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Vol. 285, Issue 2, 561-567, May 1998
Department of Pharmacology and Therapeutics, University of Manitoba Faculty of Medicine, Winnipeg, Manitoba, R3E 0T6, Canada
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Abstract |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Glutamate release after ischemia, hypoxia and seizure activity plays an
important role in stimulating adenosine production and release. We
characterized the ionotropic glutamate receptor subtype that regulates
adenosine levels in vivo and investigated the role of
nitric oxide and free radicals in mediating
N-methyl-D-aspartate (NMDA)-induced increases in adenosine
levels. Rats received unilateral intrastriatal injections and were
sacrificed 15 min postinjection by high-energy focused microwave
irradiation (10 kW, 1.25 s). Adenosine levels were measured by
high-performance liquid chromatography in ipsilateral and contralateral
striata. NMDA and kainic acid dose-dependently increased levels of
adenosine whereas (±)-
-amino-3-hydroxy-5-methyl-4-isoxazol proprionic acid had no effect. The NMDA- and kainic acid-induced increases were blocked by dizocilpine, and the kainic acid response was
decreased by 6-cyano-7-nitroquinoxaline-2,3-dione. The effects of NMDA
and kainic acid on levels of adenosine were not additive. Intrastriatal
L-arginine decreased, and the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester,
increased basal adenosine levels. Coadministration of NMDA with
L-arginine or NG-nitro-L-arginine
methyl ester did not significantly affect NMDA-induced increases in
levels of adenosine. N-Tert-butyl-phenylnitrone, a free
radical scavenger, reversed L-arginine-induced decreases and NMDA-induced increases in levels of adenosine. Together, these results indicate that NMDA-type ionotropic receptors play an important role in regulating in vivo levels of adenosine in rat
striatum and that free radicals, but not nitric oxide, apparently are
involved in NMDA-induced increases in levels of adenosine. Conversely, nitric oxide, but not free radicals, apparently exert tonic control over basal levels of endogenous adenosine.
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Introduction |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Levels
of glutamate, the major excitatory transmitter in the CNS, increase
greatly during pathological conditions such as ischemia and hypoxia
(Phillis et al., 1991
). Glutamate exerts its actions through
G-protein-linked metabotropic receptors or cation-selective ionotropic
receptors. On the basis of agonist selectivity, ionotropic receptors
have been classified as being either NMDA or non-NMDA. Non-NMDA
receptors are classified further as kainate- or AMPA-preferring
excitatory amino acid receptors (for reviews see Barnes and Henley,
1992; Bettler and Mulle, 1995). Excessive glutamate receptor
stimulation, in particular NMDA receptors, has been implicated as a
major common pathway leading to neuronal damage in a variety of
pathologies (Beal, 1992; Lipton and Rosenberg, 1994; Lynch and Dawson,
1994).
NMDA receptor activation can lead to stimulation of NOS (EC
1.14.13.39), the enzyme responsible for the formation of NO (Bredt and
Snyder, 1989). NO mediates many of the effects of NMDA, including
neurotransmitter release from striatum and cerebral cortex (Montague
et al., 1994; Sandor et al., 1995). In addition to formation of NO, itself a free radical, NMDA receptor activation can
lead to the generation of other free radical species (Lafon-Cazal et al., 1993; Beckman et al., 1990). Treatment
with free radical scavengers, such as the spin trap agent PBN, can
prevent NMDA-induced neurotoxicity and protect against neuronal damage
in a variety of in vitro and in vivo models
(Schultz et al., 1995b; Nakao et al., 1996;
Oliver et al., 1990; Cao and Phillis, 1994).
Levels of adenosine, a mainly inhibitory neuromodulator in the CNS,
increase in response to glutamate and selective ligands for ionotropic
glutamate receptors both in vitro (Hoehn and White, 1990;
Pedata et al., 1991; White, 1996) and in vivo
(Jhamandas and Dumbrille, 1980; Perkins and Stone, 1983; Chen et
al., 1992; Carswell et al., 1997). Adenosine through
its actions to depress basal and evoked neuronal firing, decrease
calcium uptake and inhibit release of excitatory neurotransmitters such
as glutamate (Wu et al., 1982; Corradetti et al.,
1984; Dunwiddie and Diao, 1994) promotes neuroprotective effects, and
these actions are mimicked by adenosine receptor agonists and
inhibitors of adenosine metabolism and uptake (Rudolphi et
al., 1992; von Lubitz et al., 1995; see Geiger et
al., 1997).
In many in vivo studies, microdialysis is used to measure
extracellular levels of adenosine. However, one of the problems with
microdialysis is the large increases observed after implantation of
microdialysis probes; adenosine levels may increase more than 75-fold
and 24 hr may be required for levels to decline to near basal values
(Ballerin et al., 1991). We have circumvented this problem
by use of high-energy focused microwave irradiation (10 kW) to prevent
postmortem metabolism of adenine nucleotides to adenosine. This method
allows for accurate and precise measurement of in vivo
levels of adenosine akin to those obtained by freeze-blowing with the
added advantage that measurements can be made in discrete brain regions
(Delaney and Geiger, 1996).
We previously reported that intrastriatal injections of NMDA increased
levels of endogenous adenosine by more than 2-fold (Delaney and Geiger,
1995). The major aim of this study was to characterize more fully the
subtype of ionotropic glutamate receptors responsible for regulating
levels of adenosine in rat striatum and the involvement of NO and other
free radicals in regulating adenosine levels under basal and
NMDA-stimulated conditions.
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Materials and Methods |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Animals. Male Sprague-Dawley rats were obtained from the University of Manitoba Central Animal Care breeding facility. All procedures followed Canadian Council on Animal Care guidelines and were approved by the Animal Care Committee at the University of Manitoba. Rats used for intrastriatal injections weighed 170 to 190 g, and rats used for NOS assays weighed 200 to 220 g.
Intrastriatal injections.
Animals were anesthetized with 74 mg/kg sodium pentobarbital (i.p.) and placed in a stereotaxic frame;
unilateral intrastriatal injections were performed with the coordinates
(in mm): AP 9.0, ML 3.0 and DV 4.5 (Paxinos and Watson, 1986). Drugs
were administered in a volume of 0.5 µl with a 30-gauge needle for a
2-min period. The needle was left in place for 1 min postinjection to
allow diffusion of the drug away from the injection site. NMDA and
kainic acid, dissolved in 50 mM Tris-HCl with the pH adjusted to 7.4 with NaOH, were administered at doses ranging from 5 to 150 nmol and
0.125 to 8 nmol, respectively. AMPA was dissolved in 6 N HCl, the pH
was adjusted to 7.4 with NaOH and the volume was adjusted with 50 mM
Tris-HCl (pH 7.4) for administration of doses ranging from 12 to 30 nmol. Control rats received unilateral injections of 0.5 µl 50 mM
Tris-HCl, pH 7.4. Rats were sacrificed by high-energy focused microwave
irradiation (Cober Instruments, South Norwalk, CT) at a power level of
10 kW for 1.25 s. Rats were sacrificed 15 min postinjection except
in time-course studies when rats were allowed to survive for times
ranging from 5 to 45 min after receiving intrastriatal injections.
Brains were removed, and striata were dissected and analyzed separately
for tissue adenosine content by HPLC with fluorescence detection
(Delaney and Geiger; 1996). Protein was determined by the method of
Lowry et al. (1951) with bovine serum albumin as standard.
Tissue adenosine content was expressed as either picomoles per
milligram protein or as a percentage of uninjected contralateral
striatum.
; Nakao
et al., 1996). Vehicle controls were used throughout all
these studies.
NOS assay.
To confirm inhibition of NOS, rats were injected
unilaterally into the striatum with 500 µg L-NAME and
sacrificed by decapitation 15 min postinjection. Uninjected and
injected striata were excised and analyzed separately for NOS activity
by a method adapted from Iadecola et al. (1994). Striata
were homogenized (20 strokes) in 1.2 ml 0.32 M sucrose containing 20 mM
HEPES, pH 7.4, 1 mM dithiothreitol and 0.5 mM EDTA. Duplicate aliquots
of 425 µl tissue were incubated at 37°C for 6 min with 0.45 mM
CaCl2·2H2O, 2 mM NADPH, 2 mM L-arginine and 0.2 µCi
L-[3H]arginine in a final volume of
500 µl. Enzyme activity was terminated by the addition of 2 ml 20 mM
HEPES, pH 5.5, containing 2 mM EDTA. Samples were applied to a Dowex AG
50W-X8 (Na+) column, and the eluent plus the
effluent from a 2-ml wash of H2O containing
[3H]citrulline were collected and radioactivity
was determined by scintillation spectrometry (Beckman LS6000TA).
Reaction blanks contained everything except NADPH and were treated
exactly as above. Enzyme activity was expressed as picomoles per
milligram protein per minute.
Chemicals. Adenosine was obtained from Fisher Scientific (Pittsburgh, PA) as was HPLC grade KH2PO4. Chloracetaldehyde was purchased from Fluka (Ronkonkoma, NY). Burdick & Jackson (Muskegon, MI) supplied HPLC-grade methanol. NMDA, kainic acid, AMPA, HEPES, EDTA, L-arginine, D-arginine, PBN and peanut oil were obtained from Sigma (St. Louis, MO). (+)-MK801 hydrogen maleate (dizocilpine maleate), CNQX and L-NAME were supplied by Research Biochemicals International (Natick, MA) and L-[2,3-3H]arginine (35.7 Ci/mmol) by New England Nuclear (Boston, MA). Dowex AG 50W-X8 was bought from Bio-Rad Laboratories (Mississauga, ONT). All other chemicals were of analytical grade.
Data analyses. Adenosine levels (pmol/mg protein) in injected striata were calculated as a percentage of levels in uninjected contralateral striata and expressed as mean ± S.E.M. for each drug treatment group. Levels in injected striata were compared with those in the uninjected side with Student's paired t tests. Differences between drug treatment groups were analyzed either by one-way analysis of variance followed by Tukey-Kramer's multiple comparisons test or by Student's unpaired t test. For all tests, statistical significance was considered to be at the P < .05 level.
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Results |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Levels of endogenous adenosine in uninjected striata of rats were 117 ± 30 pmol/mg protein whereas those in striata injected with buffer were 127 ± 38 pmol/mg protein (data not shown). When calculated as a percentage of uninjected contralateral striatum, levels of adenosine in buffer-injected striata were 105 ± 15%.
Both NMDA and kainic acid when injected unilaterally into striata dose-dependently increased levels of adenosine (fig. 1). Although NMDA and kainic acid showed similar efficacies (adenosine levels were increased by approximately 6-fold), kainic acid with its apparent ED50 of 0.3 nmol was 167 times more potent than was NMDA with its apparent ED50 of 50 nmol. The minimum dose of NMDA required to produce a statistically significant increase in adenosine levels to 238 ± 21% (P < .05) was 10 nmol, and maximal increases of 613 ± 91% were obtained with 150 nmol NMDA. The minimum dose of kainic acid required to produce a statistically significant increase in adenosine levels to 350 ± 67% (P < .05) was 0.25 nmol, and a maximal increase of 591 ± 31% was obtained with 5 nmol of kainic acid. AMPA injected into striata at doses ranging from 12 to 30 nmol did not produce statistically significant increases in levels of endogenous adenosine; maximal increases of 192 ± 30% (P = .26) were observed at 19 nmol (data not shown).
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Having found that NMDA and kainic acid both significantly increased levels of endogenous adenosine, we performed time-course experiments to determine the time at which maximal increases would be observed. The doses of 25 nmol NMDA and 0.25 nmol kainic were chosen on the basis of results from initial experiments which suggested that these doses approximately represented ED50 values. Maximally increased levels (271 ± 35%) were observed 15 min after administration of NMDA and levels returned to basal values 45 min postinjection (fig. 2). For kainic acid, maximally increased levels (350 ± 67%) were observed similarly 15 min postinjection and levels returned to basal values 30 min postinjection (fig. 2).
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Dizocilpine (4 mg/kg) decreased significantly (P < .05) basal levels of endogenous adenosine to 38 ± 11% regardless of whether data were compared with buffer-injected striata (i.e., control rats) or with uninjected striata (fig. 3). The levels of endogenous adenosine of 75 ± 22 pmol/mg protein in uninjected contralateral striata were not significantly different from those in control rats even though these rats received half doses of anesthetic (37 mg/kg). Dizocilpine (4 mg/kg) decreased significantly (P < .05) the increase in levels of adenosine induced by 25 nmol NMDA from 271 ± 35% to 82 ± 18% (fig. 3). At high doses (100 nmol) of NMDA, dizocilpine at 4 mg/kg and 6 mg/kg significantly decreased levels from 539 ± 69% to 238 ± 53% (P < .05) and to 104 ± 38% (P < .01), respectively (fig. 3).
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The competitive kainate/AMPA receptor antagonist CNQX (5 nmol) decreased significantly (P < .05) basal levels of adenosine to 43 ± 14% compared with vehicle-injected striata and decreased kainic acid-induced increases in levels of adenosine from 350 ± 67% to 154 ± 56% (fig. 4). Because kainic acid can release glutamate and subsequently activate NMDA receptors, we administered dizocilpine (4 mg/kg i.p.) 30 min before intrastriatal injection of kainic acid (0.25 nmol) and observed a significant (P < .01) reduction in levels of adenosine from 350 ± 67% to 40 ± 2% (fig. 4). In a separate series of experiments, NMDA (50 nmol) and kainic acid (0.25 nmol) administered in combination had no additive effects on levels of adenosine when compared with the effects of each drug separately (fig. 5).
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The following two series of experiments were performed to determine the involvement of free radicals in general, and nitric oxide in particular, in regulating levels of adenosine under basal and NMDA-stimulated conditions. Adenosine levels decreased significantly (P < .01) with 100 nmol L-arginine, but not with the inactive stereoisomer D-arginine (fig. 6). L-NAME (500 µg) significantly (P < .05; Student's unpaired t test) increased adenosine levels to 230 ± 54% (fig.6) and inhibited NOS activity by 86% (P < .001, table 1). In testing for the involvement of free radicals in L-arginine-induced decreases in adenosine levels, adenosine levels in rats pretreated with PBN (150 mg/kg i.p.) 1 hr before L-arginine was injected intrastriatally were not significantly different from control values (fig. 6). The absolute levels of adenosine (pmol/mg protein) in uninjected contralateral striata of rats that received PBN were slightly, but not significantly, elevated to 167 ± 24 (n = 14).
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Coadministration of NMDA with either 100 nmol L-arginine or 500 µg L-NAME did not affect NMDA-induced increases in levels of adenosine (fig. 7). However, levels of adenosine in rats pretreated with PBN were decreased significantly (P < .05). Preinjection (i.p.) of peanut oil, the vehicle for PBN, followed by intrastriatal NMDA, increased levels of adenosine by 249 ± 61%, which indicates a lack of vehicle effects (data not shown).
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Discussion |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Adenosine is produced in and released from CNS tissue preparations
in response to a variety of conditions including, but not limited to,
depolarization, ATP metabolism and glutamate release (Berne et
al., 1974; Van Wylen, 1986; Phillis et al., 1991).
Evidence that adenosine analogs and inhibitors of adenosine transport
and metabolism can inhibit glutamate release and protect against
pathological consequences associated with glutamate release (Corradetti
et al., 1984; Finn et al., 1991) suggests that
adenosine may represent a "brake" against prolonged glutamate
receptor activation and excitotoxicity. We reported previously that
NMDA injected into rat striatum increased levels of endogenous
adenosine (Delaney and Geiger, 1995), and here, we characterized the
ionotropic glutamate receptor subtype that regulates adenosine levels
in vivo and investigated the role of NO and free radicals in
regulating adenosine levels under basal and NMDA-stimulated conditions.
In all these studies, our in vivo protocol required the use
of pentobarbital anesthesia. However, the anesthetic did not affect adenosine values because adenosine levels in nonanesthetized, noninjected rats or rats receiving half doses (37 mg/kg) of sodium pentobarbital were not significantly different from levels in rats
receiving a full dose of anesthetic. Also, although the doses of NMDA
and kainate used here were similar to those that produce, over much
longer times (3-7 days), excitotoxic lesions in striatum (Finn
et al., 1991; Globus et al., 1995; Schultz
et al., 1995a), our findings that total tissue levels of
adenosine peaked 15 min postinjection and returned to basal levels by
45 min postinjection argues against a nonspecific release of adenosine
because of cell death. Indeed, release of adenosine from hippocampus
in vivo showed a similar time scale for peak release and
return to basal levels in response to glutamate receptor agonists (Chen
et al., 1992; Carswell et al., 1997).
Furthermore, AMPA in doses similar to those used here can cause
excitotoxic damage in striatum (McDonald et al., 1992), but
did not significantly increase adenosine levels.
NMDA-induced increases in levels of adenosine apparently were mediated
through specific ionotropic NMDA receptors because the responses to
NMDA were blocked by the noncompetitive use-dependent NMDA receptor
antagonist dizocilpine. Although systemic administration of dizocilpine
did not alter levels of adenosine in uninjected striata, it did
significantly decrease levels of adenosine in buffer-injected striata.
These findings suggest that there was no tonic regulation of adenosine
levels by glutamate in uninjected striata, but that mechanical injury
from needle insertion may have decreased the voltage-dependent
Mg++ blockade of NMDA receptors (Zhang et
al., 1996), thereby increasing their basal activity and/or
increased the release of endogenous glutamate. Because dizocilpine acts
use-dependently, "priming" of the NMDA receptor in injected
striata, regardless of how or at what point during the surgery it
occurs, would lead to a greater degree of blockade than in uninjected
striata. Thus, the final outcome of decreased NMDA receptor activity in
the injected striatum was decreased adenosine levels.
Kainic acid increased levels of adenosine, but apparently did so
through activation of kainate and subsequently NMDA receptors, because
CNQX and dizocilpine decreased levels of adenosine in buffer-injected
striata and blocked kainate-induced increases in adenosine. Dizocilpine
was tested because kainate induces glutamate release (Ferkany and
Coyle, 1983; Young et al., 1988) and the effects of kainate
then may be mediated through other glutamate receptor subtypes
(reviewed by Coyle, 1983). Although dizocilpine at 4 mg/kg was more
effective than 5 nmol CNQX, it is difficult to compare the potencies of
these two antagonists because dizocilpine works use-dependently and
CNQX is a competitive blocker. Even though CNQX may block glycine sites
on the NMDA receptor (Lester et al., 1989), we believe that
CNQX was acting at kainate sites because the concentrations required to
block NMDA receptors are much higher than those required to inhibit
AMPA/kainate receptor activation and because CNQX would have to compete
with endogenous glycine, which is thought to be present in
supersaturating concentrations in vivo (Birch et
al., 1988). Findings that dizocilpine blocked the effects of
kainate, combined with data showing that the effects of NMDA and
kainate were not additive, suggests that in striatum, at least,
glutamate released by kainate activates NMDA receptors and results in
higher levels of adenosine. This seems to contrast with results showing
that NMDA receptors do not appear to play a major role in
kainate-induced increases in levels of adenosine in hippocampus
in vivo (Carswell et al., 1997). However, our
data do support and extend findings that kainic acid and NMDA increase adenosine release in vitro (Hoehn and White, 1990; Craig and
White, 1993b; Manzoni et al., 1994), as well as in
vivo (Perkins and Stone, 1983; Chen et al., 1992;
Carswell et al., 1997).
In contrast to NMDA and kainate, intrastriatal injection of AMPA did
not affect adenosine levels significantly. Similarly, AMPA had minimal
effects on adenosine release from hippocampal slices (Pedata et
al., 1991). However, AMPA and quisqualate (a nonselective agonist
with activity at the AMPA receptor) increased adenosine release in
cortical slices (Hoehn and White, 1990; Craig and White, 1993b; White,
1996). Even though cortical, striatal and hippocampal brain regions all
contain NMDA, AMPA and kainate receptors (Cotman and Monaghan, 1986;
Young and Fagg, 1990), there appear to be regional differences in the
ability of these receptor subtypes to evoke increases in levels of
adenosine. There is evidence for a common site of action for kainic
acid and AMPA as well as for kainate acting at a unique set of
receptors (Barnes and Henley, 1992); however, because AMPA had no
significant effect on levels of adenosine, it seems clear that kainate
acted through its own receptors.
The second aim of this study was to test the hypothesis that NO and/or
free radicals regulate levels of endogenous adenosine under basal as
well as NMDA-stimulated conditions. A reciprocal relationship appears
to exist in vivo between levels of NO and basal levels of
endogenous adenosine such that L-arginine decreased whereas
L-NAME increased levels of adenosine. Confidence in our results is raised by findings that L-arginine and
L-NAME had opposite effects on basal levels of adenosine
and that the inactive stereoisomer of L-arginine,
D-arginine, had no effect. In addition, the effects of
L-arginine were blocked by the spin trap agent PBN that
scavenges free radicals by forming more stable free radical adducts
(Knecht and Mason, 1993). Furthermore, our findings with
L-NAME are consistent with findings in guinea pig brain and
rabbit heart, which show that basal levels of adenosine increase after
perfusion with L-NAME (Kostic and Schrader, 1992; Woolfson
et al., 1995), and that the NOS inhibitor
NG-nitro-L-arginine potentiated
adenosine-mediated hypocapnic vasodilation in rat brain (Fabricius and
Lauritzen, 1994). Results with NO donors are less clear. NO donors
reportedly decrease (Craig and White, 1993a) and increase (Fallahi
et al., 1996) adenosine release in vitro,
increase adenosine release in vivo (Fischer et
al., 1995) and inhibit (Siegfried et al., 1996) and
stimulate (Minamino and Hasegawa, 1995) the activity of the
adenosine-producing enzyme 5'-nucleotidase. Although we did not study
the relationship between striatal levels of adenosine, NO and blood
flow, our results may suggest that levels of NO and adenosine are
altered reciprocally as a mechanism for controlling local cerebral
blood flow.
Many effects of NMDA, including neurotransmitter release, are mediated
by NO (Montague et al., 1994; Sandor et al.,
1995). However, this does not appear to be the case with adenosine
levels, because neither L-arginine nor L-NAME
affected levels of endogenous adenosine increased by NMDA. Similarly,
NO apparently did not mediate NMDA-evoked adenosine release from rat
cortical slices (Craig and White, 1993a). However, it is possible that
in the presence of NMDA a role for NO regulation of adenosine levels, as seen under basal conditions, may still exist, but the effects have
been masked by other processes subsequent to receptor activation.
Administration of PBN reversed L-arginine-induced decreases
in basal levels of endogenous adenosine possibly through the removal of
NO (itself a free radical). With respect to NMDA, determining the
nature of the free radical(s) involved is more complicated because NMDA
generates superoxide radicals (Lafon-Cazal et al., 1993),
which in combination with NO results in the formation of peroxynitrite,
thus leading to lipid peroxidation and production of additional free
radicals (Beckman et al., 1990). Free radicals inhibit
glutamate uptake (Volterra et al., 1994) as well as disrupt mitochondrial function and ATP production (Brown and Squier, 1996), and
these effects could lead to increases in levels of endogenous adenosine. PBN can trap carbon-centered free radicals such as those
resulting from lipid peroxidation (Knecht and Mason, 1993) as well as
combine, although less readily, with hydroxyl radicals (Thomas et
al., 1994). Despite such ambiguities in the identity of the free
radicals involved, it remains clear that PBN is very effective at
preventing NMDA-induced neuronal damage (Schultz et al.,
1995a; Nakao et al., 1996; Lafon-Cazal, 1993) and, perhaps in doing so, blocks the stimulus that leads to increased levels of
adenosine.
In an in vivo preparation such as ours, it would be expected
that both endothelial and neuronal isoforms of NOS would contribute to
NO production, and because L-NAME is an inhibitor of both, there would be no distinction between the relative contributions of
each isoform. Under basal conditions, endothelial NOS activity might be
the predominantly active isoform, whereas under NMDA-stimulated conditions, neuronal NOS might be activated, in addition to endothelial NOS, as part of the signal transduction system. Changes in the relative
activities of each isoform can produce vastly different effects (Globus
et al., 1995), and if neuronal NOS activity increases levels
of adenosine, this might counteract endothelial NOS-induced decreases
in levels of adenosine and produce no overall changes in adenosine
levels. The importance of different isoforms of NOS is being recognized
increasingly when looking at the effects of NO (Iadecola 1997; Snyder,
1995; Globus et al., 1995), and the effects of NO on
adenosine levels may be another example where this must be considered.
Adenosine agonists prevent both NMDA and kainate striatal toxicity
in vivo (Finn et al., 1991). Thus, through
application of adenosinergic strategies, excitotoxic damage may be
preventable and/or treatable especially when directed at events in
which NMDA and possibly kainate receptors are involved.
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Footnotes |
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Accepted for publication January 23, 1998.
Received for publication August 28, 1997.
1 These studies were supported by a grant from the Medical Research Council of Canada (to J.D.G.).
2 Recipient of a Medical Research Council of Canada Studentship. Current address: Department of Pharmacology, University of North Carolina, Chapel Hill, NC 27599.
3 Recipient of a National Science and Engineering Council Studentship award.
4 Recipient of a Medical Research Council of Canada Scientist award.
Send reprint requests to: Dr. J. D. Geiger, Department of Pharmacology and Therapeutics, University of Manitoba, 753 McDermot Avenue, Winnipeg, Manitoba, R3E 0T6 Canada.
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Abbreviations |
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AMPA, (±)--amino-3-hydroxy-5-methyl-4-isoxazol proprionic acid;
CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione;
L-NAME, NG-nitro-L-arginine methyl ester;
NMDA, N-methyl-D-aspartate;
NO, nitric oxide;
NOS, nitric oxide
synthase;
PBN, N-tert-butyl-phenylnitrone;
HPLC, high-performance liquid chromatography;
HEPES, N-2-hydroxyethylpiperaine-N'-ethanesulfonic acid;
EDTA, ethylenediaminetetraacetic acid;
CNS, central nervous system.
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References |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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-Phenyl-tert-butyl-nitrone reduces cortical infarct and edema in rats subjected to focal ischemia.
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