Dose and schedule determinants of cocaine choice under concurrent variable-interval schedules in rhesus monkeys

Drug abuse can be characterized as a condition in which the choice to self-administer a drug is excessive, even exclusive of the choice of other reinforcers. Under concurrent interval schedules of reinforcement, subjects typically distribute behavior to match reinforcement allocation. However, research has shown that when behavior is maintained by different doses of cocaine under concurrent variable-interval (conc VI) schedules, exclusive choice of the higher dose is the rule.

The present study was designed to examine the generality of this finding to other behavioral conditions.

Rhesus monkeys ( n=5) lever pressed under a conc VI 60-s VI 60-s or a conc VI 600-s VI 600-s schedule of cocaine (i.v.) presentation. Doses differing by 4-fold (0.025 versus 0.1, 0.05 versus 0.2 mg/kg per injection) were available for lever pressing.

Monkeys responded more on the lever associated with the higher dose when saline or a lower dose was the alternative. The distribution of responses was well predicted by relative drug intake, but with consistent undermatching. Exclusive high-dose responding was seen in about half of the individual session intervals under the shorter schedule, rarely under the longer schedule, and was not seen over the session.

Under conc VI schedules, behavior was apportioned between two different doses in a manner that favored the higher dose but undermatched relative intake. Exclusive high-dose choice may occur when cocaine is frequently available but is not an invariable outcome of the choice between a low and a high dose of cocaine.