Figure 10. Effect of MTSET⁺ on the PCCP⁻ and picrotoxin inhibition of wild type and mutants GABA_A receptors.

Currents were elicited with GABA EC₁₀. The concentration of PCCP⁻ or picrotoxin was chosen such as to inhibit about 50% of the late current response in the corresponding receptor. Subsequently oocytes were treated with the cysteine-reactive reagent MTSET⁺ and inhibition by the same concentration of PCCP⁻ or picrotoxin was determined. The ratio of the inhibition after treatment divided by inhibition before treatment is shown as a bar. A, C, MTSET⁺ was applied in the absence of GABA. B, D, MTSET⁺ was applied in the presence of 100 µM GABA. The circle symbol on top of the bar for the T260C and T264C mutations indicates formation of an open channel after MTSET⁺ treatment when applied in the presence of GABA. The white bars show results of experiments where the MTSET⁺ + GABA treatment was performed in the presence of 1 mM PCCP⁻ or 1 mM picrotoxin, in order to see if the covalent reaction could be prevented by the channel blockers. The asterisks sign (*) indicates that 1 mM picrotoxin was not able to suppress formation of open channels. Mean ± SD is shown. The number of oocytes for each experimental condition is either three or six.