Kremer E, Lev-Tov A
Department of Anatomy and Cell Biology, The Hebrew University Medical School, Jerusalem 91120, Israel.
J Neurophysiol. 1998 May;79(5):2581-92. doi: 10.1152/jn.1998.79.5.2581.
Dorsal root afferent depolarization and antidromic firing were studied in isolated spinal cords of neonatal rats. Spontaneous firing accompanied by occasional bursts could be recorded from most dorsal roots in the majority of the cords. The afferent bursts were enhanced after elevation of the extracellular potassium concentration ([K+]e) by 1-2 mM. More substantial afferent bursts were produced when the cords were isolated with intact brain stems. Rhythmic afferent bursts could be recorded from dorsal roots in some of the cords during motor rhythm induced by bath-applied serotonin and N-methyl--aspartate (NMDA). Bilaterally synchronous afferent bursts were produced in pairs of dorsal roots after replacing the NaCl in the perfusate with sodium-2-hydroxyethansulfonate or after application of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline with or without serotonin (5-HT) and NMDA. Antidromic afferent bursts also could be elicited under these conditions by stimulation of adjacent dorsal roots, ventrolateral funiculus axons, or ventral white commissural (VWC) fibers. The antidromic bursts were superimposed on prolonged dorsal root potentials (DRPs) and accompanied by a prolonged increase in intraspinal afferent excitability. Surgical manipulations of the cord revealed that afferent firing in the presence of bicuculline persisted in the hemicords after hemisection and still was observed after removal of their ventral horns. Cutting the VWC throughout its length did not perturb the bilateral synchronicity of the discharge. These findings suggest that the activity of dorsal horn neurons is sufficient to produce the discharge and that the bilateral synchronicity can be maintained by cross connectivity that is relayed from side to side dorsal to the VWC. Antagonists of GABAB, 5-HT2/5-HT1C, or glutamate metabotropic group II and III receptors could not abolish afferent depolarization in the presence of bicuculline. Depolarization comparable in amplitude to DRPs, could be produced in tetrodotoxin-treated cords by elevation of [K+]e to the levels reported to develop in the neonatal rat spinal cord in response to dorsal root stimulation. A mechanism involving potassium transients produced by neuronal activity therefore is suggested to be the major cause of the GABA-independent afferent depolarization reported in our study. Possible implications of potassium transients in the developing and the adult mammalian spinal cord are discussed.
在新生大鼠的离体脊髓中研究了背根传入神经去极化和逆向发放。在大多数脊髓的大多数背根中可记录到伴有偶尔爆发的自发放电。细胞外钾浓度（[K⁺]e）升高1 - 2 mM后，传入爆发增强。当脊髓与完整脑干一起分离时，会产生更大量的传入爆发。在浴用5-羟色胺和N-甲基-D-天冬氨酸（NMDA）诱导的运动节律期间，在一些脊髓的背根中可记录到节律性传入爆发。在用2-羟乙磺酸钠替代灌注液中的氯化钠后，或在应用γ-氨基丁酸-A（GABAA）受体拮抗剂荷包牡丹碱（无论有无5-羟色胺（5-HT）和NMDA）后，成对的背根中会产生双侧同步传入爆发。在这些条件下，通过刺激相邻背根、腹外侧索轴突或腹侧白质连合（VWC）纤维，也可引发逆向传入爆发。逆向爆发叠加在延长的背根电位（DRP）上，并伴有脊髓内传入兴奋性的持续增加。对脊髓的手术操作表明，在存在荷包牡丹碱的情况下，传入放电在半切后的脊髓半侧中持续存在，并且在去除其腹角后仍可观察到。沿其全长切断VWC并不会扰乱放电的双侧同步性。这些发现表明，背角神经元的活动足以产生放电，并且双侧同步性可通过在VWC背侧从一侧传递到另一侧的交叉连接来维持。在存在荷包牡丹碱的情况下，GABAB、5-HT2/5-HT1C或谷氨酸代谢型II和III类受体的拮抗剂不能消除传入去极化。在河豚毒素处理的脊髓中，通过将[K⁺]e升高到据报道新生大鼠脊髓在对背根刺激作出反应时所达到的水平，可产生幅度与DRP相当的去极化。因此，提示一种涉及神经元活动产生的钾瞬变的机制是我们研究中报道的不依赖GABA的传入去极化的主要原因。讨论了钾瞬变在发育中和成年哺乳动物脊髓中的可能意义。
