Stereoselective stimulus effects of 3-methylflunitrazepam and pentobarbital.

Rats trained to discriminate 3.0 mg/kg of diazepam from saline in a two-lever operant choice task were challenged with the racemic mixture and optical isomers of 3- methylflunitrazepam or pentobarbital. Generalization of the diazepam stimulus was found to occur to (+/-)- and S(+)-3- methylflunitrazepam , with the S(+)-isomer being twice as active as the racemate. Diazepam stimulus generalization also occurred to (+/-)-, S(-)-, and R(+)-pentobarbital, with the S(-)-isomer being approximately twice as active as (+/-)- or R(+)-pentobarbital. In addition, the administration of the imidazobenzodiazepine Ro 15-1788, a selective benzodiazepine receptor antagonist, prior to benzodiazepine or barbiturate administration competitively antagonized the discriminative stimulus properties of the benzodiazepines but was completely ineffective in attenuating the discriminative stimulus effect of the barbiturates. The results of this study suggest that benzodiazepines exert their stimulus effects by a stereoselective interaction at a benzodiazepine receptor and that stereochemical factors are important in evaluating the stimulus properties of benzodiazepines or barbiturates.