Subthreshold Membrane Potential Research Articles

IA Channels Encoded by Kv1.4 and Kv4.2 Regulate Neuronal Firing in the Suprachiasmatic Nucleus and Circadian Rhythms in Locomotor Activity

Neurons in the suprachiasmatic nucleus (SCN) display coordinated circadian changes in electrical activity that are critical for daily rhythms in physiology, metabolism, and behavior. SCN neurons depolarize spontaneously and fire repetitively during the day and hyperpolarize, drastically reducing firing rates, at night. To explore the hypothesis that rapidly activating and inactivating A-type (I(A)) voltage-gated K(+) (Kv) channels, which are also active at subthreshold membrane potentials, are critical regulators of the excitability of SCN neurons, we examined locomotor activity and SCN firing in mice lacking Kv1.4 (Kv1.4(-/-)), Kv4.2 (Kv4.2(-/-)), or Kv4.3 (Kv4.3(-/-)), the pore-forming (α) subunits of I(A) channels. Mice lacking either Kv1.4 or Kv4.2 α subunits have markedly shorter (0.5 h) periods of locomotor activity than wild-type (WT) mice. In vitro extracellular multi-electrode recordings revealed that Kv1.4(-/-) and Kv4.2(-/-) SCN neurons display circadian rhythms in repetitive firing, but with shorter periods (0.5 h) than WT cells. In contrast, the periods of wheel-running activity in Kv4.3(-/-) mice and firing in Kv4.3(-/-) SCN neurons were indistinguishable from WT animals and neurons. Quantitative real-time PCR revealed that the transcripts encoding all three Kv channel α subunits, Kv1.4, Kv4.2, and Kv4.3, are expressed constitutively throughout the day and night in the SCN. Together, these results demonstrate that Kv1.4- and Kv4.2-encoded I(A) channels regulate the intrinsic excitability of SCN neurons during the day and night and determine the period and amplitude of circadian rhythms in SCN neuron firing and locomotor behavior.

Opposing effects of intrinsic conductance and correlated synaptic input on Vm-fluctuations during network activity

Neurons often receive massive concurrent bombardment of synaptic inhibition and excitation during functional network activity. This increases membrane conductance and causes fluctuations in membrane potential (Vm) and spike timing. The conductance increase is commonly attributed to synaptic conductance, but also includes the intrinsic conductances recruited during network activity. These two sources of conductance have contrasting dynamic properties at sub-threshold membrane potentials. Synaptic transmitter gated conductance changes abruptly and briefly with each presynaptic action potential. If the spikes arrive at random times the changes in synaptic conductance are therefore stochastic and rapid during intense network activity. In comparison, sub-threshold intrinsic conductances vary smoothly in time. In the present study this discrepancy is investigated using two conductance-based models: a (1) compartment model and a (2) compartment with realistic slow intrinsic conductances. We examine the effects of varying the relative contributions of non-fluctuating intrinsic conductance with fluctuating concurrent inhibitory and excitatory synaptic conductance. For given levels of correlation in the synaptic input we find that the magnitude of the membrane fluctuations uniquely determines the relative contribution of synaptic and intrinsic conductance. We also quantify how Vm-fluctuations vary with synaptic correlations for fixed ratios of synaptic and intrinsic conductance. Interestingly, the levels of Vm -fluctuations and conductance observed experimentally during functional network activity leave little room for intrinsic conductance to contribute. Even without intrinsic conductances the variance in Vm -fluctuations can only be explained by a high degree of correlated firing among presynaptic neurons.

Chronic imaging of cortical sensory map dynamics using a genetically encoded calcium indicator

In vivo optical imaging can reveal the dynamics of large-scale cortical activity, but methods for chronic recording are limited. Here we present a technique for long-term investigation of cortical map dynamics using wide-field ratiometric fluorescence imaging of the genetically encoded calcium indicator (GECI) Yellow Cameleon 3.60. We find that wide-field GECI signals report sensory-evoked activity in anaesthetized mouse somatosensory cortex with high sensitivity and spatiotemporal precision, and furthermore, can be measured repeatedly in separate imaging sessions over multiple weeks. This method opens new possibilities for the longitudinal study of stability and plasticity of cortical sensory representations.

Role of TRP channels in dendritic integration and subthreshold membrane potential plateaus

Cortical as well as subcortical neurons display plateau properties [1]. The plateaus may be voltage and/or calcium gated and depend on Na, Ca or mixed cation (TRP) currents. Depending on the nature of the synaptic input, neurons may respond by entering one of several multiple stable states, in the form of either subthreshold or persistent firing. We have studied the activation of a TRP current, in which synaptic excitatory input activates metabotropic glutamate receptors in turn leading to a TRP-mediated slow EPSP, both experimentally [2] and computationally [3]. In the computational work we have studied the contribution of the TRP current on dendritic integration of input of low (1-10 Hz) frequency. The model is based on a CA1 pyramidal cell model [4] developed to study dendritic processing. The spatial compartmentalization had been obtained from a digitized neuron and contains 183 compartments. It includes ion channels described by a Hodgkin-Huxley formalism and has the following channels: Na, NaP, CaR, CaT, CaL, Kdr, KA, KM, K(Ca)BK, K(Ca)slow, Kleak, h. To this model we added models for a metabotropic glutamate receptor as well as a TRP channel, the latter being adapted from a CAN channel model. Continuing our computational work, we have recently studied how the dendritic integration properties depend on the time constant of the calcium activating the TRP-current. In the model calcium comes from L-type Cav1.3 channels with a low threshold but there may also be contributions from T-type channels. We find that the effective integration time constant, the decay time constant of the neuronal membrane, increases when the amplitude of the calcium concentration increases. Moreover, recent experimental work [5] has shown that the decay time constant of calcium increases as a result of increased calcium concentration. This prompted us to study effects of adding a slower decay time constant in addition to the one present in the model used so far. Intriguingly, we find that a stable subthreshold plateau appears when a slower (200 ms) calcium decay time constant is present but not when only the original faster (30 ms) is present. This plateau of the resting potential is stable for weaker synaptic excitatory or inhibitory input, but stronger input can switch the membrane between the two levels, see figure ​figure1.1. We find that the subthreshold plateau is essentially local to individual dendritic branches, and that the cell enters a persistent spiking [1] mode when several branches enters plateau mode. This bistability in membrane potential due to intrinsic conductances may be compared to the network driven changes of subthreshold membrane potential during cortical up and down states, suggested to switch the neuron between different modes of integration/operation. Figure 1 The membrane voltage (Vm) is resting close to -70 mV. At the arrival of an excitatory synaptic stimulus, Vm enters a more depolarized, subthreshold plateau, which is stable to weak inhibitory input (t = 3000 ms). Vm returns to its resting state only if ...

Striatal cellular properties conserved from lampreys to mammals

The striatum of the lamprey, the first vertebrate group to appear in evolution, shows striking similarities to that of mammals with respect to histochemical markers, afferent and efferent projections and the effect of dopamine depletion, which leads to hypokinetic motor symptoms. The cellular properties of lamprey striatal neurons were studied here using patch-clamp recordings in acute striatal slices. Sixty-five per cent of recorded neurons were characterised by a prominent inward rectification due to a K+ conductance of the Kir type. They had a ramping response with a long delay to the first action potential due to activation of a low-voltage-activated A-type K+ current. Many such inwardly rectifying neurons (IRNs) had a hyperpolarised resting membrane potential and some had spiny dendrites. The remaining 35% of the neurons (non-IRNs) represent a heterogeneous group, including some with characteristics similar to the fast-spiking interneuron of the mammalian striatum. They showed short-lasting, large after hyperpolarisations (AHPs) and discharged action potentials at high frequency. None of the recorded neurons were spontaneously active but they received GABAergic and glutamatergic synaptic input. The fact that most lamprey striatal neurons display inward rectification indicates that this is a conserved characteristic of striatal neurons throughout vertebrate phylogeny. This is a cellular property of critical importance for the operations of the striatum in mammals.

Quadratic sinusoidal analysis of voltage clamped neurons

Nonlinear biophysical properties of individual neurons are known to play a major role in the nervous system, especially those active at subthreshold membrane potentials that integrate synaptic inputs during action potential initiation. Previous electrophysiological studies have made use of a piecewise linear characterization of voltage clamped neurons, which consists of a sequence of linear admittances computed at different voltage levels. In this paper, a fundamentally new theory is developed in two stages. First, analytical equations are derived for a multi-sinusoidal voltage clamp of a Hodgkin-Huxley type model to reveal the quadratic response at the ionic channel level. Second, the resulting behavior is generalized to a novel Hermitian neural operator, which uses an algebraic formulation capturing the entire quadratic behavior of a voltage clamped neuron. In addition, this operator can also be used for a nonlinear identification analysis directly applicable to experimental measurements. In this case, a Hermitian matrix of interactions is built with paired frequency probing measurements performed at specific harmonic and interactive output frequencies. More importantly, eigenanalysis of the neural operator provides a concise signature of the voltage dependent conductances determined by their particular distribution on the dendritic and somatic membrane regions of neurons. Finally, the theory is concretely illustrated by an analysis of an experimentally measured vestibular neuron, providing a remarkably compact description of the quadratic responses involved in the nonlinear processing underlying the control of eye position during head rotation, namely the neural integrator.

Intracellular Determinants of Hippocampal CA1 Place and Silent Cell Activity in a Novel Environment

For each environment a rodent has explored, its hippocampus contains a map consisting of a unique subset of neurons, called place cells, that have spatially tuned spiking there, with the remaining neurons being essentially silent. Using whole-cell recording in freely moving rats exploring a novel maze, we observed differences in intrinsic cellular properties and input-based subthreshold membrane potential levels underlying this division into place and silent cells. Compared to silent cells, place cells had lower spike thresholds and peaked versus flat subthreshold membrane potentials as a function of animal location. Both differences were evident from the beginning of exploration. Additionally, future place cells exhibited higher burst propensity before exploration. Thus, internal settings appear to predetermine which cells will represent the next novel environment encountered. Furthermore, place cells fired spatially tuned bursts with large, putatively calcium-mediated depolarizations that could trigger plasticity and stabilize the new map for long-term storage. Our results provide new insight into hippocampal memory formation.

Mechanisms of closed‐state inactivation in voltage‐gated ion channels

Inactivation of voltage-gated ion channels is an intrinsic auto-regulatory process necessary to govern the occurrence and shape of action potentials and establish firing patterns in excitable tissues. Inactivation may occur from the open state (open-state inactivation, OSI) at strongly depolarized membrane potentials, or from pre-open closed states (closed-state inactivation, CSI) at hyperpolarized and modestly depolarized membrane potentials. Voltage-gated Na(+), K(+), Ca(2+) and non-selective cationic channels utilize both OSI and CSI. Whereas there are detailed mechanistic descriptions of OSI, much less is known about the molecular basis of CSI. Here, we review evidence for CSI in voltage-gated cationic channels (VGCCs) and recent findings that shed light on the molecular mechanisms of CSI in voltage-gated K(+) (Kv) channels. Particularly, complementary observations suggest that the S4 voltage sensor, the S4S5 linker and the main S6 activation gate are instrumental in the installment of CSI in Kv4 channels. According to this hypothesis, the voltage sensor may adopt a distinct conformation to drive CSI and, depending on the stability of the interactions between the voltage sensor and the pore domain, a closed-inactivated state results from rearrangements in the selectivity filter or failure of the activation gate to open. Kv4 channel CSI may efficiently exploit the dynamics of the subthreshold membrane potential to regulate spiking properties in excitable tissues.

Active dendrites: adaptation to spike-based communication

Event Abstract Back to Event Active dendrites: adaptation to spike-based communication B. Ujfalussy1* and M. Lengyel2 1 KFKI Res Inst for Particle and Nuclear Physics, Hungary 2 University of Cambridge, Department of Engineering, United Kingdom Dendritic trees possess a plethora of voltage-dependent conductances that render them sophisticated computational devices well beyond the realm of traditional cable theory. Branch specific regulation of dendritic excitability and synaptic plasticity suggests that local nonlinear processes play an essential role in single-cell and network computations. Indeed, much of classical theoretical work focussed on how units that perform complex, nonlinear mappings from their inputs to their outputs contribute to cognitively relevant computations. However, the variables relevant for such computations have often been assumed to be represented by analogue quantities (e.g., somatic membrane potential), ignoring that neurons communicate by spikes, and spike generation loses information about subthreshold membrane potential dynamics. Here we studied how the nonlinear properties of dendrites may be adapted to the computational demands of spike-based communication. We found that the optimal implementation of even purely linear dendritic computations requires the interplay of many independent nonlinear subunits within the postsynaptic dendritic tree. We demonstrate that nonlinear dendritic trees bring significant benefits to single-neuron computation across a wide range of input correlations, even if the individual synapses are optimal estimators of the corresponding presynaptic membrane potentials. Keywords: Neurophysiology, Neuroscience Conference: 13th Conference of the Hungarian Neuroscience Society (MITT), Budapest, Hungary, 20 Jan - 22 Jan, 2011. Presentation Type: Abstract Topic: Neurophysiology Citation: Ujfalussy B and Lengyel M (2011). Active dendrites: adaptation to spike-based communication. Front. Neurosci. Conference Abstract: 13th Conference of the Hungarian Neuroscience Society (MITT). doi: 10.3389/conf.fnins.2011.84.00063 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Mar 2011; Published Online: 23 Mar 2011. * Correspondence: Dr. B. Ujfalussy, KFKI Res Inst for Particle and Nuclear Physics, Budapest, Hungary, ubi@rmki.kfki.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers B. Ujfalussy M. Lengyel Google B. Ujfalussy M. Lengyel Google Scholar B. Ujfalussy M. Lengyel PubMed B. Ujfalussy M. Lengyel Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Role of active dendritic conductances in subthreshold input integration

Dendrites of many types of neurons contain voltage-dependent conductances that are active at subthreshold membrane potentials. To understand the computations neurons perform it is key to understand the role of active dendrites in the subthreshold processing of synaptic inputs. We examine systematically how active dendritic conductances affect the time course of postsynaptic potentials propagating along dendrites, and how they affect the interaction between such signals. Voltage-dependent currents can be classified into two types that have qualitatively different effects on subthreshold input responses: regenerative dendritic currents boost and broaden EPSPs, while restorative currents attenuate and narrow EPSPs. Importantly, the effects of active dendritic currents on EPSP shape increase as the EPSP travels along the dendrite. The effectiveness of active currents in modulating the EPSP shape is determined by their activation time constant: the faster it is, the stronger the effect on EPSP amplitude, while the largest effects on EPSP width occur when it is comparable to the membrane time constant. We finally demonstrate that the two current types can differentially improve precision and robustness of neural computations: restorative currents enhance coincidence detection of dendritic inputs, whereas direction selectivity to sequences of dendritic inputs is enhanced by regenerative dendritic currents.

A‐type K+ current dominates somatic excitability of delayed firing neurons in rat substantia gelatinosa

Substantia gelatinosa neurons display three main types of intrinsic firing behavior: tonic, adapting, and delayed onset. Here, voltage-gated currents expressed by delayed firing neurons were studied in nucleated patches obtained in spinal cord slices of 3-5 weeks-old rats. Inward Na+ current was negligible under these conditions and was usually occluded by superposition of much larger outward currents. Two kinds of outward currents were found, an A-type (K(A) ) and delayed rectifier (K(DR) ) potassium currents. K(A) activated rapidly (<1.5 ms at >-20 mV) and operated at subthreshold membrane potentials; voltages of steady-state half-maximal activation and inactivation were -38.7 and -87.2 mV, respectively. Inactivation was biexponential with a dominant fast component (~90%, time constant ∼8 ms). K(DR) activated more slowly (<8 ms at >-20 mV), half-maximal activation was -23.6 mV, and decayed mono-exponentially with a time constant 70-110 ms. Maximal amplitudes of K(A) were almost 10-times larger than those of K(DR) , their respective densities were 8.5 and 0.97 μS μm⁻². Tetraethylammonium, 5 mM, blocked K(DR) but not K(A) , whereas both currents were depressed by 5 mM 4-aminopyridine. In current-clamp recordings, 4-action potential but not tetraethylammonium abolished firing delay suggesting the causative role of K(A) . Thus, the predominance of fast K(A) over other somatic currents is a distinctive feature of delayed firing neurons among all other types of substantia gelatinosa neurons and likely explains the appearance of their typical firing delay.

Support for a synaptic chain model of neuronal sequence generation

SummaryIn songbirds, the remarkable temporal precision of song is generated by a sparse sequence of bursts in the premotor nucleus HVC (proper name). To distinguish between two possible classes of models of neural sequence generation, we carried out intracellular recordings of HVC neurons in singing birds. We found that the subthreshold membrane potential is characterized by a large rapid depolarization 5–10 ms prior to burst onset, consistent with a synaptically-connected chain of neurons in HVC. We found no evidence for the slow membrane potential modulation predicted by models in which burst timing is controlled by subthreshold dynamics. Furthermore, bursts ride on an underlying depolarization of ~10ms duration, likely the result of a regenerative calcium spike within HVC neurons that could facilitate the propagation of activity through a chain network with high temporal precision. Our results shed light on the fundamental mechanisms by which neural circuits can generate complex sequential behaviours.

Cascade-induced synchrony in stochastically driven neuronal networks

Perfect spike-to-spike synchrony is studied in all-to-all coupled networks of identical excitatory, current-based, integrate-and-fire neurons with delta-impulse coupling currents and Poisson spike-train external drive. This synchrony is induced by repeated cascading "total firing events," during which all neurons fire at once. In this regime, the network exhibits nearly periodic dynamics, switching between an effectively uncoupled state and a cascade-coupled total firing state. The probability of cascading total firing events occurring in the network is computed through a combinatorial analysis conditioned upon the random time when the first neuron fires and using the probability distribution of the subthreshold membrane potentials for the remaining neurons in the network. The probability distribution of the former is found from a first-passage-time problem described by a Fokker-Planck equation, which is solved analytically via an eigenfunction expansion. The latter is found using a central limit argument via a calculation of the cumulants of a single neuronal voltage. The influence of additional physiological effects that hinder or eliminate cascade-induced synchrony are also investigated. Conditions for the validity of the approximations made in the analytical derivations are discussed and verified via direct numerical simulations.

Isoform-specific Prolongation of Kv7 (KCNQ) Potassium Channel Opening Mediated by New Molecular Determinants for Drug-Channel Interactions

Kv7 channels, especially Kv7.2 (KCNQ2) and Kv7.3 (KCNQ3), are key determinants for membrane excitability in the brain. Some chemical modulators of KCNQ channels are in development for use as anti-epileptic drugs, such as retigabine (D-23129, N-(2-amino-4-(4-fluorobenzylamino)-phenyl)), which was recently approved for clinical use. In addition, several other compounds were also reported to potentiate activity of the Kv7 channels. It is therefore of interest to investigate compound-channel interactions, so that more insights may be gained to aid future development of therapeutics. We have conducted a screen of 20,000 compounds for KCNQ2 potentiators using rubidium flux combined with atomic absorption spectrometry. Here, we report the characterization of a series of new structures that display isoform specificity and induce a marked reduction of deactivation distinct from that of retigabine. Furthermore, KCNQ2(W236L), a previously reported mutation that abolishes sensitivity to retigabine, remains fully sensitive to these compounds. This result, together with mutagenesis and other studies, suggests that the reported compounds confer a unique mode of action and involve new molecular determinants on the channel protein, consistent with the idea of recognizing a new site on channel protein.

Role of active dendritic conductances in subthreshold input integration

A large body of data has demonstrated the presence of voltage-dependent conductances in the dendrites of many types of neurons. A subset of these conductances are active at subthreshold membrane potentials and can greatly affect the integration of synaptic inputs. To understand the computations neurons perform it is key to understand the role of active dendrites in subthreshold input processing. Here, we examine how active dendritic conductances affect postsynaptic potentials that propagate along dendrites and the interaction between such signals. We focus in particular on coincidence detection, one of the most basic operations a neuron can perform, in which a neuron needs to detect the occurrence of two or more EPSPs in a short time interval (e.g. down to ~10 µs in auditory brainstem). To systematically study the effects of active dendritic conductances on synaptic inputs, we make use of the so-called quasi-active description of dendritic cables [1], an extension of classical passive cable theory, which relies on linearizing the voltage-dependent conductances. Though the linearized description does not capture the full dynamics of the active currents, the results can serve as a solid reference for the effects of active dendritic currents on propagating EPSPs. This approach allows us to categorize active dendritic currents into two types: restorative currents (e.g. Ih), which function as a negative feedback and counteract changes of the membrane potential, and regenerative currents (e.g. INaP), which act as a positive feedback and amplify membrane potential changes. The two types have qualitatively different effects on subthreshold EPSP propagation and interaction. Compared to a passive cable’s response, the EPSP halfwidth is decreased by restorative currents and increased by regenerative currents. Moreover, these effects increase as the EPSP propagates along the active cable. Interestingly, restorative dendritic currents can maintain a constant EPSP halfwidth or even gradually narrow the EPSP as it moves along a dendrite. We find there is an optimal activation time constant of the active dendritic currents to exert their maximal effect on the EPSP halfwidth. Finally, since narrow EPSPs will only summate on short intervals, coincidence detection of dendritic inputs is enhanced by restorative currents: the coincidence window is narrower and is less dependent on the exact locations of the inputs along the dendrite. Conversely, coincidence detection is less precise when the dendrites have regenerative currents, which cause a wider coincidence window and stronger dependence of the window on input locations.

Synaptic Mechanisms of Direction Selectivity in Primary Auditory Cortex

Frequency modulation (FM) is a prominent feature in animal vocalization and human speech. Although many neurons in the auditory cortex are known to be selective for FM direction, the synaptic mechanisms underlying this selectivity are not well understood. Previous studies of both visual and auditory neurons have suggested two general mechanisms for direction selectivity: (1) differential delays of excitatory inputs across the spatial/spectral receptive field and (2) spatial/spectral offset between excitatory and inhibitory inputs. In this study, we have examined the contributions of both mechanisms to FM direction selectivity in rat primary auditory cortex. The excitatory and inhibitory synaptic inputs to each cortical neuron were measured by in vivo whole-cell recording. The spectrotemporal receptive field of each type of inputs was mapped with random tone pips and compared with direction selectivity of the neuron measured with FM stimuli. We found that both the differential delay of the excitatory input and the spectral offset between excitation and inhibition are positively correlated with direction selectivity of the neuron. Thus, both synaptic mechanisms are likely to contribute to FM direction selectivity in the auditory cortex. Finally, direction selectivity measured from the spiking output is significantly stronger than that based on the subthreshold membrane potentials, indicating that the selectivity is further sharpened by the spike generation mechanism.

Cav1.3 Channel Voltage Dependence, Not Ca2+Selectivity, Drives Pacemaker Activity and Amplifies Bursts in Nigral Dopamine Neurons

Ca(v)1.3 (alpha 1D) L-type Ca(2+) channels have been implicated in substantia nigra (SN) dopamine (DA) neuron pacemaking and vulnerability to Parkinson's disease. These effects may arise from the depolarizing current and cytoplasmic Ca(2+) elevation produced by Ca(v)1.3 channels at subthreshold membrane potentials. However, the assumption that the Ca(2+) selectivity of Ca(v)1.3 channels is essential has not been tested. In this study the properties of SN DA neuron L-type Ca(2+) channels responsible for driving pacemaker activity in juvenile rat brain slices were probed by replacing native channels blocked with the dihydropyridine nimodipine with virtual channels generated by dynamic clamp. Surprisingly, virtual L-type channels that mimic native and recombinant Ca(v)1.3 channels supported pacemaker activity even though dynamic clamp currents are not carried by Ca(2+). This effect is specific because pacemaker activity could not be restored by tonic current injection, virtual nonselective leak channels or virtual NMDA receptors, which share with L-type channels a negative slope conductance region in their current-voltage (I-V) curve. Altering virtual channels showed that the production of pacemaker activity depended on the characteristic voltage dependence of DA neuron L-type channels, while activation kinetics and reversal potential were not critical parameters. Virtual L-type channels also supported slow oscillatory potentials and enhanced firing rate during evoked bursts. Thus, Ca(v)1.3 channel voltage dependence, rather than Ca(2+) selectivity, drives pacemaker activity and amplifies bursts in SN DA neurons.

A nerve model of greatly increased energy-efficiency and encoding flexibility over the Hodgkin–Huxley model

A mammalian “RGC model” (retinal ganglion cells) is distinguished from the Hodgkin–Huxley model by the virtual absence of K-current during, and the virtual absence of Na-current after, the regenerative (rising) phase of the action potential. Both Na- and K-currents remain negligible throughout the interspike interval, whose control is therefore relinquished to stimulus currents. These properties yield a highly flexible and energy-efficient nerve impulse encoder. For the Hodgkin–Huxley model, in contrast, only 15% of the Na-ions enter the axon regeneratively during the action potential (squid giant axon); a wasteful 85% enter during the falling phase. Further, early activation of K-current causes the Na- and K-currents of the action potential to dominate over stimulus currents in controlling the sub-threshold membrane potential (interspike interval). This property makes the Hodgkin–Huxley model an intractable high frequency oscillator, which cannot be converted to flexible impulse encoding. The temperature difference between the squid giant axon (6.3 °C) and RGCs (37 °C) is bridged by a Q10 analysis, which suggests that an additional molecular gating mechanism of high Q10 – which is not present in the squid – is active in RGCs.

Whole-Cell Recording and Voltage-Sensitive Dye Imaging In Vivo: Figure 1.

INTRODUCTIONThis protocol describes an in vivo technique for combining whole-cell (WC) recordings of membrane potentials of individual neurons with voltage-sensitive dye (VSD) imaging of the ensemble neocortical network dynamics. If careful experimental measurements and controls are performed, VSD imaging can be used to define the ensemble spatiotemporal subthreshold membrane potential dynamics within which the membrane potential changes of individual neurons are embedded. Data from these individual neurons can be recorded simultaneously with the ensemble dynamics using the WC technique. This approach has been tested on rodent barrel cortex, but it is likely to be applicable, with minor modifications, to other superficial brain areas (including other neocortical areas, cerebellum, and olfactory bulb) and other species (such as cat or monkey).

Convergent Modulation of Kv4.2 Channel α Subunits by Structurally Distinct DPPX and KChIP Auxiliary Subunits

Kv4.2 is the major voltage-gated K(+) (Kv) channel alpha subunit responsible for the somatodendritic transient or A-type current I(SA) that activates at subthreshold membrane potentials. Stable association of Kv4.2 with diverse auxiliary subunits and reversible Kv4.2 phosphorylation regulate I(SA) function. Two classes of auxiliary subunits play distinct roles in modulating the biophysical properties of Kv4.2: dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S, and cytoplasmic Ca(2+) binding proteins known as K(+) channel interacting proteins (KChIPs). Here, we characterize the convergent roles that DPPX-S and KChIPs play as component subunits of Kv4.2 channel complexes. We coexpressed DPPX-S with Kv4.2 in heterologous cells and found a dramatic redistribution of Kv4.2, releasing it from intracellular retention and allowing plasma membrane expression, as well as altered Kv4.2 phosphorylation, detergent solubility, and stability. These changes are remarkably similar to those obtained upon coexpression of Kv4.2 with the structurally distinct KChIPs1-3 auxiliary subunits. KChIP4a, which negatively affects the impact of other KChIPs on Kv4.2, also inhibits the effects of DPPX-S, consistent with the formation of a ternary complex of Kv4.2, DPPX-S, and KChIPs early in channel biosynthesis. Tandem MS analyses reveal that coexpression with DPPX-S or KChIP2 leads to a pattern of Kv4.2 phosphorylation in heterologous cells similar to that observed in brain, but lacking in cells expressing Kv4.2 alone. In conclusion, transmembrane DPPX-S and cytoplasmic KChIPs, despite having distinct structures and binding sites on Kv4.2, exert similar effects on Kv4.2 trafficking, but distinct effects on Kv4.2 gating.