Substance P Infusion into Striatum Relieves Neuropathic Pain via Muscarinic Receptor Pathway | Research

The takeaway

Continuous substance P infusion into the rat striatum dose-dependently relieved chronic neuropathic pain through NK1 receptors activating muscarinic cholinergic neurons — a novel pain pathway.

Novel NK1→muscarinic pain pathway
Substance P relieves neuropathic pain through striatal NK1 receptors activating muscarinic cholinergic neurons

What the researchers found

Continuous striatal substance P infusion dose-dependently relieved neuropathic pain via NK1 receptors activating muscarinic cholinergic neurons — a novel brain-based analgesic pathway.

Why it matters

Neuropathic pain is notoriously difficult to treat. This study identifies a new brain circuit — striatal substance P activating cholinergic neurons — that could be targeted for chronic pain management.

The numbers in context

SP dose-dependent at 0.2-0.8 ug/mL; NK1R-dependent (CP96345); muscarinic-dependent (atropine); nicotinic-independent (mecamylamine); acute/ipsilateral ineffective

How the study worked

Animal study using reverse microdialysis to deliver continuous substance P into rat striatum after partial sciatic nerve ligation, with pharmacological dissection using NK1 antagonist, atropine, and mecamylamine.

Who was studied

Rats with partial sciatic nerve ligation (neuropathic pain model); contralateral striatal SP infusion

What this study cannot tell us

Invasive brain infusion delivery — not clinically practical as-is; rat model; only mechanical hypersensitivity tested; acute injection was ineffective, requiring continuous administration.

How to read the evidence

Strong mechanistic animal study with dose-response, receptor-specific antagonism, and multiple controls, but limited to invasive delivery and one pain model.

When this study was published

Published in 2020; complements parallel study on substance P in inflammatory pain, together mapping striatal neuropeptide analgesia.

The bigger picture

This challenges the conventional view of substance P as purely pro-pain. In the brain's striatum, it acts as an analgesic through a cholinergic pathway, suggesting the neuropeptide has context-dependent roles.

Questions still open

Could this NK1-muscarinic pathway be targeted with less invasive approaches?
Why does only continuous contralateral infusion produce analgesia?
Do muscarinic agonists alone provide similar neuropathic pain relief?

Common questions

Is substance P a pain-causing or pain-relieving peptide?

Both — in peripheral nerves it promotes pain signaling, but when infused into the brain striatum it activates cholinergic pathways that reduce pain, showing context-dependent effects.

What is neuropathic pain?

Pain caused by nerve damage (like sciatic nerve injury), often presenting as heightened sensitivity to touch. It is difficult to treat with standard painkillers and often becomes chronic.

Read the original research

Continuous infusion of substance P into rat striatum relieves mechanical hypersensitivity caused by a partial sciatic nerve ligation via activation of striatal muscarinic receptors.

Behavioural brain research, 391, 112714

View on PubMed ↗Publisher’s paper ↗

Citation

Nakamura, Yoki; Fukushige, Ryo; Watanabe, Kohei; Kishida, Yuki; Hisaoka-Nakashima, Kazue; Nakata, Yoshihiro; Morioka, Norimitsu. (2020). Continuous infusion of substance P into rat striatum relieves mechanical hypersensitivity caused by a partial sciatic nerve ligation via activation of striatal muscarinic receptors.. Behavioural brain research, 391, 112714. https://doi.org/10.1016/j.bbr.2020.112714