Behavioral dependence on caffeine and phencyclidine in rhesus monkeys: interactive effects.

Five rhesus monkeys were trained to self-administer orally-delivered phencyclidine (PCP) and water under concurrent fixed-ratio (FR) 8 schedules. Liquid deliveries were contingent upon lip-contact responses on solenoid-operated drinking spouts, and food pellet delivery was contingent upon responses on a centrally-located lever. Food was available during three 1-hr periods each day under an FR 64 or FR 80 schedule. The liquids were available during three 6.5-hr periods after each food component. In the first experiment caffeine (4 or 8 mg) was added to each 6-g food pellet, and after responding stabilized, noncaffeinated pellets were substituted for the caffeinated pellets for eight days. There were no differences in food-, water- or PCP-maintained behavior due to caffeine concentration (4 vs. 8 mg/pellet) although the monkeys consumed twice as much caffeine at the higher concentration. Food-maintained responding was reliably reduced by 25-50 percent the first day of caffeine removal, and there was a recovery of responding characterized by intermittent cycles of low response rates over the next 7 days. Water and PCP intake were not systematically disrupted when caffeine access was terminated. In the second experiment the monkeys were tested with caffeinated (6 mg/pellet) and noncaffeinated pellets under conditions of PCP removal (water substitution) and reinstatement. Under both food conditions, when PCP access was terminated, pellet deliveries decreased by about 50 percent and gradually recovered over the 8-day water substitution phase. However, behavioral disruptions were more severe under conditions in which monkeys received caffeinated pellets, suggesting an interactive effect due to termination of PCP access and decreased caffeine intake. These results indicate that disruptions in operant baselines are sensitive indicators of the effects of discontinuing caffeine access; however, the severity and time course of behavioral disruptions due to caffeine removal are considerably less than after termination of PCP access.