Structure of α 6 β 3 δ GABA A receptors and their lack of ethanol sensitivity

Delta (δ) subunit containing GABA A receptors are expressed extra‐synaptically and mediate tonic inhibition. In cerebellar granule cells, they often form a receptor together with α 6 subunits. We were interested to determine the architecture of these receptors. We predefined the subunit arrangement of 24 different GABA A receptor pentamers by subunit concatenation. These receptors (composed of α 6 , β 3 and δ subunits) were expressed in Xenopus oocytes and their electrophysiological properties analyzed. Currents elicited in response to GABA were determined in presence and absence of 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one and to 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]‐pyridin‐3‐ol. α 6 ‐β 3 ‐α 6 /δ receptors showed a substantial response to GABA alone. Three receptors, β 3 ‐α 6 ‐δ/α 6 ‐β 3 , α 6 ‐β 3 ‐α 6 /β 3 ‐δ and β 3 ‐δ‐β 3 /α 6 ‐β 3 , were only uncovered in the combined presence of the neurosteroid 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one with GABA. All four receptors were activated by 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]‐pyridin‐3‐ol. None of the functional receptors was modulated by physiological concentrations (up to 30 mM) of ethanol. GABA concentration response curves indicated that the δ subunit can contribute to the formation of an agonist site. We conclude from the investigated receptors that the δ subunit can assume multiple positions in a receptor pentamer composed of α 6 , β 3 and δ subunits.