Effect of Electroacupuncture on Neuropathic Pain and its Mechanism

Acupuncture has long been used to relieve pain. Clinical trials have shown that acupuncture has significant analgesic effects on chronic pain induced by peripheral neuropathy including facial nerve neuropathy, sciatic nerve neuropathy, herpetic trigeminal ganglionic neuropathy, diabetic neuropathy and HIV related neuropathy etc. Clinical and experimental studies have shown that clectroacupuncture (EA) plays its analgesic role via activating the endogenous pain modulating systems, for example, the opioid system. Orphanin FQ/nociceptin (OFQ), a member of opioids family found in 1995, is an endogenous ligand of opioid receptor like receptor 1 (ORL1), a member of opioid receptor family. OFQ is a 17 amino-acid peptide (FGGFTGARKSARKLANQ). Our previous work demonstrated that spinal OFQ produced analgesia and potentiated morphine and endomorphin-1 induced analgesia but supraspinal OFQ induced hyperalgesia and antagonized morphine and endomorphin-1 induced analgesia, indicating that spinal and supraspinal OFQ play different roles in pain modulation. Neuropathic pain is a common chronic pain in clinics, and it has been shown that EA had potent analgesic effect in neuropathic pain patients. However, the role of OFQ during neuropathic pain and EA analgesia is unclear, and the therapeutic effect of EA remains to be enhanced. Therefore, the aim of this study was to investigate the role of spinal OFQ in the neuropathic pain of rats and EA analgesia, and the changes of expression of ppOFQ mRNA in the spinal dorsal horn of rats during neuropathic pain and EA analgesia. Chronic constriction injury (CCI) to sciatic nerve in rats was used as an animal model of neuropathic pain in our experiment. In CCI rats, ipsilateral paw withdrawal latency (PWL) decreased significantly and hyperalgesia score became negative, which lasted more than 28 days. EA treatment: Two acupoints ‘Huan-Tiao’ (GB-30) and ‘Yang-Ling-Quan’ (GB-34) were selected. Using the current stimulation with alternating strings of dense-sparse frequencies (60 Hz for 1.05 s and 2 Hz for 2.85 s alternately, lasting for 30 min), the intensity was adjusted to induce slight twitch of the hindlimb (≤1 mA, 12 volt).