The Role of Beta Subunit Variant on the Properties of Delta-Containing Extrasynaptic GABAA Receptors

Abstract

Muscimol, a psychoactive constituent of Amanita muscaria, activates all GABAA receptor subtypes with exceptionally high affinity to αβδ. This is explained by δ subunit role on muscimol extremely slow dissociation from these receptors. However, the contributions of GABAAR β subunit variant to the affinity state of αβδ receptors and function were not described in similar detail as α subunit variant in the literature. The extrasynaptic δ-containing receptors are in a great deal responsible for the so-called tonic inhibition in the brain. This study aims to understand the role of β1,2,3 subunits in muscimol binding to δ-containing extrasynaptic GABAA receptors. We performed transient expression of recombinant GABAAR subtypes in HEK293 cells and determined the association and dissociation rates of [3H]muscimol binding to recombinant α6β1δ, α6β2δ and α6β3δ receptors using radiolabeled binding kinetics approaches. α6β1,2,3δ recombinant receptors showed distinct binding kinetics. In α6β1δ and α6β2δ recombinant receptors, [3H]muscimol association was similar and significantly slower than α6β3δ. Moreover, dissociation of the binding was very slow especially to α6β2δ recombinant receptor compared to α6β1δ and α6β3δ that were similar. The remarkable slow dissociation of [3H]muscimol from all three receptor subtypes confirms the studies indicating α6β1-3δ high affinity state being independent of β subunit. Further electrophysiological studies are needed to reveal how different β subunits affect binding site of αβδ GABAA receptors in terms of ligand sensitivity and efficacy. This leads to an improved pharmacological characterization of extrasynaptic αβδ receptors and possible development of selective allosteric modulators with therapeutic significance for alcoholism, anesthesia and epilepsy.