The first two-dimensional map of iris nerves showed that CGRP and Substance P — peptides linked to pain and inflammation — make up the majority of the iris nerve supply.
61–69% CGRP-positive fibersThe majority of iris nerve fibers contain the pain and inflammation peptide CGRP, suggesting a major role in ocular inflammatory disease
What the researchers found
This is the first complete two-dimensional map of iris nerve architecture. CGRP-positive nerve fibers constituted approximately 61% of anterior and 69% of posterior iris nerve content, while Substance P-positive fibers made up about 30.5% (anterior) and 20% (posterior). All SP-positive neurons in the trigeminal ganglia were also CGRP-positive, indicating co-expression. The iris has a complex nerve architecture with 4–5 stromal nerve rings, a superficial anterior network, and radial posterior nerve bundles running along the dilator muscle. The pupillary margin had the densest innervation. Non-neuronal SP-positive cells were also found in the anterior stroma.
Why it matters
The iris is richly innervated by neuropeptide-containing nerve fibers, and these peptides — CGRP and Substance P — are known mediators of inflammation and pain. Their high expression in the iris suggests they play important roles in eye conditions like uveitis, glaucoma, cataracts, and chronic ocular pain. Understanding this anatomy could guide the development of peptide-targeted therapies for these diseases.
The numbers in context
CGRP: ~61% anterior, ~69% posterior · SP: ~30.5% anterior, ~20% posterior · 4–5 stromal nerve rings · pupillary margin densest · all SP neurons co-express CGRP
How the study worked
Whole-mount immunohistochemistry of rabbit irises using antibodies against βIII-tubulin (total nerve marker), CGRP, and Substance P. Time-lapse imaging built two-dimensional maps of the complete iris nerve architecture. Computer-assisted quantitative analysis measured relative nerve fiber densities for each peptide. Trigeminal ganglia were also examined for neuropeptide expression.
Who was studied
New Zealand rabbit irises and trigeminal ganglia (animal anatomy study)
What this study cannot tell us
Rabbit iris anatomy may differ from human iris innervation in some respects. The study provided relative peptide proportions but not absolute fiber counts. Functional roles of CGRP and SP in the iris were inferred from their known roles elsewhere but not directly tested in this study. Only two neuropeptides were examined; other peptides may also be present.
How to read the evidence
Well-executed anatomical study providing the first comprehensive iris nerve map. Quantitative computer-assisted analysis adds rigor. However, it is an animal study (rabbit) with functional roles of the peptides inferred rather than directly tested.
When this study was published
Published in 2015, this foundational anatomy study remains relevant as CGRP-targeted therapies continue to expand and interest in ocular neuropeptide biology grows.
The bigger picture
CGRP has become a major therapeutic target — CGRP-blocking antibodies are already approved for migraine prevention. This detailed mapping of CGRP and Substance P in the iris suggests these same peptide pathways could be targeted for eye conditions. As anti-CGRP therapies expand, understanding neuropeptide distribution in ocular tissues becomes increasingly relevant.
Questions still open
- Could anti-CGRP antibodies (like those used for migraine) be repurposed for treating uveitis or ocular pain?
- Does the neuropeptide distribution in the human iris mirror what was found in rabbits?
- Do changes in CGRP or Substance P levels in the iris correlate with severity of glaucoma or uveitis?
Common questions
What are CGRP and Substance P?
How might these findings help treat eye diseases?
Read the original research
Morphology and neurochemistry of rabbit iris innervation.
Experimental eye research, 135, 182-91
Citation
He, Jiucheng; Bazan, Haydee E P. (2015). Morphology and neurochemistry of rabbit iris innervation.. Experimental eye research, 135, 182-91. https://doi.org/10.1016/j.exer.2015.03.005