Persistent sodium currents through brain sodium channels induced by G protein betagamma subunits

Antagonism of 5-HT2 receptors attenuates self-sustained firing of human motor units

5-HT2 receptors on motoneurones play a critical role in facilitating persistent inward currents (PICs). Although facilitation of PICs can enhance self-sustained firing after periods of excitation, the relationship between 5-HT2 receptor activity and self-sustained firing in human motor units (MUs) has not been resolved. MU activity was assessed from the tibialis anterior of 10 healthy adults (24.9 ± 2.8 years) during two contraction protocols. Both protocols featured steady-state isometric contractions with constant descending drive to the motoneurone pool. However, one protocol also included an additional phase of superimposed descending drive. Adding and then removing descending drive in the middle of steady-state contractions altered MU firing behaviour across the motor pool, where newly recruited units in the superimposed phase were unable to switch off (P = 0.0002), and units recruited prior to additional descending drive reduced their discharge rates (P < 0.0001, difference in estimated marginal means (∆) = 2.24 pulses/s). The 5-HT2 receptor antagonist, cyproheptadine, was then administered to determine whether changes in MU firing were mediated by serotonergic mechanisms. 5-HT2 receptor antagonism caused reductions in MU discharge rate (P < 0.001, ∆ = 1.65 pulses/s), recruitment threshold (P = 0.00112, ∆ = 1.09% maximal voluntary contraction) and self-sustained firing duration (P < 0.0001, ∆ = 1.77s) after the additional descending drive was removed in the middle of the steady-state contraction. These findings indicate that serotonergic neuromodulation plays a key role in facilitating discharge and self-sustained firing of human motoneurones, where adaptive changes in MU recruitment must occur to meet the demands of the contraction. KEY POINTS: Animal and cellular preparations indicate that somato-dendritic 5-HT2 receptors regulate the intrinsic excitability of motoneurones. 5-HT2 receptor antagonism reduces estimates of persistent inward currents in motoneurones, which contribute to self-sustained firing when synaptic inputs are reduced or removed. This human study employed a contraction task that slowly increased (and then removed) the additional descending drive in the middle of a steady-state contraction where marked self-sustained firing occurred when the descending drive was removed. 5-HT2 receptor antagonism caused widespread reductions in motor unit (MU) discharge rates during contractions, which was accompanied by reduced recruitment threshold and attenuation of self-sustained firing duration after the removal of the additional descending drive to motoneurones. These findings support the role that serotonergic neuromodulation is a key facilitator of MU discharge and self-sustained firing of human motoneurones, where adaptative changes in MU recruitment must occur to meet the demands of the contraction.

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

Post-stroke epileptogenesis is associated with altered intrinsic properties of hippocampal pyramidal neurons leading to increased theta resonance

Brain insults like stroke, trauma or infections often lead to blood-brain barrier-dysfunction (BBBd) frequently resulting into epileptogenesis. Affected patients suffer from seizures and cognitive comorbidities that are potentially linked to altered network oscillations. It has been shown that a hippocampal BBBd in rats leads to in vivo seizures and increased power at theta (3-8 Hz), an important type of network oscillations. However, the underlying cellular mechanisms remain poorly understood. At membrane potentials close to the threshold for action potentials (APs) a subpopulation of CA1 pyramidal cells (PCs) displays intrinsic resonant properties due to an interplay of the muscarine-sensitive K⁺-current (I_M) and the persistent Na⁺-current (I_NaP). Such resonant neurons are more excitable and generate more APs when stimulated at theta frequencies, being strong candidates for contributing to hippocampal theta oscillations during epileptogenesis. We tested this hypothesis by characterizing changes in intrinsic properties of hippocampal PCs one week after post-stroke epileptogenesis, a model associated with BBBd, using slice electrophysiology and computer modeling. We find a higher proportion of resonant neurons in BBBd compared to sham animals (47 vs. 29%), accompanied by an increase in their excitability. In contrast, BBBd non-resonant neurons showed a reduced excitability, presented with lower impedance and more positive AP threshold. We identify an increase in I_M combined with either a reduction in I_NaP or an increase in I_Leak as possible mechanisms underlying the observed changes. Our results support the hypothesis that a higher proportion of more excitable resonant neurons in the hippocampus contributes to increased theta oscillations and an increased likelihood of seizures in a model of post-stroke epileptogenesis.

Nonlinear Input-Output Functions of Motoneurons

All movements are generated by the activation of motoneurons, and hence their input-output properties define the final step in processing of all motor commands. A major challenge to understanding this transformation has been the striking nonlinear behavior of motoneurons conferred by the activation of persistent inward currents (PICs) mediated by their voltage-gated Na+ and Ca2+ channels. In this review, we focus on the contribution that these PICs make to motoneuronal discharge and how the nonlinearities they engender impede the construction of a comprehensive model of motor control.

Computational Modeling of Genetic Contributions to Excitability and Neural Coding in Layer V Pyramidal Cells: Applications to Schizophrenia Pathology

Pyramidal cells in layer V of the neocortex are one of the most widely studied neuron types in the mammalian brain. Due to their role as integrators of feedforward and cortical feedback inputs, they are well-positioned to contribute to the symptoms and pathology in mental disorders-such as schizophrenia-that are characterized by a mismatch between the internal perception and external inputs. In this modeling study, we analyze the input/output properties of layer V pyramidal cells and their sensitivity to modeled genetic variants in schizophrenia-associated genes. We show that the excitability of layer V pyramidal cells and the way they integrate inputs in space and time are altered by many types of variants in ion-channel and Ca²⁺ transporter-encoding genes that have been identified as risk genes by recent genome-wide association studies. We also show that the variability in the output patterns of spiking and Ca²⁺ transients in layer V pyramidal cells is altered by these model variants. Importantly, we show that many of the predicted effects are robust to noise and qualitatively similar across different computational models of layer V pyramidal cells. Our modeling framework reveals several aspects of single-neuron excitability that can be linked to known schizophrenia-related phenotypes and existing hypotheses on disease mechanisms. In particular, our models predict that single-cell steady-state firing rate is positively correlated with the coding capacity of the neuron and negatively correlated with the amplitude of a prepulse-mediated adaptation and sensitivity to coincidence of stimuli in the apical dendrite and the perisomatic region of a layer V pyramidal cell. These results help to uncover the voltage-gated ion-channel and Ca²⁺ transporter-associated genetic underpinnings of schizophrenia phenotypes and biomarkers.

MrgprX1 Mediates Neuronal Excitability and Itch Through Tetrodotoxin-Resistant Sodium Channels

In this study, we sought to elucidate the molecular mechanism underlying human Mas-related G protein-coupled receptor X1 (MrgprX1) mediated itch sensation. We found that activation of MrgprX1 by BAM8-22 triggered robust action potential discharges in dorsal root ganglion (DRG) neurons. This neuronal excitability is not mediated by Transient receptor potential (TRP) cation channels, M-type potassium channels, or chloride channels. Instead, activation of MrgprX1 lowers the activation threshold of TTX-resistant sodium channels and induces inward sodium currents. These MrgprX1-elicited action potential discharges can be blocked by Pertussis toxin (PTX) and a Gβγ inhibitor - Gallein. Behavioral results showed that Nav1.9 knockout but not Trpa1 knockout significantly reduced BAM8-22 evoked scratching behavior. Collectively, these data suggest that activation of MrgprX1 triggers itch sensation by increasing the activity of TTX-resistant voltage-gated sodium channels.

Rapid evolution of a voltage-gated sodium channel gene in a lineage of electric fish leads to a persistent sodium current

Most weakly electric fish navigate and communicate by sensing electric signals generated by their muscle-derived electric organs. Adults of one lineage (Apteronotidae), which discharge their electric organs in excess of 1 kHz, instead have an electric organ derived from the axons of specialized spinal neurons (electromotorneurons [EMNs]). EMNs fire spontaneously and are the fastest-firing neurons known. This biophysically extreme phenotype depends upon a persistent sodium current, the molecular underpinnings of which remain unknown. We show that a skeletal muscle–specific sodium channel gene duplicated in this lineage and, within approximately 2 million years, began expressing in the spinal cord, a novel site of expression for this isoform. Concurrently, amino acid replacements that cause a persistent sodium current accumulated in the regions of the channel underlying inactivation. Therefore, a novel adaptation allowing extreme neuronal firing arose from the duplication, change in expression, and rapid sequence evolution of a muscle-expressing sodium channel gene.

The electrical properties of excitable cells, such as those in muscle and nervous tissue, were enabled in large part by the evolution of voltage-gated ion channel genes. The regulated conduction of ions through these channels results in the propagation of electrical signals, facilitating communication between cells. Here, we investigated how voltage-gated sodium (Na_v) channels contributed to the evolution of a novel electric organ system in the Apteronotids—a lineage of weakly electric fish. This organ is developmentally derived from motor neurons and used for communication between individual fish, as well as for probing their nocturnal environment. We used transcriptomic data to show that the gene encoding a broadly conserved muscle-specific sodium channel was duplicated in an ancestral fish. One duplicated gene copy subsequently gained expression in the spinal cord, where the electric organ is located. Through evolutionary analysis and biophysical experiments, we demonstrate that sequence changes in this new sodium channel transformed its function to cause novel electrical properties that can facilitate spontaneous high-frequency action potentials. This study shows that duplicate genes can gain highly novel expression patterns and quickly adapt to contribute to the phenotypic evolution of novel organ systems.

Competition between Persistent Na+ and Muscarine-Sensitive K+ Currents Shapes Perithreshold Resonance and Spike Tuning in CA1 Pyramidal Neurons

Neurons from many brain regions display intrinsic subthreshold theta-resonance, responding preferentially to theta-frequency oscillatory stimuli. Resonance may contribute to selective communication among neurons and to orchestrate brain rhythms. CA1 pyramidal neurons receive theta activity, generating place fields. In these neurons the expression of perithreshold frequency preference is controversial, particularly in the spiking regime, with evidence favoring either non-resonant (integrator-like) or resonant behavior. Perithreshold dynamics depends on the persistent Na⁺ current I_NaP developing above −70 mV and the muscarine-sensitive K⁺ current I_M activating above −60 mV. We conducted current and voltage clamp experiments in slices to investigate perithreshold excitability of CA1 neurons under oscillatory stimulation. Around 20% of neurons displayed perithreshold resonance that is expressed in spiking. The remaining neurons (~80%) acted as low-pass filters lacking frequency preference. Paired voltage clamp measurement of I_NaP and I_M showed that perithreshold activation of I_M is in general low while I_NaP is high enough to depolarize neurons toward threshold before resonance expression, explaining the most abundant non-resonant perithreshold behavior. Partial blockade of I_NaP by pharmacological tools or dynamic clamp changed non-resonant to resonant behavior. Furthermore, shifting I_M activation toward hyperpolarized potentials by dynamic clamp also transformed non-resonant neurons into resonant ones. We propose that the relative levels of I_NaP and I_M control perithreshold behavior of CA1 neurons constituting a gating mechanism for theta resonance in the spiking regime. Both currents are regulated by intracellular signaling and neuromodulators which may allow dynamic switching of perithreshold behavior between resonant and non-resonant.

Model systems for studying cellular mechanisms of SCN1A-related epilepsy

Mutations in SCN1A, the gene encoding voltage-gated sodium channel NaV1.1, cause a spectrum of epilepsy disorders that range from genetic epilepsy with febrile seizures plus to catastrophic disorders such as Dravet syndrome. To date, more than 1,250 mutations in SCN1A have been linked to epilepsy. Distinct effects of individual SCN1A mutations on neuronal function are likely to contribute to variation in disease severity and response to treatment in patients. Several model systems have been used to explore seizure genesis in SCN1A epilepsies. In this article we review what has been learned about cellular mechanisms and potential new therapies from these model systems, with a particular emphasis on the novel model system of knock in Drosophila and a look toward the future with expanded use of patient-specific induced pluripotent stem cell-derived neurons.

Muscarinic receptor control of pyramidal neuron membrane potential in the medial prefrontal cortex (mPFC) in rats

Damage to the cholinergic input to the prefrontal cortex has been implicated in neuropsychiatric disorders. Cholinergic endings release acetylcholine, which activates nicotinic and/or G-protein-coupled muscarinic receptors. Muscarinic receptors activate transduction systems, which control cellular effectors that regulate the membrane potential in medial prefrontal cortex (mPFC) neurons. The mechanisms responsible for the cholinergic-dependent depolarization of mPFC layer V pyramidal neurons in slices obtained from young rats were elucidated in this study. Glutamatergic and GABAergic transmission as well as tetrodotoxin (TTX)-sensitive Na(+) and voltage-dependent Ca(++) currents were eliminated. Cholinergic receptor stimulation by carbamoylcholine chloride (CCh; 100 μM) evoked depolarization (10.0 ± 1.3 mV), which was blocked by M1/M4 (pirenzepine dihydrochloride, 2 μM) and M1 (VU 0255035, 5 μM) muscarinic receptor antagonists and was not affected by a nicotinic receptor antagonist (mecamylamine hydrochloride, 10 μM). CCh-dependent depolarization was attenuated by extra- (20 μM) or intracellular (50 μM) application of an inhibitor of the βγ-subunit-dependent transduction system (gallein). It was also inhibited by intracellular application of a βγ-subunit-binding peptide (GRK2i, 10μM). mPFC pyramidal neurons express Nav1.9 channels. CCh-dependent depolarization was abolished in the presence of antibodies against Nav1.9 channels in the intracellular solution and augmented by the presence of ProTx-I toxin (100 nM) in the extracellular solution. CCh-induced depolarization was not affected by the following reagents: intracellular transduction system blockers, including U-73122 (10 μM), chelerythrine chloride (5 μM), SQ 22536 (100 μM) and H-89 (2 μM); channel blockers, including Ba(++) ions (200 μM), apamin (100 nM), flufenamic acid (200 μM), 2-APB (200 μM), SKF 96365 (50 μM), and ZD 7288 (50 μM); and a Na(+)/Ca(++) exchanger blocker, benzamil (20 μM). We conclude that muscarinic M1 receptor-dependent depolarization in mPFC pyramidal neurons is evoked by the activation of Nav1.9 channels and that the signal transduction pathway involves G-protein βγ subunits.

Amyloid precursor protein enhances Nav1.6 sodium channel cell surface expression

Amyloid precursor protein (APP) is commonly associated with Alzheimer disease, but its physiological function remains unknown. Nav1.6 is a key determinant of neuronal excitability in vivo. Because mouse models of gain of function and loss of function of APP and Nav1.6 share some similar phenotypes, we hypothesized that APP might be a candidate molecule for sodium channel modulation. Here we report that APP colocalized and interacted with Nav1.6 in mouse cortical neurons. Knocking down APP decreased Nav1.6 sodium channel currents and cell surface expression. APP-induced increases in Nav1.6 cell surface expression were Go protein-dependent, enhanced by a constitutively active Go protein mutant, and blocked by a dominant negative Go protein mutant. APP also regulated JNK activity in a Go protein-dependent manner. JNK inhibition attenuated increases in cell surface expression of Nav1.6 sodium channels induced by overexpression of APP. JNK, in turn, phosphorylated APP. Nav1.6 sodium channel surface expression was increased by T668E and decreased by T668A, mutations of APP695 mimicking and preventing Thr-668 phosphorylation, respectively. Phosphorylation of APP695 at Thr-668 enhanced its interaction with Nav1.6. Therefore, we show that APP enhances Nav1.6 sodium channel cell surface expression through a Go-coupled JNK pathway.

Cell-attached single-channel recordings in intact prefrontal cortex pyramidal neurons reveal compartmentalized D1/D5 receptor modulation of the persistent sodium current

The persistent Na⁺ current (I_Nap) is believed to be an important target of dopamine modulation in prefrontal cortex (PFC) neurons. While past studies have tested the effects of dopamine on I_Nap, the results have been contradictory largely because of difficulties in measuring I_Nap using somatic whole-cell recordings. To circumvent these confounds we used the cell-attached patch-clamp technique to record single Na⁺ channels from the soma, proximal dendrite (PD) or proximal axon (PA) of intact prefrontal layer V pyramidal neurons. Under baseline conditions, numerous well resolved Na⁺ channel openings were recorded that exhibited an extrapolated reversal potential of 73 mV, a slope conductance of 14–19 pS and were blocked by tetrodotoxin (TTX). While similar in most respects, the propensity to exhibit prolonged bursts lasting >40 ms was many fold greater in the axon than the soma or dendrite. Bath application of the D1/D5 receptor agonist SKF81297 shifted the ensemble current activation curve leftward and increased the number of late events recorded from the PD but not the soma or PA. However, the greatest effect was on prolonged bursting where the D1/D5 receptor agonist increased their occurrence 3 fold in the PD and nearly 7 fold in the soma, but not at all in the PA. As a result, D1/D5 receptor activation equalized the probability of prolonged burst occurrence across the proximal axosomatodendritic region. Therefore, D1/D5 receptor modulation appears to be targeted mainly to Na⁺ channels in the PD/soma and not the PA. By circumventing the pitfalls of previous attempts to study the D1/D5 receptor modulation of I_Nap, we demonstrate conclusively that D1/D5 receptor activation can increase the I_Nap generated proximally, however questions still remain as to how D1/D5 receptor modulates Na⁺ currents in the more distal initial segment where most of the I_Nap is normally generated.

Persistent sodium current properties in hippocampal CA1 pyramidal neurons of young and adult rats

Persistent tetrodotoxin-sensitive sodium current (INaP) plays an important role in cellular and neuronal network excitability in physiological conditions and under different pathological circumstances. However, developmental changes in INaP properties remain largely unclear. In the present study using whole cell patch clamp technique we evaluated INaP properties in CA1 hippocampal pyramidal neurons isolated from young (postnatal day (P) 12-16) and adult (P60-75) rats. We show that the INaP density is substantially larger in the adult group. Although INaP inactivation characteristics were found to be similar in both groups, voltage dependence of INaP activation is shifted to more negative membrane potentials (young: -48.6±0.5mV vs. adult: -52.4±0.2mV, p<0.01). Our data indicates the increase of INaP contribution in the basal membrane sodium conductivity in the mature hippocampus.

Sodium leak channels in neuronal excitability and rhythmic behaviors

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

The chemokine CCL2 increases Nav1.8 sodium channel activity in primary sensory neurons through a Gβγ-dependent mechanism

Changes in function of voltage-gated sodium channels in nociceptive primary sensory neurons participate in the development of peripheral hyperexcitability that occurs in neuropathic and inflammatory chronic pain conditions. Among them, the tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.8, primarily expressed by small- and medium-sized dorsal root ganglion (DRG) neurons, substantially contributes to the upstroke of action potential in these neurons. Compelling evidence also revealed that the chemokine CCL2 plays a critical role in chronic pain facilitation via its binding to CCR2 receptors. In this study, we therefore investigated the effects of CCL2 on the density and kinetic properties of TTX-R Na(v)1.8 currents in acutely small/medium dissociated lumbar DRG neurons from naive adult rats. Whole-cell patch-clamp recordings demonstrated that CCL2 concentration-dependently increased TTX-resistant Na(v)1.8 current densities in both small- and medium-diameter sensory neurons. Incubation with CCL2 also shifted the activation and steady-state inactivation curves of Na(v)1.8 in a hyperpolarizing direction in small sensory neurons. No change in the activation and inactivation kinetics was, however, observed in medium-sized nociceptive neurons. Our electrophysiological recordings also demonstrated that the selective CCR2 antagonist INCB3344 [N-[2-[[(3S,4S)-1-E4-(1,3-benzodioxol-5-yl)-4-hydroxycyclohexyl]-4-ethoxy-3-pyrrolidinyl]amino]-2-oxoethyl]-3-(trifluoromethyl)benzamide] blocks the potentiation of Na(v)1.8 currents by CCL2 in a concentration-dependent manner. Furthermore, the enhancement in Na(v)1.8 currents was prevented by pretreatment with pertussis toxin (PTX) or gallein (a Gβγ inhibitor), indicating the involvement of Gβγ released from PTX-sensitive G(i/o)-proteins in the cross talk between CCR2 and Na(v)1.8. Together, our data clearly demonstrate that CCL2 may excite primary sensory neurons by acting on the biophysical properties of Na(v)1.8 currents via a CCR2/Gβγ-dependent mechanism.

Alternative splicing modulates inactivation of type 1 voltage-gated sodium channels by toggling an amino acid in the first S3-S4 linker

Voltage-gated sodium channels underlie the upstroke of action potentials and are fundamental to neuronal excitability. Small changes in the behavior of these channels are sufficient to change neuronal firing and trigger seizures. These channels are subject to highly conserved alternative splicing, affecting the short linker between the third transmembrane segment (S3) and the voltage sensor (S4) in their first domain. The biophysical consequences of this alternative splicing are incompletely understood. Here we focus on type 1 sodium channels (Nav1.1) that are implicated in human epilepsy. We show that the functional consequences of alternative splicing are highly sensitive to recording conditions, including the identity of the major intracellular anion and the recording temperature. In particular, the inactivation kinetics of channels containing the alternate exon 5N are more sensitive to intracellular fluoride ions and to changing temperature than channels containing exon 5A. Moreover, Nav1.1 channels containing exon 5N recover from inactivation more rapidly at physiological temperatures. Three amino acids differ between exons 5A and 5N. However, the changes in sensitivity and stability of inactivation were reproduced by a single conserved change from aspartate to asparagine in channels containing exon 5A, which was sufficient to make them behave like channels containing the complete exon 5N sequence. These data suggest that splicing at this site can modify the inactivation of sodium channels and reveal a possible interaction between splicing and anti-epileptic drugs that stabilize sodium channel inactivation.

Human voltage-gated sodium channel mutations that cause inherited neuronal and muscle channelopathies increase resurgent sodium currents

Inherited mutations in voltage-gated sodium channels (VGSCs; or Nav) cause many disorders of excitability, including epilepsy, chronic pain, myotonia, and cardiac arrhythmias. Understanding the functional consequences of the disease-causing mutations is likely to provide invaluable insight into the roles that VGSCs play in normal and abnormal excitability. Here, we sought to test the hypothesis that disease-causing mutations lead to increased resurgent currents, unusual sodium currents that have not previously been implicated in disorders of excitability. We demonstrated that a paroxysmal extreme pain disorder (PEPD) mutation in the human peripheral neuronal sodium channel Nav1.7, a paramyotonia congenita (PMC) mutation in the human skeletal muscle sodium channel Nav1.4, and a long-QT3/SIDS mutation in the human cardiac sodium channel Nav1.5 all substantially increased the amplitude of resurgent sodium currents in an optimized adult rat-derived dorsal root ganglion neuronal expression system. Computer simulations indicated that resurgent currents associated with the Nav1.7 mutation could induce high-frequency action potential firing in nociceptive neurons and that resurgent currents associated with the Nav1.5 mutation could broaden the action potential in cardiac myocytes. These effects are consistent with the pathophysiology associated with the respective channelopathies. Our results indicate that resurgent currents are associated with multiple channelopathies and are likely to be important contributors to neuronal and muscle disorders of excitability.

Regulation of persistent Na current by interactions between beta subunits of voltage-gated Na channels

The beta subunits of voltage-gated Na channels (Scnxb) regulate the gating of pore-forming alpha subunits, as well as their trafficking and localization. In heterologous expression systems, beta1, beta2, and beta3 subunits influence inactivation and persistent current in different ways. To test how the beta4 protein regulates Na channel gating, we transfected beta4 into HEK (human embryonic kidney) cells stably expressing Na(V)1.1. Unlike a free peptide with a sequence from the beta4 cytoplasmic domain, the full-length beta4 protein did not block open channels. Instead, beta4 expression favored open states by shifting activation curves negative, decreasing the slope of the inactivation curve, and increasing the percentage of noninactivating current. Consequently, persistent current tripled in amplitude. Expression of beta1 or chimeric subunits including the beta1 extracellular domain, however, favored inactivation. Coexpressing Na(V)1.1 and beta4 with beta1 produced tiny persistent currents, indicating that beta1 overcomes the effects of beta4 in heterotrimeric channels. In contrast, beta1(C121W), which contains an extracellular epilepsy-associated mutation, did not counteract the destabilization of inactivation by beta4 and also required unusually large depolarizations for channel opening. In cultured hippocampal neurons transfected with beta4, persistent current was slightly but significantly increased. Moreover, in beta4-expressing neurons from Scn1b and Scn1b/Scn2b null mice, entry into inactivated states was slowed. These data suggest that beta1 and beta4 have antagonistic roles, the former favoring inactivation, and the latter favoring activation. Because increased Na channel availability may facilitate action potential firing, these results suggest a mechanism for seizure susceptibility of both mice and humans with disrupted beta1 subunits.

The genomic determinants of alcohol preference in mice

Searches for the identity of genes that influence the levels of alcohol consumption by humans and other animals have often been driven by presupposition of the importance of particular gene products in determining positively or negatively reinforcing effects of ethanol. We have taken an unbiased approach and performed a meta-analysis across three types of mouse populations to correlate brain gene expression with levels of alcohol intake. Our studies, using filtering procedures based on QTL analysis, produced a list of eight candidate genes with highly heritable expression, which could explain a significant amount of the variance in alcohol preference in mice. Using the Allen Brain Atlas for gene expression, we noted that the candidate genes' expression was localized to the olfactory and limbic areas as well as to the orbitofrontal cortex. Informatics techniques and pathway analysis illustrated the role of the candidate genes in neuronal migration, differentiation, and synaptic remodeling. The importance of olfactory cues, learning and memory formation (Pavlovian conditioning), and cortical executive function, for regulating alcohol intake by animals (including humans), is discussed.

How do mutant Nav1.1 sodium channels cause epilepsy?

Voltage-gated sodium channels comprise pore-forming alpha subunits and auxiliary beta subunits. Nine different alpha subtypes, designated Nav1.1-Nav1.9 have been identified in excitable cells. Nav1.1, 1.2 and 1.6 are major subtypes in the adult mammalian brain. More than 200 mutations in the Nav1.1 alpha subtype have been linked to inherited epilepsy syndromes, ranging in severity from the comparatively mild disorder Generalized Epilepsy with Febrile Seizures Plus to the epileptic encephalopathy Severe Myoclonic Epilepsy of Infancy. Studies using heterologous expression and functional analysis of recombinant Nav1.1 channels suggest that epilepsy mutations in Nav1.1 may cause either gain-of-function or loss-of-function effects that are consistent with either increased or decreased neuronal excitability. How these diverse effects lead to epilepsy is poorly understood. This review summarizes the data on sodium channel mutations and epilepsy and builds a case for the hypothesis that most Nav1.1 mutations have their ultimate epileptogenic effects by reducing Nav1.1-mediated whole cell sodium currents in GABAergic neurons, resulting in widespread loss of brain inhibition, an ideal background for the genesis of epileptic seizures.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Differentiated pattern of sodium channel expression in dissociated Purkinje neurons maintained in long-term culture

Cerebellar Purkinje neurons in vivo exhibit high frequency and multi-spike action potentials with transient (INaT), resurgent (INaR) and persistent (INaP) Na+ currents arising from voltage-gated Na+ channels, which play important roles in shaping the action potentials and electrical activity of these cells. However, little is known about Na+ channel expression in cultured Purkinje neurons despite the use of in vitro approaches to study these cells. Therefore, GFP-expressing Purkinje neurons isolated from transgenic mice were analysed after four weeks in culture, when, coincident with distinct axonal and dendritic morphologies, cultured Purkinje neurons exhibited dendrite-specific MAP2 expression characteristic of polarized neurons. In cell-attached patch clamp recordings, Na+ currents occurred at significantly higher frequencies and amplitudes in patches from the soma and axon than from dendrites, similar to the polarized distribution observed in vivo. INaT, INaR and INaP Na+ currents with properties similar to those observed in acutely isolated Purkinje neurons were detected in nucleated outside-out patches from cultured Purkinje cells. RT-PCR analysis detected Nav1.1, Nav1.2 and Nav1.6, but not Nav1.3, Nav1.4, Nav 1.5 or Nav1.8 Na+ channel alpha subunit gene expression in cultured Purkinje neurons, as observed in vivo. Together, the results indicate that key aspects of Na+ channel expression in mature Purkinje neurons in vivo occur in vitro.

The molecular basis for T-type Ca2+ channel inhibition by G protein beta2gamma2 subunits

Gbetagamma, a ubiquitous second messenger, relays external signals from G protein-coupled receptors to networks of intracellular effectors, including voltage-dependent calcium channels. Unlike high-voltage-activated Ca(2+) channels, the inhibition of low-voltage-activated Ca(2+) channels is subtype-dependent and mediated selectively by Gbeta(2)-containing dimers. Yet, the molecular basis for this exquisite selectivity remains unknown. Here, we used pure recombinant Gbetagamma subunits to establish that the Gbeta(2)gamma(2) dimer can selectively reconstitute the inhibition of alpha(1H) channels in isolated membrane patches. This inhibition is the result of a reduction in channel open probability that is not accompanied by a change in channel expression or an alteration in active-channel gating. By exchanging residues between the active Gbeta(2) subunit and the inactive Gbeta(1) subunit, we identified a cluster of amino acids that functionally distinguish Gbeta(2) from other Gbeta subunits. These amino acids on the beta-torus identify a region that is distinct from those regions that contact the Galpha subunit or other effectors.

G-protein activation modulates pseudo-periodic oscillation of Na channel

We have shown before that the duration and amplitude of both prolonged (1-160 s) and short (100-1000 ms) depolarizing prepulse altered all the steady-state and kinetic parameters of rNav1.2a voltage-gated sodium channel in a pseudo-oscillatory fashion with variable time period and amplitude, often superimposed on a linear trend. In this study, we have examined the effect of G-protein activation on pseudo-oscillatory properties of the rNav1.2a sodium channel alpha subunit, heterologously expressed in Chinese hamster ovary cells. G-protein modification caused insignificant changes in the slow pseudo-periodic oscillation of the activation properties of sodium channel; only the time period of the oscillation was altered from approximately 30 to 21s. In contrast, G-protein activation abolished the faster component of pseudo-periodic oscillation in steady-state inactivation properties of sodium channel; the conditioning duration dependence of steady-state inactivation becomes monotonic in nature.

Motor learning changes GABAergic terminals on spinal motoneurons in normal rats

The role of spinal cord plasticity in motor learning is largely unknown. This study explored the effects of H-reflex operant conditioning, a simple model of motor learning, on GABAergic input to spinal motoneurons in rats. Soleus motoneurons were labeled by retrograde transport of a fluorescent tracer and GABAergic terminals on them were identified by glutamic acid decarboxylase (GAD)67 immunoreactivity. Three groups were studied: (i) rats in which down-conditioning had reduced the H-reflex (successful HRdown rats); (ii) rats in which down-conditioning had not reduced the H-reflex (unsuccessful HRdown rats) and (iii) unconditioned (naive) rats. The number, size and GAD density of GABAergic terminals, and their coverage of the motoneuron, were significantly greater in successful HRdown rats than in unsuccessful HRdown or naive rats. It is likely that these differences are due to modifications in terminals from spinal interneurons in lamina VI-VII and that the increased terminal number, size, GAD density and coverage in successful HRdown rats reflect and convey a corticospinal tract influence that changes motoneuron firing threshold and thereby decreases the H-reflex. GABAergic terminals in spinal cord change after spinal cord transection. The present results demonstrate that such spinal cord plasticity also occurs in intact rats in the course of motor learning and suggest that this plasticity contributes to skill acquisition.

Molecular determinants for modulation of persistent sodium current by G-protein betagamma subunits

Voltage-gated sodium channels are responsible for the upstroke of the action potential in most excitable cells, and their fast inactivation is essential for controlling electrical signaling. In addition, a noninactivating, persistent component of sodium current, I(NaP), has been implicated in integrative functions of neurons including threshold for firing, neuronal bursting, and signal integration. G-protein betagamma subunits increase I(NaP), but the sodium channel subtypes that conduct I(NaP) and the target site(s) on the sodium channel molecule required for modulation by Gbetagamma are poorly defined. Here, we show that I(NaP) conducted by Na(v)1.1 and Na(v)1.2 channels (Na(v)1.1 > Na(v)1.2) is modulated by Gbetagamma; Na(v)1.4 and Na(v)1.5 channels produce smaller I(NaP) that is not regulated by Gbetagamma. These qualitative differences in modulation by Gbetagamma are determined by the transmembrane body of the sodium channels rather than their cytoplasmic C-terminal domains, which have been implicated previously in modulation by Gbetagamma. However, the C-terminal domains determine the quantitative extent of modulation of Na(v)1.2 channels by Gbetagamma. Studies of chimeric and truncated Na(v)1.2 channels identify molecular determinants that affect modulation of I(NaP) located between amino acid residue 1890 and the C terminus at residue 2005. The last 28 amino acid residues of the C terminus are sufficient to support modulation by Gbetagamma when attached to the proximal C-terminal domain. Our results further define the sodium channel subtypes that generate I(NaP) and identify crucial molecular determinants in the C-terminal domain required for modulation by Gbetagamma when attached to the transmembrane body of a responsive sodium channel.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Computer simulations of high-pass filtering in zebrafish larval muscle fibres

Larval somatic muscle of the zebrafish, Danio rerio, like that of some other organisms, responds to a sustained depolarization with one, and only one, action potential. Here, we report computer simulations, using the NEURON simulation programme, of sodium and potassium currents of somatic muscle of larval Danio rerio to investigate their possible contribution to once-only firing. Our computer model incorporated simulated sodium and potassium ion channels based on steady-state and kinetic parameters derived from a recent electrophysiological study. The model responded to sustained depolarizations with a single action potential at all levels of depolarization above threshold. By varying several parameters of the sodium and potassium currents systematically, the minimum changes necessary to produce repetitive firing were found to be a positive shift in the half-inactivation and a negative shift in the half-activation potentials for the sodium current, accompanied by a slowing of the rate of inactivation to half of the experimentally observed values. This suggests that once-only spiking can be attributed to the steady-state values of activation and inactivation of the sodium current, along with a slower rate of inactivation. Mapping of the resultant firing properties against steady-state and kinetic ion channel parameters revealed a high safety factor for once-only firing and showed that the time constant of inactivation of the sodium current was the key determinant of once-only or repetitive firing. The rapidly inactivating potassium current does not influence once-only firing or the maximum rate of firing in response to periodic excitation in these simulations. Although a contribution of other currents to produce once-only firing has not been excluded, this model suggests that the properties of the sodium current are sufficient to account for once-only firing.

Modal gating of human CaV2.1 (P/Q-type) calcium channels: I. The slow and the fast gating modes and their modulation by beta subunits

The single channel gating properties of human Ca_V2.1 (P/Q-type) calcium channels and their modulation by the auxiliary β_1b, β_2e, β_3a, and β_4a subunits were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing human Ca_V2.1 channels. These calcium channels showed a complex modal gating, which is described in this and the following paper (Fellin, T., S. Luvisetto, M. Spagnolo, and D. Pietrobon. 2004. J. Gen. Physiol. 124:463–474). Here, we report the characterization of two modes of gating of human Ca_V2.1 channels, the slow mode and the fast mode. A channel in the two gating modes differs in mean closed times and latency to first opening (both longer in the slow mode), in voltage dependence of the open probability (larger depolarizations are necessary to open the channel in the slow mode), in kinetics of inactivation (slower in the slow mode), and voltage dependence of steady-state inactivation (occurring at less negative voltages in the slow mode). Ca_V2.1 channels containing any of the four β subtypes can gate in either the slow or the fast mode, with only minor differences in the rate constants of the transitions between closed and open states within each mode. In both modes, Ca_V2.1 channels display different rates of inactivation and different steady-state inactivation depending on the β subtype. The type of β subunit also modulates the relative occurrence of the slow and the fast gating mode of Ca_V2.1 channels; β_3a promotes the fast mode, whereas β_4a promotes the slow mode. The prevailing mode of gating of Ca_V2.1 channels lacking a β subunit is a gating mode in which the channel shows shorter mean open times, longer mean closed times, longer first latency, a much larger fraction of nulls, and activates at more positive voltages than in either the fast or slow mode.

Electrophysiological properties of two axonal sodium channels, Nav1.2 and Nav1.6, expressed in mouse spinal sensory neurones

Sodium channels Na(v)1.2 and Na(v)1.6 are both normally expressed along premyelinated and myelinated axons at different stages of maturation and are also expressed in a subset of demyelinated axons, where coexpression of Na(v)1.6 together with the Na(+)/Ca(2+) exchanger is associated with axonal injury. It has been difficult to distinguish the currents produced by Na(v)1.2 and Na(v)1.6 in native neurones, and previous studies have not compared these channels within neuronal expression systems. In this study, we have characterized and directly compared Na(v)1.2 and Na(v)1.6 in a mammalian neuronal cell background and demonstrate differences in their properties that may affect neuronal behaviour. The Na(v)1.2 channel displays more depolarized activation and availability properties that may permit conduction of action potentials, even with depolarization. However, Na(v)1.2 channels show a greater accumulation of inactivation at higher frequencies of stimulation (20-100 Hz) than Na(v)1.6 and thus are likely to generate lower frequencies of firing. Na(v)1.6 channels produce a larger persistent current that may play a role in triggering reverse Na(+)/Ca(2+) exchange, which can injure demyelinated axons where Na(v)1.6 and the Na(+)/Ca(2+) exchanger are colocalized, while selective expression of Na(v)1.2 may support action potential electrogenesis, at least at lower frequencies, while producing a smaller persistent current.

PGE2 increases the tetrodotoxin-resistant Nav1.9 sodium current in mouse DRG neurons via G-proteins

Inflammation caused by tissue damage results in pain, reflecting an increase in excitability of the primary afferent neurons innervating the area. There is some evidence to suggest that altered function of voltage-gated sodium channels is responsible for the hyperexcitability produced by inflammatory agents, possibly acting through G-proteins, but the role of different channel subtypes has not been fully explored. The tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.9 is expressed selectively in C- and A-fibre nociceptive-type units and is upregulated by G-protein activation. In this study, we examined the effects of the inflammatory agent prostaglandin-E(2) (PGE(2)) on Na(v)1.9 current in both Na(v)1.8-null and wild-type (WT) mice and explored the role of specific G-proteins in modulation. PGE(2) caused a twofold increase in Na(v)1.9 current (p<0.05) in both systems. Steady-state activation was shifted in a hyperpolarizing direction by 6-8 mV and availability of channels by 12 mV. No differences in the activation and inactivation kinetics could be detected. The increase in current was blocked by pertussis toxin (PTX) but not cholera toxin (CTX), showing involvement of G(i/o) but not G(s) subunits. Our data indicate that Na(v)1.9 current can be increased during inflammation via a G-protein dependent mechanism and suggest that this could contribute to the regulation of electrogenesis in dorsal root ganglia (DRG) neurons.

Persistent tetrodotoxin-sensitive sodium current resulting from U-to-C RNA editing of an insect sodium channel

The persistent tetrodotoxin (TTX)-sensitive sodium current, detected in neurons of many regions of mammalian brains, is associated with many essential neuronal activities, including boosting of excitatory synaptic inputs, acceleration of firing rates, and promotion of oscillatory neuronal activities. However, the origin and molecular basis of the persistent current have remained controversial for decades. Here, we provide direct evidence that U-to-C RNA editing of an insect sodium channel transcript generates a sodium channel with a persistent current. We detected a persistent TTX-sensitive current in a splice variant of the cockroach sodium channel gene BgNa(v) (formerly para(CSMA)). Site-directed mutagenesis experiments revealed that an F-to-S change at the C-terminal domain of this variant was responsible for the persistent current. We demonstrated that this F-to-S change was the result of a U-to-C RNA editing event, which also occurred in the Drosophila para sodium channel transcript. Our work provides direct support for the hypothesis that posttranscriptional modification of a conventional transient sodium channel produces a persistent TTX-sensitive sodium channel.

Kinetic diversity of single-channel burst openings underlying persistent Na(+) current in entorhinal cortex neurons

The kinetic diversity of burst openings responsible for the persistent Na(+) current (I(NaP)) in entorhinal cortex neurons was examined by separately analyzing single bursts. Although remarkable kinetic variability was observed among bursts in terms of intraburst opening probability and mean open and closed times, the values of time constants describing intraburst open times (tau(o(b))s) and closed times (tau(c(b))s) were distributed around well-defined peaks. At -40 mV, tau(o(b)) peaks were found at approximately 0.34 (tau(o(b))1) and 0.77 (tau(o(b))2) ms, and major tau(c(b)) peaks were found at approximately 0.24 (tau(c(b))1) and 0.54 (tau(c(b))2) ms. In approximately 80% of the bursts two preferential gating modes were found that consisted of a combination of either tau(o(b))1 and tau(c(b))2 ("intraburst mode 1"), or tau(o(b))2 and tau(c(b))1 ("intraburst mode 2"). Individual channels could switch between different gating modalities, but normally tended to maintain a specific gating mode for long periods. Mean burst duration also displayed considerable variability. At least three time constants were found to describe burst duration, and the frequencies at which each of the corresponding "bursting states" occurred varied in different channels. Short-lasting bursting states were preferentially associated with intraburst mode 1, whereas very-long-lasting bursts tended to gate according to mode 2 only or other modes that included considerably longer mean open times. These results show that I(NaP) channels can generate multiple intraburst open and closed states and bursting states, but these different kinetic states tend to combine in definite ways to produce a limited number of prevalent, well-defined gating modalities. Modulation of distinct gating modalities in individual Na(+) channels may be a powerful form of plasticity to influence neuronal excitability and function.

Persistent sodium current in subicular neurons isolated from patients with temporal lobe epilepsy

The persistent sodium current is a common target of anti-epileptic drugs and contributes to burst firing. Intrinsically burst firing subicular neurons are involved in the generation and spread of epileptic activity. We measured whole-cell sodium currents in pyramidal neurons isolated from the subiculum resected in drug-resistant epileptic patients and in rats. In half of the cells from both patients and rats, the sodium current inactivated within 500 ms at -30 mV. Others displayed a tetrodotoxin-sensitive slowly or non-inactivating sodium current of up to 53% of the total sodium current amplitude. Compared with the transient sodium current in the same cells, this persistent sodium current activated with normal kinetics but its voltage-dependent activation occurred 7 mV more hyperpolarized. Depolarizing voltage steps that lasted 10 s completely inactivated the persistent sodium current. Its voltage dependence did not differ from that of the transient sodium current but its slope was less steep. The voltage dependence and kinetics of the persistent sodium current in cells from patients were not different from that in subicular cells from rats. The current density and the relative amplitude contribution were 3-4 times greater in neurons from drug-resistant epilepsy patients. The abundant presence of persistent sodium current in half of the subicular neurons could lead to a larger number of neurons with intrinsic burst firing. The extraordinarily large amplitude of the persistent sodium current in this subset of subicular neurons might explain why these patients are susceptible to seizures and hard to treat pharmacologically.

Effects of alcohols and anesthetics on recombinant voltage-gated Na+ channels

Voltage-gated Na(+) channels (Na(+) channels) mediate the rising phase of action potentials in neurons and excitable cells. Nine subtypes of the alpha subunit (Na(v)1.1-Na(v)1.9) have been shown to form functional Na(+) channels to date. Recently, anesthetic concentrations of volatile anesthetics and ethanol were reported to inhibit Na(+) channel functions, but it is not known whether all subtypes are inhibited by anesthetics. To investigate possible subtype-specific effects of anesthetics on Na(+) channels, mRNA of Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.8 alpha subunit-encoded genes were injected individually or together with a beta subunit mRNA into Xenopus oocytes. Na(+) currents were recorded using the two-electrode voltage-clamp technique. Isoflurane, at clinically relevant concentrations, inhibited the currents produced by Na(v)1.2, Na(v)1.4, and Na(v)1.6 by approximately 10% at the holding potential of -90 mV and by approximately 30% at -60 mV, but it did not affect the Na(v)1.8-mediated current. An anesthetic fluorocyclobutane (1-chloro-1,2,2-trifluorocyclobutane) also inhibited the Na(v)1.2 channel, whereas the nonanesthetic fluorocyclobutane (1,2-dichlorohexafluorocyclobutane) had no effect. The perfluorinated heptanol [CF(3)(CF(2))(5)CH(2)OH], which produces anesthesia, inhibited the Na(v)1.2 channel like other alcohols tested (ethanol, heptanol, and CF(3)CH(2)OH), even though this compound does not affect GABA, glycine, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, or kainate receptors. In contrast, most intravenous anesthetics did not have significant effects on the Na(v)1.2 channel at clinically relevant concentrations although urethane inhibited. These results show that isoflurane inhibits the Na(+) channel functions except Na(v)1.8 in a voltage-dependent manner. These findings indicate that the Na(+) channel is a neuronal target for anesthetic action.

Cooperative activation of dopamine D1 and D2 receptors increases spike firing of nucleus accumbens neurons via G-protein betagamma subunits

Dopamine in the nucleus accumbens modulates both motivational and addictive behaviors. Dopamine D1 and D2 receptors are generally considered to exert opposite effects at the cellular level, but many behavioral studies find an apparent cooperative effect of D1 and D2 receptors in the nucleus accumbens. Here, we show that a dopamine-induced enhancement of spike firing in nucleus accumbens neurons in brain slices required both D1 and D2 receptors. One intracellular mechanism that might underlie cooperativity of D1 and D2 receptors is activation of specific subtypes of adenylyl cyclases by G-protein betagamma subunits (Gbetagamma) released from the Gi/o-linked D2 receptor in combination with Galpha(s)-like subunits from the D1 receptor. In this regard, dopaminergic enhancement of spike firing was prevented by inhibitors of protein kinase A or Gbetagamma. Furthermore, intracellular perfusion with Gbetagamma enabled D1 receptor activation but not D2 receptor activation to enhance spike firing. Finally, our data suggest that these pathways may increase spike firing by inhibition of a slow A-type potassium current. These results provide evidence for a novel cellular mechanism through which cooperative action of D1 and D2 receptors in the nucleus accumbens could mediate dopamine-dependent behaviors.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.

GTP-induced tetrodotoxin-resistant Na+ current regulates excitability in mouse and rat small diameter sensory neurones

Peripheral pain thresholds are regulated by the actions of inflammatory mediators. Some act through G-protein-coupled receptors on voltage-gated sodium channels. We have found that a low-threshold, persistent tetrodotoxin-resistant Na+ current, attributed to NaV1.9, is upregulated by GTP and its non-hydrolysable analogue GTP-gamma-S, but not by GDP. Inclusion of GTP-gamma-S (500 microM) in the internal solution led to an increase in maximal current amplitude of > 300 % within 5 min. In current clamp, upregulation of persistent current was associated with a more negative threshold for action potential induction (by 15-16 mV) assessed from a holding potential of -90 mV. This was not seen in neurones without the low-threshold current or with internal GDP (P < 0.001). In addition, persistent current upregulation depolarized neurones. At -60 mV, internal GTP-gamma-S led to the generation of spontaneous activity in initially silent neurones only when persistent current was upregulated. These findings suggest that regulation of the persistent current has important consequences for nociceptor excitability.

Overview of the voltage-gated sodium channel family

Selective permeation of sodium ions through voltage-dependent sodium channels is fundamental to the generation of action potentials in excitable cells such as neurons. These channels are large integral membrane proteins and are encoded by at least ten genes in mammals. The different sodium channels have remarkably similar functional properties, but small changes in sodium-channel function are biologically relevant, as underscored by mutations that cause several human diseases of hyperexcitability.

Voltage-gated sodium channels in epilepsy

Animal experiments, and particularly functional investigations on human chronically epileptic tissue as well as genetic studies in epilepsy patients and their families strongly suggest that some forms of epilepsy may share a pathogenetic mechanism: an alteration of voltage-gated sodium channels. This review summarizes recent data on changes of sodium channel expression, molecular structure and function associated with epilepsy, as well as on the interaction of new and established antiepileptic drugs with sodium currents. Although it remains to be determined precisely how and to what extent altered sodium-channel functions play a role in different epilepsy syndromes, future promising therapy approaches may include drugs modulating sodium currents, and particularly substances changing their inactivation characteristics.

G protein modulates thyroid hormone-induced Na(+) channel activation in ventricular myocytes

To evaluate the effects of liothyronine (3,5,3'-triiodo-L-thyronine, T(3)) on Na(+) channel current (I(Na)) properties, I(Na) was recorded in adult guinea pig ventricular myocytes. T(3) (1 nM) acutely increased whole cell I(Na) and shifted the steady-state I(Na) inactivation curve dose dependently. When the pipette solution contained 100 microM GTP or GTPgammaS, the effect of T(3) on the whole cell I(Na) was increased two- to threefold. This effect was almost completely abolished by pertussis toxin preincubation. In the cell-attached patch, T(3) increased the open probability of single I(Na) by reducing the null probability. In the inside-out patch, T(3) effect was 10 times faster than that in whole cell and cell-attached patches while GTPgammaS was present and could be completely washed out. T(3) alone slightly increased the channel open probability by increasing the closed state to open state rate constant (k(CO)) and reducing the null probability. GTPgammaS exposure only increased the number of functional channels. T(3) and GTPgammaS synergistically enhanced the channel open probability 5.8 +/- 0.5-fold by increasing k(CO), decreasing the open state to absorbing inactivated state rate constant, and greatly reducing the null probability. These results demonstrate that T(3) acts on the cytosolic side of the membrane and acutely activates I(Na). Pertussis toxin-sensitive G protein modulation greatly magnifies the T(3) effects on the channel kinetics and null probability, thereby increasing the channel open probability.

Phosphorylation-dependent and phosphorylation-independent modes of modulation of shaker family voltage-gated potassium channels by SRC family protein tyrosine kinases

Modulation of voltage-gated potassium (Kv) channels by protein phosphorylation plays an essential role in the regulation of the membrane properties of cells. Protein-protein binding domains, such as Src homology 3 (SH3) domains, direct ion channel modulation by coupling the channels with intracellular signaling enzymes. The conventional view is that protein kinase binding to ion channels leads to modulation by bringing the channel substrate into physical proximity to the enzyme, thereby fostering covalent modification of the channel. The SH3 domain binding-dependent functional suppression of Kv1.5 currents by Src family protein tyrosine kinases (PTKs) is considered a canonical example of this type of mechanism. In the present study we address whether the SH3-dependent binding of Src family PTKs to Shaker family Kvs mediates modulatory events that are independent of and/or dependent on Src-catalyzed tyrosine phosphorylation of the channel. We find that Src binding and tyrosine phosphorylation are each able to modulate Kv1 family macroscopic channel currents independently. SH3-dependent binding of Src leads to the suppression of both Kv1.5 and Kv1.4 (modified to contain proline-rich SH3 domain binding sites) macroscopic currents even in the absence of Src-catalyzed tyrosine phosphorylation, whereas binding-independent tyrosine phosphorylation by Src leads to the suppression of Kv1.5 macroscopic currents and the modulation of Kv1.4 inactivation kinetics.

Molecular cloning, distribution and functional analysis of the NA(V)1.6. Voltage-gated sodium channel from human brain

We have cloned and expressed the full-length human Na(V)1.6 sodium channel cDNA. Northern analysis showed that the hNa(V)1.6 gene, like its rodent orthologues, is abundantly expressed in adult brain but not other tissues including heart and skeletal muscle. Within the adult brain, hNa(V)1.6 mRNA is widely expressed with particularly high levels in the cerebellum, occipital pole and frontal lobe. When stably expressed in human embryonic kidney cells (HEK293), the hNa(V)1.6 channel was found to be very similar in its biophysical properties to human Na(V)1.2 and Na(V)1.3 channels [Eur. J. Neurosci. 12 (2000) 4281-4289; Pflügers Arch. 441 (2001) 425-433]. Only relatively subtle differences were observed, for example, in the voltage dependence of gating. Like hNa(V)1.3 channels, hNa(V)1.6 produced sodium currents with a prominent persistent component when expressed in HEK293 cells. These persistent currents were similar to those reported for the rat Na(V)1.2 channel [Neuron 19 (1997) 443-452], although they were not dependent on over-expression of G protein betagamma subunits. These data are consistent with the proposal that Na(V)1.6 channels may generate the persistent currents observed in cerebellar Purkinje neurons [J. Neurosci. 17 (1997) 4157-4536]. However, in our hNa(V)1.6 cell line we have been unable to detect the resurgent currents that have also been described in Purkinje cells. Although Na(V)1.6 channels have been implicated in producing these resurgent currents [Neuron 19 (1997) 881-891], our data suggest that this may require modification of the Na(V)1.6 alpha subunit by additional factors found in Purkinje neurons but not in HEK293 cells.

Role of the C-terminal domain in inactivation of brain and cardiac sodium channels

Inactivation is a fundamental characteristic of Na(+) channels, and small changes cause skeletal muscle paralysis and myotonia, epilepsy, and cardiac arrhythmia. Brain Na(v)1.2a channels have faster inactivation than cardiac Na(v)1.5 channels, but minor differences in inactivation gate structure are not responsible. We constructed chimeras in which the C termini beyond the fourth homologous domains of Na(v)1.2a and Na(v)1.5 were exchanged. Replacing the C-terminal domain (CT) of Na(v)1.2a with that of Na(v)1.5 (Na(v)1.2/1.5CT) slowed inactivation at +40 mV approximately 2-fold, making it similar to Na(v)1.5. Conversely, replacing the CT of Na(v)1.5 with that of Na(v)1.2a (Nav1.5/1.2CT) accelerated inactivation, making it similar to Na(v)1.2a. Activation properties were unaffected. The voltage dependence of steady-state inactivation of Na(v)1.5 is 16 mV more negative than that of Na(v)1.2a. The steady-state inactivation curve of Na(v)1.2a was shifted +12 mV in Na(v)1.2/1.5CT, consistent with destabilization of the inactivated state. Conversely, Na(v)1.5/1.2CT was shifted -14 mV relative to Na(v)1.5, consistent with stabilization of the inactivated state. Although these effects of exchanging C termini were consistent with their effects on inactivation kinetics, they magnified the differences in the voltage dependence of inactivation between brain and cardiac channels rather than transferring them. Thus, other parts of these channels determine the basal difference in steady-state inactivation. Deletion of the distal half of either the Na(v)1.2 or Na(v)1.5 CTs accelerated open-state inactivation and negatively shifted steady-state inactivation. Thus, the C terminus has a strong influence on kinetics and voltage dependence of inactivation in brain Na(v)1.2 and cardiac Na(v)1.5 channels and is primarily responsible for their differing rates of channel inactivation.

AMPA receptor activation induces association of G-beta protein with the alpha subunit of the sodium channel in neurons

Glutamatergic transmission is mediated by ionotropic receptors that directly gate cationic channels and metabotropic receptors that are coupled to second messenger generating systems and to ionic channels via heterotrimeric guanine-nucleotide binding- (G) proteins. This distinction cannot be made for the ionotropic receptor subclass activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which has been shown to be physically associated with the alpha-subunit of Gi1 protein and activates this G-protein. Here, we report that, in addition to a Ca2+ influx, AMPA induces the mobilization of Ca2+ from the mitochondrial pool by reversing the mitochondrial Na+/Ca2+ exchanger in mouse neurons in primary culture. Both processes required the activation of tetrodotoxin-sensitive Na+ channels. AMPA receptor activation modified the gating properties of the Na+ channel, independently of the AMPA current, suggesting a G-protein-mediated process. Indeed, co-immunoprecipitation experiments indicated that AMPA receptor activation induced the association of Gbeta with the alpha-subunit of the Na+ channel. These results suggest that, in addition to its ionic channel function, the AMPA receptor is coupled to Na+ channels through G-proteins and that this novel metabotropic function is involved in the control of neuronal excitability.