Sandler V M, Puil E, Schwarz D W
Department of Surgery, Faculty of Medicine, University of British Columbia, Vancouver, Canada.
Neuroscience. 1998 Apr;83(3):891-904. doi: 10.1016/s0306-4522(97)00415-6.
In trigeminal neurons, the spike rate, modulated by input parameters, may serve as a code for sensory information. We investigated intrinsic response properties that affect rate coding in neurons of nucleus principalis trigemini (young gerbils). Using the whole-cell recording technique and neurobiotin staining in slices, we found bursting behaviour in approximately 50% of the neurons. These neurons fired spike bursts, spontaneously, as well as at the onset of depolarizing, and offset of hyperpolarizing, current pulses. The spike rate within an initial burst was independent of stimulus strength, in contrast to single spike firing that occurred later in the response to current pulse injection. The spikes within a burst were superimposed on slow depolarizing humps. Under favourable conditions, these led to "plateau potentials", that lasted for hundreds of milliseconds at membrane potentials near approximately -20 mV. Occasionally, plateau potentials were spontaneous or evoked under control conditions: usually, they were evoked by current pulse injection during blockade of Ca2+ influx with Co2+ or Cd2+ in Ca(2+)-free extracellular media, or during blockade of K+ currents with tetraethylammonium. The plateau potentials recorded during internal Cs+ (132.5 mM) substitution of K+ had more positive amplitudes (near +20 mV). Despite relatively stable depolarization levels, the plateau potentials decreased in duration and decayed in amplitude during application of tetrodotoxin (0.6-1.8 nM). Higher tetrodotoxin concentrations (5-60 nM) eliminated the plateau potentials despite well-maintained, fast action potentials. A reduction of external [Na+] reduced the amplitudes of the spikes and plateau potentials. A hyperpolarization of long duration (> 3 s) followed a plateau potential, or a depolarizing response that was subthreshold for plateau generation. Tetrodotoxin application blocked this after-effect. We suggest that a persistent Na+ influx is a major contributor to the bursts and plateau potentials and that it mediates the hyperpolarization. Depending on Ca2+ influx, K+ conductances may regulate the amplitudes of these long-lasting depolarizations. A Ca2+ conductance, blockable by Ni2+, may support burst initiation in these neurons. In very young animals (P2-P9), we found only non-bursting neurons. Both bursting and non-bursting neurons with elongated dendritic fields showed inward rectification on hyperpolarization. The bursts in nucleus principalis trigemini neurons emphasize the onsets of stimulus transients, at the expense of using firing rate as a sensory code. Our studies describe neurons with a surprising ability to distort sensory signals, transforming depolarizing inputs into bursts of spikes by virtue of a Na(+)-conductance activation. The principal trigeminal nucleus also contains neurons with tonic firing ability, compatible with simple rate coding.
在三叉神经元中，由输入参数调制的放电频率可能作为感觉信息的一种编码方式。我们研究了影响三叉神经主核（幼年沙鼠）神经元频率编码的内在反应特性。利用全细胞记录技术和切片中的神经生物素染色，我们发现约50%的神经元存在爆发式放电行为。这些神经元会自发地以及在去极化电流脉冲开始时和超极化电流脉冲结束时发放动作电位爆发。与对电流脉冲注入反应后期出现的单个动作电位发放不同，初始爆发内的动作电位频率与刺激强度无关。爆发内的动作电位叠加在缓慢的去极化波峰上。在有利条件下，这些去极化波峰会导致“平台电位”，在膜电位接近约 -20 mV时持续数百毫秒。偶尔，平台电位在对照条件下是自发的或可诱发的：通常，它们是在无钙细胞外介质中用Co2+或Cd2+阻断Ca2+内流期间，或在用四乙铵阻断K+电流期间通过电流脉冲注入诱发的。在内部用Cs+（132.5 mM）替代K+时记录到的平台电位具有更正的幅度（接近 +20 mV）。尽管去极化水平相对稳定，但在应用河豚毒素（0.6 - 1.8 nM）期间，平台电位的持续时间缩短且幅度衰减。更高浓度的河豚毒素（5 - 60 nM）尽管动作电位维持良好且快速，但消除了平台电位。降低细胞外[Na+]会降低动作电位和平台电位的幅度。在平台电位或低于平台产生阈值的去极化反应之后会出现长时间（> 3 s）的超极化。应用河豚毒素可阻断这种后效应。我们认为持续性Na+内流是爆发和平台电位的主要促成因素，并且它介导了超极化。根据Ca2+内流情况，K+电导可能调节这些长时去极化的幅度。一种可被Ni2+阻断的Ca2+电导可能支持这些神经元的爆发起始。在非常年幼的动物（P2 - P9）中，我们只发现了非爆发式放电的神经元。具有细长树突野的爆发式和非爆发式放电神经元在超极化时均表现出内向整流。三叉神经主核中的爆发式放电强调了刺激瞬变的起始，而牺牲了将放电频率用作感觉编码的方式。我们的研究描述了具有惊人能力的神经元，它们能够扭曲感觉信号，通过激活Na+电导将去极化输入转化为动作电位爆发。三叉神经主核中也包含具有紧张性放电能力的神经元，这与简单的频率编码相一致。
