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Updated: Oct 23, 2025
Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Microglial ablation does not affect opioid-induced hyperalgesia in rodents
Xin Liu1, Bo-Long Liu1,2, Qing Yang1
1Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, TX, United States.
Abstract:
Opioids are the frontline analgesics in pain management. However, chronic use of opioid analgesics causes paradoxical pain that contributes to the decrease of their efficacy in pain control and the escalation of dose in long-term management of pain. The underling pathogenic mechanism is not well understood. Microglia have been commonly believed to play a critical role in the expression of opioid-induced hyperalgesia in animal models. We performed microglial ablation experiments using either genetic (CD11b-diphtheria toxin receptor transgenic mouse) or pharmacological (colony-stimulating factor-1 receptor inhibitor PLX5622) approaches. Surprisingly, ablating microglia using these specific and effective approaches did not cause detectable impairment in the expression of hyperalgesia induced by morphine. We confirmed this conclusion with a behavioral test of mechanical and thermal hyperalgesia, in male and female mice, and with different species (mouse and rat). These findings raise caution about the widely assumed contribution of microglia to the development of opioid-induced hyperalgesia.
Insights
Challenging the role of microglia in opioid-induced hyperalgesia, this study found that removing these immune cells did not affect pain sensitivity in mice and rats. These findings question the established understanding of opioid pain mechanisms.
Area of Science:
- Neuroscience
- Immunology
- Pharmacology
Background:
- Opioid analgesics are primary pain relievers, but chronic use can paradoxically increase pain, reducing treatment efficacy.
- The exact mechanisms behind opioid-induced hyperalgesia (OIH) remain unclear.
- Microglia are widely suspected to be key players in OIH development.
Purpose of the Study:
- To investigate the role of microglia in the development of opioid-induced hyperalgesia.
- To determine if microglial ablation prevents or reduces OIH.
Main Methods:
- Microglial ablation was achieved using genetic (CD11b-diphtheria toxin receptor transgenic mice) and pharmacological (PLX5622) methods.
- Behavioral tests assessed mechanical and thermal hyperalgesia in male and female mice and rats.
- The impact of microglial depletion on morphine-induced hyperalgesia was evaluated.
Main Results:
- Effective microglial ablation did not impair the expression of morphine-induced hyperalgesia.
- These results were consistent across different methods of ablation, sexes, and species (mouse and rat).
Conclusions:
- The study challenges the widely held belief in microglia's critical role in OIH.
- Findings suggest that other mechanisms may be more significant in the development of opioid-induced hyperalgesia.
- Further research is needed to elucidate the precise pathways involved in OIH.
