Reinforcing effects of a pentobarbital-ethanol combination relative to each drug alone

The reinforcing effects of an orally delivered combination of 1 mg/ml pentobarbital plus 1% ethanol were evaluated in four rhesus monkeys. The drug combination and another liquid (either water, 1 mg/ml pentobarbital, or 1% ethanol) were concurrently available under identical fixed-ratio (FR) schedules. Substantially higher response rates were maintained by the drug combination than by any of the three other liquids. Thus, the reinforcing effects of the pentobarbital-ethanol combination were greater than those of either component drug. In a second experiment, water (the vehicle) was concurrently available with one other liquid (1 mg/ml pentobarbital, 1% ethanol, or the pentobarbital-ethanol combination). All three drug solutions functioned as reinforcers since they maintained much higher response rates than water. These results demonstrate that the greater relative reinforcing effects of the drug combination in the first experiment were not due to a lack of reinforcing effects of the 1 mg/ml pentobarbital or 1% ethanol solutions. In a final experiment, the drug combination was scheduled concurrently with 1 mg/ml pentobarbital, and FR size was systematically varied. The drug combination was then scheduled concurrently with 1% ethanol, and FR size was again varied. As FR size increased, the relative amount of responding maintained by the drug combination increased. Thus, differences in relative reinforcing effects that were evident in the first experiment were again evident in the final experiment when appropriate schedule-parameter values were used.