ALTERATIONS OF THE BENZODIAZEPINE SITE OF RAT ALPHA-6-BETA-2-GAMMA-2-GABA(A) RECEPTOR BY REPLACEMENT OF SEVERAL DIVERGENT AMINO-TERMINAL REGIONS WITH THE ALPHA-1 COUNTERPARTS

1 The benzodiazepine site of the alpha 6 beta 2 gamma 2 subtype of gam ma-aminobutyric acid(A) (GABA(A)) receptors is distinguishable from th at of the alpha 1 beta 2 gamma 2 subtype by its inability to interact with classical benzodiazepines (i.e., diazepam) and its agonistic resp onse to Ro 15-1788, which behaves as an antagonist in the alpha 1 beta 2 gamma 2 subtype. 2 The point mutation of Arg 100 of the alpha 6 sub unit to histidine (the corresponding residue in alpha 1) has been show n to enable the alpha 6 beta 2 gamma 2 subtype to interact with diazep am but failed in this study to abolish the ability of Ro 15-1788 to en hance GABA-induced Cl- currents. 3 Here we identified the segment of P 161 to L187 of alpha 6 to contain the functional region responsible fo r the agonistic action of Ro 15-1788. Its replacement with the corresp onding alpha 1 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 benzodiaz epine site ligands such as diazepam, U-92330 and ,7-dimethoxy-4-ethyl- beta-carboline-3-carboxylate. These data support the evidence that the functionality of a given ligand could arise from a single region of t he benzodiazepine site, not shared by others. 4 To addition we have le arned that several residues in the N-terminal region of alpha 6, such as R100, V142 and N143, have the ability to influence GABA-dependent C l- current induction probably by allosterically modulating low affinit y GABA sites.