Microglia-neuron signalling in the spinal cord is a key mediator of mechanical allodynia caused by peripheral nerve injury. We recently reported sex differences in microglia in pain signalling in mice: spinal mechanisms underlying nerve injury-induced allodynia are microglial dependent in male but not female mice. Whether this sex difference in pain hypersensitivity mechanisms is conserved in other species is unknown. Here, we show that in rats, the spinal mechanisms of nerve injury-induced hypersensitivity in males differ from those in females, with microglial P2X4 receptors (P2X4Rs) being a key point of divergence. In rats, nerve injury produced comparable allodynia and reactive microgliosis in both sexes. However, inhibiting microglia in the spinal cord reversed allodynia in male rats but not female rats. In addition, pharmacological blockade of P2X4Rs, by an intrathecally administered antagonist, attenuated pain hypersensitivity in male rats only. Consistent with the behavioural findings, nerve injury increased cell surface expression and function of P2X4Rs in acutely isolated spinal microglia from male rats but not from female rats. Moreover, in microglia cultured from male rats, but not in those from female rats, stimulating P2X4Rs drove intracellular signalling through p38 mitogen-activated protein kinase. Furthermore, chromatin immunoprecipitation-qPCR revealed that the transcription factor IRF5 differentially binds to the P2rx4 promoter region in female rats vs male rats. Finally, mechanical allodynia was produced in otherwise naive rats by intrathecally administering P2X4R-stimulated microglia from male rats but not those from female rats. Together, our findings demonstrate the existence of sexually dimorphic pain signalling in rats, suggesting that this sex difference is evolutionarily conserved, at least across rodent species.

Background and Purpose: Chronic pain is a devastating problem affecting 1 in 5 individuals around the globe, with neuropathic pain the most debilitating and poorly treated type of chronic pain. Advances in transcriptomics and data mining have contributed to cataloging diverse cellular pathways and transcriptomic alterations in response to peripheral nerve injury but have focused on phenomenology and classifying transcriptomic responses. Experimental approach: Here, with the goal of identifying new types of pain-relieving agents, we compared transcriptional reprogramming changes in the dorsal spinal cord after peripheral nerve injury cross-sex and cross-species and imputed commonalities, as well as differences in cellular pathways and gene regulation. Key Results: We identified 93 transcripts in the dorsal horn that were increased by peripheral nerve injury in male and female mice and rats. Following gene ontology and transcription factor analyses, we constructed a pain interactome for the proteins encoded by the differentially expressed genes, discovering new, conserved signaling nodes. We interrogated the interactome with the Drug-Gene database to predict FDA-approved medications that may modulate key nodes within the network. The top hit from the analysis was fostamatinib, the molecular target of which is the non-receptor tyrosine kinase Syk, which our analysis had identified as a key node in the interactome. Conclusions & Implications : We found that intrathecally administrating the active metabolite of fostamatinib, R406, significantly reversed pain hypersensitivity in both sexes. Thus, we have identified and shown the efficacy of an agent that could not have been previously predicted to have analgesic properties.

Background:The beneficial treatment effect surgery demonstrates over conservative care for radiculopathy secondary to acute lumbar disc herniation (LDH), occurs in the first 3 to 6 months; thereafter outcomes are recognized to be similar. This is not surprising given the favourable natural history; 90% will experience gradual resolution of their symptoms within 4 months. In Canada, owing to the inherent wait time to see a surgeon and the referring physician's expectation that most patients will improve without surgery, symptomatic patients presenting to surgeons are often the 10% that have remained symptomatic longer than the expected 4 months. The purpose is to determine whether surgery is superior to conservative care in a patient population that has had persistent symptoms for more than 4 months, and therefore create a study population which is generalizable to the Canadian health care experience. Methods: This single blinded (assessor) RCT enrolled 18-to 60-year-old patients with a unilateral, single radiculopathy from a posterolateral L4-5 or L5-S1 disc herniation. Radiculopathy duration was longer than 4 months but less than 12 months. Patients on a waiting list to see surgeons at 1 academic hospital centre were randomized to early microdiscectomy or standardized nonoperative care, including medications, education, physiotherapy and steroid injections. Patients were excluded if they had previously received these conservative modalities. The primary outcome was intensity of sciatica (scale 0-10) measured at 6 months following randomization. Secondary outcome measures included back pain, Oswestry Disability Index (ODI), SF-36, work status and satisfaction. Results: This interim analysis reports on 40 nonoperative and 39 surgical patients. No difference existed between their demographic or preoperative data. At 6 months follow-up 32 of 39 surgical patients and 36 of 40 nonoperative patients had data available. Treatment effect for all outcome measures favoured surgery for the intent-to-treat, as-treat and last-value carried forward analysis (p < 0.05). To date 13 of 40 nonoperative patients have undergone microdiscectomy (performed after the primary outcome measure of 6 mo); they have had persistent inferior scores than early surgical patients (p < 0.05). Conclusion: At the interim analysis microdiscectomy is superior to nonoperative care for patients presenting with sciatica secondary to LDH. This study will continue to confirm robustness and validity of results.