Flexible Nanopipettes for Minimally Invasive Intracellular Electrophysiology In Vivo

Site-specific seeding of Lewy pathology induces distinct pre-motor cellular and dendritic vulnerabilities in the cortex

Circuit-based biomarkers distinguishing the gradual progression of Lewy pathology across synucleinopathies remain unknown. Here, we show that seeding of α-synuclein preformed fibrils in mouse dorsal striatum and motor cortex leads to distinct prodromal-phase cortical dysfunction across months. Our findings reveal that while both seeding sites had increased cortical pathology and hyperexcitability, distinct differences in electrophysiological and cellular ensemble patterns were crucial in distinguishing pathology spread between the two seeding sites. Notably, while beta-band spike-field-coherence reflected a significant increase beginning in Layer-5 and then spreading to Layer-2/3, the rate of entrainment and the propensity of stochastic beta-burst dynamics was markedly seeding location-specific. This beta dysfunction was accompanied by gradual superficial excitatory ensemble instability following cortical, but not striatal, preformed fibrils injection. We reveal a link between Layer-5 dendritic vulnerabilities and translaminar beta event dysfunction, which could be used to differentiate symptomatically similar synucleinopathies.

Parvalbumin interneuron impairment causes synaptic transmission deficits and seizures in SCN8A developmental and epileptic encephalopathy

SCN8A developmental and epileptic encephalopathy (DEE) is a severe epilepsy syndrome resulting from mutations in the voltage-gated sodium channel Nav1.6, encoded by the gene SCN8A. Nav1.6 is expressed in excitatory and inhibitory neurons, yet previous studies primarily focus on how SCN8A mutations affect excitatory neurons, with limited studies on the importance of inhibitory interneurons. Parvalbumin (PV) interneurons are a prominent inhibitory interneuron subtype that expresses Nav1.6. To assess PV interneuron function within SCN8A DEE, we used 2 mouse models harboring patient-derived SCN8A gain-of-function variants, Scn8aD/+, where the SCN8A variant N1768D is expressed globally, and Scn8aW/+-PV, where the SCN8A variant R1872W is selectively expressed in PV interneurons. Expression of the R1872W SCN8A variant selectively in PV interneurons led to development of spontaneous seizures and seizure-induced death. Electrophysiology studies showed that Scn8aD/+ and Scn8aW/+-PV interneurons were susceptible to depolarization block and exhibited increased persistent sodium current. Evaluation of synaptic connections between PV interneurons and pyramidal cells showed synaptic transmission deficits in Scn8aD/+ and Scn8aW/+-PV interneurons. Together, our findings indicate that PV interneuron failure via depolarization block along with inhibitory synaptic impairment likely elicits an overall inhibitory reduction in SCN8A DEE, leading to unchecked excitation and ultimately resulting in seizures and seizure-induced death.

Epilepsy insights revealed by intravital functional optical imaging

Despite an abundance of pharmacologic and surgical epilepsy treatments, there remain millions of patients suffering from poorly controlled seizures. One approach to closing this treatment gap may be found through a deeper mechanistic understanding of the network alterations that underly this aberrant activity. Functional optical imaging in vertebrate models provides powerful advantages to this end, enabling the spatiotemporal acquisition of individual neuron activity patterns across multiple seizures. This coupled with the advent of genetically encoded indicators, be them for specific ions, neurotransmitters or voltage, grants researchers unparalleled access to the intact nervous system. Here, we will review how in vivo functional optical imaging in various vertebrate seizure models has advanced our knowledge of seizure dynamics, principally seizure initiation, propagation and termination.

Probing multiplexed basal dendritic computations using two-photon 3D holographic uncaging

Basal dendrites of layer 5 cortical pyramidal neurons exhibit Na+ and N-methyl-D-aspartate receptor (NMDAR) regenerative spikes and are uniquely poised to influence somatic output. Nevertheless, due to technical limitations, how multibranch basal dendritic integration shapes and enables multiplexed barcoding of synaptic streams remains poorly mapped. Here, we combine 3D two-photon holographic transmitter uncaging, whole-cell dynamic clamp, and biophysical modeling to reveal how synchronously activated synapses (distributed and clustered) across multiple basal dendritic branches are multiplexed under quiescent and in vivo-like conditions. While dendritic regenerative Na+ spikes promote millisecond somatic spike precision, distributed synaptic inputs and NMDAR spikes regulate gain. These concomitantly occurring dendritic nonlinearities enable multiplexed information transfer amid an ongoing noisy background, including under back-propagating voltage resets, by barcoding the axo-somatic spike structure. Our results unveil a multibranch dendritic integration framework in which dendritic nonlinearities are critical for multiplexing different spatial-temporal synaptic input patterns, enabling optimal feature binding.

Membrane-coated glass electrodes for stable, low-noise electrophysiology recordings in Drosophila central neurons

BACKGROUND

Electrophysiological recording with glass electrodes is one of the best techniques to measure membrane potential dynamics and ionic currents of voltage-gated channels in neurons. However, artifactual variability of the biophysical state variables that determine recording quality can be caused by insufficient affinity between the electrode and cell membrane during the recording.

NEW METHOD

We introduce a phospholipid membrane coating on glass electrodes to improve intracellular electrophysiology recording quality. Membrane-coated electrodes were prepared with a tip-dip protocol for perforated-patch, sharp-electrode current-clamp, and cell-attached patch-clamp recordings from specific circadian clock neurons in Drosophila. We perform quantitative comparisons based on the variability of functional biophysical parameters used in various electrophysiological methods, and advanced statistical comparisons based on the degree of stationariness and signal-to-noise ratio.

RESULTS

Results indicate a dramatic reduction in artifactual variabilities of functional parameters from enhanced stability. We also identify significant exclusions of a statistically estimated noise component in a time series of membrane voltage signals, improving signal-to-noise ratio.

COMPARISON WITH EXISTING METHODS

Compared to standard glass electrodes, using membrane-coated glass electrodes achieves improved recording quality in intracellular electrophysiology.

CONCLUSIONS

Electrophysiological recordings from Drosophila central neurons can be technically challenging, however, membrane-coated electrodes will possibly be beneficial for reliable data acquisition and improving the technical feasibility of axonal intracellular activities measurements and single-channel recordings. The improved electrical stability of the recordings should also contribute to increased mechanical stability, thus facilitating long-term stable measurements of neural activity. Therefore, it is possible that membrane-coated electrodes will be useful for any model system.

Intracellular neural control of an active feeding structure in Aplysia using a carbon fiber electrode array

BACKGROUND

To study neural control of behavior, intracellular recording and stimulation of many neurons in freely moving animals would be ideal. However, current technologies limit the number of neurons that can be monitored and manipulated. A new technology has become available for intracellular recording and stimulation which we demonstrate in the tractable nervous system of Aplysia.

NEW METHOD

Carbon fiber electrode arrays (whose tips are coated with platinum-iridium) were used with an in vitro feeding preparation to intracellularly record from and to control the activity of multiple neurons during feeding movements.

RESULTS

In an in vitro feeding preparation, the carbon fiber electrode arrays recorded action potentials and subthreshold synaptic potentials during feeding movements. Depolarizing or hyperpolarizing currents activated or inhibited identified neurons (respectively), manipulating the movements of the feeding apparatus.

COMPARISON WITH EXISTING METHOD(S)

Standard glass microelectrodes that are commonly used for intracellular recording are stiff, liable to break in response to movement, and require many micromanipulators to be precisely positioned. In contrast, carbon fiber arrays are less sensitive to movement, but are capable of multiple channels of intracellular recording and stimulation.

CONCLUSIONS

Carbon fiber arrays are a novel technology for intracellular recording that can be used in moving preparations. They can record both action potentials and synaptic activity in multiple neurons and can be used to stimulate multiple neurons in complex patterns.

Parvalbumin Interneuron Impairment Leads to Synaptic Transmission Deficits and Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy (EE) is a severe epilepsy syndrome resulting from de novo mutations in the voltage-gated sodium channel Na v 1.6, encoded by the gene SCN8A . Na v 1.6 is expressed in both excitatory and inhibitory neurons, yet previous studies have primarily focused on the impact SCN8A mutations have on excitatory neuron function, with limited studies on the importance of inhibitory interneurons to seizure onset and progression. Inhibitory interneurons are critical in balancing network excitability and are known to contribute to the pathophysiology of other epilepsies. Parvalbumin (PV) interneurons are the most prominent inhibitory neuron subtype in the brain, making up about 40% of inhibitory interneurons. Notably, PV interneurons express high levels of Na v 1.6. To assess the role of PV interneurons within SCN8A EE, we used two mouse models harboring patient-derived SCN8A gain-of-function mutations, Scn8a D/+ , where the SCN8A mutation N1768D is expressed globally, and Scn8a W/+ -PV, where the SCN8A mutation R1872W is selectively expressed in PV interneurons. Expression of the R1872W SCN8A mutation selectively in PV interneurons led to the development of spontaneous seizures in Scn8a W/+ -PV mice and seizure-induced death, decreasing survival compared to wild-type. Electrophysiology studies showed that PV interneurons in Scn8a D/+ and Scn8a W/+ -PV mice were susceptible to depolarization block, a state of action potential failure. Scn8a D/+ and Scn8a W/+ -PV interneurons also exhibited increased persistent sodium current, a hallmark of SCN8A gain-of-function mutations that contributes to depolarization block. Evaluation of synaptic connections between PV interneurons and pyramidal cells showed an increase in synaptic transmission failure at high frequencies (80-120Hz) as well as an increase in synaptic latency in Scn8a D/+ and Scn8a W/+ -PV interneurons. These data indicate a distinct impairment of synaptic transmission in SCN8A EE, potentially decreasing overall cortical network inhibition. Together, our novel findings indicate that failure of PV interneuron spiking via depolarization block along with frequency-dependent inhibitory synaptic impairment likely elicits an overall reduction in the inhibitory drive in SCN8A EE, leading to unchecked excitation and ultimately resulting in seizures and seizure-induced death.

Recent advances of nanopore technique in single cell analysis

Single cells and their dynamic behavior are closely related to biological research. Monitoring their dynamic behavior is of great significance for disease prevention. How to achieve rapid and non-destructive monitoring of single cells is a major issue that needs to be solved urgently. As an emerging technology, nanopores have been proven to enable non-destructive and label-free detection of single cells. The structural properties of nanopores enable a high degree of sensitivity and accuracy during analysis. In this article, we summarize and classify the different types of solid-state nanopores that can be used for single-cell detection and illustrate their specific applications depending on the size of the analyte. In addition, their research progress in material transport and microenvironment monitoring is also highlighted. Finally, a brief summary of existing research challenges and future trends in nanopore single-cell analysis is tentatively provided.

Scanning ion conductance microscopy revealed cisplatin-induced morphological changes related to apoptosis in single adenocarcinoma cells

The studies of drug-induced apoptosis play a vital role in the identification of potential drugs that could treat diseases such as cancer. Alterations in the native morphology of cancer cells following treatment with anticancer drugs serve as one of the indicators that reveal drug efficacy. Various techniques such as optical microscopy, electron microscopy (EM), and atomic force microscopy (AFM) have been used to map the three dimensional (3D) morphological changes in cells induced with drugs. However, caution should be exercised when interpreting morphological data from techniques that might alter the native morphology of cells, caused by phototoxicity, electron beam invasiveness, intrusive sample preparation, and cell membrane deformation. Herein, we have used scanning ion conductance microscopy (SICM) to study the 3D morphology and roughness of A549 adenocarcinoma cells under physiological conditions before and after cisplatin induced apoptosis, where we observed an increase in height, overall shrinkage of the cells, and irregular features form on the cell membrane. Tracking the morphology of the same single A549 cells exposed to cisplatin unveiled heterogeneity in response to the drug, formation of membrane blebs, and an increase in membrane roughness. We have also demonstrated the use of SICM for studying the effect of cisplatin on the dynamic changes in the volume of A549 cells over days. SICM is demonstrated as a technique for studying the effect of drug induced apoptosis in the same cells over time, and for multiple different single cells.

A debate on the neuronal origin of focal seizures

A critical question regarding how focal seizures start is whether we can identify particular cell classes that drive the pathological process. This was the topic for debate at the recent International Conference for Technology and Analysis of Seizures (ICTALS) meeting (July 2022, Bern, CH) that we summarize here. The debate has been fueled in recent times by the introduction of powerful new ways to manipulate subpopulations of cells in relative isolation, mostly using optogenetics. The motivation for resolving the debate is to identify novel targets for therapeutic interventions through a deeper understanding of the etiology of seizures.

Diversity of dynamic voltage patterns in neuronal dendrites revealed by nanopipette electrophysiology

Dendrites and dendritic spines are the essential cellular compartments in neuronal communication, conveying information through transient voltage signals. Our understanding of these compartmentalized voltage dynamics in fine, distal neuronal dendrites remains poor due to the difficulties inherent to accessing and stably recording from such small, nanoscale cellular compartments for a sustained time. To overcome these challenges, we use nanopipettes that permit long and stable recordings directly from fine neuronal dendrites. We reveal a diversity of voltage dynamics present locally in dendrites, such as spontaneous voltage transients, bursting events and oscillating periods of silence and firing activity, all of which we characterized using segmentation analysis. Remarkably, we find that neuronal dendrites can display spontaneous hyperpolarisation events, and sustain transient hyperpolarised states. The voltage patterns were activity-dependent, with a stronger dependency on synaptic activity than on action potentials. Long-time recordings of fine dendritic protrusions show complex voltage dynamics that may represent a previously unexplored contribution to dendritic computations.

The Modification and Applications of Nanopipettes in Electrochemical Analysis

Nanopipette, which is fabricated by glasses and possesses a nanoscale pore in the tip, has been proven to be immensely useful in electrochemical analysis. Numerous nanopipette-based sensors have emerged with improved sensitivity, selectivity, ease of use, and miniaturization. In this minireview, we provide an overview of the recent developments of nanopipette-based electrochemical sensors based on different types of nanopipettes, including single-nanopipettes, self-referenced nanopipettes, dual-nanopipettes, and double-barrel nanopipettes. Several important modification materials for nanopipette functionalization are highlighted, such as conductive materials, macromolecular materials, and functional molecules. These materials can improve the sensing performance and targeting specificities of nanopipettes. We also discuss examples of related applications and the future development of nanopipette-based strategies.

Integration of reconfigurable microchannels into aligned three-dimensional neural networks for spatially controllable neuromodulation

Anisotropically organized neural networks are indispensable routes for functional connectivity in the brain, which remains largely unknown. While prevailing animal models require additional preparation and stimulation-applying devices and have exhibited limited capabilities regarding localized stimulation, no in vitro platform exists that permits spatiotemporal control of chemo-stimulation in anisotropic three-dimensional (3D) neural networks. We present the integration of microchannels seamlessly into a fibril-aligned 3D scaffold by adapting a single fabrication principle. We investigated the underlying physics of elastic microchannels' ridges and interfacial sol-gel transition of collagen under compression to determine a critical window of geometry and strain. We demonstrated the spatiotemporally resolved neuromodulation in an aligned 3D neural network by local deliveries of KCl and Ca2+ signal inhibitors, such as tetrodotoxin, nifedipine, and mibefradil, and also visualized Ca2+ signal propagation with a speed of ~3.7 μm/s. We anticipate that our technology will pave the way to elucidate functional connectivity and neurological diseases associated with transsynaptic propagation.

3D flow field measurements outside nanopores

We demonstrate a non-stereoscopic, video-based particle tracking system with optical tweezers to study fluid flow in 3D in the vicinity of glass nanopores. In particular, we used the quadrant interpolation algorithm to extend our video-based particle tracking to displacements out of the trapping plane of the tweezers. This permitted the study of flow from nanopores oriented at an angle to the trapping plane, enabling the mounting of nanopores on a micromanipulator with which it was then possible to automate the mapping procedure. Mapping of the voltage driven flow in 3D volumes outside nanopores revealed polarity dependent flow fields. This is in agreement with the model of voltage driven flow in conical nanopores depending on the interaction of distinct flows within the nanopore and along the outer walls.

Seizures initiate in zones of relative hyperexcitation in a zebrafish epilepsy model

Seizures are thought to arise from an imbalance of excitatory and inhibitory neuronal activity. While most classical studies suggest excessive excitatory neural activity plays a generative role, some recent findings challenge this view and instead argue that excessive activity in inhibitory neurons initiates seizures. We investigated this question of imbalance in a zebrafish seizure model with two-photon imaging of excitatory and inhibitory neuronal activity throughout the brain using a nuclear-localized calcium sensor. We found that seizures consistently initiated in circumscribed zones of the midbrain before propagating to other brain regions. Excitatory neurons were both more prevalent and more likely to be recruited than inhibitory neurons in initiation as compared with propagation zones. These findings support a mechanistic picture whereby seizures initiate in a region of hyper-excitation, then propagate more broadly once inhibitory restraint in the surround is overcome.

Patch Clamp: The First Four Decades of a Technique That Revolutionized Electrophysiology and Beyond

Forty years ago, the introduction of a new electrophysiological technique, the patch clamp, revolutionized the fields of Cellular Physiology and Biophysics, providing for the first time the possibility of describing the behavior of a single protein, an ion-permeable channel of the cell plasma membrane, in its physiological environment. The new approach was actually much more potent and versatile than initially envisaged, and it has evolved into several different modalities that have radically changed our knowledge of how cells (not only the classical "electrically excitable "ones, such as nerves and muscles) use electrical signaling to modulate and organize their activity. This review aims at telling the history of the background from which the new technique evolved and at analyzing some of its more recent developments.

Simultaneous two-photon imaging of action potentials and subthreshold inputs in vivo

To better understand the input-output computations of neuronal populations, we developed ArcLight-ST, a genetically-encoded voltage indicator, to specifically measure subthreshold membrane potentials. We combined two-photon imaging of voltage and calcium, and successfully discriminated subthreshold inputs and spikes with cellular resolution in vivo. We demonstrate the utility of the method by mapping epileptic seizures progression through cortical circuits, revealing divergent sub- and suprathreshold dynamics within compartmentalized epileptic micronetworks. Two-photon, two-color imaging of calcium and voltage enables mapping of inputs and outputs in neuronal populations in living animals.

Learning from the brain's architecture: bioinspired strategies towards implantable neural interfaces

While early neural interfaces consisted of rigid, monolithic probes, recent implantable technologies include meshes, gels, and threads that imitate various properties of the neural tissue itself. Such mimicry brings new capabilities to the traditional electrophysiology toolbox, with benefits for both neuroscience studies and clinical treatments. Specifically, by matching the multi-dimensional mechanical properties of the brain, neural implants can preserve the endogenous environment while functioning over chronic timescales. Further, topological mimicry of neural structures enables seamless integration into the tissue and provides proximal access to neurons for high-quality recordings. Ultimately, we envision that neuromorphic devices incorporating functional, mechanical, and topological mimicry of the brain may facilitate stable operation of advanced brain machine interfaces with minimal disruption of the native tissue.

Somatostatin-Positive Interneurons Contribute to Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy is a devastating epilepsy syndrome caused by mutant SCN8A, which encodes the voltage-gated sodium channel Na_V1.6. To date, it is unclear if and how inhibitory interneurons, which express Na_V1.6, influence disease pathology. Using both sexes of a transgenic mouse model of SCN8A epileptic encephalopathy, we found that selective expression of the R1872W SCN8A mutation in somatostatin (SST) interneurons was sufficient to convey susceptibility to audiogenic seizures. Patch-clamp electrophysiology experiments revealed that SST interneurons from mutant mice were hyperexcitable but hypersensitive to action potential failure via depolarization block under normal and seizure-like conditions. Remarkably, GqDREADD-mediated activation of WT SST interneurons resulted in prolonged electrographic seizures and was accompanied by SST hyperexcitability and depolarization block. Aberrantly large persistent sodium currents, a hallmark of SCN8A mutations, were observed and were found to contribute directly to aberrant SST physiology in computational modeling and pharmacological experiments. These novel findings demonstrate a critical and previously unidentified contribution of SST interneurons to seizure generation not only in SCN8A epileptic encephalopathy, but epilepsy in general.SIGNIFICANCE STATEMENT SCN8A epileptic encephalopathy is a devastating neurological disorder that results from de novo mutations in the sodium channel isoform Na_v1.6. Inhibitory neurons express Na_V1.6, yet their contribution to seizure generation in SCN8A epileptic encephalopathy has not been determined. We show that mice expressing a human-derived SCN8A variant (R1872W) selectively in somatostatin (SST) interneurons have audiogenic seizures. Physiological recordings from SST interneurons show that SCN8A mutations lead to an elevated persistent sodium current which drives initial hyperexcitability, followed by premature action potential failure because of depolarization block. Furthermore, chemogenetic activation of WT SST interneurons leads to audiogenic seizure activity. These findings provide new insight into the importance of SST inhibitory interneurons in seizure initiation, not only in SCN8A epileptic encephalopathy, but for epilepsy broadly.

Time for NanoNeuro

The study of electronic properties of materials at the nanoscale has unveiled physical laws and generated materials such as nanoparticles, quantum dots, nanodiamonds, nanoelectrodes, and nanoprobes. Independently, large-scale public and private neuroscience programs have been launched to develop methods to measure and manipulate neural circuits in living animals and humans. Here, we review an upcoming field, NanoNeuro, defined as the intersection of nanoscience and neuroscience, that aims to develop nanoscale methods to record and stimulate neuronal activity. Because of their unique physical properties, nanomaterials have intrinsic advantages as biosensors and actuators, and they may be applicable to humans without the need for genetic modifications. Thus, nanoscience could make major methodological contributions to the future of neuroscience and, more generally, to biomedical sciences.

Nanopipette-Based Nanosensor for Label-Free Electrochemical Monitoring of Cell Membrane Rupture under H2O2 Treatment

H₂O₂ is an essential signaling molecule in living cells that can cause direct damage to lipids, proteins, and DNA, resulting in cell membrane rupture. However, current studies mostly focus on probe-based sensing of intracellular H₂O₂, and these methods usually require sophisticated probe synthesis and instruments. In particular, local H₂O₂ treatment induces cell membrane rupture, but the level of cell membrane destruction is unknown because the mechanical properties of the cell membrane are difficult to accurately determine. Therefore, highly sensitive and label-free methods are required to measure and reflect mechanical changes in the cell membrane. Here, using an ultrasmall quartz nanopipette with a tip diameter less than 90 nm as a nanosensor, label-free and noninvasive electrochemical single-cell measurement is achieved for real-time monitoring of cell membrane rupture under H₂O₂ treatment. By spatially controlling the nanopipette tip to precisely approach a specific location on the membrane of a single living cell, stable cyclic membrane oscillations are observed under a constant direct current voltage. Specifically, upon nanopipette advancement, the mechanical status of the cell membrane can be sensibly displayed by continuous current versus time traces. The electrical signals are collected and processed, ultimately revealing the mechanical properties of the cell membrane and the degree of cell apoptosis. This nanopipette-based nanosensor paves the way for developing a facile, label-free, and noninvasive strategy to assay the mechanical properties of the cell membrane during external stimulation at the single-cell level.

Modeling the voltage distribution in a non-locally but globally electroneutral confined electrolyte medium: applications for nanophysiology

The distribution of voltage in sub-micron cellular domains remains poorly understood. In neurons, the voltage results from the difference in ionic concentrations which are continuously maintained by pumps and exchangers. However, it not clear how electro-neutrality could be maintained by an excess of fast moving positive ions that should be counter balanced by slow diffusing negatively charged proteins. Using the theory of electro-diffusion, we study here the voltage distribution in a generic domain, which consists of two concentric disks (resp. ball) in two (resp. three) dimensions, where a negative charge is fixed in the inner domain. When global but not local electro-neutrality is maintained, we solve the Poisson-Nernst-Planck equation both analytically and numerically in dimension 1 (flat) and 2 (cylindrical) and found that the voltage changes considerably on a spatial scale which is much larger than the Debye screening length, which assumes electro-neutrality. The present result suggests that long-range voltage drop changes are expected in neuronal microcompartments, probably relevant to explain the activation of far away voltage-gated channels located on the surface membrane.

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.

Nanotechnology: new opportunities for the development of patch-clamps

The patch-clamp technique is one of the best approaches to investigate neural excitability. Impressive improvements towards the automation of the patch-clamp technique have been made, but obvious limitations and hurdles still exist, such as parallelization, volume displacement in vivo, and long-term recording. Nanotechnologies have provided opportunities to overcome these hurdles by applying electrical devices on the nanoscale. Electrodes based on nanowires, nanotubes, and nanoscale field-effect transistors (FETs) are confirmed to be robust and less invasive tools for intracellular electrophysiological recording. Research on the interface between the nanoelectrode and cell membrane aims to reduce the seal conductance and further improve the recording quality. Many novel recording approaches advance the parallelization, and precision with reduced invasiveness, thus improving the overall intracellular recording system. The combination of nanotechnology and the present intracellular recording framework is a revolutionary and promising orientation, potentially becoming the next generation electrophysiological recording technique and replacing the conventional patch-clamp technique. Here, this paper reviews the recent advances in intracellular electrophysiological recording techniques using nanotechnology, focusing on the design of noninvasive and greatly parallelized recording systems based on nanoelectronics.

Tb3+-doped fluorescent glass for biology

Optical investigation and manipulation constitute the core of biological experiments. Here, we introduce a new borosilicate glass material that contains the rare-earth ion terbium(III) (Tb³⁺), which emits green fluorescence upon blue light excitation, similar to green fluorescent protein (GFP), and thus is widely compatible with conventional biological research environments. Micropipettes made of Tb³⁺-doped glass allowed us to target GFP-labeled cells for single-cell electroporation, single-cell transcriptome analysis (Patch-seq), and patch-clamp recording under real-time fluorescence microscopic control. The glass also exhibited potent third harmonic generation upon infrared laser excitation and was usable for online optical targeting of fluorescently labeled neurons in the in vivo neocortex. Thus, Tb³⁺-doped glass simplifies many procedures in biological experiments.

Nanoneedle-Based Materials for Intracellular Studies

Nanoneedles, defined as high aspect ratio structures with tip diameters of 5 to approximately 500 nm, are uniquely able to interface with the interior of living cells. Their nanoscale dimensions mean that they are able to penetrate the plasma membrane with minimal disruption of normal cellular functions, allowing researchers to probe the intracellular space and deliver or extract material from individual cells. In the last decade, a variety of strategies have been developed using nanoneedles, either singly or as arrays, to investigate the biology of cancer cells in vitro and in vivo. These include hollow nanoneedles for soluble probe delivery, nanocapillaries for single-cell biopsy, nano-AFM for direct physical measurements of cytosolic proteins, and a wide range of fluorescent and electrochemical nanosensors for analyte detection. Nanofabrication has improved to the point that nanobiosensors can detect individual vesicles inside the cytoplasm, delineate tumor margins based on intracellular enzyme activity, and measure changes in cell metabolism almost in real time. While most of these applications are currently in the proof-of-concept stage, nanoneedle technology is poised to offer cancer biologists a powerful new set of tools for probing cells with unprecedented spatial and temporal resolution.

Identification and quantification of neuronal ensembles in optical imaging experiments

Recent technical advances in molecular biology and optical imaging have made it possible to record from up to thousands of densely packed neurons in superficial and deep brain regions in vivo, with cellular subtype specificity and high spatiotemporal fidelity. Such optical neurotechnologies are enabling increasingly fine-scaled studies of neuronal circuits and reliably co-active groups of neurons, so-called ensembles. Neuronal ensembles are thought to constitute the basic functional building blocks of brain systems, potentially exhibiting collective computational properties. While the technical framework of in vivo optical imaging and quantification of neuronal activity follows certain widely held standards, analytical methods for study of neuronal co-activity and ensembles lack consensus and are highly varied across the field. Here we provide a comprehensive step-by-step overview of theoretical, experimental, and analytical considerations for the identification and quantification of neuronal ensemble dynamics in high-resolution in vivo optical imaging studies.

Flexible Electronics for Monitoring in vivo Electrophysiology and Metabolite Signals

Numerous efforts have been made to develop efficient biosensors for detecting analytes in the human body. However, biosensors are often developed on rigid materials, which limits their application on skin, organs, and other tissues in the human body where good flexibility is required. Developing flexible materials for biosensors that can be used on soft and irregularly shaped surfaces would significantly expand the clinical application of biosensors. In this review, we will provide a selective overview of recently developed flexible electronic devices and their applications for monitoring in vivo metabolite and electrophysiology signals. The article provides guidelines for the development of an in vivo signal monitoring system and emphasizes research from various disciplines for the further development of flexible electronics that can be used in more biomedical applications in the future.

Evolution of in vivo dopamine monitoring techniques

The brain dopamine system is central to numerous behavioral processes, including movement, learning, and motivation. Accordingly, disruptions of this neural system underlie numerous neurological and psychiatric disorders. Current understanding of how dopamine neurotransmission contributes to behavior and its dysfunction has been driven by technological advancements that permit spatiotemporally-defined measurements of dopaminergic signaling in behaving animals. In this review, we will discuss the evolution of in vivo neural monitoring technologies for measuring dopamine neuron function. We focus on the dopamine system for two reasons: (1) the central role of dopamine neurotransmission in normal behavior and disease, and (2) dopamine neuron measurements have long been at the forefront of in vivo neural monitoring technologies. We will provide a brief overview of standard techniques for monitoring dopamine function, including electrophysiology, microdialysis, and voltammetry. Then, we will discuss recent advancements in optical technologies using genetically-encoded fluorescent proteins (GEFPs), including a critical evaluation of their advantages and limitations.

How is flexible electronics advancing neuroscience research?

Innovative neurotechnology must be leveraged to experimentally answer the multitude of pressing questions in modern neuroscience. Driven by the desire to address the existing neuroscience problems with newly engineered tools, we discuss in this review the benefits of flexible electronics for neuroscience studies. We first introduce the concept and define the properties of flexible and stretchable electronics. We then categorize the four dimensions where flexible electronics meets the demands of modern neuroscience: chronic stability, interfacing multiple structures, multi-modal compatibility, and neuron-type-specific recording. Specifically, with the bending stiffness now approaching that of neural tissue, implanted flexible electronic devices produce little shear motion, minimizing chronic immune responses and enabling recording and stimulation for months, and even years. The unique mechanical properties of flexible electronics also allow for intimate conformation to the brain, the spinal cord, peripheral nerves, and the retina. Moreover, flexible electronics enables optogenetic stimulation, microfluidic drug delivery, and neural activity imaging during electrical stimulation and recording. Finally, flexible electronics can enable neuron-type identification through analysis of high-fidelity recorded action potentials facilitated by its seamless integration with the neural circuitry. We argue that flexible electronics will play an increasingly important role in neuroscience studies and neurological therapies via the fabrication of neuromorphic devices on flexible substrates and the development of enhanced methods of neuronal interpenetration.

Nanobubble-controlled nanofluidic transport

Nanofluidic platforms offering tunable material transport are applicable in biosensing, chemical detection, and filtration. Prior studies have achieved selective and controllable ion transport through electrical, optical, or chemical gating of complex nanostructures. Here, we mechanically control nanofluidic transport using nanobubbles. When plugging nanochannels, nanobubbles rectify and occasionally enhance ionic currents in a geometry-dependent manner. These conductance effects arise from nanobubbles inducing surface-governed ion transport through interfacial electrolyte films residing between nanobubble surfaces and nanopipette walls. The nanobubbles investigated here are mechanically generated, made metastable by surface pinning, and verified with cryogenic transmission electron microscopy. Our findings are relevant to nanofluidic device engineering, three-phase interface properties, and nanopipette-based applications.

Bioinspired Materials for In Vivo Bioelectronic Neural Interfaces

The success of in vivo neural interfaces relies on their long-term stability and large scale in interrogating and manipulating neural activity after implantation. Conventional neural probes, owing to their limited spatiotemporal resolution and scale, face challenges for studying the massive, interconnected neural network in its native state. In this review, we argue that taking inspiration from biology will unlock the next generation of in vivo bioelectronic neural interfaces. Reducing the feature sizes of bioelectronic neural interfaces to mimic those of neurons enables high spatial resolution and multiplexity. Additionally, chronic stability at the device-tissue interface is realized by matching the mechanical properties of bioelectronic neural interfaces to those of the endogenous tissue. Further, modeling the design of neural interfaces after the endogenous topology of the neural circuitry enables new insights into the connectivity and dynamics of the brain. Lastly, functionalization of neural probe surfaces with coatings inspired by biology leads to enhanced tissue acceptance over extended timescales. Bioinspired neural interfaces will facilitate future developments in neuroscience studies and neurological treatments by leveraging bidirectional information transfer and integrating neuromorphic computing elements.

Dynamic single-cell intracellular pH sensing using a SERS-active nanopipette

Glass nanopipettes have shown promise for applications in single-cell manipulation, analysis, and imaging. In recent years, plasmonic nanopipettes have been developed to enable surface-enhanced Raman spectroscopy (SERS) measurements for single-cell analysis. In this work, we developed a SERS-active nanopipette that can be used to perform long-term and reliable intracellular analysis of single living cells with minimal damage, which is achieved by optimizing the nanopipette geometry and the surface density of the gold nanoparticle (AuNP) layer at the nanopipette tip. To demonstrate its ability in single-cell analysis, we used the nanopipette for intracellular pH sensing. Intracellular pH (pHi) is vital to cells as it influences cell function and behavior and pathological conditions. The pH sensitivity was realized by simply modifying the AuNP layer with the pH reporter molecule 4-mercaptobenzoic acid. With a response time of less than 5 seconds, the pH sensing range is from 6.0 to 8.0 and the maximum sensitivity is 0.2 pH units. We monitored the pHi change of individual HeLa and fibroblast cells, triggered by the extracellular pH (pHe) change. The HeLa cancer cells can better resist pHe change and adapt to the weak acidic environment. Plasmonic nanopipettes can be further developed to monitor other intracellular biomarkers.

Bioinspired bio-voltage memristors

Memristive devices are promising candidates to emulate biological computing. However, the typical switching voltages (0.2-2 V) in previously described devices are much higher than the amplitude in biological counterparts. Here we demonstrate a type of diffusive memristor, fabricated from the protein nanowires harvested from the bacterium Geobacter sulfurreducens, that functions at the biological voltages of 40-100 mV. Memristive function at biological voltages is possible because the protein nanowires catalyze metallization. Artificial neurons built from these memristors not only function at biological action potentials (e.g., 100 mV, 1 ms) but also exhibit temporal integration close to that in biological neurons. The potential of using the memristor to directly process biosensing signals is also demonstrated.

Bioelectronics for Millimeter-Sized Model Organisms

Advances in microfabrication technologies and biomaterials have enabled a growing class of electronic devices that can stimulate and record bioelectronic signals. Many of these devices have been developed for humans or vertebrate animals, where miniaturization allows for implantation within the body. There are, however, another class of bioelectronic interfaces that exploit microfabrication and nanoelectronics to record signals from tiny, millimeter-sized organisms. In these cases, rather than implanting a device inside an animal, animals themselves are loaded in large numbers into bioelectronic devices for neural circuit and behavioral interrogation. These scalable interfaces provide platforms to develop new therapeutics as well as better understand basic principles of bioelectronic communication, neuroscience, and behavior. Here we review recent progress in these bioelectronic technologies and describe how they can complement on-chip optical, mechanical, and chemical interrogation methods to achieve high-throughput, multimodal studies of millimeter-sized small animals.

Focus on the Focus

Acute Focal Seizures Start as Local Synchronizations of Neuronal Ensembles

Wenzel M, Hamm JP, Peterka DS, Yuste R. J Neurosci. 2019;39(43):8562-8575. doi:10.1523/JNEUROSCI.3176-18.2019.

Understanding seizure formation and spread remains a critical goal of epilepsy research. We used fast in vivo 2-photon calcium imaging in male mouse neocortex to reconstruct, with single-cell resolution, the dynamics of acute (4-aminopyridine) focal cortical seizures as they originate within a spatially confined seizure initiation site (intrafocal region) and subsequently propagate into neighboring cortical areas (extrafocal region). We find that seizures originate as local neuronal ensembles within the initiation site. This abnormal hyperactivity engages increasingly larger areas in a saltatory fashion until it breaks into neighboring cortex, where it proceeds smoothly and is then detected electrophysiologically (local field potential). Interestingly, PV inhibitory interneurons have spatially heterogeneous activity in intra- and extrafocal territories, ruling out a simple role of inhibition in seizure formation and spread. We propose a 2-step model for the progression of focal seizures, where neuronal ensembles activate first, generating a microseizure, followed by widespread neural activation in a travelling wave through neighboring cortex during macroseizures.

Acute Focal Seizures Start As Local Synchronizations of Neuronal Ensembles

Understanding seizure formation and spread remains a critical goal of epilepsy research. We used fast in vivo two-photon calcium imaging in male mouse neocortex to reconstruct, with single-cell resolution, the dynamics of acute (4-aminopyridine) focal cortical seizures as they originate within a spatially confined seizure initiation site (intrafocal region), and subsequently propagate into neighboring cortical areas (extrafocal region). We find that seizures originate as local neuronal ensembles within the initiation site. This abnormal hyperactivity engages increasingly larger areas in a saltatory fashion until it breaks into neighboring cortex, where it proceeds smoothly and is then detected electrophysiologically (LFP). Interestingly, PV inhibitory interneurons have spatially heterogeneous activity in intrafocal and extrafocal territories, ruling out a simple role of inhibition in seizure formation and spread. We propose a two-step model for the progression of focal seizures, where neuronal ensembles activate first, generating a microseizure, followed by widespread neural activation in a traveling wave through neighboring cortex during macroseizures.SIGNIFICANCE STATEMENT We have used calcium imaging in mouse sensory cortex in vivo to reconstruct the onset of focal seizures elicited by local injection of the chemoconvulsant 4-aminopyridine. We demonstrate at cellular resolution that acute focal seizures originate as increasingly synchronized local neuronal ensembles. Because of its spatial confinement, this process may at first be undetectable even by nearby LFP electrodes. Further, we establish spatial footprints of local neural subtype activity that correspond to consecutive steps of seizure microprogression. Such footprints could facilitate determining the recording location (e.g., inside/outside an epileptogenic focus) in high-resolution studies, even in the absence of a priori knowledge about where exactly a seizure started.

Nanoscale Fluid Vortices and Nonlinear Electroosmotic Flow Drive Ion Current Rectification in the Presence of Concentration Gradients

Ion current rectification (ICR) is a transport phenomenon in which an electrolyte conducts unequal currents at equal and opposite voltages. Here, we show that nanoscale fluid vortices and nonlinear electroosmotic flow (EOF) drive ICR in the presence of concentration gradients. The same EOF can yield negative differential resistance (NDR), in which current decreases with increasing voltage. A finite element model quantitatively reproduces experimental ICR and NDR recorded across glass nanopipettes under concentration gradients. The model demonstrates that spatial variations of electrical double layer properties induce the nanoscale vortices and nonlinear EOF. Experiments are performed in conditions directly related to scanning probe imaging and show that quantitative understanding of nanoscale transport under concentration gradients requires accounting for EOF. This characterization of nanopipette transport physics will benefit diverse experimentation, pushing the resolution limits of chemical and biophysical recordings.

Methods for single-cell recording and labeling in vivo

Targeting individual neurons in vivo is a key method to study the role of single cell types within local and brain-wide microcircuits. While novel technological developments now permit assessing activity from large number of cells simultaneously, there is currently no better solution than glass micropipettes to relate the physiology and morphology of single-cells. Sharp intracellular, juxtacellular, loose-patch and whole-cell approaches are some of the configurations used to record and label individual neurons. Here, we review procedures to establish successful electrophysiological recordings in vivo followed by appropriate labeling for post hoc morphological analysis. We provide operational recommendations for optimizing each configuration and a generic framework for functional, neurochemical and morphological identification of the different cell-types in a given region. Finally, we highlight emerging approaches that are challenging our current paradigms for single-cell recording and labeling in the living brain.

Approaches and Limitations in the Investigation of Synaptic Transmission and Plasticity

The numbers and strengths of synapses in the brain change throughout development, and even into adulthood, as synaptic inputs are added, eliminated, and refined in response to ongoing neural activity. A number of experimental techniques can assess these changes, including single-cell electrophysiological recording which offers measurements of synaptic inputs with high temporal resolution. Coupled with electrical stimulation, photoactivatable opsins, and caged compounds, to facilitate fine spatiotemporal control over release of neurotransmitters, electrophysiological recordings allow for precise dissection of presynaptic and postsynaptic mechanisms of action. Here, we discuss the strengths and pitfalls of various techniques commonly used to analyze synapses, including miniature excitatory/inhibitory (E/I) postsynaptic currents, evoked release, and optogenetic stimulation. Together, these techniques can provide multiple lines of convergent evidence to generate meaningful insight into the emergence of circuit connectivity and maturation. A full understanding of potential caveats and alternative explanations for findings is essential to avoid data misinterpretation.

Advances in the automation of whole-cell patch clamp technology

Electrophysiology is the study of neural activity in the form of local field potentials, current flow through ion channels, calcium spikes, back propagating action potentials and somatic action potentials, all measurable on a millisecond timescale. Despite great progress in imaging technologies and sensor proteins, none of the currently available tools allow imaging of neural activity on a millisecond timescale and beyond the first few hundreds of microns inside the brain. The patch clamp technique has been an invaluable tool since its inception several decades ago and has generated a wealth of knowledge about the nature of voltage- and ligand-gated ion channels, sub-threshold and supra-threshold activity, and characteristics of action potentials related to higher order functions. Many techniques that evolve to be standardized tools in the biological sciences go through a period of transformation in which they become, at least to some degree, automated, in order to improve reproducibility, throughput and standardization. The patch clamp technique is currently undergoing this transition, and in this review, we will discuss various aspects of this transition, covering advances in automated patch clamp technology both in vitro and in vivo.

Site-Specific Cytosol Sampling from a Single Cell in an Intact Tumor Spheroid Using an Electrochemical Syringe

A multicellular tumor aggregate, known as a spheroid, is an indispensable tool to study cancer biology. Owing to its three-dimensional organization, a spheroid exhibits an inherent gradient of nutrients, oxygen, and metabolites within itself. The spheroid provides culture conditions that resemble the microenvironment of certain cancer cells and causes these cells to acquire characteristics relevant to tumors in our body. However, site-specific gene expression analysis in an intact spheroid with single-cell resolution has not been explored. Recently, some types of electrochemical syringes were developed to extract cellular materials from living single cells for transcriptomic analysis. Here, we investigated whether an electrochemical syringe could be used to evaluate site-specific gene expression in a spheroid. A small amount of cytosol (roughly 540-1480 fL, less than the volume of a single cell) was successfully collected from the first, second, and third layers of the spheroid using an electrochemical syringe without causing damage to the spheroid architecture. We found that the CCNB1 and CCNA2 expression levels were different between the surface and the average of the entire spheroid, indicating that there are heterogeneous cellular functions across different regions of the spheroid. This method provides opportunities to improve our understanding of spatial gene expression of single cells in a three-dimensional environment.

Single-cell micro- and nano-photonic technologies

Since the advent of optogenetics, the technology development has focused on new methods to optically interact with single nerve cells. This gave rise to the field of photonic neural interfaces, intended as the set of technologies that can modify light radiation in either a linear or non-linear fashion to control and/or monitor cellular functions. This set includes the use of plasmonic effects, up-conversion, electron transfer and integrated light steering, with some of them already implemented in vivo. This article will review available approaches in this framework, with a particular emphasis on methods operating at the single-unit level or having the potential to reach single-cell resolution.