Expression and Treatment of Pain-Related Depression of Fixed-Ratio and Progressive-Ratio Food-Maintained Behavior in Rats

Abstract

Increasingly, preclinical studies on the expression, mechanisms, and treatment of pain have been aimed at improving understanding of pain-related interference with behavior. Positively reinforced operant behaviors are sensitive to depression by physiologically relevant pain stimuli. Most studies using operant conditioning procedures to examine pain-related depression of behavior have used fixed-ratio schedules of reinforcement. The primary dependent variable in these studies is the rate of behavior. In contrast, the primary dependent variable in studies using progressive-ratio schedules of reinforcement is breakpoint. Breakpoint is defined as the total number of reinforcers earned, and is thought to be related to the subject's motivation to obtain the reinforcer. This study examined effects of pain and analgesic manipulations on behavior maintained under fixed-ratio and progressive-ratio schedules of behavior. Intraperitoneal injection of dilute lactic acid was more potent at depressing behavior under the fixed-ratio schedule compared to the progressive-ratio schedule. Ketoprofen was equipotent at blocking pain-related depression of behavior maintained under both schedules. These findings support the validity of operant procedures astools to examine candidateanalgesics for the treatment of pain-related depression of behavior. Moreover, the use of diverse schedules of reinforcement may yield important scientific information on the mechanisms underlying pain-related interference with behavior.