EFFECTS OF HIGH HELIUM PRESSURE ON INTRACELLULAR AND FIELD POTENTIAL RESPONSES IN THE CA1 REGION OF THE IN-VITRO RAT HIPPOCAMPUS

We have investigated the effects of high helium pressure (up to 13.3 M Pa) on the electrophysiological responses of CA1 neurons in rat hippoc ampal slices using a purpose built pressure chamber, designed to facil itate field potential and intracellular recording. In field potential experiments, near threshold orthodromic responses were depressed by mo dest pressure (0.4 MPa). At higher stimulus intensities, orthodromic a nd antidromic population spike amplitudes were not increased at 5 MPa but were significantly enhanced at 10 MPa, and multiple population spi kes were observed in some experiments. Orthodromic paired pulse potent iation was not affected by pressure up to 10 MPa. In intracellular exp eriments there were no significant differences between mean values obt ained for resting membrane potential and input resistance at atmospher ic pressure and pressures of up to 10 MPa. However, spontaneous depola rizing membrane potential excursions, decreased slow afterhyperpolariz ation responses and reduced accommodation properties were observed at pressure (5-10 MPa). One possibility is that the SK and M potassium ch annel systems may be more sensitive to high pressure than other membra ne ion channels. These effects may contribute towards the high pressur e neurological syndrome observed in vivo.