Two different Ca2+-dependent inhibitory mechanisms of spontaneous firing by glutamate in dopamine neurons

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

In multipolar nigral dopamine (DA) neurons, the highly excitable proximal dendritic compartments (PDCs) and two Na⁺ -permeable leak channels, TRPC3 and NALCN, play a key role in pacemaking. However, the causal link between them is unknown. Here we report that the proximal dendritic localization of NALCN underlies pacemaking and burst firing in DA neurons. Our morphological analysis of nigral DA neurons reveals that TRPC3 is ubiquitously expressed in the whole somatodendritic compartment, but NALCN is localized within the PDCs. Blocking either TRPC3 or NALCN channels abolished pacemaking. However, only blocking NALCN, not TRPC3, degraded burst discharges. Furthermore, local glutamate uncaging readily induced burst discharges within the PDCs, compared with other parts of the neuron, and NALCN channel inhibition dissipated burst generation, indicating the importance of NALCN to the high excitability of PDCs. Therefore, we conclude that PDCs serve as a common base for tonic and burst firing in nigral DA neurons. KEY POINTS: Midbrain dopamine (DA) neurons are slow pacemakers that can generate tonic and burst firings, and the highly excitable proximal dendritic compartments (PDCs) and two Na⁺ -permeable leak channels, TRPC3 and NALCN, play a key role in pacemaking. We find that slow tonic firing depends on the basal activity of both the NALCN and TRPC3 channels, but that burst firing does not require TRPC3 channels but relies only on NALCN channels. We find that TRPC3 is ubiquitously expressed in the entire somatodendritic compartment, but that NALCN exists only within the PDCs in nigral DA neurons. We show that NALCN channel localization confers high excitability on PDCs and is essential for burst generation in nigral DA neurons. These results suggest that PDCs serve as a common base for tonic and burst firing in nigral DA neurons.

Balance between the proximal dendritic compartment and the soma determines spontaneous firing rate in midbrain dopamine neurons

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.

Valproic acid suppresses Nrf2/Keap1 dependent antioxidant protection through induction of endoplasmic reticulum stress and Keap1 promoter DNA demethylation in human lens epithelial cells

Recent epidemiological studies confirm the prevalence of cataract in epileptic patients. Similarly, the drugs used to treat epilepsy also show the connection with increased cataract formation. In this present study, we investigated the suppression of Nrf2/Keap1 dependent antioxidant protection through induction of endoplasmic (ER) stress and Keap1 promoter DNA demethylation in human lens epithelial cells (HLECs) treated with valproic acid (VPA), an antiepileptic drug. 20 mM VPA induces ER stress and activates the unfolded protein response (UPR) within 4 h by activating the ER stress sensor proteins, such as PERK, IRE1α, and ATF6 in HLECs. Consequently, the integrated ER stress signals, such as eIF2α, ATF4, BiP, and CHOP are altered accordingly to induce ER-Ca2+ release, reactive oxygen species (ROS) overproduction, and cell death in HLECs treated with VPA. VPA also suppresses the Nrf2, catalase, and glutathione reductase expressions with significant increases in Keap1 protein. Bisulphite genomic DNA sequencing reveals the promoter DNA demethylation in the Keap1 promoter, which results in the overexpression of Keap1 mRNA and protein in HLECs treated with 20 mM VPA. VPA also alters the expression profiles of passive DNA demethylation pathway enzymes such Dnmt1, Dnmt3a, Dnmt3b, and active DNA demethylation pathway enzyme, TET1 leading to DNA demethylation in the Keap1 promoter of HLECs. Overexpressed Keap1 decreases the Nrf2 level, thereby abolishing the Nrf2 dependent antioxidant protection. This might be responsible for lenticular proteins oxidation and cataract formation.

Homeostatic regulation mechanism of spontaneous firing determines glutamate responsiveness in the midbrain dopamine neurons

Autonomous tonic firing of the midbrain dopamine neuron is essential for maintenance of ambient dopamine level in the brain, in which intracellular Ca2+ concentration ([Ca2+]c) plays a complex but pivotal role. However, little is known about Ca2+ signals by which dopamine neurons maintain an optimum spontaneous firing rate. In the midbrain dopamine neurons, we here show that spontaneous firing evoked [Ca2+]c changes in a phasic manner in the dendritic region but a tonic manner in the soma. Tonic levels of somatic [Ca2+]c strictly tallied with spontaneous firing rates. However, manipulatory raising or lowering of [Ca2+]c with caged compounds from the resting firing state proportionally suppressed or raised spontaneous firing rate, respectively, suggesting presence of the homeostatic regulation mechanism for spontaneous firing rate via tonic [Ca2+]c changes of the soma. More importantly, abolition of this homeostatic regulation mechanism significantly exaggerated the responses of tonic firings and high-frequency phasic discharges to glutamate. Therefore, we conclude that this Ca(2+)-dependent homeostatic regulation mechanism is responsible for not only maintaining optimum rate of spontaneous firing, but also proper responses to glutamate. Perturbation of this mechanism could cause dopamine neurons to be more vulnerable to glutamate and Ca2+ toxicities.

Decreased sensitivity of NMDA receptors on dopaminergic neurons from the posterior ventral tegmental area following chronic nondependent alcohol consumption

BACKGROUND

The mesocorticolimbic dopamine system mediates the reinforcing effects of salient stimuli, including drugs of abuse. Nondependent chronic alcohol consumption modifies this system, resulting in an increased number of spontaneously active dopamine neurons in the posterior ventral tegmental area (VTA) of alcohol-preferring (P) rats. Enhanced responses of postsynaptic glutamate receptors may contribute to the increase in active dopamine neurons. Thus, excitations of putative dopamine neurons to locally applied N-methyl-d-aspartic acid (NMDA; glutamate receptor subtype agonist) were evaluated.

METHODS

P rats were assigned to alcohol naïve (water only) or alcohol drinking (continuous access to 15% alcohol and water for 8 consecutive weeks) groups. Responses of 23 putative dopamine neurons from naïve rats and 19 putative dopamine neurons from drinking rats were assessed in vivo using microiontophoretically applied NMDA. Current-response curves for firing frequency and burst activity were constructed using nonlinear mixed effects models. Between-group comparisons were made for EC(50) (effective current producing a half maximal excitatory response), E(max) (maximal excitatory effect), and C(DB) (the current at which depolarization block-marked decrease in neuronal activity-occurred).

RESULTS

Drinking P rats steadily consumed alcohol over the 8-week protocol and did not exhibit signs of dependence or withdrawal. Putative dopamine neurons from drinking rats exhibited resistance to depolarization block (higher C(DB) values) and required larger doses of NMDA to elicit moderate excitatory responses (higher EC(50) values), consistent with decreased receptor affinity. Maximal excitatory responses (E(max) ) did not differ between the groups, consistent with no change in receptor number. Blood alcohol was at undetectable levels at the time of experimentation.

CONCLUSIONS

NMDA receptor sensitivity is decreased on posterior VTA putative dopamine neurons in P rats on a nondependent schedule of alcohol consumption. Mechanisms underlying increased spontaneous dopamine neuron activity may be independent of changes in NMDA receptor function. Decreased NMDA receptor sensitivity may precede the development of dependence.

Functional organization of dendritic Ca2+ signals in midbrain dopamine neurons

Dendritic Ca2+ plays an important role not only in synaptic integration and synaptic plasticity, but also in dendritic excitability in midbrain dopamine neurons. However, the functional organization of dendritic Ca2+ signals in the dopamine neurons remains largely unknown. We therefore investigated dendritic Ca2+ signals by measuring glutamate-induced Ca2+ increases along the dendrites of acutely isolated midbrain dopamine neurons. Maximal doses of glutamate induced a [Ca2+]c rise with similar amplitudes in proximal and distal dendritic regions of a dopamine neuron. Glutamate receptors contributed incrementally to the [Ca2+]c rise according to their distance from the soma, with a reciprocal decrement in the contribution of voltage-operated Ca2+ channels (VOCCs). The contribution of AMPA and NMDA receptors increased with dendritic length, but that of metabotropic glutamate receptors decreased. At low doses of glutamate at which spontaneous firing was sustained, the [Ca2+]c rise was higher in the distal than the proximal regions of a dendrite, possibly due to the increased spontaneous firing rate. These results indicate that functional organization of Ca2+ signals in the dendrites of dopamine neurons requires different combination of VOCCs and glutamate receptors according to dendritic length, and that regional Ca2+ rises in dendrites respond differently to applied glutamate concentration.

Regulation of dopaminergic neuron firing by heterogeneous dopamine autoreceptors in the substantia nigra pars compacta

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, D(1), D(2)S, D(2)L, D(3), and D(5) receptor mRNA were heterogeneously expressed in the order of D(2)L > D(2)S > D(3) > D(5) > D(1). Stimulation of D(2) receptors with quinpirole suppressed spontaneous firing similarly among all neurons expressing mRNA solely for D(2)S, D(2)L, or D(3) receptors. However, quinpirole most strongly suppressed spontaneous firing in the neurons expressing mRNA for both D(2) and D(3) receptors. These data suggest that D(2) S, D(2)L, and D(3) receptors are able to equally suppress firing activity, but that D(2) and D(3) receptors synergistically suppress firing. This diversity in DA autoreceptors could explain the various actions of DA in the brain.

Modulation of firing activity by ATP in dopamine neurons of the rat substantia nigra pars compacta

ATP acts as a neurotransmitter or co-neurotransmitter in many areas of the CNS and peripheral nervous systems; however, little is known about the expression and functional role of purinoceptors (P2) in midbrain dopaminergic neurons. Therefore, we investigated P2X receptor expression and regulation of spontaneous firing activity in dopaminergic neurons of the substantia nigra pars compacta (SNc) in rats using patch-clamp and Ca(2+)-imaging techniques. In most neurons, application of ATP (1 microM-1 mM) increased firing rate dose-dependently (EC(50)=1.26+/-0.26 microM, n=45). When the P2-receptor agonists such as 2-methylthio-adenosine 5'-triphosphate (2-MeSATP) or ATPgammaS were applied or pressure-applied to the neuron, the firing activity increased together with a rise in cytosolic Ca(2+) concentration ([Ca(2+)]c), but application of beta,gamma-methylene ATP (P2X(1, 3) agonist) or methylthio-adenosine 5'-diphosphate (P2Y(1) agonist) had no effect. In many neurons, the effect of ATP was abolished by the application of the P2-receptor antagonists, suramin or pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). When ATP was applied in a Ca(2+)-free solution, there was no detectable change in [Ca(2+)]c, suggesting that ATP does not release Ca(2+) from intracellular stores. In the single-cell reverse transcription polymerase chain reaction (RT-PCR), we found that 65% of dopaminergic neurons expressed mRNAs for P2X receptors; positive amplifications of P2X(6) (57.1%), P2X(2/6) (25.0%), and P2X(4) mRNA (17.9%), respectively. From the above results, we could conclude that ATP modulates firing activities in the rat SNc dopaminergic neurons, possibly via P2X(2), P2X(2/6), and/or P2X(4) receptors.

Voltage-operated Ca2+ channels regulate dopamine release from somata of dopamine neurons in the substantia nigra pars compacta

Dopamine (DA) neurons release DA not only from axon terminals at the striatum, but from their somata and dendrites at the substantia nigra pars compacta (SNc). Released DA may auto-regulate further DA release or modulate non-DA cells. However, the actual mechanism of somatodendritic DA release, especially the Ca(2+) dependency of the process, remains controversial. In this study, we used amperometry to monitor DA release from somata of acutely isolated rat DA neurons. We found that DA neurons spontaneously released DA in the resting state. Removal of extracellular Ca(2+) and application of blockers for voltage-operated Ca(2+) channels (VOCCs) suppressed the frequency of secretion events. Activation of VOCCs by stimulation with K(+)-rich saline increased the frequency of secretion events, which were also sensitive to blockers for L- and T-type Ca(2+) channels. These results suggest that Ca(2+) influx through VOCCs regulates DA release from somata of DA neurons.

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

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.

Synaptic activation of dendritic AMPA and NMDA receptors generates transient high-frequency firing in substantia nigra dopamine neurons in vitro

Transient high-frequency activity of substantia nigra dopamine neurons is critical for striatal synaptic plasticity and associative learning. However, the mechanisms underlying this mode of activity are poorly understood because, in contrast to other rapidly firing neurons, high-frequency activity is not evoked by somatic current injection. Previous studies have suggested that activation of dendritic N-methyl-d-aspartate (NMDA) receptors and/or G-protein-coupled receptor (GPCR)-mediated reduction of action potential afterhyperpolarization and/or activation of cation channels underlie high-frequency activity. To address their relative contribution, transient high-frequency activity was evoked using local electrical stimulation (1 s, 10-100 Hz) in brain slices prepared from p15-p25 rats in the presence of GABA and D2 dopamine receptor antagonists. The frequency, pattern, and morphology of action potentials evoked under these conditions were similar to those observed in vivo. Evoked activity and reductions in action potential afterhyperpolarization were diminished greatly by application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or NMDA receptor selective antagonists and abolished completely by co-application of AMPA and NMDA antagonists. In contrast, application of glutamatergic and cholinergic GPCR antagonists moderately enhanced evoked activity. Dendritic pressure-pulse application of glutamate evoked high-frequency activity that was similarly sensitive to antagonism of AMPA or NMDA receptors. Taken together, these data suggest that dendritic AMPA and NMDA receptor-mediated synaptic conductances are sufficient to generate transient high-frequency activity in substantia nigra dopamine neurons by rapidly but transiently overwhelming the conductances underlying action potential afterhyperpolarization and/or engaging postsynaptic voltage-dependent ion channels in a manner that overcomes the limiting effects of afterhyperpolarization.

Regional interaction of endoplasmic reticulum Ca2+ signals between soma and dendrites through rapid luminal Ca2+ diffusion

The endoplasmic reticulum (ER) Ca2+ store plays a key role in integration and conveyance of Ca2+ signals in highly polarized neurons. The interconnected ER network in neurons generates Ca2+ signals in local domains, but the regional interaction is unclear. Here, we show that continuous or repetitive applications of caffeine produced robust Ca2+ release from the ER Ca2+ store in dendritic areas without severe store depletion, but that similar stimuli applied to soma caused rapid store depletion in acutely isolated midbrain dopamine neurons. Partial emptying of the ER Ca2+ store within a dendrite caused a similar level of store depletion in unstimulated dendrites, as well as in soma. Photobleaching and local stimulation experiments revealed that Ca2+ and the dye trapped within the ER diffused rapidly from the soma to dendrites up to 90 microm, which we could resolve, suggesting that the ER network acts as a functional tunnel for rapid Ca2+ transport. These data imply that the ER in soma acts as a Ca2+ reservoir supplying Ca2+ to the dendritic store, and that the dendritic store, hence, is able to respond to Ca2+-mobilizing input signals endurably.