Cameron W E, Núñez-Abades P A, Kerman I A, Hodgson T M
Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
J Neurophysiol. 2000 Nov;84(5):2330-9. doi: 10.1152/jn.2000.84.5.2330.
The role of potassium conductances in determining input resistance was studied in 166 genioglossal (GG) motoneurons using sharp electrode recording in brain stem slices of the rats aged 5-7 days, 13-15 days, and 19-24 days postnatal (P). A high magnesium (Mg(2+); 6 mM) perfusate was used to block calcium-mediated synaptic release while intracellular or extracellular cesium (Cs(+)) and/or extracellular tetraethylammonium (TEA) or barium (Ba(2+)) were used to block potassium conductances. In all cases, the addition of TEA to the high Mg(2+) perfusate generated a larger increase in both input resistance (R(n)) and the first membrane time constant (tau(0)) than did high Mg(2+) alone indicating a substantial nonsynaptic contribution to input resistance. With intracellular injection of Cs(+), GG motoneurons with lower resistance (<40 MOmega), on the average, showed a larger percent increase in R(n) than cells with higher resistance (>40 MOmega). There was also a significant increase in the effect of internal Cs(+) on R(n) and tau(0) with age. The largest percent increase (67%) in the tau(0) due to intracellular Cs(+) occurred at P13-15, a developmental stage characterized by a large reduction in specific membrane resistance. Addition of external Cs(+) blocked conductances (further increasing R(n) and tau(0)) beyond those blocked by the TEA perfusate. Substitution of external calcium with 2 mM barium chloride produced a significant increase in both R(n) and tau(0) at all ages studied. The addition of either intracellular Cs(+) or extracellular Ba(2+) created a depolarization shift of the membrane potential. The amount of injected current required to maintain the membrane potential was negatively correlated with the control R(n) of the cell at most ages. Thus low resistance cells had, on the average, more Cs(+)- and Ba(2+)-sensitive channels than their high resistance counterparts. There was also a disproportionately larger percent increase in tau(0) as compared with R(n) for both internal Cs(+) and external Ba(2+). Based on a model by Redman and colleagues, it might be suggested that the majority of these potassium conductances underlying membrane resistance are initially located in the distal dendrites but become more uniformly distributed over the motoneuron surface in the oldest animals.
利用尖锐电极记录技术，在出生后5 - 7天、13 - 15天和19 - 24天的大鼠脑干切片中的166个颏舌肌（GG）运动神经元上，研究了钾电导在决定输入电阻中的作用。使用高镁（Mg(2 +); 6 mM）灌流液来阻断钙介导的突触释放，同时使用细胞内或细胞外铯（Cs(+)）和/或细胞外四乙铵（TEA）或钡（Ba(2 +)）来阻断钾电导。在所有情况下，与单独使用高镁灌流液相比，向高镁灌流液中添加TEA会使输入电阻（R(n)）和第一个膜时间常数（tau(0)）都有更大幅度的增加，这表明对输入电阻有相当大的非突触贡献。通过细胞内注射Cs(+)，平均而言，电阻较低（<40 MΩ）的GG运动神经元的R(n)增加百分比比电阻较高（>40 MΩ）的细胞更大。随着年龄增长，内部Cs(+)对R(n)和tau(0)的影响也显著增加。由于细胞内Cs(+)导致的tau(0)最大增加百分比（67%）出现在P13 - 15，这是一个以比膜电阻大幅降低为特征的发育阶段。添加细胞外Cs(+)会阻断比TEA灌流液所阻断的更多的电导（进一步增加R(n)和tau(0)）。用2 mM氯化钡替代细胞外钙在所有研究的年龄段都会使R(n)和tau(0)显著增加。添加细胞内Cs(+)或细胞外Ba(2 +)都会使膜电位产生去极化偏移。在大多数年龄段，维持膜电位所需的注入电流量与细胞的对照R(n)呈负相关。因此，平均而言，低电阻细胞比高电阻细胞具有更多对Cs(+)和Ba(2 +)敏感的通道。与R(n)相比，内部Cs(+)和外部Ba(2 +)导致的tau(0)增加百分比也不成比例地更大。根据Redman及其同事的模型，可能会认为这些构成膜电阻的钾电导大多数最初位于远端树突，但在最年长的动物中会更均匀地分布在运动神经元表面。
