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

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.

Rat hippocampi were incubated at room temperature for 2 hr in artificial cerebrospinal fluid (aCSF) equilibrated with 95% O2 plus 5% CO2. 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 degrees C and baseline data recorded before 2 mM H2O2 was added. For some studies, mannitol or inhibitors of glutamate receptors or the volume-regulated anion channel (VRAC) were added before and during H2O2 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.

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 H2O2 exposure. The alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate antagonist, 25 microM CNQX, but not the N-methyl-D-aspartate (NMDA) receptor antagonist, 10 microM MK-801, inhibited the increase in IOS during H2O2 treatment. Taurine-treated slices exposed to H2O2 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 microM DCPIB, no increase in the taurine efflux rate was observed during H2O2 exposure. Taurine-treated slices perfused with 10 microM MK-801 during H2O2 exposure showed a 4.6+/-1.9% increase in IOS but no increase in the taurine efflux rate.

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 H2O2. NMDA receptor activation plays an important role in taurine release during oxidative stress.