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The Journal of Neuroscience, March 1, 1999, 19(5):1657-1662
Ischemic Tolerance in Murine Cortical Cell Culture: Critical Role
for NMDA Receptors
Margaret C.
Grabb and
Dennis W.
Choi
Center for the Study of Nervous System Injury and Department of
Neurology, Washington University School of Medicine, St. Louis,
Missouri 63110
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ABSTRACT |
Murine cortical cultures containing both neurons and glia (days
in vitro 13-15) were exposed to periods of
oxygen-glucose deprivation (5-30 min) too brief to induce neuronal
death. Cultures "preconditioned" by sublethal oxygen-glucose
deprivation exhibited 30-50% less neuronal death than controls when
exposed to a 45-55 min period of oxygen-glucose deprivation 24 hr
later. This preconditioning-induced neuroprotection was specific
in that neuronal death induced by exposure to excitotoxins or to
staurosporine was not attenuated. Neuroprotection was lost if the time
between the preconditioning and severe insult were decreased to 7 hr or
increased to 72 hr and was blocked if the NMDA antagonist 100 µM
3-((D)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid
was applied during the preconditioning insult. This was true even if
the duration of preconditioning was increased as far as possible (while
still remaining sublethal). A similar preconditioning effect was also
produced by sublethal exposure to high K+,
glutamate, or NMDA but not to kainate or
trans-1-aminocyclopentane-1,3-dicarboxylic acid.
Key words:
glutamate; cerebral preconditioning; NMDA; kainate; metabotropic; ischemia
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INTRODUCTION |
Kitagawa et al. (1990) found that
gerbils subjected to sublethal transient global ischemia exhibited
reduced hippocampal CA1 neuronal death after a more severe ischemic
insult 24-48 hr later, a phenomenon they called "ischemic
tolerance." Similar findings were reported by others (Liu et al.,
1992 ; Simon et al., 1993 ; Gidday et al., 1994 ; Glazier et al., 1994 ;
Chen et al., 1996 ; Cai et al., 1997 ). A neuroprotective effect was also
induced in vivo by application of KCl (Kawahara et al.,
1995 ; Kobayashi et al., 1995 ; Matsushima et al., 1996 ; Taga et al.,
1997 ) or systemic injection of the mitochondrial inhibitor
3-nitroproprionic acid (3-NPA) (Riepe et al., 1997 ).
The mechanistic basis of ischemic tolerance has not been fully
delineated. A role for NMDA receptor activation was suggested by Kato
et al. (1992) , because MK801 blocked the development of ischemic
tolerance in gerbils (although as those authors pointed out, this
treatment likely attenuated the preconditioning insult itself). In
addition, Kasischke et al. (1996) made the curious observation that
3-NPA-induced preconditioning in hippocampal slices could be abolished
by exposing the slices to an NMDA antagonist before hypoxic test
challenge. Recently, Heurteaux et al. (1995) suggested that the
activation of adenosine A1 receptors or KATP channels may
be involved.
Alterations in gene expression are likely also required for the
development of ischemic tolerance. Expression of c-JUN increases in CA1
neurons after preconditioning ischemia (Sommer et al., 1995 ), possibly
contributing to the induction of other survival proteins such as HSP70
(Kirino et al., 1991 ; Kitagawa et al., 1991 ; Liu et al., 1993 ; Glazier
et al., 1994 ; Chen et al., 1996 ; but see Abe and Nowak, 1996 ),
superoxide dismutase (Toyoda et al., 1997 ), and bcl-2 (Shimazaki et
al., 1994 ). Furthermore, ischemic stress enhances expression of the
glucose transporter GLUT1 in microvessels and astrocytes (McCall et
al., 1996 ), heme-oxygenase-1 in neurons and glia (Paschen et al., 1994 ;
Geddes et al., 1996 ; Takeda et al., 1996 ), and growth factors in
neurons, astrocytes, and endothelial cells (Takeda et al., 1993 ;
Krupinski et al., 1996 ; Lee et al., 1996 ; Lin et al., 1997 ; Cobbs et
al., 1998 ).
To gain leverage on elucidating underlying mechanisms, several
investigators have modeled ischemic tolerance in vitro.
Murine cortical cell cultures exposed to sublethal oxygen-glucose
deprivation exhibited reduced neuronal death after subsequent more
severe test challenge the next day (Grabb and Choi, 1995 ). Sakaki et al. (1995) described a neuroprotective effect of sublethal hypoxia on
hypoxic neuronal death in young [days in vitro (DIV)
1-3] cortical cultures and correlated this effect with increased
basic fibroblast growth factor expression. Recently, Bruer et al.
(1997) described a neuroprotective effect of sublethal oxygen-glucose
deprivation or Na+/K+ ATPase
inhibition in cortical cultures. Neuronal death in this latter study
predominantly exhibited apoptosis, perhaps reflecting the relatively
young cultures used (DIV 10-12), a feature likely to attenuate
excitotoxicity (Choi et al., 1987 ; Frandsen and Schousboe, 1990 ) and
potentiate apoptosis (McDonald et al., 1997 ).
We report here a pharmacological characterization of ischemic
tolerance in murine cortical cell cultures, aimed at testing the
hypothesis that NMDA receptor activation is specifically required for
its induction.
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MATERIALS AND METHODS |
Cortical cell culture preparation. Brain cortices
were dissected from fetal Swiss Webster mice (14-16 d gestation) and
plated at a density of 3.75 hemispheres per 24 well culture, on
established glial monolayers (see below), as previously described (Rose
et al., 1993 ). The plating medium consisted of Eagle's Minimum
Essential Medium (MEM; Life Technologies, Gaithersburg, MD) lacking
bicarbonate and glutamine, supplemented with 5% fetal bovine serum,
5% horse serum (serum was obtained from HyClone, Logan, UT), 2 mM glutamine, 26.2 mM bicarbonate, and glucose
added to bring the solution to a final glucose concentration of 20 mM. Cells were fed twice a week with MEM containing 5%
horse serum, and at DIV 5-7 they were treated with cytosine
arabinoside (10 µM) to eliminate the dividing microglia
from the cultures. The cultures were used for experiments at 13-15 d
in culture.
Glial cultures were prepared using the same procedure, except the brain
cortices were dissected from mice, postnatal 1-3 d, in plating medium
similar to above (with 10% horse serum, 10% fetal bovine serum, and
10 nM epidermal growth factor). Neurons were plated on
glial cultures after the glia had become confluent, ~2-4 weeks in culture.
All chemicals used were obtained from Sigma (St. Louis, MO) except
where noted.
Oxygen-glucose deprivation. Mixed cortical cultures were
subjected to oxygen-glucose deprivation injury using protocols
previously described (Goldberg and Choi, 1993 ). Cultures were placed in
an anaerobic chamber (Forma Scientific) and washed three times with balanced salt solution (BSS: 116 mM NaCl, 5.4 mM KCl, 0.8 mM MgSO4, 1.0 mM NaH2PO4, 26.2 mM NaHCO3, 1.8 mM
CaCl2, 0.01 mM glycine, and 10 mg/l
phenol red) lacking glucose and aerated with an anaerobic gas mix (85%
N2/5% CO2/10%
H2) to remove residual oxygen. Cell cultures were
incubated in the solution at 37°C for a designated period to produce
either mild or lethal oxygen-glucose deprivation. Control cell
cultures not deprived of oxygen and glucose were placed in BSS
containing 20 mM glucose, and not bubbled with anaerobic gas, during the experiment. To terminate the oxygen-glucose
deprivation, cells were carefully washed with MEM (containing 2 mM glutamine, 20 mM glucose, and 10 µM glycine) and then removed from the anaerobic chamber.
For preconditioning, cortical cultures were deprived of oxygen and
glucose for 5-30 min, an insult that did not induce neuronal death, as
measured by lactate dehydrogenase (LDH) release or trypan blue
staining. In some cultures, the NMDA receptor antagonist 3-((D)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid
(D-CPP, 100-300 µM; Research Biochemicals,
Natick, MA) was used to block NMDA receptor activation during the
preconditioning episode, and in a subset of these cultures, mild
ischemia was extended to 40-50 min. At a designated period after the
preconditioning stimulus (7, 24, 48, or 72 hr), the cultures were again
deprived of oxygen and glucose, this time for a period long enough to
cause ~50% neuronal death in control cell cultures (45-55 min),
where the longer deprivation periods were used with less dense
cultures. We have previously noted that the neuronal death that occurs
after oxygen-glucose deprivation varies somewhat as a function of
neuronal density, with high-density cultures exhibiting greater
vulnerability than low-density cultures (Goldberg and Choi, 1993 ; and
unpublished observations). The cultures used within any given
experiment (e.g., preconditioned vs control) were always sister
cultures derived from a single plating and hence were exposed to a
single insult duration.
Excitatory amino acid exposure. Mixed cortical cultures were
subjected to excitotoxic injury by treatment with glutamate or the
glutamate receptor agonists kainate and NMDA. To induce ~50% neuronal death, brief exposure to glutamate (200 µM) or
NMDA (300 µM) was performed at room temperature by
rapidly exchanging the growth media with a HEPES-buffered balanced salt
solution (HBBSS) containing 5.5 mM glucose (we lowered the
glucose for 5-15 min exposures) and 10 µM glycine for 5 min. To terminate the exposure, cultures were washed by triple exchange
of the media with MEM (containing glutamine and glycine). For lethal
kainate exposures, 50 µM kainate and 10 µM
MK801 (Research Biochemicals; used to block secondary NMDA
receptor-induced neuronal death) were applied to the cells for the full
24 hr in an MEM-based solution.
Sublethal exposures of glutamate (10, 20, and 30 µM),
NMDA (5, 7.5, and 10 µM), or kainate (10, 30, and 45 µM) were applied in a BSS solution containing 20 mM glucose for 30 min at 37°C, 24 hr before oxygen
glucose deprivation. The highest concentrations of each agonist tested
represented maximal sublethal exposures, i.e., the most intense
exposures that did not alone induce neuronal cell death. In addition,
the broad spectrum mGluR agonist
trans-1-aminocyclopentane-1,3-dicarboxylic acid
(trans-ACPD, 100-200 µM; Research
Biochemicals) was added to the cultures using the same paradigm.
Potassium-induced depolarization. HBBSS, containing 45 mM KCl and 76.4 mM NaCl, was added to cortical
cultures at room temperature for 15 min. Cultures were then washed back
into the MEM-based solution. The NMDA receptor antagonist
D-CPP (100 µM) was used to block NMDA
receptor activation during the preconditioning episode in a subset of cultures.
Staurosporine-induced apoptosis. Cell cultures were exposed
to staurosporine (150 nM) in an MEM-based solution to
induce apoptotic cell death in neurons (Koh et al., 1995 ). To confirm
the neuronal death as apoptotic, the protein synthesis inhibitor
cycloheximide (0.5 µg/ml) was administered with staurosporine in an
additional set of cultured cells.
Quantitation of neuronal injury. Neuronal injury was
assessed by examination of the neurons with phase-contrast microscopy and by measurement of LDH efflux into the bathing medium 24 hr after
every sublethal and lethal insult (excluding neuronal apoptosis, which
was assessed after 48 hr). It has previously been established that LDH
release correlates linearly with the number of damaged or dying neurons
after both excitotoxic (Koh and Choi, 1987 ) and apoptotic injuries (Koh
et al., 1995 ). Sister cultures were treated with 300 µM
NMDA for 24 hr to induce near complete neuronal death without glial death.
LDH values of maximally injured, preconditioned, and control cultures
were compared in all experiments. In addition, cell death was confirmed
in some experiments by the trypan blue exclusion assay.
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RESULTS |
As previously reported, cultures exposed to 45-55 min of combined
oxygen-glucose deprivation developed widespread neuronal cell body
swelling by the end of the exposure period and widespread neuronal
degeneration without glial degeneration by 24 hr later (Goldberg and
Choi, 1993 ). In contrast, cultures "preconditioned" by previous
nonlethal exposure to 20-30 min oxygen-glucose deprivation on the
previous day exhibited only approximately half as much neuronal death
24 hr after the same 45-55 min test challenge (Fig. 1). This protective effect of
preconditioning persisted when cultures were examined 48 hr after test
challenge (data not shown). The protective efficacy of preconditioning
exposure depended on both the duration of preconditioning exposure
(Fig. 2; maximal protective effect was
achieved with 30 min of preconditioning, because 40-50 min exposures
became intrinsically damaging) and the interval between preconditioning
exposure and test challenge. Specifically, no protective effect was
seen with an interval between preconditioning and test challenge of 7 or 72 hr (Fig. 3).

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Figure 1.
Morphological evidence for preconditioning
in vitro. A, Phase-contrast micrograph of
control cortical cultures 24 hr after sham wash. Scale bar, 50 µm.
B, Same but 24 hr after 50 min oxygen-glucose
deprivation. C, Sister cultures exposed to the same
insult as in B, but preconditioned with a previous 30 min exposure to oxygen-glucose deprivation 24 hr before the test
insult.
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Figure 2.
Neuroprotection is dependent on the duration of
preconditioning exposure. Cortical cultures were preconditioned with
5-30 min of oxygen-glucose deprivation as indicated and then exposed
24 hr later to oxygen glucose deprivation for 45-55 min. Twenty-four
hours later, resultant neuronal death was assessed by LDH release to
the bathing medium, scaled to the release associated with near complete
neuronal death without glial death (=100, induced by exposure to 300 µM NMDA for 24 hr). Mean LDH + SE is displayed;
n = 8-20 cultures per condition pooled from five
experiments. *p < 0.05 versus control (no
preconditioning) by one-way ANOVA followed by Student-Newman-Keuls
multiple comparisons test.
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Figure 3.
Neuronal protection is dependent on the time
between the two episodes of oxygen-glucose deprivation
(OGD). The timing between the preconditioning
oxygen-glucose deprivation (10-30 min) and severe oxygen-glucose
deprivation was varied from 7 to 72 hr. Mean + SE is displayed;
n = 12-20 cultures per condition pooled from three
to five experiments. *p < 0.05 versus control (no
preconditioning) by one-way ANOVA followed by Student-Newman-Keuls
multiple comparisons test.
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To determine whether the same preconditioning stimulus could protect
neurons from other forms of excitotoxic injury, cultures were exposed
to toxic levels of NMDA (300 µM for 5 min), kainate (50 µM with 10 µM MK801 for 24 hr) or glutamate
(200 µM for 5 min) after a 10-30 min preconditioning
exposure 24 hr earlier. In each case, the excitotoxin exposure was
chosen to produce ~50% neuronal death in control cultures. This
preconditioning exposure produced no significant changes in either
rapidly triggered NMDA- or glutamate-induced excitotoxic neuronal death
or slowly triggered kainate-induced excitotoxic neuronal death (Table
1).
We next examined whether preconditioning exposure to oxygen-glucose
deprivation could attenuate neuronal vulnerability to apoptotic cell
death. Control cultures exposed for 24 hr to the nonspecific kinase
inhibitor staurosporine (150 nM) developed intermediate
levels of neuronal apoptosis by the end of this exposure, characterized
by cell body shrinkage, internucleosomal DNA fragmentation, and
sensitivity to protein synthesis inhibitors or caspase inhibitors (Falcieri et al., 1993 ; Jacobson and Raff, 1995 ; Koh et al., 1995 ). Optimal preconditioning exposure (20-30 min 24 hr earlier) did not
alter this staurosporine-induced apoptosis (Fig.
4).

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Figure 4.
Preconditioning does not protect neurons from
staurosporine-induced apoptosis. Cultures were preconditioned with 20 or 30 min oxygen-glucose deprivation and then 24 hr later were exposed
to 150 nM staurosporine for 48 hr. Neuronal death was
assessed at the end of the staurosporine exposure by LDH release to the
bathing medium (mean + SE; n = 8 cultures per
condition pooled from 2 experiments). *p < 0.05 versus control (no preconditioning) by one-way ANOVA followed by
Student-Newman-Keuls multiple comparisons test. Concurrent addition
of 0.5 µg/ml cycloheximide (CHX), but not
preconditioning, attenuated staurosporine-induced apoptosis.
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Although combined oxygen-glucose deprivation produces multiple
perturbations that could account for the observed preconditioning effect, we tested the hypothesis that the associated excitotoxic stress
bore primary responsibility. Therefore, we tried to induce tolerance by
exposing cultures for 30 min to maximal sublethal concentrations of
NMDA (5-10 µM) or kainate (10-45 µM), as
well as 100-200 µM trans-ACPD (a nontoxic
agonist), 24 hr before oxygen-glucose deprivation (Table
2). Application of NMDA did induce a mild protective effect in the cortical cultures; however,
trans-ACPD or kainate did not. In fact, kainate pretreatment
actually enhanced the death induced by subsequent oxygen-glucose
deprivation. Glutamate application for 30 min (10-20 µM)
required a change in the standard preconditioning protocol, because
glutamate levels dropped to 50% of the original levels in the culture
media after 10 min of 30 µM glutamate exposure as
assessed by HPLC analysis (M. C. Grabb, D. Lobner, and D. W. Choi,
unpublished observations), presumably reflecting cellular uptake.
Therefore, glutamate was added again at 10 and 20 min after the initial
exposure onset, to a final media concentration of 10 µM.
Both an initial concentration of 10 µM and an initial
concentration of 20 µM glutamate produced a mild
neuroprotective effect against subsequent oxygen-glucose deprivation
challenge, although the latter higher concentration of glutamate did
itself produce some neuronal cell death (Table 2).
Preconditioning-induced neuroprotection could also be induced by the
application of 45 mM K+ for 15 min, an
exposure that was just below a toxic level in most cultures (Fig.
5). This concentration of KCl was
selected on the basis of dose-finding experiments. Application of 60 mM KCl itself produced >10% neuronal death; 30 mM KCl did not induce tolerance (data not shown).

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Figure 5.
An NMDA antagonist blocks preconditioning
in vitro. Cells were preconditioned with 30 min of
oxygen-glucose deprivation (OGD) or 15 min of 45 mM KCl, either alone or with 100 µM
D-CPP in the bathing media; any added drug was washed out
at the end of the preconditioning period. Twenty-four hours later,
cells were exposed to test challenge (oxygen-glucose deprivation for
45-55 min). D-CPP alone (added for 30 min and then washed
out 24 hr before test challenge) did not reduce neuronal death.
Neuronal death was assessed 24 hr later by LDH release to the bathing
medium (mean + SE; n = 16 cultures per condition
pooled from 4 experiments). *p < 0.05 versus
control (no preconditioning) by one-way ANOVA followed by
Student-Newman-Keuls multiple comparisons test.
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Application of the NMDA receptor antagonist D-CPP (100 µM) during the preconditioning exposure to high
K+ or sublethal oxygen-glucose deprivation
completely blocked the subsequent development of resistance to
oxygen-glucose deprivation-induced death (Fig. 5). Application of
D-CPP alone without high K+ or
sublethal oxygen-glucose deprivation did not itself induce a
preconditioning effect (Fig. 5). In addition, the competitive NMDA
receptor antagonist D-( )-2-amino-5-phosphonopentanoic
acid also blocked the preconditioning effect at a concentration of 100 µM (data not shown).
To test the possibility that this NMDA antagonist effect was explained
by a nonspecific reduction in preconditioning injury, we examined
whether increasing the duration of preconditioning exposure could
overcome the ability of D-CPP to block
preconditioning-induced neuroprotection. We found that a
preconditioning exposure duration of 50 min was the longest that could
be tolerated without itself producing lethal injury, even if the
concentration of D-CPP were increased to 300 µM to ensure that NMDA receptors were fully blocked (the
D-CPP concentration-response relationship for
neuroprotection against oxygen-glucose deprivation in our system
reaches maximum at 100 µM drug; data not shown). However,
even this just sublethal preconditioning stress, minus NMDA receptor
activation, failed to produce any protective effect against a
subsequent lethal oxygen-glucose deprivation insult (data not shown).
 |
DISCUSSION |
We describe here a cell culture model of ischemic tolerance, in
which mouse cortical neurons preconditioned by a sublethal exposure to
oxygen-glucose deprivation were rendered resistant to injury induced
by a subsequent longer exposure to oxygen-glucose deprivation.
Demonstration of such preconditioning-induced tolerance in
vitro supports the view that ischemic tolerance in vivo
may be predominantly explained by alterations in brain parenchymal rather than by alterations in blood flow or systemic response to
ischemia and fits with observations that preconditioning does not alter
regional cerebral blood flow associated with a subsequent lethal
ischemic insult (Matsushima and Hakim, 1995 ; Chen et al., 1996 ).
Using the experimental leverage gained in such a reductionist model
system, we demonstrated here a critical role for NMDA receptor
activation in mediating preconditioning-induced tolerance. Besides
oxygen-glucose deprivation, several other insults capable of
increasing NMDA receptor activation increasing extracellular K+ or adding exogenous glutamate or NMDA
itself induced tolerance, whereas activating AMPA/kainate or
metabotropic glutamate receptors failed to induce tolerance.
Furthermore, we found that the ability of an NMDA antagonist to block
oxygen-glucose deprivation-induced preconditioning could not be
overcome by increasing insult duration, even to a maximal sublethal
level, arguing that the NMDA antagonist effect was not mediated by a
nonspecific reduction in cellular stress and injury as Kato et al.
(1992) had postulated (see above). Finally, we showed evidence that
oxygen-glucose deprivation-induced tolerance was specific for a
subsequent lethal oxygen-glucose deprivation insult. Tolerance did not
extend to staurosporine-induced apoptosis and, unexpectedly, also did
not extend to glutamate, NMDA, or kainate neurotoxicity. The present
data are consistent with previous reports of preconditioning-induced
tolerance in vivo or in vitro but make several
novel points: (1) the specificity of NMDA receptor activation versus
AMPA/kainate or metabotropic glutamate receptor activation in inducing
tolerance; (2) the inability of overall stress and injury reduction to
explain NMDA antagonist reduction of tolerance; and (3) the specificity
of tolerance with regard to subsequent oxygen-glucose
deprivation-induced insults versus other apoptosis or excitotoxicity paradigms.
Besides the demonstrations by Kato et al. (1992) that MK801 blocked
ischemic tolerance in gerbils, the finding that tolerance could be
induced by the application of high K+ concentrations
(Kawahara et al., 1995 ; Kobayashi et al., 1995 ; Matsushima et al.,
1996 ; Taga et al., 1997 ) fits with a postulated key role for NMDA
receptor activation. Application of K+ would be
expected to depolarize neurons and increase neurotransmitter glutamate
release, leading to enhanced NMDA receptor activation. In these studies
of high K+-induced ischemic tolerance, the
application of K+-triggered spreading depression is
also dependent on NMDA receptor activation (McLachlan, 1992 ; Nellgard
and Wieloch, 1992 ). In fact, Taga et al. (1997) used an experiment
comparable with that of Kato et al. (1992) when they blocked both the
spreading depression and subsequent tolerance to ischemia using the
NMDA antagonist ketamine.
As in the initial gerbil studies by Kitagawa et al. (1990) and Kato et
al. (1991) , we observed that tolerance developed slowly over many hours
after the preconditioning episode, suggesting that changes in gene
expression may be involved. We would have liked to have determined
whether inhibitors of protein synthesis would block the development of
tolerance but did not do this experiment, because the inhibitors
themselves attenuate neuronal death after oxygen-glucose deprivation
(Lobner and Choi, 1996 ).
The observation that oxygen-glucose deprivation-induced tolerance was
not associated with a reduction in neuronal vulnerability to NMDA,
kainate, or glutamate toxicity argues against the possibility that this
tolerance was explained by postsynaptic changes in glutamate receptor
structure. Previous studies have observed alterations in hippocampal
neuronal glutamate receptor expression consequent to hypoxic and
ischemic insults, including selective downregulation of NMDA receptor
subunits NR2A and 2B (Zhang et al., 1997 ), selective downregulation of
the Ca2+ gatekeeper AMPA receptor subunit
GluR2/GluRB (Pellegrini-Giampietro et al., 1992 ; Gorter et al., 1997 ),
and selective upregulation of AMPA receptor subunit GluR4/GluRD (Ying
et al., 1997 ). The current failure to observe altered vulnerability to
excitotoxicity after conditioning oxygen-glucose deprivation may
reflect differences in the regulation of glutamate receptor expression
in cortical versus hippocampal neurons.
At face value, the present observation that preconditioning
oxygen-glucose deprivation did not protect cortical neurons against subsequent staurosporine-induced apoptosis is at odds with the prominence of DNA fragmentation associated with the hypoxic neuronal death in vitro studied by Bruer et al. (1997) , an injury
paradigm that was successfully attenuated by hypoxic preconditioning.
However, those investigators used younger cortical neuronal cultures
(rat DIV 8-10) than used here (mouse DIV 13-15), a feature that
likely enhances the propensity to undergo programmed cell death (see above). It is therefore possible that their experiments, and our experiments, induced similar shifts in the mechanisms responsible for
hypoxic neuronal death but that the ultimate phenotype of this death,
necrosis versus apoptosis, was determined by age. Although further
experiments will be needed to delineate the basis of oxygen-glucose
deprivation-induced tolerance in the present model system, the observed
absence of associated changes in neuronal vulnerability to
excitotoxicity or staurosporine-induced apoptosis favors the
possibility of changes in glutamate release or uptake.
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FOOTNOTES |
Received Sept. 16, 1998; revised Dec. 10, 1998; accepted Dec. 18, 1998.
This work was supported by National Institutes of Health Grant NS 30337 (D.W.C.) and National Institutes of Health Fellowship NS 10291 (M.C.G.). We thank Min Tian for technical assistance and Doug Lobner
and Debra Babcock for helpful discussions.
Correspondence should be addressed to Dennis W. Choi, Washington
University School of Medicine, Department of Neurology, Box 8111, 660 South Euclid, St. Louis, MO 63110.
 |
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