Contribution of G-Protein α-Subunits to Analgesia, Hyperalgesia, and Hyperalgesic Priming Induced by Subanalgesic and Analgesic Doses of Fentanyl and Morphine

While opioids produce both analgesia and side effects by action at μ-opioid receptors (MORs), at spinal and supraspinal sites, the potency of different opioids to produce these effects varies. While it has been suggested that these differences might be because of bias for signaling via β-arrestin versus G-protein α-subunits (Gα), recent studies suggest that G-protein-biased MOR agonists still produce clinically important side effects. Since bias also exists in the role of Gα subunits, we evaluated the role of Gα subunits in analgesia, hyperalgesia, and hyperalgesic priming produced by fentanyl and morphine, in male rats. We found that intrathecal treatment with oligodeoxynucleotides antisense (AS-ODN) for Gα2, Gα3, and Gα markedly attenuated hyperalgesia induced by subanalgesic dose (sub-AD) fentanyl, while AS-ODN for Gα1, as well as Gα2 and Gα3, but not Gα, prevented hyperalgesia induced by sub-AD morphine. AS-ODN for Gα1 and Gα2 unexpectedly enhanced analgesia induced by analgesic dose (AD) fentanyl, while Gα1 AS-ODN markedly reduced AD morphine analgesia. Hyperalgesic priming, assessed by prolongation of prostaglandin E-induced hyperalgesia, was not produced by systemic sub-AD and AD fentanyl in Gα3 and Gα AS-ODN-treated rats, respectively. In contrast, none of the Gα AS-ODNs tested affected priming induced by systemic sub-AD and AD morphine. We conclude that signaling by different Gα subunits is necessary for the analgesia and side effects of two of the most clinically used opioid analgesics. The design of opioid analgesics that demonstrate selectivity for individual Gα may produce a more limited range of side effects and enhanced analgesia. Biased μ-opioid receptor (MOR) agonists that preferentially signal through G-protein α-subunits over β-arrestins have been developed as an approach to mitigate opioid side effects. However, we recently demonstrated that biased MOR agonists also produce hyperalgesia and priming. We show that oligodeoxynucleotide antisense to different Gα subunits play a role in hyperalgesia and analgesia induced by subanalgesic and analgesic dose (respectively), of fentanyl and morphine, as well as in priming. Our findings have the potential to advance our understanding of the mechanisms involved in adverse effects of opioid analgesics that could assist in the development of novel analgesics, preferentially targeting specific G-protein α-subunits.