White matter of the mammalian CNS suffers irreversible injury when subjected to
anoxia/
ischemia. However, the mechanisms of anoxic injury in central myelinated tracts are not well understood. Although white matter injury depends on the presence of extracellular Ca2+, the mode of entry of Ca2+ into cells has not been fully characterized. We studied the mechanisms of anoxic injury using the in vitro rat optic nerve, a representative central white matter tract. Functional integrity of the nerves was monitored electrophysiologically by quantitatively measuring the area under the compound action potential, which recovered to 33.5 +/- 9.3% of control after a standard 60 min anoxic insult. Reducing Na+ influx through voltage-gated Na+ channels during
anoxia by applying Na+ channel blockers (TTX,
saxitoxin) substantially improved recovery; TTX was protective even at concentrations that had little effect on the control compound action potential. Conversely, increasing Na+ channel permeability during
anoxia with
veratridine resulted in greater injury. Manipulating the transmembrane Na+ gradient at various times before or during
anoxia greatly affected the degree of resulting injury; applying zero-Na+
solution (
choline or Li+ substituted) before
anoxia significantly improved recovery; paradoxically, the same
solution applied after the start of
anoxia resulted in more injury than control. Thus, ionic conditions that favored reversal of the normal transmembrane Na+ gradient during
anoxia promoted injury, suggesting that Ca2+ loading might occur via reverse operation of the Na+)-Ca2+ exchanger. Na(+)-Ca2+ exchanger blockers (
bepridil,
benzamil,
dichlorobenzamil) significantly protected the optic nerve from anoxic injury. Together, these results suggest the following sequence of events leading to anoxic injury in the rat optic nerve:
anoxia causes rapid depletion of
ATP and membrane depolarization leading to Na+ influx through incompletely inactivated Na+ channels. The resulting rise in the intracellular [Na+], coupled with membrane depolarization, causes damaging levels of Ca2+ to be admitted into the intracellular compartment through reverse operation of the Na(+)-Ca2+ exchanger. These observations emphasize that differences in the pathophysiology of gray and white matter anoxic injury are likely to necessitate multiple strategies for optimal CNS protection.