Ketamine fails to protect against ischaemic neuronal necrosis in the rat

Stroke and Translational Research - Review of Experimental Models with a Focus on Awake Ischaemic Induction and Anaesthesia

Animal models are an indispensable tool in the study of ischaemic stroke with hundreds of drugs emerging from the preclinical pipeline. However, all of these drugs have failed to translate into successful treatments in the clinic. This has brought into focus the need to enhance preclinical studies to improve translation. The confounding effects of anaesthesia on preclinical stroke modelling has been raised as an important consideration. Various volatile and injectable anaesthetics are used in preclinical models during stroke induction and for outcome measurements such as imaging or electrophysiology. However, anaesthetics modulate several pathways essential in the pathophysiology of stroke in a dose and drug dependent manner. Most notably, anaesthesia has significant modulatory effects on cerebral blood flow, metabolism, spreading depolarizations, and neurovascular coupling. To minimise anaesthetic complications and improve translational relevance, awake stroke induction has been attempted in limited models. This review outlines anaesthetic strategies employed in preclinical ischaemic rodent models and their reported cerebral effects. Stroke related complications are also addressed with a focus on infarct volume, neurological deficits, and thrombolysis efficacy. We also summarise routinely used focal ischaemic stroke rodent models and discuss the attempts to induce some of these models in awake rodents.

Volatile and Intravenous Anesthetics for Brain Protection in Cardiac Surgery: Does the Choice of Anesthesia Matter?

Postoperative neurologic complications have a significant effect on morbidity, mortality, and long-term disability in patients undergoing cardiac surgery. The etiology of brain injury in patients undergoing cardiac surgery is multifactorial and remains unclear. There are several perioperative causative factors for neurologic complications, including microembolization, hypoperfusion, and systemic inflammatory response syndrome. Despite technologic advances and the development of new anesthetic drugs, there remains a high rate of postoperative neurologic complications. Moreover, despite the strong evidence that volatile anesthesia exerts cardioprotective effects in patients undergoing cardiac surgery, the neuroprotective effects of volatile agents remain unclear. Several studies have reported an association of using volatile anesthetics with improvement of biochemical markers of brain injury and postoperative neurocognitive function. However, there is a need for additional studies to define the optimal anesthetic drug for protecting the brain in patients undergoing cardiac surgery.

Hypothermic preconditioning but not ketamine reduces oxygen and glucose deprivation induced neuronal injury correlated with downregulation of COX-2 expression in mouse hippocampal slices

Hypothermic preconditioning is an effective treatment for limiting ischemic injury, but the mechanism is poorly understood. This study was aimed to explore the effect of hypothermic and ketamine preconditioning on oxygen and glucose deprivation (OGD) induced neuronal injury in mouse hippocampal slices, and to investigate its possible mechanism. The population spike (PS) was recorded in the CA1 region of mouse hippocampal slices using extracellular recordings, Na⁺/K⁺ ATPase activity in slices was determined by spectrophotometer and the expression of Cyclooxygenase-2 (COX-2) was measured by Western blot. Ten min of OGD induced a poor recovery of PS in slices after reoxygenation. Hypothermic (33 °C) preconditioning delayed the appearance of transient recovery (TR) of PS and improved the recovery amplitude of PS after reoxygenation. Hypothermic preconditioning also decreased the expression of COX-2 and increased Na⁺/K⁺ ATPase activity in slices. Pretreatment of ketamine, a non-competitive NMDA receptor antagonist at a subanesthetic dose has no effect on OGD induced neuronal injury. Moreover, the protection of hypothermic preconditioning was not added by ketamine. The downregulation of COX-2 expression and the increase of Na⁺/K⁺ ATPase activity may be associated with the effectiveness of hypothermic preconditioning in increasing tolerance to an OGD insult.

Anesthesia in Experimental Stroke Research

Anesthetics have enabled major advances in development of experimental models of human stroke. Yet, their profound pharmacologic effects on neural function can confound the interpretation of experimental stroke research. Anesthetics have species-, drug-, and dose-specific effects on cerebral blood flow and metabolism, neurovascular coupling, autoregulation, ischemic depolarizations, excitotoxicity, inflammation, neural networks, and numerous molecular pathways relevant for stroke outcome. Both preconditioning and postconditioning properties have been described. Anesthetics also modulate systemic arterial blood pressure, lung ventilation, and thermoregulation, all of which may interact with the ischemic insult as well as the therapeutic interventions. These confounds present a dilemma. Here, we provide an overview of the anesthetic mechanisms of action and molecular and physiologic effects on factors relevant to stroke outcomes that can guide the choice and optimization of the anesthetic regimen in experimental stroke.

Ketamine and thiopental sodium: individual and combined neuroprotective effects on cortical cultures exposed to NMDA or nitric oxide

BACKGROUND

An N-methyl-D-aspartate (NMDA) blocker, ketamine, has been shown to be neuroprotective both in vivo and in vitro. However, ketamine is not commonly recommended for use in patients suffering from cerebral ischaemia because of its adverse neurological effects. We hypothesized that combined administration of ketamine and thiopental sodium (TPS) would be highly effective in protecting cerebral cortical neurones from ischaemia, with possibly reduced dosages.

METHODS

We examined the degree of neuroprotection provided by various concentrations of ketamine and TPS, alone and in combination, in cortical cultures exposed to NMDA or a nitric oxide-releasing compound (NOC-5) for 24 h. The survival rate (SR) of E16 Wistar rat cortical neurones was evaluated using photomicrographs before and after exposure to these compounds.

RESULTS

The SRs of cortical neurones exposed to 30 microM NMDA or NOC-5 were 15.0 (3.8)%, 12.8 (3.1)%, respectively. Higher doses (5, 10 and 50 microM) but not lower doses (<1 microM) of ketamine improved SRs [57.9 (2.2)%, 61.1 (5.4)%, 76.7 (3.0)%, respectively] against NMDA but not NOC. Enhanced survival was observed with combined administration of 5 or 10 microM ketamine and 50 microM TPS [SR 71.3 (4.8)%, 74.7 (3.7)%, respectively, P<0.05 if ketamine alone, P<0.01 if TPS alone], against NMDA-induced neurotoxicity in vitro. Only the highest dose of TPS (50 microM) improved survival after NOC exposure. This neuroprotection was not influenced by ketamine.

CONCLUSIONS

These data indicate that a low, clinically relevant dose of ketamine offer significant neuroprotection during prolonged exposure to NMDA but not to NOC. Combinations of reduced doses of ketamine and TPS exhibited enhanced neuroprotection against NMDA-induced neurotoxicity. Hence, combinations of these two common i.v. anaesthetics agents could be developed to protect the brain from ischaemia.

Ion channels involved in stroke

Ion channels are membrane proteins that flicker open and shut to regulate the flow of ions down their electrochemical gradient across the membrane and consequently regulate cellular excitability. Every living cell expresses ion channels, as they are critical life-sustaining proteins. Ion channels are generally either activated by voltage or by ligand interaction. For each group of ion channels the channels' molecular biology and biophysics will be introduced and the pharmacology of that group of channels will be reviewed. The in vitro and in vivo literature will be reviewed and, for ion channel groups in which clinical trials have been conducted, the efficacy and therapeutic potential of the neuroprotective compounds will be reviewed. A large part of this article will deal with glutamate receptors, focusing specifically on N-methyl-D-aspartate (NMDA) receptors. Although the outcome of clinical trials for NMDA receptor antagonists as therapeutics for acute stroke is disappointing, the culmination of these failed trials was preceded by a decade of efforts to develop these agents. Sodium and calcium channel antagonists will be reviewed and the newly emerging efforts to develop therapeutics targeting potassium channels will be discussed. The future development of stroke therapeutics targeting ion channels will be discussed in the context of the failures of the last decade in hopes that this decade will yield successful stroke therapeutics.

Brain protection during neurosurgery

The initial concept of brain protection during neurosurgery is based on research done in the 1970s-1980 which established the concept that by decreasing cerebral metabolic rate the brain could survive longer periods of ischemia. The first section of this chapter reviews some of this initial research that promoted the use of barbiturates for cerebral protection. The second section reviews current anesthetic drugs and their potential for cerebral protection in addition to the benefits of blood pressure, temperature and glucose control. The final section discusses the "new mechanisms of cerebral protection" and the role old and new drugs may play in the future for brain protection during neurosurgery.

Neuroprotection of S(+) ketamine isomer in global forebrain ischemia

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine can block the action of excitotoxic amino acids in the central nervous system. S(+) ketamine has a 2-3 times higher anesthetic potency compared with the ketamine-racemate and also shows a higher neuroprotective efficacy in vitro. To determine the neuroprotective activity of S(+) ketamine compared with its R(-) stereoisomer in vivo, we examined the functional and neurohistological outcome in rats treated 15 min after global forebrain ischemia with S(+) ketamine in different dosages compared with R(-) ketamine. Influence of the treatment on regional cerebral blood flow (rCBF) and cortical oxygen saturation (HbO2) was monitored over 1 h after the ischemia using laser doppler flowmetry and microphotospectrometry respectively. Sixty and ninety mg/kg of S(+) ketamine but not R(-) ketamine significantly reduced neuronal cell loss in the cortex compared with the saline treated group. No significant neuroprotection was observed in the hippocampus. Although no significant change in rCBF was found, S(+) ketamine restored the cortical HbO2 to preischemic values. These results indicate that S(+) ketamine in higher dosages can reduce neuronal damage in the cortex after cerebral ischemia, possibly by improving the ratio of oxygen supply to consumption in the postischemic tissue.

Intravenous anesthetics differentially reduce neurotransmission damage caused by oxygen-glucose deprivation in rat hippocampal slices in correlation with N-methyl-d-aspartate receptor inhibition

OBJECTIVE

To examine the relation between the effect of intravenous anesthetics on ischemic neurotransmission damage and their actions on N-methyl-d-aspartate (NMDA) receptors in an in vitro cerebral ischemic model.

DESIGN

Prospective, randomized study in freshly prepared rat hippocampal slices.

SETTING

University research laboratory.

SUBJECTS

Hippocampal slices were prepared from male Wistar rats (4-5 wks old).

INTERVENTIONS AND MEASUREMENTS

In vitro ischemia was induced by exposing slices to glucose-free Krebs solution gassed with 95% N2 /5% CO2 at 37.1-37.3 degrees C. Ischemic neurotransmission damage was indicated by the amplitudes of population spikes (PS) recorded from the CA1 pyramidal layer after stimulation of the Schaffer collaterals. The effect of anesthetics on NMDA receptors was determined by measuring the NMDA-mediated changes in intracellular calcium in the CA1 pyramidal layer with a calcium indicator, fura-2.

RESULTS

Following 4, 6, and 7.5 mins ischemia in vitro, the recoveries of PS (% control) were 100%, 17.5 +/- 21.8%, and 5.4 +/- 2.1%, respectively. 3-(R)-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 5 microM), an NMDA receptor antagonist, increased the recovery of PS to 88.3 +/- 24.5% after 6 mins ischemia, and to 42.1 +/- 18.7% after 7.5 mins ischemia. Thiopental (400 microM), thiamylal (400 microM), and ketamine (100 microM), but not propofol (100 microM) and etomidate (10 microM), improved the recovery of PS after 6 and 7.5 mins ischemia; the degrees of their protection were comparable to that of 5 microM CPP. The NMDA-mediated increases in intracellular calcium were almost completely inhibited by thiamylal, reduced to half by ketamine and thiopental, augmented by propofol, and not affected by etomidate.

CONCLUSIONS

The results indicate that the efficacy of intravenous anesthetics in attenuating ischemic neuronal damage varies among agents, relating to their effects on NMDA receptors.

Effect of ketamine on hypoxic-ischemic brain damage in newborn rats

The present study tests the hypothesis that ketamine, a dissociative anesthetic known to be a non-competitive antagonist of the NMDA receptor, will attenuate hypoxic-ischemic damage in neonatal rat brain. Studies were performed in 7-day-old rat pups which were divided into four groups. Animals of the first group, neither ligated nor exposed to hypoxia, served as controls. The second group was exposed to hypoxic-ischemic conditions and sacrificed immediately afterwards. Animals of the third and fourth groups were treated either with saline or ketamine (20 mg/kg, i.p.) in four doses following hypoxia. Hypoxic-ischemic injury to the left cerebral hemisphere was induced by ligation of the left common carotid artery followed by 1 h of hypoxia with 8% oxygen. Measurements of high energy phosphates (ATP and phosphocreatine) and amino acids (glutamate and glutamine) and neuropathological evaluation of the hippocampal formation were used to assess the effects of hypoxia-ischemia. The combination of common carotid artery ligation and exposure to an hypoxic environment caused major alterations in the ipsilateral hemisphere. In contrast, minor alterations in amino acid concentrations were observed after the end of hypoxia in the contralateral hemisphere. These alterations were restored during the early recovery period. Post-treatment with ketamine was associated with partial restoration of energy stores and amino acid content of the left cerebral hemisphere. Limited attenuation of the damage to the hippocampal formation as demonstrated by a reduction in the number of damaged neurons was also observed. These findings demonstrate that systemically administered ketamine after hypoxia offers partial protection to the newborn rat brain against hypoxic-ischemic injury.

Ketamine antagonizes hypoxia-induced dopamine release in rat striatum

The purpose of this study is to investigate the hypothesis that ketamine, a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuates hypoxia-induced striatal dopamine release in vivo. High-speed chronoamperometric recording techniques, using Nafion-coated carbon fiber electrodes, were used to evaluate extracellular dopamine (DA) concentration in the striatum. KCl and DA were locally applied directly to the striatum of urethane-anesthetized Sprague-Dawley rats, in order to measure release and clearance, respectively, of DA. These anesthetized animals were paralyzed with D-tubocurarine and connected to a respirator to allow controlled respiration. Systemic concentrations of oxygen and carbon dioxide were altered by changing the partial pressure of O2, CO2, N2 of inspired air and the rate of the respirator. Our data indicate that lowering the respiratory rate from 90 to 20 times/min for 5 min, in room air, caused a decrease in blood O2 while increasing the CO2 concentration. These changes in blood gas concentration were reversible and reproducible. We also found that lowering the respiratory rates potentiated K(+)-induced DA release but not DA clearance in the striatum. In an attempt to induce hypercapnia, the room air was replaced with high CO2-containing air (15% CO2 + 20% O2 + 65% N2), and this change resulted in increased blood CO2 levels without lowering O2 concentration. The hypercapnia did not alter K(+)-induced DA release in the striatum. Next, we attempted to simulate anoxic hypoxia in the absence of hypercapnia. Respiration with pure N2 for 30 s resulted in lowering blood O2 without increasing CO2 levels. Both basal and K(+)-evoked DA releases were increased during N2-induced anoxic hypoxia. These data suggested that transient hypoxia facilitates DA release in the striatum. It has been suggested that NMDA is involved in many hypoxia-mediated responses. We also found that systemic application of ketamine, which itself did not affect blood O2 or CO2 levels, antagonized hypoxia-induced electrochemical responses. These data suggest that the increase in DA release in vivo during short-term hypoxia may probably be mediated through NMDA receptors.

Influence of ketamine on the neuronal death caused by NMDA in the rat hippocampus

The protection provided by ketamine against the neuronal cytotoxicity of NMDA was investigated and compared with that provided by dizocilpine (MK 801). A massive anaesthetic dose of ketamine (180 mg/kg) was required for substantial protection (about 70%) of rat dorsal hippocampal neurons. Protection was markedly decreased if the ketamine was given in three divided doses of 60 mg/kg over a period of 2 hr, rather than as a bolus injection of 180 mg/kg. A lower dose (60 mg/kg i.p.) gave no protection when given 10 min prior to NMDA, but some protection (up to 30%) was found when administration was delayed for 1-2 hr. After 3 hr, ketamine at this dose did not protect. In comparison, the toxicity of NMDA was reduced by about 70% by prior treatment with dizocilpine at 1 mg/kg, and completely eliminated at 10 mg/kg. The lack of protection when ketamine at 60 mg/kg was administered prior to NMDA may be due to a proconvulsant action of ketamine, as diazepam in the presence but not in the absence of ketamine significantly reduced the toxicity of NMDA. However, there was no behavioural or histological evidence of increased seizure activity in the presence of ketamine. Neuroprotectant effects may prevail with massive anaesthetic doses of ketamine or when diffusion has reduced the concentration of NMDA. The heroic doses of ketamine required for protection diminish its attractiveness as a potential anti-ischaemic agent.

[Cerebral protection: contribution of intravenous anesthetic agents]

The administration of an intravenous anaesthetic agent before experimental cerebral ischaemia in animals improves the functional and histological outcome. Cerebral ischaemia may be global or focal, complete or incomplete. Intravenous anaesthetic agents reduce the cerebral metabolic demand for oxygen (CMRO2) and abolish electrophysiological activity. This reflects a discontinuation of the functional neuronal activity with maintenance of its basic metabolic activity. The oxygen spared by the decrease in consumption, while reducing the functional activity, might be used by the neurons to sustain longer periods of ischaemia. This protective effect is also observed after pretreatment with either lidocaine or volatile agents, but their potentially deleterious vasodilating effect must be considered. Ketamine has recently been shown to antagonize NMDA receptors. The protective effect of barbiturates was experimentally demonstrated more than 30 years ago. They are still used as a reference. They reduce CMRO2, optimise the ratio between oxygen consumption and oxygen delivery and thus reduce cerebral blood flow and cerebral blood volume, as a result of the decrease of the metabolic demand. This might explain why a protective effect is seen in case of global or focal hypoxia with increased intracranial pressure, while no protection is documented in case of global cerebral ischaemia, such as after cardiac arrest, where EEG is immediately flat and ICP low. However, at doses required to obtain a protective effect, barbiturates induce deleterious side effects such as severe arterial hypotension, which limits their use. Cerebrovascular and cardiac surgery or surgery of the carotids are characterised by potentially ischaemic episodes which can be predicted.(ABSTRACT TRUNCATED AT 250 WORDS)

[Pharmacologic brain protection: specific agents]

Dysfunctional sodium influx is the first step in the ischaemic cascade. It has been recently demonstrated that reducing ionic flux through voltagegated Na channels shortens the NMDA receptor activity of cultured hippocampal slices in which oxidative phosphorylation and glycolysis have been blocked. The implication of this finding is that blocking initial events in the ischaemic cascade, events which do not directly cause neuronal damage, will reduce the damage done by downstream events. It also seems intuitively reasonable to suppose that truncating initial steps of the ischaemic cascade, as distinct from blocking glutamate receptors and scavening free radicals, will reduce the probability of interfering with endogenous mechanisms of repair. Clinically useful, substantive, prophylactic, pharmacological cerebral protection will come from drugs that work upstream. And for pharmacological protection that can only be initiated subsequent to an ischaemic event, the more we learn about endogenous repair, or genetic pharmacology, the closer we will come to maximizing the benefits and minimizing the costs of downstream intervention.

Hypothermia, metabolic stress, and NMDA-mediated excitotoxicity

Isolated embryonic retinas were metabolically stressed by inhibition of glycolysis either with iodoacetate (IOA) or by glucose withdrawal plus 10 mM 2-deoxy-D-glucose, and the effects of hypothermia were examined. Incubation at 30 versus 37 degrees C during 30 min of hypoglycemia with IOA completely reduced the rapid swelling-related GABA release [6 +/- 2 vs. 68 +/- 10 nmol/100 mg of protein (mean +/- SEM) for 30 and 37 degrees C, respectively]. Histology of the retina immediately following 30 min of metabolic stress at 30 degrees C appeared normal, whereas that at 37 degrees C showed a pattern of acute edema, characteristic of NMDA-mediated acute excitotoxicity. Coincubation with a competitive or noncompetitive NMDA antagonist, respectively, CGS-19755 (10 microM) or MK-801 (1 microM), during 30 min of hypoglycemia at 37 degrees C completely prevented tissue swelling, whereas extracellular GABA content remained at basal levels, indicating that the cytotoxic effects of IOA treatment for 30 min at 37 degrees C were NMDA receptor mediated. Longer periods of hypoglycemia at 37 degrees C produced acute toxicity that was only partially NMDA receptor mediated. Hypothermia delayed the onset of NMDA-mediated toxicity by 30-60 min. At 30 degrees C, the rate of loss of ATP was slowed during the first several minutes of hypoglycemia (82 and 58% of maximal tissue levels at 30 and 37 degrees C, respectively, at 5 min, but by 10 min, ATP levels were comparably reduced. After a transient exposure of retina to 50 microM NMDA in Mg(2+)-free medium, hypothermia significantly attenuated acute GABA release by 30%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral pharmacodynamics of anaesthetic and subanaesthetic doses of ketamine in the normoventilated pig

There are still divergent opinions regarding the pharmacodynamic effects of ketamine on the brain. In this study, the cerebral blood flow (CBF), cerebral metabolic rate for oxygen (CMRO2) and electroencephalographic (EEG) activity were sequentially assessed over 80 min in 17 normoventilated pigs following rapid i.v. infusions of anaesthetic (10.0 mg.kg-1; n = 7) or subanaesthetic (2.0 mg.kg-1; n = 7) doses of ketamine or of its major metabolite norketamine (10.0 mg.kg-1; n = 3). The animals were continuously anaesthetized with fentanyl, nitrous oxide and pancuronium. CBF was determined by the intra-arterial 133Xe technique. Ketamine (10.0 mg.kg-1) induced an instant, gradually reverting decrease in CBF, amounting to -26% (P < 0.01) at 1 min and -13% (P < 0.05) at 10 min, a delayed increase in CMRO2 by 42% (P < 0.01) at 10 min and a sustained rise in low- and intermediate-frequency EEG voltage by 87% at 1 and 97% at 10 min (P < 0.0001). It is concluded that metabolically formed norketamine does not contribute to these effects. Considering the dissociation of CBF from CMRO2 found 10-20 min after ketamine (10.0 mg.kg-1) administration, it is suggested that ketamine should be used with caution for anaesthesia in patients with suspected cerebral ischaemia in order not to increase the vulnerability of brain tissue to hypoxic injury. Ketamine (2.0 mg.kg-1) had no significant effects on CBF, CMRO2 or EEG. It therefore seems that up to one fifth of the minimal anaesthetic i.v. dose can be used safely for analgesia, provided that normocapnaemia is preserved.

Effects of hyperthermia on the effectiveness of MK-801 treatment in the gerbil hippocampus following transient forebrain ischemia

The effects of dizocilipine maleate (MK-801), a noncompetitive N-methyl-D-aspartate (NMDA) receptor/channel antagonist, were tested on the dysfunction of neurotransmitter and signal transduction systems and morphological damage 7 days after transient forebrain ischemia in gerbils. Neurotransmitter system (adenosine A1, muscarinic cholinergic receptor) and signal transduction system (inositol 1,4,5-trisphosphate receptor: IP3, protein kinase C: PKC, L-type calcium channels) binding sites were mapped by in vitro quantitative receptor autoradiography. All ligands used in the present study decreased significantly in the CA1 subfield 7 days after ischemia. In normothermic animals, pretreatment with MK-801 failed to protect against decreased receptor binding in the hippocampus 7 days after ischemia. Moreover, in a morphological study, pre- and posttreatment of MK-801 failed to show protective effects against ischemic neuronal damage. On the other hand, pretreatment of MK-801, without maintaining body temperature, prevented the neuronal death of CA1 subfield 7 days after ischemia. These results weaken the hypothesis that NMDA receptor/channel may play a pivotal role in the pathogenesis of neuronal damage after transient forebrain ischemia.

Inhibition of glutamate release in rat hippocampus by kynurenic acid does not protect CA1 cells from forebrain ischemia

We assessed the effect of a broad spectrum glutamatergic receptor antagonist, kynurenic acid (500 mg/kg) on ischemia-induced hippocampal glutamate release and neuronal damage. Kynurenic acid significantly decreased glutamate release during ischemia but had no effect on the hippocampal lesion. Some protection was observed in the cortex and in the striatum. These data suggested that the extracellular accumulation of glutamate during forebrain ischemia does not play a major role in the hippocampus.

Efflux of glutamate produced by short ischemia of varied severity in rat striatum

Evidence has accumulated suggesting that ischemia-induced neuronal damage may be linked to an extracellular overflow of glutamate. The purpose of this study was to provide new information about the time course of the increase in extracellular glutamate concentration associated with moderate and severe ischemia, and its relationship with electrical changes including anoxic depolarization.

Changes in the extracellular concentration of glutamate were continuously monitored in the rat striatum by microdialysis. Ischemia was induced by four-vessel occlusion for 3 or 5 minutes, and in some cases its severity was increased with a neck tourniquet. The severity of ischemia was assessed by electroencephalogram and direct current potential recording to detect anoxic depolarization.

In all experiments, the extracellular glutamate concentration began to increase shortly after the onset of ischemia and steadily rose throughout the ischemic period. Increases up to 35.0 mumol/l (2-3 mumol/l baseline; p less than 0.005) were observed when ischemia provoked the rapid occurrence of a large and sustained anoxic depolarization. Relatively smaller but still significant increases (6.9 mumol/l; p less than 0.005) were observed in penumbral conditions (electroencephalogram loss without anoxic depolarization). Glutamate began to be cleared immediately after reperfusion and 90% of released glutamate was cleared within 5 minutes, even when the preceding ischemia had been severe.

We propose that the extracellular glutamate concentration may not reach critical levels during short episodes of penumbral ischemia, but this might happen with a longer ischemic period.

Effects of ketamine on outcome from temporary middle cerebral artery occlusion in the spontaneously hypertensive rat

This experiment evaluated the potential for ketamine HCl, a non-competitive glutamate antagonist, to minimize injury resulting from temporary focal cerebral ischemia. Male spontaneously hypertensive rats were randomly assigned to receive either ketamine (n = 13) or halothane anesthesia (n = 12) during 2 h of reversible middle cerebral artery occlusion (MCAO). Ketamine was administered as a 50 mg/kg i.v. loading dose followed by a continuous 1.25 mg/kg/min i.v. infusion beginning 25 min prior to ischemia and continued until 30 min after reperfusion. An additional group of rats (ketamine-shams, n = 8) underwent craniectomy and ketamine administration (as above) but the middle cerebral artery was not ligated. Physiologic values were similar between groups with the exception of plasma glucose which was elevated in the halothane-MCAO group. After 4 days recovery, rats in all groups were neurologically evaluated. There were no differences between the two groups undergoing MCAO for neurologic grading or open field behavior, although both groups performed worse than did ketamine-shams (P less than 0.05). In contrast, motor performance revealed more severe deficits in the ketamine-MCAO rats vs either the halothane-MCAO or ketamine-sham groups (P less than 0.05). Cerebral infarct volume was then planimetrically measured after triphenyl tetrazolium chloride (TTC) staining of fresh brain sections. Mean +/- S.D. infarct volume was not different between the halothane-MCAO (134 +/- 51 mm3) and ketamine-MCAO (131 +/- 64 mm3) groups. Seven of 8 sham rats were free of TTC demarcated injury and in the remaining rat injury was minimal.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroblastoma cells for testing neuroprotective drug effects

An attempt was made to use neuroblastoma cells for testing neuroprotective drug effects. To achieve cellular damage, cytotoxic hypoxia was induced in neuroblastoma cells after 10 days in culture by addition of sodium cyanide (1 mmol/L) to the culture medium and was terminated after 6 hr by replacing the cyanide-containing fresh nutrient medium. During this hypoxic period cells were additionally deprived of glucose. They were allowed to recover for another 7 days. Drugs were available to the cells from 30 min prior to hypoxia until 24 hr after hypoxia. Cell concentration of high-energy phosphates and culture protein content were determined as representatives for the posthypoxic development of cell damage, cell activity and viability. While barbiturates and phenytoin revealed neurotoxic effects when applied in doses higher than 300 mumol/L, chlorpromazine, dizocilpine, ketamine, ketazocine, naftidrofuryl, and flunarizine protected neuroblastoma cells against hypoxic damage. These results were comparable to those obtained from primary cultures of neurons under similar experimental conditions. In addition, they were in keeping with neuroprotective drug effects obtained from in vivo experiments. It is suggested that neuroblastoma cells are suitable for testing neuroprotective drug effects.

The neuroprotective action of dizocilpine (MK-801) in the rat middle cerebral artery occlusion model of focal ischaemia

1. An acute model of focal ischaemia, which involves permanent occlusion of the middle cerebral artery of the rat with 4 h survival, was used to find the minimum effective plasma concentration of dizocilpine (MK-801) and to determine its dose-effect relationship. 2. MK-801 was administered at the time of occlusion and was given as an i.v. bolus followed by an infusion for 4 h to maintain a steady state plasma concentration of the drug throughout the study. MK-801 was given at 3 dose levels; 0.04 mg kg-1 i.v. bolus + 0.6 micrograms kg-1 min-1 infusion; 0.12 mg kg-1 i.v. bolus + 1.8 micrograms kg-1 min-1 infusion; 0.4 mg kg-1 i.v. bolus + 6 micrograms kg-1 min-1 infusion, which gave mean plasma levels over the 4 h of 8.0 ng ml-1, 18.9 ng ml-1 and 113.2 ng ml-1 respectively. 3. MK-801 at 8.0 ng ml-1 gave 10% reduction in the volume of ischaemic brain damage in the cerebral cortex which just reached significance. The middle dose of MK-801 (18.9 ng ml-1) gave a highly significant reduction in the volume of ischaemic brain damage in the cerebral cortex and hemisphere, volumes of ischaemic tissue being reduced by 60% and 50% compared to saline-treated animals, respectively. The highest plasma concentration of MK-801 (113.2 ng ml-1) resulted in a 35% reduction in the volume of hemispheric damage and a 40% reduction in the volume of cortical damage, which were significant.4. The reduction in the amount of protection afforded by the highest dose of MK-801 may be due to the hypotensive effect of this dose. There was no protection against the volume of damage in the caudate nucleus for any of the doses of MK-801 tested.5. Therefore the minimum effective plasma concentration of MK-801 was 8.0 ngml1, although the greatest protection was seen with a plasma level of 18.9 ng ml- 1. This correlates well with the concentration of MK-801 required to block N-methyl-D-aspartate (NMDA) receptors and prevent NMDA receptor mediated neurotoxicity in vitro.

Forebrain ischemia induces selective behavioral impairments associated with hippocampal injury in rats

Two groups of rats were tested on a variety of motor and cognitive tasks after either 10 minutes of two-vessel occlusion forebrain ischemia (n = 8) or sham operative procedures (n = 6). Histological injury was absent in the sham-operated group. In the ischemic group, hippocampal injury was restricted to field CA1, while damage in the neocortex and caudoputamen was sparse. Motor tests performed on postoperative days 18 and 28 revealed no significant differences between the ischemic and sham-operated groups. Retention performance of a radial maze discrimination task was impaired, with a significant but transient increase in both working and reference memory errors. Passive avoidance acquisition and retention were not significantly affected, although conclusions concerning the utility of this task must be reserved because of variability in the behavior of the sham-operated rats. Morris maze spatial navigation (place learning) and open-field activity were insensitive to treatment group. These functional results are consistent with the observed histological injury and what is known about hippocampal injury and behavior, and they provide further guidance for the development of neurological assays appropriate for discriminating outcome from forebrain ischemia in rats.

Age-related differences in recovery of blood flow and metabolism after cerebral ischemia in swine

We tested two hypotheses: 1) that cerebral blood flow, oxygen consumption, and evoked potentials recover to preischemic values at 120 minutes of reperfusion more completely in 1-2-week-old piglets than in 6-10-month-old pigs after complete ischemia; and 2) that recovery of cerebral blood flow, oxygen consumption, and electrical function in piglets and pigs at 120 minutes of reperfusion is better after incomplete than after complete ischemia. During 30 minutes of ischemia produced by intracranial pressure elevation, cerebral blood flow determined by the microspheres technique was decreased to 0-1 ml/min/100 g with complete ischemia, to 1-10 ml/min/100 g with severe incomplete ischemia, or to 10-20 ml/min/100 g with moderate incomplete ischemia. During reperfusion after complete ischemia, both piglets and pigs demonstrated hyperemia but delayed hypoperfusion occurred in more brain regions in pigs, oxygen consumption returned to preischemic values in piglets but not in pigs (70 +/- 10% of preischemic values), and evoked potentials recovered better in piglets than in pigs (24 +/- 4% and 9 +/- 4% of preischemic values, respectively). Both piglets and pigs had fewer brain areas with hyperemia and hypoperfusion and improved oxygen consumption and electrical function during recovery from incomplete than from complete ischemia. We speculate that piglets tolerate complete ischemia better than pigs because of decreased reperfusion injury and that both groups recover better from incomplete than complete ischemia because of improved substrate supply during ischemia.

Ketamine promotes hippocampal CA1 pyramidal neuron loss after a short-duration ischemic insult in rats

The neuroprotective activity of the N-methyl-D-aspartate (NMDA) antagonist ketamine was examined in two groups of rats subjected to forebrain ischemia of differing durations. Ketamine provided a limited degree of protection against hippocampal CA1 pyramidal cell loss following a 10 min ischemic insult. In contrast after 6 min ischemia, which in non-treated control animals resulted in less CA1 neuron loss than was seen after 10 min ischemia, ketamine at the same dose worsened outcome. The data indicate that the possession by a drug of NMDA antagonist activity does not alone determine its neuroprotective activity in vivo.

Pre- and post-treatment with MK-801 but not pretreatment alone reduces neocortical damage after focal cerebral ischemia in the rat

The effect of treatment with the potent, non-competitive NMDA receptor-channel antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d] cyclohepten-5,10-imine maleate (MK-801) on ischemia-induced brain damage was studied in a well-characterized model of focal neocortical infarction in spontaneously hypertensive rats. Anesthesia exposure was minimized to the surgical procedure and the infarcts were allowed to mature over a 24-h period. Pretreatment with 5 mg/kg i.p. MK-801 (n = 11 control, n = 12 treated animals) 30 min before induction of focal cerebral ischemia had no statistically significant influence on infarct volumes. However, pre- and post-treatment with MK-801 5 mg/kg i.p. 30 min before induction of ischemia and 2.5 mg/kg each at 8 and 16 h after onset of ischemia, reduced infarct volumes in two separate studies by 29% (investigator J.T., n = 5 control and n = 7 treated animals) and 20% (investigator U.D., n = 8 control and n = 8 treated animals), respectively. The combined reduction in infarct volume in MK-801-treated animals for both investigators was 23% (P = 0.016, ANOVA). The findings indicate a smaller neuroprotective effect of MK-801 in spontaneously hypertensive rats subjected to focal ischemia than in previous reports using normotensive animals.

The NMDA antagonist MK-801 improved metabolic recovery after 10 minutes global cerebral ischaemia in rats measured with 31 phosphorous magnetic resonance spectroscopy

The blockade of postsynaptic receptors for excitatory amino acids is a promising new field for the possible treatment of cerebral ischaemia. The most important receptor seems to be the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptors and MK-801 is a potent non-competitive antagonist to the NMDA receptor. 31P NMR Spectroscopy was used to measure the recovery of intracellular pH and the high energy phosphates Phosphocreatine (PCr) and ATP after ten minutes of temporary global cerebral ischaemia in the rat. Cerebral ischaemia was obtained by combining bilateral carotid ligation and systemic hypotension (2 vessel occlusion model). Two intervention groups with intravenous injection of MK-801 in doses of 0.25 mg/kg and 0.5 mg/kg 15 minutes before onset of ischaemia were compared to a control group. Both intervention groups showed a more rapid recovery of PCr and ATP than the control group, but there were no significant differences for intracellular pH.