Ecophysiology of neuronal metabolism in transiently oxygen depleted environments
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The interactions between coral reef topography, tide cycles and photoperiod provided a selection pressure that resulted in adaptive physiological changes enabling sheltered hypoxic niches to be exploited by suitably specialised tropical reef fish, including at least one species of shark. The epaulette shark (Hemiscyllium ocellatum) can withstand temporary cyclic hypoxia in its natural environment, several hours of experimental hypoxia at 5% of saturation (Wise et al., 1998) and even exposure to anoxia (Renshaw et al., 2002). The epaulette responds to cyclic exposure to 5% of saturation by entering a state of hypoxia-induced neuronal hypometabolism (Renshaw and Mulvey, 2000). Our data from GABA immunochemistry, HPLC analysis and receptor binding studies revealed that, in the cerebellum, there was an accumulation of GABA within neurons with no change in the concentration of GABA and this was accompanied by a significant increase in receptor density without any decrease in receptor binding affinity. While all hypoxia and anoxia tolerant teleosts examined so far, respond with significant elevations in GABA, the phylogenetically older epaulette shark did not, indicating that the novel mechanism may be used to elicit energy conservation. The increased receptor density is likely to protect the cerebellum from re-oxygenation damage.
Comparative Biochemistry and Physiology. Part A: Molecular & Integrative Physiology
Physiology not elsewhere classified