Excitatory amino acid neurotoxicity in the hippocampal slice preparation

Age-dependent changes in NMDA-induced excitotoxicity and neuromodulatory effects of kynurenic acid and its analogue in mouse brain slices

Kynurenic acid (KYNA) is one of the main neuroprotective substances of the kynurenine pathway. KYNA plays an important role in various neurodegenerative and psychiatric diseases. Although KYNA has been shown to have neuroprotective effects, it cannot be used as a peripherally administered drug due to its poor ability to cross the blood-brain barrier. To address this limitation, chemically modified KYNA analogues are being developed: SZR72 is one such analogue and has been shown to be protective in various animal models. Glutamate-induced excitotoxicity is a key factor in many neurodegenerative diseases. Therefore, we used the N-methyl-D-aspartate (NMDA)-induced excitotoxicity model to investigate the neuromodulatory agents. Using acute hippocampal slices from mouse brains, we investigated the potential neuroprotective effect of KYNA and its analogue, SZR72 on NMDA-induced excitotoxicity across different age groups of mice. The degree of tissue damage was assessed using biochemical and histological methods. In young animals (1- and 4-week-old), NMDA treatment caused no significant changes, and the cells were found to be resistant. However, in older animals (8-week-old and 1-year-old), NMDA caused significant damage in cells and tissue structure, which was reduced by KYNA and SZR72 treatment. To our knowledge, this is the first study to compare the neuroprotective effects of KYNA and SZR72 in animals of different ages using an in vitro NMDA excitotoxicity model.

Developmental changes in NMDA neurotoxicity reflect developmental changes in subunit composition of NMDA receptors

Excitotoxicity is generally studied in dissociated neurons, cultured hippocampal slices, or intact animals. However, the requirements of dissociated neurons or cultured slices to use prenatal or juvenile rats seriously limit the advantages of these systems, whereas the complexity of intact animals prevents detailed molecular investigations. In the present experiments, we studied developmental changes in NMDA neurotoxicity in acute hippocampal slices with lactate dehydrogenase (LDH) release in medium, propidium iodide (PI) uptake, and Nissl staining as markers of cell damage. Calpain-mediated spectrin degradation was used to test calpain involvement in NMDA neurotoxicity. NMDA treatment produced increased LDH release, PI uptake, and spectrin degradation in slices from juvenile rats but not adult rats. NMDA-induced changes in slices from young rats were blocked completely by the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801) and by the antagonists of NR2B receptor ifenprodil and R-(R, S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol and were partly blocked by calpain inhibitor III but were not affected by the NR2A-specific antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid. NMDA-induced changes in Nissl staining were also different in slices from young and adult rats and blocked by NR2B but not NR2A antagonists. In contrast to NMDA treatment, oxygen/glucose deprivation (OGD) induced neurotoxicity in slices from both young and adult rats, although OGD-induced toxicity was attenuated by MK-801 only in slices from young rats. Our results are consistent with the idea that NMDA-mediated toxicity is caused by activation of NR2B- but not NR2A-containing NMDA receptors leading to calpain activation and that developmental changes in NMDA toxicity reflect developmental changes in NMDA receptor subunit composition.

Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys

Previous studies showed that dorsal psalterium (PSD) volleys to the entorhinal cortex (ENT) activated in layer II perforant path neurons projecting to the dentate gyrus. The discharge of layer II neurons was followed by the sequential activation of the dentate gyrus (DG), field CA3, field CA1. The aim of the present study was to ascertain whether in this experimental model field, CA2, a largely ignored sector, is activated either directly by perforant path volleys and/or indirectly by recurrent hippocampal projections. Field potentials evoked by single-shock PSD stimulation were recorded in anesthetized guinea pigs from ENT, DG, fields CA2, CA1, and CA3. Current source-density (CSD) analysis was used to localize the input/s to field CA2. The results showed the presence in field CA2 of an early population spike superimposed on a slow wave (early response) and of a late and smaller population spike, superimposed on a slow wave (late response). CSD analysis during the early CA2 response showed a current sink in stratum lacunosum-moleculare, followed by a sink moving from stratum radiatum to stratum pyramidale, suggesting that this response represented the activation and discharge of CA2 pyramidal neurons, mediated by perforant path fibers to this field. CSD analysis during the late response showed a current sink in middle stratum radiatum of CA2 followed by a sink moving from inner stratum radiatum to stratum pyramidale, suggesting that this response was mediated by Schaffer collaterals from field CA3. No early population spike was evoked in CA3. However, an early current sink of small magnitude was evoked in stratum lacunosum-moleculare of CA3, suggesting the presence of synaptic currents mediated by perforant path fibers to this field. The results provide novel information about the perforant path system, by showing that dorsal psalterium volleys to the entorhinal cortex activate perforant path neurons that evoke the parallel discharge of granule cells and CA2 pyramidal neurons and depolarization, but no discharge of CA3 pyramidal neurons. Consequently, field CA2 may mediate the direct transfer of ENT signals to hippocampal and extrahippocampal structures in parallel with the DG-CA3-CA1 system and may provide a security factor in situations in which the latter is disrupted.

Effects of early environment on field CA2 pyramidal neurons in the guinea-pig

Previous work showed that isolation rearing produces remarkable changes in the dendritic pattern and soma of the principal neurons in the dentate gyrus and hippocampal fields CA3 and CA1 of the guinea-pig. The aim of the present study was to obtain information about the effects of early postnatal isolation on neuron morphology in field CA2, the "resistant sector" of the hippocampal formation. Male and female guinea-pigs were assigned at 6-7 days of age to either a control (social) or an isolated environment where they remained for 80-90 days. The apical and basal dendritic trees and the soma of CA2 pyramidal neurons were analyzed and quantified in Golgi-stained brains. The results showed that in both males and females early isolation caused no effects on the length and dendritic branching density of the apical tree of field CA2 pyramidal neurons. In males but not in females isolation caused a spine density reduction in the inner apical tree. Isolation notably influenced the morphology of the basal tree, but in males only. Isolated males exhibited a significant reduction in the length of the basal tree and number of dendritic branches accompanied by a reduction in spine density. The comparison of animals reared in the same environment showed that in the control environment males had more apical and basal dendritic branches and a larger neuron soma than females. In the isolated environment the sex differences in the apical tree disappeared and those in the basal tree changed direction.The results demonstrate structural changes in field CA2 pyramidal neurons following neonatal isolation, with a specific reactivity to environment of the basal tree of males. The dendritic atrophy in field CA2 of isolated males is in line with previous evidence that males react to isolation mainly with dendritic atrophy, though field CA2 neurons appear to be less damaged than those of the other hippocampal fields. This is in line with the resistance of this field to neurodegeneration. The absence of structural changes in field CA2 of isolated females confirms, once again, that males are more liable to be endangered by early isolation than females.

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.

Comparison of excitotoxic profiles of ATPA, AMPA, KA and NMDA in organotypic hippocampal slice cultures

The excitotoxic profiles of (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionic acid (ATPA), (RS)-2-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainic acid (KA) and N-methyl-D-aspartate (NMDA) were evaluated using cellular uptake of propidium iodide (PI) as a measure for induced, concentration-dependent neuronal damage in hippocampal slice cultures. ATPA is in low concentrations a new selective agonist of the glutamate receptor subunit GluR5 confined to KA receptors and also in high concentrations an AMPA receptor agonist. The following rank order of estimated EC(50) values was found after 2 days of exposure: AMPA (3.7 mM)>NMDA (11 mM)=KA (13 mM)>ATPA (33 mM). Exposed to 30 microM ATPA, 3 microM AMPA and 10 microM NMDA, CA1 was the most susceptible subfield followed by fascia dentata and CA3. Using 8 microM KA, CA3 was the most susceptible subfield, followed by fascia dentata and CA1. In 100 microM concentrations, all four agonists induced the same, maximal PI uptake in all hippocampal subfields, corresponding to total neuronal degeneration. Using glutamate receptor antagonists, like GYKI 52466, NBQX and MK-801, inhibition data revealed that AMPA excitotoxicity was mediated primarily via AMPA receptors. Similar results were found for a high concentration of ATPA (30 microM). In low GluR5 selective concentrations (0.3-3 microM), ATPA did not induce an increase in PI uptake or a reduction in glutamic acid decarboxylase (GAD) activity of hippocampal interneurons. For KA, the excitotoxicity appeared to be mediated via both KA and AMPA receptors. NMDA receptors were not involved in AMPA-, ATPA- and KA-induced excitotoxicity, nor did NMDA-induced excitotoxicity require activation of AMPA and KA receptors. We conclude that hippocampal slice cultures constitute a feasible test system for evaluation of excitotoxic effects and mechanisms of new (ATPA) and classic (AMPA, KA and NMDA) glutamate receptor agonists. Comparison of concentration-response curves with calculation of EC(50) values for glutamate receptor agonists are possible, as well as comparison of inhibition data for glutamate receptor antagonists. The observation that the slice cultures respond with more in vivo-like patterns of excitotoxicity than primary neuronal cultures, suggests that slice cultures are the best model of choice for a number of glutamate agonist and antagonist studies.

Novel characteristics of glutamate-induced cell death in primary septohippocampal cultures: relationship to calpain and caspase-3 protease activation

Studies examined the phenotypic characteristics of glutamate-induced cell death and their relationship to calpain and caspase-3 activation. Cell viability was assessed by fluorescein diacetate and propidium iodide staining and lactate dehydrogenase release. Calpain and caspase-3 activity was inferred from signature proteolytic fragmentation of alpha-spectrin. Characterization of cell death phenotypes was assessed by Hoechst 33258 and DNA fragmentation assays. Exposure of septohippocampal cultures to 1.0, 2.0, and 4.0 mmol/L glutamate induced a dose-dependent cell death with an LD50 of 2.0 mmol/L glutamate after 24 hours of incubation. Glutamate treatment induced cell death in neurons and astroglia and produced morphological alterations that differed from necrotic or apoptotic changes observed after maitotoxin or staurosporine exposure, respectively. After glutamate treatment, cell nuclei were enlarged and eccentrically shaped, and aggregated chromatin appeared in a diffusely speckled pattern. Furthermore, no dose of glutamate produced evidence of internucleosomal DNA fragmentation. Incubation with varying doses of glutamate produced calpain and caspase-3 activation. Calpain inhibitor II (N-acetyl-Leu-Leu-methionyl) provided protection only with a narrow dose range, whereas carbobenzoxy-Asp-CH2-OC(O)-2,6-dichlorobenzene (Z-D-DCB; pan-caspase inhibitor) and MK-801 (N-methyl-D-aspartate receptor antagonist) were potently effective across a wider dose range. Cycloheximide did not reduce cell death or protease activation.

Autoradiographic measurements of protein synthesis in hippocampal slices from rats and guinea pigs

Protein synthesis is an extremely important cell function and there is now good evidence that changes in synthesis play important roles both in neuronal cell damage from ischemic insults and in neural plasticity though the mechanisms of these effects are not at all clear. The brain slice, and particularly the hippocampal slice, is an excellent preparation for studying these effects although, as with all studies on slices, caution must be exercised in that regulation in the slice may be different from regulation in vivo. Studies on neural tissue need to take into account the heterogeneity of neural tissue as well as the very different compartments within neurons. Autoradiography at both the light and electron microscope levels is a very powerful method for doing this. Successful autoradiography depends on many factors. These include correct choice of precursor amino acid, mechanisms for estimating changes in the specific activity of the precursor amino acid pool, and reliable methods for quantitation of the autoradiographs. At a more technical level these factors include attention to detail in processing tissue sections so as to avoid light contamination during exposure and developing and, also, appropriate choices of the various parameters such as exposure time and section thickness. The power of autoradiography is illustrated here by its ability to discern effects of ischemia and of plasticity-related neural input on distinct cell types and also in distinct compartments of neurons. Ischemia inhibits protein synthesis in principal neurons but activates synthesis in other cell types of the brain slice. Plasticity-related neural input immediately enhances protein synthesis in dendrites but does not affect cell bodies.

Intrinsic NMDA-induced oscillations in motoneurons of an adult vertebrate spinal cord are masked by inhibition

Low-frequency membrane potential oscillations were induced in motoneurons (MNs) of isolated hemisected frog spinal cords during N-methyl-D-aspartate (NMDA) application. Oscillations required the presence of physiological Mg2+ and preincubation with strychnine, whereas incubation with bicuculline or phaclofen was not effective. Oscillations were evident in intracellular recordings from single MNs and simultaneous extracellular recordings from lumbar ventral roots. In Mg(2+)-free solution, MNs exhibited irregular transient membrane potential depolarizations that were blocked by D,L-2-amino-5-phosphonopentanoic acid (APV) but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Generation and maintenance of membrane potential oscillations required specific NMDA receptor activation. Oscillations were antagonized by APV but not by CNQX. Strychnine preincubation was required for NMDA to induce oscillations, but was not critical in maintaining them, because oscillations persisted after removal of strychnine. Therefore oscillations are suggested to be an inherent property of the spinal neuronal circuitry. Tetrodotoxin (TTX) blocked spike activity and had a bimodal effect on membrane potential oscillations. Oscillations initially were blocked by TTX, but reappeared spontaneously after 10-40 min. This suggests that maintenance of oscillations, once evoked, does not involve MN firing. Na+ entry through TTX-insensitive Na+ channels and/or NMDA receptor channels, trans-membrane Ca2+ flux, Ca2+ release from intracellular stores, and Ca2+ activated K+ channels were critical in controlling the amplitude and frequency of membrane potential oscillations. It is hypothesized that these unmasked intrinsic oscillations in adult frog spinal cord MNs may represent a premetamorphic spinal oscillator involved in tadpole swimming that becomes suppressed during metamorphosis as strychnine-sensitive inhibition becomes more pronounced.

Apoptosis in neurodegenerative disorders

Although the exact mechanism of nigral cell death in Parkinson's disease (PD) is not known, increasing evidence suggests the presence of apoptotic cell death in PD. When we applied the TUNEL method to detect DNA fragmentation, four out of seven late onset sporadic patients with PD showed TUNEL-positive neurons. The percentages of those neurons among the remaining melanin containing neurons were 0.6 to 4.8% (average 2.1%). But TUNEL-positive neurons could not be detected in control subjects as well as four patients with young onset (under 40 years of the age) PD. Numbers of nigral toxins such as MPTP, complex I inhibitors, and mitochondrial respiratory inhibitors have been reported to induced apoptotic cell death. These findings suggest that apoptosis is involved in nigral cell cleath in PD at least in part and warrant further studies on apoptosis-related substances in PD.

Schwann cell apoptosis during normal development and after axonal degeneration induced by neurotoxins in the chick embryo

In the present work, we show that chick embryo Schwann cells die by apoptosis both during normal development and after axonal degeneration induced by neurotoxin treatment. Schwann cell apoptosis during development takes place during a period roughly coincidental with normally occurring motoneuron death. Administration of NMDA to chick embryos on embryonic day 7 induces extensive excitotoxic motoneuronal damage in the spinal cord without any apparent effects on neurons in the dorsal root ganglia (DRG). The death of Schwann cells in ventral nerve roots after NMDA treatment causes degenerative changes that display ultrastructural features of apoptosis and exhibit in situ detectable DNA fragmentation. By contrast, NMDA treatment does not increase the death of Schwann cells in dorsal nerve roots. In situ detection of DNA fragmentation in combination with the avian Schwann cell marker 1E8 antibody demonstrates that dying cells in ventral nerve roots are in the Schwann cell lineage. Administration of cycloheximide does not prevent the toxic effects of NMDA on motoneurons, but dramatically reduces the number of pyknotic Schwann cells and DNA fragmentation profiles in the ventral nerve roots. In ovo administration of various tissue extracts (muscle, brain, and spinal cord) from the chick embryo or of the motoneuron conditioned medium fails to prevent Schwann cell apoptosis in NMDA-treated embryos. Intramuscular administration of the snake toxin beta-bungarotoxin produces a massive death of both lateral motor column motoneurons and DRG neurons, resulting in a substantial increase in the number of pyknotic Schwann cells in both ventral and dorsal nerve roots. It is concluded that during development, axonal-derived trophic signals are involved in the regulation of Schwann cell survival in peripheral nerves.

Time-related neuronal changes following middle cerebral artery occlusion: implications for therapeutic intervention and the role of calpain

Changes in neocortex and striatum were characterized over time following focal ischemia to the brain. Rats were subjected to permanent middle cerebral artery occlusion (MCA-O) and sacrificed 1, 3, 6, 12, or 24 h later. The affected tissue was processed for tetrazolium chloride (TTC) and cresyl violet staining, as well as for Western blots to detect calpain-induced spectrin proteolysis. Significant changes in cell size and spectrin breakdown occurred within the first hour of occlusion, with further, dramatic changes in these two early markers continuing over time. Initial evidence of cell loss was noted at 1 h postocclusion in the striatum and at 3 h in the neocortex. However, even in the center of the most affected portion of the neocortex, the majority of cells appeared to be intact through 6 h. By this time, a significant TTC-defined infarct also emerged. These quantitative data indicate that calpain-induced proteolysis occurs very soon after the ischemic insult, is correlated with earliest changes in cell hypotrophy, and precedes or occurs in tandem with evidence of significant cell loss. They also demonstrate that, while some cell loss occurs earlier than previously believed, the majority of cells remains morphologically intact well beyond what is typically thought to be the window of opportunity for intervention. The results thus raise the question of how long after the ischemic event pharmaceutic intervention might be employed to salvage substantial numbers of neurons.

Neuropathological changes in two lines of mice carrying a transgene for mutant human Cu,Zn SOD, and in mice overexpressing wild type human SOD: a model of familial amyotrophic lateral sclerosis (FALS)

Two different lines of mice, G1 and G20, carrying a transgene for a mutant form of Cu,Zn SOD, found in a family with familial amyotrophic lateral sclerosis (FALS), develop clinical and pathological changes which are, in their late stages, strikingly similar to those in human disease. We have analyzed the distribution and characteristics of lesions in the central and peripheral nervous systems of such mice. The most affected structure was the spinal cord, followed by the medulla, pons and midbrain. The early stages of the disease were characterized by vascular degeneration of anterior horn neurons and their processes, while, in the late stages, the main changes consisted of neuronal loss and atrophy of the anterior horns and the deposition in these areas of multiple filamentous inclusions resembling Lewy bodies. In the late stages of the disease, the white matter of the spinal cord was also involved, particularly in the anterior and lateral columns. Posterior columns were also involved, but to a much lesser degree. The brainstem structures also showed vacuolar degeneration of several motor nuclei and of several groups neurons in the reticular formation. Anterior roots and peripheral nerves showed the classical features of Wallerian degeneration. The dorsal root ganglia, with rare exceptions, were unremarkable. The posterior roots showed mild changes in the most severely affected mice. Changes in these two affected lines were compared to changes in mice overexpressing wild type, rather than mutant human Cu,Zn SOD. These mice never developed clinical disease, although, pathologically, they developed very mild vacuolar changes in the anterior horns of the spinal cord and in motor axons. This study shows that although simple overexpression of SOD may be injurious to motor neurons, albeit very mildly, the mutant form is necessary to produce both clinical disease and severe pathological changes which, in the chronic stage of the disease, have striking similarities to human familial ALS. A dominant gain of function, therefore, is the most likely pathogenesis of tissue injury induced by mutations in Cu,Zn SOD.

Immunocytochemical evidence that quisqualate is selectively internalized into a subset of hippocampal neurons

Quisqualic acid (QUIS) has been shown to interact with several glutamate receptor subtypes and uptake sites. We have previously demonstrated that a brief exposure of hippocampal cells to QUIS sensitizes them to depolarization by the alpha-amino-omega-phosphonate analogues of glutamate, AP4, AP5, and AP6. This QUIS-induced sensitization is accompanied by the active uptake of QUIS into hippocampal slices. In order to localize the sites of QUIS uptake into rat hippocampal slices, a polyclonal antibody against QUIS was raised in rabbits. Utilizing immunocytochemical techniques, we have identified immunoreactive axons and dendrites after brief exposure times to QUIS, and perikarya after longer exposure times to QUIS. The intensity of the QUIS immunoreactivity increased as the exposure time to QUIS increased. QUIS immunoreactivity was primarily found in stratum oriens and stratum radiatum, of regions CA1, CA2, and CA3 of the hippocampus as well as in the hilus and molecular layer of the dentate gyrus. The distribution and morphology of QUIS immunoreactive cells appeared to be similar to those of GABAergic interneurons. Glial fibrillary acidic protein (GFAP) did not co-localize with the QUIS-internalizing cells suggesting that they are not glia. Ultrastructural analysis revealed QUIS immunoreactive profiles within the stratum radiatum. Immunostained profiles at both the light and EM levels appeared, in many cases, to be swollen and showed signs of degeneration. Such changes were only evident in tissue exposed to QUIS. These data demonstrate that QUIS is taken up by a select group of neurons in the rat hippocampus.

Distribution of cholinergic, GABAergic and serotonergic neurons in the medial medullary reticular formation and their projections studied by cytotoxic lesions in the cat

As part of a larger study concerning the role of neurons in the medial medullary reticular formation in sleep-wake states, the distribution and projections of cholinergic, GABAergic and serotonergic neurons were studied within the lower brainstem of the cat. Cells were plotted with the aid of an image analysis system through the medullary reticular formation and raphe in adjacent sections immunostained for choline acetyltransferase, glutamic acid decarboxylase and serotonin. Immunostained fibres and varicosities were examined and quantified by microdensitometry in regions of the medulla, pons and upper spinal cord in normal and quisqualate-injected animals to assess the loss of local and distant projections following cytotoxic destruction of neurons in the medial medullary reticular formation. Choline acetyltransferase-immunoreactive neurons are unevenly and sparsely distributed, though none the less in significant numbers (estimated at approximately 9080 in total), through the medial medullary reticular formation, and are present in all tegmental fields, including the gigantocellular (approximately 3700) and magnocellular (approximately 1760) rostrally and the ventral (approximately 3240) and paramedian (approximately 380) caudally, and are absent in the midline raphe. Glutamic acid decarboxylase-immunoreactive neurons are more evenly and densely distributed in large numbers (estimated at approximately 18,720) through the medial medullary reticular formation, being present in the gigantocellular (approximately 5960), magnocellular (approximately 8260), ventral (approximately 2280) and paramedian (approximately 2220) tegmental fields, and are also numerous within the raphe magnus and pallidus-obscurus nuclei (approximately 3880). Serotonin-immunoreactive cells are sparse in the medial medullary reticular formation (estimated to total approximately 1540), where they are mainly located in the magnocellular tegmental field (approximately 1340), and are concentrated in larger numbers within the raphe nuclei (approximately 8060). Cholinergic varicose fibres were moderately densely distributed through the medial medullary reticular formation, as well as through more distant lateral, rostral and caudal brainstem and upper spinal regions. After cytotoxic lesions focussed in the gigantocellular and magnocellular tegmental fields, a loss of approximately 55% of the cholinergic neurons in the medial medullary reticular formation was associated with a minor decrease (approximately 35% in optical density measures) of local cholinergic fibres. Small and variable reductions in varicose fibres (and their optical density measures) were detected in distant structures (including the pontine lateral, gigantocellular and subcoerular tegmental fields and the caudal spinal trigeminal nucleus), that were none the less correlated with the number of intact medial medullary cholinergic cells, suggesting that these cells may project to distant brainstem targets, in addition to providing a minor proportion of the local cholinergic innervation of the medial medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Aurintricarboxylic acid protects hippocampal neurons from glutamate excitotoxicity in vitro

Aurintricarboxylic acid (ATA), an endonuclease inhibitor, has been shown to protect several cell types from an apoptotic form of cell death. We tested ATA for protective effects against glutamate excitotoxicity in 2-week-old cultured hippocampal neurons. Cell viability was determined 24 h after glutamate exposure either by trypan blue exclusion or by measurement of lactate dehydrogenase release. When ATA was added during exposure to glutamate, there was a dramatic increase in the number of viable neurons compared with cultures that did not receive ATA. If ATA was added after glutamate exposure, the rate of survival approached 100%. Several cellular processes may be the targets for ATA action. If the mechanisms of ATA protection are similar for excitotoxicity and apoptosis, then these distinct forms of cell death may share a common intracellular pathway.

High-affinity kainate binding sites in living slices of rat neocortex: characterization and regulation

We have characterized a high-affinity kainate binding site in in vitro living rat neocortical slices using [3H]kainate. [3H]Kainate labelled at least two binding sites, the higher affinity site with a Kd of 7.1 nM and a Bmax of 71.2 fmol/mg protein. This high-affinity binding site showed a pharmacology consistent with a kainate receptor with competition by kainate and domoic acid, as well as the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline. Increases in cellular depolarization induced by 2-h preincubations in veratridine and glutamate led to a significant 55% average decrease in [3H]kainate binding in adult cortex. Similarly, preincubation in kainate led to a significant average 26% decrease in binding. In both instances, Eadie-Hofstee analysis of saturation binding data revealed that the decreased binding reflected changes in receptor number. At different postnatal ages, increases in cellular depolarization significantly decreased binding (< 20 days postnatal age, -86%; > 60 days, -48%). Kainate treatment also significantly decreased binding at all ages (-64% at < 20 days; > 60 days, -18%), with significant differences noted between ages. These age-dependent effects are unlike those previously described for either N-methyl-D-aspartate [Lanius and Shaw (1992) Anat. Rec. 232, 54(A)] or (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate high affinity receptors [Shaw and Lanius (1992) Devl Brain Res. 68, 225-233].(ABSTRACT TRUNCATED AT 250 WORDS)

Serine released from hippocampal slices during deprivation of oxygen and glucose enhances the effects of glutamate on neuronal function

Using guinea-pig hippocampal slices, we determined the amount of various amino acids released into the medium during deprivation of oxygen and glucose. Within 10 min of slices being deprived of O2 and glucose, the amounts of serine, aspartate, alanine, glycine, GABA, taurine and threonine released into the medium increased up to 1.7 (serine), 1.6 (aspartate), 1.6 (alanine), 1.9 (glycine), 2.0 (GABA), 1.4 (taurine) and 1.8 (threonine) times the control levels, respectively. The amount of serine released 10 min after O2 and glucose deprivation was four times as great as that of glutamate. The dose-response effects of glutamate and serine were studied on the population spikes evoked in the granular cell layer. Bath application of 100 microM serine elevated the amplitude of the population spike to 117% and at 10 mM depressed it completely. The dose-response curve for glutamate displayed a similar pattern but the effectiveness was 10 times higher than that of serine. The combined application of glutamate (300 microM) and serine (2 mM) produced a dramatic reduction in and depression of the amplitude of the population spike, although 300 microM glutamate and 2 mM serine individually failed to show a significant effect. The population spike was depressed by the addition of 1 mM glutamate but, after washing, it recovered completely. On the other hand treatment with 1 mM glutamate together with 5 mM serine caused no recovery of the population spike even after removal of the agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of transient [3H]kainic acid-binding sites in the central nervous system of the developing mouse: an autoradiographic study

[3H]kainate-binding site distribution in mouse brain was studied by in vitro autoradiography during postnatal development. Sites, highly concentrated at early postnatal ages and undetectable at adult ages, were observed in deep cerebellar nuclei, inferior olive, pontine nuclei, inferior colliculus and stratum lacunosum moleculare of the area CA1 in the hippocampus as well as in previously described rat brain areas. It is suggested that the molecules carrying these sites play a role in the development of the regions where they are transiently expressed.

Localization of synapses in rat cortical cultures

Astrocyte-rich and astrocyte-poor cultures derived from embryonic rat cerebral cortex were compared to determine whether differences in the location of neuronal somas, dendrites, axons, synapses or astrocytes, relative to the bulk culture medium, could help to explain the large difference in neuronal susceptibility to glutamate toxicity between the two culture systems. The cultures were processed for electron microscopy, thin sectioned across their depths, and photomontaged. In astrocyte-rich cultures, most of the dendrites, axons and synapses were sequestered from the medium by a nearly continuous layer of astrocyte cell bodies and processes. In contrast, astrocytes did not cover the synapses or neuronal processes in astrocyte-poor cultures. In neither culture system were neuronal cell somas covered by glia. Since neuronal cell somas are freely exposed to the medium in both culture conditions, it seems unlikely that receptors on the somal membrane mediate the greater susceptibility of neurons in astrocyte-poor cultures to glutamate toxicity. The layer of astrocytes in the astrocyte-rich cultures may provide a physical buffer that could hinder diffusion of substances from the medium to the interstitium of the neuropil. This physical buffer combined with avid glutamate uptake mechanisms might allow astrocytes to maintain a sufficiently low concentration of glutamate in the local extracellular space to protect dendrites and synapses in the astrocyte-rich, but not in the astrocyte-poor cultures, from the excitotoxic effects of glutamate. The results of this study demonstrate that local sequestering of neurites and synapses by a physical buffer of astrocytes may help to explain the relative resistance of neurons cultured with astrocytes to glutamate toxicity. A similar physical sequestering by astrocytes, of sensitive regions of neurons in the brain, may help protect neurons from glutamate toxicity in vivo.

A simplified approach to retrograde/anterograde axonal labeling using combined injections of horseradish peroxidase and ibotenic acid

Combined injections of ibotenic acid and horseradish peroxidase (HRP) were made into the region of the mouse ventrobasal thalamus that is related to the large mystacial vibrissae. Examination 4 and 5 days later of the corresponding area of the primary somatosensory cortex (i.e., barrel cortex), in thick and in thin sections, showed it to contain numerous corticothalamic projection cells the somata, dendrites and axons of which were densely labeled by the retrograde transport of HRP. Analysis of serial thin sections showed that thalamocortical axon terminals, which had degenerated in response to the injection of ibotenic acid, formed approximately 20% of the asymmetrical synapses in barrel cortex. The fine structure and distribution in cortex of degenerating thalamocortical axon terminals and of intrinsic HRP-labeled corticothalamic axon terminals were identical to those reported in previous studies in which the injection of HRP into the thalamus was combined with the making of electrolytic lesions. This indicates that injecting ibotenic acid is an effective replacement for electrolytic lesioning of the thalamus. The combined injection of ibotenic acid and HRP represents a new and efficient approach for studying reciprocal projection pathways.

Adenosine and glutamate modulate each other's release from rat hippocampal synaptosomes

In rat hippocampal synaptosomes, adenosine decreased the K+ (15 mM) or the kainate (1 mM) evoked release of glutamate and aspartate. An even more pronounced effect was observed in the presence of the stable adenosine analogue, R-phenylisopropyladenosine. All these effects were reversed by the selective adenosine A1 receptor antagonist 8-cyclopentyltheophylline. In the same synaptosomal preparation, K+ (30 mM) strongly stimulated the release of the preloaded [3H]adenosine in a partially Ca(2+)-dependent and tetrodotoxin (TTX)-sensitive manner. Moreover, in the same experimental conditions, both L-glutamate and L-aspartate enhanced the release of [3H]adenosine derivatives ([3H]ADD). The glutamate-evoked release was dose dependent and appeared to be Ca2+ independent and tetrodotoxin insensitive. This effect was not due to metabolism because even the nonmetabolizable isomers D-glutamate and D-aspartate were able to stimulate [3H]ADD release. In contrast, the specific glutamate agonists N-methyl-D-aspartate, kainate, and quisqualate failed to stimulate [3H]ADD release, suggesting that glutamate and aspartate effects were not mediated by known excitatory amino acid receptors. Moreover, NMDA was also ineffective in the absence of Mg2+ and L-glutamate-evoked release was not inhibited by adding the specific antagonists 2-amino-5-phosphonovaleric acid or 6-7-dinitroquinoxaline-2,3-dione. The stimulatory effect did not appear specific for only excitatory amino acids, as gamma-aminobutyric acid stimulated [3H]ADD release in a dose-related manner. These results suggest that, at least in synaptosomal preparations from rat hippocampus, adenosine and glutamate modulate each other's release. The exact mechanism of such interplay, although still unknown, could help in the understanding of excitatory amino acid neurotoxicity.

NMDA-, kainate- and quisqualate-stimulated release of taurine from electrophysiologically monitored rat hippocampal slices

While excitatory amino acids (EAAs) are known to evoke the release of taurine in the hippocampus, we have found that taurine is localized primarily in dendrites and only to a lesser extent in terminals in this region. To determine whether taurine is released as a neurotransmitter by non-toxic concentrations of EAAs, or exclusively as a neuroprotectant in response to excitotoxicity, we monitored the release of amino acids from hippocampal slices during simultaneous electrophysiological recording in the CA1 region to assess tissue viability. N-methyl-D-aspartate (NMDA) was the most potent of the EAA agonists tested for stimulating release of taurine. Exposure of slices to 120 microM NMDA increased the concentration of taurine in the perfusate to 1325% of its basal value. Kainate (KA) at a concentration of 128 microM increased taurine to 543% of baseline while quisqualate (Quis) at a concentration of 120 microM increase taurine to only 202% of its baseline value. Release of taurine in response to NMDA and KA peaked during the period when the concentration of the agonist was declining in the bath and did not return to its baseline value until 20 min after removal of the agonist. Increases in release of taurine were associated with concentrations of NMDA, KA, and Quis that caused an incomplete recovery of the CA1 field potential. These results suggest that taurine is primarily released by concentrations of glutamate receptor agonists that exhibit evidence of excitotoxicity in the CA1 region.

Enhanced aspartate release from hippocampal slices of epileptic (El) mice

The release of putative neurotransmitters [aspartate, glutamate, and gamma-aminobutyric acid (GABA)] was studied in hippocampal slices from adult normal C57BL/6J (B6) and El (epileptic) mice. The El mice, a genetic model of temporal lobe epilepsy, had an average of 86 seizures. Sets of B6 and El hippocampal slices (400 microns thick) were incubated in a series of normal and high potassium (60 mM) buffers in the presence or absence of calcium. The calcium-dependent and calcium-independent potassium-induced release of amino acids was compared in each mouse strain. Release of endogenous amino acids was measured using liquid chromatography with electrochemical detection and was expressed as picomoles of amino acid released per milliliter of incubation buffer per minute of incubation per slice +/- SEM. No significant differences were found between the El and B6 mice for the calcium-dependent potassium-evoked release of glutamate (18.20 +/- 2.62 and 15.41 +/- 3.56), or GABA (17.28 +/- 2.90 and 12.73 +/- 1.37), respectively. Aspartate release, however, was significantly higher in the El mice (6.62 +/- 0.69) than in the B6 mice (3.31 +/- 0.72). These findings suggest that enhanced aspartate release may be related to seizure expression in El mice.

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.

Blockade of desensitization augments quisqualate excitotoxicity in hippocampal neurons

Glutamate neurotoxicity is thought to play a role in the pathogenesis of several neurodegenerative diseases. While prolonged activation of either NMDA or non-NMDA receptors causes neuronal damage, NMDA receptors appear to mediate most of the glutamate toxicity. The reasons why NMDA toxicity predominates are uncertain but may relate to more effective neuroprotective mechanisms acting at non-NMDA receptors. To determine whether desensitization is one such mechanism, we studied the effects of the lectin wheat germ agglutinin (WGA) on quisqualate currents and toxicity in cultured postnatal rat hippocampal neurons. After WGA treatment, quisqualate currents exhibit little desensitization and a 4- to 8-fold increase in steady-state amplitude. WGA also markedly augments the degree of acute, quisqualate-induced neuronal degeneration. These results suggest that non-NMDA desensitization serves a neuroprotective function in hippocampal neurons.

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

N-methyl-D-aspartate, kainate and quisqualate release endogenous adenosine from rat cortical slices

N-Methyl-D-aspartate, kainate, and quisqualate released endogenous adenosine from superfused slices of rat parietal cortex. N-Methyl-D-aspartate-evoked adenosine release was blocked by D,L-2-amino-5-phosphono-valeric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), indicating that it was receptor-mediated, although it did not show the expected potentiation in the absence of Mg2+. In contrast, N-methyl-D-aspartate-evoked release of [3H]noradrenaline from the same slices was markedly potentiated in Mg2(+)-free medium. Therefore, the lack of Mg2+ modulation of N-methyl-D-aspartate-evoked adenosine release was not due to depolarization-induced alleviation of the Mg2+ block in the slices. Kainate-evoked adenosine release was diminished by the non-specific excitatory amino acid antagonist, gamma-D-glutamyl-glycine, and kainate- and quisqualate-evoked adenosine release was diminished by 6,7-dinitroquinoxaline-2,3-dione, indicating that these agonists release adenosine by acting at non-N-methyl-D-aspartate receptors. Tetrodotoxin decreased N-methyl-D-aspartate- and kainate-evoked adenosine release by 40% and 19% respectively, indicating that release was mediated in part by propagated action potentials in the slices. Total release of adenosine by N-methyl-D-aspartate, kainate or quisqualate was not diminished in the absence of Ca2+. A second exposure to kainate following restoration of Ca2+ to slices previously depolarized in the absence of Ca2+ resulted in an amount of adenosine release equal to an initial release by slices in the presence of Ca2+, a result suggesting the presence of separate Ca2(+)-dependent and Ca2(+)-independent pools of adenosine. The present experiments demonstrate that activation of all three major subtypes of excitatory amino acid receptors in the cortex releases adenosine, possibly from separate Ca2(+)-dependent and -independent pools. Adenosine released from the cortex following excitatory amino acid stimulation may, by acting at inhibitory P1 purinoceptors, diminish excitatory neurotransmission and protect against excitotoxicity.

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.