Synapse-Level Determination of Action Potential Duration by K + Channel Clustering in Axons

Abstract

Summary In axons, an action potential (AP) is thought to be broadcast as an unwavering binary pulse over its arbor driving neurotransmission uniformly at release sites. Yet, by recording from axons of cerebellar stellate cell (SC) interneurons, we show that AP width varies between presynaptic bouton sites, even within the same axon branch. The varicose geometry of SC boutons, alone, does not impose differences in spike duration. Rather, axonal patching revealed heterogeneous peak conductance densities of currents mediated mainly by fast-activating Kv3-type potassium channels, with clustered hot-spots at boutons and restricted expression at adjoining shafts. Blockade of Kv channels at individual boutons indicates that currents immediately local to a release site directs spike repolarization at that location. Thus, the clustered arrangement and variable expression density of Kv3 channels at boutons are key determinants underlying compartmentalized control of AP width in a near synapse-by-synapse manner, multiplying the signaling capacity of these structures.

Rapid State-Dependent Alteration in Kv3 Channel Availability Drives Flexible Synaptic Signaling Dependent on Somatic Subthreshold Depolarization

“…To determine whether K v 3 activity is required for depolarization-induced spike broadening, we measured AP duration in boutons in the presence of low-concentration TEA (500 µM) (Baranauskas et al, 2003; Ishikawa et al, 2003). As previously reported (Rowan et al, 2016; Rowan et al, 2014), TEA induced an increase in basal AP duration in boutons (126.9 ± 5.6 % of control AP duration; n = 9; p < 0.05; paired t-test) indicating loss of K v 3 activity during spiking. Importantly, in this condition, somatic depolarization no longer induced spike broadening (Figure 3A).…”

“…In SCs, fast-activating K v 3 channels are particularly enriched at boutons and are the predominant K v channel conductance setting spike duration (Laube et al, 1996; Rowan et al, 2016; Southan and Robertson, 2000). To determine whether K v 3 activity is required for depolarization-induced spike broadening, we measured AP duration in boutons in the presence of low-concentration TEA (500 µM) (Baranauskas et al, 2003; Ishikawa et al, 2003).…”

“…The average increase in spike width induced by somatic depolarization was similar to that induced pharmacologically in matched examples (AP width 116.0 ± 2.4 and 119.2 ± 5.1% of control by depolarization and K v 3 blockers, respectively; n = 11; p > 0.05; paired t-test), however, analog modulation of spike width at individual bouton sites was not correlated with the width change induced by K v 3 block (p > 0.05; n = 11; Pearson correlation) (Figure 3B). This may be reflective of underlying differences in the K v 3 channels whose density and subunit composition vary amongst presynaptic sites throughout the axon (Rowan et al, 2016). …”

“…In the axons of excitatory neocortical and hippocampal cells, activity-dependent modification of AP duration is mediated through changes in K v 1 channel availability through slow accumulation of inactivation including during subthreshold depolarization (Bialowas et al, 2015; Kole et al, 2007; Shu et al, 2006). However AP duration is controlled primarily by K v 3-type channels at SC boutons, similar to other inhibitory interneurons (Rowan et al, 2016). Thus AP broadening in SC axons likely involves K v 3 channel modulation but, paradoxically, K v 3 channels are thought to be essential for maintaining AP waveform fidelity (Lien and Jonas, 2003; Whim and Kaczmarek, 1998).…”

Dynamic Control of Neurotransmitter Release by Presynaptic Potential

“…The study of functional properties of ion channels in the axon is only at its beginning. And the recent development of direct recordings from thin axons and presynaptic terminals (Novak et al, 2013; Kawaguchi and Sakaba, 2015; Begum et al, 2016; Rowan et al, 2016) together with the development of genetically-encoded voltage indicators (Hoppa et al, 2014) will certainly open new investigation opportunities about the role and function of ion channels in the presynaptic compartment.…”