Opioid-mediated pain sensitivity in mice bred for high voluntary wheel running

We tested the hypothesis that thermal tail-flick latency, a common measure of pain sensitivity in rodents, would be altered in lines of mice that had been selectively bred for high voluntary wheel-running behavior. Specifically, we predicted that the selected (High-Runner) lines would show decreased pain sensitivity relative to their control (C; randombred) lines, and would respond differently to drugs that block opioid receptors. We first compared tail-flick latency between High-Runner and C female mice during the day (no wheel access) and at night (with wheel access). Second, we compared effects of the opioid antagonist naloxone (10 mg/kg, i.p.) on tail-flick latency during the day (no wheel access). Third, we compared effects of naloxone (5 and 10 mg/kg, i.p.) and naltrexone, a longer-lasting opioid antagonist (0.1, 1, 5, 10, 50, and 100 mg/kg, i.p.), on voluntary wheel running. Tail-flick latencies were longer at night (when mice were active on wheels), but mice from High-Runner and C lines did not differ during the day or night. Administration of naloxone (10 mg/kg, i.p.) decreased tail-flick latency measured during the day, equally in High-Runner and C mice. Naloxone (5 and 10 mg/kg, i.p.) and high doses of naltrexone (50 and 100 mg/kg, i.p.) decreased wheel running equally in High-Runner and C mice. Further studies will be required to determine whether other types of pain sensitivity have also failed to evolve in association with increased voluntary wheel running. (C) 2004 Elsevier Inc. All rights reserved.