Summary
A bioRxiv preprint reports that mice developed opioid-induced hyperalgesia even when spinal microglia were absent. The findings also separate changes in microglial appearance from a necessary role in the measured opioid effects.
A bioRxiv preprint reports that spinal microglia were not necessary for opioid-induced hyperalgesia, analgesic tolerance or somatic withdrawal symptoms in mice. The researchers also found that opioid-related changes in microglial appearance could occur even though the cells’ gene-expression changes were modest.
Microglia are immune cells in the central nervous system. Opioid-induced hyperalgesia (OIH) is increased sensitivity to pain associated with opioid exposure. The findings challenge the idea that spinal microglia must drive the development of this and the other measured opioid-related effects in mice.
Microglial changes did not prevent the effects
The researchers examined spinal microglia from opioid-withdrawn and saline-treated mice using morphology analysis and single-cell transcriptomics, which measures gene activity in individual cells. They report that the cells showed morphological changes, while the transcriptomic changes were modest.
The team also compared opioids restricted to peripheral tissues with opioids restricted to the central nervous system. The abstract reports that peripheral, but not central, administration was sufficient to produce both hyperalgesia and microglial morphological changes in mice.
Most directly, mice in the Csf1rΔΔFIRE model, in which microglia were completely absent, still developed OIH, analgesic tolerance and somatic withdrawal symptoms. That result suggests that the observed microglial changes were not required for these outcomes in the model.
What the result changes
The study separates a visible cellular response from a necessary cause: microglia changed in appearance in some conditions, but removing them did not stop the measured effects. This makes microglia a less compelling explanation for how these opioid-related phenomena develop in the experiments reported here.
The work is a preprint describing mouse research. It offers a mechanistic finding in that model; whether human opioid-related hyperalgesia follows the same biology remains an open question.