INTRAVENOUS SELF-ADMINISTRATION OF COCAINE UNDER CONCURRENT VI-SCHEDULES OF REINFORCEMENT

Under concur-rent VI (cone VI) schedules of reinforcement, organisms m atch the proportion of responses to the proportion of reinforcers obta ined from the available options. This result is the basis of the match ing law, a major theoretical view of the control of choice between and among available reinforcers. The present experiment examined the exte nt to which IV cocaine self-administration by monkeys under cone VI sc hedules of reinforcement was predicted by the matching law. One group of rhesus monkeys (n=4) was prepared with chronic indwelling IV cathet ers and allowed to respond in a two-lever situation under cone VI sche dules of reinforcement for cocaine injections. Three doses of cocaine (0.025, 0.05 and 0.1 mg/kg per injection) were made available under va rious cone VI schedules with an average inter-injection interval of 3 min. The same injection was available for each response, the differenc e between the options being the schedule of reinforcement. Each dose a nd schedule condition was in effect for at least ten consecutive sessi ons and until responding was stable. In a second group of monkeys (n=3 ), a comparable experiment was conducted with responding maintained by the delivery of 1-g food pellets. Overall response rate maintained by cocaine was inversely related to dose. In addition, response rate dec reased over the course of a session, apparently due to accumulation of cocaine. With regard to choice, more responding was maintained by the schedule that arranged more frequent injections. Choice was well pred icted by the matching law, with a consistent tendency toward undermatc hing but no consistent bias toward one or the ether option. Results we re similar for behavior maintained by food, though two of three monkey s showed an unexplained bias toward the left lever. With regard to dru g self-administration, these results demonstrate that in a choice situ ation, with all other variables being equal, injections that are avail able more frequently in time maintain behavior more strongly than less frequently available injections. Undermatching implies that the relat ive proportion of behavior maintained by the two options is somewhat l ess than the relative proportion of injections obtained. The finding t hat choice maintained by cocaine under cone VI schedules was comparabl e to choice maintained by food extends the generality of the conclusio n that drugs and non-drug reinforcers control behavior by similar mech anisms.