Role of the rostral medial medulla in the development of primary and secondary hyperalgesia after incision in the rat.

BACKGROUND Descending influences from the rostral medial medulla (RMM) contribute to secondary hyperalgesia in persistent inflammatory, neuropathic, and visceral pain models. The current study examined if descending inhibition or facilitation from the RMM modulates primary and secondary hyperalgesia after incision in the rat hind limb. METHODS Bilateral RMM lesions were produced using the soma-selective neurotoxin ibotenic acid, and the effect of RMM lesion was examined on primary and secondary hyperalgesia 5 days after a plantar or gastrocnemius incision, respectively. RESULTS Plantar incision reduced withdrawal thresholds to von Frey filaments applied adjacent to the incision (primary punctate hyperalgesia). The withdrawal thresholds were the same in RMM-lesioned and sham-operated rats. The response frequency to a blunt mechanical stimulus after plantar incision was increased (primary nonpunctate hyperalgesia) in both groups. Nonpunctate hyperalgesia was greater in lesioned rats on postoperative day 2 only; all other measures were not different. Primary heat hyperalgesia after plantar incision was not modulated by RMM lesion. Secondary punctate hyperalgesia after gastrocnemius incision was not affected by RMM lesion. Gastrocnemius incision did not produce secondary nonpunctate or heat hyperalgesia in either RMM lesion or sham rats. CONCLUSION Primary and secondary hyperalgesia after an incision were not modulated by descending influence from the RMM. The lack of contribution of descending facilitatory influences from the RMM to secondary hyperalgesia after gastrocnemius incision supports the notion that incision-induced pain involves dissimilar mechanisms compared with inflammatory and neuropathic pain.