Dual use of rectangular and triangular waveforms in voltammetry using a carbon fiber microelectrode to differentiate norepinephrine from dopamine

Highlights

• FSCV using carbon fiber is the major detection tool for dopamine (DA) in vivo.

• Conventional FSCV is unable to distinguish Norepinephrine (NE) from DA.

• Voltammetry with rectangular pulses is potentially useful to differentiate NE from DA

Abstract

While dopamine (DA) may be clearly detected by voltammetric techniques such as fast-scan cyclic voltammetry (FSCV) in the heavily-innervated striatum, the differentiation of DA from other monoamines, including norepinephrine (NE) and serotonin (5-HT), is crucial for further applications outside of the striatum. We show that using normal pulse voltammetry (NPV) with 0.1 V to 0.3 V rectangular pulses can differentiate between NE and DA. While the major electrochemical current of NE and DA is obtained on 0.2 V rectangular pulse of NPV, the relative current on 0.1 V pulse was higher for DA than NE. It was possible to differentiate NE from DA by electrochemical current on three NPV pulses using simple mathematics of sequential equation. Since FSCV obtains a larger current value and less noisy results than NPV, we alternately recorded both. The electrochemical current data of NPV was combined with FSCV for principal component regression (PCR) analysis. The estimated value was evaluated by percent error and correlation coefficient from the true concentration, and the slope of the linear regression line. Estimation by sequential equation using NPV results could differentiate NE from DA, while PCR without NPV current data could not. PCR including NPV current data also could differentiate NE from DA. In this study, we compared the performance of electrochemical recordings with rectangular and triangular waveforms under the same conditions, and the advantages and disadvantages became very clear. Together with our previous finding of significant differentiation of pH changes from monoamines on rectangular pulses of NPV, the combined use of rectangular and triangular pulses would improve the molecular identification of monoamines in the brain using voltammetric recording.

Introduction

The usefulness of fast-scan cyclic voltammetry (FSCV) for the detection of dopamine (DA) release in the rat striatum has been established [1], [2], [3]. It is not only widely used today in rats [4], [5], but it is also capable of detecting the reward-associated DA release in behaving mice [6] and monkey striatum [7]. Important cognitive roles of DA in the prefrontal cortex (PFC) have been suggested [8], [9], [10], particularly in primates. In addition to the PFC, the rapid DA release in various locations of the brain, such as the hippocampus [10], [11], are awaiting a means of detection. For its application in the PFC or the hippocampus, however, DA should be distinguished from other monoamines like norepinephrine (NE) and serotonin (5-HT). In the striatum, the monoamine-like signal has simply been assumed to be DA because of its overwhelming concentration. However, the relative concentration of DA is close to those of NE and 5-HT in the PFC or the hippocampus [12].

The segregation of NE from DA is impossible with FSCV because the voltage-current waveforms of NE and DA are identical [3]. The voltammetric detection of NE on a carbon fiber microelectrode has been performed by FSCV [13], [14], [15], along with other voltammetric techniques [16], [17], [18]. Efforts to record NE and DA in the different part of the brain has also been tried [15], but the differential recording of two monoamines at the same recording location has not been possible by standard techniques.

While FSCV applies triangular potential scan, rectangular potentials are also used for voltammetric techniques, including normal-pulse voltammetry (NPV), differential pulse voltammetry and chronoamperometry. We have shown the usefulness of voltammetry using rectangular pulse for fine chemical differentiation [19], [20], [21], [22], [23], [24]. Rectangular pulse is useful for the segregation of changes in 5-HT from DA [19], [20], [24]. The voltage dependency of an electrochemical reaction is the basic principle behind chemical identification by voltammetric recordings, and NPV is more effective for finer identifications of voltage dependencies, because the NPV waveform has a period with constant voltage. With fast voltage sweeping in FSCV, the voltage selectivity can be temporally skewed due to electrochemical kinetics, although the peak current amplitude is enhanced. When fine chemical identification is required, NPV would be helpful, and it may assist in differentiating NE and DA. During our previous study (Yoshimi 2014 [20]), we noticed a minor difference in the responses of NE and DA on a 0.1 V rectangular pulse (unpublished observation). In the present study, we sought to use this method for the quantitative estimation of NE and DA. In this study, to take advantage of the high sensitivity of fast-scan cyclic voltammetry (FSCV) and the fine chemical identification of normal pulse voltammetry (NPV), we attempted to combine both techniques on a single carbon fiber. Here, we show the differentiation of NE from DA using an alternative dual recording of FSCV and NPV.