Changes in turtle brain neurotransmitters and related substances during anoxia.

Freshwater turtles (Pseudemys scripta elegans) were exposed to 0.5-13 h of anoxia at 25 degrees C, whereupon the brain concentrations of 14 amino acid and monoamine neurotransmitters and related substances were measured. Monoamines are of particular interest, because their synthesis and (in part) degradation require molecular oxygen. During anoxia, the level of the inhibitory transmitter gamma-aminobutyric acid (GABA) increased (2.3-fold after 13 h) and the level of the excitatory transmitter Glu fell. Furthermore, anoxia caused increases in the levels of Ala (14 times after 13 h), Tau, Gly, and Ser, whereas the Gln level fell. The increase in Ala is likely to inhibit pyruvate kinase, thereby mediating the decreased rate of glycolysis seen after prolonged anoxia. The increased level of Tau might protect the turtle brain against Ca2(+)-mediated anoxic damage. The monoamine metabolites almost vanished within a few hours of anoxia, indicating a halt in monoamine synthesis and breakdown, and the dopamine level fell. Nevertheless, serotonin, norepinephrine, and epinephrine levels were maintained during 13 h of anoxia, at levels extremely high compared with mammals, suggesting adaptive mechanisms such as stockpiling. It is hypothesized that the pattern of change in levels of amino acids (notably GABA and Glu) and monoamines is of functional significance, because it promotes the decrease in brain activity and energy consumption seen in anoxic turtles.