Electrical activity, cAMP, and cytosolic calcium regulate mRNA encoding sodium channel alpha subunits in rat muscle cells

Empagliflozin treatment rescues abnormally reduced Na+ currents in ventricular cardiomyocytes from dystrophin-deficient mdx mice

Cardiac arrhythmias significantly contribute to mortality in Duchenne muscular dystrophy (DMD), a severe muscle illness caused by mutations in the gene encoding for the intracellular protein dystrophin. A major source for arrhythmia vulnerability in patients with DMD is impaired ventricular impulse conduction, which predisposes for ventricular asynchrony, decreased cardiac output, and the development of reentrant circuits. Using the dystrophin-deficient mdx mouse model for human DMD, we previously reported that the lack of dystrophin causes a significant loss of peak Na+ current (INa) in ventricular cardiomyocytes. This finding provided a mechanistic explanation for ventricular conduction defects and concomitant arrhythmias in the dystrophic heart. In the present study, we explored the hypothesis that empagliflozin (EMPA), an inhibitor of sodium/glucose cotransporter 2 in clinical use to treat type II diabetes and nondiabetic heart failure, rescues peak INa loss in dystrophin-deficient ventricular cardiomyocytes. We found that INa of cardiomyocytes derived from mdx mice, which had received clinically relevant doses of EMPA for 4 wk, was restored to wild-type level. Moreover, incubation of isolated mdx ventricular cardiomyocytes with 1 µM EMPA for 24 h significantly increased their peak INa. This effect was independent of Na+-H+ exchanger 1 inhibition by the drug. Our findings imply that EMPA treatment can rescue abnormally reduced peak INa of dystrophin-deficient ventricular cardiomyocytes. Long-term EMPA administration may diminish arrhythmia vulnerability in patients with DMD.NEW & NOTEWORTHY Dystrophin deficiency in cardiomyocytes leads to abnormally reduced Na+ currents. These can be rescued by long-term empagliflozin treatment.

Osmoregulation and the Hypothalamic Supraoptic Nucleus: From Genes to Functions

Due to the relatively high permeability to water of the plasma membrane, water tends to equilibrate its chemical potential gradient between the intra and extracellular compartments. Because of this, changes in osmolality of the extracellular fluid are accompanied by changes in the cell volume. Therefore, osmoregulatory mechanisms have evolved to keep the tonicity of the extracellular compartment within strict limits. This review focuses on the following aspects of osmoregulation: 1) the general problems in adjusting the "milieu interieur" to challenges imposed by water imbalance, with emphasis on conceptual aspects of osmosis and cell volume regulation; 2) osmosensation and the hypothalamic supraoptic nucleus (SON), starting with analysis of the electrophysiological responses of the magnocellular neurosecretory cells (MNCs) involved in the osmoreception phenomenon; 3) transcriptomic plasticity of SON during sustained hyperosmolality, to pinpoint the genes coding membrane channels and transporters already shown to participate in the osmosensation and new candidates that may have their role further investigated in this process, with emphasis on those expressed in the MNCs, discussing the relationships of hydration state, gene expression, and MNCs electrical activity; and 4) somatodendritic release of neuropeptides in relation to osmoregulation. Finally, we expect that by stressing the relationship between gene expression and the electrical activity of MNCs, studies about the newly discovered plastic-regulated genes that code channels and transporters in the SON may emerge.

Low-frequency Stimulation Decreases Hyperexcitability Through Adenosine A1 Receptors in the Hippocampus of Kindled Rats

INTRODUCTION

In this study, the role of A1 adenosine receptors in improving the effect of Low-Frequency Electrical Stimulation (LFS) on seizure-induced hyperexcitability of hippocampal CA1 pyramidal neurons was investigated.

METHODS

A semi-rapid hippocampal kindling model was used to induce seizures in male Wistar rats. Examination of the electrophysiological properties of CA1 pyramidal neurons of the hippocampus using whole-cell patch-clamp recording 48 h after the last kindling stimulation revealed that the application of LFS as two packages of stimulations at a time interval of 6 h for two consecutive days could significantly restore the excitability CA1 pyramidal neurons evidenced by a decreased in the of the number of evoked action potentials and enhancement of amplitude, maximum rise slope and decay slope of the first evoked action potential, rheobase, utilization time, adaptation index, first-spike latency, and post-AHP amplitude. Selective locked of A1 receptors by the administration of 8-Cyclopentyl-1,3-dimethylxanthine (1 μM, 1 μl, i.c.v.) before applying each LFS package, significantly reduced LFS effectiveness in recovering these parameters.

RESULTS

On the other hand, selective activation of A1 receptors by an injection of N6-cyclohexyladenosine (10 μM, 1 μl, i.c.v.), instead of LFS application, could imitate LFS function in improving these parameters.

CONCLUSION

It is suggested that LFS exerts its efficacy on reducing the neuronal excitability, partially by activating the adenosine system and activating its A1 receptors.

Functional interactions of ion channels with the actin cytoskeleton: does coupling to dynamic actin regulate NMDA receptors?

Synapses are enriched in the cytoskeletal protein actin, which determines the shape of the pre- and postsynaptic compartments, organizes the neurotransmitter release machinery, and provides a framework for trafficking of components. In the postsynaptic compartment, interactions with actin or its associated proteins are also critical for the localization and activity of synaptic neurotransmitter receptors and ion channels. Actin binding proteins, including spectrin and α-actinin, serve as molecular linkages between the actin cytoskeleton and a diverse collection of receptors, including the NMDA receptor (NMDAR) and voltage-gated Na⁺ channels. The actin cytoskeleton can regulate neurotransmitter receptors and ion channels by controlling their trafficking and localization at the synapse and by directly gating receptor channel opening. We highlight evidence that synaptic actin couples physically and functionally to the NMDAR and supports its activity. The molecular mechanisms by which actin regulates NMDARs are only just emerging, and recent advancements in light and electron microscopy-based imaging techniques should aide in elucidating these mechanisms.

Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity

Recent papers have attributed arrhythmic substrate in murine RyR2‐P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)‐mediated ryanodine receptor‐2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff‐perfused wild‐type mouse ventricles before pharmacological intervention. The Epac activator 8‐CPT (8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine‐3′,5′‐cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8‐CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8‐CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8‐CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8‐CPT but not dantrolene, whether alone or combined with 8‐CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.

Epac-induced ryanodine receptor type 2 activation inhibits sodium currents in atrial and ventricular murine cardiomyocytes

Acute RyR2 activation by exchange protein directly activated by cAMP (Epac) reversibly perturbs myocyte Ca²⁺ homeostasis, slows myocardial action potential conduction, and exerts pro‐arrhythmic effects. Loose patch‐clamp studies, preserving in vivo extracellular and intracellular conditions, investigated Na⁺ current in intact cardiomyocytes in murine atrial and ventricular preparations following Epac activation. Depolarising steps to varying test voltages activated typical voltage‐dependent Na⁺ currents. Plots of peak current against depolarisation from resting potential gave pretreatment maximum atrial and ventricular currents of −20.23 ± 1.48 (17) and −29.8 ± 2.4 (10) pA/μm² (mean ± SEM [n]). Challenge by 8‐CPT (1 μmol/L) reduced these currents to −11.21 ± 0.91 (12) (P < .004) and −19.3 ± 1.6 (11) pA/μm² (P < .04) respectively. Currents following further addition of the RyR2 inhibitor dantrolene (10 μmol/L) (−19.91 ± 2.84 (13) and −26.6 ± 1.7 (17)), and dantrolene whether alone (−19.53 ± 1.97 (8) and −27.6 ± 1.9 (14)) or combined with 8‐CPT (−19.93 ± 2.59 (12) and −29.9 ± 2.5(11)), were indistinguishable from pretreatment values (all P >> .05). Assessment of the inactivation that followed by applying subsequent steps to a fixed voltage 100 mV positive to resting potential gave concordant results. Half‐maximal inactivation voltages and steepness factors, and time constants for Na⁺ current recovery from inactivation in double‐pulse experiments, were similar through all the pharmacological conditions. Intracellular sharp microelectrode membrane potential recordings in intact Langendorff‐perfused preparations demonstrated concordant variations in maximum rates of atrial and ventricular action potential upstroke, (dV/dt)_max. We thus demonstrate an acute, reversible, Na⁺ channel inhibition offering a possible mechanism for previously reported pro‐arrhythmic slowing of AP propagation following modifications of Ca²⁺ homeostasis, complementing earlier findings from chronic alterations in Ca²⁺ homeostasis in genetically‐modified RyR2‐P2328S hearts.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Flecainide exerts paradoxical effects on sodium currents and atrial arrhythmia in murine RyR2-P2328S hearts

Aims

Cardiac ryanodine receptor mutations are associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), and some, including RyR2-P2328S, also predispose to atrial fibrillation. Recent work associates reduced atrial Na_v1.5 currents in homozygous RyR2-P2328S (RyR2^S/S) mice with slowed conduction and increased arrhythmogenicity. Yet clinically, and in murine models, the Na_v1.5 blocker flecainide reduces ventricular arrhythmogenicity in CPVT. We aimed to determine whether, and how, flecainide influences atrial arrhythmogenicity in RyR2^S/S mice and their wild-type (WT) littermates.

Methods

We explored effects of 1 μm flecainide on WT and RyR2^S/S atria. Arrhythmic incidence, action potential (AP) conduction velocity (CV), atrial effective refractory period (AERP) and AP wavelength (λ = CV × AERP) were measured using multi-electrode array recordings in Langendorff-perfused hearts; Na⁺ currents (I_Na) were recorded using loose patch clamping of superfused atria.

Results

RyR2^S/S showed more frequent atrial arrhythmias, slower CV, reduced I_Na and unchanged AERP compared to WT. Flecainide was anti-arrhythmic in RyR2^S/S but pro-arrhythmic in WT. It increased I_Na in RyR2^S/S atria, whereas it reduced I_Na as expected in WT. It increased AERP while sparing CV in RyR2^S/S, but reduced CV while sparing AERP in WT. Thus, RyR2^S/S hearts have low λ relative to WT; flecainide then increases λ in RyR2^S/S but decreases λ in WT.

Conclusions

Flecainide (1 μm) rescues the RyR2-P2328S atrial arrhythmogenic phenotype by restoring compromised I_Na and λ, changes recently attributed to increased sarcoplasmic reticular Ca²⁺ release. This contrasts with the increased arrhythmic incidence and reduced I_Na and λ with flecainide in WT.

In vitro Differentiation of Functional Human Skeletal Myotubes in a Defined System

In vitro human skeletal muscle systems are valuable tools for the study of human muscular development, disease and treatment. However, published in vitro human muscle systems have so far only demonstrated limited differentiation capacities. Advanced differentiation features such as cross-striations and contractility have only been observed in co-cultures with motoneurons. Furthermore, it is commonly regarded that cultured human myotubes do not spontaneously contract, and any contraction has been considered to originate from innervation. This study developed a serum-free culture system in which human skeletal myotubes demonstrated advanced differentiation. Characterization by immunocytochemistry, electrophysiology and analysis of contractile function revealed these major features: A) well defined sarcomeric development, as demonstrated by the presence of cross-striations. B) finely developed excitation-contraction coupling apparatus characterized by the close apposition of dihydropyridine receptors on T-tubules and Ryanodine receptors on sarcoplasmic reticulum membranes. C) spontaneous and electrically controlled contractility. This report not only demonstrates an improved level of differentiation of cultured human skeletal myotubes, but also provides the first published evidence that such myotubes are capable of spontaneous contraction. Use of this functional in vitro human skeletal muscle system would advance studies concerning human skeletal muscle development and physiology, as well as muscle-related disease and therapy.

Loss of Nav1.5 expression and function in murine atria containing the RyR2-P2328S gain-of-function mutation

AIMS

Recent studies reported slowed conduction velocity (CV) in murine hearts homozygous for the gain-of-function RyR2-P2328S mutation (RyR2(S/S)) and associated this with an increased incidence of atrial and ventricular arrhythmias. The present experiments determined mechanisms contributing to the reduced atrial CV.

METHODS AND RESULTS

The determinants of CV were investigated in murine RyR2(S/S) hearts and compared with those in wild-type (WT) and slow-conducting Scn5a(+/-) hearts. Picrosirius red staining demonstrated increased fibrosis only in Scn5a(+/-) hearts. Immunoblot assays showed similar expressions of Cx43 and Cx40 levels in the three genotypes. In contrast, Nav1.5 expression was reduced in both RyR2(S/S) and Scn5a(+/-) atria. These findings correlated with intracellular microelectrode and loose-patch-clamp studies. Microelectrode measurements showed reduced maximum rates of depolarization in Scn5a(+/-) and RyR2(S/S) atria compared with WT, despite similar diastolic membrane potentials. Loose-patch-clamp measurements demonstrated reduced peak Na(+) currents (INa) in the Scn5a(+/-) and RyR2(S/S) atria relative to WT, with similar normalized current-voltage relationships. In WT atria, reduction in INa could be produced by treatment with high extracellular Ca(2+), caffeine, or cyclopiazonic acid, each expected to produce an acute increase in [Ca(2+)]i.

CONCLUSION

RyR2(S/S) atria show reduced levels of Nav1.5 expression and Na(+) channel function. Reduced Na(+) channel function was also observed in WT atria, following acute increases in [Ca(2+)]i. Taken together, the results suggest that raised [Ca(2+)]i produces both acute and chronic inhibition of Na(+) channel function. These findings may help explain the relationship between altered Ca(2+) homeostasis, CV, and the maintenance of common arrhythmias such as atrial fibrillation.

Determinants of myocardial conduction velocity: implications for arrhythmogenesis

Slowed myocardial conduction velocity (θ) is associated with an increased risk of re-entrant excitation, predisposing to cardiac arrhythmia. θ is determined by the ion channel and physical properties of cardiac myocytes and by their interconnections. Thus, θ is closely related to the maximum rate of action potential (AP) depolarization [(dV/dt)_max], as determined by the fast Na⁺ current (I_Na); the axial resistance (r_a) to local circuit current flow between cells; their membrane capacitances (c_m); and to the geometrical relationship between successive myocytes within cardiac tissue. These determinants are altered by a wide range of pathophysiological conditions. Firstly, I_Na is reduced by the impaired Na⁺ channel function that arises clinically during heart failure, ischemia, tachycardia, and following treatment with class I antiarrhythmic drugs. Such reductions also arise as a consequence of mutations in SCN5A such as those occurring in Lenègre disease, Brugada syndrome (BrS), sick sinus syndrome, and atrial fibrillation (AF). Secondly, r_a, may be increased due to gap junction decoupling following ischemia, ventricular hypertrophy, and heart failure, or as a result of mutations in CJA5 found in idiopathic AF and atrial standstill. Finally, either r_a or c_m could potentially be altered by fibrotic change through the resultant decoupling of myocyte–myocyte connections and coupling of myocytes with fibroblasts. Such changes are observed in myocardial infarction and cardiomyopathy or following mutations in MHC403 and SCN5A resulting in hypertrophic cardiomyopathy (HCM) or Lenègre disease, respectively. This review defines and quantifies the determinants of θ and summarizes experimental evidence that links changes in these determinants with reduced myocardial θ and arrhythmogenesis. It thereby identifies the diverse pathophysiological conditions in which abnormal θ may contribute to arrhythmia.

The axon initial segment in nervous system disease and injury

The axon initial segment (AIS), with its dense clusters of voltage-gated ion channels decorating the axonal membrane, regulates action potential initiation and modulation. The AIS also functions as a barrier to maintain axodendritic polarity, and its precise axonal location contributes to the fine-tuning of neuronal excitability. Therefore, it is not surprising that mutations in AIS-related genes, disruption of the molecular organization of the AIS and altered AIS ion channel expression, function, location and/or density are emerging as key players in neurological disorders. Here, we consider the role of the AIS in nervous system disease and injury.

Sodium channels, the electrogenisome and the electrogenistat: lessons and questions from the clinic

In the six decades that have followed the work of Hodgkin and Huxley, multiple generations of neuroscientists and biophysicists have built upon their pivotal contributions. It is now clear that, in mammals, nine genes encode nine distinct voltage-gated sodium channels with different amino acid sequences and different physiological and pharmacological properties. The different sodium channel isoforms produce a multiplicity of distinct sodium currents with different time-dependent characteristics and voltage dependencies, which interact with each other and with the currents produced by other channels (including calcium and potassium channels) to shape neuronal firing patterns. Expression of these sodium channel isoforms is highly dynamic, both in the normal nervous system, and in the injured nervous system. Recent research has shed light on the roles of sodium channels in human disease, a development that may open up new therapeutic strategies. This article examines the pain-signalling system as an example of a neuronal network where multiple sodium channel isoforms play complementary roles in electrogenesis and a strong link with human disease has been established. Recent research suggests that it may be possible to target specific sodium channel isoforms that drive hyperexcitability in pain-signalling neurons, thereby providing new therapeutic strategies for chronic pain, and providing an illustration of the impact of the Hodgkin-Huxley legacy in the clinical domain.

INa and IKir are reduced in Type 1 hypokalemic and thyrotoxic periodic paralysis

We evaluated voltage-gated Na(+) (I(Na)) and inward rectifier K(+) (I(Kir)) currents and Na(+) conductance (G(Na)) in patients with Type 1 hypokalemic (HOPP) and thyrotoxic periodic paralysis (TPP). We studied intercostal muscle fibers from five subjects with HOPP and one with TPP. TPP was studied when the patient was thyrotoxic (T-toxic) and euthyroid. We measured: (1) I(Kir), (2) action potential thresholds, (3) I(Na), (4) G(Na), (5) intracellular [Ca(2+)], and (6) histochemical fiber type. HOPP fibers had lower I(Na), G(Na), and I(Kir) and increased action potential thresholds. Paralytic attack frequency correlated with the action potential threshold, G(Na) and I(Na), but not with I(Kir). G(Na), I(Na), and [Ca(2+)] varied with fiber type. HOPP fibers had increased [Ca(2+)]. The subject with TPP had values for G(Na), I(Na), action potential threshold, I(Kir), and [Ca(2+)] that were similar to HOPP when T-toxic and to controls when euthyroid. HOPP T-toxic TPP fibers had altered G(Na), I(Na), and I(Kir) associated with elevation in [Ca(2+)].

Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis

PURPOSE

Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity-dependent plasticity of hippocampal circuits.

METHODS

We studied kindling, a form of abnormal activity-dependent facilitation. We investigated: (1) sodium channel protein expression by immunocytochemistry and sodium channel messenger RNA (mRNA) by in situ hybridization, (2) sodium current by patch clamp recordings, and (3) rate of kindling by analysis of seizure behavior. The initiation, development, and expression of kindling in wild-type mice were compared to Nav1.6 +/-med(tg) mice, which have reduced expression of Nav1.6.

RESULTS

We found that kindling was associated with increased expression of Nav1.6 protein and mRNA, which occurred selectively in hippocampal CA3 neurons. Hippocampal CA3 neurons also showed increased persistent sodium current in kindled animals compared to sham-kindled controls. Conversely, Nav1.6 +/-med(tg) mice resisted the initiation and development of kindling.

DISCUSSION

These findings suggest an important mechanism for enhanced excitability, in which Nav1.6 may participate in a self-reinforcing cycle of activity-dependent facilitation in the hippocampus. This mechanism could contribute to both normal hippocampal function and to epilepsy and other common nervous system disorders.

Pumilio binds para mRNA and requires Nanos and Brat to regulate sodium current in Drosophila motoneurons

Homeostatic regulation of ionic currents is of paramount importance during periods of synaptic growth or remodeling. Our previous work has identified the translational repressor Pumilio (Pum) as a regulator of sodium current (I(Na)) and excitability in Drosophila motoneurons. In this current study, we show that Pum is able to bind directly the mRNA encoding the Drosophila voltage-gated sodium channel paralytic (para). We identify a putative binding site for Pum in the 3' end of the para open reading frame (ORF). Characterization of the mechanism of action of Pum, using whole-cell patch clamp and real-time reverse transcription-PCR, reveals that the full-length protein is required for translational repression of para mRNA. Additionally, the cofactor Nanos is essential for Pum-dependent para repression, whereas the requirement for Brain Tumor (Brat) is cell type specific. Thus, Pum-dependent regulation of I(Na) in motoneurons requires both Nanos and Brat, whereas regulation in other neuronal types seemingly requires only Nanos but not Brat. We also show that Pum is able to reduce the level of nanos mRNA and as such identify a potential negative-feedback mechanism to protect neurons from overactivity of Pum. Finally, we show coupling between I(Na) (para) and I(K) (Shal) such that Pum-mediated change in para results in a compensatory change in Shal. The identification of para as a direct target of Pum represents the first ion channel to be translationally regulated by this repressor and the location of the binding motif is the first example in an ORF rather than in the canonical 3'-untranslated region of target transcripts.

Activity-dependent regulation of voltage-gated Na+ channel expression in Mat-LyLu rat prostate cancer cell line

We have shown previously that voltage-gated Na(+) channels (VGSCs) are up-regulated in human metastatic disease (prostate, breast and small-cell lung cancers), and that VGSC activity potentiates metastatic cell behaviours. However, the mechanism(s) regulating functional VGSC expression in cancer cells remains unknown. We investigated the possibility of activity-dependent (auto)regulation of VGSC functional expression in the strongly metastatic Mat-LyLu model of rat prostate cancer. Pretreatment with tetrodotoxin (TTX) for 24-72 h subsequently suppressed peak VGSC current density without affecting voltage dependence. The hypothesis was tested that the VGSC auto-regulation occurred via VGSC-mediated Na(+) influx and subsequent activation of protein kinase A (PKA). Indeed, TTX pretreatment reduced the level of phosphorylated PKA, and the PKA inhibitor KT5720 decreased, whilst the adenylate cyclase activator forskolin and the Na(+) ionophore monensin both increased the peak VGSC current density. TTX reduced the mRNA level of Nav1.7, predominant in these cells, and VGSC protein expression at the plasma membrane, although the total VGSC protein level remained unchanged. TTX pretreatment eliminated the VGSC-dependent component of the cells' migration in Transwell assays. We concluded that the VGSC activity in Mat-LyLu rat prostate cancer cells was up-regulated in steady-state via a positive feedback mechanism involving PKA, and this enhanced the cells' migratory potential.

Determinants of electrical properties in developing neurons

The regulatory mechanisms that orchestrate the developmental acquisition of electrical properties in embryonic neurons are poorly understood. Progress in this important area is dependent on the availability of preparations that allow electrophysiology to be married with genetics. The powerful genetics of the fruitfly Drosophila melanogaster has long been exploited to describe fundamental mechanisms associated with structural neuronal development (i.e. axon guidance). It has not, however, been fully employed to study the final stages of embryonic neural development and in particular the acquisition of electrical activity. This review focuses on the recent development of a Drosophila preparation that allows central neurons to be accessed by patch electrodes at both embryonic and larval stages. This preparation, which allows electrophysiology to be coupled with genetics, offers the prospect of making significant advances in our understanding of functional neuron development.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Regulation of muscle Cav1.1 channels by long-term depolarization involves proteolysis of the alpha1s subunit

The effects of long-term depolarization on frog skeletal muscle Cav1.1 channels were assessed. Voltage-clamp and Western-blot experiments revealed that long-term depolarization brings about a drastic reduction in the amplitude of currents flowing through Cav1.1 channels and in the levels of the alpha1s subunit, the main subunit of muscle L-type channels. The decline of both phenomena was prevented by the action of the protease inhibitors E64 (50 microM) and leupeptin (50 microM). In contrast, long-term depolarization had no effect on beta1, the auxiliary subunit of alpha1s. The levels of mRNAs coding the alpha1s and the beta1 subunits were measured by RNase protection assays. Neither the content of the alpha1s nor the beta1 subunit mRNAs were affected by long-term depolarization, indicating that the synthesis of Cav1.1 channels remained unaffected. Taken together, our experiments suggest that the reduction in the amplitude of membrane currents and in the alpha1s subunit levels is caused by increased degradation of this subunit by a Ca2+-dependent protease.

Cerebellar dysfunction in multiple sclerosis: evidence for an acquired channelopathy

Cerebellar dysfunction in multiple sclerosis (MS) is a significant contributor to disability, is relatively refractory to symptomatic therapy, and often progresses despite treatment with disease-modifying agents. Thus, there is a need for better understanding of its pathophysiology. This chapter reviews a growing body of evidence which suggests that mis-tuning of Purkinje cells, due to expression of an abnormal repertoire of sodium channels, contributes to cerebellar deficits in MS. Within the normal nervous system, sodium channel Na(v)1.8 is expressed in a highly specific manner within spinal sensory and trigeminal neurons, and is not present within Purkinje cells, Na(v)1.8 mRNA and protein are, however, expressed within Purkinje cells both in models of MS (experimenal autoimmume encephalomyelitis; EAE), and in postmortem tissue from humans with MS. Expression of Na(v)1.8 within Purkinje cells in vitro alters electrogenesis in these cells in several ways: first, by increasing duration and amplitude of action potentials; second, by decreasing the proportion of action potentials that are conglomerate and the number of spikes per conglomerate action potential; and third, by supporting sustained, pacemaker-like impulse trains in response to depolarization, which are not seen in the absence of Na(v)1.8. Similar changes are observed in recordings from Purkinje cells in vivo from mice with EAE. Taken together, these results suggest that expression of Na(v)1.8 within Purkinje cells distorts their pattern of firing in MS.

The effects of intracellular Ca2+ on cardiac K+ channel expression and activity: novel insights from genetically altered mice

We tested the hypothesis that chronic changes in intracellular Ca(2+) (Ca(2+)(i)) can result in changes in ion channel expression; this represents a novel mechanism of crosstalk between changes in Ca(2+) cycling proteins and the cardiac action potential (AP) profile. We used a transgenic mouse with cardiac-specific overexpression of sarcoplasmic reticulum Ca(2+) ATPase (SERCA) isoform 1a (SERCA1a OE) with a significant alteration of SERCA protein levels without cardiac hypertrophy or failure. Here, we report significant changes in the expression of a transient outward K(+) current (I(to,f)), a slowly inactivating K(+) current (I(K,slow)) and the steady state current (I(SS)) in the transgenic mice with resultant prolongation in cardiac action potential duration (APD) compared with the wild-type littermates. In addition, there was a significant prolongation of the QT interval on surface electrocardiograms in SERCA1a OE mice. The electrophysiological changes, which correlated with changes in Ca(2+)(i), were further corroborated by measuring the levels of ion channel protein expression. To recapitulate the in vivo experiments, the effects of changes in Ca(2+)(i) on ion channel expression were further tested in cultured adult and neonatal mouse cardiac myocytes. We conclude that a primary defect in Ca(2+) handling proteins without cardiac hypertrophy or failure may produce profound changes in K(+) channel expression and activity as well as cardiac AP.

Activity-dependent regulation of the potassium channel subunits Kv1.1 and Kv3.1

Afferent activity, especially in young animals, can have profound influences on postsynaptic neuronal structure, function and metabolic processes. Most studies evaluating activity regulation of cellular components have examined the expression of ubiquitous cellular proteins as opposed to molecules that are specialized in the neurons of interest. Here we consider the regulation of two proteins (voltage-gated potassium channel subunits Kv1.1 and Kv3.1) that auditory brainstem neurons in birds and mammals express at uniquely high levels. Unilateral removal of the avian cochlea leads to rapid and dramatic reduction in the expression of both proteins in the nucleus magnocellularis (NM; a division of the avian cochlear nucleus) neurons as detected by immunocytochemistry. Uniform downregulation of Kv1.1 was reliable by 3 hours after cochlea removal, was sustained through 96 hours, and returned to control levels in the surviving neurons by 2 weeks. The activity-dependent changes in Kv3.1 appear to be bimodal and are more transient, being observed at 3 hours after cochlea removal and recovering to control levels within 24 hours. We also explored the functional properties of Kv1.1 in NM neurons deprived of auditory input for 24 hours by whole-cell recordings. Low-threshold potassium currents in deprived NM neurons were not significantly different from control neurons in their amplitude or sensitivity to dendrotoxin-I, a selective K+ channel antagonist. We conclude that the highly specialized abundant expression of Kv1.1 and 3.1 channel subunits is not permanently regulated by synaptic activity and that changes in overall protein levels do not predict membrane pools.

Fiber type conversion alters inactivation of voltage-dependent sodium currents in murine C2C12 skeletal muscle cells

Each skeletal muscle of the body contains a unique composition of "fast" and "slow" muscle fibers, each of which is specialized for certain challenges. This composition is not static, and the muscle fibers are capable of adapting their molecular composition by altered gene expression (i.e., fiber type conversion). Whereas changes in the expression of contractile proteins and metabolic enzymes in the course of fiber type conversion are well described, little is known about possible adaptations in the electrophysiological properties of skeletal muscle cells. Such adaptations may involve changes in the expression and/or function of ion channels. In this study, we investigated the effects of fast-to-slow fiber type conversion on currents via voltage-gated Na+ channels in the C(2)C(12) murine skeletal muscle cell line. Prolonged treatment of cells with 25 nM of the Ca2+ ionophore A-23187 caused a significant shift in myosin heavy chain isoform expression from the fast toward the slow isoform, indicating fast-to-slow fiber type conversion. Moreover, Na+ current inactivation was significantly altered. Slow inactivation less strongly inhibited the Na+ currents of fast-to-slow fiber type-converted cells. Compared with control cells, the Na+ currents of converted cells were more resistant to block by tetrodotoxin, suggesting enhanced relative expression of the cardiac Na+ channel isoform Na(v)1.5 compared with the skeletal muscle isoform Na(v)1.4. These results imply that fast-to-slow fiber type conversion of skeletal muscle cells involves functional adaptation of their electrophysiological properties.

Patterned electrical activity modulates sodium channel expression in sensory neurons

Peripheral nerve injury induces changes in the level of gene expression for sodium channels Nav1.3, Nav1.8, and Nav1.9 within dorsal root ganglion (DRG) neurons, which may contribute to the development of hyperexcitability, ectopic neuronal discharge, and neuropathic pain. The mechanism of this change in sodium channel expression is unclear. Decreased availability of neurotrophic factors following axotomy contributes to these changes in gene transcription, but the question of whether changes in intrinsic neuronal activity levels alone can trigger changes in the expression of these sodium channels has not been addressed. We examined the effect of electrical stimulation on the expression of Nav1.3, Nav1.8, and Nav1.9 by using cultured embryonic mouse sensory neurons under conditions in which nerve growth factor (NGF) was not limiting. Expression of Nav1.3 was not significantly changed following stimulation. In contrast, we observed activity-dependent down-regulation of Nav1.8 and Nav1.9 mRNA and protein levels after stimulation, as demonstrated by quantitative polymerase chain reaction and immunocytochemistry. These results show that a change in neuronal activity can alter the expression of sodium channel genes in a subtype-specific manner, via a mechanism independent of NGF withdrawal.

Postsynaptic protein kinase A reduces neuronal excitability in response to increased synaptic excitation in the Drosophila CNS

Previous work has identified a role for synaptic activity in the development of excitable properties of motoneurons in the Drosophila embryo. In this study the underlying mechanism that enables two such neurons, termed aCC and RP2, to respond to increased exposure to synaptic excitation is characterized. Synaptic excitation is increased in genetic backgrounds that lack either a cAMP-specific phosphodiesterase (EC:3.1.4, dunce) or acetylcholinesterase (EC:3.1.1.7, ace), the enzyme that terminates the endogenous cholinergic excitation of these motoneurons. Analysis of membrane excitability in aCC/RP2, in either background, shows that these neurons have a significantly reduced capability to fire action potentials (APs) in response to injection of depolarizing current. Analysis of underlying voltage-gated currents show that this effect is associated with a marked reduction in magnitude of the voltage-dependent inward Na+ current (INa). Partially blocking INa in these motoneurons, using low concentrations of TTX, demonstrates that a reduction of INa is, by itself, sufficient to reduce membrane excitability. An analysis of firing implicates an increased AP threshold to underlie the reduction in membrane excitability observed because of heightened exposure to synaptic excitation. Genetic or pharmacological manipulations that either elevate cAMP or increase protein kinase A (PKA) activity in wild-type aCC/RP2 mimic both the reductions in membrane excitability and INa. In comparison, increasing cAMP catabolism or inhibition of PKA activity is sufficient to block the induction of these activity-dependent changes. The induced changes in excitability can be rapid, occurring within 5 min of exposure to a membrane-permeable cAMP analog, indicative that threshold can be regulated in these neurons by a post-translational mechanism that is dependent on phosphorylation.

Visually driven regulation of intrinsic neuronal excitability improves stimulus detection in vivo

Neurons adapt their electrophysiological properties to maintain stable levels of electrical excitability when faced with a constantly changing environment. We find that exposing freely swimming Xenopus tadpoles to 4-5 hr of persistent visual stimulation increases the intrinsic excitability of optic tectal neurons. This increase is correlated with enhanced voltage-gated Na+ currents. The same visual stimulation protocol also induces a polyamine synthesis-dependent reduction in Ca2+-permeable AMPAR-mediated synaptic drive, suggesting that the increased excitability may compensate for this reduction. Accordingly, the change in excitability was prevented by blocking polyamine synthesis during visual stimulation and was rescued when Ca2+-permeable AMPAR-mediated transmission was selectively reduced. The changes in excitability also rendered tectal cells more responsive to synaptic burst stimuli, improving visual stimulus detection. The synaptic and intrinsic adaptations function together to keep tectal neurons within a constant operating range, while making the intact visual system less responsive to background activity yet more sensitive to burst stimuli.

Differential effects of bupivacaine enantiomers, ropivacaine and lidocaine on up-regulation of cell surface voltage-dependent sodium channels in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, (+/-)-bupivacaine inhibited veratridine-induced 22Na(+) influx (IC(50) 6.8 microM). The IC(50) of (+)-bupivacaine (2.8 microM) was 6.2-, 7.4-, and 17.1-fold lower than those of (-)-bupivacaine (17.3 microM), (-)-ropivacaine (20.6 microM), and lidocaine (47.8 microM). Chronic (i.e. 3-h) treatment of cells with (+/-)-bupivacaine increased cell surface [3H]saxitoxin ([3H]STX) binding capacity by 48% (EC(50) of 233 microM; t(1/2)=7.4 h), without changing the K(d) value. Treatment for 24 h with either (+)- or (-)-bupivacaine, or (-)-ropivacaine elevated [3H]STX binding, whereas 24-h treatment with lidocaine had no effect. The rise of [3H]STX binding by (+/-)-bupivacaine was prevented by cycloheximide, an inhibitor of protein synthesis, or brefeldin A, an inhibitor of cell surface vesicular exit from the trans-Golgi network; however, (+/-)-bupivacaine did not increase Na(+) channel alpha- and beta(1)-subunit mRNA levels. In cells subjected to (+/-)-bupivacaine treatment (1 mM for 24 h) followed by 3-h washout, veratridine-induced 22Na(+) influx was enhanced, even when measured in the presence of ouabain, an inhibitor of Na(+),K(+)-ATPase. Ptychodiscus brevis toxin-3 potentiated veratridine-induced 22Na(+) influx by 2.3-fold in the (+/-)-bupivacaine-treated cells, as in non-treated cells. These results suggest that lipophilic bupivacaine enantiomers or (-)-ropivacaine acutely inhibit Na(+) channel gating, whereas its chronic treatment up-regulates cell surface expression of Na(+) channels via translational and externalization events.

Changes in the active membrane properties of rat supraoptic neurones during pregnancy and lactation

To better understand the plasticity of intrinsic membrane properties of supraoptic magnocellular neuroendocrine cells associated with reproductive function, intracellular recordings were performed in oxytocin (OT) and vasopressin (VP) neurones from virgin, late pregnant (E19-22), and lactating (8-12 days of lactation) rats in vitro, using hypothalamic explants. OT neurones from virgin rats displayed a narrower spike width than neurones from pregnant and lactating rats, characterized by faster rise and decay times. Spike width changes in VP neurones were not as prominent as those observed in OT neurones. In OT neurones, the amplitude and the decay of the afterhyperpolarization following spike trains was significantly larger and faster, respectively, in pregnant and lactating rats compared to virgin rats. These properties did not change during pregnancy and lactation in VP neurones. The incidence of the depolarizing afterpotential following spikes significantly increased from approximately 20% in virgin rats to 40-50% during pregnancy and lactation in OT neurones, but was stable (80-90%) across states in VP neurones. Repetitive firing properties (frequency adaptation, the first interspike interval frequency and frequency-current (F-I) relationship) were altered during pregnancy and lactation in OT neurones, but not VP neurones. The increased incidence of depolarizing afterpotentials in OT neurones enhances excitability, while the increased afterhyperpolarization results in suppression of firing rate. Thus, the changes may favour the short bursting activity seen in OT neurones during lactation. These results confirmed reproductive state-dependent changes in intrinsic membrane properties of OT neurones during lactation, and suggest these changes are in place during late pregnancy. This argues that the plasticity in the electrical properties in OT neurones associated with lactation is not instigated by suckling.

Sodium channel expression in hypothalamic osmosensitive neurons in experimental diabetes

Vasopressin is synthesized by neurons in the supraoptic nucleus of the hypothalamus and its release is controlled by action potentials produced by specific subtypes of voltage-gated sodium channels expressed in these neurons. The hyperosmotic state associated with uncontrolled diabetes mellitus causes elevated levels of plasma vasopressin, which are thought to contribute to the pathologic changes of diabetic nephropathy. We demonstrate here that in the rodent streptozotocin model of diabetes there are increases in expression of mRNA and protein for two sodium channel alpha-subunits and two beta-subunits in the neurons of the supraoptic nucleus. Transient and persistent sodium currents show parallel increases in these diabetic neurons. In the setting of chronic uncontrolled diabetes, these changes in sodium channel expression in the supraoptic nucleus may be maladaptive, contributing to the development of secondary renal complications.

Changes in the mRNAs encoding voltage-gated sodium channel types II and III in human epileptic hippocampus

Studies with animal seizure models have indicated that changes in temporal and spatial expression of voltage-gated sodium channels may be important in the pathology of epilepsy. Here, by using in situ hybridisation with previously characterised subtype-selective oligonucleotide probes [Whitaker et al. (2000) J. Comp. Neurol. 422, 123-139], we have compared the cellular expression of all four brain alpha-subunit sodium channel mRNAs in "normal" and epileptic hippocampi from humans. Neuronal cell loss was observed in all regions of the hippocampus of diseased patients, indicating that sclerosis had occurred. Losses of up to 40% compared to post-mortem controls were observed which were statistically significant in all regions studied (dentate gyrus, hilus, and CA1-3). To assess mRNA levels of the different alpha-subtypes in specific subregions, control and diseased tissue sections were hybridised to subtype-specific probes. To quantify any changes in expression while allowing for cell loss, the sections were processed for liquid emulsion autoradiography and grain counts were performed on populations of individual neurones in different subregions. No significant differences were found in the expression of type I and VI mRNAs. In contrast, a significant down-regulation of type II mRNA was observed in the epileptic tissue in the remaining pyramidal cells of CA3 (71+/-7% of control, P<0.01), CA2 (81+/-8% of control, P<0.05) and CA1 (72+/-6% of control, P<0.05) compared with control tissue. Additionally, a significant up-regulation in type III mRNA in epileptic CA4 pyramidal cells (145+/-7% of control, P<0.05) was observed. It is not clear whether these changes play a causal role in human epilepsy or whether they are secondary to seizures or drug treatment; further studies are necessary to investigate these alternatives. However, it is likely that such changes would affect the intrinsic excitability of hippocampal neurones.

Distinct signal transduction pathways for GABA-induced GABA(A) receptor down-regulation and uncoupling in neuronal culture: a role for voltage-gated calcium channels

Changes in GABA receptor (GABA(A)R) gene expression are detected in animal models of epilepsy, anxiety and in post-mortem schizophrenic brain, suggesting a role for GABA(A)R regulation in neurological disorders. Persistent (48 h) exposure of brain neurons in culture to GABA results in down-regulation of GABA(A)R number and uncoupling of GABA and benzodiazepine (BZD) binding sites. Given the central role of GABA(A)Rs in fast inhibitory synaptic transmission, GABA(A)R down-regulation and uncoupling are potentially important mechanisms of regulating neuronal excitability, yet the molecular mechanisms remain unknown. In this report we show that treatment of brain neurons in culture with tetrodotoxin, glutamate receptor antagonists, or depolarization with 25 mM K(+) fails to alter GABA(A)R number or coupling. Changes in neuronal activity or membrane potential are therefore not sufficient to induce either GABA(A)R down-regulation or uncoupling. Nifedipine, a voltage-gated Ca(2+) channel (VGCC) blocker, inhibits both GABA-induced increases in [Ca(2+)](i) and GABA(A)R down-regulation, suggesting that VGCC activation is required for GABA(A)R down-regulation. Depolarization with 25 mM K(+) produces a sustained increase in intracellular [Ca(2+)] without causing GABA(A)R down-regulation, suggesting that activation of VGCCs is not sufficient to produce GABA(A)R down-regulation. In contrast to GABA(A)R down-regulation, nifedipine and 25 mM K(+) fail to inhibit GABA-induced uncoupling, demonstrating that GABA-induced GABA(A)R down-regulation and uncoupling are mediated by independent molecular events. Therefore, GABA(A)R activation initiates at least two distinct signal transduction pathways, one of which involves elevation of intracellular [Ca(2+)] through VGCCs.

The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.

Altered electrical properties in Drosophila neurons developing without synaptic transmission

We examine the role of synaptic activity in the development of identified Drosophila embryonic motorneurons. Synaptic activity was blocked by both pan-neuronal expression of tetanus toxin light chain (TeTxLC) and by reduction of acetylcholine (ACh) using a temperature-sensitive allele of choline acetyltransferase (Cha(ts2)). In the absence of synaptic activity, aCC and RP2 motorneurons develop with an apparently normal morphology and retain their capacity to form synapses. However, blockade of synaptic transmission results in significant changes in the electrical phenotype of these neurons. Specifically, increases are seen in both voltage-gated inward Na(+) and voltage-gated outward K(+) currents. Voltage-gated Ca(2+) currents do not change. The changes in conductances appear to promote neuron excitability. In the absence of synaptic activity, the number of action potentials fired by a depolarizing ramp (-60 to +60 mV) is increased and, in addition, the amplitude of the initial action potential fired is also significantly larger. Silencing synaptic input to just aCC, without affecting inputs to other neurons, demonstrates that the capability to respond to changing levels of synaptic excitation is intrinsic to these neurons. The alteration to electrical properties are not permanent, being reversed by restoration of normal synaptic function. Whereas our data suggest that synaptic activity makes little or no contribution to the initial formation of embryonic neural circuits, the electrical development of neurons that constitute these circuits seems to depend on a process that requires synaptic activity.

Heterogeneous increases of cytoplasmic calcium: distinct effects on down-regulation of cell surface sodium channels and sodium channel subunit mRNA levels

1. Long-term (> or = 12 h) treatment of cultured bovine adrenal chromaffin cells with A23187 (a Ca(2+) ionophore) or thapsigargin (TG) [an inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)] caused a time- and concentration-dependent reduction of cell surface [(3)H]-saxitoxin (STX) binding capacity, but did not change the K:(D:) value. In A23187- or TG-treated cells, veratridine-induced (22)Na(+) influx was reduced (with no change in veratridine EC(50) value) while it was enhanced by alpha-scorpion venom, beta-scorpion venom, or Ptychodiscus brevis toxin-3, like in nontreated cells. 2. The A23187- or TG-induced decrease of [(3)H]-STX binding was diminished by BAPTA-AM. EGTA also inhibited the decreasing effect of A23187. A23187 caused a rapid, monophasic and persistent increase in intracellular concentration of Ca(2+) ([Ca(2+)](i)) to a greater extent than that observed with TG. 2,5-Di-(t-butyl)-1,4-benzohydroquinone (DBHQ) (an inhibitor of SERCA) produced only a rapid monophasic increase in [Ca(2+)](i), without any effect on [(3)H]-STX binding. 3. Reduction in [(3)H]-STX binding capacity induced by A23187 or TG was attenuated by Gö6976 (an inhibitor of conventional protein kinase C) or calpastatin peptide (an inhibitor of calpain). When the internalization rate of cell surface Na(+) channels was measured in the presence of brefeldin A (an inhibitor of vesicular exit from the trans-Golgi network), A23187 or TG accelerated the reduction of [(3)H]-STX binding capacity. 4. Six hours treatment with A23187 lowered Na(+) channel alpha- and beta(1)-subunit mRNA levels, whereas TG had no effect. 5. These results suggest that elevation of [Ca(2+)](i) caused by A23187, TG or DBHQ exerted differential effects on down-regulation of cell surface functional Na(+) channels and Na(+) channel subunit mRNA levels.

Sensory neuron-specific sodium channel SNS is abnormally expressed in the brains of mice with experimental allergic encephalomyelitis and humans with multiple sclerosis

Clinical abnormalities in multiple sclerosis (MS) have classically been considered to be caused by demyelination and/or axonal degeneration; the possibility of molecular changes in neurons, such as the deployment of abnormal repertoires of ion channels that would alter neuronal electrogenic properties, has not been considered. Sensory Neuron-Specific sodium channel SNS displays a depolarized voltage dependence, slower activation and inactivation kinetics, and more rapid recovery from inactivation than classical "fast" sodium channels. SNS is selectively expressed in spinal sensory and trigeminal ganglion neurons within the peripheral nervous system and is not expressed within the normal brain. Here we show that sodium channel SNS mRNA and protein, which are not present within the cerebellum of control mice, are expressed within cerebellar Purkinje cells in a mouse model of MS, chronic relapsing experimental allergic encephalomyelitis. We also demonstrate SNS mRNA and protein expression within Purkinje cells from tissue obtained postmortem from patients with MS, but not in control subjects with no neurological disease. These results demonstrate a change in sodium channel expression in neurons within the brain in an animal model of MS and in humans with MS and suggest that abnormal patterns of neuronal ion channel expression may contribute to clinical abnormalities such as ataxia in these disorders.

Protein kinase C-alpha and -epsilon down-regulate cell surface sodium channels via differential mechanisms in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, our [3H]saxitoxin ([3H]STX) binding, immunoblot, and northern blot analyses specified protein kinase C (PKC) isoform-specific posttranscriptional and posttranslational mechanisms that direct down-regulation of cell surface Na channels. Immunoblot analysis showed that among 11 PKC isoforms, adrenal chromaffin cells contained only conventional (c)PKC-alpha, novel (n)PKC-epsilon, and atypical (a)PKC-zeta. Treatment of adrenal chromaffin cells with 100 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) or 100 nM phorbol 12,13-dibutyrate (PDBu) caused a rapid (<15 min) and sustained (>15 h) translocation of PKC-alpha and -epsilon (but not -zeta) from cytosol to membranes, whereas a biologically inactive 4alpha-TPA had no effect. Thymeleatoxin (TMX), an activator of cPKC, produced similar membrane association of only PKC-alpha at 100 nM, with the potency of TMX being comparable with those of TPA and PDBu. Treatment with either 100 nM TPA or 100 nM TMX reduced cell surface [3H]STX binding to a comparable extent at 3, 6, and 12 h, whereas TPA lowered the binding to a greater extent than TMX at 15, 18, and 24 h; at 15 h, Gö6976, a specific inhibitor of cPKC, completely blocked TMX-induced decrease of [3H]STX binding while preventing by merely 57% TPA-induced decrease of [3H]STX binding. Treatment with 100 nM TPA lowered the Na channel alpha-subunit mRNA level between 3 and 12 h, with its maximum 52% fall at 6 h, and it was accompanied by a subsequent 61 % rise of the beta1-subunit mRNA level at 24 h. Gö6976 failed to prevent TPA-induced reduction of the alpha-subunit mRNA level; TMX did not change the alpha- and beta1-subunit mRNA levels throughout the 24-h treatment. Brefeldin A, an inhibitor of vesicular exit from the trans-Golgi network, augmented TPA- and TMX-induced decrease of [3H]STX binding at 1 and 3 h. Our previous and present studies suggest that PKC down-regulates cell surface Na channels without altering the allosteric gating of Na channels via PKC isoform-specific mechanisms; cPKC-alpha promotes Na channel internalization, whereas nPKC-epsilon decreases the alpha-subunit mRNA level by shortening the half-life of alpha-subunit mRNA without changing its gene transcription.

Changes in Ca(2+) cycling proteins underlie cardiac action potential prolongation in a pressure-overloaded guinea pig model with cardiac hypertrophy and failure

Ventricular arrhythmias are common in both cardiac hypertrophy and failure; cardiac failure in particular is associated with a significant increase in the risk of sudden cardiac death. We studied the electrophysiologic changes in a guinea pig model with aortic banding resulting in cardiac hypertrophy at 4 weeks and progressing to cardiac failure at 8 weeks using whole-cell patch-clamp and biochemical techniques. Action potential durations (APDs) were significantly prolonged in banded animals at 4 and 8 weeks compared with age-matched sham-operated animals. APDs at 50% and 90% repolarization (APD(50) and APD(90) in ms) were the following: 4 week, banded, 208+/-51 and 248+/-49 (n = 15); 4 week, sham, 189+/-68 and 213+/-69 (n = 16); 8 week, banded, 197+/-40 and 226+/-40 (n = 21); and 8 week, sham, 156+/-42 and 189+/-45 (n = 22), respectively; P<0.05 comparing banded versus sham-operated animals. We observed no significant differences in the K(+) currents between the 2 groups of animals at 4 and 8 weeks. However, banded animals exhibited a significant increase in Na(+) and Na(+)-Ca(2+) exchange current densities compared with controls. Furthermore, we have found a significant attenuation in the Ca(2+)-dependent inactivation of the L-type Ca(2+) current in the banded compared with sham-operated animals, likely as a result of the significant downregulation of the sarcoplasmic reticulum Ca(2+) ATPase, which has been documented previously in the heart failure animals. Our data provide an alternate mechanism for APD prolongation in cardiac hypertrophy and failure and support the notion that there is close interaction between Ca(2+) handling and action potential profile.

The neuron as a dynamic electrogenic machine: modulation of sodium-channel expression as a basis for functional plasticity in neurons

Neurons signal each other via regenerative electrical impulses (action potentials) and thus can be thought of as electrogenic machines. Voltage-gated sodium channels produce the depolarizations necessary for action potential activity in most neurons and, in this respect, lie close to the heart of the electrogenic machinery. Although classical neurophysiological doctrine accorded 'the' sodium channel a crucial role in electrogenesis, it is now clear that nearly a dozen genes encode distinct sodium channels with different molecular structures and functional properties, and the majority of these channels are expressed within the mammalian nervous system. The transcription of these sodium-channel genes, and the deployment of the channels that they encode, can change significantly within neurons following various injuries. Moreover, the transcription of these genes and the deployment of various types of sodium channels within neurons of the normal nervous system can change markedly as neurons respond to changing milieus or physiological inputs. As a result of these changes in sodium-channel expression, the membranes of neurons may be retuned so as to alter their transductive and/or encoding properties. Neurons within the normal and injured nervous system can thus function as dynamic electrogenic machines with electroresponsive properties that change not only in response to pathological insults, but also in response to shifting functional needs.

Effect of prolonged O2 deprivation on Na+ channels: differential regulation in adult versus fetal rat brain

Neuronal Na+ channels are functionally inhibited in the adult in response to acute O2 deprivation. Since prolonged hypoxia may not only affect channel function, but also its expression, we hypothesized that long-term hypoxia alters Na+ channel density. This alteration may depend on age, because we have found major differences in neuronal responses to hypoxia between the immature and adult. In the present work, we used northern blots, slot blots, saxitoxin binding and autoradiography to ask whether: (i) prolonged hypoxia alters Na+ channel messenger RNA and protein levels in the brain; (ii) there is a difference between the adult and prenatal brains regarding Na+ channel expression with hypoxic exposure; and (iii) regional differences in Na+ channel expression occur in hypoxia-exposed brains. Our results show the following. (1) Na+ channel messenger RNA and saxitoxin binding density decreased after prolonged hypoxia in adult brain homogenates; this is in sharp contrast to the changes observed in fetal brains, which tended to increase Na+ channel messenger RNA and protein after hypoxia. (2) Changes in saxitoxin binding density are related to alterations in the number of saxitoxin binding sites and not to binding affinity, since there was no major change in Kd values between the hypoxia and naive groups. (3) The hypoxia-induced Na+ channel expression was heterogeneous, with major differences between rostral regions (e.g., the cortex) and caudal regions (e.g., the medulla and pons). We speculate that down-regulation of Na+ channels during long-term hypoxia in mature brains is an adaptive cellular response, aimed at minimizing the mismatch between energy supply and demand, since maintenance of Na+ gradients is a major energy-requiring process. However, the prenatal brain does not depend on this adaptive mechanism in response to hypoxic stress.

L-type calcium channel activity regulates sodium channel levels in rat pituitary GH3 cells

1. The effects of chronic pharmacological modulation of L-type Ca2+ channel activity on the cell surface expression of Na+ channels were examined in GH3 cells. 2. Prolonged inhibition (4-5 days) of L-channels with nimodipine caused a 50-60 % decrease in the peak amplitude of whole-cell Na+ currents recorded with the patch-clamp technique. On the contrary, prolonged exposure to the L-channel agonist Bay K 8644 induced an approximately 2.5-fold increase in peak Na+ current. In both cases, there were only minor changes in cell capacitance and no significant changes in Na+ channel gating properties. 3. Measurements of the specific binding of radiolabelled saxitoxin to intact cells showed that nimodipine treatment reduced the number of cell surface Na+ channels, whereas treatment with Bay K 8664 produced the opposite effect. The dual regulation of Na+ channel abundance explained the mentioned changes in Na+ current amplitude. 4. Plasma membrane Na+ channels had a half-life of approximately 17 h both in control cells and in cells treated with Bay K 8644, as estimated from the rate of decay of peak Na+ current after inhibition of protein synthesis with cycloheximide. Actinomycin D, an inhibitor of gene transcription, and also cycloheximide, occluded the stimulatory effect of Bay K 8644 on Na+ current density when measured over a 24 h period. 5. These findings indicate that the entry of Ca2+ through L-type channels influences in a positive way the number of functional Na+ channels in GH3 cells, and suggest that Ca2+ influx stimulates either Na+ channel gene expression or the expression of a regulatory protein that promotes translocation of pre-assembled Na+ channels into the plasma membrane.

Protein kinase C and the opposite regulation of sodium channel alpha- and beta1-subunit mRNA levels in adrenal chromaffin cells

Our previous [3H]saxitoxin binding and 22Na influx assays showed that treatment of cultured bovine adrenal chromaffin cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) or phorbol 12,13-dibutyrate (PDBu), an activator of protein kinase C (PKC), decreased the number of cell surface Na channels (IC50 = 19 nM) but did not alter their pharmacological properties; Na channel down-regulation developed within 3 h, reached the peak decrease of 53% at 15 h, and was mediated by transcriptional/translational events. In the present study, treatment with 100 nM TPA lowered the Na channel alpha-subunit mRNA level by 34 and 52% at 3 and 6 h, followed by restoration to the pretreatment level at 24 h, whereas 100 nM TPA elevated the Na channel beta1-subunit mRNA level by 13-61% between 12 and 48 h. Reduction of alpha-subunit mRNA level by TPA was concentration-dependent (IC50 = 18 nM) and was mimicked by PDBu but not by the biologically inactive 4alpha-TPA; it was prevented by H-7, an inhibitor of PKC, but not by HA-1004, a less active analogue of H7, or by H-89, an inhibitor of cyclic AMP-dependent protein kinase. Treatment with cycloheximide, an inhibitor of protein synthesis, per se sustainingly increased the alpha-subunit mRNA level and decreased the beta1-subunit mRNA level for 24 h; also, the TPA-induced decrease of alpha-subunit mRNA and increase of beta1-subunit mRNA were both totally prevented for 24 h by concurrent treatment with cycloheximide. Nuclear run-on assay showed that TPA treatment did not alter the transcriptional rate of the alpha-subunit gene. A stability study using actinomycin D, an inhibitor of RNA synthesis, revealed that TPA treatment shortened the t(1/2) of alpha-subunit mRNA from 18.8 to 3.7 h. These results suggest that Na channel alpha- and beta-subunit mRNA levels are differentially down- and up-regulated via PKC; the process may be mediated via an induction of as yet unidentified short-lived protein(s), which may culminate in the destabilization of alpha-subunit mRNA without altering alpha-subunit gene transcription.

Plasticity in the intrinsic excitability of cortical pyramidal neurons

During learning and development, the level of synaptic input received by cortical neurons may change dramatically. Given a limited range of possible firing rates, how do neurons maintain responsiveness to both small and large synaptic inputs? We demonstrate that in response to changes in activity, cultured cortical pyramidal neurons regulate intrinsic excitability to promote stability in firing. Depriving pyramidal neurons of activity for two days increased sensitivity to current injection by selectively regulating voltage-dependent conductances. This suggests that one mechanism by which neurons maintain sensitivity to different levels of synaptic input is by altering the function relating current to firing rate.