Mu opioid receptor-containing neurons mediate electroacupuncture-produced anti-hyperalgesia in rats with hind paw inflammation

Adiponectin regulates electroacupuncture-produced analgesic effects in association with a crosstalk between the peripheral circulation and the spinal cord

Neurotransmitter-mediated acupuncture analgesia has been widely studied in nervous systems. It remains largely unclear if peripheral substances are involved the acupuncture analgesia. Adiponectin (APN), a circulating adipokine, shows analgesic effects. The study aimed to examine whether APN regulates analgesic effects of electroacupuncture (EA) in the complete Freund's adjuvant (CFA)-induced mouse model. APN wild type (WT) and knockout (KO) mouse were employed in the study. We found that EA attenuates the CFA-induced pain as demonstrated by the Hargreaves thermal test and the von Frey filament test. The deletion of APN significantly reduced the acupuncture analgesia in the CFA-treated APN KO mice while the intrathecal administration of APN mimicked the analgesic effects of EA. We further revealed that EA produced analgesic effects mainly via APN/AdipoR2-mediated AMPK pathway by the siRNA inhibitions of APN receptors (adipoR1/2) in the spinal cord. The immunofluorescence staining analysis showed that EA increased the APN accumulation in spinal cord through the blood circulation. In conclusion, the study indicates a novel mechanism that acupuncture produces analgesic effects at least partially via APN/AdipoR2-AMPK pathway in the spinal cord.

Electroacupuncture decreases Netrin-1-induced myelinated afferent fiber sprouting and neuropathic pain through μ-opioid receptors

Purpose: We determined whether electroacupuncture (EA) reduces Netrin-1-induced myelinated primary afferent nerve fiber sprouting in the spinal cord and pain hypersensitivity associated with postherpetic neuralgia (PHN) through activation of μ-opioid receptors.

Methods: PHN was induced by systemic injection of resiniferatoxin (RTX) in rats. Thirty-six days after RTX injection, a μ-opioid receptor antagonist, beta-funaltrexamine (β-FNA) or a κ-opioid receptor antagonist, nor Binaltorphimine (nor-BNI), was injected intrathecally 30 mins before EA, once every other day for 4 times. Mechanical allodynia was tested with von Frey filaments. The protein expression level of Netrin-1 and its receptors (DCC and UNC5H2) were quantified by using western blotting. The myelinated primary afferent nerve fiber sprouting was mapped with the transganglionic tracer cholera toxin B-subunit (CTB).

Results: Treatment with 2 Hz EA at “Huantiao” (GB30) and “Yanglingquan” (GB34) decreased the mechanical allodynia at 22 days and the myelinated primary afferent nerve fiber preternatural sprouting into the lamina II of the spinal dorsal horn at 42 days after RTX injection. Also, treatment with 2 Hz EA reduced the protein levels of DCC and Netrin-1 and promoted the expression of UNC5H2 in the spinal dorsal horn 42 days after RTX injection. Furthermore, the μ-opioid receptor antagonist β-FNA, but not the κ-opioid receptor antagonist nor-BNI, reversed the effect of EA on neuropathic pain caused by RTX. In addition, morphine inhibited the Netrin-1 protein level induced by RTX in SH-SY5Y cells.

Conclusions: Through activation of μ-opioid receptors, treatment with EA reduces the expression level of DCC and Netrin-1 and changes a growth-permissive environment in spinal dorsal horn into an inhibitory environment by increasing UNC5H2, thus decreasing RTX-caused primary afferent nerve sprouting in the spinal dorsal horn and neuropathic pain.

Opioidergic effects of transcutaneous electrical nerve stimulation on pain and inflammatory edema in a rat model of ankle sprain

ABSTRACT Although transcutaneous electrical nerve stimulation (TENS) has been proposed to modulate pain and the mechanisms underlying analgesia remain poorly understood, evidence of anti-inflammatory effect is more limited. The purpose of this study was to examine the opioidergic mechanisms of TENS effects in two different frequencies on pain and inflammatory edema in the ankle sprain model in rats. Threshold to mechanical stimulation was utilized to examine the changes produced by intraperitoneal injection of non-selective opioid antagonist naloxone on the antihyperalgesic effect induced by a 20-min period of 2Hz or 100Hz TENS in the ankle sprain model, produced by manually overextending the lateral ligaments. Ankle sprain induced a long-lasting reduction in paw withdrawn latency (PWL) after 30 minutes for up to 24 hours in sham TENS (SH-TENS) treated rats. The reduced PWL after the induction of ankle sprain was restored partially at 0,1,2,3 and 6, but not 24 hours, after the termination of 2 Hz-TENS (LF-TENS). In 100Hz (HF-TENS) the reduction in PWL was shorter than LF-TENS and both LF and HF effects were fully blocked in naloxone-treated rats. LF- and HF-TENS treated rats did not reach the elevation of edema and presented a progressive edema reduction for over 24 hours when compared to SH-TENS group. Both effects were reduced by naloxone. TENS-induced antihyperalgesic and anti-edematous effects observed in ankle sprain model were mediated by the endogenous opioid system.

100-Hz Electroacupuncture but not 2-Hz Electroacupuncture is Preemptive Against Postincision Pain in Rats

Preemptive analgesia involves introducing an analgesic before noxious stimulation. Electroacupuncture (EA) activates descending mechanisms that modulate nociceptive inputs into the spinal dorsal horn. This study evaluated whether preoperative EA is more effective than postoperative EA in reducing incision pain in rats. The nociceptive threshold to mechanical stimulation was utilized to examine the effects of an intraperitoneal injection of saline (0.1 mL/kg) or naloxone (1 mg/kg) on antinociception induced by a 20-minute period of 2-Hz or 100-Hz EA applied to the Zusanli (ST36) and Sanyinjiao (SP6) acupoints before surgical incision, or 10 minutes after or 100 minutes after surgical incision of the hind paw. The extent of mechanical hyperalgesia after the incision was significantly attenuated by the application of 100-Hz EA preoperatively, but not by its application at 10 minutes or 100 minutes postoperatively. By contrast, 2-Hz EA was effective against postoperative hyperalgesia when applied 10 minutes or 100 minutes after surgery but not when it was applied preoperatively. Only the effect of 2-Hz EA applied 10 minutes after surgery was sensitive to naloxone. The present study showed for the first time that 100-Hz EA, but not 2-Hz EA, exerts a nonopioidergic preemptive effect against postincision pain in rats.

Spinal p38 activity and analgesic effect after low- and high-intensity electroacupuncture stimulation in a plantar incision rat model

AIMS

Postoperative pain is a major problem. Electroacupuncture (EA) has been accepted as a useful and low-risk complementary therapy for post-operative pain. Animal studies indicate that surgical incision activates p38 MAPK in the spinal microglia, which critically contributes to post-incisional nociceptive development. How EA affects incision-induced p38 activation is important but yet to be fully elucidated.

METHODS

Male adult rats received plantar incision (PI) at the right hind paw followed by 30-min EA of 4-Hz, one of two intensities (3 and 10mA), and at right ST36 (Zusanli) acupoint immediately after PI and for 3 successive days. EA analgesia was evaluated by von Frey fibers and Hargreaves' tests. Spinal p38 activation was examined by immunostaining. In separate groups, SB203580, a p38 inhibitor, was intrathecally injected alone or with EA to test the combining effect on nociception and spinal phospho-p38.

KEY FINDINGS

EA of 10-mA significantly ameliorated mechanical allodynia, but 3-mA did not. None of them altered thermal hyperalgesia. Repeated EA could not inhibit phospho-p38 in the PI rats, contrarily, EA per se significantly induced phospho-p38 in the normal rats. Intrathecal SB203580 injection dose-dependently prevented PI-induced allodynia. Combination of low-dose SB203580 and 3-mA EA, which were ineffective individually, profoundly reduce post-PI allodynia.

SIGNIFICANCE

We demonstrated that 10-mA EA exerts a significant inhibition against post-PI mechanical hypersensitivity via a p38-independent pathway. Importantly, co-treatment with low-dose p38 inhibitor and 3-mA EA can counteract spinal phospho-p38 to exert strong analgesic effect. Our finding suggests a novel strategy to improve EA analgesic quality.

Effect of electro-acupuncture stimulation of Ximen (PC4) and Neiguan (PC6) on remifentanil-induced breakthrough pain following thoracal esophagectomy

The clinical analgesic effect of electro-acupuncture (EA) stimulation (EAS) on breakthrough pain induced by remifentanil in patients undergoing radical thoracic esophagectomy, and the mechanisms were assessed. Sixty patients (ASAIII) scheduled for elective radical esophagectomy were randomized into three groups: group A (control) receiving a general anesthesia only; group B (sham) given EA needles at PC4 (Ximen) and PC6 (Neiguan) but no stimulation; and group C (EAS) electrically given EAS of the ipsilateral PC4 and PC6 throughout the surgery. The EAS consisting of a disperse-dense wave with a low frequency of 2 Hz and a high frequency of 20 Hz, was performed 30 min prior to induction of general anesthesia and continued through the surgery. At the emergence, sufentanil infusion was given for postoperative analgesia with loading dose of 7.5 μg, followed by a continuous infusion of 2.25 μg/h. The patient self-administration of sufentanil was 0.75 μg with a lockout of 15 min as needed. Additional breakthrough pain was treated with dezocine (5 mg) intravenously at the patient's request. Blood samples were collected before (T1), 2 h (T2), 24 h (T3), and 48 h (T4) after operation to measure the plasma β-EP, PGE2, and 5-HT. The operative time, the total dose of sufentanil and the dose of self-administration, and the rescue doses of dezocine were recorded. Visual Analogue Scale (VAS) scores at 2, 12, 24 and 48 h postoperatively and the incidence of apnea and severe hypotension were recorded. The results showed that the gender, age, weight, operative time and remifentanil consumption were comparable among 3 groups. Patients in EAS group had the lowest VAS scores postoperatively among the three groups (P<0.05). The total dose of sufentanil was 115±6.0 μg in EAS group, significantly lower than that in control (134.3±5.9 μg) and sham (133.5±7.0 μg) groups. Similarly, the rescue dose of dezocine was the least in EAS group (P<0.05) among the three groups. Plasma β-EP levels in EAS group at T3 (176.90±45.73) and T4 (162.96±35.00 pg/mL) were significantly higher than those in control (132.33±36.75 and 128.79±41.24 pg/mL) and sham (136.56±45.80 and 129.85±36.14 pg/mL) groups, P<0.05 for all. EAS could decrease the release of PGE2. Plasma PGE2 levels in EAS group at T2 and T3 (41±5 and 40±5 pg/mL respectively) were significantly lower than those in control (64±5 and 62±7 pg/mL) and sham (66±6 and 62±6 pg/mL) groups. Plasma 5-HT levels in EAS group at T2 (133.66±40.85) and T3 (154.66±52.49 ng/mL) were significantly lower than those in control (168.33±56.94 and 225.28±82.03) and sham (164.54±47.53 and 217.74±76.45 ng/mL) groups. For intra-group comparison, plasma 5-HT and PGE2 levels in control and sham groups at T2 and T3, and β-EP in EAS group at T3 and T4 were significantly higher than those at T1 (P<0.05); PGE2 and 5-HT levels in EAS group showed no significant difference among the different time points (P>0.05). No apnea or severe hypotension was observed in any group. It was concluded that intraoperative ipsilateral EAS at PC4 and PC6 provides effective postoperative analgesia for patients undergoing radical esophagectomy with remifentanil anesthesia and significantly decrease requirement for parental narcotics. The underlying mechanism may be related to stimulation of the release of endogenous β-EP and inhibition of inflammatory mediators (5-HT and PGE2).

CXCL10 controls inflammatory pain via opioid peptide-containing macrophages in electroacupuncture

Acupuncture is widely used for pain treatment in patients with osteoarthritis or low back pain, but molecular mechanisms remain largely enigmatic. In the early phase of inflammation neutrophilic chemokines direct opioid-containing neutrophils in the inflamed tissue and stimulate opioid peptide release and antinociception. In this study the molecular pathway and neuroimmune connections in complete Freund's adjuvant (CFA)-induced hind paw inflammation and electroacupuncture for peripheral pain control were analyzed. Free moving Wistar rats with hind paw inflammation were treated twice with electroacupuncture at GB30 (Huan Tiao - gall bladder meridian) (day 0 and 1) and analyzed for mechanical and thermal nociceptive thresholds. The cytokine profiles as well as the expression of opioid peptides were quantified in the inflamed paw. Electroacupuncture elicited long-term antinociception blocked by local injection of anti-opioid peptide antibodies (beta-endorphin, met-enkephalin, dynorphin A). The treatment altered the cytokine profile towards an anti-inflammatory pattern but augmented interferon (IFN)-gamma and the chemokine CXCL10 (IP-10: interferon gamma-inducible protein) protein and mRNA expression with concomitant increased numbers of opioid peptide-containing CXCR3⁺ macrophages. In rats with CFA hind paw inflammation without acupuncture repeated injection of CXCL10 triggered opioid-mediated antinociception and increase opioid-containing macrophages. Conversely, neutralization of CXCL10 time-dependently decreased electroacupuncture-induced antinociception and the number of infiltrating opioid peptide-expressing CXCR3⁺ macrophages. In summary, we describe a novel function of the chemokine CXCL10 - as a regulator for an increase of opioid-containing macrophages and antinociceptive mediator in inflammatory pain and as a key chemokine regulated by electroacupuncture.

Efeitos da eletroacupuntura, aquapuntura e farmacopuntura em cadelas anestesiadas com isofluorano e submetidas à ovário-histerectomia

Avaliaram-se os efeitos analgésicos da eletroacupuntura, aquapuntura e farmacopuntura com morfina nos acupontos VB41 e TA5 de 24 cadelas hígidas submetidas à ovário-histerectomia. Os animais foram distribuídos em quatro grupos (G) de igual número - GDest, GMorf, GElet e GC - e anestesiados com acepromazina, propofol e isofluorano. Após a estabilização do plano anestésico, os animais do GDest receberam 0,5mL de água destilada em cada acuponto; os do GMorf receberam 0,1mg/kg de morfina distribuído nos quatro acupontos; os do GElet foram submetidos à eletroacupuntura; e os do GC, acupuntura em pontos sham. Os animais do GC receberam, após o término do procedimento cirúrgico e antes do início da avaliação pelas escalas de dor, 2,0mg/kg de tramadol. Foram avaliadas: frequências cardíaca e respiratória, temperatura retal e glicemia. A dor foi avaliada por duas escalas, uma de analogia numérica e outra contagem variável, por três observadores. A avaliação iniciou-se imediatamente após a extubação e foi realizada a cada 15 minutos, durante duas horas. Não houve diferença entre os tratamentos em todas as variáveis. Pode-se concluir que eletroacupuntura, aquapuntura e farmacopuntura com morfina resultaram em analgesia similar ao tramadol no pós-operatório imediato de cadelas submetidas à ovário-histerectomia eletiva.

Effect of electroacupuncture on visceral hyperalgesia, serotonin and fos expression in an animal model of irritable bowel syndrome

Background/Aims

While it is well established that acupuncture relieves somatic pain, its therapeutic effect on visceral pain such as irritable bowel syndrome (IBS) is unclear. We evaluated the effect of acupuncture in treating visceral hyperalgesia in an animal model.

Methods

Sprague-Dawley rats (n = 8 per group) with prior neonatal maternal separation stress were randomly allocated to receive 3-day treatment of either electroacupuncture (EA) or sham acupuncture at acupoint ST-36. Another group of rats without prior maternal separation was included as non-handled controls. Colorectal distension was performed on the day after acupuncture treatment. The 3 groups were compared for pain threshold as determined by abdominal withdrawal reflex and visceromotor response as measured by electromyogram. Colon, spinal cord, and brainstem were sampled for topographic distribution and quantitative assessment of serotonin and Fos expression by immunohistochemistry.

Results

Rats in EA group had significantly higher pain threshold compared to those in sham acpuncture group (25.0 ± 5.7 mmHg vs 18.7 ± 5.2 mmHg, p = 0.01) and it was comparable with that of non-handled treatment naïve controls (29.4 ± 9.3 mmHg, p = 0.28). They also had lower visceromotor response as measured by electromyogram compared to those received sham acupuncture at all colorectal distension pressures. EA significantly suppressed Fos expression in doral raphe nuclei of brainstem, superficial dorsal horn of spinal cord and colonic epithelium but suppressed 5-HT expression only in brainstem and spinal cord.

Conclusions

Electro acupuncture attenuates visceral hyperlagesia through down-regulation of central serotonergic activities in the brain-gut axis.

The status and future of acupuncture mechanism research

On November 8-9, 2007, the Society for Acupuncture Research (SAR) hosted an international conference to mark the tenth anniversary of the landmark NIH [National Institutes of Health] Consensus Development Conference on Acupuncture. More than 300 acupuncture researchers, practitioners, students, funding agency personnel, and health policy analysts from 20 countries attended the SAR meeting held at the University of Maryland School of Medicine, Baltimore, MD. This paper summarizes important invited lectures in the area of basic and translational acupuncture research. Specific areas include the scientific assessment of acupuncture points and meridians, the neural mechanisms of cardiovascular regulation by acupuncture, mechanisms for electroacupuncture applied to persistent inflammation and pain, basic and translational research on acupuncture in gynecologic applications, the application of functional neuroimaging to acupuncture research with specific application to carpal-tunnel syndrome and fibromyalgia, and the association of the connective tissue system to acupuncture research. In summary, mechanistic models for acupuncture effects that have been investigated experimentally have focused on the effects of acupuncture needle stimulation on the nervous system, muscles, and connective tissue. These mechanistic models are not mutually exclusive. Iterative testing, expanding, and perhaps merging of such models will potentially lead to an incremental understanding of the effects of manual and electrical stimulation of acupuncture needles that is solidly rooted in physiology.

Inhibition of the ubiquitin-proteasome activity prevents glutamate transporter degradation and morphine tolerance

Glutamate transporters play a crucial role in physiological glutamate homeostasis and neurotoxicity. Recently, we have shown that downregulation of glutamate transporters after chronic morphine exposure contributed to the development of morphine tolerance. In the present study, we examined whether regulation of the glutamate transporter expression with the proposed proteasome inhibitor MG-132 would contribute to the development of tolerance to repeated intrathecal (twice daily x 7 days) morphine administration in rats. The results showed that MG-132 (5 nmol) given intrathecally blocked morphine-induced glutamate transporter downregulation and the decrease in glutamate uptake activity within the spinal cord dorsal horn. Co-administration of morphine (15 nmol) with MG-132 (vehicle=1<2.5<5=10 nmol) also dose-dependently prevented the development of morphine tolerance in rats. These findings suggest that prevention of spinal glutamate transporter downregulation may regulate the glutamatergic function that has been implicated in the development of morphine tolerance. The possible relationship between MG-132-mediated regulation of glutamate transporters, ubiquitin-proteasome system, and the cellular mechanisms of morphine tolerance is discussed in light of these findings.

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.

Spinal mu-opioid receptor-expressing dorsal horn neurons: role in nociception and morphine antinociception

The role of spinal cord mu-opioid receptor (MOR)-expressing dorsal horn neurons in nociception and morphine analgesia is incompletely understood. Using intrathecal dermorphin-saporin (Derm-sap) to selectively destroy MOR-expressing dorsal horn neurons, we sought to determine the role of these neurons in (1) normal baseline reflex nocifensive responses to noxious thermal stimulation (hotplate, tail flick) and to persistent noxious chemical stimulation (formalin) and (2) the antinociceptive activity of intrathecal and systemic morphine in the same tests. Lumbar intrathecal Derm-sap (500 ng) produced (1) partial loss of lamina II MOR-expressing dorsal horn neurons, (2) no effect on MOR-expressing dorsal root ganglion neurons, and (3) no change in baseline tail-flick and hotplate reflex nocifensive responses. Derm-sap treatment attenuated the antinociceptive action of both intrathecal and systemic morphine on hotplate responses. Derm-sap treatment had two effects in the formalin test: (1) increased baseline nocifensive responding and (2) reduced antinociceptive action of systemic morphine. We conclude that MOR-expressing dorsal horn neurons (1) are not essential for determining nocifensive reflex responsiveness to noxious thermal stimuli, (2) are necessary for full antinociceptive action of morphine (intrathecal or systemic) in these tests, and (3) play a significant role in the endogenous modulation of reflex nocifensive responses to persistent pain in the formalin test. Thus, one would predict that altering the activity of MOR-expressing dorsal horn neurons would be antinociceptive and of interest in the search for new approaches to management of chronic pain.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).