TRPC3 and NALCN channels drive pacemaking in substantia nigra dopaminergic neurons

Abstract

Midbrain dopamine (DA) neurons are slow pacemakers that maintain extracellular DA levels. During the interspike intervals, subthreshold slow depolarization underlies autonomous pacemaking and determines its rate. However, the ion channels that determine slow depolarization are unknown. Here we show that TRPC3 and NALCN channels together form sustained inward currents responsible for the slow depolarization of nigral DA neurons. Specific TRPC3 channel blockade completely blocked DA neuron pacemaking, but the pacemaking activity in TRPC3 knock-out (KO) mice was perfectly normal, suggesting the presence of compensating ion channels. Blocking NALCN channels abolished pacemaking in both TRPC3 KO and wild-type mice. The NALCN current and mRNA and protein expression are increased in TRPC3 KO mice, indicating that NALCN compensates for TRPC3 currents. In normal conditions, TRPC3 and NALCN contribute equally to slow depolarization. Therefore, we conclude that TRPC3 and NALCN are two major leak channels that drive robust pacemaking in nigral DA neurons.

Proximal dendritic localization of NALCN channels underlies tonic and burst firing in nigral dopaminergic neurons

“…During the single neuron dissociation procedures, the dissociated DA neurons should lose more distal parts of their dendrites, probably leading to loss of more TRPC3 channels than NALCN channels, which could explain the different results from the current injection experiments in acutely dissociated DA neurons and midbrain slices. Consistent with that interpretation, the input resistance measured in acutely dissociated DA neurons (361.23 ± 12.17 M , n = 7) was about 48% higher than that of DA neurons in midbrain slices (244.44 ± 17.00 M , Um et al, 2021). In addition, NALCN channel inhibition with L703,606 increased input resistance significantly more than TRPC3 channel inhibition with pyr10 (L703606, n = 7; Pyr10, n = 7, P = 0.014, Fig.…”

“…Blocking either NALCN or TRPC3 with L703606 or pyr10, respectively, in acutely dissociated DA neurons, as in midbrain slices (Um et al, 2021), completely abolished spontaneous tonic firing, and the injection of a leak-like depolarizing current revived tonic firing completely (L703606: n = 12, P = 0.003; Pyr10: n = 9, P = 0.004, Fig. 6A-C).…”

“…1A-E). Given that the whole-cell NALCN current equals the whole-cell TRPC3 current in intact nigral DA neurons in midbrain slices (Um et al, 2021), the current density of NALCN localized within the PDCs should be higher than that of TRPC3. During the single neuron dissociation procedures, the dissociated DA neurons should lose more distal parts of their dendrites, probably leading to loss of more TRPC3 channels than NALCN channels, which could explain the different results from the current injection experiments in acutely dissociated DA neurons and midbrain slices.…”

“…Very recently, we reported that two non‐selective cation channels, the sodium leak channel (NALCN) and transient receptor potential‐canonical 3 (TRPC3) channel, produce a sustained leak‐like inward current that is responsible for the slow and regular pacemaking of nigral DA neurons (Um et al., 2021). NALCN is a Na + ‐permeable leak channel that produces background Na + ‐leak conductance and determines the resting membrane potential and excitability of many central neurons (Hahn et al., 2020; Lu et al., 2007; Lutas et al., 2016; Shi et al., 2016).…”

“…NALCN is a Na + ‐permeable leak channel that produces background Na + ‐leak conductance and determines the resting membrane potential and excitability of many central neurons (Hahn et al., 2020; Lu et al., 2007; Lutas et al., 2016; Shi et al., 2016). TRPC3 is another leak channel that opens constitutively, leading to slow depolarization in many neurons (Dietrich et al., 2003; Um et al., 2021; Zhou et al., 2008). Although NALCN and TRPC3, as subthreshold leak channels, underlie the slow depolarization responsible for pacemaker activity in nigral DA neurons (Um et al., 2021), little is known about the relationships between these two pacemaker ion channels and the highly excitable PDCs (Jang et al., 2014).…”