A descending glycinergic circuit drives opioid-resistant mechanical pain via spinal GluN3A excitatory glycine receptors in mice.

Mechanical pain that resists even systemic opioids is a major clinical challenge, yet its underlying mechanisms remain poorly understood. Here, we identify a glycinergic descending circuit from GlyT2-positive neurons in the rostral ventromedial medulla (RVMGlyT2+) to somatostatin-positive neurons in the spinal dorsal horn (SDHSOM+). This RVMGlyT2+→SDHSOM+ circuit is both necessary and sufficient for widespread morphine-resistant mechanical allodynia in mice. Craniofacial pain recruits this circuit through trigeminal and limbic inputs. Mechanistically, glycine from RVMGlyT2+ terminals activates GluN3A-containing NMDA receptors on SDHSOM+ neurons, where they function as unconventional excitatory glycine receptors. This "Glycine-GluN3A" signaling bypasses opioid suppression, unmasks latent mechanical allodynia, and is required for the opioid-resistant pain state. Our findings uncover an unexpected excitatory glycinergic mechanism in the spinal cord, define a dedicated circuit for morphine-resistant pain, and highlight GluN3A-containing NMDARs as potential therapeutic targets for intractable pain.