Characterization of a Multiple-Scan-Rate Voltammetric Waveform for Real-Time Detection of Met-Enkephalin

Dioxythiophene/Nafion Polymer Composite Membranes for Tunable Size-Based Selectivity in the Voltammetric Detection of Small Neuropeptides

Carbon-fiber microelectrodes are proven and powerful sensors for electroanalytical measurements in a variety of environments, including complex systems such as the brain. They are used to detect and quantify a range of biological molecules, including neuropeptides, which are of broad interest for understanding physiological function. The enkephalins (met- and leu-) are endogenous opioid peptides that are involved in both pain and motivated behavior. Each is comprised of only five amino acids including tyrosine, an electroactive species. Electroanalytical measurements targeting tyrosine can reveal the dynamics of endogenous enkephalin transients in live tissue. However, when using electrochemistry in a biological system, selectivity is always a concern. Many larger neuropeptides also contain tyrosine. As such, they could generate a redox signature similar to that of the enkephalins, potentially confounding the measurement. In this work, three distinctly sized dioxythiophene monomers were mixed with Nafion and electrodeposited onto cylindrical carbon-fiber microelectrodes to form composite polymer films that allow for the tunable, size-based exclusion of larger molecules. The dioxythiophene monomers 3,4-ethylenedioxythiophene (EDOT), 3,4-propylenedioxythiophene (ProDOT), and 3,4-(2',2'-diethylpropylene) dioxythiophene (ProDOT-Et2) were used to create nanostructured pores of increasing size. The dioxythiophene/Nafion modified electrodes were characterized in the voltammetric detection of dopamine, a classic small molecule neurotransmitter, and a series of tyrosine containing neuropeptides of increasing size: met-enkephalin (M-ENK; 5 residues), oxytocin (OXY; 9 residues), neurotensin (NT; 13 residues), and neuropeptide Y (NPY; 36 residues). The modified electrodes exhibited enhanced selectivity for smaller peptide species over larger peptides in a manner consistent with the size of the dioxythiophene monomer incorporated into the polymeric film, allowing for tunability in terms of size-based selective detection.

Voltammetric detection of Neuropeptide Y using a modified sawhorse waveform

The hormone Neuropeptide Y (NPY) plays critical roles in feeding, satiety, obesity, and weight control. However, its complex peptide structure has hindered the development of fast and biocompatible detection methods. Previous studies utilizing electrochemical techniques with carbon fiber microelectrodes (CFMEs) have targeted the oxidation of amino acid residues like tyrosine to measure peptides. Here, we employ the modified sawhorse waveform (MSW) to enable voltammetric identification of NPY through tyrosine oxidation. Use of MSW improves NPY detection sensitivity and selectivity by reducing interference from catecholamines like dopamine, serotonin, and others compared to the traditional triangle waveform. The technique utilizes a holding potential of -0.2 V and a switching potential of 1.2 V that effectively etches and renews the CFME surface to simultaneously detect NPY and other monoamines with a sensitivity of 5.8 ± 0.94 nA/µM (n = 5). Furthermore, we observed adsorption-controlled, subsecond NPY measurements with CFMEs and MSW. The effective identification of exogenously applied NPY in biological fluids demonstrates the feasibility of this methodology for in vivo and ex vivo studies. These results highlight the potential of MSW voltammetry to enable fast, biocompatible NPY quantification to further elucidate its physiological roles.

Electrochemical Quantification of Enkephalin Peptides Using Fast-Scan Cyclic Voltammetry

Endogenous opioid neuropeptides serve as important chemical signaling molecules in both the central and peripheral nervous systems, but there are few analytical tools for directly monitoring these molecules in situ. The opioid peptides share the amino acid motif, Tyr-Gly-Gly-Phe-, at the N-terminus. Met-enkephalin is a small opioid peptide comprised of only five amino acids with methionine (Met) incorporated at the C-terminus. Tyrosine (Tyr) and Met are electroactive, and their distinct electrochemical signatures can be utilized for quantitative molecular monitoring. This work encompasses a thorough voltammetric characterization of Tyr and Met redox chemistry as individual amino acids and when incorporated into small peptide fragments containing the shared Tyr-Gly-Gly-Phe- motif. NMR spectroscopy was used to determine the structure and conformation at near-physiological conditions. Voltammetric data demonstrate how the peak oxidation potential and the rate of electron transfer are dependent on the local chemical environment. Both the proximity of the electroactive residue to the C- or N-terminus and the hydrophobicity of the additional nonelectroactive amino acids profoundly affect sensitivity. Finally, the work uses the electrochemical signal for individual amino acids in a "training set", with a combination of principal component analysis and least-squares regression to accurately predict the voltammetric signal for short peptides comprising different combinations of those amino acids. Overall, this study demonstrates how fast-scan cyclic voltammetry can be utilized to discriminate between peptides with small differences in the chemical structure, thus establishing a framework for reliable quantification of small peptides in a complex signal, broadly speaking.

Synchronous Measurements of Extracellular Action Potentials and Neurochemical Activity with Carbon Fiber Electrodes in Nonhuman Primates

Measuring the dynamic relationship between neuromodulators, such as dopamine, and neuronal action potentials is imperative to understand how these fundamental modes of neural signaling interact to mediate behavior. We developed methods to measure concurrently dopamine and extracellular action potentials (i.e., spikes) in monkeys. Standard fast-scan cyclic voltammetric (FSCV) electrochemical (EChem) and electrophysiological (EPhys) recording systems are combined and used to collect spike and dopamine signals, respectively, from an array of carbon fiber (CF) sensors implanted in the monkey striatum. FSCV requires the application of small voltages at the implanted sensors to measure redox currents generated from target molecules, such as dopamine. These applied voltages create artifacts at neighboring EPhys measurement sensors which may lead to misclassification of these signals as physiological spikes. Therefore, simple automated temporal interpolation algorithms were designed to remove these artifacts and enable accurate spike extraction. We validated these methods using simulated artifacts and demonstrated an average spike recovery rate of 84.5%. We identified and discriminated cell type-specific units in the monkey striatum that were shown to correlate to specific behavioral task parameters related to reward size and eye movement direction. Synchronously recorded spike and dopamine signals displayed contrasting relations to the task variables, suggesting a complex relationship between these two modes of neural signaling. Future application of our methods will help advance our understanding of the interactions between neuromodulator signaling and neuronal activity, to elucidate more detailed mechanisms of neural circuitry and plasticity mediating behaviors in health and in disease.

Fabrication and Optimization of a Molecularly Imprinted Carbon Fiber Microelectrode for Selective Detection of Met-enkephalin Using Fast-Scan Cyclic Voltammetry

Methionine-enkephalin (Met-Enk) is an endogenous opioid peptide that is involved in various physiological processes including memory. A technological gap in the understanding of Met-Enk's role in memory is the lack of rapid measurement tools to selectively quantify Met-Enk concentrations in situ. Here, we integrate molecularly imprinted polymers (MIPs) with carbon fiber microelectrodes (CFMs) to selectively detect Met-Enk by using fast-scan cyclic voltammetry (FSCV). We report two MIP conditions that yield 2-fold and 5-fold higher selectivity toward Met-Enk than the tyrosine-containing hexapeptide fragment angiotensin II (3-8). We demonstrate that MIP technology can be combined with FSCV at CFMs to create rapid and selective sensors for Met-Enk. This technology is a promising platform for creating selective sensors for other peptides and biomarkers.

Perspective-Advances in Voltammetric Methods for the Measurement of Biomolecules

Voltammetry is a powerful electroanalytical tool that makes fast, real-time measurements of neurotransmitters and other molecules. Electroanalytical methods like cyclic, pulse, and stripping voltammetry are useful for qualitative and quantitative examination. Neurochemical sensing has been enhanced using carbon-based electrodes and waveform modification methods that improve sensitivity and stability of electrode performance. Voltammetry has revolutionized neurochemical monitoring by providing real-time information on neurotransmitter dynamics for neurochemical studies. Selectivity and electrode fouling remain issues for biomolecule detection, but recent advances promise new methods of analysis for other applications to enhance spatiotemporal resolution, sensitivity, selectivity, and other important considerations.

Maximizing Electrochemical Information: A Perspective on Background-Inclusive Fast Voltammetry

This perspective encompasses a focused review of the literature leading to a tipping point in electroanalytical chemistry. We tie together the threads of a "revolution" quietly in the making for years through the work of many authors. Long-held misconceptions about the use of background subtraction in fast voltammetry are addressed. We lay out future advantages that accompany background-inclusive voltammetry, particularly when paired with modern machine-learning algorithms for data analysis.

Subsecond Codetection of Dopamine and Estradiol at a Modified Sharkfin Waveform

17β-Estradiol (E2) is a ubiquitously expressed hormone that is active in a wide range of neuroprotective and regenerative roles throughout the brain. In particular, it is a well-known dopamine (DA) regulator and is responsible for modulating the expression of dopaminergic receptors and transporters. Recent studies point to E2 release occurring on a rapid time scale and having impacts on DA activity within seconds to minutes. As such, tools capable of monitoring the release of both E2 and DA in real time are essential for developing an accurate understanding of their interactive roles in neurotransmission and regulation. Currently, no analytical techniques capable of codetection of both analytes with high sensitivity, spatiotemporal resolution, extended monitoring, and minimal tissue damage exist. We describe a modified waveform using fast-scan cyclic voltammetry that is capable of low nanomolar detection of both DA and E2 on a subsecond time scale. Both analytes have limits of detection at or below 30 nM and high sensitivity: 11.31 ± 0.55 nA/μM for DA and 9.47 ± 0.36 nA/μM for E2. The waveform is validated in a tissue matrix, confirming its viability for measurement in a biologically relevant setting. This is the first method capable of codetection of fluctuations in DA and E2 with the temporal, spatial, and sensitivity requirements necessary for studying real-time neurochemical signaling.

Editors' Choice-Review-The Future of Carbon-Based Neurochemical Sensing: A Critical Perspective

Carbon-based sensors have remained critical materials for electrochemical detection of neurochemicals, rooted in their inherent biocompatibility and broad potential window. Real-time monitoring using fast-scan cyclic voltammetry has resulted in the rise of minimally invasive carbon fiber microelectrodes as the material of choice for making measurements in tissue, but challenges with carbon fiber's innate properties have limited its applicability to understudied neurochemicals. Here, we provide a critical review of the state of carbon-based real-time neurochemical detection and offer insight into ways we envision addressing these limitations in the future. This piece focuses on three main hinderances of traditional carbon fiber based materials: diminished temporal resolution due to geometric properties and adsorption/desorption properties of the material, poor selectivity/specificity to most neurochemicals, and the inability to tune amorphous carbon surfaces for specific interfacial interactions. Routes to addressing these challenges could lie in methods like computational modeling of single-molecule interfacial interactions, expansion to tunable carbon-based materials, and novel approaches to synthesizing these materials. We hope this critical piece does justice to describing the novel carbon-based materials that have preceded this work, and we hope this review provides useful solutions to innovate carbon-based material development in the future for individualized neurochemical structures.

Untapped Potential: Real-Time Measurements of Opioid Exocytosis at Single Cells

The endogenous opioid system is commonly targeted in pain treatment, but the fundamental nature of neuropeptide release remains poorly understood due to a lack of methods for direct detection of specific opioid neuropeptides in situ. These peptides are concentrated in, and released from, large dense-core vesicles in chromaffin cells. Although catecholamine release from these neuroendocrine cells is well characterized, the direct quantification of opioid peptide exocytosis events has not previously been achieved. In this work, a planar carbon-fiber microelectrode served as a "postsynaptic" sensor for probing catecholamine and neuropeptide release dynamics via amperometric monitoring. A constant potential of 500 mV was employed for quantification of catecholamine release, and a higher potential of 1000 mV was used to drive oxidation of tyrosine, the N-terminal amino acid in the opioid neuropeptides released from chromaffin cells. By discriminating the results collected at the two potentials, the data reveal unique kinetics for these two neurochemical classes at the single-vesicle level. The amplitude of the peptidergic signals decreased with repeat stimulation, as the halfwidth of these signals simultaneously increased. By contrast, the amplitude of catecholamine release events increased with repeat stimulation, but the halfwidth of each event did not vary. The chromogranin dense core was identified as an important mechanistic handle by which separate classes of transmitter can be kinetically modulated when released from the same population of vesicles. Overall, the data provide unprecedented insight into key differences between catecholamine and opioid neuropeptide release from isolated chromaffin cells.

Measuring tryptophan dynamics using fast scan cyclic voltammetry at carbon fiber microelectrodes with improved sensitivity and selectivity

Despite the fact that tryptophan (Trp) is an essential amino acid that humans typically obtain through diet, there are several interesting tryptophan dynamics at play in the body. Quantifying and understanding these dynamics are crucial in studies of depression, autism spectrum disorder, and other disorders that involve neurotransmitters directly synthesized from tryptophan. Here we detail the optimization of waveform parameters in fast scan cyclic voltammetry at carbon fiber microelectrodes to yield four-fold higher sensitivity and six-fold higher selectivity compared to previously reported methods. We demonstrate the utility of our method in measuring (1) exogenous Trp dynamics from administration of Trp to PC-12 cells with and without overexpression of tryptophan hydroxylase-2 and (2) endogenous Trp dynamics in pinealocyte cultures with and without stimulation via norepinephrine. We observed interesting differences in Trp dynamics in both model systems, which demonstrate that our method is indeed sensitive to Trp dynamics in different applications.

Exploiting Microelectrode Geometry for Comprehensive Detection of Individual Exocytosis Events at Single Cells

Carbon fiber microelectrodes are commonly used for real-time monitoring of individual exocytosis events at single cells. Since the nature of an electrochemical signal is fundamentally governed by mass transport to the electrode surface, microelectrode geometry can be exploited to achieve precise and accurate measurements. Researchers traditionally pair amperometric measurements of exocytosis with a ∼10-μm diameter, disk microelectrode in an "artificial synapse" configuration to directly monitor individual release events from single cells. Exocytosis is triggered, and released molecules diffuse to the "post-synaptic" electrode for oxidation. This results in a series of distinct current spikes corresponding to individual exocytosis events. However, it remains unclear how much of the material escapes detection. In this work, the performance of 10- and 34-μm diameter carbon fiber disk microelectrodes was directly compared in monitoring exocytosis at single chromaffin cells. The 34-μm diameter electrode was more sensitive to catecholamines and enkephalins than its traditional, 10-μm diameter counterpart, and it more effectively covered the entire cell. As such, the larger sensor detected more exocytosis events overall, as well as a larger quantal size, suggesting that the traditional tools underestimate the above measurements. Both sensors reliably measured l-DOPA-evoked changes in quantal size, and both exhibited diffusional loss upon adjustment of cell-electrode spacing. Finite element simulations using COMSOL support the improved collection efficiency observed using the larger sensor. Overall, this work demonstrates how electrode geometry can be exploited for improved detection of exocytosis events by addressing diffusional loss─an often-overlooked source of inaccuracy in single-cell measurements.

Current and emerging methods for probing neuropeptide transmission

Neuropeptides comprise the most diverse category of neurochemicals in the brain, playing critical roles in a wide range of physiological and pathophysiological processes. Monitoring neuropeptides with high spatial and temporal resolution is essential for understanding how peptidergic transmission is regulated throughout the central nervous system. In this review, we provide an overview of current non-optical and optical approaches used to detect neuropeptides, including their design principles, intrinsic properties, and potential limitations. We also highlight the advantages of using G protein‒coupled receptor (GPCR) activation‒based (GRAB) sensors to monitor neuropeptides in vivo with high sensitivity, good specificity, and high spatiotemporal resolution. Finally, we present a promising outlook regarding the development and optimization of new GRAB neuropeptide sensors, as well as their potential applications.

Carbon Electrode Sensor for the Measurement of Cortisol with Fast-Scan Cyclic Voltammetry

Cortisol is a vital steroid hormone that has been known as the "stress hormone", which is elevated during times of high stress and anxiety and has a significant impact on neurochemistry and brain health. The improved detection of cortisol is critically 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 they are relatively slow. In this study, we developed an assay to measure cortisol with carbon fiber microelectrodes (CFMEs) and fast-scan cyclic voltammetry (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, subsecond 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 to -1.2 V at 400 V/s to electro-reduce cortisol at the surface of CFMEs. The sensitivity of cortisol was found to be 0.87 ± 0.055 nA/μM (n = 5) and was found to be adsorption controlled on the surface of CFMEs and stable over several hours. Cortisol was 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 and impact on brain health.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

An electrochemical approach for rapid, sensitive, and selective detection of dynorphin

The endogenous opioid peptide systems are critical for analgesia, reward processing, and affect, but research on their release dynamics and function has been challenging. Here, we have developed microimmunoelectrodes (MIEs) for the electrochemical detection of opioid peptides using square-wave voltammetry. Briefly, a voltage is applied to the electrode to cause oxidation of the tyrosine residue on the opioid peptide of interest, which is detected as current. To provide selectivity to these voltammetric measurements, the carbon fiber surface of the MIE is coated with an antiserum selective to the opioid peptide of interest. To test the sensitivity of the MIEs, electrodes are immersed in solutions containing different concentrations of opioid peptides, and peak oxidative current is measured. We show that dynorphin antiserum-coated electrodes are sensitive to increasing concentrations of dynorphin in the attomolar range. To confirm selectivity, we also measured the oxidative current from exposure to tyrosine and other opioid peptides in solution. Our data show that dynorphin antiserum-coated MIEs are sensitive and selective for dynorphin with little to no oxidative current observed in met-enkephalin and tyrosine solutions. Additionally, we demonstrate the utility of these MIEs in an in vitro brain slice preparation using bath application of dynorphin as well as optogenetic activation of dynorphin release. Future work aims to use MIEs in vivo for real-time, rapid detection of endogenous opioid peptide release in awake, behaving animals.

Genetically encoded tools for in vivo G-protein-coupled receptor agonist detection at cellular resolution

G-protein-coupled receptors (GPCRs) are the most abundant receptor type in the human body and are responsible for regulating many physiological processes, such as sensation, cognition, muscle contraction and metabolism. Further, GPCRs are widely expressed in the brain where their agonists make up a large number of neurotransmitters and neuromodulators. Due to the importance of GPCRs in human physiology, genetically encoded sensors have been engineered to detect GPCR agonists at cellular resolution in vivo. These sensors can be placed into two main categories: those that offer real-time information on the signalling dynamics of GPCR agonists and those that integrate the GPCR agonist signal into a permanent, quantifiable mark that can be used to detect GPCR agonist localisation in a large brain area. In this review, we discuss the various designs of real-time and integration sensors, their advantages and limitations, and some in vivo applications. We also discuss the potential of using real-time and integrator sensors together to identify neuronal circuits affected by endogenous GPCR agonists and perform detailed characterisations of the spatiotemporal dynamics of GPCR agonist release in those circuits. By using these sensors together, the overall knowledge of GPCR-mediated signalling can be expanded.

Emerging approaches for decoding neuropeptide transmission

Neuropeptides produce robust effects on behavior across species, and recent research has benefited from advances in high-resolution techniques to investigate peptidergic transmission and expression throughout the brain in model systems. Neuropeptides exhibit distinct characteristics which includes their post-translational processing, release from dense core vesicles, and ability to activate G-protein-coupled receptors (GPCRs). These complex properties have driven the need for development of specialized tools that can sense neuropeptide expression, cell activity, and release. Current research has focused on isolating when and how neuropeptide transmission occurs, as well as the conditions in which neuropeptides directly mediate physiological and adaptive behavioral states. Here we describe the current technological landscape in which the field is operating to decode key questions regarding these dynamic neuromodulators.

Probing Neuropeptide Volume Transmission In Vivo by Simultaneous Near-Infrared Light-Triggered Release and Optical Sensing

Neuropeptides are abundant signaling molecules in the central nervous system. Yet remarkably little is known about their spatiotemporal spread and biological activity. Here, we developed an integrated optical approach using Plasmonic nAnovesicles and cell-based neurotransmitter fluorescent engineered reporter (CNiFER), or PACE, to probe neuropeptide signaling in the mouse neocortex. Small volumes (fL to pL) of exogenously supplied somatostatin-14 (SST) can be rapidly released under near-infrared light stimulation from nanovesicles implanted in the brain and detected by SST2 CNiFERs with nM sensitivity. Our measurements reveal reduced but synchronized SST transmission within 130 μm, and markedly smaller and delayed transmission at longer distances. These measurements enabled a quantitative estimation of the SST loss rate due to peptide degradation and binding. PACE offers a new tool for determining the spatiotemporal scales of neuropeptide volume transmission and signaling in the brain.

New Technologies to Investigate Neuropeptides at Scale

Neuropeptides are some of the most elusive molecules to monitor in neuroscience. Detecting their release and spread in brain tissue requires the development and use of advanced technologies that enable specific neuropeptide measurements with high spatial and temporal resolution. This Viewpoint highlights some of the emerging tools and techniques that are already advancing our knowledge of neuropeptide physiology and discusses possible future developments.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Challenges and new opportunities for detecting endogenous opioid peptides in reward

The endogenous opioid peptide system, comprised of enkephalins, endorphins, dynorphins, and nociceptin, is a highly complex neurobiological system. Opioid peptides are derived from four precursor molecules and undergo several processing events yielding over 20 unique opioid peptides. This diversity together with low in vivo concentration and complex processing and release dynamics has challenged research into each peptide's unique function. Despite the subsequent challenges in detecting and quantifying opioid peptides in vivo, researchers have pioneered several techniques to directly or indirectly assay the roles of opioid peptides during behavioral manipulations. In this review, we describe the limitations of the traditional techniques used to study the role of endogenous opioid peptides in food and drug reward and bring focus to the wealth of new techniques to measure endogenous opioid peptides in reward processing.

Letting the little light of mind shine: Advances and future directions in neurochemical detection

Synaptic transmission via neurochemical release is the fundamental process that integrates and relays encoded information in the brain to regulate physiological function, cognition, and emotion. To unravel the biochemical, biophysical, and computational mechanisms of signal processing, one needs to precisely measure the neurochemical release dynamics with molecular and cell-type specificity and high resolution. Here we reviewed the development of analytical, electrochemical, and fluorescence imaging approaches to detect neurotransmitter and neuromodulator release. We discussed the advantages and practicality in implementation of each technology for ease-of-use, flexibility for multimodal studies, and challenges for future optimization. We hope this review will provide a versatile guide for tool engineering and applications for recording neurochemical release.

Optimized Sawhorse Waveform for the Measurement of Oxytocin Release in Zebrafish

Oxytocin is a nonapeptide hormone involved in numerous physiological functions. Real-time electrochemical measurements of oxytocin in living tissue are challenging due to electrode fouling and the large potentials needed to oxidize the tyrosine residue. Here, we used fast-scan cyclic voltammetry at carbon-fiber microelectrodes and flow injection analysis to optimize a waveform for the measurement of oxytocin. This optimized waveform employed an accumulation potential of -0.6 V, multiple scan rates, and a 3 ms holding potential at a positive, oxidizing potential of +1.4 V before linearly scanning the potential back to -0.6 V (versus Ag/AgCl). We obtained a limit of quantitation of 0.34 ± 0.02 μM, and our electrodes did not foul upon multiple injections. Moreover, to demonstrate the utility of our method, we measured the release of oxytocin, evoked by light application and mechanical perturbation, in whole brains from genetically engineered adult zebrafish that express channelrhodopsin-2 selectively on oxytocinergic neurons. Collectively, this work expands the toolkit for the measurement of peptides in living tissue preparations.

Current and future techniques for detecting oxytocin: Focusing on genetically-encoded GPCR sensors

Oxytocin is a neuropituitary hormone that is involved in a wide range of psychosocial behaviors. Despite its psychophysiological importance as a neuromodulator in the CNS, effective techniques capable of monitoring oxytocin dynamics or testing related behavioral consequences are limited. Along with an explosive advancement in synthetic biology, high-performance genetically-encoded neuromodulator sensors are being developed. Here we comprehensively review the current methodologies available for detecting oxytocin in neuroscience. Their strengths and weaknesses are discussed, and a graphical summary is plotted for better comparison of techniques. We also suggest future directions for next generation oxytocin sensor development and their working principles.

Modified Sawhorse Waveform for the Voltammetric Detection of Oxytocin

Carbon fiber microelectrodes (CFMEs) have been used to detect neurotransmitters and other biomolecules using fast-scan cyclic voltammetry (FSCV) for the past few decades. This technique measures neurotransmitters such as dopamine and, more recently, physiologically relevant neuropeptides. Oxytocin, a pleiotropic peptide hormone, is physiologically important for adaptation, development, reproduction, and social behavior. This neuropeptide functions as a stress-coping molecule, an anti-inflammatory agent, and serves as an antioxidant with protective effects especially during adversity or trauma. Here, we measure tyrosine using the Modified Sawhorse Waveform (MSW), enabling enhanced electrode sensitivity for the amino acid and oxytocin peptide. Applying the MSW, decreased surface fouling and enabled codetection with other monoamines. As oxytocin contains tyrosine, the MSW was also used to detect oxytocin. The sensitivity of oxytocin detection was found to be 3.99 ± 0.49 nA/μM, (n=5). Additionally, we demonstrate that applying the MSW on CFMEs allows for real time measurements of exogenously applied oxytocin on rat brain slices. These studies may serve as novel assays for oxytocin detection in a fast, sub-second timescale with possible implications for in vivo measurements and further understanding of the physiological role of oxytocin.

Multiplexing neurochemical detection with carbon fiber multielectrode arrays using fast-scan cyclic voltammetry

Carbon fiber microelectrodes (CFMEs) have been extensively used to measure neurotransmitters with fast-scan cyclic voltammetry (FSCV) due to their ability to adsorb cationic monoamine neurotransmitters. Although FSCV, in tandem with CFMEs, provides high temporal and spatial resolution, only single-channel potentiostats and electrodes have been primarily utilized. More recently, the need and use of carbon fiber multielectrode arrays has risen to target multiple brain regions. Previous studies have shown the ability to detect dopamine using multielectrode arrays; however, they are not readily available to the scientific community. In this work, we interfaced a carbon fiber multielectrode array (MEA or multielectrode array), to a commercially available four-channel potentiostat for multiplexing neurochemical measurements. The MEA's relative performance was compared to single CFMEs where dopamine detection was found to be adsorption controlled to the electrode's surface. Multiple waveforms were applied to each fiber of the multielectrode array simultaneously to detect different analytes on each electrode of the array. A proof of concept ex vivo experiment showed that the multielectrode array could record redox activity in different areas within the mouse caudate putamen and detect dopamine in a 3-mm2 area. To our knowledge, this is the first use of the multielectrode array paired with a commercially available multichannel potentiostat for multi-waveform application and neurotransmitter co-detection. This novel array may aid in future studies to better understand complex brain heterogeneity, the dynamic neurochemical environment, and how disease states or drugs affect separate brain areas concurrently. Graphical abstract.

Simultaneous serotonin and dopamine monitoring across timescales by rapid pulse voltammetry with partial least squares regression

Many voltammetry methods have been developed to monitor brain extracellular dopamine levels. Fewer approaches have been successful in detecting serotonin in vivo. No voltammetric techniques are currently available to monitor both neurotransmitters simultaneously across timescales, even though they play integrated roles in modulating behavior. We provide proof-of-concept for rapid pulse voltammetry coupled with partial least squares regression (RPV-PLSR), an approach adapted from multi-electrode systems (i.e., electronic tongues) used to identify multiple components in complex environments. We exploited small differences in analyte redox profiles to select pulse steps for RPV waveforms. Using an intentionally designed pulse strategy combined with custom instrumentation and analysis software, we monitored basal and stimulated levels of dopamine and serotonin. In addition to faradaic currents, capacitive currents were important factors in analyte identification arguing against background subtraction. Compared to fast-scan cyclic voltammetry-principal components regression (FSCV-PCR), RPV-PLSR better differentiated and quantified basal and stimulated dopamine and serotonin associated with striatal recording electrode position, optical stimulation frequency, and serotonin reuptake inhibition. The RPV-PLSR approach can be generalized to other electrochemically active neurotransmitters and provides a feedback pipeline for future optimization of multi-analyte, fit-for-purpose waveforms and machine learning approaches to data analysis.

A genetically encoded sensor with improved fluorescence intensity for opioid detection at cellular resolution

The mu-opioid receptor (MOR) regulates the neuronal pathways involved in pain, reward, and respiration. To increase our understanding of MOR's roles in these pathways, there is a need to detect opioids at cellular resolution. Here, we engineered an improved opioid-sensor, called M-SPOTIT2, which is 11x brighter than our previously engineered M-SPOTIT1.1. We engineered M-SPOTIT2 by adding the amino acids YNSH, located near the fluorophore of the enhanced green fluorescent protein, to the circular permuted green fluorescent protein in M-SPOTIT2. M-SPOTIT2 is 11x brighter than our previously engineered M-SPOTIT1.1 in HEK293T cell culture and 2.7x brighter in neuronal culture. M-SPOTIT2 will potentially be useful for the detection of opioids in cell culture for drug screening and the detection of opioids at cellular resolution in animal tissues. By using M-SPOTIT2, researchers can gain more understanding about the mechanisms of addiction, respiratory suppression, and pain-modulation involved in opioid signaling.

Review-Recent Advances in FSCV Detection of Neurochemicals via Waveform and Carbon Microelectrode Modification

Fast scan cyclic voltammetry (FSCV) is an analytical technique that was first developed over 30 years ago. Since then, it has been extensively used to detect dopamine using carbon fiber microelectrodes (CFMEs). More recently, electrode modifications and waveform refinement have enabled the detection of a wider variety of neurochemicals including nucleosides such as adenosine and guanosine, neurotransmitter metabolites of dopamine, and neuropeptides such as enkephalin. These alterations have facilitated the selectivity of certain biomolecules over others to enhance the measurement of the analyte of interest while excluding interferants. In this review, we detail these modifications and how specializing CFME sensors allows neuro-analytical researchers to develop tools to understand the neurochemistry of the brain in disease states and provide groundwork for translational work in clinical settings.

Opioid Growth Factor and its Derivatives as Potential Non-toxic Multifunctional Anticancer and Analgesic Compounds

Despite significant research progress on the pathogenesis, molecular biology, diagnosis, treatment, and prevention of cancer, its morbidity and mortality are still high around the world. The emerging resistance of cancer cells to anticancer drugs remains still a significant problem in oncology today. Furthermore, an important challenge is the inability of anticancer drugs to selectively target tumor cells thus sparing healthy cells. One of the new potential options for efficient and safe therapy can be provided by opioid growth factor (OGF), chemically termed Met-enkephalin. It is an endogenous pentapeptide (Tyr-Gly-Gly-Phe-Met) with antitumor, analgesic, and immune-boosting properties. Clinical trials have demonstrated that OGF therapy alone, as well as in combination with standard chemotherapies, is a safe, non-toxic anticancer agent that reduces tumor size. In this paper, we review the structure-activity relationship of OGF and its analogues. We highlight also OGF derivatives with analgesic, immunomodulatory activity and the ability to penetrate the blood-brain barrier and may be used as safe agents enhancing chemotherapy efficacy and improving quality of life in cancer patients. The reviewed papers indicate that Met-enkephalin and its analogues are interesting candidates for the development of novel, non-toxic, and endowed with an analgesic activity anticancer drugs. More preclinical and clinical studies are needed to explore these opportunities.

Recent Advances in In Vivo Neurochemical Monitoring

The brain is a complex network that accounts for only 5% of human mass but consumes 20% of our energy. Uncovering the mysteries of the brain's functions in motion, memory, learning, behavior, and mental health remains a hot but challenging topic. Neurochemicals in the brain, such as neurotransmitters, neuromodulators, gliotransmitters, hormones, and metabolism substrates and products, play vital roles in mediating and modulating normal brain function, and their abnormal release or imbalanced concentrations can cause various diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease. A wide range of techniques have been used to probe the concentrations of neurochemicals under normal, stimulated, diseased, and drug-induced conditions in order to understand the neurochemistry of drug mechanisms and develop diagnostic tools or therapies. Recent advancements in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. However, direct in vivo measurements require a robust sensor that is highly sensitive and selective with minimal fouling and reduced inflammatory foreign body responses. Here, we review recent advances in neurochemical sensor development for in vivo studies, with a focus on electrochemical and optical probes. Other alternative methods are also compared. We discuss in detail the in vivo challenges for these methods and provide an outlook for future directions.

Detection of neuropeptides in vivo and open questions for current and upcoming fluorescent sensors for neuropeptides

During a stress response, various neuropeptides are secreted in a spatiotemporally coordinated way in the brain. For a precise understanding of peptide functions in a stress response, it is important to investigate when and where they are released, how they diffuse, and how they are broken down in the brain. In the past two decades, genetically encoded fluorescent calcium indicators have greatly advanced our knowledge of the functions of specific neuronal activity in regulation of behavioral changes and physiological responses during stress. In addition, various kinds of structural information on G-protein-coupled receptors (GPCRs) for neuropeptides have been revealed. Recently, genetically encoded fluorescent sensors have been developed for detection of neurotransmitters by making use of conformational changes induced by ligand binding. In this review, we summarize the recent and upcoming advances of techniques for detection of neuropeptides and then present several open questions that will be solved by application of recent or upcoming technical advances in detection of neuropeptides in vivo.

Endogenous Opioids at the Intersection of Opioid Addiction, Pain, and Depression: The Search for a Precision Medicine Approach

Opioid addiction and overdose are at record levels in the United States. This is driven, in part, by their widespread prescription for the treatment of pain, which also increased opportunity for diversion by sensation-seeking users. Despite considerable research on the neurobiology of addiction, treatment options for opioid abuse remain limited. Mood disorders, particularly depression, are often comorbid with both pain disorders and opioid abuse. The endogenous opioid system, a complex neuromodulatory system, sits at the neurobiological convergence point of these three comorbid disease states. We review evidence for dysregulation of the endogenous opioid system as a mechanism for the development of opioid addiction and/or mood disorder. Specifically, individual differences in opioid system function may underlie differences in vulnerability to opioid addiction and mood disorders. We also review novel research, which promises to provide more detailed understanding of individual differences in endogenous opioid neurobiology and its contribution to opioid addiction susceptibility.

Five Decades of Research on Opioid Peptides: Current Knowledge and Unanswered Questions

In the mid-1970s, an intense race to identify endogenous substances that activated the same receptors as opiates resulted in the identification of the first endogenous opioid peptides. Since then, >20 peptides with opioid receptor activity have been discovered, all of which are generated from three precursors, proenkephalin, prodynorphin, and proopiomelanocortin, by sequential proteolytic processing by prohormone convertases and carboxypeptidase E. Each of these peptides binds to all three of the opioid receptor types (μ, δ, or κ), albeit with differing affinities. Peptides derived from proenkephalin and prodynorphin are broadly distributed in the brain, and mRNA encoding all three precursors are highly expressed in some peripheral tissues. Various approaches have been used to explore the functions of the opioid peptides in specific behaviors and brain circuits. These methods include directly administering the peptides ex vivo (i.e., to excised tissue) or in vivo (in animals), using antagonists of opioid receptors to infer endogenous peptide activity, and genetic knockout of opioid peptide precursors. Collectively, these studies add to our current understanding of the function of endogenous opioids, especially when similar results are found using different approaches. We briefly review the history of identification of opioid peptides, highlight the major findings, address several myths that are widely accepted but not supported by recent data, and discuss unanswered questions and future directions for research. SIGNIFICANCE STATEMENT: Activation of the opioid receptors by opiates and synthetic drugs leads to central and peripheral biological effects, including analgesia and respiratory depression, but these may not be the primary functions of the endogenous opioid peptides. Instead, the opioid peptides play complex and overlapping roles in a variety of systems, including reward pathways, and an important direction for research is the delineation of the role of individual peptides.

Interpreting Dynamic Interfacial Changes at Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy

Carbon-fiber microelectrodes are instrumental tools in neuroscience used for the electroanalysis of neurochemical dynamics and recordings of neural activity. However, performance is variable and dependent on fabrication strategies, the biological response to implantation, and the physical and chemical composition of the recording environment. This presents an analytical challenge, as electrode performance is difficult to quantitatively assess in situ, especially when electrodes are permanently implanted or cemented in place. We previously reported that electrode impedance directly impacts electrochemical performance for molecular sensing. In this work, we investigate the impacts of individual components of the electrochemical system on impedance. Equivalent circuit models for glass- and silica-insulated carbon-fiber microelectrodes were determined using electrochemical impedance spectroscopy (EIS). The models were validated based on the ability to assign individual circuit elements to physical properties of the electrochemical system. Investigations were performed to evaluate the utility of the models in providing feedback on how changes in ionic strength and carbon fiber material alter impedance properties. Finally, EIS measurements were used to investigate the electrode/solution interface prior to, during, and following implantation in live brain tissue. A significant increase in impedance and decrease in capacitance occur during tissue exposure and persist following implantation. Electrochemical conditioning, which occurs continually during fast-scan cyclic voltammetry recordings, etches and renews the carbon surface, mitigating these effects. Overall, the results establish EIS as a powerful method for characterization of carbon-fiber microelectrodes, providing unprecedented insight into how real-world factors affect the electrode/solution interface.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

A microfluidic electrochemical flow cell capable of rapid on-chip dilution for fast-scan cyclic voltammetry electrode calibration

Here, we developed a microfluidic electrochemical flow cell for fast-scan cyclic voltammetry which is capable of rapid on-chip dilution for efficient and cost-effective electrode calibration. Fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes is a robust electroanalytical technique used to measure subsecond changes in neurotransmitter concentration over time. Traditional methods of electrode calibration for FSCV require several milliliters of a standard. Additionally, generating calibration curves can be time-consuming because separate solutions must be prepared for each concentration. Microfluidic electrochemical flow cells have been developed in the past; however, they often require incorporating the electrode in the device, making it difficult to remove for testing in biological tissues. Likewise, current microfluidic electrochemical flow cells are not capable of rapid on-chip dilution to eliminate the requirement of making multiple solutions. We designed a T-channel device, with microchannel dimensions of 100 μm × 50 μm, that delivered a standard to a 2-mm-diameter open electrode sampling well. A waste channel with the same dimensions was designed perpendicular to the well to flush and remove the standard. The dimensions of the T-microchannels and flow rates were chosen to facilitate complete mixing in the delivery channel prior to reaching the electrode. The degree of mixing was computationally modeled using COMSOL and was quantitatively assessed in the device using both colored dyes and electrochemical detection. On-chip electrode calibration for dopamine with FSCV was not significantly different than the traditional calibration method demonstrating its utility for FSCV calibration. Overall, this device improves the efficiency and ease of electrode calibration. Graphical abstract.

Fundamentals of fast-scan cyclic voltammetry for dopamine detection

Fast-scan cyclic voltammetry (FSCV) is used with carbon-fiber microelectrodes for the real-time detection of neurotransmitters on the subsecond time scale. With FSCV, the potential is ramped up from a holding potential to a switching potential and back, usually at a 400 V s-1 scan rate and a frequency of 10 Hz. The plot of current vs. applied potential, the cyclic voltammogram (CV), has a very different shape for FSCV than for traditional cyclic voltammetry collected at scan rates which are 1000-fold slower. Here, we explore the theory of FSCV, with a focus on dopamine detection. First, we examine the shape of the CVs. Background currents, which are 100-fold higher than faradaic currents, are subtracted out. Peak separation is primarily due to slow electron transfer kinetics, while the symmetrical peak shape is due to exhaustive electrolysis of all the adsorbed neurotransmitters. Second, we explain the origins of the dopamine waveform, and the factors that limit the holding potential (oxygen reduction), switching potential (water oxidation), scan rate (electrode instability), and repetition rate (adsorption). Third, we discuss data analysis, from data visualization with color plots, to the automated algorithms like principal components regression that distinguish dopamine from pH changes. Finally, newer applications are discussed, including optimization of waveforms for analyte selectivity, carbon nanomaterial electrodes that trap dopamine, and basal level measurements that facilitate neurotransmitter measurements on a longer time scale. FSCV theory is complex, but understanding it enables better development of new techniques to monitor neurotransmitters in vivo.

Recent advances in fast-scan cyclic voltammetry

Fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes (CFMEs) is a versatile electrochemical technique to probe neurochemical dynamics in vivo. Progress in FSCV methodology continues to address analytical challenges arising from biological needs to measure low concentrations of neurotransmitters at specific sites. This review summarizes recent advances in FSCV method development in three areas: (1) waveform optimization, (2) electrode development, and (3) data analysis. First, FSCV waveform parameters such as holding potential, switching potential, and scan rate have been optimized to monitor new neurochemicals. The new waveform shapes introduce better selectivity toward specific molecules such as serotonin, histamine, hydrogen peroxide, octopamine, adenosine, guanosine, and neuropeptides. Second, CFMEs have been modified with nanomaterials such as carbon nanotubes or replaced with conducting polymers to enhance sensitivity, selectivity, and antifouling properties. Different geometries can be obtained by 3D-printing, manufacturing arrays, or fabricating carbon nanopipettes. Third, data analysis is important to sort through the thousands of CVs obtained. Recent developments in data analysis include preprocessing by digital filtering, principal components analysis for distinguishing analytes, and developing automated algorithms to detect peaks. Future challenges include multisite measurements, machine learning, and integration with other techniques. Advances in FSCV will accelerate research in neurochemistry to answer new biological questions about dynamics of signaling in the brain.

Real-time in vivo detection techniques for neurotransmitters: a review

Functional synapses in the central nervous system depend on a chemical signal exchange process that involves neurotransmitter delivery between neurons and receptor cells in the neuro system.