Objective: To study the effects of electroacupuncture (EA) on the immunoreactivity of focal cutaneous cannabinoid receptor 2 (CB2) positive cells in adjuvant arthritis (AA) rats.
Methods: A total of 48 adult female SD rats were randomly divided into control group (n = 12), model group (n = 12), acupoint group (n = 12), and non-acupoint group (n = 12). Arthritis model was established by hypodermic injection of complete Freund's adjuvant (CFA, 50 microL) into the left ankle joint. EA (2/ 15 Hz, 1 mA) was applied to "Huantiao" (GB 30) and "Yanglingquan" (GB 34) or two control points 5 mm left to GB30 and GB34 on the diseased side for 30 min, once every other day from the second day on after injection of CFA. Behavioral performance (pain test score) was assesed by using dorsiflexion and plantarflexion pain tests. On the 6th and 16th day after injection of CAF, the animals were anesthetized with 20% urethane (1 g/kg) for collecting the focal skin and subcutaneous tissue samples which were cut into sections (5 microm) to be stained with HE (haematoxylin & cosin) method and immunohistochemical technique respectively for observing changes of the focal cells of the inflamatory tissue and the immunoactivity of the focal cutaneous CB receptor positive cells.
Results: 1) In comparison with control group, the scores of both dorsiflexion and plantarflexion pain tests in model, acupoint and non-acupoint groups increased evidently after modeling (P < 0.05). Compared with model group, the scores of dorsiflexion test on the 3rd day and 5th day and plantarflexion test on the 5th day in EA-acupoint group were considerably lower (P < 0.05), and the scores of the two tests on the 3rd day and 5th day of acupoint group were also markedly lower than those of non-acupoint group (P < 0.05), suggesting a marked pain-relief after EA; while no significant differences were found between non-acupoint and model groups. 2) HE staining showed that the inflammatory cells in the dermal layer of the focus of acupoint group were evidently fewer than those of model and non-acupoint groups on the 6th day and 16th day after modeling. 3) The immunohistochemical results revealed that compared with control group, on the 6th day, the percentages of CB2 receptor positive cell area in the focus of model, non-acupoint and acupoint groups were significantly higher (P < 0.05), and that of acupoint group was markedly higher than those of model and non-acupoint groups (P < 0.05), suggesting further upregulation of the expression of CB2 receptor positive cells; no significant differences of percentages of CB2 receptor immunoreaction positive cell area among model, non-acupoint and acupoint groups were found on the 16th day (P > 0.05).
Conclusion: EA of GB30 and GB34 can raise the immunoactivity of cutaneous CB2 receptor positive cells in the inflammatory tissue which maybe contribute to its effects in relieving inflammatory pain and suppressing focal inflammation and adjusting the balance of nociception and antinociception in AA rats.