Altered receptor subtypes in the forebrain of GABA A receptor δ subunit-deficient mice: recruitment of γ2 subunits

Abstract

A GABA A receptor δ subunit-deficient mouse line was created by homologous recombination in embryonic stem cells to investigate the role of the subunit in the brain GABA A receptors. High-affinity [ 3H]muscimol binding to GABA sites as studied by ligand autoradiography was reduced in various brain regions of δ −/− animals. [ 3H]Ro 15-4513 binding to benzodiazepine sites was increased in δ −/− animals, partly due to an increment of diazepam-insensitive receptors, indicating an augmented forebrain assembly of γ2 subunits with α4 subunits. In the western blots of forebrain membranes of δ −/− animals, the level of γ2 subunit was increased and that of α4 decreased, while the level of α1 subunits remained unchanged. In the δ −/− forebrains, the remaining α4 subunits were associated more often with γ2 subunits, since there was an increase in the α4 subunit level immunoprecipitated by the γ2 subunit antibody. The pharmacological properties of t-butylbicyclophosphoro[ 35S]thionate binding to the integral ion-channel sites were slightly altered in the forebrain and cerebellum, consistent with elevated levels of α4γ2 and α6γ2 subunit-containing receptors, respectively. The altered pharmacology of forebrain GABA A receptors and the decrease of the α4 subunit level in δ subunit-deficient mice suggest that the δ subunit preferentially assembles with the α4 subunit. The δ subunit seems to interfere with the co-assembly of α4 and γ2 subunits and, therefore, in its absence, the γ2 subunit is recruited into a larger population of α4 subunit-containing functional receptors. These results support the idea of subunit competition during the assembly of native GABA A receptors.