Tian Li-Juan, DU Yi-Ru, Xiao Yong, Lv Zhuo-Min, Yu Yao-Qing, Cui Xiu-Yu, Chen Jun
Institute for Biomedical Sciences of Pain, Capital Medical University, Beijing 100069, China.
Sheng Li Xue Bao. 2009 Oct 25;61(5):404-16.
The formalin test is a commonly used animal model of acute and tonic pain. However, the molecular targets of formaldehyde (FA, the main ingredient of the formalin solution) on primary nociceptor cells remain controversial. In this report, the effects of FA on electrophysiologically-identified primary nociceptor cells were evaluated in vitro and the roles of the vanilloid receptor TRPV1 in FA-produced activation of primary nociceptors were also examined at both cellular and behavioral levels. Of 92 acutely dissociated dorsal root ganglion (DRG) cells recorded by current patch-clamp technique, 34% were discharged by FA application with the mean onset latencies of the first action potential (AP) being (367.34+/-32.96) s. All the FA-sensitive cells were identified as nociceptor cells by their distinguishable features of AP including longer duration, existence of a hump (a shoulder or inflection) on the repolarizing phase, and longer after-hyperpolarization of APs. Co-application of capsazepine (CPZ), a competitive antagonist of TRPV1 receptors, could block FA-evoked firing with partial inhibition on the membrane depolarization of all cells tested. Of another 160 cells examined by confocal calcium imaging, 32% were shown to respond to FA with an intracellular Ca(2+) rise. Of 51 FA-sensitive cells, 67% were suppressed by CPZ, suggesting partial involvement of TRPV1 in mediation of the FA-evoked intracellular Ca(2+) rise. Under voltage-clamp mode, 41% of DRG cells were evoked to give rise to inward current with the remaining 59% being unchanged. In separate experiments on the other 56 FA-sensitive cells, concentration-dependent increase in the FA-evoked current amplitude was demonstrated. In comparison with controls, the FA-evoked inward current could be significantly suppressed by CPZ that was further enhanced by HC-030031, a TRPA1 selective antagonist. Finally, local effects of CPZ were confirmed in the formalin test and it was shown that the formalin-induced paw flinches were strongly suppressed by CPZ in phase 1 but with phase 2 being significantly suppressed only during 25-55 min. It is therefore concluded that FA can directly activate a subpopulation of primary nociceptor cells and the FA-induced AP discharges are likely to contribute mainly to phase 1, but not phase 2 of the formalin-induced nociception. The activation of primary nociceptor cells by FA is likely to be mediated, at least in part, through TRPV1 and/or TRPA1 receptors.
福尔马林试验是一种常用的急性和紧张性疼痛动物模型。然而，甲醛（福尔马林溶液的主要成分）在初级伤害感受器细胞上的分子靶点仍存在争议。在本报告中，体外评估了甲醛对电生理鉴定的初级伤害感受器细胞的影响，并在细胞和行为水平上研究了香草酸受体TRPV1在甲醛引起的初级伤害感受器激活中的作用。在通过电流膜片钳技术记录的92个急性解离的背根神经节（DRG）细胞中，34%的细胞在施加甲醛后放电，第一个动作电位（AP）的平均起始潜伏期为（367.34±32.96）秒。所有对甲醛敏感的细胞通过其AP的可区分特征被鉴定为伤害感受器细胞，这些特征包括持续时间更长、复极化阶段存在驼峰（肩部或拐点）以及AP的超极化后时程更长。共同应用TRPV1受体的竞争性拮抗剂辣椒素（CPZ）可以阻断甲醛诱发的放电，并对所有测试细胞的膜去极化有部分抑制作用。在通过共聚焦钙成像检查的另外160个细胞中，32%的细胞显示对甲醛有细胞内Ca(2+)升高的反应。在51个对甲醛敏感的细胞中，67%被CPZ抑制，表明TRPV1部分参与介导甲醛诱发的细胞内Ca(2+)升高。在电压钳模式下，41%的DRG细胞被诱发产生内向电流，其余59%保持不变。在对另外56个对甲醛敏感的细胞进行的单独实验中，证明了甲醛诱发电流幅度的浓度依赖性增加。与对照组相比，甲醛诱发的内向电流可被CPZ显著抑制，而TRPA1选择性拮抗剂HC-030031可进一步增强这种抑制作用。最后，在福尔马林试验中证实了CPZ的局部作用，结果表明CPZ在第1阶段强烈抑制福尔马林诱导的爪部退缩，但仅在25-55分钟内第2阶段受到显著抑制。因此得出结论，甲醛可直接激活一部分初级伤害感受器细胞，甲醛诱导的AP放电可能主要促成福尔马林诱导的伤害感受的第1阶段，而非第2阶段。甲醛对初级伤害感受器细胞的激活可能至少部分通过TRPV1和/或TRPA1受体介导。
