GABAr1yGABAAa1 receptor chimeras to study receptor desensitization

g-Aminobutyrate type C (GABAC) 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 GABAA antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric r1 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 r1subunit and of the a1-subunit of the GABAA receptor. When expressed in Xenopus oocytes, one chimeric gene (r1ya1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABAC 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 GABAA receptors, in striking contrast to the almost nil desensitization of wild-type r1 (wt r1) 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 r1 by that of the a1-subunit of GABAA. Although the a1-subunit forms fast-desensitizing receptors when coexpressed with other GABAA subunits, the sole transfer of the a1TM2 segment to r1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of r1 receptors is determined by a combination of several interacting domains along the molecule.