[Observation on the analgesic effect of repeated electroacupuncture and its relation to changes of plasma beta-EP, ACTH and COR levels].

OBJECTIVE

To observe the regularity of electroacupuncture (EA) analgesia and to analyze its relation to hypothalamus-pituitary-adrenal cortex axis function in chronic constrictive injury (CCI) and ovariectomy (OVX) rats.

METHODS

A total of 110 Wistar rats anesthetized with mixture solution of 1.5% chloralose (50 mg/kg) and 25% urethrane (420 mg/kg, i.p.) were randomized into control (n=10), CCI (n =10), CCI + EA (n=30), OVX + CCI (n=30), OVX + CCI + EA (n=30) groups, and the later 3 groups were further divided into 2 times (2 t), 2 weeks (2 w) and 3 w time courses, with 10 cases in each. Forty-five days after OVX, Morris water maze test was conducted to evaluate the OVX rats' memory-learning ability. Chronic pain model was established by ligating the right sciatic nerve with a piece of suture. EA (2/15 Hz, 1 mA) was applied to bilateral "Zusanli" (ST 36)-"Yanglingquan" (GB 34) for 30 min, once daily for 2 t, 2 w, 3 w respectively. The paw withdrawal latency (PWL) was detected to be used as the pain threshold. Under deep anesthesia, the rat was decapitated for collecting blood samples. Plasma adrenocorticotropic hormone (ACTH), beta-endorphine (EP), cortisone (COR) contents were detected using radioimmunoassay.

RESULTS

In comparison with control group, PWL difference values (PWLDV) of CCI + EA, OVX + CCI and OVX + CCI + EA groups increased significantly (P<0.05), suggesting a marked decrease of pain threshold. Compared with CCI group, PWLDV of CCI + EA group at 3 w decreased significantly (P<0.05), being close to normal control group (P>0.05). Compared with OVX + CCI group, PWLDV of OVX + CCI + EA at 2 w and 3 w lowered significantly (P<0.05), suggesting an evident pain relief. In comparison with CCI + EA group, PWLDV of OVX + CCI + EA group at most time courses were considerably higher (P<0.05), showing a better cumulative analgesic effect in CCI rats without OVX. In CCI group, plasma beta-EP increased slightly, ACTH had no clear changes, and COR lowered lightly at 2 t. No significant differences were found between control and CCI + EA groups in beta-EP and ACTH levels at 2 w and 3 w (P>0.05), only COR levels of CCI + EA group at 2 w and 3 w increased significantly (P<0.05). Compared with control group, beta-EP levels of OVX + CCI group at 2 w and 3 w increased significantly (P<0.05); plasma ACTH had no marked changes (P>0.05); COR contents of OVX + CCI group decreased evidently at 2 t and increased markedly at 2 w (P<0.05). Compared with OVX + CCI group, in OVX + CCI + EA group, beta-EP levels of 2 t and 2 w decreased significantly (P<0.05); ACTH levels increased evidently at 2 t and 3 w, and decreased markedly at 2 w (P<0.05); and COR level at 2 w decreased significantly (P<0.05). It indicated that when cumulative EA analgesia appeared, in CCI animals, only plasma COR level increased pronouncedly, while in OVX + CCI rats, both plasma beta-EP and COR contents decreased evidently.

CONCLUSION

Both plasma beta-EP and COR are involved in the accumulative analgesic effect of EA in the rat, and decline of learning memory ability may weaken the analgesic effect of EA to a certain degree, but no clear correlation tendency is found between changes of plasma beta-EP and COR and the improvement of pain behavior.