GABAρ1/GABA(A)α1 receptor chimeras to study receptor desensitization

γ-Aminobutyrate type C (GABA(C)) receptors are ligand-gated ion channels that are expressed preponderantly in the vertebrate retina and are characterized, among other things, by a very low rate of desensitization and resistance to the specific GABA(A) antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric ρ1 receptor, we used a combination of gene chimeras and electrophysiology of receptors expressed in Xenopus oocytes. Two chimeric genes were constructed, made up of portions of the ρ1-subunit and of the α1-subunit of the GABA(A) receptor. When expressed in Xenopus oocytes, one chimeric gene (ρ1/α1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABA(C) antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid and by zinc. Moreover, these chimeric receptors had a fast-desensitizing component, even faster than that of heterooligomeric GABA(A) receptors, in striking contrast to the almost nil desensitization of wild-type ρ1 (wt ρ1) receptors. To see whether the fast-desensitizing characteristic of the chimera was determined by the amino acids forming the ion channels, we replaced the second transmembrane segment (TM2) of ρ1 by that of the α1-subunit of GABA(A). Although the α1-subunit forms fast-desensitizing receptors when coexpressed with other GABA(A) subunits, the sole transfer of the α1TM2 segment to ρ1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of ρ1 receptors is determined by a combination of several interacting domains along the molecule.