Meredith Frances L, Rennie Katherine J
Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, United States.
Department of Physiology & Biophysics, University of Colorado School of Medicine, Aurora, CO, United States.
Front Cell Neurosci. 2018 Nov 14;12:423. doi: 10.3389/fncel.2018.00423. eCollection 2018.
The vestibular system relays information about head position afferent nerve fibers to the brain in the form of action potentials. Voltage-gated Na channels in vestibular afferents drive the initiation and propagation of action potentials, but their expression during postnatal development and their contributions to firing in diverse mature afferent populations are unknown. Electrophysiological techniques were used to determine Na channel subunit types in vestibular calyx-bearing afferents at different stages of postnatal development. We used whole cell patch clamp recordings in thin slices of gerbil crista neuroepithelium to investigate Na channels and firing patterns in central zone (CZ) and peripheral zone (PZ) afferents. PZ afferents are exclusively dimorphic, innervating type I and type II hair cells, whereas CZ afferents can form dimorphs or calyx-only terminals which innervate type I hair cells alone. All afferents expressed tetrodotoxin (TTX)-sensitive Na currents, but TTX-sensitivity varied with age. During the fourth postnatal week, 200-300 nM TTX completely blocked sodium currents in PZ and CZ calyces. By contrast, in immature calyces [postnatal day (P) 5-11], a small component of peak sodium current remained in 200 nM TTX. Application of 1 μM TTX, or Jingzhaotoxin-III plus 200 nM TTX, abolished sodium current in immature calyces, suggesting the transient expression of voltage-gated sodium channel 1.5 (Nav1.5) during development. A similar TTX-insensitive current was found in early postnatal crista hair cells (P5-9) and constituted approximately one third of the total sodium current. The Nav1.6 channel blocker, 4,9-anhydrotetrodotoxin, reduced a component of sodium current in immature and mature calyces. At 100 nM 4,9-anhydrotetrodotoxin, peak sodium current was reduced on average by 20% in P5-14 calyces, by 37% in mature dimorphic PZ calyces, but by less than 15% in mature CZ calyx-only terminals. In mature PZ calyces, action potentials became shorter and broader in the presence of 4,9-anhydrotetrodotoxin implicating a role for Nav1.6 channels in firing in dimorphic afferents.
前庭系统以动作电位的形式将有关头部位置的信息通过传入神经纤维传递给大脑。前庭传入神经中的电压门控钠通道驱动动作电位的起始和传播，但其在出生后发育过程中的表达以及对不同成熟传入神经群体放电的贡献尚不清楚。我们使用电生理技术来确定出生后不同发育阶段前庭带花萼传入神经中的钠通道亚基类型。我们在沙鼠嵴神经上皮薄片中使用全细胞膜片钳记录来研究中央区（CZ）和外周区（PZ）传入神经中的钠通道和放电模式。PZ传入神经仅为双形，支配I型和II型毛细胞，而CZ传入神经可形成双形或仅花萼终末，仅支配I型毛细胞。所有传入神经均表达对河豚毒素（TTX）敏感的钠电流，但TTX敏感性随年龄而变化。在出生后第四周，200 - 300 nM TTX可完全阻断PZ和CZ花萼中的钠电流。相比之下，在未成熟花萼[出生后第（P）5 - 11天]中，200 nM TTX作用下仍有一小部分钠电流峰值。应用1 μM TTX或京蝎毒素 - III加200 nM TTX可消除未成熟花萼中的钠电流，提示发育过程中电压门控钠通道1.5（Nav1.5）的短暂表达。在出生后早期的嵴毛细胞（P5 - 9）中也发现了类似的对TTX不敏感的电流，约占总钠电流的三分之一。Nav1.6通道阻滞剂4,9 - 脱水河豚毒素可减少未成熟和成熟花萼中钠电流的一个成分。在100 nM 4,9 - 脱水河豚毒素作用下，P5 - 14花萼中钠电流峰值平均降低20%，成熟双形PZ花萼中降低37%，但在成熟的仅含CZ花萼终末中降低不到15%。在成熟的PZ花萼中，4,9 - 脱水河豚毒素存在时动作电位变得更短更宽，这表明Nav1.6通道在双形传入神经放电中起作用。
