Nernst Equilibrium Potential Research Articles

The Long Journey from Animal Electricity to the Discovery of Ion Channels and the Modelling of the Human Brain.

This retrospective begins with Galvani's experiments on frogs at the end of the 18th century and his discovery of 'animal electricity'. It goes on to illustrate the numerous contributions to the field of physical chemistry in the second half of the 19th century (Nernst's equilibrium potential, based on the work of Wilhelm Ostwald, Max Planck's ion electrodiffusion, Einstein's studies of Brownian motion) which led Bernstein to propose his membrane theory in the early 1900s as an explanation of Galvani's findings and cell excitability. These processes were fully elucidated by Hodgkin and Huxley in 1952 who detailed the ionic basis of resting and action potentials, but without addressing the question of where these ions passed. The emerging question of the existence of ion channels, widely debated over the next two decades, was finally accepted and, a decade later, many of them began to be cloned. This led to the possibility of modelling the activity of individual neurons in the brain and then that of simple circuits. Taking advantage of the remarkable advances in computer science in the new millennium, together with a much deeper understanding of brain architecture, more ambitious scientific goals were dreamed of to understand the brain and how it works. The retrospective concludes by reviewing the main efforts in this direction, namely the construction of a digital brain, an in silico copy of the brain that would run on supercomputers and behave just like a real brain.

Theoretical analysis of mixed open-circuit potential for high temperature electrochemical cells electrodes

The Nernst equilibrium potential calculates the theoretical OCV, which estimates the best performance achievable by an electrochemical cell. When multiple semi-reactions (or multiple ionic species) are active in one of the electrodes, the calculation of the theoretical OCV is not straightforward, since different Nernst potentials are associated to each semi-reaction. In this paper, analytical equations for calculation of the theoretical OCV are developed, using the mixed potential theory. The case of H2 and CO co-oxidation (or H2O and CO2 co-reduction) in solid oxide cells is used as a reference case, but similar conclusions can be drawn for other equivalent cases. OCV data from literature are used to calibrate and validate the model. The relative reaction rate of H2 and CO semi-reactions is estimated within the calibration process, and the result is in line with assumptions and suggestions given by other authors. The validation procedure shows predicted OCV values in line with experimental literature data, except for mixtures with relatively large CH4 concentration (e.g., 8%), for which the OCV is significantly underestimated. This is expected when thermochemical reactions, in parallel to electrochemical reactions occur, since the additional H2 produced by internal steam methane reforming is not accounted within the local mixed potential model. A fuel cell polarization model is developed based on the results from the calibration procedure, and it is used to predict the polarization behavior of an SOFC fed with a H2-H2O-CO-CO2 fuel mixture. It is found that either H2 or CO may be reduced rather than oxidized via an equivalent electrochemical water-gas-shift reaction.

Relationship between inorganic ion distribution, resting membrane potential, and the ΔG' of ATP hydrolysis: a new paradigm.

Cell membrane potential and inorganic ion distributions are currently viewed from a kinetic electric paradigm, which ignores thermodynamics. The resting membrane potential is viewed as a diffusion potential. The 9 major inorganic ions found in blood plasma (Ca2+, Na+, Mg2+, K+, H+, Cl-, HCO3-, H2PO4-, and HPO42-) are distributed unequally across the plasma membrane. This unequal distribution requires the energy of ATP hydrolysis through the action of the Na+-K+ ATPase. The cell resting membrane potential in each of 3 different tissues with widely different resting membrane potentials has been shown to be equal to the Nernst equilibrium potential of the most permeant inorganic ion. The energy of the measured distribution of the 9 major inorganic ions between extra- and intracellular phases was essentially equal to the independently measured energy of ATP hydrolysis, showing that the distribution of these 9 major ions was in near-equilibrium with the ΔG' of ATP. Therefore, thermodynamics does appear to play an essential role in the determination of the cell resting membrane potential and the inorganic ion distribution across the plasma membrane.-Veech, R. L., King, M. T., Pawlosky, R., Bradshaw, P. C., Curtis, W. Relationship between inorganic ion distribution, resting membrane potential, and the ΔG' of ATP hydrolysis: a new paradigm.

Trimetallic PtRhNi alloy nanoassemblies as highly active electrocatalyst for ethanol electrooxidation

Although nanostructures based on noble metal alloys are widely utilized in (electro)catalysis, their low-temperature synthesis remains an enormous challenge due to the different Nernst equilibrium potentials of metal precursors. Herein, we describe the successful synthesis of trimetallic PtRhNi alloy nanoassemblies (PtRhNi-ANAs) with tunable Pt/Rh ratios using a simple mixed cyanogel reduction method and provide a detailed characterization of their chemical composition, morphology, and structure. Additionally, the electrochemical properties of PtRhNi-ANAs are examined by cyclic voltammetry, revealing composition-dependent electrocatalytic activity in the ethanol oxidation reaction (EOR). Compared to a commercial Pt black electrocatalyst, optimized Pt3Rh1Ni2-ANAs display remarkably enhanced EOR electrocatalytic performance in alkaline media.

The Expanding Family of Natural Anion Channelrhodopsins Reveals Large Variations in Kinetics, Conductance, and Spectral Sensitivity

Natural anion channelrhodopsins (ACRs) discovered in the cryptophyte alga Guillardia theta generate large hyperpolarizing currents at membrane potentials above the Nernst equilibrium potential for Cl− and thus can be used as efficient inhibitory tools for optogenetics. We have identified and characterized new ACR homologs in different cryptophyte species, showing that all of them are anion-selective, and thus expanded this protein family to 20 functionally confirmed members. Sequence comparison of natural ACRs and engineered Cl−-conducting mutants of cation channelrhodopsins (CCRs) showed radical differences in their anion selectivity filters. In particular, the Glu90 residue in channelrhodopsin 2, which needed to be mutated to a neutral or alkaline residue to confer anion selectivity to CCRs, is nevertheless conserved in all of the ACRs identified. The new ACRs showed a large variation of the amplitude, kinetics, and spectral sensitivity of their photocurrents. A notable variant, designated “ZipACR”, is particularly promising for inhibitory optogenetics because of its combination of larger current amplitudes than those of previously reported ACRs and an unprecedentedly fast conductance cycle (current half-decay time 2–4 ms depending on voltage). ZipACR expressed in cultured mouse hippocampal neurons enabled precise photoinhibition of individual spikes in trains of up to 50 Hz frequency.

Deposition of Nickel on Electrodeposited Cu2O Films at Potentials More Positive than the Nernst Potential of Ni2+/Ni0

Deposition of nickel on electrodeposited Cu2O films was investigated at potentials more positive than the estimated Nernst equilibrium potential of Ni2+/Ni0 for the purpose of developing a monolayer deposition method that will act as a protective and catalytic overlayer on the oxide films. Cu2O films were subjected to 0.08 mM nickel acetate solutions for 60 s at potentials more positive than the estimated Nernst equilibrium potential of Ni2+/Ni0. The quantities of Ni recovered from the Cu2O films were determined by atomic absorption spectrometry. The oxidation state of Ni deposited on the Cu2O was determined to be +3 via X-ray photoelectron spectroscopy. The absence of evidence of reduction of Ni2+ to Ni0 suggests that the nature of the nickel deposition is adsorption, and it is not characterized as underpotential deposition. Photoelectrochemical measurements showed that deposited Ni protected Cu2O films from the photodecomposition to Cu0 by interfering with H+ binding to the O2− surface sites, and indicated the possibility of catalytic activity of deposited nickel for H+ reduction. The catalytic activity of deposited nickel was explained by the shift of the Cu2O Fermi level by 50 mV in negative direction upon nickel deposition.

Paralysis and heart failure precede ion balance disruption in heat-stressed European green crabs

Acute exposure of ectotherms to critically high temperatures causes injury and death, and this mortality has been associated with a number of physiological perturbations including impaired oxygen transport, loss of ion and water homeostasis, and neuronal failure. It is difficult to discern which of these factors, if any, is the proximate cause of heat injury because, for example, loss of ion homeostasis can impair neuromuscular function (including cardiac function), and conversely impaired oxygen transport reduces ATP supply and can thus reduce ion transport capacity. In this study we investigated if heat stress causes a loss of ion homeostasis in marine crabs and examined if such loss is related to heart failure. We held crabs (Carcinus maenas) at temperatures just below their critical thermal maximum and measured extracellular (hemolymph) and intracellular (muscle) ion concentrations over time. Analysis of Arrhenius plots for heart rates during heating ramps revealed a breakpoint temperature below which heart rate increased with temperature, and above which heart rate declined until complete cardiac failure. As hypothesised, heat stress reduced the Nernst equilibrium potentials of both K+ and Na+, likely causing a depolarization of the membrane potential. To examine whether this loss of ion balance was likely to cause disruption of neuromuscular function, we exposed crabs to the same temperatures, but this time measured ion concentrations at the individual-specific times of complete paralysis (from which the crabs never recovered), and at the time of cardiac failure. Loss of ion balance was observed only after both paralysis and complete heart failure had occurred; indicating that the loss of neuromuscular function is not caused by a loss of ion homeostasis. Instead we suggest that the observed loss of ion balance may be linked to tissue damage related to heat death.

Tight Junction Elaboration Aids Hypersaline Secretion in Euryhaline Teleost Fish

We examined how the Na+ conductive paracellular pathway between ionocytes and accessory cells is modified in hypersaline conditions to allow Na+ secretion in extreme environments. Mummichogs (Fundulus heteroclitus) were acclimated to seawater (SW) or hypersaline conditions (2SW, 64 ‰) for 30 days. In 2SW fish, invasive cellular projections of accessory cells with increased area of simple tight junctions were detected by the punctate distribution of CFTR immunofluorescence and by TEM ultrastructure of the opercular epithelium (OE), a gill‐like tissue rich in mitochondrion‐rich ionocytes. Punctate CFTR distribution was not explained by membrane raft organization, as chlorpromazine (50 μM) and filipin (1.5 μM) were ineffective in changing electrophysiology of OE. Isolated OE from SW fish with SW on the mucosal side had a transepithelial potential (Vt) of +40.1 + 0.9 mV (n = 24), sufficient to allow passive Na+ secretion (Nernst equilibrium potential ≡ ENa = +24.11 mV). OE from fish acclimated to 2SW and bathed in 2SW (mucosal side) had significantly higher Vt of +45.1 + 1.2 mV (n = 24; P < 0.001) and plasma Na+ was slightly elevated, sufficient for passive Na+ secretion (ENa = +40.74 mV), but with a severely diminished net driving force. Paradoxically, estimates of shunt conductance from epithelial conductance (Gt ) vs. short‐circuit current (Isc) plots (extrapolation to zero Isc) revealed significant reduction in total epithelial shunt conductance in 2SW acclimated fish. We conclude that whereas most epithelial tight junctions become less conductive in hypersaline conditions, those localized to the paracellular Na+ exit pathway become more conductive, compensating for lower driving force. Funded by NSERC and CFI.

Functional identification of an outwardly rectifying pH‐ and anesthetic‐sensitive leak K+ conductance in hippocampal astrocytes

Astrocytes function as spatial K(+) buffers by expressing a rich repertoire of K(+) channels. Earlier studies suggest that acid-sensitive tandem-pore K(+) channels, mainly TWIK-related acid-sensitive K(+) (TASK) channels, mediate part of the passive astroglial membrane conductance. Here, using a combination of electrophysiology and pharmacology, we investigated the presence of TASK-like conductance in hippocampal astrocytes of rat brain slices. Extracellular pH shifts to below 7.4 (or above 7.4) induced a prominent inward (or outward) current in astrocytes in the presence of tetrodotoxin, a Na(+) channel blocker, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonate, a co-transporter blocker. The pH-sensitive current was insensitive to quinine, a potent blocker of tandem-pore K(+) channels including TWIK-1 and TREK-1 channels. Voltage-clamp analysis revealed that the pH-sensitive current exhibited weak outward rectification with a reversal potential of -112 mV, close to the Nernst equilibrium potential for K(+) . Furthermore, the current-voltage relationship was well fitted with the Goldman-Hodgkin-Katz current equation for the classical open-rectifier 'leak' K(+) channel. The pH-sensitive K(+) current was potentiated by TASK channel modulators such as the volatile anesthetic isoflurane but depressed by the local anesthetic bupivacaine. However, unlike TASK channels, the pH-sensitive current was insensitive to Ba(2+) and quinine. Thus, the molecular identity of the pH-sensitive leak K(+) channel is unlikely to be attributable to TASK channels. Taken together, our results suggest a novel yet unknown leak K(+) channel underlying the pH- and anesthetic-sensitive background conductance in hippocampal astrocytes.

Sodium current of neurosecretory cells in the eyestalk from the shrimp Penaeus japonicus

The properties of the inward current of medulla terminalis-X-organ (MTXO) cells isolated from the Penaeus japonicus eyestalk were studied with the whole-cell clamp technique in the presence of Ca 2+ and K + channel blockers. The inward currents had a threshold at about −50 mV and peaked at −10 mV. The reversed potential ( V rev) was very close to V Na, the theoretical Nernst equilibrium potential for Na +. V rev followed V Na when the external Na + concentration was varied and the currents were entirely suppressed by 30 nM tetrodotoxin (TTX), indicating that it was carried by Na +. The smooth line of concentration-dependent inhibition of sodium currents by TTX represented the best fit with the Hill equation, yielding an IC 50 of 2.1 ± 0.1 nM. The values of the half-maximal activation voltage V h were −20.6 ± 0.5 and −19.3 ± 0.5 mV, respectively, in the absence and presence of 2 nM TTX. TTX had no significant effect on the voltage dependence of steady-state activation and inactivation of I Na. Taken together, the results suggest that the inward current recorded under our experimental conditions was carried by sodium ions flowing through fast voltage-dependent Na + channels.

The temperature-induced changes in membrane potential.

Excitability and response properties of a neuron may vary in different environmental conditions of temperature. Increase/decrease of membrane potential has been observed under increase/decrease of temperature in the external side of membrane compared with internal temperature, i.e. the internal cell environment becomes more electronegative/electropositive in relation to the external one. Changes in temperature affected the amplitude of action potentials, measured as the voltage difference between the threshold and the peak, and their duration, measured as the width of the action potential at the threshold. As the temperature is increased, the amplitude of action potential is decreased and its duration is reduced. This parameter may influence the functioning of a neuron through the temperature dependence of ion channel conductance and time constants of channel activation/inactivation factors. Alterations in temperature affect the rates of diffusion through ion channels, the rates of conformational changes that lead to their activation and inactivation, and the rates of the biochemical reactions with which ion channels are modulated and transported into and out of membranes. Measurements of the propagated action potentials at different temperatures show that temperature has a double effect on the action potential: an increase of the Nernst equilibrium potentials when the absolute temperature is decreased and a change of the rate constants by a temperature factor. Temperature induced changes in the equilibrium potentials alone have little effect on the duration and amplitude of any action potentials but, because the cell membranes are permeable to more than one ion, the relation between the membrane potential and the Nernst equilibrium potentials shows different importance of the different ionic species in determining both resting and action potentials. In contrast, the temperature induced scaling of the rate constants can have quite dramatic effects on the duration of the action potential. As well as affecting the rate of action potential propagation, temperature influences the rate of neuron firing: the changes of action potential frequencies with temperature are associated, although not in a simple manner, with changes in resting potentials. Cooling reduces the resting potential (depolarization) and this leads to a rise in action potential frequencies; but certain nerve cells show a frequency increase when temperature is raised.

Stabilizing High-Energy Crystal Structure in Silver Nanowires with Underpotential Electrochemistry

We demonstrate that commonly face-centered cubic (fcc) metallic nanowires can be stabilized in hexagonal structures even when their surface energy contribution is relatively small. With a modified electrochemical growth process, we have grown purely single-crystalline 4H silver nanowires (AgNWs) of diameters as large as 100 nm within nanoporous anodic alumina and polycarbonate templates. The growth process is not limited by the/Ag Nernst equilibrium potential, and time-resolved imaging with high-resolution transmission electron microscopy (TEM) indicates a kinematically new mechanism of nanowire growth. Most importantly, our experiments aim to separate the effects of confinement and growth conditions on the crystal structure of nanoscale systems.

Molecular cloning of a GABA receptor subunit from Laodelphax striatella (Fallén) and patch clamp analysis of the homo-oligomeric receptors expressed in a Drosophila cell line

A cDNA encoding a gamma-aminobutyric acid (GABA) receptor subunit was cloned from the small brown planthopper Laodelphax striatella. The L. striatella GABA receptor subunit was found to have high amino acid sequence similarity to the bd-type splice variant of the Drosophila GABA receptor Rdl subunit and several other GABA receptor subunits, with identities of over 70%. The cDNA was inserted into the expression vector pAc5.1-lac-Hygro. Clonal cell lines stably expressing homo-oligomeric L. striatella GABA receptors were generated by transfecting the vector into D.mel-2 cells. Expression of functional GABA receptors in the cell lines was demonstrated by whole-cell patch clamp recordings. GABA induced inward currents with an EC(50) value of 29 microM and a Hill coefficient of 1.7. The GABA-evoked responses reversed close to the Nernst equilibrium potential for chloride ions. The amplitudes of agonist-induced currents were found to be in the order muscimol (100 microM) >/= GABA (100 microM) > isoguvacine (100 microM) > cis-4-aminocrotonic acid (CACA) (100 microM) > 5-(4-piperidyl)-3-isoxazolol (4-PIOL) (1 mM). Antagonists such as fipronil (100 nM), 4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) (100 nM), dieldrin (100 nM) and SR95531 (gabazine) (1 microM) suppressed GABA-induced currents. The functional expression of a GABA receptor from an agricultural pest presents a unique opportunity to discover new molecules active at this important target site.

A new framework for physically based modeling of solid oxide fuel cells

Current solid oxide fuel cell (SOFC) cell-level models are based on the subtraction of overpotentials from the Nernst equilibrium potential. This approach has two limitations: First, the Nernst equation cannot be applied to non-equilibrium gas feeds (e.g., CH 4/H 2O mixtures). Second, the Butler–Volmer equations used to describe current/overpotential relationships do not account for the complex electrochemical processes at the three-phase boundaries. We present here a new modeling approach that is based on the combination of (1) the elementary-kinetic description of electrochemistry, where multi-step chemical mechanisms account for coupled charge-transfer and surface chemistry, (2) the physical representation of electric potential steps due to interfacial double layers, allowing to calculate the cell voltage without using the Nernst equation, (3) a quasi-three-dimensional multi-scale transport model describing surface transport towards the three-phase boundaries (nanoscopic scale), continuum mass and charge transport in the porous electrodes (microscopic scale), and gas-phase channel transport (macroscopic scale), and (4) a fully transient representation of all model equations for the prediction of electrochemical impedance spectra. Simulation results are presented for an internal-reforming SOFC operated on CH 4/H 2O mixtures. The predicted cell voltage is considerably below the value resulting from full equilibration. Distributions of current density, electric potential, and species concentrations are discussed.

Cannabinoid-induced hyperphagia: Correlation with inhibition of proopiomelanocortin neurons?

We tested the hypothesis that cannabinoids modulate feeding in male guinea pigs, and correlated cannabinoid-induced changes in feeding behavior with alterations in glutamatergic synaptic currents impinging upon proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus. Feeding experiments were performed as follows: after a three-day acclimation period, animals were weighed and injected with either the CB1 receptor agonist WIN 55,212-2 (1 mg/kg, s.c.), antagonist AM251 (3 mg/kg, s.c.) or their cremophore/ethanol/saline vehicle (1:1:18; 1 ml/kg, s.c.) each day for seven days. WIN 55,212-2 increased, whereas AM251 decreased, the rate of cumulative food intake. The agonist effect was manifest primarily by increases in meal frequency and the amount of food eaten per meal. By contrast, the antagonist effect was associated with decreases in meal frequency, duration and weight loss. For the electrophysiological experiments, we performed whole-cell patch-clamp recordings from POMC neurons in hypothalamic slices. WIN 55,212-2 decreased the amplitude of evoked, glutamatergic excitatory postsynaptic currents (eEPSCs) and increased the S2:S1 ratio. Conversely, AM251 increased eEPSC amplitude per se, and blocked the inhibitory effects of the agonist. WIN 55,212-2 also decreased miniature EPSC (mEPSC) frequency; whereas AM251 increased mEPSC frequency per se, and again blocked the inhibitory effect of the agonist. A subpopulation of cells exhibited an agonist-induced outward current, which was blocked by AM251, associated with increased conductance and reversed polarity near the Nernst equilibrium potential for K+. These data demonstrate that cannabinoids regulate appetite in the guinea pig in part through both presynaptic and postsynaptic actions on anorexigenic POMC neurons.

The anodic oxidation of silver in 1 M NaOH: electrochemistry, ex situ XPS and in situ X‐ray absorption spectroscopy

AbstractWe have investigated the electrochemical formation of oxide layers on silver in 1 M NaOH (pH 13.8) using a combination of electrochemical techniques, X‐ray photoelectron spectroscopy (XPS) and surface sensitive grazing incidence X‐ray absorption spectroscopy (XAS). While the XPS experiments were performed after the removal of the samples from the electrolyte and transfer into an ultra‐high vacuum (UHV) system under inert conditions, the XAS experiments were performed in situ and under full potential control. The application of these techniques leads to a detailed, consistent picture of the anodic oxide layer formation process on silver. All the applied techniques proved that the anodic oxidation starts at potentials lower than the corresponding Nernst potential. Extended X‐ray absorption fine structure (EXAFS) investigations suggest that the underpotential Ag+ oxide layer of some few nanometres thickness has a rather disordered structure. Time resolved XPS and EXAFS experiments have shown that the growth of these disordered layers is diffusion controlled, i.e. the thickness increases with the square root of the oxidation time. For moderately higher potentials, i.e. slightly above the Nernst equilibrium potential, the thickness of the silver(I) oxide layer increases rapidly with potential and the structure is very similar to that of crystalline Ag2O. Copyright © 2006 John Wiley & Sons, Ltd.

The role of intrinsic and agonist-activated conductances in determining the firing patterns of preoptic area neurons in the guinea pig

Whole-cell and intracellular recordings were made in coronal hypothalamic slices prepared from ovariectomized female guinea pigs. 62% of preoptic area (POA) neurons fired action potentials in a bursting manner, and exhibited a significantly greater afterhyperpolarization (AHP) than did non-bursting POA neurons. The majority (70%) of POA neurons ( n=76) displayed a time-dependent inward rectification ( I h) that was blocked by CsCl (3 mM) or by ZD 7288 (30 μM). In addition, 51% of the cells expressed a low-threshold spike (LTS) associated with a transient inward current ( I T) that was blocked by NiCl 2 (200 μM). A smaller percentage of POA neurons (29%) expressed a transient outward, A-type K + current that was antagonized by a high concentration of 4-aminopyridine (3 mM). Moreover, POA neurons responded to bath application of the μ-opioid receptor agonist DAMGO (93%) or the GABA B receptor agonist baclofen (83%) with a membrane hyperpolarization or an outward current. These responses were accompanied by a decrease in input resistance or an increase in conductance, respectively, and were attenuated by BaCl 2 (100 μM). In addition, the reversal potential for these responses closely approximated the Nernst equilibrium potential for K +. These results suggest that POA neurons endogenously express to varying degrees an AHP, an I h, an I T and an A-type K + current. The vast majority of these neurons also are inhibited upon μ-opioid or GABA B receptor stimulation via the activation of an inwardly-rectifying K + conductance. Such intrinsic and transmitter-activated conductances likely serve as important determinants of the firing patterns of POA neurons.

Physics of Electrical Effects and Energy Conversion on Membranes

The living cell's electric potential and the electromotive force of a galvanic cell are described here using a nonvolume work thermodynamics and are shown to develop and operate on the same principle, which is that in both external forces perform work at the expense of the system's free energy. The role of the membrane as external-force transmitter could be allowed for only after the momentum conservation principle had been incorporated into the thermostatic description of membrane equilibria and transport. For that it was necessary to introduce an additional term to a species' chemical potential that accounts for its potential interaction with the membrane. Then membrane equilibria, in particular the Nernst equilibrium potential, the Donnan potential, and osmotic pressure, could be properly described. The electric equivalent circuit method supplemented with an electric energy diagram is applied for describing steady-state transport across ion-selective membranes. Then it becomes evident that the system's...

A novel tetrodotoxin-resistant sodium current from an immortalized neuroepithelial cell line.

1. Voltage-gated ionic currents were recorded from cells of an immortalized neuroepithelial cell line named V1. The cell line was produced by insertion of the temperature-sensitive tsA58 allele of the SV40 large T-antigen into embryonic day 14 mouse hypothalamic cells. V1 cells display a mixed immature neural-glial phenotype and have two phenotypes, round and flat. 2. Recordings from round V1 cells demonstrate voltage-gated Na+ and K+ currents (n = 297), while no voltage-gated currents were observed in flat V1 cells (n = 45). Voltage-gated currents were recorded from cells cultured at both permissive and restrictive temperatures. 3. Internal Cs+ and external tetraethylammonium (TEA) were used to suppress outward currents. The remaining inward current has rapid activation and inactivation time constants which decreased as the test potential increased. In different cells, the amplitude of the peak inward current varied from about 50 pA to as large as 4500 pA (in 120 mM external Na+). The reversal potential for the inward current was close to the predicted Nernst equilibrium potential for Na+. Both the magnitude and reversal potential of the inward current were dependent on the external Na+ concentration. It is therefore considered to be a Na+ current, INa. 4. INa was found to be TTX resistant. About 5% of the INa was blocked by 200 nM TTX and 20 microM TTX fully suppressed the Na+ current. The apparent Kd for TTX blockade was estimated to be 1.49 microM. 5. The activation kinetics of INa could be described by a Hodgkin-Huxley model with an m3 variable. The time constants of activation and inactivation of INa are fast, similar to those of the TTX-resistant and TTX-sensitive Na+ currents in central nervous system neurons and glial cells. 6. The divalent and trivalent cations Cd2+, Co2+, Ni2+, Zn2+ and La3+ shifted the activation of INa to more positive potentials and decreased the maximal conductance in a dose-dependent manner. The apparent Kd values for blockade of the INa by Cd2+, Co2+, Ni2+, Zn2+ and La3+ were 430, 3500, 1900, 83 and 202 microM, respectively. 7. The addition of phorbol myristate acetate, an activator of protein kinase C, consistently produced a reduction in the amplitudeof INa without affecting the time course of activation or inactivation. 8. INa in V1 cells expresses a unique combination of voltage and time kinetics and sensitivity to blockade by TTX and cations. We hypothesize that this Na+ current may be expressed transiently during development of the central nervous system at the stage of development represented by the V1 cell line.

Noradrenaline increases K-conductance and reduces glutamatergic transmission in the mouse entorhinal cortex by activation of alpha 2-adrenoreceptors.

The entorhinal cortex is a gateway to the hippocampus; it receives inputs from several cortical associative areas as well as subcortical areas. Since there is evidence showing that noradrenaline reduces the epileptic activity generated in the entorhinal cortex, we have examined the action of noradrenaline in the superficial layer of the entorhinal cortex, which is the main source of afferents to the hippocampus. In a previous study we showed that noradrenaline hyperpolarized layer II entorhinal cortex neurons and reduced global synaptic transmission via alpha 2-adrenoreceptors. Here we present a detailed analysis of the effect of noradrenaline on membrane resistance and on the pharmacologically isolated postsynaptic potentials in layer II entorhinal cortex neurons of mice. Noradrenaline (50 microM) hyperpolarized most layer II entorhinal cortex neurons. This hyperpolarization corresponded to an outward current with a reversal potential following the Nernst equilibrium potential for potassium. The hyperpolarizing effect of noradrenaline was blocked by 10 microM yohimbine. These observations suggest that noradrenaline activates a potassium conductance via an alpha 2-adrenoreceptor. Noradrenaline (10-50 microM) reversibly reduced the amplitude of the pharmacologically isolated excitatory potentials mediated by both NMDA and alpha-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) receptors, the former being more strongly affected. Again this effect was blocked by 10 microM yohimbine. In contrast, GABAA-mediated synaptic transmission was virtually unaffected by noradrenaline. Thus, noradrenaline appears to strongly inhibit the glutamate-mediated synaptic transmission in the entorhinal cortex without affecting inhibitory post-synaptic potentials. These observations suggest that alpha 2-adrenergic receptor agonists may exert a beneficial effect in the control of hyperexcitability in temporal lobe epilepsy.