Neurochemistry and electroanalytical probes

Copper-Based Electrochemical Sensor Derived from Thiosemicarbazide for Selective Detection of Neurotransmitter Dopamine

This paper presents the synthesis of the ligand 1-picolinoyl-4-cyclohexyl-3-thiosemicarbazide (H2pctc) and new metal complexes [Ni(Hpctc)2] (1), [Cu(Hpctc)Cl] (2), and [Cd(Hpctc)2] (3). The synthesized metal complexes were precisely characterized using single crystal X-ray diffraction (SC-XRD). In addition, complexes 1-3 were also characterized by UV-vis, fluorescence, and infrared spectroscopy. SC-XRD data confirmed the distorted octahedral geometry around the Ni(II) and Cd(II) centers and the distorted square planar geometry around the Cu(II) center. Data derived from the emission spectra depict that higher fluorescence intensity was exhibited by complexes 1, 2, and 3 in comparison to that of the free ligand H2pctc, and complex 3 showed the maximum intensity. Further, these metal complex-modified GCEs (glassy carbon electrodes) were utilized for electrochemical sensing of dopamine (DPM). The electrochemical studies of these complexes were performed using modified glassy carbon electrodes supported by electrical impedance spectroscopy (EIS) and cyclic voltammetry (CV) methods. In contrast to complexes 1 and 3, complex 2 is a superior electrode material with a high effective surface area for the electrochemical oxidation of DPM, according to the electrochemical response results. The derived sensor had a wide linear detection range of 1 to 1400 μM, an acceptable sensitivity of 0.01531 μA cm-2 μM-1, and a low LOD of 0.38 μM. The proposed approach was free of the interfering effects of ascorbic acid, uric acid, aminophenol, and other substances. During the successive scans, no fouling of the electrode surface was observed. The proposed electrochemical sensor had excellent stability, sensitivity, and a low detection limit making it suitable for the analysis of a variety of real samples. Additionally, it was proven to be useful for analyzing biological fluids.

Carbon Electrode Sensor for the Measurement of Cortisol with Fast-Scan Cyclic Voltammetry

Cortisol is a vital steroid hormone that has been known as the "stress hormone", which is elevated during times of high stress and anxiety and has a significant impact on neurochemistry and brain health. The improved detection of cortisol is critically important as it will help further our understanding of stress during several physiological states. Several methods exist to detect cortisol; however, they suffer from low biocompatibility and spatiotemporal resolution, and they are relatively slow. In this study, we developed an assay to measure cortisol with carbon fiber microelectrodes (CFMEs) and fast-scan cyclic voltammetry (FSCV). FSCV is typically utilized to measure small molecule neurotransmitters by producing a readout cyclic voltammogram (CV) for the specific detection of biomolecules on a fast, subsecond timescale with biocompatible CFMEs. It has seen enhanced utility in measuring peptides and other larger compounds. We developed a waveform that scanned from -0.5 to -1.2 V at 400 V/s to electro-reduce cortisol at the surface of CFMEs. The sensitivity of cortisol was found to be 0.87 ± 0.055 nA/μM (n = 5) and was found to be adsorption controlled on the surface of CFMEs and stable over several hours. Cortisol was co-detected with several other biomolecules such as dopamine, and the waveform was fouling resistant to repeated injections of cortisol on the surface of the CFMEs. Furthermore, we also measured exogenously applied cortisol into simulated urine to demonstrate biocompatibility and potential use in vivo. The specific and biocompatible detection of cortisol with high spatiotemporal resolution will help further elucidate its biological significance and further understand its physiological importance and impact on brain health.

Moving Fast-Scan Cyclic Voltammetry toward FDA Compliance with Capacitive Decoupling Patient Protection

Carbon-fiber microelectrodes allow for high spatial and temporal measurements of electroactive neurotransmitter measurements in vivo using fast-scan cyclic voltammetry (FSCV). However, common instrumentation for such measurements systems lack patient safety precautions. To add safety precautions as well as to overcome chemical and electrical noise, a two-electrode FSCV headstage was modified to introduce an active bandpass filter on the electrode side of the measurement amplifier. This modification reduced the measured noise and ac-coupled the voltammetric measurement and moved it from a classical direct current response measurement. ac-coupling not only reduces the measured noise, but also moves FSCV toward compliance with IEC-60601-1, enabling future human trials. Here, we develop a novel ac-coupled voltammetric measurement method of electroactive neurotransmitters. Our method allows for the modeling of a system to then calculate a waveform to compensate for added impedance and capacitance for the system. We describe how first by measuring the frequency response of the system and modeling the analogue response as a digital filter we can then calculate a predicted waveform. The predicted waveform, when applied to the bandpass filter, is modulated to create a desired voltage sweep at the electrode interface. Further, we describe how this modified FSCV waveform is stable, allowing for the measurement of electroactive neurotransmitters. We later describe a 32.7% sensitivity enhancement for dopamine with this new measurement as well as maintaining a calibration curve for dopamine, 3,4-dihydroxyphenylacetic acid, ascorbic acid, and serotonin in vitro. We then validate dopamine in vivo with stimulated release. Our developed measurement method overcame the added capacitance that would traditionally make a voltammetric measurement impossible, and it has wider applications in electrode sensor development, allowing for measurement with capacitive systems, which previously would not have been possible.

A Cobalt (II) Oxide Carbon Nanotube Composite to Assay Dopamine

A cobalt (II) oxide/carboxylic acid functionalized multiwalled carbon nanotube (CoO/COOH-MWNT) composite was fabricated for the biochemical detection of dopamine (DA). CoO particles were tethered to COOH-MWNTs by sonication. The current response versus different concentration was measured using cyclic voltammetry. Various parameters, including sonication time, pH, and loading were varied for the best current response. The composite with optimum current response was formed using a 30-min sonication time, at pH 5.0 and a 0.89 µg/mm2 loading onto the glassy carbon electrode surface. Good sensitivity with a limit of detection of 0.61 ± 0.03 μM, and dynamic range of 10–100 µM for DA is shown, applicable for neuroblastoma screening. The sensor was selective against ascorbic and uric acids.

Mass Spectrometric Quantification of Arousal Associated Neurochemical Changes in Single Honey Bee Brains and Brain Regions

Honey bee foragers show a strong diurnal rhythm of foraging activity, and such behavioral changes are likely under the control of specific neuromodulators. To identify and quantify neuromodulators involved in regulating rest and arousal in honey bees, we established a mass spectrometric method for quantifying 14 different neurochemicals and precursor molecules. We measured forager type and brain region specific differences in amine levels from individual honey bee brains and brain regions. The observed differences in amine levels between resting and aroused foragers resemble findings in other species indicating a conserved molecular mechanism by glutamate and GABA in regulating arousal. Subesophageal ganglion specific changes in the histaminergic system and global increases in aspartate during arousal suggest a possible role of histamine and aspartate in feeding and arousal, respectively. More aminergic systems were significantly affected due to arousal in nectar foragers than in pollen foragers, implying that forager phenotypes differ not only in their food preference but also in their neuromodulatory signaling systems (brain states). Finally, we found that neurotransmitter precursors were better at distinguishing brain states in the central brain, while their end products correlated with arousal associated changes in sensory regions like the optic and antennal lobes.

A baseline drift detrending technique for fast scan cyclic voltammetry

Fast scan cyclic voltammetry (FSCV) has been commonly used to measure extracellular neurotransmitter concentrations in the brain. Due to the unstable nature of the background currents inherent in FSCV measurements, analysis of FSCV data is limited to very short amounts of time using traditional background subtraction. In this paper, we propose the use of a zero-phase high pass filter (HPF) as the means to remove the background drift. Instead of the traditional method of low pass filtering across voltammograms to increase the signal to noise ratio, a HPF with a low cutoff frequency was applied to the temporal dataset at each voltage point to remove the background drift. As a result, the HPF utilizing cutoff frequencies between 0.001 Hz and 0.01 Hz could be effectively used to a set of FSCV data for removing the drifting patterns while preserving the temporal kinetics of the phasic dopamine response recorded in vivo. In addition, compared to a drift removal method using principal component analysis, this was found to be significantly more effective in reducing the drift (unpaired t-test p < 0.0001, t = 10.88) when applied to data collected from Tris buffer over 24 hours although a drift removal method using principal component analysis also showed the effective background drift reduction. The HPF was also applied to 5 hours of FSCV in vivo data. Electrically evoked dopamine peaks, observed in the nucleus accumbens, were clearly visible even without background subtraction. This technique provides a new, simple, and yet robust, approach to analyse FSCV data with an unstable background.

Carbon Nanoelectrodes for the Electrochemical Detection of Neurotransmitters

Carbon-based electrodes have been developed for the detection of neurotransmitters over the past 30 years using voltammetry and amperometry. The traditional electrode for neurotransmitter detection is the carbon fiber microelectrode (CFME). The carbon-based electrode is suitable for in vivo neurotransmitter detection due to the fact that it is biocompatible and relatively small in surface area. The advent of nanoscale electrodes is in high demand due to smaller surface areas required to target specific brain regions that are also minimally invasive and cause relatively low tissue damage when implanted into living organisms. Carbon nanotubes (CNTs), carbon nanofibers, carbon nanospikes, and carbon nanopetals among others have all been utilized for this purpose. Novel electrode materials have also required novel insulations such as glass, epoxy, and polyimide coated fused silica capillaries for their construction and usage. Recent research developments have yielded a wide array of carbon nanoelectrodes with superior properties and performances in comparison to traditional electrode materials. These electrodes have thoroughly enhanced neurotransmitter detection allowing for the sensing of biological compounds at lower limits of detection, fast temporal resolution, and without surface fouling. This will allow for greater understanding of several neurological disease states based on the detection of neurotransmitters.

Hydrogel Based Sensors for Biomedical Applications: An Updated Review

Biosensors that detect and convert biological reactions to a measurable signal have gained much attention in recent years. Between 1950 and 2017, more than 150,000 papers have been published addressing the applications of biosensors in different industries, but to the best of our knowledge and through careful screening, critical reviews that describe hydrogel based biosensors for biomedical applications are rare. This review discusses the biomedical application of hydrogel based biosensors, based on a search performed through Web of Science Core, PubMed (NLM), and Science Direct online databases for the years 2000⁻2017. In this review, we consider bioreceptors to be immobilized on hydrogel based biosensors, their advantages and disadvantages, and immobilization techniques. We identify the hydrogels that are most favored for this type of biosensor, as well as the predominant transduction strategies. We explain biomedical applications of hydrogel based biosensors including cell metabolite and pathogen detection, tissue engineering, wound healing, and cancer monitoring, and strategies for small biomolecules such as glucose, lactate, urea, and cholesterol detection are identified.

Bioapplications of Electrochemical Sensors and Biosensors

Recent progress in the electrochemical field enabled development of miniaturized sensing devices that can be used in biological settings to obtain fundamental and practical biochemically relevant information on physiology, metabolism, and disease states in living systems. Electrochemical sensors and biosensors have demonstrated potential for rapid, real-time measurements of biologically relevant molecules. This chapter provides an overview of the most recent advances in the development of miniaturized sensors for biological investigations in living systems, with focus on the detection of neurotransmitters and oxidative stress markers. The design of electrochemical (bio)sensors, including their detection mechanism and functionality in biological systems, is described as well as their advantages and limitations. Application of these sensors to studies in live cells, embryonic development, and rodent models is discussed.

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.

Development of a versatile in vitro platform for studying biological systems using micro-3D printing and scanning electrochemical microscopy

We report a novel strategy for studying a broad range of cellular behaviors in real time by combining two powerful analytical techniques, micro-3D printing and scanning electrochemical microscopy (SECM). This allows one, in microbiological studies, to isolate a known number of cells in a micrometer-sized chamber with a roof and walls that are permeable to small molecules and observe metabolic products. In such studies, the size and spatial organization of a population play a crucial role in cellular group behaviors, such as intercellular interactions and communication. Micro-3D printing, a photolithographic method for constructing cross-linked protein microstructures, permits one to compartmentalize a small population of microbes by forming a porous roof and walls around cells in situ. Since the roof and walls defining the microchamber are porous, any small molecules can freely diffuse from the chamber to be detected and quantified using SECM. The size of the chamber and the roof permeability can be obtained by SECM using a small probe molecule, ferrocenemethanol (FcMeOH). The chamber permeability to FcMeOH can be tuned by varying printing parameters that influence the cross-linking density of the proteinaceous material. These analyses establish a versatile strategy as a sensitive platform to quantitatively monitor small molecules produced by microbes.

Nanomaterial-based electrochemical sensing of neurological drugs and neurotransmitters

Nanomaterial-modified detection systems represent a chief driver towards the adoption of electrochemical methods, since nanomaterials enable functional tunability, ability to self-assemble, and novel electrical, optical and catalytic properties that emerge at this scale. This results in tremendous gains in terms of sensitivity, selectivity and versatility. We review the electrochemical methods and mechanisms that may be applied to the detection of neurological drugs. We focus on understanding how specific nano-sized modifiers may be applied to influence the electron transfer event to result in gains in sensitivity, selectivity and versatility of the detection system. This critical review is structured on the basis of the Anatomical Therapeutic Chemical (ATC) Classification System, specifically ATC Code N (neurotransmitters). Specific sections are dedicated to the widely used electrodes based on the carbon materials, supporting electrolytes, and on electrochemical detection paradigms for neurological drugs and neurotransmitters within the groups referred to as ATC codes N01 to N07. We finally discuss emerging trends and future challenges such as the development of strategies for simultaneous detection of multiple targets with high spatial and temporal resolutions, the integration of microfluidic strategies for selective and localized analyte pre-concentration, the real-time monitoring of neurotransmitter secretions from active cell cultures under electro- and chemotactic cues, aptamer-based biosensors, and the miniaturization of the sensing system for detection in small sample volumes and for enabling cost savings due to manufacturing scale-up. The Electronic Supporting Material (ESM) includes review articles dealing with the review topic in last 40 years, as well as key properties of the analytes, viz., pKa values, half-life of drugs and their electrochemical mechanisms. The ESM also defines analytical figures of merit of the drugs and neurotransmitters. The article contains 198 references in the main manuscript and 207 references in the Electronic Supporting Material. Figureᅟ

Development of micropatterned cell-sensing surfaces

Microfabricated surfaces have been widely utilized for defining adhesion of single cells or groups of cells of various kinds. Beyond simple control of cell attachment, it is often important to monitor the molecules released by cells. Co-immobilizing miniature sensors alongside cells enables more sensitive detection of secreted factors and may allow for such detection to happen within the context of local microenvironment. Methods for interfacing cells and sensors are central to the notion of local in situ detection of cell function. This chapter describes the use of hydrogel photolithography for integrating cells and sensing elements on culture surfaces. Two types of micropatterned sensing surfaces are described: (1) arrays of microwells for single cell capture that contain antibodies against secreted proteins and (2) entrapment of enzymes inside hydrogel microstructures for local detection of cell metabolism. In both cases, poly(ethylene glycol) hydrogel lithography was employed to control cell attachment, in the second approach hydrogel structures also carried enzymes and functioned as sensors. The development of robust cell/sensor interfaces has implications for diagnostics, tissue engineering, and drug screening.

A planar microelectrode array for simultaneous detection of electrically evoked dopamine release from distinct locations of a single isolated neuron

Neurotransmission is a key process of communication between neurons. Although much is known about this process and the influence it has on the function of the body, little is understood about the dynamics of signalling from structural regions of a single neuron. In this study we have fabricated and characterised a microelectrode array (MEA) which was utilised for simultaneous multi-site recordings of dopamine release from an isolated single neuron. The MEA consisted of gold electrodes that were created in plane with the insulation layer using a chemical mechanical planarization process. The detection limit for dopamine measurements was 11 ± 3 nM and all the gold electrodes performed in a consistent fashion during amperometric recordings of 100 nM dopamine. Fouling of the gold electrode was investigated, where no significant change in the current was observed over 4 hours when monitoring 100 nM dopamine. The MEA was accessed using freshly isolated dopaminergic somas from the pond snail, Lymnaea stagnalis, where electrically evoked dopamine release was clearly observed. Measurements were conducted at four structural locations of a single isolated neuron, where electrically evoked dopamine release was observed from the cell body, axonal regions and the terminal. Over time, the release of dopamine varied over the structural regions of the neuron. Such information can provide an insight into the signalling mechanism of neurons and how they potentially form synaptic connections.

New trends in the electrochemical sensing of dopamine

Since the early 70s electrochemistry has been used as a powerful analytical technique for monitoring electroactive species in living organisms. In particular, after extremely rapid evolution of new micro and nanotechnology it has been established as an invaluable technique ranging from experiments in vivo to measurement of exocytosis during communication between cells under in vitro conditions. This review highlights recent advances in the development of electrochemical sensors for selective sensing of one of the most important neurotransmitters--dopamine. Dopamine is an electroactive catecholamine neurotransmitter, abundant in the mammalian central nervous system, affecting both cognitive and behavioral functions of living organisms. We have not attempted to cover a large time-span nor to be comprehensive in presenting the vast literature devoted to electrochemical dopamine sensing. Instead, we have focused on the last five years, describing recent progress as well as showing some problems and directions for future development.

Rapid, sensitive detection of neurotransmitters at microelectrodes modified with self-assembled SWCNT forests

Carbon nanotube (CNT) modification of microelectrodes can result in increased sensitivity without compromising time response. However, dip coating CNTs is not very reproducible and the CNTs tend to lay flat on the electrode surface which limits access to the electroactive sites on the ends. In this study, aligned CNT forests were formed using a chemical self-assembly method, which resulted in more exposed CNT ends to the analyte. Shortened, carboxylic acid functionalized single-walled CNTs were assembled from a dimethylformamide (DMF) suspension onto a carbon-fiber disk microelectrode modified with a thin iron hydroxide-decorated Nafion film. The modified electrodes were highly sensitive, with 36-fold higher oxidation currents for dopamine using fast-scan cyclic voltammetry than bare electrodes and 34-fold more current than electrodes dipped in CNTs. The limit of detection (LOD) for dopamine was 17 ± 3 nM at a 10 Hz repetition rate and 65 ± 7 nM at 90 Hz. The LOD at 90 Hz was the same as a bare electrode at 10 Hz, allowing a 9-fold increase in temporal resolution without a decrease in sensitivity. Similar increases were observed for other cationic catecholamine neurotransmitters, and the increases in current were greater than for anionic interferents such as ascorbic acid and 3,4-dihydroxyphenylacetic acid (DOPAC). The CNT forest electrodes had high sensitivity at 90 Hz repetition rate when stimulated dopamine release was measured in Drosophila . The sensitivity, temporal resolution, and spatial resolution of these CNT forest modified disk electrodes facilitate enhanced electrochemical measurements of neurotransmitter release in vivo.

Inhibitory neuromuscular transmission to ileal longitudinal muscle predominates in neonatal guinea pigs

BACKGROUND

Inhibitory neurotransmission to the longitudinal muscle is more prominent in the neonatal than in the adult guinea pig ileum.

METHODS

Inhibitory neuromuscular transmission was investigated using in vitro ileal longitudinal muscle myenteric plexus (LMMP) preparations made from neonatal (< or =48 h postnatal) and adult ( approximately 4 weeks postnatal) guinea pigs.

KEY RESULTS

Amperometric measurements of nicotine-induced nitric oxide (NO) release (measured as an oxidation current) from myenteric ganglia revealed larger currents in neonatal (379 +/- 24 pA) vs adult (119 +/- 39 pA, P < 0.05) tissues. Nicotine-induced oxidation currents were blocked by the nitric oxide synthase (NOS) inhibitor, nitro-l-arginine (NLA, 100 micromol L(-1)). Nicotine-induced, NLA-sensitive oxidation currents could be detected in the tertiary plexus of neonatal but not adult tissues. Immunohistochemistry demonstrated stronger NOS immunoreactivity in neonatal compared with adult myenteric ganglia. Western blot studies revealed higher levels of NOS in neonatal compared with adult LMMP. Cell counts revealed that the total number of myenteric neurons in the small intestine was greater in adults than in neonatal guinea pigs, however, the ratio of NOS : Calbindin neurons was significantly higher in neonatal compared with adult tissues.

CONCLUSIONS & INFERENCES

Nitric oxide signaling to the longitudinal muscle is stronger in neonatal compared with adult guinea pig ileum. Nitric oxide synthase-containing neurons are diluted postnatally by cholinergic and other, as yet unidentified neuronal subtypes.

Desorption electrospray ionization imaging mass spectrometry of lipids in rat spinal cord

Imaging mass spectrometry allows for the direct investigation of tissue samples to identify specific biological compounds and determine their spatial distributions. Desorption electrospray ionization (DESI) mass spectrometry has been used for the imaging and analysis of rat spinal cord cross sections. Glycerophospholipids and sphingolipids, as well as fatty acids, were detected in both the negative and positive ion modes and identified through tandem mass spectrometry (MS/MS) product ion scans using collision-induced dissociation and accurate mass measurements. Differences in the relative abundances of lipids and free fatty acids were present between white and gray matter areas in both the negative and positive ion modes. DESI-MS images of the corresponding ions allow the determination of their spatial distributions within a cross section of the rat spinal cord, by scanning the DESI probe across the entire sample surface. Glycerophospholipids and sphingolipids were mostly detected in the white matter, while the free fatty acids were present in the gray matter. These results show parallels with reported distributions of lipids in studies of rat brain. This suggests that the spatial intensity distribution reflects relative concentration differences of the lipid and fatty acid compounds in the spinal cord tissue. The "butterfly" shape of the gray matter in the spinal cord cross section was resolved in the corresponding ion images, indicating that a lateral resolution of better than 200 mum was achieved. The selected ion images of lipids are directly correlated with anatomic features on the spinal cord corresponding to the white and the gray matter.

In vivo measurement of somatodendritic release of dopamine in the ventral tegmental area

The ventral tegmental area (VTA), the locus of mesolimbic dopamine cell bodies, contains dopamine. Experiments in brain slices have demonstrated that VTA dopamine can be released by local electrical stimulation. Measurements with both push-pull cannula and microdialysis in intact animals have also obtained evidence for releasable dopamine. Here we demonstrate that dopamine release in the VTA can be evoked by remote stimulations of the medial forebrain bundle (MFB) in the anesthetized rat. In initial experiments, the MFB was electrically stimulated while a carbon-fiber electrode was lowered to the VTA, with recording by fast-scan cyclic voltammetry. While release was not observed with the carbon fiber 4-6 mm below dura, a voltammetric response was observed at 6-8 mm below dura, but the voltammogram was poorly defined. At lower depths, in the VTA, dopamine release was evoked. Immunohistochemistry experiments with antibodies for tyrosine hydroxylase (TH) confirmed that dopamine processes were primarily found below 8 mm. Similarly, tissue content determined by liquid chromatography revealed serotonin but not dopamine dorsal to 8 mm with both dopamine and serotonin at lower depths. Evaluation of the VTA signal by pharmacological means showed that it increased with inhibitors of dopamine uptake, but release was not altered by D2 agents. Dopamine release in the VTA was frequency dependent and could be exhausted by stimulations longer than 5 s. Thus, VTA dopamine release can be evoked in vivo by remote stimulations and it resembles release in terminal regions, possessing a similar uptake mechanism and a finite releasable storage pool.

High-throughput capillary-electrophoresis analysis of the contents of a single mitochondria

We present a technique for labeling the contents of acidic organelles and rapidly releasing, separating, and detecting their labeled contents with laser-induced fluorescence. We have performed solution-phase separation of the contents of single mitochondria and single 100 nm vesicles, which represents a demonstration of an analyzed volume of approximately 1 aL. Our strategy to label the acidic contents of the mitochondrion relies on the use of the membrane-permeable dye, Oregon Green diacetate succinimidyl ester, and a membrane-permeable base to raise intramitochondrial pH. In order to measure the contents, we utilized a glass microfluidic chip and high voltage gradient for millisecond capillary electrophoresis separation after single-mitochondrion photolysis. We observed heterogeneity among a population of mitochondria with respect to a constituent chemical component.

Enzyme-containing hydrogel micropatterns serving a dual purpose of cell sequestration and metabolite detection

The integration of sensing elements with small groups of cells is a critical step towards miniaturization of cell cultivation and analysis. This paper describes the development of an optical, enzyme-based biosensor for local detection of hydrogen peroxide (H(2)O(2)) secreted by stimulated macrophages. Photolithographic patterning of horseradish peroxidase (HRP)-containing poly (ethylene glycol) (PEG) hydrogel microstructures was used to create sensing structures on the glass surface. Importantly, enzyme-entrapping hydrogel micropatterns did not support protein or cell deposition and allowed to guide attachment of macrophages next to the sensing elements. Amplex Red, an organic molecule that becomes fluorescent in the presence of H(2)O(2) and HRP, was either immobilized inside hydrogel elements alongside enzyme molecules or added into the cell culture media during cell activation. The production of H(2)O(2) after mitogenic stimulation of macrophages resulted in appearance of fluorescence in the HRP-containing hydrogel microstructures, with fluorescence intensity being a strong function of analyte concentration. The novel cell culture system with integrated sensing elements described here may be enhanced in the future by incorporating additional biorecognition elements to enable multi-metabolite detection at the site of a cell.

A Nonoxidative Electrochemical Sensor Based on a Self-Doped Polyaniline/Carbon Nanotube Composite for Sensitive and Selective Detection of the Neurotransmitter Dopamine: A Review

Most of the current techniques for in vivo detection of dopamine exploit the ease of oxidation of this compound. The major problem during the detection is the presence of a high concentration of ascorbic acid that is oxidized at nearly the same potential as dopamine on bare electrodes. Furthermore, the oxidation product of dopamine reacts with ascorbic acid present in samples and regenerates dopamine again, which severely limits the accuracy of the detection. Meanwhile, the product could also form a melanin-like insulating film on the electrode surface, which decreases the sensitivity of the electrode. Various surface modifications on the electrode, new materials for making the electrodes, and new electrochemical techniques have been exploited to solve these problems. Recently we developed a new electrochemical detection method that did not rely on direct oxidation of dopamine on electrodes, which may naturally solve these problems. This approach takes advantage of the high performance of our newly developed poly(anilineboronic acid)/carbon nanotube composite and the excellent permselectivity of the ion-exchange polymer Nafion. The high affinity binding of dopamine to the boronic acid groups of the polymer affects the electrochemical properties of the polyaniline backbone, which act as the basis for the transduction mechanism of this non-oxidative dopamine sensor. The unique reduction capability and high conductivity of single-stranded DNA functionalized single-walled carbon nanotubes greatly improved the electrochemical activity of the polymer in a physiologically-relevant buffer, and the large surface area of the carbon nanotubes increased the density of the boronic acid receptors. The high sensitivity and selectivity of the sensor show excellent promise toward molecular diagnosis of Parkinson's disease. In this review, we will focus on the discussion of this novel detection approach, the new interferences in this detection approach, and how to eliminate these interferences toward in vivo and in vitro detection of the neurotransmitter dopamine.

Electrochemical monitoring of nitric oxide released by myenteric neurons of the guinea pig ileum

Nitric oxide (NO) released by myenteric neurons in isolated segments of guinea pig ileum was monitored in vitro using continuous amperometry. NO was detected as an oxidation current recorded with a boron-doped diamond microelectrode held at 1 V vs a Ag|AgCl reference electrode. This potential was sufficient to oxidize NO. Longitudinal muscle-myenteric plexus (LMMP) and circular muscle strip preparations were used. In the LMMP preparation, NO release was evoked by superfusion of 1 mumol L(-1) nicotine, which activates nicotinic acetylcholine receptors expressed by myenteric neurons and myenteric nerve endings. The oxidation current was ascribed to NO based on the following observations: (i) no response was detected at less positive potentials (0.75 V) at which only catecholamines and biogenic amines are oxidized, (ii) the current was abolished in the presence of the nitric oxide synthase antagonist, N-nitro-l-arginine (l-NNA) and (iii) oxidation currents were attenuated by addition of the NO scavenger, myoglobin, to the superfusing solution. In the LMMP preparation, stimulated release produced a maximum current that corresponded nominally to 46 nmol L(-1) of NO. The oxidation currents decreased to 10 and 2 nmol L(-1), respectively, when the tissue was perfused with tetrodotoxin and l-NNA. Oxidation currents recorded from circular muscle strips (stimulated using nicotine) were threefold larger than those recorded from the LMMP. This study shows that NO release can be detected from various in vitro preparations of the guinea pig ileum using real-time electroanalytical techniques.

Dynamic changes in dopamine tone during self-stimulation of the ventral tegmental area in rats

In a prior study, phasic release of dopamine (DA) in the nucleus accumbens (NAc) was only transiently and rarely detected by means of fast-scan cyclic voltammetry (FCSV) in rats already trained to work for electrical stimulation of the ventral tegmental area (VTA) on a continuous reinforcement schedule. However, in rats receiving rewarding electrical stimulation via lateral hypothalamic (LH) electrodes, elevated DA tone in the NAc terminal field was detected via microdialysis for up to 2h, even when short (1.5s) inter-train intervals were employed. To better characterize the similarities and differences between the FSCV and microdialysis measurements, we trained rats to self-administer VTA stimulation under conditions similar to those employed in the initial FSCV study. The results resemble those obtained by means of microdialysis in rats receiving LH stimulation but differed from the prior FSCV data. Although the concentration of DA in dialysate obtained from NAc probes did fall after having peaked at the 30 min mark, this decline set in much later than in the FSCV studies, and elevated DA tone could still be detected after 110 min of self-stimulation. The stimulation-induced peak in DA tone could be restored by a 30 min rest period, a manipulation that was ineffective previously in restoring the FSCV measure of phasic release. These findings are discussed in terms of the differential sensitivity of the FSCV and microdialysis methods to phasic and tonic signaling by DA neurons and to different transitions between their activity states.

Quantitative and real-time detection of secretion of chemical messengers from individual platelets

Carbon-fiber microelectrochemical methods were utilized in this study to measure individual exocytotic events of secretion of serotonin and histamine from washed rabbit platelets. The quantal release of serotonin was quantitatively characterized with a delta-granule serotonin concentration of 0.6 M and secretion time course of 7 ms. Additionally, extracellular osmolarity influences quantal size, causing quantal size increases under hypotonic conditions, presumably due to the influx of cytosolic serotonin into the halo region of the delta-granules.

Electrochemical Dopamine Detection: Comparing Gold and Carbon Fiber Microelectrodes using Background Subtracted Fast Scan Cyclic Voltammetry

Electrochemical detection is becoming increasingly important for the detection of biological species. Most current biological research with electrochemical detection is done with carbon fiber electrodes due to their many beneficial properties. The ability to build electrochemical sensor from noble metals instead of carbon fibers may be beneficial in developing inexpensive multiplexed electrochemical detection schemes. To advance understanding and to test the feasibility of using noble metal electrochemical sensors the detection of dopamine, a biologically important small molecule was studied here. Specifically, dopamine detection on gold microelectrodes was characterized and compared to P-55 carbon fiber microelectrodes of the same geometry, using background subtracted fast scan cyclic voltammetry. While not as sensitive to dopamine as carbon fibers, it was observed that gold microelectrodes have six times the saturation coverage per area and 40 times the linear working range. Selectivity to dopamine, in comparison to several other neurotransmitters and their derivatives, is also quantitatively described.

Pre-synaptic dopaminergic compensation after moderate nigrostriatal damage in non-human primates

Despite a dramatic loss of nigrostriatal dopaminergic neurons in Parkinson's disease, clinical symptoms only arise with 70-80% reduction of striatal dopamine. The mechanisms responsible for this functional compensation are currently under debate. Although initial studies showed an enhanced pre-synaptic dopaminergic function with nigrostriatal degeneration, more recent work suggests that functional compensation is not dopamine-mediated. To address this issue, we used cyclic voltammetry to directly measure endogenous dopamine release from striatal slices of control monkeys and animals with a moderate or severe MPTP-induced dopaminergic lesion. The moderately lesioned monkeys were asymptomatic, while the severely lesioned animals were parkinsonian. In monkeys with a moderate lesion, a 300% increase was obtained in endogenous striatal dopamine release. In contrast, in striatal slices from severely lesioned animals, a small % of evoked dopamine signals were similar in amplitude to control while the greater majority were undetectable. These findings suggest that pre-synaptic dopaminergic compensation develops in residual dopaminergic terminals with moderate lesioning, but that this response is lost with severe nigrostriatal damage. Such an interpretation is supported by the results of dopamine turnover studies. This enhanced pre-synaptic dopaminergic activity may be important in maintaining normal motor function during the initial stages of Parkinson's disease.

Diamond microelectrodes for in vitro electroanalytical measurements: current status and remaining challenges

An emerging research field in electrochemistry today is the preparation, characterization and application of diamond microelectrodes for electroanalytical measurements in biological media. Interest in this new electrode material stems from its outstanding properties: (i) hardness, (ii) low, stable and pH-independent background current, (iii) morphological and microstructural stability over a wide range of potentials, (iv) good electrochemical responsiveness for multiple redox analytes without any conventional pre-treatment and (v) weak molecular adsorption of polar molecules that leads to a high level of resistance to response deactivation and electrode fouling. Diamond electrodes have advanced in recent years from being simply a scientific curiosity into a viable material for electroanalysis. In this article, we highlight the current state of progress by our laboratory and others on the preparation, study of the basic electrochemical properties, and application of this new type of microelectrode for in vitro electroanalytical measurements, and discuss some of the remaining challenges.

Temperature-dependent differences between readily releasable and reserve pool vesicles in chromaffin cells

Statistical differences between amperometric traces recorded from chromaffin cells using K(+) and Ba(2+) secretagogues support the assertion that readily releasable pool (RRP) and reserve pool (RP) vesicles can be probed with pool-specific secretagogues. Release from the RRP was evoked by K(+) while release from the RP was evoked by Ba(2+). Similar temperature-dependent changes in spike area and half-width for both pools suggest that the content of RRP and RP vesicles is similar and packaged in the same way. Differences between the vesicle pools were revealed in the temperature dependence of spike frequency. While the burst spike frequency of the RRP, which is comprised of pre-docked and primed vesicles, increased 2.8% per degrees C, the RP spike frequency increased 12% per degrees C. This difference is attributed to a temperature-dependent mobilization of the RP. Furthermore, the RP exhibited more foot events at room temperature than the RRP but this difference was not apparent at 37 degrees C. This trend suggests that RP vesicle membranes have a compromised surface tension compared to RRP vesicles. Collectively, the changes of release characteristics with temperature reveal distinctions between the RRP and the RP.

A Nonoxidative Sensor Based on a Self-Doped Polyaniline/Carbon Nanotube Composite for Sensitive and Selective Detection of the Neurotransmitter Dopamine

Most of the current techniques for detection of dopamine exploit its ease of oxidation. However, the oxidative approaches suffer from a common problem. The products of dopamine oxidation can react with ascorbic acid present in samples and regenerate dopamine again, which severely limits the accuracy of detection. In this paper, we report a nonoxidative approach to electrochemically detect dopamine with high sensitivity and selectivity. This approach takes advantage of the high performance of our newly developed poly(anilineboronic acid)/carbon nanotube composite and the excellent permselectivity of the ion-exchange polymer Nafion. The binding of dopamine to the boronic acid groups of the polymer with large affinity affects the electrochemical properties of the polyaniline backbone, which act as the transduction mechanism of this nonoxidative dopamine sensor. The unique reduction capability and high conductivity of single-stranded DNA functionalized, single-walled carbon nanotubes greatly improved the electrochemical activity of the polymer in physiological buffer, and the large surface area of the carbon nanotubes largely increased the density of the boronic acid receptors. The high sensitivity along with the improved selectivity of this sensing approach is a significant step forward toward molecular diagnosis of Parkinson's disease.

Biological applications of scanning electrochemical microscopy: chemical imaging of single living cells and beyond

Recent applications of scanning electrochemical microscopy (SECM) to studies of single biological cells are reviewed. This scanning probe microscopic technique allows the imaging of an individual cell on the basis of not only its surface topography but also such cellular activities as photosynthesis, respiration, electron transfer, single vesicular exocytosis and membrane transport. The operational principles of SECM are also introduced in the context of these biological applications. Recent progress in techniques for high-resolution SECM imaging are also reviewed. Future directions, such as single-channel detection by SECM, high-resolution imaging with nanometer-sized probes, and combined SECM techniques for multidimensional imaging are also discussed.

Transport model of chemical secretion process for tracking exocytotic event dynamics using electroanalysis

A unified model is developed to analyze the key features of the chemical secretion process observed in experimental studies of various vesicles with application to electroanalytical measurements of vesicular exocytosis. The intimately coupled dynamics and kinetics are simultaneously resolved based on continuum fluid flow, mass transport, and linear elasticity theories combined with biomembrane mechanics. We report three case studies of exocytosis, including a large electroporated granule of the mast cell, a small and clear synaptic vesicle, and a medium size vesicle in the chromaffin cell. The simulation results for each case are compared with electroanalytical measurements from the literature. The results provide a theoretical ground for defining the rate-controlling step(s) of an exocytotic sequence, allowing interpretation of electroanalysis data. Thus, it provides a tool for theoretical verification of competing hypotheses of what controls/limits messenger release during exocytosis. Simulations show that the pore size, the pore opening velocity, and the swelling dynamics of the granule matrix play the key roles in controlling the messenger release kinetics.

Simultaneous determination of biogenic monoamines in rat brain dialysates using capillary high-performance liquid chromatography with photoluminescence following electron transfer

Simultaneous determination of biogenic monoamines such as dopamine, serotonin, and 3-methoxytyramine in brain is important in understanding neurotransmitter activity. This study presents a sensitive determination of biogenic monoamines in rat brain striatum microdialysates using capillary high-performance liquid chromatography with the photoluminescence following electron-transfer detection technique. Separation conditions were optimized by changing the concentration of an ion-interaction agent and the percentage of an organic modifier. The high concentration of ion-interaction agent enabled the amines as a class to be separated from interfering acids, but also made the separation very long. To shorten the separation time, 10% (v/v) acetonitrile was used as the organic modifier. Eight chromatographic runs during a 3-h period were analyzed in terms of retention times, peak heights, and peak widths. Chromatograms are very reproducible, with less than 1% changes in peak height over 3 h. Typical concentration detection limits at the optimum separation conditions were less than 100 pM for metabolic acids and approximately 200 pM for monoamines. The injection volume of the sample was 500 nL. Thus, the mass detection limits were less than 50 amol for metabolic acids and approximately 100 amol for monoamines. Typical separation time was less than 10 min. To validate the technique, the separation method was applied to the observation of drug-induced changes of monoamine concentrations in rat brain microdialysis samples. Local perfusion of tetrodotoxin, a sodium channel blocker, into the striatum of an anesthetized rat decreased dopamine, 3-methoxytyramine, and serotonin concentrations in dialysates. Successive monitoring of striatal dialysates at a temporal resolution of 7.7 min showed that the injection of nomifensine transiently increased dopamine and 3-methoxytyramine concentrations in rat brain dialysate.

Carbon-fiber microelectrodes modified with 4-sulfobenzene have increased sensitivity and selectivity for catecholamines

Elliptical and cylindrical geometries of carbon-fiber microelectrodes were modified by covalent attachment of 4-sulfobenzenediazonium tetrafluoroborate following its electroreduction. Elliptical electrodes fabricated from Thornel P-55 carbon fibers show the highest amount of 4-sulfobenzene attached to the electrode. Fast-scan cyclic voltammetry was used to compare the response to dopamine and other neurochemicals at these modified carbon-fiber microelectrodes. The grafted layer causes an increased sensitivity to dopamine and other positively charged analytes that is due to increased adsorption of analyte in the grafted layer. However, this layer remains permeable to negatively charged compounds. Modified electrodes retain the increased sensitivity for dopamine during measurements in mouse brain tissue.

Filtration disrupts synaptosomes during radiochemical analysis of serotonin uptake: comparison with chronoamperometry in SERT knockout mice

Radiochemical methods have failed to reveal decreases in synaptosomal serotonin uptake in mice lacking one functional copy of the serotonin transporter (SERT) gene. By contrast, uptake rates determined by chronoamperometry in synaptosomes from SERT+/- mice show gene-related reductions. We revisited [(3)H]5-HT uptake in SERT knockout mice to determine the effects of inclusion of O(2) in the incubation buffer on the kinetic parameters obtained by this method. In oxygenated synaptosomes prepared from frontal cortex and striatum, modest 25 and 35% reductions in radiolabeled 5-HT uptake were detected in SERT+/- versus SERT+/+ mice. However, even in the presence of O(2), no differences in [(3)H]5-HT uptake were detected between SERT+/- and SERT+/+ mice in brain stem in contrast to 60% reductions determined by chronoamperometry. Moreover, while inclusion of O(2) modestly increased the rates of [(3)H]5-HT uptake, rates determined by chronoamperometry in the presence of O(2) were 40-fold greater than those determined radiochemically. We present evidence that the filtration process used in the radiochemical method leads to substantial loss of transported 5-HT resulting in lower apparent uptake rates. These findings explain the relative insensitivity of radiochemical methods for determining biologically important alterations in uptake such as those occurring between SERT+/- and SERT+/+ mice and in response to O(2).