PACAP Peptide Is Essential for Nerve Pain — Mice Without It Don't Develop Neuropathic Hyperalgesia | Research

The takeaway

PACAP-deficient mice failed to develop neuropathic pain after nerve injury, while tachykinin-deficient mice still developed pain normally, identifying PACAP as a critical mediator of neuropathic hyperalgesia.

No hyperalgesia in PACAP-knockouts
While wildtype mice developed 30-40% mechanical hyperalgesia after nerve ligation, PACAP-deficient mice showed no pain hypersensitivity — substance P-deficient mice developed pain normally

What the researchers found

Using partial sciatic nerve ligation in gene-deficient mice:

- PACAP(-/-) mice: Did NOT develop mechanical hyperalgesia (30-40% in wildtype), identifying PACAP as crucial for neuropathic pain

- Tac1(-/-) mice (lacking SP/NKA): Developed hyperalgesia normally — tachykinins are NOT required for neuropathic pain

- Tacr1(-/-) mice (lacking NK1 receptor): Also developed hyperalgesia normally

- Motor coordination: Impaired in both PACAP(-/-) and Tac1(-/-) mice, unaffected in Tacr1(-/-)

- Basal skin blood flow: Reduced in Tac1(-/-) and Tacr1(-/-), normal in PACAP(-/-)

- Neurogenic vasodilation (mustard oil): Significantly reduced in PACAP(-/-), normal in Tac1(-/-) and Tacr1(-/-)

Conclusion: PACAP and tachykinins have distinct, non-overlapping roles in pain, vascular regulation, and motor function.

Why it matters

Neuropathic pain affects millions and is notoriously difficult to treat. Identifying PACAP as a crucial mediator — when substance P (long thought to be the key pain peptide) turned out not to be required — fundamentally shifts the drug target landscape. Developing PACAP antagonists could open new treatment approaches for chronic nerve pain.

How the study worked

Gene-deficient (knockout) mice for PACAP, Tac1 (substance P/NKA), and Tacr1 (NK1 receptor) underwent partial sciatic nerve ligation to model neuropathic pain. Mechanical pain threshold was measured with dynamic plantar aesthesiometry, motor coordination with Rota-Rod, and skin blood flow with laser Doppler imaging. Neurogenic vasodilation was tested with mustard oil stimulation.

What this study cannot tell us

Mouse models of neuropathic pain may not fully recapitulate human conditions. Compensatory mechanisms in knockout mice could mask or exaggerate the roles of individual peptides. Only one model of neuropathic pain (partial sciatic nerve ligation) was tested. The downstream targets of PACAP mediating neuropathic pain were not identified in this study.

How to read the evidence

This is a well-designed preclinical study using three gene-knockout mouse lines with multiple functional assessments. The clear dissociation between PACAP and tachykinin roles provides strong mechanistic evidence, though results need human validation.

When this study was published

Published in 2013, this study was among the early demonstrations of PACAP's crucial role in neuropathic pain. PACAP has since become a recognized drug target, with antagonists in early development.

The bigger picture

For decades, substance P was considered the primary pain neuropeptide, leading to the development of NK1 receptor antagonists for pain — which largely failed in clinical trials. This study helps explain why: PACAP, not substance P, is the critical mediator of neuropathic pain. This redirection of drug development targets could lead to more effective neuropathic pain treatments.

Questions still open

Which PACAP receptor subtype (PAC1, VPAC1, or VPAC2) mediates the neuropathic pain effect?
Would PACAP antagonists effectively treat neuropathic pain in humans?
Does PACAP play the same critical role in other types of chronic pain beyond traumatic neuropathy?

Common questions

What is PACAP and why is it important for pain?

PACAP (pituitary adenylate cyclase-activating polypeptide) is a neuropeptide found in sensory nerves. This study showed it's essential for developing neuropathic pain — mice without it simply don't develop the pain hypersensitivity that normally follows nerve injury. This makes PACAP a promising new drug target for chronic pain conditions that don't respond well to current treatments.

Why did substance P turn out not to be needed for nerve pain?

Despite decades of research on substance P as a pain peptide, mice lacking substance P still developed neuropathic pain normally in this study. This helps explain why NK1 receptor antagonists (substance P blockers) failed in clinical trials for pain — they were targeting the wrong peptide. PACAP, not substance P, appears to be the critical driver of neuropathic pain.

Read the original research

Role of Pituitary Adenylate-Cyclase Activating Polypeptide and Tac1 gene derived tachykinins in sensory, motor and vascular functions under normal and neuropathic conditions.

Peptides, 43, 105-12

View on PubMed ↗Publisher’s paper ↗

Citation

Botz, Bálint; Imreh, András; Sándor, Katalin; Elekes, Krisztián; Szolcsányi, János; Reglődi, Dóra; Quinn, John P; Stewart, James; Zimmer, Andreas; Hashimoto, Hitoshi; Helyes, Zsuzsanna. (2013). Role of Pituitary Adenylate-Cyclase Activating Polypeptide and Tac1 gene derived tachykinins in sensory, motor and vascular functions under normal and neuropathic conditions.. Peptides, 43, 105-12. https://doi.org/10.1016/j.peptides.2013.03.003