Abstract 2029 Poster Session II, Sunday, 5/2 (poster 238)

Excessive activation of glutamate-induced ion channels (NMDA and AMPA) has been shown to play a major role in the pathogenesis of neuronal excitotoxicity. Increased extracellular concentration of glutamate ( [GLU]ec ) has been implicated both in vivo and in vitro as a key triggering factor for this neuronal excitotoxicity. We have shown that [GLU]ec selectively increases after pyramidal cell damage produced by the injection of Quinolinic Acid (QA), an excitotoxic convulsant brain metabolite (Pediatr. Res. 43:322A;1998). Recent in vitro studies suggest that mitochondrial glutaminase is a significant contributor to [GLU]ec after neuronal injury (J.Biol.Chem. 272,11276;1998). To investigate the importance of glutamine (GLN) to glutamate hydrolysis in the pathogenesis of neuronal excitotoxicity in vivo, we measured the [14C-GLU]ec during infusion of artificial CSF containing 14C-GLN in the QA-injured hippocampus of unanesthetized free moving rats by microdialysis and High Performance Liquid Chromatography analysis. Samples were collected every 30 minutes for 5 hrs immediately following QA injection or for 5 hrs the day after the injection (n = 20 cases and 20 controls). L-trans-pyrrolidine-2,4-dicarboxylate (L-PDC), a potent GLU uptake inhibitor, was added to the perfusate.

Results: Infusion of 100 nmoles of QA was followed by an immediate increase in [GLU]ec that persisted 24 hrs later. This increase was enhanced by L-PDC. After QA injection, recovery of 14C-GLU was 141 ± 27% and 177 ± 25%(S.E.) greater at 5 hrs and 24 hrs, respectively, when compared to the original perfusate (p<0.01), and was 88 ± 27% and 76 ± 25% greater when compared to sham operated controls (p<0.03). Since the amino acid concentrations in the microdialysis eluate are between 9 and 15% of their actual concentrations in the extracellular fluid, the measured [14C-GLU]ec presumably reflects a much higher GLU production via glutamine hydrolysis. The measured 14C-GLU produced following neuronal injury thus represented a 7-9% conversion of the estimated 14C-GLU produced following neuronal

Conclusion: Considering the high concentration of glutamine normally present in the extracellular fluid, the measured glutamine to glutamate conversion suggests that glutamine hydrolysis plays a significant role in the increase in [GLU]ec after neuronal injury in vivo.

(Funded in part by NIH, HD16596)