GABA increases both the conductance and mean open time of recombinant GABA_A channels co-expressed with GABARAP

The single channel properties of recombinant γ-aminobutyric acid type A (GABA_A) αβγ receptors co-expressed with the trafficking protein GABARAP were investigated using membrane patches in the outside-out patch clamp configuration from transiently transfected L929 cells. In control cells expressing αβγ receptors alone, GABA activated single channels whose main conductance was 30 picosiemens (pS) with a subconductance state of 20 pS, and increasing the GABA concentration did not alter their conductance. In contrast, when GABA_A receptors were co-expressed with GABARAP, the GABA-activated single channels displayed multiple, high conductances (≥40 pS), and GABA (≥10 μM) was able to increase their conductance, up to a maximum of 60 pS. The mean open time of GABA-activated channels in control cells expressing αβγ receptors alone was 2.3 ± 0.1 ms for the main 30-pS channel and shorter for the subconductance state (20 pS, 0.8 ± 0.1 ms). Similar values were measured for the 30- and 20-pS channels active in patches from cells co-expressing GABARAP. However higher conductance channels (≥40 pS) remained open longer, irrespective of whether GABA or GABA plus diazepam activated them. Plotting mean open times against mean conductances revealed a linear relationship between these two parameters. Since high GABA concentrations increase both the maximum single channel conductance and mean open time of GABA_A channels co-expressed with GABARAP, trafficking processes must influence ion channel properties. This suggests that the organization of extrasynaptic GABA_A receptors may provide a range of distinct inhibitory currents in the brain and, further, provide differential drug responses.