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

摘要

Background: Electroacupuncture has been elicited to effectively alleviate the pain sensation. Preemptive analgesic effect of pre-EA has also been suggested in recent studies, while the underlying analgesic mechanism of pre-electroacupuncture (pre-EA) requires further investigation. This study aimed to explore the preemptive analgesia of pre-EA in formalin-induced acute inflammatory pain model. Methods: Forty rats were randomly divided into control, model, pre-EA and post-EA group. Inflammatory pain model was induced via injecting 50 ul 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-EA group were treated with EA at ipsilateral Zusanli (ST36) and Weizhong (BL40) acupoints (2 Hz, 1 mA) for 30 min before formalin injection, while received the same EA treatment immediately after formalin injection in the post-EA 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 Iba1 and c-fos in spinal cord. Moreover, enzyme linked immunosorbent assay (ELISA) was applied to measure the secretion of IL-6, IFN-γ, IL-4, substance P (SP) and calcitonin gene-related peptide (CGRP) in spinal cord. Results: Paw flinching and licking were obviously induced by formalin injection. Iba-1, c-fos, proinflammatory cytokines (IL-6 and IFN-γ), pain neurotransmitters (SP and CGRP) were dramatically increased in the L4-5 spinal cord after formalin injection, while anti-inflammatory cytokine IL-4 was decreased. Pre-EA and post-EA administration significantly attenuated formalin-induced nociceptive effects, spinal microglia and neurons activation, proinflammatory cytokines and pain neurotransmitters upregulation, and upregulated the anti-inflammatory cytokine. Further, these effects of pre-EA were more significant than that of post-EA. Conclusions: This study illustrates the potential therapeutic effect of pre-EA against acute inflammatory pain and reveals the mechanism underlying pre-EA mediated analgesia, thus provides a novel preemptive analgesic treatment.