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Anatomical and functional dichotomy of ocular itch and pain

Nature Medicine volume 24, pages 1268–1276 (2018)Cite this article

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Abstract

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.

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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.

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Acknowledgements

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.).

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Author notes

  1. Cheng-Chiu Huang

    Present address: Merck Research Laboratories, South San Francisco, CA, USA

  2. Bo Duan

    Present address: Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI, USA

  3. These authors contributed equally: Cheng-Chiu Huang, Weishan Yang.

Authors and Affiliations

  1. Department of Anesthesiology and Center for the Study of Itch, Washington University School of Medicine, St. Louis, MO, USA

    Cheng-Chiu Huang, Weishan Yang, Changxiong Guo, Haowu Jiang, Fengxian Li, Maolei Xiao & Qin Liu

  2. Department of Anesthesiology, Zhujiang Hospital, Southern Medical University, Guangzhou, People’s Republic of China

    Fengxian Li

  3. Department of Anesthesiology and Pain Research Center, University of Cincinnati College of Medicine, Cincinnati, OH, USA

    Steve Davidson

  4. School of Medicine and Life Sciences, Nanjing University of Chinese Medicine, Jiangsu, China

    Guang Yu

  5. Dana-Farber Cancer Institute and Department of Neurobiology, Harvard Medical School, Boston, MA, USA

    Bo Duan & Tianwen Huang

  6. Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO, USA

    Andrew J. W. Huang & Qin Liu

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Contributions

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.

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Correspondence to Qin Liu.

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Huang, CC., Yang, W., Guo, C. et al. Anatomical and functional dichotomy of ocular itch and pain. Nat Med 24, 1268–1276 (2018). https://doi.org/10.1038/s41591-018-0083-x

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