The use of fast-scan cyclic voltammetry Is explored as an analytical technique for the detection of biogenic amines. Cyclic voltammograms were recorded at a scan rate of 200 V s"1 at carbon-fiber electrodes with and without coating of a perfluorinated Ion-exchange material. Voltammograms were

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Cyclic voltammetry of Nafion-coated, carbon-fiber electrodes is used to detect trace concentrations of dopamine, both in a flow injection apparatus and in the brain of an anaesthetized rat. To improve signal-to-noise ratios, the sources of noise during cyclic voltammetry have been determined and strategies have been developed to decrease the noise. With the potentiostat employed, the measured noise is comparable to that expected for Johnson noise from the feedback resistor of the current transducer. Additional noise arises from the waveform generator employed and, in some cases, line noise. Line noise is discriminated against by starting each cyclic voltammogram either in phase or 180 degrees out of phase with the line frequency. When used in vivo, additional noise also arises from the physiological activity of the animal. Detection limits are found to closely correspond to those predicted on the basis of simulation of the voltammetric shape and the measured noise. Detection limits are improved by the use of appropriate analog and digital filtering, ensemble averaging, and appropriate timing of repetitive cyclic voltammograms. The combined use of these techniques enables the in vivo detection of approximately 100 nM of dopamine with a signal-to-noise ratio of 25.

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Abstract:As in the preceding study, electrical stimulation was used to effect release overflow of norepinephrine in the rat thalamus. Using a weak electrochemical pretreatment of a carbon fiber electrode, it was possible to "tune in" the electrochemical response signal for norepinephrine without metabolite interference. This reasonably selective signal was then used to study the degradation of norepinephrine release ability caused by prolonged stimulation. Further, the signals were modeled by the method used successfully for stimulated dopamine overflow, providing hitherto unavailable information on the temporal and spatial characteristics of norepinephrine release overflow. Pertinent comparisons between the release characteristics of the dopamine and norepinephrine systems show that the half-life for norepinephrine in the extracellular fluid space is -1 s in thalamus compared with 33 ms for dopamine in caudate. Key Words:Norepinephrine -Release -Reuptake -Electrochemical monitoring. Capella P. et al. Real-time monitoring of electrically stimulated norepinephrine release in rat thalamus: 11. Modeling of release and reuptake characteristics of stimulated norepinephrine overflow. J. Neurochem. 60,449-453 (1993). In the previous report, pretreated carbon fiber electrodes were used to follow stimulated overflow in the rat thalamus of norepinephrine (NA) and the metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) (Ghasemzadeh et al., 1993). Using a weaker electrochemical pretreatment, it was shown that the electrode response time and sensitivity could be "tuned" to follow reliably just the fast NA peak overflow release and reuptake, essentially eliminating the slower DO-PAC plateau response. These faster responding electrodes have been used to study the ability of NA terminals to sustain prolonged excitation with only minimal (2 min) rest periods between stimulations. In a collaborative study, the rapid NA responses were modeled by the approach for electrically stimulated striatal dopamine (DA) overflow used by Wightman and Zimmerman (1990). The NA system has not been modeled previously in this fashion, and some distinct differences between it and the DA system were observed. These provide information on the temporal and spatial characteristics of NA overflow release. These results and other significant comparisons between the DA and NA systems are summarized below. MATERIALS AND METHODSThe carbon fiber electrodes (250-pm length) used in this study were given a weak electrochemical pretreatment and Nafion dip coating. They are designated as CFE-W and their preparation was detailed earlier (Ghasemzadeh et al., 1993). The measurement of response times of fiber electrodes in a switched flow system and their calibration has been described (Capella et al., 1990). All electrodes used in this work had a t,,* (time to 50% of maximum response to a step flow pulse of 1 pA4NA) of 0.5-1 f 0.25 s.The animal surgery, electrode placements, and measure-