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

Fxr1 Deletion from Cortical Parvalbumin Interneurons Modifies Their Excitatory Synaptic Responses

Fragile X autosomal homolog 1 (FXR1), a member of the fragile X messenger riboprotein 1 family, has been linked to psychiatric disorders including autism and schizophrenia. Parvalbumin (PV) interneurons play critical roles in cortical processing and have been implicated in FXR1-linked mental illnesses. Targeted deletion of FXR1 from PV interneurons in mice has been shown to alter cortical excitability and elicit schizophrenia-like behavior. This indicates that FXR1 regulates behaviorally relevant electrophysiological functions in PV interneurons. We therefore expressed a genetically encoded hybrid voltage sensor in PV interneurons and used voltage imaging in slices of mouse somatosensory cortex to assess the impact of targeted FXR1 deletion. These experiments showed that PV interneurons lacking FXR1 had excitatory synaptic potentials with larger amplitudes and shorter latencies compared with wild type. Synaptic potential rise-times, decay-times, and half-widths were also impacted to degrees that varied between cortical layer and synaptic input. Thus, FXR1 modulates the responsiveness of PV interneurons to excitatory synaptic inputs. This will enable FXR1 to control cortical processing in subtle ways, with the potential to influence behavior and contribute to psychiatric dysfunction.

Inter and intralaminar excitation of parvalbumin interneurons in mouse barrel cortex

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

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.

Inter and Intralaminar Excitation of Parvalbumin Interneurons in Mouse Barrel Cortex

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

Unitary synaptic responses of parvalbumin interneurons evoked by excitatory neurons in the mouse barrel cortex

The mammalian cortex integrates and processes information to transform sensory inputs into perceptions and motor outputs. These operations are performed by networks of excitatory and inhibitory neurons distributed through the cortical layers. Parvalbumin interneurons (PVIs) are the most abundant type of inhibitory cortical neuron. With axons projecting within and between layers, PVIs supply feedforward and feedback inhibition to control and modulate circuit function. Distinct populations of excitatory neurons recruit different PVI populations, but the specializations of these synapses are poorly understood. Here, we targeted a genetically encoded hybrid voltage sensor to PVIs and used fluorescence imaging in mouse somatosensory cortex slices to record their voltage changes. Stimulating a single visually identified excitatory neuron with small-tipped theta-glass electrodes depolarized multiple PVIs, and a common threshold suggested that stimulation elicited unitary synaptic potentials in response to a single excitatory neuron. Excitatory neurons depolarized PVIs in multiple layers, with the most residing in the layer of the stimulated neuron. Spiny stellate cells depolarized PVIs more strongly than pyramidal cells by up to 77%, suggesting a greater role for stellate cells in recruiting PVI inhibition and controlling cortical computations. Response half-width also varied between different excitatory inputs. These results demonstrate functional differences between excitatory synapses on PVIs.

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

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

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

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

Direct synaptic excitation between hilar mossy cells revealed with a targeted voltage sensor

The dentate gyrus not only gates the flow of information into the hippocampus, it also integrates and processes this information. Mossy cells (MCs) are a major type of excitatory neuron strategically located in the hilus of the dentate gyrus where they can contribute to this processing through networks of synapses with inhibitory neurons and dentate granule cells. Some prior work has suggested that MCs can form excitatory synapses with other MCs, but the role of these synapses in the network activity of the dentate gyrus has received little attention. Here, we investigated synaptic inputs to MCs in mouse hippocampal slices using a genetically encoded hybrid voltage sensor (hVOS) targeted to MCs by Cre-lox technology. This enabled optical recording of voltage changes from multiple MCs simultaneously. Stimulating granule cells and CA3 pyramidal cells activated well-established inputs to MCs and elicited synaptic responses as expected. However, the weak blockade of MC responses to granule cell layer stimulation by DCG-IV raised the possibility of another source of excitation. To evaluate synapses between MCs as this source, single MCs were stimulated focally. Stimulation of one MC above its action potential threshold evoked depolarizing responses in neighboring MCs that depended on glutamate receptors. Short latency responses of MCs to other MCs did not depend on release from granule cell axons. However, granule cells did contribute to the longer latency responses of MCs to stimulation of other MCs. Thus, MCs transmit their activity to other MCs both through direct synaptic coupling and through polysynaptic coupling with dentate granule cells. MC-MC synapses can redistribute information entering the dentate gyrus and thus shape and modulate the electrical activity underlying hippocampal functions such as navigation and memory, as well as excessive excitation during seizures.

Activity of a subset of vesicular GABA-transporter neurons in the ventral zona incerta anticipates sleep onset

STUDY OBJECTIVES

Sleep and wake are opposing behavioral states controlled by the activity of specific neurons that need to be located and mapped. To better understand how a waking brain falls asleep it is necessary to identify activity of individual phenotype-specific neurons, especially neurons that anticipate sleep onset. In freely behaving mice, we used microendoscopy to monitor calcium (Ca2+) fluorescence in individual hypothalamic neurons expressing the vesicular GABA transporter (vGAT), a validated marker of GABA neurons.

METHODS

vGAT-Cre mice (male = 3; female = 2) transfected with rAAV-FLEX-GCaMP6M in the lateral hypothalamus were imaged 30 days later during multiple episodes of waking (W), non-rapid-eye movement sleep (NREMS) or REMS (REMS).

RESULTS

372 vGAT neurons were recorded in the zona incerta. 23.9% of the vGAT neurons showed maximal fluorescence during wake (classified as wake-max), 4% were NREM-max, 56.2% REM-max, 5.9% wake/REM max, while 9.9% were state-indifferent. In the NREM-max group, Ca2+ fluorescence began to increase before onset of NREM sleep, remained high throughout NREM sleep, and declined in REM sleep.

CONCLUSIONS

We found that 60.2% of the vGAT GABA neurons in the zona incerta had activity that was biased towards sleep (NREM and REMS). A subset of vGAT neurons (NREM-max) became active in advance of sleep onset and may induce sleep by inhibiting the activity of the arousal neurons. Abnormal activation of the NREM-max neurons may drive sleep attacks and hypersomnia.

Genetically Encoded Voltage Indicators

Optogenetic approaches combine the power to allocate optogenetic tools (proteins) to specific cell populations (defined genetically or functionally) and the use of light-based interfaces between biological wetware (cells and tissues) and hardware (controllers and recorders). The optogenetic toolbox contains two main compartments: tools to interfere with cellular processes and tools to monitor cellular events. Among the latter are genetically encoded voltage indicators (GEVIs). This chapter outlines the development, current state of the art and prospects of emerging optical GEVI imaging technologies.

Parvalbumin-Positive Neuron Loss and Amyloid-β Deposits in the Frontal Cortex of Alzheimer's Disease-Related Mice

Alzheimer's disease (AD) has several hallmark features including amyloid-β (Aβ) plaque deposits and neuronal loss. Here, we characterized Aβ plaque aggregation and parvalbumin-positive (PV) GABAergic neurons in 6-9-month-old 5xFAD mice harboring mutations associated with familial AD. We used immunofluorescence staining to compare three regions in the frontal cortex-prelimbic (PrL), cingulate (Cg, including Cg1 and Cg2), and secondary motor (M2) cortices-along with primary somatosensory (S1) cortex. We quantified the density of Aβ plaques, which showed significant laminar and regional vulnerability. There were more plaques of larger sizes in deep layers compared to superficial layers. Total plaque burden was higher in frontal regions compared to S1. We also found layer- and region-specific differences across genotype in the density of PV interneurons. PV neuron density was lower in 5xFAD mice than wild-type, particularly in deep layers of frontal regions, with Cg (-50%) and M2 (-39%) exhibiting the largest reduction. Using in vivo two-photon imaging, we longitudinally visualized the loss of frontal cortical PV neurons across four weeks in the AD mouse model. Overall, these results provide information about Aβ deposits and PV neuron density in a widely used mouse model for AD, implicating deep layers of frontal cortical regions as being especially vulnerable.

Two-Photon Voltage Imaging of Spontaneous Activity from Multiple Neurons Reveals Network Activity in Brain Tissue

Recording the electrical activity of multiple neurons simultaneously would greatly facilitate studies on the function of neuronal circuits. The combination of the fast scanning by random-access multiphoton microscopy (RAMP) and the latest two-photon-compatible high-performance fluorescent genetically encoded voltage indicators (GEVIs) has enabled action potential detection in deep layers in in vivo brain. However, neuron connectivity analysis on optically recorded action potentials from multiple neurons in brain tissue has yet to be achieved. With high expression of a two-photon-compatible GEVI, ASAP3, via in utero electroporation and RAMP, we achieved voltage recording of spontaneous activities from multiple neurons in brain slice. We provide evidence for the developmental changes in intralaminar horizontal connections in somatosensory cortex layer 2/3 with a greater sensitivity than calcium imaging. This method thus enables investigation of neuronal network connectivity at the cellular resolution in brain tissue.

Recent Progress of Hybrid Optical Probes for Neural Membrane Potential Imaging

The neural membrane potential of nerve cells is the basis of neural activity production, which controls advanced brain activities such as memory, emotion, and learning. In the past decades, optical voltage indicator has emerged as a promising tool to decode neural activities with high-fidelity and excellent spatiotemporal resolution. In particular, the hybrid optical probes can combine the advantageous photophysical properties of different components such as voltage-sensitive molecules, highly fluorescent fluorophores, membrane-targeting tags, and optogenetic materials, thus showing numerous advantages in improving the photoluminescence intensity, voltage sensitivity, photostability, and cell specificity of probes. In this review, the current state-of-the-art hybrid probes are highlighted, that are designed by using fluorescent proteins, organic dyes, and fluorescent nanoprobes as the fluorophores, respectively. Then, the design strategies, voltage-sensing mechanisms and the in vitro and in vivo neural activity imaging applications of the hybrid probes are summarized. Finally, based on the current achievements of voltage imaging studies, the challenges and prospects for design and application of hybrid optical probes in the future are presented.

Enhanced genetically encoded voltage indicators advance their applications in neuroscience

Genetically encoded voltage indicators report membrane voltage with high spatiotemporal resolution. Extensive recent efforts to improve the GEVIs' brightness, sensitivity, and kinetics have greatly increased the GEVIs' signal-to-noise performance over ten-fold and lowered their response time to the sub-millisecond regime. Such capabilities have broadened the GEVIs' ability to measure membrane voltage of neural populations at cellular resolution in vitro and in vivo, all at high speeds. The GEVIs' high voltage fidelity and fast response have revealed novel physiological phenomena in multiple neuroscientific applications. Such applications portend future targeted studies of voltage activity that take advantage of the GEVIs' ability to report rapid dynamics from genetically-targeted neural populations.

Optical voltage imaging in neurons: moving from technology development to practical tool

A central goal in neuroscience is to determine how the brain's neuronal circuits generate perception, cognition and emotions and how these lead to appropriate behavioural actions. A methodological platform based on genetically encoded voltage indicators (GEVIs) that enables the monitoring of large-scale circuit dynamics has brought us closer to this ambitious goal. This Review provides an update on the current state of the art and the prospects of emerging optical GEVI imaging technologies.

Engineering Photoactivatability in Genetically Encoded Voltage and pH Indicators

Genetically-encoded indicators of neuronal activity enable the labeling of a genetically defined population of neurons to optically monitor their activities. However, researchers often find difficulties in identifying relevant signals from excessive background fluorescence. A photoactivatable version of a genetically encoded calcium indicator, sPA-GCaMP6f is a good example of circumventing such an obstacle by limiting the fluorescence to a region of interest defined by the user. Here, we apply this strategy to genetically encoded voltage (GEVI) and pH (GEPI) indicators. Three photoactivatable GEVI candidates were considered. The first one used a circularly-permuted fluorescent protein, the second design involved a Förster resonance energy transfer (FRET) pair, and the third approach employed a pH-sensitive variant of GFP, ecliptic pHluorin. The candidate with a variant of ecliptic pHluorin exhibited photoactivation and a voltage-dependent fluorescence change. This effort also yielded a pH-sensitive photoactivatable GFP that varies its brightness in response to intracellular pH changes.

Optical Studies of Action Potential Dynamics with hVOS probes

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

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.

Dynamic Network Activation of Hypothalamic MCH Neurons in REM Sleep and Exploratory Behavior

Most brain neurons are active in waking, but hypothalamic neurons that synthesize the neuropeptide melanin-concentrating hormone (MCH) are claimed to be active only during sleep, particularly rapid eye movement (REM) sleep. Here we use deep-brain imaging to identify changes in fluorescence of the genetically encoded calcium (Ca²⁺) indicator GCaMP6 in individual hypothalamic neurons that contain MCH. An in vitro electrophysiology study determined a strong relationship between depolarization and Ca²⁺ fluorescence in MCH neurons. In 10 freely behaving MCH-cre mice (male and female), the highest fluorescence occurred in all recorded neurons (n = 106) in REM sleep relative to quiet waking or non-REM sleep. Unexpectedly, 70% of the MCH neurons had strong fluorescence activity when the mice explored novel objects. Spatial and temporal mapping of the change in fluorescence between pairs of MCH neurons revealed dynamic activation of MCH neurons during REM sleep and activation of a subset of the same neurons during exploratory behavior. Functional network activity maps will facilitate comparisons of not only single-neuron activity, but also network responses in different conditions and disease.SIGNIFICANCE STATEMENT Functional activity maps identify brain circuits responding to specific behaviors, including rapid eye movement sleep (REM sleep), a sleep phase when the brain is as active as in waking. To provide the first activity map of individual neurons during REM sleep, we use deep-brain calcium imaging in unrestrained mice to map the activity of hypothalamic melanin-concentrating hormone (MCH) neurons. MCH neurons were found to be synchronously active during REM sleep, and also during the exploration of novel objects. Spatial mapping revealed dynamic network activation during REM sleep and activation of a subset of the neurons during exploratory behavior. Functional activity maps at the cellular level in specific behaviors, including sleep, are needed to establish a brain connectome.

Single-Neuron Level One-Photon Voltage Imaging With Sparsely Targeted Genetically Encoded Voltage Indicators

Voltage imaging of many neurons simultaneously at single-cell resolution is hampered by the difficulty of detecting small voltage signals from overlapping neuronal processes in neural tissue. Recent advances in genetically encoded voltage indicator (GEVI) imaging have shown single-cell resolution optical voltage recordings in intact tissue through imaging naturally sparse cell classes, sparse viral expression, soma restricted expression, advanced optical systems, or a combination of these. Widespread sparse and strong transgenic GEVI expression would enable straightforward optical access to a densely occurring cell type, such as cortical pyramidal cells. Here we demonstrate that a recently described sparse transgenic expression strategy can enable single-cell resolution voltage imaging of cortical pyramidal cells in intact brain tissue without restricting expression to the soma. We also quantify the functional crosstalk in brain tissue and discuss optimal imaging rates to inform future GEVI experimental design.

Spying on Neuronal Membrane Potential with Genetically Targetable Voltage Indicators

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

Mapping of excitatory and inhibitory postsynaptic potentials of neuronal populations in hippocampal slices using the GEVI, ArcLight

To understand the circuitry of the brain, it is essential to clarify the functional connectivity among distinct neuronal populations. For this purpose, neuronal activity imaging using genetically-encoded calcium sensors such as GCaMP has been a powerful approach due to its cell-type specificity. However, calcium (Ca²⁺) is an indirect measure of neuronal activity. A more direct approach would be to use genetically encoded voltage indicators (GEVIs) to observe subthreshold, synaptic activities. The GEVI, ArcLight, which exhibits large fluorescence transients in response to voltage, was expressed in excitatory neurons of the mouse CA1 hippocampus. Fluorescent signals in response to the electrical stimulation of the Schaffer collateral axons were observed in brain slice preparations. ArcLight was able to map both excitatory and inhibitory inputs projected to excitatory neurons. In contrast, the Ca²⁺ signal detected by GCaMP6f, was only associated with excitatory inputs. ArcLight and similar voltage sensing probes are also becoming powerful paradigms for functional connectivity mapping of brain circuitry.