Study on presynaptic action potential waveform in hippocampal neuronal culture models of episodic ataxia type 1 using scanning ion conductance microscopy

Abstract

Background: Episodic ataxia type 1 (EA1) is an autosomal dominant disorder characterized by paroxysmal cerebellar incoordination and interictal myokymia (Rajakulendran et al., 2007). It is caused by dominant negative mutations of KCNA1, which encodes the presynaptic and axonal potassium channel subunit Kv1.1. EA1 mutations increase neuronal excitability and neurotransmitter release in neuronal cultures (Heeroma et al., 2009). Although this could be explained by impaired action potential repolarization leading to an increase in presynaptic calcium influx, the action potential duration measured at the cell body is unaffected. Objective: Does EA1 prolong the action potential repolarization phase at presynaptic terminals thus causing increased neurotransmitter release? Method: We used Scanning Ion Conductance Microscopy to acquire super-resolution images of very small synaptic boutons (approximately 1 micrometer diameter) in mouse hippocampal neuronal cultures and gain direct electrophysiological recording from these boutons (Novak et al, 2013). We compared wild type, KCNA1a-/- knock-out and KCNA1a(V408A/+) knock-in mice (Herson et al., 2003). Action potentials were either elicited by injecting depolarizing current at the bouton or via a second pipette at the soma. Results: Presynaptic action potential half width was larger in KCNA1a-/- and KCNA1a(V408A/+) compared to their wild type litter mates, both when directly elicited at the bouton and when elicited by depolarising the cell body. A prolonged presynaptic action potential in EA1 may be responsible for increased neurotransmitter release. Because Kv1.1 is abundantly expressed in cerebellar basket cells, this may lead to excessive inhibition of cerebellar Purkinje cells and ataxia.