Inhibition of ischemia-induced glutamate release in rat striatum by dihydrokinate and an anion channel blocker

Abstract

Background and Purpose - Increased activation of excitatory amino acid (EAA) receptors is considered a major cause of neuronal damage. Possible sources and mechanisms of ischemia-induced EAA release were investigated pharmacologically with microdialysis probes placed bilaterally in rat striatum. Methods - Forebrain ischemia was induced by bilateral carotid artery occlusion and controlled hypotension in halothane-anesthetized rats. During 30 minutes of ischemia, microdialysate concentrations of glutamate and aspartate were measured in the presence of a nontransportable blocker of the astrocytic glutamate transporter GLT-1, dihydrokinate (DHK), or an anion channel blocker, 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS), administered separately or together through the dialysis probe. Results - In control striata during ischemia, glutamate and aspartate concentrations increased 44 ±13 (mean±SEM) times and 19 ± 5 times baseline, respectively, and returned to baseline values on reperfusion. DHK (1 mmol/L in perfusate; n=8) significantly attenuated EAA increases compared with control (glutamate peak, 9.6 ±1.7 versus control, 15.4 ±2.6 pmol/μL). EAA levels were similarly decreased by 10 mmol/L DHK. DNDS (1 mmol/L; n=5) also suppressed EAA peak increases (glutamate peak, 5.8 ± 1.1 versus control, 10.1 ± 0.7 pmol/μL). At a higher concentration, DNDS (10 mmol/L; n=7) further reduced glutamate and aspartate release and also inhibited ischemia-induced taurine release. Together, 1 mmol/L DHK and 10 mmol/L DNDS (n=5) inhibited 83% of EAA release (glutamate peak, 2.7 ± 0.7 versus control, 10.9 ± 1.2 pmol/μL). Conclusions - These findings support the hypothesis that both cell swelling- induced release of EAAs and reversal of the astrocytic glutamate transporter are contributors to the ischemia-induced increases of extracellular EAAs in the striatum as measured by microdialysis.