In comparison with WT rats, hSOD1
G93A animals showed enhanced thermal hypersensitivity following PSNL, while the increase in mechanical hypersensitivity was only mild and did not reach statistical significance. This discrepancy may be explained by a biased floor effect in the assessment of mechanical sensitivity, as nerve injury induced-PWTs are typically low in WT animals. Alternatively, a differential mechanism may underlie thermal as opposed to mechanical hypersensitivity. It is indeed proposed that mechanical hypersensitivity requires a specific population of unmyelinated fibers [
36] while thermal hypersensitivity is dependent on the expression of a specific protein in nociceptors [
37]. Thermal and mechanical hypersensitivities were moreover shown to be regulated by different pathways in an aquaporin-4 KO animal model [
38] or after pharmacological treatment with ROS scavengers [
10].
While microglial activation is known to contribute to the development of neuropathic pain [
3,
39], it has also been suggested to participate in the maintenance of this hypersensitive condition [
40]. Hence, the increased PSNL induced-hypersensitivity in transgenic rats may reflect the activation state adopted by microglia in the dorsal horn of the spinal cord. Indeed, different activation states of microglia have been documented, suggesting that their response is specialized and dictated by the nature of the stimulus [
41]. Only some of these phenotypic changes have been directly linked to neuropathic pain and have been qualified as "pain-related" [
8]. Hence, it may be proposed that the activation state reached by microglia after PSNL differs between WT and transgenic rats, with distinct consequences on the severity of pain symptoms. Indeed, hSOD1
G93A rats did not only show increased microglial activation, as evidenced with Iba1 gene and protein up-regulation, but also an increased gene expression of TLR4, which was absent in WT ligated rats. Because microglial TLR4 is linked to inflammation and pain [
9], its high expression at late time-points supports the idea that hSOD1
G93A microglia adopt an exacerbated "pain-related" activation state after nerve injury. PSNL was moreover found to induce increased expression of the pro-inflammatory cytokine IL-1β in transgenic, but not in WT animals after 3 weeks. This cytokine was also clearly linked to neuropathic pain [
42,
43] and may therefore participate in the enhanced pain hypersensitivity observed in transgenic rats. Finally, ROS production associated with Nox2 activity was previously shown to be critical for pain hypersensitivity, but also for the production of pro-inflammatory mediators [
10]. Because transgenic rats show enhanced Nox2 gene expression after PSNL as compared to WT ligated rats, we conclude that the increased ROS production together with the exacerbated inflammatory reactions may contribute to the enhanced pain hypersensitivity. However, we cannot exclude the possibility that beside glial cells, inherent changes in dorsal horn sensory neurons due to hSOD1
G93A expression, are to some extent involved in the increased pain effects.
Surprisingly, we did not observe mirror-pain after PSNL. Although PSNL was originally associated with mirror-pain [
29], works from different laboratories frequently led to inconsistent results [
44‐
46]. Interestingly, we observed a modest microglial Iba1 up-regulation in the contralateral dorsal horn of transgenic rats. This observation supports the concept dissociating microglial responses and mirror-pain in the PSNL model [
44].