Comparison of behavior maintained by intravenous cocaine and d-amphetamine in dogs.

The effectiveness of i.v. injections of cocaine and d-amphetamine in maintaining schedule-controlled behavior was compared directly in dogs. Behavior was maintained under either a fixed-ratio (FR) 15, fixed-interval (FI)5-min, FI5 -min ( FR5 :S), FI15 -min, FI15 -min ( FR5 :S) or FI45 -min ( FR10 :S) schedule of reinforcement during daily experimental sessions where the maximum number of injections per session was limited to 2 to 11, depending on the schedule employed. Each drug injection was followed by a 10-min (FR schedule) or 5-min (interval schedules) timeout period to reduce the direct effects of the drugs on responding. Both cocaine and d-amphetamine maintained temporal patterns of responding characteristic of each of the schedules. Similar rates of responding were maintained by cocaine and d-amphetamine under the FR and longer (15- and 45-min) interval schedules, but cocaine maintained higher rates than did d-amphetamine under the shorter (5-min) interval schedules. Brief stimulus presentations intermittently contiguous with drug injections did not always maintain higher response rates under the second-order schedules as compared to rates maintained under the simple FI schedules, but higher rates were observed more often when stimuli were paired with cocaine than when stimuli were paired with d-amphetamine. These results suggest that cocaine and d-amphetamine can function differently as reinforcers and the differences depend, at least in part, on the schedule of reinforcement under which the drugs are presented.