To observe the effect of electroacupuncture (EA) at “Shangjuxu” (ST37) on the visceral hypersensitivity, expressions of colonic 5-hydroxytryptamine 7 receptor (5-HT₇) and nerve growth factor (NGF) in rats with irritable bowel syndrome (IBS), so as to explore its mechanisms underlying the improvement of visceral hypersensitivity of IBS.

A total of 18 male SD rats were randomly divided into normal control, model and EA groups, with 6 rats in each group. The IBS model was established by intracolonic administration of 2,4,6-trinitrobenzenesulfonic acid (0.8 mL, 80 mg/kg, in 50% ethanol) for 4 weeks. EA (2 Hz, 1.0 mA) was applied to bilateral ST37 for 30 min, once daily for 10 d. The visceral hypersensitivity was assessed by visceromotor responses (abdominal withdrawal reflex [AWR] to 20, 40, 60, and 80 mmHg colorectal distension pressure [CRD], simultaneously). Changes of the electromyography (EMG) activities of the musculus obliquus externus abdominis were synchronously recorded after CRD. Histopathological changes of the colonic tissue were observed after HE staining. The activity of myeloperoxidase (MPO, an inflammatory marker) in the colon tissue was detected by colorimetry. The co-expressions of 5-HT₇, NGF and pan-neural marker (PGP9.5) in the colon tissue was detected by double immunofluorescence staining. The protein expression levels of colonic tumor necrosis factor-α (TNF-α), interleukin-10 (IL-10), 5-HT₇, NGF and tropomyosin receptor kinase A (TrkA) were detected by Western blot.

In comparison with the normal control group, the model group showed a striking increase in the AWR scores and EMG activities in response to 20, 40, 60, and 80 mmHg CRD (P<0.05), suggesting a visceral hypersensitivity after CRD. In addition, the activity of MPO, expression levels of TNF-α, 5-HT₇, TrkA and NGF proteins, and the positive cell rates of 5-HT₇/PGP9.5 dual staining, and NGF/PGP9.5 dual staining in the colon tissue were significantly increased (P<0.05), while the expression of colonic IL-10 protein was obviously down-regulated in the model group than in the normal control group (P<0.05). After EA intervention, both the AWR scores and EMG activities were considerably down-regulated (P<0.05), suggesting an apparent reduction of the visceral pain. Correspondingly, both the increase of the activity of MPO, expression levels of TNF-α, 5-HT₇, TrkA and NGF proteins, and the positive cell rates of 5-HT₇/PGP9.5 and NGF/PGP9.5 dual staining, and the decrease of IL-10 protein expression were reversed by EA (P<0.05). HE staining displayed disordered arrangement of the epithelial cells of the colon mucosa, interstitial edema, and inflammatory cell infiltration in the submucosa in the model group. While in the EA group, the epithelial cells were comparatively closely arranged, the muscular layer structure was relatively complete, and the inflammatory cell infiltration was reduced.

EA at ST37 can alleviate visceral hypersensitivity and regulate low-grade inflammation in the colon tissue of IBS rats, which may be related to its functions in inhibiting the over-activation of 5-HT₇ to down-regulate NGF expression, affecting the colonic nerve activities.