Loss of RGS Control at Gα_o Reveals a Balance Between Nociceptin and Mu‐opioid Receptor Systems

Abstract

Regulator of G‐protein signaling (RGS) proteins bind to the active GTP‐bound Gα subunit of heterotrimeric G‐proteins to accelerate hydrolysis of GTP and limit signaling downstream of G‐protein coupled receptors (GPCRs). Studies have shown that mice expressing Gαo protein that is insensitive to modulation by RGS proteins (RGSi Gαo) are less hyperalgesic due to enhanced signaling at the mu‐opioid receptor (MOPR). In contrast, data suggest these mice have a hyperalgesic response to mechanical stimulation. The goal of the present study was to determine the mechanism behind this apparent contradiction. Baseline mechanical hypersensitivity in naive RGSi Gαo mice was reversed by pretreatment with systemic or central administration of the nociceptin/orphanin FQ (N/OFQ) receptor (NOPR) selective antagonist J‐113397, while the opioid antagonist naltrexone further intensified the hypersensitivity. Intraplantar injection of λ‐carrageenan produced mechanical hypersensitivity, which was reversed by J‐113397 in wild‐type mice and exacerbated by naltrexone in mutant mice. Whole brain homogenates from RGSi Gαo mice showed no change from wild‐type littermates in levels of NOPR as determined by [3H]N/OFQ saturation binding and no change in the affinity of [3H]N/OFQ for NOPR, while N/OFQ had unaltered potency to activate G‐protein as measured by [35S]GTPγS binding. N/OFQ‐induced inhibition of presynaptic GABA release in the periaqueductal gray was reduced in the mutant mice compared to their wild‐type controls. Together, these results indicate that increased signaling downstream of NOPR leads to hyperalgesia in the RGSi Gαo knock‐in mice, while increased signaling downstream of MOPR reverses NOPR‐mediated hyperalgesia. Similar opposing actions of the systems are present after inflammatory pain in wild‐type mice, signifying that in both cases the balance between MOPR and NOPR signaling is disturbed. These data highlight a delicate homeostatic balance between two receptor systems that is dependent on pain state. Support or Funding Information Funded by R01 DA035316 and T32 DA007268. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.