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Updated: Aug 5, 2026
Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Morphine analgesia in mice of different ages
Abstract:
The analgesic response to 5 mg per kg morphine sulfate was examined in male C57BL/6J mice aged from 2 months to 28 months by the tail-flick assay. The baseline latency of old mice was slightly lower than that of young mice. There was a smaller increase in latency after morphine in old mice than in young mice. Decreased analgesic response in old mice was accompanied by a slower rate of uptake of labeled morphine into the blood after i.p. injection and a longer half-life of the drug. There was no difference between young and old mice in the number of narcotic receptors in the brain.
Insights
Older mice show a reduced pain relief response to morphine sulfate due to slower drug uptake and longer half-life, despite similar brain narcotic receptor levels.
Area of Science:
- Pharmacology
- Gerontology
- Neuroscience
Background:
- Aging can alter drug responses and pharmacokinetics.
- Morphine is a widely used analgesic, but its efficacy may vary with age.
Purpose of the Study:
- To investigate the age-related differences in the analgesic effects of morphine sulfate in mice.
- To explore the pharmacokinetic factors contributing to any observed age-related changes in morphine efficacy.
Main Methods:
- Male C57BL/6J mice aged 2 to 28 months were used.
- The tail-flick assay was employed to measure the analgesic response to morphine sulfate (5 mg/kg).
- Blood samples were analyzed to determine morphine uptake and half-life after intraperitoneal injection.
Main Results:
- Older mice exhibited a diminished analgesic response to morphine compared to young mice.
- A slower rate of morphine uptake into the blood and a longer drug half-life were observed in older mice.
- No significant differences in the number of brain narcotic receptors were found between young and old mice.
Conclusions:
- Aging in mice is associated with a decreased analgesic efficacy of morphine sulfate.
- Altered morphine pharmacokinetics, specifically slower absorption and prolonged elimination, contribute to the reduced pain relief in older mice.
- The number of brain narcotic receptors does not appear to be a limiting factor in the age-related decline of morphine analgesia.
