Analgesia induced by 2- or 100-Hz electroacupuncture in the rat tail-flick test depends on the anterior pretectal nucleus

Electroacupuncture effects on trigeminal neuralgia with comorbid anxiety and depression: The role of frequency and acupoint specificity

This study aimed to investigate the effects of electroacupuncture (EA) at specific acupoints (DU20 and ST36) and different frequencies (2 and 100 Hz) on brain regions associated with trigeminal neuralgia, anxiety, and depression. Chronic trigeminal neuralgia was induced by the chronic constriction of the infraorbital nerve (CION). Anxiety and depression were assessed through behavioral tests. The effects of high-frequency (100 Hz) and low-frequency (2 Hz) EA at DU20 and ST36 were compared using immunofluorescence staining to evaluate their impact on pain, anxiety, depression, and brain activity. CION induced prominent trigeminal neuralgia in mice, accompanied by anxiety- and depression-like behaviors. Two weeks post-CION surgery increased neural activity was observed in the Prl, Cg1, CeA, BLA, TRN, CA3, CA1, vlPAG, PC5, and LPB brain regions, while reduced activity was noted in the PVN, VTA, and LDTgv regions. EA at 100 Hz applied to DU20 and ST36 rapidly alleviated pain and specifically reduced despair behavior, a depressive-like phenotype. In contrast, 2 Hz EA at the same acupoints addressed both anxiety- and depression-like behaviors, modulating a broader range of brain regions, including the PrL, BLA, PVN, VTA, vlPAG, and LDTgv, compared to 100 Hz EA. Repeated 2 Hz EA exclusively at DU20 was sufficient for analgesia and improvement of anxiety and depression, demonstrating a more extensive modulation of brain activity, particularly in the VTA and LDTgv, than EA at ST36. The study reveals that CION induces significant trigeminal neuralgia, accompanied by anxiety and depression, characterized by distinct neural activity patterns. EA at 2 Hz exhibits greater effectiveness in alleviating anxiety and depression, exerting broad modulation across various brain regions. Notably, EA at DU20 demonstrates superior modulation of brain activity and enhanced antidepressant and analgesic effects compared to ST36. These findings provide valuable insights into the nuanced therapeutic effects of EA on the interplay between chronic pain and affective disorders, suggesting potential clinical strategies for intervention.

Potential mechanisms of acupuncture for neuropathic pain based on somatosensory system

Neuropathic pain, caused by a lesion or disease of the somatosensory system, is common and distressing. In view of the high human and economic burden, more effective treatment strategies were urgently needed. Acupuncture has been increasingly used as an adjuvant or complementary therapy for neuropathic pain. Although the therapeutic effects of acupuncture have been demonstrated in various high-quality randomized controlled trials, there is significant heterogeneity in the underlying mechanisms. This review aimed to summarize the potential mechanisms of acupuncture on neuropathic pain based on the somatosensory system, and guided for future both foundational and clinical studies. Here, we argued that acupuncture may have the potential to inhibit neuronal activity caused by neuropathic pain, through reducing the activation of pain-related ion channels and suppressing glial cells (including microglia and astrocytes) to release inflammatory cytokines, chemokines, amongst others. Meanwhile, acupuncture as a non-pharmacologic treatment, may have potential to activate descending pain control system via increasing the level of spinal or brain 5-hydroxytryptamine (5-HT), norepinephrine (NE), and opioid peptides. And the types of endogenously opioid peptides was influenced by electroacupuncture-frequency. The cumulative evidence demonstrated that acupuncture provided an alternative or adjunctive therapy for neuropathic pain.

The role of the anterior pretectal nucleus in pain modulation: A comprehensive review

Descending pain modulation involves multiple encephalic sites and pathways that range from the cerebral cortex to the spinal cord. Behavioral studies conducted in the 1980s revealed that electrical stimulation of the pretectal area causes antinociception dissociation from aversive responses. Anatomical and physiological studies identified the anterior pretectal nucleus and its descending projections to several midbrain, pontine, and medullary structures. The anterior pretectal nucleus is morphologically divided into a dorsal part that contains a dense neuron population (pars compacta) and a ventral part that contains a dense fiber band network (pars reticulata). Connections of the two anterior pretectal nucleus parts are broad and include prominent projections to and from major encephalic systems associated with somatosensory processes. Since the first observation that acute or chronic noxious stimuli activate the anterior pretectal nucleus, it has been established that numerous mediators participate in this response through distinct pathways. Recent studies have confirmed that at least two pain inhibitory pathways are activated from the anterior pretectal nucleus. This review focuses on rodent anatomical, behavioral, molecular, and neurochemical data that have helped to identify mediators of the anterior pretectal nucleus and pathways related to its role in pain modulation.

Acupuncture for Pain Management: Molecular Mechanisms of Action

Acupuncture reduces pain by activating specific areas called acupoints on the patient’s body. When these acupoints are fully activated, sensations of soreness, numbness, fullness, or heaviness called De qi or Te qi are felt by clinicians and patients. There are two kinds of acupuncture, manual acupuncture and electroacupuncture (EA). Compared with non-acupoints, acupoints are easily activated on the basis of their special composition of blood vessels, mast cells, and nerve fibers that mediate the acupuncture signals. In the spinal cord, EA can inhibit glial cell activation by down-regulating the chemokine CX3CL1 and increasing the anti-inflammatory cytokine interleukin-10. This inhibits P38 mitogen-activated protein kinase and extracellular signal-regulated kinase pathways, which are associated with microglial activation of the C-Jun N-terminal kinase signaling pathway and subsequent astrocyte activation. The inactivation of spinal microglia and astrocytes mediates the immediate and long-term analgesic effects of EA, respectively. A variety of pain-related substances released by glial cells such as the proinflammatory cytokines tumor necrosis factor [Formula: see text], interleukin-1[Formula: see text], interleukin-6, and prostaglandins such as prostaglandins E2 can also be reduced. The descending pain modulation system in the brain, including the anterior cingulated cortex, the periaqueductal gray, and the rostral ventromedial medulla, plays an important role in EA analgesia. Multiple transmitters and modulators, including endogenous opioids, cholecystokinin octapeptide, 5-hydroxytryptamine, glutamate, noradrenalin, dopamine, [Formula: see text]-aminobutyric acid, acetylcholine, and orexin A, are involved in acupuncture analgesia. Finally, the “Acupuncture [Formula: see text]” strategy is introduced to help clinicians achieve better analgesic effects, and a newly reported acupuncture method called acupoint catgut embedding, which injects sutures made of absorbable materials at acupoints to achieve long-term effects, is discussed.

Developing Improved Translational Models of Pain: A Role for the Behavioral Scientist

The effective management of pain is a longstanding public health concern. Although opioids have been frontline analgesics for decades, they also have well-known undesirable effects that limit their clinical utility, such as abuse liability and respiratory depression. The failure to develop better analgesics has, in some ways, contributed to the escalating opioid epidemic that has claimed tens of thousands of lives and has cost hundreds of billions of dollars in health-care expenses. A paradigm shift is needed in the pharmacotherapy of pain management that will require extensive efforts throughout biomedical science. The purpose of the present review is to highlight the critical role of the behavioral scientist to devise improved translational models of pain for drug development. Despite high heterogeneity of painful conditions that involve cortical-dependent pain processing, current models often feature an overreliance on simple reflex-based measures and an emphasis on the absence, rather than presence, of behavior as evidence of analgesic efficacy. Novel approaches should focus on the restoration of operant and other CNS-mediated behavior under painful conditions.

Electroacupuncture Relieves CCI-Induced Neuropathic Pain Involving Excitatory and Inhibitory Neurotransmitters

Neuropathic pain caused by peripheral tissue injuries to the higher brain regions still has no satisfactory therapy. Disruption of the balance of excitatory and inhibitory neurotransmitters is one of the underlying mechanisms that results in chronic neuropathic pain. Targeting neurotransmitters and related receptors may constitute a novel approach for treating neuropathic pain. We investigated the effects of electroacupuncture (EA) on chronic constriction injury- (CCI-) induced neuropathic pain. The mechanical allodynia and thermal hyperalgesia pain behaviors were relieved by 15 Hz EA but not by 2 and 50 Hz. These phenomena were associated with increasing γ-amino-butyric acid (GABA) receptors in the hippocampus and periaqueductal gray (PAG) but not N-methyl-D-aspartate receptors. Furthermore, excitatory neurotransmitter glutamate was decreased in the hippocampus and inhibitory neurotransmitter GABA was increased in the PAG under treatment with EA. These data provide novel evidence that EA modulates neurotransmitters and related receptors to reduce neuropathic pain in the higher brain regions. This suggests that EA may be a useful therapy option for treating neuropathic pain.

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.

Involvement of hippocampal acetylcholinergic receptors in electroacupuncture analgesia in neuropathic pain rats

BACKGROUND

Cumulating evidence has shown a close correlation between electroacupuncture stimulation (EAS) frequency-specific analgesic effect and central opioid peptides. However, the actions of hippocampal acetylcholinergic receptors have not been determined. This study aims to observe the effect of different frequencies of EAS on the expression of hippocampal muscarinic and nicotinic acetylcholinergic receptors (mAChRs, nAChRs) in neuropathic pain rats for revealing their relationship.

METHODS

Forty male Wistar rats were randomly and equally divided into sham, CCI model, 2, 2/15 and 100 HzEA groups. The neuropathic pain model was established by ligature of the left sciatic nerve to induce chronic constriction injury (CCI). EAS was applied to bilateral Zusanli (ST36) and Yanglingquan (GB34) for 30 min, once daily for 14 days except weekends. The mechanical pain thresholds (withdrawal latencies, PWLs) of bilateral hindpaws were measured. The expression levels of hippocampal M1 and M2 mAChR, and α4 and β2 nAChR genes and proteins were detected by quantitative RT-PCR and Western blot, separately. The involvement of mAChR and nAChR in the analgesic effect of EAS was confirmed by intra-hippocampal microinjection of M1mAChR antagonist (Pirenzepine) and α4β2 nAChR antagonist (dihydro-beta-erythroidine) respectively.

RESULTS

Following EAS, the CCI-induced increase of difference values of bilateral PWLs on day 6 and 14 was significantly reduced (P < 0.05), with 2/15 Hz being greater than 100 Hz EAS on day 14 (P < 0.05). After 2 weeks' EAS, the decreased expression levels of M1 mAChR mRNA of both 2 and 2/15 Hz groups and M1 mAChR protein of the three EAS groups, α4 AChR mRNA of the 2/15 Hz group and β2 nAChR protein of the three EAS groups were considerably increased (P < 0.05), suggesting an involvement of M1 mAChR and β2 nAChR proteins in EAS-induced pain relief. No significant changes were found in the expression of M2 mAChR mRNA and protein, α4 nAChR protein and β2 nAChR mRNA after CCI and EAS (P > 0.05). The analgesic effect of EAS was abolished by intra-hippocampal microinjection of M1mAChR and α4β2 nAChR antagonists respectively.

CONCLUSIONS

EAS of ST36-GB34 produces a cumulative analgesic effect in neuropathic pain rats, which is frequency-dependent and probably mediated by hippocampal M1 mAChR and β2 nAChR proteins.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.