EMBRYONIC RAT HIPPOCAMPAL-NEURONS AND GABA(A) RECEPTOR SUBUNIT-TRANSFECTED NONNEURONAL CELLS RELEASE GABA TONICALLY

We used patch-clamp recording techniques to investigate the contributi on of GABA to baseline membrane properties in cultured embryonic rat h ippocampal neurons. Almost all of the neurons recorded with Cl--filled pipettes and clamped at negative potentials exhibited baselines that were noticeably noisy, with microscopic fluctuations superimposed on t he macroscopic holding current. A gentle steam of saline applied to th e neuronal surface rapidly and reversibly reduced the baseline current and fluctuations, both of which were completely eliminated by bicucul line. Fluctuation analysis showed that the variance in the baseline cu rrent signal was exponentially distributed with estimated kinetics com parable to those activated by submicromolar concentrations of exogenou s GABA, The kinetics of Cl- channels activated by endogenous GABA disp layed a potential sensitivity comparable to those activated by exogeno us GABA, Non-neuronal cells stably transfected with alpha(1) and gamma (2) GABA(A) receptor subunits exhibited little baseline current varian ce when recorded with Cl--filled pipettes. Addition of micromolar GABA to the extracellular saline or to the pipette solution induced a sali ne- and bicuculline-sensitive baseline current signal comparable to th at recorded in hippocampal neurons. Thus, both intra- and extracellula r sources of GABA could contribute to the baseline properties recorded in these cultured neurons.