Alterations of the benzodiazepine site of rat α6β2γ2-GABAAreceptor by replacement of several divergent amino-terminal regions with the α1 counterparts

摘要

class="enumerated" style="list-style-type:decimal">The benzodiazepine site of the α6β2γ2 subtype of γ-aminobutyric acidA (GABAA) receptors is distinguishable from that of the α1β2γ2 subtype by its inability to interact with classical benzodiazepines (i.e., diazepam) and its agonistic response to Ro 15-1788, which behaves as an antagonist in the α1β2γ2 subtype.The point mutation of Arg 100 of the α6 subunit to histidine (the corresponding residue in α1) has been shown to enable the α6β2γ2 subtype to interact with diazepam but failed in this study to abolish the ability of Ro 15-1788 to enhance GABA-induced Cl⁻ currents.Here we identified the segment of P161 to L187 of α6 to contain the functional region responsible for the agonistic action of Ro 15-1788. Its replacement with the corresponding α1 sequence abolished the ability of Ro 15-1788 to enhance GABA currents without appreciable effects on its binding affinity to the benzodiazepine site or on the functionality of the other benzodiazepine site ligands such as diazepam, U-92330 and 6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate. These data support the evidence that the functionality of a given ligand could arise from a single region of the benzodiazepine site, not shared by others.In addition we have learned that several residues in the N-terminal region of α6, such as R100, V142 and N143, have the ability to influence GABA-dependent Cl⁻ current induction probably by allosterically modulating low affinity GABA sites.