Itch, also known as pruritus, is a common, intractable symptom of several skin diseases, such as atopic dermatitis and xerosis. This chronic condition erodes quality of life.
The authors of this publication have made an important discovery that could prove a target for treating chronic itch. They also used 3 of our markers to confirm this discovery-guinea pig anti-TRPV1, guinea pig anti-SP antibody and rabbit anti-CGRP antibodies: Tong Liu, Temugin Berta, Zhen-Zhong Xu,Chul-Kyu and Ru-Rong Ji. TLR3 deficiency impairs spinal cord synaptic transmission, central sensitization, and pruritus in mice. J Clin Invest. 2012 June 1; 122(6): 2195–2207. Published online 2012 May 8. doi:10.1172/JCI45414.
Highlights: Scratching behaviors induced by histamine-dependent and -independent pruritogens are markedly reduced in mice lacking the Tlr3 gene. TLR3 is expressed mainly by small-sized primary sensory neurons in dorsal root ganglions (DRGs) that coexpress the itch signaling pathway components transient receptor potential subtype V1 and gastrin-releasing peptide. Ttreatment with a TLR3 agonist induces inward currents and action potentials in DRG neurons and elicited scratching in WT mice but not Tlr3–/– mice. Furthermore, excitatory synaptic transmission in spinal cord slices and long-term potentiation in the intact spinal cord were impaired in Tlr3–/– mice but not Tlr7–/– mice. Consequently, central sensitization–driven pain hypersensitivity, but not acute pain, was impaired in Tlr3–/– mice. In addition, TLR3 knockdown in DRGs also attenuated pruritus in WT mice. Finally, chronic itch in a dry skin condition was substantially reduced in Tlr3–/– mice. This demonstrates a critical role of TLR3 in regulating sensory neuronal excitability, spinal cord synaptic transmission, and central sensitization.
Images: Impaired scratching behaviors and reduced c-Fos expression in the spinal cords in Tlr3–/– mice. (A and B) Scratches in every 5 minutes (left) and 0–30 minutes (right) induced by intradermal injection of 50 μl compound 48/80 (100 μg) and CQ (200 μg). Note a reduction of both histaminergic (compound 48/80) and nonhistaminergic (CQ) itch in Tlr3–/– mice. *P < 0.05, Student’s t test; n = 11–13 mice for each group. Mean ± SEM. Two-way repeated-measures ANOVA analysis also shows a significant difference in the time course of compound 48/80– and CQ-induced scratching between the 2 groups (P < 0.05). (C) c-Fos–like immunoreactivity in the dorsal horn of the cervical spinal cord in WT and Tlr3–/– mice 2 hours after intradermal injection of compound 48/80 (48/80) or CQ. Right panels show the number of c-Fos–positive neurons in the dorsal horn. Scale bars, 100 μm. *P < 0.05, Student’s t test; n = 4–6 mice. All the data are mean ± SEM.
Images: Expression of TLR3 in a subset of small-sized DRG neurons. (A) Single-cell RT-PCR analysis from dissociated small-sized DRG neurons showing the distinct and overlapped distribution patterns of TLR3 and TLR7 in DRG neurons. The lanes were run on the same gel but were noncontiguous. M, marker; NC, negative control. (B) Single-cell RT-PCR analysis from dissociated small-sized DRG neurons showing colocalization of TLR3 with TPRV1 and GRP. Similar results were obtained from 3 independent experiments in 30 cells collected from different animals. (C) Double immunostaining in DRGs showing co-colocalization of TLR3 and GRP. Red and yellow arrows indicate GRP+ only and double-labeled neurons, respectively. Scale bars: 50 μm. (D) Cell size distribution frequency of TLR3+ and GRP+ neurons. (E) Double immunostaining in cultured DRG neurons showing co-colocalization of TLR3 with TRPV1 but not with NF200. Green arrows indicate NF200+ or TRPV1+ neurons, red arrows indicate TLR3+ neurons, and yellow allows indicate double-labeled neurons. Scale bars: 50 μm. (F) A Venn diagram showing the relationship of TLR3+, GRP+, and TRPV1+ populations in a DRG. Note that all TLR3+ cells also express GRP and TRPV1.
Nociceptive DRG neurons are involved in itch. TRPV1 and CGRP are indispensible for itch sensation. Given that all TRL3+ cells express these proteins, TRL3 could be a viable target for treating itch.