A novel model of primary and secondary hyperalgesia after mild thermal injury in the rat.

Secondary hyperesthesia was investigated in a rat thermal injury model. After a mild focal thermal injury (52 degrees C/45 s) to the rat heel, the response latency for a thermal stimulus directed at the injured site was reduced (10 --> 6 s; e.g. primary thermal hyperalgesia) but no change was seen at the distal site. Conversely, tactile threshold at the distal site was significantly reduced (15 --> 5 g; e.g. secondary tactile allodynia) but much less so at the injured site. Magnitude of the secondary tactile allodynia paralleled the severity of the primary injury. Accordingly, this model has the same characteristics seen in human post-tissue-injury hyperesthetic states and provides a tool for the study of mechanisms underlying primary and secondary hyperesthesia.