Oral pentobarbital intake in rhesus monkeys: effects of drug concentration under conditions of food deprivation and satiation.

Pentobarbital-reinforced behavior was studied in four rhesus monkeys. A pentobarbital solution and water were concurrently available during 3-hr sessions; water was freely available between sessions. Both pentobarbital concentration and feeding conditions (deprivation versus satiation) were varied. In two food-restricted monkeys subsequent food satiation eliminated pentobarbital-maintained responding. In two other food-restricted monkeys the effects of food satiation varied with the drug concentration. At the highest concentration, 4 mg/ml, food satiation did not alter responding, whereas at 2 mg/ml a moderate decrease occurred and at 1 mg/ml responding was greatly reduced. During the food satiation phase, when the concentration was 4 mg/ml, responding was well maintained under several fixed-ratio sizes. Large quantities of pentobarbital were consumed, and intoxication was observed. Water-maintained responding occurred at low rates and did not vary across feeding conditions or drug concentration. The results support an interpretation in terms of a behavioral mechanism of action. Specifically, the effects of food deprivation on drug self-administration are to increase the magnitude of the reinforcing effects of the drug.