Roles of Subthreshold Calcium Current and Sodium Current in Spontaneous Firing of Mouse Midbrain Dopamine Neurons

Abstract

We used a preparation of acutely dissociated neurons to quantify the ionic currents driving the spontaneous firing of substantia nigra pars compacta neurons, isolated from transgenic mice in which the tyrosine hydroxylase promoter drives expression of human placental alkaline phosphatase (PLAP) on the outer surface of the cell membrane. Dissociated neurons identified by fluorescent antibodies to PLAP showed firing properties similar to those of dopaminergic neurons in brain slice, including rhythmic spontaneous firing of broad action potentials and, in some cells, rhythmic oscillatory activity in the presence of tetrodotoxin (TTX). Spontaneous activity in TTX had broader, smaller spikes than normal pacemaking and was stopped by removal of external calcium. Normal pacemaking was also consistently silenced by replacement of external calcium by cobalt and was slowed by more specific calcium channel blockers. Nimodipine produced a slowing of pacemaking frequency. Pacemaking was also slowed by the P/Q-channel blocker -Aga-IVA, but the N-type channel blocker -conotoxin GVIA had no effect. In voltage-clamp experiments, using records of pacemaking as command voltage, cobalt-sensitive current and TTX-sensitive current were both sizeable at subthreshold voltages between spikes. Cobalt-sensitive current was consistently larger than TTX-sensitive current at interspike voltages from Ϫ70 to Ϫ50 mV, with TTX-sensitive current larger at voltages positive to Ϫ45 mV. These results support previous evidence for a major role of voltage-dependent calcium channels in driving pacemaking of midbrain dopamine neurons and suggest that multiple calcium channel types contribute to this function. The results also show a significant contribution of subthreshold TTX-sensitive sodium current.

Dopamine neurons change the type of excitability in response to stimuli

“…The subthreshold Ca 2+ -K + oscillatory mechanism underlies the generation of low frequency background firing in a significant subpopulation of DA neurons [7][8][9][10][11][12][13][14][15][16][17][18]51]. However, a number of studies suggest a contribution of Ca 2+ -independent currents to oscillations [26,19,25,27,46]. In accord with these studies, we found that, if considered in isolation, the Ca 2+ -K + oscillatory mechanism provides type II excitability, which is incompatible with the experiments reproduced above.…”

“…Persistent sodium current is known to amplify subthreshold oscillations [69] and increases neural excitability of DA neurons by contributing to spontaneous depolarization in between the spikes [25]. Consistent with experimental observations, the subthreshold sodium current population receiving the tonic synaptic inputs.…”

“…In our model, spike-producing currents (fast sodium and the delayed rectifier potassium) play a mostly subordinate role in this dynamic, adding a spike on top of the oscillations without significant changes to the period or shape of voltage and calcium oscillations, as in the study by Wilson and Callaway 2000 [4]. A number of studies suggest an additional Ca 2+ -independent oscillatory mechanism [25][26][27]. In particular, they emphasize the contribution of sodium currents to pacemaking.…”

“…Even though the persistent subthreshold sodium current is much smaller than the transient spike-producing current, it influences the firing pattern and the frequency of the DA neuron by contributing to depolarization below the spike threshold [25]. We modeled the voltage dependence of the subthreshold sodium current as follows:…”

A novel design of a Phase III trial of isradipine in early Parkinson disease (STEADY‐PD III)

“…Neuroprotective effects of isradipine are achieved at the plasma concentration that is obtained within the safe dose range for human administration10, 11 and consistent with the tolerable dosage identified in our phase II study of isradipine in PD (STEADY‐PDII) 12. Isradipine is the most potent of the clinically available Cav1.3 DHPs and has excellent central nervous system penetration suggesting it is the optimal DHP to target this novel mechanism of neuroprotection 13, 14…”