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Inability of naloxone to change brain morphine levels in tolerant mice
Abstract:
The effect of naloxone on brain morphine concentrations was measured in naive and morphine-dependent mice using radioimmunoassay and gas-liquid chromatography. No displacement of morphine from the brain by naloxone could be observed in naive mice acutely injected with morphine or in pellet-implanted mice at increasing intervals after removal of the morphine pellets. The suggestion of a change in affinity of opiate receptors during the development of tolerance/dependence, which had been made on the basis of the displacement of morphine by naloxone found by other workers, is thus not supported by the present results.
Insights
Naloxone did not displace morphine in the brains of mice, challenging previous findings. This suggests opiate receptor affinity does not change with morphine tolerance or dependence.
Area of Science:
- Pharmacology
- Neuroscience
- Toxicology
Background:
- Morphine dependence and tolerance are significant clinical challenges.
- Previous studies suggested naloxone could displace morphine from brain receptors, implying altered receptor affinity.
Purpose of the Study:
- To investigate the effect of naloxone on brain morphine concentrations in naive and morphine-dependent mice.
- To re-evaluate the hypothesis of altered opiate receptor affinity during morphine tolerance and dependence.
Main Methods:
- Radioimmunoassay and gas-liquid chromatography were used to quantify brain morphine levels.
- Experiments were conducted on naive mice and mice with chronic morphine exposure (pellet implantation).
Main Results:
- Naloxone did not displace morphine from the brain in acutely injected naive mice.
- No displacement was observed in mice at various intervals after morphine pellet removal, indicating no change in receptor affinity.
Conclusions:
- The study's findings do not support the theory that naloxone displacement indicates altered opiate receptor affinity during morphine tolerance/dependence.
- Results challenge previous interpretations of naloxone's interaction with brain morphine in dependent states.