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

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 pharmacolo gically with microdialysis probes placed bilaterally in rat striatum. Methods-Forebrain ischemia was induced by bilateral carotid artery occlusio n and controlled hypotension in halothane-anesthetized rats. During 30 minu tes of ischemia, microdialysate concentrations of glutamate and aspartate w ere measured in the presence of a nontransportable blocker of the astrocyti c glutamate transporter GLT-1, dihydrokinate (DHK), or an anion channel blo cker, 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS), administered separat ely or together through the dialysis probe. Results-In control striata during ischemia, glutamate and aspartate concent rations 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/mu L). 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 versu s control, 10.1 +/- 0.7 pmol/mu L). At a higher concentration, DNDS (10 mmo l/L; n = 7) further reduced glutamate and aspartate release and also inhibi ted 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 vers us control, 10.9 +/- 1.2 pmol/mu L). Conclusions-These findings support the hypothesis that both cell swelling-i nduced release of EAAs and reversal of the astrocytic glutamate transporter are contributors to the ischemia-induced increases of extracellular EAAs i n the striatum as measured by microdialysis.