High-speed, random-access fluorescence microscopy: I. High-resolution optical recording with voltage-sensitive dyes and ion indicators

Acousto-optic holography for pseudo-two-dimensional dynamic light patterning

Optical systems use acousto-optic deflectors (AODs) mostly for fast angular scanning and spectral filtering of laser beams. However, AODs may transform laser light in much broader ways. When time-locked to the pulsing of low repetition rate laser amplifiers, AODs permit the holographic reconstruction of 1D and pseudo-two-dimensional (ps2D) intensity objects of rectangular shape by controlling the amplitude and phase of the light field at high (20-200 kHz) rates for microscopic light patterning. Using iterative Fourier transformations (IFTs), we searched for AOD-compatible holograms to reconstruct the given ps2D target patterns through either phase-only or complex light field modulation. We previously showed that phase-only holograms can adequately render grid-like patterns of diffraction-limited points with non-overlapping diffraction orders, while side lobes to the target pattern can be cured with an apodization mask. Dense target patterns, in contrast, are typically encumbered by apodization-resistant speckle noise. Here, we show the denoised rendering of dense ps2D objects by complex acousto-optic holograms deriving from simultaneous optimization of the amplitude and phase of the light field. Target patterns lacking ps2D symmetry, although not translatable into single holograms, were accessed by serial holography based on a segregation into ps2D-compatible components. The holograms retrieved under different regularizations were experimentally validated in an AOD random-access microscope. IFT regularizations characterized in this work extend the versatility of acousto-optic holography for fast dynamic light patterning.

A silent two-photon imaging system for studying in vivo auditory neuronal functions

Two-photon laser-scanning microscopy has become an essential tool for imaging neuronal functions in vivo and has been applied to different parts of the neural system, including the auditory system. However, many components of a two-photon microscope, such as galvanometer-based laser scanners, generate mechanical vibrations and thus acoustic artifacts, making it difficult to interpret auditory responses from recorded neurons. Here, we report the development of a silent two-photon imaging system and its applications in the common marmoset (Callithrix Jacchus), a non-human primate species sharing a similar hearing range with humans. By utilizing an orthogonal pair of acousto-optical deflectors (AODs), full-frame raster scanning at video rate was achieved without introducing mechanical vibrations. Imaging depth can be optically controlled by adjusting the chirping speed on the AODs without any mechanical motion along the Z-axis. Furthermore, all other sound-generating components of the system were acoustically isolated, leaving the noise floor of the working system below the marmoset's hearing threshold. Imaging with the system in awake marmosets revealed many auditory cortex neurons that exhibited maximal responses at low sound levels, which were not possible to study using traditional two-photon imaging systems. This is the first demonstration of a silent two-photon imaging system that is capable of imaging auditory neuronal functions in vivo without acoustic artifacts. This capacity opens new opportunities for a better understanding of auditory functions in the brain and helps isolate animal behavior from microscope-generated acoustic interference.

Fast optical recording of neuronal activity by three-dimensional custom-access serial holography

Optical recording of neuronal activity in three-dimensional (3D) brain circuits at cellular and millisecond resolution in vivo is essential for probing information flow in the brain. While random-access multiphoton microscopy permits fast optical access to neuronal targets in three dimensions, the method is challenged by motion artifacts when recording from behaving animals. Therefore, we developed three-dimensional custom-access serial holography (3D-CASH). Built on a fast acousto-optic light modulator, 3D-CASH performs serial sampling at 40 kHz from neurons at freely selectable 3D locations. Motion artifacts are eliminated by targeting each neuron with a size-optimized pattern of excitation light covering the cell body and its anticipated displacement field. Spike rates inferred from GCaMP6f recordings in visual cortex of awake mice tracked the phase of a moving bar stimulus with higher spike correlation between intra compared to interlaminar neuron pairs. 3D-CASH offers access to the millisecond correlation structure of in vivo neuronal activity in 3D microcircuits.

Comprehensive Imaging of Sensory-Evoked Activity of Entire Neurons Within the Awake Developing Brain Using Ultrafast AOD-Based Random-Access Two-Photon Microscopy

Determining how neurons transform synaptic input and encode information in action potential (AP) firing output is required for understanding dendritic integration, neural transforms and encoding. Limitations in the speed of imaging 3D volumes of brain encompassing complex dendritic arbors in vivo using conventional galvanometer mirror-based laser-scanning microscopy has hampered fully capturing fluorescent sensors of activity throughout an individual neuron's entire complement of synaptic inputs and somatic APs. To address this problem, we have developed a two-photon microscope that achieves high-speed scanning by employing inertia-free acousto-optic deflectors (AODs) for laser beam positioning, enabling random-access sampling of hundreds to thousands of points-of-interest restricted to a predetermined neuronal structure, avoiding wasted scanning of surrounding extracellular tissue. This system is capable of comprehensive imaging of the activity of single neurons within the intact and awake vertebrate brain. Here, we demonstrate imaging of tectal neurons within the brains of albino Xenopus laevis tadpoles labeled using single-cell electroporation for expression of a red space-filling fluorophore to determine dendritic arbor morphology, and either the calcium sensor jGCaMP7s or the glutamate sensor iGluSnFR as indicators of neural activity. Using discrete, point-of-interest scanning we achieve sampling rates of 3 Hz for saturation sampling of entire arbors at 2 μm resolution, 6 Hz for sequentially sampling 3 volumes encompassing the dendritic arbor and soma, and 200–250 Hz for scanning individual planes through the dendritic arbor. This system allows investigations of sensory-evoked information input-output relationships of neurons within the intact and awake brain.

Action Potential: A Vortex Phenomena; Driving Membrane Oscillations

Hodgkin-Huxley (HH) model has been one of the most successful electrical interpretation of nerve membrane which led to revolutions in the field of computational neuroscience. On the contrary, experimental observations indicate that, an Action Potential (AP) is accompanied with certain physiological changes in the nerve membrane such as, production and absorption of heat; variation of axon diameter, pressure and length. Although, in the early 1900's a Pressure Wave Theory was proposed by E. Wilke, but, due to lack of sophisticated experimental techniques it was left uncharted. Until recently, when Heimburg-Jackson, Hady-Machta and Rvachev, independently proposed Soliton Theory (thermodynamic interpretation of nerve membrane), Mechanical Surface Waves theory (electro-mechanical interpretation) and Rvachev Model (mechano-electrical activation of voltage gated sodium ion channels) respectively; encouraging a deviation from the traditional HH interpretation with justification for the physical changes in the nerve membrane observed experimentally. But, these theories lead to a “hit and miss” scenario because, they do explain certain features (increase/decrease in axon diameter) but miss to explain, correlation between the strength of stimuli and spike rate of AP. Bio-physical models of nerve membrane are thus important for enhancing our understanding regarding the governing dynamics of neural activities encompassing the experimental observations. A novel theory is proposed here which, unravels vortex ring formation due to ion currents in the intracellular and extracellular region leading to variation of pressure causing the increment/decrement in axon diameter. These formations manifest as membrane oscillations which are used to establish a correlation between the strength of stimuli and spike rate of AP. The theory proposed in this paper, brings a paradigm shift in our understanding of neural dynamics from a thorough bio-physical and physiological perspective with promising applications.

Enhancement of the Temporal Resolution of Fluorescent Signals Acquired by the Confocal Microscope

Here, we describe a method of acquisition of fast fluorescent signals with the help of the laser scanning confocal microscope (LSCM). Our method permits an increase in the temporal resolution of acquired signals. The method is based on LSCM recordings of fast fluorescent signals with the shortest achievable time sweep, which are performed with the help of a proprietary algorithm. A series of recordings is made in multiple steps; at each step, the fluorescent signal is incremented by a time interval smaller than the time sweep of the frame of LSCM. The size of the increment determines the achievable time resolution. The convolution of the recorded images results in a signal with the temporal resolution determined by the chosen time increment. This method was applied to register the change in fluorescence (calcium transient) of calcium dye preloaded into peripheral nerve endings by electrical stimulation of the motor nerve. Calculated parameters of the calcium transient were identical to the parameters obtained earlier with the help of a high-speed camera and photodiode. We conclude that the method described here can be applied for the registration of fast fluorescent signals by LSCM with a high spatial and temporal resolution.

Continuous focal translation enhances rate of point-scan volumetric microscopy

Two-Photon Laser-Scanning Microscopy is a powerful tool for exploring biological structure and function due to its ability to optically section through a sample with a tight focus. While it is possible to obtain 3D image stacks by moving a stage, this per-frame imaging process is time consuming. Here, we present a method for an easy-to-implement and inexpensive modification of an existing two-photon microscope to rapidly image in 3D using an electrically tunable lens to create a tessellating scan pattern which repeats with the volume rate. Using appropriate interpolating algorithms, the volumetric imaging rate can be increased by a factor up to four-fold. This capability provides the expansion of the two-photon microscope into the third dimension for faster volumetric imaging capable of visualizing dynamics on timescales not achievable by traditional stage-stack methods.

Kilohertz frame-rate two-photon tomography

Point-scanning two-photon microscopy enables high-resolution imaging within scattering specimens such as the mammalian brain, but sequential acquisition of voxels fundamentally limits its speed. We developed a two-photon imaging technique that scans lines of excitation across a focal plane at multiple angles and computationally recovers high-resolution images, attaining voxel rates of over 1 billion Hz in structured samples. Using a static image as a prior for recording neural activity, we imaged visually evoked and spontaneous glutamate release across hundreds of dendritic spines in mice at depths over 250 µm and frame rates over 1 kHz. Dendritic glutamate transients in anesthetized mice are synchronized within spatially contiguous domains spanning tens of micrometers at frequencies ranging from 1-100 Hz. We demonstrate millisecond-resolved recordings of acetylcholine and voltage indicators, three-dimensional single-particle tracking and imaging in densely labeled cortex. Our method surpasses limits on the speed of raster-scanned imaging imposed by fluorescence lifetime.

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.

Automating Event-detection of Brain Neuron Synaptic Activity and Action Potential Firing in vivo using a Random-access Multiphoton Laser Scanning Microscope for Real-time Analysis

Determining how a neuron computes requires an understanding of the complex spatiotemporal relationship between its input (e.g. synaptic input as a result of external stimuli) and action potential output. Recent advances in in vivo, laser-scanning multiphoton technology, known as random-access microscopy (RAM), can capture this relationship by imaging fluorescent light, emitted from calcium-sensitive biosensors responding to synaptic and action potential firing in a neuron's full dendritic arbor and cell body. Ideally, a continuous output of fluorescent intensities from the neuron would be converted to a binary output (`event', 'or no-event'). These binary events can be used to correlate temporal and spatial associations between the input and output. However, neurons contain hundreds-to-thousands of synapses on the dendritic arbors generating an enormous quantity of data composed of physiological signals, which vary greatly in shape and size. Thus, automating data-processing tasks is essential to support high-throughput analysis for real-time/post-processing operations and to improve operators' comprehension of the data used to decipher neuron computations. Here, we describe an automated software algorithm to detect brain neuron events in real-time using an acousto-optic, multiphoton, laser scanning RAM developed in our laboratory. The fluorescent light intensities, from a genetically encoded, calcium biosensor (GCAMP 6m), are measured by our RAM system and are input to our 'event-detector', which converts them to a binary output meant for real-time applications. We evaluate three algorithms for this purpose: exponentially weighted moving average, cumulative sum, and template matching; present each algorithm's performance; and discuss user-feasibility of each. We validated our system in vivo, using the visual circuit of the Xenopus laevis.

Optogenetic stimulation of complex spatio-temporal activity patterns by acousto-optic light steering probes cerebellar granular layer integrative properties

Optogenetics provides tools to control afferent activity in brain microcircuits. However, this requires optical methods that can evoke asynchronous and coordinated activity within neuronal ensembles in a spatio-temporally precise way. Here we describe a light patterning method, which combines MHz acousto-optic beam steering and adjustable low numerical aperture Gaussian beams, to achieve fast 2D targeting in scattering tissue. Using mossy fiber afferents to the cerebellar cortex as a testbed, we demonstrate single fiber optogenetic stimulation with micron-scale lateral resolution, >100 µm depth-penetration and 0.1 ms spiking precision. Protracted spatio-temporal patterns of light delivered by our illumination system evoked sustained asynchronous mossy fiber activity with excellent repeatability. Combining optical and electrical stimulations, we show that the cerebellar granular layer performs nonlinear integration, whereby sustained mossy fiber activity provides a permissive context for the transmission of salient inputs, enriching combinatorial views on mossy fiber pattern separation.

Enhanced volumetric imaging in 2-photon microscopy via acoustic lens beam shaping

Three-dimensional imaging at high-spatiotemporal resolutions and over large penetration depths is key for unmasking the dynamics and structural organization of complex biological systems. However, the need to axially shift the focus, with consequent limitations in imaging speed, and signal degradation at large depths due to scattering effects, makes this task challenging. Here, we present a novel approach in 2-photon excitation microscopy that allows fast volumetric imaging and enhanced signal-to-background (S/B) in thick tissue. Our technique is based on ultrafast beam shaping at each pixel by means of an acoustic optofluidic lens. Shaping the excitation beam with different phase profiles enables both high-speed axial focus shifting, for continuous volumetric imaging, and controlled aberrated imaging, advantageous for out-of-focus background removal. We provide a theoretical description of our approach, and demonstrate volumetric imaging of neuronal cells from a mouse brain slice with enhancements in S/B up to a factor of 10 over a depth of 600 μm.

Technologies for imaging neural activity in large volumes

Neural circuitry has evolved to form distributed networks that act dynamically across large volumes. Conventional microscopy collects data from individual planes and cannot sample circuitry across large volumes at the temporal resolution relevant to neural circuit function and behaviors. Here we review emerging technologies for rapid volume imaging of neural circuitry. We focus on two critical challenges: the inertia of optical systems, which limits image speed, and aberrations, which restrict the image volume. Optical sampling time must be long enough to ensure high-fidelity measurements, but optimized sampling strategies and point-spread function engineering can facilitate rapid volume imaging of neural activity within this constraint. We also discuss new computational strategies for processing and analyzing volume imaging data of increasing size and complexity. Together, optical and computational advances are providing a broader view of neural circuit dynamics and helping elucidate how brain regions work in concert to support behavior.

Efficient multi-site two-photon functional imaging of neuronal circuits

Two-photon imaging using high-speed multi-channel detectors is a promising approach for optical recording of cellular membrane dynamics at multiple sites. A main bottleneck of this technique is the limited number of photons captured within a short exposure time (~1ms). Here, we implement temporal gating to improve the two-photon fluorescence yield from holographically projected multiple foci whilst maintaining a biologically safe incident average power. We observed up to 6x improvement in the signal-to-noise ratio (SNR) in Fluorescein and cultured hippocampal neurons showing evoked calcium transients. With improved SNR, we could pave the way to achieving multi-site optical recording of fluorogenic probes with response times in the order of ~1ms.

Historical Overview and General Methods of Membrane Potential Imaging

Voltage imaging was first conceived in the late 1960s and efforts to find better organic voltage sensitive dyes began in the 1970s and continue until today. At the beginning it was difficult to measure an action potential signal from a squid giant axon in a single trial. Now it is possible to measure the action potential in an individual spine. Other chapters will discuss advances in voltage imaging technology and applications in a variety of biological preparations. The development of genetically encoded voltage sensors has started. A genetically encoded sensor could provide cell type specific expression and voltage recording (see Chap. 20). Optimizing the signal-to-noise ratio of an optical recording requires attention to several aspects of the recording apparatus. These include the light source, the optics and the recording device. All three have improved substantially in recent years. Arc lamp, LED, and laser sources are now stable, more powerful, and less expensive. Cameras for recording activity have frames rates above 1 kHz and quantum efficiencies near 1.0 although they remain expensive. The sources of noise in optical recordings are well understood. Both the apparatus and the noise sources are discussed in this chapter.

Random-Access Multiphoton Microscopy for Fast Three-Dimensional Imaging

Studies in several important areas of neuroscience, including analysis of single neurons as well as neural networks, continue to be limited by currently available experimental tools. By combining molecular probes of cellular function, such as voltage-sensitive or calcium-sensitive dyes, with advanced microscopy techniques such as multiphoton microscopy, experimental neurophysiologists have been able to partially reduce this limitation. These approaches usually provide the needed spatial resolution along with convenient optical sectioning capabilities for isolating regions of interest. However, they often fall short in providing the necessary temporal resolution, primarily due to their restrained laser scanning mechanisms. In this regard, we review a method of laser scanning for multiphoton microscopy that overcomes the temporal limitations of pervious approaches and allows for what is known as 3D Random Access Multiphoton (3D RAMP) microscopy, an imaging technique that supports full three dimensional recording of many sites of interest on physiologically relevant time scales.

Two-Photon Excitation of Fluorescent Voltage-Sensitive Dyes: Monitoring Membrane Potential in the Infrared

Functional imaging microscopy based on voltage-sensitive dyes (VSDs) has proven effective for revealing spatio-temporal patterns of activity in vivo and in vitro. Microscopy based on two-photon excitation of fluorescent VSDs offers the possibility of recording sub-millisecond membrane potential changes on micron length scales in cells that lie upwards of one millimeter below the brain's surface. Here we describe progress in monitoring membrane voltage using two-photon excitation (TPE) of VSD fluorescence, and detail an application of this emerging technology in which action potentials were recorded in single trials from individual mammalian nerve terminals in situ. Prospects for, and limitations of this method are reviewed.

Combining Membrane Potential Imaging with Other Optical Techniques

Membrane potential imaging using voltage-sensitive dyes can be combined with other optical techniques for a variety of applications. Combining voltage imaging with Ca2+ imaging allows correlating membrane potential changes with intracellular Ca2+ signals or with Ca2+ currents. Combining voltage imaging with uncaging techniques allows analyzing electrical signals elicited by photorelease of a particular molecule. This approach is also a useful tool to calibrate the change in fluorescence intensity in terms of membrane potential changes from different sites permitting spatial mapping of electrical activity. Finally, combining voltage imaging with optogenetics, in particular with channelrhodopsin stimulation, opens the gate to novel investigations of brain circuitries by allowing measurements of synaptic signals mediated by specific sets of neurons. Here we describe in detail the methods of membrane potential imaging in combination with other optical techniques and discus some important applications.

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.

A micron resolution optical scanner for characterization of silicon detectors

The emergence of high position resolution (∼10 μm) silicon detectors in recent times have highlighted the urgent need for the development of new automated optical scanners of micron level resolution suited for characterizing microscopic features of these detectors. More specifically, for the newly developed silicon photo-multipliers (SiPM) that are compact, possessing excellent photon detection efficiency with gain comparable to photo-multiplier tube. In a short time, since their invention the SiPMs are already being widely used in several high-energy physics and astrophysics experiments as the photon readout element. The SiPM is a high quantum efficiency, multi-pixel photon counting detector with fast timing and high gain. The presence of a wide variety of photo sensitive silicon detectors with high spatial resolution requires their performance evaluation to be carried out by photon beams of very compact spot size. We have designed a high resolution optical scanner that provides a monochromatic focused beam on a target plane. The transverse size of the beam was measured by the knife-edge method to be 1.7 μm at 1 - σ level. Since the beam size was an order of magnitude smaller than the typical feature size of silicon detectors, this optical scanner can be used for selective excitation of these detectors. The design and operational details of the optical scanner, high precision programmed movement of target plane (0.1 μm) integrated with general purpose data acquisition system developed for recording static and transient response photo sensitive silicon detector are reported in this paper. Entire functionality of scanner is validated by using it for selective excitation of individual pixels in a SiPM and identifying response of active and dead regions within SiPM. Results from these studies are presented in this paper.

Micromirror structured illumination microscope for high-speed in vivo drosophila brain imaging

Genetic tools and especially genetically encoded fluorescent reporters have given a special place to optical microscopy in drosophila neurobiology research. In order to monitor neural networks activity, high speed and sensitive techniques, with high spatial resolution are required. Structured illumination microscopies are wide-field approaches with optical sectioning ability. Despite the large progress made with the introduction of the HiLo principle, they did not meet the criteria of speed and/or spatial resolution for drosophila brain imaging. We report on a new implementation that took advantage of micromirror matrix technology to structure the illumination. Thus, we showed that the developed instrument exhibits a spatial resolution close to that of confocal microscopy but it can record physiological responses with a speed improved by more than an order a magnitude.

Fast flexible multiphoton fluorescence lifetime imaging using acousto-optic deflector

We present a fast and flexible fluorescence lifetime imaging microscopy which uses a two-dimensional acousto-optic deflector to achieve fast beam scanning across the sample and provides random access to the regions of interests (ROI). Experimental results using standard fluorescent dye and biological samples show that this system can make addressable fluorescence lifetime measurements and perform fast and flexible fluorescence lifetime imaging particularly to the discontinuous ROI in the sample.

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.

Anomalous diffusion of single particles in cytoplasm

The crowded intracellular environment poses a formidable challenge to experimental and theoretical analyses of intracellular transport mechanisms. Our measurements of single-particle trajectories in cytoplasm and their random-walk interpretations elucidate two of these mechanisms: molecular diffusion in crowded environments and cytoskeletal transport along microtubules. We employed acousto-optic deflector microscopy to map out the three-dimensional trajectories of microspheres migrating in the cytosolic fraction of a cellular extract. Classical Brownian motion (BM), continuous time random walk, and fractional BM were alternatively used to represent these trajectories. The comparison of the experimental and numerical data demonstrates that cytoskeletal transport along microtubules and diffusion in the cytosolic fraction exhibit anomalous (nonFickian) behavior and posses statistically distinct signatures. Among the three random-walk models used, continuous time random walk provides the best representation of diffusion, whereas microtubular transport is accurately modeled with fractional BM.

Advances in multiphoton microscopy technology

Multiphoton microscopy has enabled unprecedented dynamic exploration in living organisms. A significant challenge in biological research is the dynamic imaging of features deep within living organisms, which permits the real-time analysis of cellular structure and function. To make progress in our understanding of biological machinery, optical microscopes must be capable of rapid, targeted access deep within samples at high resolution. In this Review, we discuss the basic architecture of a multiphoton microscope capable of such analysis and summarize the state-of-the-art technologies for the quantitative imaging of biological phenomena.

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator

We present an addressable, large-field second harmonic generation microscope by combining a 2D acousto-optical deflector with a spatial light modulator. The SLM shapes an incoming mode-locked, near-infrared Ti:Sapphire laser beam into a multifocus array, which can be rapidly scanned by changing the incident angle of the laser beam using a 2D acousto-optical deflector. Compared to the single-beam-scan technique, the multifocus array scan can increase the scanning rate and the field-of-view size with the multi-region imaging ability.

Acousto-optical deflector-based patterned ultraviolet uncaging of neurotransmitter for the study of neuronal integration

The method of patterned photoactivation is a natural fit for the study of neuronal dendritic integration. Photoactivatable molecules that influence a wide range of extracellular and intracellular neurophysiological functions are available. The choice of photosensitive molecules depends on the research question and will influence the design of the experimental apparatus. This article describes an acousto-optical deflector (AOD)-based system for rapid ultraviolet (UV) photolysis in arbitrary spatial and temporal patterns. Some basics of caged neurotransmitters and the theory of operation of AODs are covered, as are descriptions for implementing the system.

Action potential propagation in transverse-axial tubular system is impaired in heart failure

The plasma membrane of cardiac myocytes presents complex invaginations known as the transverse-axial tubular system (TATS). Despite TATS's crucial role in excitation-contraction coupling and morphological alterations found in pathological settings, TATS's electrical activity has never been directly investigated in remodeled tubular networks. Here we develop an ultrafast random access multiphoton microscope that, in combination with a customly synthesized voltage-sensitive dye, is used to simultaneously measure action potentials (APs) at multiple sites within the sarcolemma with submillisecond temporal and submicrometer spatial resolution in real time. We find that the tight electrical coupling between different sarcolemmal domains is guaranteed only within an intact tubular system. In fact, acute detachment by osmotic shock of most tubules from the surface sarcolemma prevents AP propagation not only in the disconnected tubules, but also in some of the tubules that remain connected with the surface. This indicates that a structural disorganization of the tubular system worsens the electrical coupling between the TATS and the surface. The pathological implications of this finding are investigated in failing hearts. We find that AP propagation into the pathologically remodeled TATS frequently fails and may be followed by local spontaneous electrical activity. Our findings provide insight on the relationship between abnormal TATS and asynchronous calcium release, a major determinant of cardiac contractile dysfunction and arrhythmias.

A reconfigurable real-time compressive-sampling camera for biological applications

Many applications in biology, such as long-term functional imaging of neural and cardiac systems, require continuous high-speed imaging. This is typically not possible, however, using commercially available systems. The frame rate and the recording time of high-speed cameras are limited by the digitization rate and the capacity of on-camera memory. Further restrictions are often imposed by the limited bandwidth of the data link to the host computer. Even if the system bandwidth is not a limiting factor, continuous high-speed acquisition results in very large volumes of data that are difficult to handle, particularly when real-time analysis is required. In response to this issue many cameras allow a predetermined, rectangular region of interest (ROI) to be sampled, however this approach lacks flexibility and is blind to the image region outside of the ROI. We have addressed this problem by building a camera system using a randomly-addressable CMOS sensor. The camera has a low bandwidth, but is able to capture continuous high-speed images of an arbitrarily defined ROI, using most of the available bandwidth, while simultaneously acquiring low-speed, full frame images using the remaining bandwidth. In addition, the camera is able to use the full-frame information to recalculate the positions of targets and update the high-speed ROIs without interrupting acquisition. In this way the camera is capable of imaging moving targets at high-speed while simultaneously imaging the whole frame at a lower speed. We have used this camera system to monitor the heartbeat and blood cell flow of a water flea (Daphnia) at frame rates in excess of 1500 fps.

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.

Advances in light microscopy for neuroscience

Since the work of Golgi and Cajal, light microscopy has remained a key tool for neuroscientists to observe cellular properties. Ongoing advances have enabled new experimental capabilities using light to inspect the nervous system across multiple spatial scales, including ultrastructural scales finer than the optical diffraction limit. Other progress permits functional imaging at faster speeds, at greater depths in brain tissue, and over larger tissue volumes than previously possible. Portable, miniaturized fluorescence microscopes now allow brain imaging in freely behaving mice. Complementary progress on animal preparations has enabled imaging in head-restrained behaving animals, as well as time-lapse microscopy studies in the brains of live subjects. Mouse genetic approaches permit mosaic and inducible fluorescence-labeling strategies, whereas intrinsic contrast mechanisms allow in vivo imaging of animals and humans without use of exogenous markers. This review surveys such advances and highlights emerging capabilities of particular interest to neuroscientists.

Invited review article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

We review the current state of multiphoton microscopy. In particular, the requirements and limitations associated with high-speed multiphoton imaging are considered. A description of the different scanning technologies such as line scan, multifoci approaches, multidepth microscopy, and novel detection techniques is given. The main nonlinear optical contrast mechanisms employed in microscopy are reviewed, namely, multiphoton excitation fluorescence, second harmonic generation, and third harmonic generation. Techniques for optimizing these nonlinear mechanisms through a careful measurement of the spatial and temporal characteristics of the focal volume are discussed, and a brief summary of photobleaching effects is provided. Finally, we consider three new applications of multiphoton microscopy: nonlinear imaging in microfluidics as applied to chemical analysis and the use of two-photon absorption and self-phase modulation as contrast mechanisms applied to imaging problems in the medical sciences.

Uniform action potential repolarization within the sarcolemma of in situ ventricular cardiomyocytes

Previous studies have speculated, based on indirect evidence, that the action potential at the transverse (t)-tubules is longer than at the surface membrane in mammalian ventricular cardiomyocytes. To date, no technique has enabled recording of electrical activity selectively at the t-tubules to directly examine this hypothesis. We used confocal line-scan imaging in conjunction with the fast response voltage-sensitive dyes ANNINE-6 and ANNINE-6plus to resolve action potential-related changes in fractional dye fluorescence (DeltaF/F) at the t-tubule and surface membranes of in situ mouse ventricular cardiomyocytes. Peak DeltaF/F during action potential phase 0 depolarization averaged -21% for both dyes. The shape and time course of optical action potentials measured with the water-soluble ANNINE-6plus were indistinguishable from those of action potentials recorded with intracellular microelectrodes in the absence of the dye. In contrast, optical action potentials measured with the water-insoluble ANNINE-6 were significantly prolonged compared to the electrical recordings obtained from dye-free hearts, suggesting electrophysiological effects of ANNINE-6 and/or its solvents. With either dye, the kinetics of action potential-dependent changes in DeltaF/F during repolarization were found to be similar at the t-tubular and surface membranes. This study provides what to our knowledge are the first direct measurements of t-tubule electrical activity in ventricular cardiomyocytes, which support the concept that action potential duration is uniform throughout the sarcolemma of individual cells.

Optical monitoring of neuronal activity at high frame rate with a digital random-access multiphoton (RAMP) microscope

Two-photon microscopy offers the promise of monitoring brain activity at multiple locations within intact tissue. However, serial sampling of voxels has been difficult to reconcile with millisecond timescales characteristic of neuronal activity. This is due to the conflicting constraints of scanning speed and signal amplitude. The recent use of acousto-optic deflector scanning to implement random-access multiphoton microscopy (RAMP) potentially allows to preserve long illumination dwell times while sampling multiple points-of-interest at high rates. However, the real-life abilities of RAMP microscopy regarding sensitivity and phototoxicity issues, which have so far impeded prolonged optical recordings at high frame rates, have not been assessed. Here, we describe the design, implementation and characterisation of an optimised RAMP microscope. We demonstrate the application of the microscope by monitoring calcium transients in Purkinje cells and cortical pyramidal cell dendrites and spines. We quantify the illumination constraints imposed by phototoxicity and show that stable continuous high-rate recordings can be obtained. During these recordings the fluorescence signal is large enough to detect spikes with a temporal resolution limited only by the calcium dye dynamics, improving upon previous techniques by at least an order of magnitude.

Three-dimensional random access multiphoton microscopy for functional imaging of neuronal activity

The dynamic ability of neuronal dendrites to shape and integrate synaptic responses is the hallmark of information processing in the brain. Effectively studying this phenomenon requires concurrent measurements at multiple sites on live neurons. Substantial progress has been made by optical imaging systems that combine confocal and multiphoton microscopy with inertia-free laser scanning. However, all of the systems developed so far restrict fast imaging to two dimensions. This severely limits the extent to which neurons can be studied, as they represent complex three-dimensional structures. Here we present a new imaging system that utilizes a unique arrangement of acousto-optic deflectors to steer a focused, ultra-fast laser beam to arbitrary locations in three-dimensional space without moving the objective lens. As we demonstrate, this highly versatile random-access multiphoton microscope supports functional imaging of complex three-dimensional cellular structures such as neuronal dendrites or neural populations at acquisition rates on the order of tens of kilohertz.

Combining optical imaging and computational modeling to analyze structure and function of living neurons

We are investigating the computational properties of principal neurons in the mammalian brain. To manage the small size and intricate structure of neuronal dendrites, we employ advanced optical imaging techniques in combination with automatic image reconstruction and computational modeling to study their complex spatio-temporal pattern of activity.

Analysis of the dispersion compensation of acousto-optic deflectors used for multiphoton imaging

The acousto-optic deflector (AOD) is highly preferred in laser scanning microscopy for its fast scanning ability and random-addressing capability. However, its application in two-photon microscopy is frustrated by the dispersion of the AOD, which results in beam distortion and pulse lengthening. We report the analysis of simultaneous compensation for the angular dispersion and temporal dispersion of the AOD by merely introducing a single dispersive element such as a prism or a grating. Besides serving as a scanner, the AOD is also a part of the compressor pair by integrating the dispersive nature of the AO interaction. This compensation principle is effective for both one-dimensional (1-D) AOD and two-dimensional (2-D) AOD scanning. Switching from a 1-D to a 2-D system requires proper optical alignment with the compensation element, but does not involve any new components. Analytical expressions are given to illustrate the working principle and to help with understanding the design of the system. Fluorescence images of beads and cells are shown to demonstrate the performance of two-photon microscopy when applying this compensated 2-D AOD as scanner.

Towards automatic reconstruction of dendrite morphology from live neurons

The recent advent of optical imaging methods to capture both structural and functional data from living neurons holds the promise of improving our understanding of neuronal function. An important technical advance would be to use computer simulations to determine the optimal locations for high-speed functional imaging and electrical recording on an individual neuron. However, such simulations require a precise reconstruction of the dendritic morphology. Thus, the currently available time-consuming, semi-manual methods for morphological reconstruction prevent integration of cell-specific simulations into the experiments. Our work focuses on implementing a fast, robust method for extracting dendrite morphology from confocal or two-photon images to enable concurrent simulation and functional imaging of individual neurons. We discuss the current implementation of the morphological reconstruction, our approach to validation, and our initial results.

Position of the prism in a dispersion-compensated acousto-optic deflector for multiphoton imaging

The performance of a dispersion-compensated acousto-optic deflector (AOD) for steering femtosecond laser pulses was examined with the prism located before or after the AOD, which is regarded as prism-AOD and AOD-prism, respectively. Comparisons are made over parameters including the spot spatial pattern, output pulse width, scanning linearity, the field of view, and the transmission rate. Fluorescence images of 170 nm diameter beads and cells were measured to provide an overall evaluation for these femtosecond laser beam scanning configurations. On the basis of these experiments, the prism-AOD configuration is concluded to be more advantageous for the purpose of simultaneous compensation for the spatial and temporal dispersion.

New methods and uses for fast optical scanning

Advanced optical imaging techniques used in neurobiology commonly employ fluorescent molecules for studying the structure and function of neural tissue. To obtain adequate spatio-temporal resolution, sophisticated scanning schemes are used to manage the excitation light going to and emission light coming from objects under observation. Although the fundamental principles of these techniques remain the same, such as scanning point illumination and point detection for confocal imaging, their physical implementation is the subject of technological advance, for example, the advent of inertia-free discontinuous scanning schemes. In general, the aims of these technological advances are to improve the spatio-temporal resolution of and/or reduce potential photodamage caused by optical imaging in live neural tissue. The number of recent advances in scanning methods indicates their increasing importance in imaging techniques.

Imaging membrane potential in dendrites and axons of single neurons

This review focuses on the use of imaging techniques to record electrical signaling in the fine processes of neurons such as dendrites and axons. Voltage imaging began with the use and development of externally applied voltage-sensitive dyes. With the introduction of internally applied dyes and advances in detection technology, it is now possible to record supra-threshold action potential responses, as well as sub-threshold synaptic potentials, in fine neuronal processes including dendritic spines. The development of genetically coded sensors, as well as variants of laser scanning microscopy such as second harmonic generation, offers promise for further advances in this field. Through the use and further development of these methods, optical imaging of membrane potential will continue to be a valuable tool for investigators wishing to explore the electrical events underlying single neuronal computation.

High-speed addressable confocal microscopy for functional imaging of cellular activity

Due to cellular complexity, studying fast signaling in neurons is often limited by: 1. the number of sites that can be simultaneously probed with conventional tools, such as patch pipettes, and 2. the recording speed of imaging tools, such as confocal or multiphoton microscopy. To overcome these spatiotemporal limitations, we develop an addressable confocal microscope that permits concurrent optical recordings from multiple user-selected sites of interest at high frame rates. Our system utilizes acousto-optic deflectors (AODs) for rapid positioning of a focused laser beam and a digital micromirror device (DMD) for addressable spatial filtering to achieve confocality. A registration algorithm synchronizes the AODs and DMD such that point illumination and point detection are always colocalized in conjugate image planes. The current system has an adjustable spatial resolution of approximately 0.5 to 1 microm. Furthermore, we show that recordings can be made at an aggregate frame rate of approximately 40 kHz. The system is capable of optical sectioning; this property is used to create 3-D reconstructions of fluorescently labeled test specimens and visualize neurons in brain slices. Additionally, we use the system to record intracellular calcium transients at several sites in hippocampal neurons using the fluorescent calcium indicator Oregon Green BAPTA-1.

Efficient estimation of detailed single-neuron models

Biophysically accurate multicompartmental models of individual neurons have significantly advanced our understanding of the input-output function of single cells. These models depend on a large number of parameters that are difficult to estimate. In practice, they are often hand-tuned to match measured physiological behaviors, thus raising questions of identifiability and interpretability. We propose a statistical approach to the automatic estimation of various biologically relevant parameters, including 1) the distribution of channel densities, 2) the spatiotemporal pattern of synaptic input, and 3) axial resistances across extended dendrites. Recent experimental advances, notably in voltage-sensitive imaging, motivate us to assume access to: i) the spatiotemporal voltage signal in the dendrite and ii) an approximate description of the channel kinetics of interest. We show here that, given i and ii, parameters 1-3 can be inferred simultaneously by nonnegative linear regression; that this optimization problem possesses a unique solution and is guaranteed to converge despite the large number of parameters and their complex nonlinear interaction; and that standard optimization algorithms efficiently reach this optimum with modest computational and data requirements. We demonstrate that the method leads to accurate estimations on a wide variety of challenging model data sets that include up to about 10(4) parameters (roughly two orders of magnitude more than previously feasible) and describe how the method gives insights into the functional interaction of groups of channels.

Simultaneous compensation for spatial and temporal dispersion of acousto-optical deflectors for two-dimensional scanning with a single prism

The dispersive nature of the acousto-optical deflector (AOD) presents a challenge to applications of two sequential orthogonal AODs (a two-dimensional AOD) as XY scanners in multiphoton microscopy. Introducing a prism before the two-dimensional (2D) AOD allows both temporal and spatial dispersion to be compensated for simultaneously. A 90 fs laser pulse was broadened to 572 fs without compensation, and 143 fs with compensation. The ratio of long axis to short axis of the exiting laser beam spot was 3.50 without compensation and 1.14 with compensation. The insertion loss was 37%. Two-photon fluorescence microscopy used the compensated 2D AOD scanner to image a fluorescent microsphere, which improves signal intensity -15-fold compared with the uncompensated scanner.

Photometric recording of transmembrane potential in outer hair cells

Cochlear outer hair cells (OHCs) are polarized epithelial cells that have mechanoelectrical transduction channels within their apical stereocilia and produce electromotile force along their lateral wall. Phase shifts, or time delays, in the transmembrane voltage occurring at different axial locations along the cell may contribute to our understanding of how these cells operate at auditory frequencies. We developed a method to optically measure the phase of the OHC transmembrane potential using the voltage-sensitive dye (VSD) di-8-ANEPPS. The exit aperture of a fibre-optic light source was driven in two dimensions so that a 24 microm spot of excitation light could be positioned along the length of the OHC. We used the whole-cell patch-clamp technique in the current-clamp mode to stimulate the OHC at the base. The photometric response and the voltage response were monitored with a photodetector and patch-clamp amplifier, respectively. The photometric response was used to measure the regional changes in the membrane potential in response to maintained (dc) and sinusoidal (ac) current stimuli applied at the base of the cell. We used a neutral density filter to lower the excitation light intensity and reduce phototoxicity. A sensitive detector and lock-in amplifier were used to measure the small ac VSD signal. This permitted measurements of the ac photometric response below the noise floor of the static fluorescence. The amplitude and phase components of the photometric response were recorded for stimuli up to 800 Hz. VSD data at 400-800 Hz show the presence of a small phase delay between the stimulus voltage at the base of the cell and the local membrane potential measured along the lateral wall. Results are consistent with the hypothesis that OHCs exhibit inhomogeneous membrane potentials that vary with position in analogy with the voltage in nerve axons.

Ultrafast random-access scanning in two-photon microscopy using acousto-optic deflectors

Two-photon scanning microscopy (TPSM) is a powerful tool for imaging deep inside living tissues with sub-cellular resolution. The temporal resolution of TPSM is however strongly limited by the galvanometric mirrors used to steer the laser beam. Fast physiological events can therefore only be followed by scanning repeatedly a single line within the field of view. Because acousto-optic deflectors (AODs) are non-mechanical devices, they allow access at any point within the field of view on a microsecond time scale and are therefore excellent candidates to improve the temporal resolution of TPSM. However, the use of AOD-based scanners with femtosecond pulses raises several technical difficulties. In this paper, we describe an all-digital TPSM setup based on two crossed AODs. It includes in particular an acousto-optic modulator (AOM) placed at 45 degrees with respect to the AODs to pre-compensate for the large spatial distortions of femtosecond pulses occurring in the AODs, in order to optimize the spatial resolution and the fluorescence excitation. Our setup allows recording from freely selectable point-of-interest at high speed (1kHz). By maximizing the time spent on points of interest, random-access TPSM (RA-TPSM) constitutes a promising method for multiunit recordings with millisecond resolution in biological tissues.