Na +–K +-ATPase inhibition and depolarization induce glutamate release via reverse Na +-dependent transport in spinal cord white matter

Abstract

Excitotoxic mechanisms involving α-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)/kainate receptors play an important role in mediating cellular damage in spinal cord injury. However, the precise cellular mechanisms of glutamate release from non-synaptic white matter are not well understood. We examined how the collapse of transmembrane Na + and K + gradients induces reverse operation of Na +-dependent glutamate transporters, leading to glutamate efflux and injury to rat spinal dorsal columns in vitro. Compound action potentials were irreversibly reduced to 43% of control after ouabain/high K +/low Na + exposure (500 μM ouabain for 30 min to increase [Na +] i, followed by 1 h ouabain+high K + (129 mM)/low Na + (27 mM), to further reverse transmembrane ion gradients) followed by a 2 h wash. Ca 2+-free perfusate was very protective (compound action potential amplitude recovered to 87% vs. 43%). The broad spectrum glutamate antagonist kynurenic acid (1 mM) or the selective AMPA antagonist GYKI52466 (30 μM) were partially protective (68% recovery). Inhibition of Na +-dependent glutamate transport with L- trans-pyrrolidine-2,4-dicarboxylic acid (1 mM) also provided significant protection (71% recovery), similar to that seen with glutamate receptor antagonists. Blocking reverse Na +–Ca 2+ exchange with KB-R7943 (10 μM) however, was ineffective in this paradigm (49% recovery). Semiquantitative glutamate immunohistochemistry revealed that levels of this amino acid were significantly depleted in axon cylinders and, to a lesser degree, in oligodendrocytes (but not in astrocytes) by ouabain/high K +/low Na +, which was largely prevented by glutamate transport inhibition. Our data show that dorsal column white matter contains the necessary glutamate pools and release mechanisms to induce significant injury. When Na + and K + gradients are disrupted, even in the absence of reduced cellular energy reserves, reverse operation of Na +-dependent glutamate transport will release enough endogenous glutamate to activate AMPA receptors and cause substantial Ca 2+-dependent injury. This mechanism likely plays an important role during ischemic and traumatic white matter injury, where collapse of transmembrane Na + and K + gradients occurs.