Loop 2 structure in glycine and GABA(A) receptors plays a key role in determining ethanol sensitivity

The present study tests the hypothesis that the structure of extracellular domain Loop 2 can markedly affect ethanol sensitivity in glycine receptors (GlyRs) and gamma-aminobutyric acid type A receptors (GABA(A)Rs). To test this, we mutated Loop 2 in the alpha1 subunit of GlyRs and in the gamma subunit of alpha1beta2gamma2GABA(A)Rs and measured the sensitivity of wild type and mutant receptors expressed in Xenopus oocytes to agonist, ethanol, and other agents using two-electrode voltage clamp. Replacing Loop 2 of alpha1GlyR subunits with Loop 2 from the deltaGABA(A)R (deltaL2), but not the gammaGABA(A)R subunit, reduced ethanol threshold and increased the degree of ethanol potentiation without altering general receptor function. Similarly, replacing Loop 2 of the gamma subunit of GABA(A)Rs with deltaL2 shifted the ethanol threshold from 50 mm in WT to 1 mm in the GABA(A) gamma-deltaL2 mutant. These findings indicate that the structure of Loop 2 can profoundly affect ethanol sensitivity in GlyRs and GABA(A)Rs. The deltaL2 mutations did not affect GlyR or GABA(A)R sensitivity, respectively, to Zn(2+) or diazepam, which suggests that these deltaL2-induced changes in ethanol sensitivity do not extend to all allosteric modulators and may be specific for ethanol or ethanol-like agents. To explore molecular mechanisms underlying these results, we threaded the WT and deltaL2 GlyR sequences onto the x-ray structure of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC). In addition to being the first GlyR model threaded on GLIC, the juxtaposition of the two structures led to a possible mechanistic explanation for the effects of ethanol on GlyR-based on changes in Loop 2 structure.