The Contribution of Resurgent Sodium Current to High-Frequency Firing in Purkinje Neurons: An Experimental and Modeling Study

Abstract

Purkinje neurons generate high-frequency action potentials and express voltage-gated, tetrodotoxin-sensitive sodium channels with distinctive kinetics. Their sodium currents activate and inactivate during depolarization, as well as reactivate during repolarization from positive potentials, producing a "resurgent" current. This reopening of channels not only generates inward current after each action potential, but also permits rapid recovery from inactivation, leading to the hypothesis that resurgent current may facilitate high-frequency firing. Mutant med mice are ataxic and lack expression of the Scn8a gene, which encodes the NaV1.6 protein. In med Purkinje cells, transient sodium current inactivates more rapidly than in wild-type cells, and resurgent current is nearly abolished. To investigate how NaV1.6-specific kinetics influence firing patterns, we recorded action potentials of Purkinje neurons isolated from wild-type and med mice. We also recorded non-sodium currents from Purkinje cells of both genotypes to test whether the Scn8a mutation induced changes in other ion channels. Last, we modeled action potential firing by simulating eight currents directly recorded from Purkinje cells in both wild-type and med mice. Regular, high-frequency firing was slowed in med Purkinje neurons. In addition to disrupted sodium currents, med neurons had small but significant changes in potassium and leak currents. Simulations indicated that these modified non-sodium currents could not account for the reduced excitability of med cells but instead slightly facilitated spiking. The loss of NaV1.6-specific kinetics, however, slowed simulated spontaneous activity. Together, the data suggest that across a range of conditions, sodium currents with a resurgent component promote and accelerate firing.

Reduced Sodium Current in Purkinje Neurons from NaV1.1 Mutant Mice: Implications for Ataxia in Severe Myoclonic Epilepsy in Infancy

“…Partial blockade of Na V 1.1 and Na V 1.6 affects the amplitude of the action potential and the threshold for action potential generation, but selective deletion of Na V 1.1 channel did not affect these electrophysiological parameters. This finding suggests that Na V 1.6 current determines the threshold voltage for action potential generation and the amplitude of the action potential in these neurons, consistent with previous data indicating that Purkinje cells from mutant mice lacking Na V 1.6 channels were not capable of sustaining a constant rate of firing (Khaliq et al, 2003).…”

“…Persistent sodium current conducted by Na V 1.1 channels in Purkinje neurons is 1.4% of peak sodium current, similar to the lower end of the range of persistent sodium current recorded in transfected non-neuronal cells. Persistent and resurgent sodium currents are known to support the generation of rapid action potential firing in neurons (Raman and Bean, 1999a;Khaliq et al, 2003). Our finding that these two forms of sodium current are substantially reduced in Purkinje neurons of Na V 1.1 mutant mice provides a biophysical basis for the reduced capacity for repetitive action potential firing of these neurons.…”

Axonal Propagation of Simple and Complex Spikes in Cerebellar Purkinje Neurons

“…Currents through these channels activate and deactivate rapidly, thereby limiting Na channel inactivation, facilitating Na channel recovery, and permitting depolarization soon after a spike (Gähwiler and Llano, 1989;Raman and Bean, 1999;Southan and Robertson, 2000;Womack and Khodakhah, 2002;Edgerton and Reinhart, 2003;Khaliq et al, 2003). The present results are suggestive of axonal K channels with similarly rapid gating kinetics.…”

“…For example, somatic voltage-gated Na channels have "resurgent" kinetic properties that promote rapid recovery from inactivation even at potentials that are only moderately hyperpolarized (Raman and Bean, 2001). This rapid recovery appears to be facilitated primarily by Na V 1.6 ␣ subunits associated with a putative protein that limits inactivation Grieco et al, 2002;Khaliq et al, 2003;Grieco and Raman, 2004). Although Na V 1.6 is the primary Na channel ␣ subunit in nodes in most parts of the nervous system Krzemien et al, 2000), it is unknown whether the nodal Purkinje Na channels share the distinctive kinetics of the somatic channels.…”

Production of Resurgent Current in NaV1.6-Null Purkinje Neurons by Slowing Sodium Channel Inactivation with β-Pompilidotoxin

“…Incorporating this difference in inactivation into a kinetic model of Purkinje sodium current demonstrated that the faster the rate of conventional inactivation, the less successfully the open-channel blocker competed with the inactivation gate, thereby reducing the amount of resurgent current (Khaliq et al, 2003).…”