On the mechanism of GABA-induced currents in cultured rat cortical neurons

We applied the perforated-patch-clamp technique to cultured cortical neuron s of the rat to characterize the ionic basis of membrane potential changes and membrane currents induced by gamma-aminobutyric acid (GABA). Gramicidin was used as the membrane-perforating agent, to allow the recording of whol e-cell currents without impairing the intracellular Cl- concentration ([Cl- ](i)). In current-clamp experiments in the presence of 26 mM HCO3- the appl ication of 50 mu M GABA evoked changes in the membrane potential of neurons including depolarizations (19%), hyperpolarizations (38%) and biphasic cha nges in membrane potential (31%), characterized by a transient hyperpolariz ation followed by a sustained depolarization. Accordingly, GABA (50-200 mu M) induced inward, outward or biphasic current responses under voltage-clam p. Inward and biphasic currents as well as depolarizations and biphasic mem brane potential responses, respectively, occurred more frequently in the pr esence of 26 mM HCO3-. The second phase of the biphasic membrane potential reduced when the preparation was bathed in a HCO3--free saline, indicating a contribution from HCO3-. The reversal potential of the GABA-induced curre nts (E-GABA) determined with the gramicidin-perforated-patch mode and in th e nominal absence of HCO3- was -73 mV, while it was shifted to -59 mV in th e presence of HCO3-. Combined patch-clamp and microfluorimetric measurement s using the Cl- sensitive dye 6-methoxy-1-(3-sulphonatopropyl)quinolinium ( SPQ) showed that GABA evoked an increase of [Cl-](i) in 54% (n = 13) of the neurons. We conclude that this increase of [Cl-](i) in combination with th e efflux of HCO, results in a shift of E-GABA above the resting membrane po tential that gives rise to GABA-mediated depolarizations.