Reduction of postincisional allodynia by subcutaneous bupivacaine: findings with a new model in the hairy skin of the rat.
BACKGROUND
An incision of hairy skin of the rat's back provides a new model for postincisional pain to determine the importance of cutaneous anesthesia.


METHODS
Male Sprague-Dawley rats were anesthetized with sevoflurane and given a 0.6-ml subcutaneous injection of bupivacaine (0.25%) under the incision site or the medial lumbar dorsum or at the nuchal midline, 30 min before a 1.0-cm skin incision. Mechanical stimuli (von Frey hairs, 18-250 mN) were applied to measure nociception, indicated by twitching of local subcutaneous muscles, the cutaneus trunci muscle reflex. A graded response score, averaging the twitches weighted by their vigor, or a population response score, measuring the fraction of rats that showed any response, was assessed for 3 days before and over 7 days after incision. von Frey hairs were applied 0.5 cm from the incision to test primary hyperalgesia and 2.0 cm contralateral to the incision for secondary hyperalgesia.


RESULTS
Incision induced responses to stimuli that had no effect on intact skin (allodynia) and also enhanced responses to forces that normally gave less than the full reflex (hyperalgesia). Hyperalgesia was present 30 min after surgery, peaked at 3-6 h, and persisted through the week; allodynia had a similar onset but was briefer. Both changes were transiently reversed by subcutaneous morphine (2.5 mg/kg intraperitoneal). Subcutaneous bupivacaine (0.25%), injected preoperatively at the incision site and anesthetizing skin for 2-3 h, suppressed primary allodynia for 1 week but had no effect on hyperalgesia. Secondary allodynia was obliterated, and secondary hyperalgesia attenuated by this treatment. Bupivacaine injected subcutaneously at the nuchal midline before surgery was also effective in abbreviating primary and secondary allodynia, with no signs of sedation, ataxia, or preconvulsive behavior.


CONCLUSIONS
Incision of rat hairy skin changes pain responses, similar to pain in humans. Preincisional subcutaneous bupivacaine selectively suppresses and shortens allodynia for times far outlasting its local anesthesia, an effect largely from systemic actions.
Pain Research Center, Department of Anesthesiology, Perioperative and Pain Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
Adriana M
AM
Duarte
Eva
E
Pospisilova
Erin
E
Reilly
Florence
F
Mujenda
Yoshihiro
Y
Hamaya
Gary R
GR
Strichartz
D000818:Animals; D002045:Bupivacaine / Q000008:administration & dosage*; D018859:Hair Follicle / Q000294:innervation*; D006930:Hyperalgesia / Q000188:drug therapy* / Q000503:physiopathology; D007279:Injections, Subcutaneous; D008297:Male; D010147:Pain Measurement / Q000187:drug effects* / Q000379:methods; D010149:Postoperative Pain / Q000188:drug therapy / Q000503:physiopathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012867:Skin / Q000187:drug effects* / Q000294:innervation; D013997:Time Factors
D002045:Bupivacaine