Nonselective cation channels are essential for maintaining intracellular Ca2+ levels and spontaneous firing activity in the midbrain dopamine neurons

Abstract

Intracellular Ca2+ and Ca2+-permeable ion channels are important in regulating the firing activity and pattern of midbrain dopamine neurons, but the role of Ca2+-permeable nonselective cation channels (NSCCs) on spontaneous firing activity is unclear. Therefore, we investigated how Ca2+-permeable NSCCs modulate spontaneous firing activity and cytosolic Ca2+ concentration ([Ca2+]c) in acutely isolated midbrain dopamine neurons of the rat. Applications of voltage-dependent Ca2+ channels antagonists failed to abolish spontaneous firing activity completely, but they decreased firing rate and [Ca2+]c. However, a blockade of NSCCs by 2-APB or SKF96365 more potently suppressed spontaneous firings with a depolarization of membrane potential and strong decreases in basal [Ca2+]c levels. The depolarization of membrane potentials was attenuated by intracellular dialysis with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). NSCCs blockers inhibited oscillatory potentials and decreased basal [Ca2+]c in the presence of tetrodotoxin. Apamin, a small-conductance Ca2+-activated K+ channel inhibitor, depolarized membrane potentials and enhanced firing rates. From these data, we conclude that NSCCs not only make up the tonic Ca2+ entry pathways to uphold basal [Ca2+]c levels but also contribute to generation of spontaneous firings, thereby regulating spontaneous firing activities of the midbrain dopamine neurons.

Cellular Mechanisms Underlying Burst Firing in Substantia Nigra Dopamine Neurons

“…Although application of dihydropyridines significantly reduced the intensity of burst firing in SN DA neurons, we observed a less consistent effect of Ca v 1 channel blockade on autonomous activity than some (Nedergaard et al, 1993; Mercuri et al, 1994) but not all studies (Fujimura and Matsuda, 1989; Shepard and Stump, 1999; Kim et al, 2007; Puopolo et al, 2007) have reported. After 10–15 minute dihydropyridine application the frequency of autonomous activity decreased in 8 neurons and increased in 8 neurons and the regularity of firing decreased in 11 neurons and increased in 5 neurons.…”

“…In concordance with some studies (Fujimura and Matsuda, 1989; Shepard and Stump, 1999; Kim et al, 2007; Puopolo et al, 2007) but not all studies (Nedergaard et al, 1993; Mercuri et al, 1994) but not all studies (Nedergaard et al, 1993; Mercuri et al, 1994) the effects of Ca v 1 channel blockade on spontaneous/autonomous activity were variable. Indeed, it is becoming increasingly clear that a variety of channels contribute to autonomous firing in these neurons including Na v , Ca v 2.1, HCN and non-selective cation channels (Grace and Onn, 1989; Neuhoff et al, 2002; Chan et al, 2007; Kim et al, 2007; Puopolo et al, 2007; Guzman et al, 2009).…”

“…Indeed, it is becoming increasingly clear that a variety of channels contribute to autonomous firing in these neurons including Na v , Ca v 2.1, HCN and non-selective cation channels (Grace and Onn, 1989; Neuhoff et al, 2002; Chan et al, 2007; Kim et al, 2007; Puopolo et al, 2007; Guzman et al, 2009). …”

“…Voltage-dependent Na + (Na v ), Ca 2+ (Ca v ) and K + (K v ) channels, mixed cation channels and small conductance Ca 2+ -dependent K + (SK) channels regulate the activity of SN DA neurons (Shepard and Bunney, 1988; Grace and Onn, 1989; Harris et al, 1989; Nedergaard et al, 1993; Mercuri et al, 1994; Liss et al, 2001; Neuhoff et al, 2002; Chan et al, 2007; Kim et al, 2007; Puopolo et al, 2007). Na v , Ca v , hyperpolarization-activated cyclic nucleotide-gated cation (HCN) and non-selective cation channels underlie depolarizing currents, whereas A-type K v and SK channels underlie hyperpolarizing currents.…”

Robust Pacemaking in Substantia Nigra Dopaminergic Neurons

“…One interpretation of these results is that the dendrites or axon initial segment (AIS), which are sheared off during acute isolation, contribute to pacemaking, rendering it sensitive to antagonism of L-type Ca 2+ channels. Underscoring the importance of these regions to the pacemaking process, SNc DA neurons in tissue slices also were insensitive to TRP channel antagonists 2-APB (20 μM, n=4, p>0.05) and SKF 96365 (20 μM, n=4, p>0.05), in contrast to the situation reported in acutely isolated DA neurons (Kim et al, 2007). …”