Background & Aims

The use of opioids in chronic pain management is limited by adverse side effects, including negative affect and hyperalgesia. The emergence of a pronounced neuroinflammatory state following chronic opioid use has been linked to the development of increased pain sensitivity and dysphoria [1]. The immune system is intimately connected to the gut microbiome via a gut-brain signalling axis, and chronic opioid exposure shifts the composition of the gut microbiome [2,3]. Consequently, the gut microbiome represents a novel route through which opioid-induced inflammation can be generated and sustained. Given the wide expression of opioid receptors throughout the gastrointestinal tract, opioid-induced dysbiosis likely emerges as a function of alterations in gut motility. We hypothesize that the effects of opioids on gut motility shift the taxonomic profile of the gut microbiome, generating a state of dysbiosis that contributes to the development and persistence of opioid-induced hyperalgesia.

Methods

Male and female C57Bl6J mice were treated with escalating doses of morphine (10-40 mg/kg, i.p.) for 4 days. Pain behaviour, spinal tissue collection, and 16S rRNA sequencing of microbiome samples was performed 12 hours- 4 weeks after morphine treatment to assess acute and protracted withdrawal syndromes, alongside measurements of gut motility and permeability. To assess the role of the gut microbiome, the same opioid regimen was used for germ-free mice, followed by pain behaviour and tissue collection. To assess the role of gut motility, drug-naïve mice were treated with tegaserod, a peripherally-restricted serotonin agonist that increases gut motility, to determine if increasing gut motility was sufficient to replicate opioid-induced dysbiosis and withdrawal symptoms. To determine if peripheral opioid receptors were required to produce an opioid withdrawal syndrome, chronic centrally-restricted administration of the opioid peptide, DAMGO, was delivered to mice via i.c.v. cannulation.

Results

Microbiome sequencing revealed that opioid withdrawal induced significant alterations in microbial diversity that persisted for up to 1-week post-treatment and correlated with expression of hyperalgesia, neuroinflammation, and impaired gut motility and permeability. Germ-free mice displayed acute hypersensitivity to chronic morphine treatment that resolved prior to 1-week post-treatment, and demonstrated no accompanying neuroinflammatory response. Pain behaviour and inflammation could be elicited in the absence of opioids by tegaserod treatment of drug-naïve mice, while chronic centrally-restricted opioid treatment failed to generate significant pain behaviour or central inflammation.

Conclusions

Opioid withdrawal produces profound and protracted changes in gut microbiota, motility, and permeability in mice, which correlated with the development and duration of withdrawal-induced inflammation and hyperalgesia. Chronic opioid treatment and withdrawal in germ-free mice, lacking a gut microbiome, does not produce spinal inflammation nor does it result in persistent hyperalgesia. Imitating the increase in gut motility during withdrawal via pharmacological means is sufficient to elicit pain and neuroinflammation, while limiting opioid actions to the central nervous system prevents these withdrawal symptoms.

References

1.Cahill, C. M., & Taylor, A. M. (2017). Neuroinflammation-a co-occurring phenomenon linking chronic pain and opioid dependence. Current Opinion in Behavioral Sciences, 13, 171–177. https://doi.org/10.1016/j.cobeha.2016.12.003
2.Farooq, R. K., Alamoudi, W., Alhibshi, A., Rehman, S., Sharma, A. R., & Abdulla, F. A. (2022). Varied Composition and Underlying Mechanisms of Gut Microbiome in Neuroinflammation. Microorganisms 2022, Vol. 10, Page 705, 10(4), 705. https://doi.org/10.3390/MICROORGANISMS10040705
3.Lee, K., Vuong, H. E., Nusbaum, D. J., Hsiao, E. Y., Evans, C. J., & Taylor, A. M. W. (2018). The gut microbiota mediates reward and sensory responses associated with regimen-selective morphine dependence. Neuropsychopharmacology?: Official Publication of the American College of Neuropsychopharmacology, 43(13), 2606–2614. https://doi.org/10.1038/S41386-018-0211-9

Presenting Author

Julia Nickols

Poster Authors

Julia Nickols

BSc(Hons)

University of Alberta

Lead Author

E-mail

Andrew Forgie

PhD

University of Alberta

Lead Author

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Ethan Chen BSc

University of Alberta

Lead Author

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Nyal Mirza BSc

University of Alberta

Lead Author

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Adam Wass BSc

University of Alberta

Lead Author

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Ben Willing PhD

University of Alberta

Lead Author

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Topics

  • Treatment/Management: Pharmacology: Opioid