In primates, β-endorphin and gastrin-releasing peptide each cause itch through distinct receptor systems, while dynorphin A selectively suppresses itch without affecting pain — revealing separable neuropeptide circuits for itch and pain in the spinal cord.
Dynorphin A blocks itch without affecting painDynorphin A selectively suppressed itch triggered by two completely different pathways (β-endorphin and GRP) through kappa-opioid receptors, without altering pain processing — demonstrating a clean separation between itch and pain circuits in the primate spinal cord.
What the researchers found
The study mapped distinct neuropeptide-receptor systems for itch and pain in primate spinal cord:
**Itch-inducing systems:**
- β-endorphin (10-100 nmol) → mu-opioid receptor (MOP) → robust scratching
- Gastrin-releasing peptide (1-10 nmol) → BB2 receptor → equal scratching
- β-endorphin also attenuates inflammatory pain; GRP does not affect pain
**Pain-inhibiting only (no itch):**
- Enkephalins (100-1000 nmol) → pain inhibition
- Nociceptin-orphanin FQ (3-30 nmol) → pain inhibition
**Itch-inhibiting (no pain effect):**
- Dynorphin A(1-17) (10-100 nmol) → kappa-opioid receptor (KOP) → suppresses both β-endorphin- and GRP-induced itch
- Anti-itch effect reversed by KOP receptor antagonist nor-binaltorphimine
Why it matters
Chronic itch affects millions of people with conditions like eczema, liver disease, and kidney failure, and current treatments are limited. Opioid painkillers notoriously cause itching as a side effect. Understanding that separate neuropeptide pathways control itch and pain in primates — the closest model to humans — could enable development of anti-itch drugs that don't affect pain processing, and pain drugs that don't cause itch. The identification of the dynorphin A-KOP receptor system as a selective anti-itch pathway is particularly promising.
How the study worked
Awake, behaving rhesus monkeys (Macaca mulatta) received intrathecal (spinal) injections of various neuropeptides at different doses. Behavioral responses — scratching (itch) and pain behaviors — were quantified. Receptor antagonists were used to confirm which receptor mediated each effect. The dose-response relationships were established for each peptide.
What this study cannot tell us
The study was conducted in nonhuman primates with intrathecal (spinal) peptide delivery, which is not a practical therapeutic route for most patients. The number of animals used is not specified in the abstract. While rhesus monkeys are the closest available model to humans, species differences in neuropeptide systems may exist. The study focused on acute itch and pain responses and did not assess chronic conditions.
How to read the evidence
This is a preclinical study in nonhuman primates (rhesus monkeys), which provides the highest-quality animal evidence available. The use of awake, behaving animals with dose-response analyses and receptor antagonist confirmation provides rigorous pharmacological evidence, though translation to human therapy requires clinical studies.
When this study was published
Published in 2015, this study laid important groundwork for the development of kappa-opioid receptor-targeted anti-itch therapies. Since publication, difelikefalin — a KOP agonist — has been approved for chronic itch in dialysis patients, validating the pathway identified in this research.
The bigger picture
This study provides some of the strongest primate evidence that itch and pain are processed by separable neuropeptide circuits in the spinal cord — a concept that has transformed our understanding of sensory neuroscience. The finding that kappa-opioid receptor activation selectively suppresses itch has since contributed to the development of drugs like difelikefalin (Korsuva), a KOP receptor agonist approved for chronic itch in dialysis patients. This study helped establish the scientific foundation for that drug class.
Questions still open
- Can peripherally administered kappa-opioid agonists replicate the anti-itch effects of spinal dynorphin A without central nervous system side effects?
- Is the gastrin-releasing peptide (GRP) itch pathway also active in chronic itch conditions in humans, and could BB2 receptor antagonists be developed as anti-itch drugs?
- Why does β-endorphin cause itch while other opioid peptides (enkephalins, nociceptin) do not — what structural features determine itch-inducing versus non-itch-inducing opioid signaling?
Common questions
Why do opioid painkillers cause itching?
What is gastrin-releasing peptide and why is it important for itch?
Read the original research
Distinct functions of opioid-related peptides and gastrin-releasing peptide in regulating itch and pain in the spinal cord of primates.
Scientific reports, 5, 11676
Citation
Lee, Heeseung; Ko, Mei-Chuan. (2015). Distinct functions of opioid-related peptides and gastrin-releasing peptide in regulating itch and pain in the spinal cord of primates.. Scientific reports, 5, 11676. https://doi.org/10.1038/srep11676