AIM: To explore the effect of N-methyl-D-aspartate, non-specific antagonist ketamine on behavior changes of mice with inflammatory pain due to formalin and the changes of TrkA expression of nerve growth factor receptor of dorsal root ganglion. METHODS: The experiment was conducted at the Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology from February to May 2005. Twenty-four adult health Qunming mice were selected and randomly divided into normal control group, formalin group and formalin+ketamine group with 8 mice in each group. 100 μL formalin of the volume fraction of 0.05 was injected into right hindfoot of mice in the formalin group and the formalin+ketamine group, respectively to establish acute inflammatory pain models. The mice in the formalin+ketamine group were injected with 40 mg/kg ketamine through abdominal cavity immediately after injecting with formaldehyde. The mice in the normal control group were not interfered. The changes in the behavior of inflammatory pain mice following the injection of formalin were observed. The mice were killed two hours after formalin-induced inflammation. Expression of TrkA receptor of dorsal root was detected with immunohistochemical ABC method. RESULTS: A total of 24 mice were involved in the result analysis after compensation. ①Changes of behavior in mice with inflammatory pain: After injecting for 1 hour the mice showed significant nociceptive behavior: throwing leg, licking foot, limping, and the inflammatory foot could not touch the ground.
The injected foot was swelling highly. Righting reflex disappeared immediately after injecting with ketamine in the mice of the formalin+ketamine group. The reaction number of pain behavior at the first time phase (0-5 minutes after injection) and second time phase (20-60 minutes after injection) after inducing inflammation was less obviously than that in the formalin group (6.0±2.5),(113.0±9.5) times;(3±0),(75.0±4.5) times;P < 0.01. ②Mean gray value of positive neuron of TrkA receptor of dorsal root: It was higher in the injected side of the formalin group than that in the formalin+ketamine group and the normal control group (205.3±11.9,121.6±7.9,116.5±7.7,P < 0.01). The difference was insignificant between the formalin+ketamine group and the normal control group (P > 0.05). CONCLUSION: Ketamine can inhibit the nociceptive behavior and reduce the TrkA expression in dorsal root ganglion following acute inflammatory pain, and this effect may be related to the reduced primary nociceptive afferent stimulation from the down-regulated TrkA receptor to the spinal dorsal horn.