Combining voltage and calcium imaging from neuronal dendrites

Simultaneous recording of multiple cellular signaling events by frequency- and spectrally-tuned multiplexing of fluorescent probes

Fluorescent probes that change their spectral properties upon binding to small biomolecules, ions, or changes in the membrane potential (Vm) are invaluable tools to study cellular signaling pathways. Here, we introduce a novel technique for simultaneous recording of multiple probes at millisecond time resolution: frequency- and spectrally-tuned multiplexing (FASTM). Different from present multiplexing approaches, FASTM uses phase-sensitive signal detection, which renders various combinations of common probes for Vm and ions accessible for multiplexing. Using kinetic stopped-flow fluorimetry, we show that FASTM allows simultaneous recording of rapid changes in Ca2+, pH, Na+, and Vm with high sensitivity and minimal crosstalk. FASTM is also suited for multiplexing using single-cell microscopy and genetically encoded FRET biosensors. Moreover, FASTM is compatible with optochemical tools to study signaling using light. Finally, we show that the exceptional time resolution of FASTM also allows resolving rapid chemical reactions. Altogether, FASTM opens new opportunities for interrogating cellular signaling.

Measuring Absolute Membrane Potential Across Space and Time

Membrane potential (Vmem) is a fundamental biophysical signal present in all cells. Vmem signals range in time from milliseconds to days, and they span lengths from microns to centimeters. Vmem affects many cellular processes, ranging from neurotransmitter release to cell cycle control to tissue patterning. However, existing tools are not suitable for Vmem quantification in many of these areas. In this review, we outline the diverse biology of Vmem, drafting a wish list of features for a Vmem sensing platform. We then use these guidelines to discuss electrode-based and optical platforms for interrogating Vmem. On the one hand, electrode-based strategies exhibit excellent quantification but are most effective in short-term, cellular recordings. On the other hand, optical strategies provide easier access to diverse samples but generally only detect relative changes in Vmem. By combining the respective strengths of these technologies, recent advances in optical quantification of absolute Vmem enable new inquiries into Vmem biology.

Fluorescence lifetime imaging reveals regulation of presynaptic Ca2+ by glutamate uptake and mGluRs, but not somatic voltage in cortical neurons

Brain function relies on vesicular release of neurotransmitters at chemical synapses. The release probability depends on action potential-evoked presynaptic Ca2+ entry, but also on the resting Ca2+ level. Whether these basic aspects of presynaptic calcium homeostasis show any consistent trend along the axonal path, and how they are controlled by local network activity, remains poorly understood. Here, we take advantage of the recently advanced FLIM-based method to monitor presynaptic Ca2+ with nanomolar sensitivity. We find that, in cortical pyramidal neurons, action potential-evoked calcium entry (range 10-300 nM), but not the resting Ca2+ level (range 10-100 nM), tends to increase with higher order of axonal branches. Blocking astroglial glutamate uptake reduces evoked Ca2+ entry but has little effect on resting Ca2+ whereas both appear boosted by the constitutive activation of group 1/2 metabotropic glutamate receptors. We find no consistent effect of transient somatic depolarization or hyperpolarization on presynaptic Ca2+ entry or its basal level. The results unveil some key aspects of presynaptic machinery in cortical circuits, shedding light on basic principles of synaptic connectivity in the brain.

Optical measurement of physiological sodium currents in the axon initial segment

KEY POINTS

Τhe axonal Na⁺ fluorescence underlying an action potential in the axon initial segment was optically measured at unprecedented temporal resolution. The measurement allowed resolution of the kinetics of the Na⁺ current at different axonal locations. The distinct components of the Na⁺ current were correlated with the kinetics of the action potential. NEURON simulations from a modified published model qualitatively predicted the experimentally measured Na⁺ current. The present method permits the direct investigation of the kinetic behaviour of native Na⁺ channels under physiological and pathological conditions.

ABSTRACT

In most neurons of the mammalian central nervous system, the action potential (AP) is generated in the axon initial segment (AIS) by a fast Na⁺ current mediated by voltage-gated Na⁺ channels. While the axonal Na⁺ signal associated with the AP has been measured using fluorescent Na⁺ indicators, the insufficient resolution of these recordings has not allowed tracking the Na⁺ current kinetics underlying this fundamental event. In this article, we report the first optical measurement of Na⁺ currents in the AIS of pyramidal neurons of layer 5 of the somatosensory cortex from brain slices of the mouse. This measurement was obtained by achieving a temporal resolution of 100 μs in the Na⁺ imaging technique, with a pixel resolution of 0.5 μm, and by calculating the time-derivative of the Na⁺ change corrected for longitudinal diffusion. We identified a subthreshold current before the AP, a fast-inactivating current peaking during the rise of the AP and a non-inactivating current during the AP repolarization. We established a correlation between the kinetics of the non-inactivating current at different distances from the soma and the kinetics of the somatic AP. We quantitatively compared the experimentally measured Na⁺ current with the current obtained by computer simulation of published NEURON models, demonstrating how the present approach can lead to the correct estimate of the native behaviour of Na⁺ channels. Finally, we discuss how the present approach can be used to investigate the physiological or pathological function of different channel types during AP initiation and propagation.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Imaging Native Calcium Currents in Brain Slices

Imaging techniques may overcome the limitations of electrode techniques to measure locally not only membrane potential changes, but also ionic currents. Here, we review a recently developed approach to image native neuronal Ca²⁺ currents from brain slices. The technique is based on combined fluorescence recordings using low-affinity Ca²⁺ indicators possibly in combination with voltage sensitive dyes. We illustrate how the kinetics of a Ca²⁺ current can be estimated from the Ca²⁺ fluorescence change and locally correlated with the change of membrane potential, calibrated on an absolute scale, from the voltage fluorescence change. We show some representative measurements from the dendrites of CA1 hippocampal pyramidal neurons, from olfactory bulb mitral cells and from cerebellar Purkinje neurons. We discuss the striking difference in data analysis and interpretation between Ca²⁺ current measurements obtained using classical electrode techniques and the physiological currents obtained using this novel approach. Finally, we show how important is the kinetic information on the native Ca²⁺ current to explore the potential molecular targets of the Ca²⁺ flux from each individual Ca²⁺ channel.

Two Distinct Sets of Ca2+ and K+ Channels Are Activated at Different Membrane Potentials by the Climbing Fiber Synaptic Potential in Purkinje Neuron Dendrites

In cerebellar Purkinje neuron dendrites, the transient depolarization associated with a climbing fiber (CF) EPSP activates voltage-gated Ca²⁺ channels (VGCCs), voltage-gated K⁺ channels (VGKCs), and Ca²⁺-activated SK and BK K⁺ channels. The resulting membrane potential (V _m) and Ca²⁺ transients play a fundamental role in dendritic integration and synaptic plasticity of parallel fiber inputs. Here we report a detailed investigation of the kinetics of dendritic Ca²⁺ and K⁺ channels activated by CF-EPSPs, based on optical measurements of V _m and Ca²⁺ transients and on a single-compartment NEURON model reproducing experimental data. We first measured V _m and Ca²⁺ transients associated with CF-EPSPs at different initial V _m, and we analyzed the changes in the Ca²⁺ transients produced by the block of each individual VGCCs, of A-type VGKCs and of SK and BK channels. Then, we constructed a model that includes six active ion channels to accurately match experimental signals and extract the physiological kinetics of each channel. We found that two different sets of channels are selectively activated. When the dendrite is hyperpolarized, CF-EPSPs mainly activate T-type VGCCs, SK channels, and A-type VGKCs that limit the transient V _m ∼ <0 mV. In contrast, when the dendrite is depolarized, T-type VGCCs and A-type VGKCs are inactivated and CF-EPSPs activate P/Q-type VGCCs, high-voltage activated VGKCs, and BK channels, leading to Ca²⁺ spikes. Thus, the potentially activity-dependent regulation of A-type VGKCs, controlling the activation of this second set of channels, is likely to play a crucial role in signal integration and plasticity in Purkinje neuron dendrites.SIGNIFICANCE STATEMENT The climbing fiber synaptic input transiently depolarizes the dendrite of cerebellar Purkinje neurons generating a signal that plays a fundamental role in dendritic integration. This signal is mediated by two types of Ca²⁺ channels and four types of K⁺ channels. Thus, understanding the kinetics of all of these channels is crucial for understanding PN function. To obtain this information, we used an innovative strategy that merges ultrafast optical membrane potential and Ca²⁺ measurements, pharmacological analysis, and computational modeling. We found that, according to the initial membrane potential, the climbing fiber depolarizing transient activates two distinct sets of channels. Moreover, A-type K⁺ channels limit the activation of P/Q-type Ca²⁺ channels and associated K⁺ channels, thus preventing the generation of Ca²⁺ spikes.

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

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

A generalised method to estimate the kinetics of fast Ca(2+) currents from Ca(2+) imaging experiments

BACKGROUND

Fast Ca(2+) imaging using low-affinity fluorescent indicators allows tracking Ca(2+) neuronal influx at high temporal resolution. In some systems, where the Ca(2+)-bound indicator is linear with Ca(2+) entering the cell, the Ca(2+) current has same kinetics of the fluorescence time derivative. In other systems, like cerebellar Purkinje neuron dendrites, the time derivative strategy fails since fluorescence kinetics is affected by Ca(2+) binding proteins sequestering Ca(2+) from the indicator.

NEW METHOD

Our novel method estimates the kinetics of the Ca(2+) current in cells where the time course of fluorescence is not linear with Ca(2+) influx. The method is based on a two-buffer and two-indicator model, with three free parameters, where Ca(2+) sequestration from the indicator is mimicked by Ca(2+)-binding to the slower buffer. We developed a semi-automatic protocol to optimise the free parameters and the kinetics of the input current to match the experimental fluorescence change with the simulated curve of the Ca(2+)-bound indicator.

RESULTS

We show that the optimised input current is a good estimate of the real Ca(2+) current by validating the method both using computer simulations and data from real neurons. We report the first estimates of Ca(2+) currents associated with climbing fibre excitatory postsynaptic potentials in Purkinje neurons.

COMPARISON WITH EXISTING METHODS

The present method extends the possibility of studying Ca(2+) currents in systems where the existing time derivative approach fails.

CONCLUSIONS

The information available from our technique allows investigating the physiological behaviour of Ca(2+) channels under all possible conditions.

Functional coupling of diverse voltage-gated Ca(2+) channels underlies high fidelity of fast dendritic Ca(2+) signals during burst firing

KEY POINTS

In neurons, the Ca(2+) signal associated with the dendritic back-propagating action potential codes a chemical message to the different dendritic sites, playing a crucial role in electrical signalling, synaptic transmission and synaptic plasticity. The study of the underlying Ca(2+) current, mediated by different types of voltage-gated Ca(2+) channels, cannot be achieved by using the patch clamp technique. In this article, we used a recently developed cutting-edge optical technique to investigate the physiological behaviour of local Ca(2+) currents along the apical dendrite of CA1 hippocampal pyramidal neurons. We directly measure, for the first time, the synergistic activation and deactivation of the diverse dendritic voltage-gated Ca(2+) channels operating during bursts of back-propagating action potentials to precisely control the Ca(2+) signal. We demonstrate that the Ca(2+) loss via high-voltage-activated channels is compensated by the Ca(2+) entry via the other channels translating in high fidelity of Ca(2+) signalling.

ABSTRACT

In CA1 hippocampal pyramidal neurons, the dendritic Ca(2+) signal associated with somatic firing represents a fundamental activation code for several proteins. This signal, mediated by voltage-gated Ca(2+) channels (VGCCs), varies along the dendrites. In this study, using a recent optical technique based on the low-affinity indicator Oregon Green 488 BAPTA-5N, we analysed how activation and deactivation of VGCCs produced by back-propagating action potentials (bAPs) along the apical dendrite shape the Ca(2+) signal at different locations in CA1 hippocampal pyramidal neurons of the mouse. We measured, at multiple dendritic sites, the Ca(2+) transients and the changes in membrane potential associated with bAPs at 50 μs temporal resolution and we estimated the kinetics of the Ca(2+) current. We found that during somatic bursts, the bAPs decrease in amplitude along the apical dendrite but the amplitude of the associated Ca(2+) signal in the initial 200 μm dendritic segment does not change. Using a detailed pharmacological analysis, we demonstrate that this effect is due to the perfect compensation of the loss of Ca(2+) via high-voltage-activated (HVA) VGCCs by a larger Ca(2+) component via low-voltage-activated (LVA) VGCCs, revealing a mechanism coupling the two VGCC families of K(+) channels. More distally, where the bAP does not activate HVA-VGCCs, the Ca(2+) signal is variable during the burst. Thus, we demonstrate that HVA- and LVA-VGCCs operate synergistically to stabilise Ca(2+) signals associated with bAPs in the most proximal 200 μm dendritic segment.

Shift and Mean Algorithm for Functional Imaging with High Spatio-Temporal Resolution

Understanding neuronal physiology requires to record electrical activity in many small and remote compartments such as dendrites, axon or dendritic spines. To do so, electrophysiology has long been the tool of choice, as it allows recording very subtle and fast changes in electrical activity. However, electrophysiological measurements are mostly limited to large neuronal compartments such as the neuronal soma. To overcome these limitations, optical methods have been developed, allowing the monitoring of changes in fluorescence of fluorescent reporter dyes inserted into the neuron, with a spatial resolution theoretically only limited by the dye wavelength and optical devices. However, the temporal and spatial resolutive power of functional fluorescence imaging of live neurons is often limited by a necessary trade-off between image resolution, signal to noise ratio (SNR) and speed of acquisition. Here, I propose to use a Super-Resolution Shift and Mean (S&M) algorithm previously used in image computing to improve the SNR, time sampling and spatial resolution of acquired fluorescent signals. I demonstrate the benefits of this methodology using two examples: voltage imaging of action potentials (APs) in soma and dendrites of CA3 pyramidal cells and calcium imaging in the dendritic shaft and spines of CA3 pyramidal cells. I show that this algorithm allows the recording of a broad area at low speed in order to achieve a high SNR, and then pick the signal in any small compartment and resample it at high speed. This method allows preserving both the SNR and the temporal resolution of the signal, while acquiring the original images at high spatial resolution.

Using simultaneous voltage and calcium imaging to study fast Ca(2+) channels

The combination of fluorescence measurements of membrane potential and intracellular [Formula: see text] concentration allows correlating the electrical and calcium activity of a cell with spatial precision. The technical advances allowing this type of measurement were achieved only recently and represent an important step in the progress of the voltage imaging approach pioneered over 40 years ago by Lawrence B. Cohen. Here, we show how this approach can be used to investigate the function of [Formula: see text] channels using the foreseen possibility to extract [Formula: see text] currents from imaging experiments. The kinetics of the [Formula: see text] current, mediated by voltage-gated [Formula: see text] channels, can be accurately derived from the [Formula: see text] fluorescence measurement using [Formula: see text] indicators with [Formula: see text] that equilibrate in [Formula: see text]. In this respect, the imaging apparatus dedicated to this application is described in detail. Next, we illustrate the mathematical procedure to extract the current from the [Formula: see text] fluorescence change, including a method to calibrate the signal to charge flux density. Finally, we show an example of simultaneous membrane potential and [Formula: see text] optical measurement associated with an action potential at a CA1 hippocampal pyramidal neuron from a mouse brain slice. The advantages and limitations of this approach are discussed.

Contribution of sublinear and supralinear dendritic integration to neuronal computations

Nonlinear dendritic integration is thought to increase the computational ability of neurons. Most studies focus on how supralinear summation of excitatory synaptic responses arising from clustered inputs within single dendrites result in the enhancement of neuronal firing, enabling simple computations such as feature detection. Recent reports have shown that sublinear summation is also a prominent dendritic operation, extending the range of subthreshold input-output (sI/O) transformations conferred by dendrites. Like supralinear operations, sublinear dendritic operations also increase the repertoire of neuronal computations, but feature extraction requires different synaptic connectivity strategies for each of these operations. In this article we will review the experimental and theoretical findings describing the biophysical determinants of the three primary classes of dendritic operations: linear, sublinear, and supralinear. We then review a Boolean algebra-based analysis of simplified neuron models, which provides insight into how dendritic operations influence neuronal computations. We highlight how neuronal computations are critically dependent on the interplay of dendritic properties (morphology and voltage-gated channel expression), spiking threshold and distribution of synaptic inputs carrying particular sensory features. Finally, we describe how global (scattered) and local (clustered) integration strategies permit the implementation of similar classes of computations, one example being the object feature binding problem.

Imaging fast calcium currents beyond the limitations of electrode techniques

The current understanding of Ca(2+) channel function is derived from the use of the patch-clamp technique. In particular, the measurement of fast cellular Ca(2+) currents is routinely achieved using whole-cell voltage-clamp recordings. However, this experimental approach is not applicable to the study of local native Ca(2+) channels during physiological changes of membrane potential in complex cells, since the voltage-clamp configuration constrains the membrane potential to a given value. Here, we report for the first time to our knowledge that Ca(2+) currents from individual cells can be quantitatively measured beyond the limitations of the voltage-clamp approach using fast Ca(2+) imaging with low-affinity indicators. The optical measurement of the Ca(2+) current was correlated with the membrane potential, simultaneously measured with a voltage-sensitive dye to investigate the activation of Ca(2+) channels along the apical dendrite of the CA1 hippocampal pyramidal neuron during the back-propagation of an action potential. To validate the method, we analyzed the voltage dependence of high- and low-voltage-gated Ca(2+) channels. In particular, we measured the Ca(2+) current component mediated by T-type channels, and we investigated the mechanisms of recovery from inactivation of these channels. This method is expected to become a reference approach to investigate Ca(2+) channels in their native physiological environment.

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.

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.

Combined optogenetics and voltage sensitive dye imaging at single cell resolution

Information processing in the central nervous system makes use of densely woven networks of neurons with complex dendritic and axonal arborizations. Studying signaling in such a network requires precise control over the activity of specific neurons and an understanding how the synaptic signals are integrated. We established a system using a recently published red-shifted voltage sensitive dye in slices from mice expressing channelrhodopsin (Ch) in GABAergic neurons. Using a focused 473 nm laser for Ch activation and 635 nm laser wide field illumination for voltage sensitive dye excitation we were able to simultaneously measure dendritic voltage transients and stimulate inhibitory synaptic connections. The combination of these techniques provides excellent spatiotemporal control over neuron activation and high resolution information on dendritic signal processing.

Firing of hippocampal neurogliaform cells induces suppression of synaptic inhibition

Little is known about how neuron firing recorded in vivo retrogradely influences synaptic strength. We injected the firing of a rat hippocampal neurogliaform cell (NGFC), a widely expressed GABAergic neuron type, detected in vivo during theta rhythm, into NGFCs of rat or neuronal nitric oxide synthase (nNOS)-Cre-tdTomato mouse recorded in vitro. We found that the "in vivo firing pattern" produced a transient firing-induced suppression of synaptic inhibition (FSI) evoked by a presynaptic NGFC. Imaging experiments demonstrate that FSI was associated with action potential backpropagation (bAP) and a supralinear increase in dendritic Ca(2+). The application of the L-type Ca(2+) channel antagonist nimodipine blocked FSI. Further pharmacological experiments, such as the application of a nitric oxide-sensitive guanylyl cyclase (NO-sGC) receptor antagonist, a NOS inhibitor, and NO donors, suggested that NO released from postsynaptic cells mediated FSI and likely activated presynaptic receptors to inhibit GABA release. The in vivo firing pattern modulated the size of unitary EPSPs impinging on NGFCs through FSI and not via a direct effect on excitatory synaptic transmission. Our data demonstrate: (1) retrograde signaling initiated by in vivo firing pattern, (2) interneuron bAPs detected with fast temporal resolution, and (3) a novel role for NO expressed by specific interneuron types.

Combining calcium imaging with other optical techniques

Ca(2+) imaging is a commonly used approach for measuring Ca(2+) signals at high spatial resolution. The method is often combined with electrode recordings to correlate electrical and chemical signals or to investigate Ca(2+) signals following an electrical stimulation. To obtain information on electrical activity at the same spatial resolution, Ca(2+) imaging must be combined with membrane potential imaging. Similarly, stimulation of subcellular compartments requires photostimulation. Thus, combining Ca(2+) imaging with an additional optical technique facilitates the study of a number of physiological questions. The aim of this article is to introduce some basic principles regarding the combination of Ca(2+) imaging with other optical techniques. We discuss the design of the optics, the design of experimental protocols, the optical characteristics of Ca(2+) indicators used in combination with an optical probe, and the affinity of the Ca(2+) indicator in relation to the type of measurement. This information will enable the reader to devise an optimal strategy for combined optical experiments.

Combining Ca2+ and membrane potential imaging in single neurons

The ability to monitor Ca(2+) signals and membrane potential simultaneously at multiple locations on the same neuron facilitates further progress in our understanding of neuronal function. In particular, this method allows correlation of electrical and chemical signals from multiple sites, including those inaccessible to microelectrodes. This protocol describes a procedure for loading cells with two indicators, a Ca(2+)-sensitive Fura dye and voltage-sensitive JPW1114, together with the equipment required for detecting and imaging the two signals. Potential problems are discussed as well as the capabilities and limitations of the technique.

Hippocampal feedforward inhibition focuses excitatory synaptic signals into distinct dendritic compartments

Feedforward inhibition controls the time window for synaptic integration and ensures temporal precision in cortical circuits. There is little information whether feedforward inhibition affects neurons uniformly, or whether it contributes to computational refinement within the dendritic tree. Here we demonstrate that feedforward inhibition crucially shapes the integration of synaptic signals in pyramidal cell dendrites. Using voltage-sensitive dye imaging we studied the transmembrane voltage patterns in CA1 pyramidal neurons after Schaffer collateral stimulation in acute brain slices from mice. We observed a high degree of variability in the excitation-inhibition ratio between different branches of the dendritic tree. Many dendritic segments showed no depolarizing signal at all, especially the basal dendrites that received predominantly inhibitory signals. Application of the GABA_A receptor antagonist bicuculline resulted in the spread of depolarizing signals throughout the dendritic tree. Tetanic stimulation of Schaffer collateral inputs induced significant alterations in the patterns of excitation/inhibition, indicating that they are modified by synaptic plasticity. In summary, we show that feedforward inhibition restricts the occurrence of depolarizing signals within the dendritic tree of CA1 pyramidal neurons and thus refines signal integration spatially.

Light sources and cameras for standard in vitro membrane potential and high-speed ion imaging

Membrane potential and fast ion imaging are now standard optical techniques routinely used to record dynamic physiological signals in several preparations in vitro. Although detailed resolution of optical signals can be improved by confocal or two-photon microscopy, high spatial and temporal resolution can be obtained using conventional microscopy and affordable light sources and cameras. Thus, standard wide-field imaging methods are still the most common in research laboratories and can often produce measurements with a signal-to-noise ratio that is superior to other optical approaches. This paper seeks to review the most important instrumentation used in these experiments, with particular reference to recent technological advances. We analyse in detail the optical constraints dictating the type of signals that are obtained with voltage and ion imaging and we discuss how to use this information to choose the optimal apparatus. Then, we discuss the available light sources with specific attention to light emitting diodes and solid state lasers. We then address the current state-of-the-art of available charge coupled device, electron multiplying charge coupled device and complementary metal oxide semiconductor cameras and we analyse the characteristics that need to be taken into account for the choice of optimal detector. Finally, we conclude by discussing prospective future developments that are likely to further improve the quality of the signals expanding the capability of the techniques and opening the gate to novel applications.

Two-photon compatibility and single-voxel, single-trial detection of subthreshold neuronal activity by a two-component optical voltage sensor

Minimally invasive measurements of neuronal activity are essential for understanding how signal processing is performed by neuronal networks. While optical strategies for making such measurements hold great promise, optical sensors generally lack the speed and sensitivity necessary to record neuronal activity on a single-trial, single-neuron basis. Here we present additional biophysical characterization and practical improvements of a two-component optical voltage sensor (2cVoS), comprised of the neuronal tracer dye, DiO, and dipicrylamine (DiO/DPA). Using laser spot illumination we demonstrate that membrane potential-dependent fluorescence changes can be obtained in a wide variety of cell types within brain slices. We show a correlation between membrane labeling and the sensitivity of the magnitude of fluorescence signal, such that neurons with the brightest membrane labeling yield the largest ΔF/F values per action potential (AP; ∼40%). By substituting a blue-shifted donor for DiO we confirm that DiO/DPA works, at least in part, via a Förster resonance energy transfer (FRET) mechanism. We also describe a straightforward iontophoretic method for labeling multiple neurons with DiO and show that DiO/DPA is compatible with two-photon (2P) imaging. Finally, exploiting the high sensitivity of DiO/DPA, we demonstrate AP-induced fluorescence transients (fAPs) recorded from single spines of hippocampal pyramidal neurons and single-trial measurements of subthreshold synaptic inputs to granule cell dendrites. Our findings suggest that the 2cVoS, DiO/DPA, enables optical measurements of trial-to-trial voltage fluctuations with very high spatial and temporal resolution, properties well suited for monitoring electrical signals from multiple neurons within intact neuronal networks.

Somatic membrane potential and Kv1 channels control spike repolarization in cortical axon collaterals and presynaptic boutons

The shape of action potentials invading presynaptic terminals, which can vary significantly from spike waveforms recorded at the soma, may critically influence the probability of synaptic neurotransmitter release. Revealing the conductances that determine spike shape in presynaptic boutons is important for understanding how changes in the electrochemical context in which a spike is generated, such as subthreshold depolarization spreading from the soma, can modulate synaptic strength. Utilizing recent improvements in the signal-to-noise ratio of voltage-sensitive dye imaging in mouse brain slices, we demonstrate that intracortical axon collaterals and en passant presynaptic terminals of layer 5 pyramidal cells exhibit a high density of Kv1 subunit-containing ion channels, which generate a slowly inactivating K(+) current critically important for spike repolarization in these compartments. Blockade of the current by low doses of 4-aminopyridine or α-dendrotoxin dramatically slows the falling phase of action potentials in axon collaterals and presynaptic boutons. Furthermore, subthreshold depolarization of the soma broadened action potentials in collaterals bearing presynaptic boutons, an effect abolished by blocking Kv1 channels with α-dendrotoxin. These results indicate that action potential-induced synaptic transmission may operate through a mix of analog-digital transmission owing to the properties of Kv1 channels in axon collaterals and presynaptic boutons.

Combining membrane potential imaging with L-glutamate or GABA photorelease

Combining membrane potential imaging using voltage sensitive dyes with photolysis of l-glutamate or GABA allows the monitoring of electrical activity elicited by the neurotransmitter at different sub-cellular sites. Here we describe a simple system and some basic experimental protocols to achieve these measurements. We show how to apply the neurotransmitter and how to vary the dimension of the area of photolysis. We assess the localisation of photolysis and of the recorded membrane potential changes by depolarising the dendrites of cerebellar Purkinje neurons with l-glutamate photorelease using different experimental protocols. We further show in the apical dendrites of CA1 hippocampal pyramidal neurons how l-glutamate photorelease can be used to calibrate fluorescence changes from voltage sensitive dyes in terms of membrane potential changes (in mV) and how GABA photorelease can be used to investigate the phenomenon of shunting inhibition. We also show how GABA photorelease can be used to measure chloride-mediated changes of membrane potential under physiological conditions originating from different regions of a neuron, providing important information on the local intracellular chloride concentrations. The method and the proof of principle reported here open the gateway to a variety of important applications where the advantages of this approach are necessary.

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.

Bulk electroporation and population calcium imaging in the adult mammalian retina

The optical recording of light-evoked activity in populations of neurons in the mammalian retina offers several benefits over the use of multielectrode arrays. However, population imaging has been hindered by the effective loading of synthetic fluorescent indicators, especially in the mature tissue. We have therefore developed an electroporation method to label the complete ganglion cell layer of the adult mammalian retina. We optimized the protocol such that the retina recovers from electroporation and generates responses to visual stimuli. The method can be used with a diverse set of indicators with a range of affinities and emission wavelengths. It therefore can be combined with transgenic animals expressing fluorescent markers to target specific neuronal types. Importantly, the ganglion cell layer remains accessible for subsequent intracellular recording and morphological identification.

Second-harmonic generation voltage imaging at subcellular resolution in rat hippocampal slices

Action potential (AP) is a major signaling mechanism in the neuronal networks. Dendritic AP propagation is important for information processing within the individual neurons. Due to limitations of electrode-based techniques most research on subcellular AP propagation has been restricted to soma and proximal parts of the primary dendrites. Development of voltage-sensitive dyes (VSD) has opened up a possibility to measure voltage changes in the oblique dendrites and the spines. Membrane-bound organic VSD can be used both for fluorescent imaging and imaging of second-harmonic generation (SHG). Both phenomena are voltage dependent and can be used for measuring membrane potential changes. However, changes in SHG are linear to the change in the local membrane potential and its slope is constant across different compartments of cells. Although SHG demonstrates reasonable change with membrane voltage (over 10% per 100 mV), the signal-to-noise (S/N) ratio is currently lower in SHG measurement than in fluorescent methods.

High-resolution simultaneous voltage and Ca2+ imaging

Combining voltage and Ca(2+) imaging allows the correlation of electrical and chemical activity at sub-cellular level. Here we describe a novel apparatus designed to obtain simultaneous voltage and Ca(2+) measurements with single-trial resolution from sites as small as a few microns. These measurements can be obtained with negligible optical cross-talk between the two signals and negligible photo-damage of the preparation. The capability of the technique was assessed recording either from individual neurons in brain slices or from networks of cultured neurons. The present achievements open the gate to many novel physiological investigations requiring simultaneous measurement of voltage and Ca(2+) signals.

Imaging inhibitory synaptic potentials using voltage sensitive dyes

Studies of the spatio-temporal distribution of inhibitory postsynaptic potentials (IPSPs) in a neuron have been limited by the spatial information that can be obtained by electrode recordings. We describe a method that overcomes these limitations by imaging IPSPs with voltage-sensitive dyes. CA1 hippocampal pyramidal neurons from brain slices were loaded with the voltage-sensitive dye JPW-1114 from a somatic patch electrode in whole-cell configuration. After removal of the patch electrode, we found that neurons recover their physiological intracellular chloride concentration. Using an improved voltage-imaging technique, dendritic GABAergic IPSPs as small as 1 mV could be resolved optically from multiple sites with spatial averaging. We analyzed the sensitivity of the technique, in relation to its spatial resolution. We monitored the origin and the spread of IPSPs originating in different areas of the apical dendrite and reconstructed their spatial distribution. We achieved a clear discrimination of IPSPs from the dendrites and from the axon. This study indicates that voltage imaging is a uniquely suited approach for the investigation of several fundamental aspects of inhibitory synaptic transmission that require spatial information.

Action potentials initiate in the axon initial segment and propagate through axon collaterals reliably in cerebellar Purkinje neurons

Purkinje neurons are the output cells of the cerebellar cortex and generate spikes in two distinct modes, known as simple and complex spikes. Revealing the point of origin of these action potentials, and how they conduct into local axon collaterals, is important for understanding local and distal neuronal processing and communication. By using a recent improvement in voltage-sensitive dye imaging technique that provided exceptional spatial and temporal resolution, we were able to resolve the region of spike initiation as well as follow spike propagation into axon collaterals for each action potential initiated on single trials. All fast action potentials, for both simple and complex spikes, whether occurring spontaneously or in response to a somatic current pulse or synaptic input, initiated in the axon initial segment. At discharge frequencies of less than approximately 250 Hz, spikes propagated faithfully through the axon and axon collaterals, in a saltatory manner. Propagation failures were only observed for very high frequencies or for the spikelets associated with complex spikes. These results demonstrate that the axon initial segment is a critical decision point in Purkinje cell processing and that the properties of axon branch points are adjusted to maintain faithful transmission.

Somatic Ca2+ signaling in cerebellar Purkinje neurons

Activity-driven Ca(2+) signaling plays an important role in a number of neuronal functions, including neuronal growth, differentiation, and plasticity. Both cytosolic and nuclear Ca(2+) has been implicated in these functions. In the current study, we investigated membrane-to-nucleus Ca(2+) signaling in cerebellar Purkinje neurons in culture to gain insight into the pathways and mechanisms that can initiate nuclear Ca(2+) signaling in this neuronal type. Purkinje neurons are known to express an abundance of Ca(2+) signaling molecules such as voltage-gated Ca(2+) channels, ryanodine receptors, and IP3 receptors. Results show that membrane depolarization evoked by brief stimulation with K(+) saline elicits a prominent Ca(2+) signal in the cytosol and nucleus of the Purkinje neurons. Ca(2+) influx through P/Q- and L-type voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores contributed to the Ca(2+) signal, which spread from the plasma membrane to the nucleus. At strong K(+) stimulations, the amplitude of the nuclear Ca(2+) signal exceeded that of the cytosolic Ca(2+) signal, suggesting the involvement of a nuclear amplification mechanism and/or differences in Ca(2+) buffering in these two cellular compartments. An enhanced nuclear Ca(2+) signal was more prominent for Ca(2+) signals elicited by membrane depolarization than for Ca(2+) signals elicited by activation of the metabotropic glutamate receptor pathway (mGluR1), which is linked to Ca(2+) release from intracellular stores controlled by the IP3 receptor.

Spike integration and cellular memory in a rhythmic network from Na+/K+ pump current dynamics

The output of a neural circuit results from an interaction between the intrinsic properties of neurons within the circuit and the features of the synaptic connections between them. The plasticity of intrinsic properties has been primarily attributed to modification of ion channel function and/or number. In this study, we demonstrate a mechanism for intrinsic plasticity in rhythmically active Drosophila neurons that is not conductance-based. Larval motor neurons show a long lasting sodium-dependent afterhyperpolarization (AHP) following bursts of action potentials that is mediated by the electrogenic activity of Na⁺/K⁺ ATPase. This AHP persists for multiple seconds following volleys of action potentials and is able to function as a pattern-insensitive integrator of spike number that is independent of external calcium. This current also interacts with endogenous Shal K⁺ conductances to modulate spike timing for multiple seconds following rhythmic activity, providing a cellular memory of network activity on a behaviorally relevant time scale.

Neuregulin signaling is dispensable for NMDA- and GABA(A)-receptor expression in the cerebellum in vivo

Neuregulin-1s (NRG-1s) are a family of growth and differentiation factors with multiple roles in the development and function in different organs including the nervous system. Among the proposed functions of NRG-1s in the nervous system is the regulation of genes encoding certain neurotransmitter receptors during synapse formation as well as of other aspects of synaptic function. Here, we have examined, in granule cells of the cerebellum in vivo, the role of NRGs in the induction of NMDA receptor (NMDA-R) and GABA(A) receptor (GABA(A)-R), which are thought to be induced by NRG-1 secreted by the synaptic inputs. To this end, we used the Cre/loxP system to genetically ablate the NRG receptors ErbB2 and ErbB4 selectively in these cells, thus eliminating all NRG-mediated signaling to them. Unlike previous reports using cultured granule cells to address the same question, we found that the developmental expression patterns of the mRNAs encoding the NR2C subunit of the NMDA-R and the beta2-subunit of the GABA(A)-R is normal in mice lacking the NRG receptors ErbB2 and ErbB4. Likewise, no alterations in cerebellar morphology nor in certain aspects of cerebellar wiring were resolved in these mutants. We conclude that NRG/ErbB signaling to the granule cells is dispensable for the normal development of their synaptic inputs.

Dendritic spike saturation of endogenous calcium buffer and induction of postsynaptic cerebellar LTP

The architecture of parallel fiber axons contacting cerebellar Purkinje neurons retains spatial information over long distances. Parallel fiber synapses can trigger local dendritic calcium spikes, but whether and how this calcium signal leads to plastic changes that decode the parallel fiber input organization is unknown. By combining voltage and calcium imaging, we show that calcium signals, elicited by parallel fiber stimulation and mediated by voltage-gated calcium channels, increase non-linearly during high-frequency bursts of electrically constant calcium spikes, because they locally and transiently saturate the endogenous buffer. We demonstrate that these non-linear calcium signals, independently of NMDA or metabotropic glutamate receptor activation, can induce parallel fiber long-term potentiation. Two-photon imaging in coronal slices revealed that calcium signals inducing long-term potentiation can be observed by stimulating either the parallel fiber or the ascending fiber pathway. We propose that local dendritic calcium spikes, evoked by synaptic potentials, provide a unique mechanism to spatially decode parallel fiber signals into cerebellar circuitry changes.