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

Physiological studies in the anoxic rat optic nerve indicate that irre versible loss of function, measured by the compound action potential, is due to depolarization and run-down of the transmembrane Na+ gradien t which triggers Ca2+ entry through reverse Na+-Ca2+ exchange. EM stud ies in the anoxic optic nerve have demonstrated characteristic changes , including mitochondrial swelling and dissolution of cristae, submyel inic vacuoles, detachment of perinodal oligodendrocyte-axon loops, and severe cytoskeletal damage with loss of microtubules and neurofilamen ts within the axoplasm. To further examine the coupling between Na+ in flux 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 interf ace brain slice chamber, were exposed to a 60-min period of anoxia. TT X (1 mu M) was introduced 10 min before the onset of anoxia. Nerves we re 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 axo ns exposed to TTX exhibited a normal ultrastructure. In optic nerves e xposed to TTX studied at the end of a 60-min period of anoxia, mitocho ndria showed swelling and loss of cristae, and terminal oligodendrogli al loops were detached from the nodal axon membrane. Cytoskeletal arch itecture was preserved in anoxic optic nerve axons treated with TTX, a nd axonal microtubules and neurofilaments maintained their continuity. Submyelinic empty spaces were not present. Perinodal astrocyte proces ses often appeared to be replaced by cellular remnants containing mult iple membranous profiles; clusters of shrunken astrocytic processes we re present between myelinated axons. These findings indicate that, in the rat optic nerve, the axonal cytoskeleton can be protected from ano xic injury by exposure to 1 mu M TTX. Since Ca2+ influx can damage the axonal cytoskeleton, these observations provide a demonstration, at t he ultrastructural level, of coupling of Ca2+-mediated axonal injury a nd Na+ in myelinated axons within anoxic white matter.