Fast-Scan Cyclic Voltammetry (FSCV) Reveals Behaviorally Evoked Dopamine Release by Sugar Feeding in the Adult Drosophila Mushroom Body

Tailoring the Electrocatalytic Properties of Porphyrin Covalent Organic Frameworks for Highly Selective Oxygen Sensing In Vivo

In vivo selective sensing of oxygen (O2) dynamics in the central nervous system could provide insights into energy metabolism and neural activities. Although the electrocatalytic four-electron oxygen reduction reaction (ORR) paves an effective way to the electrochemical sensing of O2 in vivo, the concurrent hydrogen peroxide reduction reaction (HPRR) within the potential windows for four-electron ORR unfortunately poses a great challenge to the conventional mechanism employed for selective electrochemical O2 sensing. In this work, we find that regulation of the linkers within the skeleton of porphyrin-based covalent organic frameworks (COFs) could improve the selectivity of the O2 sensor against hydrogen peroxide (H2O2). The electrochemical results reveal that the Co porphyrin active sites facilitate the direct four-electron pathway for ORR and that the Co porphyrin-based COF, enriched with pyrene units, shows enhanced four-electron ORR kinetics and better tolerance to HPRR. The theoretical calculation suggests that introducing pyrene units essentially weakens the adsorption of H2O2, leading to suppression of the HPRR. The microsensor fabricated with the Co porphyrin-based COF as the electrocatalyst features a high selectivity for real-time monitoring of O2 in a living rat brain.

Micro-invasive probes for monitoring electrical and chemical neural activity in nonhuman primates

We leveraged carbon fiber materials for creating sensors that provide dual neurochemical and electrical neural activity recording at micro-invasive (10 μm) spatial footprints proximal to recording sites, and enabling these measurements from deep brain targets of primates with conventional cranial chambers. These shaft-assisted micro-invasive probes (s-μIPs) are approximately 10 μm in diameter along the distal length (1 - 15 mm) immediately surrounding the targeted recording site. This micro-invasive portion ensures that the recording site is isolated from tissue damage induced by the wider shaft portion of the device. The shaft (150 - 165 μm in diameter) within the device stiffens the remaining length of the probe (> 100 mm), and provides compatibility with standard intracranial insertion protocols (e.g., guide tubes and chamber setups) that require a sufficiently rigid and long shaft for deep brain insertion in monkeys. The s-μIP was further expanded to provide dual-channel chemical and electrical neural activity recording with micrometer spatial resolution. Measurements of reward- and movement- related dopamine, spikes, and local field potentials were made from single and dual-channel s-μIPs implanted in task-performing monkeys. Recordings from chronically implanted s-μIPs display the capability of functional multi-modal (chemical and electrical) neural activity measurements over 1-year post-implantation from micro-invasive devices.

Synaptic enrichment and dynamic regulation of the two opposing dopamine receptors within the same neurons

Dopamine can play opposing physiological roles depending on the receptor subtype. In the fruit fly Drosophila melanogaster, Dop1R1 and Dop2R encode the D1- and D2-like receptors, respectively, and are reported to oppositely regulate intracellular cAMP levels. Here, we profiled the expression and subcellular localization of endogenous Dop1R1 and Dop2R in specific cell types in the mushroom body circuit. For cell-type-specific visualization of endogenous proteins, we employed reconstitution of split-GFP tagged to the receptor proteins. We detected dopamine receptors at both presynaptic and postsynaptic sites in multiple cell types. Quantitative analysis revealed enrichment of both receptors at the presynaptic sites, with Dop2R showing a greater degree of localization than Dop1R1. The presynaptic localization of Dop1R1 and Dop2R in dopamine neurons suggests dual feedback regulation as autoreceptors. Furthermore, we discovered a starvation-dependent, bidirectional modulation of the presynaptic receptor expression in the protocerebral anterior medial (PAM) and posterior lateral 1 (PPL1) clusters, two distinct subsets of dopamine neurons, suggesting their roles in regulating appetitive behaviors. Our results highlight the significance of the co-expression of the two opposing dopamine receptors in the spatial and conditional regulation of dopamine responses in neurons.

In Vivo Electrochemical Biosensing Technologies for Neurochemicals: Recent Advances in Electrochemical Sensors and Devices

In vivo electrochemical sensing of neurotransmitters, neuromodulators, and metabolites plays a critical role in real-time monitoring of various physiological or psychological processes in the central nervous system. Currently, advanced electrochemical biosensors and technologies have been emerging as prominent ways to meet the surging requirements of in vivo monitoring of neurotransmitters and neuromodulators ranging from single cells to brain slices, even the entire brain. This review introduces the fundamental working principles and summarizes the achievements of in vivo electrochemical biosensing technologies including voltammetry, amperometry, potentiometry, field-effect transistor (FET), and organic electrochemical transistor (OECT). According to the elaborate feature of sensing technology, versatile strategies have been devoted to solve critical issues associated with the sensing of neurochemicals under an intricate physiological environment. Voltammetry is a universal technique to investigate electrochemical processes in complex matrices which could realize the miniaturization of electrodes, while amperometry serves as a well-suited approach offering high temporal resolution which is favorable for the fast oxidation-reduction kinetics of neurochemicals. Potentiometry realizes quantitative analysis by recording the potential difference with reduced invasiveness and high compatibility. FET and OECT serve as amplification strategies with higher sensitivity than traditional technologies. Furthermore, we point out the current shortcomings and address the challenges and perspectives of in vivo electrochemical biosensing technologies.

Utilizing a Disposable Sensor with Polyaniline-Doped Multi-Walled Carbon Nanotubes to Enable Dopamine Detection in Ex Vivo Mouse Brain Tissue Homogenates

Disposable sensors are inexpensive, user-friendly sensing tools designed for rapid single-point measurements of a target. Disposable sensors have become more and more essential as diagnostic tools due to the growing demand for quick, easy-to-access, and reliable information related to the target. Dopamine (DA), a prevalent catecholamine neurotransmitter in the human brain, is associated with central nervous system activities and directly promotes neuronal communication. For the sensitive and selective estimation of DA, an enzyme-free amperometric sensor based on polyaniline-doped multi-walled carbon nanotubes (PANI-MWCNTs) drop-coated disposable screen-printed carbon electrodes (SPCEs) was fabricated. This PANI-MWCNTs-2/SPCE sensor boasts exceptional accuracy and sensitivity when working directly with ex vivo mouse brain homogenates. The sensor exhibited a detection limit of 0.05 μM (S/N = 3), and a wide linear range from 1.0 to 200 μM. The sensor's high selectivity to DA amidst other endogenous interferents was recognized. Since the constructed sensor is enzyme-free yet biocompatible, it exhibited high stability in DA detection using ex vivo mouse brain homogenates extracted from both Parkinson's disease and control mice models. This research thus presents new insights into understanding DA release dynamics at the tissue level in both of these models.

Electrochemical and biosensor techniques to monitor neurotransmitter changes with depression

Depression is a common mental illness. However, its current treatments, like selective serotonin reuptake inhibitors (SSRIs) and micro-dosing ketamine, are extremely variable between patients and not well understood. Three neurotransmitters: serotonin, histamine, and glutamate, have been proposed to be key mediators of depression. This review focuses on analytical methods to quantify these neurotransmitters to better understand neurological mechanisms of depression and how they are altered during treatment. To quantitatively measure serotonin and histamine, electrochemical techniques such as chronoamperometry and fast-scan cyclic voltammetry (FSCV) have been improved to study how specific molecular targets, like transporters and receptors, change with antidepressants and inflammation. Specifically, these studies show that different SSRIs have unique effects on serotonin reuptake and release. Histamine is normally elevated during stress, and a new inflammation hypothesis of depression links histamine and cytokine release. Electrochemical measurements revealed that stress increases histamine, decreases serotonin, and leads to changes in cytokines, like interleukin-6. Biosensors can also measure non-electroactive neurotransmitters, including glutamate and cytokines. In particular, new genetic sensors have shown how glutamate changes with chronic stress, as well as with ketamine treatment. These techniques have been used to characterize how ketamine changes glutamate and serotonin, and to understand how it is different from SSRIs. This review briefly outlines how these electrochemical techniques work, but primarily highlights how they have been used to understand the mechanisms of depression. Future studies should explore multiplexing techniques and personalized medicine using biomarkers in order to investigate multi-analyte changes to antidepressants.

A Potentiometric Dual-Channel Microsensor Reveals that Fluctuation of H2 S is Less pH-Dependent During Spreading Depolarization in the Rat Brain

Spreading depolarization (SD) is one of the most common neuropathologic phenomena in the nervous system, relating to numerous diseases. However, real-time monitoring the rapid chemical changes during SD to probe the molecular mechanism remains a great challenge. We develop a potentiometric dual-channel microsensor for simultaneous monitoring of H2 S and pH featuring excellent selectivity and spatiotemporal resolution. Using this microsensor we first observe real time changes of H2 S and pH in the rat brain induced by SD. This changes of H2 S are completely suppressed when the rat pre-treats with aminooxyacetic acid (AOAA), a blocker to inhibit the H2 S-producing enzyme, indicating H2 S fluctuation might be related to enzyme-dependent pathway during SD and less pH-dependent. This study provides a new perspective for studying the function of H2 S and the molecular basis of SD-associated diseases.

Enzymatic Galvanic Redox Potentiometry for In Vivo Biosensing

Redox potentiometry has emerged as a new platform for in vivo sensing, with improved neuronal compatibility and strong tolerance against sensitivity variation caused by protein fouling. Although enzymes show great possibilities in the fabrication of selective redox potentiometry, the fabrication of an enzyme electrode to output open-circuit voltage (EOC) with fast response remains challenging. Herein, we report a concept of novel enzymatic galvanic redox potentiometry (GRP) with improved time response coupling the merits of the high selectivity of enzyme electrodes with the excellent biocompatibility and reliability of GRP sensors. With a glucose biosensor as an illustration, we use flavin adenine dinucleotide-dependent glucose dehydrogenase as the recognition element and carbon black as the potential relay station to improve the response time. We find that the enzymatic GRP biosensor rapidly responds to glucose with a good linear relationship between EOC and the logarithm of glucose concentration within a range from 100 μM to 2.65 mM. The GRP biosensor shows high selectivity over O2 and coexisting neurochemicals, good reversibility, and sensitivity and can in vivo monitor glucose dynamics in rat brain. We believe that this study will pave a new platform for the in vivo potentiometric biosensing of chemical events with high reliability.

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Challenges and strategies faced in the electrochemical biosensing analysis of neurochemicals in vivo: A review

Our brain is an intricate neuromodulatory network, and various neurochemicals, including neurotransmitters, neuromodulators, gases, ions, and energy metabolites, play important roles in regulating normal brain function. Abnormal release or imbalance of these substances will lead to various diseases such as Parkinson's and Alzheimer's diseases, therefore, in situ and real-time analysis of neurochemical interactions in pathophysiological conditions is beneficial to facilitate our understanding of brain function. Implantable electrochemical biosensors are capable of monitoring neurochemical signals in real time in extracellular fluid of specific brain regions because they can provide excellent temporal and spatial resolution. However, in vivo electrochemical biosensing analysis mainly faces the following challenges: First, foreign body reactions induced by microelectrode implantation, non-specific adsorption of proteins and redox products, and aggregation of glial cells, which will cause irreversible degradation of performance such as stability and sensitivity of the microsensor and eventually lead to signal loss; Second, various neurochemicals coexist in the complex brain environment, and electroactive substances with similar formal potentials interfere with each other. Therefore, it is a great challenge to design recognition molecules and tailor functional surfaces to develop in vivo electrochemical biosensors with high selectivity. Here, we take the above challenges as a starting point and detail the basic design principles for improving in vivo stability, selectivity and sensitivity of microsensors through some specific functionalized surface strategies as case studies. At the same time, we summarize surface modification strategies for in vivo electrochemical biosensing analysis of some important neurochemicals for researchers' reference. In addition, we also focus on the electrochemical detection of low basal concentrations of neurochemicals in vivo via amperometric waveform techniques, as well as the stability and biocompatibility of reference electrodes during long-term sensing, and provide an outlook on the future direction of in vivo electrochemical neurosensing.

Scalable, flexible carbon fiber electrode thread arrays for three-dimensional probing of neurochemical activity in deep brain structures of rodents

We developed a flexible "electrode-thread" array for recording dopamine neurochemicals from a lateral distribution of subcortical targets (up to 16) transverse to the axis of insertion. Ultrathin (∼10 μm diameter) carbon fiber (CF) electrode-threads (CFETs) are clustered into a tight bundle to introduce them into the brain from a single-entry point. The individual CFETs splay laterally in deep brain tissue during insertion due to their innate flexibility. This spatial redistribution allows navigation of the CFETs towards deep brain targets spreading horizontally from the axis of insertion. Commercial "linear" arrays provide single-entry insertion but only allow measurements along the axis of insertion. Horizontally configured arrays inflict separate penetrations for each individual channel. We tested functional performance of our CFET arrays in vivo for recording dopamine and for providing lateral spread to multiple distributed sites in the rat striatum. Spatial spread was further characterized in agar brain phantoms as a function of insertion depth. We also developed protocols to slice the embedded CFETs within fixed brain tissue using standard histology. This method allowed extraction of the precise spatial coordinates of the implanted CFETs and their recording sites as integrated with immunohistochemical staining for surrounding anatomical, cytological, and protein expression labels. Our CFET array has the potential to unlock a wide range of applications, from uncovering the role of neuromodulators in synaptic plasticity, to addressing critical safety barriers in clinical translation towards diagnostic and adaptive treatment in Parkinson's disease and major mood disorders.

Fast-Scan Cyclic Voltammetry (FSCV) Reveals Behaviorally Evoked Dopamine Release by Sugar Feeding in the Adult Drosophila Mushroom Body

Drosophila melanogaster, the fruit fly, is an excellent model organism for studying dopaminergic mechanisms and simple behaviors, but methods to measure dopamine during behavior are needed. Here, we developed fast-scan cyclic voltammetry (FSCV) to track in vivo dopamine during sugar feeding. First, we employed acetylcholine stimulation to evaluate the feasibility of in vivo measurements in an awake fly. Next, we tested sugar feeding by placing sucrose solution near the fly proboscis. In the mushroom body medial tip, 1 pmol acetylcholine and sugar feeding released 0.49±0.04 μM and 0.31±0.06 μM dopamine, respectively but sugar-evoked release lasted longer than with acetylcholine. Administering the dopamine transporter inhibitor nisoxetine or D2 receptor antagonist flupentixol significantly increased sugar-evoked dopamine. This study develops FSCV to measure behaviorally evoked release in fly, enabling Drosophila studies of neurochemical control of reward, learning, and memory behaviors.