The role of reinforcement loss in tolerance to chronic delta9-tetrahydrocannabinol effects on operant behavior of rhesus monkeys.

Two monkeys were trained on a multiple fixed-interval (FI) 120 sec, differential reinforcement of low rate (DRL) 120 sec schedule of food reinforcement for lever pressing in which the two schedules, each correlated with a distinctive cue, alternated throughout an experimental session. Under chronic daily treatment with delta-9-tetrahydrocannabinol in a dose of 7 mg per os (1 mg/kg), for 40 consecutive days, responding increased in both schedules. Performance on the DRL schedule was affected less dramtically than that on the FI schedule. Even though reinforcement frequency on the DRL schedule remained suppressed and FI reinforcement frequency was unaffected during chronic drug treatment, DRL performance showed greater tolerance than FI performance.