Immunohistochemical localization of voltage-gated calcium channels in substantia nigra dopamine neurons

High-Resolution Proteomics Unravel a Native Functional Complex of Cav1.3, SK3, and Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels in Midbrain Dopaminergic Neurons

Pacemaking activity in substantia nigra dopaminergic neurons is generated by the coordinated activity of a variety of distinct somatodendritic voltage- and calcium-gated ion channels. We investigated whether these functional interactions could arise from a common localization in macromolecular complexes where physical proximity would allow for efficient interaction and co-regulations. For that purpose, we immunopurified six ion channel proteins involved in substantia nigra neuron autonomous firing to identify their molecular interactions. The ion channels chosen as bait were Cav1.2, Cav1.3, HCN2, HCN4, Kv4.3, and SK3 channel proteins, and the methods chosen to determine interactions were co-immunoprecipitation analyzed through immunoblot and mass spectrometry as well as proximity ligation assay. A macromolecular complex composed of Cav1.3, HCN, and SK3 channels was unraveled. In addition, novel potential interactions between SK3 channels and sclerosis tuberous complex (Tsc) proteins, inhibitors of mTOR, and between HCN4 channels and the pro-degenerative protein Sarm1 were uncovered. In order to demonstrate the presence of these molecular interactions in situ, we used proximity ligation assay (PLA) imaging on midbrain slices containing the substantia nigra, and we could ascertain the presence of these protein complexes specifically in substantia nigra dopaminergic neurons. Based on the complementary functional role of the ion channels in the macromolecular complex identified, these results suggest that such tight interactions could partly underly the robustness of pacemaking in dopaminergic neurons.

PI4KIIIβ-Mediated Phosphoinositides Metabolism Regulates Function of the VTA Dopaminergic Neurons and Depression-Like Behavior

Phosphoinositides, including phosphatidylinositol-4,5-bisphosphate (PIP2), play a crucial role in controlling key cellular functions such as membrane and vesicle trafficking, ion channel, and transporter activity. Phosphatidylinositol 4-kinases (PI4K) are essential enzymes in regulating the turnover of phosphoinositides. However, the functional role of PI4Ks and mediated phosphoinositide metabolism in the central nervous system has not been fully revealed. In this study, we demonstrated that PI4KIIIβ, one of the four members of PI4Ks, is an important regulator of VTA dopaminergic neuronal activity and related depression-like behavior of mice by controlling phosphoinositide turnover. Our findings provide new insights into possible mechanisms and potential drug targets for neuropsychiatric diseases, including depression. Both sexes were studied in basic behavior tests, but only male mice could be used in the social defeat depression model.

Role of voltage-dependent calcium channels on the striatal in vivo dopamine release induced by the organophosphorus pesticide glyphosate

In the present study, we investigated the role of voltage-sensitive calcium channels (VSCCs) on the striatal dopamine release induced by the pesticide glyphosate (GLY) using selective VSCC inhibitors. The dopamine levels were measured by in vivo cerebral microdialysis coupled to HPLC-ED. Nicardipine (L-type VSCC antagonist) or ω-conotoxin MVIIC (non-selective P/Q-type antagonist) had no effect on dopamine release induced by 5 mM GLY. In contrast, flunarizine (T-type antagonist) or ω-conotoxin GVIA (neuronal N-type antagonist) significantly reduced GLY-stimulated dopamine release. These results suggest that GLY-induced dopamine release depends on extracellular calcium and its influx through the T- and N-type VSCCs. These findings were corroborated by molecular docking, which allowed us to establish a correlation between the effect of GLY on blocked VSCC with the observed dopamine release. We propose new molecular targets of GLY in the dorsal striatum, which could have important implications for the assessment of pesticide risks in non-target organisms.

Role of voltage-sensitive Ca2+ channels in the in vivo dopamine release induced by the organophosphorus pesticide glufosinate ammonium in rat striatum

The possible role of voltage-sensitive calcium channels (VSCC) activation in the glufosinate ammonium (GLA)-induced dopamine release was investigated using selective VSCC blockers and the dopamine levels were measured by HPLC from samples obtained by in vivo cerebral microdialysis. While pretreatment with 10 μM flunarizine (T-type VSCC antagonist) or nicardipine (L-type VSCC antagonist) had no statistically significant effect on dopamine release induced by 10 mM GLA, pretreatment with 100 μM of both antagonists, or 20 μM ω-conotoxin MVIIC (non-selective P/Q-type VSCC antagonist) significantly decreased the GLA-induced dopamine release over 72.2%, 73%, and 70.2%, respectively. Administration of the specific antagonist of neuronal N-type VSCCs, the ω-conotoxin GVIA (20 μM), produced an almost complete blockade of in vivo dopamine release induced by GLA. These results show that GLA-induced dopamine release could be produced by the activation of a wide range of striatal VSCC located at the synaptic terminals and axons of striatal dopaminergic neurons, especially N-type VSCC.

Ca2+ channels couple spiking to mitochondrial metabolism in substantia nigra dopaminergic neurons

How do neurons match generation of adenosine triphosphate by mitochondria to the bioenergetic demands of regenerative activity? Although the subject of speculation, this coupling is still poorly understood, particularly in neurons that are tonically active. To help fill this gap, pacemaking substantia nigra dopaminergic neurons were studied using a combination of optical, electrophysiological, and molecular approaches. In these neurons, spike-activated calcium (Ca2+) entry through Cav1 channels triggered Ca2+ release from the endoplasmic reticulum, which stimulated mitochondrial oxidative phosphorylation through two complementary Ca2+-dependent mechanisms: one mediated by the mitochondrial uniporter and another by the malate-aspartate shuttle. Disrupting either mechanism impaired the ability of dopaminergic neurons to sustain spike activity. While this feedforward control helps dopaminergic neurons meet the bioenergetic demands associated with sustained spiking, it is also responsible for their elevated oxidant stress and possibly to their decline with aging and disease.

Calcium channels and iron metabolism: A redox catastrophe in Parkinson's disease and an innovative path to novel therapies?

Autonomously spiking dopaminergic neurons of the substantia nigra pars compacta (SNpc) are exquisitely specialized and suffer toxic iron-loading in Parkinson's disease (PD). However, the molecular mechanism involved remains unclear and critical to decipher for designing new PD therapeutics. The long-lasting (L-type) Ca_V1.3 voltage-gated calcium channel is expressed at high levels amongst nigral neurons of the SNpc, and due to its role in calcium and iron influx, could play a role in the pathogenesis of PD. Neuronal iron uptake via this route could be unregulated under the pathological setting of PD and potentiate cellular stress due to its redox activity. This Commentary will focus on the role of the Ca_V1.3 channels in calcium and iron uptake in the context of pharmacological targeting. Prospectively, the audacious use of artificial intelligence to design innovative Ca_V1.3 channel inhibitors could lead to breakthrough pharmaceuticals that attenuate calcium and iron entry to ameliorate PD pathology.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Unraveling correlative roles of dopamine transporter (DAT) and Parkin in Parkinson's disease (PD) - A road to discovery?

Parkinson's disease (PD) is a neurodegenerative disorder accompanied by depletion of dopamine(DA) and loss of dopaminergic (DAergic) neurons in the brain that is believed to be responsible for the motor and non-motor symptoms of PD. Dopamine Transporter (DAT) is essential for reuptake of DA into the presynaptic terminal, thereby controlling the availability and spatial activity of released DA. Parkin interacts with proteins involved in the endosomal pathway, suggesting that presynaptic Parkin could regulate the expression of DAT in the plasma membrane. Parkin mutations lead to early synaptic damage and it appears as a crucial gene having a vast functioning area. PD-specific induced pluripotent stem cells (iPSCs) derived DA neurons exist as a potential tool for in-vitro modeling of PD, as they can recapitulate the pathological features of PD. The exact mechanism of PARKIN influenced DAT variations and changes in DA reuptake by DAT remain unknown. Hence, DAT and PARKIN mutated PD-specific iPSCs-derived DA neurons could provide important clues for elucidating the pathogenesis and mechanism of PD. This mysterious and hidden connection may prove to be a boon in disguise, hence, here we review the influence of PARKIN and DAT on DA mechanism and will discuss how these findings underpin the concept of how downregulation or upregulation of DAT is influenced by PARKIN. We conclude that the establishment of new model for PD with a combination of DAT and PARKIN would have a high translational potential, which includes the identification of drug targets and testing of known and novel therapeutic agents.

The Potential of L-Type Calcium Channels as a Drug Target for Neuroprotective Therapy in Parkinson's Disease

The motor symptoms of Parkinson's disease (PD) mainly arise from degeneration of dopamine neurons within the substantia nigra. As no disease-modifying PD therapies are available, and side effects limit long-term benefits of current symptomatic therapies, novel treatment approaches are needed. The ongoing phase III clinical study STEADY-PD is investigating the potential of the dihydropyridine isradipine, an L-type Ca²⁺ channel (LTCC) blocker, for neuroprotective PD therapy. Here we review the clinical and preclinical rationale for this trial and discuss potential reasons for the ambiguous outcomes of in vivo animal model studies that address PD-protective dihydropyridine effects. We summarize current views about the roles of Cav1.2 and Cav1.3 LTCC isoforms for substantia nigra neuron function, and their high vulnerability to degenerative stressors, and for PD pathophysiology. We discuss different dihydropyridine sensitivities of LTCC isoforms in view of their potential as drug targets for PD neuroprotection, and we conclude by considering how these aspects could guide further drug development.

Altered function of neuronal L-type calcium channels in ageing and neuroinflammation: Implications in age-related synaptic dysfunction and cognitive decline

The rapid developments in science have led to an increase in human life expectancy and thus, ageing and age-related disorders/diseases have become one of the greatest concerns in the 21st century. Cognitive abilities tend to decline as we get older. This age-related cognitive decline is mainly attributed to aberrant changes in synaptic plasticity and neuronal connections. Recent studies show that alterations in Ca²⁺ homeostasis underlie the increased vulnerability of neurons to age-related processes like cognitive decline and synaptic dysfunctions. Dysregulation of Ca²⁺ can lead to dramatic changes in neuronal functions. We discuss in this review, the recent advances on the potential role of dysregulated Ca²⁺ homeostasis through altered function of L-type voltage gated Ca²⁺ channels (LTCC) in ageing, with an emphasis on cognitive decline. This review therefore focuses on age-related changes mainly in the hippocampus, and with mention of other brain areas, that are important for learning and memory. This review also highlights age-related memory deficits via synaptic alterations and neuroinflammation. An understanding of these mechanisms will help us formulate strategies to reverse or ameliorate age-related disorders like cognitive decline.

Cav1.2, but not Cav1.3, L-type calcium channel subtype mediates nicotine-induced conditioned place preference in mice

Nicotine use is one of the most common forms of drug addiction. Although L-type calcium channels (LTCCs) are involved in nicotine addiction, the contribution of the two primary LTCC subtypes (Ca_v1.2 and 1.3) is unknown. This study aims to determine the contribution of these two LTCC subtypes to nicotine-induced conditioned place preference (CPP) responses by using transgenic mouse models that do not express Ca_v1.3 (Ca_v1.3^-/-) or contain a mutation in the dihydropyridine (DHP) site of the Ca_v1.2 (Ca_v1.2DHP^-/-). We found a hyperbolic dose dependent nicotine (0.1-1mg/kg; 0.5mg/kg optimum) effect on place preference in wild type (WT) mice, that could be prevented by the DHP LTCC blocker nifedipine pretreatment. Similarly, Ca_v1.3^-/- mice showed nicotine-induced place preference which was antagonized by nifedipine. In contrast, nifedipine pretreatment of Ca_v1.2DHP^-/- mice had no effect on nicotine-induced CPP responses, suggesting an involvement of Ca_v1.2 subtype in the nicotine-induced CPP response. Nifedipine alone failed to produce either conditioned place aversion or CPP in WT mice. These results collectively indicate Ca_v1.2, but not Ca_v1.3 LTCC subtype regulates, at least in part, the reinforcing effects of nicotine use.

Environment- and activity-dependent dopamine neurotransmitter plasticity in the adult substantia nigra

The ability of neurons to change the amount or type of neurotransmitter they use, or 'neurotransmitter plasticity', is an emerging new form of adult brain plasticity. For example, it has recently been shown that neurons in the adult rat hypothalamus up- or down-regulate dopamine (DA) neurotransmission in response to the amount of light the animal receives (photoperiod), and that this in turn affects anxiety- and depressive-like behaviors (Dulcis et al., 2013). In this Chapter I consolidate recent evidence from my laboratory suggesting neurons in the adult mouse substantia nigra pars compacta (SNc) also undergo DA neurotransmitter plasticity in response to persistent changes in their electrical activity, including that driven by the mouse's environment or behavior. Specifically, we have shown that the amounts of tyrosine hydroxylase (TH, the rate-limiting enzyme in DA synthesis) gene promoter activity, TH mRNA and TH protein in SNc neurons increases or decreases after ∼20h of altered electrical activity. Also, infusion of ion-channel agonists or antagonists into the midbrain for 2 weeks results in ∼10% (∼500 neurons) more or fewer TH immunoreactive (TH+) SNc neurons, with no change in the total number of SNc neurons (TH+ and TH-). Targeting ion-channels mediating cell-autonomous pacemaker activity in, or synaptic input and afferent pathways to, SNc neurons are equally effective in this regard. In addition, exposing mice to different environments (sex pairing or environment enrichment) for 1-2 weeks induces ∼10% more or fewer TH+ SNc (and ventral tegmental area or VTA) neurons and this is abolished by concurrent blockade of synaptic transmission in midbrain. Although further research is required to establish SNc (and VTA) DA neurotransmitter plasticity, and to determine whether it alters brain function and behavior, it is an exciting prospect because: (1) It may play important roles in movement, motor learning, reward, motivation, memory and cognition; and (2) Imbalances in midbrain DA cause symptoms associated with several prominent brain and behavioral disorders such as schizophrenia, addiction, obsessive-compulsive disorder, depression, Parkinson's disease and attention-deficit and hyperactivity disorder. Midbrain DA neurotransmitter plasticity may therefore play a role in the etiology of these symptoms, and might also offer new treatment options.

Amphetamine activates calcium channels through dopamine transporter-mediated depolarization

Amphetamine (AMPH) and its more potent enantiomer S(+)AMPH are psychostimulants used therapeutically to treat attention deficit hyperactivity disorder and have significant abuse liability. AMPH is a dopamine transporter (DAT) substrate that inhibits dopamine (DA) uptake and is implicated in DA release. Furthermore, AMPH activates ionic currents through DAT that modify cell excitability presumably by modulating voltage-gated channel activity. Indeed, several studies suggest that monoamine transporter-induced depolarization opens voltage-gated Ca(2+) channels (CaV), which would constitute an additional AMPH mechanism of action. In this study we co-express human DAT (hDAT) with Ca(2+) channels that have decreasing sensitivity to membrane depolarization (CaV1.3, CaV1.2 or CaV2.2). Although S(+)AMPH is more potent than DA in transport-competition assays and inward-current generation, at saturating concentrations both substrates indirectly activate voltage-gated L-type Ca(2+) channels (CaV1.3 and CaV1.2) but not the N-type Ca(2+) channel (CaV2.2). Furthermore, the potency to achieve hDAT-CaV electrical coupling is dominated by the substrate affinity on hDAT, with negligible influence of L-type channel voltage sensitivity. In contrast, the maximal coupling-strength (defined as Ca(2+) signal change per unit hDAT current) is influenced by CaV voltage sensitivity, which is greater in CaV1.3- than in CaV1.2-expressing cells. Moreover, relative to DA, S(+)AMPH showed greater coupling-strength at concentrations that induced relatively small hDAT-mediated currents. Therefore S(+)AMPH is not only more potent than DA at inducing hDAT-mediated L-type Ca(2+) channel currents but is a better depolarizing agent since it produces tighter electrical coupling between hDAT-mediated depolarization and L-type Ca(2+) channel activation.

Gating of dopamine transmission by calcium and axonal N-, Q-, T- and L-type voltage-gated calcium channels differs between striatal domains

KEY POINTS

The voltage-gated Ca(2+) channels (VGCCs) that catalyse striatal dopamine transmission are critical to dopamine function and might prime subpopulations of neurons for parkinsonian degeneration. However, the VGCCs that operate on mesostriatal axons are incompletely defined; previous studies encompassed channels on striatal cholinergic interneurons that strongly influence dopamine transmission. We define that multiple types of axonal VGCCs operate that extend beyond classic presynaptic N/P/Q channels to include T- and L-types. We reveal differences in VGCC function between mouse axon types that in humans are vulnerable versus resistant to Parkinson's disease. We show for the first time that this is underpinned by different sensitivity of dopamine transmission to extracellular Ca(2+) and by different spatiotemporal intracellular Ca(2+) microdomains. These data define key principles of how Ca(2+) and VGCCs govern dopamine transmission in the healthy brain and reveal differences between neuron types that might contribute to vulnerability in disease.

ABSTRACT

The axonal voltage-gated Ca(2+) channels (VGCCs) that catalyse dopamine (DA) transmission are incompletely defined. Yet, they are critical to DA function and might prime subpopulations of DA neurons for parkinsonian degeneration. Previous studies of VGCCs will have encompassed those on striatal cholinergic interneurons, which strongly influence DA transmission. We identify which VGCCs on DA axons govern DA transmission, we determine their dynamic properties and reveal an underlying basis for differences between the caudate putamen (CPu) and nucleus accumbens (NAc). We detected DA release evoked electrically during nicotinic receptor blockade or optogenetically by light activation of channel rhodopsin-expressing DA axons in mouse striatal slices. Subtype-specific VGCC blockers indicated that N-, Q-, T- and L-VGCCs govern DA release in CPu, but in NAc, T and L-channels are relatively silent. The roles of the most dominant channels were inversely frequency-dependent, due to low-pass filtering of DA release by Ca(2+)-dependent relationships between initial release probability and short-term plasticity. Ca(2+) concentration-response curves revealed that differences between CPu and NAc were due to greater underlying Ca(2+) sensitivity of DA transmission from CPu axons. Functions for 'silent' L- and T-channels in NAc could be unmasked by elevating extracellular [Ca(2+)]. Furthermore, we identified a greater coupling between BAPTA-sensitive, fast Ca(2+) transients and DA transmission in CPu axons, and evidence for endogenous fast buffering of Ca(2+) in NAc. These data reveal that a range of VGCCs operate dynamically on DA axons, depending on local driving forces. Furthermore, they reveal dramatic differences in Ca(2+) handling between axonal subpopulations that show different vulnerability to parkinsonian degeneration.

Nimodipine enhances neurite outgrowth in dopaminergic brain slice co-cultures

Calcium ions (Ca(2+)) play important roles in neuroplasticity and the regeneration of nerves. Intracellular Ca(2+) concentrations are regulated by Ca(2+) channels, among them L-type voltage-gated Ca(2+) channels, which are inhibited by dihydropyridines like nimodipine. The purpose of this study was to investigate the effect of nimodipine on neurite growth during development and regeneration. As an appropriate model to study neurite growth, we chose organotypic brain slice co-cultures of the mesocortical dopaminergic projection system, consisting of the ventral tegmental area/substantia nigra and the prefrontal cortex from neonatal rat brains. Quantification of the density of the newly built neurites in the border region (region between the two cultivated slices) of the co-cultures revealed a growth promoting effect of nimodipine at concentrations of 0.1μM and 1μM that was even more pronounced than the effect of the growth factor NGF. This beneficial effect was absent when 10μM nimodipine were applied. Toxicological tests revealed that the application of nimodipine at this higher concentration slightly induced caspase 3 activation in the cortical part of the co-cultures, but did neither affect the amount of lactate dehydrogenase release or propidium iodide uptake nor the ratio of bax/bcl-2. Furthermore, the expression levels of different genes were quantified after nimodipine treatment. The expression of Ca(2+) binding proteins, immediate early genes, glial fibrillary acidic protein, and myelin components did not change significantly after treatment, indicating that the regulation of their expression is not primarily involved in the observed nimodipine mediated neurite growth. In summary, this study revealed for the first time a neurite growth promoting effect of nimodipine in the mesocortical dopaminergic projection system that is highly dependent on the applied concentrations.

Aging decreases L-type calcium channel currents and pacemaker firing fidelity in substantia nigra dopamine neurons

Substantia nigra dopamine neurons are involved in behavioral processes that include cognition, reward learning, and voluntary movement. Selective deterioration of these neurons is responsible for the motor deficits associated with Parkinson's disease (PD). Aging is the leading risk factor for PD, suggesting that adaptations occurring in dopamine neurons during normal aging may predispose individuals to the development of PD. Previous studies suggest that the unique set of ion conductances that drive spontaneous, rhythmic firing of action potentials could predispose substantia nigra dopamine neurons to selective neurodegeneration. Here we show, using patch-clamp electrophysiological recordings in brain slices, that substantia nigra dopamine neurons from mice 25-30 months of age (old) have comparable membrane capacitance and input resistance to neurons from mice 2-7 months of age (young). However, neurons from old mice exhibit slower firing rates, narrower spike widths, and more variable interspike intervals compared with neurons from young mice. Dopamine neurons from old mice also exhibit smaller L-type calcium channel currents, providing a plausible mechanism that likely contributes to the changes in impulse activity. Age-related decrements in the physiological function of dopamine neurons could contribute to the decrease in voluntary movement and other dopamine-mediated behaviors observed in aging populations. Furthermore, as pharmacological antagonism of L-type calcium channels has been proposed as a potential treatment for the early stages of PD, our results could point to a limited temporal window of opportunity for this therapeutic intervention.

Cav1.2 and Cav1.3 L-type calcium channels regulate dopaminergic firing activity in the mouse ventral tegmental area

Dopaminergic projections from the ventral tegmental area (VTA) constitute the mesolimbocortical system that underlies addiction and psychosis primarily as a result of increased dopaminergic transmission. Dopamine release is spike dependent. L-type calcium channels (LTCCs) play an important role in regulating firing activities, but the contribution of specific subtypes remains unclear. This article describes different functions of Cav1.2 and Cav1.3 subtypes in regulating firing properties with two transgenic mouse strains. For basal firing, Cav1.3-deficient (Cav1.3(-/-)) mice had a lower basal firing frequency. The dihydropyridine (DHP) channel blocker nifedipine reduced single-spike firing in mice expressing DHP-insensitive Cav1.2 channels (Cav1.2DHP(-/-) mice), confirming the significant contribution from the Cav1.3 subtype in basal firing. Moreover, the DHP channel activator (S)-(-)-Bay K8644 and the non-DHP channel activator FPL 64176 converted firing patterns from single spiking to bursting in Cav1.2DHP(-/-) mice. Nifedipine inhibited burst firing induced by both activators, suggesting that Cav1.3 also serves an essential role in burst firing. However, FPL 64176 also induced bursting in Cav1.3(-/-) mice. These results indicate that the Cav1.3 subtype is crucial to regulation of basal single-spike firing, while activation of both Cav1.2 and Cav1.3 can support burst firing of VTA neurons.

Somatodendritic ion channel expression in substantia nigra pars compacta dopaminergic neurons across postnatal development

Dopaminergic neurons of the substantia nigra pars compacta (SNc) are involved in the control of movement, sleep, reward, learning, and nervous system disorders and disease. To date, a thorough characterization of the ion channel phenotype of this important neuronal population is lacking. Using immunohistochemistry, we analyzed the somatodendritic expression of voltage-gated ion channel subunits that are involved in pacemaking activity in SNc dopaminergic neurons in 6-, 21-, and 40-day-old rats. Our results demonstrate that the same complement of somatodendritic ion channels is present in SNc dopaminergic neurons from P6 to P40. The major developmental changes were an increase in the dendritic range of the immunolabeling for the HCN, T-type calcium, Kv4.3, delayed rectifier, and SK channels. Our study sheds light on the ion channel subunits that contribute to the somatodendritic delayed rectifier (Kv1.3, Kv2.1, Kv3.2, Kv3.3), A-type (Kv4.3) and calcium-activated SK (SK1, SK2, SK3) potassium currents, IH (mainly HCN2, HCN4), and the L- (Cav1.2, Cav1.3) and T-type (mainly Cav3.1, Cav3.3) calcium currents in SNc dopaminergic neurons. Finally, no robust differences in voltage-gated ion channel immunolabeling were observed across the population of SNc dopaminergic neurons for each age examined, suggesting that differing levels of individual ion channels are unlikely to distinguish between specific subpopulations of SNc dopaminergic neurons. This is significant in light of previous studies suggesting that age- or region-associated variations in the expression profile of voltage-gated ion channels in SNc dopaminergic neurons may underlie their vulnerability to dysfunction and disease.

A single compartment model of pacemaking in dissasociated substantia nigra neurons: stability and energy analysis

Spontaneous oscillations in the mid-brain dopaminergic neurons are an important feature of motor control. The degeneration of these neurons is involved in movement disorders, particularly Parkinson's Disease. Modelling of this activity is an important part of developing an understanding of the pathogenic process. We develop a mathematical paradigm to describe this activity with a single compartment approach and a CellML version is made publicly available. The model explicitly describes the dynamics of the transmembrane potential with changes in the levels of important cations and is consistent with two major observations in the literature regarding its behaviour in the presence of channel blockers. Stability of the model behaviour is determined from the properties of its Monodromy matrix. We also discuss from the perspective of energy, a pharmacological intervention suggested in the treatment of Parkinson's Disease.

Ca(v)1.3 and BK channels for timing and regulating cell firing

L-type Ca(2+) channels (LTCCs, Ca(v)1) open readily during membrane depolarization and allow Ca(2+) to enter the cell. In this way, LTCCs regulate cell excitability and trigger a variety of Ca(2+)-dependent physiological processes such as: excitation-contraction coupling in muscle cells, gene expression, synaptic plasticity, neuronal differentiation, hormone secretion, and pacemaker activity in heart, neurons, and endocrine cells. Among the two major isoforms of LTCCs expressed in excitable tissues (Ca(v)1.2 and Ca(v)1.3), Ca(v)1.3 appears suitable for supporting a pacemaker current in spontaneously firing cells. It has steep voltage dependence and low threshold of activation and inactivates slowly. Using Ca(v)1.3(-/-) KO mice and membrane current recording techniques such as the dynamic and the action potential clamp, it has been possible to resolve the time course of Ca(v)1.3 pacemaker currents that regulate the spontaneous firing of dopaminergic neurons and adrenal chromaffin cells. In several cell types, Ca(v)1.3 is selectively coupled to BK channels within membrane nanodomains and controls both the firing frequency and the action potential repolarization phase. Here we review the most critical aspects of Ca(v)1.3 channel gating and its coupling to large conductance BK channels recently discovered in spontaneously firing neurons and neuroendocrine cells with the aim of furnishing a converging view of the role that these two channel types play in the regulation of cell excitability.

Regulation of firing frequency in a computational model of a midbrain dopaminergic neuron

Dopaminergic (DA) neurons of the mammalian midbrain exhibit unusually low firing frequencies in vitro. Furthermore, injection of depolarizing current induces depolarization block before high frequencies are achieved. The maximum steady and transient rates are about 10 and 20 Hz, respectively, despite the ability of these neurons to generate bursts at higher frequencies in vivo. We use a three-compartment model calibrated to reproduce DA neuron responses to several pharmacological manipulations to uncover mechanisms of frequency limitation. The model exhibits a slow oscillatory potential (SOP) dependent on the interplay between the L-type Ca(2+) current and the small conductance K(+) (SK) current that is unmasked by fast Na(+) current block. Contrary to previous theoretical work, the SOP does not pace the steady spiking frequency in our model. The main currents that determine the spontaneous firing frequency are the subthreshold L-type Ca(2+) and the A-type K(+) currents. The model identifies the channel densities for the fast Na(+) and the delayed rectifier K(+) currents as critical parameters limiting the maximal steady frequency evoked by a depolarizing pulse. We hypothesize that the low maximal steady frequencies result from a low safety factor for action potential generation. In the model, the rate of Ca(2+) accumulation in the distal dendrites controls the transient initial frequency in response to a depolarizing pulse. Similar results are obtained when the same model parameters are used in a multi-compartmental model with a realistic reconstructed morphology, indicating that the salient contributions of the dendritic architecture have been captured by the simpler model.

Cav1.3 channel voltage dependence, not Ca2+ selectivity, drives pacemaker activity and amplifies bursts in nigral dopamine neurons

Ca(v)1.3 (alpha 1D) L-type Ca(2+) channels have been implicated in substantia nigra (SN) dopamine (DA) neuron pacemaking and vulnerability to Parkinson's disease. These effects may arise from the depolarizing current and cytoplasmic Ca(2+) elevation produced by Ca(v)1.3 channels at subthreshold membrane potentials. However, the assumption that the Ca(2+) selectivity of Ca(v)1.3 channels is essential has not been tested. In this study the properties of SN DA neuron L-type Ca(2+) channels responsible for driving pacemaker activity in juvenile rat brain slices were probed by replacing native channels blocked with the dihydropyridine nimodipine with virtual channels generated by dynamic clamp. Surprisingly, virtual L-type channels that mimic native and recombinant Ca(v)1.3 channels supported pacemaker activity even though dynamic clamp currents are not carried by Ca(2+). This effect is specific because pacemaker activity could not be restored by tonic current injection, virtual nonselective leak channels or virtual NMDA receptors, which share with L-type channels a negative slope conductance region in their current-voltage (I-V) curve. Altering virtual channels showed that the production of pacemaker activity depended on the characteristic voltage dependence of DA neuron L-type channels, while activation kinetics and reversal potential were not critical parameters. Virtual L-type channels also supported slow oscillatory potentials and enhanced firing rate during evoked bursts. Thus, Ca(v)1.3 channel voltage dependence, rather than Ca(2+) selectivity, drives pacemaker activity and amplifies bursts in SN DA neurons.

Endogenous calcium buffering capacity of substantia nigral dopamine neurons

Dopamine (DA)-containing cells from the substantia nigra pars compacta (SNc) play a major role in the initiation of movement. Loss of these cells results in Parkinson's disease (PD). Changes in intracellular calcium ion concentration ([Ca(2+)](i)) elicit several events in DA cells, including spike afterhyperpolarizations (AHPs) and subthreshold oscillations underlying autonomous firing. Continuous Ca(2+) load due to Ca(2+)-dependent rhythmicity has been proposed to cause the death of DA cells in PD and normal aging. Because of the physiological and pathophysiological importance of [Ca(2+)](i) in DA cells, we characterized their intrinsic Ca(2+)-buffering capacity (K(S)) in brain slices. We introduced a fluorescent Ca(2+)-sensitive exogenous buffer (200 microM fura-2) and cells were tracked from break-in until steady state by stimulating with a single action potential (AP) every 30 s and measuring the Ca(2+) transient from the proximal dendrite. DA neurons filled exponentially with a tau of about 5-6 min. [Ca(2+)](i) was assumed to equilibrate between the endogenous Ca(2+) buffer and the exogenous Ca(2+) indicator buffer. Intrinsic buffering was estimated by extrapolating from the linear relationships between the amplitude or time constant of the Ca(2+) transients versus [fura-2]. Extrapolated Ca(2+)-transients in the absence of fura-2 had mean peak amplitudes of 293.7 +/- 65.3 nM and tau = 124 +/- 13 ms (postnatal day 13 [P13] to P17 animals). Intrinsic buffering increased with age in DA neurons. For cells from animals P13-P17, K(S) was estimated to be about 110 (n = 20). In older animals (P25-P32), the estimate was about 179 (n = 10). These relatively low values may reflect the need for rapid Ca(2+) signaling, e.g., to allow activation of sK channels, which shape autonomous oscillations and burst firing. Low intrinsic buffering may also make DA cells vulnerable to Ca(2+)-dependent pathology.

Mobilization of calcium from intracellular stores facilitates somatodendritic dopamine release

Somatodendritic dopamine (DA) release in the substantia nigra pars compacta (SNc) shows a limited dependence on extracellular calcium concentration ([Ca(2+)](o)), suggesting the involvement of intracellular Ca(2+) stores. Here, using immunocytochemistry we demonstrate the presence of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) that sequesters cytosolic Ca(2+) into the endoplasmic reticulum (ER), as well as inositol 1,4,5-triphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) in DAergic neurons. Notably, RyRs were clustered at the plasma membrane, poised for activation by Ca(2+) entry. Using fast-scan cyclic voltammetry to monitor evoked extracellular DA concentration ([DA](o)) in midbrain slices, we found that SERCA inhibition by cyclopiazonic acid (CPA) decreased evoked [DA](o) in the SNc, indicating a functional role for ER Ca(2+) stores in somatodendritic DA release. Implicating IP(3)R-dependent stores, an IP(3)R antagonist, 2-APB, also decreased evoked [DA](o). Moreover, DHPG, an agonist of group I metabotropic glutamate receptors (mGluR1s, which couple to IP(3) production), increased somatodendritic DA release, whereas CPCCOEt, an mGluR1 antagonist, suppressed it. Release suppression by mGluR1 blockade was prevented by 2-APB or CPA, indicating facilitation of DA release by endogenous glutamate acting via mGluR1s and IP(3)R-gated Ca(2+) stores. Similarly, activation of RyRs by caffeine increased [Ca(2+)](i) and elevated evoked [DA](o). The increase in DA release was prevented by a RyR blocker, dantrolene, and by CPA. Importantly, the efficacy of dantrolene was enhanced in low [Ca(2+)](o), suggesting a mechanism for maintenance of somatodendritic DA release with limited Ca(2+) entry. Thus, both mGluR1-linked IP(3)R- and RyR-dependent ER Ca(2+) stores facilitate somatodendritic DA release in the SNc.

Electrophysiological characteristics of dopamine neurons: a 35-year update

This chapter consists of four sections. The first section provides a general description of the electrophysiological characteristics of dopamine (DA) neurons in both the substantia nigra and ventral tegmental area. Emphasis is placed on the differences between DA and neighboring non-DA neurons. The second section discusses the ionic mechanisms underlying the generation of action potential in DA cells. Evidence is provided to suggest that these mechanisms differ not only between DA and non-DA neurons but also between DA cells located in different areas, with different projection sites and at different developmental stages. Some of the differences may play a critical role in the vulnerability of a DA neuron to cell death. The third section describes the firing patterns of DA cells. Data are presented to show that the current "80/160 ms" criteria for burst identification need to be revised and that the burst firing, originally described by Bunney et al., can be described as slow oscillations in firing rate. In the ventral tegmental area, the slow oscillations are, at least partially, derived from the prefrontal cortex and part of prefrontal information is transferred to DA cells indirectly through inhibitory neurons. The final section focuses on the feedback regulation of DA cells. New evidence suggests that DA autoreceptors are coupled to multiple effectors, and both D1 and D2-like receptors are involved in long-loop feedback control of DA neurons. Because of the presence of multiple feedback and nonfeedback pathways, the effect of a drug on a DA neuron can be far more complex than an inhibition or excitation. A better understanding of the intrinsic properties of DA neurons and their regulation by afferent input will, in time, help to point to the way to more effective and safer treatments for disorders including schizophrenia, drug addiction, and Parkinson's disease.

Distribution of calcium channel Ca(V)1.3 immunoreactivity in the rat spinal cord and brain stem

The function of local networks in the CNS depends upon both the connectivity between neurons and their intrinsic properties. An intrinsic property of spinal motoneurons is the presence of persistent inward currents (PICs), which are mediated by non-inactivating calcium (mainly Ca(V)1.3) and/or sodium channels and serve to amplify neuronal input signals. It is of fundamental importance for the prediction of network function to determine the distribution of neurons possessing the ion channels that produce PICs. Although the distribution pattern of Ca(V)1.3 immunoreactivity (Ca(V)1.3-IR) has been studied in some specific central nervous regions in some species, so far no systematic investigations have been performed in both the rat spinal cord and brain stem. In the present study this issue was investigated by immunohistochemistry. The results indicated that the Ca(V)1.3-IR neurons were widely distributed across different parts of the spinal cord and the brain stem although with variable labeling intensities. In the spinal gray matter large neurons in the ventral horn (presumably motoneurons) tended to display higher levels of immunoreactivity than smaller neurons in the dorsal horn. In the white matter, a subset of glial cells labeled by an oligodendrocyte marker was also Ca(V)1.3-positive. In the brain stem, neurons in the motor nuclei appeared to have higher levels of immunoreactivity than those in the sensory nuclei. Moreover, a number of nuclei containing monoaminergic cells, for example the locus coeruleus, were also strongly immunoreactive. Ca(V)1.3-IR was consistently detected in the neuronal perikarya regardless of the neuronal type. However, in the large neurons in the spinal ventral horn and the cranial motor nuclei the Ca(V)1.3-IR was clearly detectable in first and second order dendrites. These results indicate that in the rat spinal cord and brain stem Ca(V)1.3 is probably a common calcium channel used by many kinds of neurons to facilitate the neuronal information processing via certain intracellular mechanisms, for instance, PICs.

Cholinergic excitation of dopaminergic cells depends on sequential activation of protein kinase C and the L-type calcium channel in ventral tegmental area slices

Dopaminergic projections from the ventral tegmental area (VTA) constitute the mesolimbocortical system that underlies addiction and psychosis primarily as the result of increased dopaminergic transmission. Dopaminergic neurons in the VTA receive glutamatergic and cholinergic innervations that regulate their firing activities. Both transmitter systems can activate protein kinase C (PKC) by increasing intracellular calcium and lipid second messengers, however, whether PKC mediates increased firing following glutamatergic and cholinergic activation remains unknown. This paper examined the effects of acute PKC inhibition on firing responses to carbachol, NMDA or AMPA using patch clamp recordings from brain slices. The three ligands all induced a reversible increase in firing, however, only carbachol-induced increase in firing was attenuated by the PKC inhibitors chelerythrine or GF 109203X. The L-type calcium channel blocker nifedipine partially blocked carbachol-induced excitation similar to PKC inhibitors. PKC inhibition and L-type channel blockade did not significantly alter NMDA- or AMPA-induced excitation. Concurrent blockade of PKC and L-type channels with chelerythrine and nifedipine did not additively suppress carbachol-induced excitation indicating they were sequential events in the same signaling pathway. Furthermore, preincubation with the PKC inhibitor GF 109203X reduced the carbachol-induced increase in nifedipine-sensitive high-voltage gated calcium currents. These results indicate that cholinergic activation enhances PKC activity, which in turn facilitates L-type channel opening to excite dopaminergic cells, a finding that is in line with reports of increased PKC in the VTA in animals displaying addictive behavior.

Computational model predicts a role for ERG current in repolarizing plateau potentials in dopamine neurons: implications for modulation of neuronal activity

Blocking the small-conductance (SK) calcium-activated potassium channel promotes burst firing in dopamine neurons both in vivo and in vitro. In vitro, the bursting is unusual in that spiking persists during the hyperpolarized trough and frequently terminates by depolarization block during the plateau. We focus on the underlying plateau potential oscillation generated in the presence of both apamin and TTX, so that action potentials are not considered. We find that although the plateau potentials are mediated by a voltage-gated Ca(2+) current, they do not depend on the accumulation of cytosolic Ca(2+), then use a computational model to test the hypothesis that the slowly voltage-activated ether-a-go-go-related gene (ERG) potassium current repolarizes the plateaus. The model, which includes a material balance on calcium, is able to reproduce the time course of both membrane potential and somatic calcium concentration, and can also mimic the induction of plateau potentials by the calcium chelator BAPTA. The principle of separation of timescales was used to gain insight into the mechanisms of oscillation and its modulation using nullclines in the phase space. The model predicts that the plateau will be elongated and ultimately result in a persistent depolarization as the ERG current is reduced. This study suggests that the ERG current may play a role in burst termination and the relief of depolarization block in vivo.

Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity

It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.

Roles of subthreshold calcium current and sodium current in spontaneous firing of mouse midbrain dopamine neurons

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 omega-Aga-IVA, but the N-type channel blocker omega-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 -70 to -50 mV, with TTX-sensitive current larger at voltages positive to -45 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.

Calcium influx through L-type channels generates protein kinase M to induce burst firing of dopamine cells in the rat ventral tegmental area

Enhanced activity of the dopaminergic system originating in the ventral tegmental area is implicated in addictive and psychiatric disorders. Burst firing increases dopamine levels at the synapse to signal novelty and salience. We have previously reported a calcium-dependent burst firing of dopamine cells mediated by L-type channels following cholinergic stimulation; this paper describes a cellular mechanism resulting in burst firing following L-type channel activation. Calcium influx through L-type channels following FPL 64176 or (S)-(-)-Bay K8644 induced burst firing independent of dopamine, glutamate, or calcium from the internal stores. Burst firing induced as such was completely blocked by the substrate site protein kinase C (PKC) inhibitor chelerythrine but not by the diacylglycerol site inhibitor calphostin C. Western blotting analysis showed that FPL 64176 and (S)-(-)-Bay K8644 increased the cleavage of PKC to generate protein kinase M (PKM) and the specific calpain inhibitor MDL28170 blocked this increase. Prevention of PKM production by inhibiting calpain or depleting PKC blocked burst firing induction whereas direct loading of purified PKM into cells induced burst firing. Activation of the N-methyl-D-aspartic acid type glutamate or cholinergic receptors known to induce burst firing increased PKM expression. These results indicate that calcium influx through L-type channels activates a calcium-dependent protease that cleaves PKC to generate constitutively active and labile PKM resulting in burst firing of dopamine cells, a pathway that is involved in glutamatergic or cholinergic modulation of the central dopamine system.

Limited regulation of somatodendritic dopamine release by voltage-sensitive Ca channels contrasted with strong regulation of axonal dopamine release

The mechanism underlying somatodendritic release of dopamine (DA) appears to differ from that of axon-terminal release. Specifically, somatodendritic DA release in the substantia nigra pars compacta (SNc) persists in low extracellular Ca2+ concentrations that are insufficient to support axonal release in striatum, suggesting that limited Ca2+ entry is necessary to trigger somatodendritic release. Here, we compared the role of voltage-dependent Ca2+ channels in mediating DA release in striatum versus SNc using specific blockers of N-, P/Q-, T-, R- and L-type Ca2+ channels individually and in combination. Release of DA evoked by a single stimulus pulse in the dorsal striatum and SNc of guinea-pig brain slices was monitored in real time using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Single-pulse evoked DA release was shown to be independent of regulation by concurrently released glutamate or GABA acting at ionotropic receptors in both regions. Under these conditions, striatal DA release was completely prevented by an N-type channel blocker, omega-conotoxin GVIA (100 nm), and was decreased by 75% by the P/Q-type channel blocker omega-agatoxin IVA (200 nm). Blockade of T-type channels with Ni2+ (100 microm) or R-type channels with SNX-482 (100 nm) decreased axonal release in striatum by 25%, whereas inhibition of L-type channels with nifedipine (20 microm) had no effect. By contrast, none of these Ca2+-channel blockers altered the amplitude of somatodendritic DA release in the SNc. Even a cocktail of all blockers tested did not alter release-signal amplitude in the SNc, although the duration of the release response was curtailed. The limited involvement of voltage-dependent Ca2+ channels in somatodendritic DA release provides further evidence that minimal Ca2+ entry is required to trigger the release process, compared with that required for axon-terminal release.

Autonomous pacemakers in the basal ganglia: who needs excitatory synapses anyway?

Autonomous pacemakers are crucial elements in many neural circuits. This is particularly true for the basal ganglia. This richly interconnected group of nuclei is rife with both fast- and slow-spiking pacemakers. Our understanding of the ionic mechanisms underlying pacemaking in these neurons is rapidly evolving, yielding new insights into the normal functioning of this network and how it goes awry in pathological states such as Parkinson's disease.

L-type Ca2+ channels mediate adaptation of extracellular signal-regulated kinase 1/2 phosphorylation in the ventral tegmental area after chronic amphetamine treatment

L-type Ca2+ channels (LTCCs) play an important role in chronic psychostimulant-induced behaviors. However, the Ca2+ second messenger pathways activated by LTCCs after acute and recurrent psychostimulant administration that contribute to drug-induced molecular adaptations are poorly understood. Using a chronic amphetamine treatment paradigm in rats, we have examined the role of LTCCs in activating the mitogen-activated protein (MAP) kinase pathway in the ventral tegmental area (VTA), a primary target for the reinforcing properties of psychostimulants. Using immunoblot and immunohistochemical analyses, we find that in chronic saline-treated rats a challenge injection of amphetamine increases phosphorylation of MAP [extracellular signal-regulated kinase 1/2 (ERK1/2)] kinase in the VTA that is independent of LTCCs. However, in chronic amphetamine-treated rats there is no increase in amphetamine-mediated ERK1/2 phosphorylation unless LTCCs are blocked, in which case there is robust phosphorylation in VTA dopamine neurons. Examination of the expression of phosphatases reveals an increase in calcineurin [protein phosphatase 2B (PP2B)] and MAP kinase phosphatase-1 (MKP-1) in the VTA. Using in situ hybridization histochemistry and immunoblot analyses, we further examined the mRNA and protein expression of the LTCC subtypes Ca(v)1.2 and Ca(v)1.3 in VTA dopamine neurons in drug-naive animals and in rats after chronic amphetamine treatment. We found an increase in Ca(v)1.2 mRNA and protein levels, with no change in Ca(v)1.3. Together, our results suggest that one aspect of LTCC-induced changes in second messenger pathways after chronic amphetamine exposure involves activation of the MAP kinase phosphatase pathway by upregulation of Ca(v)1.2 in VTA dopaminergic neurons.

Developmental expression of Ca(v)1.3 (alpha1d) calcium channels in the mouse inner ear

Voltage-gated calcium channels are important for neurotransmission at the level of inner hair cells (IHCs) and outer hair cells (OHCs). These channels open when mechanical stimulation depolarises the hair cell membrane and the resulting calcium influx triggers neurotransmitter release. Voltage-gated calcium channels expressed in hair cells are known to be of the L-type with a predominance of the Ca(v)1.3 subunit. The present study describes the developmental expression of the Ca(v)1.3 protein in the cochlea and the vestibular system using immunohistochemical technique. In the adult organ of Corti (OC), Ca(v)1.3 was localized in both sensory and non-sensory cells with a more intense expression in IHCs and Deiters cells when compared to OHCs. In both hair cell types, immunoreactivity was observed in the apical pole, basolateral membrane and at the basal pole (synaptic zone). Similar results were obtained in the vestibular organs. During development, Ca(v)1.3 immunoreactivity was observed in the cochlea as early as embryonic day 15, with expression increasing at birth. At these early stages of cochlear development, Ca(v)1.3 was expressed in all cell types surrounding the scala media. In the OC, the labeling was observed in IHCs, OHCs and supporting cells. The Ca(v)1.3 expression reached an adult-like pattern by the end of the second postnatal week. The present findings suggested that, in addition to their implication in hair cells synaptic transmission, Ca(v)1.3 calcium channels also play an important role in vesicle recycling and transport, as suggested by their extrasynaptic location at the apical pole of the hair cells. The Ca(v)1.3 channels in Deiters cells could participate in active calcium-induced changes in micromechanics of these supporting cells. An early expression during development suggested that these calcium channels are in addition important in the development of the cochlear and vestibular sensory epithelium.

A Modeling Study Suggests Complementary Roles for GABAA and NMDA Receptors and the SK Channel in Regulating the Firing Pattern in Midbrain Dopamine Neurons

Midbrain dopaminergic (DA) neurons in vivo exhibit two major firing patterns: single-spike firing and burst firing. The firing pattern expressed is dependent on both the intrinsic properties of the neurons and their excitatory and inhibitory synaptic inputs. Experimental data suggest that the activation of N-methyl-d-aspartate (NMDA) and GABAA receptors is a crucial contributor to the initiation and suppression of burst firing, respectively, and that blocking Ca2+-activated potassium SK channels can facilitate burst firing. A multi-compartmental model of a DA neuron with a branching structure was developed and calibrated based on in vitro experimental data to explore the effects of different levels of activation of NMDA and GABAA receptors as well as the modulation of the SK current on the firing activity. The simulated tonic activation of GABAA receptors was calibrated by taking into account the difference in the electrotonic properties in vivo versus in vitro. Although NMDA-evoked currents are required for burst generation in the model, currents evoked by GABAA-receptor activation can also regulate the firing pattern. For example, the model predicts that increasing the level of NMDA receptor activation can produce excessive depolarization that prevents burst firing, but a concurrent increase in the activation of GABAA receptors can restore burst firing. Another prediction of the model is that blocking the SK channel current in vivo will facilitate bursting, but not as robustly as blocking the GABAA receptors.

Glutamate-mediated [Ca2+]c dynamics in spontaneously firing dopamine neurons of the rat substantia nigra pars compacta

The mechanism by which glutamate regulates the cytosolic free Ca2+ concentration ([Ca2+]c) in spontaneously firing dopamine neurons is not clear. Thus we have investigated the glutamate-mediated [Ca2+]c dynamics in the acutely isolated dopamine neurons from the rat substantia nigra pars compacta by measuring [Ca2+]c and spontaneously occurring action potentials (SAPs). The freshly isolated dopamine neurons showed tetrodotoxin (TTX)-sensitive spontaneous firing of 2-3 Hz and the resting [Ca2+]c decreased with abolition of the SAPs. The level of [Ca2+]c was affected by the spontaneous firing rate. In the presence of the Na+ channel antagonist, TTX (0.5 microM), glutamate increased [Ca2+]c by activating different glutamate receptors depending on the glutamate concentration used. Addition of glutamate at low concentrations (<3 microM) raised [Ca2+]c mainly by activating metabotropic glutamate receptors (mGluR), whereas at high concentrations (>10 microM) it raised [Ca2+]c mainly by activating AMPA/kainate receptors. The contribution of NMDA receptors to the glutamate-mediated [Ca2+]c rises was largest at intermediate concentrations of glutamate. Activation of mGluR elicited a Ca2+ release from intracellular Ca2+ stores and continuous Ca2+ influx out of the cell. The spontaneous firing activities were highly enhanced by submicromolar levels of glutamate and abolished at levels above 10 microM. From these results, we conclude that at low glutamate concentrations the [Ca2+]c in the dopamine neurons is mainly governed by mGluR and the firing activities, whose rate is regulated at submicromolar glutamate concentrations, but at higher glutamate concentrations [Ca2+]c is dominantly affected by AMPA/kainate receptors.

Expression of voltage-dependent calcium channels in the embryonic rat midbrain

The diversity of expression of high-voltage activated voltage-dependent calcium channels (VDCC) was investigated with whole-cell voltage-clamp recordings from dissociated embryonic rat ventral mesencephalic cells over a 7-day culture period. Cell phenotype was identified post-recording by fluorescent immunocytochemistry as tyrosine hydroxylase positive (TH+) or glutamic acid decarboxylase positive (GAD+). Both TH+ and GAD+ cells displayed high-threshold calcium (Ca(2+)) currents activated by depolarisations positive to -60 mV. In both cell types, pharmacological dissection using selective VDCC inhibitors, omega-agatoxin IVA (Aga IVA), omega-conotoxin GVIA (GVIA) and nifedipine demonstrated the existence of P/Q-, N- and L-type VDCC, respectively. The remaining residual current could be blocked by cadmium. It was found that the contribution to the whole-cell current by the N-type channel was greater in TH+ cells than GAD+ cells at each time point examined, whilst the contribution to the whole-cell current by the L-type channel was greater in GAD+ cells than TH+ cells. However, over the 7-day culture period, the expression of VDCC types in both cell phenotypes changed in a similar fashion, with the contribution to the whole-cell current from the N-type current decreasing, and the contribution from the R-type current increasing. Our data could provide new insights into a range of neurodevelopmental mechanisms related to Ca(2+) homeostasis in developing mesencephalic neurons.

Selective coupling of T-type calcium channels to SK potassium channels prevents intrinsic bursting in dopaminergic midbrain neurons

Dopaminergic midbrain (DA) neurons display two principal activity patterns in vivo, single-spike and burst firing, the latter coding for reward-related events. We have shown recently that the small-conductance calcium-activated potassium channel SK3 controls pacemaker frequency and precision in DA neurons of the substantia nigra (SN), and previous studies have implicated SK channels in the transition to burst firing. To identify the upstream calcium sources for SK channel activation in DA SN neurons, we studied the sensitivity of SK channel-mediated afterhyperpolarization (AHP) currents to inhibitors of different types of voltage-gated calcium channels in perforated patch-clamp recordings. Cobalt-sensitive AHP currents were not affected by L-type and P/Q-type calcium channel inhibitors and were reduced slightly (26%) by the N-type channel inhibitor omega-conotoxin-GVIA. In contrast, AHP currents were blocked substantially (85-94%) by micromolar concentrations of nickel (IC50, 33.75 microm) and mibefradil (IC50, 4.83 microm), indistinguishable from the nickel and mibefradil sensitivities of T-type calcium currents (IC50 values, 33.86 and 4.59 microm, respectively). These results indicate that SK channels are activated selectively via T-type calcium channels in DA SN neurons. Consequently, SK currents displayed use-dependent inactivation with similar time constants when compared with those of T-type calcium currents and generated a transient rebound inhibition. Both SK and T-type channels were essential for the stability of spontaneous pacemaker activity, and, in some DA SN neurons, T-type channel inhibition was sufficient to induce intrinsic burst firing. The functional coupling of SK to T-type channels has important implications for the temporal integration of synaptic input and might help to understand how DA neurons switch between pacemaker and burst-firing modes in vivo.