test whether the glutamate dehydrogenase (GDH) deficiency observed in some MSA patients could be found also in any of the murine mutants. GDH activity was assayed in several organs of these mutants, and no general deficiency was detected. By contrast, the level was found to be elevated in the cerebellum. The GDH gene was localized on mouse chromosome 14 and does not map close to any known neurological mutation in the mouse. We conclude, for the moment, that none of these cerebellar mutant mice can be considered as an animal model for GDH-deficient MSA.

Metabolic action of N-methyl-D-aspartate in newborn rat brain ex vivo: 31p magnetic resonance spectroscopy

N-methyl-D-aspartate (NMDA) is an agonist used to identify neuronal receptive sites for dicarboxylic amino acid neurotransmitters; NMDA receptors are implicated in neuronal damage of ischemic or hypoglycemic origin in newborns although involved mechanisms remain to be identified. In the present study, 31P magnetic resonance spectroscopy with fast (6/min) data acquisition was used in newborn rat brain slices to measure changes of intracellular phosphocreatine and nucleotide triphosphate levels following extracellular NMDA applications. The rapid exhaustion of phosphocreatine stores in 50% of the total population of brain cells was induced in all cases by application of NMDA (30-45 s, 25-100 mM). It was not reproduced by other excitatory agents: potassium ions (24.6 mM, 4 min), isobutylxanthine (1mM), muscarine (10 mM), serotonin (0.1 mM) or substance P (10 microM). Such an effect of NMDA was not modified after tetrodotoxin (1 microM) and was reduced by extracellular 2-amino-5-phosphonovalerate (50 microM) or magnesium ions (2.2 mM). However it did develop during NMDA-induce neuronal excitations and was reversible within 10-30 min. This action of NMDA was followed by an irreversible decrease of phosphorus metabolites if mitochondrial creatine kinase and adenosine triphosphatase were decoupled by atractyloside (50 microM). Experiments revealed a link between selective NMDA action at neuronal plasma membranes, neurotoxicity and energy production by mitochondria.

Neurotoxicity of excitatory amino acid receptor agonists in young rat hippocampal slices

Hippocampal slices from young (8-day-old) rats were evaluated as a model for investigating the mechanisms underlying the neurotoxic action of excitatory amino acid receptor agonists. The slices were exposed to the agonists for up to 30 min and were then postincubated for 90 min in order to allow irreversibly damaged cells to become visibly necrotic. Under control conditions (greater than or equal to 3 h incubation) all regions of the hippocampus and dentate gyrus displayed good preservation. Exposure of the slices to N-methyl-D-aspartate (NMDA) resulted in widespread, oedematous necrosis of all neuronal types (except undifferentiated granule cells) which was maximal after 20 min exposure to a concentration of 100 microM. With 30 min exposure, the EC50 for NMDA was 30 microM; 10 min exposure to NMDA at a concentration of 100 microM was sufficient to destroy 50% of the neurones. Quisqualate produced a degeneration of most (98%) of the CA3 neurones, a proportion (65%) of CA1 neurons and some (25%) of the dentate granule cells. The occurrence of "dark cell degeneration" was prevalent. Half maximal effects on CA3 neurones were estimated to be produced by a concentration of 15 microM (with 30 min exposure) or by 8 min exposure (at 100 microM concentration). Incubation of the slices with kainate (100 microM for 30 min) did not cause widespread damage but led to the necrosis of a small population of cells scattered in all regions of the hippocampus and dentate gyrus. The patterns of toxicity of the different agonists resemble closely those found after their administration in vivo. It is suggested that the hippocampal slices provide a valuable new model system for studying excitatory amino acid toxicity.

Quisqualate neurotoxicity: a delayed, CNQX-sensitive process triggered by a CNQX-insensitive mechanism in young rat hippocampal slices

Exposure of slices of young rat hippocampus for 30 min to the glutamate receptor agonist, quisqualate (QA, 30 microM), led, after a 90 min recovery period, to severe 'dark cell degeneration' of pyramidal neurones, most extensively those in CA3. When present during the exposure, 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX, 10 microM), an antagonist with preferential action on non-N-methyl-D-aspartate receptors, did not prevent this toxic effect of QA. However, it was effective when included either during the recovery period as well or, indeed, only during recovery. Comparable results were obtained with kynurenate (3 mM), but not with D,L-2-amino-5-phosphonovalerate (100 microM) or with tetrodotoxin (0.5 microM). Grease-gap recordings showed that CNQX markedly inhibited QA-induced depolarization. It is concluded that QA toxicity is not triggered by QA-induced depolarization but instead involves CNQX-resistant QA receptors, possibly those linked to phospholipid metabolism. The induction mechanism does not itself cause irreversible injury but subsequently, a delayed form of damage takes place which is mediated by activation of CNQX/kynurenate-sensitive receptors.

Differential dependence on Ca2+ of N-methyl-D-aspartate and quisqualate neurotoxicity in young rat hippocampal slices

Exposure of slices of young (8 days old) rat hippocampus to 100 microM N-methyl-D-aspartate (NMDA) for 20 min followed by 90 min recovery, resulted in widespread, oedematous necrosis of all classes of neurones. The NMDA antagonist, D,L-2-amino-5-phosphonovalerate (APV) or omission of Ca2+ from the exposing solution prevented this cell death, but a large reduction in Cl- was ineffective. Quisqualate (100 microM, 20 min) led to a different pathological pattern characterised most strikingly by large numbers of cells undergoing 'dark cell degeneration'. Numerically, the neurones were affected in the order CA3 greater than CA1 greater than dentate granule cells. Quisqualate toxicity was not prevented by APV nor by reducing Ca2+ or Cl-. It is concluded that, as in cerebellar slices (but unlike in cultures of hippocampal neurones) NMDA toxicity in hippocampal slices is Ca2+-dependent and Cl- -independent. However, quisqualate exerts its pathological effects through a different mechanism. This mechanism may be primarily metabolic rather than ionic.

Metabolic acidosis induced by N-methyl-D-aspartate in brain slices of the neonatal rat: 31P- and 1H-magnetic resonance spectroscopy

1H- and 31P-magnetic resonance spectroscopy was used to monitor intracellular lactate, phosphorus metabolites and pH in superfused brain slices from 2- to 9-day-old rats. N-Methyl-D-aspartate (NMDA) (100 microM, 0.5-3 min) was applied in the extracellular magnesium-free perfusion medium. NMDA induced intracellular metabolic acidosis, i.e., an increase of freely mobile lactate levels and an 0.3 pH unit acidification. This was abolished when the extracellular glucose supply was reduced. Experiments also indicate that acidosis is not responsible for the cell damage resulting from activation of NMDA receptors in hypoglycemic conditions.

Cyclic GMP and cell death in rat cerebellar slices

Incubated slices of young rat cerebellum were used to examine the possible relationship between the neurotoxic effects of excitatory amino acids and their ability to elicit large increases in the levels of cyclic GMP in this tissue. No cell death was detectable following exposure of the slices to the guanylate cyclase activator, nitroprusside (up to 0.3 mM), the phosphodiesterase inhibitor, isobutylmethylxanthine (0.5 mM), or to cyclic GMP (10 mM) and its dibutyryl and 8-bromo derivatives (0.5 mM). However, incubation of the slices with tbe guanylate cyclase inhibitors, N-methylhydroxylamine and hydroxylamine (0.1-1 mM), methylene blue (10-100 microM), ethacrynic acid (300 microM) and retinol (1 mM) caused a progressive destruction of the differentiating cells. The damage induced by N-methylhydroxylamine and hydroxylamine was inhibited by nitroprusside, cyclic GMP and isobutylmethylxanthine. It could also be reduced by lowering the partial pressure of oxygen, by oxygen radical scavenging enzymes and by omitting Ca2+ from the medium. Oxygen radical generating enzyme systems mimicked the pattern of toxicity of the guanylate cyclase inhibitors but their effects were not reduced by nitroprusside or omission of Ca2+. The results indicate that guanylate cyclase/cyclic GMP does not mediate amino acid neurotoxicity but, instead, may be part of a protective mechanism against oxygen free radicals.