Neuronal communication relies on vesicular neurotransmitter release from signaling neurons and detection of these molecules by neighboring neurons. Glutamate, the main excitatory neurotransmitter in the mammalian brain, is involved in nearly all brain functions. However, glutamate has suffered from detection schemes that lack temporal and spatial resolution allowed by electrochemistry. Here we show an amperometric, novel, ultrafast enzyme-based nanoparticle modified sensor, measuring random bursts of hundreds to thousands of rapid spontaneous glutamate exocytotic release events at approximately 30 Hz frequency in the nucleus accumbens of rodent brain slices. Characterizing these single submillisecond exocytosis events revealed a great diversity in spike shape characteristics and size of quantal release, suggesting variability in fusion pore dynamics controlling the glutamate release by cells in this brain region. Hence, this novel biosensor allows recording of rapid single glutamate exocytosis events in the brain tissue and offers insight on regulatory aspects of exocytotic glutamate release, which is critical to understanding of brain glutamate function and dysfunction.

Acetylcholine is a highly abundant nonelectroactive neurotransmitter in the mammalian central nervous system. Neurochemical release occurs on the millisecond time scale, requiring a fast, sensitive sensor such as an enzymatic amperometric electrode. Typically, the enzyme used for enzymatic electrochemical sensors is applied in excess to maximize signal. Here, in addition to sensitivity, we have also sought to maximize temporal resolution, by designing a sensor that is sensitive enough to work at near monolayer enzyme coverage. Reducing the enzyme layer thickness increases sensor temporal resolution by decreasing the distance and reducing the diffusion time for the enzyme product to travel to the sensor surface for detection. In this instance, the sensor consists of electrodeposited gold nanoparticle modified carbon fiber microelectrodes (CFMEs). Enzymes often are sensitive to curvature upon surface adsorption; thus, it was important to deposit discrete nanoparticles to maintain enzyme activity while depositing as much gold as possible to maximize enzyme coverage. To further enhance sensitivity, the enzymes acetylcholinesterase (AChE) and choline oxidase (ChO) were immobilized onto the gold nanoparticles at the previously determined optimal ratio (1:10 AChE/ChO) for most efficient sequential enzymatic activity. This optimization approach has enabled the rapid detection to temporally resolve single vesicle acetylcholine release from an artificial cell. The sensor described is a significant advancement in that it allows for the recording of acetylcholine release on the order of the time scale for neurochemical release in secretory cells.

Neuronal activity and brain glucose metabolism are tightly coupled, where triggered neurotransmission leads to a higher demand for glucose. To better understand the regulation of neuronal activity and its relation to high-speed metabolism, development of analytical tools that can temporally resolve the transients of vesicular neurotransmitter release and fluctuations of metabolites such as glucose in the local vicinity of the activated neurons is needed. Here we present an amperometric biosensor design for rapid co-detection of glucose and the neurotransmitter dopamine. The sensor is based on the immobilization of an ultra-thin layer of glucose oxidase on to a gold-nanoparticle-covered carbon fiber microelectrode. Our electrode, by altering the potential applied at the sensor surface, allows for the high-speed recording of both glucose and dopamine. We demonstrate that, even though glucose is electrochemically detected indirectly through the enzymatic product and the electroactive dopamine is sensed directly, when exposing the sensor surface to a mixture of the two analytes, fluctuations in glucose and dopamine concentrations can be visualized with similar speed and at a millisecond time scale. Hence, by minimizing the enzyme coating thickness at the sensor surface, dual detection of glucose and dopamine can be realized at the same sensor surface and at time scales necessary for monitoring fast metabolic alterations during neurotransmission.

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To get insight into the stimulus-dependent translocation of mRNA encoding neuropeptides to the axonal compartment of neurons, we investigated this process in the egg-laying hormone producing caudodorsal cells of the mollusk Lymnaea stagnalis. The axonal compartment including the nerve terminals of these neurons harbors high amounts of mRNA encoding the egg-laying hormone precursor. We determined how a sensory stimulus, that results in egg-laying, affected the amount of egg-laying hormone encoding transcripts in the axon endings. Four hours after stimulation high amounts of transcripts were detected in the axonal compartment and maximum values were reached after 8 h. Transcript levels in the somata were affected in a similar fashion, although the increase was not as pronounced as in the axons. Next, we investigated the ultrastructural localization of egg-laying hormone encoding transcripts in axons and axon terminals by means of electron microscopic in situ hybridization and showed that transcripts were localized in the axoplasm. By means of conventional electron microscopy we showed that axon terminals of egg-laying hormone producing neurons contained large amounts of polyribosomes. Together, these data support the notion that egg-laying hormone encoding transcripts are translated in the axonal compartment.

To immobilize enzymes at the surface of a nanoparticle-based electrochemical sensor is a common method to construct biosensors for non-electroactive analytes. Studying the interactions between the enzymes and nanoparticle support is of great importance in optimizing the conditions for biosensor design. This can be achieved by using a combination of analytical methods to carefully characterize the enzyme nanoparticle coating at the sensor surface while studying the optimal conditions for enzyme immobilization. From this analytical approach, it was found that controlling the enzyme coverage to a monolayer was a key factor to significantly improve the temporal resolution of biosensors. However, these characterization methods involve both tedious methodologies and working with toxic cyanide solutions. Here we introduce a new analytical method that allows direct quantification of the number of immobilized enzymes (glucose oxidase) at the surface of a gold nanoparticle coated glassy carbon electrode. This was achieved by exploiting an electrochemical stripping method for the direct quantification of the density and size of gold nanoparticles coating the electrode surface and combining this information with quantification of fluorophore-labeled enzymes bound to the sensor surface after stripping off their nanoparticle support. This method is both significantly much faster compared to previously reported methods and with the advantage that this method presented is non-toxic. Graphical abstractA new analytical method for direct quantification of the number of enzymes immobilized at the surface of gold nanoparticles covering a glassy carbon electrode using anodic stripping and fluorimetry

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