A design of an action potential generator for electrical neurons

The capsaicin receptor transient receptor potential cation channel vanilloid 1 (TRPV1) is activated by various noxious stimuli, and the stimuli are converted into electrical signals in primary sensory neurons. It is believed that cation influx through TRPV1 causes depolarization, leading to the activation of voltage-gated sodium channels, followed by the generation of action potential. Here we report that the capsaicin-evoked action potential could be induced by two components: a cation influx-mediated depolarization caused by TRPV1 activation and a subsequent anion efflux-mediated depolarization via activation of anoctamin 1 (ANO1), a calcium-activated chloride channel, resulting from the entry of calcium through TRPV1. The interaction between TRPV1 and ANO1 is based on their physical binding. Capsaicin activated the chloride currents in an extracellular calcium-dependent manner in HEK293T cells expressing TRPV1 and ANO1. Similarly, in mouse dorsal root ganglion neurons, capsaicin-activated inward currents were inhibited significantly by a specific ANO1 antagonist, T16Ainh-A01 (A01), in the presence of a high concentration of EGTA but not in the presence of BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. The generation of a capsaicin-evoked action potential also was inhibited by A01. Furthermore, pain-related behaviors in mice treated with capsaicin, but not with αβ-methylene ATP, were reduced significantly by the concomitant administration of A01. These results indicate that TRPV1-ANO1 interaction is a significant pain-enhancing mechanism in the peripheral nervous system.

Membranes in all cell types regulate the extra- and intracellular ionic environment. In neurons and other excitable cells, regulation of the ionic environment is also crucial for the development and maintenance of the specific signaling pathway for these cells, known as the electrical signaling system. This system is composed of two basic elements: i) a lipid bimolecular diffusion barrier, termed lipid bilayer, that separates cells from their environment, and ii) two classes of macromolecule proteins, known as ion channels and ion carriers, that regulate the movement and distribution of ions across the lipid barrier in the plasma membrane, as well as in the endoplasmic reticulum (ER) and nuclear membranes (Fig. 1). Neurons, like other cell types, also signal through nonelectrical plasma membrane-dependent mechanisms, independently of ions and proteins responsible for ion transport, and this receptor-mediated signaling frequently interacts with the electrical signaling system. This chapter focuses on electrical signaling in neurons. For receptor-mediated signaling in neurons, see Chapter 5.KeywordsExcitable CellIntercellular ChannelNeuronal nAChRsNonexcitable CellElectrical Signaling SystemThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The traditional thoughts hold that action potential is a basis of neural information transmission. Previous studies have found that microtubules are structurally connected to some ion channels such as the subunits of sodium, potassium and calcium channels on axons, suggesting that microtubules may be related to the propagation of action potential. Moreover, recent studies have demonstrated that microtubule system network in the brain may be involved in the mechanism of photon quantum brain and the origin of consciousness. These studies indicate that the structural integrity of microtubules is closely related to the action potential. However, the detailed relationship between microtubules and action potential is not clear. Here, we found that the compound action potentials of bullfrog sciatic nerve were inhibited significantly by colchicine, a microtubule depolymerizer. The inhibitory effects presented time-dependent changes and even reached to a decrease of 57% after treatment for 480 min with 20 mM Colchicine. These results suggest that the propagation and transmission of action potentials are related to the stability of microtubule system and may involve in the neural quantum mechanism.

Advances in microfabrication technology have enabled the production of devices containing arrays of thousands of closely spaced recording electrodes, which afford subcellular resolution of electrical signals in neurons and neuronal networks. Rationalizing the electrode size and configuration in such arrays demands consideration of application-specific requirements and inherent features of the electrodes. Tradeoffs among size, spatial density, sensitivity, noise, attenuation, and other factors are inevitable. Although recording extracellular signals from neurons with planar metal electrodes is fairly well established, the effects of the electrode characteristics on the quality and utility of recorded signals, especially for small, densely packed electrodes, have yet to be fully characterized. Here, we present a combined experimental and computational approach to elucidating how electrode size, and size-dependent parameters, such as impedance, baseline noise, and transmission characteristics, influence recorded neuronal signals. Using arrays containing platinum electrodes of different sizes, we experimentally evaluated the electrode performance in the recording of local field potentials (LFPs) and extracellular action potentials (EAPs) from the following cell preparations: acute brain slices, dissociated cell cultures, and organotypic slice cultures. Moreover, we simulated the potential spatial decay of point-current sources to investigate signal averaging using known signal sources. We demonstrated that the noise and signal attenuation depend more on the electrode impedance than on electrode size, per se, especially for electrodes <10 μm in width or diameter to achieve high-spatial-resolution readout. By minimizing electrode impedance of small electrodes (<10 μm) via surface modification, we could maximize the signal-to-noise ratio to electrically visualize the propagation of axonal EAPs and to isolate single-unit spikes. Due to the large amplitude of LFP signals, recording quality was high and nearly independent of electrode size. These findings should be of value in configuring in vitro and in vivo microelectrode arrays for extracellular recordings with high spatial resolution in various applications.

Eukaryotic voltage gated sodium-selective channels (VGSCs) enable influx of Na+ into excitable cells in response to a change in the transmembrane potential. This movement of ions causes the membrane depolarization occurring during the rising phase of the action potential and, as such, underlies propagation of electrical signals in neurons. The transmembrane region of VGSCs is characterized by a fourfold pseudosymmetrical architecture. In particular, the channel is constituted of four homologous repeats (referred to as domains, DI through DIV), each comprising six helical segments (S1 through S6). The first four helices (S1–S4) of each domain assemble into a separate helix bundle, the so-called voltage sensor domain, which undergoes a conformational transition in response to membrane depolarization. The remaining S5 and S6 helices from all of the domains form a tetrameric assembly, the pore domain, containing a lumen in its center. The latter constitutes a pathway connecting the extracellular and intracellular compartments, enabling diffusion of water molecules and ions across the membrane. Crucial milestones along this pathway are the selectivity filter, a section permeable to Na+ but not K+, and the activation gate, a hydrophobic plug that hinders the passage of waters and ions when the channel is in the closed state. The major features of this biological nanomachine are remarkably conserved along evolution: Voltage-gated ion channels from all kingdoms of life share a common “blueprint” with the same architecture and basic rules of functioning. In particular, VGSCs are members of a large phylogenetic family, the six-transmembrane family, also containing VGSCs from bacteria. Despite the large degree of sequence similarity, the structure of prokaryotic VGSCs is less complex than … [↵][1]1Email: vincenzo.carnevale{at}temple.edu. [1]: #xref-corresp-1-1

Electrical signaling in neurons is mediated by the opening and closing of large numbers of individual ion channels. The ion channels' state transitions are stochastic and introduce fluctuations in the macroscopic current through ion channel populations. This creates an unavoidable source of intrinsic electrical noise for the neuron, leading to fluctuations in the membrane potential and spontaneous spikes. While this effect is well known, the impact of channel noise on single neuron dynamics remains poorly understood. Most results are based on numerical simulations. There is no agreement, even in theoretical studies, on which ion channel type is the dominant noise source, nor how inclusion of additional ion channel types affects voltage noise. Here we describe a framework to calculate voltage noise directly from an arbitrary set of ion channel models, and discuss how this can be use to estimate spontaneous spike rates.

The geometric and subcellular organization of axon arbors distributes and regulates electrical signaling in neurons and networks, but the underlying mechanisms have remained elusive. In rodent cerebellar cortex, stellate interneurons elaborate characteristic axon arbors that selectively innervate Purkinje cell dendrites and likely regulate dendritic integration. We used GFP BAC transgenic reporter mice to examine the cellular processes and molecular mechanisms underlying the development of stellate cell axons and their innervation pattern. We show that stellate axons are organized and guided towards Purkinje cell dendrites by an intermediate scaffold of Bergmann glial (BG) fibers. The L1 family immunoglobulin protein Close Homologue of L1 (CHL1) is localized to apical BG fibers and stellate cells during the development of stellate axon arbors. In the absence of CHL1, stellate axons deviate from BG fibers and show aberrant branching and orientation. Furthermore, synapse formation between aberrant stellate axons and Purkinje dendrites is reduced and cannot be maintained, leading to progressive atrophy of axon terminals. These results establish BG fibers as a guiding scaffold and CHL1 a molecular signal in the organization of stellate axon arbors and in directing their dendritic innervation.

Voltage-gated ion channels are essential for electrical signaling in neurons and other excitable cells. Among them, voltage-gated sodium and calcium channels are four-domain proteins, and ion selectivity is strongly influenced by a ring of amino acids in the pore regions of these channels. Sodium channels contain a DEKA motif (i.e., amino acids D, E, K, and A at the pore positions of domains I, II, III, and IV, respectively), whereas voltage-gated calcium channels contain an EEEE motif (i.e., acidic residues, E, at all four positions). Recently, a novel family of ion channel proteins that contain an intermediate DEEA motif has been found in a variety of invertebrate species. However, the physiological role of this new family of ion channels in animal biology remains elusive. DSC1 in Drosophila melanogaster is a prototype of this new family of ion channels. In this study, we generated two DSC1 knockout lines using ends-out gene targeting via homologous recombination. DSC1 mutant flies exhibited impaired olfaction and a distinct jumpy phenotype that is intensified by heat shock and starvation. Electrophysiological analysis of the giant fiber system (GFS), a well-defined central neural circuit, revealed that DSC1 mutants are altered in the activities of the GFS, including the ability of the GFS to follow repetitive stimulation (i.e., following ability) and response to heat shock, starvation, and pyrethroid insecticides. These results reveal an important role of the DSC1 channel in modulating the stability of neural circuits, particularly under environmental stresses, likely by maintaining the sustainability of synaptic transmission.

Faculty Opinions recommendation of Evaluating the potential of using quantum dots for monitoring electrical signals in neurons.

We see the world around us when light bounces off of objects and hits the retina at the back of our eyes. This triggers electrical signals in neurons called retinal ganglion cells (RGCs), which have long structures called axons that extend out from the retina and into the parts of the brain where the signals are interpreted. As the axons grow, various ‘guidance’ molecules direct the axons to the correct part of the brain. One molecule that is important for the growth of retinal ganglion cells' axons is a protein called Vax1. This protein is a transcription factor and binds to DNA to control how and when the molecular templates used to make proteins are made—a process called transcription. Vax1 is not produced in retinal ganglion cells, but it does control the extension of these cells' axons into part of the brain called the ventral hypothalamus. In this study, the axons cross to the other side of the brain by forming a structure called optic chiasm. Humans and mice lacking Vax1 are unable to develop the optic chiasm, and the axons of their retinal ganglion cells do not reach their targets in the brain. These defects were thought to occur because the guidance molecules whose transcription is normally controlled by Vax1 were not produced in the correct amounts when Vax1 is absent. Kim et al. now challenge this view by creating a mutant version of Vax1 that cannot bind to DNA or regulate the transcription of other proteins. Retinal ganglion cell axons could still grow correctly when they were put close to cells expressing this version of the Vax1 protein. This contradicts a hypothesis that Vax1 supports axonal growth by transcribing guidance molecules. Kim et al. followed up these results by examining developing mice and reached the unexpected conclusion that Vax1 is secreted from cells in the ventral hypothalamus and binds to a type of sugar molecule found on the surface of the axons. Once bound, Vax1 can enter the axons where it appears to stimulate the production of proteins inside axons, which helps the axons to grow. These findings reveal unconventional functions for Vax1 that occur in addition to its role as a transcription factor. Vax1 is known to regulate the development of several structures in the brain, so the work of Kim et al. also raises new questions about how Vax1 controls these processes.

Ion channels are responsible for numerous physiological functions ranging from transport to chemical and electrical signaling. Although static ion channel structure has been studied following a structural biology approach, spatiotemporal investigation of the dynamic molecular mechanisms of operational ion channels has not been achieved experimentally. In particular, the role of water remains elusive. Here, we perform label-free spatiotemporal second harmonic (SH) imaging and capacitance measurements of operational voltage-gated alamethicin ion channels in freestanding lipid membranes surrounded by aqueous solution on either side. We observe changes in SH intensity upon channel activation that are traced back to changes in the orientational distribution of water molecules that reorient along the field lines of transported ions. Of the transported ions, a fraction of 10-4 arrives at the hydrated membrane interface, leading to interfacial electrostatic changes on the time scale of a second. The time scale of these interfacial changes is influenced by the density of ion channels and is subject to a crowding mechanism. Ion transport along cell membranes is often associated with the propagation of electrical signals in neurons. As our study shows that this process is taking place over seconds, a more complex mechanism is likely responsible for the propagation of neuronal electrical signals than just the millisecond movement of ions.

Field-induced reorganization of the neural membrane lipid bilayer: a proposed role in the regulation of ion-channel dynamics

This article proposes that electrostatic interaction between transiently polarized neural‐membrane ethenes and charged residues of an unfolded ion‐channel protein regulate channel closing and electrical signaling in neurons. Field effects are confined by a cytoskeleton corral that gates movement of membrane lipids from one corralled region to another. Cytoskeleton gating permits stepwise changes in the concentration of unsaturated lipids and thereby modulates ion‐channel activity. The system is hypothesized to operate at axonal branch points where impulse conduction has a low safety factor. Throughout the discussion the A‐current delayed‐rectifier potassium channel is used as an example. Implications of the model for molecular networks are briefly discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

G protein-gated inwardly rectifying potassium (GIRK) channels play essential roles in electrical signaling in neurons and muscle cells. Nonequilibrium environments provide crucial driving forces behind many cellular events. Here, we apply the antiparallel alignment double bilayer model to study GIRK2 in response to the time-dependent membrane potential. Using molecular dynamics and umbrella sampling, we examined the time-dependent environmental impact on the ion conduction, energy basis, and primary motions of GIRK2 in different complex states with phosphatidylinositol-4,5-bisphosphate (PIP2) and G-protein βγ subunits (Gβγ). The antiparallel alignment double bilayer model enables us to study the transport performance in inward and outward K+ and mixed K+ and Na+. We obtained the recoverable discharge process of GIRK2 complexed with both PIP2 and Gβγ, compared with occasional conduction under PIP2-only regulation. Calculations of potential of mean force suggest different regulation by the helix bundle crossing (HBC) gate and G-loop gate regarding different complex states and under a membrane potential. In a nonequilibrium environment, distinct functional rocking motions of GIRK2 were identified under strengthened correlations between the transmembrane helices and downstream cytoplasmic domains with binding of PIP2, cations, and Gβγ. The findings suggest the potential domain motions and dynamics associated with a nonequilibrium environment and highlight the application of the antiparallel alignment double bilayer model to investigate factors in an asymmetric environment.

Platon G. Kostyuk: Un survol unique de l’océan du temps (20/08/1924–10/05/2010)