To elucidate the mechanism by which hyperbaric oxygen (HBO2) induces electrical discharge, changes in the extracellular concentrations of GABA and glutamate were measured every 5 min using a microdialysis technique in rats during a period of exposure to HBO2 (5 atm abs). Electrical discharge was observed at 28 +/- 4 min after the onset of exposure. Though the extracellular concentrations of glutamate remained unchanged, the extracellular GABA concentrations (pre-exposure level, 0.026 +/- 0.005 microM in dialysate) began to decrease 15 min after the onset of exposure and reached their lowest level (74 +/- 14%, 0.019 +/- 0.004 microM) at the time of appearance of the discharge. There was a close logistic relationship between extracellular GABA concentrations and the discharge incidence, and the extracellular concentrations of GABA causing electrical discharge in 50% of the animals were estimated to be 80% of the pre-exposure level. These results suggest a possible mechanism that HBO2 exposure-induced discharge is caused by the decrease in extracellular concentration of GABA.
