Raman I M, Bean B P
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA.
J Neurosci. 1999 Mar 1;19(5):1663-74. doi: 10.1523/JNEUROSCI.19-05-01663.1999.
Acutely dissociated cell bodies of mouse Purkinje neurons spontaneously fired action potentials at approximately 50 Hz (25 degrees C). To directly measure the ionic currents underlying spontaneous activity, we voltage-clamped the cells using prerecorded spontaneous action potentials (spike trains) as voltage commands and used ionic substitution and selective blockers to isolate individual currents. The largest current flowing during the interspike interval was tetrodotoxin-sensitive sodium current (approximately -50 pA between -65 and -60 mV). Although the neurons had large voltage-dependent calcium currents, the net current blocked by cobalt substitution for calcium was outward at all times during spike trains. Thus, the electrical effect of calcium current is apparently dominated by rapidly activated calcium-dependent potassium currents. Under current clamp, all cells continued firing spontaneously (though approximately 30% more slowly) after block of T-type calcium current by mibefradil, and most cells continued to fire after block of all calcium current by cobalt substitution. Although the neurons possessed hyperpolarization-activated cation current (Ih), little current flowed during spike trains, and block by 1 mM cesium had no effect on firing frequency. The outward potassium currents underlying the repolarization of the spikes were completely blocked by 1 mM TEA. These currents deactivated quickly (<1 msec) after each spike. We conclude that the spontaneous firing of Purkinje neuron cell bodies depends mainly on tetrodotoxin-sensitive sodium current flowing between spikes. The high firing rate is promoted by large potassium currents that repolarize the cell rapidly and deactivate quickly, thus preventing strong hyperpolarization and restoring a high input resistance for subsequent depolarization.
小鼠浦肯野神经元的急性解离细胞体在25摄氏度时以约50赫兹的频率自发发放动作电位。为了直接测量自发活动背后的离子电流，我们使用预先记录的自发动作电位（尖峰序列）作为电压指令对细胞进行电压钳制，并使用离子替代和选择性阻滞剂来分离单个电流。在峰间期流动的最大电流是河豚毒素敏感的钠电流（在-65至-60毫伏之间约为-50皮安）。尽管神经元具有较大的电压依赖性钙电流，但在用钴替代钙时所阻断的净电流在尖峰序列期间始终是外向的。因此，钙电流的电效应显然主要由快速激活的钙依赖性钾电流主导。在电流钳制下，在用米贝地尔阻断T型钙电流后，所有细胞仍继续自发发放（尽管速度约慢30%），并且在用钴替代阻断所有钙电流后，大多数细胞仍继续发放。尽管神经元具有超极化激活的阳离子电流（Ih），但在尖峰序列期间几乎没有电流流动，并且用1毫摩尔铯阻断对发放频率没有影响。动作电位复极化过程中的外向钾电流被1毫摩尔四乙铵完全阻断。这些电流在每次动作电位后迅速失活（<1毫秒）。我们得出结论，浦肯野神经元细胞体的自发发放主要取决于峰间期流动的河豚毒素敏感的钠电流。高发放率由大的钾电流促进，这些钾电流使细胞迅速复极化并迅速失活，从而防止强烈的超极化并恢复高输入电阻以进行后续去极化。
