Drug states as modulators of conditioned immobility in a latent discrimination procedure

Midazolam, amphetamine, and flesinoxan were used in four rat experiments to examine the usefulness of a latent Pavlovian discrimination procedure to assess the discriminative-stimulus, or occasion-setting, properties of drugs. Experiment 1 first assessed the unconditioned effect of each of the drugs on the dependent measure used, which was immobility. Relative to saline, midazolam enhanced immobility, whereas flesinoxan, and especially amphetamine decreased it. In each of the Experiments 2–4, subjects received a limited number of training sessions during which they consistently received a footshock in a distinctive box after a drug but not after saline, or vice versa. Subsequently, non-reinforced test sessions were performed in the drug and saline states in both the conditioning box and a novel box. Relative to the saline state, rats previously shocked under midazolam were less mobile in the conditioning box under midazolam, whereas rats previously not shocked under amphetamine or flesinoxan were more mobile under the drug. The remaining animals did not show differential responding. The response profiles were accounted for in terms of the combined operation of an associative, or occasion-setting, effect and a non-associative effect of the drug-induced states.