Exploration of Sensory and Spinal Neurons Expressing GRP in Itch and Pain

Gastrin-releasing peptide (GRP) is a putative itch-specific neurotransmitter, but definite evidence in the dorsal root ganglion (DRG) and spinal cord is lacking. We generated and validated a Grp-Cre knock-in (GrpCre-KI) mouse line whereby Grp neurons are genetically labeled. Cre-dependent marking analysis revealed exclusive innervation of the upper epidermis of the skin by GRP fibers. Importantly, optical stimulation of Grp fibers expressing channel rhodopsin (ChR2) in the skin evoked itch but not pain-related scratching behaviors, while conditional deletion of Grp in sensory neurons attenuated non-histaminergic itch. In contrast, intersectional genetic ablation of spinal Grp neurons did not affect itch nor pain transmission. Our study demonstrates a role of GRP in sensory neurons in itch and suggests that GRP sensory neurons are dedicated to itch transmission. GrpCre-KI mice provide a long-sought avenue for investigating peripheral coding mechanism of itch and further interrogation of itch-nerve fibers in the skin under chronic pruritus.