Labelling of diazepam-sensitive and -insensitive benzodiazepine receptors with [3H]tert-butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-a][1,4]benzodiazepine 3-carboxylate (ZG-63).
A diazepam-insensitive subtype of benzodiazepine receptor has been identified in the cerebella of several species, including man. t-Butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine 3-carboxylate (ZG-63) was recently described as a selective, high affinity ligand at diazepam-insensitive benzodiazepine receptors. This compound was tritiated, and its properties as a radioligand evaluated in rat brain membranes. Consistent with the high affinity and selectivity described for the non-radioactive form of this compound, saturation analyses of [3H]ZG-63 binding to cerebellar diazepam-insensitive and other, diazepam-sensitive benzodiazepine receptors revealed Kd values of 2.6 +/- 0.2 nM and 10.6 +/- 1.4 nM, respectively. The density (Bmax) of cerebellar diazepam-insensitive receptors labelled with [3H]ZG-63 was not significantly different from values obtained with the prototypical diazepam-insensitive receptor ligand [3H]Ro 15-4513, representing approximately 30% of total cerebellar benzodiazepine receptors. [3H]ZG-63 also labelled cortical diazepam-sensitive benzodiazepine receptors, with Bmax values that were not significantly different from those obtained with [3H]flunitrazepam. Diazepam-insensitive benzodiazepine receptors in rat cerebral cortex could be detected with [3H]ZG-63, but the densities of these sites are a very minor component (< or = 5%) of total benzodiazepine receptors. In the presence of GABA, [3H]ZG-63 behaved as a 'gamma-aminobutyric acid (GABA) -positive', 'GABA-negative', and 'GABA-neutral' ligand at cortical diazepam-sensitive receptors, cerebellar diazepam-sensitive receptors, and cerebellar diazepam-insensitive benzodiazepine receptors, respectively. This profile differs from the prototype diazepam-insensitive receptor ligand, [3H]Ro 15-4513. Competition studies demonstrated a very high correlation (r2 = 0.98; P < 0.002) between the potencies of a series of benzodiazepine receptor ligands to inhibit [3H]ZG-63 and [3H]Ro 15-4513 binding to cerebellar diazepam-insensitive receptors. The high affinity and selectivity of [3H]ZG-63 for diazepam-insensitive receptors (diazepam-insensitive/diazepam-sensitive ratio of approximately 0.25) together with a GABA-shift profile which differs from Ro 15-4513 suggests that this compound may be useful in elucidating the function(s) of this benzodiazepine receptor subtype.
Wong
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Laboratory of Neuroscience, National Institute of Diabetes, Digestive, and Kidney Diseases National Institutes of Health, Bethesda, MD 20892.
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de Costa
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Skolnick
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D000818:Animals; D001570:Benzodiazepinones / Q000378:metabolism*; D001665:Binding Sites / Q000187:drug effects; D002531:Cerebellum / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism*; D003975:Diazepam / Q000494:pharmacology*; D008024:Ligands; D008297:Male; D011869:Radioligand Assay; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000187:drug effects / Q000378:metabolism*; D013696:Temperature; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D001570:Benzodiazepinones; D008024:Ligands; D011963:Receptors, GABA-A; C084850:ZG 63; D005680:gamma-Aminobutyric Acid; D003975:Diazepam