Evaluation of anxiety-like behaviour in a rat model of acute postoperative pain

BACKGROUND Unrelieved acute postoperative pain can lead to a wide range of adverse effects, such as anxiety, depression, restlessness and sleep deprivation. OBJECTIVE To investigate anxiety-like behaviour in a postoperative pain model. INTERVENTIONS AND MAIN OUTCOME MEASURES Mechanical hypersensitivity was assessed with an electronic von Frey device, whereas anxiety-like behaviour was measured with light/dark testing and elevated plus maze testing. RESULTS Rats developed significant mechanical hyperalgesia on 1, 3 and 8 days postsurgery compared with sham-operated rats. There was no reduction in motility between preincision and postincision when animals were allowed to move freely in an open field locomotion test. In light-dark tests, incised animals spent significantly less time than sham rats in the light compartment on the 1st and 3rd postoperative days. However, in an elevated plus maze test, differences between sham and incised rats were only observed on the 8th postoperative day as they spent significantly more time in the open arms. Pretreatment with morphine significantly increased withdrawal thresholds compared with treatment with saline (0.9% NaCl), but had no effect on light or open arm avoidance behaviour. CONCLUSION We report that a rat model of acute postoperative pain is associated with anxiety-like increased light and open arm avoidance behaviour.