Enhancement of NMDA receptor-mediated neurotoxicity in the hippocampal slice by depolarization and ischemia

Neuroprotection against supra-lethal 'stroke in a dish' insults by an anti-excitotoxic receptor antagonist cocktail

The goal of this study was to identify cocktails of drugs able to protect cultured rodent cortical neurons against increasing durations of oxygen-glucose deprivation (OGD). As expected, a cocktail composed of an NMDA and AMPA receptor antagonists and a voltage gated Ca²⁺ channel blocker (MK-801, CNQX and nifedipine, respectively) provided complete neuroprotection against mild OGD. Increasingly longer durations of OGD necessitated increasing the doses of MK-801 and CNQX, until these cocktails ultimately failed to provide neuroprotection against supra-lethal OGD, even at maximal drug concentrations. Surprisingly, supplementation of any of these cocktails with blockers of TRPM7 channels for increasing OGD durations was not neuroprotective, unless these blockers possessed the ability to inhibit NMDA receptors. Supplementation of the maximally effective cocktail with other NMDA receptor antagonists augmented neuroprotection, suggesting insufficient NMDAR blockade by MK-801. Substitution of MK-801 in cocktails with high concentrations of a glycine site NMDA receptor antagonist caused the greatest improvements in neuroprotection, with the more potent SM-31900 superior to L689,560. Substitution of CQNX in cocktails with AMPA receptor antagonists at high concentrations also improved neuroprotection, particularly with the combination of SYM 2206 and NBQX. The most neuroprotective cocktail was thus composed of SM-31900, SYM2206, NBQX, nifedipine and the antioxidant trolox. Thus, the cumulative properties of antagonist potency and concentration in a cocktail dictate neuroprotective efficacy. The central target of supra-lethal OGD is excitotoxicity, which must be blocked to the greatest extent possible to minimize ion influx.

G protein-coupled receptor 37-like 1 modulates astrocyte glutamate transporters and neuronal NMDA receptors and is neuroprotective in ischemia

We show that the G protein‐coupled receptor GPR37‐like 1 (GPR37L1) is expressed in most astrocytes and some oligodendrocyte precursors in the mouse central nervous system. This contrasts with GPR37, which is mainly in mature oligodendrocytes. Comparison of wild type and Gpr37l1^–/– mice showed that loss of GPR37L1 did not affect the input resistance or resting potential of astrocytes or neurons in the hippocampus. However, GPR37L1‐mediated signalling inhibited astrocyte glutamate transporters and – surprisingly, given its lack of expression in neurons – reduced neuronal NMDA receptor (NMDAR) activity during prolonged activation of the receptors as occurs in ischemia. This effect on NMDAR signalling was not mediated by a change in the release of D‐serine or TNF‐α, two astrocyte‐derived agents known to modulate NMDAR function. After middle cerebral artery occlusion, Gpr37l1 expression was increased around the lesion. Neuronal death was increased by ∼40% in Gpr37l1^–/– brain compared to wild type in an in vitro model of ischemia. Thus, GPR37L1 protects neurons during ischemia, presumably by modulating extracellular glutamate concentration and NMDAR activation.

AMPA receptor regulation mechanisms: future target for safer neuroprotective drugs

The post-synaptic AMPA receptors play an important role in mediating fast excitatory transmission in the mammalian brain. Over-activated AMPA receptors induce excitotoxicity, implicated in a number of Chronic neurodegenerative disorders such as Parkinson's disease, Huntington's disease, and AIDS encephalitis. AMPA receptor antagonists offer protection against neurodegeneration in the experimental models even if they are given 24 h after the injury. Because AMPA receptors seem to be involved in the neurodegenerative diseases, modulating the activity of the AMPA receptors could be an attractive approach to reduce or prevent excitotoxicity. Studies conducted recently have exhibited a number of new mechanisms for AMPA receptor regulation. Modulations of these were found to have protective implications. AMPA receptor depolarization and desensitization are protective to the neurons. Receptor desensitization depends on the receptor subunit composition. The R/G editing site and the flip/flop cassettes in AMPA receptor subunits contribute to a great extent in receptor desensitization and recovery rates. Molecules that could quicken receptor desensitization or delay recovery could be of use. AMPA receptors limit neuronal entry of Ca2+ ions by regulating Ca2+-permeability. Ca2+-permeable receptor channels are made up of GluR1, GluR3, or GluR4 subunits, whereas presence of the GluR2 subunit restricts Ca2+ entry and renders the receptor Ca2+-impermeable. GluR2 levels, however, experience a fall after neuronal insult rendering the AMPA receptors Ca2+-permeable, thus factors that could interfere with this event might prove to be very beneficial against excitotoxicity. AMPA receptor clusters are stabilized by PSD-95, which requires palmitoylation at two sites. Targeting palmitoylation of the PSD-95 can also be a useful approach to disperse AMPA clusters at the synapse. In the perisynaptic region, mGluRs are present a little away from the synapse and are among the glutamate transporters, which require high-frequency firing for activation. On activation they might enhance the activity of NMDA receptors at the synapse to increase the levels of AMPA receptors. AMPA receptors surfaced at this juncture can contribute to heavy Ca2+ influx. Thus, blocking this pathway could be of considerable importance in preventing the excitotoxicity. A number of proteins such as the GRIP, PICK, and NSF also modulate the functions of AMPA receptors. Polyamines also block Ca2+ permeable AMPA receptors and thus are protective. NO and cGMP also play an important role in negatively regulating AMPA receptors and thus could offer protection. Modulation of AMPA receptor by different mechanisms has been discussed in the present review to implicate importance of these targets/pathways for safer and future neuroprotective drugs.

Different mechanisms of hypoxic injury on white matter and gray matter as revealed by dynamic changes in glucose metabolism in rats

Fresh rat brain slices were incubated with [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) in oxygenated Krebs-Ringer solution at 36 degrees C, and the fractional rate constant (=k3*) of [18F]FDG proportional to the cerebral glucose metabolic rate in white matter and gray matter was investigated with positron autoradiography. In both white matter and gray matter, the k3* value with > or = 20 min hypoxia was markedly lower than the unloaded control value, indicating irreversible hypoxic injury. Next, the neuroprotective effect against hypoxia induced by the addition of an N-methyl-D-aspartate receptor antagonist or a free radical scavenger was assessed by determining whether a decrease in the k3* value after hypoxia loading was prevented. In gray matter, both agents exhibited a neuroprotective effect against 20 min hypoxia. In white matter, however, only the free radical scavenger was effective. These results suggest a similarity in the degree of vulnerability to hypoxia between white matter and gray matter as well as a difference in the developmental mechanism of hypoxic injury, i.e. the involvement of both glutamate and free radicals in gray matter, and the more selective involvement of free radicals in white matter.

Brain edema induced by in vitro ischemia: causal factors and neuroprotection

Decreased cerebral blood flow, hence decreased oxygen and glucose, leads to ischemic brain injury via complex pathophysiological events, including excitotoxicity, mitochondrial dysfunction, increased intracellular Ca2+, and reactive oxygen species (ROS) generation. Each of these could also contribute to cerebral edema, which is the primary cause of patient mortality after stroke. In vitro brain slices are widely used to study ischemia. Here we introduce a slice model to investigate ischemia-induced edema. Significant water gain was induced in coronal slices of rat brain by 5 min of oxygen and glucose deprivation (OGD) at 35 degrees C, with progressive edema formation after return to normoxic, normoglycemic medium. Edema increased with increasing injury severity, determined by OGD duration (5-30 min). Underlying factors were assessed using glutamate-receptor antagonists (AP5/CNQX), blockade of mitochondrial permeability transition [cyclosporin A (CsA) versus FK506], inhibition of Na+/Ca2+ exchange (KB-R7943), and ROS scavengers (ascorbate, Trolox, dimethylthiourea, Tempol). All agents except KB-R7943 and FK506 significantly attenuated edema when applied after OGD; KB-R7943 was effective when applied before OGD. Significantly, complete prevention of ischemia-induced edema was achieved with a cocktail of AP5/CNQX, CsA and Tempo applied after OGD, which demonstrates the involvement of multiple, additive mechanisms. The efficacy of this cocktail further shows the potential value of combination therapies for the treatment of cerebral ischemia.

Inflammatory neurodegeneration mediated by nitric oxide from activated glia-inhibiting neuronal respiration, causing glutamate release and excitotoxicity

Glia undergo inflammatory activation in most CNS pathologies and are capable of killing cocultured neurons. We investigated the mechanisms of this inflammatory neurodegeneration using a mixed culture of neurons, microglia, and astrocytes, either when the astrocytes were activated directly with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) or LPS/IFN-gamma-activated microglia were added to mixed neuronal cultures. In either case, activated glia caused 75-100% necrotic cell death within 48 hr, which was completely prevented by inhibitors of inducible nitric oxide synthase (iNOS) (aminoguanidine or 1400W). Activated astrocytes or microglia produced nitric oxide (NO) (steady-state level approximately 0.5 microm), which immediately inhibited the cellular respiration of cocultured neurons, as did authentic NO. NO donors also decreased ATP levels and stimulated lactate production by neurons, consistent with NO-induced respiratory inhibition. NO donors or a specific respiratory inhibitor caused rapid (<1 min) release of glutamate from neuronal and neuronal-astrocytic cultures and subsequent neuronal death that was blocked by an antagonist of NMDA receptor (MK-801). MK-801 also blocked neuronal death induced by activated glia. High oxygen also prevented NO-induced neuronal death, consistent with death being induced by NO inhibition of cytochrome c oxidation in competition with oxygen. Thus activated glia kill neurons via NO from iNOS, which inhibits neuronal respiration resulting in glutamate release and subsequent excitotoxicity. This may contribute to neuronal cell death in inflammatory, infectious, ischemic, and neurodegenerative diseases.

Long-term cellular dysfunction after focal cerebral ischemia: in vitro analyses

The long-term (< or = six months) functional consequences of permanent middle cerebral artery occlusion were studied with in vitro extra- and intracellular recording techniques in adult mouse neocortical slices. After survival times of one to three days, 28 days and six months, intracellular recordings from layers II/III pyramidal cells in the vicinity of the infarct did not reveal any statistically significant changes in the intrinsic membrane properties when compared to age-matched control animals. However, a pronounced hyperexcitability could be observed upon orthodromic synaptic stimulation in neocortical slices obtained from mice 28 days after induction of ischemia. Low-intensity electrical stimulation of the afferents elicited particularly in this group epileptiform extracellular field potential responses and intracellular excitatory postsynaptic potentials, that were longer in duration as compared to the controls. When the N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potential was pharmacologically isolated in a bathing solution containing 0.1 mM Mg2+ and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione, the synaptic responses were longer and larger in the ischemic cortex as compared to the controls. Higher stimulus intensities evoked in normal medium a biphasic inhibitory postsynaptic potential, that contained in the 28 days post-ischemia group a prominent amino-phosphonovaleric acid-sensitive component, indicating a strong concurrent activation of a N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potential. This pronounced co-activation could only be observed in the 28 days ischemic group, and neither after one to three days or six months post-ischemia nor in the controls. The quantitative analysis of the efficiency of stimulus- evoked inhibitory postsynaptic potentials recorded in amino-phosphono-valeric acid revealed a reduction of GABA-mediated inhibition in ischemic cortex. Although this reduction in intracortical inhibition may already contribute to an augmentation of N-methyl-D-aspartate receptor-mediated excitation, our results do also indicate that the function of N-methyl-D-aspartate receptors is transiently enhanced in the ischemic cortex. This transient hyperexcitability does not only cause cellular dysfunction in the vicinity of the infarct, but may also contribute to neuronal damage due to excitotoxicity.

Regional differences in expression of transcripts for Na+/Ca2+ exchanger isoforms in rat brain

The Na+/Ca2+ exchanger has a primary role in maintaining intraneuronal Ca2+ homeostasis. There are three distinct Na+/Ca2+ exchanger isoforms cloned from rat brain, NCX1, NCX2 and NCX3, which are the products of three different genes. In the present study, isoform expression in different regions of rat brain was determined by using reverse transcription PCR (RT-PCR) and Northern analysis. RT-PCR detected all three Na+/Ca2+ exchanger isoforms in each region studied (brainstem/spinal cord, cerebellum, cerebral cortex, striatum/septum and hippocampus). Northern analysis was performed to determine the steady-state mRNA levels of each isoform. NCX1 had two transcripts, 14 and 7 kb, and the 7-kb transcript was predominant in brainstem/spinal cord, cerebellum and hippocampus. NCX2 expression (4.8-kb transcript) was an order of magnitude higher than NCX1 or NCX3 expression in all the five areas except brainstem/spinal cord where the 4.8-kb transcript was nearly absent. The third isoform (NCX3) had two transcripts, one was 6 kb and the other was 4 kb. The 6-kb transcript was predominant in brainstem/spinal cord and cerebellum. The results suggest that Na+/Ca2+ exchanger isoforms are expressed ubiquitously in rat brain but that each isoform shows a unique distribution within the brain. The exchanger probably participates in the regulation of intracellular calcium homeostasis in a wide range of cell types within the brain. Furthermore, individual cells may contain more than one type of exchanger isoform with distinct subcellular distributions.

Alzheimer's amyloid-beta peptide inhibits sodium/calcium exchange measured in rat and human brain plasma membrane vesicles

Na+/Ca2+ exchange activity was measured by monitoring vesicular Ca2+ content after incubation in buffers containing 45Ca2+. When Na+-loaded vesicles were placed into Na+-free buffer, vesicular Ca2+ content increased rapidly and reached a plateau after two to three minutes. Only preaggregated amyloid-beta1-40 (Abeta1-40) and Abeta25-35 reduced vesicular Ca2+ content. Both peptides produced a maximal reduction in Ca2+ content of approximately 50%. The peptides reduced Ca2+ content with similar potency and half maximal effects were seen at less than 10 microM for Abeta25-35. Calcium-loaded vesicles mediate a rapid Ca2+/Ca2+ exchange, which also was inhibited by aggregated Abeta25-35. Aggregated Abeta25-35 did not affect the passive Ca2+ permeability of the vesicles. Aggregated Abeta25-35 reduced Ca2+ content in plasma membrane vesicles isolated from normal and Alzheimer's disease frontal cortex with less potency but the same efficacy as seen in rat brain. Aggregated Abeta25-35 did not produce nonspecific effects on vesicle morphology such as clumping or loss of intact vesicles. When placed in the buffer used to measure Ca2+ content, Congo Red at molar ratios of less than one blocked the inhibitory effect of preaggregated Abeta25-35. When added in equimolar concentrations to freshly dissolved and unaggregated Abeta25-35, Congo Red also was effective at blocking the inhibitory effect on Ca2+ content. In contrast, vitamin E (antioxidant) and N-tert-butyl-alpha-phenylnitrone (spin trapping agent) failed to block the inhibitory action of aggregated Abeta25-35. The exact mechanisms of Abeta-induced neurotoxicity in cell culture has yet to be solved. Accumulation of free radicals play a necessary role, but disruptions of Ca2+ homeostasis are also important. The data presented here are consistent with a proposed mechanism where aggregated Abeta peptides directly interact with hydrophobic surfaces of the exchanger protein and/or lipid bilayer and interfere with plasma membrane Ca2+ transport.

Ischemic and excitotoxic damage to brain slices from normal and microencephalic rats

Brain slices (olfactory cortex, fronto-parietal cortex and hippocampus) taken from normal or microencephalic rats, obtained by gestational administration of the DNA-alkylating agent methylazoxymethanol acetate (MAM), were subjected to in vitro simulated ischemia or exposed to glutamate (5 mM) or kainate (1 mM). All these neurotoxic insults resulted in decreased viability of the slices, as quantitatively assessed by decrease in the rate of protein synthesis. Hippocampal slices subjected to ischemia and olfactory cortex slices exposed to glutamate or kainate were significantly less sensitive to the neurotoxic insult in microencephalic rats than in controls. The increased efflux of neurotransmitter amino acids (glutamate, aspartate and GABA) in the medium from slices subjected to ischemia or exposed to kainate, showed no significant differences among microencephalic and control rats. The present results suggest that the decreased excitotoxic sensitivity of microencephalic rats is, at least in part, related to intrinsic structural and/or functional alterations of some brain regions which undergo decrease in size as a consequence of the gestational treatment.

No effect of glutamate on metabolic disturbances in hippocampal slices of mature fetal guinea pigs after transient in vitro ischemia

The involvement of glutamate in the development of cerebral metabolic disturbances in mature fetuses after transient ischemia was studied using a hippocampal slice model. We investigated the effects of exogenously applied glutamate or glutamate antagonists on the recovery of energy metabolism and protein synthesis rate (PSR) in hippocampal slices of mature guinea pigs after in vitro ischemia. The slices were incubated in a thermostatically controlled flow-through chamber and gassed with carbogen (95% O2/5% CO2). In vitro ischemia was induced by transferring the slices to an aglycemic, artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. In a first set of experiments slices were exposed to 10 mM glutamate during a 20-40 min period of in vitro ischemia. In a second set slices were incubated in aCSF containing MK-801 (100 microM) or kynurenic acid (0.5 mM) 30 min before, during and 2 h after in vitro ischemia. After a 12 h recovery phase, the concentrations of adenylates in the slices were measured by HPLC after extraction with perchloric acid. PSR was calculated from the rate of incorporation of [14C]leucine into tissue proteins. Neither glutamate nor glutamate antagonists had any effect on the postischemic recovery of energy metabolism and PSR when applied during in vitro ischemia. It is therefore concluded that glutamate does not play a major role in the development of metabolic disturbances in hippocampal slices from mature guinea pig fetuses subjected to transient in vitro ischemia.

Mechanisms and effects of intracellular calcium buffering on neuronal survival in organotypic hippocampal cultures exposed to anoxia/aglycemia or to excitotoxins

Neuronal calcium loading attributable to hypoxic/ischemic injury is believed to trigger neurotoxicity. We examined in organotypic hippocampal slice cultures whether artificially and reversibly enhancing the Ca2+ buffering capacity of neurons reduces the neurotoxic sequelae of oxygen-glucose deprivation (OGD), whether such manipulation has neurotoxic potential, and whether the mechanism underlying these effects is pre- or postsynaptic. Neurodegeneration caused over 24 hr by 60 min of OGD was triggered largely by NMDA receptor activation and was attenuated temporarily by pretreating the slices with cell-permeant Ca2+ buffers such as 1, 2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM). This pretreatment produced a transient, reversible increase in intracellular buffer content as demonstrated autoradiographically using slices loaded with 14C-BAPTA-AM and by confocal imaging of slices loaded with the BAPTA-AM analog calcium green-acetoxymethyl ester (AM). The time courses of 14C-BAPTA retention and of neuronal survival after OGD were identical, indicating that increased buffer content is necessary for the observed protective effect. Protection by Ca2+ buffering originated presynaptically because BAPTA-AM was ineffective when endogenous transmitter release was bypassed by directly applying NMDA to the cultures, and because pretreatment with the low Ca2+ affinity buffer 2-aminophenol-N,N,O-triacetic acid acetoxymethyl ester, which attenuates excitatory transmitter release, attenuated neurodegeneration. Thus, in cultured hippocampal slices, enhancing neuronal Ca2+ buffering unequivocally attenuates or delays the onset of anoxic neurodegeneration, likely by attenuating the synaptic release of endogenous excitatory neurotransmitters (excitotoxicity).

Glycine causes increased excitability and neurotoxicity by activation of NMDA receptors in the hippocampus

Glycine is an inhibitory neurotransmitter in the spinal cord and also acts as a permissive cofactor required for activation of the N-methyl-D-aspartate (NMDA) receptor. We have found that high concentrations of glycine (10 mM) cause marked hyperexcitability and neurotoxicity in organotypic hippocampal slice cultures. The hyperexcitability, measured using intracellular recording in CA1 pyramidal neurons was completely blocked by the NMDA receptor antagonist MK-801 (10 microM), but not by the AMPA receptor antagonist DNQX (100 microM). The neurotoxicity caused by glycine occurred in all regions of hippocampal cultures but was most marked in area CA1. There was significant CA1 neuronal damage in cultures exposed to 10 mM glycine for 30 min or longer (P < 0.01) or those exposed to 4 mM glycine for 24 h compared to control cultures (P < 0.01). The NMDA antagonists MK-801 (10 microM) and APV (100 microM) significantly reduced glycine-induced neuronal damage in all hippocampal subfields (P < 0.01). The AMPA antagonists CNQX, DNQX, and NBQX (100 microM) had no effect on glycine-induced neuronal damage. High concentrations of glycine therefore appear to enhance the excitability of hippocampal slices in an NMDA receptor-dependent manner. The neurotoxic actions of glycine are also blocked by NMDA receptor antagonists.

In vivo amino acid release from the striatum of aging rats: adenosine modulation

The release of glutamate, aspartate, GABA, and taurine from the striatum of young (3 months), mature (12 months), and old (22 months), freely moving male rats was investigated by using a microdialysis fiber inserted transversally in the striatum. In old rats basal extracellular glutamate and aspartate levels were decreased vs. young rats (-38 and -49%, respectively). GABA and taurine levels were unmodified by age. In the presence of the adenosine receptor antagonist 8-phenyltheophilline (8-pT) at the concentration of 50 microM, both K(+)-evoked releases of glutamate and aspartate were more than doubled in young, but not in mature and old rats. 8-pT at the concentration of 500 microM significantly decreased glutamate basal levels and K(+)-evoked aspartate release in old rats only. GABA and taurine releases were not affected by 8-pT at either dose. Our findings indicate a modified adenosine modulation on glutamate and aspartate release in aged rats, that could result from a change in the balance between A1 and A2a adenosine receptor density or an alteration of A1 and A2a receptor-effector coupling.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Neurochemical mechanisms in brain injury and treatment: a review

This article reviews cellular energy transformation processes and neurochemical events that take place at the time of brain injury and shortly thereafter emphasizing hypoxia-ischemia, cerebrovascular accident, and traumatic brain injury. New interpretations of established concepts, such as diffuse axonal injury, are discussed; specific events, such as free radical production, excess production of excitatory amino acids, and disruption of calcium homeostasis, are reviewed. Neurochemically-based interventions are also presented: calcium channel blockers, excitatory amino acid antagonists, free radical scavengers, and hypothermia treatment. Concluding remarks focus on the role of clinical neuropsychologists in validation of treatment interventions.

Ontogenetic differences in energy metabolism and inhibition of protein synthesis in hippocampal slices during in vitro ischemia and 24 h of recovery

The present study was designed to clarify whether ontogenetic differences in the vulnerability of the brain towards hypoxic-ischemic insults are only caused by the low cerebral energy demand of immature animals or whether there are additional mechanisms, such as protein synthesis (PSR), that may be involved in this phenomenon. We therefore measured tissue levels of adenylates and PSR in hippocampal slices from immature (E40) and mature (E60) guinea pigs fetuses and from adult guinea pigs during in vitro ischemia and 24 h of recovery using a recently modified method. Hippocampal slices were incubated in a temperature controlled flow-through chamber, gassed with 95% O2/5% CO2. In vitro ischemia was induced by transferring slices to a glucose-free artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. The duration of ischemia ranged from 10 to 40 min. Adenylates were measured by HPLC after extraction with perchloric acid. PSR was evaluated as the incorporation rate of [14C]leucine into proteins. Under control conditions, tissue levels in adenylates did not change, whereas PSR increased slightly in hippocampal slices from mature fetuses and adult animals during a 24-h control incubation period. In slices from immature fetuses ATP levels were only maintained for 2 h. During in vitro ischemia the decline in ATP, total adenylate pool, and adenylate energy charge was much slower in slices from immature fetuses than in slices from mature fetuses or adults. After in vitro ischemia, ATP and the total adenylate pool did not completely recover in mature fetuses and adults, whereas adenylate energy charge almost returned to control values independently of the developmental stage. Two hours after in vitro ischemia PSR was undisturbed in slices from immature fetuses, but severely inhibited in slices from mature fetuses and adults. With ongoing recovery, PSR in mature fetuses returned to control values, while in adults it was still inhibited even 24 h after in vitro ischemia. From these results we conclude that hippocampal slices prepared from mature guinea pig fetuses as well as from adult guinea pigs can be held metabolically stable during long-term incubation using a recently modified technique. However, in slices from immature fetuses a stable energy state could not be maintained for more than 2 h. We further conclude that postischemic disturbances in PSR closely reflect the ontogenetic changes in the vulnerability of the brain to ischemia and that low energy metabolism is certainly not the only cause of the increased vulnerability of the fetal brain to ischemia.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Inhibition of murine retrovirus-induced neurodegeneration in the spinal cord by explant culture

The neurovirulent chimeric mouse ecotropic retrovirus FrCasE causes a rapid neurodegenerative disease of the central nervous system (CNS) characterized by the appearance of spongiform lesions in motor areas 10 days after neonatal inoculation. To study the details of the pathogenic process, we examined the ability of an ex vivo spinal cord model to recapitulate disease. Organotypic spinal cord slice cultures were established from IRW mice 7 days after neonatal inoculation. This corresponds to a time when virus expression in the CNS is first detectable but spongiform changes have yet to evolve. Infectivity associated with these cultures peaked at 7 days in vitro and persisted at this level for 6 weeks. FrCasE infection of the spinal cord slices was primarily found associated with microglial cells. Infection of neurons, astrocytes, oligodendroglia, and endothelial cells was not observed; however, significant astrogliosis was found. Despite the presence of extensive microglial infection in close association with spinal motor neurons in organotypic cultures, no virus-specific spongiform degenerative changes were observed. These results suggest that removal of motor neurons from the developing CNS, despite maintaining the local cytoarchitectural relationships, prevents the virus from eliciting its pathological effects. Possible reasons for the interruption of lesion development are discussed.

NBQX blocks acute and late epileptogenic effects of perinatal hypoxia

Clinically, and in experimental models, perinatal hypoxic encephalopathy is commonly associated with seizures. We previously described a rat model in which hypoxia induces seizures and permanently increases in seizure susceptibility in immature rats [postnatal day (P) 10-12] but not in older rats. In the present study, we compared the effect of pretreatment with the excitatory amino acid antagonists MK-801 and NBQX versus lorazepam in our rat model of perinatal hypoxia. Animals exposed to hypoxia at P10 without treatment have frequent seizures during hypoxia and subsequently exhibit increased seizure susceptibility to flurothyl. Treatment with 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX 20 mg/kg) effectively suppressed hypoxia-induced seizures in immature rats and also protected against permanent changes in flurothyl threshold in adulthood, whereas treatment with MK-801 (1 mg/kg) or lorazepam (LZP 1 mg/kg) did not prevent these hypoxia-related epileptogenic effects. These results suggest that activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA) receptors may partly mediate the age-dependent epileptogenic effect of hypoxia in the perinatal period.

Protection from neuronal damage induced by combined oxygen and glucose deprivation in organotypic hippocampal cultures by glutamate receptor antagonists

Organotypic hippocampal cultures were exposed to defined periods (30 and 60 min) of combined oxygen and glucose deprivation, mimicking transient ischemic conditions. The involvement of different glutamate receptors in individual hippocampal subfields (CA1, CA3 and dentate gyrus) was studied using antagonists of NMDA (dizocilpine) and AMPA/kainate receptors (CNQX and GYKI 52466). Staining with the fluorescent dye propidium iodide (PI) allowed detection of damaged cells. For quantitative determination of neuronal damage, fluorescence intensity was measured after a 22 h recovery period and was related to maximal fluorescence intensity measured after fixation and PI restaining of the cultures at the end of the experiment. Dizocilpine (10 microM), CNQX (100 microM) and GYKI 52466 (100 microM) provided complete protection in CA1, CA3 and dentate gyrus following the moderate ischemic insult, when the antagonists were present permanently. This indicates that none of the ionotropic glutamate receptor subtypes dominated toxicity in the most sensitive subpopulation of neurons. When applied only during the recovery period protection with dizocilpine (10 microM) or CNQX (100 microM) was drastically reduced by about 60% in the most sensitive area (CA1), but only slightly by 15% in CA3. Therefore the onset of irreversible damage seems to occur earlier in CA1 than in CA3. Blockade of AMPA/kainate receptors by GYKI 52466 (100 microM) offered no neuroprotection if the compound was applied only during the recovery period.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative measurement of neuronal degeneration in organotypic hippocampal cultures after combined oxygen/glucose deprivation

Organotypic hippocampal cultures were used to study cell degeneration during the recovery period after defined periods (30 and 60 min) of combined oxygen/glucose deprivation mimicking transient ischemic conditions. Staining with the fluorescent dye propidium iodide allowed detection of damaged cells. Fluorescence intensity was measured by an image analysis system and used to quantify cell damage at different time points during the recovery period (up to 22 h). At 30 min of oxygen/glucose deprivation cells in the CA1 area were relatively more sensitive compared to CA3 and dentate gyrus cells, with respect to the time course of degeneration and the percentage of affected cells. Expanding the oxygen/glucose deprivation period from 30 to 60 min drastically increased the percentage of cells dying in all hippocampal areas. Still, however, cells in CA1 degenerated faster compared to those in the CA3 area and dentate gyrus. A histological analysis of toluidine blue as well as MAP2-immunostained sections revealed that almost all neurons degenerated in all hippocampal areas following the 60-min deprivation period, whereas GFAP-stained astrocytes appeared to be unaffected. Therefore, neuronal degeneration could be quantified by taking the fluorescence intensity values 22 h after 60 min of oxygen/glucose deprivation as 100% neuronal damage. The possibility to quantify neuronal damage in organotypic cultures offers a useful tool for detailed studies on mechanisms of neuronal cell death in a cell culture system which is closer to in situ conditions than monolayer cell cultures.

Simultaneous blockade of non-NMDA ionotropic receptors and NMDA receptor-associated ionophore partially protects hippocampal slices from protein synthesis impairment due to simulated ischemia

A large body of evidence exists to demonstrate that excitatory amino acids (EAA) and their receptors are involved in the pathophysiological mechanisms linking several acute brain insults, such as cerebral ischemia, to neuronal degeneration and death. Accordingly, the use of EAA receptor antagonists can be beneficial in attenuating or preventing the neuronal irreversible damage subsequent to various neuropathological syndromes. We have investigated the effect of 15 min of simulated ischemic conditions, i.e., oxygen/glucose deprivation, on hippocampal slices preparation measuring, as neurotoxicity indexes, both the amino acids efflux in the incubation medium, detected by HPLC, and the inhibition of protein synthesis, evaluated as 3H-Leucine incorporation into proteins. Accumulation of neurotransmitter amino acids was measured in the medium during the "ischemic" period. Glutamate increased 30-fold over the basal level while aspartate was sevenfold and GABA 12-fold higher than in normal conditions. After a reoxygenation period of 30 min, the rate of protein synthesis of hippocampal slices subjected to "ischemia" was reduced to 35-50% of controls. The non-competitive NMDA antagonist MK-801 (100 microM) and the competitive NMDA antagonist CGP 39551 (100-250 microM) as well as the non-NMDA receptor antagonists NBQX (100 microM) and AP3 (300 microM) were unable to counteract the metabolic impairment when they were present alone in the incubation fluid during simulated "ischemia." An incomplete, but highly significant (p < 0.001), protection from protein synthesis impairment was achieved in the presence of an equimolar concentration (100 microM) of MK-801 and NBQX.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the mechanism underlying increased Na+/Ca2+ exchange activity in Alzheimer's disease brain

The Na+/Ca2+ exchanger was characterized in plasma membrane vesicles derived from frozen human postmortem tissues. The frontal cortex, temporal cortex and cerebellum of control and Alzheimer's disease (AD) tissues were compared. Na+/Ca2+ exchange activity was defined as the change in vesicular Ca2+ content seen after Na+ loaded vesicles were diluted into choline buffer. The time course of changes in Ca2+ content after dilution was similar in all three regions of control brain. In AD brain, both frontal and temporal cortex vesicles showed elevated Ca2+ content, most evident as an increased peak Ca2+ content at 2 min. The AD cerebellar cortex time course was similar to control and did not show an elevated peak at 2 min. No differences were seen in the passive permeability to Ca2+ when comparing plasma membrane vesicles prepared from control and AD brain. Vesicles from the frontal and temporal cortex of AD brain showed increases in the Vmax of the initial velocity of Ca2+ uptake when compared to control brain, whereas, the cerebellum did not. There were no significant effects of AD on the Km for Ca2+ activation of the initial velocity. Ca2+ influx measured during the rise in vesicular Ca2+ content was elevated in vesicles from AD temporal cortex when compared to control. Two known inhibitors (exchange inhibitory peptide and dichlorobenzamil) of the cardiac Na+/Ca2+ exchanger inhibited the human brain exchanger equally well in control and AD vesicles. Increased Na+/Ca2+ exchange activity was not due to astrocytic gliosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous adenosine exerts inhibitory effects upon the development of spreading depression and glutamate release induced by microdialysis with high K+ in rat hippocampus

Spreading depression (SD) is known to be involved in the N-methyl-D-aspartate receptor-mediated neuronal damage. In urethane-anesthetized rats, we examined the release of adenosine and glutamate during SD induced by microdialysis of high K+ perfusate through the hippocampal CA1 area. The effects of endogenous adenosine upon SD were studied by applying an adenosine antagonist, theophylline (1 mM) and by a simultaneous application of adenosine uptake blockers, dipyridamole (DPR) (100 microM) and nitrobenzylthioinosine (NBI) (50 microM). The dialysates were sampled every 5 or 10 min and analyzed by HPLC. SD was identified by flattening of background EEg and disappearance of population spikes recorded from the pyramidal cell layer of CA1 area by a glass microelectrode. Adenosine and glutamate release was enhanced significantly in association with the occurrence of SD. Theophylline increased the release of glutamate and the incidence of SD and decreased the latency of the SD occurrence. DPR+NBI decreased the release of glutamate and the occurrence of SD, but increased extracellular adenosine concentration. The effects of DPR+NBI were blocked by application of a selective antagonist of adenosine A1 receptor, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microM). These findings suggest that endogenous adenosine exerts inhibitory influences upon the development of SD and the glutamate release through the A1 receptor in rat hippocampus.

Neuroprotective effects of riluzole on N-methyl-D-aspartate- or veratridine-induced neurotoxicity in rat hippocampal slices

The neuroprotective activity of riluzole has been studied on N-methyl-D-aspartate (NMDA)- or veratridine-induced toxicity in immature rat hippocampal slices. Neurodegeneration was assessed by the measurement of LDH release and histology. Veratridine-induced LDH release can be inhibited by 100 microM riluzole (-90% and by tetrodotoxin (1 microM). Riluzole markedly reduced (-59%) the NMDA-induced LDH release and this protective effect was confirmed by histology. Riluzole inhibited the NMDA-induced LDH release in the presence of tetrodotoxin. Moreover, a pretreatment with pertussis toxin (1 microgram/ml) abolished the effect of riluzole against NMDA-induced neurotoxicity. These results support the view that the neuroprotective properties of riluzole could be exerted via two distinct mechanisms of action.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

Delayed protection by MK-801 and tetrodotoxin in a rat organotypic hippocampal culture model of ischemia

BACKGROUND AND PURPOSE

The hippocampus demonstrates a regional pattern of vulnerability to ischemic injury that depends on its characteristic differentiation and intrinsic connections. We now describe a model of ischemic injury using organotypic hippocampal culture, which preserves the anatomic differentiation of the hippocampus in long-term tissue culture.

METHODS

Ischemic conditions were modeled by metabolic inhibition. Cultures were briefly exposed to potassium cyanide to block oxidative phosphorylation and 2-deoxyglucose to block glycolysis. The fluorescent dye propidium iodide was used to observe membrane damage in living cultures during recovery.

RESULTS

2-Deoxyglucose/potassium cyanide incubation resulted in dose-dependent, regionally selective neuronal injury in CA1 and the dentate hilus, which began slowly after 2 to 6 hours of recovery. Subsequent histological examination of cultures after 1 to 7 days of recovery demonstrated neuronal pyknosis that was correlated with the early, direct observation of membrane damage with propidium. Both propidium staining and histological degeneration were prevented by the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 when administered 30 minutes after the end of the exposure to 2-deoxyglucose and potassium cyanide. Tetrodotoxin, which blocks voltage-dependent sodium channels, had protective effects that were greatest during the period of 2-deoxyglucose and potassium cyanide incubation but also produced protection against the mildest conditions of metabolic inhibition when administered after 30 minutes of recovery.

CONCLUSIONS

This in vitro model reproduced elements of the time course, regional vulnerability, and pharmacologic sensitivities of in vivo ischemic hippocampal injury. Inhibition of metabolism in organotypic culture provides a rapid, easily controlled injury and reproduces the in vitro pattern of hippocampal regional vulnerability to ischemia. It is the first in vitro model of ischemia to exhibit complete protection by delayed administration of an NMDA receptor antagonist during recovery from a brief insult. The protective effects of tetrodotoxin suggest that an early period of sodium entry into cells during and after ATP depletion may be responsible for the more prolonged period of toxic NMDA receptor activation.

Oxidative stress, glutamate, and neurodegenerative disorders

There is an increasing amount of experimental evidence that oxidative stress is a causal, or at least an ancillary, factor in the neuropathology of several adult neurodegenerative disorders, as well as in stroke, trauma, and seizures. At the same time, excessive or persistent activation of glutamate-gated ion channels may cause neuronal degeneration in these same conditions. Glutamate and related acidic amino acids are thought to be the major excitatory neurotransmitters in brain and may be utilized by 40 percent of the synapses. Thus, two broad mechanisms--oxidative stress and excessive activation of glutamate receptors--are converging and represent sequential as well as interacting processes that provide a final common pathway for cell vulnerability in the brain. The broad distribution in brain of the processes regulating oxidative stress and mediating glutamatergic neurotransmission may explain the wide range of disorders in which both have been implicated. Yet differential expression of components of the processes in particular neuronal systems may account for selective neurodegeneration in certain disorders.

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)

Direct observation of the agonist-specific regional vulnerability to glutamate, NMDA, and kainate neurotoxicity in organotypic hippocampal cultures

Excessive activation of excitatory amino acid receptors has been implicated in the neuronal degeneration caused by ischemia, hypoglycemia, and prolonged seizures. We have observed directly the time course and regional vulnerability of hippocampal neurons to glutamate receptor-mediated injury in organotypic hippocampal cultures, a preparation which combines accessibility and long-term survival with preservation of regional differentiation and neuroanatomic organization. Cultures were incubated with the fluorescent dye propidium iodide which selectively enters and stains cells only after membrane damage. After 5 to 10 min of a 30-min exposure to kainate (100 microM), large neurons in the hilus of the dentate were first to become brightly fluorescent. Propidium staining subsequently appeared in the other regions of the hippocampus and increased to a maximum over the first 6 h of recovery. NMDA (10 microM) caused propidium staining that was limited to CA1 and the dentate gyrus of the cultures, sparing CA3, consistent with the regions of highest NMDA receptor density in vivo. Glutamate (1 mM) caused a delayed, progressive pattern of staining that began in CA1 (2 to 4 h after exposure), then extended to include CA3 and finally the dentate gyrus over the next 24 h. Release of LDH activity into the media was slower and less sensitive than propidium staining. Histologic degeneration was limited to neurons 24 h after agonist exposure and was consistent with the propidium staining. NMDA, kainate, and glutamate each produced a unique pattern of neuronal injury. Most notably, glutamate had low potency as a toxin and its pattern of neuronal injury was not reproduced by NMDA.