A pentasymmetric open channel blocker for cys-loop receptor channels

γ-Aminobutyric acid type A receptors (GABA_Areceptors) are chloride ion channels composed of five subunits, mediating fast synaptic and tonic inhibition in the mammalian brain. These receptors show near five-fold symmetry that is most pronounced in the second trans-membrane domain M2 lining the Cl^-ion channel. To take advantage of this inherent symmetry, we screened a variety of aromatic anions with matched symmetry and found an inhibitor, pentacyanocyclo-pentdienyl anion (PCCP^-) that exhibited all characteristics of an open channel blocker. Inhibition was strongly dependent on the membrane potential. Through mutagenesis and covalent modification, we identified the region α₁V256-α₁T261 in the rat recombinant GABA_Areceptor to be important for PCCP^-action. Introduction of positive charges into M2 increased the affinity for PCCP^-while PCCP^-prevented the access of a positively charged molecule into M2. Interestingly, other anion selective cys-loop receptors were also inhibited by PCCP^-, among them the Drosophila RDL GABA_Areceptor carrying an insecticide resistance mutation, suggesting that PCCP^-could serve as an insecticide.