LABELING OF DIAZEPAM-SENSITIVE AND DIAZEPAM-INSENSITIVE BENZODIAZEPINE RECEPTORS WITH [H-3] -METHYL-6-OXO-4H-IMIDAZO[1,5-A][1,4]BENZODIAZEPINE 3-CARBOXYLATE (ZG-63)

A diazepam-insensitive subtype of benzodiazepine receptor has been ide ntified in the cerebella of several species, including man. -methyl-6- oxo-4H-imidazo[1,5-a][1,4]benzodiazepine 3-carboxylate (ZG-63) was rec ently described as a selective, high affinity ligand at diazepam-insen sitive benzodiazepine receptors. This compound was tritiated, and its properties as a radioligand evaluated in rat brain membranes. Consiste nt with the high affinity and selectivity described for the non-radioa ctive form of this compound, saturation analyses of [H-3]ZG-63 binding to cerebellar diazepam-insensitive and other, diazepam-sensitive benz odiazepine receptors revealed K(d) values of 2.6 +/- 0.2 nM and 10.6 /- 1.4 nM, respectively. The density (B(max)) of cerebellar diazepam-i nsensitive receptors labelled with [H-3]ZG-63 was not significantly di fferent from values obtained with the prototypical diazepam-insensitiv e receptor ligand [H-3]Ro 15-4513, representing approximately 30% of t otal cerebellar benzodiazepine receptors. [H-3]ZG-63 also labelled cor tical diazepam-sensitive benzodiazepine receptors, with B(max) values that were not significantly different from those obtained with [H-3]fl unitrazepam. Diazepam-insensitive benzodiazepine receptors in rat cere bral cortex could be detected with [H-3]ZG-63, but the densities of th ese sites are a very minor component (less-than-or-equal-to 5%) of tot al benzodiazepine receptors. In the presence of GABA, [H-3]ZG-63 behav ed as a 'gamma-aminobutyric acid (GABA) -positive', 'GABA-negative', a nd 'GABA-neutral' ligand at cortical diazepam-sensitive receptors, cer ebellar diazepam-sensitive receptors, and cerebellar diazepam-insensit ive benzodiazepine receptors, respectively. This profile differs from the prototype diazepam-insensitive receptor ligand, [H-3]Ro 15-4513. C ompetition studies demonstrated a very high correlation (r2 = 0.98; P < 0.002) between the potencies of a series of benzodiazepine receptor ligands to inhibit [H-3]ZG-63 and [H-3]Ro 15-4513 binding to cerebella r diazepam-insensitive receptors. The high affinity and selectivity of [H-3]ZG-63 for diazepam-insensitive receptors (diazepam-insensitive/d iazepam-sensitive ratio of approximately 0.25) together with a GABA-sh ift profile which differs from Ro 15-4513 suggests that this compound may be useful in elucidating the function(s) of this benzodiazepine re ceptor subtype.