Primary and secondary hyperalgesia in a rat model for human postoperative pain

Previously, the authors developed and characterized a rat model for postoperative pain to learn more about pain produced by incisions. In this study, the responses to heat and mechanical stimuli were evaluated directly on or adjacent to the incision and at varying distances from the incision.

Rats were anesthetized with halothane and incisions were made at different locations in the plantar aspect of the foot. The response frequency to a blunt mechanical stimulus, the withdrawal threshold to von Frey filaments (15-522 mN), and the withdrawal latency to radiant heat were measured. Rats were tested before surgery, 2 h later, and then daily through postoperative day 9.

After plantar incision, persistent hyperalgesia was observed immediately adjacent to or directly on the incision to punctate and blunt mechanical stimuli, respectively. The withdrawal threshold to punctate stimuli applied 1 cm from the incision was decreased through postoperative day 1. In a transitional area, between the distant and adjacent sites, the withdrawal threshold was intermediate and the duration of hyperalgesia was transient. Heat hyperalgesia was persistent but present when the stimulus was applied to the site of injury but not to a distant site.

Robust primary hyperalgesia to punctate and blunt mechanical stimuli was present. Hyperalgesia distant to the wound, or secondary hyperalgesia, occurred in response to punctate mechanical stimuli, was short-lived, and required greater forces. These results suggest that the most persistent pain behaviors in this model are largely primary hyperalgesia.