Coincident Regulation of PLCβ Signaling by Gq-Coupled and μ-Opioid Receptors Opposes Opioid-Mediated Antinociception

Pain management is an important problem worldwide. The current frontline approach for pain management is the use of opioid analgesics. The primary analgesic target of opioids is the mu-opioid receptor (MOR). Deletion of phospholipase C beta 3 (PLC beta 3) or selective inhibition of G beta gamma regulation of PLC beta 3 enhances the potency of the antinociceptive effects of morphine suggesting a novel strategy for achieving opioid-sparing effects. Here we investigated a potential mechanism for regulation of PLC signaling downstream of MOR in human embryonic kidney 293 cells and found that MOR alone could not stimulate PLC but rather required a coincident signal from a Gq-coupled receptor. Knockout of PLC beta 3 or pharmacological inhibition of its upstream regulators, G beta gamma or Gq, ex vivo in periaqueductal gray slices increased the potency of the selective MOR agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin acetate salt in inhibiting presynaptic GABA release. Finally, inhibition of Gq- G protein-coupled receptor coupling in mice enhanced the antinociceptive effects of morphine. These data support a model where Gq and G beta gamma-dependent signaling cooperatively regulate PLC activation to decrease MOR-dependent antinociceptive potency. Ultimately, this could lead to identification of new non-MOR targets that would allow for lower-dose utilization of opioid analgesics. SIGNIFICANCE STATEMENT Previous work demonstrated that deletion of phospholipase C beta 3 (PLC beta 3) in mice potentiates mu-opioid receptor (MOR)-dependent antinociception. How PLC beta 3 is regulated downstream of MOR had not been clearly defined. We show that PLC-dependent diacylglycerol generation is cooperatively regulated by MOR-G beta gamma and Gq-coupled receptor signaling through PLC beta 3 and that blockade of either Gq-signaling or G beta gamma signaling enhances the potency of opioids in ex vivo brain slices and in vivo. These results reveal potential novel strategies for improving opioid analgesic potency and safety.