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Fig. 2. A diagrammatic overview of some of the factors involved in negotiating
anoxia survival in the turtle brain. Constitutive factors that predispose for
anoxia tolerance include enhanced levels of glycogen stores, elevated heat
shock protein, increased densities of protective receptors and elevated
antioxidant capacities. Three distinct phases are involved in surviving and
recovering from anoxia. (1) A coordinated downregulation of ATP demand
processes to 
20% of normoxic levels. This phase, which takes 1–2 h,
includes a reduction in ion channel and electrical activities, a reduction in
glutamate and dopamine release, a reduction in protein synthesis and a
substantial increase in Hsp72 and Hsc73 levels. (2) Long-term survival (days)
at basal levels of ATP expenditure. Neuronal network integrity is preserved
through the continued operation of core activities. These include periodic
electrical activity, an increased release of GABA and a reduced but continued
release of glutamate and dopamine. There is a further increase in Hsc73,
indicating a `housekeeping' role for this protein during this period. (3) When
oxygen becomes available there is a rapid upregulation of neuronal processes
to restore full function, together with the activation of protection
mechanisms against reperfusion-generated reactive oxygen species (ROS).
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