GLIAL GLUTAMATE TRANSPORTER GLT-1 DOES NOT REVERSE ITS FUNCTION IN ISCHEMIC PENUMBRA.
Functional glutamate uptake systems are critical for maintenance of normal synaptic transmission. Several studies have shown that in severely ischemic brain regions, massive efflux of excitatory amino acids (EAAs) results, in large portion, from the reversal of Na+-dependent glutamate transporters. This study was designed to examine the function of the predominant glial transporter, GLT-1, in a brain area representative of ischemic penumbra. Spontaneously hypertensive rats were subjected to a 45 min transient focal ischemia by intraluminal occlusion of the middle cerebral artery (MCAO). In this model, the core ischemic region resides in the medial striatum while the surrounding parietal cortex represents a penumbral area. Temporal profiles of extracellular levels of glutamate [Glu]e and aspartate [Asp]e were assessed using microdialysis in the parietal cortex. Preischemic [Glu]e and [Asp]e were constant and no changes were observed in sham operated animals. MCAO induced rapid efflux of EAAs with the peak of 7 and 9 fold increases in [Glu]e and [Asp]e at 20 min post MCAO, respectively. Levels of both EAAs returned to baseline prior to reperfusion. In contrast, a local infusion (reverse microdialysis) of the GLT-1 selective nontransportable inhibitor, dihydrokainate (DHK, 1 mM) during the ischemic insult,prolonged and increased the efflux of both EAAs throughout MCAO interval resulting in peak 9 and 16 fold increases (p>0.05) in [Glu]e and [Asp]e, respectively, at 40 min post MCAO. The total ischemic efflux of Glu and Asp (areas under the curve, AUC) was increased by 70% and 98%. Infusion of the voltage dependent Na+ channel blocker tetrodotoxin (TTX, 10 μM) during the ischemic period attenuated the initial MCAO-induced Glu and Asp efflux from 7 to 2 fold and from 9 to 2 fold, respectively. These data provide the first evidence that within the ischemic brain penumbra, GLT-1 remains functional and displays no evidence of reverse transport. In addition, the attenuation of MCAO-induced EAA efflux by TTX indicates that the source of penumbral release is primarily neuronal.