Acupuncture promotes tear secretion by up-regulating VIP/cAMP/PKA/AQP5 signaling in guinea pigs with aqueous tear deficiency dry eye.

Objectives: To observe the effect of acupuncture on the ocular surface symptoms and the protein expression of vasoactive intestinal peptide (VIP) / cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) / aquaporin 5(AQP5) signaling pathway in lacrimal gland tissue of aqueous tear deficiency (ATD) type dry eye model, so as to investigate its mechanism underlying improvement of ATD.

Methods: British shorthair guinea pigs were randomly divided into blank control, model, acupuncture, sham-acupuncture and medication group, with 8 guinea pigs in each group. The ATD model was established by subcutaneous injection of scopolamine hydrobromide (0.6 mg/dose, 4 times/d for 10 days). For guinea pigs of the acupuncture group, filiform needles were inserted into bilateral "Jingming"(BL1), "Cuanzhu"(BL2), "Sizhukong"(TE23), "Taiyang"(EX-HN5), and "Tongziliao"(GB1) for 15 min. For guinea pigs of the sham-acupuncture group, a blunt filiform needle was used to repeatedly prick (not pierce) the skin of the same acupoints mentioned above. The treatment in the above two groups was conducted once daily for 14 days. The guinea pigs in the medication group received administration of sodium hyaluronate eye drops in both eyes, three times a day for 14 days. The objective tests of tear film break-up time (BUT), corneal fluorescein staining score (FLS) and phenol red thread (PRT) test were conducted before and after modeling and after the intervention. After the intervention, the lacrimal index (weight of lacrimal gland/body weight) was calculated. Histopathological changes of the lacrimal gland were observed after H.E. staining. The expression of AQP5 in the lacrimal gland were detected by immunofluorescence, and the contents of VIP and AQP5 in the lacrimal gland were measured by ELISA, the protein expression levels of VIP, cAMP, PKA, p-PKA and AQP5 in the lacrimal gland were detected by Western blot.

Results: In comparison with the blank control group, the PRT, BUT, lacrimal index, AQP5 immunoactivity, contents of VIP and AQP5, and protein expression levels of VIP, cAMP, PKA, p-PKA and AQP5 were significantly decreased(P<0.01, P<0.05), and FLS was obviously increased (P<0.01) in the model group . Compared to the model group, the PRT, BUT, lacrimal index, AQP5 immunoactivity, contents of VIP and AQP5, and expression levels of VIP and AQP5 in both acupuncture and medication groups, and the expression levels of cAMP, PKA, p-PKA in the acupuncture group were considerably increased (P<0.01, P<0.05), while the FLS was markedly decreased in both acupuncture and medication groups (P<0.01, P<0.05). Compared with the medication group, the acupuncture group had increased PRT (P<0.05).

Conclusions: Acupuncture intervention is effective in reducing ocular surface damage and promoting tear secretion in guinea pigs with ATD, which may be related to its function in activating VIP/cAMP/PKA signaling, and promoting the expression of AQP5 in the lacrimal gland.