Action potential firing rates are generally limited by the refractory period, which depends on the recovery from inactivation of voltage-gated Na channels. In cerebellar Purkinje neurons, the kinetics of Na channels appear specialized for rapid firing. Upon depolarization, an endogenous open-channel blocker rapidly terminates current flow but prevents binding of the “fast” inactivation gate. Upon repolarization, unbinding of the blocker produces “resurgent” Na current while allowing channels to recover rapidly. Because other cerebellar neurons, including granule cells, unipolar brush cells, and neurons of the cerebellar nuclei, also fire rapidly, we tested whether these cells might also express Na channels with resurgent kinetics. Neurons were acutely isolated from mice and rats, and TTX-sensitive Na currents were recorded under voltage clamp. Unlike Purkinje cells, the other cerebellar neurons produced only tiny resurgent currents in solutions optimized for voltage-clamping Na currents (50 mM Na+; Co2+ substitution for Ca2+). Under more physiological ionic conditions (155 mM Na+; 2 mM Ca2+ with 0.03 mM Cd2+), however, granule cells, unipolar brush cells, and cerebellar nuclear cells all produced robust resurgent currents. The increase in resurgent current, which was greater than predicted by the Goldman-Hodgkin-Katz equation, appeared to result from a combination of knock-off of open-channel blockers by permeating ions as well as relief of divalent block at negative potentials. These results indicate that resurgent current is typical of many cerebellar neurons and suggest that rapid open-channel block and unblock may be a widespread mechanism for restoration of Na channel availability in rapidly firing neurons.

Cerebellar Purkinje neurons express voltage-gated, tetrodotoxin (TTX)-sensitive sodium channels that not only open and inactivate rapidly during depolarization but also reopen during repolarization, carrying an unusual "resurgent" sodium current. Expression of Na(V)1.6 alpha subunits appears necessary but not sufficient to generate this component of current; Purkinje cells without Na(V)1.6 lack resurgent current, but resurgent current is absent from many other Na(V)1.6-expressing neurons. These observations raise the question of how modulation or modification of the Na(V)1.6 subunit may lead to production of resurgent current. Previous studies have suggested that sodium channels of Purkinje neurons are subject to a rapid, voltage-dependent, open channel block by an endogenous particle whose unbinding allows resurgent current to flow. To investigate the nature of this block, we recorded TTX-sensitive sodium currents in outside-out patches from Purkinje cells acutely isolated from mice. In all patches, step depolarizations evoked transient current, and step repolarizations evoked resurgent current. The amplitudes of the transient and resurgent currents were highly correlated across patches (R(2) > 0.99), suggesting that the blocking agent is closely associated with the channel. Intracellular protease eliminated fast inactivation, indicating that the blocking element, like the fast inactivation gate, may be proteinaceous. Intracellular application of alkaline phosphatase abolished resurgent current and significantly slowed inactivation of transient current. The phosphatase inhibitor vanadate reduced these effects. Together, the results suggest that constitutive phosphorylation of the sodium channel complex of Purkinje neurons is necessary to maintain a functional blocking element and produce resurgent sodium current.