title: Opioid-mediated pain sensitivity in mice bred for high voluntary wheel running.
abstract: 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.
authors.affiliation: Department of Zoology, University of Wisconsin-Madison, Madison, WI 53706, USA.
authors.forename: Guo
authors.initials: G
authors.lastname: Li
authors.forename: Justin S
authors.initials: JS
authors.lastname: Rhodes
authors.forename: Isabelle
authors.initials: I
authors.lastname: Girard
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Gammie
authors.forename: Theodore
authors.initials: T
authors.lastname: Garland
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D000831:Animals, Newborn; D001522:Behavior, Animal / Q000187:drug effects; D001947:Breeding / Q000379:methods; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005260:Female; D008297:Male; D051379:Mice; D009043:Motor Activity / Q000187:drug effects*; D009270:Naloxone / Q000494:pharmacology; D009271:Naltrexone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D010147:Pain Measurement / Q000379:methods; D017288:Pain Threshold / Q000187:drug effects* / Q000502:physiology; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D012420:Running / Q000502:physiology*; D012641:Selection, Genetic; D013045:Species Specificity; D013997:Time Factors
chemical_list: D009292:Narcotic Antagonists; D009294:Narcotics; D009270:Naloxone; D009271:Naltrexone