Control of neuronal excitability by phosphorylation and dephosphorylation of sodium channels

Spike desensitisation as a mechanism for high-contrast selectivity in retinal ganglion cells

In the vertebrate retina, several dozens of parallel channels relay information about the visual world to the brain. These channels are represented by the different types of retinal ganglion cells (RGCs), whose responses are rendered selective for distinct sets of visual features by various mechanisms. These mechanisms can be roughly grouped into synaptic interactions and cell-intrinsic mechanisms, with the latter including dendritic morphology as well as ion channel complement and distribution. Here, we investigate how strongly ion channel complement can shape RGC output by comparing two mouse RGC types, the well-described ON alpha cell and a little-studied ON cell that is EGFP-labelled in the Igfbp5 mouse line and displays an unusual selectivity for stimuli with high contrast. Using patch-clamp recordings and computational modelling, we show that a higher activation threshold and a pronounced slow inactivation of the voltage-gated Na+ channels contribute to the distinct contrast tuning and transient responses in ON Igfbp5 RGCs, respectively. In contrast, such a mechanism could not be observed in ON alpha cells. This study provides an example for the powerful role that the last stage of retinal processing can play in shaping RGC responses.

Increased Excitability and Synaptic Plasticity of Drd1- and Drd2-Expressing Prelimbic Neurons Projecting to Nucleus Accumbens after Heroin Abstinence Are Reversed by Cue-Induced Relapse and Protein Kinase A Inhibition

Dysregulation of the input from the prefrontal cortex (PFC) to the nucleus accumbens (NAc) contributes to cue-induced opioid seeking but the heterogeneity in, and regulation of, prelimbic (PL)-PFC to NAc (PL->NAc) neurons that are altered has not been comprehensively explored. Recently, baseline and opiate withdrawal-induced differences in intrinsic excitability of Drd1+ (D1+) versus Drd2+ (D2+) PFC neurons have been demonstrated. Thus, here we investigated physiological adaptations of PL->NAc D1+ versus D2+ neurons after heroin abstinence and cue-induced relapse. Drd1-Cre+ and Drd2-Cre+ transgenic male Long-Evans rats with virally labeled PL->NAc neurons were trained to self-administer heroin followed by 1 week of forced abstinence. Heroin abstinence significantly increased intrinsic excitability in D1+ and D2+ PL->NAc neurons and increased postsynaptic strength selectively in D1+ neurons. These changes were normalized by cue-induced relapse to heroin seeking. Based on protein kinase A (PKA)-dependent changes in the phosphorylation of plasticity-related proteins in the PL cortex during abstinence and cue-induced relapse to cocaine seeking, we assessed whether the electrophysiological changes in D1+ and D2+ PL->NAc neurons during heroin abstinence were regulated by PKA. In heroin-abstinent PL slices, application of the PKA antagonist (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP-cAMPs) reversed intrinsic excitability in both D1+ and D2+ neurons and postsynaptic strength in only D1+ neurons. Additionally, in vivo bilateral intra-PL infusion of RP-cAMPs after abstinence from heroin inhibited cue-induced relapse to heroin seeking. These data reveal that PKA activity in D1+ and D2+ PL->NAc neurons is not only required for abstinence-induced physiological adaptations but is also required for cue-induced relapse to heroin seeking.SIGNIFICANCE STATEMENT Neuronal plasticity in the medial prefrontal cortex is thought to underlie relapse to drug seeking, yet the subpopulation of neurons that express this plasticity to functionally guide relapse is unclear. Here we show cell type-specific adaptations in Drd1-expressing versus Drd2-expressing prelimbic pyramidal neurons with efferent projections to nucleus accumbens. These adaptations are bidirectionally regulated during abstinence versus relapse and involve protein kinase A (PKA) activation. Furthermore, we show that disruption of the abstinence-associated adaptations via site-specific PKA inhibition abolishes relapse. These data reveal the promising therapeutic potential of PKA inhibition for preventing relapse to heroin seeking and suggest that cell type-specific pharmacologies that target subpopulations of prefrontal neurons would be ideal for future therapeutic developments.

Dendritic Excitability and Synaptic Plasticity In Vitro and In Vivo

Much of our understanding of dendritic and synaptic physiology comes from in vitro experimentation, where the afforded mechanical stability and convenience of applying drugs allowed patch-clamping based recording techniques to investigate ion channel distributions, their gating kinetics, and to uncover dendritic integrative and synaptic plasticity rules. However, with current efforts to study these questions in vivo, there is a great need to translate existing knowledge between in vitro and in vivo experimental conditions. In this review, we identify discrepancies between in vitro and in vivo ionic composition of extracellular media and discuss how changes in ionic composition alter dendritic excitability and plasticity induction. Here, we argue that under physiological in vivo ionic conditions, dendrites are expected to be more excitable and the threshold for synaptic plasticity induction to be lowered. Consequently, the plasticity rules described in vitro vary significantly from those implemented in vivo.

Control of neuronal excitability by GSK-3beta: Epilepsy and beyond

Glycogen synthase kinase 3beta (GSK-3β) is an enzyme with a variety of cellular functions in addition to the regulation of glycogen metabolism. In the central nervous system, different intracellular signaling pathways converge on GSK-3β through a cascade of phosphorylation events that ultimately control a broad range of neuronal functions in the development and adulthood. In mice, genetically removing or increasing GSK-3β cause distinct functional and structural neuronal phenotypes and consequently affect cognition. Precise control of GSK-3β activity is important for such processes as neuronal migration, development of neuronal morphology, synaptic plasticity, excitability, and gene expression. Altered GSK-3β activity contributes to aberrant plasticity within neuronal circuits leading to neurological, psychiatric disorders, and neurodegenerative diseases. Therapeutically targeting GSK-3β can restore the aberrant plasticity of neuronal networks at least in animal models of these diseases. Although the complete repertoire of GSK-3β neuronal substrates has not been defined, emerging evidence shows that different ion channels and their accessory proteins controlling excitability, neurotransmitter release, and synaptic transmission are regulated by GSK-3β, thereby supporting mechanisms of synaptic plasticity in cognition. Dysregulation of ion channel function by defective GSK-3β activity sustains abnormal excitability in the development of epilepsy and other GSK-3β-linked human diseases.

Environmental Enrichment and Social Isolation Mediate Neuroplasticity of Medium Spiny Neurons through the GSK3 Pathway

Resilience and vulnerability to neuropsychiatric disorders are linked to molecular changes underlying excitability that are still poorly understood. Here, we identify glycogen-synthase kinase 3β (GSK3β) and voltage-gated Na+ channel Nav1.6 as regulators of neuroplasticity induced by environmentally enriched (EC) or isolated (IC) conditions-models for resilience and vulnerability. Transcriptomic studies in the nucleus accumbens from EC and IC rats predicted low levels of GSK3β and SCN8A mRNA as a protective phenotype associated with reduced excitability in medium spiny neurons (MSNs). In vivo genetic manipulations demonstrate that GSK3β and Nav1.6 are molecular determinants of MSN excitability and that silencing of GSK3β prevents maladaptive plasticity of IC MSNs. In vitro studies reveal direct interaction of GSK3β with Nav1.6 and phosphorylation at Nav1.6T1936 by GSK3β. A GSK3β-Nav1.6T1936 competing peptide reduces MSNs excitability in IC, but not EC rats. These results identify GSK3β regulation of Nav1.6 as a biosignature of MSNs maladaptive plasticity.

Reappraising striatal D1- and D2-neurons in reward and aversion

The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion. In this review we will discuss pharmacological, genetic and optogenetic studies that indicate that there is still controversy to what concerns the role of striatal D1- and D2-MSNs in this type of behaviors, highlighting the need to reconsider the early view that they mediate solely opposing aspects of valenced behaviour.

Low-level laser therapy (LLLT) reduces oxidative stress in primary cortical neurons in vitro

Low-level laser (light) therapy (LLLT) involves absorption of photons being in the mitochondria of cells leading to improvement in electron transport, increased mitochondrial membrane potential (MMP), and greater ATP production. Low levels of reactive oxygen species (ROS) are produced by LLLT in normal cells that are beneficial. We exposed primary cultured murine cortical neurons to oxidative stressors: hydrogen peroxide, cobalt chloride and rotenone in the presence or absence of LLLT (3 J/cm², CW, 810 nm wavelength laser, 20 mW/cm²). Cell viability was determined by Prestoblue™ assay. ROS in mitochondria was detected using Mito-sox, while ROS in cytoplasm was detected with CellRox™. MMP was measured with tetramethylrhodamine. In normal neurons LLLT elevated MMP and increased ROS. In oxidatively-stressed cells LLLT increased MMP but reduced high ROS levels and protected cultured cortical neurons from death. Although LLLT increases ROS in normal neurons, it reduces ROS in oxidatively-stressed neurons. In both cases MMP is increased. These data may explain how LLLT can reduce clinical oxidative stress in various lesions while increasing ROS in cells in vitro.

Localisation and role of activin receptor-interacting protein 1 in mouse brain

Activin A, a stimulator of follicle-stimulating hormone secretion from the pituitary, acts as a neurotrophic and neuroprotective factor in the central nervous system. Activin receptor-interacting protein 1 (ARIP1) has been identified as a cytoplasmic protein that interacts with the type II receptor of activin (ActRII). However, the distribution pattern and function of ARIP1 are not well characterised in the brain. In the present study, we confirmed the existence of mRNA and protein of ARIP1 in the mouse brain, and found that ARIP1 was mainly localised at the hippocampus and hypothalamus in the cerebrum, granular layers in the cerebellum (especially in Purkinje cells of the cerebellum) and choroid epithelial cells by immunohistochemical staining. Furthermore, in contrast to the significant increase of activin A mRNA, ARIP1 mRNA and protein expression decreased in the mechanically lesioned brain of the mouse. Using neuroblastoma-derived Neuro-2a cells to investigate the function of ARIP1, we found that overexpression of ARIP1 down-regulated the activin A-induced signal transduction and significantly decreased the voltage-gated Na(+) current (I(Na) ). These data indicate that ARIP1 is a key molecule for the regulation of the action of activin in neurones, and also that decreased ARIP1 expression in the lesioned brain may be beneficial to the neurotrophic and neuroprotective roles of activin A in recovery after brain injury.

Nicotine suppresses hyperexcitability of colonic sensory neurons and visceral hypersensivity in mouse model of colonic inflammation

Recently, we reported that nicotine in vitro at a low 1-μM concentration suppresses hyperexcitability of colonic dorsal root ganglia (DRG; L(1)-L(2)) neurons in the dextran sodium sulfate (DSS)-induced mouse model of acute colonic inflammation (1). Here we show that multiple action potential firing in colonic DRG neurons persisted at least for 3 wk post-DSS administration while the inflammatory signs were diminished. Similar to that in DSS-induced acute colitis, bath-applied nicotine (1 μM) gradually reduced regenerative multiple-spike action potentials in colonic DRG neurons to a single action potential in 3 wk post-DSS neurons. Nicotine (1 μM) shifted the activation curve for tetrodotoxin (TTX)-resistant sodium currents in inflamed colonic DRG neurons (voltage of half-activation changed from -37 to -32 mV) but did not affect TTX-sensitive currents in control colonic DRG neurons. Further, subcutaneous nicotine administration (2 mg/kg b.i.d.) in DSS-treated C57Bl/J6 male mice resulted in suppression of hyperexcitability of colonic DRG (L(1)-L(2)) neurons and the number of abdominal constrictions in response to intraperitoneal injection of 0.6% acetic acid. Collectively, the data suggest that neuronal nicotinic acetylcholine receptor-mediated suppression of hyperexcitability of colonic DRG neurons attenuates reduction of visceral hypersensitivity in DSS mouse model of colonic inflammation.

Modulation of striatal projection systems by dopamine

The basal ganglia are a chain of subcortical nuclei that facilitate action selection. Two striatal projection systems--so-called direct and indirect pathways--form the functional backbone of the basal ganglia circuit. Twenty years ago, investigators proposed that the striatum's ability to use dopamine (DA) rise and fall to control action selection was due to the segregation of D(1) and D(2) DA receptors in direct- and indirect-pathway spiny projection neurons. Although this hypothesis sparked a debate, the evidence that has accumulated since then clearly supports this model. Recent advances in the means of marking neural circuits with optical or molecular reporters have revealed a clear-cut dichotomy between these two cell types at the molecular, anatomical, and physiological levels. The contrast provided by these studies has provided new insights into how the striatum responds to fluctuations in DA signaling and how diseases that alter this signaling change striatal function.

Dopaminergic modulation of striatal neurons, circuits, and assemblies

In recent years, there has been a great deal of progress toward understanding the role of the striatum and dopamine in action selection. The advent of new animal models and the development of optical techniques for imaging and stimulating select neuronal populations have provided the means by which identified synapses, cells, and circuits can be reliably studied. This review attempts to summarize some of the key advances in this broad area, focusing on dopaminergic modulation of intrinsic excitability and synaptic plasticity in canonical microcircuits in the striatum as well as recent work suggesting that there are neuronal assemblies within the striatum devoted to particular types of computation and possibly action selection.

Temporal lobe epilepsy induces intrinsic alterations in Na channel gating in layer II medial entorhinal cortex neurons

Temporal lobe epilepsy (TLE) is the most common form of adult epilepsy involving the limbic structures of the temporal lobe. Layer II neurons of the entorhinal cortex (EC) form the major excitatory input into the hippocampus via the perforant path and consist of non-stellate and stellate neurons. These neurons are spared and hyper-excitable in TLE. The basis for the hyper-excitability is likely multifactorial and may include alterations in intrinsic properties. In a rat model of TLE, medial EC (mEC) non-stellate and stellate neurons had significantly higher action potential (AP) firing frequencies than in control. The increase remained in the presence of synaptic blockers, suggesting intrinsic mechanisms. Since sodium (Na) channels play a critical role in AP generation and conduction we sought to determine if Na channel gating parameters and expression levels were altered in TLE. Na channel currents recorded from isolated mEC TLE neurons revealed increased Na channel conductances, depolarizing shifts in inactivation parameters and larger persistent (I(NaP)) and resurgent (I(NaR)) Na currents. Immunofluorescence experiments revealed increased staining of Na(v)1.6 within the axon initial segment and Na(v)1.2 within the cell bodies of mEC TLE neurons. These studies provide support for additional intrinsic alterations within mEC layer II neurons in TLE and implicate alterations in Na channel activity and expression, in part, for establishing the profound increase in intrinsic membrane excitability of mEC layer II neurons in TLE. These intrinsic changes, together with changes in the synaptic network, could support seizure activity in TLE.

Arrhythmic risk biomarkers for the assessment of drug cardiotoxicity: from experiments to computer simulations

In this paper, we illustrate how advanced computational modelling and simulation can be used to investigate drug-induced effects on cardiac electrophysiology and on specific biomarkers of pro-arrhythmic risk. To do so, we first perform a thorough literature review of proposed arrhythmic risk biomarkers from the ionic to the electrocardiogram levels. The review highlights the variety of proposed biomarkers, the complexity of the mechanisms of drug-induced pro-arrhythmia and the existence of significant animal species differences in drug-induced effects on cardiac electrophysiology. Predicting drug-induced pro-arrhythmic risk solely using experiments is challenging both preclinically and clinically, as attested by the rise in the cost of releasing new compounds to the market. Computational modelling and simulation has significantly contributed to the understanding of cardiac electrophysiology and arrhythmias over the last 40 years. In the second part of this paper, we illustrate how state-of-the-art open source computational modelling and simulation tools can be used to simulate multi-scale effects of drug-induced ion channel block in ventricular electrophysiology at the cellular, tissue and whole ventricular levels for different animal species. We believe that the use of computational modelling and simulation in combination with experimental techniques could be a powerful tool for the assessment of drug safety pharmacology.

Pinostrobin from Cajanus cajan (L.) Millsp. inhibits sodium channel-activated depolarization of mouse brain synaptoneurosomes

This investigation focuses on the in vitro neuroactive properties of pinostrobin, a substituted flavanone from Cajanus cajan (L.) Millsp. of the Fabaceae family. We demonstrate that pinostrobin inhibits voltage-gated sodium channels of mammalian brain (IC(50)=23 µM) based on the ability of this substance to suppress the depolarizing effects of the sodium channel-selective activator veratridine in a synaptoneurosomal preparation from mouse brain. The resting membrane potential of synaptoneurosomes was unaffected by pinostrobin. The pharmacological profile of pinostrobin resembles that of depressant drugs that block sodium channels.

The role of dopamine in modulating the structure and function of striatal circuits

Dopamine (DA) is a key regulator of action selection and associative learning. The striatum has long been thought to be a major locus of DA action in this process. Although all striatal cell types express G protein-coupled receptors for DA, the effects of DA on principal medium spiny neurons (MSNs) understandably have received the most attention. In the two principal classes of MSN, DA receptor expression diverges, with striatonigral MSNs robustly expressing D(1) receptors and striatopallidal MSNs expressing D(2) receptors. In the last couple of years, our understanding of how these receptors and the intracellular signalling cascades that they couple to modulate dendritic physiology and synaptic plasticity has rapidly expanded, fuelled in large measure by the development of new optical and genetic tools. These tools also have enabled a rapid expansion of our understanding of the striatal adaptations in models of Parkinson's disease. This chapter highlights some of the major advances in these areas.

GPD1L links redox state to cardiac excitability by PKC-dependent phosphorylation of the sodium channel SCN5A

The SCN5A-encoded cardiac sodium channel underlies excitability in the heart, and dysfunction of sodium current (I(Na)) can cause fatal ventricular arrhythmia in maladies such as long QT syndrome, Brugada syndrome (BrS), and sudden infant death syndrome (SIDS). The gene GPD1L encodes the glycerol phosphate dehydrogenase 1-like protein with homology to glycerol phosphate dehydrogenase (GPD1), but the function for this enzyme is unknown. Mutations in GPD1L have been associated with BrS and SIDS and decrease I(Na) through an unknown mechanism. Using a heterologous expression system, we show that GPD1L associated with SCN5A and that the BrS- and SIDS-related mutations in GPD1L caused a loss of enzymatic function resulting in glycerol-3-phosphate PKC-dependent phosphorylation of SCN5A at serine 1503 (S1503) through a GPD1L-dependent pathway. The direct phosphorylation of S1503 markedly decreased I(Na). These results show a function for GPD1L in cell physiology and a mechanism linking mutations in GPD1L to sudden cardiac arrest. Because the enzymatic step catalyzed by GPD1L depends upon nicotinamide adenine dinucleotide, this GPD1L pathway links the metabolic state of the cell to I(Na) and excitability and may be important more generally in cardiac ischemia and heart failure.

Role of Ca(2+) in injury-induced changes in sodium current in rat skeletal muscle

Characteristics of voltage-dependent sodium current recorded from adult rat muscle fibers in loose patch mode were rapidly altered following nearby impalement with a microelectrode. Hyperpolarized shifts in the voltage dependence of activation and fast inactivation occurred within minutes. In addition, the amplitude of the maximal sodium current decreased within 30 min of impalement. Impalement triggered a sustained elevation of intracellular Ca(2+). However, buffering Ca(2+) by loading fibers with AM-BAPTA did not affect the hyperpolarized shifts in activation and inactivation, although it did prevent the reduction in current amplitude. Surprisingly, the rise in intracellular Ca(2+) occurred even in the absence of extracellular Ca(2+). This result indicated that the injury-induced Ca(2+) increase came from an intracellular source, but it was not blocked by an inhibitor of release from the sarcoplasmic reticulum, which suggested involvement of mitochondria. Ca(2+) release from mitochondria triggered by carbonyl cyanide 3-chlorophenylhydrazone was sufficient to cause a reduction in sodium current amplitude but had little effect of the voltage dependence of activation and fast inactivation. Our data suggest the effects of muscle injury can be separated into a Ca(2+)-dependent reduction in amplitude and a largely Ca(2+)-independent shift in activation and fast inactivation. Together, the impalement-induced changes in sodium current reduce the number of sodium channels available to open at the resting potential and may limit further depolarization and thus promote survival of muscle fibers following injury.

Proteomics of membrane receptors and signaling

Receptors represent an abundant class of integral membrane proteins that transmit information on various types of signals within the cell. Assemblages of receptors and their interacting proteins (receptor complexes) have emerged as important units of signal transduction for various types of receptors including G protein coupled, ligand-gated ion channel, and receptor tyrosine kinase. This review aims to summarize the major approaches and findings of receptor proteomics. Isolation and characterization of receptor complexes from cells has become common using the methods of immunoaffinity-, ligand-, and tag-based chromatography followed by MS for the analysis of enriched receptor preparations. In addition, tools such as stable isotope labeling have contributed to understanding quantitative properties and PTMs to receptors and their interacting proteins. As data from studies on receptor-protein interactions considerably expands, complementary approaches such as bioinformatics and computational biology will undoubtedly play a significant role in defining cellular and network functions for various types of receptor complexes. Findings from receptor proteomics may also shed light on the mechanism of action for pharmacological drugs and can be of value in understanding molecular pathologies of disease states.

Action potential initiation and propagation: upstream influences on neurotransmission

Axonal action potentials initiate the cycle of synaptic communication that is key to our understanding of nervous system functioning. The field has accumulated vast knowledge of the signature action potential waveform, firing patterns, and underlying channel properties of many cell types, but in most cases this information comes from somatic intracellular/whole-cell recordings, which necessarily measure a mixture of the currents compartmentalized in the soma, dendrites, and axon. Because the axon in many neuron types appears to be the site of lowest threshold for action potential initiation, the channel constellation in the axon is of particular interest. However, the axon is more experimentally inaccessible than the soma or dendrites. Recent studies have developed and applied single-fiber extracellular recording, direct intracellular recording, and optical recording techniques from axons toward understanding the behavior of the axonal action potential. We are starting to understand better how specific channels and other cellular properties shape action potential threshold, waveform, and timing: key elements contributing to downstream transmitter release. From this increased scrutiny emerges a theme of axons with more computational power than in traditional conceptualizations.

Channel density distributions explain spiking variability in the globus pallidus: a combined physiology and computer simulation database approach

Globus pallidus (GP) neurons recorded in brain slices show significant variability in intrinsic electrophysiological properties. To investigate how this variability arises, we manipulated the biophysical properties of GP neurons using computer simulations. Specifically, we created a GP neuron model database with 100,602 models that had varying densities of nine membrane conductances centered on a hand-tuned model that replicated typical physiological data. To test the hypothesis that the experimentally observed variability can be attributed to variations in conductance densities, we compared our model database results to a physiology database of 146 slice recordings. The electrophysiological properties of generated models and recordings were assessed with identical current injection protocols and analyzed with a uniform set of measures, allowing a systematic analysis of the effects of varying voltage-gated and calcium-gated conductance densities on the measured properties and a detailed comparison between models and recordings. Our results indicated that most of the experimental variability could be matched by varying conductance densities, which we confirmed with additional partial block experiments. Further analysis resulted in two key observations: (1) each voltage-gated conductance had effects on multiple measures such as action potential waveform and spontaneous or stimulated spike rates; and (2) the effect of each conductance was highly dependent on the background context of other conductances present. In some cases, such interactions could reverse the effect of the density of one conductance on important excitability measures. This context dependence of conductance density effects is important to understand drug and neuromodulator effects that work by affecting ion channels.

D1 and D2 dopamine-receptor modulation of striatal glutamatergic signaling in striatal medium spiny neurons

Dopamine shapes a wide variety of psychomotor functions. This is mainly accomplished by modulating cortical and thalamic glutamatergic signals impinging upon principal medium spiny neurons (MSNs) of the striatum. Several lines of evidence suggest that dopamine D1 receptor signaling enhances dendritic excitability and glutamatergic signaling in striatonigral MSNs, whereas D2 receptor signaling exerts the opposite effect in striatopallidal MSNs. The functional antagonism between these two major striatal dopamine receptors extends to the regulation of synaptic plasticity. Recent studies, using transgenic mice in which cells express D1 and D2 receptors, have uncovered unappreciated differences between MSNs that shape glutamatergic signaling and the influence of DA on synaptic plasticity. These studies have also shown that long-term alterations in dopamine signaling produce profound and cell-type-specific reshaping of corticostriatal connectivity and function.