CGRPα-Expressing Sensory Neurons Respond to Stimuli that Evoke Sensations of Pain and Itch

Calcitonin gene-related peptide (CGRP alpha, 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 CGRP alpha-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%) CGRP alpha-GFP(+) neurons responded to the TRPM8-selective cooling agent icilin. In adult mice, CGRP alpha-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 CGRP alpha-GFP(+) DRG neurons expressed TRPV1, similar to 25% expressed neurofilament-200, <10% contained nonpeptidergic markers (IB4 and Prostatic acid phosphatase) and almost none (<1%) expressed TRPM8. CGRP alpha-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 CGRP alpha(+) 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 CGRP alpha-expressing neurons to evaluate sensory and non-sensory functions for these neurons.