Electroacupuncture in rats: evidence for naloxone and naltrexone potentiation of analgesia

An exploratory review of the electroacupuncture literature: clinical applications and endorphin mechanisms

Electroacupuncture (EA) is widely used in clinical practice and research, as well as in experimental investigations into the mechanisms of acupuncture. This study explores publication trends in clinical and experimental studies of EA (1975-2011) for pain and non-pain research; EA use for different clinical conditions (1974-2012); and the relation of EA research, including stimulation frequency, to opioid peptide mechanisms. Appropriate PubMed 'all fields' searches were conducted, identified studies were classified using PubMed filters and manually, and data extracted into tables. A total of 2916 clinical studies were located, of which 19% involved EA. Additionally, 3344 animal studies were located, of which 48% involved EA. The publication rate of EA studies per year has risen over time, but the percentage of studies of pain has fallen from 60% to 25%. The conditions most commonly treated with EA are musculoskeletal, neurological, obstetric and gastrointestinal, along with intraoperative and postoperative analgesia. EA studies, particularly with low frequency stimulation, are more likely to support the role of endogenous opioid mechanisms than manual acupuncture studies, and opioid release is more likely in the central nervous system than the circulation. EA is increasingly used in clinical and especially experimental research, particularly for non-pain conditions. Acupuncture does release endogenous opioids, but this probably depends on 'dosage', with the evidence more consistent and convincing for EA than for manual acupuncture. Different frequencies of EA appear to activate different endogenous opioid mechanisms.

Peripheral muscarinic receptors mediate the anti-inflammatory effects of auricular acupuncture

BACKGROUND

The cholinergic and opioid systems play important roles in modulating inflammation. This study tests whether auricular acupuncture (AA) produces anti-inflammatory effects via opioid and peripheral cholinergic receptors in a rat model.

METHODS

Rats were anesthetized with chloral hydrate and inflammation was induced by intraplantar injection of carrageenan. Electroacupuncture was performed at auricular points bilaterally. The severity of inflammation was assessed using changes in paw volume and thermal and mechanical pain thresholds of the rats during recovery from anesthesia.

RESULTS

Electroacupuncture at selected auricular acupoints significantly reduced paw edema and mechanical hyperalgesia, with no significant effect on thermal hyperalgesia. The anti-edematous and analgesic effects of AA were abolished by blockade of peripheral cholinergic muscarinic receptors with methyl atropine. Blockade of local muscarinic receptors at the inflamed site with a small dose of atropine also antagonized the anti-edematous effect of AA. By contrast, systemic opioid receptor blockade with naloxone did not antagonize the anti-inflammatory effects of AA.

CONCLUSION

This study discovers a role of peripheral muscarinic receptors in mediating the anti-inflammatory effects of AA. The cholinergic muscarinic mechanism appears to be more important than the opioid mechanism in the anti-inflammatory action of AA.

Electroacupuncture analgesia in rat ankle sprain pain model: neural mechanisms

OBJECTIVES

Acupuncture, an alternative medical therapy with a long history, is appealing because it can activate endogenous analgesic mechanisms by minimally invasive means. The mechanisms of acupuncture, however, are not well understood yet. The following sentence was removed from our original manuscript. One of the major problems impeding understanding of the acupuncture mechanism is lack of experimental models that mimic various forms of persistent pain that respond to acupuncture in humans.

METHODS

In this review, we summarize and discuss previous and recent findings regarding electroacupuncture-induced analgesia in an ankle sprain pain model and the potential underlying mechanisms of acupuncture.

RESULTS

A novel model of ankle sprain pain is introduced recently and the mechanism of electroacupuncture-induced analgesia in this model has been explored. The following sentence was removed from our original manuscript. This model provides a reproducible and quantifiable index of persistent pain at the ankle joint in rats. Acupuncture at a remote site produces long-lasting and powerful analgesia. The consistent analgesic effect of acupuncture in this model has allowed us to pursue the underlying neural mechanisms.

CONCLUSIONS

These studies provide insight into the mechanisms of acupuncture analgesia in one particular form of persistent pain, and hopefully will allow us to expand our knowledge to other painful conditions.

DNIC-mediated analgesia produced by a supramaximal electrical or a high-dose formalin conditioning stimulus: roles of opioid and alpha2-adrenergic receptors

Background

Diffuse noxious inhibitory controls (DNIC) can be produced by different types of conditioning stimuli, but the analgesic properties and underlying mechanisms remain unclear. The aim of this study was to differentiate the induction of DNIC analgesia between noxious electrical and inflammatory conditioning stimuli.

Methods

First, rats subjected to either a supramaximal electrical stimulation or an injection of high-dose formalin in the hind limb were identified to have pain responses with behavioral evidence and spinal Fos-immunoreactive profiles. Second, suppression of tail-flick latencies by the two noxious stimuli was assessed to confirm the presence of DNIC. Third, an opioid receptor antagonist (naloxone) and an α2-adrenoreceptor antagonist (yohimbine) were injected, intraperitoneally and intrathecally respectively, before conditioning noxious stimuli to test the involvement of descending inhibitory pathways in DNIC-mediated analgesia.

Results

An intramuscular injection of 100 μl of 5% formalin produced noxious behaviors with cumulative pain scores similar to those of 50 μl of 2% formalin in the paw. Both electrical and chemical stimulation significantly increased Fos expression in the superficial dorsal horns, but possessed characteristic distribution patterns individually. Both conditioning stimuli prolonged the tail-flick latencies indicating a DNIC response. However, the electrical stimulation-induced DNIC was reversed by yohimbine, but not by naloxone; whereas noxious formalin-induced analgesia was both naloxone- and yohimbine-reversible.

Conclusions

It is demonstrated that DNIC produced by different types of conditioning stimuli can be mediated by different descending inhibitory controls, indicating the organization within the central nervous circuit is complex and possibly exhibits particular clinical manifestations.

Opioidergic orofacial antinociception induced by electroacupuncture at acupoint St36

The participation of opioids in the antinociceptive effect of electroacupuncture was evaluated in terms of nociception produced by thermal stimuli applied to the face of male Wistar rats, weighing 180-230 g. Electrical stimulation (bipolar and asymmetric square wave with 0.5 mA intensity for 20 min) of acupoint St36, located in the anterior tibial muscle 10 mm distal to the knee joint, induced antinociception in the present model, which was maintained for 150 min. Acupoint LI4, located in the junction of the first and second metacarpal bones, did not achieve antinociception at any frequency studied (5 Hz: 1.7 +/- 0.1; 30 Hz: 1.8 +/- 0.1; 100 Hz: 1.7 +/- 0.1 vs 1.4 +/- 0.2). The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency 100 Hz (3.0 +/- 0.2 vs 1.0 +/- 0.1) was used, and not with 5 or 30 Hz (1.2 +/- 0.2 and 0.7 +/- 0.1 vs 1.0 +/- 0.1). The antinociceptive effect of acupuncture occurred by opioid pathway activation, since naloxone (1 and 2 mg/kg, subcutaneously) antagonized it (1.8 +/- 0.2 and 1.7 +/- 0.2 vs 3.0 +/- 0.1).

Electroacupuncture-induced analgesia in a rat model of ankle sprain pain is mediated by spinal alpha-adrenoceptors

In a previous study, we showed that electroacupuncture (EA) applied to the SI-6 point on the contralateral forelimb produces long-lasting and powerful analgesia in pain caused by ankle sprain in a rat model. To investigate the underlying mechanism of EA analgesia, the present study tested the effects of various antagonists on known endogenous analgesic systems in this model. Ankle sprain was induced in anesthetized rats by overextending their right ankle with repeated forceful plantar flexion and inversion of the foot. When rats developed pain behaviors (a reduction in weight-bearing of the affected hind limb), EA was applied to the SI-6 point on the contralateral forelimb for 30 min under halothane anesthesia. EA significantly improved the weight-bearing capacity of the affected hind limb for 2h, suggesting an analgesic effect. The alpha-adrenoceptor antagonist phentolamine (2mg/kg, i.p. or 30 microg, i.t.) completely blocked the EA-induced analgesia, whereas naloxone (1mg/kg, i.p.) failed to block the effect. These results suggest that EA-induced analgesia is mediated by alpha-adrenoceptor mechanisms. Further experiments showed that intrathecal administration of yohimbine, an alpha(2)-adrenergic antagonist, reduced the EA-induced analgesia in a dose-dependent manner, whereas terazosin, an alpha(1)-adrenergic antagonist, did not produce any effect. These data suggest that the analgesic effect of EA in ankle sprain pain is, at least in part, mediated by spinal alpha(2)-adrenoceptor mechanisms.

Pain-relieving effects of acupuncture and electroacupuncture in an animal model of arthritic pain

The effects of acupuncture and electroacupuncture on an animal model of arthritic pain were examined. Under halothane anesthesia, arthritic pain was induced by the injection of carrageenan into the knee joint cavity of male Sprague-Dawley rats. Behavioral performance was tested before and after the termination of acupuncture or electroacupuncture. Electrophysiologically, the responses of afferents to a movement cycle were recorded before and after acupuncture or electroacupuncture. After the acupuncture procedure, the weight-bearing force of the rats was significantly improved and the neural responses to noxious movement stimulation were reduced. Electroacupuncture significantly improved weight-bearing behavior and inhibited neural responses of articular afferents to noxious stimulation. These results indicate that acupuncture and electroacupuncture may provide a potent strategy in relieving arthritic pain.

Development of opioid tolerance with repeated transcutaneous electrical nerve stimulation administration

The analgesia produced by low and high frequency transcutaneous electrical nerve stimulation (TENS) is mediated by the release of mu- or delta-opioids, respectively in the central nervous system. Repeated administration of either mu- or delta-opioid agonists induce opioid analgesic tolerance. Thus, we tested if repeated administration of TENS (either low or high frequency) in rats leads to a development of tolerance to its antihyperalgesic effects with a corresponding cross-tolerance to mu- and delta-opioid agonists. Unilateral knee joint inflammation (3% carrageenan) was induced in adult Sprague-Dawley rats. Either low (4 Hz) or high frequency (100 Hz) TENS was administered for 6 days (20 min daily) to the inflamed knee joint under halothane anesthesia. The no TENS controls were administered anesthesia only for the same period. Withdrawal threshold to mechanical stimuli was measured before and after administration of TENS on each day and also on the sixth day. A separate group of animals was tested for tolerance to either the mu-opioid agonist, morphine (1.32, 3.95, 13.2 nmol/10 ml, intrathecal (i.t.)) or the delta-opioid agonist, SNC-80 (6, 20, 60, 120 nmol/10 ml, i.t.) 30 min after i.t. administration. The reduced mechanical withdrawal threshold following the induction of inflammation was reversed by the application of TENS. However, repeatedly administering either low or high frequency TENS for 6 days, lead to a diminution in its effectiveness in reversing the ipsilateral secondary mechanical hyperalgesia by the fourth day. The effects of morphine in the low and SNC-80 in the high frequency TENS groups were significantly less than the group that did not receive TENS. On the other hand, morphine and SNC-80 were similar to the no TENS control in the high and low frequency TENS groups, respectively. Thus, repeated administration of low and high frequency TENS leads to a development of opioid tolerance with a corresponding cross-tolerance to i.t. administered mu- and delta-opioid agonists, respectively. Clinically, it can be inferred that a treatment schedule of repeated daily TENS administration should be avoided to possibly obviate the induction of tolerance.

Acupuncture analgesia in a new rat model of ankle sprain pain

The lack of suitable experimental animal models for persistent pain showing clear acupuncture analgesia, has been the major stumbling block in the investigation of the physiological mechanisms of acupuncture analgesia. The present study developed a new rat model of ankle sprain pain and the effect of electroacupuncture (EA) on this model was examined. A common source of persistent pain in humans is the lateral ankle sprain. To model this condition, the rat's right ankle was bent repeatedly, overextending lateral ligaments, for 4 min under halothane anesthesia. The rat subsequently showed swelling of the ankle and a reduced stepping force of the affected limb for the next several days. The reduced stepping force of the limb was presumably due to a painful ankle since systemic injection of morphine produced temporary improvement of weight bearing. EA was applied to the SI-6 acupuncture point on the contralateral forelimb for 30 min under halothane anesthesia. After the termination of EA, behavioral tests measuring stepping force were periodically conducted during the next 4h. EA produced a 40% recovery in the stepping force of the sprained foot lasting for at least 2h. The magnitude of this improvement was equivalent to that obtained after a systemic injection of 2mg/kg of morphine and this improvement of stepping pressure was interpreted as an analgesic effect. The analgesic effect was specific to the acupuncture point since (1). the analgesic effect on the ankle sprain pain model could not be mimicked by EA applied to a nearby point, LI-4 and (2). EA applied to the SI-6 point was not effective in the knee arthritis pain model. The analgesic effect could not be blocked by systemic injection of opioid antagonists naloxone or naltrexone. These data suggest that EA produces a potent analgesic effect on the ankle sprain pain model in the rat. This analgesic effect is produced by applying EA to a site remote from the painful area in a stimulus point-specific way. The present study provides a powerful experimental animal model that can be used for investigating the unique physiological mechanisms involved in acupuncture analgesia.

Acupuncture for chronic low back pain: a randomized placebo-controlled study with long-term follow-up

OBJECTIVE

The authors sought to determine whether a series of needle acupuncture treatments produced long-term relief of chronic low back pain.

DESIGN

A blinded placebo-controlled study with an independent observer. The patients were randomized to receive manual acupuncture, electroacupuncture, or active placebo (mock transcutaneous electrical nerve stimulation). Subjects were examined and monitored by an investigator who was blinded to the treatment given.

SETTING

A tertiary-level pain clinic at a Swedish university hospital.

PATIENTS

Fifty consecutive patients (33 women, 17 men; mean age, 49.8 years) with chronic low back pain (mean pain duration, 9.5 years) and without rhizopathy or history of acupuncture treatment were included in the study.

INTERVENTIONS

Treatments were given once per week for 8 weeks. Two further treatments were given during the follow-up assessment period of 6 months or longer.

OUTCOME MEASURES

The independent observer made a global assessment of the patients 1, 3, and 6 months after treatment. The patients kept pain diaries to score pain intensity twice daily, analgesic intake, and quality of sleep daily, and activity level weekly.

RESULTS

At the 1-month independent assessment, 16 of 34 patients in the acupuncture groups and 2 of 16 patients in the placebo group showed improvement (p <0.05). At the 6-month follow-up assessment, 14 of 34 patients in the acupuncture groups and 2 of 16 patients in the placebo group showed improvement (p <0.05). A significant decrease in pain intensities occurred at 1 and 3 months in the acupuncture groups compared with the placebo group. There was a significant improvement in return to work, quality of sleep, and analgesic intake in subjects treated with acupuncture.

CONCLUSIONS

The authors found a long-term pain-relieving effect of needle acupuncture compared with true placebo in some patients with chronic nociceptive low back pain.

The effect of genotype on sensitivity to electroacupuncture analgesia

Individual differences in sensitivity to pain and analgesia are well appreciated, and increasing evidence has pointed towards a role of inherited genetic factors in explaining some proportion of such variability. It has long been known by practitioners of acupuncture, an ancient modality of analgesia, that some patients are 'responders' and others 'non-responders.' The present research was aimed at defining the inherited genetic influence on acupuncture analgesia in the mouse, using 10 common inbred strains. Two pairs of metallic needles were inserted into acupoints ST 36 and SP 6, fixed in situ and then connected to the output channel of an electric pulse generator. Electroacupuncture (EA) parameters were set as constant current output (intensity: 1.0-1.5-2.0 mA, 10 min each; frequency: 2 or 100 Hz) with alteration of a positive and negative square wave, 0.3 ms in pulse width. Tail-flick latencies evoked by radiant heat were measured before, during and after EA stimulation. Narrow-sense heritability estimates of 2 and 100 Hz EA were 0.37 and 0.16, respectively. We found that the C57BL/10 strain was the most sensitive, and the SM strain was the least sensitive to both 2 and 100 Hz EA. However, the relative sensitivities of other strains to these two EA frequencies suggested some genetic dissociation between them as well. These results demonstrate a role of inherited genetic factors in EA sensitivity in the mouse, although the low-to-moderate heritability estimates suggest that environmental factors may be of greater importance in predicting who will benefit from this analgesic modality.

Anti-hyperalgesic effect of electroacupuncture in a model of post-incisional pain in rats

Electroacupuncture has been proposed to be a low cost and practical method that allows effective pain management with minimal collateral effects. In this study we have examined the effect of electroacupuncture against the hyperalgesia developed in a model of post-incisional pain in rats. A 1-cm longitudinal incision was made through the skin and fascia of the plantar region of the animal hind paw. Mechanical hyperalgesia in the incision was evaluated 135 min after the surgery with von Frey filaments. The tension threshold was reduced from 75 g (upper limit of the test) to 1.36 +/- 0.36 g (mean +/- SEM) in control rats. It is shown that a 15-min period of electroacupuncture applied 120 min after surgery to the Zusanli (ST36) and Sanyinjiao (SP6) points, but not to non-acupoints, produces a significant and long-lasting reduction of the mechanical hyperalgesia induced by the surgical incision of the plantar surface of the ipsilateral hind paw. The tension threshold was reduced from 75 to 27.6 +/- 4.2 g in animals soon after the end of electroacupuncture. The mechanical threshold in this group was about 64% less than in control. Electroacupuncture was ineffective in rats treated 10 min earlier with naloxone (1 mg/kg, ip), thus confirming the involvement of opioid mechanisms in the antinociceptive effects of such procedure. The results indicate that post-incisional pain is a useful model for studying the anti-hyperalgesic properties of electroacupuncture in laboratory animals.

The initial effects of a cervical spine manipulative physiotherapy treatment on the pain and dysfunction of lateral epicondylalgia

Manipulative therapy is frequently used in the management of musculoskeletal pain. A frequently reported clinical feature of this treatment is the immediacy with which it appears to initiate improvement in pain and function. A randomised, double blind, placebo controlled, repeated measures design was employed to study the initial effects of a cervical spine treatment technique in a group of 15 patients with lateral epicondylalgia. Pressure pain threshold, pain-free grip strength, upper limb neurodynamics, pain and function were assessed prior to and following application of either a treatment, placebo or control condition. All subjects received all three conditions. Differences between the pre-post measures were used as indicators of change in subject's symptom profiles. The treatment condition produced significant improvement in pressure pain threshold, pain-free grip strength, neurodynamics and pain scores relative to placebo and control conditions (P < 0.05). In summary, this study demonstrates that manipulative therapy is capable of eliciting a rapid hypoalgesic effect.

Opioid antagonists: indirect antagonism of morphine analgesia by spinal dynorphin A

Naloxone and norbinaltorphimine when given ICV to mice can antagonize IT morphine-induced analgesia indirectly by releasing spinal dynorphin A(1-17) (Dyn A). Dyn A produces an antianalgesic action against IT morphine. In the present study, drugs with varying amounts of opioid antagonist to agonist action (nalbuphine, levallorphan, naltrexone, and naltrindole) were given ICV to determine whether they antagonized IT morphine-induced inhibition of the tail-flick response as an indication of spinal Dyn A release. Additional pharmacological tests were used as criteria for Dyn A release: a) Small doses of the opioid antagonists naloxone and norbinaltorphimine administered IT inhibited the antagonistic action; b) dynorphin antiserum given IT blocked the action of Dyn A; c) desensitization to the effect of Dyn A was produced by 3-h pretreatment with morphine, 10 mg/kg SC, or by pretreatment with the agents themselves. When given ICV, nalbuphine, levallorphan, and naltrexone released Dyn A in the spinal cord to produce an antianalgesic effect. Naltrindole, a delta-receptor antagonist, did not release Dyn A. Dyn A release did not appear to involve delta-receptors. Thus, a number of opioid antagonists inhibit the analgesic action of opioid agonists indirectly through Dyn A release.

Electroacupuncture analgesia in rats: naltrexone antagonism is dependent on previous exposure

Inhibition of pain by low frequency electroacupuncture (EA) has been thought to be mediated by endogenous opioids. We reported in a previous paper, however, that naloxone (NAL) and naltrexone (NTX) either potentiated or had no effect on analgesia in EA-naive rats, independent of origin (American or Chinese), sex, weight, geographic location (the U.S.A. or China), or numerous variations of experimental methodology. In the present study, we hypothesized that the number of exposures to EA treatment may be responsible for the discrepant effect of opiate antagonists. We found, as previously demonstrated, analgesia in EA-naive rats was potentiated by NTX. After two pre-exposures to EA, however, NTX antagonized analgesia. These results indicate that, in rats: (1) pre-exposure is a key variable for opiate antagonists to produce antagonism or potentiation of analgesia; (2) environmental cues paired with the initial analgesic manipulation may be responsible for converting analgesia from non-opioid to opioid, as already demonstrated with classically conditioned and learned helplessness induced analgesia; and (3) EA analgesia in rats is a multidimensional manipulation which can be influenced by subtle environmental changes.

Electroacupuncture analgesia in naive rats: effects of brainstem and spinal cord lesions, and role of pituitary-adrenal axis

Recent studies have shown that analgesia is potentiated by naltrexone (NTX) and naloxone (NAL) pretreatment in rats exposed for the first time to electroacupuncture (EA). In the present study, we have investigated the role of the pituitary-adrenal axis and of brainstem and spinal cord structures in EA analgesia and its potentiation by NTX. The pituitary and adrenal glands do not participate in the production of EA analgesia, but may produce a non-opioid substance which interferes with the development of EA analgesia. Spinalization or dorsolateral funiculi lesions blocked EA analgesia, and intrathecal NTX had no effect. These results indicate that supraspinal structures are necessary to produce and potentiate EA analgesia. Contrary to their critical role in morphine and other models of environmentally produced analgesia nucleus raphe alatus and raphe structures dorsal to it are not necessary for the development of EA analgesia. These structures, however, may contain opiate synapses on which NTX may act as an agonist to potentiate analgesia. The various components which appear to participate in the production of EA analgesia imply a complex circuit of pain modulation systems and indicate that an organism can adapt to distinct environmental conditions with versatile means to avoid pain.