Action Potentials Of Single Neurons Research Articles

Comparison of unitary synaptic currents generated by indirect and direct pathway neurons of the mouse striatum.

Two subtypes of striatal spiny projection neurons, iSPNs and dSPNs, whose axons form the "indirect" and "direct" pathways of the basal ganglia, respectively, both make synaptic connections in the external globus pallidus (GPe) but are usually found to have different effects on behavior. Activation of the terminal fields of iSPNs or dSPNs generated compound currents in almost all GPe neurons. To determine whether iSPNs and dSPNs have the same or different effects on pallidal neurons, we studied the unitary synaptic currents generated in GPe neurons by action potentials in single striatal neurons. We used optogenetic excitation to elicit repetitive firing in a small number of nearby SPNs, producing sparse barrages of inhibitory postsynaptic currents (IPSCs) in GPe neurons. From these barrages, we isolated sequences of IPSCs with similar time courses and amplitudes, which presumably arose from the same SPN. There was no difference between the amplitudes of unitary IPSCs generated by the indirect and direct pathways. Most unitary IPSCs were small, but a subset from each pathway were much larger. To determine the effects of these unitary synaptic currents on the action potential firing of GPe neurons, we drove SPNs to fire as before and recorded the membrane potential of GPe neurons. Large unitary potentials from iSPNs and dSPNs perturbed the spike timing of GPe neurons in a similar way. Most SPN-GPe neuron pairs are weakly connected, but a subset of pairs in both pathways are strongly connected.NEW & NOTEWORTHY This is the first study to record the synaptic currents generated by single identified direct or indirect pathway striatal neurons on single pallidal neurons. Each GPe neuron receives synaptic inputs from both pathways. Most striatal neurons generate small synaptic currents that become influential when occurring together, but a few are powerful enough to be individually influential.

Development of a microelectrode for simultaneous <i>in vivo</i> calcium and electrophysiological recording of hippocampal neuronal activity

Calcium (Ca2+) imaging is a commonly utilized neuroscience technique for in vivo recording of neuronal activity. It involves the optical measurement of calcium concentration using genetically encoded calcium indicators (GECI) [1]. However, the kinetics of changes in the fluorescence of GECI are relatively slow and limited by the biophysics of the calcium binding [2]. In response to single action potentials (APs) in pyramidal neurons, most of the widely used GECI have a fluorescence half-life of approximately 100 ms [3]. As a result, GECI cannot provide complete information about the dynamics of neural ensembles. To address this issue, new variants of GECI, such as jGCaMP7 [3], and jGCaMP8 [4], have been developed, or genetically encoded voltage indicators (GEVI), such as JEDI-2P [5], have been utilized. However, the speed of GECI or GEVI is still lower than that of electrophysiological registration methods. Thus, we have designed a microelectrode that can be utilized with a gradient lens for in vivo calcium imaging with a miniscope. The miniscope is a miniature microscope for single photon epifluorescence Ca2+ imaging, which enables recording of neuronal activity in freely moving laboratory animals, unlike the traditionally used two-photon imaging technique. The miniscope utilizes gradient-index (GRIN) lenses that are implanted directly into the brain of a laboratory animal, instead of a conventional lens. The gradient lens is a transparent cylinder with a diameter of 1.8 mm and a length of 3.8 mm. To facilitate electrophysiological recording, we developed a microelectrode that can be aligned with a GRIN lens. The microelectrode is a three-layer structure consisting of: 1) a polyimide film, 2) conductive copper tracks deposited through thermoforming, and 3) a polyimide film with cutouts for pads. On one side of the microelectrode, there are 12 gold-plated conductive contacts for registering local field potentials, while on the other side, a similar number of conductive tracks are present for connecting to a connector that transmits data to the processing board. The flexible microelectrode is wrapped around a gradient lens and fixed using thermoforming, after which it is implanted in the animal's brain. Using the developed microelectrode, our aim is to perform a comparative analysis of the calcium and electrophysiological activity of hippocampal neurons in freely moving wild-type mice and in a mouse model of Alzheimer's disease. This study will enable the identification of any abnormalities in Alzheimer's disease at the level of neural ensembles and may suggest new treatment approaches or mechanisms for the development of the progressive memory loss pathology associated with this disease. We would like to express our gratitude to Anastasia Viktorovna Bolshakova for her administrative assistance, and to the staff of the Laboratory of Molecular Neurodegeneration for their invaluable help and advices.

Open-Source Real-Time Closed-Loop Electrical Threshold Tracking for Translational Pain Research.

Nociceptors are a class of primary afferent neurons that signal potentially harmful noxious stimuli. An increase in nociceptor excitability occurs in acute and chronic pain conditions. This produces abnormal ongoing activity or reduced activation thresholds to noxious stimuli. Identifying the cause of this increased excitability is required for the development and validation of mechanism-based treatments. Single-neuron electrical threshold tracking can quantify nociceptor excitability. Therefore, we have developed an application to allow such measurements and demonstrate its use in humans and rodents. APTrack provides real-time data visualization and action potential identification using a temporal raster plot. Algorithms detect action potentials by threshold crossing and monitor their latency after electrical stimulation. The plugin then modulates the electrical stimulation amplitude using an up-down method to estimate the electrical threshold of the nociceptors. The software was built upon the Open Ephys system (V0.54) and coded in C++ using the JUCE framework. It runs on Windows, Linux, and Mac operating systems. The open-source code is available (https://github.com/Microneurography/APTrack). The electrophysiological recordings were taken from nociceptors in both a mouse skin-nerve preparation using the teased fiber method in the saphenous nerve and in healthy human volunteers using microneurography in the superficial peroneal nerve. Nociceptors were classified by their response to thermal and mechanical stimuli, as well as by monitoring the activity-dependent slowing of the conduction velocity. The software facilitated the experiment by simplifying the action potential identification through the temporal raster plot. We demonstrate real-time closed-loop electrical threshold tracking of single-neuron action potentials duringin vivohuman microneurography, for the first time, and duringex vivomouse electrophysiological recordings of C-fibers and Aδ-fibers. We establish proof of principle by showing that the electrical threshold of a human heat-sensitive C-fiber nociceptor is reduced by heating the receptive field. This plugin enables the electrical threshold tracking of single-neuron action potentials and allows the quantification of changes in nociceptor excitability.

Engineering of NEMO as calcium indicators with large dynamics and high sensitivity

Genetically encoded calcium indicators (GECIs) are indispensable tools for real-time monitoring of intracellular calcium signals and cellular activities in living organisms. Current GECIs face the challenge of suboptimal peak signal-to-baseline ratio (SBR) with limited resolution for reporting subtle calcium transients. We report herein the development of a suite of calcium sensors, designated NEMO, with fast kinetics and wide dynamic ranges (>100-fold). NEMO indicators report Ca2+ transients with peak SBRs around 20-fold larger than the top-of-the-range GCaMP6 series. NEMO sensors further enable the quantification of absolution calcium concentration with ratiometric or photochromic imaging. Compared with GCaMP6s, NEMOs could detect single action potentials in neurons with a peak SBR two times higher and a median peak SBR four times larger in vivo, thereby outperforming most existing state-of-the-art GECIs. Given their high sensitivity and resolution to report intracellular Ca2+ signals, NEMO sensors may find broad applications in monitoring neuronal activities and other Ca2+-modulated physiological processes in both mammals and plants.

Feasibility study of magnetic sensing for detecting single-neuron action potentials

Understanding the magnitude of the local magnetic fields generated by neurons is critical to assessing the feasibility of novel magnetic field sensors to record in vivo neuronal activities at cellular resolution. However, the strength of the magnetic fields induced by individual neurons and neuronal networks has not been systematically studied. This step is critical for evaluating and benchmarking the ability of different magnetic field sensors to record neuronal activities with far better spatial and temporal resolution. Herein, FEM exemplary models and open-source computational libraries are used to calculate the magnetic fields generated by individual neurons and neuronal networks at micrometer distances. Our theoretical results show that the magnetic field generated by a single-neuron action potential can be detected by ultra-high sensitivity sub-pT magnetic field sensors, which opens the door to future in vivo decoding of neuronal activities through custom neural networks. We anticipate that the identification of single-neuron signals with high-sensitivity magnetic devices will allow the interface of nanoscale devices to interpret biological signals supported by machine-learning techniques capable of monitoring and predicting the localized activities underlying brain computations.

Stochasticity Versus Determinacy in Neurobiology: From Ion Channels to the Question of the "Free Will".

If one accepts that decisions are made by the brain and that neuronal mechanisms obey deterministic physical laws, it is hard to deny what some brain researchers postulate, such as “We do not do what we want, but we want what we do” and “We should stop talking about freedom. Our actions are determined by physical laws.” This point of view has been substantially supported by spectacular neurophysiological experiments demonstrating action-related brain activity (readiness potentials, blood oxygen level–dependent signals) occurring up to several seconds before an individual becomes aware of his/her decision to perform the action. This report aims to counter the deterministic argument for the absence of free will by using experimental data, supplemented by computer simulations, to demonstrate that biological systems, specifically brain functions, are built on principle randomness, which is introduced already at the lowest level of neuronal information processing, the opening and closing of ion channels. Switching between open and closed states follows physiological laws but also makes use of randomness, which is apparently introduced by Brownian motion – principally unavoidable under all life-compatible conditions. Ion-channel stochasticity, manifested as noise, function is not smoothed out toward higher functional levels but can even be amplified by appropriate adjustment of the system’s non-linearities. Examples shall be given to illustrate how stochasticity can propagate from ion channels to single neuron action potentials to neuronal network dynamics to the interactions between different brain nuclei up to the control of autonomic functions. It is proposed that this intrinsic stochasticity helps to keep the brain in a flexible state to explore diverse alternatives as a prerequisite of free decision-making.

BK potassium currents contribute differently to action potential waveform and firing rate as rat hippocampal neurons mature in the first postnatal week.

The large-conductance calcium-activated potassium (BK) channel is a critical regulator of neuronal action potential firing and follows two distinct trends in early postnatal development: an increase in total expression and a shift from the faster activating STREX isoform to the slower ZERO isoform. We analyzed the functional consequences of developmental trends in BK channel expression in hippocampal neurons isolated from neonatal rats aged 1 to 7 days. Following overnight cultures, action potentials and currents were recorded using whole cell patch-clamp electrophysiology. These neurons undergo a steady increase in excitability during this time, and the effect of blockade of BK channel activity with 100 nM iberiotoxin changes as the neurons mature. BK currents contribute significantly more to total potassium current and single action potentials in neurons of 1-day old rats (with BK blockade extending action potential duration by 0.46 ± 0.12 ms) than in those of 7-day old rats (with BK blockade extending action potential duration by 0.17 ± 0.05 ms). BK currents contribute consistently to maintain firing rates in neurons of 1-day old rats throughout extended action potential firing; BK blockade evenly depresses firing frequency across action potential trains. In neurons from 7-day old rats, BK blockade initially increases firing frequency and then progressively decreases frequency as firing continues, ultimately depressing neuronal firing rates to a greater extent than in the neurons from 1-day-old animals. These results are consistent with a transition from low expression of a fast-activating BK isoform (STREX) to high expression of a slower activating isoform (ZERO).NEW & NOTEWORTHY This work describes the early developmental trends of large-conductance calcium-activated potassium (BK) channel activity. Early developmental trends in expression of BK channels, both total expression and relative isoform expression, have been previously reported, but little work describes the effect of these changes in expression patterns on excitability. Here, we show that early changes in BK channel expression patterns lead to changes in the role of BK channels in determining the action potential waveform and neuronal excitability.

Hybrid diamond/ carbon fiber microelectrodes enable multimodal electrical/chemical neural interfacing

Implantable medical devices are now in regular use to treat or ameliorate medical conditions, including movement disorders, chronic pain, cardiac arrhythmias, and hearing or vision loss. Aside from offering alternatives to pharmaceuticals, one major advantage of device therapy is the potential to monitor treatment efficacy, disease progression, and perhaps begin to uncover elusive mechanisms of diseases pathology. In an ideal system, neural stimulation, neural recording, and electrochemical sensing would be conducted by the same electrode in the same anatomical region. Carbon fiber (CF) microelectrodes are the appropriate size to achieve this goal and have shown excellent performance, in vivo. Their electrochemical properties, however, are not suitable for neural stimulation and electrochemical sensing. Here, we present a method to deposit high surface area conducting diamond on CF microelectrodes. This unique hybrid microelectrode is capable of recording single-neuron action potentials, delivering effective electrical stimulation pulses, and exhibits excellent electrochemical dopamine detection. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.

Rapid Voltage Sensing with Single Nanorods via the Quantum Confined Stark Effect.

Properly designed colloidal semiconductor quantum dots (QDs) have already been shown to exhibit high sensitivity to external electric fields via the quantum confined Stark effect (QCSE). Yet, detection of the characteristic spectral shifts associated with the effect of the QCSE has traditionally been painstakingly slow, dramatically limiting the sensitivity of these QD sensors to fast transients. We experimentally demonstrate a new detection scheme designed to achieve shot-noise-limited sensitivity to emission wavelength shifts in QDs, showing feasibility for their use as local electric field sensors on the millisecond time scale. This regime of operation is already potentially suitable for detection of single action potentials in neurons at a high spatial resolution.

Temporal coupling of field potentials and action potentials in the neocortex.

The local field potential (LFP) is an aggregate measure of group neuronal activity and is often correlated with the action potentials of single neurons. In recent years, investigators have found that action potential firing rates increase during elevations in power high-frequency band oscillations (50-200Hz range). However, action potentials also contribute to the LFP signal itself, making the spike-LFP relationship complex. Here, we examine the relationship between spike rates and LFP in varying frequency bands in rat neocortical recordings. We find that 50-180Hz oscillations correlate most consistently with high firing rates, but that other LFP bands also carry information relating to spiking, including in some cases anti-correlations. Relatedly, we find that spiking itself and electromyographic activity contribute to LFP power in these bands. The relationship between spike rates and LFP power varies between brain states and between individual cells. Finally, we create an improved oscillation-based predictor of action potential activity by specifically utilizing information from across the entire recorded frequency spectrum of LFP. The findings illustrate both caveats and improvements to be taken into account in attempts to infer spiking activity from LFP.

ELECTROPHYSIOLOGICAL PHENOTYPE CHARACTERIZATION OF HUMAN IPSC-DERIVED DOPAMINERGIC NEURONAL LINES BY MEANS OF HIGH-RESOLUTION MICROELECTRODE ARRAYS

Event Abstract Back to Event ELECTROPHYSIOLOGICAL PHENOTYPE CHARACTERIZATION OF HUMAN IPSC-DERIVED DOPAMINERGIC NEURONAL LINES BY MEANS OF HIGH-RESOLUTION MICROELECTRODE ARRAYS Michele Fiscella1, 2*, Noelle Leary1, Ebru Acun1, Silvia Ronchi1 and Andreas Hierlemann1 1 ETH Zürich, Switzerland 2 MaxWell Biosystems, Switzerland High-resolution-microelectrode-array (MEA) technology enables to study neuronal dynamics at different scales, ranging from axonal physiology to network connectivity (Müller et. al, Lab on a Chip, 2015). We have used this MEA technology to characterize and compare the electrical phenotypes of commercially available human dopaminergic neurons (iCell DopaNeurons, MyCell DopaNeurons A53T α-synuclein, Cellular Dynamics International, Madison, WI, US). Furthermore, we have studied the effect of human astrocytes (iCell Astrocytes, Cellular Dynamics International, Madison, WI, US) on neural culture development. Astrocyte/neuron co-cultures showed higher signal amplitudes and higher firing rates than neural cultures without astrocytes. Adding astrocytes to neural cultures changed the whole culture morphology by promoting cell clustering. Interestingly, astrocyte/neuron co-cultures showed a lower sample-to-sample variability across multiple MEA recording sessions compared to neural cultures without astrocytes. We compared velocities of action potential propagation along axons between dopaminergic A53T α-synuclein neurons and the wild-type isogenic control cell line. We found that in both, wild-type and disease-model neurons, axonal action potential propagation velocities were lower than, for example, in rat primary cortical neurons (Bakkum et. al, Nature Communications, 2013). Furthermore, we found different axonal action-potential-velocity development profiles of A53T α-synuclein dopaminergic neurons and the wild-typecell line. Finally, we were able to precisely and reproducibly evoke action potentials in individual single human IPSC-derived neurons through subcellular-resolution electrical stimulation. High-resolution MEA systems enable to access novel electrophysiological parameters of iPSC-derived neurons, which can be potentially used as biomarkers for phenotype screening and drug testing. Acknowledgements EU, ERC Advanced Grant “neuroXscales” contract number 694829 EU, ERC-PoC “MwHresEP" contract number 755383 CH, Project CTI-No. 25933. 2 PFLS-LS “Multi-well electrophysiology platform for high-throughput cell-based assays” References J. Müller, M. Ballini, P. Livi, Y. Chen, M. Radivojevic, A. Shadmani, V. Viswam, I. L. Jones, M. Fiscella, R. Diggelmann, A. Stettler, U. Frey, D. J. Bakkum, A. Hierlemann, "High-resolution CMOS MEA platform to study neurons at subcellular, cellular, and network levels", Lab Chip, 2015, 15, pp. 2767-2780 (DOI: 10.1039/C5LC00133A) Douglas J. Bakkum, Urs Frey, Milos Radivojevic, Thomas L. Russell, Jan Müller, Michele Fiscella, Hirokazu Takahashi, Andreas Hierlemann, "Tracking Axonal Action Potential Propagation on a High-density Microelectrode Array across Hundreds of Sites", Nature Communications, 2013, 4:2181 (DOI: 10.1038/ncomms3181). Keywords: CMOS-MEAs, IPSC-derived neurons, Parkinson Disease, microelectrode arrays, phenotyping Conference: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays, Reutlingen, Germany, 4 Jul - 6 Jul, 2018. Presentation Type: Poster Presentation Topic: Stem cell-derived applications Citation: Fiscella M, Leary N, Acun E, Ronchi S and Hierlemann A (2019). ELECTROPHYSIOLOGICAL PHENOTYPE CHARACTERIZATION OF HUMAN IPSC-DERIVED DOPAMINERGIC NEURONAL LINES BY MEANS OF HIGH-RESOLUTION MICROELECTRODE ARRAYS. Conference Abstract: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays. doi: 10.3389/conf.fncel.2018.38.00014 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: 05 Mar 2018; Published Online: 17 Jan 2019. * Correspondence: Dr. Michele Fiscella, ETH Zürich, Zurich, Switzerland, michele.fiscella@mxwbio.com 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 Michele Fiscella Noelle Leary Ebru Acun Silvia Ronchi Andreas Hierlemann Google Michele Fiscella Noelle Leary Ebru Acun Silvia Ronchi Andreas Hierlemann Google Scholar Michele Fiscella Noelle Leary Ebru Acun Silvia Ronchi Andreas Hierlemann PubMed Michele Fiscella Noelle Leary Ebru Acun Silvia Ronchi Andreas Hierlemann 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.

Optical magnetic detection of single-neuron action potentials using quantum defects in diamond

Magnetic fields from neuronal action potentials (APs) pass largely unperturbed through biological tissue, allowing magnetic measurements of AP dynamics to be performed extracellularly or even outside intact organisms. To date, however, magnetic techniques for sensing neuronal activity have either operated at the macroscale with coarse spatial and/or temporal resolution-e.g., magnetic resonance imaging methods and magnetoencephalography-or been restricted to biophysics studies of excised neurons probed with cryogenic or bulky detectors that do not provide single-neuron spatial resolution and are not scalable to functional networks or intact organisms. Here, we show that AP magnetic sensing can be realized with both single-neuron sensitivity and intact organism applicability using optically probed nitrogen-vacancy (NV) quantum defects in diamond, operated under ambient conditions and with the NV diamond sensor in close proximity (∼10 µm) to the biological sample. We demonstrate this method for excised single neurons from marine worm and squid, and then exterior to intact, optically opaque marine worms for extended periods and with no observed adverse effect on the animal. NV diamond magnetometry is noninvasive and label-free and does not cause photodamage. The method provides precise measurement of AP waveforms from individual neurons, as well as magnetic field correlates of the AP conduction velocity, and directly determines the AP propagation direction through the inherent sensitivity of NVs to the associated AP magnetic field vector.

Mapping Micro-Circuits with Extra-Cellular Arrays in Three Layered Cortex

Mapping Micro-Circuits with Extra-Cellular Arrays in Three Layered Cortex

Reactive species modify NaV1.8 channels and affect action potentials in murine dorsal root ganglion neurons.

Dorsal root ganglion (DRG) neurons are important relay stations between the periphery and the central nervous system and are essential for somatosensory signaling. Reactive species are produced in a variety of physiological and pathophysiological conditions and are known to alter electric signaling. Here we studied the influence of reactive species on the electrical properties of DRG neurons from mice with the whole-cell patch-clamp method. Even mild stress induced by either low concentrations of chloramine-T (10μM) or low-intensity blue light irradiation profoundly diminished action potential frequency but prolonged single action potentials in wild-type neurons. The impact on evoked action potentials was much smaller in neurons deficient of the tetrodotoxin (TTX)-resistant voltage-gated sodium channel NaV1.8 (NaV1.8(-/-)), the channel most important for the action potential upstroke in DRG neurons. Low concentrations of chloramine-T caused a significant reduction of NaV1.8 peak current and, at higher concentrations, progressively slowed down inactivation. Blue light had a smaller effect on amplitude but slowed down NaV1.8 channel inactivation. The observed effects were less apparent for TTX-sensitive NaV channels. NaV1.8 is an important reactive-species-sensitive component in the electrical signaling of DRG neurons, potentially giving rise to loss-of-function and gain-of-function phenomena depending on the type of reactive species and their effective concentration and time of exposure.

Neural substrates of tinnitus in animal and human cortex : cortical correlates of tinnitus.

Animal models of tinnitus complement human findings and potentially deepen our insight into the neural substrates of tinnitus. The fact that animal data are largely based on recordings from the auditory system, in particular from subcortical structures, makes comparison with human electrophysiological data from predominantly cortical areas difficult. Electro/magnetoencephalography and imaging data extend beyond the auditory cortex. The most challenging link to be made is the one between the macroscopic data in humans and the microscopic (single neuron action potentials) and mesoscopic (local field potentials) results obtained in animal models. Since invasive recordings in humans are rare, a bridge needs to be built on the basis of changes in brain rhythms in animals with putative tinnitus.

In quest of the oscillator(s) in tremor: are we getting closer?

This scientific commentary refers to ‘The nature of tremor circuits in parkinsonian and essential tremor’ by Cagnan et al. (10.1093/brain/awu250). Periodic oscillatory activity is abundant in the nervous system (Gray, 1994), both in health and in disease. Ion channels, membrane potentials, action potentials of single neurons and macroscopic brain regions all show periodic oscillations (hereafter ‘oscillations’). Within this framework, the neuronal oscillators that generate tremor, an oscillatory phenomenon in its own right, are being sought. However, attempts to define the neuronal substrate/s that generate rest and postural tremors in Parkinson’s disease and essential tremor face a number of conceptual difficulties. Correlation does not imply causality, thus strong coherence between tremor and neuronal oscillations could be the result of peripheral feedback to the nervous system. Many researchers have therefore focused on active manipulation of tremor phenomena. However, the ability to attenuate or modulate tremor by stimulating a certain brain structure does not necessarily prove that this locus is the tremor generator. What features can be used to distinguish between the tremor generator and the downstream neuronal pathways that transmit the oscillatory signal? In this issue of Brain , Cagnan et al. (2014) address this …

A Molecular Voltmeter Based on Fluorescence Dynamics

In this issue, Hou et al. (1) introduce an elegant method for measuring the absolute voltage across a cell membrane with an accuracy of 10 mV. Their study presents an incisive approach to voltage imaging in which the voltage value is encoded in the temporal dynamics of fluorescence rather than in the raw fluorescence intensity. Together with an engineered microbial rhodopsin with properties amenable to the approach, the method provides a genetically encoded optical read-out independent of the reporter’s expression level and insensitive to variations in common experimental parameters. Their work presents a conceptually new avenue toward developing probes for membrane voltage useful for myriad systems from bacteria to mammalian tissue.

Application of floating silicon-based linear multielectrode arrays for acute recording of single neuron activity in awake behaving monkeys

One of the fundamental challenges in behavioral neurophysiology in awake animals is the steady recording of action potentials of many single neurons for as long as possible. Here, we present single neuron data obtained during acute recordings mainly from premotor cortices of three macaque monkeys using a silicon-based linear multielectrode array. The most important aspect of these probes, compared with similar models commercially available, is that, once inserted into the brain using a dedicated insertion device providing an intermediate probe fixation by means of vacuum, they can be released and left floating in the brain. On the basis of our data, these features appear to provide (i) optimal physiological conditions for extracellular recordings, (ii) good or even excellent signal-to-noise ratio depending on the recorded brain area and cortical layer, and (iii) extreme stability of the signal over relatively long periods. The quality of the recorded signal did not change significantly after several penetrations into the same restricted cortical sector, suggesting limited tissue damage due to probe insertion. These results indicate that these probes offer several advantages for acute neurophysiological experiments in awake monkeys, and suggest the possibility to employ them for semichronic or even chronic studies.

Kv2 channels regulate firing rate in pyramidal neurons from rat sensorimotor cortex

The largest outward potassium current in the soma of neocortical pyramidal neurons is due to channels containing Kv2.1 α subunits. These channels have been implicated in cellular responses to seizures and ischaemia, mechanisms for intrinsic plasticity and cell death, and responsiveness to anaesthetic agents. Despite their abundance, knowledge of the function of these delayed rectifier channels has been limited by the lack of specific pharmacological agents. To test for functional roles of Kv2 channels in pyramidal cells from somatosensory or motor cortex of rats (layers 2/3 or 5), we transfected cortical neurons with DNA for a Kv2.1 pore mutant (Kv2.1W365C/Y380T: Kv2.1 DN) in an organotypic culture model to manipulate channel expression. Slices were obtained from rats at postnatal days (P7-P14) and maintained in organotypic culture. We used biolistic methods to transfect neurons with gold 'bullets' coated with DNA for the Kv2.1 DN and green fluorescent protein (GFP), GFP alone, or wild type (WT) Kv2.1 plus GFP. Cells that fluoresced green, contained a bullet and responded to positive or negative pressure from the recording pipette were considered to be transfected cells. In each slice, we recorded from a transfected cell and a control non-transfected cell from the same layer and area. Whole-cell voltage-clamp recordings obtained after 3-7 days in culture showed that cells transfected with the Kv2.1 DN had a significant reduction in outward current (∼45% decrease in the total current density measured 200 ms after onset of a voltage step from -78 to -2 mV). Transfection with GFP alone did not affect current amplitude and overexpression of the Kv2.1 WT resulted in greatly increased currents. Current-clamp experiments were used to assess the functional consequences of manipulation of Kv2.1 expression. The results suggest roles for Kv2 channels in controlling membrane potential during the interspike interval (ISI), firing rate, spike frequency adaptation (SFA) and the steady-state gain of firing. Specifically, firing rate and gain were reduced in the Kv2.1 DN cells. The most parsimonious explanation for the effects on firing is that in the absence of Kv2 channels, the membrane remains depolarized during the ISIs, preventing recovery of Na(+) channels from inactivation. Depolarization and the number of inactivated Na(+) channels would build with successive spikes, resulting in slower firing and enhanced spike frequency adaptation in the Kv2.1 DN cells.

Simultaneous stability and sensitivity in model cortical networks is achieved through anti-correlations between the in- and out-degree of connectivity

Neuronal networks in rodent barrel cortex are characterized by stable low baseline firing rates. However, they are sensitive to the action potentials of single neurons as suggested by recent single-cell stimulation experiments that reported quantifiable behavioral responses in response to short spike trains elicited in single neurons. Hence, these networks are stable against internally generated fluctuations in firing rate but at the same time remain sensitive to similarly-sized externally induced perturbations. We investigated stability and sensitivity in a simple recurrent network of stochastic binary neurons and determined numerically the effects of correlation between the number of afferent (“in-degree”) and efferent (“out-degree”) connections in neurons. The key advance reported in this work is that anti-correlation between in-/out-degree distributions increased the stability of the network in comparison to networks with no correlation or positive correlations, while being able to achieve the same level of sensitivity. The experimental characterization of degree distributions is difficult because all pre-synaptic and post-synaptic neurons have to be identified and counted. We explored whether the statistics of network motifs, which requires the characterization of connections between small subsets of neurons, could be used to detect evidence for degree anti-correlations. We find that the sample frequency of the 3-neuron “ring” motif (1→2→3→1), can be used to detect degree anti-correlation for sub-networks of size 30 using about 50 samples, which is of significance because the necessary measurements are achievable experimentally in the near future. Taken together, we hypothesize that barrel cortex networks exhibit degree anti-correlations and specific network motif statistics.