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 turtle (Chrysemys and Trachemys species). Comparisons are made with the hypoxic tolerant mammalian neonate brain. In the anoxic tolerant species 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, increased blood supply to the brain, the suppression of electrical activity, increased release of inhibitory neuromodulators and neurotransmitters, 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, with its lower energy requirements, slower decline in ATP and lower excitability levels acting to delay depolarization.