Ketamine inhibits the excitability induced by formalin in spinal dorsal horn neurons of rats / 中国组织工程研究

ABSTRACT

BACKGROUND: It is indistinct that whether ketamine can exert antinociceptive effect througb influencing the transmission of nocuous information in spinal cord; Nitric oxide (NO) in spinal cord participates mainly in the formation and development of hyperalgesia, and it can also induce Fos protein expression. It is still controversal whether it contributes to the transmission and mediation of ketamine to pain signal.OBJECTIVE: To observe the response to formalin stimulation in spinal cord of the rats and the effect of ketamine.DESIGN: Balanced randomized animal trial.SETTING: Department of Anesthesiology, Affiliated Hospital of Xuzhou Medical College; Jiangsu Provincial Key Laboratory of Anesthesiology.MATERIALS: This trial was carried out in the Jiangsu Provincial Key Laboratory of Anesthesiology, Xuzhou Medical College from January to March 2000. Totally 30 Sprague-Dawley rats were chosen and balanced randomized into 6 groups: formalin group (n=6), formalin + ketamine group (n=6), ketamine +formalin group (n=6), ketamine group (n=6), formalin+normal saline group (n=3) and normal saline group (n=3). The gender ratio was the same in each group.METHODS: Formalin group:The rats were stimulated for one hour by subcutaneous injection of 0.05 volume fraction of 200 μL in the center of palm of unilateral fore-claw. Formalin +ketamine group: The rats were stimulated for 10 minutes by formalin, then for one hour by intraperitoneal injection of 100 rg/kg ketamine. Ketamine + formalin group: The rats were injected with ketamine for 10 minutes, then with formalin for one hour. Ketamine group: the same dosage of ketamine was intraperitoneally injected into the rats for one hour. Formalin + normal saline group: The rats were stimulated for 10 minutes by formalin, then intraperitoneally given 10 mL/kg normal saline for one hour. Normal saline group: the same volume of normal saline was intraperitoneally injected into the rats for one hour.MAIN OUTCOME MEASURES: ① Behavioral performance of the rats in each group. ② Spinal sections were chosen, and stained with c-fos genetic immunohistochemical and NADPH-d histochemical methods. The changes of the number of Fos-like immuno-positive neurons (FLI) and FLI/nitric oxide synthase (NOS) double-labeled neurons in the 4-layer sections (layer Ⅰ -Ⅱ ,layer Ⅲ-Ⅳ ,layerⅤ-Ⅵ ,layer Ⅶ-X )of spinal dorsal horn of the rats were observed.RESULTS: All the thirty rats entered the stage of result analysis. ① Behavioral changes: The rats of formalin group and formalin+ normal salinegroup had apparent pain response; Several minutes after injection with ketamine, righting reflex disappeared and did not recover at perfusion period.Prolonged sleep was found without obvious pain response performance. ② FLI neuron expression: A lot of FLI positive neurons were found in the spinal dorsal horn of injec tion side of the rats in the formalin group and formalin+ normal saline group, and they distributed principally in the layer Ⅰ - Ⅱ of spinal dorsal horn.The distribution in the ketamine + formalin group and formalin + ketamine group was basically similar to that in the formalin group and formalin + normal saline group, but positive neuron counts were significantly reduced (P ＜ 0.01). ③ The expression of FLI/NOS double-labeled neurons: The number of double-labeled neurons in the spinal dorsal horn layer Ⅰ - Ⅱ of the rats in the ketamine+ formalin group and formalin+ ketamine group were significantly less than that in the formalin group and formalin+normal saline group [(1±1), (1±1), (7±3), (8±3),P ＜ 0.01].CONCLUSION: Some neurons of ipsilateral corresponding spinal segments participate in the transmission and mediation of pain signal. Ketamine can suppress the activities of these neurons and exert antinociceptive effect. The antinococeptive function of ketamine may be caused by the activity depression of the NOS-positive neurons in spinal cord.