Real-Time Measurement of Stimulated Dopamine Release in Compartments of the Adult Drosophila melanogaster Mushroom Body

Electrochemistry at the Edge of a van der Waals Heterostructure

Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V s-1), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.

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.

Perspective-Assessing Electrochemical, Aptamer-Based Sensors for Dynamic Monitoring of Cellular Signaling

Electrochemical, aptamer-based (E-AB) sensors provide a generalizable strategy to quantitatively detect a variety of targets including small molecules and proteins. The key signaling attributes of E-AB sensors (sensitivity, selectivity, specificity, and reagentless and dynamic sensing ability) make them well suited to monitor dynamic processes in complex environments. A key bioanalytical challenge that could benefit from the detection capabilities of E-AB sensors is that of cell signaling, which involves the release of molecular messengers into the extracellular space. Here, we provide a perspective on why E-AB sensors are suited for this measurement, sensor requirements, and pioneering examples of cellular signaling measurements.

Parkin Knockdown Modulates Dopamine Release in the Central Complex, but Not the Mushroom Body Heel, of Aging Drosophila

Parkinson's disease (PD) is characterized by progressive degeneration of dopaminergic neurons leading to reduced locomotion. Mutations of parkin gene in Drosophila produce the same phenotypes as vertebrate models, but the effect of parkin knockdown on dopamine release is not known. Here, we report age-dependent, spatial variation of dopamine release in the brain of parkin-RNAi adult Drosophila. Dopamine was repetitively stimulated by local application of acetylcholine and quantified by fast-scan cyclic voltammetry in the central complex or mushroom body heel. In the central complex, the main area controlling locomotor function, dopamine release is maintained for repeated stimulations in aged control flies, but lower concentrations of dopamine are released in the central complex of aged parkin-RNAi flies. In the mushroom body heel, the dopamine release decrease in older parkin-RNAi flies is similar to controls. There is not significant dopaminergic neuronal loss even in older parkin knockdown flies, which indicates that the changes in stimulated dopamine release are due to alterations of neuronal function. In young parkin-RNAi flies, locomotion is inhibited by 30%, while in older parkin-RNAi flies it is inhibited by 85%. Overall, stimulated dopamine release is modulated by parkin in an age and brain region dependent manner. Correlating the functional state of the dopaminergic system with behavioral phenotypes provides unique insights into the PD mechanism. Drosophila can be used to study dopamine functionality in PD, elucidate how genetics influence dopamine, and test potential therapies to maintain dopamine release.

Delineating Purinergic Signaling in Drosophila

Simplistic models can aid in discovering what is important in the context of normal and pathological behavior. First recognized as a genetic model more than 100 years ago, to date, fruit flies (Drosophila melanogaster) still remain an astonishingly good laboratory stand-in for scientists to study development and physiology and to investigate the molecular mechanisms of human diseases. This is because fruit flies indeed represent a simplistic model. Furthermore, about 75% of human disease-related genes have their counterparts in the Drosophila genome, added to the fact that fruit flies are inexpensive and extremely easy to maintain, being invertebrates and, moreover, lacking any ethical concern issues. Purinergic signaling is, by definition, mediated by extracellular purinergic ligands, among which ATP represents the prototype molecule. A key feature that has progressively emerged when dissecting the purinergic mechanisms is the multilayer and dynamic nature of the signaling sustained by purinergic ligands. Indeed, these last are sequentially metabolized by several different ectonucleotidases, which generate the ligands that simultaneously activate several different purinergic receptors. Since significant purinergic actions have also been described in Drosophila, the aim of the present work is to provide a comprehensive picture of the purinergic events occurring in fruit flies.

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.

SSRI antidepressants differentially modulate serotonin reuptake and release in Drosophila

Selective serotonin reuptake inhibitor (SSRI) antidepressants are commonly prescribed treatments for depression, but their effects on serotonin reuptake and release are not well understood. Drosophila melanogaster, the fruit fly, expresses the serotonin transporter (dSERT), the major target of SSRIs, but real-time serotonin changes after SSRIs have not been characterized in this model. The goal of this study was to characterize effects of SSRIs on serotonin concentration and reuptake in Drosophila larvae. We applied various doses (0.1-100 μM) of fluoxetine (Prozac), escitalopram (Lexapro), citalopram (Celexa), and paroxetine (Paxil), to ventral nerve cord (VNC) tissue and measured optogenetically-stimulated serotonin release with fast-scan cyclic voltammetry (FSCV). Fluoxetine increased reuptake from 1 to 100 μM, but serotonin concentration only increased at 100 μM. Thus, fluoxetine occupies dSERT and slows clearance but does not affect concentration. Escitalopram and paroxetine increased serotonin concentrations at all doses, but escitalopram increased reuptake more. Citalopram showed lower concentration changes and faster reuptake profiles compared with escitalopram, so the racemic mixture of citalopram does not change reuptake as much as the S-isomer. Dose response curves were constructed to compare dSERT affinities and paroxetine showed the highest affinity and fluoxetine the lowest. These data demonstrate SSRI mechanisms are complex, with separate effects on reuptake or release. Furthermore, dynamic serotonin changes in Drosophila are similar to previous studies in mammals. This work establishes how antidepressants affect serotonin in real-time, which is useful for future studies that will investigate pharmacological effects of SSRIs with different genetic mutations in Drosophila.

Temporally and Spatially Localized PKA Activity within Learning and Memory Circuitry Regulated by Network Feedback

Dynamic functional connectivity within brain circuits requires coordination of intercellular signaling and intracellular signal transduction. Critical roles for cAMP-dependent protein kinase A (PKA) signaling are well established in the Drosophila mushroom body (MB) learning and memory circuitry, but local PKA activity within this well-mapped neuronal network is uncharacterized. Here, we use an in vivo PKA activity sensor (PKA-SPARK) to test spatiotemporal regulatory requirements in the MB axon lobes. We find immature animals have little detectable PKA activity, whereas postcritical period adults show high field-selective activation primarily in just 3/16 defined output regions. In addition to the age-dependent PKA activity in distinct α'/β' lobe nodes, females show sex-dependent elevation compared with males in these same restricted regions. Loss of neural cell body Fragile X mental retardation protein (FMRP) and Rugose [human Neurobeachin (NBEA)] suppresses localized PKA activity, whereas overexpression (OE) of MB lobe PKA-synergist Meng-Po (human SBK1) promotes PKA activity. Elevated Meng-Po subverts the PKA age-dependence, with elevated activity in immature animals, and spatial-restriction, with striking γ lobe activity. Testing circuit signaling requirements with temperature-sensitive shibire (human Dynamin) blockade, we find broadly expanded PKA activity within the MB lobes. Using transgenic tetanus toxin to block MB synaptic output, we find greatly heightened PKA activity in virtually all MB lobe fields, although the age-dependence is maintained. We conclude spatiotemporally restricted PKA activity signaling within this well-mapped learning/memory circuit is age-dependent and sex-dependent, driven by FMRP-Rugose pathway activation, temporally promoted by Meng-Po kinase function, and restricted by output neurotransmission providing network feedback.