title: CGRPα-expressing sensory neurons respond to stimuli that evoke sensations of pain and itch.
abstract: Calcitonin gene-related peptide (CGRPα, encoded by Calca) is a classic marker of nociceptive dorsal root ganglia (DRG) neurons. Despite years of research, it is unclear what stimuli these neurons detect in vitro or in vivo. To facilitate functional studies of these neurons, we genetically targeted an axonal tracer (farnesylated enhanced green fluorescent protein; GFP) and a LoxP-stopped cell ablation construct (human diphtheria toxin receptor; DTR) to the Calca locus. In culture, 10-50% (depending on ligand) of all CGRPα-GFP-positive (+) neurons responded to capsaicin, mustard oil, menthol, acidic pH, ATP, and pruritogens (histamine and chloroquine), suggesting a role for peptidergic neurons in detecting noxious stimuli and itch. In contrast, few (2.2±1.3%) CGRPα-GFP(+) neurons responded to the TRPM8-selective cooling agent icilin. In adult mice, CGRPα-GFP(+) cell bodies were located in the DRG, spinal cord (motor neurons and dorsal horn neurons), brain and thyroid-reproducibly marking all cell types known to express Calca. Half of all CGRPα-GFP(+) DRG neurons expressed TRPV1, ∼25% expressed neurofilament-200, <10% contained nonpeptidergic markers (IB4 and Prostatic acid phosphatase) and almost none (<1%) expressed TRPM8. CGRPα-GFP(+) neurons innervated the dorsal spinal cord and innervated cutaneous and visceral tissues. This included nerve endings in the epidermis and on guard hairs. Our study provides direct evidence that CGRPα(+) DRG neurons respond to agonists that evoke pain and itch and constitute a sensory circuit that is largely distinct from nonpeptidergic circuits and TRPM8(+)/cool temperature circuits. In future studies, it should be possible to conditionally ablate CGRPα-expressing neurons to evaluate sensory and non-sensory functions for these neurons.
authors.affiliation: Department of Cell and Molecular Physiology, UNC Neuroscience Center, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.
authors.forename: Eric S
authors.initials: ES
authors.lastname: McCoy
authors.forename: Bonnie
authors.initials: B
authors.lastname: Taylor-Blake
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Zylka
mesh_terms: D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D015740:Calcitonin Gene-Related Peptide / Q000235:genetics / Q000378:metabolism*; D002211:Capsaicin / Q000633:toxicity; D002478:Cells, Cultured; D002738:Chloroquine / Q000633:toxicity; D005260:Female; D005727:Ganglia, Spinal / Q000166:cytology / Q000378:metabolism / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006632:Histamine / Q000633:toxicity; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D009132:Muscles / Q000187:drug effects / Q000294:innervation / Q000378:metabolism; D009149:Mustard Plant / Q000633:toxicity; D010146:Pain / Q000139:chemically induced / Q000503:physiopathology*; D010938:Plant Oils / Q000633:toxicity; D020671:Posterior Horn Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D011537:Pruritus / Q000139:chemically induced / Q000503:physiopathology*; D011984:Sensory Receptor Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D012867:Skin / Q000187:drug effects / Q000294:innervation / Q000378:metabolism; D050053:TRPM Cation Channels / Q000378:metabolism; D050916:TRPV Cation Channels / Q000378:metabolism
chemical_list: D010938:Plant Oils; D050053:TRPM Cation Channels; C492297:TRPM8 protein, mouse; D050916:TRPV Cation Channels; C493921:TRPV1 protein, mouse; D049452:Green Fluorescent Proteins; D006632:Histamine; D002738:Chloroquine; D015740:Calcitonin Gene-Related Peptide; D002211:Capsaicin; C027793:mustard oil