Submillisecond optical reporting of membrane potential in situ using a neuronal tracer dye

Inter and intralaminar excitation of parvalbumin interneurons in mouse barrel cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition and control rhythmic activity. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes in many L2/3 and L4 PV interneurons simultaneously, with stimulation applied to either L2/3 or L4. A semi-automated procedure was developed to identify small regions of interest corresponding to single responsive PV interneurons. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus, PV interneurons in different cortical layers of BC respond in a layer specific and input specific manner, and these differences have potential roles in cortical computations.

Optical recordings of organellar membrane potentials and the components of membrane conductance in lysosomes

The eukaryotic cell is highly compartmentalized with organelles. Owing to their function in transporting metabolites, metabolic intermediates and byproducts of metabolic activity, organelles are important players in the orchestration of cellular function. Recent advances in optical methods for interrogating the different aspects of organellar activity promise to revolutionize our ability to dissect cellular processes with unprecedented detail. The transport activity of organelles is usually coupled to the transport of charged species; therefore, it is not only associated with the metabolic landscape but also entangled with membrane potentials. In this context, the targeted expression of fluorescent probes for interrogating organellar membrane potential (Ψorg) emerges as a powerful approach, offering less-invasive conditions and technical simplicity to interrogate cellular signalling and metabolism. Different research groups have made remarkable progress in adapting a variety of optical methods for measuring and monitoring Ψorg. These approaches include using potentiometric dyes, genetically encoded voltage indicators, hybrid fluorescence resonance energy transfer sensors and photoinduced electron transfer systems. These studies have provided consistent values for the resting potential of single-membrane organelles, such as lysosomes, the Golgi and the endoplasmic reticulum. We can foresee the use of dynamic measurements of Ψorg to study fundamental problems in organellar physiology that are linked to serious cellular disorders. Here, we present an overview of the available techniques, a survey of the resting membrane potential of internal membranes and, finally, an open-source mathematical model useful to interpret and interrogate membrane-bound structures of small volume by using the lysosome as an example.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Velocity of conduction between columns and layers in barrel cortex reported by parvalbumin interneurons

Inhibitory interneurons expressing parvalbumin (PV) play critical roles throughout the brain. Their rapid spiking enables them to control circuit dynamics on a millisecond time scale, and the timing of their activation by different excitatory pathways is critical to these functions. We used a genetically encoded hybrid voltage sensor to image PV interneuron voltage changes with sub-millisecond precision in primary somatosensory barrel cortex (BC) of adult mice. Electrical stimulation evoked depolarizations with a latency that increased with distance from the stimulating electrode, allowing us to determine conduction velocity. Spread of responses between cortical layers yielded an interlaminar conduction velocity and spread within layers yielded intralaminar conduction velocities in different layers. Velocities ranged from 74 to 473 μm/ms depending on trajectory; interlaminar conduction was 71% faster than intralaminar conduction. Thus, computations within columns are more rapid than between columns. The BC integrates thalamic and intracortical input for functions such as texture discrimination and sensory tuning. Timing differences between intra- and interlaminar PV interneuron activation could impact these functions. Imaging of voltage in PV interneurons reveals differences in signaling dynamics within cortical circuitry. This approach offers a unique opportunity to investigate conduction in populations of axons based on their targeting specificity.

Inter and Intralaminar Excitation of Parvalbumin Interneurons in Mouse Barrel Cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition, control rhythmic activity, and have been linked to disorders including autism spectrum and schizophrenia. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes simultaneously in many L2/3 and L4 PV interneurons to stimulation in either L2/3 or L4. Decay-times were consistent across L2/3 and L4. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus PV interneurons in different cortical layers of BC show differences in response properties with potential roles in cortical computations.

Imaging Voltage Globally and in Isofrequency Lamina in Slices of Mouse Ventral Cochlear Nucleus

The cochlear nuclei (CNs) receive sensory information from the ear and perform fundamental computations before relaying this information to higher processing centers. These computations are performed by distinct types of neurons interconnected in circuits dedicated to the specialized roles of the auditory system. In the present study, we explored the use of voltage imaging to investigate CN circuitry. We tested two approaches based on fundamentally different voltage sensing technologies. Using a voltage-sensitive dye we recorded glutamate receptor-independent signals arising predominantly from axons. The mean conduction velocity of these fibers of 0.27 m/s was rapid but in range with other unmyelinated axons. We then used a genetically-encoded hybrid voltage sensor (hVOS) to image voltage from a specific population of neurons. Probe expression was controlled using Cre recombinase linked to c-fos activation. This activity-induced gene enabled targeting of neurons that are activated when a mouse hears a pure 15-kHz tone. In CN slices from these animals auditory nerve fiber stimulation elicited a glutamate receptor-dependent depolarization in hVOS probe-labeled neurons. These cells resided within a band corresponding to an isofrequency lamina, and responded with a high degree of synchrony. In contrast to the axonal origin of voltage-sensitive dye signals, hVOS signals represent predominantly postsynaptic responses. The introduction of voltage imaging to the CN creates the opportunity to investigate auditory processing circuitry in populations of neurons targeted on the basis of their genetic identity and their roles in sensory processing.

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.

Optical probing of local membrane potential with fluorescent polystyrene beads

The study of electrical activity in single cells and local circuits of excitable cells, such as neurons, requires an easy-to-use, high-throughput methodology that allows for the measurement of membrane potential. Investigating the electrical properties in specific subcompartments of neurons, or in a specific type of neurons, introduces additional complexity. An optical voltage-imaging technique that allows high spatial and temporal resolution could be an ideal solution. However, most valid voltage-imaging techniques are nonspecific. Those that are more site-directed require a lot of preliminary work and specific adaptations, among other drawbacks. Here, we explore a new method for membrane voltage imaging, based on Förster resonance energy transfer between fluorescent polystyrene (FPS) beads and dipicrylamine. Not only has it been shown that fluorescence intensity correlates with membrane potential, but more importantly, the membrane potential from individual particles can be detected. Among other advantages, FPS beads can be synthesized with surface functional groups and can be targeted to specific proteins by conjugation of recognition molecules. Therefore, in the presence of dipicrylamine, FPS beads represent single-particle detectors of membrane potential that can be localized to specific membrane compartments. This new and easily accessible platform for targeted optical voltage imaging can further elucidate the mechanisms of neuronal electrical activity.

Bringing together the best of chemistry and biology: hybrid indicators for imaging neuronal membrane potential

Membrane potential is an indispensable biophysical signal in neurobiology. Imaging neuronal electrical signals with fluorescent indicators allows for non-invasive recording at high spatial resolution. Over the past decades, both genetically encoded voltage indicators (GEVIs) and organic voltage sensing dyes (OVSDs) have been developed to achieve imaging membrane potential dynamics in cultured neurons and in vivo. More recently, hybrid voltage indicators have gained increasing attention due to their superior fluorescent quantum yield and photostability as compared to conventional GEVIs. In this mini-review, we summarize the design, characterization and biological applications of hybrid voltage indicators, and discuss future improvements.

Point-localized, site-specific membrane potential optical recording by single fluorescent nanodiscs

Nanodisc technology was implemented as a platform for voltage nanosensors. A fluorescence (Förster) resonance energy transfer (FRET)- based voltage-sensing scheme employing fluorescent nanodiscs and the hydrophobic ion dipicrylamine was developed and utilized to optically record membrane potentials on the single-nanodisc level. Ensemble and single-nanosensor recordings were demonstrated for HEK293 cells and primary cortical neuron cells. Conjugation of nanodiscs to anti-GABA_A antibodies allowed for site-specific membrane potential measurements from postsynaptic sites.

Strobe photography mapping of cell membrane potential with nanosecond resolution

The ability to directly observe membrane potential charging dynamics across a full microscopic field of view is vital for understanding interactions between a biological system and a given electrical stimulus. Accurate empirical knowledge of cell membrane electrodynamics will enable validation of fundamental hypotheses posited by the single shell model, which includes the degree of voltage change across a membrane and cellular sensitivity to external electric field non-uniformity and directionality. To this end, we have developed a high-speed strobe microscopy system with a time resolution of ~ 6 ns that allows us to acquire time-sequential data for temporally repeatable events (non-injurious electrostimulation). The imagery from this system allows for direct comparison of membrane voltage change to both computationally simulated external electric fields and time-dependent membrane charging models. Acquisition of a full microscope field of view enables the selection of data from multiple cell locations experiencing different electrical fields in a single image sequence for analysis. Using this system, more realistic membrane parameters can be estimated from living cells to better inform predictive models. As a proof of concept, we present evidence that within the range of membrane conductivity used in simulation literature, higher values are likely more valid.

A dark quencher genetically encodable voltage indicator (dqGEVI) exhibits high fidelity and speed

Voltage sensing with genetically expressed optical probes is highly desirable for large-scale recordings of neuronal activity and detection of localized voltage signals in single neurons. Here we describe a method for a two-component (hybrid) genetically encodable fluorescent voltage sensing in neurons. The approach uses a glycosylphosphatidylinositol-tagged fluorescent protein (enhanced green fluorescent protein) that ensures the fluorescence to be specifically confined to the outside of the plasma membrane and D3, a voltage-dependent quencher. Previous hybrid genetically encoded voltage sensing approaches relied on a single quenching molecule, dipycrilamine (DPA), which is toxic, increases membrane capacitance, interferes with neurotransmitters, and is explosive. Our method uses a nontoxic and nonexplosive compound that performs better than DPA in all aspects of fluorescent voltage sensing.

Voltage sensing with genetically expressed optical probes is highly desirable for large-scale recordings of neuronal activity and detection of localized voltage signals in single neurons. Most genetically encodable voltage indicators (GEVI) have drawbacks including slow response, low fluorescence, or excessive bleaching. Here we present a dark quencher GEVI approach (dqGEVI) using a Förster resonance energy transfer pair between a fluorophore glycosylphosphatidylinositol–enhanced green fluorescent protein (GPI-eGFP) on the outer surface of the neuronal membrane and an azo-benzene dye quencher (D3) that rapidly moves in the membrane driven by voltage. In contrast to previous probes, the sensor has a single photon bleaching time constant of ∼40 min, has a high temporal resolution and fidelity for detecting action potential firing at 100 Hz, resolves membrane de- and hyperpolarizations of a few millivolts, and has negligible effects on passive membrane properties or synaptic events. The dqGEVI approach should be a valuable tool for optical recordings of subcellular or population membrane potential changes in nerve cells.

Insights Into Spinal Dorsal Horn Circuit Function and Dysfunction Using Optical Approaches

Somatosensation encompasses a variety of essential modalities including touch, pressure, proprioception, temperature, pain, and itch. These peripheral sensations are crucial for all types of behaviors, ranging from social interaction to danger avoidance. Somatosensory information is transmitted from primary afferent fibers in the periphery into the central nervous system via the dorsal horn of the spinal cord. The dorsal horn functions as an intermediary processing center for this information, comprising a complex network of excitatory and inhibitory interneurons as well as projection neurons that transmit the processed somatosensory information from the spinal cord to the brain. It is now known that there can be dysfunction within this spinal cord circuitry in pathological pain conditions and that these perturbations contribute to the development and maintenance of pathological pain. However, the complex and heterogeneous network of the spinal dorsal horn has hampered efforts to further elucidate its role in somatosensory processing. Emerging optical techniques promise to illuminate the underlying organization and function of the dorsal horn and provide insights into the role of spinal cord sensory processing in shaping the behavioral response to somatosensory input that we ultimately observe. This review article will focus on recent advances in optogenetics and fluorescence imaging techniques in the spinal cord, encompassing findings from both in vivo and in vitro preparations. We will also discuss the current limitations and difficulties of employing these techniques to interrogate the spinal cord and current practices and approaches to overcome these challenges.

Near-Infrared Voltage Nanosensors Enable Real-Time Imaging of Neuronal Activities in Mice and Zebrafish

Optical voltage sensors with the ability to monitor neuronal activities are invaluable tools for studying information processing of the brain. However, the current genetically encoded voltage indicators usually require high-power visible light for excitation and are limited to genetically addressable model animals. Here, we report a near-infrared (NIR)-excited nongenetic voltage nanosensor that achieves stable recording of neuronal membrane potential in intact animals. The nanosensor is composed of a Förster resonance energy transfer (FRET) pair, the outer membrane-anchored upconversion nanoparticle (UCNP), and the membrane-embedded dipicrylamine (DPA). The negative charge of DPA allows membrane potential fluctuation to affect the distance between the DPA and UCNP, therefore changing the FRET efficiency. Consequently, the emission intensity of the nanosensor can report the membrane potential. Using the nanosensor, we monitor not only electrically evoked changes in the membrane potential of cultured cells but also sensory responses of neurons in intact zebrafish and brain state-modulated subthreshold activities of cortical neurons in intact mice.

Optical Studies of Action Potential Dynamics with hVOS probes

The detection of action potentials and the characterization of their waveform represent basic benchmarks for evaluating optical sensors of voltage. The effectiveness of a voltage sensor in reporting action potentials will determine its usefulness in voltage imaging experiments designed for the study of neural circuitry. The hybrid voltage sensor (hVOS) technique is based on a sensing mechanism with a rapid response to voltage changes. hVOS imaging is thus well suited for optical studies of action potentials. This technique detects action potentials in intact brain slices with an excellent signal-to-noise ratio. These optical action potentials recapitulate voltage recordings with high temporal fidelity. In different genetically-defined types of neurons targeted by cre-lox technology, hVOS recordings of action potentials recapitulate the expected differences in duration. Furthermore, by targeting an hVOS probe to axons, imaging experiments can follow action potential propagation and document dynamic changes in waveform resulting from use-dependent plasticity.

Spying on Neuronal Membrane Potential with Genetically Targetable Voltage Indicators

Methods for optical measurement of voltage dynamics in living cells are attractive because they provide spatial resolution surpassing traditional electrode-based measurements and temporal resolution exceeding that of widely used Ca²⁺ imaging. Chemically synthesized voltage-sensitive dyes that use photoinduced electron transfer as a voltage-sensing trigger offer high voltage sensitivity and fast-response kinetics, but targeting chemical indicators to specific cells remains an outstanding challenge. Here, we present a new family of readily functionalizable, fluorescein-based voltage-sensitive fluorescent dyes (sarcosine-VoltageFluors) that can be covalently attached to a genetically encoded cell surface receptor to achieve voltage imaging from genetically defined neurons. We synthesized four new VoltageFluor derivatives that possess carboxylic acid functionality for simple conjugation to flexible tethers. The best of this new group of dyes was conjugated via a polyethylene glycol (PEG) linker to a small peptide (SpyTag, 13 amino acids) that directs binding and formation of a covalent bond with its binding partner, SpyCatcher (15 kDa). The new VoltageSpy dyes effectively label cells expressing cell-surface SpyCatcher, display good voltage sensitivity, and maintain fast-response kinetics. In cultured neurons, VoltageSpy dyes enable robust, single-trial optical detection of action potentials at neuronal soma with sensitivity exceeding genetically encoded voltage indicators. Importantly, genetic targeting of chemically synthesized dyes enables VoltageSpy to report on action potentials in axons and dendrites in single trials, tens to hundreds of micrometers away from the cell body. Genetic targeting of synthetic voltage indicators with VoltageSpy enables voltage imaging with low nanomolar dye concentration and offers a promising method for allying the speed and sensitivity of synthetic indicators with the enhanced cellular resolution of genetically encoded probes.

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.

Visualization of Dynamic Sub-microsecond Changes in Membrane Potential

Direct observation of rapid membrane potential changes is critical to understand how complex neurological systems function. This knowledge is especially important when stimulation is achieved through an external stimulus meant to mimic a naturally occurring process. To enable exploration of this dynamic space, we developed an all-optical method for observing rapid changes in membrane potential at temporal resolutions of ∼25 ns. By applying a single 600-ns electric pulse, we observed sub-microsecond, continuous membrane charging and discharging dynamics. Close agreement between the acquired results and an analytical membrane-charging model validates the utility of this technique. This tool will deepen our understanding of the role of membrane potential dynamics in the regulation of many biological and chemical processes within living systems.

Relative positioning of Kv11.1 (hERG) K+ channel cytoplasmic domain-located fluorescent tags toward the plasma membrane

Recent cryo-EM data have provided a view of the KCNH potassium channels molecular structures. However, some details about the cytoplasmic domains organization and specially their rearrangements associated to channel functionality are still lacking. Here we used the voltage-dependent dipicrylamine (DPA)-induced quench of fluorescent proteins (FPS) linked to different positions at the cytoplasmic domains of KCNH2 (hERG) to gain some insights about the coarse structure of these channel parts. Fast voltage-clamp fluorometry with HEK293 cells expressing membrane-anchored FPs under conditions in which only the plasma membrane potential is modified, demonstrated DPA voltage-dependent translocation and subsequent FRET-triggered FP quenching. Our data demonstrate for the first time that the distance between an amino-terminal FP tag and the intracellular plasma membrane surface is shorter than that between the membrane and a C-terminally-located tag. The distances varied when the FPs were attached to other positions along the channel cytoplasmic domains. In some cases, we also detected slower fluorometric responses following the fast voltage-dependent dye translocation, indicating subsequent label movements orthogonal to the plasma membrane. This finding suggests the existence of additional conformational rearrangements in the hERG cytoplasmic domains, although their association with specific aspects of channel operation remains to be established.

TI Workbench, an integrated software package for electrophysiology and imaging

TI Workbench is a software package that serves as a control and analysis center for cellular imaging and electrophysiological experiments. It is unique among general-purpose software packages where it integrates the control of cellular imaging and electrophysiological devices, as well as sophisticated data analyses, which provides superior usability in imaging experiments combined with electrophysiology. During the development over the last 20 years, the range of supported image acquisition devices has expanded from cooled charge-coupled device (CCD) cameras to multi-photon microscope systems. In this review, I outline the concept of TI Workbench together with its unique functions and features derived from ideas emerging during daily experiments in my own lab and in those of my collaborators over the last 20 years. TI Workbench includes standard functions required for time-lapse multicolor fluorescence imaging and electrophysiological experiments, in addition to specialized functions such as random-scan or conventional raster-scan two-photon microscopy packages and fluorescence life time imaging (FLIM) utilities. Data analysis modules, e.g. digital data filters for temporal waveforms of time-lapse image data and electrophysiology and for 2-D image data, and fluorescence correlation spectroscopy (FCS) analysis functions, are well integrated with data acquisition functions. A notebook function holds formatted text, graphs, image and movie data altogether, which are linked to the actual data files. TI Workbench uses Igor Pro software as a back-end output for publishing. In addition, TI Workbench imports several different formats of image and electrophysiology data, serving as a general-purpose data analysis software package.

Beyond Fluorescent Proteins: Hybrid and Bioluminescent Indicators for Imaging Neural Activities

Optical biosensors have been invaluable tools in neuroscience research, as they provide the ability to directly visualize neural activity in real time, with high specificity, and with exceptional spatial and temporal resolution. Notably, a majority of these sensors are based on fluorescent protein scaffolds, which offer the ability to target specific cell types or even subcellular compartments. However, fluorescent proteins are intrinsically bulky tags, often insensitive to the environment, and always require excitation light illumination. To address these limitations, there has been a proliferation of alternative sensor scaffolds developed in recent years, including hybrid sensors that combine the advantages of synthetic fluorophores and genetically encoded protein tags, as well as bioluminescent probes. While still in their early stage of development as compared with fluorescent protein-based sensors, these novel probes have offered complementary solutions to interrogate various aspects of neuronal communication, including transmitter release, changes in membrane potential, and the production of second messengers. In this Review, we discuss these important new developments with a particular focus on design strategies.

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.

Imaging Voltage in Genetically Defined Neuronal Subpopulations with a Cre Recombinase-Targeted Hybrid Voltage Sensor

Genetically encoded voltage indicators create an opportunity to monitor electrical activity in defined sets of neurons as they participate in the complex patterns of coordinated electrical activity that underlie nervous system function. Taking full advantage of genetically encoded voltage indicators requires a generalized strategy for targeting the probe to genetically defined populations of cells. To this end, we have generated a mouse line with an optimized hybrid voltage sensor (hVOS) probe within a locus designed for efficient Cre recombinase-dependent expression. Crossing this mouse with Cre drivers generated double transgenics expressing hVOS probe in GABAergic, parvalbumin, and calretinin interneurons, as well as hilar mossy cells, new adult-born neurons, and recently active neurons. In each case, imaging in brain slices from male or female animals revealed electrically evoked optical signals from multiple individual neurons in single trials. These imaging experiments revealed action potentials, dynamic aspects of dendritic integration, and trial-to-trial fluctuations in response latency. The rapid time response of hVOS imaging revealed action potentials with high temporal fidelity, and enabled accurate measurements of spike half-widths characteristic of each cell type. Simultaneous recording of rapid voltage changes in multiple neurons with a common genetic signature offers a powerful approach to the study of neural circuit function and the investigation of how neural networks encode, process, and store information.SIGNIFICANCE STATEMENT Genetically encoded voltage indicators hold great promise in the study of neural circuitry, but realizing their full potential depends on targeting the sensor to distinct cell types. Here we present a new mouse line that expresses a hybrid optical voltage sensor under the control of Cre recombinase. Crossing this line with Cre drivers generated double-transgenic mice, which express this sensor in targeted cell types. In brain slices from these animals, single-trial hybrid optical voltage sensor recordings revealed voltage changes with submillisecond resolution in multiple neurons simultaneously. This imaging tool will allow for the study of the emergent properties of neural circuits and permit experimental tests of the roles of specific types of neurons in complex circuit activity.

Label-free optical detection of action potential in mammalian neurons

We describe an optical technique for label-free detection of the action potential in cultured mammalian neurons. Induced morphological changes due to action potential propagation in neurons are optically interrogated with a phase sensitive interferometric technique. Optical recordings composed of signal pulses mirror the electrical spike train activity of individual neurons in a network. The optical pulses are transient nanoscale oscillatory changes in the optical path length of varying peak magnitude and temporal width. Exogenous application of glutamate to cortical neuronal cultures produced coincident increase in the electrical and optical activity; both were blocked by application of a Na-channel blocker, Tetrodotoxin. The observed transient change in optical path length in a single optical pulse is primarily due to physical fluctuations of the neuronal cell membrane mediated by a yet unknown electromechanical transduction phenomenon. Our analysis suggests a traveling surface wave in the neuronal cell membrane is responsible for the measured optical signal pulses.

Differential Dendritic Integration of Synaptic Potentials and Calcium in Cerebellar Interneurons

Dendritic voltage integration determines the transformation of synaptic inputs into output firing, while synaptic calcium integration drives plasticity mechanisms thought to underlie memory storage. Dendritic calcium integration has been shown to follow the same synaptic input-output relationship as dendritic voltage, but whether similar operations apply to neurons exhibiting sublinear voltage integration is unknown. We examined the properties and cellular mechanisms of these dendritic operations in cerebellar molecular layer interneurons using dendritic voltage and calcium imaging, in combination with synaptic stimulation or glutamate uncaging. We show that, while synaptic potentials summate sublinearly, concomitant dendritic calcium signals summate either linearly or supralinearly depending on the number of synapses activated. The supralinear dendritic calcium triggers a branch-specific, short-term suppression of neurotransmitter release that alters the pattern of synaptic activation. Thus, differential voltage and calcium integration permits dynamic regulation of neuronal input-output transformations without altering intrinsic nonlinear integration mechanisms.

Photoacoustic imaging of voltage responses beyond the optical diffusion limit

Non-invasive optical imaging of neuronal voltage response signals in live brains is constrained in depth by the optical diffusion limit, which is due primarily to optical scattering by brain tissues. Although photoacoustic tomography breaks this limit by exciting the targets with diffused photons and detecting the resulting acoustic responses, it has not been demonstrated as a modality for imaging voltage responses. In this communication, we report the first demonstration of photoacoustic voltage response imaging in both in vitro HEK-293 cell cultures and in vivo mouse brain surfaces. Using spectroscopic photoacoustic tomography at isosbestic wavelengths, we can separate voltage response signals and hemodynamic signals on live brain surfaces. By imaging HEK-293 cell clusters through 4.5 mm thick ex vivo rat brain tissue, we demonstrate photoacoustic tomography of cell membrane voltage responses beyond the optical diffusion limit. Although the current voltage dye does not immediately allow in vivo deep brain voltage response imaging, we believe our method opens up a feasible technical path for deep brain studies in the future.

Whole-GUV patch-clamping

Studying how the membrane modulates ion channel and transporter activity is challenging because cells actively regulate membrane properties, whereas existing in vitro systems have limitations, such as residual solvent and unphysiologically high membrane tension. Cell-sized giant unilamellar vesicles (GUVs) would be ideal for in vitro electrophysiology, but efforts to measure the membrane current of intact GUVs have been unsuccessful. In this work, two challenges for obtaining the "whole-GUV" patch-clamp configuration were identified and resolved. First, unless the patch pipette and GUV pressures are precisely matched in the GUV-attached configuration, breaking the patch membrane also ruptures the GUV. Second, GUVs shrink irreversibly because the membrane/glass adhesion creating the high-resistance seal (>1 GΩ) continuously pulls membrane into the pipette. In contrast, for cell-derived giant plasma membrane vesicles (GPMVs), breaking the patch membrane allows the GPMV contents to passivate the pipette surface, thereby dynamically blocking membrane spreading in the whole-GMPV mode. To mimic this dynamic passivation mechanism, beta-casein was encapsulated into GUVs, yielding a stable, high-resistance, whole-GUV configuration for a range of membrane compositions. Specific membrane capacitance measurements confirmed that the membranes were truly solvent-free and that membrane tension could be controlled over a physiological range. Finally, the potential for ion transport studies was tested using the model ion channel, gramicidin, and voltage-clamp fluorometry measurements were performed with a voltage-dependent fluorophore/quencher pair. Whole-GUV patch-clamping allows ion transport and other voltage-dependent processes to be studied while controlling membrane composition, tension, and shape.

Using Biophysics to Monitor the Essential Protonmotive Force in Bacteria

Protonmotive force is an essential biological energy format in all levels of cells. Protonmotive force comprises electrical and chemical potential difference across biological membrane. In bacteria, protonmotive force couples to metabolism and ATP production. Moreover, protonmotive force directly provides driving energy of bacterial flagellar motor that is critical for bacterial motility and infection. Due to the small size of bacterial cells, there were limited experimental tools to measure protonmotive force in bacteria. Recent developments of optical membrane potential and intracellular pH indicators provide valuable information on bacterial studies. These new biophysical techniques allow us to monitor the protonmotive force even in single bacterial cell level that shed the light of next generation single-cell physiological experiments towards the understanding of bacterial infection process.

Small molecule fluorescent voltage indicators for studying membrane potential

Voltage imaging has the potential to unravel the contributions that rapid changes in membrane voltage make to cellular physiology, especially in the context of neuroscience. In particular, small molecule fluorophores are especially attractive because they can, in theory, provide fast and sensitive measurements of membrane potential dynamics. A number of classes of small molecule voltage indicators will be discussed, including dyes with improved two-photon voltage sensing, near infrared optical profiles for use in in vivo applications, and newly developed electron-transfer based indicators, or VoltageFluors, that can be tuned across a range of wavelengths to enable all-optical voltage manipulation and measurement. Limitations and a 'wish-list' for voltage indicators will also be discussed.

High throughput physiological screening of iPSC-derived cardiomyocytes for drug development

Cardiac drug discovery is hampered by the reliance on non-human animal and cellular models with inadequate throughput and physiological fidelity to accurately identify new targets and test novel therapeutic strategies. Similarly, adverse drug effects on the heart are challenging to model, contributing to costly failure of drugs during development and even after market launch. Human induced pluripotent stem cell derived cardiac tissue represents a potentially powerful means to model aspects of heart physiology relevant to disease and adverse drug effects, providing both the human context and throughput needed to improve the efficiency of drug development. Here we review emerging technologies for high throughput measurements of cardiomyocyte physiology, and comment on the promises and challenges of using iPSC-derived cardiomyocytes to model disease and introduce the human context into early stages of drug discovery. This article is part of a Special Issue entitled: Cardiomyocyte biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Dipicrylamine Modulates GABAρ1 Receptors through Interactions with Residues in the TM4 and Cys-Loop Domains

Dipicrylamine (DPA) is a commonly used acceptor agent in Förster resonance energy transfer experiments that allows the study of high-frequency neuronal activity in the optical monitoring of voltage in living cells. However, DPA potently antagonizes GABAA receptors that contain α1 and β2 subunits by a mechanism which is not clearly understood. In this work, we aimed to determine whether DPA modulation is a general phenomenon of Cys-loop ligand-gated ion channels (LGICs), and whether this modulation depends on particular amino acid residues. For this, we studied the effects of DPA on human homomeric GABAρ1, α7 nicotinic, and 5-HT3A serotonin receptors expressed in Xenopus oocytes. Our results indicate that DPA is an allosteric modulator of GABAρ1 receptors with an IC50 of 1.6 µM, an enhancer of α7 nicotinic receptors at relatively high concentrations of DPA, and has little, if any, effect on 5-HT3A receptors. DPA antagonism of GABAρ1 was strongly enhanced by preincubation, was slightly voltage-dependent, and its washout was accelerated by bovine serum albumin. These results indicate that DPA modulation is not a general phenomenon of LGICs, and structural differences between receptors may account for disparities in DPA effects. In silico modeling of DPA docking to GABAρ1, α7 nicotinic, and 5-HT3A receptors suggests that a hydrophobic pocket within the Cys-loop and the M4 segment in GABAρ1, located at the extracellular/membrane interface, facilitates the interaction with DPA that leads to inhibition of the receptor. Functional examinations of mutant receptors support the involvement of the M4 segment in the allosteric modulation of GABAρ1 by DPA.

Optical detection of neuron connectivity by random access two-photon microscopy

BACKGROUND

Knowledge about the distribution, strength, and direction of synaptic connections within neuronal networks are crucial for understanding brain function. Electrophysiology using multiple electrodes provides a very high temporal resolution, but does not yield sufficient spatial information for resolving neuronal connection topology. Optical recording techniques using single-cell resolution have provided promise for providing spatial information. Although calcium imaging from hundreds of neurons has provided a novel view of the neural connections within the network, the kinetics of calcium responses are not fast enough to resolve each action potential event with high fidelity. Therefore, it is not possible to detect the direction of neuronal connections.

NEW METHOD

We took advantage of the fast kinetics and large dynamic range of the DiO/DPA combination of voltage sensitive dye and the fast scan speed of a custom-made random-access two-photon microscope to resolve each action potential event from multiple neurons in culture.

RESULTS

Long-duration recording up to 100min from cultured hippocampal neurons yielded sufficient numbers of spike events for analyzing synaptic connections. Cross-correlation analysis of neuron pairs clearly distinguished synaptically connected neuron pairs with the connection direction.

COMPARISON WITH EXISTING METHOD

The long duration recording of action potentials with voltage-sensitive dye utilized in the present study is much longer than in previous studies. Simultaneous optical voltage and calcium measurements revealed that voltage-sensitive dye is able to detect firing events more reliably than calcium indicators.

CONCLUSIONS

This novel method reveals a new view of the functional structure of neuronal networks.

Multifocal fluorescence microscope for fast optical recordings of neuronal action potentials

In recent years, optical sensors for tracking neural activity have been developed and offer great utility. However, developing microscopy techniques that have several kHz bandwidth necessary to reliably capture optically reported action potentials (APs) at multiple locations in parallel remains a significant challenge. To our knowledge, we describe a novel microscope optimized to measure spatially distributed optical signals with submillisecond and near diffraction-limit resolution. Our design uses a spatial light modulator to generate patterned illumination to simultaneously excite multiple user-defined targets. A galvanometer driven mirror in the emission path streaks the fluorescence emanating from each excitation point during the camera exposure, using unused camera pixels to capture time varying fluorescence at rates that are ∼1000 times faster than the camera's native frame rate. We demonstrate that this approach is capable of recording Ca(2+) transients resulting from APs in neurons labeled with the Ca(2+) sensor Oregon Green Bapta-1 (OGB-1), and can localize the timing of these events with millisecond resolution. Furthermore, optically reported APs can be detected with the voltage sensitive dye DiO-DPA in multiple locations within a neuron with a signal/noise ratio up to ∼40, resolving delays in arrival time along dendrites. Thus, the microscope provides a powerful tool for photometric measurements of dynamics requiring submillisecond sampling at multiple locations.

A Small-Molecule Photoactivatable Optical Sensor of Transmembrane Potential

This paper discloses the design, synthesis, and imaging applications of the first member of a new class of SPOTs, small-molecule photoactivatable optical sensors of transmembrane potential. SPOT2.1.Cl features an established voltage-sensitive dye, VoltageFluor2.1.Cl--or VF--capped with a dimethoxy-o-nitrobenzyl (DMNB) caging group to effectively diminish fluorescence of the VF dye prior to uncaging. SPOT2.1.Cl localizes to cell membranes and displays weak fluorescence until photoactivated. Illumination generates the parent VF dye which then optically reports on changes in the membrane voltage. After photoactivation with spatially restricted light, SPOT2.1.Cl-loaded cells display bright, voltage-sensitive fluorescence associated with the plasma membrane, while neighboring cells remain dark. Activated SPOT reports on action potentials in single trials. SPOT can be activated in neuron cell bodies or uncaged in dendrites to enable structural tracing via "backfilling" of the dye to the soma, followed by functional imaging in the labeled cell. The combination of cellular specificity achieved through spatially defined patterns of illumination, coupled with the fast, sensitive, and noncapacitive voltage sensing characteristics of VF dyes makes SPOT2.1.Cl a useful tool for interrogating both structure and function of neuronal systems.

A Photostable Silicon Rhodamine Platform for Optical Voltage Sensing

This paper describes the design and synthesis of a photostable, far-red to near-infrared (NIR) platform for optical voltage sensing. We developed a new, sulfonated silicon rhodamine fluorophore and integrated it with a phenylenevinylene molecular wire to create a Berkeley Red Sensor of Transmembrane potential, or BeRST 1 ("burst"). BeRST 1 is the first member of a class of far-red to NIR voltage sensitive dyes that make use of a photoinduced electron transfer (PeT) trigger for optical interrogation of membrane voltage. We show that BeRST 1 displays bright, membrane-localized fluorescence in living cells, high photostability, and excellent voltage sensitivity in neurons. Depolarization of the plasma membrane results in rapid fluorescence increases (24% ΔF/F per 100 mV). BeRST 1 can be used in conjunction with fluorescent stains for organelles, Ca(2+) indicators, and voltage-sensitive fluorescent proteins. In addition, the red-shifted spectral profile of BeRST 1, relative to commonly employed optogenetic actuators like ChannelRhodopsin2 (ChR2), which require blue light, enables optical electrophysiology in neurons. The high speed, sensitivity, photostability and long-wavelength fluorescence profiles of BeRST 1 make it a useful platform for the noninvasive, optical dissection of neuronal activity.

Improved PeT molecules for optically sensing voltage in neurons

VoltageFluor (VF) dyes have the potential to measure voltage optically in excitable membranes with a combination of high spatial and temporal resolution essential to better characterize the voltage dynamics of large groups of excitable cells. VF dyes sense voltage with high speed and sensitivity using photoinduced electron transfer (PeT) through a conjugated molecular wire. We show that tuning the driving force for PeT (ΔGPeT + w) through systematic chemical substitution modulates voltage sensitivity, estimate (ΔGPeT + w) values from experimentally measured redox potentials, and validate the voltage sensitivities in patch-clamped HEK cells for 10 new VF dyes. VF2.1(OMe).H, with a 48% ΔF/F per 100 mV, shows approximately 2-fold improvement over previous dyes in HEK cells, dissociated rat cortical neurons, and medicinal leech ganglia. Additionally, VF2.1(OMe).H faithfully reports pharmacological effects and circuit activity in mouse olfactory bulb slices, thus opening a wide range of previously inaccessible applications for voltage-sensitive dyes.

Design and Use of Organic Voltage Sensitive Dyes

The chemistry and the physics of voltage sensitive dyes (VSDs) should be understood and appreciated as a prerequisite for their optimal application to problems in neuroscience cardiology. This chapter provides a basic understanding of the properties of the large variety of available organic VSDs. The mechanisms by which the dyes respond to voltage guides the best set up of the optics for recording or imaging electrophysiological activity. The physical and chemical properties of the dyes can be tuned to optimize delivery to and staining of the cells in different experimental preparations. The aim of this chapter is to arm the experimentalists who use the dyes with enough information and data to be able to intelligently choose the best dye for their specific requirements.

Imaging Submillisecond Membrane Potential Changes from Individual Regions of Single Axons, Dendrites and Spines

A central question in neuronal network analysis is how the interaction between individual neurons produces behavior and behavioral modifications. This task depends critically on how exactly signals are integrated by individual nerve cells functioning as complex operational units. Regional electrical properties of branching neuronal processes which determine the input-output function of any neuron are extraordinarily complex, dynamic, and, in the general case, impossible to predict in the absence of detailed measurements. To obtain such a measurement one would, ideally, like to be able to monitor, at multiple sites, subthreshold events as they travel from the sites of origin (synaptic contacts on distal dendrites) and summate at particular locations to influence action potential initiation. It became possible recently to carry out this type of measurement using high-resolution multisite recording of membrane potential changes with intracellular voltage-sensitive dyes. This chapter reviews the development and foundation of the method of voltage-sensitive dye recording from individual neurons. Presently, this approach allows monitoring membrane potential transients from all parts of the dendritic tree as well as from axon collaterals and individual dendritic spines.

Single-trial imaging of spikes and synaptic potentials in single neurons in brain slices with genetically encoded hybrid voltage sensor

Genetically encoded voltage sensors expand the optogenetics toolkit into the important realm of electrical recording, enabling researchers to study the dynamic activity of complex neural circuits in real time. However, these probes have thus far performed poorly when tested in intact neural circuits. Hybrid voltage sensors (hVOS) enable the imaging of voltage by harnessing the resonant energy transfer that occurs between a genetically encoded component, a membrane-tethered fluorescent protein that serves as a donor, and a small charged molecule, dipicrylamine, which serves as an acceptor. hVOS generates optical signals as a result of voltage-induced changes in donor-acceptor distance. We expressed the hVOS probe in mouse brain by in utero electroporation and in transgenic mice with a neuronal promoter. Under conditions favoring sparse labeling we could visualize single-labeled neurons. hVOS imaging reported electrically evoked fluorescence changes from individual neurons in slices from entorhinal cortex, somatosensory cortex, and hippocampus. These fluorescence signals tracked action potentials in individual neurons in a single trial with excellent temporal fidelity, producing changes that exceeded background noise by as much as 16-fold. Subthreshold synaptic potentials were detected in single trials in multiple distinct cells simultaneously. We followed signal propagation between different cells within one field of view and between dendrites and somata of the same cell. hVOS imaging thus provides a tool for high-resolution recording of electrical activity from genetically targeted cells in intact neuronal circuits.

Control and plasticity of the presynaptic action potential waveform at small CNS nerve terminals

The steep dependence of exocytosis on Ca(2+) entry at nerve terminals implies that voltage control of both Ca(2+) channel opening and the driving force for Ca(2+) entry are powerful levers in sculpting synaptic efficacy. Using fast, genetically encoded voltage indicators in dissociated primary neurons, we show that at small nerve terminals K(+) channels constrain the peak voltage of the presynaptic action potential (APSYN) to values much lower than those at cell somas. This key APSYN property additionally shows adaptive plasticity: manipulations that increase presynaptic Ca(2+) channel abundance and release probability result in a commensurate lowering of the APSYN peak and narrowing of the waveform, while manipulations that decrease presynaptic Ca(2+) channel abundance do the opposite. This modulation is eliminated upon blockade of Kv3.1 and Kv1 channels. Our studies thus reveal that adaptive plasticity in the APSYN waveform serves as an important regulator of synaptic function.

Hybrid voltage sensor imaging of eGFP-F expressing neurons in chicken midbrain slices

BACKGROUND

Dendritic computation is essential for understanding information processing in single neurons and brain circuits. Optical methods are suited best to investigate function and biophysical properties of cellular compartments at high spatial and temporal resolution. Promising approaches include the use of voltage sensitive dyes, genetically encoded voltage sensors, or hybrid voltage sensors (hVoS) consisting of fluorescent proteins and voltage-dependent quenchers that, so far, are not available in avian neuroscience.

NEW METHOD

We have adapted a hVoS system for a chicken midbrain slice preparation by combining genetically expressed farnesylated eGFP with dipicrylamine (DPA). Depending on the cellular potential, DPA is shifted in the membrane, resulting in quenching of eGFP fluorescence linearly to the membrane potential by Förster resonance electron transfer.

RESULTS

In ovo electroporation resulted in labelled neurons throughout the midbrain with a high level of fine structural detail. After application of DPA, we were able to optically record electrically evoked action potentials with high signal-to-noise ratio and high spatio-temporal resolution.

COMPARISON WITH EXISTING METHODS

Standard methods available for avian neuroscience such as whole-cell patch clamp yield insufficient data for the analysis of dendritic computation in single neurons. The high spatial and temporal resolution of hVoS data overcomes this limitation. The results obtained by our method are comparable to hVoS data published for mammals.

CONCLUSIONS

With the protocol presented here, it is possible to optically record information processing in single avian neurons at such high spatial and temporal resolution, that cellular and subcellular events can be analysed.

Confocal spot detection of presynaptic Ca²⁺ domains

Advances in rapid imaging of fluorescence probes are crucial to the study of neuronal signaling and information processing within the nervous system. Fluorescence indicators of cellular signaling (e.g., Ca²⁺ or voltage) can report changes on the millisecond timescale and signals can be localized to submicrometer structures. There are significant methodological challenges, however, involved in making measurements at such high temporal and spatial resolution. I describe here the principle and implementation of a confocal spot detection method for rapid and localized detection of fluorescence changes, which we have used for the detection of action potential-induced presynaptic Ca²⁺ domains.

Confocal imaging of transmembrane voltage by SEER of di-8-ANEPPS

Imaging, optical mapping, and optical multisite recording of transmembrane potential (V_m) are essential for studying excitable cells and systems. The naphthylstyryl voltage-sensitive dyes, including di-8-ANEPPS, shift both their fluorescence excitation and emission spectra upon changes in V_m. Accordingly, they have been used for monitoring V_m in nonratioing and both emission and excitation ratioing modes. Their changes in fluorescence are usually much less than 10% per 100 mV. Conventional ratioing increases sensitivity to between 3 and 15% per 100 mV. Low sensitivity limits the value of these dyes, especially when imaged with low light systems like confocal scanners. Here we demonstrate the improvement afforded by shifted excitation and emission ratioing (SEER) as applied to imaging membrane potential in flexor digitorum brevis muscle fibers of adult mice. SEER—the ratioing of two images of fluorescence, obtained with different excitation wavelengths in different emission bands—was implemented in two commercial confocal systems. A conventional pinhole scanner, affording optimal setting of emission bands but less than ideal excitation wavelengths, achieved a sensitivity of up to 27% per 100 mV, nearly doubling the value found by conventional ratioing of the same data. A better pair of excitation lights should increase the sensitivity further, to 35% per 100 mV. The maximum acquisition rate with this system was 1 kHz. A fast “slit scanner” increased the effective rate to 8 kHz, but sensitivity was lower. In its high-sensitivity implementation, the technique demonstrated progressive deterioration of action potentials upon fatiguing tetani induced by stimulation patterns at >40 Hz, thereby identifying action potential decay as a contributor to fatigue onset. Using the fast implementation, we could image for the first time an action potential simultaneously at multiple locations along the t-tubule system. These images resolved the radially varying lag associated with propagation at a finite velocity.

Monitoring synaptic and neuronal activity in 3D with synthetic and genetic indicators using a compact acousto-optic lens two-photon microscope

•

We expand the utility of acousto-optic lens (AOL) 3D 2P microscopy.

•

We show rapid, simultaneous monitoring of synaptic inputs distributed in 3D.

•

First use of genetically encoded indicators with AOL 3D functional imaging.

•

Measurement of sensory-evoked neuronal population activity in 3D in vivo.

•

Strategies for improving the measurement of the timing of neuronal signals.

Background

Two-photon microscopy is widely used to study brain function, but conventional microscopes are too slow to capture the timing of neuronal signalling and imaging is restricted to one plane. Recent development of acousto-optic-deflector-based random access functional imaging has improved the temporal resolution, but the utility of these technologies for mapping 3D synaptic activity patterns and their performance at the excitation wavelengths required to image genetically encoded indicators have not been investigated.

New method

Here, we have used a compact acousto-optic lens (AOL) two-photon microscope to make high speed [Ca²⁺] measurements from spines and dendrites distributed in 3D with different excitation wavelengths (800–920 nm).

Results

We show simultaneous monitoring of activity from many synaptic inputs distributed over the 3D arborisation of a neuronal dendrite using both synthetic as well as genetically encoded indicators. We confirm the utility of AOL-based imaging for fast in vivo recordings by measuring, simultaneously, visually evoked responses in 100 neurons distributed over a 150 μm focal depth range. Moreover, we explore ways to improve the measurement of timing of neuronal activation by choosing specific regions within the cell soma.

Comparison with existing methods

These results establish that AOL-based 3D random access two-photon microscopy has a wider range of neuroscience applications than previously shown.

Conclusions

Our findings show that the compact AOL microscope design has the speed, spatial resolution, sensitivity and wavelength flexibility to measure 3D patterns of synaptic and neuronal activity on individual trials.

Measuring membrane voltage with fluorescent proteins

Measuring signal transduction in large numbers of cells with high spatial and temporal resolution is fundamental to studying information processing in the nervous system. DNA-encoded sensors have an advantage in that they can be introduced into an organism noninvasively and targeted to specific brain regions, cell types, or subcellular compartments. A variety of chimeric proteins that report transmembrane voltage have been developed. The prototype sensor, FlaSh, is a green fluorescent protein fused to a voltage-sensitive K(+) channel, where voltage-dependent rearrangements in the channel induce changes in the protein's fluorescence. Subsequent sensors have refined this basic design using a monomeric voltage-sensing phosphatase domain from Ciona intestinalis and pairs of fluorescent proteins to produce a larger fluorescent signal. These sensors and their uses are discussed here.

Encoded multisite two-photon microscopy

The advent of scanning two-photon microscopy (2PM) has created a fertile new avenue for noninvasive investigation of brain activity in depth. One principal weakness of this method, however, lies with the limit of scanning speed, which makes optical interrogation of action potential-like activity in a neuronal network problematic. Encoded multisite two-photon microscopy (eMS2PM), a scanless method that allows simultaneous imaging of multiple targets in depth with high temporal resolution, addresses this drawback. eMS2PM uses a liquid crystal spatial light modulator to split a high-power femto-laser beam into multiple subbeams. To distinguish them, a digital micromirror device encodes each subbeam with a specific binary amplitude modulation sequence. Fluorescence signals from all independently targeted sites are then collected simultaneously onto a single photodetector and site-specifically decoded. We demonstrate that eMS2PM can be used to image spike-like voltage transients in cultured cells and fluorescence transients (calcium signals in neurons and red blood cells in capillaries from the cortex) in depth in vivo. These results establish eMS2PM as a unique method for simultaneous acquisition of neuronal network activity.