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Updated: Aug 29, 2026
Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
The development of morphine antinociceptive tolerance in nitric oxide synthase-deficient mice
Erin L Heinzen1, Gary M Pollack
1Division of Drug Delivery and Disposition, School of Pharmacy, University of North Carolina, Kerr Hall CB 7360, Chapel Hill, NC 27599-7360, USA.
Unlabelled:
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.
Insights
Neuronal nitric oxide synthase (nNOS) plays a key role in morphine tolerance. Nitric oxide (NO) mediates this tolerance, though a secondary pathway may also contribute.
Area of Science:
- Pharmacology
- Neuroscience
- Pain Research
Background:
- Elevated nitric oxide (NO) levels are linked to morphine antinociceptive tolerance.
- Understanding the specific roles of NO synthase (NOS) isoforms is crucial for elucidating tolerance mechanisms.
Purpose of the Study:
- To investigate the involvement of endothelial NOS (eNOS) and neuronal NOS (nNOS) isoforms in the development of morphine tolerance.
- To determine the contribution of NO to morphine tolerance using genetically modified mice.
Main Methods:
- Utilized three groups of mice: eNOS-deficient, nNOS-deficient, and NOS-competent controls.
- Assessed analgesic response to morphine before and after tolerance induction via chronic morphine or L-arginine administration.
- Measured tolerance by the reduction in antinociceptive effect of a challenge morphine dose.
Main Results:
- nNOS-deficient mice showed reduced morphine tolerance compared to controls.
- eNOS-deficient mice exhibited tolerance levels similar to controls.
- L-arginine administration induced significant morphine tolerance in NOS-competent and eNOS-deficient mice, but not in nNOS-deficient mice.
Conclusions:
- The neuronal NOS isoform is pivotal in mediating morphine antinociceptive tolerance.
- Morphine tolerance is primarily NO-dependent but may involve secondary non-NO pathways.
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