In the brain, oxygen is required for both the synthesis and the degradation of monoamine transmitters, so monoaminergic systems can be expected to be strongly affected by anoxia. However, crucian carp (Carassius carassius L.) may survive anoxia for many days or even weeks. In the present study, crucian carp were exposed to anoxia for 22, 76, and 160 h at 8°C. All survived and were found to excrete ethanol at a constant rate. The brain concentrations of serotonin and its two main metabolites, 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophol, were analysed after each experiment. In a preliminary experiment, it was found that the brain of the crucian carp contained about the same amount of serotonin and 5-HIAA as two species less tolerant to anoxia - the common carp and the rainbow trout. The levels of the serotonin metabolites decreased drastically (by 80–90%) during anoxia, whereas serotonin levels were only slightly reduced (by 15% or less). These results suggest a complete or nearly complete stop in serotonin metabolism during anoxia.

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