Discussion
In the present study, we found a decreased pain threshold with the progression of OA, increased levels of SP and IL-6 in the serum of OA rats, activated M1-type microglia in the spinal cord, and increased expression of pain neurotransmitters. We also demonstrated that HIIT remarkably down-regulated pain neurotransmitters and proinflammatory cytokines in OA rats, driven by the M2-type transformation of microglia. Moreover, the Jak2/Stat3 pathway plays a critical role in microgliosis and pain. C-A1 can overexpress Jak2 and Stat3, and in rats treated with OA+C-A1+HIIT, the downregulation of pain neurotransmitters was significantly less compared to rats treated with OA+HIIT alone. Therefore, HIIT promotes the phenotypic transformation of microglia through the Jak2/Stat3 signaling pathway and mitigates inflammatory responses.
Microglia activation plays a key role in the development of pain. At the early stage of injury, microglia usually differentiate into M1-type and release proinflammatory cytokines. While M2-type microglia can inhibit inflammation and increase antiinflammatory mediators, promoting tissue repair and regulating the neuronal release of pain-causing substances [28-30]. In this study, we found that there was an M1-type of microglia in the CNS of OA rats and that HIIT promoted the pheno-typic transformations of microglia, thereby alleviating the OA-induced neuroinflammatory response and suppressing pain. These findings suggest that HIIT exerts a pain-relieving effect by regulating the pheno-typic transformations of microglia and balancing anti-inflammatory and pro-inflammatory cytokines.
We found the differential proteins, such as Stat1, Stat2, Stat5b, Stat6, Sos1, and Pdgfrb, through the PPI method in OA and OA+HIIT rats, and most genes were related to the Stat family. Then we analyzed the top 20 pathways between the two groups by KEGG enrichment, among which the pathway Jak2/Stat3 was screened. Previous studies have shown that microglial activation was affected by multiple pathways, and activation of the Jak2/Stat3 pathway may be the main pathway leading to microglial transformation [31, 32]. We found that HIIT can reduce the expression of Jak2 and Stat3. To investigate that HIIT relieves chronic pain by driving the pheno-typic transformations of microglia via the Jak2/Stat3 pathway in OA rats. We overexpressed the Jak/Stat pathway by intraperitoneal injection of C-A1 and then provided HIIT intervention. Our results suggest that HIIT promoted the transformation of microglia from the M1 to the M2 type and that the expression levels of inflammatory factors and pain neurotransmitters were down-regulated. The pain neurotransmitters in OA+C-A1+HIIT rats showed a significant decrease compared to those in OA+HIIT rats. Although C-A1 might exacerbate the inflammatory response in the short term, HIIT could potentially counteract these effects over a longer period or with increased exercise intensity, thereby yielding significant anti-inflammatory outcomes. Taken together, HIIT promotes the pheno-typic transformations of microglia through the Jak2/Stat3 signalling pathway and affects the release of pain-causing neurotransmitters and proinflammatory cytokines.
A major limitation of this study is the lack of effects of proinflammatory cytokines in the joint synovium of OA rats on pain. Pain caused by OA is mediated by a variety of factors, including changes in joint structure [33], secretion of proinflammatory cytokines in articular cartilage, synovium, and muscles [34, 35], and release of neurotransmitters from peripheral and central neurons [36], microglia phenotype transformation, and ion channel changes [37]. In the future study, we will continue to explore the effects of HIIT on the muscle, synovial membrane and synovial fluid of OA rats and the underlying mechanisms. Furthermore, male rats were chosen for this study due to their greater behavioral and physiological stability compared to female rats, as well as their lesser vulnerability to hormonal fluctuations. As a simple and non-invasive treatment method, HIIT can not only reduce levels of inflammatory cytokines and pain neurotransmitters in the CNS, but also improve muscle strength and further delay the degeneration of articular cartilage [38, 39]. Compared with regular exercise, HIIT has the highest level of exercise efficiency and exercise performance [40], providing new insights into the molecular mechanisms and diagnosis and treatment for OA. It is worth noting that weight-bearing running is not suitable for patients with severe deformities of OA. Therefore, we could prescribe HIIT exercise on a anti-gravity treadmill or power bicycle in future clinical trials.
In conclusion, our findings provide substantial evidence that HIIT improves OA-associated inflammation by regulating the phenotypic transformations of microglia via the Jak2/Stat3 signaling pathway.