Hitchhiker's Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry

Abstract

Fast-scan cyclic voltammetry (FSCV) has been used for over 20 years to study rapid neurotransmission in awake and behaving animals. These experiments were first carried out with carbon-fiber microelectrodes (CFMs) encased in borosilicate glass, which can be inserted into the brain through micromanipulators and guide cannulas. More recently, chronically implantable CFMs constructed with small diameter fused-silica have been introduced. These electrodes can be affixed in the brain with minimal tissue response, which permits longitudinal measurements of neurotransmission in single recording locations during behavior. Both electrode designs have been used to make novel discoveries in the fields of neurobiology, behavioral neuroscience, and psychopharmacology. The purpose of this Review is to address important considerations for the use of FSCV to study neurotransmitters in awake and behaving animals, with a focus on measurements of striatal dopamine. Common issues concerning experimental design, data collection, and calibration are addressed. When necessary, differences between the two methodologies (acute vs chronic recordings) are discussed. The topics raised in this Review are particularly important as the field moves beyond dopamine toward new neurochemicals and brain regions.

Keywords: Fast-scan cyclic voltammetry; carbon-fiber microelectrodes; chemometrics; dopamine; principal component regression.

Figure 1

Designs of (a) borosilicate glass (b) fused silica CFMs. (a) Carbon fibers are aspirated through borosilicate glass under vacuum. A seal is created by heating and pulling the capillary to a fine tip. The protruding fiber is then trimmed, typically between 75 and 125 μm. For optimal electrochemical performance, epoxy resin is used to fill any leaks in the seal that occur during electrode fabrication. Left panel: Electron micrograph of CFM. Reprinted with permission from ref 113. Copyright 2003 American Association for Clinical Chemistry. Middle panel: A rat with dual cannulas for later acute implantation of a CFM and reference electrode. The rat is tethered to a swivel and commutator via fastening of the headstage to an implanted stimulating electrode. Right panel: Side view of cannula for acute implantation of electrodes (left) and a micromanipulator for precise driving of the CFM during in vivo recordings (right). (b) Carbon fibers are threaded through a small diameter fused silica capillary under isopropyl alcohol. After drying, epoxy is placed on the fiber and wicked into the fused silica capillary to create a hemispherical seal (inset image). The protruding carbon fiber is trimmed between 150 and 200 μm long. Electrical connection is established between a silver pin and the fiber with silver epoxy, which is later insulated with clear epoxy. Reprinted with permission from ref 59. Copyright 2010 Nature Publishing Group.

Figure 2

Comparison of concentrations measured at acute and chronic CFMs without optimization for dopamine release sites. In a study with acute CFMs (Owesson-White et al., ref 87), electrode placement was optimized for extracellular electrophysiological signals rather than dopamine release, resulting in recording locations without (top) and with (middle) phasic dopamine release. The concentration profile corresponds well with values from chronically implanted CFMs that were not optimized for recording location (bottom), indicating the lower concentrations measured with chronic CFM may be a result of recording site selection. Reprinted with permissions from ref 86. Copyright 2012 PNAS.

Figure 3

Dopamine transients at chronically implanted CFMs. (a) Pharmacologically induced dopamine transients at a chronic CFM in response to i.v. cocaine infusion (red bar, 1.5 s duration). Background subtraction is denoted by the white dashed line. (b) Measurements at a chronic CFM during a behavioral session of Pavlovian conditioning. Spontaneous dopamine transients are observed preceding cue onset (white asterisks). Moreover, both cue onset (left red dotted line) and reward delivery immediately following cue offset (right red dotted line) evoked phasic dopamine release. Background subtraction is denoted by the white dashed line. Dopamine traces were extracted with PCR using a standard training set. Both measurements were made in the nucleus accumbens core.

Figure 4

Example of the use of principal component analysis to analyze cocaine-induced dopamine transients. (a) A 30 second color plot following cocaine (20 mg/kg) administration in an awake rat shows overlapping dopamine and pH changes. The dopamine (black) and pH (blue) changes are separated by PCA, and quantitated using inverse-least-squares regression. pH changes have a maximum contribution of +0.019 pH units (−0.76 nA) at 8.3 s, while dopamine maximizes at 262 nM (3.13 nA) at 28.9 s. (b) Residual analysis confirms that the PCA model is valid for analysis of this data. Q_t values (black) fall below the model specific tolerance level (Q_α, 379 nA²) for the data shown in panel (a). A residual color plot displays current uncaptured by the model.

Figure 5

Training sets built with data from separate electrodes could capture qualitative information. Dopamine CVs from a training set built at the same electrode as the collected data were replaced with dopamine CVs from a separate electrode, while pH CVs were left unaltered. Analysis with this composite training set resulted in underestimation of signal, but tracked qualitative information for this electrical stimulation (red bar). Reproduced with permission from ref 123. Copyright 2011 American Chemical Society.