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Characterization of the axon initial segment of mice substantia nigra dopaminergic neurons
Authors:Cristian González‐Cabrera  Rodrigo Meza  Lorena Ulloa  Paulina Merino‐Sepúlveda  Valentina Luco  Ana Sanhueza  Alejandro Oñate‐Ponce  J. Paul Bolam  Pablo Henny
Affiliation:1. Laboratorio de Neuroanatomía, Departamento de Anatomía, and Centro Interdisciplinario de Neurociencia, NeuroUC, Escuela de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile;2. Departamento de Fisiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile;3. MRC Brain Network Dynamics Unit, Department of Pharmacology, University of Oxford, Oxford, United Kingdom
Abstract: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 (Nav) 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 Nav subunits revealed the expression of Nav1.2 by most SNc neurons and a small proportion expressing Nav1.6. 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 Nav1.2 by the majority of dopaminergic neurons may explain their high threshold for firing and their low discharge rate.
Keywords:axon initial segment  dopamine  substantia nigra  RRID: AB_2289736  RRID: AB_2619897  RRID: AB_2040204  RRID: SCR_014329  RRID: SCR_002526  RRID: AB_2201518  RRID: SCR_002716  RRID: nlx_84100  RRID: nlx_84530  RRID: OMICS_02343  RRID: AB_514497
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