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.