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.

The Mas-related G protein-coupled receptor D (Mrgprd) is selectively expressed in nonpeptidergic nociceptors that innervate the outer layers of mammalian skin. The function of Mrgprd in nociceptive neurons and the physiologically relevant somatosensory stimuli that activate Mrgprd-expressing (Mrgprd(+)) neurons are currently unknown. To address these issues, we studied three Mrgprd knock-in mouse lines using an ex vivo somatosensory preparation to examine the role of the Mrgprd receptor and Mrgprd(+) afferents in cutaneous somatosensation. In mouse hairy skin, Mrgprd, as marked by expression of green fluorescent protein reporters, was expressed predominantly in the population of nonpeptidergic, TRPV1-negative, C-polymodal nociceptors. In mice lacking Mrgprd, this population of nociceptors exhibited decreased sensitivity to cold, heat, and mechanical stimuli. Additionally, in vitro patch-clamp studies were performed on cultured dorsal root ganglion neurons from Mrgprd(-/-) and Mrgprd(+/-) mice. These studies revealed a higher rheobase in neurons from Mrgprd(-/-) mice than from Mrgprd(+/-) mice. Furthermore, the application of the Mrgprd ligand beta-alanine significantly reduced the rheobase and increased the firing rate in neurons from Mrgprd(+/-) mice but was without effect in neurons from Mrgprd(-/-) mice. Our results demonstrate that Mrgprd influences the excitability of polymodal nonpeptidergic nociceptors to mechanical and thermal stimuli.

PMID:

19571152

[PubMed – indexed for MEDLINE

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.

PMID:

21666106

[PubMed – indexed for MEDLINE]

PMCID:

PMC3169882

Free PMC Article

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.