Chronic CRPS Pain Shifts From Peripheral Neuropeptide Signaling to Central Spinal Cord Mechanisms in Rats | Research

The takeaway

In a rat model of complex regional pain syndrome, substance P and inflammatory mediators drive acute pain peripherally, but chronic pain persists through central spinal cord mechanisms even after peripheral inflammation resolves.

90% acute → pain-only chronic
While 90% of rats showed full CRPS symptoms at 4 weeks, by 16 weeks only pain behaviors remained — peripheral inflammation resolved but spinal cord neuropeptide signaling persisted.

What the researchers found

At 4 weeks post-fracture (acute phase), 90% of rats showed pain behaviors (allodynia, unweighting), warmth, edema, and epidermal thickening. Substance P, NK1 receptor, TNFα, IL-1β, IL-6, and nerve growth factor (NGF) were all elevated in sciatic nerve and/or skin. By 16 weeks (chronic phase), only pain behaviors persisted — all peripheral inflammatory markers had returned to normal.

Critically, spinal cord levels of NK1 receptor, TNFα, IL-1β, and NGF remained elevated at both 4 and 16 weeks. Peripheral administration of IL-1 receptor antagonist (anakinra) or anti-NGF blocked pain at 4 weeks but not 16 weeks. However, intrathecal (spinal) delivery of NK1 receptor antagonist, anakinra, or anti-NGF reduced pain at both timepoints, demonstrating that central mechanisms sustain chronic CRPS pain.

Why it matters

CRPS is notoriously difficult to treat, partly because the mechanisms maintaining chronic pain are poorly understood. This study demonstrates a clear shift from peripheral to central nervous system mechanisms as CRPS becomes chronic — explaining why treatments targeting peripheral inflammation often fail in long-standing cases. The finding that substance P/NK1 receptor signaling persists in the spinal cord provides a specific peptide-based target for developing therapies for chronic CRPS.

How the study worked

Researchers used a tibial fracture and cast immobilization model in male Sprague-Dawley rats to induce CRPS-like symptoms. They assessed nociceptive behaviors (von Frey testing for allodynia, unweighting), vascular changes (skin temperature and thickness), and molecular markers using immunoassays and Western blotting in skin, sciatic nerve, and spinal cord at 4 and 16 weeks. Pharmacological interventions included systemic (peripheral) and intrathecal (spinal) administration of NK1 receptor antagonist (LY303870), IL-1 receptor antagonist (anakinra), and anti-NGF antibody. Data were analyzed by one-way ANOVA with Newman-Keuls post hoc tests.

What this study cannot tell us

The study was conducted in male Sprague-Dawley rats, and CRPS mechanisms may differ in females and in humans. The fracture/immobilization model reproduces some but not all features of human CRPS. Specific sample sizes per group were not stated in the abstract. The study did not explore the cellular mechanisms driving sustained spinal cord inflammation or whether longer treatment could reverse central sensitization. The 16-week timepoint, while chronic for rats, may not fully represent years-long human CRPS.

How to read the evidence

This is a well-designed preclinical animal study with comprehensive molecular, vascular, and behavioral assessments at two timepoints, plus pharmacological validation using both peripheral and central drug delivery. The dual-timepoint design with multiple outcome measures is a strength, though it remains an animal model requiring human validation.

When this study was published

Published in 2016, this study is about 10 years old. The concept of peripheral-to-central transition in CRPS remains highly relevant and has been supported by subsequent research. NK1 receptor antagonists for pain management continue to be an area of investigation.

The bigger picture

This study illustrates a broader principle in chronic pain: conditions that begin with peripheral tissue injury can transition into centrally maintained pain states where the spinal cord itself becomes a driver of ongoing symptoms. The neuropeptide substance P and its NK1 receptor are key players in this transition. Understanding this shift has implications not just for CRPS but for other chronic pain conditions like fibromyalgia and neuropathic pain, where central sensitization plays a similar role.

Questions still open

Could intrathecal NK1 receptor antagonists be developed as targeted treatments for chronic CRPS in humans?
At what point during the acute-to-chronic transition does the shift from peripheral to central pain mechanisms occur?
Would early aggressive treatment of peripheral substance P signaling prevent the development of central sensitization and chronic CRPS?

Common questions

What is CRPS and why is it so hard to treat?

Complex regional pain syndrome (CRPS) is a chronic pain condition that usually develops after an injury, surgery, or fracture. The pain is disproportionate to the original injury and can persist for years. It's hard to treat because, as this study shows, the mechanisms driving pain change over time — what works early (targeting peripheral inflammation) may not work later when the spinal cord has taken over as the pain generator.

What role does substance P play in chronic pain?

Substance P is a neuropeptide — a chemical messenger used by nerve cells — that transmits pain signals and promotes inflammation. In this study, substance P and its receptor (NK1) were elevated in the spinal cord during both acute and chronic CRPS phases. Even after peripheral inflammation resolved, persistent substance P signaling in the spinal cord continued to drive pain, making it a key target for potential chronic pain treatments.

Read the original research

Acute versus chronic phase mechanisms in a rat model of CRPS.

Journal of neuroinflammation, 13, 14

View on PubMed ↗Publisher’s paper ↗

Citation

Wei, Tzuping; Guo, Tian-Zhi; Li, Wen-Wu; Kingery, Wade S; Clark, John David. (2016). Acute versus chronic phase mechanisms in a rat model of CRPS.. Journal of neuroinflammation, 13, 14. https://doi.org/10.1186/s12974-015-0472-8