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J Neurosci. 2019 Apr 26. pii: 2781-18. doi: 10.1523/JNEUROSCI.2781-18.2019. [Epub ahead of print]

Robustness to axon initial segment variation is explained by somatodendritic excitability in rat substantia nigra dopaminergic neurons.

Author information

1
UMR_S 1072, Aix Marseille Université, INSERM, Faculté de Médecine Secteur Nord, Marseille, FRANCE.
2
Laboratory of Neurophysiology, GIGA- Neurosciences, Université de Liège, Liège, BELGIQUE.
3
UMR_S 1072, Aix Marseille Université, INSERM, Faculté de Médecine Secteur Nord, Marseille, FRANCE jean-marc.goaillard@univ-amu.fr.

Abstract

In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. Interestingly, the axon of rat substantia nigra pars compacta (SNc) dopaminergic (DA) neurons displays a highly variable location and most often arises from an axon-bearing dendrite (ABD). We combined current-clamp somatic and dendritic recordings, outside-out recordings of dendritic sodium and potassium currents, morphological reconstructions and multi-compartment modelling on male and female rat SNc DA neurons to determine cell-to-cell variations in AIS and ABD geometry and their influence on neuronal output (spontaneous pacemaking frequency, AP shape). Both AIS and ABD geometries were found to be highly variable from neuron to neuron. Surprisingly, we found that AP shape and pacemaking frequency were independent of AIS geometry. Modelling realistic morphological and biophysical variations helped us clarify this result: in SNc DA neurons, the complexity of the ABD combined with its excitability predominantly define pacemaking frequency and AP shape, such that large variations in AIS geometry negligibly affect neuronal output, and are tolerated.SIGNIFICANCE STATEMENTIn many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. In the current study, we describe large cell-to-cell variations in AIS length or distance from the soma in rat substantia nigra pars compacta dopaminergic neurons. Using neuronal reconstruction and electrophysiological recordings, we show that this morphological variability does not seem to affect their electrophysiological output, as neither action potential properties nor pacemaking frequency are correlated with AIS morphology. Realistic multicompartment modelling suggests that this robustness to AIS variation is mainly explained by the complexity and excitability of the somatodendritic compartment.

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