Self-administration of the isomers of pentobarbital and secobarbital by rhesus monkeys.

Previous studies have shown that the isomers of pentobarbital and secobarbital have behavioral effects that are qualitatively similar to those of the racemic mixture, but that the S-(-) isomers are more potent than the R-(+) isomers. The present study was designed to compare the reinforcing effects of the isomers of these compounds to those of the racemic mixtures in monkeys experienced in the intravenous self-administration of barbiturates. Rhesus monkeys (N = 3) were prepared with indwelling intravenous catheters and allowed to self-administer racemic pentobarbital in 1-hour sessions under a fixed ratio 5 schedule. When responding was stable, various doses of (+,-) pentobarbital, (+,-) secobarbital and single doses of both isomers of these compounds were substituted for the baseline drug in a mixed order. All of the compounds functioned as positive reinforcers in all monkeys. R-(+) isomer were self-administered at higher rates than the racemic mixtures which were self-administered at higher rates than the S-(-) isomers. The results demonstrate that both isomers of these barbiturates can function as positive reinforcers.