Identification of a novel residue within the second transmembrane domain that confers use-facilitated block by picrotoxin in glycine alpha 1 receptors.

The central nervous system convulsant picrotoxin (PTX) inhibits GABA(A) and glutamate-gated Cl(minus sign) channels in a use-facilitated fashion, whereas PTX inhibition of glycine and GABA(C) receptors displays little or no use-facilitated block. We have identified a residue in the extracellular aspect of the second transmembrane domain that converted picrotoxin inhibition of glycine alpha1 receptors from non-use-facilitated to use-facilitated. In wild type alpha1 receptors, PTX inhibited glycine-gated Cl(minus sign) current in a competitive manner and had equivalent effects on peak and steady-state currents, confirming a lack of use-facilitated block. Mutation of the second transmembrane domain 15'-serine to glutamine (alpha1(S15'Q) receptors) converted the mechanism of PTX blockade from competitive to non-competitive. However, more notable was the fact that in alpha1(S15'Q) receptors, PTX had insignificant effects on peak current amplitude and dramatically enhanced current decay kinetics. Similar results were found in alpha1(S15'N) receptors. The reciprocal mutation in the beta2 subunit of alpha1beta2 GABA(A) receptors (alpha1beta2(N15'S) receptors) decreased the magnitude of use-facilitated PTX inhibition. Our results implicate a specific amino acid at the extracellular aspect of the ion channel in determining use-facilitated characteristics of picrotoxin blockade. Moreover, the data are consistent with the suggestion that picrotoxin may interact with two domains in ligand-gated anion channels.