Real-time chemical measurements of dopamine release in the brain

Optimized Fabrication of Carbon-Fiber Microbiosensors for Codetection of Glucose and Dopamine in Brain Tissue

Dopamine (DA) signaling is critically important in striatal function, and this metabolically demanding process is fueled largely by glucose. However, DA and glucose are typically studied independently and, as such, the precise relationship between DA release and glucose availability remains unclear. Fast-scan cyclic voltammetry (FSCV) is commonly coupled with carbon-fiber microelectrodes to study DA transients. These microelectrodes can be modified with glucose oxidase (GOx) to generate microbiosensors capable of simultaneously quantifying real-time and physiologically relevant fluctuations of glucose, a nonelectrochemically active substrate, and DA, which is readily oxidized and reduced at the electrode surface. A chitosan hydrogel can be electrodeposited to entrap the oxidase enzyme on the sensor surface for stable, sensitive, and selective codetection of glucose and DA using FSCV. This strategy can also be used to entrap lactate oxidase on the carbon-fiber surface for codetection of lactate and DA. However, these custom probes are individually fabricated by hand, and performance is variable. This study characterizes the physical nature of the hydrogel and its effects on the acquired electrochemical data in the detection of glucose (2.6 mM) and DA (1 μM). The results demonstrate that the electrodeposition of the hydrogel membrane is improved using a linear potential sweep rather than a direct step to the target potential. Electrochemical impedance spectroscopy data relate information on the physical nature of the electrode/solution interface to the electrochemical performance of bare and enzyme-modified carbon-fiber microelectrodes. The electrodeposition waveform and scan rate were characterized for optimal membrane formation and performance. Finally, codetection of both DA/glucose and DA/lactate was demonstrated in intact rat striatum using probes fabricated according to the optimized protocol. Overall, this work improves the reliable fabrication of carbon-fiber microbiosensors for codetection of DA and important energetic substrates that are locally delivered to the recording site to meet metabolic demand.

Modular Aptamer Switches for the Continuous Optical Detection of Small-Molecule Analytes in Complex Media

Aptamers are a promising class of affinity reagents because signal transduction mechanisms can be built into the reagent, so that they can directly produce a physically measurable output signal upon target binding. However, endowing the signal transduction functionality into an aptamer remains a trial-and-error process that can compromise its affinity or specificity and typically requires knowledge of the ligand binding domain or its structure. In this work, a design architecture that can convert an existing aptamer into a "reversible aptamer switch" whose kinetic and thermodynamic properties can be tuned without a priori knowledge of the ligand binding domain or its structure is described. Finally, by combining these aptamer switches with evanescent-field-based optical detection hardware that minimizes sample autofluorescence, this study demonstrates the first optical biosensor system that can continuously measure multiple biomarkers (dopamine and cortisol) in complex samples (artificial cerebrospinal fluid and undiluted plasma) with second and subsecond-scale time responses at physiologically relevant concentration ranges.

Challenges and strategies faced in the electrochemical biosensing analysis of neurochemicals in vivo: A review

Our brain is an intricate neuromodulatory network, and various neurochemicals, including neurotransmitters, neuromodulators, gases, ions, and energy metabolites, play important roles in regulating normal brain function. Abnormal release or imbalance of these substances will lead to various diseases such as Parkinson's and Alzheimer's diseases, therefore, in situ and real-time analysis of neurochemical interactions in pathophysiological conditions is beneficial to facilitate our understanding of brain function. Implantable electrochemical biosensors are capable of monitoring neurochemical signals in real time in extracellular fluid of specific brain regions because they can provide excellent temporal and spatial resolution. However, in vivo electrochemical biosensing analysis mainly faces the following challenges: First, foreign body reactions induced by microelectrode implantation, non-specific adsorption of proteins and redox products, and aggregation of glial cells, which will cause irreversible degradation of performance such as stability and sensitivity of the microsensor and eventually lead to signal loss; Second, various neurochemicals coexist in the complex brain environment, and electroactive substances with similar formal potentials interfere with each other. Therefore, it is a great challenge to design recognition molecules and tailor functional surfaces to develop in vivo electrochemical biosensors with high selectivity. Here, we take the above challenges as a starting point and detail the basic design principles for improving in vivo stability, selectivity and sensitivity of microsensors through some specific functionalized surface strategies as case studies. At the same time, we summarize surface modification strategies for in vivo electrochemical biosensing analysis of some important neurochemicals for researchers' reference. In addition, we also focus on the electrochemical detection of low basal concentrations of neurochemicals in vivo via amperometric waveform techniques, as well as the stability and biocompatibility of reference electrodes during long-term sensing, and provide an outlook on the future direction of in vivo electrochemical neurosensing.

Simultaneous, Real-Time Detection of Glutamate and Dopamine in Rat Striatum Using Fast-Scan Cyclic Voltammetry

Glutamate and dopamine (DA) represent two key contributors to striatal functioning, a region of the brain that is essential to motor coordination and motivated behavior. While electroanalytical techniques can be utilized for rapid, spatially resolved detection of DA in the interferent-rich brain environment, glutamate, a nonelectroactive analyte, cannot be directly detected using electroanalytical techniques. However, it can be probed using enzyme-based sensors, which generate an electroactive reporter in the presence of glutamate. The vast majority of glutamate biosensors have relied on amperometric sensing, which is an inherently nonselective detection technique. This approach necessitates the use of complex and performance-limiting modifications to ensure the desired single-analyte specificity. Here, we present a novel glutamate microbiosensor fabricated on a carbon-fiber microelectrode substrate and coupled with fast-scan cyclic voltammetry (FSCV) to enable the simultaneous quantification of glutamate and DA at single recording sites in the brain, which is impossible when using typical amperometric approaches. The glutamate microbiosensors were characterized for sensitivity, stability, and selectivity by using a voltammetric waveform optimized for the simultaneous detection of both species. The applicability of these sensors for the investigation of neural circuits was validated in the rat ventral striatum. Electrically evoked glutamate and DA release were recorded at single-micrometer-scale locations before and after pharmacological manipulation of glutamatergic signaling. Our novel glutamate microbiosensor advances the state of the art by providing a powerful tool for probing coordination between these two species in a way that has previously not been possible.

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Neurotransmitter Readily Escapes Detection at the Opposing Microelectrode Surface in Typical Amperometric Measurements of Exocytosis at Single Cells

For decades, carbon-fiber microelectrodes have been used in amperometric measurements of neurotransmitter release at a wide variety of cell types, providing a tremendous amount of valuable information on the mechanisms involved in dense-core vesicle fusion. The electroactive molecules that are released can be detected at the opposing microelectrode surface, allowing for precise quantification as well as detailed kinetic information on the stages of neurotransmitter release. However, it remains unclear how much of the catecholamine that is released into the artificial synapse escapes detection. This work examines two separate mechanisms by which released neurotransmitter goes undetected in a typical amperometric measurement. First, diffusional loss is assessed by monitoring exocytosis at single bovine chromaffin cells using carbon-fiber microelectrodes fabricated in a recessed (cavity) geometry. This creates a microsampling vial that minimizes diffusional loss of analyte prior to detection. More molecules were detected per exocytotic release event when using a recessed cavity sensor as compared to the conventional configuration. In addition, pharmacological inhibition of the norepinephrine transporter (NET), which serves to remove catecholamine from the extracellular space, increased both the size and the time course of individual amperometric events. Overall, this study characterizes distinct physical and biological mechanisms by which released neurotransmitter escapes detection at the opposing microelectrode surface, while also revealing an important role for the NET in "presynaptic" modulation of neurotransmitter release.

Miniaturized probe on polymer SU-8 with array of individually addressable microelectrodes for electrochemical analysis in neural and other biological tissues

An SU-8 probe with an array of nine, individually addressable gold microband electrodes (100 μm long, 4 μm wide, separated by 4-μm gaps) was photolithographically fabricated and characterized for detection of low concentrations of chemicals in confined spaces and in vivo studies of biological tissues. The probe's shank (6 mm long, 100 μm wide, 100 μm thick) is flexible, but exhibits sufficient sharpness and rigidity to be inserted into soft tissue. Laser micromachining was used to define probe geometry by spatially revealing the underlying sacrificial aluminum layer, which was then etched to free the probes from a silicon wafer. Perfusion with fluorescent nanobeads showed that, like a carbon fiber electrode, the probe produced no noticeable damage when inserted into rat brain, in contrast to damage from an inserted microdialysis probe. The individual addressability of the electrodes allows single and multiple electrode activation. Redox cycling is possible, where adjacent electrodes serve as generators (that oxidize or reduce molecules) and collectors (that do the opposite) to amplify signals of small concentrations without background subtraction. Information about electrochemical mechanisms and kinetics may also be obtained. Detection limits for potassium ferricyanide in potassium chloride electrolyte of 2.19, 1.25, and 2.08 μM and for dopamine in artificial cerebral spinal fluid of 1.94, 1.08, and 5.66 μM for generators alone and for generators and collectors during redox cycling, respectively, were obtained.

Biocompatible reference electrodes to enhance chronic electrochemical signal fidelity in vivo

In vivo electrochemistry is a vital tool of neuroscience that allows for the detection, identification, and quantification of neurotransmitters, their metabolites, and other important analytes. One important goal of in vivo electrochemistry is a better understanding of progressive neurological disorders (e.g., Parkinson's disease). A complete understanding of such disorders can only be achieved through a combination of acute (i.e., minutes to hours) and chronic (i.e., days or longer) experimentation. Chronic studies are more challenging because they require prolonged implantation of electrodes, which elicits an immune response, leading to glial encapsulation of the electrodes and altered electrode performance (i.e., biofouling). Biofouling leads to increased electrode impedance and reference electrode polarization, both of which diminish the selectivity and sensitivity of in vivo electrochemical measurements. The increased impedance factor has been successfully mitigated previously with the use of a counter electrode, but the challenge of reference electrode polarization remains. The commonly used Ag/AgCl reference electrode lacks the long-term potential stability in vivo required for chronic measurements. In addition, the cytotoxicity of Ag/AgCl adversely affects animal experimentation and prohibits implantation in humans, hindering translational research progress. Thus, a move toward biocompatible reference electrodes with superior chronic potential stability is necessary. Two qualifying materials, iridium oxide and boron-doped diamond, are introduced and discussed in terms of their electrochemical properties, biocompatibilities, fabrication methods, and applications. In vivo electrochemistry continues to advance toward more chronic experimentation in both animal models and humans, necessitating the utilization of biocompatible reference electrodes that should provide superior potential stability and allow for unprecedented chronic signal fidelity when used with a counter electrode for impedance mitigation.

New chemical biopsy tool for spatially resolved profiling of human brain tissue in vivo

It is extremely challenging to perform chemical analyses of the brain, particularly in humans, due to the restricted access to this organ. Imaging techniques are the primary approach used in clinical practice, but they only provide limited information about brain chemistry. Solid-phase microextraction (SPME) has been presented recently as a chemical biopsy tool for the study of animal brains. The current work demonstrates for the first time the use of SPME for the spatially resolved sampling of the human brain in vivo. Specially designed multi-probe sampling device was used to simultaneously extract metabolites from the white and grey matter of patients undergoing brain tumor biopsies. Samples were collected by inserting the probes along the planned trajectory of the biopsy needle prior to the procedure, which was followed by metabolomic and lipidomic analyses. The results revealed that studied brain structures were predominantly composed of lipids, while the concentration and diversity of detected metabolites was higher in white than in grey matter. Although the small number of participants in this research precluded conclusions of a biological nature, the results highlight the advantages of the proposed SPME approach, as well as disadvantages that should be addressed in future studies.

Dopamine, vocalization, and astrocytes

Dopamine, the main catecholamine neurotransmitter in the brain, is predominately produced in the basal ganglia and released to various brain regions including the frontal cortex, midbrain and brainstem. Dopamine's effects are widespread and include modulation of a number of voluntary and innate behaviors. Vigilant regulation and modulation of dopamine levels throughout the brain is imperative for proper execution of motor behaviors, in particular speech and other types of vocalizations. While dopamine's role in motor circuitry is widely accepted, its unique function in normal and abnormal speech production is not fully understood. In this perspective, we first review the role of dopaminergic circuits in vocal production. We then discuss and propose the conceivable involvement of astrocytes, the numerous star-shaped glia cells of the brain, in the dopaminergic network modulating normal and abnormal vocal productions.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Simultaneous voltammetric detection of glucose and lactate fluctuations in rat striatum evoked by electrical stimulation of the midbrain

Glucose and lactate provide energy for cellular function in the brain and serve as an important carbon source in the synthesis of a variety of biomolecules. Thus, there is a critical need to quantitatively monitor these molecules in situ on a time scale commensurate with neuronal function. In this work, carbon-fiber microbiosensors were coupled with fast-scan cyclic voltammetry to monitor glucose and lactate fluctuations at a discrete site within rat striatum upon electrical stimulation of the midbrain projection to the region. Systematic variation of stimulation parameters revealed the distinct dynamics by which glucose and lactate responded to the metabolic demand of synaptic function. Immediately upon stimulation, extracellular glucose and lactate availability rapidly increased. If stimulation was sufficiently intense, concentrations then immediately fell below baseline in response to incurred metabolic demand. The dynamics were dependent on stimulation frequency, such that more robust fluctuations were observed when the same number of pulses was delivered at a higher frequency. The rates at which glucose was supplied to, and depleted from, the local recording region were dependent on stimulation intensity, and glucose dynamics led those of lactate in response to the most substantial stimulations. Glucose fluctuated over a larger concentration range than lactate as stimulation duration increased, and glucose fell further from baseline concentrations. These real-time measurements provide an unprecedented direct comparison of glucose and lactate dynamics in response to metabolic demand elicited by neuronal activation. Graphical abstract.

Needle-type organic electrochemical transistor for spatially resolved detection of dopamine

In this work, the advantages of carbon nanoelectrodes (CNEs) and orgonic electrochemical transistors (OECTs) were merged to realise nanometre-sized, spearhead OECTs based on single- and double-barrel CNEs functionalised with a conducting polymer film. The needle-type OECT shows a high aspect ratio that allows its precise positioning by means of a macroscopic handle and its size is compatible with single-cell analysis. The device was characterised with respect to its electrolyte-gated behaviour and was employed as electrochemical sensor for the proof-of-concept detection of dopamine (DA) over a wide concentration range (10^-12-10^-6 M). Upon application of fixed drain and gate voltages (V_d = - 0.3 V, V_g = - 0.9 V, respectively), the nano-sized needle-type OECT sensor exhibited a linear response in the low pM range and from 0.002 to 7 μM DA, with a detection limit of 1 × 10^-12 M. Graphical abstract.

Mitigating the Effects of Electrode Biofouling-Induced Impedance for Improved Long-Term Electrochemical Measurements In Vivo

Biofouling is a prevalent issue in studies that involve prolonged implantation of electrochemical probes in the brain. In long-term fast-scan cyclic voltammetry (FSCV) studies, biofouling manifests as a shift in the peak oxidative potential of the background signal that worsens over days to weeks, diminishing sensitivity and selectivity to neurotransmitters such as dopamine. Using open circuit potential (OCP) measurements, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), and electrochemical impedance spectroscopy (EIS), we examined the biofouling-induced events that occur due to electrode implantation. We determined that the FSCV background signal shift results from cathodic polarization of the Ag/AgCl-wire reference electrode and increased electrochemical impedance of both the Ag/AgCl-wire reference electrode and carbon-fiber working electrode. These events are likely caused collectively by immune response-induced electrode encapsulation. A headstage utilizing a three-electrode configuration, designed to compensate for the impedance component of biofouling, reduced the FSCV background signal shift in vivo and preserved dopamine sensitivity at artificially increased impedance levels in vitro. In conjunction with a stable reference electrode, this three-electrode configuration will be critical in achieving reliable neurotransmitter detection for the duration of long-term FSCV studies.

Interpreting Dynamic Interfacial Changes at Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy

Carbon-fiber microelectrodes are instrumental tools in neuroscience used for the electroanalysis of neurochemical dynamics and recordings of neural activity. However, performance is variable and dependent on fabrication strategies, the biological response to implantation, and the physical and chemical composition of the recording environment. This presents an analytical challenge, as electrode performance is difficult to quantitatively assess in situ, especially when electrodes are permanently implanted or cemented in place. We previously reported that electrode impedance directly impacts electrochemical performance for molecular sensing. In this work, we investigate the impacts of individual components of the electrochemical system on impedance. Equivalent circuit models for glass- and silica-insulated carbon-fiber microelectrodes were determined using electrochemical impedance spectroscopy (EIS). The models were validated based on the ability to assign individual circuit elements to physical properties of the electrochemical system. Investigations were performed to evaluate the utility of the models in providing feedback on how changes in ionic strength and carbon fiber material alter impedance properties. Finally, EIS measurements were used to investigate the electrode/solution interface prior to, during, and following implantation in live brain tissue. A significant increase in impedance and decrease in capacitance occur during tissue exposure and persist following implantation. Electrochemical conditioning, which occurs continually during fast-scan cyclic voltammetry recordings, etches and renews the carbon surface, mitigating these effects. Overall, the results establish EIS as a powerful method for characterization of carbon-fiber microelectrodes, providing unprecedented insight into how real-world factors affect the electrode/solution interface.

Carbon-Fiber Nanoelectrodes for Real-Time Discrimination of Vesicle Cargo in the Native Cellular Environment

Carbon-fiber microelectrodes have proven to be an indispensable tool for monitoring exocytosis events using amperometry. When positioned adjacent to a cell, a traditional microdisc electrode is well suited for quantification of discrete exocytotic release events. However, the size of the electrode does not allow for intracellular electrochemical measurements, and the amperometric approach cannot distinguish between the catecholamines that are released. In this work, carbon nanoelectrodes were developed to permit selective electrochemical sampling of nanoscale vesicles in the cell cytosol. Classical voltammetric techniques and electron microscopy were used to characterize the nanoelectrodes, which were ∼5 μm long and sharpened to a nanometer-scale tip that could be wholly inserted into individual neuroendocrine cells. The nanoelectrodes were coupled with fast-scan cyclic voltammetry to distinguish secretory granules containing epinephrine from other catecholamine-containing granules encountered in the native cellular environment. Both vesicle subtypes were encountered in most cells, despite prior demonstration of populations of chromaffin cells that preferentially release one of these catecholamines. There was substantial cell-to-cell variability in relative epinephrine content, and vesicles containing epinephrine generally stored more catecholamine than the other vesicles. The carbon nanoelectrode technology thus enabled analysis of picoliter-scale biological volumes, revealing key differences between chromaffin cells at the level of the dense-core granule.

High frequency, real-time neurochemical and neuropharmacological measurements in situ in the living body

The beautiful and complex brain machinery is perfectly synchronized, and our bodies have evolved to protect it against a myriad of potential threats. Shielded physically by the skull and chemically by the blood brain barrier, the brain processes internal and external information so that we can efficiently relate to the world that surrounds us while simultaneously and unconsciously controlling our vital functions. When coupled with the brittle nature of its internal chemical and electric signals, the brain's "armor" render accessing it a challenging and delicate endeavor that has historically limited our understanding of its structural and neurochemical intricacies. In this review, we briefly summarize the advancements made over the past 10 years to decode the brain's neurochemistry and neuropharmacology in situ, at the site of interest in the brain, with special focus on what we consider game-changing emerging technologies (eg, genetically encoded indicators and electrochemical aptamer-based sensors) and the challenges these must overcome before chronic, in situ chemosensing measurements become routine.

Characterization of a Multiple-Scan-Rate Voltammetric Waveform for Real-Time Detection of Met-Enkephalin

Opioid peptides are critically involved in a variety of physiological functions necessary for adaptation and survival, and as such, understanding the precise actions of endogenous opioid peptides will aid in identification of potential therapeutic strategies to treat a variety of disorders. However, few analytical tools are currently available that offer both the sensitivity and spatial resolution required to monitor peptidergic concentration fluctuations in situ on a time scale commensurate with that of neuronal communication. Our group has developed a multiple-scan-rate waveform to enable real-time voltammetric detection of tyrosine containing neuropeptides. Herein, we have evaluated the waveform parameters to increase sensitivity to methionine-enkephalin (M-ENK), an endogenous opioid neuropeptide implicated in pain, stress, and reward circuits. M-ENK dynamics were monitored in adrenal gland tissue, as well as in the dorsal striatum of anesthetized and freely behaving animals. The data reveal cofluctuations of catecholamine and M-ENK in both locations and provide measurements of M-ENK dynamics in the brain with subsecond temporal resolution. Importantly, this work also demonstrates how voltammetric waveforms can be customized to enhance detection of specific target analytes, broadly speaking.

NMDA Receptor-Dependent Cholinergic Modulation of Mesolimbic Dopamine Cell Bodies: Neurochemical and Behavioral Studies

Substance abuse disorders are devastating, costly, and difficult to treat. Identifying the neurochemical mechanisms underlying reinforcement promises to provide critical information in the development of effective treatments. Several lines of evidence suggest that striatal dopamine (DA) release serves as a teaching signal in reinforcement learning, and that shifts in DA release from the primary reward to reward-predicting stimuli play a critical role in the self-administration of both natural and non-natural rewards. However, far less is known about the reinforcing effects of motivationally neutral sensory stimuli, or how these signals can facilitate self-administration behavior. Thus, we trained rats ( n = 7) to perform a visual stimulus-induced instrumental task, which involved lever pressing for activation of a stimulus light. We then microinfused vehicle (phosphate buffered saline), carbachol (acetylcholine receptor agonist), or carbachol in the presence of an N-methyl-d-aspartate (NMDA) receptor-specific drug (NMDA itself, or the antagonist, AP5) into the ventral tegmental area (VTA). This enabled us to directly evaluate how chemical modulation of dopamine cell bodies affects the instrumental behavior, as well as the nature of extracellular dopamine transients recorded in the nucleus accumbens shell (NAc shell) using fast-scan cyclic voltammetry (FSCV). Intra-VTA infusion of carbachol enhanced the magnitude and frequency of dopamine transients in the NAc shell and potentiated active lever responding without altering inactive lever responding, as compared to infusion of vehicle. Coinfusion of carbachol with AP5 abolished dopamine transients recorded in the NAc and attenuated active lever responding without altering inactive lever responding. Finally, coadministration of carbachol and NMDA into the VTA restored both lever pressing and dopaminergic signals recorded in the striatum. Together, these results suggest that acetylcholine and glutamate synergistically act at dopamine cells in the VTA to modulate VTA-NAc shell dopaminergic output, and this underlies motivation to lever press for a motivationally neutral visual stimulus.

A high spatiotemporal study of somatic exocytosis with scanning electrochemical microscopy and nanoITIES electrodes

Extra-synaptic exocytosis is an essential component of cellular communication. A knowledge gap exists in the exocytosis of the non-redox active transmitter acetylcholine. Using the nano-interface between two immiscible electrolyte solutions and scanning electrochemical microscopy (SECM), a high resolution spatiotemporal study of acetylcholine exocytosis is shown from an individual neuronal soma. The nanoelectrode was positioned ∼140 nm away from the release sites on the soma using an SECM. The quantitative study enables the obtaining of key information related to cellular communication, including extracellular concentration of the neurotransmitter, cellular permeability, Ca2+ dependence on somatic release, vesicle density, number of molecules released and the release dynamics. Measurements were achieved with a high signal to noise ratio of 6-19. The released neurotransmitter with a concentration of 2.7 (±1.0) μM was detected at the nanoelectrodes with radii of 750 nm to 860 nm.

Selective and Mechanically Robust Sensors for Electrochemical Measurements of Real-Time Hydrogen Peroxide Dynamics in Vivo

Hydrogen peroxide (H2O2) is an endogenous molecule that plays several important roles in brain function: it is generated in cellular respiration, serves as a modulator of dopaminergic signaling, and its presence can indicate the upstream production of more aggressive reactive oxygen species (ROS). H2O2 has been implicated in several neurodegenerative diseases, including Parkinson's disease (PD), creating a critical need to identify mechanisms by which H2O2 modulates cellular processes in general and how it affects the dopaminergic nigrostriatal pathway, in particular. Furthermore, there is broad interest in selective electrochemical quantification of H2O2, because it is often enzymatically generated at biosensors as a reporter for the presence of nonelectroactive target molecules. H2O2 fluctuations can be monitored in real time using fast-scan cyclic voltammetry (FSCV) coupled with carbon-fiber microelectrodes. However, selective identification is a critical issue when working in the presence of other molecules that generate similar voltammograms, such as adenosine and histamine. We have addressed this problem by fabricating a robust, H2O2-selective electrode. 1,3-Phenylenediamine (mPD) was electrodeposited on a carbon-fiber microelectrode to create a size-exclusion membrane, rendering the electrode sensitive to H2O2 fluctuations and pH shifts but not to other commonly studied neurochemicals. The electrodes are described and characterized herein. The data demonstrate that this technology can be used to ensure the selective detection of H2O2, enabling confident characterization of the role this molecule plays in normal physiological function as well as in the progression of PD and other neuropathies involving oxidative stress.

Spectroelectrochemical Characterization of the Dynamic Carbon-Fiber Surface in Response to Electrochemical Conditioning

The effects of electrochemical preconditioning of P-55 pitch-based carbon-fiber microelectrodes were quantitatively examined in this study. Microstructural characterization of the electrode surface was done using Raman spectroscopy and scanning electron microscopy. Electrochemical performance was evaluated using cyclic voltammetry. The data show that application of positive potentials provides beneficial structural modifications to the electrode surface. Electrodes that were preconditioned using a static potential of +1.0 V exhibited enhanced sensitivity and electron transfer properties when compared to electrodes conditioned for the same amount of time with dynamic (triangular) waveforms reaching +1.0 V. Conditioning elicited microstructural changes to the electrode surface that were dependent on the amount of time spent at potentials greater than ∼1.0 V. Importantly, the data demonstrate that the carbon-fiber microstructure is dynamic. It is able to quickly and continuously undergo rapid structural reorganization as potential is applied, repeatedly alternating between a relatively ordered state and one that exhibits greater disorder in response to applied electrochemical potentials that span the range commonly used in voltammetric experiments.

Background Signal as an in Situ Predictor of Dopamine Oxidation Potential: Improving Interpretation of Fast-Scan Cyclic Voltammetry Data

Background-subtracted fast-scan cyclic voltammetry (FSCV) has emerged as a powerful analytical technique for monitoring subsecond molecular fluctuations in live brain tissue. Despite increasing utilization of FSCV, efforts to improve the accuracy of quantification have been limited due to the complexity of the technique and the dynamic recording environment. It is clear that variable electrode performance renders calibration necessary for accurate quantification; however, the nature of in vivo measurements can make conventional postcalibration difficult, or even impossible. Analyte-specific voltammograms and scaling factors that are critical for quantification can shift or fluctuate in vivo. This is largely due to impedance changes, and the effects of impedance on these measurements have not been characterized. We have previously reported that the background current can be used to predict electrode-specific scaling factors in situ. In this work, we employ model circuits to investigate the impact of impedance on FSCV measurements. Additionally, we take another step toward in situ electrode calibration by using the oxidation potential of quinones on the electrode surface to accurately predict the oxidation potential for dopamine at any point in an electrochemical experiment, as both are dependent on impedance. The model, validated both in adrenal slice and live brain tissue, enables information encoded in the shape of the background voltammogram to determine electrochemical parameters that are critical for accurate quantification. This improves data interpretation and provides a significant next step toward more automated methods for in vivo data analysis.

Simultaneous Voltammetric Measurements of Glucose and Dopamine Demonstrate the Coupling of Glucose Availability with Increased Metabolic Demand in the Rat Striatum

Cerebral blood flow ensures delivery of nutrients, such as glucose, to brain sites with increased metabolic demand. However, little is known about rapid glucose dynamics at discrete locations during neuronal activation in vivo. Acute exposure to many substances of abuse elicits dopamine release and neuronal activation in the striatum; however, the concomitant changes in striatal glucose remain largely unknown. Recent developments have combined fast-scan cyclic voltammetry with glucose oxidase enzyme modified carbon-fiber microelectrodes to enable the measurement of glucose dynamics with subsecond temporal resolution in the mammalian brain. This work evaluates several waveforms to enable the first simultaneous detection of endogenous glucose and dopamine at single recording sites. These molecules, one electroactive and one nonelectroactive, were found to fluctuate in the dorsal striatum in response to electrical stimulation of the midbrain and systemic infusion of cocaine/raclopride. The data reveal the second-by-second dynamics of these species in a striatal microenvironment, and directly demonstrate the coupling of glucose availability with increased metabolic demand. This work provides a foundation that will enable detailed investigation of local mechanisms that regulate the coupling of cerebral blood flow with metabolic demand under normal conditions, and in animal studies of drug abuse and addiction.

Unmasking the Effects of L-DOPA on Rapid Dopamine Signaling with an Improved Approach for Nafion Coating Carbon-Fiber Microelectrodes

L-DOPA has been the gold standard for symptomatic treatment of Parkinson's disease. However, its efficacy wanes over time as motor complications develop. Very little is known about how L-DOPA therapy affects the dynamics of fluctuating dopamine concentrations in the striatum on a rapid time scale (seconds). Electrochemical studies investigating the effects of L-DOPA treatment on electrically evoked dopamine release have reported conflicting results with significant variability. We hypothesize that the uncertainty in the electrochemical data is largely due to electrode fouling caused by polymerization of L-DOPA and endogenous catecholamines on the electrode surface. Thus, we have systematically optimized the procedure for fabricating cylindrical, Nafion-coated, carbon-fiber microelectrodes. This has enabled rapid and reliable detection of L-DOPA's effects on striatal dopamine signaling in intact rat brain using fast-scan cyclic voltammetry. An acute dose of 5 mg/kg L-DOPA had no significant effect on dopamine dynamics, demonstrating the highly efficient regulatory mechanisms at work in the intact brain. In contrast, administration of 200 mg/kg L-DOPA significantly increased the amplitude of evoked dopamine release by ∼200%. Overall, this work describes a reliable tool that allows a better measure of L-DOPA augmented dopamine release in vivo, measured using fast-scan cyclic voltammetry. It provides a methodology that improves the stability and performance of the carbon-fiber microelectrode when studying the molecular mechanisms underlying L-DOPA therapy and also promises to benefit a wide variety of studies because Nafion is so commonly used in electroanalytical chemistry.

Modulating dopamine release by optogenetics in transgenic mice reveals terminal dopaminergic dynamics

Dopamine (DA) release and uptake dynamics in the nucleus accumbens (NAc) have important implications for neurological diseases and mammalian animal behaviors. We demonstrate here the use of cell-type-specific optogenetic targeting in conjunction with fast-scan cyclic voltammetry applied to brain slices prepared from specifically tailored transgenic mice, which conditionally express channelrhodopsin-2 (ChR2) through dopamine transporter (DAT)-Cre. Terminal dopaminergic dynamics and the direct manipulation of induced DA release level by controlling light intensity, pulse width, and the shape of stimulation waveforms were studied. Effective cell terminal-targeting optogenetic induction of DA release at physiological levels in NAc is demonstrated and discussed. It was found that delivering more light energy by increasing stimulation intensity and length is not the only way to control DA release; the temporal shape of the stimulus waveform at light onset is also critically related to induced DA concentrations. In addition, DA uptake dynamics as well as the recovery of the presynaptic releasable DA pool are studied and modeled. More broadly, our experimental findings provide important further evidence for effectively applying optogenetics to induce neurotransmitter release in the behaviorally relevant region of the brain in a highly cell-type selective context.

Sawhorse waveform voltammetry for selective detection of adenosine, ATP, and hydrogen peroxide

Fast-scan cyclic voltammetry (FSCV) is an electrochemistry technique which allows subsecond detection of neurotransmitters in vivo. Adenosine detection using FSCV has become increasingly popular but can be difficult because of interfering agents which oxidize at or near the same potential as adenosine. Triangle shaped waveforms are traditionally used for FSCV, but modified waveforms have been introduced to maximize analyte sensitivity and provide stability at high scan rates. Here, a modified sawhorse waveform was used to maximize the time for adenosine oxidation and to manipulate the shapes of cyclic voltammograms (CVs) of analytes which oxidize at the switching potential. The optimized waveform consists of scanning at 400 V/s from −0.4 to 1.35 V and holding briefly for 1.0 ms followed by a ramp back down to −0.4 V. This waveform allows the use of a lower switching potential for adenosine detection. Hydrogen peroxide and ATP also oxidize at the switching potential and can interfere with adenosine measurements in vivo; however, their CVs were altered with the sawhorse waveform and they could be distinguished from adenosine. Principal component analysis (PCA) was used to determine that the sawhorse waveform was better than the triangle waveform at discriminating between adenosine, hydrogen peroxide, and ATP. In slices, mechanically evoked adenosine was identified with PCA and changes in the ratio of ATP to adenosine were observed after manipulation of ATP metabolism by POM-1. The sawhorse waveform is useful for adenosine, hydrogen peroxide, and ATP discrimination and will facilitate more confident measurements of these analytes in vivo.

In situ electrode calibration strategy for voltammetric measurements in vivo

Technological advances have allowed background-subtracted fast-scan cyclic voltammetry to emerge as a powerful tool for monitoring molecular fluctuations in living brain tissue; however, there has been little progress to date in advancing electrode calibration procedures. Variability in the performance of these handmade electrodes renders calibration necessary for accurate quantification; however, experimental protocol makes standard postcalibration difficult or in some cases impossible. We have developed a model that utilizes information contained in the background charging current to predict electrode sensitivity to dopamine, ascorbic acid, hydrogen peroxide, and pH shifts at any point in an electrochemical experiment. Analysis determined a high correlation between predicted sensitivity and values obtained using the traditional postcalibration method, across all analytes. To validate this approach in vivo, calibration factors obtained with this model at electrodes in brain tissue were compared to values obtained at these electrodes using a traditional ex vivo calibration. Both demonstrated equal power of predictability for dopamine concentrations. This advance enables in situ electrode calibration, allowing researchers to track changes in electrode sensitivity over time and eliminating the need to generalize calibration factors between electrodes or across multiple days in an experiment.

Carbon nanotube yarn electrodes for enhanced detection of neurotransmitter dynamics in live brain tissue

This work demonstrates the potential of nanoscale carbon electrode materials for improved detection of electroactive neurotransmitter dynamics in the brain. Individual multiwalled carbon nanotubes were synthesized via chemical vapor deposition, spun into yarns, and used in the fabrication of disk microelectrodes that were subsequently characterized using scanning electron and atomic force microscopies. The carbon nanotube yarn electrodes were coupled with fast-scan cyclic voltammetry and used to discriminately detect rapid neurotransmitter fluctuations in acute brain slices. The results demonstrate that the distinct structural and electronic properties of the nanotubes result in improved selectivity, sensitivity, and spatial resolution, as well as faster apparent electron transfer kinetics when compared to the conventional carbon-fiber microelectrodes typically used in vivo.

Quantitation of hydrogen peroxide fluctuations and their modulation of dopamine dynamics in the rat dorsal striatum using fast-scan cyclic voltammetry

The dopaminergic neurons of the nigrostriatal dopamine (DA) projection from the substantia nigra to the dorsal striatum become dysfunctional and slowly degenerate in Parkinson's disease, a neurodegenerative disorder that afflicts more than one million Americans. There is no specific known cause for idiopathic Parkinson's disease; however, multiple lines of evidence implicate oxidative stress as an underlying factor in both the initiation and progression of the disease. This involves the enhanced generation of reactive oxygen species, including hydrogen peroxide (H2O2), whose role in complex biological processes is not well understood. Using fast-scan cyclic voltammetry at bare carbon-fiber microelectrodes, we have simultaneously monitored and quantified H2O2 and DA fluctuations in intact striatal tissue under basal conditions and in response to the initiation of oxidative stress. Furthermore, we have assessed the effect of acute increases in local H2O2 concentration on both electrically evoked DA release and basal DA levels. Increases in endogenous H2O2 in the dorsal striatum attenuated electrically evoked DA release, and also decreased basal DA levels in this brain region. These novel results will help to disambiguate the chemical mechanisms underlying the progression of neurodegenerative disease states, such as Parkinson's disease, that involve oxidative stress.