Microelectrode Arrays of Diamond-Insulated Graphitic Channels for Real-Time Detection of Exocytotic Events from Cultured Chromaffin Cells and Slices of Adrenal Glands

Methodologies for Detecting Quantal Exocytosis in Adrenal Chromaffin Cells Through Diamond-Based MEAs

Diamond-based multiarray sensors are suitable to detect in real-time exocytosis and action potentials from cultured, spontaneously firing chromaffin cells, primary hippocampal neurons, and midbrain dopaminergic neurons. Here, we focus on how amperometric measurements of catecholamine release are performed on micrographitic diamond multiarrays (μG-D-MEAs) with high temporal and spatial resolution by 16 electrodes simultaneously.

On-Chip Cyclic Voltammetry Measurements Using a Compact 1024-Electrode CMOS IC

Complementary metal-oxide-semiconductor (CMOS) microelectrode arrays integrate amplifier arrays with on-chip electrodes, offering high-throughput platforms for electrochemical sensing with high spatial and temporal resolution. Such devices have been developed for highly parallel constant voltage amperometric detection of transmitter release from multiple cells with single-vesicle resolution. Cyclic voltammetry (CV) is an electrochemical method that applies voltage waveforms, which provides additional information about electrode properties and about the nature of analytes. A 16-channel, 64-electrode-per-channel CMOS integrated circuit (IC) fabricated in a 0.5 μm CMOS process for CV is demonstrated. Each detector consists of only 11 transistors and an integration capacitor with a unit dimension of 0.0015 mm². The device was postfabricated using Pt as the working electrode material with a shifted electrode design, which makes it possible to redefine the size and the location of working electrodes. The system incorporating cell-sized (8 μm radius) microelectrodes was validated with dopamine injection tests and CV measurements of potassium ferricyanide at a 1 V/s scanning rate. The cyclic voltammograms were in excellent agreement with theoretical predictions. The technology enables rigorous characterization of electrode performance for the application of CMOS microelectrode arrays in low-noise amperometric measurements of quantal transmitter release as well as other biosensing applications.

Recent Progress in Quantitatively Monitoring Vesicular Neurotransmitter Release and Storage With Micro/Nanoelectrodes

Exocytosis is one of the essential steps for chemical signal transmission between neurons. In this process, vesicles dock and fuse with the plasma membrane and release the stored neurotransmitters through fusion pores into the extracellular space, and all of these steps are governed with various molecules, such as proteins, ions, and even lipids. Quantitatively monitoring vesicular neurotransmitter release in exocytosis and initial neurotransmitter storage in individual vesicles is significant for the study of chemical signal transmission of the central nervous system (CNS) and neurological diseases. Electrochemistry with micro/nanoelectrodes exhibits great spatial-temporal resolution and high sensitivity. It can be used to examine the exocytotic kinetics from the aspect of neurotransmitters and quantify the neurotransmitter storage in individual vesicles. In this review, we first introduce the recent advances of single-cell amperometry (SCA) and the nanoscale interface between two immiscible electrolyte solutions (nanoITIES), which can monitor the quantity and release the kinetics of electrochemically and non-electrochemically active neurotransmitters, respectively. Then, the development and application of the vesicle impact electrochemical cytometry (VIEC) and intracellular vesicle impact electrochemical cytometry (IVIEC) and their combination with other advanced techniques can further explain the mechanism of neurotransmitter storage in vesicles before exocytosis. It has been proved that these electrochemical techniques have great potential in the field of neuroscience.

Advanced real-time recordings of neuronal activity with tailored patch pipettes, diamond multi-electrode arrays and electrochromic voltage-sensitive dyes

To understand the working principles of the nervous system is key to figure out its electrical activity and how this activity spreads along the neuronal network. It is therefore crucial to develop advanced techniques aimed to record in real time the electrical activity, from compartments of single neurons to populations of neurons, to understand how higher functions emerge from coordinated activity. To record from single neurons, a technique will be presented to fabricate patch pipettes able to seal on any membrane with a single glass type and whose shanks can be widened as desired. This dramatically reduces access resistance during whole-cell recording allowing fast intracellular and, if required, extracellular perfusion. To simultaneously record from many neurons, biocompatible probes will be described employing multi-electrodes made with novel technologies, based on diamond substrates. These probes also allow to synchronously record exocytosis and neuronal excitability and to stimulate neurons. Finally, to achieve even higher spatial resolution, it will be shown how voltage imaging, employing fast voltage-sensitive dyes and two-photon microscopy, is able to sample voltage oscillations in the brain spatially resolved and voltage changes in dendrites of single neurons at millisecond and micrometre resolution in awake animals.

Microelectrode-Based Electrochemical Sensing Technology for in Vivo Detection of Dopamine: Recent Developments and Future Prospects

Dopamine (DA) is an essential type of neurotransmitter in the central nervous system. DA neurons usually exist as nuclei which are mainly found in the ventral tegmental area (VTN) and substantia nigra pars compacta (SNc). Parkinson's disease, epilepsy, schizophrenia and other diseases are all related to the abnormal metabolism of DA. Compared with traditional DA detection methods such as spectrophotometry and electrophoresis, electrochemical sensing technology has high detection efficiency, high sensitivity, fast and convenient real-time detection, which is recognized as the most effective method for measuring neurotransmitters in vivo. The working electrode of an electrochemical sensor can be generally divided into the conventional electrode and the microelectrode according to its size. The microelectrode shows excellent properties such as high sensitivity, high temporal resolution, and high spatial resolution while detecting DA, which makes it possible to detect neurotransmitters in vivo. In order to further investigate the role of DA in regulating action, emotion, and cognition, and to further clarify the relationship between DA abnormalities or lack and neurological diseases such as Parkinson, more and more researchers apply microelectrode-based electrochemistry sensing technology to detect DA in vivo. This article reviews recent applications of microelectrodes and the latest researches in DA detection in vivo, focusing on the following three types of microelectrodes: (1) non-nanomaterial-modified carbon fiber microelectrodes (CFE); (2) nanomaterial-modified microelectrodes; (3) microelectrode arrays (MEA).

Electrochemistry of Single-Vesicle Events

Neuronal transmission relies on electrical signals and the transfer of chemical signals from one neuron to another. Chemical messages are transmitted from presynaptic neurons to neighboring neurons through the triggered fusion of neurotransmitter-filled vesicles with the cell plasma membrane. This process, known as exocytosis, involves the rapid release of neurotransmitter solutions that are detected with high affinity by the postsynaptic neuron. The type and number of neurotransmitters released and the frequency of vesicular events govern brain functions such as cognition, decision making, learning, and memory. Therefore, to understand neurotransmitters and neuronal function, analytical tools capable of quantitative and chemically selective detection of neurotransmitters with high spatiotemporal resolution are needed. Electrochemistry offers powerful techniques that are sufficiently rapid to allow for the detection of exocytosis activity and provides quantitative measurements of vesicle neurotransmitter content and neurotransmitter release from individual vesicle events. In this review, we provide an overview of the most commonly used electrochemical methods for monitoring single-vesicle events, including recent developments and what is needed for future research.

Triggering Neurotransmitters Secretion from Single Cells by X-ray Nanobeam Irradiation

The employment of ionizing radiation is a powerful tool in cancer therapy, but beyond targeted effects, many studies have highlighted the relevance of its off-target consequences. An exhaustive understanding of the mechanisms underlying these effects is still missing, and no real-time data about signals released by cells during irradiation are presently available. We employed a synchrotron X-ray nanobeam to perform the first real-time simultaneous measurement of both X-ray irradiation and in vitro neurotransmitter release from individual adrenal phaeochromocytoma (PC12) cells plated over a diamond-based multielectrode array. We have demonstrated that, in specific conditions, X-rays can alter cell activity by promoting dopamine exocytosis, and such an effect is potentially very attractive for a more effective treatment of tumors.

Microfabricated electrochemical sensing devices

Electrochemistry provides possibilities to realize smart microdevices of the next generation with high functionalities. Electrodes, which constitute major components of electrochemical devices, can be formed by various microfabrication techniques, and integration of the same (or different) components for that purpose is not difficult. Merging this technique with microfluidics can further expand the areas of application of the resultant devices. To augment the development of next generation devices, it will be beneficial to review recent technological trends in this field and clarify the directions required for moving forward. Even when limiting the discussion to electrochemical microdevices, a variety of useful techniques should be considered. Therefore, in this review, we attempted to provide an overview of all relevant techniques in this context in the hope that it can provide useful comprehensive information.

Simultaneous multisite detection of quantal release from PC12 cells using micro graphitic-diamond multi electrode arrays

Micro graphitic - diamond - multi electrode arrays (μG-D-MEAs) are suitable for measuring multisite quantal dopamine (DA) release from PC12 cells. Following cell stimulation with high extracellular KCl and electrode polarization at +650 mV, amperometric spikes are detected with a mean frequency of 0.60 ± 0.16 Hz. In each recording, simultaneous detection of secretory events is occurred in approximately 50% of the electrodes. Kinetic spike parameters and background noise are preserved among the different electrodes. Comparing the amperometric spikes recorder under control conditions with those recorders from PC12 cells previously incubated for 30 min with the dopamine precursor Levodopa (L-DOPA, 20 μM) it appears that the quantal size of amperometric spikes is increased by 250% and the half-time width (t_1/2) by over 120%. On the contrary, L-DOPA has no effect on the frequency of secretory events. Overall, these data demonstrate that the μG-D-MEAs represent a reliable bio-sensor to simultaneously monitor quantal exocytotic events from different cells and in perspective can be exploited as a drug-screening tool.

Latest Trends in Electrochemical Sensors for Neurotransmitters: A Review

Neurotransmitters are endogenous chemical messengers which play an important role in many of the brain functions, abnormal levels being correlated with physical, psychotic and neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's disease. Therefore, their sensitive and robust detection is of great clinical significance. Electrochemical methods have been intensively used in the last decades for neurotransmitter detection, outclassing more complicated analytical techniques such as conventional spectrophotometry, chromatography, fluorescence, flow injection, and capillary electrophoresis. In this manuscript, the most successful and promising electrochemical enzyme-free and enzymatic sensors for neurotransmitter detection are reviewed. Focusing on the activity of worldwide researchers mainly during the last ten years (2010-2019), without pretending to be exhaustive, we present an overview of the progress made in sensing strategies during this time. Particular emphasis is placed on nanostructured-based sensors, which show a substantial improvement of the analytical performances. This review also examines the progress made in biosensors for neurotransmitter measurements in vitro, in vivo and ex vivo.

Quantal Release of Dopamine and Action Potential Firing Detected in Midbrain Neurons by Multifunctional Diamond-Based Microarrays

Micro-Graphitic Single Crystal Diamond Multi Electrode Arrays (μG-SCD-MEAs) have so far been used as amperometric sensors to detect catecholamines from chromaffin cells and adrenal gland slices. Besides having time resolution and sensitivity that are comparable with carbon fiber electrodes, that represent the gold standard for amperometry, μG-SCD-MEAs also have the advantages of simultaneous multisite detection, high biocompatibility and implementation of amperometric/potentiometric protocols, aimed at monitoring exocytotic events and neuronal excitability. In order to adapt diamond technology to record neuronal activity, the μG-SCD-MEAs in this work have been interfaced with cultured midbrain neurons to detect electrical activity as well as quantal release of dopamine (DA). μG-SCD-MEAs are based on graphitic sensing electrodes that are embedded into the diamond matrix and are fabricated using MeV ion beam lithography. Two geometries have been adopted, with 4 × 4 and 8 × 8 microelectrodes (20 μm × 3.5 μm exposed area, 200 μm spacing). In the amperometric configuration, the 4 × 4 μG-SCD-MEAs resolved quantal exocytosis from midbrain dopaminergic neurons. KCl-stimulated DA release occurred as amperometric spikes of 15 pA amplitude and 0.5 ms half-width, at a mean frequency of 0.4 Hz. When used as potentiometric multiarrays, the 8 × 8 μG-SCD-MEAs detected the spontaneous firing activity of midbrain neurons. Extracellularly recorded action potentials (APs) had mean amplitude of ∼-50 μV and occurred at a mean firing frequency of 0.7 Hz in 67% of neurons, while the remaining fired at 6.8 Hz. Comparable findings were observed using conventional MEAs (0.9 and 6.4 Hz, respectively). To test the reliability of potentiometric recordings with μG-SCD-MEAs, the D₂-autoreceptor modulation of firing was investigated by applying levodopa (L-DOPA, 20 μM), and comparing μG-SCD-MEAs, conventional MEAs and current-clamp recordings. In all cases, L-DOPA reduced the spontaneous spiking activity in most neurons by 70%, while the D₂-antagonist sulpiride reversed this effect. Cell firing inhibition was generally associated with increased APs amplitude. A minority of neurons was either insensitive to, or potentiated by L-DOPA, suggesting that AP recordings originate from different midbrain neuronal subpopulations and reveal different modulatory pathways. Our data demonstrate, for the first time, that μG-SCD-MEAs are multi-functional biosensors suitable to resolve real-time DA release and AP firing in in vitro neuronal networks.

Real-time characterization of uptake kinetics of glioblastoma vs. astrocytes in 2D cell culture using microelectrode array

Extracellular measurement of uptake/release kinetics and associated concentration dependencies provides mechanistic insight into the underlying biochemical processes. Due to the recognized importance of preserving the natural diffusion processes within the local microenvironment, measurement approaches which provide uptake rate and local surface concentration of adherent cells in static media are needed. This paper reports a microelectrode array device and a methodology to measure uptake kinetics as a function of cell surface concentration in adherent 2D cell cultures in static fluids. The microelectrode array simultaneously measures local concentrations at five positions near the cell surface in order to map the time-dependent concentration profile which in turn enables determination of surface concentrations and uptake rates, via extrapolation to the cell plane. Hydrogen peroxide uptake by human astrocytes (normal) and glioblastoma multiforme (GBM43, cancer) was quantified for initial concentrations of 20 to 500 μM over time intervals of 4000 s. For both cell types, the overall uptake rate versus surface concentration relationships exhibited non-linear kinetics, well-described by a combination of linear and Michaelis-Menten mechanisms and in agreement with the literature. The GBM43 cells showed a higher uptake rate over the full range of concentrations, primarily due to a larger linear component. Diffusion-reaction models using the non-linear parameters and standard first-order relationships are compared. In comparison to results from typical volumetric measurements, the ability to extract both uptake rate and surface concentration in static media provides kinetic parameters that are better suited for developing reaction-diffusion models to adequately describe behavior in more complex culture/tissue geometries. The results also highlight the need for characterization of the uptake rate over a wider range of cell surface concentrations in order to evaluate the potential therapeutic role of hydrogen peroxide in cancerous cells.

Boron Doped Diamond: A Designer Electrode Material for the Twenty-First Century

Boron doped diamond (BDD) is continuing to find numerous electrochemical applications across a diverse range of fields due to its unique properties, such as having a wide solvent window, low capacitance, and reduced resistance to fouling and mechanical robustness. In this review, we showcase the latest developments in the BDD electrochemical field. These are driven by a greater understanding of the relationship between material (surface) properties, required electrochemical performance, and improvements in synthetic growth/fabrication procedures, including material postprocessing. This has resulted in the production of BDD structures with the required function and geometry for the application of interest, making BDD a truly designer material. Current research areas range from in vivo bioelectrochemistry and neuronal/retinal stimulation to improved electroanalysis, advanced oxidation processes, supercapacitors, and the development of hybrid electrochemical-spectroscopic- and temperature-based technology aimed at enhancing electrochemical performance and understanding.

Surface-modified CMOS IC electrochemical sensor array targeting single chromaffin cells for highly parallel amperometry measurements

Amperometry is a powerful method to record quantal release events from chromaffin cells and is widely used to assess how specific drugs modify quantal size, kinetics of release, and early fusion pore properties. Surface-modified CMOS-based electrochemical sensor arrays allow simultaneous recordings from multiple cells. A reliable, low-cost technique is presented here for efficient targeting of single cells specifically to the electrode sites. An SU-8 microwell structure is patterned on the chip surface to provide insulation for the circuitry as well as cell trapping at the electrode sites. A shifted electrode design is also incorporated to increase the flexibility of the dimension and shape of the microwells. The sensitivity of the electrodes is validated by a dopamine injection experiment. Microwells with dimensions slightly larger than the cells to be trapped ensure excellent single-cell targeting efficiency, increasing the reliability and efficiency for on-chip single-cell amperometry measurements. The surface-modified device was validated with parallel recordings of live chromaffin cells trapped in the microwells. Rapid amperometric spikes with no diffusional broadening were observed, indicating that the trapped and recorded cells were in very close contact with the electrodes. The live cell recording confirms in a single experiment that spike parameters vary significantly from cell to cell but the large number of cells recorded simultaneously provides the statistical significance.

Electrochemical measurement of quantal exocytosis using microchips

Carbon-fiber electrodes (CFEs) are the gold standard for quantifying the release of oxidizable neurotransmitters from single vesicles and single cells. Over the last 15 years, microfabricated devices have emerged as alternatives to CFEs that offer the possibility of higher throughput, subcellular spatial resolution of exocytosis, and integration with other techniques for probing exocytosis including microfluidic cell handling and solution exchange, optical imaging and stimulation, and electrophysiological recording and stimulation. Here we review progress in developing electrochemical electrode devices capable of resolving quantal exocytosis that are fabricated using photolithography.

Low pHo boosts burst firing and catecholamine release by blocking TASK-1 and BK channels while preserving Cav1 channels in mouse chromaffin cells

KEY POINTS

Mouse chromaffin cells (MCCs) generate spontaneous burst-firing that causes large increases of Ca²⁺ -dependent catecholamine release, and is thus a key mechanism for regulating the functions of MCCs. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 6.6 induces cell depolarizations of 10-15 mV that generate bursts of ∼330 ms at 1-2 Hz and a 7.4-fold increase of cumulative catecholamine-release. Burst-firing originates from the inhibition of the pH-sensitive TASK-1-channels and a 60% reduction of BK-channel conductance at pH_o 6.6. Blockers of the two channels (A1899 and paxilline) mimic the effects of pH_o 6.6, and this is reverted by the Cav1 channel blocker nifedipine. MCCs act as pH-sensors. At low pH_o , they depolarize, undergo burst-firing and increase catecholamine-secretion, generating an effective physiological response that may compensate for the acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

ABSTRACT

Mouse chromaffin cells (MCCs) generate action potential (AP) firing that regulates the Ca²⁺ -dependent release of catecholamines (CAs). Recent findings indicate that MCCs possess a variety of spontaneous firing modes that span from the common 'tonic-irregular' to the less frequent 'burst' firing. This latter is evident in a small fraction of MCCs but occurs regularly when Nav1.3/1.7 channels are made less available or when the Slo1β2-subunit responsible for BK channel inactivation is deleted. Burst firing causes large increases of Ca²⁺ -entry and potentiates CA release by ∼3.5-fold and thus may be a key mechanism for regulating MCC function. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pH_o ) from 7.4 to 7.0 and 6.6 induces cell depolarizations of 10-15 mV that generate repeated bursts. Bursts at pH_o 6.6 lasted ∼330 ms, occurred at 1-2 Hz and caused an ∼7-fold increase of CA cumulative release. Burst firing originates from the inhibition of the pH-sensitive TASK-1/TASK-3 channels and from a 40% BK channel conductance reduction at pH_o 7.0. The same pH_o had little or no effect on Nav, Cav, Kv and SK channels that support AP firing in MCCs. Burst firing of pH_o 6.6 could be mimicked by mixtures of the TASK-1 blocker A1899 (300 nm) and BK blocker paxilline (300 nm) and could be prevented by blocking L-type channels by adding 3 μm nifedipine. Mixtures of the two blockers raised cumulative CA-secretion even more than low pH_o (∼12-fold), showing that the action of protons on vesicle release is mainly a result of the ionic conductance changes that increase Ca²⁺ -entry during bursts. Our data provide direct evidence suggesting that MCCs respond to low pH_o with sustained depolarization, burst firing and enhanced CA-secretion, thus mimicking the physiological response of CCs to acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.

Planar Diamond-Based Multiarrays to Monitor Neurotransmitter Release and Action Potential Firing: New Perspectives in Cellular Neuroscience

High biocompatibility, outstanding electrochemical responsiveness, inertness, and transparency make diamond-based multiarrays (DBMs) first-rate biosensors for in vitro detection of electrochemical and electrical signals from excitable cells together, with potential for in vivo applications as neural interfaces and prostheses. Here, we will review the electrochemical and physical properties of various DBMs and how these devices have been employed for recording released neurotransmitter molecules and all-or-none action potentials from living cells. Specifically, we will overview how DBMs can resolve localized exocytotic events from subcellular compartments using high-density microelectrode arrays (MEAs), or monitoring oxidizable neurotransmitter release from populations of cells in culture and tissue slices using low-density MEAs. Interfacing DBMs with excitable cells is currently leading to the promising opportunity of recording electrical signals as well as creating neuronal interfaces through the same device. Given the recent increasingly growing development of newly available DBMs of various geometries to monitor electrical activity and neurotransmitter release in a variety of excitable and neuronal tissues, the discussion will be limited to planar DBMs.