Effects of high-pressure helium on gamma-[3H]aminobutyric acid release from the isolated frog spinal cord.

The application of high pressure in vivo causes a hyperexcitability syndrome involving tremors and convulsions. Drugs that potentiate GABA transmission protect animals against this syndrome. It is possible that changes in gamma-aminobutyric acid (GABA) transmission may underlie the hyperexcitability. We have therefore investigated the effects of pressure on the components of GABA transmission in vitro. After incubation with [3H]GABA, hemisected frog spinal cords were superfused inside a pressure chamber and perfusate fractions were collected every 10 min. Helium, at 50 or 100 atm, did not alter the spontaneous release of GABA, but if electrical stimulation had been applied previously, then pressure caused a prolonged increase in GABA release. Helium at 50 atm did not alter the evoked release during electrical stimulation, but at 100 atm this was increased. This increase was smaller in the absence of calcium. No corresponding changes in [14C]urea efflux were seen, suggesting that the effects were not due to nonselective membrane permeability changes. The results are consistent with the known effects of pressure on neuronal activity, such as repetitive firing, but they do not suggest a selective action on the GABA release process.