Biasing G protein βγ subunit downstream signaling enhances the analgesic effects of endogenous opioid receptor agonists during nitroglycerin-induced thermal hypersensitivity.

μ-Opioid receptor (MOR) agonists are a mainstay in acute pain management. However, they also produce adverse effects and are frequently misused, increasing susceptibility for opioid use disorder. Thus, a strategy for improving the safety of opioid analgesics is needed. Gallein-mediated inhibition of Gβγ signaling to a subset of inhibitory effector feedback systems, such as phospholipase C (PLC) β3 and G protein-coupled receptor kinase 2, potentiates the antinociceptive effects of morphine without altering its rewarding effects in vivo. In this study, we examined effectiveness of gallein in the context of persistent pain using a nitroglycerin (NTG)-induced thermal hypersensitivity assay. In the warm water tail withdrawal assay, NTG (10 mg/kg, i.p.) decreased tail withdrawal latencies by 90%, from 30 to 3 seconds, indicating a hyperalgesic state. Gallein alone fully reversed NTG-induced decreases in withdrawal latencies. Furthermore, a low dose of gallein (3.2 mg/kg, i.p.) that was ineffective alone was sufficient to potentiate the antihyperalgesic effects of morphine. Pretreatment with the nonselective opioid antagonist naloxone (1.0 mg/kg, i.p.) attenuated both the antihyperalgesic effects of gallein alone and gallein-mediated potentiation of morphine. NTG did not decrease tail withdrawal latencies in PLCβ3