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

The preemptive analgesia of pre-electroacupuncture in rats with formalin-induced acute inflammatory pain

Background

Electroacupuncture has been elicited to effectively alleviate the pain sensation. Preemptive analgesic effect of pre-electroacupuncture has also been suggested in recent studies, while the underlying analgesic mechanism of pre-electroacupuncture requires further investigation. This study aimed to explore the preemptive analgesia of pre-electroacupuncture in formalin-induced acute inflammatory pain model.

Methods

Forty rats were randomly divided into control, model, pre-electroacupuncture, and post-electroacupuncture group. Inflammatory pain model was induced via injecting 50 µl 5% formalin into the plantar surface of right hind paw, while the equal volume of saline injection in the control group. Rats in the pre-electroacupuncture group were treated with electroacupuncture at ipsilateral Zusanli (ST36) and Weizhong (BL40) acupoints (2 Hz, 1 mA) for 30 min before formalin injection, while received the same electroacupuncture treatment immediately after formalin injection in the post-electroacupuncture group. Flinching number and licking time were recorded during 60 min after formalin injection. Immunofluorescence and Western blot were used to detect the expression of ionized calcium binding adapter molecule 1 (Iba1) and c-fos in spinal cord. Moreover, enzyme-linked immunosorbent assay was applied to measure the secretion of IL-6, IFN-γ, IL-4, substance P, and calcitonin gene-related peptide in spinal cord.

Results

Paw flinching and licking were obviously induced by formalin injection. Iba1, c-fos, proinflammatory cytokines (IL-6 and IFN-γ), and pain neurotransmitters (substance P and calcitonin gene-related peptide) were dramatically increased in the L4-5 spinal cord after formalin injection, while anti-inflammatory cytokine IL-4 was decreased. Pre-electroacupuncture and post-electroacupuncture administration significantly attenuated formalin-induced nociceptive effects, spinal microglia and neurons activation, proinflammatory cytokines and pain neurotransmitters upregulation, and upregulated the anti-inflammatory cytokine. Furthermore, these effects of pre-electroacupuncture were more significant than that of post-electroacupuncture.

Conclusions

This study illustrates the potential therapeutic effect of pre-electroacupuncture against acute inflammatory pain and reveals the mechanism underlying pre-electroacupuncture mediated analgesia, thus providing a novel preemptive analgesic treatment.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Electroacupuncture Stimulation Alleviates CFA-Induced Inflammatory Pain Via Suppressing P2X3 Expression

Chronic inflammatory pain is one of the most common complaints that seriously affects patients’ quality of life. Previous studies have demonstrated that the analgesic effect of electroacupuncture (EA) stimulation on inflammatory pain is related to its frequency. In this study, we focused on whether the analgesic effects of EA are related to the period of stimulation. Purinergic receptor P2X3 (P2X3) is involved in the pathological process underlying chronic inflammatory pain and neuropathic pain. We hypothesized that 100 Hz EA stimulation alleviated Freund’s complete adjuvant (CFA) induced inflammatory pain via regulating P2X3 expression in the dorsal root ganglion (DRG) and/or spinal cord dorsal horn (SCDH). We also assumed that the analgesic effect of EA might be related to the period of stimulation. We found that both short-term (three day) and long-term (14 day) 100 Hz EA stimulation effectively increased the paw withdrawal threshold (PWT) and reversed the elevation of P2X3 in the DRG and SCDH of CFA rats. However, the analgesic effects of 100 Hz EA were not dependent on the period of stimulation. Moreover, P2X3 inhibition or activation may contribute to or attenuate the analgesic effects of 100 Hz EA on CFA-induced inflammatory pain. This result indicated that EA reduced pain hypersensitivity through P2X3 modulation.