Abstract
PURPOSE: Remifentanil is one of the most commonly used opioids intraoperatively. Previous reports indicate that long-term use of opioids may lead to cross-tolerance to remifentanil, which poses a challenge in the control of acute pain intraoperatively. However, there is limited information regarding cross-tolerance to remifentanil, especially in visceral pain. Therefore, this study aimed to examine cross-tolerance to remifentanil in somatic and visceral tolerance using morphine-tolerant rats. METHODS: Six male Sprague-Dawley rats were allocated to the morphine and saline groups each. Tolerance to the antinociceptive effect of morphine was induced in rats in the morphine group. Remifentanil was continuously infused intravenously at 10 mcg/kg/min for 120 min to assess cross-tolerance from morphine to remifentanil. The antinociceptive effects on somatic and visceral nociceptive stimuli were measured using the tail-flick (TF) and colorectal distension (CD) tests, respectively. The antinociceptive efficacy was evaluated by converting the response threshold to the percentage maximal possible effect (%MPE). RESULTS: Remifentanil increased the %MPE in the morphine and saline groups in both the tests; however, the increase in %MPE was attenuated significantly in the morphine group compared with that in the saline group at 60, 90, and 120 min (all P < 0.01) in the TF test and at 90 and 120 min in the CD test (all P <0.05). CONCLUSION: Our results indicate that morphine-tolerant rats exhibit cross-tolerance to remifentanil's acute antinociceptive effects on somatic and visceral stimuli. Cross-tolerance to remifentanil should be considered in the perioperative management of patients using morphine.