Effects Of Clobazam In Benzodiazepine-Receptor Binding Assays

In benzodiazepine-receptor binding assays using membrane preparations from rat brain cortex, the inhibitor affinity constants, K1, of clobazam and diazepam in displacing 3H-flunitrazepam binding are 170.0 and 7.0 nM, respectively. Thus, the in vitro activity of clobazam in the receptor assay does not seem to correlate with its in vivo potency. With both drugs, identical Hill coefficients (0.94) near unity have been obtained, indicating an equal affinity to possibly existing receptor subtypes under usual experimental conditions. When membrane suspensions in either sodium phosphate buffer or TRIS-HCl buffer are preincubated at 60°C for 60 min, inactivation of one or two binding components (BZ1 and BZ2, respectively) occurs. After preincubation in TRIS-HCl, a third subtype (BZ3) of binding sites remains. Clobazam reveals a high affinity to subtype BZ3, similar to that of diazepam, but is much less active in displacing 3H-flunitrazepam from subtypes BZ1 and BZ2, whereas diazepam also shows a high affinity to BZ1. The possible relevance of these findings is discussed, and an involvement of subtype BZ3 in mediating anxiolytic drug effects is suggested.