The TGR5 receptor mediates bile acid–induced itch and analgesia
Farzad Alemi, … , Nigel W. Bunnett, Carlos U. Corvera
Farzad Alemi, … , Nigel W. Bunnett, Carlos U. Corvera
Published March 25, 2013
Citation Information: J Clin Invest. 2013;123(4):1513-1530. https://doi.org/10.1172/JCI64551.
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Research Article Hepatology

The TGR5 receptor mediates bile acid–induced itch and analgesia

Abstract

Patients with cholestatic disease exhibit pruritus and analgesia, but the mechanisms underlying these symptoms are unknown. We report that bile acids, which are elevated in the circulation and tissues during cholestasis, cause itch and analgesia by activating the GPCR TGR5. TGR5 was detected in peptidergic neurons of mouse dorsal root ganglia and spinal cord that transmit itch and pain, and in dermal macrophages that contain opioids. Bile acids and a TGR5-selective agonist induced hyperexcitability of dorsal root ganglia neurons and stimulated the release of the itch and analgesia transmitters gastrin-releasing peptide and leucine-enkephalin. Intradermal injection of bile acids and a TGR5-selective agonist stimulated scratching behavior by gastrin-releasing peptide– and opioid-dependent mechanisms in mice. Scratching was attenuated in Tgr5-KO mice but exacerbated in Tgr5-Tg mice (overexpressing mouse TGR5), which exhibited spontaneous pruritus. Intraplantar and intrathecal injection of bile acids caused analgesia to mechanical stimulation of the paw by an opioid-dependent mechanism. Both peripheral and central mechanisms of analgesia were absent from Tgr5-KO mice. Thus, bile acids activate TGR5 on sensory nerves, stimulating the release of neuropeptides in the spinal cord that transmit itch and analgesia. These mechanisms could contribute to pruritus and painless jaundice that occur during cholestatic liver diseases.

Authors

Farzad Alemi, Edwin Kwon, Daniel P. Poole, TinaMarie Lieu, Victoria Lyo, Fiore Cattaruzza, Ferda Cevikbas, Martin Steinhoff, Romina Nassini, Serena Materazzi, Raquel Guerrero-Alba, Eduardo Valdez-Morales, Graeme S. Cottrell, Kristina Schoonjans, Pierangelo Geppetti, Stephen J. Vanner, Nigel W. Bunnett, Carlos U. Corvera

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Figure 1

TGR5 expression and localization in mouse DRG.

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TGR5 expression and localization in mouse DRG. (A) Single-cell RT-PCR an...
(A) Single-cell RT-PCR analysis of DRG neurons from C57BL/6 mice. Small-diameter neurons were selected, and Tgr5, Grp, Trpa1, and Trpv1 mRNA was amplified. Results from 10 neurons are shown (78 neurons, 7 mice). Neurons 1, 2, 6, and 8 coexpressed Tgr5, Grp, Trpa1, and Trpv1. No transcripts were amplified from bath fluid. (B) Proportion of small-diameter neurons expressing Tgr5 (36%), Grp (50%), Trpa1 (59%), and Trpv1 (77%). Of the Tgr5-expressing neurons, 39% coexpressed Grp, 41% coexpressed Trpa1, and 32% coexpressed Trpv1. Tgr5, Grp, Trpa1, and Trpv1 were all coexpressed by 22% of small-diameter neurons. (C) Localization of TGR5-IR, Hu-IR, CGRP-IR, SP-IR, GRP-IR, and IB4-FITC binding in DRG (thoracic, lumbar, and sacral) of C57BL/6 mice. Arrowheads denote neurons coexpressing markers; arrowheads with asterisks denote lack of marker coexpression. TGR5-IR was prominently expressed in small-diameter Hu-positive neurons, most of which coexpressed CGRP-IR, SP-IR, or GRP-IR. TGR5-IR was rarely expressed in neurons that bound IB4-FITC. (D) Cross-sectional area of the TGR5-IR population (50-μm2 bins), which indicated that 50% of TGR5-IR neurons were 150–250 μm2. (E) Controls for specific detection of TGR5-IR. TGR5-IR was prominently detected in small-diameter DRG neurons of Tgr5-WT mice (arrowheads). TGR5-IR of small-diameter neurons of Tgr5-KO mice was markedly diminished (arrowheads with asterisks), although the background fluorescence of larger-diameter neurons was retained. Preadsorption of the TGR5 antibody with the receptor fragment used for immunization abolished TGR5-IR in DRG of C57BL/6 mice. There was no staining when the primary antibody was replaced with normal rabbit (Rb) IgG. Scale bars: 50 μm.