While opioids produce both analgesia and side-effects by action at mu-opioid receptors (MOR), at spinal and supraspinal sites, potency of different opioids to produce these effects vary. While it has been suggested that these differences might be due to bias for signaling via β-arrestin versus G protein alpha (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 Gαi/o 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 Gαi2, Gαi3 and Gαo markedly attenuated hyperalgesia induced by sub-analgesic dose (sub-AD) fentanyl, while AS-ODN for Gαi1, as well as Gαi2 and Gαi3, but not Gαo, prevented hyperalgesia induced by sub-AD morphine. AS-ODN for Gαi1 and Gαi2 unexpectedly enhanced analgesia induced by analgesic dose (AD) fentanyl, while ...