A choice procedure for drug reinforcers: cocaine and methylphenidate in the rhesus monkey

A choice procedure was developed to compare the reinforcing efficacy of drug solutions delivered via intravenous catheters to rhesus monkeys. Choices were arranged between doses of cocaine or methylphenidate and saline, different doses of the same drug and doses of both drugs. In each session, monkeys were allowed to self-inject one solution five times in the presence of a stimulus. Thirty minutes after the fifth injection, a second solution could be self-injected five times in the presence of a different stimulus. Thirty minutes later, choice trials began in which both stimuli were present and monkeys could choose one of the two solutions. Rate of responding decreased with increases in dose for both cocaine (0.05-1.5 mg/kg) and methylphenidate (0.075-0.7 mg/kg). Response rates maintained by cocaine were 2 to 3 times higher than those maintained by methylphenidate. Drug was always chosen over saline. Higher doses of cocaine were preferred to lower doses except when both were above 0.5 mg/kg, when no preference was shown. Higher doses of methylphenidate were preferred over low doses, but compared to cocaine, a greater absolute difference between dose magnitude was required to demonstrate preference. When equal doses of cocaine and methylphenidate were compared, no preference was shown. On other comparisons between the drugs, the higher dose was generally preferred regardless of the drug. The reinforcing efficacy of drugs must be considered not only in terms of response rate maintained or reinforcement schedule but also with reference to concurrently available drugs.