Postischemic therapy with MK-801 (dizocilpine) in a primate model of transient focal brain ischemia

Inducible glutamate oxaloacetate transaminase as a therapeutic target against ischemic stroke

SIGNIFICANCE

Glutamate serves multi-faceted (patho)physiological functions in the central nervous system as the most abundant excitatory neurotransmitter and under pathological conditions as a potent neurotoxin. Regarding the latter, elevated extracellular glutamate is known to play a central role in ischemic stroke brain injury.

RECENT ADVANCES

Glutamate oxaloacetate transaminase (GOT) has emerged as a new therapeutic target in protecting against ischemic stroke injury. Oxygen-sensitive induction of GOT expression and activity during ischemic stroke lowers glutamate levels at the stroke site while sustaining adenosine triphosphate levels in brain. The energy demands of the brain are among the highest of all organs underscoring the need to quickly mobilize alternative carbon skeletons for metabolism in the absence of glucose during ischemic stroke. Recent work builds on the important observation of Hans Krebs that GOT-mediated metabolism of glutamate generates tri-carboxylic acid (TCA) cycle intermediates in brain tissue. Taken together, outcomes suggest GOT may enable the transformative switch of otherwise excitotoxic glutamate into life-sustaining TCA cycle intermediates during ischemic stroke.

CRITICAL ISSUES

Neuroprotective strategies that focus solely on blocking mechanisms of glutamate-mediated excitotoxicity have historically failed in clinical trials. That GOT can enable glutamate to assume the role of a survival factor represents a paradigm shift necessary to develop the overall significance of glutamate in stroke biology.

FUTURE DIRECTIONS

Ongoing efforts are focused to develop the therapeutic significance of GOT in stroke-affected brain. Small molecules that target induction of GOT expression and activity in the ischemic penumbra are the focus of ongoing studies.

Multimodal assessment of neuroprotection applied to the use of MK-801 in the endothelin-1 model of transient focal brain ischemia

Transient focal ischemia produced by local infusion of endothelin-1 (ET1) in the territory of the middle cerebral artery has been proposed as a potentially useful model for the screening of drugs developed for the treatment of thrombo-embolic stroke. However, most of the data rely exclusively on the assessment of the infarct volume, which is only a partial predictor of the neurological outcome of stroke. Here, we have validated the model using a multimodal approach for the assessment of neuroprotection, which includes (i) determination of the infarct volume by 2,3,5-triphenyltetrazolium chloride staining; (ii) an in-depth behavioral analysis of the neurological deficit; and (iii) an EEG analysis of electrophysiological abnormalities in the peri-infarct somatosensory forelimb cortical area, S1FL. The non-competitive NMDA receptor antagonist, MK-801 (3 mg/kg, injected i.p. 20 min after ET1 infusion in conscious rats) could reduce the infarct volume, reverse the EEG changes occurring at early times post-ET1, and markedly improve the neurological deficit in ischemic animals. The latter effect, however, was visible at day 3 post-ET1, because the drug itself produced substantial behavioral abnormalities at earlier times. We conclude that a multimodal approach can be applied to the ET1 model of focal ischemia, and that MK-801 can be used as a reference compound to which the activity of safer neuroprotective drugs should be compared.

Suppression of post-hypoxic-ischemic EEG transients with dizocilpine is associated with partial striatal protection in the preterm fetal sheep

In vitro studies suggest that glutamate receptor activation is important in the genesis of post-hypoxic preterm brain injury, but there are limited data on post-hypoxic N-methyl-D-aspartate (NMDA) receptor activation. We therefore examined an infusion of the specific, non-competitive NMDA receptor antagonist dizocilpine (2 mg kg(-1) bolus plus 0.07 mg kg(-1) h(-1) i.v.) from 15 min to 4 h after severe hypoxia-ischemia induced by umbilical cord occlusion for 25 min in fetal sheep at 70% of gestation. Dizocilpine suppressed evolving epileptiform transient activity in the first 6 h after reperfusion (2.3 +/- 0.9 versus 9.3 +/- 2.3 maximal counts min(-1), P < 0.05) and mean EEG intensity up to 11 h after occlusion (P < 0.05). Fetal extradural temperature transiently increased during the dizocilpine infusion (40.1 +/- 0.2 versus 39.3 +/- 0.1 degrees C, P < 0.05). After 3 days recovery, treatment was associated with a significant reduction in neuronal loss in the striatum (31 +/- 7 versus 58 +/- 2%, P < 0.05), expression of cleaved caspase-3 (111+/-7 versus 159 +/- 10 counts area(-1), P < 0.05) and numbers of activated microglia (57 +/- 9 versus 92 +/- 16 counts area(-1), P < 0.05); there was no significant effect in other regions or on loss of immature O4-positive oligodendrocytes. In conclusion, abnormal NMDA receptor activation in the first few hours of recovery from hypoxia-ischemia seems to contribute to post-hypoxic striatal damage in the very immature brain.

The neuropharmacological basis for the use of memantine in the treatment of Alzheimer's disease

Memantine has been demonstrated to be safe and effective in the symptomatic treatment of Alzheimer's disease (AD). While the neurobiological basis for the therapeutic activity of memantine is not fully understood, the drug is not a cholinesterase inhibitor and, therefore, acts differently from current AD therapies. Memantine can interact with a variety of ligand-gated ion channels. However, NMDA receptors appear to be a key target of memantine at therapeutic concentrations. Memantine is an uncompetitive (channel blocking) NMDA receptor antagonist. Like other NMDA receptor antagonists, memantine at high concentrations can inhibit mechanisms of synaptic plasticity that are believed to underlie learning and memory. However, at lower, clinically relevant concentrations memantine can under some circumstances promote synaptic plasticity and preserve or enhance memory in animal models of AD. In addition, memantine can protect against the excitotoxic destruction of cholinergic neurons. Blockade of NMDA receptors by memantine could theoretically confer disease-modifying activity in AD by inhibiting the "weak" NMDA receptor-dependent excitotoxicity that has been hypothesized to play a role in the progressive neuronal loss that underlies the evolving dementia. Moreover, recent in vitro studies suggest that memantine abrogates beta-amyloid (Abeta) toxicity and possibly inhibits Abeta production. Considerable attention has focused on the investigation of theories to explain the better tolerability of memantine over other NMDA receptor antagonists, particularly those that act by a similar channel blocking mechanism such as dissociative anesthetic-like agents (phencyclidine, ketamine, MK-801). A variety of channel-level factors could be relevant, including fast channel-blocking kinetics and strong voltage-dependence (allowing rapid relief of block during synaptic activity), as well as reduced trapping (permitting egress from closed channels). These factors may allow memantine to block channel activity induced by low, tonic levels of glutamate--an action that might contribute to symptomatic improvement and could theoretically protect against weak excitotoxicity--while sparing synaptic responses required for normal behavioral functioning, cognition and memory.

Intrastriatal microinjection of sodium nitroprusside induces cell death and reduces binding of dopaminergic receptors

Rat striatum was microinjected with 50 nmol sodium nitroprusside (SNP) and neural cell death as well as the binding of dopaminergic receptors were followed for 24 h after the infusion using TTC staining, cresyl violet staining, and quantitative autoradiography. Striatal cell death was observed 3 h after the infusion of SNP. A widespread area of cell death, including part of the cerebral cortex, was seen at 24 h after the infusion. A decrease of more than 80% in dopamine D1 receptor binding was seen in rat brain slices prepared 2 h after the infusion of SNP, whereas only a slight decrease in dopamine D2 receptor binding and almost no changes in dopamine transporter binding were observed. One day after the infusion, less than 10% of the binding of all three types of dopaminergic receptors remained in a widespread area in the infused side of the striatum and part of the cerebral cortex. Microinjection of either NOC-18 (50 nmol), another type of NO donor, or sodium cyanide (50 nmol) did not caused cell death. In addition, microinjection of FeCl2 (50 nmol) into the striatum caused cell death and reduction in dopamine D() receptor binding. These results suggest that iron-related radical reactions, but not NO itself, might have important roles on SNP-caused cell death. The current receptor binding study also indicated that dopamine D1 receptor binding is the most sensitive indicator for detection of cell death or cell damage induced by radical reactions in the rat striatum.

Quantitative electroencephalography spectral analysis and topographic mapping in a rat model of middle cerebral artery occlusion

Electroencephalography (EEG) has a long history in clinical evaluations of cerebrovascular disease. Distinct EEG abnormalities, such as increased slow delta activity, voltage depression and epileptiform discharge, have been identified in stroke patients. However, preclinical use of EEG analysis of cerebral ischaemia is less documented. We report a new rat model of EEG topographic mapping during permanent and transient middle cerebral artery occlusion. Ten EEG electrodes were implanted on the rat skull, symmetrically covering the cortical regions of two hemispheres. Monopolar EEG recordings were acquired from each animal at multiple time points during the initial 24 h, and again once daily for 7 days. Traditional EEG examinations, quantitative EEG (qEEG) spectral analysis and topographic EEG mapping were employed for comprehensive data analyses. Several distinct spatiotemporal EEG abnormalities were identified in the ischaemic rat brain. In the ipsilateral hemisphere, pronounced increase in delta activity was observed in each recorded area within 24 h of injury. While sharp waves and spike complexes dominated the parietal region, a nearly isoelectric EEG state was seen in the temporal region. After 48 h, spontaneous, albeit incomplete, recovery of EEG activities developed in all rats. Reperfusion appeared to promote delta and alpha recovery more efficiently. The contralateral EEG changes were also recorded in two phases: an acute moderate increase in delta activities with intermittent rhythmic activities, followed by a delayed and significant increase in beta activities across the hemisphere. The similarities of rat qEEG profiles identified in this study to that of stroke patients and the application of topographic mapping broaden our research technology for preclinical experimental studies of brain injury.

Non-pharmacologic (physiologic) neuroprotection in the treatment of brain ischemia

Clinical trials for ischemic stroke have been characterized by a disappointing series of negative results, using a panoply of pharmacologic agents. This paper emphasizes five physiologic measures that can be taken to mitigate ischemic brain damage. These are (1) hypothermia, (2) insulin, (3) arterial hyperoxemia, (4) blood pressure control and (5) magnesium. Hypothermia is protective in both focal and global ischemia, even postischemically protecting against selective neuronal necrosis and infarction. The total equation for protection includes the (i) postischemic delay, (ii) depth, and (iii) duration of hypothermia. Insulin operates by lowering glucose levels to the normal range in focal ischemia. It is possible that very low glucose levels are detrimental in focal ischemia with paradoxical augmentation of the infarct size, and that spreading depression plays a role in this. Controlled arterial hyperoxemia seems effective experimentally in reducing infarct size, operating mechanistically by either a direct effect of oxygen, or vasoconstriction causing shunting of blood into the infarct, or both. Blood pressure is a critical determinant of infarct size, and raising blood pressure improves collateral blood flow and reduces stroke size. To be used clinically, however, hemorrhage must be ruled out. The most dramatic clinical effects of blood pressure are seen in aneurysm patients with vasospasm, where minor increases in blood pressure reverse temporary hemiparesis by reducing ischemia. Magnesium is likely the safest NMDA antagonist, with a long history of safe administration to pregnant women with eclampsia. There is potential interaction with insulin, in that magnesium causes hyperglycemia, which requires insulin to counteract it. Magnesium and insulin together have been shown effective in experimental brain ischemia. In the absence of safe and effective pharmacologic neuroprotection agents, clinical trials should be designed and launched to test these physiologic measures, singly and in combination, to reduce brain damage after ischemia.

A review of the nonmedical use of ketamine: use, users and consequences

Ketamine is a dissociative anesthetic with an accepted place in human medicine. Ketamine also has psychedelic properties, and there has been a recent increase in nonmedical use linked with the growth of the "dance culture." This has attracted little comment in the formal literature but has been the subject of many reports in the media. Myths and misunderstandings are common. The psychedelic properties of ketamine have also led to its use as an adjunct to psychotherapy. This review is intended as a resource for the wide range of persons now requesting accurate information about the nonmedical use of ketamine. It accepts the current necessity of sometimes referring to anecdotal reports while seeking to encourage an increase in formal research. The review includes the history of ketamine, its growing role as a "dance drug," the sought-after effects (including the near-death experience) for which it is taken in a nonmedical context, how these are produced, common mental and physical adverse effects, and the ketamine model of schizophrenia.

Glutamic acid, twenty years later

This review examines progress in understanding the physiologic functions of glutamic acid in the body since the first symposium on glutamic acid physiology and biochemistry was held at the Mario Negri Institute in Milan in 1978. The topics reviewed, although not exhaustive, include the metabolism of glutamic acid, umami taste, the role of glutamic acid as a neurotransmitter, glutamate safety and the development of new drugs resulting from the knowledge of the neurodegeneration induced by high doses of glutamic acid.

Behavioural evaluation of long-term neurotoxic effects of NMDA receptor antagonists

High doses of NMDA antagonists e.g. (+)MK-801 evoke neurodegeneration in retrosplenial cortex in rodents. To assess functional consequences of such treatment, three paradigms of two-way active avoidance learning (with visual or auditory conditioned stimuli) and additionally a spatial learning paradigm - radial maze - were used. Female rats were treated i.p. with 5 mg/kg of (+)MK-801. Recumbence, severe hypothermia and loss of body weight were observed for 3-7 days. Despite that, there were no statistically significant differences in performance of avoidance reaction between saline and (+)MK-801 treated animals trained 10-40 days after the drug administration. However, in the radial maze test (+)MK-801 impaired reference (but not working) memory in the experiment that started 8 days after the treatment. Similar effect was observed on reversal learning. The clinically used NMDA receptor antagonist memantine at the doses of 20 and 40 mg/kg had also no such long term negative effect on working memory during training (even positive effect was seen at 20 mg/kg) but at 40 mg/kg impaired learning on the first day of reversal. This indicates that (+)MK-801 neurotoxicity in the retrosplenial cortex is connected with subtle alterations in the learning performance that may be seen in some tests only. Moreover, memantine doses greatly exceeding therapeutically relevant range produce minimal functional alteration. An additional experiment revealed that the same dose of memantine results in two fold higher serum levels of the antagonist in female than male rats. Hence, considering that profiling studies are done in male rats, a safety factor of over 16 fold can be calculated for memantine.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

The effect of 4 beta-phorbol-12,13-dibutyrate and staurosporine on the extracellular glutamate levels during ischemia in the rat striatum

Hypothermia diminishes the ischemia-induced protein kinase C (PKC) translocation and inhibition, and also reduces transmitter release during ischemia. To study the role of PKC in the mechanism of glutamate release during ischemia, we measured extracellular glutamate levels in the striatum with the microdialysis technique, in the presence and absence in the dialysate of the PKC activator 4 beta-phorbol-12,13-dibutyrate (PDBu) and the protein kinase inhibitor staurosporine. We confirm that hypothermia attenuates the elevation of extracellular levels of glutamate in the striatum during ischemia. In the presence of PDBu, the glutamate levels in the dialysate increased from 0.3 mumol/L to an end ischemic level of 4.8 mumol/L during hypothermic ischemia (33 degrees C). These levels were significantly higher than in hypothermic ischemia (33 degrees C) without added PDBu. Staurosporine significantly mitigated the glutamate levels during normothermic ischemia. Our data suggest that PKC is involved in the temperature-dependent elevations of extracellular glutamate levels in the striatum during ischemia, and we propose that compounds preventing PKC activation may mimic the hypothermic protective action against ischemic brain damage.