Blockade of selective Gβγ signaling pathway by gallein decreases development of opioid tolerance

Abstract

<b>Abstract ID 18253</b> <b>Poster Board 406</b> The μ-opioid receptor (MOR) is a G_i-protein-coupled receptor (GPCR) responsible for opioid-induced analgesia and undesired effects such as constipation, respiratory depression, and addiction. Upon binding of an agonist to MOR, intracellular Ga_i and the Gβγ subunits dissociate to signal, and the signaling cascade that follows Gβγ activation by MOR is necessary for opioid-mediated antinociception. However, phospholipase-Cβ3 (PLCβ3)—one effector of Gβγ and Gaq —has been shown to limit the antinociceptive effects of morphine since genetic deletion of PLCβ3 potentiates the antinociceptive effects of morphine in mice. Gallein is a small-molecule that binds to Gβγ, and it prevents protein-protein interactions between Gβγ and select effectors such as PLCβ and GRK2. We hypothesize that preventing Gβγ-PLC interactions potentiates antinociceptive effects of opioids and decreases the development of morphine tolerance. The warm water tail withdrawal (WWTW) assay is used to measure antinociceptive response in mice by timing the latency of tail withdrawal from 55C water. In these studies, we used a paradigm of repeated administration of morphine or saline (3 times daily) for 6 days to induce morphine tolerance. After repeated morphine administration, the development of opioid tolerance is determined by a rightward shift in the morphine dose-response on day 6 compared to day 1. Repeated administration of 10 mg/kg morphine produced a 5x-fold rightward shift in ED50 values comparing day 1 and day 6. Gallein (50mg/kg) treatment on day 1 and day 3 of repeated morphine administration prevented the rightward shift in the morphine dose-response curve on day 6. We also tested gallein’s ability to potentiate morphine antinociception in a physiological state of tolerance. Gallein (100mg/kg) administered 15hrs prior to day 6 assay shifted the morphine dose-response curve to the left, increasing the potency and efficacy of morphine. These data suggest that gallein treatment prevents the development of opioid tolerance, and that after development of tolerance, gallein can enhance opioid-mediated antinociception. Overall, these studies show that inhibition of Gβγ-PLC pathway may have therapeutic potential to decrease the development of opioid tolerance and/or provide opioid-sparing effects. Support/Funding Information: DA 0418625