Mitogen activated protein kinase (MAPK) transduces a broad range of extracellular stimuli into diverse intracellular response by producing changes in transcription and post-transcriptional modification of target proteins. Expression of phospho-ERK (pERK) in neurons of dorsal root ganglion (DRG) and superficial dorsal horn can be induced by noxious stimuli on the skin and activation of ERK in spinal neurons may contribute to hyperalgesia. In this study, the author pursued altered expression of pERK in spinal neurons and microglia with time course in the neuropathic pain model of the rat induced by L5-L6 spinal neurotomy.

Rats were divided into sham control and neurotomy groups. In neurotomy group, L5 and L6 spinal nerves were cut 1-2 mm distal to intervertebral foramina and sacrificed at 1day, 1 week and 2 week after surgery. The expression of pERK in dorsal horn and its colocalization with NK1 and OX-42, was studied using single or multiple labeling immunohistochemistry and confocal microscopy.

At 1 day after neurotomy, pERK positive cells in ipsilateral dorsal horn were NK1-positive neurons in lamina I and OX-42-positive microglia in lamina I to V. Expression of pERK in NK1-positive neurons were sustained until 2 weeks after neurotomy. The number of pERK- and OX2-positive microglia were maximal at 1 day after surgery and decreased thereafter, while immunofluorescent reaction (IR) for OX-2 kept increased until 2 weeks.

In conclusion, it is suggested that expression of pERK in microglia play one part of roles in the development of neuropathic pain in early phase, and that activation of pERK in NK1-positive neurons is responsible for sustained hyperalgesia in the neuropathic pain.