Lisa Loram

A model of incisional pain: the effects of dermal tail incision on pain behaviours of Sprague Dawley rats

Hyperalgesia, a component of post-operative pain, is an enhanced responsiveness to painful challenges after the tissue damage caused by an incision. It is important to understand the mechanisms involved in the development of incisional pain, in order to treat the condition appropriately. The aim of this study was to develop a model of post-operative pain using the rats tail. Under halothane-induced anaesthesia, female Sprague Dawley rats underwent 10mm longitudinal incisions through skin and fascia (n = 10) or 20 mm incisions through skin, fascia and muscle (n = 10) of the mid-portion of the tail. Control rats were only anaesthetised (n = 14). Withdrawal latencies to noxious mechanical and thermal challenges were recorded daily. A bar algometer was placed onto and 15 mm proximal to the incision with a force of 4N and the tail was immersed in 49 degrees C water. Daily withdrawal latencies were compared to pre-incision values using one way analysis of variance (ANOVA) with Dunnetts post-hoc test. Primary mechanical hyperalgesia lasted for 6 days after the 10 mm incisions (P < 0.0001) and for 7 days after the 20 mm incisions (P < 0.0001). Secondary mechanical hyperalgesia persisted for 1 day after the 10 mm incisions (P = 0.0013) and for 2 days after the 20 mm incisions (P = 0.0028). Thermal hyperalgesia was not elicited. This model is suitable to examine the mechanisms involved in post-operative pain.

Postoperative Administration of the Analgesic Tramadol, but Not the Selective Cyclooxygenase-2 Inhibitor Parecoxib, Abolishes Postoperative Hyperalgesia in a New Model of Postoperative Pain in Rats

Background/Aims: Using a new animal model of postoperative pain we recently developed, we investigated whether the selective cyclooxygenase-2 (COX-2) inhibitor pare-coxib sodium, and the analgesic tramadol hydrochloride, attenuated mechanical primary hyperalgesia induced by minor surgery on the rat tail. Methods: For surgery, rats were anesthetized with isoflurane, a 20-mm-long incision was made through the skin and fascia of their tails, and the wound was sutured. Immediately after surgery, rats were injected intraperitoneally with parecoxib sodium (10 or 20 mg·kg–1), tramadol hydrochloride (10 mg·kg–1), or sterile saline (0.1 ml·kg–1). Hyperalgesia was assessed by measuring rats’ response latencies to a blunt noxious mechanical stimulus (4 N) applied to their tails. Nociceptive testing was performed before surgery and 90 min after surgery. Results: Hyperalgesia was present in all saline-injected animals within 90 min of surgery. This hyperalgesia was not attenuated by postoperative injection of parecoxib. However, administration of tramadol completely prevented the development of postoperative hyperalgesia. Conclusion: We have shown that the hyperalgesia in our model of postoperative pain is responsive to treatment with the analgesic tramadol, but it is not responsive to the selective COX-2 inhibitor parecoxib at the doses we used.

Gender and site of injection do not influence intensity of hypertonic saline-induced muscle pain in healthy volunteers

The aim of the study was to determine whether the same stimulus to different muscles results in comparable pain and whether gender has any influence on the pain. We compared the quality and intensity of muscle pain induced by a hypertonic saline injection into the tibialis anterior (leg) muscle to that after an injection into the lumbar erector spinae (back) muscle in both male (n=10) and female (n=10) volunteers. Hypertonic or isotonic saline was injected into the leg and back muscles and pain intensity (visual analogue scale, VAS) and pain quality (McGill Pain Questionnaire) were measured. Pressure pain tolerance around the site of injection and on the contralateral side was measured. Hypertonic saline injection induced significant muscle pain in the back and leg compared to isotonic saline (P<0.05, ANOVA). The site of injection did not influence the quality of pain but there was a gender bias in the descriptive words chosen (chi(2) test, P<0.05) and female subjects were more sensitive to pressure than male subjects. Experimentally induced muscle pain is equivalent in intensity and quality in the leg and back muscle. Gender does not influence muscle pain intensity but does influence sensitivity to pressure and the description of pain.