Tissue injury models of persistent nociception in rats

The purpose of this chapter is to provide guidance to the novice investigator as to two models of ongoing nociception in rats. The models described herein are the formalin test, in which an irritant is injected subcutaneously into a dorsal paw and the numbers of flinches produced over 60 min are counted, and a mild burn model that produces a transitory primary and secondary thermal and mechanical hyperalgesia lasting approx 90 min. These models allow assessment of spinal sensitization, which may be an important factor when considering plasticity associated with human pain states. Detailed protocols using both manual and automated counting for the formalin test are included, as are methods concerning data analysis.