Neurotransmitter levels and energy status in brain of fish species with and without the survival strategy of metabolic depression

Abstract

The effects of anoxia were studied in the whole brain of three fish species, each with a specific metabolic strategy for anoxic survival. Goldfish ( Carassius auratus) combine a lactate to ethanol conversion with a metabolic depression, tilapia ( Oreochromis mossambicus) use an anaerobic glycolysis with the strategy of metabolic depression, and carp ( Cyprinus carpio) use an increased anaerobic glycolysis for energy production. Tilapia and carp were exposed to anoxia until they lost equilibrium and exhibited escape reactions, this occurred after 2 hours of anoxia for tilapia and 30 minutes of anoxia for carp. Goldfish were exposed to a selected period of 8 hours anoxia. The energy status and neurotransmitter (amino acid) levels in whole brain tissue were measured after anoxia exposure. The energy status was affected in all three groups exposed to anoxia. Lactic acid levels increased five- to six-fold in all three groups. No direct correlation was observed between energy status and survival strategy. Remarkably, the changes in the amino acid patterns in whole fish brains show the greatest changes in the anoxia-tolerant goldfish, an intermediate pattern in tilapia, and nearly no changes in the anoxia-intolerant carp. The changes in amino acid are probably dependent on the period of anoxia exposure. For goldfish, the lactate-ethanol conversion primarily determines anoxic survival, but the strategy of metabolic depression observed in goldfish and tilapia 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 prolong survival.