NEUROTRANSMITTERS AND ANOXIC SURVIVAL OF THE BRAIN - A COMPARISON OF ANOXIA-TOLERANT AND ANOXIA-INTOLERANT VERTEBRATES

The effects of anoxia on brain amino acid and monoamine levels were measured in the brown anole lizard (Anolis sagrei) and loggerhead sea turtle (Caretta caretta) and compared to previous measurements on a freshwater turtle (Pseudemys scripta elegans), the crucian carp (Carassius carassius), and mammals. With regard to amino acids, anoxia-tolerant species (loggerhead turtle, freshwater turtle, and crucian carp) display similar changes in response to anoxia. Thus, in these species, anoxia causes increased levels of gamma-aminobutyric acid (GABA) and glycine, while the levels of glutamate and glutamine decrease. In both turtle species, the level of taurine also increases. In contrast, in the anoxia-intolerant species (anole lizard and dog), GABA is the only inhibitory amino acid that shows an increase, while glutamate and glutamine remain unchanged or even increase. It is hypothesized that the increased levels of the inhibitory amino acids GABA, glycine, and taurine, in combination with the decreased concentration of the excitatory amino acid glutamate (and its precursor glutamine), mediate or facilitate the lowering of brain activity and energy consumption during anoxia, a key strategy for anoxic survival. With the possible exception of GABA, these mechanisms seem not to function in the anoxia-intolerant species. With regard to monoamines, for which synthesis demands molecular O2, a 40% fall is observed in the brain level of serotonin in anole lizards exposed to 40 min of anoxia, while turtles and crucian carp maintain their monoamine levels for several hours of anoxia.