ANOXIA TOLERANT ANIMALS FROM A NEUROBIOLOGICAL PERSPECTIVE

This paper discusses the mechanisms for brain anoxia survival seen in crucian carp (Carassius carassius) and a few species of freshwater tur tle (Chrysemys and Trachemys species). Comparisons are made with the h ypoxic tolerant mammalian neonate brain. In the anoxic tolerant specie s the basic strategy for anoxia survival appears to be the maintenance of ion gradients, and thereby the avoidance of anoxic depolarization. Important facilitating factors involve having huge glycogen stores, i ncreased blood supply to the brain, the suppression of electrical acti vity, increased release of inhibitory neuromodulators ana neurotransmi tters, upregulation of inhibitory neuroreceptors, the down-regulation of excitatory ion conductance and the down-regulation of Ca2+ channels . By contrast, for the mammalian neonate the most important causes of its increased hypoxia tolerance may be just simple consequences of the comparatively undifferentiated state of the brain of the newborn, wit h its lower energy requirements, slower decline in ATP and lower excit ability levels acting to delay depolarization.