Multidimensional Analgesia of Acupuncture by Increasing Expression of MD2 in Central Nervous System.

Objective: To investigate changes of myeloid differentiation factor 2 (MD2) in inflammation-induced pain and acupuncture-mediated analgesia.

Methods: Mice were randomly divided into three groups by a random number table method: saline group (n=16), complete Freund's adjuvant (CFA) group (n=24) and CFA+electroacupuncture (EA) group (n=26). Inflammation-induced pain was modelled by injecting CFA to the plantar surface of the hind paw of mice and EA was applied to bilateral Zusanli (ST 36) to alleviate pain. Only mice in the CFA+EA group received EA treatment (30 min/d for 2 weeks) 24 h after modelling. Mice in the saline and CFA groups received sham EA. von-Frey test and Hargreaves test were used to assess the pain threshold. Brain and spinal tissues were collected for immunofluorescence staining or Western blotting to quantify changes of MD2 expression.

Results: CFA successfully induced plantar pain and EA significantly alleviated pain 3 days after modelling (P<0.01). Compared with the CFA group, the number of MD2⁺/c-fos⁺ neurons was significantly increased in the dorsal horn of the spinal cord 7 and 14 days after EA, especially in laminae I - II_o (P<0.01). The proportion of double positive cells to the number of c-fos positive cells and the mean fluorescence intensity of MD2 neurons were also significantly increased in laminae I - II_o (P<0.01). Western blotting showed that the level of MD2 was significantly decreased by EA only in the hippocampus on day 7 and 14 (both P<0.01) and no significant changes were observed in the cortex, thalamus, cerebellum, or the brainstem (P<0.05). Fluorescence staining showed significant decrease in the level of MD2 in periagueductal gray (PAG) and locus coeruleus (LC) after CFA injection on day 7 (P<0.01 for PAG, P<0.05 for LC) and EA significantly reversed this decrease (P<0.01 for PAG, P<0.05 for LC).

Conclusion: The unique changes of MD2 suggest that EA may exert the analgesic effect through modulating neuronal activities of the superficial laminae of the spinal cord and certain regions of the brain.