Chronically Implanted, Nafion-Coated Ag/AgCl Reference Electrodes for Neurochemical Applications

Understanding the different effects of fouling mechanisms on working and reference electrodes in fast-scan cyclic voltammetry for neurotransmitter detection

Fast-scan cyclic voltammetry (FSCV) is a widely used technique for detecting neurotransmitters. However, electrode fouling can negatively impact its accuracy and sensitivity. Fouling refers to the accumulation of unwanted materials on the electrode surface, which can alter its electrochemical properties and reduce its sensitivity and selectivity. Fouling mechanisms can be broad and may include biofouling, the accumulation of biomolecules on the electrode surface, and chemical fouling, the deposition of unwanted chemical species. Despite individual studies discussing fouling effects on either the working electrode or the reference electrode, no comprehensive study has been conducted to compare the overall fouling effects on both electrodes in the context of FSCV. Here, we examined the effects of biofouling and chemical fouling on the carbon fiber micro-electrode (CFME) as the working electrode and the Ag/AgCl reference electrode with FSCV. Both fouling mechanisms significantly decreased the sensitivity and caused peak voltage shifts in the FSCV signal with the CFME, but not with the Ag/AgCl reference electrode. Interestingly, previous studies have reported peak voltage shifts in FSCV signals due to the fouling of Ag/AgCl electrodes after implantation in the brain. We noticed in a previous study that energy-dispersive spectroscopy (EDS) spectra showed increased sulfide ion concentration after implantation. We hypothesized that sulfide ions may be responsible for the peak voltage shift. To test this hypothesis, we added sulfide ions to the buffer solution, which decreased the open circuit potential of the Ag/AgCl electrode and caused a peak voltage shift in the FSCV voltammograms. Also, EDS analysis showed that sulfide ion concentration increased on the surface of the Ag/AgCl electrodes after 3 weeks of chronic implantation, necessitating consideration of sulfide ions as the fouling agent for the reference electrodes. Overall, our study provides important insights into the mechanisms of electrode fouling and its impact on FSCV measurements. These findings could inform the design of FSCV experiments, with the development of new strategies for improving the accuracy and reliability of FSCV measurements in vivo.

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.

Electrochemistry in a Two- or Three-Electrode Configuration to Understand Monopolar or Bipolar Configurations of Platinum Bionic Implants

Electrodes are used in vivo for chemical sensing, electrophysiological recording, and stimulation of tissue. The electrode configuration used in vivo is often optimised for a specific anatomy and biological or clinical outcomes, not electrochemical performance. Electrode materials and geometries are constrained by biostability and biocompatibility issues and may be required to function clinically for decades. We performed benchtop electrochemistry, with changes in reference electrode, smaller counter-electrode sizes, and three- or two-electrode configurations. We detail the effects different electrode configurations have on typical electroanalytical techniques used on implanted electrodes. Changes in reference electrode required correction by application of an offset potential. In a two-electrode configuration with similar working and reference/counter-electrode sizes, the electrochemical response was dictated by the rate-limiting charge transfer step at either electrode. This could invalidate calibration curves, standard analytical methods, and equations, and prevent use of commercial simulation software. We provide methods for determining if an electrode configuration is affecting the in vivo electrochemical response. We recommend sufficient details be provided in experimental sections on electronics, electrode configuration, and their calibration to justify results and discussion. In conclusion, the experimental limitations of performing in vivo electrochemistry may dictate what types of measurements and analyses are possible, such as obtaining relative rather than absolute measurements.

Accurate and stable chronic in vivo voltammetry enabled by a replaceable subcutaneous reference electrode

In vivo sensing of neurotransmitters has provided valuable insight into both healthy and diseased brain. However, chronically implanted Ag/AgCl reference electrodes suffer from degradationgradation, resulting in errors in the potential at the working electrode. Here, we report a simple, effective way to protect in vivo sensing measurements from reference polarization with a replaceable subcutaneously implanted reference. We compared a brain-implanted reference and a subcutaneous reference and observed no difference in impedance or dopamine redox peak separation in an acute preparation. Chronically, peak background potential and dopamine oxidation potential shifts were eliminated for three weeks. Scanning electron microscopy shows changes in surface morphology and composition of chronically implanted Ag/AgCl electrodes, and postmortem histology reveals extensive cell death and gliosis in the surrounding tissue. As accurate reference potentials are critical to in vivo electrochemistry applications, this simple technique can improve a wide and diverse assortment of in vivo preparations.

Voltammetric Approach for Characterizing the Biophysical and Chemical Functionality of Human Induced Pluripotent Stem Cell-Derived Serotonin Neurons

Depression is quickly becoming one of the world's most pressing public health crises, and there is an urgent need for better diagnostics and therapeutics. Behavioral models in animals and humans have not adequately addressed the diagnosis and treatment of depression, and biomarkers of mental illnesses remain ill-defined. It has been very difficult to identify biomarkers of depression because of in vivo measurement challenges. While our group has made important strides in developing in vivo tools to measure such biomarkers (e.g., serotonin) in mice using voltammetry, these tools cannot be easily applied for depression diagnosis and drug screening in humans due to the inaccessibility of the human brain. In this work, we take a chemical approach, ex vivo, to introduce a human-derived system to investigate brain serotonin. We utilize human induced pluripotent stem cells differentiated into serotonin neurons and establish a new ex vivo model of real-time serotonin neurotransmission measurements. We show that evoked serotonin release responds to stimulation intensity and tryptophan preloading, and that serotonin release and reuptake kinetics resemble those found in vivo in rodents. Finally, after selective serotonin reuptake inhibitor (SSRI) exposure, we find dose-dependent internalization of the serotonin reuptake transporters (a signature of the in vivo response to SSRI). Our new human-derived chemical model has great potential to provide an ex vivo chemical platform as a translational tool for in vivo neuropsychopharmacology.

Defining a Path Toward the Use of Fast-Scan Cyclic Voltammetry in Human Studies

Fast Scan Cyclic Voltammetry (FSCV) has been used for decades as a neurochemical tool for in vivo detection of phasic changes in electroactive neurotransmitters in animal models. Recently, multiple research groups have initiated human neurochemical studies using FSCV or demonstrated interest in bringing FSCV into clinical use. However, there remain technical challenges that limit clinical implementation of FSCV by creating barriers to appropriate scientific rigor and patient safety. In order to progress with clinical FSCV, these limitations must be first addressed through (1) appropriate pre-clinical studies to ensure accurate measurement of neurotransmitters and (2) the application of a risk management framework to assess patient safety. The intent of this work is to bring awareness of the current issues associated with FSCV to the scientific, engineering, and clinical communities and encourage them to seek solutions or alternatives that ensure data accuracy, rigor and reproducibility, and patient safety.

Simultaneous fMRI and fast-scan cyclic voltammetry bridges evoked oxygen and neurotransmitter dynamics across spatiotemporal scales

The vascular contributions of neurotransmitters to the hemodynamic response are gaining more attention in neuroimaging studies, as many neurotransmitters are vasomodulatory. To date, well-established electrochemical techniques that detect neurotransmission in high magnetic field environments are limited. Here, we propose an experimental setting enabling simultaneous fast-scan cyclic voltammetry (FSCV) and blood oxygenation level-dependent functional magnetic imaging (BOLD fMRI) to measure both local tissue oxygen and dopamine responses, and global BOLD changes, respectively. By using MR-compatible materials and the proposed data acquisition schemes, FSCV detected physiological analyte concentrations with high temporal resolution and spatial specificity inside of a 9.4 T MRI bore. We found that tissue oxygen and BOLD correlate strongly, and brain regions that encode dopamine amplitude differences can be identified via modeling simultaneously acquired dopamine FSCV and BOLD fMRI time-courses. This technique provides complementary neurochemical and hemodynamic information and expands the scope of studying the influence of local neurotransmitter release over the entire brain.

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.

Carbon Nanotube Microelectrode Set: Detection of Biomolecules to Heavy Metals

An ultrasensitive electrochemical microelectrode set (μ-ES), where all three electrodes are made of highly densified carbon nanotube fiber (HD-CNTf) cross sections (length ∼40 μm), embedded in an inert polymer matrix, and exposed open-ended CNTs at the interface, is presented here. Bare open ends of HD-CNTf rods were used as the working (∼40 μm diameter) and counter (∼94 μm diameter) electrodes, while the cross section of a ∼94 μm diameter was electroplated with Ag/AgCl and coated with Nafion to employ as a quasi-reference electrode. The Ag/AgCl/Nafion-coated HD-CNTf rod quasi-reference electrode provided a very stable potential comparable to the commercial porous-junction Ag/AgCl reference electrode. The HD-CNTf rod μ-ES has been evaluated by electrochemical determination of biologically important analytes, i.e., dopamine (DA), β-nicotinamide adenine dinucleotide (NADH), a diuretic drug, i.e., furosemide, and a heavy metal, i.e., lead ions (Pb²⁺). Different voltammetric techniques were employed during the study, i.e., cyclic voltammetry (CV), square wave voltammetry (SWV), amperometry, and square wave anodic stripping voltammetry (SWASV). The direct metallic connection to CNTs gives access to the exceptional properties of highly ordered open-ended CNTs as electrochemical sensors. The distinct structural and electronic properties of aligned HD-CNTf rods in the μ-ES demonstrate fast electron transfer kinetics and offer excellent detection performance during testing for different analytes with wide linear ranges, excellent sensitivity, and very low limits of detection.

Implantable platinum nanotree microelectrode with a battery-free electrochemical patch for peritoneal carcinomatosis monitoring

As a severe stage of cancers, peritoneal carcinomatosis should be frequently monitored by means of ascites analysis. Nevertheless, the analysis process is traumatic and time-consuming in clinical practice. In this study, an implantable platinum nanotree microelectrode with a wireless, battery-free and flexible electrochemical patch was developed for in vivo and real-time peritoneal glucose detection to monitor peritoneal carcinomatosis. As the core of implantable microelectrode, platinum trees were synthesized by one-step electrodeposition method and highly sensitive to glucose detection. The platinum nanotree microelectrode was implantable in peritoneal cavity in minimally invasive way. A flexible circuit patch could execute electrochemical test and realize wireless power harvesting and data interaction with a near field communication (NFC)-enabled smartphone. The whole system could detect glucose dynamics in vivo in rat peritoneal cavity. Furthermore, the accuracy of this system was validated in ascites of patients. In this way, the system could offer hassle-free, rapid and minimally invasive opportunities toward peritoneal carcinomatosis monitoring.

Phasic oxygen dynamics confounds fast choline-sensitive biosensor signals in the brain of behaving rodents

Cholinergic fast time-scale modulation of cortical physiology is critical for cognition, but direct local measurement of neuromodulators in vivo is challenging. Choline oxidase (ChOx)-based electrochemical biosensors have been used to capture fast cholinergic signals in behaving animals. However, these transients might be biased by local field potential and O₂-evoked enzymatic responses. Using a novel Tetrode-based Amperometric ChOx (TACO) sensor, we performed highly sensitive and selective simultaneous measurement of ChOx activity (COA) and O₂. In vitro and in vivo experiments, supported by mathematical modeling, revealed that non-steady-state enzyme responses to O₂ give rise to phasic COA dynamics. This mechanism accounts for most of COA transients in the hippocampus, including those following locomotion bouts and sharp-wave/ripples. Our results suggest that it is unfeasible to probe phasic cholinergic signals under most behavioral paradigms with current ChOx biosensors. This confound is generalizable to any oxidase-based biosensor, entailing rigorous controls and new biosensor designs.

Dopaminergic Signaling in the Nucleus Accumbens Modulates Stress-Coping Strategies during Inescapable Stress

Maladaptation to stress is a critical risk factor in stress-related disorders, such as major depression and post-traumatic stress disorder (PTSD). Dopamine signaling in the nucleus accumbens (NAc) has been shown to modulate behavior by reinforcing learning and evading aversive stimuli, which are important for the survival of animals under environmental challenges such as stress. However, the mechanisms through which dopaminergic transmission responds to stressful events and subsequently regulates its downstream neuronal activity during stress remain unknown. To investigate how dopamine signaling modulates stress-coping behavior, we measured the subsecond fluctuation of extracellular dopamine concentration and pH using fast scanning cyclic voltammetry (FSCV) in the NAc, a postsynaptic target of midbrain dopaminergic neurons, in male mice engaged in a tail suspension test (TST). The results revealed a transient decrease in dopamine concentration and an increase in pH levels when the animals changed behaviors, from being immobile to struggling. Interestingly, optogenetic inhibition of dopamine release in NAc, potentiated the struggling behavior in animals under the TST. We then addressed the causal relationship of such a dopaminergic transmission with behavioral alterations by knocking out both the dopamine receptors, i.e., D1 and D2, in the NAc using viral vector-mediated genome editing. Behavioral analyses revealed that male D1 knock-out mice showed significantly more struggling bouts and longer struggling durations during the TST, while male D2 knock-out mice did not. Our results therefore indicate that D1 dopaminergic signaling in the NAc plays a pivotal role in the modulation of stress-coping behaviors in animals under tail suspension stress.SIGNIFICANCE STATEMENT The tail suspension test (TST) has been widely used as a despair-based behavioral assessment to screen the antidepressant so long. Despite its prevalence in the animal studies, the neural substrate underlying the changes of behavior during the test remains unclear. This study provides an evidence for a role of dopaminergic transmission in the modulation of stress-coping behavior during the TST, a despair test widely used to screen the antidepressants in rodents. Taking into consideration the fact that the dopamine metabolism is upregulated by almost all antidepressants, a part of which acts directly on the dopaminergic transmission, current results would uncover the molecular mechanism through which the dopaminergic signaling mediates antidepressant effect with facilitation of the recovery from the despair-like behavior in the TST.

Real-time glucose monitoring system containing enzymatic sensor and enzymatic reference electrodes

Every electrochemical biosensor uses two or three electrode setup, which involves sensing electrode for a specific reaction, metal/salt reference electrode (i.e., Ag/AgCl or Hg/Hg₂Cl₂) for the control of the potential and, is some cases, counter electrode for the compensation of the current. This setup has significant flaws related to metal/salt reference electrodes: they are bulky and difficult to miniaturize, leak electrolyte to the medium, lose the ability to define the electrochemical potential precisely in time, consequently, have to be updated or replaced. This causes problems when the biosensor cannot be easily replaced (e.g., implanted electronics). Here we present a fully enzymatic real-time glucose monitoring system capable of referencing its own electrochemical potential. Using sensing electrode composed of wired glucose dehydrogenase and enzymatic reference electrode composed of wired laccase we have created a stable and accurate electrode system, which measured fluxes in concentration of glucose in a physiological range (3-8 mM), and demonstrated performance of the designed system in undiluted human serum. In addition, our designed enzymatic reference electrode is universal and may be applied for other biosensors, thus open possibilities for the new generation of implantable devices for healthcare monitoring.

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.

Biocompatibility of common implantable sensor materials in a tumor xenograft model

Real-time monitoring of tumor microenvironment parameters using an implanted biosensor could provide valuable information on the dynamic nature of a tumor's biology and its response to treatment. However, following implantation biosensors may lose functionality due to biofouling caused by the foreign body response (FBR). This study developed a novel tumor xenograft model to evaluate the potential of six biomaterials (silicon dioxide, silicon nitride, Parylene-C, Nafion, biocompatible EPOTEK epoxy resin, and platinum) to trigger a FBR when implanted into a solid tumor. Biomaterials were chosen based on their use in the construction of a novel biosensor, designed to measure spatial and temporal changes in intra-tumoral O2 , and pH. None of the biomaterials had any detrimental effect on tumor growth or body weight of the murine host. Immunohistochemistry showed no significant changes in tumor necrosis, hypoxic cell number, proliferation, apoptosis, immune cell infiltration, or collagen deposition. The absence of biofouling supports the use of these materials in biosensors; future investigations in preclinical cancer models are required, with a view to eventual applications in humans. To our knowledge this is the first documented investigation of the effects of modern biomaterials, used in the production of implantable sensors, on tumor tissue after implantation. © 2018 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials published by Wiley Periodicals, Inc. J Biomed Mater Res Part B, 2018. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1620-1633, 2019.

Polymer Modified Carbon Fiber-Microelectrodes and Waveform Modifications Enhance Neurotransmitter Metabolite Detection

Carbon-fiber microelectrodes (CFMEs) have been used for several years for the detection of neurotransmitters such as dopamine. Dopamine is a fundamentally important neurotransmitter and is also metabolized at a subsecond timescale. Recently, several metabolites of dopamine have been shown to be physiologically important such as 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA). Many of these neurotransmitter metabolites are currently only detected with microdialysis coupled with liquid chromatography with relatively low temporal and spatial resolution. Current electrochemical methods such as the dopamine waveform (scanning from -0.4 to 1.3 V at 400 V/sec) are utilized to electrostatically repel anions such as DOPAC and promote dopamine adsorption to the surface of the electrode. Moreover, polymer coatings such as Nafion have been shown to electrostatically repel anions such as 5-hydroxyindoleacetic acid (5-HIAA). In this study, we develop novel polymer and waveform modifications for enhanced DOPAC detection. Applying the DOPAC waveform (scanning from 0 to 1.3 V at 400 V/sec) enhances DOPAC detection significantly because it does not include the negative holding potential of the dopamine waveform. Moreover, positively charged cationic polymers such as polyethyleneimine (PEI) allow for the preconcentration of DOPAC to the surface of the carbon fiber through an electrostatic attraction. The limit of detection for DOPAC for PEI coated CFMEs with the DOPAC waveform applied is 58.2 ± 2 nM as opposed to 291 ± 10 nM for unmodified electrodes applying the dopamine waveform (n = 4). This work offers promise for the development of novel electrode materials and waveforms for the specific detection of several important biomolecules such as dopamine metabolite neurotransmitters.

Improvement of the Biocompatibility and Potential Stability of Chronically Implanted Electrodes Incorporating Coating Cell Membranes

The development of chronically implanted electrodes attracts much attention since these electrodes are much favorable for monitoring changes of neurotransmitters in brain science. The research in this field focused mainly on chemical modification to improve the potential stability and less on the biocompatibility. In this work, for the first time, we proposed the concept of cell-membrane electrodes based on a basic hypothesis using animal's self-cell membrane to reduce animal exclusiveness (hyperacute rejection and chronic rejection). As a proof of concept, we first studied cell-membrane reference electrodes for chronically implanted electrodes. Red cell membrane (RCM) was extracted from rat blood and coated on the chemically modified Ag/AgCl electrodes. It was found that ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium hexafluorophosphate (BDMI) showed good performance rather than Nafion used as coating film for protection of silver chloride on Ag wire and support of the cell membrane. Electrochemical impedance spectra supported that charge-transfer resistance nearly kept constant before and after the electrodes were implanted into the rat's brain tissues for 28 days. Immunohistochemical analysis of the implant sites in the rat's brain tissues indicated that the extent of glial scarring arising from the Ag/AgCl/BDMI/RCM electrodes was smaller than that of both Ag/AgCl/Nafion and Ag/AgCl/Nafion/RCM electrodes after 28 days of implantation. The RCM-coated Ag/AgCl/IL electrodes showed a relatively potential stability compared to RCM-noncoated Ag/AgCl/IL electrodes after 28 days of implantation. Additionally, the current-voltage curve demonstrated that the RCM-coated electrodes can be used as polarized electrodes. This work demonstrated that the RCM, which was coated on the Ag/AgCl/IL electrodes, can significantly improve the biocompatibility and potential stability of the RCM-noncoated Ag/AgCl/IL electrodes implanted in the rat brain. The cell-membrane-coated electrodes will serve as a lighthouse in guiding the design of chronically implanted electrodes for in vivo electrochemical detection.

Hitchhiker's Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry

Fast-scan cyclic voltammetry (FSCV) has been used for over 20 years to study rapid neurotransmission in awake and behaving animals. These experiments were first carried out with carbon-fiber microelectrodes (CFMs) encased in borosilicate glass, which can be inserted into the brain through micromanipulators and guide cannulas. More recently, chronically implantable CFMs constructed with small diameter fused-silica have been introduced. These electrodes can be affixed in the brain with minimal tissue response, which permits longitudinal measurements of neurotransmission in single recording locations during behavior. Both electrode designs have been used to make novel discoveries in the fields of neurobiology, behavioral neuroscience, and psychopharmacology. The purpose of this Review is to address important considerations for the use of FSCV to study neurotransmitters in awake and behaving animals, with a focus on measurements of striatal dopamine. Common issues concerning experimental design, data collection, and calibration are addressed. When necessary, differences between the two methodologies (acute vs chronic recordings) are discussed. The topics raised in this Review are particularly important as the field moves beyond dopamine toward new neurochemicals and brain regions.

Biocompatible PEDOT:Nafion composite electrode coatings for selective detection of neurotransmitters in vivo

A Nafion and poly(3,4-ethylenedioxythiophene) (PEDOT) containing composite polymer has been electropolymerized on carbon-fiber microelectrodes with the goal of creating a mechanically stable, robust, and controllable electrode coating that increases the selectivity and sensitivity of in vivo electrochemical measurements. The coating is deposited on carbon-fiber microelectrodes by applying a triangle waveform from +1.5 V to -0.8 V and back in a dilute solution of ethylenedioxythiophene (EDOT) and Nafion in acetonitrile. Scanning electron microscopy demonstrated that the coating is uniform and ∼100 nm thick. Energy-dispersive X-ray spectroscopy demonstrated that both sulfur and fluorine are present in the coating, indicating the incorporation of PEDOT (poly(3,4-ethylenedioxythiophene) and Nafion. Two types of PEDOT:Nafion coated electrodes were then analyzed electrochemically. PEDOT:Nafion-coated electrodes made using 200 μM EDOT exhibit a 10-90 response time of 0.46 ± 0.09 s versus 0.45 ± 0.11 s for an uncoated fiber in response to a 1.0 μM bolus of dopamine. The electrodes coated using a higher EDOT concentration (400 μM) are slower with a 10-90 response time of 0.84 ± 0.19 s, but display increased sensitivity to dopamine, at 46 ± 13 nA/μM, compared to 26 ± 6 nA/μM for the electrodes coated in 200 μM EDOT and 13 ± 2 nA/μM for an uncoated fiber. PEDOT:Nafion-coated electrodes were lowered into the nucleus accumbens of a rat, and both spontaneous and electrically evoked dopamine release were measured. In addition to improvements in sensitivity and selectivity, the coating dramatically reduces acute in vivo biofouling.

An update of the classical and novel methods used for measuring fast neurotransmitters during normal and brain altered function

To understand better the cerebral functions, several methods have been developed to study the brain activity, they could be related with morphological, electrophysiological, molecular and neurochemical techniques. Monitoring neurotransmitter concentration is a key role to know better how the brain works during normal or pathological conditions, as well as for studying the changes in neurotransmitter concentration with the use of several drugs that could affect or reestablish the normal brain activity. Immediate response of the brain to environmental conditions is related with the release of the fast acting neurotransmission by glutamate (Glu), γ-aminobutyric acid (GABA) and acetylcholine (ACh) through the opening of ligand-operated ion channels. Neurotransmitter release is mainly determined by the classical microdialysis technique, this is generally coupled to high performance liquid chromatography (HPLC). Detection of neurotransmitters can be done by fluorescence, optical density, electrochemistry or other detection systems more sophisticated. Although the microdialysis method is the golden technique to monitor the brain neurotransmitters, it has a poor temporal resolution. Recently, with the use of biosensor the drawback of temporal resolution has been improved considerably, however other inconveniences have merged, such as stability, reproducibility and the lack of reliable biosensors mainly for GABA. The aim of this review is to show the important advances in the different ways to measure neurotransmitter concentrations; both with the use of classic techniques as well as with the novel methods and alternant approaches to improve the temporal resolution.

Voltammetric and mathematical evidence for dual transport mediation of serotonin clearance in vivo

The neurotransmitter serotonin underlies many of the brain's functions. Understanding serotonin neurochemistry is important for improving treatments for neuropsychiatric disorders such as depression. Antidepressants commonly target serotonin clearance via serotonin transporters and have variable clinical effects. Adjunctive therapies, targeting other systems including serotonin autoreceptors, also vary clinically and carry adverse consequences. Fast scan cyclic voltammetry is particularly well suited for studying antidepressant effects on serotonin clearance and autoreceptors by providing real-time chemical information on serotonin kinetics in vivo. However, the complex nature of in vivo serotonin responses makes it difficult to interpret experimental data with established kinetic models. Here, we electrically stimulated the mouse medial forebrain bundle to provoke and detect terminal serotonin in the substantia nigra reticulata. In response to medial forebrain bundle stimulation we found three dynamically distinct serotonin signals. To interpret these signals we developed a computational model that supports two independent serotonin reuptake mechanisms (high affinity, low efficiency reuptake mechanism, and low affinity, high efficiency reuptake system) and bolsters an important inhibitory role for the serotonin autoreceptors. Our data and analysis, afforded by the powerful combination of voltammetric and theoretical methods, gives new understanding of the chemical heterogeneity of serotonin dynamics in the brain. This diverse serotonergic matrix likely contributes to clinical variability of antidepressants.

Emerging trends in in vivo neurochemical monitoring by microdialysis

Mapping chemical dynamics in the brain of live subjects is a challenging but highly rewarding goal because it allows neurotransmitter fluctuations to be related to behavior, drug effects, and disease states. A popular method for such measurements is microdialysis sampling coupled to analytical measurements. This method has become well-established for monitoring low molecular weight neurotransmitters, metabolites, and drugs, especially in pharmacological and pharmacokinetic studies. Recent technological developments which improve the temporal and spatial resolution of the methods will enable it to be used for studying behavior and small brain nuclei. Better assays allow monitoring more neurotransmitters simultaneously. Extension to analysis of aggregating proteins like amyloid β is proving extremely useful for uncovering the roles of these molecules and how they contribute to neurodegenerative diseases.

Implantable enzyme amperometric biosensors

The implantable enzyme amperometric biosensor continues as the dominant in vivo format for the detection, monitoring and reporting of biochemical analytes related to a wide range of pathologies. Widely used in animal studies, there is increasing emphasis on their use in diabetes care and management, the management of trauma-associated hemorrhage and in critical care monitoring by intensivists in the ICU. These frontier opportunities demand continuous indwelling performance for up to several years, well in excess of the currently approved seven days. This review outlines the many challenges to successful deployment of chronically implantable amperometric enzyme biosensors and emphasizes the emerging technological approaches in their continued development. The foreign body response plays a prominent role in implantable biotransducer failure. Topics considering the approaches to mitigate the inflammatory response, use of biomimetic chemistries, nanostructured topographies, drug eluting constructs, and tissue-to-device interface modulus matching are reviewed. Similarly, factors that influence biotransducer performance such as enzyme stability, substrate interference, mediator selection and calibration are reviewed. For the biosensor system, the opportunities and challenges of integration, guided by footprint requirements, the limitations of mixed signal electronics, and power requirements, has produced three systems approaches. The potential is great. However, integration along the multiple length scales needed to address fundamental issues and integration across the diverse disciplines needed to achieve success of these highly integrated systems, continues to be a challenge in the development and deployment of implantable amperometric enzyme biosensor systems.