title: Endogenous opioid inhibition of chronic low-back pain influences degree of back pain relief after morphine administration.
abstract: OBJECTIVE
  Factors underlying differential responsiveness to opioid analgesic medications used in chronic pain management are poorly understood. We tested whether individual differences in endogenous opioid inhibition of chronic low-back pain were associated with the magnitude of acute reductions in back pain ratings after morphine administration.
  
  
  METHODS
  In randomized counterbalanced order over three sessions, 50 chronic low-back pain patients received intravenous naloxone (8 mg), morphine (0.08 mg/kg), or placebo. Back pain intensity was rated predrug and again after peak drug activity was achieved using the McGill Pain Questionnaire-Short Form (Sensory and Affective subscales, VAS Intensity measure). Opioid blockade effect measures to index degree of endogenous opioid inhibition of back pain intensity were derived as the difference between predrug to postdrug changes in pain intensity across placebo and naloxone conditions, with similar morphine responsiveness measures derived across placebo and morphine conditions.
  
  
  RESULTS
  Morphine significantly reduced back pain compared with placebo (McGill Pain Questionnaire-Short Form Sensory, VAS; P < 0.01). There were no overall effects of opioid blockade on back pain intensity. However, individual differences in opioid blockade effects were significantly associated with the degree of acute morphine-related reductions in back pain on all measures, even after controlling for effects of age, sex, and chronic pain duration (P < 0.03). Individuals exhibiting greater endogenous opioid inhibition of chronic back pain intensity reported less acute relief of back pain with morphine.
  
  
  CONCLUSIONS
  Morphine appears to provide better acute relief of chronic back pain in individuals with lower natural opioidergic inhibition of chronic pain intensity. Possible implications for personalized medicine are discussed.
authors.affiliation: From the *Department of Anesthesiology, Vanderbilt University School of Medicine, Nashville, TN; †Department of Behavioral Science, Rush University, Chicago, IL; ‡Department of Anesthesiology, Rush University, Chicago, IL; and §Department of Psychology, Ohio University, Athens, OH.
authors.forename: Stephen
authors.initials: S
authors.lastname: Bruehl
authors.forename: John W
authors.initials: JW
authors.lastname: Burns
authors.forename: Rajnish
authors.initials: R
authors.lastname: Gupta
authors.forename: Asokumar
authors.initials: A
authors.lastname: Buvanendran
authors.forename: Melissa
authors.initials: M
authors.lastname: Chont
authors.forename: Erik
authors.initials: E
authors.lastname: Schuster
authors.forename: Christopher R
authors.initials: CR
authors.lastname: France
mesh_terms: D000328:Adult; D018592:Cross-Over Studies; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D017116:Low Back Pain / Q000175:diagnosis* / Q000188:drug therapy*; D008297:Male; D008875:Middle Aged; D009020:Morphine / Q000494:pharmacology / Q000627:therapeutic use*; D018847:Opioid Peptides / Q000494:pharmacology / Q000627:therapeutic use*; D010147:Pain Measurement / Q000187:drug effects* / Q000379:methods
chemical_list: D018847:Opioid Peptides; D009020:Morphine