Relative contributions of axonal and somatic Na channels to action potential initiation in cerebellar purkinje neurons

Zayd M. Khaliq, Indira M. Raman*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

117 Scopus citations

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 β-pompilidotoxin (β-PMTX). Dual somatic and axonal recordings indicated that spike thresholds and input- output curves were sensitive to TTX or β-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.

Original languageEnglish (US)
Pages (from-to)1935-1944
Number of pages10
JournalJournal of Neuroscience
Volume26
Issue number7
DOIs
StatePublished - Feb 15 2006

Keywords

  • Axon
  • Initial segment
  • Na current
  • Node of Ranvier
  • Propagation
  • Resurgent

ASJC Scopus subject areas

  • Neuroscience(all)

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