Mechanisms of failure and survival in the anoxic fish brain : With focus on the crucian carp

Sammanfattning: Neural responses to anoxia were studied in two fish species, the rainbow trout(Oncorhynchus mykiss) and the crucian carp (Carassius carassius). The rainbow troutbrain reacted to anoxia by a massive release of excitatory amino acids and K+ into theextracellular compartment, in a fashion very much like the brain of mammals duringanoxia. By contrast, the crucian carp is extremely tolerant to anoxia, which is probablyrelated to an ability to down-regulate the ATP use in order to avoid a fall in ATP levelsand save on the glycogen stores. This study shows that the crucian carp lowers itsspontaneous locomotor activity by ~50% in response to anoxia, which corresponds to anestimated fall in body energy consumption by 35-40%. Furthermore, in anoxia the heatproduction of isolated telencephalic slices was found to be reduced by 37%,corresponding to a 31% reduction in ATP turnover rate, although energy charge wasmaintained. The anoxic crucian carp did not down regulate the rate of tram-membraneK+ flux in brain ("channel arrest"), a phenomenon that thought to be an energy savingstrategy in the anoxia-tolerant fresh water turtle Trachemys scripta. In normoxia thecrucian carp maintained ion-homeostasis even though the glycolysis was inhibited withiodoacetate, whilst in anoxia, a functioning glycolysis was found to be totally essential.The opening of KATP-channels in mammalian neurons has been suggested to prolong thesurvival time of energetically depressed neurons during short term energy deficiency bycounteracting depolarisation and glutamate release. However, I was unable to find anyevidence for such channels in the brains of crucian carp and rainbow trout. Finally, it wasfound that in anoxia, the crucian carp retina suppresses the electric response to visualstimulation by ~80%, and that there is a corresponding decrease in evoked potential in theoptic tectum of the same magnitude. These changes were reversible and are likely toreflect a strategy for reducing ATP consumption during anoxia.