Background: Previous studies have demonstrated that p38 MAPK signal transduction pathway plays an important role in the development and maintenance of inflammatory pain. Electroacupuncture (EA) can suppress the inflammatory pain. However, the relationship between EA effect and p38 MAPK signal transduction pathway in inflammatory pain remains poorly understood. It is our hypothesis that p38 MAPK/ATF-2/VR-1 and/or p38 MAPK/ATF-2/COX-2 signal transduction pathway should be activated by inflammatory pain in CFA-injected model. Meanwhile, EA may inhibit the activation of p38 MAPK signal transduction pathway. The present study aims to investigate that anti-inflammatory and analgesic effect of EA and its intervention on the p38 MAPK signal transduction pathway in a rat model of inflammatory pain. Results: EA had a pronounced anti-inflammatory and analgesic effect on CFA-induced chronic inflammatory pain in rats. EA could quickly raise CFA-rat's paw withdrawal thresholds (PWTs) and maintain good and long analgesic effect, while it subdued the ankle swelling of CFA rats only at postinjection day 14. EA could down-regulate the protein expressions of p-p38 MAPK and p-ATF-2, reduced the numbers of p-p38 MAPK-IR cells and p-ATF-2-IR cells in spinal dorsal horn in CFA rats, inhibited the expressions of both protein and mRNA of VR-1, but had no effect on the COX-2 mRNA expression. Conclusions: The present study indicates that inhibiting the activation of spinal p38 MAPK/ATF-2/VR-1 pathway may be one of the main mechanisms via central signal transduction pathway in the process of anti-inflammatory pain by EA in CFA rats.

The effect of electroacupuncture (EA) is affected by both the acupuncture point selection and the frequency of stimulation. However, little is known regarding acupuncture point and simulation frequency selection. Neuronal activation of the nucleus of the solitary tract (NTS) is one of the important targets of EA for modulating gastrointestinal function. This study investigated the effects of various combinations of EA frequencies and acupuncture points on NTS neurons. Rats were randomly divided into normal, 2 Hz EA, 100 Hz EA and the alternate 2/100 Hz EA groups. Then rats in each group were randomly divided into the following two subgroups according to the acupuncture point: ST 36 group and ST 25 group. All the rats underwent electrode implantation surgery. Rats in all EA groups received one treatment with EA (a constant square wave at, 2 Hz,100 Hz or 2/100 Hz frequencies with intensities ranging from 1 to 2 mA), and NTS neuronal activation was recorded before and after EA treatment. Finally, to confirm the effect of EA on the NTS, minimal acupuncture was administered and its effect on NTS was detected. ST 36 stimulated with 2 Hz EA significantly increased the population of excited NTS neurons and spike frequency. However, ST 36 stimulated with 100 Hz or 2/100 Hz EA produced only a transient effect on the activity of NTS neurons and did not induce any effect on the spike frequency. Furthermore, the excitatory effect of 100 Hz or 2/100 Hz EA on NTS neurons in the ST 36 group was lower than 2 Hz EA at the same point. When applied to ST 25, 2 Hz EA had no significant excitatory effect on NTS neurons or spike frequency. However, 100 Hz EA or 2/100 Hz EA at ST 25 decreased both NTS neuronal excitability and spike frequency. By comparing the effects of different EA combinations, it was shown 2 Hz EA applied to ST 36 had the strongest excitatory effect on NTS neurons, while 100 Hz EA applied to ST 25 had the greatest inhibitory effect. Minimal acupuncture stimulation produced no effect on NTS neurons. EA’s effects on NTS were mainly affected by the acupuncture point selection, but the frequency of EA also played a role. Different combinations of acupuncture points and frequency selection may lead to different EA effects on NTS neuronal excitability.

Activation of extracellular signal-regulated kinase1/2 (ERK1/2) in dorsal horn of the spinal cord by peripheral inflammation is contributed to inflammatory pain hypersensitivity. Although electroacupuncture (EA) has been widely used to alleviate various kinds of pain, the underlying mechanism of EA analgesia requires further investigation. This study investigated the relationship between EA-induced analgesia and ERK signaling involved in pain hypersensitivity. The rats were randomly divided into control, model, EA and sham EA groups. Inflammatory pain model was induced by injecting of 100 μl Complete Freund’s adjuvant (CFA) into the plantar surface of a hind paw. Rats in the EA group were treatment with EA (constant aquare wave, 2 Hz and 100 Hz alternating frequencies, intensities ranging from 1-2 mA) at 5.5 h, 24.5 h and 48.5 h. Paw withdrawal thresholds (PWTs) were measured before modeling and at 5 h, 6 h, 25 h and 49 h after CFA injection. Rats were killed and ipsilateral side of the lumbar spinal cords were harvested for detecting the expressions of p-ERK1/2, Elk1, COX-2, NK-1 and CREB by immunohistochemistry, real-time PCR, western blot analysis and EMSA. Finally, the analgesic effect of EA plus U0126, a MEK (ERK kinase) inhibitor, on CFA rats was examined. Inflammatory pain was induced in rats by hindpaw injection of CFA and significantly increased phospho-ERK1/2 positive cells and protein levels of p-ERK1/2 in the ipsilateral spinal cord dorsal horn (SCDH). CFA up-regulated of cyclooxygenase-2 (COX-2) mRNA and protein expression at 6 h after injection and neurokinin-1 receptor (NK-1) expression at 49 h post-injection, in the SCDH. EA, applied to Zusanli (ST36) and Kunlun (BL60), remarkably increased the pain thresholds of CFA injected rats, significantly suppressed ERK1/2 activation and COX-2 protein expression after a single treatment, and decreased NK-1 mRNA and protein expression at 49 h. EA decreased the DNA binding activity of cAMP response element binding protein (CREB), a downstream transcription factor of ERK1/2, at 49 h after CFA injection. Moreover, EA and U0126 synergistically inhibited CFA-induced allodynia. The present study suggests that EA produces analgesic effect by preventing the activation of ERK1/2-COX-2 pathway and ERK1/2-CREB-NK-1 pathway in CFA rats.

Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacologic treatment for pain relief. In previous animal studies, TENS effectively alleviated Complete Freund’s Adjuvant (CFA)- or carrageenan-induced inflammatory pain. Although TENS is known to produce analgesia via opioid activation in the brain and at the spinal level, few reports have investigated the signal transduction pathways mediated by TENS. Prior studies have verified the importance of the activation of extracellular signal-regulated kinase (ERK) signal transduction pathway in the spinal cord dorsal horn (SCDH) in acute and persistent inflammatory pains. Here, by using CFA rat model, we tested the efficacy of TENS on inhibiting the expressions of p-ERK1/2 and of its downstream cyclooxygenase-2 (COX-2) and the level of prostaglandin E2 (PGE2) at spinal level. Rats were randomly divided into control, model and TENS groups, and injected subcutaneously with 100 μl CFA or saline in the plantar surface of right hind paw. Rats in the TENS group were treated with TENS (constant aquare wave, 2 Hz and 100 Hz alternating frequencies, intensities ranging from 1 to 2 mA, lasting for 30 min each time) at 5 h and 24 h after injection. Paw withdrawal thresholds (PWTs) were measured with dynamic plantar aesthesiometer at 3d before modeling and 5 h, 6 h, and 25 h after CFA injection. The ipsilateral sides of the lumbar spinal cord dosral horns were harvested for detecting the expressions of p-ERK1/2 and COX-2 by western blot analysis and qPCR, and PGE2 by ELISA. CFA-induced periphery inflammation decreased PWTs and increased paw volume of rats. TENS treatment significantly alleviated mechanical hyperalgesia caused by CFA. However, no anti-inflammatory effect of TENS was observed. Expression of p-ERK1/2 protein and COX-2 mRNA was significantly up-regualted at 5 h and 6 h after CFA injection, while COX-2 and PGE2 protein level only increased at 6 h after modeling. Furthermore, the high expression of p-ERK1/2 and COX-2, and over-production of PGE2 induced by CFA, were suppressed by TENS administration. TENS may be an effective therapy in controlling inflammatory pain induced by CFA. Its analgesic effect may be associated with the inhibition of activation of the spinal ERK1/2-COX-2 pathway.

Complex regional pain syndrome type‐I (CRPS‐I) represents a type of neurovascular condition featured by severe pain in affected extremities. Few treatments have proven effective for CRPS‐I. Electroacupuncture (EA) is an effective therapy for pain relief. We explored the mechanism through which EA ameliorates pain in a rat CRPS‐I model. The chronic postischemic pain (CPIP) model was established using Sprague‐Dawley rats to mimic CRPS‐I. We found that oxidative stress‐related biological process was among the predominant biological processes in affected hindpaw of CPIP rats. Oxidative stress occurred primarily in local hindpaw but not in the spinal cord or serum of model rats. Antioxidant N‐acetyl cysteine (NAC) attenuated mechanical allodynia and spinal glia overactivation in CPIP model rats, whereas locally increasing oxidative stress is sufficient to induce chronic pain and spinal glia overactivation in naive rats. EA exerted remarkable antiallodynia on CPIP rats by reducing local oxidative stress via enhancing nuclear factor erythroid 2‐related factor 2 (Nrf2) expression. Pharmacological blocking Nrf2 abolished antioxidative and antiallodynic effects of EA. EA reduced spinal glia overactivation, attenuated the upregulation of inflammatory cytokines, reduced the enhanced TRPA1 channel activity in dorsal root ganglion neurons innervating the hindpaws, and improved blood flow dysfunction in hindpaws of CPIP rats, all of which were mimicked by NAC treatment. Thus, we identified local oxidative injury as an important contributor to pathogenesis of animal CRPS‐I model. EA targets local oxidative injury by enhancing endogenous Nrf2‐mediated antioxidative mechanism to relieve pain and inflammation. Our study indicates EA can be an alternative option for CRPS‐I management.