Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) abounds in the central nervous system and represents an important component of the postsynaptic density. Recently, evidence points to a key role of CaMKII in nociceptive transmission. CaMKII alpha, a major isoform, is an essential mediator of activity-dependent synaptic plasticity. CaMKII alpha is preferentially localized in pain-processing regions in the CNS. Current evidence shows that CaMKII is upregulated in the superficial laminae of the dorsal horn after inflammation. Furthermore, mutant mice exhibiting autophosphorylation targeted at threonine 286 have been used to demonstrate the importance of the autonomous phosphorylation activity and Ca²⁺/calmodulin-binding capacity of CaMKII alpha not only in the induction of long-term potentiation (LTP) but also in persistent neuropathic pain. However, kinase activity, a critical enzymatic role of CaMKII alpha, has not been fully elucidated yet.
In the present study, to investigate the localization of CaMKII in the superficial laminae of the dorsal horn, we transected the L5 spinal nerve in the spinal cord of wild type and kinase-dead knock-in mice, which effectively caused neuropathic pain. One week after surgery, the spinal cord of mice was fixed for pre-embedding immuno electron microscopy with gold enhancement. Gold particles labeling CaMKII or its phosphorylated form in the laminae II-III were counted. In wild type mice, the number of gold particles for CaMKII and the phosphorylated form of CaMKII were upregulated in the spinal cord, and preferentially increased near the postsynaptic membrane, respectively. In contrast, neither upregulation nor translocation occurred in the kinase-dead knock-in mouse. These results show that phosphorylation of CaMKII occurs during maintenance of neuropathic pain and that the phosphorylated CaMKII is recruited and localized at the synaptic sites, depending on the kinase activity of CaMKII. The regulation of CaMKII activity or CaMKII-mediated signaling may be an innovative therapeutic target for the treatment of chronic pain.