Relative Contributions of Axonal and Somatic Na Channels to Action Potential Initiation in Cerebellar Purkinje Neurons

Abstract

Neuronal excitability is likely to be regulated by the site of action potential initiation, the location on a neuron that crosses threshold first. Although initiation is axonal in many neurons, in Purkinje cells, somatic conductances can generate spontaneous action potentials, suggesting that the perisomatic region (soma and/or initial segment) contributes to spike initiation and may regulate firing. To identify directly the cellular regions at which Na channel modulation significantly influences firing, we measured spontaneous and evoked action potentials in Purkinje cells in cerebellar slices from postnatal day 14 -28 mice while applying drugs locally to either the soma/initial segment or the first node of Ranvier. Na currents were decreased by tetrodotoxin (TTX) or increased by ␤-pompilidotoxin (␤-PMTX). Dual somatic and axonal recordings indicated that spike thresholds and input-output curves were sensitive to TTX or ␤-PMTX at the perisomatic region but were unchanged by either drug at the first node. When perisomatic Na channel availability was reduced with subsaturating TTX, however, the input-output curve became shallower during additional TTX block of nodal channels, revealing a latent role for nodal Na channels in facilitating firing. In perisomatic TTX, axons failed to generate spontaneous or evoked spike trains. In contrast, choline block of the initial segment alone altered normal input-output curves. The data suggest that, although the first node reliably follows action potentials, spike initiation in Purkinje neurons occurs in the initial segment. Moreover, Purkinje cell output depends on the density, availability, and kinetics of perisomatic Na channels, a characteristic that may distinguish spontaneously firing from quiescent neurons.

High Threshold, Proximal Initiation, and Slow Conduction Velocity of Action Potentials in Dentate Granule Neuron Mossy Fibers

“…One criterion for the location of action potential initiation is sensitivity of action potential threshold to pharmacologically reduced sodium channel density (Colbert and Johnston 1996;Khaliq and Raman 2006;Meeks and Mennerick 2007;Palmer and Stuart 2006). We found that local tetrodotoxin application to the proximal axon of granule neurons (15-30 m) produced a shift in spike threshold, assessed by phase plot analysis (Fig.…”

Effects of Axonal Topology on the Somatic Modulation of Synaptic Outputs

“…Although eAPs recorded from local field potentials (LFPs) are thought to reflect the derivative of intracellularly recorded APs (iAPs), the origin of eAPs monitored by cell-attached recording is controversial (Raastad and Shepherd, 2003;Khaliq and Raman, 2006;Perkins, 2006). Therefore, we simultaneously recorded and compared the waveforms of iAPs (whole cell) and eAPs (cell attached) from cell bodies of CA3 pyramidal cells and eAPs recorded in LFPs (Fig.…”

Postsynaptic Density-93 Clusters Kv1 Channels at Axon Initial Segments Independently of Caspr2

“…The generation, modulation, and duration of axonal action potentials depend on both the high densities and the specific types of ion channels found at the AIS (Hedstrom and Rasband, 2006;Khaliq and Raman, 2006;Shu et al, 2007). Kv1 channels are prominent in axons and nerve terminals (Gu et al, 2003;Trimmer and Rhodes, 2004) and modulate action potentials (Kole et al, 2007).…”