thiourea and pyrrolidine-2-4-dicarboxylic-acid

Excitotoxic mechanisms involving alpha-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 microM 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 microM) 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 microM) 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.

Topics: alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Anti-Anxiety Agents; Anti-Arrhythmia Agents; Benzodiazepines; Calcium; Dicarboxylic Acids; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Kynurenic Acid; Male; Membrane Potentials; Nerve Fibers, Myelinated; Neurotransmitter Uptake Inhibitors; Ouabain; Potassium; Pyrrolidines; Rats; Rats, Long-Evans; Receptors, AMPA; Sodium; Sodium-Potassium-Exchanging ATPase; Spinal Cord; Thiourea