Tail-flick test. I: Impact of a suprathreshold exposure to radiant heat on pain reactivity in rats.

Radiant heat applied to the tail elicits a vigorous tail-flick response in rats. This tail-flick reflex is frequently used to measure pain reactivity. Prior research has shown that a suprathreshold exposure to the radiant heat used to elicit this reflex causes a reduction in tail-flick latencies in pentobarbital anesthetized rats. Experiment 1 replicated this observation and showed that the effect is not due to a change in tail temperature. Experiment 2 showed that reduced latencies (hyperalgesia) are not observed if the spinal cord is transected prior to testing. Experiment 3 revealed that pentobarbital anesthesia is required to observe thermal-induced hyperalgesia, for this effect is not observed in awake subjects. Experiments 4 and 5 extended this observation by showing that awake rats also fail to exhibit hyperalgesia if others measures of pain reactivity are employed (shock-induced vocalization and motor reactivity). Implications of the results are discussed.