Key pointsr The electrical coupling between the soma and dendritic compartments has been regarded as a key determinant for the spontaneous firing rate (SFR) in midbrain dopamine neurons.r Isolated nigral dopamine neurons showed a wide range of soma size and a variable number of primary dendrites but preserved a quite consistent SFR.r The SFR was not correlated with soma size or with the number of primary dendrites, but it was strongly correlated with the area ratios of the proximal dendritic compartments to the somatic compartment.r Tetrodotoxin puff and local Ca 2+ perturbation experiments, computer simulation, and local glutamate uncaging experiments suggest the importance of the proximal dendritic compartments in pacemaker activity.r These results lead us to a novel conclusion that the proximal dendritic compartments, not the whole dendritic compartments, play a key role in the somatodendritic balance that determines the SFR in dopamine neurons.Abstract Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that require the elaborate composition of many ion channels in the somatodendritic compartments. Understanding the major determinants of the spontaneous firing rate (SFR) of midbrain DA neurons is important because they determine the basal DA levels in target areas, including the striatum. As spontaneous firing occurs synchronously at the soma and dendrites, the electrical coupling between the soma and dendritic compartments has been regarded as a key determinant for the SFR. However, it is not known whether this somatodendritic coupling is served by the whole dendritic compartments or only parts of them. In the rat substantia nigra pars compacta (SNc) DA neurons, we demonstrate that the balance between the proximal dendritic compartment and the soma determines the SFR. Isolated SNc DA neurons showed a wide range of soma size and a variable number of primary dendrites but preserved a quite consistent SFR. The SFR was not correlated with soma size or with the number of primary dendrites, but it was strongly correlated with the area ratios of the proximal dendritic compartments to the somatic compartment. Tetrodotoxin puff and local Ca 2+ perturbation experiments, computer simulation, and local glutamate uncaging experiments suggest the importance of the proximal dendritic compartments in pacemaker activity. These data indicate that the proximal dendritic compartments, not the whole dendritic compartments, play a key role in the somatodendritic balance that determines the SFR in DA neurons.

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.

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Background and Purpose NALCN is a Na+ leak, GPCR‐activated channel that regulates the resting membrane potential and neuronal excitability. Despite numerous possible roles for NALCN in both normal physiology and disease processes, lack of specific blockers hampers further investigation. Experimental Approach The effect of N‐benzhydryl quinuclidine compounds on NALCN channels was demonstrated using whole‐cell patch‐clamp recordings in HEK293T cells overexpressing NALCN and acutely isolated nigral dopaminergic neurons that express NALCN endogenously. Src kinase activity was measured using a Src kinase assay kit, and voltage and current‐clamp recordings from nigral dopaminergic neurons were used to measure NALCN currents and membrane potentials. Key Results N‐benzhydryl quinuclidine compounds inhibited NALCN channels without affecting TRPC channels, another important route for Na+ leak. In HEK293T cells overexpressing NALCN, N‐benzhydryl quinuclidine compounds potently suppressed muscarinic M3 receptor‐activated NALCN currents. Structure–function relationship studies suggest that the quinuclidine ring with a benzhydryl group imparts the ability to inhibit NALCN currents regardless of Src family kinases. Moreover, N‐benzhydryl quinuclidine compounds inhibited not only GPCR‐activated NALCN currents but also background Na+ leak currents and hyperpolarized the membrane potential in native midbrain dopaminergic neurons that express NALCN endogenously. Conclusion and Implications These findings suggest that N‐benzhydryl quinuclidine compounds have a pharmacological potential to directly inhibit NALCN channels and could be a useful tool to investigate functions of NALCN channels.

J. Neurochem. (2011) 116, 966–974. Abstract Dopamine (DA) receptors generate many cellular signals and play various roles in locomotion, motivation, hormone production, and drug abuse. According to the location and expression types of the receptors in the brain, DA signals act in either stimulatory or inhibitory manners. Although DA autoreceptors in the substantia nigra pars compacta are known to regulate firing activity, the exact expression patterns and roles of DA autoreceptor types on the firing activity are highly debated. Therefore, we performed individual correlation studies between firing activity and receptor expression patterns using acutely isolated rat substantia nigra pars compacta DA neurons. When we performed single‐cell RT‐PCR experiments, D1, D2S, D2L, D3, and D5 receptor mRNA were heterogeneously expressed in the order of D2L > D2S > D3 > D5 > D1. Stimulation of D2 receptors with quinpirole suppressed spontaneous firing similarly among all neurons expressing mRNA solely for D2S, D2L, or D3 receptors. However, quinpirole most strongly suppressed spontaneous firing in the neurons expressing mRNA for both D2 and D3 receptors. These data suggest that D2S, D2L, and D3 receptors are able to equally suppress firing activity, but that D2 and D3 receptors synergistically suppress firing. This diversity in DA autoreceptors could explain the various actions of DA in the brain.