(Invited) Voltammetric Measurement of Neuropeptides and Neurohormones with Carbon-Fiber Microelectrode Biosensors

Abstract

Carbon fiber microelectrodes (CFMEs) have been used to detect neurotransmitters and other biomolecules using fast-scan cyclic voltammetry (FSCV) for the past several years. These assays typically measure small molecule neurotransmitters such as dopamine and serotonin. The carbon fiber is relatively small, biocompatible, and makes minimally invasive measurements at high spatial and temporal resolution. Carbon fiber multielectrode arrays have been utilized to measure multiple neurotransmitters in several brain regions simultaneously with multi-waveform application on each electrode. We have extended this work to measure larger molecule neuropeptides such as Neuropeptide Y and Oxytocin, pleiotropic peptide hormones, that are physiologically important for adaptation, development, reproduction, and social behavior. These neuropeptides function as a stress-coping molecules, an anti-inflammatory agents, and serves as antioxidants with protective effects especially during adversity or trauma. Here, we measure tyrosine within these neuropeptides using the Modified Sawhorse Waveform (MSW), enabling enhanced electrode sensitivity for the amino acid and peptide, decreased surface fouling, and codetection with other catecholamines. As both oxytocin and Neuropeptide Y contain tyrosine, the MSW was also used to detect these neuropeptides. Additionally, we demonstrate that applying the MSW on CFMEs allows for real time measurements of exogenously applied neuropeptides on rat brain slices. These results may serve as novel assays for neuropeptide detection in a fast, sub-second timescale with possible implications for in vivo measurements and further understanding of the physiological role of neuropeptides such as Neuropeptide Y and oxytocin.Cortisol is also a vital steroid neurohormone that has been known as the “stress hormone,” which is elevated during times of high period of anxiety which is biologically critical. The enhanced detection of cortisol is fundamentally important as it will help further our understanding of stress during several physiological states. Several methods exist to detect cortisol, however, they suffer from low biocompatibility and spatiotemporal resolution and are relatively slow. In this study, we developed an assay to detect cortisol CFMEs and FSCV. FSCV is typically utilized to measure small molecule neurotransmitters by producing a readout cyclic voltammogram (CV) for the specific detection of biomolecules on a fast, sub second timescale with biocompatible CFMEs. It has seen enhanced utility in measuring peptides and other larger compounds. We developed a waveform that scanned from -0.5 V to -1.2 V at 400 V/sec to electro-reduce cortisol at the surface of CFMEs. The sensitivity of cortisol was found to be 0.87 +/- 0.05 nA/μM (n=5) and was found to be adsorption controlled on the surface of CFMEs and stable over several hours. Cortisol was able to be co-detected with several other biomolecules such as dopamine, and the waveform was fouling resistant to repeated injections of cortisol on the surface of the CFMEs. Furthermore, we also measured exogenously applied cortisol into simulated urine to demonstrate biocompatibility and potential use in vivo. The specific and biocompatible detection of cortisol with high spatiotemporal resolution will help further elucidate its biological significance and further understand its physiological importance.