Effect of analgesic standards on persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR)

Various common surgeries such as thoracotomy and inguinal hernia repair involve essential prolonged tissue retraction, often causing persistent postoperative pain. A new model was developed to mimic this clinical scenario, whereby skin/muscle incision and retraction (SMIR) in the medial thigh evoked persistent postoperative pain (Flatters (2008) [Pain 135:119-130]). This study examines the response of SMIR-evoked mechanical hypersensitivity to analgesic standards commonly used as positive controls in behavioural pain studies. Rats were anaesthetised, the skin and superficial muscle of the medial thigh was then incised and retracted for 1 h. In separate experiments, morphine, gabapentin and MK-801 were intraperitoneally administered to SMIR-operated rats, at maximally tolerated doses, on postoperative day 9–13. Mechanical hypersensitivity was measured by withdrawal responses to von Frey stimulation of the plantar hindpaws. Morphine (6 mg/kg) and gabapentin (100 mg/kg) elicited an almost complete reversal of SMIR-evoked mechanical hypersensitivity. In contrast, MK-801 (0.1 mg/kg) did not affect SMIR-evoked mechanical hypersensitivity. Contralateral hindpaw responses to von Frey stimulation were unaffected by SMIR surgery or any drug treatment. In conclusion, the SMIR model displays persistent mechanical hypersensitivity that is reversible by morphine or gabapentin treatment. As previously demonstrated, SMIR-evoked pain is not driven by neuronal damage and these data show that NMDA receptor activation does not play a role in the maintenance of SMIR-evoked pain. This study further demonstrates the value of the SMIR model as a tool to understand persistent postoperative/postsurgical pain mechanisms and evaluate potential treatments.