Local depletion of monoamines induced with in vivo voltammetry in the cat brain

A review of flux considerations for in vivo neurochemical measurements

The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

Effects of sampling rate on the interpretation of cellular transport measurements

Electrochemical sensing techniques are increasingly used to study biological processes by monitoring concentration changes of the molecule of interest close to cells. The measured concentration is the result of cellular transport across the cell membrane and diffusion of the released molecules from the cells to the sensing electrode. The objective of such experiments is to understand the cellular processes underlying the observed changes in concentration. Thus, the influence of mass transport on the measured concentration trace has to be removed. This is done by deconvolution of the impulse response function of diffusion from the concentration data. We have recently observed that measuring concentration at a sampling rate that satisfies the Nyquist criterion for the observed concentration dynamics may not be sufficient to correctly reconstruct cellular flux. This is because the impulse response function of diffusion also has to be represented with sufficient temporal resolution. We discuss this problem here using the example of monitoring drug efflux from a monolayer of cancer cells with microvoltammetry, and chloride secretion from an epithelial cell monolayer monitored with an ion-selective electrode.

Sustained depolarisation induces changes in the extracellular concentrations of purine and pyrimidine nucleosides in the rat thalamus

ATP and adenosine are well-known neuroactive compounds. Other nucleotides and nucleosides may also be involved in different brain functions. This paper reports on extracellular concentrations of nucleobases and nucleosides (uracil, hypoxanthine, xanthine, uridine, 2'-deoxycytidine, 2'-deoxyuridine, inosine, guanosine, thymidine, adenosine) in response to sustained depolarisation, using in vivo brain microdialysis technique in the rat thalamus. High-potassium solution, the glutamate agonist kainate, and the Na(+)/K(+) ATPase blocker ouabain were applied in the perfusate of microdialysis probes and induced release of various purine and pyrimidine nucleosides. All three types of depolarisation increased the level of hypoxanthine, uridine, inosine, guanosine and adenosine. The levels of measured deoxynucleosides (2'-deoxycytidine, 2'-deoxyuridine and thymidine) decreased or did not change, depending on the type of depolarisation. Kainate-induced changes were TTX insensitive, and ouabain-induced changes for inosine, guanosine, 2'-deoxycytidine and 2'-deoxyuridine were TTX sensitive. In contrast, TTX application without depolarisation decreased the extracellular concentrations of hypoxanthine, uridine, inosine, guanosine and adenosine. Our data suggest that various nucleosides may be released from cells exposed to excessive activity and, thus, support several different lines of research concerning the regulatory roles of nucleosides.

Short-range differential pulse voltammetry for fast, selective analysis of basal levels of cerebral compounds in vivo

Differential pulse voltammetry (DPV) with pretreated biosensors (carbon fibre microelectrodes (mCFE), 10-30 microns diameter) allows selective in vivo measurement of basal endogenous levels of dopamine (DA), serotonin (5-HT), their metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5-HIAA), and neuropeptides. We have now modified DPV in order to reduce the time of analysis from tens of seconds to 1-2 s without losing selectivity. We call this newly reported method short-range differential pulse voltammetry (SRDPV). Simply, while in DPV the complete oxidation peak is recorded, SRDPV measures only the top of each oxidation peak. For example, to monitor peak 2 which corresponds to the in vivo oxidation of extracellular DOPAC and occurs at approximately +85 +/- 10 mV, the initial (Ei) and final (Ef) potentials applied with DPV were -100 mV and +200 mV, respectively, while they were +75 mV (Ei) and +95 mV (Ef) with SRDPV. At the typical scan range of 10 mV.s-1, the effective time of measurement was 30 s for DPV and 2 s for SRDPV. A similar procedure was performed to analyze peak 3 (5-HIAA, occurring at +230 +/- 11 mV) with Ei + 50 mV and Ef + 350 mV for DPV, or +220 mV and +240 mV for SRDPV. DPV and SRDPV were compared in vitro by quantitating DOPAC and 5-HIAA in solutions of increasing concentrations (chosen on the basis of the suggested in vivo content of these two compounds). Data indicated that similar sensitivity and selectivity were obtained with both methods at all concentrations, supporting the applicability of SRDPV for in vitro studies. In vivo experiments were performed in anesthetized adult male rats prepared for voltammetry by inserting the electrically pretreated biosensor (mCFE) into the striatum. DPV measurements were performed automatically every 3-5 min and were alternated every 10-20 min with a sequence of 5-10 SRDPV scans performed every 10-30 s. Subsequent pharmacological or electrical manipulations of the two biogenic amine systems studied were monitored by alternate use of DPV and SRDPV. The data presented support the capability of SRDPV with pretreated biosensors to measure in vivo electroactive compounds with selectivity and sensitivity comparable to that of DPV, but with improved time resolution.