ANOMALOUS RECTIFYING PROPERTIES OF DIAZEPAM-INSENSITIVE GABA(A) RECEPTORS

Studies using recombinant systems indicate that 'diazepam-insensitive' GABA(A) receptors in the central nervous system contain alpha 4 and a lpha 6 subunits while 'diazepam-sensitive' GABA(A) receptors contain a lpha 1, alpha 2, alpha 3 and alpha 5 subunits. Both native and recombi nant diazepam-sensitive GABA(A) receptors typically exhibit large, out wardly rectifying currents. For example, in patch clamp studies, Human Embryonic Kidney (HEK) 293 cells transfected with cDNAs encoding alph a 1 beta 2 gamma 2 subunits exhibit a rectification ratio (I+60 mV/I-6 0 mV) of 1.95 +/- 0.21. However, anomalous rectification was observed in recombinant diazepam-insensitive GABA(A) receptors composed of eith er alpha 4 beta 2 gamma 2 (rectification ratio, 0.74 +/- 0.09) or alph a 6 beta 2 gamma 2 (rectification ratio, 0.67 +/- 0.11) subunits. Base d on sequence differences between diazepam-sensitive and -insensitive GABA(A) receptor alpha subunits in the vicinity of the putative channe l lining, a point mutation was introduced at His(273) on the alpha 4 s ubunit. The rectification ratio in cells expressing a mutated alpha 4( Asn(273))beta 2 gamma 2 receptor increased to 1.92+/-0.17. Moreover, m utation of the homologous residue in the alpha 1 subunit to histidine reduced the rectification ratio of alpha 1(His(274))beta 2 gamma 2 to 1.02 +/- 0.12. The affinities of benzodiazepine site ligands at diazep am-sensitive and -insensitive GABA(A) receptors were unaffected by the se mutations. Thus, the electrophysiological properties of diazepam-se nsitive and -insensitive GABA, receptors may be as divergent as their pharmacological characteristics. (C) 1998 Elsevier Science B.V.