Characterization of the axon initial segment of mice substantia nigra dopaminergic neurons

• Published in: Journal of comparative neurology (1911) 2017-11, Vol.525 (16), p.3529-3542
• Main Authors: González‐Cabrera, Cristian, Meza, Rodrigo, Ulloa, Lorena, Merino‐Sepúlveda, Paulina, Luco, Valentina, Sanhueza, Ana, Oñate‐Ponce, Alejandro, Bolam, J. Paul, Henny, Pablo
• Format: Article
• Language: English
• Subjects: Action potential; Action Potentials - physiology; Animals; Ankyrins; Ankyrins - metabolism; Ankyrins - ultrastructure; axon initial segment; Axon Initial Segment - physiology; Axon Initial Segment - ultrastructure; dopamine; Dopamine receptors; Dopaminergic Neurons - cytology; Ecstasy; Firing pattern; Gene Expression - physiology; Hydroxylase; Imaging, Three-Dimensional; Innervation; Localization; Male; Mice; Mice, Inbred C57BL; Microscopy, Electron, Transmission; Microscopy, Immunoelectron; mRNA; NAV1.2 Voltage-Gated Sodium Channel - genetics; NAV1.2 Voltage-Gated Sodium Channel - metabolism; NAV1.2 Voltage-Gated Sodium Channel - ultrastructure; NAV1.6 Voltage-Gated Sodium Channel - genetics; NAV1.6 Voltage-Gated Sodium Channel - metabolism; NAV1.6 Voltage-Gated Sodium Channel - ultrastructure; Neuroimaging; Neurons; Nodes of Ranvier; RNA, Messenger - metabolism; Rodents; RRID: AB_2040204; RRID: AB_2201518; RRID: AB_2289736; RRID: AB_2619897; RRID: AB_514497; RRID: nlx_84100; RRID: nlx_84530; RRID: OMICS_02343; RRID: SCR_002526; RRID: SCR_002716; RRID: SCR_014329; Sodium channels (voltage-gated); Substantia nigra; Substantia Nigra - cytology; Tapering; Tyrosine 3-monooxygenase; Tyrosine 3-Monooxygenase - metabolism; Tyrosine 3-Monooxygenase - ultrastructure
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.24288

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. Diagrammatic depiction of a typical AIS from a SNc dopaminergic neuron, its average dimensions, presence of Nav channels on its membrane, and synaptic contacts which occurred very scantly. Ultrastructurally, ankyrin‐G (gold particles) is confined to a submembranous location in TH‐positive (black precipitate), dopaminergic AIS. In situ hybridization indicates that Nav1.2 is the main Nav subunit expressed by SNc neurons.