[Electroacupuncture and transcutaneous electrical acupoint stimulation relieve muscular inflammatory pain by activating afferent nerve fibers in different layers of "Liangqiu"(ST34) in rats].

OBJECTIVE To compare the effect of electroacupuncture (EA) and transcutaneous electrical acupoint stimulation (TEAS) of skin and muscle layers of Liangqiu (ST34) area on inflammatory muscular pain in rats, so as to study the role of different-layer afferent nerve fibers in acupuncture analgesia. METHODS A total of 120 male SD rats were used in the present study, including 8 rats used for determining the excitatory threshold of Aδ(Ta) and C (Tc) afferent nerve fibers, 48 employed for comparing the analgesic effect of EA and TEAS at intensities of Ta and Tc, and 64 for observing the effect of EA and TAES stimulation of ipsilateral (Ipsi), contralateral (Contra) ST34 and ipsilateral LI4 at Ta and Tc intensities. Inflammatory muscle pain was induced by injection of complete Freund's adjuvant (CFA) into the right biceps femoris muscle. In the second part of the present study, 48 rats were randomly and equally divided into control, model, TEAS-Ta, TEAS-Tc, EA-Ta and EA-Tc groups, while in the 3rd part, 64 rats were randomly and equally divided into control, model, Ipsi-ST34-TEAS, Contra-ST34-TEAS, Ipsi-ST34-EA, Contra-ST34-EA, Ipsi-LI4-TEAS and Ipsi-LI4-EA groups. TEAS or EA was applied to the skin and muscle layers, respectively. Before and after modeling, the animal was forced to stand on a bipedal equilibrator, the difference in body mass distribution of both feet (bearing difference) was used to assess the pain severity. The frequency of myoelectrical discharges of the right bicep femoris muscle in responding to electrical stimulation of the spot between the 4th and 5th toes of the ipsilateral hindlimb at an intensity of two-folds of C-fiber excitatory threshold was recorded. RESULTS 1) The bearing difference between the bilateral hindlimbs was markedly higher in the model group than in the control group (P<0.01), and significantly lower at the 2nd and 3rd day in the EA-Ta and EA-Tc, and TEAS-Tc groups relevant to the model group (P<0.05, P<0.01). 2) The frequency of C-fiber reflex induced electromyogram (EMG) activities were significantly decreased at 0 and 1 min after TEAS of both ipsilateral ST34 at Tc, and 0 min after TEAS of the contralateral ST34 at Tc (rather than at Ta and not LI4 even at Tc) in comparison with pre-TEAS, and 0, 1 and 2 min after TEAS of ipsilateral ST34 at Tc, and 0 min after TEAS of contra-ST34 at Tc compared with the model group, respectively (P<0.01, P<0.05). In comparison with pre-EA, the frequency of C-fiber reflex induced EMG activities were significantly decreased at 0 and 1 min after EA of the ipsilateral ST34 at Ta, and 0 min after EA of the contra-ST34 at Ta. In addition, the frequency of C-fiber reflex induced EMG activities were decreased at 0, 1 and 2 min after EA of the ipsilateral ST34 at Tc, and 0 and 1 min after EA of the contra-ST34 at Tc, as well as 0 min after EA of LI4 at Tc (P<0.01, P<0.05).In comparison with the model group, the frequency of C-fiber reflex induced EMG activities are significantly decreased at 0, and 1 min after EA of the ipsilateral ST34 at Ta, and 0 min after EA of the contra-ST34 at Ta. In addition, the frequency of C-fiber reflex induced EMG activities were decreased at 0, 1 ,2,and 3 min after EA of the ipsilateral ST34 at Tc, and 0 and 1 min after EA of the contra-ST34 at Tc, as well as 0 min after EA of LI4 at Tc, respectively (P<0.01, P<0.05). CONCLUSION TEAS-ST34 at Tc and EA-ST34 at both Ta and Tc can alleviate pain behavior in inflammatory pain rats, which may be related to its effect in activating the afferent nerve fiber in different layers of ST34 area.