NEUROTRANSMITTER LEVELS AND ENERGY STATUS IN BRAIN OF FISH SPECIES WITH AND WITHOUT THE SURVIVAL STRATEGY OF METABOLIC DEPRESSION

The effects of anoxia were studied in the whole brain of three fish sp ecies, each with a specific metabolic strategy for anoxic survival. Go ldfish (Carassius auratus) combine a lactate to ethanol conversion wit h a metabolic depression, tilapia (Oreochromis mossambicus) use an ana erobic glycolysis with the strategy of meta belie depression, and carp (Cyprinus carpio) use an increased anaerobic glycolysis for energy pr oduction. Tilapia and carp were exposed to anoxia until they lost equi librium and exhibited escape reactions, this occurred after 2 hours of anoxia for tilapia and 30 minutes of anoxia for carp. Goldfish were e xposed to a selected period of 8 hours anoxia. The energy status and n eurotransmitter (amino acid) levels in whole brain tissue were measure d after anoxia exposure. The energy status was affected in all three g roups exposed to anoxia. Lactic acid levels increased five- to six-fol d in all three groups. No direct correlation was observed between ener gy status and survival strategy. Remarkably, the changes in the amino acid patterns in whole fish brains show the greatest changes in the an oxia-tolerant goldfish, an intermediate pattern in tilapia, and nearly no changes in the anoxia-intolerant carp. The changes in amino acid a re probably dependent on the period of anoxia exposure. For goldfish, the lactate-ethanol conversion primarily determines anoxic survival, b ut the strategy of metabolic depression observed in goldfish and tilap ia may contribute secondarily to anoxic tolerance. It is hypothesized that a decrease Of excitatory neurotransmitters (mainly glutamate), in combination with an increase of inhibitory neurotransmitters (mainly GABA), may contribute to the process of metabolic depression and prolo ng survival.