Relation between choice of ethanol concentration and response rates under progressive- and fixed-ratio schedules: studies with rhesus monkeys.

Rationale: A fundamental problem in the study of drugs as reinforcers is the evaluation of a drug's relative reinforcing effects and changes in such effects. Relative reinforcing effects can be measured by determining the preference for one drug dose relative to another drug dose. However, in IV drug self-administration studies technical limitations make direct comparisons between drug doses difficult. An alternative procedure is to measure the relative persistence of behavior across increases in schedule size.

Objective: To develop a more rapid method to measure the relative persistence of behavior. Instead of increasing the schedule size across sessions, schedule size was increased within sessions by use of a progressive-ratio schedule (PR).

Methods: Male rhesus monkeys orally self-administered ethanol during daily 3-h sessions. At each concentration responding was measured with fixed-ratio (FR) 8 schedules to obtain baseline values. Subsequently behavior was studied with a PR schedule. Relative persistence of behavior was calculated by dividing the mean response rate under the PR schedule by the mean response rate under the FR8 schedules. To compare these findings with results of choice between concentrations, monkeys were given concurrent access to pairs of ethanol concentrations.

Results: The relative persistence of behavior increased with increases in drug concentration. When two concentrations were concurrently available, the higher concentration maintained higher response rates.

Conclusions: The relative persistence of behavior can be efficiently measured by dividing the response rate under the PR schedule by the response rate under the FR schedule. Measures of relative persistence corresponded well with measures of choice and show that relative reinforcing effects increase as dose increases.