Characterization of hind paw licking and lifting to noxious radiant heat in the rat with and without chronic inflammation

The paw withdrawal latency to thermal radiant heat stimuli is a widely used nociceptive measure to study hyperalgesic mechanisms. In the present study, in addition to the paw withdrawal latency, two behavioral components of pain behaviors, paw licking and paw lifting have been characterized and quantified. The thermal stimuli were successively applied to the plantar surface of the rat hind paws and recorded the behavioral responses to each of the stimuli. Noxious radiant heat significantly increased the frequency of paw lifting behavior in naïve rats. On the other hand, the thermal stimuli significantly evoked the occurrence of paw licking behavior in inflamed paws and it was maintained at all time points of measurements. The paw withdrawal latency decreased in inflamed rats in comparison with control rats. These data informs that noxious radiant heat specifically evokes the frequency of paw lifting behavior in normal physiological condition, and paw licking behavior in a pathological inflammatory condition. These findings suggest that in addition to the measurement of PWL, scoring of paw licking and lifting behaviors will improve the sensitivity of this pain test.