DECREASED AGONIST SENSITIVITY OF HUMAN GABA(A) RECEPTORS BY AN AMINO-ACID VARIANT, ISOLEUCINE TO VALINE, IN THE ALPHA(1) SUBUNIT

Recombinant human GABA(A) receptors were investigated in vitro by coex pression of cDNAs coding for alpha(1), beta(2), and gamma(2) subunits in the baculovirus/Sf-9 insect cell system. We report that a single am ino acid exchange (isoleucine 121 to valine 121) in the N-terminal, ex tracellular part of the alpha(1) subunit induces a marked decrease in agonist GABA(A) receptor ligand sensitivity. The potency of muscimol a nd GABA to inhibit the binding of the GABA(A) receptor antagonist [H-3 ]SR 95531 oxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide) was higher in receptor complexes of alpha(1)(ile 121) beta(2) gamma(2) than in those of alpha(1)(val 121) beta(2) gamma(2) (IC50 values were 32-fold and 26-fold lower for muscimol and GABA, respectively). The a pparent affinity of the GABA(A) receptor antagonist bicuculline methio dide to inhibit the binding of [H-3]SR 95531 did not differ between th e two receptor complex variants. Electrophysiological measurements of GABA induced whole-cell Cl- currents showed a ten-fold decrease in the GABA(A) receptor sensitivity of alpha(1)(val 121) beta(2) gamma(2) as compared to alpha(1)(ile 121) beta(2) gamma(2) receptor complexes. Th us, a relatively small change in the primary structure of the alpha(1) subunit leads to a decrease selective for GABA(A) receptor sensitivit y to agonist ligands, since no changes were observed in a GABA(A) rece ptor antagonist affinity and benzodiazepine receptor binding. (C) 1997 Elsevier Science B.V.