Objective: To observe the effect of electroacupuncture (EA) at different acupoints on the expression of gamma aminobutyric (GABA) receptor (R) subunit genes in the cervical spinal cord in rats with thyroid regional inflammatory pain so as to analyze its analgesic mechanism for thyroid surgery.
Methods: A total of 50 Wistar rats were randomized into control, model, Futu (LI 18), Hegu (LI 4)-Neiguan (PC 6, LI 4-PC 6), Zusanli (ST 36)-Yanglingquan (GB 34, ST 36-GB 34) groups, with 10 rats in each group. Thyroid regional pain model was established by subcutaneous injection of 2.5% formalin (100 microL). Ten minutes after modeling, EA (2 Hz/15 Hz, 1 mA) was applied to LI 18, LI 4-PC 6 and ST 36-GB 34 for 30 min, respectively. The animals' face-grooming (FG) times in 5 min and thermal pain threshold (paw withdrawal latency, PWL) were recorded. The expression of GABA(A) R, GABA(B) R1, and GABA(B) R2 genes in the cervical 1-3 segments of the spinal cord 80 min after modeling was detected by using reversed transcription-polymerase chain reaction (RT-PCR). Histological changes of the tissues of the thyroid region were observed by using H. E. staining.
Results: After subcutaneous injection of formalin, the animals' FG times in 5 min were increased considerably and the thermal pain threshold was decreased obviously in the model group (P < 0.05). Concomitantly, the expression levels of GABA(B) R1, and GABA(B) R2 genes of the cervical spinal cord were upregulated significantly in the model group (P < 0.05), and GABA(A) R expression was increased slightly (P > 0.05). Compared with the model group, the FG times in 5 min in the LI 18 and LI 4-PC 6 groups at 40 min and 70 min after modeling were decreased significantly (P < 0.05), and their thermal pain threshold values were increased markedly (P < 0.05). The expression levels of GABA(B) R1 mRNA, GABA(B) R2 mRNA and GABAA R mRNA were significantly higher in the LI 18 and LI 4-PC 6 groups than in the model group (P < 0.05). The expression level of GABA(B) R1 mRNA of the ST 36-GB 34 group was obviously higher than that of the model group (P < 0.05). The inflammatory cells in the formalin-injected thyroid region were relatively fewer in the LI 18 and LI 4-PC 6 groups than in the model group.
Conclusion: Both EA of LI 18 and LI 4-PC 6 can significantly suppress formalin-injection induced pain reactions in the thyroid region, which may be closely associated with its effects in upregulating expression levels of cervico-spinal GABA(B) R1 mRNA, GABA(B) R2 mRNA and GABA(A) R mRNA, and reduce regional inflammatory reactions in the thyroid region.