An NK1 receptor antagonist that blocks Substance P signaling reduced tendon thickening, muscle and skin fibrosis, and pain-related behaviors in a rat model of repetitive overuse injury.
Multi-tissue protectionNK1R blockade reduced fibrosis and pain across three tissue types — tendons, muscles, and skin — indicating Substance P drives a systemic fibrogenic response to overuse injury
What the researchers found
NK1R antagonist treatment (L-732,138) in rats performing a high repetition high force (HRHF) task produced multiple beneficial effects compared to untreated HRHF rats:
- Improved grip strength (reversed task-induced decline)
- Reduced mechanical sensitivity and temperature aversion (pain measures)
- Reduced flexor digitorum epitendon thickening
- Decreased HRHF-induced increases of fibrotic markers TGFβ1, CCN2/CTGF, and collagen type 1 in flexor digitorum muscles
- Reduced task-induced collagen deposition in forepaw upper dermis
These findings demonstrate that Substance P-NK1R signaling drives fibrogenic responses and associated pain in overuse injuries across multiple tissue types (tendon, muscle, skin).
Why it matters
Repetitive strain injuries (carpal tunnel syndrome, tendinitis, trigger finger) affect millions of workers and athletes, and current treatments often provide only temporary relief. This research identifies Substance P as a treatable cause of the tissue scarring that drives chronic pain and disability in these conditions, opening the door to NK1R antagonist therapy as a disease-modifying treatment rather than just symptom management.
How the study worked
Young adult Sprague-Dawley rats learned to pull at high force levels over 5 weeks, then performed a high repetition high force (HRHF) task for 3 weeks (2 hours/day, 3 days/week). Groups included untreated HRHF, NK1RA-treated HRHF (L-732,138 IP in weeks 2-3), and controls with vehicle or NK1RA. Outcomes assessed: grip strength, mechanical sensitivity, temperature aversion, epitendon thickness, muscle fibrotic markers (TGFβ1, CCN2/CTGF, collagen type 1), and dermal collagen deposition.
What this study cannot tell us
The study was conducted in young adult rats performing a controlled task, which may not fully replicate the complexity and chronicity of human repetitive strain injuries. NK1RA treatment was given for only 2 weeks during an 8-week protocol, so long-term treatment effects are unknown. The study did not assess whether fibrotic changes reversed after longer treatment or whether benefits persisted after stopping the drug.
How to read the evidence
This is a well-designed preclinical animal study using a validated overuse injury model with appropriate controls (vehicle, NK1RA-only). The multi-tissue, multi-outcome assessment provides comprehensive evidence, though translation to human repetitive strain injuries requires clinical validation.
When this study was published
Published in 2020, this study builds on a body of work from this research group on Substance P's role in overuse injuries, contributing to the rationale for clinical trials of NK1R antagonists for musculoskeletal conditions.
The bigger picture
This study expands the known pathological roles of Substance P beyond pain signaling into tissue fibrosis and scarring. The finding that a single neuropeptide drives both pain and fibrosis across tendons, muscles, and skin in overuse injuries suggests that NK1R antagonists could serve as comprehensive treatments — addressing both the cause (fibrosis) and the symptom (pain) simultaneously.
Questions still open
- Could NK1R antagonists (like aprepitant, already approved for nausea) be repurposed for treating repetitive strain injuries in humans?
- Would earlier or longer NK1R blockade prevent fibrosis development rather than just reducing established fibrosis?
- Does Substance P drive fibrosis in other overuse conditions like rotator cuff tendinopathy or Achilles tendinitis?
Common questions
How does Substance P cause tissue scarring from repetitive injuries?
Could this lead to new treatments for carpal tunnel or tendinitis?
Read the original research
Blocking substance P signaling reduces musculotendinous and dermal fibrosis and sensorimotor declines in a rat model of overuse injury.
Connective tissue research, 61(6), 604-619
Citation
Barbe, M F; Hilliard, B A; Fisher, P W; White, A R; Delany, S P; Iannarone, V J; Harris, M Y; Amin, M; Cruz, G E; Popoff, S N. (2020). Blocking substance P signaling reduces musculotendinous and dermal fibrosis and sensorimotor declines in a rat model of overuse injury.. Connective tissue research, 61(6), 604-619. https://doi.org/10.1080/03008207.2019.1653289