Gαq signaling in primary sensory neurons shifts opioid analgesia to NMDA receptor-driven tolerance and hyperalgesia.

Opioids relieve pain by activating μ-opioid receptors (MORs), which inhibit communication between pain-sensing neurons (nociceptors) and the spinal cord. However, prolonged opioid use can paradoxically lead to increased pain sensitivity (hyperalgesia) and reduced analgesic efficacy (tolerance), partly because of the activation of NMDA-type glutamate receptors (NMDARs) at the central terminals of primary sensory neurons in the spinal cord. Here, we identified a critical role for the G protein Gαq in this paradox. Pharmacological inhibition of Gαq in rats reversed morphine-induced increases in NMDAR phosphorylation, synaptic trafficking, and activity at sensory neuron terminals and reduced morphine-induced excitatory nociceptive input to spinal dorsal horn neurons. Morphine enhanced Gαq coupling specifically to metabotropic glutamate receptor 5 (mGluR5) dimers in the spinal cord. Furthermore, targeted knockdown of Gαq in dorsal root ganglion neurons in mice normalized NMDAR-related changes and prevented NMDAR-mediated synaptic potentiation triggered by MOR activation. In addition, either pharmacological or genetic disruption of Gαq signaling enhanced morphine's analgesic effects while reducing hyperalgesia and tolerance. These findings reveal that Gαq signaling contributes to opioid-induced NMDAR hyperactivity at nociceptor central terminals by promoting MOR-mGluR5 cross-talk. Targeting this pathway may improve the safety and efficacy of opioid-based pain management.