title: The development of morphine antinociceptive tolerance in nitric oxide synthase-deficient mice.
abstract: Elevations in nitric oxide (NO) have been implicated in the development of morphine antinociceptive tolerance. This study was conducted to establish the role of specific isoforms of NO synthase (NOS) in morphine tolerance development using genetically modified mice.
  
  
  METHODS
  Three groups of mice (endothelial NOS [eNOS]-deficient, neuronal NOS [nNOS]-deficient, and NOS-competent) were used in this experiment. On Day 1, the analgesic response (radiant heat tail-flick) to a challenge dose of morphine (4 mg/kg) was determined over 3 hr. Tolerance was induced on Days 1-5 by administering morphine subcutaneously (10 mg/kg) or L-arginine, a NO precursor, intraperitoneally (200 mg/kg), twice daily. Analgesic response to the challenge dose was determined again on Day 6.
  
  
  RESULTS
  Following sustained morphine administration, nNOS-deficient mice exhibited less tolerance development when compared to the control group, although measurable tolerance still occurred. Mice deficient in eNOS evidenced a degree of tolerance similar to that of control. Prolonged L-arginine administration produced significant functional tolerance to morphine in NOS-competent and eNOS-deficient mice. The loss of morphine responsivity after L-arginine administration was similar to that after morphine pretreatment. L-Arginine did not affect the antinociceptive response to morphine in mice deficient in nNOS, suggesting that the small degree of morphine-induced tolerance in this group occurs through an alternate pathway.
  
  
  CONCLUSIONS
  These data demonstrate the pivotal role of the neuronal isoform of NOS in development of morphine antinociceptive tolerance. Furthermore, tolerance development appears to be predominantly a NO-mediated process, but likely is augmented by a secondary (non-NO) pathway.
authors.affiliation: Division of Drug Delivery and Disposition, School of Pharmacy, University of North Carolina, Kerr Hall CB 7360, Chapel Hill, NC 27599-7360, USA.
authors.forename: Erin L
authors.initials: EL
authors.lastname: Heinzen
authors.forename: Gary M
authors.initials: GM
authors.lastname: Pollack
mesh_terms: D000700:Analgesics / Q000009:adverse effects*; D000818:Animals; D004195:Disease Models, Animal; D004361:Drug Tolerance / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009020:Morphine / Q000009:adverse effects*; D009021:Morphine Dependence / Q000201:enzymology; D019001:Nitric Oxide Synthase / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*
chemical_list: D000700:Analgesics; D009020:Morphine; D019001:Nitric Oxide Synthase