Nicotinic receptor-mediated catecholamine secretion from individual chromaffin cells. Chemical evidence for exocytosis

Lead exerts a depression of neurotransmitter release through a blockade of voltage dependent calcium channels in chromaffin cells

The present work, using chromaffin cells of bovine adrenal medullae (BCCs), aims to describe what type of ionic current alterations induced by lead (Pb2+) underlies its effects reported on synaptic transmission. We observed that the acute application of Pb2+ lead to a drastic depression of neurotransmitters release in a concentration-dependent manner when the cells were stimulated with both K+ or acetylcholine, with an IC50 of 119,57 μM and of 5,19 μM, respectively. This effect was fully recovered after washout. Pb2+ also blocked calcium channels of BCCs in a time- and concentration-dependent manner with an IC50 of 6,87 μM. This blockade was partially reversed upon washout. This compound inhibited the calcium current at all test potentials and shows a shift of the I-V curve to more negative values of about 8 mV. The sodium current was not blocked by acute application of high Pb2+ concentrations. Voltage-dependent potassium current was also shortly affected by high Pb2+. Nevertheless, the calcium- and voltage-dependent potassium current was drastically depressed in a dose-dependent manner, with an IC50 of 24,49 μM. This blockade was related to the prevention of Ca2+ influx through voltage-dependent calcium channels coupled to Ca2+-activated K+-channels (BK) instead a direct linking to these channels. Under current-clamp conditions, BCCs exhibit a resting potential of -52.7 mV, firing spontaneous APs (1-2 spikes/s) generated by the opening of Na+ and Ca2+-channels, and terminated by the activation of K+ channels. In spite of the effect on ionic channels exerted by Pb2+, we found that Pb2+ didn't alter cellular excitability, no modification of the membrane potential, and no effect on action potential firing. Taken together, these results point to a neurotoxic action evoked by Pb2+ that is associated with changes in neurotransmitter release by blocking the ionic currents responsible for the calcium influx.

Single vesicle chemistry reveals partial release happens at the mechanical stress-induced exocytosis

Neuronal activity can be modulated by mechanical stress in the central nervous system (CNS) in neurodegenerative diseases, for example Alzheimer's disease. However, the impact of mechanical stress on chemical signal transmission, especially the storage and release of neurotransmitter in neuron vesicles, has not been fully clarified. In this study, a nanotip conical carbon fiber microelectrode (CFME) and a disk CFME are placed in and on a cell, respectively. The nanotip conical CFME functions for both the mechanical stress and the quantification of transmitter storage in single vesicles, while the disk CFME is used to monitor the transmitter release during exocytosis induced by mechanical stress at the same cell. By comparing the vesicular transmitter storage with its release during mechanical stress-induced exocytosis at the same cell, we find the release ratio of transmitter in chromaffin cells varies from 27 % to 100 %, while for PC12 cells from 30 % to 100 %. Our results indicate that the exocytosis of cells responding to mechanical stress shows individual difference obviously, with a significant population exhibiting partial release mode. The variation of Ca2+ channels and mechanosensitive ion channels on cell membrane may both contribute to this variation. Our discovery not only shows mechanical stress can change the transmission of cellular chemical signals at the vesicle level, but also provides an important reference perspective for the study of nervous system regulation and nervous system diseases.

Reconstruction of Vesicle Assemblies with DNA Nanorulers for Resolving Heterogeneity of Vesicles in Live Cells

Nanoscale vesicles such as synaptic vesicles play a pivotal role in efficient interneuronal communications in vivo. However, the coexistence of single vesicle and vesicle clusters in living cells increases the heterogeneity of vesicle populations, which largely complicates the quantitative analysis of the vesicles. The high spatiotemporal monitoring of vesicle assemblies is currently incompletely resolved. Here, this work uses synthetic vesicles and DNA nanorulers to reconstruct in vitro the vesicle assemblies that mimic vesicle clusters in living cells. DNA nanorulers program the lateral distance of vesicle assemblies from 3 to 10 nm. This work uses the carbon fiber nanoelectrode (CFNE) to amperometric monitor artificial vesicle assemblies with sub-10 nm interspaces, and obtain a larger proportion of complex events. This work resolves the heterogeneity of individual vesicle release kinetics in PC12 cells with the temporal resolution down to ≈0.1 ms. This work further analyzes the aggregation state of intracellular vesicles and the exocytosis of living cells with electrochemical vesicle cytometry. The results indicate that the exocytosis of vesicle clusters is critically dependent on the size of clusters. This technology has the potential as a tool to shed light on the heterogeneity analysis of vesicle populations.

Omega-3 and -6 Fatty Acids Alter the Membrane Lipid Composition and Vesicle Size to Regulate Exocytosis and Storage of Catecholamines

The two essential fatty acids, alpha-linolenic acid and linoleic acid, and the higher unsaturated fatty acids synthesized from them are critical for the development and maintenance of normal brain functions. Deficiencies of these fatty acids have been shown to cause damage to the neuronal development, cognition, and locomotor function. We combined electrochemistry and imaging techniques to examine the effects of the two essential fatty acids on catecholamine release dynamics and the vesicle content as well as on the cell membrane phospholipid composition to understand how they impact exocytosis and by extension neurotransmission at the single-cell level. Incubation of either of the two fatty acids reduces the size of secretory vesicles and enables the incorporation of more double bonds into the cell membrane structure, resulting in higher membrane flexibility. This subsequently affects proteins regulating the dynamics of the exocytotic fusion pore and thereby affects exocytosis. Our data suggest a possible pathway whereby the two essential fatty acids affect the membrane structure to impact exocytosis and provide a potential treatment for diseases and impairments related to catecholamine signaling.

Analytical Determination of Serotonin Exocytosis in Human Platelets with BDD-on-Quartz MEA Devices

Amperometry is arguably the most widely used technique for studying the exocytosis of biological amines. However, the scarcity of human tissues, particularly in the context of neurological diseases, poses a challenge for exocytosis research. Human platelets, which accumulate 90% of blood serotonin, release it through exocytosis. Nevertheless, single-cell amperometry with encapsulated carbon fibers is impractical due to the small size of platelets and the limited number of secretory granules on each platelet. The recent technological improvements in amperometric multi-electrode array (MEA) devices allow simultaneous recordings from several high-performance electrodes. In this paper, we present a comparison of three MEA boron-doped diamond (BDD) devices for studying serotonin exocytosis in human platelets: (i) the BDD-on-glass MEA, (ii) the BDD-on-silicon MEA, and (iii) the BDD on amorphous quartz MEA (BDD-on-quartz MEA). Transparent electrodes offer several advantages for observing living cells, and in the case of platelets, they control activation/aggregation. BDD-on-quartz offers the advantage over previous materials of combining excellent electrochemical properties with transparency for microscopic observation. These devices are opening exciting perspectives for clinical applications.

Exploiting Microelectrode Geometry for Comprehensive Detection of Individual Exocytosis Events at Single Cells

Carbon fiber microelectrodes are commonly used for real-time monitoring of individual exocytosis events at single cells. Since the nature of an electrochemical signal is fundamentally governed by mass transport to the electrode surface, microelectrode geometry can be exploited to achieve precise and accurate measurements. Researchers traditionally pair amperometric measurements of exocytosis with a ∼10-μm diameter, disk microelectrode in an "artificial synapse" configuration to directly monitor individual release events from single cells. Exocytosis is triggered, and released molecules diffuse to the "post-synaptic" electrode for oxidation. This results in a series of distinct current spikes corresponding to individual exocytosis events. However, it remains unclear how much of the material escapes detection. In this work, the performance of 10- and 34-μm diameter carbon fiber disk microelectrodes was directly compared in monitoring exocytosis at single chromaffin cells. The 34-μm diameter electrode was more sensitive to catecholamines and enkephalins than its traditional, 10-μm diameter counterpart, and it more effectively covered the entire cell. As such, the larger sensor detected more exocytosis events overall, as well as a larger quantal size, suggesting that the traditional tools underestimate the above measurements. Both sensors reliably measured l-DOPA-evoked changes in quantal size, and both exhibited diffusional loss upon adjustment of cell-electrode spacing. Finite element simulations using COMSOL support the improved collection efficiency observed using the larger sensor. Overall, this work demonstrates how electrode geometry can be exploited for improved detection of exocytosis events by addressing diffusional loss─an often-overlooked source of inaccuracy in single-cell measurements.

Hofmeister Series: From Aqueous Solution of Biomolecules to Single Cells and Nanovesicles

Hofmeister effects play a critical role in numerous physicochemical and biological phenomena, including the solubility and/or accumulation of proteins, the activities of enzymes, ion transport in biochannels, the structure of lipid bilayers, and the dynamics of vesicle opening and exocytosis. This minireview focuses on how ionic specificity affects the physicochemical properties of biomolecules to regulate cellular exocytosis, vesicular content, and nanovesicle opening. We summarize recent progress in further understanding Hofmeister effects on biomacromolecules and their applications in biological systems. These important steps have increased our understanding of the Hofmeister effects on cellular exocytosis, vesicular content, and nanovesicle opening. Increasing evidence is firmly establishing that the ions along the Hofmeister series play an important role in living organisms that has often been ignored.

The dynamic nature of exocytosis from large secretory vesicles. A view from electrochemistry and imaging

In this brief review, we discuss the factors that modulate the quantum size and the kinetics of exocytosis. We also discuss the determinants which motivate the type of exocytosis from the so-called kiss-and-run to full fusion and along the intermediate mode of partial release. Kiss-and-run release comprises the transient opening of a nanometer (approx. 2 nm diameter) fusion pore between vesicle and plasma membrane allowing a small amount of release. Partial release comprises a larger more extended opening of the pore to allow a larger fraction of released vesicle content and is what is observed as normal full release in most electrochemical measurements. Partial release appears to be dominant in dense core vesicles and perhaps synaptic vesicles. The concept of partial release leads to the fraction released as a plastic component of exocytosis. Partial vesicular distension and the kinetics of exocytosis can be modulated by second messengers, physiological modulators, and drugs. This concept adds a novel point of regulation for the exocytotic process.

Aluminum alters excitability by inhibiting calcium, sodium, and potassium currents in bovine chromaffin cells

Aluminum (Al³⁺ ) has long been related to neurotoxicity and neurological diseases. This study aims to describe the specific actions of this metal on cellular excitability and neurotransmitter release in primary culture of bovine chromaffin cells. Using voltage-clamp and current-clamp recordings with the whole-cell configuration of the patch clamp technique, online measurement of catecholamine release, and measurements of [Ca²⁺ ]_c with Fluo-4-AM, we have observed that Al³⁺ reduced intracellular calcium concentrations around 25% and decreased catecholamine secretion in a dose-dependent manner, with an IC₅₀ of 89.1 μM. Al³⁺ blocked calcium currents in a time- and concentration-dependent manner with an IC₅₀ of 560 μM. This blockade was irreversible since it did not recover after washout. Moreover, Al³⁺ produced a bigger blockade on N-, P-, and Q-type calcium channels subtypes (69.5%) than on L-type channels subtypes (50.5%). Sodium currents were also inhibited by Al³⁺ in a time- and concentration-dependent manner, 24.3% blockade at the closest concentration to the IC₅₀ (399 μM). This inhibition was reversible. Voltage-dependent potassium currents were low affected by Al³⁺ . Nonetheless, calcium/voltage-dependent potassium currents were inhibited in a concentration-dependent manner, with an IC₅₀ of 447 μM. This inhibition was related to the depression of calcium influx through voltage-dependent calcium channels subtypes coupled to BK channels. In summary, the blockade of these ionic conductance altered cellular excitability that reduced the action potentials firing and so, the neurotransmitter release and the synaptic transmission. These findings prove that aluminum has neurotoxic properties because it alters neuronal excitability by inhibiting the sodium currents responsible for the generation and propagation of impulse nerve, the potassium current responsible for the termination of action potentials, and the calcium current responsible for the neurotransmitters release.

Vitamin D Inhibits the Early Aggregation of α-Synuclein and Modulates Exocytosis Revealed by Electrochemical Measurements

Alpha-synuclein (α-Syn) localizes at presynaptic terminal and modulates synaptic functions. Increasing evidence demonstrate that α-Syn oligomers, forming at the early of aggregation, are cytotoxic and is thus related to brain neurodegenerative diseases. Herein, we find that vitamin D (VD) can reduce neurocytotoxicity. The reduced neurocytotoxicity might be attributed to the less amount of large-sized α-Syn oligomers inhibited by VD, measured by electrochemical collision at single particle level, which are not observable with traditionally ensembled method. Single-cell amperometry (SCA) results show that VD can recover the amount of neurotransmitter release during exocytosis induced by α-Syn oligomers, further verifying the neuroprotection of VD. Our study reveals the neuroprotective role of VD through inhibiting α-Syn aggregation, which is envisioned to be of great importance in treatment and prevention of the neurodegenerative diseases.

Nano-analysis reveals high fraction of serotonin release during exocytosis from a gut epithelium model cell

Electrochemical methods were used to explore the exocytotic nature of serotonin (5‐HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut‐derived 5‐HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5‐HT release in BON cells is 80 % compared to previous work in pancreatic beta cells (34 %). The fractional release increased from 80 % in control BON cells to 87 % with 5‐HT preincubation and nearly 100 % with the combination of 5‐HT and the 5‐HT₄ autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5‐HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.

Plasmodium chabaudi Affects Mast Cell Degranulation as Measured by Carbon-Fiber Microelectrode Amperometry

Mast cells (MCs) are effector cells of the immune system commonly known for their role in asthma and allergy. They are present throughout biological systems in various tissues, serving as an interface between the biological system and environment. Previous work characterizing the impact of malaria on MCs revealed contradictory results, showing minimal to strong correlation between MC degranulation and disease progression. This work seeks to reveal how MC degranulation is impacted in the presence of malaria, induced by Plasmodium chabaudi, using a mouse model and a single cell measurement technique that reveals exquisite biophysical detail about any impacts to the degranulation process. It was hypothesized that the malaria parasites would impact MC degranulation response during live infection, and the differences would be revealed via carbon-fiber microelectrode amperometry. In fact, the data collected show that different stages of malaria infection affect MC degranulation differently, affirming the importance of considering different infection stages in future studies of malarial immune response. Furthermore, a comparison of MC degranulation response to that measured from platelets under similar circumstances shows similar trends in quantitative degranulation, suggesting that MC and platelet exocytosis machinery are affected similarly despite their distinct biological roles. However, based on the small number of mouse replicates, the studies herein suggest that there should be further study about cellular and disease processes. Overall, the work herein reveals important details about the role of MCs in malaria progression, relevant during treatment decisions, as well as a potentially generalizable impact on chemical messenger secretion from cells during malarial progression.

Simultaneous detection of vesicular content and exocytotic release with two electrodes in and at a single cell

We developed a technique employing two electrodes to simultaneously and dynamically monitor vesicular neurotransmitter storage and vesicular transmitter release in and at the same cell. To do this, two electrochemical techniques, single-cell amperometry (SCA) and intracellular vesicle impact electrochemical cytometry (IVIEC), were applied using two nanotip electrodes. With one electrode being placed on top of a cell measuring exocytotic release and the other electrode being inserted into the cytoplasm measuring vesicular transmitter storage, upon chemical stimulation, exocytosis is triggered and the amount of release and storage can be quantified simultaneously and compared. By using this technique, we made direct comparison between exocytotic release and vesicular storage, and investigated the dynamic changes of vesicular transmitter content before, during, and after chemical stimulation of PC12 cells, a neuroendocrine cell line. While confirming that exocytosis is partial, we suggest that chemical stimulation either induces a replenishment of the releasable pool with a subpool of vesicles having higher amount of transmitter storage, or triggers the vesicles within the same subpool to load more transiently at approximately 10–20 s. Thus, a time scale for vesicle reloading is determined. The effect of l-3,4-dihydroxyphenylalanine (l-DOPA), the precursor to dopamine, on the dynamic alteration of vesicular storage upon chemical stimulation for exocytosis was also studied. We found that l-DOPA incubation reduces the observed changes of vesicular storage in regular PC12 cells, which might be due to an increased capacity of vesicular transmitter loading caused by l-DOPA. Our data provide another mechanism for plasticity after stimulation via quantitative and dynamic changes in the exocytotic machinery.

Simultaneous measurements of IVIEC and SCA by two nanotip electrodes allows direct and dynamic comparison between vesicular transmitter content and vesicular transmitter release to shed light on stimulation-induced plasticity.

Electrogenerated chemiluminescence detection of single entities

Electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), is an electrochemically induced production of light by excited luminophores generated during redox reactions. It can be used to sense the charge transfer and related processes at electrodes via a simple visual readout; hence, ECL is an outstanding tool in analytical sensing. The traditional ECL approach measures averaged electrochemical quantities of a large ensemble of individual entities, including molecules, microstructures and ions. However, as a real system is usually heterogeneous, the study of single entities holds great potential in elucidating new truths of nature which are averaged out in ensemble assays or hidden in complex systems. We would like to review the development of ECL intensity and imaging based single entity detection and place emphasis on the assays of small entities including single molecules, micro/nanoparticles and cells. The current challenges for and perspectives on ECL detection of single entities are also discussed.

Electrochemical Quantification of Neurotransmitters in Single Live Cell Vesicles Shows Exocytosis is Predominantly Partial

Exocytosis plays an essential role in the communication between cells in the nervous system. Understanding the regulation of neurotransmitter release during exocytosis and the amount of neurotransmitter content that is stored in vesicles is of importance, as it provides fundamental insights to understand how the brain works and how neurons elicit a certain behavior. In this minireview, we summarize recent progress in amperometric measurements for monitoring exocytosis in single cells and electrochemical cytometry measurements of vesicular neurotransmitter content in individual vesicles. Important steps have increased our understanding of the different mechanisms of exocytosis. Increasing evidence is firmly establishing that partial release is the primary mechanism of release in multiple cell types.

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.

Comparison of Disk and Nanotip Electrodes for Measurement of Single-Cell Amperometry during Exocytotic Release

We compare single-cell amperometric measurements of exocytosis from pheochromocytoma (PC12) cells between two types of electrodes, carbon fiber disk microelectrodes and nanotip conical-shape carbon fiber microelectrodes. During the exocytotic process, individual exocytotic release events, measured as current spikes at the electrode, offer quantitative and dynamic information about the chemical release from cells. Using two electrodes gives rise to an unequal distance between the fusion pore and the electrode as well as fusion pore size, which leads to different average spike shapes. Nanotip electrodes show a slightly higher and narrower spike than disk electrodes when measuring exocytosis. The estimated pore-electrode distance and fusion pore size for disk electrodes are 239 and 11.5 nm, while for nanotip electrodes, these are 215 and 18.2 nm, respectively. The data show that nanotip electrodes, despite showing slightly different dynamics for release, are quantitative in measuring the number of molecules released and can be used for quantitative comparison between exocytosis and vesicular content in intracellular vesicle impact electrochemical cytometry.

Vesicular Transmitter Content in Chromaffin Cells Can Be Regulated via Extracellular ATP

The energy carrying molecule adenosine triphosphate (ATP) has been implicated for its role in modulation of chemical signaling for some time. Despite this, the precise effects and mechanisms of action of ATP on secretory cells are not well-known. Here, bovine chromaffin cells have been used as a model system to study the effects of extracellular ATP in combination with the catecholamine transmitter norepinephrine (NE). Both transmitter storage and exocytotic release were quantified using complementary amperometric techniques. Although incubation with NE alone did not cause any changes to either transmitter storage or release, coincubation with NE and ATP resulted in a significant increase that was concentration dependent. To probe the potential mechanisms of action, a slowly hydrolyzable version of ATP, ATP-γ-S, was used either alone or together with NE. The result implicates two different behaviors of ATP acting on both the purinergic autoreceptors and as a source of the energy needed to load chromaffin cell vesicles.

Regulation of catecholamine release from the adrenal medulla is altered in deer mice (Peromyscus maniculatus) native to high altitudes

High-altitude natives have evolved to overcome environmental hypoxia and provide a compelling system to understand physiological function during reductions in oxygen availability. The sympathoadrenal system plays a key role in responses to acute hypoxia, but prolonged activation of this system in chronic hypoxia may be maladaptive. Here, we examined how chronic hypoxia exposure alters adrenal catecholamine secretion and how adrenal function is altered further in high-altitude natives. Populations of deer mice (Peromyscus maniculatus) native to low and high altitudes were each born and raised in captivity at sea level, and adults from each population were exposed to normoxia or hypobaric hypoxia for 5 mo. Using carbon fiber amperometry on adrenal slices, catecholamine secretion evoked by low doses of nicotine (10 µM) or acute hypoxia (Po₂ ∼15-20 mmHg) was reduced in lowlanders exposed to hypobaric hypoxia, which was attributable mainly to a decrease in quantal charge rather than event frequency. However, secretion evoked by high doses of nicotine (50 µM) was unaffected. Hypobaric hypoxia also reduced plasma epinephrine and protein expression of 3,4-dihydroxyphenylalanine (DOPA) decarboxylase in the adrenal medulla of lowlanders. In contrast, highlanders were unresponsive to hypobaric hypoxia, exhibiting typically low adrenal catecholamine secretion, plasma epinephrine, and DOPA decarboxylase. Highlanders also had consistently lower catecholamine secretion evoked by high nicotine, smaller adrenal medullae with fewer chromaffin cells, and a larger adrenal cortex compared with lowlanders across both acclimation environments. Our results suggest that plastic responses to chronic hypoxia along with evolved changes in adrenal function attenuate catecholamine release in deer mice at high altitude.

Electrochemistry at the Synapse

Electrochemical measurements of neurotransmitters provide insight into the dynamics of neurotransmission. In this review, we describe the development of electrochemical measurements of neurotransmitters and how they started with extrasynaptic measurements but now are pushing toward synaptic measurements. Traditionally, biosensors or fast-scan cyclic voltammetry have monitored extrasynaptic levels of neurotransmitters, such as dopamine, serotonin, adenosine, glutamate, and acetylcholine. Amperometry and electrochemical cytometry techniques have revealed mechanisms of exocytosis, suggesting partial release. Advances in nanoelectrodes now allow spatially resolved, electrochemical measurements in a synapse, which is only 20-100 nm wide. Synaptic measurements of dopamine and acetylcholine have been made. In this article, electrochemical measurements are also compared to optical imaging and mass spectrometry measurements, and while these other techniques provide enhanced spatial or chemical information, electrochemistry is best at monitoring real-time neurotransmission. Future challenges include combining electrochemistry with these other techniques in order to facilitate multisite and multianalyte monitoring.

Extracellular ATP Regulates the Vesicular Pore Opening in Chromaffin Cells and Increases the Fraction Released During Individual Exocytosis Events

Adenosine triphosphate (ATP) is the main energy source for cellular metabolism. Besides that, ATP is a neurotransmitter and a cotransmitter that acts on purinergic receptors present either pre- or postsynaptically. Almost all types of secretory vesicles from any neuron or animal species contain high concentrations of ATP, being an essential factor in the accumulation of neurotransmitters. In this work, we studied the effects of ATP on quantum catecholamine release and vesicular storage in chromaffin cells. We combined three electrochemical methods: conventional amperometry with intracellular vesicle impact electrochemical cytometry and vesicle impact electrochemical cytometry. We found that extracellular ATP increased the released quantal fraction of catecholamine but not its vesicular content. Studying the dynamics of exocytosis events in ATP treated cells showed that ATP affects the release fusion pore. To elucidate the mechanisms of the observed ATP effects, cells and vesicles were pharmacologically treated with suramin (a purinergic blocker) and ARL-67156 (an antagonist of ecto-ATPases). The data indicate that the catecholamine content of vesicles increased compared to control after these drugs. Our data suggest that ATP acting on purinergic receptors increases the quantum releasable size through an increased fusion pore opening and that ARL-67156 and/or suramin protect the vesicle from neurotransmitter leakage by functioning as competitive inhibitors to ATP.

Altered exocytosis in chromaffin cells from mouse models of neurodegenerative diseases

Chromaffin cells from the adrenal gland (CCs) have extensively been used to explore the molecular structure and function of the exocytotic machinery, neurotransmitter release and synaptic transmission. The CC is integrated in the sympathoadrenal axis that helps the body maintain homoeostasis during both routine life and in acute stress conditions. This function is exquisitely controlled by the cerebral cortex and the hypothalamus. We propose the hypothesis that damage undergone by the brain during neurodegenerative diseases is also affecting the neurosecretory function of adrenal medullary CCs. In this context, we review here the following themes: (i) How the discharge of catecholamines is centrally and peripherally regulated at the sympathoadrenal axis; (ii) which are the intricacies of the amperometric techniques used to study the quantal release of single-vesicle exocytotic events; (iii) which are the alterations of the exocytotic fusion pore so far reported, in CCs of mouse models of neurodegenerative diseases; (iv) how some proteins linked to neurodegenerative pathologies affect the kinetics of exocytotic events; (v) finally, we try to integrate available data into a hypothesis to explain how the centrally originated neurodegenerative diseases may alter the kinetics of single-vesicle exocytotic events in peripheral adrenal medullary CCs.

Simultaneous voltammetric detection of 5-hydroxyindole-3-acetic acid and 5-hydroxytryptamine using a glassy carbon electrode modified with conducting polymer and platinised carbon nanofibers

The authors describe a method for simultaneous voltammetric determination of 5-hydroxytryptamine (serotonin; 5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). A glassy carbon electrode was modified with poly(pyrrole-3-carboxylic acid) and with platinised carbon nanofibers to obtain a sensor that can quantify 5-HT and 5-HIAA with detection limits of 10 nM and 20 nM, respectively. The peak currents, best measured at voltages of 170 mV and 500 mV (vs. Ag/AgCl) for 5-HT and 5-HIAA, increase linearly in the 0.01-100 μM concentration range for both analytes. The method was successfully applied to the quantitation of 5-HT and 5-HIAA in spiked artificial urine samples, and the sensor can be used up to 10 days. Graphical abstract A new electroanalytical device was developed for separation and quantitation of 5-hydroxytryptamine (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA), based on stripping square wave voltammetry, exploiting conducting polymer surfaces on platinised carbon nanofiber supports.

Characterization of Membrane Patch-Ion Channel Probes for Scanning Ion Conductance Microscopy

Integration of dual-barrel membrane patch-ion channel probes (MP-ICPs) to scanning ion conductance microscopy (SICM) holds promise of providing a revolutionized approach of spatially resolved chemical sensing. A series of experiments are performed to further the understanding of the system and to answer some fundamental questions, in preparation for future developments of this approach. First, MP-ICPs are constructed that contain different types of ion channels including transient receptor potential vanilloid 1 and large conductance Ca2⁺ -activated K⁺ channels to establish the generalizability of the methods. Next, the capability of the MP-ICP platforms in single ion channel activity measurements is proved. In addition, the interplay between the SICM barrel and the ICP barrel is studied. For ion channels gated by uncharged ligands, channel activity at the ICP barrel is unaffected by the SICM barrel potential; whereas for ion channels that are gated by charged ligands, enhanced channel activity can be obtained by biasing the SICM barrel at potentials with opposite polarity to the charge of the ligand molecules. Finally, a proof-of-principle experiment is performed and site-specific molecular/ionic flux sensing is demonstrated at single-ion-channel level, which show that the MP-ICP platform can be used to quantify local molecular/ionic concentrations.

The unfolding landscape of the congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is impaired by one or more specific mechanisms. Since the advent of next-generation sequencing methods, the discovery of novel CMS targets and phenotypes has proceeded at an accelerated rate. Here, we review the current classification of CMS and describe our findings in five of these targets identified and investigated in our laboratory in the past 5 years. Defects in LRP4 hinder synaptic development and maintenance; the defects in PREPL are predicted to diminish filling of the synaptic vesicle with acetylcholine; and defects in SNAP25, Munc13-1, and synaptotbrevin-1 impede synaptic vesicle exocytosis.

Old and emerging concepts on adrenal chromaffin cell stimulus-secretion coupling

The chromaffin cells (CCs) of the adrenal medulla play a key role in the control of circulating catecholamines to adapt our body function to stressful conditions. A huge research effort over the last 35 years has converted these cells into the Escherichia coli of neurobiology. CCs have been the testing bench for the development of patch-clamp and amperometric recording techniques and helped clarify most of the known molecular mechanisms that regulate cell excitability, Ca²⁺ signals associated with secretion, and the molecular apparatus that regulates vesicle fusion. This special issue provides a state-of-the-art on the many well-known and unsolved questions related to the molecular processes at the basis of CC function. The issue is also the occasion to highlight the seminal work of Antonio G. García (Emeritus Professor at UAM, Madrid) who greatly contributed to the advancement of our present knowledge on CC physiology and pharmacology. All the contributors of the present issue are distinguished scientists who are either staff members, external collaborators, or friends of Prof. García.

Amperometry methods for monitoring vesicular quantal size and regulation of exocytosis release

Chemical signaling strength during intercellular communication can be regulated by secretory cells through controlling the amount of signaling molecules that are released from a secretory vesicle during the exocytosis process. In addition, the chemical signal can also be influenced by the amount of neurotransmitters that is accumulated and stored inside the secretory vesicle compartment. Here, we present the development of analytical methodologies and cell model systems that have been applied in neuroscience research for gaining better insights into the biophysics and the molecular mechanisms, which are involved in the regulatory aspects of the exocytosis machinery affecting the output signal of chemical transmission at neuronal and neuroendocrine cells.

v-SNARE function in chromaffin cells

Vesicle fusion is elementary for intracellular trafficking and release of signal molecules, thus providing the basis for diverse forms of intercellular communication like hormonal regulation or synaptic transmission. A detailed characterization of the mechanisms underlying exocytosis is key to understand how the nervous system integrates information and generates appropriate responses to stimuli. The machinery for vesicular release employs common molecular players in different model systems including neuronal and neuroendocrine cells, in particular members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) protein family, Sec1/Munc18-like proteins, and other accessory factors. To achieve temporal precision and speed, excitable cells utilize specialized regulatory proteins like synaptotagmin and complexin, whose interplay putatively synchronizes vesicle fusion and enhances stimulus-secretion coupling. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. Specifically, we will focus on the role of vesicle associated membrane proteins, also referred to as vesicular SNAREs, in fusion and rapid cargo discharge. We will further discuss the functions of SNARE regulators during exocytosis and focus on chromaffin cell as a model system of choice that allows for detailed structure-function analyses and direct measurements of vesicle fusion under precise control of intracellular [Ca]i.

Novel synaptobrevin-1 mutation causes fatal congenital myasthenic syndrome

Objective

To identify the molecular basis and elucidate the pathogenesis of a fatal congenital myasthenic syndrome.

Methods

We performed clinical electrophysiology studies, exome and Sanger sequencing, and analyzed functional consequences of the identified mutation.

Results

Clinical electrophysiology studies of the patient revealed several‐fold potentiation of the evoked muscle action potential by high frequency nerve stimulation pointing to a presynaptic defect. Exome sequencing identified a homozygous c.340delA frameshift mutation in synaptobrevin 1 (SYB1), one of the three SNARE proteins essential for synaptic vesicle exocytosis. Analysis of both human spinal cord gray matter and normal human muscle revealed expression of the SYB1A and SYB1D isoforms, predicting expression of one or both isoforms in the motor nerve terminal. The identified mutation elongates the intravesicular C‐terminus of the A isoform from 5 to 71, and of the D isoform from 4 to 31 residues. Transfection of either mutant isoform into bovine chromaffin cells markedly reduces depolarization‐evoked exocytosis, and transfection of either mutant isoform into HEK cells significantly decreases expression of either mutant compared to wild type.

Interpretation

The mutation is pathogenic because elongation of the intravesicular C‐terminus of the A and D isoforms increases the energy required to move their C‐terminus into the synaptic vesicle membrane, a key step for fusion of the synaptic vesicle with the presynaptic membrane, and because it is predicted to reduce expression of either isoform in the nerve terminal.

Cholesterol Alters the Dynamics of Release in Protein Independent Cell Models for Exocytosis

Neurons communicate via an essential process called exocytosis. Cholesterol, an abundant lipid in both secretory vesicles and cell plasma membrane can affect this process. In this study, amperometric recordings of vesicular dopamine release from two different artificial cell models created from a giant unilamellar liposome and a bleb cell plasma membrane, show that with higher membrane cholesterol the kinetics for vesicular release are decelerated in a concentration dependent manner. This reduction in exocytotic speed was consistent for two observed modes of exocytosis, full and partial release. Partial release events, which only occurred in the bleb cell model due to the higher tension in the system, exhibited amperometric spikes with three distinct shapes. In addition to the classic transient, some spikes displayed a current ramp or plateau following the maximum peak current. These post spike features represent neurotransmitter release from a dilated pore before constriction and show that enhancing membrane rigidity via cholesterol adds resistance to a dilated pore to re-close. This implies that the cholesterol dependent biophysical properties of the membrane directly affect the exocytosis kinetics and that membrane tension along with membrane rigidity can influence the fusion pore dynamics and stabilization which is central to regulation of neurochemical release.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

The evidence for open and closed exocytosis as the primary release mechanism

Exocytosis is the fundamental process by which cells communicate with each other. The events that lead up to the fusion of a vesicle loaded with chemical messenger with the cell membrane were the subject of a Nobel Prize in 2013. However, the processes occurring after the initial formation of a fusion pore are very much still in debate. The release of chemical messenger has traditionally been thought to occur through full distention of the vesicle membrane, hence assuming exocytosis to be all or none. In contrast to the all or none hypothesis, here we discuss the evidence that during exocytosis the vesicle-membrane pore opens to release only a portion of the transmitter content during exocytosis and then close again. This open and closed exocytosis is distinct from kiss-and-run exocytosis, in that it appears to be the main content released during regular exocytosis. The evidence for this partial release via open and closed exocytosis is presented considering primarily the quantitative evidence obtained with amperometry.

Measuring Exocytosis Rate Using Corrected Fluorescence Recovery After Photoconversion

Exocytosis plays crucial roles in regulating the distribution and function of plasma membrane (PM) and extracellular matrix proteins. However, measuring the exocytosis rate of a specific protein by conventional methods is very difficult because of exocytosis-independent trafficking such as endocytosis, which also affects membrane protein distribution. Here, we describe a novel method, corrected fluorescence recovery after photoconversion, in which exocytosis-dependent and -independent trafficking events are measured simultaneously to accurately determine exocytosis rate. In this method, the protein-of-interest is tagged with Dendra2, a green-to-red photoconvertible fluorescent protein. Following the photoconversion of PM-localized Dendra2, both the recovery of the green signal and the changes in the photoconverted red signal are measured, and the rate of exocytosis is calculated from the changing rates of these two signals.

Somatodendritic dopamine release: recent mechanistic insights

Dopamine (DA) is a key transmitter in motor, reward and cogitative pathways, with DA dysfunction implicated in disorders including Parkinson's disease and addiction. Located in midbrain, DA neurons of the substantia nigra pars compacta project via the medial forebrain bundle to the dorsal striatum (caudate putamen), and DA neurons in the adjacent ventral tegmental area project to the ventral striatum (nucleus accumbens) and prefrontal cortex. In addition to classical vesicular release from axons, midbrain DA neurons exhibit DA release from their cell bodies and dendrites. Somatodendritic DA release leads to activation of D2 DA autoreceptors on DA neurons that inhibit their firing via G-protein-coupled inwardly rectifying K(+) channels. This helps determine patterns of DA signalling at distant axonal release sites. Somatodendritically released DA also acts via volume transmission to extrasynaptic receptors that modulate local transmitter release and neuronal activity in the midbrain. Thus, somatodendritic release is a pivotal intrinsic feature of DA neurons that must be well defined in order to fully understand the physiology and pathophysiology of DA pathways. Here, we review recent mechanistic aspects of somatodendritic DA release, with particular emphasis on the Ca(2+) dependence of release and the potential role of exocytotic proteins.

Electrochemical nanoprobes for the chemical detection of neurotransmitters

Neurotransmitters, acting as chemical messengers, play an important role in neurotransmission, which governs many functional aspects of nervous system activity. Electrochemical probes have proven a very useful technique to study neurotransmission, especially to quantify and qualify neurotransmitters. With the emerging interests in probing neurotransmission at the level of single cells, single vesicles, as well as single synapses, probes that enable detection of neurotransmitters at the nanometer scale become vitally important. Electrochemical nanoprobes have been successfully employed in nanometer spatial resolution imaging of single nanopores of Si membrane and single Au nanoparticles, providing both topographical and chemical information, thus holding great promise for nanometer spatial study of neurotransmission. Here we present the current state of electrochemical nanoprobes for chemical detection of neurotransmitters, focusing on two types of nanoelectrodes, i.e. carbon nanoelectrode and nano-ITIES pipet electrode.

Nanopipet-Based Liquid-Liquid Interface Probes for the Electrochemical Detection of Acetylcholine, Tryptamine, and Serotonin via Ionic Transfer

A nanoscale interface between two immiscible electrolyte solutions (ITIES) provides a unique analytical platform for the detection of ionic species of biological interest such as neurotransmitters and neuromodulators, especially those that are otherwise difficult to detect directly on a carbon electrode without electrode modification. We report the detection of acetylcholine, serotonin, and tryptamine on nanopipet electrode probes with sizes ranging from a radius of ≈7 to 35 nm. The transfer of these analytes across a 1,2-dichloroethane/water interface was studied by cyclic voltammetry and amperometry. Well-defined sigmoidal voltammograms were observed on the nanopipet electrodes within the potential window of artificial seawater for acetylcholine and tryptamine. The half wave transfer potential, E1/2, of acetylcholine, tryptamine, and serotonin were found to be -0.11, -0.25, and -0.47 V vs E(1/2,TEA) (term is defined later in experimental), respectively. The detection was linear in the range of 0.25-6 mM for acetylcholine and of 0.5-10 mM for tryptamine in artificial seawater. Transfer of serotonin was linear in the range of 0.15-8 mM in LiCl solution. The limit of detection for serotonin in LiCl on a radius ≈21 nm nanopipet electrode was 77 μM, for acetylcholine on a radius ≈7 nm nanopipet electrode was 205 μM, and for tryptamine on a radius ≈19 nm nanopipet electrode was 86 μM. Nanopipet-supported ITIES probes have great potential to be used in nanometer spatial resolution measurements for the detection of neurotransmitters.

Carbon fiber ultramicrodic electrode electrodeposited with over-oxidized polypyrrole for amperometric detection of vesicular exocytosis from pheochromocytoma cell

Vesicular exocytosis is ubiquitous, but it is difficult to detect within the cells' communication mechanism. For this purpose, a 2 μm ultramicrodic carbon fiber electrode was fabricated in this work based on electrodeposition with over-oxidized polypyrrole nanoparticle (PPyox-CFE), which was applied successfully for real-time monitoring of quantal exocytosis from individual pheochromocytoma (PC12) cells. PPyox-CFE was evaluated by dopamine (DA) solutions through cyclic voltammetry and amperometry electrochemical methods, and results revealed that PPyox-CFE improved the detection limit of DA. In particular, the sensitivity of DA was improved to 24.55 μA·μM⁻¹·μm⁻² using the PPyox-CFE. The ultramicrodic electrode combined with the patch-clamp system was used to detect vesicular exocytosis of DA from individual PC12 cells with 60 mM K⁺ stimulation. A total of 287 spikes released from 7 PC12 cells were statistically analyzed. The current amplitude (I_max) and the released charge (Q) of the amperometric spikes from the DA release by a stimulated PC12 cell is 45.1 ± 12.5 pA and 0.18 ± 0.04 pC, respectively. Furthermore, on average ∼562,000 molecules were released in each vesicular exocytosis. PPyox-CFE, with its capability of detecting vesicular exocytosis, has potential application in neuron communication research.