Effect Of Electroacupuncture On Activation Of P38MAPK In The Central Descending Pathway In Rats With Inflammatory Pain

Pain is an unpleasant feeling associating with many diseases and difficult to solve. Management of pain has become a very thorny work in veterinary medicine, which includes pharmacologic treatment and nonpharmacologic modality. Sometimes pharmacological treatment is unsatisfactory with side effects. The nonpharmacologic treatments are worth popularizing with the advantages of convenience, safe, reliable, less damage, less side effects and prominent effect. Electroacupuncture(EA) is relatively simple to operate and its parameters can be easily contolled, so it is more appropriate for clinical application. Electrocupuncture analgesia involves a variety of neural and hormonal factors in peripheral and central nervous system. Researching the central modulating mechanism of electroacupuncture analgesia is becoming a hot topic. Accumulating evidence shows that EA can modulate central descending pathway to release neurotransmitters(serotonin, norepinephrine and glutamate) or neuromodulators(opioids and CCK8) to suppress nociception. These neurotransmitters or neuromodulators rely on cellular signal transduction to modulate pain. The influence of EA on signal molecules and signal pathways has been relatively less investigated. There is little concern and studying of EA on signal transduction in centralnervous system cells. Studies show that activation of MAPK pathways contribute to pain sensitization after tissue and nerve injury, and p38 MAPK activation is in early phase. Inhibition of MAPK pathways has been shown to attenuate inflammatory and neuropathic pain in different animal models. However, it is not clear whether EA involves dampening p38 MAPK activation in neural descending inhibitory pathway. Present study applied complete Freund’s adjuvant(CFA) inflammatory pain model to explore the effect of EA on p38 MAPK activation in periaqueductal gray matter(PAG), rostral ventromedial medulla(RVM) and spinal cord(SCDH), and to probe into the central mechanism of EA on inflammatory pain.In this study, one hundred and forty-four SD rats(weighing 200±20g) were chosen for research. The rats were randomly divided into four groups: Saline group, CFA group, CFA+EA group and CFA + sham group. Each group had 36 rats. Saline group rats were injected 0.1m L saline into left hind paw, while rats in the other three groups were injected 0.1m L CFA. EA was applied to rats in the CFA+EA group bilateral “Zusanli” and “Kunlun” acupoints for 30 min at 30 min, 24.5h, 48.5 and 72.5h after CFA injection. CFA + sham group rats were received the same needle insertion into “Zusanli” and “Kunlun”, but without electrical stimulation. To eliminate the stress effect, rats in Saline group and CFA group were immobilized same as CFA+EA group. Paw withdrawl threshold(PWTs) and paw volume of rats were measured at 1h, 3h, 5h, 7h, 25 h and 73 h after CFA injeciton. And 6 rats were chosen to take samples at every time point mentioned above. The expression of phosphorylated p38MAPK(p-p38MAPK) was checked in relevant area(PAG, RVM and SCDH) by immunohistochemistry.The results shows that PWTs of rats in CFA group were obviously lower than saline group at 1h~73h(p<0.001). PWTs of rats in CFA+EA group increased significantly as compared with CFA and CFA +sham group(p<0.001) at 1h, 3h, 25 h and73h after CFA injection, which means EA significantly alleviates CFA-induced mechanical hyperalgesia not only in transient but also accumulating effect. The paw volume in CFA group and CFA+sham gourp were higher than that in Saline group at all timepoints(p<0.001). The paw edema in CFA+EA group markedly decreased at 5h~73h as compared with CFA group(p<0.01), and significantly lower than CFA+sham group at 3h~73h(p<0.05), which infers the anti-inflammatory effect of EA. CFA injection induced increase of p-p38 MAPK in PAG and RVM at 1h~5h(p<0.001) as compared with Saline group. In CFA+EA group, the numbers of p-p38 MAPK cells in PAG and RVM were markedly decreased at 3h~5h(p<0.001). The number of p-p38 MAPK cells in SCDH was significantly increased than Saline group after CFA injection(p<0.001). In CFA+EA group, the number of p-p38 MAPK cells was smaller than CFA+sham group(p<0.05), and have significant differences with CFA group and CFA+sham group at 5h~73h. In CFA+EA group, p-p38 MAPK expression has no difference as compared with Saline group at 5h(p>0.05). The results of immunohistochemistry present that EA decreases the p38 MAPK activation in PAG, RVM and SCDH.Present study applied CFA inflammatory pain model to investigate the effect of EA on PWTs, paw volume and p38 MAPK activation in PAG-RVM-SCDH system. We can conclude that EA exhibits anti- hyperalgesia and anti-inflammatory effect by inhibiting p38 MAPK activation in neural descending inhibitory pathway. This is beneficial to uncover the neural mechanism of EA analgesia, and promote the neuroscience development as well as application in veterinary medicine.