Individual differences in rhesus monkeys' demand for drugs of abuse

A relatively small percentage of humans who are exposed to drugs of abuse eventually become addicted to or dependent on those drugs. These individual differences in likelihood of developing drug addiction may reflect behavioral, neurobiological or genetic correlates of drug addiction and are therefore important to model. Behavioral economic measures of demand establish functions whose overall elasticity (rate of decrease in consumption as price increases) reflects the reinforcing effectiveness of various stimuli, including drugs. Using these demand functions, we determined the reinforcing effectiveness of five drugs of abuse (cocaine, remifentanil, ketamine, methohexital and ethanol) in 10 rhesus monkeys with histories of intravenous drug-taking. There was a continuum of reinforcing effectiveness across the five drugs, with cocaine and remifentanil showing the most reinforcing effectiveness. There was also a continuum of sensitivity of the monkeys; two of the 10 animals, in particular, showed greater demand for the drugs than did the remaining eight monkeys. In addition, monkeys that demonstrated greater demand for one drug tended to show greater demand for all drugs but did not show a similar relatively greater demand for sucrose pellets. These findings suggest that the tendency to find drugs to be reinforcing is a general one, not restricted to particular drugs and also, that a minority of animals show a substantially enhanced sensitivity to the reinforcing effects of drugs. The possibility that differences in responsiveness to the reinforcing effects of drugs may form the basis of individual differences in drug-taking in humans should be considered.