title: Importance of sex and relative efficacy at the mu opioid receptor in the development of tolerance and cross-tolerance to the antinociceptive effects of opioids.
abstract: BACKGROUND
  Recent studies indicate that mu opioids are generally more potent and effective as antinociceptive agents in male than female rodents.
  
  
  OBJECTIVE
  To evaluate the influence of sex on the development of tolerance to the antinociceptive effects of morphine and cross-tolerance to the lower efficacy mu opioids buprenorphine and dezocine in F344 and Lewis rats.
  
  
  METHODS
  Using a warm-water tail-withdrawal procedure, the antinociceptive effects of morphine, buprenorphine and dezocine were determined before and during chronic morphine (5, 10 and 20 mg/kg, b.i.d., for 7 and 14 days) administration.
  
  
  RESULTS
  Under acute conditions, morphine was more potent in males and during chronic morphine administration tolerance development was generally greater in males. As males were more sensitive to the acute effects of morphine, the functional chronic morphine dose (i.e., chronic morphine dose/acute morphine ED50) administered to males was larger than in females. Analyses of the relationship between the functional chronic morphine dose and tolerance indicated that morphine tolerance development was comparable in males and females. Under acute conditions, buprenorphine and dezocine were more potent and effective in males. During chronic morphine administration, cross-tolerance was conferred to these opioids as evidenced by rightward, and in some cases downward, shifts in their dose-effect curves. Decreases in the maximal effects produced by buprenorphine and dezocine were more frequently observed in females.
  
  
  CONCLUSIONS
  That comparable levels of morphine tolerance were obtained in males and females when the functional chronic morphine dose was taken into consideration suggests that the mechanism underlying tolerance is not sex-dependent. Sex differences in the effectiveness of buprenorphine and dezocine when administered acutely and during chronic morphine administration further suggest that these opioids have lower efficacy at the mu opioid receptor in females.
authors.affiliation: Department of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3270, USA. barrett3@email.unc.edu
authors.forename: A C
authors.initials: AC
authors.lastname: Barrett
authors.forename: C D
authors.initials: CD
authors.lastname: Cook
authors.forename: J M
authors.initials: JM
authors.lastname: Terner
authors.forename: R M
authors.initials: RM
authors.lastname: Craft
authors.forename: M J
authors.initials: MJ
authors.lastname: Picker
mesh_terms: D000700:Analgesics / Q000494:pharmacology*; D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance / Q000502:physiology*; D005260:Female; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D010147:Pain Measurement / Q000187:drug effects; D051381:Rats; D011916:Rats, Inbred F344; D011917:Rats, Inbred Lew; D017450:Receptors, Opioid, mu / Q000502:physiology*; D012727:Sex Characteristics; D012737:Sex Factors
chemical_list: D000700:Analgesics; D000701:Analgesics, Opioid; D009294:Narcotics; D017450:Receptors, Opioid, mu; D009020:Morphine