A Glra3 phosphodeficient mouse mutant establishes the critical role of protein kinase A-dependent phosphorylation and inhibition of glycine receptors in spinal inflammatory hyperalgesia

Glycinergic neurons and glycine receptors (GlyRs) exert a critical control over spinal nociception. Prostaglandin E-2 (PGE(2)), a key inflammatory mediator produced in the spinal cord in response to peripheral inflammation, inhibits a certain subtype of GlyRs (alpha 3GlyR) that is defined by the inclusion of alpha 3 subunits and distinctly expressed in the lamina II of the spinal dorsal horn, ie, at the site where most nociceptive nerve fibers terminate. Previous work has shown that the hyperalgesic effect of spinal PGE(2) is lost in mice lacking alpha 3GlyRs and suggested that this phenotype results from the prevention of PGE(2)-evoked protein kinase A (PKA)-dependent phosphorylation and inhibition of alpha 3GlyRs. However, direct proof for a contribution of this phosphorylation event to inflammatory hyperalgesia was still lacking. To address this knowledge gap, a phospho-deficient mouse line was generated that carries a serine to alanine point mutation at a strong consensus site for PKA-dependent phosphorylation in the long intracellular loop of the GlyR alpha 3 subunit. These mice showed unaltered spinal expression of GlyR alpha 3 subunits. In behavioral experiments, they showed no alterations in baseline nociception, but were protected from the hyperalgesic effects of intrathecally injected PGE(2) and exhibited markedly reduced inflammatory hyperalgesia. These behavioral phenotypes closely recapitulate those found previously in GlyR alpha 3-deficient mice. Our results thus firmly establish the crucial role of PKA-dependent phosphorylation of alpha 3GlyRs in inflammatory hyperalgesia.