Altered receptor subtypes in the forebrain of GABAA receptor δ subunit-deficient mice:: Recruitment of γ2 subunits

A GABA(A) receptor delta 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 [H-3]muscimol binding to GABA sites as studied by ligand autoradiography was reduced in various brain regions of delta(-/-) animals. [H-3]Ro 154513 binding to benzodiazepine sites was increased in delta(-/-) animals, partly due to an increment of diazepam-insensitive receptors, indicating an augmented forebrain assembly of gamma2 subunits with alpha4 subunits. In the western blots of forebrain membranes of delta(-/-) animals, the level of gamma2 subunit was increased and that of a4 decreased, while the level of alpha1 subunits remained unchanged. In the delta(-/-) forebrains, the remaining alpha4 subunits were associated more often with gamma2 subunits, since there was an increase in the a4 subunit level immunoprecipitated by the gamma2 subunit antibody. The pharmacological properties of t-butylbicyclophosphoro[S-35]thionate binding to the integral ion-channel sites were slightly altered in the forebrain and cerebellum, consistent with elevated levels of alpha4gamma2 and alpha6gamma2 subunit-containing receptors, respectively. The altered pharmacology of forebrain GABAA receptors and the decrease of the a4 subunit level in 5 subunit-deficient mice suggest that the 8 subunit preferentially assembles with the alpha4 subunit. The delta subunit seems to interfere with the co-assembly of alpha4 and gamma2 subunits and, therefore, in its absence, the gamma2 subunit is recruited into a larger population of alpha4 subunit-containing functional receptors. These results support the idea of subunit competition during the assembly of native GABA(A) receptors. (C) 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved.