Activity-dependent silencing reveals functionally distinct itch-generating sensory neurons

David P Roberson1,2, Sagi Gudes3,4,8, Jared M Sprague1,5,8, Haley A W Patoski1, Victoria K Robson1, Felix Blasl3, Bo Duan2,6, Seog Bae Oh1,7, Bruce P Bean2, Qiufu Ma2,6, Alexander M Binshtok3,4 & Clifford J Woolf1,2

nn.3404 nn.3404-S1

The peripheral terminals of primary sensory neurons detect histamine and non-histamine itch-provoking ligands through molecularly distinct transduction mechanisms. It remains unclear, however, whether these distinct pruritogens activate the same or different afferent fibers. Using a strategy of reversibly silencing specific subsets of murine pruritogen-sensitive sensory axons by targeted delivery of a charged sodium-channel blocker, we found that functional blockade of histamine itch did not affect the itch evoked by chloroquine or SLIGRL-NH2, and vice versa. Notably, blocking itch-generating fibers did not reduce pain-associated behavior. However, silencing TRPV1+ or TRPA1+ neurons allowed allyl isothiocyanate or capsaicin, respectively, to evoke itch, implying that certain peripheral afferents may normally indirectly inhibit algogens from eliciting itch. These findings support the presence of functionally distinct sets of itch-generating neurons and suggest that targeted silencing
of activated sensory fibers may represent a clinically useful anti-pruritic therapeutic approach for histaminergic and non- histaminergic pruritus.

The TGR5 receptor mediates bile acid– induced itch and analgesia

64551

Farzad Alemi,1 Edwin Kwon,1 Daniel P. Poole,2 TinaMarie Lieu,3 Victoria Lyo,1 Fiore Cattaruzza,1 Ferda Cevikbas,4 Martin Steinhoff,4 Romina Nassini,5 Serena Materazzi,5 Raquel Guerrero-Alba,6 Eduardo Valdez-Morales,6 Graeme S. Cottrell,7 Kristina Schoonjans,8 Pierangelo Geppetti,5 Stephen J. Vanner,6 Nigel W. Bunnett,3 and Carlos U. Corvera1

1Department of Surgery, UCSF, San Francisco, California, USA. 2Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia. 3Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia. 4Department of Dermatology, UCSF, San Francisco, California, USA. 5Department of Preclinical and Clinical Pharmacology, University of Florence, Florence, Italy. 6Gastrointestinal Diseases Research Unit, Division of Gastroenterology, Queen’s University, Kingston, Ontario, Canada. 7Department of Pharmacy and Pharmacology, The University of Bath, Bath, United Kingdom. 8Laboratory of Integrative and Systems Physiology, Institute of Bioengineering, School of Life Sciences, Lausanne, Switzerland.

Patients with cholestatic disease exhibit pruritus and analgesia, but the mechanisms underlying these symp- toms are unknown. We report that bile acids, which are elevated in the circulation and tissues during cho- lestasis, 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 neu- rons 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.

Peptidergic CGRPa Primary Sensory Neurons Encode Heat and Itch
and Tonically Suppress Sensitivity to Cold

Eric S. McCoy,1 Bonnie Taylor-Blake,1 Sarah E. Street,1 Alaine L. Pribisko,1 Jihong Zheng,1 and Mark J. Zylka1,* 1Department of Cell Biology and Physiology, UNC Neuroscience Center, The University of North Carolina at Chapel Hill, CB #7545, Chapel Hill, NC 27599, USA
*Correspondence: zylka@med.unc.edu

http://dx.doi.org/10.1016/j.neuron.2013.01.030

mmc1, 1-s2.0-S0896627313000962-main

Despite its clinical importance, the mechanisms that mediate or generate itch are poorly defined. The identification of pruritic com- pounds offers insight into understanding the molecular and cellular basis of itch. Imiquimod (IQ) is an agonist of Toll-like receptor 7 (TLR7) used to treat various infectious skin diseases such as genital warts, keratosis, and basal cell carcinoma. Itch is reportedly one of the major side effects developed during IQ treatments. We found that IQ acts as a potent itch-evoking compound (pruritogen) in mice via direct excitation of sensory neurons. Combined studies of scratching behavior, patch-clamp recording, and Ca2+ response re- vealed the existence of a unique intracellular mechanism, which is independent of TLR7 as well as different from the mechanisms exploited by other well-characterized pruritogens. Nevertheless, as for other pruritogens, IQ requires the presence of transient re- ceptor potential vanilloid 1 (TRPV1)-expressing neurons for itch- associated responses. Our data provide evidence supporting the hypothesis that there is a specific subset of TRPV1-expressing neu- rons that is equipped with diverse intracellular mechanisms that respond to histamine, chloroquine, and IQ.

A subpopulation of nociceptors specifically linked to itch

nn.3289-S1, nn.3289

Liang Han1, Chao Ma2,3, Qin Liu1,4, Hao-Jui Weng1,4, Yiyuan Cui5, Zongxiang Tang1,4, Yushin Kim1, Hong Nie3,6, Lintao Qu3, Kush N Patel1,4, Zhe Li1, Benjamin McNeil1, Shaoqiu He7, Yun Guan7, Bo Xiao5, Robert H LaMotte3 & Xinzhong Dong1,4

Itch-specific neurons have been sought for decades. The existence of such neurons has been doubted recently as a result of the observation that itch-mediating neurons also respond to painful stimuli. We genetically labeled and manipulated MrgprA3+ neurons in the dorsal root ganglion (DRG) and found that they exclusively innervated the epidermis of the skin and responded to multiple pruritogens. Ablation of MrgprA3+ neurons led to substantial reductions in scratching evoked by multiple pruritogens and occurring spontaneously under chronic itch conditions, whereas pain sensitivity remained intact. Notably, mice in which TRPV1 was exclusively expressed in MrgprA3+ neurons exhibited itch, but not pain, behavior in response to capsaicin. Although MrgprA3+ neurons were sensitive to noxious heat, activation of TRPV1 in these neurons by noxious heat did not alter pain behavior. These data suggest that MrgprA3 defines a specific subpopulation of DRG neurons mediating itch. Our study opens new avenues for studying itch and developing anti-pruritic therapies.

TRPA1 underlies a sensing mechanism for O2

nchembio.640 , nchembio.640-S1

nobuaki takahashi1–3, tomoyuki Kuwaki4, shigeki Kiyonaka1,2,5, tomohiro numata1,2, daisuke Kozai1,2, Yusuke Mizuno1,2, shinichiro Yamamoto1,2, shinji naito6, ellen Knevels7,8, peter Carmeliet7,8, toru Oga9, shuji Kaneko10, seiji suga1, toshiki nokami1, Jun-ichi Yoshida1 & Yasuo Mori1,2,5*

Oxygen (O2) is a prerequisite for cellular respiration in aerobic organisms but also elicits toxicity. To understand how animals cope with the ambivalent physiological nature of O2, it is critical to elucidate the molecular mechanisms responsible for O2 sens- ing. Here our systematic evaluation of transient receptor potential (TRP) cation channels using reactive disulfides with differ- ent redox potentials reveals the capability of TRPA1 to sense O2. O2 sensing is based upon disparate processes: whereas prolyl hydroxylases (PHDs) exert O2-dependent inhibition on TRPA1 activity in normoxia, direct O2 action overrides the inhibition via the prominent sensitivity of TRPA1 to cysteine-mediated oxidation in hyperoxia. Unexpectedly, TRPA1 is activated through relief from the same PHD-mediated inhibition in hypoxia. In mice, disruption of the Trpa1 gene abolishes hyperoxia- and hypoxia-induced cationic currents in vagal and sensory neurons and thereby impedes enhancement of in vivo vagal discharges induced by hyperoxia and hypoxia. The results suggest a new O2-sensing mechanism mediated by TRPA1.