The effects of radient heat on pain reactivity

Abstract

Prior research has shown that an aversive event can produce either a decrease (hypoalgesia) or an increase in pain reactivity (hyperalgesia). The present study explores the impact of a suprathreshold exposure to radiant heat on pain reactivity. Rats served as subjects and the thermal stimulus was applied to the tail. In the first three experiments, pain reactivity was measured by applying radiant heat to the tail and recording the latency of a flick response (the "tail-flick test"). Experiment I showed that a suprathreshold exposure to radiant heat induced hyperalgesia in pentobarbital anesthetized rats, and that this effect was not due to a change in tail temperature. Experiment 2 showed that a spinal transaction eliminated the hyperalgesia. Experiment 3 revealed that pentobarbital anesthesia was required to observe thermalinduced hyperalgesia, for this effect was not observed in awake subjects. Experiment 4 extended this observation by demonstrating that a suprathreshold exposure to radiant heat had little effect on two other measures of pain reactivity, shock-induced vocalization and motor activity, in awake rats. Experiment 5 looked at whether endogenous opioids play a role in the production of these effects. Implications of the results are discussed.