Chapter 2 Quantitative experimental assessment of pain and hyperalgesia in animals and underlying neural mechanisms

Abstract

The primary aim of this chapter is to critically examine the use of acute noxious stimuli to assess pain in normal animals and in animal models of persistent pain. The new strategies are suggested to examine nomifensine behavior evoked by controlled stimuli over a wide range of stimulus intensities in assessing underlying neural mechanisms and treatments. As pain is normally a conscious perception, a more realistic animal model might incorporate a cognitive-evaluative component associated with nocifensive behavior. For this reason, this chapter develops the operant response model that requires the rat to perceive and respond to a noxious stimulus with a bar-press. It is interesting that rats respond over a higher range of stimulus intensities in this paradigm compared to the measure of tail flick reflex force. One might speculate that the operant response is a measure of the motivation of the animal working to avoid the noxious stimuli or of the probability that the animal will or will not tolerate it. On the other hand, in human studies using identical noxious radiant heat stimuli, the threshold for reflexive limb withdrawal was higher compared to that for pain sensation. However, the fact that subjects knew the stimuli would not cause tissue damage may have influenced their withdrawal threshold. These observations further underscore the need for caution in extrapolating from measures.