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Is turtle longevity linked to enhanced mechanisms for surviving brain anoxia and reoxygenation?
Experimental Gerontology 38 (2003) 797–800 www.elsevier.com/locate/expgero
Is turtle longevity linked to enhanced mechanisms for surviving brain anoxia and reoxygenation? Peter L. Lutza,*, Howard M. Prenticeb, Sarah L. Miltona a
Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA b Department of Biomedical Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA
Abstract We suggest that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity since many of them are linked to age related neurodegeneration. In the turtle the mechanisms for conserving ion channel function are particularly robust. The anoxic turtle brain avoids excitatory neurotransmitter toxicity by maintaining a balance between dopamine and glutamate-release and still active uptake mechanisms. In the anoxic turtle brain the inhibitory tone is strengthened through a sustained rise in extracellular GABA, and a corresponding increase in the density of GABA(A) receptors. The turtle has enhanced mechanisms that protect against the formation of ROS and mechanisms to protect from ROS damage. As many of these may be selectively activated during anoxia and recovery, the turtle could serve as a useful model to identify and investigate mechanisms for activating key protection and rescue mechanisms implicated in aging. q 2003 Elsevier Science Inc. All rights reserved. Keywords: Turtle; Anoxia; Reoxygenation; Longevity; Neurodegeneration; Reactive oxygen species
1. Introduction Freshwater turtles are extraordinary in their ability to endure brain anoxia, being able to preserve neuronal functional capacity for days at 25 8C and months at 3 8C in the complete absence of oxygen (Lutz et al., 2003). In addition the turtle brain must survive a massive increase in reactive oxygen species (ROS) when it is reoxygenated after such extreme exposure to anoxia (Lutz et al., 2003). Turtles are also known for their exceptional longevity, and can serve as a model of ‘negligible senescence’ (Finch, 1990). Since brain aging is associated with increased synaptic dysfunction and as ROS are implicated in the pathogenesis of various age related neurodenerative disorders (Mattson et al., 2002), we suggest that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity. 2. Adaptive neuroprotective mechanisms in anoxia and aging The vertebrate brain is extremely sensitive to a reduction in oxygen supply and suffers energy failure with consequent * Corresponding author. Tel.: þ 1-561-297-2886; fax: þ1-561-297-2749. E-mail address: [email protected] (P.L. Lutz).
irreversible damage within minutes of anoxia (Lutz et al., 2003). The turtle brain is one of the few exceptions being able to survive days without oxygen. This is achieved by a carefully orchestrated reduction in major brain energy demanding activities while maintaining neuronal ATP levels and ionic gradients (Lutz et al., 2003). The result is an 80% depression in turtle brain energy consumption to a level where metabolic demand can be fully met by anaerobic glycolysis. Key processes involved in this metabolic suppression include the down-regulation of ion channel activities, the enhancement of brain inhibitory processes and the suppression of excitatory processes (Lutz et al., 2003); these processes may also be involved in protection against age-related neuropathologies.
2.1. Ion channels Age related deleterious functional changes in ion channels, particularly Naþ, Kþ and Ca2þ channels, are thought to be an important part of the aging process (Annunziato et al., 2002). Indeed, it has been suggested that voltage-gated cationic channels are key factors in the genesis and progression of brain aging and neuronal diseases and that these are particularly susceptible to
0531-5565/03/$ - see front matter q 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0531-5565(03)00111-6
798
P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
oxidative modification from excessive ROS (Annunziato et al., 2002). In the turtle the mechanisms for conserving ion channel function are particularly robust. The shift to the metabolically depressed anoxic state involves the ‘arrest’ of ion channels which results in a decrease in excitability and a reduction in the costs of transmembrane ion pumping. Within the first hours of anoxia there is a reduction in the density of voltage gated Naþ channels (Lutz et al., 2003), a decrease in the mRNA for voltage gated Kþ channels (Prentice et al., submitted), a decrease in Kþ leak channels (Pek and Lutz, 1998) and a silencing of the Ca2þ permeable NMDA receptor (Bickler et al., 2000). Adenosine and KATP channels appear to be involved in the downregulation of the activities of Kþ channels (Pek and Lutz, 1998) and NMDA receptor activity (Buck and Bickler, 1998). The turtle therefore has efficient mechanisms not only to functionally down regulate ion channel activities but also to maintain their integrity during prolonged anoxia and to reactivate these systems when oxygen supply is restored.
3. Antioxidants A few minutes of anoxia is sufficient to reduce the mitochondrial respiratory chain electron carriers to such an extent that toxic amounts of ROS will be generated when oxygen supply is restored (Lutz et al., 2003). Clearly the turtle brain is in a prime condition to experience massive amounts of ROS when it is reoxygenated after many hours of anoxia. As the turtle survives this insult (Fig. 1), it either has mechanisms that protect against the formation of ROS and/or mechanisms to protect from ROS damage (Fig. 2). There is gathering evidence that the turtle indeed possesses such capacities. Turtles maintain high tissue levels of antioxidants. The freshwater turtle (Trachemys scripta elegans) has high constitutive activities of catalase, superoxide dismutase, and alkyl hydroperoxide reductase (Willmore and Storey, 1997a). The turtle brain has greater concentrations of ascorbic acid compared to mammals with cortex levels
2.2. Neurotransmitters and receptors One of the major destructive events in the energy deprived brain is a massive and uncontrolled increase in excitatory neurotransmitters (EAA), such as glutamate and dopamine, into the extracellular space where they act as powerful neurotoxins (Lutz et al., 2003). An increased susceptibility to EAAs may be an important factor in age related neurodegenerative diseases (Brewer, 1998). For example, neurons from old rats show a 5– 10% higher death rate with glutamate compared to young rats (Brewer, 1998). The dopaminergic signaling pathway is consistently impaired during aging (Mattson et al., 2002). The anoxic turtle brain avoids the EAA challenge by maintaining a balance between dopamine and glutamaterelease and still active uptake mechanisms (Milton and Lutz, 1998; Milton et al., 2002). GABA, the major inhibitory neurotransmitter, may have an important role in aging since GABA protects neurons in some experimental models of age related neurodegenerative disorders (Mattson et al., 2002). In the turtle brain there is a sustained rise in extracellular GABA during anoxia, and a corresponding increase in the density of GABA(A) receptors (Lutz et al., 2003). The result will be a strengthening of the inhibitory tone. Interestingly, an upregulation of GABA(A) receptor efficacy is seen in medial vestibular nucleus neurons of aged rats, possibly in compensation for a reduction in inhibitory inputs from neuronal loss in the aged brain (Him et al., 2001). Age related alterations in GABA(A) receptor function has also been related to the sleep disturbances of the elderly (Mathias et al., 2001)
Fig. 1. Mixed brain cultures survive prolonged anoxia and reoxygenation: (a) normoxia, (b) 2 days of anoxia, (c) 2 days of anoxia with 1 day of reoxygenation.
P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
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Fig. 2. Primary targets for enhanced oxidative stress response in turtle (a) lower rate of ROS production, (b) altered pattern of ROS induced transcription and signaling factors, (c) activation of neurogenesis, (d) increased production of antioxidants, (e) protection against ROS oxidation, (f) enhanced repair mechanisms, (g) protection of vulnerable targets.
being 2 –3 times greater than in the mammalian cortex (Rice et al., 1995). The turtle may also have enhanced defenses against oxidative damage. For example, in contrast to mammals, there is little evidence in the turtle of lipid peroxidation damage during anoxia or recovery (Willmore and Storey, 1997b). The expression of key adaptive molecules associated with brain hypoxia tolerance may contribute to survival during subsequent reoxygenation. In particular, the immediate early gene products c-fos and c-jun and the heart shock protein HSP-70, which protect the mammalian brain in hypoxia are elevated in the turtle brain during anoxia (Lutz and Prentice, 2002). In rats, age diminishes the stress induced activity levels of the transcription factor NF-kappaB, an important transcription regulator of many genes that play a role in recovery from acute or chronic trauma (Toliver-Kinsky et al., 2002). Interestingly, in the turtle NF-kappaB shows maximal binding to a promoter consensus sequence during anoxia (Lutz and Prentice, 2002). Mechanisms that protect against ROS are of particular interest in aging studies since there is considerable evidence implicating oxidative stress in the aging process (Sohal et al., 2002). For example, it has been suggested that part of the aging process might be deleterious functional changes in ion channel activity induced by ROS and reactive nitrogen species (Annunziato et al., 2002). Comparative studies indicate that the rate of generation of endogenous oxidative damage determines, at least in part, the rate of aging in
animals (Barja, 2002). And it has been suggested that a low rate of free radical production near DNA, together with a high rate of DNA repair, can be responsible for the slow rate of accumulation of DNA damage and thus the slow aging rate of longevous animals (Barja, 2002). This may very well be the case in the turtle, with the addition that in the turtle these processes may be selectively switched on and off as the animal experiences and recovers from long periods of anoxia.
4. Conclusion The evidence suggests that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity. As many of these may be selectively activated during anoxia and recovery the turtle could serve as a useful model to identify and investigate mechanisms for activating key protection and rescue mechanisms implicated in aging.
References Annunziato, L., Pannaccione, A., Cataldi, M., Secondo, A., Castaldo, P., Di Renzo, G., Taglialatela, M., 2002. Modulation of ion channels by reactive oxygen and nitrogen species: a pathophysiological role in brain aging? Neurobiol. Aging 23, 819 –834. Barja, G., 2002. Rate of generation of oxidative stress-related damage and animal longevity. Free Radic. Biol. Med. 33, 1167– 1172.
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P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
Bickler, P.E., Donohoe, P.H., Buck, L.T., 2000. Hypoxia-induced silencing of NMDA receptors in Turtle neurons. J. Neurosci. 20, 3522–3528. Brewer, G., 1998. Age-related toxicity to lactate, glutamate, and b-amyloid in cultured adult neurons. Neurobiol. Aging 19, 561 –568. Buck, L.T., Bickler, P.E., 1998. Adenosine and anoxia reduce N-methyl-D aspartate receptor open probability in turtle cerebrocortex. J. Exp. Biol. 210, 289–297. Finch, C.E., 1990. Longevity, Senescence, and the Genome, University of Chicago Press, Chicago. Him, A., Johnston, A.R., Yau, J.L., Seckl, J., Dutia, M.B., 2001. Tonic activity and GABA responsiveness of medial vestibular nucleus neurons in aged rats. Neuroreport 12, 3965–3968. Lutz, P.L., Prentice, H.M., 2002. Sensing and responding to hypoxia, molecular and physiological mechanisms. Integr. Comp. Biol. 42, 436– 468. Lutz, P.L., Nilsson, G.E., Prentice, H.M., 2003. The Brain without Oxygen: Causes of Failure Molecular and Physiological Mechanisms for Survival, Third ed., Kluwer, Dordrecht. Mathias, S., Wetter, T.C., Steiger, A., Lancel, M., 2001. The GABA uptake inhibitor tiagabine promotes slow wave sleep in normal elderly subjects. Neurobiol. Aging 22, 247–253. Mattson, M.P., Chan, S.L., Duan, W., 2002. Modification of brain aging and neurodegenerative disorders by genes, diet and behavior. Physiol. Rev. 82, 637 –672.
Milton, S.L., Lutz, P.L., 1998. Low extracellular dopamine levels are maintained in the anoxic turtle brain. J. Cereb. Blood Flow Metab. 18, 803 –807. Milton, S.L., Thompson, J.W., Lutz, P.L., 2002. Mechanisms for maintaining extracellular glutamate in the anoxic turtle striatum. Am. J. Physiol. 282, R1317–R1323. Pek, M., Lutz, P.L., 1998. K þ ATP channel activation provides transient protection in anoxic turtle brain. Am. J. Physiol. 44, R2023–R2027. Prentice, H.M., Milton, S.L., Scheurle, D., Lutz, P.L., The gene transcription of voltage gated potassium channels is reversibly regulated by oxygen supply. Am. J. Physiol. Submitted for publication Rice, M.E., Lee, E.J., Choy, Y., 1995. High levels of ascorbic acid, not glutathione, in the CNS of anoxia-tolerant reptiles contrasted with levels in anoxia-intolerant species. J. Neurochem. 64, 1790–1799. Sohal, R.S., Mockett, R.J., Orr, W.C., 2002. Mechanisms of aging: an appraisal of the oxidative stress hypothesis. Free Radic. Biol. Med. 33, 575 –586. Toliver-Kinsky, T., Rassin, D., Perez-Polo, J.R., 2002. NF-kappaB activity decreases in basal forebrain of young and aged rats after hyperoxia. Neurobiol. Aging 23, 899– 905. Willmore, W.G., Storey, K.B., 1997a. Antioxidant systems and anoxia tolerance in a freshwater turtle Trachemys scripta elegans. Mol. Cell Biochem. 170, 177 –185. Willmore, W.G., Storey, K.B., 1997b. Glutathione systems and anoxia tolerance in turtles. Am. J. Physiol. 273, R219–R225.
Is turtle longevity linked to enhanced mechanisms for surviving brain anoxia and reoxygenation? Peter L. Lutza,*, Howard M. Prenticeb, Sarah L. Miltona a
Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA b Department of Biomedical Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA
Abstract We suggest that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity since many of them are linked to age related neurodegeneration. In the turtle the mechanisms for conserving ion channel function are particularly robust. The anoxic turtle brain avoids excitatory neurotransmitter toxicity by maintaining a balance between dopamine and glutamate-release and still active uptake mechanisms. In the anoxic turtle brain the inhibitory tone is strengthened through a sustained rise in extracellular GABA, and a corresponding increase in the density of GABA(A) receptors. The turtle has enhanced mechanisms that protect against the formation of ROS and mechanisms to protect from ROS damage. As many of these may be selectively activated during anoxia and recovery, the turtle could serve as a useful model to identify and investigate mechanisms for activating key protection and rescue mechanisms implicated in aging. q 2003 Elsevier Science Inc. All rights reserved. Keywords: Turtle; Anoxia; Reoxygenation; Longevity; Neurodegeneration; Reactive oxygen species
1. Introduction Freshwater turtles are extraordinary in their ability to endure brain anoxia, being able to preserve neuronal functional capacity for days at 25 8C and months at 3 8C in the complete absence of oxygen (Lutz et al., 2003). In addition the turtle brain must survive a massive increase in reactive oxygen species (ROS) when it is reoxygenated after such extreme exposure to anoxia (Lutz et al., 2003). Turtles are also known for their exceptional longevity, and can serve as a model of ‘negligible senescence’ (Finch, 1990). Since brain aging is associated with increased synaptic dysfunction and as ROS are implicated in the pathogenesis of various age related neurodenerative disorders (Mattson et al., 2002), we suggest that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity. 2. Adaptive neuroprotective mechanisms in anoxia and aging The vertebrate brain is extremely sensitive to a reduction in oxygen supply and suffers energy failure with consequent * Corresponding author. Tel.: þ 1-561-297-2886; fax: þ1-561-297-2749. E-mail address: [email protected] (P.L. Lutz).
irreversible damage within minutes of anoxia (Lutz et al., 2003). The turtle brain is one of the few exceptions being able to survive days without oxygen. This is achieved by a carefully orchestrated reduction in major brain energy demanding activities while maintaining neuronal ATP levels and ionic gradients (Lutz et al., 2003). The result is an 80% depression in turtle brain energy consumption to a level where metabolic demand can be fully met by anaerobic glycolysis. Key processes involved in this metabolic suppression include the down-regulation of ion channel activities, the enhancement of brain inhibitory processes and the suppression of excitatory processes (Lutz et al., 2003); these processes may also be involved in protection against age-related neuropathologies.
2.1. Ion channels Age related deleterious functional changes in ion channels, particularly Naþ, Kþ and Ca2þ channels, are thought to be an important part of the aging process (Annunziato et al., 2002). Indeed, it has been suggested that voltage-gated cationic channels are key factors in the genesis and progression of brain aging and neuronal diseases and that these are particularly susceptible to
0531-5565/03/$ - see front matter q 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S0531-5565(03)00111-6
798
P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
oxidative modification from excessive ROS (Annunziato et al., 2002). In the turtle the mechanisms for conserving ion channel function are particularly robust. The shift to the metabolically depressed anoxic state involves the ‘arrest’ of ion channels which results in a decrease in excitability and a reduction in the costs of transmembrane ion pumping. Within the first hours of anoxia there is a reduction in the density of voltage gated Naþ channels (Lutz et al., 2003), a decrease in the mRNA for voltage gated Kþ channels (Prentice et al., submitted), a decrease in Kþ leak channels (Pek and Lutz, 1998) and a silencing of the Ca2þ permeable NMDA receptor (Bickler et al., 2000). Adenosine and KATP channels appear to be involved in the downregulation of the activities of Kþ channels (Pek and Lutz, 1998) and NMDA receptor activity (Buck and Bickler, 1998). The turtle therefore has efficient mechanisms not only to functionally down regulate ion channel activities but also to maintain their integrity during prolonged anoxia and to reactivate these systems when oxygen supply is restored.
3. Antioxidants A few minutes of anoxia is sufficient to reduce the mitochondrial respiratory chain electron carriers to such an extent that toxic amounts of ROS will be generated when oxygen supply is restored (Lutz et al., 2003). Clearly the turtle brain is in a prime condition to experience massive amounts of ROS when it is reoxygenated after many hours of anoxia. As the turtle survives this insult (Fig. 1), it either has mechanisms that protect against the formation of ROS and/or mechanisms to protect from ROS damage (Fig. 2). There is gathering evidence that the turtle indeed possesses such capacities. Turtles maintain high tissue levels of antioxidants. The freshwater turtle (Trachemys scripta elegans) has high constitutive activities of catalase, superoxide dismutase, and alkyl hydroperoxide reductase (Willmore and Storey, 1997a). The turtle brain has greater concentrations of ascorbic acid compared to mammals with cortex levels
2.2. Neurotransmitters and receptors One of the major destructive events in the energy deprived brain is a massive and uncontrolled increase in excitatory neurotransmitters (EAA), such as glutamate and dopamine, into the extracellular space where they act as powerful neurotoxins (Lutz et al., 2003). An increased susceptibility to EAAs may be an important factor in age related neurodegenerative diseases (Brewer, 1998). For example, neurons from old rats show a 5– 10% higher death rate with glutamate compared to young rats (Brewer, 1998). The dopaminergic signaling pathway is consistently impaired during aging (Mattson et al., 2002). The anoxic turtle brain avoids the EAA challenge by maintaining a balance between dopamine and glutamaterelease and still active uptake mechanisms (Milton and Lutz, 1998; Milton et al., 2002). GABA, the major inhibitory neurotransmitter, may have an important role in aging since GABA protects neurons in some experimental models of age related neurodegenerative disorders (Mattson et al., 2002). In the turtle brain there is a sustained rise in extracellular GABA during anoxia, and a corresponding increase in the density of GABA(A) receptors (Lutz et al., 2003). The result will be a strengthening of the inhibitory tone. Interestingly, an upregulation of GABA(A) receptor efficacy is seen in medial vestibular nucleus neurons of aged rats, possibly in compensation for a reduction in inhibitory inputs from neuronal loss in the aged brain (Him et al., 2001). Age related alterations in GABA(A) receptor function has also been related to the sleep disturbances of the elderly (Mathias et al., 2001)
Fig. 1. Mixed brain cultures survive prolonged anoxia and reoxygenation: (a) normoxia, (b) 2 days of anoxia, (c) 2 days of anoxia with 1 day of reoxygenation.
P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
799
Fig. 2. Primary targets for enhanced oxidative stress response in turtle (a) lower rate of ROS production, (b) altered pattern of ROS induced transcription and signaling factors, (c) activation of neurogenesis, (d) increased production of antioxidants, (e) protection against ROS oxidation, (f) enhanced repair mechanisms, (g) protection of vulnerable targets.
being 2 –3 times greater than in the mammalian cortex (Rice et al., 1995). The turtle may also have enhanced defenses against oxidative damage. For example, in contrast to mammals, there is little evidence in the turtle of lipid peroxidation damage during anoxia or recovery (Willmore and Storey, 1997b). The expression of key adaptive molecules associated with brain hypoxia tolerance may contribute to survival during subsequent reoxygenation. In particular, the immediate early gene products c-fos and c-jun and the heart shock protein HSP-70, which protect the mammalian brain in hypoxia are elevated in the turtle brain during anoxia (Lutz and Prentice, 2002). In rats, age diminishes the stress induced activity levels of the transcription factor NF-kappaB, an important transcription regulator of many genes that play a role in recovery from acute or chronic trauma (Toliver-Kinsky et al., 2002). Interestingly, in the turtle NF-kappaB shows maximal binding to a promoter consensus sequence during anoxia (Lutz and Prentice, 2002). Mechanisms that protect against ROS are of particular interest in aging studies since there is considerable evidence implicating oxidative stress in the aging process (Sohal et al., 2002). For example, it has been suggested that part of the aging process might be deleterious functional changes in ion channel activity induced by ROS and reactive nitrogen species (Annunziato et al., 2002). Comparative studies indicate that the rate of generation of endogenous oxidative damage determines, at least in part, the rate of aging in
animals (Barja, 2002). And it has been suggested that a low rate of free radical production near DNA, together with a high rate of DNA repair, can be responsible for the slow rate of accumulation of DNA damage and thus the slow aging rate of longevous animals (Barja, 2002). This may very well be the case in the turtle, with the addition that in the turtle these processes may be selectively switched on and off as the animal experiences and recovers from long periods of anoxia.
4. Conclusion The evidence suggests that the processes that protect the turtle brain against anoxia and subsequent reoxygenation might also contribute to turtle longevity. As many of these may be selectively activated during anoxia and recovery the turtle could serve as a useful model to identify and investigate mechanisms for activating key protection and rescue mechanisms implicated in aging.
References Annunziato, L., Pannaccione, A., Cataldi, M., Secondo, A., Castaldo, P., Di Renzo, G., Taglialatela, M., 2002. Modulation of ion channels by reactive oxygen and nitrogen species: a pathophysiological role in brain aging? Neurobiol. Aging 23, 819 –834. Barja, G., 2002. Rate of generation of oxidative stress-related damage and animal longevity. Free Radic. Biol. Med. 33, 1167– 1172.
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P.L. Lutz et al. / Experimental Gerontology 38 (2003) 797–800
Bickler, P.E., Donohoe, P.H., Buck, L.T., 2000. Hypoxia-induced silencing of NMDA receptors in Turtle neurons. J. Neurosci. 20, 3522–3528. Brewer, G., 1998. Age-related toxicity to lactate, glutamate, and b-amyloid in cultured adult neurons. Neurobiol. Aging 19, 561 –568. Buck, L.T., Bickler, P.E., 1998. Adenosine and anoxia reduce N-methyl-D aspartate receptor open probability in turtle cerebrocortex. J. Exp. Biol. 210, 289–297. Finch, C.E., 1990. Longevity, Senescence, and the Genome, University of Chicago Press, Chicago. Him, A., Johnston, A.R., Yau, J.L., Seckl, J., Dutia, M.B., 2001. Tonic activity and GABA responsiveness of medial vestibular nucleus neurons in aged rats. Neuroreport 12, 3965–3968. Lutz, P.L., Prentice, H.M., 2002. Sensing and responding to hypoxia, molecular and physiological mechanisms. Integr. Comp. Biol. 42, 436– 468. Lutz, P.L., Nilsson, G.E., Prentice, H.M., 2003. The Brain without Oxygen: Causes of Failure Molecular and Physiological Mechanisms for Survival, Third ed., Kluwer, Dordrecht. Mathias, S., Wetter, T.C., Steiger, A., Lancel, M., 2001. The GABA uptake inhibitor tiagabine promotes slow wave sleep in normal elderly subjects. Neurobiol. Aging 22, 247–253. Mattson, M.P., Chan, S.L., Duan, W., 2002. Modification of brain aging and neurodegenerative disorders by genes, diet and behavior. Physiol. Rev. 82, 637 –672.
Milton, S.L., Lutz, P.L., 1998. Low extracellular dopamine levels are maintained in the anoxic turtle brain. J. Cereb. Blood Flow Metab. 18, 803 –807. Milton, S.L., Thompson, J.W., Lutz, P.L., 2002. Mechanisms for maintaining extracellular glutamate in the anoxic turtle striatum. Am. J. Physiol. 282, R1317–R1323. Pek, M., Lutz, P.L., 1998. K þ ATP channel activation provides transient protection in anoxic turtle brain. Am. J. Physiol. 44, R2023–R2027. Prentice, H.M., Milton, S.L., Scheurle, D., Lutz, P.L., The gene transcription of voltage gated potassium channels is reversibly regulated by oxygen supply. Am. J. Physiol. Submitted for publication Rice, M.E., Lee, E.J., Choy, Y., 1995. High levels of ascorbic acid, not glutathione, in the CNS of anoxia-tolerant reptiles contrasted with levels in anoxia-intolerant species. J. Neurochem. 64, 1790–1799. Sohal, R.S., Mockett, R.J., Orr, W.C., 2002. Mechanisms of aging: an appraisal of the oxidative stress hypothesis. Free Radic. Biol. Med. 33, 575 –586. Toliver-Kinsky, T., Rassin, D., Perez-Polo, J.R., 2002. NF-kappaB activity decreases in basal forebrain of young and aged rats after hyperoxia. Neurobiol. Aging 23, 899– 905. Willmore, W.G., Storey, K.B., 1997a. Antioxidant systems and anoxia tolerance in a freshwater turtle Trachemys scripta elegans. Mol. Cell Biochem. 170, 177 –185. Willmore, W.G., Storey, K.B., 1997b. Glutathione systems and anoxia tolerance in turtles. Am. J. Physiol. 273, R219–R225.