Identification of a domain in the δ subunit (S238-V264) of the α4β3δ GABAA receptor that confers high agonist sensitivity

We have expressed the alpha 4 beta 3 delta and alpha 4 beta 3 gamma 2L subtypes of the rat GABA(A) receptor in Xenopus oocytes and have investigated their agonist activation properties. GABA was a more potent agonist of the alpha 4 beta 3 delta receptor (EC50 approximate to 1.4 mu mol/L) than of the alpha 4 beta 3 gamma 2L subtype (EC50 approximate to 27.6 mu mol/L). Other GABA(A) receptor agonists (muscimol, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, imidazole-4-amino acid) displayed similar subtype selectivity. The structural determinants underlying these differences have been investigated by co-expressing chimeric delta/gamma 2L subunits with alpha 4 and beta 3 subunits. A stretch of amino acids in the delta subunit, S238-V264, is shown to play an important role in determining both agonist potency and the efficacies of full or partial agonists. This segment includes transmembrane domain 1 and the short intracellular loop that leads to the second transmembrane domain. The effects of the competitive antagonists, bicuculline and SR95531, and the channel blocker, picrotoxin, were not significantly affected by the incorporation of chimeric subunits. As the delta and gamma 2L subunits have not been previously implicated directly in agonist binding, we suggest that the effects are likely to arise from changes in the transduction mechanisms that link agonist binding to channel activation.