Membrane Potential

Potential-sensitive fluorescent probes oxonol V and oxonol VI were employed for monitoring membrane potential (Dw) generated by the Schizosaccharomyces pombe plasma membrane H +-ATPase reconstituted into vesicles. Oxonol VI was used for quantitative measurements of the Dw because its response to membrane potential changes can be easily calibrated, which is not possible with oxonol V. However, oxonol V has a superior sensitivity to Dw at very low concentration of reconstituted vesicles, and thus it is useful for testing quality of the reconstitution. Oxonol VI was found to be a good emission-ratiometric probe. We have shown that the reconstituted H +-ATPase generates Dw of about 160 mV on the vesicle membrane. The generated Dw was stable at least over tens of minutes. An influence of the H + membrane permeability on the Dw buildup was demonstrated by manipulating the H + permeability with the protonophore CCCP. Ratiometric measurements with oxonol VI thus offer a promising tool for studying processes accompanying the yeast plasma membrane H +-ATPasemediated Dw buildup.

öTo elucidate the role of dendritic morphology in signal transfer, the passive propagation of somatic and dendritic potentials was compared in multi-compartment models of three interneuron subpopulations in the CA1 region. Nine calbindin-, 15 calretinin-and 10 parvalbumin-containing cells were modelled incorporating the detailed geometry, the currents of the action potentials in the soma, and the AMPA, N-methyl-D-aspartate and GABA-B receptor-mediated postsynaptic currents in the dendrites. The cable properties show characteristic di¡erences among the subpopulations. The morphotonic length of calbindin and calretinin cell dendrites is larger than of parvalbumin cells. Thus parvalbumin cells are more compact than calbindin or calretinin cells unless the ratio of their axial and membrane resistivities exceeds the ratios of the other two cell types by more than 33%. In calbindin cells, the distal parts of the extremely long dendrites that invade the alveus are virtually isolated from the soma for passively propagating signals. The synaptic potentials evoked at a given morphotonic distance from the soma show larger di¡erences locally on the dendrites than on the soma in all subpopulations. Both the somatic and dendritic amplitude ratios are the smallest in PV cells. In calbindin cells the somatic amplitude of synaptic potentials evoked at the same morphotonic distance from the soma is similar regardless of the number of branchpoints along their path. In calretinin and parvalbumin cells, from dendrites with long primary segments synaptic potentials reach the soma with larger amplitude than from dendrites that are branching close to the soma. The dendrites with the larger impact on somatic membrane potential are usually the dendrites that enter the stratum lacunosum-moleculare. These results indicate that dendritic morphology plays a role in changing the e¡ectiveness of synaptic potentials evoked at di¡erent dendritic locations, and in this way is likely to be an important factor in determining the integrative properties of the di¡erent neuron populations.

A qualitative mathematical model is presented that examines membrane potential feedback on synthesis of inositol 1,4,5-trisphosphate (IP 3 ), and its role in generation and modulation of slow waves. Previous experimental studies indicate that slow waves show voltage dependence, and this is likely to result through membrane potential modulation of IP 3 . It is proposed that the observed response of the tissue to current pulse, pulse train, and maintained current injection can be explained by changes in IP 3 , modulated through a voltage-IP 3 feedback loop. Differences underlying the tissue responses to current injections of opposite polarities are shown to be due to the sequence of events following such currents. Results from this model are consistent with experimental findings and provide further understanding of these experimental observations. Specifically, we find that membrane potential can induce, abolish, and modulate slow wave frequency by altering the excitability of the tissue through the voltage-IP 3 feedback loop.

Generation of free radicals is thought to mediate the cytotoxic action of alloxan on the pancreatic ␤-cell. In this investigation, the early effects of alloxan on cell function were studied. When INS-1D insulinoma cells were exposed to alloxan (1 mM) for 45 min followed by a 3-hr recovery period, the drug increased basal insulin release while abolishing the effect of glucose in static incubations. This was associated with impaired stimulation of cellular metabolism by glucose and reduced viability, both monitored colorimetrically with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). These alterations were largely counteracted by the antioxidant butylated hydroxyanisol (BHA). Similar changes occurred when glucose was added directly after 5 min of alloxan treatment, whereas KCl-induced secretion was only partially inhibited. In perifusion, alloxan caused transient insulin secretion to 50% of the rates obtained with glucose 30 min later. Under these conditions, epinephrine abolished the stimulation due to both agents. Membrane potential and cytosolic calcium concentrations ([Ca 2ϩ ] i ) were recorded to clarify the action of alloxan. Alloxan-induced insulin release correlated with depolarization of INS-1D cells and a rise in [Ca 2ϩ ] i . Alloxan did not augment [Ca 2ϩ ] i in the presence of BHA or the absence of extracellular calcium. Nickel chloride blocked the effect of alloxan on [Ca 2ϩ ] i , whereas verapamil was ineffective. This suggests that alloxan promotes Ca 2ϩ influx through channels distinct from L-type channels, perhaps through non-selective cation channels. Thus, alloxan causes changes in INS-1D cells prevented by antioxidant treatment, suggesting that free radicals may modulate the ionic permeability leading to functional activation.

Cardiac contractile dysfunction is frequently reported in human patients and experimental animals with type-1 diabetes mellitus. The aim of this study was to investigate the voltage-dependence of contraction in ventricular myocytes from the streptozotocin (STZ)-induced diabetic rat. STZ-induced diabetes was characterised by hyperglycaemia and hypoinsulinaemia. Other characteristics included reduced body and heart weight and raised blood osmolarity. Isolated ventricular myocytes were patched in whole cell, voltage-clamp mode after correcting for membrane capacitance and series resistance. From a holding membrane potential of −40 mV, test pulses were applied at potentials between −30 and +50 mV in 10 mV increments. L-type Ca 2+ current (I Ca,L ) density and contraction was measured simultaneously using a video-edge detection system. Membrane capacitance was not significantly altered between control and STZ-induced diabetic myocytes. The I Ca,L density was significantly (p < 0.05) reduced throughout voltage ranges (−10 mV to +10 mV) in myocytes from STZ-treated rats compared to age-matched controls. Moreover, the amplitude of contraction was significantly reduced (p < 0.05) in myocytes from STZ-treated rats at all test potentials between −20 mV and +30 mV. However, in electrically field-stimulated (1 Hz) myocytes, the amplitude of contraction was not altered by STZ-treatment. It is suggested that in field-stimulated myocytes taken from STZ-induced diabetic hearts, prolonged action potential duration may promote increased Ca 2+ influx via the sodium-calcium exchanger (NCX), which may compensate for a reduction in Ca 2+ trigger through L-type-Ca 2+ -channels and lead to normalised contraction. (Mol Cell Biochem xxx: 1-9, 2004) 7

Successful defense depends on the ability of the plant to recognize an attacking 'enemy' as early as possible. Early defense responses require enemy-initiated signaling cascades. Their activation ensures an induced response that is quantitative, timely and coordinated with other activities of the host cells. Damage-induced ion imbalances and modulations of channel activities are the first events occurring in the plasma membrane and result in rapid perturbations of the plasma membrane potential (V m ) involving variations of cytosolic Ca 2+ concentrations. Interacting downstream networks of kinases and phytohormones mediate the signal and result in concerted gene activation. Here we review and discuss early events occurring before herbivore attack-related gene expression that are responsible for cascades of events and signal transductions, eventually leading to indirect and direct plant responses.

To document synaptic interactions between neurons in the precentral cortex of macaque monkeys, we recorded in vivo the intracellular (IC) membrane potentials of cortical neurons simultaneously with extracellular (EC) action potentials of neighboring cells. The synaptic potentials correlated with EC spikes were obtained by spike-triggered averages (STA) of the IC membrane potentials for 373 cell pairs recorded in anesthetized and awake behaving monkeys. Sixty-three STAs (17%) showed excitatory postsynaptic potentials (EPSPs), beginning after the trigger spike. Pure EPSPs had onset latencies of 0.9 +/- 0.7 msec (mean +/- SD) and amplitudes of 226 +/- 130 microV. Sixteen STAs (4%) showed postspike inhibitory postsynaptic potentials (IPSPs), with onset latencies of 0.4 +/- 0.4 msec and amplitudes of -274 +/- 188 microV. The most common waveform, observed in 82% of the STAs with features, was a broad depolarization straddling the trigger spikes, reflecting synchronized synaptic input to ...

In this paper, a new computer software package, Yalzer, is introduced for simulating single-compartmental model of neurons. Passive or excitable membranes with voltage-gated ion channels can be modeled, and current clamp and voltage clamp experiments can be simulated. In the Yalzer, first-order differential equations used to define the dynamics of the gate variables and the membrane potential are solved by two separate integration methods with variable time steps: forward Euler and exponential Euler methods. Outputs of the simulation are shown on a spreadsheet template for allowing flexible data manipulation and can be graphically displayed. The user can define the model in detail, and examine the excitability of the model and the dynamics of voltage-gated ion channels. The software package addresses to ones who want to run simple simulations of neurons without need to any programming language skills or expensive software. It can also be used for educational purposes.

Abstract² Optogenetic technology based on light activation of genetically targeted single component opsins such as Channelrhodopsin-2 (ChR2) has been changing the way neuroscience research is conducted. This technology is becoming increasingly important for neural engineering as well. The efficiency of neural stimulation with ChR2 drops at high frequencies, often before the natural limit of the neuron is reached. This study aims to investigate the underlying mechanisms that limit the efficiency of the stimulation at high frequencies. The study analyzes the dynamics of the spikes induced by ChR2 in comparison to control stimulations using patch clamp current injection. It shows that the stimulation dynamics is limited by two mechanisms: 1) a frequency independent reduction in the conductance-to-irradiance yield due to the ChR2 light adaptation process and 2) a frequency dependent reduction in the conductance-to-current yield due to a decrease in membrane re-polarization level between spikes that weakens the ionic driving force. The effect of the first mechanism can be minimized by using ChR2 mutants with lower irradiance threshold. In contrast the effect of the second mechanism is fundamentally limited by the rate the native ion channels re-polarize the membrane potential.

1 Aerobic respiration is how bacteria generate metabolic energy. It implicates a four-electron reduction of molecular oxygen ( O 2 ) to water, which can be hazardous to the cell. In particular, an abnormal electron can deviate from the electron transport chain, or cellular enzymes to O 2 can induce the production of reactive oxygen intermediates (ROI) that includes superoxide ( ), hydrogen peroxide ( H 2 O 2 ), and hydroxyl radical (

The present study analyzes the expression of the thioredoxin/thioredoxin reductase (Trx/TR) system in UVA-irradiated human skin fibroblasts. Irradiation increases the intracellular level of Trx and a time-dependent increase of Trx mRNA is observed. Our data indicate that Trx translocates from the cytoplasm to the nucleus. In addition, UV exposure results in an increase in TR synthesis. In order to evaluate the function of Trx/TR system, we investigated the antioxidant role of Trx in transient transfected cells. The ROS accumulation in UVA irradiated cells was assessed using flow cytometry. A 3-fold decrease in ROS production was observed in transiently transfected fibroblasts. These results indicate that Trx acts as an antioxidant protein in UVA irradiated fibroblasts. As ROS are inducers of cell death, this raises the question as to whether Trx is able to protect cells from apoptosis and/or necrosis induced by UVA. Six hours after UVA-irradiation, 29.92% of cells were annexin-V positive. This population was significantly reduced in Trxtransfected cells (8.58%). Moreover, this work demonstrates that Trx prevents the loss of the membrane potential of the mitochondria, the depletion of cellular ATP content, and the loss of cell viability induced by irradiation.

Assessment of the vigour, vitality or viability of microorganisms must be done on an individual basis and thus requires the non-invasive interrogation of single organisms. For suspended organisms, flow cytometry provides a powerful means of measurement of a wide range of characteristics. Similar information for microbes in aggregates or growing on surfaces may be obtained by use of confocal scanning laser microscopy. For instance, membrane potential-sensitive fluorophores can distinguish between vigorous, frail and dead cells.

including acetycholine, norepinephrine, serotonin, dopamine and a range of peptides alter the processing characteristics of cortical networks through effects on excitatory and inhibitory synaptic transmission, on the adaptation of cortical pyramidal cells, on membrane potential, on the rate of synaptic modification, and on other cortical parameters. Computational models of self-organization and associative memory function in cortical structures such as the hippocampus, piriform cortex and neocortex provide a theoretical framework in which the role of these neuromodulatory effects can be analyzed. Neuromodulators such as acetylcholine and norepinephrine appear to enhance the influence of synapses from afferent fibers arising outside the cortex relative to the synapses of intrinsic and association fibers arising from other cortical pyramidal cells. This provides a continuum between a predominant influence of external stimulation to a predominant influence of internal recall (extrinsic vs. intrinsic). Modulatory influence along this continuum may underlie effects described in terms of learning and memory, signal to noise ratio, and attention. * Corresponding author. Fax: 495-3728. ' Invited review. 0166-4328/95/$9.30 0 1995 Elsevier Science B.V. All rights reserved SSDI 0166-4328(94)00113-8

Normal fetal brain maturation can be studied by in vivo magnetic resonance imaging (MRI) from the 18th gestational week (GW) to term, and relies primarily on T2-weighted and diffusion-weighted (DW) sequences. These maturational changes must be interpreted with a knowledge of the histological background and the temporal course of the respective developmental steps. In addition, MR presentation of developing and transient structures must be considered. Signal changes associated with maturational processes can mainly be ascribed to the following changes in tissue composition and organization, which occur at the histological level: (1) a decrease in water content and increasing cell-density can be recognized as a shortening of T1- and T2-relaxation times, leading to increased T1-weighted and decreased T2-weighted intensity, respectively; (2) the arrangement of microanatomical structures to create a symmetrical or asymmetrical environment, leading to structural differences that may be demonstrated by DW-anisotropy; (3) changes in non-structural qualities, such as the onset of a membrane potential in premyelinating axons. The latter process also influences the appearance of a structure on DW sequences. Thus, we will review the in vivo MR appearance of different maturational states of the fetal brain and relate these maturational states to anatomical, histological, and in vitro MRI data. Then, the development of the cerebral cortex, white matter, temporal lobe, and cerebellum will be reviewed, and the MR appearance of transient structures of the fetal brain will be shown. Emphasis will be placed on the appearance of the different structures with the various sequences. In addition, the possible utility of dynamic fetal sequences in assessing spontaneous fetal movements is discussed.

Pain caused by nerve injury (i.e. neuropathic pain) is associated with development of neuronal hyperexcitability at several points along the pain pathway. Within primary afferents, numerous injury-induced changes have been identified but it remains unclear which molecular changes are necessary and sufficient to explain cellular hyperexcitability. To investigate this, we built computational models that reproduce the switch from a normal spiking pattern characterized by a single spike at the onset of depolarization to a neuropathic one characterized by repetitive spiking throughout depolarization. Parameter changes that were sufficient to switch the spiking pattern also enabled membrane potential oscillations and bursting, suggesting that all three pathological changes are mechanistically linked. Dynamical analysis confirmed this prediction by showing that excitability changes co-develop when the nonlinear mechanism responsible for spike initiation switches from a quasi-separatrix-crossing to a subcritical Hopf bifurcation. This switch stems from biophysical changes that bias competition between oppositely directed fast-and slow-activating conductances operating at subthreshold potentials. Competition between activation and inactivation of a single conductance can be similarly biased with equivalent consequences for excitability. ''Bias'' can arise from a multitude of molecular changes occurring alone or in combination; in the latter case, changes can add or offset one another. Thus, our results identify pathological change in the nonlinear interaction between processes affecting spike initiation as the critical determinant of how simple injury-induced changes at the molecular level manifest complex excitability changes at the cellular level. We demonstrate that multiple distinct molecular changes are sufficient to produce neuropathic changes in excitability; however, given that nerve injury elicits numerous molecular changes that may be individually sufficient to alter spike initiation, our results argue that no single molecular change is necessary to produce neuropathic excitability. This deeper understanding of degenerate causal relationships has important implications for how we understand and treat neuropathic pain.

The main function of background two-pore potassium (K(2P)) channels KCNK3/9/15 is to stabilize the cell membrane potential. We previously observed that membrane potential depolarization enhances the release of HIV-1 viruses. Because membrane polarization affects the biomembrane directly, here we examined the effects of KCNK3/9/15 on the budding of nonviral vesicles. We found that depolarization by knocking down endogenous KCNK3/9/15 promoted secretion of cell-derived vesicles. We further used Vpu (an antagonist of KCNK3) as a model for the in vivo study of depolarization-stimulated secretion. Vpu is a HIV-1-encoded, ion channel-like protein (viroporin) capable of enhancing virus release and depolarizing the cell membrane potential. We found that Vpu could also promote nonviral vesicle release, perhaps through a similar mechanism that Vpu utilizes to promote viral particle release. Notably, T cells expressing Vpu alone became pathologically low in intracellular K(+) and insensitive to extracellular K(+) or membrane potential stimulation. In contrast, heterologous expression of KCNK3 in T cells stabilized the cell potentials by maintaining intracellular K(+). We thus concluded that KCNK3/9/15 expression limits membrane depolarization and depolarization-induced secretion at least in part by maintaining intracellular K(+).

The feasibility of the use of focused ultrasound for stimulation of the superficial and deep-seated receptor structures of humans and animals are presented in this review article. Applications of this method in physiology, for research into somatosensory and hearing perception, and also in clinical medicine for the diagnosis of neurological, dermatological and hearing disorders involving changes in perception of sensations different from normal, are discussed. It is proposed that the main effective factor of focused ultrasound as a stimulus of neural structures is a mechanical one. Such a mechanical effect could produce a change in membrane potential resulting in the stimulation of neural structures, which is related to the origin of tactile, thermal and hearing sensations. The direct action of sign-altering ultrasonic oscillations during the use of comparatively long ultrasound stimuli could possibly be the main effective factor for the induction of pain sensations and can also change the thresholds of other sensations (thermal, hearing and so forth).

From the venom of the scorpion Androctonus australis, we have isolated a new bioactive polypeptide termed AaBTX-L1. When tested on the insect voltage-gated Na+ channel (para) of the fruit fly, this toxin was able to induce a clear shift in activation (V1/2), resulting in the opening of the channel at more negative membrane potentials. Furthermore, AaBTX-L1 was totally devoid of toxicity when injected into mice intracerebroventricularly and did not compete with radiolabeled voltage-gated K+ and Na+ channel toxins in binding experiments on rat brain synaptosomes. Using its N-terminal amino acid sequence to design degenerate primers, several clones were amplified by PCR from the A. australis venom gland cDNA library. As a consequence, seven full oligonucleotide sequences encoding “long-chain” polypeptides with only three disulfide bridges have been cloned for the first time and are described here. Remarkably, they share high similarity with the anti-insect toxin Birtoxin from Parabuthus transvaalicus.

The integrate-and-fire neuron model describes the state of a neuron in terms of its membrane potential, which is determined by the synaptic inputs and the injected current that the neuron receives. When the membrane potential reaches a threshold, an action potential (spike) is generated. This review considers the model in which the synaptic input varies periodically and is described by an inhomogeneous Poisson process, with both current and conductance synapses. The focus is on the mathematical methods that allow the output spike distribution to be analyzed, including first passage time methods and the Fokker-Planck equation. Recent interest in the response of neurons to periodic input has in part arisen from the study of stochastic resonance, which is the noise-induced enhancement of the signalto-noise ratio. Networks of integrate-and-fire neurons behave in a wide variety of ways and have been used to model a variety of neural, physiological, and psychological phenomena. The properties of the integrate-and-fire neuron model with synaptic input described as a temporally homogeneous Poisson process are reviewed in an accompanying paper (Burkitt in Biol Cybern, 2006).

Drinking water ATP a b s t r a c t Fast and accurate monitoring of chemical and microbiological parameters in drinking water is essential to safeguard the consumer and to improve the understanding of treatment and distribution systems. However, most water utilities and drinking water guidelines still rely solely on time-requiring heterotrophic plate counts (HPC) and plating for faecal indicator bacteria as regular microbiological control parameters. The recent development of relative simple bench-top flow cytometers has made rapid and quantitative analysis of cultivation-independent microbial parameters more feasible than ever before. Here we present a study using a combination of cultivation-independent methods including fluorescence staining (for membrane integrity, membrane potential and esterase activity) combined with flow cytometry and total adenosine tri-phosphate (ATP) measurements, to assess microbial viability in drinking water. We have applied the methods to different drinking water samples including non-chlorinated household tap water, untreated natural spring water, and commercially available bottled water. We conclude that the esterase-positive cell fraction, the total ATP values and the high nucleic acid (HNA) bacterial fraction (from SYBR Ò Green I staining) were most representative of the active/viable population in all of the water samples. These rapid methods present an alternative way to assess the general microbial quality of drinking water as well as specific events that can occur during treatment and distribution, with equal application possibilities in research and routine analysis. ª A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / w a t r e s ARTICLE IN PRESS 0043-1354/$ -see front matter ª

Many studies have shown that a large number of terpenoids and aromatic compounds contained in essential oils have significant anticancer activities, both on cell lines and on tumors in animals. The activity of these constituents is related to the activation of cell death (apoptosis) induced by the caspases proteins in cancer cells, with minor modifications of healthy cells. Many phenomena seem to occur, among which are as follows: overexpression and regulation of liver detoxification enzymes, changes in the membrane potential of cancer cells and mitochondria, production of free radicals in cancer cells, inhibition of angiogenesis, and modification of tumor-inducing genes. These active essential oil constituents appear to act synergistically with conventional chemotherapy and radiotherapy, and some clinical studies in humans are beginning to be realized. In this review, we discuss about the antitumoral activity of 13 essential oil components selected among the most studied in the literature, with a focus on their possible mode of action. We also report current data on the anticancer properties of several total essential oils.

Flow cytometry is a general method for rapidly analyzing large numbers of cells individually using light-scattering, fluorescence, and absorbence measurements. The power of this method lies both in the wide range of cellular parameters that can be determined and in the ability to obtain information on how these parameters are distributed in the cell population. Flow cytometric assays have been developed to determine both cellular characteristics such as size, membrane potential, and intracellular pH, and the levels of cellular components such as DNA, protein, surface receptors, and calcium. Measurements that reveal the distribution of these parameters in cell populations are important for biotechnology, because they better describe the population than the average values obtained from traditional techniques. This Mini-Review provides an overview of the principles of flow cytometry, with descriptions of methods used to measure various cellular parameters and examples of the application of flow cytometry in biotechnology. Finally, a discussion of the challenges and limitations of the method is presented along with a future outlook.

The aim of the work was early identification of preventable risk factors connected with the consumers usage of products of everyday use, such as cosmetics, toys and children products, and other materials intended for contact with human skin. The risk factor is represented by substances with irritation potential and subsequent possible sensitisation, resulting in negative impact on human physical and psychical health with social and societal consequences. The legislation for cosmetics, chemical substances and other products requires for hazard identification the application of alternative toxicological methods in vitro without the use of animals. For this reason we used a battery of alternative assays in vitro, based on cell cultures. Progressive methods of molecular biology, based on fluorimetry and fluorescence, were employed for identification of early morphological and functional changes on cellular level. Four colorants frequently used in cosmetics (P-WS Caramel, Chlorophyllin, Unicert Red K 7054-J and Unicert Red K 7008-J) were tested on cell line NIH3T3 (mouse fibroblast cell) and 3T3 Balb/c with/without UV irradiation (dose 5 J cm À2 ). Fluorescence methods for the study of cell damage using fluorescence probes offer results for the evaluation of cytotoxicity and cell viability of adherent cells. We detected intracellular production of ROS investigated by molecular probe CM-H 2 DCFDA, which is primarily sensitive to the increased production of hydrogen peroxide or its downstream products. Toxic effects on the cellular level were identified by viability tests using Neutral Red uptake and MTT assay, where the live cells reduce yellow soluble 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to insoluble formazan crystals. The reaction was investigated on mitochondrial membrane of living cells and the type of cell death was determined using Apoptosis detection kit. Cytotoxicity tests revealed health risks of using Chlorophyllin and Unicert Red K 7054-J.

The charge of ceramic UF membranes is studied in NaCl and CaCl 2 media from streaming potential and membrane potential measurements. The amphoteric behaviour of these materials is observed with both methods. The apparent transport numbers of cations in the membrane are determined from cell potential measurements. Streaming potential measurements and the study of the transport properties lead to similar isoelectric points of the membrane. It also appears that the presence of Ca 2 cations leads to a more positive net charge of the membrane at pH lower than the isoelectric point and a less negative charge for the higher pH. #

Condition assessment or controlling systems for cathodic protection of reinforced concrete structures are usually based on potential measurements. In case of permanent monitoring systems, reference electrodes are embedded in the concrete, whereas for occasional investigations external reference electrodes are normally used and placed on the concrete surface. However, ionic concentration gradients between the inner solution of the reference electrode and the concrete pore solution give rise to liquid junction potentials. In addition, internal gradients in pH or chloride content, as well as permselective properties of the cement paste lead to membrane potentials. Although these phenomena are usually not considered to be significant, they might be underestimated in many cases and lead to misinterpretation of measurement results. This is especially important with regard to applications of potentiometric sensors such as chloride or pH sensors, where accurate potential readings are fundamental. This article presents the theoretical background and summarises available literature with regard to diffusion potentials in concrete. Liquid junction potentials between various solutions have been measured experimentally. Practical consequences and possible attempts to minimise measurement errors are discussed.

Time-resolved electron transfer and electrogenic H ؉ translocation have been compared in a bd-type quinol oxidase from Escherichia coli and its E445A mutant. The high-spin heme b 595 is found to be retained by the enzyme in contrast to the original proposal, but it is not reducible even by excess of dithionite. When preincubated with the reductants, both the WT (b 558 2؉ , b595 2؉ , d 2؉ ) and E445A mutant oxidase (b 558 2؉ , b595 3؉ , d 2؉ ) bind O2 rapidly, but formation of the oxoferryl state in the mutant is Ϸ100-fold slower than in the WT enzyme. At the same time, the E445A substitution does not affect intraprotein electron re-equilibration after the photolysis of CO bound to ferrous heme d in the one-electron-reduced enzyme (the so-called ''electron backflow''). The backflow is coupled to membrane potential generation. Electron transfer between hemes d and b 558 is electrogenic. In contrast, electron transfer between hemes d and b 595 is not electrogenic, although heme b595 is the major electron acceptor for heme d during the backflow, and therefore is not likely to be accompanied by net H ؉ uptake or release. The E445A replacement does not alter electron distribution between hemes b 595 and d in the one-electron reduced cytochrome bd [E m(d) > Em(b595), where Em is the midpoint redox potential]; however, it precludes reduction of heme b 595, given heme d has been reduced already by the first electron. Presumably, E445 is one of the two redox-linked ionizable groups required for charge compensation of the di-heme oxygen-reducing site (b 595 , d) upon its full reduction by two electrons.

Large-conductance Ca2+-activated K+ channels (BKCa or maxiK channels) are expressed in different cell types. They play an essential role in the regulation of various cell functions. In particular, BKCa channels have been extensively studied in vascular smooth muscle cells, where they contribute to the control of vascular tone. They facilitate the feedback regulation against the rise of intracellular Ca2+, membrane depolarization and vasoconstriction. BKCa channels promote a K+ outward current and lead to membrane hyperpolarization. In endothelial cells expression and function of BKCa channels play an important role in the regulation of the vascular smooth muscle activity. Endothelial BKCa channels modulate the biosyntheses and release of various vasoactive modulators and regulate the membrane potential. Because of their regulatory role in vascular tone, endothelial BKCa channels have been suggested as therapeutic targets for the treatment of cardiovascular diseases. Hypertension, atherosclerosis, and diabetes are associated with altered current amplitude, open probability, and Ca2+-sensing of BKCa channels. The properties of BKCa channels and their role in endothelial and vascular smooth muscle cells would address them as potential therapeutic targets. Further studies are necessary to identify the detailed molecular mechanisms of action and to investigate selective BKCa channels openers as possible therapeutic agents for clinical use.

Understanding the mechanisms of episodic memory requires linking behavioral data and lesion effects to data on the dynamics of cellular membrane potentials and population interactions within brain regions. Linking behavior to specific membrane channels and neurochemicals has implications for therapeutic applications. Lesions of the hippocampus, entorhinal cortex and subcortical nuclei impair episodic memory function in humans and animals, and unit recording data from these regions in behaving animals indicate episodic memory processes. Intracellular recording in these regions demonstrates specific cellular properties including resonance, membrane potential oscillations and bistable persistent spiking that could underlie the encoding and retrieval of episodic trajectories. A model presented here shows how intrinsic dynamical properties of neurons could mediate the encoding of episodic memories as complex spatiotemporal trajectories. The dynamics of neurons allow encoding and retrieval of unique episodic trajectories in multiple continuous dimensions including temporal intervals, personal location, the spatial coordinates and sensory features of perceived objects and generated actions, and associations between these elements. The model also addresses how cellular dynamics could underlie unit firing data suggesting mechanisms for coding continuous dimensions of space, time, sensation and action.

We describe a low cost design of an electronic neuron, which is designed to represent the dynamical properties of the membrane potential of biological neurons by modeling the states of the membrane channels. This electronic neuron can be used to study the nonlinear properties of the membrane voltage dynamics and to develop and analyze small neuronal circuits using electronic neurons as building blocks.

A position dependent modulation of the membrane potential difference is induced when an electric field is applied to a cell. When cells are submitted to short lived electric field pulses with an overcritical intensity, a local membrane alteration is induced, which may reseal. Its molecular definition remains unknown. A free exchange of hydrophilic molecules takes place across the membrane. A leakage of cytosolic metabolites is present. However, a loading of polar drugs into the cytoplasm is obtained. A short description of the processes affecting the cell membrane organization is given. Lipids appear as the primary target of the field effect as in the case of liposomes. Nevertheless membrane proteins appear to be affected by a direct or by a back effect. The permeabilized state is long lived. The cell metabolism plays indeed a critical role in the recovery. The cell viability can be nevertheless preserved.

A circle criterion observer is designed for estimating the unmeasured membrane potential of neuronal populations using the electroencephalogram (EEG) from a class of parameterised models that replicates patterns seen on the EEG. Compared to existing similar designs, we provide a less conservative linear matrix inequality (LMI) condition that is shown to be fulfilled for the neural models we consider. The designed observer is robust towards disturbances in the input and measurement, as well as model uncertainty. We show that the observer can be designed for a model that reproduces alpha rhythms in the EEG as an illustrative example.

In this paper, we use a numerical method to solve boundary-value problems for a singularly-perturbed differential-difference equation of mixed type, i.e., containing both terms having a negative shift and terms having a positive shift. Similar boundary-value problems are associated with expected first exit time problems of the membrane potential in models for the neuron. The stability and convergence analysis of the method is given. The effect of a small shift on the boundary-layer solution is shown via numerical experiments. The numerical results for several test examples demonstrate the efficiency of the method.

Caribbean ciguatoxin-1 (C-CTX-1) induced, after about 1 h exposure, muscle membrane depolarisation and repetitive post-synaptic action potentials (APs) in frog neuromuscular preparations. This depolarising effect was also observed in a Ca 2þ -free medium with a strong enhancement of spontaneous quantal transmitter release, compared with control conditions. The ciguatoxin-induced increase in release could be accelerated when Ca 2þ was present in the extracellular medium. C-CTX-1 also enhanced nerve-evoked quantal acetylcholine (ACh) release. At normal neuromuscular junctions loaded with the fluorescent dye FM1-43, C-CTX-1 induced swelling of nerve terminals, an effect that was reversed by hyperosmotic D-mannitol. In myelinated axons, C-CTX-1 increased nodal membrane excitability, inducing spontaneous and repetitive APs. Also, the toxin enlarged the repolarising phase of APs in control and tetraethylammonium-treated axons. Overall, our data suggest that C-CTX-1 affects nerve excitability and neurotransmitter release at nerve terminals. We conclude that C-CTX-1-induced up-regulation of Na þ channels and the inhibition of K þ channels, at low nanomolar concentrations, produce a variety of functional dysfunctions that are in part responsible for the human muscle skeletal symptoms observed in ciguatera. All these dysfunctions seem to result from the subtle balance between ionic currents, intracellular Na þ and Ca 2þ concentrations, and engaged second messengers.

The integrate-and-fire neuron model is one of the most widely used models for analyzing the behavior of neural systems. It describes the membrane potential of a neuron in terms of the synaptic inputs and the injected current that it receives. An action potential (spike) is generated when the membrane potential reaches a threshold, but the actual changes associated with the membrane voltage and conductances driving the action potential do not form part of the model. The synaptic inputs to the neuron are considered to be stochastic and are described as a temporally homogeneous Poisson process. Methods and results for both current synapses and conductance synapses are examined in the diffusion approximation, where the individual contributions to the postsynaptic potential are small. The focus of this review is upon the mathematical techniques that give the time distribution of output spikes, namely stochastic differential equations and the Fokker-Planck equation. The integrate-and-fire neuron model has become established as a canonical model for the description of spiking neurons because it is capable of being analyzed mathematically while at the same time being sufficiently complex to capture many of the essential features of neural processing. A number of variations of the model are discussed, together with the relationship with the Hodgkin-Huxley neuron model and the comparison with electrophysiological data. A brief overview is given of two issues in neural information processing that the integrate-and-fire neuron model has contributed to -the irregular nature of spiking in cortical neurons and neural gain modulation.

Spermine is shown to uncouple isolated mitochondria and to trigger the selective release of cytochrome c. Pargyline, an inhibitor of amine oxidase (AO), fully prevented these effects of spermine, which instead were potentiated by exogenous AO. Hydrogen peroxide, an oxidation product of spermine, mimicked the effects of spermine on mitochondria, while the addition of catalase prevented them. Spermidine and putrescine also caused mitochondrial uncoupling and triggered cytochrome c release, with a potency which correlated with the substrate preference of mitochondrial AO. Pargyline protected human lymphoma U937 cells against UVB-induced apoptosis, by reducing AO activity, mitochondrial uncoupling and cytochrome c release. ß 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

In the present work, we studied the mitochondrial function and cell death pathway(s) in heterozygous and homozygous immortalized cell lines from patients with Huntington's disease (HD). Heterozygosis was characterized by specific alterations in mitochondrial membrane potential, a constitutive hyperpolarization state of mitochondria, and was correlated with an increased susceptibility to apoptosis. Lymphoblasts from homozygous patients, on the other hand, were characterized by a significant percentage of cells displaying autophagic vacuoles. These cells also demonstrated a striking attitude towards significant cannibalistic activity. Considering the pathogenic role of cell death in HD, our study provides new and useful insights into the role of mitochondrial dysfunction, i.e. hyperpolarization, in hijacking HD heterozygous cells towards apoptosis and HD homozygous cells towards a peculiar phenotype characterized by both self-and xenocannibalism. These events can, however, be viewed as an ultimate attempt to survive rather than a way to die. The present work underlines the possibility that HD-associated mitochondrial defects could tentatively be by-passed by the cells by activating cellular 'phagic' activities, including socalled 'mitophagy' and 'cannibalism', that only finally lead to cell death.