Further characterization of adjunctive behavior generated by schedules of cocaine self-administration

Abstract

Rhesus monkeys (n = 5), surgically implanted with double-lumen catheters, were allowed to self-administer cocaine (0.1 or 0.3mg/kg/injection, i.v.) on one lever (COC lever) under several fixed-interval schedules of reinforcement. Responding on a second lever (SAL lever) delivered saline (i.v.) under a fixed-ratio 1 schedule. Responding on both levers was a bitonic function of interval value and cocaine dose. A variety of experimental conditions were examined to determine whether SAL lever responding could be considered to be adjunctive in nature. SAL lever responding did not change when saline injections were discontinued, suggesting that SAL lever responding was not maintained by interoceptive stimuli associated with the injection. Discontinuation of various exteroceptive stimulus changes that had occurred as a consequence of SAL lever responding also did not affect the frequency of SAL lever responding. However, when there was no stimulus change following a SAL response, response rates on that lever decreased by approximately 40-60% indicating that stimulus change played some role in the maintenance of the behavior. The introduction of change-over-delays (2-16 s) between responding on the SAL and COC levers had little or no effect on responding on the SAL lever, suggesting that SAL lever responding was not maintained by adventitious reinforcement by cocaine injections. SAL lever responding also occurred in these same monkeys when cocaine was available under fixed-time or variable-interval schedules of reinforcement. These results confirm that presentation of cocaine under interval schedules of reinforcement can generate substantial amounts of behavior (pressing the SAL lever) that is not necessary for obtaining the drug. Further, the results strongly suggest that the behavior can be classified as an adjunctive behavior that is similar to adjunctive behaviors generated by the intermittent presentation of other positive reinforcers.