Pan F, Beam K G
Department of Anatomy, Colorado State University, Fort Collins, CO, USA.
Brain Res. 1999 Dec 4;849(1-2):162-8. doi: 10.1016/s0006-8993(99)02060-0.
The Scn8a gene encodes a neuronal, voltage-gated sodium channel, which is highly expressed in both cerebellar Purkinje neurons and spinal motoneurons [D.L. Burgess, D.C. Kohrman, J. Galt, N.W. Plummer, J.M. Jones, B. Spear, M.H. Meisler, Mutation of a new sodium channel gene, Scn8a, in the mouse mutant 'motor endplate disease', Nature Genetics 10 (1995) 461-465; K.L. Schaller, D.M. Krzemien, P.J. Yarowsky, B.K. Krueger, J.H. Caldwell, A novel, abundant sodium channel expressed in neurons and glia, J. Neurosci. 15 (1995) 3231-3242]. Sodium channels in Purkinje cells produce an unusual, "resurgent" current when the cells are repolarized to intermediate potentials (-60 to -20 mV) following a strong depolarization that completely inactivates transient sodium current [I.M. Raman, L.K. Sprunger, M.H. Meisler, B.P. Bean, Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice, Neuron 19 (1997) 881-891; I.M. Raman, B.P. Bean, Resurgent sodium current and action potential formation in dissociated cerebellar Purkinje neurons, J. Neurosci. 17 (1997) 4517-4526]. Here, we have examined whether large spinal neurons (predominantly motoneurons), isolated from P6-P8 mice and cultured overnight, produce sodium currents resembling those either of Purkinje cells or of Xenopus oocytes after heterologous expression of Scn8a. We found that P10-P14 Purkinje cells exhibited resurgent current (ranging from -3.6 to -15.4 pA/pF in 16 cells at -40 mV), but cultured spinal neurons had little or no such current (<0.5 pA/pF in 13 of 16 cells; -1.2 to -2.3 pA/pF in three of 16 cells). Furthermore, unlike Scn8a channels heterologously expressed in Xenopus oocytes [M.R. Smith, R.D. Smith, N.W. Plummer, M.H. Meisler, A.L. Goldin, Functional analysis of the mouse Scn8a sodium channel. J. Neurosci. 18 (1998) 6093-6102], there was not a prominent component of persistent sodium current in either Purkinje neurons or large spinal neurons. Based on analysis of cells from mice with a Scn8a null mutation, Scn8a channels appear to contribute significantly to total sodium current in both in P10-P14 Purkinje cells (approximately 40%; [21]) and cultured P7-P8 spinal motoneurons (approximately 70% [K.D. García, L.K. Sprunger, M.H. Meisler, K.G. Beam, The sodium channel Scn8a is the major contributor to the postnatal developmental increase of sodium current density in spinal motoneurons, J. Neurosci. 18 (1998) 5234-5239]). Thus, the presence or absence of resurgent current, and of persistent sodium current, appears to depend on cellular factors other than the mere presence of the Scn8a transcript.
Scn8a基因编码一种神经元电压门控钠通道，该通道在小脑浦肯野神经元和脊髓运动神经元中均高度表达[D.L. 伯吉斯、D.C. 科尔曼、J. 高尔特、N.W. 普拉默、J.M. 琼斯、B. 斯皮尔、M.H. 梅斯勒，小鼠突变体“运动终板病”中新钠通道基因Scn8a的突变，《自然遗传学》10 (1995) 461 - 465；K.L. 沙勒、D.M. 克热米恩、P.J. 亚罗夫斯基、B.K. 克鲁格、J.H. 考德威尔，一种在神经元和神经胶质细胞中表达的新型丰富钠通道，《神经科学杂志》15 (1995) 3231 - 3242]。当浦肯野细胞在强烈去极化后复极化至中间电位（-60至-20 mV）时，其钠通道会产生一种不寻常的“复苏”电流，这种强烈去极化会使瞬时钠电流完全失活[I.M. 拉曼、L.K. 斯普伦格、M.H. 梅斯勒、B.P. 比恩，Scn8a突变小鼠浦肯野神经元阈下钠电流改变及放电模式紊乱，《神经元》19 (1997) 881 - 891；I.M. 拉曼、B.P. 比恩，解离的小脑浦肯野神经元中的复苏钠电流与动作电位形成，《神经科学杂志》17 (1997) 4517 - 4526]。在此，我们研究了从P6 - P8小鼠分离并培养过夜的大型脊髓神经元（主要是运动神经元）是否会产生类似于浦肯野细胞或非洲爪蟾卵母细胞在异源表达Scn8a后产生的钠电流。我们发现，P10 - P14浦肯野细胞表现出复苏电流（在-40 mV时，16个细胞中的电流范围为-3.6至-15.4 pA/pF），但培养的脊髓神经元几乎没有这种电流（16个细胞中有13个细胞的电流<0.5 pA/pF；16个细胞中有3个细胞的电流为-1.2至-2.3 pA/pF）。此外，与在非洲爪蟾卵母细胞中异源表达的Scn8a通道不同[M.R. 史密斯、R.D. 史密斯、N.W. 普拉默、M.H. 梅斯勒、A.L. 戈尔丁，小鼠Scn8a钠通道的功能分析。《神经科学杂志》18 (1998) 6093 - 6102]，浦肯野神经元或大型脊髓神经元中均不存在显著的持续性钠电流成分。基于对Scn8a基因敲除突变小鼠细胞的分析，Scn8a通道似乎对P10 - P14浦肯野细胞（约40%；[21]）和培养的P7 - P8脊髓运动神经元（约70% [K.D. 加西亚、L.K. 斯普伦格、M.H. 梅斯勒、K.G. 比姆，钠通道Scn8a是脊髓运动神经元钠电流密度出生后发育性增加的主要贡献者，《神经科学杂志》18 (1998) 5234 - 5239]）中的总钠电流有显著贡献。因此，复苏电流和持续性钠电流的有无似乎取决于除Scn8a转录本的单纯存在之外的细胞因素。
