Ionic Currents and Spontaneous Firing in Neurons Isolated from the Cerebellar Nuclei

Abstract

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly (ϳ20 Hz), with shallow interspike hyperpolarizations (approximately Ϫ60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections.During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltageindependent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage-and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mM TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Mechanism of Spontaneous Firing in Dorsomedial Suprachiasmatic Nucleus Neurons

“…The outward shift of resting currents on replacing Na ϩ with NMDG ϩ is consistent with a resting sodium conductance (with TTX present), similar to previous results in deep cerebellar nuclear cells (Raman et al, 2000) and orexinergic neurons (Eggermann et al, 2003). The outward shift of currents after Na ϩ removal could also represent enhancement of an outward Na/K ATPase electrogenic pump current (owing to reduction of the sodium gradient) or elimination of Na/Ca exchanger current.…”

“…Some types of spontaneously active neurons appear to have resting potentials (measured when the cell is silenced by TTX) that are relatively depolarized (Raman et al, 2000;Eggerman et al, 2003). In principle, if the "resting" potential is depolarized with respect to the threshold for firing a spike, spontaneous firing might occur even in the absence of any subthreshold voltagedependent conductances.…”

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

“…Resurgent current is not unique to Purkinje cells, however. It is also present in neurons of the subthalamic nuclei (Do and Bean, 2003) and has been observed or hypothesized to exist in other cerebellar neurons, including unipolar brush cells (Mossadeghi and Slater, 1998), granule cells (D'Angelo et al, 2001), and some neurons of the cerebellar nuclei (Raman et al, 2000). Although the profile of resurgent current is qualitatively similar across cell types, different neurons may or may not achieve resurgent kinetics by identical mechanisms.…”

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

“…Voltage-gated sodium currents underlie repetitive firing of cerebellar Purkinje neurons (Raman and Bean, 1999a;Raman et al, 2000). To evaluate whether the reduction of sodium current caused by deletion of Na V 1.1 alters spontaneous firing, we recorded spontaneous activity of acutely isolated Purkinje neurons without applying any extrinsic holding or depolarizing current, and compared firing rate, threshold, peak, and minimum voltages of the action potential.…”