Electroacupuncture suppresses capsaicin-induced secondary hyperalgesia through an endogenous spinal opioid mechanism.
Central sensitization, caused either by tissue inflammation or peripheral nerve injury, plays an important role in persistent pain. An animal model of capsaicin-induced pain has well-defined peripheral and central sensitization components, thus is useful for studying the analgesic effect on two separate components. The focus of this study is to examine the analgesic effects of electroacupuncture (EA) on capsaicin-induced secondary hyperalgesia, which represents central sensitization. Capsaicin (0.1%, 20 microl) was injected into the plantar side of the left hind paw, and foot withdrawal thresholds in response to von Frey stimuli (mechanical sensitivity) were determined for both primary and secondary hyperalgesia in rats. EA (2 Hz, 3 mA) was applied to various pairs of acupoints, GB30-GB34, BL40-BL60, GV2-GV6, LI3-LI6 and SI3-TE8, for 30 min under isoflurane anesthesia and then the effect of EA on mechanical sensitivity of paw was determined. EA applied to the ipsilateral SI3-TE8, but to none of the other acupoints, significantly reduced capsaicin-induced secondary hyperalgesia but not primary hyperalgesia. EA analgesic effect was inhibited by a systemic non-specific opioid receptor (OR) antagonist or an intrathecal mu- or delta-OR antagonist. EA analgesic effect was not affected by an intrathecal kappa-OR antagonist or systemic adrenergic receptor antagonist. This study demonstrates that EA produces a stimulation point-specific analgesic effect on capsaicin-induced secondary hyperalgesia (central sensitization), mediated by activating endogenous spinal mu- and delta-opioid receptors.
Kim
Hee Young
HY
Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, TX 77555-1069, USA.
Wang
Jigong
J
Lee
Inhyung
I
Kim
Hee Kee
HK
Chung
Kyungsoon
K
Chung
Jin Mo
JM
D015669:Acupuncture Points; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000378:metabolism*; D000704:Analysis of Variance; D000818:Animals; D002211:Capsaicin / Q000494:pharmacology; D004195:Disease Models, Animal; D015671:Electroacupuncture / Q000379:methods*; D007839:Functional Laterality / Q000187:drug effects / Q000502:physiology; D006930:Hyperalgesia / Q000139:chemically induced / Q000473:pathology / Q000628:therapy*; D008297:Male; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects / Q000379:methods; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology*; D010646:Phentolamine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism*; D013997:Time Factors; D014110:Touch
D000317:Adrenergic alpha-Antagonists; D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D002211:Capsaicin; D010646:Phentolamine