Volume-regulated anion channels are the predominant contributors to release of excitatory amino acids in the ischemic cortical penumbra

Abstract

Background and Purpose - Release of excitatory amino acids (EAA) is considered a cause of neuronal damage in ischemia. We investigated the sources and mechanisms of EAA release using microdialysis in regions of incomplete ischemia where perfusion was reduced by 50% to 80%, by applying inhibitors of volume-regulated anion channels (VRACs) and the GLT-1 glutamate transporter. Methods - Reversible middle cerebral artery occlusion (rMCAo) was induced in anesthetized rats using the intraluminal suture technique. Microdialysate concentrations of glutamate, aspartate, and taurine were measured before, during 2 hours of rMCAo, and for 2 hours after rMCAo. Vehicle, dihydrokainate (DHK, 1 mmol/L), a GLT-1 inhibitor, or tamoxifen (50 μmol/L), a VRAC inhibitor, were administered continuously via the dialysis probes starting one hour prior to ischemia. Results - During incomplete ischemia, dialysate glutamate levels averaged 1.74±0.31 μmol/L (SEM) in the control group (n=8), 2.08±0.33 μmol/L in the DHK group (n=7), and were significantly lower at 0.88±0.30 μmol/L in the tamoxifen group (n=9; P<0.05). As perfusion returned toward baseline levels, EAA levels declined in the vehicle and tamoxifen-treated animals but they remained elevated in the DHK-treated animals. Conclusion - In contrast to previous results in severely ischemic regions, DHK did not reduce EAA release in less severely ischemic brain, suggesting a diminished role for transporter reversal in these areas. These findings also support the hypothesis that in regions of incomplete ischemia, release of EAAs via VRACs may play a larger role than reversal of the GLT-1 transporter.