Capillary gel electrophoresis with sinusoidal voltammetric detection: a strategy to allow four-"color" DNA sequencing

A novel detection strategy for DNA sequencing applications that utilizes a frequency-based electrochemical method is reported. Sinusoidal voltammetry is used to selectively identify four unique redox molecules that are covalently attached to the 5'-end of a 20-base sequencing primer. The tags used in this work are ferrocene derivatives with different substituents attached to the ferrocene ring, where the electron-donating or -withdrawing character of the substituent alters the half-wave potential of the modified ferrocene. Therefore, each tag has a unique SV frequency spectrum that can be easily identified in the frequency domain. In this work, the discrimination of one tag versus all others is accomplished through a "phase-nulling" technique. The signal for each tag is selectively eliminated while the other three responses remain virtually unchanged. This analysis scheme allows for the selective identification of each tagged oligonucleotide eluting in sieving polymer capillary gel electrophoresis with a separation efficiency of 2 x 10(6) theoretical plates per meter. This separation efficiency is sufficient to perform "low-resolution" DNA sequencing; the conditions used in this work have not yet been optimized for high-resolution sequencing applications.

Detection of native amino acids and peptides utilizing sinusoidal voltammetry

Native amino acids and peptides were detected at a copper microelectrode using sinusoidal voltammetry (SV). Traditionally, these molecules can only be measured after derivatization with either a fluorescent or electroactive tag. In this work, an electrocatalytic oxidation reaction at copper is used to detect underivatized peptides and amino acids. The oxidation reaction is somewhat independent of peptide structure (i.e., it is not limited to the detection of aromatic amino acids) and is therefore able to produce nanomolar detection limits for all amino acids and peptides tested. A scanning technique, sinusoidal voltammetry, is used to provide the sensitivity of constant-potential techniques but also provide selectivity gained through utilization of the frequency domain. The frequency spectrum due to the oxidation of each molecule has a unique "fingerprint" response resulting from the kinetics of oxidation at the electrode surface. Through examination of the frequency spectra, even structurally similar molecules can be easily distinguished from one another. Flow injection analysis is used to demonstrate the sensitive and selective detection of a variety of amino acids and peptides. This technique can also be easily coupled to a separation step, i.e., high-performance liquid chromatography or capillary electrophoresis without electrode fouling from the adsorption of the analytes.

Laser interference pattern ablation of a carbon fiber microelectrode: biosensor signal enhancement after enzyme attachment

Fluorescence microscopy was used to visualize the accumulated fluorescent product of the enzyme alkaline phosphatase to indicate where active covalently bound enzyme remained on the surface after application of a Nd: YAG laser interference pattern to a surface that was first globally derivatized with the covalently bound enzyme. The electrochemical kinetics of the same carbon fiber surface were examined through the electrogenerated chemiluminescence of Ru(bpy)(3)2+ to determine that electron-transfer sites were indeed segregated from the enzyme-binding sites. The enzyme-derivatized areas are determined to be separate and distinct from the areas of enhanced electron transfer. Two other enzymes, glucose oxidase and malic dehydrogenase, were then covalently bound to carbon fiber microelectrode surfaces in order to verify the change in detection limit of their respective cofactors, NADH or H2O2, under a variety of surface conditions. The S/N of an enzyme-modified electrode after laser interference pattern photoablation and electrocatalytic treatment is improved by more than 1 order of magnitude over that observed at an electrode that is globally enzyme modified.

Segregation of micrometer-dimension biosensor elements on a variety of substrate surfaces

With the rapid development of micro total analysis systems and sensitive biosensing technologies, it is often desirable to immobilize biomolecules to small areas of surfaces other than silicon. To this end, photolithographic techniques were used to derivatize micrometer-sized, spatially segregated biosensing elements on several different substrate surfaces. Both an interference pattern and a dynamic confocal patterning apparatus were used to control the dimensions and positions of immobilized regions. In both of these methods, a UV laser was used to initiate attachment of a photoactive biotin molecule to the substrate surfaces. Once biotin was attached to a substrate, biotin/avidin/biotin chemistry was used to attach fluorescently labeled or nonlabeled avidin and biotinylated sensing elements such as biotinylated antibodies. Dimensions of 2-10 microm were achievable with these methods. A wide variety of materials, including glassy carbon, quartz, acrylic, polystyrene, acetonitrile-butadiene-styrene, polycarbonate, and poly(dimethylsiloxane), were used as substrates. Nitrene- and carbene-generating photolinkers were investigated to achieve the most homogeneous films. These techniques were applied to create a prototype microfluidic sensor device that was used to separate fluorescently labeled secondary antibodies.

Micrometer dimension derivatization of biosensor surfaces using confocal dynamic patterning

Using laser scanning confocal optics in conjunction with avidin/biotin technology, micrometer-sized patterns of biomolecules were fabricated on glassy-carbon and fused-silica surfaces. Photoactive biotin was immobilized using the 325-nm line of a Helium-Cadmium laser, which was focused through a 25x or 100x quartz microscope objective. A three-dimensional piezoelectric micromanipulator was used to position the sample surface in the focal plane of the microscope objective and to create patterns on the focused surface. Biotin patterns with line widths of 5-20 microns were produced by varying the scan speed of the micromanipulator while exposing the surface to the laser. The integrity of the immobilized biotin was confirmed by subsequent derivatization with fluorescently labeled avidin. Fluorescence microscopy with a cooled charge coupled device (CCD) imaging system was used to visualize the distribution of biotin and fluorescent avidin within the patterns created by the laser.

Preservation of NADH voltammetry for enzyme-modified electrodes based on dehydrogenase

Minimizing overpotential and generating high faradaic currents are critical issues for fast-scan voltammetry of beta-nicotinamide adenine dinucleotide (NADH) for the sensitivity of enzyme-modified electrodes based on dehydrogenases. Although NADH voltammetry exhibits high overpotential and poor voltammetric peak shape at solid electrode surfaces, modification of the electrode surface can improve the electrochemical response at carbon fibers. However, these improvements are severely degraded upon the covalent attachment of enzyme. The creation of improved electron-transfer properties and the retention of these properties throughout the enzyme attachment process is the focus of this study. A novel polishing and electrochemical pretreatment method was developed which generated a decreased overpotential and a high faradaic current at carbon-fiber electrodes for NADH. Factors that lead to a degradation of voltammetric response during the enzyme fabrication were investigated, and both the aging and the covalent modification of the pretreated surface contributed to this degradation. Attachment processes that minimized the preparation time, in turn, maximized the retention of the facile electron-transfer properties. These attachment processes included varying the surface attachment reactions for the enzyme. Preparation time reduction techniques included modeling existing techniques and then improving kinetic and mass transport issues where possible. Alternate covalent attachment methods included a direct electrochemical amine reaction and an electrochemically reductive hydrazide reaction. The surface attachment and retention of electron-transfer properties of these probes were confirmed by fluorescence and electrochemical studies.

Separation of double- and single-stranded DNA restriction fragments: capillary electrophoresis with polymer solutions under alkaline conditions

Capillary electrophoresis in buffers containing hydroxyethyl cellulose (HEC) was used to separate double- and single-stranded DNA restriction fragments under neutral and alkaline conditions in epoxy-coated capillaries. It was found that better resolution was achieved using highly entangled HEC solutions for a narrow range of DNA fragment sizes, while lower resolution was obtained over a wide separation range using diluted HEC solutions. Optimal resolution of these DNA fragments was obtained using buffers containing 0.5% HEC at pH 11 with plate numbers exceeding 3 x 10(6) plates/m. It was also found that the diffusion coefficients and electrophoretic mobilities of DNA fragments decreased with increasing pH. This may indicate a more extended DNA conformation and, therefore, enhancement of transient entanglement coupling between DNA and HEC polymers under alkaline condition. At pH 12, ss-DNA were well separated in entangled HEC solutions; however, the resolution of ss-DNA was significantly decreased in diluted polymer solution.

Development of sub-micron patterned carbon electrodes for immunoassays

Sub-micron sized domains of a carbon surface are derivatized with antibodies using biotin/avidin technology. These sites are spatially-segregated from, and directly adjacent to, electron transfer sites on the same electrode surface. The distance between these electron transfer sites and enzyme-loaded domains are kept to a minimum (e.g. less than a micron) to maintain the high sensitivity required for the measurement of enzyme-linked cofactors in an enzyme-linked immunoassay (ELISA). This is accomplished through the use of photolithographic attachment of photobiotin using an interference pattern from a UV laser generated at the electrode surface. This allows the construction of microscopic arrays of active ELISA sites on a carbon substrate while leaving other sites underivatized to facilitate electron transfer reactions of redox mediators; thus maximizing sensitivity and detection of the enzyme mediator. The carbon electrode surface is characterized with respect to its chemical structure and electron transfer properties following each step of the antibody immobilization process. The characterization of specific modifications of micron regions of the carbon surface requires analytical methodology that has both high spatial resolution and sensitivity. We have used fluorescence microscopy with a cooled CCD imaging system to visualize the spatial distribution of enzyme immobilization sites (indicated by fluorescence from Texas-Red labeled antibody) across the carbon surface. The viability of the enzyme attached to the surface in this manner was demonstrated by imaging the distribution of an insoluble, fluorescent product.

Background-subtraction of fast-scan cyclic staircase voltammetry at protein-modified carbon-fiber electrodes

Background-subtraction techniques were applied to the voltammetry of nicotinamide adenine dinucleotide (NADH) at protein-modified carbon-fiber microelectrodes. The background currents at carbon-fiber electrodes were stable and voltammetric scans immediately before or after the analyte were effectively used for background subtraction. Digital step-potential waveforms were used to excite these carbon-fiber electrodes, where the resulting voltammetric analysis assessed the optimal switching and initial potentials and the electrochemical response time was determined. The initial potential was 0.0 V and the switching potential 1.1 V (versus Ag/AgCl) and the response time was approximately 300 ms. Some sensitivity to NADH was lost and voltammetric prescans were required at protein-modified electrodes to obtain a stable baseline. Current versus time was assessed by the average current of the faradaic region from each voltammogram and by differential current; the average current minus the current from a non-faradaic potential range. Differential current assessments discriminated against artifacts caused by pH (as high as 1.0 pH unit) and ionic strength flux (100 mM). These background-subtraction techniques allowed the faradaic information to be obtained quickly and conveniently while maximizing sensitivity and maintaining selectivity.

Electron transfer kinetics at a biotin/avidin patterned glassy carbon electrode

Photolithographic techniques using a laser interference pattern were used to attach photobiotin to micron-sized stripes on the surface of a carbon electrode. Fluorophore-tagged avidin was attached to this spatially-patterned biotin with essentially no loss in spatial resolution. The kinetics of the glassy carbon surface were examined to see if electron transfer sites could indeed be segregated from the attachment sites of photobiotin-immobilized avidin. The ECL of luminol and SECM were used to verify the segregation between underivatized sites (which exhibit normal electron transfer kinetics) and extensively derivatized biotin/avidin surfaces (which presumably exhibit slow electron transfer kinetics). Both techniques were found to be capable of differentiating the protein-covered surface from bare carbon with sufficient resolution to tell whether a significant portion of the carbon surface is still active and available to detect the product of an enzyme generated analyte. These results indicate that extensive biotin/avidin derivatization of the surface does decrease the electron transfer rate of a carbon electrode, and that the photolithographic approach was able to modify specific sections of the electrode surface, while leaving other regions untouched and available for facile electron transfer. This leads to a more general protocol for the construction of enzyme-based biosensors which utilize diffusable mediators.

Localized avidin/biotin derivatization of glassy carbon electrodes using SECM

Different forms of the microreagent mode of SECM were used to attach biotin or make "clean" spots on micron-sized regions on the surface of a carbon electrode. In the direct-write mode, the SECM probe tip is used as an electrochemical "pen" depositing biotin in micron-sized lines on the carbon substrate as it is scanned across its surface. In the negative microreagent mode, the SECM probe tip is used as an electrochemical "eraser" cleaning of the surface attached molecules and leaving clean spots on the surface of a globally derivatized carbon surface. This type of simple micromodification of the surface of a carbon electrode will allow the fabrication of biosensors that can potentially be tailor-made for a variety of applications.

A laser ablation method for the spatial segregation of enzyme and redox sites on carbon fiber microelectrodes

A laser-generated interference pattern was used to remove enzyme from micrometer-wide stripes on an enzyme-covered carbon fiber microelectrode surface to create regions of facile electron transfer. Fluorescence microscopy was used to visualize fluorophore-tagged enzyme to indicate where the adsorbed enzyme remained on the surface. The electrochemical kinetics of the carbon fiber surface were examined to see if electron-transfer sites could indeed be segregated from enzyme adsorbed across the entire surface. CCD imaging of the electrochemical luminescence of Ru(bpy)3(2+) was used to verify the segregation between photoablated sites (with facile electron-transfer kinetics) and surfaces with adsorbed enzyme (which exhibit slow electron-transfer kinetics). The laser-ablated surface could also be distinguished from the enzyme-covered carbon surface with atomic force microscopy. Thus, photoablation of the surface of a protein-covered carbon fiber microelectrode with an interference pattern generated by a Nd:YAG laser allows the activation of 1.7-micron-wide bands of the electrode surface (available for facile electron transfer) while leaving 2.6-micron-wide enzyme-modified areas intact, thereby producing electroactive regions directly adjacent to enzyme modified regions of the same surface.

Ultrasensitive voltammetric detection of underivatized oligonucleotides and DNA

Electrochemical detection of nucleotides, ssDNA, and dsDNA was accomplished by using sinusoidal voltammetric detection at copper microelectrodes. Generally, detection of these molecules utilizes the electroactive nature of adenine and guanine residues at most electrode surfaces. The detection approach used in this study is based on the electrocatalytic oxidation of sugars and amines at copper surfaces. All nucleotides and DNA molecules comprise a ribose sugar backbone and primary amines present on the different nucleobases. Consequently, the detection approach is universal to all types of nucleotides. As the number of sugar moieties increases with the length of an oligonucleotide, the detection sensitivity is enhanced for bigger oligonucleotides. Irreversible adsorption of these oligonucleotides and other biomacromolecules like dsDNA on the electrode surface was avoided with sinusoidal voltammetry since it is a scanning electrochemical technique. The sensitivity of the detection strategy is, however, still preserved due to the effective decoupling of the faradaic signal from the capacitive background currents in the frequency domain. The ssDNA and dsDNA were detected in the picomolar concentration range. The electrochemical signal due to dsDNA is actually higher than that due to ssDNA due to the larger number of easily accessible sugars on the outer perimeter of a dsDNA double helix compared to those on a ssDNA of the same size. This is in contrast to the existing electrochemical detections techniques based on the electroactivity of the nucleobase.

Generation of biotin/avidin/enzyme nanostructures with maskless photolithography

Micrometer-sized domains of a carbon surface are modified to allow derivatization to attach redox enzymes with biotin/avidin technology. These sites are spatially segregated from and directly adjacent to electron transfer sites on the same electrode surface. The distance between these electron transfer sites and enzyme-loaded domains must be kept to a minimum (e.g., less than 5 microns) to maintain the fast response time and high sensitivity required for the measurement of neurotransmitter dynamics. This is accomplished through the use of photolithographic attachment of photobiotin using an interference pattern from a UV laser generated at the electrode surface. This will allow the construction of microscopic arrays of active enzyme sites on a carbon fiber substrate while leaving other sites underivatized to facilitate electron transfer reactions of redox mediators, thus maximizing enzyme activity and detection of the enzyme mediator. The ultimate sensitivity of these sensors will be realized only through careful characterization of the carbon electrode surface with respect to its chemical structure and electron transfer properties following each step of the enzyme immobilization process. The characterization of specific modifications of micrometer regions of the carbon surface requires analytical methodology that has both high spatial resolution and sensitivity. We have used fluorescence microscopy with a cooled CCD imaging system to visualize the spatial distribution of enzyme immobilization sites (indicated by fluorescence from Texas Red-labeled avidin) across the carbon surface. The viability of the enzyme attached to the surface in this manner was demonstrated by imaging the distribution of an insoluble, fluorescent product. An atomic force microscope was used to obtain high-resolution images that probe the heterogeneity of the enzyme sites.

Electrocatalytic surface for the oxidation of NADH and other anionic molecules of biological significance

A simple electrochemical treatment of a carbon fiber electrode surface has been found to dramatically improve the voltammetry of NADH and several other anionic molecules under steady-state and fast scan (100 V/s) conditions. The electrocatalytic surface is generated through the electrochemical oxidation of NADH on a carbon fiber electrode that exhibits product adsorption. The oxidative product is reacted with ascorbic acid at elevated temperatures to create a surface which has very little overpotential for the oxidation of dopamine and many metabolites such as NADH, DOPAC, uric acid, and ascorbate. The electrochemical properties of the modified surface were examined voltammetrically at both slow and fast scan rates. The surface shown in this paper shifts the oxidation overpotentials different magnitudes for each analyte tested, thus allowing discrimination between analytes of interest and their major interferences. Another benefit of this new electrocatalytic wave is that it decreases the limit of detection for NADH by approximately 1 order of magnitude. Therefore, this new carbon surface not only gives better discrimination between two analytes but also gives better detection limits for certain analytes of interest.

Characterization of the chemical architecture of carbon-fiber microelectrodes. 3. Effect of charge on the electron-transfer properties of ECL reactions

Two techniques, cyclic voltammetry and the microscopic imaging of electrochemically generated chemiluminescence (ECL), have been used to evaluate the effectiveness of various electrochemical pretreatments on the electron-transfer properties of carbon-fiber microelectrodes. The surfaces of carbon-fiber microelectrodes were electrochemically treated to produce different levels of surface oxides in the following manner: after normal polishing and cleaning in hot toluene and water, the carbon surface was activated by applying a cyclic potential from -0.2 to 2.0 V at a frequency of 50 Hz for 3 s in solutions of varying pH: 1M HCl, pH 7.4 and pH 12.0 phosphate buffers. Cyclic voltammetry was employed to elucidate the effect of the surface pretreatment on the overall voltammetric properties of the different pretreatment methods. The ECL emission intensity was imaged with resolution on the submicrometer scale with a conventional fluorescence microscope equipped with a cooled, slow-scan CCD camera. In addition to investigating pretreatment effects with luminol, we have also examined the ECL properties of a positively charged species, ruthenium tris (2,2'-bipyridine) dichloride hexahydrate, whose luminescent properties are also well documented. Such information is not only invaluable for the rational design of surface-modified ultramicroelectrodes, but it can also yield considerable information concerning the surface interactions influencing organic electron-transfer reactions at carbon surfaces.

Characterization of the chemical architecture of carbon-fiber microelectrodes. 2. Correlation of carboxylate distribution with electron-transfer properties

Correlation of the chemical architecture of the surface of 10-microns-diameter carbon-fiber microelectrodes (illustrated by the fluorescence intensity of FITC-labeled carboxylates) and the rate of electron transfer of the surface (illustrated by the intensity of the electrogenerated chemiluminescence of luminol) allows the development of quantitative relationships between the chemical structure of an electrode surface and its electron-transfer properties. A fluorescence microscope equipped with a Peltier-cooled charge-coupled device was used to image these electrode surfaces with submicron spatial resolution. The total fluorescence emission observed at electrochemically treated electrodes was higher than that of controls while the voltammetric behavior and integrated ECL intensity of luminol were very similar. Imaging spectroscopy with submicron spatial resolution was able to demonstrate the microscopic heterogeneity of these surfaces and to assess the effect of the production of carboxylates on the rate of electron transfer of luminol.

Dehydrogenase-modified carbon-fiber microelectrodes for the measurement of neurotransmitter dynamics. 1. NADH voltammetry

The voltammetry of NADH has been characterized at carbon-fiber microelectrodes at scan rates up to 100 V/s. Electrochemical pretreatment of the electrode dramatically changed the properties of the modified electrode. Anodic pretreatment of the surface resulted in an adsorptive wave for NADH oxidation, while less adsorption was evident under more moderate conditions. The pH of the buffer used for the anodization played a critical role in determining the voltammetric peak shape. Oxidation of NADH at slow scan rates (< 10 V/s) fouled the electrode. In contrast, consistent and reproducible voltammetry of NADH was observed at faster scan rates (100 V/s). This voltammetric measurement was used to monitor NADH generated during the oxidative deamination of glutamate catalyzed by glutamate dehydrogenase. A 150-microns-l.d. microdialysis fiber was used to entrap the enzyme near the microelectrode tip, forming the dehydrogenase-modified carbon-fiber microelectrode.

Dehydrogenase-modified carbon-fiber microelectrodes for the measurement of neurotransmitter dynamics. 2. Covalent modification utilizing avidin-biotin technology

Dehydrogenase enzymes are immobilized onto carbon-fiber microelectrode surfaces via avidin-biotin technology and a covalently linked hydrophilic tether. The avidin-biotin coupling strategy allows the selectivity of the electrochemical measurement to be easily changed without reoptimizing the immobilization conditions. Optimized derivatization conditions are demonstrated for dimer, tetramer, and hexamer dehydrogenases. Nonelectroactive substrates (ethanol, glucose-6-phosphate, and glutamate) are quantitated through the detection of enzyme-generated NADH by fast scan cyclic staircase voltammetry (100 V/s). The glutamate dehydrogenase-modified microelectrode possesses a 300-ms response time with a detection limit of 0.5 mM glutamate and a 1-60 mM linear concentration range.

Characterization of the chemical architecture of carbon-fiber microelectrodes. 1. Carboxylates

A new method to characterize the chemical architecture of a carbon-fiber microelectrode surface is described. Derivatization of carboxyl groups on the carbon surface with a poly(oxyalkalene)diamine (Jeffamine ED-600), followed by biotinylation of the free amine, allowed the attachment of a fluorescein isothiocyanate (FITC) conjugate of ExtrAvidin. The fluorescence observed after excitation at 488 nm was imaged with a fluorescence microscope equipped with a CCD camera, yielding a spatial map of the distribution of modified carboxyl groups on the surface of the carbon fiber with 0.5-micron resolution. Colloidal gold particles (15 nm diameter) coated with ExtrAvidin were used in place of the FITC-ExtrAvidin, and the carbon-fiber surface was imaged with scanning electron microscopy on a submicron scale. This selective information regarding surface-bound functional groups (i.e. carboxylates) has proven invaluable toward the rational design of novel sensors based on surface-modified ultramicroelectrodes.

In vivo identification and quantitative evaluation of carrier-mediated transport of lactate at the cellular level in the striatum of conscious, freely moving rats

Intracerebral dialysis has allowed the continuous, on-line measurement of lactate in the extracellular fluid (ECF) of conscious, freely moving rats. The rapid time response of the technique allows the direct determination of the time course of changes in lactate in ECF following externally imposed stimuli. The time course of lactate appearance in ECF was found to be considerably slower than that observed in tissue following electroconvulsive shock or during ischemia following cardiac arrest. The ECF data could be fit to an integrated Michaelis-Menten model that assumed reversible transport of lactate across the cell membrane. This transport was found to act only when energy supplies could maintain membrane integrity and function, since ECF levels of lactate failed to follow tissue levels after cardiac arrest when energy resources are depleted. The calculated rate of cellular lactate transport was two orders of magnitude faster than transport of lactate across the blood-brain barrier in the adult rat, and passive diffusion of lactate was not found to contribute significantly across either cell or blood-brain barriers. Probenecid, an inhibitor of acid transport, was able to block both the efflux of lactate from cell to ECF and the consequent reuptake of lactate by cells in the striatum of the rat following electroconvulsive shock or ischemia.

N-methyl-D-aspartate receptor involvement in lactate production following ischemia or convulsion in rats

Intracerebral dialysis in conscious freely moving rats was used to examine the involvement of the N-methyl-D-aspartate (NMDA) receptor in the formation of lactic acid and its consequent appearance in extracellular fluid. Local administration of NMDA in the striatum of conscious, freely moving rats was found to produce a transient increase in extracellular lactate. Alternatively, administration of the NMDA antagonists 2-amino-7-phosphonoheptanoic acid or 2-amino-5-phosphonopentanoic acid delayed the onset of and attenuated the magnitude of the lactate production induced by an electroconvulsive seizure. Pretreatment of the striatum with either of these antagonists reduced the total amount of lactate observed in extracellular fluid following ischemia induced by cardiac arrest, but did not affect the time course of the appearance of lactate in the extracellular space.

Differentiation of dopamine overflow and uptake processes in the extracellular fluid of the rat caudate nucleus with fast-scan in vivo voltammetry

Stimulated overflow of dopamine (DA) into the extracellular fluid of the rat caudate nucleus was measured with fast-scan cyclic voltammetry. DA concentrations were sampled in less than 10 ms at 100-ms intervals with a Nafion-coated, carbon-fiber microelectrode. Overflow of DA was induced by electrical stimulation of the medial forebrain bundle with 300-microA pulses of various duration and frequency. Stimulated overflow was measured as a function of stimulus duration before and after administration of benztropine, bupropion, and amphetamine. These results were correlated with simulated curves based on a simple uptake/overflow model. The observed overflow was assumed to be a function of [DA]p, the concentration of DA which overflows per stimulus pulse, and the kinetics of cellular uptake of DA. Correlation of experimental with stimulated results was obtained at the 95% confidence limit for the duration studies; however, it was not possible to distinguish between the effects of pharmacological agents on uptake and overflow. In contrast, modulation of stimulus frequency did permit such distinction. Simulations of an increase in [DA]p fit results following dihydroxyphenylalanine methyl ester at 95% confidence limits, whereas an equivalent change in the apparent Km did not fit. An increase in the apparent value of Km correlated with results obtained at different frequencies following nomifensine and bupropion administration at the 95% confidence limit, whereas an equivalent increase in [DA]p did not fit. The effects of GBR 12909 best correlated with an increase in the DA available for overflow.

MPP+-induced efflux of dopamine and lactate from rat striatum have similar time courses as shown by in vivo brain dialysis

Perfusion of rat striatum with 10 mM 1-methyl-4-phenylpyridinium (MPP+) during 1 min caused a rapid reversible increase in dopamine (DA) efflux (maximal after 4 min) that was similar to the effects of 1 or 15 min 10 mM amphetamine. A massive, prolonged, and irreversible release of DA occurred after 15 min 10 mM MPP+, showing a biphasic trend: the rapid amphetamine-like increase was followed by a slower and larger DA efflux, reaching its maximum after 25 min. One minute perfusions with 1-100 mM MPP+ caused a gradual increase in the overflow of striatal lactate, reaching a maximum effect after 20-30 min. The similarity between the time courses of the MPP+-induced efflux of DA and lactate suggests that both effects are probably consequences of inhibition of aerobic glycolysis by MPP+. A comparison with the courses of DA and lactate efflux after death of control rats showed a delay of several minutes before the toxic effects of MPP+ become apparent. These in vivo data are consistent with the hypothesis that MPP+ accumulates and then kills dopaminergic cells, possibly by the irreversible inhibition of mitochondrial respiration.

Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in less than 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of L-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz, suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis-Menten model for a single uptake site with Km = 200 nM and Vmax = 5 microM/s. The increase in stimulated overflow observed after L-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in modelled by an increase in Km. In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.

The use of in vivo brain dialysis of dopamine, acetylcholine, amino acids and lactic acid in studies on the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The use of intracerebral brain dialysis in freely moving rats in neurochemical and neurotoxicological research is discussed and exemplified by studies on the neurotoxin MPTP. Intrastriatal administration of its toxic metabolite MPP+, via the dialysis tube, induced massive changes in the release of neurotransmitters and metabolites. Release enhancing effects could not be repeated by a second MPP+ perfusion and decreases in neurotransmitter or metabolite output were persistent. This indicates that MPP+ has irreversible, toxic effects on various neuortransmitter systems. The MPP+-induced release of DA has been characterized by studying the effect of pretreatment with various drugs, as well by comparison of the time courses of MPP+-induced DA release with those of amphetamine-induced DA release and of MPP+-induced lactate overflow.

The effect of L-dopa on in vivo dopamine release from nigrostriatal bundle neurons

We have examined the impact of L-dihydroxyphenylalanine (L-DOPA) on the in vivo release of dopamine (DA) in rat striatum using carbon fiber voltammetry. L-DOPA caused a large increase in the DA released by nigrostriatal bundle stimulation. This was reduced by 94% in animals pretreated with intraventricular 6-hydroxydopamine. We conclude that L-DOPA increases depolarization-induced DA release from DA neurons.

Extracellular lactic acid as an indicator of brain metabolism: continuous on-line measurement in conscious, freely moving rats with intrastriatal dialysis

Lactic acid was measured continuously in the dialysis perfusate emerging from the striatum of conscious, freely moving rats. The continuous measurement utilized a specific enzymatic/fluorometric detector that provided temporal information about the changes in the concentration of lactate in extracellular fluid (ECF). The level of lactate in extracellular fluid was found to be directly linked to local cellular metabolism. Inhibition of glycolysis with 2-deoxyglucose decreased the ECF level of lactate, whereas increased lactate production was observed after uncoupling mitochondrial electron transport with 2,4-dinitrophenol. A transient increase in the extracellular level of lactate was found after neuronal stimulation (e.g., electroconvulsive shock or local administration of kainic acid). The response to electroconvulsive shock could be attenuated by inhibiting the electrical activity of neurons with tetrodotoxin. Thus, this system is capable of providing novel information about transient changes in the extracellular concentration of lactic acid in real time, and these changes can be related to changes in metabolism and neuronal activity.

Dopaminergic neurons: simultaneous measurements of dopamine release and single-unit activity during stimulation of the medial forebrain bundle

Simultaneous electrical and chemical recordings have been made of dopamine neuronal activity in the rat brain during electrical stimulation of the medial forebrain bundle. Tungsten recording electrodes were placed at the level of the substantia nigra and carbon-fiber, Nafion-coated, voltammetric electrodes were placed in the neostriatum. Dopamine units, verified by histology to be in the zona compacta of the substantia nigra, were identified by previously established electrophysiological criteria. Dopamine release was detected by fast-scan cyclic voltammetry, a technique which allows dopamine to be determined in vivo on a sub-second time scale. The majority of dopamine cells examined (7 out of 10) were antidromically activated by 60 Hz stimulation of the medial forebrain bundle. The same stimulus also elicits dopamine overflow in the caudate nucleus. Following stimulation, dopamine concentrations in the extracellular fluid of the neostriatum rapidly declined to prestimulus levels. In addition, impulse flow in dopaminergic neurons was inhibited for 20 s following stimulation. These measurements represent the first direct observation from a neuronal tract of simultaneous unit activity and chemical release of a neurotransmitter in real time.

Real-time measurement of dopamine release in rat brain

Dopamine release at the submicromolar level has been observed in the striatum of an anesthetized rat on a millisecond time scale. Fast-scan cyclic voltammetry with Nafion-coated microelectrodes has been synchronized with electrical stimulation of the medial forebrain bundle, and synaptic overflow is observed following a burst of 15 impulses. The rapid appearance of dopamine following this stimulus indicates that the source of dopamine is very close (approximately 10 micron) to the electrode. The rapid disappearance of released dopamine reflects the potency of cellular uptake for dopamine. Inhibition of dopamine uptake with nomifensine allows the measurement of dopamine overflow as a result of a single stimulus impulse or with low-frequency stimulations, both comparable to physiological dopaminergic impulse flow.

In vivo comparison of the regulation of releasable dopamine in the caudate nucleus and the nucleus accumbens of the rat brain

In vivo voltammetry has been used to measure the release of dopamine evoked by electrical stimulation of the medial forebrain bundle (MFB). Simultaneous measurements have been made with voltammetric-sensing electrodes ipsilateral to the stimulating electrode in the nucleus accumbens and the caudate nucleus of the anesthetized rat. During the stimulation, the species observed in both regions is voltammetrically identical to dopamine. Further evidence for the identity of dopamine is provided by anatomical, physiological, pharmacological, and postmortem data. Postmortem analysis of these brain regions after a single stimulation demonstrates that dopamine levels are unchanged, while dihydroxyphenylacetic acid (DOPAC) levels are increased in both regions. Systemic application of synthesis inhibitors results in a decrease in evoked release for each brain region. Amfonelic acid results in a restoration of stimulated release after synthesis inhibition. Evoked release is affected differently by pargyline in the two brain regions. The evoked release of dopamine is significantly elevated in the nucleus accumbens as a result of pargyline administration, but similar effects are not seen in the caudate nucleus. Tissue levels of dopamine are increased in both brain regions by pargyline, but the increase is significantly greater in the accumbens. Electrolytic lesions of the striatonigral pathway or systemic administration of picrotoxin eliminates the pargyline-induced difference in evoked release of dopamine. Amphetamine causes a reduction in stimulated release in the caudate nucleus with little effect on that observed in the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

Amphetamine attenuates the stimulated release of dopamine in vivo

Carbon-fiber voltammetric electrodes have been used to measure the release of dopamine in the caudate nucleus of an anesthetized rat. Release is induced by electrical stimulation of the medial forebrain bundle. The amplitude of the observed release is attenuated by i.p. injection of amphetamine. A similar attenuation is induced by reserpine; however, at a slower rate. The combined regimen of amphetamine (1 or 10 mg/kg) and electrical stimulation does not deplete striatal dopamine levels and thus the decreased release of dopamine is not a result of depleted dopamine stores. Benztropine (25 mg kg-1) is able to cause a short term inhibition of the action of amphetamine (1 mg kg-1). The dopamine agonist pergolide (0.5 mg kg-1) does not affect the stimulated release. Haloperidol (1.0 mg kg-1) increases the amount of DA release, but is unable to attenuate the inhibition caused by amphetamine. Thus, it appears that the actions induced by amphetamine are a result of interaction with the neuronal uptake carrier and subsequent transport of dopamine from a functional to nonfunctional pool. In isolated striatal synaptic vesicles, amphetamine is found to block dopamine uptake and induce its release. This in vitro evidence provides a possible mechanism for the observed in vivo actions of amphetamine.

Effects of nicotinamide and structural analogs on DNA synthesis and cellular replication of rat hepatoma cells

The effects of nicotinamide and structural analogs on DNA synthesis were studied in rat hepatoma (HTC) cells. Inhibitory effects of these compounds were observed on DNA synthesis as judged by the incorporation of [3H]-thymidine into DNA. Evidence for a marked effect on DNA integrity after preincubation with 1 mM methyl methanesulfonate was provided by a fluorometric technique with ethidium bromide. There was only a small or insignificant enhancement of this effect when hepatoma cells were incubated with nicotinamide. At concentrations of 2-20 mM, 3-aminobenzamide was observed to cause greater effects than nicotinamide on DNA synthesis and integrity and on cellular proliferation in HTC cells. Comparison of the effects of nicotinamide and 3-aminobenzamide with those of N'-methylnicotinamide suggested that some of the effects on DNA synthesis may not be mediated through inhibition of poly(ADP-ribose) synthetase. Inhibition of HTC cell proliferation was observed at a concentration of 3-aminobenzamide, 2 mM, which has been reported to be nontoxic for other cell types.

Monitoring the stimulated release of dopamine with in vivo voltammetry. I: Characterization of the response observed in the caudate nucleus of the rat

Microvoltammetric electrodes were employed in the brain of an anesthetized rat to monitor chemical substances in extracellular fluid following electrical stimulation of the medial forebrain bundle. An increase in concentration of an easily oxidized substance is observed in the caudate nucleus and in the nucleus accumbens. A large amount of evidence suggests that the substance that is observed following stimulation is dopamine. (1) The location of the stimulating electrode must be in known dopaminergic tracts to induce release. (2) Release is most easily observed in brain regions that contain significant numbers of dopamine-containing neurons. (3) Two voltammetric electrodes with very different electrochemical responses provide voltammograms of the released species that are unique for catechols in one case and catecholamines in another case. (4) The amount of 3,4-dihydroxyphenylacetic acid found in striatal tissue by postmortem analysis correlates with the calculated amount of dopamine released. (5) Inhibition of tyrosine hydroxylase, and thus dopamine synthesis, decreases the observed release while inhibition of monoamine oxidase, and thus formation of dopamine metabolites, does not. (6) The dependence of release on stimulation parameters agrees with results obtained with perfusion techniques. Thus, a new method has been developed to characterize endogenous dopamine release in the rat brain and can be used on a time scale of seconds.