Passive Electrical Membrane Properties Research Articles

Identification of Capacitance Distribution in Neuronal Membranes from a Fractional-Order Electrical Circuit and Whole-Cell Patch-Clamped Cells

Passive electrical membrane properties are key determinants of signaling processes in brain cells, influencing input-output responses as well as action potential firing. We propose a novel interpretation of patch-clamp recordings that brings out the fractional dynamic of the electrical properties of cell membranes and provides a better knowledge of their microscopic behavior. The passive electrical properties of the cell membrane were modeled using an electrical equivalent circuit (EEC) consisting of a constant phase element (CPE) in parallel with a resistor. The Mittag-Leffler function was used to describe the non-exponential behavior of the voltage transients that are attributable to the processes of charging and discharging the membrane capacitance. The procedure proposed, based on circuit theory and fractional calculus, was used to study the voltage transients obtained in response to low-amplitude hyperpolarizing current pulses applied to cultured mice dorsal root ganglion (DRG) neurons under whole-cell current-clamp configuration. To further validate the method, we also analyzed the voltage transients obtained from hippocampal pyramidal neurons and glial cells recorded in mice brain slices in vitro using the short-time behavior of the resulting membrane voltages.

Persistent alterations in active and passive electrical membrane properties of regenerated nerve fibers of man and mice.

Excitability of regenerated fibers remains impaired due to changes in both passive cable properties and alterations in the voltage-dependent membrane function. These abnormalities were studied by mathematical modeling in human regenerated nerves and experimental studies in mice. In three adult male patients with surgically repaired complete injuries of peripheral nerves of the arm 22 months-26 years prior to investigation, deviation of excitability measures was explained by a hyperpolarizing shift in the resting membrane potential and an increase in the passive 'Barrett and Barrett' conductance (GBB) bridging the nodal and internodal compartments. These changes were associated with an increase in the 'fast' K(+) conductance and the inward rectifier conductance (GH). Similar changes were found in regenerated mouse tibial motor axons at 1 month after a sciatic crush lesion. During the first 5 months of regeneration, GH showed partial recovery, which paralleled that in GBB. The internodal length remained one-third of normal. Excitability abnormalities could be reversed by the energy-dependent Na(+)/K(+) pump blocker ouabain resulting in membrane depolarization. Stressing the Na(+) pumping system during a strenuous activity protocol triggered partial Wallerian degeneration in regenerated nerves but not in control nerves from age-matched mice. The current data suggest that the nodal voltage-gated ion channel machinery is restored in regenerated axons, although the electrical separation from the internodal compartment remains compromised. Due to the persistent increase in number of nodes, the increased activity-dependent Na(+) influx could lead to hyperactivity of the Na(+)/K(+) pump resulting in membrane hyperpolarization and neurotoxic energy insufficiency during strenuous activity.

Xenopus laevis Oocytes as a Model System for Studying the Interaction Between Asbestos Fibres and Cell Membranes.

The mode of interaction of asbestos fibres with cell membranes is still debatable. One reason is the lack of a suitable and convenient cellular model to investigate the causes of asbestos toxicity. We studied the interaction of asbestos fibres with Xenopus laevis oocytes, using electrophysiological and morphological methods. Oocytes are large single cells, with a limited ability to endocytose molecular ligands; we therefore considered these cells to be a good model for investigating the nature of asbestos/membrane interactions. Electrophysiological recordings were performed to compare the passive electrical membrane properties, and those induced by applying positive or negative voltage steps, in untreated oocytes and those exposed to asbestos fibre suspensions. Ultrastructural analysis visualized in detail, any morphological changes of the surface membrane caused by the fibre treatment. Our results demonstrate that Amosite and Crocidolite-type asbestos fibres significantly modify the properties of the membrane, starting soon after exposure. Cells were routinely depolarized, their input resistance decreased, and the slow outward currents evoked by step depolarizations were dramatically enhanced. Reducing the availability of surface iron contained in the structure of the fibres with cation chelators, abolished these effects. Ultrastructural analysis of the fibre-exposed oocytes showed no evidence of phagocytic events. Our results demonstrate that asbestos fibres modify the oocyte membrane, and we propose that these cells represent a viable model for studying the asbestos/cell membrane interaction. Our findings also open the possibly for finding specific competitors capable of hindering the asbestos-cell membrane interaction as a means of tackling the long-standing asbestos toxicity problem.

Multiple forms of activity‐dependent intrinsic plasticity in layer V cortical neurones in vivo

Synaptic plasticity is classically considered as the neuronal substrate for learning and memory. However, activity-dependent changes in neuronal intrinsic excitability have been reported in several learning-related brain regions, suggesting that intrinsic plasticity could also participate to information storage. Compared to synaptic plasticity, there has been little exploration of the properties of induction and expression of intrinsic plasticity in an intact brain. Here, by the means of in vivo intracellular recordings in the rat we have examined how the intrinsic excitability of layer V motor cortex pyramidal neurones is altered following brief periods of repeated firing. Changes in membrane excitability were assessed by modifications in the discharge frequency versus injected current (F-I) curves. Most (approximately 64%) conditioned neurones exhibited a long-lasting intrinsic plasticity, which was expressed either by selective changes in the current threshold or in the slope of the F-I curve, or by concomitant changes in both parameters. These modifications in the neuronal input-output relationship led to a global increase or decrease in intrinsic excitability. Passive electrical membrane properties were unaffected by the intracellular conditioning, indicating that intrinsic plasticity resulted from modifications of voltage-gated ion channels. These results demonstrate that neocortical pyramidal neurones can express in vivo a bidirectional use-dependent intrinsic plasticity, modifying their sensitivity to weak inputs and/or the gain of their input-output function. These multiple forms of experience-dependent intrinsic changes, which expand the computational abilities of individual neurones, could shape new network dynamics and thus might participate in the formation of mnemonic motor engrams.

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na +, Ca 2+ and K + currents, altered membrane densities of Na + and Ca 2+ channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

The mesangial cell culture: a tool for the study of the electrophysiological and pharmacological properties of the glomerular mesangial cell

Abstract Cultured rat glomerular mesangial cells (MC) were evaluated as a tool for reliable electrophysiological measurements as well as for fluorimetric determinations of intracellular Ca ++. They had a resting potential similar to that observed in cultured vascular smooth muscle cells (VSMCs), in VSMCs of mouse kidney arterioles, or in glomerular - presumably mesangial - cells of kidney slices. The comparison with the other cell types was carried out in order to look for features distinguishing them from these cells, e.g., active and passive electrical membrane properties or electrical membrane responses to vasoactive pharmacological agents. In MCs, as well as in the other cell types, the average membrane potential was approx. − 50 mV. The vasoconstrictor peptides angiotensin II (ANG II) and arginine-vasopressin (AVP) caused depolarizations that could be blocked by the respective specific inhibitors of these compounds. The agonist-induced depolarizations have to be attributed, at least in part, to a Ca + + inward current. Norepi-nephrine, if any, had only a weak action upon MCs, whereas isoproterenol either did not influence the membrane potential or hyperpolarized the cells. Other substances tested, which had no influences upon the membrane potential, were neuropeptide Y and atriopeptin 3. As to their resting electrical properties and their responses to pharmacological agents, cultured mesangial cells did not differ from glomerular, i.e., most probably mesangial, cells in the kidney slice. The difference between mesangial cells and VSMCs consists in their reaction to noradrenaline. Whereas VSMCs respond with a marked depolarization, the noradrenaline effect upon MCs in culture and in the kidney slice is either absent or very weak. Repeated passage of the cells (more than six passages) led to a gradual loss of their responsiveness to the agonists, indicating reduced receptor expression which may be interpreted as dedifferentiation. This held for both cultured MCs and VSMCs. Fluorimetric measurements using the Ca + +-specific indicators quin-2 and fura-2 were performed with a purpose-developed, ultrasensitive photon-counting microspectro-fluorimeter. Individual MCs as well as isolated glomeruli responded to the vasoconstrictors ANG II and AVP with an increase in Ca + +-dependent fluorescence indicating that these agents indeed depolarize the cells partly via a Ca + + influx and increase cytosolic free Ca + + Owing to the absence of visual criteria for the discrimination of MCs from VSMCs, the establishment of electrophysiological differences, e.g., in the response to noradrenaline, may prove to be very helpful for cell identification, especially when questions are addressed that require recording from the cells in their microenvironment, e.g., in the kidney slice.

Cable models and their use for analysis of neuronal integration

Cable theory provides the mathematical methods that allow to evaluate the role of passive electrical membrane properties and dendritic geometry in neuronal information processing. Some of the cable models frequently employed in neurophysiology are reviewed, with special stress laid on the segmental cable model. It is concluded that a dendritic neuron can comprise a multitude of computational units, in contrast to the traditional view on the neuron as the functional elementary unit of the central nervous system.

Passive electrical membrane properties of rat neostriatal neurons in an in vitro slice preparation

The passive electrical membrane properties of rat neostriatal neurons were studied in in vitro slice preparations. The data are only from neurons having stable resting membrane potentials of more than 50 mV and able to generate action potentials of amplitudes greater than 70 mV evoked by local or intracellular stimulation. All neurons measured for current-voltage relationship (n= 52), showed non-linearity of the input resistance in the hyperpolarizing direction. The mean input resistance at the resting membrane potential was 16.6 M/om. Depolarizing postsynaptic potentials evoked by local stimulation were decreased both in their amplitude and half-decay time by inward current injections exceeding more than 1 nA due to the strong membrane rectification at these levels of hyperpolarization. The mean membrane time constant (τ 0) was 5.3 ms, as measured from the semilogarithmic plots of transmembrane potential shift produced by small hyperpolarizing current pulses. In some neurons, the equalizing term (τ 1) could be determined as well and had a mean value of 1.0 ms. Measurement of (τ 0) using the strength-latency relation showed a similar value (5.0 ms) to that measured from the voltage transients. Intracellular labeling of the recorded neurons with horseradish peroxidase suggested that the recording were obtained from medium spiny neurons.

Measurement of the electrotonic length constant in cultured muscle fibres

The measurement of passive electrical membrane properties in cultured muscle cells is complicated by the fact that the electrotonic length in these cells often exceeds their actual length, leading to serious deviations from the simple infinite cable behaviour. By using very short current pulses for these measurements and integrating the responses, the cells can be made to behave as infinite cables.