Immune cells at injury sites release the body's own opioid peptides — Met-enkephalin, β-endorphin, and dynorphin A — through a calcium-regulated signaling pathway to produce pain relief.
3 opioid peptides released by immune cellsMet-enkephalin, β-endorphin, and dynorphin A are secreted by leukocytes at nerve injury sites to produce local analgesia
What the researchers found
Opioid receptors on immune cells (leukocytes) can trigger the release of three opioid peptides — Met-enkephalin, β-endorphin, and dynorphin A (1-17) — which then act on nearby nerve cell receptors to relieve neuropathic pain. This challenges the classical view that opioid pain relief works only through receptors on neurons.
The researchers mapped the complete signaling pathway: activation of leukocyte opioid receptors triggers a Gαi/o-Gβγ protein cascade through phospholipase C and IP3 receptors, releasing intracellular calcium, which drives opioid peptide secretion. When leukocytes were depleted, pain relief from exogenous opioids was significantly reduced and could only be restored by transplanting wild-type immune cells — not those lacking opioid receptors.
Why it matters
This research reveals that immune cells are active participants in pain control, not just bystanders. By showing that opioid receptors on leukocytes drive the release of the body's own painkilling peptides, this work opens a fundamentally new approach to pain management — one that could potentially harness the immune system's natural analgesic capacity rather than relying solely on drugs that act on nerve cells.
The numbers in context
3 opioid peptides released (Met-enkephalin, β-endorphin, dynorphin A) · δ-, μ-, and κ-opioid receptor subtypes involved · Gαi/o-Gβγ-PLC-IP3-Ca²⁺ signaling pathway
How the study worked
Researchers used a mouse model of neuropathic pain created by chronic constriction injury of the sciatic nerve. They injected opioid receptor agonists at the damaged nerve site and measured pain relief using von Frey filaments (which test sensitivity to touch). They systematically blocked or removed components of the signaling pathway — depleting leukocytes, using genetic knockouts, and applying pharmacological inhibitors — to identify the mechanism. They also tested opioid peptide release from immune cells ex vivo.
Who was studied
Mice with sciatic nerve chronic constriction injury (neuropathic pain model)
What this study cannot tell us
This was a mouse study, so findings need to be confirmed in humans. The chronic constriction injury model, while well-established, may not fully replicate all forms of human neuropathic pain. The relative contributions of different leukocyte subtypes to opioid peptide release were not fully characterized.
How to read the evidence
This is a preclinical animal study with rigorous mechanistic investigation including genetic knockouts, pharmacological blockade, cell transfer experiments, and ex vivo validation. The evidence is strong for the mouse model but requires human translation.
When this study was published
Published in 2016, this study established a key mechanism in immune cell-mediated analgesia. Subsequent research has likely built on these findings in the active field of neuroimmune pain modulation.
The bigger picture
The opioid crisis has driven intense interest in understanding exactly how pain relief works at the molecular level. This research reveals that immune cells aren't just passive targets of opioid drugs — they actively participate in pain modulation by releasing the body's own opioid peptides. This could lead to entirely new pain management strategies that enhance the immune system's natural analgesic function rather than flooding neuronal receptors with synthetic opioids.
Questions still open
- Could therapies that enhance leukocyte opioid peptide release provide pain relief with fewer side effects than traditional opioids?
- Do different types of immune cells contribute differently to opioid peptide release at injury sites?
- Is this immune-mediated pain relief pathway impaired in chronic pain conditions that resist conventional opioid treatment?
Common questions
What are opioid peptides and why do immune cells release them?
Could this research lead to new pain treatments?
Read the original research
Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides.
Brain, behavior, and immunity, 57, 227-242
Citation
Celik, Melih Ö; Labuz, Dominika; Henning, Karen; Busch-Dienstfertig, Melanie; Gaveriaux-Ruff, Claire; Kieffer, Brigitte L; Zimmer, Andreas; Machelska, Halina. (2016). Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides.. Brain, behavior, and immunity, 57, 227-242. https://doi.org/10.1016/j.bbi.2016.04.018