Mice genetically lacking both Substance P and CGRPα — two neuropeptides considered essential for pain — showed completely normal pain responses, overturning decades of assumptions about pain transmission.
0 pain responses affectedComplete genetic elimination of both Substance P and CGRPα — the two most-studied pain neuropeptides — produced no measurable change in any type of pain tested
What the researchers found
Mice genetically lacking both Substance P (Tac1 knockout) and CGRPα (Calca knockout) — two neuropeptides long considered central to pain transmission — displayed largely intact pain responses across every type of pain tested. Mechanical, thermal, chemical, and visceral pain were all preserved. Chronic inflammatory pain and neurogenic inflammation were unaffected. Even neuropathic pain from nerve injury or chemotherapy treatment persisted normally.
Both peptides were confirmed completely absent throughout the nervous system, eliminating the possibility of residual signaling. This definitively shows that even combined loss of these two major pain-associated neuropeptides is not sufficient to block pain transmission.
Why it matters
Substance P and CGRP have been the two most prominent neuropeptide targets for pain treatment for decades. Billions of dollars were spent developing drugs targeting each individually, and both failed as pain treatments in clinical trials. The prevailing hypothesis was that their co-expression meant both needed to be blocked simultaneously. This study demolishes that hypothesis — even eliminating both completely doesn't stop pain. This fundamentally reshapes our understanding of pain neurobiology and redirects the search for pain targets away from these neuropeptides.
The numbers in context
2 neuropeptides eliminated (Substance P + CGRPα) · Both undetectable throughout nervous system · Normal responses in mechanical, thermal, chemical, visceral pain · Inflammatory pain intact · Neuropathic pain intact · Itch intact
How the study worked
Researchers generated Tac1/Calca double knockout (DKO) mice lacking both Substance P and CGRPα. Complete peptide absence was confirmed throughout the nervous system. Pain behavior was assessed using a comprehensive battery: mechanical stimuli, thermal stimuli, chemical irritants, visceral pain models, itch assays, chronic inflammatory pain, neurogenic inflammation, nerve injury-induced neuropathic pain, and chemotherapy-induced neuropathic pain.
Who was studied
Tac1/Calca double knockout mice and wild-type controls tested across multiple pain assays
What this study cannot tell us
This is a preprint (bioRxiv), not yet peer-reviewed. Genetic knockouts from birth may allow compensatory mechanisms to develop that mask the peptides' roles in normally functioning adults. CGRPβ (from the Calcb gene) was not eliminated and could provide partial compensation. The study cannot address whether acute pharmacological blockade of both peptides in adult animals would have different results than lifelong genetic absence. Mouse pain models may not fully recapitulate human pain experiences.
How to read the evidence
This is a preprint (bioRxiv) of a preclinical mouse genetics study. The comprehensive battery of pain tests and confirmed complete peptide elimination strengthen the findings, but the lifetime-knockout approach leaves open the possibility of developmental compensation. Peer review is pending.
When this study was published
Posted as a preprint in 2024, this study challenges fundamental assumptions in the neuropeptide and pain research fields. Its findings have immediate implications for understanding why CGRP-targeting migraine drugs work and for redirecting pain drug development.
The bigger picture
This is potentially one of the most important findings in pain neuroscience in years. It explains the clinical failure of Substance P receptor (NK1) antagonists for pain and suggests that CGRP-targeting migraine drugs (erenumab, fremanezumab) work through non-pain mechanisms — perhaps by reducing neurogenic inflammation or vascular changes rather than blocking pain signals directly. It also redirects the search for pain targets toward other molecules and pathways.
Questions still open
- If Substance P and CGRP aren't required for pain, what are their actual roles in the pain-related neurons where they're so abundant?
- Do CGRP-targeting migraine drugs work through vascular or inflammatory mechanisms rather than pain signal blockade?
- Could compensatory mechanisms in lifetime-knockout mice mask an acute role for these peptides in adult pain processing?
Common questions
If CGRP isn't needed for pain, why do CGRP-blocking drugs help migraines?
Why did drugs targeting Substance P fail for pain if it was supposed to be essential?
Read the original research
Pain persists in mice lacking both Substance P and CGRPα signaling.
bioRxiv : the preprint server for biology
Citation
MacDonald, Donald Iain; Jayabalan, Monessha; Seaman, Jonathan; Balaji, Rakshita; Nickolls, Alec; Chesler, Alexander. (2024). Pain persists in mice lacking both Substance P and CGRPα signaling.. bioRxiv : the preprint server for biology. https://doi.org/10.1101/2023.11.15.567208