Mechanisms underlying axonal plasticity and computational abilities

神经功能依赖于神经元的通信能力，这种通信方式被认为依赖于神经元整合冲击其胞体和树突的输入并通过其轴突传递信息的能力，如果膜电位的变化使轴突初始段(AIS)达到阈值的话。在AIS产生的动作电位(AP)被认为是在轴突树上均匀传播的，但越来越多的证据激发了这些“教条”的修改。研究表明：1)AIS是可塑性的：过度或过度刺激神经元会导致其长度、位置和离子通道组成的改变。2)轴突不是被动传导AP：轴突主干上的递质受体的激活可以改变AP的传播；3)通过控制离子细胞外环境、髓鞘厚度和Ranvier结节的大小，神经胶质细胞影响AP的传播速度(从而影响整体的同步性)。形成髓鞘的少突胶质细胞通过缝隙连接与Ranvier结节周围的星形胶质细胞相互作用，耦合的星形胶质细胞和少突胶质细胞形成所谓的束网络。在这里，我们建议用电生理学、光遗传学、成像和免疫组织化学的方法来研究神经胶质细胞和苍白球网络在以下方面的作用：1)AIS的可塑性；2)三叉神经系膜核(NVmes)初级传入神经元轴杆的区域化；3)AP传导速度的调节。NVME是解决这些问题的独特机会，因为它们的大轴突可以维护，很容易在脑干切片中记录。在目标1)中，我们将使用局部改变细胞外钙离子浓度的方法或使用Na+指示剂来研究它们的活性的短期增加如何影响其AIS的长度和位置。活体测量将通过使用Ankyrin G或BetaIV血影蛋白抗体的特殊后免疫组织化学标记来确认。用这些方法获得的AIS变化将被比较星形胶质细胞激活或失活和/或它们与少突胶质细胞偶联前后的变化。目标2和3)将研究星形胶质细胞/少突胶质细胞的相互作用如何以分支特有的方式影响AP的繁殖和传导速度。有证据表明，NVme轴突的不同隔室之间存在GABA依赖的解偶联。在这里，我们将研究星形胶质细胞和/或少突胶质细胞对NVmes细胞不同轴突分支中顺行和逆行AP传播的影响，并试图确定任何已观察到的影响背后的胶质递质。这些实验提出了新的和原创的问题，可能会对我们理解决定最基本的计算和通信神经元功能的因素有广泛的影响。

Nervous functions rely on the ability of neurons to communicate, and this communication mode is thought to rely on the capacity of neurons to integrate inputs impinging on their soma and dendritic processes and to transmit this information through their axonal arbors if the changes in membrane potential bring the axon initial segment (AIS) to threshold. Action potentials (APs) generated at the AIS are thought to propagate uniformly across the axonal tree, but growing evidence incite revisions of these "dogmas". Studies have shown that: 1) AIS are plastic: Under or over stimulation of neurons lead to changes in their length, position and ion channel composition. 2) Axons are not "passively" conducting APs: Activation of transmitter receptors located on the axonal trunk can alter AP propagation, and 3) By controlling the ionic extracellular environment, the thickness of the myelin and the size of Ranvier nodes, glial cells affect AP speed of propagation (with consequences on synchrony in an ensemble). Oligodendrocytes forming the myelin interact with (perinodal) astrocytes at Ranvier nodes through gap junctions and coupled astrocytes and oligodendrocytes form what are called panglial networks. Here we propose to use electrophysiology, optogenetic, imaging and immunohistochemistry to examine how glial cells and panglial networks contribute to: 1) plasticity of the AIS and 2) Compartmentalization of the axonal arbor of primary afferent neurons located in the mesenscephalic trigeminal nucleus (NVmes), and 3) Regulation of conduction velocity of APs. NVmes represent a unique opportunity to address these questions because of their large axons that can be maintained and easily targeted for recording in brainstem slices. In Aim 1) we will examine how short term increase of their activity affects the length and position of their AIS using a method to vary focally the extracellular Ca2+ concentration or using Na+ indicators. Live measurements will be confirmed with post-hoc immuno-histochemical labelling with antibodies against Ankyrin G or BetaIV spectrin. AIS changes obtained with these methods will be compared before and after activation or inactivation of astrocytes and/or their coupling with oligodendrocytes. Aims 2 and 3) will examine how astrocytes/oligodendrocytes interactions can affect propagation and conduction velocity of APs in a branch specific manner. There is evidence of GABA dependent decoupling between different compartments of NVmes axons. Here we will examine the effect of astrocytes and/or oligodendrocytes manipulations on propagation of orthodromic and antidromic APs in different axonal branches of NVmes cells and will try to identify the gliotransmitter underlying any observed effect. These experiments raise novel and original questions that may have wide implications on our understanding of factors determining the most basic neuronal functions of computation and communication.