Subunit-specific action of an anticonvulsant thiobutyrolactone on recombinant glycine receptors involves a residue in the M2 membrane-spanning region

The anticonvulsant alpha-ethyl, alpha-methyl-gamma-thiobutyrolactone (alpha EMTBL) potentiates the response to a submaximal concentration of glycine p roduced by receptors composed of human glycine alpha 1-subunits but reduces the response of receptors composed of rat glycine alpha 3-subunits. Both t he potentiating and blocking actions of alpha EMTBL are reduced by higher c oncentrations of glycine. The subunit specificity of alpha EMTBL block is c onferred by a residue in the second membrane-spanning region (M2), which is alanine in the alpha 3-subunit (A254) and glycine in the alpha 1-subunit. The mutation A254G in the alpha 3-subunit removes blocking by alpha EMTBL a nd reveals potentiation. Picrotin, a picrotoxinin analog, blocks responses of receptors composed of either alpha 1 or alpha 3-subunits. Blocking of al pha 3 receptors by picrotin is reduced in the presence of alpha EMTBL, indi cating that the mechanisms interact at some point, but the mutation alpha 3 A254G does not remove block by picrotin. However, mutation of a nearby res idue alpha 3 T258F does remove block by picrotin, picrotoxinin and alpha EM TBL. These observations suggest that alpha EMTBL and picrotin act on glycin e alpha 3 receptors to produce block by an allosteric mechanism that involv es overlapping sets of residues in the M2 region. Coexpression of the alpha 3subunit with the beta-subunit of the glycine receptor also removes block by alpha EMTBL and reveals potentiation, suggesting that receptors containi ng either alpha 3 or alpha 1 glycine receptor subunits are potentiated in t he adult brain.