Pain-Modulating Nociceptin Receptors Found on Endometriosis Nerve Fibers — A New Treatment Target? | Research

The takeaway

Nociceptin/orphanin FQ (NOP) receptors are present and upregulated on nerve fibers in endometriosis tissue, suggesting NOP agonists could be a new approach to treating endometriosis-associated chronic pain.

NOP receptors increased on EM nerve fibers
Nociceptin receptors were found on both sensory and autonomic nerve fibers in endometriosis tissue, and their expression was elevated compared to healthy controls

What the researchers found

NOP receptor expression was detected on peritoneal nerve fibers in both endometriosis patients and healthy controls, but expression was increased in endometriosis-associated nerve fibers. NOP frequently colocalized with substance P, CGRP, tyrosine hydroxylase, and VIP-positive nerve fibers, indicating it is expressed on both sensory (pain-transmitting) and autonomic nerve fibers.

This is the first study to characterize NOP receptor expression specifically in endometriosis-associated nerve fibers, establishing a molecular basis for testing NOP agonists as analgesics in this condition.

Why it matters

Endometriosis affects millions of women and chronic pelvic pain is one of its most debilitating symptoms. Current pain management options — NSAIDs, hormonal therapies, surgery — are often insufficient. The nociceptin/NOP system is distinct from traditional opioid pathways and doesn't carry the same addiction risk. Finding NOP receptors on the very nerve fibers responsible for endometriosis pain suggests a highly targeted treatment approach that could offer relief without the drawbacks of conventional opioids.

The numbers in context

n=94 (73 EM, 21 controls); NOP colocalized with SP, CGRP, TH, VIP; NOP expression increased in EM-associated nerve fibers

How the study worked

Laparoscopically excised peritoneal tissue from 94 symptomatic women (73 with endometriosis, 21 controls) was analyzed using immunohistochemistry for NOP, PGP9.5, substance P, CGRP, tyrosine hydroxylase, and VIP. Colocalization of NOP with sensory and autonomic nerve fiber markers was assessed.

Who was studied

94 symptomatic women (73 with endometriosis, 21 controls) undergoing laparoscopic surgery

What this study cannot tell us

Observational tissue study only — no drug tested. Immunohistochemistry shows presence, not function. Correlation between NOP expression and pain severity not assessed. No NOP agonist intervention.

How to read the evidence

This is a well-designed human tissue study with a meaningful sample size (94 women) and appropriate controls. However, it demonstrates receptor expression only — not therapeutic effect — placing it at moderate evidence strength for treatment potential.

When this study was published

Published in 2023, this is recent research that establishes a new therapeutic rationale. NOP agonists are in clinical development for other pain conditions, so this finding is timely and could influence trial design for endometriosis-specific studies.

The bigger picture

This research connects two active fields: endometriosis pain research and nociceptin pharmacology. NOP receptor agonists are already being studied for other pain conditions, and this study provides the tissue-level evidence needed to justify testing them specifically in endometriosis. It also adds to our understanding of how the nociceptin peptide system interacts with sensory and autonomic nerve fibers in chronic inflammatory conditions.

Questions still open

Do NOP receptor agonists actually reduce pain when administered to women with endometriosis?
Is the degree of NOP receptor upregulation correlated with the severity of endometriosis pain?
Could NOP agonists be delivered locally to endometriosis lesions to minimize systemic side effects?

Common questions

What is the nociceptin system and how does it relate to pain?

Nociceptin (also called orphanin FQ) is a naturally occurring peptide that modulates pain through its own receptor (NOP), which is related to but distinct from traditional opioid receptors. Activating NOP receptors can dampen pain signaling, potentially without the addiction and tolerance issues seen with drugs that target classical opioid receptors like morphine.

Could this finding lead to a new treatment for endometriosis pain?

It provides a strong rationale to test NOP receptor agonists for endometriosis pain. The fact that these receptors are present and upregulated on the specific nerve fibers in endometriosis tissue suggests the target is there. However, clinical trials testing NOP agonists in endometriosis patients are needed before any treatment conclusions can be drawn.

Read the original research

Nociceptin/Orphanin FQ Opioid Peptide-Receptor Expression in the Endometriosis-Associated Nerve Fibers-Possible Treatment Option?

Cells, 12(10)

View on PubMed ↗Publisher’s paper ↗

Citation

Guan, Qihui; Velho, Renata Voltolini; Jordan, Alice; Pommer, Sabrina; Radde, Irene; Sehouli, Jalid; Mechsner, Sylvia. (2023). Nociceptin/Orphanin FQ Opioid Peptide-Receptor Expression in the Endometriosis-Associated Nerve Fibers-Possible Treatment Option?. Cells, 12(10). https://doi.org/10.3390/cells12101395