PROTECTION OF THE AXONAL CYTOSKELETON IN ANOXIC OPTIC-NERVE BY DECREASED EXTRACELLULAR CALCIUM

Since CNS white matter tracts contain axons, oligodendrocytes and astr ocytes but not synapses, it is likely that anoxic injury of white matt er is mediated by cellular mechanisms that do not involve synapses. In order to test the hypothesis, that anoxic injury of white matter is m ediated by an influx of Ca2+ into the intracellular compartment of axo ns, we compared the ultrastructure of axons in rat optic nerve exposed to 60 min of anoxia in artificial cerebrospinal fluid (aCSF) containi ng normal (2 mM) Ca2+, and in aCSF containing zero-Ca2+ together with 5 mM EGTA. Optic nerves fixed at the end of 60 min of anoxia in 2 mM C a2+ exhibit extensive ultrastructural alterations including disruption of microtubules and neurofilaments within the axonal cytoskeleton, de velopment of membranous profiles and empty spaces between the axon and the ensheathing myelin, and swelling of mitochondria with loss of cri stae. Bathing the nerves in zero-Ca2+ aCSF during anoxia protected the axons from cytoskeletal changes; after 60 min of anoxia, optic nerve axons retained normal-appearing microtubules and neurofilaments. Membr anous profiles were rare, and empty spaces between axons and myelin di d not develop in anoxic optic nerves bathed in zero-Ca2+ aCSF. Disorga nization of cristae in axonal mitochondria was observed in anoxic opti c nerves even when Ca2+ was omitted from the medium. Because Ca2+-medi ated injury is known to disrupt the axonal cytoskeleton, these results support the hypothesis that anoxia triggers an abnormal influx of Ca2 + into myelinated axons in CNS white matter.