Mechanisms of Glutamate Release in the Rat Spinal Cord Slices during Metabolic Inhibition

Abstract

Glutamate toxicity is a viable hypothesis to explain the expanding tissue degeneration occurring after traumatic or ischemic spinal cord injury. One important component in this process is the acute, excessive release of glutamate. In the current communication, the glycolytic inhibitor iodoacetate was used to induce metabolic inhibition in spinal cord slices and thereby provide an in vitro model to study the mechanisms of pathological glutamate release in the spinal cord. The evoked glutamate release was not Ca²⁺-dependent. Exclusion of NaCl reduced the evoked release of endogenous glutamate by 56%, while excluding Na⁺ increased release. Glutamate release was also reduced by the PLA₂ inhibitors indomethacin (40%), arachidonyltrifluoromethyl ketone (45%) and 4-bromophenacyl bromide (36%). Blocking reverse glutamate transport by preincubation with 1 mM dihydrokainic acid reduced evoked release by 41%. However, when the dihydrokainic acid and arachidonyltrifluoromethyl ketone treatments were combined, no additive effect of the two substances was seen. These findings suggest that glutamate is released by three mechanisms from the energy compromised spinal cord: (1) in response to cellular swelling, most likely by the regulatory volume decrease, (2) by PLA₂-mediated breakdown of the cell membrane and diffusion of glutamate down its concentration gradient, and (3) through reversal of the glutamate transporter.