Anatomical and functional dichotomy of ocular itch and pain


Itch and pain are refractory symptoms of many ocular conditions. Ocular itch is generated mainly in the conjunctiva and is absent from the cornea. In contrast, most ocular pain arises from the cornea. However, the underlying mechanisms remain unknown. Using genetic axonal tracing approaches, we discover distinct sensory innervation patterns between the conjunctiva and cornea. Further genetic and functional analyses in rodent models show that a subset of conjunctival-selective sensory fibers marked by MrgprA3 expression, rather than corneal sensory fibers, mediates ocular itch. Importantly, the actions of both histamine and nonhistamine pruritogens converge onto this unique subset of conjunctiva sensory fibers and enable them to play a key role in mediating itch associated with allergic conjunctivitis. This is distinct from skin itch, in which discrete populations of sensory neurons cooperate to carry itch. Finally, we provide proof of concept that selective silencing of conjunctiva itch-sensing fibers by pruritogen-mediated entry of sodium channel blocker QX-314 is a feasible therapeutic strategy to treat ocular itch in mice. Itch-sensing fibers also innervate the human conjunctiva and allow pharmacological silencing using QX-314. Our results cast new light on the neural mechanisms of ocular itch and open a new avenue for developing therapeutic strategies.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: There are distinct sensory innervation patterns between the cornea and conjunctiva.
Fig. 2: Conjunctival MrgprA3+ sensory neurons mediate acute ocular itch.
Fig. 3: MrgprA3+ sensory neurons are required for mast cell-dependent allergic ocular itch.
Fig. 4: Central NMB–NMBR signaling is required for conjunctival itch transmission.
Fig. 5: Pharmacological silencing of MrgprA3+ neurons reduces both acute and allergic ocular itch.
Fig. 6: MrgprX1+ sensory afferents selectively innervate human conjunctiva and mediate itch in a humanized mouse model.


  1. 1.

    Ciprandi, G., Buscaglia, S., Cerqueti, P. M. & Canonica, G. W. Drug treatment of allergic conjunctivitis. A review of the evidence. Drugs 43, 154–176 (1992).

    CAS  PubMed  Google Scholar 

  2. 2.

    Abelson, M. B., Smith, L. & Chapin, M. Ocular allergic disease: mechanisms, disease sub-types, treatment. Ocul. Surf. 1, 127–149 (2003).

    PubMed  Google Scholar 

  3. 3.

    Wong, A. H., Barg, S. S. & Leung, A. K. Seasonal and perennial allergic conjunctivitis. Recent Pat. Inflamm. Allergy Drug Discov. 3, 118–127 (2009).

    CAS  PubMed  Google Scholar 

  4. 4.

    Ono, S. J. & Abelson, M. B. Allergic conjunctivitis: update on pathophysiology and prospects for future treatment. J. Allergy Clin. Immunol. 115, 118–122 (2005).

    CAS  PubMed  Google Scholar 

  5. 5.

    Yeniad, B., Alparslan, N. & Akarcay, K. Eye rubbing as an apparent cause of recurrent keratoconus. Cornea 28, 477–479 (2009).

    PubMed  Google Scholar 

  6. 6.

    Nagaki, Y., Hayasaka, S. & Kadoi, C. Cataract progression in patients with atopic dermatitis. J. Cataract Refract. Surg. 25, 96–99 (1999).

    CAS  PubMed  Google Scholar 

  7. 7.

    Ambati, B. K. et al. Corneal avascularity is due to soluble VEGF receptor-1. Nature 443, 993–997 (2006).

    CAS  PubMed  PubMed Central  Google Scholar 

  8. 8.

    Cursiefen, C. Immune privilege and angiogenic privilege of the cornea. Chem. Immunol. Allergy 92, 50–57 (2007).

    CAS  PubMed  Google Scholar 

  9. 9.

    Paus, R., Schmelz, M., Bíró, T. & Steinhoff, M. Frontiers in pruritus research: scratching the brain for more effective itch therapy. J. Clin. Invest. 116, 1174–1186 (2006).

    CAS  PubMed  PubMed Central  Google Scholar 

  10. 10.

    Oetjen, L. K. et al. Sensory neurons co-opt classical immune signaling pathways to mediate chronic itch. Cell 171, 217–228.e13 (2017).

    CAS  PubMed  PubMed Central  Google Scholar 

  11. 11.

    Bonini, S. et al. Conjunctival provocation test as a model for the study of allergy and inflammation in humans. Int. Arch. Allergy Appl. Immunol. 88, 144–148 (1989).

    CAS  PubMed  Google Scholar 

  12. 12.

    Leonardi, A. The central role of conjunctival mast cells in the pathogenesis of ocular allergy. Curr. Allergy Asthma Rep. 2, 325–331 (2002).

    PubMed  Google Scholar 

  13. 13.

    Belmonte, C., Acosta, M. C., Schmelz, M. & Gallar, J. Measurement of corneal sensitivity to mechanical and chemical stimulation with a CO2 esthesiometer. Invest. Ophthalmol. Vis. Sci. 40, 513–519 (1999).

    CAS  PubMed  Google Scholar 

  14. 14.

    Müller, L. J., Marfurt, C. F., Kruse, F. & Tervo, T. M. Corneal nerves: structure, contents and function. Exp. Eye Res. 76, 521–542 (2003).

    PubMed  Google Scholar 

  15. 15.

    Tanelian, D. L. & Beuerman, R. W. Responses of rabbit corneal nociceptors to mechanical and thermal stimulation. Exp. Neurol. 84, 165–178 (1984).

    CAS  PubMed  Google Scholar 

  16. 16.

    MacIver, M. B. & Tanelian, D. L. Structural and functional specialization of A delta and C fiber free nerve endings innervating rabbit corneal epithelium. J. Neurosci. 13, 4511–4524 (1993).

    CAS  PubMed  PubMed Central  Google Scholar 

  17. 17.

    Beuerman, R. W. & Tanelian, D. L. Corneal pain evoked by thermal stimulation. Pain 7, 1–14 (1979).

    CAS  PubMed  Google Scholar 

  18. 18.

    Huang, C. C. et al. A histamine-independent itch pathway is required for allergic ocular itch. J. Allergy Clin. Immunol. 137, 1267–1270.e6 (2016).

    CAS  PubMed  Google Scholar 

  19. 19.

    Bautista, D. M. et al. TRPA1 mediates the inflammatory actions of environmental irritants and proalgesic agents. Cell 124, 1269–1282 (2006).

    CAS  PubMed  Google Scholar 

  20. 20.

    Wilson, S. R. et al. TRPA1 is required for histamine-independent, Mas-related G protein–coupled receptor–mediated itch. Nat. Neurosci. 14, 595–602 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  21. 21.

    Usoskin, D. et al. Unbiased classification of sensory neuron types by large-scale single-cell RNA sequencing. Nat. Neurosci. 18, 145–153 (2015).

    CAS  PubMed  Google Scholar 

  22. 22.

    Cavanaugh, D. J. et al. Distinct subsets of unmyelinated primary sensory fibers mediate behavioral responses to noxious thermal and mechanical stimuli. Proc. Natl Acad. Sci. USA 106, 9075–9080 (2009).

    CAS  PubMed  Google Scholar 

  23. 23.

    Kremer, A. E. et al. Lysophosphatidic acid is a potential mediator of cholestatic pruritus. Gastroenterology 139, 1008–1018.e1 (2010).

    CAS  PubMed  Google Scholar 

  24. 24.

    Liu, Q. et al. Mechanisms of itch evoked by β-alanine. J. Neurosci. 32, 14532–14537 (2012).

    CAS  PubMed  PubMed Central  Google Scholar 

  25. 25.

    Liu, Q. et al. Sensory neuron-specific GPCR Mrgprs are itch receptors mediating chloroquine-induced pruritus. Cell 139, 1353–1365 (2009).

    PubMed  PubMed Central  Google Scholar 

  26. 26.

    Han, L. et al. A subpopulation of nociceptors specifically linked to itch. Nat. Neurosci. 16, 174–182 (2013).

    CAS  PubMed  Google Scholar 

  27. 27.

    Liu, Q. et al. The distinct roles of two GPCRs, MrgprC11 and PAR2, in itch and hyperalgesia. Sci. Signal. 4, ra45 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  28. 28.

    Sikand, P., Dong, X. & LaMotte, R. H. BAM8-22 peptide produces itch and nociceptive sensations in humans independent of histamine release. J. Neurosci. 31, 7563–7567 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  29. 29.

    Reddy, V. B. et al. Redefining the concept of protease-activated receptors: cathepsin S evokes itch via activation of Mrgprs. Nat. Commun. 6, 7864 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  30. 30.

    Stantcheva, K. K. et al. A subpopulation of itch-sensing neurons marked by Ret and somatostatin expression. EMBO Rep. 17, 585–600 (2016).

    CAS  PubMed  PubMed Central  Google Scholar 

  31. 31.

    Seal, R. P. et al. Injury-induced mechanical hypersensitivity requires C-low threshold mechanoreceptors. Nature 462, 651–655 (2009).

    CAS  PubMed  PubMed Central  Google Scholar 

  32. 32.

    Lou, S., Duan, B., Vong, L., Lowell, B. B. & Ma, Q. Runx1 controls terminal morphology and mechanosensitivity of VGLUT3-expressing C-mechanoreceptors. J. Neurosci. 33, 870–882 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  33. 33.

    Zylka, M. J., Rice, F. L. & Anderson, D. J. Topographically distinct epidermal nociceptive circuits revealed by axonal tracers targeted to Mrgprd. Neuron 45, 17–25 (2005).

    CAS  PubMed  Google Scholar 

  34. 34.

    Knop, E., Korb, D. R., Blackie, C. A. & Knop, N. The lid margin is an underestimated structure for preservation of ocular surface health and development of dry eye disease. Dev. Ophthalmol. 45, 108–122 (2010).

    PubMed  Google Scholar 

  35. 35.

    Cevikbas, F. et al. A sensory neuron-expressed IL-31 receptor mediates T helper cell–dependent itch: involvement of TRPV1 and TRPA1. J. Allergy Clin. Immunol. 133, 448–460 (2014).

    CAS  PubMed  Google Scholar 

  36. 36.

    Lee, M. G. et al. Agonists of the MAS-related gene (Mrgs) orphan receptors as novel mediators of mast cell-sensory nerve interactions. J. Immunol. 180, 2251–2255 (2008).

    CAS  PubMed  Google Scholar 

  37. 37.

    Morita, T. et al. HTR7 mediates serotonergic acute and chronic itch. Neuron 87, 124–138 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  38. 38.

    Kim, Y. S. et al. Central terminal sensitization of TRPV1 by descending serotonergic facilitation modulates chronic pain. Neuron 81, 873–887 (2014).

    CAS  PubMed  PubMed Central  Google Scholar 

  39. 39.

    Otsuki, J. A., Grassick, R., Seymour, D. & Kind, L. S. The use of 3H serotonin release from mast cells of the mouse as an assay for mediator liberation. Immunol. Commun. 5, 27–39 (1976).

    CAS  PubMed  Google Scholar 

  40. 40.

    Razin, E. et al. IgE-mediated release of leukotriene C4, chondroitin sulfate E proteoglycan, β-hexosaminidase, and histamine from cultured bone marrow-derived mouse mast cells. J. Exp. Med. 157, 189–201 (1983).

    CAS  PubMed  Google Scholar 

  41. 41.

    Panneton, W. M., Hsu, H. & Gan, Q. Distinct central representations for sensory fibers innervating either the conjunctiva or cornea of the rat. Exp. Eye Res. 90, 388–396 (2010).

    CAS  PubMed  Google Scholar 

  42. 42.

    Zhao, Z. Q. et al. Cross-inhibition of NMBR and GRPR signaling maintains normal histaminergic itch transmission. J. Neurosci. 34, 12402–12414 (2014).

    PubMed  PubMed Central  Google Scholar 

  43. 43.

    Fleming, M. S. et al. The majority of dorsal spinal cord gastrin releasing peptide is synthesized locally whereas neuromedin B is highly expressed in pain- and itch-sensing somatosensory neurons. Mol. Pain 8, 52 (2012).

    CAS  PubMed  PubMed Central  Google Scholar 

  44. 44.

    Mishra, S. K., Holzman, S. & Hoon, M. A. A nociceptive signaling role for neuromedin B. J. Neurosci. 32, 8686–8695 (2012).

    CAS  PubMed  PubMed Central  Google Scholar 

  45. 45.

    Binshtok, A. M., Bean, B. P. & Woolf, C. J. Inhibition of nociceptors by TRPV1-mediated entry of impermeant sodium channel blockers. Nature 449, 607–610 (2007).

    CAS  PubMed  Google Scholar 

  46. 46.

    Roberson, D. P. et al. Activity-dependent silencing reveals functionally distinct itch-generating sensory neurons. Nat. Neurosci. 16, 910–918 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  47. 47.

    McNeil, B. D. et al. Identification of a mast-cell-specific receptor crucial for pseudo-allergic drug reactions. Nature 519, 237–241 (2015).

    CAS  PubMed  Google Scholar 

  48. 48.

    Mousli, M. et al. Activation of rat peritoneal mast cells by substance P and mastoparan. J. Pharmacol. Exp. Ther. 250, 329–335 (1989).

    CAS  PubMed  Google Scholar 

  49. 49.

    Parra, A. et al. Ocular surface wetness is regulated by TRPM8-dependent cold thermoreceptors of the cornea. Nat. Med. 16, 1396–1399 (2010).

    CAS  PubMed  Google Scholar 

  50. 50.

    Belmonte, C., Aracil, A., Acosta, M. C., Luna, C. & Gallar, J. Nerves and sensations from the eye surface. Ocul. Surf. 2, 248–253 (2004).

    PubMed  Google Scholar 

  51. 51.

    Abila, B., Ezeamuzie, I. C., Igbigbi, P. S., Ambakederemo, A. W. & Asomugha, L. Effects of two antihistamines on chloroquine and histamine induced weal and flare in healthy African volunteers. Afr. J. Med. Med. Sci. 23, 139–142 (1994).

    CAS  PubMed  Google Scholar 

  52. 52.

    McCoy, E. S. et al. Peptidergic CGRPα primary sensory neurons encode heat and itch and tonically suppress sensitivity to cold. Neuron 78, 138–151 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  53. 53.

    Liu, Q. et al. Molecular genetic visualization of a rare subset of unmyelinated sensory neurons that may detect gentle touch. Nat. Neurosci. 10, 946–948 (2007).

    CAS  PubMed  Google Scholar 

  54. 54.

    Dell, R. B., Holleran, S. & Ramakrishnan, R. Sample size determination. ILAR J. 43, 207–213 (2002).

    CAS  PubMed  PubMed Central  Google Scholar 

Download references


We are grateful to M. W. Panneton, H. Hu, B. Kim, Z. F. Chen, T. P. Margolis and X. Dong for insightful discussions and comments on the manuscript, and we thank A. S. Yoo and Y. Liu for technical support. Mrgpra3GFP-cre, PirtGCaMP3/+ and MRGPRX1;Mrgpr-clusterΔ−/− mice were generous gifts from X. Dong of Johns Hopkins University. MrgprdeGFP/+ mice were from D. J. Anderson of the California Institute of Technology. Trpm8GFP/+ mice were from G. Story. Nav1.8cre, Nmb−/−, Nmbr−/− and NmbrGFP transgenic mice were from Z. -F. Chen of Washington University in St. Louis. Slc17a8Cre/+ tissues were from Q. Ma of Dana-Farber Cancer Institute. This work was supported by the “Research to Prevent Blindness” (RPB) unrestricted grant to the Department of Ophthalmology (A.J.W.H. and Q.L.), and the National Institutes of Health (R01EY024704 and 1R01AI125743; Q.L.) and the Pew Scholar Award (Q.L.).

Author information




C.-C.H. performed genetic axonal tracing, pharmacological and behavioral assays, calcium imaging experiments, immunofluorescence staining and data analysis and participated in manuscript preparation. W.Y. conducted retrograde tracing of ocular afferent neurons and single-cell picking, pharmacological and behavioral assays, calcium imaging, immunofluorescence and H&E staining and data analysis and participated in manuscript preparation. C.G. conducted single-cell qRT-PCR and immunofluorescence staining and assisted with mouse breeding strategy design, genetic ablation tests and manuscript preparation. H.J. performed electrophysiological recordings and data analysis. F.L. conducted TRPM8-GFP axonal tracing and ocular pain tests. M.X., in collaboration with W.Y, examined itch-sensing afferent fibers in human conjunctiva. S.D. performed the electrophysiological recordings and data analysis. G.Y. conducted calcium imaging of culture DRG neurons. B.D. and T.H. perfused Slc17a8cre/+; Rosa26tdTomato/+ mice and provided tissues for imaging. A.J.W.H. provided human tissues and contributed to experimental design and manuscript preparation. Q.L. planned and directed all of the experiments and wrote the paper.

Corresponding author

Correspondence to Qin Liu.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Text and Figures

Supplementary Figures 1–10 and Supplementary Tables 1 and 2

Reporting Summary

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Huang, CC., Yang, W., Guo, C. et al. Anatomical and functional dichotomy of ocular itch and pain. Nat Med 24, 1268–1276 (2018).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing