Probing Cellular Chemistry in Biological Systems with Microelectrodes

Sensitization of rapid dopamine signaling in the nucleus accumbens core and shell after repeated cocaine in rats

Repeated cocaine exposure and withdrawal leads to long-term changes, including behavioral and dopamine sensitization to an acute cocaine challenge, that are most pronounced after long withdrawal periods. However, the changes in dopamine neurotransmission after short withdrawal periods are less well defined. To study dopamine neurotransmission after 1-day withdrawal, we used fast-scan cyclic voltammetry (FSCV) to determine whether repeated cocaine alters rapid dopamine release and uptake in the nucleus accumbens (NAc) core and shell. FSCV was performed in urethane anesthetized male Sprague-Dawley rats that had previously received one or seven daily injections of saline or cocaine (15 mg/kg, ip). In response to acute cocaine, subjects showed increased dopamine overflow that resulted from both increased dopamine release and slowed dopamine uptake. One-day cocaine pre-exposure, however, did not alter dopaminergic responses to a subsequent cocaine challenge. In contrast, 7-day cocaine-treated subjects showed a potentiated rapid dopamine response in both the core and shell after an acute cocaine challenge. In addition, kinetic analysis during the cocaine challenge showed a greater increase in apparent K(m) of 7-day cocaine exposed subjects. Together, the data provide the first in vivo demonstration of rapid dopamine sensitization in the NAc core and shell after a short withdrawal period. In addition, the data clearly delineate cocaine's release and uptake effects and suggest that the observed sensitization results from greater uptake inhibition in cocaine pre-exposed subjects.

Voltammetric detection of hydrogen peroxide at carbon fiber microelectrodes

Hydrogen peroxide is a reactive oxygen species that is implicated in a number of neurological disease states and that serves a critical role in normal cell function. It is commonly exploited as a reporter molecule enabling the electrochemical detection of nonelectroactive molecules at electrodes modified with substrate-specific oxidative enzymes. We present the first voltammetric characterization of rapid hydrogen peroxide fluctuations at an uncoated carbon fiber microelectrode, demonstrating unprecedented chemical and spatial resolution. The carbon surface was electrochemically conditioned on the anodic scan and the irreversible oxidation of peroxide was detected on the cathodic scan. The oxidation potential was dependent on scan rate, occurring at +1.2 V versus Ag/AgCl at a scan rate of 400 V.s(-1). The relationship between peak oxidation current and concentration was linear across the physiological range tested, with deviation from linearity above 2 mM and a detection limit of 2 muM. Peroxide was distinguished from multiple interferents, both in vitro and in brain slices. The enzymatic degradation of peroxide was monitored, as was peroxide evolution in response to glucose at a glucose oxidase modified carbon fiber electrode. This novel approach provides the requisite sensitivity, selectivity, spatial and temporal resolution to study dynamic peroxide fluctuations in discrete biological locations.

Specific oxygen-containing functional groups on the carbon surface underlie an enhanced sensitivity to dopamine at electrochemically pretreated carbon fiber microelectrodes

The in vivo use of carbon-fiber microelectrodes for neurochemical investigation has proven to be selective and sensitive when coupled with background-subtracted fast-scan cyclic voltammetry (FSCV). Various electrochemical pretreatments have been established to enhance the sensitivity of these sensors; however, the fundamental chemical mechanisms underlying these enhancement strategies remain poorly understood. We have investigated an electrochemical pretreatment in which an extended triangular waveform from -0.5 to 1.8 V is applied to the electrode prior to the voltammetric detection of dopamine using a more standard waveform ranging from -0.4 to 1.3 V. This pretreatment enhances the electron-transfer kinetics and significantly improves sensitivity. To gain insight into the chemical mechanism, the electrodes were studied using common analytical techniques. Contact atomic force microscopy (AFM) was used to demonstrate that the surface roughness was not altered on the nanoscale by electrochemical pretreatment. Raman spectroscopy was utilized to investigate oxide functionalities on the carbon surface and confirmed that carbonyl and hydroxyl functional groups were increased by electrochemical conditioning. Spectra collected after the selective chemical modification of these groups implicate the hydroxyl functionality, rather than the carbonyl, as the major contributor to the enhanced electrochemical signal. Finally, we have demonstrated that this electrochemical pretreatment can be used to create carbon microdisc electrodes with sensitivities comparable to those associated with larger, conventionally treated cylindrical carbon fiber microelectrodes.

Analytical approaches to investigate transmitter content and release from single secretory vesicles

The vesicle serves as the primary intracellular unit for the highly efficient storage and release of chemical messengers triggered during signaling processes in the nervous system. This review highlights conventional and emerging analytical methods that have used microscopy, electrochemistry, and spectroscopy to resolve the location, time course, and quantal content characteristics of neurotransmitter release. Particular focus is on the investigation of the synaptic vesicle and its involvement in the fundamental molecular mechanisms of cell communication.

Scanning mass spectrometry probe: a scanning probe electrospray ion source for imaging mass spectrometry of submerged interfaces and transient events in solution

The scanning mass spectrometry (SMS) probe is a new electrospray ion source. Motivated by the need for untargeted chemical imaging of dynamic events in solution, we have exploited an approach to electrospray ionization (ESI) that allows continuous sampling from a highly localized volume (approximately picoliters) in a liquid environment, softly ionizes molecules in the sample to render them amenable for mass spectrometric analysis, and sends the ions to the mass spectrometer. The key underlying concepts for our approach are (1) treating the electrospray capillary inlet as a chemical scanning probe and (2) locating the electrospray point as close as possible to the sampling point, thus providing the shortest response time possible. This approach enables chemical monitoring or imaging of submerged interfaces, providing access to details of spatial heterogeneity and temporal changes within liquid samples. It also permits direct access to liquid/ liquid interfaces for ESI-MS analysis. In this letter we report the first demonstrations of these capabilities of the SMS probe and describe some of the probe's basic characteristics.

Improved surface-patterned platinum microelectrodes for the study of exocytotic events

Surface-patterned platinum microelectrodes insulated with 300 nm thick fused silica were fabricated using contact photolithography. These electrodes exhibit low noise and were used for monitoring single vesicle exocytosis from chromaffin cells by constant potential amperometry as well as fast-scan cyclic voltammetry. Amperometric spike parameters were consistent with those obtained with conventional carbon fiber electrodes. Catecholamine voltammograms acquired with platinum electrodes exhibited redox peaks with full width at half-maximum of approximately 45 mV, much sharper than those of carbon fiber electrode recordings. The time course of voltammetrically measured release events was similar for platinum and carbon fiber electrodes. The fused-silica-insulated platinum electrodes could be cleaned and reused repetitively and allowed incorporation of micrometer precision surface-patterned poly-D-lysine. Poly-D-lysine-functionalized devices were applied to stimulate mast cells and record single release events without serotonin preloading. Microfabricated platinum electrodes are thus able to record single exocytotic events with high resolution and should be suitable for highly parallel electrode arrays allowing simultaneous measurements of single events from multiple cells.

Boron-Doped Diamond Microelectrodes Reveal Reduced Serotonin Uptake Rates in Lymphocytes from Adult Rhesus Monkeys Carrying the Short Allele of the 5-HTTLPR

Uptake resolved by high-speed chronoamperometry on a second-by-second basis has revealed important differences in brain serotonin transporter function associated with genetic variability. Here, we use chronoamperometry to investigate variations in serotonin transport in primary lymphocytes associated with the rhesus serotonin transporter gene-linked polymorphism (rh5-HTTLPR), a promoter polymorphism whose orthologs occur only in higher order primates including humans. Serotonin clearance by lymphocytes is Na(+)-dependent and inhibited by the serotonin-selective reuptake inhibitor paroxetine (Paxil®), indicative of active uptake by serotonin transporters. Moreover, reductions in serotonin uptake rates are evident in lymphocytes from monkeys with one or two copies of the short 's' allele of the rh5-HTTLPR (s/s Collapse

Key Words

• serotonin transporter
• promoter polymorphism
• nonhuman primate
• peripheral blood mononuclear cells
• chronoamperometry
• voltammetry
• carbon fiber microelectrode

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MESH Headings Collapse

Grants

• R01 MH064756 NIMH NIH HHS

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Affiliation(s)

Yogesh S. Singh Departments of Chemistry
Lauren E. Sawarynski Bioengineering
Heather M. Michael Department of Microbiology and Molecular Genetics
Robert E. Ferrell Department of Human Genetics
Michael A. Murphey-Corb Department of Microbiology and Molecular Genetics
Greg M. Swain Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
Bhavik A. Patel Department of Bioengineering, Imperial College London, London, U.K. SE7 2AZ
Anne M. Andrews Departments of Chemistry Veterinary and Biomedical Sciences Huck Institutes of Life Sciences Department of Psychiatry & Biobehavioral Sciences and California NanoSystems Institute, University of California, Los Angeles, California 90024

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Real-time monitoring of oxidative burst from single plant protoplasts using microelectrochemical sensors modified by platinum nanoparticles

Oxidative bursts from plants play significant roles in plant disease defense and signal transduction; however, it has not hitherto been investigated on individual living plant cells. In this article, we fabricated a novel sensitive electrochemical sensor based on electrochemical deposition of Pt nanoparticles on the surface of carbon fiber microdisk electrodes via a nanopores containing polymer matrix, Nafion. The numerous hydrophilic nanochannels in the Nafion clusters coated on the electrode surface served as the molecular template for the deposition and dispersion of Pt, which resulted in the uniform construction of small Pt nanoparticles. The novel sensor displayed a high sensitivity for detection of H(2)O(2) with a detection limit of 5.0 x 10(-9) M. With the use of this microelectrochemical sensor, the oxidative burst from individual living plant protoplasts have been real-time monitored for the first time. The results showed that oxidative burst from single protoplasts triggered by a pathogen analogue were characterized by quanta release with a large number of "transient oxidative microburst" events, and protoplasts from the transgenic plants biologically displayed better disease-resistance and showed a distinguished elevation and longer-lasting oxidative burst.

Simultaneous monitoring of dopamine concentration at spatially different brain locations in vivo

When coupled with a microelectrode, background-subtracted fast scan cyclic voltammetry (FSCV) allows fast, sensitive and selective determination of analytes within a small spatial location. For the past 30 years experiments using this technique have been largely confined to recordings at a single microelectrode. Arrays with closely separated microelectrodes would allow researchers to gain more informative data as well as probe regions in close spatial proximity. This work presents one of the first FSCV microelectrode arrays (MEA) implemented in vivo with the ability to sample from different regions in close spatial proximity (equidistant within 1mm). The array is manufactured from fused silica capillaries and a microfabricated electrode spacer. The functionality of the array is assessed by simultaneously monitoring electrically stimulated dopamine (DA) release in the striatum of anesthetized rat. As expected, heterogeneous dopamine release was simultaneously observed. Additionally, the pharmacological effect of raclopride (D(2) receptor antagonist) and cocaine (monoamine uptake blocker) on the heterogeneity of DA release, in spatially different brain regions was shown to alter neurotransmitter release at all four electrode sites.

Regional specificity in the real-time development of phasic dopamine transmission patterns during acquisition of a cue-cocaine association in rats

Drug seeking is significantly regulated by drug-associated cues and associative learning between environmental cues and cocaine reward is mediated by dopamine transmission within the nucleus accumbens (NAc). However, dopamine transmission during early acquisition of a cue-cocaine association has never been assessed because of the technical difficulties associated with resolving cue-evoked and cocaine-evoked dopamine release within the same conditioning trial. Here, we used fast-scan cyclic voltammetry to measure sub-second fluctuations in dopamine concentration within the NAc core and shell during the initial acquisition of a cue-cocaine Pavlovian association. Within the NAc core, cue-evoked dopamine release developed during conditioning. However, within the NAc shell, the predictive cue appeared to cause an unconditioned decrease in dopamine concentration. The pharmacological effects of cocaine also differed between sub-regions, as cocaine increased phasic dopamine release events within the NAc shell but not the core. Thus, real-time measurements not only revealed the initial development of a conditioned neurochemical response but also demonstrated differential phasic dopamine transmission patterns across NAc sub-regions during the acquisition of a cue-cocaine association.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.

Amperometric assessment of functional changes in nanoparticle-exposed immune cells: varying Au nanoparticle exposure time and concentration

A mast cell/fibroblast co-culture system is used as a model to assess the toxicity of Au nanoparticles over the course of 72 hours of exposure. Cellular uptake of nanoparticles was found to increase over the 72 hr exposure period and the nanoparticles localized within granular bodies of the primary culture mast cells. These granules were found to increase in volume with the addition of nanoparticles. There was no decrease in viability for 24 hr exposed cells but a slight viability decrease was found after 48 and 72 hr exposure. Carbon-fiber amperometry analysis of exocytosis of serotonin from mast cells revealed changing release profiles over the time course of exposure. In early exposure times, granular secretion of serotonin increased with exposure to Au nanoparticles whereas 72 hr exposure showed decreased secretion of serotonin with nanoparticle exposure. The kinetics of this release was also found to be affected by Au colloid exposure where the rate of serotonin expulsion decreased with increasing nanoparticle exposure. These results illustrate the dynamic nature of nanoparticle-cell interactions and the critical changes in cell behavior even when viability is unaffected.

Release of adenosine and ATP during ischemia and epilepsy

Eighty years ago Drury & Szent-Györgyi described the actions of adenosine, AMP (adenylic acid) and ATP (pyrophosphoric or diphosphoric ester of adenylic acid) on the mammalian cardiovascular system, skeletal muscle, intestinal and urinary systems. Since then considerable insight has been gleaned on the means by which these compounds act, not least of which in the distinction between the two broad classes of their respective receptors, with their many subtypes, and the ensuing diversity in cellular consequences their activation invokes. These myriad actions are of course predicated on the release of the purines into the extracellular milieu, but, surprisingly, there is still considerable ambiguity as to how this occurs in various physiological and pathophysiological conditions. In this review we summarise the release of ATP and adenosine during seizures and cerebral ischemia and discuss mechanisms by which the purines adenosine and ATP may be released from cells in the CNS under these conditions.

Simultaneous decoupled detection of dopamine and oxygen using pyrolyzed carbon microarrays and fast-scan cyclic voltammetry

Microfabricated structures utilizing pyrolyzed photoresist have been shown to be useful for monitoring electrochemical processes. These previous studies, however, were limited to constant-potential measurements and slow-scan voltammetry. The work described in this paper utilizes microfabrication processes to produce devices that enable multiple fast-scan cyclic voltammetry (FSCV) waveforms to be applied to different electrodes on a single substrate. This enabled the simultaneous, decoupled detection of dopamine and oxygen. In this paper we describe the fabrication process of these arrays and show that pyrolyzed photoresist electrodes possess surface chemistry and electrochemical properties comparable to PAN-type, T-650, carbon fiber microelectrodes using background-subtracted FSCV. The functionality of the array is discussed in terms of the degree of cross talk in response to flow injections of physiologically relevant concentrations of dopamine and oxygen. Finally, other applications of pyrolyzed photoresist microelectrode arrays are shown, including spatially resolved detection of analytes and combining FSCV with amperometry for the detection of dopamine.

The dopaminergic basis of human behaviors: A review of molecular imaging studies

This systematic review describes human molecular imaging studies which have investigated alterations in extracellular DA levels during performance of behavioral tasks. Whilst heterogeneity in experimental methods limits meta-analysis, we describe the advantages and limitations of different methodological approaches. Interpretation of experimental results may be limited by regional cerebral blood flow (rCBF) changes, head movement and choice of control conditions. We revisit our original study of striatal DA release during video-game playing [Koepp, M.J., Gunn, R.N., Lawrence, A.D., Cunningham, V.J., Dagher, A., Jones, T., Brooks, D.J., Bench, C.J., Grasby, P.M., 1998. Evidence for striatal dopamine release during a video game. Nature 393, 266-268] to illustrate the potentially confounding influences of head movement and alterations in rCBF. Changes in [(11)C]raclopride binding may be detected in extrastriatal as well as striatal brain regions-however we review evidence which suggests that extrastriatal changes may not be clearly interpreted in terms of DA release. Whilst several investigations have detected increases in striatal extracellular DA concentrations during task components such as motor learning and execution, reward-related processes, stress and cognitive performance, the presence of potentially biasing factors should be carefully considered (and, where possible, accounted for) when designing and interpreting future studies.

Acquiring local field potential information from amperometric neurochemical recordings

Simultaneous acquisition of in vivo electrophysiological and neurochemical information is essential for understanding how endogenous neurochemicals modulate the dynamics of brain activity. However, up to now such a task has rarely been accomplished due to the major technical challenge of operating two independent recording systems simultaneously in real-time. Here we propose a simpler solution for achieving this goal by using only a standard electrochemical technique--amperometry. To demonstrate its feasibility, we compared amperometric signals with simultaneously recorded local field potential (LFP) signals. We found that the high frequency component (HFC) of the amperometric signals did not reflect neurochemical fluctuations, but instead it resembled LFPs in several aspects, including: (1) coherent spectral fluctuations; (2) clear characterization of different brain states; (3) identical hippocampal theta depth profile. As such, our findings provide the first demonstration that both LFP and local neurochemical information can be simultaneously acquired from electrochemical sensors alone.

Quantitative evaluation of serotonin release and clearance in Drosophila

Serotonin signaling plays a key role in the regulation of development, mood and behavior. Drosophila is well suited for the study of the basic mechanisms of serotonergic signaling, but the small size of its nervous system has previously precluded the direct measurements of neurotransmitters. This study demonstrates the first real-time measurements of changes in extracellular monoamine concentrations in a single larval Drosophila ventral nerve cord. Channelrhodopsin-2-mediated, neuronal type-specific stimulation is used to elicit endogenous serotonin release, which is detected using fast-scan cyclic voltammetry at an implanted microelectrode. Release is decreased when serotonin synthesis or packaging are pharmacologically inhibited, confirming that the detected substance is serotonin. Similar to tetanus-evoked serotonin release in mammals, evoked serotonin concentrations are 280-640nM in the fly, depending on the stimulation length. Extracellular serotonin signaling is prolonged after administering cocaine or fluoxetine, showing that transport regulates the clearance of serotonin from the extracellular space. When ChR2 is targeted to dopaminergic neurons, dopamine release is measured demonstrating that this method is broadly applicable to other neurotransmitter systems. This study shows that the dynamics of serotonin release and reuptake in Drosophila are analogous to those in mammals, making this simple organism more useful for the study of the basic physiological mechanisms of serotonergic signaling.

Theoretical modelling to understand the neurotransmission mechanism in the gastrointestinal tract

In this paper we apply a novel experimental and theoretical method to study the neurotransmitter signalling process. This method allows the understanding of changes in uptake between different tissue types from the gastrointestinal tract. The reaction-diffusion model we used has shown that by changing the uptake rate, the slope of the response changed when the levels of 5-HT released were held constant. Experimental data from the ileum and colon obtained at one distance from the tissue shows no significant difference in the release profile, however measurements at multiple sites show different current slopes for the two tissues. The response based upon the theoretical data indicates that colon has a higher uptake rate than the ileum. These results show that the combination of a theoretical and experimental approach to study biological processes can provide a mean of gaining further inside into the mechanisms involved and can have important clinical applications.

Entrapment and measurement of a biologically functionalized microbead with a microwell electrode

A chip with integrated electrophoretic and electrochemical systems was developed to manipulate either an individual microbead or a cell inside a microwell electrode (MWE) for electrochemical measurement. The optimal MWE geometry (30 microm diameter and 25 microm depth) was designed to accommodate the micro particles according to the simulated results. A chip device was sequentially built from a slide patterned with Pt electrodes, an adhesive tape defined with a flow channel (200 microm in width and 25 microm in height), and an indium tin oxide (ITO) cover. The MWE not only generated an active electrophoretic force to trap the particle but also provided a low flow velocity area (LFVA) to stabilize the trapped bead or cell in a continuous flow. Scanning electrochemical microscopy (SECM) theory was employed to explain the electrochemical behaviors of the MWE. An enhanced current was confirmed as the redox recycling effect on the conductive ITO cover. The catalytic reaction of an individual alkaline phosphatase coated microbead (ALP-bead) was electrochemically detected with the MWE after being trapped. The ALP on the trapped ALP-bead catalyzed the hydrolysis of p-aminophenylphosphate (PAPP) to p-aminophenol (PAP), and then a decaying amperogram (+0.3 V vs. Ag/AgCl) due to a tiny PAP quantity around the MWE was observed.

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.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

The chemistrode: a droplet-based microfluidic device for stimulation and recording with high temporal, spatial, and chemical resolution

Microelectrodes enable localized electrical stimulation and recording, and they have revolutionized our understanding of the spatiotemporal dynamics of systems that generate or respond to electrical signals. However, such comprehensive understanding of systems that rely on molecular signals-e.g., chemical communication in multicellular neural, developmental, or immune systems-remains elusive because of the inability to deliver, capture, and interpret complex chemical information. To overcome this challenge, we developed the "chemistrode," a plug-based microfluidic device that enables stimulation, recording, and analysis of molecular signals with high spatial and temporal resolution. Stimulation with and recording of pulses as short as 50 ms was demonstrated. A pair of chemistrodes fabricated by multilayer soft lithography recorded independent signals from 2 locations separated by 15 mum. Like an electrode, the chemistrode does not need to be built into an experimental system-it is simply brought into contact with a chemical or biological substrate, and, instead of electrical signals, molecular signals are exchanged. Recorded molecular signals can be injected with additional reagents and analyzed off-line by multiple, independent techniques in parallel (e.g., fluorescence correlation spectroscopy, MALDI-MS, and fluorescence microscopy). When recombined, these analyses provide a time-resolved chemical record of a system's response to stimulation. Insulin secretion from a single murine islet of Langerhans was measured at a frequency of 0.67 Hz by using the chemistrode. This article characterizes and tests the physical principles that govern the operation of the chemistrode to enable its application to probing local dynamics of chemically responsive matter in chemistry and biology.

Microelectrodes for studying neurobiology

Microelectrodes have emerged as an important tool used by scientists to study biological changes in the brain and in single cells. This review briefly summarizes the ways in which microelectrodes as chemical sensors have furthered the field of neurobiology by reporting on changes that occur on the subsecond time scale. Microelectrodes have been used in a variety of fields including their use by electrophysiologists to characterize neuronal action potentials and develop neural prosthetics. Here we restrict our review to microelectrodes that have been used as chemical sensors. They have played a major role in many important neurobiological findings.

Preferential enhancement of dopamine transmission within the nucleus accumbens shell by cocaine is attributable to a direct increase in phasic dopamine release events

Preferential enhancement of dopamine transmission within the nucleus accumbens (NAc) shell is a fundamental aspect of the neural regulation of cocaine reward. Despite its importance, the nature of this effect is poorly understood. Here, we used fast-scan cyclic voltammetry to examine specific transmission processes underlying cocaine-evoked increases in dopamine transmission within the NAc core and shell. Initially, we examined altered terminal dopamine concentrations after global autoreceptor blockade. This was the first examination of autoreceptor regulation of naturally occurring phasic dopamine transmission and provided a novel characterization of specific components of dopamine neurotransmission. Comparison of increased dopamine signaling evoked by autoreceptor blockade and cocaine administration allowed robust resolution between increased frequency, concentration, and duration of phasic dopamine release events after cocaine delivery. Cocaine increased dopamine transmission by slowed uptake and increased concentration of dopamine released in the core and shell. However, an additional increase in the number phasic release events occurred only within the NAc shell, and this increase was eliminated by inactivation of midbrain dopaminergic neurons. This represents the first evidence that cocaine directly increases the frequency of dopamine release events and reveals that this is responsible for preferentially increased dopamine transmission within the NAc shell after cocaine administration. Additionally, cocaine administration resulted in a synergistic increase in dopamine concentration, and subregion differences were abolished when cocaine was administered in the absence of autoregulation. Together, these results demonstrate that cocaine administration results in a temporally and regionally specific increase in phasic dopamine release that is significantly regulated by dopamine autoreceptors.

Fabrication of size-controllable ultrasmall-disk electrode: monitoring single vesicle release kinetics at tiny structures with high spatio-temporal resolution

Size-controllable micron or nano-disk carbon fiber electrode (CFE) is prepared and demonstrated to be excellent for extra-cellular transmitter release detection at tiny structures and vesicle fusion kinetics analysis with high spatio-temporal resolution. An improved electrochemical etching procedure was employed, for the first time, to fabricate cylindrical fiber with controlled micron or nano-diameter. Afterwards, a facile insulation with polypropylene sheath was employed to completely insulate the whole body of the thinned fiber, and an ultrasmall-disk sensing area was finally produced by cutting of the insulated fibers. Scanning electron microscopy (SEM) was employed to characterize the ultrasmall geometry size of the fabricated electrode and to show the tight adherence of the insulation sheath on the fiber. The cut ends of the electrodes were also shown to be smooth, clean and without obvious jagged layer. The fabricated micron or nano-disk carbon electrodes show ideal steady-state voltammetric behavior with satisfying reversibility. Subsequently, the performance of the ultrasmall-disk CFE for amperometric detection of cell secretion was characterized. Results showed that, compared to the conventional micro-disk CFE, the etched small disk CFE possesses higher sensitivity due to its obviously improved signal-to-noise level, which enables minute amounts of 3000 oxidizable molecules to be detectable. The nano-disk CFE was shown to be particularly ideal for analysis of fusion kinetics, due to its avoidance of diffusion broadening of the detected spikes, which is the inherent defect of the conventional micro-CFE technique.

Theory of Polymer Entrapped Enzyme Ultramicroelectrodes: Application to Glucose and Adenosine Triphosphate Detection

We validate, by comparison with experimental data, a theoretical description of the amperometric response of microbiosensors formed via enzyme entrapment. The utility of the theory is further illustrated with two relevant examples supported by experiments: (1) quantitative detection of glucose and (2) quantitative detection of adenosine triphosphate (ATP).

In Vitro Electrochemistry of Biological Systems

This article reviews recent work involving electrochemical methods for in vitro analysis of biomolecules, with an emphasis on detection and manipulation at and of single cells and cultures of cells. The techniques discussed include constant potential amperometry, chronoamperometry, cellular electroporation, scanning electrochemical microscopy, and microfluidic platforms integrated with electrochemical detection. The principles of these methods are briefly described, followed in most cases with a short description of an analytical or biological application and its significance. The use of electrochemical methods to examine specific mechanistic issues in exocytosis is highlighted, as a great deal of recent work has been devoted to this application.

Potential applications of nanotechnologies to Parkinson's disease therapy

Nanotechnology will play a key role in developing new diagnostic and therapeutic tools. Nanotechnologies use engineered materials with the smallest functional organization on the nanometre scale in at least one dimension. Some aspects of the material can be manipulated resulting in new functional properties. Nanotechnology could provide devices to limit and reverse neuropathological disease states, to support and promote functional regeneration of damaged neurons, to provide neuroprotection and to facilitate the delivery of drugs and small molecules across the blood-brain barrier. All of them are relevant to improve current therapy of Parkinson's disease (PD).

Spatially and temporally resolved single-cell exocytosis utilizing individually addressable carbon microelectrode arrays

We report the fabrication and characterization of carbon microelectrode arrays (MEAs) and their application to spatially and temporally resolve neurotransmitter release from single pheochromocytoma (PC12) cells. The carbon MEAs are composed of individually addressable 2.5-mum-radius microdisks embedded in glass. The fabrication involves pulling a multibarrel glass capillary containing a single carbon fiber in each barrel into a sharp tip, followed by beveling the electrode tip to form an array (10-20 microm) of carbon microdisks. This simple fabrication procedure eliminates the need for complicated wiring of the independent electrodes, thus allowing preparation of high-density individually addressable microelectrodes. The carbon MEAs have been characterized using scanning electron microscopy, steady-state and fast-scan voltammetry, and numerical simulations. Amperometric results show that subcellular heterogeneity in single-cell exocytosis can be electrochemically detected with MEAs. These ultrasmall electrochemical probes are suitable for detecting fast chemical events in tight spaces, as well as for developing multifunctional electrochemical microsensors.

Fully automated microchip system for the detection of quantal exocytosis from single and small ensembles of cells

A lab-on-a-chip device that enables positioning of single or small ensembles of cells on an aperture in close proximity to a mercaptopropionic acid (MPA) modified sensing electrode has been developed and characterized. The microchip was used for the detection of Ca(2+)-dependent quantal catecholamine exocytosis from single as well as small assemblies of rat pheochromocytoma (PC12) cells. The frequency of events increased considerably upon depolarization of the PC12 cell membrane using a high extracelluar concentration of potassium. The number of recorded events could be correlated with the number of cells immobilized on the electrode. Quantal characteristics, such as the number of released molecules per recorded event, are equivalent to data obtained using conventional carbon fiber microelectrodes. The detection sensitivity of the device allows for the detection of less than 10 000 dopamine molecules in a quantal release. The distribution of peak rise-time and full width at half maximum was constant during measurement periods of several minutes demonstrating the stability of the MPA modified surface.