title: Endocannabinoid and Opioid System Interactions in Exercise-Induced Hypoalgesia.
abstract: The purpose of this study was to examine the interaction between the endogenous opioid and endocannabinoid (eCB) systems in a pain modulatory process known as exercise-induced hypoalgesia (EIH).
  
  
  
  Randomized controlled trial.
  
  
  
  Clinical research unit in a hospital.
  
  
  
  Fifty-eight healthy men and women (mean age = 21 ± 3 years) participated in this study.
  
  
  
  Participants were administered (randomized, double-blind, counterbalanced procedure) an opioid antagonist (i.e., naltrexone) and a placebo prior to performing pain testing and isometric exercise.
  
  
  
  Results indicated that 2-arachidonoylglycerol (2-AG) and 2-oleoylglycerol (2-OG) increased significantly (P < 0.05) following exercise in both placebo and naltrexone conditions. In comparison, N-arachidonylethanolamine (AEA) and oleoylethanolamine (OEA) increased significantly (P < 0.05) following exercise in the placebo condition but not the naltrexone condition. There were no significant (P > 0.05) differences in palmitolethanolamine (PEA) between the placebo and naltrexone conditions.
  
  
  
  As reductions in pain (i.e., EIH) were observed following both conditions, these results suggest that the opioid system may not be the primary system involved in exercise-induced hypoalgesia and that 2-AG and 2-OG could contribute to nonopioid exercise-induced hypoalgesia. Moreover, as exercise-induced increases in AEA and OEA were blocked by naltrexone pretreatment, this suggests that the opioid system may be involved in the increase of AEA and OEA following exercise.
authors.affiliation: Department of Kinesiology, University of Wisconsin-Madison, Madison, Wisconsin.
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Crombie
authors.affiliation: Department of Kinesiology, University of Wisconsin-Madison, Madison, Wisconsin.
authors.forename: Angelique G
authors.initials: AG
authors.lastname: Brellenthin
authors.affiliation: Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
authors.forename: Cecilia J
authors.initials: CJ
authors.lastname: Hillard
authors.affiliation: Department of Kinesiology, University of Wisconsin-Madison, Madison, Wisconsin.
authors.forename: Kelli F
authors.initials: KF
authors.lastname: Koltyn
mesh_terms: D004311:Double-Blind Method; D063388:Endocannabinoids / Q000097:blood*; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009271:Naltrexone / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D058748:Pain Perception / Q000187:drug effects / Q000502:physiology*; D017288:Pain Threshold / Q000187:drug effects; D055815:Young Adult
chemical_list: D063388:Endocannabinoids; D009292:Narcotic Antagonists; D009271:Naltrexone
keywords: 2-AG; AEA; EIH Mechanisms; Isometric Exercise; Pain Modulation