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.