Effects of different food-reinforcement histories on cocaine self-administration by rhesus monkeys

The purpose of the present study was to examine whether a history of responding under food reinforcement schedules that generated either high or low response rates would influence the acquisition and maintenance of cocaine self-administration. Eight experimentally naive rhesus monkeys were initially trained to respond on the right lever under either a fixed-ratio (FR) 50 or interresponse times (IRT) > 30-s schedule of food reinforcement. After 65 sessions of food-maintained responding, monkeys were surgically prepared with indwelling intravenous catheters and cocaine 0.03 mg/kg per injection (IV) was available on the left lever under a fixed-interval (FI) 5-min schedule. After at least 60 consecutive sessions at this dose, a cocaine dose-response curve (saline, 0.01–0.3 mg/kg per injection) was determined. The FR 50 schedule generated high rates of food-maintained responding (90.1±6.2 responses/min), while response rates under the IRT >30-s schedule were low (1.9±0.1 responses/min). Across the 60 consecutive sessions under the FI 5-min schedule, linear changes in response rates and cocaine intake were significantly different between FR- and IRT-history monkeys. FR-history monkeys responded at higher rates than IRT-history subjects, while cocaine intake during the first 15 sessions was lower in FR- compared to IRT-history monkeys. Rates of cocaine-maintained responding after food-reinforcement histories were compared to response rates of monkeys initially trained to self-administer cocaine under an FI 5-min schedule (Nader and Reboussin 1994). Response rates were higher in this latter group compared to rates generated by either group of monkeys after food-reinforcement histories. Furthermore, a significant interaction between behavioral history and cocaine dose on response rates was observed. Results from the present study indicate that a history of responding maintained by a nondrug reinforcer can have significant and long-lasting effects on response rates and total cocaine intake under an FI schedule. Furthermore, these results indicate that prior experiences may produce different effects on acquisition and maintenance of cocaine self-administration.