Primary, and secondary hyperalgesia in a rat model for human postoperativepain

Background Previously, the authors developed and characterized a rat model for postoperative pain to learn more about pain produced by incisions. In t his study, the responses to heat and mechanical stimuli were evaluated dire ctly on or adjacent to the incision and at varying distances from the incis ion. Methods: Rats were anesthetized with halothane and incisions were made at d ifferent locations in the plantar aspect of the foot. The response frequenc y to a blunt mechanical stimulus, the withdrawal threshold to von Frey fila ments (15-522 mN), and the withdrawal latency to radiant heat were measured . Rats were tested before surgery, 2 h later, and then daily through postop erative day 9, Results: After plantar incision, persistent hyperalgesia was observed Immed iately adjacent to or directly on the incision to punctate and blunt mechan ical stimuli, respectively. The withdrawal threshold to punctate stimuli ap plied 1 cm from the incision was decreased through postoperative day 1, In a transitional area, between the distant and adjacent sites, the withdrawal al threshold was intermediate and the duration of hyper algesia was transi ent. Heat hyperalgesia was persistent but present when the stimulus was app lied to the site of injury but not to a distant site. Conclusion Robust primary hyperalgesia to punctate and blunt mechanical sti muli was present. Hyperalgesia distant to the wound or secondary hyperalges ia, occurred in response to punctate mechanical stimuli, was short-lived, a nd required greater forces. These results suggest that the most persistent pain behaviors in this model are largely primary hyperalgesia.