A newly discovered MRGPRA3 agonist by high-throughput screening induces scratching behaviors

Abstract

The Mas-related receptor A3 (MRGPRA3), an orphan GPCR, is expressed specifically in the dorsal root ganglion (DRG) sensory neurons, and it has recently been attracting much attention as an itch inducer. While MRGPRA3 responds to chloroquine which is an anti-malaria drug and causes strong itch, chloroquine requires high concentrations to activate MRGPRA3 and also produces MRGPRA3-independent responses. Therefore, in order to unveil the role of MRGPRA3, a new tool that enables selective manipulation of MRGPRA3 function is needed. In this study, we screened a series of small-molecule chemical libraries (1,084 compounds) to explore agonists for MRGPRA3 by high-throughput Ca²⁺ imaging. We identified that papaverine, an opium alkaloid, specifically evoked Ca²⁺ responses in cells expressing MRGPRA3 without affecting responses of other MRGPRs subtypes. Papaverine evoked Ca²⁺ responses in a subpopulation of DRG neurons that responded to chloroquine. In addition, we found that intradermal injection of papaverine to the cheek produced scratching behavior in a histamine-independent manner and did not produce nociceptive wiping behavior. Furthermore, the papaverine-induced scratching behavior was suppressed by a selective ablation of MRGPRA3-expressing primary afferent neurons or a genetic knockout of gastrin-releasing peptide receptors (GRPR: a crucial receptor for spinal itch transmission). Taken together, these results uncover a new pharmacological action of papaverine that is a potent and selective agonistic effect for MRGPRA3, which could be a powerful tool for research investigating the biological role of MRGPRA3 and the physiology and pathology of itch.