Behavioral antecedents of cocaine-induced stereotypy

The effect of cocaine on discriminative behavior was determined in a study in which the subjects' ongoing behavior at the time of drug administration was manipulated. Cats were trained in a discrimination task where bar-pressing when a tone was present (S+) resulted in milk reinforcement. When the tone was absent (S-) milk was not delivered. The duration of these periods varied randomly during 30-min sessions and responses during S- delayed the next trial. Once a high level of discrimination was achieved, drug testing was begun. Ongoing behavior was manipulated by scheduling either a 5-min S+ or S- period immediately after cocaine injection (1 mg/kg). When S+ followed cocaine, stereotyped bar-pressing developed with markedly increased responding during the remainder of the session. In contrast, when S- followed cocaine, suppression of bar-pressing developed. The effect could be reversed in most subjects when stimulus conditions were later reversed. A similar experiment using rats yielded similar results. These results indicate that ongoing behavior and stimulus conditions are critically important in determining the behavioral effects of cocaine.