Abstract
A neurochemical hypothesis of acupuncture analgesia suggests that the pain relief effect of acupuncture is primarily due to activation of a central endorphin system. It has been shown that the primary afferent sensory fibers, a mesolymbic neural circuit, and a descending inhibitory pathway are critical in acupuncture analgesia. The therapeutic effects of electroacupuncture (EA) and related techniques, such as transcutaneous electroacupoint stimulation (TEAS), are frequency-dependent: different frequencies of EA activate different brain regions and release different neuropeptides. EA and TEAS have been used successfully to treat heroin addiction. Activation of endorphin gene expression and release by TEAS can explain the dramatic attenuation of withdrawal syndrome and prolongation of retention time during and after detoxification treatment in patients who are addicted to heroin. However, repeated EA at high intensity should be avoided because it can induce a gradual loss of the analgesic effect. Opioid-receptor desensitization occurs and is manifested as decreased ligand-binding affinity and second-messenger detachment. Repeated large doses of morphine induce morphine tolerance. Cross-tolerance between morphine and EA suggests similar underlying mechanisms. Recent studies have demonstrated that excessive activation of cholecystokinin (CCK), an antiopioid peptide, appears to be responsible. CCK-receptor subtype B (CCKBR) and opioid μ-receptor are co-expressed in the dorsal-horn neurons. Activation of CCKBR promotes formation of heteromerization of morphine-receptor and CCKBR. Interaction of the third transmembrane domain between the 2 receptors resulted in the reduced binding affinity of the opioid receptor.