Classification of benzodiazepine receptor agonists, inverse agonists and antagonists using bicuculline in an in vitro test.

The mechanism by which a substance that binds to the benzodiazepine receptor acts as an agonist, an inverse agonist (e.g. methyl-beta-carboline-3-carboxylate (beta-CCM] or an antagonist (e.g. Ro 15-1788) was investigated. For this purpose, we studied the influence of bicuculline, an antagonist of gamma-aminobutyric acid (GABA), on the binding of these substances in crude synaptosomal preparation (P2 fraction) containing high levels of endogenous GABA. Displacement curves were performed, using 3H-flunitrazepam in the absence and in the presence of a high concentration (7.10(-5) M) of bicuculline. The ratios of IC50 values with and without bicuculline were significantly higher than 1 for all benzodiazepine agonists investigated (e.g. 1.91 +/- 0.11 (n = 3) for diazepam), about 1 for Ro 15-1788 (0.94 +/- 0.06 (n = 4)) and lower than 1 for beta-CCE (0.55 +/- 0.05 (n = 4)). Statistically significant differences were also observed among benzodiazepine agonists e.g. between flunitrazepam (a sedative-hypnotic drug) and clonazepam (an anticonvulsant drug) or lorazepam (an anxiolytic drug). These data indicate that the ratios of IC50 values with and without bicuculline might provide the basis for an in vitro, pharmacologically relevant, classification of drugs acting on the benzodiazepine receptor. This procedure does not require extensive washing of the membrane preparation, in contrast to the method in which the ratios of IC50 values were determined with and without addition of GABA.