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 inv estigate the reinforcing effects of drug combinations. Objectives: To deter mine the absolute and relative response rates maintained by orally delivere d methadone, cocaine, and their combinations under sequential and concurren t access. Choice between drug combinations containing different concentrati ons of cocaine was also determined. Methods: Oral intake of methadone, coca ine, and their combinations was studied with rhesus monkeys during daily 3- h sessions. Lip contact (the operant response) was reinforced by delivery o f liquid contingent upon completion of a fixed-ratio schedule. In one serie s, the drugs and drug combinations were studied sequentially with the water vehicle concurrently available. In the next series, the drugs and drug com binations were concurrently available. In the third series, pairs of drug c ombinations containing different concentrations of cocaine were also concur rently available. Results: Methadone, cocaine and their combinations functi oned as reinforcers. Under sequential access, response rates for the drug c ombinations and the component drugs were often similar. However, under conc urrent access, response rates for the drug combinations were greater than r esponse rates for the component drugs at the highest FR size for each condi tion. Also, drug combinations containing higher cocaine concentrations were preferred to combinations containing lower cocaine concentrations. Conclus ions: Combinations of methadone and cocaine have relatively greater reinfor cing effects than the component drugs, and these greater reinforcing effect s are better detected with concurrent measures than with sequential measure s.