Surface Heterogeneities Matter in Fast Scan Cyclic Voltammetry Investigations of Catecholamines in Brain with Carbon Microelectrodes of High-Aspect Ratio: Dopamine Oxidation at Conical Carbon Microelectrodes

Alexander Oleinick,Irina Svir,Christian Amatore,Elena E Ferapontova,Isabel Álvarez-Martos

doi:10.1149/2.0071812jes

Abstract

Fast Scan Cyclic Voltammetry (FSCV) at high aspect ratio carbon microelectrodes shows adequate high temporal and spatial resolution for in vivo analysis of catecholamines. Though the presence of their surface heterogeneities has been recognized since their earliest introduction for in vivo measurements in the brain, the kinetic consequences on the measurements have not been investigated and FSCV measurements are treated based on pre- and post-calibrations. We establish here that surface heterogeneities play a consequent dynamic role on the oxidation of dopamine taken as an example of catecholamines. Hence, the FSCV current peak intensities do not scale with the scan rate v or its square root. This is rationalized with a simple model involving a co-existence of at least two types of surface nanodomains with different electrochemical reactivities and different time responses. At low scan rates (<100 V s−1) dopamine molecules that initially adsorbed onto non-electroactive nanodomains have enough time to migrate toward highly electroactive ones so all molecules initially adsorbed on the whole electrode surface may be oxidized during one FSCV cycle. Current peak intensities then increase proportionally to the scan rate. However, above 100 V s−1, dopamine migration between sites starts to be kinetically limited so that FSCV current peak intensities do not increase any more proportionally to the scan rate. Ultimately, i.e., above 1000 V s−1, the dopamine exchange between sites is almost totally blocked so only dopamine molecules initially adsorbed on the electroactive surface nanodomains may be oxidized; the current peak intensities then increase again proportionally with the scan rate though with a smaller slope than that observed at small scan rates. Since carbon fibers with large aspect ratios are frequently used in brain investigations, this effect should be a concern when extracting quantitative results even when each carbon fiber response is properly pre- or post-calibrated using the exact CV waveform and scan rate used during the in vivo measurements.

Highlights

Release and modulation of neurotransmitters fluxes and concentrations are essential in complex organisms since they regulate many key functions by transferring information between cells such as neurons in the brain, neurons and muscle cells as well as through stimulating specific actions in endocrine-related systems
Where R is the ohmic resistance determined from the initial slope of the background currents at v = 5 kV.s−1,40 viz., R = 72 k ; Ebpack−corr(v) is the oxidation or reduction peak potential measured at a given scan rate v from background-subtracted CVs; itpot(v) is the maximum current intensity of the CV wave measured at the same scan rate before background subtraction
The general assumption that at fast scan rates current intensities scale linearly with the scan rate may not be always correct. This seems to justify the common practice of pre- and postcalibrating high aspect ratio carbon fiber electrodes using fast-scan cyclic voltammetry (FSCV) conditions identical to those used during measurements in the brain.[36]

Summary

Introduction

Release and modulation of neurotransmitters fluxes and concentrations are essential in complex organisms since they regulate many key functions by transferring information between cells such as neurons in the brain, neurons and muscle cells as well as through stimulating specific actions in endocrine-related systems. Albeit their unique and ubiquitous functions are well recognized today, the very existence and identification of neurotransmitters has long been a matter of debate up to the end of the second third of the past century,[1,2,3] and still needs to be better understood This explains why characterizing fluxes and nature of neurotransmitters release is still stimulating a quest for analytical methods sufficiently accurate, specific and sensitive to investigate dynamic effluxes and flows of neurotransmitters in vivo (e.g., within a living brain)[4,5,6] or ex vivo (e.g., at the single endocrine cell level,[7,8,9] or intrasynaptically10–12) with an adequate temporal resolution. In contrast with the near-homogeneity of electroactive properties of their cross-sections,[17,18] carbon fiber shafts are known

Results

Discussion

Conclusion