Glutamate receptor-mediated taurine release from the hippocampus during oxidative stress

BACKGROUND: Hippocampal slices swell and release taurine during oxidative stress. The influence of cellular signalling pathways on this process is unclear. Glutamate signalling can facilitate volume regulation in other CNS preparations. Therefore, we hypothesize activation of taurine release by oxidative stress results from tissue swelling and is coupled to activation of glutamate receptors. METHODS: Rat hippocampi were incubated at room temperature for 2 hr in artificial cerebrospinal fluid (aCSF) equilibrated with 95% O(2) plus 5% CO(2). For some slices, 1 mM taurine was added to the aCSF to maintain normal tissue taurine content. Slices then were perfused with aCSF at 35° C and baseline data recorded before 2 mM H(2)O(2) was added. For some studies, mannitol or inhibitors of glutamate receptors or the volume-regulated anion channel (VRAC) were added before and during H(2)O(2) treatment. The intensity of light transmitted through the slice (the intrinsic optical signal, IOS) was determined at 1-min intervals. Samples of perfusate were collected at 2-min intervals and amino acid contents determined by HPLC. Data were analyzed by repeated measures ANOVA and post hoc Dunnett’s test with significance indicated for p<0.05. RESULTS: IOS of slices prepared without taurine treatment increased significantly by 3.3±1.3% (mean±SEM) during oxidative stress. Little taurine was detected in the perfusate of these slices and the rate of taurine efflux did not change during H(2)O(2) exposure. The α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate antagonist, 25 µM CNQX, but not the N-methyl-D-aspartate (NMDA) receptor antagonist, 10 µM MK-801, inhibited the increase in IOS during H(2)O(2) treatment. Taurine-treated slices exposed to H(2)O(2) showed no change in IOS; however, taurine efflux increased by 335±178%. When these slices were perfused with hypertonic aCSF (350 mOsm) or exposed to the VRAC inhibitor, 20 µM DCPIB, no increase in the taurine efflux rate was observed during H(2)O(2) exposure. Taurine-treated slices perfused with 10 µM MK-801 during H(2)O(2) exposure showed a 4.6±1.9% increase in IOS but no increase in the taurine efflux rate. CONCLUSIONS: Taurine efflux via VRAC is critical for volume regulation of hippocampal slices exposed to oxidative stress. This increased taurine efflux does not result from direct activation of the taurine release pathway by H(2)O(2). NMDA receptor activation plays an important role in taurine release during oxidative stress.