494 Mechanisms underlying MOR-DOR analgesic synergy in epidermal nerve fibers

Abstract

We recently published electrophysiological data showing that the combination of the mu-opioid (MOR) agonist loperamide and the delta-opioid (DOR) agonist oxymorphindole (Lo-OMI), applied topically to mouse toe pads, prevents activation of nociceptive epidermal nerve fibers with high potency and efficacy (Uhelski et al., 2020). The present study tested the hypotheeis that i) heterodimeric opioid receptors mediate this analgesic action, and ii) inhibitory G proteins mediate this action via activation of G protein-coupled inwardly rectifying potassium (GIRK) channels. Recent studies have demonstrated that G protein-biased ligands demonstrate reduced side effects such as respiratory depression and tolerance. Using the complete Freund’s adjuvant model of inflammatory pain and the Hargreaves assay for thermal nociception, the anti-hyperalgesic effect of Lo-OMI in male and female mice was challenged by co-administration of a recently developed antagonist against MOR-DOR heteromers, D24M (Olson, Streicher et al., 2018). Then, mice were given an injection of Lo, OMI, or their combination with and without pertussis toxin in order to assess the involvement of Gai/o signaling on the behavioral anti-hyperalgesia. Finally, in naïve mice, the role of GIRK channels, a downstream target of G proteins, was investigated using the peptide inhibitor tertiapin-Q, as well as GIRK KO mice. The data demonstrate that D24M was significantly more potent in antagonizing the Lo-OMI combination (p<0.01) than either drug alone, supporting the heterodimeric opioid receptor hypothesis, and that pertussis toxin significantly reduces the anti-hyperalgesic effect of the combination (p<0.05), supporting the Gai/o signaling hypothesis. Finally, both pharmacologic and genomic inhibition of GIRK2 function results in a significant reduction of the combination’s anti-nociceptive effect (p<0.0001), supporting the requirement for GIRK channels.