According to the rules of GABAA receptor (GABAAR) subunit assembly, α4 and α6 subunits are considered to be the natural partners of δ subunits. These GABAARs are a preferred target of low, sobriety-impairing concentrations of ethanol. Here we demonstrate a new naturally occurring GABAAR subunit partnership: δ subunits of hippocampal interneurons are coexpressed and colocalized with α1 subunits, but not with α4, α6 or any other α subunits. Ethanol potentiates the tonic inhibition mediated by such native α1/δ GABAARs in wild-type and in α4 subunit–deficient (Gabra4−/−) mice, but not in δ subunit–deficient (Gabrd−/−) mice. We also ruled out any compensatory upregulation of α6 subunits that might have accounted for the ethanol effect in Gabra4−/− mice. Thus, α1/δ subunit assemblies represent a new neuronal GABAAR subunit partnership present in hippocampal interneurons, mediate tonic inhibitory currents and are highly sensitive to low concentrations of ethanol. [ABSTRACT FROM AUTHOR]