| Electroacupuncture (EA) is a therapeutic method through transmitting the current to the acupoints with needles, and extensively used in the clinic practice to treat painful diseases and to alleviate the pain in various operations because it has better analgesic effect. Since1960s, many scientists have investigated the mechanism by which electroacupuncture induces analgesic effect. Early studies showed that analgesia induced by EA was involved in modulations of neurotransmitters (serotonin, acetylcholine, catecholamine, etc) in the central nerve system (CNS). Latter, studies verified that neuromodulators, especially some endogenous opioid peptides (enkephalin, β-endorphin and dynorphin), played a more important role in EA-induced analgesia.δ-receptors, μ-receptors and κ-receptors are three important opioid receptors in the CNS. EA-induced immediate analgesia is mediated by M-ENK via δ-receptors, β-EP via δ-and μ-receptors, and DYN via κ-receptors.EA not only induces an "immediate analgesia", but also causes an analgesic aftereffect (analgesia lasts for a while after EA is discontinued). This aftereffect plays an important role in the treatment of painful diseases, and is conducive to the recovery from the surgery. So far the mechanisms by which acupuncture induces analgesic aftereffect have not been fully studied. Some reports showed that EA induced the gene expression of opioid peptide precursors and opioid receptors in the CNS of rats. It suggests that EA-induced analgesic aftereffect may be caused by the initiation of endogenous opioid peptides gene expressions which can replenish the consumed opioid peptides. However, the roles of the gene expressions of endogenous opioid peptides and their receptors in EA-induced aftereffect are not completely confirmed.Analgesia induced by EA has been proven to vary in animal species. Studies show that the analgesic effect induced by EA in goats (a ruminant) is superior to that in rats or human. Therefore, ruminants should be optimal model animals for researches on the mechanisms of EA-induced analgesia (including analgesic aftereffect).One hundred and eight healthy hybrid male goats, weighing23-28kg, were used in this experiment (54goats for the measurement of gene expression levels of proenkephalin (PENK) and opioid receptors using quantitative real-time PCR, another54goats for the measurement of M-ENK levels using immunohistochemistry). Goats were restrained in right recumbency, and stimulated with EA of60Hz for0.5h at a set of Baihui, Qijia, Ergen, and Sanyangluo points.The sham control goats which were only dealt with needles left in the acupoints for0.5h without electricity were restrained as the experimental goats. At0.5h before and at0,2,4,6,8,12and24h after EA, the pain threshold was measured using potassium iontophoresis (n=6), and then the samples were taken, including nucleus accumbens (ACB), caudate nucleus (CAU), amygdala (AMY), supraoptic nucleus (SON), paraventricular nucleus of hypothalamus (PVH), ventromedial nucleus of hypothalamus (VMH), arcuate nucleus (ARC), paraventricular nucleus of thalamus (PVT), periaqueductal gray (PAG), dorsal raphe nucleus (DR), habenular nucleus (HB), parabrachial nucleus (PBN), nucleus raphe magnus (NRM), gigantocellular reticular nucleus (GI), solitary nucleus (SOL), neurohypophysis (NH) and spinal cord dorsal horn (SCD). The gene expression levels of proenkephalin and opioid receptors and M-ENK levels were measured in the samples to study their relationship with the pain threshold and their function in the EA-induced analgesic aftereffect.Results of pain threshold determination showed:The pain threshold of experimental goats at0.5h before EA was not different (p=1.00) from that of sham control goats. After EA stimulated the goats, the pain threshold increased and reached the peak at0h. Then the pain threshold gradually decreased, began to rebound at6h, came to the second peak at8h, and then fell gradually again. The pain threshold during the time from0to12h after EA was higher (p<0.05) than that at0.5h before EA, which showed that the EA-induced analgesic aftereffect lasted for at least12h in goats.Results of PENK mRNA measurement showed:After EA stimulated the goats, the mRNA levels of PENK increased (p<0.01) with the peak at6h in ACB, PAG, DR, NH, CAU, VMH, PBN, SOL, and PVH, at8h in GI, at12h in AMY or24h in HB. PENK mRNAs remained higher levels (p<0.05) at24h after EA than at0.5h before EA in the measured nuclei or areas except ACB.Results of8-receptors mRNA measurement showed:The changes of8-receptors mRNA level were similar with those of PENK mRNA levels. The8-receptor mRNA levels at the peak in NH, PAG, DR, GI, VMH, PVH, PBN, SOL, AMY, HB, ACB and CAU increased by3.24,2.30,1.44,1.36,1.24,1.22,1.20,1.10,1.10,0.96,0.71and0.71times, respectively.Results of μ-receptors mRNA measurement showed:The mRNA levels of μ-receptor increased (p<0.05) at0h after EA, and then fluctuated with one (4h or6h)or two apparent peaks (2h and8h,4h and8h,4h and12h) in the measured nuclei or areas. μ-receptor mRNA kept higher levels (p<0.05) at24h after EA than at0.5h before EA in all the measured nuclei or areas.Results of κ-receptors mRNA measurement showed:The mRNA levels of κ-receptor increased (p<0.05) at0h, slightly decreased at8h, then increased quickly, and reached the peak at12h after EA was terminated in all the measured nuclei or areas. Thereafter, κ-receptor mRNA levels declined again, but remained higher levels (p<0.05) at24h, compared with those at0.5h before EA.Results of M-ENK level measurement showed:M-ENK immunoactivities positively correlated (p<0.01) with the pain threshold in the measured nuclei and areas. Electroacupuncture induced M-ENK immunoactivities to increase (p<0.05), to reach the peak at0h, and then to fall down gradually to the lowest levels at4-6h (but still higher than those at0.5h before EA) in most measured nuclei or areas.The results above clearly exhibited the dynamic processes of mRNA levels of proenkephalin and opioid receptors (δ-, μ-,κ-receptors) induced by EA in the CNS of goats, which suggested that the initiation of gene expression of opioid peptides, and the three receptors may participate in the regulation of EA-induced analgesic aftereffect. This is helpful to clarify the mechanism of EA-induced analgesic aftereffect, and to apply the EA in veterinary medicine. |
PDF Full Text Request