Electroacupuncture decreases Netrin-1-induced myelinated afferent fiber sprouting and neuropathic pain through μ-opioid receptors

Abstract

Hong-Ping Li,1 Wen Su,2 Yang Shu,3 Xiao-Cui Yuan,1 Li-Xue Lin,1 Teng-Fei Hou,1 Hong-Chun Xiang,1 He Zhu,1 Xue-Fei Hu,1 Li Pan,1 Jing-Nan Wu,1 Xian-Fang Meng,1 Hui-Lin Pan,4 Cai-Hua Wu,1,2 Man Li11Department of Neurobiology, School of Basic Medicine, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, People’s Republic of China; 2Department of Acupuncture, Wuhan First Hospital, Wuhan, People’s Republic of China; 3Department of Central Laboratory, Affiliated Hospital of Jiangsu University, Zhenjiang 212000, People’s Republic of China; 4Department of Anesthesiology and Perioperative Medicine, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USAPurpose: We determined whether electroacupuncture (EA) reduces Netrin-1-induced myelinated primary afferent nerve fiber sprouting in the spinal cord and pain hypersensitivity associated with postherpetic neuralgia (PHN) through activation of μ-opioid receptors.Methods: PHN was induced by systemic injection of resiniferatoxin (RTX) in rats. Thirty-six days after RTX injection, a μ-opioid receptor antagonist, beta-funaltrexamine (β-FNA) or a κ-opioid receptor antagonist, nor Binaltorphimine (nor-BNI), was injected intrathecally 30 mins before EA, once every other day for 4 times. Mechanical allodynia was tested with von Frey filaments. The protein expression level of Netrin-1 and its receptors (DCC and UNC5H2) were quantified by using western blotting. The myelinated primary afferent nerve fiber sprouting was mapped with the transganglionic tracer cholera toxin B-subunit (CTB).Results: Treatment with 2 Hz EA at “Huantiao” (GB30) and “Yanglingquan” (GB34) decreased the mechanical allodynia at 22 days and the myelinated primary afferent nerve fiber preternatural sprouting into the lamina II of the spinal dorsal horn at 42 days after RTX injection. Also, treatment with 2 Hz EA reduced the protein levels of DCC and Netrin-1 and promoted the expression of UNC5H2 in the spinal dorsal horn 42 days after RTX injection. Furthermore, the μ-opioid receptor antagonist β-FNA, but not the κ-opioid receptor antagonist nor-BNI, reversed the effect of EA on neuropathic pain caused by RTX. In addition, morphine inhibited the Netrin-1 protein level induced by RTX in SH-SY5Y cells.Conclusions: Through activation of μ-opioid receptors, treatment with EA reduces the expression level of DCC and Netrin-1 and changes a growth-permissive environment in spinal dorsal horn into an inhibitory environment by increasing UNC5H2, thus decreasing RTX-caused primary afferent nerve sprouting in the spinal dorsal horn and neuropathic pain.Keywords: postherpetic neuralgia, analgesia, mechanical hyperalgesia, rat, μ-opioid receptors