Repeated Negative DC Deflections in Rat Cortex Following Middle Cerebral Artery Occlusion Are Abolished by MK-801: Effect on Volume of Ischemic Injury

Journal of Cerebral Blood Flow and Metabolism 12:727-733 1992 The International Society of Cerebral Blood Flow and Metabolism Published by Raven Press, Ltd., New York Repeated Negative DC Deflections in Rat Cortex Following Middle Cerebral Artery Occlusion Are Abolished by MK-801: Effect on Volume of Ischemic Injury T. Iijima, G. Mies, and K.-A. Hossmann Department of Experimental Neurology, Max Planck Institute for Neurological Research, Cologne, Germany Summary: Following permanent occlusion of the left middle cerebral artery (MCA) in rats, electrophysiological and hemodynamic characteristics of the periinfarct border zone were investigated in sham-operated (n = 6), untreated (n = 6), and MK-801-treated 0.0 mg/kg; n = 6) animals. For this purpose, direct current potential (DC), EEG, and blood flow (laser-doppler flowmetry) were recorded from the cortex in the periphery of the MCA territory. In sham-operated rats, a single negative cortical DC deflection was observed after electrocoagulation of the cortex, whereas in untreated MCA-occluded animals, three to eight transient DC det1ections were monitored during the initial 3 h of ischemia. The duration of these cortical DC shifts gradually increased from 1.2 ± 0.3 to 3.7 ± 2.7 min (mean ± SD; p < 0.05) during this time. In animals treated intraperitoneally with MK-801 (3.0 mgt kg) immediately after MCA occlusion, the number of cortical DC shifts significantly declined to one to three deflections (p < 0.005). The EEG of the treated animals revealed low-amplitude burst-suppression activity. In the untreated and treated experimental group, the reduction of cortical blood flow amounted to 69 ± 25 and 49 ± 13% of control, respectively. Despite the more pronounced cortical oligemia, MK-801 treatment resulted in a significant decrease of the volume of the ischemically injured tissue from 108 ± 38.5 (untreated group) to 58 ± 11.5 (p < 0.05) mm3. Our results suggest that repetitive cortical DC deflections in the periinfarct border zone contribute to the expansion of ischemic brain infarcts. Key Words: Focal cerebral ischemia-ischemic penumbra-n-methyl-daspartate antagonist MK-801-Spreading depression. According to the classic concept of the threshold relationship between the density of ischemia and neuronal injury, different flow thresholds have been identified for the suppression of membrane potentials and the suppression of neuronal transmission (Astrup et ai., 1977). Tissue perfused at a flow rate between these thresholds is referred to as the ischemic penumbra because the neurons in this area are thought to be functionally inactive but still viable (Astrup et ai., 1981 ). Threshold determinations of brain metabolism have revealed that the flow rate Received December 10, 1991; final revision received April 6, 1992; accepted April 6, 1992. Address correspondence and reprint requests to Prof. Dr. K. A. Rossmann at Abteilung fur experimentelle Neurologie, Max Planck Institut fur Neurologische Forschung, Gleueler Str. 50, D-5000 Koln 41 (Lindenthal), Germany. Abbreviations used: DC, direct current; LDF, laser-doppler flowmetry; MCA, middle cerebral artery. below which energy metabolism breaks down is equivalent to that of membrane failure (Mies et ai., 1991 ). The threshold for the suppression of protein synthesis, in contrast, is much higher and exceeds even that of transmission failure (Xie et ai., 1989; Mies et ai., 1991 ). Obviously, the persistent suppression of protein synthesis is not compatible with neuronal survival (Hossmann, 1992). This raises the question as to whether neurons in the penumbra zone of an infarct are able to survive for an extended period of time. In fact, the ischemic thresholds of energy metabolism (Mies et ai., 1991) and tissue damage (Heiss and Rosner, 1983) steadily increase with the duration of ischemia, indicating that the capacity of the brain tissue to survive at low flow rates is limited. The reason for the suppression of protein synthesis at a flow rate above the threshold of membrane or energy failure is unknown. A possible ex plana- 727

728 T. IIJIMA ET AL. tion is a disturbance of transmitter and/or calcium homeostasis, in analogy to the pathobiochemical changes leading to neuronal death in selectively vulnerable areas of the brain after transient global ischemia. In fact, permanent occlusion of middle cerebral artery (MCA) results in spreading depressionlike shifts of the cortical steady direct current potential (DC) in periinfarct areas (N edergaard and Astrup, 1986), which are presumably associated with increased conductance of calcium ions. Spreading depression does not cause irreversible neuronal injury under normoxic conditions (Nedergaard and Hansen, 1988), but may be injurious if blood supply is restricted. It is therefore conceivable that the therapeutic suppression of such transients ameliorates the effects of flow reduction. A recent observation supports this hypothesis. Spreading depression can be blocked by glutamate antagonists (Marrannes et al., 1988), and the same drugs are also able to reduce the size of an infarct following permanent MCA occlusion (Ozyurt et al., 1988; Park et al., 1988; Dirnagl et al., 1990). Interestingly, the therapeutic effect is not associated with any changes in blood flow (Park et al., 1988, 1989), indicating that the drugs do not interfere with oxygen delivery to the tissue. The present study was designed to investigate the effect of a noncompetitive glutamate antagonist (MK-80l) on the cortical steady potential and the volume of ischemically damaged tissue following permanent MCA occlusion in rats. The results obtained demonstrate that there is, in fact, a relationship between cortical depolarizations and ischemic injury that may explain the beneficial effect of this drug. MATERIAL AND METHODS Experimental groups Experiments were performed in male rats of the CDF 344 strain weighing 200-280 g. Prior to MCA occlusion, rats were deprived of food for 16 h but had free access to drinking water. Animals were assigned to the following experimental groups: sham-operated group (n = 6), MCA-occluded group without treatment (n = 6), and MCA-occluded group with intraperitoneal MK-801 treatment (3 mg/kg; n = 6). Surgical procedures After induction of anesthesia with 2.5% halothane and 70% nitrous oxide (remainder oxygen), a tracheal tube was inserted, and the femoral arteries and veins were cannulated for continuous measurement of arterial blood pressure and intravenous administration of drugs and fluid, respectively. The animals were then immobilized with pancuronium bromide (0.3 mg/kg) and thereafter artificially ventilated. Rectal temperature was kept at 37 C with a feedback-controlled heating pad. Rats were mounted in a stereotaxic frame, and the temporal bone was carefully removed for electrophysiological recording from the cortex. The stem of the left MCA was exposed by subtemporal craniotomy (Yamamoto et ai., 1988) and was electrocoagulated medial to the olfactory tract. In the sham-operated group, an area adjacent to the MCA stem was electrocoagulated. After termination of surgery, halothane was reduced to 0.8%, and animals were kept in this state for the rest of the experiment. Three minutes after sham operation or after MCA occlusion, animals received an intraperitoneal injection of I ml physiological saline or were treated with 3 mg/kg MK-801 dissolved in I ml physiological saline given intraperitoneally. Physiological recordings A miniature calomel electrode was placed on the frontal skull bone as the indifferent electrode. Two other calomel electrodes were positioned on the dura at the following stereotaxic coordinates: 2 mm anterior and 3 mm lateral to bregma, 0.5 mm posterior and 1 mm lateral to bregma. The connection between electrodes and tissue was established with a thin saline-rinsed cotton wick. From these electrodes, the cortical steady potential and the EEG were recorded prior to and during MCA occlusion and for 3 h after onset of focal ischemia. EEG stored on digital tape was processed off-line by fast Fourier frequency analysis. The power spectrum of the EEG was derived from the square roots of the Fourier coefficients covering the frequency range from 1 to 20 Hz. Blood flow was measured continuously by laser Doppler f10wmetry (LDF). The LDF probe was located between the cortical calomel electrodes, and flow was expressed as a percentage of the preocclusion value. Morphological evaluation Three hours after MCA occlusion, animals were killed by intravenous injection of saturated potassium chloride to ensure terminal cortical depolarization. Brains were immersed in 4% paraformaldehyde and embedded in paraffin. Regions with acute ischemic injury were identified on cresyl violet- and hematoxylin-eosin-stained coronal sections by the appearance of spindle-shaped condensed neurons and the marked paleness of the neuropil. The cross-sectional area of the ischemically injured tissue was measured by a blinded observer at eight levels, and the volume of injury was calculated by integration. In addition, the difference between the volume of the ischemic and the nonischemic hemisphere was calculated for assessing the volume of ischemic brain edema. Statistics Values were assessed by analysis of variance, and statistical differences between experimental groups were determined by Fischer's protected least-squares difference test, which allows multiple comparisons. A p value <0.05 was regarded as statistically significant. RESULTS Physiological variables Table 1 summarizes the physiological parameters at the beginning of the experiment. Before MCA occlusion, mean arterial blood pressure, arterial blood gases, and arterial plasma glucose did not differ in sham-operated and experimental groups. MK- J Cereb Blood Flow Metab. Vol. /2, No.5, 1992

PREVENTION OF DC DEFLECTIONS IN FOCAL CEREBRAL ISCHEMIA 729 TABLE 1. Physiological variables MCA occlusion ---- Sham MK-80 1 operation Untreated treated (n = 6) (n = 6) (n = 6) -,--_._-- MABP (mmhg) 97 ± 6 105 ± 22 98 ± 7 Arterial ph (unit) 7.42 ± 0.04 7.4 1 ± 0.02 7.4 1 ± 0.04 Arterial Peo2 (mmhg) 39 ± 6 38 ± 4 40 ± 2 Arterial P02 (mmhg) 138 ± 37 157 ± 37 146 ± 25 Plasma glucose (mg/dl) 138 ± 23 144 ± 48 150 ± 20 Values are means ± SD. MCA, middle cerebral artery. No significant differences between groups. 80l-treated animals produced a small but significant decrease in mean arterial blood pressure during the initial 2 h after vascular occlusion (Table 2). Blood pressure in the untreated group did not differ from that of the sham-operated group. Cortical blood flow LDF measurements were recorded from an area between the periphery of the MCA territory close to the position of the calomel electrodes. In the shamoperated animals, blood flow remained constant throughout the 3-h observation period (Table 3). In untreated rats, MCA occlusion caused an initial reduction of cortical blood flow at the recording site to 57% of control, followed by a secondary rise to 69%. This value did not change significantly over the next 3 h. In MK-80l-treated animals, MCA occlusion also caused an initial reduction of blood flow to 57% of control, which, however, was followed by a further transient decline to 49% of control in parallel with the drop in blood pressure. This decline was slowly reversed, and after 3 h cortical blood flow stabilized at the same level as in the untreated animals. During the passage of DC deflections (see also below), blood flow transiently increased. This increase ranged from 3 to 67% of control value, and it followed the onset of the DC shift with a delay of between 0.5 and 2.0 min. Blood flow usually returned to the baseline after reversal of the DC deflection. With increasing duration of ischemia, the increase in blood flow declined (Fig. 1), and in two cases paradoxical reductions of blood flow were seen during passage of the DC deflection. Cortical DC recordings Cortical steady (DC) potentials were recorded from two locations in the border zone of the MCA territory. Representative recordings from the electrode adjacent to the infarct are illustrated in Fig. 2. Both sham operation (i.e., a small electrocoagulation lesion adjacent to the MCA stem) and MCA occlusion evoked a transient negative DC deflection of 10 m V in amplitude lasting 1.0-2.3 min. In the untreated MCA-occluded animals, three to eight DC deflections followed at irregular intervals during the subsequent 3 h. The amplitude of these deflections was slightly higher than the initial DC shift, and the duration significantly increased from 1.2 ± 0.3 to 3.7 ± 2.7 min after 3 h (range 1-10 min; p < 0.05). The passage of DC deflection was accompanied by a transient suppression of EEG activity (Fig. 1). MK-801 treatment of MCA-occluded animals did not influence the initial DC deflection after MCA occlusion, but almost completely suppressed the subsequent repetitive DC shifts. In total, three DC deflections were observed: in one of the six animals during the first and third hour and in another animal during the first hour after MCA occlusion. The difference between the number of the DC shifts in treated and untreated MCA-occluded animals was statistically highly significant (p < 0.005) (Table 4). Recordings from the calomel electrode at the position more distant from the infarct were obtained from only four of six animals of the untreated group for technical reasons. The number of DC deflections was similar to that recorded from the electrode closer to the infarct except that the onset was either 0.3-1.7 min earlier (1 5 transients) or 0.3-0.5 min later (3 transients). These variations indicate that the cortical DC alterations originate from different parts of the infarct and spread multidirectionally into the periinfarct surrounding. TABLE 2. Effect of MK-801 on MABP After MCA occlusion Control 1 h 2h 3 h Sham operation 96.8 ± 6.8 95.8 ± 9.7 92.8 ± 11.0 83.4 ± 9.5 MeA occlusion group Untreated 105.0 ± 22.0 95. 1 ± 26.5 97.6 ± 21.3 96.0 ± 27.2 MK-801 treated 98.2 ± 7.4 83.1 ± 4.5a 82.6 ± 4.2a 87.0 ± 8.9 Values are means ± SD. MCA, middle cerebral artery. a p < 0.05, significantly different from values of the sham-operated and untreated groups. J Cereb Blood Flow Me/ab, Vol. 12, No.5, 1992

730 T. IIJIMA ET AL. TABLE 3. Effect of MK-801 on cerebral blood flow (LDF) After MCA occlusion Control 1 h 2h 3h Sham operation 113 ± 31 114 ± 30 114 ± 31 MCA occlusion group Untreated 57 ± 23 69 ± 25a 59 ± 41a 61 ± 31 MK-801 treated 57 ± 17 49 ± l 3a, b 55 ± 12a 63 ± 20a Values are expressed as percentage of control (means ± SD). LDF, laser-doppler flowmetry; MCA, middle cerebral artery. a p < 0.05, significantly different from values of sham-operated group. b p < 0.05, significantly different from value at 3 h after MCA occlusion. EEG recordings Typical EEG recordings following MCA occlusion are shown in Fig. 3. In both treated and untreated animals, MCA occlusion resulted in a marked decrease of the amplitude of EEG, recorded from both calomel electrodes. In the untreated animals, continuous slow-wave activity returned after 25-45 min. Amplitude and frequency of the EEG gradually improved during the 3-h observation period. In MK-801 -treated animals, EEG recordings revealed low-amplitude burst-suppression activity with little tendency to improve. The different patterns of EEG were confirmed by EEG frequency analysis, which revealed a significantly faster improvement of EEG magnitude in untreated animals. Volume of ischemic territory The ipsilateral hemispheric volumes of MK-801 - treated (525.9 ± 0.02 mm 3 ) and untreated (520.5 ± 0.08 mm 3 ) MCA-occluded animals did not differ significantly. In paraformaldehyde-fixed brains, therefore, differences in edema volumes of treated and untreated animals were minimal. The volume of ischemic tissue injury in the untreated group was EEG DC LDF l, l',,,, [ [ 500,uV 100 % '-f _ _ 50 MCA occlusion 10 min FIG. 1. Recording of EEG, steady-state direct current (DC) potential, and laser-doppler flowmetry (LDF) from the cortex of an untreated animal. The EEG recording shows a sudden suppression of EEG amplitude with the onset and a slow recovery after the passage of the spreading depression-like DC deflection. Note that several cortical DC deflections are observed and that the duration of DC deflection increases with time after occlusion of the middle cerebral artery (MCA). Initially, blood flow reveals a moderate reactive increase during the phase of DC deflection, which becomes less pronounced with MCA occlusion time ( t ). o 108 ± 38.5 mm 3 or 21 ± 7.6% of the volume of the MCA-occluded hemisphere. Treatment with MK- 801 after MCA occlusion decreased the volume of ischemic injury to 58 ± 11.5 mm 3 or 11 ± 2.2% of hemispheric volume (p < 0.05) (Fig. 4). DISCUSSION The noncompetitive antagonist of the N-methyl D-aspartate subtype of the glutamate receptor MK- 801 has been previously shown to reduce the volume of experimental infarcts (Ozyurt et ai., 1988; Park et ai., 1988; Dirnagl et ai., 1990). Interestingly, the therapeutic effect seems to be independent of major changes in blood flow (Park et ai., 1989). The present study confirms that 3 mg/kg MK-801 given intraperitoneally shortly after the onset of permanent occlusion of the MCA in the rat reduces the volume of ischemically injured tissue by 47% despite a transient reduction of arterial blood pressure and cortical blood flow as compared with untreated animals. It follows that this drug improves the re- Sham operation DC r--- II I MeA occlusion untreated -------------------------- [ 10mV '---r-r-r----- [+ r 10mV MK-801 treated ----[ MeA occlusion 10 mv 10min FIG.2. Direct current (DC) potential recordings from the cortex of a sham-operated animal and of an untreated and an MK-801-treated (3 mg/kg Lp.) animal 3 h after middle cerebral artery (MCA) occlusion. In sham-operated animals one spreading depression-like deflection was consistently recorded after manipulation of the MCA. In MCA-occluded untreated animals, repeated spreading depression-like DC deflections additionally occurred. Note that the duration of the DC deflections increased following MCA occlusion. In MCAoccluded MK-801-treated (3 mg/kg Lp.) animals, only one DC deflection after MCA occlusion was observed, as in shamoperated controls. J Cereb Blood Flow Metab, Vol. 12, No. 5, 1992

PREVENTION OF DC DEFLECTIONS IN FOCAL CEREBRAL ISCHEMIA 731 TABLE 4. DC deflections after MCA occlusion MCA occlusion Sham MK-801 operation Untreated treated First DC deflection Amplitude (m V) 12.0 ± 8.5 12.5 ± 3.3 8.7 ± 3.9 Duration (min) 1.0 ± 0.1 1.2 ± 0.3 1.7 ± 0.4 5-59 min No. (median and range) 0 2.5 (2-5)' 1 (0-1) Amplitude (m V) 13.8 ± 3.2 8.5 ± 1.8 Duration (min) 1.9 ± 1.0 2.3 ± 1.4 60-119 min No. (median and range) 0 1(0-2) 0 Amplitude (m V) 17.3 ± 4.3 Duration (min) 2.7 ± 1.1 120-180 min No. (median and range) 0 1 (1-2)' o (0-1)" Amplitude (m V) 13.0 ± 6. 4 10.0 Duration (min) 3.7 ± 2.7h 2.6 Amplitude and duration are expressed as means ± SD. DC, direct current; MCA, middle cerebral artery. a DC deflection in only one animal. b p < 0.05, significantly different from first DC deflection. c p < 0.005, significantly different between experimental groups. sistance of the brain to ischemia by mechanisms that do not depend on the amelioration of oxygen or substrate delivery to the tissue. MK-801 has been shown to interfere with a variety of pathological processes. Besides its remarkable beneficial effect on infarct size after permanent vascular occlusion (Ozyurt et ai., 1988; Park et ai., 1988; Dirnagl et ai., 1990), it reduces neuronal injury after severe hyperglycemia (Papagapiou and Auer, 1990) and improves the survival ratio of pyramidal neurons in the CAl sector of hippocampus after brief periods of global ischemia (Gill et ai., 1988; Kass et ai., 1989; Swan and Meldrum, 1990), although the latter has been related mainly to its temperature-lowering effect (Buchan and Pulsinelli, 2 h MeA occlusion untreated.11/,:" MK-801 treated "' {::/,,"-..,. -,"('-----y!"'v( :>' [500)JV 5min 1s FIG. 3. Representative cortical EEG recording before, during, and 2 h after occlusion of the middle cerebral artery (MeA). Following electrocoagulation of the MeA, a sudden decrease in EEG amplitude occurs. Note that 2 h after MeA occlusion, a dominant slow EEG pattern is observed in untreated animals, but a burst-suppression pattern in animals treated with MK-801 (3 mg/kg Lp.). % 60 50 1.0 30 20 10 o Info ret volu me i i i i 0-0 untreated.--. MK-801 treated 11.0 115 9.0 6.5 1..0 1.5-10 mm i i i FIG. 4. Effect of MK-801 treatment on the volume of ischemically injured tissue. Values (means ± SD) represent ischemic brain damage as percentage of hemispheric cross-sectional area at various stereotaxic planes (distance from the interaural line). Note that a significant reduction of volume of injured tissue predominantly occurs in the posterior parts of hemispheres (*p < 0.05, significantly different from untreated group). 1990; Corbett et ai., 1990). It inhibits spontaneous electrocortical activity in a dose-dependent way (Marquis et ai., 1989), and it suppresses cortical spreading depression induced by electrical stimulation (Marrannes et ai., 1989). The minimal dose required to obtain a therapeutic effect varies between 0.5 mg/kg i.v. (Park et ai., 1988) and 3 mg/kg i.p. for reducing infarct size and for suppressing spreading depression (Marrannes et ai., 1989). At high doses (above 10 mg/kg) toxic side-effects of MK- 801 may prevail, but up to 5 mg/kg its beneficial effect on infarct size is prominent (Dirnagl et ai., 1990). We therefore chose a dose of 3 mg/kg to make sure that toxic side-effects were avoided. The demonstration of a marked reduction of the volume of ischemically injured tissue after permanent MCA occlusion despite the further decline of blood flow supports the hypothesis that MK-801 interferes with functional or metabolic processes that render the ischemic tissue more resistant to the pathological changes. Our electrophysiological recordings suggest that two processes may be involved: the suppression of repetitive cortical depolarizations in the periphery of the ischemic territory and the reduction of spontaneous electrocortical background activity. Cortical de polarizations in the vicinity of focal ischemia were first described by Nedergaard and Astrup (1986), who used intracortical microelectrodes for the recording of the DC potential. The authors noted that the DC deflections may last as long as 90 min after MCA occlusion. The present study demonstrates that such DC deflections persist for at least 3 h when recorded with surface elec- J Cereb Blood Flow Metab, Vol. 12, No.5, 1992

732 T. IIJIMA ET AL. trodes. The cortical DC deflections are presumably generated in the periphery of the ischemic territory in response to pathological events occurring at the center of the infarct. The most likely cause is the rise of extracellular potassium (Hansen and Nedergaard, 1988) and glutamate, which are released from the intracellular compartment at flow values below 20 mllloo g/min (Astrup et ai., 1977; Shimada et ai., 1990) and are known to evoke spreading cortical depression (Van Harreveld and Fifkova, 1973). Obviously, the flow rate of brain regions into which cortical depolarization spreads must be above the threshold for membrane failure because the tissue spontaneously repolarizes. In the present study, LDF revealed a flow value of 57% of control in the vicinity of the recording electrodes. In the halothane-anesthetized rat, the normal cortical flow rate is 120 ml/loo g/min (Mies et ai., 1991 ); blood flow at the periphery of the infarct thus amounted to 70 mlll00 g/min. This flow value is far above the threshold for energy depletion and does not generally produce ischemic injury. However, it is conceivable that the combination of a marginal ischemia with repeated cell depolarizations creates a pathophysiological situation that may result in irreversible cell damage. In fact, in the intact animal, spreading depression is associated with an increased metabolic rate for glucose (Shinohara et al., 1979), which is coupled to a similar rise in blood flow (Kocher, 1990; Mayevsky and Weiss, 1991 ). The increase in blood flow ensures adequate tissue oxygenation and is probably the main factor preventing spreading depression from producing irreversible neuronal injury in normal animals (Nedergaard and Hansen, 1988). After MCA occlusion, in contrast, the regulatory capacity of the already dilated vasculature is limited, and the increase in blood flow is less pronounced than in the normal animal. It is therefore possible that the tissue becomes transiently hypoxic during these episodes. This could explain why, in the present study, the duration of periinfarct DC deflections was as long as 10 min, i.e., much longer than the DC deflection during spreading depression in normal cortex, which lasts only 2 min (Mies and Paschen, 1984; Kocher, 1990). This observation is in line with an earlier study by Nedergaard and Astrup (1 986), who reported that DC deflections in the periinfarct area last for up to 8 min. The suppression of periinfarct depolarizations by MK-801 may therefore protect the cortex against transient episodes of hypoxia that result from temporary increases in the metabolic workload of the cortex. An alternative mechanism for the potentially deleterious effect of periinfarct DC deflection is re- peated calcium flooding of the cytosol. Depolarization of neuronal membranes causes an opening of voltage-gated calcium channels and a rise of cytoplasmic Ca2+ activity (Hofmeier and Lux, 1981). One of the numerous biological effects of increased calcium activity is the suppression of protein synthesis that in turn may be involved in the manifestation of ischemic neuronal death (A varia and Krivanek, 1973; Krivanek, 1978). Measurements of protein synthesis in focal ischemia revealed that the flow threshold of this metabolic pathway is much higher than that of energy metabolism (Xie et ai., 1989; Mies et ai., 1991 ); this is in line with the flow values observed in the regions with transient depolarizations. It is therefore conceivable that the suppression of the depolarizations by MK-801 reduces the impairment of protein synthesis and by this mechanism reduces infarct size. An unexpected finding of the present study was the effect of MK-801 on EEG activity. It is widely held that burst-suppression activity, as observed in the MK-801 -treated animals (Marquis et ai., 1989; Uematsu et ai., 1991 ), indicates more severe injury than the continuous slow-wave activity seen in untreated ischemic animals. However, it can also be argued that the occurrence of a burst-suppression pattern signals reduced energy metabolism (see Steer, 1982) and that the suppression of electrical activity minimizes the mismatch between the energy needs and the reduced oxygen delivery to the ischemic tissue. The effect of the glutamate antagonist MK-801 on spontaneous electrocortical activity may therefore be therapeutically just as significant as the relief from intermittent episodes of membrane depolarization. In conclusion, the present study demonstrates that MK-801 substantially modifies the electrophysiological events associated with acute MeA occlusion and that this effect may interfere with the pathophysiological sequelae of ischemic cell injury. In particular, reversal or prevention of disturbances of energy or protein metabolism may be of importance. It would therefore be of interest to investigate tliis relationship and to establish whether MK- 801 or any other glutamate antagonists are able to improve metabolic activity at the periphery of the ischemic infarct. REFERENCES Astrup J, Symon L, Branston NM, Lassen NA (1977) Cortical evoked potential and extracellular K + and H + at critical levels of brain ischemia. Stroke 8:51-57 Astrup J, Symon L, Siesjo BK (1981) Thresholds in cerebral ischemia-the ischemic penumbra. 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