| Spinal cord injury (SCI) is a devastating condition, with much of the clinical disability resulting from disruption of ascending and descending white matter tracts. Recent reports suggest that a component of axonal dysfunction during SCI involves glutamate-mediated white matter damage, but the cellular targets of excitotoxicity and the precise mechanisms of glutamate release from non-synaptic white matter are not understood. In the present study, using combined techniques including electrophysiology, pharmacology, immunohistochemistry and confocal microscopy, we demonstrate that myelinated axons in isolated dorsal columns are vulnerable to irreversible excitotoxic injury, which is primarily dependent on Ca2+-permeable AMPA receptors. The cellular components susceptible to glutamate include oligodendrocytes, astrocytes and the myelin sheath, consistent with the distribution of GluR3 and GluR4 in these cell types in situ, but not GluR2. We also demonstrate that reduced transmembrane Na+ and K+ gradients induced by inhibiting Na+-K+-ATPase with ouabain plus high K+ could drive Na+-dependent glutamate transporters to operate in a reverse mode, resulting in glutamate release from intracellular compartments and functional failure in white matter tracts by activation of AMPA receptors. Using injury models of anoxia or trauma, we further show that the ionic and membrane potential perturbations induced during in vitro anoxia or trauma are sufficient to cause toxic glutamate efflux via reverse Na+-dependent glutamate transport, resulting in damage to the myelin sheath and possibly other structures by activation of AMPA receptors. Semiquantitative measurement of intracellular glutamate indicates that axon cylinders, and to a lesser extent oligodendrocytes, are the major cellular sources of endogenous glutamate release. Our findings are consistent with the immunolocalization of Na+-dependent transporters (GLT1, EAAC1 and GLAST) in dorsal columns. We conclude that white matter, especially glial elements including myelin, is vulnerable to excitotoxins acting via AMPA receptors; release of glutamate by reversal of Na+-dependent glutamate transport with subsequent activation of these receptors is an important mechanism in anoxic and traumatic injury of spinal cord white matter. |
