DIFFERENTIAL AFFINITY OF DIHYDROIMIDAZOQUINOXALINES AND DIIMIDAZOQUINAZOLINES TO THE ALPHA-1-BETA-2-GAMMA-2 AND ALPHA-6-BETA-2-GAMMA-2 SUBTYPES OF CLONED GABA-A RECEPTORS

1 In this study, we compared two series of newly discovered ligands for their selectivity to benzodiazepine sites in the alpha1beta2gamma2 and the alpha6beta2gamma2 subtypes of cloned gamma-aminbutyric acid(A) (GABA(A)) receptors, the latter being unique in not interacting with classical benzodiazepines. 2 The prototype compounds, U-85575 (12-chloro-5-(5-cyclopropyl-1',2',4'- oxadiazol-3'-yl)-2,3-dihydro-diimidazo[1,5-a;1,2-c]quinazoline), and U-92330 (5-acetyl-3-(5'-cyclopropyl-1',2',4'-oxadiazole-3'-yl)-7-chloro-4,5-dihydro[1,5-a]quinoxaline), appear to share an overlapping recognition site with classical benzodiazepines on the GABA(A) receptor, because their potentiation of GABA-mediated Cl- currents in both subtypes were sensitive to Ro 15-1788, a classical benzodiazepine antagonist. 3 Minor changes in the ring substituents of the drugs reduced their affinity to the alpha6beta2gamma2 subtype more pronouncedly than to the alpha1beta2gamma2 subtype. The diimidazoquinazoline containing a 2-methyl group which projected below the plane of the rigid ring showed a markedly lower affinity to the alpha6beta2gamma2 subtype as compared to its stereoisomer having the methyl group above the plane of the ring. Also, the dihydroimidazoquinoxalines containing the 5-benzoyl group showed a lower affinity to the alpha6beta2gamma2 subtype than the 5-acetyl counterpart. In particular, the 5-benzoyl analogue containing a 6-fluoro group showed no interaction with the alpha6beta2gamma2 subtype even at the concentration of 10 muM, probably due to stabilization of the benzoyl group in the out-of-plane region by the steric and electrostatic effects of the 6-fluoro group. 4 We propose that the benzodiazepine site of the alpha6beta2gamma2 subtype shares overlapping regions with that of the alpha1beta2gamma2 subtype, but has a sterically restricted out-of-plane region, which may be also incompatible with the 5-phenyl group of classical benzodiazepines.