Coincident Regulation of PLC β 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 -opioid receptor (MOR). Deletion of phospholipase C3 (PLC3) or selective inhibition of G regulation of PLC3 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 PLC3 or pharmacological inhibition of its upstream regulators, G or Gq, ex vivo in periaqueductal gray slices increased the potency of the selective MOR agonist [D-Ala, N-Me-Phe, Gly-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-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 C3 (PLCβ3) in mice potentiates -opioid receptor (MOR)-dependent antinociception. How PLCβ3 is regulated downstream of MOR had not been clearly defined. We show that PLC-dependent diacylglycerol generation is cooperatively regulated by MOR-Gβγ and Gq-coupled receptor signaling through PLCβ3 and that blockade of either Gq-signaling or Gβγ 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.