Mrgprd Enhances Excitability in Specific Populations of Cutaneous Murine Polymodal Nociceptors

Department of Neurobiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 08/2009; 29(26):8612-9. DOI: 10.1523/JNEUROSCI.1057-09.2009
Source: PubMed


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.

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Available from: Michael P Jankowski, Oct 01, 2015
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    • "to branch as they ascended through the epidermis , and terminated in the stratum granulosum . In contrast , terminals of CGRP+ fibers terminated lower in the epidermis ( in the stratum spinosum ) as simple endings like those reported in our study . Both MrgprD+ and CGRP+ neurons supplying glabrous skin have been shown to be polymodal nociceptors ( Rau et al . , 2009 ) . However , selective ablation of the MrgprD+ neurons indicates that they contribute to behavioral responses associated with noxious mechanical stimuli , but not to noxious heat , whilst selective block of signaling by TRPV1 expressing peptidergic neurons impairs the behavioral responses to noxious heat stimuli ( Cavanaugh et al . , 2"
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    ABSTRACT: The cornea is innervated by 3 main functional classes of sensory neurons: polymodal nociceptors, pure mechano-nociceptors and cold-sensing neurons. Here we explored transient receptor potential cation channel subfamily V member 1 (TRPV1) expression in guinea pig corneal sensory neurons, a widely used molecular marker of polymodal nociceptors. We used retrograde tracing to identify corneal afferent neurons in the trigeminal ganglion and double label in situ hybridization and/or immunohistochemistry to determine their molecular profile. In addition, we used immunohistochemistry to reveal the neurochemistry and structure of TRPV1 expressing nerve endings in the corneal epithelium. Approximately 45% of corneal afferent neurons expressed TRPV1, 28% expressed Piezo2 (a marker of putative pure mechano-nociceptors) and 8% expressed the transient receptor potential cation channel subfamily M member 8 (TRPM8; a marker of cold-sensing neurons). There was no co-expression of TRPV1 and Piezo2 in corneal afferent neurons, but 6% of TRPV1 neurons co-expressed TRPM8. The TRPV1 expressing corneal afferent neurons could be divided into 3 subpopulations on the basis of calcitonin gene-related peptide (CGRP) and/or or glial cell line-derived neurotrophic factor family receptor alpha3 (GFRα3) co-expression. In the corneal epithelium, the TRPV1 axons that co-expressed CGRP and GFRα3 ended as simple unbranched endings in the wing cell layer. In contrast, those that only co-expressed GFRα3 had ramifying endings that branched and terminated in the squamous cell layer, whereas those that only co-expressed CGRP had simple endings in the basal epithelium. This study shows that the majority of TRPV1 expressing corneal afferent neurons (>90%) are likely to be polymodal nociceptors. Furthermore, TRPV1 expressing corneal afferent neurons can be subdivided into specific subpopulations based on their molecular phenotype, nerve terminal morphology and distribution in the corneal epithelium.
    Frontiers in Neuroanatomy 06/2015; 9. DOI:10.3389/fnana.2015.00071 · 3.54 Impact Factor
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    • "This discrepancy between Lawson's study and our present study suggests that physiology alone may not be sufficient to define the function of somatosensory neurons. Indeed, using a different physiological preparation, Rau and colleagues found that Mrgprd-expressing sensory neurons were polymodal and could be activated by noxious heat and mechanical stimuli (Rau et al., 2009); however, when these neurons were ablated, only mechanosensory behaviors were impaired (Cavanaugh et al., 2009). We previously found that <10% of all CGRPa-expressing DRG neurons (defined by expression of a knocked in GFP reporter) were IB4 + (McCoy et al., 2012). "
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    ABSTRACT: Calcitonin gene-related peptide (CGRP) is a classic molecular marker of peptidergic primary somatosensory neurons. Despite years of research, it is unknown whether these neurons are required to sense pain or other sensory stimuli. Here, we found that genetic ablation of CGRPα-expressing sensory neurons reduced sensitivity to noxious heat, capsaicin, and itch (histamine and chloroquine) and impaired thermoregulation but did not impair mechanosensation or β-alanine itch-stimuli associated with nonpeptidergic sensory neurons. Unexpectedly, ablation enhanced behavioral responses to cold stimuli and cold mimetics without altering peripheral nerve responses to cooling. Mechanistically, ablation reduced tonic and evoked activity in postsynaptic spinal neurons associated with TRPV1/heat, while profoundly increasing tonic and evoked activity in spinal neurons associated with TRPM8/cold. Our data reveal that CGRPα sensory neurons encode heat and itch and tonically cross-inhibit cold-responsive spinal neurons. Disruption of this crosstalk unmasks cold hypersensitivity, with mechanistic implications for neuropathic pain and temperature perception.
    Neuron 03/2013; 78(1). DOI:10.1016/j.neuron.2013.01.030 · 15.05 Impact Factor
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    • "Furthermore, Mrg receptors have been described to be preferentially expressed in lectin B4? and glial cell line-derived neurotrophic factor co-receptor c-Ret? primary afferent neurons, and to be involved in nociception (Bender et al. 2002; Cox et al. 2008; Dong et al. 2001; Grazzini et al. 2004; Gustafson et al. 2005; Hager et al. 2008; Lembo et al. 2002; Milasta et al. 2006; Rau et al. 2009; Shinohara et al. 2004; Wilson et al. 2011; Zhang et al. 2005; Wang and Zylka 2009; Zylka et al. 2005). In mice, due to the species-specific atypical expansion, the Mrg family consists of three large clades or subfamilies (22 MrgA, 13 MrgB, and 14 MrgC subtypes) and six single-copy genes (MrgD–H and Mas1/ Mas), which, together, comprise a large group of more than 50 individual members, a substantial number of which are true receptor-encoding genes (Dong et al. 2001; Ross et al. 1990; Zhang et al. 2005; Zylka et al. 2003). "
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    ABSTRACT: Mas-related gene (Mrg) receptors constitute a subfamily of G protein-coupled receptors that are implicated in nociception, and are as such considered potential targets for pain therapies. Furthermore, some Mrgs have been suggested to play roles in the regulation of inflammatory responses to non-immunological activation of mast cells and in mast cell-neuron communication. Except for MrgD, E and F, whose changed expression has been revealed during inflammation in the mouse intestine in our earlier studies, information concerning the remaining cloned mouse Mrg subtypes in the gastrointestinal tract during (patho) physiological conditions is lacking. Therefore, the present study aimed at identifying the presence and putative function of these remaining cloned Mrg subtypes (n = 19) in the (inflamed) mouse intestine. Using reverse transcriptase-PCR, quantitative-PCR and multiple immunofluorescence staining with commercial and newly custom-developed antibodies, we compared the ileum and the related dorsal root ganglia (DRG) of non-inflamed mice with those of two models of intestinal inflammation, i.e., intestinal schistosomiasis and 2,4,6-trinitrobenzene sulfonic acid-induced ileitis. In the non-inflamed ileum and DRG, the majority of the Mrg subtypes examined were sparsely expressed, showing a neuron-specific expression pattern. However, significant changes in the expression patterns of multiple Mrg subtypes were observed in the inflamed ileum; for instance, MrgA4, MrgB2and MrgB8 were expressed in a clearly increased number of enteric sensory neurons and in nerve fibers in the lamina propria, while de novo expression of MrgB10 was observed in enteric sensory neurons and in newly recruited mucosal mast cells (MMCs). The MrgB10 expressing MMCs were found to be in close contact with nerve fibers in the lamina propria. This is the first report on the expression of all cloned Mrg receptor subtypes in the (inflamed) mouse intestine. The observed changes in the expression and cellular localization of the Mrg subtypes suggest that these receptors are involved in the mediation of primary afferent responses, mast cell responses, and in neuroimmune communication during intestinal inflammation.
    Histochemie 03/2013; 139(5). DOI:10.1007/s00418-013-1086-9 · 3.05 Impact Factor
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