Figure 6. Currents evoked by GABA_A receptor agonists in the whole-cell and gramicidin-perforated patch configurations.

A, time course of the GABA-evoked current amplitude within the first seconds following whole-cell access. Chloride concentration in the patch pipette solution was 6 mM (E_Cl= -75 mV). Left, the arrow indicates the onset of whole-cell access. Holding potential was -40 mV. Repeated 3 s pulses of GABA (10 μM, filled bar) were delivered 1, 6, 11, 16 and 21 s after membrane rupture. Note that the first pulse of GABA induced an inward current while the following pulses generated outward currents. Right, time course of the GABA-evoked current amplitude in three cells clamped at holding potentials of -80 (○), -40 (□) or -35 mV (▵). The data were fitted to monoexponential functions yielding time constants of 5.2, 4.6 and 4.3 s, respectively. Note that the initial GABA-evoked current was inwardly directed at -80 and -40 mV, whilst it was outwardly directed at -35 mV. B, chloride concentration in the patch pipette solution was 115 mM (E_Cl= -0.6 mV). Isoguvacine (10 μM) was pressure-ejected (5 s, filled bar) in the vicinity of a melanotroph clamped at -20 (top traces) or -50 mV (bottom traces). The perforated patch recording (left) was subsequently converted into a whole-cell recording (right) by a suction pulse-induced rupture of the underlying plasma membrane. Isoguvacine-induced currents were studied within the first minute after gaining whole-cell access to avoid gramicidin perforation of the whole-cell membrane. In the gramicidin-perforated patch recording, the currents evoked by isoguvacine at -20 and -50 mV were opposite, whilst in the whole-cell configuration they both remained inwardly directed.