Wenner P, O'Donovan M J
Laboratory of Neural Control, Section on Developmental Neurobiology, National Institute of Neurological Disorders and Stroke/NIH, 49 Convent Drive, Bethesda, MD 20892, USA.
J Neurophysiol. 2001 Sep;86(3):1481-98. doi: 10.1152/jn.2001.86.3.1481.
Many developing networks exhibit a transient period of spontaneous activity that is believed to be important developmentally. Here we investigate the initiation of spontaneous episodes of rhythmic activity in the embryonic chick spinal cord. These episodes recur regularly and are separated by quiescent intervals of many minutes. We examined the role of motoneurons and their intraspinal synaptic targets (R-interneurons) in the initiation of these episodes. During the latter part of the inter-episode interval, we recorded spontaneous, transient ventral root depolarizations that were accompanied by small, spatially diffuse fluorescent signals from interneurons retrogradely labeled with a calcium-sensitive dye. A transient often could be resolved at episode onset and was accompanied by an intense pre-episode (approximately 500 ms) motoneuronal discharge (particularly in adductor and sartorius) but not by interneuronal discharge monitored from the ventrolateral funiculus (VLF). An important role for this pre-episode motoneuron discharge was suggested by the finding that electrical stimulation of motor axons, sufficient to activate R-interneurons, could trigger episodes prematurely. This effect was mediated through activation of R-interneurons because it was prevented by pharmacological blockade of either the cholinergic motoneuronal inputs to R-interneurons or the GABAergic outputs from R-interneurons to other interneurons. Whole-cell recording from R-interneurons and imaging of calcium dye-labeled interneurons established that R-interneuron cell bodies were located dorsomedial to the lateral motor column (R-interneuron region). This region became active before other labeled interneurons when an episode was triggered by motor axon stimulation. At the beginning of a spontaneous episode, whole-cell recordings revealed that R-interneurons fired a high-frequency burst of spikes and optical recordings demonstrated that the R-interneuron region became active before other labeled interneurons. In the presence of cholinergic blockade, however, episode initiation slowed and the inter-episode interval lengthened. In addition, optical activity recorded from the R-interneuron region no longer led that of other labeled interneurons. Instead the initial activity occurred bilaterally in the region medial to the motor column and encompassing the central canal. These findings are consistent with the hypothesis that transient depolarizations and firing in motoneurons, originating from random fluctuations of interneuronal synaptic activity, activate R-interneurons, which then trigger the recruitment of the rest of the spinal interneuronal network. This unusual function for R-interneurons is likely to arise because the output of these interneurons is functionally excitatory during development.
许多发育中的神经网络会经历一段自发活动的过渡时期，人们认为这段时期在发育过程中很重要。在此，我们研究了胚胎期鸡脊髓中节律性活动自发发作的起始机制。这些发作有规律地反复出现，且间隔数分钟的静止期。我们研究了运动神经元及其脊髓内突触靶点（R-中间神经元）在这些发作起始过程中的作用。在发作间期的后半段，我们记录到自发的、短暂的腹根去极化，同时伴有来自用钙敏染料逆行标记的中间神经元的小范围、空间扩散的荧光信号。在发作开始时常常能分辨出一个瞬变过程，并且伴有发作前强烈的（约500毫秒）运动神经元放电（特别是在内收肌和缝匠肌中），但从腹外侧索（VLF）监测到的中间神经元放电则没有。运动轴突的电刺激足以激活R-中间神经元，且能过早触发发作，这一发现表明发作前运动神经元放电具有重要作用。这种效应是通过激活R-中间神经元介导的，因为对R-中间神经元的胆碱能运动神经元输入或R-中间神经元向其他中间神经元的GABA能输出进行药理学阻断均可阻止该效应。对R-中间神经元的全细胞记录以及对钙染料标记的中间神经元的成像显示，R-中间神经元的胞体位于外侧运动柱（R-中间神经元区域）的背内侧。当通过运动轴突刺激触发发作时，该区域比其他标记的中间神经元更早变得活跃。在自发发作开始时，全细胞记录显示R-中间神经元会产生高频脉冲发放，光学记录表明R-中间神经元区域比其他标记的中间神经元更早变得活跃。然而，在存在胆碱能阻断的情况下，发作起始变慢，发作间期延长。此外，从R-中间神经元区域记录到的光学活动不再领先于其他标记的中间神经元。相反，初始活动在运动柱内侧且围绕中央管的区域双侧出现。这些发现与以下假设一致：运动神经元中的瞬变去极化和放电源于中间神经元突触活动的随机波动，激活了R-中间神经元，进而触发脊髓中间神经元网络其余部分的募集。R-中间神经元的这种特殊功能可能是因为这些中间神经元在发育过程中的输出在功能上是兴奋性的。
