Abstract

γ-Aminobutyric acid_A (GABA_A) receptors (GABARs) are responsible for most fast inhibitory neurotransmission in the mammalian brain. The GABARs contain several allosteric modulatory sites, many of which are useful clinically. The activity of most of these modulators depends upon the subunit composition of the receptor. The diuretic amiloride was previously reported to inhibit GABARs in frog sensory neurons. We measured its effects on recombinant GABARs to determine its mechanism of action at mammalian receptors and to examine the effect of subunit composition. Amiloride acted primarily as a competitive antagonist, reducing the sensitivity of the receptor to GABA without affecting the maximal current amplitude. Receptors containing an α6 subunit were about 10-fold more sensitive to amiloride than those containing other α subunits. In contrast, the identity of the β or γ subtype had little effect on amiloride sensitivity. Although several other modulators have specific effects at α6-containing receptors, amiloride is the first inhibitor to be reported with no additional dependence on the identity of the β or γ subunit. Therefore, it probably represents a unique modulatory site on the GABAR, which could be useful for developing drugs targeting these receptors. The selective activity of amiloride could also be helpful for isolating the contribution of receptors composed of α6 subtypes in heterogeneous native GABAR populations.