BRAIN NEUROTRANSMITTER CHANGES ASSOCIATED WITH EXERCISE AND STRESS IN A TELEOST FISH (SCIAENOPS-OCELLATUS)

Changes in the brain content of selected neuroactive monoamines and amino acids [dopamine, serotonin, epinephrine, norepinephrine, glutamate, c-aminobutyric acid (GABA), glycine, serine, alanine] and related metabolites were investigated in a teleost fish during sustained swimming in a tunnel respirometer (below the maximum sustainable speed U-crit), following swimming to exhaustion (above U-crit) and after exhaustion caused by chasing. Sustained exercise was associated with increases in the turnover rate of serotonin and with decreases in inhibitory GABA and norepinephrine levels. In the fish swum to exhaustion there was an increase in the activity of the dopaminergic system, which may be involved in the central control of endurance, and the serotogenic system. Concentrations of the excitatory neurotransmitter glutamate increased. A post exercise increase in GABA may facilitate a protective decrease in brain energy requirements during this period of energy crisis, as evidenced by the activation of anaerobic glycolysis. By contrast, fish chased to immobility showed no increase in activity of the dopaminergic system nor was there any increase in glutamate levels. There was, however, a marked increase in serotogenic metabolism which with a post-exercise increase in GABA, may be involved in protectively conserving energy. The observation that the brains remained fully aerobic while the muscles switched to anaerobic glycolysis suggests that (unlike swimming exhaustion) not all of the energy reserves have been called upon in the stress immobilized fish. Epinephrine, was detected in the brains of all of the stressed fish. We suggest that the patterns of changes seen reflect aspects of regulatory control by neurotransmitters on exercise activity in fish. The different responses to exhaustion produced by chasing stress versus that produced by exercise indicate a separate aetiology for each condition.