PROBING NEUROCHEMISTRY WITH FAST-SCAN CYCLIC VOLTAMMETRY

Takmakov, Pavel

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March 22, 2019

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Takmakov, Pavel
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

Fast-scan cyclic voltammetry (FSCV) with carbon-fiber microelectrodes is a prominent analytical technique for rapid and sensitive detection of electrochemically active analytes in mammalian brain. In recent years this technique became very popular among neuroscientists. However, many improvements for FSCV are possible. Chapter 1 introduces the technique and provides brief description of recent improvements in fast-scan cyclic voltammetry. During voltammetric experiments, potential applied to electrodes changes carbon surface. Chapter 2 describes investigation of the changes induced by waveforms with anodic potential limits of 1.0 V, 1.3 V and 1.4 V. Instrumental methods of analysis such as XPS, AFM and SEM together with electrochemical studies were used. It was observed, that for waveforms with high anodic potential (1.3 V and 1.4 V) carbon electrode surface continuously oxidizes and etches away. Thus, the electrode surface which has surface groups that promote adsorption of catechols is constantly renewed. A benefit of surface renewal is sustainability to chemical fouling. Carbon electrode surface has electrochemically active chemical groups which are oxidized and reduced during voltammetric potential ramps. Electrochemical reactions for these groups involve protons, thus changes in pH of solution generate characteristic cyclic voltammograms. Hence, FSCV can be used to sample rapid pH fluctuations in the brain that are associated with metabolism and changes in the cerebral blood flow. However, cyclic voltammograms for pH changes recorded in brain in vivo and in the flow cell have different shapes, which compromises the identity of pH signal. Chapter 3 describes investigation of the peaks in cyclic voltammogram for pH which led to the conclusion that adsorption of electrochemically inert species to electrode surface is responsible for the interference and the mismatch. Identity of pH signal in brain in vivo was confirmed by inducing acidosis by increasing concentration of carbon dioxide in breathing mixture (hypercapnia). Acidic pH shift with characteristic cyclic voltammogram was recorded with FSCV in hypercapnia. Traditionally, FSCV experiments are done with a single carbon-fiber microelectrode. These electrodes are produced manually and they are very fragile. Chapter 4 describes alternative microfabricated microelectrode arrays (MEAs) which are more robust than glass-encased carbon fibers and can be produced using batch fabrication methods. Microelectrodes in MEAs are distant from each other, thus heterogeneity in analyte concentration such as difference in dopamine release in the brain can be studied. Also, microelectrodes in the array are independently addressable which means that multiplexed detection of different analytes in brain can be performed simultaneously. Instrumentation for FSCV experiments in freely moving animals is custom made which limits the dissemination of this technique. Chapter 5 describes instrumentation for FSCV experiments for combined electrochemical and electrophysiological measurements in details. All electronic components are documented and layouts of electronic circuits are provided in this chapter. Recording of brain functions with multiple electrodes is beneficial in providing information about interconnections of different brain regions as well as synchronization of their activity. This approach was limited to electrophysiological recordings. Chapter 6 describes recordings of endogenous and pharmacologically induced activity of dopaminergic neurons in separate brain hemispheres of anesthetized rat. Synchronization of activity of dopaminergic neurons between two separate and symmetrical systems is observed. Possible link to slow wave oscillations that occur in brain cortex during sleep is discussed.

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