[Effects of different intensities of electroacupuncture and manual acupuncture at "Shangjuxu" (ST37) on acute inflammatory visceral pain and their relationship with α7nAChR/PI3K/AKT signaling pathway].

OBJECTIVES: To compare the effects of electroacupuncture (EA) and manual acupuncture (MA) at different stimulation intensities on acute inflammatory visceral pain, and to analyze their mechanisms underlying relief of inflammatory visceral pain mediated by α7 nicotinic acetylcholine receptor (α7nAChR)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway.

METHODS: Male SD rats were randomly divided into normal control, visceral pain model, EA 1 mA, EA 2 mA, EA 3 mA, EA 4 mA, EA 6 mA, and MA groups (with the same frequency as EA, 2 times/s, for 2 min), with 12 rats in each group. The acute inflammatory visceral pain model was established by intracolonic instillation of 2% acetic acid. EA (2 Hz, 1, 2, 3, 4, 6 mA) or MA (120 twirlings/min, with the needle rotated at a magnitude of about 90°-180°) was applied to the right "Shangjuxu" (ST37) for 2 min. The abdominal withdrawal reflex (AWR) score and external oblique muscle electromyogram amplitude were used to assess the visceral pain reaction. Histopathological changes of the colonic mucosal tissue were detected by H.E. staining. For observing the involvement of colonic α7nAChR in visceral pain relief of both EA and MA, intraperitoneal injection of methylaconitine (MLA, α7nAChR blocker, 4 mg/kg) was given to rats of MA+MLA and EA 2 mA+MLA groups (with 6 rats in each group) before each intervention. The contents of serum tumor necrosis factor-α(TNF-ɑ) and interleukin-1β(IL-1β) were detected by enzyme-linked immunosorbent assay (ELISA). The expression levels of α7nAChR, PI3K and AKT in the colon tissue were detected by Western blot.

RESULTS: In comparison with the normal control group, both the AWR scores and the amplitude of EMG at colorectal dilation (CRD) pressure of 20, 40, 60 and 80 mmHg were significantly increased in the model group (P<0.05). Compared with the model group, both the AWR scores and the amplitude of EMG at CRD pressure of 20, 40, 60 and 80 mmHg were significantly decreased in the EA 1 mA, EA 2 mA and MA groups (P<0.05), rather than in the EA 3 mA, EA 4 mA and EA 6 mA groups, suggesting that the higher stimulation intensity of EA had no analgesic effect. H.E. staining showed slightly hyperemic and edematous injury of the colonic mucosa tissue, with obvious inflammatory infiltration and abnormal morphological changes in the model group, which was obviously milder in the injury in the MA and EA 1 mA, EA 2 mA groups, rather than in the EA 3 mA, 4 mA and 6 mA groups. In comparison with the normal control group, the contents of serum TNF-ɑ and IL-1β, and expression levels of PI3K and AKT proteins in colon were significantly increased (P<0.05), and the expression of α7nAChR protein was remarkably down-regulated (P<0.05) in the model group. Compared with the model group, the contents of serum TNF-ɑ and IL-1β, and the expression levels of PI3K and AKT proteins were strikingly down-regulated (P<0.05), and the expression of α7nAChR was considerably up-regulated (P<0.05) in both EA 2 mA and MA groups. Moreover, the effects of reducing the corresponding AWR scores at various CRD pressure levels, serum TNF-α and IL-1β contents, and the protein expressions of PI3K and AKT in colon, as well as increasing the protein expression of α7nAChR in colon in rats of MA and EA 2 mA groups can be inhibited by MLA (P<0.05), a blocker of α7nAChR.

CONCLUSIONS: Both MA and 2 mA EA (not 3, 4, 6 mA EA) stimulation of ST37 have a similar analgesic effect in acute inflammatory visceral pain rats, which may be related to their function in suppressing the activities of α7nAChR/PI3K/AKT signaling pathway.