(Stroke. 1998;29:2141-2148.)
© 1998 American Heart Association, Inc.
YM872, a Highly Water-Soluble AMPA Receptor Antagonist, Preserves the Hemodynamic Penumbra and Reduces Brain Injury After Permanent Focal Ischemia in Rats
Masao Shimizu-Sasamata, PhD;
Tsuneo Kano, MD;
Jadwiga Rogowska, PhD;
Gerald L. Wolf, PhD, MD;
Michael A. Moskowitz, MD;
Eng H. Lo, PhD
From the Departments of Neurology and Radiology, Neuroprotection Research
Laboratory (T.K., E.H.L.); Departments of Neurosurgery and Neurology, Stroke
and Neurovascular Regulation Laboratory (M.S-S., M.A.M.); and Department of
Radiology, Center for Imaging and Pharmaceutical Research (J.R., G.L.W.),
Massachusetts General Hospital, Harvard Medical School, Charlestown, Mass.
Correspondence to Eng H. Lo, PhD, Departments of Neurology and Radiology, Neuroprotection Research Laboratory, Harvard Medical School, Massachusetts General Hospital, 149 13th St, Room 2322, Charlestown, MA 02129. E-mail eng{at}cipr.mgh.harvard.edu
 |
Abstract
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Background and Purpose—We
recently described an image analysis technique based on the
temporal correlation mapping (TCM) of injected contrast agents that can
be used to distinguish the hemodynamic core and
hemodynamic penumbra after focal ischemia. In
this study we used this technique for the first time to investigate the
effects of the water-soluble AMPA receptor antagonist YM872
in permanent focal ischemia.
Methods—Fischer 344 rats were subjected to permanent occlusion of
the middle cerebral artery. Approximately 30 minutes after
ischemia, functional CT images were collected with the use of a
dynamic scanning protocol with bolus injections of nonionic contrast
agent iohexol (1 mL/kg). TCM analysis defined the distributions
of hemodynamic core and hemodynamic
penumbra. Cerebral perfusion indices were calculated on the basis of
the area under the first-pass transit curves. One hour after
ischemia, animals were randomly treated with YM872 (n=8, 20
mg/kg per hour over 4 hours) or normal saline (n=10). Twenty-four hours
later, neurological deficits were evaluated, and conventional CT and
triphenyltetrazolium chloride staining were
used to define volumes of ischemic damage.
Results—At 24 hours after ischemia, hypodense lesions
were visible on conventional CT scans that were highly correlated with
triphenyltetrazolium chloride lesion
volumes. YM872 improved neurological deficits and reduced volumes of
ischemic damage in cortex (90±14 versus 170±16
mm3 in controls) but not striatum (57±14 versus 79±6
mm3 in controls). Comparison of early TCM images with
conventional CT scans of ischemic injury showed that the
hemodynamic core was always damaged in all rats. In
controls, 54% of the tissue within the hemodynamic
penumbra evolved into ischemic damage compared with 24% in
YM872-treated rats. Furthermore, the perfusion index corresponding to
the ischemic damage threshold was significantly reduced by
YM872 (28±2% versus 37±2% in controls).
Conclusions—These results indicate that YM872 is a
neuroprotective compound that ameliorates the deterioration of the
hemodynamic penumbra after focal ischemia.
Key Words: cerebral ischemia, focal • neuroprotection • penumbra • receptor antagonist, AMPA • tomography, emission computed • rats
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Introduction
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In a wide variety of animal models of cerebral
ischemia, AMPA
(
-amino-3-hydroxy-5-methylisoxazole-4-propionic
acid) receptor antagonists have been shown to be
neuroprotective.1 2 3 4 5 6 However, first-generation
compounds were poorly soluble in water,6 which
limited their use in patients. YM872
([2,3-dioxo-7-(1H-imidazol-1-yl)-6-nitro-1,2,3,4-tetrahydro-1-quinoxalinyl]-acetic
acid monohydrate) is a novel AMPA receptor antagonist that
is highly water soluble. In in vitro experiments, YM872 significantly
antagonized kainate neurotoxicity
(IC50=1.1 µmol/L) and decreased
AMPA-induced intracellular calcium accumulation
(IC50=0.83 µmol/L) in rat hippocampal
neurons.7 In addition, YM872 has been shown to reduce brain damage after focal
ischemia in rats8 and
cats.9
Recently, we described a class of image analysis techniques
that are based on the temporal correlation mapping (TCM) of injected
contrast agents into the brain.10 11 We showed
that the TCM approach can quantitatively assess the
hemodynamic gradients that are present after focal
cerebral ischemia by segmenting perfusion patterns into a
hemodynamic core and hemodynamic
penumbra.12 13 In the present study we used
this technique in a rat model of permanent focal ischemia to
test the hypothesis that neuroprotection by YM872 is accompanied by an
attenuation of ischemic injury that otherwise occurs in the
hemodynamic penumbra. Some of these data have been
previously presented in abstract
form.14
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Materials and Methods
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Rat Focal Cerebral Ischemia Model
All procedures were conducted following an institutionally
approved protocol in accordance with guidelines set by the National
Institutes of Health Guide for the Care and Use of Laboratory
Animals. Male Fischer 344 rats (weight, 260 to 330 g; Charles
River) were subjected to focal cerebral ischemia under
halothane anesthesia by means of face masks (induction,
1.5%; maintenance, 1% in 70% N2O/30%
O2). The femoral artery was cannulated for
monitoring mean arterial blood pressure and for sampling
blood gases and pH. The jugular vein was cannulated for contrast agent
injection and drug administration. Both arterial and venous
catheters were then subcutaneously externalized through the dorsal neck
region for easy access and drug delivery (see below). Rectal
temperature was maintained at 37±1°C with a thermostatically
controlled heating pad.
The proximal portion of the left middle cerebral artery (MCA) was
permanently occluded by a microsurgical technique, as described by
Tamura et al.15 16 Briefly, the temporal muscle
was retracted through a trans-retro-orbital approach without removal of
the temporal muscle and zygomatic arch, and a left subtemporal
craniectomy was performed. The dura was incised with a sharp needle,
and the stem of the MCA was electrocauterized just medial to the
olfactory tract, then cut to ensure the completeness of the
occlusion.
Thirty minutes after MCA occlusion, rats were subjected to dynamic CT
scanning to map the hemodynamic core and
hemodynamic penumbra, as described below. Immediately
after CT scanning, rats were returned to their cages. After recovery
from anesthesia (1 hour after ischemia), rats were
randomly treated with either YM872 (20 mg/kg per hour over 4 hours;
n=8) or normal saline (n=10). In this dosing protocol, YM872 did not
induce abnormal behavior including ataxia, hyperactivity, catalepsy,
and agitation. Drug infusion was accomplished under conscious and
freely moving conditions. After surgery, animals were placed in
individual cages for drug administration. The tip of the catheter
coming from the dorsal neck was connected to a cannula swivel device
(375/23, Insstech Laboratories, Inc) in the roof of the
individual cage. This device was used to allow bidirectional rotation
while fluid could be continuously passed between 2 cannulas.
Polyethylene tubing (PE-50) extending from the other end of the device
was joined to a disposable syringe that was fixed to an infusion
apparatus (STC-525, Terumo). This was considered close to
freely moving conditions since the rats could move in all 3 orthogonal
directions. In addition, the cannula for monitoring blood pressure and
blood gases was connected to a cannula swivel and joined to
probe for monitoring blood pressure.
Dynamic CT Scanning Procedure
Rats were inserted into a custom-made head holder and placed
into a slip-ring CT scanner (TCT-900S/X, Toshiba Medical Systems). All
CT images were obtained with 150-mA and 120-kV settings. Sagittal scout
images were collected to localize the brain, and 5 contiguous axial
slices (2 mm thick, 150x150-µm in-plane resolution) were
selected to cover the brain. A dynamic scanning protocol that has been
previously described10 12 13 was used. Briefly,
dynamic scans were collected at a rate of 1 image every second. A
1.0-mL/kg bolus of nonionic contrast agent iohexol (Omnipaque-350,
Sterling-Winthrop) was injected through the jugular vein after 4 to 5
seconds of scanning, and images were collected for 35 seconds total.
The same procedure was then repeated for each of the 5 axial slices.
Therefore, for a typical 300-g rat, this entailed 
1.5 mL of total
contrast administered. Image analysis was performed with
DIPStation software (Hyden Image Processing Group) with custom-designed
modules on a Macintosh platform.
Behavioral Evaluation
Rats were allowed to survive 24 hours after ischemia.
Neurological deficits were then evaluated as described by Bederson et
al17 with some modifications. The following were
assessed: (1) the degree of spontaneous activity, (2) right forepaw
hemiplegia, (3) failure to extend right forepaw when the rat was lifted
by its tail, (4) resistance to lateral push, (5) inclined posture to
the right, (6) circling to the right, and (7) response to vibrissae
touch. Each sign was scored according to the following criteria: grade
0, no abnormality; grade 1, mild abnormality; and grade 2, severe
abnormality. The scores were summed into a total, with the lowest
possible score of 0 and a highest possible score of 14.
Lesion Quantification with Triphenyltetrazolium Chloride Staining
and Conventional CT Scanning
After behavioral observation at 24 hours after ischemia,
rats were reanesthetized with 1.5% halothane in 70%
N2O/30% O2 and placed in
the CT ring for conventional CT scanning. As before, sagittal scout
images were used to localize the brain, and 5 axial slices were imaged
without contrast. Areas of ischemic damage were identified as
hypodense lesions on these 24-hour CT scans and quantified as percent
areas of ipsilateral hemisphere. Immediately after the end of
conventional CT scanning, rats were killed with a lethal
intravenous injection of sodium pentobarbital. Brains were
removed, and the forebrains were sliced into 5 coronal (2-mm) sections
with the use of a rat brain matrix (RBM-2000C, Activational System).
Slices were placed in 2%
triphenyltetrazolium chloride (TTC)
solution, followed by 10% formalin overnight. Infarcted areas were
visualized as regions lacking the typical brick-red staining of normal
brain tissue. These areas were quantified with an image
analysis system (Bioquant IV; R&M Biometrics), and lesion
volumes were calculated by integrating areas in all slices.
Quantitative Analysis of Dynamic CT Data
Each dynamic CT data set describes the cerebral transit profile
of the injected iodinated contrast agent, which remains
restricted to the intravascular compartment during the hyperacute phase
of ischemia. Opening of the blood-brain barrier typically
occurs within minutes of reperfusion after transient
ischemia.18 19 20 With permanent
ischemia, however, disruption of the blood-brain barrier as
assessed by Evans blue permeability does not occur until much later, 12
to 24 hours after occlusion.19 21 In the
present study, contrast CT was performed at a very early stage, 30
minutes after ischemia. No parenchymal leakage of contrast
agents was observed in our experiments.
Alterations in cerebral hemodynamics after focal
ischemia change the shape of the cerebral transit profile.
These hemodynamic alterations were quantitatively
analyzed with TCM, as previously
described.12 13 Briefly, for each pixel in the
brain, a normalized correlation coefficient was calculated with the
transit profile from contralateral cortex used as a normal reference
curve. Each pixel in the resulting TCM image thus has a value that
quantifies how similar the shape of the transit profile is compared
with normal transit profiles in unaffected brain. Statistical
analysis was used to distinguish normal from abnormal
hemodynamics. The first cutoff was set at the minimum
value obtained from the contralateral hemisphere; any pixel in the
ipsilateral hemisphere with correlation coefficients below the minimum
level found in the contralateral side was deemed abnormal and thus part
of the ischemic distribution. A second cutoff was selected on
the basis of a P<0.01 threshold (1-tailed t
distribution) comparing the shape of transit profiles from normal
versus ipsilateral brain pixels. As previously described and validated,
this approach defines the hemodynamic core as regions
with no detectable transit profile and the hemodynamic
penumbra as regions where bolus transit was not eliminated but delayed
so that the shape of the transit profile was different from that in
normal brain. The change in transit profile shape encompasses all
aspects of the curve, including peak height, peak arrival time, and
bolus width. Color look-up tables were constructed to display the TCM
images with normal brain appearing green, the
hemodynamic core appearing black, and the
hemodynamic penumbra appearing as an intermediate
reddish zone surrounding the core.
In addition to the TCM analysis, a cerebral perfusion index for
each image pixel was also calculated on the basis of the area under the
first-pass transit curves. The index is expressed as a percentage of
mean contralateral levels so that 100% is normal and 0%
represents no flow. This approach has been widely used to
indirectly estimate perfusion and includes both blood flow and blood
volume influences.22 For our purposes, this index
was used to compare the thresholds for ischemic damage between
control versus treated rats.
Laser-Doppler Flowmetry of Cerebral Blood Flow in
Normal Brain
In a separate set of experiments, the effects of YM872 on
regional cerebral blood flow were determined in normal
nonischemic brain with the use of laser-Doppler
flowmetry. Under halothane anesthesia, catheters
were placed into femoral arteries and veins in Fischer rats (n=10).
Laser-Doppler fiberoptic flow probes (Omega FLO-N1, Neuroscience
Instruments) were positioned onto the parietal cortex (from bregma:
3 mm lateral, 3 mm posterior). Care was taken to ensure that
the probes were placed away from large surface vessels. These rats were
then randomly infused with either normal saline or YM872 (20 mg/kg per
hour over 4 hours). Changes in heart rate, mean arterial
blood pressure, and laser-Doppler cerebral blood flow were
monitored. Laser-Doppler blood flow values were expressed as a
percentage of predrug baseline levels.
Statistical Comparisons
Data were expressed as mean±SEM. Comparisons of lesion size
between controls and treated animals were performed with unpaired
2-tailed Student's t tests. Multiple comparisons of
systemic parameters were performed with ANOVA. Linear
regression analysis was used to examine the relationship
between conventional CT and TTC lesion volumes. Neurological scores
were compared with nonparametric Mann-Whitney tests. Values
of P<0.05 were considered statistically significant.
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Results
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Systemic Parameters
Physiological parameters including
blood pressure, heart rate, blood gases, pH, and rectal temperature for
both groups were within normal limits before MCA occlusion, 5 minutes
after the end of drug administration, and 24 hours after MCA occlusion
(Table
). YM872 did not appear to have any
effects on temperature or blood pressure.
Volume of Ischemic Damage at 24 Hours
YM872, when administered 1 hour after ischemia,
significantly reduced the total volume of ischemic brain damage
measured at 24 hours with TTC staining (P<0.01) (Figure 1). Most of the neuroprotection was found
in the cortex, where ischemic damage was reduced by almost 47%
(P<0.01). Lesion sizes were not significantly different in
the striatum. Conventional CT scans also showed clearly demarcated
regions of hypodensity that were highly correlated with TTC lesion
areas (Figure 2).
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Figure 1. Neuroprotective effects of YM872 (20 mg/kg per
hour over 4 hours) on the volume of ischemic brain damage at 24
hours after MCA occlusion. YM872 reduced the volume of the
ischemic damage in the cerebral cortex but not in the striatum.
**P<0.01 (2-tailed t test comparing
controls vs YM872-treated rats).
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Figure 2. Linear regression analysis demonstrates
the significant correlation (P<0.05) between areas of
ischemic tissue damage (TTC staining) and area of hypodense
lesions (conventional CT scanning) at 24 hours after MCA occlusion.
y=0.98x-0.82 (for controls);
y=0.81x+2.14 (for YM872-treated
rats).
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Neurological Deficits and Lack of Renal Precipitation of
YM872
Neurological deficits were reduced by YM872 treatment at 24 hours
after MCA occlusion (5.7±0.8 versus 9.4±0.5 in controls;
P<0.01). Specifically, YM872-treated animals showed
improved right forepaw extension and posture and less hemiplegia. Under
light microscopy of hematoxylin-eosin–stained sections, no crystals of
YM872 were found in the kidney medulla or cortex in the rats
tested.
TCM and Perfusion Index Analysis
TCM analysis showed high and stable correlation
coefficients in the contralateral hemisphere where
hemodynamics would be normal, as expected, and there
were no differences in hemodynamic patterns between
controls and YM872-treated rats. In the ipsilateral hemisphere, TCM
images obtained at 30 minutes after ischemia showed
hemodynamic core regions located primarily in the
striatum and ventral cortex surrounded by regions of
hemodynamic penumbra that typically extended into the
overlying dorsolateral cortex (Figure 3
and Figure 4A and 4B). Comparison of
early TCM images at 30 minutes after MCA occlusion with conventional CT
scans of ischemic injury at 24 hours showed that the
hemodynamic core was always damaged in all rats (Figure 3 and Figure 4A through 4C). In contrast, only 54±10% of the
hemodynamic penumbra decayed into ischemic
injury over 24 hours in untreated controls in this model (Figure 4C).
YM872 appeared to significantly (P<0.01) ameliorate this
process so that only 24±6% of the hemodynamic
penumbra became damaged in treated rats (Figure 4C).
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Figure 3. Representative examples of TCM
images (top and bottom left) and TTC-stained sections (top and bottom
right) in control and YM872-treated rats. For the TCM images, normal
hemodynamic status is shown as gradations of green.
Regions with abnormal hemodynamics are
represented with a red-to-black sliding color scale.
Therefore, the hemodynamic core appears black, and
hemodynamic penumbra is the reddish rim surrounding the
core (see Materials and Methods for details). Note that 30 minutes
after focal ischemia, an extensive hemodynamic
penumbral region is still present. By 24 hours after occlusion,
most of the hemodynamic penumbra in control rats has
become damaged, as seen in the matching TTC-stained sections. However,
in YM872-treated rats, the regions corresponding to the
hemodynamic penumbra are relatively preserved over
time. The tissue missing in the TTC sections is an artifact due to
damage during processing.
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In control rats, brain regions with a cerebral perfusion index
<37±2% became damaged at 24 hours after ischemia. YM872
significantly lowered this threshold for ischemic damage to
28±2% (P<0.05) (Figure 5).
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Figure 5. Effects of YM872 on the relative cerebral
perfusion index threshold required for ischemic damage. In
control rats, brain regions with perfusion index levels <37% of mean
levels in the contralateral hemisphere at 30 minutes after
ischemia corresponded to areas of ischemic injury at 24
hours. In YM872-treated rats, perfusion index thresholds were
significantly reduced, and only regions with perfusion index levels
<28% corresponded to damaged tissue at 24 hours.
*P<0.05 (2-tailed t test comparing
controls vs YM872-treated rats).
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Effects of YM872 on Normal Cerebral Blood Flow
In normal nonischemic brain, YM872 did not alter cerebral
blood flow as measured with laser-Doppler flowmetry (Figure 6). There were also no detectable effects
on heart rates or mean arterial blood pressure. Heart rates
were 444±5 in controls and 436±13 in YM872-treated rats. Blood
pressures were 118±4 mm Hg in controls and 122±5 mm Hg in
YM872-treated rats.
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Figure 6. No effects of YM872 on regional cerebral blood
flow were detected as measured by laser-Doppler flowmetry
in normal nonischemic brain. Control rats treated with normal
saline (n=5) were compared with those infused with 20 mg/kg per hour of
YM872 intravenously over 4 hours (n=5).
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Discussion
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Blockade of the AMPA-type glutamate receptor appears to be a
promising approach for treating for acute ischemic
stroke.3 However, most first-generation compounds
were nephrotoxic as a result of their poor solubility in water. For
example, NBQX
(2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-F-quinoxalone) has been
shown to precipitate into the renal tubules after
intravenous administration.6 The
compound used in the present study, YM872, is a
quinoxallinedione-type AMPA receptor antagonist that is
highly water soluble.7 Others have previously
shown that YM872 is neuroprotective after focal ischemia in
rats8 and cats.9 Here we
provide evidence that the neuroprotective properties of YM872 may be
reflected by its ability to ameliorate the deterioration of the
hemodynamic penumbra after permanent focal
ischemia in rats. Moreover, YM872 also significantly decreased
neurological deficits compared with untreated controls, suggesting that
histological neuroprotection may be associated with
functional improvement as well. Whether the changes we observed reflect
a direct action of YM872 on the vasculature or indeed reflect AMPA
receptor antagonist action in the parenchyma requires
further study. However, YM872 did not alter blood flow in normal
nonischemic brain.
The evolution of brain damage after focal ischemia
follows a complex spatiotemporal profile.23 In
central or core regions with severe deficits in cerebral blood flow, a
rapid progression to irreversible pannecrosis typically occurs.
However, in the peripheral or penumbral zones where the
ischemic insult may be moderate or mild, tissue damage may
evolve more slowly and gradually over several hours or even
days.23 24 25 26 While salvage of the core may not be
possible without the return of blood flow, targeting 1 or more steps in
the ischemic cascade within the penumbra constitutes a rational
strategy for stroke therapy. It is therefore important to develop
methods that can directly and quantitatively assess neuroprotection in
the ischemic core and penumbra.
We have previously described an image analysis approach based
on the TCM of injected boluses of contrast agents into the
brain.10 11 Our previous studies showed that this
approach was able to spatially resolve the hemodynamic
core and the hemodynamic penumbra after focal
ischemia.12 13 The
hemodynamic core was operationally defined as regions
with ischemia so severe that no detectable transit profiles of
injected contrast agents were observed. The hemodynamic
penumbra was operationally defined as regions where the shapes of the
transit profiles were significantly different from those found in
normal brain. In the hemodynamic penumbra, cerebral
perfusion levels were typically in the 30% to 40% range compared with
normal or contralateral levels.12 13 However,
since the TCM approach measures alterations in overall
hemodynamics and not absolute blood flow rates per se,
it is likely that the hemodynamic penumbra will be
mainly composed of regions where vasodilation and/or collateral
recruitment have combined to compensate and alter the shape of the
cerebral transit profile.27 28 29 30 This idea is
supported in part by results obtained in knockout mice deficient in
endothelial nitric oxide production. These
animals show more severe ischemia and smaller
hemodynamic penumbras than wild-type mice after focal
ischemia.12 Nitric oxide generated by the
endothelium promotes vasodilation and/or collateral
recruitment,31 32 and removal of this source
results in larger cores and more restricted hemodynamic
penumbras in the knockout mice.
In the present study we coupled TCM analysis to dynamic CT
scans with bolus contrast injections to examine the neuroprotective
effects of the AMPA antagonist YM872 in a rat model of
permanent focal ischemia. Infarct volumes at 24 hours assessed
with both TTC staining and conventional CT imaging of hypodense lesions
showed that treatment with YM872 at 1 hour after ischemia led
to significant neuroprotection. When these late measurements were
compared with the early (30 minutes after ischemia) TCM images,
the tissue corresponding to entire hemodynamic core had
completed the transition into ischemic damage in all 18 rats by
24 hours. In contrast, only 54% of the hemodynamic
penumbra became damaged in control rats. In YM872-treated rats,
preservation of the hemodynamic penumbra was evident;
only 24% of the tissue had progressed to ischemic damage by 24
hours. When cerebral perfusion indices were calculated, the threshold
corresponding to ischemic damage was also significantly reduced
by YM872 from 37% in controls to 28% in treated rats. The perfusion
threshold of 37% is higher than ischemic blood flow thresholds
that have been measured by others in rat focal ischemia.
Ginsberg and colleagues33 have reported that for
a P<0.04 probability of ischemic damage, the flow
threshold was 20% of contralateral levels. For a P<0.08
probability of ischemic damage, the threshold was higher, ie,
30% of contralateral levels. Two critical differences between this
study and ours should be noted. First, Ginsberg's group used a
transient 2-hour occlusion, whereas we used a permanent occlusion of
the MCA. Thus, it is conceivable that our thresholds are slightly
higher since our ischemic insults were more severe. Second, our
perfusion index includes a complex mix of blood flow and blood volume
influences. Therefore, it cannot be directly compared with the
"pure" measurements of blood flow that were conducted by Ginsberg
and colleagues. This limitation of the dynamic bolus imaging approach
is well known.22
The results from this study are consistent with the idea that
gradients in perfusion and tissue injury exist after focal
ischemia,34 35 36 37 and these gradients can
provide a useful index for assessing long-term tissue viability in the
presence or absence of treatment. In a previous study we reached
similar conclusions using a different analytical approach and imaging
modality. Apparent diffusion coefficient probability distribution
functions derived from diffusion-weighted MRI showed that gradients in
cell swelling existed after focal ischemia in rats, and
successful treatment with a glutamate antagonist
ameliorated the worsening in apparent diffusion coefficient gradients
over time.38
The TCM method is highly sensitive to but not specific for each of the
myriad hemodynamic effects of arterial
occlusion. As discussed above, these include effects on blood flow and
blood volume, compensatory vasodilation, and collateral recruitment.
Therefore, the hemodynamic penumbra as defined here
most likely differs from the penumbra defined with the use of absolute
blood flow values.25 26 39 The high sensitivity
of the technique may be one reason why, even in untreated rats, only
54% of the hemodynamic penumbra decays into regions
of ischemic damage by 24 hours after ischemia. This may
be due to an overestimation of what constituted a potentially lethal
ischemic challenge, or alternatively, this may indicate that
further delayed decay of the hemodynamic penumbra might
occur over periods >24 hours.24 Many recent
efforts have been aimed at obtaining truly quantitative in vivo imaging
measurements of either blood flow40 41 42 or blood
volume43 44 in the brain. It will be critical for
future studies to assess neuroprotection in the penumbra using these
quantitative blood flow and/or blood volume parameters in
vivo.
In conclusion, we have demonstrated that early TCM analysis may
be used to directly examine the effects of neuroprotective therapy in
the hemodynamic penumbra. These findings provide
evidence that the AMPA receptor antagonist YM872
ameliorates the deterioration of the hemodynamic
penumbra after focal ischemia and reduces the perfusion
threshold for ischemic damage.
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Acknowledgments
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This study was supported in part by NIH grant NS32806 (Dr
Lo), American Heart Association Grant 95-011-360 (Dr Lo), NIH
grant NS10828 (Dr Moskowitz), a grant from the Whitaker
Foundation (Dr Rogowska), and a grant from Yamanouchi Pharmaceuticals
Co Ltd (Dr Shimizu-Sasamata). The authors thank Allen Pierce, Marek
Trocha, and Mary-Theresa Shore for assistance with the CT scanning
protocols.
Received April 14, 1998;
revision received July 14, 1998;
accepted July 14, 1998.
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Editorial Comment
Costantino Iadecola, MD, Guest Editor
Laboratory
of Cerebrovascular Biology and Stroke,
Department of Neurology,
University of Minnesota,
Minneapolis, Minnesota
|
Introduction
|
|---|
Pharmacological inhibition of glutamate receptors is a
well-established experimental strategy for neuroprotection following
cerebral ischemia.1 Glutamate receptors include NMDA, AMPA,
kainate, and metabotropic receptors.2 NMDA receptor
antagonists, while conferring substantial protection in animal models
of focal cerebral ischemia, may be of limited clinical usefulness
because of their psychomimetic effects.3 The discovery by
Sheardown et al4 that AMPA receptor antagonists reduce
cerebral ischemic damage raised the possibility that inhibition of AMPA
receptors could be useful in the therapy of ischemic stroke. However,
the first generation of AMPA receptor antagonists were difficult to use
in vivo because their poor water solubility resulted in precipitation
in the kidneys and nephrotoxicity.5
In the accompanying article, Shimizu-Sasamata and colleagues
demonstrate that the water-soluble AMPA receptor antagonist YM872
reduces brain damage and neurological deficits in a rat model of
permanent focal cerebral ischemia. Using a recently introduced dynamic
CT scanning technique, they were able to obtain a qualitative estimate
of cerebral blood flow in the ischemic territory and to correlate the
degree of flow reduction with tissue outcome in a topographic fashion.
They found that in regions surrounding the ischemic core, comparable
degrees of ischemia resulted in brain damage in untreated rats but not
in rats treated with YM872. In addition, they demonstrated that YM872
does not influence resting cerebral blood flow in intact rats,
indicating that effects of YM872 on postischemic blood flow are
unlikely to play a role in the mechanism of the protection. These
observations, collectively, suggest that YM872 renders the brain tissue
more resistant to the deleterious effects of cerebral ischemia.
Activation of glutamate receptors is thought to contribute to ischemic
injury by increasing intracellular calcium concentration, which in turn
leads to cell death by activating an array of destructive enzymatic
systems.6 However, only a small subset of AMPA receptors is
highly permeable to calcium.2 Therefore, the mechanisms of
the protection exerted by AMPA receptor antagonists is not entirely
clear. One possibility is that activation of AMPA receptors increases
intracellular calcium indirectly, for example, through voltage-gated
calcium channels activated by depolarization or by reverse operation of
the sodium-calcium exchanger.1 Increases in calcium
permeability may also result from disruption of editing at the
"Q/R" site of the GluR2 subunit of the AMPA receptor or from
decreases in the expression of the GluR2 subunit itself.7
Irrespective of the mechanisms of the effect, the careful and
well-controlled study of Shimizu-Sasamata et al provides convincing
evidence that water-soluble AMPA receptor antagonists are promising
compounds for the treatment of ischemic brain injury.
Received April 14, 1998;
revision received July 14, 1998;
accepted July 14, 1998.
|
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Abstract
Introduction
). C, In untreated rats, all of
the hemodynamic core and 
