HCN2通过调控外周C纤维“传导丢峰”参与慢性炎性痛敏形成的作用及机制

Chronic inflammatory hyperalgesia is a common disease which is difficult to treat, affecting many aspects of life and severely limiting the patient's daily functions. Peripheral nociceptive unmyelinated C fibers are an indispensable pathway in the transmission of pain signals from the periphery to the center. The axon has long been considered as a neuronal process which functions like a cable between neurons, ensuring the conduction of information from the cell body to the nerve terminal. However, mounting experimental evidence has exhibited the necessity of reexamination of the functions of fibers. We have found that C fibers from the coccygeal nerves of rats had conduction failure. Besides, we further proved that the degree of conduction failure had significantly decreased in diabetic rats than controls. These results indicated that C fibers may be involved in the formation of hyperalgesia by regulating the afferent number of peripheral pain signals (Brain, 2012). Despite these observations, we also found that the slowing of conduction velocity could result in conduction failure, but the precise role and mechanisms still remain elusive (Pain, 2016)..Increasing evidence indicated that the sustained hyperpolarization of axon membrane potential induced by repetitive stimulation of C fibers is the key factor of the slowing of conduction velocity. HCN2 is a hyperpolarization-activated cyclic nucleotide-gated cation channel which produces an inward current (Ih) when activated. This inward current can inhibit the level of hyperpolarization of axon membrane. In addition, chronic inflammation can activate HCN2 channels. Based on these results, we proposed that HCN2 participates in the formation of chronic inflammatory hyperalgesia by regulating the degree of conduction failure of C fibers under the condition of chronic inflammation. With the aim of illuminating the role and mechanisms of conduction failure of C fibers in hyperalgesia, this project hopes to shed new light, if possible, on the theoretical basis of the occurrence of hyperalgesia, and to move forward to provide relatively new strategy and specific target for the clinical peripheral analgesia.

痛敏是慢性炎性痛的一种常见却难以治疗的并发症，严重影响病人的生活质量。C纤维是痛信号由外周传递到中枢的必经之路。传统观点认为，神经纤维像“电缆”一样忠实地传递神经信号。而我们的研究发现C纤维存在传导丢峰现象，且在糖尿病痛敏条件下传导丢峰程度降低，意味着由外周传入的动作电位数量增加，这可能是导致痛敏形成的关键因素（Brain, 2012）。进一步研究发现，传导速度减慢导致C纤维传导丢峰，但其机制尚未阐明（Pain, 2016）。研究表明，传导速度减慢是由轴突膜电位超极化所致，而激活HCN2通道产生的Ih电流可减小轴突膜电位超极化。并且，慢性炎症可激活HCN2通道。因此我们推测，在慢性炎症条件下，HCN2通道通过降低C纤维传导丢峰程度导致痛敏的形成。本项目拟阐明HCN2调控C纤维传导丢峰参与痛敏形成的作用及机制，以干预HCN2通道调控传导丟峰为切入点，为临床外周镇痛提供新策略和新靶点。

传统观点认为，神经纤维像电缆一样忠实地传递神经信号，我们前期的研究发现，多觉型伤害性C纤维存在传导丟峰现象，而C纤维主要传递痛信号。本项目的研究发现，CFA慢性炎性痛后C纤维传导丟峰程度显著降低，并伴随着传导速度减慢程度下降；HCN2通道阻断剂ZD7288可增加C纤维传导丟峰程度和传导速度减慢程度但并不影响A纤维的传导，此外神经干施加ZD7288还可降低C纤维自发放电的频率；通过行为学实验发现，坐骨神经周围注射ZD7288可显著缓解动物的机械痛敏、热痛敏和自发痛等行为，但是并不影响动物的心率和运动功能；对传导丟峰的离子通道机制进行探讨，发现CFA慢性炎性痛后HCN2蛋白在C纤维和DRG小细胞上表达均上调；电生理的结果也发现，CFA组DRG小细胞HCN2产生的Ih电流密度增加，动作电位超极化后电位的幅度和半宽均减小，施加ZD7288可降低Ih电流密度，进而增加超极化后电位的幅度及半宽。综合以上研究结果表明，CFA慢性炎性痛后，HCN2表达上调致使Ih电流增加，通过降低动作电位超极化后电位降低C纤维传导丟峰程度，增加由外周传递到中枢痛信号的数量，进而诱发中枢痛敏的形成。本项目的研究结果提示，HCN2可作为外周镇痛的理想靶点，为临床药物研发提供理论依据。