Simultaneous optical measurements of electrical activity from multiple sites on processes of cultured neurons

Intrinsic and Synaptic Contributions to Repetitive Spiking in Dentate Granule Cells

Repetitive firing of granule cells (GCs) in the dentate gyrus (DG) facilitates synaptic transmission to the CA3 region. This facilitation can gate and amplify the flow of information through the hippocampus. High-frequency bursts in the DG are linked to behavior and plasticity, but GCs do not readily burst. Under normal conditions, a single shock to the perforant path in a hippocampal slice typically drives a GC to fire a single spike, and only occasionally more than one spike is seen. Repetitive spiking in GCs is not robust, and the mechanisms are poorly understood. Here, we used a hybrid genetically encoded voltage sensor to image voltage changes evoked by cortical inputs in many mature GCs simultaneously in hippocampal slices from male and female mice. This enabled us to study relatively infrequent double and triple spikes. We found GCs are relatively homogeneous and their double spiking behavior is cell autonomous. Blockade of GABA type A receptors increased multiple spikes and prolonged the interspike interval, indicating inhibitory interneurons limit repetitive spiking and set the time window for successive spikes. Inhibiting synaptic glutamate release showed that recurrent excitation mediated by hilar mossy cells contributes to, but is not necessary for, multiple spiking. Blockade of T-type Ca2+ channels did not reduce multiple spiking but prolonged interspike intervals. Imaging voltage changes in different GC compartments revealed that second spikes can be initiated in either dendrites or somata. Thus, pharmacological and biophysical experiments reveal roles for both synaptic circuitry and intrinsic excitability in GC repetitive spiking.

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.

Voltage Imaging in Drosophila Using a Hybrid Chemical-Genetic Rhodamine Voltage Reporter

We combine a chemically-synthesized, voltage-sensitive fluorophore with a genetically encoded, self-labeling enzyme to enable voltage imaging in Drosophila melanogaster. Previously, we showed that a rhodamine voltage reporter (RhoVR) combined with the HaloTag self-labeling enzyme could be used to monitor membrane potential changes from mammalian neurons in culture and brain slice. Here, we apply this hybrid RhoVR-Halo approach in vivo to achieve selective neuron labeling in intact fly brains. We generate a Drosophila UAS-HaloTag reporter line in which the HaloTag enzyme is expressed on the surface of cells. We validate the voltage sensitivity of this new construct in cell culture before driving expression of HaloTag in specific brain neurons in flies. We show that selective labeling of synapses, cells, and brain regions can be achieved with RhoVR-Halo in either larval neuromuscular junction (NMJ) or in whole adult brains. Finally, we validate the voltage sensitivity of RhoVR-Halo in fly tissue via dual-electrode/imaging at the NMJ, show the efficacy of this approach for measuring synaptic excitatory post-synaptic potentials (EPSPs) in muscle cells, and perform voltage imaging of carbachol-evoked depolarization and osmolarity-evoked hyperpolarization in projection neurons and in interoceptive subesophageal zone neurons in fly brain explants following in vivo labeling. We envision the turn-on response to depolarizations, fast response kinetics, and two-photon compatibility of chemical indicators, coupled with the cellular and synaptic specificity of genetically-encoded enzymes, will make RhoVR-Halo a powerful complement to neurobiological imaging in Drosophila.

Current Review of Optical Neural Interfaces for Clinical Applications

Neural interfaces, which enable the recording and stimulation of living neurons, have emerged as valuable tools in understanding the brain in health and disease, as well as serving as neural prostheses. While neural interfaces are typically based on electrical transduction, alternative energy modalities have been explored to create safe and effective approaches. Among these approaches, optical methods of linking neurons to the outside world have gained attention because light offers high spatial selectivity and decreased invasiveness. Here, we review the current state-of-art of optical neural interfaces and their clinical applications. Optical neural interfaces can be categorized into optical control and optical readout, each of which can be divided into intrinsic and extrinsic approaches. We discuss the advantages and disadvantages of each of these methods and offer a comparison of relative performance. Future directions, including their clinical opportunities, are discussed with regard to the optical properties of biological tissue.

The chemical tools for imaging dopamine release

Dopamine is a modulatory neurotransmitter involved in learning, motor functions, and reward. Many neuropsychiatric disorders, including Parkinson's disease, autism, and schizophrenia, are associated with imbalances or dysfunction in the dopaminergic system. Yet, our understanding of these pervasive public health issues is limited by our ability to effectively image dopamine in humans, which has long been a goal for chemists and neuroscientists. The last two decades have witnessed the development of many molecules used to trace dopamine. We review the small molecules, nanoparticles, and protein sensors used with fluorescent microscopy/photometry, MRI, and PET that shape dopamine research today. None of these tools observe dopamine itself, but instead harness the biology of the dopamine system-its synthetic and metabolic pathways, synaptic vesicle cycle, and receptors-in elegant ways. Their advantages and weaknesses are covered here, along with recent examples and the chemistry and biology that allow them to function.

Photoacoustic Neuroimaging - Perspectives on a Maturing Imaging Technique and its Applications in Neuroscience

A prominent goal of neuroscience is to improve our understanding of how brain structure and activity interact to produce perception, emotion, behavior, and cognition. The brain's network activity is inherently organized in distinct spatiotemporal patterns that span scales from nanometer-sized synapses to meter-long nerve fibers and millisecond intervals between electrical signals to decades of memory storage. There is currently no single imaging method that alone can provide all the relevant information, but intelligent combinations of complementary techniques can be effective. Here, we thus present the latest advances in biomedical and biological engineering on photoacoustic neuroimaging in the context of complementary imaging techniques. A particular focus is placed on recent advances in whole-brain photoacoustic imaging in rodent models and its influential role in bridging the gap between fluorescence microscopy and more non-invasive techniques such as magnetic resonance imaging (MRI). We consider current strategies to address persistent challenges, particularly in developing molecular contrast agents, and conclude with an overview of potential future directions for photoacoustic neuroimaging to provide deeper insights into healthy and pathological brain processes.

Genetic voltage indicators

As a "holy grail" of neuroscience, optical imaging of membrane potential could enable high resolution measurements of spiking and synaptic activity in neuronal populations. This has been partly achieved using organic voltage-sensitive dyes in vitro, or in invertebrate preparations yet unspecific staining has prevented single-cell resolution measurements from mammalian preparations in vivo. The development of genetically encoded voltage indicators (GEVIs) and chemogenetic sensors has enabled targeting voltage indicators to plasma membranes and selective neuronal populations. Here, we review recent advances in the design and use of genetic voltage indicators and discuss advantages and disadvantages of three classes of them. Although genetic voltage indicators could revolutionize neuroscience, there are still significant challenges, particularly two-photon performance. To overcome them may require cross-disciplinary collaborations, team effort, and sustained support by large-scale research initiatives.

Imaging membrane potential changes from dendritic spines using computer-generated holography

Electrical properties of neuronal processes 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 electrical subthreshold events as they travel from synapses on distal dendrites and summate at particular locations to initiate action potentials. It is now possible to carry out these measurements at the scale of individual dendritic spines using voltage imaging. In these measurements, the voltage-sensitive probes can be thought of as transmembrane voltmeters with a linear scale, which directly monitor electrical signals. Grinvald et al. were important early contributors to the methodology of voltage imaging, and they pioneered some of its significant results. We combined voltage imaging and glutamate uncaging using computer-generated holography. The results demonstrated that patterned illumination, by reducing the surface area of illuminated membrane, reduces photodynamic damage. Additionally, region-specific illumination practically eliminated the contamination of optical signals from individual spines by the scattered light from the parent dendrite. Finally, patterned illumination allowed one-photon uncaging of glutamate on multiple spines to be carried out in parallel with voltage imaging from the parent dendrite and neighboring spines.

Long-Term Spatiotemporal Reconfiguration of Neuronal Activity Revealed by Voltage-Sensitive Dye Imaging in the Cerebellar Granular Layer

Understanding the spatiotemporal organization of long-term synaptic plasticity in neuronal networks demands techniques capable of monitoring changes in synaptic responsiveness over extended multineuronal structures. Among these techniques, voltage-sensitive dye imaging (VSD imaging) is of particular interest due to its good spatial resolution. However, improvements of the technique are needed in order to overcome limits imposed by its low signal-to-noise ratio. Here, we show that VSD imaging can detect long-term potentiation (LTP) and long-term depression (LTD) in acute cerebellar slices. Combined VSD imaging and patch-clamp recordings revealed that the most excited regions were predominantly associated with granule cells (GrCs) generating EPSP-spike complexes, while poorly responding regions were associated with GrCs generating EPSPs only. The correspondence with cellular changes occurring during LTP and LTD was highlighted by a vector representation obtained by combining amplitude with time-to-peak of VSD signals. This showed that LTP occurred in the most excited regions lying in the core of activated areas and increased the number of EPSP-spike complexes, while LTD occurred in the less excited regions lying in the surround. VSD imaging appears to be an efficient tool for investigating how synaptic plasticity contributes to the reorganization of multineuronal activity in neuronal circuits.

Monitoring Integrated Activity of Individual Neurons Using FRET-Based Voltage-Sensitive Dyes

Pairs of membrane-associated molecules exhibiting fluorescence resonance energy transfer (FRET) provide a sensitive technique to measure changes in a cell's membrane potential. One of the FRET pair binds to one surface of the membrane and the other is a mobile ion that dissolves in the lipid bilayer. The voltage-related signal can be measured as a change in the fluorescence of either the donor or acceptor molecules, but measuring their ratio provides the largest and most noise-free signal. This technology has been used in a variety of ways; three are documented in this chapter: (1) high throughput drug screening, (2) monitoring the activity of many neurons simultaneously during a behavior, and (3) finding synaptic targets of a stimulated neuron. In addition, we provide protocols for using the dyes on both cultured neurons and leech ganglia. We also give an updated description of the mathematical basis for measuring the coherence between electrical and optical signals. Future improvements of this technique include faster and more sensitive dyes that bleach more slowly, and the expression of one of the FRET pair genetically.

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.

An optically stabilized fast-switching light emitting diode as a light source for functional neuroimaging

Neuroscience research increasingly relies on optical methods for evoking neuronal activity as well as for measuring it, making bright and stable light sources critical building blocks of modern experimental setups. This paper presents a method to control the brightness of a high-power light emitting diode (LED) light source to an unprecedented level of stability. By continuously monitoring the actual light output of the LED with a photodiode and feeding the result back to the LED's driver by way of a proportional-integral controller, drift was reduced to as little as 0.007% per hour over a 12-h period, and short-term fluctuations to 0.005% root-mean-square over 10 seconds. The LED can be switched on and off completely within 100 s, a feature that is crucial when visual stimuli and light for optical recording need to be interleaved to obtain artifact-free recordings. The utility of the system is demonstrated by recording visual responses in the central nervous system of the medicinal leech Hirudo verbana using voltage-sensitive dyes.

Optophysiological approach to resolve neuronal action potentials with high spatial and temporal resolution in cultured neurons

Cell to cell communication in the central nervous system is encoded into transient and local membrane potential changes (ΔVm). Deciphering the rules that govern synaptic transmission and plasticity entails to be able to perform V_m recordings throughout the entire neuronal arborization. Classical electrophysiology is, in most cases, not able to do so within small and fragile neuronal subcompartments. Thus, optical techniques based on the use of fluorescent voltage-sensitive dyes (VSDs) have been developed. However, reporting spontaneous or small ΔV_m from neuronal ramifications has been challenging, in part due to the limited sensitivity and phototoxicity of VSD-based optical measurements. Here we demonstrate the use of water soluble VSD, ANNINE-6plus, with laser-scanning microscopy to optically record ΔV_m in cultured neurons. We show that the sensitivity (>10% of fluorescence change for 100 mV depolarization) and time response (sub millisecond) of the dye allows the robust detection of action potentials (APs) even without averaging, allowing the measurement of spontaneous neuronal firing patterns. In addition, we show that back-propagating APs can be recorded, along distinct dendritic sites and within dendritic spines. Importantly, our approach does not induce any detectable phototoxic effect on cultured neurons. This optophysiological approach provides a simple, minimally invasive, and versatile optical method to measure electrical activity in cultured neurons with high temporal (ms) resolution and high spatial (μm) resolution.

Random insertion of split-cans of the fluorescent protein venus into Shaker channels yields voltage sensitive probes with improved membrane localization in mammalian cells

FlaSh-YFP, a fluorescent protein (FP) voltage sensor that is a fusion of the Shaker potassium channel with yellow fluorescent protein (YFP), is primarily expressed in the endoplasmic reticulum (ER) of mammalian cells, possibly due to misfolded monomers. In an effort to improve plasma membrane expression, the FP was split into two non-fluorescent halves. Each half was randomly inserted into Shaker monomers via a transposon reaction. Shaker subunits containing the 5' half were co-expressed with Shaker subunits containing the 3' half. Tetramerization of Shaker subunits is required for re-conjugation of the FP. The misfolded monomers trapped in ER are unlikely to tetramerize and reconstitute the beta-can structure, and thus intracellular fluorescence might be reduced. This split-can transposon approach yielded 56 fluorescent probes, 30 (54%) of which were expressed at the plasma membrane and were capable of optically reporting changes in membrane potential. The largest signal from these novel FP-sensors was a -1.4% in ΔF/F for a 100 mV depolarization, with on time constants of about 15 ms and off time constants of about 200 ms. This split-can transposon approach has the potential to improve other multimeric probes.

Simultaneous illumination method: application in a two-channel emission fluorometer with multi-wavelength excitation

We developed a new illumination method called the simultaneous illumination method. This method does not require synchronization between light sources and sensor signals, which drastically simplifies the instrumentation. As a proof-of-concept, we applied this method to an oceanographic fluorometer. In principle, using this method, one can easily increase the number of characterized emission wavelengths by mounting optical sensors for as many emission wavelengths as needed. Our fluorometer has two emission-wavelength channels and twelve excitation wavelengths. The aim of this prototype is to demonstrate a viable in situ N-channel emission fluorometer with multiple wavelengths of excitation, which has not been previously realized.

Local establishment of repetitive long-term potentiation-induced synaptic enhancement in cultured hippocampal slices with divided input pathways

Long-term potentiation (LTP) in the rodent hippocampus is a popular model for synaptic plasticity, which is considered the cellular basis for brain memory. Because most LTP analysis involves acutely prepared brain slices, however, the longevity of single LTP has not been well documented. Using stable hippocampal slice cultures for long-term examination, we previously found that single LTP disappeared within 1 day. In contrast, repeated induction of LTP led to the development of a distinct type of plasticity that lasted for more than 3 weeks and was accompanied by the formation of new synapses. Naming this novel plastic phenomenon repetitive LTP-induced synaptic enhancement (RISE), we proposed it as a model for the cellular processes involved in long-term memory formation. However, because in those experiments LTP was induced pharmacologically in the whole slice, it is not known whether RISE has input-pathway specificity, an essential property for memory. In this study, we divided the input pathway of CA1 pyramidal neurons by a knife cut and induced LTP three times, the third by tetanic stimulation in one of the divided pathways to express RISE specifically. Voltage-sensitive dye imaging and Golgi-staining performed 2 weeks after the three LTP inductions revealed both enhanced synaptic strength and increased dendritic spine density confined to the tetanized region. These results demonstrate that RISE is a feasible cellular model for long-term memory.

Imaging voltage in neurons

In the last decades, imaging membrane potential has become a fruitful approach to study neural circuits, especially in invertebrate preparations with large, resilient neurons. At the same time, particularly in mammalian preparations, voltage imaging methods suffer from poor signal to noise and secondary side effects, and they fall short of providing single-cell resolution when imaging of the activity of neuronal populations. As an introduction to these techniques, we briefly review different voltage imaging methods (including organic fluorophores, SHG chromophores, genetic indicators, hybrid, nanoparticles, and intrinsic approaches) and illustrate some of their applications to neuronal biophysics and mammalian circuit analysis. We discuss their mechanisms of voltage sensitivity, from reorientation, electrochromic, or electro-optical phenomena to interaction among chromophores or membrane scattering, and highlight their advantages and shortcomings, commenting on the outlook for development of novel voltage imaging methods.

Intracellular long-wavelength voltage-sensitive dyes for studying the dynamics of action potentials in axons and thin dendrites

In CNS neurons most of synaptic integration takes place in thin dendritic branches that are difficult to study with conventional physiological recording techniques (electrodes). When cellular compartments are too small, or too many, for electrode recordings, optical methods bring considerable advantages. Here we focused our experimental effort on the development and utilization of new kinds of voltage-sensitive dyes (VSD). The new VSDs have bluish appearance in organic solvents, and hence are dubbed "blue dyes". They have preferred excitation windows for voltage recording that are shifted to longer wavelengths (approximately 660nm). Excitation in deep red light and emission in the near-infrared render "blue VSDs" potentially useful in measurements from fluorescent structures below the tissue surface because light scattering is minimized at longer wavelengths. Seven new molecules were systematically tested using intracellular injection. In comparison to the previously used red dye (JPW-3028) the blue dyes have better sensitivity (DeltaF/F) by approximately 40%. Blue dyes take little time to fill the dendritic tree, and in this aspect they are comparable with the fastest red dye JPW-3028. Based on our results, blue VSDs are well suited for experimental exploration of thin neuronal processes in semi intact preparations (brain slice). In some cases only six sweeps of temporal averaging were needed to acquire excellent records of individual action potentials in basal and oblique dendritic branches, or in axons and axon collaterals up to 200microm away from the cell body. Signal-to-noise ratio of these recordings was approximately 10. The combination of blue dyes and laser illumination approach imposed little photodynamic damage and allowed the total number of recording sweeps per cell to exceed 100. Using these dyes and a spot laser illumination technique, we demonstrate the first recording of action potentials in the oblique dendrite and distal axonal segment of the same pyramidal cell.

Fast wave propagation in auditory cortex of an awake cat using a chronic microelectrode array

We investigated fast wave propagation in auditory cortex of an alert cat using a chronically implanted microelectrode array. A custom, real-time imaging template exhibited wave dynamics within the 33-microwire array (3 mm(2)) during ten recording sessions spanning 1 month post implant. Images were based on the spatial arrangement of peri-stimulus time histograms at each recording site in response to auditory stimuli consisting of tone pips between 1 and 10 kHz at 75 dB SPL. Functional images portray stimulus-locked spiking activity and exhibit waves of excitation and inhibition that evolve during the onset, sustained and offset period of the tones. In response to 5 kHz, for example, peak excitation occurred at 27 ms after onset and again at 15 ms following tone offset. Variability of the position of the centroid of excitation during ten recording sessions reached a minimum at 31 ms post onset (sigma = 125 microm) and 18 ms post offset (sigma = 145 microm), suggesting a fine place/time representation of the stimulus in the cortex. The dynamics of these fast waves also depended on stimulus frequency, likely reflecting the tonotopicity in auditory cortex projected from the cochlea. Peak wave velocities of 0.2 m s(-1) were also consistent with those purported across horizontal layers of cat visual cortex. The fine resolution offered by microimaging may be critical for delivering optimal coding strategies used with an auditory prosthesis. Based on the initial results, future studies seek to determine the relevance of these waves to sensory perception and behavior.

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.

Genetic targeting of individual cells with a voltage-sensitive dye through enzymatic activation of membrane binding

Optical recording of the electrical activity of individual neurons in culture or in a tissue requires cell-selective staining with a fluorescent voltage-sensitive dye. In a proof-of-principle experiment, we implement a novel approach to genetically targeted staining. The method relies on a water-soluble precursor dye and an overexpressed cell-surface enzyme that transforms the precursor into a hydrophobic dye that binds to the targeted cell. We fused an alkaline phosphatase to a specifically designed general-purpose membrane anchor, and the fusion protein was expressed on the surface of HEK293 cells, as was corroborated by immuno- and histochemical staining. We next synthesised an amphiphilic hemicyanine dye containing two enzymatically cleavable phosphate groups at its hydrocarbon tails. When the phosphate groups were removed, the binding to membranes was enhanced by a factor of a thousand, as shown by titration with lipid vesicles. We observed selective staining of enzymatically active cells by fluorescence microscopy in a mixed population of phosphatase-transfected and untransfected HEK293 cells. The critical parameters of enzyme-induced cell-selective staining were elucidated by a simple kinetic model to guide further developments of the method.

VSDI: a new era in functional imaging of cortical dynamics

During the last few decades, neuroscientists have benefited from the emergence of many powerful functional imaging techniques that cover broad spatial and temporal scales. We can now image single molecules controlling cell differentiation, growth and death; single cells and their neurites processing electrical inputs and sending outputs; neuronal circuits performing neural computations in vitro; and the intact brain. At present, imaging based on voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution for imaging neocortical functions in the living brain, and has paved the way for a new era in the functional imaging of cortical dynamics. It has facilitated the exploration of fundamental mechanisms that underlie neocortical development, function and plasticity at the fundamental level of the cortical column.

Second harmonic imaging of membrane potential of neurons with retinal

We present a method to optically measure and image the membrane potential of neurons, using the nonlinear optical phenomenon of second harmonic generation (SHG) with a photopigment retinal as the chromophore [second harmonic retinal imaging of membrane potential (SHRIMP)]. We show that all-trans retinal, when adsorbed to the plasma membrane of living cells, can report on the local electric field via its change in SHG. Using a scanning mode-locked Ti-sapphire laser, we collect simultaneous two-photon excited fluorescence (TPEF) and SHG images of retinal-stained kidney cells and cultured pyramidal neurons. Patch clamp experiments on neurons stained with retinal show an increase of 25% in SHG intensity per 100-mV depolarization. Our data are the first demonstration of optical measurements of membrane potential of mammalian neurons with SHG. SHRIMP could have wide applicability in neuroscience and, by modifying rhodopsin, could in principle be subject for developing genetically engineered voltage sensors.

Cell-transistor hybrid systems and their potential applications

Electrogenic cells fire spontaneous or triggered action potentials (transient changes of their membrane potentials) and can be electronically coupled to external electrodes (arrays). Signals from rat heart-muscle cells were recorded by a field-effect transistor and the results described on the basis of an equivalent circuit. This technique has potential applications in drug screening, such as measuring the dose-response curve of isoproterenol, a beta-adrenergic agonist with a positive chronotropic effect.

Optical measurements of synchronized activity in isolated mammalian cerebellum

An experimental system that combines optical imaging of voltage-sensitive dyes with an in vitro isolated cerebellum preparation is described. The optical imaging system is based on a photodiode array and two rather simple amplification stages. The isolated cerebellum preparation preserves the integrity of the neuronal circuitry, thus allowing the exploration of the path of information flow. In this study, we characterize the nature and sources of the optical signal evoked in the cerebellar cortex by surface stimulation. We show that this signal reflects inhibitory and excitatory synaptic potentials generated by cell bodies and dendrites of cortical neurons, whereas action potentials of the parallel fibers are not detected by the system. The spatial distribution of the optical signals agrees with the classical view of cerebellar activity following surface stimulation. Hence, this experimental system provides a powerful means to explore the functional organization, in time and space, of the cerebellar cortex.

Detecting action potentials in neuronal populations with calcium imaging

The study of neural circuits requires methods for simultaneously recording the activity of populations of neurons. Here, using calcium imaging of neocortical brain slices we take advantage of the ubiquitous distribution of calcium channels in neurons to develop a method to reconstruct the action potentials occurring in a population of neurons. Combining calcium imaging with whole-cell or perforated patch recordings from neurons loaded with acetoxymethyl ester or potassium salt forms of calcium indicators, we demonstrate that each action potential produces a stereotyped calcium transient in the somata of pyramidal neurons. These signals are detectable without averaging, and the signal-to-noise is sufficient to carry out a reconstruction of the spiking pattern of hundreds of neurons, up to relatively high firing frequencies. This technique could in principle be applied systematically to follow the activity of neuronal populations in vitro and in vivo.

Voltage-sensitive dyes for monitoring multineuronal activity in the intact central nervous system

Optical monitoring of activity provides new kinds of information about brain function. Two examples are discussed in this article. First, the spike activity of many individual neurons in small ganglia can be determined. Second, the spatiotemporal characteristics of coherent activity in the brain can be directly measured. This article discusses both general characteristics of optical measurements (sources of noise) as well as more methodological aspects related to voltage-sensitive dye measurements from the nervous system.

Fast optical measurement of membrane potential changes at multiple sites on an individual nerve cell

In the past 15 years, there has been renewed interest in the detailed spatial analyses of signalling in individual neurons. The behaviour of many nerve cells is difficult to understand on the basis of microelectrode measurements from the soma. Regional electrical properties of neurons have been studied using sharp microelectrode and patch-electrode recordings from neuronal processes, high-resolution multisite optical recordings of Ca2+ concentration changes and by using models to predict the distribution of membrane potential in the entire neuronal arborization. Additional, direct evidence about electrical signalling in neuronal processes of individual cells in situ can now be obtained by recording of membrane potential changes using voltage-sensitive dyes. A number of recent studies have shown that active regional electrical properties of individual neurons are extraordinarily complex, dynamic and, in the general case, impossible to predict by present models. This places a great significance on measuring capabilities in experiments studying the detailed functional organization of individual neurons. The main difficulty in obtaining a more accurate description was that experimental techniques for studying regional electrical properties of neurons were not available. With this motivation, we worked on the development of multisite voltage-sensitive dye recording as a potentially powerful approach. The results described here demonstrate that the sensitivity of voltage-sensitive dye recording from branches of individual neurons was brought to a level at which it can be used routinely in physiologically relevant experiments. The crucial figure-of-merit in this approach, the signal-to-noise ratio from neuronal processes in intact ganglia, has been improved by a factor of roughly 150 over previously available signals. The improvement in the sensitivity allowed, for the first time, direct investigation of several important aspects of the functional organization of an individual neuron: (1) the direction and the velocity of action potential propagation in different neuronal processes in the neuropile was determined; and (2) the interaction of two independent action potentials (spike collision) was monitored directly in a neurite in the neuropile; (3) it was demonstrated that several action potentials are initiated in the same neuron at different sites (multiple spike trigger zones) by a single stimulus; (4) the exact location and the size of one of the remote spike trigger zones was determined; (5) the spread of passive subthreshold signals was followed in the neurites in the neuropile. This kind of information was not previously available. Preliminary experiments on vertebrate neurons indicate partial success in the effort to use intracellularly applied voltage-sensitive dyes to record from neurons in a mammalian brain slice preparation. The results suggest that, with further improvements, it may be possible to follow optically synaptic integration and spike conduction in the dendrites of vertebrate nerve cells. The main impact of these results is a demonstration of a new way of analysing how individual neurons are functionally organized. Limitations and prospects for the further refinement of the technique are discussed mostly in terms of the signal-to-noise ratio; both improvements in the apparatus and design of more sensitive dyes are addressed.

Cable properties of dendrites in hippocampal neurons of the rat mapped by a voltage-sensitive dye

Dendrites of pyramidal neurons from embryonic rat hippocampus are investigated in culture using a voltage-sensitive fluorescent dye. The electrical response to somatic stimulation is observed as a time-resolved map with a resolution of 0.9 microm at a time constant of 0.4 ms without signal averaging. The data are interpreted in terms of a tapering cable with Hodgkin-Huxley parametrization. The spread of short hyperpolarizing transients is damped by capacitive shunting. The invasion of an action potential is boosted by voltage-gated conductances of a low density. No irregularity is observed at a bifurcation. The passive cable parameters of internal resistance and membrane resistance at resting voltage are Ri = 300 omega cm and Rm = 40 (k)omega cm2 respectively, at a maximum sodium conductance of approximately 4.4 mS/cm2. The electrotonic length constant and the dynamic length constant at 1 kHz are 580 and 90 microm respectively. These results are compatible with electrophysiological data of dendrites in slices of adult hippocampus and with optical data of narrow processes of leech neurons in culture. The functional implications of boosting an action potential by voltage-gated channels of low density are considered.

Comparison of fluorescent voltage-sensitive dyes for multisite optical recording in hamster cerebral cortex by measurement of bicuculline-induced epileptiform events

Two fluorescent voltage-sensitive dyes, RH795 and DI-2-ANEPPQ, were compared for in vivo multisite optical recording from the gustatory insular cortex of the golden Syrian hamster, the first reported use of DI-2-ANEPPQ in a mammalian brain preparation. The exposed cortex of the anesthetized hamster was stained with a 500 microM solution of either dye, and the cortical surface was imaged onto a 124-element photodiode array by an epifluorescence microscope equipped with a 2.5x objective (0.08 na) and appropriate excitation and emission filters for each dye. Background fluorescence was recorded with amplifiers in DC-coupled mode, and the bathing solution changed to one containing 100 microM bicuculline methiodide. Large, widespread epileptiform events were recorded optically, and with a surface electrode, within 2 min. Three experiments were carried out with each dye. The 10 recorded events from each experiment (and five detector records from each of these 10) having the largest signals were selected for comparison. Five measures were derived from the data: (1) The ratio of peak signal fluorescence to background fluorescence (delta F/F), (2) signal-to-noise power ratio (S/N), (3) root mean square noise (RMS noise), (4) an amplitude ratio (AR) consisting of the signal height divided by RMS noise, and (5) the peak value of the surface electrode record (SER). The results indicate a twofold increase in delta F/F and a fivefold increase in S/N (twofold increase in AR) with DI-2-ANEPPQ. No significant difference was found between dyes in the RMS noise or SERs. In addition, signals did not decline detectably over time with DI-2-ANEPPQ, but declined about 25%/h with RH795.

Detection and modulation of acetylcholine release from neurites of rat basal forebrain cells in culture

1. Nicotinic acetylcholine (ACh) receptor-rich patches prepared from rat myotubes were used as focal ACh detectors to record the release of ACh from magnocellular basal forebrain (MBF) neurones from 11- to 14-day-old postnatal rats maintained in dissociated cell culture. 2. An action potential generated by intracellularly stimulating the MBF cell soma through a patch electrode induced a brief (mean tau(decay), 6.3 ms) short latency (1.35-5.1 ms; median 3.1 ms) burst of nicotinic channel openings in the detector patch when the latter was positioned at discrete loci along the MBF neurites. Detected ACh concentrations ranged from approximately 480 nM to > 50 microM. Concentrations increased markedly during the first 14 days in vitro and were inversely related to response latency. 3. Sites of release were generally confined to the more proximal neurites within 100 microm of the cell body and were invariably associated with the presence of small (2-3 microm diameter) phase-dark puncta located at discrete intervals along the length of the neurites or at points where short collaterals branched from the main process. Release was never detected from the cell soma except under extreme non-physiological conditions but could occasionally be elicited from growth cones at the ends of the shorter thicker neurites in the absence of a target cell. 4. Evoked release was abolished by tetrodotoxin (0.5 microM) and by superfusing with low Ca(2+)-high Mg(2+)-containing solutions (0.25 mM Ca(2+), 5 mM Mg(2+)). Myotube patch responses were antagonized by d-tubocurarine (3 microM). 5. Muscarine (10 microM) inhibited release by 70 +/- 3% (n = 12 cells). This effect was antagonized by 100 nM methoctramine but not by 100 nM pirenzepine, indicating that it was mediated by M(2) muscarinic ACh receptors. 6. These results indicate that ACh release from the processes of magnocellular cholinergic basal forebrain neurones arises from highly specialized and discrete sites, and that it can be inhibited through activation of muscarinic receptors. It is suggested that the latter results from inhibition of presynaptic Ca(2+) channels and that it might be responsible for feedback autoinhibition of ACh release from cortical afferents of nucleus basalis neurones in vivo.

Advantages of using microfabricated extracellular electrodes for in vitro neuronal recording

We describe fabrication methods and the characterisation and use of extracellular microelectrode arrays for the detection of action potentials from neurons in culture. The 100 microns2 platinised gold microelectrodes in the 64 electrode array detect the external current which flows during an action potential with S:N ratios of up to 500:1, giving a maximum recorded signal of several millivolts. The performance of these electrodes is enhanced if good sealing of the cells over the electrodes is obtained and further enhanced if the electrodes and the cells lie in a deep groove in the substratum. The electrodes can be used for both recording and stimulation of activity in cultured neurons and for recording from multiple sites on a single cell. The use of such electrodes to obtain recordings from invertebrate neurons is described. The particular advantages of these electrodes, their long term stability, non-invasive nature, high packing density, and utility in stimulation, are demonstrated.

Cable properties of a straight neurite of a leech neuron probed by a voltage-sensitive dye

We measured a time-resolved map of electrical activity in a thin straight neurite (1.5 microns thick, 500 microns long) at a resolution of 8 microns and 0.4 ms. The neurite was obtained by guided outgrowth of an identified neuron of the leech on lanes of extracellular matrix protein. The electrical signals were detected by a fluorescent voltage-sensitive dye. We observed the voltage that was caused by an action potential elicited at the soma and by a Gaussian hyperpolarization induced at the soma, respectively. We compared the data with numerical solutions of the cable equation using the Hodgkin-Huxley parametrization. We could attribute the experimental results of depolarization and of hyperpolarization to the propagation of an action potential along an "active" cable and to the spread along a "passive" cable, respectively, if we assigned rather high specific resistances to the cytoplasm (RI = 250 omega.cm) and to the membrane (RM = 22 k omega.cm2). This assignment explained the slow velocity of 150 microns/ms of a pulse by active propagation and the limited range of 200 microns of a pulse by passive spread.

Cable properties of arborized Retzius cells of the leech in culture as probed by a voltage-sensitive dye

Retzius cells of Hirudo medicinalis were cultivated on extracellular matrix protein so that extended arborizations were formed. The propagation of voltage transients along 1-microns-thick neurites was observed at a resolution of 8 microns at 10 kHz by use of a voltage-sensitive dye. Delay and width of the fluorescence transients caused by hyperpolarization of the soma are described by passive spread of voltage in a homogeneous cable (time constant, 10 ms; space constant, 320 microns). The local sensitivity of the dye was determined from a comparison of the amplitudes of fluorescence and of fitted voltage. The fluorescence transients caused by depolarization were scaled using the sensitivity profile. Action potentials were found to pervade the neurites without significant change of amplitude but with enhanced pulse width.

Optical imaging of architecture and function in the living brain sheds new light on cortical mechanisms underlying visual perception

Long standing questions related to brain mechanisms underlying perception can finally be resolved by direct visualization of the architecture and function of mammalian cortex. This advance has been accomplished with the aid of two optical imaging techniques with which one can literally see how the brain functions. The upbringing of this technology required a multi-disciplinary approach integrating brain research with organic chemistry, spectroscopy, biophysics, computer sciences, optics and image processing. Beyond the technological ramifications, recent research shed new light on cortical mechanisms underlying sensory perception. Clinical applications of this technology for precise mapping of the cortical surface of patients during neurosurgery have begun. Below is a brief summary of our own research and a description of the technical specifications of the two optical imaging techniques. Like every technique, optical imaging also suffers from severe limitations. Here we mostly emphasize some of its advantages relative to all alternative imaging techniques currently in use. The limitations are critically discussed in our recent reviews. For a series of other reviews, see Cohen (1989).

Noninvasive detection of changes in membrane potential in cultured neurons by light scattering

We report a procedure to detect electrical activity in cultured neurons by changes in their intrinsic optical properties. Using dark-field microscopy to detect scattered light, we observe an optical signal that is linearly proportional to the change in the membrane potential. Action potentials can be recorded without signal averaging. We use the dark-field method to show that there are substantial time delays between activity in the soma and in fine distal processes of identified Aplysia neurons. The biophysical basis for the change in optical properties of the neuron was deduced from measurements of the angular distribution of scattered laser light. An analysis of the data indicates that the radial component of the index of refraction of the membrane increases and the tangential components decrease concomitant with an increase in membrane potential. This is suggestive of a rapid reorientation of dipoles in the membrane during an action potential.

Optical measurement of conduction in single demyelinated axons

Demyelination was initiated in Xenopus sciatic nerves by an intraneural injection of lysolecithin over a 2-3-mm region. During the next week macrophages and Schwann cells removed all remaining damaged myelin by phagocytosis. Proliferating Schwann cells then began to remyelinate the axons, with the first few lamellae appearing 13 d after surgery. Action potentials were recorded optically through the use of a potential-sensitive dye. Signals could be detected both at normal nodes of Ranvier and within demyelinated segments. Before remyelination, conduction through the lesion occurred in only a small fraction of the fibers. However, in these particular cases we could demonstrate continuous (nonsaltatory) conduction at very low velocities over long (greater than one internode) lengths of demyelinated axons. We have previously found through loose patch clamp experiments that the internodal axolemma contains voltage-dependent Na+ channels at a density approximately 4% of that at the nodes. These channels alone, however, are insufficient for successful conduction past the transition point between myelinated and demyelinated regions. Small improvements in the passive cable properties of the axon, adequate for propagation at this site, can be realized through the close apposition of macrophages and Schwann cells. As the initial lamellae of myelin appear, the probability of success at the transition zone increases rapidly, though the conduction velocity through the demyelinated segment is not appreciably changed. A detailed computational model is used to test the relative roles of the internodal Na+ channels and the new extracellular layer. The results suggest a possible mechanism that may contribute to the spontaneous recovery of function often seen in demyelinating disease.

Optical recording of synaptic potentials from processes of single neurons using intracellular potentiometric dyes

To record post synaptic potentials or electrical activity from processes of single cells in a central nervous system (CNS) preparation in situ, voltage sensitive dyes can be injected intracellularly, thereby staining only the cell under investigation. We report the structure, evaluation, and synthesis of 11 fluorescent styryl dyes developed for iontophoretic injection. The optical signals that represent small synaptic potentials from single processes of iontophoretically injected cells are expected to be very small and, therefore, such measurements are not easy. We report the methodology that permitted the optical recording of action potentials from a 3-micron axon and the recording of small synaptic potentials from the processes of single cells in the segmental ganglia of the leech. The same dyes also proved useful for optical recording of action potentials of anterogradely labeled axons, following local extracellular injection at a remote site in a mammalian CNS preparation.

Regional distribution of calcium influx into bursting neurons detected with arsenazo III

Absorbance changes of the metallochromic indicator arsenazo III were used in conjunction with an array of 100 photodiodes to measure changes in intracellular calcium concentration at many positions simultaneously in identified neurons of the crab stomatogastric ganglion. When stimulated with intrasomatically injected current, several of these neurons showed calcium changes all over the cell, indicating that calcium channels were distributed widely in the neuropil and on the soma. When the membrane potential was allowed to oscillate without stimulation, absorbance oscillations were detected all over the neuropil but not in the soma. A comparison between the membrane potential recorded in the soma and the calcium signal in the neuropil shows that calcium entry followed the slow voltage oscillation with the peak calcium signal detected 50-150 msec after the end of the voltage plateau.

Morphological properties and membrane channels of the growth cones induced in PC12 cells by nerve growth factor

Large growth cones were produced in vitro by nerve growth factor (NGF) treatment of multinucleate cells produced by chemical fusion of cells of the neuron-like clone PC12. These endings were studied both at the light microscopic and ultrastructural levels. The activity of ionic channels at growth cones was recorded with intracellular microelectrodes, patch recording of single channels, and whole cone recording from mechanically isolated growth cones. Morphologically, these large growth cones were characterized by the presence of microspikes and filopodia, by the presence of actin demonstrated immunohistochemically, and by the presence of catecholamine fluorescence. At the ultrastructural level they contained a broad spectrum of organelles with a distribution characteristic of neuronal growth cones, including dense core vesicles, abundant smooth membrane cisternae, microtubules, and a filamentous network. The presence of channels capable of generating action potentials was revealed by intracellular microelectrode recording from the growth cone in the presence of locally applied tetraethylammonium (TEA). TEA appeared to block outward current channels that could effectively shunt inward current activated by depolarization. Action potentials elicited by depolarizing current in the presence of TEA could be blocked reversibly by Cd2+, a specific blocker of Ca channels. These action potentials were often followed by a long after-hyperpolarization lasting hundreds of milliseconds. This after-hyperpolarization was similar to that recorded in the cell body of PC12 cells where it appears to be mediated by Ca-activated K current. Single channel recording from outside-out excised patches of membrane from the growth cones perfused with KF revealed the presence of voltage sensitive Na channels, Ca-activated K channels, and K channels resembling delayed rectifier K channels. Macroscopic currents recorded from mechanically isolated growth cones in the "whole cone" configuration showed rapid inward currents at potentials greater than or equal to -40 mV, followed by delayed outward currents at more positive potentials, a finding providing additional evidence for the presence of Na and K channels in growth cones.

Fluorescence monitoring of electrical responses from small neurons and their processes

To improve the sensitivity of fluorescence measurements of electrical responses from small cells and their processes, we have optimized the optical measuring system. The fluorescence intensity from a stained cell was increased 40-fold relative to our previous apparatus. The increased fluorescence intensity permits the use of an inexpensive photodiode (or a photodiode array) that has a approximately 10-fold higher quantum efficiency relative to a photomultiplier. Utilizing the improved apparatus, we optically recorded an action potential of a 2 microns wide neuronal process with a signal-to-noise ratio of approximately 50 (root mean square noise) without averaging. We also report the design of an improved fluorescence voltage-sensitive probe; the fractional change of the fluorescence signal under optimal conditions was 21%/100 mV.