Slow death in the leopard frog Rana pipiens: neurotransmitters and anoxia tolerance
Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA
* Author for correspondence (e-mail: smilton{at}fau.edu)
Accepted 6 August 2003
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Summary |
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Key words: anoxia, frog, Rana pipiens, excitatory amino acid, dopamine, slow death
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Introduction |
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Some frog species (Rana temporaria, Rana pipiens) demonstrate an
intermediate anoxia tolerance, their brains able to survive 4-5 h without
oxygen at room temperature (Knickerbocker
and Lutz, 2001; Lutz and
Reiners, 1997
) and at least 30 h without oxygen at 5°C
(Hermes-Lima and Storey,
1996
). This tolerance to anoxia appears to be accomplished through
an overall metabolic depression, primarily via hypoperfusion of the
skeletal muscle (Donohoe and Boutilier,
1998
; Donohoe et al.,
1998
), which allows the frog to maintain ATP levels in certain
organ systems. However, unlike truly anoxia-tolerant vertebrates, the frog
brain does not defend ATP levels, and when energy stores reach a critical low,
(approximately 35% of normoxic levels), ion homeostatic mechanisms are
compromised and extracellular K+ levels begin to increase
(Knickerbocker and Lutz,
2001
). Extracellular [K+] reaches a critical threshold
within an additional 1-2 h, after which there is a rapid K+ efflux,
indicating anoxic depolarization
(Knickerbocker and Lutz,
2001
), accompanied by a massive loss of neurotransmitters
(Lutz and Reiners, 1997
). This
pattern of ATP loss, ion leakage and depolarization in response to anoxia in
the frog brain is the same as that observed in the anoxic/ischemic mammalian
brain, but critically low ATP levels are reached in only a few minutes in the
mammalian brain, versus approximately 3 h in the frog brain
(Lutz and Reiners, 1997
;
Knickerbocker and Lutz, 2001
).
The enhanced anoxia tolerance of the frog brain thus appears to be a case of
`slow death', with the same sequence of events that occur catastrophically in
the mammalian brain but on a greatly extended time scale. This could make the
frog brain a model of particular interest for investigating the processes of
anoxic/ischemic failure.
There is, however, a lack of basic information on adaptations exhibited by
the frog that enable them to extend anoxic death over a period of hours. The
present study examines the effects of anoxia on tissue and extracellular
neurotransmitter levels, as anoxia is known to cause very substantial changes
in these compounds in the vertebrate brain. Anoxia-tolerant species such as
the crucian carp and freshwater turtle tend to exhibit similar patterns of
changes in these compounds, i.e. increases in whole brain and extracellular
levels of inhibitory compounds such as GABA, glycine and taurine, and
decreases in the levels of excitatory amino acids such as glutamate and
glutamine (Nilsson et al.,
1990). By contrast, anoxia-intolerant animals (brown anole,
neonatal and adult rat) demonstrate increases only in tissue GABA and/or
alanine levels, while whole brain levels of glutamate and glutamine may even
increase (Nilsson et al.,
1991
; Lutz et al.,
1994
; Erecinska et al.,
1984
); neurotransmitters are then released indiscriminately upon
depolarization (Lutz et al.,
2003
).
There is little known about the effect of anoxia on neurotransmitter
release in the frog brain. Lutz and Reiners
(1997) found a massive
increase in extracellular glutamate and GABA levels upon anoxic depolarization
in the frog brain, but numerous other neuroactive compounds have not yet been
investigated. These include neuroprotective amino acids such as taurine and
alanine as well as neurotoxic compounds such as dopamine. In the turtle,
extracellular glutamate and dopamine are maintained at basal levels during
extended anoxia (Nilsson and Lutz,
1991
; Milton and Lutz,
1998
), while levels of inhibitory neuroactive compounds such as
GABA and glycine increase (Nilsson and
Lutz, 1991
). By contrast, extracellular neurotransmitter levels
increase indiscriminately in anoxia-intolerant animals: dopamine, aspartate,
glutamate, GABA and taurine all increase in the hypoxic or anoxic neonatal rat
and adult rat brain (Huang et al.,
1994
; Perez-Pinzon et al.,
1993
; Young et al.,
1993
), while glutamate, aspartate, GABA, and taurine are all
released from hippocampal slices exposed to hypoxia, chemical ischemia or
hydrogen peroxide (Saransaari and Oja,
1998
).
Dopamine is of particular interest because this compound is neurotoxic at
high levels, but unlike other excitotoxins such as glutamate, dopamine levels
increase in the mammalian extracellular compartment well before anoxic or
ischemic depolarization (Huang et al.,
1994; Globus et al.,
1988
). By contrast, extracellular dopamine levels do not increase
in the anoxic turtle brain due to both decreased release (S. L. Milton and P.
L. Lutz, manuscript in preparation) and continued reuptake
(Milton and Lutz, 1998
).
The present study measured the effects of anoxia on levels of neurotransmitters in the whole brain and extracellular space in the frog Rana pipiens, in order to determine whether neurotransmitter responses during `slow death' more closely resemble the anoxia-vulnerable (mammalian) or anoxia-tolerant (turtle, carp) response to anoxia.
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Materials and methods |
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Tissue sampling: whole brain amino acids
All experiments were performed at room temperature (25±1°C).
Control animals were removed from their holding tank and decapitated. The
brains were dissected out in less than 1 min and frozen in liquid nitrogen.
Anoxic animals were held in a sealed polyethylene chamber under positive-flow
nitrogen (99.99% nitrogen, County Welding, Pompano Beach, FL, USA) for 1, 2 or
4 h anoxic exposure. At these time points, the frogs were removed from the
anoxic chamber, and their brains removed and treated as above. Samples were
held at -80°C.
While still frozen, brain sections were weighed and homogenized in 20 volumes of 4% (w:v) ice-cold perchloric acid buffered with 0.2% EDTA and 0.05% sodium bisulfate, using a Sorval Teflon-glass homogenizer. The homogenate was cold-centrifuged at 15 000 g for 5 min and the supernatant stored at -20°C until analysis.
Tissue sampling: microdialysis
The frogs were placed in a 500 ml sealed polyethylene box containing saline
in which was dissolved the anesthetic tricaine methanesulfonate, MS-222
(250-300 g l-1) buffered to a pH of 7.4. Until surgery the chamber
was continuously aerated with 100% O2. After a surgical plane was
achieved (as indicated by lack of ocular reflex), an area (4 mm x 5 mm)
of the skull directly over the cerebral hemispheres was removed; a small
incision through the dura mater exposed the cerebral hemispheres. A
stereotaxic instrument and guide were used to insert a CMA 12 microdialysis
probe (1 mm membrane length, Bioanalytical Systems, Inc., Acton, MA, USA) into
the hemispheres (2 mm depth from the cerebral surface). After a 1 h
stabilization period during which no sampling occurred, the probe was perfused
with frog ACSF solution (100 mmol l-1 NaCl, 3.5 mmol l-1
KCl, 26 mmol l-1 NaHCO3, 1.25 mmol l-1
NaH2PO4, 2.0 mmol l-1 CaCl2, 2.0
mmol l-1 MgSO4 and 2.0 mmol l-1 glucose, pH
7.4) at 2.0 µl min-1 with a CMA/100 microdialysis pump (Carnegie
Medecin, Solna, Sweden). Anoxia was induced in experimental groups by changing
the aerating gas to 100% nitrogen; animals remained anoxic for 4 h. Control
animals remained in oxygen-aerated chambers for 4 h; all experiments were
performed at 15-18°C. After all experiments, Methylene Blue was injected
through the microdialysis probe to identify the probe location. Data were
utilized only from those animals in which the probe location was verified.
Probe recovery (amino acids and dopamine) was determined from known standards
in vitro (Huang et al.,
1994); mean recoveries were 20-23%.
Amino acid analysis
The amounts of amino acids present in the tissue supernatants obtained
after centrifugation and in dialysate samples were quantified by
reversed-phase high-performance liquid chromatography (HPLC) with fluorescent
detection. The HPLC system consisted of a Knauer HPLC pump 64 (Sonntek, Inc.,
Upper Saddle River, NJ, USA) and E-lab gradient controller (OMS Tech, Miami,
FL, USA), a reversed-phase column (Adsorbosphere OPA 5u, 150 mm x 4.6
mm, Alltech, Deerfield, IL, USA), and an RF-535 fluorescence detector
(Shimadzu, Kyoto, Japan). For microdialysis samples, 20 µl dialysate was
mixed with 30 µl complete o-phthaldialdehyde reagent solution
(Sigma) at 25°C; after exactly 1 min, 30 µl was injected onto the HPLC
column. Data are presented as percentage of control due to baseline
variability between animals.
Tissue amino acid concentrations are given as µmol g-1 wet mass brain tissue. All values are expressed as mean ± S.E.M. Statistical significance of changes for all data was determined using one-way analysis of variance (ANOVA/Student's t-test) utilizing the SAS/JMP statistical package (Cary, NC, USA); in cases of unequal variance, data were treated non-parametrically (Kruskal-Wallis test for unequal variances). P<0.05 was considered to be statistically significant. Normoxia (control) was calculated as the sample mean for the hour immediately preceding anoxic exposure, 1 h of anoxia as the mean of samples from time zero (start of anoxia) to 1 h of anoxia, and 4 h of anoxia as the mean of samples collected from 3 h to 4 h of anoxic exposure.
Dopamine analysis
Samples were analyzed for monoamine content using HPLC with electrochemical
detection, as adapted from Nilsson
(1990). A 20 µl sample of
dialysate was injected into a 510 HPLC pump (Waters, Milford, MA, USA; flow
rate 1.3 ml min-1). The mobile phase consisted of 100 mmol
l-1 NaH2PO4, 9% (v/v) methanol, 0.63 mmol
l-1 sodium octyl sulfate and 0.2 mmol l-1 EDTA, pH 3.6.
Samples were separated on a Catecholamine C18 column (3 µm, 100 mm x
4.6 mm; Alltech) and detected by an LC-3 electrochemical detector with a
glassy carbon working electrode set at +750 mV. Concentrations were determined
by integrating the area under the peak compared to known standards.
Integrations were performed using the Dynamax MacIntegrator II Integrator and
software (Rainin Instrument Corp., Woburn, MA, USA).
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Results |
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Whole brain amino acids
Amino acid concentrations in the frog forebrain resemble the levels
observed in other vertebrate brains (e.g. neonate rat,
Lutz et al., 1994; brown anole
and Trachemys, Nilsson et al.,
1991
) with most in the range of 1-3 µmol g-1;
glutamine and glutamate predominate with concentrations of 13.68±1.964
and 9.00±0.805 µmol g-1, respectively
(Fig. 1). Changes in whole
brain amino acid levels over a 4 h period of anoxia appear to be intermediate
on the anoxia-tolerant to anoxia-intolerant scale. Changes that were similar
to those seen in anoxia-tolerant species included increases in the
concentrations of serine, glycine, alanine and GABA. Remarkably, no
significant changes occurred over the first hour of anoxia, though a slight
increase in alanine was detected. By 2 h of anoxia, alanine levels had
increased significantly to 220% of control while GABA increased by 40%. By 4 h
of anoxia, alanine had increased further to 330% of control levels, with no
additional changes in GABA. Glycine and serine levels were 193% and 225% of
control, respectively, after 4 h of anoxia. Aspartate levels decreased by 54%
over 4 h of anoxia; changes in whole brain aspartate levels in response to
anoxia have not been previously reported.
|
Whole brain amino acid levels that were not altered by 4 h anoxia included glutamine, taurine and glutamate (Fig. 2).Increases in whole brain taurine and decreases in glutamate and glutamine have been reported in anoxia-tolerant animals, but do not change in anoxia-sensitive species.
|
Extracellular amino acids
Extracellular levels of aspartate, taurine and GABA increased in a
statistically significant manner, while glutamate decreased
(Fig. 3). Extracellular
aspartate levels increased to 145±6% of control by 1 h of anoxia; this
increase was significant by 4 h of anoxia, when mean levels were
215±41% of control. Taurine levels increased by 24±5.5% over 4 h
of anoxia, while GABA levels increased by 45% after 1 h of anoxia and
increased significantly to 302±78% of control by 4 h. On the other
hand, unlike anoxia-intolerant organisms, glutamate levels did not increase,
and in fact decreased by approximately 25% over a 4 h period of anoxia.
Extracellular amino acid levels that did not change during anoxia included
serine, glutamine, glycine and alanine
(Fig. 4).
|
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Dopamine
The largest contrast between anoxia-tolerant vertebrates and R.
pipiens, however, was in extracellular dopamine (DA). Mean DA levels
prior to anoxic exposure (17.0±2.49 ng ml-1) did not
significantly differ from control levels (12.9±1.6 ng ml-1),
nor did control DA levels increase over a 3 h period of normoxia.
Extracellular DA levels began to rise immediately upon anoxic exposure,
however, and were significantly different from pre-anoxic levels by 2 h of
anoxia. By 4 h of anoxia, dopamine had increased 12-fold over normoxic control
levels (Fig. 5).
|
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Discussion |
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By 2 h of anoxia in the frog brain, however, significant changes have occurred in both intra- and extracellular neurotransmitter levels.
Alanine
Since alanine serves as an end product of glycolysis
(Hochachka and Somero, 2002),
the increase in alanine and its continued rise indicates an early and
sustained activation of glycolysis in the anoxic frog brain. Besides being an
alternative anaerobic end product to lactate, alanine may also mediate a
decrease in glycolytic rate by inhibiting pyruvate kinase, such that even
small amounts may act as an important regulator of the hypometabolic state
(Hochachka, 1980
). In the
turtle Trachemys scripta, alanine concentrations between 0.16 mmol
l-1 (heart and liver) and 1.08 mmol l-1 (white muscle)
decreased pyruvate kinase activity by 50%
(Brooks and Storey, 1989
). An
increase in brain [alanine] during anoxia/ischemia appears to be a common
vertebrate response, having been found in the anoxic turtle
(Nilsson and Lutz, 1991
)
crucian carp (Hylland and Nilsson,
1999
) and neonate rat (Lutz et
al., 1994
) as well as the ischemic adult rat brain
(Erecinska et al., 1984
;
Young et al., 1993
). It has
been suggested that increasing alanine levels may even be a preferred
indicator of severe hypoxia, as lactate is a reliable indicator of only mild
hypoxia (Ben-Yoseph et al.,
1993
). However, the increase in alanine concentration in true
facultative anaerobes (approximately tenfold in the carp and turtle) is far
greater than occurs in the anoxia/ischemic mammalian brain (
60% in the
neonate rat). The threefold increase in alanine concentration in the frog
brain over a 4 h period of anoxia, then, is greater than observed in
anoxia-sensitive species, if not of the magnitude seen in the carp or turtle.
Interestingly, extracellular alanine levels did not increase despite the
threefold increase in intracellular levels; it may be that intracellular
alanine is conserved to serve as a brain energy substrate when oxygen supply
is restored; extracellular alanine levels may be biologically less important
than intracellular changes, as it is unlikely that alanine acts as a
neurotransmitter (Nilsson et al.,
1990
).
GABA and glycine
The increase in both whole brain and extracellular GABA levels seen in the
anoxic frog brain may also be common to anoxic/ischemic vertebrate brains,
having been found in anoxic/ischemic mammals as well as the crucian carp and
turtle (Nilsson and Lutz,
1990; Nilsson,
1990
). GABA is a well-established inhibitory neurotransmitter in
the vertebrate brain that increases chloride influx into the neuron and
reduces hypoxic/ischemic damage, in part by decreasing glutamate release
(Johns et al., 2000
).
Extracellular increases in GABA levels, as in the anoxic turtle, appear to be
due to a sustained release from intracellular sources rather than mere
overflow. The 41% increase in total GABA concentration in the frog echoes the
45-60% increase in turtle whole brain GABA concentration, but extracellular
levels increase to a much greater degree (to 300% of control by 4 h) than
intracellular levels. This increase is still quite small compared to that in
the turtle (which shows a 90-fold increase over 6 h;
Nilsson and Lutz, 1991
), thus
the utility of GABA as an inhibitory neurotransmitter in the frog may be a
case of `too little, too late'. As in the anoxic mammal, the greatest
increases in extracellular GABA concentration occur after depolarization
(Lutz and Reiners, 1997
), when
the response is pathological rather than adaptive, though even at that point
high concentrations of inhibitory compounds released concomitantly with
excitotoxic ones may constitute a protective mechanism against excess
excitatory amino acids (Saransaari and
Oja, 1998
). Such protective mechanisms simply appear to be
expressed earlier and more robustly in anoxia-tolerant organisms like the
turtle.
Like GABA, glycine is a well-described inhibitory neurotransmitter in the
brainstem and spinal cord of mammals, where it opens post-synaptic
glycine-gated chloride channels (Wheeler
et al., 1999), and may also lead to the internalization of
N-methyl-D-aspartate (NMDA) receptors
(Nong et al., 2003
), though it
is also excitatory in the higher brain as an allosteric activator of glutamate
(NMDA) receptors (Jones and Szatkowski,
1995
) and is thus likely to contribute to ischemic neuronal damage
(Saransaari and Oja, 2001
).
The increase in glycine concentration seen after a 4 h period of anoxia in the
frog brain, however, was not reflected in extracellular changes, thus it is
unlikely to have any significant neuroactive effect. In anoxia-tolerant
species an increase in both intracellular and extracellular brain glycine
levels is common (Nilsson et al.,
1991
; Nilsson and Lutz,
1991
), while extracellular increases in the mammalian brain are
common under anoxic or ischemic conditions
(Jones and Szatkowski, 1995
;
Tan et al., 1996
;
Li et al., 1999
;
Saransaari and Oja, 2001
) and
upon depolarization (Fabricius et al.,
1993
), and are also seen in anoxia-intolerant ectotherms
(Hylland et al., 1995
).
Taurine
As with changes in GABA and alanine, an increase in extracellular taurine
levels appears to be a common vertebrate response to anoxia, though unlike in
the anoxia-tolerant turtle, tissue levels did not increase in R.
pipiens, nor do they increase in the rat or anole. The increase observed
here in the anoxic frog is similar to those seen in other animals:
extracellular taurine levels increased approximately 34% over 4 h anoxia in
the crucian carp (Hylland and Nilsson,
1999), while 4 h anoxia in the frog resulted in a 24% increase.
Taurine is thought to play a variety of protective roles in the
hypoxic/ischemic brain; hypoxia, anoxia, and ischemia all increase taurine
release in mammals (al-Bekairi,
1989
; Saransaari and Oja,
1998
).
Aspartate, glutamate and glutamine
No significant changes in brain glutamate or glutamine concentrations or in
extracellular levels was observed, while aspartate showed a significant
decrease by 4 h in the whole brain coupled to a significant increase in the
extracellular space. This lack of change in tissue levels is similar to
mammals and other anoxia-intolerant animals during anoxia/ischemia
(Wallin et al., 2000); both
glutamate and glutamine levels decline in the brain tissue of T.
scripta (Nilsson et al.,
1990
) as well as in other hypoxia-tolerant animals like the shore
crab Carcinus maenas (Nilsson and
Winberg, 1993
). The lack of extracellular increase, however, is
noteworthy, as pathological increases in extracellular glutamate levels are
thought to play a central role in hypoxic/ischemic neuronal degeneration and
reperfusion injuries (Globus et al.,
1988
; Choi, 1992
).
Extracellular glutamate levels may increase as much as 37-fold in the ischemic
rat brain (Goda et al., 1998
),
while Young et al. (1993
)
found that levels in anoxic rat brain slices more than doubled in 10 min.
Excess glutamate then activates ionotropic and metabotropic receptors,
triggering a cascade of events resulting in neuronal death. In the frog,
glutamate is apparently released only after depolarization
(Lutz and Reiners, 1997
),
which suggests that glutamate is either not initially released in the anoxic
frog or that uptake mechanisms are still active even as ATP is depleted. Such
continued release and reuptake, albeit at reduced levels, was recently shown
to occur in the turtle (Milton et al.,
2002
). It may also be that an increase in extracellular glutamate
levels is in part prevented by the continued conversion of glutamate into GABA
via glutamate decarboxylase; a slight but statistically insignificant
(14%) decrease in tissue glutamate levels did occur in the anoxic frog brain.
In the frog, however, these processes eventually fail and a massive,
mammalian-like increase in extracellular glutamate levels occurs; it would be
interesting to determine if this is a cause or an effect of
depolarization.
The fall in tissue aspartate concentration, corresponding to a rise in the
extracellular compartment, suggests there may be a shift from the intra- to
the extracellular compartment over a 4 h period of anoxia. Extracellular
aspartate levels increase in anoxia-intolerant animals, including the rat,
during cortical spreading depression and upon anoxic depolarization
(Fabricius et al., 1993) and
in the anoxic rainbow trout (Hylland et
al., 1995
). Aspartate is considered to be neurotoxic when released
under cell-damaging conditions, compounding the effects of other excitatory
amino acids such as glutamate (Saransaari
and Oja, 1998
; Zeitlow et al.,
2002
).
Dopamine
The most striking difference we found, however, between anoxia-tolerant
animals and the frog was in the frog's lack of ability to regulate
extracellular dopamine levels. The uncontrolled release of dopamine (DA) into
the extracellular space of the mammalian brain has been identified as one
major cause of hypoxic/ischemic brain damage, and unlike the widespread
release of excitatory amino acids, which occurs only upon brain depolarization
(Rothman and Olney, 1986;
Baker et al., 1991
), DA release
is seen well before high energy stores are fully depleted
(Huang et al., 1994
). While
severe hypoxia or ischemia can increase extracellular levels as much as
500-fold (Globus et al., 1987
,
1988
), even mild hypoxia (11%
cortical oxygen levels) can cause neuronal damage, with DA increases as high
as 200% (Huang et al., 1994
).
In hypoxia-vulnerable mammals, this DA increase is highly toxic and causes
neuronal apoptosis in the striatum
(McLaughlin et al., 1998
) and
in cell cultures (Zietlow et al., 2002). Extracellular DA may contribute to
neuronal damage by altering cerebral blood flow and glucose metabolism
(Globus et al., 1987
), through
the production of reactive oxygen species
(Remblier et al., 1999
), by
modulating the release of excitatory amino acids
(Morari et al., 1998
),
especially by its interactions with glutamatergic systems
(Hoyt et al., 1997
;
Kalivas and Duffy, 1997
) and
directly via DA oxidation to a quinone moiety
(Stokes et al., 1999
). Like
the hypoxic/ischemic mammal, the frog is also unable to prevent massive DA
release into the extracellular space during anoxia. DA levels had nearly
tripled within the first hour of anoxia (though the time course of this
response was variable and thus not statistically significant); by 2 h, DA
levels had increased to 392% of basal, and to more than 1200% of basal by 4 h
of anoxia. The dramatic but rather slow increase (vs. depolarization)
in extracellular DA levels may reflect ATP depletion; such a relationship has
been demonstrated in rat nerve terminals in which the decline in ATP levels
and inhibition of Na+/K+-ATPase promoted a reversal of
neurotransmitter transporters (Santos et
al., 1996
). (Though is interesting to note that this does not
appear to occur with glutamate, whose uptake is also ATP-dependent). The DA
increase is quite distinct from what is seen in the anoxia-tolerant turtle,
which prevents such a catastrophic increase by a combination of reduced efflux
and continued uptake (Milton and Lutz,
1998
; S. L. Milton and P. L. Lutz, manuscript in preparation). The
frog model may thus be an interesting model in which to examine the mechanisms
of DA failure in early anoxia, which occur so rapidly in the mammal but over a
period of hours in the frog.
In summary, then, the anoxic frog is a potentially rich model of `slow death' in which to examine the mechanisms of anoxic neuronal failure that are so similar to those which occur in the hypoxic/ischemic mammalian brain. The extended time course of ATP depletion, excitotoxin release and anoxic depolarization in the frog compared to the mammal provides a large window of opportunity within which to define and manipulate the critical stages of anoxic failure. While there are some significant differences between the mammalian and frog brain, such as the lack of increase in extracellular glutamate levels, even these differences could provide a rich area for further investigation. The frog brain, at any rate, is clearly not like those of true facultative anaerobes such as the crucian carp and freshwater turtle, which defend ATP levels and ion homeostasis and thus avoid the catastrophe of anoxic vulnerability.
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References |
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---|
al-Bekairi, A. M. (1989). Effect of hypoxia and/or cold stress on plasma and brain amino acids in rat. Res. Commun. Chem. Pathol. Pharmacol. 64,287 -297.[Medline]
Baker, A. J., Zornow, M. H., Scheller, M. S., Yaksh, T. L., Skilling, S. R., Smullin, D. H., Larson, A. A. and Kuczenski, R. (1991). Changes in extracellular concentrations of glutamate, aspartate, glycine, dopamine, serotonin, and dopamine metabolites after transient global ischemia in the rabbit brain. J. Neurochem. 57,1370 -1379.[Medline]
Ben-Yoseph, O., Badar-Goffer, R. S., Morris, P. G. and Bachelard, H. S. (1993). Glycerol 3-posphate and lactate as indicators of the cerebral cytoplasmic redoc state in severe and mild hypoxia respectively: a 13C- and 31P-N.M.R. study. Biochem. J. 291,915 -919.[Medline]
Bickler, P. E., Donohoe, P. H. and Buck, L. T.
(2002). Molecular adaptations for survival during anoxia: lessons
from lower vertebrates. Neuroscientist
8, 234-242.
Brooks, S. P. J. and Storey, K. B. (1989). Regulation of glycolytic enzymes during anoxia in the turtle Pseudemys scripta. Am. J. Physiol. 257,R278 -R283.[Medline]
Choi, D. W. (1992). Excitotoxic cell death. J. Neurobiol. 23,1261 -1276.[Medline]
Donohoe, P. H. and Boutilier, R. G. (1998). The protective effects of metabolic rate depression in hypoxic cold submerged frogs. Resp. Pysiol. 111,325 -336.[CrossRef]
Donohoe, P. W., West, T. G. and Boutilier, R. G. (1998). Respiratory, metabolic, and acid-base correlates of aerobic metabolic rate reduction in overwintering frogs. Am. J. Physiol. 274,R704 -R710.[Medline]
Erecinska, M., Nelson, D., Wilson, D. F., and Silver, I. A. (1984). Neurotransmitter amino acids in the CNS. I. Regional changes in amino acid levels in rat brain during ischemia and reperfusion. Brain Res. 304,9 -22.[CrossRef][Medline]
Fabricius, M., Jensen, L. H. and Lauritzen, M. (1993). Microdialysis of interstitial amino acids during spreading depression and anoxic depolarization in rat neocortex. Brain Res. 612,61 -69.[CrossRef][Medline]
Globus, M. Y.-T., Busto, R., Dietrich, W. D., Martinez, E., Valdes, I. and Ginsberg, M. D. (1988). Effect of ischemia on the in vivo release of striatal dopamine, glutamate and alpha-amino-butyric-acid studied by intracerebral microdialysis. J. Neurochem. 51,1455 -1464.[Medline]
Globus, M. Y.-T., Ginsberg, M. D., Harik, S. I., Busto, R. and Dietrich, W. D. (1987). Role of dopamine in ischemic striatal injury: metabolic evidence. Neurol. 37,1712 -1719.[Abstract]
Goda, H., Ooboshi, H., Nakane, H., Ibayasi, S., Sadoshima, S. and Fujishima, M. (1998). Modulation of ischemia-evoked release of excitatory and inhibitory amino acids by adenosine A1 receptor agonist. Eur. J. Pharmacol. 357,149 -155.[CrossRef][Medline]
Hermes-Lima, M. and Storey, K. B. (1996). Relationship between anoxia exposure and antioxidant status in the frog Rana pipiens. Am. J. Physiol. 271,R918 -R925.[Medline]
Hochachka, P. W. (1980). Living Without Oxygen. Cambridge: Harvard University Press.181 pp.
Hochachka, P. W. and Somero, G. N. (2002). Biochemical Adaptation. New York: Oxford University Press.
Hoyt, K. R., Reynolds, I. J. and Hastings, G. (1997). Mechanisms of dopamine-induced cell death in cultured rat forebrain neurons, interactions with and differences from glutamate-induced cell death. Exp. Neurol. 143,269 -281.[CrossRef][Medline]
Huang, C. C., Najevardi, N. S., Tammela, O., Pastruszko, A., Delivoria-Papadopoulos, M. and Wilson, D. F. (1994). Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure. Neurochem. Res. 19,6499 -6655.
Hylland, P. and Nilsson, G. E. (1999). Extracellular levels of amino acid neurotransmitters during anoxia and forced energy deficiency in crucian carp brain. Brain Res. 823, 49-58.[CrossRef][Medline]
Hylland, P., Nilsson, G. E. and Lutz, P. L. (1994). Time course of anoxia induced increase in cerebral blood flow rate in turtles: evidence for a role of adenosine. J. Cereb. Blood Flow Metab. 14,877 -881.[Medline]
Hylland, P., Nilsson, G. E. and Johansson, D. (1995). Anoxic brain failure in an ectothermic vertebrate: release of amino acids and K+ in rainbow trout thalamus. Am. J. Physiol. 269,R1077 -R1084.[Medline]
Johns, L., Sinclair, A. J. and Davies, J. A. (2000). Hypoxia/hypoglycemia-induced amino acid release is decreased in vitro by preconditioning. Biochem. Biophys. Res. Commun. 276,134 -136.[CrossRef][Medline]
Jones, M. G. and Szatkowski, M. S. (1995). The role of glycine in anoxia/aglycaemia-induced potentiation of N-methyl-D-aspartate receptor-mediated postsynaptic potentials in the rat hippocampus. Neurosci. Lett. 201,227 -230.[CrossRef][Medline]
Kalivas, P. W. and Duffy, P. (1997). Dopamine regulation of extracellular glutamate in the nucleus accumbens. Brain Res. 761,173 -177.[CrossRef][Medline]
Knickerbocker, D. L. and Lutz, P. L. (2001). Slow ATP loss and the defense of ion homeostasis in the anoxic frog brain. J. Exp. Biol. 204,3547 -3551.[Medline]
Li, X., Wallin, C., Weber, S. G. and Sandberg, M. (1999). Net efflux of cysteine, glutathione, and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glumly transpeptidase. Bran Res. 815, 81-88.[CrossRef]
Lutz, P. L. and Reiners, R. (1997). Survival of
energy failure in the anoxic frog brain: delayed release of glutamate.
J. Exp. Biol. 200,2913
-2917.
Lutz, P. L., Nilsson, G. E. and Prentice, H. (2003). The Brain Without Oxygen: Causes of Failure and Molecular Mechanisms for Survival. 3rd edn. Boston: Kluwer Academic Publishers.
Lutz, P. L., Ortiz, M., Leone-Kabler, S. and Schulman, A. (1994). Regional changes in amino acid levels of the neonate rat brain during anoxia and recovery. Neurochem. Res. 19,1283 -1287.[Medline]
McLaughlin, B. A., Nelson, D., Erecinska, M. and Chesselet, M. F. (1998). Toxicity of dopamine to striatal neurons in vitro and potentiation of cell death by a mitochondrial inhibitor. J. Neurochem. 70,2406 -2415.[Medline]
Milton, S. L. and Lutz, P. L. (1998). Low extracellular dopamine levels are maintained in the anoxic turtle (Trachemys scripta) striatum. J. Cereb. Blood Flow Metab. 18,803 -807.[Medline]
Milton, S. L., Thompson, J. W. and Lutz, P. L.
(2002). Mechanisms for maintaining extracellular glutamate levels
in the anoxic turtle striatum. Am. J. Physiol. Reg. Integr. Comp.
Physiol. 282,R1317
-R1323.
Morari, M., Marti, M., Sbrenna, S., Fuxe, K., Bianchi, C. and Beani, L. (1998). Reciprocal dopamine-glutamate modulation of release in the basal ganglia. Neurochem. Int. 33,383 -397.[CrossRef][Medline]
Nilsson, G. E. (1990). Long-term anoxia in crucian carp: changes in the levels of amino acid and monoamine neurotransmitters in the brain, catecholamines in chromaffin tissue and liver glycogen. J. Exp. Biol. 150,295 -320.[Abstract]
Nilsson, G. E. and Lutz, P. L. (1991). Release of inhibitory neurotransmitters in response to anoxia in turtle brain. Am. J. Physiol. 261,R32 -R37.[Medline]
Nilsson, G. E. and Winberg, S. (1993). Changes in the brain levels of GABA and related amino acids in anoxic shore crab (Carcinus maenas). Am. J. Physiol. 264,R733 -R737.[Medline]
Nilsson, G. E., Alfaro, A. and Lutz, P. L.
(1990). Changes in turtle brain neurotransmitters and related
substances during anoxia. Am. J. Physiol. Reg. Integr. Comp.
Physiol. 259,R376
-R384.
Nilsson, G. E., Lutz, P. L., and Jackson, T. L. (1991). Neurotransmitters and anoxic survival of the brain: A comparison of anoxia-tolerant and anoxia-intolerant vertebrates. Physiol. Zool. 64,638 -652.
Nong, Y., Huang, Y. Q., Ju, W., Kalia, L. V., Ahmadian, G., Wang, Y. T. and Salter, M. W. (2003). Glycine binding primes NMDA receptor internalization. Nature 422,302 -307.[CrossRef][Medline]
Perez-Pinzon, M. A., Nilsson, G. E. and Lutz, P. L. (1993). Relationship between ion gradients and neurotransmitter release in the newborn rat striatum during anoxia. Brain Res. 602,228 -233.[CrossRef][Medline]
Remblier, C., Pontchamaud, R., Tallineaie, C., Piriou, A. and Huguet, F. (1999). Lactic acid induced increase of extracellular dopamine measured by microdialysis in rat striatum: evidence for glutamatergic and oxidative mechanisms. Brain Res. 837, 22-28.[CrossRef][Medline]
Rothman, S. M. and Olney, J. W. (1986). Glutamate and the pathophysiology of hypoxic-ischemic brain damage. Ann. Neurol. 19,105 -111.[Medline]
Santos, M. S., Moreno, A. J. and Carvalho, A. P.
(1996). Relationships between ATP depletion, membrane potential,
and the release of neurotransmitters in rat nerve terminals. An in vitro study
under conditions that mimic anoxia, hypoglycemia, and ischemia.
Stroke 27,941
-950.
Saransaari, P. and Oja, S. S. (1998). Release of endogenous glutamate, aspartate, GABA, and taurine from hippocampal slices from adult and developing mice under cell-damaging conditions. Neurochem. Res. 23,563 -570.[CrossRef][Medline]
Saransaari, P. and Oja, S. S. (2001). Characteristics of hippocampal glycine release in cell-damaging conditions in the adult and developing mouse. Neurochem. Res. 26,845 -852.[CrossRef][Medline]
Stokes, A. H., Hastings, T. G. and Vrana, K. E. (1999). Cytotoxic and genotoxic potential of dopamine. J. Neurosci. Res. 55,659 -665.[CrossRef][Medline]
Tan, W. K., Williams, C. E., During, M. J., Mallard, C. E., Gunning, M. I., Gunn, A. J. and Gluckman, P. D. (1996). Accumulation of cytotoxins during development of seizures and edema after hypoxic-ischemic injury during late gestation fetal sheep. Pediatr. Res. 39,791 -797.[Abstract]
Wallin, C., Puka-Sundvall, M., Hagberg, H., Weber, S. G., and Sandberg, M. (2000). Alterations in glutathione and amino acid concentrations after hypoxia-ischemia in the immature rat brain. Brain Res. Dev. Brain Res. 125, 51-60.[Medline]
West, T. G. and Boutilier, R. G. (1998). Metabolic suppression in anoxic frog muscle. J. Comp. Physiol. B 168,273 -280.[CrossRef][Medline]
Wheeler, M. D., Ikejema, K., Enomoto, N., Stacklewitz, R. F., Seabra, V., Zhong, Z., Yin, M., Schemmer, P., Rose, M. L., Rusyn, I., Bradford, B. and Thurman, R. G. (1999). Glycine: a new anti-inflammatory immunonutrient. Cell. Mol. Life Sci. 56,843 -856.[CrossRef][Medline]
Young, R. S., During, M. J., Donnelly, D. F., Aquila, W. J., Perry, V. L. and Haddad, G. G. (1993). Effect of anoxia on excitatory amino acids in brain slices of rats and turtles: in vitro microdialysis. Am. J. Physiol. 264,R716 -R719.[Medline]
Zeitlow, R., Sinclair, S. R., Schwiening, C. J., Dunnett, S. B. and Fawcett, J. W. (2002). The release of excitatory amino acids, dopamine, and potassium following transplantation of embryonic mesencephalic dopaminergic grafts to the rat striatum, and their effects on dopaminergic neuronal survival in vitro. Cell Transplant 11,637 -652.[Medline]