Modulation of N-type Ca2+ channel current kinetics by PMA in rat sympathetic neurons

Fast Inactivation of CaV2.2 Channels Is Prevented by the Gβ1 Subunit in Rat Sympathetic Neurons

Voltage-dependent regulation of Ca_V2.2 channels by G-proteins is performed by the β (Gβ) subunit. Most studies of regulation by G-proteins have focused on channel activation; however, little is known regarding channel inactivation. This study investigated inactivation of Ca_V2.2 channels in superior cervical ganglion neurons that overexpressed Gβ subunits. Ca_V2.2 currents were recorded by whole-cell patch clamping configuration. We found that the Gβ₁ subunit reduced inactivation, while Gβ₅ subunit did not alter at all inactivation kinetics compared to control recordings. Ca_V2.2 current decay in control neurons consisted of both fast and slow inactivation; however, Gβ₁-overexpressing neurons displayed only the slow inactivation. Fast inactivation was restored by a strong depolarization of Gβ₁-overexpressing neurons, therefore, through a voltage-dependent mechanism. The Gβ₁ subunit shifted the voltage dependence of inactivation to more positive voltages and reduced the fraction of Ca_V2.2 channels resting in the inactivated state. These results support that the Gβ₁ subunit inhibits the fast inactivation of Ca_V2.2 channels in SCG neurons. They explain the long-observed sustained Ca²⁺ current under G-protein modulation.

Voltage-Independent Inhibition of the Tetrodotoxin-Sensitive Sodium Currents by Oxotremorine and Angiotensin II in Rat Sympathetic Neurons

Tetrodotoxin-sensitive Na(+) currents have been extensively studied because they play a major role in neuronal firing and bursting. In this study, we showed that voltage-dependent Na(+) currents are regulated in a slow manner by oxotremorine (oxo-M) and angiotensin II in rat sympathetic neurons. We found that these currents can be readily inhibited through a signaling pathway mediated by G proteins and phospholipase C (PLC) β1. This inhibition is slowly established, pertussis toxin-insensitive, partially reversed within tens of seconds after oxo-M washout, and not relieved by a strong depolarization, suggesting a voltage-insensitive mechanism of inhibition. Specificity of the M1 receptor was tested by the MT-7 toxin. Activation and inactivation curves showed no shift in the voltage dependency under the inhibition by oxo-M. This inhibition is blocked by a PLC inhibitor (U73122, 1-(6-{[(17β)-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2,5-dione), and recovery from inhibition is prevented by wortmannin, a PI3/4 kinase inhibitor. Hence, the pathway involves Gq/11 and is mediated by a diffusible second messenger. Oxo-M inhibition is occluded by screening phosphatidylinositol 4,5-bisphosphate (PIP2)-negative charges with poly-l-lysine and prevented by intracellular dialysis with a PIP2 analog. In addition, bisindolylmaleimide I, a specific ATP-competitive protein kinase C (PKC) inhibitor, rules out that this inhibition may be mediated by this protein kinase. Furthermore, oxo-M-induced suppression of Na(+) currents remains unchanged when neurons are treated with calphostin C, a PKC inhibitor that targets the diacylglycerol-binding site of the kinase. These results support a general mechanism of Na(+) current inhibition that is widely present in excitable cells through modulation of ion channels by specific G protein-coupled receptors.

Gβ₂ mimics activation kinetic slowing of CaV2.2 channels by noradrenaline in rat sympathetic neurons

Several neurotransmitters and hormones acting through G protein-coupled receptors elicit a voltage-dependent regulation of CaV2.2 channels, having profound effects on cell function and the organism. It has been hypothesized that protein-protein interactions define specificity in signal transduction. Yet it is unknown how the molecular interactions in an intracellular signaling cascade determine the specificity of the voltage-dependent regulation induced by a specific neurotransmitter. It has been suspected that specific effector regions on the Gβ subunits of the G proteins are responsible for voltage-dependent regulation. The present study examines whether a neurotransmitter's specificity can be revealed by simple ion-current kinetic analysis likely resulting from interactions between Gβ subunits and the channel-molecule. Noradrenaline is a neurotransmitter that induces voltage-dependent regulation. By using biochemical and patch-clamp methods in rat sympathetic neurons we examined calcium current modulation induced by each of the five Gβ subunits and found that Gβ2 mimics activation kinetic slowing of CaV2.2 channels by noradrenaline. Furthermore, overexpression of the Gβ2 isoform reproduces the effect of noradrenaline in the willing-reluctant model. These results advance our understanding on the mechanisms by which signals conveying from a variety of membrane receptors are able to display precise homeostatic responses.

A Conus regularis conotoxin with a novel eight-cysteine framework inhibits CaV2.2 channels and displays an anti-nociceptive activity

A novel peptide, RsXXIVA, was isolated from the venom duct of Conus regularis, a worm-hunting species collected in the Sea of Cortez, México. Its primary structure was determined by mass spectrometry and confirmed by automated Edman degradation. This conotoxin contains 40 amino acids and exhibits a novel arrangement of eight cysteine residues (C-C-C-C-CC-CC). Surprisingly, two loops of the novel peptide are highly identical to the amino acids sequence of ω-MVIIA. The total length and disulfide pairing of both peptides are quite different, although the two most important residues for the described function of ω-MVIIA (Lys2 and Tyr13) are also present in the peptide reported here. Electrophysiological analysis using superior cervical ganglion (SCG) neurons indicates that RsXXIVA inhibits Ca_V2.2 channel current in a dose-dependent manner with an EC₅₀ of 2.8 μM, whose effect is partially reversed after washing. Furthermore, RsXXIVA was tested in hot-plate assays to measure the potential anti-nociceptive effect to an acute thermal stimulus, showing an analgesic effect in acute thermal pain at 30 and 45 min post-injection. Also, the toxin shows an anti-nociceptive effect in a formalin chronic pain test. However, the low affinity for Ca_V2.2 suggests that the primary target of the peptide could be different from that of ω-MVIIA.

A novel CaV2.2 channel inhibition by piracetam in peripheral and central neurons

No mechanistic actions for piracetam have been documented to support its nootropic effects. Voltage-gated calcium channels have been proposed as a promising pharmacological target of nootropic drugs. In this study, we investigated the effect of piracetam on Ca(V)2.2 channels in peripheral neurons, using patch-clamp recordings from cultured superior cervical ganglion neurons. In addition, we tested if Ca(V)2.2 channel inhibition could be related with the effects of piracetam on central neurons. We found that piracetam inhibited native Ca(V)2.2 channels in superior cervical ganglion neurons in a dose-dependent manner, with an IC(50) of 3.4 μmol/L and a Hill coefficient of 1.1. GDPβS dialysis did not prevent piracetam-induced inhibition of Ca(V)2.2 channels and G-protein-coupled receptor activation by noradrenaline did not occlude the piracetam effect. Piracetam altered the biophysical characteristics of Ca(V)2.2 channel such as facilitation ratio. In hippocampal slices, piracetam and ω-conotoxin GVIA diminished the frequency of excitatory postsynaptic potentials and action potentials. Our results provide evidence of piracetam's actions on Ca(V)2.2 channels in peripheral neurons, which might explain some of its nootropic effects in central neurons.

Voltage-independent inhibition of Ca(V)2.2 channels is delimited to a specific region of the membrane potential in rat SCG neurons

Neurotransmitters and hormones regulate Ca(V)2.2 channels through a voltage-independent pathway which is not well understood. It has been suggested that this voltage-independent inhibition is constant at all membrane voltages. However, changes in the percent of voltage-independent inhibition of Ca(V)2.2 have not been tested within a physiological voltage range. Here, we used a double-pulse protocol to isolate the voltage-independent inhibition of Ca(V)2.2 channels induced by noradrenaline in rat superior cervical ganglion neurons. To assess changes in the percent of the voltage-independent inhibition, the activation voltage of the channels was tested between -40 and +40 mV. We found that the percent of voltage-independent inhibition induced by noradrenaline changed with the activation voltage used. In addition, voltage-independent inhibition induced by oxo-M, a muscarinic agonist, exhibited the same dependence on activation voltage, which supports that this pattern is not exclusive for adrenergic activation. Our results suggested that voltage-independent inhibition of Ca(V)2.2 channels depends on the activation voltage of the channel in a physiological voltage range. This may have relevant implications in the understanding of the mechanism involved in voltage-independent inhibition.

Augmentation of Cav1 channel current and action potential duration after uptake of S100A1 in sympathetic ganglion neurons

S100A1, a 21-kDa dimeric Ca2+-binding protein of the EF-hand type, is expressed in cardiomyocytes and is an important regulator of heart function. During ischemia, cardiomyocytes secrete S100A1 to the extracellular space. Although the effects of extracellular S100A1 have been documented in cardiomyocytes, it is unclear whether S100A1 exerts modulatory effects on other tissues in proximity with cardiac cells. Therefore, we sought to investigate the effects of exogenous S100A1 on Ca2+ signals and electrical properties of superior cervical ganglion (SCG) neurons. Immunostaining and Western blot assays indicated no endogenous S100A1 in SCG neurons. Cultured SCG neurons took up S100A1 when it was present in the extracellular medium. Inside the cell exogenous S100A1 localized in a punctate pattern throughout the cytoplasm but was excluded from the nuclei. S100A1 partially colocalized with markers for both receptor- and non-receptor-mediated endocytosis, indicating that in SCG neurons multiple endocytotic pathways are involved in S100A1 internalization. In compartmentalized SCG cultures, axonal projections were capable of uptake and transport of S100A1 toward the neuronal somas. Exogenous S100A1 applied either extra- or intracellularly enhanced Cav1 channel currents in a PKA-dependent manner, prolonged action potentials, and amplified action potential-induced Ca2+ transients. NMR chemical shift perturbation of Ca2+-S100A1 in the presence of a peptide from the regulatory subunit of PKA verifies that S100A1 directly interacts with PKA, and that this interaction likely occurs in the hydrophobic binding pocket of Ca2+-S100A1. Our results suggest the hypothesis that in sympathetic neurons exogenous S100A1 may lead to an increase of sympathetic output.

Gating charges per channel of Ca(V)2.2 channels are modified by G protein activation in rat sympathetic neurons

It has been suggested that voltage-dependent G protein modulation of Ca(V)2.2 channels is carried out at closed states of the channel. Our purpose was to estimate the number of gating charges of Ca(V)2.2 channel in control and G protein-modulated conditions. By using a Cole-Moore protocol we observed a significant delay in Ca(V)2.2 channel activation according to a transit of the channel through a series of closed states before channel opening. If G protein voltage-dependent modulation were carried out at these closed states, then we would have expected a greater Cole-Moore lag in the presence of a neurotransmitter. This prediction was confirmed for noradrenaline, while no change was observed in the presence of angiotensin II, a voltage-insensitive G protein modulator. We used the limiting slope method for calculation of the gating charge per channel. Effective charge z was 6.32+/-0.65 for Ca(V)2.2 channels in unregulated conditions, while GTPgammaS reduced elementary charge by approximately 4 e(0). Accordingly, increased concentration of noradrenaline induced a gradual decrease on z, indicating that this decrement was due to a G protein voltage-sensitive modulation. This paper shows for the first time a significant and reversible decrease in charge transfer of Ca(V)2.2 channels under G protein modulation, which might depend on the activated G protein inhibitory pathway.

PMA counteracts G protein actions on CaV2.2 channels in rat sympathetic neurons

Protein kinase C (PKC)-induced phosphorylation and G protein-mediated inhibition of Ca(V)2.2 N-type Ca2+ channels counteract exerting opposing modulatory responses at the channel level. At present, the most striking question remaining is whether prominent enhancement of the Ca2+ current (I(Ca)) observed under PKC activation arises from relief of G-protein tonic inhibition. Here, by using patch-clamp methods in superior cervical ganglion (SCG) neurons of rat, we show the following: First, that PKC activation by phorbol-12-myristate-13-acetate (PMA) not only counteracts mutually with noradrenaline (NA) and GTPgammaS-induced I(Ca) inhibition, but also reverses current inhibition by Gbetagamma subunits over-expression. Second, that PMA increases I(Ca) beyond the enhancement expected by sole removal of the G protein-mediated tonic inhibition. Accordingly, PMA increases conductance through N-type Ca2+ channels, unlike the G protein inhibitor GDPbetaS. Together, our results support that PMA-induced phosphorylation produces changes in I(Ca) that cannot be accounted for by prevention of G protein inhibition. They may have important implications in reinterpretation of existing data with PMA. Furthermore, counteracting modulation of ion channels and reversibility within a short time frame are better support for a dynamic system with short-term adaptive responses.

G protein activation inhibits gating charge movement in rat sympathetic neurons

G protein-coupled receptors (GPCRs) control neuronal functions via ion channel modulation. For voltage-gated ion channels, gating charge movement precedes and underlies channel opening. Therefore, we sought to investigate the effects of G protein activation on gating charge movement. Nonlinear capacitive currents were recorded using the whole cell patch-clamp technique in cultured rat sympathetic neurons. Our results show that gating charge movement depends on voltage with average Boltzmann parameters: maximum charge per unit of linear capacitance (Q(max)) = 6.1 +/- 0.6 nC/microF, midpoint (V(h)) = -29.2 +/- 0.5 mV, and measure of steepness (k) = 8.4 +/- 0.4 mV. Intracellular dialysis with GTPgammaS produces a nonreversible approximately 34% decrease in Q(max), a approximately 10 mV shift in V(h), and a approximately 63% increase in k with respect to the control. Norepinephrine induces a approximately 7 mV shift in V(h) and approximately 40% increase in k. Overexpression of G protein beta(1)gamma(4) subunits produces a approximately 13% decrease in Q(max), a approximately 9 mV shift in V(h), and a approximately 28% increase in k. We correlate charge movement modulation with the modulated behavior of voltage-gated channels. Concurrently, G protein activation by transmitters and GTPgammaS also inhibit both Na(+) and N-type Ca(2+) channels. These results reveal an inhibition of gating charge movement by G protein activation that parallels the inhibition of both Na(+) and N-type Ca(2+) currents. We propose that gating charge movement decrement may precede or accompany some forms of GPCR-mediated channel current inhibition or downregulation. This may be a common step in the GPCR-mediated inhibition of distinct populations of voltage-gated ion channels.

Ca2+ signal summation and NFATc1 nuclear translocation in sympathetic ganglion neurons during repetitive action potentials

NFATc-mediated gene expression constitutes a critical step during neuronal development and synaptic plasticity. Although considerable information is available regarding the activation and functionality of specific NFATc isoforms, in neurons little is known about how sensitive NFAT nuclear translocation is to specific patterns of electrical activity. Here we used high-speed fluo-4 confocal imaging to monitor action potential (AP)-induced cytosolic Ca2+ transients in rat sympathetic neurons. We have recorded phasic and repetitive AP patterns, and corresponding Ca2+ transients initiated by either long (100-800 ms) current-clamp pulses, or single brief (2 ms) electrical field stimulation. We address the functional consequences of these AP and Ca2+ transient patterns, by using an adenoviral construct to express NFATc1-CFP and evaluate NFATc1-CFP nuclear translocation in response to specific patterns of electrical activity. Ten Hertz trains stimulation induced nuclear translocation of NFATc1, whereas 1 Hz trains did not. However, 1 Hz train stimulation did result in NFATc1 translocation in the presence of 2 mM Ba2+, which inhibits M-currents and promotes repetitive firing and the accompanying small (approximately 0.6 DeltaF/F0) repetitive and summating Ca2+ transients. Our results demonstrate that M-current inhibition-mediated spike frequency facilitation enhances cytosolic Ca2+ signals and NFATc1 nuclear translocation during trains of low frequency electrical stimulation.

The effect of PKC activity on the TTX-R sodium currents from rat nodose ganglion neurons

To determine how protein kinase C (PKC) activity influences properties of the tetrodotoxin-resistant sodium current (TTX-R I(Na)) in neonatal rat nodose ganglion (NG) neurons, we assessed the effects of phorbol,-12-myristate,13-acetate (PMA), one of the PKC activators, and staurosporine, one of the PKC inhibitors, on the current. PMA (30 and 100 nM) induced an increase in the peak current amplitude of normalized current-voltage curves, a leftward shift in the potential for half activation (V(1/2)) of normalized conductance-voltage curves and a leftward shift of V(1/2) potential for steady-state inactivation curves. The effects of staurosporine (0.1 and 1 muM) on the peak current amplitude and the V(1/2) potential for activation were opposite compared with those seen after PMA application. Staurosporine (1 muM) antagonized PMA (100 nM)-induced modification of TTX-R I(Na). These results suggest that the basal TTX-R I(Na) obtained from neonatal NG neurons is controlled by the level of PKC activity.

Different pathways leading to activation of extracellular signal-regulated kinase and p38 MAP kinase by formyl-methionyl-leucyl-phenylalanine or platelet activating factor in human neutrophils

The signaling pathways leading to extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) activation by N-formyl-Met-Leu-Phe (fMLP) or platelet activating factor (PAF) in human neutrophils were examined. Previously, we found that changes of intracellular Ca2+ ([Ca2+]i) stimulated by PAF and fMLP were due to Ca2+ influx and internal Ca2+ release, respectively. To further determine the mechanism of MAPK activation and its relation with Ca2+ influx, blood from healthy human volunteers was taken by venous puncture. Human polymorphonuclear cells (PMNs) were isolated and incubated with protein kinase C (PKC) inhibitor Calphostin C, PKC-gamma isoform inhibitor GF109203X, phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002, phospholipase C (PLC) inhibitor U73122, phospholipase A2 (PLA2) inhibitor Aristolochic acid, store-operated calcium (SOC) channel inhibitor SKF96365, or extracellular calcium chelator EGTA followed by fMLP or PAF treatment. Phosphorylation of ERK p38 was determined by immunoblotting analysis. Our data indicate that neutrophil MAPK signaling pathways mediated by fMLP and PAF are different. PAF-induced ERK phosphorylation is mediated by PI3K, PKC, PLA2, PLC, and extracellular calcium, whereas fMLP-induced ERK phosphorylation does not involve the PKC-gamma isoform and extracellular calcium. PAF-induced p38 phosphorylation involves PLA2, whereas fMLP-induced p38 activation is PLC dependent.

Activation of metabotropic glutamate receptors inhibits high-voltage-gated calcium channel currents of chicken nucleus magnocellularis neurons

Using whole cell patch-clamp recordings, we pharmacologically characterized the voltage-gated Ca2+ channel (VGCC) currents of chicken nucleus magnocellularis (NM) neurons using barium as the charge carrier. NM neurons possessed both low- and high-voltage-activated Ca2+ channel currents (HVA I(Ba2+)). The N-type channel blocker (omega-conotoxin-GVIA) inhibited more than half of the total HVA I(Ba2+), whereas blockers of L- and P/Q-type channels each inhibited a small fraction of the current. Metabotropic glutamate receptor (mGluR)-mediated modulation of the HVA I(Ba2+) was examined by bath application of glutamate (100 microM), which inhibited the HVA I(Ba2+) by an average of 16%. The inhibitory effect was dose dependent and was partially blocked by omega-conotoxin-GVIA, indicating that mGluRs modulate N and other type HVA I(Ba2+). The nonspecific mGluR agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarbosylic acid (1S,3R-ACPD), mimicked the inhibitory effect of glutamate on HVA I(Ba2+). Group I-III mGluR agonists showed inhibition of the HVA current with the most potent being the group III agonist L(+)-2-amino-4-phosphonobutyric acid. 1S,3R-ACPD (200 microM) had no effect on K+ or Na+ currents. The firing properties of NM neurons were also not altered by 1S,3R-ACPD. We propose that the inhibition of VGCC currents by mGluRs limits depolarization-induced Ca2+ entry into these highly active NM neurons and regulates their Ca2+ homeostasis.