A naphthyl analog of the aminostyryl pyridinium class of potentiometric membrane dyes shows consistent sensitivity in a variety of tissue, cell, and model membrane preparations

Scanless two-photon voltage imaging

Two-photon voltage imaging has long been heralded as a transformative approach capable of answering many long-standing questions in modern neuroscience. However, exploiting its full potential requires the development of novel imaging approaches well suited to the photophysical properties of genetically encoded voltage indicators. We demonstrate that parallel excitation approaches developed for scanless two-photon photostimulation enable high-SNR two-photon voltage imaging. We use whole-cell patch-clamp electrophysiology to perform a thorough characterization of scanless two-photon voltage imaging using three parallel illumination approaches and lasers with different repetition rates and wavelengths. We demonstrate voltage recordings of high-frequency spike trains and sub-threshold depolarizations from neurons expressing the soma-targeted genetically encoded voltage indicator JEDI-2P-Kv. Using a low repetition-rate laser, we perform multi-cell recordings from up to fifteen targets simultaneously. We co-express JEDI-2P-Kv and the channelrhodopsin ChroME-ST and capitalize on their overlapping two-photon absorption spectra to simultaneously evoke and image action potentials using a single laser source. We also demonstrate in vivo scanless two-photon imaging of multiple cells simultaneously up to 250 µm deep in the barrel cortex of head-fixed, anaesthetised mice.

An NIR-IIb emissive transmembrane voltage nano-indicator for the optical monitoring of electrophysiological activities in vivo

In vivo transmembrane-voltage detection reflected the electrophysiological activities of the biological system, which is crucial for the diagnosis of neuronal disease. Traditional implanted electrodes can only monitor limited regions and induce relatively large tissue damage. Despite emerging monitoring methods based on optical imaging have access to signal recording in a larger area, the recording wavelength of less than 1000 nm seriously weakens the detection depth and resolution in vivo. Herein, a Förster resonance energy transfer (FRET)-based nano-indicator, NaYbF4:Er@NaYF4@Cy7.5@DPPC (Cy7.5-ErNP) with emission in the near-infrared IIb biological window (NIR-IIb, 1500-1700 nm) is developed for transmembrane-voltage detection. Cy7.5 dye is found to be voltage-sensitive and is employed as the energy donor for the energy transfer to the lanthanide nanoparticle, NaYbF4:Er@NaYF4 (ErNP), which works as the acceptor to achieve electrophysiological signal responsive NIR-IIb luminescence. Benefiting from the high penetration and low scattering of NIR-IIb luminescence, the Cy7.5-ErNP enables both the visualization of action potential in vitro and monitoring of Mesial Temporal lobe epilepsy (mTLE) disease in vivo. This work presents a concept for leveraging the lanthanide luminescent nanoprobes to visualize electrophysiological activity in vivo, which facilitates the development of an optical nano-indicator for the diagnosis of neurological disorders.

Monitoring the response of a model protocell to dye and surfactant molecules through second harmonic generation and fluorescence imaging

Probing the interaction between molecules and protocells is crucial for understanding the passive transport of functional molecules in and out of artificial and real cells. Second-harmonic generation (SHG) has been proven to be a powerful method for analyzing the adsorption and cross-membrane transport of molecules on lipid bilayers. In this study, we used SHG and two-photon fluorescence (TPF) imaging to study the interaction of charged dye molecules (D289) with a lipid vesicle. Unexpectedly, it was observed that the transport of D289 at a relatively high concentration is not as efficient as that at a lower dye concentration. Periodic shrinking of the model protocell and discharging of D289 out from the vesicle were revealed by combined analyses of SHG and TPF images. The response of the vesicle to a surfactant was also analyzed with D289 as a probe. This work demonstrates that the combined SHG and TPF imaging method is a unique approach that can provide detailed information on the interaction of molecules and lipids (both morphology and molecular kinetics). Determining these subtle interfacial kinetics in molecules is important for understanding the mechanism of many biophysical processes occurring on lipids.

Life Detection on Icy Moons Using Flow Cytometry and Exogenous Fluorescent Stains

Flow cytometry is a potential technology for in situ life detection on icy moons (such as Enceladus and Europa) and on the polar ice caps of Mars. We developed a method for using flow cytometry to positively identify four classes of biomarkers using exogenous fluorescent stains: nucleic acids, proteins, carbohydrates, and lipids. We demonstrated the effectiveness of exogenous stains with six known organisms and known abiotic material and showed that the cytometer is easily able to distinguish between the known organisms and the known abiotic material using the exogenous stains. To simulate a life-detection experiment on an icy world lander, we used six natural samples with unknown biotic and abiotic content. We showed that flow cytometry can identify all four biomarkers using the exogenous stains and can separate the biotic material from the known abiotic material on scatter plots. Exogenous staining techniques would likely be used in conjunction with intrinsic fluorescence, clustering, and sorting for a more complete and capable life-detection instrument on an icy moon lander.

Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes (1a-1b) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b, exhibited interesting photophysical properties including, longer wavelength excitation (λex ≈ 530-560 nm), bright near-infrared emission (λem ≈ 690-720 nm), high-fluorescence quantum yields (ϕfl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes (1a-1b) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a, due to the regio-effect. Probes (1a-1b) exhibited an excellent ability to visualize bacteria (Bacillus megaterium, Escherichia coli), and yeast (Saccharomyces cerevisiae) via fluorescence microscopy.

Membrane Dipole Potential: An Emerging Approach to Explore Membrane Organization and Function

Biological membranes are complex organized molecular assemblies of lipids and proteins that provide cells and membrane-bound intracellular organelles their individual identities by morphological compartmentalization. Membrane dipole potential originates from the electrostatic potential difference within the membrane due to the nonrandom arrangement (orientation) of amphiphile and solvent (water) dipoles at the membrane interface. In this Feature Article, we will focus on the measurement of dipole potential using electrochromic fluorescent probes and highlight interesting applications. In addition, we will focus on ratiometric fluorescence microscopic imaging technique to measure dipole potential in cellular membranes, a technique that can be used to address novel problems in cell biology which are otherwise difficult to address using available approaches. We envision that membrane dipole potential could turn out to be a convenient tool in exploring the complex interplay between membrane lipids and proteins and could provide novel insights in membrane organization and function.

Lipids-Fluorophores Interactions Probed by Combined Nonlinear Polarized Microscopy

Studying the structural dynamics of lipid membranes requires methods that can address both microscopic and macroscopic characteristics. Fluorescence imaging is part of the most used techniques to study membrane properties in various systems from artificial membranes to cells: It benefits from a high sensitivity to local properties such as polarity and molecular orientational order, with a high spatial resolution down to the single-molecule level. The influence of embedded fluorescent lipid probes on the lipid membrane molecules is however poorly known and relies most often on molecular dynamics simulations, due to the challenges faced by experimental approaches to address the molecular-scale dimension of this question. In this work we develop an optical microscopy imaging method to probe the effect of fluorophores embedded in the membrane as lipid probes, on their lipid environment, with a lateral resolution of a few hundreds of nanometers. We combine polarized-nonlinear microscopy contrasts that can independently address the lipid probe, by polarized two-photon fluorescence, and the membrane lipids, by polarized coherent Raman scattering. Using trimethylamino derivative 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS) as model probes, we show that both probes tend to induce an orientational disorder of their surrounding lipid CH-bonds in 1,2-dipalmitoylphosphatidylcholine (DPPC) lipids environments, while there is no noticeable effect in more disordered 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid membranes.

Fluorescence sensors for imaging membrane lipid domains and cholesterol

Lipid membrane domains are supramolecular lateral heterogeneities of biological membranes. Of nanoscopic dimensions, they constitute specialized hubs used by the cell as transient signaling platforms for a great variety of biologically important mechanisms. Their property to form and dissolve in the bulk lipid bilayer endow them with the ability to engage in highly dynamic processes, and temporarily recruit subpopulations of membrane proteins in reduced nanometric compartments that can coalesce to form larger mesoscale assemblies. Cholesterol is an essential component of these lipid domains; its unique molecular structure is suitable for interacting intricately with crevices and cavities of transmembrane protein surfaces through its rough β face while "talking" to fatty acid acyl chains of glycerophospholipids and sphingolipids via its smooth α face. Progress in the field of membrane domains has been closely associated with innovative improvements in fluorescence microscopy and new fluorescence sensors. These advances enabled the exploration of the biophysical properties of lipids and their supramolecular platforms. Here I review the rationale behind the use of biosensors over the last few decades and their contributions towards elucidation of the in-plane and transbilayer topography of cholesterol-enriched lipid domains and their molecular constituents. The challenges introduced by super-resolution optical microscopy are discussed, as well as possible scenarios for future developments in the field, including virtual ("no staining") staining.

Studying Synaptically Evoked Cortical Responses ex vivo With Combination of a Single Neuron Recording (Whole-Cell) and Population Voltage Imaging (Genetically Encoded Voltage Indicator)

In a typical electrophysiology experiment, synaptic stimulus is delivered in a cortical layer (1-6) and neuronal responses are recorded intracellularly in individual neurons. We recreated this standard electrophysiological paradigm in brain slices of mice expressing genetically encoded voltage indicators (GEVIs). This allowed us to monitor membrane voltages in the target pyramidal neurons (whole-cell), and population voltages in the surrounding neuropil (optical imaging), simultaneously. Pyramidal neurons have complex dendritic trees that span multiple cortical layers. GEVI imaging revealed areas of the brain slice that experienced the strongest depolarization on a specific synaptic stimulus (location and intensity), thus identifying cortical layers that contribute the most afferent activity to the recorded somatic voltage waveform. By combining whole-cell with GEVI imaging, we obtained a crude distribution of activated synaptic afferents in respect to the dendritic tree of a pyramidal cell. Synaptically evoked voltage waves propagating through the cortical neuropil (dendrites and axons) were not static but rather they changed on a millisecond scale. Voltage imaging can identify areas of brain slices in which the neuropil was in a sustained depolarization (plateau), long after the stimulus onset. Upon a barrage of synaptic inputs, a cortical pyramidal neuron experiences: (a) weak temporal summation of evoked voltage transients (EPSPs); and (b) afterhyperpolarization (intracellular recording), which are not represented in the GEVI population imaging signal (optical signal). To explain these findings [(a) and (b)], we used four voltage indicators (ArcLightD, chi-VSFP, Archon1, and di-4-ANEPPS) with different optical sensitivity, optical response speed, labeling strategy, and a target neuron type. All four imaging methods were used in an identical experimental paradigm: layer 1 (L1) synaptic stimulation, to allow direct comparisons. The population voltage signal showed paired-pulse facilitation, caused in part by additional recruitment of new neurons and dendrites. "Synaptic stimulation" delivered in L1 depolarizes almost an entire cortical column to some degree.

Unveiling the Molecular Dynamics in a Living Cell to the Subcellular Organelle Level Using Second-Harmonic Generation Spectroscopy and Microscopy

Second-harmonic generation (SHG) microscopy has been proved to be a powerful method for investigating the structures of biomaterials. SHG spectra were also generally used to probe the adsorption and cross-membrane transport of molecules on lipid bilayers in situ and in real time. In this work, we applied SHG and two-photon fluorescence (TPF) spectra to investigate the dynamics of an amphiphilic ion with an SHG and TPF chromophore, D289 (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide), on the surface of human chronic myelogenous leukemia (K562) cells and the subcellular structures inside the cells. The adsorption and cross-membrane transport of D289 into the cells and then into the organelles such as mitochondria were revealed. SHG images were also recorded and used to demonstrate their capability of probing molecular dynamics in organelles in K562 cells. This work demonstrated the first SHG investigation of the cross-membrane transport dynamics on the surface of subcellular organelles. It may also shed light on the differentiation of different types of subcellular structures in cells.

Advanced real-time recordings of neuronal activity with tailored patch pipettes, diamond multi-electrode arrays and electrochromic voltage-sensitive dyes

To understand the working principles of the nervous system is key to figure out its electrical activity and how this activity spreads along the neuronal network. It is therefore crucial to develop advanced techniques aimed to record in real time the electrical activity, from compartments of single neurons to populations of neurons, to understand how higher functions emerge from coordinated activity. To record from single neurons, a technique will be presented to fabricate patch pipettes able to seal on any membrane with a single glass type and whose shanks can be widened as desired. This dramatically reduces access resistance during whole-cell recording allowing fast intracellular and, if required, extracellular perfusion. To simultaneously record from many neurons, biocompatible probes will be described employing multi-electrodes made with novel technologies, based on diamond substrates. These probes also allow to synchronously record exocytosis and neuronal excitability and to stimulate neurons. Finally, to achieve even higher spatial resolution, it will be shown how voltage imaging, employing fast voltage-sensitive dyes and two-photon microscopy, is able to sample voltage oscillations in the brain spatially resolved and voltage changes in dendrites of single neurons at millisecond and micrometre resolution in awake animals.

A simultaneous optical and electrical in-vitro neuronal recording system to evaluate microelectrode performance

Objectives

In this paper, we aim to detail the setup of a high spatio-temporal resolution, electrical recording system utilising planar microelectrode arrays with simultaneous optical imaging suitable for evaluating microelectrode performance with a proposed ′performance factor′ metric.

Methods

Techniques that would facilitate low noise electrical recordings were coupled with voltage sensitive dyes and neuronal activity was recorded both electrically via a customised amplification system and optically via a high speed CMOS camera. This technique was applied to characterise microelectrode recording performance of gold and poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) coated electrodes through traditional signal to noise (SNR) calculations as well as the proposed performance factor.

Results

Neuronal activity was simultaneously recorded using both electrical and optical techniques and this activity was confirmed via tetrodotoxin application to inhibit action potential firing. PEDOT/PSS outperformed gold using both measurements, however, the performance factor metric estimated a 3 fold improvement in signal transduction when compared to gold, whereas SNR estimated an 8 fold improvement when compared to gold.

Conclusion

The design and functionality of a system to record from neurons both electrically, through microelectrode arrays, and optically via voltage sensitive dyes was successfully achieved.

Significance

The high spatiotemporal resolution of both electrical and optical methods will allow for an array of applications such as improved detection of subthreshold synaptic events, validation of spike sorting algorithms and a provides a robust evaluation of extracellular microelectrode performance.

Recent progress in optical voltage-sensor technology and applications to cardiac research: from single cells to whole hearts

The first workshop on Novel Optics-based approaches for Cardiac Electrophysiology (NOtiCE) was held in Florence Italy in 2018. Here, we learned how optical approaches have shaped our basic understanding of cardiac electrophysiology and how new technologies and approaches are being developed and validated to advance the field. Several technologies are being developed that may one day allow for new clinical approaches for diagnosing cardiac disorders and possibly intervening to treat human patients. In this review, we discuss several technologies and approaches to optical voltage imaging with voltage-sensitive dyes. We highlight the development and application of fluorinated and long wavelength voltage-sensitive dyes. These optical voltage sensors have now been applied and well validated in several different assays from cultured human stem cell-derived cardiomyocytes to whole hearts in-vivo. Imaging concepts such as dual wavelength ratiometric techniques, which are crucial to maximizing the information from optical sensors by increasing the useful signal and eliminating noise and artifacts, are presented. Finally, novel voltage sensors including photoacoustic voltage-sensitive dyes, their current capabilities and potential advantages, are introduced.

Liquid-Ordered Phase Formation by Mammalian and Yeast Sterols: A Common Feature With Organizational Differences

Here, biophysical properties of membranes enriched in three metabolically related sterols are analyzed both in vitro and in vivo. Unlike cholesterol and ergosterol, the common metabolic precursor zymosterol is unable to induce the formation of a liquid ordered (l _o) phase in model lipid membranes and can easily accommodate in a gel phase. As a result, Zym has a marginal ability to modulate the passive membrane permeability of lipid vesicles with different compositions, contrary to cholesterol and ergosterol. Using fluorescence-lifetime imaging microscopy of an aminostyryl dye in living mammalian and yeast cells we established a close parallel between sterol-dependent membrane biophysical properties in vivo and in vitro. This approach unraveled fundamental differences in yeast and mammalian plasma membrane organization. It is often suggested that, in eukaryotes, areas that are sterol-enriched are also rich in sphingolipids, constituting highly ordered membrane regions. Our results support that while cholesterol is able to interact with saturated lipids, ergosterol seems to interact preferentially with monounsaturated phosphatidylcholines. Taken together, we show that different eukaryotic kingdoms developed unique solutions for the formation of a sterol-rich plasma membrane, a common evolutionary trait that accounts for sterol structural diversity.

Optogenetic Manipulation of Postsynaptic cAMP Using a Novel Transgenic Mouse Line Enables Synaptic Plasticity and Enhances Depolarization Following Tetanic Stimulation in the Hippocampal Dentate Gyrus

cAMP is a positive regulator tightly involved in certain types of synaptic plasticity and related memory functions. However, its spatiotemporal roles at the synaptic and neural circuit levels remain elusive. Using a combination of a cAMP optogenetics approach and voltage-sensitive dye (VSD) imaging with electrophysiological recording, we define a novel capacity of postsynaptic cAMP in enabling dentate gyrus long-term potentiation (LTP) and depolarization in acutely prepared murine hippocampal slices. To manipulate cAMP levels at medial perforant path to granule neuron (MPP-DG) synapses by light, we generated transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC) in DG granule neurons. Using these Tg(CMV-Camk2a-RFP/bPAC)3Koka mice, we recorded field excitatory postsynaptic potentials (fEPSPs) from MPP-DG synapses and found that photoactivation of PAC during tetanic stimulation enabled synaptic potentiation that persisted for at least 30 min. This form of LTP was induced without the need for GABA receptor blockade that is typically required for inducing DG plasticity. The paired-pulse ratio (PPR) remained unchanged, indicating the cAMP-dependent LTP was likely postsynaptic. By employing fast fluorescent voltage-sensitive dye (VSD: di-4-ANEPPS) and fluorescence imaging, we found that photoactivation of the PAC actuator enhanced the intensity and extent of dentate gyrus depolarization triggered following tetanic stimulation. These results demonstrate that the elevation of cAMP in granule neurons is capable of rapidly enhancing synaptic strength and neuronal depolarization. The powerful actions of cAMP are consistent with this second messenger having a critical role in the regulation of synaptic function.

Anionic Conjugated Polyelectrolytes for FRET-based Imaging of Cellular Membrane Potential

We report a Förster resonance energy transfer (FRET)-based imaging ensemble for the visualization of membrane potential in living cells. A water-soluble poly(fluorene-cophenylene) conjugated polyelectrolyte (FsPFc10) serves as a FRET donor to a voltage-sensitive dye acceptor (FluoVolt^™ ). We observe FRET between FsPFc10 and FluoVolt^™ , where the enhancement in FRET-sensitized emission from FluoVolt^™ is measured at various donor/acceptor ratios. At a donor/acceptor ratio of 1, the excitation of FluoVolt^™ in a FRET configuration results in a three-fold enhancement in its fluorescence emission (compared to when it is excited directly). FsPFc10 efficiently labels the plasma membrane of HEK 293T/17 cells and remains resident with minimal cellular internalization for ~ 1.5 h. The successful plasma membrane-associated colabeling of the cells with the FsPFc10-FluoVolt^™ donor-acceptor pair is confirmed by dual-channel confocal imaging. Importantly, cells labeled with FsPFc10 show excellent cellular viability with no adverse effect on cell membrane depolarization. During depolarization of membrane potential, HEK 293T/17 cells labeled with the donor-acceptor FRET pair exhibit a greater fluorescence response in FluoVolt^™ emission relative to when FluoVolt^™ is used as the sole imaging probe. These results demonstrate the conjugated polyelectrolyte to be a new class of membrane labeling fluorophore for use in voltage sensing schemes.

Electrophysiological characterization of drug response in hSC-derived cardiomyocytes using voltage-sensitive optical platforms

INTRODUCTION

Voltage-sensitive optical (VSO) sensors offer a minimally invasive method to study the time course of repolarization of the cardiac action potential (AP). This Comprehensive in vitro Proarrhythmia Assay (CiPA) cross-platform study investigates protocol design and measurement variability of VSO sensors for preclinical cardiac electrophysiology assays.

METHODS

Three commercial and one academic laboratory completed a limited study of the effects of 8 blinded compounds on the electrophysiology of 2 commercial lines of human induced pluripotent stem-cell derived cardiomyocytes (hSC-CMs). Acquisition technologies included CMOS camera and photometry; fluorescent voltage sensors included di-4-ANEPPS, FluoVolt and genetically encoded QuasAr2. The experimental protocol was standardized with respect to cell lines, plating and maintenance media, blinded compounds, and action potential parameters measured. Serum-free media was used to study the action of drugs, but the exact composition and the protocols for cell preparation and drug additions varied among sites.

RESULTS

Baseline AP waveforms differed across platforms and between cell types. Despite these differences, the relative responses to four selective ion channel blockers (E-4031, nifedipine, mexiletine, and JNJ 303 blocking I_Kr, I_CaL, I_Na, and I_Ks, respectively) were similar across all platforms and cell lines although the absolute changes differed. Similarly, four mixed ion channel blockers (flecainide, moxifloxacin, quinidine, and ranolazine) had comparable effects in all platforms. Differences in repolarisation time course and response to drugs could be attributed to cell type and experimental method differences such as composition of the assay media, stimulated versus spontaneous activity, and single versus cumulative compound addition.

DISCUSSION

In conclusion, VSOs represent a powerful and appropriate method to assess the electrophysiological effects of drugs on iPSC-CMs for the evaluation of proarrhythmic risk. Protocol considerations and recommendations are provided toward standardizing conditions to reduce variability of baseline AP waveform characteristics and drug responses.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Tethered Bichromophoric Fluorophore Quencher Voltage Sensitive Dyes

Voltage sensitive dyes (VSDs) are used for in vitro drug screening and for imaging of patterns of electrical activity in tissue. Wide application of this technology depends on the availability of sensors with high sensitivity (percent change of fluorescence per 100 mV), high fluorescence quantum yield, and fast response kinetics. A promising approach uses a two-component system consisting of anionic membrane permeable quenchers with fluorophores labeling one side of the membrane; this produces voltage-dependent fluorescence quenching. However, the quencher must be kept at low concentrations to minimize pharmacological effects, thus limiting sensitivity. By developing tethered bichromophoric fluorophore quencher (TBFQ) dyes, where the fluorophore and quencher are covalently connected by a long hydrophobic chain, the sensitivity is maximized and is independent of VSD concentration. A series of 13 TBFQ dyes based on the aminonaphthylethenylpyridinium (ANEP) fluorophore and the dipicrylamine anion (DPA) quencher have been synthesized and tested in an artificial lipid bilayer apparatus. The best of these, TBFQ1, shows a 2.5-fold change in fluorescence per 100 mV change in membrane potential, and the response kinetics is in the 10-20 ms range. This sensitivity is an order of magnitude better than that of commonly used VSDs. However, the fluorescence quantum yield is only 1.6%, which may make this first generation of TBFQ VSDs impractical for in vivo electrical imaging. Nevertheless, the design principles established here can serve as foundation for improved TBFQ VSDs. We believe this approach promises to greatly enhance our ability to monitor electrical activity in cells and tissues.

Overall Assay of Neuronal Signal Propagation Pattern With Long-Term Potentiation (LTP) in Hippocampal Slices From the CA1 Area With Fast Voltage-Sensitive Dye Imaging

Activity-dependent changes in the input-output (I-O) relationship of a neural circuit are central in the learning and memory function of the brain. To understand circuit-wide adjustments, optical imaging techniques to probe the membrane potential at every component of neurons, such as dendrites, axons and somas, in the circuit are essential. We have been developing fast voltage-sensitive dye (VSD) imaging methods for quantitative measurements, especially for single-photon wide-field optical imaging. The long-term continuous measurements needed to evaluate circuit-wide modifications require stable and quantitative long-term recordings. Here, we show that VSD imaging (VSDI) can be used to record changes in circuit activity in association with theta-burst stimulation (TBS)-induced long-term potentiation (LTP) of synaptic strength in the CA1 area. Our optics, together with the fast imaging system, enabled us to measure neuronal signals from the entire CA1 area at a maximum frame speed of 0.1 ms/frame every 60 s for over 12 h. We also introduced a method to evaluate circuit activity changes by mapping the variation in recordings from the CA1 area to coordinates defined by the morphology of CA1 pyramidal cells. The results clearly showed two types of spatial heterogeneity in LTP induction. The first heterogeneity is that LTP increased with distance from the stimulation site. The second heterogeneity is that LTP is higher in the stratum pyramidale (SP)-oriens region than in the stratum radiatum (SR). We also showed that the pattern of the heterogeneity changed according to the induction protocol, such as induction by TBS or high-frequency stimulation (HFS). We further demonstrated that part of the heterogeneity depends on the I-O response of the circuit elements. The results show the usefulness of VSDI in probing the function of hippocampal circuits.

Characterization of di-4-ANEPPS with nano-black lipid membranes

We report a platform based on lateral nano-black lipid membranes (nano-BLMs), where electrical measurements and fluorescence microscopy setup are combined, for the calibration of di-4-ANEPPS, a common voltage sensitive dye (VSD). The advantage of this setup is (1) its flexibility in the choice of lipids and applied voltages, (2) its high stability that enables a high voltage (500 mV) application and long-time measurements and (3) its fluorescence microscopy readout, which can be directly correlated with other fluorescence microscopy experiments using VSDs (e.g. membrane potential measurements in living cells). Using this setup, we observed that the calibration curve of di-4-ANEPPS is strongly dependent on the net electric charge of the lipids. The developed setup can be used to calibrate VSDs in different lipid environments in order to better understand their fundamental voltage-sensing mechanism in the future.

Optical imaging of the rat brain suggests a previously missing link between top-down and bottom-up nervous system function

Optical imaging with voltage-sensitive dyes enables the visualization of extensive yet highly transient coalitions of neurons (assemblies) operating throughout the brain on a subsecond time scale. We suggest that operating at the mesoscale level of brain organization, neuronal assemblies may provide a functional link between "bottom-up" cellular mechanisms and "top-down" cognitive ones within anatomically defined regions. We demonstrate in ex vivo rat brain slices how varying spatiotemporal dynamics of assemblies reveal differences not previously appreciated between: different stages of development in cortical versus subcortical brain areas, different sensory modalities (hearing versus vision), different classes of psychoactive drugs (anesthetics versus analgesics), different effects of anesthesia linked to hyperbaric conditions and, in vivo, depths of anesthesia. The strategy of voltage-sensitive dye imaging is therefore as powerful as it is versatile and as such can now be applied to the evaluation of neurochemical signaling systems and the screening of related new drugs, as well as to mathematical modeling and, eventually, even theories of consciousness.

Quantum Dot-Peptide-Fullerene Bioconjugates for Visualization of in Vitro and in Vivo Cellular Membrane Potential

We report the development of a quantum dot (QD)-peptide-fullerene (C60) electron transfer (ET)-based nanobioconjugate for the visualization of membrane potential in living cells. The bioconjugate is composed of (1) a central QD electron donor, (2) a membrane-inserting peptidyl linker, and (3) a C60 electron acceptor. The photoexcited QD donor engages in ET with the C60 acceptor, resulting in quenching of QD photoluminescence (PL) that tracks positively with the number of C60 moieties arrayed around the QD. The nature of the QD-capping ligand also modulates the quenching efficiency; a neutral ligand coating facilitates greater QD quenching than a negatively charged carboxylated ligand. Steady-state photophysical characterization confirms an ET-driven process between the donor-acceptor pair. When introduced to cells, the amphiphilic QD-peptide-C60 bioconjugate labels the plasma membrane by insertion of the peptide-C60 portion into the hydrophobic bilayer, while the hydrophilic QD sits on the exofacial side of the membrane. Depolarization of cellular membrane potential augments the ET process, which is manifested as further quenching of QD PL. We demonstrate in HeLa cells, PC12 cells, and primary cortical neurons significant QD PL quenching (ΔF/F0 of 2-20% depending on the QD-C60 separation distance) in response to membrane depolarization with KCl. Further, we show the ability to use the QD-peptide-C60 probe in combination with conventional voltage-sensitive dyes (VSDs) for simultaneous two-channel imaging of membrane potential. In in vivo imaging of cortical electrical stimulation, the optical response of the optimal QD-peptide-C60 configuration exhibits temporal responsivity to electrical stimulation similar to that of VSDs. Notably, however, the QD-peptide-C60 construct displays 20- to 40-fold greater ΔF/F0 than VSDs. The tractable nature of the QD-peptide-C60 system offers the advantages of ease of assembly, large ΔF/F0, enhanced photostability, and high throughput without the need for complicated organic synthesis or genetic engineering, respectively, that is required of traditional VSDs and fluorescent protein constructs.

Fast flip–flop of halogenated cobalt bis(dicarbollide) anion in a lipid bilayer membrane

Halogenation dramatically affects the flip–flop of cobalt bis(dicarbollide) across the lipid membrane causing acceleration (Cl, Br, I) or deceleration (F).

Disulfide mapping the voltage-sensing mechanism of a voltage-dependent potassium channel

Voltage-dependent potassium (Kv) channels allow for the selective permeability of potassium ions in a membrane potential dependent manner, playing crucial roles in neurotransmission and muscle contraction. Kv channel is a tetramer, in which each subunit possesses a voltage-sensing domain (VSD) and a pore domain (PD). Although several lines of evidence indicated that membrane depolarization is sensed as the movement of helix S4 of the VSD, the detailed voltage-sensing mechanism remained elusive, due to the difficulty of structural analyses at resting potential. In this study, we conducted a comprehensive disulfide locking analysis of the VSD using 36 double Cys mutants, in order to identify the proximal residue pairs of the VSD in the presence or absence of a membrane potential. An intramolecular SS-bond was formed between 6 Cys pairs under both polarized and depolarized environment, and one pair only under depolarized environment. The multiple conformations captured by the SS-bond can be divided by two states, up and down, where S4 lies on the extracellular and intracellular sides of the membrane, respectively, with axial rotation of 180°. The transition between these two states is caused by the S4 translocation of 12 Å, enabling allosteric regulation of the gating at the PD.

Layer-specific potentiation of network GABAergic inhibition in the CA1 area of the hippocampus

One of the most important functions of GABAergic inhibition in cortical regions is the tight control of spatiotemporal activity of principal neuronal ensembles. However, electrophysiological recordings do not provide sufficient spatial information to determine the spatiotemporal properties of inhibitory plasticity. Using Voltage Sensitive Dye Imaging (VSDI) in mouse hippocampal slices, we demonstrate that GABA_A-mediated field inhibitory postsynaptic potentials undergo layer-specific potentiation upon activation of metabotropic glutamate receptors (mGlu). VSDI recordings allowed detection of pharmacologically isolated GABA_A-dependent hyperpolarization signals. Bath-application of the selective group-I mGlu receptor agonist, (S)-3,5-Dihydroxyphenylglycine (DHPG), induces an enhancement of the GABAergic VSDI-recorded signal, which is more or less pronounced in different hippocampal layers. This potentiation is mediated by mGlu₅ and downstream activation of IP₃ receptors. Our results depict network GABAergic activity in the hippocampal CA1 region and its sub-layers, showing also a novel form of inhibitory synaptic plasticity tightly coupled to glutamatergic activity.

Voltage-sensitive styryl dyes as singlet oxygen targets on the surface of bilayer lipid membrane

Photosensitizers are widely used as photodynamic therapeutic agents killing cancer cells by photooxidation of their components. Development of new effective photosensitive molecules requires profound knowledge of possible targets for reactive oxygen species, especially for its singlet form. Here we studied photooxidation of voltage-sensitive styryl dyes (di-4-ANEPPS, di-8-ANEPPS, RH-421 and RH-237) by singlet oxygen on the surface of bilayer lipid membranes commonly used as cell membrane models. Oxidation was induced by irradiation of a photosensitizer (aluminum phthalocyanine tetrasulfonate) and monitored by the change of dipole potential on the surface of the membrane. We studied the drop of the dipole potential both in the case when the dye molecules were adsorbed on the same side of the lipid bilayer as the photosensitizer (cis-configuration) and in the case when they were adsorbed on the opposite side (trans-configuration). Based on a simple model, we determined the rate of oxidation of the dyes from the kinetics of change of the potential during and after irradiation. This rate is proportional to steady-state concentration of singlet oxygen in the membrane under irradiation. Comparison of the oxidation rates of various dyes reveals that compounds of ANEPPS series are more sensitive to singlet oxygen than RH type dyes, indicating that naphthalene group is primarily responsible for their oxidation.

Electric Field Modulation of Semiconductor Quantum Dot Photoluminescence: Insights Into the Design of Robust Voltage-Sensitive Cellular Imaging Probes

The intrinsic properties of quantum dots (QDs) and the growing ability to interface them controllably with living cells has far-reaching potential applications in probing cellular processes such as membrane action potential. We demonstrate that an electric field typical of those found in neuronal membranes results in suppression of the QD photoluminescence (PL) and, for the first time, that QD PL is able to track the action potential profile of a firing neuron with millisecond time resolution. This effect is shown to be connected with electric-field-driven QD ionization and consequent QD PL quenching, in contradiction with conventional wisdom that suppression of the QD PL is attributable to the quantum confined Stark effect.

Monitoring Population Membrane Potential Signals from Neocortex

Voltage-sensitive dyes (VSDs) and optical imaging are useful tools for studying spatiotemporal patterns of population neuronal activity in cortex. Because fast VSDs respond to membrane potential changes with microsecond temporal resolution, these are better suited than calcium indicators for recording rapid neural signals. Here we describe methods for using a 464 element photodiode array and fast VSDs to record signals ranging from large scale network activity in brain slices and in vivo mammalian preparations with sensitivity comparable to local field potential (LFP) recordings. With careful control of dye bleaching and phototoxicity, long recording times can be achieved. Absorption dyes have less photo-toxicity than fluorescent dyes. In brain slices, the total recording time in each slice can be 1,000-2,000 s, which can be divided into hundreds of short recording trials over several hours. In intact brains when fluorescent dyes are used, reduced light intensity can also increase recording time. In this chapter, we will discuss technical details for the methods to achieve reliable VSD imaging with high sensitivity and long recording time.

Optical stimulation enables paced electrophysiological studies in embryonic hearts

Cardiac electrophysiology plays a critical role in the development and function of the heart. Studies of early embryonic electrical activity have lacked a viable point stimulation technique to pace in vitro samples. Here, optical pacing by high-precision infrared stimulation is used to pace excised embryonic hearts, allowing electrophysiological parameters to be quantified during pacing at varying rates with optical mapping. Combined optical pacing and optical mapping enables electrophysiological studies in embryos under more physiological conditions and at varying heart rates, allowing detection of abnormal conduction and comparisons between normal and pathological electrical activity during development in various models.

Absolute spectroscopic determination of cross-membrane potential

Spectroscopic determination of the cross-membrane electric potential has been used for more than 20 years. This method, which usually employs absorption or fluorescence measurements, allows for a rapid and noninvasive study of the electrical properties of the membranes of cells and liposomes. However, the usual fluorescence techniques preferably allow monitoring changes in the potential on triggerable or excitable membranes, and not the absolute value of the potential. They also do not provide means for measuring the potential on single cells. This paper reviews three methods that solve these issues. Nernstian dyes which partition between intra-and extracompartmental volumes enable a fluorescence microscopic determination of a single cell and even a single organelle. Dual-wavelength ratiometric recording from membrane-staining dyes also provides means for measuring the field on a single cell. Resonance Raman probes provide a spectroscopic method with a natural internal standard for the absolute measurement of membrane potential.

A new nonscanning confocal microscopy module for functional voltage-sensitive dye and Ca2+ imaging of neuronal circuit activity

Recent advances in fluorescent confocal microscopy and voltage-sensitive and Ca(2+) dyes have vastly improved our ability to image neuronal circuits. However, existing confocal systems are not fast enough or too noisy for many live-cell functional imaging studies. Here, we describe and demonstrate the function of a novel, nonscanning confocal microscopy module. The optics, which are designed to fit the standard camera port of the Olympus BX51WI epifluorescent microscope, achieve a high signal-to-noise ratio (SNR) at high temporal resolution, making this configuration ideal for functional imaging of neuronal activities such as the voltage-sensitive dye (VSD) imaging. The optics employ fixed 100- × 100-pinhole arrays at the back focal plane (optical conjugation plane), above the tube lens of a usual upright microscope. The excitation light travels through these pinholes, and the fluorescence signal, emitted from subject, passes through corresponding pinholes before exciting the photodiodes of the imager: a 100- × 100-pixel metal-oxide semiconductor (MOS)-type pixel imager with each pixel corresponding to a single 100- × 100-μm photodiode. This design eliminated the need for a scanning device; therefore, acquisition rate of the imager (maximum rate of 10 kHz) is the only factor limiting acquisition speed. We tested the application of the system for VSD and Ca(2+) imaging of evoked neuronal responses on electrical stimuli in rat hippocampal slices. The results indicate that, at least for these applications, the new microscope maintains a high SNR at image acquisition rates of ≤0.3 ms per frame.

Probing the orientational distribution of dyes in membranes through multiphoton microscopy

Numerous dyes are available or under development for probing the structural and functional properties of biological membranes. Exogenous chromophores adopt a range of orientations when bound to membranes, which have a drastic effect on their biophysical behavior. Here, we present a method that employs optical anisotropy data from three polarization-imaging techniques to establish the distribution of orientations adopted by molecules in monolayers and bilayers. The resulting probability density functions, which contain the preferred molecular tilt μ and distribution breadth γ, are more informative than an average tilt angle [φ]. We describe a methodology for the extraction of anisotropy data through an image-processing technology that decreases the error in polarization measurements by about a factor of four. We use this technique to compare di-4-ANEPPS and di-8-ANEPPS, both dipolar dyes, using data from polarized 1-photon, 2-photon fluorescence and second-harmonic generation imaging. We find that di-8-ANEPPS has a lower tilt but the same distributional width. We find the distribution of tilts taken by di-4-ANEPPS in two phospholipid membrane models: giant unilamellar vesicles and water-in-oil droplet monolayers. Both models result in similar distribution functions with average tilts of 52° and 47°, respectively.

An evaluation of in vivo voltage-sensitive dyes: pharmacological side effects and signal-to-noise ratios after effective removal of brain-pulsation artifacts

In the current study, we investigated pharmacological side effects and signal-to-noise ratios (SNRs) of two commonly used voltage-sensitive dyes (VSDs): the blue dye RH-1691 (1 mg/ml) and the red dye di-4-ANEPPS (0.1 mg/ml), applied in vivo to the rat barrel cortex. Blue dyes are often favored over red dyes in in vivo studies due to their apparent superior SNR, partly because their fluorescence spectrum is farther away from the hemoglobin absorption spectrum, making them less prone to heartbeat-associated brain-pulsation artifacts (BPA). We implemented a previously reported template-based BPA removal algorithm and evaluated its applicability to di-4-ANEPPS before comparing characteristics of the two dyes. Somatosensory-evoked potentials (SEPs) were also recorded. Whereas SEPs recorded before and after application of di-4-ANEPPS failed to exhibit demonstrable differences, RH-1691 caused a significant and prolonged increase in SEP amplitude for several hours. In contrast, neither dye influenced the spontaneous cortical activity as assessed by the spectral content of the EEG. Both dyes turned out to be strikingly similar with respect to changes in fractional fluorescence as a function of SEP response amplitude, as well as regarding shot noise characteristics after removal of the BPA. Thus there is strong evidence that the increased SNR for RH-1691 is a consequence of an artificially increased signal. When applying an appropriate BPA removal algorithm, di-4-ANEPPS has proven to be suitable for single-trial in vivo VSD imaging (VSDI) and produces no detectable neurophysiological changes in the system under investigation. Taken together, our data argue for a careful re-evaluation of pharmacological side effects of RH-1691 and support the applicability of di-4-ANEPPS for stable single-trial in vivo VSDI recordings.

Palette of fluorinated voltage-sensitive hemicyanine dyes

Optical recording of membrane potential permits spatially resolved measurement of electrical activity in subcellular regions of single cells, which would be inaccessible to electrodes, and imaging of spatiotemporal patterns of action potential propagation in excitable tissues, such as the brain or heart. However, the available voltage-sensitive dyes (VSDs) are not always spectrally compatible with newly available optical technologies for sensing or manipulating the physiological state of a system. Here, we describe a series of 19 fluorinated VSDs based on the hemicyanine class of chromophores. Strategic placement of the fluorine atoms on the chromophores can result in either blue or red shifts in the absorbance and emission spectra. The range of one-photon excitation wavelengths afforded by these new VSDs spans 440-670 nm; the two-photon excitation range is 900-1,340 nm. The emission of each VSD is shifted by at least 100 nm to the red of its one-photon excitation spectrum. The set of VSDs, thus, affords an extended toolkit for optical recording to match a broad range of experimental requirements. We show the sensitivity to voltage and the photostability of the new VSDs in a series of experimental preparations ranging in scale from single dendritic spines to whole heart. Among the advances shown in these applications are simultaneous recording of voltage and calcium in single dendritic spines and optical electrophysiology recordings using two-photon excitation above 1,100 nm.

Reversible cardiac conduction block and defibrillation with high-frequency electric field

Electrical impulse propagation is an essential function in cardiac, skeletal muscle, and nervous tissue. Abnormalities in cardiac impulse propagation underlie lethal reentrant arrhythmias, including ventricular fibrillation. Temporary propagation block throughout the ventricular myocardium could possibly terminate these arrhythmias. Electrical stimulation has been applied to nervous tissue to cause reversible conduction block, but has not been explored sufficiently in cardiac tissue. We show that reversible propagation block can be achieved in cardiac tissue by holding myocardial cells in a refractory state for a designated period of time by applying a sustained sinusoidal high-frequency alternating current (HFAC); in doing so, reentrant arrhythmias are terminated. We demonstrate proof of concept using several models, including optically mapped monolayers of neonatal rat ventricular cardiomyocytes, Langendorff-perfused guinea pig and rabbit hearts, intact anesthetized adult rabbits, and computer simulations of whole-heart impulse propagation. HFAC may be an effective and potentially safer alternative to direct current application, currently used to treat ventricular fibrillation.

Measurements and implications of the membrane dipole potential

There are three kinds of membrane potentials: the surface potentials, resulting from the accumulation of charges at the membrane surfaces; the transmembrane potential, determined by imbalance of charge in the aqueous solutions; and the dipole potential, a membrane-internal potential from the dipolar components of the phospholipids and interface water. The absolute value of the dipole potential has been very difficult to measure, although its value has been estimated to be in the range of 200-1,000 mV from ion translocation rates (determined by the planar lipid bilayer method), the surface potential of lipid monolayers (determined by the lipid monolayer method), molecular-dynamics calculations, and electron scattering using cryoelectron microscopy (cryo-EM). Spectroscopy methods have also been used to monitor the dipole potential changes on the basis of the observed fluorescence changes of voltage-sensitive probes. The dipole potential accounts for the much larger permeability of a bare phospholipid membrane to anions than cations and affects the conformation and function of membrane proteins.

Deciphering Arrhythmia Mechanisms - Tools of the Trade

Pathophysiological remodeling of cardiac function occurs at multiple levels, spanning the spectrum from molecular and sub-cellular changes to those occurring at the organ-system levels. Of key importance to arrhythmias are changes in electrophysiological and calcium handling properties at the tissue level. In this review, we discuss how high-resolution optical action potential and calcium transient imaging has advanced our understanding of basic arrhythmia mechanisms associated with multiple cardiovascular disorders, including the long QT syndrome, heart failure, and ischemia-reperfusion injury. We focus on the role of repolarization gradients (section 1) and calcium mediated triggers (section 2) in the initiation and maintenance of complex arrhythmias in these settings.

High-precision recording of the action potential in isolated cardiomyocytes using the near-infrared fluorescent dye di-4-ANBDQBS

The use of voltage-sensitive fluorescent dyes (VSD) for noninvasive measurement of the action potential (AP) in isolated cells has been hindered by low-photon yield of the preparation, dye toxicity, and photodynamic damage. Here we used a new red-shifted VSD, di-4-ANBDQBS, and a fast electron-multiplied charge-coupled device camera for optical AP (OAP) recording in guinea pig cardiac myocytes. Loading di-4-ANBDQBS did not alter APs recorded with micropipette. With short laser exposures (just enough to record one OAP every 1-5 min), di-4-ANBDQBS yielded fluorescent signals with very high signal-to-background ratios (change in fluorescence on depolarization/fluorescence at resting potential: 19.2 ± 4.1%) and signal-to-noise ratios (40 ± 13.2). Quantum chemical calculations comparing the ANBDQ chromophore to the conventional ANEP chromophore showed that the higher wavelength and the greater voltage sensitivity of the former have the same electro-optical origin: a longer path for electron redistribution in the excited state. OAP closely tracked simultaneously recorded electrical APs, permitting measurement of AP duration within 1% error. Prolonged laser exposure caused progressive AP duration prolongation and instability. However, these effects were alleviated or abolished by reducing the dye concentration and by perfusion with antioxidants. Thus the presented technique provides a unique opportunity for noninvasive AP recording in single cardiomyocytes.

Recordings from human myenteric neurons using voltage-sensitive dyes

Voltage-sensitive dye (VSD) imaging became a powerful tool to detect neural activity in the enteric nervous system, including its routine use in submucous neurons in freshly dissected human tissue. However, VSD imaging of human myenteric neurons remained a challenge because of limited visibility of the ganglia and dye accessibility. We describe a protocol to apply VSD for recordings of human myenteric neurons in freshly dissected tissue and myenteric neurons in primary cultures. VSD imaging of guinea-pig myenteric neurons was used for reference. Electrical stimulation of interganglionic fiber tracts and exogenous application of nicotine or elevated KCl solution was used to evoke action potentials. Bath application of the VSDs Annine-6Plus, Di-4-ANEPPS, Di-8-ANEPPQ, Di-4-ANEPPDHQ or Di-8-ANEPPS revealed no neural signals in human tissue although most of these VSD worked in guinea-pig tissue. Unlike methylene blue and FM1-43, 4-Di-2-ASP did not influence spike discharge and was used in human tissue to visualize myenteric ganglia as a prerequisite for targeted intraganglionic VSD application. Of all VSDs, only intraganglionic injection of Di-8-ANEPPS by a volume controlled injector revealed neuronal signals in human tissue. Signal-to-noise ratio increased by addition of dipicrylamine to Di-8-ANEPPS (0.98±0.16 vs. 2.4±0.62). Establishing VSD imaging in primary cultures of human myenteric neurons led to a further improvement of signal-to-noise ratio. This allowed us to routinely record spike discharge after nicotine application. The described protocol enabled reliable VSD recordings from human myenteric neurons but may also be relevant for the use of other fluorescent dyes in human tissues.

Wnt11 patterns a myocardial electrical gradient through regulation of the L-type Ca(2+) channel

Electrical gradients are critical for many biological processes, including the normal function of excitable tissues, left-right patterning, organogenesis and wound healing. The fundamental mechanisms that regulate the establishment and maintenance of such electrical polarities are poorly understood. Here we identify a gradient of electrical coupling across the developing ventricular myocardium using high-speed optical mapping of transmembrane potentials and calcium concentrations in the zebrafish heart. We excluded a role for differences in cellular excitability, connexin localization, tissue geometry and mechanical inputs, but in contrast we were able to demonstrate that non-canonical Wnt11 signals are required for the genesis of this myocardial electrical gradient. Although the traditional planar cell polarity pathway is not involved, we obtained evidence that Wnt11 acts to set up this gradient of electrical coupling through effects on transmembrane Ca(2+) conductance mediated by the L-type calcium channel. These data reveal a previously unrecognized role for Wnt/Ca(2+) signalling in establishing an electrical gradient in the plane of the developing cardiac epithelium through modulation of ion-channel function. The regulation of cellular coupling through such mechanisms may be a general property of non-canonical Wnt signals.

Wide-field and two-photon imaging of brain activity with voltage- and calcium-sensitive dyes

This review presents three examples of using voltage- or calcium-sensitive dyes to image the activity of the brain. Our aim is to discuss the advantages and disadvantages of each method with particular reference to its application to the study of the brainstem. Two of the examples use wide-field (one-photon) imaging; the third uses two-photon scanning microscopy. Because the measurements have limited signal-to-noise ratio, the paper also discusses the methodological aspects that are critical for optimizing the signal. The three examples are the following. (i) An intracellularly injected voltage-sensitive dye was used to monitor membrane potential in the dendrites of neurons in in vitro preparations. These experiments were directed at understanding how individual neurons convert complex synaptic inputs into the output spike train. (ii) An extracellular, bath application of a voltage-sensitive dye was used to monitor population signals from different parts of the dorsal brainstem. We describe recordings made during respiratory activity. The population signals indicated four different regions with distinct activity correlated with inspiration. (iii) Calcium-sensitive dyes can be used to label many individual cells in the mammalian brain. This approach, combined with two-photon microscopy, made it possible to follow the spike activity in an in vitro brainstem preparation during fictive respiratory rhythms. The organic voltage- and ion-sensitive dyes used today indiscriminatively stain all of the cell types in the preparation. A major effort is underway to develop fluorescent protein sensors of activity for selectively staining individual cell types.