Effect And Its Underlying Mechanism Of Electroacupuncture On Remifentanil-induced Postoperative Hyperalgesia In Rats

Objective:Opioids,serving as the major potent analgesic agents used in clinic,can paradoxically activate the pronociceptive systems,causing opioid-induced hyperalgesia(OIH)and even transforming acute pain into chronic pain.Remifentanil is an ultra-short-acting?-opioid receptor agonist with rapid onset and predictable rapid recovery profile,and has been widely used in clinical anesthesia.However,many studies indicated that remifentanil could induce remifentanil-induced hyperalgesia(RIH)more rapidly,prominently and frequently than other opioids.Although extensive studies have been conducted and considerable progress has been made concerning the mechanisms underlying OIH/RIH,a clear understanding in this domain is still lacking.Accumulating studies have suggested that multiple factors may be involved in OIH/RIH,including spinal glial activation,proinflammatory cytokine upregulation,mitogenactivated protein kinase(MAPK)activation,and N-methyl-D-aspartate(NMDA)receptor activation.Acupuncture,used in China for the past 3,000 yrs,is growing in popularity world-wide at present,and interests in the clinical and basic research of acupuncture are also increasing.Acupuncture is often used as a safe,economic,and effective tool for preventing and treating a spectrum of diseases,especially pain.Acupuncture has also been used to prevent various opioid-induced adverse effects,such as addiction,tolerance,nausea,vomiting,and locomotor sensitization.Nowadays,the antinociceptive mechanisms of acupuncture are still unclear and considered multifactorial,including glial cells,proinflammatory cytokines,MAPKs,and NMDA receptors.In this study,we investigated whether intraoperative electroacupuncture(EA)could relieve remifentanil-induced postoperative hyperalgesia,and detected astrocytic marker glial fibrillary acidic protein(GFAP),microglial marker Ibal,proinflammatory cytokines,phosphorylated MAPKs(p-MAPKs)and p-NR2B in the spinal cord with Western blot and/or immunofluorescence,for the purpose of revealing the specific mechanisms of anti-hyperalgesia of EA for RIH.Methods:The study consisted of two independent experiments:(1)A rat model of remifentanil-induced postoperative hyperalgesia was used in this study.Adult male Sprague-Dawley rats were subcutaneously administered with remifentanil(0.04 mg/kg,0.4 ml)at a rate of 0.8 ml/h over a period of 30 min using an infusion pump.The surgical incision of the right hind paw was started during the infusion of remifentanil.EA during surgery was conducted at bilateral Zusanli(ST36)acupoints,and different EA parameters(2 Hz and 100 Hz in frequency;0.5 mA,1 mA and 2 mA in intensity)were used.Behavior tests,including mechanical allodynia and thermal hyperalgesia,were performed at 2 h,1 d,2 d,3 d,4 d,5 d,6 d,7 d,8 d,9 d,and 10 d after the surgical procedure;(2)The same rat model of remifentanil-induced postoperative hyperalgesia as above was used in this study.EA during surgery was conducted at bilateral Zusanli(ST36)acupoints,and EA parameters(2 Hz in frequency;1 mA in intensity)were used.Astrocytic marker GFAP,microglial marker Iba1,interleukin-1?(IL-1?),tumor necrosis factor-?(TNF-a),phosphorylated MAPKs(p-p38,p-JNK,and p-ERK1/2),and p-NR2B in the spinal cord were detected by Western blot and/or immunofluorescence at 1 d after the surgical procedure.Results:(1)Mechanical allodynia and thermal hyperalgesia were induced by both surgical incision and remifentanil infusion,and remifentanil infusion significantly exaggerated and prolonged incision-induced pronociceptive effects;(2)When the intensity(1 mA)was kept constant,EA with low frequency(2 Hz,1 mA)significantly attenuated incision-and/or remifentanil induced pronociceptive effects,and decreased the duration of hyperalgesia induced by combination of surgical incision and remifentanil infusion;EA with high frequency(100 Hz,1 mA)only attenuated incision-induced pronociceptive effects,but neither attenuated remifentanil-induced pronociceptive effects nor decreased the duration of hyperalgesia induced by combination of surgical incision and remifentanil infusion;(3)When the frequency(2 Hz)was kept constant,EA with different intensity(0.5 mA,1 mA and 2 mA)all significantly attenuated incision-and/or remifentanil-induced pronociceptive effects;(4)GFAP,Iba1,IL-1?,TNF-??p-p38,p-JNK,p-ERK1/2 and p-NR2B were upregulated after surgical incision,remifentanil infusion,and especially after their combination at 1 d after the surgical procedure.Intraoperative EA(2 Hz,1 mA)significantly attenuated incision-and/or remifentanil-induced spinal glial activation,proinflammatory cytokine upregulation,MAPK activation and p-NR2B upregulation.Conclusions:Remifentanil-induced postoperative hyperalgesia in rats can be relieved by intraoperative EA via inhibiting the activation of spinal glial cells,the upregulation of spinal proinflammatory cytokines,the activation of spinal MAPKs,and the activation of spinal NMDA receptors.The antihyperalgesia effect of EA on RIH is frequency-dependent,and low frequency is superior to high frequency.