CONTRASTING STRATEGIES FOR ANOXIC BRAIN SURVIVAL - GLYCOLYSIS UP OR DOWN

Anoxia-tolerant turtles and carp (Carassius) exhibit contrasting strat egies for anoxic brain survival, In the turtle brain, the energy consu mption is deeply depressed to the extent of producing a comatose-like state, Brain metabolic depression is brought about by activating chann el arrest to reduce ion flux and through the release of inhibitory gam ma-aminobutyric acid (GABA) and the upregulation of GABA(A) receptors, Key glycolytic enzymes are down-regulated during prolonged anoxia, Th e result is a suppression of neurotransmission and a substantial depre ssion in brain electrical activity. By contrast, Carassius remain acti ve during anoxia, though at a reduced level, As in the turtle, there i s an adenosine-mediated increase in brain blood flow but, in contrast to the turtle, this increase is sustained throughout the anoxic period . Key glycolytic enzymes are up-regulated and anaerobic glycolysis is enhanced. There is no evidence of channel arrest in Carassius brain, T he probable result is that electrical activity in the brain is not sup pressed but instead maintained at a level sufficient to regulate and c ontrol the locomotory and sensory activities of the anoxic carp. The k ey adaptations permitting the continued high level of glycolysis in Ca rassius are the production and excretion of ethanol as the glycolytic end-product, which avoids self-pollution by lactate produced during gl ycolysis that occurs in other vertebrates.