SYNTHESIS AND PHARMACOLOGICAL PROPERTIES OF NOVEL 8-SUBSTITUTED IMIDAZOBENZODIAZEPINES - HIGH-AFFINITY, SELECTIVE PROBES FOR ALPHA-5-CONTAINING GABA(A) RECEPTORS

The synthesis and pharmacological properties of imidazobenzodiazepines with both high affinity and selectivity for alpha 5-containing GABAA receptors are described. Four of these compounds (5, 6, 8, and 9) inhi bited [H-3]flunitrazepam binding to recombinant alpha 5 beta 2 gamma 2 GABA(A) receptors with IC50 values between similar to 0.4 and 5 nM. T hese compounds were greater than or equal to 24-75-fold more selective for recombinant receptors containing alpha 5 subunits compared to oth er, ''diazepam-sensitive'' GABA(A) receptors containing either alpha 1 , alpha 2, or alpha 3 subunits. Imidazobenzodiazepine 9 (used as the p rototypical alpha 5 selective ligand) inhibited [H-3]flunitrazepam bin ding to hippocampal membranes with high- and low-affinity components ( IC50 0.6 +/- 0.2 and 85.6 +/- 13.1 nM, respectively), representing sim ilar to 16% and similar to 84% of the receptor pool. Inhibition of [H- 3]flunitrazepam binding to cerebellar membranes with imidazobenzodiaze pine 9 was best fitted to a single population of sites with an IC50 of 79.8 +/- 18.3 nM. These imidazobenzodiazepines behaved as GABA negati ve ligands in recombinant GABA(A) receptors expressed in Xenopus oocyt es and were convulsant in mice after parenteral administration. The re lative potencies of flumazenil and zolpidem in blocking convulsions in duced by 9 and DMCM, respectively, indicated that occupation of alpha 5-containing GABA(A) receptors substantially contributed to the convul sant properties of acetylene analog 9. These 8-substituted imidazobenz odiazepines (5, 6, 8, and 9) should prove useful in examining the phys iological roles of GABA(A) receptors bearing an alpha 5 subunit and ma y also lead to the development of novel, subtype selective therapeutic agents.