Effects of local anesthetics on fixed-interval responding in rhesus monkeys

Several local anesthetics of both the ester and amide type were administered IM to rhesus monkeys trained to respond on a fixed-interval 5 min schedule of food delivery. With the exception of procainamide, all local anesthetics produced dose-related decreases in response rates. Effects on pattern of responding varied between local anesthetics. With some (cocaine, dimethocaine and lidocaine), rate-dependent effects were apparent. When control rates were low, these compounds increased rates; when control rates were high, they decreased rates. However, with others (procaine, chloroprocaine, tetracaine and propoxycaine) no rate-dependent effects were noted; i.e., these compounds had little or no effect on the pattern of responding, even at doses that substantially reduced response rates. Consistent with other experiments with these compounds, cocaine was the most potent of the group. In several instances, local anesthetics which had similar stimulus properties in other behavioral paradigms differed in terms of their effects on fixed-interval behavior.