While the mammalian brain exhibits an exquisite sensitivity to low oxygen levels, some vertebrate animals are able to survive extended periods of hypoxia (low oxygen), while others are able to live for hours to days in the complete absence of oxygen (anoxia). Many vertebrates encounter low oxygen tensions either acutely (exercise, diving), or chronically (hibernation, burrowing, high altitudes), but a few extremophiles have developed elaborate cerebral defense mechanisms to survive complete anoxia. The most anoxia tolerant vertebrates include certain species of North American pond turtles (Chrysemys picta and Trachemys scripta), and the crucian carp (Carassius carassius) of northern Europe, which can go for days without oxygen at room temperature and live weeks to months at 3°C. Survival is the result of complex physiological and molecular adaptations that defend the turtle against the stress of oxygen deprivation, with the suppression of pathological processes and the robust expression of protective mechanisms. These changes occur in three phases in the turtle: an initial inhibition to a highly depressed metabolic state, the long term maintenance of brain function in the absence of oxygen, and recovery in the face of potential oxidative stress. Metabolic changes occur against a backdrop of “constitutive preconditioning”; many protective mechanisms are seen even in normoxia, providing ready protection in the face of low oxygen. Early anoxia is characterized by increases in adenosine and cerebral blood flow, and the suppression of energy intensive processes such as excitatory neurotransmitter release, RNA transcription, and protein synthesis. Decreased ion permeability (channel arrest), and the suppression of action potentials (spike arrest) also decrease energy requirements; together, the reductions in ion flow and neurotransmitter release result in a reversible “coma”. This abrogated electrical activity is accompanied by increases in protective MAPK and heat shock proteins (HSPs) that tilt the balance in favor of cell survival and away from apoptosis. The maintenance of long term anoxia, interestingly, allows for the continued release and reuptake of neurotransmitters, albeit at greatly reduced rates. Many neuroprotective pathways remain activated, and antioxidant levels may even increase. In contrast to mammalian survival mechanisms, cellular protection does not appear to be related to a strong HIF response. The crucian carp follows a similar strategy of reducing energy demand to meet the decreased energy supplied by anaerobic glycolysis. However, the carp remains mildly active during anoxia; channel arrest and GABA levels increase to a lesser extent than in the turtle, and HSPs vary by organ and temperature. In the carp, HSP increases with cold temperatures may precondition the fish for the anoxia that results from iced-over winter ponds. Recent work in my laboratory has focused on the “Recovery” phase of anoxia; in the mammalian brain subjected to severe hypoxia or ischemia, reperfusion is followed by an overproduction of oxygen free radicals including superoxides, hydrogen peroxide, and hydroxyl radicals. These reactive oxygen species (ROS) damage cellular constituents including proteins, lipids, and nucleic acids and can result in more brain damage than the original hypoxic insult. Remarkably, the freshwater turtle recovers from extended anoxia without ROS overproduction due to both constitutively high levels of antioxidants, which continue to increase during anoxia, and the suppression of ROS formation linked in part to Hsp72. One potential antioxidant mechanism in T. scripta is the Methionine Sulfoxide Reductase system (MsrA and MsrB), which is upregulated in the turtle brain during anoxia/reoxygenation. Methionine (Met) is one of the most readily oxidized amino acids and Msr may restore the activity of damaged proteins by the reduction of oxidized Met. Msr has been shown in mammalian models to catalytically scavenge ROS before they damage cellular constituents by the reversible oxidation/ reduction of readily available protein Met, and increased cell death and ROS damage occur when Msr levels are reduced. The turtle is the first vertebrate model in which Msr transcript and protein levels are induced by low oxygen concentration, which makes T. scripta radically different from other animal models, and provides a unique opportunity to investigate the function and regulation of this peptide which may play a critical role in protection against oxidative damage. We are currently investigating the mechanism of Msr regulation in response to oxygen levels, which may involve the FOXO3a transcription factor; in C. elegans DAF-16/FOXO3a affects oxidative stress resistance and longevity, and activates the human MsrA promoter in cell culture.