Reversal of ERK activation in the dorsal horn after decompression in chronic constriction injury

Injury-induced neuropathic pain is related to changes in the central terminals of dorsal root ganglia neurons, i.e., dorsal horn plasticity. We investigated the influences of decompression by removing ligatures producing chronic constriction injury (CCI) in Sprague-Dawley rats at postoperative week (POW) 4, the decompression group; for comparison, all ligatures remained through the experimental period in the CCI group. The effect was evaluated with extracellular signal-regulated kinase (ERK) activation in the dorsal horn, i.e., number of phosphorylated ERK (+) cells in the dorsal horn. At POW 1, the dorsal horn indexes had increased to a similar degree in both groups (2.40 ± 0.58 vs. 2.27 ± 0.36, p = 0.73). At POW 8, thermal hyperalgesia and mechanical allodynia had completely disappeared with a normalization of dorsal horn index (1.17 ± 0.11 vs. 1.02 ± 0.12 at POW 0, p = 0.07) in the decompression group; in contrast, the dorsal horn index remained elevated in the CCI group (2.48 ± 0.30, p < 0.001) with persistent neuropathic pain behaviors at POW 8. This report suggests that ERK activation in the dorsal horn is correlated with neuropathic pain behaviors and its normalization reflects the reversal of neuropathic pain behaviors after decompression.