Differential cross talk of ROD compounds with the benzodiazepine binding site

We have recently identified a novel class of allosteric modulators of GABA( A) receptors, the ROD compounds that are structurally related to bicucullin e. Here, the relationship of their site of action relative to other known m odulatory sites of this receptor was investigated. Two types of ROD compoun ds, R1 (ROD164A, ROD185) and R2 (ROD222 and ROD259) could be differentiated . R1 compounds competitively inhibited binding of benzodiazepines in alpha1 beta2 gamma2 receptors, and their functional effects were partially inhibi ted by the benzodiazepine antagonist Ro15-1788 in a noncompetitive manner. The enhancement by an R1 compound was not additive with that by diazepam. R 2 compounds in contrast failed to inhibit binding of benzodiazepines; the R 2 compounds' functional effects were not inhibited by the benzodiazepine an tagonist. The enhancement by an R2 compound was additive with that by diaze pam. In contrast to benzodiazepines, both R1 and R2 type compounds were sti ll able to enhance alpha1 beta2 receptors. ROD164A in alpha1 beta2 gamma2 r eceptors was found to be partially antagonized by Ro15-1788 in a noncompeti tive way. ROD178B did not affect gamma -aminobutyric acid induced currents, but was able to inhibit both enhancement by R1 and R2 type compounds as we ll as enhancement by diazepam. R1 and R2 type compounds as well as diazepam enhanced pentobarbital-induced currents in a Ro15-1788-sensitive way. We c onclude that R1 type compounds act at the benzodiazepine binding site and a dditionally at a different R1 site, and that the R1, but not the R2 site is allosterically coupled to the benzodiazepine binding site. ROD178B is a co mpetitive antagonist at the R1 site in that it shows allosteric interaction with the benzodiazepine binding site and displacement of benzodiazepines, and a negative allosteric modulator at the R2 site.