SPINAL cord ischemia remains a devastating complication after repair of thoracoabdominal aortic aneurysms. Spinal cord motor neurons are especially sensitive to mild or short ischemia.1,2
This type of injury results in paraplegia.3
Growing evidence suggests that release of excitatory neurotransmitters, into the extracellular space of the central nervous system by ischemic cells, may contribute to cell death in both cerebral and spinal cord injury.4–8
There have been extensive studies of various drugs, including postsynaptic N-methyl-D-aspartate (NMDA) receptor antagonists, as possible neuroprotectants after hypoperfusion or ischemia.9,10
Magnesium sulfate, a noncompetitive NMDA receptor antagonist, has demonstrated a variety of neuroprotective actions in the central nervous system after induction of experimental ischemia11–14
and in clinical studies.15
The majority of these studies have focused on how magnesium prevented or attenuated necrotic phenomenon in the gray matter of spinal cord, and preserved motor function partially or completely. This antagonistic action is speculated to occur from a direct postsynaptic effect. However, the effects of magnesium on the extracellular concentration of glutamate during a spinal cord ischemic event have not been demonstrated in vivo
In the present study, we sought to determine the degree to which magnesium sulfate affects the concentration of glutamate in cerebrospinal fluid and enhances motor neuron survival in both the foci and the penumbra of spinal cord experimental ischemia. By investigating the relationship between motor neuron survival after a combination of ischemia and magnesium sulfate (MgSO4) at different levels of the spinal cord, we provide evidence that the beneficial effects of a one-time administration of MgSO4 are not limited to only the ischemic lesion foci but can also affect neuronal degeneration several days later in adjacent penumbral nervous tissue.
Materials and Methods
After approval by our institutional animal care and use committee (Loyola University Medical Center, Maywood, IL) with adherence to American Physiology Society/National Institute Health guidelines, male New Zealand rabbits (n = 52) weighing 2–3 kg at the beginning of experiments were used. They were housed under controlled laboratory conditions consisting of a 12-h light/dark cycle with lights turned on at 6:00 am, a temperature of 22 ± 1°C, and 55 ± 5% humidity. Food pellets and water were supplied ad libitum. Under anesthesia with isoflurane, a catheter was inserted into the auricular vein and fixed on the ear for drug administration. Saline (10 ml·kg-1·h-1) was administrated intravenously to prevent dehydration. A butterfly catheter (0.5 × 19 mm × 304.8 mm tubing infusion set) filled with heparinized saline was inserted into the auricular artery and fixed on the ear for constant monitoring of blood pressure and heart rate during surgery. The rectal temperature of animals was maintained between 37 and 37.5°C with a heating pad before and after aortic occlusion.
A total of 52 rabbits were randomly assigned to one of seven treatment groups with 6–10 animals per group. One group of animals served as sham controls by undergoing the surgical procedure without aortic occlusion followed by 24 h of survival. The other six groups underwent a 30-min aortic artery occlusion followed by a survival time of 1, 2, or 7 days. Three of these groups (1, 2, and 7 d postoperative survival) received placebo treatment, which consisted of artificial cerebrospinal fluid intrathecal administration 15 min before the occlusion. The remaining three groups received MgSO4 intrathecal administration 15 min before the occlusion and were allowed to survive for either 1, 2, or 7 days.
Experimentally Induced Spinal Cord Ischemia
Rabbits were anesthetized via an intravenous sedative agent, propofol (10 mg/kg). A mask was placed over the rabbit’s muzzle for further induction of anesthesia using isoflurane 4% in oxygen; tracheal intubation was then performed. The lungs were mechanically ventilated with isoflurane 1–3% in oxygen throughout the surgery and experimental procedures to maintain anesthesia.
Spinal cord ischemic injury was produced according to a previously described method.4,13,16
Briefly, the abdominal cavity was entered through a midline incision, and the aorta was exposed at the level of the left renal artery. The artery was then isolated 1 cm below the renal bifurcation. A polyethylene suture (PE10 tubing) was passed underneath the aorta, and a button combined with larger plastic tubing (PE190, 10 cm in length) was used to protect the aortic artery wall during ligation. The abdominal incision was then sutured. Arterial occlusion was achieved by pulling the suture outside the abdomen to obstruct blood flow for 30 min. A Doppler probe flow device (Ultrasonic Flow Detectors Models 811-B/811-BTS; Medical Electronics, Inc., Aloha, OR) pointing down against the skin surface of the groin area was used for repeated measuring of the femoral artery pulse to ascertain confirmation of occlusion and reperfusion. Heparin 150 U/kg was administered intravenously 5 min before aortic occlusion to prevent clot formation. The sham procedure involved conducting a similar surgical procedure without aortic occlusion. Surgery was performed with aseptic technique.
Experiment 1: Effect of MgSO4 on Cerebral Spinal Fluid Levels of Glutamate.
This experiment examined the effects of MgSO4 on the level of glutamate present in the cerebral spinal fluid before (60 min), during (30 min), and after the period of aortic occlusion (150 min). Rabbits were randomly assigned to undergo either MgSO4 treatment (3 mg/0.05 ml/kg) or placebo (artificial cerebral spinal fluid) (n = 12 rabbits placebo plus occlusion; n = 12 MgSO4 plus occlusion; n = 4 sham-treated controls).
Spinal Cord Microdialysis and Microinjection
A microdialysis probe (outside diameter, 200 μm; length, 70 mm) along with microinjection tubing (outside diameter, 152 μm; length, 60 mm) manufactured in our laboratory was inserted through a 22-gauge guide needle placed between lumbar vertebral levels L4 and L5 into the subarachnoid space. The recovery efficiency of the microdialysis probe was approximately 15%, as previously determined in vitro
. The permeable section of the probe was positioned cephalad between the T12 and L4 levels. During the experiment, artificial cerebrospinal fluid (125 mm NaCl, 0.86 mm CaCl2
, 3 mm KCl, 0.89 mm MgCl2
, 25 nm NaHCO3
, 0.5 nm NaH2
, and 0.25 nm Na2
), which served as a perfusion medium, was infused via
a syringe pump at a rate of 3 μl/min to collect perfusate before and after drug treatment and at onset and offset of ligation procedure. Each sample was collected for 30 min, and two samples were collected at baseline before aortic occlusion. The duration of total collection was 4 h. MgSO4
(3 mg/0.05 ml/kg) or placebo (artificial cerebrospinal fluid), which was determined based on a previously reported study on dogs,13
was injected intrathecally with a syringe pump set at 0.01 ml/min. The total duration of MgSO4
infusion was based on the body weight (5 min/kg) and was performed approximately 15 min before the onset of ischemia.
Each experiment lasted between 6 and 7 h. At the end of the experiment, the arterial and venous lines were removed. Anesthetic administration was discontinued, and the lungs were ventilated with 100% oxygen. Extubation of the trachea took place when vigorous spontaneous ventilation and movement occurred. After extubation and cessation of the experiment, the animal was randomly assigned to recover for 1, 2, or 7 days. Cephazolin 10 mg/kg intramuscularly was administered once per day for 2 d to prevent infection, and lactated Ringer’s solution 40 ml was administered subcutaneously to prevent dehydration. Buprenorphine 0.03 mg/kg was given subcutaneous to control pain twice per day. If animals showed signs of pain, such as grooming or guarding, 2 d after surgery, then additional pain medication was administered. Bladder contents were expressed manually twice per day.
Measurement of glutamate from the spinal cord dialysate was performed using high-performance liquid chromatography (Waters Corp., Milford, MA).17
The system consists of a solvent delivery system (Waters 515 HPLC pump, Waters Corp.), an octadecyl disufonate C18
3 Fm 100 × 3.2-mm phase II column, and a fluorescent detector (Waters 474, Water Corp.) with excitement set at 330 λ and emission set at 425 λ. The mobile phase consisted of 0.1 m sodium acetate in 10% acetronitrile, pH 6.8 as buffer I and 70% acetronile as buffer II. Precolumn derivatization with O-phthalaldehyde reagent solution was performed manually. After derivatization, each standard solution and prepared sample was injected into the system. Glutamate and aspartate samples in a standard solution and biologic samples were separated as they migrated along the phase II column (retention times, Asp = 2.69 min; glutamate 4.65 min.). Biologic samples were quantified against a standard curve of known solutions run with each sample set. The area of glutamate peak was converted to an actual concentration (picomole/microliter) using linear regression curves obtained from the series of standard samples. The baseline concentration was calculated by averaging the actual concentration of each component in the initial two 30-min samples.
Experiment 2: Effect of MgSO4 on Neurologic Outcome as Determined by Behavior and Neuronal Survival.
This experiment examined the effects of MgSO4 on hind limb motor function and neuron survival after the period of aortic occlusion (see Aortic Occlusion section above). Rabbits were randomly assigned to undergo either MgSO4 treatment (3 mg/0.05 ml/kg) or placebo (artificial cerebral spinal fluid) (n = 13 rabbits placebo plus occlusion; n = 17 MgSO4 plus occlusion; n = 7 sham-treated controls).
Animal hind limb motor function was observed and scored daily for 1, 2, or 7 d after the ischemic event. A Tarlov five-point grading scale was used: 4 = normal; 3 = ability to draw legs under body and hop, but not normal; 2 = some lower extremity function with good antigravity strength but inability to draw legs under body; 1 = poor lower extremity motor function, weak antigravity movement only; and 0 = paraplegic with no lower extremity motor function muscle tone or contraction. The experimenter was blinded as to the condition of the animal.
Histology and Cell Counting Analysis
Rabbits were euthanized by an overdose of pentobarbital 100 mg/kg intravenously 1, 2, or 7 d after the ischemic event,. The spinal cord tissue was fixed by transcardial perfusion of 4% paraformaldehyde and processed for paraffin embedding. Four sections from each spinal cord level (T7–L5) were stained with Nissl stain and photographed on a light microscope (BH2-RFCA; Olympus, Center Valley, PA). Neurons were distinguished within the spinal cord ventral horn based on cell morphology, a clear nucleus, and prominent nucleolus. The ventral horn cells with a clear nucleus were counted and traced. There was a 400-μM distance between each stained tissue section. Based on spinal cord levels of hind limb muscular innervation, spinal cord levels L3–L6 were pooled together, as were spinal cord levels of non–hindlimb-associated spinal cord segments, T7-L2. Results are presented as mean ± SEM of the total cells in the two prespecified regions of spinal cord segments. The experimenter was blinded as to the treatment condition of the animal.
SAS software (SAS Institute Inc., Cary, NC) was used for the following statistical analyses. A repeated-measures analysis of variance was used to determine whether glutamate levels present in spinal cord dialysate differed among treatment groups and differed within subjects over time. A Fisher exact test was performed to analyze differences among the neurologic behavioral outcome groups. For the cell counting analysis, a SAS code was created and applied to the raw data to sort and group the data based on the cell size range in each spinal cord segment. The frequency averages and standard deviations were computed for each subgroup. After grouping results based on the variable treatments, a nonparametric Wilcoxon rank sums test was performed to compare the significance within the region among the treatment groups using χ2
analysis. The paired t
test compared the mean of the differences in the observations between two regions. Linear regression analysis was using to compare hindlimb motor function (Tarlov scores) with cell survival (cell count of viable cells). Model assumptions (normality and equality of variances) were assessed using graphical (i.e.
, q-q plots) and analytical methods (i.e.
, kurtosis/skewness-based tests18
for normality, and Bartlett’s test19
to verify equality of variances among groups).
Physiologic values are presented in table 1
. There were no physiologically relevant differences among the groups for all monitored variables with the exception of rectal temperatures. Placebo- and MgSO4
-treated rabbits showed similar values in mean blood pressure, heart rate, and blood glucose levels. The rectal temperatures, however, were significantly reduced 0.4-0.5°C during aortic artery occlusion in placebo- and MgSO4
-treated groups compared with the sham group, but they returned to the preocclusion baseline level after reperfusion (table 1
Before the ischemic challenge, baseline levels (mean ± SEM) of glutamate were similar in all three groups (2.13 ± 0.18 μm in the placebo group, 2.65 ± 0.41 μm in the MgSO4
group, and 3.31 ± 1.23 μm in the sham group). The baseline values were considered as 100%, and each animal served as its own control. Glutamate levels in cerebrospinal fluid increased by 212% immediately after ischemia plus placebo and remained elevated for at least 2 h (P
= 0.0001) after the offset of ischemia (n = 12) (fig. 1
). In contrast, pretreatment with intrathecal MgSO4
before ischemia produced glutamate levels similar to those recorded for sham-injured animals (n = 12 and n = 4, respectively) (fig. 1
Animals subjected to all treatment conditions were neurologically evaluated on a daily basis using a Tarlov five-point scoring system. Of the placebo-treated animals, 86% exhibited complete paraplegia with rigidly extended hind limbs and no movement (Tarlov 0) (fig. 2
). The remaining animals had slight lower extremity function (Tarlov 1). MgSO4
-treated animals had better neurologic outcomes (Tarlov 0–3). Twelve percent demonstrated good antigravity strength (Tarlov 3), 18% had some antigravity strength (Tarlov 2), 35% had weak antigravity strength (Tarlov 1), and 35% demonstrated Tarlov 0 scores, whereas 100% of the sham animals had completely normal examinations (fig. 2
). The data from Days 2 and 7 are presented together in figure 2
. The animal’s motor function was not different between Day 2 and Day 7. The worst neurologic outcomes had already been obtained (placebo-treated) on Day 2 even for the animals that survived 7 days. Overall, there was a significant difference between sham and either placebo- or MgSO4- treated group. There is also a significant difference when the MgSO4
-treated group was compared with the placebo group.
Cells with motor neuron-like morphology were assessed from segments T7 to L6 (fig. 3
). Neuronal loss was greatest in the lower lumbar spinal segments L3–L6 on Day 1 in the ischemic group receiving the placebo treatment (fig. 3b
). In contrast, animals receiving MgSO4
in combination with ischemic treatment exhibited significantly lower loss of motor neurons on Day 1 compared with the placebo-treated group (fig. 3b
There was a parallel, albeit delayed, loss of neurons in the spinal cord adjacent to L3–L6. Spinal cord levels T7-L2 displayed a loss of neurons by Day 7 (fig. 3a
). The differences between the two regions (T7-L2 and L3-L6) were statistically significant in the placebo group (P
= 0.0086, 0.0005, and 0.0001, respectively for the three time points). The cell loss present on Day 7 did not approach the degree of damage present in L3–L6. However, the numbers of remaining neurons were significantly lower when compared with sham-treated tissue and Postinjury Days 1 and 2 (fig. 3a
). Not unlike the neuroprotective effects in spinal segments L3–L6 in placebo-treated ischemic animals, the addition of MgSO4
just before the ischemic event largely eliminated the loss of neurons in T7-L2 (fig. 4
Linear regression analysis comparing hind limb motor function (Tarlov scores) including the data from Days 2 and 7 with cell survival (cell count of viable cells) demonstrated a significant coefficient (r2
= 0.81, P
< 0.001), indicating a considerable relationship between motor function and integrity of motor neurons in the spinal cord (fig. 5
Transient ischemia in the spinal cord arising as a consequence of thoracic surgery, trauma, or ischemia that is experimentally induced elicits increased release of extracellular glutamate. Ischemia-enhanced concentrations of this neurotransmitter may lead to excitotoxicity and neuronal cell death via
calcium-mediated NMDA receptor-dependent intracellular processes.10
Spinal cord neurons, especially motor neurons in the ventral horn, are extremely sensitive to excitotoxicity and constitute one of the main neural elements damaged after spinal cord ischemia.1,3,20,21
In this article, we provide direct evidence that the addition of MgSO4
just before experimentally induced ischemia effectively eliminates injury-induced glutamate increases in cerebrospinal fluid. MgSO4
-treated animals subjected to spinal cord ischemia also exhibit significantly increased numbers of motor neurons in both the lumbar and lower thoracic levels of the spinal cord compared with the ischemia/placebo treatment group. In addition, there was a strong linear correlation between decreased neuronal cell death and hindlimb mobility when ischemic animals were pretreated with intrathecal MgSO4
Our data corroborate results in a variety of animal models4,5,7,8
and in humans.6
However, it seems that there are two mechanisms of cell death operating within the ischemic spinal cord, as most of the hindlimb motor neurons present in the lumbar spinal cord are lost with the first 24 h, whereas there is no histologic evidence of degeneration in the lower thoracic/upper lumbar region of the spinal cord until after 48 h. These differences in regional cell death may not differ from a mixed form of concurrent apoptosis and necrosis in the ischemic cerebrum termed hybrid death.22–23
Moreover, it would seem that regardless of the mechanism of cell death, a one-time administration of MgSO4
provides significant benefits for both regions of the injured spinal cord.
As there is benefit from MgSO4
administration in both the reduction of cerebrospinal fluid glutamate concentrations and reduced neuronal loss after ischemia, cell death mechanisms are expected to exhibit some dependence on glutamate-specific receptors, such as the calcium (Ca2+
)-permeable NMDA-type receptors.9,10
In this regard, the role of MgSO4
is likely as a noncompetitive antagonist of the NMDA receptors present on both presynaptic and postsynaptic neurons.24
Regardless of site of receptor action, it seems that injury-induced increases in glutamate accumulation are likely to overstimulate neurons, leading to a vicious cycle in which massive increases in extracellular glutamate eventually lead to glutamate receptor overactivation (excitotoxicity) and rapid postsynaptic Ca2+
Evidence of such a condition has been demonstrated using the NMDA antagonist, D-APV, which effectively reduces neuronal calcium accumulation in a manner proportional to its protective effect.26
Neuronal cell death after ischemia is not limited to glutamate-mediated influx of calcium via
ionotrophic NMDA receptors, as there is growing evidence that expression of novel α-amino-3-hydroxymethylisoxazole-4-propionate (AMPA) receptors may also undermine neuronal survival after nervous system ischemic events. Under normal circumstances, it is thought that AMPA receptors mediate the majority of fast excitatory transmission in the mammalian central nervous system via
-impermeable tetrameric assemblies of subunits GluR1-427,28
as relatively few neuron populations exhibit Ca2+
-permeable AMPA receptors.29
After global ischemia, numerous groups have demonstrated that pyramidal cells in the hippocampus upregulate expression of AMPA receptors that lack the GluR2 subunit. This particular AMPA receptor isoform is permeable to both Ca2+
a feature likely to contribute to neuronal cell death.33
Recent studies on the susceptibility of neurons to either ischemia or disease pathologies reveal that spinal motor neurons can also exhibit these AMPA receptors lacking GluR2.34
Moreover, the presence of tumor necrosis factor-α, a pro-inflammatory cytokine typically present during the initial stages of ischemic-induced neuroinflammation, facilitates additional membrane insertion of Ca2+
-permeable AMPA channels into motor neurons35–37
and may profoundly contribute to changes in Ca2+
-permeable AMPA receptors after ischemia.38
Taken together, glutamate receptor-mediated changes in neuronal levels of Ca2+
likely lead to mitochondrial disruption, generation of reactive oxygen species, release of apoptotic mediators such as cytochrome C, and neuronal cell death.39
Despite a strong potential for attenuation of Ca2+
translocation after a combination of ischemia and magnesium, biologically relevant in vivo
testing has yet to be conducted.
Several mechanisms independent of neuronal glutamate receptors may also be modified in the extracellular space after ischemia, which may contribute to superfluous amounts of glutamate. Normally glutamate released into the extracellular space is quickly removed and transported into the astroglial syncytium by sodium-dependent astrocytic Glu transporters (GLT-1).40–42
GLT-1 transports one glutamate anion coupled to the co-transport of three Na+
and one H+
, as well as to the countertransport of one K+
GLT-1 uses steep ionic gradients across the membrane to accumulate a high intracellular concentration of glutamate in astrocytes. The ionic gradients are mainly maintained by Na+
-ATPase, which excludes Na+
in exchange for extracellular K+
, in turn energizing other secondary ion transporters (e.g.
exchanger). After ischemia, changing ionic gradients may negate the driving force for glutamate uptake, resulting in a reversed operation of astrocytic GLTs and release of neuropathological levels of glutamate, potentially contributing to the death of neurons.44–46
Not unlike neurons, astrocytes are also known to exhibit both metabotropic and ionotropic glutamate receptors, which may facilitate astrocytic detection of changes in extracellular glutamate levels.47
However, the effects of magnesium on GLT-1 reversal are not apparent given that the NMDA receptors present on astrocytes seem to be insensitive to the presence of up to 10 mm magnesium.47
The benefits of magnesium treatment during ischemia may not be limited to neuron and glial support cells as magnesium produces relaxation of vascular smooth muscle,48
effectively enhancing cerebral blood flow.49
The increase in blood flow secondary to vasodilation could improve perfusion to ischemic neurons and effectively reduce glutamate release. This magnesium-dependent mechanism may involve antagonism of the vasoconstrictive mediator, endothelin-1.50
Magnesium has also been shown to attenuate vasoconstriction induced by norephinphrine, angiotensin II, and serotonin51,52
and to prevent thrombosis by inhibiting platelet reactivity.53
Additional evidence that local vasodilation and improvement in blood flow may be a major mechanism in reducing glutamate release is demonstrated by studies that show magnesium has a minimal effect on improving neurologic function if administered peripherally.54
Magnesium may not penetrate the blood-brain barrier sufficiently to increase cerebrospinal fluid concentrations to a level that would produce local vasodilation. In the present study, magnesium was administered intrathecally and probably produced local concentrations much higher than if given by parenteral infusion. These effects seem beneficial for cell survival after ischemic insult and have an undetermined impact on our study.
Only one animal study55
has questioned the safety of intrathecal magnesium administration by observing neurotoxicity in a dose-dependent manner including histopathologic changes and motor dysfunction (Tarlov score 1-0) 7 days post-administration in rabbits. This study further demonstrated that intrathecal magnesium (0.3 or 1 mg/kg) did not improve hindlimb motor function and histopathologic outcomes induced by aortic occlusion and that higher concentrations of magnesium (2–3 mg/kg) administered intrathecally caused direct damage on spinal cord neurons. This result is contrary to a majority of reports from magnesium studies in which neuroprotective effects of magnesium have been confirmed in different animal models by different investigators.11,12
More recent clinical evidence suggests that the use of intravenous magnesium (5 days treatment) did not differ from placebo treatment in individuals with a combination of moderate to severe traumatic brain injury (within 8 h) and low serum magnesium.56
This outcome is suggestive that the neuroprotective effect of magnesium may be dependent on the route of administration or be limited only to individuals with certain clinical criteria (thoracoabdominal aneurysm repair) or neurologic injury.
Recent studies demonstrated that glutamate-induced calcium entry and energy failure has played a deleterious role for the delayed secondary spinal cord injury. It was reported that the initial excitotoxic events associated with necrosis also activate microglia and produce pro-inflammatory cytokines, which may be crucial for inducing necrotic and apoptotic cell death. New approaches are focusing on using treatments that target the nonneuronal cellular components of the central nervous system inflammatory response and long-term secondary events after initial injury. It is likely that the key to postischemic treatment will include the control of factors that modulate the inflammatory response.57–59
However, the effects of MgSO4
and other potential neuroprotective agents on the ischemia-induced inflammatory response are unknown.
Taken together, the present study has demonstrated that a single intrathecal administration of MgSO4 attenuates immediate release of glutamate and reduces neurologic injury after transient ischemia both in the foci and associated pneumbra of the spinal cord. Further investigations will focus on magnesium activation of glutamate transport proteins and inhibition of the inflammatory process.
1. DeGirolami U, Zivin JA: Neuropathology of experimental spinal cord ischemia in the rabbit. J Neuropathol Exp Neurol 1982; 41:129–49
2. Sakurai M, Aoki M, Abe K, Sadahiro M, Tabayashi K: Selective motor neuron death and heat shock protein induction after spinal cord ischemia in rabbits. J Thorac Cardiovasc Surg 1997; 113:159–64
3. Moore WM Jr, Hollier LH: The influence of severity of spinal cord ischemia in the etiology of delayed-onset paraplegia. Ann Surg 1991; 213:427–31
4. Simpson RK Jr, Robertson CS, Goodman JC: Spinal cord ischemia-induced elevation of amino acids: Extracellular measurement with microdialysis. Neurochem Res 1990; 15:635–9
5. Marsala M, Sorkin LS, Yaksh TL: Transient spinal ischemia in rat: Characterization of spinal cord blood flow, extracellular amino acid release, and concurrent histopathological damage. J Cereb Blood Flow Metab 1994; 14:604–14
6. Brock MV, Redmond JM, Ishiwa S, Johnston MV, Baumgartner WA, Laschinger JC, Williams GM: Clinical markers in CSF for determining neurologic deficits after thoracoabdominal aortic aneurysm repairs. Ann Thorac Surg 1997; 64:999–1003
7. Ishikawa T, Marsala M: Hypothermia prevents biphasic glutamate release and corresponding neuronal degeneration after transient spinal cord ischemia in the rat. Cell Mol Neurobiol 1999; 19:199–208
8. Wakamatsu H, Matsumoto M, Nakakimura K, Sakabe T: The effects of moderate hypothermia and intrathecal tetracaine on glutamate concentrations of intrathecal dialysate and neurologic and histopathologic outcome in transient spinal cord ischemia in rabbits. Anesth Analg 1999; 88:56–62
9. Vink R, Nimmo AJ: Novel therapies in development for the treatment of traumatic brain injury. Expert Opin Investig Drugs 2002; 11:1375–86
10. Arundine M, Tymianski M: Molecular mechanisms of glutamate-dependent neurodegeneration in ischemia and traumatic brain injury. Cell Mol Life Sci 2004; 61:657–68
11. Kaplan S, Ulus AT, Tutun U, Aksoyek A, Ozgencil E, Saritas Z, Apaydin N, Pamuk K, Can Z, Surucu S, Katircioglu SF: Effect of Mg2SO4 usage on spinal cord ischemia-reperfusion injury: electron microscopic and functional evaluation. Eur Surg Res 2004; 36:20–5
12. Lang-Lazdunski L, Heurteaux C, Dupont H, Widmann C, Lazdunski M: Prevention of ischemic spinal cord injury: Comparative effects of magnesium sulfate and riluzole. J Vasc Surg 2000; 32:179–89
13. Simpson JI, Eide TR, Schiff GA, Clagnaz JF, Hossain I, Tverskoy A, Koski G: Intrathecal magnesium sulfate protects the spinal cord from ischemic injury during thoracic aortic cross-clamping. Anesthesiology 1994; 81:1493–9
14. Vacanti FX, Ames A III: Mild hypothermia and Mg++ protect against irreversible damage during CNS ischemia. Stroke 1984; 15:695–8
15. Lang-Lazdunski L, Bachet J: Pharmacological spinal cord protection with magnesium during replacement of the thoracic and thoracoabdominal aorta. Ann Thorac Surg 2001; 72:2180–1
16. Johnson SH, Kraimer JM, Graeber GM: Effects of flunarizine on neurological recovery and spinal cord blood flow in experimental spinal cord ischemia in rabbits. Stroke 1993; 24:1547–53
17. Jellish WS, Murdoch J, Kindel G, Zhang X, White FA: The effect of clonidine on cell survival, glutamate, and aspartate release in normo- and hyperglycemic rats after near complete forebrain ischemia. Exp Brain Res 2005; 167:526–34
18. D’Agostino RB, Balanger A: A suggestion for using powerful and informative test of normality. Am Stat 1990; 44:316–21
19. Bartlett MS: Properties of sufficiency and statistical test. Proc R Soc 1937; 160:268–82
20. Juvonen T, Biancari F, Rimpilainen J, Satta J, Rainio P, Kiviluoma K: Strategies for spinal cord protection during descending thoracic and thoracoabdominal aortic surgery: Up-to-date experimental and clinical results. A review. Scand Cardiovasc J 2002; 36:136–60
21. Marsala J, Sulla I, Santa M, Marsala M, Zacharias L, Radonak J: Mapping of the canine lumbosacral spinal cord neurons by Nauta method at the end of the early phase of paraplegia induced by ischemia and reperfusion. Neuroscience 1991; 45:479–94
22. Martin LJ, Al-Abdulla NA, Brambrink AM, Kirsch JR, Sieber FE, Portera-Cailliau C: Neurodegeneration in excitotoxicity, global cerebral ischemia, and target deprivation: A perspective on the contributions of apoptosis and necrosis. Brain Res Bull 1998; 46:281–309
23. Xiao AY, Wei L, Xia S, Rothman S, Yu SP: Ionic mechanism of ouabain-induced concurrent apoptosis and necrosis in individual cultured cortical neurons. J Neurosci 2002; 22:1350–62
24. McKee JA, Brewer RP, Macy GE, Borel CO, Reynolds JD, Warner DS: Magnesium neuroprotection is limited in humans with acute brain injury. Neurocrit Care 2005; 2:342–51
25. Hartley DM, Kurth MC, Bjerkness L, Weiss JH, Choi DW: Glutamate receptor-induced 45Ca2+ accumulation in cortical cell culture correlates with subsequent neuronal degeneration. J Neurosci 1993; 13:1993–2000
26. Benveniste H, Jorgensen MB, Diemer NH, Hansen AJ: Calcium accumulation by glutamate receptor activation is involved in hippocampal cell damage after ischemia. Acta Neurol Scand 1988; 78:529–36
27. Dingledine R, Borges K, Bowie D, Traynelis SF: The glutamate receptor ion channels. Pharmacol Rev 1999; 51:7–61
28. Geiger JR, Melcher T, Koh DS, Sakmann B, Seeburg PH, Jonas P, Monyer H: Relative abundance of subunit mRNAs determines gating and Ca2+ permeability of AMPA receptors in principal neurons and interneurons in rat CNS. Neuron 1995; 15:193–204
29. Petralia RS, Wang YX, Mayat E, Wenthold RJ: Glutamate receptor subunit 2-selective antibody shows a differential distribution of calcium-impermeable AMPA receptors among populations of neurons. J Comp Neurol 1997; 385:456–76
30. Burnashev N, Monyer H, Seeburg PH, Sakmann B: Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit. Neuron 1992; 8:189–98
31. Verdoorn TA, Burnashev N, Monyer H, Seeburg PH, Sakmann B: Structural determinants of ion flow through recombinant glutamate receptor channels. Science 1991; 252:1715–8
32. Hollmann M, Hartley M, Heinemann S: Ca2+ permeability of KA-AMPA–gated glutamate receptor channels depends on subunit composition. Science 1991; 252:851–3
33. Tanaka H, Grooms SY, Bennett MV, Zukin RS: The AMPAR subunit GluR2: still front and center-stage. Brain Res 2000; 886:190–207
34. Kawahara Y, Ito K, Sun H, Aizawa H, Kanazawa I, Kwak S: Glutamate receptors: RNA editing and death of motor neurons. Nature 2004; 427:801
35. Beattie EC, Stellwagen D, Morishita W, Bresnahan JC, Ha BK, Von Zastrow M, Beattie MS, Malenka RC: Control of synaptic strength by glial TNFalpha. Science 2002; 295:2282–5
36. Ogoshi F, Yin HZ, Kuppumbatti Y, Song B, Amindari S, Weiss JH: Tumor necrosis-factor-alpha (TNF-alpha) induces rapid insertion of Ca2+-permeable alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainate (Ca-A/K) channels in a subset of hippocampal pyramidal neurons. Exp Neurol 2005; 193:384–93
37. Stellwagen D, Beattie EC, Seo JY, Malenka RC: Differential regulation of AMPA receptor and GABA receptor trafficking by tumor necrosis factor-alpha. J Neurosci 2005; 25:3219–28
38. Gorter JA, Petrozzino JJ, Aronica EM, Rosenbaum DM, Opitz T, Bennett MV, Connor JA, Zukin RS: Global ischemia induces downregulation of Glur2 mRNA and increases AMPA receptor-mediated Ca2+ influx in hippocampal CA1 neurons of gerbil. J Neurosci 1997; 17:6179–88
39. Van der Schyf CJ, Gal S, Geldenhuys WJ, Youdim MB: Multifunctional neuroprotective drugs targeting monoamine oxidase inhibition, iron chelation, adenosine receptors, and cholinergic and glutamatergic action for neurodegenerative diseases. Expert Opin Investig Drugs 2006; 15:873–86
40. Haugeto O, Ullensvang K, Levy LM, Chaudhry FA, Honore T, Nielsen M, Lehre KP, Danbolt NC: Brain glutamate transporter proteins form homomultimers. J Biol Chem 1996; 271:27715–22
41. Rothstein JD, Dykes-Hoberg M, Pardo CA, Bristol LA, Jin L, Kuncl RW, Kanai Y, Hediger MA, Wang Y, Schielke JP, Welty DF: Knockout of glutamate transporters reveals a major role for astroglial transport in excitotoxicity and clearance of glutamate. Neuron 1996; 16:675–86
42. Lehre KP, Levy LM, Ottersen OP, Storm-Mathisen J, Danbolt NC: Differential expression of two glial glutamate transporters in the rat brain: quantitative and immunocytochemical observations. J Neurosci 1995; 15:1835–53
43. Levy LM, Warr O, Attwell D: Stoichiometry of the glial glutamate transporter GLT-1 expressed inducibly in a Chinese hamster ovary cell line selected for low endogenous Na+-dependent glutamate uptake. J Neurosci 1998; 18:9620–8
44. Benveniste H, Jorgensen MB, Sandberg M, Christensen T, Hagberg H, Diemer NH: Ischemic damage in hippocampal CA1 is dependent on glutamate release and intact innervation from CA3. J Cereb Blood Flow Metab 1989; 9:629–39
45. Mitani A, Tanaka K: Functional changes of glial glutamate transporter GLT-1 during ischemia: an in vivo study in the hippocampal CA1 of normal mice and mutant mice lacking GLT-1. J Neurosci 2003; 23:7176–82
46. Hamann M, Rossi DJ, Marie H, Attwell D: Knocking out the glial glutamate transporter GLT-1 reduces glutamate uptake but does not affect hippocampal glutamate dynamics in early simulated ischaemia. Eur J Neurosci 2002; 15:308–14
47. Lalo U, Pankratov Y, Kirchhoff F, North RA, Verkhratsky A: NMDA receptors mediate neuron-to-glia signaling in mouse cortical astrocytes. J Neurosci 2006; 26:2673–83
48. Teo KK, Yusuf S, Collins R, Held PH, Peto R: Effects of intravenous magnesium in suspected acute myocardial infarction: Overview of randomised trials. BMJ 1991; 303:1499–503
49. Chi OZ, Pollak P, Weiss HR: Effects of magnesium sulfate and nifedipine on regional cerebral blood flow during middle cerebral artery ligation in the rat. Arch Int Pharmacodyn Ther 1990; 304:196–205
50. Torregrosa G, Perales AJ, Salom JB, Miranda FJ, Barbera MD, Alborch E: Different effects of Mg2+ on endothelin-1- and 5-hydroxytryptamine-elicited responses in goat cerebrovascular bed. J Cardiovasc Pharmacol 1994; 23:1004–10
51. Ram Z, Sadeh M, Shacked I, Sahar A, Hadani M: Magnesium sulfate reverses experimental delayed cerebral vasospasm after subarachnoid hemorrhage in rats. Stroke 1991; 22:922–7
52. Kemp PA, Gardiner SM, March JE, Rubin PC, Bennett T: Assessment of the effects of endothelin-1 and magnesium sulphate on regional blood flows in conscious rats, by the coloured microsphere reference technique. Br J Pharmacol 1999; 126:621–6
53. Reinhart RA, Marx JJ Jr, Broste SK, Haas RG: Myocardial magnesium: relation to laboratory and clinical variables in patients undergoing cardiac surgery. J Am Coll Cardiol 1991; 17:651–6
54. McKee JA, Brewer RP, Macy GE, Phillips-Bute B, Campbell KA, Corell CO, Reynolds JD, Warner DS: Analysis of the brain bioavailability of peripherally administered magnesium sulfate: A study in humans with acute brain injury undergoing prolonged induced hypermagnesemia. Crit Care Med 2005; 33:661–6
55. Saeki H, Matsumoto M, Kaneko S, Tsuruta S, Cui YJ, Ohtake K, Ishida K, Sakabe T: Is intrathecal magnesium sulfate safe and protective against ischemic spinal cord injury in rabbits? Anesth Analg 2004; 99:1805–12
56. Temkin NR, Anderson GD, Winn HR, Ellenbogen RG, Britz GW, Schuster J, Lucas T, Newell DW, Mansfield PN, Machamer JE, Barber J, Dikmen SS: Magnesium sulfate for neuroprotection after traumatic brain injury: A randomised controlled trial. Lancet Neurol 2007; 6:29–38
57. Liu Z, Fan Y, Won SJ, Neumann M, Hu D, Zhou L, Weinstein PR, Liu J: Chronic treatment with minocycline preserves adult new neurons and reduces functional impairment after focal cerebral ischemia. Stroke 2007; 38:146–52
58. Wenq YC, Kriz J: Differential neuroprotective effects of a minocycline-based drug cocktail in transient and permanent focal cerebral ischemia. Exp Neurol 2007; 204:433–42
59. Beattie MS: Inflammation and apoptosis: linked therapeutic targets in spinal cord injury. Trends Mol Med 2004; 10:580–3
© 2008 American Society of Anesthesiologists, Inc.