Investigating Mu-Opioid Receptor Selectivity of Potentiation of Antinociceptive Effects by a Gβγ Inhibitor, Gallein

Abstract ID 99657 Poster Board 311 Chronic pain imposes a formidable cost on society – in terms of lost economic productivity, healthcare costs, and with respect to individual well-being. Mu-opioid receptor (MOR) agonist-induced activation of MORs leads to the dissociation of the Gi/o protein heterotrimers into G αi/o and Gβγ subunits. Gβγ engages with several distinct effectors through unique binding interactions. Of particular importance among Gβγ effectors is PLCβ3 which has been shown to negatively regulate MORs. In fact, inhibition of PLCβ3 activation by Gβγ with the small molecule, gallein, and PLCβ3 knockout potentiates morphine-induced antinociception. Thus, Gβγ inhibition may be a promising therapeutic strategy for improving the therapeutic index of MOR agonists. However, it is unknown whether the effects of gallein to potentiate antinociception are specific to opioid receptors. I sought to investigate this question by evaluating the ability of gallein to potentiate morphine-induced antinociception in MOR knockout mice or following naloxone pretreatment. To determine whether the effects of gallein to potentiate MOR-mediated antinociception are specific to morphine, I also examined how gallein pretreatment affected buprenorphine-, nalbuphine-, and fentanyl-induced antinociception. Furthermore, considering the importance of Gi/o-coupled GPCRs in mediating Gβγ effects, I sought to evaluate the ability of gallein to potentiate antinociception induced by the α2-adrenergic receptor agonist clonidine. Antinociception was measured in female and male C57BL/6 mice using the warm water tail withdrawal assay (WWTW, 55°C). Intraperitoneal (i.p.) gallein pretreatments were administered 24 hrs prior to and intracerebroventricular gallein pretreatments were administered 30 min prior to buprenorphine (0.32 mg/kg), nalbuphine (32 mg/kg), fentanyl (0.1 mg/kg), or morphine (3.2 mg/kg). Enhancements in morphine-induced antinociception by gallein were absent in MOR knockout mice and were blocked by naloxone pretreatment. Additionally, gallein potentiated antinociception induced by MOR agonists other than morphine. Last, no potentiation of clonidine-induced antinociception by gallein was observed. These data suggest that the ability of gallein to potentiate morphine-induced antinociception is MOR-dependent. Also, the effects of gallein to potentiate MOR-mediated antinociception are not specific to morphine. While the inability of gallein to potentiate clonidine-induced antinociception may indicate a selective effect for gallein at MORs, future work should continue to investigate other analgesic agents.