Reyes A D, Rubel E W, Spain W J
Department of Medical Genetics, University of Washington School of Medicine, Seattle 98195.
J Neurosci. 1994 Sep;14(9):5352-64. doi: 10.1523/JNEUROSCI.14-09-05352.1994.
Neurons of the avian nucleus magnocellularis (NM) relay auditory information from the VIIIth nerve to other parts of the auditory system. To examine the cellular properties that permit NM neurons to transmit reliably the temporal characteristics of the acoustic stimulus, we performed whole-cell recordings in neurons of the chick NM using an in vitro thin slice preparation. NM neurons exhibited strong outward rectification near resting potential; the voltage responses to depolarizing current steps were substantially smaller than to equivalent hyperpolarizing steps. Suprathreshold current steps evoked only a single action potential at the start of the step. In contrast, stimulation with trains of brief current pulses evoked repetitive firing that was phase-locked to the stimulus cycle. The number of action potentials evoked by the pulses during the train decreased with increasing stimulus rate. Voltage-clamp experiments revealed a rapidly activating, slowly inactivating, outward current with a threshold near -65 mV. During depolarizing voltage steps, the outward current rose sigmoidally to a peak and then decayed slowly, reaching steady state within 5 sec. Application of 200 microM 4-aminopyridine (4-AP) reduced the peak of the outward current by 84%, leaving a small, persistent component. Under current clamp, application of 200 microM 4-AP reduced the outward rectification and increased the amplitude and duration of the action potentials. Moreover, NM neurons could no longer sustain firing during high rates of stimulation with the current pulses: increased temporal summation of the potentials caused sufficient depolarization to inactivate the sodium conductance underlying the action potential. These results suggest that the outward current is necessary for NM neurons to transmit well-timed events reliably for the duration of an acoustic stimulus.
鸟类大细胞神经核（NM）的神经元将来自第八对脑神经的听觉信息传递至听觉系统的其他部分。为了研究使NM神经元能够可靠传递声刺激时间特征的细胞特性，我们使用体外薄片制备技术对雏鸡NM神经元进行了全细胞记录。NM神经元在静息电位附近表现出强烈的外向整流；对去极化电流阶跃的电压反应明显小于同等幅度的超极化阶跃。阈上电流阶跃仅在阶跃开始时诱发单个动作电位。相反，用一串短暂电流脉冲刺激可诱发与刺激周期锁相的重复放电。一串脉冲期间诱发的动作电位数量随刺激频率增加而减少。电压钳实验揭示了一种快速激活、缓慢失活的外向电流，其阈值接近-65 mV。在去极化电压阶跃期间，外向电流呈S形上升至峰值，然后缓慢衰减，在5秒内达到稳态。施加200 microM 4-氨基吡啶（4-AP）使外向电流峰值降低84%，仅留下一个小的持续成分。在电流钳模式下，施加200 microM 4-AP可减少外向整流，并增加动作电位的幅度和持续时间。此外，在高频率电流脉冲刺激期间，NM神经元无法再维持放电：电位的时间总和增加导致足够的去极化，使动作电位的钠电导失活。这些结果表明，外向电流对于NM神经元在声刺激持续期间可靠传递适时事件是必需的。
