ANOXIC INJURY OF RAT OPTIC-NERVE - ULTRASTRUCTURAL EVIDENCE FOR COUPLING BETWEEN NA+ INFLUX AND CA2+-MEDIATED INJURY IN MYELINATED CNS AXONS

Physiological studies in the anoxic rat optic nerve indicate that irreversible loss of function, measured by the compound action potential, is due to depolarization and run-down of the transmembrane Na+ gradient which triggers Ca2+ entry through reverse Na+-Ca2+ exchange. EM studies in the anoxic optic nerve have demonstrated characteristic changes, including mitochondrial swelling and dissolution of cristae, submyelinic vacuoles, detachment of perinodal oligodendrocyte-axon loops, and severe cytoskeletal damage with loss of microtubules and neurofilaments within the axoplasm. To further examine the coupling between Na+ influx and Ca2+-mediated injury in myelinated axons within anoxic white matter, we have examined the ultrastructural effects of tetrodotoxin (TTX), in the anoxic optic nerve. Optic nerves, maintained in an interface brain slice chamber, were exposed to a 60-min period of anoxia. TTX (1 mu M) was introduced 10 min before the onset of anoxia. Nerves were examined at the end of the anoxic period, or after 80 min in 1 mu M TTX for normoxic controls. Under normoxic conditions, optic nerve axons exposed to TTX exhibited a normal ultrastructure. In optic nerves exposed to TTX studied at the end of a 60-min period of anoxia, mitochondria showed swelling and loss of cristae, and terminal oligodendroglial loops were detached from the nodal axon membrane. Cytoskeletal architecture was preserved in anoxic optic nerve axons treated with TTX, and axonal microtubules and neurofilaments maintained their continuity. Submyelinic empty spaces were not present. Perinodal astrocyte processes often appeared to be replaced by cellular remnants containing multiple membranous profiles; clusters of shrunken astrocytic processes were present between myelinated axons. These findings indicate that, in the rat optic nerve, the axonal cytoskeleton can be protected from anoxic injury by exposure to 1 mu M TTX. Since Ca2+ influx can damage the axonal cytoskeleton, these observations provide a demonstration, at the ultrastructural level, of coupling of Ca2+-mediated axonal injury and Na+ in myelinated axons within anoxic white matter.