Botulinum Toxin B Injected Into the Foot Travels to the Spinal Cord and Reduces Inflammatory Pain by Blocking NMDA Signaling | Research

The takeaway

Intraplantar botulinum toxin-B reduces inflammatory pain in mice not by acting locally but by traveling to the spinal cord and blocking NMDA-mediated phosphorylation of pain-facilitating proteins.

Pain reduced, inflammation unchanged
BoNT-B blocked allodynia without reducing paw edema, proving its pain-relieving effect is central (spinal cord) not peripheral (anti-inflammatory)

What the researchers found

Intraplantar BoNT-B (1 U) reduced carrageenan-induced mechanical allodynia without affecting paw edema, confirming a central rather than local anti-inflammatory mechanism. The toxin decreased spinal phosphorylation of GluA1 (glutamate receptor subunit) at serine 845 and Akt at serine 473, as well as c-Fos expression — all markers of spinal nociceptive facilitation. BoNT-B inhibited NMDA-evoked but not substance P-evoked phosphorylation of GluA1 and Akt in the dorsal horn, demonstrating selective modulation of glutamate-mediated versus tachykinin-mediated spinal pain pathways.

Why it matters

Chronic inflammatory pain is a massive clinical burden, and current treatments (NSAIDs, opioids) have significant side effects. Botulinum toxin is already FDA-approved for chronic migraine, and understanding its spinal mechanism of action could expand its use to other pain conditions. The selective blockade of NMDA-mediated (but not substance P-mediated) spinal pain pathways suggests botulinum toxin acts on specific aspects of central sensitization — the process by which the spinal cord amplifies pain signals.

How the study worked

Male C57BL/6 mice received unilateral intraplantar BoNT-B (1 U, 30 μL) or saline followed by intraplantar carrageenan (2%) to induce inflammation. Additional groups received intrathecal NMDA or substance P to probe specific spinal pathways. Outcomes included mechanical allodynia (behavioral), paw edema, and spinal dorsal horn biochemistry (pGluA1, pAkt, c-Fos immunostaining via Western blot and immunohistochemistry).

What this study cannot tell us

Only male mice were studied, and sex differences in pain processing are well-documented. The study does not directly visualize or trace BoNT-B transport from the foot to the spinal cord. The 1 U dose may not be directly translatable to human dosing. The carrageenan model represents acute inflammatory pain; whether BoNT-B has similar effects in chronic pain models is unknown. The selective blockade of NMDA versus substance P pathways may reflect dose or timing rather than absolute selectivity.

How to read the evidence

This is a rigorous preclinical mechanistic study combining behavioral pharmacology with spinal cord biochemistry in a well-established inflammatory pain model. The selective NMDA/substance P dissociation adds mechanistic depth. However, findings are in mice only.

When this study was published

Published in 2016, this study contributed to the growing understanding of botulinum toxin's central mechanisms of action for pain, which continues to inform clinical research.

The bigger picture

Botulinum toxin research is expanding far beyond cosmetics and muscle spasticity. The finding that BoNT can travel from peripheral injection sites to the spinal cord and modulate central pain processing represents a paradigm shift in understanding how this peptide toxin works for pain. If BoNT selectively targets NMDA-mediated central sensitization — a key driver of chronic pain — it could become an important tool for conditions where central sensitization maintains persistent pain even after the original injury heals.

Questions still open

Could peripheral botulinum toxin injections treat chronic pain conditions driven by spinal NMDA-mediated central sensitization?
What is the mechanism by which BoNT-B travels retrogradely from the injection site to the spinal dorsal horn?
Would BoNT-B be effective for neuropathic pain conditions that involve NMDA receptor-mediated sensitization?

Common questions

How does botulinum toxin relieve pain if it's injected far from the brain?

This study shows that botulinum toxin injected into the skin doesn't just work locally — it travels along nerve fibers all the way to the spinal cord. Once there, it blocks the activation of proteins that normally amplify pain signals, a process called central sensitization. This explains why botulinum toxin injections in the forehead can prevent migraine headaches that originate deeper in the brain — the toxin is transported centrally.

Why does BoNT-B block NMDA pain signals but not substance P signals?

Botulinum toxins work by blocking the release of molecules from nerve endings. NMDA receptor signaling at the spinal cord involves the insertion of receptor subunits into cell membranes — a process that requires vesicular transport, which BoNT can block. Substance P signaling may use different release mechanisms that are less dependent on the molecular machinery that BoNT disrupts. This selectivity could be therapeutically useful, as NMDA-mediated sensitization is a major driver of chronic pain.

Read the original research

Effects of intraplantar botulinum toxin-B on carrageenan-induced changes in nociception and spinal phosphorylation of GluA1 and Akt.

The European journal of neuroscience, 44(1), 1714-22

View on PubMed ↗Publisher’s paper ↗

Citation

Sikandar, Shafaq; Gustavsson, Ynette; Marino, Marc J; Dickenson, Anthony H; Yaksh, Tony L; Sorkin, Linda S; Ramachandran, Roshni. (2016). Effects of intraplantar botulinum toxin-B on carrageenan-induced changes in nociception and spinal phosphorylation of GluA1 and Akt.. The European journal of neuroscience, 44(1), 1714-22. https://doi.org/10.1111/ejn.13261