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Anesthesiology 2002; 96:450 –7

? 2002 American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc.

Effect of the 2-Agonist Dexmedetomidine on Cerebral Neurotransmitter Concentrations during Cerebral Ischemia in Rats
Kristin Engelhard, M.D.,* Christian Werner, M.D.,? Susanne Kaspar, B.S.,? Oliver M?llenberg, M.D.,§ Manfred Blobner, M.D.,? Monika Bachl, Cand.Med., Eberhard Kochs, M.D.

Background: This study investigates whether neuroprotection seen with dexmedetomidine is associated with suppression of peripheral or central sympathetic tone. Methods: Thirty fasted male Sprague-Dawley rats were intubated and ventilated with iso?urane and N2O/O2 (fraction of inspired oxygen 0.33). Catheters were inserted into the right femoral artery and vein and into the right jugular vein. Cerebral blood ?ow was measured using laser Doppler ?owmetry. Bilateral microdialysis probes were placed into the cortex and the dorsal hippocampus. At the end of preparation, the administration of iso?urane was replaced by fentanyl (bolus: 10 g/kg; infusion: 25 g · kg 1 · h 1). Animals were randomly assigned to one of the following groups: group 1 (n 10): control animals; group 2 (n 10): 100 g/kg dexmedetomidine administered intraperitoneally 30 min before ischemia; group 3 (n 10): sham-operated rats. Ischemia (30 min) was produced by unilateral carotid artery occlusion plus hemorrhagic hypotension to a mean arterial blood pressure of 30 –35 mmHg to reduce ipsilateral cerebral blood ?ow by 70%. Pericranial temperature, arterial blood gases, and pH were maintained constant. Cerebral catecholamine and glutamate concentrations and plasma catecholamine concentrations were analyzed using high-performance liquid chromatography. Results: During ischemia, dexmedetomidine suppressed circulating norepinephrine concentrations by 95% compared with control animals. In contrast, brain norepinephrine and glutamate concentrations were increased irrespective of dexmedetomidine infusion before ischemia. Conclusions: The current data show that the increase of circulating catecholamine concentrations during cerebral ischemia was suppressed with dexmedetomidine. In contrast, dexmedetomidine does not suppress elevation in brain norepinephrine and glutamate concentration associated with cerebral ischemia. This suggests that the neuroprotective effects of dexmedetomidine are not related to inhibition of presynaptic norepinephrine or glutamate release in the brain.

ripheral catecholamine concentrations, e.g., ganglionic blocking agents, 2-agonists, and anesthetics improve neurologic outcome and reduce histopathologic damage.1–3 Although these data generally suggest that neuroprotection is related to a reduced sympathetic tone, it is unclear whether this is a function of suppressed plasma catecholamine or brain catecholamine concentrations. Therefore, this study investigates the effect of the 2-agonist dexmedetomidine on circulating and cerebral catecholamine concentrations during incomplete cerebral ischemia in rats.

Materials and Methods
Preparation After obtaining approval from the institutional animal care committee (Government of Bavaria), 30 male Sprague-Dawley rats (weighing 300 – 420 g) were anesthetized in a bell jar saturated with iso?urane. Rats were tracheally intubated and mechanically ventilated with 1.5 vol% iso?urane in nitrous oxide and oxygen (fraction of inspired oxygen 0.33). Catheters were inserted into the right femoral artery and vein and into the right jugular vein for blood withdrawal, administration of drugs, and blood sampling. A loose ligature was placed around the right common carotid artery for later clamping. The rats were then placed in a stereotactic “U”frame with nonpenetrating ear bars (Model 962; David Kopf Instruments, Tujunga, CA). After incision of the skull, penetrating burr holes (1 mm in diameter) were drilled into the cranium 4.2 mm posterior and 2.5 mm lateral of the bregma over both hemispheres according to the stereotaxis coordinates of the rat brain.4 The tip of the drill was continuously ?ushed with saline to avoid thermal injury. After incision of the dura, the microdialysis probes (CMA12, 4.0 mm length, 0.5 mm diameter; CMA/Microdialysis AB, Solna, Sweden) were carefully inserted into the cortex and the dorsal hippocampus. The probes were then ?xed using a zinc polycarboxylate cement (Poly-F Plus; Dentsply, York, PA) and perfused with Ringer’s solution (Boehringer Ingelheim Delta Pharm GmbH, Pfullingen, Germany; 147 mM Na , 2.25 mM Ca2 , 4 mM K , 155.5 mM Cl ) at a rate of 1.0 l/min. Small collector vials were ?lled with 10 l of 0.5 M perchloric acid to stabilize catecholamines and placed in a refrigerated fraction collector (CMA 170; CMA/Microdialysis AB). Ninety minutes after implantation of the microdialysis probes, sample fractions of 30 min were 450

STUDIES in rats subjected to incomplete cerebral ischemia have shown that the preischemic administration of agents suppressing the ischemia-induced increase of pe-

* Resident, ? Professor, § Assistant Professor, Professor and Chairman, Klinik für Anaesthesiologie, ? Medical Technician, Institut für klinische Chemie und Pathobiochemie. Received from the Klinik für Anaesthesiologie and Institut für klinische Chemie und Pathobiochemie, Technische Universit?t München, Klinikum rechts der Isar, Munich, Germany. Submitted for publication December 5, 2000. Accepted for publication September 26, 2001. Supported in part by grants from Abbott GmbH, Wiesbaden, Germany, and Else Kr?ner-Fresenius-Stiftung, Bad Homburg v.d. H?he, Germany. Presented in part at the annual meetings of the American Society of Anesthesiologists, San Francisco, California, October 16, 2000, and the Society of Neurosurgical Anesthesiology and Critical Care, San Francisco, California, October 13, 2000. Address reprint requests to Dr. Engelhard: Klinik für Anaesthesiologie, Technische Universit?t München, Klinikum rechts der Isar, Ismaninger Stra e 22, 81675 München, Germany. Address electronic mail to: k.engelhard@lrz.tu-muenchen.de. Individual article reprints may be purchased through the Journal Web site, www.anesthesiology.org.

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collected and subsequently stored at 70°C. Nonpenetrating burr holes were drilled 0.5 mm anterior and 1 mm lateral of the bregma into the cranium over both hemispheres for continuous measurement of erythrocyte ?ow velocity using a laser Doppler ?owmeter (PeriFlux System 4001; Perimed, J?rf?lla, Sweden). Local cerebral blood ?ow (CBF) was continuously measured and expressed in arbitrary perfusion units, which were sampled over 0.3 s. The laser Doppler ?ow probes (Probe 403; Perimed) were placed over both hemispheres and ?xed using the stereotactic frame. Care was taken to place the probes over a tissue area devoid of large blood vessel visible through the thinned bone, and correct placement of laser Doppler probes was con?rmed by transient hypoventilation. Pericranial temperature was measured with a 22-gauge stainless steel needle thermistor (model 73A, Yellow Springs Instrument Co., YSI Temperature Controller; Yellow Springs, OH) placed beneath the right temporal muscle and was maintained constant at 37.5°C throughout the experiment by a servomechanism using an overhead heating lamp and a heating pat. Cerebral Ischemia At the end of the preparation, all surgical incisions were in?ltrated with 0.5% bupivacaine, and the administration of iso?urane was replaced by fentanyl (bolus: 10 g/kg; infusion: 25 g · kg 1 · h 1) while ventilation was continued with nitrous oxide and oxygen (fraction of inspired oxygen 0.33). Mechanical ventilation was adjusted to maintain arterial carbon dioxide tension at 38 – 42 mmHg. The nonpenetrating ear bars of the stereotactic frame were released. During cerebral ischemia, arterial pH was maintained at physiologic levels by intravenous infusion of bicarbonate. Vecuronium was given as a continuous infusion (0.1 mg · kg 1 · min 1) to maintain neuromuscular blockade. Animals were randomly assigned to one of the following treatment groups. Group 1 (n 10) represents the control group, with no additional treatment. Animals in group 2 (n 10) received 100 g/kg dexmedetomidine administered intraperitoneally 30 min before the onset of ischemia. Animals in group 3 (n 10) were sham-operated (i.e., complete instrumentation, no ischemia) with no additional treatment. After an equilibration period of 2 h, cerebral ischemia was induced by hemorrhagic hypotension and clip occlusion of the right common carotid artery. Mean arterial blood pressure was maintained within the range of 30 –35 mmHg to reduce CBF in the ischemic hemisphere by 70%. After 30 min of cerebral ischemia, the clip was released and the shed blood was reinfused over 15 min. Arterial blood gases and plasma glucose concentrations were analyzed at baseline, 30 min during ischemia, 15 min after ischemia (reperfusion), and 90 min after ischemia (recovery). Blood samples for measurement of plasma catecholamine concentrations
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were collected at baseline, at the end of ischemia, and 90 min after cerebral ischemia. The blood samples were centrifuged at 4°C for 10 min, and the plasma was stored at 70°C. Brains were removed 4 h after ischemia and placed in tissue-freezing medium (Jung Tissue Freezing Medium; Leica Instruments GmbH, Nussloch, Germany), frozen in methylbutan on dry ice, and stored at 70°C. The correct position of the microdialysis probes was veri?ed, and histologic damage caused by the microdialysis probes was evaluated in 7- m brain slices, stained with hematoxylin and eosin, and animals were excluded from the study in case of major bleeding. High-performance Liquid Chromatography Samples for plasma norepinephrine, epinephrine, and dopamine analyses were processed using the ClinRep test kit for high-performance liquid chromatography analysis of catecholamines (Recipe Chemicals and Instruments GmbH, Munich, Germany). Plasma samples (0.25 ml) were mixed with 50 l dehydroxybenzylamine (internal standard). The mixture was passed through the sample preparation column ?lled with aluminum oxide. The remaining particles of plasma proteins were washed out. Norepinephrine, epinephrine, and dopamine were eluted by adding 120 l elution buffer. No pretreatment was necessary to analyze cerebral dialysate for extracellular norepinephrine and dopamine concentration. Samples were placed into a cooled autosampler (AS2000A; Merck Hitachi, Darmstadt, Germany). Sixty microliters of the plasma elute or 10 l of the cerebral dialysate was injected into the high-performance liquid chromatography circulation system (mobile phase ClinRep; Recipe Chemicals and Instruments GmbH) for electrochemical detection (0.5 V potential). The elute and the cerebral dialysate was passed over the analytical column (ClinRep; Recipe Chemicals and Instruments GmbH) with a ?ow of 1.0 ml/min to the electrochemical detector (Waters 460; Waters, Milford, MA). The whole system was controlled and data were stored by the high-performance liquid chromatography systems manager software (Merck Hitachi, Darmstadt, Germany). The system was calibrated with a catecholamine standard (ClinRep; Recipe Chemicals and Instruments GmbH). For analysis of the cerebral glutamate and aspartate concentration, the microdialysis samples were placed into an autoinjector (Gina 50 Probengeber; Dionex, Germering, Germany). Ortho-phthaldialdehyd (20 l), diluted with boracic buffer (1:10), was mixed with 10 l cerebral dialysate for derivatization. This mixture was injected into the high-performance liquid chromatography circulation system (Pumpensystem M480; Dionex) for ?uorometric detection and was passed over the analytical column (Grom-Sil OAA-2, 250 4 mm; Grom, Herrenberg, Germany) with a ?ow of 0.8 ml/min to the ?uorescence detector (Fluoreszenzdetektor RF-2000; Dionex; wavelength: extinction 280 nm–emission 475 nm).

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The mobile phase A consisted of 23 mM sodium acetate adjusted with HCl to a pH of 6.0. The mobile phase B consisted of 600 ml methanol and 50 ml acetonitiril. The gradient was changed as follows: after beginning, 100% phase A– 0% phase B; after 29.8 min, 79% phase A–21% phase B; after 32.5 min, 47% phase A–53% phase B; after 34 min, 0% phase A–100% phase B. The analysis refers to a four-point standard curve of a custom-made standard. Statistical Analysis Data (mean SD) of four consecutive time points were evaluated: before hemorrhagic hypotension (baseline), at 30 min of cerebral ischemia (ischemia), 15 min after cerebral ischemia upon reinfusion of the withdrawn blood (reperfusion), and 90 min after cerebral ischemia (recovery). Data of variables that are available from two hemispheres and at each time point (CBF and cerebral neurotransmitter concentrations) were subjected to a twoway repeated-measurements analysis of variance with the two within-group factors (time2 and hemisphere), the between-groups factor (group), and all possible interaction terms (time2 hemisphere; time2 group; 2 hemisphere group; time hemisphere group). To evaluate the following three hypotheses, respective post hoc analyses were performed in a stepwise manner, if the respective interaction terms of this global test were signi?cant (P 0.05). Differences between groups during ischemia: Once “time2 group” or “time2 hemisphere group” proved to be signi?cant in the global test (P 0.05), another two-way repeated-measurements analysis of variance was performed with the within-groups factor (time2), the between-groups factor (group), and their interaction term separately for each hemisphere. Once “time2 group” proved to be signi?cant (P 0.05), values during ischemia of the respective hemisphere were analyzed with a factorial analysis of variance, with group as independent factor and, if signi?cant (P 0.05), followed by three unpaired t tests (P 0.05/3 0.016 for multiple comparison correction). Differences between baseline and ischemia within each group and each hemisphere: Once “time2 group” or “time2 hemisphere group” proved to be signi?cant in the global test (P 0.05), another two-way repeated-measurements analysis of variance was performed with the within-groups factor (time2), the between-groups factor (group), and their interaction term separately for each hemisphere. Once “time2 group” proved to be signi?cant (P 0.05), values at baseline and during ischemia were compared using paired t tests in each group and each hemisphere (P 0.05/6 0.008 for multiple comparison correction). Differences between hemispheres during ischemia within each group: Once “time2 hemisphere” or “time2 hemisphere group” proved to be signi?cant
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in the global test (P 0.05), another two-way repeatedmeasurement analysis of variance was performed with the within-groups factor (hemisphere), the betweengroups factor (group), and their interaction term. Once “hemisphere group” proved to be signi?cant (P 0.05), values during ischemia of the ipsilateral and the contralateral side were compared using paired t tests separately in each group (P 0.05/3 0.016 for multiple comparison correction). Data of variables that are available at each time point (physiologic variables and plasma catecholamine concentrations) were subjected to a two-way repeated-measurements analysis of variance with the within-groups factors (time2), the between-groups factor (group), and their interaction term (time2 group). To evaluate the following two hypotheses, respective post hoc analyses were performed in a stepwise manner if “time2 group” was signi?cant (P 0.05). Differences between groups during ischemia: Values during ischemia were analyzed with a factorial analysis of variance, with “group” as independent factor and, if signi?cant (P 0.05), followed by three unpaired t tests (P 0.05/3 0.016 for multiple comparison correction). Differences between baseline and ischemia within each group: Values at baseline and during ischemia were compared using paired t tests in each group (P 0.05/3 0.016 for multiple comparison correction). All variables are presented as mean SD. Statistical analyses were performed using SPSS 10.0 for Windows (SPSS Inc., Chicago, IL).

Results
Table 1 shows the physiologic variables. According to the study protocol, mean arterial blood pressure was decreased in control animals (group 1) and in dexmedetomidine-treated animals (group 2) during cerebral ischemia compared with sham-operated animals (group 3). There were no differences for arterial oxygen and carbon dioxide tensions within (baseline vs. ischemia) and between groups. In control animals and animals treated with dexmedetomidine, plasma glucose concentration decreased during ischemia compared with baseline. During ischemia, plasma glucose concentration was higher with sham-operated animals compared with control animals and dexmedetomidine-treated animals. Table 2 shows the results of the statistical analysis testing the differences between both hemispheres. As expected, the decrease of cortical CBF (laser Doppler ?owmetry) was more severe in the ischemic hemisphere compared with the nonischemic hemisphere in control animals (group 1) and animals treated with dexmedetomidine (group 2). Cerebral norepinephrine and glutamate concentration was signi?cantly higher in the ischemic hemisphere compared with the nonisch-

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Table 1. Mean Arterial Blood Pressure (MAP), Arterial Blood Gas Tensions, and Plasma Glucose Concentration during Baseline, Ischemia, Reperfusion, and Recovery (90 min after Cerebral Ischemia)
Time Group Baseline Ischemia Reperfusion Recovery Time Group Signi?cance Baseline vs. Ischemia

MAP (mmHg)

PaO2 (mmHg)

PaCO2 (mmHg)

Glucose (mg/dl)

Control Dexmedetomidine Sham operated Control Dexmedetomidine Sham operated Control Dexmedetomidine Sham operated Control Dexmedetomidine Sham operated

118 133 120 135 116 108 40 42 40 55 77 57

8 8 9 25 15 16 2 1 4 9 6 6

33 31 122 140 122 95 37 41 41 44 50 60

2? 1? 11* 16 6 22 4 5 3 5? 6? 5*

127 128 123 145 110 103 40 43 40 50 56 60

20 21 10 35 21 10 3 3 4 7 7 11

98 70 125 137 100 109 39 41 42 71 69 61

16 6 10 32 4 9 4 4 3 9 10 11

P

0.001

P P

ns

ns P P

P

0.001

0.001 0.001 ns na na na na na na 0.05 0.001 ns

Mean ns

SD; * P

0.05 compared with control group during ischemia; ? P not applicable.

0.05 compared with sham operated animals during ischemia.

not signi?cant; na

emic hemisphere in control animals (group 1) and animals receiving dexmedetomidine (group 2). Figure 1 shows the cortical CBF in the ischemic (?g. 1A) and the nonischemic (?g. 1B) hemisphere before, during, and after cerebral ischemia. CBF was measured in arbitrary perfusion units. According to the study protocol, during cerebral ischemia CBF was decreased in control animals (group 1) and in animals treated with the dexmedetomidine (group 2) but not sham-operated animals (group 3). This decrease in CBF was also observed in the nonischemic hemisphere. Figure 2A shows the plasma norepinephrine concentration before, during, and after cerebral ischemia. In
Table 2. Statistical Analysis of Differences between Both Hemispheres for Local Cerebral Blood Flow and Neurotransmitter Concentrations in the Dialysate
Signi?cance Hemisphere Group

Group

Hemisphere

Local Cerebral Blood ?ow Cerebral Norepinephrine Concentration Cerebral Dopamine Concentration Cerebral Glutamate Concentration Cerebral Aspartate Concentration

Control Dexmedetomidine Sham-operated Control Dexmedetomidine Sham-operated Control Dexmedetomidine Sham-operated Control Dexmedetomidine Sham-operated Control Dexmedetomidine Sham-operated

P

0.05

P P P P

0.05 0.05 ns 0.001 0.001 ns na na na

control animals, plasma norepinephrine concentration was increased during cerebral ischemia compared with baseline. In contrast, treatment with dexmedetomidine decreased plasma norepinephrine concentration compared with control animals during cerebral ischemia. Plasma epinephrine concentration was also decreased with dexmedetomidine compared with control animals (?g. 2B) during cerebral ischemia, whereas plasma dopamine concentrations did not change in any group (data not shown). Figure 3 shows norepinephrine (?g. 3A) and glutamate concentrations (?g. 3B) in the cerebral cortex and hippocampus before, during, and after cerebral ischemia in the ischemic and nonischemic hemisphere. Compared with baseline, the cerebral norepinephrine and glutamate concentrations were increased during ischemia in the ipsilateral but not in the contralateral hemisphere in control animals. Dexmedetomidine did not affect the ischemia-induced elevation of cerebral norepinephrine and glutamate concentrations. Likewise, brain aspartate was elevated in the control and dexmedetomidinetreated animals (data not shown). The cerebral dopamine concentrations did not change over time in any group (data not shown).

P

0.05

Discussion
Consistent with previous investigations using this animal model, the current results show that cerebral ischemia (induced by hemorrhagic hypotension and clipping of the right common carotid artery) causes an increase in plasma norepinephrine and epinephrine but not in dopamine concentrations. This elevation in peripheral sympathetic tone did not occur in the presence of the 2-agonist dexmedetomidine. Likewise, cerebral norepinephrine, glutamate, and aspartate, but not dopamine concentrations were increased during ischemia. How-

ns P P

P

0.05

0.05 0.05 ns na na na

ns

time2 hemisphere or time2 hemisphere global test (P 0.05) for all tested variables. ns not signi?cant; na not applicable.

group was signi?cant in the

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Fig. 1. Cortical cerebral blood ?ow (CBF) in the ischemic (A) and nonischemic (B) hemisphere before, during, and after cerebral ischemia. *P < 0.05 compared with control group during ischemia; §P < 0.05 compared with sham-operated animals during ischemia; #P < 0.05, baseline versus ischemia within control group; $P < 0.05, baseline versus ischemia within dexmedetomidineanesthetized animals.

ever, the ischemia-induced increase of cerebral norepinephrine and glutamate could not be suppressed by administration of dexmedetomidine. This suggests that neuroprotection observed with dexmedetomidine1,5 is not related to presynaptic inhibition of catecholamine and glutamate release in the brain. Cerebral ischemia is associated with an increase in circulating and extracellular brain catecholamine concentrations.1,2,6,7 Interventions to reduce sympathetic tone (e.g., administration of ganglionic blocking agents, anesthetics, or 2 agonists) improved neurologic outcome. Neuroprotection observed with reduced sympathetic activity may be related to several mechanisms. (1) Catecholamines stimulate cerebral metabolic rate for ox-

ygen,8,9 an effect that further imbalances the ratio between cerebral oxygen demand and oxygen supply. (2) High catecholamine concentrations also increase the sensitivity of pyramidal neurons to excitatory neurotransmitters such as glutamate,10 which results in elevated intracellular Ca2 concentrations with consecutive activation of intracellular catabolic enzymes (excitotoxicity). (3) Catecholamines may exert a direct neurotoxic effect when exposed to neuronal tissue in excessive concentrations.11 (4) It is also possible that increased sympathetic activity decreases perfusion in the ischemic penumbra because ischemic hypotension would produce greater decrease in CBF in sympathetically intact rats (i.e., sympathetic vasoconstriction) as compared

Fig. 2. Plasma norepinephrine (A) and epinephrine (B) concentration before, during, and after cerebral ischemia. Dexmedetomidine suppressed the plasma norepinephrine and epinephrine concentration during cerebral ischemia. *P < 0.05 compared with control group during ischemia; §P < 0.05 compared with sham-operated animals during ischemia; #P < 0.05, baseline versus ischemia within control group; $P < 0.05, baseline versus ischemia within dexmedetomidine-treated animals; P < 0.05, baseline versus ischemia within sham-operated animals. Anesthesiology, V 96, No 2, Feb 2002

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Fig. 3. Cerebral norepinephrine concentration (A) and glutamate concentration (B) in the ischemic and nonischemic hemisphere before, during, and after cerebral ischemia. Both cerebral norepinephrine and glutamate concentrations increased in the ischemic hemisphere during ischemia irrespective of treatment. *P < 0.05 compared with control group during ischemia; §P < 0.05 compared with sham-operated animals during ischemia; #P < 0.05, baseline versus ischemia within control group; $P < 0.05, baseline versus ischemia within dexmedetomidine-anesthetized animals.

with animals subjected to ganglionic blockade.12 Therefore, suppression of catecholamine concentrations may be neuroprotective by economizing the ratio between cerebral oxygen demand and oxygen supply, reducing excitotoxicity, reducing toxic effects, or improving perfusion in the ischemic penumbra. We expected a suppression of the ischemia-induced increase in cerebral catecholamine concentration in the presence of dexmedetomidine. This assumption was based on the current understanding of the pharmacologic mechanisms of 2-adrenergic agonists and a study in rabbits subjected to global ischemia in which norepinephrine concentration in the striatum was less with dexmedetomidine compared with control animals.13 However, dexmedetomidine did not attenuate the stress response to incomplete hemispheric ischemia (group 2)
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in the current study. It is possible that the different results between the two studies are related to differences in regions of interest (i.e., the striatum represents dopaminergic territory) or the use of different ischemia models. The data also suggest that presynaptic stimulation of 2-adrenergic receptors with consecutive decrease in extracellular catecholamine concentrations occurs to a lesser extent than previously believed. The excitatory amino acid neurotransmitter glutamate triggers neuronal death when released in excessive concentrations during cerebral ischemia.14 In hippocampal rat brain slices, dexmedetomidine suppressed the excessive glutamate release during potassium chloride– evoked depolarization or hypoxic stress.15 It was therefore expected that dexmedetomidine would decrease the ischemia-induced glutamate release in the current

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study. However, during cerebral ischemia, the elevation of extracellular brain glutamate concentrations was similar between rats treated with dexmedetomidine compared with control animals. Consistently, dexmedetomidine did not decrease the hippocampal glutamate concentration in a model of global cerebral ischemia in rabbits.16 These data suggest that the neuroprotective potential of dexmedetomidine is not related to changes in extracellular glutamate concentrations. An in vitro study in cultured astrocytes has shown that dexmedetomidine activates the oxidative metabolism of glutamine, a precursor of the neurotoxic glutamate, and thereby reduces the glutamate availability.17 However, our results did not demonstrate a reduction in cerebral glutamate concentration, an effect that would have been expected from the mechanism of glutamine disposal suggested by Huang et al.,17 and, therefore, this mechanism is not supposed to be a major factor of brain protection with dexmedetomidine. Experiments using isolated canine stellate ganglia have shown that dexmedetomidine inhibited the synaptic conduction by decreasing the postsynaptic response to a late, slow excitatory presynaptic stimulation through an 18 In addition, studies in 2-receptor–mediated process. isolated primary cortical neurons (i.e., tissue deprived from glial supporting layers and presynaptic receptors) demonstrate neuroprotection with dexmedetomidine, an effect that was reversible by the 2-receptor antagonist yohimbine.19 It is therefore possible that dexmedetomidine is neuroprotective by postsynaptic action with consecutive reduction in Ca2 currents. In the current model of incomplete cerebral ischemia in which decreased sympathetic tone was always associated with improved outcome,1–3 plasma but not extracellular brain catecholamine concentrations were suppressed. Similarly, intraischemic plasma catecholamine concentrations were lower with the neuroprotective anesthetics iso?urane or ketamine compared with control rats anesthetized with fentanyl–nitrous oxide that were subjected to incomplete or near-complete forebrain ischemia,20 whereas brain catecholamine concentrations were elevated with any background anesthetic technique. Consistently, in rats in which brain norepinephrine stores were depleted 7 days before forebrain ischemia, no differences in plasma catecholamine concentrations and histopathologic damage were evident when compared with control animals, despite signi?cant differences in cerebral norepinephrine concentrations.21 These results support the view that circulating catecholamines rather than cerebral catecholamine concentrations mediate neuroprotection after cerebral ischemia. The purpose of the current study was to investigate whether neuroprotection observed with dexmedetomidine in previous investigations1 using the same ischemia model involves changes of cerebral neurotransmitter concentrations. To enable comparison between the curAnesthesiology, V 96, No 2, Feb 2002

rent and previous investigations, we repeated the study protocol with respect to the background anesthetic technique (fentanyl–nitrous oxide) and the dosage of dexmedetomidine. The use of this background anesthetic technique has the advantage of preserved cerebrovascular autoregulation in rats,22 but concern was expressed as to the adequacy of analgesia and sedation. However, there is strong evidence that fentanyl–nitrous oxide provides adequate anesthesia in nonstressed animals, as rats given fentanyl–nitrous oxide demonstrate an identical electrophysiologic pattern (electroencephalogram decreased to frequency) compared with animals receiving 1.0 minimum alveolar concentration iso?urane or des?urane.3 At the end of preparation before termination of iso?urane, all surgical incisions were in?ltrated with 0.5% bupivacaine. Thereafter, rats were not exposed to any painful stimuli. Therefore, fentanyl– nitrous oxide seems to provide suf?cient analgesia and sedation in rats not exposed to surgical stress. The dose of dexmedetomidine used during the current study results from previous experiments using this ischemia model, where 10 g/kg dexmedetomidine produced moderate and 100 g/kg dexmedetomidine produced profound improvement of neurologic outcome. Likewise, studies in rats have shown that a hypnotic anesthetic effect, as characterized by loss of righting re?ex, occurs at doses greater than 100 g/kg dexmedetomidine intraperitoneal.23 We therefore decided to assess cerebral and peripheral neurotransmitter concentrations using a dose of 100 g/kg dexmedetomidine based on our previous experiences and the dose-dependent hypnotic anesthetic action of dexmedetomidine via activation of central 2-adrenoceptors.23 In conclusion, dexmedetomidine did not inhibit the intraischemic increase of cerebral extracellular catecholamine or glutamate concentrations, whereas peripheral catecholamine concentrations were suppressed. These data indicate that modulation of cerebral catecholamine and glutamate release is not related to the neuroprotective effect of dexmedetomidine previously demonstrated in this model.
The authors thank Doris Droese (Medical Technician, Klinik für Anaesthesiologie, Technische Universit?t, Munich, Germany), Bianca Matthes (Medical Technician, Institut für klinische Chemie und Pathobiochemie, Technische Universit?t, Munich, Germany), and J?rg Eriskat, M.D. (Resident, Institut für Chirurgische Forschung, Ludwig-Maximilian-Universit?t, Munich, Germany), for technical assistance and expertise.

References
1. Hoffman WE, Kochs E, Werner C, Thomas C, Albrecht RF: Dexmedetomidine improves neurologic outcome from incomplete ischemia in the rat. ANESTHESIOLOGY 1991; 75:328 –32 2. Werner C, Hoffman WE, Thomas C, Miletich DJ, Albrecht RF: Ganglionic blockade improves neurologic outcome from incomplete ischemia in rats: Partial reversal by exogenous catecholamines. ANESTHESIOLOGY 1990; 73:923–9 3. Engelhard K, Werner C, Reeker W, Lu H, M?llenberg O, Mielke L, Kochs E: Des?urane and iso?urane improve neurological outcome after incomplete cerebral ischaemia in rats. Br J Anaesth 1999; 83:415–21

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457

4. The Rat Brain in Stereotaxic Coordinates, 4th Edition. Edited by Paxinos G, Watson C. San Diego, Academic Press, 1998. 5. Jokkonen J, Puurunen K, Koistinaho R, Kauppinen R, Haapalinna A, Nieminen L, Sivenius J: Neuroprotection by the 2-adrenoreceptor agonist, dexmedetomidine, in rat focal cerebral ischemia. Eur J Pharmacol 1999; 372:31– 6 6. Globus MYT, Busto R, Dietrich WD, Martinez E, Valdés I, Ginsberg MD: Direct evidence for acute and massive norepinephrine release in the hippocampus during transient ischemia. J Cereb Blood Flow Metab 1989; 9:892– 6 7. Bhardwaj A, Brannan TS, Martinez-Tica J, Weinberger J: Ischemia in the dorsal hippocampus is associated with acute extracellular release of dopamine and norepinephrine. J Neural Transm 1990; 80:195–201 8. Nemoto EM, Klementavicius R, Melick JA, Yonas H: Norepinephrine activation of basal cerebral metabolic rate for oxygen (CMRO2) during hypothermia in rats. Anesth Analg 1996; 83:1262–7 9. Meyer JS, Welch KMA, Okamoto S, Shimazu K: Disordered neurotransmitter function. Brain 1974; 97:655– 64 10. Madison DV, Nicoll RA: Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro. J Physiol 1986; 372:221– 44 11. Stein SC, Cracco RQ: Cortical injury without ischemia produced by topical monoamines. Stroke 1982; 13:74 – 83 12. Busija DW: Sympathetic nerves reduce cerebral blood ?ow during hypoxia in awake rabbits. Am J Phys 1984; 247:H446-51 13. Matsumoto M, Zornow MH, Rabin BC, Maze M: The 2 adrenergic agonist, dexmedetomidine, selectively attenuates ischemia-induced increases in striatal norepinephrine concentrations. Brain Res 1993; 627:325–9 14. Graham SH, Chen J, Sharp FR, Simon RP: Limiting ischemic injury by inhibition of excitatory amino acid release. J Cereb Blood Flow Metab 1993; 13:88 –97 15. Talke P, Bickler PE: Effects of dexmedetomidine on hypoxia-evoked glu-

tamate release and glutamate receptor activity in hippocampal slices. ANESTHESIOLOGY 1996; 85:551–7 16. Kim HK, Zornow M, Strnat MAP, Maze M: Dexmedetomidine does not attenuate increases in excitatory amino acids after transient global ischemia in the rabbit. J Neurosug Anesth 1996; 8:230 –5 17. Huang R, Chen Y, Yu ACH, Hertz L: Dexmedetomidine-induced stimulation of glutamine oxidation in astrocytes: A possible mechanism for its neuroprotective activity. J Cereb Blood Flow Metab 2000; 20:895– 8 18. McCallum JB, Boban N, Hogan Q, Schmeling WT, Kampine JP, Bosnjak ZJ: The mechanism of 2-adrenergic inhibition of sympathetic ganglion transmission. Anesth Analg 1998; 87:503–10 19. Laudenbach V, Mantz J, Evrard P, Gressens P: Dexmedetomidine protects against neonatal excitotoxic brain injury (online abstract). ANESTHESIOLOGY 2000; A-732 20. Miura Y, Mackensen B, Nellgard B, Pearlstein RD, Bart RD, Dexter F, Warner DS: Effects of iso?urane, ketamine, and fentanyl/N2O on concentrations of brain and plasma catecholamines during near-complete cerebral ischemia in the rat. Anesth Analg 1999; 88:787–92 21. Nellg?rd BMG, Miura Y, Mackensen GB, Pearlstein RD, Warner DS: Effect of intracerebral norepinephrine depletion on outcome from severe forebrain ischemia in the rat. Brain Res 1999; 847:262–9 22. Hoffman WE, Werner C, Kochs E, Segil L, Edelman G, Albrecht RF: Cerebral and spinal cord blood ?ow in awake and fentanyl-N2O anesthetized rats: Evidence for preservation of blood ?ow autoregulation during anesthesia. J Neurosurg Anesth 1992; 4:31–5 23. Doze vA, Chen B-X, Maze M: Dexmedetomidine produces a hypnoticanesthetic action in rats via activation of central alpha-2 adrenoreceptors. ANESTHESIOLOGY 1989; 71:75–9

Anesthesiology, V 96, No 2, Feb 2002


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