Decreased agonist sensitivity of human GABAA receptors by an amino acid variant, isoleucine to valine, in the α1 subunit

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Abstract

Recombinant human GABAA receptors were investigated in vitro by coexpression of cDNAs coding for α1, β2, and γ2 subunits in the baculovirus/Sf-9 insect cell system. We report that a single amino acid exchange (isoleucine 121 to valine 121) in the N-terminal, extracellular part of the α1 subunit induces a marked decrease in agonist GABAA receptor ligand sensitivity. The potency of muscimol and GABA to inhibit the binding of the GABAA receptor antagonist [3H]SR 95531 (2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide) was higher in receptor complexes of α1(ile 121)β2γ2 than in those of α1(val 121)β2γ2 (IC50 values were 32-fold and 26-fold lower for muscimol and GABA, respectively). The apparent affinity of the GABAA receptor antagonist bicuculline methiodide to inhibit the binding of [3H]SR 95531 did not differ between the two receptor complex variants. Electrophysiological measurements of GABA induced whole-cell Cl currents showed a ten-fold decrease in the GABAA receptor sensitivity of α1(val 121)β2γ2 as compared to α1(ile 121)β2γ2 receptor complexes. Thus, a relatively small change in the primary structure of the α1 subunit leads to a decrease selective for GABAA receptor sensitivity to agonist ligands, since no changes were observed in a GABAA receptor antagonist affinity and benzodiazepine receptor binding.

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