mGluR5/ERK signaling regulated the phosphorylation and function of glycine receptor α1(ins) subunit in spinal dorsal horn of mice

Inhibitory glycinergic transmission in adult spinal cord is primarily mediated by glycine receptors (GlyRs) containing the α1 subunit. Here, we found that α1(ins), a longer α1 variant with 8 amino acids inserted into the intracellular large loop (IL) between transmembrane (TM)3 and TM4 domains, was expressed in the dorsal horn of the spinal cord, distributed at inhibitory synapses, and engaged in negative control over nociceptive signal transduction. Activation of metabotropic glutamate receptor 5 (mGluR5) specifically suppressed α1(ins)-mediated glycinergic transmission and evoked pain sensitization. Extracellular signal-regulated kinase (ERK) was critical for mGluR5 to inhibit α1(ins). By binding to a D-docking site created by the 8-amino–acid insert within the TM3–TM4 loop of α1(ins), the active ERK catalyzed α1(ins) phosphorylation at Ser380, which favored α1(ins) ubiquitination at Lys379 and led to α1(ins) endocytosis. Disruption of ERK interaction with α1(ins) blocked Ser380 phosphorylation, potentiated glycinergic synaptic currents, and alleviated inflammatory and neuropathic pain. These data thus unraveled a novel, to our knowledge, mechanism for the activity-dependent regulation of glycinergic neurotransmission.