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

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 (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.

Robustness to Axon Initial Segment Variation Is Explained by Somatodendritic Excitability in Rat Substantia Nigra Dopaminergic Neurons

“…Conductance values were set according to our own measurements or published values (Table 1). Consistent with the literature (Zhou et al, 1998;Kole et al, 2008;Hu et al, 2009;González-Cabrera et al, 2017), g Na and g KDR densities are higher in the AIS than in the rest of the neuron so that AP always initiates in the AIS. According to Gentet and Williams (2007), I A density and inactivation kinetics were higher and depolarized, respectively, in the soma compared with the dendritic arbor.…”

“…Moreover, as shown in Figure 2D, most of the variation in soma-AIS distance was due to variation in ABD stem length. Therefore, consistent with previous studies (Häusser et al, 1995;Blythe et al, 2009;González-Cabrera et al, 2017;Meza et al, 2018), we found large variations in AIS length, soma-AIS distance, but also in dendrite and axon diameter between SNc DA neurons.…”

Brief sensory deprivation triggers cell type-specific structural and functional plasticity in olfactory bulb neurons

“…We found that OB AISes are of reasonably consistent length (coefficient of variation, CV = 0.34 in control cells), but can be situated at highly variable distances from the soma (control CV = 0.75). Contrary to findings in midbrain DA cells (González-Cabrera et al, 2017;Meza et al, 2018) and in OB dissociated cultures (Chand et al, 2015) we found no consistent relationship between these parameters in bulbar DA neurons (Spearmen coefficient of AIS length vs soma distance: Ctrl r=0.03, Occl, r=0.04; both p>0.73). We also noted that that the AIS of an OB DA neuron can be located either on a process that directly emanates from the soma ("soma-origin" AIS), or on a process separated from the soma by one or more branch nodes ("dendrite-origin" AIS, Ctrl n=31, Occl n=33; Fig.…”

“…We also noted that that the AIS of an OB DA neuron can be located either on a process that directly emanates from the soma ("soma-origin" AIS), or on a process separated from the soma by one or more branch nodes ("dendrite-origin" AIS, Ctrl n=31, Occl n=33; Fig. 8A-B) (González-Cabrera et al, 2017;Höfflin et al, 2017;Houston et al, 2017;Kosaka et al, 2019;Thome et al, 2014;Yang et al, 2019). While this peculiar axonal arrangement challenges the traditional view on neuronal input-output transformation (Kaifosh and Losonczy, 2014), it is not unique to bulbar DA neurons.…”

“…While this peculiar axonal arrangement challenges the traditional view on neuronal input-output transformation (Kaifosh and Losonczy, 2014), it is not unique to bulbar DA neurons. Indeed, midbrain DA neurons have been shown to carry "dendrite-origin" AISes (González-Cabrera et al, 2017;Yang et al, 2019), and recently the overall variability in AIS length and location in these neurons has been proposed to play a key role in the maintenance of an appropriate pacemaking rhythm in the context of variable dendritic branching (Moubarak et al, 2019). Moreover, "dendriteorigin" AISes are not exclusive to DA neurons: common in invertebrates (Triarhou, 2014), they have also been described in cat and mouse cortex (Hamada et al, 2016;Höfflin et al, 2017;Meyer and Wahle, 1988), in hippocampal pyramidal cells (Thome et al, 2014), and in cerebellar granule cells (Houston et al, 2017).…”