Reinforcement by orally delivered methadone, cocaine, and methadone-cocaine combinations in rhesus monkeys: Are the combinations better reinforcers?

Rationale: Polydrug abuse is a problem that has been infrequently examined. In the present study, drug self-administration procedures were used to investigate the reinforcing effects of drug combinations. Objectives: To determine the absolute and relative response rates maintained by orally delivered methadone, cocaine, and their combinations under sequential and concurrent access. Choice between drug combinations containing different concentrations of cocaine was also determined. Methods: Oral intake of methadone, cocaine, and their combinations was studied with rhesus monkeys during daily 3-h sessions. Lip contact (the operant response) was reinforced by delivery of liquid contingent upon completion of a fixed-ratio schedule. In one series, the drugs and drug combinations were studied sequentially with the water vehicle concurrently available. In the next series, the drugs and drug combinations were concurrently available. In the third series, pairs of drug combinations containing different concentrations of cocaine were also concurrently available. Results: Methadone, cocaine and their combinations functioned as reinforcers. Under sequential access, response rates for the drug combinations and the component drugs were often similar. However, under concurrent access, response rates for the drug combinations were greater than response rates for the component drugs at the highest FR size for each condition. Also, drug combinations containing higher cocaine concentrations were preferred to combinations containing lower cocaine concentrations. Conclusions: Combinations of methadone and cocaine have relatively greater reinforcing effects than the component drugs, and these greater reinforcing effects are better detected with concurrent measures than with sequential measures.