Electrochemistry at the Synapse

Abstract

Electrochemical measurements of neurotransmitters provide insight into the dynamics of neurotransmission. In this review, we describe the development of electrochemical measurements of neurotransmitters and how they started with extrasynaptic measurements but now are pushing toward synaptic measurements. Traditionally, biosensors or fast-scan cyclic voltammetry have monitored extrasynaptic levels of neurotransmitters, such as dopamine, serotonin, adenosine, glutamate, and acetylcholine. Amperometry and electrochemical cytometry techniques have revealed mechanisms of exocytosis, suggesting partial release. Advances in nanoelectrodes now allow spatially resolved, electrochemical measurements in a synapse, which is only 20-100 nm wide. Synaptic measurements of dopamine and acetylcholine have been made. In this article, electrochemical measurements are also compared to optical imaging and mass spectrometry measurements, and while these other techniques provide enhanced spatial or chemical information, electrochemistry is best at monitoring real-time neurotransmission. Future challenges include combining electrochemistry with these other techniques in order to facilitate multisite and multianalyte monitoring.

Keywords: amperometry; dopamine; glutamate; microelectrode; nanoelectrodes; voltammetry.

Figure 1

Schematics of neurotransmission. (a) Direct synaptic transmission. The neurotransmitter is released from the vesicle into the synaptic cleft and interacts with the receptor on the postsynaptic terminal. (b) Volume transmission. Two mechanisms are pictured: synaptic spillover (green) and extrasynaptic exocytosis (blue). Here, neurotransmitters diffuse and act at a more distant neuron not forming a synapse.

Figure 2

Chronic and multimodal carbon-fiber microelectrodes. A schematic of (a) the chronic microelectrode, which is made of a carbon fiber encased in a polyimide-fused silica. (b) FSCV data of food-evoked dopamine release measured at the chronic microelectrode. Panels a, b adapted with permission from Reference 33. Copyright 2010, Springer Nature. (c) Multimodal microelectrode with iontophoresis barrels. (d) FSCV data and histogram of firing rate (insert) of cue-evoked dopamine transient during ICSS. Panels c, d adapted with permission from Reference 37. Copyright 2016, CCC Republication. Abbreviations: FSCV, fast-scan cyclic voltammetry; ICSS, intracranial self-stimulation.

Figure 3

Nanoelectrode amperometric monitoring of neurotransmitter release inside a SCG–SMC synapse. (a) SEM of a carbon-fiber nanoelectrode. (b) Brightfield image of the nanoelectrode inserted inside a synapse between a varicosity of an SCG neuron and an SMC. (c) Schematics of in vivo-like neuromuscular junction in a microfluidic device. (d) Schematics of a CFNE inside a synapse and a glass nanopipette inside an SMC. Figure adapted with permission from Reference 101. Copyright 2015, John Wiley and Sons. Abbreviations: CFNE, carbon-fiber nanoelectrode; SCG, superior cervical ganglion; SEM, scanning electron microelectrode; SMC, smooth muscle cell.

Figure 4

Comparisons of electrochemistry to other techniques. A. Electrochemistry. a. FSCV is typically used in vivo to measure neurochemical dynamics, such as measurements here of adenosine during ischemia. This method has 100 millisecond response times and the color plots allow different molecules to be distinguished. Figure adapted with permission from Reference 64. Copyright 2018, the authors (Venton group). b. FSCAV is used for basal measurements, and the waveform is turned off for 10 s to allow DA to adsorb to the electrode. The CV of the first few scans after the waveform is resumed give information about the ambient levels. This method has ~10 s time resolution and can distinguish between molecules with the voltammogram. Figure adapted with permission from Reference 51. Copyright 2016. Royal Society of Chemistry. B. Mass spectrometry. Mass spectrometry experiments are typically performed in brain slices, not in vivo, and require surface preparation, such as nanoparticles. Here, three types of mass spectrometry are compared a. gas cluster ion beam secondary ion mass spectrometry (GCIB SIMS), b. matrix-assisted laser desorption ionization (MALDI) and c. nanoparticle laser desorption ionization (NP-LDI) and they give different lipid profiles. Mass spectrometry data is rich in chemical information but there is no data for changes over time. Figure adapted with permission from Reference 118. Copyright 2016, Springer Nature. C. Imaging. a. GPCR-based sensors are expressed in different neurons and b. fluoresce when the target molecule is present. Here, DA1m fluoresces in the red and DA1h fluoresces blue, although the camera is not in color, so all fluorescence appears green. c. The time response of these sensors is fast, on the order of 100 ms for the rise time and 1–2 s for the decay time. These sensors are specific to the target molecule but pharmacology experiments are more difficult than with electrochemistry. Reference adapted with permission from Reference 111. Copyright 2018, Elsevier.