González-Cabrera Cristian, Meza Rodrigo, Ulloa Lorena, Merino-Sepúlveda Paulina, Luco Valentina, Sanhueza Ana, Oñate-Ponce Alejandro, Bolam J Paul, Henny Pablo
Laboratorio de Neuroanatomía, Departamento de Anatomía, and Centro Interdisciplinario de Neurociencia, NeuroUC, Escuela de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile.
Departamento de Fisiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile.
J Comp Neurol. 2017 Nov 1;525(16):3529-3542. doi: 10.1002/cne.24288. Epub 2017 Aug 18.
The axon initial segment (AIS) is the site of initiation of action potentials and influences action potential waveform, firing pattern, and rate. In view of the fundamental aspects of motor function and behavior that depend on the firing of substantia nigra pars compacta (SNc) dopaminergic neurons, we identified and characterized their AIS in the mouse. Immunostaining for tyrosine hydroxylase (TH), sodium channels (Na ) and ankyrin-G (Ank-G) was used to visualize the AIS of dopaminergic neurons. Reconstructions of sampled AIS of dopaminergic neurons revealed variable lengths (12-60 μm) and diameters (0.2-0.8 μm), and an average of 50% reduction in diameter between their widest and thinnest parts. Ultrastructural analysis revealed submembranous localization of Ank-G at nodes of Ranvier and AIS. Serial ultrathin section analysis and 3D reconstructions revealed that Ank-G colocalized with TH only at the AIS. Few cases of synaptic innervation of the AIS of dopaminergic neurons were observed. mRNA in situ hybridization of brain-specific Na subunits revealed the expression of Na 1.2 by most SNc neurons and a small proportion expressing Na 1.6. The presence of sodium channels, along with the submembranous location of Ank-G is consistent with the role of AIS in action potential generation. Differences in the size of the AIS likely underlie differences in firing pattern, while the tapering diameter of AIS may define a trigger zone for action potentials. Finally, the conspicuous expression of Na 1.2 by the majority of dopaminergic neurons may explain their high threshold for firing and their low discharge rate.
轴突起始段（AIS）是动作电位起始的部位，并影响动作电位波形、发放模式和频率。鉴于运动功能和行为的基本方面依赖于黑质致密部（SNc）多巴胺能神经元的发放，我们在小鼠中鉴定并表征了它们的AIS。使用酪氨酸羟化酶（TH）、钠通道（Na）和锚蛋白-G（Ank-G）的免疫染色来可视化多巴胺能神经元的AIS。多巴胺能神经元采样AIS的重建显示出可变的长度（12 - 60μm）和直径（0.2 - 0.8μm），并且其最宽和最窄部分之间的直径平均减少了50%。超微结构分析显示Ranvier结和AIS处Ank-G的膜下定位。连续超薄切片分析和三维重建显示Ank-G仅在AIS处与TH共定位。观察到少数多巴胺能神经元AIS的突触支配情况。脑特异性Na亚基的mRNA原位杂交显示大多数SNc神经元表达Na 1.2，一小部分表达Na 1.6。钠通道的存在以及Ank-G的膜下定位与AIS在动作电位产生中的作用一致。AIS大小的差异可能是发放模式差异的基础，而AIS逐渐变细的直径可能定义了动作电位的触发区。最后，大多数多巴胺能神经元中Na 1.2的显著表达可能解释了它们的高发放阈值和低放电频率。
