The nuclear kinases mitogen- and stress-activated kinase 1 and 2 (MSK1 and MSK2), through regulating transcriptional processes, are pivotal for various adaptive responses, including inflammation, learning and addiction. Transcriptional alterations in neurons and glia cells within the pain signal-processing (nociceptive) pathway, including the superficial spinal dorsal horn (SSDH), are critical for the development and persistence of inflammatory pain that results from tissue injuries and subsequent inflammatory reactions. While previous reports have indicated that MSK1 contributes to transcriptional changes in inflamed tissues, the impact of MSK1 on nociceptive processing in the SSDH are poorly understood at present. Here, we report MSK1 immunoreactivity (IR) in a group of excitatory and inhibitory neurons as well as in microglia and oligodendrocytes in the SSDH. Injecting Complete Freund’s Adjuvant into the mouse hind paw produced robust non-evoked pain-related behavior, which was significantly attenuated by global depletion of MSK1. In wild-type mice, the inflammatory pain was accompanied by transient MSK1-dependent phosphorylation of the MSK1 downstream effector histone 3 at serine 10 at one hour but not two days after the injection; still, the number of nuclei exhibiting activated MSK1 expression remained highly restricted even at 1 h post-injection. Our data indicate that MSK1 contributes to inflammatory pain via epigenetic and transcriptional alterations; however, once initiated, MSK1’s downstream effects do not require further drive from the persistent activity of the MSK1 signaling pathway in the SSDH.

Introduction: The nuclear mitogen- and stress-activated kinases (MSKs) play a critical role in the development and persistence of pain after tissue injury. Objectives: Here, we ascertained the MSK isoform, the cells and mechanisms, which mediate MSKs' pronociceptive function. Methods: Nocifensive behaviour evoked by subcutaneous formalin injection into the paw was quantified in wild type (WT), MSK1 and MSK2 global knock out (MSK1^−/− and MSK2^−/−) mice, and a month after injecting adeno-associated viral vector carrying short-hairpin (sh) RNA directed towards the MSK1-encoding gene Rps6ka5 mRNA or scrambled shRNA into the sciatic nerve of WT mice. Rps6ka5 expression in nociceptors was ascertained by analysing publicly available single cell and single nucleus RNA sequencing datasets on primary sensory neurons and reverse transcription polymerase chain reaction on dorsal root ganglia (DRG). Mitogen- and stress-activated kinase 1 expression was verified by immunofluorescent staining on DRG sections. Results: MSK1^−/− but not MSK2^−/− mice exhibited significantly attenuated evoked nocifensive behaviour specifically in the second but not the first phase of the formalin test. Downregulating Rps6ka5 in nociceptors by the viral vector tool attenuated formalin-induced pain behaviour to the same extent as observed in MSK1^−/− animals. Rps6ka5 expression was found in DRG and various transcriptionally defined groups of nociceptive primary sensory neurons (nociceptors). Immunofluorescence confirmed the presence of MSK1 predominantly in peptidergic nociceptors. Conclusion: MSK1 constitutes the principal MSK isoform, which is critically important for regulating cellular components that enable the transient activation of a specific subpopulation of nociceptors by formalin.