MUTATIONS AFFECTING THE GLYCINE RECEPTOR AGONIST TRANSDUCTION MECHANISM CONVERT THE COMPETITIVE ANTAGONIST, PICROTOXIN, INTO AN ALLOSTERIC POTENTIATOR

Contrary to its effects on the gamma-aminobutyric acid type A receptor, picrotoxin antagonism of the alpha 1 subunit of the human glycine receptor is shown to be competitive, not use-dependent, and nonselective between the picrotoxin components, picrotin, and picrotoxinin. Competitive antagonism and non-use dependence are consistent with picrotoxin binding to a site in the extracellular domain. The mutations Arg --> Leu or Arg --> Gln at residue 271 of the glycine receptor alpha 1 subunit, which are both associated with human startle disease, have previously been demonstrated to disrupt the transduction process between agonist binding and channel activation, We show here that these mutations also transform picrotoxin from an allosterically acting competitive antagonist to an allosteric potentiator at low (0.01-3 mu M) concentrations and to a noncompetitive antagonist at higher (greater than or equal to 3 mu M) concentrations. This demonstrates that arginine 271 is involved in the transduction process between picrotoxin binding and its mechanism of action, Thus, the allosteric transduction pathways of both agonists and antagonists converge at a common residue prior to the activation gate of the channel, suggesting that this residue may act as an integration point for information from various extracellular ligand binding sites.