Protein kinase C gamma-mediated phosphorylation of GluA1 in the postsynaptic density of spinal dorsal horn neurons accompanies neuropathic pain, and dephosphorylation by calcineurin is associated with prolonged analgesia.

Loss of calcineurin (protein phosphatase 3) activity and protein content in the postsynaptic density (PSD) of spinal dorsal horn neurons was associated with pain behavior after chronic constriction injury (CCI) of the rat sciatic nerve, and intrathecal administration of the phosphatase provided prolonged analgesia (Miletic et al. 2013). In this study, we examined whether one consequence of the loss of calcineurin was the persistent phosphorylation of the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropioinic acid (AMPAR) receptors in the PSD. This would allow continual activation of AMPAR receptors at the synapse to help maintain a long-lasting enhancement of synaptic function, ie, neuropathic pain. We also investigated if the phosphorylation was mediated by protein kinase A (PKA), protein kinase C gamma (PKCγ), or calcium-calmodulin dependent kinase II (CaMKII), and if the prolonged calcineurin analgesia was associated with GluA1 dephosphorylation. Mechanical thresholds and thermal latencies were obtained before CCI. Seven days later, the behavioral testing was repeated before saline, calcineurin, or the specific peptide inhibitors of PKA (PKI-tide), PKCγ (PKC 19-31), or CaMKII (autocamtide-2-related inhibitory peptide) were injected intrathecally. The behavior was retested before the animals were euthanized and their PSD isolated. All CCI animals developed mechanical and thermal hypersensitivity. This was associated with phosphorylation of GluA1 in the ipsilateral PSD at Ser831 (but not Ser845) by PKCγ and not by PKA or CaMKII. Intrathecal treatment with calcineurin provided prolonged analgesia, and this was accompanied by GluA1 dephosphorylation. Therapy with calcineurin may prove useful in the prolonged clinical management of well-established neuropathic pain.