OPRM1/MRGPRX1 heterodimers drive opioid-induced itch through a peripheral mechanism

Opioid-induced itch is a common and distressing side effect of opioid analgesics, yet its underlying mechanisms remain poorly understood. While central µ-opioid receptor (OPRM1) signaling has been implicated, emerging evidence suggests that peripheral mechanisms also contribute, although their specific roles have not been clearly defined.

We investigated the interaction between OPRM1 and the itch-specific receptor MRGPRX1 in sensory neurons using bimolecular fluorescence complementation (BiFC), calcium and cAMP imaging, siRNA knockdown, and pharmacological inhibition assays. Behavioral assays in mice were conducted to assess scratching responses. We also employed immunohistochemistry, RT-qPCR, and ELISA to evaluate gene and protein expression levels in dorsal root ganglia (DRG) and skin tissues, including a mouse model of atopic dermatitis (AD).

OPRM1 formed heterodimers with MRGPRX1 in HEK293T cells and sensory neurons, triggering a signaling switch from Gα_i/o-mediated cAMP inhibition to Gα_q/11-driven calcium mobilization upon activation with DAMGO or endogenous opioids. This heterodimerization elicited robust intracellular calcium responses and scratching behavior in mice, which were attenuated by OPRM1 or MRGPRX1 antagonists. In the AD mouse model, increased OPRM1 expression and β-endorphin levels were observed in DRG neurons, correlating with heightened scratching and calcium responses. In contrast, although the δ-opioid receptor (OPRD1) associated with MRGPRX2, it did not trigger mast cell degranulation, suggesting a limited contribution to peripheral itch signaling.

Our findings identify a novel peripheral mechanism of opioid-induced itch mediated by OPRM1/MRGPRX1 heterodimers in sensory neurons. This receptor complex promotes calcium signaling and itch behavior, distinct from central or mast cell-dependent pathways. Targeting this heterodimer may offer new therapeutic strategies to alleviate opioid-induced itch without impairing analgesia.