| Electroacupuncture (EA) is a modern method of electrical stimulation instead of the traditional hand acupuncture to achieve analgesia. Numerous studies have shown that EA not only have a good analgesia effect, but aslo have a bidrectional regulation. Under EA, the animal's physiological index was stable during the operation, and recovered well after the operation. Since the 1960s, the EA had been widely used in the animals'operations. It has obtained good effect in the ruminant and overcame the problems of tympany and regurgitation which were caused by simple drug, and it had a good prospect.Studies in the small animals (such as rats) have revealed that endogenous opioid peptides(enkephalin,β-endorphin and dynorphin) in the central nervous system play an essential role in mediating the analgesic effect of EA. Different frequencies produce different analgesic effects by releasing different kind of analgesic substances.2 Hz stimulation mainly causes the relseaing of enkephalin andβ-endorphine, whereas 100 Hz sitmulation chooses to elicit an increase of dynorphin in the spinal cord. Stimulations with 2 Hz and 100 Hz alternatively can produce these three opioid peptides. Veterinary experience has shown that high frequency (30-100 Hz) is the optimal for the ruminant, which is higher than that for the human or for the small animals (2-15 Hz), and that the analgesic effect for ruminants under the high frequency is better, too. However, whether the mechanism of the EA in the runimant was the same as that in the human and the small laboratory animals or not is still unclear. The theory study on acupuncture analgesia made the acupuncture be accepted by Western medicine, and widely used in the clinical practice home and abroad. Nowadays, the lack of study on the mechanisms of veterinary acupuncture has become a bottleneck in its popularization and application in the world.Enkephalin is one of the main central opioid peptides which is involved in the regulation of acupuncture analgesia, and its release is frequency-dependent. At present, people still don't elaborate the law of the enkephalin-releaing frequencies in goats. This study was designed to detect the changes of pain threshold, and the expressing and the distribution of the enkephalin induced by different frequencies in goats, to determine the the best analgesic frequency for goats, to elucidate the mechanism of enkephalin participating in the ruminant's acupuncture analgesia, and to provide a theoretical basis for the application of the acupuncture analgesia in venterinary practice.Thirty five healthy adult male goats were used (weight23-27 kg). the goats were randomly devided into five groups (including controls,2 Hz,40 Hz,60 Hz and 100 Hz). The goats in EA groups are stimulated at "baihui-qijia" and "ergen-sanyangluo" acupionts for 30 min. The pain threshold was detected immediately after EA. Then the goats were perfused with formalin and their brains were removed. The distribution and expressing levels of the enkephalin were observed in the mainly analgesic nuclei (accumbens, septal area, caudate putamen nucleus, amygdala, supraoptic nucleus, hypothalamic ventromedial nucleus, hypothalamic paraventricular nucleus, periaqueductal gray, dorsal raphe nucleus, locus ceruleus, sSubstaintia nigra, raphe magnus nucleus and paraventeicular nucleus) and spinal cord dorsal horn) by SABC immunohistochemistry.The pain threshold elicited by 60 Hz increased greatest, improved by 91% compared with the basic pain threshold, and had significant difference from that by other frequencies. The pain threshold induce by 40 Hz was secondary to that by 60 Hz, increased by 69% of the pain threshold, and had significant difference from that by 2 Hz or 100 Hz. The pain threshold caused by 100 Hz, improved by 41%, was greater than, but not different from that by 2 Hz (improved by 35%).60 Hz and 40 Hz increased the release of enkephalin in the accumbens, septal area, caudate putamen nucleus, amygdala, hypothalamic ventromedial nucleus, hypothalamic paraventricular nucleus, periaqueductal gray, locus ceruleus, dorsal raphe nucleus and substaintia nigra greater compared with the control did (P<0.05). 60 Hz induced the expressing of enkephalin in accumbens, septal area, caudate putamen nucleus, amygdala, hypothalamic paraventricular nucleus and locus ceruleus higher than 40 Hz. There was significant difference in amygdala and periaqueductal gray between 60 Hz and 40 Hz (P<0.05).100 Hz increased the enkephalin releasing in paraventeicular nucleus greater than the other EA groups, have significant difference from the control group (P<0.05).2 Hz EA stimulation increased the enkephalin releasing in the spinal cord dorsal horn more than the other groups did, had significant difference from the control group(P<0.05).The result suggests that EA can actually increase the pain threshold of goats, and 60 Hz is the optimal frequency which induces analgesia. The enkephalin is involved in the regulation for the EA of the goat, and its release in different nuclei dependent on the specific frequency. The accumbens, septal area, caudate putamen nucleus, amygdala, hypothalamic ventromedial nucleus, hypothalamic paraventricular nucleus, dorsal raphe nucleus, periaqueductal gray, substaintia nigra, paraventeicular nucleus and locus ceruleus are sensitive to the high frequency, and mediate the EA by releasing a large number of enkephalin, whereas the spinal cord horn releases enkephalin induced by the low frequency.
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