Differences in the Stimulus Properties of Barbital and Hallucinogens

The present investigation sought to determine whether drugs which produce markedly different perceptual effects in man, barbital and mescaline or LSD produce different stimuli in rats. In a standard 2 lever operant test chamber, rats received sweetened milk for correct responses according to a variable interval schedule. All sessions were preceded by 1 of 2 treatments; following Treatment A, only responses on Lever A were reinforced and, in a similar fashion, Lever B was correct following Treatment B. No responses were reinforced during the first 5 min of a daily 30 min session. It was found that barbital can serve as a discriminative stimulus when this drug is paired with saline. Previously, we had demonstrated that mescaline or LSD can serve as discriminative stimuli. When barbital was administered to animals trained to discriminate mescaline or LSD and saline, they made either random responses or responses appropriate to saline treatment. This observation suggests that the stimulus properties of barbital are difference from those of the hallucinogens. A subsequent demonstration of discriminated responding when barbital and mescaline or LSD were paired as discriminative stimuli supports this conclusion. The present results extend previous findings which suggest that the degree of similarity or difference of drug stimuli in rats is congruent with the degree of similarity or difference of perceptual drug effects in man.