Analgesic effect of electric stimulation of peripheral nerves with different electric frequencies using the formalin test

Duration of the preemptive analgesic effects of low- and high-frequency transcutaneous electrical nerve stimulation in rats with acute inflammatory pain

Transcutaneous electrical nerve stimulation (TENS) activates various pathways to induce antinociceptive effects, based on the frequencies used. This study evaluates the preemptive analgesic effects and their duration of low- (LT: 4 Hz) and high-frequency TENS (HT: 100 Hz) using a rat model of acute inflammatory pain. Acute inflammation was induced by injecting 1% formalin into the hind paws of rats. LT or HT was applied for 30 min before formalin injection. Pain-related behaviors, such as licking, flinching, and lifting, were recorded for 60 min postinjection. Immunohistochemistry was used to assess the number of phosphorylated extracellular signal-regulated kinase (pERK)- and c-fos-positive cells in the spinal cord. Naloxone, a μ-opioid receptors (MORs) antagonist, and naltrindole, a δ-opioid receptors (DORs) antagonist, were administered before TENS application. Pain behavior duration and pERK- and c-fos-positive cell expression were then measured. LT and HT pretreatment significantly reduced both pain behaviors and the number of pERK- and c-fos-positive cells postformalin injection. Naloxone and naltrindole partially reversed the effects of LT and HT, respectively. Notably, HT's analgesic effect lasted up to 120 min whereas that of LT persisted for 90 min. LT and HT effectively exerted their preemptive analgesic effects on acute inflammatory pain by inhibiting pERK and c-fos expression in the spinal cord. HT presented a longer-lasting effect compared to LT. MOR and DOR activation may contribute to LT and HT's analgesic mechanisms, respectively.

Electroacupuncture analgesia with different frequencies is mediated via different opioid pathways in acute visceral hyperalgesia rats

AIMS

To investigate the efficacy of electroacupuncture (EA) alleviation of acute visceral hyperalgesia, the frequency dependence of this efficacy, and the difference in endogenous opioid pathways as underlying mechanism explaining the frequency dependence.

MAIN METHODS

A visceral hyperalgesia model was established by colorectal instillation of 2% acetic acid (AA) in adult rats. EA treatment at 2Hz, 100Hz, 2/100Hz and sham EA were performed at two bilateral acupoints, ST-36 and ST-37, in the hind-limbs. Naloxone (NLX, 2mg/kg) was administered intraperitoneally 5min before the application of EA. The visceral sensation was quantified by scores of abdominal withdrawal reflex (AWR) and values of rectus abdominis electromyograms (EMGs) in response to colorectal distension (CRD).

KEY FINDINGS

Acute visceral hyperalgesia was produced by instillation of AA. The hyperalgesia reached the peak at 2h, and maintained steadily for >6h. EA treatment at 2Hz, 100Hz and 2/100Hz attenuated the acute hyperalgesia, and the attenuation lasted for 2.5h, 2h and 3h, respectively. Sham EA produced no such effect. The analgesic potencies of EA treatment at 2Hz, 100Hz and 2/100Hz were completely blocked, almost not affected, and partially blocked by NLX. In the latter two frequencies, the analgesic durations were shortened to 1.5h and 2h, respectively.

SIGNIFICANCE

EA can alleviate acute visceral hyperalgesia. Effective EA analgesia at different frequencies are potentially mediated via different endogenous opioid pathways.

Clinical effectiveness of acupuncture for carpal tunnel syndrome

Acupuncture and electroacupuncture treatments of symptomatic carpal tunnel syndrome (CTS) may improve symptoms and aid nerve repair as well as improve sensory and motor functions. However, limited evidence is available regarding the effects of these treatments based on comprehensive evaluation methods. This research completed the treatment and evaluation of 26 patients with confirmed CTS. Participants were divided into two treatment groups based on a modified neurophysiological grading scale. Of the total number of participants, 15 received acupuncture and 11 received electroacupuncture on both upper limbs. Acupoints were PC-7 (Daling) and PC-6 (Neiguan) along the pericardial meridian compatible with the median nerve tract. The treatment program consisted of 24 sessions of 15 min duration over 6 weeks. After electroacupuncture treatments, symptom severity was evaluated using the short clinical questionnaire by Lo and Chiang, which indicated improvements in the respective symptom severity score. After the acupuncture treatment, grip strength in the major symptomatic side in CTS patients could be significantly increased. Electrophysiology evaluation likewise indicated a significant increase in the distal median motor amplitude of the palm-wrist segment. In addition, Tinel's sign significantly decreased in the major symptomatic side. Our findings indicated that electroacupuncture could improve symptomatology, while acupuncture could exert positive therapeutic effects for CTS patients, as evidenced by improved symptomatology, grip strength, electrophysiological function, and physical provocation sign.

Successful practice of electroacupuncture analgesia in equine surgery

Electroacupuncture analgesia was used for surgery in horses and donkeys. A KWD-808 electrical stimulator was used to incrementally induce a dense, dispersed wave output at frequencies from 20 to 55 Hz, which was maintained at a frequency of 55 Hz, and to change the amplitude of the wave to the best grading number for the suggested operation in each animal. Induction of analgesia lasted for 20-30 minutes, and the effect of analgesia was maintained for 20-45 minutes depending on the type of surgery performed. The exhibited clinical signs, physical examination data, and the responses of all animals were used for evaluating the periods of analgesia. Although the majority of the cases (95%) had no response to strong surgical pain, they experienced significant increases in heart rates and respiratory rates during induction. The lack of pain, relaxed surgical procedures, reduced intraoperative bleeding, and improved healing without complications were all definite benefits of using electroacupuncture analgesia in surgery. Thus, this study has provided surgical evidence supporting the effectiveness of electroacupuncture analgesia, as well as confirming its reliability, in the field of equine anesthesia and surgery.

Acupuncture analgesia: a review of its mechanisms of actions

The mechanism of acupuncture analgesia (AA) has been widely explored since the 1970s. Early studies investigated the relationship between acupuncture and endogenous opiates (beta-endorphin, enkephalin, endomorphin and dynorphin). Before the 1990s, most experts agreed on the concept that in normal animal models, lower frequency electroacupuncture (EA) stimulates the release of beta-endorphin, enkephalin and endomorphin, which in turn activates the mu- and delta-opioid receptors, and that higher frequency EA stimulates dynorphin which activates the kappa-opioid receptor. Besides endogenous opiates, our studies have focused on serotonin. The serotoninergic descending inhibitory pathway is suggested to be an important mechanism of acupuncture analgesic, collaborating with endogenous opiates. Many efforts have been made to clarify these mechanisms, but to date no satisfactory consensus has been reached. In the late 1990s, researchers began to focus on the different analgesic effects of EA between normal and hyperalgesic animal models. Published data from these studies imply that normal and hyperalgesic animals respond differently to EA. Results from experiments on the anti-hyperalgesia effect of EA have raised a new issue about the influences of EA on receptors to excitatory amino acid in the spinal cord level. Results from various studies have shown that these receptors play a role in the mechanism of AA. Recently, research on the autonomic nervous system (ANS) seem to indicate its connection with acupuncture. The inflammatory reflex (via the ANS) might be a crucial part of anti-hyperalgesia elicited by acupuncture, and this reflex, which regulates the immune system in the organism, can elucidate not only the mechanism of AA but also the mechanism of acupuncture applied to other inflammatory conditions. Innovation of functional image study enables us to analyze the responses of cortex on living human body to acupuncture. However, results of these experiments are still controversial. After 30 years of acupuncture research, there are still many puzzles left to be solved regarding the mechanism of AA.

Effect of electroacupuncture on thalamic neuronal response to visceral nociception

The thalamus has been shown to play an important role in somatovisceral integration. This study set out to examine thalamic neuronal responses to visceral nociception when electrical stimulation was applied to the skin receptive field (RF) or to ST(36), an acupoint most frequently used for abdominal pain conventionally. Single neuronal recordings were carried out extracellularly in the thalamic ventrobasal nucleus of anaesthetized rats. Among numerous neurons responding to tactile stimulation, 72 units were found responsive not only to innocuous stimulation on skin RF (60 activated, 12 inhibited) but also to noxious colorectal distension (CRD). Electrical stimulation (2 Hz, 1 mA) of the neuronal somatic receptive field center reduced the subsequent neuronal responses to CRD in 40 neurons tested. High frequency stimulation (100 Hz) produced stronger inhibition than low frequency (2 Hz) stimulation at RF. The inhibition on visceral nociceptive response occurred immediately after the stimulation. In comparison with the effect of RF stimulation, the inhibitory effect was less at either ipsilateral or contralateral ST(36). Our data suggest that, at single thalamic neuron level, stimulation at conventional acupoint is not necessarily as effective as stimulation at neuronal skin receptive field, and high frequency is more effective than low frequency stimulation for the inhibition of visceral nociception.

Effect of varying frequency and duration of electroacupuncture stimulation on carrageenan-induced hyperalgesia

OBJECTIVE

Although electroacupuncture (EA) therapy is used to relieve various kinds of pain, the optimal frequency and duration of EA remain unclear. We investigated the effect of varying frequency and duration of EA during hyperalgesia elicited by carrageenan-induced inflammation.

METHODS

Carrageenan was administered by subcutaneous intraplantar injection to induce inflammation. Nociceptive thresholds were measured using the paw pressure threshold (PPT) (Randall-Selitto Test). EA was applied at 3, 15, or 100Hz to the left anterior tibial muscles for 1, 15, or 60 minutes. Intensities used were chosen within the known tolerance of the animal, and increased up to 3mA for 3Hz, and up to 1.5mA for 15 and 100Hz. EA was started three hours after carrageenan injection.

RESULTS

Three hours after carrageenan injection, a marked ipsilateral inflammatory response appeared and PPT decreased significantly. This decrease persisted for at least 24 hours after carrageenan injection. EA at 3Hz (60 minutes) resulted in significant increases of PPT which persisted for 24 hours after injection. EA at 3Hz (15 minutes) also induced PPT elevations immediately and for one hour after EA compared to the control group. However, no other variety of EA significantly increased PPT.

CONCLUSION

These results show that EA produces electroacupuncture analgesia of carrageenan-induced hyperalgesia. These findings also suggest that, among the frequencies and durations tested, EA at 3Hz (60 minutes) is the most suitable frequency and duration for carrageenan-induced inflammation. It seems that EA has different analgesic effects and mechanisms according to the parameters of stimulation. For EA in the clinical induction of analgesia, it is especially important that an effective frequency and duration are selected.

Effects of intensity of electroacupuncture upon experimental pain in healthy human volunteers: a randomized, double-blind, placebo-controlled study

Electroacupuncture is commonly used for pain relief. Despite an extensive evidence-base guiding the selection of stimulation parameters, little methodologically robust research exists regarding the level of intensity required to provide effective doses. This study investigated the hypoalgesic effects of two stimulation intensities compared to placebo on pressure pain thresholds (PPTs) in pain-free humans. Forty-eight acupuncture-naïve volunteers (mean age 23), stratified by gender, were screened for relevant contraindications and randomly allocated to four groups: control, placebo, high-intensity ("to tolerance but sub-noxious") or low-intensity ("strong but comfortable"). True or placebo electroacupuncture, using the Streitberger placebo needle, was administered to acupoints on dominant forearm (LI10, TH5) and ipsilateral leg (GB34, ST38). True needles (30 mm long, 0.3 mm diameter) were inserted 20-25 mm and "de-qi" was elicited from active groups, prior to administering 30 min of 4 Hz, 200 micros electroacupuncture. No electrical stimulation was performed on control and placebo groups, and placebo needles were used. After the intervention period, all needles were removed. Volunteers were monitored for 30 further minutes. Two PPT measurements were taken bilaterally from muscle bellies of first dorsal interosseous by an independent rater, at baseline and at six subsequent 10-min intervals. Square-root transformed data were analysed using repeated-measures ANOVA, with baseline data as covariate. The high-intensity group was significantly different from the placebo group for both measurement sites (p=.020, p=.033). The control group displayed stable PPT readings over time. No significant differences were observed between the placebo and control groups. These findings suggest that high-intensity levels may be important in optimal dose selection.

Suppression of neuropathic pain by peripheral electrical stimulation in rats: μ-opioid receptor and NMDA receptor implicated

Peripheral electrical stimulation (PES) has been utilized to manage chronic pain associated with nerve injury. However, the data on clinical effectiveness are conflicting and the neurophysiological mechanism is not well known. This study was designed to assess whether PES relieved neuropathic pain and its possible mechanisms. The neuropathic pain model was made with lumbar 5th (L5) and 6th (L6) spinal nerve ligations in rats. Nociceptive responses of the rats were assessed by the cold plate test (the number and duration of paw lifts that occurred in 5 min on a 5 +/- 1 degrees C cold plate). PES with a frequency of 2 Hz and at increasing strengths was given for 30 min via stainless-steel needles inserted into standard acupoints on the leg and back, respectively. Immunochemistry was used to examine the immunoreactivity of the NMDA receptor 1 (NR1) subunit in the spinal cord dorsal horn. The results are as follows: (1) PES relieved neuropathic pain and the effect was blocked by 1.0 mg/kg naloxone. (2) The effect of one session of PES lasted up to 12 h. (3) Repetitive PES showed a cumulative effect and no tolerance was observed. (4) There was a significant increase of NR1 immunoreactivity in the superficial laminae of the spinal cord of neuropathic pain rats as compared with naive rats. This increase could be reversed by repetitive 2 Hz PES. These results suggest that PES can relieve neuropathic pain, and that mu-opioid receptors and NMDA receptors are involved in the effect of PES.